Linear MEMS variable optical attenuator using reflective elliptical mirror

We propose an improved design of variable optical attenuator (VOA) using a micromachined elliptical mirror, which offers the advantages of both the shutter-type VOAs and the flat-mirror reflection-type VOAs while overcoming their problems. This design positions the input and output fibers at the two focal centers of the reflective elliptical mirror. Since the ellipse can focus the light from one center to the other, the VOA enjoys low insertion loss while using the normally cleaved single-mode fibers. As the mirror is displaced, the input beam is rapidly defocused, producing a large attenuation range without requiring a large mirror displacement. This VOA also has low polarization dependence loss (PDL), low wavelength dependence loss (WDL), and low back reflection. It is fabricated by the deep reactive ion etching process and achieves 44-dB attenuation at 10.7-V driving voltage. The PDL is 0.8 dB at the 40-dB attenuation level and the WDL is 1.2 dB at the 20-dB level for 100-nm wavelength change. More importantly, the measurement shows that attenuation has a nearly linear relationship with the mirror displacement over a 30-dB attenuation range.