An Optical Frequency Shifter Based on High-Order Optical Single-Sideband Modulation and Polarization Multiplexing

We propose and experimentally demonstrate a novel electro-optic frequency shifter with frequency multiplication operation based on high-order optical single-sideband (SSB) modulation and polarization multiplexing using an integrated dual-polarization quadrature phase shift keying (DP-QPSK) modulator. By changing the dc bias phases of the modulator and the phase difference between the input RF signals, two-high-order SSB modulation signals are generated from the two QPSK modulators. After that, the two optical signals are polarization multiplexed and then projected to one direction to remove the low-order sidebands while only the high-order sideband is reserved. The scheme is numerically analyzed with the modulation indices of the modulator for optimal operation. A distinct advantage of this approach is the capability to generate a frequency shifting with a continuous and ultrawide tunable range (more than ±100 GHz), and the tuning operation is accurate, simple, and flexible. An experiment is carried out to achieve a frequency-doubled optical frequency shifter. A continuous frequency shifting ranged from -36 to 36 GHz is experimentally verified.