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 14â dB. Moreover, the output power of the fiber oscillator is improved to 3.5â 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.
We present a robust chirped and tilted fiber Bragg grating (CTFBG) in a large-mode-area double-cladding fiber (LMA-DCF) written by a femtosecond (fs) laser. By implementing the fs-CTFBG into the output end of a high-power fiber laser for Raman filtering, a power handling capability of 4â kW is achieved with a Raman filtering ratio of â¼13â dB. To the best of our knowledge, this is the maximum handling power of a CTFBG for Raman filtering. The signal loss of the fs-CTFBG is 0.03â dB, which has little effect on the output laser beam quality. The air-cooled fs-CTFBG has a minimum temperature slope of 7.8°C/kW due to a self-annealing effect. This work proves the excellent performance of the fs-CTFBG, promoting the development of high-power CTFBGs.
Fiber Optic Technology , Refractometry , Equipment Design , Equipment Failure Analysis , Lasers
Here, we have experimentally demonstrated the selection principle of the seed power in a narrow linewidth fiber amplifier seeded by fiber oscillator based on a pair of fiber Bragg gratings. During the study on the selection of seed power, the spectral instability of the amplifier is found when a low power seed with bad temporal characteristics is amplified. This phenomenon is thoroughly analyzed from seed itself and the influence of the amplifier. Increasing the seed power or isolating the backward light of amplifier could effectively eliminate the spectral instability. Based on this point, we optimize the seed power and utilize a band pass filter circulator to isolate the backward light and filter the Raman noise. Finally, a 4.2â kW narrow linewidth output power is achieved with signal to noise ratio of 35â dB, which has exceeded the value under the highest output power reported in this type of narrow linewidth fiber amplifiers. This work provides a solution for high power and high signal to noise ratio narrow-linewidth fiber amplifiers seeded by FBGs-based fiber oscillator.
Chirped and tilted fiber Bragg gratings (CTFBGs) are important all-fiber filtering components in high-power fiber lasers for stimulated Raman scattering (SRS) suppression. The fabrication of CTFBGs in large-mode-area double-cladding fibers (LMA-DCFs) by femtosecond (fs) laser is reported for the first time to the best of our knowledge. The chirped and tilted grating structure is obtained by scanning the fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. By this method, the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated, and the maximum rejection depth and bandwidth are â¼25â dB and â¼12â nm, respectively. To test the performance of the fabricated CTFBGs, one is inserted between the seed laser and the amplifier stage of a 2.7â kW fiber amplifier, and an SRS suppression ratio of â¼4â dB is achieved with no reduction in laser efficiency and degradation in beam quality. This work provides a highly fast and flexible method to fabricate large-core CTFBGs, which is of great significance to the development of high-power fiber laser systems.
The stimulated Raman scattering (SRS) process in gas-filled hollow-core fiber is mostly used to realize the wavelength conversion, which has the potential to produce narrow-linewidth and high-power fiber laser output. However, limited by the coupling technology, the current research is still at a few watts power level. Here, through the fusion splicing between the end-cap and the hollow-core photonics crystal fiber, several hundred watts pump power can be coupled into the hollow core. Homemade narrow-linewidth continuous wave (CW) fiber oscillators with different 3â dB linewidths are used as the pump sources, then the influences of the pump linewidth and the hollow-core fiber length are studied experimentally and theoretically. As the hollow-core fiber length is 5 m the H2 pressure is 30 bar, 109 W 1st Raman power is obtained with a Raman conversion efficiency 48.5%. This study is significant for the development of high-power gas SRS in hollow-core fibers.
In a previous study, we proposed a measuring method for the reflectivity of weak-reflection large-mode-area fiber Bragg gratings by using scale gratings. We experimentally found that the interference between two scale fiber Bragg gratings (FBGs) is beneficial for increasing reflectivity scales, which can improve the measurement accuracy. Therefore, in this study, we designed and fabricated FBG-based Fabry-Perot cavities (FBG-FP) in single-mode fibers by two inscription methods, namely ultraviolet (UV) laser exposure and femtosecond-laser direct writing. Then, a large-mode-area double-clad (LMA-DC) FBG of weak reflectivity was measured by these two scales, and the experimental results show that the Bragg resonance reflectivity is less than 4.28% and 1.14% â¼ 2.28%, respectively. This method of measuring the weak grating reflectivity based on FBG-FP scales is convenient, efficient, and accurate. It is also worth mentioning that the method of femtosecond-laser direct writing eliminates the period limitation of the phase mask, thereby expanding the measurement wavelength range of FBGs. In the future, with the improvement of fiber grating fabrication technology, it is expected that more accurate results can be obtained.
Accurate fundamental-mode (FM) reflectivity measurements of FM weak reflection large-mode-area (LMA) FBGs constitute a challenging problem owing to high-order modes (HMs). In this paper, we propose a novel measurement method that uses scale gratings fabricated in a single-mode fiber. The weak reflectivity of the measured FBGs was achieved by comparing the peak reflection resonance with that of the scale gratings. The measured minimum reflectivity of the scale grating was 1.3%, giving the measurement accuracy. The accuracy can be improved further by increasing the number of reflectivity scales. A Fabry-Perot-interferometer-based scale grating was proposed, designed, and fabricated using a chirped phase mask to achieve soaring scale numbers. The minimum reflectivity of the scales decreased by 0.37%. Several LMA double-cladding FBGs with weak reflectivity were measured. The results show that this novel measurement method is convenient and efficient, does not depend on the transmission spectrum of the grating, and can circumvent the influence of HM. It is anticipated that weak reflectivity can be measured more accurately by improving grating fabrication technology.
Line-by-line direct writing by femtosecond laser has been proved to be a simple and effective method for the fabrication of low-loss fiber Bragg gratings (FBGs), and is more flexible compared with the traditional ultraviolet exposure method. In this paper, the line-position-dependent characteristics of cladding modes coupling in line-by-line FBGs have been studied, to the best of our knowledge, for the first time. Both theoretical and experimental results show that off-center inscribing could compress the bandwidth of the Bragg resonance and excite more abundant cladding mode coupling, in which the core-guided fundamental mode would couple to the cladding-guided LP0n and LP1n simultaneously. By aligning the line positions across the core region, the first apodized line-by-line FBG was achieved. This work enriches the theories of line-by-line FBGs and provides an inscription guidance to meet different application requirements.
In this paper, we studied the basic characteristics of tilted fiber Bragg gratings (TFBGs), inscribed line-by-line. Experimental results showed that if the TFBGs were located within different planes parallel to the fiber axis, the spectra performed differently. For 2°TFBG, if it was located near the central plane, the Bragg resonance was stronger than ghost mode resonance, and the order reversed if it was located near the boundary between core and cladding. As the tilted angle increased, the range of cladding mode resonance increased. When the tilted angle was larger than 12°, the birefringence effect was observed. Based on the birefringence phenomenon, torsion characteristics were experimentally studied; the sensitivity was about 0.025 dB/degree in the linear variation range. The harmonic order of TFBGs also affected the transmission spectrum. Leaky mode resonance was observed in the 8th order TFBG, and torsion (or polarization) influenced the spectrum of the 8th order TFBG. Our research represented the theory of line-by-line inscribed TFBGs and provided an inscription guidance for TFBGs.
We study here the spectral characteristics of square-wave-modulated type II long-period fiber gratings (LPFGs) inscribed by a femtosecond laser. Both theoretical and experimental results indicate that higher-order harmonics refractive index (RI) modulation commonly exists together with the fundamental harmonic RI modulation in such LPFGs, and the duty cycle of a square wave has a great influence on the number and amplitudes of higher-order harmonics. A linear increase in the duty cycle in a series of square wave pulses will induce another LPFG with a minor difference in periods, which is useful for expanding the bandwidth of LPFGs. We also propose a method to reduce insertion loss by fabricating type II LPFGs without higher-order harmonic resonances. This work intensifies our comprehension of type II fiber gratings with which novel optical fiber sensors can be fabricated.
Recently, chirped and tilted fiber Bragg gratings (CTFBGs) have received great attention because they can realize suppression of stimulated Raman scattering (SRS) in high-power fiber lasers. In this study, the possible coupling between the core modes and cladding modes in CTFBGs inscribed in large-mode-area double-cladding fibers is investigated for the first time. Theoretical results show that the coupling between the LP11 mode and cladding modes would destroy the transmission spectra envelope only considering the coupling of LP01 for single-mode CTFBGs, which will degenerate the SRS suppression performance. This was confirmed experimentally by measuring the spectral response under different mode excitations. A reliable method is demonstrated to ease the LP11-excitation-induced spectral deterioration by choosing an appropriate chirp rate for the inscription of CTFBGs, which is useful for improving the Raman suppression effect of large-mode-area double-cladding CTFBGs in high-power fiber lasers.
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.
