Yongheng Dai

Photonic crystal fiber based Mach-Zehnder interferometer for DPSK signal demodulation

We demonstrate a novel fiber-based in-line DPSK demodulator using an in-fiber Mach-Zehnder interferometer (MZI). The device is fabricated by mismatch splicing of a photonic crystal fiber (PCF) with standard single mode fibers. The spectral characteristics at different PCF lengths are analyzed. The envelope of the interference fringes show a period that is inversely proportional to the PCF length, and is attributed to the periodic coupling between the core mode and the cladding mode. Error free demodulations of 10-Gb/s RZ- and NRZ-DPSK signals have been demonstrated using the in-fiber PCF-MZI demodulator with only 3-m PCF to introduce 91-ps delay. Wideband DPSK demodulation has also been achieved.

All-Optical RZ-to-NRZ and NRZ-to-PRZ Format Conversions Based on Delay-Asymmetric Nonlinear Loop Mirror

We perform all-optical return-to-zero (RZ) to nonreturn-to-zero (NRZ) and NRZ to pseudoreturn-to-zero (PRZ) format conversions at tunable bit rates, both by using the same delay-asymmetric nonlinear loop mirror. The temporal and spectral characteristics of the input and output signals are analyzed. Bit-error-rate measurements of RZ-to-NRZ conversion at 10 and 12.5 Gb/s show error-free performance with power penalties less than 3 dB with reference to the input RZ signals.

Wavelength Interchange of Phase-Shift-Keying Signal

All-optical wavelength interchange for differential phase-shift-keying signal is proposed and demonstrated using an optical parametric loop mirror with a phase-controlled optical pump. The performance of wavelength interchange is experimentally evaluated at 10 Gb/s. Successful swapping of phase information is achieved with error-free performance for 231-1 pseudorandom binary sequence encoded signals.

Ultrawideband monocycle pulse generation based on delayed interference ofπ/2 phase-shift keying signal

We propose a scheme to generate ultrawideband (UWB) monocycle pulses using delayed interference of a π/2 phase-shift keying (PSK) signal. The PSK signal is generated with an optical phase modulator. A fiber-based delay interferometer is used to provide the delay together with a ±π/2 phase shift between the signals in its two arms before the interference. The two opposite phase shifts correspond to the generation of a pair of polarity-reversed UWB monocycle pulses. The full width at half maximum of the pulses is ~68 ps and the 10 dB fractional bandwidth is 168%.

Bit-rate variable DPSK demodulation based on cascaded four-wave mixing

We report a demodulator for DPSK signals at variable bit rates based on cascaded four-wave mixing (FWM). The demodulation utilizes two FWM processes in a photonic crystal fiber (PCF) with in-between dispersion in a chirped fiber Bragg grating (CFBG). The first FWM generates a wavelength-tunable idler carrying phase information of the signal. A tunable optical delay between the signal and the idler is then introduced by dispersion. The signal, the idler, and the pump are reflected by the CFBG with a reflectance of 99% back to the PCF to initiate the second FWM process. In the second FWM, the phase relationship between the signal and the one-bit-delayed idler determines an amplification or attenuation of the idler, converting phase modulation to intensity modulation. Error-free demodulations have been successfully demonstrated for both NRZ and RZ-DPSK signals at 5 and 10 Gb/s.

Reconfigurable two-channel demultiplexing using a single baseband control pulse train in a dispersion asymmetric NOLM.

We demonstrate a unique solution to use a one-pulse control to achieve simultaneous two-channel all-optical demultiplexing that usually requires a two-pulse control or a two-step operation based on the conventional approaches. By applying a dispersion asymmetric nonlinear optical loop mirror (DA-NOLM) to introduce cross phase modulation (XPM) in both the clockwise (CW) and the counter clockwise (CCW) propagating branches, we have demonstrated reconfigurable, error-free 40-to-10 Gb/s two-channel demultiplexing (DEMUX) for OTDM OOK signals. Switchable operation between two-channel and single-channel DEMUX is also realized based on the proposed all-optical DEMUX configuration.

Widely Tunable Polarization-Insensitive Nondegenerate Four-Wave Mixing Wavelength Conversion for DPSK Signal

We demonstrate all-optical wavelength conversion for return-to-zero differential phase-shift keying (RZ-DPSK) signal by dual-pump four-wave mixing in a highly nonlinear photonic crystal fiber. By exploiting the residual birefringence in the fiber, the polarization of the first pump can be adjusted to achieve polarization-insensitive operation. The wavelength of the second pump is free to tune to broaden the conversion bandwidth. Open eyes of the converted signals are obtained for polarization-scrambled 10-Gb/s RZ-DPSK signal over a down-conversion bandwidth of 27 nm. Bit-error measurement shows a maximum power penalty of 4.5 dB at the error-free detection level.

Reconfigurable photonic ultrawideband pulse generation from an optically injected semiconductor laser

We demonstrate reconfigurable photonic ultrawideband (UWB) pulse generation by injecting a phase-modulated optical wave into a semiconductor laser. By adjusting the injection power level and phase modulation depth, the photonic UWB generator is capable of generating either a single-optical-carrier monocycle or power-efficient UWB pulses. A UWB pulse with spectral power efficiency (SPE) of 36.10% has been achieved by optimizing the injection parameters. Transmission in an 8.4 km single mode fiber results in a degradation of SPE to 27.55% without significant pulse distortion.

Bit-Rate Flexible Demodulation of DPSK Signals Based on Phase Sensitive Gain in Cascaded Four-Wave Mixing

We demonstrate bit-rate flexible demodulation of differential phase-shift keying (DPSK) signals in a linear structure. The demodulator utilizes a tunable optical delay based on the combination of wavelength conversion in a photonic crystal fiber and dispersion in a short section of single-mode fiber. Error-free demodulations for return-to-zero-DPSK signals at both 25 and 40 Gb/s have been successfully achieved.

RZ-to-NRZ and NRZ-to-PRZ format conversions using a photonic crystal fiber based Mach-Zehnder interferometer

We have fabricated a photonic crystal fiber based Mach-Zehnder interferometer and demonstrated its use in both RZ-to-NRZ and NRZ-to-PRZ pulse format conversions for 10-Gb/s OOK signals. The PCF length is only ∼165 cm.