Sophisticated packaging architectures have been developed that enable low cost, very high average power, long lived pumping of solid state lasers. Single water cooled manifolds now provide slab pumping of up to 2.5 kW of average optical power, while low cost yet flexible bar mounting techniques allow burn-in that enables very long lifetimes. Architecture modification allows for high peak power of up to 80 kW per water cooled pump manifold. Specialized high brightness packaging now allows approximately 20 watt cw bars to be lensed into less than 200 micrometers diameter spot sizes (approximately 54 kW/cm2).
Diode pumping of solid state lasers promises many advantages compared with flashlamp pumping. Such as better efficiency, lower thermal stress, compact design, no high voltage and longer lifetime. In between several possibilities of pumping configurations direct coupling of laserdiodes and crystal seems to have the highest efficiency. Stacked arrays as pumping modules for Nd:YAG slab and rod lasers are investigated. The stack consists of 28 monolithic arrays mounted on copper microchannel coolers. The dimensions of the emitting area are 50 X 9 mm2 resulting in a maximum pump power density of 107 W/cm2. Using a slab crystal with dimensions of 3 X 16 X 65 mm3 and a doping concentration of 0.7 at% Nd, an output power of 140 W cw has been achieved. The optical efficiency has been 31%. With the same stacked array a rod of 50 mm length and 4 mm diameter has been pumped. The pump light has been concentrated into the rod by a cusp shape reflector. Using a stable resonator 170 W output power in free running operation and 100 W average power in high repetition rate Q-switch operation has been achieved.
The primary intent of this research was to determine the influence of three common degradation mechanisms, dark area defects, facet degradation and contact degradation, on the operational lifetime of GaAs edge-emitting semiconductor lasers driven by continuous current at 100 degree(s)C. Inherent to this work was the quantified characterization of the lasers during their operation. Power vs. current characterizations were conducted at room temperature on each laser before and after exposure to 100 degree(s)C as well as at the beginning and end of each laser's exposure to 100 degree(s)C. An additional means of examining laser degradation came from measuring the current required over time to maintain a constant power output of 5, 7, or 10 mW at the elevated temperature. The research demonstrated that facet degradation and contact degradation were minor contributors to the bulk of the data base's degradation. Dark area defects were thus the primary degradation mechanism as the data's gradually increasing current necessary to maintain constant output will attest. An HF acid rinse on one laser, reacting aggressively to local crystal defects, highlighted the growth of dark area defects toward the lasing cavity due to continued lasing. One trend was left unexplained. As a whole, the lasers performed with higher slope efficiencies at elevated temperature, contrary to previously documented research. This topic deserves future research.
We report on passivation of AlxGa1-xAs/GaAs surfaces using different sulfur and chlorine based treatments: These include ammonium sulfide solution, arsenic sulfide vapor and hydrochloric acid treatments. Enhancements in the intensity of near band-gap photoluminescence (PL) peaks, coupled with peak half-width reduction on treatment were attributed to a reduction in the density of surface states. Pre-etching using sulfuric acid- and ammonium hydroxide-based solutions prior to sulfur passivation was also found to contribute significantly to the overall success of a passivation treatment. The best sulfur-passivation results for all x (0 < x < 0.38) were found when sulfuric acid-peroxide-deionized water (Caros) solution pre-etching was followed by ammonium sulfide solution treatment at 65 degree(s)C for 25 min.
A monolithically integrated array of InGaAs/AlGaAs flared amplifiers driven by a single DBR laser through a power splitter network and individually addressed phase modulators is described. Phase adjustment of > 2(pi) per element by free-carrier effects is verified by monitoring the interference pattern of 4 emitters, and typically requires < 15 mA of current to obtain a 2(pi) phase shift. Phase matching is achieved among all four diffraction-limited emitters at a pulsed output power of > 5 W, and, combined with the proper external lensing, could therefore result in an ultra-narrow, single-lobed far-field pattern whose width is determined by the extended aperture of the array. This architecture is capable of providing single-mode, diffraction-limited performance from each emitter and is scalable to unprecedented power levels. Over 20 W of pulsed, spectrally coherent emission is generated at 955 nm from a 4-element array, and 39 W is obtained from an 8-element array.
The design, fabrication, and testing of a two-dimensional array of 900 laser diode power amplifiers injected by a single semiconductor master oscillator is discussed. This array extends the results of the 200-emitter array reported on last year. The array consists of a stack of ten 100-emitter linear arrays. Nine of the 100-emitter arrays were successfully stacked and injected by a semiconductor master oscillator laser by way of a 1 to 10 semiconductor ridge- waveguide splitter and amplifier array. The tenth 100-emitter array was included in the stack but the fiber optic used to connect it to the 1 to 10 splitter broke before final strain-relief was added. The 900 amplifiers produced approximately 36 W of coherent power in the near field. Poor fill optics allowed only 15.6 W to be delivered to the far field of which 5.5 W were phase aligned into a narrow beam in one axis and into 5 major spots in the other axis. Improved optics and higher power emitters are the next steps in moving this array architecture to high power and brightness.
The development of high power laser diodes using surface emitting distributed feedback (SEDFB) techniques has matured to the point where serious marketing analyses have been conducted. While development of the base technology continues, the initiation of systems applications and manufacturing engineering has begun. This effort, in direct response to growing market demand, is the critical bridge between research and the development of viable products for commercial applications. This paper addresses the history of laser technology development, the current status of high powered laser diode development, the forces defining current and future markets and the role of `conventional wisdom' in laser technology and market development.
The paper summarizes activities of the two Fraunhofer-Institutes ILT and IPT concerning the development of high-power laser-diode stacks and their direct industrial applications. With microchannel coolers in copper technology and ultra-precision machined micro-optics a stack of 330 - 400 W total power with a maximum intensity of the focused beam of 2 104 W/cm2 has been built and tested in first applications. By further improvements of the lens-fabrication and -alignment technology as well as increase of the number of stacked diodes an output power in the kW-range and intensities up to about 105 W/cm2 shall be achieved in the near future. Applications of such laser sources in surface technology, in the processing of plastics, in laser-assisted machining and in brazing are discussed.
Applied Optronics Corp. has designed and built a portable rechargeable battery operated 10 watt air cooled diode laser system with a small physical size that could be run cw. Heat dissipation, reliability, portability, power consumption, and survivability were important considerations that were incorporated in the system design. This novel design approach relies on individual 980 nm lasers, each mounted on its own miniature heatsink, and each coupled to a single fiber. After all fibers are coupled to the individual laser diodes, the fibers are combined together into a compact bundle 600 micrometers in diameter. The individual diodes are mounted in a package just slightly larger than a cigarette package. The lasers are connected in series. This design concept is in contrast to the established approach of coupling a linear array of emitting elements into a single fiber using optics. The low current required by the series lasers allow the module to be run on rechargeable batteries. The module has an electrical to `optical out of the fiber' efficiency of 25%. This high efficiency allows prolonged use from the battery pack. The module can be run for short periods of time without any additional heatsinking. This is due to the heat dissipation of the package, and the 980 nm lasers' ability to lase at temperatures above 100 degree(s)C.
High pulse energy diode array laser systems have been constructed for therapeutic and diagnostic medical applications. Two systems are described. One system, constructed for therapeutic application in dermatology, uses 45 bars to generate > 10 joules of energy at approximately 800 nm in a 5-millisecond pulse. This system uses simple microlenses and non- imaging condensers to uniformly illuminate areas of 0.1 to 0.4 cm2 at fluences up to 40 joules per cm2. Cooling, power, and control electronics are housed within the control console, and the laser and condensing optics are housed in the handpiece connected to the control console by means of a two-meter umbilical. The complete system, including closed- cycle cooling, weighs under 14 kg and uses < 2 amps at 110 V. A second system, which is being developed as a burn diagnostic, utilizes a 15-bar diode laser array. The array generates over 3 joules of optical energy. The output light is homogenized and projected using microlenses, a non-imaging condenser, and projection optics. With this system an area of approximately 1000 cm2 can be uniformly illuminated at an intensity of one millijoule per square centimeter. The system, including receiving optics, can be battery-powered and packaged into a hand-held unit.
High-power laser-diode arrays have been demonstrated to be viable pump sources for solid- state lasers. The diode bars (fill factor of 0.7) were bonded to silicon microchannel heatsinks for high-average-power operation. Over 12 W of cw output power was achieved from a one cm AlGaInP tensile-strained single-quantum-well laser diode bar. At 690 nm, a compressively strained single-quantum-well laser-diode array produced 360 W/cm2 per emitting aperture under cw operation, and 2.85 kW of pulsed power from a 3.8 cm2 emitting- aperture array. InGaAs strained single-quantum-well laser diodes emitting at 900 nm produced 2.8 kW pulsed power from a 4.4 cm2 emitting-aperture array.
Kink-free, high coupled power of 148 mW into a single mode fiber has been realized by narrowing the vertical beam divergence in 0.98 micrometers laser diodes. It has been found that the high-refractive index GaAs layers have crucial influence on the far field patterns as well as lasing spectrum, since the GaAs is transparent to the emission wavelength of 0.98 micrometers .
The gain-dependent polarization properties of vertical-cavity surface emitting lasers and methods for polarization control and modulation are discussed. The partitioning of power between the two orthogonal eigen polarizations is shown to depend upon the relative spectral alignment of the nondegenerate polarization cavity resonances with the laser gain spectrum. A dominant polarization can thus be maintained by employing a blue-shifted offset of the peak laser gain relative to the cavity resonance wavelength. Alternatively, the polarization can be controlled through use of anisotropic transverse cavity geometries. The orthogonal eigen polarizations are also shown to enable polarization modulation. By exploiting polarization switching transitions in cruciform lasers, polarization modulation of the fundamental mode up to 50 MHz is demonstrated. At lower modulation frequencies, complementary digital polarized output or frequency doubling of the polarized output is obtained. Control and manipulation of vertical-cavity laser polarization may prove valuable for present and future applications.
The fabrication and performance characteristics of InGaAs/GaAsInGaP strained quantum well lasers are described. Lasers with low threshold current, high output power and excellent reliability have been fabricated. In0.2Ga0.8As/GaAs lasers emitting near 1 micrometers are useful as pump sources for erbium doped optical fiber amplifiers.
Temperature dependent efficiency and modulation characteristics of strained quantum well (QW) InGaAs/InGaAsP/InGaP 980 nm laser diodes of various designs are analyzed using self consistent carrier transport analysis including stimulated emission. The decrease of the differential efficiency of 980 nm laser diodes with temperature is found to be caused by an increased modal loss attributed to the free carrier (electron and hole) absorption. The obtained results agree well with experimentally observed increase of internal loss at higher temperatures. Modulation characteristics are determined mainly by drift-diffusion in separate confinement region along with processes of carrier capture and escape in QWs. At high temperatures modulation bandwidth is reduced because of the decrease in differential gain. Graded index separate confinement heterostructure and multi-QW lasers show superior efficiency and modulation behavior at high temperatures.
ARROW-type diode lasers show strong single-mode selectivity with stable beam operation to high powers (500 mW) due to the large built-in index profile. Triple-core ARROW lasers have potential for 1 W single-spatial mode operation. Preliminary results are 550 mW (diffraction- limited beam) from a 20 micron aperture triple-core ARROW laser fabricated using the InGaP based material system.
A variety of broad-area semiconductor laser devices are currently providing high-power with near diffraction-limited performance. These new devices share an important design element: the suppression of filamentation tendencies. Specific designs include traveling-wave amplifiers and reflective-wave amplifiers, tapered amplifiers, unstable resonator oscillators and most recently, curved or angled grating DBR/DFB oscillators. These latter designs offer the advantages of filament suppression and angular spectrum filtering, while providing strong spectral filtering as well. We review the motivation and design principles for several of the new broad-area DBR/DFB devices and describe the algorithms that have been developed in order to provide accurate numerical simulations and performance predictions.
The optical noise behavior of a 1.55 micrometer distributed feedback laser (DFB) has been investigated, in a medium frequency range (10 kHz-1 MHz). This noise characterization allows us to bring to the fore a high noise level due to mode competition. The presence of this process is also extremely notable on the analysis of the electrical noise, related to the laser voltage fluctuations. The tight link between both spectral densities, electrical and optical, is strengthened by the measurement of the correlation function. Finally, a theoretical study, based on rate equations, confirms the noise evolution in the absence of the feedback process.
Laser absorption spectroscopy using III-V semiconductor laser diodes has several advantages for gas sensing applications, as compared with traditional methods employing tunable dye laser and II-VI (e.g., lead salt) laser sources. These advantages include room-temperature operation, reduced cost, and compact size. Limited coverage of spectroscopy wavelengths by high-performance III-V lasers has prevented their widespread application to gas sensing. At those fixed wavelengths, performance of commercially available devices has been limited by multimode emission and/or inadequate wavelength tuning and mode hops. These spectra can, however, be greatly improved by incorporating frequency-selective structures. We have developed single-mode distributed-feedback (DFB) GaAs/AlGaAs quantum well lasers applicable to laser spectroscopy of molecules absorbing in the wavelength interval from 760 to 840 nm. These devices exhibit low threshold current (< 20 mA), high efficiency (> 40%), high output power (> 25 mW), and narrow linewidth (< 3.0 MHz). The lasers display smooth, continuous, single-mode wavelength tuning over 5 nm. Typical temperature and current wavelength-tuning coefficients are 0.065 nm/ degree(s)C and 0.0075 nm/mA (approximately -3.5 GHz/mA), respectively. In preliminary tests, they have been applied to the detection of H2O vapor and O2 gas.
Continuously and arbitrarily chirped distributed feedback (DFB) gratings of ultrahigh spatial precision for photonic components are implemented using bent waveguides on homogeneous grating fields. Choosing special bending functions, individual chirping functions and distributed phase shifts (PSs) are generated. Thus, additional degrees of freedom are obtained to tailor and improve specific device performances. This paper focuses on bending function design with respect to PS region extension, PS amount, bending radii, maximum tilt angles, threshold gain, photon density profile homogenization and side mode suppression ratio (SMSR). Continuously distributed PSs were implemented in DFB lasers revealing in the experiment reduced photon pile-ups, higher single axial mode stability, higher SMSR and higher yield than conventional abruptly phase-shifted (PS'ed) lasers. Second, multiple-section DFB lasers are implemented showing 5.5 nm wavelength tuning.
GaAs MESFET dynamic photoresponse under optical illumination by AM light of a laser diode has been investigated both theoretically and experimentally. The possibility of MESFET gain coefficient measurement via laser probing is shown in the frequency range from 100 MHz to 8 GHz.
A gain-guided high-power 1.55 micrometers large optical cavity (LOC) laser structure was successfully prepared based on considerations of laser loss mechanisms such as auger electron recombination, carrier leakage over the heterobarrier, inter-valence-band absorption and so forth. The structure was grown by a unique liquid phase epitaxy (LPE) and was modified by growing an intrinsic InGaAsP waveguiding layer which replaced its n-type counterpart as in usual LOC devices and provided a higher temperature stability than that of conventional DH lasers. Using the LOC structure, we have obtained 1.55 micrometers laser diodes with threshold currents comparable to commercial lasers (Jth < 2.7 KA/cm2) but with the characteristic temperature To near to those of GaAs-AlGaAs lasers (140 K). Especially, we have attained the peak output power up to higher than 2 W per facet in pulsed operation at room temperature.
The current status of GaInAsSb/AlGaAsSb quantum-well (QW) lasers emitting at approximately 2 micrometers and InAsSb/InAlAsSb QW lasers emitting at approximately 4 micrometers is described. At room temperature, GaInAsSb/AlGaAsSb lasers have exhibited pulsed threshold current density as low as 143 A/cm2 and cw power of 1.3 W from a 300- micrometers aperture. Ridge-waveguide lasers have operated cw up to 130 degree(s)C, with a maximum power of 100 mW in a single lobe at 20 degree(s)C. From a tapered laser, diffraction-limited cw power of 200 mW has been obtained. The InAsSb/InAlAsSb QW broad- stripe lasers have operated pulsed up to 165 K, with a characteristic temperature of 30 K up to 120 K. At 80 K, cw power up to 30 mW/facet has been obtained. Ridge-waveguide lasers have exhibited cw threshold current of 35 mA at 80 K, and operated cw up to 128 K.
Recent advances in GaSb-based crystal growth technology have led to the demonstration of high performance quantum well lasers which emit at mid-infrared wavelengths. In fabricating such lasers, techniques are utilized which are processing intensive and time consuming. In this work, we report on the use of a pulsed anodic oxidation (anodization) technique to fabricate low-ridge, wide-stripe GaInAsSb/AlGaAsSb lasers operating near 2 micrometers . The low-ridge stripe areas are defined in one, 5-minute processing step which converts the p+ layer outside the stripe area into a uniform, stable native oxide.
A number of double heterostructure and quantum well lasers with wavelengths approximately 3.1, 3.2, 3.4, 3.85 - 4.1, and 4.5 micrometers have been realized in InAsSb/GaSb and HgCdTe/CdZnTe material systems. Peak powers at the few W level and average power at the few hundred mW-level were obtained from optically pumped broad-area lasers at >= 80 K. Threshold, efficiency, internal loss, and gain saturation studies are reported. A compact laser package was built, using a high-power diode array for pumping and a Stirling pump for cooling. Its performance with a 4-micrometers laser is described.
Vertical-cavity lasers have demonstrated the ability to produce mW level output powers in a low divergence circular beam. This has made them attractive sources for high density applications such as free space interconnects and optical computing. Moving from device demonstration to practical applications, however, poses additional demands. High density arrays require very low power consumption for thermal management while the interconnect lines must not introduce excessive stray capacitance. We have developed vertical cavity lasers with intra-cavity contacts on semi-insulating substrates which provide solutions to these practical problems. The intra-cavity design allows the use of ring contacts on the same surface with either top or bottom emission. The lasers can thus be connected in a raster interconnect pattern or any other contacting configuration without impairing the high speed performance. On chip resistors can be incorporated into the circuit so that simple voltage drives can be used. Using a current constriction to provide electrical and optical confinement has removed the limitations of surface recombination, allowing the development of high external efficiency, sub-milliamp threshold vertical cavity lasers. The 7 micrometers diameter lasers have output powers of 1 mW at a bias current of 3 mA with modulation bandwidths above 8 GHz while consuming only 10 mW of power. The lasers can therefore be driven with full on/off current modulation to transmit data at rates exceeding 1 Gbit/s, simplifying the driver electronics considerably. The structure and fabrication sequence is discussed along with bit error rate measurements including on/off modulation conditions.
Spectra of some ridge-waveguide lasers grown by metal-organic chemical vapor deposition (MOCVD) undergo a reversible transformation at a certain value of drive current -- usually from 5 to 10 thresholds. When the current is increased past this point, the spectrum abruptly widens and its amplitude drops correspondingly. In the widened spectrum a structure with period equal to longitudinal mode separation can be seen. We call this effect `spectral collapse.' The effect seems to be typical for ridge-waveguide lasers with ternary active regions and independent of active region strain. Data on `collapse' in both cw and pulsed modes at different temperatures suggest its connection with active region overheating. The intensity noise versus current dependence for some of the lasers reveals two peaks, one near the threshold and the other near the `spectral collapse' point. This led us to a suggestion that the `collapse' can be explained by nonlinear mode interaction. Some LPE-grown InGaAsP/GaAs lasers of similar design also exhibit spectral collapse while other samples from the same wafers do not, which may be evidence of a competition between nonlinear effects that cause spectral collapse and continuous widening of the spectrum with current due to spinodal decomposition in quaternary active region.
Coherent optical-fiber communication systems that are used in conjunction with multiple in- line optical amplifiers are very important for achieving point to point long distance communication as well as compensation of tap-losses in distribution networks. In this paper, the performance of in-line heterodyne synchronous detection type of coherent optical fiber communication systems for binary and M-ary ASK signaling schemes are investigated. This extends the previously published results on ASK asynchronous demodulation in the presence of optical amplifiers in cascade. The local oscillator spontaneous emission beat noise is the dominant source of noise in these systems. Expressions for bit error rate are derived and compared with the case where no amplifier is used. The results show that the value of optical amplifier input power significantly affects the system performance. Bit error floors are observed at about 10-9 for the total spontaneous emission parameter nsp equals 55 for binary synchronous ASK signal detection and nsp equals 20 and 8 for 4-ASK and 8- ASK signalling schemes when the amplifier input power is assumed to be -35 dBm at a bit rate of 1.2 Gb/sec.
Many simple molecules, such as H2O, CO2, CO, N2O, CH4, and HCN, have strong absorption bands at wavelengths between 2 and 3.5 micrometers . We are developing InGaAsSb/AlGaAsSb multi-quantum-well diode lasers operating from 2 to 3.5 micrometers as sources for trace-gas monitors. These devices are grown by molecular beam epitaxy, and they generally comprise four or five InGaAsSb quantum wells separated by AlGaAsSb barriers. The cladding layers are high-Al-content AlGaAsSb layers. Our longest-wavelength, room- temperature (20 degree(s)C) lasers operate at 2.78 micrometers in the pulsed mode, delivering 95 mW peak power. The highest temperature for pulsed-mode operation is 60 degree(s)C, at which the wavelength is 2.9 micrometers . Between 78 and 200 K they operate cw, and at 200 K the output is 3 mW at 2.66 micrometers in a dominant single mode. We discuss the properties of these lasers along with some initial applications to water-vapor detection.
We demonstrate midwave infrared (MID-IR) diode lasers that span most of the 3 - 4 micrometers range. Laser active regions are multiple quantum well (MQW) structures with GaInSb/InAs, type-II, broken gap superlattices for the wells and GaInAsSb for the barriers. The superlattice constituents and dimensions were tailored to reduce losses from auger recombination. AlSb/InAs superlattices are used for both n-type and p-type laser cladding regions.
The new concept of a single-mode resonator for single-frequency distributed feedback (DFB) lasers is presented. This concept is based on an embedded nonharmonic distributed Bragg structure with a sinusoidally modulated coupling coefficient, which is a combination of two superimposed sinusoidal Bragg gratings of equal heights and slightly different periods. Resonant frequencies and corresponding threshold gains of such a distributed resonator are calculated theoretically by using the coupled-mode theory. The designed resonator provides stable single-frequency oscillation and has lasing characteristics (frequencies and thresholds) very similar to those of a well-known distributed resonator with quarter-wavelength shift. The developed concept of a resonator with a sinusoidally modulated coupling coefficient can be applied both to semiconductor laser diodes with incorporated Bragg relief grating and to DFB fiber lasers with refractive index grating. The important advantage of designed new single- mode Bragg structure, as compared to quarter-wavelength-shifted structure, is that it can be fabricated very easily on semiconductor surfaces and in photosensitive fibers by direct three- beam holographic writing.