In this paper we summarize recent developments in the experimental study of the intrinsic gain and recombination characteristics of GaInP quantum wells. Derivation of gain spectra from spontaneous emission spectra observed through a top-contact window is limited to radiation of TE polarization an dit is necessary to assume the carrier system is in quasi equilibrium to calibrate the data into real units. These difficulties are overcome by deriving the gain and spontaneous emission spectra from the amplified spontaneous emission spectra observed from the end of the structure as a function of the pumped stripe length. The emission spectra can be calibrated by identifying the region where the carrier distributions are fully inverted, without assuming that quasi-equilibrium conditions are established. We have determined the modal gain and spontaneous emission spectra for both TE and TM polarization for a tensile strained GaInP quantum well structure, and have obtained the TM and TE_y gains as functions of the total experimentally-determined radiative recombination current.

This paper analyses the gain and carrier-induced refractive index change in group-III nitride quantum wells. An approach based on the semiconductor Bloch equations with carrier-carrier collisions treated at the level of quantum kinetic theory is used. The influences of the strong carrier-carrier Coulomb interaction and the quantum-confined Stark effect on laser threshold and output beam quality are discussed.

We have successfully developed GaInN-based 400nm lasers for DVR-blue systems and GaInP-based 650nm lasers for DVD+/- RW systems. The high-performance blue-violet laser developed here has low relative intensity noise (RIN) of -128 dB/Hz, low aspect ratio of 2.3, and a nominal lifetime of 15000 h at 60 degree(s)C and 30 mW output power. The 650nm red laser was developed for DVD+/- RW systems, which require red lasers with output power exceeding 90 mW in order to increase the data transfer speed. The high-power red lasers developed here are capable of 90 to 120 mW output power with high reliability at 60 to 70 degree(s)C and have a low aspect ratio of 2.3.

Long wavelength-GaInNAsSb quantum well lasers that include small amount of Sb were successfully grown by gas-source molecular beam epitaxy (GSMBE). We confirmed that Sb reacts in GaInNAs/GaAs system like a surfactant, which increase the critical thickness at which the growth mode changes from the 2-dimensional (2-D) growth to the 3-dimensional (3-D) growth. The GaInNAsSb/GaAs lasers oscillated under CW operation at 1.258micrometers at room temperature. The low CW threshold current of 12.4mA and high characteristic temperature (T₍)) of 157K were obtained for GaInNAsSb/GaAs lasers, which is the best result for GaInNAs- based narrow stripe lasers. To extend the lasing wavelength over 1.3micrometers with keeping the threshold density low, we adopted GaNAs barriers instead of GaAs barriers. We obtained the very low threshold current density of 570A/cm² at 900micrometers -long cavity with the lasing wavelength of 1.308micrometers . We can say that GaInNAsSb lasers are very promising material for realizing peltier-free devices for access network.

We report the growth of GaInAsN heterostructures on GaAs substrates by conventional molecular beam epitaxy (MBE) using a radio frequency plasma source. Lattice-matched bulk samples and several strained single quantum well (SQW) and multiple quantum well (MQW) structures were grown. The QWs were sandwiched between two GaAsN strain-compensating layers (SCL) and AlGaAs cladding layers. By the aid of SCLs the photoluminescence (PL) wavelength red-shifted as much as 88 nm with the same intensity. GaInAsN strain-mediating layers (SML), having less strain than QW, were also used to obtain red shift and improved luminescence properties. The structures were studied by room temperature (RT) PL, x-ray diffraction (XRD) measurements and atomic force microscopy (AFM). The indium and nitrogen compositions of the QWs varied from 34 to 38 % and 1.3 to 3.5 %, respectively. Most of the studied structures showed PL peak wavelength at over 1.3 mm. Depending on the structure and thermal annealing treatment conditions the wavelength blue shifted up to 55 nm and intensity increased ~45 times. Furthermore, an AFM image of a five QW sample showed very smooth surface indicating together with PL measurements that high quality MQWs can be realized. In addition, 1.32-micrometers continuous-wave GaInAsN edge-emitting lasers were demonstrated.

We present a new structure with nitrogen incorporation in barrier and new material with antimony for developing post-annealed long wavelength material. This new structure and new material result in a shift of the post-annealed luminescence out to 1.6 um. The new structure, nitrogen in barrier, reduces the blue-shift of the emission spectrum by suppressing nitrogen out-diffusion from the quantum wells (QWs) and decreasing carrier confinement between barriers and QWs. GaNAs or GaNAsSb barriers can also reduce the overall strain of the active region because the high indium mole fraction QWs are compressively strained and the barriers with nitrogen are tensely strained. By adding small amount of antimony, we were able to incorporate up to 46% indium. We will present results of high efficiency long wavelength multiple QW GaInNAs ridge-waveguide laser diodes using GaNAs barriers. We will also show GaInNAsSb QWs with GaNAsSb barriers ridge waveguide laser emitting at 1.465 um. We have observed photoluminescence up to 1.6 um with different indium and antimony concentrations. Our GaInNAs and GaInNAsSb ridge waveguide laser diodes have broad emission spectra covering 1.27 um to 1.465 um with pulsed operation up to 90 degree(s)C. The maximum output power at room temperature, under pulsed operation was 350 mW with a differential efficiency of 0.67 W/A.

High-power (>100mW) 820 nm-band distributed Bragg reflector (DBR) laser diodes (LDs) with stable fundamental transverse mode operation and continuous wavelength tuning characteristics have been developed. To obtain high-power LDs with a stable fundamental transverse mode in 820 nm wavelength range, an AlGaAs narrow stripe (2.0 micrometers ) real refractive-index-guided self-aligned (RISA) structure is utilized. In the RISA structure, the index step between inside and outside the stripe region ((Delta) n) can be precisely controlled in the order of 10^-3). To maintain a stable fundamental transverse mode up to an output power over 100 mW, (Delta) n is designed to be 4x10^-3. Higher-order transverse modes are effectively suppressed by a narrow stripe geometry. Further, to achieve continuous wavelength tuning capability, the three-section LD structure, which consists of the active (700micrometers ), phase control (300micrometers ), and DBR(500micrometers ) sections, is incorporated. Our DBR LDs show a maximum output power over 200mW with a stable fundamental transverse mode, and wavelength tuning characteristics ((Delta) (lambda) ~2nm) under 100 mW CW operation.

We have demonstrated a 60 nm-tunable, 160 psec-width, optical pulses from a 980 nm Grating Coupled Surface Emitting Laser (GCSEL) in an external cavity under nanosecond pump pulses. GCSEL is in-plane laser monolithically integrated with grating outcoupler. The grating was detuned from second order Bragg condition and it served as an efficient interface between planar waveguide and free space. Wavelength tuning was simply achieved by tilting an external flat mirror provided wavelength selective feedback to the GCSEL chip. Gain switched pulses with wavelength linewidth less than 0.1 nm and peak power of 200 mW have been obtained. In our experiments we measured a shortest optical pulses by reducing a distance between external mirror and laser diode chip. This corresponded with decreasing the effective laser cavity length and the cavity round-trip time as well.

Ridge waveguide distributed Bragg reflector (RW-DBR) lasers having an active gain section and passive phase shift and Bragg sections were used to generate high power optical pulses with small spectral widths and repetition rates in the GHz range at an emission wavelength of 1060nm. The Ohmic p-contacts of the passive sections were deposited on the ridge as small metal stripes with a resistance per length of about 10(Omega) /mm. If an electrical current flows along the stripes, a selective heating and hence a controllable increase of the absorption can be achieved. If the absorption is strong enough, even for large currents injected into the gain section no lasing can be observed. With small current pulses through the absorbing region Q- switched ps-pulses with high peak power of 1W can be generated. Theoretical investigations based on the solution of the time-dependent traveling-wave equations and carrier density rate equation yield a good agreement with experimental results.

High d/G allows high total emitted power and avoids high power density in the active QW layer where power induced degradation processes occur. d/G is scaled up to 0.84 micrometers in a series of four 7 nm thick single QW AlGaAs structures, designed for 810 nm wavelength. Two structures have an asymmetric design that includes an optical trap next to the active region. The optical trap captures a part of the total radiation flux and reduces the confinement factor. One of asymmetrical structures includes an optical wall built into the p cladding layer that further pushes the radiation flux toward the trap. Device lengths are inversely proportional to the confinement factor; therefore all lasers optimally operate at the same current density. All have approximately the same threshold current density equal to 250 A/cm². For the 0.84 micrometers d/G case, the nominal length is 2.8 mm, the attenuation coefficient is 0.7 cm^-1 and the slope efficiency is 0.96 W/A. The thermal rollover maximum power is greater than 8 W/100micrometers .

High bit rate, WDM, networks use intensively Er or Er/Yb doped fibre amplifiers. Reliable, high power laser diodes at 980nm and 1480nm are key devices for pumping these amplifiers. We have developed different structures of laser diodes at 980nm, using Aluminium free materials. Our laser structure shows low optical losses together with a low threshold current density and a high external differential efficiency. We demonstrate a mini-bar of broad area laser diodes (emissive width of 2.6mm) with an optical output power of 19W at 25A under CW operation. We have also developed a mini-bar of small angle index guided tapered laser diodes (W=2.6mm). We demonstrate 17W at 27.6A under CW operation at 20 degree(s)C. Slow axis far field has a Gaussian single mode shape, with a FWHM of 3.3 degree(s) (at 15A, 11W), which is two times less than obtained on multimode broad area lasers. With such a device, we expect to couple 10W into a 100micrometers diameter fiber. We also demonstrate, on a large aperture gain-guided tapered laser, an output power of 1.3W with an M² of 3.3

In this paper we summarize the results on the development of high power 1300 nm ridge waveguide Fabry-Perot and distributed-feedback (DFB) lasers. Improved performance of MOCVD grown InGaAsP/InP laser structures and optimization of the ridge waveguide design allowed us to achieve more than 800 mW output power from 1300 nm single mode Fabry-Perot lasers. Despite the fact that the beam aspect ratio for ridge lasers (30 degree(s) x 12 degree(s)) is higher than that for buried devices, our modeling and experiments demonstrated that the fiber coupling efficiency of about 75-80% could be routinely achieved using a lensed fiber or a simple lens pair. Fiber power of higher than 600 mW was displayed. Utilizing similar epitaxial structures and device geometry, the 1300 nm DFB lasers with output power of 500 mW have been fabricated. Analysis of the laser spectral characteristics shows that the high power DFB lasers can be separated into several groups. The single frequency spectral behavior was exhibited by about 20% of all studied DFB lasers. For these lasers, side-mode suppression increases from 45 dB at low current up to 60 dB at maximum current. About 30% of DFB lasers, at all driving currents, demonstrate multi-frequency spectra consisting of 4-8 longitudinal modes with mode spacing larger than that for Fabry-Perot lasers of the same cavity length. Both single frequency and multi frequency DFB lasers exhibit weak wavelength-temperature dependence and very low relative intensity noise (RIN) values. Fabry-Perot and both types of DFB lasers can be used as pump sources for Raman amplifiers operating in the 1300 nm wavelength range where the use of EDFA is not feasible. In addition, the single-mode 1300 nm DFB lasers operating in the 500 mW power range are very attractive for new generation of the cable television transmission and local communication systems.

A frequency stable semiconductor laser source based on InGaAsP distributed feedback (DFB) laser technology is described. The device enables tight tolerances to be maintained on the absolute accuracy and long-term stability of laser sources for the next generation of wavelength division multiplexed (WDM) fiber optic networks. The improved DFB laser frequency stability is achieved by utilizing electrical feedback to stabilize the optical frequency relative to the transmission peak of a Fabry-Perot reference cavity. The novel design achieves stable loop operation while resolving fundamentally incompatible loop gain and bandwidth requirements imposed by the respectively high gain and wide linewidth of the free-running laser. Laboratory measurements of the resulting laser spectrum are presented, demonstrating the impact of the electrical feedback on the short and long-term laser frequency stability. Heterodyne measurements are presented depicting relative laser frequency stability including the Allan variance of the long-term spectral drift.

Wide wavelength control over a range of 370 nm for simultaneously formed optical waveguides is achieved by atmospheric-pressure narrow-stripe (<2 micrometers ) selective MOVPE for a selectively grown InGaAsP/InGaAsP multiple quantum well with a small mask width variation (0-30 micrometers ). This shift is four times that obtained with growth at 75 Torr. Moreover, high-quality MQWs are obtained over a wide range of InGaAsP compositions by optimizing growth conditions. We successfully apply this growth technique to 4-channel DFB-LD arrays with 20-nm wavelength spacing for 1.3-micrometers CWDM network systems. To adjust the wide-range gain peak wavelength, we control the composition of each channel by this atmospheric-pressure narrow-stripe selective MOVPE growth. Furthermore, to control the emission wavelength, we form a precise pitch-controlled Bragg grating by electron beam lithography. These two techniques enable us to precisely control the detuning value in each channel. As a result, we obtain high-speed characteristics up to 5 GHz for all channels of the array and throughput of more than 10 Gb/s in the DFB-LD array. Moreover, low electrical cross-talk between neighboring LDs in the array- as low as 25 dB up to 5 GHz- is obtained using an electrically isolated structure with a deep trench reaching the semi-insulating substrate for each channel.

An 1550 nm semiconductor optical amplifier (SOA) with a very thin tensile-strained bulk active layer and active width-tapered spot-size converters was developed. The SOA module exhibited a record high saturation output power of +17 dBm together with a low noise figure of 7 dB, large gain of 19 dB and small polarization sensitivity of 0.2 dB. A good eye pattern without waveform distortion due to the pattern effect was obtained for amplified 10 Gb/s NRZ signals up to an average output power of +12 dBm.

Semiconductor lasers with InGaAsP/InP nonidentical multiple quantum wells (MQWs) for optical communication are experimented to show the improved temperature characteristics. With proper layout of the nonidentical MQWs, the characteristic temperature of the laser diodes is increased. Also, the differential quantum efficiency increases to around 40% for the temperature increasing from 30 degree(s)C to 40 degree(s)C and approximately remains at this value for temperature above 40 degree(s)C. The reason is attributed to the carrier redistribution in the nonidentical MQWs as temperature increases. The change in temperature causes certain QWs to have increased carriers. Therefore their corresponding gain increases to overcome other effects that degrade temperature characteristics. With proper design of nonidentical MQWs, significant improvement on temperature characteristics of semiconductor lasers is possible.

The realization of Quantum Cascade lasers at very long wavelengths is of particular interest due to the lack of narrow-band, powerful sources in the far-infrared range of the electromagnetic spectrum. We report Quantum Cascade lasers operating above 20micrometers (at (lambda) =21.5micrometers and (lambda) =24micrometers ) wavelengths, with pulsed operation up to 140 K and with a peak power of few milliwatts at cryogenic temperatures. Increased accuracy in the band-structure design becomes one of the key factors to assure high electron injection efficiency and to prevent hot-carrier effects. For this reason we developed a technique which allows the observation of intersubband spontaneous emission in unipolar Quantum Cascade lasers above threshold, a helpful instrument for device optimization. Finally, at these very long wavelengths various types of waveguide concepts have to be adopted in order to reduce the otherwise prohibitive layer thickness, enhance the optical confinement and control the waveguide loss. We report Quantum Cascade lasers with double metal-semiconductor waveguide resonators for operating wavelengths of 19, 21 and 24micrometers . The waveguides are based on surface-plasmon modes confined at the metal-semiconductor interfaces on both sides of the active region/injector stack and are not restricted by a cut-off wavelength for the TM polarized intersubband radiation.

Although mid infrared research into sources and detectors has made considerable progress in recent years, requirements for gas sensing purposes for source power and the detectivity of diode detectors - particularly in combination - remain to be convincingly demonstrated in an industrial context. Published results are often confusing in that they apply to a variety of pulse lengths and duty cycles. We suggest a standardized approach in terms of an averaged cw power output. Parameters such as radiance, drive current and electrical power are also important. We discuss the relative merits of lasers and LEDs, i.e. use of line or band absorption on gas sensing. We report recent advances in the use of immersion optics leading to detectors with D*~5x10⁹ cmHz^1/2W^-1 at 5.4micrometers , LEDs with outputs improved by a factor of 5 and an LED/Diode/White cell gas sensing demonstration giving 2ppm sensitivity for NO₂ with an electrical power requirement of only 0.25mW. Further consideration includes sensitivity of gas sensing, path length and volume, time constant and temperature stability. Latest results are assessed on the basis of the above and combined with some market indications.

We present recent progress achieved in the development of type-I GaInAsSb/AlGaAsSb quantum-well (QW) lasers covering the 1.74-2.34micrometers spectral range. Diode lasers based on the broadened waveguide design comprising 3 Qws have been studied in detail. Laser structures emitting at 2.23 micrometers exhibited a record high internal quantum efficiency of 89%, internal loss of 6.8cm^-1, and threshold current density at infinite cavity length as low as 120 A/cm², indicating the superior quality of these devices. For the 2micrometers lasers a high characteristic temperature of 179K for the threshold current was achieved for temperatures between 250 and 280 K. In order to investigate the heterobarrier leakage associated with thermally activated carriers, laser structures emitting at 2.23micrometers with different Al- concentrations in the barriers and separate confinement regions have been studied. While the structure with 40% Al revealed the highest T_o of 103K, the laser with 20% Al yielded the best power efficiency, with a maximum value of 30%. 1.7W in cw mode at room temperature has been achieved for broad area single emitters at (lambda) =2 micrometers , with high-reflection/antireflection coated mirror facets, mounted epi-side down. As an application, tunable diode lasers absorption spectroscopy (TDLAS) sensing small concentrations of methane has been demonstrated using our 2.3micrometers diode laser.

Gain in broad area mid-infrared diode W lasers ((lambda) =3- 3.1micrometers ) has been measured using lateral mode spatial filtering combined with the Hakki-Paoli approach. The internal optical loss of approximately equals 19cm^-1 determined from the gain spectra was the same for devices with either 10- or 5-period active regions and nearly constant in the temperature range between 80 and 160K. Analysis of the differential gain and spontaneous emission spectra shows that the main contribution to the temperature dependence of the threshold current is Auger recombination, which dominates within almost the entire temperature range studied (80-160K).

There is considerable interest in the realization of room temperature mid-infrared diode lasers for a variety of applications, including remote gas sensing, infrared countermeasures and molecular spectroscopy. However the maximum temperature of operation in narrow gap III-V component alloys is limited by strong non-radiative Auger recombination and various band structure engineering techniques are being investigated to provide Auger suppression. In our work we are investigating the possibility of obtaining a practical 3.3micrometers laser by making use of radiative recombination across single type II hetero-interfaces. Because transitions occur between confined electron and hole states localized on either side of the heterojunction where the potential wells are triangular, there exists the possibility of tailoring the wave-function overlap to give good Auger suppression while still maintaining high radiative output. At the same time growth form the liquid phase offers potentially lower SRH recombination. We compared two such heterojunctions (InAs_0.94Sb_0.06/InAs and Ga_0.96In_0.04As_0.11Sb_0.89/ InAs) grown by rapid slider LPE and report on the photoluminescence and electroluminescence from the interfaces. The dependence of these interface transitions on temperature, excitation intensity, band offset and polarization is reported, with a view towards incorporating these in the active region of a practical laser.

We present a novel hybrid laser structure based on III-V and II-VI compounds combining some advantages of type I and type II heterojunctions in one heterostructure. Such design allows the achievement of large energy offsets at the interface in the conduction and the valence band exceeding of 1.0 eV in order to provide good electron and hole confinement. P-AlAsSb/n-InAs/N-Cd(Mg)Se laser heterostructures were grown on p-InAs substrates by original technology of MBE method in two separate growth chambers consequently. Photoluminescence spectra included tow emission bands at hv=0.41 eV and hv=2.08 eV associated with InAs and CdMgSe bulk recombination transitions, respectively. Intense electroluminescence was observed at (lambda) =2.73micrometers (77K) and (lambda) =3.12micrometers (300K). Weak temperature dependence of spontaneous emission indicated the effective carrier confinement in the InAs layer due to large potential barriers ((Delta) sE_c=1.28eV and (Delta) E_V=1.68eV). Proposed hybrid III-V/II-VI heterostructure is very promising for creation the mid-infrared lasers with improved performances operating in the spectral range of 3- 5micrometers .

Optically thin dielectric slabs, in which a fully etched through two-dimensional patterning is applied, can be used to form high-Q optical cavities with modal volumes approaching the theoretical limit of a cubic half-wavelength. A cavity design strategy based upon simple group theoretical techniques is presented in which emphasis is placed upon a momentum space description of the resonant modes. It is shown that photonic crystal laser cavities can be designed with a particular wavelength, polarization, and radiation pattern using these methods.

In this article, we report on a two-dimensional (2D) photonic crystal (PhC) laser with a surface-emitting function. First of all, 2D PhC laser with triangular-lattice structure is described. A uniform 2D coherent lasing oscillation based on coupling of lightwaves propagating to six equivalent Gamma-X directions is successfully demonstrated. A large area 2D lasing oscillation over 300 micrometers in diameter and correspondingly a very narrow divergence angle less than 2 degree are observed. Then, the properties of a square-lattice PhC laser are described where the geometry of unit cell structure is designed appropriately to control the polarization mode of emitted light of the device. As the band diagram of the square-lattice photonic crystal is influenced by the unit cell structure, the electromagnetic field distributions at individual band edges are strongly modified and become unified or linear by changing the structure from a circular to an elliptical geometry. We fabricate a laser with a square-lattice PhC of which unit cell structures are elliptical. In spite of the very large diameter, single wavelength at 1.3micrometers and linear polarization mode are observed at representative positions. These results encourage us to realize lasers with desirable features such as perfect single-mode emission over a large area, high output power, and surface emission with a very narrow divergence angle.

Active photonic lattices (APLs) of large index step ((Delta) n>=0.05 for spatial-mode stability) have been used, as early as 1988, for effective lateral-mode control in large-aperture (>=100micrometers ) high-power coherent devices. Photonic-bandpass (PBP) structures, so called ROW arrays, have allowed stable, near-diffraction-limited beam operation to powers as high as 1.6W CW and 10W peak pulsed. Photonic-bandgap (PBG) structures, so called ARROW lasers, have provided up to 0.5W peak-pulsed stable, single-mode power and hold the potential for 1W CW reliable single-mode operation from apertures ~10micrometers wide. The solution for high efficiency surface emission, from 2nd order DFB/DBR lasers, in an othonormal, single-lobe beam pattern was found in 2000. That opens the way for the realization fo 2-D surface-emitting, 2nd order APLs for the stable generation of watts of CW single-lobe, single-mode power form large 2-D apertures.

Photonic-crystal distributed-feedback (PCDFB) lasers, in which the DFB grating is defined on a two-dimensional lattice, have the potential to provide near-diffraction-limited, spectrally pure sources of radiation. The conventional 1D DFB laser and also the angled-grating DFB (a-DFB) laser are special cases of the PCDFB geometry. For a first proof-of-principle demonstration, optical lithography and dry etching were used to pattern a 2nd-order two-dimensional rectangular lattice whose grating was tilted by 20 degree(s) relative to the facet normal. The antimonide type-II W active region emitted at (lambda) = 4.6-4.7 micrometers . For pulsed optical pumping, the emission line was much narrower (7-10 nm) than those of Fabry-Perot and (alpha) -DFB lasers fabricated from the same wafer, and the beam quality was enhanced by as much as a factor of 5 compared with the (alpha) -DFB. The observation of two distinct lines in the PCDFB spectrum is attributed to a near-degeneracy of grating resonances at two different symmetry points of the Brillouin zone for the rectangular lattice. Quantum-cascade (QC) PCDFB lasers are shown to be particularly attractive in the mid-IR spectral range since their linewidth enhancement factor, which governs the carrier-induced refractive index change, is close to zero. Using a time-domain Fourier-transform algorithm, we estimate that rectangular-lattice QC lasers should emit in a single mode up to a stripe width of approximately equals may be employed to maintain spectral and spatial coherence over stripes as wide as 3 mm.

We present a simple broad area semiconductor laser which uses a current spreading layer to modify the transverse gain profile. The device exhibits excellent spatial coherence to total output powers of 2.5 W under pulsed operation. Devices have been focused down to a spot size of approximately 5 micrometers FWHM at 2.5 W with the beam profile and position remaining stable over the entire range of operation. Under CW operation, thermal effects reduce spatial coherence leading to a significantly increased spot size and loss of beam stability. This work demonstrates the advantages of modifying the transverse gain profile and how it can be used to produce high brightness devices required for single mode fiber coupling.

Beam quality and output power of mostly 2-dimensional stacked diode laser systems are insufficient for the demands of materials processing. To increase the output power at almost constant beam-quality, superimposition of diode laser bars of different wavelengths as well as polarization-multiplexing of s- and p-polarized laser beams is possible. Different techniques for wavelength-multiplexing have been developed. The so-called multi-filter concept of a spanned coated etalon with edge-filters has turned out best. The concept features a modular design, simple adjustment and easy add-on of more wavelengths. Concerning the polarization-multiplexing we take advantage of the almost linear polarized diode laser bars. Ordinary used beam splitter cubes with a cemented structure are less qualified for high radiance. Hence the beam combination is achieved with beam displacers made of a birefringent crystal (YVO₄) which provide high transmittance and convenient adaptation. Finally an experimental set-up with 8 diode laser bars of 4 different wavelengths, i.e. 8-times beam superimposition, is realized. The set-up called multiplexer obtains a radiance of about 4 x 10⁶ W cm^-2 sr^-1 and outnumbers all other comparable high power diode laser systems.

The effect of carrier-carrier relaxation and carrier - phonon relaxation on threshold characteristics of quantum well (QW) lasers is studied. Carrier relaxation time considerably depends on temperature, carrier density, and quantum well width. It is shown that in this case the gain coefficient becomes a more pronounced function of temperature and carrier density.

A mechanism of electric current-induced cooling of heterosystems is proposed and analyzed. The conditions are studied of electric current flow through a heterostructure with two quantum wells, with electrons from one quantum well passing into the other via phonon-assisted indirect tunneling. As a result, the system is cooled by the flowing current, with the temperature of the system depending on the current nonmonotonically. A universal law for the maximal cooling temperature is derived.

Physics and applications of recent quantum cascade laser active region designs are discussed. Specifically, the use of bound-to-continuum and two-phonon resonance active regions for high temperature, high duty cycle operation is reviewed. Threshold current densities as low as 3kA/cm² at T=300K, operation with a peak power of 90mW at 425K, and single mode, high power operation up to temperatures above 330K at (lambda) approximately equals 16micrometers are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300MHz free space optical link between two buildings separated by 350m.

Unipolar Quantum Cascade (QC) lasers are easily recognized by the cascading scheme, in which electrons traverse a stack of many, typically 30 but sometimes up to 100, active regions alternated with injector regions, rather than only a single active region, as in conventional semiconductor lasers. So far, QC-lasers shared the characteristic, that all stages of the cascade were essentially identical. This makes perfect sense for lasers with optimized performance, with a low threshold current density and high optical output power. The possibility of heterogeneous cascades was sometimes discussed. However, it was uncertain if optimal operating conditions could be achieved for all components of the cascade. Here, we experimentally discuss three types of QC-lasers with heterogeneous cascades. The first type contains two sub-stacks, each using a previously optimized QC structure, connected by a thin InGaAs layer. This results in a QC-laser emitting simultaneously at 5.2 and 8.0 micrometers wavelength, with performance levels similar to those of the respective homogeneous stack lasers. It was not necessary to adjust the design electric field of the two stacks to match each other. Each sub-stack is apportioned the appropriate fraction of the applied bias. In addition, an etch-stop layer inserted between the two sub-stacks allowed fabrication of a tap into the cascade. The latter was used to selectively manipulate the laser threshold of one sub-stack, turning the 8.0 micrometers laser on and off while the adjacent 5.2 micrometers QC-laser was operating undisturbed. We also fabricated a doubly-single mode QC-distributed feedback laser with single-mode emission at 5.0 and 7.5 micrometers with simultaneous single-mode tunability. The second type of QC-laser contains a waveguide core with an interdigitated cascade of two different active regions with matching injectors and emitting at 8.0 and 9.5 micrometers wavelength simultaneously. Finally, the third type of QC-laser with heterogeneous cascade was designed to generate a broadband continuum. We observe gain from 5 to 8 micrometers and laser action continuously from 6 to 8 micrometers .

980 nm GaInAs/(Al)GaAs quantum dot laser structures with a high internal quantum efficiency n_i of about 90%, and a low internal absorption <2.5 cm^-1 and a threshold current densities as low as 140 A/cm² have been fabricated. Laser diodes that show output powers of up to 4W from a single facet of a 100micrometers broad laser diode after AR/HR coating at 10 degree(s)C have been processed. The highest total output power for lasers with cleaved facets was obtained with a 200micrometers broad laser diode. This laser shows a maximum output power of 5W. Through the use of short period superlattices in the inner waveguide region we could significantly improve the high temperature properties compared to earlier devices. The lasers have characteristic temperatures T₀ of above 100 K up to an operation temperature of 110 degree(s)C. As a result our lasers show output powers as high as 1 W even at temperatures as high as 100 degree(s)C. In addition to quite competitive laser properties the rather broad gain profile of a quantum dot ensemble allows the fabrication of laser diodes with a reduced temperature-induced wavelength shift. Understanding the underlying effects and optimizing the cavity design, quantum dot lasers with wavelength shifts of 0.16 nm/K have been realized which is only half the value of a typical GaInAs/(Al)GaAs quantum well laser.

We report on the growth of GaInNAs materials and lasers by molecular beam epitaxy (MBE) using a rf-plasma source. Optimal GaInNAs quantum well (QW) structures have been designed and grown in order to achieve the brightest and narrowest photoluminescence (PL) spectra beyond 1.30 um. State-of-the-art GaInNAs/GaAs SQW lasers operating at 1.32 um have been demonstrated. For a broad area oxide stripe, uncoated Fabry-Perot laser with a cavity length of 1600 um, the threshold current density is 546 A/cm2 at room temperature. Optical output up to 40 mW per facet under continuous wave operation was achieved for these uncoated lasers at room temperature.