Mode selective ber Bragg gratings - Macquarie University version (ope… · Mode selective ber...

Mode selective fiber Bragg gratings

Jens U. Thomasa, Christian Voigtlandera, Stefan Noltea, Andreas Tunnermanna,b,Nemanja Jovanovicc, Graham D. Marshallc, Michael J. Withfordc, Michael Steelc

aFriedrich-Schiller-University, Institute of Applied Physics,Max-Wien-Platz 1, 07743 Jena, Germany;

bFraunhofer Institute for Applied Optics and Precision Engineering,Albert-Einstein-Str. 7, 07745 Jena, Germany;

cMacquarie University, MQ Photonics Research Centre,North Ryde, New South Wales 2109, Australia

ABSTRACT

Focussing ultrashort laser pulses allows for inscribing fiber Bragg gratings (FBGs) directly into rare earth dopedfiber cores - without prior photosensitivity treatment. High reflective FBGs can be written into active LargeMode Area (LMA) Fibers with 20 micron core diameter using a phase mask scanning technique. Here, wedemonstrate fiber Bragg gratings (FBGs), which cover only a fraction of the core. With this additional degreeof freedom it is possible to taylor the reflectivity of individual modes. We show for example how those FBGscan be used in few mode LMA fibers to suppress reflections into higher order modes.

Keywords: fiber Bragg gratings, fiber laser, ultrafast applications

1. INTRODUCTION

High power fiber lasers have conquered a large market share within solid state lasers, because of their compactnessand high brilliance.1 For improved cost and stability monolithic setups become more and more important.Therefore one strives to replace as many bulk components with fiber integrated devices. Most prominently,fiber Bragg gratings (FBGs) compete with dielectric mirrors and volume Bragg gratings as resonator mirrors.However, many high power fiber laser systems rely on large mode area (LMA) fibers. FBG inscription byconventional means is challenging here, because for large, actively doped fiber cores, the necessary prior photosensitivity treatment comes to its limits. Additionally, fibers with larger core diameter often support more thanone transversal mode. For FBG based laser setups this results in modal instabilities, thus power scaling ishampered. More sophisticated FBGs could filter or convert higher order modes. However, since the transversalcross section of conventional fabricated FBG cannot be controlled, efficient elements could only be realized withslanted gratings.2–6

These limits can be overcome by using an ultrashort laser for FBG inscription: Because of the nonlinearabsorbtion of ultrashort pulses within the femtosecond (fs) range, refractive index changes can be obtained innon photosensitive fiber cores. FBGs have been successfully inscribed by fs pulses in standard Ge-doped fibercores7,8 as well as in Er-doped9 and Yb-doped10,11 fibers. In all cases, the rare-earth doped fibers could beoperated as one-piece-laser.9–11 In a Yb-doped, single mode LMA fiber with 10 micron diameter, 100 W couldbe obtained in cw operation.11 Moreover, with a point-by-point (PbP) approach,12 it is possible to target smallsubsections of the fiber core.

In the first part of this paper we apply the coupled mode theory to show, how multi mode reflections of FBGscan be tailored by partially modified core cross sections. In the second part, we use the PbP technique (Figure1a)) to show the impact of partial core modification on core-cladding mode coupling. We also demonstrate crosscoupling in such FBGs, which results in mode conversion. Finally, with a phase mask scanning (Figure 1b))written FBG we obtain a cross coupling free spectrum that enables stable few mode operation.

Further author information: [email protected]

Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications X, edited by Alexander Heisterkamp,Joseph Neev, Stefan Nolte, Rick P. Trebino, Proc. of SPIE Vol. 7589, 75890J · © 2010 SPIE · CCC code:

0277-786X/10/$18 · doi: 10.1117/12.843805

Proc. of SPIE Vol. 7589 75890J-1

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of their transversal field components Eti with the dielectric pertubation Δε(x, y). When the coupling constants

κij are known, reflection and transmission spectra can be computed by numerically solving the coupled modeequations.16–18

x

y

w

h

a1

z

Λ

Figure 2. Simplified model of the fs induced modification within the fiber core

In case of a FBG in a single mode fiber, evaluation of equation (3) leads to the well known Bragg reflectionat

λ = 2neffΛ/m, (5)

where the effective refractive index neff of the mode is computed from its propagation constant β = 2πneff/λ.In a fiber that supports N modes however, a single period FBG exhibits not only N reflection peaks

λi = 2neff,iΛ/m, (6)

but also N(N − 1)/2 cross coupling peaks at

λi,j = (neff,i + neff,j)Λ/m. (7)

Thus, a FBG in a N mode fiber has up to N(N + 1)/2 reflection peaks. The magnitude of such peaks heavilydepends on the cross section Δnmod(x, y), since it governs the coupling constant (equation 4).



3. EXPERIMENTAL

In this paper we use two different techniques for FBG inscription with ultrashort pulses (Figure 1): while thephase mask scanning technique is efficient for structuring large cross sections,19 the strength of the PbP methodlies in the ability to probe selected parts of the core.12,20

3.1 Cladding mode coupling of PbP-FBG in single mode fiber

For the grating inscription we used a femtosecond laser (Spectra Hurrican) that delivers pulses of 110 fs lengthat 800 nm with a repetion rate f of 1kHz. A 20× oil immersion objective (NA = 0.8) is used for focusing theultrashort pulses as well as for imaging the fiber core before and after inscription of the grating. This allowsfor positioning the modification of the FBG within the fiber core with an error of less than one micron. Pulseenergies between 200 and 275 nJ cause a micro explosion within the focal volume, that leaves a micro void afterone pulse. Micro void chains of period Λ = v/f are written by pulling the fiber under the microscope objectivewith a velocity v.20,21 All gratings were written in second order m = 2 with the polymer coating stripped intoSMF-28e fibers (a1 = 4.5μm). The length of the gratings was 20 mm.

The inscribed FBGs were probed in transmission: The light of a swept wavelength laser system (SWS) with arange of λ = 1520 . . . 1560nm was launched into the fiber. Then, the signal at the end of the fiber was measuredwith a photo diode that was synchronized with the SWS. The resulting cladding spectra are plotted in Figure 3and 5. The domiant peak on the long wavelength side of the picture results from the expected core-core modecoupling (see equation 5). The comb of transmissions dips on the short wavelength side of the Bragg peak iscaused by coupling of the core mode to cladding modes. Two envelope functions classify the cladding modereflections (Figure 3). While the stronger peaks result from coupling into cladding modes of azimuthal orderl = 1, the less stronger resonances in between result from coupling into cladding modes of higher azimuthalorder.

1535 1540 1545 1550 1555−20

−10

0

wavelength λ [nm]

Tra

nsm

issi

on [d

B]

Figure 3. Transmission spectrum of a point by point written FBG, where the modification is located approximately 1-2microns off the center of the fiber core.

By evalutation of equations 3 and 4 and subsequent solving of the coupled mode equation, we could computethe transmission spectra of the micro void FBGs.17 In Figure 4 the spectra are plotted for different transversaldisplacements of the micro void. The more the micro void is off center, the lower is the overall reflectivity andthe stronger the coupling to higher order modes. For a complete suppression of higher order modes, the microvoid has to be inscribed exactly in the center of the fiber core. Even small deviations cause the higher azimuthalorder comb to rise.

Therefore, in order to suppress higher order cladding modes, the modification has to be centered in the fibercore as good as possible. The spectrum of such a grating is shown in Figure 5. Here, reflection to the higherazimuthal cladding modes could be avoided in a range of over 10nm. Only below 1530nm, a small contributionof the light couples into higher azimuthal order modes.



0

1

2

3

4

1520 1525 1530 1535 1540 1545 1550 1555

0

-10

-20

wavelength λ (nm)displacement (μm)

Tra

nsm

issi

on (

dB)

Figure 4. Location dependent evolution of the Transmission spectrum

1525 1530 1535 1540−20

−10

0

wavelength λ [nm]

Tra

nsm

issi

on [d

B]

Figure 5. Transmission spectrum of a point by point written FBG, where the modification is centered at the core withinthe experimental error.



3.2 Coupling of PbP FBGs in multi mode fiber

In order to investigate for multi mode reflection, we wrote second order PbP FBGs in a two mode LMA fiberwith 15μm core diameter (Nufern LMA GSF 15/123). In order to favor higher order reflections, two parallellines of micro voids were written, each placed 2μm off the core center. The gratings were written for a reflectionwavelength of 1079nm. For probing we used a broad band home built Amplified Spontaneous Emission (ASE)source, that delivered 97mW, with a center wavelength at 1060nm. This time, the transmission spectrum wastaken with a free beam grating-spectrograph (Oriel 77200). The spectrum of the grating is shown in Figure6. The top picture shows the spectrally decomposed mode. The spatial information of the beam is maintainedvertically. With integrating different areas of the raw spectra, mode selective transmission spectra can beobtained: While the top area (I) shows the transmission spectra of the fundamental mode, the bottom area (II)shows contributions of both the fundamental and the higher order mode. We identify the reflection peaks asfollows: 1) is the reflection of the higher order mode into itself, 3) the fundamental Bragg peak and 2) the crosscoupling peak. At the cross coupling wavelength, mode conversion from the fundamental to the higher ordermode and backward occurs. Since the micro voids cover only a fraction of the core, the overall reflectivity is only20 percent.

I

II

1078 1078.5 1079 1079.50.60.8

1

T

Wavelength (nm)

I 1 2 3

1078 1078.5 1079 1079.50.60.8

1

T

Wavelength (nm)

II 1 2 3

Figure 6. Spectrum of a multi mode FBG: before integration (top) the spatial information of the modes is verticallymaintained. Mode selective spectra are obtained by integrating over area I (middle) and area II (bottom)

3.3 Cross coupling suppressed LMA FBG written via phase mask scanning

Since cross coupling of the fiber mode hampers laser stability it has to be suppressed in LMA fibers for highpower lasers. One way would be to center the PbP FBG as good as possible. However, experimentally this isnot easy feasible. To overcome these limits, we chose to write a transversally homogeneous FBG. Thus, onehas to integrate over the whole core area, when computing coupling coefficients (equation 4). Cross couplingcoefficients are zero, because their integral kernel exhibits an azimuthal dependence.



For the experimental realization we used a commercial amplified Ti Sapphire laser (Spectra physics Spitfire)with pulse energies of up to 700μJ, a central wavelength of 800nm and a repetition rate of 1kHz. For phasemask scanning, a phase mask of period 1.485μm is placed above the fiber. Hence, phase mask and fiber areilluminated with the line focus of a cylindrical lens (focal length 20mm). Due to ”‘order walkoff”’ of the diffractedultrashort pulses, a pure two beam interference pattern modifies the core.19,22 In order to elongate and widenthe area of modification, both fiber and phase mask are translated with respect to the beam with a velocityof v = 0.5 mm/min.19 The FBG was inscribed into a Yb-doped LMA fiber with 20μm core diameter (NufernLMA-YDF-20/400); it is 10mm long and 40μm wide, thus covering the core and parts of the cladding. Forinscription, we used a pulse energy of 250μJ. The transmission spectrum is shown in Figure 7. More than 80percent of the fundamental mode (peak 3) are reflected. The cross coupling peak 2 is much weaker in contrastto Figure 6, thus the FBG can be used for obtaining stable laser operation.

1074 1075 1076 1077 1078 1079 1080 1081 1082−10

−8

−6

−4

−2

0

wavelength λ (nm)

tran

smis

sion

(dB

)

1

3

2

Figure 7. Transmission spectra of a FBG written with the phase mask scanning technique.

4. CONCLUSION

By using an ultrashort laser and the PbP inscription method, we investigated the coupling behavior of transver-sally inhomogeneous FBG. We described in experiment and theory, how coupling into higher order modes can besteered with a partial modification of the core. Furthermore we accurately rendered the transmission spectra byapplying the coupled mode theory. In a few mode fiber, we demonstrated how the FBG causes mode conversion.Since this is usually undesirable for FBGs used in a laser, we finally wrote a transversally homogeneous FBGusing a phase mask scanning technique directly into an Yb-doped LMA fiber. A monolithic fiber laser based onthis fiber delivered up to 215W of output power and is also presented at Photonic West 2010.23 Ongoing workis on the realization of a Fabry-Perot cavity in order to also damp reflection of the higher order modes.



ACKNOWLEDGMENTS

The authors would like to thank Ed Grace for help in probing the fiber. This work was produced with fundingfrom the German Federal Ministry of Education and Research (BMBF) and the Australian Research Councilunder the ARC Centres of Excellence and LIEF programs. Jens Thomas acknowledges funding by the DAAD,grant D/0846673.

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