Differential Phase-shift Keying Signals Research Articles

Impact of Receiver on In-Band Crosstalk-Induced Penalties in Differentially Phase-Modulated Signals

The impact of optical receiver configuration on in-band crosstalk-induced penalty has been investigated in both theoretical and experimental analyses, for differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) signals. Previously it has been shown that DPSK signals are ~6 dB more tolerant to in-band crosstalk than on-off keying (OOK) signals. However, we find that the tolerance difference between the two signals is reduced to ~3 dB when the decision threshold of the receiver is optimized to minimize the bit-error rate for each signal. Then we derive simple equations for the in-band crosstalk-induced penalty in DPSK and DQPSK signals with two different optical receiver configurations: balanced and single-ended direct-detection receivers. We confirm that the penalties obtained from our simple equations agree well with the measured results.

Broad-Band All-Optical Wavelength Conversion of Differential Phase-Shift Keyed Signal Using an SOA-Based Nonlinear Polarization Switch

Broad-band all-optical wavelength conversion of differential phase-shift keyed (DPSK) signal is experimentally demonstrated. This scheme is composed of a one-bit delay interferometer demodulation stage followed by a semiconductor optical amplifier (SOA) based nonlinear polarization switch. A wavelength converter for the 10 Gb/s DPSK signal is presented, which has a wide wavelength range of more than 30 nm. The converted signals experience small power penalties less than 1.4 dB compared with the original signal, at a bit error rate of 10−9. Additionally, the optical spectra, the measured waveforms and the open eye diagrams of the converted signals show a high quality wavelength conversion performance.

Optical data exchange of differential quadrature phase-shift keying based on four-wave mixing using a semiconductor optical amplifier

We propose to use a semiconductor optical amplifier as the nonlinear medium to realize data exchange of two differential quadrature phase-shift keying (DQPSK) signals based on the nondegenerate four-wave mixing (FWM) process. This scheme exploits the parametric depletion effect of the FWM process, when proper pump power and wavelength result in almost negligible noise of the original signal for the converted signal after data exchange. Optical data exchange of two DQPSK signals at 10 Gbps is achieved with a power penalty of less than 4 dB at the bit-error rate of 10−9.

40 Gb/s reconfigurable optical logic gates based on FWM in silicon waveguide.

Here we experimentally demonstrate reconfigurable logic gates via four-wave-mixing (FWM) in silicon waveguide with an operating speed of up to 40 Gb/s. After demodulated by a 40-GHz delay interferometer (DI), four non-return-to-zero differential phase shift keying (NRZ-DPSK) signals with carefully selected wavelengths are launched into the waveguide at the same time. Thanks to the effective FWM in silicon nano-waveguide, a full set of the two-input logic minterms can be generated simultaneously, and arbitrary combinational logic functions are able to be realized by properly combining these minterms.

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.

DP-DQPSK Optical Access System Integrated with Fiber and Free-Space Downlink for High Spectral Efficiency Application

In this letter, we present the generation, the balanced detection, and the transmission performance evaluation of dual polarization differential quadrature phase shift keying (DP-DQPSK) signals in optical access system integrated with fiber and free-space downlink. Polarization-multip- lexed (POLMUX) technique is introduced in the system for high spectral efficiency access utilization. 10 Gb/s DP-DQPSK downlink signals are successfully transmitted over 50 km SMF-28 and a 800 m wireless optical channel under the bad weather condition, such as fog and haze. The results show that the potentiality of DP-DQPSK optical access system is integrated with fiber and free- space downlink for providing flexible user access with high bandwidth efficiency.

Specific Emitter Identification Based on Nonlinear Dynamical Characteristics

Specific emitter identification (SEI) designates the unique transmitter of a given signal, using only external feature measurements called the RF fingerprints of the signal. SEI is often used in military and civilian spectrum-management operations. The SEI technique has also been applied to enhance the security of wireless network, such as VHF radio networks, Wi-Fi networks, cognitive radios, and cellular networks. A novel SEI method based on nonlinear dynamical characteristics is proposed in this paper. The method works based on the actual signal's inherent nonlinear dynamical characteristics. The permutation entropy is extracted as the signal's RF fingerprint to identify the unique transmitter. The quadrature phase-shift keying (QPSK) signals from four wireless network cards and differential quadrature phase-shift keying (DQPSK) signals from three digital radios are utilized to evaluate the performance of the method. Experimental results demonstrate that the proposed method is effective. On the other hand, the proposed method is convenient to implement in a PC.

Pump linewidth requirements for processing dispersion-altered DQPSK signals using FWM

We report on a potentially deleterious issue regarding the four-wave mixing (FWM)-based processing of dispersion-altered signals. We estimate the baudrate-dependent pump linewidth tolerances by calculating the extra optical signal to noise ratio (OSNR) penalty with respect to the propagation distance. We find that the issue is not important for 10Gbaud differential quadrature phase shift keying (DQPSK) signals, though for 28Gbaud (and 56Gbaud) DQPSK signals we find that the pump linewidth requirements to implement FWM based optical signal processing needs to be in the sub-MHz range in order to avoid excessive OSNR penalties for the case of dispersion-altered signals. These results are pertinent for systems employing FWM, which could be all—optical wavelength converters for packet switching or mid-span spectral inversion techniques.

On-chip WDM mode-division multiplexing interconnection with optional demodulation function.

We propose and fabricate a wavelength-division-multiplexing (WDM) compatible and multi-functional mode-division-multiplexing (MDM) integrated circuit, which can perform the mode conversion and multiplexing for the incoming multipath WDM signals, avoiding the wavelength conflict. An phase-to-intensity demodulation function can be optionally applied within the circuit while performing the mode multiplexing. For demonstration, 4 × 10 Gb/s non-return-to-zero differential phase shift keying (NRZ-DPSK) signals are successfully processed, with open and clear eye diagrams. Measured bit error ratio (BER) results show less than 1 dB receive sensitivity variation for three modes and four wavelengths with demodulation. In the case without demodulation, the average power penalties at 4 wavelengths are -1.5, -3 and -3.5 dB for TE₀-TE₀, TE₀-TE₁ and TE₀-TE₂ mode conversions, respectively. The proposed flexible scheme can be used at the interface of long-haul and on-chip communication systems.

Simple DQPSK Signal Regenerator Using Electrical Limiting Amplifiers

A simple optoelectronic regenerator for differential quadrature phase-shift keying (QPSK) signals is demonstrated. The regenerator consists of two parallel delay interferometers followed by balanced detectors, limiting and driving amplifiers, and a dual-parallel Mach–Zehnder modulator that regenerates noise-suppressed optical QPSK signals. We experimentally show that the regenerator improves the receiver sensitivity by about 8 dB at an input optical signal-to-noise ratio (OSNR) of 19 dB at a signal speed of 10 Gbaud. Numerical simulation is conducted for higher speed operation at 56 Gbaud to study the impact of the bandwidth of electrical circuits inside the regenerator on the regenerator performance. Electrical power consumption of the regenerator is assessed, and it is shown that the optoelectronic regenerator consumes a similar amount of electrical power as that consumed by an all-optical version of the regenerator using semiconductor optical amplifiers as a nonlinear element.

On the Error and Outage Performance of Decode-and-Forward Cooperative Selection Diversity System With Correlated Links

A cooperative diversity system improves the communication reliability and throughput in a multipath fading environment with the help of relay nodes. We consider a cooperative diversity system with a source (S), a decode-and-forward relay (R), and a selection-combining-enabled destination (D). In this paper, we derive the exact end-to-end symbol error probability (SEP) for $M$ -ary phase-shift keying (MPSK) signaling using a paired error approach and differential binary phase-shift keying (DBPSK) signaling over correlated, nonidentical, slow, and flat Nakagami- $m$ fading channels by assuming the correlation between SD–RD, SR–SD, and SR–RD fading channels. In addition, we perform the exact outage probability analysis for the same scenario. Furthermore, we provide the asymptotic expressions for SEP and outage probability by removing the infinite-series terms to investigate the diversity order. In a nutshell, the numerical results reveal that the SR–RD correlated channel scenario outperforms the uncorrelated channel scenario in terms of SEP. In addition, SEP performance degradation is observed in the case of other correlated channel scenarios compared with the uncorrelated channel scenario. Moreover, marginal performance improvement in outage probability is observed in the low-signal-to-noise-ratio (SNR) regime of the SR–RD correlated channel scenario compared with the uncorrelated channel scenario. Finally, Monte Carlo simulations are performed to validate the theoretical results.

Implications of in-band crosstalk on DQPSK signals in ROADM-based metropolitan optical networks

Metropolitan optical networks can be designed to transport a multitude of signals with different bit rates and modulations formats. In this way, in-band crosstalk signals, originated from imperfect isolation inside ROADM (Reconfigurable Optical Add and Drop Multiplexer)-based optical nodes, will potentially have a different modulation format than the primary signal. In this paper, the origin of in-band crosstalk in a typical ROADM is analysed and its impact on differential quadrature phase-shift keying (DQPSK) signals is assessed through an analytical formalism based on the moment generating function (MGF) of the receiver decision variable. Various scenarios are analysed including the case of multiple interfering terms with different modulation formats, namely the on-off keying (OOK). It is concluded that the OOK interferer is more detrimental than the DQPSK interferer for DQPSK systems, or than the differential phase-shift keying (DPSK) interferer for DPSK systems.

Bit-error-rate improvement of binary phase-shift keying signals with an optoelectronic regenerator based on the feed-forward control of a LiNbO3 intensity modulator

This study evaluates the performance of an optoelectronic regenerator based on feed-forward control of a LiNbO3 intensity modulator for optical binary phase-shift keying (BPSK) signals. First, in order to confirm the operation principle, we measure the transfer function of the regenerator in terms of the optical power, and observe the power limiting effect. Next, we numerically and experimentally evaluate the performance of the regenerator for 10-Gbit/s optical BPSK signals where BER is measured for various receiver OSNR. We obtain a received power improvement of approximately 1dB at a bit error rate of 10−4. Finally, we compare the regenerator performance for BPSK signals with that for differential phase-shift keying (DPSK) signals by calculating the BERs with offline digital signal processor for identical experimental data. It is shown that the regenerator is more effective for BPSK signals than for DPSK signals because DPSK signals are more fragile against phase fluctuation that cannot be suppressed by our regenerator.

25 Gbit/s differential phase-shift-keying signal generation using directly modulated quantum-dot semiconductor optical amplifiers

Error-free generation of 25-Gbit/s differential phase-shift keying (DPSK) signals via direct modulation of InAs quantum-dot (QD) based semiconductor optical amplifiers (SOAs) is experimentally demonstrated with an input power level of −5 dBm. The QD SOAs emit in the 1.3-μm wavelength range and provide a small-signal fiber-to-fiber gain of 8 dB. Furthermore, error-free DPSK modulation is achieved for constant optical input power levels from 3 dBm down to only −11 dBm for a bit rate of 20 Gbit/s. Direct phase modulation of QD SOAs via current changes is thus demonstrated to be much faster than direct gain modulation.

All-optical pulsewidth-tunable wavelength conversion of return-to-zero differential phase-shift keying signal

We demonstrate the wavelength conversion of return-to-zero differential phase-shift keying (RZ-DPSK) signal with the pulsewidth-tunable operation using a Raman amplifier (RA)-based ultrashort pulse compressor and a highly nonlinear fiber (HNLF)-based four-wave mixing (FWM) switch. The wavelength of the input RZ-DPSK signal is converted to new wavelengths owing to the FWM effect in HNLF. On the other hand, the pulsewidth of the wavelength-converted RZ-DPSK signal is tuned from 10 to 3 ps by using the ultrashort pulse source based on RA-based adiabatic soliton pulse compression. The bit error rate characteristics of the input RZ-DPSK and the wavelength-converted RZ-DPSK signals with different pulsewidths are measured and error-free operations are achieved in all cases. The optical spectra, autocorrelation traces, and clear and opening eye diagrams show high conversion performance with wide-range pulsewidth tunability.

Wide range operation of an all-optical NRZ-DPSK-to-RZ-DPSK regenerative waveform–wavelength conversion with flexible width-tunability

For the first time, we demonstrated a wide range operation of an all-optical non-return-to-zero (NRZ)-differential-phase-shift-keying (DPSK)-to-return-to-zero (RZ)-DPSK waveform–wavelength conversion with flexible picosecond width-tunablity and regenerative functionality. The scheme is based on a Raman amplifier soliton compressor (RASC) and a fiber-based four-wave mixing (FWM) AND-gate. In the first demonstration, we demonstrated waveform–wavelength conversion of a 10-Gb/s DPSK signal without input signal degradation over 54-nm input–output wavelength ranges. The measurement results of the converted RZ-DPSK signal are pedestal-free, and its converted pulse width from 13.0 to 2.87-ps can be adjusted by tuning the Raman pump power between 0.20 and 0.90 W. An investigation of the regenerative waveform–wavelength conversion is further conducted at different Raman pump power settings over 40-km standard single-mode fibers without dispersion compensation. Error-free operation with a low power penalty less than $$-1.95$$ dB is obtained for the RZ-DPSK regenerated converted signal.

Multi-Format Wavelength Conversion Using Quantum Dash Mode-Locked Laser Pumps

We investigate and compare the performance of wavelength conversion for two different non-return-to-zero (NRZ) modulation formats at 40 Gb/s: on off keying (OOK) and differential phase-shift keying (DPSK). To achieve wide wavelength coverage and integrability, we use a dual pump scheme exploiting four-wave mixing in semiconductor optical amplifiers. For phase stability, we use a quantum-dash mode-locked laser (QD-MLL) as a multi-wavelength source for the dual pumps, with tunability provided by the output filter. The significant sidelobes of the DPSK spectrum (relative to OOK) require the balancing of the pump proximity to the original signal (facilitating high conversion efficiency) with the signal degradation from the pump spectrum overlapping the converted DPSK signal. We achieve a conversion efficiency near –3.6 dB for OOK and –5.4 dB for DPSK across a 12 nm tuning range with low input powers (1 dBm). We measure bit error rate (BER) and obtain error free transmission (BER < 10−9) with a power penalty less than 2 dB for OOK and 3 dB for DPSK.

A DSP-Based Control Method For a Nonlinear Mach–Zehnder Interferometer DPSK Regenerator

A frequency offset and phase control method is described for a differential-phase-shift-keying (DPSK) 2R (reamplification and reshaping) regenerator based on a nonlinear Mach–Zehnder interferometer. The controller is comprised of a coherent receiver, analog-to-digital converters (ADCs), a digital signal processor, and an optical frequency/phase shifter. For a 40 Gb/s DPSK signal, simulation results demonstrate that an ADC sampling rate of only 335.94 MSa/s is needed to adequately estimate the frequency offset and phase noise, and adjust the pump signal accordingly. The cascadability of the 2R regenerator is evaluated in recirculating loop simulations for different regenerator spacings, using both ideal DPSK signals and captured experimental DPSK signals. The influence of residual span dispersion on the performance is investigated.

Phase regeneration for polarization-division multiplexed signals based on vector dual-pump nondegenerate phase sensitive amplification.

The polarization-division multiplexing (PDM) technology is a practical method to double the transmission capacity, and the corresponding phase regeneration (PR) for PDM signals is meaningful and necessary to extend the transmission distance and increase the transparency for the phase-encoded PDM system. Those reported PDM PR schemes either utilized polarization-diversity technique or required special PDM format. In order to overcome these issues, the PR for the PDM phase-modulated signals is proposed and theoretically demonstrated in this paper, based on the vector dual-pump nondegenerate phase sensitive amplification (PSA). The theoretical model is established and the detailed characteristics are investigated to optimize the PR performance. Results show an obvious phase squeezing for the degraded 80 Gbit/s PDM differential phase-shift keying (DPSK) signals, and the error vector magnitude (EVM) of the regenerated signals on dual polarization states can be improved from 22.58% and 21.39% to 4.57% and 4.63%, respectively. Furthermore, the applicability of the proposed scheme for PDM quaternary-phase shift keying (QPSK) signals is investigated. The proposed scheme can be useful and promising in current PDM based coherent fiber-optic communication.

Investigation of OSNR monitoring technique based on the beat noise detection

We propose an optical signal-to-noise ratio (OSNR) monitoring technique based on the beat noise detection. The signal under test (SUT) is filtered by a band pass filter (BPF) at the signal center frequency. The incident power of the monitor module is adjusted to keep constant. Through measuring the noise power at low radio frequencies (RF), OSNR can be monitored. The monitoring performance for 80Gb/s non-return-to-zero differential quadrature phase shift keying (NRZ-DQPSK) signal is experimentally demonstrated. OSNR of 8~33.5dB can be monitored with a 25.4dB dynamic range (DR) at 100MHz RF. The impacts of the BPF bandwidth, received power, RF position, chromatic dispersion (CD), and first order polarization mode dispersion (PMD) on the monitoring performance are experimentally investigated. The bandwidths of the receivers can be reduced to tens of MHz.