We successfully demonstrated overwriting of differential quadrature phase-shift keying (DQPSK) on inverse return-to-zero (RZ) pulses for simple 3-bit/symbol operation at a 10-Gb/s symbol rate (30-Gb/s bit rate). We adopted cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) for inverse-RZ generation, which allows both low and high levels of RZ optical signal to have a finite pulse energy in a bit time slot. We verified a wide tolerance of 20% of the bit-slot for time slot alignment between amplitude-shift keying and differential phase-shift keying modulation in the proposed scheme. We also demonstrated wide dynamic range characteristics at the extinction ratio for both 2- and 3-bit/symbol operation, compared to the conventional scheme. The proposed scheme allows a cross-modulation penalty, due to the intensity to phase modulation, of less than 1.5 dB in 2-bit/symbol and less than 5 dB in 3-bit/symbol operation.
[1]
Robert A. Griffin,et al.
Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission
,
2002,
Optical Fiber Communication Conference and Exhibit.
[2]
Masataka Nakazawa,et al.
Generation of a pseudorandom dark soliton data train and its coherent detection by one-bit-shifting with a Mach-Zehnder interferometer
,
1995
.
[3]
R. Brenot,et al.
Low input power all-optical 3R regenerator based on SOA devices for 42.66 Gbit/s ULH WDM RZ transmissions with 23 dB span loss and all-EDFA amplification
,
2003,
OFC 2003 Optical Fiber Communications Conference, 2003..
[4]
J. Speidel,et al.
Quaternary optical ASK-DPSK and receivers with direct detection
,
2003,
IEEE Photonics Technology Letters.