Abstract
1 Introduction
For the advantage of high slope efficiency, good beam quality, superior thermal management property and compactness, fiber lasers are widely used in the field of laser marking, material processing, medicine, communication and many other industrial applications[
Compared with traditional fiber laser oscillator, the main oscillator power amplifier (MOPA) system, which can boost the progress of fiber laser power scaling, offers an effective way to acquire high power fiber laser source with excellent beam quality by employing cascaded structure. However, further power scaling of MOPA system requires a considerable effort to suppress nonlinearities, which usually has contradiction in fiber type selection with the requirement of good beam quality. Compared to uniform LMA active fiber, T-DCF shows numerous unique advantages when being employed as gain medium of optical amplifier, such as larger mode area, higher pump absorption, suppression to nonlinear effects, maintaining good beam quality and so on[
In this manuscript, we have constructed an all-fiberized fiber amplifier, which is based on a piece of ytterbium-doped T-DCF. The fiber amplifier was operated at 1080 nm wavelength under CW regime. The maximum output power of the system reached 1.47 kW, which, to the best of our knowledge, is the record power of long tapered fiber based fiber laser system. The factor of the amplifier at 1.39 kW output power is . Our result successfully verified the potential of power scalability and all-fiberized capability of long tapered fiber, and the performance of our system can be further enhanced by fiber design optimization.
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2 Experimental setup
The schematic diagram of the long tapered fiber amplifier, with all-fiber connections fusion spliced, is shown in Figure
As shown in Figure
The gain fiber of amplifier stage is a piece of ytterbium-doped long tapered fiber, which is 33 m long with core/clad diameters of and at narrow and wide end, respectively, and the core/clad NA are 0.06/0.46. The narrow end of T-DCF is connected with the output port of the combiner, which means both the seed power and pump power will be injected from the narrow end. The nominal total absorption coefficient to 976 nm pump of the fiber is . The wide end of T-DCF is spliced to a piece of passive fiber, and the status of splicing point is shown in Figure
In order to lead out residual pump laser, the cladding modes are stripped on the passive fiber. Finally, the passive fiber is connected to an endcap to eliminate probable harmful feedbacks at the output facet. The pump sources of the amplifier are six pigtailed LDs, each of which can generate pump power at 976 nm wavelength, and they are coupled into the amplifier through the pump ports of the combiner.
After the endcap, we put power meter, optical spectrum analyzer (OSA) and photoelectric detector (PD) to record power, optical spectrum and time domain data, respectively. The bandwidths of both PD and oscilloscope are 1 GHz in our experiment. In the time trace measurement process, the PD with a pinhole of 1.5 mm diameter is placed in the center of the collimated beam to monitor the onset of transverse modal instability (TMI). From previous researches, we can know that the identification of the presence of TMI is the time domain fluctuation with frequency less than 5 kHz[
3 Results and discussion
Using the experimental setup which we mentioned above, the output characteristics of the amplifier are carefully investigated. Figure
From Figure
The spectrum of the output laser of the amplifier is measured using an OSA with 0.05 nm resolution. Figure
From Figure
In order to investigate the characteristics of TMI, we also measure the time domain data of the amplifier, which is plotted in Figure
Figure
To further discuss TMI optimization of T-DCF, we have to define some parameters that we use to describe tapered fibers. We define the core radius of the narrow end and wide end of the tapered fiber as and , respectively. Assume that the total length of tapered fiber is , so an average tapering angle could be defined as
In the theoretical research of TMI in fiber amplifier, the growth of HOM power ratio is a key parameter in the onset judgment of TMI threshold and it could be calculated using the following equation[
Second, better guided HOM will cause another affection that modal distribution of HOM will be better overlapped with fiber core, which means the nonlinear coupling coefficient in thicker fiber will also be bigger than that in thinner fiber. In order to verify this conclusion theoretically, we have simulated TMI threshold variation with change of parabolic shaping factor using Equation (
From all of the above, we can conclude two possible solutions for TMI issue in T-DCF fiber laser system. First, T-DCF with smaller average core size will have a better performance in suppression of TMI. The design of the fiber we use in this case is . Fabricating a piece of concave fiber () which can be used in fiber amplifier with better performance in TMI is the next step of our research on T-DCF. Second, we can use tandem pumping scheme in order to reduce thermal load in gain fiber. The physical origin of TMI is the heat load that generated inside the doped area of gain fiber in amplifying procedure, and quantum defect (QD) is the main heat source among all thermal effects. Because of that, the pumping wavelength is close to signal wavelength, and QD in tandem pumping scheme is much lower than that in LD pumping scheme. Lower QD lead to lower overall thermal load in gain fiber, so that the threshold of TMI can be effectively improved in tandem pumping configuration.
4 Conclusion
In summary, we have constructed an all-fiberized fiber amplifier which is based on a piece of ytterbium-doped T-DCF. The seed laser of the whole system is a home-made fiber laser oscillator which can generate CW seed laser. The fiber amplifier is operated under CW regime. The maximum output power of the system reached 1.47 kW. To the best of our knowledge, it is the highest output power of long tapered fiber based fiber laser system. Our result successfully verifies the potential of power scalability and all-fiberized capability of long tapered fiber, and the performance of our system can be further enhanced by fiber design optimization.
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