Nikita Yu. Gordeev

Negative characteristic temperature of InGaAs quantum dot injection laser

The range of negative characteristic temperatures in temperature dependences of threshold current density of low-threshold (In, Ga)As/(Al, Ga)As quantum dot injection lasers has been observed. A model describing the decrease in threshold current density with temperature at low temperatures is proposed.

InGaAs/GaAs Quantum Dot Lasers with Ultrahigh Characteristic Temperature (T 0= 385 K) Grown by Metal Organic Chemical Vapour Deposition

Low threshold current density (AlInGa)As/GaAs lasers based on InGaAs quantum dots (QDs) are grown by metal organic chemical vapour deposition (MOCVD). Quantum dots deposited at 490° C and covered with GaAs are directly revealed in the active region. On a transmission electron microscopy (TEM) image of the laser structure no large clusters or dislocations are found over a macroscopic distance. We show that the properties of QD lasers can be strongly improved if the QDs are confined by Al0.3Ga0.7As barriers and the cladding layers are grown at high temperature. Optimisation of the laser structure geometry allows extension of the range of ultrahigh temperature stability (T0=385 K) of the threshold current to 50° C.

High-power high-brightness semiconductor lasers based on novel waveguide concepts

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, ηwp ~ 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°.

Vertically coupled quantum dot lasers: First device oriented structures with high internal quantum efficiency

Main mechanisms of internal carrier losses and leakage from the ground state of quantum dots have been studied in heterostructure lasers based on vertically coupled quantum dots. It has been shown that the threshold current density may be reduced down to 15 A/cm 2 at room temperature by reducing the non-radiative recombination and improving the carrier localization.

Multi-Stacked InAs/InGaAs/InP Quantum Dot Laser (Jth=11 A/cm2, λ=1.9 µm (77 K))

Self-organized InAs quantum dots inserted in an (In, Ga)As matrix lattice matched to InP substrate were used as an active region of an injection laser. Low threshold (11 A/cm2) lasing at 1.9 nm (77 K) via the quantum dot states was realized. Temperature dependencies of the main laser parameters demonstrate the important role of the nonradiative recombination. An analysis of basic mechanisms of leakage shows that the Auger recombination share is negligible.

Tilted Wave Lasers: A Way to High Brightness Sources of Light

Semiconductor laser diodes are conventionally based on a relatively thin waveguide structure grown epitaxially on a thick single crystalline substrate, wherein the latter serves as a medium for carrier flow and as mechanical support and plays no role in optics. Although earlier attempts to provide the outcoupling of light through a transparent substrate in leaky lasers realized a narrow leaky emission beam, either significant leakage losses led to the deterioration of the laser performance or/and a large fraction of the output optical power was concentrated in a co-existing angularly broad emission peak originating from the narrow active waveguide. Our solution, a tilted wave laser (TWL), includes polishing the back side of the substrate under the stripe providing mirror-like reflection for the leaky mode which can thus exhibit multiple reflection and amplification cycles before exiting the device from the substrate facet. Fulfillment of phase matching conditions allows wavelength-stabilized operation. At a wavelength of 1060 nm TWLs are shown to exhibit a very small thermal shift of the emission wavelength of 0.03 nm/K. A cw output power of 3.3 W for 2 mm long cavities with uncoated facets is obtained, wherein the entire power is concentrated in a single vertical lobe having a full width at half maximum of 0.8°. The scattering of the tilted optical wave by the back substrate surface roughness is modeled and found to be the main mechanism limiting the differential efficiency, wherein the scattering contributes up to 10 cm-1 to the losses for a present roughness of ~30 nm. The free carrier absorption in the n-doped substrate ( ~3 cm-1 for n ~1018 cm-3) dominates for a roughness <; 10 nm.

High brilliance photonic band crystal lasers

High concentration of optical power in a narrow exit angle is extremely important for numerous applications of laser diodes, for example, for low-cost fiber pumping and coupling, material processing, direct frequency conversion, etc. Lasers based on the longitudinal photonic band crystal (PBC) concept allow a robust and controllable extension of the fundamental mode over a thick multi-layer waveguide region to achieve a very large vertical optical mode spot size and, consequently, a very narrow vertical beam divergence. Many undesirable effects like beam filamentation, lateral multimode operation and catastrophic optical mirror damage (COMD) are strongly reduced. 650 nm GaInP/GaAlInP PBC lasers show narrow far field pattern (FWHM~7°) stable up to the highest output powers. Differential efficiency up to 85% is demonstrated. Total single mode output power as high as 150 mW is achieved in 4 μm-wide stripes in continuous wave operation, being limited by COMD due to not passivated facets. The lateral far field FWHM is 4 degrees. 840 nm GaAs/GaAlAs PBC lasers show a vertical beam divergence of 8° (FWHM) and a high differential efficiency up to 95% (L=500 μm). A total single mode CW power approaches 500 mW for 1 mm-long 4 μm-wide stripes devices at ~500 mA current, being COMD-limited. The lateral far field FWHM is 5 degrees. Another realization of a longitudinal PBC laser allows lasing in a single high-order vertical mode, a so-called tilted mode, which provides wavelength selectivity and substantially extends the possibility to control the thermal shift of the lasing wavelength. In a multilayer laser structure, where the refractive index of each layer increases upon temperature, it is possible to reach both a red shift of the lasing wavelength for some realizations of the structures, and a blue shift for some others. Most important, the absolute thermal stabilization of the lasing wavelength of a semiconductor laser can be realized.

Modeling of photonic-crystal-based high-power high-brightness semiconductor lasers

The concepts, features, modeling and practical realizations of high power high brightness semiconductor diode lasers having ultrathick and ultrabroad waveguides and emitting in the single vertical single lateral mode are analyzed. Ultrathick vertical waveguide can be realized as a photonic band crystal with an embedded filter of high order modes. In a second approach a tilted wave laser enables leakage of the optical wave from the active waveguide to the substrate and additional feedback from the back substrate side. Both designs provide high power and low divergence in the fast and the slow axis, and hence an increased brightness. Lateral photonic crystal enables coherent coupling of individual lasers and the mode expansion over an ultrabroad lateral waveguide. Experimental results are presented. Obtained results demonstrate a possibility for further expansion of the concept and using the single mode diodes having an ultrabroad waveguide to construct single mode laser bars and stacks.

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.