电反馈对FM光发射机信噪比性能的改善

本文研究了电反馈半导体激光器作为FM光发射机的信噪比性能。分析表明:电反馈可以改善FM光发射机的信噪比性能。这种改善在适当的反馈条件下可达近30dB。反馈环路存在着最佳增益使信噪比达到最大。     

Design theory of electrically frequency-controlled narrow-linewidth semiconductor lasers for coherent optical communication systems

We describe a design rule for the automatic frequency control of semiconductor lasers by electrical feedback for coherent optical communications. The linewidth achieved by the feedback control is not a good measure for evaluating the system performance, but the bandwidth of the feedback loop is the most important parameter. For example, a 500-MHz loop bandwidth is required for a heterodyne DPSK system when the free-running linewidth of the laser is 10 MHz.     

Electrical feedback and its network analysis for linewidthreduction of a semiconductor laser

The center frequency of the field spectrum of a 1.5-μm InGaAsP distributed-feedback laser was stabilized by negative electrical feedback. The resultant residual frequency fluctuation was σ=3.6×10^-11 for an integration time of 10 s. The linewidth of the field spectrum was simultaneously reduced by using another electrical feedback loop. Its minimum value was 360 kHz, which was 1/27 that of the free-running condition. This linewidth was narrower than the one determined by the magnitude of the spontaneous emission of the free-running laser. The field spectrum was very stable, and the value of the linewidth was constant for more than 40 h. Network analysis was carried out to realize further reductions of the linewidth     

Carrier synchronization for homodyne and heterodyne detection ofoptical quadriphase-shift keying

Through analysis and simulation, the authors investigated the performance of four carrier-synchronization techniques suitable for both homodyne and heterodyne detection of optical quadriphase-shift keying: the discrete-time decision-directed loop, the analog decision-directed loop, the Costas quadriphase loop, and the fourth-power phase-locked loop. Accounting for shot noise, laser phase noise, and feedback delay, they optimize the loop natural frequency and specify laser-linewidth requirements. The performance discrepancy between the best and worst of these loops is found to be small; accounting for inherent loop delays only, the linewidth requirements range from ΔvT<2.5×10^-5 to ΔvT<5.2×10^-5, where Δv is the beat laser linewidth and T is the baud interval. Hence other considerations, such as ease of implementation, will govern the design choice for most practical systems. For the case when propagation delays in the feedback loop are significant, a simple and accurate method for estimating the laser-linewidth requirement and corresponding optimal natural frequency is presented     

Effective linewidth enhancement factor and spontaneous emissionrate of DFB lasers with gain coupling

A field rate equation governing the noise and dynamic properties of a DFB (distributed feedback) laser with gain coupling is presented. Analytic expressions for the effective linewidth enhancement factor and spontaneous emission rate are derived. It is shown numerically that the linewidth contribution from spontaneous emission can be substantially reduced in DFB lasers with gain coupling     

Damping factor influence on linewidth requirements for optical PSKcoherent detection systems

Spectral linewidth requirements for optical phase-shift-keying (PSK) coherent detection systems are found to depend on the phase-locked loop (PLL) parameters. Until now, the damping factor of the PLL has been assumed to be 1/√2 when deriving the required spectral linewidth of a light source, because it is at this value that an electrical PLL offers near optimum performance in many cases. By increasing the PLL damping factor above 1/√2, it is shown that there exists a maximum value of the required linewidth that achieves a received optical power penalty of 1 dB at a bit error rate of 10^-10. The required beat linewidths so obtained are 50% larger than previously reported results (which assume a damping factor of 1/√2). As for PLL frequency acquisition performance, it is shown that raising the camping factor above 1/√2 does not seriously affect the hold-in limit or the pull-in limit. It is also shown that the normalized loop gain that optimizes PLL performance is roughly one half the normalized loop gain at which the PLL oscillation commences     

The dynamic properties and stability analysis for vertical-cavitysurface-emitting lasers

In this paper, a phenomenal analysis on the wavelength dependence of the gain temperature coefficient is presented. Based on this model, the relation between the dynamic properties and the lasing wavelength within the gain regime are investigated using the rate equations, including the wavelength dependence of both the differential gain and linewidth enhancement factor. Our calculation shows that when the lasing mode occurs in the negative damping rate regime, the relative intensity noise (RIN) is more than 43 dB higher, and the linewidth is broadened out more than 30 dB, which indicates laser being in a chaotic state. The stability of the vertical-cavity surface-emitting lasers with distant external optical feedback is analyzed numerically. The results for different cavity designs are compared. The effects of the nonlinear gain coefficient on RIN and spectral linewidth of the VCSEL's with external optical feedback is investigated     

Reduction of effective linewidth enhancement factor αeff of DFB lasers with complex coupling coefficients

The effective linewidth enhancement factor α_eff of DFB lasers which have both gain and index coupling coefficients is theoretically analyzed. The unique reduction mechanism of α_eff due to the optical negative feedback of modal phase and modal gain in gain coupled DFB lasers was found for the first time. The numerical result showed that α_eff can be reduced to almost one half of the material defined linewidth enhancement factor α when there exists an index coupling coefficient comparable to the gain coupling coefficient     

The influence of resonator structure on the linewidth enhancementfactor of semiconductor lasers

The phenomenon of intensity-phase coupling in semiconductor lasers is studied in the linear regime in terms of an effective linewidth enhancement factor. The circumstances under which the longitudinal laser structure of a distributed feedback laser may influence this coupling are determined. For the case of uniform gain and carrier density no effect is expected from a periodic or quasi-periodic corrugation. However, when the gain layer is corrugated or a periodic gain is otherwise implemented, the influence may be considerable. It is shown that the degree of gain coupling between right- and left-travelling waves and the specific mechanism by which variations in the carrier density influence this coupling are of key importance. It is found that depending on the degree and nature of the gain coupling the effective linewidth enhancement factor may be considerably reduced or enhanced     

Optical feedback stabilization of the intensity oscillations inultrahigh-frequency passively modelocked monolithic quantum-well lasers

The effect of optical feedback on the stability of the modelocking frequency of monolithic, passively modelocked semiconductor lasers at ~50 GHz is investigated. Optical feedback leads to linewidth narrowing, frequency pulling, and multiple modes of the intensity modulations in modelocked lasers, reminiscent of similar effects on the optical frequency in conventional continuous wave (CW) lasers. The narrowest observed line has a full-width-at-half-maximum (FWHM) of 56 kHz, which represents close to two orders of magnitude reduction of the linewidth compared to the situation without feedback. The effects of the optical feedback vary periodically as the length of the feedback delay is varied, with a periodicity that roughly matches the mode-locking wavelength. This wavelength is on the order of millimeters, which makes the mechanical stability of the optical feedback loop relatively uncritical     

Effect of stimulated Brillouin scattering on distributederbium-doped fiber amplifier

Stimulated Brillouin scattering (SBS) causes backward scattering of the pump light and this in turn causes a reduction in the gain of a distributed erbium-doped fiber amplifier (DEDFA). A theoretical analysis of the influence of SBS on a DEDFA and illustrative numerical results are presented. It is shown that there exists a threshold value of pump laser linewidth: when the pump laser linewidth is smaller than this threshold value the gain of the DEDFA is reduced greatly. This analysis and the results presented facilitate the design and optimization of DEDFAs     

A tool to calculate the linewidth of complicated semiconductor lasers

A theory of the low-frequency phase fluctuations in the output of a semiconductor laser due to spontaneous emission is developed. The theory can be used as a tool to numerically calculate the linewidth of complicated laser structures, e.g., by use of the transfer matrix formulation. The results for the single Fabry-Perot laser are shown to be in exact agreement with the most accurate treatments published so far. Results are then presented for both DFB and F-P lasers with external optical feedback showing how the linewidth varies with the threshold gain, with the coupling coefficient, and with the external feedback conditions.     

Dynamic and noise properties of tunable multielectrodesemiconductor lasers including spatial hole burning and nonlinear gain

A general formalism based on the Green's function method is given for multielectrode semiconductor lasers. The effects of both spatial hole burning and nonlinear gain are included in this formalism. An effective nonlinear gain is introduced by taking into account the influence of the laser structure and the associated distribution of the mode intensity along the cavity length and the frequency and intensity modulation properties of multielectrode semiconductor lasers are studied. A general linewidth expression which includes contributions from spontaneous emission and carrier shot noise is given. It is found that the effective α-factor affecting the linewidth is in general different from its counterpart affecting modulation and injection locking properties due to spatial hole burning and nonlinear gain. The linewidth due to various contributions is calculated for both uniform intensity distributed lasers and phase-shifted distributed feedback (DFB) lasers     

A comparison of amplitude-phase coupling and linewidth enhancementin semiconductor quantum-well and bulk lasers

The amplitude-phase coupling factor α (linewidth enhancement factor) is compared for typical semiconductor quantum-well and bulk double heterostructure lasers. As a direct consequence of the reduction of the differential gain, there is no reduction of α in single-quantum-well lasers compared to bulk lasers. The number of quantum wells strongly affects the amplitude-phase coupling in quantum-well lasers. It is shown that the interband transition induced amplitude-phase coupling dominates that induced by the plasma effect of carriers in typical quantum-well lasers. By considering the spontaneous emission factor in the spectral linewidth, the authors show that there is an optimal number of quantum wells for achieving the narrowest spectral linewidth     

A 250 Hz spectral linewidth 1.5 mu m MQW-DFB laser diode with negative-electrical-feedback

A 1.5 mu m corrugation-pitch-modulated MQW-DFB laser diode (LD) with multielectrodes was frequency stabilized using the negative electrical feedback technique. The FM response of the LD was precisely measured and used for the feedback loop design. The FM noise of the LD was reduced and reached 10 Hz/sup 2//Hz at 1 kHz>     

光纤光栅外腔半导体激光器特性的研究

对谐振式集成光学角速率传感器 IORS 的关键器件之一——大功率、窄线宽光源进行了分析与实验 .分析结果表明 ,为达到优于 1°/ h的分辨率 ,光源应选用线宽小于 1MHz的强反馈光纤光栅外腔半导体激光器 FBG- L D ;研究了 FBG- LD的单纵模机制和线宽压窄特性 ,着重考虑了反馈端面的残留反射率对 FBG- L D输出功率的影响 ,以及耦合效率对 FBG- L D线宽的影响 ;在实验研究中采用了大功率、窄线宽、单纵模的 FBG- L D,并应用于光纤谐振腔 ,实测的谐振腔的清晰度与理论计算值是一致的 .     

电反馈半导体激光器强度调制的最佳设计

本文研究了电反馈对半导体激光器强度调制的影响。在适当的反馈条件下,电反馈可以较好地改善半导体激光器强度调制的性能。并且存在着最佳反馈环路传递函数,使得其强度调制性能达到最佳。     

AM noise enhancement in a semiconductor laser with electricalfeedback

A model of a self-heterodyne-type electrical feedback semiconductor laser is presented. The effects of electrical feedback on the AM noise of a semiconductor laser are discussed on the basis of the model. It is shown that the FM noise can be greatly reduced by electrical feedback. However, this reduction is always accompanied by AM noise enhancement. The AM noise can also be reduced by this scheme under some conditions with the enhancement of the FM noise. This means that AM noise reduction and FM noise reduction can be interchanged by using this method. The influence of feedback parameters on the noise is discussed     

Instability of coherence in semiconductor laser due to externalfeedback in hybrid-integrated optical waveguide components

The coherence instability of oscillation-frequency shift and spectrum-linewidth fluctuation of a distributed feedback (DFB) semiconductor laser due to reflected feedback from input or output facets in a butt-coupled waveguide device is theoretically analyzed. A low-loss butt-coupling method between a semiconductor laser and a waveguide device for hybrid-integrated optical components used in coherent optical-fiber systems is presented. The calculated results indicate that (1) the coherence instability of the DFB lasers is sensitive to external optical feedback, (2) feedback from the waveguide input facet affects the laser frequency and gain shifts, and (3) feedback from the output facet intensifies the linewidth fluctuations     

Spectral Narrowing of a Varactor-Integrated Resonant-Tunneling-Diode Terahertz Oscillator by Phase-Locked Loop

Spectral narrowing of a resonant-tunneling-diode (RTD) terahertz oscillator, which is useful for various applications of terahertz frequency range, such as an accurate gas spectroscopy, a frequency reference in various communication systems, etc., was achieved with a phase-locked loop system. The oscillator is composed of an RTD, a slot antenna, and a varactor diode for electrical frequency tuning. The output of the RTD oscillating at 610 GHz was down-converted to 400 MHz by a heterodyne detection. The phase noise was transformed to amplitude noise by a balanced mixer and fed back into the varactor diode. The loop filter for a stable operation is discussed. The spectral linewidth of 18.6 MHz in free-running operation was reduced to less than 1 Hz by the feedback.