Analysis of DFB semiconductor lasers with external optical feedback

A theoretical analysis of the sensitivity of DFB semiconductor lasers to external optical feedback is presented. Numerical simulations allow the authors to determine the optimum facet reflectivity, for a given κL value, to minimise the sensitivity to external optical feedback     

Sensitivity to external feedback for gain-coupled DFB semiconductor lasers

Performances of DFB semiconductor lasers with both index and gain spatial modulation are theoretically analysed in terms of threshold gain, emission wavelength, model discrimination and sensitivity to external optical feedback. It is found that gain coupling provides low threshold gain, high spectral selection in favour of the mode oscillating at the Bragg wavelength with a relative immunity from facet reflection, and the sensitivity to external feedback is not significantly reduced in comparison with the case of pure index coupling.<>     

Effects of structural parameters on the external optical feedbacksensitivity in DFB semiconductor lasers

External optical feedback sensitivity in distributed feedback (DFB) semiconductor lasers is analyzed with special attention to phase-shifted and complex-coupled lasers. The effects of various structural parameters such as coupling strength, facet reflectivity, and corrugation phase angle on external optical feedback sensitivity are studied. The λ/4 phase-shifted index-coupled DFB laser exhibits low external optical feedback sensitivity for large index-coupling coefficient and high facet reflectivity. Pure gain-coupled DFB lasers perform better than the phase-shiftless uniform index-coupled DFB lasers but worse than λ/4 phase-shifted index-coupled lasers with high coupling strengths. External optical feedback sensitivity of complex-coupled lasers depends significantly on the index-to-gain coupling ratio, the phase between the index and gain gratings, and the total coupling     

Effect of external optical feedback on spectral properties of external cavity semiconductor lasers

The effects of external optical feedback on the spectral properties of single-mode external cavity semiconductor lasers that use a graded-index (GRIN) lens in the optical cavity have been investigated. Mode rejection ratio and intensity drop-out rate of the dominant mode as a function of optical strength have been measured. These measurements show that a laser with a short (160 ?m) GRIN-lens external cavity can tolerate optical feedback as large as ?20 dB without significant penalty. This minimum optical feedback can be larger when lasers with shorter cavity length are used.     

Suppression of external cavity modes in DFB lasers with a highendurance against optical feedback

This paper clarifies that the external cavity modes deform optical output response in the DFB lasers by enhancing the relaxation oscillation under NRZ modulation. It is shown that a DFB laser with an asymmetric structure in reflectivity of facets and in position of a λ/4 phase shift can suppress the influence of the external cavity modes     

Gain measurements on semiconductor lasers by optical feedback from an external grating cavity

We report on new methods to determine the curvature around maximum of the mode gain curve of homogeneously line-broadened semiconductor lasers, either by measuring the wavelength dependence of threshold current of the laser coupled to an external grating cavity or by measuring the tuning range of the laser versus the amount of optical feedback. We experimentally found the mode gain width and the current-induced peak-gain wavelength shift of a GaAlAs CSP laser to 175 Å and 2.1 Å/ mA, respectively.     

半导体激光器的外光学反馈效应的理论分析

本文研究了半导体激光器的外光学反馈效应，应用Lang-Kobayashi方程，分析求解了半导体激光器在外腔反馈条件下激光频率和光强的输出变化特性。     

Criterion of external feedback sensitivity in index-coupled andgain-coupled DFB semiconductor lasers to be free from excess intensitynoise

Sensitivities of index-coupled as well as gain-coupled DFB semiconductor lasers to external optical feedback are theoretically investigated in this work. The conditions of getting free operation from excess intensity noise in these two types of DFB lasers are obtained. It is found that the sensitivity of external feedback for these two types of DFB laser depends significantly on the structure such as the internal grating, the facet reflection, and the phase relation between the grating and the facet. The condition of making a DFB laser less sensitive to external optical feedback is to adjust the lasing wavelength to the Bragg wavelength in both index-coupled and gain-coupled structures. A phase-adjusting mechanism such as the λ/4 shift region in the index-coupled structure is effective to avoid the external feedback     

Transform-limited pulses from low chirp DFB lasers with external feedback

Transform-limited pulses with time-bandwidth product as low as 0.34 are obtained from a microwave modulated low-chirp MQW-DFB laser diode coupled to a long, external cavity. Pulsewidths from 20 to 30 ps are measured for repetition frequencies tunable from 2.5 to 5 GHz in steps of approximately 100 MHz.<>     

The influence of external frequency shifted feedback on a DFB semiconductor laser

To investigate the possibility of optical isolation of a semiconductor laser using acoustooptic interaction, the influence of frequency shifted feedback on a DFB laser was studied experimentally. The isolation concept is based on a frequency shift of the laserlight by means of an acoustooptic Bragg deflection. This concept offers the opportunity of optical integration when Surface Acoustic Waves are used. In the experiments, a clear difference in laser performance between regular feedback and frequency shifted feedback was observed.     

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.     

Spectral characteristics of semiconductor lasers with optical feedback

Optical feedback-induced changes in the output spectra of several GaAlAs lasers operating at 0.83 μm are described. The feedback radiation obtained from a mirror 60 cm away from the laser is controlled in intensity and phase. Spectral line narrowing or broadening is observed in each laser depending on the feedback conditions. Minimum linewidths observed with feedback are less than 100 kHz. Improved wavelength stability is also obtained with optical feedback resulting in 15 dB less phase noise. An analytical model for the three-mirror cavity is developed to explain these observations.     

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.     

Mode partition in semiconductor lasers with optical feedback

The authors point out that random noise processes induce mode partition fluctuations in semiconductor lasers. Mode partition depends on laser parameters and modulation current. However, external optical feedback can also increase mode partition noise. Here, a numerical solution of multimode noise-driven rate equations with time-delayed terms is utilized to investigate mode partition in semiconductor lasers with reflecting feedback. Photon statistics of the main and side modes in semiconductor lasers under both CW operation and dynamic operation are considered. Probability-density curves for the main and side modes are shown. The feedback-induced change of photon statistics of the main and side modes is clearly seen. Numerical results indicate that, if the laser used is exposed to reflections, a more stringent mode discrimination requirement for suppressing the buildup of laser-cavity longitudinal side modes may result. If mode discrimination is insufficient for avoiding the excitation of side modes, the feedback-induced power penalty depends on the fiber dispersion     

基于半导体环形激光器的光反馈动力学研究

为了研究半导体环形激光器在外部交叉光反馈情况下的非线性动力学特性, 基于环形激光器的速率方程, 采用数值仿真的方法, 得到了各种反馈参量下的时间序列及功率谱, 并研究了反馈系数对混沌带宽及时间延迟特性的影响。结果表明, 在各种不同外部反馈参量的情况下, 环形激光器展现了单周期、多周期、低频反相波动以及混沌态等丰富的动力学态; 在对称光反馈时, 可得到最大为3.7GHz的混沌带宽; 在时间不对称光反馈时, 产生的混沌信号的时延特征被很好地抑制。该研究结果可为环形激光器的实际应用提供一定的理论参考。     

Self-pulsation in two-section DFB semiconductor lasers and itssynchronization to an external signal

A model of self-pulsation in two-section distributed feedback (DFB) lasers without a saturable absorber is developed by using generalized rate equations. The introduction of an effective differential gain in our model allows us to take into consideration both material and structural effects. The self-pulsation conditions are derived from a linear stability analysis. A mechanism based on a negative effective differential gain is proposed to explain the origin of self-pulsation in such lasers. By considering an injected optical signal, the optical synchronization of self-pulsating lasers is studied using nonlinear simulations. This leads to the determination of some locking-range properties, which are then compared to experimental and analytical results     

Noise properties of semiconductor lasers due to optical feedback

It has been recognized for some time that reflections back into the cavity of an injection laser from a mechanically unstable external mirror cause noise in the output of the laser (reflection noise). In this paper, noise measurements are presented for index-guided lasers as functions of the output power, the degree of optical isolation and external cavity length (5-15 cm), while current modulation (using 2-4 mA at 50-200 MHz) has been used to reduce the noise levels by as much as 20 dB. It is demonstrated that the mechanism of noise reduction by modulation is the frequency modulation of the laser's spectrum and its interaction with the modes of the external cavity. It is shown that using an optical isolator can give a relative intensity noise of -110 dB while using modulation in addition can reduce this to -125 dB; the bandwidth performance of optical storage systems is discussed in terms of these two schemes.     

A novel analytical expression of sensitivity to external opticalfeedback for DFB semiconductor lasers

A novel analytical expression for DFB laser sensitivity to external optical feedback is derived based on coupled-wave theory. The expression reveals that the sensitivity is proportional to the ratio between the output power from the reflector-side facet and the magnitude of Lagrangian of the electromagnetic field in the cavity. The simplicity of the expression helps in estimating the sensitivity of DFB lasers with complex cavity structures     

Subharmonic modulation distortions in semiconductor lasers with optical feedback

Experimental studies of subharmonic modulation distortions of semiconductor lasers with optical feedback are described. These distortions were found to exhibit almost resonant frequency characteristics and threshold behaviour against the modulation frequency and its depth, respectively. It was inferred that they would be excited by parametric interactions caused by some laser diode parameter changes due to modulation.     

Quantifying optical feedback into semiconductor lasers via thermal profiling

We show that thermal profiling can be used to monitor optical feedback into semiconductor lasers in photonic integrated circuits, where direct optical access is impractical. The optical output power of a laser, both free-running and when exposed to optical feedback, can be quantitatively determined from thermal measurements alone, without recourse to direct optical measurements. Furthermore, the shift in threshold current resulting from optical feedback can be determined from the thermal measurements, enabling quantitative determination of the feedback coupling efficiency into the laser.