Retro-reflection VOA using parabolic mirror for low insertion loss and linear attenuation relationship

This paper reports a new design of MEMS variable optical attenuator (VOA) that makes use of a pair of parabolic mirrors as the retro-reflector in the horizontal plane and a normal fiber as the rod lens to focus in the vertical plane. It is advantageous over the conventional co-axial VOAs since it allows the fibers to be arranged in parallel and is thus convenient for the packaging. The parabolic mirrors improve the optical coupling and yield an insertion loss of only 1.4 dB even when only the normally-cleaved single mode fibers are used. The attenuation can go up to 63 dB when the mirror pair is rotated by 2.6 degrees. More importantly, a linear relationship is well maintained over the whole range, unlike the nonlinear relationship in many conventional VOAs. At the 20-dB level, the VOA measures a PDL of 0.2 dB and a WDL of 0.5 dB, which are superior to the typically values of 1-2 dB in the conventional VOAs.