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Abstract
We characterized high-power continuous-wave (CW) and pulsed mid-infrared (mid-IR) fiber amplifiers at a wavelength of 3.1 µm in acetylene-filled hollow-core fibers (HCFs) with a homemade seed laser. A maximum CW power of 7.9 W was achieved in a 4.2-m HCF filled with 4-mbar acetylene, which was 11% higher than the power without the seed. The maximum average power of the pulsed laser was 8.6 W (pulse energy of 0.86 µJ) at 7-mbar acetylene pressure, a 16% increase over the power without the seed. To the best of our knowledge, backward characteristics are reported for the first time for fiber gas lasers, and the backward power accounted for less than 5% of the forward power. The optimum acetylene pressure and HCF length for the highest mid-IR output are discussed based on theoretical simulations. This study provides significant guidance for high-power mid-infrared (mid-IR) output in gas-filled HCFs.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mid-IR lasers have wide applications in many areas, such as laser surgery, spectroscopy, defense, manufacturing, and free-space communication [1,2]. Among the many methods of generating mid-IR lasers, fiber lasers have the unique advantages of superior beam quality, good thermal management, high efficiency, and compactness [2]. Generally, mid-IR fiber lasers are generated in fluoride and chalcogenide fibers, which have limited chemical resistance and thermal stability compared with silica fibers. With the development of hollow-core fiber (HCF) [3,4], a new type of fiber laser named “fiber gas laser (FGL)” appears. The unique design of the microstructure allows mid-IR transmission in silica-based HCF. Instead of doped ions, gases were added to the HCF as a gain medium. Because gases have a wider emission band than rare-earth ions, FGLs have advantages in extending the laser wavelength [5].
In the past decade, FGLs have advanced greatly in mid-IR regions between 3–5 µm based on population inversion of various gases [6–10]. Among these gases, acetylene is most widely studied because of its suitable absorption lines (1.5 µm) within the telecommunication C-band and emission lines (3.1 µm) whose loss is smaller than 4 µm in current HCFs. The first breakthrough in many aspects of mid-IR FGLs was achieved using acetylene-filled HCFs. The first reported mid-IR FGL was realized in an acetylene-filled HCF [6]. A mid-IR FGL with a ring-cavity configuration was also reported in acetylene-filled HCFs [11]. The first wavelength-tunable mid-IR FGLs [12] and quasi-all-fiber FGLs [13,14] were based on acetylene-filled HCF. In 2017, Xu et al. reported a mid-IR FGL with CW output of 1.1 W in an acetylene-filled HCF and reported that the ultimate low fiber attenuation is the key to efficient laser emission [15]. In addition, efficient and stable coupling at low pressure and high temperature is the key to achieving a laser output with higher power. In 2022, we realized high-power FGLs based on acetylene-filled HCFs in a single-pass configuration by optimizing the coupling method [16,17]. Subsequently, using a quasi-all-fiber FGL as a seed source, we demonstrated a FGL based on an acetylene-filled HCF in an amplifier configuration [18]. That study focused only on the low-power result of CW pumping, thereby showing the efficiency improvement of seed injection for the FGL. It is important to realize high-power FGLs based on amplifier configuration.
In this study, we investigated the characteristics of high-power mid-IR fiber amplifiers at 3.1 µm in a 4.2-m acetylene-filled HCF in CW and pulsed conditions using a CW quasi-all-fiber FGL as a seed source. Pumped by high-power CW and pulsed erbium-doped fiber amplifiers (EDFAs), the maximum average power of CW laser and pulsed laser were achieved at approximately 7.9 W and 8.6 W, respectively, which is 0.8 W and 1.2 W higher than that obtained in single-pass configuration without the seed. The output power and pulse evolution characteristics were analyzed and compared. Meanwhile, to the best of our knowledge, backward characteristics are reported for FGLs for the first time. The optimum acetylene pressure and HCF length for the highest mid-IR output are discussed based on theoretical simulations.
2. Experimental setup
The experimental setup of the fiber-gas amplifier is shown in Fig. 1. The 1.5-µm pump laser and 3.1-µm seed laser are coupled into the HCF through a dichromatic mirror (DM1; > 99% transmittance at 3 µm and > 99% reflectance at 1.5 µm). A CaF2 lens (L2) focuses the pump and the seed lasers. The position of L2 is adjusted to maximize the coupling efficiency of the pump laser because the focal length is different for different wavelengths. The input end of the HCF is sealed in a water-cooled gas cell. The water-cooling design is to reduce the temperature of the sealing point to maintain coupling stability. To prevent the reflected light from returning to the pump source, the input window (W1, WG41050-C, Thorlabs) is tilted at an angle of 8°. The output end of HCF1 is sealed in a general gas cell with a tilted output window (W2, WG61050, Thorlabs). Both gas cells were filled with several millibars of acetylene through the vacuum bellows. The output laser from the HCF is collimated using a CaF2 lens (L4), and another dichromatic mirror (DM2) separates the signal and residual pump lasers.
Fig. 1. Schematic of the experimental setup. DM, dichromatic mirror; M, mirror; W, window; SMF, single-mode fiber; HCF, hollow-core fiber; L, lens. Insets shows schematic of a tapered SMF inserted into an HCF and scanning electron micrography image of the cross section of HCF.
The 3.1-µm seed is generated in another HCF of the same type, as shown in Fig. 1. The output end of HCF2 was sealed in a small gas cell with a half-inch window, which could be easily placed on 3-axis stages. The input end of HCF2 is connected to a single-mode fiber (SMF) through fiber-tapering technology [19]. Potting silicon was used to seal the connections. The HCF2 was filled with acetylene through a small gas cell. This structure rendered the seed laser portable. The pump source of the seed laser consists of a CW tunable 1.5-µm laser (CoBrite DX1, ID Photonics) and EDFAs. The pigtail of the EDFAs was SMF-28, which was directly fused to the tapered SMF. The pump source in the amplification configuration is similar to that in the seed configuration. It consists of the same CW-tunable laser, an electro-optic modulator (EOM), and customized high-power EDFAs with an average output power of up to 50 W.
The fiber used in the experiment is a node-less anti-resonant HCF with a core diameter of approximately 70 µm and a cladding diameter of approximately 250 µm, as shown in the inset of Fig. 1, which is of the same type as that used in our previous work [18]. The losses at 1.5 µm and 3.1 µm were approximately 0.08 dB/m and 0.14 dB/m, respectively, according to the previously measured results using the cut-back method. The lengths of the HCF used in the seed and amplifier configurations were approximately 3 m and 4.2 m, respectively.
3. Results of CW pump
In the experiment, HCF2 was filled with approximately 1.7 mbar acetylene, and a seed laser of approximately 50 mW was generated. Two power meters were used to measure the signal laser and residual pump laser of the amplifier. The power at the output end of the HCF can be obtained by eliminating the transmission losses L4 and W2; the results are shown in Fig. 2(a). At low pressures of 2.5 and 3 mbar, the signal power saturates with an increase in coupled pump power. The saturation can be solved by increasing the acetylene pressure; however, the slope efficiency also decreases with an increase in pressure as the collisional relaxation becomes stronger at high pressures. Therefore, an optimum acetylene pressure exists for obtaining the maximum signal power.
Fig. 2. Evolution of signal power and residual pump power with coupled pump power at different acetylene pressures under CW pumping (a) with and (b) without the seed injection. (c) Evolution of maximum signal power and residual pump power with acetylene pressure under CW pumping with and without the seed injection.
When the 3.1-µm seed laser was blocked between L3 and DM1, the power of the FGL in single-pass configuration could be measured, which is shown in Fig. 2(b). The power curve is similar to that of the FGL with seed injection, and the threshold is observed at approximately 1 W. Figure 2(c) plots the evolution of the maximum signal power and residual pump power with acetylene pressure. The seed significantly increased the signal power and had no influence on the optimum pressure. The maximum CW signal power of approximately 7.9 W was achieved at 4 mbar acetylene pressure, which is 11% higher than the maximum power of approximately 7.1 W without the seed.
Interestingly, the seed injection also increased the residual pump power. Because the seed enhances the stimulated emission and inhibits collisional relaxation, the absorption of the stimulated emission appears weaker than that of the pure collisional relaxation of the upper energy level. This significant phenomenon revealed that the collisional relaxation of the upper energy level was stronger than the stimulated emission. We believe that the rate of stimulated emissions was reduced because of the relatively long lifetime of the lower energy level such that a considerable portion of the population of the upper energy level was lost through collisional relaxation [20]. The strong collisional relaxation of the upper energy level is the main reason for the low efficiency of the acetylene-filled FGL.
Figure 3 displays the plots of the measured M2 of the CW signal laser at power of 1 W after the DM2 using a mid-IR camera (WinCamD-IR-BB, DataRay). The insets show the beam profile at the waist. A near-diffraction-limited beam quality of approximately 1.14 is obtained. No significant difference was observed in the output beam quality between the amplifier structure and single-pass structure. The high loss of high-order mode compared with the fundamental mode in the HCF determines the high beam quality of the output laser beam.
4. Results of pulsed pump
When the pump source is operated in the pulsed mode, we can obtain a 3.1-µm pulsed laser while the seed is still a CW laser. The average values of output power of the pulsed laser with and without seed injection are shown in Figs. 4(a) and 4(b), respectively, where the EOM is modulated by a square wave signal with a repetition frequency of 10 MHz and a pulse width of 20 ns. We can see that the saturation was more evident for pulsed pumping than for CW pumping. Overall, pulsed pumping was more efficient than CW pumping, as demonstrated in our previous work [17]. The CW seed contributes to power boost, as shown in Fig. 4(c), which shows the evolution of the maximum signal power and residual pump power with acetylene pressure. A maximum average power of approximately 8.6 W (corresponding to a pulse energy of approximately 0.86 µJ) is obtained at the optimum pressure of 7 mbar, which is 16% higher than the maximum power of approximately 7.4 W without the seed.
Fig. 4. Evolution of signal power and residual pump power with coupled pump power at different acetylene pressures under pulsed pumping (a) with and (b) without the seed injection. (c) Evolution of maximum signal power and residual pump power with acetylene pressure under pulsed pumping with and without the seed injection.
In contrast to CW pumping, the injection of seeds leads to a larger signal power increment in pulsed pumping. This is because the backward power under pulsed pumping is considerably higher than that under CW pumping, as described later. Because the injection of the seed inhibits the backward signals, the original backward power is transferred to the forward power. Thus, for pulsed pumping, the forward signal power exhibits a significant increase with the seed injection. Similar to the results of CW pumping shown in Fig. 2(c), seed injection not only increased the signal power but also increased the residual pump power. This indicates that for pulsed pumping with a pulse width of 20 ns, collisional relaxation was still extremely stronger than stimulated emission. If the pulse width is less than the average collision time, the collisional relaxation has little effect on the laser performance, and the slope efficiency becomes independent of the acetylene pressure [21].
When HCF1 was filled with 6.5 mbar acetylene, a photodetector (PDAVJ10, bandwidth 100 MHz, Thorlabs) was placed after DM2 to measure the temporal profiles of the signal pulses, and the results are shown in Fig. 5. The pump pulse is a square pulse with a width of approximately 20 ns, as shown in the inset, measured by another photodetector (ET-5000, bandwidth >12.5 GHz, EOT). All the measured pulses were normalized to the maximum value. The oscillation after the leading and trailing edges was caused by the low bandwidth of the detector. Thus, the pulse shape may not be measured precisely; however, information such as the pulse duration is still valuable. Figure 5(a) shows the evolution of the signal pulse profile with the measured signal power. We can see that the signal pulse width gradually increases with an increase in the output power and reaches approximately 20 ns at a measured signal power of approximately 1.7 W. As the power further increases, the pulse width remains constant and is limited by the pump pulse width. Figure 5(b) shows the evolution of the pulse profiles with the measured signal power when the CW seed was injected into the HCF1. It can be observed that the pulse widths were almost the same at different values of output power. This is because the CW seed is amplified by the pump pulse from the leading edge. Therefore, the pump energy at the leading edge can be fully utilized by the injection of seeds, which significantly increases laser efficiency. With the injection of the CW seed, the pulse width of the signal laser depends on the pulse width of the pump laser.
Fig. 5. Evolution of signal temporal profile with the measured signal power (a) with and (b) without the seed injection. Inset shows the pulse shape of the pump pulse at 50 W.
Similar to the M2 measurement results of the CW signal laser, the M2 of the pulsed signal laser at 1-W power was measured using the same mid-IR camera, and the results are shown in Fig. 6. The beam profile at the waist is shown in the inset of the figure, which indicates that the acetylene-filled HCF outputs the fundamental mode. Near-diffraction-limited beam quality of approximately 1.14 is obtained. No significant difference is observed in the output beam quality between the amplifier and single-pass structures. In addition, on comparing Figs. 3 and 6, we found no significant difference in the output beam quality of the CW and pulsed lasers. It should be noted that all the optical elements did not move during the measurement process, and only the output state of the pump laser changed. The results show that the gas-filled HCF laser maintains good beam quality regardless of whether it has a single-pass structure or an amplifier structure.
A power meter was placed between DM1 and L3, and the backward power of the FGL at 3.1 µm transmitting through the DM1 were measured; the results are shown in Fig. 7(a). The backward result of the CW pumping is also included in the figure. The backward signal power at the input end of HCF1 was obtained by eliminating the transmission losses of the window and lens. We observe that a backward laser exists in the single-pass configuration in both pulsed and CW pumping, which has not been reported previously. However, the power of the backward laser was extremely low, accounting for less than 5% of the forward power. Because no signal could be reflected by the tilted windows at both ends of HCF1, the backward signal can be assumed to be directly generated in the HCF. The backward signal power under CW pumping was extremely small and unstable. When HCF1 is filled with 5.5 mbar acetylene, the backward CW power first increases and then decreases to zero with an increase in the pump power, as shown in Fig. 7(a). This is because the backward signal cannot compete with the forward signal with increasing pump power. For the pulsed pumping, the average power of the backward signal was significantly higher than that for the CW pumping. As the pump power increases, the backward power increases and then becomes saturated. Similar to CW pumping, this may be because an increase in the forward power inhibits the increase in the backward power. In addition, an increase in acetylene pressure increases the backward power.
Fig. 7. (a) Backward signal power as a function of pump power for CW pumping and pulsed pumping. (b) Evolution of temporal profile of backward signal with the backward power.
Figure 7(b) plots the evolution of temporal profile of backward signal with the backward power, at an acetylene pressure of 7.5 mbar. All measured pulses are normalized by the maximum value. The pulse widths of the backward pulses are less than 10 ns, which are considerably smaller than the forward pulses. Because of the low bandwidth of the detector, the accurate pulse widths of backward pulses cannot be obtained. On comparing the time of the trailing edge of each curve, it can be inferred that the backward pulse width gradually increases with increasing power.
The large difference in the backward and forward characteristics of the single-pass FGL, particularly for CW pumping, shows that, although the forward and backward signal lasers originate from amplified spontaneous emission (ASE), the ASE has evident directivity. If the ratio of spontaneous emissions in both directions is the same, the backward power is nearly the same as the forward power from the numerical calculation of the rate equations [10]. Considering that the backward power depends on pressure, the directivity of the ASE may be related to the acetylene pressure, that is, to the particle number density. The mechanism underlying this backward signal requires further investigation.
5. Discussion
Figure 8 shows the comparison between the simulated and measured maximum signal and residual pump power as a function of acetylene pressure under 50-W pumping, using the rate equations described in [10] and [22]. For CW pumping, the population distribution was considered steady state. Therefore, the equations for the population density and laser were independent of time. Thus, these equations can be calculated numerically using the finite-difference method. For pulsed pumping, we used the finite-difference time-domain method to perform numerical simulations. Owing to the complexity of the acetylene polyatomic molecular structure and the lack of accurate energy-level parameters, simulations cannot provide accurate quantitative results. However, based on the rate equations combined with the approximation of the relaxation process and optimization of uncertain parameters, the simulation can provide qualitative rules to a certain extent. Figure 8 shows that the simulated results reproduce the variation in the laser power with the acetylene pressure. This indicates that numerical simulations facilitate better understanding of the laser output law.
Fig. 8. Simulated and measured maximum signal and residual pump power as a function of acetylene pressure under 50-W pumping with seed injection for (a) CW and (b) pulsed pumping.
Figure 9 shows the plots of the simulated signal power as a function of fiber length and acetylene pressure of acetylene-filled FGL without the seed injection under 50-W CW pumping. The signal power saturates at low pressure or short fiber length owing to insufficient particle numbers. The increase in pressure or fiber length can increase the signal power; however, the excessive increase in pressure and fiber length decreases the signal power because of excessive collisional relaxation loss and fiber loss, respectively. In longer fibers with lower optimum pressure, the increased fiber loss outweighs the reduced relaxation loss. In shorter fibers with higher optimum pressure, the increased relaxation loss outweighs the reduced fiber loss. Therefore, a suitable combination of fiber length and acetylene pressure is needed to obtain the highest power output. Figure 9 shows the maximum signal power in an approximately 3-m HCF filled with approximately 5 mbar acetylene pressure.
Fig. 9. Thermodynamic chart of simulated signal power as a function of fiber length and acetylene pressure under CW pumping without the seed injection.
The transmission loss of HCF is a key factor that affects the optimum combination of fiber length and acetylene pressure. If the fiber loss decreases, the optimum fiber length increases and the optimum acetylene pressure decreases. The maximum signal power can be increased using a longer HCF filled with lower pressure. In addition, heat management is particularly important for achieving higher power output. As the absorption and emission cross sections decrease with increasing temperature [23], a high temperature causes a decrease in efficiency. The use of a long fiber for better heat management is important for achieving a high-power output. Therefore, the use of an HCF with a lower transmission loss and a longer optimum length is the key to obtaining a more efficient output with higher power.
6. Conclusions
This study demonstrated high power mid-IR fiber amplifiers at 3.1 µm in a 4.2-m acetylene-filled HCF pumped with high-power CW and pulsed EDFAs. For CW pumping, a maximum power of 7.9 W was achieved in an HCF filled with 4-mbar acetylene. For pulsed pumping, a maximum average power of 8.6 W, corresponding to the pulse energy of 0.86 µJ, was obtained in the HCF filled with 7-mbar acetylene. Compared with the FGL without a seed, the seed injection demonstrated power increments of 11% and 16% for the CW and pulsed outputs, respectively; this further improves the output power. In addition, a backward signal was reported for the first time in a single-pass FGL without seed injection. The backward power was considerably higher for pulsed pumping than that for CW pumping but accounted for less than 5% of the forward signal power. The large difference between the backward and forward powers indicated that the ASE in the acetylene-filled HCF had directivity. For further improvement of signal power, the use of an HCF with lower transmission loss and a longer optimum length, in addition to the heat management of the HCF, is the key to obtaining a more efficient output with higher power. We believe that this study provides significant guidance for the study of high-power FGLs.
Funding
National Natural Science Foundation of China (11974427, 12004431); Science and Technology innovation program of Hunan Province (2021RC4027); State Key Laboratory of Pulsed Power Laser Technology (SKL2020ZR05, SKL2021ZR01).
Disclosures
The authors declare no conflicts of interest.
Data availability
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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