M. Pochet

Modeling the Injection-Locked Behavior of a Quantum Dash Semiconductor Laser

Using the conventional rate equations describing an injection-locked system, a novel modulation response function is derived, which implicitly incorporates nonlinear gain through the free-running relaxation oscillation frequency and damping rate of the slave laser. In this paper, it is shown that the model presented can be used to extract the characteristic parameters of the coupled system from experimental data. The number of fitting parameters in the model is reduced by determining the fundamental slave parameters through the conventional free-running response function; these parameters are considered to be constant during the curve-fitting of the injection-locked system. Furthermore, in order to reduce the number of possible solutions generated during the least-squares-fitting process, the remaining fitting parameters are tightly constrained based on the physical limits of the coupled system. By reducing the number of unknown fitting parameters and constraining the remaining terms, a stronger confidence in the extracted parameters is achieved. Using a series of response curves measured from an injection-locked quantum dash laser, characteristic parameters of the system are extracted and validity of the model is examined. The verified model is used to analyze the impact of the linewidth enhancement factor on the characteristics of the response function in the microwave domain.

Variation of the feedback sensitivity in a 1.55 μm InAs/InP quantum-dash Fabry–Perot semiconductor laser

Dynamic feedback properties of a 1.55 μm InAs/InP quantum dash laser are reported. The ground state linewidth enhancement factor (αH-factor) is found to be enhanced from ∼1 to ∼14 as the bias current is increased beyond the threshold value. As a consequence of the variation in the αH-factor, the feedback sensitivity of the quantum dash semiconductor laser is dramatically affected over the entire range of operational currents. The onset of its critical feedback regime, which is incompatible with data transmission, is shown to exhibit a variation of approximately 20 dB for the quantum dash device.

Optical feedback instabilities in a monolithic InAs/GaAs quantum dot passively mode-locked laser

The impact of optical feedback on the direct performance of a monolithic InAs/GaAs quantum dot passively mode-locked laser intended for applications such as multigigahertz interchip/intrachip clock distribution is experimentally investigated. Evaluation of the feedback resistance is an important feature, as the laser is to be monolithically integrated on chip with other devices, in which case optical isolation is difficult. This work shows that a feedback level on the order of −24 dB is detrimental for mode-locking operation, enhancing noise in the rf electrical signal, strongly narrowing the useful mode-locking region as well as causing central frequency shift, and severe instabilities.

Manipulation of the linewidth enhancement factor in an injection-locked Quantum-Dash Fabry-Perot laser at 1550nm

The impact of ultra-strong optical-injection on the linewidth enhancement factor through the threshold gain shift of a Quantum-Dash Fabry-Perot laser at zero-detuning condition is analyzed using theoretical predictions and verified with experimental observations.

Bandwidth enhancement in an injection-locked quantum dot laser operating at 1.31-μm

The high-speed modulation characteristics of an injection-locked quantum dot Fabry-Perot (FP) semiconductor laser operating at 1310-nm under strong injection are investigated experimentally with a focus on the enhancement of the modulation bandwidth. The coupled system consists of a directly-modulated Quantum Dot (QD) slave injected-locked by a distributed feedback (DFB) laser as the master. At particular injection strengths and zero detuning cases, a unique modulation response is observed that differs from the typical modulation response observed in injection-locked systems. This unique response is characterized by a rapid low-frequency rise along with a slow high-frequency roll-off that enhances the 3-dB bandwidth of the injection-locked system at the expense of losing modulation efficiency of about 20 dB at frequencies below 1 GHz. Such behavior has been previously observed both experimentally and theoretically in the high-frequency response characteristic of an injection-locked system using an externally-modulated master; however, the results shown here differ in that the slave laser is directly-modulated. The benefit of the observed response is that it takes advantage of the enhancement of the resonance frequency achieved through injection-locking without experiencing the low frequency dip that significantly limits the useful bandwidth in the conventional injection-locked response. The second benefit of this unique response is the improvement in the high frequency roll-off that extends the bandwidth. Finally a 3-dB bandwidth improvement of greater than 8 times compared to the free-running slave laser has been achieved.

Linewidth enhancement factor and dynamical response of an injection-locked quantum-dot Fabry-Perot laser at 1310nm

This work investigates the linewidth enhancement factor (alpha-factor) and stability of an optically-injected InAs/InGaAs quantum-dot Fabry-Perot laser. Using the injection-locking technique, the above threshold alpha-factor is measured to be as low as 0.6 at 1.3X the threshold current. The below threshold alpha-factor is also measured using the Hakki-Paoli technique. The measured alpha-factor values are used to simulate the dynamic response (stable locking, period-one, period-doubling, or chaos) in the context of single-mode rate equations under zero-detuning injection conditions for external injected power ratios ranging from -11dB to +15dB and slave current bias levels of 1.3X, 2X, and 2.6X threshold. Legacy literature has shown that optically-injected diode lasers typically follow the period-doubling route into a chaotic region as the injection level is increased. Simulations show that at 2X the threshold current, a small region of period-one operation will be observed followed by stable-locking as the injection ratio is increased. This predominantly stable behavior is driven largely by the low alpha-factor. Experimental results support this prediction, where under zero-detuning conditions, only unlocked and stable-locking operation is observed. Experimentally, periodone operation was not observed at a slave laser bias current of 2X threshold, as it was predicted to occur below an external power ratio of -20 dB, a level which was not attainable in this work. Such findings suggest that a quantum-dot device can be employed in an optically-injected configuration for photonic tunable-clock applications.

Optical injection of quantum dash semiconductor lasers at 1550nm for tunable photonic oscillators

In this manuscript, we will theoretically compute and experimentally investigate the dynamics of an optically injected nanostructure laser as a function of the injection strength and the optical detuning frequency. A model describing the optically-injected semiconductor laser is derived in dimensionless format that accounts for the large nonlinear gain component associated with nanostructure semiconductor lasers through the gain coefficients derivative with respect to perturbations in the carrier and photon density. The nonlinear gain (carrier) relaxation rate and gain compression coefficient account for the perturbation in the slave lasers photon density, and are theoretically shown to have a strong impact on the level of stability exhibited by the optically-injected laser. The theoretical model is experimentally verified through the optical-injection of a quantum-dash Fabry-Perot laser at an operating wavelength of 1550 nm. The quantum-dash lasers large damping rate and sufficiently small linewidth enhancement factor are observed to inhibit period-doubling and chaotic operation under zero frequency-detuning conditions. Additionally, the optically injected quantum-dash laser is found to exhibit a large period-one operational state as the injection strength and the detuning frequency are varied, resulting in a highly tunable microwave frequency in the coupled systems equilibrium state. The enhanced and undamped relaxation oscillations of the period-one state are discussed as a building block for tunable photonic oscillators in various RF photonics applications. Finally a path towards realizing an optically-injected diode laser with a THz resonance frequency will be reviewed.

Temperature effects on the modulation response of an injection-locked InAs/InP Quantum-Dash laser

The impact of device temperature variations on the modulation response of an injection-locked Quantum-Dash laser is analyzed. Lower slave operating temperatures result in a large reduction in the undesirable pre-resonance dip in the modulation resp onse.

Modulation Response of an Injection Locked Quantum-Dash Fabry Perot Laser at 1550nm

The microwave domain modulation response of an injection-locked laser system is analyzed in the context of a Quantum Dash Fabry-Perot laser. This work demonstrates the applicability of a newly-derived modulation response function by using it to least-squares fit data collected on an injection-locked system with a Quantum-Dash Fabry-Perot semiconductor slave laser. The maximum injection strength, linewidth enhancement factor, coupled phase between the master and slave, and field enhancement factor characterizing the deviation of the locked slave laser from its freerunning value are extracted by least-squares fitting the collected data with the function. The extracted values are then compared with theoretically expected values under the given detuning conditions. The correlation between the frequency of the resonance peak of the modulation response at the positive frequency detuning edge and a pole in the modulation response function under this detuning condition is illustrated. The calculation of the injection strength based on the experimental operating conditions is verified by applying the modulation response function to the experimental data. With the modulation response function, injection-locked behaviors can be accurately simulated in the microwave domain and used to predict operating conditions ideal for high-performance RF links.

Systematic Investigation of the Alpha Parameter Influence on the Critical Feedback Level in QD Lasers

The dramatic variation in the linewidth enhancement factor (αΗ-factor) that has been reported for quantum dot lasers makes them an interesting subject for optical feedback studies. A low αΗ-factor combined with a high damping factor is especially interesting because it should increase the tolerance to optical feedback in these devices and may offer potential advantages for direct modulation. In the particular case of QD lasers, the carrier density is not clearly clamped at threshold. The lasing wavelength can switch from the ground state (GS) to the excited state (ES) as the current injection increases meaning that a carrier accumulation occurs in the ES even though lasing in the GS is still occurring. The filling of the ES inevitably enhances the αΗ-factor of the GS above threshold as experimentally and numerically shown. Consequently, this strong variation of the GS αΗ-factor in comparison to QW devices, should theoretically produce a significant variation in the onset of coherence collapse due to feedback. This coherence collapse regime, in which the laser is subject to instabilities, is incompatible with data transmission because of the induced high bit-error rate. One method to investigate the tolerance to optical feedback is to compare experiment with the theoretical work introduced by Petermann. It will be presented that under specific conditions, i.e., in the case of a strong enhancement in the αΗ-factor, the feedback sensitivity of the laser can vary by as much as 10dB within the same device.