Limiting factors for short-wavelength CW QCL designs are discussed. A model is used to predict the short-wavelength cutoff for strain-balanced QCL structures. High performance is predicted at wavelengths as short as 3.0 micron based on a conduction band offset of 0.9 eV in the GaInAs/AlInAs materials. Recent work is presented on the growth of strained materials using gas-source molecular beam epitaxy to investigate the model predictions. Advanced material characterization, including high-resolution x-ray diffraction, photoluminescence, atomic force microscopy, and wafer-scale uniformity and repeatability are demonstrated for highly-strained and strain-balanced QCL structures. Laser testing results are presented for QCLs operating near the short wavelength limit at 3.66 micron. Lastly, predictions for further performance improvement at short wavelengths are discussed.

We describe the design and implementation of a broad-gain and low-threshold (Jth = 860 A/cm² at 8 K) quantum-cascade laser based on strain-compensated In_0.73Ga_0.27As-AlAs on InP. Laser emission between 3.7 and 4.2 um is achieved because of the very large Gamma-valley conduction band discontinuity with narrow quantum wells, allowing large intersubband energy differences. Furthermore, the design inhibits carrier loss from the upper lasing state into the continuum even at elevated operating temperatures, resulting in room-temperature operation. Laser operation in pulsed mode is achieved up to a temperature of 330 K with maximum single-facet output peak powers of 6 W at 8 K and 240 mW at 296 K. The temperature coefficient T0 is 119K. The 30-period structure exhibits an external differential efficiency of 13 (40% per period) at low temperatures and a maximum wall-plug efficiency of 24%. The lasing transition takes place from several upper states to several lower states, resulting in a relatively broad (300 cm^-1) gain spectrum and could allow the design to be used in external tuning configurations.

High power highly-efficient AlGa(In)As/GaAs multimode diodes operating at 915 nm range have been developed. 2 mm long-cavity Chips-On-Submount demonstrate 72% peak power conversion efficiency at 25°C heatsink temperature. Peak power efficiency over 50% was recorded up to 130°C heatsink temperature. CW power launched at 7°C heatsink temperature from 20 um wide aperture devices is around 8.5 W and is thermally limited. This output power level translates into record-high brightness on the facet: > 65 MW/cm². Device reliability assessment indicates high potential of such devices to operate reliably at high electrical and optical power densities.

Quantum well intermixing (QWI) of the facet regions of a semiconductor laser can significantly improve the COD of the device giving high kink power and high reliability. A novel epitaxy design incorporating a graded 'V-profile' layer allows for a reduced vertical far-field and simultaneously suppresses higher order modes to give high power operation. Furthermore, the 'V-profile' layer provides a robust design to improve the ridge etch tolerance to give excellent device performance uniformity across an array. Very large arrays of individually addressable lasers (up to 100 elements) are reported with small pitch size (~100 μm), high single mode power (up to 300 mW) and high uniformity.

High power and reliable 980nm single mode lasers are required for pumping EDFAs and 915, 940, 975nm broad area lasers are demanded for pumping double clad fiber lasers and amplifiers. While we have demonstrated high power lasers by using decoupled confinement heterostructure (DCH) , we employed a window structure due to impurity free vacancy disordering (IFVD.) 980nm single mode lasers showed complete rollover with I=4A(CW). For 12000 hour lifetest at 700mW-40C, no sudden failure was observed. And the average gradual degradation rate was zero. Also the window structure applied to 975nm broad area laser successfully. The degradation rate during 500 hours operation was decreased to zero.

Al-free active diode lasers emitting near 970 nm wavelength have been optimized for high electrical-to-optical power conversion efficiency. There are numerous key contributors such as scattering and absorption losses, band alignment, Joule heating, carrier leakage and below-threshold losses that contribute to power loss mechanisms. We report on improvement from 50% to a record-high 73% power conversion efficiency for a 1 cm bar at 10C, resulting from a multi-pronged approach that has been taken to minimize each of the loss mechanisms as to improve the overall power conversion efficiency.

Nine years has passed since the initial development of GaN-based violet laser diodes (LDs) in the 405 nm wavelength range in 1995. Starting with next-generation high-density optical discs, the commercial use of violet LDs has been adopted in many new fields, such as biomedical, reprographic, and exposure fields. Recently, lasing wavelength has broadened to cover from the ultraviolet (UV) to blue-green regions, which enabled other new applications. In this paper, the current status of GaN-based LDs from the UV to blue-green regions is reported.

The degradation mechanism of blue-violet laser diodes (BV-LDs) fabricated on epitaxial lateral overgrowth (ELO) GaN on sapphire substrates is investigated and the characteristics of recent BV-LDs fabricated on GaN substrates are reported. The lifetime of lasers on ELO-GaN/sapphire is shown to be strongly dependent on the dislocation density of the basal GaN layer, with the degradation rate being roughly proportional to the square root of the operating time. Electron microscopy confirms that the degradation is not due to the multiplication of threading dislocation but rather the diffusion of defects toward dislocations in the active layer, which results in an increase in the capture cross-section for non-radiative recombination centers. BV-LDs on GaN substrates are demonstrated to have a lifetime of over 2000 h under 160 mW pulsed operation at 80 °C and to achieve a high critical power density at catastrophic optical damage (COD) of 40.8 MWcm^-2.

High pressure grown GaN bulk crystals, because of their low defect density, are atractive for the use as substrates for blue-violet laser diode fabrication. These laser diodes are characterized by a low density of dislocations (8×10⁴-1×10⁵ cm^-2) and thus they possibly have the best crystalline quality ever reported for this type of nitride devices. Previously, we demonstrated that these lasers are able to emit a very high optical power under pulse operation. In the present paper we will demonstrate the details of their room temperature CW operation, giving good prognostics for the further development of these devices. Preliminary estimation of the internal losses indicated a very low internal absorption in the range of 5 cm^-1. The characterization of the aged devices did not reveal any dark lines or facet degradation. A correlation between the device lifetime and p-type layers growth methods will be suggested here.

Nonlinear light generation in quantum-cascade lasers (QCLs) has the potential to extend the operating wavelength of these devices outside the limits imposed by the fundamental properties of the materials of choice. The giant nonlinear susceptibilities of resonant intersubband transitions have been studied intensively both theoretically and experimentally over the past twenty years. However, the practical applications have been limited by the lack of efficient laser pumping and of convenient phase matching techniques. The first obstacle was overcome by monolithically integrating the nonlinear intersubband transitions within the active region of a quantum cascade (QC) laser. Sum-frequency and second-harmonic (SH) generation were the first nonlinear processes observed in QCLs. The optimization of the second-harmonic generation in InGaAs/InAlAs QCLs will be discussed in detail. The second challenge for achieving high efficiency nonlinear power conversion is the phase-matching of the fundamental and nonlinear light. We have developed a technique for modal phase-matching that takes advantage of the flexibility in the design of the QCL waveguide. An additional degree of freedom for tuning into exact phase-matching conditions is provided by the dependence of the refractive indices on the laser ridge width. Record nonlinear power of 2 mW at 4.45 mm was achieved using an InP top-cladding waveguide and high-reflectance coating on the laser back facet. Reaching the milliwatt power range is significant as such power levels are sufficient for trace gas point sensors using mid-infrared light sources. The practical limitations of the phase-matching method will be evaluated and the experimental results will be compared with theoretical predictions.

Oscillators based on Raman gain, known in the literature as Raman lasers, have until now necessitated an external optical pump. A powerful multi-watt fundamental laser radiation is usually required to offset a small value of the Raman gain coefficient, typically several 10^-9cm/W in most Raman-active media. For example, these limitations in solid-state Raman lasers which employ scattering off a vibrational (phonon) resonance in a crystal or fiber have led to high thresholds and small conversion efficiencies. If these could be overcome such lasers would find important applications as compact light sources due to their spectral agility and could extend the range of semiconductor lasers. An important step in this direction was the recent demonstration of near infrared Raman lasers, in which ultralow threshold was achieved thanks to the use of high quality-factor dielectric microsphere resonators. Here we demonstrate a compact injection Raman laser operating in the mid-infrared, in which low threshold and high conversion efficiency are achieved by integrating within the active region of a quantum cascade region a Raman section designed to resonantly enhance the Raman gain by many orders of magnitude.

We overview coherent nonlinear optical phenomena that occur in active quantum-cascade structures which support both laser action and, at the same time, nonlinear self-conversion of laser light into coherent radiation at different frequencies. In other words, the laser field serves as an intracavity optical pump for the desired nonlinear optical process. In such systems, resonant absorption of the pump field is overcome by laser gain, and huge resonant nonlinearities of the intersubband transitions can be fully exploited. The proposed approach holds promise to extend the operating wavelength of QC lasers to the spectral regions where they become less efficient, constrained by material limitations, or operate only at cryogenic temperatures. It may also lead to the development of broadly tunable injection-pumped sources.

Dilute nitride type-II "W" structures have potential for lasing at 1.55 microns (on GaAs substrates) and in the mid-infrared (3-6 microns, on InP substrates). The former active regions utilize (In)GaAsN/GaAsSb/(In)GaAsN/GaAs quantum wells, whereas the latter are based on InAsN/GaAsSb/InAsN/GaInP structures. Following a review of the theoretical rationale, we will present some preliminary MOCVD growth results for the GaAs-based type-II structures, along with their characterization by x-ray, TEM, and photoluminescence. The experimental energy gaps corresponding to the layer compositions determined from characterization are in good agreement with calculations based on the 10-band k×p formalism.

We review here our results concerning laser diodes emitting at 2.38 µm and 2.60 μm. We present an original method allowing to determine the monomolecular, radiative and Auger recombination coefficients A, B and C, as well as the transparency carrier density N_tr, the internal loss α_i and the gain coefficient g_o from the differential efficiency and the threshold current density obtained with different laser diodes. We show how these parameters can be used to optimize the number of quantum wells and explain the differences existing between laser diodes emitting at 2.38 and 2.60 μm. At 2.38 μm, we obtained a threshold current density of 76 A/cm² with a single quantum well laser diode and at 2.60 μm, a J_th of 152 A/cm² with a double quantum well laser diode. These threshold current densities can be compared favorably to the best reported values in the 0.85-3.0 mu;m range.

We propose a novel design of an electrically tunable type II mid-IR light source based on a InAs/AlSb/GaInAsSb/GaInSb heterostructure. The design combines the advantages of strong wavelength tuning due to the linear Stark effect and the presence of separate charge accumulation layers, which enables laser wavelength tuning without a change of the optical loss. Experiment shows a blue shift of the electroluminescence (EL) line at increasing bias current, commensurate with that expected form the linear Stark effect. The laser generation was observed at higher currents. The EL wavelength shifts from 2.79um to 2.38um ( ~ 80meV) at T=80K as the bias current increases from 97mA to 418mA, which provides the record combination of the wide tuning range and low relative change of the bias current.

We present results on low beam divergence, low threshold current GaSb-based quantum-well diode lasers emitting in the 1.9 - 2.4 μm wavelength range. By carefully designing the active quantum-well region, low threshold current densities in the range of 148 to 190 A/cm² could be achieved in the entire wavelength range. A novel structure for the epitaxial waveguide was designed and realized experimentally, leading to a reduced beam divergence in the fast axis of 44° full width at half maximum (FWHM), compared to 67° FWHM of a conventional broadened waveguide design. This improvement was achieved without any sacrifice in the laser performance, i.e. the novel laser structure showed the same threshold current and differential quantum effciency as the standard one. Ridge-waveguide lasers employing the new waveguide design and emitting at 2.3 μm were operated in an external cavity configuration. Due to the improved coupling effciency of the laser beam into the collimating optic, a wide tuning range of 130nm could be achieved, limited only by the gain bandwidth of the active material.

Quantum cascade (QC) lasers operating at terahertz frequencies were demonstrated two and a half years ago, and, since then, their development has proceeded at a very rapid pace. Most of the research has focused on new concepts for the quantum design of the gain medium as well as on improving device structure by optimizing fabrication and waveguide technology. These efforts by various groups have led to maximum operating temperatures of about 140 K in pulsed mode, output powers of up to 50 mW, and lasing in continuous wave up to 93 K. Such advances are making THz QC lasers more and more appealing for applications in various fields like chemical sensing, astronomy, spectroscopy, and imaging. For their successful implementation, specific requirements have to be addressed, particularly concerning the spectral properties of the emission. Here we report some latest developments in this direction. We demonstrate the perfect control of laser design for the realization of devices with precisely defined emission frequency in the whole range from 2.3 THz to 4.8 THz. Additionally, single-mode THz lasers with distributed feedback resonators have been achieved and a new technique involving surface plasmon gratings has been developed to improve performances. The latter offers also the possibility of constructing distributed Bragg gratings as a replacement for high-reflection coatings. Finally, solutions allowing broad tuneability are examined, with preliminary results illustrating the viability of external cavity set-ups.

In this work we demonstrate for the first time that terahertz (THz) quantum cascade lasers can be realised in a buried-waveguide geometry. In our prototype devices the optical mode is a surface plasmon bound at the interface between the top contact and the semiconductor, providing for both vertical and lateral confinement without the need to define a cavity ridge. Proton-implanted high-resistivity sections are used to define the current channel where electrons can be injected into the active region. This way the electrical and optical confinement can be controlled independently: the former is defined by the non-implanted regions and the latter by the width of the top contact metal strip. Compared to standard ridge waveguides this technique allowed for a narrowing of the device effective cross section without introducing additional losses and improving the thermal conductivity, resulting in an increase of the maximum operating temperature up to 77K in continuous wave at 2.9 THz. In addition, we present preliminary results from buried-waveguide THz quantum cascade lasers obtained by combining a double-metal waveguide geometry with proton implantation. Initial results are promising, yielding record low threshold currents of 19mA at 4.2K in both pulsed and continuous wave operation.

Quantum cascade lasers that operate in the underdeveloped terahertz spectral range (1-10 THz) promise to contribute to applications in sensing, spectroscopy, and imaging. We describe our development of terahertz quantum cascade lasers based on the resonant-phonon depopulation concept and that use low-loss metal-metal waveguides for optical confinement. Two- and three-dimensional finite-element simulations of terahertz metal-metal waveguides are used to demonstrate their high modal confinement even for very narrow ridges. Also, simulations predict high facet reflectivities due to the modal impedance mismatch with free space at the sub-wavelength waveguide aperture of these metal-metal waveguides. Finally, we report the demonstration of a 2.8 THz laser that operates up to 97 K in continuous-wave mode fabricated using a Cu-Cu thermocompression bonding technique.

We achieved 1.5-um CW SQW GaInNAsSb lasers with GaNAs barriers grown by MBE on GaAs substrates with typical room temperature threshold densities below 600A/cm², external quantum efficiencies above 50%, and output powers exceeding 200mW from both facets for 20x1222um devices tested epitaxial-side up. In pulsed mode, 450A/cm², 50%, and 1100mW were realized. Longer devices yielded over 425mW of total CW power and thresholds below 450A/cm². These results are comparable to high quality GaInNAs/GaAs lasers at 1.3um. Z-parameter measurements revealed that these improvements in the performance metrics of approximately 40-60% over previous results are primarily due to reduced monomolecular recombination. The large differential gain of GaInNAsSb/GaNAs/GaAs lasers at 1.5um of approximately 1.2x10-15cm² was mostly squandered in previous devices due to large quantities of monomolecular recombination. The characteristic temperatures for threshold current, T0, and for efficiency, T1, were 66K and 132K, respectively. These reduced values, compared to prior measurements of 106K and 208K, respectively, indicate carrier leakage. Since monomolecular recombination is temperature insensitive, the temperature stability of device operation was adversely affected.

Wide stripe and single-mode ridge guide dilute nitride quantum well lasers exhibit high performance in the 1.3 micron wavelength region. Optimizing the MOCVD growth of InGaAsN has yielded devices displaying reduced temperature sensitivity compared with conventional InP-based structures. Current annealing effects in InGaAsN QW lasers are minimized through the use of a nitrogen only cool down after growth of the laser structure. Device studies reveal the impact of nitrogen content and quantum well barrier materials on the optical gain, carrier confinement, and laser characteristics. Extension of the emission wavelength in MOCVD grown structures remains challenging. Higher N content results in a significant increase in threshold current density and higher temperature sensitivity.

High power and single mode InGaAsN ridge waveguide lasers were developed. The pulsed the maximum output power was 240 mW at room temperature (RT). The threshold was 15 mA at 20°C. The ridge waveguide laser could work beyond 120°C. For cw operation, the lasers show a maximum output up to 40 mW RT. The broad area lasers using the same materials has been working under continuous-wave operation at constant current (80% of maximum output) for more than 42,800 device-hours at 30°C with as-cleaved facets. They are still working well.

We have fabricated UV-emitters such as UV-light emitting diode (UV-LED) and UV-laser diode (UV-LD) on sapphire substrates. The combination of low-temperature-deposited AlN interlayer and lateral seeding epitaxy (Hetero-ELO) yielded crack-free and low-dislocation-density AlGaN. The light output power of GaN/AlGaN multi-quantum wells active layer based UV-LED monotonically decreased with the increase of threading dislocations. Moreover, we have demonstrated a UV-LD grown on this low-dislocation-density AlGaN. The lasing wavelength under pulsed current injection at room temperature was 350.9 nm. We also present violet and UV-LEDs grown on ZrB2 substrate. The violet LED exhibits excellent linearity of L-I characteristic and sharp single spectrum, and vertical conduction through nitride and ZrB2 interface has been confirmed in the UV-LED. We also present the growth of AlN single crystals by sublimation method.

Current-injection spiral-shaped microcavity disk laser diodes emitting in the ultraviolet (UV) are reported. We have previously demonstrated spiral microdisk lasers with InGaN multiple quantum well (MQW) gain media emitting near 400 nm and microresonators with an AlGaAs single quantum well (SQW) active regions emitting around 730 nm. In this paper we will extend this concept to UV emitting microdisk laser diodes and demonstrate room-temperature laser operation of InAlGaN heterostructures with emission wavelength near 368 nm and pulsed threshold current densities as low as 6.7 kA/cm².

For blue (Al,In)GaN laser diodes we measure the evolution of the laser beam during its propagation from near-field to far-field. We reconstruct the near-field phase distribution of the waveguide mode. The dynamic behaviour is a tilting of the phase front and a far-field beam-steering on a 10 ns to 100 ns time scale. These observations are discussed in the context of the carrier induced change of the refractive index associated with the antiguiding factor and of thermal dynamics.

With increasing demands for the development of high power GaN-based blue-violet laser diodes (LDs), thermal management has become an important issue. We present a new method to determine junction temperature of GaN-based LDs for simple, fast, and reliable characterization of thermal performances. The large change of forward operation voltage with temperature is advantageously used to measure junction temperature. Using this method, we compare junction temperature of LD structures with different substrates and chip mounting methods. It is found that the junction temperature can be reduced considerably by employing GaN substrates or epi-down bonding. For epi-down bonded LDs, as much as two-fold reduction in junction temperature is achieved compared to epi-up bonded ones and temperature increase in this case is only about 13 degrees for more than 100 mW-output power.

Semiconductor nitrides have many applications for optoelectronic devices; particularly, blue-violet laser diodes (LDs) are required for blu-ray optical disc systems. Molecular beam epitaxy (MBE) with its fine control of growth parameters and capability for in-situ growth monitoring is a well-established technique for depositing III-V heterostructures. Indeed, many commercial infrared LDs are grown very successfully by MBE. However, MBE-growth of nitrides is much more difficult, because providing enough nitrogen atoms at the growth surface, sustaining the high growth temperatures as well as finding the right growth parameters have proved to be very challenging. We recently reported the first InGaN LDs by MBE, showing that those problems can be solved in practice as well as demonstrating the capability of MBE to produce high-quality optoelectronic devices. As the efficiency of nitrides depends strongly on the growth process, structural differences of MBE over metal-organic vapor phase epitaxy (MOVPE)-material may also lead to device advantages. Our first InGaN LDs were grown on sapphire templates, with a pulsed room-temperature threshold current-density of 30 kA/cm² and a threshold voltage of 33 V. Here, we report on MBE-grown 405 nm InGaN LDs on freestanding GaN substrates, with threshold current-densities <10 kA/cm² and threshold voltages <10 V, approaching state-of-the-art values. We will report on details of the material quality and LD structure; and will discuss the advantages of MBE-grown LDs over MOVPE-LDs, resulting from fine growth control and no requirement for p-dopant activation. Therefore, the MBE-growth of nitrides has opened a new approach to efficient optoelectronic devices.

Tunable laser diodes with a tuning range of several tens of nanometers are generally being acknowledged as key components for future generation optical networks. However, all presently available devices suffer from several serious drawbacks. The most well-known issue is the time-consuming calibration procedure that has to be carried out for every single device. Recently the so-called sampled or superstructure grating tunable twin-guide or (S)SG-TTG laser diode has been suggested to overcome some of the prevailing problems. In this paper we will present tuning characteristics of first devices and discuss the influence of facet reflections on the tuning behaviour. The widely tunable twin-guide laser diodes operate around 1.55 μm and have a continuous tuning range of ~ 2 nm. However, by utilizing Vernier-effect tuning, the overall quasi-continuous tuning range is extended to 28 nm. Within this tuning range, five supermodes can be continuously tuned without the occurrence of any mode hops. The side-mode suppression ratio is kept between 25 and 37 dB.

Among various types of tunable lasers, distributed Bragg reflection lasers employing chirped gratings have shown a wide tuning range, but they are difficult to fabricate and require sophisticated control electronics. On the other hand, distributed feedback lasers, although their tuning range is narrower than DBR lasers, do not suffer from the aforementioned problems. Thermally tunable 1.5um DFB and DBR lasers with thin film heaters have been reported, but their heater currents were too high to find practical use. We report on novel tunable DFB lasers at 1.3um with thin film heaters deposited by focused ion beam that require much smaller heater currents than previously reported. The lasers were InGaAsP/InP multiple quantum well DFB lasers grown by metal-organic chemical vapor deposition. Pt heater strips were deposited by FIB processing on the top surface of the completed lasers. Our FIB process can control Pt film resistivities between 24 and 105 ohms-μm. The CW laser threshold was 12mA at room temperature without the heater current, the lasing wavelength was 1309.5nm at 27mW, and the sidemode suppression ratio was over 40dB. As the heater current increased to 13mA, the lasing wavelength shifted to 1314.4nm. The tuning efficiency of 5.9nm/W was almost twice that reported previously. FIB was also employed to deposit Au pads for the lasers. To the best of the authors' knowledge, this is the first application of FIB deposition processes to semiconductor lasers.

High temperature continuous-wave (CW) operation of 1310-nm single-frequency grating-outcoupled surface emitting (GSE) semiconductor lasers with output powers exceeding 3.0 mW into a multi-mode fiber, threshold currents below 30 mA and with > 30 dB side-mode suppression ratios at temperatures of up to 85 C are reported. These lasers consist of a 400 um long horizontal cavity, and a 15 um long second-order outcoupler grating sandwiched between 200 um long first-order distributed Bragg reflector (DBRs) gratings. Higher output powers can be achieved with longer outcoupler gratings. These GSE lasers operate at 3.125 Gbps and have a full-width at half-maximum (FWHM) beam divergence of 7 x 13 degrees.

Microresonators are of considerable interest in wavelength division multiplexing (WDM) applications due to their small feature size and versatile functionality. When combined with other high-Q passive and active elements connected through a common bus line, microresonator lasers will enable the fabrication of sophisticated photonic integrated circuits (PICs) that take full advantage of compact chip layouts. Circular microresonator lasers are attractive sources for PICs because of their cleavage-free cavity and excellent wavelength selectivity. We have demonstrated an InP/InGaAsP microdisk resonator laser, where a high-Q microdisk lasing mode is vertically coupled out through a straight bus waveguide. The vertically coupled design is realized by using wafer-boding techniques. By connecting multiple microdisk lasers through a common output bus line, we have demonstrated an 8-channel laser arrays with 1.6 nm (200 GHz) spectral channel spacing. The channel spacing is achieved by varying the disk resonator radii from 10.6 to 10.95 mm. Typical threshold current of ~ 7 mA is observed under CW lasing operation near 1510 nm. In this presentation we will discuss the operating characteristics of microresonator lasers and the use of resonators in laser cavities as wavelength selection and stabilization elements.

The microdisk laser is a microcavity laser, which strongly confines the whispering gallery mode and realizes the ultralow threshold lasing and large scale integration. This paper presents two recent topics related with this laser. The first topic is the microdisk with InAs quantum-dot active layer. It is not only promising for ultimate low threshold operation but also has the potential of a high-speed single photon emitter because of the atom-like electron transition in quantum-dots and the large Purcell effect in the microcavity. In this study, we obtained the room temperature operation in this device, for the first time, by photopumping with a low effective threshold of less than 1 milliwatt. The second topic is the photonic molecule consisting of evanescent-coupled multiple microdisks based on GaInAsP materials. As well as the unique behavior of coupled modes in this structure, we also observed the bistable lasing by the nonuniform photopumping, which was considered to be arising from the saturable absorption. Due to its low threshold of 70 microwatts, this device is expected to be an optical memory and flip-flop device by photonic integration.

For high performance operation of semiconductor lasers consisting of low-dimensional electron systems as the active medium, a good size uniformity and a formation of low-damage interfaces are essentially required. In this paper, we review our recent results obtained by GaInAsP/InP based long-wavelength lasers consisting of quantum-wire structures fabricated by electron beam lithography, CH4/H2-reactive ion etching and 2-step organo-metallic vapor-phase-epitaxial growth processes. Good size uniformity of vertically-stacked multiple-quantum-wire structures was obtained with a standard deviation of less than ±2 nm. By using a strain-compensated quantum-well structure as an initial wafer, non-radiative recombinations at etched/regrown interfaces were fairly reduced, which resulted in a room-temperature continuous-wave (RT-CW) operation of a quantum-wire laser for more than 15,000 hours. By taking an advantage of this fabrication method, 1540 nm wavelength quantum-wire distributed feedback (DFB) lasers were also realized for the first time with relatively low threshold current and high differential quantum efficiency under a RT-CW condition. Moreover, by utilizing the energy blue shift due to the lateral quantum confinement effect, a low threshold current operation with a stable single-mode property has been successfully demonstrated for distributed-reflector (DR) lasers consisting of a DFB section with wirelike active regions and a passive distributed-Bragg-reflector (DBR) section with narrow quantum-wire active regions.

We present an overview of the dual QC laser spectrometer developed at Aerodyne Research and various examples of its application for atmospheric trace gas detection. The instrument incorporates two pulsed QC lasers, a compact 76-m (or 56-m) multipass absorption cell, a dual HgCdTe detector, and a sophisticated signal generation, data acquisition and processing system. Recent findings and hardware innovations are highlighted. Our results show that the precision and minimal detectable absorbance obtainable with pulsed QC lasers are comparable to those achieved with cryogenically cooled CW Pb-salt lasers in spite of the broader laser linewidths inherent to pulsed operation. This is demonstrated through in situ measurements of several trace gases, including methane, nitrous oxide, carbon monoxide, formaldehyde, formic acid, nitrous acid and ethylene. Recent measurements of HCHO and HCOOH on board a NOAA aircraft are presented. The precision, stability and intrinsic accuracy of the instrument were assessed through inter-comparisons measuring CH4 and CO. These measurements were made either comparing two QC lasers sweeping over different transitions or comparing the dual QCL spectrometer and a standard instrument (NDIR CO). The absorbance precision achieved is typically 2x10^-5 Hz^-1/2. For long-lived species, such as CH₄ and N₂O, this implies 1-Hz fractional precisions of 0.1% or better, which fulfill the requirement for meaningful measurements from aircraft platforms. Spectroscopically derived mixing ratios are accurate within 5% or better. The spectrometer is equipped to perform automatic, periodic calibrations with zero and span gases whenever higher accuracy is required.

Quantum dots have demonstrated improved performance relative to quantum wells in lasers and amplifiers for structures where the total optical loss, and hence the gain required from the dot active material, has been kept low. In many applications higher gain and/or high differential gain are required and high gain structures must be routinely produced if quantum dots are to replace quantum wells in more than a few niche applications. The obvious approach is to use multiple layers of quantum dots in the active region of the laser or amplifier. However, stacking multiple quantum dot layers modifies the growth of subsequent layers and in the extreme case leads to defect formation. In this work we study an approach where the negative effects caused by the introduction of multiple layers of quantum dots are minimised using a high growth temperature spacer layer (HGTSL) to planarize the surface before deposition of the subsequent layer of dots. We show that this has a dramatic affect on the threshold current of our 1.3μm emitting lasers and by use of detailed characterisation show that this is due to 4 physical effects. Samples containing the HGTSL exhibit less inhomogenous broadening, have an increased dot density, a lower internal optical mode loss and contain fewer defects than samples containing a conventional spacer layer. Our results demonstrate the importance of going beyond an approach based on defect reduction alone.

We investigate tunneling injection quantum-dot (QD) lasers both theoretically and experimentally. Our laser structure consists of two tensile-strained quantum wells (QWs) coupled to a compressive-strained QD layer. The QWs serve as efficient carrier collectors and as a medium to inject electrons into the QDs by tunelling. Polarization-resolved amplified spontaneous emission (ASE) spectroscopy is used to extract the transverse-electric (TE) and transverse-magnetic (TM) polarized optical gain spectra at very low to near threshold injection currents. At a low bias current, the TE polarized ASE from the ground state of the QD layer is observed. At an intermediate current level, the coupling of the QW ground state to the QD excited state becomes important and an increase of the TM polarized emission from the tensile-strained QWs at a higher energy level becomes significant. Near threshold current, we observe TE gain narrowing due to the QD excited-state activation and the pinning of TM gain with subsequent TE lasing above threshold. We explain the physics of tunneling injection from the QWs into the QDs and how the tunneling injection affects the polarization resolved optical gain spectra as the injection current level increases.

Quantum-dot index-guided tapered lasers and laser arrays of seven tapered emitters were developed for improved beam quality and wavelength stability. They are compared with one quantum well devices. These lasers have less than 30 μm output width and do not show any measurable astigmatism (<±10 μm). The structures were grown at the University of Wuerzburg using Molecular Beam Epitaxy (MBE). The quantum dot tapered laser emits 1W CW at 1.6 A with 0.74 W/A external differential efficiency, 39% maximum wall plug efficiency, and 110 mA threshold current. The quantum well tapered laser emits 1.1W CW at 1.5 A with 0.87 W/A external differential efficiency, 49% maximum wall plug efficiency, and 52 mA threshold current. The beam quality parameter at 1W is better for the quantum dot tapered emitter (M²=2.9) than for quantum well (M²=3.5). Quantum well lasers have the highest power and efficiency, but quantum dot lasers show better beam quality and a high wavelength stability.

We address basic issues related to the dynamical properties of low dimensionality semiconductor lasers. We concentrate on quantum dash lasers but many of the properties hold, with very few changes, to quantum dot lasers. We formulate a self consistent semiclassical model for a multimode laser field which interacts with an inhomogeneously broadened assembly of quantum dashes. The key advantage of this model is the fact that it spectrally resolves the gain and multimode laser field. We show that the differential gain and the nonlinear damping can not be optimized simultaneously.

We report on the development of a new cost-effective, small form-factor laser source at a wavelength of 980 nm. The laser module is based on proven technology commonly used for pump laser modules deployed in fiber amplifiers of telecommunication networks. The package uses a state-of-the-art 14-pin butterfly housing with a footprint of 30x15 mm² with a Fabry-Perot AlGaAs-InGaAs pump laser diode mounted inside having an anti-reflection coating on its front facet. The light is coupled into a single-mode polarization-maintaining fiber with a mode-field diameter of 6.6 micrometer. The spectral properties of the source are defined by a fiber Bragg grating (FBG) that provides feedback in a narrow reflection band. The laser back facet and the FBG form a long resonant cavity of 1.7 m length in which laser light with a low coherence length of a few cm is generated. This configuration with the laser being operated in the coherence-collapse regime has the advantage of being robust against variations in the optical path, thus enabling stable and mode-hop free emission. The laser module has the following properties: a continuous-wave fiber output power exceeding 800 mW, a spectral bandwidth of less than 50 pm, a root-mean square power variation of less than 0.2 % from DC to 2 MHz over the entire power operating range, and a polarization extinction ratio of more than 20 dB. This is a compact, low noise, high power source for frequency conversion with nonlinear optical materials, such as blue light generation.

High-power semiconductor lasers with good beam quality are important for many applications. Angled broad-area waveguide structure could increase the output power and the beam quality is maintained at good quality. In this paper, we report the in-depth investigation on the geometrical factors that influence the performance of angled broad-area semiconductor lasers. The angled broad-area laser diodes are based on InGaAsP/InP materials. The waveguide is oriented at an angle from the normal of the cleaved facets. Our investigation shows that the waveguide width, the device length, and the tilted angle have to follow a certain relation in order to have the light propagate along a zigzag path inside the broad-area waveguide. When such a mode oscillates, the output light emits along the normal of the cleaved facet, showing no filamentation. Over 1 W of output power can be obtained with a good beam quality. Our investigation also discovers that devices with 100μm waveguide width perform better than devices with 50μm waveguide width.

We have developed Fabry-Perot lasers at λ=852nm, using an aluminium free active region with the aim to develop a single-frequency and single spatial mode device for atomic clocks and interferometry applications. The device is a separate confinement heterostructure with a GaInP large optical cavity and a 8nm compressive-strained GaInAsP quantum well. The broad-area (100μm wide) laser diodes are characterised by low internal losses (<3 cm^-1), a high internal efficiency (94%) and a low transparency current density (100A/cm²) which illustrates the quality of the laser structure. For an AR/HR coated 2mm long broad area laser diodes we measure a low threshold current density (245A/cm²) and a high slope efficiency (0.9 W/A). We obtain an optical power of more than 5.5W (I= 8.5A), under CW operation at 15°C, with a maximum wall-plug efficiency of 0.45. The lasing emission is achieved up to at least 115°C An optical power of more than 1.4W is obtained at 100°C (I=3.6A). A power of 1.2W (I=1.7A, 15°C) is achieved at 852nm. For an AR/HR coated 2mm long 4μm wide ridge waveguide laser diode, we obtain a low threshold current (46mA) and a high slope efficiency (0.9W/A). We obtain 852nm wavelength at 145mW (I=200mA, 15°C). We measure an optical power of 180mW (I=240mA) in a single spatial mode with the beam quality parameter M²=1.5. At 180mW both near and far field are gaussian-shaped with respective full widths at 1/e² of 6μm and 12°.

High-power spatially single-mode diode lasers at 1.4 - 1.5 μm wavelength are of interest as pump lasers for Raman and rare-earth doped fiber amplifiers as well as for material processing and for Light Detection and Ranging (LIDAR) at eye-safe wavelengths. A cost-efficient way to realize high-power high-brightness devices is the tapered resonator concept. We demonstrate InGaAsP/InP based diode lasers with compressively strained quantum wells and wavelengths around 1480 nm which were grown by solid source MBE. From broad area lasers with variations in quantum well number and waveguide layer thickness, parameters for the logarithmic gain model are deduced. With their implementation in 2-dimensional BPM simulations, an optimized resonator geometry is derived. Devices employ a 500 μm ridge section followed by a 2000 μm taper section with 6° angle. Continuous-wave (cw) output powers reach more than 1.5 W. Beam quality is characterized in terms of near field and far field distribution, M², and astigmatism. An excellent agreement is found between measurement and simulation. For narrow-linewidth operation, devices are provided with anti-reflection coatings on both facets and spectrally stabilized with an external grating. We achieve 0.7 W single mode power and a side mode suppression ratio (SMSR) of 42 dB. Reliability is tested in terms of facet stability and lifetime. Pulsed measurements reveal a power stability up to more than 5 MW/cm². From cw aging tests at 1 W output power, lifetimes of about 6,000 h are extrapolated.

Experimental investigations on RW and BA DFB lasers emitting in the wavelength range between 760 and 790 nm are presented. The maximum output powers are 300 mW and 2.4 W for the RW and BA devices, respectively. The optical spectra of the RW DFB lasers show single mode emission with a side-mode suppression ratio of about 50 dB. The profile of the lateral far field reveals stable lasing of the fundamental lateral mode without any beam steering up to 250 mW power. The spectral linewidth of the RW devices is < 2 MHz and sufficiently small for spectroscopic applications (e.g. D₂ line of rubidium vapor). The BA devices have a full l/e² width of the spectrum of 0.08 nm at 0.5 W and 0.16 nm at 2 W.