A. S. Payusov

A 1.33 µm InAs/GaAs quantum dot laser with a 46 cm−1 modal gain

We report on 1.33 µm quantum dot (QD) lasers grown on GaAs substrates that show a modal gain of 45 cm−1, low threshold current density of 150 A cm−2 and room-temperature continuous wave output power of 2.5 W. The active region is based on ten InAs/InGaAs/GaAs quantum dot layers formed by activated phase separation. High structural quality of the active region is achieved, owing to minimization of the total amount of strained material per QD layer. The optical confinement factor is increased by exploiting high Al composition (80%) in the cladding layers. A modal gain over 20 cm−1 in the 1315–1345 nm wavelength range is revealed by the Hakki–Paoli technique at a low current density of 500 A cm−2.

Wavelength-stabilized tilted wave lasers with a narrow vertical beam divergence

We studied laser diodes grown in the tilted wave geometry with cleaved facets. In this approach a cavity with the gain medium is coupled to the second cavity, while the phase matching of the modes of the two cavities results in the wavelength stabilization. The mode separation can be controlled by the tilt angle of the leaky wave emission and the thickness of the coupled cavity. In one case a ~100 µm thick transparent substrate with a polished and dielectric-coated back surface was used as a coupled waveguide. In the second case, a 10 µm thick GaAs layer followed by an InGaP evanescent reflector was applied. We observed an increase in the lasing mode wavelength spacing and the width of the vertical far-field lobes from ~0.7° to 5° (full width at half maximum, FWHM) with the reduction of the thickness of the coupled cavity, in agreement with expectations. The FWHM numbers correspond to the diffraction limit for 100 and 10 µm thick coupled waveguides, respectively. A high temperature stability of the lasing wavelengths (0.1 nm K−1) was revealed. The results indicate that a new generation of wavelength-stabilized lasers for applications requiring ultrahigh brightness and wavelength stabilization can be developed.

Transverse single-mode edge-emitting lasers based on coupled waveguides.

We report on the transverse single-mode emission from InGaAs/GaAs quantum well edge-emitting lasers with broadened waveguide. The lasers are based on coupled large optical cavity (CLOC) structures where high-order vertical modes of the broad active waveguide are suppressed due to their resonant tunneling into a coupled single-mode passive waveguide. The CLOC lasers have shown stable Gaussian-shaped vertical far-field profiles with a reduced divergence of ∼22° FWHM (full width at half-maximum) in CW (continuous-wave) operation.

Edge-emitting InGaAs/GaAs laser with high temperature stability of wavelength and threshold current

We have investigated an edge-emitting tilted wave laser (TWL) with the active region based on GaInAs/GaAs quantum wells. In the TWL the wavelength stabilization is based on the coupling of the laser active waveguide cavity to a specially introduced thick epitaxial layer and the emission wavelength is defined by the combined cavity mode preferably by a single dominating mode. The TWL wafer has been grown by metal-organic chemical vapour deposition. Laser parameters have been investigated both in pulsed and CW mode in the temperature range of 15–60 °C. In the temperature window of 20–50 °C under CW excitation the lasers have shown high wavelength temperature stability with the temperature shift of 0.05 nm K−1 and threshold current stability with the characteristic temperature of 500 K. The data obtained prove the concept of thermal stability in tilted wave lasers.

Optical properties of hybrid quantum-confined structures with high absorbance

The methods of photoluminescence and photoconductivity spectroscopy and the spectroscopy of photocurrent of a p–i–n structure are used to study samples with hybrid quantum-confined medium “quantum well–dots” (QWD) structures grown on GaAs substrates. The significant contribution of QWD states, which extends the GaAs absorption range to 1075 nm, is found in the photoconductivity and photocurrent spectra. The absorption and luminescence of the quantum-confined structures possess characteristic features of quantum wells. Analysis of the photocurrent and photoconductivity spectra demonstrate that the excitation of carriers from localized QWD states has a combined nature: at temperatures lower than 100 K and an electric-field strength of below 40 kV/cm, excitation is possible via tunneling to the matrix, while at higher temperatures thermal activation coming into play. Also, lateral photoconductivity is observed in the layers of quantum-confined structures.

Optical mode engineering and high power density per facet length (>8.4 kW/cm) in tilted wave laser diodes

Tilted Wave Lasers (TWLs) based on optically coupled thin active waveguide and thick passive waveguide offer an ultimate solution for thick–waveguide diode laser, preventing catastrophic optical mirror damage and thermal smile in laser bars, providing robust operation in external cavity modules thus enabling wavelength division multiplexing and further increase in brightness enabling direct applications of laser diodes in the mainstream material processing. We show that by proper engineering of the waveguide one can realize high performance laser diodes at different tilt angles of the vertical lobes. Two vertical lobes directed at various angles (namely, +/–27° or +/–9°) to the junction plane are experimentally realized by adjusting the compositions and the thicknesses of the active and the passive waveguide sections. The vertical far field of a TWL with the two +/–9° vertical beams allows above 95% of all the power to be concentrated within a vertical angle below 25°, the fact which is important for laser stack applications using conventional optical coupling schemes. The full width at half maximum of each beam of the value of 1.7° evidences diffraction– limited operation. The broad area (50 μm) TWL chips at the cavity length of 1.5 mm reveal a high differential efficiency ~90% and a current–source limited pulsed power >42W for as–cleaved TWL device. Thus the power per facet length in a laser bar in excess of 8.4 kW/cm can be realized. Further, an ultimate solution for the smallest tilt angle is that where the two vertical lobes merge forming a single lobe directed at the zero angle is proposed.

Passive cavity laser and tilted wave laser for Bessel-like beam coherently coupled bars and stacks

Ultralarge output apertures of semiconductor gain chips facilitate novel applications that require efficient feedback of the reflected laser light. Thick (10-30 μm) and ultrabroad (>1000 μm) waveguides are suitable for coherent coupling through both near-field of the neighboring stripes in a laser bar and by applying external cavities. As a result direct laser diodes may become suitable as high-power high-brightness coherent light sources. Passive cavity laser is based on the idea of placing the active media outside of the main waveguide, for example in the cladding layers attached to the waveguide, or, as in the case of the Tilted Wave Laser (TWL) in a thin waveguide coupled to the neighboring thick waveguide wherein most of the field intensity is localized in the broad waveguide. Multimode or a single vertical mode lasing is possible depending on the coupling efficiency. We demonstrate that 1060 nm GaAs/GaAlAs–based Tilted Wave Lasers (TWL) show wall-plug efficiency up to ~55% with the power concentrated in the two symmetric vertical beams having a full width at half maximum (FWHM) of 2 degrees each. Bars with pitch sizes in the range of 25–400 μm are studied and coherent operation of the bars is manifested with the lateral far field lobes as narrow as 0.1° FWHM. As the near field of such lasers in the vertical direction represents a strongly modulated highly periodic pattern of intensity maxima such lasers or laser arrays generate Bessel-type beams. These beams are focusable similar to the case of Gaussian beams. However, opposite to the Gaussian beams, such beams are self-healing and quasi non-divergent. Previously Bessel beams were generated using Gaussian beams in combination with an axicon lens or a Fresnel biprism. A new approach does not involve such complexity and a novel generation of laser diodes evolves.

Light-emitting and photovoltaic devices based on quantum well-dots hybrid nanostructures

We report on optoelectronic devices based on novel type of active region - quantum well-dots (QWD) hybrid nanostructures. This hybrid type of the active region can be described as a quantum well, which has an ultradense array of narrow-gap In-rich regions with the size of 20-30 nm, which serve as the localization centers of charge carriers. Such QWD structures can be formed spontaneously during the MOVPE (metalorganic vapor phase epitaxy) deposition of InxGa1-xAs (0.3<; x<0.5) on GaAs substrate. Optimal average thickness and composition of InxGa1-xAs to achieve maximal PL intensity and photocurrent in QWD structures are determined. Characteristics of edge-emitting lasers based on 5 QWD layers are described. Advantages of using QWD medium in light-emitting and photovoltaic devices are discussed.

Novel concepts for designing semiconductor lasers

We review novel concepts and demonstrate recent experimental data for edge- emitting semiconductor lasers with broad vertical waveguide. The ultimate case for waveguide extension in the vertical direction can be implemented by using the Tilted Wave Laser (TWL) approach. A TWL is composed of a thin active waveguide (typically 0.3-2 μm) optically coupled to a thick passive waveguide (10-150 μm). A TWL with a 26 μm-thick passive waveguide demonstrated low internal loss of 1.4 cm−1, maximum pulsed power 18 W and maximum CW power 4.7 W. Vertical far field of the TWL consists of two tilted narrow lobs of 2 degrees full width at half maximum each.

Thermally stable surface-emitting tilted wave laser

Novel lasing modes in a vertical-cavity surface-emitting laser (VCSEL)-type structure based on an antiwaveguding cavity are studied. Such a VCSEL cavity has an effective refractive index in the cavity region lower than the average index of the distributed Bragg reflectors (DBRs). Such device in a stripe geometry does not support in–plane waveguiding mode, and all modes with a high Q-factor are exclusively VCSEL-like modes with similar near field profile in the vertical direction. A GaAlAs–based VCSEL structure studied contains a resonant cavity with multiple GaInAs quantum wells as an active region. The VCSEL structure is processed as an edge-emitting laser with cleaved facets and top contact representing a non–alloyed metal grid. Rectangular-shaped ~400x400 µm pieces are cleaved with perpendicular facets. The contact grid region has a total width of ~70 μm. 7 μm–wide metal stripes serve as non–alloyed metal contact and form periodic rectangular openings having a size of 10x40 μm. Surface emission through the windows on top of the chip is measured at temperatures from 90 to 380 K. Three different types of modes are observed. The longest wavelength mode (mode A) is a VCSEL–like mode at ~854 nm emitting normal to the surface with a full width at half maximum (FWHM) of the far field ~10°. Accordingly the lasing wavelength demonstrates a thermal shift of the wavelength of 0.06 nm/K. Mode B is at shorter wavelengths of ~840 nm at room temperature, emitting light at two symmetric lobes at tilt angles ~40° with respect to the normal to the surface in the directions parallel to the stripe. The emission wavelength of this mode shifts at a rate 0.22 nm/K according to the GaAs bandgap shift. The angle of mode B with respect to the normal reduces as the wavelength approaches the vertical cavity etalon wavelength and this mode finally merges with the VCSEL mode. Mode B hops between different lateral modes of the VCSEL forming a dense spectrum due to significant longitudinal cavity length, and the thermal shift of its wavelength is governed by the shift of the gain spectrum. The most interesting observation is Mode C, which shifts at a rate 0.06 nm/K and has a spectral width of ~1 nm. Mode C matches the wavelength of the critical angle for total internal reflection for light impinging from semiconductor chip on semiconductor/air interface and propagates essentially as an in–plane mode. According to modeling data we conclude that the lasing mode represents a coupled state between the TM–polarized surface–trapped optical mode and the VCSEL cavity mode. The resulting mode has an extended near field zone and low propagation losses. The intensity of the mode drastically enhances once is appears at resonance with Mode B. A clear threshold is revealed in the L–I curves of all modes and there is a strong competition of the lasing mechanisms once the gain maximum is scanned over the related wavelength range by temperature change.