Abstract

We propose an optical tunable filter for wavelength division multiplexing (WDM) applications. The filter consists of a classical fan Šolc stack placed in a ring cavity, with the polarizer outside and the analyzer inside it. Each birefringent plate is formed with the nematic-phase liquid crystal and is associated with a compensation birefringent plate, which introduces a constant phase retardation between the two eigen axes. In such an arrangement, the incident light beam undergoes a multipass through the Šolc filter in the same direction owing to the ring structure [1], in which the inner surfaces of the input and the output mirrors are coated with dielectric layers of high intensity reflectivity. By using the compensation birefringent plates, the wavelength operation window around 1.55 μm can be adjusted readily even with small optical thicknesses. The input wavelengths can be tuned continually by an electric field (about 1 V/μm) applied to each Šolc plate with a tuning speed of milliseconds. We note that compared with a classical one-pass Šolc filter, this structure reduces considerably the 3-dB peak bandwidth as well as the cross-talk, since the multipass of a coherent light beam in a Šolc filter can minimize the sidelobes around each main transmission peak.

© 2000 IEEE