Experimental Study on Transverse Mode Instability of All-Fiber Single-Frequency Amplifier Based on Tapered Yb-Doped Fiber

Abstract

We experimentally studied the transverse mode instability (TMI) threshold of a linearly polarized single-frequency fiber laser amplifier constructed with tapered ytterbium-doped fiber (TYDF) under different bending diameters. The TMI threshold increased from 333 W to 451 W by reducing the bending diameter from 16 cm to 12 cm, which was accompanied by the deterioration of the beam quality from 1.47 to 1.67. The anomalous characteristics between the TMI threshold, bending diameter, and beam quality are mainly attributed to the decreased bending loss of higher-order mode (HOM) content as a result of the increased system heat loads caused by a tight bending-induced loss of amplification efficiency. It is believed that the presented results will provide useful guidelines for the design of high-power single-frequency fiber amplifiers.

1. Introduction

High-power linearly polarized single-frequency fiber amplifiers (SFFAs) have been attracting tremendous interest in recent years for potential applications in the area of gravitational wave detection (GWD) [1], coherent beam combination [2], nonlinear frequency conversion [3], coherent LIDAR detection [4] and so on. However, owing to the high spectral intensity of single-frequency lasers, the stimulated Brillouin scattering (SBS) effect, which generates a backward-scattered Stokes signal with a frequency downshift in the fiber core when the operating power exceeds a certain threshold, seriously restricts further improvements of power and even introduces damage to the entire system [5,6]. Various experimental techniques aiming to suppress the SBS effect have been carried out, mainly including the employment of large-mode-area (LMA) active fibers [7,8], tailoring the gain and acoustics of the active fiber [9], incorporating longitudinal strains or temperature gradients to the fiber [10,11], etc. Particularly, an output power of 811 W has been achieved based on a bulk optics configuration by adopting a specially designed acoustic- and gain-tailored Yb-doped photonic crystal fiber (PCF) accompanied by a thermal gradient, which, up to now, has provided the highest output power in terms of a SFFA [9]. However, its free-space coupling system is susceptible to external environmental disturbances. In contrast, a SFFA with an all-fiber structure is more favorable in many applications due to its high compactness, flexibility and reliability.

Up to now, the highest output power of an all-fiber linearly polarized SFFA by employing conventional uniform active fiber was 414 W, which was achieved with the assistance of step-distributed longitudinal strain gradient modulation along the fiber for SBS suppression [10]. Nonetheless, these kinds of externally imposed physical gradients might cause inconvenience and sacrifice the stability of the fiber laser system in practical applications. An alternative method for addressing the SBS effect is to develop special fiber structures, such as chirally coupled core fibers, all-solid photonic bandgap fibers, confined-doped fibers, and tapered fibers [12,13,14,15]. Among all the fiber designs for SBS mitigation, the strategy of adopting tapered fibers has attracted extensive attention due to their outstanding geometry properties, relatively simple structure, and good compatibility with passive fiber components. The mechanism of how tapered fibers increase the SBS threshold is that by gradually increasing the core/cladding diameter, the frequency shift of SBS changes accordingly, resulting in a broadened SBS gain spectrum with reduced magnitude [16]. By leveraging a tapered YDF with a core/cladding diameter of 35/250 μm at the narrow end and 56/400 μm at the wide end, the output power of a linearly polarized SFFA was significantly scaled to 650 W [14]. Furthermore, further power enhancement was not limited by SBS but by the TMI effect, which induced the degradation of output beam quality.

TMI is a phenomenon that involves dynamic energy transfer between the fundamental and higher-order transverse modes of the fiber caused by thermo-optical scattering, resulting in output power stagflation and beam distortion [17,18,19,20]. In view of this, a series of techniques have been proposed to mitigate the TMI effect, such as modulating the pump noise [21], utilizing a hybrid wavelength pumping scheme [22], coiling the active fiber [23,24], engineering the pumping configuration [25,26], etc. A common feature of all of those approaches is to excite the fundamental mode of the fiber as much as possible while reducing the excitation and amplification of HOMs. However, recent advances in high-power fiber amplifiers based on few-mode LMA fibers have demonstrated that HOM components are conducive to improving the TMI threshold due to the reduced frequency-shifted Stokes HOMs caused by the inter-mode gain competition mechanism [27,28,29,30]. Among the attempts to increase the HOM component for a LMA fiber amplifier, changing the bending diameter of the gain fiber is the most convenient and effective way. In addition, there are also theoretical works that have suggested that bend loss of HOMs is sensitive to local heat load and can be significantly reduced at high heat loads [31,32].

Although extensive research has been devoted to investigating TMI mitigation methods in high-power fiber amplifiers, the exact relationship between the TMI threshold, beam quality, and the HOMs of high-power SFFAs, especially those constructed with tapered gain fiber, has not been investigated yet. In this article, we present an experimental investigation of the TMI characteristics of an all-fiber linearly polarized single-frequency amplifier based on TYDF. The TYDF was coiled in a racetrack shape, and it is shown that the TMI threshold of the amplifier increases monotonically with the bending diameter changing from 16 cm to 12 cm; however, the corresponding output beam quality demonstrated a degradation trend at the same time.

2. Experimental Setup

The schematic diagram of the all-fiber linearly polarized single-frequency amplifier based on TYDF is depicted in Figure 1. The laser system employed a master oscillation power amplification (MOPA) structure containing a seed laser and three-stage fiber amplifiers. The seed laser operates at 1064 nm with an output power of 60 mW and a linewidth of 15 kHz. The seed laser was first amplified to 350 mW by adopting a single-mode first-stage pre-amplifier that employed a gain fiber with a core/cladding diameter of 6/125 µm (absorption coefficient 250 dB/m @ 976 nm). After that, a polarization-maintained (PM) isolator (ISO) was connected to block the back-reflected laser signal. The boosted laser was then injected into the second pre-amplifier utilizing gain fiber with a core/inner cladding diameter of 10/125 µm and an absorption coefficient of 4.8 dB/m @ 976 nm and the pumping of a 976 nm laser diode via a (2 + 1) × 1 pump/signal combiner. A PM band pass filter (BPF) was subsequently connected to remove the amplified spontaneous emission (ASE) around 1030 nm. Meanwhile, a PM circulator (CIR) was employed to monitor the backward power from the main amplifier and protect the forestage amplification chain. The signal power after the CIR was measured to be about 8.5 W.

In the power amplifier stage, two series of wavelength-stabilized 976 nm LD groups were utilized as the pump source, with each group producing a maximum output power of ~390 W. The pump power was coupled into the amplifier stage through a (2 + 1) × 1 pump/signal combiner, of which the input and output signal ports are both based on LMA fiber with a core/cladding diameter of 30/250 µm. For the purpose of implementing a monolithic system with a minimized insertion loss in fusion splicing, a mode field adapter (MFA) transitioning the fiber core/cladding diameter from 10/125 µm to 30/250 µm was adopted to connect the CIR and the combiner. A piece of 2.5 m-long PM TYDF was utilized as the gain fiber with a core/inner-cladding diameters of 36/250 µm at the narrow end and 56/400 µm at the wide end, respectively. The tapered region is 0.7 m-long with an approximately linear longitudinal profile, and the lengths of the uniform parts at the narrow and wide ends are 0.5 m and 1.3 m, respectively [as shown in Figure 2a]. The nominal pump absorption coefficient of the TYDF is ~10 dB/m at 976 nm, and the core NA is 0.07. A cladding light striper (CLS) was spliced to the wide end of the TYDF to strip out the residual cladding light and connected with a quartz block holder (QBH) for beam delivery. All of the components were placed on a water-cooled aluminum sink for efficient thermal management.

In order to analyze the impact of the bending effect on the TMI threshold, the TYDF was coiled in the racetrack groove with three different bending diameters (shown in Figure 2b), each of which has a bending diameter ranging from 16 cm to 15 cm (input end to output end), 14 cm to 12.8 cm, and 12 cm to 10.4 cm. Throughout the whole experiment process, all the fusion points, particularly those between the combiner, the TYDF, and the CLS, remained unchanged. Moreover, it should also be noted that the fusion splicing point between the combiner and the TYDF was consistently placed in the middle of the straight track to guarantee that HOM excitation at the input port was nearly identical.

3. Experimental Results and Discussions

In the experiment, the influence of the bending diameter of the tapered fiber on the TMI threshold performance of the TYDF amplifier was investigated. At first, the narrow end of the TYDF was coiled with a bending diameter of 16 cm, which gradually decreased to 15 cm at the wide end. To verify the emergency of TMI, the time trace of the output laser intensity was monitored using a high-speed photodetector (PD) with a 150 MHz bandwidth. During the experimental process, the pump power was increased continuously until temporal instability was recorded. Figure 3a,b show the time traces during a period of 100 ms and the corresponding Fourier transform (FFT) spectra before and after the onset of the TMI effect, respectively. At an operating power of 300 W, it can be seen that the temporal signal of the output laser remains quite stable, and no frequency peaks appear in the corresponding FFT spectrum. Whereas with an increased output power of 333 W, a remarkable temporal intensity fluctuation emerges; this was induced by the fast energy coupling between the fundamental mode and HOMs. Moreover, in the frequency domain, there are feature frequency components of TMI that are mainly distributed in the range of 0–2 kHz.

Subsequently, the main amplifier was reoperated with the bending diameter of the TYDF decreased to 14–12.8 cm from the input end to the output end. Figure 3c,d depict the time trace and FFT spectra of the output beam at output powers of 332 W and 378 W, respectively. The temporal intensity was kept stable, and no characteristic frequency peaks were observed until the output power increased to 332 W, resulting in the absence of the TMI effect. However, the temporal trace shows a strong fluctuation when the output power was enhanced to 378 W. According to the corresponding FFT spectrum, the main characteristic frequency is about 2 kHz, accompanied by several other individual characteristic peaks at frequencies below 2 kHz. It can be preliminarily concluded that the threshold of the TMI effect might increase with the decreasing the coil diameter of the TYDF, and this was further verified by coiling the TYDF more tightly, i.e., the bending radius starts at 12 cm at the input end and decreases towards 10.4 cm at the output end. Relevant TMI performances were recorded; they are presented in Figure 3e,f. It can be observed from Figure 3e that kHz-level oscillation frequency peaks were absent at an operating output power of 415 W; however, they rose significantly when the output power reached 451 W. In Figure 3f, the corresponding FFT spectrum presents slightly increased feature frequency components at 2.7 kHz, indicating the occurrence of TMI. From the above results, it can be seen that decreasing the bending diameter of the TYDF could not only improve the TMI threshold of the SFFA but also increase the corresponding output intensity oscillation frequency, which is consistent with the results presented in Ref. [33].

The dependence of the output power and monitored backward Stokes light power on the pump power under different bending diameters of the TYDF is depicted in Figure 4a, while the corresponding beam quality (M² factor) of the fiber amplifier as a function of the output power was measured and subsequently illustrated in Figure 4b. At a bending diameter of 16 cm, the maximum output power of the TYDF amplifier was measured to be ~333 W, with an optical-to-optical efficiency of 85%. In addition, the measured M² factors at the selected operation power below 200 W almost maintain at about 1.3, indicating the single-mode laser output of the TYDF amplifier. However, the M² factor began to increase with further increasing output power, and it reached 1.47 after the emergence of TMI. When decreasing the bending diameter to 14 cm, the slope efficiency of the fiber amplifier obviously declined to 74%, owing to the decreased effective mode area and increased bending loss of the fundamental mode of the TYDF. In the meantime, the assessed beam quality demonstrates overall degradation compared to the case with a larger bending diameter, and the correspondingly measured M² factors ranged from 1.3 to 1.5. At an even smaller bending diameter of 12 cm, a slope efficiency of only 67% was measured, and the M² degraded further from 1.3 to 1.67 as the power increased.

Basically, the above results demonstrate two unusual phenomena that have never been found in single-frequency fiber amplifiers, i.e., tighter bending of the gain fiber leads to higher HOM content and a larger M² factor of the amplifier output, and a higher HOM content leads to a larger TMI threshold. For the former, the most probable reason is that the bending of the TYDF induces significant losses to both the fundamental and higher-order modes, and these can be inferred from the reduced slope efficiency of the amplifier from 85% to 67%. As a result, the sacrifice of efficiency causes remarkable heat depositing, which, in turn, increases the numerical aperture due to the increased refractive index, leading to a reduced bending loss in the HOM [32]. Considering the latter, it is attributed to the decreased frequency-shifted Stokes HOM mode due to the transverse modes competition effect with increased HOM components [25,28,29,34]. Although a thorough theoretical analysis is needed to illustrate the above process more explicitly, it is fairly reasonable to conclude that for a single-frequency fiber amplifier constructed with TYDF, the TMI threshold could be considerably increased via a tight bending of the gain fiber, with a specific degree of sacrifice in terms of efficiency and output beam quality. In addition, it can be observed from Figure 4a that the backward propagating power under different bending diameters is relatively low, and there is no sign of nonlinear growth, meaning that the SBS threshold is far from being reached. Therefore, it is believed that a higher TMI threshold and, thus, output power is obtainable by further decreasing the bending diameter of the TYDF. As a relatively low slope efficiency of the amplifier is acceptable since higher pump power is readily available, the d method of realizing a high-power single-frequency laser demonstrated in the current work should be attractive for applications that have loose requirements in terms of beam quality.

Under TYDF bending diameters of 16 cm, 14 cm, and 12 cm, the optical spectra at the maximum output power were recorded and are shown in Figure 5a. It can be seen that the final output has signal-to-noise ratios of 46.6 dB, 47.8 dB, and 48 dB, respectively, indicating the good ASE suppression of the system. In addition, the polarization degree evolution of the output laser with increasing operation power was also investigated by using a λ/2 wavelength plate and a polarization beam splitter, as shown in Figure 5. The polarization degree gradually decreased from 95% to 91.5% during power enhancement, which was primarily due to the thermally induced polarization degradation in the gain fiber. In addition, the longitudinal-mode characteristic of the TYDF amplifier was examined using a scanning Fabry–Perot interferometer, with a free spectral range (FSR) of 4 GHz and a resolution of 10 MHz. As shown in the inset of Figure 5b, it can be seen that the final output laser was operated with a strict single longitudinal mode.

4. Conclusions

In this work, the dependence of the TMI threshold and output beam quality on the bending diameter of gain fiber in a high-power linearly polarized single-frequency fiber amplifier based on large-mode-area TYDF was systematically studied. The experimental results demonstrated that tightly bending the TYDF increases heat accumulation in the system; therefore, it increases the HOM content, resulting in a higher TMI threshold and degraded beam quality. In the experiment, when the bending diameter of the TYDF decreased from 16 to 12 cm, the TMI threshold increased from 333 W to 451 W, while the corresponding beam quality deteriorated from 1.47 to 1.67. We believe this work provides a good reference and practical solution for robust power scaling in high-power single-frequency fiber laser amplifiers.