Differential Phase-shift Keying Demodulator Research Articles

Electro-optic delay oscillator with nonlocal nonlinearity: Optical phase dynamics, chaos, and synchronization

We demonstrate experimentally how nonlinear optical phase dynamics can be generated with an electro-optic delay oscillator. The presented architecture consists of a linear phase modulator, followed by a delay line, and a differential phase-shift keying demodulator (DPSK-d). The latter represents the nonlinear element of the oscillator effecting a nonlinear transformation. This nonlinearity is considered as nonlocal in time since it is ruled by an intrinsic differential delay, which is significantly greater than the typical phase variations. To study the effect of this specific nonlinearity, we characterize the dynamics in terms of the dependence of the relevant feedback gain parameter. Our results reveal the occurrence of regular GHz oscillations (approximately half of the DPSK-d free spectral range), as well as a pronounced broadband phase-chaotic dynamics. Beyond this, the observed dynamical phenomena offer potential for applications in the field of microwave photonics and, in particular, for the realization of novel chaos communication systems. High quality and broadband phase-chaos synchronization is also reported with an emitter-receiver pair of the setup.

A DPSK Receiver With Enhanced CD Tolerance Through Optimized Demodulation and MLSE

We experimentally demonstrate that a differential phase-shift keying (DPSK) receiver employing a Mach-Zehnder delay interferometer (MZDI) with a bit delay of less than one bit combined with maximum likelihood sequence estimation can considerably enhance the tolerance to chromatic dispersion (CD). Furthermore, we analyze the impact of sampling phase and optimize the MZDI bit delay for 10.7-Gb/s nonreturn-to-zero (NRZ)-DPSK demodulation. Our investigation shows that a 4000-ps/nm CD tolerance at 2-dB penalty is feasible for 10.7-Gb/s NRZ-DPSK.

Constellation diagram analysis of DPSK signal regeneration in a saturated parametric amplifier

The regeneration of 10 Gbit/s differential phase shift keying (DPSK) signals in a saturated fiber optic parametric amplifier (FOPA) acting as an optical intensity regenerator is investigated. The regeneration properties are investigated in amplitude and phase separately by means of coherent detection and constellation diagram analysis. A significant reduction of the amplitude fluctuations is observed and the measured signal-to-noise ratio (SNR) after DPSK demodulation is increased up to 10 dB. However, from the constellation analysis it is found that the FOPA introduces phase noise to the signal. The FOPA induced phase noise originates from pump amplitude fluctuations that are transferred to the signal through cross-phase modulation.

Tolerances and Receiver Sensitivity Penalties of Multibit Delay Differential-Phase Shift-Keying Demodulation

Multibit delay demodulation of differential-phase shift-keying (DPSK) is finding applications in polarization interleaved modulation, optical time-domain multiplexing (OTDM), and multisymbol DPSK demodulation. Little attention has been paid to the degradation in tolerance and power penalty associated with multibit delay demodulation. We assess experimentally, numerically, and analytically the power penalties and tolerances associated with multibit delay DPSK demodulation. Numerical and analytical results show that the power penalty scales by a small factor of 0.2-0.35 dB per integer bit delay due to laser linewidth (LW) while experimental back-to-back results show a significant 1.2 dB per integer bit delay due to frequency offset penalty of longer bit delays. Frequency offset tolerance scales as 1/bit-delay and the delay-mismatch tolerance decreases by 20% for delays longer than 1 bit. A simple analytic model accounts for the combined effect of LW, frequency offset, and amplified spontaneous emission.

Phase-Tunable Low-Loss, $S$-, $C$-, and $L$-Band DPSK and DQPSK Demodulator

Differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) are touted as performers and reliable advanced modulation formats for next-generation optical transmission systems. One key device enabling such systems is the delay interferometer, converting the signal phase information into intensity modulation to be detected by the photodiodes. We developed an all-fiber delay-line interferometer for DPSK and DQPSK demodulation in the S-, C-, and L-band with low insertion loss, low-birefringence, and greater than 30 dB of extinction ratio over 100 nm and 20 dB from 1460 to 1640 nm in a single device. The device also features insensitivity to mechanical vibration, very low port imbalance (0.1 dB), and very low time delay between all outputs (0.1 ps). The device is highly reliable with a demonstrated failure-in-time rate of less than 100.

Microring-based modulation and demodulation of DPSK signal

Ultra-small modulator and demodulator for 10 Gb/s differential phase-shift-keying (DPSK), using silicon-based microrings, are proposed. A single-waveguide microring modulator with over-coupling between ring and waveguide generates a DPSK signal, while a double-waveguide microring filter enables balanced DPSK detection. These modulator and demodulator are characterized. A trade-off between pattern dependence of the Duobinary signal and alternate-mark inversion signal power in demodulator design is discussed. Power penalty of the proposed approach is 0.8 dB relative to baseline using conventional modulation and demodulation techniques.

Optical DPSK demodulator based on /spl pi/-phase-shifted fiber Bragg grating with an optically tunable phase shifter

We propose and demonstrate a novel optical differential phase-shift keying (DPSK) demodulator with an optically tunable phase shifter. The proposed DPSK demodulator is implemented by using a pi-phase-shifted fiber Bragg grating and an Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -Al <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+ </sup> codoped optical fiber. A 10-Gb/s DPSK signal was successfully demodulated by the proposed demodulator, showing clearly open eye diagrams as well as bit-error-free performance. Moreover, the phase of delayed optical signal can be tuned by the phase shifter that is controlled by a pumping light at around 980nm

DPSK demodulator based on optical discriminator filter

A 10-Gb/s differential phase-shift-keying (DPSK) demodulator is demonstrated based on an ultranarrow 6.2-GHz optical discriminator filter. The DPSK demodulator operates with a conventional direct detection p-i-n receiver, while still outperforming on-off keying by 1.2 dB.

Optimum multiuser noncoherent DPSK detection in generalized diversity rayleigh-fading channels

The jointly optimum multiuser noncoherent detector for differential phase-shift keying (DPSK) modulation over the generalized diversity Rayleigh-fading (GDRF) channel is derived and analyzed. The GDRF channel includes time/frequency/receiver antenna diversity and allows fading correlations between the various diversity branches of each user. Noncoherent detection here refers to the case where the receiver has neither knowledge of the instantaneous phases nor of the envelopes of the users' channels. Upper and lower bounds on the bit-error probability of the optimum detector are derived for a given user. For fast fading, when the fading coefficients vary from one symbol interval to the next (but are still essentially constant over one symbol interval), the detector asymptotically (for high signal-to-noise ratios (SNRs)) reaches an error floor, which is bounded from below and above for different fast fading scenarios. For slow fading, when the channel is constant for at least two consecutive symbol intervals, the upper bound is shown to converge asymptotically to the lower bound. Thus, the asymptotic efficiency of optimum multiuser DPSK detection can be determined and is found to be positive. In contrast to coherent detection, however, it is smaller than unity in general. Since the asymptotic efficiency is independent of the interfering users' signal strengths, the optimum detector is near-far resistant. While optimum multiuser detection is exponentially complex in the number of users, its performance provides the benchmark for suboptimal detectors. In particular, it is seen that the previously suggested post-decorrelative detectors can be far from satisfactory.

Impact of filtering on RZ-DPSK reception

We discuss the influence of optical and electrical filtering on the performance of beat-noise limited balanced and single-ended direct detection of return-to-zero differential phase-shift keying (DPSK). Our simulations, supported by 40-Gb/s measurements, show that balanced DPSK detection outperforms both its single-ended equivalent and ON-OFF keying by /spl sim/2.7 dB, with higher gains at narrower optical filter bandwidths.

Comments on "Differential detection with IIR filter for improving DPSK detection performance"

Hamamoto (see ibid., vol.44, no. 8, p.959-66, 1996) presented a detection technique for differential phase-shift keying (DPSK) with improved performance over conventional differential coherent detection. Leib (see ibid., vol.43, no.2/3/4, p.722-25, 1995) comments that this technique had been introduced much earlier. As correctly stated by Hamamoto the essence of the new DPSK detection is based on a technique introduced by Leib and Pasupathy (1988). Differential coherent detection can be viewed as a scheme that uses the previous data symbol as a phase reference. The problem is that this phase reference suffers from the channel noise in very much the same way as the data symbol does and, therefore, it results in a performance degradation.

3 Gbit/s optically preamplified direct detection DPSK receiver with 116 photon/bit sensitivity

For the first time experimental bit error rate curves are presented for an optically preamplified direct detection differential phase shift keying (DPSK) communication link. DPSK offers ̃6 dB peak power sensitivity improvement over more traditionally optically preamplified on/off keying (OOK). Using an erbium doped fibre preamplifier, a fibre Fabry–Perot filter, an optical DPSK demodulator consisting of a fibre-optic Mach–Zehnder interferometer with a 1 bit differential delay, and a balanced receiver, a sensitivity of 116 photon/bit was obtained. To the authors&apos; knowledge these results represent the first demonstration of optically preamplified DPSK with better sensitivity than previously reported multigigabit per second heterodyne DPSK and ̃3 dB more sensitivity on a peak power basis than previously reported preamplified OOK systems.

A differential-delay SAW correlator for combined DSSS despreading and DPSK demodulation

The design of a novel SAW (surface-acoustic-wave) component for the noncoherent detection of DS/SS (direct-sequence spread-spectrum) signals with a DPSK (differential phase-shift keying) modulation format is presented. In the considered differential-delay correlator, both the correlation function for despreading and the delay function for DPSK demodulation are merged into a single device. Due to the absence of a separate delay line, no bandwidth-limiting effects no high insertion losses occur in the delay branch of the DPSK demodulator. In addition, an exact match between the correlator output and its one-symbol-delayed replica is obtained. Experiments on an ST-quartz differential-delay correlator with a center frequency of 98.5 MHz, a chip rate of 12.3 MHz and a symbol rate of 195.4 kb/s demonstrated the operation of the device. >

Reduction of AM-induced penalty in DPSK receivers by sum-square demodulation

A DPSK (differential phase shift keying) demodulator which is insensitive to the amplitude modulation induced by semiconductor optical amplifier phase modulators is proposed. The demodulator consists of only two additional power dividers/combiners, compared to a traditional DPSK demodulator. Analysis shows that the receiver penalty caused by amplitude modulation can be reduced from 2-4 dB to zero. The demodulator is demonstrated in a 2.5-Gb/s DPSK system experiment using an optical amplifier as phase modulator. >

On the achievable information rates of DPSK

The achievable information rates of differential phase shift keying (DPSK) with differential detection are investigated. DPSK detection creates a dependency between two consecutive receiver outputs, thus introducing memory in the corresponding channel model. With interleaving the performance of DPSK over the additive white Gaussian noise (AWGN) channel is substantially degraded compared to that of coherent PSK. The authors use the suboptimal phase-difference receiver to show that in this case noninterleaved DPSK achieves the performance of coherent detection (employing independent increment phase keying) when the channel phase is assumed constant throughout the coded block. Both capacity and cutoff rate are examined, and the results are compared to the (known) respective expressions evaluated for the interleaved case. The generalised cutoff rate for a scheme employing multiple-symbol detection over (short) sub-sequences is also examined.

BER evaluation for phase and polarization diversity optical homodyne receivers using noncoherent ASK and DPSK demodulation

An analysis of the performance of phase diversity receivers using amplitude-shift keying (ASK) and differential phase-shift keying (DPSK) is presented. Both (2*2) and (3*3) multiport receivers are investigated. Asymptotic methods are used to estimate the bit error rate (BER) and signal-to-noise power ratio (SNR) dependence for each type of the receiver. The analysis favors the squarers as the demodulators for ASK whose performance approaches that of the ideal heterodyne detector in the limit of large SNR. A modification of the ASK ((3*3)) receiver which cancels the local oscillator intensity noise is proposed. Receivers which comprise polarization and phase diversity techniques are also investigated for both ASK and DPSK. Their performance is independent of the polarization state of the received signal, and the value of SNR required to obtain the BER of 10/sup -9/ is only a few tenths of a decibel greater than that needed by the phase diversity receivers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>