High-Power 125-μm-Optical-Fiber Cladding Light Stripper

 

Aleshkina S. S.1, Kochergina T. A.1, Bobkov K. K.1, Bubnov M. M.1, Park J.2* , Likhachev M. E.1**

1. FiberOpticsResearchCenteroftheRussianAcademyofSciences,Moscow119333,RussianFederation 2. FOSPIA Co.,Ltd, 53, Jiwon-ro, Danwon-gu, Ansan-si, Gyeonggi-do 15619, Korea

* e-mail: jwpark@fospia.com ** e-mail: likhachev@fo.gpi.ru,

 

Abstract: An improved cladding-light-stripper design is developed. Continuous operation of it

has been demonstrated during 170 hours at power injured in the fiber with outer diameter of 125

μm as high as 60 W. OCIScodes:060.2340;230.2285.

 

1. Introduction

Utilization of unabsorbed pump power is a significant problem of many fiber lasers designs. In some cases power of unwanted light in the cladding could exceed tens and even hundreds of Watts. To preserve monolithic all-fiber laser construction a fiber cladding light stripper (CLS) must be integrated in the schemes.

By now there are three different approaches for removing high average light power from the fiber cladding. The best results were demonstrated with method based on fiber cladding etching and creation of scattering centers on the fiber surface [1, 2]. Operation at more than 500 W was demonstrated with such CLS design. The main disadvantage of this method is strong decreasing of fiber mechanical reliability due to induced defects: the fiber used in [1] was destroyed by bending on a spool with diameter of about 150-160 mm. Small stresses during package of such fibers would strongly deteriorate long-term mechanical reliability of the CLS. The second method is free of this problem and based on coating of bare fiber with a high thermal conductivity absorbing layer (for example, some metal). Up to 150 W was successfully removed from the fiber cladding due to this technique [3]. A third approach is based on jacketing of the fiber by various transparent materials (low-melting glass [4], the polymer coatings [5]) with a refractive index higher than refractive index of the silica glass. In this case it is possible to remove about 100 W of the cladding power.

It should be emphasizing that the CLSs discussed above were fabricated on the base of fibers with a large outer silica cladding (200 μm for the case of [4] and larger than 400 μm for the case of [1-3, 5]). This choice is evident as the optical load on the fiber surface decreases as square of fiber cladding diameter (Figure 1a). The optical load inside CLS in [1-3, 5] did not exceed 320 W/mm2. In fiber laser schemes based on 125 μm fibers utilization a CLS based on fibers with outer diameter larger than 150 μm is unreasonable because of it is not compatible with standard equipment (fusion splicer, fiber cleaver, etc). At the same time, utilization of 125 μm fibers would increase thermal stress on the fiber surface by an order of magnitude as compared to the 400 μm fibers. It means that CLS designs would not allow operating at more than 20-40 W power in the cladding. For example, our experience shows that CLS on the basis of aluminum-coated 125 μm fibers is burned even at 20-30 Watts cladding power.

In the present paper an improved design of CLS is proposed and the power of 60 W was successfully removed from the fiber cladding with outer diameter of 125 μm. Long-term reliability of the device was tested for 170 hours of continuous operation. The optical load on the fiber surface exceeded of 630 W/mm2 in this case, which is to the best of our knowledge, is the highest ever reported value.

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2.CLS design

Design of the developed CLS is depicted in Figure 1b. The method similar to [5] based on evacuation of cladding light to the high-refractive index polymer was used. It is usual demand to CLS in high power lasers to use large mode area single-mode fibers that are inherently highly bend-sensitive. Moreover to reveal package loss in the single-mode core a 20/125 μm double-clad fiber (core NA = 0.08) was used. Small fiber length (about 4 cm) was cleared from low-index polymer coating and the bare fiber was placed on a light transparent heatsink. The fiber was glued to heatsink with a high refractive index polymer. The best results were achieved by utilization of high optical transparent high temperature epoxy resin cured with anti-hybrid hardener in the presence of special catalyst developed by Saint-Petersburg State Institute of Technology Laboratory of Polymer's Physics. It must be noted that primary low-refractive index coating near the pump stripping point is also subject of high thermal load and its properties strongly influence on CLS maximum operating power. In our experiments we tested several fibers with different polymer coatings: standard commercially available acrylate coating with outer diameter of 250 μm and 400 μm and fiber coated with a specially developed high temperature resistance acrylate coating (Fospia Co.,Ltd XPC-373 [6], designed to operate at temperatures above 100°C) with outer diameter of 250 μm. The numerical aperture of the fibers second cladding exceeded of 0.45. Excess core loss in the CLS was controlled at wavelength of 1.55 μm and it did not exceed 0.1-0.2 dB.

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3.Experiment

Irradiation from two semiconductor diodes (pigtailed with 105/125 μm, NA=0.15 fiber) with total power of 60 W was coupled into the developed CLS with a help of 2+1Æ 1 pump combiner. The NA of radiation at input of CLS was about 0.45. The CLS was placed to aluminum heat dissipater with cooling fan. During the CLSs test the power of cladding light was increased every several hours by 5 W.

It was found that the CLS based on 400 μm fiber with standard acrylate coating was burned at light power in the cladding as low as 20-35 W (according to data of 5 samples). The CLS based on fiber with outer diameter of 250 μm was burned at pump power of 40-50 W (according to data of 7 samples). Most likely that this difference in maximum strip power is associated with better heat dissipation in the CLS with 250 μm polymer coating (due to the smaller distance between the fiber and heatsink). The best results were achieved with the CLS based on the fiber coated with high temperature resistant acrylate coating (Fospia Co.,Ltd XPC-373 [6]): 100% of the tested samples survived under full available pump power stress. The long term stability of the manufactured samples was tested by 170 hours continuous operation at the maximum available pump power of 60 W. The estimated pump power density affected on the cladding surface exceeded 630 W/mm2. This value exceeds the optical load in [1, 4] by more than 1.5-2 times and more than 4-7 times value in [2-3, 5]. Moreover to the best of our knowledge obtained result (60 W of stripped light) is a record value for CLSs on the basis of fibers with 125 μm outer silica cladding diameter.