Frequency Comb-Based Seed Sources for High-Power Spectral Beam Combining

Abstract

In recent years, the need for high-power laser systems has grown across multiple industries. Lasers with high output power and excellent beam quality are used in material processing and directed energy weapons for defense. Fiber lasers, in particular, have emerged as a leading platform for power scaling due to their efficient thermal management and beam quality. However, the output power of a single fiber laser is fundamentally limited by nonlinear effects such as Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS), and Transverse Mode Instabilities (TMI). To overcome these limitations while preserving beam quality, beam combining techniques are employed. Among them, Spectral Beam Combining (SBC) stands out due to its architectural simplicity. However, traditional SBC systems require multiple stabilized laser sources, each with independent control units for current, temperature, and linewidth management, leading to significant system complexity and cost. This thesis explores an optical frequency comb (OFC)-based architecture as an alternative to conventional SBC systems, generating multiple seed wavelengths from a single laser source through electro-optic modulation and advanced filtering techniques. This approach significantly reduces system complexity while enabling scalable, narrow-linewidth beam combining. In the first part of this work, a frequency comb-based seed architecture is demonstrated in the telecom C-band (around 1550 nm). Using electro-optic phase modulation and white noise-driven linewidth broadening, we generate multiple comb lines with tunable linewidths, from a single laser source. These lines are separated using high-resolution de-multiplexers and combined in a grating-based beam combiner. SBS is a primary nonlinear limitation when scaling narrow-linewidth fiber lasers to higher powers. To have superior SBS suppression, the comb lines are spectrally shaped by leveraging the suppression band of the de-multiplexer. Through line-shaping an enhancement of over 50% in SBS threshold is experimentally observed, enabling 1.5× power scaling, which is a substantial number in high-power fiber amplifiers. In the second part, we extend the architecture to the 1 μm wavelength region, where highly efficient gain media like Yb-doped fiber amplifiers (YDFA) are available. However, this wavelength region presents challenges: (1) limited availability of phase modulators capable of generating high repetition-rate combs, and (2) lack of high-resolution de-multiplexers, such as arrayed-waveguide gratings. In response, we explore two parallel efforts: one in integrated photonics and another in fiber-based architecture. First, we develop and evaluate an on-chip OFC source at 1 μm using silicon nitride waveguides. This platform offers advantages such as compactness, dispersion engineering, and low power operation. To interface with fiber systems, we design and fabricate high-efficiency grating couplers operating in the 1030-1070 nm band. Two apodized grating designs: air-clad and oxide-clad are considered, with the air-clad variant achieving coupling losses as low as 2.4 dB per coupler. These results enable low-loss fiber-to-chip coupling, a critical requirement for integrated comb-based SBC systems. However, on-chip comb generation at 1 μm remains limited by fabrication challenges and power handling issues. Therefore, we return to fiber-based EO combs at 1 μm and address the challenge of de-multiplexing densely spaced comb lines by proposing a novel de-multiplexing scheme using the Vernier effect in stimulated Brillouin amplification process. Two combs are derived from a single-frequency laser diode; one is amplified through a YDFA and counter-propagated with the other in a Brillouin gain medium. The carefully chosen repetition rates lead to selective amplification of certain comb lines of the seed comb, resulting in a de-interleaved seed comb with 4x spacing. These widely spaced lines are de-multiplexed using a custom-designed grating-based system, achieving > 20 dB inter-channel suppression. The de-multiplexed channels are well-isolated and, after individual amplification, are well-suited for integration into SBC systems. Together, these efforts present a coherent pathway towards simplified, scalable, and integration-ready architectures for high-power laser systems based on optical frequency comb technologies.