Open Access
Add to BibSonomy
Abstract
Realizing stimulated Raman scattering (SRS) suppression is a key topic for high-power fiber lasers. Here, we report an effective and simple strategy for SRS suppression using chirped and tilted fiber Bragg gratings (CTFBGs) in high-power fiber oscillators while maintaining the compactness and stability of the system. The CTFBG is inserted on the side of a cavity mirror FBG without cutting the gain fiber. To improve power handling capability, the CTFBG and cavity mirror FBGs are inscribed by femtosecond (fs) lasers. The optimal SRS suppression effect can be realized when the CTFBG is inserted into the resonant cavity and on the side of the output coupler FBG. The SRS threshold is increased by approximately 11% with an SRS suppression ratio of nearly 14 dB. Moreover, the output power of the fiber oscillator is improved to 3.5 kW, which is the maximum power achieved in fiber oscillators with SRS suppression using CTFBGs, to the best of our knowledge. The temperature of the air-cooled CTFBG is 50.2 °C, which has the potential to handle higher power. This work provides new insights for suppressing SRS in fiber oscillators, promoting the application of CTFBGs in high-power lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-power fiber lasers have significant application value in many fields, especially in the field of material processing with a remarkably fast-increasing business [1,2]. Compared with high-power fiber amplifiers, high-power fiber oscillators are more popular in the material processing field due to their advantages of strong anti-refection ability, simple structure, and easy control logic [3]. With the increase in output power of fiber oscillators, nonlinear effects such as stimulated Raman scattering (SRS) become significant. SRS can lead to a decrease in the conversion efficiency of signal power, affecting material processing quality. SRS is more likely to occur in two typical scenarios when using fiber oscillators for material processing. One is delivering the high-power laser to the processing area via a long passive fiber, which lowers the SRS threshold due to the extended interaction length. The other is that the surface of the material can reflect a part of the laser into the fiber oscillator. resulting in a decrease in the SRS threshold because the Raman light component in the reflected laser provides the feedback for SRS. Thus, there is an urgent need to suppress SRS in high-power fiber oscillators. Recently, long-period fiber gratings (LPFGs) [4–8] or chirped and tilted fiber Bragg gratings (CTFBGs) [9–17] as band rejection all-fiber filters have been successfully used to suppress SRS in fiber lasers. However, LPFGs are sensitive to the external environment, causing their unstable filtering characteristics when handling high-power lasers. Accordingly, the LPFGs have only been utilized in hundred-watt-level fiber oscillators to suppress SRS because the highest handling power of LPFGs is less than 1 kW [5,6]. In contrast, CTFBG with good stability is successfully commercialized, which is more suitable for the application of SRS suppression in high-power fiber oscillators. In 2019, Jiao et al. fabricated the first kW-level CTFBG by using UV lasers and inserted it into the output end of a fiber oscillator to suppress SRS [9]. In 2020, Lin et al. inserted a CTFBG into the resonant cavity of a fiber oscillator to suppress SRS, increasing the output power to 1.8 kW [12]. Afterward, they further improved the output power to 2 kW by increasing the filtering bandwidth using two CTFBGs [14]. The above works of literature have fully demonstrated the feasibility of using CTFBG for SRS suppression in high-power fiber oscillators, but there are still two shortcomings. On the one hand, the insertion position of CTFBG in the fiber oscillator needs to be comprehensively considered. In Ref. [12,14], the CTFBG is located within the ytterbium-doped fiber (YDF) of the resonant cavity, resulting in the YDF being cut into two sections and fusion-spliced with the CTFBG. Because the octagonal cladding shape of YDF is different from that of passive fiber, the fusion-splice point between YDF and passive fiber is usually more fragile. Thus, this CTFBG insertion strategy makes the fiber oscillator structure more complicated and excessively sacrifices its compactness and stability. On the other hand, the power handling capability of CTFBG needs to be further improved. Although the CTFBG inscribed by UV-laser has achieved a maximum handling power of 3.4 kW [13], the fabrication of high-power CTFBG is very complex and time-consuming due to the utilization of a special annealing method, multiplexed inscription method, and powerful cooled package, which limits the application of CTFBG in high-power fiber oscillators.
The purpose of this study is to explore methods that will help address the shortcomings of previous research. To minimize the impact of CTFBG on the stability and compactness of the fiber oscillator, the insertion position of CTFBG was chosen to be on the side of the cavity mirror FBGs. With this insertion strategy, the cutting of YDF can be avoided, and the insertion of CTFBG only introduces one additional fusion-splice point in the fiber oscillator system. Moreover, in order to improve the power handling capability of CTFBG, the fs-laser was used to inscribe CTFBGs. Unlike UV-lasers, fs-lasers can directly inscribe FBGs in the fiber without hydrogen loading or thermal annealing treatment [18–20]. Thus, the fabrication period of FBG is significantly reduced, and the significant heating of FBG caused by incomplete thermal annealing can also be avoided. Thus, fs-lasers are extremely suitable for the fabrication of high-power FBGs [21–24]. Recently, we have demonstrated that fs-written CTFBG has good high-power handling capability and stability [17]. However, the fs-written CTFBG has not been tested in fiber oscillators. Consequently, it is meaningful to investigate whether fs-written CTFBG is more suitable than UV-written CTFBG for suppressing SRS in high-power fiber oscillators.
In this paper, a CTFBG and a pair of cavity mirror FBGs are inscribed in 20/400 µm large-mode-area double-cladding fibers (LMA-DCFs) based on the fs-laser phase mask scanning technology. A high-power fiber oscillator is constructed using cavity mirror FBGs, and the CTFBG is inserted into the fiber oscillator at three different insertion positions to suppress SRS. The optimal SRS suppression effect can be achieved when the CTFBG is inserted into the resonant cavity and on the side of the output coupler (OC) FBG. The SRS suppression width of CTFBG is larger than 55 nm, which is more than 3 times larger than the 3-dB bandwidth of the measured transmission spectrum of CTFBG. Moreover, the SRS threshold of the fiber oscillator is increased by more than 500 W, and the output power is improved to 3.5 kW. To the best of our knowledge, this is the maximum power in fiber oscillators with SRS suppression using CTFBG. The air-cooled fs-written CTFBG and cavity mirror FBGs have small thermal slopes of less than 9 °C/kW, indicating their potential to handle higher power.
2. Experimental setup
2.1 FBG fabrication
Based on our fs-laser inscription setup, a pair of cavity mirror FBGs and a CTFBG are fabricated using two different scanning strategies [16,24], as shown in Figs. 1(a) and 1(b), respectively. The green patterns are the interference fringes that are focused inside the fiber core. These fringes are formed after a 515 nm fs-laser with a beam diameter of approximately 3 mm passes through a cylindrical lens with a focal length of 25 mm and a linearly chirped phase mask along the Y-axis. The pitch period for the CTFBG is 1586 nm with a chirp rate of 2 nm/cm, while the pitch period for the cavity mirror FBG is 1488 nm with a chirp rate of 2 nm/cm. The size of the interference fringes along the Z-axis is approximately 3.7 µm, which is much smaller than the core diameter of the 20/400 µm LMA-DCFs. Thus, two fs-laser scanning strategies are used to expand the region of refractive index modulation induced by interference fringes. The black dashed patterns represent the scanning paths of interference fringes, which could be seen as formed grating planes. The scanning strategy shown in Fig. 1(a) is used to inscribe the cavity mirror FBG. The fs-laser interference fringes are scanned perpendicular to the fiber axis, so the formed grating planes are also perpendicular to the fiber axis. In Fig. 1(b), the fs-laser interference fringes are scanned obliquely to the fiber axis, forming tilted grating planes for inscribing the CTFBG.
Fig. 1. The schematic illustration of fs-laser scanning strategy for inscribing (a) cavity mirror FBG and (b) CTFBG, respectively.
The FBGs were inscribed in 20/400 µm LMA-DCFs (core NA of 0.065, cladding NA of ≥0.46, manufactured by Yangtze Optical Fibre and Cable Joint Stock Limited Company) with the coating stripped before inscription. The FBGs were measured using a typical setup consisting of a broadband light source, a fiber circulator, two mode field adapters, and an optical spectrum analyzer. The transmission and reflection spectra of high reflectivity (HR) FBG and OCFBG are shown in Figs. 2(a) and 2(b), respectively. The transmission peak depth of HRFBG is greater than 20 dB, corresponding to a reflectivity of over 99%. The center wavelength of the HRFBG is 1080.5 nm, with a 3-dB bandwidth of 3.6 nm and a grating length of 20 mm. The reflectivity of OCFBG is approximately 13% with a transmission peak depth of approximately 0.6 dB. The center wavelength of OCFBG is 1079.2 nm, which is slightly smaller than that of HRFBG because high-power lasers induce a temperature increase in OCFBG, leading to a redshift of its center wavelength. Moreover, the OCFBG has a narrow 3-dB bandwidth of 0.36 nm, which is achieved with a grating length of 3 mm. Because the narrow bandwidth of OCFBG could reduce the threshold of SRS in fiber oscillators [25], which facilitates the investigation of SRS suppression by CTFBG. Figure 2(c) presents the measured spectra of CTFBG. The long-period end of CTFBG is used as the input end of the broadband light to minimize the bandwidth of the Bragg reflection [12]. The center wavelength of CTFBG is 1133 nm with a 3-dB bandwidth of 17.3 nm, corresponding to the wavelength of 1st-order Raman light converted from 1080 nm signal light. During the CTFBG inscription process, a tandem inscription method was used to increase the bandwidth [15]. As a result, the CTFBG with a total grating length of 33 mm was inscribed with two different tilted angles of 7.4° and 6.6°, corresponding to grating lengths of 17 mm and 16 mm, respectively. The insertion loss of CTFBG is approximately 2%, which was measured by the cutback method utilizing a 1080 nm fiber laser and a power meter.
Fig. 2. The measured transmission and reflection spectra of fs-written (a) HRFBG, (b) OCFBG, and (c) CTFBG, respectively.
2.2 High-power fiber oscillator
A high-power fiber oscillator was constructed using the fs-written HRFBG and OCFBG, as shown in Fig. 3. The fiber oscillator employs a bi-pumping scheme, and the pump sources are wavelength-stabilized laser diodes (LDs) at 976 nm. The two ends of a 20-meter-long 20/400 µm YDF (core NA of 0.065, cladding NA of ≥0.46, pump absorption coefficient of 1.2 dB/m at 976 nm, manufactured by Jiangsu Fasten Optoelectronics Technology Co., Ltd.) are fusion-spliced with two pump/signal combiners. The output signal fibers of combiners are fusion-spliced with HRFBG and OCFBG, respectively. Forward and backward laser output through the endcap. Without cutting the YDF, the CTFBG was inserted in three different positions of the fiber oscillator to study the SRS suppression effect, including P1 (in the cavity and on the side of HRFBG), P2 (in the cavity and on the side of OCFBG), and P3 (out of the cavity and on the side of OCFBG). Specifically, when inserting CTFBG in three different positions, the long-period end of CTFBG was used as the input end of the forward signal laser. Additionally, a section of passive fiber of the same length was cut to ensure that the total fiber length in the fiber oscillator remains unchanged.
Fig. 3. The configurations of the high-power fiber oscillator with CTFBG inserted in three different positions.
It is worth noting that the surface of the fiber cladding near the CTFBG is etched using the cladding light stripping technology [26], as shown in Fig. 4. The cladding light stripping technology was adopted for two purposes. The main purpose is to strip the backward Raman laser through the etched region. When the forward signal light in the fiber core passes through the CTFBG, the Raman light component in the signal light can be coupled from the forward core modes to the backward cladding modes by the CTFBG. However, the backward Raman light not only generates heat at the coating-cladding interface of CTFBG [9], but it can also be injected into the pump source through the pump/signal combiner, potentially damaging both the CTFBG and pump source. Thus, it is necessary to strip the backward Raman light. Another purpose is that the etched region can also strip the residual cladding pump laser, improving the purity of the signal light in the output laser.
3. Results and discussion
Figure 5(a)∼5(d) presents the forward output spectra of four fiber oscillator configurations at different pump powers, respectively. As the pump power increases, the intensity of Raman light increases, and the linewidth of the signal and Raman light also becomes larger. To avoid damaging the fiber oscillator due to excessive Raman light, the pump power is not increased when the intensity difference between the signal and Raman light is 20 dB. At this time, the pump powers in Fig. 5(a)∼5(d) are 4778 W, 4778 W, 5520 W, and 5079 W, respectively. Obviously, the CTFBG hardly suppresses SRS when inserted at P1. However, when the CTFBG is at P2 or P3, a significant SRS suppression effect is achieved, so the pump power can be further increased. To compare the SRS suppression effect of CTFBG at different insertion positions, Fig. 5(e) presents the forward output spectra at a pump power of 4778 W. The inset in Fig. 5(e) shows the difference between the forward output spectra without and with CTFBG. This can be seen as the actual SRS suppression spectrum of CTFBG at different insertion positions. The forward output spectrum remains largely unchanged, and the actual depth of the SRS suppression spectrum is almost 0 dB when CTFBG is inserted at P1. When CTFBG is inserted at P2 or P3, both exhibit almost the same SRS suppression depth of approximately 14 dB. However, the SRS suppression width of the former is much larger than that of the latter (the SRS suppression width is defined as the bandwidth at a spectral intensity of -3 dB). The SRS suppression width of CTFBG at P2 is larger than 55 nm, which reduces the overall Raman light intensity in the forward output spectrum, while the SRS suppression width of CTFBG at P3 is approximately 17 nm. Furthermore, the backward output spectra at a pump power of 4778 W were also measured, as shown in Fig. 5(f). The inset in Fig. 5(f) is a zoom in the Raman light waveband. A distinct Raman peak at 1135 nm is observed in the backward output spectrum without CTFBG. When CTFBG is inserted at P1 or P2, there is no Raman peak in the backward output spectra, indicating that CTFBG effectively suppresses backward Raman light. However, when CTFBG is inserted at P3, not only is there a Raman peak at 1135 nm, but also a new Raman peak can be observed at 1149.1 nm, which is caused by the Bragg reflection of CTFBG (see Fig. 2(c)) [12,27]. Obviously, the backward Raman light is not suppressed but rather enhanced when CTFBG is at P3. To summarize, when comparing the forward and backward output spectra, it is evident that the insertion of CTFBG at P2 results in an optimal suppression of SRS.
Fig. 5. The forward output spectra at different pump powers (a) without CTFBG and with CTFBG at (b) P1, (c) P2, (d) P3; (e) the forward output spectra of four fiber oscillator configurations at the same pump power, inset: the difference between the output spectra without and with CTFBG; (f) the backward output spectra of four fiber oscillator configurations at the same pump power, inset: zoom in the Raman light waveband.
Considering the steady-state and transient processes of the power distribution in the resonant cavity of the fiber oscillator, we further discuss the differences in the SRS suppression effect under various CTFBG insertion positions. Figure 6(a) presents the steady-state power distribution over the active fiber length in the resonant cavity simulated according to the rate equation, and the simulated parameters are the actual parameters of the fiber oscillator without CTFBG (co-pumping power is 1.3 kW and counter-pumping power is 3.5 kW). Ignoring the fiber length of CTFBG, it can be considered that P1 and P2 are the positions of 0 m and 20 m in the resonant cavity, respectively. It can be seen that the forward signal light is mainly amplified in the second half of the resonant cavity. The Raman light can be converted from the signal light only when the signal power is large enough and reaches the SRS threshold, so the Raman light is also mainly generated in the second half of the resonant cavity. Thus, when CTFBG is inserted at P1, almost no Raman light is filtered by CTFBG in the forward signal light, but CTFBG can filter the Raman light in the backward output spectrum (see Fig. 5(f)). In comparison, when inserting CTFBG at P2 and P3, because the Raman light converted from the signal light is relatively considerable, the SRS suppression effect of CTFBG is more significant. Next, the difference in the SRS suppression effect of inserting CTFBG at P2 or P3 is further analyzed. The transient process of power distribution in the cavity needs to be considered when CTFBG is inserted at P2 in the cavity. Figure 6(b) presents the transient process of multiple round-trip signal light propagation in the cavity. Indeed, the OC-FBG not only reflects signal light but also Raman light. Due to the change in refractive index, light that does not coincide with the Bragg resonance wavelength also experiences weak reflection at each of the grating planes, and this reflection accumulates over the grating length [28]. Since the Raman power is considerable near OC-FBG (Raman light power exceeds 30 W at the maximum pump power without CTFBG in Fig. 7 below), the weak Raman reflection of OC-FBG cannot be disregarded as it can provide feedback for enhancing SRS, particularly in the resonant cavity of multiple round-trip propagation [29–31]. We can speculate that when CTFBG is inserted at P2, a portion of the Raman light component of the signal light is filtered out by CTFBG at each oscillation, resulting in a reduction of the Raman light reflected by OC-FBG. As a result, the SRS induced by weak reflection in the cavity is effectively suppressed, and the Raman suppression width is more than 55 nm (see Fig. 5(e) inset), which is 3 times the width of the CTFBG transmission spectrum (see Fig. 2(c)). In comparison, when CTFBG is inserted at P3, CTFBG only filters out the Raman light output from the cavity, so the SRS suppression width is essentially the same as the width of the CTFBG transmission spectrum. Moreover, the change in SRS intensity in the cavity can also be observed from the backward output spectra in Fig. 5(f). The 1135 nm Raman peak disappears in the backward spectrum after inserting CTFBG at P2, indicating that the SRS is suppressed and the Raman light becomes weak in the cavity. By contrast, when CTFBG is inserted at P3, not only does the 1135 nm Raman peak still exist, but a new Raman peak at 1149 nm appears. Consequently, the SRS in the cavity is not suppressed, and the 1149 nm Raman light is reflected into the cavity by the Bragg reflection of CTFBG and amplified during the backward transmission [12].
Fig. 6. (a) Simulation of pump and signal power distribution in the resonant cavity of the fiber oscillator without CTFBG; (b) the schematic illustration of signal laser oscillating and amplifying in the resonant cavity.
Fig. 7. The variation of (a) output power, (b) signal ratio, (c) Raman ratio, and (d) M2 factor with the pump power. Inset: (a) zoom near the maximum output power, (d) output laser beam profile.
Figure 7(a) presents the variation of output power with pump power under four different configurations, and the inset is a zoom near the maximum output power. There is almost no change in slope efficiency when inserting CTFBG at P1, while the slope efficiency decreases slightly when CTFBG is inserted at P2 or P3. This is because the signal power at P1 is relatively low and the signal power is mainly amplified in the second half of the resonant cavity (see Fig. 6(a)), making the impact of CTFBG insertion loss relatively small. Furthermore, when CTFBG is inserted at P2, the slope efficiency decreases more than that of CTFBG at P3 because CTFBG loses signal light in the resonant cavity during each oscillation (see Fig. 6(b)). Nevertheless, when CTFBG is at P2, the pump power can be further increased due to the effective suppression of SRS, and the maximum output power is increased to 3.5 kW, which is approximately 150 W higher than that without CTFBG. In comparison, the SRS suppression effect of CTFBG at P3 is somewhat worse compared to that of CTFBG at P2, so the maximum output power does not exceed that without CTFBG. Moreover, the ratio of signal or Raman light in the output power is analyzed by integrating the output spectra at different pump powers, as shown in Figs. 7(b) and 7(c), respectively. It can be seen from Fig. 7(b) that when CTFBG is inserted at P2, the ratio of the signal light is maximized at the same pump power, so the purity of the signal light is also improved by CTFBG. In Fig. 7(c), logarithmic coordinates are used for the vertical coordinates to facilitate comparison of the growth rate of the Raman ratio in different configurations. We can see that the SRS threshold without CTFBG is below 4778 W, while the SRS threshold exceeds 5220 W when inserting CTFBG at P2, indicating that the SRS threshold is increased by more than 500 W, corresponding to an improvement of approximately 11% (the SRS threshold is defined as the value of the pump power when the Raman ratio reached 1% (-20 dB) of the total output power). In comparison, the increase of the Raman threshold is relatively less when CTFBG is at P3, which can be attributed to the difference in the growth rate of the Raman ratio. The slope efficiency of the Raman ratio with CTFBG at P3 is basically the same as that without CTFBG, indicating that the growth rates of the Raman ratio are also essentially the same for both. However, the slope efficiency of the Raman ratio (i.e., Raman growth rate) decreases when CTFBG is inserted at P2. This further confirms the analysis that SRS in the cavity is effectively suppressed by inserting CTFBG at P2, but CTFBG at P3 only filters out the Raman light output from the cavity and does not affect the Raman growth rate. Furthermore, the output laser beam quality factor M2 was measured, as shown in Fig. 7(d). When CTFBG is not inserted, the ratio of Raman light increases with the increase of pump power, resulting in a degradation of beam quality [32]. Similarly, it has almost no effect on the beam quality when inserted at P1. However, when inserting CTFBG at P2 or P3, there is a slight degradation in the beam quality at low pump power levels, and the degradation of beam quality is relatively more significant because the P2 position is inside the resonant cavity. It is worth noting that the SRS suppression by CTFBG can reduce the ratio of Raman light, which helps to optimize the beam quality [33]. Thus, as the pump power increases to approximately 4.5 kW, the output laser beam quality is almost the same in the four different configurations. The inset in Fig. 7(d) shows the measured output laser beam profile at the maximum output power when CTFBG is inserted at P2. The beam profile is good in near-single-mode operation.
The temperature of fs-written FBGs was measured by a thermal camera during power scaling, as shown in Fig. 8. The solid pattern represents the measured data, and the dashed line represents the linearly fitted value. It should be noted that the temperature data in Fig. 8 were measured after fs-written FBGs had been fully self-annealed, indicating that the thermal slopes of fs-written FBGs are stable. Indeed, there is no significant difference in the self-annealing process between fs-written CTFBGs and cavity mirror FBGs. The thermal slopes of both gradually decrease with an increase in the number of fiber laser runs and eventually stabilize [17,21,23]. Moreover, the fs-written FBGs were only air-cooled and not recoated or packaged for accurate temperature measurement. Figure 8(a) presents the variation of CTFBG temperature with the output power at three different insertion positions. It can be seen that the thermal slope of CTFBG varies at different positions, mainly depending on the signal power handled by CTFBG. Since the handling signal power of CTFBG is low at P1, its temperature is only 25.9 °C, which is comparable to room temperature. Similarly, the thermal slope of CTFBG at P2 and P3 varies depending on the handling signal power. Indeed, the thermal slope of 8.5 °C/kW at P2 is relatively high because the signal power in the cavity is greater than the measured output signal power. Accordingly, the thermal slope of 7.1 °C/kW at P3 is more accurate and realistic than that at P2. The measured temperature of HRFBG and OCFBG is also influenced by the handling signal power, as shown in Fig. 8(b). The HRFBG temperature is only 26.8 °C, but the thermal slope of OCFBG is 8.4 °C/kW. Obviously, the CTFBG, HRFBG, and OCFBG written by fs-lasers have good power-temperature characteristics. Furthermore, the fs-written cavity mirror FBGs and CTFBGs also exhibit excellent long-term stability when operating at high power [17,23]. As a result, fs-written FBGs have great potential for applications in high-power fiber oscillators.
Fig. 8. The variation of fs-written FBG temperature with the output power. (a) CTFBG; (b) HRFBG and OCFBG.
Table 1 presents the reported experimental results of SRS suppression in the fiber oscillator using CTFBG. We can see that the CTFBG is primarily used in high-power fiber oscillators based on 20/400 µm DCFs for suppressing SRS. The pump wavelength is either 915 nm or 976 nm, and the pumping scheme is either co-pumping or bi-pumping. Indeed, although the LDs at 915 nm as pump sources have a low cost, the pump absorption coefficient at 915 nm is one-third of that at 976 nm. This implies that a longer YDF is required to achieve sufficient absorption, which is unfavorable for suppressing SRS. Moreover, compared to the bi-pumping scheme, the co-pumping scheme is simpler, but it can lead to a decrease in the SRS threshold. In this work, the fs-written CTFBG is first used in the fiber oscillator to suppress SRS. To better investigate the application of fs-written CTFBG in high-power oscillators, the 976 nm LD pump source and the bi-pumping scheme are employed to improve the SRS threshold, so that the output power of the fiber oscillator is over 3 kW when reaching the SRS threshold. The maximum output power is improved to 3.5 kW by using fs-written CTFBG, which is a record power in fiber oscillators with CTFBG for suppressing SRS.
Table 1. SRS suppression in fiber oscillators by CTFBG
4. Conclusion
We demonstrate an effective and convenient strategy for SRS suppression via CTFBG in high-power fiber oscillators. A fiber oscillator is constructed using fs-written HRFBG and OCFBG, and an fs-written CTFBG is also inserted in the fiber oscillator to suppress SRS at three different positions. When the CTFBG is inserted into the resonant cavity and on the side of the OCFBG, the SRS can be optimally suppressed while maintaining the compactness of the fiber oscillator. The SRS suppression width of CTFBG is larger than 55 nm, which is more than 3 times larger than the 3-dB bandwidth of the measured transmission spectrum of CTFBG. The SRS threshold of the fiber oscillator is increased by more than 500 W, with an SRS suppression ratio of nearly 14 dB. The output power is increased by 150 W to 3.5 kW, and the purity of signal power is also improved. To the best of our knowledge, this is the maximum power in fiber oscillators with SRS suppression using CTFBG. The thermal slope of CTFBG without a cooling package is 8.5 °C/kW, indicating its potential to handle higher power. The insights gained from this work may be of assistance in promoting the application of CTFBG in high-power fiber oscillators. Future work involves inscribing a CTFBG and an OCFBG through the coating in the same passive fiber using fs-lasers, which can further enhance the mechanical strength of the FBGs [36] and improve the compactness of the fiber oscillator system.
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.
References
1. J. Gabzdyl, “Fibre lasers make their mark,” Nat. Photonics 2(1), 21–23 (2008). [CrossRef]
2. C. Jauregui, J. Limpert, and A. Tunnermann, “High-power fibre lasers,” Nat. Photonics 7(11), 861–867 (2013). [CrossRef]
3. K. Shima, S. Ikoma, K. Uchiyama, et al., “5-kW single stage all-fiber Yb-doped single-mode fiber laser for materials processing,” Proc. SPIE 10512, 11 (2018). [CrossRef]
4. D. Nodop, C. Jauregui, F. Jansen, et al., “Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers,” Opt. Lett. 35(17), 2982–2984 (2010). [CrossRef]
5. K. Jiao, H. Shen, Z. Guan, et al., “Suppressing stimulated Raman scattering in kW-level continuous-wave MOPA fiber laser based on long-period fiber gratings,” Opt. Express 28(5), 6048–6063 (2020). [CrossRef]
6. Q. Hu, X. Zhao, X. Tian, et al., “Raman suppression in high-power fiber laser oscillator by long period fiber grating,” Results Phys. 26, 104460 (2021). [CrossRef]
7. Q. Hu, X. Tian, X. Zhao, et al., “Fabrication of cascaded wideband LPFGs by CO2 laser and Raman suppression in a 5 kW one-stage MOPA fiber laser,” Opt. Laser Technol. 150, 107984 (2022). [CrossRef]
8. Q. Hu, X. Zhao, X. Tian, et al., “Raman suppression in 5 kW fiber amplifier using long period fiber grating fabricated by CO2 laser,” Opt. Laser Technol. 145, 107484 (2022). [CrossRef]
9. K. Jiao, J. Shu, H. Shen, et al., “Fabrication of kW-level chirped and tilted fiber Bragg gratings and filtering of stimulated Raman scattering in high-power CW oscillators,” High Power Laser Sci. Eng. 7, e31 (2019). [CrossRef]
10. M. Wang, L. Liu, Z. Wang, et al., “Mitigation of stimulated Raman scattering in kilowatt-level diode-pumped fiber amplifiers with chirped and tilted fiber Bragg gratings,” High Power Laser Sci. Eng. 7, e18 (2019). [CrossRef]
11. M. Wang, Z. Wang, L. Liu, et al., “Effective suppression of stimulated Raman scattering in half 10 kW tandem pumping fiber lasers using chirped and tilted fiber Bragg gratings,” Photonics Res. 7(2), 167–171 (2019). [CrossRef]
12. W. Lin, M. Desjardins-Carriere, B. Sevigny, et al., “Raman suppression within the gain fiber of high-power fiber lasers,” Appl. Opt. 59(31), 9660–9666 (2020). [CrossRef]
13. H. Song, D. Yan, W. Wu, et al., “SRS suppression in multi-kW fiber lasers with a multiplexed CTFBG,” Opt. Express 29(13), 20535–20544 (2021). [CrossRef]
14. W. Lin, M. Desjardins-Carrière, V. L. Iezzi, et al., “Simple design of Yb-doped fiber laser with an output power of 2 kW,” Opt. Laser Technol. 156, 108448 (2022). [CrossRef]
15. X. Zhao, X. Tian, M. Wang, et al., “Design and fabrication of wideband chirped tilted fiber Bragg gratings,” Opt. Laser Technol. 148, 107790 (2022). [CrossRef]
16. H. Li, M. Wang, B. Wu, et al., “Femtosecond laser fabrication of chirped and tilted fiber Bragg gratings for stimulated Raman scattering suppression in kilowatt-level fiber lasers,” Opt. Express 31(8), 13393–13401 (2023). [CrossRef]
17. H. Li, X. Ye, M. Wang, et al., “Robust femtosecond-written chirped and tilted fiber Bragg gratings for Raman filtering in multi-kW fiber lasers,” Opt. Lett. 48(14), 3697–3700 (2023). [CrossRef]
18. J. Thomas, C. Voigtländer, R. G. Becker, et al., “Femtosecond pulse written fiber gratings: a new avenue to integrated fiber technology,” Laser Photonics Rev. 6(6), 709–723 (2012). [CrossRef]
19. S. J. Mihailov, D. Grobnic, C. Hnatovsky, et al., “Extreme Environment Sensing Using Femtosecond Laser-Inscribed Fiber Bragg Gratings,” Sensors 17(12), 2909 (2017). [CrossRef]
20. J. He, B. Xu, X. Xu, et al., “Review of Femtosecond-Laser-Inscribed Fiber Bragg Gratings: Fabrication Technologies and Sensing Applications,” Photonic Sens. 11(2), 203–226 (2021). [CrossRef]
21. R. G. Kramer, F. Moller, C. Matzdorf, et al., “Extremely robust femtosecond written fiber Bragg gratings for an ytterbium-doped fiber oscillator with 5 kW output power,” Opt. Lett. 45(6), 1447–1450 (2020). [CrossRef]
22. S. Pelletier-Ouellet, L. Talbot, A. Mailloux, et al., “Femtosecond inscription of large-area fiber Bragg gratings for high-power cladding pump reflection,” Opt. Lett. 47(19), 4989–4992 (2022). [CrossRef]
23. H. Li, X. Tian, H. Li, et al., “Fiber oscillator of 5 kW using fiber Bragg gratings inscribed by a visible femtosecond laser,” Chin. Opt. Lett. 21(2), 021404 (2023). [CrossRef]
24. H. Li, B. Yang, M. Wang, et al., “Femtosecond laser fabrication of large-core fiber Bragg gratings for high-power fiber oscillators,” APL Photonics 8(4), 046101 (2023). [CrossRef]
25. Y. Ye, B. Yang, X. Wang, et al., “Experimental study of SRS threshold dependence on the bandwidths of fiber Bragg gratings in co-pumped and counter-pumped fiber laser oscillator,” J. Opt. 21(2), 025801 (2019). [CrossRef]
26. L. Yin, M. Yan, Z. Han, et al., “High power cladding light stripper using segmented corrosion method: theoretical and experimental studies,” Opt. Express 25(8), 8760–8776 (2017). [CrossRef]
27. X. Tian, X. Zhao, M. Wang, et al., “Influence of Bragg reflection of chirped tilted fiber Bragg grating on Raman suppression in high-power tandem pumping fiber amplifiers,” Opt. Express 28(13), 19508–19517 (2020). [CrossRef]
28. A. Othonos, K. Kalli, and Fiber Bragg Gratings, Fundamentals and Applications in Telecommunications and Sensing (Artech House, 1999).
29. Y. Wang, “Stimulated Raman scattering in high-power double-clad fiber lasers and power amplifiers,” Opt. Eng. 44(11), 114202 (2005). [CrossRef]
30. Y. Ye, X. Xi, C. Shi, et al., “Experimental Study of 5-kW High-Stability Monolithic Fiber Laser Oscillator With or Without External Feedback,” IEEE Photonics J. 11(4), 1–8 (2019). [CrossRef]
31. Q. Chen, S. Ren, W. Liu, et al., “Raman Oscillation Induced by Weak Internal Reflections of Fiber Devices in High-Power Fiber Amplifiers,” J. Lightwave Technol. 1–6 (2023).
32. W. Liu, P. Ma, C. Shi, et al., “Theoretical analysis of the SRS-induced mode distortion in large-mode area fiber amplifiers,” Opt. Express 26(12), 15793–15803 (2018). [CrossRef]
33. W. Gao, W. Fan, P. Ju, et al., “Effective suppression of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers,” High Power Laser Sci. Eng. 9, e20 (2021). [CrossRef]
34. X. Zhao, X. Tian, M. Wang, et al., “Fabrication of 2 kW-Level Chirped and Tilted Fiber Bragg Gratings and Mitigating Stimulated Raman Scattering in Long-Distance Delivery of High-Power Fiber Laser,” Photonics 8(9), 369 (2021). [CrossRef]
35. X. Zhao, X. Tian, Q. Hu, et al., “Raman suppression in a high-power single-mode fiber oscillator using a chirped and tilted fiber Bragg grating,” Laser Phys. Lett. 18(3), 035103 (2021). [CrossRef]
36. M. Bernier, F. Trepanier, J. Carrier, et al., “High mechanical strength fiber Bragg gratings made with infrared femtosecond pulses and a phase mask,” Opt. Lett. 39(12), 3646–3649 (2014). [CrossRef]
