Ytterbium-doped fiber (YDF) loaded with deuterium is used herein to mitigate mode instability. Experimental results reveal that this method can increase the mode instability threshold in a laser oscillator. Specifically, when the YDF was loaded with deuterium over two- and four-week periods, the mode instability threshold power increased from ∼459 W to ∼533 W (16%) and to ∼622 W (35%), respectively, but the respective laser efficiencies were almost unaffected (71.5% vs. 72.9% and 75.4%). In conclusion, deuterium loading is effective in the mitigation of mode instability. It is envisaged to be applied in the power scaling of high-power fiber lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Fiber lasers exhibit high-beam quality and increased output power levels characteristics. Accordingly, fiber lasers are extensively used in industrial processing, military defense, and scientific research applications [1,2]. The success of fiber lasers is primarily based on the superior properties of the fiber as the active gain medium. These superior features of optical fibers are mainly derived from the waveguide structure of the fiber itself, and fiber lasers exhibit single-pass, high-gain advantages, with an excellent, almost power-independent, beam quality . Furthermore, fiber lasers exhibit superior thermal management performance owing to the large fiber surface-to-volume ratios. Given that ytterbium is applied as the dopant material, the signal wavelength is similar to the pump wavelength, and the quantum defect heating is thus relatively low. However, with further increases in output power, fiber lasers will also encounter thermal management problems, including mode instability (MI). MI causes the mode contents to suffer from considerable time-dependent fluctuations that limit additional power scaling and beam quality improvements in high-power fiber lasers, and impose severe negative impacts on the properties of fiber lasers [3–9].
The mechanism of MI is mainly attributed to various physical factors (mainly heat in active fibers) that modulate the refractive index of the fiber and generate induced refractive index gratings in fibers [4–6]. It is these induced gratings that cause mutual propagating mode couplings. In addition, mode coupling requires a phase shift between the mode interference and refractive index grating . To solve this problem, many mitigation methods have been proposed. These methods can be categorized into active and passive types [10,11]. Active strategies include acousto–optic deflection, pump modulation [12–15], and passive strategies include fiber core numerical aperture/normalized frequency parameter (V-parameter) optimization, bending method improvement, pump schemes and distributions variations and photodarkening suppression [16–26]. Compared with active strategies, passive strategies eliminated the requirement for additional external modulation devices. Therefore, passive strategies are more extensively used in the mitigation of MI. H.J. Otto et al. have proposed that photodarkening effect has significant impact on mode instability threshold, which indicates photodarkening suppression is beneficial to MI mitigation . B. Kumar et al have reported that deuterium loading is effective in background loss reduction of fused silica at IR wavelength . At 2019, N. Zhao et al. have found that photodarkening effect could be eliminated in deuterium pre-loaded fiber . However, these researches have not directly investigated correlation between deuterium loading and MI. Thus it is meaningful that investigation of correlation between deuterium loading and MI.
To find a novel MI mitigation approach, we have proposed a method to load deuterium in optical fibers in this study. This mitigation method is convenient to implement, and could be easily realized after the fiber fabrication process. Additionally, deuterium loading is easier to implement and control than the adjustment of parameters of active fibers. To-this-date, few studies have been conducted on the impact of deuterium loading on the MI of the ytterbium-doped fiber (YDF) oscillator.
Herein, we have a) conducted an experimental study on deuterium loading on the MI of the YDF oscillator and b) analyzed and discussed the mechanism associated with deuterium loading on the mitigation of MI. We have employed a 20/400 step-index, Yb-doped, all-fiberized laser oscillator, pumped by 976 nm laser diodes which operated at a wavelength of 1080 nm. The laser efficiency and MI threshold were measured carefully before and after deuterium loading over two- and four-week periods, respectively.
2. Experimental setup
The active YDF is designed and fabricated with the traditional modified chemical vapor deposition (MCVD) and solution doping technology, and possesses a double-clad, step index, refractive index profile. The inner cladding of the active YDF is octagonal in shape, the inner cladding diameter is 400 um, and the core diameter is 20 um with core NA = 0.063. The Yb and Al concentrations are approximately 0.45at% and 0.26at%, respectively. The fiber pump absorption at 976 nm is 1.20 dB/m and fiber length used in the experiment is approximately 19 m. The active fiber was then set in a chamber to be fully exposed to deuterium for two- and four-week periods at 293 K and at a pressure of 3 bar. After the loading of deuterium, measurements were carried out immediately to avoid considerable deuterium diffusion. The experimental setup of the laser oscillator is shown in Fig. 1. As indicated, the pump sources of the all-fiber oscillator comprise several center wavelengths associated with high-power laser diodes (LDs, 976 nm), which are fed to the (6 + 1)×1 pump combiner. The pump light is then injected into the pump combiner and is then passed through the a) high reflectivity fiber Bragg grating (HR–FBG), b) the active ytterbium-doped fiber (YDF), and c) the output coupler fiber Bragg grating (OC–FBG). The passive fibers on the combiner signal port (HR/OC FBG) all possess double-clad, step-index refractive index profile characteristics, and their core and cladding diameters are 20/400 um, respectively. The center wavelength of the HR–FBG is 1080 nm, the reflectivity is approximately 99%, and the 3 dB bandwidth is approximately 3 nm. The active fiber is embedded in the spiral groove of water-cooled aluminum heat-sink. The heat-sink consists of two straight parts 20 cm in length and 2 semi-circle with inner bending diameter of 15 cm and outer diameter of 25 cm and 25 cm fiber end is spliced to HR-FBG and 15 cm fiber end is spliced to OC-FBG. Additionally, the center wavelength of OC–FBG is 1080 nm, the reflectivity is approximately 10% and the 3 dB bandwidth is approximately 1 nm. A cladding light stripper (CLS) is attached to the output end of OC–FBG to strip the cladding light of the laser oscillator. An end-cap was spliced to the output end of CLS to avoid damaging the laser oscillator by backward reflection light. The passive fibers on OC–FBG and CLS were matched completely, and the laser output from the end-cap was fed into the power meter to measure the output power. A photo-detector was installed near the target face of the power meter to measure the reflected light signal, convert it into an electrical signal, and input it in an oscilloscope. All devices mounted on the heat sink were cooled by circulating cooling water to ensure adequate heat dissipation.
3. Experimental results and discussion
We have carried out several experiments to investigate laser performance of pristine fiber and deuterium loaded fiber. Firstly, the output power and MI threshold of pristine fiber were measured as a function of pump power. Subsequently, we measured the output power and the MI threshold of the post-processed fiber at different deuterium loading levels, and compared it to the pristine fiber. And then, the longevity of deuterium loading on increased mode instability power threshold were measured. Finally, the probable mechanism that drove this process was discussed.
3.1 Laser performance of pristine fiber
We have used a laser oscillator to measure the laser performance of pristine fiber, as shown in Fig. 1. Accordingly, the output power and corresponding standard deviation versus the launched pump power is plotted in Fig. 2(a) and Fig. 2(b). When the launched pump power reached a level of 642 W, the output power was 459 W, and a correlation existed between the output power and the launched pump power. The corresponding standard deviation was 0.02 compared to the launched power level of 642 W. However, when the launched pump power was further increased and exceeded 642 W, the output power started to occur roll-over behavior, and the pump power increase resulted in a drop of the output power. Furthermore, the standard deviation increased suddenly to 0.05 and 0.06 as the pump power was further increased. This means that the output signal suffered a severe fluctuation. Additionally, MI was induced when the launched pump power exceeded the level of 642 W. As the standard deviation suddenly increased, the temperature on the CLS also started to increase rapidly. This justified the occurrence of MI.
For further investigations of the MI properties of pristine fiber, we measured the scattered signal from the power meter target plane of the output power with a photo-detector and an oscilloscope. As shown in Fig. 3(a), the comparison of P1, P2, and P3, shows increased and obvious fluctuations in the time domain signal. This means that when the laser oscillator operates at P2 and P3, the mode content in the oscillator is fluctuating, and modal coupling occurs at P2 and P3. Additionally, the corresponding standard deviation reveals steep increases from P1 to P2, as plotted in Fig. 2(a) and Fig. 2(b). Fourier transformation was also applied to investigate the frequency components of the output signal, as shown in Fig. 3(b). At P1, there is only a direct current (DC) component in the frequency spectrum, which indicates that the signal is stable at that time. If the laser oscillator operates at P2, higher frequency components appear. However, if the laser oscillator operates at P3, the intensity peaks of higher frequency components are stronger than P2 and start to broaden. This indicates that the signal at P3 is more chaotic than P2. As shown in Figs. 2(a)–2(b) and Figs. 3(a)–3(b), it can be concluded that MI occurred at P2, and the corresponding MI threshold power is approximately 459 W.
3.2 Laser performance after deuterium loading
To investigate the impact of deuterium loading on laser performance and on the MI threshold of the laser oscillator, YDF loaded with deuterium was also measured following two- and four-week loading periods. The experimental setup is shown in Fig. 1, and the same pristine fiber measurement conditions were used. The variation of the output power of the YDF which was loaded with deuterium over two- and four-week periods as a function of pump power are shown in Figs. 4(a)–4(c). As shown in Fig. 4(a), when the pump power exceeded ∼642 W, the output power of the pristine fiber started to roll over and yielded a roll-over behavior. Based on these observations, it was verified that MI occurred, as analyzed in section 3.1. The measurement for the YDF loaded with deuterium for two and four weeks had been carried out within 24 h after the onset of deuterium loading. When the pump power exceeded ∼642 W, the output power was still increasing in linear manner, and no obvious roll-over was observed. At the pump power level of ∼731 W, the maximum output power of the fiber which was loaded with deuterium over a two-week period was 533 W. If the pump power was increased further to values larger than ∼731 W, the output power also appeared to occur a roll-over behavior. The MI threshold was at the output power level of 533 W compared to the pristine fiber power level of 459 W. The MI threshold increase that was equal to 74 W exhibited an increase of 16.1%. For the fiber which was loaded with deuterium over a four week period, the output power was still increased linearly and no obvious roll-over was observed if the pump power was increased to values in excess of ∼731 W. The pump power increased to ∼825 W, and the maximum output power of the fiber loaded with deuterium over a period of four weeks was ∼622 W. If the pump power exceeded 825 W, the output power also appeared to occur a roll-over behavior. As analyzed above, the MI threshold of the fiber loaded with deuterium over a four week period was ∼622 W compared to pristine fiber. Moreover, the MI threshold power was raised by ∼163 W and thus exhibited an increase of 35.5%. Furthermore, the optical–optical efficiency of the fiber loaded with deuterium over two- and four-week periods before MI were 72.9% and 75.4% respectively, as respectively depicted in Fig. 4(b) and Fig. 4(c). For pristine fiber, the optical–optical efficiency before MI was 71.5%.
To pursue additional investigations of the mode instability properties of the post-processed fiber loaded with deuterium over two- and four-week periods, the photo-detector and the oscilloscope were used to measure the time and frequency domain signals of the post-processed fibers. As analyzed in section 3.1 and depicted in Figs. 5(a)–5(d), comparison of the signals at P1, P2, and P3, shows obvious and increased fluctuations in the time domain signals, as respectively depicted in Fig. 5(a) and Fig. 5(c). Furthermore, the Fourier transform has also been carried out to investigate the frequency component of the output signal of the fibers loaded with deuterium over two- and four-week periods. At P1, as shown in Fig. 5(b) and Fig. 5(d), only DC components appeared in the frequency spectra. When the oscillator operated in P2, higher frequency components appeared. At P3, a peak appears that comprises higher frequency components which have increased intensities compared to those at P2, and the frequency spectrum begins to broaden. This verifies the occurrence of MI.
Additionally, the longevity of deuterium loading on increased mode instability power threshold has also been measured. The first measurement on Yb-doped fiber with deuterium loaded for two and four weeks were applied within 24 hours respectively, and experimental results were depicted in Fig. 4(a). After first measurement within 24 hours, we conducted second and third measurements with 7 days and 14 days later, and experimental results were shown in Figs. 6(a)–6(b). As depicted in Fig. 6(a) and Fig. 6(b), mode instability power threshold of Yb-doped fiber loaded two weeks deuterium of measurements of 24 hours, 7 days and 14 days were 533W, 534W and 532W, and mode instability power threshold of Yb-doped fiber loaded four weeks deuterium of measurements of 24 hours, 7 days and 14 days were 622W, 627W and 625W, the cause of watt-level power fluctuation was due to system noise including mechanical vibration of cooling chiller. The experimental result shown that there was almost no change of laser performance of second and third measurements conducted with 7 and 14 days later of fiber loaded with deuterium for two and four weeks respectively. The experimental results indicated that the increase mode instability power threshold would not decay in 14 days. It can be concluded that deuterium loading does not have a negative impact on the laser efficiency of the YDF. This indicates that deuterium loading is useful and effective in the mitigation of the mode instability.
3.3 Hypothesis of mode instability mitigation mechanism with deuterium loading
It is commonly accepted by the scientific community that mode instability in high-power fiber lasers is mainly attributed to a thermally-induced refractive index grating. It is this thermally induced grating that results in mode-coupling in high-power fiber lasers. It has been reported that hydrogen/deuterium loading can be effective in enhancing photosensitivity in optical fibers [30–32]. In other words, the refractive index of fibers loaded with hydrogen/deuterium can be modulated by an external electric field in an easier manner. Antipov et al. have suggested that the thermally induced refractive index grating is not the only reason for the occurrence of mode instability. Instead, the outer electric field will also result in electronically induced refractive index grating, and the trends between the thermally induced refractive index grating and the electronically induced refractive index grating are opposite [23,33,34]. Mode instability mitigation may be explained by the fact that thermally induced refractive index grating is slightly weakened by the electronically induced refractive index grating owing to the photosensitivity enhancement of YDF due to deuterium loading.
Additionally, it has been reported that photodarkening can be suppressed by hydrogen loading [35,36,37]. Accordingly, photodarkening is another prominent heat source in active fibers that considerably degrades the MI threshold of the fiber lasers and amplifiers . Thus, the mitigation of mode instability may be explained by the fact that the deuterium loading could also suppress the photodarkening of the measured active fiber, and could thus increase the mode instability threshold power.
It has been experimentally proven that the mode instability mechanism is attributed to the thermally induced refractive index grating that results in mode-coupling in high-power fiber lasers. Thus, phase differences between mode interference and the thermally induced refractive index grating is a key factor for energy coupling between modes. Accordingly, active mitigation strategy based on the use of pump modulation has been proven useful [13,14]. However, enforcement of a passive mitigation strategy will need additional, in-depth studies. This constitutes ongoing work. Other related detrimental effects, such as Stimulated Raman Scattering and Photodarkening, and their effects and interdependence with mode instability still require further exploration. In view of these, we plan to continue to study mode instability mitigation strategies.
In conclusion, we have proposed an approach based on deuterium loading on YDF to suppress mode instability in high-power fiber lasers. Experimental results have revealed that deuterium loading on YDF over two- and four-week periods can increase the mode instability threshold power by more than 16% and 35%, respectively. The optical–optical efficiencies were 71.5% vs. 72.9% and 75.4%, before and after deuterium loading over two- and four-week periods, respectively. No laser efficiency deterioration was observed during the entire measurement process of deuterium-loaded YDF. The results have indicated that deuterium loading on YDF is an effective method for the suppression of mode instability and the achievement of further power scaling in high-power fiber lasers.
Ministry of Science and Technology of the People's Republic of China (2017YFB1104400); National Natural Science Foundation of China (61735007).
This work has been supported by The People’s Republic of China Ministry of Science and Technology, Grant No. (2017YFB1104400) and the National Natural Science Foundation of China, Grant No. (61735007)
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