Fiber Amplification of Diode Lasers

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Department of Science and TechnologyLaser Physics and Nonlinear Optics

RF mode locking and fiberamplification of diode lasers

M.G. Hekelaar

Graduation thesis

Enschede, June 2005


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University of Twente

Department of Science and Technology

Laser Physics and Nonlinear Optics

RF mode locking and fiberamplification of diode lasers

M.G. Hekelaar

Graduation committee:

Prof. Dr. K.-J. Boller

Dr. P. Gro

Dr. ir. H.L. Offerhaus

Enschede, June 2005


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Abstract

Semiconductor lasers are compact and very efficient sources of coherent radiation. By modulatingthe driving current of such lasers, picosecond pulses can be generated. One important applicationof ultrashort pulses is that the efficiency of frequency of conversion stages, like optical parametric

oscillators (OPOs), can be greatly enhanced. By a synchronously pumped optical parametricoscillator access into the mid-infrared spectral region can be created. For an efficient wavelengthconversion process, these OPOs require very high pump powers of several watts.

In this work, we present the first mode-locked diode laser around 1.06 m wavelength withsubsequent amplification of the pulses in an ytterbium-doped fiber amplifier.

To obtain ultrashort pulses, the diode laser has been actively mode-locked by modulation of theinjection current. The antireflection coated diode laser has been placed in an external cavity with itslength adjusted to match the modulation frequency. Difficulties caused by the residual reflectivityof the antireflection coated facet have been resolved by spectral limitation of the emitted light. Adiffraction grating provides selective feedback, after which emission of nearly Fourier-limited pulsesresults. The pulses have a duration of approximately 35 ps at a repetition rate of 1.4 GHz.

For power amplification of the mode-locked pulses from the diode laser, we use a claddingpumped double-clad ytterbium-doped fiber. After amplification, the average output power is morethan 9 W. With slightly shorter pulse durations than in the unamplified case, this corresponds topeak powers of more than 100 W.

By grating tuning the diode laser, the fiber output is tunable from 1050 to 1085 nm with averageoutput powers of more than 9 W and corresponding peak powers of more than 100 W. Based uponour previous experience, these output parameters provide ideal conditions to synchronously pumpmid-IR OPOs. We note that no other experiments so far have shown such high-power pulses incombination with such large wavelength tunability.

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Acknowledgement

Ten months ago I started my graduation period at the Laser Physics and Nonlinear Optics groupat the University of Twente. Starting from almost zero, my knowledge of experimental optical workhas grown tremendously during these months. The results presented in this thesis would have been

almost impossible without the help of some members of the group (and, of course, the nature ofthe universe).First of all I would like to thank my supervisor, Petra Gro, for her time, explanation, support

and elitehaver. Secondly, Balaji Adhimoolam, thank you for the lot of joyful time we spent in thelab. Further I would like to thank Ian Lindsay and Prof. Klaus Boller for their useful suggestionsand advices.

I also wish to thank Remco Nieuwland, Rolf Loch, Arie Irman and Ab Nieuwenhuis for themany discussions and their experimental help. I am grateful to Martin Fransen for his assistancein revealing some mysteries of the RF electronics. Besides the graduation students from the group,Rolf Loch, Marten de Wit, Arjan Verkerk, Mark Luttikhof, Willem Beeker, Edip Can and Johan-Martijn ten Hove, also Marieke van As, Remco Nieuwland, Bert Borger and Maarten van Zalk

often contributed to very pleasant lunch breaks.Finally, I would like to thank the graduation committee members for reading my thesis, espe-cially Herman Offerhaus from the Optical Techniques group for willing to be the external committeemember.

Thijs Hekelaar,June 2005

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Contents

1 Introduction 6

2 Theory 8

2.1 Diode lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.1 Lasers in general . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.2 Principle of operation of diode lasers . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Ultrashort pulse generation in diode lasers . . . . . . . . . . . . . . . . . . . . . . . . 102.2.1 Mode locking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.2 Active mode locking in diode lasers . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Pulse amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Measurement 17

3.1 Ultrashort pulse measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.1.1 Intensity autocorrelation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.2 Interferometric autocorrelation . . . . . . . . . . . . . . . . . . . . . . . . . . 183.1.3 Rotating-mirror autocorrelator . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.2 Spectral measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.2.1 Optical spectrum analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.2.2 Scanning Fabry-Perot interferometer . . . . . . . . . . . . . . . . . . . . . . . 24

4 Experimental results 26

4.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.1.1 Diode laser setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264.1.2 RF electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2 Continuous wave diode laser characteristics . . . . . . . . . . . . . . . . . . . . . . . 29

4.3 Measurements on mode-locked diode lasers . . . . . . . . . . . . . . . . . . . . . . . 324.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.3.2 Spectral measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3.3 Autocorrelation measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 364.3.4 Power measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.3.5 Wavelength tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.3.6 Influence of parameter changes on pulse duration . . . . . . . . . . . . . . . . 394.3.7 Summary of experimental results . . . . . . . . . . . . . . . . . . . . . . . . . 40

5 Pulse amplification 41

5.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.2 Measurements on amplified mode-locked pulses . . . . . . . . . . . . . . . . . . . . . 435.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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CONTENTS

5.2.2 Spectral measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435.2.3 Autocorrelation measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.2.4 Power measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465.2.5 Wavelength tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465.2.6 Summary of experimental results . . . . . . . . . . . . . . . . . . . . . . . . . 47

6 Summary 49

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Chapter 1

Introduction

Diode lasers are highly efficient, compact sources of coherent radiation, which are widely used inscience and technology, for example in CD players, for communication technology or for opticalpumping of solid-state lasers.

By modulating the driving current of such diode lasers, pulses with picosecond duration canbe generated, thereby enabling an even larger range of other applications [3]. One importantpossibility is that the efficiency of frequency of conversion stages (e.g. second harmonic generation,optical parametric oscillation, etc.) can be greatly enhanced.

Over the last decade, there has been considerable interest in powerful mode-locked light sourcesin the wavelength range around 1 m. Such sources are ideal for sensitive environmental monitoringand for test and measurement purposes. Pulses of picosecond duration, for example, are suitablefor ultrafast spectroscopy within the lifetime of molecular energy levels. Access further into the

mid-infrared spectral region can be readily created by efficient wavelength conversion processes,such as synchronously pumped optical parametric oscillators (OPOs) [4, 5].

Interest in the 1 m region has been boosted by recent developments in cladding pumpedytterbium-doped silica fiber lasers and amplifiers. Offering a broad gain bandwidth and a high pumpconversion efficiency, they form an almost ideal gain medium for the generation and amplification ofwavelength-flexible and short pulses in the spectral region between 1000 and 1100 nm. Short pulsegeneration has been demonstrated using ytterbium-doped fiber lasers, followed up by a ytterbium-based amplification [6, 7, 8]. However, due to the high dispersion in such a system, rather complexcavities are required. Additionally, the extended fiber lengths lead to an inherently low repetitionrate (tens of MHz), compelling large cavity lengths for synchronous pumping of, e.g., OPOs.

Mode-locked semiconductor lasers are a highly attractive alternative to mode-locked bulk and

fiber solid state laser oscillators. Such diodes, in addition to the simple cavity design, offer high(GHz) and variable repetition rates, suitable for driving compact synchronously pumped OPOs.The electronically controllable repetition rate can be employed for rapid OPO tuning [9], whilethe narrow spectral bandwidth of pulses in the picosecond range ensure efficient conversion in longnonlinear crystals. In addition, the limited spectral bandwidth of these pulses yields a spectralresolution suitable for spectroscopic detection schemes in ambient pressure-broadened molecularspecies.

Surprisingly, despite the potential as seed sources for Yb-doped fiber amplifiers, there is onlya single experimental demonstration of a mode-locked laser at wavelengths above 1 m [10], inwhich an internal absorber section was utilized to produce pulses of around 1 ps duration. Fiberamplification of a gain-switched Fabry-Perot (FP) diode laser with 20 ps pulse duration has been

demonstrated very recently [11]. However, none of the experiments presented until today exploits

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CHAPTER 1. INTRODUCTION

the superior wavelength flexibility of mode-locked diode lasers in connection with amplification tohigh power over a large spectral range.

In this work, we present the first mode-locked diode laser around 1.06 m wavelength with subse-quent amplification of the pulses in an ytterbium-doped fiber amplifier. The pulses are wavelength-tunable over 45 nm and have a pulse duration of approximately 35 ps at a repetition rate of 1.4 GHz.The average power of around 15 mW from the mode-locked diode laser is amplified to a maximumaverage power of 9.5 W, corresponding to a peak power of more than 100 W.

The basic principles of lasers and diode lasers in particular are described in Chapter 2. Modelocking, the technique that is employed in this work to generate short pulses, is explained and howto achieve this in diode lasers is discussed in more detail. Furthermore, a short introduction to thebasic properties of ytterbium-doped fiber amplifiers is given, as such a fiber is used to boost thepower level of the generated pulses.

To characterize the pulses before and after amplification, an autocorrelator is designed. InChapter 3, first the autocorrelation technique is clarified, before the specific autocorrelator thatis used in the actual work is explained in detail. After that, the instruments to perform spectralmeasurements are listed and details about the scanning Fabry-Perot interferometer are given.

The experimental realization of the mode-locked diode lasers is described in Chapter 4. First theexperimental setup is presented and characteristics of the diode lasers in continuous-wave operationare determined. During the course of the measurements with the mode-locked diode laser, spectraland temporal characteristics are presented, together with output power and wavelength tuningproperties.

Chapter 5 deals with the amplification of the mode-locked pulses in an ytterbium-doped fiber.First, changes in the experimental setup are described, after which the results obtained from am-

plification of the mode-locked pulses are presented.

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Chapter 2

Theory

Overview In this chapter a brief theoretical characterization of diode lasers and ultra-short pulsegeneration in diode lasers is given. First, in section 2.1, the fundamental concepts of lasers in gen-eral are described and of diode lasers in particular, which provides basic understanding of how diodelasers emit a coherent beam. In section 2.2 several methods of pulse generation in diode lasers arebriefly examined. Mode locking is set out theoretically in detail, after which specific details aboutmode locking in diode lasers are outlined. Unless indicated otherwise, the majority of this theory istaken from [12] and [13]. Finally, a short introduction to ytterbium-doped fiber amplifiers is givenin section 2.3.

2.1 Diode lasers

2.1.1 Lasers in general

The word laser is an acronym and stands for Light Amplification by Stimulated Emission of Radi-ation. This explains roughly how a laser actually emits light.

In its simplest form a laser is formed by the amplifying medium and a set of mirrors to provideoptical feedback of the light into the amplifier for growth of the light intensity. The amplifyingmedium gains energy from a pump source, e.g. an electric current or a flash light. The light thatis created in the active medium travels back and forth between the mirrors and develops into abeam with unique properties by an amplifying light-matter interaction, called stimulated emission.

A part of this light is transmitted through one of the mirrors and can be used for the specificapplication.

A laser is quite different from a light bulb. Radiation from a light bulb is emitted in all directions,whereas a laser concentrates the light that would usually be radiated in all directions into a singlehigh intensity beam in one direction. This is one of the unique properties of stimulated emission.In addition to the direction, also the frequency, phase and polarization are all equal for all photonsin the beam. Another difference, as compared to the light bulb, are the photon statistics. In alight bulb, the output intensity fluctuates randomly, because all light is generated by spontaneousemission. In a laser, a repetitive pattern of output intensity is emitted because of the repetitivestimulated emission occurring in the resonator once per round trip.

Pumping the gain medium excites, e.g., atoms, molecules or ions to higher energy levels. Elec-

trons decaying to lower levels can emit photons. These photons can be absorbed again by themedium to excite another atom, but can also leave the medium. When they are not absorbed, they

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CHAPTER 2. THEORY

can, on their way out, stimulate other exited atoms, molecules or ions to emit phase-coherently.Without this process there would be no laser. When the rate of emission exceeds the rate of ab-

sorption, i.e. when is the gain in the system is greater than the total loss (absorption, outcoupling,scattering, etc.), stimulated emission develops into a coherent beam. The dimension of the laserresonator forms a condition for this emission, namely that a wave must overlap with itself afterone round trip, i.e. constructive interference within the resonator. Fields that satisfy this conditionbecome standing electromagnetic waves in the cavity and are called eigenmodes or simply modes.Depending on the gain bandwidth, the number of longitudinal modes may be very large or as smallas only a few or just one, which is sometimes very advantageous. Studying the transverse beamcross section, the light intensity can be found in different distributions. Such patterns are calledtransverse electromagnetic modes (TEM). To indicate the TEM-modes, three indices can be used,TEMnpq, where n is the number of longitudinal field nodes and p and q are the number of radialand angular nodes respectively. Since the longitudinal mode number is generally very large for

optical frequencies, usually only the two latter transverse indices are used to specify a TEM-mode.Higher order transverse modes are more difficult to focus, so often the TEM00 mode, also namedfundamental Gaussian beam, is preferred.

2.1.2 Principle of operation of diode lasers

Semiconductor diode lasers distinguish themselves from the various other types of lasers by theirextremely small size and high efficiency. With typical dimensions of less than a millimeter in alldirections, these solid state devices form compact, low-cost sources of coherent light. Due to theirhigh efficiency, optical output powers of several tens of milliwatts can be produced by relativelylow electrical input powers.

As lasers in general, diode lasers consist of a gain medium and two mirrors. The active mediumof a diode laser is an advanced form of a p-n-junction, pumped by an electric current. Usually themirrors are the facets of the semiconductor chip.

When forward biased, the p-n-junction starts radiating due to the recombination of holes fromthe p-region and electrons from the n-region. Additional non-radiative recombination expressesitself as heat. The narrow depletion region of the junction in which the recombination processtakes place is called the active region. In principle, the spontaneous emitted light is incoherent andtherefore no lasing occurs. Electrons and holes can also absorb the emitted radiation, at which anelectron-hole pair is generated. Stimulated emission develops when the gain exceeds the loss, whichhappens above the threshold injection current. A population inversion is achieved as the holesand electrons in the active region can coexist for a rather long time (several nanoseconds) beforethey recombine. If during this time a photon of exactly the right frequency passes, it forces thehole and the electron to recombine and another photon is emitted with exactly the same properties,frequency, direction, phase and polarization as the incident photon. Photons resonating between thetwo mirroring edges of the junction maintain the stimulated emission and thus keep the laser fieldoscillating. When the diode is lasing, the optical output rises faster with increasing input currentthan below the threshold current. This can be explained as follows. Increasing the injection currentcauses an increase of injected charge carriers (electrons and holes) into the active region. Below thethreshold, this results in an increased charge carrier density, whereas above threshold the chargecarrier density remains the same. In the latter case, the increased injection current only favors theoptical output power.

With gain coefficients generally between 5.000 m1 and 10.000 m1 and losses of approximately2.000 m1 [12], semiconductor lasers show much higher single pass gains than other types of lasers.Therefore the size of diode lasers can be extremely small, usually less than a millimeter. In case the

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CHAPTER 2. THEORY

facets of the diode form the resonator, the free spectral range of the laser is very large (1/sep =c/2nd = 50100 GHz), i.e. the modes show a large spectral separation. Also the gain bandwidth of

these lasers is extremely broad (tens of nm), so generally multi-longitudinal mode operation results.

2.2 Ultrashort pulse generation in diode lasers

Pulses with picosecond duration are of high interest since decades. They enable a large rangeof applications, see e.g. [3]. For example, nonlinear frequency conversion (e.g. second harmonicgeneration, optical parametric oscillation (OPO), etc.) can be greatly enhanced in efficiency [14, 9].

Several methods to generate short pulses in diode lasers are available, among them Q-switching,gain switching and mode locking. With the Q-switching technique, the laser medium is pumpedwhile feedback from the resonator is prevented (low Q value of the resonator). A populationinversion occurs and a large amount of energy is stored in the gain medium. As the amount of

energy reaches a maximum level because of losses by spontaneous emission, the Q-switch (to a highQ value) suddenly allows feedback and the intensity of light in the resonator builds up very quicklybecause of stimulated emission. As a results, the energy stored in the medium then depletes veryquickly and a short pulse of light is emitted from the laser. Q-switching leads to pulses in thenano- or picosecond regime with repetition rates of up to a few MHz. In gain switching, the gain ismodulated (in stead of the losses), bringing the carrier density within the active region from belowto above the lasing threshold. Typical pulse durations in the picosecond regime are obtained fromgain-switched diode lasers. A common feature of Q-switching and gain-switching is that subsequentpulses lack phase relation, so they are suitable only for a limited number of experiments.

In particular, the synchronous pumping of OPOs require a phase relation between subsequentpump pulses, thus a different approach to pulse generation is required. Mode locking of diode lasers

can provide the required phase relation. This is why mode locking is elaborated below.

2.2.1 Mode locking

The longitudinal modes in a Fabry-Perot laser cavity are separated by sep = c/2nd, in which ndis the optical length of the cavity with the refractive index n and length d. If the gain bandwidthis broader than this mode spacing, more than one longitudinal mode can, in principle, oscillate.Assuming a cavity containing N light modes of, e.g., equal amplitude, E0, each of which has afrequency and a phase , the total amplitude can be mathematically expressed as

E(t) = E0

N

n=1

ei(nt+n) (2.1)

Normally the phases are independent of each other and for large N an intensity is expected,which fluctuates to some, possibly large, extent around an average value of N times the intensityof one mode.

I(t) = |E(t)|2 = E20Nn=1

ei(nt+n)

2

= NE20 (2.2)

The fluctuation around this value has a repeating character induced by the modes circulatingthrough the cavity, producing the same output every round trip. For small N, beat patterns may

occur due to interference of the few modes.

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Now consider a fixed linear relationship between the phases of the laser modes. All oscillatingmodes then are phase-locked to each other, and the process is termed mode locking. For this to

happen, an intra-cavity loss or gain modulator operating synchronously with the cavity round tripfrequency, sep, is required. The modulation generates side bands which give rise to an energytransport between neighbouring modes. With it, phase information is transferred and after severalround trips, all modes agree on a common phase.

Inserting the linear phase relationship, e.g. as a constant phase offset from mode to mode, = n+1 n, into Eq. (2.1), the combined field amplitude can be rewritten as

E(t) = E0ei(Nt+N)

1 eiN(t+)

1 eit+

(2.3)

and the intensity becomes

I(t) = E20

1 eiN(t+)1 eit+

2

= E20sin2 (N(t + ) /2)

sin2 ((t + ) /2)(2.4)

where = n+1 n and = n+1 n.

0Time

Intensity

50 modes

5 modescw

resonator round trip time

Figure 2.1: Locked modes (upper), showing periodic maxima when the phases of all modes

match (lower).

From Eq. (2.1) and Eq. (2.4) and by taking, e.g., n = 0 and = 0, Fig. 2.1 can be constructed.The upper part shows the real part of the complex field of five laser modes as function of time(green). The lower part shows the intensity as function of time for two different numbers of modes,5 (magenta) and 50 (blue), and the normalized intensity in case the laser is operated in a singlemode with continuous wave (cw) output (red). The fives modes show an equal phase once perround trip (tsep = 1/sep), whereas at all other times the modes interfere destructively. Also,once per round trip, a pulse with large intensity occurs, containing the same amount of energyas the continuous output per round trip, but concentrated in a small time window. More modes

have a shorter time of coincidence, which results in narrower peaks with larger peak intensities. By

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CHAPTER 2. THEORY

inspecting Eq. (2.4), one obtains that the maximum intensity is increased by a factor ofN over theaverage intensity.

I(t)max = N2E20 (2.5)

The shortest pulse duration at full width half maximum (FWHM) that can be generated isrelated to the optical cavity length, nd, and the number of oscillating modes, N, as follows

tp,min =2nd

Nc=

2

N=

1

sepN 1

gain bandwidth(2.6)

and is ideally as short as the reciprocal of the gain bandwidth. Experimentally generated pulsesthat fulfill this condition, i.e. pulses with a duration of the inverse gain bandwidth, are referred toas bandwidth-limited pulses. For example, in case of pulses with a Gaussian temporal shape, the

minimum pulse duration is tp,min = 0.441/N. The value 0.441 is known as the time-bandwidthproduct and depends on the pulse shape. The minimum attainable time-bandwidth product of asecant hyperbolic squared pulse shape is 0.315 and 0.11 for a single sided exponential shape [15].The actual pulse shape can be estimated by fitting an experimental autocorrelation trace to atheoretical one assuming a certain pulse shape.

Mode locking techniques

A number of mode locking techniques is available, which roughly can be divided into three groups.

Active mode lockingIn active mode locking, the gain or loss of the laser is modulated. An external high frequency,

usually a radio frequency (RF) signal is applied to modulate either the laser gain or lossdirectly or to drive an intra-cavity active device. Modulating the gain or loss must be doneexactly synchronized to the pulse round trip time through the cavity, tsep = 2nd/c. Thismodulation causes sidebands to develop, with which phase information is transferred. Asthis relates the phases of all modes, pulses are formed as described in the current section.This technique enables ultrashort pulses in the picosecond regime to develop, with high phasestability between successive pulses.

Passive mode lockingMode locking can be obtained with a passive medium like a saturable absorber or a Kerr lensinside the cavity. These intensity-dependent shutters are transmitting light if the intensity

is high, such that they require no external control (thus the form passive) because they arecontrolled by the arrival time of the pulse itself. Mode locking is initiated by peaks in theat first randomly fluctuating intensity inside the laser cavity. Since the higher intensities aretransmitted, the peaks, consisting of modes with equal phase, grow and eventually lead toa single high-intensity peak traveling through the cavity by filtering out all other (wronglyphased) mode superpositions. The nonlinear effect of saturable absorbers enables ultrashortpulses in the femtosecond regime, providing that the gain profile is sufficiently large.

Hybrid mode lockingHybrid mode locking is a combination of active and passive mode locking, where both an RFsignal and a passive medium are used to produce ultrashort pulses. It takes advantage of

active mode-locked stability and the saturable absorbers pulse shortening mechanisms.

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2.2.2 Active mode locking in diode lasers

In active mode locking, modulation the loss or gain of a diode laser at a frequency equal to the

intermodal spacing, sep, results in optical sidebands to emerge. Each of these modes is beingdriven by the modulation sidebands of its neighbours [16], and consequently, the phases of themodes are locked to each other.

The intermodal spacing of diode lasers (typically 50 to over 100 GHz) is at the upper end ofexisting RF technology due to the very small dimensions of the laser medium, which makes directmodulation technically complicated. To decrease the spacing between the modes, lengthening thecavity is an easy possibility, so mode locking can be performed at a much lower standard radiofrequency. Lengthening the cavity can be achieved by an external cavity, created by removing onemirror from the diode laser via an antireflection (AR) coating and replacing it by an external oneat some suitable distance.

M DL

M L DL DG L DL

F DL FBG DL

a)

b)

c)

d)

e)

f)

M FP L DL

Figure 2.2: External-cavity configurations of mode-locked diode lasers. DL diode laser, M mirror, L lens, FP Fabry-Perot etalon, DG diffraction grating, F fiber, FBG Fiber

Bragg grating

Several commonly used external-cavity configurations are schematically illustrated in Fig. 2.2.The first experimentally mode-locked diode lasers used a simple cavity, where a curved mirrorreflects the light back into the diode. Such a setup is drawn in (a). It has the disadvantage thatthe that mirror curvature has to be chosen according to the applied frequency or vice versa. Asolution to this limitation can be realized using a collimating lens and a plane mirror (b) or a fiber(c). For additional selection of operation wavelength or for bandwidth reduction, implementing aFabry-Perot etalon (d), a diffraction grating (e) or a fiber Bragg grating (f) are commonly usedoptions.

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Spectral properties

To reduce the required modulation frequency from 50100 GHz to lower values, the diode laser has

to be placed in an external cavity configuration. However, this causes additional complications.A finite residual reflectivity of the front facet, which always remains after antireflection coating,usually results in the formation of a three-mirror cavity. Such a composite cavity consists of thetwo diode laser facets and the external reflector. Then, also the reflectivity of the front diode facetdetermines the resultant spectrum of laser emission.

The composite Fabry-Perot resonator effect implies a non-trivial optical output spectrum, whichis a convolution of two Fabry-Perot cavity spectra. An example of such a spectrum of two combinedcavities is given in Fig. 2.3, where the intensity of the modes is plotted as function of the opticalfrequency. The external mirror with the diode rear facet usually form a high-Finesse cavity with

Frequency

Intensity

d

c

Figure 2.3: Typical output spectrum of a mode-locked diode laser with an imperfect antire-flection coating on the front diode facet. The clusters of external cavity modes are separatedby the intermodal spacing of the diode laser.

larger mirror spacing, i.e. the resulting spectrum consists of rather sharp modes with a smallspacing, c. This spectrum is convoluted with the spectrum generated by the two laser diodefacets. As one of the facets is AR coated, the facets from a short, low-Finesse cavity, whichresults in a spectrum of broad modes with a large spacing, d. The resulting spectrum exhibits aclustered structure of the modes with the clusters spaced by d. Although the modes within sucha cluster are locked, separate clusters are not locked to each other, since they rise from statisticallyindependent noise sources of spontaneous emission. Generated pulses with such a spectrum have atemporal substructure and are far from bandwidth limited.

Former studies have shown that a high quality AR coating with a residual facet reflectivityof less than 104 [17] is required to prevent mode cluster formation. Otherwise, a weak feedbackin combination with the high single pass gain is sufficient to drastically reduce the lockable gainbandwidth to less than the intermodal spacing of the diode cavity [16]. In that case, the modes ofonly one of the clusters in Fig. 2.3 should be locked. If the pump power is high enough for modes ofmore than one cluster to reach threshold, only the modes within a cluster are locked. Such emissionconsisting of several clusters cannot produce bandwidth-limited pulses. A solution is to restrict thevery broad optical spectrum to a single mode cluster by a Fabry-Perot etalon, diffraction gratingor a fiber Bragg grating, as shown in Fig. 2.2 (d), (e) and (f).

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Reducing the spectral bandwidth is one option to avoid the effects of the residual reflectivity ofthe front facet. Another solution is to use an angled-stripe semiconductor laser [18], in which the

gain channel is not exactly perpendicular with respect to the facets. This prevents the formationof the second resonator between the diode facets. A third solution to the issue is to lock more thanone mode cluster, which can be realized by hybridly mode locking the laser by placing a saturableabsorber in the cavity. A broader spectrum and thus shorter pulses can be achieved.

Pulse quality

The time-bandwidth product, tp, is often used to indicate the quality of optical pulses. Band-width or Fourier-limited pulses are theoretically the shortest pulses that can be obtained from agiven amplitude spectrum. From the mode locking theory follows that a linear phase spectrumresults in the shortest pulse. This minimized pulse duration yields the smallest possible time-bandwidth product. For this number to give an indication of the pulse quality, the pulse shapemust be known. In the case of a sinusoidal gain modulation, Gaussian pulse shapes have been ob-served in actively mode-locked solid state lasers. In diode lasers however, the strong carrier photoninteractions [13] and the carrier lifetimes in the order of the modulation period, causes significantdifferent gain modulation profiles, resulting in secant hyperbolic squared [13, 19] of single sidedexponential [15] temporal pulse shapes. In the case of mode-locked diode lasers with an imperfectAR coating, which show optical spectra like in Fig. 2.3, the random phases of the mode clusterslead to excessive values for the time-bandwidth product. Reducing the lasing spectrum to a singlemode cluster by inserting a Fabry-Perot etalon in the cavity or by replacing the external mirror bya diffraction grating can result in nearly bandwidth-limited pulses, with tp in the range of 0.3to 0.6 [13, 15].

The time-bandwidth product often is reduced due to a wavelength chirp, which is frequentlyobserved in experimental work on actively mode-locked diode lasers [20]. Such a wavelength chirpcan be caused by refractive index changes. The carrier density (concentration of electrons andholes) determines the refractive index of the diode and during the optical pulse emission the carrierdensity is affected. The instantaneous frequency over the produced mode-locked pulse typicallydecreases (red chirp) [20]. Hence the time-bandwidth product is far from limited. Because thechange of refractive index with carrier density generates a relatively linear chirp [20], the chirp isfound to be linear near the peak of the pulse [21]. Towards the wings of the optical pulse, therefractive index change due to the electron temperature becomes more dominant, which results ina nonlinear chirp [20]. According to [21], the amount of chirp and its linearity depends on thewavelength and the spectral bandwidth.

To counteract the wavelength chirp, dispersion compensating optical components can be placedinto the external cavity like, e.g., a chirped mirror [22]. Linearly chirped pulses can be effectivelycompressed using well-known optical pulse compression techniques, like a grating, prism pair orsuch chirped mirrors.

2.3 Pulse amplification

High power pulses in the near-infrared (NIR) region, 13 m, are interesting for many applica-tions, such as spectroscopy, laser and amplifier pumping and frequency conversion. Diode-pumpedytterbium-doped fiber amplifiers offer a large gain-bandwidth, high efficiency and high reliability.Such optical amplifiers are perfectly suitable to reach high powers in the 1-1.1 m region utilizing

low power seed sources [23, 24]. Pumping an OPO with pulses in he wavelength region 1-1.1 mcan give access to the mid-IR region [4, 5, 25], which is interesting for, e.g., spectroscopy purposes.

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CHAPTER 2. THEORY

Ytterbium-doped (Yb-doped) silica has very broad absorption and emission spectra. Bothabsorption and emission as function of wavelength are shown in the right-hand side of Fig. 2.4.

Both spectra show a narrow peak at 975 nm. The left-hand side of Fig. 2.4 shows the energy levels

0

4000

8000

12000

E(cm

1)

gfe

dcb

a

2F5/2

2F7/2

Figure 2.4: Energy levels (left) and absorption and emission spectra [26](right) of ytterbium-doped silica.

of Yb-doped silica. The main absorption and emission peak at 975 nm corresponds to an energytransition between the lowest levels of both manifolds in the energy level structure. Absorptionbelow this wavelength mainly results from transitions from level a to f and g, whereas transitionsfrom level e to b, c and d are responsible for the emission at higher wavelengths [27].

When amplification of a particular wavelength is required, a pumping range of 8001064 nmis available. However, gain is generated only at wavelengths longer than the pump wavelength, soamplification of pulses in the 1-1.1 m region requires the pump wavelength to be below 1 m.

High output powers can be obtained from so-called double-clad fibers. A single mode Yb-dopedcore is surrounded by a larger multi-mode undoped inner cladding. The pump light is sent intothe multi-mode inner cladding and due to the overlap these modes have with the doped inner core,pump light can be absorbed there. A drawback of cladding pumping is that three-level transitionscause significant reabsorption of signal in weakly pumped regions and hardly any amplification isachieved. However, for wavelengths above 1040 nm, four-level transitions dominate and efficientcladding pumping is possible [28].

The lower pump absorption per unit length, by cladding pumping in stead of core pumping,

can be overcome by a longer fiber length or a larger absorption cross-section. Because of its largeabsorption cross-section, the sharp absorption line at 975 nm is an ideal pump wavelength forcladding pumped Yb-doped fiber amplifiers.

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Chapter 3

Measurement

Overview In Chapter 2 mode locking as a method of pulse generation was introduced. Thepresent chapter will discuss how these ultrashort pulses can be measured using the autocorrelationtechnique (section 3.1). A general theoretical description is given and certain practical issues arediscussed. Finally, the design of an autocorrelator is worked out and the details on the actualexperimental setup used in this work are given. The theoretical approach of autocorrelation usedin the following sections is mainly based on [29]. In addition to the measurement of the temporalproperties, Chapter 3 also deals with the measurement of spectral properties of diode lasers. Insection 3.2, two ways of doing this are presented, namely the optical spectrum analyzer and theFabry-Perot interferometer.

3.1 Ultrashort pulse measurement

Short light pulses of durations down to about one picosecond, can today be measured by optoelec-tronic methods. However, to visualize the signal from a photodiode with a response time of a fewhundred femtoseconds, an oscilloscope with a response time of the same order is required. Anotherpossibility is to use a streak camera, a high precision device that not only gives the pulse width,but also details of the temporal profile.

The main drawback of these methods is that they require very expensive equipment. Becauseof that, other techniques are considered to measure ultrashort events. One such method is opti-cal autocorrelation, a widely used diagnostic technique to determine whether a laser is actually

producing short pulses.

3.1.1 Intensity autocorrelation

In order to measure the temporal profile of an ultrashort event, an even shorter reference event ofknown shape is required. Let Is(t) be the unknown ultrashort pulse and Ir(t) be a reference pulse.With the delay parameter , the intensity cross-correlation is defined as

Ac() =

Is(t)Ir(t )dt (3.1)

The measured signal, Ac(), differs from zero only if the pulse to be measured and its referencepulse overlap temporally. The reference pulse represents a measurement window which is shifted

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CHAPTER 3. MEASUREMENT

across Is(t) in order to scan the temporal profile. With the idealistic assumption of Ir(t) being adelta-function, Ac() is identical to Is(t). However, all phase information of the analyzed pulse is

lost in this intensity cross-correlation through conjugate multiplication.The prerequisite of a reference signal shorter than the optical pulse to be analyzed is often notavailable. Even if it is, the shape of this signal must be determined. A very short signal in the formof the optical pulse itself however, is available at all time. Although the pulse of course is not shorterthan itself, one should consider to use it as the reference signal. In this case Ir(t) = Is(t) = I(t)and Eq. (3.1) becomes the intensity autocorrelation.

A() =

I(t)I(t )dt (3.2)

As can be understood from Eq. (3.2), the autocorrelation function is always symmetric. De-spite the fact that the autocorrelation provides little information about the pulse shape and no

information about the phase, the pulse duration can be estimated. For this, most often a certainpulse shape is assumed and fitted to the autocorrelation trace. With the help of the known ratiobetween the FWHM of the autocorrelation and that of an assumed pulse shape, the approximatepulse duration is determined.

3.1.2 Interferometric autocorrelation

In practice, an optical autocorrelator is often realized with a setup based on the Michelson in-terferometer. Two replicas of the input pulse are generated, e.g. using a 50% beamsplitter, andthen overlapped again with a variable delay, . In the Michelson interferometer, one replica is sentback to the beamsplitter via a static arm, i.e. E(t), while the other travels a variable path length,

E(t ).From the theory of the Michelson interferometer, one can find the field autocorrelation, defined

as

A() + c.c =

E(t)E(t )dt + c.c. (3.3)

A rather slow detector at the output of the interferometer can be used to experimentally measurethis field autocorrelation. In fact, for pulse trains, the detector gives an output signal which isaveraged over many pulses. The signal is integrated, which results in the measured intensity being,expressed as

G1() =

|E(t) + E(t

)

|2 dt

=

{I(t) + I(t )} dt + A() + c.c.,(3.4)

where the term A() + c.c. represents the field autocorrelation as given in Eq. (3.3). From this firstorder autocorrelation still no reliable information about the pulse duration can be obtained. Noisesources, chirped pulses or ultrashort pulses may produce exactly the same output, as it reveals onlythe coherence time of the input signal, which is short in all these cases.

To acquire a correct pulse duration from the interferometer a nonlinear process in front of thedetector is necessary. An often used, suitable nonlinear process is a second harmonic generating(SHG) crystal, in which the second harmonic is proportional to E2. The second order nonlinear

effects can be accredited to an energy transfer between electromagnetic fields of different frequencies

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within the material [30]. Second harmonic generation is a special case of the more general conversion = + , namely that = , so:

= 2 (3.5)

In this case the incoming light at the fundamental frequency, , is converted into light with twicethat frequency, . Second order nonlinearities only occur in materials with a noncentrosymmetriccrystal structure such as Beta Barium Borate (BBO).

Employing such a second order process, the detected signal changes to the second order inter-ferometric autocorrelation.

G2() =

|E(t) + E(t )|2

2dt (3.6)

Decomposition of this equation after substituting the complex fields by the real amplitude, ,

and phase dependence, , via E(t) = (t)expi(t), shows that the second order autocorrelationhas four components.

G2() =

constant

I2(t) + I2(t ) dt+

intensity autocorrelation 4

I(t)I(t )dt

+ Re

2

I2 + I2(t ) (t)(t )eiei((t)(t))dt + c.c.

+ Re

2(t)2(t )e2iei((t)(t))dt + c.c.

interferogram of second harmonic

(3.7)

These four components of the interferometric autocorrelation of an arbitrary pulse as a function

00

2

4

6

8

Delay

Amplitude(a.u.)

04

2

0

2

4

Delay

Amplitude(a.u.)

Figure 3.1: (left) The interferometric autocorrelation (blue) and its intensity autocorrelation(gray). (right) The four components of the interferometric autocorrelation: the constant back-ground (blue), the intensity autocorrelation (cyan), the field autocorrelation (green) and thesecond harmonic autocorrelation (magenta).

of the delay are graphically represented on the right-hand side of Fig. 3.1 as function of the delay,

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in addition to the combined trace on the left. The four components are centered around threeoscillation frequencies, namely: zero, and 2. The first two components of the expansion are

at zero frequency and together these are referred to as the intensity autocorrelation (cyan) withbackground (blue). At = 0, a maximum value of 64dt is reached, whereas at = a

constant background value of 24dt is found. The peak-to-background ratio of this intensity

autocorrelation is thus 3 to 1 (gray line in the left figure). The other two terms represent theoscillating components, of which the former is the field autocorrelation (green), oscillating at ,and the latter the second harmonic autocorrelation (magenta), at 2, with maxima of 8

4dt and

24dt respectively.Constructive interference of all components together results in a maximum value of 16

4dt

at = 0. Taking into account the constant background level of 24dt explains the peak to

background ratio of 8 to 1 for the interferometric autocorrelation (blue). The sum-frequency signalas a function of relative time delay is proportional to the shape of the pulse. As any autocorrelation,

the interferometric autocorrelation is also symmetrical.Unlike the intensity autocorrelation the interferometric autocorrelation does contain some phase

information. The ability to quantitatively measure a linear chirp is the main advantage of this,which is demonstrated below.

Linear chirp

Unchirped Gaussian pulses show correlation traces like in Fig. 3.1. The lower and upper envelopessplit evenly from the constant background level, indicating the coherence time of the pulse is aslong as the pulse itself.

Next, consider a chirped Gaussian pulse, (t) = 0 exp

(1 + ia)(t/G)2

. Depending on the

amount of chirp, defined by the chirp parameter a, the interference pattern is smaller and wingsappear at both sides. These wings exactly correspond to the shape of the intensity autocorrelationand the width of the interference pattern points out the shorter coherence time. The level at whichthe interference pattern starts in relation to the peak level indicates to what extent the pulse ischirped.

The interferometric autocorrelation of a linearly chirped Gaussian pulse is expressed as

G2() = 1 + 2 exp

G

2+ 4 exp

a

2 + 3

4

G

2cos

a

2

G

2cos

+exp

(1 + a2)

G2

cos2

(3.8)

and is shown in Fig. 3.2. Equation 3.8 shows the same constant background and the same intensityautocorrelation component as the unchirped Gaussian pulse (gray line). A difference is noticed forthe other two terms, which include the chirp factor a. This term causes the interference patternto narrow (magenta line) and the intensity autocorrelation to partially appear. The right-handside of Fig. 3.2 shows the envelopes of the autocorrelation for three values of the chirp factor. Itdemonstrates that a larger chirp factor causes a larger narrowing the interference pattern.

As was demonstrated in this section, interferometric autocorrelation is capable of revealingmore information about the pulse than the intensity autocorrelation technique. For a practicalapplication, usually this type is preferred.

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00

2

4

6

8

Delay

Amplitude(a.u.)

00

2

4

6

8

Delay

Amplitude(a.u.)

a=0

a=2

a=8

Figure 3.2: Interferometric (magenta) en corresponding intensity autocorrelation (gray) ofa chirped Gaussian pulse (left) and upper and lower envelopes for three values of the chirpparameter a (right).

Types of autocorrelators

Practical measurement of picosecond or femtosecond pulses requires one variable arm of the Michel-son interferometer. The most common method to produce an optical path difference is mountinga mirror on a linear motion device like an audio loudspeaker or a motorized translation stage anddriving that with a sinusoidal signal. A second method utilizes a material with a significantly largerrefractive index than air, which is rotated to induce a delay. Another method relies on a rotatingplatform containing one or more optical components to produce a varying path length [31].

Pulses longer than a few picoseconds require large scan ranges. For example, for a delay of100 ps, a path length difference of 3 cm has to be scanned. The scan range is limited by the typeof autocorrelator. Up to 300 ps of delay can be obtained by a rotating-mirror autocorrelator, whilelarger scan ranges can be reached with a motorized linear translator, although the size of such adevice can be considerable.

Because of the large scan range required to measure the expected picosecond pulses from ourdiode laser and the flexibility to adapt to different pulse lengths, we decided to build a rotating-mirror autocorrelator, which is explained below.

3.1.3 Rotating-mirror autocorrelator

The rotating-mirror autocorrelator consists of a Michelson interferometer where the variable armemploys two antiparallel mirrors mounted on a rotating platform symmetrically to its rotation axis.The setup is schematically illustrated in Fig. 3.3.

Splitting the incoming pulses by means of a beam splitter is the first step in the autocorrelator.One beam is transmitted to mirror M4 and the other is reflected to the rotating platform. The twomirrors on the platform, M1 and M2, steer the beam to a third fixed mirror, M3, from which it isretroreflected back to the beam splitter, again via the two rotating mirrors. The returned pulsesare combined with the ones returning from M4 and focussed into a SHG crystal where the second

harmonic signal is generated. The power of the SHG signal depends on the extent of overlap. After

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PMT

M M

BS SHGcrystal

delay

L L

h

d

r

p4

p3

p1p2

Figure 3.3: Schematic setup of a rotating-mirror autocorrelator, utilizing a rotating platformwith two mirrors.

the fundamental wavelength is filtered out, the second harmonic signal is sent onto a detector. Thesecond harmonic is usually very weak, so often the detector is chosen to be a photomultiplier tube.

Due to the rotation of the platform, the path length, L, changes. The path length should bevaried around the point at which the path length in both arms is equal, i.e. the point at which themaximum overlap in the crystal occurs. Calculation of the path length of the variable arm as afunction of the rotation angle of the platform, , is relatively easy, once four crucial spacial pointsare defined. These are four points at which the beam intercepts the beam splitter, the mirrors onthe platform and the fixed mirror, p1p4 in Fig. 3.3. Taking the origin to be the axis of rotation,a mathematical expression of these points in their x and y coordinates can be formulated as

xp() =

C1h tan( + ) + r [sin() cos()tan( + )]

h tan( + ) + r

3 4cos2( + ) [sin() cos()tan( + )]C2

(3.9)

yp() =

hh

h

4r sin()sin( + )h 4r sin()sin( + )

(3.10)

in which represents the mirror angle with respect to the line, of length 2r, through the centersof both mirrors [32]. h is assigned to the incoming beam height, that is the vertical distance in theview of Fig. 3.3 from the center of the beam splitter to the origin of rotation. The locations of thebeam splitter and of M3 determine the constants C1 and C2, but they do not affect the path lengthdifference L. Making use of these coordinates, the path length, L(), can be easily derived.

L() =

[xp1() xp0()]2 + [y1() y0()]2 +

[xp2() xp1()]2 + [y2() y1()]2

+

[xp3() xp2()]2 + [y3() y2()]2

(3.11)

From [32] the optimum mirror angle and and separation can be taken to maximize the pathlength difference, L. It appears that L is maximum for a mirror separation of little more than

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twice the mirror width. For any mirror angle and separation, the optimum incoming beam height,hopt, can be calculated to maximize L [32]:

hopt =2r sin()

(d/2)2 + dr cos() + r2

d

2+ r cos()

(3.12)

Usually, the angular velocity of the rotating platform is constant, leading to the drawback ofa non-uniform rate of delay. The smaller the mirror separation, the higher the nonlinearity. In[31], the nonlinearity NL over the whole scan is estimated to be NL d/8r, leading to a changeof delay rate of 25% for d = 2r. In contrast to pulses in the fs regime, for pulses in the psregime this nonlinearity cannot be neglected if a highly accurate autocorrelation trace is required.Compensation is possible using an interferogram of a cw laser.

Experimental realization

For the characterization of the ultrashort pulses generated in de course of this work, we constructedhome-made rotating mirror autocorrelator. Pulse durations of several tens of picoseconds, i.e. aspatial length of about 2 cm, are to be measured, so a path length difference of a least a threefoldof that is required. The mirrors on the rotating platform have a diameter of d = 6.3 cm and areseparated by 2r = 7 cm. The angle at which the mirrors are mounted on the rotating platform is = 55. These values allow a path length difference corresponding to about 200 ps to be traced.

The SHG crystal used is a rather long BBO crystal with dimensions of 5x5x12 mm, cut under anangle of 22.8, for type I second harmonic generation of 1064 nm to 532 nm. Instead of a fundamen-tal filter and a photomultiplier tube, a PerkinElmer channel photomultiplier (CPM) module (typeMD983) is used. Its spectral sensitivity is in the range 185650 nm, so the fundamental wavelength

of around 1060 nm remains undetected, making the fundamental filter abundant. This module is anewly developed detector, as an enhancement of a conventional photomultiplier tube. It offers anextremely low dark current and a very high anode sensitivity, both one order of magnitude betteras compared to conventional PMTs. The CPM uses a unique detection principle by converting avery low light level into electrons via a semitransparent photocathode. On their way to the anode,the electrons move through a narrow, curved semiconductive channel, hit the inner wall and emitsecondary electrons upon every bounce. This process repeats itself several times along the pathresulting an avalanche effect with a very high gain.

For calibration of the autocorrelator, an event of a known temporal duration is required ascalculation of the path difference gives only an estimation of the practical scan range. In this work,the pulses generated within the semiconductor chip itself have been used for that purpose. Their

temporal separation can be verified from spectral measurements. This will be explained in detailin section 4.3.3.

Evaluation of the recorded traces

From the recorded traces by the autocorrelator, the pulse durations have to be estimated. Thetraces are recorded by an oscilloscope (Tektronix TDS2022) and stored in data files. These datafiles contain the time axis of the oscilloscope and the measured output of the CPM in Volts. We havewritten a MATLAB program to analyze the data files. It generates a plot of the autocorrelationtrace, fits the data by a chosen function (see section 2.2.2) and calculates the pulse duration basedon that fit. Once the scan setup and trace calibration parameters are entered, our program can

calculate the delay time and the normalized signal levels directly from the data file. A plot of the

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autocorrelation trace is generated as well as a plot of a fit for a Gaussian or secant hyperbolicsquared function.

From the autocorrelation traces recorded with this autocorrelator, no clear pulse profile can bedetermined by comparing it to fits with Gaussian or sech2 shaped pulse. A Gaussian pulse shapeassumption results in longer pulse durations than with a secant hyperbolic squared assumption.To avoid overestimation of the shortness of the pulses, all traces in this thesis are fitted assuminga Gaussian pulse shape, even though the actual pulse shape may be closer to the sech2 function.The pulse duration for a sech2 function, calculated using the same fitting procedure, is mentionedseveral times for comparison.

3.2 Spectral measurements

Apart from an estimation of the phase and temporal pulse properties, information about the op-

tical spectrum is desired for a better characterization of the laser. For coarse, but quantitativemeasurements, an optical spectrum analyzer (OSA) is used, whereas finer details are revealed by ascanning Fabry-Perot interferometer.

3.2.1 Optical spectrum analyzer

Quantitative measurements of the optical spectrum are performed using two optical spectrumanalyzers1. The spectral bandwidth of these grating-based devices covers the wavelength rangefrom 600 nm to 1750 nm. The incorporated detectors have a huge dynamic range, which enablesthe measurement of light powers from 20 dBm (100 mW) down to -90 dBm with 0.3 dB accuracy.The first of the two (Ando AQ6317) has a wavelength resolution of 0.015 nm with an accuracy of

0.05 nm. The second (Ando AQ6315A) differs from the first by its resolution of 0.05 nm and itsspectral range, which is enhanced by 250 nm into the UV.

Even though the resolution is not sufficient to resolve the single longitudinal modes of a laserresonator of several centimeters in length, it is sufficient for the large intermodal spacing of thediode laser itself, which in this case is of particular importance. To be able to make fully use of thehigh resolution, the light is coupled into the spectrum analyzer using a single mode fiber. Theseoutstanding features allow high resolution and sensitivity measurements of the optical spectrum.

3.2.2 Scanning Fabry-Perot interferometer

In order to resolve the longitudinal modes of the external cavity that we set up, we use a scanningFabry-Perot interferometer. Such an instrument usually consists of two partially transmitting mir-rors, in between which light waves bounce back and forth [ 33]. This multiple-beam interferometer isilluminated along the axis, such that incident light can interfere with itself within the Fabry-Perotcavity. To get a signal, the interferometer cavity has to be filled with light. This can only be thecase if there is positive interference of the waves that oscillate between the mirrors. Standing wavesare formed as a result of the optical distance between the mirrors, nd, being an integer multiple ofhalf a wavelength. With m being the order of interference, this can be expressed as

nd = m/2, (3.13)

which shows that the separation between the mirrors determines the transmitted wavelength. Bysmoothly varying the position of one of the mirrors along the optical axis, this characteristic feature

1Which OSA was actually used for an experiment was based on the availability at that specific day.

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can be exploited to continuously tune the interferometer. Usually, the mirror separation is scannedover several wavelengths by a piezoelectric transducer, leading to a repetitive (because of the

different orders of constructive interference) output pattern. The frequency difference betweenadjacent peaks of two successive interference orders of the same incident wavelength is termed thefree spectral range (FSR). For a plane mirror Fabry-Perot interferometer, i.e. one that uses twoflat mirrors aligned exactly parallel, the FSR is given by FSR = c/2nd. It defines the frequencybandwidth over which a measurement is possible without overlapping different interference orders,which is normally in the order of some 100 MHz to several GHz. Meaningful measurements canonly be obtained if the FSR is greater than the spectral bandwidth of the input beam. On theother hand the highest resolution measurement requires the FSR to be as small as possible.

Another type of Fabry-Perot interferometer uses two concave mirrors with radii of curvatureequal to their separation. This type is called a confocal mirror Fabry-Perot interferometer. TheFSR of these is directly related to the mirror curvature by FSR = c/4nd. In this case the FSR

differs from that of a plane mirror Fabry-Perot interferometer, since the incident light falls backupon itself only after four traversals in stead of two, which means that the transmitted spectrumis reproduced after changing the mirror distance a quarter of a wavelength.

A common measure to quantify the performance of the Fabry-Perot interferometer is the ratioof the distance between two adjacent maxima to the width of the maximum (FWHM), which iscalled the finesse of the cavity, F. The finesse is highly influenced by the reflectance, R, of themirrors. As the distance between two maxima corresponds to a wavelength difference of, or to afrequency difference of 2, the finesse can be calculated as follows:

F =R

1 R (3.14)

Higher finesse values enable higher resolution measurements, but reduce the transmission of theincident light as a consequence, so choosing an appropriate mirror reflection is eminent.

Experimental realization

Two confocal mirrors with a radius of curvature of 25 mm in a confocal setup form the cavity of thescanning Fabry-Perot interferometer we use. It has a free spectral range of 3 GHz, corresponding toa wavelength range little more than 0.01 nm at a wavelengths of about 1 m. One of the mirrors ismounted on a piezoelectric activator, allowing it to be shifted several micrometers. A high-voltagesupply, fed by a signal generator, drives this mirror in a triangular way back and forth, varying thecavity length to scan the frequency range of 3 GHz. With a mirror reflectance of 98%, the finesse

can be calculated to be around 150, leading to a minimum resolvable bandwidth of approximately20 MHz. In practice however, the minimum resolvable bandwidth reduces to about 100 MHz, butis still very suitable to distinguish laser modes of about 1 GHz apart.

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Chapter 4

Experimental results

Overview This chapter describes the experimental setup, the process of mode locking a diodelaser and the measurement results. At first the experimental setup is explained in detail. A com-mercially available single stripe diode laser first is characterized in continuous operation, after whichit is actively mode-locked, which should lead to pulses with picosecond duration. Optical propertiesand pulse durations are examined.

4.1 Experimental setup

4.1.1 Diode laser setup

Active mode locking of a diode laser by modulating the input current at RF frequencies requiresa longer cavity than that of the semiconductor chip itself. Therefore the diode laser has to bepositioned in an external cavity configuration. The diverging output beam is collimated using alens, CL (Thorlabs C220 TM-B, f= 11 mm AR coated for wavlengths 6001050 nm). Figure 4.1shows the diode laser placed in an external cavity, where feedback is provided by a diffractiongrating in Littrow configuration. Such an external Littrow cavity provides a coarse selection of theoperation wavelength by adjusting the angle of the feedback grating.

For our experiments, presented in the following section, two diode lasers of identical structureand similar specifications have been compared and tested for their suitability for generating mode-locked pulses. The two GaInAs diode lasers are SAL1060-60 AR coated single stripe diode lasers

from Sacher Lasertechnik, SN281 and SN286. The front facet of these diode lasers is anti-reflectioncoated, necessary to facilitate mode locking in diode lasers, as was discussed in section 2.2.2. Withresidual reflectivities specified to be R = 4 105 for the SN281 and R = 2 106 for the SN286,both seem to fulfill the minimum requirement of R = 1 104 [3], demanded because of the largesingle pass gain of diode lasers.

The output of the external cavity laser is sent through an optical Faraday isolator (Linos FI1040-TI), to prevent undesirable optical feedback into the diode laser. Around the Faraday isolator, twoplano-convex cylindrical lenses, ZL1 (f= 100 mm, AR810) and ZL2 (f= 50 mm, AR1064) form atelescope to shape the beam and reduce the beam diameter. This beam shaping is required suchthat it can be more easily analyzed or, later, focussed into the fiber amplifier core. The lens infront of the Faraday isolator is tilted slightly to avoid reflections back into the diode.

Research on the optical properties of the output is performed by three measurement devices, therotating mirror autocorrelator, an optical spectrum analyzer and the Fabry-Perot interferometer.

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CHAPTER 4. EXPERIMENTAL RESULTS

Opticalspectrumanalyzer

Bias-TDG

M

HR AR

RF

DC

M

BS

BS

ZL1

M

ScanningFabry-Perot

interferometer

Autocorrelator

ZL2FI

CLDL

Figure 4.1: Experimental setup for mode locking a diode laser and characterizing its output.DC DC current source, RF RF current source, DL diode laser, HR Highly reflective,AR Antireflection coated, CL Collimating lens, DG diffraction grating, M mirror,ZL Cylindrical lens, FI Faraday isolator, BS Beam splitter.

All these are described in Chapter 3. A noteworthy, unique feature of our setup is that all threeoptical instruments can be monitored simultaneously, so realigning can be kept minimal.

4.1.2 RF electronics

As mentioned before, active mode locking a diode laser by modulating the gain requires an RFmodulation signal to be superimposed onto a DC bias current. A device capable of doing thatis a so called bias-T, from which the combined signal has to be coupled into the diode laser. Toensure an excellent coupling and to avoid reflections back into the RF source, the circuit has to beimpedance matched to the RF source at the desired frequency. A home-made circuit has been usedfor both purposes. The circuit to supply the combined current to the diode laser contains a bias-Tand a part to take care of the impedance matching the diode laser to the input RF frequency. Thiscircuit is drawn in Fig. 4.2. The DC bias current is supplied by an ILX Lightwave LDC-37248laser diode controller. Temperature stabilization of the diode laser by means of a peltier element

integrated in the housing, is also achieved using this controller. The RF signal is provided by asynthesized sweep generator (Anritsu 69147A, 0.0120 GHz, max. 20 dBm). To avoid possible

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reflections to reach the generator, a Philips 2272 162 03951 circulator (1.72.3 GHz, -20 dB), actingas an isolator due to a 50 termination on the third port, is used. The RF signal is amplified by

a Trontech P1020-33 amplifier (12 GHz, 33 dB) to reach high modulation powers.

DC

RFGND C1 Cb

Ca

LD

L

Bias-T Impedance match

Figure 4.2: Bias-T circuit including an impedance matching addition to ensure high coupling.DC DC current source, RF RF current source, C Capacity, L Inductor, LD Laserdiode.

Impedance is defined as the total opposition to current flow in an alternating current circuit.It differs from simple resistance in that it takes into account a possible phase offset between thecurrent and the voltage, i.e. it can change value with frequency. Usually, the impedance is expressedin complex notation, Z= R+jX, where j is the imaginary unit, R the ohmic resistance and X thereactance. In case of a sinusoidal signal of angular frequency , inductors have an impedance of

jL and capacitors 1/jC, thus the higher the frequency, the larger the inductive and the smallerthe capacitive impedance.Maximum power transfer in high-frequency circuits happens when the impedances of different

devices are matched. In this case the impedance of the diode laser has to be matched to theimpedance of the RF source. Apart from the maximum power transfer, unwanted reflections areanother important reason to match the impedances. High power reflections back into the RF sourcemay cause damage to that apparatus, as well as low power reflections may distort the input signal.Two adjustable capacitors are used for this, Ca and Cb in Fig. 4.2. The first, Ca, is to compensatethe induction originated from the parallel inductor and capacity, L and C1 respectively. Thisway, the impedance is matched to that of the input signal. The second adjustable capacity, Cb,compensates for the induction in the supply channels to the diode. Summarized, the two adjustable

capacitors are required to let the circuit accept the chosen input frequency.Superimposing the RF signal on top of a DC current is accomplished by means of a bias-T,integrated in the circuit of Fig. 4.2. Via this bias-T, the DC and RF sources are connected tothe laser diode by separate paths. That for the DC signal is equipped with an inductor, L, whichtransmits low frequency signals, but is of high impedance for high frequency signals. The RFsource on the other hand, is connected through a capacitor, C1, which acts just the opposite, lowfrequencies undergo a high impedance whereas high frequencies can transmit easily. This way, thesignals from each source are prevented from entering the other source, which otherwise can loadeach other in stead of the diode and possible leads to the damaging of one of the sources.

To correctly adjust the two capacities Ca and Cb, we use setup in Fig. 4.3. The circuit isfrequency swept by a very low RF power of 30 dBm, transmitted through a directional coupler.Backward reflections in the transmission line at all frequencies in the sweep are partially separated,through a known coupling loss. The source of the frequency sweep is integrated in the RF spec-

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Directional coupler

RF sourceRF spectrum analyzer

Bias-TDC

DL

reflections RF

0.5 1 1.5 290

80

70

60

50

Frequency (GHz)

Reflectedpower(dBm)

Figure 4.3: (left) Setup for impedance matching the diode laser to an RF frequency. DC DC current source, DL Diode laser. (right) Reflected power with a steep dip at the impedancematch frequency.

trum analyzer (Agilent E4407B), which also measures the reflected power. The result of such ameasurement is presented in the right-hand side of the figure. It shows the power reflections ona logarithmic scale over a frequency range of 500 MHz2 GHz. At 1.4 GHz a steep dip of morethan 25 dB below the surrounding powers is observed. At this frequency, minimal reflections occur,indicating that the impedances of the source and the diode are matched. By adjusting the twocapacitors, a steep dip in the RF spectrum can be positioned exactly at the desired frequency any-where between 1 and 1.5 GHz for this particular diode (SN286). Once the impedances are matched,a large RF power may be applied at that specific frequency without the danger of damaging thesource.

4.2 Continuous wave diode laser characteristics

For preliminary characterization, the properties of the diode lasers are first investigated in continu-ous wave (cw) operation, i.e. no RF power is supplied to the diode. The power-current characteristicand the output spectrum are of particular importance. From the power-current curve, the thresh-old value can be deduced, whereas the tunability and mode distance can be determined from the

spectrum.First of all the laser is activated without any other optical components besides the collimating

lens. The temperature of the diode is stabilized at 19C. While increasing the injection current,the optical output power is monitored using a power meter (Newport 1815-C with detector head883-SL). Figure 4.4 shows the power-current curves for two cases, namely for the free running laser,i.e. without feedback (purple), and for the case that grating feedback is provided (blue).

In both plots, the power remains low (below 1 mW) until a certain injection current is reached.Beyond that threshold value, the power increases approximately linear with the current. Also theslope increases, indicating the output has changed from spontaneous to stimulated emission. Withgrating feedback, the threshold value lowers from 43 mA to 33 mA. Looking back on section 2.1.2,the shift of the threshold current can be explained. Optical feedback allows stimulated emission

to develop at a lower injection current because of the higher optical density due to the feedback.Secondly, feedback affects the slope in the lasing regime. With feedback, the slope decreases because

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0 20 40 60 80 1000

5

10

15

20

25

30

35

40

Injection current (mA)

Opticaloutputpower(mW)

no feedback

feedback

Figure 4.4: Power-current characteristics of the SAL 1060-60 SN286 diode laser for the caseof the diode laser without feedback (purple) and with grating feedback (blue) at 1040 nm.

of the charge carrier density is still relatively low at the threshold, but stays constant when furtherincreasing the injection current. This results in a less rapidly growing output power as comparedto the case without external feedback. At 100 mA, a power of 30 mW with and of 34 mW withoutfeedback is reached, despite the lower threshold injection current in the case of optical feedback.

Furthermore, the optical spectra with and without optical feedback are recorded using the OSA(Ando AQ6317), at a resolution of 0.2 nm. Both spectra, which are plotted in Fig. 4.5, representthe output of the laser at 40 mA injection current. The power in a logarithmic scale is plottedas function of wavelength for the case of no feedback (purple) and with grating feedback (blue).Either spectrum covers the huge range from 10001100 nm. While the output of the diode laserwithout feedback is relatively flat, the spectrum of the diode laser with feedback shows a high peakat a specific wavelength, 1040 nm in the figure. The very broad and relatively flat output spectrum

is typical for a diode laser below threshold. Amplified spontaneous emission (ASE) dominates thespectrum as no lasing occurs as a consequence of the absence of high optical intensities. Abovethreshold, the laser lases around a wavelength 1025 nm, but still shows a broad spectrum. Thetendency towards the 1025 nm already appears in the spectrum, paying attention to higher outputlevels in that region. In contrast to the case without feedback, an extremely high peak is foundwhen the laser is in the external cavity configuration. The selective character of the diffractiongrating emerges. With over 30 dB of power difference and a spectral width of less than 0.2 nm, byfar most of the optical power is concentrated within this narrow peak, suppressing the ASE to someextend. As evident from Fig. 4.4, a higher output power at the same injection current is observed.

The tuning range of the SN286 laser diode in Littrow configuration has been measured with theOSA. The operation wavelength has been changed via the angle of feedback, while the current is

kept constant at 75 mA. From the very broad ASE spectrum a large tuning range can be predicted.For the SN286 diode laser, the tuning range proves to be 1010 nm to 1085 nm.

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1000 1020 1040 1060 1080 110090

80

70

60

50

40

30

20

Wavelength (nm)

Power(dBm)

no feedback

feedback

Figure 4.5: Optical output spectra of the SAL 1060-60 SN286 diode laser running at 40 mAinjection current for the case without feedback (purple) and for the case there is (blue). In thepresence of optical feedback by the diffraction grating, a narrow peak in the spectrum is greatlyintensified, at the expense of the amplified spontaneous emission.

Below 1040 nm a ripple in the spectrum can be perceived. Careful examination shows thatthe ripples appear with a period of approximately 0.2 nm. Taking into consideration the verysmall dimensions of diode lasers, this 0.2 nm may be the separation of individual laser modes. Acorresponding frequency spacing of 57 GHz used in sep = c/2nd leads to an optical length ofthe cavity of 2.6 mm. The refractive index is in the order of n = 3, which implies a plausiblediode length of less than a millimeter. This supports the assumption that the ripple is the resultof a residual facet reflectivity, indicating that the quality of the AR coating is less at wavelengthsshorter than 1040 nm.

The diode laser SN281 has been characterized analogously to the investigations above. In

comparison to the SN286, this laser shows a slightly lasing threshold of 33 mA without feedbackand its tuning range of 10201070 nm is narrower. Both characteristics can be attributed to aslightly higher residual reflectivity of the AR coated facet.

To get an impression of the quality of the AR coating of the front facet of the diode laser,we explored the number of composite cavity modes spontaneously willing to run. To observe themodes, the diode is placed in an external cavity with a 70% reflective plane mirror. The wavelengthis tuned by slowly increasing the DC current and monitored using a wave meter (HP 86120B). Thecavity length corresponds to an intermodal spacing of 1.4 GHz, and a wavelength jump of around0.005 nm between two successive modes is expected.

Figure 4.6 shows a portion of the possible wavelengths for both the SN281 and the SN286 diodelasers. At wavelengths at which a resonating mode was detected, a circle is plotted. Clearly, the

possible modes form clusters, with approximately 0.005 nm separation between two modes andapproximately 0.12 nm between the centers of two mode clusters. This 0.12 nm differs from the

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1058.6 1058.7 1058.8 1058.9 1059Wavelength (nm)

1042.3 1042.4 1042.5 1042.6 1042.7Wavelength (nm)

Figure 4.6: Possible free running wavelengths in an 107 mm external cavity setup for theSN281 diode laser (upper) and the SN286 diode laser (lower). The modes caused by thecomposite cavity cluster around the original modes of the diode lasers.

0.2 nm mentioned above due to the dispersion of the diode chip. On average, the SN286 diode laserappears to allow oscillation of a few more modes per cluster than the SN281 diode laser. Broadermodes from the resonator of the two facets can be explained by the less residual reflectivity of ARcoating of the front facet of this laser. Presumably a number of additional modes can be excited bythe sideband generation from the modulation of the injection current, but it cannot necessarily be

expected that the gap between two clusters can be completely filled up and that clusters becomelocked to each other. In this case no phase information can be transferred between the modeclusters owing to the absence of modes in the gap. However, based upon the number of modeswilling to run spontaneously, the SN286 diode laser is expected to be able to generate the shortestpulses of the two.

4.3 Measurements on mode-locked diode lasers

As section 4.2 has shown, additional modes apart from the modes allowed in the diode chip itselfare generated. In this section, the effect of modulating the gain by a large RF modulation isinvestigated, after which the characteristics of the output of the diode laser in mode-locked operation

are explored.

4.3.1 Introduction

Gain modulation in diode lasers is often achieved by modulating the injection current. Active modelocking requires a high modulation depth, so a high RF power is needed. Before driving the diodelaser with large RF modulation powers, the impedances of the RF source and the diode have beenmatched at 1.4 GHz. The optical cavity length corresponding to this frequency is 107 mm andexternal configurations having this length are set up.

Initial experiments are carried out using the SN281 diode laser in an external cavity setup usinga 70% reflective plane mirror. The diode laser is operated at a bias current of 52 mA and 27 dBm of

modulation power is applied to modulate the gain. As the initial experiments did not result in highquality pulses, a second series of experiments is carried out with the same diode laser employing a

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1800 lines/mm diffraction grating in a Littrow configuration to limit the lasing bandwidth. A biascurrent of 60 mA and a RF power of 30 dBm have been used in this case. Further improvement in

the pulse duration is gained utilizing SN286 diode laser, which, according to the specifications, hasa lower residual reflectivity, in a Littrow configuration with a 1200 lines/mm diffraction grating.The laser was biassed at 75 mA, and the RF power was set to 30 dBm again. Wavelength tuningand amplification of the output pulses is also performed using this setup.

For optimal pulse generation, the cavity length has to be matched exactly to the modulationfrequency, so the sidebands generated from one mode will exactly fall into the next possible cavitymode. Once this condition is fulfilled, pulses are expected. Average output powers will be thesame for all cavity lengths, whether there are pulses or not. However, utilizing a second orderprocess, a distinction between pulsating or continuous output can be made. The BBO secondharmonic generating crystal and the channel photomultiplier from the autocorrelator are used toinvestigate. The cavity length is optimized by expanding or shortening it, while monitoring the

output of the second harmonic detector. For the plane mirror external cavity, varying the cavitylength over a range of 5 mm around the optimal position, shows a change of output signal from45 mV to 180 mV at an RF power of 27 dBm. At the highest output level, the optimum cavitylength has been established. Considering that the repetition rate of output pulses is equal to themodulation frequency of 1.4 GHz and expected pulse durations of several picoseconds, this change isinsufficient. However, the pulse shape and duration are unknown, as well as possibly some constantbackground. From this small change, the pulse duration can be estimated to be around 200 pswhen no background is taken into account.

4.3.2 Spectral measurements

Due to the modulation of the injection current of the diode laser, optical sidebands are generated.This causes changes in the optical spectrum, which can be observed by the scanning Farby-Perotinterferometer and the optical spectrum analyzer.

Scanning Farby-Perot interferometer

The development of sideband generation can be easily visualized by the scanning Fabry-Perotinterferometer. Figure 4.7 shows the output of the interferometer at three different modulationstrengths of the SN281 diode laser at a bias current of 60 mA with grating feedback.

A frequency span of the FSR of 3 GHz of the interferometer on the horizontal axis is definedby the two vertical large intensity peaks in left-hand side of the figure. The other two subfiguresare equally scaled. Operating the diode laser above threshold current in the external cavity setup

results in a single peak

Fiber Amplification of Diode Lasers

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