This PDF file contains the front matter associated with SPIE Proceedings Volume 8640, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Based on a novel quantum dot (QD) growth technique, high density dot-like QDs were grown on (100)InAlGaAs/InP surfaces, which resulted in a strongly improved modal gain in 1.55 μm QD lasers. The influence of the number of QD layers on static properties, e.g., modal gain, threshold current density and spectral properties, are presented and discussed. For a large number of QD layers, e.g., 6 QD layers, a high modal gain of > 70cm^-1 could be obtained. By reducing the number of QD layers, i.e., lowering the modal gain, the wavelength shift with temperature can be reduced to < 0.2 nm/K. Systematic dependence of laser properties on structural parameters is observed.

We report on the atypical emission dynamics of InAs/AlGaInAs/InP quantum dash (Qdash) lasers employing varying AlGaInAs barrier thickness (multilayer-chirped structure). The analysis is carried out via fabry-perot (FP) ridge (RW) and stripe waveguide (SW) laser characterization corresponding to the index and gain guided waveguiding mechanisms, respectively, and at different current pulse width operations. The laser emissions are found to emerge from the size dispersion of the Qdash ensembles across the four Qdash-barrier stacks, and governed by their overlapping quasi-zero dimensional density of states (DOS). The spectral characteristics demonstrated prominent dependence on the waveguiding mechanism at quasi-continuous wave (QCW) operation (long pulse width). The RW geometry showed unusual spectral split in the emission spectra on increasing current injection while the SW geometry showed typical broadening of lasing spectra. These effects were attributed to the highly inhomogeneous active region, the nonequilibrium carrier distribution and the energy exchange between Qdash groups across the Qdash-barrier stacks. Furthermore, QCW operation showed a progressive red shift of emission spectra with injection current, resulted from active region heating and carrier depopulation, which was observed to be minimal in the short pulse width (SPW) operation. Our investigation sheds light on the device physics of chirped Qdash laser structure and provides guidelines for further optimization in obtaining broad-gain laser diodes.

Novel monolithic widely tunable laser diodes in the 2.1μm wavelength region based on GaSb / AlGaAsSb are presented. Using the concept of a lateral binary superimposed (BSG) grating structures and multisegment Verniertuning, stable single-mode output is realized at discrete wavelength channels in the 2060 nm – 2140 nm region. A total tuning above 80 nm in six channels is demonstrated. In every wavelength channel, the output wavelength can be tuned by current and temperature. Each wavelength channel offers up to 6 nm of mode hop free tuning, making this novel widely tunable laser highly attractive as a monolithic light source for multiple-gas sensing or liquid detection purposes. The wavelength channels can be arbitrarily placed within the material gain allowing BSG lasers to sweep e.g. over several gas absorption line within 80 nm. Within a wavelength channel, the widely tunable lasers show DFB like spectral performance with average side-mode suppression-ratios above 40 dB, output power of up to 15 mW at 25°C. Also temperature and current tuning coefficients are comparable to those of DFB lasers. This paper will present an overview of laser concept, performance data and applications.

There has been much interest in developing low-cost laser sources for applications such as photonics integrated circuits and advanced coherent optical communications. The ultimate objectives in this development include wide wavelength tunability, a narrow linewidth, and an ease of integration with other devices. For this purpose, semiconductor surface grating distributed feedback (SG-DFB) lasers have been introduced. SG-DFB manufacturing consists of a unique sequence of planar epitaxial growth resulting in a major simplification to the fabrication process. SG-DFB lasers are highly monolithically integrate-able with other devices due to their small footprint. The segmentation of the built-in top electrode helps to alleviate the adverse spatial-hole burning effects encountered in single-electrode devices and brings hence significant enhancements to the laser performance. For the first time, we report here on the design, fabrication, and characterization of InGaAsP/InP multiple-quantum-well (MQW) SG-DFB lasers with uniform third-order surface grating etched by means of stepper lithography and inductively-coupled reactive-ion. The uncoated device reported here is 750 μm-long SG-DFB laser whose central and lateral top electrodes are 244 μmlongs each, separated by two 9 μm-long grooves. The experimental characterization shows stable single mode operation at room temperature under uniform and non-uniform injection. High side mode suppression ratios (SMSRs) (50-55dB) under a wide range of injection current have been discerned as well. A relatively broad wavelength tuning (<4nm) has also been observed. Moreover, a narrow linewidth (<300 kHz) has been recorded for different injection currents.

Red-emitting diode lasers having a large coherence length with a tunable wavelength and a narrow spectral linewidth with an emission power in the 10 mW range are sought for a variety of techniques in applications such as spectroscopy, interferometry and holography. Currently, helium-neon lasers or diode lasers with external wavelength stabilization are widely used for these applications. By integrating a wavelength selective element into the ridge waveguide (RW) of the diode laser chip itself a high degree of miniaturization and stability can be reached. To this end, we have developed RW lasers with deeply etched surface distributed Bragg reflector (DBR) gratings in order to achieve a high-yield, singleepitaxy manufacturing process. These DBR lasers consist of a 1.5 mm RW gain section and a 500 μm grating section, which has a reflectivity of about 60%. The facets of the lasers were coated to achieve a reflectivity of 30% at the front and smaller than 0.1% at the rear facet. The diode lasers achieve an optical output power of 20 mW at an injection current of 150 mA and a heat-sink temperature of 15°C at a wavelength of 633 nm. The DBR enables single longitudinal mode operation over a wide range of operation conditions. Self-delayed heterodyne measurements were performed to measure the emission linewidth of these lasers using a 1 km long fiber, which gives a spectral resolution of about 100 kHz. A linewidth of less than 1 MHz was obtained. In reliability tests at 14 mW a lifetime of more than 1,700 h could be demonstrated, dedicating these devices to the above mentioned applications.

Compared to diode lasers emitting in the near infrared, the development of high power diode lasers in the red spectral range is more challenging due to the applicable compound semiconductors, the limited stability of the laser facets, and the small barrier heights for electrons and holes. For CW applications, their mounting requires excellent heat removal or expansion matched submounts. For QCW operation with small duty cycles and about 2 W per 100 μm stripe width emitter, like for the pumping of Q-switched alexandrite (Cr³⁺:BeAl₂O₄) lasers at 654 nm, a compromise is the application of aluminum nitride as heat sink. The presented broad area (BA) lasers are based on a GaInP single quantum well embedded in AlGaInP waveguide layers. The structure provides a vertical far field angle of 31° (FWHM). The material data can be compiled as follows: transparency current density j_T = 220 A/cm², internal efficiency ƞ_i = 0.83, internal losses α_i = 1.0 cm^-1. BA lasers with a stripe width of 100 μm and a length of 1.5 mm were fabricated, facet coated including a passivation procedure, and mounted on AlN submounts. In QCW operation (100 μs, 35 Hz) at 15°C, the devices had threshold currents of about 600 mA, slope efficiencies up to 1.3 W/A and conversion efficiencies of 0.36. A maximal output of 6.3 W was measured. At lower temperatures of -10°C the maximal peak power was determined to 7.6 W, i.e. a facet load of 76 mW/μm. The devices showed reliable operation over 1,000 h at a peak power of 2.7 W.

Laser based displays, as pico to cinema laser projectors have gathered much attention because of wide gamut, low power consumption, and so on. Laser light sources for the displays are operated mainly in CW, and heat management is one of the big issues. Therefore, highly efficient operation is necessitated. Also the light sources for the displays are requested to be highly reliable. 638 nm broad stripe laser diode (LD) was newly developed for high efficiency and highly reliable operation. An AlGaInP/GaAs red LD suffers from low wall plug efficiency (WPE) due to electron overflow from an active layer to a p-cladding layer. Large optical confinement factor (Γ) design with AlInP cladding layers is adopted to improve the WPE. The design has a disadvantage for reliable operation because the large Γ causes high optical density and brings a catastrophic optical degradation (COD) at a front facet. To overcome the disadvantage, a window-mirror structure is also adopted in the LD. The LD shows WPE of 35% at 25°C, highest record in the world, and highly stable operation at 35°C, 550 mW up to 8,000 hours without any catastrophic optical degradation.

We have demonstrated continuous wave operation of BeZnCdSe quantum well laser diodes at room temperature in the green to yellow spectrum range. The laser diodes structures were grown by molecular beam epitaxy. To overcome low doping ability of a p-cladding layer materials of BeMgZnSe, a short-period superlattice of BeMgZnSe/ZnSe:N was employed. High-power lasing over 50mW at a peak wavelength of 536 nm was achieved. By employing highly strained BeZnCdSe quantum wells, continuous wave lasing up to 570 nm has been also achieved. The threshold current density of 20-μm-wide lasers was found to be sufficiently low (less than 0.85 kA/cm2) in the wavelength range of 545 nm to 570 nm.

InGaN based green laser diodes with output powers up to 50mW are now well established for variety of applications ranging from leveling to special lighting effects and mobile projection of 12lm brightness. In future the highest market potential for visible single mode profile lasers might be laser projection of 20lm. Therefore direct green single-mode laser diodes with higher power are required. We found that self heating was the limiting factor for higher current operation. We present power-current characteristics of improved R and D samples with up to 200mW in cw-operation. An optical output power of 100mW is reached at 215mA, a current level which is suitable for long term operation. Blue InGaN laser diodes are also the ideal source for phosphor based generation of green light sources of high luminance. We present a light engine based on LARP (Laser Activated Remote Phosphor) which can be used in business projectors of several thousand lumens on screen. We discuss the advantages of a laser based systems in comparison with LED light engines. LARP requires highly efficient blue power laser diodes with output power above 1W. Future market penetration of LARP will require lower costs. Therefore we studied new designs for higher powers levels. R and D chips with power-current characteristics up to 4W in continuous wave operation on C-mount at 25°C are presented.

We demonstrate picosecond pulse generation in the blue-violet wavelength region by passive intra-cavity mode-locking in GaN-based ridge waveguide laser diodes with monolithically integrated absorbers. For cavity lengths of 1.2 and 0.6 mm we observe repetition frequencies of 40 and 90 GHz, and pulse lengths of 7 and 4 ps, respectively. The results are explained by an extremely short, tunneling dominated carrier life time in the saturable absorber at high negative bias. The fast depletion of the charge carriers in the absorber is investigated by bias-dependent life-time measurements in the absorber.

Blue- and green-emitting laser heterostructures, incorporating InGaN/GaN quantum dots as the active medium have been grown by molecular beam epitaxy. The quantum dot growth parameters have been optimized to obtain the highest photoluminescence intensity and radiative efficiency in the blue (λ=420 nm) and green (λ=545 nm). The blue and green lasers are characterized by threshold current densities of 930 A/cm² and 1.65 kA/cm², respectively, under quasi-continuous wave bias. To further reduce the threshold current density in the green-emitting devices, a tunnel injection scheme is used to inject cold holes into the quantum dot lasing states. These devices are characterized by a reduced threshold current density of 945 A/cm². The measured differential gain in the blue-emitting lasers is 2 x 10^-16 cm². Slope efficiencies of 0.41 W/A and 0.25 W/A have been measured, corresponding to differential quantum efficiencies of 13.9% and 11.3%, in the blue and green lasers, respectively.

Interband cascade (IC) lasers take advantage of the broken band-gap alignment in type-II quantum wells to reuse injected electrons in cascade stages for photon generation with high quantum efficiency, while retaining interband transitions for photon emission without involving fast phonon scattering. As such, the threshold current density can be significantly lowered with high voltage efficiency, resulting in low power consumption. After about 18 years of exploration and development, IC lasers have now been proven to be capable of continuous wave operation at room temperature and above for a wide wavelength range of 2.9 to 5.7 μm in the mid-infrared spectral region. Here, we present our recent progress in InAs-based IC lasers, which use plasmon cladding layers to replace superlattice cladding layers, resulting in improved thermal dissipation and extended lasing wavelengths.

Quantum cascade (QC) lasers are promising light sources for many chemical sensing applications in the mid-infrared spectral range. For industrial applications, broadband wavelength tuning of external-cavity QC lasers with very broad gain-width has been demonstrated. QC lasers based on anti-crossed dual-upper-state (DAU) designs are one of the promising candidates because of its broad bandwidth as well as high device performances. In fact, wide wavelength tuning of external cavity QC lasers with the anti-crossed DAU designs has been exhibited in several wavelengths: the tuning range of ~25% in pulsed mode and <17% in cw mode at room temperature. Here we report conspicuous temperature performances of continuous wave quantum cascade lasers with broad gain bandwidths. The lasers with the anti-crossed DAU designs, characterized by strong super-linear current-light output curves, exhibit the extremely high characteristic temperature for threshold current density, T0~750 K above room temperature. In addition, its slope efficiency is growing with increasing temperature (negative T1-value). For the pulsed operation of a short 1 mm length laser, the temperature coefficient reaches the surprisingly high value of 1085 K over 340-380 K temperature range. The distinctive characteristics of the DAU lasers are attributable to the optical absorption quenching which has been clarified to take place in indirect pumped QC lasers. Such high characteristic temperatures of the DAU-QC lasers provide great advantages for practical applications, in addition to its potential of broadband tuning.

In the hybrid silicon platform, active III/V based components are integrated on a silicon-on-insulator photonic integrated circuit by means of wafer bonding. This is done in a self-aligned back-end process at low temperatures, making it compatible with CMOS-based silicon processing. This approach allows for low cost, high volume, high quality and reproducible chip fabrication. Such features make the hybrid silicon platform an attractive technology for applications like optical interconnects, microwave photonics and sensors operating at wavelengths around 1.3 μm and 1.55 μm. For these applications energy efficient operation is a key parameter. In this paper we present our efforts to bring the III/V components in the hybrid silicon platform, such as lasers and optical amplifiers, on par with the far more mature monolithic InP-based integration technology. We present our development work to increase hybrid silicon laser and amplifier wall-plug efficiency. This is done by careful optimization of III/V mesa geometry and guiding silicon waveguide width. We also discuss current injection efficiency and thermal performance. Furthermore we show the characterization of the low-loss and low-reflection mode converters that couple the hybrid III/V components to silicon waveguides. Reflections below -41 dB and passive loss of 0.3 dB per converter were obtained.

The performance of injection schemes among recently demonstrated electrically pumped photonic crystal nanolasers has been investigated numerically. The computation has been carried out at room temperature using a commercial semiconductor simulation software. For the simulations two electrical injection schemes have been compared: vertical pi- n junction through a current post structure as in1 and lateral p-i-n junction with either uniform material as in2 or with a buried heterostructure (BH) as in3. To allow a direct comparison of the three schemes the same active material composition consisting of 3 InGaAsP QWs on an InP substrate has been chosen for the modeling. In the simulations the main focus is on the electrical and optical properties of the nanolasers i.e. electrical resistance, threshold voltage, threshold current and wallplug efficiency. In the current flow evaluation the lowest threshold current has been achieved with the lateral electrical injection through the BH; while the lowest resistance has been obtained from the current post structure even though this model shows a higher current threshold because of the lack of carrier confinement. Final scope of the simulations is the analyses of advantages and disadvantages of different electrical injection schemes for the development of the optimal device design for the future generation of electrically pumped nanolasers for terabit communication.

For a given average power, the energy per pulse of a mode-locked laser increases with increasing cavity length, lowering the repetition rate. Photonic crystal slow light optical waveguides can be used to address the high repetition rates and resulting low pulse energies of conventional semiconductor lasers by substantially increasing the effective optical cavity length while keeping the device compact. Such a device could enable a semiconductor laser to power two-photon microscopy, an advanced non-linear technique for time-resolved deep-tissue imaging. We present a design for realizing a monolithic two-segment quantum dot passively mode-locked photonic crystal laser. The cavity consists of a novel photonic crystal waveguide designed for low dispersion and wide bandwidth by engineering the photonic crystal lattice structure. Group velocity dispersion of 2x104 ps2/km, more than an order of magnitude lower than similar dispersion engineered photonic crystal waveguides, is achieved over 2% bandwidth, more than sufficient for mode-locking. Gain is achieved by optically pumping epitaxially grown InAs/GaAs quantum dots in part of the photonic crystal waveguide, and the saturable absorber section is reversed biased to enable pulse shaping. A cladding scheme is used to apply reverse bias to the saturable absorber and shorten its recovery time. Devices are fabricated using a combination of electron beam lithography, anisotropic etching, and selective under-etching processes, similar to standard photonic crystal waveguides. The low-dispersion, wide bandwidth waveguide, combined with the fast dynamics of InAs quantum dots could enable a compact, low repetition rate mode-locked laser to be realized.

High temperature operation of terahertz (THz) sources based on quantum cascade lasers (QCLs) is discussed. THz QCLs are compact, powerful sources but can only operate at cryogenic temperatures. State-of-the art THz QCLs are made with GaAs/AlGaAs heterostructures and use a single composition of AlGaAs for the barrier material. It was recently shown that multi-composition barriers in the band structure can result in gain > loss at temperature as high as ~240K. We demonstrate early experimental results that yield QCLs that operate up to 184K – similar to QCLs based on single composition barrier designs. An alternative method of producing room-temperature THz is based on intra-cavity difference-frequency generation (DFG) in mid-infrared (mid-IR) QCLs. Here we report devices with record conversion efficiency. THz DFG QCLs reported previously are highly inefficient since THz radiation produced more than ~100 μm away from the exit facet is fully absorbed due to high THz losses in the QCL waveguide. Our lasers use a non-collinear Cherenkov DFG scheme to extract THz radiation from the active region. Dual-color mid-IR quantum cascade lasers with integrated giant optical nonlinearity are grown on semi-insulating (S.I.) InP substrates. THz radiation is emitted at an angle into the substrate with respect to the mid-infrared pumps. Since S.I. InP is virtually lossless to THz radiation, this scheme allows for efficient extraction of THz radiation along the whole waveguide length. As a result, our sources demonstrate large mid-infrared-to-THz conversion efficiency. Devices tested at room-temperature produced 18μW peakpower and 75μW/W² conversion efficiency.

Long-wavelength (λ ≈ 12 - 16 μm) Quantum Cascade (QC) lasers are crucial devices for improving the detection sensitivity of QC-laser based sensing for important gases including BTEX (benzene, toluene, ethylbenzene, and xylenes) or uranium hexafluoride. A high-performance QC laser emitting at ~ 14 μm is reviewed, optimized by employing a diagonal optical transition and a “two-phonon-continuum” depletion scheme. It shows a low threshold current density of 2.0 kA/cm², a peak power of 336 mW, all at 300 K, as well as a high characteristic temperature ~ 310 K over a wide temperature range around room temperature (240- 390 K). Single-mode operation is demonstrated with short cavities, with a mode-hop-free continuous tuning range of ~ 5.5 cm^-1. The ridge-width dependence of threshold of ~ 14 μm QC lasers by both wet etching and dry etching is studied. The main challenge for narrowing wet-etched ridges is the high loss caused by mode coupling to surface plasmon modes at the insulator/metal interface of sloped sidewalls. Conversely, dryetched ridges avoid surface plasmon mode coupling due to the absence of transverse magnetic polarization for the vertical insulator and metal layers. To further improve the efficiency of QC lasers, a same-wavelength cascaded transition approach is developed, with two sequential cascaded transitions at the same wavelength ~ 14.2 μm in each stage. This same-wavelength cascaded-transition QC gain medium was inserted between two conventional QC stacks at the same wavelength. Slope efficiency is increased by 46% when laser operation changes from the single-transition region to the cascaded-transition region.

Using a range of grating pitches, we obtained distributed feedback (DFB) quantum cascade lasers (QCL) operating CW at room temperature over the 9-11μm range. Single mode CW operation is demonstrated up to 10.8 μm, and up to 40 °C. To the best of our knowledge, it is the longest wavelength reported for a DFB QCL operating CW above room temperature. The wavelength coverage per wafer is larger than 110 cm-1. DFB QCLs from two wafers which have different gain peaks have lasing wavelengths near optical absorption peaks of glucose.

We present recent work towards the realization of a nanowire-based terahertz quantum cascade laser. Nanowires offer an additional quantum mechanical confinement of electrons in the plane of a two-dimensional quantum cascade structure. The additional quantization can greatly increase the lifetimes of intersubband transitions and therefore increase the optical gain and also the maximum operating temperature of terahertz quantum cascade lasers. We outline a fabrication process that is fully scalable from nanowire to micropillar devices and present measurements of micropillar arrays in a double metal waveguide. The results are very promising and also show the main technological challenges for realizing nanowire-based devices.

A strain-balanced, AlInAs/InGaAs/InP quantum cascade laser structure, designed for light emission near 9μm, was grown by molecular beam epitaxy. Laser devices were processed in buried heterostructure geometry. Maximum pulsed and continuous wave room temperature optical power of 4.5 and 2W and wallplug efficiency of 16% and 10%, respectively, were demonstrated for a 3mm by 10μm laser mounted epi-side down on an AlN/SiC composite submount. Pulsed laser characteristics were shown to be self-consistently described by a simple model based on rate equations using measured 70% injection efficiency for the upper laser level.

Quantum cascade (QC) lasers are compact and versatile light sources suitable for a broad range of absorption spectroscopy based molecular sensing applications. However, for most of such sensing applications, single-mode operation of QC lasers is a prerequisite. Conventional single-mode QC lasers, e.g., distributed feedback (DFB) [1] or external cavity QC lasers [2], have much higher cost than multi-mode simple ridge QC lasers, mainly due to their complicated and demanding device fabrication or time-consuming system integration and alignment processes. In order to achieve more cost-effective single-mode QC lasers, we demonstrate a novel type of laser cavity design which consists of an asymmetric Mach-Zehnder (AMZ) interferometer structure monolithically integrated in a conventional Fabry-Perot (FP) cavity with simple ridge waveguide and as-cleaved facets. Strong wavelength selectivity is introduced by the properly designed AMZ interferometer whose transmission spectrum comprises equidistantly spaced narrow peaks, which in turn selects a specific FP mode associated with the entire laser cavity near the optical gain spectrum peak, effectively facilitating single-mode operation of the laser. Continuously wavelength-tunable single-mode operation of QC lasers is achieved in pulsed mode from 80 K to room temperature and in continuous-wave (CW) mode with high side-mode suppression ratio (SMSR) up to ~35 dB. The observed spectral characteristics of the tested lasers are described with satisfying accuracy by our model developed for such cavity structures. The fabrication process for such AMZ interferometer type cavities is identical to that for simple ridge lasers, therefore providing a promising solution to achieving more cost-effective single-mode QC lasers.

Mechanisms are addressed limiting the reliability high-power diode lasers. An overview is given on the kinetics of the Catastrophic Optical Damage (COD) process, which is related to highest output powers. It involves fast defect growth fed by re-absorption of laser light. Local temperatures reach the order of the melting temperature of the waveguide of the device. The process starts either at a facet or at any weak point, e.g., at extended defects in the interior of the cavity.

Reliability and degradation processes in broad-area InGaAs-AlGaAs strained quantum well (QW) lasers are under investigation because these lasers are indispensible as pump lasers for fiber lasers and amplifiers that have found an increasing number of industrial applications in recent years. Extensive efforts by a number of groups to develop InAs-GaAs quantum dot (QD) lasers have recently led to significant improvement in performance characteristics, but due to a short history of commercialization, high power QD lasers lacks studies in reliability and degradation processes. For the present study, we investigated reliability and degradation processes in MOCVD-grown broad-area InGaAs-AlGaAs strained QW lasers as well as in MBE-grown broad-area InAs-GaAs QD lasers using various failure mode analysis (FMA) techniques. Dots for the QD lasers were formed via a self-assembly process during MBE growth. We employed two different methods to degrade lasers during accelerated life-testing: commercial lifetester and our newly developed time-resolved electroluminescence (TR-EL) set-up. Our TR-EL set-up allows us to observe formation of a hot spot and subsequent formation and progression of dark spots and dark lines through windowed n-contacts during entire accelerated life-tests. Deep level transient spectroscopy (DLTS) and time resolved photoluminescence (TR-PL) techniques were employed to study trap characteristics and carrier dynamics in pre- and post-stressed QW and QD lasers to identify the root causes of catastrophic degradation processes in these lasers. We also employed electron beam induced current (EBIC), focused ion beam (FIB), and high resolution TEM to study dark line defects and crystal defects in post-aged QW and QD lasers at different stages of degradation.

The facets of InP/(Al)GaInP/GaAs quantum dot laser active regions offer superior resistance to catastrophic optical mirror damage at high facet power densities. These structures degrade by bulk damage. We have used a new range of techniques to identify changes occurring during damage in working devices: thermography through windows in the nmetallization, photoluminescence via p-metallization windows and photocurrent studies. Devices were aged with single very high current pulses or pulses of increasing size and monitored during this process with these techniques. Previous investigation with panchromatic cathodoluminescence revealed dark non-radiative spots throughout the plane of the active region. The dark spots, which were present even in unprocessed material, increased in size in the pumped regions only during lasing action. The spots and background regions darkened throughout the pumped stripe area only for the whole duration of the current pulse. Thermography after successive pulses confirmed damage originating from a point in the bulk rather than at the facet. p-windows observations of light and dark regions showed a blue shift in the photoluminescence spectra of the dark regions. Photocurrent studies of more gently aged devices showed a greater decrease in signal in the region associated in previous work with defective very large dots. Identification of such spectral regions, which were previously found to be influenced by changes in structure design and growth conditions offer a route to control degradation mechanisms by this means.

Reliable single emitters delivering >10W in the 9xx nm spectral range, are common building blocks for fiber laser pumps. As facet passivation techniques can suppress or delay catastrophic optical mirror damage (COMD) extending emitter reliability into hundreds of thousands of hours, other, less dominant, failure modes such as intra-chip catastrophic optical bulk damage (COBD) become apparent. Based on our failure statistics in high current operation, only ~52% of all failures can be attributed to COMD. Imaging through a window opened in the metallization on the substrate (n) side of a p-side down mounted emitter provides valuable insight into both COMD and COBD failure mechanisms. We developed a laser ablation process to define a window on the n-side of an InGaAs/AlGaAs 980nm single emitter that is overlaid on the pumped 90μm stripe on the p-side. The ablation process is compatible with the chip wire-bonding, enabling the device to be operated at high currents with high injection uniformity. We analyzed both COMD and COBD failed emitters in the electroluminescence and mid-IR domains supported by FIB/SEM observation. The ablated devices revealed branching dark line patterns, with a line origin either at the facet center (COMD case) or near the stripe edge away from the facet (COBD case). In both cases, the branching direction is always toward the rear facet (against the photon density gradient), with SEM images revealing a disordered active layer structure. Absorption levels between 0.22eV – 0.55eV were observed in disordered regions by FT-IR spectroscopy. Temperature mapping of a single emitter in the MWIR domain was performed using an InSb detector. We also report an electroluminescence study of a single emitter just before and after failure.

Highly brilliant diode lasers at 1120nm with a high optical output power, nearly diffraction limited beam and narrow spectral line width are increasingly important for non-linear frequency conversion to 560 nm. We present experimental results about edge-emitting distributed Bragg reflector (DBR) tapered diode lasers emitting at 1120 nm. The investigated lasers show an output power of up to 8W with a conversion efficiency of about 40%, and a lifetime of more than 5000 h at 5 W. The lasers also exhibit a small vertical divergence <15° full width at half maximum (FWHM), a nearly diffraction limited beam quality, and a narrow spectral line width with FWHM smaller than 10pm. These properties allow the lasers to be used for future second harmonic (SH) generation.

There is great interest in the development of high-power, high-efficiency InP-based broad area pump diode lasers operating in the 14xx-15xx nm band to be used for resonant-pumping of Er-doped solid state lasers. Cryogenic cooling of diode lasers can provide great benefit to performance, arising from the dramatic reduction in the threshold current and the increase in the diode’s slope efficiency. These improvements are attributed to reduction in the non-radiative losses and leakage current associated with thermionic emission of carriers from the quantum well. This is, however, at the expense of a large increase in the diode voltage, limiting the power conversion efficiency at cryogenic temperatures. In this work, we report on the development of high-power, high-efficiency diode lasers and stacked arrays operating at 15xx-nm, which are specifically designed and optimized for operation at cryogenic temperatures. We show that the diode voltage defects under cryogenic operation can be greatly reduced through reducing the energy band offsets at the hetero-interface, and through material change to reduce the dopant ionization energy, effectively mitigating carrier freeze-out at low temperatures. Optical cavity designs and band engineering optimization are also explored for low intrinsic optical loss and low carrier leakage. A peak power conversion efficiency of >74% was demonstrated at a temperature of ~100K in a 15xx-nm single emitter. Record high peak conversion efficiency of 71% and peak power of > 500 W were also demonstrated in a stacked array, under QCW pulses of 1 ms and 10 Hz.

We study experimentally the dynamic properties of a fully integrated high power master-oscillator power-amplifier emitting at 1.5 μm under continuous wave and gain-switching conditions. High peak power (2.7 W) optical pulses with short duration (~ 110 ps) have been generated by gain switching the master-oscillator. We show the existence of working points at very close driving conditions with stable or unstable regimes caused by the compound cavity effects. The optical and radio-frequency spectra of stable and unstable operating points are analyzed.

In this paper we present detailed experimental results of the impact of the amplitude and the widths of current pulses injected into a gain-switched distributed feedback (DFB) laser emitting at a wavelength of 1064 nm. The laser with a InGaAs triple quantum well active region has a 3 μm wide ridge waveguide (RW) and a cavity length of 1.5 mm. Gainswitching is achieved by injecting current pulses with a width of 50 ns, a repetition frequency of 200 kHz and a very high amplitude up to 40 times the threshold current (2.5 A). Time resolved investigations show, that depending on the amplitude and the duration of the current pulses, the optical power exhibits different types of oscillatory behavior during the pulses, accompanied by changes in the lateral near field intensity profiles and optical spectra. Three different types of instabilities can be distinguished: Mode beating with frequencies between 25 GHz and 30 GHz, switching between different lateral modes and self-sustained oscillations with a frequency of about 4 GHz. Our results are relevant for the utilization of gain-switched DFB-RW lasers as seed lasers for fiber laser systems and in other applications, which require high optical power.

High-power diode lasers in the mid-infrared wavelength range between 1.8μm and 3.0μm have emerged new possibilities for solid-state pumping and direct material applications based on water absorption with favoured wavelengths at 1.94μm and 2.9μm. GaSb based diode lasers are naturally predestined for this wavelength range. We will present results on MBE grown (AlGaIn)(AsSb) quantum-well diode laser single emitters and laser arrays at different wavelengths between 1.8μm and 3.0μm. At 1.94μm different epitaxial designs have been investigated in order to optimize the GaSb based diode lasers for higher wall-plug efficiency and higher brightness. More than 30% maximum wall-plug efficiency in cw operation for single emitters could be demonstrated for resonator lengths of 1mm. At 2.25μm a high wall-plug efficiency of 24% has been measured. For 2mm resonator length by using asymmetric waveguide structures the wall-plug efficiency could be doubled. Fast axis far field widths of 70 degree (95% power included) have been demonstrated. At 2.9μm emitting wavelength broad-area lasers with 2mm resonator length with 360mW at 10°C heat sink temperature are presented. We have also started to transfer the concepts for higher brightness to this wavelength regime. 19-emitter laser arrays emitting at 1.94μm have been packaged on actively cooled heat sinks. Comparable high wallplug efficiencies have been measured with p-side down and p-side up packaging. In all configurations far field widths are well below 90 degree (95% power included). Finally a record value of 140W have been measured for a stack built of 10x 20% fill factor bars emitting at 1.91μm.

The leakage current in two quantum-cascade (QCL) structures is measured and analyzed. The structures illustrate a new design feature, exploiting the interface roughness scattering at the well/barrier interfaces to intentionally shorten the lifetime of the lower laser state while increasing that of the upper laser state. By using low barriers where the upper laser state has its maximum probability and high barriers where the lower laser state has its maximal probability in strain-compensated designs for short wavelength emission, the lifetime of the upper laser state can be increased, while decreasing the lifetime of the lower laser state. First realizations of this design result in J_th = 1.7 kA/cm² at 300 K, slope efficiency η = 1.4 W/A, T₀ = 175 K, and T₁ = 550 K for lasers emitting at 3.9 μm. A further analysis allows the extraction of the leakage current into higher minibands from the temperature dependence of the threshold current density and to reconstruct the energies of the higher-lying states from this current. The modeling includes the thermal population of LO phonons that drive the leakage.

This paper reports on the mode-locked operation of a 980-nm external-cavity passively mode-locked laser with extremely narrow RF linewidth. Optical pulses with 10-ps pulse duration were generated at a repetition rate of 955 MHz, with an average output power of 39.3 mW – which corresponds to a peak power of 4.1 W, generated directly from the oscillator. The RF spectrum displays a -3dB RF linewidth of only ~40 Hz, as well as a 60-dB dynamic contrast, revealing the exceptionally low-noise fundamental mode-locked operation of this laser. At a repetition rate of ~1 GHz, the highest peak power of 5.26 W was achieved, albeit with an increased -3dB RF linewidth of ~100 Hz. The two-section chip incorporated an active region with a dual InGaAs quantum well sandwiched by an asymmetrical waveguide, and was operated at room temperature. By taking advantage of the broad tunability of the repetition rate which externalcavity lasers can afford, we also investigated the limits of stable fundamental mode-locked operation at the lowest repetition rates (or maximum external cavity lengths).

Quantum Cascade (QC) lasers are semiconductor devices operating in the mid-infrared and terahertz regions of the electromagnetic spectrum. Since their first demonstration in 1994, they have evolved rapidly into high power devices. However, they also have intrinsic challenges, such as beam steering at high power. Such phenomenon has been observed in QC lasers and attributed to the interaction between the two lowest transverse modes in the laser cavity.

In this project, we have used COMSOL Multiphysics simulations to first investigate how transverse mode propagation can be controlled with cavity spoilers. We have modeled this effect by creating short and lossy lateral constrictions from the top of the laser ridge to perturb the modes distributed more toward the sides of the laser ridge, while leaving the fundamental mode intact. After obtaining optimized dimensions for the constrictions, we have utilized focused ion beam (FIB) milling to etch two small trenches from the top of several laser ridges to create the simulated effect on our devices. We, then, filled them with platinum in an effort to completely suppress the propagation of higher order transverse modes in the cavity. The results obtained show minimal effect on threshold and a Gaussian far-field distribution at various current levels, indicating a complete suppression of the higher order transverse modes.