This PDF file contains the front matter associated with SPIE Proceedings Volume 7616, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The authors report on the nanowires-like and nanodots-like lasing behaviors in addition to multiple-wavelength interband transitions from InAs/InAlGaAs quantum dash (Qdash) lasers in the range of ~1550 nm. The presence of lasing actions simultaneously from two different dash ensembles, after postgrowth intermixing for crystalline quality improvement, indicate the absence of optical phonon emission due to the small variation in quantized interband transition energies. This effect is reproducible and shows different lasing characteristics from its quantum dot and quantum wire laser counterparts. Furthermore, the small energy spacing of only 25 nm (at center lasing wavelength of ~1550 nm) and the subsequent quenching of higher energy transition states at higher bias level in Qdash lasers suggest the absence of excited-state transition in highly inhomogeneous self-assembled Qdash structures. However, the appearance of a second lasing line in a certain range of high injection level, which is due to the presence of different sizes of dash assembles, corresponds to the transition from smaller size of Qdash ensembles in different planar active medium. This unique transition mechanism will affect the carrier dynamics, relaxation process in particular and further indicates localized finite carrier lifetime in all sizes of Qdash ensembles. These phenomena will lead to important consequences for the ground-state lasing efficiency and frequency modulation response of Qdash devices. In addition, these imply that proper manipulation of the Qdash ensembles will potentially result in localized nanolasers from individual ensemble and thus contributing towards enormously large envelope lasing coverage from semiconductor devices.

The processes which control the occupation of quantum dot (QD) states have a major influence on the temperature dependence of threshold current and the modulation speed of QD dot lasers. Using variable stripe length measurements we have investigated the occupation of dot states of different energy as a function of temperature using structures which have a bimodal size distribution which allows us to distinguish emission from two groups of dots ("large" and "small") with different energy states located in different dots, and emission from ground and excited states in the same size group being of different energy but located in the same dot. Between 200 K and 80 K spontaneous emission from higher states on the small dots increases relative to the emission at lower energy from larger dots which indicates a transition to nonthermal population. At 80 K and 20 K, we observe a linear relation between the emission of the ground states of the small and large dots as a function of current, even though they are many (kT) apart and the slope indicates that states of different energy are populated with the same probability. From measured gain spectra we find a local minimum at 180 K in the radiative threshold current density, which is due to an increase in recombination from higher lying states of the small dots as the temperature is reduced. The computed threshold current for thermal occupation at 300 K and random population at 20 K is in excellent agreement with these results.

Quantum Dot lasers exhibit the novel phenomenon of dual state lasing where population inversion can be achieved on two optical transitions within the dots. In principle this might occur if a phonon bottleneck exists to impede relaxation of carriers from the higher energy state. Here we present an alternative explanation whereby different lasing modes compete for carriers and are spatially separable. Evidence comes from a comparison of electrical and optical measurements made on the devices. The evolution of a particular lasing mode depends on diffusion of carriers between dots and we show how, using an equivalent circuit model, this is consistent with our measurements.

Interest in quantum dot mode-locked lasers (QD MLLs) has grown in recent years since their first demonstration in 2001 as applications for optical time domain multiplexing, arbitrary waveform generation, and optical clocking are anticipated. Ultrafast pulses below 1 ps have been reported from QD MLLs using intensity autocorrelation techniques, but so far detailed characterization examining the pulse shape, duration, chirp, and degree of coherence spiking in these lasers has not been carried out. We describe the first direct frequency-resolved optical gating (FROG) measurements on a QD MLL operating at a repetition rate of 5 GHz.

This paper reports the improvements and limitations of MBE grown 1.3μm GaAsSb/GaAs single QW lasers. At room temperature, the devices show a low threshold current density (J_th) of 253 Acm^-2, a transparent current density of 98 Acm^-2, an internal quantum efficiency of 71%, an optical loss of 18 cm^-1 and a characteristic temperature (T₀) = 51K. The defect related recombination in these devices is negligible and the primary non-radiative current path has a stronger dependence on the carrier density than the radiative current contributing to ~84% of the threshold current at RT. From high hydrostatic pressure dependent measurements, a slight decrease followed by the strong increase in threshold current with pressure is observed, suggesting that the device performance is limited to both Auger recombination and carrier leakage.

We demonstrate how GaAs/AlGaAs regrowth upon patterned InGaP can be utilised to realise self-aligned lasers, window structured superluminescent diodes and distributed feedback lasers. Such realisation demonstrates the promise of this methodology for GaAs-based opto-electronic integrated circuits through new capability for buried waveguides, low reflectivity facets and gratings structures.

In this paper we review the recent progress on the development of novel quantum-dot structures and laser devices. The investigation of novel regimes of ultrashort pulse generation in quantum-dot edge-emitting lasers will be presented. We illustrate how new functionalities have been opened up, such as dual-wavelength mode-locking and enhanced tunability, through the exploitation of the excited-state transitions in the quantum dots as an additional degree of freedom in these ultrafast lasers. Progress on novel design rules for quantum-dot based vertical external cavity lasers and SESAMs are also considered.

Higher-order harmonic repetition rate generation in quantum dot mode-locked lasers (QDMLLs) was realized using a double interval technique. Using this approach, a wider operation range and improved mode-locking performance was demonstrated for generating the 6th harmonic of the fundamental repetition rate. Without changing the layout of the device, mode-locking at a repetition rate of 60 GHz, which corresponds to the 10th harmonic of the fundamental frequency of the QDMLL, was achieved which cannot be realized utilizing the single absorber technique.

This work investigates the linewidth enhancement factor (alpha-factor) and stability of an optically-injected InAs/InGaAs quantum-dot Fabry-Perot laser. Using the injection-locking technique, the above threshold alpha-factor is measured to be as low as 0.6 at 1.3X the threshold current. The below threshold alpha-factor is also measured using the Hakki-Paoli technique. The measured alpha-factor values are used to simulate the dynamic response (stable locking, period-one, period-doubling, or chaos) in the context of single-mode rate equations under zero-detuning injection conditions for external injected power ratios ranging from -11dB to +15dB and slave current bias levels of 1.3X, 2X, and 2.6X threshold. Legacy literature has shown that optically-injected diode lasers typically follow the period-doubling route into a chaotic region as the injection level is increased. Simulations show that at 2X the threshold current, a small region of period-one operation will be observed followed by stable-locking as the injection ratio is increased. This predominantly stable behavior is driven largely by the low alpha-factor. Experimental results support this prediction, where under zero-detuning conditions, only unlocked and stable-locking operation is observed. Experimentally, periodone operation was not observed at a slave laser bias current of 2X threshold, as it was predicted to occur below an external power ratio of -20 dB, a level which was not attainable in this work. Such findings suggest that a quantum-dot device can be employed in an optically-injected configuration for photonic tunable-clock applications.

Mobile laser projection is of great commercial interest. Today, a key parameter in embedded mobile applications is the optical output power and wall plug efficiency. We report improvements of the performance of true blue single mode InGaN laser at 450nm with output power of more than 200mW in cw operation for temperatures between 20°C and 60°C. We succeeded in temperature independent high wall plug efficiency of 15-18% for stable output power levels from 100 to 200mW with estimated life times >4000h in cw operation. Furthermore, we present pioneering studies on long green InGaN laser diodes. The technological challenge is to achieve InGaN-quantum wells with sufficiently high material quality for lasing. We investigate the density of non radiative defects by electro-optical measurements confirming that low defect densities are essential for stimulated emission. Laser operation at 516nm with more than 50mW output power in cw operation is demonstrated in combination with a high wall plug efficiency of 2.7%.

Results for long-wavelength emitters are presented for semi-polar InGaN/AlGaN/GaN heterostructures grown on GaN(1122)/m-sapphire templates by metalorganic chemical vapor deposition. The semi-polar GaN layers were 10 to 25 μm thick and grown by HVPE on sapphire substrates. X-ray diffraction measurements indicated high crystallographic quality that approaches that of GaN(0001) layers on sapphire. A comparison based on optical pumping experiments, low- and high-density excitation photoluminescence experiments, and atomic force microscopy is drawn between InGaN/GaN quantum well laser heterostructures grown by metalorganic vapor phase epitaxy either on either polar GaN(0001)/c-sapphire or on semi-polar GaN(1122)/m-sapphire. C-plane InGaN/GaN/sapphire structures exhibited low threshold pump power densities < 500 kW/cm² for emission wavelengths up to 450 nm. For laser structures beyond 450 nm the threshold pump power density rapidly increased resulting in a maximum lasing wavelength of 460 nm. Semipolar InGaN/GaN(1122)/m-sapphire structures showed a factor of 2-4 higher threshold pump power densities at wavelengths below 440 nm which is partly due to lower crystalline perfection of the semi-polar GaN/sapphire templates. However, at longer wavelengths > 460 nm the threshold power density for lasing of semi-polar heterostructures is less than that for c-plane heterostructures which enabled rapid progress to demonstration of lasing at 500 nm wavelength on semi-polar heterostructures. The absence of V-type defects in semi-polar, long-wavelength InGaN/GaN structures which are usually present in long-wavelength c-plane InGaN/GaN structures is attributed to this phenomenon.

Mid-infrared intersubband absorption in InGaAs/GaAsSb multiple quantum wells grown lattice-matched to InP substrates by molecular beam epitaxy has been studied experimentally. Intersubband absorption in a broad wavelength region (5.8 - 11.6 μm) is observed in samples with well widths ranging between 4.5 and 12 nm. A conduction band offset at the InGaAs/GaAsSb heterointerface of 360 meV gives an excellent agreement between the theoretically calculated ISB transition energies and the Fourier-transform infrared spectroscopy measurements over the whole range of well widths under investigation. Two kinds of intersubband devices based on the InGaAs/GaAsSb material system are presented: a quantum well infrared photodetector operating at a wavelength of 5.6μm and an aluminum-free quantum cascade laser. The presented quantum cascade laser emits at a wavelength of 11.3 μm, with a threshold current density of 1.7 kA/cm² at 78 K.

In this work we show that by using both deep quantum wells and tall barriers in the active regions of quantum cascade (QC)-laser structures and by tapering the conduction-band edge of both injector an extractor regions one can significantly reduce the leakage of the injected carriers. Threshold-current, J_th and differential-quantum efficiency, η_d characteristic temperatures, T₀ and T₁, values as high as 278 K and 285 K are obtained to 90 °C heatsink temperature, which means that J_th and η_d vary ~ 2.5 slower over the 20-90 °C temperature range than in conventional QC devices. Modified equations for J_th and η_d are derived. In particular, the equation for η_d includes, for the first time, its dependence on heatsink temperature. A model for the thermal excitation of injected carriers from the upper lasing level to upper active-region energy states from where they relax to lower active-region energy states or get scattered to the upper Γ miniband is employed to estimate carrier leakage. Good agreement with experiment is obtained for both conventional QC lasers and deep-well (DW)-QC lasers.

We report a surface-emitting THz source based on intracavity difference-frequency generation in dual-wavelength midinfrared quantum cascade lasers with integrated giant second-order nonlinear susceptibility. The THz light is coupled out of the waveguide by a second-order grating etched into the laser ridges. In contrast to sources where the difference-frequency radiation is emitted from the facet, this approach enables extraction of the THz emission from the whole length of the device even when the coherence length is small. We also studied the properties of the mid-infrared pump beams and found that due to gain competition, mid-infrared modes tend to start lasing in higher order lateral modes. The mid-infrared mode with the lower threshold current reduces population inversion for the second laser with the higher threshold current due to stimulated emission. We developed a rate equation model to quantitatively describe mode interactions due to mutual gain depletion.

The integration of thin film edge emitting lasers onto silicon enables the realization of planar photonic structures for interconnection and for miniaturized optical systems that can be integrated in their entirety at the chip scale. These thin film emitters are compound semiconductor lasers that are optimized for operation without the growth substrate. Removal of the laser growth substrate, coupled with bonding to the silicon host substrate, enable the integration of high quality edge emitting lasers with silicon. This paper explores the challenges, approaches, fabrication processes, and progress in the integration of thin film edge emitting lasers integrated onto silicon.

In this paper we review the recent progress in developing compact microring lasers on the hybrid silicon platform. A simplified self-aligned process is used to fabricate devices as small as 15 μm in diameter. The optically-pumped, continuous wave (cw) devices show low threshold carrier density, comparable to the carrier density to reach material transparency. In the electrically-pumped lasers, the short cavity length leads to the minimum laser threshold less than 5 mA in cw operation. The maximum cw lasing temperature is up to 65 °C. Detailed studies in threshold as a function of coupling coefficient and bus waveguide width are presented. Surface recombination at the dry-etched exposed interface is investigated qualitatively by studying the current-voltage characteristics. Ring resonator-based figures of merits including good spectral purity and large side-mode suppression ratio are demonstrated. Thermal impedance data is extracted from temperature-dependent spectral measurement, and buried oxide layer in silicon-on-insulator wafer is identified as the major thermal barrier to cause high thermal impedance for small-size devices. The demonstrated compact hybrid ring lasers have low power consumption, small footprint and dynamic performance. They are promising for Si-based optical interconnects and flip-flop applications.

The material system comprising GaSb, InAs, AlSb and their related alloys are an impressive toolbox for device designers, as they offer a very large choice of band-gaps and band offsets. Molecular beam epitaxy (MBE) and device processing have been improving quickly over recent years, allowing the fabrication of high performance devices as quantum cascade lasers, mid-infrared (MIR) edge emitting and surface emitting lasers, superlattice infrared photodetectors, but also very high speed / low consumption AlSb/InAs field effect transistors. Efforts have been made to monolithically grow these devices onto larger and cheaper substrates like GaAs and Si, to improve the yield / decrease the cost of this technology and possibly integrate the devices with CMOS technology. We recently fabricated a 2.3 μm edge emitting laser grown by MBE on a Si substrate, and demonstrated roomtemperature pulsed operation. Lasers emitting at this wavelength are of particular interest for gas sensing. Challenges to further improve the device include the substrate preparation, optimization of the nucleation layer quality, but also the conduction band engineering in order to facilitate the electronic transport at the Si/III-Sb interface.

The basic design considerations for a spectrally-stable DBR semiconductor laser specifically designed for pulsing in the nanosecond regime is presented, along with test results from devices fabricated according to these design parameters. Results show excellent mode selection and spectral stability over an extremely large range of conditions, including temperature ranges of 15-60°C and peak drive current ranges from threshold to 880 mA. These lasers exhibit peak output powers of greater than 500 mW for DBR semiconductor lasers at 976 nm and 1064 nm while remaining spectrally stable. Chirp data shows the chirp can be effectively tuned from approximately 1 GHz to greater than 20 GHz by varying the pulse width and peak drive current.

For the past several years, we have been developing a new class of high-power, low-noise semiconductor optical gain medium based on the slab-coupled optical waveguide (SCOW) concept. The key characteristics of the SCOW design are (i) large (> 5 x 5 μm), symmetric, fundamental-transverse-mode operation attained through a combination of coupledmode filtering and low index-contrast, (ii) very low optical confinement factor (Γ ~ 0.3-0.5%), and (iii) low excessoptical loss (αi ~ 0.5 cm^-1). The large transverse mode and low confinement factor enables SCOW lasers (SCOWLs) and amplifiers (SCOWAs) having Watt-class output power. The low confinement factor also dictates that the waveguide length be very large (0.5-1 cm) to achieve useful gain, which provides the benefits of small ohmic and thermal resistance. In this paper, we review the operating principles and performance of the SCOW gain medium, and detail its use in 1550-nm single-frequency SCOW external cavity lasers (SCOWECLs). The SCOWECL consists of a doublepass, curved-channel InGaAlAs quantum-well SCOWA and a narrowband (2.5 GHz) fiber Bragg grating (FBG) external cavity. We investigate the impact of the cavity Q on SCOWECL performance by varying the FBG reflectivity. We show that a bench-top SCOWECL having a FBG reflectivity of R = 10% (R = 20%) has a maximum output power of 450 mW (400 mW), linewidth of 52 kHz (28 kHz), and RIN at 2-MHz offset frequency of -155 dB/Hz (-165 dB/Hz).

Low levels of intensity noise in semiconductor lasers is a key feature for numerous applications such as high resolution spectroscopy, fiber-optic sensors, signal distribution in broadband analog communications as CATV, and more generally for microwave photonics systems. In particular, a DFB laser with very low relative intensity noise (RIN) levels from 0.1 to 20 GHz is a key component as it correspond to the whole frequency bandwidth of interest for radars. Several approaches have been reported but most suffer from the compromise between RIN level and power out level and stability, with RIN level in the range -150 dB.Hz^-1 to -155 dB.Hz^-1 in this frequency range [1,2]. We report here results from a new AlGaInAs DFB laser developed at 3S PHOTONICS. Excellent device performance is observed across an operating range from the laser threshold up to the thermal roll-over. Pure longitudinal single mode at 1545 nm is obtained over the whole current operating range with side mode suppression ratio higher than 50dB. The maximum output power reaches up to 130 mW. In these conditions, RIN levels below -160 dB.Hz^-1 is obtained in up to 20 GHz. These are the best results to our knowledge combining such high single mode output power with such low RIN level in the frequency range 0.4-20 GHz.

The development of techniques such as atom optical pumping, for atomics clocks or precise gyroscopes, requires laser diodes with high power and excellent spectral (narrow linewidth) and spatial qualities together with high reliability. We have realized a six months ageing test on Al-free DFB lasers emitting at 852nm for Cs pumping. Ten DFB lasers were aged at 40°C and 20mW. The extrapolated lifetimes at 40°C, based on 20mW operating current, of our DFB lasers are higher than 500000 hours which confirms the excellent potential of this Al-free technology for long life spatial mission. Furthermore, the evolution of the operating current (initially around 70mA), after six months, is less than 5% (corresponding to 3mA). We obtain a very good stability of optical spectra: an average variation of the Side Mode Suppression Ratio (SMSR) of less than 2dB and a variation of the wavelength of less than 0.12 nm. We also measured the linewidth of our DFB lasers with the delayed self-heterodyne method after the six months ageing: we obtain a very narrow linewidth at 25°C (measurement temperature) around 215kHz (lorentzian fit, white noise) or 330kHz (gaussian fit, 1/f noise).

We present high-power ridge waveguide (RW) distributed feedback (DFB) lasers and DFB master optical power amplifiers (DFB-MOPAs) with high-quality beams optimized for pulsed operation and current modulation. A Bragg grating ensures stable longitudinal single-mode emission around 1064 nm. Furthermore, vertical and lateral structures of the devices were optimized for stable fundamental-mode operation. The slope efficiency of 1 mm DFB lasers slightly above threshold is as high as 0.95 W/A and the continuous wave (cw) optical output power is almost 400 mW at a current of 500 mA. For the DFB-MOPAs a cw output power of 1 W has been obtained. Due to low ellipticity, low divergence and low beam steering of the output beams the devices are well suited for efficient coupling to single mode waveguides.

We review the development of high performance, short wavelength (3 μm < λ < 3.8 μm) quantum cascade lasers (QCLs) based on the deep quantum well InGaAs/AlAsSb/InP materials system. Use of this system has enabled us to demonstrate room temperature operation at λ ~ 3.1 μm, the shortest room temperature lasing wavelength yet observed for InP-based QCLs. We demonstrate that significant performance improvements can be made by using strain compensated material with selective incorporation of AlAs barriers in the QCL active region. This approach provides reduction in threshold current density and increases the maximum optical power. In such devices, room-temperature peak output powers of up to 20 W can be achieved at λ ~ 3.6 μm, with high peak powers of around 4 W still achievable as wavelength decreases to 3.3 μm.

A new theoretical method for describing QCLs is presented. The method extends the familiar incoherent rate equations to include coherence. Smooth output power and voltage curves as function of current are obtained at very modest computational effort. Coherence is shown to play an important role in QCLs. The method is derived by imposing the following requirements on the equations of motion of the density matrix: 1. Expectation values should be independent of the choice of basis; 2. The density matrix should be positive definite; 3. The model should reduce to existing rate-equation models when coherences are omitted.

We demonstrate surface-emitting distributed-feedback (DFB) quantum cascade (QC) lasers - operating in the midinfrared - with double-slit 2nd order DFB gratings which are implemented via the sole patterning of the top metallic layer, directly at the top of the active region. The small-slit design allows one to reduce the high plasmonic losses which would be induced by a standard 2nd order grating. The devices operate at room temperature, in pulsed mode, and exhibit single mode emission at 7.2 μm wavelength. The device far-field consists of an almost single-lobed emission with a low divergence (<1 degree) in the grating direction. This result shows that sub-wavelength patterning of the top metallization layer of mid-infrared surface-plasmon QC lasers allows one to reduce the losses even for higher-order DFB grating devices.

We report on the measurement of both transverse and in-plane heat transfer speed in GaAs/AlGaAs quantum cascade lasers via time-resolved micro-probe photoluminescence. We found approximately a one order of magnitude reduction in the heat transverse speed with respect to bulk values that we ascribed to the heat interface boundary resistance. We also compared the non equilibrium population of phonons via anti-Stokes Raman scattering in different active region configurations.

We report the fabrication and measurement results of an integrated distributed Bragg reflector (DBR) tunable buriedheterstructure (BH) quantum-cascade-laser (QCL) emitting around λ ~ 4.7 μm. We also demonstrated a special laser device bonding technique by utilizing two submounts for the gain and DBR grating sections of the laser. As a result, separated temperature controls of the two sections were successfully accomplished. At 293K, a wavelength tuning from 4.605 μm to 4.670 μm was obtained when the grating section temperature was changed by a miniature thermoelectric (TE) cooler. This is corresponding to a tuning range of 65 nm or equivalently 29.831 cm^-1. The tuning efficiency is 0.0325 nm/mA. This technology is particularly desirable for chemical sensing, spectroscopic measurements and medical applications.

We review the state-of-the-art performance of interband cascade lasers emitting in the 3-5 μm spectral band and discuss the prospects for future improvements. New five-stage designs produce a combination of pulsed roomtemperature threshold current densities of 400-500 A/cm² and internal losses as low as ≈ 6 cm^-1 for broad-area devices. A 4.4-μm-wide ridge fabricated from one of these wafers and emitting at 3.7 μm lased cw to 335 K, which is the highest cw operating temperature for any semiconductor laser in the 3.0-4.6 μm spectral range. A 10-μm-wide ridge with high-reflection and anti-reflection facet coatings produced up to 59 mW of cw power at 298 K, and displayed a maximum wall-plug efficiency of 3.4%. Corrugated-sidewall distributed-feedback lasers from similar material produce 45 mW of cw power in a single spectral mode at -20°C, with maximum wall-plug efficiency of 7.6%. The current tuning range for temperatures between 0 and 25°C is ≥11 nm.

We demonstrate room temperature electroluminescence from intersublevel transitions in self-assembled InAs quantum dots in GaAs/AlGaAs heterostructures. The quantum dot devices are grown on GaAs substrates in a Varian Gen II molecular beam epitaxy system. The device structure is designed specifically to inject carriers into excited conduction band states in the dots and force an optical transition between the excited and ground states of the dots. A downstream filter is designed to selectively extract carriers from the dot ground states. Electroluminescence measurements were made by Fourier Transform Infrared Spectroscopy in amplitude modulation step scan mode. Current-Voltage measurements of the devices are also reported. In addition, both single period and multi-period devices are grown, fabricated, characterized, and compared to each other. Finally, we discuss the use of plasmonic output couplers for these devices, and discuss the unique emission observed when the quantum dot layer sits in the near field of the plasmonic top contacts.

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. Over the past several years, significant progress has been made in developing efficient IC lasers, particularly in the 3-4 μm region where continuous wave (cw) operation was achieved at above room temperature with low power consumption. In this paper, we report our recent efforts in the development of IC lasers at longer wavelengths (4.3-7.5 μm) based on InAs substrates and plasmon-waveguide structures. Cw operation of plasmon-waveguide IC lasers has been achieved at temperatures up to 184 K near 6 μm. Also, improved thermal dissipation has been demonstrated with the use of the plasmon waveguide structure.

Tapered semiconductor lasers have demonstrated both high power and good beam quality, and are of primary interest for those applications demanding high brightness optical sources. The complex non-linear interaction between the optical field and the active material requires accurate numerical simulations to improve the device design and to understand the underlying physics. In this work we present results on the design and simulation of tapered lasers by means of a Quasi- 3D steady-state single-frequency model. The results are compared with experiments on Al-free active region devices emitting at 1060 nm. The performance of devices based on symmetric and asymmetric epitaxial designs is compared and the influence of the design on the beam properties is analyzed. The role of thermal effects on the beam properties is experimentally characterized and analyzed by means of the numerical simulations. Tapered lasers with separate electrical contacts in the straight and tapered sections, based on symmetrical and asymmetrical epitaxial designs are also presented and analyzed.

High-power (more than 500 mW) and high-speed (more than 1 Gbps) tapered lasers at 1060 nm are required in freespace optical communications and (at lower frequencies of around 100 MHz) display applications for frequency doubling to the green. On a 4 mm-long tapered laser, we have obtained an open eye diagram at 700 Mbps, together with a high extinction ratio of 19 dB, a high optical modulation amplitude of 1.6 W, and a very high modulation efficiency of 19 W/A. On a 3 mm long tapered laser, we have obtained an open eye diagram at 1 Gbps, together with a high extinction ratio of 11 dB, an optical modulation amplitude of 530 mW, and a high modulation efficiency of 13 W/A.

The catastrophic optical mirror damage (COMD) effect is analyzed for 808 nm emitting diode lasers in single-pulse operation. During each single pulse, both nearfield of the laser emission and thermal image of the laser facet are monitored with cameras being sensitive in the respective spectral regions. A temporal resolution in the μs-range is achieved. The COMD is unambiguously related to the occurrence of a 'thermal flash' detected by thermal imaging. A one-by-one correlation between emission nearfield, 'thermal flash', thermal runaway, and structural damage is observed. As a consequence of the single-pulse-excitation technique, the propagation of 'dark bands' as observed in photo- or cathodoluminescence maps in the plane of the active region from the front facet is halted after the first pulse. Because of the rapidness of the thermal runaway, we propose the single-pulse technique for testing the facet stability and the intentional preparation of early stages of COMD; even for diode lasers that regularly fail by other mechanisms.

650 nm tapered laser diodes with nearly diffraction limited beam quality are requested for laser display applications. In this paper, results for 2 mm long 650 nm tapered lasers diodes with different lateral geometries will be presented. The vertical structure is based on a 5 nm thick InGaP single quantum well embedded in AlGaInP waveguide and n- AlInP and p-AlGaAs cladding layers. The ridge waveguides of lengths LRW = 200 μm, 300 μm, 500 μm, and 750 μm were fabricated using selective etching. The contact window for the tapered section was defined applying ion implantation. Devices with a taper angle of 4° were manufactured. The facets were passivated. The rear side was high reflection coated and the taper side anti reflection coated. The devices were mounted p-side down on CVDdiamond heat spreaders and standard C-mounts. All devices reached a maximal output power larger than 1 W. They had a threshold current density of about 700 A/cm² and a slope efficiency of 0.8 W/A. The best conversion efficiency was 20%. The devices with the shortest RW-length LRW = 200 μm had the best beam quality (beam waist width 7 μm, far field angle 8.8°, 85% of the emitted power in central lobe, M² of 1.3 (all values measured at 1/e²-level)) at P = 1 W. The beam quality for devices with longer RW-section deteriorates up to M² = 4.4 for a LRW = 750 μm laser.

We have designed, fabricated and measured the performance of two types of edge emitting lasers with unconventional waveguides and lateral arrays thereof. Both designs provide high power and low divergence in the fast and the slow axis, and hence an increased brightness. The devices are extremely promising for new laser systems required for many scientific and commercial applications. In the first approach we use a broad photonic crystal waveguide with an embedded higher order mode filter, allowing us to expand the ground mode across the entire waveguide. A very narrow vertical far field of ~ 7° is resulting. 980 nm single mode lasers show in continuous wave operation more than 2 W, η_wp ~ 60%, M² ~ 1.5, beam parameter product of 0.47 mm×mrad and a brightness ~ 1×10⁸ Wsr^-1cm^-2 respectively. First results on coherent coupling of several lasers are presented. In the second approach we use leaky designs with feedback. The mode leaks from a conventional waveguide into a transparent substrate and reflects back, such that only one mode at a selected wavelength is enhanced and builds up, others are suppressed by interference. 1060 nm range devices demonstrate an extremely narrow vertical far field divergence of less than 1°.

Experimental results from a Q-switched three section tapered laser, with an InGaAs triple quantumwell (TQW) active region, consisting of a 2000 μm long tapered element, a 1000 μm ridge waveguide (RW) section and a 1000 μm long passive grating section with a 6^th order surface grating were fabricated and investigated. For the generation of short optical pulses the tapered section was biased with a dc-current and the RW absorber section was modulated by a 1 GHz sinusoidal current. By biasing the tapered section with 4 A cw current and GHz sinusoidal modulation of the RW section, short pulses of ~75 ps width and a pulse power of 6.3 W corresponding to 470 pJ pulse energy is reached. The Q-switched pulses are single mode with a central emission wavelength of ~1064 nm and a FWHM spectral width ≤ 0.05 nm.

Single-transverse-mode semiconductor laser diodes with broad emission spectrum in pulsed or CW regime are attractive as seed sources in fiber laser systems. Stimulated Brillouin scattering can be a limiting factor in such systems, causing damaging high power pulses to reverse propagate in the fibre. The effect can be significantly mitigated by broadening the linewidth of the seed laser. Here we report on such a seed source capable of operating in either CW or pulsed mode with a center wavelength at around 1060 nm and spectral full width at half maximum of greater than 10 nm. The new source is based on well-established ridge waveguide pump laser technology, modified for operation in a superluminescent regime. A coupling efficiency of ~80 % into a single mode fiber is achieved. Our time resolved spectral studies show that the device is demonstrating fast modulation rate and very high peak optical power up to 1 W while maintaining a broad emission spectrum greater than 10 nm.

The most promising concept to achieve high-output power together with a good beam quality is the tapered laser consisting of a straight ridge waveguide (RW) section and a tapered gain-region. The RW section should support only the fundamental guided mode and should suppress higher order modes. The taper angle has to be selected with respect to the lateral divergence of the beam propagating from the RW to the tapered section. High brightness tapered devices in the wavelength range between 635 nm and 1085 nm will be presented. For red emitting tapered lasers around 650 nm, the output power is limited to about 1 W due to the properties of the laser material. At this output power a beam propagation ratio M2 of 1.3 and a brightness of 100 MW•cm^-2•sr^-1 will be shown. Devices made from laser structures with low vertical divergence down to 25° (95% power included) without a significant deterioration of device parameters will be presented for the longer wavelength range near 1 μm. For tapered lasers manufactured from these structures, nearly diffraction limited output powers larger than 10 W and a brightness of 1 GW•cm^-2•sr^-1 were measured.

We describe the design, simulation, fabrication and operation of ring cavity surface emitting lasers (RCSEL) based on quantum cascade structures for the midinfrared (MIR) and terahertz (THz) spectral range. MIR RCSELs facilitate an enhancement of optical power and a reduction in threshold current density, as compared to Fabry-Perot (FP) lasers. In continuous wave operation the maximum temperature of ring based devices is 50 K higher than in FP emitters. Also in THz QCLs a twofold increase in radiation efficiency is observed when compared to FP lasers. The emitters exhibit a robust single-mode operation around 8 μm and 3.2 THz, with a side mode suppression ratio of 30 dB. The ring-shaped resonator forms symmetric far-field profiles, represented by a lobe separation of ~1.5° and ~15° for MIR and THz lasers, respectively.

We report in this work the near-Infrared optical quenching effects on mid-Infrared Quantum Cascade Lasers (QCLs). The quenching effect is both intensity and wavelength dependent. A group of strain-compensated InGaAs/InAlAs, 4.8μm mid-IR QCLs, were used in the experiment. The pump lasers are near-IR lasers with wavelengths ranging from 1550nm to visible wavelength around 500nm. When the pump lasers have their lasing wavelengths shorter than 950 nm, it seems that majority of the generated electrons are excited above the conduction band edge of the InAlAs material and be swept off as photoconductive current without significantly affecting the QCL intersubband operations. So their quenching effect is weaker. We also observe that a near-IR laser with good quenching ability can modulate the mid-IR laser with speeds way above 100 MHz, which excludes the possibility of a thermal origin of these results.

We describe the effects created by selective oxidation of high aluminium content AlGaAs layers at the facets of 5-stack quantum dot edge-emitting 50μm stripe lasers. The steam oxidation affects only the facet areas of the devices, where unpumped sections are created. These unpumped regions alone enable reduction of the width of the lasing near-field spatial profile of up to 65% and the reduction of threshold in long devices by up to 30%. These effects are attributed to saturable absorber-type behaviour, where the absorber saturates first at the location of highest optical intensity, so allowing lasing over a smaller spatial area. Secondly, a combination of self-heating at the facets and the saturable absorption generates novel saw-toothed wavelength-time profiles. A model for the behaviour behind all of these results is proposed and backed up with experimental data.