
- Photonics Research
- Vol. 13, Issue 6, 1631 (2025)
Abstract
1. INTRODUCTION
Due to the excellent advantages, such as high conversion efficiency, good temporal-spatial coherence, and robust structure, high power narrow linewidth fiber amplifiers have been widely employed in tremendous applications [1–4]. Power scaling of narrow linewidth fiber amplifiers is mainly restricted by stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) [1,5,6]. With the comprehensive understanding of their underlying physical mechanism, effective mitigation strategies have been proposed and demonstrated, which results in the power soaring up dramatically, and >5 kW narrow linewidth fiber amplifiers have already been reported [7–9]. During the power boosting process, another phenomenon, named TMI or mode distortion (MD) induced by SBS, has been observed in narrow linewidth fiber amplifiers [10,11], which occurs after the onset of SBS, and renders the single mode beam into a multi-mode operation with severely deteriorated spatial brightness. In Ref. [10], it shows that the TMI threshold was almost the same as that of SBS, and the suppression of SBS increased the TMI threshold. Recently, Liao
In this paper, experimental investigation has been carried out to unveil the origins of SBS-induced MD. Firstly, a high power narrow linewidth amplifier has been setup, and the physical phenomena of SBS-induced MD was re-examined from the perspective of spectral and spatial domains, which revealed some clues about the underlying physical mechanism. Secondly, following the revealed clues from the physical phenomena, the origin of the SBS-induced MD has been investigated in detail by analyzing the temporal and frequency domain characteristics, and comparative investigation between a narrow linewidth amplifier and a single frequency one has been carried out to further confirm the origins.
2. EXPERIMENTAL SETUP
An all-fiber laser system based on a master oscillator power amplifier (MOPA) was established, as shown in Fig. 1. A single frequency (SF) narrow linewidth linearly polarized seed centered at 1064 nm served as the seed, and the linewidth of the seed was broadened by a phase modulation [12]. Then the broadened seed laser was injected into two pre-amplification stages, which boosted the seed laser to 19 W. Then a polarization maintaining (PM) isolator (ISO) was utilized to isolate the backward light from the main amplifier, and protect the preceding stages. The backward light was dumped at the multi-mode-fiber (MMF) port, which was used to determine the occurrence of SBS. A PM mode field adapter (MFA) was applied to deliver the near-diffraction-limited signal laser from 10/125 μm (core diameter/cladding diameter) fiber to 20/400 μm fiber in the main amplifier stage [13]. The main amplifier stage was made up of a piece of 8 m large mode area (LMA) 20/400 μm PM ytterbium-doped fiber (YDF), a counter
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Figure 1.Experimental setup of the laser system.
The characteristics of the laser system were analyzed by a measurement system as shown in Fig. 1, in which two power meters, an optical spectrum analyzer (OSA) with the resolution of 0.02 nm, a beam quality analyzer, a photodiode (PD), and a charge-coupled device (CCD) with a 20 Hz sampling rate were applied. The wedge mirrors (WMs) with the reflectivity of 4% were used to sample the output laser beam. The beam quality analyzer and CCD were employed for measuring the beam quality and the beam profile, which show the spatial characteristics. The
3. EXPERIMENTAL RESULTS
A. Re-observation of SBS-Induced Mode Distortion
The physical phenomenon of mode distortion accompanied with the SBS effect was observed in the narrow linewidth PM amplifier, and the measured results are shown in Fig. 2. One can see in Fig. 2(a) that, below the pump power of 300 W, the output power of the fiber amplifier increases almost linearly as a function of the pump power with slope efficiency being
Figure 2.(a) The output power and backward power of laser system versus pump power. (b) The output beam quality and the
Similar to Ref. [11], the spectral evolution has also been investigated, and Figs. 3(a) and 3(b) show the spectral characteristics versus different output powers. The backward optical spectra are shown in Fig. 3(a). It reveals that the first-Stokes light became observable at the output power of 153 W. With the increase of output power, the intensity of first-Stokes light is the same as that of backward Rayleigh scattering signal at the output power of 173 W, which means that the presence of SBS signal became non-negligible. As the output power scaled higher, first-Stokes light intensified, and the backward random self-pulses were observed as it reached 248 W, which generally was a sign of a strong SBS effect. The output spectral evolution is plotted in Fig. 3(b). No spectral component has been observed in the region of Raman light (1100–1030 nm) below 248 W. The first-order Stokes component of SRS showed up at the output power of 256 W. As the output power reached 262 W, the intensity of first-Raman light increased dramatically, and the inter-modal four-wave-mixing (IM-FWM) effect was also observed, which was similar to the phenomena in Refs. [17,18]. During the process of the IM-FWM, phase-matching condition must be realized, which means that a high order mode photon is produced while a high order mode photon is consumed [17]. No extra high order mode photon will be produced, which means that IM-FWM is just a manifestation of the beam quality deterioration, and has no influence on high order mode content or beam quality deterioration. Compared with the beam quality results in Fig. 2(b), one can conclude that the appearance of Raman light is related with the MD instead of the SBS signal.
Figure 3.(a) The backward and (b) output optical spectra versus output power.
To further reveal the relationship between MD and nonlinear effects, the SBS signal fraction in the backward light power and Raman light ratio were calculated and are plotted in Fig. 4(a). The red curve describes the power ratio of the backward Stokes light over the total backward light, which is calculated via the multi-peak fitting and numerical integration. In concrete terms, after determining the Rayleigh scattering signal and the first-Stokes light central wavelength, we separated the two signal peaks by Gaussian fitting, and finally numerically integrated the two signal peaks to get the SBS signal fraction. The blue curve depicts the power ratio of the Raman light over the output power, which is calculated through spectral integration [19]. In Fig. 4(a), one can see that, although the SBS signal fraction increases nonlinearly beyond 153 W, the MD has not appeared at this power. For the Raman ratio, it increased rapidly beyond 250 W, and the corresponding power was coincident with the MD threshold shown in Fig. 2(a), which indicates the relevance between the SRS effect and MD. In addition, a sample of the Gaussian multi-peak fitting process is shown in Fig. 4(b). The ordinate of the original spectrum was adjusted to a linear scale during fitting, since the Gaussian multi-peak method requires linear coordinates.
Figure 4.(a) The SBS signal fraction and Raman light ratio versus output power and (b) the fitting sample graph.
B. Origin of SBS-Induced Mode Distortion
Figure 5.The backward temporal characteristics (a) around the SBS effect and (b) the MD; the synchronous forward temporal characteristics (c) around the SBS effect and (d) the MD. (e) The pulse width of one single forward giant pulse (inset: the local magnification of the pulse at the same time location).
It is well known that there are clear frequency components at 0–15 kHz in the forward Fourier transform spectrum (Fts) after the occurrence of the TMI effect [27] while tens of MHz for the forward SBS effect [28,29]. To further investigate the origin of the MD, the Fourier analysis with high and low frequency ranges was applied to the time domain characteristics to calculate the corresponding Fts, which is plotted in Fig. 6. As shown in Fig. 6(c), no characteristic frequency component of TMI is clearly observed after the onset of MD except for an overall improvement of the intensity. One can also see from Fig. 6(d) that there is no sign of forward SBS except for the frequency component corresponding to the pulses of the second-order SBS. After the onset of MD, there were forward high peak power pulses, which were induced by the cascaded SBS effect. It is shown that the characteristic frequency components of those pulses fall within the MHz range [30]. So the characteristic frequency components after the onset of MD are distributed in the range of 0–20 MHz. Fourier spectra for the cases before and after the onset of SBS have also been shown. One can see from Figs. 6(a) and 6(b) that, after the onset of the SBS effect, no new frequency component shows up other than the electrical noise. One can conclude that the MD is neither the thermal-induced mode instability nor the forward SBS.
Figure 6.The forward Fts around the SBS effect with the range of (a) kHz and (b) MHz. The forward Fts around the MD with the range of (c) kHz and (d) MHz.
To further investigate the physical connection between SBS and MD, the spectrum broadening module was removed to directly employ the SF, and the results are shown in Fig. 7. Because the linewidth of the SF was 3.3 kHz, the SBS effect was triggered at a much lower power. The backward optical spectrum at several typical output powers is shown in Fig. 7(a). One can see that the first-Stokes light is first observed at the output power of 3.3 W, and the SBS signal is as high as the Rayleigh backscattering at 5.3 W. However, no deterioration of the beam quality has been observed even at 9 W, shown in Fig. 7(b), where the SBS signal is 20 dB higher than the Rayleigh backscattering. It revealed that there is no MD effect after the onset of first-order SBS. In addition, the temporal traces and output optical spectrum at the maximal output power were measured and are shown in Figs. 7(c) and 7(d), respectively. In Fig. 7(c), one can see that forward pulses have been stimulated, but the peak power is not more than 250 W, which is obviously lower than the theoretical SRS threshold. According to Fig. 7(d), no SRS peak has been observed. Although SBS is also a core-pumped effect, SBS-induced beam cleanup, which results in the beam quality deterioration of the pump laser and MD, occurs only in graded-index fibers, and has not been observed in step-index fibers [31]. So the SBS effect could not stimulate MD directly. One can conclude that MD is not directly related to the SBS effect.
Figure 7.(a) The backward optical spectrum and (b) the output beam quality at several typical output powers. The forward (c) temporal trace and (d) optical spectrum at the maximal output power.
Based on the above results, one can conclude that the MD is not a manifestation of SBS, but an indirect and new phenomenon of SBS at high power operation. As the SBS effect intensifies, tremendous forward giant pulses were stimulated. If the peak power of the forward giant pulses becomes higher than the SRS threshold, the SRS effect is stimulated, which results in MD due to the core-pumped Raman effect [32]. Due to that the SBS thresholds are lower for narrow linewidth lasers, the phenomenon of SBS-induced MD is mainly observed in narrow linewidth ones. However, for those with broadband linewidths, the SBS thresholds are relatively higher, and other MD, such as SRS-induced MD, occurs earlier. If the SRS thresholds were improved higher than SBS thresholds, SBS-induced MD would also occur theoretically.
4. CONCLUSION
The phenomenon of SBS-induced MD has been investigated in a high power narrow linewidth fiber amplifier. By comprehensively investigating the spectral evolution, spatial beam profiles, and time-frequency domain characteristics, the origin of SBS-induced MD was verified. Analyzing the spectral evolution and spatial characteristics, it revealed that the observed MD is dependent on the show-up of the SRS effect, and the temporal characteristics pointed out that the occurrence of the SRS effect was induced by forward giant pulses. The onset process of SBS-induced MD is that SBS-induced high peak power forward giant pulses trigger the SRS effect, and then the SRS effect causes the beam deterioration of the signal laser, which means that SRS is the origin of the MD observed after the onset of SBS.
References
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