Raman fiber lasers have been demonstrated to reach power levels of several hundred watts. However, the wavelength-division-multiplexing (WDM) component that usually combines the pump and seed laser in the Raman fiber is the bottleneck to reaching higher powers. Likewise, 1120 nm ytterbium (Yb)-doped fiber lasers that are used to pump 1178 nm Raman fiber amplifiers (frequency-doubled to 589 nm for the creation of sodium guide stars in the upper atmosphere for adaptive optical telescopes) suffer from amplified spontaneous emission (ASE) and parasitic lasing at 1060 nm that seriously limit the 1120 nm fiber-laser output.
To address these problems and create a high-power 1120 nm fiber-laser source, researchers at the Shanghai Institute of Optics and Fine Mechanics (SIOM; see story on p. 33 or http://bit.ly/1iGPyrg) at the Chinese Academy of Sciences (Shanghai, China) have developed an integrated Yb-Raman fiber amplifier (YRFA) architecture that solves power-scaling issues in Raman fiber lasers and/or long-wavelength Yb fiber lasers.1 The amplifier uses a section of Yb-doped fiber followed by a Raman gain fiber, with both sections seeded by a dual-wavelength laser to avoid parasitic lasing and the need for WDM components.
Multiwavelength seeding
The YRFA contains a 4-m-long section of Yb-doped gain fiber and a 20-m-long section of germanium (Ge)-doped fiber that serves as the Raman converter and is seeded with an 1120 nm Yb fiber laser that also contains 1070 nm light. This dual-wavelength emitter—with adjustable power levels for the 1120 and 1070 nm wavelengths—is combined with six 976 nm laser diodes in a (6 +1) X 1 commercially available polarization-maintaining combiner that is then spliced directly to the amplification fibers.
The pump power after the combiner is 390 W at 976 nm; the 976 nm diodes pump the Yb fiber section to amplify the 1070 nm light. At the output of the YRFA, a home-made cladding mode stripper removes the residual 976 nm pump light, and an 8° cleave on the output fiber suppresses parasitic oscillations.
Simulations of the Yb-only fiber amplifier show that using an 1120-nm-only seed laser yields forward and backward ASE that are only 31 and 22 dB lower, respectively, than the 1120 nm laser output. However, when the seed laser's output is adjusted to 38 W of 1120 nm light and 2 W of 1070 nm light, ASE is suppressed to levels of 55 dB and 42 dB lower than the signal, indicating the multiwavelength seed laser effectively quells ASE.
In this integrated amplifier setup, the Yb fiber section basically acts as the input to the Ge fiber section. For the 38 W/2 W (1120 nm/1070 nm) dual-wavelength seed, both the 1070 and 1120 nm laser inputs are amplified in the first 2.7 m of the Yb fiber. After that, the 1070 nm signal reaches a maximum and begins to be Raman-converted to 1120 nm. This Raman shift continues along the Ge fiber length such that the signal is nearly 99% 1120 nm light at the output. In the experiments, the 1120 nm light reaches a power level of 301 W, limited only by the pump power. The initial 1120 nm seed laser linewidth of 1.6 nm broadens to 3.3 nm after full amplification due to four-wave mixing of longitudinal modes within the fiber sections.
Currently, Yb fiber lasers can produce kilowatts or even tens of kilowatts of diffraction-limited output, typically using a master-oscillator power-amplifier architecture. By replacing the master oscillator with a dual-wavelength laser and adding a Raman fiber section at the end of the power amplifier, the YRFA architecture is complete; the researchers believe that greater than kilowatt-level Raman fiber lasers are possible.
"Indeed, very recently, we have already achieved a kilowatt Raman fiber laser at 1120 nm," says Yan Feng, a professor of laser technology at the SIOM. "The proposed laser architecture allows further power scaling of Raman fiber lasers and the ability to generate high-power lasers at virtually any wavelength from 1 to 2 μm due to the wavelength versatility of the Raman fiber laser."
REFERENCE
1. Lei Zhang et al., Opt. Lett., 39, 7, 1933–1936 (2014).
