Error-rate floor in optical ASK heterodyne systems caused by nonzero (semiconductor) laser linewidth

Using a simple model to account for the effects of non-negligible linewidths for the transmitter and local oscillator lasers in a heterodyne ASK system, we show that there is an error probability floor which is a function of the IF bandwidth and the relative decision threshold level. We describe the physical reason for this behaviour and present some calculated results for total error probability in terms of signal power, IF linewidth and bandwidth, and threshold setting. The implications of this `floor? for practical receiver design are discussed.     

Reliability of semiconductor lasers for undersea optical transmission systems

The reliability of laser diodes developed for undersea optical transmission systems is analyzed. Up to 1000 device samples of two types (DC-PBH and VSB) are used. An aging test with constant light power operation of 5 mW is carried out at 10, 50, and 70°C for 10 000 h. The median lifetime at 10°C is conservatively estimated to be 1.6 × 10⁶hours. Failure rate at 10°C is estimated at 250 FIT's at 25 years of service for the wear-out failure mode and at less than 50 FIT's for the random failure mode with a sufficient margin. Furthermore, it is determined that reliability can be further improved by selection of long-life lasers through an additional third step screening aging.     

Reliability of semiconductor lasers for undersea optical transmission systems

The reliability of laser diodes developed for undersea optical transmission systems is analyzed. Up to 1000 device samples of two types (DC-PBH and VSB) are used. An aging test with constant light power operation of 5 mW is carried out at 10, 50, and 70°C for 10 000 h. The median lifetime at 10°C is conservatively estimated to be1.6 times 10^{6}hours. Failure rate at 10°C is estimated at 250 FIT's at 25 years of service for the wear-out failure mode and at less than 50 FIT's for the random failure mode with a sufficient margin. Furthermore, it is determined that reliability can be further improved by selection of long-life lasers through an additional third step screening aging.     

Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; ifeta = 0.7, then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency andlambda = 1.5 mum, the above quoted numberers give 0.9 pW/ kHz foreta rightarrow inftyand 1.35 pW/kHz fornu = 0.7. The loop bandwidth required is also evaluated and found to be155 Deltanu, whereDeltanuis the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be2.26 cdot 10^{-3} R_{b}where Rbis the system bit rate. ForR_{b}=100Mbit/s, this leads toDeltanu < 226kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerateDeltanuup to 0.09 Rb. At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth (Deltanu < 3 cdot 10^{4} R_{b}).     

Influence of (semiconductor) laser linewidth on the error-rate floor in dual-filter optical FSK receivers

We discuss an error-rate floor in dual-filter optical FSK receivers in terms of the IF linewidth and the frequency shift between 0s and 1s, using a comprehensive model for the receiver which takes into account nonzero laser linewidth, noise correlation and signal overlap between the filters. We present calculated results for total error probability in terms of signal power, IF linewidth and frequency shift, illustrating the effect of the floor on receiver sensitivity, and we contrast it with a related effect in heterodyne ASK systems.     

Theoretical analysis of external optical feedback on DFB semiconductor lasers

The effect of external optical feedback on resonant frequency, threshold gain, and spectral linewidth of distributed feedback (DFB) semiconductor lasers is theoretically analyzed. The analysis applies to any type of laser cavity formed by a corrugated waveguide limited by partially reflecting facets. It is shown that the sensitivity to optical feedback on a facet is closely related to the power emitted through this facet. Numerical results on wavelength selectivity and on sensitivity to optical feedback are given for conventional DFB lasers having an AR-coated facet and for quarter-wave-shifted (QWS) DFB lasers with AR-coatings on both facets. Both laser types are found to be more sensitive to optical feedback on their AR-coated facet than Fabry-Perot lasers for lowkL. On the other hand, QWS-DFB lasers are found to be relatively insensitive to optical feedback for largekL.     

BER for optical heterodyne DPSK receivers using delay demodulationand integration detection

Previous work on the bit error rate (BER) performance of differential phase-shift keying (DPSK) including the effects of additive white Gaussian noise (AWGN) and phase noise has concentrated on the delay demodulator with narrow-band intermediate frequency (IF) bandpass filtering (BPF) and sampling detection. No similar analysis has yet been performed for the delay demodulator with wideband IF bandpass filtering and integration detection. Phase noise is an important consideration in coherent optical communication systems and the most widely accepted model is a Brownian motion process. A closed-form BER expression along with detailed Monte Carlo simulation results are presented for the DPSK delay demodulator with wideband IF bandpass filtering and integration detection filtering including phase noise effects using the Brownian motion model. Analytic expressions are also obtained for the moments of the phase noise component of the decision variable. Using these moments, estimates of the phase noise BER floor are produced. It is found that this receiver has noise performance comparable to receivers with narrowband IF bandpass filtering and sampling detectors for signal-to-noise ratio (SNR) and phase noise in the range of practical interest, but with potentially less degradation due to intersymbol interference (ISI)     

Exact evaluation of laser linewidth requirements for optical PSKhomodyne communication systems with balanced PLL receivers

The power penalty induced by imperfect phase recovery in PSK homodyne communication systems with balanced phase-locked loop receivers are exactly evaluated. Optimum phase deviations between the mark-state and the space-state bits are used in this study. This study for the first time shows the imperfect-phase-recovery-induced power penalty as a function of laser linewidth with optimum phase deviations considered. It can be estimated from the theoretical result that an optimal balanced PLL receiver requires the laser linewidth as Δν⩽1.15×10^-6× (bit rate) in contrast to the previous reported one Δν⩽5.88×10^-6× (bit rate). We also point out here that the previously reported laser linewidth requirement was wrongly estimated     

Spectral linewidth reduction in semiconductor lasers by an external cavity with weak optical feedback

The linewidth narrowing of a semiconductor laser due to weak optical feedback is analysed, taking into account both phase condition and threshold change for the feedback-induced modes. The achievable linewidth reduction lies in between two limiting cases, 1/(1 + X ?(1 + ?2))2 and 1/(1 + X)2, where ? and X are the linewidth enhancement factor and the feedback parameter, respectively.     

Novel measurement method of linewidth enhancement factor in semiconductor lasers by optical self-locking

The authors propose a simple, accurate and novel method to measure the value of the linewidth enhancement factor in semiconductor lasers. It utilises the asymmetry of the frequency locking range of a semiconductor laser with the optical feedback from the external CFP cavity. The measured value for a 0.8 mu m CSP-type AlGaAs laser was 3.07+or-0.26, which showed good agreement with those measured by other conventional methods.<>     

Frequency stabilisation of semiconductor lasers for heterodyne-type optical communication systems

In the somewhat distant future, heterodyne-type optical communications will appear, increasing channel capacity and repeater separation. At present, the greatest difficulty is the frequency drift of lasers. The letter describes an a.f.c. (automatic frequency control) experiment using a Peltier cooling element; it is shown that the frequency fluctuation can be reduced to below 10 MHz.     

Real-time implementation of optical OFDM transmitters and receivers for practical end-to-end optical transmission systems

Real-time optical OFDM transceivers are successfully demonstrated for the first time, which support 1.5 Gbit/s transmission over 500 m 62.5/125 μm multimode fibres in an intensity-modulation and direct-detection system involving a directly modulated DFB laser. The implemented transceivers only use standard commercially available components.     

Decision-driven phase-locked loop for optical homodyne receivers: Performance analysis and laser linewidth requirements

Optical homodyne receivers based on decision-driven phase-locked loops are investigated. The performance of these receivers is affected by two phase noises due to the laser transmitter and laser local oscillator, and by two shot noises due to the two detectors employed in the receiver. The impact of these noises is minimized if the loop bandwidthBis chosen optimally. The value of Boptand the corresponding optimum loop performance are evaluated in this paper. It is shown that second-order phase-locked loops require at least 0.8 pW of signal power per every kilohertz of laser linewidth (this number refers to the system with the detector responsivity 1 A/W, dumping factor 0.7, and rms phase error 10°). This signal power is used for phase locking, and is, therefore, lost from the data receiver. Further, the maximum permissible laser linewidthDelta vis evaluated and for second order loops with the dumping factor 0.7 found to be 3.1 × 10^-4. Rb, where Rb(bit/s) is the system bit rate. ForR_{b} = 100Mbit/s, this leads toDelta v = 31kHz. For comparison, heterodyne receivers with noncoherent postdetection processing only requireDelta v = 0.72-9MHz forR_{b} = 100Mbit/s. Thus, the homodyne systems impose much more stringent requirements on the laser linewidth than the heterodyne systems. However, homodyne systems have several important advantages over heterodyne systems, and the progress of laser technology may make homodyning increasingly attractive. Even today, homodyne reception is feasible with experimental external cavity lasers, which have been demonstrated to haveDelta vas low as 10 kHz.     

基于半导体激光器窄线宽光子微波信号获取

为了研究光注入半导体激光器(SL)产生的光子微波信号的性能, 基于SL的速率方程和光纤布喇格光栅(FBG)滤波理论, 采用数值仿真的方法进行了理论分析, 得到了各种注入参量下的光谱、功率谱和线宽, 并讨论了反馈参量对微波线宽的影响, 考虑到光注入下产生的微波线宽较宽, 进一步引入FBG光反馈窄化了微波信号的线宽。结果表明, 当SL仅在光注入作用下时, 通过改变注入参量, 可实现微波频率连续可调谐和微波强度最大化; 微波线宽随着反馈强度的增加逐渐变窄, 通过适当调节反馈参量可将微波线宽压缩到10kHz以下。该研究结果可为半导体激光器在光生微波中的应用提供一定的理论参考。     

Decision-driven phase-locked loop for optical homodyne receivers: Performance analysis and laser linewidth requirements

Optical homodyne receivers based on decision-driven phase-locked loops are investigated. The performance of these receivers is affected by two phase noises due to the laser transmitter and laser local oscillator, and by two shot noises due to the two detectors employed in the receiver. The impact of these noises is minimized if the loop bandwidthBis chosen optimally. The value of Boptand the corresponding optimum loop performance are evaluated in this paper. It is shown that second-order phase-locked loops require at least 0.8 pW of signal power per every kilohertz of laser linewidth (this number refers to the system with the detector responsivity 1 A/W, dumping factor 0.7, and rms phase error 10°). This signal power is used for phase locking, and is, therefore, lost from the data receiver. Further, the maximum permissible laser linewidthDeltanuis evaluated and for second order loops with the dumping factor 0.7 found to be3.1 times 10^{-4} cdot R_{b}, where Rb(bit/s) is the system bit rate. ForR_{b} = 100Mbit/s, this leads toDeltanu = 31kHz. For comparison, heterodyne receivers with noncoherent postdetection processing only requireDeltanu = 0.72-9MHz forR_{b} = 100Mbit/s. Thus, the homodyne systems impose much more stringent requirements on the laser linewidth than the heterodyne systems. However, homodyne systems have several important advantages over heterodyne systems, and the progress of laser technology may make homodyning increasingly attractive. Even today, homodyne reception is feasible with experimental external cavity lasers, which have been demonstrated to haveDeltanuas low as 10 kHz.     

The effect of spurious intensity modulation in semiconductor diode lasers on the performance of optical heterodyne frequency shift-keyed communications systems

Analytical expressions are derived for the bit-error rate (BER) of anM-ary frequency shift-keyed (FSK), heterodyne, optical communication system with noncoherent demodulation in the presence of spurious intensity modulation (SIM) and frequency noise. The SIM degradation of an FSK system, implemented with semiconductor diode lasers, is estimated for lasers with zero and nonzero linewidths and will be discussed for a distributed feedback laser operating at 1.5μm and a channeled substrate planer laser operating at 0.83 μm. The SIM power penalty is typically less than 1 dB, but can exceed 1 dB for 2-, 4-, and 8-ary FSK at data rates above 1 Gbit/s.     

Theoretical performance of multigigabit-per-second lightwavesystems using injection-locked semiconductor lasers

The locking conditions for multigigabit-per-second modulation are examined, and the dependence of the receiver sensitivity on the fiber dispersion coefficient-length product is investigated. With a 4.8-Gb/s NRZ (nonreturn-to-zero) modulation, a 1-dB penalty in receiver sensitivity occurs for a transmission distance of 68 km. The injected power is 0.4 mW and the frequency detuning is -35 GHz. With 10-Gb/s NRZ modulation, the allowable transmission distance is 12.5 km for an injected power of 1.0 mW and a frequency detuning of -35 GHz. These results represent increases in the transmission distances obtained with a solitary laser by factors of 3.7 at 4.8 Gb/s and 2.7 at 10 Gb/s     

Influence of self-sustained pulsation in semiconductor lasers on optical transmission system

Degradations in semiconductor lasers caused by defects, such as dark lines or dark spots in an active region, sometimes introduce self-sustained pulsations. In this paper, dynamic properties of a modulated self-sustained pulsation InGaAsP/InP semiconductor laser are presented and the influence on the optical transmission system is described. In high-speed pulse modulation above the pulsation frequency, output waveforms showed a marked pattern effect. When the pulsing semiconductor laser is modulated below the pulsation frequency, the laser output had relatively large noise which had flat noise spectra. Therefore, error-rate characteristics were influenced by this noise. These results show that the self-sustained pulsations in semiconductor lasers sometimes cause transmission characteristics degradation on the optical transmission system. Therefore, careful attention should be paid to this matter.     

Measurement of the linewidth enhancement factor of semiconductor lasers based on the optical feedback self-mixing effect

A new method for the measurement of the linewidth enhancement factor of semiconductor lasers is presented, based on the interferometric self-mixing effect. It is a fast and easy to perform method that does not require radio frequency nor optical spectrum measurements. A small fraction of the emitted light is backreflected into the laser cavity by a remote target driven by a sine waveform. The mixing of the returned and the lasing fields generates a modulation of the optical output power in the form of an interferometric waveform, with a shape that depends on the optical feedback strength and the linewidth enhancement factor /spl alpha/, according to the well-known Lang-Kobayashi theory. We show that the value of /spl alpha/ can be retrieved from a simple measurement of two characteristic time intervals of the interferometric waveform. Experimental results obtained on different laser diodes show an accuracy of /spl plusmn/6.5%.