Static and dynamic properties of injection-locked semiconductorlasers

The static and dynamic properties of injection-locked semiconductor lasers considering the influence of nonlinear gain are presented systematically. Depending on locking conditions, the modulation bandwidth of a semiconductor laser may be increased or decreased by external light injection. However, the relaxation resonance frequency and the damping rate as defined for a solitary Fabry-Perot (FP) laser are always enhanced by injection locking. That is, contrary to that in a solitary FP laser, the modulation bandwidth in an injection-locked laser is not determined solely by the relaxation resonance frequency, because an injection-locked laser is a third-order system. Therefore, a new definition of the modulation bandwidth is presented for such a laser. The performances of injection-locked distributed feedback (DFB) lasers are also discussed. The theory is in good agreement with the experiments     

Improved semiconductor-laser dynamics from induced population pulsation

This paper investigates theoretically the modification of dynamical properties in a semiconductor laser by a strong injected signal. It is found that enhanced relaxation oscillations are governed by the pulsations of the intracavity field and population at frequencies determined by the injected field and cavity resonances. Furthermore, the bandwidth enhancement is associated with the undamping of the injection-induced relaxation oscillation and strong population pulsation effects. There are two limitations to the modulation-bandwidth enhancement: Overdamping of relaxation oscillation and degradation of flat response at low frequencies. The injected-laser rate-equations used in the investigation reproduce the relevant aspects of modulation-bandwidth enhancement found in the experiment on injection-locked vertical-cavity surface-emitting lasers.     

Phase locking and chaos synchronization in injection-locked semiconductor lasers

We theoretically studied synchronization of chaotic oscillation in semiconductor lasers with chaotic light injection. Feedback-induced chaotic light generated from a master semiconductor laser was injected into a solitary slave semiconductor laser. The slave laser subsequently exhibited synchronized chaotic output for a wide parameter range with strong injection and frequency detuning within the injection-locking regime. Our numerical simulation revealed that the synchronized slave laser exhibits remarkable phase locking, even for chaotic light injection. Consequently, synchronization in phase fluctuations becomes dominant over intensity fluctuations. We found that there exists a parameter range where the slave can synchronize in phase only, with no intensity synchronization. However, synchronization can be completely destroyed, both in phase and in intensity, when the phase locking becomes unstable due to four-wave mixing or excited resonance oscillation. The phase locking was studied analytically and the correspondence between numerical and analytical results was shown. We also analytically examined chaos synchronization based on a linear stability analysis from the viewpoint of modulation response of injection-locked semiconductor lasers to a chaotic light signal. As a result, we verified that such injection-locking-induced chaos synchronization results from a quasilinear response of the bandwidth-broadened slave laser due to strong optical injection.     

Locking characteristics of a side-mode injected semiconductor laser

This paper is focused on an experimental study of the injection-locking bandwidth and the relaxation oscillation frequency in a side-mode injection-locked semiconductor laser. It is shown that, for such a laser, the injection-locking bandwidth and the relaxation oscillation frequency are related to the injection power and the wavelength of the target injection mode. At fixed launched injection power, the injection-locking bandwidth and relaxation oscillation frequency can be increased when the injection wavelength is detuned to the short-wavelength side of the free-running wavelength, while the relaxation oscillation frequency decreases when the injection wavelength is set to the long-wavelength side of the free-running wavelength. The results are in good agreement with theoretical predictions     

35-GHz intrinsic bandwidth for direct modulation in 1.3-/spl mu/m semiconductor lasers subject to strong injection locking

Significant enhancement in modulation bandwidth of semiconductor lasers subject to strong optical injection is experimentally and theoretically studied. At least two folds of improvement is achieved under study. By using an optical probing method, a modulation bandwidth of 35 GHz that is free from electrical parasitic effects is observed in the injection-locked laser system. The achieved bandwidth approaches the maximum modulation bandwidth set by the K factor for the free-running laser. Discussions are presented for an even larger modulation bandwidth using the injection-locking technique.     

Characteristics of chaotic masking in synchronized semiconductor lasers

Modulations imposed on a chaotic optical signal generated by a semiconductor laser can be suppressed by injecting the signal into another similar laser under conditions for chaos synchronization. This filter effect could be used to recover messages hidden in chaotic carriers for robust and secure communications. We use a numerical model to examine the filter properties and show that the filter can be described in terms of differences in characteristic transmission functions for imposed signal and chaotic carrier in the output of the synchronized laser. The filter effect is shown to be larger for lower frequencies and decreases as frequencies approach the relaxation oscillation frequency of the laser in the gigahertz regime, similar to the response of steady-state injection-locked lasers to small-signal modulation. The filter properties are confirmed in experiments using both single and multimode lasers.     

Direct Modulation Characteristics of Composite Resonator Vertical-Cavity Lasers

We report the small-signal modulation characteristics of a monolithic dual resonator vertical cavity surface emitting laser. The modulation response is described by a system of rate equations with two independent carrier populations and a single longitudinal optical mode. The independent optical overlaps and differential gains of the two active regions can each be adjusted to maximize the output response. We show that under certain conditions, the composite resonator may achieve a higher bandwidth than a single cavity laser with the same photon density. We find the relaxation oscillation frequency to depend mainly on the total photon density and not the individual currents in the two cavities. With appropriate current injection, the composite resonator laser achieves a maximum -3-dB bandwidth of 12.5 GHz and a maximum modulation current efficiency factor of approximately 5GHz/ma^1/2      

An optically driven phased array antenna utilizing heterodynetechniques

An optically driven phased array antenna system that utilizes heterodyne techniques to generate the microwave carrier frequency and phases is proposed. The heterodyning at each antenna element requires three injection-locked lasers. The microwave carrier frequency is determined by the order of the modulated sideband used in the injection-locking scheme. A design is also presented for an integrated optical fast Fourier transform (FFT). This device can perform the necessary phase processing for both beam steering and detection-of-arrival angle. Recent results on the noise properties of injection-locked semiconductor lasers are applied to the proposed antenna system. The effect of noise on the gain, signal-to-noise ratio, and the bandwidth of the antenna system is examined. It is shown that the performance degradation of the system, due to the noise in the lasers, is minimal, and therefore the bandwidth promise of optical drive could well be achieved with the use of heterodyne laser locking techniques     

Reduction of nonlinear distortion in directly modulatedsemiconductor lasers by coherent light injection

A theoretical investigation of the second- and third-order intermodulation distortions (IMDs) in an injection-locked semiconductor laser under small-signal modulation is presented. The results show that a substantial reduction of the laser nonlinearity can be obtained, depending on both the injection level and frequency detuning between the master and slave lasers. The intensity modulation frequency response is also reported and shows that the injection-locked laser may also have a significantly improved behavior over the same free-running laser, revealed mostly in the reduced resonance peak and the broadening of the modulation bandwidth available     

强光注入锁定半导体激光器的理论分析

通过速率方程建立了强光注入半导体激光器的理论模型,研究了注入光强与从激光器注入锁定频率范围的关系.结果表明,强光注入增大了半导体激光器的注入锁定范围.同时对半导体激光器进行了时间特性的数值模拟,计算结果表明,无论是强光注入还是弱光注入,激光器的输出最终都趋于稳定,通过强光注入锁定条件与弱光注入条件下的对比,得出强光注入能有效地抑制弛豫振荡,缩短达到稳定的时间.     

Frequency Response Enhancement of Optical Injection-Locked Lasers

The modulation response of injection-locked lasers has been carefully analyzed, theoretically and experimentally, with a focus on the strong optical injection regime. We derive closed-form solutions to the relaxation oscillation (resonance) frequency and damping term, as well as the low-frequency damping term, and discuss design rules for maximizing resonance frequency and broadband performance. A phasor model is described in order to better explain the enhancement of the resonance frequency. Experimental curves match closely to theory. Record resonance frequency of 72 GHz and broadband results are shown.     

Nonlinear gain coefficients in semiconductor lasers: effects ofcarrier heating

Nonlinear gain coefficients due to the effects of carrier heating are derived from the rate equations of carrier energy transfer in semiconductor lasers. We find that, in the modulation responses of semiconductor lasers, stimulated recombination heating will affect the resonant frequency and damping rate in a same form as the effects of spectral hole burning, while free carrier absorption heating will only affect the damping rate. The effects of injection heating and nonstimulated recombination heating are also discussed. The carrier energy relaxation time is calculated from first principles by considering the interactions between carriers and polar optical phonons, deformation potential optical phonons, deformation potential acoustic phonons, piezoelectric acoustic phonons. At the same time, the hot phonon effects associated with the optical phonons are evaluated because their negligible group velocity and finite decay time. We show that the carrier-polar longitudinal optical phonon interaction is the major channel of carrier energy relaxation processes for both electron and holes. We also point out the importance of the longitudinal optical phonon lifetime in evaluating the carrier energy relaxation time. Neglecting the finite decay time of longitudinal optical phonons will significantly underestimate the carrier energy relaxation time, this not only contradicts the experimental results but also severely underestimates the nonlinear gain coefficients due to carrier heating. The effects of spectral hole burning, stimulated recombination heating, and free carrier absorption heating on limiting the modulation bandwidth in semiconductor lasers are also discussed     

Locking bandwidth and relaxation oscillations of an injection-locked semiconductor laser

Detailed observations of an injection-locked semiconductor laser have allowed a verification of theoretical predictions for the locking bandwidth. In parallel, the fine structure of the intensity spectrum has been investigated and has revealed a strong modification of relaxation oscillation damping when locking is achieved.     

Four-wave mixing and optical modulation in a semiconductor laser

There is a direct connection between nearly degenerate four-wave mixing in a semiconductor laser and optical modulation in the laser field. It can be understood using a model of an unlocked, optically injected laser, which emphasizes the effect of the laser resonator on the optical interactions. This model correctly describes the observed spectral characteristics and their dependence on the intrinsic parameters of the semiconductor laser. This is used to develop a simple and accurate technique using a single experimental setup for the parasitic-free characterization of the intrinsic laser parameters, including the relaxation resonance frequency, the total relaxation rate, the nonlinear relaxation rate, and the linewidth enhancement factor. Other parameters, such as the spontaneous carrier lifetime, the photon lifetime, the differential and nonlinear gain parameters, and the K factor, are determined from the power dependencies of these parameters. This technique requires only two CW lasers closely matched in wavelength and is applicable to semiconductor lasers of any wavelength and any dynamic bandwidth     

Low-frequency intensity noise resonance in an external cavity GaAslaser for possible laser characterization

The dependence of the (low-frequency) resonance of light intensity noise in an external-cavity GaAs laser on cavity length and bias current, combined with the dependence of the threshold current on the magnitude of the light feedback, is used to determine simultaneously the carrier recombination lifetime, the carrier injection at transparency, and the optical loss. The key to this technique is understanding the correction term to the relaxation oscillation frequency due to finite carrier density at transparency and the influence of the long external cavity transit time on the photon lifetime in the three-mirror cavity. The technique uses commercial lasers without modifying them and does not require fast electronics     

Theoretical analysis of longitudinal mode coupling in external cavity semiconductor lasers

The single longitudinal mode behavior in long external cavity semiconductor lasers is discussed. Experimentally, the laser exhibits a single frequency oscillation even for an external cavity length of 100 cm. The mode selectivity of a composite cavity is shown to be insufficient to explain the experiments. Longitudinal mode coupling in semiconductor lasers is found to arise from an interference effect between the modes on the interband transition probability of electrons. Mode coupling equations are derived, which indicates that the single mode oscillation in long external cavity semiconductor lasers is brought about when the coupling strength goes beyond a critical value. It is shown that the effect of the hole burning in the external cavity semiconductor lasers is similar to that in the solitary laser.     

Frequency noise reduction of visible InGaAlP laser diodes bydifferent optical feedback methods

The emission spectra of several commercial InGaAlP laser diodes operating in the visible range were investigated. Strong relaxation sidebands at frequency spacings up to 2 GHz were observed in the field spectra. By using optical feedback from an external high-finesse resonator, the laser line-width was reduced from a few hundred megahertz to less than one megahertz. However, only a small part of the laser power was found within the narrow bandwidth, whereas most of the power finds itself in relaxation sidebands a few gigahertz apart from the carrier. The frequency noise reduction was investigated by using an external cavity configuration. With a short cavity length the relaxation oscillation sidebands were completely suppressed, and the laser linewidth was reduced to a few megahertz. Additional strong reduction in the laser linewidth to roughly 50 kHz was obtained using a combination of the external cavity laser with optical feedback from an external high-finesse resonator     

Small signal analysis of THz optical-frequency conversion in aninjection-locked semiconductor laser

A theory of terahertz optical-frequency conversion using highly nondegenerate four-wave mixing (FWM) in an injection-locked semiconductor laser is presented, using small-signal analysis. The optical frequency conversion can be realized through the use of cavity-enhanced highly nondegenerate FWM in an injection-locked semiconductor laser in a range of 1-THz detuning frequency between the pump and the probe waves, when the probe frequency is tuned close to one of the resonance modes. The frequency conversion is mainly attributed to the nonlinear gain effect. The maximum bandwidth of the converted signal is increased by shortening the laser cavity length. The frequency conversion efficiency is asymmetrical with respect to both the zero detuning frequency and the resonance mode where the converted signal appears. The theoretical results agree with experiments     

光纤环形腔半导体激光器弛豫振荡的分析

通过建立光纤环形腔半导体激光器的单模速率方程,用小信号方法讨论了弛豫振荡与环形腔长度的关系,以及直接电流调制下的调制响应与调制带宽。分析发现,在确定的参数下,存在环形腔长度的临界值。只有当环形腔长度小于该临界值时,激光器才发生弛豫振荡。     

Power and frequency dependence of hysteresis in optically bistableinjection locked VCSELs

Optical bistability in an injection-locked vertical cavity surface emitting semiconductor laser subject to parallel optical injection has been observed. It is shown that the width of the bistability loop may be enhanced by increasing the injection power and the frequency detuning