The laser configuration is shown in Fig. 1. It includes a tunable Yb doped fiber master oscillator power amplifier as pump source, followed by a random distributed feedback Raman fiber laser. The output wavelength of the Yb doped fiber master oscillator can be tuned from 1000 to 1099 nm by the intracavity bandpass filter. After the Yb fiber power amplifier and isolator, more than 37.5 W can be obtained from 1020 to 1080 nm (Supplementary Information S1). The output from the tunable pump laser is optically isolated and injected into a 2000 m long piece of Raman fiber (OFS Raman optical fiber) after a wavelength division multiplexer. A broadband fiber pigtailed metallic mirror is attached to the rear free end of the wavelength division multiplexer, which forms a “half-open” random laser cavity together with the long piece of Raman fiber (Supplementary Information S2). The half-open configuration can greatly reduce the random laser threshold (Supplementary Information S3). The far end of Raman fiber is angle cleaved to minimize the back reflection. The randomly distributed Rayleigh scattering in the core of Raman fiber provides necessary feedback for the laser action.

Figure 1: The configuration of the tunable fiber laser. It contains two main sections: a tunable Yb fiber laser as pump source and a random distributed feedback Raman fiber laser. YDF, Yb doped fiber; LD, laser diode; T-BPF, tunable bandpass filter; ISO, isolator; CMS, cladding mode stripper; WDM, wavelength division multiplexer. Full size image

The laser setup is examined first with fixed pump wavelength. At low pump laser power, broadband Raman amplified spontaneous emission is observed. With increasing pump power, the output spectrum narrows suddenly, showing a clear threshold behavior as one proof random lasing. Further increasing the pump power, Raman Stokes light can be generated cascadedly. In the case of 1025 nm pumping, up to 10th order Stokes light at 1.9 μm can be obtained (Supplementary Information S4). Output spectra of the laser optimized for the 6th and 9th Stokes light are shown in Fig. 2(a) and (b) as examples. Figure 2(c) and (d) summarize the output power and efficiency of different Raman Stokes light with respect to the pump power. The pump power is limited by allowed input of the optical isolator. The power and efficiency curves show obvious threshold behavior. A gradual power curve would be observed if the Raman Stokes light is generated by amplified spontaneous emission.

Figure 2: Cascaded Raman Stokes light generation with increasing pump power. The pump wavelength is 1025 nm in this case. (a) and (b) are the output spectra of the laser when optimized for the 6th and 9th order Raman Stokes. (c) and (d) are the output powers and efficiencies for different order Raman Stokes lights with respect to pump power. Full size image

Notably, higher order Stokes light is generated almost successively, which means specific Stokes light can be selected just by tuning the pump power. In the experiments, more than 80% power purity can be readily achieved for up to 9th order Stokes light. This is a rather unique property of random Raman fiber laser among different cascaded Raman Stokes generation processes. In nested Raman oscillators and cascaded Raman fiber amplifiers, similar spectral purity can only be obtained within a narrow parameter range12,24. In spontaneous cascaded Raman amplification process pumped by a pulsed laser, supercontinuum-like multiple Raman Stokes output is usually obtained25.

The maximum conversion efficiency from pump to different Stokes orders ranges from 30 to 40%. A recent work on cascaded random lasing in polarization maintaining fiber reported about 2 times higher efficiency, where three Raman Stokes was achieved26. The lower efficiency here is due to the 2 times longer fiber used the experiments, which is necessary for achieving random lasing up to 10th Raman Stokes. Another interesting observation is that the maximum optical efficiency is seen for the 4th Stokes light, even though more Raman shifting processes are required than the lower orders light. This is due to the loss spectrum of the germanium doped silica Raman fiber, which has a minimum around 1.5 μm, typically for all silica fibers. Because of the long fiber, the passive power loss predominates the quantum defect induced loss for the first 4 Raman Stokes.

When the pump wavelength varies, the wavelengths of the cascaded Raman Stokes light changes accordingly. The Raman shift is 440 cm−1 for germanium doped silica fiber. At 1 μm, it corresponds to a wavelength change of 46 nm. Therefore, if one adjusts the wavelength of a pump laser at 1 μm over 50 nm range, gapless tuning of cascaded Raman Stokes light can be realized.

Figure 3 shows the result of continuously wavelength tuning from 1 to 1.9 μm. A time-lapse of the continuous wavelength tuning is shown in Supplementary Movie. For each output, the wavelength is determined by tuning the pump wavelength, and the spectral purity is optimized by adjusting the pump power. The output power increases with respect to the wavelength, because higher power is required to generate higher Stokes. The 3 dB laser linewidth increases from 2 nm to about 5 nm with increasing Stokes order and wavelength. And the spectral purity is higher than 80%. These behavior changes suddenly at about 1.8 μm, which is the 10th Stokes. The output power drops, and the conversion efficiency is low. The spectral purity decreases, and only 50% power ratio is achieved. This is due to the increased fiber loss at longer wavelength. The Raman fiber is specified to work at wavelength from 1.1 to 1.7 μm.

Figure 3: Continuously wavelength tuning from 1 to 1.9 μm. (a) Output spectra plotted for every 20 nm from 1 to 1.9 μm. (b) Output power and inband power ratio as a function of wavelength. Full size image

To further extend into the longer wavelength, a 200 m long piece of UHNA7 fiber (Nufern inc.) is spliced after the Raman fiber. The UHNA7 fiber has a high germanium doping of ~58 wt.%, a core diameter of 2.4 μm and NA of 0.41, leading to a high Raman gain and low fiber loss at around 2 μm. As seen in Fig. 4, the wavelength is further extended to 1.94 μm. But the output is only 2.5 W due to the greater fiber loss. A further Raman shift into >2 μm regime would require higher injected pump power.