Glass Fibers with Antireflective Coating for Phase Conjugation by hcj


									          Multimode Fibers with Antireflective Coating for SBS Phase
             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 [1]. 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
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 [2].
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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