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Abstract
We report here, to the best of our knowledge, the first 1.5 µm methane-filled fiber Raman laser pumped by a fiber laser. Based on the narrow-linewidth pulsed Yb-doped fiber laser pump source and a 15 m hollow-core fiber filled with 2.5 bar methane, the maximum power of 2.06 W Stokes wave at 1543 nm is obtained. The output laser has a narrow linewidth of 2.3 GHz, and the pulse repetition frequency can be adjusted flexibly. The output shows excellent near-diffraction-limited beam quality with a M2 factor of ∼1.09. This work proves the advantage of the fiber laser pump source with modest peak power and flexible temporal characteristics in 1.5 µm fiber gas Raman laser emission, providing good guidance for generating pulsed fiber source with narrow linewidth and high beam quality.
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1. Introduction
Stimulated Raman scattering (SRS) in gas has been proved to be an effective method for expending laser wavelength, which has drawn widespread interest. Since the first report in 1963 [1], the laser based on this principle has realized the output covering from ultraviolet to infrared band [2–5]. Typically, the conventional gas cell in Raman gas laser is slightly bulky, giving a relatively weak interaction intensity between incident light and gas molecules. Moreover, the effective interaction distance between them is somewhat short. This has raised higher requirements for pump source, even requiring megawatt-class pump power. In addition, there are always multiple Raman lines consisting the output, resulting in the inefficiency of the desired Raman lines. The advent of the hollow-core fiber (HCF) provides feasible solutions for these problems [6] as well as gives birth to the novel Raman gas lasers based on the HCF, namely fiber gas Raman laser (FGRL). This microstructured fiber can confine the light inside its tiny hollow core, which dramatically improves the pumping intensity and greatly enhances the interaction intensity. By employing longer fibers, the interaction distance can be further extended. Moreover, the controllable transmission spectrum of the HCF makes it workable to generate targeted Raman line productively. Since the first demonstration of hydrogen SRS inside the HCF by Benabid et al in 2002 [7], a great many of works have been reported based on various kinds of gases and HCFs [8–15].
Fiber laser operating at 1.5 µm band is widely used in the fields of remote sensing, medical treatment and optical communication et al [16–18], while FGRL provides a unique method for generating laser in this band. In 2016, we reported the first FGRL based on an ethane-filled anti-resonance hollow-core fiber (AR-HCF) at 1552.7 nm [19]. By employing a pulsed microchip laser, the first-order Stokes wave with peak power of 400 kW and laser linewidth of 6.3 GHz was achieved, and the maximum optical-to-optical conversion efficiency was 38%. Subsequently, by injecting a narrow-linewidth continuous wave seed laser in the single-pass configuration, we realized an ultra-efficient gas Raman amplifier in a 2-meter-long AR-HCF. The quantum efficiency approached 96.3%, which near the quantum-limit efficiency [20]. In 2018, a 1543.9 nm methane-filled FGRL operating at atmospheric pressure was reported [21]. By optimizing the fiber length, a maximum output power of 0.83 W was obtained as well as the corresponding peak power is 60 kW. Apart from these works, Cao et al. realized 1.5 µm first-order Stokes laser output along with a cascaded SRS mid-infrared laser output [22], while the quantum efficiency is merely 14% at 1544 nm. Although Zhang et al. improved output power and conversion efficiency by optimizing pump source and fiber loss based on their previous work [23], the 3-dB linewidth of the first-order Stokes wave at 1.5 µm is more than 2 nm, and the beam quality is unsatisfactory with a beam quality factor M2 of 2.5, which is somewhat restrictive for the applications. A summary of the major works on FGRL in 1.5 µm band is shown in Table 1. Up to now, all the related works are based on the solid-state laser pumping with extremely short pulse duration and low pulse repetition frequency (RF), resulting in shorter Raman pulses with inflexible RF. Besides, realizing a Watt-level laser output with good beam quality and narrow linewidth in this way still need to be resolved.
Table 1. Characteristics of 1.5 µm FGRL
In this work, we report a 1.5 µm gas-filled fiber Raman laser pumped by a Yb-doped fiber laser for the first time, to the best of our knowledge. A1064 nm narrow-linewidth pulsed fiber laser works as pump source, offering a 4 ns-width and adjustable-RF pump pulses. When a 2.5 bar methane-filled AR-HCF is pumped, the first-order Stokes wave at 1543 nm is obtained based on methane vibrational SRS. The maximum output power of 2.06 W is obtained with a laser linewidth of 2.3 GHz. The output laser has a near diffraction-limited beam quality with a M2 factor of 1.09. This work demonstrates the advantage of fiber lasers as pump sources, which is beneficial for obtaining 1.5 µm nanosecond pulse output with good beam quality and high average power.
2. Experimental setup
Figure 1 shows the schematic of the experimental setup, which is similar to our previous works but simpler in structure [19,21]. A narrow linewidth pulsed fiber amplifier is employed as the pump source, while the specific properties of which will be detailed introduced later. Its pigtail (a core diameter of 25 µm) is cleaved at an angle of 8° in order to prevent damage to the pump source from backward laser, then a plano-convex lens (Lens 1) with a focal length of 20 mm locates behind it to collimate the output beam. Subsequently, a dichroic mirror (the transmittance of 98.78% at 1064 nm and reflectance of 98% at 1500 nm) is employed with a view to monitoring the characteristics of the backward 1.5 µm Stokes wave which may be generated inside the AR-HCF. A reversible silver mirror is placed in the optical path for the convenience of monitoring real-time pump power. Another plano-convex lens (Lens 2) with a focal length of 25 mm is used to couple the pump laser into the AR-HCF.
Fig. 1. The schematic of the experimental setup. DM, dichroic mirror; PM, power meter; OSA, optical spectrum analyzer.
The 15-m AR-HCF used in the experiment is nodeless with a core diameter of ∼35 µm, and the scanning electron micrograph of its cross section is shown in Fig. 2(a). The transmission band of the AR-HCF covers 1 µm and 1.5 µm, as depicted in Fig. 2(b). The transmission loss at 1064 nm and 1543 nm are ∼0.025 dB/m and ∼0.078 dB/m, respectively. Both ends of the AR-HCF are sealed into the specially designed gas cell, which can help creates a vacuum inside the AR-HCF then fill it with methane at various gas pressures. Limited by the beam quality of the pump light (the beam quality factor M2 is 2.56 at the maximum pump power), the coupling efficiency is estimated to be 50%. The output laser is collimated by another lens (Lens 3) with a focal length of 25 mm then passes through another identical dichromic mirror, for the purpose of separating pump light and the first-order Stokes light. After that, detailed characteristics tests can be carried out.
Fig. 2. (a) The scanning electron micrograph of the AR-HCF’s cross section. (b) The spectrum of the AR-HCF’s transmitted band.
The pump source used in our work is a narrow-linewidth fiber amplifier, consisting of a 1064 nm single-frequency seed, a modulation module and a power amplifier, as depicted in dashed box in Fig. 1. The seed offers a continuous-wave 1064 nm signal light with a maximum power of 50 mW and the linewidth is narrower than 20 kHz. The modulation module based on the principle of electro-optic modulation modulates continuous-wave seed light into pulsed light, and the pulsed width is set to be 4 ns while the RF is adjustable from kHz to MHz. Subsequently, by employing a customed narrow-linewidth Yb-doped fiber amplifier, the power of µW-level pulsed signals with different RF all can be amplified to ∼16 W. Figure 3 shows the output spectrum of the pump source at the maximum output power when the RF is 250 kHz, 500 kHz and 1 MHz, respectively. It can be seen that, although the pump source has higher peak power at low RF, it is also more likely to induce SRS in the solid-core fiber inside the amplifier, resulting a decreasing of the 1064 nm signal power. Therefore, the pulse RF for the pump is supposed to be set appropriately. We also measured the 3-dB linewidth with the OSA and all the results are closed to the highest resolution of 0.02 nm, indicating its narrow-linewidth output.
3. Results and discussion
3.1 Output spectrum
Figure 4 depicts the output spectrum when the pulse RF is 500 kHz and the pump power is at the maximum. The gas pressure inside the AR-HCF is 5 bar. Firstly, we measured the spectrum by the OSA (wavelength range of 600-1700nm), as shown in the Fig. 4(a). It can be seen that there are only two lines in this range, which are the residual pump light at 1064.1 nm and the first-order Stokes light at 1543.1 nm, respectively. This Stokes line is converted by the pump light based on the vibrational SRS of the methane molecule, which corresponds to a Raman frequency shift coefficient of ∼2917 cm-1, and the first-order anti-Stokes line at ∼812 nm is not observed. To further investigate whether cascaded SRS occurs, we measured the output spectrum using another OSA (wavelength range of 1-12 µm) after filtering out the pump light, and the results are shown in Fig. 4(b). It is clear that there is only the first-order Stokes line, whereas high-order Stokes lines such as the second-order Stokes line at 2805 nm is not generated. This result suggests a higher threshold for cascaded SRS. Additionally, we also measured the fine spectrum of the pump light before coupled into the AR-HCF and the first-order Stokes light with the highest resolution of 0.02 nm, which is shown in Fig. 4(c) and 4(d), respectively. Limited by the resolution, the accurate laser linewidth is unobtainable by the OSA and we will discuss its linewidth characteristics in the Part 3.3.
Fig. 4. (a) The output spectrum when the gas pressure is 5 bar and the pulse is 4 ns width with the RF of 500 kHz. (b) The spectrum by using a OSA with wider wavelength range. The fine spectrum of (c) the pump light and (d) the Stokes light.
3.2 Output pulse shape
When the gas pressure inside the AR-HCF is 5 bar, we fixed the pulse width as 4 ns and the RF is set to be 500 kHz, then measured the output pulse shape at different pump powers, as shown in the Fig. 5. When the coupled pump power is ∼2 W, only the pump pulse is detected by the high speed InGaAs photodetector at the output, indicting the pump power is below the threshold power as well as the SRS conversion process does not occur, as shown in Fig. 5(a). Figure 5(b) displays the pulse shapes when the coupled pump power is ∼2.6 W, which exceeds the Raman threshold power slightly. The pump pulse depressed a little meanwhile the weak Stokes pulse can be detected. Due to the relatively low pump power at this time, only the part with higher energy in a pump pulse can be converted into a Raman pulse, resulting a little dip in the middle of the pump pulse. The first-order Stokes pulse is relatively weak, with a pulse width within 1 ns. With the continuous increase of the pump power, much more energy gets involved in the Raman conversion leading to the wider and deeper dip in the profile of the pump pulse. Figure 5(c) presents the pulse shapes when the coupled pump power is at the maximum. It can be seen that the Stokes pulse is strong enough and the pulse width is almost 2.75 ns, which is close to the dip of the pump pulse. All these curves show similar trends to our previous studies [13,24,25], presenting the pulse shape changes at different stages of the conversion process.
Fig. 5. Pulse shapes of the output pulses when the pump power is (a) ∼2 W, (b) ∼2.6 W, (c) the maximum.
3.3 Output power and conversion efficiency
Figure 6 reveals the evolutions of the output power characteristics. When the gas pressure inside the AR-HCF is 5 bar, we tested the power of Raman laser and residual pump light with the increasing coupled pump power in different pulse RF, as shown in Fig. 6(a) and 6(b). It is clear that for the same width of the pump pulse, when the average power is kept as a constant, the peak power of the pump pulse at lower RF is much higher, which is easier to exceed the Raman threshold peak power and get into the SRS conversion. The maximum output Raman power of 1.44 W is obtained when the RF is 1 MHz. The residual pump power drops gradually with the increasing of the Raman power, except for the case when the RF is 500 kHz. We estimate that the output laser for the pulse RF of 500 kHz is partly composed of continuous wave components, which is useless in the conversion while its power increases with the increasing of the coupled pump power.
Fig. 6. The (a) output power and (b) residual pump power with coupled pump power at different pulse RF. The (c) output power, (d) residual pump power and (e) conversion efficiency with coupled pump power at different gas pressure. The (f) output power characteristics when the Raman power is at the maximum.
To further investigate the output power characteristics, we changed the gas pressure of methane inside the hollow-core fiber by varying it among 1-10 bar with the pulse RF of 1 MHz. The measured result is plotted in Fig. 6(c)∼(e). For Fig. 6(c), it can be noticed that higher the gas pressure is, lower the Raman threshold average power is. This is mainly due to the coefficient of Raman gain increases with the increasing of the gas pressure, which has been proved in our previous works [24]. The minimum Raman threshold of average power obtained is 2.25 W when the gas pressure is 10 bar. The corresponding peak power is estimated to be 0.56 kW, which is more than 5 times lower than that in the previous work [21]. However, different from the case with gas pressure of 1 bar, 2.5 bar, 5 bar, the output power curves at 7.5 bar and 10 bar tend to flatten and the Raman power stops increasing as the coupled pump power increases. We speculate this can be attributed to the characteristics of methane molecules during the SRS conversion. During the SRS process, each Stokes photon corresponds to a vibrational excited state particle, while these excited state particles transfer into the ground state through the relaxation will release a large amount of thermal energy. Therefore, the temperature gradient and refractive index distribution inside the HCF may be changed, leading to the divergence of the pump light. Then the interaction strength between the methane molecules and pump light is reduced, making it challenging to generate new Stokes photon. Thus, it will also be quite noticeable when the gas pressure is higher as the number of methane gas particles is denser. The optimized gas pressure in this work is 2.5 bar and the maximum output power of 1.91 W is obtained. The curves of the residual pump power show the similar trends as that in Fig. 6(b), except for the cases with 7.5 bar and 10 bar gas pressure. Moreover, the corresponding conversion efficiency with respect to the coupled pump power is calculated and plotted in Fig. 6(e). Due to the stagnation of the Raman power, the efficiency under the circumstances of 7.5 bar and 10 bar get decreased with the increasing pump power. In comparison, the efficiency keeps rising as the conversion is gradually sufficient when the gas pressure is lower.
Based on the aforementioned results, we keep the optimized gas pressure of 2.5 bar inside the HCF and set the pulse RF to be 1 MHz. By further optimizing coupling efficiency, a 2.06 W first-order Stokes power is obtained, as depicted in Fig. 6(f). The pulse width is estimated to be 2.7 ns and the corresponding peak power exceeds 750 W. The evolutions of the residual pump power and the conversion efficiency have been also plotted in the figure. The maximum optical-to-optical conversion efficiency is ∼28.30%, which corresponds to the quantum efficiency of 41.04%.
At the maximum Raman power output, we measured the laser linewidth of the pump light before coupled into the gas cell and the first-order Stokes laser (the setup used in the measurement is similar to our previous work [19]), respectively. Obviously, the pump source has a narrow-linewidth laser output and the linewidth is around ∼10 MHz, which is close to the resolution of the F-P interferometer (∼7.5 MHz), as illustrated in Fig. 7(a). The linewidth of the output Stokes laser is almost ∼2.3 GHz showing in Fig. 7(b), which is much wider than the pump linewidth. This is mainly due to the fact that the obtained first-order Stokes laser in our work is arisen from the amplifier of spontaneous Raman scattering inside the HCF. By further introducing a single-frequency seed at 1543 nm, the linewidth of the output laser can be much narrower.
Subsequently, we measured the beam quality of the output Stokes light at the maximum power, and the results are plotted in Fig. 8. The corresponding beam quality factor M2 is estimated to be 1.09, and the inset in the figure shows the beam profile of the Stokes beam, indicating excellent beam quality and the near diffraction-limited output.
4. Conclusion
In this study, we have fabricated a 1.5 µm methane-filled fiber Raman laser pumped by a Yb-doped fiber amplifier. By employing a 15-meter-long AR-HCF filled with 2.5 bar methane, 1064 nm pump light can be converted to 1543 nm Raman laser based on the vibrational SRS of methane molecules. Benefit from the flexibility of fiber lasers, the output pulse with different RF can be obtained easily. The maximum output power of 2.06 W is realized when the pump power is at the maximum, and the maximum quantum efficiency estimates to be 41.04%. The laser linewidth of the output pulse at the maximum power is 2.3 GHz, and the corresponding beam quality is excellent with the M2 factor of 1.09. By further optimizing the fiber length and the coupling efficiency, the output power can be dramatically improved. This promising work provides significant insight into 1.5 µm gas-filled Raman fiber laser through fiber laser pumping, showing great potential in emitting narrow-linewidth flexible laser output with good beam quality in this method.
Funding
National Natural Science Foundation of China (11974427); Science and Technology Program of Hunan Province (2021RC4027); Postgraduate Scientific Research Innovation Project of Hunan Province (CX20220004).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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