Carrier-induced index change in AlGaAs double-heterostructure lasers

A large carrier-induced index change is reported for conventional 8 ?m-stripe oxide-isolated AlGaAs double-heterostructure lasers. At threshold, the index change of the active layer is ?0.05 to ?0.07, which is a factor of 5 to 10 larger than previously reported. It is accompanied by an even greater change in dispersion. These effects cannot be explained by a free-carrier effect, and are most likely caused by a carrier-induced shift of the absorption edge.     

AlGaAs/GaAs双异质结激光器退化的研究

观察了室温下工作的DH激光器退化现象,发现在短时间内退化的器件,除阈值升高外尚伴随着微分量子效率下降。对热稳定性良好,并对于较长时间(>250小时)阈值上升率(△J_(th)/J_(th)千小时)小于3％的器件,高温老化证明它们的寿命都能超过万小时量级。     

Digital optical signal processing with polarization-bistable semiconductor lasers

The operations of a complete set of optical AND, NAND, OR, and NOR gates and clocked opticalS-R, D, J-K,andTflip-flops are demonstrated, based on direct polarization switching and polarization bistability, which we have recently observed in InGaAsP/InP semiconductor lasers. By operating the laser in the direct-polarization-switchable mode, the output of the laser can be directly switched between the TM00and TE00modes with high extinction ratios by changing the injection-current level, and optical logic gates are constructed with two optoelectronic switches or photodetectors. In the polarization-bistable mode, the laser exhibits controllable hysteresis loops in the polarization-resolved power versus current characteristics. When the laser is biased in the middle of hysteresis loop, the light output can be switched between the two polarization states by injection of short electrical or optical pulses, and clocked optical flip-flops are constructed with a few optoelectronic switches and/or photodetectors. The 1 and 0 states of these devices are defined through polarization changes of the laser and direct complement functions are obtainable from the TE and TM output signals from the same laser. Switching of the polarization-bistable lasers with fast-rising current pulses has an instrument-limited mode-switching time on the order of 1 ns. With fast optoelectronic switches and/or fast photodetectors, the overall switching speed of the logic gates and flip-flops is limited by the polarization-bistable laser to < 1 ns. We have demonstrated the operations of these devices using optical signals generated by semiconductor lasers. The proposed schemes of our devices are compatible with monolithic integration based on current fabrication technology and are applicable to other types of bistable semiconductor lasers.     

Gain and internal losses in InGaAsSb/InAsSbP double-heterostructure lasers

We report on a study characterizing internal losses and the gain in InGaAsSb/InAsSbP diode-heterostructure lasers emitting in the mid-infrared (3–4 μm). Numerical simulations of the current dependence of the intensity of spontaneous emission above the laser threshold and of the differential quantum efficiency allowed us to determine the intraband absorption k ₀ ≈5.6×10⁻¹⁶ cm². The cavity-length dependence of the threshold current is used to estimate the internal losses at zero injection current α ₀≈5 cm⁻¹. Calculations of the internal losses at laser threshold showed that they increase more than fourfold when the cavity length is decreased from 500 μm to 100 μm. The temperature dependence of the differential quantum efficiency is explained on the assumption that intraband absorption with hole transitions into a split-off band occurs. It is shown that the maximum operating temperature of “short-cavity” lasers is determined by the intraband absorption rather than by Auger recombination. The internal losses are shown to have a linear current dependence. The separation of the quasi-Fermi levels as a function of current demonstrates an absence of voltage saturation of the p-n junction above threshold. Fiz. Tekh. Poluprovodn. 33, 759–763 (June 1999)     

On the polarisation of radiation from double-heterostructure injection lasers

It is shown that the radiation from double heterostructure injection lasers is polarised with the electric field in the plane of the junction (TE waves), owing to mirror reflectivities at the nonnormal angles of incidence which are peculiar to these devices.     

Coupling coefficients for distributed feedback single- and double-heterostructure diode lasers

We compute coupling coefficients for TE modes as a function of tooth height, active layer thickness, and Bragg scattering order in single-heterostructure (SH) and double-hetetostructute (DH) distributed feedback (DFB) diode lasers for a variety of corrugation shapes. In particular, equations for rectangular, sinusoidal, and triangular shapes are evaluated; the last both in the symmetric and saw-tooth cases. It is shown that the coupling coefficient for rectangular and sawtooth gratings decreases much less rapidly with increasing Bragg order than do the sinusoidal and symmetric triangular.     

TE and TM coupling coefficients in distributed feedback double-heterostructure PbSnTe lasers

TE and TM coupling coefficients for double-heterostructure distributed feedback (DFB) Pb1-xSnxTe diode lasers are calculated. Results are presented for rectangular, sinusoidal, symmetric-triangular, and sawtooth gratings for both intermode and intramode coupling. Based on these results, the design considerations of PbSnTe DFB lasers are also discussed.     

An optical filter based on carrier nonlinearities for optical RFchannelizing and spectrum analysis

We experimentally demonstrate filtering based on the finite time response of the interband carrier nonlinearities in a direct bandgap semiconductor. The filter is implemented in a mixed strain polarization insensitive multiple-quantum-well, broad-area semiconductor amplifier. The key to the filter implementation is the separation of a four-wave mixing generated sideband from the linearly amplified inputs based on spatial filtering. Both spectrum analysis and channelizing of RF modulated optical carriers are demonstrated     

Zn-diffused, stripe-geometry, double-heterostructure GaInAsP/InP diode lasers

Deep Zn diffusion from a ZnP2source has been used to fabricate stripe-geometry double-heterostructure GaInAsP/InP diode lasers with emission wavelengths in the1.2-1.3 mum range. These devices exhibit good electrical and optical confinement. For sufficiently narrow stripe widths (<10 mum), emission is usually single mode and linear. CW outputs up to ∼8 mW per facet have been observed. In initial life tests, two Zn-diffused lasers have operated CW at room temperature for over 5000 h without appreciable degradation.     

Lateral waveguiding in stripe-geometry double-heterostructure lasers below the lasing threshold

Structure in the far-field pattern of current-confined stripe-geometry double-heterostructure lasers in the angle rangepm 20degis observed at current levels of less thanfrac{1}{4}of threshold value. The behavior is explained by leaky-mode guiding due to a gain maximum under the stripe and to a negative refractive index step on the order of -0.01.     

Calculation of the far-field halfpower width and mirror reflection coefficients of double-heterostructure lasers

A comparison is made between theoretical and experimental values of the beamwidth of a d.h. injection laser. The importance of the obliquity factor is pointed out. The good agreement between theory and experiment permits the determination of material properties from laser-diode measurements. Further, mirror reflection coefficients for lasers immersed in different liquids are calculated. Marked differences in the values of the Fresnel reflection coefficients, for normal incidence, are found.     

Chaos synchronization and chaotic signal masking in semiconductor lasers with optical feedback

Theoretical and experimental investigations of chaos synchronization and its application to chaotic data transmissions in semiconductor lasers with optical feedback are presented. Two schemes of chaos synchronization-complete and generalized synchronization-are discussed in the delay differential systems. The conditions for chaos synchronization in the systems and the robustness for the parameter mismatches are studied. The possibility of secure communications based on the chaos masking technique in semiconductor lasers with optical feedback is also discussed, and message transmission of a 1.5-GHz sinusoidal signal is demonstrated. The method of bandwidth enhancement of chaotic carriers is proposed for broad-band chaos communications.     

Thermal resistance and temperature distribution in double-heterostructure lasers: Calculations and experimental results

The thermal resistance and temperature distribution in double-heterostructure lasers have been calculated taking into account the characteristics of the different layers, the internal quantum efficiency, and allotment of the dissipated power, in order to optimize their structure. The influence of the different layers in the heterostructure and of the electrical contact is analyzed. Thermal resistance of CW, shallow proton-implanted lasers has been determined experimentally using the technique that relies upon a null measurement of the wavelength of a single Fabry-Perot mode. Statistical results on some hundreds of lasers with different stripe widths (6-125 mum), mounted on different heat sinks (copper, silicon, beryllium oxide) are given and compared to theoretical values. The model we propose gives good agreement with experimental results. The 6 μm width stripe laser is of special interest because this laser is transverse monomode up to an optical power of 6 mW. A value of 22° C/W has been achieved in a reproducible manner for6 times 300 mum lasers mounted on copper heat sinks. The effectiveness of the bonding technique is demonstrated. Si and BeO heat sinks are suitable for many applications because of their chemical (V grove etching in Si) and thermal properties (better linear expansion coefficient match to GaAs). We show that the increase of thermal resistance so introduced is still compatible with long CW operation.     

Gain nonlinearities in semiconductor lasers: Theory and application to distributed feedback lasers

The gain spectrum in semiconductor lasers is affected by the intensity-dependent nonlinear effects taking place due to a finite intraband relaxation time of charge carriers. We obtain an analytic expression for the nonlinear gain in multimode semiconductor lasers using the density-matrix formalism. In general, the nonlinear gain is found to consist of the symmetric and asymmetric components. The asymmetry does not have its origin in the carrier-induced index change, but is related to details of the gain spectrum. The general expression for the nonlinear gain is used to discuss the range of single-longitudinal-mode operation of distributed feedback lasers. It is also used to obtain an analytic expression for the self-saturation coefficient and to compare the predicted value to the experimental value for both GaAs and InGaAsP lasers. The agreement between the theoretical and the experimental values supports the hypothesis that spectral hole burning is the dominant mechanism for the gain nonlinearities in semiconductor lasers.     

Narrow-beam divergence of the emission from low-threshold GaInAsP/InP double-heterostructure lasers

The angles of beam divergence perpendicular to the junction plane have been studied for low-threshold GaInAsP/InP double-heterostructure lasers with a thin active layer emitting at 1.31 ?m. The narrowest beam divergence obtained was 23° for a laser diode with an active-layer thickness of 0.05 ?m. At an active-layer thickness of 0.13 ?m, the beam divergence was 40° with a threshold current density of 770 A/cm2.     

Calculated spontaneous emission factor for double-heterostructure injection lasers with gain-induced waveguiding

The fraction of spontaneous emission going into an oscillating laser mode has been calculated. It is shown that this fraction strongly depends on the strength of astigmatism in the laser output beam. Therefore the spontaneous emission factor in planar stripe lasers with narrow stripe is in the order of 10^-4and by one order of magnitude larger than in injection lasers with a comparable active layer volume and with a built-in index waveguide. It is shown that the spontaneous emission factor is approximately proportional to the solid angle of laser radiation and nearly independent of the transverse active layer dimensions. Owing to the large spontaneous emission factor, the spectral width of narrow planar stripe lasers is significantly broader compared to narrow stripe lasers with a built-in index waveguide. In addition the large spontaneous emission coefficient also yields a much stronger damping of relaxation oscillations.     

Molecular-beam epitaxially grown 1.3 ?m GaInAsP/InP double-heterostructure lasers

We report the first successful preparation of current-injection GaInAsP/InP double-heterostructure laser lasing at 1.3 ?m by molecular beam epitaxy. For broad-area Fabry-Perot diodes of 380 ?m×200 ?m and an active layer thickness of 0.2 ?m, we have observed threshold current densities Jth as low as 1.8 kA/cm2 and a median Jth?3.5 kA/cm2.     

Observation of second derivatives of the electrical characteristics of double-heterostructure junction lasers

Measurements of the second current derivative of the external voltage d²V/dI²generated by stripe-geometry (AlGa)As junction lasers are reported and correlated with corresponding measurements of the first derivative dV/dI as a function of current. In the vicinity of the lasing threshold, a pronounced negative peak occurs in the second-derivative signal as a result of the voltage saturation induced by the stimulated emission. With further increases in current, the second derivative assumes nonzero positive values for lasers with incomplete saturation of the junction voltage, while complete saturation is characterized by a near-zero value for the derivative. A pronounced fine structure observed in the second derivative at currents above threshold indicates the presence of perturbations in the saturated state of a nominally well-behaved laser.