Literature and Theory
Fiber lasers have attract attention due to their high efficiency, low-cost and high power [1]. The integration process of two fibers, which is splicing, should be done precisely in order to eliminate the splicing loss. If the alignment of the two fibers does not set precisely, there will be splicing losses. For example, the core light could be coupled to the cladding. This situation ends up with the unwanted cladding light. Also, if the pump light (cladding light) could not be absorbed by the active atoms located in the active fiber, there will be residual cladding light. The unwanted cladding light can cause some problems such as low M2 value, fiber degradation due to thermal load [2]. The unwanted cladding light could reach hundreds of Watts for the kW-regime fiber laser systems. If the cladding light could not be extracted from the system, it would started to damage the fiber laser components. Thus, the unwanted cladding light should be extracted from the system without disturbing. The cladding light strippers became an important fiber optical equipment for the kW-regime fiber laser systems. Sévigny B. et.al., could manage to strip hundreds of Watts cladding light from the system with the high index polymer coating technique. They achieved 20dB attenuation with high index polymer coated cladding light stripper [3].
Fig.1: The blue arrows show the pump light and the green arrows show the signal light. (a) TIR (total internal reflection) inside an optical fiber, (b) violation of TIR by coating the cladding with high index polymer.
High index polymer coating violates the TIR for the boundary at cladding and polymer coating. Therefore, unwanted light couples out of the fiber cladding. Only remaining light will be the core light (signal light) as expected. Another technique is the etching of the cladding surface and make the cladding light scattered from the rough surface. Tünnerman A. et.,al., achieved to strip 500W cladding light with the etched fiber [4].
Fig.2: The blue arrows show the pump light and the green arrows show the signal light. (a) TIR (total internal reflection) inside an optical fiber, (b) the light behaviour for the etched cladding fiber.
The cladding light travels with TIR inside the fiber as represented in Fig.2.a. The light incident to the rough surface scatters randomly. The etched (rough) fiber surface scatters the cladding light as shown in Fig.2.b.[1] D. J. Richardson, J. Nilsson, and W. A. Clarkson, High power fiber lasers: Current status and future perspectives, J. Opt.Soc. Amer. B, Opt. Phys., vol. 27, no. 11, pp. 63–92, Nov. 2010.
[2] Wei Guo, Zilun Chen, Hang Zhou, Jie Li, and Jing Hou, ‘’Cascaded Cladding Light Extracting Strippers for High Power Fiber Lasers and Amplifiers’’, IEEE Photonics Journal, Vol. 6, No. 3, June 2014.
[3] Alexandre Wetter, Mathieu Faucher, Benoit Sévigny, High power cladding light strippers, Proc. of SPIE Vol. 6873, 2008.
[4] A. Kliner, Kai-Chung Hou, M. Plötner, Ch. Hupel, Th. Stelzner, T. Schreiber, R. Eberhardt, A. Tünnermann, Fabrication and evaluation of a 500 W cladding-light stripper, Proc. of SPIE Vol. 8616 86160N-1.
High Power Cladding Light Strippers Fabricated in UNAM Labs
In our studies, we focused on high power cladding light stripper developments. We fabricated two types of cladding light strippers which are very effective. One of them is high index polymer coated, the other one is etched cladding light strippers. We achieved 18dB attenuation with high index polymer coated cladding light stripper with 40mm length and aproximately 14dB attenuation with etched and high index polymer coated cladding light stripper with 20mm length. In order to characterize our home-built cladding light stripper, the launched power from laser diode and power output from cladding light strippers were measured. The attenuation values were calculated due to power measurements.
Light Deflection from Cladding Light Strippers
IR camera images were taken for both of the cladding light strippers. The light extracted from the etched or high index polymer coated region as shown in Fig.3a and b.
Fig.3: Light deflection at the CLS region for (a) etched and high index polymer coated and (b) high index polymer coated cladding light strippers.
Power Measurements
Power was measured for the diode output only first. Then, the cladding light strippers were spliced to the diode output and measured. The power was decreased due to the performance of the cladding light strippers.
Fig.4: Power measurements for etched and high index polymer coated cladding light stripper.
The etched and high index polymer coated cladding light stripper extracted 125W when the launched power was 140W as shown in Fig.4. It was cooled with a heat sink in the power measurement experiments.
Fig.5: Power measurements for high index polymer coated cladding light stripper.
The high index polymer coated cladding light stripper extracted 135W when the launched power was 140W as shown in Fig.5. It was cooled with a heat sink in the power measurement experiments.
Attenuation Calculation
Fig.6: Attenuation calculations for high index polymer coated and etched & high index polymer coated cladding light stripper.
The attenuation of both of the cladding light strippers were calculated by using power measurement data. The attenuation values achieved as 18dB for high index polymer coated and 14dB for etched & high index polymer coated cladding light strippers represented in Fig.6.