Open Access
Add to BibSonomy
Abstract
The key remaining technological challenge to the realization of further power scaling for high-power fiber laser systems is overcoming the stimulated Raman scattering (SRS) effect. In past years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to be a simple and effective way to suppress SRS in high-power fiber amplifiers. However, the weak reflection at the Bragg wavelength could be strongly amplified, which not only limits the power and efficiency but also degrades the beam quality. We report here, for the first time to the best of our knowledge, the influence of the residual Bragg reflection of CTFBGs on SRS suppression in high-power fiber laser systems. Two groups of CTFBGs with different Bragg reflection wavelengths are fabricated and used for the comparison experiments. Test results show that the CTFBGs of longer Bragg wavelengths have a better suppression effect, in particular, at a higher power level. By further moving the Bragg wavelength of a CTFBG out of the Raman gain spectral range, a better suppression effect and a promotion in laser efficiency could be achieved, which is very useful for further power scaling.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power fiber lasers have been widely applied to various fields due to their diffraction-limited beam quality, compactness, high efficiency, stability and robustness [1–3]. In the past years, due to the great improvement of laser diodes (LDs) brightness, the output power of direct LD pumped fiber amplifiers has been scaled deep into kW regime, and some 10 kW master oscillator power amplification (MOPA) systems have been reported [4,5]. However, with the further increase of the output power, these systems will increasingly rely on the output power and brightness of LDs. In addition, thermal management is also a challenge. In comparison, tandem pumping scheme, in which the pumping LDs are replaced with in-band fiber lasers of higher brightness and higher injection power, is a more promising choice for such case [6–11]. Up to now, the highest power record of 20 kW for quasi-single-mode fiber laser systems was achieved using tandem pumping scheme by IPG Inc. in 2013 [10]. However, stimulated Raman scattering (SRS) is still a major factor limiting the further power scaling. And the backward Stokes light is more dangerous to the seed and fiber components at higher laser power level. By now, many methods have been used to suppress the SRS in high-power fiber laser systems, among which using lumped spectral filters like long-period fiber gratings (LPFGs) and chirped and tilted fiber Bragg gratings (CTFBGs) has been intensively studied in the past years [11–16]. We firstly reported SRS suppression in fiber amplifier by CTFBGs in 2017 [11], then effective SRS suppression in a 5 kW tandem pumping fiber amplifier was achieved in 2019 [14]. However, further research shows that the residual Bragg reflection of CTFBG has a great influence on SRS suppression, especially at high power, as the weak reflection at the Bragg wavelength could be strongly enhanced by the amplifier, which not only limits the power and efficiency increase, but also degrades the beam quality.
Here, we firstly investigate the influence of the residual Bragg reflection of CTFBGs on SRS suppression in high-power fiber laser systems. Two groups of CTFBGs with different Bragg wavelengths are designed and fabricated, then used in three 5 kilowatt-level tandem pumping fiber amplifiers of different Raman levels for comparison. Experimental results show that the CTFBGs of longer Bragg wavelengths have better SRS suppression effect at higher power level than those of shorter Bragg wavelengths, which will easily excite laser oscillation at the Bragg wavelength at high power, and then decrease the efficiency and degrade the beam quality. With two CTFBGs of longer Bragg wavelengths being involved, no laser oscillation was observed even the amplifier has the highest Raman level among the three comparing systems, and the effective laser output power was improved from ∼4 kW to ∼5 kW with an increasing ratio of 25%. By further improving the spectral properties of the CTFBGs and the power of the 1018 nm pump source, a promotion in laser efficiency and power could be achieved in the future.
2. Principles and grating simulation
The schematic structure of CTFBG is shown in Fig. 1(a). There is not only an angle between the fiber axis and the grating plane, but also a non-uniform in its period along fiber length, which is call chirp. Due to the tilted angle of grating plane, the forward fundamental core mode will couple not only to the backward fundamental core mode just like in a normal FBG, which is called the residual Bragg reflection, but also to various orders of cladding modes.
Fig. 1. (a) Schematic diagram of the structure of CTFBG; (b) calculated spectrum of CTFBG with tilt angle of 6.5°.
If the chirp is linear, then the grating period Λg can be expressed as
where Λ0 is the initial period of the grating, F is the chirp rate and z is the position. Then according to the fiber grating theories, the wavelengths of the residual Bragg reflection and the i-th resonance cladding mode of the CTFBG can be expressed as [17] where ncore and nclad,i are the refractive index of the fundamental core mode and the i-th cladding mode, respectively. And the corresponding bandwidth can be expressed asTherefore, in the transmission spectrum of a CTFBG, there will be a series of discrete cladding-mode resonances, however, due to the chirp each of the resonance will broaden and overlap with the neighboring resonances, resulting in a wide rejection band, as show in Fig. 1(b). If we design a CTFBG and just let the Raman gain peak of the signal laser fall into the trough of the rejection band, meanwhile the signal wavelength is outside the rejection band at the short direction, then it can operate as a good wide-band optical filter for the Stokes wave due to SRS, which is the basic principle of SRS suppression by CTFBGs.
Figure 2(a) shows the normalized Raman gain spectrum of fused quartz [18]. It can be seen that the Raman gain coefficient is largest near the frequency shift of about 13.2 THz, which corresponding to 1135 nm when the incident laser operates at 1080 nm. CTFBGs with filtering center matching with the wavelength of Stokes wave can significantly reduce the Raman noise power level of the seed laser when placed between the seed and fiber amplifier stage, so as to increase Raman threshold and suppress the SRS in high-power fiber amplifier systems [19]. However, the residual Bragg wavelength would better be outside the spectral range of high Raman gain because even a weak Bragg reflection maybe excite strong Random Raman emission at high power. Therefore, a CTFBG of longer Bragg wavelength is expected to achieve a better SRS suppression.
Fig. 2. (a) The normalized Raman gain spectrum of fused quartz [18] (b)-(d) the simulated spectra of CTFBGs with different tilt angles, grating periods and chirp rates, respectively.
For better design and fabrication of CTFBGs, we study the influences of major parameters on the optical spectra by simulation in LMA-GDF-15/130 fiber. The grating length is 30 mm and the index modulation amplitude is 0.001. Figure 2(b) shows the change of the transmission and reflection spectra with the tilt angles, where the grating period is 392 nm and the chirp rate is 0.6 nm/cm. It can be seen that with the tilt angle getting bigger, the filtering center moves to shorter wavelength while the residual Bragg resonance move to longer wavelength, the rejection depth and the FBG reflectivity decreases, meanwhile the rejection bandwidth increases due to more cladding mode resonances. From Fig. 2(c), it can be seen that grating period only influences the central wavelengths of the rejection band and the residual Bragg reflection, and both of them move towards longer wave direction with increasing of grating period. Figure 2(d) is the simulation spectra of CTFBGs with different chirp rates, where the tilt angle is 7° and the grating period is 0.392. We can see that the rejection bandwidth increases while the depth decreases with increasing of chirp rate.
From the simulations, we can know that CTFBGs of longer Bragg wavelength can be obtained by increasing the tilt angle or the period of the phase mask. However, in terms of fabrication, there are many limits on how longer the Bragg reflection wavelength can be pushed. For example, a too large tilt angle will make it more difficult to inscribe for a deep and broad cladding resonance, it must be with a concomitant increase in chirp ratio. In addition, a big grating period will cause mismatch between cladding mode resonance center and SRS central wavelength. Therefore, we have to make a balance among the tilt angle, grating period and chirp ratio to achieve the best Raman suppression effect.
3. Fabrication of CTFBGs
Figure 3 shows the fiber grating inscription and online measurement system. The excimer laser (COMPexPro110, made by Coherent Corporation, using KrF) produces 248 nm UV light. Two mirrors are used to adjust the height of the light path. After a collimation system, a better energy distribution is achieved. Finally, the light is focused on a chirped phase mask by a cylindrical lens. As for tilting, we keep the phase mask and the fiber perpendicular to the incident inscribing beam, and only rotate the phase mask around the axis of the light beam, causing an angle between the phase-mask grooves and fiber [11]. All the inscription system are places on an air floatation optical platform, which ensures the stability of UV exposure for fibers. And we have optimized the inscribing parameters of the UV laser to achieve a low insertion loss of CTFBG by plenty of explorations. Meanwhile, an amplified spontaneous emission (ASE) source is used to monitoring the transmission and reflection spectra in real time by coupling the light into the CTFBG through a circulator and a mode field adapter (MFA), and then coupled into the optical spectrum analyzer (OSA) by another MFA.
Fig. 3. Total output and output end coupling efficiency changes with tapered waist diameters of (a) input and (b) output tapered SMF-28 fibers at 1568 nm.
To further study the influence of the residual Bragg wavelength on SRS suppression, two groups of CTFBGs with different Bragg wavelengths are designed and inscribed for the following compariation experiments. The CTFBGs are inscribed in fiber LMA-GDF-15/130-M, which had been hydrogen-loaded with 12 MPa for 15 days at room temperature. Based on our simulations and requirements, the first group of CTFBGs (CTFBG Ι-1 and CTFBG Ι-2) are inscribed by using a linearly chirped phase mask of a period of 785.8 nm and a chirp rate of 0.4 nm/cm. The second group of CTFBGs (CTFBG Π-1 and CTFBG Π-2) are inscribed by using another linearly chirped phase mask of a period of 791 nm and a chirp rate of 0.6 nm/cm. During inscription, both the tilt angles of the phase masks are 4°, giving a grating plane tilt angle of about 6.5° [11]. The measured optical spectra of the two groups of CTFBGs are shown in Fig. 4, and the detail parameters are summarized in Table 1. The residual Bragg reflection is too small to be measured by OSA. By comparing the reflection peak with Fresnel reflection, all the Bragg reflectivity is estimated to be smaller than 5%.
Fig. 4. The measured transmission and reflection spectra of the two groups of CTFBGs: (a) CTFBG Ι-1; (b) CTFBG Ι-2; (c) CTFBG Π-1; (b) CTFBG Π-2.
Table 1. Characteristic parameters of the used CTFBGs
After inscription, an annealing method, combining constant-low-temperature and step-variable high-temperature, was used to reduce the thermal slope [15]. The CTFBGs were annealed in constant temperature of 150°C for 15 hours to remove residual dissociative hydrogen molecules, and then were annealed at various high temperatures as shown in Fig. 5. The gratings were slowly heated to 300°C in intervals of 50°C, from room temperature, and remains at 300°C for 15 min, before being gradually reduced to room temperature. The purpose of stepped heating and cooling is to avoid residual thermal stress inside the grating caused by a drastic change of temperature. Further, a number of processes of variable high-temperature annealing will also reduce overheating in the coating of CTFBGs. When these CTFBGs are placed on a water-cooled plate with the signal laser of 150 W passing through respectively, the temperature distribution is uniform and can be stably controlled within 35°C, which meets the requirements of long-term high-power operation. Besides, we also tested the central wavelength shift with the temperature and the temperature enhanced with the transmission laser power. Results show that the CTFBGs have good temperature stability with a wavelength shift factor of about 0.006 nm/°C, which has almost no influence on SRS suppression effect.
4. Experimental results and discussion
The experimental setup of 5 kilowatt-level tandem pumping fiber amplifier with two CTFBGs for SRS suppression is shown in Fig. 6. The seed is a fiber Bragg grating-based laser oscillator of 1080 nm with an output fiber having a core/inner-cladding diameter of 15/130 µm. Two CTFBGs are inserted between the seed and the amplifier stage to reduce the Raman noise level of the seed. A (6 + 1) × 1 pump and signal combiner is used. The signal fiber for input is 15/130 µm while that for output is 25/250 µm, matching with the seed output fiber and the gain fiber respectively. A 33 m gain fiber with a core/cladding diameter of 30/250 µm (LMA-YDF-30/250-M, Nufern Inc.) is used to ensure adequate pump absorption. The gain fiber is coiled in circles with diameters of about 16 cm and is put in a water-cooling plate with slot. Two self-made combined 3000 W 1018 nm fiber lasers are used as the pump source, whose center wavelength is 1018 nm (±1 nm) and the 3 dB bandwidth is <1 nm. The amplified signal power is led out by a pigtailed endcap, which is spliced to the gain fiber to eliminate probable harmful feedback at the output facet. A cladding pump stripper (CPS) is made online before the connection point of the endcap to provide protection to the endcap. Power meter and optical spectrum analyzer are to record the power and optical spectrum after the endcap respectively.
To compare the SRS suppression effect of CTFBGs with different Bragg wavelengths, three similar experiments were arranged using the two groups of CTFBGs shown in Fig. 4, respectively. For better demonstration of the benefit of CTFBGs with longer residual Bragg wavelength in SRS suppression, three seed lasers with similar structures but different Raman noise levels were used respectively. And the beam quality factor M2 of seed is about 1.15 in the first two experiments and is about 1.6 in the third experiment at their full powers. Meanwhile, considering the thermal slope of the first group of CTFBGs (CTFBG Ι-1 and CTFBG Ι-2), we fix the effective seed power (injecting into the amplifier) about 100 W to ensure the safety of the system in all the three experiments.
Both the first and the second experimental systems have very high Raman thresholds, weak Stokes wave with peak near 1135 nm, corresponding the maximum frequency shift of about 13.2 THz, start to appear at 3800 W without CTFBGs, then grows rapidly with the increasing laser power as usual, so we don’t show the results here. When we put CTFBG Ι-1 and CTFBG Ι-2 into the two experimental systems, the measured changing optical spectra are shown in Figs. 7(a) and (b) respectively. The strong peak near 1141 nm, which is caused by the residual Bragg reflection of the CTFBGs near 1141 nm (as shown Fig. 4), can be clearly observed at high power level. In Figs. 7(a) and (c), the strong burr-like Raman spectrum can be seen at 4690 W, which is due to the transverse mode instability (TMI) [20]. Although the CTFBGs can suppress the SRS to a certain extent at relative low power level, however, the laser oscillation with peak wavelength near 1141 nm will be introduced and become very strong at high power, which will seriously degrades the beam quality, decreases the laser efficiency, and limits the effective output laser power [20].
Fig. 7. Changing spectra at different output laser power with CTFBG I-1 and CTFBG I-2 in (a) the first group and (b) the second group experiments; (c) and (d) the corresponding zoomed in spectra near Stokes wave wavelength.
Figure 8 shows the results of the third set of experiment with CTFBGΠ-1 and CTFBGΠ-2. Comparing with Fig. 7, it can be seen that the seed used here has much higher Raman noise level than that used in the above two sets of experiments. Figure 8(a) shows the output spectra of the fiber amplifier system at different power without CTFBG. We can see that the Stokes light near 1135 nm could be clearly observed when the output power reaches 2274 W and then increases rapidly with the laser power further increasing. The difference between the signal and Stokes light is 25 dB at the output power of 3653 W. And then TMI occurs near 4 kW. Although the difference of the spectral intensity between the signal and the Stokes light is still bigger than 20 dB, the beam quality greatly degrades [20], which is about 2.2 at maximum output power. With CTFBG Π-1 and CTFBG Π-2 being inserted, the output spectra are shown in Fig. 8(b). It can see that the level of Stokes light is lower than that without CTFBG at the same laser power level and the Stokes light near 1135 nm is clearly observed until the laser reaches 2597 W. Different from the above experimental results shown in Fig. 7, there are no peak near 1141 nm or 1152 nm shown in spectra with the increase of output power even at the maximum output power of 4983 W, which is owing to the longer Bragg reflection wavelength near 1152 nm of the used CTFBGs, and the Raman gain is much lower at 1152 nm than 1141 nm. The difference of the spectral intensity between the signal and the Stokes light is still as big as 24 dB at the maximum output power. Although subtle TMI can be observed at 4985 W, the beam quality is good with a factor M2 of about 1.85. The effective laser output power is increased from ∼4 kW to ∼5 kW with an increasing ratio of 25%. By further improving the properties of the CTFBGs and the 1018 nm pump source, a promotion in laser efficiency and power could be achieved.
Fig. 8. Changing spectra at different output laser power in the third experiments (a) without and (b) with CTFBGΠ-1 and CTFBGΠ-2, insert: the measured beam quality at the maximum laser power; (c) and (d) the corresponding zoomed in spectra near Stokes wave wavelength.
5. Conclusions
We have reported the influence of the Bragg reflection of CTFBGs on SRS suppression in high-power fiber laser systems for the first time. Two groups of CTFBGs with different Bragg wavelengths are designed and fabricated, then used in three 5 kW tandem pumping 1080 nm fiber amplifiers with the same structure but different Raman levels for comparison. Experimental results show that if the Bragg wavelength is at the Raman gain spectrum range of relative high gain, the very weak Bragg reflection could be significantly amplified at high power, which will seriously affect the laser output power, efficiency and beam quality. While using CTFBGs of longer Bragg wavelength, no amplified Bragg reflection are observed even the system has a much lower Raman threshold. Effective SRS suppression and much higher TMI threshold are achieved, which helps to enhance the effective output power from ∼4 kW to ∼5 kW. A promotion in laser efficiency and power could be achieved by further improving the properties of the CTFBGs and the 1018 nm pump source in the future.
Funding
Natural Science Foundation of Hunan Province (2019JJ20023); National Natural Science Foundation of China (11974427).
Disclosures
The authors declare no conflicts of interest.
References
1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, “High power fiber lasers: current status and future perspectives,” J. Opt. Soc. Am. B 27(11), B63–B92 (2010). [CrossRef]
2. J. Nilsson, S. Ramachandran, T. Shay, and A. Shirakawa, “High power fiber lasers,” IEEE J. Sel. Top. Quantum Electron. 15(1), 1–2 (2009). [CrossRef]
3. C. Jauregui, J. Limpert, and A. Tünnermann, “High-power fibre lasers,” Nat. Photonics 7(11), 861–867 (2013). [CrossRef]
4. H. H. Lin, X. Tang, C. Y. Li, C. Guo, Y. Liu, P. F. Zhao, J. J. Wang, and F. Jing, “The localization single-fiber laser system obtained 10.6 kW laser output,” Zhongguo Jiguang 45(3), 0315001 (2018). [CrossRef]
5. X. L. Chen, F. G. Lou, Y. He, M. Wang, Z. W. Xu, X. C. Guo, R. Ye, L. Zhang, C. L. Yu, L. L. Yu, B. He, and J. Zhou, “Home-made 10 kW fiber laser with high efficiency,” Acta Opt. Sin. 39(3), 0336001 (2019). [CrossRef]
6. X. J. Wang, P. Yan, Z. H. Wang, and Y. S. Huang, “The 5.4 kW output power of the ytterbium-doped tandem-pumping fiber amplifier,” CLEO: Applications and Technology AM2M.5 (2018).
7. C. A. Codemard, J. K. Sahu, and J. Nilsson, “Tandem Cladding-pumping for control of excess gain in ytterbium-doped fiber amplifiers,” IEEE J,” Quantum Electron. 46(12), 1860–1869 (2010). [CrossRef]
8. J. J. Zhu, P. Zhou, Y. X. Ma, X. J. Xu, and Z. J. Liu, “Power scaling analysis of tandem-pumped Yb-doped fiber lasers and amplifiers,” Opt. Express 19(19), 18645–18654 (2011). [CrossRef]
9. Z. H. Wang, Q. R. Xiao, X. J. Wang, Y. Q. Yi, L. Pang, R. Pan, Y. S. Huang, J. D. Tian, D. Li, P. Yan, and M. L. Ma, “3000 W tandem pumped all-fiber laser based on domestic fiber,” Acta Phys. Sin. 67(2), 024205 (2018). [CrossRef]
10. B. Shiner. “The impact of fiber laser technology on the world wide material processing market,” CLEO: Applications and Technology AF2J.1 (2013)
11. M. Wang, Y. J. Zhang, Z. F. Wang, J. J. Sun, J. Q. Cao, J. Y. Leng, X. J. Gu, and X. J. 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]
12. M. Heck, V. Bock, R. G. Krämer, D. Richter, T. A. Goebel, C. Matzdorf, A. Liem, T. Schreiber, A. Tünnermann, and S. Nolte, “Mitigation of stimulated Raman scattering in high power fiber lasers using transmission gratings,” Proc. SPIE 10512, 105121I (2018). [CrossRef]
13. M. Wang, L. Liu, Z. Wang, X. Xi, and X. Xu, “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(1), e18 (2019). [CrossRef]
14. M. Wang, Z. F. Wang, L. Liu, Q. H. Hu, H. Xiao, and X. J. Xu, “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]
15. K. R. Jiao, J. Shu, H. Shen, Z. W. Guan, F. Y. Yang, and R. H. Zhu, “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(2), e31 (2019). [CrossRef]
16. K. R. Jiao, H. Shen, Z. W. Guan, F. Y. Yang, and R. H. Zhu, “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]
17. G. Laffont and P. Ferdinand, “Tilted short-period fibre-Bragg-grating-induced coupling to cladding modes for accurate refractometer,” Meas. Sci. Technol. 12(7), 765–770 (2001). [CrossRef]
18. D. Hollenbeck and C. D. Cantrell, “Multiple-vibrational-mode model for fiber-optic Raman gain spectrum and response function,” J. Opt. Soc. Am. B 19(12), 2886 (2002). [CrossRef]
19. H. Ying, J. Cao, Y. Yu, and M. Wang, “Raman-noise enhanced stimulated Raman scattering in high-power continuous-wave fiber amplifier,” Optik 144, 163–171 (2017). [CrossRef]
20. R. M. Tao, H. Xiao, H. W. Zhang, J. Y. Leng, X. L. Wang, P. Zhou, and X. J. Xu, “Dynamic characteristics of stimulated Raman scattering in high power fiber amplifiers in the presence of mode instabilities,” Opt. Express 26(19), 25098 (2018). [CrossRef]
