Laser Systems and Applications

Cristina Masoller Research group on Dynamics, Nonlinear Optics and Lasers (DONLL)

Departament de Física i Enginyeria Nuclear Universitat Politècnica de Catalunya

cristina.masoller@upc.edu www.fisica.edu.uy/~cris

MSc in Photonics & Europhotonics

Outline Block 1: Low power laser systems

Introduction

Photons in semiconductors

Semiconductor light sources

Models

Dynamical effects

Applications

SEMICONDUCTOR LIGHT SOURCES LEDS, AMPLIFIERS AND DIODE LASERS

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Outline

• Fabrication

• LI curve (efficiency, threshold)

• Characteristics (optical spectrum, thermal effects)

• Single-mode lasers

• Novel types of lasers

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The simplest cavity: Fabry-Perot (FP), formed by the

cleaved facets of the semiconductor material.

FP laser=Edge-Emitting laser (EEL)

Fabrication steps:

– epitaxial growth,

– wafer processing,

– facet treatment,

– packaging.

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Semiconductor laser = semiconductor material + optical cavity

Cleaved facets

Each mirror consists of a high-to-low refractive index

step, as seen from the inside of the cavity

10/12/2014 6 Adapted from J. Faist, ETHZ

The final step: packaging

• Allows integrating laser diodes

in devices

– Mechanical and optical coupling to

an optical fiber.

– Temperature stabilization.

– Photodiode for monitoring of the

optical power.

– Optical Isolation to avoid back

reflections.

• Increases the fabrication cost.

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A laser diode with the case cut away.

The laser diode chip is the small black

chip at the front; a photodiode at the

back is used to control output power.

Source: Wikipedia

Laser diode: just the laser; diode laser: the complete system

Diode lasers vs LEDs

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Diode laser LED

Note the different scales

Resonator losses and optical confinement factor

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r = i - ln(R1R2)/2L

Laser threshold condition: net gain = losses (mirror + other) the laser

threshold is above transparency.

Losses caused by optical spread are accounted for, phenomenologically,

by the optical confinement factor < 1 that either increases the losses or

reduces the net gain:

= r/

a = p (gain coefficient)

i represents other sources of loss (the absorption in the

semiconductor material is accounted for by the net gain coefficient)

QW lasers vs DH lasers

Density of states

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Peak gain coefficient

In QWs JT is 4 - 5 times smaller

than comparable DHs.

Quantum dot lasers (QDLs)

• While QWs are thin layers of semiconductor material,

quantum dots are (as the name suggests), dots or islands of

a material surrounded by another material.

• The dots have a lower-energy bandgap than the surrounding

material.

• The lasing wavelength is determined by the size and shape

of the QDs.

• By controlling the size and shape of the QDs, QDLs can

span a large range of wavelengths.

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To further reduce the threshold

10/12/2014 12 Adapted from J. Ohtsubo

Gain guided (carrier induced small n)

Index guided

(build-in larger n)

Improve the confinement of photons and carriers via a built-in

lateral waveguide.

The lasing threshold: a long way from the beginning

10/12/2014 13 Adapted from H. Jäckel, ETHZ

4 orders of

magnitude

Power conversion efficiency (PCE)

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• Is the slope of the LI curve,

PCE =ΔP0/ΔI

• Typically 50% at 50 C.

• Other efficiency measures:

─ quantum efficiency

─ overall efficiency

LI curve

Thermal effects at high currents

lead to saturation (more latter)

Quantum efficiency • Laser optical power: P= h (in Watts)

: flux of emitted photons (photons per unit time).

• = d (I-Ith)/e (I, Ith in amperes)

d : quantum efficiency. It accounts for the fact that only a

fraction of the electron-hole recombinations are radiative

(internal efficiency) + only part of the emitted photons are

useful (emission efficiency)

P = h d (I-Ith)/e =1.24 d (I-Ith)/

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Example:

• Ith= 21 mA

• =1.3 m (InGaAsP)

• d =( /1.24) P/(I-Ith)=0.4

Overall efficiency

• Ratio optical power to electrical power, = P/IV

• P = (h/e) d (I-Ith)

• V = Vk + RdI

Vk is the kink voltage (related to the separation of quasi-

Fermi energies)

Rd = dV/dI is the differential resistance

• = (h/e) d (I-Ith) /I(Vk + RdI)

• The efficiency is maximum when the injected current is

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Optical spectrum: how many modes?

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(free-space wavelength

spacing, measured with an

Optical Spectrum Analyzer)

n = 3.5, L = 1 mm:

Δλ = 0.05 nm @ 635 nm

Δλ = 0.3 nm @ 1550 nm

Δ = c/(2nL)

Δλ = (λo)2/(2nL)

m = m (c/n)/(2L)

(mirrors)

• The semiconductor gain spectrum is broad

supports many longitudinal modes.

Exercise

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Consider a InGaAsP (n=3.5) laser with a FP cavity of length

L = 400 m. If the gain spectral width is 1.2 THz, how many

longitudinal modes may oscillate? If the central wavelength is

1.3 m, which is the wavelength spacing?

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Gain + cavity determine the optical spectrum

• The number of lasing

modes and their

relative power depends

on gain (current and

temperature) and on

the type of laser.

• It is often possible to

adjust I and T for

single-mode operation,

but it can be achieved

over a limited I and T

range.

Thermal properties: variation of the emission wavelength

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Multimode: Mode hopping Single-mode laser

0.4 nm/C

Thermal effects in the LI curve

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Thermal variation of threshold current

Thermal

saturation

Why?

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With increasing current, increasing temperature (Joule

heating).

Temperature affects:

• the gain (the peak and the width)

• the refractive index

Kramers-Kronig: gain ─ Im(), n Re()

The temperature modifies n which in turn modifies the

cavity resonance.

Compensation: engineered gain off-set

Single-mode emission is important

Low power lasers:

• High-data-rate optical fiber transmission requires lasers that

emit single mode.

• This is because each mode travels with its own group velocity.

Optical pulses emitted by a multimode laser broaden with

propagation distance (and the distinction between 'zero' and

'one' is gradually lost).

High power lasers:

• Single-mode emission is needed in applications that require

high-quality beam (multimode emission results in low beam

quality).

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Can we fabricate single-mode lasers?

Dynamically stable?

Yes! By using a mode-selective cavity

• An External mirror – External Cavity Laser (ECL)

• A Bragg-Grating (BG) mirror

– Distributed Feedback (DFB)

– Distributed Bragg Reflector (DBR)

– Vertical Cavity Surface Emitting Lasers (VCSEL)

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External Cavity Laser

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• With controlled feedback conditions the laser emission

“locks” to one of the modes of the “compound” cavity.

Advantage: decrease of the threshold current (reduced

cavity loss) and reduced line-width.

• Drawback: uncontrolled feedback conditions can lead to

instabilities and chaotic output (more latter).

Bragg-Grating (BG) mirror

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DBR

(1972)

DFB

• Peak reflectivity for a specific frequency (the Bragg-frequency)

via coherent addition of distributed reflections.

VCSEL

(mid 1980s)

Optical spectra

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FP laser (EEL) vs VCSEL

L 300-500 m

The semiconductor

facets serve as mirrors

Two DBRs serve as mirrors

L=1-10 m

Edge-Emitting Laser (EEL)

VCSEL

Wide divergent output

Δλ = (λo)2/(2nL)

EELs and VCSELs have very different cavity lengths and mirror

reflectivities (30%, 99%), but similar photon lifetimes.

single-longitudinal-mode.

L

Exercise

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InGaAsP (n=3.5) VCSEL laser with L = 5 m. If the gain

spectral width is 1.2 THz, how many longitudinal modes may

oscillate? If the DBRs reflectivity is 99%, what is the photon

lifetime? Compare with an EEL of L = 400 m.

VCSELs

• First demonstrated by K. Iga et al. in late 70s, cw operation in late 80s.

• VCSELs nowadays used in optical computer mouse (but also LEDs). The

low power consumption of VCSELs is attractive for cordless, battery-

driven mice.

• VCSELs have replaced EELs for use in multimode fiber-based Gbit/s

speed optical data transmission for the interconnection of data centers

and computer clusters.

• Arrays of individually addressable VCSELs are used in laser printers, to

simultaneously improve resolution and speed.

• Early GaAs-based VCSELs (850 nm) used metal and dielectric mirrors.

• Next development: full monolithic InGaAs–GaAs lasers (960 nm)

incorporating epitaxial distributed Bragg reflectors (DBRs).

• The production volume nowadays is about 100 million lasers per year.

C. Masoller, LSA 2013 30

Advantages

• single-longitudinal-mode emission

• low threshold currents (< 1mA), high efficiency (> 50%)

• circular beam profiles with small divergence angles, simplifying the design

of beam-shaping optics.

• Wide temperature operation (+125◦C); long lifetimes (>106 hours at RT).

• High data transmission speed (850 nm, 40 Gbit/s up to 80 C –Sep. 2014).

• The active diameter of the VCSEL can be reduced to just a few μms in

order to obtain single-transverse-mode operation together with lowest

threshold currents in the sub-100 μA range.

• It can also exceed 100 μm to get high output powers beyond 100 mW.

• Can be designed for continuous wavelength tuning for gas sensing

applications.

• Device testing at the wafer level, reducing fabrication cost.

• Straightforward fabrication of homogeneous 1D and 2D laser arrays for

compact space division multiplexed data transmission,

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The cost of manufacturing optics

10/12/2014 33 Source: OPN Oct. 2014

In absolute terms, however, wafer fabrication costs just as

much for integrated photonics as for conventional

electronics—the high cost of assembly is in addition to that.

10/12/2014 34 Laser focus world Jan. 2014

Emitting in the 800-1100 nm range, size: from 500 x 500 m to 5 x 5 mm.

Output power from 500 mW to several 100 Ws

VCSELs can provide high output power

Scalability

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Chip with an array of

thousands of

VCSELs

Building blocks of increasing power and size.

Submodule with 12 x 14

chips on sub-mounts on

a micro-channel cooler

System of 3.5 kW

consisting of 24

sub-modules.

Kolb et al 2013

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Lateral/transverse modes

Edge-Emitting Lasers: VCSELs:

• The circular

profile allows

easy coupling to

an optical fiber.

• But single-

transverse mode

emission limited

to few mW.

Solutions of the

Helmholz equation

Adapted from A. Larsson, JSTQE 2011

Fundamental mode operation can be achieved by

matching the mode area to the active gain area.

Complex transverse patterns

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Near-field intensity distributions at different injection currents (9 mA 15

mA 20 mA and 25 mA).

How does a VCSEL work?

10/12/2014 38 Adapted from K. Iga, JLT 2008

Blue indicates n-type

material and red

indicates p-type

Adapted from A. Larsson, JSTQE 2011

The small cavity length requires highly-reflective DBRs,

which are doped to facilitate the injection of electrons/holes

The active region is

composed of a few QWs

Current confinement

Mesa etching (air-post)

Drawback: scattering

losses of the optical

field and reliability

problems if the active

region is exposed to air.

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Selective oxidation

introduces less

optical losses in the

cavity and improves

performance

Ion implantation

(predominantly

using protons)

creates highly

resistive

semiconductor

regions

Distributed Bragg Reflector (DBR)

• Peak reflectivities of 980nm GaAs–AlAs Bragg reflectors versus the

number of mirror pairs.

• In VCSELs DBRs typically have 30 pairs.

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DBR reflectivity vs wavelength

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• Intensity reflection coefficient R and the phase of the

amplitude reflection coefficient vs (MB =16).

• The outer left-hand ordinate presents details in the center,

revealing substantial curvature of R(λ)

Combined effect of two DBRs

• 980nm VCSEL with 16 top and

22.5 bottom GaAs–AlAs Bragg

mirror pairs.

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Cavity resonance

• Optical mode.

DBRs drawbacks for long wavelength VCSELs

• GaAs-based devices benefit from a large index difference

between GaAs and AlAs that allows to fabricate high-

reflective Bragg mirrors even with small numbers of layers.

• Long-wavelength VCSELs based on InP suffer from almost

a factor of two smaller index contrast of the InGaAsP or

InGaAlAs mirror layers. Thus, larger numbers of layer pairs

are required for reasonable mirror reflectivity.

• Also a problem: larger layer thicknesses (due to the longer

wavelength), the epitaxial mirror stacks of the InP-based

VCSELs become rather thick.

• Also a problem: heat due to the high thermal resistance of

DBRs.

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MQWs within the DBRs

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• Typically, the cavity length

of a VCSEL is of few ½s.

• Inside the cavity the

electromagnetic waves

form standing wave

patterns with nodes and

anti-nodes.

• Because the VCSEL cavity

is very short, proper

alignment of the QWs

enhance the coupling and

reduce the threshold.

QWs

VCSEL Design

• Align MQWs within the DBRs to maximize the coupling of the

longitudinal mode to the gain region.

• Because in VCSELs thermal effects are more pronounced

than in EEL, VCSELs are often engineered with a ‘gain offset’.

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Alternative solution: MEMs (micro-electro-mechanical systems)

10/12/2014 46 Adapted from K. Iga, JLT 2008

Extended-cavity VCSELs (VECSELs, also referred to as Semiconductor Disk Lasers)

Extended cavity: 0.1 mm – few cms

• Optically or electrically pumped

Advantages:

• Piezo-wavelength tuning of

single longitudinal mode

• excellent beam quality

Drawbacks:

• less efficient,

• not monolithic.

47

Kuznetsov et al IEEE JSTQE 1999

• Applications: intra-cavity

spectroscopy (gases, etc.)

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Another type of cavity: Ring

• “whispering-gallery” modes, discovered by Raman in

1920 while visiting London Cathedral.

OPN July 2013

10/12/2014 49 Adapted from Sorel et al, JQE 2003

Semiconductor ring lasers Two “whispering-gallery” modes

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• Like a whispering gallery mode, the light

propagates around the edges of the

pillars, but in a helix rather than a circle.

• Although light propagating downward is

absorbed by the substrate, there was

enough gain in the upward propagating

light to provide lasing.

• The semiconductor cavity mode alone

provides enough confinement (no need

of metal cavity).

• sub-wavelength lasing: the pillars are

smaller on a side than the wavelength

they emit.

See also OPN May 2011

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Optically pumped by using a mode-locked Ti:sapphire laser

A limit for the wavelength of semiconductor lasers

• In conventional semiconductor lasers, when electrons

from the conduction band relax to the valence band, the

energy is typically transferred to a photon.

• At longer wavelengths, depending on the band structure

and temperature, this energy is often re-absorbed by

another charge carrier and eventually transferred to heat.

• Thus, the emission wavelength of conventional, inter-

band lasers is limited to about 3 m.

• Solution: inter-sub-band transitions.

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Cascade: one electron

generates many photons

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QCLs wavelengths

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Optics and Photonics News, July/August 2008

QCLs applications

Summarizing, the design goals of semiconductor light sources are

To optimize carrier injection properties

To optimize optical confinement

To minimize optical loss and heating

To obtain maximum gain at a given injection power

To obtain high-quality spatial profile and spectral purity

To cover a wide range of wavelengths

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TF test In EELs the Fabry-Perot cavity is formed by the cleaved facets of

the semiconductor material.

EELs and VCSELs have active regions of comparable sizes.

Bragg-Grating lasers (DFBs and DBRs) emit a multimode

spectrum.

The goal of diode laser design is to improve the confinement of

photons and carriers, which allows lowering the threshold current.

The threshold of diode lasers and amplifiers is at transparency,

when the rate of stimulated emission is equal to absorption.

Thermal heating is responsible for the saturation of the LI curve and

the shift of the emission wavelength with increasing current.

Bulk lasers are as efficient as QW lasers.

Quantum cascade lasers operate on inter-sub-band transitions.

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THANK YOU FOR YOUR ATTENTION !

<cristina.masoller@upc.edu>

Universitat Politecnica de Catalunya

http://www.fisica.edu.uy/~cris/