Open Access
Add to BibSonomy
Abstract
Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) is a newly developed configuration with many advantages like compactness and good reliability. In this work, a B-OAIFL with a low time-stabilized threshold was constructed by employing a pair of side pump/signal combiner in the oscillating section, which demonstrates smooth temporal characteristics with no pulse detected by the photodetector at the output power level of only a few of tens Watts. We investigated the effect of side pumping on the Raman Stokes light and verified its contribution to mitigating the temporal-chaos-induced stimulated Raman scattering (SRS). The phenomenon of co-SRS caused by the mutual excitation of backward Stokes light from two amplifying sections under bidirectional pumping was first reported and studied. A pair of chirped and tilted fiber Bragg gratings (CTFBGs) were applied between the oscillating and amplifying sections to suppress the co-SRS, and the effect of the number of CTFBGs on the suppression of co-SRS was studied in detail experimentally. Finally, we successfully suppressed the co-SRS, and achieved a 3kW × 2 ports laser output, with a near-single-mode beam quality of ${M_A}^2\sim 1.3,{M_B}^2\sim 1.4$. In contrast, without the use of CTFBG, only a 2 kW-level output was obtained from each port, limited by co-SRS (with an SRS suppression ratio of less than 15 dB). The maximum output power of end A and end B is 3133 W and 3213 W, with the SRS suppression ratio of about 27.6 dB and 28.1 dB, respectively. No TMI features were observed under bidirectional pumping. The results demonstrate a significant potential for further power scaling based on this configuration. To the best of our knowledge, it is the highest output power achieved based on the B-OAIFL.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High power fiber lasers, as their advantages of good beam quality and reliable stability, have been broadly favored in the industry. Nonlinear effects, pump brightness, and thermal lensing effects have chronically been regarded as the main limiting factors of power scaling for fiber lasers [1–3]. Prior to the discovery of transverse mode instability (TMI), stimulated Raman Scattering (SRS) was once regarded as the chief physical limitation factor to the power scaling of high power wide-linewidth fiber lasers with the lowest threshold [1–3]. Extensive and in-depth researches on the suppression of SRS were considerably conducted, and numerous methods have been proposed: the enlargement of the effective mode area of the fiber [4,5], the optimization of the pump structure [6,7], the reduction of the power overlap factor of Raman Stokes light and signal light [8–10], the increase of the Raman Stokes light loss [11,12], and the removal of the Raman Stokes light from the signal [13,14], etc. Moreover, it has been discovered that the SRS is closely associated with the temporal characteristics of lasers, and research on this topic has garnered increasing attention in recent years. In 2014, T. Schreiber et al. experimentally reported that a wider bandwidth of the output coupler fiber Bragg grating contributes to the suppression of SRS in kW fiber oscillators [15]. In 2016, Liu et al. conducted theoretical research on the phenomenon of temporal-chaos-induced SRS [16]. Additionally, it has been reported that by optimizing the spectral shape of the FBG, employing temporally stable phase-modulated seed lasers, or introducing passive fiber, the SRS could be suppressed effectively due to the improved temporal features in lasers [17–20].Since the TMI was first proposed in 2010 [21,22], it took a short time until TMI revealed itself as one of the strongest limitations for the average power scaling of the near-single-mode fiber laser [22–24]. Especially in the experiment with the large mode area (LMA) fiber laser, the threshold of TMI performs even lower than that of SRS, representing as the strongest limitation factor [25,26]. Masses of attention dedicated from not only science community but also industry led to a significant effort being devoted worldwide to further understand and mitigate the TMI, both theoretically and experimentally. Over the years, a multitude of methods to mitigate TMI have been proposed: improving the seed power [27], optimizing the pumping wavelength [28], optimizing the pumping configuration [20], bending to suppress high-order modes [29], reducing the photodarkening [30], and designing novel fibers [31,32], etc. Moreover, the phenomenon of SRS-induced TMI was found in 2018 [33], indicating the profound connection between SRS and TMI.
In 2018, Hejaz et al. proposed a novel structure that came to be named as the oscillating-amplifying integrated fiber laser (OAIFL), and successfully boosted the TMI threshold by approximately 26% [34]. It is a configuration that directly fusion spliced the oscillating section and the amplifying section, which requires neither an isolator nor a cladding light stripper (CLS) between them. Compared with the conventional fiber laser amplifier based on the master oscillation power amplification (MOPA), the simpler structure shortens the length of the passive fiber as well as the active fiber, and relatively contributes to the suppression of SRS [35]. Additionally, the pumping share between the two sections, as well as the absence of the reflection loss at the output coupler fiber Bragg grating (OC-FBG), allow a fuller utilization of pumping sources and lower loss of the signal, hence it performs a higher efficiency compared to the customary fiber laser oscillator. Based on this configuration, Zeng et al. achieve a record power of 6 kW in 2023 [36], with the only 14.7 dB signal-SRS suppression. In 2022, Zhong et al. proposed a bidirectional linear cavity all-fiber laser [37], shotting the outstanding features of lower cost, volume, and weight. It replaces the high-reflectivity fiber Bragg grating (HR-FBG) with an OC-FBG, by utilizing the two OC-FBGs, the bidirectional output can be achieved. However, both OC-FBGs led to fewer round-trip times of laser in the cavity, eventually resulting in low utilization of pumping. They achieved a 2 × 2 kW laser output with a beam quality ${M^2}$ factor of about 1.5, and no further reports regarding higher output power were published. In 2022, we proposed a new structure called bidirectional output oscillating-amplifying integrated fiber laser(B-OAIFL) [38], with the goal of combining the advantages of the abovementioned fiber laser structures. Apart from the inheritance of merits of the OAIFL and bidirectional linear-cavity laser oscillator, the standout features of B-OAIFL lies in the achievement of bidirectional laser output based on one oscillator, and little reciprocal effect of pump light between the two ends contributes to realizing a simple adjustment of bidirectional output power by regulating the pump power. Besides, sufficient absorption of pump light by the amplifying sections allows for a minimized length of oscillating section as much as possible under the consideration of the amplified spontaneous emission (ASE), which diminishes the requirement for the fabrication of FBG. We finally reported a bidirectional laser output of 2 × 2 kW with a beam quality ${M^2}$ factor of ${M_A}^2\sim 1.30,{M_B}^2\sim 1.43$. However, further scaling of the output power is limited by TMI, and SRS occurs on both ends, which is believed to be caused not by the overlong fiber length or the small core diameter, but by the temporal chaos of the oscillating section.
In this manuscript, we report our continuous striving for improving the TMI threshold and mitigating the intracavity temporal chaos through the optimization of pumping wavelength and pumping configuration. By utilizing the configuration of side pumping, we have reduced the time-stabilized threshold of B-OAIFL, which is judged by pulse-free temporal properties detected by the photodetector, from a level of 1 kW to less than 100 W. We first report and study the unconventional SRS phenomenon (which we call co-SRS) in the B-OAIFL, characterized by the rapid surge in Stokes light intensity under bidirectional pumping. Based on the CTFBG, we successfully suppressed the co-SRS and finally realized a level of 2 ports × 3-kW B-OAIFL. A total output power of 6346 W (3133 W of end A, 3213 W of end B) was achieved, with a near-single-mode beam quality of ${M_A}^2\sim 1.3,{M_B}^2\sim 1.4$.
2. Experimental setup
A monolithic B-OAIFL consists of an oscillating section (OS) and two amplifying sections (AS-A and AS-B), and all the listed components are all-fiberized spliced in sequence, as is sketched in Fig. 1. A piece of 8-meter-long double cladding ytterbium-doped fiber (DCYDF) (DCYDFO) and a pair of FBGs (OCFBG-A, OCFBG-B) with the center wavelength of about 1080 nm form the OS. The DCYDFO was fixed on a water-cooled plate and coiled symmetrically with a small coiling diameter ranging from 8.5 cm to 9.5 cm for better mode control, held a core/cladding diameter of 20/400 µm with the numerical aperture (NA) of 0.065/0.46 and a nominal cladding pump absorption coefficient of 0.44 dB/m at 915 nm. The 3-dB bandwidth and reflectivity of OCFBG-A are 1.91 nm and 9.27%, respectively, while those of OCFBG-B are 1.98 nm and 9.99%. As for the amplifying sections, they were mainly constructed from a piece of 19-meter-long DCYDF (DCYDFA, DCYDFB), with a 22 µm/0.065 NA core and 400 µm/0.46 NA inner cladding and a nominal cladding pump absorption coefficient of 0.54 dB/m at 915 nm. The (2 + 1) × 1 side pump/signal combiner (SPSC) was fusion-spliced with the OCFBG to directly apply to the cavity, and the (18 + 1) × 1 backward pump/signal combiner (BPSC) was employed for the further amplification of the laser. A group of wavelength-stabilized 976 nm LDs were coupled into the cavity through SPSC, ensuring sufficient power of the signal in the cavity to improve the temporal stability. 18 non-wavelength-stabilized 976 nm LDs were coupled into the AS through BPSC to boost the TMI threshold of the laser [39].
Fig. 1. Original experimental setup of the B-OAIFL. (PM: power meter, OSA: optical spectrum analyzer, PD: photodetector, AS-A: amplifying section A, AS-B: amplifying section B, BQM: beam quality meter, BPF: bandpass filter, HRFM: high reflectivity flat mirror)
The laser signal generated by the OS was further scaled as it transmitted through the AS-A and AS-B. A piece of 3-meter-long passive fiber with about 15 cm-long cladding light stripper (CLS) was fusion-spliced with the BPSC to fully dump the residual pumping as well as signal light leaked into the cladding. The laser first passes through a piece of passive fiber integrated with CLS and then through commercial-made endcap that is utilized to eliminate facet reflection, and is then guided into the measuring system, where the characteristics of output power, spectrum, temporal properties, and beam quality were analyzed. Specifically, the majority of output signal light is reflected by a HRFM to the PM for power measurement. And the remaining small portion of transmitted light enters the BQM for beam quality analysis. The temporal characteristic is measured by employing a PD with a bandwidth of 12 MHz. It receives the signal light reflected from the end face of PM, and converts it into an electrical signal, which is then amplified by the amplification circuit, and analyzed and recorded by the oscilloscope.
3. Experimental results and discussion
3.1 Laser performance with unidirectional pumping
Benefiting from the symmetric structure of the B-OAIFL, the characteristics of output laser under unidirectional pumping can be researched by representing with one end. In this section, using end B as a reference, the laser performance with B-pumping was studied. To stabilize the temporal characteristic, a side pump of 525 W was initially introduced, followed by the subsequent addition of backward pumping from end B. The dependence of the recorded output power and the optical-to-optical (O-O) efficiency on the pump power are shown in Fig. 2(a). The different background colors in the figure distinguish the side and counter pumping configurations. A notable point is that the output power of end A initially increased linearly but then decreased after the pump power at end B reached 3245W. This decline could be attributed to the increased pump absorption by the AS-B and oscillating section, as well as the signal reabsorption in AS-A. As the operating current increases, the center wavelength of LDs gradually shifts towards 976 nm, accompanying an increase in the pump absorption coefficient. The increased absorption of AS-B and oscillating section resulted in a reduced amount of pump light could be transmitted to AS-A. As a result, the signal light of AS-A was less amplified, causing a decrease in the output power of end A. In addition, the reabsorption of signal light by the active fiber in AS-A might also contribute to this drop. As the pumping light reaching AS-A decreased, a portion of the signal light was absorbed by the DCYDF-A, resulting in the output reduction of end A. In previous studies, it was reported that the time-stabilized threshold of B-OAIFL was up to 1 kW, accompanying pulses with unfixed repetition rates [38]. It was regarded as a handicap of the B-OAIFL. Therefore, an optimization of the configuration of side pumping was implemented in this study. A side pumping with a power of 219 W was first added preliminarily to stabilize the cavity in the temporal domain, with the output power of 66 W of end A and 57 W of end B. At this point, a smooth temporal signal of PD and its corresponding fast-Fourier transform (FFT) were visualized on the oscilloscope, indicating a relatively stable state of the laser, as is shown in Fig. 2(b). With the increase of the pump power, there is a simultaneous improvement in both output power and efficiency. Limited by obvious TMI shown in Fig. 2(c), the maximum output power of end B reaches 3275 W, with 177 W of end A and a total O-O efficiency of 81.6%. Figure 2(d) depicts a slight Raman Stokes light of about 34.4 dB lower than the signal. Likewise, under the condition of A-Pumping, an output power of 3220 W of end A and 273 W of end B is obtained, with no signs of Stokes light, but the characteristics of TMI were observed at end A. However, at this time, TMI, not SRS, plays the role as the main limitation for power scaling.
Fig. 2. Laser performance with unidirectional pumping (B-Pumping). (a) variation of output power and O-O efficiency with pump power. (b) temporal characteristic and corresponding FFT at 57 W of end B. (a relatively stable state of the cavity) (c) temporal characteristic and corresponding FFT at 3275 W of end B. (d) laser spectra at different output power.
A more detailed investigation was conducted on the effect of side pumping on the laser characteristics of the cavity. Directly adding the counter pumping without side pumping, the intensity of Raman Stokes light is approximately 23.1 dB lower than the signal at an output power of 2979 W, which is 11.3 dB stronger at the same power level than that in the aforementioned condition where the side pumping was first added, as is depicted in Fig. 3. Afterward, side pumping was gradually added, leading to a corresponding increase in output power. However, it is noteworthy that at this point, the intensity of SRS diminished piece by piece. As the output power of end B reaches 3135 W, only a faint intensity of Raman Stokes light could be observed, with an intensity of about 47.3 dB below the signal. Compared with the case without side pumping initially, the intensity of SRS decreased by 24.2 dB, indicating a remarkable impact of side-pumping-employed on the suppression of SRS under the condition of unidirectional pumping. In the structure of B-OAIFL, the simultaneous counter-pumping and side-pumping configurations of the single-end oscillating and amplifying sections can be considered as a bidirectional pumping setup. According to the published reports, classical SRS was believed to be more effectively suppressed under the counter-pumping configuration compared to bidirectional pumping, illustrating a contradictory conclusion with our experiment. It is believed that SRS observed here is primarily induced by the temporal chaos within the cavity. The introduction of side pumping increases the power inside the cavity, contributing to a smoother temporal property. Consequently, a weaker SRS can be attained.
3.2 Phenomenon of co-SRS with bidirectional pumping
According to the results in section 3.1, considering that there is no mutual influence between AS-A and AS-B, a dual-end output of 3 kW can be predictively achieved based on the current configuration. Similarly, a side pumping with a power of 525 W was supplied to stabilize the temporal characteristic inside the cavity before adding the counter pumping. The output power and O-O efficiency improved as anticipated with the increase of pumping power, as is shown in Fig. 4(a). However, surprisingly and unexpectedly, a weak Raman Stokes light was observed at around 1900 W of each port, and then sharply intensified to only 12.6 dB and 11.7 dB lower than the signal at an output power of 2203 W of end A and 2234 W of end B, respectively, as is illustrated in Fig. 4(b) and (c).
Fig. 4. Results with bidirectional pumping (side pumping of 525 W was first added). (a) variation of output power and O-O efficiency with pump power. (b) laser spectra of end A at different output power. (c) laser spectra of end B at different output power.
In order to further study the generation mechanism behind the sudden appearance of SRS, the pumping sequence was altered. Initially, counter pumping was applied, considering it as a potential consequence caused by the temporal chaos within the cavity. As anticipated, a rapid boost in the intensity of SRS was observed, with 17.8 dB and 15.3 dB below the signal at the output power of 2242 W of end A and 2293 W of end B, respectively, after the initial appearance of Stokes light at a similar power level of about 1900 W at each port. Side pumping was then introduced, and the output power increased accordingly. However, the intensity of SRS showed no signs of weakening, instead, it continued to escalate. Specifically, as the output power at end A increased from 2242W to 2266W with the increase of side-pumping power, the corresponding intensity of SRS also increased from 17.8 dB below the signal to 13.3 dB below it, and a similar trend was also observed at end B, as were revealed in Fig. 5(a) and (b).
Fig. 5. Results with bidirectional pumping (counter pumping was first added). (a) laser spectra of end A at different output power. (b) laser spectra of end B at different output power.
In the above section, it is demonstrated that the effective suppression, and even elimination, of temporal-chaos-induced SRS can be achieved through the introduction of side pumping. However, in this section, we have verified that under bidirectional pumping, the rapid growth of SRS cannot be mitigated by this method, indicating that SRS in this particular case is not predominantly determined by the temporal characteristic of the cavity. According to the publicized studies, it has been reported that the external feedback provided by one end of HR-FBG can lead to a stronger SRS within the cavity in the structure of the fiber oscillator [40]. Similar phenomena have also been observed in MOPA-based fiber laser amplifiers [41]. Enlightened by these findings, it is reasonable to think that the abrupt increase in Stokes light observed in our experiment may be attributed to the backward Raman Stokes light from one end acting as external feedback for the other end. The mutual feedback between the Stokes light of each end finally leads to the rapid growth of SRS. We refer to this phenomenon as “co-SRS”.
3.3 Co-SRS suppression based on CTFBG
As the generation of co-SRS relied on the mutual positive feedback of backward-transmitted Stokes light between AS-A and AS-B, the measures for mitigation should be aimed at interrupting this feedback loop. Among the various existing strategies for SRS suppression, both long-period gratings and CTFBGs have been proven effective in filtering out Stokes light from the signal [13,14]. Considering the specific circumstances of our study, we chose to utilize CTFBGs for this purpose. Referring to published reports, CTFBG is commonly employed between the seed and the amplification stages in MOPA-based fiber laser amplifiers or at the output end of lasers. Considering the generation mechanism of co-SRS, we placed the CTFBG before the amplifying section to prevent the transmission of back-propagated Stokes light between the two amplifying sections. In accordance with fiber-matching principles, it was determined to insert the CTFBG between the SPSC and the OC-FBG, as depicted in Fig. 6.
Fig. 6. Latest experimental setup of the B-OAIFL (CTFBG is introduced) (PM: power meter, OSA: optical spectrum analyzer, PD: photodetector).
Maintaining the above settings, a pair of commercial CTFBGs inscribed in fiber LMA-GDF20/400-M with a central depth deeper than -20 dB at 1132 nm were employed. These CTFBGs were strategically inserted between the OCFBG and the side pumping of each end to effectively filter out the Stokes light generated from the cavity, as well as the backward Stokes light from both ends. The co-SRS, as anticipated, was effectively suppressed. When the output power of end A and end B reached 3017 W and 3093 W, the SRS was approximately 27 dB and 28 dB lower than the signal, respectively, as can be seen from Fig. 7(b) and (c). According to the results in Section 3.1, the side pumping was further increased to lower the SRS. However, the intensity of SRS rose unexpectedly. When the output power only slightly increased to 3062W and 3145W at end A and end B, respectively, the intensity of SRS surged to only about 21 dB and 20.6 dB lower than the signal, respectively, indicating that the SRS at this point was not induced by unstable temporal properties demonstrated in the previous experiment, but rather the classical SRS influenced by the power level. It was verified that counter pumping offered superior suppression of classical SRS compared to bidirectional pumping. Consequently, we reverted the side pumping to its original power point of 525 W and continued to increase the counter pumping. As is depicted in Fig. 7(a), with a total pumping of 8563 W (525 W of side pumping and 8038 W of counter pumping), a bidirectional laser output of end A of 3133 W and end B of 3213 W was achieved, with a total output power of 6346 W and a total O-O efficiency of 74.1%. At the maximum output power, the intensity of SRS was approximately 27.6 dB and 28.1 dB lower than the signal, respectively. It is worth noting that the Raman Stokes light observed in this case is primarily resided around 1120 nm, as the light centered around 1135 nm which serves as the center wavelength for SRS of the signal, has been effectively filtered out by the CTFBGs. However, while the Raman scattering suppression (RSS) wavelength of CTFBG falls within the range of 1132 nm ± 10 nm, the Raman Stokes light near 1120 nm has not been filtered out thoroughly. No TMI feature appears in the output signal of the PD and its corresponding FFT, as shown in Fig. 7(d). Limited by the temperature of the BPSC that up to 70 °C, further power scaling was not pursued. The beam quality ${M^2}$ factor of the laser at the highest output power is ${M_{Ax}}^2 = 1.35,{M_{Ay}}^2 = 1.31$ of end A and ${M_{Bx}}^2 = 1.45,{M_{By}}^2 = 1.38$ of end B, as are shown in Fig. 7(e) and (f). Besides, with unidirectional pumping based on this configuration (represented by B-end pumping), a maximum output power of 147W at end A and 3112W at end B was obtained, with a higher O-O efficiency of 77.06%. The slight drop of efficiency demonstrated in bidirectional pumping could be basically considered from the perspective of signal loss. With higher power of the signal involved under bidirectional pumping, it would certainly result in increased losses during the transmission process.
Fig. 7. Results with bidirectional pumping (with two CTFBGs). (a) variation of output power and O-O efficiency with pump power. (b) laser spectra of end A at different output power. (c) laser spectra of end B at different output power. (d) temporal characteristics and corresponding FFT at the peak power. (e) beam quality measurement (${M_{Ax}}^2 = 1.35,{M_{Ay}}^2 = 1.31$) for 3133 W of end A. (f) beam quality measurement (${M_{Bx}}^2 = 1.45,{M_{By}}^2 = 1.38$) for 3213 W of end B.
Furthermore, a more detailed study on the effect of the number of CTFBGs on the suppression of co-SRS was conducted. A consistent bidirectional pumping method was executed. As shown in Fig. 8(a) and (b), when no CTFBG was introduced, co-SRS was observed at both ends at the output level of 2203W at end A and 2234W at end B. The Raman Stokes light rapidly ascended from a weak intensity to a level of about 15 dB below the signal, with a center wavelength of around 1135 nm, significantly limiting the power scaling of B-OAIFL. With the incorporation of a single CTFBG (placed at end A), co-SRS was effectively suppressed. At the output power of 2771W at end A and 2949W at end B, the SRS suppression ratios were 18.2 dB and 17.4 dB, respectively. And it should be noted that with the implementation of CTFBG, the original Stokes light centered at about 1135 nm was effectively filtered out, while the filtering effect near 1120 nm for Stokes light is not significant due to being at the edge of the CTFBG’s suppression range. For the Stokes light in this wavelength range, implementing two CTFBGs exhibited a more pronounced effect on the suppression of co-SRS compared to using only one. Figure 8(b) depicts the output spectra of end B with different numbers of CTFBGs. With a single CTFBG, the SRS suppression ratio was 17.4 dB at an output power of 2949W. However, with two CTFBGs, the SRS suppression ratio reached 26.6 dB at a similar power level of 3093W, demonstrating a 9.2 dB improvement compared to the case with a single CTFBG. With the employment of two CTFBGs, a total maximum output power of 6346 W (3133W at end A and 3213W at end B) was achieved, with the SRS intensity approximately 27.6 dB and 28.1 dB lower than the signal, respectively, still indicating a room for power scaling.
Fig. 8. Comparison of output spectra with different numbers of CTFBG under the consistent bidirectional pumping. (a) output spectra at end A. (b) output spectra at end B.
3.4 Discussion
The main experimental results are summarized in Table 1. It is evident that under unidirectional pumping, the main limitations of power scaling were attributed to the TMI, with minimal signs of SRS were observed. However, in the experiments with bidirectional pumping, the co-SRS caused by the mutual excitation of backward Stokes light from two amplifying sections significantly impeded the power scaling. Thus, the utilization of CTFBGs was indispensable. In terms of TMI, the achievement of a dual-end output of up to 3 kW in the experiment utilizing two CTFBGs, no characteristics of TMI were observed, indicating a potential for further power scaling.
Table 1. Main results in this manuscript
From the results of this manuscript, it can be seen that the SPSC and CTFBG play an important role in the power scaling of the B-OAIFL. For SPSC, on the one hand, it directly provides pump light for the OS, which can enhance the intracavity laser power, as well as lower the time-stabilized threshold, and suppress the temporal-chaos-induced SRS. This conclusion has been verified in section 3.2. On the other hand, for the structure of B-OAIFL, the OS and the AS at one end actually form a complete OAIFL, and the counter pumping and the side pumping of the other end form a bidirectional pumping configuration which has a higher TMI threshold than unidirectional pumping, thus improving the power scaling ability of the laser. For CTFBG, its core function is to suppress co-SRS. Due to the unique structure of lasers, it is inevitable to introduce SRS filtering devices to prevent the generation of co-SRS to achieve higher power. The B-OAIFL with the structure shown in Fig. 6 has great potential, and further power scaling needs to consider both TMI suppression, available pump power, and SRS suppression. Measures taken can include optimizing the pump wavelength, increasing side pumping power, and CTFBGs with wider suppressing bandwidth. Subsequent experiments can be conducted from these aspects for achieving higher power B-OAIFL with high beam quality.
4. Conclusions
In conclusion, we constructed a B-OAIFL with a low time-stabilized threshold by introducing the configuration of side pumping. The time-stabilized threshold was significantly reduced from a level of 1 kW to less than 100 W. The effect of side pumping on the Raman Stokes light was studied and its effectiveness in mitigating the temporal-chaos-induced SRS through providing additional power to stabilize the cavity was verified. During the bidirectional pumping, we found a phenomenon known as co-SRS, which resulted in a sharp boosting of the SRS. This effect was a result of the mutual excitation of backward Stokes light from two amplifying sections. By utilizing CTFBGs, we successfully suppressed the co-SRS and realized a recorded bidirectional output power of 6346 W in total (3133 W of end A, 3213 W of end B), with a near-single-mode beam quality of ${M_A}^2\sim 1.3,{M_B}^2\sim 1.4$, a total O-O efficiency of 74.1% and an SRS intensity of about 27.6 dB and 28.1 dB lower than the signal, respectively. A scope for power scaling was revealed with no signs of TMI observed. And the effect of the number of CTFBGs on the suppression of co-SRS was researched, further power scaling can be achieved by broadening the bandwidth of CTFBG to suppress the Stokes light around 1120 nm.
Funding
Training Program for Excellent Young Innovations of Changsha (kq2206006); Funds for Distinguished Youth of Hunan Provence (2023JJ10057); Basic Scientific Research Program (JCKY2021525B015).
Acknowledgments
Thanks to Xiaoyong Xu, Siliu Liu, Junyu Chai, Yun Ye, Jinming Wu, Xiangming Meng, Fengchang Li, Xinyi Ding, and Xiangyong Xiao for help with this work.
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. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. Barty, “Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power,” Opt. Express 16(17), 13240–13266 (2008). [CrossRef]
2. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives [Invited],” J. Opt. Soc. Am. B 27(11), B63 (2010). [CrossRef]
3. J. Zhu, P. Zhou, Y. Ma, X. Xu, and Z. Liu, “Power scaling analysis of tandem-pumped Yb-doped fiber lasers and amplifiers,” Opt. Express 19(19), 18645–18654 (2011). [CrossRef]
4. E. A. Shcherbakov, V. V. Fomin, A. A. Abramov, A. A. Ferin, D. V. Mochalov, and V. P. Gapontsev, “Industrial grade 100 kW power CW fiber laser,” in Advanced Solid-State Lasers Congress, (2013).
5. I. Hartl, A. L. Carter, D. Tanaka, K. Shima, M. Kashiwagi, Y. Takubo, K. Uchiyama, and S. Ikoma, “5-kW single stage all-fiber Yb-doped single-mode fiber laser for materials processing,” in Fiber Lasers XV: Technology and Systems, (2018).
6. Y. Wang, “Stimulated Raman scattering in high-power double-clad fiber lasers and power amplifiers,” Opt. Eng 44(11), 114202 (2005). [CrossRef]
7. C. A. Robin, I. Hartl, S. Ikoma, H. K. Nguyen, M. Kashiwagi, K. Uchiyama, K. Shima, and D. Tanaka, “3 kW single stage all-fiber Yb-doped single-mode fiber laser for highly reflective and highly thermal conductive materials processing,” in Fiber Lasers XIV: Technology and Systems, (2017).
8. L. Zenteno, J. Wang, D. Walton, B. Ruffin, M. Li, S. Gray, A. Crowley, and X. Chen, “Suppression of Raman gain in single-transverse-mode dual-hole-assisted fiber,” Opt. Express 13(22), 8921–8926 (2005). [CrossRef]
9. P. D. J. K. Sahu, J. Kim, C. Codemard, J. Nilsson, and D. N. Payne, “Suppression of stimulated Raman scattering in a high-peak-power pulsed 1060 nm fiber MOPA source with purely single-mode output using W-type fiber,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, (2006).
10. J. M. Fini, M. D. Mermelstein, M. F. Yan, R. T. Bise, A. D. Yablon, P. W. Wisk, and M. J. Andrejco, “Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier,” Opt. Lett. 31(17), 2550–2552 (2006). [CrossRef]
11. X. Ma, I. N. Hu, and A. Galvanauskas, “Propagation-length independent SRS threshold in chirally-coupled-core fibers,” Opt. Express 19(23), 22575–22581 (2011). [CrossRef]
12. I. Hartl, A. L. Carter, S. Nolte, A. Tünnermann, T. Schreiber, A. Liem, V. Bock, C. Matzdorf, T. A. Goebel, D. Richter, R. G. Krämer, and M. Heck, “Mitigation of stimulated Raman scattering in high power fiber lasers using transmission gratings,” in Fiber Lasers XV: Technology and Systems, (2018).
13. D. Nodop, C. Jauregui, F. Jansen, J. Limpert, and A. Tunnermann, “Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers,” Opt. Lett. 35(17), 2982–2984 (2010). [CrossRef]
14. M. Wang, Y. Zhang, Z. Wang, J. Sun, J. Cao, J. Leng, X. Gu, and X. Xu, “Fabrication of chirped and tilted fiber Bragg gratings and suppression of stimulated Raman scattering in fiber amplifiers,” Opt. Express 25(2), 1529–1534 (2017). [CrossRef]
15. T. Schreiber, A. Liem, E. Freier, C. Matzdorf, R. Eberhardt, C. Jauregui, J. Limpert, and A. Tünnermann, Analysis of stimulated Raman scattering in cw kW fiber oscillators, SPIE LASE (SPIE, 2014), Vol. 8961.
16. W. Liu, P. Ma, H. Lv, J. Xu, P. Zhou, and Z. Jiang, “General analysis of SRS-limited high-power fiber lasers and design strategy,” Opt. Express 24(23), 26715–26721 (2016). [CrossRef]
17. H. Xu, M. Jiang, C. Shi, P. Zhou, G. Zhao, and X. Gu, “Spectral shaping for suppressing stimulated-Raman-scattering in a fiber laser,” Appl. Opt. 56(12), 3538–3542 (2017). [CrossRef]
18. V. Bock, A. Liem, T. Schreiber, R. Eberhardt, and A. Tünnermann, Explanation of stimulated Raman scattering in high power fiber systems, SPIE LASE (SPIE, 2018), Vol. 10512.
19. T. Li, W. Ke, Y. Ma, Y. Sun, and Q. Gao, “Suppression of stimulated Raman scattering in a high-power fiber amplifier by inserting long transmission fibers in a seed laser,” J. Opt. Soc. Am. B 36(6), 1457 (2019). [CrossRef]
20. R. Tao, P. Ma, X. Wang, P. Zhou, and Z. Liu, “Theoretical study of pump power distribution on modal instabilities in high power fiber amplifiers,” Laser Phys. Lett. 14(2), 025002 (2017). [CrossRef]
21. W. Christian, S. Thomas, R. Miroslaw, T. Igor, P. Thomas, E. Ramona, and T. Andreas, “High-power linear-polarized narrow linewidth photonic crystal fiber amplifier,” Proc. SPIE 7580, 75801H (2017). [CrossRef]
22. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H.-J. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, “Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers,” Opt. Express 19(14), 13218–13224 (2011). [CrossRef]
23. J. P. L. J. R. Marciante, “The impact of thermal mode instability on core diameter scaling in high-power fiber amplifiers,” in 2016 Conference on Lasers and Electro-Optics (CLEO), (2016).
24. K. R. Hansen, T. T. Alkeskjold, J. Broeng, and J. Laegsgaard, “Thermally induced mode coupling in rare-earth doped fiber amplifiers,” Opt. Lett. 37(12), 2382–2384 (2012). [CrossRef]
25. X. Wang, R. Tao, H. Xiao, P. Zhou, C. Zhang, and X. Xu, “Experimental studies of mode instability and thermal induced effects in all-fiber amplifier,” in Advanced Solid-State Lasers Congress, OSA Technical Digest (online) (Optica Publishing Group, 2013), JTh2A.44.
26. Y. Ran, R. Tao, P. Ma, X. Wang, R. Su, P. Zhou, and L. Si, “560 W all fiber and polarization-maintaining amplifier with narrow linewidth and near-diffraction-limited beam quality,” Appl. Opt. 54(24), 7258–7263 (2015). [CrossRef]
27. R. Tao, P. Zhou, H. Xiao, X. Wang, L. Si, and Z. Liu, “Progress of Study on Mode Instability in High Power Fiber Amplifiers,” Laser & Optoelectronics Progress 51, (2014).
28. R. Tao, P. Ma, X. Wang, P. Zhou, and Z. Liu, “Mitigating of modal instabilities in linearly-polarized fiber amplifiers by shifting pump wavelength,” J. Opt. 17(4), 045504 (2015). [CrossRef]
29. K. Hejaz, A. Norouzey, R. Poozesh, A. Heidariazar, A. Roohforouz, R. Rezaei Nasirabad, N. Tabatabaei Jafari, A. Hamedani Golshan, A. Babazadeh, and M. Lafouti, “Controlling mode instability in a 500 W ytterbium-doped fiber laser,” Laser Phys. 24(2), 025102 (2014). [CrossRef]
30. H.-J. Otto, N. Modsching, C. Jauregui, J. Limpert, and A. Tünnermann, “Impact of photodarkening on the mode instability threshold,” Opt. Express 23(12), 15265–15277 (2015). [CrossRef]
31. F. Beier, C. Hupel, S. Kuhn, S. Hein, J. Nold, F. Proske, B. Sattler, A. Liem, C. Jauregui, J. Limpert, N. Haarlammert, T. Schreiber, R. Eberhardt, and A. Tünnermann, “Single mode 4.3 kW output power from a diode-pumped Yb-doped fiber amplifier,” Opt. Express 25(13), 14892–14899 (2017). [CrossRef]
32. C. Jauregui, H. J. Otto, S. Breitkopf, J. Limpert, and A. Tunnermann, “Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities,” Opt. Express 24(8), 7879–7892 (2016). [CrossRef]
33. X. Wang, R. Tao, Y. Baolai, C. Shi, H. Zhang, P. Zhou, and X. Xu, “Relationship Between Transverse Mode Instability and Stimulated Raman Scattering in Ytterbium Doped All-Fiber Laser Oscillator,” Chin. J. Laser 45(8), 0801008 (2018). [CrossRef]
34. K. Hejaz, M. Shayganmanesh, A. Roohforouz, R. Rezaei-Nasirabad, A. Abedinajafi, S. Azizi, and V. Vatani, “Transverse mode instability threshold enhancement in Yb-doped fiber lasers by cavity modification,” Appl. Opt. 57(21), 5992–5997 (2018). [CrossRef]
35. L. Zeng, X. Wang, B. Yang, H. Zhang, and X. Xu, “A 3.5-kW near-single-mode oscillating–amplifying integrated fiber laser,” High Power Laser Sci. Eng. 9, e41 (2021). [CrossRef]
36. L. Zeng, H. Yang, X. Xi, Y. Ye, L. Huang, B. Yang, H. Zhang, Z. Yan, X. Wang, Z. Pan, Z. Wang, and X. Xu, “Optimization and demonstration of 6 kW oscillating-amplifying integrated fiber laser employing spindle-shaped fiber to suppress SRS and TMI,” Opt. Laser Technol. 159, 108903 (2023). [CrossRef]
37. P. Zhong, L. Wang, B. Yang, H. Zhang, X. Xi, P. Wang, and X. Wang, “2 × 2 kW near-single-mode bidirectional high-power output from a single-cavity monolithic fiber laser,” Opt. Lett. 47(11), 2806–2809 (2022). [CrossRef]
38. J. Liu, L. Zeng, P. Wang, B. Yang, X. Xi, C. Shi, H. Zhang, X. Wang, and F. Xi, “A Novel Bidirectional Output Oscillating-Amplifying Integrated Fiber Laser With 2 Ports × 2 kW Level Near-Single-Mode Output,” IEEE Photonics J. 15(4), 1–8 (2023). [CrossRef]
39. Y. Wan, B. Yang, X. Xi, H. Zhang, P. Wang, X. Wang, and X. Xu, “Comparison and Optimization on Transverse Mode Instability of Fiber Laser Amplifier Pumped by Wavelength-Stabilized and Non-Wavelength-Stabilized 976 nm Laser Diode,” IEEE Photonics J. 14(1), 1–5 (2022). [CrossRef]
40. Y. Ye, X. Xi, C. Shi, H. Zhang, B. Yang, X. Wang, P. Zhou, and X. Xu, “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]
41. W. Gao, W. Fan, P. Ju, G. Li, Y. Zhang, A. He, Q. Gao, and Z. Li, “Effective suppression of mode distortion induced by stimulated Raman scattering in high-power fiber amplifiers,” High Power Laser Sci. Eng. 9, e20 (2021). [CrossRef]
