We report here the first watt-level efficient single-pass 1.54 μm fiber gas Raman source by methane-filled hollow-core fiber operating at atmospheric pressure. Pumped with a high-power MOPA (master oscillator power amplifier) structure Q-switched 1.06 μm pulsed solid-state laser, efficient 1.54 μm Stokes wave is generated in a single-pass configuration by vibrational stimulated Raman scattering of methane molecules. With an experimentally optimized fiber length of 3.2 m, we get a 1543.9 nm Stokes wave operating at atmospheric pressure with a record average power of ~0.83 W, which is about 12 times higher than the similar experiment previously reported, and the corresponding power conversion efficiency is about 45%. Operating at atmospheric pressure makes it more convenient in future applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The merits of 1.5 μm pulsed fiber sources in the fields of communication, medicine, and nonlinear wavelength conversion [1–4] have attracted more and more attentions. Nowadays, 1.5 μm pulsed fiber sources are usually obtained through Q-switched, gain-switched, or mode-locked methods [5-6] by using Er-doped fiber or Er-Yb co-doped fiber. However, diverse nonlinear effects of the solid-core fiber will limit the power scaling of lasers. Several methods have been applied to solve this problem, such as employing double-cladding Yb-free, Er-doped fiber as the main amplifier, which can avoid amplifier spontaneous emission (ASE). However, without ASE, four-wave mixing and stimulated Raman scattering (SRS) still exist, which will broaden the linewidth of the output wave and limit the power scaling . At the same time, the thermal management in the solid-core fiber is another serious problem limiting the laser’s performance in high power condition . Due to the high confinement of laser power in the hollow core area and low nonlinearity of gases, gas-filled hollow-core fibers (HCFs) offer a versatile solution for these problems [3,7,8].
Since firstly demonstrated in 2002 , fiber laser sources based on gas-filled HCFs have attracted more and more attentions in the past decade [10–23]. Recently, HCF-based gas Raman sources have been intensively studied [16-17, 19–23] thanks to the emerging of low-loss negative curvature HCFs . The advantages of HCFs include long effective interaction length, high confinement of pump light in a small area, and the possibility of controlling the gain spectrum by wavelength-dependent fiber attenuation, which can help to improve the optical conversion efficiency dramatically [9–12]. Except for the common advantages shared with the solid-core fiber laser such as high power conversion efficiency , near-diffraction limit beam quality, compactness, and so on, gas SRS lasers in HCFs have high damage threshold, low transmission attenuation, and excellent power handling characteristics, making it possible to obtain a high power and narrow linewidth laser source. SRS of gases in HCF has been proved to be an effective way to obtain narrow linewidth and high peak-power light sources covering from visible spectrum to mid-infrared [16,17,20–23]. In 2016, we reported a narrow linewidth 1.55 μm Raman laser source by ethane-filled negative curvature HCF with a maximum peak power of ~400 kW, while the average power was only ~25 mW, which was achieved in a 6 m long fiber filled with 2 bar ethane. The power conversion efficiency was ~38%, corresponding to a quantum efficiency of ~61.5% . In 2017, with a precisely tunable 1.54 μm CW seed laser, we achieved an ultra-efficient Raman amplifier with a maximum optical-to-optical conversion efficiency of ~66%, resulting in a record near quantum-limit efficiency of 96.3% in a 2 m fiber length and 2 bar gas pressure, while the maximum average power was only ~43 mW . In 2018, L. Cao et al. reported a 100-mW level 2.8 μm Raman source in methane-filled HCF by the 2nd SRS, simultaneously they got a maximum 1.54 μm first stokes power of 71 mW, while the methane pressure is high (~18 bar) and the power conversion efficiency is low (~27%), theoretically limited by the broad transmission band of the used HCF . We believe that there is a lot of room for improvement in the average output power. At the same time, working under atmospheric pressure is beneficial to the system's airtightness and can ensure long-term stable operation of the system.
Here we firstly report a watt-level average power 1.5 μm fiber gas Raman source working in a methane-filled HCF. Through optimizing the system’s structure and using a high power pulsed 1064.5 nm MOPA structure pump laser, a high average power, 1543.9 nm Stokes wave is obtained through vibrational SRS of methane molecules. With a 3.2 m fiber length and at atmospheric pressure, the maximum average output power is ~0.83 W, which is about 12 times as much as the similar experiment reported , corresponding to a maximum Raman conversion efficiency of ~45% and a quantum efficiency of ~65%. With a repetition frequency of 10 kHz and pulse duration of ~1.38 ns, the maximum peak power of the output Stokes wave is ~60 kW. We also investigate the characteristics of this system in different gas pressure and fiber length, these data could provide guidance for further improvement for the power scaling of gas Raman laser source.
2. Experimental setup
Figure 1(a) shows the experimental setup of the fiber methane Raman laser source, which is similar to our previously reported high peak-power experiments  except that the pump source is changed to a high power MOPA structure Q-switched diode pumped solid-state laser (STA-01-MOPA-1, produced by Standa in Lithuania, single longitudinal mode, pulse duration of ~1.5 ns, repetition rate of 10 kHz, and average power of 2.5 W).
As seen in Fig. 1(a), two convex-plane mirrors are used to adjust the alignment of the pump light. A continuously tunable attenuator consisting of a half-wave plate and a polarization beam splitter (PBS) is employed here to control the pump power and for the online monitoring at the same time. Through a telescope system and a silica window, the pump power is coupled into the HCF. The HCF is a little polarization-dependent, so we use another λ/2 plate, positioned after the PBS, to optimize the coupling efficiency. The optimized fiber coupling efficiency is ~80%. The output beam is collimated by lens3 (focal length of 50 mm) and then delivered to an optical spectrum analyzer (OSA) or a power meter for laser characterization. The transmission spectrum of the HCF is measured with the cut-off method and shown in Fig. 1(b). The losses at pump laser and Stokes wave are 0.12 dB/m and 0.22 dB/m respectively. A cross-section of the HCF used in our experiments is shown in Fig. 1(c), which consists of core walls with negative curvatures and a core diameter of ~46 μm.
3. Experimental results
The output spectrum is measured with two OSAs (Yokogawa, AQ6375, spectrum covering 300~1200 nm, and AQ6370D, spectrum covering 1200~2400 nm). Figure 2(a) and 2(b) show the spectra at methane pressure of 1 bar and 5 bar, respectively, in a 3.2 m long HCF. The detail spectra of the transmitted pump and Stokes wave are exhibited in Fig. 2(c) and 2(d), obtained with an OSA resolution of 0.02 nm, Fig. 2(c) is measured when there is no methane gas in the fiber, while Fig. 2(d) is measured when the pump power just reach the stimulated Raman threshold. The peak at ~1200 nm is caused by the sensitivity difference of the two OSAs. When the pump power just reaches the Raman threshold (~0.39 W), as showed in Fig. 2(a), there are only three lines: 1064.5 nm for pump wave, 1543.9 nm for the 1st Stokes wave (theoretical Raman shift of 2917 cm−1), and 812.2 nm for the 1st anti-Stokes (AS1) wave. Other Raman lines are suppressed because they are located at the ‘stop bands’ of the HCFs. With the increase of the pump power, as Fig. 2(a) shows, a series of anti-Stokes components, such as 812.2 nm, 656.6 nm, 474.8 nm, and 417.1 nm (corresponding to AS1, AS2, AS4 and AS5, respectively), can be observed. By passing through different filters, the power of these anti-Stokes waves is measured, which is less than 1% of the total output power. The reason why the AS3 (551.1 nm) does not appear in the figure, we believe, is its loss is much higher than other lines during propagating along the fiber. Figure 2(b) shows the case when gas pressure in the HCF increases to 5 bar. With the increase of the pump power, the relative intensity of the high order anti-Stokes wave is much higher than those in the low gas pressure (shown in Fig. 2(a)). An insert picture shows the visible anti-stokes lines leaked from the HCFs. Figure 2(c) and 2(d) are obtained with an OSA resolution of 0.02 nm, showing the fine spectrum characteristics of the transmitted pump and the Stokes wave.
A fast InGaAs photodetector (EOT ET5000, wavelength range 850 nm to 2150 nm, bandwidth 12.5 GHz, rise time <28 ps) and a broadband oscilloscope (LECROY Waverunner 640Zi, bandwidth 4 GHz, sampling rate 40 GSa/s) were used to detect the pulse shape and the repetition frequency of the pump and Stokes waves transmitted from the HCF. The results are shown in Fig. 3. The inconsistency of the height of the pulse shown in the picture is due to the instability of the light source. The repetition frequencies for both pump and Stokes wave are 10 kHz and the pulse shapes for both the pump and the Stokes are Gaussian-like line-type. As showed in Fig. 3(c) and 3(d), the measured pulse FWHM (Full Wave at Half Maxium) is about 1.58 ns and 1.38 ns for pump and Stokes wave respectively. By comparing Fig. 3(c) with 3(d), we can find that the rising edge of Stokes wave is steeper than that of the pump wave, supporting a pulse condensation in the SRS process, which may due to the high threshold of the methane. At the same time, note that the asymmetric pulse shape observed in Fig. 3(d) is also contributed by the pulse condensation in the SRS process .
In order to investigate the laser properties, the gas-based Raman laser have been characterized in different gas pressures and different fiber lengths. Two typical examples are picked out as shown in Fig. 4. The measured results in 4.35 m fiber length are shown in Fig. 4(a) and 4(b), while results of 3.2 m fiber length are shown in Fig. 4(c) and 4(d). From Fig. 4(a) and 4(c), it can be seen that the output Stokes power increases rapidly with the pump power when it is beyond the Raman threshold. However, under a high gas pressure, pump saturation at high pump power appears, the output Stokes power maintains almost the same value while increasing the pump power. The reason why the output Stokes power stops to increase at high gas pressure may be various. For example, as can be seen in Fig. 2(a) and 2(b), the relative intensity for anti-stokes waves is much higher at high gas pressure (5 bar) than that at low gas pressure (1 bar), proving that at high gas pressure, it becomes easier for the generation of other Raman lines, adding an extra loss for the first vibration Stokes wave. The Raman-enhanced self-focusing might contribute to the roll-over effect as well. It causes energy transformation from the fundamental mode to higher-order modes and then attenuates rapidly, leading to a strong attenuation of the transmitted Stokes laser . Note that the optimal gas pressure for maximum Stokes power output is different in different fiber length. The longer the fiber is, the lower gas pressure is required for the maximum output power. As the longer interaction length makes it easier for the first vibrational Stokes wave to transmit to other lines, while the lower gas pressure can slow down this trend, thus the optimal gas pressure is lower in the long fiber. In our experiment, the optimal gas pressure is 0.5 bar and 1 bar for the case of 4.35 m long fiber and 3.2 m long fiber, respectively. There is a little difference between the maximum output powers we obtained in these two experiments. At the same time, the Raman threshold will decrease as we increase the fiber length, as the longer interaction length for pump light and methane gas makes the Raman effect become easier to generate.
As can be seen in Fig. 4(b) and 4(d), the Raman conversion efficiency rises linearly with pump power at low power scale and then maintains a certain level at low gas pressure or decreases at high gas pressure when pump power reaches to a certain level. The curves gradually shift to left in the longer fiber due to the lower Raman threshold. The maximum Stokes power is about 0.83 W, which is obtain in a 3.2 m long fiber filled with 1 bar gas pressure, showing a maximum conversion efficiency of about 45%, corresponding to a quantum efficiency of about 65%.
The gas Raman threshold can be calculated by Eq. (1) , where g is the Raman gain coefficient given by Eq. (2) , Aeff is the effective area of the fiber modal field, L is the fiber length, αp and αs are the fiber loss for pump and Stokes wave, respectively, G is the net gain factor related to measurement condition. The Raman threshold is defined as the required coupled pump power causing the Stokes intensity Is arising from spontaneous Raman scattering to an output power level that can be observed, i.e. Isth = Is0eG .27], p is the gas pressure, ∂σ/∂Ω is the differential cross section for Raman scattering. The value of the methane Raman gain coefficient at 30 bar is given in .
As discussed above, the Raman threshold is not a constant under different circumstances. We use a fast photodetector (Thorlabs S122C, wavelength range 700-1800nm, resolution of 2 nW) to detect the Raman threshold value and the results are shown in Fig. 5. The Raman signal (1543.9 nm) is measured after passing through a long-pass filter (transmission >99.3% @1553 nm, and attenuation >30 dB @1064 nm), and the threshold criteria is set to 10 μW. As showed in Fig. 5, the measured Raman threshold is in inverse proportional to the gas pressure and well matched with the theoretical value calculated with Eq. (1). Compared with the threshold in different length of fiber, we can make the conclusion that the threshold can be reduced by increasing the length of the HCF. In the 4.35 m HCF, the Raman threshold could be reduced to less than 5 μJ.
In this paper, we demonstrated a high average power 1.54 μm fiber gas Raman source using methane-filled HCFs pumped with a 1064.5 nm pulse MOPA laser. In a 3.2 m fiber and at 1 bar gas pressure, the maximum average output is ~0.83 W, which is 12 times as much as the latest reported value. The maximum Raman laser conversion efficiency is about 45%, corresponding to a quantum efficiency of about 65%. With a repetition frequency of 10 kHz and 1.38 ns pulse duration, this laser’s peak-power reaches up to 60 kW. Operation at atmospheric pressure makes it more convenient in future applications. What’s more, we conduct a series of experiments to investigate the characteristic of this kind of 1.5 μm fiber gas Raman laser source under different circumstances, such as gas pressure and fiber length, these data could provide guidance for further power scaling for this kind of laser source.
National Natural Science Foundation of China (NSFC) (Grant No. 11504424, 11274385).
We are grateful to Prof. Jonathan C. Knight and Dr. Fei Yu from University of Bath in UK for providing the HCF for our experiments. We are also grateful to Dr. Xiaoming Xi for useful discussions.
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