Theoretical investigation of optical wavelength conversion techniques for DPSK Modulation formats using FWM in SOAs and frequency comb in 10 gb/s transmission systems

We have investigated the wavelength conversion techniques for differential phase-shift keying (DPSK) modulation formats in 10 Gb/s transmission systems, compared with the non-return-to-zero (NRZ) modulation format. For the wavelength conversion of DPSK modulation formats, we employed the wavelength converters based on the four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) and the frequency comb generated by phase modulation. The power penalty at 10/sup -9/ bit error rate was used as a measure of the system performance degraded by the wavelength conversion. Our simulation results show that the DPSK modulation formats have a smaller power penalty than the NRZ modulation format for the wavelength conversion using the FWM effect in an SOA due to a much lower pattern effect. However, as the wavelength conversion uses the frequency comb generated by phase modulation, it has a similar power penalty compared with the NRZ modulation format. It is also shown that the DPSK modulation formats are possible to obtain the power penalty less than 0.4 dB for both wavelength conversion techniques.     

Reduction of AM-induced penalty in DPSK receivers by sum-squaredemodulation

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     

Low detection penalty in an optical DPSK heterodyne system using asaturated optical amplifier as phase modulator

A model established from the rate equations of a near traveling-wave optical amplifier (NTWOA) used as a phase modulator shows that the modulation rate is limited by the carrier lifetime to 500 Mb/s and that the worst-case detection penalty is about 0.7 dB. It is then experimentally demonstrated that close control of the injected light polarization can reduce the detection penalty in a DPSK (differential phase-shift keying) heterodyne system, using an NTWOA phase modulator instead of an LiNbO₃ modulator, to 0.6 dB. This device and a balance receiver have allowed the implementation of 282-Mb/s DPSK link operating at 1.522 μm over 242 km of fiber with a 3-dB margin     

5 Gbit/s direct optical DPSK modulation of a 1530-nm DFB laser

5 Gb/s direct optical differential-phase-shift-keying (DPSK) modulation of a 1530-nm distributed feedback (DFB) laser is demonstrated using injection current modulation with a bipolar signal format. Delay demodulation is performed using an interferometer with a delay time T equal to the duration of one bit. The input and differentially encoded nonreturn to zero (NRZ) signals are shown. The bipolar modulation current signal is basically the time derivative of the NRZ signal. There was no degradation of the optical DPSK signal due to thermal frequency modulation of the laser. The direct DPSK modulation technique avoids the insertion loss and systems complexity of external DPSK modulators     

Highly efficient multichannel wavelength conversion of DPSK signals

This paper demonstrates a multichannel wavelength conversion of differential phase-shift-keyed (DPSK) signals using four-wave mixing in a highly nonlinear fiber. The wavelengths of three 10-Gb/s nonreturn-to-zero (NRZ) DPSK channels are simultaneously converted without incurring the cross-gain modulation penalty usually associated with on-off-keyed signals. A maximum conversion efficiency of 85% was achieved for both NRZ and return-to-zero DPSK signals.     

Optical Systems With High-Order DPSK and Star QAM Modulation Based on Interferometric Direct Detection

Increased interest in novel modulation formats for optical transmission has come up in recent years. Receivers with interferometric direct detection can be used to detect arbitrary modulation formats with differentially encoded phases as differential phase-shift keying (DPSK) and differentially phase-encoded star-shaped quadrature amplitude modulation (Star QAM). In this paper, two novel 16-ary modulation formats, which are the 16 DPSK and the Star 16 QAM (ASK-8 DPSK), are characterized for optical transmission for the first time. To be able to identify clear performance tendencies for high-order optical modulation, the novel formats are compared to a wide range of already investigated formats by conducting comprehensive calculations in a uniform simulation environment. The influence of different transmitter structures and decision schemes is considered, and all the systems are characterized with respect to the optical signal-to-noise ratio requirements, dispersion tolerance, and self-phase-modulation (SPM) performance for nonreturn-to-zero (NRZ) and RZ pulse shapes. Moreover, an inherent problem of Star QAM transmission concerning SPM is illustrated, and compensation techniques are examined. The results give a substantial insight into the properties of high-order optical modulation formats.     

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.     

An AWG-based 10 Gbit/s colorless WDM-PON system using a chirp-managed directly modulated laser

We propose an arrayed waveguide grating(AWG)-based 10 Gbit/s per channel full duplex wavelength division multiplexing passive optical network(WDM-PON).A chirp managed directly modulated laser with return-to-zero(RZ) differential phase shift keying(DPSK) modulation technique is utilized for downlink(DL) direction,and then the downlink signal is re-modulated for the uplink(UL) direction using intensity modulation technique with the data rate of 10 Gbit/s per channel.A successful WDM-PON transmission operation with the data rate of 10 Gbit/s per channel over a distance of 25 km without any optical amplification or dispersion compensation is demonstrated with low power penalty.     

Performance degradation of phase-modulated systems due to nonlinear phase noise

The error probability is calculated for phase-modulated systems with nonlinear phase noise. Using the assumption that the phase of amplifier noise and nonlinear phase noise are independent of each other, the error probability and penalty are calculated for both phase-shift keying (PSK) and differential phase-shift keying (DPSK) systems. The mean nonlinear phase shift must be less than about 1.00 and 0.63 rad for a penalty less than 1 dB for PSK and DPSK systems, respectively.     

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.     

Impact of laser phase noise on the performance of a {3×3}phase and polarization diversity optical homodyne DPSK receiver

An analysis of the impact of laser phase noise on the performance of a {3×3} phase- and polarization-diversity differential phase-shift keying (DPSK) receiver is done for the phase and shot-noise limited case. The results show that, for zero laser linewidths, the maximal signal power penalty of the {3×3} phase- and polarization-diversity DPSK receiver with respect to the conventional heterodyne DPSK receiver is approximately 0.7 dB for P_e =10^-9. For nonzero laser linewidths, it appears that, depending on the laser linewidth, for large signal-to-noise ratios the performance of the analyzed {3×3} phase- and polarization-diversity DPSK receiver is close to that of the ideal conventional heterodyne DPSK receiver. For a rectangular intermediate-frequency filter, the maximum allowable normalized laser linewidth (Δυ×T) for the (3×3) phase and polarization diversity DPSK receiver is found to be approximately 0.46% for a power penalty of 1 dB     

空间光DPSK发射与自相干接收系统实验

搭建通信速率为10 Gbps的光DPSK调制发射、自相干接收单元实验测试平台。采用光相位调制和自相干接收技术实现光DPSK系统设计。为稳定输出信号解决相位漂移,发射单元设计出自适应增益控制单元和交叉点自适应控制单元。在系统测试平台上,完成差分编码模块、发射单元和自相干接收单元及系统性能测试。实验室环境条件下,测得系统接收单元灵敏度为-48 dBm,通信系统误码率优于1×10-7;误差矢量幅度为7 %,通信性能较好,满足系统设计指标要求。     

Direct modulation 565 Mb/s DPSK experiment with 62.3 dB loss spanand endless polarization control

Optical DPSK at 565 Mb/s is performed by direct modulation of a narrow-linewidth 1.5-μm SL-QW-DFB (strain layer-quantum well-distributed feedback) transmitter laser. The bipolar RZ drive signal has no DC component which allows penalty-free transmission of 2 ²³-1 bit patterns. The heterodyne receiver contains an endless polarization controller with <6.3 or >12 rad/s tracking speed, depending on the tolerable intensity loss. A loss span of 62.3 dB is achieved,without degradation due to stimulated Brillouin scattering     

Intrinsic lineshape and FM response of modulated semiconductor lasers

A new method for measuring the FM response and the intrinsic linewidth of modulated semiconductor lasers is reported. No additional stochastic line broadening has been observed for direct modulation of buried-heterostructure 1.3 ?m lasers with modulation depths of less than 20%. Thus FSK systems employing heterodyne detection are expected not to exhibit any additional phase noise penalty due to direct current modulation.     

fso中一种光载波外差dpsk系统的设计

介绍了自由空间光通信中的相干通信系统,通过对相干光通信中振幅键控(ASK),频移键控(FSK),相移键控(PSK),差分相移键控(DPSK)四种光载波相干调制方式性能的分析和比较,仿真结果得出PSK调制误码性能更好,提出了一种光载波外差差分相移键控(DPSK)系统.     

Study on 8DPSK of multi-phase modulation technology based on CSRZ

A new modulation format in optical fiber communication system, the eight differential phase shift keying (8DPSK) of multi-phase modulation technology based on carrier-suppressed return-to-zero (CSRZ) is proposed in this paper. The formulae of CSRZ-8DPSK modem methods are derived theoretically and the methods are demonstrated. Spectra based on CSRZ and CSRZ-8DPSK modulation methods and their eye diagrams by simulation with MATLAB are obtained. The results show that the CSRZ-8DPSK modulation methods have narrower spectra with higher efficiency. The performance of eye diagrams is also much satisfactory after demodulation, suggesting some possible applications of the methods in the next generation of optical fiber communications.     

The effect of crosstalk and phase noise in multichannel coherentoptical DPSK systems with tight IF filtering

Results for two-channel differential-phase-shift-keying (DPSK) systems using finite integrator and raised cosine response IF filters are presented. The sensitivity of an optical receiver in a two-channel DPSK system is studied. The results are compared with previous work in the limit of no phase noise and it is shown that the agreement is good. The penalty due to crosstalk for different linewidths and filter shapes is computed, and it is shown that the minimum channel spacing is a few bit rates for an ideal integrator IF filter and is larger for an IF filter with a raised cosine impulse response. The penalty is increased somewhat by phase noise     

Design and Experimental Results for a Direct-Sequence Spread-Spectrum Radio Using Differential Phase-Shift Keying Modulation for Indoor, Wireless Communications

We report on our design and measurements that have been made for a direct-sequence spread-spectrum radio using differential phase-shift keying modulation for a wireless PBX. We describe the design and implementation of a transmitter and a receiver using a surface acoustic wave (SAW) filter matching the spread-spectrum code of a user. The receiver performance is within 1 dB of the theoretical performance of a differential phase-shift keying (DPSK) receiver in the presence of additive white Gaussian noise. We also show receiver performance in a multipath fading indoor environment with multipath fade notches of up to 50 dB depth. The indoor channel multipath fading can be overcome by using an equal gain diversity combiner which is suitable when DPSK modulation is used. We confirm that the indoor mean power level attenuation follows the inverse fourth power of the distance. Also, we investigate the multiple-access capability of the system by introducing an interfering transmitter with a different spread-spectrum code sequence.     

Reduction of four-wave mixing induced penalty in unequally spacedWDM transmission system by using optical DPSK

Optical DPSK signalling is employed in unequally spaced WDM transmission systems using dispersion-shifted fibres to reduce the penalty caused by four-wave mixing. More than 10 dB reduction in power penalty is achieved compared with the use of intensity modulation     

Transmission Performance Comparison of Direction Detection‐Based 100‐Gb/s Modulation Formats for Metro Area Optical Networks

Transmission performances of direct detection‐based 100‐Gb/s modulation formats are investigated and compared for metro area optical networks. The effects of optical signal‐to‐noise ratio sensitivity, chromatic dispersion, cross‐channel nonlinearity, and transmission distance on the performance of differential 8‐ary phase‐shift keying (D8PSK), differential phase‐shift keying plus three‐level amplitude‐shift keying (DPSK+3ASK), and dual‐carrier differential quaternary phase‐shift keying (DC‐DQPSK) are evaluated. The performance of coherent dual‐polarization quadrature phase‐shift keying (DP‐QPSK) with block phase estimation and coherent DP‐QPSK with digital differential detection are also presented for reference. According to our analysis, all three direct detection modulation formats could transmit a 100‐Gb/s signal over several hundred kilometers of a single‐mode fiber link. The results also show that DC‐DQPSK outperforms D8PSK and DPSK+3ASK, and the performance of DC‐DQPSK is comparable to that of coherent DP‐QPSK with digital differential detection. The maximum transmission distance of DC‐DQPSK is over 1,000 km, which is enough distance for metro applications.