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We report on the strong photo-bleaching of the photo-darkening (PD) induced loss under the cladding pump of a 793 nm laser diode (LD) in double clad Yb-doped fibers. Up to 68% PD loss at 810 nm was bleached. The bleaching rates under different powers show that the higher pump power corresponds to the more bleached loss within the same time. Moreover, repeatable processes of PD and photo-bleaching were observed when alternately pumped with 915 and 793 nm LDs. Furthermore, it was found that simultaneously pumping the fiber with 915 and 793 nm LDs can suppress about 80% PD loss, compared with pumping the fiber only with 915 nm LD. The mechanism of photo-bleaching under 793 nm pump was also discussed.
© 2015 Optical Society of America
1. Introduction
In recent years, ytterbium (Yb) -doped fiber lasers and amplifiers have been widely used in the fields [1, 2 ] of industry, medicine, and military for their high reliability, low cost, high beam quality, etc. However, a phenomenon called pump-induced photo-darkening (PD) effect [3] was observed in Yb-doped fibers with the laser power increasing [4–8 ] and seriously deteriorated the performance of fiber lasers or amplifiers. This effect is commonly manifested as a time-dependent broadband absorption [9, 10 ]. The PD induced absorption in the visible wavelength range with long tails stretching into near infrared range results in an increased background loss at pump and lasing wavelength. However, the mechanism of PD is still unclear. Different theories have been proposed to explain the PD process, including the formation of divalent ytterbium [11, 12 ], color center [13] and Oxygen Deficiency Center (ODC) defects in Al-silicate fibers where oxygen bonds are modified [14]. The PD effect reduces the efficiency and the lifetime of Yb-doped fiber lasers, leads to instability of the system, and finally greatly limits the applications and development of fiber lasers.
Significant efforts have been devoted to revealing PD mechanism and mitigation of PD effect in optical fibers. The common methods of mitigating PD effect include photo-bleaching (UV, 543 nm, 633 nm) [15–17 ], H2 [18] or O2 loading [14], and co-doping with ions (Al, Ce, and P) [19, 20 ]. The thermal-bleaching method should be carried out at high temperature (>300 °C) and will lead to fiber properties degradation. The method of H2 or O2 loading is very complicated to operate. Regarding to the ions co-doping, it could lead to changes in the manufacture process and on refractive index, which will bring adverse effects on guiding properties, as well as on the shape of absorption spectrum [21]. Therefore, photo-bleaching is convinced to be a preferable route to decrease the PD loss in Yb-doped fibers. However, the power of reported bleaching wavelengths (405 nm, 543 nm, 633 nm, etc) are in mW scale, which is unsuitable to apply in double clad Yb-doped high power fiber lasers.
In this work, we demonstrated the bleaching of a photo-darkened double clad Yb-doped fiber under 793 nm pump. The measurements showed that over 68% PD loss was mitigated. It was also found that within the same bleaching time, more PD loss was bleached with the power increasing of 793 nm laser diode (LD). Furthermore, the fiber shows repeatable processes of PD and photo-bleaching when pumped alternately with 915 and 793 nm LDs. Besides, simultaneous pump with 915 and 793 nm LDs also effectively suppressed the PD loss. Compared to LDs with other bleaching wavelengths, the commercial 793 nm LDs are economical pump sources with higher power and various pigtail sizes, and more suitable for the photo-bleaching of PD in double-clad Yb-doped high power fiber lasers.
2. Experimental setup
In this work, a double-clad Yb/Al co-doped silica fiber was fabricated by the modified chemical vapor deposition (MCVD) combined with the solution doping technique. The Yb3+ and Al3+ concentrations are about 0.14 at% and 0.47 at%, respectively. The core and cladding of the fiber are 10 μm and 130 μm in diameter, respectively. The numerical aperture (NA) of the fiber core is calculated to be about 0.074.
Figure 1 shows the experimental setup to measure the fiber spectral properties before and after pump with a LD (including 915 and 793 nm). A standard white-light signal source (lamp) was coupled into a single mode fiber, where the monochromator and filter were used to ensure monochromatic light output. The single mode fiber was then spliced with one of arms of a (2 + 1) × 1 commercially available combiner, whose other two arms were spliced with the pigtailed 915 and 793 nm LDs, respectively. A 10 cm fiber sample was spliced between the output end of the combiner and a 2 m single mode fiber. Besides, a mode-stripper stage was applied to remove the light propagating in the cladding. About 95% of the launched cladding power can be stripped by index matching gel in the stage. The residual cladding light can be completely removed in the following 2 m single mode fiber by the high index acrylate coating. To avoid the heat accumulation, a water-cooling system with 18 °C was adopted for the mode-stripper. The spectra were measured by a low-noise receiver and a lock-in amplifier. The launched pump power of the 915 nm LD was always kept 5 W to provide an inversion level at about 43%.
Fig. 1 Schematic of the experimental setup (LD-laser diode, SMF-single mode fiber, MMF-multi mode fiber, YDF-Yb-doped fiber)
3. Results and discussion
Several of experiments were conducted to investigate the 793 nm LD induced photo-bleaching effects. Firstly, the 793 nm LD was used to pump the darkened fiber as long as possible and the time-dependent bleaching curve was recorded. Secondly, the relationship between the bleaching rate and the pump power was studied. Then the 915 nm LD and the 793 nm LD were used to pump the Yb-doped fiber simultaneously. At last, the mechanism of this effect was discussed.
3.1 Photo-bleaching of PD under 793 nm LD pump
We used the 915 nm LD to pump the pristine Yb-doped fiber for 600 minutes. Then the 915 nm LD was switched off and the 793 nm LD was turned on to pump the photo-darkened fiber. The operation power of 793 nm LD was fixed to 5 W. Figure 2(a) shows the absorption spectra of the pristine fiber, photo-darkened fiber and photo-bleached fiber. The photo-darkened fiber presented a recovery when pumped with the 793 nm LD for 60 minutes. Figure 2(b) depicts the PD induced time-dependent loss curve at 633nm, 702nm, and 810nm, respectively. It is shown from Fig. 2(b) that the PD induced loss of the fiber increased during the pump of the 915 nm LD, and then apparently recovered when pumped with the 793 nm light. The 793 nm LD was kept operating to explore the bleaching efficiency until the loss nearly reached an equilibration. The measurements show that the fastest photo-bleaching occurs during the first 20 minutes; then the bleaching rate began to slow down with time; finally the bleaching got to an equilibration after pumped with the 793 nm LD for 1600 minutes. The bleached proportions of PD loss at wavelengths of 633 nm, 702 nm, and 810 nm are 55%, 58%, and 68%, respectively. The bleaching curve indicated that more PD loss could be bleached when keeping the 793 nm LD running for enough time.
Fig. 2 (a) Absorption spectra of the fiber. The inset displays an enlargement of the spectra between 650 nm and 850 nm; (b) evolution of photo-darkening under 915 nm pump and photo-bleaching under 793 nm pump
To provide more insight on the process of photo-bleaching, the 915 nm LD and the 793 nm LD were used to pump the fiber alternately. The power of the 793 nm LD was fixed to 3 W. Figure 2(b) shows the curve of the time-dependent kinetic process. It is clear that the PD induced loss increased or decreased with the 915 nm LD on or 793 nm LD on. This interesting phenomenon strongly demonstrates that the 793 nm light was able to bleach the PD induced loss repeatedly.
3.2 Photo-bleaching efficiency under different power of 793 nm LD
A series of experiments were conducted to investigate the relationship between the bleaching rate and pump power. Figure 3(a) depicts the bleaching dependence at 810 nm on the power of the 793 nm LD. The initial darkened losses of the fibers were kept same. The launched power of the 793 nm LD was 3W, 5W, 7W, 8W, and 9W, respectively. It is found that the PD induced loss decreases more quickly with the power increasing. It indicates the bleaching process can be accelerated with higher power LDs and will get to an equilibrium state in a shorter time.
Fig. 3 (a)Photo-darkening and bleaching evolution under different 793 nm pump power at 810 nm; (b) Bleached loss as the function of laser power of 793 nm LD
Figure 3(b) shows the relationship between the bleached loss and the 793 nm LD pump power within 400 minutes. There was a significant positive linear dependence when the 793 nm power is no more than about 7W and the fitting coefficient R2 is up to 0.987. When it was over this power, the bleaching process slowed down and the bleached loss didn’t increase linearly with the pump power. A tendency to saturation was observed. It can be predicted that the fiber will not be completely bleached even when the 793 nm power is high enough. There will still be some PD loss that cannot be bleached.
3.3 Simultaneous pump of 915 nm and 793 nm LDs
Figure 4 shows the PD loss evolution under pump of 915 nm and simultaneous pump of the 915 and 793 nm LDs, respectively. Both the powers of the two LDs are fixed to 5 W. The measurement data were fitted according to standard stretched exponential function [9] as follows:
Where α(t) is the induced loss on period of t under pump, α is the loss at the final equilibrium state, τ is the time constant, and β is the stretching parameter ranging between 0 and 1. The measurements at four wavelengths (633 nm, 702 nm, 810 nm, and 1041 nm) show that the PD loss is greatly suppressed when the fiber was under simultaneous pump of the 915 and 793 nm LDs. It is suggested that a loss mitigation of about 80% will be reached when bleaching the fiber with the 793 nm LD. The efficient suppression of PD loss shows that on-line PD loss compensation can be achieved by injecting 793 nm into an operating fiber laser device, offering another effective way to mitigate impact of PD on laser performance.Fig. 4 PD induced loss evolution at (a) 633 nm, (b) 702 nm, (c) 810 nm and (d) 1041 nm. Curves are plotted for different wavelengths.
3.4 Hypothesis of 793 nm LD photo-bleaching mechanism
To investigate the self-bleaching effect, a pristine fiber was pumped with the 915 nm LD for 400 minutes and the absorption spectrum was recorded. It was also recorded when the 915 nm pump LD was turned off for 16 hours. The subtraction value (the latter one minus the former one) of loss between the two spectra was shown in Fig. 5 . There was a slight increase of loss for the Yb-doped fiber, indicating that no self-bleaching exists in the fiber. The same phenomenon was also reported in [22].
Fig. 5 The subtraction value of loss spectrum between the photo-darkened fiber and the same fiber after about 16 hours
To eliminate the possibility of thermal bleaching, a thermal camera was used to detect the surface temperature of the Yb-doped fiber during the 793 nm LD pump. The thermal camera was put at a fixed distance and the initial surface temperature of darkened YDF was the same as room temperature 21 °C. The camera was kept operating during the whole bleaching process of 400 minutes. The temperature of the whole 10 cm Yb-doped fiber remains under 25 °C during the bleaching process, indicating that no heat accumulation existed in the fiber core and thermal bleaching can be excluded in the experiments.
In [16, 17 ], it is suggested that the PD loss reduction may be related with local charge rearrangement described as Yb2+→Yb3+. Regarding to the photo-bleaching phenomenon, one could expect that a similar mechanism may take place with 793 nm pumping. In [23], R. Peretti proposed that the traces of Tm3+ (ppb by weight) in the Yb-doped fiber strongly impact the defects creation process involved in PD. Thus we don’t exclude the possibility of Tm-related defects recovery when pumped with 793 nm. There may be some other color centers created by other mechanisms which have been stated in the introduction part and may be responsible for the residual unbleachable losses in the photo-darkened Yb-doped fiber. However, there are still many confusing on the photo-bleaching effect from 793 nm pumping, and we will go on with further investigation to pursue the mechanism.
The results above show that the 793 nm LD will be an excellent candidate for PD mitigation. It can be spliced to fiber lasers and effectively bleach the PD induced loss. Compared to other bleaching sources with power limited in watt scale, 793 nm LDs are more preferable due to their commercial feasibility and higher power over hundred of watts.
4. Conclusion
In conclusion, the photo-bleaching effect by use of the 793 nm LD in photo-darkened Yb-doped fiber was studied. The bleaching efficiency, repeatable time-dependence kinetic processes of PD and photo-bleaching, and on line 793 nm photo-bleaching processes have been investigated. The measurements show that over 68% PD loss could be bleached under the pump of the 793 nm LD. It is also found that the higher 793 nm LD pump power leads to the faster bleaching rate. The higher output power of these kinds of LDs makes them suitable to mitigate the PD loss in double-clad Yb-doped high power fiber lasers. All the above indicates that the 793 nm LDs will play very important roles in power stability and recovery of Yb-doped fiber lasers.
Acknowledgment
This work was financially supported by the National High-Technology Research and Development Program of China (Grant No. 2013AA031501).
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