Optical Differential Quadrature Phase-shift Keying Research Articles

DQPSK Directly Phase Modulated DFB for Flexible Coherent UDWDM-PONs

A bandwidth-limited distributed feedback laser directly phase modulated with a 5-Gb/s differential quadrature phase shift keying (DQPSK) signal is demonstrated in a statistical ultra-dense wavelength division multiplexing passive optical network. Optical DQPSK modulation was achieved by simply equalizing a four-level waveform. An intradyne receiver based on a 120° optical hybrid with straightforward differential decoding processing was used to detect and recover the data attaining a sensitivity of −38 dBm at $\textrm {BER}=10^{-3}$ . Negligible penalty was observed when adding a second user at 5-GHz channel separation.

Reconfigurable crystal-based demodulator for both differential-phase-shift keying and differential-quadrature-phase-shift keying formats: principle, design and performance

A reconfigurable demodulator for both differential-phase-shift keying (DPSK) and differential-quadrature-phase-shift keying (DQPSK) formats is theoretically and experimentally presented. In this crystal-based demodulator, a rotatable quarter-wave plate is implemented to switch the operation of the device between DPSK and DQPSK modes. To mitigate polarisation-dependent-frequency shift (PDFS), the authors control uniformity of optical path length differences for the orthogonal polarisation states in the delay crystal, a liquid crystal phase-tuning retarder is used to improve the tuning time. The design and fabrication consideration of this device are also discussed. The packaged device has a low PDFS of within ±0.2 GHz over C-band, fast tuning time of 20.5 ms, error-free demodulation of optical transponder unit-3 DPSK and DQPSK signals can be achieved.

Novel 2N bit bipolar photonic digital-to-analog converter based on optical DQPSK modulation coupled with differential detection

A novel 2N bit bipolar photonic digital-to-analog converter (PDAC) scenario based on the optical differential quadrature phase shift keying (ODQPSK) modulation coupled with differential detection is proposed. Compared with other proposed schemes, this bipolar PDAC has a greater dynamic range and a larger noise margin with good scalabilities both in speed and resolution. We demonstrate a 4 bit PDAC in a proof-of-principle experiment at a sampling rate of 2.5 GS/s.

High-speed addition/subtraction/complement/doubling of quaternary numbers using optical nonlinearities and DQPSK signals

We propose an innovative approach to implementing multiple arithmetic functions of quaternary numbers using optical nonlinearities and differential quadrature phase-shift keying (DQPSK) signals. By adopting 100 Gbit/s DQPSK signals (A, B) and exploiting nondegenerate four-wave mixing (FWM) for addition/subtraction and degenerate FWM for complement and doubling in a single highly nonlinear fiber, we demonstrate 50 Gbaud/s simultaneous quaternary addition (A+B), dual-directional subtraction (A-B, B-A), complement (-A, -B), and doubling (2B). Power penalties less than 4 dB (addition), 3 dB (dual-directional subtraction), 2 dB (complement), and 3.1 dB (doubling) are observed at a bit-error rate of 10(-9).

Optical DQPSK System Performance Evaluation Using Equivalent Differential Phase in Presence of Receiver Imperfections

The performance evaluation of optical differential quadrature phase-shift keying (DQPSK) systems through numerical simulation using a Gaussian approach (GA) for the equivalent differential phase (EDP) statistics is analyzed. It is shown that the GA describes quite accurately the probability density function (PDF) of EDP for a large range of DQPSK receiver (RX) imperfections. A semi-analytical simulation method (SASM) for bit error probability (BEP) evaluation based on the GA for the EDP is derived, proving to be quite accurate in presence of RX imperfections. The SASM takes into account the signal-noise beat variance dependence on signal waveform distortion and arbitrary optical and electrical filtering frequency responses. Closed-form expressions for the mean and standard deviation of EDP are derived which allows evaluating the DQPSK system performance in a quite time-efficient manner. Discrepancies not exceeding 0.1 dB on the optical SNR penalty estimated by the SASM for a BEP of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> when compared with Monte Carlo simulation are observed for the majority of the acceptable RX imperfections. Only the time-misalignment of the signals at the balanced detector input leads to higher discrepancies but not exceeding 0.3 dB even for considerable time-misalignment.

Performance Evaluation of Trellis Code Modulated oDQPSK Using the KLSE Method

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A direct detection optical differential quadrature phase-shift keying (oDQPSK) system with trellis coded modulation (TCM) is proposed and analyzed. From the results obtained for the symbol-error rate, it is observed that the proposed oDQPSK-TCM system can perform about 5 dB better than the uncoded oDQPSK system. Optical signal-to-noise ratio penalty due to first-order polarization-mode dispersion of the proposed oDQPSK-TCM system is evaluated and compared with that of unequalized as well as electrically equalized oDQPSK systems. </para>

Phase diversity receivers for homodyne detection of optical DQPSK signals

Phase diversity (PD) receivers are an elegant option to handle the phase noise at homodyne detection and are especially interesting when used as wideband tunable wavelength-division multiplexing receivers. In the late 1980s and early 1990s, they were proposed and investigated for binary modulation formats (amplitude-shift keying, differential phase-shift keying, and frequency-shift keying). In this paper, a homodyne PD receiver for differential quadrature phase shift keying (DQPSK) modulation is analyzed, which provides a promising alternative to presently investigated DQPSK-direct-detection receivers and homodyne receivers with digital phase estimation.

Optical DPASK and DQPSK: a comparative analysis for linear and nonlinear transmission

The performance of two multilevel modulation formats, differential-phase-amplitude shift-keying (DPASK) and differential quadrature-phase-shift keying (DQPSK), is compared for linear and nonlinear channels. For DPASK, the optimal extinction ratio is derived for both linear and nonlinear transmission. It is shown that for single-channel transmission, DQPSK outperforms DPASK in linear channels, whereas DPASK achieves better performance when nonlinear impairments are present

Monolithic demodulator for 40-gb/s DQPSK using a star coupler

A novel device that demodulates both quadratures of the optical differential quadrature phase-shift keying (DQPSK) format in a single interferometer by using a 4 /spl times/ 4 star coupler was demonstrated. Simultaneous measurements of both DQPSK quadratures at 42.7 Gb/s are also presented.

3-bit/symbol optical data format based on simultaneous DRZ, DPSK and PolSK orthogonal modulations

We propose a novel 3-bit/symbol optical data format based on simultaneously modulating dark-return-to-zero (DRZ), differential-phase-shift-keying (DPSK) and polarization-shift-keying (PolSK) on the same optical carrier. Transmission performance is investigated. The proposed data format has a more compact spectrum when compared with other modulation schemes (at the same data rate), such as return-to-zero (RZ), DPSK and PolSK. It is also more tolerant to phase noise when compared with optical-differential-quadrature-phase-shift-keying (ODQPSK).

Investigation of 10-Gb/s optical DQPSK systems in presence of chromatic dispersion, fiber nonlinearities, and phase noise

The impact of phase noise, chromatic dispersion, and nonlinear effects on a 10-Gb/s optical differential quadrature phase-shift keying (oDQPSK) system has been evaluated by computer simulations. Both single channel and multiple wavelength-division-multiplexing channels have been considered. The results show that the low spectral width of this format allows a reduced channel spacing as low as 25-12.5 GHz. Furthermore, the use of this format leads to an increase in dispersion tolerance together with a robustness to nonlinear effects due to the signal near constant envelope characteristics. Finally, simulations show that phase noise can be the limiting factor of oDQPSK systems.

6500 km transmission of RZ-DQPSK WDM signals

The first transoceanic transmission experiment using optical differential quadrature phase-shift keying (DQPSK) is presented. The long-haul transmission capabilities of RZ-DQPSK in a 64-channel WDM system have been demonstrated by transmitting 12.5 Gbit/s RZ-DQPSK

Analysis of impact of receiver imperfections on performance of optical DQPSK systems

The effects of key receiver impairments on the performance of differential quadrature-phase shift keying (DQPSK) are analysed. While the sensitivity to photodetector responsivity imbalance is low and comparable to the case of binary DPSK, the constraints on the maximum tolerable detuning for the interferometers are very tight. In a 40 Gbit/s system, a 1 dB penalty is already incurred with a 0.7 GHz detuning.