Simple tunable all-fiber delay line filter for dispersion compensation

A delay line filter with tunable group delay for residual dispersion compensation is presented. The device is suitable for channel bit rates of 40 Gb/s and consists mainly of two 3 /spl times/ 3 fiber couplers connected in series. Due to the all-fiber implementation, it has low polarization sensitivity and can be directly inserted in the transmission line. Its dispersion value can be tuned by simply changing the optical phase in one filter path. Thereby, a tuning range of /spl plusmn/50 ps/nm is realized with a very good agreement between simulation and measurement.

I. Introduction

Chromatic dispersion is a limiting factor in fast optical networks with channel bit rates of 40 Gb/s or higher. Fig. 1. Schematic of the proposed delay line filter. The main part of the dispersion is usually compensated by a dispersion-compensating fiber that has a fixed dispersion value. But the residual dispersion, caused by environmental changes, rerouting, and power variations has to be compensated adaptively. Therefore, several methods were already suggested. One approach is the use of chirped fiber Bragg gratings [1], where the main problem is their group delay ripple. Other approaches are based on delay line structures like ring-resonators [2], virtually imaged phased arrays [3], [4], multicavity etalons [5], and cascaded Mach–Zehnder interferometers [6], [7]. These realizations are in planar or bulk optics, which causes loss due to the coupling to the fibers. Furthermore, the planar structures suffer for high waveguide loss and non-negligible polarization-dependent loss (PDL) and polarization-mode dispersion.