The objectives of reactive chemical and nonreactive thermal processing with laser radiation are outlined giving indication that processing with laser radiation is governed by a hierarchy of time constants originating from photon-matter interaction, phase transition dynamics, laser source excitation fluctuations,, and optical feedback in combination with the influence of beam delivery systems, processing/shielding gas flow configurations, robotics, production lines and environment. The minimization of losses by heat flow, reflection and transmission and the stringent need for quality assurance require as first approach the control of processing, which is mainly due to the capability of laser radiation source. The current status of laser radiation sources is reported giving information on the state of the art of processing with laser radiation in combination with subsequent demonstration of future trends and developments with respect to radiation sources, beam delivery, beam shaping, materials, processing and quality assurance.

This review concerns optical components which are specially designed for diode. lasers.

Design considerations are given for surface-emitting lasers based on quarter-wavelength semiconductor multilayer stacks. The main problems for achieving low threshold CW operation at room temperature are discussed. A new design is proposed.

Results of spectral measurements of λ = 1.5 μm quaternary laser diodes are presented. With regards to frequency chirp, frequency drift and emission bandwidth quite often considerable discrepancies have been observed between measured values and theoretical predictions of numerical laser modelling. Besides the well known tendency of DFB lasers to exhibit mode jumps or poor side mode suppression ratio depending on the fact reflection phase and magnitude sometimes a non-monotonous dependance of the emission frequency on the laser current is observed even for perfectly single mode emitting laser diodes. This behavior may partly be caused by stochastically distributed disturbances of the light guide and the grating structure within the laser due to unsufficient control of crystal growth during laser fabrication particularly when using the LPE method. Measurement results are given and compared to numerically simulated laser behavior. An outline of the laser simulation model is also presented.

Ridge Waveguide MB lasers were fabricated by I.PF over-growth of a first order grating in the InP substrate with a sequence of planar layers consisting of a λ = 1.3 μm InGaAsP guiding layer, a A = 1.5 pm InGaAsP active layer, a λ = 1.3 μm InGaAsP guiding layer, an InP layer and an InGaAsP contact layer. Excellent control of layer thickness was obtained using a special LPE boat allowing the overgrowth of several wafers from the same set of mothermelts. The lasers obtained after etching of a 3.5 μm wide ridge, deposition of SiO₂ isolation, metallization and mounting, show threshold currents of 35 mΛ and efficiencies of 18% at 20 C. Selected devices showed single longitudinal mode operation with a sideband suppression better then 30 dB for powers up to 10 mW. Spectra recorded near threshold show a clear stopband which compares very well in width to devices with the grating located above the active layer. The Ridge Waveguide DFB laser fabricated in a single epitaxial step is a very promising structure for making opto-electronic integrated devices in a relatively simple way.

Examination of the spontaneous emission spectra of GaAs/AlGaAs quantum well lasers with 25Å wide wells shows that a realistic model of quantum well gain should include band gap renormalisation and broadening due to intraband scattering and fluctuations in the well width of ± 1 monolayer. Such a model has been used to obtain a correct description of laser wavelength as a function of gain. For parameter values describing our samples, the model predicts that the broadening due to monolayerofluctuations in the well width has a similar effect on the threshold current of a 2 x 25Å laser as the broadening due to intra-band scattering, and that the threshold current is about 2.5 times greater than without broadening. The calculations also predict a linear temperature dependence of threshold current between 200K and 400K. Since the principal effect of the broadening is to increase the threshold current, the value of the parameter To is predicted to increase to about 400K compared with about 320K without broadening. Non-radiative recombination processes in the AlGaAs barriers are shown to have the dominant effect on the temperature sensitivity above 300K, lowering the To to about 138K using a non-radiative recombination time of 3ns.

Index-guided Multiple Quantum Well Separate Confinement Heterostructure (MQW-SCH) GaAs/AlGaAs laser diodes with a Self-Aligned Structure (SAS) as well as with a Ridge Wave-guide Structure (RWS) have been fabricated from wafers grown by MOVPE at atmospheric pressure. Apart from single emitters we have realized phase-locked index-guided laser arrays with four emitters in an RWS design. We have observed that at the same emission wavelength in the range of 750-880 nm the threshold current of an MQW-SCH SAS laser diode with four GaAs wells is lower than that of an SAS laser diode with a conventional AlGaAs active layer. However, the MQW lasers may show somewhat lower To values. Uncoated MQW-SCH RWS laser diodes (cavity length 250 μm) with one and four emitters have been fabricated having CW 30°C threshold currents as low as 12 mA and 40 mA, respectively. Above threshold the phase-locked laser array emitted in a phase-coupled super-mode, indicating both a good homogeneity of the grown material and a controlled device fabrication.

The influence of the large-area forward-biased p-n homojunctions at both sides of the active stripe on the performance of PBRS laser diodes has been investigated. For that purpose, the Bridge-Contacted (BC) PBRS has been created, within which the area of these junctions is significantly reduced without changing the fundamental features of the fabrication process and the structure geometry. The comparison of both structures originating from the same wafer on an n-type substrate shows that the forward-biased junctions do not severely affect the PBRS laser performance. On p-type substrates, however, bridge-contacting is expected to improve considerably the current confinement.

The performance of parallel and Y-junction arrays has been studied experimentally; calculations of modes in Y-junctions, based on a new model are presented. A flat nearfield distribution from a parallel-stripe array has been achieved by spatially chirping the inter stripe distances. A 2/1 Y-junction array in ',ISIS structure has been realised, which lases in a stable in-phase mode up to the mirror damage level. In contrast, we show a 2/3 Y-junction ridge-guide array, with an InGaP active, layer (λ =660 nm.), which does not start in an in-phase mode.

The competition between two transverse lasing modes in ridp waveguide semiconductor lasers has been studied through a numerical model expressly developed. The model allows for spatial Me burning and its influence on the field shape. The numerical simulations have been executed employing theparamstets of InGaAsP-InP devices andvuying the cladding thickness in the interval where the field confinement changes from to gain guiding. Information on the of spatial hole burning on device efficiency, mode competition and polarization bistabifity has been obtained.

A rate equations model for the analysis and simulation of the dynamic behaviour of a Fabry-Perot semiconductor laser amplifier is presented. The model includes the incident radiation and the dependence of the refraction index on the instantaneous carrier density in the active layer and may be applied to the study of transient effects of short width pulses. Simulation and numerical results show the limitations about relaxation and switching times and their relation to amplifier supplied optical and electrical power. These times determine, respectively, the highest transmission speed and the smallest width of the amplifier input pulses. The possibility of achieving optical pulse reshaping is discussed in this work making use of the unwished transitory pick obtained during gain-switching.

In this paper, we present the effects of a phase-conjugate mirror when used as mirror of a laser resonator with a waveguiding configuration. Simulations are based on the beam-propagation method. A strong asymmetric behavior of the beams traveling in opposite directions in PCM semiconductor laser cavities is reported and a physical description of the phenomenon is given.

A microscopic approach to semiconductor injection laser dynamics is discussed to investigate the amplitude modulation with a small current signal in semiconductor lasers. An expression for the resonance frequency υ_r is obtained as a function of microscopic parameters which characterize the laser system. This expression can be compared with the one derived from a standard rate equations approach, showing the existence of an additional factor. This factor leads to the prediction of a larger resonance frequency and consequently to a better agreement with the experimental data. We investigate the problem of amplitude modulation with small current signal in semiconductor lasers deriving an expression for the resonance frequency as a function of microscopic parameters which characterize the laser system and as a function of the injected current. This approach is based on the analysis of the competition among the fundamental microscopic processes typical of light-matter interaction and of the loss and pumping mechanisms that are at work in a laser system. The starting point of the SLTMB model for the analysis of semiconductor laser modulation by a small signal of current is the couple of equations describing the temporal evolution of the two variables A² (intensity of the coherent field in the optical cavity) and D (the carrier population inversion ).

Materials characterization of semiconductor structures suited for nonlinear optical applications has been performed by absorption and photomodulated absorption spectroscopy. With respect to non-modulated absorption spectroscopy, a strong increase in room temperature resolution is observed. We present here the first photomodulated absorption studies performed on Al_xGa_1-xAs/GaAs and In_xGa_1-xAs/GaAs superlattices and AlAs/GaAs tunnel diodes, revealing excellent resolution of excitonic transitions. The temperature dependence of the spectral lineshapes arising from bulk InGaAs and AlGaAs samples is studied in the temperature range from 100K to 300K, and is compared to lineshapes arising from excitonic transitions in superlattices. Application of this technique to In_xGa_1-xAs/GaAs structures grown on GaAs substrates, which due to their favorable band structure do not require sample thinning, is most attractive.