Effects of four-wave-mixing in high-power Raman fiber amplifiers

Mutual influence of intermodal and fundamental four-wave mixing of 1.0 µm and 1.5 µm nanosecond pulses in a dual-wavelength fiber laser

We introduce the experimental and theoretical investigation of the mutual influence of two four-wave mixing (FWM) processes during the propagation of high-power nanosecond pulses at 1030 nm and 1562 nm wavelengths in the few-mode optical fiber. Both fundamental mode FWM (FM FWM) of laser pulses at these wavelengths, and intermodal FWM (IM FWM) of 1030 nm pulses, have common anti-Stokes components at 1003 nm wavelength. This leads to the mutual influence of FWM effects. As a result, the decrease of optical power of the fundamental mode is observed at both 1030 nm and 1562 nm wavelengths. The method for the suppression of such an adverse effect is proposed. It is based on the attenuation of the 1030 nm higher LP11 mode at the few-mode optical fiber input.

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.

Enhanced stimulated Raman scattering during intense laser propagation

Stimulated Raman scattering is ubiquitous in many high-intensity laser environments. Parametric four-wave mixing between the pump and Raman sidebands can affect the Raman gain, but stringent phase matching requirements and strongly nonlinear dynamics obscure clear understanding of its effects at high laser powers. Here we investigate four-wave mixing in the presence of strong self-focusing and weak ionization at laser powers above the Kerr critical power. Theoretical analysis shows that the plasma generated at focus naturally leads to phase matching conditions suitable for enhanced Raman gain, almost without regard to the initial phase mismatch. Multidimensional nonlinear optical simulations with multiphoton and collisional ionization confirm the enhancement and suggest that it may lead to significantly higher Raman losses in some high-intensity laser environments.

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.

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.

Brightness enhancement in random Raman fiber laser based on a graded-index fiber with high-power multimode pumping

A brightness-enhanced random Raman fiber laser (RRFL) with maximum power of 306 W at 1120 nm is demonstrated. A half-open cavity is built based on a graded-index (GRIN) passive fiber and single high-reflective fiber Bragg grating written in it directly. Due to the beam cleanup effect in the GRIN fiber enhanced in the half-open RRFL cavity, the output beam quality factor M² is improved from 9.15 (pump) to 1.76-2.35 (Stokes) depending on power, while the pump-Stokes brightness enhancement (BE) factor increases proportionally to output power reaching 6.1 at maximum. To the best of our knowledge, this is the highest power GRIN RRFL with BE.