Condition for short pulse generation in ultrahigh frequency mode-locking of semiconductor lasers

It is shown that although it is possible to obtain mode-locking without self-pulsation when certain criteria are satisfied, the shortest pulses are almost always generated at or close to the onset of self-pulsation. Thus, the amplitude of the optical pulse train is modulated by the (relatively) low-frequency envelop of a few gigahertz under this condition. This observation was obtained by simultaneously measuring the pulsewidth using an autocorrelator and monitoring the optical intensity using a high-speed photodiode and a microwave spectrum analyzer. It is concluded that while it is possible to generate picosecond optical pulses in ultrahigh-frequency mode-locking of quantum-well lasers, very short pulses ( to 1 ps) are almost always accompanied by self-pulsation which is manifested as low-frequency (gigahertz) envelope modulation of the optical pulse train.<>     

Dispersive self-Q-switching in self-pulsating DFB lasers

Self-pulsations reproducibly achieved in newly developed lasers with two distributed feedback sections and with an additional phase tuning section are investigated. The existence of the dispersive self-Q-switching mechanism for generating the high-frequency self-pulsations is verified experimentally for the first time. This effect is clearly distinguished from other possible self-pulsation mechanisms by detecting the single-mode type of the self-pulsation and the operation of one section near the transparency current density using it as a reflector with dispersive feedback. The operating conditions for generating this self-pulsation type are analyzed. It is revealed that the required critical detuning of the Bragg wavelengths of the two DFB sections is achieved by a combination of electronic wavelength tuning and current-induced heating. The previous reproducibility problems of self-pulsations in two-section DFB lasers operated at, in principle, suited current conditions are discussed, and the essential role of an electrical phase-control section for achieving reproducible device properties is pointed out. Furthermore, it is demonstrated that phase tuning can be used for extending the self-pulsation regime and for optimizing the frequency stability of the self-pulsation. Improved performance of the devices applied as optical clocks thus can be expected     

Development of self-pulsations due to self-annealing of proton bombarded regions during aging in proton bombarded stripe-geometry AlGaAs DH lasers grown by molecular beam epitaxy

Experimental observations indicate that the occurrence of optical self-pulsation in proton delineated stripe-geometry double-heterostructure junction lasers is related to the degree of gain guiding inherent in individual lasers. We show that an aging process occurs during lasing operation which has the effect of partially annealing the proton induced carrier removal concentration at the edges of the active stripe of the laser. In some lasers, the magnitude of this annealing effect is sufficiently large to flatten the active stripe carrier concentration profile thus reducing filament stability leading ultimately to optical self-pulsation. It is shown that the carrier concentration profile modification is due to the dual effects of decreasing then = 2nonradiative current component at the active stripe-proton bombarded interface as well as the geometric effect of increasing the laser active stripe width. This latter effect may be also responsible for some portion of laser threshold current increase observed during device operation.     

Self-Pulsating Amplified Feedback Laser Based on a Loss-Coupled DFB Laser

Monolithic self-pulsating semiconductor lasers called amplified feedback lasers (AFLs) can generate high-frequency self-pulsations according to the concept of a single-mode laser with shortly delayed optical feedback, which consist of a distributed-feedback (DFB) laser, a phase control, and an amplifier section. Since mode degeneracy of the DFB section, which should operate as a single-mode laser, affects the self-pulsation, single-mode characteristics of the DFB section are critical for the self-pulsation. The effect of a complex coupling in the DFB section on the self-pulsation is numerically analyzed to reveal that the complex coupling provides a wide operation range for the self-pulsation. Also, self-pulsating AFLs based on a loss-coupled DFB laser are experimentally demonstrated to verify the self-pulsation characteristics and the capability for all-optical clock recovery.     

Self-pulsation in multielectrode distributed feedback lasers

Measurements on multielectrode distributed feedback (DFB) lasers without a saturable absorber reveal the existence of a self-pulsation (SP) regime. In this regime, the laser remains in the same single-longitudinal mode with simultaneous intensity and frequency modulation. The laser spectrum is similar to that of a current-modulated single-mode laser. At the up-limit of the SP regime, the behavior between the output power and the injection current becomes bistable. In one branch of the bistable loop, the SP laser presents a very large spectrum without distinguishable peaks, a kind of chaotic state with coherence collapse. A qualitative explanation based on the effective differential gain is given for the origin of SP and associated phenomena in these lasers     

Analysis of differential gain in InGaAs-InGaAsP compressive andtensile strained quantum-well lasers and its application for estimationof high-speed modulation limit

A simplified model that furnishes an intuitive insight for the change in quantum-well (QW) laser gain due to QW strain and quantum confinement is presented. Differential gain for InGaAs-InGaAsP compressive and tensile strained multi-quantum-well (MQW) lasers is studied using the model. The comparison between the calculated and experimental results for lattice-matched and compressive strained MQW lasers shows that this model also gives quantitatively reasonable results. It is found that the variance-band barrier height strongly affects the differential gain, especially for compressively strained MQW lasers. The tensile strained MQW lasers are found to have quite high differential gain, due to the large dipole matrix element for the electron-light-hole transition, in spite of the large valence-band state density. Furthermore, a great improvement in the differential gain is expected by modulation p doping in the tensile strained MQW lasers. The ultimate modulation bandwidth for such lasers is studied using the above results     

Self-pulsations in strongly coupled asymmetric external cavitysemiconductor lasers

Self-pulsations in asymmetric external cavity semiconductor lasers are studied experimentally and are analyzed using improved rate equations which include multiple reflections. These equations are valid for arbitrary levels of coherent external optical feedback. The dependence of self-pulsation frequencies on injection current, external mirror tilt angle and reflectivity, and external cavity length is explained by small-signal analysis of the rate equations. By numerical integration of the rate equations, self-pulsations are demonstrated theoretically and resonant enhancement of intensity noise is shown to occur when the self-pulsation frequency is an integer fraction of the external cavity resonance frequency     

Measurement of Semiconductor Laser Gain by the Segmented Contact Method Under Strong Current Spreading Conditions

A segmented contact method for the measurement of optical gain is developed for the case of strong current spreading. A simple model of current spreading in a ridge laser with a segmented contact is proposed and analyzed. We show that current spreading effects should be taken into account in lasers with low threshold current densities and high ldquoopeningrdquo voltages. When applied to interband cascade lasers, the method gives an internal optical loss of ~ 10-17 cm^-1 and a differential gain of ~ 2.9 cm/A at 80 K, which agrees well with previously reported Hakki-Paoli data. The limitations of the technique are discussed.     

Microscopic simulation of the temperature dependence of static and dynamic 1.3-/spl mu/m multi-quantum-well laser performance

The temperature dependence of the performance of 1.3-/spl mu/m Fabry-Perot (FP) multiple-quantum-well (MQW) lasers is analyzed using detailed microscopic simulations. Both static and dynamic properties are extracted and compared to measurements. Devices with different profiles of acceptor doping in the active region are studied. The simulation takes into account microscopic carrier transport, quantum mechanical calculation of the optical and electronic quantum well properties, and the solution of the optical mode. The temperature dependence of the Auger coefficients is found to be important and is represented by an activated form. Excellent agreement between measurement and simulation is achieved as a function of both temperature and doping profile for static and dynamic properties of the lasers, threshold current density, and effective differential gain. The simulations show that the static carrier density, and hence the contribution to the optical gain, varies significantly from the quantum wells on the p-side of the active layer to those on the n-side. Furthermore, the modal differential gain and the carrier density modulation also vary. Both effects are a consequence of the carrier dynamics involved in transport through the MQW active layer. Despite the complexity of the dynamic response of the MQW laser, the resonance frequency is determined by an effective differential gain, which we show can be estimated by a gain-weighted average of the local differential gain in each well.     

Generation and quenching of intensity pulsations in semiconductor lasers coupled to external cavities

The behavior of self-pulsing and nonpulsing lasers coupled to external cavities is investigated experimentally and theoretically. We investigate the dependence of the pulsation characteristics on the external cavity length using a saturable absorber model for self-pulsing lasers. It was found that quenching of self-pulsation occurs only for a certain limited range of external cavity length, and the frequencies of external-cavity induced pulsations lies within a certain range determined by the coupling coefficient. Small-signal analysis allows these ranges to be derived analytically. Hitherto, complex pulsation phenomena can be explained very intuitively by interpreting the combined laser-external cavity system as a microwave oscillator with a limited gain band and discrete mode structure.     

Theory of gain in quantum-wire lasers grown in V-grooves

Theoretical calculations of gain, refractive index change, differential gain, and threshold current for GaAs-AlGaAs quantum-wire lasers grown in V-shaped grooves are presented. The theoretical model is based on the density-matrix formalism with intraband relaxation, and the subband structure is calculated within the effective bond-orbital model. For the quantum-wire geometry treated, agreement with the observed subband spacings is found. Because of the small overlap of the optical field with the active region for a single quantum wire, lasing threshold is reached only when several subbands are filled     

Phase correlation between longitudinal modes in semiconductor self-pulsating DBR lasers

Phase correlation leading to self-pulsation (SP) in semiconductor distributed Bragg reflector (DBR) lasers is investigated experimentally and theoretically. Under proper biasing conditions, the laser oscillates with three main modes and we observe that each two-modes beating provides SP with identical spectral linewidth. Under the same operating conditions, the measured spectral linewidths of the beating modes are much larger than the linewidth of the self-pulsating signal. These results demonstrate the natural occurrence of passive mode-locking (PML) and phase correlation in semiconductor DBR lasers. A model based on multimode coupled-wave rate equations, including four-wave mixing (FWM), is developed to describe PML and SP in the gain region of the laser cavity. This model demonstrates that the existence of phase correlation between longitudinal modes is due to FWM.     

High-speed modulation of long-wavelength In1-xGaxAsyP1-y andIn1-x-yGaxAlyAs strained quantum-welllasers

In_1-xGa_xAs_1-yP_y quantum-well (QW) lasers with compressive strain and In_1-x-yGa_xAl_yAs QW lasers with two strain types (compressively strained and lattice matched) for 1.55-μm telecommunication applications are investigated both in the steady-state and high-speed microwave modulation schemes. Under steady-state electric bias, the gain and intrinsic loss are measured based on the well-known Hakki-Paoli method from below threshold to threshold. The photon lifetime is obtained from this measurement. A comprehensive theoretical gain model with realistic band structure, including valence band mixing and many-body effects, is then used to fit the experimentally obtained modal gain profiles and extract the carrier density and, therefore, the differential gain. In the high-speed microwave modulation scheme, the experimental modulation response curves are fitted by the theory and parameters such as the differential gain and K factor are obtained. The differential gain agrees very well with the value obtained from the steady-state direct optical gain measurement. The comparison of two material systems will be important to design high-bandwidth high-performance semiconductor lasers in order to meet requirements of 1.55-μm telecommunication applications     

Dynamics of all-optical clock recovery using two-section index- and gain-coupled DFB lasers

The dynamics of coherent clock recovery (CR) using self-pulsing two-section distributed feedback (TS-DFB) lasers have been investigated. Both simulation and experimental results indicate fast lockup and walk-off of the clock-recovery process on the order of nanoseconds. Phase stability of the recovered clock from a pseudorandom bit sequence (PRBS) signal can be achieved by limiting the detuning between the frequency of free-running self-pulsation and the input bit rate. The simulation results show that all-optical clock recovery using TS-DFB lasers can maintain a better than 5% clock phase stability for large variations in power, bit rate, and optical carrier frequency of the input data and therefore is suitable for applications in optical packet switching.     

The impact of energy band diagram and inhomogeneous broadening on the optical differential gain in nanostructure lasers

We present a general theoretical model for the optical differential gain in semiconductor lasers. The model describes self assembly quantum dots (QDs), self assembly quantum wires (QWRs) and single quantum-well lasers. We have introduced the inhomogeneous broadening due to size fluctuations in the assembly cases. At each dimensionality, we have considered the carrier populations in the excited states and in the reservoirs, where conduction and valence bands are treated separately. We show that for room temperature operation the differential gain reduction due to increased size inhomogeneity is more pronounced in QDs than in QWRs. We show this reduction to be smaller than the one-order reduction attributed to state filling in conventional dot and wire assemblies operating at room temperature. The integration prefactor coefficient of the differential gain in zero-dimensional cases exceed one- and two-dimensional coefficients only for low temperatures where the homogenous broadening is considerably smaller than the thermal energy. The differential gain of QDs, QWRs, and compressively strained single quantum-well lasers operating at room temperature and close to equilibrium is nearly the same.     

High-frequency pulsations in DFB lasers with amplified feedback

We describe the basic ideas behind the concept of distributed feedback (DFB) lasers with short optical feedback for the generation of high-frequency self-pulsations and show the theoretical background describing realized devices. It is predicted by theory that the self-pulsation frequency increases with increasing feedback strength. To provide evidence for this, we propose a novel device design which employs an amplifier section in the integrated feedback cavity of a DFB laser. We present results from numerical simulations and experiments. It has been shown experimentally that a continuous tuning of the self-pulsation frequency from 12 to 45 GHz can be adjusted via the control of the feedback strength. The numerical simulations, which are in good accordance with experimental investigations, give an explanation for a self-stabilizing effect of the self-pulsations due to the additional carrier dynamic in the integrated feedback cavity.     

High-temperature operation of 650-nm wavelength AlGaInPself-pulsating laser diodes

Low-coherence self-pulsating laser diodes operating at a wavelength of 650 nm and at temperatures in excess of 70°C are required for high density optical storage systems. We report on AlGaInP lasers operating at this wavelength which exhibit stable self-pulsation up to a temperature of 100°C. The lasers are 50-μm-wide oxide-isolated stripe devices in which the saturable absorption necessary for pulsation is provided by multiple-quantum wells placed within the p-doped cladding layer. The pulsation frequency of the devices increases linearly with increasing drive current and is present up to 1.5 times, lasing threshold     

Reduced effective differential gain in diode lasers due toconfinement factor modulation

A reduced effective differential gain is shown to arise in diode lasers by including the modulation of the confinement factor with carrier density. This effective differential gain, not the material gain, is the parameter determined from conventional measurements of the differential gain. This term is in addition to the static reduction in confinement factor with carrier density, and can significantly reduce the resonance frequency and modulation bandwidth for lasers with short cavities and thin active layers     

Nonlinearities in the emission characteristics of stripe-geometry (AlGa)As double-heterostructure junction lasers

Significant changes in the optical and electrical properties of stripe-geometry (AlGa)As double-heterostructure junction lasers have been observed to accompany a nonlinearity in the current dependence of the lasing emission. Over the nonlinear range, the optical field of the lasing emission shifts continuously with current toward one boundary of the active stripe. Simultaneously, effective gain saturation is lost and additional transverse modes are excited. The increasing gain is believed to occur primarily in spatial regions where the optical intensity is reduced as a result of the transverse motion of the field. Additional observations suggest that this transverse motion results from an interaction between the intense lasing field and the active medium. Consequently, gain depletion is discussed as a possible cause for the transverse instability. Implications of this model for a qualitative understanding of the improved output performance recently obtained from lasers with narrow stripes are also considered.     

Empirical formulas for design and optimization of 1.5 μmInGaAs/InGaAsP strained-quantum-well lasers

The effects of strain and number of quantum wells on optical gain, differential gain, and nonlinear gain coefficient in 1.55-μm InGaAs/InGaAsP strained-quantum-well lasers are theoretically investigated. Well-approximated empirical expressions are proposed to model these effects. Using these formulas, one can easily and accurately predict the performance of a laser diode for a given structure. Therefore, these empirical formulas are useful tools for design and optimization of strained quantum well lasers. As a general design guideline revealed from the empirical formulas, the threshold current is reduced with the compressive strain, and the modulation bandwidth is most efficiently increased with the number of wells