The efficiency and reliability of high power (multi-kW) fiber lasers has revolutionized material processing and other applications. However, diode lasers offer the promise of even higher efficiency, yet scaling to high power while maintaining good beam quality remains a significant challenge. We discuss the challenges, advances, and potential future of this technology.
We recently demonstrated a seed modulation format that is effective in suppressing SBS in kW-class fiber amplifiers. By applying synchronous amplitude and frequency modulation, a broad low power seed spectrum can be spectrally narrowed via self-phase modulation as it is amplified, thus reducing the SBS gain and enabling narrower linewidths. Since the approach leverages the fiber Kerr nonlinearity, SBS suppression becomes more effective as fiber powers and lengths increase. We will present updated results extending this approach toward higher modulation depths, higher powers, and narrower linewidths.
State-of-the-art diffraction-limited fiber lasers are presently capable of producing kilowatts of power. Power levels
produced by single elements are gradually increasing but beam combining techniques are attractive for rapidly scaling
fiber laser systems to much higher power levels. We discuss both coherent and spectral beam combining techniques for
scaling fiber laser systems to high brightness and high power. Recent results demonstrating beam combination of 500-W
commercial fiber laser amplifiers will be presented.
Thulium-doped fiber lasers (TFLs) emitting retina-safe 2-μm wavelengths offer substantial power-scaling advantages over ytterbium-doped fiber lasers for narrow linewidth, single-mode operation. This article reviews the design and performance of a pump-limited, 600 W, single-mode, single-frequency TFL amplifier chain that balances thermal limitations against those arising from stimulated Brillouin scattering (SBS). A simple analysis of thermal and SBS limits is anchored with measurements on kilowatt class Tm and Yb fiber lasers to highlight the scaling advantage of Tm for narrow linewidth operation. We also report recent results on active phase-locking of a TFL amplifier to an optical reference as a precursor to further parallel scaling via coherent beam combining.
The power record for near-diffraction limited output from a single fiber laser amplifier is now 10 KW. However, a
single fiber appears unlikely to approach powers greater than 100 kW, which is needed for some applications.
Therefore, there is great interest in methods for combination of many high power fiber beams that maintain aggregate
beam quality. A number of methods have been proposed, including active and passive phasing, and spectral
combination. These methods have varying implementation and performance advantages. The limitations of these
methods, and approaches to address them, are discussed as applied toward combination of kilowatt-class fiber
amplifiers.
A four-stage, Tm-doped fiber amplifier chain emitted 608 W of single-frequency (SF) output power with 53 dB gain,
54% slope efficiency, and M2 = 1.05 beam quality. The output power was limited by available pump power. The final
amplifier stage preserved the input <5-MHz linewidth and imposed negligible phase noise above 3 kHz. SBS limits at
the 2040-nm operating wavelength were measured by splicing different lengths of passive fiber to the amplifier exit.
Thermal limits of the fiber were explored analytically and are consistent with the measured power performance.
Comparison of the SBS and thermal limits suggests a maximum SF power of ~750 W from this fiber configuration, with
further potential to scale past 1 kW with different fiber parameters. To our knowledge, this is the highest power reported
to date from any single-frequency, single-mode fiber laser.
Photonics Research Ontario (PRO) is an Ontario Provincial Center of Excellence supporting a broad range of laser- processing activities within its photonics program. These activities are centered at the University of Toronto, and split between an industrial-user facility and the individual research programs of principal investors. The combined effort furnishes forefront laser system and advanced optical tools to explore novel processing applications in photonic, biomedical, and microelectronic areas. Facilities include laser micromachining stations, excimer-based mask-projection stations, extremely short wavelength lasers such as the molecular fluorine laser, and ultrafast laser systems. The latter two advanced laser offer interesting advantages and contrast in processing 'difficult' materials through linear and nonlinear absorption processes, respectively. These laser systems provide fine precision and strong interaction with a wide range of materials, including 'transparent' glasses, and also ceramics and metals. Applications fall broadly into several areas: wafer-level circuit trimming, high-resolution ultrasonic transducers, and the shaping of optical waveguides and Bragg-gratings for photonic components. This paper summarizes the laser-processing infrastructure and research activities at PRO.
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