All-Fiber Variable Optical Attenuator for High-Power Applications

We have exploited a high-power-tolerant variable optical attenuator (VOA) based on the fused fiber coupler in the all-fiber structure. A newly designed VOA employs the external modulation by forcing an axial stress in the tapered region of the fused fiber coupler. In the tapered region, the axial stress changes the refractive index of silica glasses resulting in a change in the coupling coefficient of the coupler. In this paper, we explain the principle of the novel device, VOA, and the optimized fabrication of the fused fiber coupler for the attenuation. The changes of the transmission spectrum for the coupler and the optical power spectrum for pump laser diode (LD), whose center wavelength is 1.47 μm, versus the axial displacement were verified by experiment. The possibility of the wavelength uniformity less than 1 dB over the range of 1460-1500 nm was also obtained by another coupler under a different fabrication condition. The polarization-dependent loss (PDL) at 1.47 μm wavelength was 0.65 dB for a maximum displacement of 150 μm. The designed device has an attractive feature of another output port of the coupler available as a monitoring tap. The device showed a high attenuation above 34 dB and an insertion loss below 0.15 dB. The all-fiber structure can provide less alignment, which in turn provides a high power tolerance. This novel design, moreover, has a simple and cost-effective structure.