New batwing Fiber Boosts Kilowatt Laser Power Limits

In the depths of space, gravitational wave detectors function as extraordinarily sensitive "ears," capturing faint signals from distant cosmic events. To enhance their sensitivity, these instruments require exceptionally stable, high-power lasers serving as their "eyes." Similarly, in cutting-edge scientific fields like long-distance communication and precision measurement, lasers capable of producing high-quality, single-frequency, diffraction-limited beams are indispensable.

Fiber lasers, with their flexible structure, easy maintenance, and clean systems, have emerged as the most promising technology to achieve these goals. However, in the quest for higher power, single-frequency fiber lasers face a significant challenge: simultaneously suppressing two major obstacles—stimulated Brillouin scattering (SBS) and transverse mode instability (TMI).

Leading laser research centers worldwide are developing kilowatt-level, ultra-low-noise single-frequency fiber lasers for next-generation gravitational wave detection systems like LIGO and KAGRA. A 2019 Nature article highlighted LIGO's upgrade plans, which prioritize increasing the power of single-frequency fiber lasers. Yet conventional step-index fibers (SIF) face a fundamental trade-off: techniques that suppress SBS often exacerbate TMI, and vice versa.

Over the past decade, researchers have explored various specialty fiber designs to break through this bottleneck. Innovations ranging from D-shaped fibers and photonic crystal fibers (PCF) to all-solid photonic bandgap fibers (APBF), along with all-fiber systems like chiral-coupled-core fibers (CCC) and confined-doped fibers (CDF), have pushed the boundaries of what's possible.

Among these, the acoustically tailored and gain-guided photonic crystal fiber proposed by C. Robin and colleagues demonstrated remarkable SBS and TMI suppression. With additional temperature control, it achieved a 2.8-fold increase in SBS threshold and delivered 811W of single-frequency output in 2014 while maintaining excellent beam quality (M²<1.2). This record remains unbroken today.

After a decade of research combining insights from trench fibers, confined-doped fibers, and all-solid photonic bandgap fibers, our team recognized that precisely engineered refractive index profiles could overcome thermal limitations. We developed an innovative "bat-wing" refractive index distribution that achieves both high-order mode (HOM) leakage loss characteristics and significantly increased effective mode area.

This design cleverly divides the fiber core into three regions: a central ytterbium-doped zone, a step-doped region, and an undoped trench. Through meticulous optimization of refractive index differences and dimensions, the design achieves three critical improvements:

1. Balancing SBS and TMI: The equivalent numerical aperture (NA) of approximately 0.030 strikes an optimal balance between SBS and TMI effects.
2. Concentrating fundamental mode energy: Over 50% of the fundamental mode's energy remains in the central region, reducing energy coupling between fundamental and higher-order modes while improving bend resistance.
3. Enhancing higher-order mode loss: Higher-order modes (like LP₁₁) are guided toward the core's edge, where bending losses dramatically increase, further weakening their energy coupling with the fundamental mode.

Finite element simulations and actual fabrication confirmed the success of this high-order mode leakage fiber (HOMLF). Simulations showed a fundamental mode effective area of 669.0 μm² and an LP₁₁ mode bending loss coefficient of 47.5 dB/m. Actual measurements revealed 51.7% fundamental mode energy concentration and 36.4 dB/m bending loss for LP₁₁. Most impressively, during 30-meter transmission tests at 670.9 μm² effective area, the output maintained a near-perfect Gaussian profile with M²~1.15.

To demonstrate this fiber's potential, we built a master oscillator power amplifier (MOPA) system using a 1029.5 nm distributed feedback Bragg grating laser (900 Hz linewidth) as the seed. After pre-amplification, the signal entered the main amplifier—a forward-pumped system with six 370 W laser diodes at 976 nm, using just 1.5 meters of our ytterbium-doped HOMLF as the gain medium.

At 1650 W pump power, the system broke the 1015 W output barrier with 61.1% optical-to-optical efficiency. Remarkably, at this record power, backward-propagating SBS power measured only 746 mW. Spectral analysis confirmed negligible amplified spontaneous emission (ASE), with signal intensity dominating the pump spectrum.

Beam quality remained exceptional across the entire power range (30-1015 W), with M² consistently below 1.2. Detailed mode analysis showed 94.0% of energy in the fundamental LP₀₁ mode, with minimal higher-order content. The output linewidth measured 2.19 kHz (slightly broadened from the seed due to pump noise and SBS), while relative intensity noise reached -120 dB at 100 kHz—both within acceptable ranges for demanding applications.

The system demonstrated outstanding stability: 2.2% power fluctuation, 19 ± 1.5 dB polarization extinction ratio, and minimal pointing noise. These results provide crucial advancements for next-generation scientific applications like gravitational wave detection.

This breakthrough "bat-wing" design overcomes the fundamental SBS-TMI trade-off in high-power single-frequency fiber lasers while maintaining excellent beam quality. Beyond establishing a systematic framework from theoretical analysis to experimental validation, the work suggests several promising directions:

Further optimization of transverse refractive index profiles, gain distribution, and longitudinal mode field tailoring could push power limits even higher. Incorporating AI algorithms may accelerate multi-parameter optimization. The design's manufacturing compatibility also enables large-scale production, while its principles show promise for broadband amplification and ultrafast laser systems.