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UNCLASSIFIED, PUBLIC RELEASE
High-power, narrow-linewidth, reverse-pumped, Yb-doped fiber amplifier using novel 7+1:1 pump-signal combiner
Ytterbium-doped fiber lasers have become an essential component in high-power laser systems used in applications requiring excellent beam quality, high electrical-to-optical efficiency, compact size, and low weight. One approach to scaling performance of narrow-linewidth, fiber lasers suitable for Directed Energy applications is to increase HOM suppression allowing for larger core sizes, and higher fiber absorption, which increases SBS thresholds while maintaining high transverse mode instability (TMI) thresholds. Another approach is to adjust the fiber composition to reduce the thermo-optic coefficient in the fiber, enabling high TMI thresholds.
Suitable amplifier architectures can also be used to improve SBS performance. Pumping an amplifier in the backward direction is known to provide improved nonlinear performance compared to forward pumping as the path-average power is lowered for a given output power. For backward pumping, a reverse pump-signal combiner (PSC) is a critical component of the amplifier architecture.
Backward pumping at high power places significant demands on the pump-signal combiner as it must have low signal insertion loss to maintain high amplifier efficiency. Furthermore, large core sizes for the signal output fiber are desirable, but the reverse combiner must not excite higher-order modes (HOMs), as HOMs will degrade the amplifier beam quality. Finally, the reverse combiner must have very low thermal slopes as it needs to simultaneously handle both pump and signal at high power.
Recently, a prototype reverse PSC with 7 pump legs and large 35 µm core signal output fiber was demonstrated to excite very low higher-order mode content as measured by S2 imaging, when launching a low-power signal into the combiner4. The LP11 power at the combiner output was measured to be 24 dB lower than the fundamental mode, as shown in Fig 3a. In this work, we demonstrate the use of this combiner in a high-power, narrow-linewidth amplifier, suitable for directed energy applications.
The common pump/signal fiber of the reverse combiner was a 22/400 fiber. The PSC had seven, 230 µm core fibers for coupling pumps into the gain fiber in the backward direction. A 35/320 fiber was used as the output signal fiber. A cladding light stripper was fabricated into the reverse combiner signal output pigtail and the signal output pigtail was end terminated with an AR-coated end cap. The seed laser was a single frequency laser at 1070 nm, broadened with phase modulators using 10^9 PRBS, and preamplified to 20 W power.
At a broad operating linewidth of 26 GHz, the amplifier was limited by TMI at 4.7 kW signal power. At this operating point, 10.7 kW of combined pump and signal power were propagating through the combiner, demonstrating its excellent power handling capability.
Next, the SBS threshold was measured as a function of linewidth with the reverse pumped combiner. At 10 GHz linewidth, we measured an SBS threshold of 3.7 kW signal power, substantially higher than we achieved with forward pump amplifiers using OFS TrueLase™ and DirectLase™ fibers. Thanks to the reverse pumping and large diameter signal output fiber in the PSC, reverse pumping achieved significantly higher SBS threshold, even though the gain fiber was longer than the comparable forward pumped amplifier using the same fiber.
Slope efficiency of the reverse pumped amplifier was also high at > 80%. In addition, thanks to the low HOM content generated in the combiner, beam quality was excellent with M2 = 1.13.
In conclusion, using a novel, reverse pump-signal combiner with 7 pump legs, large core signal output fiber, and low HOM coupling, we demonstrated record narrow linewidth amplification in a backward pumped amplifier, making this reverse pump-signal combiner ideal for directed energy systems
UNCLASSIFIED, PUBLIC RELEASE
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