The nature of optical confinement in the laser diodes (LDs) and phase-locked laser arrays (PLLAs) with a multiple mesa-stripe structure is studied. Two specific mechanisms of optical confinement in the direction parallel to the p-n junction plain are distinguished -- (1) variation of the waveguide effective refractive index due to the mesa-stripe formation, and (2) refractive index modulation induced by heating of the heterostructure. Stable operation of the LD is achieved when either weak or strong optical coupling between the neighboring stripes is realized. A phase-locked emission is obtained only for the LDs with the strong optical coupling which is related to the heating of heterostructure. Temperature effects are found to influence the optical confinement in the direction perpendicular to p-n junction plain in the case of LDs with an 'extended' waveguide. This effect hinders the narrowing of the far field distribution of such LDs.

We present results of an experimental investigation of the temperature sensitivity of separate confinement strained InGaAs/GaAs/Al_xGa_1-xAs quantum well single-mode and multimode high power laser diodes (HPLDs). The HPLDs have been fabricated by use of MBE material and a 'wet' thermal oxidation process. Due to the high carrier confinement in the structure with two quantum wells (DQW) and Al-content x greater than 0.7 in the cladding layers an extremely high characteristic temperature T₀ equals 350 K for broad area and T₀ of 400 K around room temperature for single mode HPLDs with extremely low threshold current density were obtained. In spite of the high Al-content in the cladding layers (0.5 less than x less than 0.8) a very high catastrophic optical damage (COD) level (greater than 10 MW/cm²) and lifetimes of more than 10 kh at 100 mW (T equals 50 degrees Celsius) have been observed.

AlGaAs/GaAs graded-index separate-confinement heterostructure single quantum well (GRINSCH-SQW) lasers with different waveguide thickness have been analyzed experimentally and compared with results from modeling using transverse optical field distributions. We have found that for GRINSCH lasers the halfwidth of near-field and far-field patterns depends very weakly on the waveguide thickness due to the focusing of the optical field in the transverse direction by the graded-index waveguide. At the same time, the mode intensity in the cladding layers is reduced by two orders of magnitude as the waveguide thickness is increased from 40 nm to 1200 nm. As a result, a 20% improvement in the differential quantum efficiency ((eta) _d) is realized, while the threshold current density remains unchanged. Differential quantum efficiency as high as 78% and output power exceeding 4 W cw have been obtained for broadened waveguide lasers.

The spatio-temporal dynamics of high-power semiconductor lasers are studied theoretically on the basis of space-dependent microscopic semiconductor-Maxwell-Bloch model equations. In the free-running broad-area and multi-stripe semiconductor lasers, complex spatio-temporal patterns are observed as a result of self-focusing, filamentation and transverse modulational instabilities in space-time images of the near-field output intensity. It is the irregular, possibly chaotic, dynamics of the filaments which are responsible for the generally much inferior beam quality and spectral characteristics of the high-power broad-area lasers when compared to common semiconductor lasers with moderate output power. The inherent coupling of optical diffraction with carrier transport processes leads to the formation of the spatio-temporal patterns. The simultaneous relevance of spectral and spatial hole-burning phenomena is manifested in the space- and time-dependence of the optical near-field showing, at the same time, spatio-temporal and spectral-temporal variations in the sub-ps range.

Laser diodes with diffraction-limited beam quality offer high power densities of the order of 10⁷ - 10⁸ W/cm², but are limited in output power to some watts. Scaling to higher powers has to be realized by superposition of a number of laser diodes. Coherent superposition allows us to further increase the power density in the far field. This is realized by injection locking of three slave laser diodes (Toshiba TOLD) 9140, 20 mW, 690 nm) by one master laser diode (TOLD 9140) and superpositioning of the three slaves lasers by a lens array. The feedback of the slaves into the master is suppressed by two Faraday isolators. For superpositioning the light of the slaves while maintaining the high beam quality, the light of each diode is coupled into an optical single-mode fiber. Phase shifts due to mechanical or thermal disturbances of the single-mode fibers for frequencies up to 1 kHz are compensated by a single-mode optical fiber piezoceramic phase modulator and an electronic control circuit. A phase stability with a maximum phase error smaller than 6 degrees is kept over an hour. The power-density distribution in the focal plane of the focusing lens shows a peak power 2.6 times that of the incoherent superposition and a modulation corresponding to the Fourier transform of the nearfield distribution of the lens array.

The combination of a laser material Nd:YVO₄ and a nonlinear crystal KTiOPO₄ (KTP) is a promising method for intracavity second harmonic generation (SHG) of laser diode pumped solid state lasers. A wavelength locking system, based on an optical feedback technique, is shown to contribute to locking the lasing wavelength of laser diode to the peak absorption wavelength of Nd:YVO₄ and generating stable green light. By applying the wavelength locking system to multi-longitudinal mode laser diode, a continuous green light of 31 mW is efficiently generated for pumping power of 121 mW. The noise characteristics of generated green light are discussed in two cases of using a single or a multi-longitudinal mode laser diode as a pumping source. Moreover, SHG green laser of 3.1 cc is fabricated to gain output power of 2.8 mW using the wavelength locking with an optical bandpass filter as a wavelength selective optical component, which has advantages as more compactness and less accurate alignment.

Criteria for the applicability of multimode laser diodes (LDs) with total emission spectra as broad as 10-30 A in holography are considered. Coherence properties of the two most probable regimes of multimode operation, free-running and phase locked, are analyzed. Prospectives of the use of multimode LDs in recording deep holograms are estimated theoretically and experimentally.

The method and system's design for the laser treatment of the heart ischemia is presented. Our conceptual approach to the development of the system is based on the theoretical and experimental works about positive influence of low intensity near infrared laser irradiation by treatment of cardiovascular diseases. The method and system allow it to influence the subepicardial collateral blood circulation with near infrared (NIR) laser irradiation in wavelength ranges of 0.86 - 1.06 mkm. The presented techniques make it possible to achieve a higher effectiveness of treatment. First, due to individual choice of radiation parameters on the basis of analysis of the patient's conditions before and after laser therapy. Second, due to simultaneous influence at several points of the human body. Finally, results of the clinical tests are presented, which confirm the discussed methods.

With the availability of laser diodes in the power range of small solid state lasers the direct material processing becomes more and more interesting as a standard application. Typical applications are heating, surface modifications, drilling and cutting. The ability to control the energy deposition simply via the current makes laser diode based equipment a tool of choice for medical applications. Both medical applications and material processing require optical systems for the appropriate energy deposition. The cooling system of laser diodes depends on the environment of the application (medical vs. industrial). Several direct applications of laser diodes are discussed.

We have examined the distribution of carriers between well and barrier in 670 nm GaInP lasers by recording spontaneous emission spectra at threshold as a function of temperature through a top contact window. Assuming the bands in the active region of wells and barrier to be flat at threshold we find that these carrier populations are well described by common quasi- Fermi levels and a carrier temperature equal to the lattice temperature. The activation energy of the thermally activated leakage current at threshold agrees well with that predicted on a flat- band model for the loss of electrons through the X conduction band minima. A self-consistent simulation of the carrier distribution through the complete laser structure at threshold shows that a flat-band model is a reasonable approximation over the range of temperatures (200 K - 400 K) and for the structures considered in this work.

Polarization characteristics of TE/TM cross-polarization semiconductor laser diodes are discussed in this paper. Broad area lasers fabricated from tensile strained In_0.5+(delta )Ga_0.5-(delta )P/(AlGa)_0.5In_0.5P quantum well laser structures oscillate in TE/TM dual polarizations. Polarization dominance changes from TE to TM as the cavity length of the laser is increased from 250 micrometers to 650 micrometers. The polarization-dependent gain property of a tensile-strained quantum well laser is analyzed from a simple theoretical model. In a slightly tensile strain quantum well, where light-hole and heavy-hole ground states are nearly degenerate in the valence band due to the strain and quantization effect, gain is provided for TM and TE modes simultaneously, and the two mode gain curves cross at certain injection level. Polarization switching is made possible by changing the threshold gain of the laser. The threshold gain dependent polarization switching is utilized to fabricate closely spaced independently-addressable dual beam cross polarization lasers. Results on 650 nm broad area dual beam cross polarization laser are presented. For dual polarization infrared lasers, a dual quantum well structure in which gains for TE and TM modes are provided by lattice-matched and tensile-strained quantum wells separately is designed. Eight-hundred-thirty-five nm broad area laser fabricated from a GaAs and GaAs_0.95P_0.05 dual quantum well structure oscillating in TE/TM dual polarizations is demonstrated.

High-power, highly reliable 685 nm Zn-diffused type window structure laser diodes (LDs) are developed using 3-inch (phi) wafers. The lasers have exhibited over 7000 hours operation under the condition of 60 degrees Celsius, 50 mW. In addition, over 4000 hours operation under the condition of 60 degrees Celsius, 60 mW is also achieved. These LDs are fabricated by well controlled 3-inch full wafer processing. The window regions near both facets consist of the disordered GaInP wells formed by an open-tube solid-phase Zn-diffusion technique. It is confirmed that highly uniform characteristics are achieved over the 3-inch wafer. Moreover, low astigmatic distances of less than 3 micrometer m are obtained in the range of 3 - 50 mW in spite of the window structure.

Vertical cavity surface emitting lasers (VCSELs) are the light source of choice for a broad range of applications in sensing due to their potential for low cost manufacturability and unique optical properties. Tunable low cost, single mode lasers with an emission wavelength around 762 nm are important for applications in oxygen sensing. We present results on top- emitting AlAs/AlGaAs VCSELs with an emission wavelength of 765 nm. Minimum threshold currents of 0.6 mA and threshold voltages of 1.9 V are achieved. The dependence of these characteristics on laser size is determined. VCSELs with an emission window radius of 8 micrometer exhibit threshold currents of 1.5 mA, wallplug efficiencies of 11.2% and a slope efficiency of 0.46 W/A. The maximum output powers exceed 5 mW for large area lasers. These characteristics represent a significant improvement compared to previously published data for VCSELs operating in this wavelength range. The emission wavelength can be tuned by the operating current over a range of 2.7 nm without the appearance of higher order lateral modes. The current tuning coefficients range from 0.38 nm/mA for small area to 0.05 nm/mA for large area devices.

A liquid contact luminescence (LCL) technique is described. LCL spectral data obtained by current injection through two GaInP quantum well laser wafers are utilized to determine the internal quantum efficiency ratio for the two wafers. This measured ratio is shown to be in good agreement with the internal quantum efficiency ratio for the two wafers as determined from conventional laser slope efficiency vs. cavity length measurements.

First results from surface mode emitting (SME)-laser diodes utilizing a first-order grating are presented. The use of a first-order grating instead of a third-order grating strongly improves the radiation characteristics of surface emitting SME-laser diodes. Although a real single mode operation from SME-laser diodes is not yet achieved, the tunability of the main emission wavelength by changes of the waveguide thickness is clearly demonstrated. The crucial feature of the SME-technique is that it provides a high flexibility, when processing surface emitting laser diodes with desired radiation pattern and wavelength emission characteristics. These features are discussed to demonstrate the high application potential of the SME-laser diodes.

The analysis of light propagation, diffraction and amplification in active multilayer corrugated waveguides with incorporated nonharmonic relief gratings, carried out by Rayleigh-Fourier formalism, is presented. On the base of this theoretical analysis the new possibilities of biharmonic grating for the design of single-frequency DFB lasers are discussed. The lasing characteristics of DFB heterolasers with biharmonic resonator are determined. It is shown that the biharmonic resonator with phase shift pi and space-modulated coupling coefficiency is capable of providing stable single-frequency oscillation at Bragg wavelength. The three-beam maskless holographic method for the fabrication of submicron biharmonic relief structures on the surface of n- A^IIIB^V semiconductors is developed. The biharmonic structure of period d equals 0.5 micrometer, possessing phase shift pi, is fabricated on n-InP in the process of laser-induced maskless wet photochemical etching under action of three Ar⁺ laser beams.

Laser diodes with diffraction limited beam quality offer high power densities of the order of iO - 108 W/cm2, but are limited in output power to some Watts. Scaling to higher powers without using a solidstate laser converter has to be realized by incoherent superposition of the outputs of a number of laser diodes. For that the radiation of single-mode laser diodes is coupled into single-mode fibers which at the other end are shaped into a bundle of hexagonal symmetry. The radiation leaving the fiber bundle is collimated with an array of achromats and focused with an additional lens onto the target. With 19 fibercoupled 690-nm diodes (TOLD 915 1, 20 mW at the fiber end) a total cw power of 338 mW in a spot of 19.4 jim diameter (at I/Is = l/e2of a nearly Gaussian cross section) was achieved. The peak power density was 263 kW/cm2, which is approximately 1 .7 times that of a single fiber. Optimizing the filling factor should further increase the power density. Keywords: single-mode lasers diodes, single-mode fibers, scalable system. incoherent coupling

This paper presents an overview of the scientific challenges that need to be met in order to realize the power and brightness output potential that is offered by semiconductor diode laser arrays. Recent progress of high-power diode lasers is reviewed.

Antiresonant reflecting optical waveguide (ARROW) lasers are novel index-antiguided structures that can achieve reliable high (approximately equal to 1 W) single-spatial-mode output power. Result from 0.98 micrometer-wavelength three-core ARROW-type InGaAs/InGaAsP/InGaP diode lasers, including near-diffraction-limited beam operation to 0.7 W peak-pulsed, and 0.4 W cw are discussed.

InGaAs/GaInAsP/GaInP ridge waveguide 980-nm laser diodes for pumping light into erbium doped fiber amplifiers are reviewed. These lasers have very good performance characteristics. They exhibit kink-free, single mode emission up to a power of 250 mW with a slope efficiency of 0.7 to 0.95 W/A, a thermally limited maximum power of 450 - 500 mW, and the threshold current density of about 150 A/cm². They are relatively stable against temperature variations. A 100 mW power from a fiber-pigtail module has been demonstrated. The lasers withstand severe thermal roll-over tests without showing degradation effects. Preliminary lifetime tests indicate that their mean-time-to-failure may be very high if not limited by sudden failure, from several hundred thousand to one million hours.

Continuous wave operation of quantum cascade lasers at lambda on the order of magnitude 4.6 micrometer and 8 micrometer is reported above liquid nitrogen temperature with few miliwatts of optical power. For the devices operating at lambda on the order of 4.6 micrometer, the single mode spectrum is temperature tunable over 1.8 cm^-1. A linewidth of 115 Mhz is measured with a Fabry-Perot on free running devices. Gain measurements show evidence for ultra low linewidth enhancement factor alpha less than 0.1. These devices also operated in pulse mode with 20 mW peak power at 200 K.

Calculation of the optical field distribution in 2 micrometer AlGaAsSb/InGaAsSb/GaSb multiple quantum well (MQW) lasers shows that incorporation of about 100 nm waveguide layers between quantum wells (QW) and cladding layers increases the optical confinement factor and reduces losses caused by mode penetration into doped cladding layers. Structures of this type have been grown by MBE. Both photoluminescence studies and measurements of laser diode parameters demonstrate that excess carriers confined in the waveguide are effectively collected and recombine in the QWs despite the small valence band offset at the interface of the QW and the waveguide, which is expected to be less than kT at 300 K.

Threshold characteristics of type II QW-lasers have been investigated theoretically. The rate of non-threshold channel of Auger recombination in type II heterostructures with quantum wells has been calculated. It is shown that the AR rate is a power function of temperature rather than an exponential one as in bulk materials. Feasibility of suppression of the Auger recombination process in the type II HS is demonstrated. The Auger current has a minimum and internal quantum efficiency has a maximum at a definite ratio of the barrier heights for electrons and holes. The possibility of controlling the AR current is shown to be very important for the creation of mid-infrared lasers operating at room temperature.

Lead-salt narrow gap semiconductors are useful materials for mid infrared lasers operating at 2.5 - 20 micrometer range. In this paper, we review recent advances in the lead-salt mid- infrared lasers, and propose a new intersubband laser using type-II superlattice structure. Progress in growth methods, studies on band offsets of heterostructures including superlattices, and developments of new lead-salt materials improved the device performance such as operation temperature, threshold current, and wavelength coverage. In PbSnTe system, design of laser structures taking into account the type-I' band offsets realized high- performance long-wavelength lasers with wavelength coverage to 20 micrometer. Development of new active layer materials such as PbEuTe, PbSrSe, and PbSrS realized effective carrier confinement, and extremely increased operation temperature of the lasers in 2.5 - 6 micrometer range. Introduction of quantum wells into the active layer is giving further possibility to the lead-salt mid-infrared lasers.

A detailed overview of the design of an optical current sensor implemented with all fiber optic components is presented. Laboratory and field data representing stability with time and temperature is included. Fundamental limits and their relationship to product specifications are also discussed.

A novel laser based corrosion sensor is being developed using embedded optical fibers, near- infrared (NIR) dyes and phase resolved fluorescence spectroscopy (PRFS) to detect corrosion by-products at the incipient stage. During the initial research effort, the practicality of using PRFS and NIR dye fluorescence lifetimes for characterization of metal ions was demonstrated. We have also demonstrated fiber optic strain measuring technology which will be integrated into the sensor design. The sensor will, thus, provide both early warning of corrosion as well as structural strain information.

An intensity-based fiber optic senor has been studied, especially for use in long term monitoring of civil structures. The fiber tested was a braided polyimide multimode fiber. By braiding, the sensitivity of the fiber was enhanced due to the increased microbending. Two types of laboratory tests have been performed using the optical fibers. First, uniaxial tension tests of the fiber were performed on an Instron testing machine. The braided fiber's intensity loss versus applied strain (using various gauge lengths and pretension loads) was monitored and shown to be more sensitive than straight fibers with inconsequential hysteresis occurring. Next, the fiber was attached to a 3/4' diameter solid steel rod at varying gauge lengths, braids per inch, and loop diameters. The rod was placed in pure tension on a SATEC Universal Testing machine and electric strain gauges were attached to relate strain measurements to intensity readings. The braided fiber was also applied on the rod in a loop to take advantage of the macrobending. Results from these tests suggested a need for further test studies, which are ongoing, within a certain range of strain values to determine an optimum gauge length, number of braids of the fiber and required pretension loading and to check the validity of using this type of fiber sensor in civil engineering structures to monitor response continuously.

Fiber Bragg gratings sensors offer a unique opportunity in civil engineering. They can be configured as a low noise distributed sensor network for measuring mechanical deformations and temperature. They are ideally suited for strain measurements of high modulus structural materials such as steel and concrete. There is considerable interest in the use of these sensors for infrastructural nondestructive testing and there have been several papers on the subject. We present some results of our experiments with fiber Bragg sensors as applied to structural engineering. These include the use of fiber gratings to measure strain behavior of steel, reinforced concrete, and some preliminary results on bituminous materials, such as asphalt concrete. In nondestructive testing using fiber Bragg gratings of structural materials the packaging of the sensors is important and is discussed.

Semiconductor lasers present enormous potential for free-space laser communications. Recently, a new class of devices based on a master oscillator power amplifier configuration has emerged as the leading contender. The paper illustrates the monolithically integrated flared amplifier (MFA-MOPA) device. It incorporates an index-guided, single-lateral-mode- distributed Bragg reflector (DBR) master oscillator section that diffracts a relatively low- power, narrow spectral bandwidth signal into a tapered amplifier section. The tapered amplifier section then amplifies the output to a one-watt or greater level while the divergence precludes the formation of filaments, maintaining good beam quality during the amplification process. The final output facet of the amplifier section is anti-reflection coated so that feedback into the DBR master oscillator is minimized, while outcoupling the amplified power.

The current status of InAsSb/InAlAsSb quantum-well (QW) lasers emitting between 3 and 4 micrometer is described. QW lasers grown on GaSb substrates, with emission wavelengths at approximately 3.9 micrometer, have operated pulsed up to 165 K. At 80 K, cw power of 30 mW/facet has been obtained. Ridge-waveguide lasers have operated cw up to 128 K. QW lasers grown on InAs have emission wavelengths between 3.2 and 3.55 micrometer. Broad- stripe lasers have operated pulsed up to 225 K and ridge-waveguide lasers have operated cw to 175 K. Theoretical analysis of the laser gain using a 6 by 6 k (DOT) p model to calculate the valence subband structure is reported.

We have demonstrated distributed-feedback, 1.39 micrometer wavelength InGaAsP/InP, multiple-quantum-well, folded-cavity surface-emitting, laser diodes with a low threshold current of 25 mA. These devices have greater than 45 dB side-mode suppression. Their wavelength tunes continuously with current over an interval of 1.5 angstrom at a rate of 0.11 angstrom/mA (-1.7 GHz/mA). These devices are useful for two-dimensional spectroscopic gas-sensing applications. Employing these compact lasers, we have detected atmospheric water-vapor at 1.3925 micrometer.

High sensitive CO gas analyzer based on tunable diode laser (TDL) was used as a real time monitor of endogenous carbon monoxide in a set of breath physiology experiments. The measurements of the CO content dynamics in exhaled air with 10 ppb sensitivity were attended with detection of carbon dioxide and O₂ in breath, lung ventilation parameters, heart rate and blood analysis using conventional techniques. Variations of endogenous CO in human breath caused by hyperoxia, hypoxia, hyperventilation as well as sport loading were studied in real time. Scattering of the CO variation time constants was observed for different tested persons. Possible reasons for this scattering related with the organisms' physiology peculiarities are discussed.

Pulsed, optically pumped four-constituent Type-II (InAs-Ga_1-xIn_xSb-InAs- AlSb) quantum well lasers emitting at 3.9 - 4.1 micrometer were observed to lase up to 285 K with a characteristic temperature T₀ of 35 K for 170 K less than T_op less than 270 K. A theoretical analysis predicts dramatic improvements once the potential for suppressing Auger recombination is fully realized.