We report on what is, to the best of our knowledge, the first ultra-efficient 1.5 μm Raman amplifier in a methane-filled anti-resonance hollow-core fiber. A 1.5 μm single frequency seed laser is coupled into the hollow-core fiber together with a 1064 nm pulsed pump laser using a shortpass dichromic mirror, and then amplified by stimulated Raman scattering of methane. A maximum optical-to-optical conversion efficiency of 66.4% has been obtained, resulting in a record near quantum-limit efficiency of 96.3% in a 2 m long hollow-core fiber filled with only 2 bar methane gas. This kind of gas filled hollow-core Raman amplifier provides a potential method to obtain high efficiency mid-infrared laser sources with low threshold and narrow linewidth in various applications.
© 2017 Optical Society of America
High-power and narrow-linewidth laser sources at wavelength of 1.5 μm band have great potential applications in such as communications, medicine  and especially in generating mid-infrared laser emission by nonlinear frequency conversion or molecule absorption emission [2,3]. Due to the limitations of a variety of nonlinear effects and heat damage, the traditional routes to obtain 1.5 μm laser sources by Er-doped or Er-Yb co-doped solid-core fibers using Q-switching, gain-switching or mode-locking strongly suffer from linewidth broadening effect when scaling the output power [4,5].
Stimulated Raman scattering (SRS) in gas-filled hollow-core fiber (HCF) has been demonstrated to be an effective way to extend laser emission from ultra-violet to infrared wavelength while maintaining relatively narrow linewidth and high power output since its first report in a hydrogen-filled hollow-core fiber . Compared to solids, the gas medium with larger Raman shift provides the possibilities for more efficient infrared Raman conversion with high power and narrow linewidth laser output, which is benefited from the characteristics of higher damage threshold and weaker nonlinear processes of the gas . The low-loss HCF provides a perfect structure for nonlinear processes such as SRS with long effective interaction length and tight confinement of light within a micro-scale area, and enables the realization of efficient nonlinear conversion [8–12]. In 2014, efficient 1.9 μm Raman emission was reported by Wang et al.  in a hydrogen-filled HCF with ice-cream-cone shaped cladding, achieving a Raman conversion efficiency of 27% and a quantum efficiency of 48%. Then the optical-to-optical efficiency was improved to 33.5% by Gladyshev et al, using a revolver-type negative curvature HCF with noncontacting capillaries, and the corresponding quantum efficiency is 60% . In 2016, we achieved a high power and narrow linewidth Raman laser source at 1.5 μm in an ethane-filled hollow-core fiber with a peak-power of 400 kW and a linewidth of 6.3 GHz , and the optical-to-optical conversion efficiency was only 38%. And we believe that the efficiency can be further improved by injecting a seed laser or using a circular feed-back structure.
Here we demonstrate the first ultra-efficient 1.5 μm Raman amplifier based on SRS in a methane-filled anti-resonance hollow-core fiber with a diode seed laser. Using a 2 m HCF filled with 2 bar methane gas, a record optical-to-optical conversion efficiency of 66.4% is obtained, resulting in a near quantum-limit efficiency of 96.3%. We also found that the seed laser can affect the laser performance slightly, for example the Raman threshold has been reduced from 17.5 mW to 9.5 mW and the Stokes linewidth has been narrowed from 3.4 GHz to 2.1 GHz in the HCF filled with 1 bar methane. This type of laser has extensive applications in mid-infrared supercontinuum generation, gas absorption spectroscopy and so on.
2. Experimental setup
The experimental setup is shown in Fig. 1, which is similar to the setup of our previous single-pass configuration , except that the coupling of a CW tunable 1.5 μm DFB laser as the seed using a shortpass dichroic mirror with a cutoff wavelength of 1180 nm (transmittance of 98% at 1064 nm and reflectance of 99% at 1544 nm), together with a 1064 nm pulsed microchip laser (linewidth of ~6 pm, pulse duration of ~0.4 ns, repetition rate of 500 Hz, and average power of ~103 mW). The core diameter of the fiber is ~44 μm and a scanning electron micrograph (SEM) of the HCF’s cross section is shown in Fig. 2(a). The measured transmission losses are 0.1 dB/m and 0.05 dB/m at the pump line of 1064.4 nm and the most intensive vibrational Stokes transition line of methane 12CH4 of 1543.7 nm (vibrational Raman shift of 2917 cm−1), respectively. The wavelength of the single frequency seed laser with a linewidth of <100 kHz is adjustable from 1.529 μm to 1.568 μm, and the output power is tunable from 6 dBm to 16 dBm. The telescope systems are used to optimize the coupling efficiency and the optimal coupling efficiencies of the pump and the seed laser are 95% and 73%, respectively. Both of the coupled pump laser and the Stokes signal operate in single traversal mode along the propagation in the HCF and the measured near-field patterns of the transmitted pump laser and Stokes signal at the output of the HCF are shown in Fig. 2(b) and 2(c) using a silicon CMOS camera and an HgCdTe infrared camera, respectively. A scanning Fabry-Perot (F-P) interferometer is set at the output to measure the spectral lineshape and linewidth of the pump and the Stokes laser. And the full-width at half-maximum (FWHM) Δν of the laser lineshape can be calculated by
3. Experimental results
The measured output optical spectrum using two optical spectrum analyzers (OSA, AQ6370D and AQ6375) is shown in Fig. 3(a). With the increase of the pump power, other than the residual pump spectral line of 1064.4 nm, the first Stokes transition (S1) at 1543.7 nm and a series of anti-Stokes components, such as 812.2 nm, 656.6 nm and 551.1 nm (AS1-AS3), can be observed. Besides this, as shown in Fig. 3(b), green and blue light are also leaked from the side of the fiber, which correspond with the AS3 (551.1 nm) and AS4 (474.4 nm) anti-Stokes lines. Since they are located outside of the transmission bands of the HCF and hence attenuate or leak out quickly along the fiber. We have checked the power of the anti-Stokes waves to be less than 1% of the total output power with different filters. It is also found that the presence of the seed laser significantly reduces the threshold of the anti-Stokes transition and makes the leaked green or blue light much brighter, possibly due to the enhancement of the Stokes conversion and thus the increase of the stimulated optical phonons which is favorable for the anti-Stokes transition.
The effects of the seed laser on the output power and the efficiency of the Raman amplifier are investigated with respect to the pump and the seed laser power, and the results are shown in Fig. 4. From Fig. 4(a), we can see that when the pump power reaches the Raman threshold, the output Stokes power (excluding the seed laser power) increases linearly with the pump power, and the threshold power has been reduced from 17.5 mW to 9.5 mW under the influence of the seed laser. Unlike our previous single-pass experiments in which the Stokes was accumulated from spontaneous Raman scattering and the threshold power was very high , the transmitted pump power in the current amplifier decreases dramatically once the seed power is coupled and quickly converts to the Stokes power, resulting in an improvement of the Raman conversion efficiency, as shown in Fig. 4(b). It is also found that the Stokes power is independent on the coupled seed laser power. This is due to the fact that only a few coherent photons are needed to convert the Stokes transition accumulated from spontaneous Raman scattering into stimulated Raman amplification.
The power conversion feature of the amplifier at different methane gas pressures is also investigated and the results are shown in Fig. 5. It can be seen from Fig. 5(a) that the Raman threshold decreases with the increase of the gas pressure. And at pressure higher than 2 bar, the Stokes power saturates at high pump powers, indicating the existence of a strong nonlinear loss mechanism. These may due to the Raman-enhanced self-focusing, which causes transfer of energy from the fundamental mode to the next higher-order mode and attenuates rapidly, leading to strong attenuation of the transmitted Stokes laser . The maximum Raman conversion efficiency of 66.4% is achieved at 2 bar pressure with 50 mW coupled pump power, as shown in Fig. 5(b), resulting in a record near quantum-limit efficiency of 96.3%.
As shown in Fig. 6, we measured the lineshape of the pump and the seed laser using two scanning F-P interferometer with a free spectral range of 10 GHz (Thorlabs SA210-8B) and 1.5 GHz (Thorlabs SA200-12B), respectively, and the calculated spectral linewidthes are 1.67 GHz and ~3 MHz, respectively. The measured linewidth of the seed laser is not reliable since the actual linewidth of the seed laser is smaller than the resolution of the F-P interferometer.
The effects of the seed laser on the linewidth of the Raman amplifier are also investigated, and the results are shown in Fig. 6. As showed in Fig. 6(d), in the absence of the seed laser, the Stokes spectral line arises from the spontaneous Raman scattering, and the Stokes lineshape is the convolution of the pump lineshape and the Raman gain and becomes unstable and irregular. When the seed laser is injected and tuned to the Stokes Raman transition at 1543.7 nm, as shown in Fig. 6(a), the Stokes signal quickly arises from the coherent seed photon and is then amplified by SRS. Under the effects of the seed laser, the Stokes lineshape becomes stable and regular, and the linewidth is compressed from 3.4 GHz to 2.1 GHz, as shown in Fig. 7(c) and 7(d). Figure 7(e) shows the evolution of the Stokes linewidth as a function of the wavelength mismatch Δλ between the seed laser and the Stokes transition. The Stokes linewidth is broadened when the wavelength of the seed laser is tuned away from the Stokes Raman transition at 1543.7 nm. Because the linewidth of the Raman gain is much larger than that of the seed laser and has the shape of a Gaussian distribution, when the wavelength of the seed laser is tuned to the center of the Raman gain, the Stokes transition at the center of the Raman gain is also enhanced while the Stokes transition away from the center of Raman gain is suppressed, so the Stokes lineshape becomes tall and thin. When the wavelength of the seed laser is tuned away from the center of the Raman gain, the Stokes transition at the seed laser wavelength is enhanced while the Stokes transition at the center of the Raman gain is suppressed, then the Stokes lineshape becomes low and fat and the Stokes linewidth is increased.
In conclusion, an ultra-efficient 1.5 μm Raman amplifier in a methane-filled anti-resonance HCF has been reported for the first time. A maximum Raman conversion efficiency of 66.4% was obtained with 2 m HCF and 2 bar methane, and the corresponding quantum efficiency reached 96.3%, which almost approaches the quantum-limit efficiency. With the introduction of the seed laser, the Raman threshold has been reduced and the Stokes linewidth been narrowed to a certain extent. This kind of gas filled hollow-core Raman amplifier provides a potential method to obtain high efficiency mid-infrared laser sources.
National Natural Science Foundation of China (NSFC) (11274385).
We are grateful to Professor Jonathan C. Knight and Doctor Fei Yu from University of Bath in UK for providing the hollow-core fiber for our experiments. And we are also grateful to Doctor Zhihong Li for useful discussion on the experimental results.
References and links
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