Non-return-to-zero Differential Phase Shift Keying Research Articles

All-Optical Clock Recovery for 40-Gb/s MZM-Generated NRZ-DPSK Signals Using a Self-Pulsating DBR Laser

All-optical clock recovery for the nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) modulation format is demonstrated experimentally at 40 Gb/s using a self-pulsating distributed Bragg reflector laser. The use of a Mach-Zehnder modulator to generate the NRZ-DPSK signal yields a modulated signal spectrum with a weak clock tone. The self-pulsating laser is able to directly recover a clock signal with a root-mean-square timing jitter of 760 fs and an extinction ratio of 13 dB; a preprocessing stage to enhance the clock tone is not required. The timing jitter of the recovered clock signal is characterized for different values of the input signal optical signal-to-noise ratio and for varying amounts of waveform distortion due to polarization-mode dispersion.

Proposal and simulation for all-optical format conversion between differential phase-shift keying signals based on cascaded second-order nonlinearities

We propose and simulate simple realizations of all-optical format conversion between differential phase-shift keying (DPSK) signals based on cascaded second-order nonlinearities in a periodically poled lithium niobate (PPLN) waveguide. Four kinds of 40 Gb/s all-optical format conversion from non-return-to-zero differential phase-shift keying (NRZ-DPSK) to return-to-zero differential phase-shift keying (RZ-DPSK) are investigated based on cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG) or cascaded sum- and difference-frequency generation (cSFG/DFG). The optical spectra, temporal waveforms, eye diagrams, constellation diagrams, and time-related phase distribution are analyzed, which indicate successful implementation of NRZ-DPSK-to-RZ-DPSK format conversion. The obtained results also confirm the phase preservation characteristic of PPLN.

Proposal for PPLN-Based All-Optical NRZ-to-CSRZ, RZ-to-CSRZ, NRZ-DPSK-to-CSRZ-DPSK, and RZ-DPSK-to-CSRZ-DPSK Format Conversions

A simple realization of 40-Gb/s all-optical format conversions from nonreturn-to-zero (NRZ) to carrier-suppressed return-to-zero (CSRZ), from return-to-zero (RZ) to CSRZ, from NRZ differential phase-shift keying (NRZ-DPSK) to CSRZ-DPSK, from RZ-DPSK to CSRZ-DPSK are proposed and numerically demonstrated based on cascaded second-order nonlinearities in a periodically poled lithium niobate waveguide. A carrier-suppressed pump clock is adopted. The spectra, waveforms, eye diagrams, and constellation diagrams are investigated. Multicasting, multichannel, and ultrahigh-speed (160 Gb/s) operations are also numerically demonstrated.

A Technique for Rephasing$\,+\,$2R Regeneration of 40-Gb/s NRZ-DPSK Data

A novel technique for rephasing, reshaping, and reamplifying (Rp + 2R) of nonreturn-to-zero differential phase- shift keying (NRZ-DPSK) data is demonstrated. The regenerator operation and performance are evaluated using numerical simulations. The system can regenerate 40-Gb/s NRZ-DPSK data degraded with both random amplitude and phase noise (linear or nonlinear). Regenerated NRZ-DPSK has the same original data pattern and wavelength. The DPSK quality-factor and optical signal-to-noise ratio (OSNR) improves significantly, and the output differential phase error is very small. The bit-error ratio (BER) shows improvement of the received data OSNR ranging from 9.5 dB at 10-4 BER up to 10.5 dB at 10-9 BER.

Simultaneous Demodulation and Clock-Recovery of 40-Gb/s NRZ-DPSK Signals Using a Multiwavelength Gaussian Filter

We demonstrate a novel optical circuit that has the potential of simultaneous demodulation and all-optical clock-recovery of 40-Gb/s wavelength-division-multiplexing nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) signals. A key device of the circuit is an ad hoc periodic fiber Bragg grating filter that simultaneously demodulates the input signals and seeds a series of clock recovery circuits. We report the complete characterization of the proposed scheme in the whole-band using a tunable transmitter. The DPSK demodulated signals show enhanced resilience to chromatic dispersion with respect to the usual NRZ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON-OFF</sub> keying format. On the other hand, the recovered clock signals are very stable and have around 200-fs root-mean-square time jitter.

All-Optical Clock Recovery from NRZ-DPSK Signals at Flexible Bit Rates

We propose and demonstrate all-optical clock recovery (CR) from nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signals at different bit rates theoretically and experimentally. By pre-processing with a single optical filter, clock component can be enhanced significantly and thus clock signal can be extracted from the preprocessed signals, by cascading a CR unit with a semiconductor optical amplifier based fibre ring laser. Compared with the previous preprocessing schemes, the single filter is simple and suitable for different bit rates. The clock signals can be achieved with extinction ratio over 10 dB and rms timing jitter of 0.86 and 0.9 at 10 and 20 Gb/s, respectively. The output performances related to the bandwidth and the detuning of the filter are analysed. By simply using a filter with larger bandwidth, much higher operation can be achieved easily.

All-optical NRZ-DPSK clock recovery using linearly chirped fiber Bragg grating induced clock tone

All-optical clock recovery (CR) from 10 Gbps nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signal is experimentally demonstrated, by introducing the chromatic dispersion induced clock tone into a free-running semiconductor optical amplifier (SOA) based fiber ring laser for achieving mode-locking. Since no special component is required for NRZ-DPSK demodulation, our proposed method is very promising because of its simple configuration and better stability. The good performance of our proposed configuration is fulfilled with a linearly chirped fiber Bragg grating (LCFBG) to regenerate clock tone of the NRZ-DPSK signal. The recovered clock signal with the extinction ratio of 17 dB and the root-mean-square timing jitter of 540 fs is achieved under 2 31 − 1 pseudorandom binary sequence NRZ-DPSK signals measurement. Due to the same operation principle, our configuration has the potential to implement all-optical CR for NRZ-DPSK signal with data rate up to 40 Gbps.

Boost up of four wave mixing signal in semiconductor optical amplifier for 40 Gb/s optical frequency conversion

The 40Gb/s optical frequency converter for non-return to zero differential phase shift keying (NRZ-DPSK) signal by using four wave mixing in semiconductor optical amplifier (SOA) have achieved sucessfully. The optimized signal-to-pump ratio for NRZ-DPSK by using optimized SOA structure with enhanced FWM effect is also evaluated. The optimum signal-to-pump ratio is 12dB and 10dB with Q factor penalty of 0.685dB and 0.663dB. The dependence of four wave mixing efficiency and converted signal power on signal input power is studied and it is evaluated that four wave mixing efficiency decreases with increase in the input power. The impact of pump power, signal-to-pump ratio, and SOA parameters with Q factor penalty for 40Gb/s has been illustrated. It has shown that converted signal power increases up to the saturation power of semiconductor optical amplifier, then decreases. It is observed that for the optimum pump power, OSNR of converted signal varies little with signal input power.

All-Optical NRZ-DPSK Clock Recovery Using Chromatic-Dispersion-Induced Clock Tone

All-optical clock recovery (CR) from 10-Gb/s nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signal is demonstrated experimentally by introducing the chromatic-dispersion-induced clock tone into a free-running semiconductor optical amplifier (SOA)-based fiber ring laser for achieving mode-locking. Since no special component is required for NRZ-DPSK demodulation, our proposed method is very promising because of its simple configuration and better stability. The good performance of our proposed configuration is fulfilled with a 20-km standard single-mode fiber to regenerate clock tone of the NRZ-DPSK signal. The recovered clock signal with the extinction ratio of 17 dB and the root-mean-square timing jitter of 718 fs is achieved under 231-1 pseudorandom binary sequence NRZ-DPSK signals measurement

Chromatic dispersion and PMD mitigation at 10 Gb/s using Viterbi equalization for DPSK and DQPSK modulation formats

We explore the potential of chromatic dispersion and polarization-mode dispersion (PMD) mitigation using Viterbi equalization in 10 Gb/s nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) and differential quadrature phase-shift keying (NRZ-DQPSK) systems. We show through Monte Carlo simulations that using Viterbi equalization improves the performance of NRZ-OOK, NRZ-DPSK and NRZ-DQPSK receivers. For NRZ-DQPSK receiver with a Viterbi equalizer, the chromatic dispersion tolerance is about 5000 ps/nm and the 1st order PMD tolerance is about 160 ps at 3 dB OSNR penalty.

Measured Noise Performance for Heterodyne Detection of 10-Gb/s OOK and DPSK

Heterodyne detection of 10-Gb/s nonreturn-to-zero (NRZ) on-off keying and NRZ differential phase-shift keying is demonstrated. The noise performance of the heterodyne receiver with and without the use of an optical preamplifier is experimentally investigated

All-Optical Clock Recovery From NRZ-DPSK Signal

All-optical clock recovery (CR) from 20-Gb/s nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) signal is demonstrated, with an all-fiber delay interferometer (DI) and a mode-locked semiconductor optical amplifier (SOA) fiber laser. The tunable DI serves as an all-optical DPSK demodulator and the phase-modulated NRZ-DPSK signal is converted into the intensity-modulated pseudoreturn-to-zero (PRZ) signal, with the enhancement of the clock component. Followed SOA fiber-laser is used to achieve CR from the PRZ signal. Fixed bit pattern and 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</sup> -1 pseudorandom binary sequence NRZ-DPSK signals are used to test the performance of the proposed system. It is shown that the recovered clock signal with the extinction ratio over 10 dB and the root-mean-square timing jitter of 800 fs can be achieved

Wide-Band Optical Wavelength Converter Based on Four-Wave Mixing Using Optimized Semiconductor Optical Amplifier

We have simulated 50 nm up and down wavelength conversion for a non-return to zero differential phase shift keying (NRZ-DPSK) signal using four-wave mixing in an optimized semiconductor optical amplifier (SOA) at 10 Gb/s for the first time. For this we optimized the SOA parameters to achieve sufficient quality and enhancement in four-wave mixing effect. This can be done in such a manner that the SOA never saturates and produces maximum four-wave mixing signals with minimum gain fluctuations. The quality of the converted signal is best before the saturation of SOA. Finally, we have numerically simulated cascaded wavelength converters up to 1300 km transmission distance.

Automatic Tunable Chromatic Dispersion Compensation at 40 Gb/s in ASK and DPSK, NRZ, and CSRZ 263-km Transmission Experiments

Chromatic dispersion (CD) in single-mode optical fiber distorts pulses and is a big obstacle for the installation of long-haul dynamically routed transmission systems, especially at 40 Gb/s. Here the automatic residual CD compensation of a 263-km fiber link in the range -300 to -700 ps/nm is demonstrated for nonreturn-to-zero amplitude-shift keying (NRZ-ASK), carrier-suppressed return-to-zero (CSRZ)-ASK, NRZ differential phase-shift keying (DPSK), and CSRZ-DPSK modulation formats at 40 Gb/s. A thermally tunable dispersion compensator minimizes residual CD, which is measured by a synchronous arrival time detection scheme. The measured transmission penalty with online CD compensation is between /spl plusmn/1.2 dB for various link lengths and compensated CD values.

A comparative study of dpsk and ook wdm transmission over transoceanic distances and their performance degradations due to nonlinear phase noise

We have compared experimentally the transmission performance of return-to-zero differential phase-shift keying (RZ-DPSK) with RZ-ON-OFF keying (OOK), nonreturn-to-zero differential phase-shift keying (NRZ-DPSK), and NRZ-OOK for 100/spl times/10-Gb/s transmission with a spectral efficiency of 0.22 b/s/Hz over transoceanic distances. The Q degradation of the RZ-DPSK after transmission over 9180 km was 3 dB greater than that of RZ-OOK. The experimental results clearly showed the major cause of degradation for DPSK is not cross-phase modulation but self-phase modulation. The calculated nonlinear phase noise, i.e., the Gordon-Mollenauer effect, agreed with the experimental results. A distributed-Raman-amplifier assisted erbium-doped-fiber-amplified transmission line acted well in reducing the nonlinear phase noise.