OSNR Monitoring Technique for DPSK/DQPSK Signals Based on Self-Heterodyne Detection

We describe a new method for monitoring the “in-band” optical signal-to-noise ratio (OSNR) of differential phase-shift-keying (DPSK) and differential quadrature phase-shift-keying (DQPSK) signals. This technique estimates the OSNR by analyzing the radio frequency spectrum obtained by the self-heterodyne detection. The results show that the performance of the proposed technique is not sensitive to the effects of chromatic dispersion and polarization-mode dispersion. In addition, this technique can be used for monitoring the polarization-scrambled signals. For a demonstration, we measure the OSNR of the polarization-scrambled 10-Gb/s DPSK and 20-Gb/s DQPSK signals in a 640-km-long transmission link.      

Wavelength Conversion of High-Speed Phase and Intensity Modulated Signals Using a Highly Nonlinear Chalcogenide Glass Chip

We report all-optical wavelength conversion of high-speed differential phase-shift keyed (DPSK) and on–off keyed (OOK) signals using the nonlinear Kerr-effect in an optical chip. This was enabled by four-wave mixing (FWM) of the signal with a continuous-wave (CW) pump laser in a 7-cm-long dispersion-shifted planar rib waveguide in highly nonlinear $hbox{As}_{2} hbox{S} _{3}$ glass. Both conversion of 40-Gb/s DPSK and 160-Gb/s OOK signals by 33 and 15 nm, respectively, are shown. These are first demonstrations of signal processing by CW-pumped FWM in a chalcogenide waveguide, highlighting its capability to perform phase-preserving operations at high bit rates in chip-scale devices.      

WDM PON using 10-Gb/s DPSK downstream and re-modulated 10-Gb/s OOK upstream based on SOA

A signal remodulation scheme of 10-Gb/s differential phase-shift keying(DPSK) downstream and 10-Gb/s on-off keying(OOK) upstream using a semiconductor optical amplifier(SOA) and a Mach-Zehnder intensity modulator(MZ-IM) at the optical networking unit(ONU) side for wavelength division multiplexed passive optical network(WDM PON) is proposed.Simulation results indicate that error-free operation can be achieved in a 20-km transmission,and the receiver sensitivity of return-to-zero differential phase-shift keying(RZ-DPSK) is higher than nonreturn-to-zero differential phase-shift keying(NRZ-DPSK) in the proposed scheme.     

Optical Signal-to-Noise Ratio Estimation Using Reference Asynchronous Histograms

This paper presents and experimentally demonstrates a novel method for the estimation of optical signal-to-noise ratio (OSNR) based on the comparison of an asynchronous histogram of the signal under analysis with a reference asynchronous histogram. The latter is acquired from the signal under analysis at a calibration stage. The proposed method allows the use of optical amplification to increase the sensitivity of the optical monitoring system (OMS) by a factor 20 dB, when using an erbium doped fiber preamplifier. In addition, the use of a semiconductor optical preamplifier, initially designed for nonlinear operation at 2.5 Gb/s is used in the OMS to preamplify 40-Gb/s signals, achieving a sensitivity gain of 10 dB. It will be experimentally demonstrated that the proposed method is applicable to 40-Gb/s nonreturn to zero (NRZ) signals arbitrarily degraded by group velocity dispersion (GVD). Furthermore, accurate monitoring of the OSNR of return-to-zero (RZ) signals will also be possible using a simple RZ-to-NRZ converter based on narrow-band optical filtering within the OMS. The proposed method also allows estimating of the GVD-induced OSNR penalty between the signal under analysis and the signal at the calibration stage.     

Optical Performance Monitoring of Phase-Modulated Signals Using Asynchronous Amplitude Histogram Analysis

As optical networks continue to grow towards high capacity and high flexibility, new transmission technologies are being introduced. In order to maintain the quality of signal and control over network in the transparent domains, optical performance monitoring (OPM) systems are becoming a necessity. Phase modulation formats emerge as the solution of choice in transparent domains because of their sensitivity, spectral efficiency, and resilience to optical impairments. In this paper, we demonstrate a flexible OPM method for phase-modulated signals using asynchronous amplitude histogram analysis. We show numerically and experimentally the monitoring of optical signal-to-noise ratio (OSNR), chromatic dispersion (CD), and polarization-mode dispersion (PMD) for differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) signals. The OSNR can be measured within range of 20-35 dB and accumulated chromatic dispersion between 600 and 600 ps/nm. The asynchronous amplitude histogram monitoring method is proved to be a precise and versatile monitoring tool for high-capacity optical networks.     

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 /spl pi/-phase-shifted fiber Bragg grating and an Yb/sup 3+/-Al/sup 3+/ 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.     

Optical performance monitoring using data stream intensity autocorrelation

We demonstrate optical performance monitoring (OPM) using intensity autocorrelations of 10-Gb/s return-to-zero (RZ) data. Data stream intensity autocorrelations are used for the quantitative determination of the optical signal-to-noise ratio (OSNR) and accumulated dispersion values up to half the Talbot dispersion. Autocorrelations are performed using sensitive two-photon absorption in photon counting silicon avalanche photodiodes. Simulations and experimental results demonstrate the retrieval of optical signal-to-noise ratio values in the presence of large accumulated dispersion.     

A WDM Passive Optical Network With Polarization-Assisted Multicast Overlay Control

We propose a novel wavelength-division-multiplexed passive optical network which supports simultaneous delivery of 10-Gb/s point-to-point downstream and upstream data as well as 10-Gb/s downstream multicast data. The multicast overlay control is achieved by a polarization-assisted scheme at the optical line terminal (OLT). A separate lightpath is provided for the downstream multicast differential phase-shift keying (DPSK) data without additional light sources. The upstream amplitude-shift keying signal at the optical network unit is superimposed onto the received multicast DPSK signal before being transmitted back to the OLT.      

Repeaterless 10.7-Gb/s DPSK Transmission Over 304 km of SSMF Using a Coherent Receiver and Electronic Dispersion Compensation

Record repeaterless transmission of differential phase-shift keying (DPSK) at 10.7 Gb/s over 304 km of standard single-mode fiber (SSMF) is demonstrated using a coherent optical receiver and electronic dispersion compensation. This is the longest repeaterless 10-Gb/s transmission over SSMF in the absence of Raman amplifiers. The high receiver sensitivity and the high tolerance to nonlinearities of DPSK allow us to overcome a total link loss of 58 dB with a 3-dB system margin. Coherent detection enables linear electrical dispersion compensation and avoids the use of optical dispersion compensation.     

Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection

We study the impact of chromatic dispersion (CD) and first-order polarization-mode dispersion (PMD) on systems using binary differential phase-shift keying (2-DPSK) or quaternary DPSK (4-DPSK) with nonreturn-to-zero (NRZ) or return-to-zero (RZ) formats. These signals are received using optical preamplification, interferometric demodulation, and direct detection. We consider the linear propagation regime and compute optical power penalties at fixed bit-error ratio (BER). In order to evaluate the BER precisely taking account amplifier noise, arbitrary pulse shapes, arbitrary optical and electrical filtering, CD, and PMD, we introduce a novel model for DPSK systems and compute the BER using a method recently proposed by Forestieri for on-off keying (OOK) systems. We show that when properly applied, the method yields highly accurate results for DPSK systems. We have found that when either the NRZ or RZ format is used, 2-DPSK exhibits lower power penalties than OOK in the presence of CD and first-order PMD. RZ-2-DPSK, as compared with NRZ-2-DPSK, incurs smaller penalties due to PMD, but offers no advantage in terms of CD. 4-DPSK, as it has twice the symbol duration of OOK or 2-DPSK for a given bit rate, incurs much lower CD and PMD power penalties than either of these techniques. RZ-4-DPSK is especially promising, as it offers CD and PMD penalties significantly smaller than all other techniques, including NRZ-4-DPSK.     

差分相移键控的非归零到归零格式转换研究

提出了一种全新的基于相位-强度混合调制和色散补偿的光差分相移键控(DPSK)信号的非归零(NRZ)到归零(RZ)格式转换器,理论分析了转换器参数对转换的影响,数值研究了恶化条件下的10 Gb/s的NRZ-DPSK到RZ-DPSK的格式转换。实验展示了10 Gb/s的DPSK信号格式转换及解调后的误码性能。计算结果表明,通过设计转换器参数可获得低占空比RZ-DPSK信号,且转换后信号质量较高。实验结果表明格式转换功率代价较低,转换后RZ-DPSK信号时间抖动较原NRZ-DPSK信号减小。该格式转换器还适合光四相差分相移键控(DQPSK)的非归零到归零格式转换及多波长操作。     

RZ-DPSK transmission using a 42.7-Gb/s integrated balanced optical front end with record sensitivity

We have demonstrated a record sensitivity of -37.0 dBm (38 photons/bit) for a BER of 10/sup -9/ at 42.7-Gb/s using an integrated balanced optical front end. Results were obtained using optical preamplification of RZ-DPSK modulation and an external delay-interferometer. The OSNR requirement was measured to be 16.9 dB in a 0.1-nm bandwidth. The impact of polarization-mode dispersion (PMD) and chromatic dispersion on the optical front end performance has been measured. Performance for enhanced forward error correction has been projected based on 10/sup -3/ BER performance.     

Modulation and demodulation techniques in optical heterodyne PSKtransmission systems

Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described     

A WDM-PON with DPSK modulated downstream and OOK modulated upstream signals based on symmetric 10 Gbit/s wavelength reused bidirectional reflective SOA

We investigate a wavelength-division-multiplexing passive optical network (WDM-PON) with centralized lightwave and direct detection. The system is demonstrated for symmetric 10 Gbit/s differential phase-shift keying (DPSK) downstream signals and on-off keying (OOK) upstream signals, respectively. A wavelength reused scheme is employed to carry the upstream data by using a reflective semiconductor optical amplifier (RSOA) as an intensity modulator at the optical network unit (ONU). The constant-intensity property of the DPSK modulation format can keep high extinction ratio (ER) of downstream signal and reduce the crosstalk to the upstream signal. The bit error rate (BER) performance of our scheme shows that the proposed 10 Gbit/s symmetric WDM-PON can achieve error free transmission over 25-km-long fiber transmission with low power penalty.     

All-Optical RZ-DPSK WDM to RZ-DQPSK Phase Multiplexing Using Four-Wave Mixing in Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical phase multiplexing scheme using four-wave mixing in a highly nonlinear fiber. Two 10-Gb/s pi/2-shifted return-to-zero (RZ) differential phase-shift-keying (DPSK) wavelength-division-multiplexing (WDM) signals are successfully phase-multiplexed into a 20-Gb/s RZ differential quadrature phase-shift-keying signal with a negative 1.6-dB power penalty. With more input DPSK WDM signals, the proposed scheme can be applied to obtain a multilevel phase-shift-keying signal with increased capacity and enhanced spectral efficiency.     

Combining DPSK and duobinary for the downstream in 40-Gb/s long-reach WDM-PONs

We propose and demonstrate combining differential phase-shift keying (DPSK) and duobinary transmission for the downstream in 40-Gb/s long-reach wavelength division multiplexed-passive optical networks (WDM-PONs) in order to provide robust transmission performance in the backhaul section and simple detection at the ONUs. DPSK is deployed in the trunk span as it provides stronger robustness to fiber nonlinearity. Duobinary is used in the access span where its higher chromatic dispersion tolerance relieves the need for dispersion compensation. All-optical multichannel modulation format conversion from DPSK to duobinary is realized in the local exchange in a single delay interferometer to reduce system cost. Single and multi-channel 80-km long-reach DPSK transmission and up to 5-km duobinary access transmission are experimentally demonstrated at 40 Gb/s. The proposed approach shows great potential for future high data rate optical access networks.     

Parametric-gain approach to the analysis of single-channel DPSK/DQPSK systems with nonlinear phase noise

This paper presents a novel method based on a parametric gain (PG) approach to study the impact of nonlinear phase noise in single-channel dispersion-managed differentially phase-modulated systems. This paper first shows through Monte Carlo simulations that the received amplified spontaneous emission (ASE) noise statistics, before photodetection, can be reasonably assumed to be Gaussian, provided a sufficiently large chromatic dispersion is present in the transmission fiber. This paper then evaluates in a closed form the ASE power spectral density by linearizing the interaction between a signal and a noise in the limit of a distributed system. Even if the received ASE is nonstationary in time due to pulse shape and modulation, this paper shows that it can be approximated by an equivalent stationary process, as if the signal were continuous wave (CW). This paper then applies the CW-equivalent ASE model to bit-error-rate evaluation by using an extension of a known Karhunen-Loe/spl acute/ve method for quadratic detectors in colored Gaussian noise. Such a method avoids calculation of the nonlinear phase statistics and accounts for intersymbol interference due to a nonlinear waveform distortion and optical and electrical postdetection filtering. This paper compares binary and quaternary schemes with both nonreturn- and return-to-zero (RZ) pulses for various values of nonlinear phases and bit rates. The results confirm that PG deeply affects the system performance, especially with RZ pulses and with quaternary schemes. This paper also compares ON-OFF keying (OOK) differential phase-shifted keying (DPSK) systems, showing that the initial 3-dB advantage of DPSK is lost for increasing nonlinear phases because DPSK is less robust to PG than OOK.     

Impact of Channel Plan and Dispersion Map on Hybrid DWDM Transmission of 42.7-Gb/s DQPSK and 10.7-Gb/s OOK on 50-GHz Grid

We investigate experimentally the performance of 42.7-Gb/s return-to-zero (RZ) differential quadrature phase-shift-keyed (DQPSK) channels in a dense wavelength-division-multiplexed transmission system having 10.7-Gb/s nonreturn-to-zero (NRZ) on-off keyed (OOK) channels. Cross-phase modulation (XPM) from the OOK channels is found to be a dominating nonlinear penalty source for copropagating DQPSK channels in a dispersion-managed transmission link with multiple standard single-mode fiber spans. It is also found that the XPM penalty strongly depends on channel occupancy and residual dispersion per span (RDPS). Large RDPS effectively mitigates XPM even for the worst-case occupancy where a 42.7-Gb/s RZ-DQPSK channel is amidst several 10.7-Gb/s NRZ-OOK channels on a 50-GHz channel grid.     

Unrepeated transmission of 20-Gb/s optical quadrature phase-shift-keying signal over 200-km standard single-mode fiber based on digital processing of homodyne-detected signal for Group-velocity dispersion compensation

We demonstrate unrepeated optical transmission of 20-Gb/s quadrature phase-shift-keying (QPSK) signals over a 200-km-long standard single-mode fiber (SMF) without using any optical dispersion compensator. By employing optical homodyne detection, which can restore the entire information of the complex amplitude of the transmitted signal, group-velocity dispersion (GVD) of the SMF can be compensated electrically by a linear equalizer at the receiver. From off-line bit-error-rate measurements, we find that a simple transversal filter implemented in digital signal processing circuits after homodyne detection can effectively cancel the fiber GVD of up to 4000 ps/nm, enabling successful 20-Gb/s QPSK transmission.     

40-Gb/s NRZ wavelength conversion with 3R regeneration using an EA modulator and SOA polarization-discriminating delay interferometer

We propose and demonstrate, for the first time, to our knowledge, a 40-Gb/s nonreturn-to-zero wavelength converter with 3R regeneration based on an electroabsorption modulator and semiconductor optical amplifier polarization-discriminating delay interferometer. The results show that the root mean square timing jitter and optical signal-to-noise ratio (OSNR) of the regenerated signal are kept to 0.68 ps and higher than 40 dB/0.1 nm, respectively. In addition, the measured receiver sensitivity (BER=10/sup -9/) is improved from -19 to -25.2 dBm at the input OSNR of 26 dB/0.1 nm.