Design and performance of the bidirectional optical single-sideband modulator

We present a detailed study of the design and performance of the bidirectional optical single-sideband modulator (BOSSM). This is a new scheme to achieve optical single-sideband modulation (OSSB) that uses a standard single-electrode Mach-Zehnder modulator (MZ-EOM) and passive fiber-optic components. The design is based on a novel technique to operate electrooptic modulators in which the radio frequency (RF) electrode is bidirectionally driven. The fundamentals of this bidirectional operation are analyzed thoroughly and it is found that it requires the use of a wide-bandwidth MZ-EOM with an electrode design that provides good velocity match between the optical and microwave modes. Deriving the expressions for the optical field and power at the output of the BOSSM, the OSSB operation is shown to be independent of the MZ-EOM bias. Therefore, the optical modulation depth at the output of the device can be enhanced using minimum transmission biasing to provide suppression of the optical carrier. Moreover, it is found that the second-order distortion is unaffected by the MZ-EOM bias; hence the technique can be applied to multi-octave bandwidth systems. Finally, the performance of the BOSSM is evaluated using a prototype based on a commercial 10 Gb/s MZ-EOM. The experimental characterization of the electrooptical parameters of this device reveals that the RF electrode design is not optimized for bidirectional operation. Therefore, the performance of the prototype is limited by the particular MZ-EOM deployed. However, sideband suppression over 10 dB is measured for most frequencies up to millimeter-waves, with peaks in the 20 dB to 30 dB range for narrow bands. This performance has enabled the demonstration of a 22-km fiber link transmitting a 29-GHz subcarrier conveying binary phase-shift keying modulated data at 622 Mb/s. The BOSSM reduces the dispersion-induced power penalty in the link to less than 1.5 dB. Furthermore, the bit error rate of the link is increased by five orders of magnitude using the carrier suppression technique.