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Beam cleanup of a pulsed multimode fiber master-oscillator power-amplifier at 1.55 µm using stimulated Brillouin scattering B. Steinhausser, A. Brignon, E. Lallier, JP Huignard Thales Research & Technology, RD 128, 91767 Palaiseau Cedex, France
[email protected]
P. Georges Laboratoire Charles Fabry de l’institut d’Optique, CNRS, Univ. Paris Sud, Campus Polytechnique, RD 128, 91127 Palaiseau Cedex France
Abstract: We present a large core Er:Yb co-doped fiber amplifier followed by a beam quality recovery system. The multimode output (220µJ, M²~6) is converted in a good quality beam (M²=1.6, 110µJ) through SBS beam cleanup. ©2007 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (290.5900) Stimulated Brillouin scattering; (190.7070) Two wave mixing; (190.4370) Nonlinear optics, fibers. 1.Introduction High energy pulsed fiber sources have numerous applications, as range-finding, remote sensing and coherent Lidar systems. Their interest is due to the fact that fiber sources are robust and efficient thanks to double-clad diode pumping. However, optical damage and nonlinear effects limits the output peak power of standard single mode fibers. To overcome these limitations quasi-single-mode Large Mode Area (LMA) fibers are now widely used [1,2], but it may need a special care on fiber arrangement in order to obtain a single mode output. An original alternative is the use of large core diameter multimode (MM) fiber to extract high energy. In order to convert the MM beam into a single mode (SM) beam, we can take advantage of non linear beam reshaping techniques as stimulated Brillouin scattering (SBS) in multimode fibers [3]. However, the spectral linewidth of the generated Stokes beam may be broadened during the process owing to the Brillouin gain bandwidth. This broadening can be a limitation for coherent Lidar systems. In this paper, we propose a new seeded SBS beam cleanup scheme to convert the aberrated output of a pulsed multimode fiber MOPA while maintaining the spectral width of the master oscillator. Furthermore this scheme allows to increase the cleanup efficiency. (a)
(b)
Fig. 1. Experimental setup of high energy fiber MOPA with seeded SBS Beam Cleanup.
Fig. 2.Beam Cleanup demonstration. (a) Input multimode beam (M² ~ 6). (b) Stokes reflected beam (M² = 1.6).
2.Experimental setup The experimental setup is shown in Fig. 1. The pulsed master oscillator is a distributed feedback (DFB) laser diode with a 300kHz linewidth modulated by an electro-optic (EO) intensity modulator to generate 1 µs (FWHM) pulses at 10 kHz repetition rate. It is amplified through two single mode preamplifiers. The first one is an Erbium doped
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fiber amplifier (EDFA) and the second one is an Erbium - Ytterbium co-doped fiber amplifier (EYDFA). The output of these preamplifiers is time-gated with a synchronized acousto-optic modulator (AOM) to suppress ASE noise between pulses which limits the energy extraction in the amplifying chain. The third amplifier is a multimode double-clad EYDFA with a length of 2.9 m, a 73 µm core and a numerical aperture (NA) of 0.2 . The MM output beam is then coupled into a 30 m long passive multimode graded index (GI) fiber (62.5 µm core, 0.27 NA). The Stokes reflected beam generated in the GI fiber is finally rejected by one polarizer of the optical isolator. We studied the cleanup properties of this arrangement as well as its efficiency and spectral characteristics. As shown in Fig. 1, the GI fiber is seeded by a small amount of power taken from the primary DFB laser source. An EO phase modulator is used to translate the seed frequency into the Brillouin gain bandwidth. A Brillouin shift of 9.77 GHz has been measured and applied. The seed beam is then amplified by the multimode beam through a SBS two-wave mixing process. 3.Results The beam cleanup in the 30 m long GI fiber is demonstrated in Fig. 2. It is shown that the multimode beam [Fig. 2(a)] having a M² ~ 6 is converted into the fundamental LP01 mode [Fig. 2(b)] of the GI fiber with a M² = 1.6 . The energy efficiency of the cleanup process is presented in Fig. 3(a). Numerical simulations are also plotted for comparison. Without seeding we obtained up to 80 µJ with 220 µJ coupled energy, thus corresponding to a 40% efficiency. With a seed level of 8 mW CW power we were able to lower the SBS threshold by a factor 2 and to increase the efficiency. For the same input energy of 220 µJ, 110 µJ were back reflected, corresponding to a 50% efficiency. To increase the coupled energy in the SBS fiber a polarization independent isolator is under consideration.
(a)
(b)
Fig 3. Characterization of SBS beam cleanup. (a) Reflected Stokes energy as a function of pump energy coupled into the GI fiber. Lower curve with no seed power and upper curve with 8 mW CW seed power. (b) Electrical spectrum of SBS beat signals
Another improvement of the seeded SBS setup is the reduction of the Stokes spectral linewidth. We measured the laser linewidth with a self-heterodyne method for both the seeded and unseeded SBS configurations. These measurements are reported on Fig. 3(b). Without seeding the process, we found a spectral width of about 20 MHz corresponding to the Brillouin gain bandwidth. On the other hand, the linewidth was reduced near 1 MHz when the process is seeded with a CW power of 8 mW. 4.Conclusion We proposed and demonstrated a new SBS beam cleanup scheme to restore the beam quality after a highly multimode pulsed fiber MOPA at 1.55 µm. Additionally the spectral quality of the master oscillator is maintained after the process. This architecture is thus promising for coherent Lidar systems in which pulse energy, beam quality, and narrow linewidth are required. This work is partially founded by the European Union under Fidelio project (www.fidelio-fp6.org). 5.References [1] Y. Jeong et al., “Ytterbium-doped large-core fiber laser with 1.36 kW continuous-wave output power”, Opt. Express 12 (2004) 6088-6092. [2] C. Codemard et al., “Milijoule, high-peak power, narrow-linewidth, sub-hundred nanosecond pulsed fibré Mater-Oscillator Power-Amplifier at 1.55 µm”, C. R. Physique 7 (2006) 170-176. [3] L. Lombard, A. Brignon, J.P. Huignard, P. Georges , “Beam cleanup in a self-aligned gradient index fiber cavity for high power multimode fiber amplifiers”, Optics Letters 31, 158-160 (2006).
