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Abstract
The stimulated Raman scattering (SRS) process in gas-filled hollow-core fiber is mostly used to realize the wavelength conversion, which has the potential to produce narrow-linewidth and high-power fiber laser output. However, limited by the coupling technology, the current research is still at a few watts power level. Here, through the fusion splicing between the end-cap and the hollow-core photonics crystal fiber, several hundred watts pump power can be coupled into the hollow core. Homemade narrow-linewidth continuous wave (CW) fiber oscillators with different 3 dB linewidths are used as the pump sources, then the influences of the pump linewidth and the hollow-core fiber length are studied experimentally and theoretically. As the hollow-core fiber length is 5 m the H2 pressure is 30 bar, 109 W 1st Raman power is obtained with a Raman conversion efficiency 48.5%. This study is significant for the development of high-power gas SRS in hollow-core fibers.
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1. Introduction
Stimulated Raman scattering (SRS) is a common and effective method to realize the wavelength conversion from ultra-violet to infrared wavelength band. Since the SRS in gas materials first reported in 1963 [1], it has been regarded as a promising method to realize a narrow-linewidth and tunable laser sources of the wavelength can be obtained from conventional means. However, due to its short effective interaction length, it requires a high pump power (around 1 MW peak power) to exceed the threshold of the Raman. Besides, many unwanted lines always occurs which reduces the conversion efficiency. The emergency of the hollow-core fiber (HCF) [2–4] provides an ideal environment for the gas SRS, which not only increases the effective interaction length between the laser and the gas, but also confines the light into small core region. Besides, for SRS in fiber, high order Stokes generation can only be suppressed by controlling the fiber length. But for SRS in HCF, various methods such as controlling the transmission band of HCF, controlling the length of HCF and the gas pressure can be used to suppress the high order Stokes.
Since the first SRS in H2-filled HCF was reported in 2002 [5], a lot of SRS processes in gas-filled HCF contain rotational-SRS [6,7], vibrational-SRS [8–15] and Raman amplifier [16,17] ranged from the near-infrared to the mid-infrared had been reported in recent 20 years. However, the power level was almost watt-level. In 2010, a 55 W rotational-SRS pumped by a 92 W fiber laser with linewidth less than 100 kHz using H2-filled hollow-core photonic crystal fiber (HC-PCF) was reported [7]. In order to couple nearly 100 W pump power, the input end of the HC-PCF used above was “open-end”, H2 in the core was in a leakage state, which is insecurity and hardly for application. To further improve the pump power of the SRS in gas-filled HCF, higher power coupling into the HCF is an important problem need to be solved.
At present, the common coupling methods between the HCF and the solid-core fibers (SCF) mainly contains free space coupling, directly splicing between the HCF and SCF [18–20], tapering SCF [21,22], thermal expanding methods, et al. To achieve low-loss coupling between HCF and SCF, the matching of the mode field diameter (MFD) is important. Recently, a two-step reverse tapering approach was reported [23,24]. The single mode fiber (SMF) was first reversely tapered and then thermal expanded to realize the MFD match with the HCF. The 0.23 dB [23] loss between HCF and PCF and 0.88 dB [24] loss for the two joints SMF/HCF/SMF were realized. Apart from the above methods, HCF interconnections with 0.1 dB between HCF and SMF and 0.13 dB between HCF and itself were reported [25]. Even though a relatively lower coupling loss was obtained, the above methods are not used for hundred-watt-level power coupling, and the free-space coupling is still the most possible way currently. However, there is also some problem for injecting hundred-watt-level power into traditional gas cell. In traditional gas cell, to ensure the gas tightness of the system, rubber plug is usually used. By squeezing the rubber plug, the gap between the HCF and the gas cell is filled. However, as the increment of the injected power, the rubber plug will be deformed due to the heat, therefore, the position of the input end of HCF will change, which will lead to the reduction of the coupling efficiency. Higher temperature will melt the rubber plug and the HCF will also break. To enhance the coupling stability and seal the gases inside the HCF in fiber gas Raman lasers (FGRLs), recently we proposed an improved free-space coupling method, through splicing the input end of the HCF with end-caps, the state of the HCF is nearly unchanged and the coupling efficiency is stable as the increasing of the injected power, therefore, higher power can be coupled into the core of HCF using HCF end-cap instead of gas cell. It has the advantages of both the high-power coupling and the sealing design, above kilowatt power was successfully coupled into the HCF [26].
Here, based on the HCF end-cap, we have researched rotational-SRS process in H2-filled HC-PCF under several hundred watt power pump. Influence of the pump linewidth on the power characteristics of the SRS in gas-filled HCF is researched using homemade CW narrow-linewidth fiber oscillators. As the 3 dB linewidth of the pump is 0.16 nm and 0.12 nm, around 62 W 1st Raman power has been produced. To obtain pure 1st Raman, through shorten the fiber length from 10 m to 5 m, 2nd Raman power is greatly reduced and 109 W 1st Raman power is obtained filled with 30 bar H2 pumped by oscillator with 0.12 nm 3 dB linewidth. Besides, a numerical model is established for narrower linewidth pump, which can help us choose suitable fiber length according to pump with different linewidth and power density to obtain pure 1st Raman.
2. Pump sources
Up to now, SRS process in HCF is almost at watt power level, pump power is hardly higher than 100 W. Through our proposed HCF end-cap, the ability to couple several hundred watt pump power into HCF has already been available. In order to further research the laser characteristics of SRS in gas-filled HCF under high-power pump, we have developed several high-power CW narrow-linewidth fiber oscillators with different structures as the pump sources. At present, there is no unified definition about the “narrow-linewidth” of fiber lasers. From the spectral beam combining application, “narrow-linewidth” is defined as < 1 nm. In this paper, if the linewidth of pump we used is less than 1 nm, we call it narrow-linewidth. Compared with narrow-linewidth fiber oscillators, wide-linewidth fiber oscillator can provide higher pump power, therefore, as a contrast, a wide-linewidth CW fiber oscillator with output power above 1 kW is also used to research the influence of the pump linewidth on SRS process in HCF.
Figure 1 shows the setups of four fiber oscillators, which are used as the pump sources. From Fig. 1(a) to Fig. 1(d) are four pump structure which are marked as Setup I to IV. Setup I is the structure of the wide-linewidth fiber oscillators, both forward and backward pumping configurations are adopted. At each pump direction, six 915 nm laser diodes (LDs) with the maximum pumping power of 180 W are used as the pump and they are fusion spliced with the fiber Bragg gratings (FBGs) using (6 + 1) × 1 coupler. Setup II to IV are homemade narrow-linewidth fiber oscillators. The gain fiber is a 12 m long Yb3+-doped fiber (YDF) with a core diameter of 20 µm and a cladding diameter of 400 µm. Two FBGs and the YDF consist of the cavity. The high reflectivity (HR)-FBGs in Setup II to IV all have a full width at half maximum (FWHM) of 0.7 nm. The FWHM of output coupled (OC)-FBG in Setup II is 0.1 nm and in Setup III and IV is 0.05 nm. Two LDs operating at 976 nm with a maximum power of 250 W are used as pump using a (2 + 1) × 1 coupler. Forward pump structure is used in Setup II and III, and backward pump structure is used in Setup IV. Cladding pump stripper (CPS) is set at output end to filter the residual pump power.
Fig. 1. Setup of four pump sources. (a) Wide-linewidth fiber oscillator; (b) Forward pumping narrow-linewidth fiber oscillator with 0.1 nm OC-FBG; (c) Forward pumping narrow-linewidth fiber oscillator with 0.05 nm OC-FBG; (d) Backward pumping narrow-linewidth fiber oscillator with 0.05 nm OC-FBG.
Figure 2 shows the output spectrum of the pump corresponding to Setup I to IV. The inset figure shows the linewidth varies with the coupled pump power (which means the transmission power at the input end of the HCF). According to the measured linewidth, the power spectral density (which is calculated as output power divided by 3 dB linewidth) of 4 pump sources is obtained and shown in Fig. 3. Spectrum of Setup I is from wide-linewidth fiber oscillator, the central wavelength is at 1070 nm. The 3 dB linewidth is around 0.76 nm at 100 W and as the power increasing to 800 W, the 3 dB linewidth is around 3.38 nm, and the 20 dB linewidth is higher than 12 nm, therefore it has a low power spectral density, the maximum power spectral density is less than 0.25 W/pm. Compared with the linewidth of Setup I, the linewidth of the Setup II to IV is narrower. The central linewidth of Setup II to IV is 1080 nm. Setup II has the maximum 3 dB linewidth of 0.43 nm, and the 20 dB linewidth is 2.15 nm, which is much narrower than that of Setup I. The maximum power spectral density is around 1.23 W/pm when the coupled pump power is around 120 W, and as the increment of the coupled pump power, the power spectral density starts to decrease as shown in Fig. 3(b). As the OC-FBG is replaced to 0.05 nm, the linewidth of the pump is furtherly narrowed. At the maximum output power, the 3 dB linewidth of Setup III and IV is both less than 0.17 nm. Compared with Setup III and Setup IV, at 100 W, the 3 dB linewidth of Setup III is narrower, but when the power is higher than 200 W, the 3 dB linewidth of Setup IV is narrower, Setup III has the maximum 3 dB linewidth of 0.164 nm and Setup IV has the maximum 3 dB linewidth of 0.12 nm. For the 10 dB linewidth, Setup IV is always narrower than Setup III. Besides, for the 20 dB linewidth, as the power is lower than 200 W, Setup IV has narrower linewidth, and as the power is higher than 200 W, Setup III has narrower linewidth. In Fig. 3(c), Setup III has the maximum power spectral density of around 2.61 W/pm, and as the coupled pump power is higher than 120 W, the power spectral density has no significant change. And in Fig. 3(d), Setup IV has the maximum power spectral density of around 3.18 W/pm, and power spectral density has been increasing as the increment of coupled pump power. In the inset figure of Fig. 3 beam quality of the four pump sources is also shown.
Fig. 2. Output spectrum of four pump sources. Inset: 3 dB, 10 dB and 20 dB linewidth varies with the pump power.
Fig. 3. Power spectral density of four pump sources varies with coupled pump power. Inset: beam profile at the waist position.
3. Experiments
3.1 Setup of SRS in H2-filled HCF
Using the pump structure in section 2, the setup of SRS in H2-filled HCF is established as shown in Fig. 4. The output laser beam of the pump source is coupled into the core of the HC-PCF through the lens group. The core diameter of all four pumps is 20 µm, which has a mode field diameter (MFD) a little bigger than 20 µm. The HC-PCF we used is HC-1060-02 from NKT photonics, the core diameter of the HC-PCF is 10 µm with the MFD of 7.5 µm as shown in the inset picture of Fig. 4(b). The lens group contains four plano-convex lenses with the focal lengths of 30 mm, 30 mm, 75 mm and 30 mm. Two reflectors are used to adjust the direction of laser beam. The input end of the HC-PCF is fabricated with the end-cap and fixed on a water-cooled fixture as shown in the inset picture of Fig. 4(a). The fixture of HCF end-cap is similar to that in Ref. [26], approximately 25 cm long HCF is water cooled, which contains 10 cm without coating and 15 cm with coating. The main heating position is at the front end of coating, therefore, 10 cm region without coating is applied with matching paste to filter the cladding light, and the 15 cm region with coating is applied with epoxy adhesive to realize cooling. The HCF end-cap is the most important device in this system, as the statement in the introduction, HCF end-cap solves the problem that traditional gas cell is hard to stably couple hundreds of watts pump power into the core of HCF. Because there is an interface between silica and air as the fabrication of HCF end-cap, around 3.5% Fresnel reflection is hard to avoid [27], this is a problem that needs to be further optimized for HCF end-cap. In our system, 3.5% Fresnel reflection has no other effect except increasing the loss of the system. Considering all the loss of mirror, loss of end-cap and coupling loss, the coupling efficiency from the output power of the pump source to the power at the front end of HCF (coupled pump power) is calculated as 59%.
Fig. 4. (a) Experiment setup of SRS in H2-filled HC-PCF. L: lens; R: reflector; PM: power meter. Inset: Detailed figure of the end-cap; (b) Measured fiber loss of the HC-PCF. Inset: The cross-section of the HC-PCF; (c) Image of the splicing process between HCF and end-cap; (d) Model of the end-cap.
Figure 4(c) shows the splicing process between the HCF and the end-cap. The center of the HCF is aligned with the center of the end-cap, and the end-cap is heated to molten state through three electrodes. Then the HCF is brought into contact with the HCF at molten state. After the HCF is cooled down, HCF end-cap is finished. For details of the fabrication of HCF end-cap. Please refer to Ref. [26,27]. Figure 4(d) shows the model of the end-cap, the total length is 20 mm and the diameter is 8.2 mm. The fiber loss of the HC-PCF is measured using cut-back method and the results is shown in Fig. 4(b). At the wavelength we used in the experiment of 1080 nm, 1153 nm and 1237 nm, the measured fiber loss is 0.056 dB/m, 0.06 dB/m and 0.45 dB/m. The output laser is firstly collimated by a plano-convex lens and then sampled by a wedge mirror. The sampled laser is used to monitor the Raman power using two long wave pass filters (LPF), LPF1100 and LPF1200.
3.2 Results of different pump sources
First, Setup I is used as the pump source. As the fiber length of the HC-PCF is 5 m filled with 30 bar H2, neither power nor the spectrum of the Raman is observed even the pump power is as high as 800 W, the output spectrum is shown in Fig. 5(a). And then, the length of the HC-PCF is changed from 5 m to 24 m, which can reduce the threshold of Raman. Still, no spectrum or power is observed. Due to the low power spectral density according to Fig. 3(a), the maximum power spectral density is less than 0.25 W/pm, therefore, the pump of Setup I is hard to reach the Raman threshold, therefore, wide linewidth fiber oscillator is not suitable for the high power FGRLs.
Fig. 5. Output spectrum with 30 bar H2 pressure at 5 m long HC-PCF pump corresponding to Setup I to IV.
Then, homemade narrow-linewidth fiber oscillators are used as the pump sources. Compared with the linewidth of Setup I, the power spectral density has a significant improvement. Using 5 m long HC-PCF filled with 30 bar H2 pressure, the output spectrum of the SRS process in HC-PCF are shown in Fig. 5(b) to (d). As the pump structure is Setup II, the output spectrum is shown in Fig. 5(b), only 1st Raman line at 1153 nm is observed, however, the intensity of the 1st Raman is far less than that of the residual pump. The power of 1st Raman is also lower than 1 W. Although the linewidth is narrower than pump source in Setup I used above, the maximum power spectral density is only 1.23 W/pm as shown in Fig. 3(b), which just reach the Raman threshold. Figure 5(c) shows the output spectrum when the pump structure is Setup III. As the linewidth of the pump gets narrower furtherly, the power spectral density gets higher and except for the 1st Raman, 2nd Raman at 1237 nm is also observed. However, as the output power of Setup III increased, the power spectral density stop to increase as the coupled pump power is higher than 120 W, and as the broadening of the linewidth, 2nd Raman disappears and the intensity of 1st Raman is also decreased. Figure 5(d) shows the output spectrum when the pump structure is Setup IV. Compared with the results in Fig. 5(c), the linewidth of the 1st Raman is narrower and has higher power spectral density. Even as the pump power gets higher, the power spectral density is still increasing and 2nd Raman still exists. The 1st vibrational SRS of the pump is at around 1960nm (vibrational Raman frequency of 4155 cm-1), where the wavelength is out of the transmission band of the HC-PCF, therefore, there is no vibrational SRS observed.
The Raman power and residual pump power pumped by setup III and IV under the same conditions are taken as the comparison, and the results are shown in Fig. 6. The maximum output power of 1st Raman is around 62 W for both structures. When the coupled pump power is less than 150 W, the forward pump structure has a higher output of 1st Raman power than that of backward pump structure, however, the backward pump structure has a higher output of 2nd Raman power, which is converted by 1st Raman power. As the coupled pump power is higher than 150 W, 1st Raman power of forward pump structure start to decrease because of the broadening of the linewidth and the power spectral density stop to increase. But for the backward pump structure, although 2nd Raman power start to decrease, 1st Raman power still increases until reach the maximum output power. For the residual pump power as shown in Fig. 6(b), using backward structure can reduce the residual pump power effectively. From the current experimental results, backward pump structure with narrower linewidth is more suitable for the FGRLs.
Fig. 6. Comparison of the (a) Raman power and (b) residual pump power between the forward pump structure and the backward pump structure with 30 bar H2 pressure at 10 m HC-PCF.
3.3 Results of different HCF lengths
According to the results in Section 3.2, as the power density and pump power increased, 2nd Raman is easily produced. In order to obtain pure 1st Raman, 2nd Raman needs to be suppressed as much as possible. Through shorten the fiber length can increase the threshold of the 2nd Raman. Therefore, research of the influence on different HCF lengths is necessary. Take the results of Setup IV as example.
The output spectrum at different fiber length with 30 bar H2 pressure is shown in Fig. 7. There are three main lines observed, the residual pump at 1079.6 nm, 1st Raman at 1152.6 nm and 2nd Raman at 1236.3 nm. When the fiber length is 2 m, no 2nd Raman is observed. At 2 m, 3 m and 5 m fiber lengths, the 1st anti-stokes at 1015.3 nm (rotational Raman frequency of 587 cm-1) is observed, which is caused by four wave mixing (FWM) process. Through the integration of the spectrum, when the fiber is 5 m, it has the maximum 1st Raman intensity. For the 2nd Raman, calculated spectral composition for 5 m is 0.5% and for 10 m is 0.4%. At the maximum pump power, lower pump power spectral density leads to less 2nd Raman power generation. For 5 m fiber length, due to higher 1st Raman power, even if the 2nd Raman threshold is higher than that of 10 m, the generated 2nd Raman power is higher than that of 10 m.
Fig. 7. Measured spectrum of the output laser with different fiber length at the maximum pump power as the gas pressure is 30 bar.
Figure 8 shows the power characteristics at different lengths of HC-PCF when the H2 pressure is 30 bar. Figure 8(a) shows the 1st Raman power varies with the coupled pump power. The threshold characteristic expresses obviously, as the increases of the fiber length, the threshold decreases gradually. At 10 m fiber length, the threshold is only around 25 W coupled pump power, however, when the fiber length is 2 m, there is nearly no Raman appears. Figure 8(b) shows the relationship between the 1st Raman efficiency and the coupled pump power at different fiber length, where the 1st Raman efficiency is calculated as 1st Raman power divided by coupled pump power. As the fiber length is 5 m, it has the maximum Raman efficiency of around 57.5%. 2nd Raman power is shown in Fig. 8(c), as the increases of the fiber length, the threshold of the 2nd Raman is still reduced, therefore the 2nd Raman power with the fiber length of 10 m is higher than that of other fiber length. Comparatively, no 2nd Raman is observed when the fiber length is 3 m and 2 m. Besides, longer fiber length can help reduce the residual pump power as Fig. 8(d) shows. As the fiber length changed from 10 m to 5 m, the residual pump power should have increased, but the residual pump power of 5 m is less than that of 10 m, as the observation of the spectrum at these two fiber lengths, anti-stokes is only observed when the fiber length is 5 m. Since anti-Stokes light is mainly generated by FWM process, therefore, there is a FWM process that pump converts to anti-stokes and stokes. We guess that FWM process may have an effect on the conversion from pump to 1st stokes, which leading to the higher output of 1st Raman power at 5 m fiber length than that of 10 m. However, how much influence the FWM process has on the generation of 1st Raman needs furtherly research.
Fig. 8. (a) 1st Raman power, (b) 1st Raman efficiency, (c) 2nd Raman power and (d) residual pump power varies with the coupled pump power at different fiber length when the gas pressure of 30 bar.
4. Discussion
A numerical model is established to analyze the SRS process in the HC-PCF, compared with the model in Ref. [28], we consider the shape of the spectrum of the pump. Using the measured spectrum in Fig. 2 as the shape of the spectrum, and the pump is separated into several segments according to the sampling interval of the optical spectrum analyzer, which is 0.004 nm. According to Ref. [29] the rotational Raman gain linewidth is calculated as 0.012 nm. Since the sampling interval is smaller than Raman gain linewidth, each segment is considered as a single wavelength model, and then add all parts together to get the total power. Besides, 2nd Raman is also considered in the numerical model. Thus, a steady-state SRS model is established as
Considering the power characteristics at different fiber positions, the numerical model that power varies with the fiber length is set up as shown in Fig. 9. The ordinate represents the power of 1st Raman, 2nd Raman and residual pump transmitted in the HCF. The four cases represent four different pump powers of the pump source Setup IV. The conversion process from the pump to 1st Raman doesn’t happen at the front end of the fiber, which reflect as threshold characteristic. This threshold characteristic explains that there is nearly no 1st Raman when the fiber length is 2 m in Fig. 8. And then it needs around several meters’ transmission distance for the completely conversion from the pump to the 1st Raman, and this is the reason that 1st Raman power is low in Fig. 8(a), 3 m long fiber length is too short that the transmission from pump to Raman has not finished yet. As the fiber length gets longer, 2nd Raman starts to occur. For different pump power, it mainly influences the position that the conversion process occurs because of different power spectrum density and linewidth.
Fig. 9. Simulated 1st Raman power, residual pump power and 2nd Raman power transmission in the HCF varies with fiber length at different pump power using Setup IV as the pump source.
Experimental and simulated results of Raman and residual pump power varies with coupled pump power are shown in Fig. 10, the influence of the fiber length mainly reflected on the threshold of the Raman. In Fig. 10(a), as the fiber length is 10 m, the simulated results have a great agreement with the experiment results, 10 m fiber length has the lowest Raman threshold, and besides the 1st Raman has converted into the 2nd Raman due to the lower threshold of 2nd Raman. As the fiber is 5 m, the results are shown in Fig. 10(b). As the decrease of the fiber length, the threshold of the 1st Raman and 2nd Raman has both increased, and there is nearly no 2nd Raman produced. When the coupled pump power is under 150 W, the simulated results have a great agreement with experimental results. However, as the coupled pump power is higher, there are some differences between the measured results and the simulated results, which may be influenced by FWM as said above, more detailed reasons need to be furtherly researched. In our experiment, 1st Raman power is what we want, but the production of 2nd Raman will reduce the power of 1st Raman power. We need to choose the suitable fiber length to obtain the maximum power of 1st Raman according to the power density of the pump and the pump power.
Fig. 10. Experimental and simulated results of Raman and residual pump power varies with coupled pump power as the fiber length is (a) 10 m and (b) 5 m.
5. Conclusion
We have demonstrated hundred-watt level rotational-SRS in H2-filled HC-PCF pumped by homemade narrow-linewidth fiber oscillator based on HCF end-cap. Pumped by homemade fiber oscillators with different 3 dB linewidth, the influences of the pump linewidth and the HCF length are studied experimentally. As the fiber length is 5 m when the H2 gas pressure is 30 bar, the power of 1st Raman is 109 W with a conversion efficiency of 48.5% pumped by setup IV. A numerical model is also established for narrower-linewidth pump, which can help us choose suitable fiber length to obtain pure 1st Raman. The results paves the way for generation of high-power pure Raman for SRS in gas-filled HCF.
Although, the 1st Raman power has been improved from the reported research, there is still a lot of residual pump power, which indicates that the linewidth of the pump is still not narrow enough. There are several methods to further reduce the linewidth of the pump. The first method is using highly Yb3+-doped gain fiber to reduce the total length of the system, through the decrease of the transmission distance to prevent the broadening of the linewidth. Another method is using the structure that amplify the narrow-linewidth seed laser, which can get a high power and narrow-linewidth pump. Besides, using the gain fiber with the core of 25 µm is also a choice. Through the special design of the fiber loss of 2nd Raman to make it completely out of the transmission band can further improve the efficiency of the 1st Raman [30]. Based on the narrower linewidth and higher power density pump, the rotational-SRS can be further enhanced.
Funding
Postgraduate Scientific Research Innovation Project of Hunan Province (CX20200017); Science and Technology Innovation Program of Hunan Province (2021RC4027); National Natural Science Foundation of China (11974427, 12004431).
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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