Optical Differential Phase-shift Keying Research Articles

An MDPSK homodyne receiver with adaptive phase-diversity

ABSTRACT We propose an adaptive phase-diversity method for optical M-ary differential phase-shift keying (MDPSK) homodyne receivers, to achieve coherent optical communication. This method involves eliminating the random fluctuations in the phase differences between the signal light and local oscillator and the in-plane and quadrature signal (IQ) imbalance compensation. For the former, a phase-diversity method, which can theoretically eliminate all phase-related noise, is proposed. The gain and phase imbalances between the I and Q channels can be compensated completely by using automatic phase- and gain-control systems. The core idea is the adaptability of the receiver to the local phase shift of the local oscillator. The performance of the receiver is not sensitive to environmental factors such as temperature and vibrations. Our simulation results show that the imbalances between the IQ channels can be completely compensated over a wide imbalance range, and the baseband signals can be accurately recovered using the proposed scheme.

EDFA Anti-Irradiation Schemes for Inter-Satellite Optical DPSK Communication Systems

The uprising of rare-earth-doped fiber amplifier technology provides an effective solution to signal power compensation for interstellar optical communication systems, but significant signal power attenuation induced by spatial irradiation has ever been a significant issue to be resolved in practical applications. In order to explore the impact of radiation on the performances of high-speed differential phase shift keying (DPSK) satellite communication systems using EDFA preamplification technique, the irradiation effect on output central wavelength, half width, and noise figure (NF) of EDFA has been studied. Different anti-irradiation schemes employing Co 60 radiation sources with a total radiation dose of up to 300 krad are compared from experimental perspective to optimize the anti-irradiation scheme. Radiation-induced attenuation (RIA) as well as increase in NF of the EDFA is considered in our simulation model, by which the limitations of the physical shielding scheme could be clarified and quantitatively evaluated. We conclude that compared to the RIA effect, the increase in noise figure plays a major role in the degradation of communication systems. In addition, according to simulation results of two high-speed DPSK systems, some anti-irradiation schemes are proposed as recommendations for future applications of EDFA-assisted optical systems under radiation exposure environments.

Performance Analysis of a Non-Hermitian OFDM Optical DQPSK FSO Link over Atmospheric Turbulent Channel

Abstract A free-space optic (FSO) system model considering non-Hermitian orthogonal frequency division multiplexing (OFDM) modulation followed by optical differential quadrature phase shift keying (DQPSK) modulation is proposed. Analysis is presented for an OFDM-DQPSK FSO link considering an optical direct detection receiver based on dual detection optical QPSK demodulator. Analytical approach is developed to find the expressions for output current and signal to noise ratio (SNR) in presence of atmospheric turbulence. The conditional bit error rate (BER) for a given turbulence-induced fading is also derived. Average SNR and BER are obtained considering the turbulence-induced fading to be log-normal distribution. Results are evaluated in terms of average SNR and BER for different link distances, number of OFDM subcarriers, background current and different wavelengths. It is noticed that the proposed OFDM FSO system suffers 4 and 1.4 dB power penalty at a BER of 10−12 due to the effect of atmospheric turbulence and background radiation respectively for 512 number of OFDM subcarrier and link distance of 2,000 m. Comparison of the results is made with simulation results as well as experimental results reported earlier. The proposed OFDM FSO system provides better performance, with an almost 3 dB improvement in receiver sensitivity compared to the experimental results on OFDM-QPSK FSO link reported earlier at a BER of 10−6. Finally, the analytical results are verified by numerical simulation.

Using optical differential phase-shift keying to solve the bipolarity problem of spreading code in optical time domain reflectometer

Abstract The application of spread spectrum technology in optical time domain reflectometer (OTDR) is studied. And an innovative method for direct using bipolar spreading code in OTDR is proposed by using optical differential phase-shift keying (DPSK). The theoretical analysis and simulation of the method are carried out, and the simulation results are in good agreement with the theoretical analysis, verifying the feasibility of the method.

Modeling interchannel four-wave mixing for 8-Ary modulated dense wavelength division multiplexing systems over dispersion map

Semianalytic models are developed to deterministically calculate the variances of degenerate and nondegenerate four-wave mixing (FWM) noises for dispersion-managed dense wavelength division multiplexing (DWDM) systems with 8-Ary modulations [i.e., 8-level amplitude- and differential phase-shift keying (8APSK) and constant-amplitude optical differential 8-level phase-shift keying (D8PSK)]. The semianalytic models include various important propagation effects for exact numerical results. A 5.28-Tb/s (40-Gs/s/ch) 100-GHz-spaced 33-channel DWDM system with a dispersion map is then numerically analyzed by using the newly derived semianalytic models. It is numerically validated that FWM impacts coming from 8APSK pump channels are more severe than those coming from D8PSK ones, where pump channels denote the channels whose energies are transferred to a probe channel through the FWM process. The numerical results show that although FWM tolerance of a central channel with 8APSK is worse than that with D8PSK, a central channel with 8APSK is still superior to that with D8PSK when some linear noises and FWM noise are simultaneously taken into account for our given system conditions, which is mainly attributed to a relatively larger minimum Euclidean distance for the 8APSK constellation than the D8PSK one.

Performance of Phase Modulation in WDM-PON Transmission System

AbstractThe modulation and demodulation methods of the optical Differential Quadrature Phase Shift Keying (DQPSK), Optical Differential Phase Shift Keying (DPSK) and the different modulation formats including nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed return-to-zero (CS-RZ) are introduced, studying the ability of anti-dispersion for different optical signals in the 40 Gb/s high speed transmission system. The system is simulated in optical fiber by way of dispersion compensation. And then analyzed the compensation effect of the different dispersion compensation method for different phase modulation formats. It is shown that the ability of anti-dispersion and anti-PMD is better in the CS-RZ-DQPSK modulation format, and this format has the smallest eye-opening penalty (EOP) with a wider range of power into the optical fiber.

Performance Enhancement of 10 Gbps OCDMA Networks Using DPSK and DQPSK with Unique Code-Sequence

This paper demonstrates the achievable performance enhancement in a multi-user network using optical unique code sequences. The study is conducted in a four-user Metropolitan Area Network (MAN) with a transmission rate of 10 Gbps. This paper investigates the feasibility of implementing Differential Phase Shift Keying (DPSK) and Differential Quadrature Phase Shift Keying (DQPSK) technique to replace conventional techniques such as On-Off Keying (OOK) and Amplitude Shift Keying (ASK). The performance of the integrated formulation of optical unique code sequenceswith DPSK and DQPSKtechnique is evaluated by determining the Bit Error Rate (BER) for various configurations and transmission distances up to 100 km.

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.

Re-modulated technology of WDM-PON employing different DQPSK downstream signals

This paper proposes a kind of modulation architecture for wavelength-division-multiplexing passive optical network (WDMPON) employing optical differential quadrature phase shift keying (DQPSK) downstream signals and two different modulation formats of re-modulated upstream signals. At the optical line terminal (OLT), 10 Gbit/s signal is modulated with DQPSK. At the optical network unit (ONU), part of the downstream signal is re-modulated with on-off keying (OOK) or inverse-return-to-zero (IRZ). Simulation results show the impact on the system employing NRZ, RZ and carrier-suppressed return-to-zero (CSRZ). The analyses also reflect that the architecture can restrain chromatic dispersion and channel crosstalk, which makes it the best architecture of access network in the future.

Analysis the Speciality of Modulation Format in WDM-PON System

In the wavelength division multiplexing passive optical network (WDM-PON) system, the modulation format was important. While three different modulation formats including not-return-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed return-to-zero (CS-RZ) and the optical differential quadrature reference phase shift keying (DQPSK) modulation format of each code are introduced in the article. A method of their modulated signal generation with computer is described, and a comparison of their spectra and waveforms is made. The 40 Gbps signal transmitted in 200 km G.652 fiber by way of single channel with erbium-dope-fiber-amplifier (EDFA) is simulated for these three formats. It is showed that the ability of anti-dispersion and anti-PMD (Polarization Mode Dispersion) is better in the CS-RZ-DQPSK modulation format, and this format has the smallest eye-opening penalty with a wider range of power into the optical fiber.

Centralized light-wave WDM-PON employing DQPSK downstream and OOK remodulated upstream signals

A novel architecture for wavelength-division-multiplexing passive optical network (WDM-PON) with centralized light-wave is proposed and demonstrated. At the optical line terminal (OLT) session of this architecture, optical differential quadrature phase shift keying (DQPSK) modulated signal with constant-intensity at 10 Gbit/s is utilized for downstream transmission. At the optical network unit (ONU), part of the downstream optical power is remodulated with on-off keying (OOK) intensity modulation at 10 Gbit/s for upstream without additional laser. Simulation results show that the power penalties of the downstream for 20 km single-mode fiber (SMF) transmission and multi-channels are negligible. While for the upstream, the power penalties are obvious. The simulation results and analysis also reflect that by reducing the launch power of DQPSK transmitters, power penalties can be reduced, although the transmission distance is limited.

On the Gaussian Approximation and Margin Measurements for Optical DPSK Systems With Balanced Detection

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Based on the computed exact probability density functions (PDFs), we analyze the conventional Gaussian approximation (GA) method and the thresholding-based bit error rate (BER) measurement method in back-to-back optical differential phase shift keying (DPSK) systems with balanced detection for the purpose of margin measurement in noise-loading experiments. We show that the GA method fails when used with DPSK systems in contrast to its success in the case of on-off keyed (OOK) systems, and as a result, the thresholding method is not as robust for DPSK systems as it is for OOK systems. Still, our results show that the thresholding method, when using reasonably small threshold bias, can measure large performance margins for DPSK systems with a wide range of <formula formulatype="inline"><tex Notation="TeX">$E_b/N_0$</tex></formula> (the energy bit to noise power spectral density ratio) and optical bandpass filter (BPF) bandwidth, and receiver imperfections such as the PIN responsivity imbalance and frequency detuning. We further discuss briefly on BER estimation for nonlinear fiber transmission. </para>

Investigation of a method to use a single shared delayed interferometer for receiving optical differential phase shift keying wavelength-division multiplexing signals

A receiver configuration using a single shared delay interferometer (DI) for optical differential phase-shift keying (DPSK) wavelength-division multiplexing (WDM) signals is demonstrated and investigated in 40-Gbit/s signal transmission. Experiment comparison between the proposed and conventional configurations is performed for signal performance and a 3-dB bandwidth of the receiver sensitivity penalty. The wavelength mismatch of optical components used and the effects are measured to investigate the wavelength dependency of the proposed configuration. This demonstrates that there is no difference in performance between the two configurations and between WDM channels in the proposed configuration. Thus, it is suggested that the proposed configuration using a single DI can be applied for 20 WDM channels between 1531.116 and 1561.419 nm, with 200-GHz channel spacing in a bandwidth of >30 nm, for receiving DPSK signals.

Nearly Shot-Noise-Limited Performance of Dual-Channel Linear Optical Sampling for Ultrafast DPSK Signals

This paper describes a newly developed dual-channel linear optical sampling technique for observing ultrafast optical differential phase-shift keying (DPSK) signals. As the proposed measurement scheme offsets two parallel interferometers by a relative delay corresponding to 1-symbol length of the DPSK signal, the measured phase distribution reflects the signal quality which is determined by the phase difference between adjacent symbols. This technique, based on interferometric optical gating by local short-pulses, also offers ultrafast measurement at symbol rates of greater than 100 Gsymbol/s. Moreover, its detection sensitivity can reach the shot noise limit. The waveform degradation caused by the coherence of the light source and the pattern effect of the phase modulator is successfully observed in continuous waves and 10-Gsymbol/s nonreturn-to-zero DPSK signals, and the constellation measurement is demonstrated for a 160-Gsymbol/s return-to-zero DPSK signal. Measurement system noise is also discussed for characterizing the detection sensitivity, and the nearly shot-noise-limited performance is experimentally verified.

Self-Coherent Multisymbol Detection of Optical Differential Phase-Shift Keying

This paper treats the recently emerging self-coherent multisymbol decision-feedback-aided detection techniques for optical differential phase-shift keyed (DPSK) transmission based on both an optical preprocessing approach and an electronic postprocessing approach. We present in detail i) the generation of a synthetic low phase-noise effective local oscillator out of the modulated optical data itself, and its estimation theory interpretation; ii) the role of polyphase (interleaved) DPSK in the optical preprocessing approach in easing the decision feedback realization requirements; iii) detection of M-ary DPSK modulation formats for general M, and differential quadrature phase-shift keying in particular; iv) three alternative integrated-optoelectronic receiver implementations for the optical preprocessing approach detailing the structures of the interferometric front-end and the high-speed electronic analog and digital building blocks; and v) the principle, and alternative implementations, of the digital postprocessing approach to self-coherent detection. We finally compare the capabilities of the emerging self-coherent detection and digital coherent detection techniques.

Ultrafast Measurement of Optical DPSK Signals Using 1-Symbol Delayed Dual-Channel Linear Optical Sampling

We propose and demonstrate a 1-symbol delayed dual-channel linear optical sampling for observing optical differential phase-shift keying (DPSK) signals. As this technique is based on optical gate processing by means of interference with local short pulses, its performance allows ultrafast measurement for a symbol rate of greater than 100 Gsymbol/s. Moreover, as the new measurement apparatus employs a two-series interferometer system with a 1-symbol delay, the measured phase distribution reflects the signal quality between adjacent symbols of the optical DPSK signals. In our experiment, we successfully observe the waveform degradation caused by the coherence of the light source and the pattern effect of the phase modulator. The measurement system noise is also discussed.

Simple phase offset monitoring technique for 43 Gbit/s optical DQPSK receiver

Proposed is a simple phase offset monitoring technique that uses a simple electric limiter amplifier circuit for the optical differential quadrature phase shift keying (DQPSK) receiver. Phase offset can be monitored even if the eye pattern of the waveform of the demodulated 43 Gbit/s DQPSK signal is completely closed due to chromatic dispersion exceeding 250 ps/nm.

Generalized data-aided multi-symbol phase estimation for improving receiver sensitivity in direct-detection optical m-ary DPSK

We present a generalized framework of data-aided multi-symbol phase estimation (MSPE) for improving the receiver sensitivity of direct-detection optical m-ary differential phase-shift keying (DPSK) through the extraction of a more accurate phase reference. Derivations of the data-aided MSPE algorithms for quaternary DPSK (DQPSK), 8-ary DPSK, m-ary DPSK, and DPSK/ASK are provided. Simulations show that receiver sensitivity improvements of over 2 dB can be obtained with the data-aided MSPE, enabling the performances of direct-detection m-ary DPSK signals to approach those of coherent homodyne-detection without resorting to an optical local oscillator. Performance improvements in nonlinear transmission are briefly discussed, and an electronic nonlinear phase shift compensation scheme is introduced for DPSK/ASK. Also presented is a simple yet universal receiver platform for m-ary DPSK consisting of just one pair of optical I/Q demodulators and a digital signal processing unit.

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

Performance evaluation of optical DQPSK using saddle point approximation

In this paper, the authors derive the moment generating function (MGF) of the decision variable for a practical optical differential quadrature phase shift keying (DQPSK) receiver affected by phase noise and the amplifier spontaneous emission noise. The effect of the different optical filters, Gaussian, Fabry-Peacuterot (FP), and optical integrator, on the phase noise is examined. Using the saddle point approximation, the authors obtain numerically the bit error rate (BER) for the above system, as a function of the optical-signal-to-noise ratio (OSNR) and the source linewidth. The model is found to agree with previous published simulation results and the Monte Carlo simulations presented in this paper. The authors also calculate the BER floors for DQPSK inflicted by the phase noise and compare its performance with differential phase shift keying (DPSK). Finally, the authors examine the effect of interferometric demodulation on the relative intensity noise (RIN) spectra and the impact on the DQPSK system performance. In particular, the authors examine the influence of system dispersion on the receiver sensitivity