The output power of a 6328-Å He-Ne gas laser

Expressions are derived for the output power of a gas laser in single-mode operation. This theory is then extended to multimode operation where the mode spacing is less than the pressure-broadened "natural" atomic linewidth. Using measured values for the linewidth and saturation parameters, these formulas are used to predict 6328-Å output power for a He-Ne laser as a function of laser gain, cavity loss, and output coupling. Experiments are reported which show good agreement with the theoretical predictions, and it is concluded that the theory presented here is adequate to describe the main features of the power output behavior of a 6328-Å He-Ne gas laser.     

Computer simulation and noise analysis of the system performance of 1.55-µm single-frequency semiconductor lasers

A theoretical model for the noise analysis of the system performance of 1.55-μm single-frequency semiconductor lasers is presented. Computer simulations are used to analyze the role of various noise sources in a 1.7-Gbit/s transmission experiment where the data was transmitted over 69 km using a 1.56-μm distributed-feedback laser. The bit-error-rate curves generated from numerical simulations agree well with the results of the transmission experiment. The relative contributions of various noise sources in limiting the system performance are discussed and compared. In particular, we consider circuit noise, shot noise, laser intensity noise, mode-partition noise, parasitic reflections, and the frequency chirp.     

In situ characterization of laser diodes from wide-band electrical noise measurements

The wide-band electrical noise characteristics of 0.8-, 1.3-, and 1.5-μm laser diodes have been studied theoretically and for the first time also experimentally. The electrical noise is related to the optical intensity noise behavior, and can therefore be used for in situ measurements and characterization of laser diodes. Since the measurements are performed without any optical components, undesired optical feedback is eliminated. The results show that several important laser parameters and characteristics can be extracted from purely electrical noise measurements. Among these are the relaxation frequency, the threshold current, the emission linewidth, optical feedback properties, and longitudinal mode hopping behavior. Good agreement between the noise theory and the electrical noise measurements has been obtained. An expression for obtaining both the spectral linewidth and lineshape from electrical noise measurements is also derived.     

Dynamic range of an optical amplifier cascade in an ASK system withsignificant phase noise

The authors present detailed results concerning the dynamic range of an amplitude-shift keyed (ASK) heterodyne optical communication system where the transmitted signal is impaired by significant amounts of laser phase noise. Theoretical results are evaluated for a system with up to ten in-line optical amplifiers and a significant influence of the laser phase noise is found for the maximum allowable transmission span and the dynamic range. With ten amplifiers, a linewidth of 11% of the bit rate causes the span to be reduced by 12.2 dB and the dynamic range to be reduced by 5.7 dB. The results have been confirmed using two amplifiers in a field trial experiment at a bit rate of 636 Mb/s with an IF linewidth of 70 MHz     

Assessment of laser noise influence on direct-detection transmission system performance

A new semi-analytical method of estimating the influence of laser noise on directly modulated direct-detection transmission system performance is proposed. Its accuracy is assessed for different propagation situations and receiver noise type dominance using rigorous mixed semi-analytical simulation. Two different relative intensity noise (RIN) powers at receiver input associated with the two laser currents of the binary levels are considered, and not just a single one corresponding to the laser average current as in other published works. It is shown that, to perform rigorous simulation, suitable methods of solving stochastic laser rate equations should be used, and a number of samples high enough in order to stabilize the sensitivity estimator should be considered. It has been found that, for power penalties of about 1 dB, more than 150 samples may be necessary. For a power penalty due to laser noise of about 4.5 dB, discrepancies not exceeding 0.4 dB are observed between the results obtained using the new semi-analytical approach and using the rigorous simulation. It is shown that the new semi-analytical approach gives more precise results than other approach currently used. For a power penalty due to laser noise not exceeding 2 dB, the discrepancies between the two approaches can reach roughly 1 dB. It is shown that these discrepancies can increase with the laser linewidth. Explicit expressions for system power penalty due to the RIN at the fiber output are presented, for both signal-independent and -dependent noise dominance. It is shown that these expressions give good upper bounds of the real power penalties.     

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}).     

Performance improvement of a coherent optical fiber communication system with the phase estimation algorithm

The phase estimation based on M-th power algorithm is employed in a differential quadrature phase-shift keying (DQPSK) coherent optical fiber communication system, and the system performance improvement is investigated through numerical simulations. It is found that the application of phase estimation algorithm can restrain the phase noise induced by laser linewidth and fiber nonlinear effects, and recover the carrier phase, so the transmission distance of a 40 Gb/s DQPSK-RZ system can be extend...     

Linewidth reduction of a semiconductor laser by electrical feedback

An electrical feedback technique was proposed to stably reduce the linewidth of a semiconductor laser without changing its cavity structure. Calculations and experiments were carried out to reduce the linewidth of a 1.5 μm InGaAsP laser (DFB type) according to the following procedure. A compact Fabry-Perot interferometer was used as a freqeuncy discriminator. The minimum attainable linewidth, limited by the detector noise, was estimated as being narrower than 1 kHz when the reflectance of the interferometer used was higher than 0.9. The minimum linewidth obtained in the experiment was 330 kHz, which was 15 times as narrow as in the case of a free-running laser. The improvements of this experimental result can be expected by simultaneously reducing the AM noise of the laser.     

Receiver analysis for synchronous coherent optical fiber transmission systems

A receiver sensitivity expression applicable for both PSK homodyne and heterodyne optical fiber transmission systems is derived taking account of polarization misalignment, reduced modulation depth, preamplifier thermal noise, power coupling ratio of the fiber coupler, local oscillator excess intensity noise, and reference phase errors. From a comparison of recent studies on system performance degradation due to laser phase noise a generalized expression relating beat linewidth to phase error variance for pilot carrier and Costas phase-locked-loop receivers is defined.     

Analysis of laser phase noise to intensity noise conversion bychromatic dispersion in intensity modulation and direct detectionoptical-fiber transmission

Laser phase noise conversion to intensity noise due to fiber chromatic dispersion is analyzed by deriving the noise power spectral density. Theory predicts that the phase-modulation-amplitude-modulation conversion noise is a principal limiting factor of the gigabit-per-second nonregenerative transmission using an external modulator when the linewidth of the laser transmitter is above several tens of megahertz and the total chromatic dispersion of fibers exceeds several thousand picoseconds per nanometer. This fact is confirmed by the 2.4-Gb/s transmission experiments using multiple inline Er-doped fiber amplifiers. The system penalty due to this noise in the intensity modulation and direct detection (IM-DD) optical transmission using an external modulator is evaluated     

Quantitative evaluation of proximity effect in raster-scan exposure system for electron-beam lithography

The proximity effect in a raster-scan for electron-beam lithography system was evaluated by Monte Carlo calculation and verified by experiments. It was revealed that the reduction in the beam diameter below the scanning pitch, which links into the shortening of drawing time, is more effective in decreasing the proximity effect than the reduction in the resist thickness. From the calculated results, it was found that the error in linewidth definition due to the proximity effect was less than 10 percent at a linewidth of 1.5 µm with scanning pitch of 0.5 µm, beam diameter of 0.2 µm, and PMMA resist of 1.0-µm thickness.     

Laser phase noise impact on optical DP-MQAM: experimental investigation

Advances in optical technologies, modulation formats, and symbol transmission rates became paramount for today’s agile high spectral efficiency optical networks. Nonetheless, laser phase noise (PN) has played a major impact on the performance of these systems due to the employment of moderate-quality lasers that significantly affect the implementation of high-order modulation schemes starting from existing QPSK and growing to MQAM, \(M=16\), 32, and 64. In this paper, we report an optical dual-polarization (DP) transmitter prototype that digitally emulates the effect of the laser phase noise, according to Wiener process, applied to a different types narrow-linewidth laser sources. This enables the generation of an arbitrary laser linewidth ranging from kilohertz up to tens of megahertz. Our experimental study evaluates the effect of PN on the performance of tracking algorithms commonly implemented on practical coherent receivers. The experimental results show that, for laser linewidth \(\le \)1 MHz, practical filter recovery algorithm succeeds to make appropriate tracking for both DP-QPSK and 16 QAM at 16- and 32-Gbaud system rates. However, for 10 MHz linewidth, correct recovery is possible only for DP-QPSK.     

基于小波变换的光相移键控传输系统相位估计

针对相位噪声对光相移键控传输系统性能影响的问题,目前广泛采用滤波器进行相位估计,针对滤波器的边界效应及高精度与延时存在矛盾的问题,文中将小波变换与前向反馈技术结合用以实现相位恢复,既避免了滤波器的边界效应及延时,同时又实现了高精度的相位恢复。仿真结果显示,该算法可消除4 MHz的激光器线宽和35°激光器相位偏差对系统误码率的影响,且更适用于高速相干光通信系统实现的实时并行处理。     

大气信道下窄线宽激光器对相干通信系统性能影响研究

在空间下行相干激光通信系统中,激光器的线宽大小会影响通信系统的性能,而窄线宽激光器能有效降低激光器线宽引起的激光器相位噪声,目前已经成为相干激光通信系统的首选。光信号在大气信道传输时,大气湍流会引起光信号强度和相位的波动,从而进一步影响系统的通信性能。针对上述问题,基于四相移键控(Quadri Phase Shift Keying, QPSK)通信系统的工作原理,进一步考虑窄线宽激光器线宽和大气湍流引起的光信号强度和相位的波动,给出了空间下行QPSK通信系统误码率模型。并基于该模型,数值仿真分析了窄线宽激光器对空间下行相干激光通信系统性能的影响。结果表明：大气湍流不仅严重影响系统性能,也会弱化激光器线宽对系统性能的影响。而对于大气湍流影响而言,其引起的相位波动要大于光强波动的影响。此外,随着通信速率增大,激光器线宽对于系统性能影响也会随之降低。文中对于空间下行相干激光通信系统的优化设计和调试具有一定的实际参考意义。     

单频分布布拉格反射光纤激光器及温度传感实验研究

报道利用Er∶Yb双掺杂光纤制作的单频窄线宽分布布拉格反射 (DBR)光纤激光器。在 980nm半导体激光器抽运下 ,当抽运功率为 75mW时 ,获得了输出功率为 2 3mW的单频激光 ,其中心波长为 1 5 5 7 5 2 4nm ,线宽小于 5MHz。利用制作的单频DBR光纤激光器构成光纤有源传感系统 ,进行温度传感实验研究 ,实验结果线性度很好 ,高信噪比和高输出功率使得该系统具有波长易于检测的优点     

Configuration of the optical transmission line using stimulated Raman scattering for signal light amplification

Expansion of a repeater spacing of optical transmission line is investigated using forward stimulated Raman scattering generated by a 1.34-μm Nd:YAG laser to amplify the signal light. Achievable repeater spacing at the 1.42-μm wavelength transmission is numerically estimated on the basis of coupled power equations and experimentally obtained Raman gain for various core-cladding index difference. A transmission distance of more than 200 km is predicted for an input signal light power of 100 μw and a pump power of 200 mW using a fiber with a relative index difference of 1 percent.     

The effect of side modes with linewidth and intensity fluctuationsin semiconductor lasers

The influence of side modes on the linewidth and intensity fluctuations of the main mode is studied using Monte Carlo techniques. Explicit analytic relations are developed for the side-mode induced linewidth, using impulse responses of the laser obtained from rate equations. It is found that both the absolute level of the main-mode too side-mode power ratio and the passive loss in the side mode influence the main-mode linewidth. A typical 3-mW laser has negligible linewidth enhancement from the side mode if the modal power ratio is about 300/1 (or greater), corresponding to a side-mode loss of 0.2-0.5 dB (or more) for one round trip within the laser resonator. Lower side-mode losses yield an abrupt catastrophic increase in main-mode linewidth     

Reduction of laser modulation bandwidth requirement in FSK systems using duobinary coding and differential detection

The duobinary signalling concept is applied to direct frequency modulation of a semiconductor laser. The main goal is the achievement of a low-cost high bit-rate transmission with reduced modulation bandwidth requirement of the laser diode. The simulation and experimental work demonstrate a laser modulation bandwidth reduction of about 50%. The system exhibits a high tolerance against fibre nonlinear effects, up to 20 dBm, and phase noise, up to 170 MHz of linewidth at 10 Gbit/s.     

Comments on the identification of some xenon ion laser lines

Earlier measurements and level assignments made by the authors for some xenon ion laser lines are reevaluated in light of recent work by Gallardo and coworkers. The lines affected are the 5260.17-, 5260.43-, 4305.75-, and 3759.94-Å lines in xenon and 4647.40- and 4650.22-Å lines attributed to carbon.