Mode selective fiber Bragg gratings

Mode selective fiber Bragg gratings Jens U. Thomas a, Christian Voigtländer a, Stefan Nolte a, Andreas Tünnermann a,b, Nemanja Jovanovic c,grahamd.marshall c, Michael J. Withford c, Michael Steel c a Friedrich-Schiller-University, Institute of Applied Physics, Max-Wien-Platz 1, 7743 Jena, Germany; b Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Str. 7, 7745 Jena, Germany; c Macquarie University, MQ Photonics Research Centre, North Ryde, New South Wales 219, Australia ABSTRACT Focussing ultrashort laser pulses allows for inscribing fiber Bragg gratings (FBGs) directly into rare earth doped fiber cores - without prior photosensitivity treatment. High reflective FBGs can be written into active Large Mode Area (LMA) Fibers with 2 micron core diameter using a phase mask scanning technique. Here, we demonstrate fiber Bragg gratings (FBGs), which cover only a fraction of the core. With this additional degree of freedom it is possible to taylor the reflectivity of individual modes. We show for example how those FBGs can 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 compactness and 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 by conventional means is challenging here, because for large, actively doped fiber cores, the necessary prior photo sensitivity treatment comes to its limits. Additionally, fibers with larger core diameter often support more than one transversal mode. For FBG based laser setups this results in modal instabilities, thus power scaling is hampered. More sophisticated FBGs could filter or convert higher order modes. However, since the transversal cross section of conventional fabricated FBG cannot be controlled, efficient elements could only be realized with slanted gratings. 2 6 These limits can be overcome by using an ultrashort laser for FBG inscription: Because of the nonlinear absorbtion of ultrashort pulses within the femtosecond (fs) range, refractive index changes can be obtained in non photosensitive fiber cores. FBGs have been successfully inscribed by fs pulses in standard Ge-doped fiber cores 7, 8 as well as in Er-doped 9 and Yb-doped 1, 11 fibers. In all cases, the rare-earth doped fibers could be operated as one-piece-laser. 9 11 In a Yb-doped, single mode LMA fiber with 1 micron diameter, 1 W could be obtained in cw operation. 11 Moreover, with a point-by-point (PbP) approach, 12 it is possible to target small subsections of the fiber core. In the first part of this paper we apply the coupled mode theory to show, how multi mode reflections of FBGs can be tailored by partially modified core cross sections. In the second part, we use the PbP technique (Figure 1a)) to show the impact of partial core modification on core-cladding mode coupling. We also demonstrate cross coupling 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: thomas@iap.uni-jena.de 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, 7589J 21 SPIE CCC code: 277-786X/1/$18 doi: 1.1117/12.84385 Proc. of SPIE Vol. 7589 7589J-1

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of their transversal field components E t i with the dielectric pertubation Δɛ(x, y). When the coupling constants κ ij are known, reflection and transmission spectra can be computed by numerically solving the coupled mode equations. 16 18 w h Λ y a 1 z x 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 reflection at λ =2n eff Λ/m, (5) where the effective refractive index n eff of the mode is computed from its propagation constant β =2πn eff /λ. In a fiber that supports N modes however, a single period FBG exhibits not only N reflection peaks but also N(N 1)/2 cross coupling peaks at λ i =2n eff,i Λ/m, (6) λ i,j =(n eff,i + n eff,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 heavily depends on the cross section Δn mod (x, y), since it governs the coupling constant (equation 4). Proc. of SPIE Vol. 7589 7589J-3

3. EXPERIMENTAL In this paper we use two different techniques for FBG inscription with ultrashort pulses (Figure 1): while the phase mask scanning technique is efficient for structuring large cross sections, 19 the strength of the PbP method 12, 2 lies in the ability to probe selected parts of the core. 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 11 fs length at 8 nm with a repetion rate f of 1kHz. A 2 oil immersion objective (NA =.8) is used for focusing the ultrashort pulses as well as for imaging the fiber core before and after inscription of the grating. This allows for positioning the modification of the FBG within the fiber core with an error of less than one micron. Pulse energies between 2 and 275 nj cause a micro explosion within the focal volume, that leaves a micro void after one pulse. Micro void chains of period Λ = v/f are written by pulling the fiber under the microscope objective 2, 21 with a velocity v. All gratings were written in second order m = 2 with the polymer coating stripped into SMF-28e fibers (a 1 =4.5μm). The length of the gratings was 2 mm. The inscribed FBGs were probed in transmission: The light of a swept wavelength laser system (SWS) with a range of λ = 152...156nm was launched into the fiber. Then, the signal at the end of the fiber was measured with a photo diode that was synchronized with the SWS. The resulting cladding spectra are plotted in Figure 3 and 5. The domiant peak on the long wavelength side of the picture results from the expected core-core mode coupling (see equation 5). The comb of transmissions dips on the short wavelength side of the Bragg peak is caused by coupling of the core mode to cladding modes. Two envelope functions classify the cladding mode reflections (Figure 3). While the stronger peaks result from coupling into cladding modes of azimuthal order l = 1, the less stronger resonances in between result from coupling into cladding modes of higher azimuthal order. Transmission [db] 1 2 1535 154 1545 155 1555 wavelength λ [nm] Figure 3. Transmission spectrum of a point by point written FBG, where the modification is located approximately 1-2 microns off the center of the fiber core. By evalutation of equations 3 and 4 and subsequent solving of the coupled mode equation, we could compute the transmission spectra of the micro void FBGs. 17 In Figure 4 the spectra are plotted for different transversal displacements of the micro void. The more the micro void is off center, the lower is the overall reflectivity and the stronger the coupling to higher order modes. For a complete suppression of higher order modes, the micro void has to be inscribed exactly in the center of the fiber core. Even small deviations cause the higher azimuthal order comb to rise. Therefore, in order to suppress higher order cladding modes, the modification has to be centered in the fiber core as good as possible. The spectrum of such a grating is shown in Figure 5. Here, reflection to the higher azimuthal cladding modes could be avoided in a range of over 1nm. Only below 153nm, a small contribution of the light couples into higher azimuthal order modes. Proc. of SPIE Vol. 7589 7589J-4

Transmission (db) -2-1 1 2 3 4 displacement (μm) 152 1525 153 1535 154 1545 155 1555 wavelength λ (nm) Figure 4. Location dependent evolution of the Transmission spectrum Transmission [db] 1 2 1525 153 1535 154 wavelength λ [nm] Figure 5. Transmission spectrum of a point by point written FBG, where the modification is centered at the core within the experimental error. Proc. of SPIE Vol. 7589 7589J-5

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 fiber with 15μm core diameter (Nufern LMA GSF 15/123). In order to favor higher order reflections, two parallel lines of micro voids were written, each placed 2μm off the core center. The gratings were written for a reflection wavelength of 179nm. For probing we used a broad band home built Amplified Spontaneous Emission (ASE) source, that delivered 97mW, with a center wavelength at 16nm. This time, the transmission spectrum was taken with a free beam grating-spectrograph (Oriel 772). The spectrum of the grating is shown in Figure 6. The top picture shows the spectrally decomposed mode. The spatial information of the beam is maintained vertically. With integrating different areas of the raw spectra, mode selective transmission spectra can be obtained: 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 as follows: 1) is the reflection of the higher order mode into itself, 3) the fundamental Bragg peak and 2) the cross coupling peak. At the cross coupling wavelength, mode conversion from the fundamental to the higher order mode and backward occurs. Since the micro voids cover only a fraction of the core, the overall reflectivity is only 2 percent. I II T T 1.8.6 1.8.6 I 1 2 3 178 178.5 179 179.5 Wavelength (nm) II 1 2 3 178 178.5 179 179.5 Wavelength (nm) Figure 6. Spectrum of a multi mode FBG: before integration (top) the spatial information of the modes is vertically maintained. 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 high power lasers. One way would be to center the PbP FBG as good as possible. However, experimentally this is not easy feasible. To overcome these limits, we chose to write a transversally homogeneous FBG. Thus, one has to integrate over the whole core area, when computing coupling coefficients (equation 4). Cross coupling coefficients are zero, because their integral kernel exhibits an azimuthal dependence. Proc. of SPIE Vol. 7589 7589J-6

For the experimental realization we used a commercial amplified Ti Sapphire laser (Spectra physics Spitfire) with pulse energies of up to 7μJ, a central wavelength of 8nm and a repetition rate of 1kHz. For phase mask scanning, a phase mask of period 1.485μm is placed above the fiber. Hence, phase mask and fiber are illuminated with the line focus of a cylindrical lens (focal length 2mm). Due to order walkoff of the diffracted 19, 22 ultrashort pulses, a pure two beam interference pattern modifies the core. In order to elongate and widen the area of modification, both fiber and phase mask are translated with respect to the beam with a velocity of v =.5 mm/min. 19 The FBG was inscribed into a Yb-doped LMA fiber with 2μm core diameter (Nufern LMA-YDF-2/4); it is 1mm long and 4μm wide, thus covering the core and parts of the cladding. For inscription, we used a pulse energy of 25μJ. The transmission spectrum is shown in Figure 7. More than 8 percent of the fundamental mode (peak 3) are reflected. The cross coupling peak 2 is much weaker in contrast to Figure 6, thus the FBG can be used for obtaining stable laser operation. transmission (db) 2 4 6 1 2 8 3 1 174 175 176 177 178 179 18 181 182 wavelength λ (nm) 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 transversally inhomogeneous FBG. We described in experiment and theory, how coupling into higher order modes can be steered with a partial modification of the core. Furthermore we accurately rendered the transmission spectra by applying 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 FBG using a phase mask scanning technique directly into an Yb-doped LMA fiber. A monolithic fiber laser based on this fiber delivered up to 215W of output power and is also presented at Photonic West 21. 23 Ongoing work is on the realization of a Fabry-Perot cavity in order to also damp reflection of the higher order modes. Proc. of SPIE Vol. 7589 7589J-7

ACKNOWLEDGMENTS The authors would like to thank Ed Grace for help in probing the fiber. This work was produced with funding from the German Federal Ministry of Education and Research (BMBF) and the Australian Research Council under the ARC Centres of Excellence and LIEF programs. Jens Thomas acknowledges funding by the DAAD, grant D/846673. REFERENCES [1] Tünnermann, A., Schreiber, T., Röser, F., Liem, A., Höfer, S., Zellmer, H., Nolte, S., and Limpert, J., The renaissance and bright future of fibre lasers, Journal of Physics B: Atomic 38, 681 (May 25). [2] Zhang, J., Yu, H., Xu, C.-Q., and Huang, W.-P., Multimode optical fiber bragg gratings: modeling, simulation, and experiments, Photonics North 24: Photonic Applications in Telecommunications 5579, 435 (Nov 24). [3] Yu, H.-G., Wang, Y., Yang, C., Xu, Q.-Y., Yang, X.-L., and Xu, C.-Q., Effects of the asymmetric refractive index change profile on the reflection spectra of multimode fiber bragg gratings, Photonic Applications in Biosensing and Imaging. Edited by Chan 597, 6 (Sep 25). [4] Yu, H.-G., Yang, C., Wang, Y., Zhang, J.-S., Yang, J., Farkas, R., and Xu, C.-Q., Bragg gratings in multimode fiber, Photonics North 24: Optical Components and Devices. Edited by Armitage 5577, 354 (Oct 24). [5] Mizunami, T., Djambova, T., Niiho, T., and Gupta, S., Bragg gratings in multimode and few-mode optical fibers, Journal of Lightwave Technology 18(2), 23 (2). [6] Erdogan, T. and Sipe, J. E., Tilted fiber phase gratings, Journal of the Optical Society of America A: Optics 13, 296 (Feb 1996). [7] Mihailov, S. J., Smelser, C. W., Grobnic, D., Walker, R. B., Lu, P., Ding, H., and Unruh, J., Bragg gratings written in all-sio2 and ge-doped core fibers with 8-nm femtosecond radiation and a phase mask, Journal of Lightwave Technology 22, 94 (Jan 24). [8] Martinez, A., Dubov, M., Khrushchev, I., and Bennion, I., Direct writing of fibre bragg gratings by femtosecond laser, Electron. Lett. (Jan 24). [9] Wikszak, E., Thomas, J., Burghoff, J., Ortaç, B., Limpert, J., Nolte, S., Fuchs, U., andtünnermann, A., Erbium fiber laser based on intracore femtosecond-written fiber bragg grating, Optics Letters 31, 239 (Aug 26). [1] Wikszak, E., Thomas, J., Klingebiel, S., Ortaç, B., Limpert, J., Nolte, S., andtünnermann, A., Linearly polarized ytterbium fiber laser based on intracore femtosecond-written fiber bragg gratings, Optics Letters 32, 2756 (Jan 27). [11] Jovanovic, N., Åslund, M., Fuerbach, A., and Jackson, S., Narrow linewidth, 1 w cw yb-doped silica fiber laser with a point-by-point bragg grating..., Optics Letters (Jan 27). [12] Martinez, A., Lai, Y., Dubov, M., and Khrushchev, I., Vector bending sensors based on fibre bragg gratings inscribed by infrared femtosecond laser, Electron. Lett. (Jan 25). [13] Snyder, A. and Young, W., Modes of optical waveguides, Journal of the Optical Society of America 68(3), 297 39 (1978). [14] Saleh, B. E. A., Teich, M. C., and Goodman, J. W., Fundamentals of photonics, Book, i xix (Aug 1991). [15] Tsao, C., Payne, D., and Gambling, W., Modal characteristics of three-layered optical fiber waveguides: a modified approach, Journal of the Optical Society of America A 6(4), 555 563 (1989). [16] Erdogan, T., Fiber grating spectra, Journal of Lightwave Technology 15, 1277 (Aug 1997). [17] Erdogan, T., Cladding-mode resonances in short- and long-period fiber grating filters, Journal of the Optical Society of America A: Optics 14, 176 (Aug 1997). [18] Kogelnik, H., [Theory of Dielectric Waveguides], vol. 7 (1979). [19] Thomas, J., Wikszak, E., Clausnitzer, T., and Fuchs, U., Inscription of fiber bragg gratings with femtosecond pulses using a phase mask scanning technique, Applied Physics A: Materials Science & Processing (Jan 27). Proc. of SPIE Vol. 7589 7589J-8

[2] Marshall, G., Ams, M., and Withford, J., Point by point femtosecond laser inscription of fibre and waveguide bragg gratings for photonic device fabrication, 2nd Pacific International Conference on Appl. Of Lasers and Optics 26 (26). [21] Jovanovic, N., Thomas, J., Williams, R. J., Steel, M. J., Marshall, G. D., Fuerbach, A., Nolte, S., Tünnermann, A., and Withford, M. J., Polarization-dependent effects in point-by-point fiber bragg gratings enable simple, linearly polarized fiber lasers, Optics Express 17, 682 (Mar 29). [22] Smelser, C. W., Grobnic, D., and Mihailov, S. J., Generation of pure two-beam interference grating structures in an optical fiber with a femtosecond infrared source and a phase mask, Optics Letters 29, 173 (Aug 24). [23] Stutzki, F., Jauregui, C., Voigtländer, C., Thomas, J. U., Limpert, J., Nolte, S., and Tünnermann, A., Passively stabilized 215-w monolithic cw lma-fiber laser with innovative transversal mode filter, Photonics West 21, Paper 758 55 (21). Proc. of SPIE Vol. 7589 7589J-9