Polarization Maintaining Large Mode Area Yb Fibers for All Fiber Nanosecond Pulse Amplification

TL;DR

The paper tackles the challenge of scaling nanosecond PM fiber amplifiers by employing high-HOM-loss, large-mode-area Yb fibers with a $26\,\mu\mathrm{m}$ mode field diameter to raise thresholds against nonlinear effects and transverse mode instability. It demonstrates an all-fiber seed-to-amplifier chain based on a PM $30/400$ gain fiber, integrated CLS, and high-power 976 nm pumping, achieving high beam quality and polarization stability. The key results show $1\,\mathrm{mJ}$ pulses at $1\, \mathrm{kW}$ average and $1.5\,\mathrm{mJ}$ pulses at $750\,\mathrm{W}$ average, with PER $>20\,\mathrm{dB}$, $M^2 \approx 1.1$, and more than $99.9\%$ of energy in the main pulse, demonstrating viable all-fiber PM nanosecond amplification at elevated power and energy. This work indicates that high-HOM-loss PM LMA fibers can deliver high-stability, high-power nanosecond pulses suitable for applications like long-range coherent LIDAR, and suggests that further energy scaling is achievable through seed-pulse shaping while maintaining environment-insensitive operation.

Abstract

Polarization maintaining (PM), all-fiber amplifiers offer the benefits of alignment free and environmentally stable operation. To achieve high output powers, particularly in pulsed operation, it is necessary to balance the need to reduce deleterious nonlinear effects, often through the use of large mode area (LMA) fibers, with the onset of transverse mode instability whereby higher order modes (HOMs) mix with the desired fundamental mode output. Over the last few years, advances in high HOM loss, ytterbium-doped LMA fibers have enabled continuous wave (CW) output powers up to 5 kW and pulse energies up to 2 mJ in non-PM fibers. In CW operation, LMA PM fibers have shown up to 2 kW of average power. In this contribution, we present all-fiber nanosecond pulsed amplification in a high HOM loss, Yb-doped LMA fiber with a 26 micrometer mode field diameter and 2.4 dB/m of pump absorption at 976 nm, achieving 1 mJ of pulse energy at 1 kW of average power, and 1.5 mJ of pulse energy at 750 W of average power. The polarization extinction ratios were 20 dB or higher and the M2 was near the diffraction limit. We measured the in-pulse to out-of-pulse energy and found 99.9% or more of the measured power remained in-pulse.

Figures (15)

• Figure 1: Effective mode-field diameter and LP$_{11}$ loss as a function of core diameter for a step-index fiber operating at 1070 nm wavelength and coiled to 15 cm bend diameter.
• Figure 2: All-fiber, diffraction limited, nanosecond reports of pulse energy vs. average power of non-PM fibers. Literature data points are from Refs. Zhang2017Su2014Huang2018Fang2011Yu2014Fu2018Avdokhin2015. Figure adapted from Ref. Glick2024.
• Figure 3: Pulse energy vs. average power for published nanosecond PM Yb fiber amplifiers in the literature compared with the Lightera PM 30/400 fiber featured in this work. Literature results are taken from Refs. Khitrov2008Ye2006Lin2017huang2021Su2013Huang2018Ran2015Su2019zhang2013Roy2021Fathi2025.
• Figure 4: Schematic of seed source for power amplifier, consisting of a pulsed laser diode and three stages of pre-amplification. ISO: isolator; PBS: polarizing beam splitter; PM: polarization maintaining.
• Figure 5: Schematic of power amplifier, consisting of the seed source shown in Fig. \ref{['fig:SourceSetup']}, a fusion spliced, polarization maintaining pump signal combiner, the Yb PM 30/400 fiber under test, a cladding light stripper, and a fusion spliced angled end cap. PSC: pump signal combiner; PM: polarization maintaining; CLS: cladding light stripper; AR: anti-reflection.