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Abstract
The laser performance of a high-power ytterbium-doped fiber amplifier is mainly hindered by the onset of mode instability. In this work, the slope efficiency and mode instability threshold of the ytterbium-doped fiber under various gamma-ray radiation doses have been measured. Experimental results reveal that gamma-ray radiation-induced photodarkening degrades mode instability severely, and gamma-ray radiation-induced mode instability degradation can be partly bleached by hours of pump-light injection. It is shown that gamma-ray radiation-induced photodarkening results in a steep reduction of slope efficiency and mode instability threshold; moreover, the entire irradiated fiber can be partly bleached by hours of pump-light injection and exhibits both time and gamma-ray radiation-dose saturation properties. The experimental results indicate that mode instability mitigation can be partly realized by pump-light injection and implies photodarkening suppression is beneficial for TMI mitigation, which is very promising for the advancement of high-power fiber lasers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the past few decades, fiber lasers have developed rapidly and shown expeditious growth compared to its counterpart—traditional diode-pumped solid state laser (DPSSL). Fiber lasers offer several advantages over DPSSL, such as excellent thermal management, high conversion efficiency, compact structure, and are nearly maintenance-free. Moreover, several crucial technologies have emerged and are well developed, such as fabrication of double-clad active fiber, especially, ytterbium-doped active fiber; cladding pump technology; and high power, high-brightness pump laser diodes [1]. For continuous-wave (CW) operation, output power emitted by a single large-mode-area (LMA) fiber has reached 10 kW level [1,2]. For pulsed operation, the output power of ultra-short pulsed fiber lasers has increased to approximately kilowatt level [3]. However, such a high power level has persisted for several years and has been hindered by the recently discovered phenomenon of mode instability (MI) [3, 4]. MI will occur if the output power of the fiber laser reaches a certain power threshold; the output beam profile will fluctuate randomly and beam quality will suffer from sudden degradation [4]. MI has limited further application of high-power fiber lasers and enhancement of power amplification. Even though it is acceptable for some specific applications, which do not require near-diffraction-limit beam quality, such beam quality fluctuation and sudden degradation are not desirable. Thus, interest in studies on mode instability is increasing among researchers worldwide [4–20].
Scientific communities worldwide have gained extensive theoretical and experimental understanding in relation to MI [4–20]. Nevertheless, the exact physical origin of MI remains unknown [4]. In spite of this, it is commonly acknowledged that the thermally induced refractive index grating is mainly attributable to the mode coupling between the fundamental mode and higher-order mode [6,10,18].The occurrence of MI is mainly attributed to the heat source in an active fiber; quantum defect was known as the only heat source initially [13,15]. Moreover, as the understanding of the phenomenon is advancing, Otto et al. have reported that the photodarkening effect has significant impact on mode instability and could be the second heat source in the active fiber [20, 21]. This implies that defects induced by other physical factors (light of specific wavelength, radiation, etc.) could introduce extra optical losses in the active fiber and degrade the MI properties of the fiber amplifier. However, by measuring the thermal load of the fiber amplifier and comparing it with detailed simulations, it has been shown that in the case of their experiment configuration and fiber with specific structural parameter the photodarkening effect has a negligible impact on thermal load, and thereby, on the mode instability threshold of the fiber [22]. Thus, it is meaningful to research the connection between MI and photodarkening, which will provide promising improvements in power stability and mode stability of high-power fiber amplifiers. Also, applications such as optical link, LIDAR in satellites or spacecrafts, high energy irradiation can be hazardous to fiber laser systems. High-energy particles, such as gamma rays and X-rays, induce a color center (CC) or absorption center, which will causes radiation-induced photodarkening [23–25]. The excitation mechanism of carriers in radiation-induced photodarkening is different from that in pump-light-induced photodarkening. Nevertheless, both radiation-induced photodarkening and pump-light-induced photodarkening will induce CC and degrade the performance of active optical fibers [26, 27]. To date, studies on the impact of gamma-ray radiation-induced photodarkening on the MI of ytterbium-doped fiber amplifier has rarely been reported. In this manuscript, we propose an experimental study on the impact of gamma-ray radiation-induced photodarkening on the MI of Yb-doped fiber amplifier. We employed a 20/400 step-index Yb-doped all-fiber master oscillator power-amplifier (MOPA) pumped by 976-nm laser diodes operating at 1080 nm and investigated its slope efficiency and MI threshold after exposure to total gamma-ray radiation doses of 100 Gy, 255 Gy, and 395 Gy. In addition, the bleaching process of total gamma-ray radiation doses of 100 Gy, 255 Gy, and 395 Gy is also discussed. Experimental results revealed that gamma-ray radiation could severely degrade slope efficiency and MI threshold. Additionally, all gamma-ray radiation doses proposed in the manuscript could be partly pump-bleached. After bleaching, the MI threshold exhibited different levels of recovery, time and radiation-dose saturation effect. Besides, the process of pump bleaching and MI threshold recovery is also discussed.
2. Experimental setup
The active YDF applied in the experiment had a double-clad, step-index refractive index profile with a core diameter of 20 µm and octagonal-shaped inner cladding diameter of 400 µm, and all active ytterbium-doped fibers (YDFs) used in the experiment were fabricated with the traditional modified chemical vapor deposition method (Fig. 1). 60Co was used in the experiment as a gamma-ray radiation source with a radioactivity of 9.99*1015 Bq and energy of 1.33 MeV. During the gamma-ray radiation and test process, the active YDF was coiled and immersed in water on a moveable platform, and then the spooled active YDF and Ag2Cr2O7 dosimeter vertically mounted on a plate were put into a radiation chamber right in front of the 60Co gamma-ray radiation source [28]. The amount of gamma-ray radiation dose could be read from the Ag2Cr2O7 dosimeter. In this experiment, different gamma-ray radiation doses were set at 100 Gy, 255 Gy, and 395 Gy, and the average gamma-ray radiation rate was approximately 250 Gy/h. After the gamma-ray irradiation process, we applied the all-fiber MOPA structure containing a CW seed laser, 976-nm laser pump diode, and fiber Bragg grating (FBG) at 1080 nm and radiation-exposed active YDF mentioned above operating at 1080 nm to investigate its laser properties. At the output end of the active YDF, a cladding light stripper (CLS) was spliced to YDF to strip the cladding light of the amplifier; an end-cap is attached to the output end of the CLS to reduce backward reflection and protect amplifier from being damaged by backward light reflection. The main amplifier and CLS are mounted on a heat sink, which is cooled by circulating cooling water. The output time-domain signal of the amplifier is monitored by a photo detector and oscilloscope to observe the onset of MI [16]. All experiments were carried out at room temperature within 24 h to avoid noticeable radiation-induced absorption relaxation.
Fig. 1 Experimental Setup of all-fiber MOPA pumped by 976-nm LDs, LD: Laser Diode; FBG: Fiber Bragg Grating; CLS: Cladding Light Stripper; PD: Photo Detector.
3. Experimental results and discussion
3.1 Laser performance of 100-Gy gamma-ray-irradiated ytterbium-doped fiber
Using the experimental setup of MOPA mentioned above, we first investigated the laser properties of active YDF exposed to 100-Gy gamma-ray radiation dose. The correlation between the output power and launched pump power is shown in Fig. 2. When the launched pump power reached 412 W, the output power reached 170.5 W and the corresponding efficiency was 41.4%; the output power and corresponding efficiency of pristine fiber were 367.6 W and 74.7%, respectively. When the pump power was further increased, the output power of the 100-Gy fiber reading on the power meter exhibited serious fluctuations. The output power started to roll over at P1, as shown in Fig. 2. Furthermore, rapid temperature increase in the CLS region was also observed, which indicated that the stripped cladding light would increase steeply at the roll-over point. As for the explanation of the phenomenon, we believe that the power fluctuation is mainly attributable to MI. As shown in Fig. 2, the standard deviation at the roll-over point is relatively low (as P1 shown in Fig. 2). Nevertheless, when the pump power was further increased and exceeded P1, the output power dropped distinctly and the corresponding standard deviation was observed to rise apparently, as shown in P2 and P3 in Fig. 2. This fact suggested that the output signal underwent serious fluctuations at P2 and P3. For further investigation of the power roll-over, a photo-detector and oscilloscope were utilized to monitor the temporal dynamics of the output signal. When the laser amplifier was operated at P1, the signal on the oscillator remained stable and no obvious fluctuation was observed. Nevertheless, if the roll-over point was exceeded and the laser amplifier operated at P2 and P3, apparent fluctuation was observed in the time domain signal, as shown in Fig. 3, which meant that the output signal became unstable. By applying Fourier transform to the time domain, as shown in Fig. 4, we acquired the Fourier spectrum of P1–P3. It is clear that when the laser was operated at P1, the main frequency components were relatively low; as the pump power further increased to P2 and P3, the output became more unstable and more higher-frequency components were revealed, and their peak intensity was stronger than P1. By analyzing the laser performance and temporal dynamic of the output signal, it was verified that MI indeed occurred around the roll-over point. It can be concluded that gamma-ray radiation-induced photodarkening has a significant impact on MI, in terms of not only the output laser power but also MI threshold.
Fig. 2 Correlation between output laser power and pump power of 100-Gy-irradiated active YDF and standard deviation and pristine fiber (0Gy).
As mentioned above, the MI threshold power is merely ~170 W after exposure to a total gamma-ray radiation dose of 100 Gy. To investigate the impact of radiation-induced photodarkening on mode instability in detail, we conducted a long-time operation experiment on the laser amplifier using the experimental setup as described earlier. The laser amplifier was kept operational for a long time at constant launched pump power (~260 W), and its laser performance was tested. Threshold power versus operation time is depicted in Fig. 5. At the beginning of the operation, the MI threshold was relatively low and increased rapidly over time; however, as the laser amplifier operated over a period, the threshold power increased at a slower rate, which indicates that the process gradually saturated. This trend indicates that photodarkening behavior has an impact on MI. It is known that there is an equilibrium state between the generation and annihilation of the CC in Yb-doped fiber [29], and if the generation rate of the CC exceeds the annihilation rate, the CC will accumulate in the fiber and the fiber will darken more. In addition, the theoretical model reveals that the correlation between photodarkening loss and time is based on a stretched exponential function, which indicates that photodarkening loss will increase steeply at the start of operation and eventually increase at a slower rate, and then finally become saturated. As depicted in Fig. 5, the measurement process begins with a relatively low threshold and increases steeply first, then eventually increases at a slower rate, and finally becomes saturated, which is in accordance with the theoretical model. Otto et al. [20] proposed that pre-darkened active fibers produced vast amount of CCs, which resulted in relatively low MI threshold power. As the laser amplifier was operated, annihilated CCs exceeded the generated CCs, which implied that the MI threshold power would increase as CCs annihilated. It can be concluded that irradiated Yb-doped fiber can be partly bleached by long-time operation and the bleaching process will be saturated as time passes by. This fact indicates that gamma-ray radiation-induced photodarkening has a significant impact on MI.
3.2 Impact of multi-gamma-ray radiation doses on laser performance and annealing of MI
After the laser performance measurement of the 100-Gy active fiber, we further carried out experiments on testing the active fiber at 255 Gy and 395 Gy. In summary, after gamma-ray irradiation of 100 Gy, 255 Gy, and 395 Gy, the maximum laser output power of the post-processed fiber was 170.5 W, 74.9, W and 69 W, respectively. The laser amplifier was kept operational for 465 min and launched power was kept constant (~260 W). After the long-time operation, the maximum laser output powers of the bleached 100-Gy, 255-Gy, and 395-Gy active fiber were 379.3 W, 192.3 W, and 194.4 W, respectively. The optical-optical efficiencies of the post-processed fiber and bleached fiber were 41.4%, 22.4%, 23.3% and 62.2%, 46.7%, 46.2%, respectively. All post-processed and bleached active fibers exhibited power roll-over if the launched pump power was further increased when in the maximum output power mode. As mentioned above, further power scaling is limited by MI, and the corresponding maximum output power can be regarded as the MI threshold. As depicted in Fig. 6 and Fig. 7, compared to the active fiber at 100 Gy, the optical-optical efficiency and mode instability threshold at 255 Gy was much lower than that at 100 Gy; however, for the active fiber at 395 Gy, there was almost no difference in optical efficiency and MI threshold between 255 Gy and 395 Gy. After 465 min of operation, the active fibers at 100 Gy, 255 Gy, and 395 Gy exhibited varying degrees of bleaching, optical efficiency, and MI threshold apparently showed clear improvement. Furthermore, the active fiber with higher gamma-ray radiation dose could be bleached to give even better laser performance than pristine lower irradiated fiber. As analyzed in part 3.1 and depicted in Fig. 6 and Fig. 8, different doses for the active fiber show similar trends, which are first steeper and then become gentle during the bleaching process, which means that the bleaching process are revealing saturation properties at a given darkening level. Furthermore, along with the increase in gamma-ray radiation dose from 100 Gy to 255 Gy, the laser performance and MI threshold suffered more severe degradation; however, when the gamma-ray radiation dose increased from 255 Gy to 395 Gy, laser performance and MI threshold evolution remained almost unchanged. From the experimental results described in this section, it can be seen that the impact of radiation-induced photodarkening on MI is a complex process. On one hand, at a relatively low darkening level, the MI threshold will degrade severely as the darkening level increases. On the other hand, there is an equilibrium state for a given active fiber with fixed doping and structural parameters, and radiation-induced darkening loss will not increase unlimitedly at limited gamma-ray radiation doses. At a relatively high radiation doses, MI will not be so sensitive to the darkening level, and gamma-ray radiation induced darkening will not have a decisive impact on the MI threshold.
Fig. 6 Output laser power versus launched pump power at different gamma-ray radiation doses before and after bleaching.
Fig. 8 Normalized mode Instability threshold versus operation time at different levels of gamma-ray radiation doses.
MI and its exact mechanism still remain unknown and need further deeper investigation. More precise and detailed research will be part of our ongoing plan, and future work will mainly involve the correlation between a variety of photodarkening processes and MI, even quasi-static MI, and the impact of MI properties on photodarkening and other fiber-structure-related mitigation strategies. We will further take measures on revealing the detailed correlation between MI and photodarkening, and mitigation strategies.
4. Conclusion
In conclusion, we reported an experiment on active fibers under different radiation doses and their relation to MI properties. Radiation-induced photodarkening will severely deteriorate laser performance and MI, and can lead to partial bleaching. Slope efficiency of darkened fibers at 100 Gy, 255 Gy, and 395 Gy decreased by 36.33%, 55.29%, and 54.40%, and increased by 20.8%, 24.25%, and 22.90%, respectively, after 465 min of bleaching. Furthermore, the impact of radiation-induced photodarkening on MI is a complicated process, where on one hand, compared to pristine fiber, fibers exposed to various radiation doses degrade severely and show time-dependent saturation behavior, and on the other hand, once radiation dose increases to a certain level, laser performance and MI threshold also reveal darkening level saturation behavior. Experimental results indicate that radiation-induced photodarkening has a complicated impact on MI and shows potential in mitigating MI by photodarkening mitigation.
Funding
National Key R&D Program of China (2017YFB1104400); National Natural Science Foundation of China (Grant No. 61735007)
Acknowledgment
This work has been supported by The People’s Republic of China ministry of science and technology, Grant No. (2017YFB1104400) and National Natural Science Foundation of China, Grant No. (61735007)
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