Multimode Fibers with Antireflective Coating for SBS Phase Conjugation St. Meister, C. Scharfenorth, C. Theiss, T. Riesbeck and H. J. Eichler Optical fibers are used as phase conjugated mirrors (PCMs) in master oscillator power amplifier (MOPA) systems with high repetition rates . For this reason, Brillouin scattering (SBS) in different optical fibers has been investigated. The SBS threshold and reflectivity were measured using four different multimode glass fibers with core diameter of 200 m. In particular: 1. step index fibers with pure fused silica core and high transparency in the IR region and a numerical aperture of NA = 0.22, 2. step index fibers with Germanium doped core (NA = 0.4), 3. graded index fibers with Germanium doped core (NA = 0.28) and 4. step index fibers with hard plastic cladding (NA = 0.37). In addition the laser-induced damage threshold (LIDT) of the fiber end-faces was determined because it limits the SBS dynamic range of the fibers. The morphology of damages induced by pump- and Stokes-radiation were studied using a N-on-1 test procedure for single- and multi-shot laser-induced damage measurements. To enhance the SBS reflectivity and to reduce unwanted front-face reflections antireflective coatings for fiber end-faces with high LIDT have been developed. All fiber end-faces were cleaved and AR-coated with a two-layer-system consisting of Ta2O5 and SiO2. The coatings were deposited by electron-beam evaporation. A multi step cleaning procedure was optimized in order to achieve high quality layer systems with highest LIDT. The back reflection was reduced to –30 dB by the AR-coating . For the measurements we used a Q-switched Nd:YAG laser to produce 24 ns pulses at 1064 nm. The laser provides a maximum pulse energy of 10 mJ and can be operated with a repetition rate of 200 Hz or in a single-shot mode with beam quality close to TEM00 (M² ≈ 1.2). The SBS thresholds are significant lower for fibers with a Germanium doped core compared to fibers with a pure fused silica core. A Germanium doped graded index fiber shows the lowest SBS threshold of all tested fibers. In the hard plastic clad silica fibers strong mode mixing occur and no SBS was observed. The step index fibers with pure silica core, and with Germanium doped core are determined as well suited as phase conjugating mirrors. LIDT measurements on AR-coated and uncoated fibers show that AR-coatings on the Germanium doped fibers do not decrease the SBS dynamic range. By deceasing Fresnel-reflections, the AR-coatings enhance the performance of Germanium doped fibers as PCMs in MOPA systems. 1. Eichler, H. J., Haase, A., Kunde, J., Liu, B., Mehl, O., Fiber phase conjugator as reflecting mirror in a MOPA arrangement, Solid State Laser IV, Proc. SPIE 2986, 46 (1997). 2. Meister St., Theiss C., Scharfenorth, C., Eichler H. J., Power Transmission Limits of Different Glass Fibers with Antireflective Coating. Reliability of Optical Fiber Components, Devices, Systems, and Networks III, Proc. SPIE 6193, 215-225 (2006).
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