1.2 kW clad pumped Raman all-passive-fiber laser with brightness enhancement

Amplification of random lasing enables a 10-kW-level high-spectral-purity Yb-Raman fiber laser

By amplifying the cascaded random Raman fiber laser (RRFL) oscillator and ytterbium fiber laser oscillator, we present the first, to the best of our knowledge, demonstration of a 10-kW-level high-spectral-purity all-fiber ytterbium-Raman fiber amplifier (Yb-RFA). With a carefully designed backward-pumped RRFL oscillator structure, the parasitic oscillation between the cascaded seeds is avoided. Leveraging the RRFL with full-open-cavity as the Raman seed, the Yb-RFA realizes 10.7-kW Raman lasing at 1125 nm, which is beyond the operating wavelengths of all the reflection components used in the system. The spectral purity of the Raman lasing reaches 94.7% and the 3-dB bandwidth is 3.9 nm. This work paves a way to combine the temporal stability of the RRFL seed and the power scaling of Yb-RFA, enabling the wavelength extension of high-power fiber lasers with high spectral purity.

Power Scaling of CW Crystalline OPOs and Raman Lasers

Optical parametric oscillators (OPOs) and Raman lasers are two nonlinear-based laser technologies that extend the spectral range of conventional inversion lasers. Power and brightness scaling of lasers are significant for many applications in industry, medicine, and defense. Considerable advances have been made to enhance the power and brightness of inversion lasers. However, research around the power scaling of nonlinear lasers is lacking. This paper reviews the development and progress of output power of continuous-wave (CW) crystalline OPOs and Raman lasers. We further evaluate the power scalability of these two laser technologies by analyzing the cavity architectures and gain materials used in these lasers. This paper also discusses why diamond Raman lasers (DRLs) show tremendous potential as a single laser source for generating exceedingly high output powers and high brightness.

Comparisons of kilowatt Yb-Raman fiber amplifiers employing a superfluorescent fiber source and fiber oscillator

In this paper, we demonstrate experimental investigations on kilowatt-level Yb-Raman fiber amplifiers (YRFAs) employing a superfluorescent fiber source (SFS) or a multi-longitudinal mode fiber oscillator (OSC) as the Raman-pump laser. Through comparing the output properties of the two YRFAs, the experimental results reveal that the YRFA employing the SFS is superior to the YRFA employing the OSC in the performances of power scalability and narrow-linewidth operation. Meanwhile, about 1.16 kW Raman-signal laser at 1120 nm is obtained through the YRFA employing the SFS as the Raman-pump laser. Overall, the presentation could provide an effective solution for the design of high-power narrow linewidth YRFAs.

Kilowatt level Raman amplifier based on 100 µm core diameter multimode GRIN fiber with M2 = 1.6

In this Letter, we demonstrate a high-power Raman fiber amplifier with excellent beam quality based on graded-index fiber. The Yb-doped fiber laser (YDFL) and bandwidth-tunable amplified spontaneous emission (ASE) source are employed as the pump source to compare the laser performance separately. When the ASE with a bandwidth of 8 nm is employed, a maximum power of 943 W at 1130 nm is achieved, which is twice that pumped by YDFL. The beam quality factor M² at maximum output power is 1.6, with a brightness enhancement (BE) factor of 27. To the best of our knowledge, this is the best beam quality and BE factor based on pure Raman gain with output power of over 100 W.

Over 400 W graded-index fiber Raman laser with brightness enhancement

The power scaling on all-fiberized Raman fiber oscillator with brightness enhancement (BE) based on multimode graded-index (GRIN) fiber is demonstrated. Thanks to beam cleanup of GRIN fiber itself and single-mode selection properties of the fiber Bragg gratings inscribed in the center of GRIN fiber, the efficient BE is realized. For the laser cavity with single OC FBG, continuous-wave power of 334 W with an M² value of 2.8 and BE value of 5.6 were obtained at a wavelength of 1120 nm with an optical-to-optical efficiency of 49.6%. Furthermore, the cavity reflectivity is increased by employing two OC FBGs to scale the output power up to 443 W, while the corresponding M² is 3.5 with BE of 4.2. To our best knowledge, it is the highest power in Raman oscillator based on GRIN fiber.

Hundred-watt-level phosphosilicate Raman fiber laser with less than 1% quantum defect

Quantum defect (QD)-induced high thermal load in high-power fiber lasers can largely affect the conversion efficiency, pose a threat to the system security, and even prohibit the further power scaling. In this Letter, we investigate evolutions and influences of the reflectivity of the output coupler, the length of phosphosilicate fiber, and the pump bandwidth, and demonstrate a hundred-watt-level low-QD Raman fiber laser (RFL). The RFL enabled by the boson peak of phosphosilicate fiber achieves a maximum power of 100.9 W with a reduced QD down to 0.97%; the corresponding conversion efficiency reaches 69.8%. This Letter may offer not only an alternative scheme for a high-power, high-efficiency fiber laser, but also great potential on the suppression of thermal-induced effects such as thermal mode instability and the thermal lens effect.

Experimental analysis of Raman-induced transverse mode instability in a core-pumped Raman fiber amplifier

The effect of transverse mode instability is a limitation for the power scaling of fiber laser systems, that can originate due to heat caused by stimulated Raman scattering. In this contribution, we experimentally investigate the threshold of transverse mode instability caused by stimulated Raman scattering in a passive fiber. Both, the Stokes seed power and the fiber length of a core-pumped Raman fiber amplifier are varied to systematically study this effect. Mode resolved measurements reveal that the threshold occurs at approximately the same Stokes output power for all tested configurations, independent of the total Raman conversion efficiency. These results increase the understanding of this type of mode instability and show which parameters are important for a further power scaling of high-power Raman fiber amplifiers.

2 kW narrow-linewidth Yb-Raman fiber amplifier

In this Letter, we report a high-power narrow-linewidth Yb-Raman fiber amplifier with a high second-order Raman threshold and high intensity stability. By employing two temporally stable seed lasers, over 2 kW output power at 1120 nm is achieved at a pump power of 2.6 kW with an optical-to-optical efficiency of 76.3%. The 3 dB linewidth of the 1120 nm Raman-signal laser varies slightly from 0.41 nm to 0.53 nm, and the power ratio of the second-order Raman Stokes light is only about ${-}{46.3}\;{\rm{dB}}$ at the output power of 2 kW. The results further confirm that the technique of employing temporally stable seed lasers is superior to the power scaling of narrow-linewidth Yb-Raman fiber amplifiers. To the best of our knowledge, it is the first demonstration of an over 2 kW narrow-linewidth fiber laser operating at 1120 nm.

High-power cladding pumped Raman fiber amplifier with a record beam quality

In this Letter, a high-power, high-brightness all-fiberized Raman amplifier based on a cladding-pumping scheme is presented for the first time, to the best of our knowledge. The triple-clad passive fiber is employed as Raman gain fiber in the laser system. The maximum output power is 762.6 W emitting at 1130 nm. To the best of our knowledge, this is the highest power in the fields of cladding-pumped Raman amplifiers. Through a cladding-pumping process, the beam quality parameter ${{\rm M}^2}$M² improves from 6.12 of seed laser to 2.24 at maximum output power of 762.6 W, while the best ${{\rm M}^2}$M² is 1.9 at 267.2 W. It is also the best beam quality of Raman laser with brightness enhancement in any kind of configuration (graded-index fiber or multi-clad fiber, laser or amplifier, all-fiber or free-space configuration) with power of over 100 W.

Power scaling of Raman fiber amplifier based on the optimization of temporal and spectral characteristics

We comprehensively study the effects of temporal and spectral optimization on single-mode Raman fiber amplifiers. Amplified spontaneous emission sources and ytterbium-doped fiber lasers are employed as seed or pump lasers for comparison, and passive fibers are utilized as gain media. The influences of various parameters of the laser on 2^nd order Raman threshold and maximum output power are investigated experimentally, including bandwidth, seed power, wavelength separation between pump and seed laser, and temporal stability. With the 190 m passive fiber, the output power increases from 99.5 W to 142.4 W, corresponding to 43.1% improvement through the optimization of seed laser power, pump wavelength and temporal performance of pump source in this amplifier, which has guidance on the establishment of high-power single-mode Raman fiber amplifiers.

2 kW high-efficiency Raman fiber amplifier based on passive fiber with dynamic analysis on beam cleanup and fluctuation

In this paper, we study the power scaling in high power continuous-wave Raman fiber amplifier employing graded-index passive fiber. The maximum output power reaches 2.087 kW at 1130 nm with an optical conversion efficiency of 90.1% (the output signal power versus the depleted pump power). To the best of our knowledge, this is the highest power in the fields of Raman fiber lasers based merely on Stokes radiation. The beam quality parameter M² improves from 15 to 8.9 during the power boosting process, then beam spot distortion appears at high power level. This is the first observation and analysis on erratic dynamic properties of the transverse modes in high power Raman fiber amplifier.