Tunable all-optical format conversion for BPSK to OOK based on highly nonlinear optical loop mirror

Abstract

A scheme is proposed for tunable all-optical format conversion from BPSK to OOK signal without active phase locking under the highly nonlinear loop structure. The 20 Gbit/s BPSK is mapped to two-level amplitude signal by delay interferometer (DI). One carrier branch generated locally remodulates the amplitude information to phase based on cross-phase modulation (XPM) effect in the highly nonlinear fiber (HNLF). The other carrier propagates as a pump under symmetrical loop ensuring the stability of phase relationship. After interference and vector shifting in coupler, the wavelength-tunable OOK is obtained. Benefiting from the signal remodulation mechanism and symmetric loop setup, separate carrier extraction and additional phase-locking are not required. The feasibility of the scheme is demonstrated by constellation diagram, eye diagram and pattern transformation diagram in numerical simulations. At the BER of 10−3, the converted OOK has ∼0.7 dB and ∼1.9 dB optical signal-to-noise ratio (OSNR) improvement, respectively, compared to back-to-back (B2B) OOK under different quality of input signals. It is shown that the scheme optimizes the signal quality while converting signal formats. For low-quality BPSK with OSNR of 15 dB, the system indicates better noise tolerance. Furthermore, the scheme has potential scalability to bidirectional format conversion between BPSK and OOK.