Design and characteristics of a micromachined variable optical attenuator with a silicon optical wedge

Abstract

We have designed and characterized a novel micromachined variable optical attenuator (VOA) with a silicon optical wedge (SOW). Micromachined VOAs are being studied for effective signal balancing and device protection on a wavelength division multiplexing (WDM) network system. Several micromachined optical attenuators have been introduced using an optical shutter between two optical fibers. This reflective shutter in previous micromachined VOAs requires a troublesome sidewall metal coating to block out a part of the incident light signal. In this paper, we report on detailed design strategy and the characteristics of a refractive VOA with a wedge-shaped silicon microstructure. The proposed VOA is optimally designed to successively dissipate a blocked optical power by partial transmission and refraction outside the acceptance angle of the output fiber. The device can be simply fabricated using silicon-on-insulator (SOI) wafers and deep reactive ion etching (DRIE) without a sidewall metallization process. The fabricated VOA showed high performances, such as low insertion loss of 0.6 dB, wide attenuation range of over 43 dB with an optical response time of 6 ms. Return loss was experimentally obtained as high as 38 dB with beveled optical fibers even in an air-ambient condition. In the reliability test, the time-dependent loss (TDL), wavelength-dependent loss (WDL) and polarization-dependent loss (PDL) were measured as 0.08, 0.5 and 1.1 dB, respectively, for the attenuation level of 10 dB, and both WDL and PDL were considerably improved by the thermal oxidation and oxide removal in a high attenuation level.