Quantum-well Heterostructure Lasers Research Articles

Controlling the radiation spectrum of quantum-well heterostructure lasers by ultrasonic strain

The radiation of a semiconductor laser has been modulated in frequency by variable strain. The strain is excited by injecting bulk or surface ultrasonic waves. Dynamic and static analyses of the variations in the spectral characteristics of the radiation in the presence of sound are performed. A model is suggested and the data obtained are analyzed theoretically. The radiation frequency modulation in InGaAsP/InP heterostructures produced by surface waves is shown to be determined mainly by the band gap modulation of the active region.

Voltage distributions and nonoptical catastrophic mirror degradation in high power InGaAs/AlGaAs/GaAs lasers studied by Kelvin probe force microscopy

Kelvin probe force microscopy is used to observe the bulk potential redistribution across the high power InGaAs/AlGaAs/GaAs separate confinement heterostructure quantum-well laser diodes for a wide range of injection currents, including the lasing regime. By increasing the injection current, the development of a parasitic voltage drop is detected at initial calibration layers and the buffer layer of the laser structure. Catastrophic degradation of the laser mirror was observed at the level of injection current ∼19 times the threshold value. Atomic force microscopy images of the mirror revealed a 100 nm deep crater of maximum width ∼2.5 μm in the vicinity of the buffer/emitter interface. By combining the surface morphology results of the destructed mirror with those of Kelvin probe force microscopy in operating devices, it is concluded that the parasitic voltage drop is responsible for a substantial energy dissipation and the nonoptical degradation of the laser mirror.

Scattering (stochastic) recoupling of a coupled ten-stripe AlGaAs–GaAs–InGaAs quantum-well heterostructure laser

Data are presented on coupled ten-stripe AlGaAs–GaAs–InGaAs quantum well heterostructure (QWH) lasers recoupled stochastically at the cleaved end mirrors. Recoupling of neighboring elements of a ten-stripe laser is accomplished by the scattering (random feedback) afforded by applying ∼10-μm-diam Al powder or 0.3 μm α-Al2O3 polishing compound in microscopy immersion oil or in epoxy at the cleaved ends (mirrors). Data on QWH samples with the end mirrors coated with the scatterer (Al or Al2O3 powder in “liquid”) exhibit spectral and far-field broadening, as well as increased laser threshold because of the reduced cavity Q. Single mode operation is possible with the conventional evanescent wave coupling of the ten-stripe QWH and is destroyed, even the laser operation itself, with the scattering recoupling (dephasing) at the end mirrors, which is reversible (removable). The narrow ten-stripe QWH laser with strong end-mirror scattering, a long amplifier with random feedback, indicates that a photopumped III–V or II–VI powder (a random “wall” cavity) has little or no merit.

Pulse characterisation of a quantum-well mode-locked laser diode by two-photon absorption frequency-resolved optical gating

The intensity and phase of ultrashort optical pulses from a 40-GHz monolithic InGaAsP quantum-well heterostructure laser diode are retrieved for the first time from two-photon absorption pulse spectrograms. The pulse traces showing the mode-locked pulses from the laser are distorted due to ultrafast absorption saturation associated with quantum-confined Stark excitons. It is suggested from comparison between experimental time-integrated spectrum and reconstructed pulse spectrum that the mode-locked pulses are formed only from a part of the longitudinal modes in the laser cavity.

Mode behavior of photopumped AlGaAs–GaAs lasers confined by oxide-semiconductor distributed Bragg reflectors

Data are presented on photopumped AlGaAs–GaAs quantum-well (QW) heterostructure lasers with oxide-semiconductor distributed Bragg reflector (DBR) vertical confinement (and Fabry–Perot cleaved edges) that reduces the mode volume and influences the edge emission (EE) spectra. For narrow samples (≲20 μm), with the vertical DBRs “tuned” below the QW recombination transitions (E1 lowest, E2 higher), single mode EE operation occurs on E2 at a wavelength ∼77 meV above the lowest QW energy. For wide samples with an oxide-semiconductor vertical DBR located in a narrow band at the sample cleave edges, the DBR region acts as a spacial and spectral filter, thus increasing the possibility of single-mode EE operation.

Strained-layer InGaAs quantum-well heterostructure lasers

The incorporation of intentional strain in heterostructure lasers was almost unheard of a decade ago or so and considered a problem to be avoided. Advances in both epitaxial crystal growth technology and the understanding of the physics and reliability of these materials have led to a remarkable increase in the commercial use of strained-layer lasers. The industry has benefited from an increase in the available range of emission wavelengths from quantum-well diode lasers and dramatic improvement in their time-zero performance. In the paper, we review the characteristics of strained-layer InGaAs quantum-well heterostructure lasers that have resulted in the emergence of this important technology.

Collective resonances and shape function for homogeneous broadening of the emission spectra of quantum-well semiconductor heterostructures

An analytic expression for the shape function for homogeneous broadening of the emission spectra of semiconductor heterostructure lasers is obtained on the basis of simple expressions from the theory of the superradiance of two-level systems. Good agreement between the theory and experimental data is achieved for an InGaAs/GaAs quantum-well heterostructure laser. An estimate of the duration of the superradiance pulse is given.

Increasing the power of broad-waveguide lasers by additional selection of transverse modes

A new method is proposed for transverse mode selection in high-power, broad-waveguide, separate-confinement, heterostructure quantum-well lasers by orienting the front mirror at an angle other than normal to the direction of radiation propagation. The dependence of the mode reflection coefficients on the laser mirror parameters is investigated. Optimal values are found for the mirror inclination angles and the thicknesses and refractive indices of the dielectric coating at which the fundamental guide mode dominates. The possibility of a 1.2-fold increase in the output power of a broad-waveguide, separate-confinement, heterostructure quantum-well diode laser is demonstrated.

Power rise in broad-waveguide diode laser with inclined facet

A method of transverse mode selection in high-power broad-waveguide separate confinement heterostructure quantum-well lasers by placing the front mirror at the angle other than the normal to the radiation propagation direction is proposed. Optimal values of facet inclination angles and widths and the refractive indices of the dielectric coating at which the fundamental guided mode dominates are found. The possibility of increasing the output power 1.2 times of a broad-waveguide separate confinement heterostructure quantum-well diode laser with inclined facet is shown.

Buried tunnel contact junction AlGaAs-GaAs-InGaAs quantum well heterostructure lasers with oxide-defined lateral currents

Al x Ga 1−x As-GaAs-In y Ga 1−y As quantum well heterostructure (QWH) lasers with p+-n+ GaAs–InyGa1−yAs reverse-biased tunnel junctions (hole sources) located in the upper cladding of standard lasers and in oxide-defined cavities (requiring lateral bias currents) are demonstrated. The tunnel junctions, introduced to aid lateral current spreading, are grown at different distances from the waveguide active region in a standard QWH structure to determine first the effect of heavily doped tunnel layers on laser threshold currents. Other QWH laser crystals are oxidized to form oxide-aperture devices with, in addition, either a top confining oxide or a top and bottom oxide confining layer. Hole injection is provided between the oxide layers with the aid of the tunnel contact junction and lateral electron current. The buried tunnel contact junction is shown to be an effective method to make possible hole injection via a lateral electron current, with only a modest increase (a small penalty) in voltage drop and series resistance compared to standard devices.

14.3 W quasicontinuous wave front-facet power from broad-waveguide Al-free 970 nm diode lasers

Wide-stripe, 0.97 μm emitting Al-free InGaAs(P)/InGaP/GaAs broad-waveguide separate confinement heterostructure quantum-well lasers demonstrate a record value for quasicontinuous wave (QCW) output power: 14.3 W (100-μm-wide stripe, 100 μs-wide pulses); and reach catastrophic optical mirror damage (COMD) in QCW operation at an optical power density of 22.5 MW/cm2; that is, 40% higher than COMD levels in cw operation. The devices have low internal losses (αi=1 cm−1) and high external differential quantum efficiency (86% for 2-mm-long lasers), and exhibit only 10–20 °C temperature rises in the active region at 10 W QCW power. We also show that long-cavity, large-contact-area devices exhibit relatively little spectral broadening with increased output power.

Carrier dynamics and gain saturation in quantum-well lasers

A Monte Carlo technique has been developed to investigate the carrier dynamics and static gain saturation in graded-index separate confinement heterostructure quantumwell laser structures. The calculated electron relaxation times and gain compression coefficient show good agreement with published experiments.

Spectral investigation of the radiation emitted by strained InGaAs/GaAlAs quantum-well heterostructures

The spectral-threshold characteristics were determined in the 980 nm range for InGaAs/GaAlAs quantum-well heterostructure injection lasers. These lasers had a GaAs substrate and an anisotropically strained active layer. A systematic study was made of the spectra of subthreshold spontaneous radiation emitted at temperatures in the range 77–300 K. The dependence of the laser photon energy hν on the threshold current density consisted of two sections: the value of hν increased strongly with the current density for low-threshold samples, but above 0.6 kA cm-2 the value of hν depended weakly on the current density. This behaviour was attributed to the initial filling of the lower quantum states participating in the 1-1h active transition, followed by filling of the second quantum states of the carriers. The lower states ensured a mode gain of 40–50 cm-1.

Vertical integration of a GaAs/AlGaAs quantum-well laser and a long-wavelength quantum-well infra-red photodetector

A short-wavelength (~0.8 µm) GaAs/AlGaAs graded-index separate-confinement heterostructure quantum-well laser has been monolithically integrated with a long-wavelength (~8 µm) GaAs/AlGaAs multiquantum-well infra-red photodetector on a semi-insulating GaAs substrate by molecular beam epitaxy. The vertical integration method is used and the combined structure is a pinin structure. Both the laser and detector exhibit excellent characteristics. At room temperature, the ridge waveguide laser has an extremely low threshold current of 25 mA and a differential quantum efficiency above 65% with a stripe width of 20 µm. The quantum-well detector has a peak response at 8 µm and a responsivity of 0.7 A/W.

A comparison of amplitude-phase coupling and linewidth enhancement in semiconductor quantum-well and bulk lasers

The amplitude-phase coupling factor alpha (linewidth enhancement factor) is compared for typical semiconductor quantum-well and bulk double heterostructure lasers. As a direct consequence of the reduction of the differential gain, there is no reduction of alpha in single-quantum-well lasers compared to bulk lasers. The number of quantum wells strongly affects the amplitude-phase coupling in quantum-well lasers. It is shown that the interband transition induced amplitude-phase coupling dominates that induced by the plasma effect of carriers in typical quantum-well lasers. By considering the spontaneous emission factor in the spectral linewidth, the authors show that there is an optimal number of quantum wells for achieving the narrowest spectral linewidth. >

780 nm band TM-mode laser operation of GaAsP/AlGaAs tensile-strained quantum-well lasers

Tensile-strained active layer GaAs/AlGaAs separateconfinement- heterostructure quantum-well lasers are reported. These lasers oscillate in the 780 nm band in the TM mode by TM mode gain enhancement in the tensile-strained active layer. The threshold current density of single-quantum-well laser diodes increases rapidly with heatsink temperature. However, triple-quantum-well laser diodes with a cavity length of 485–110 μm oscillated with a threshold current density 1–4 and 30 kA/cm2.

Four wavelength distributed feedback ridge waveguide quantum-well heterostructure laser array

A four-wavelength strained-layer InGaAs-GaAs-AlGaAs distributed feedback ridge waveguide quantum-well heterostructure laser array, with a lasing emission wavelength separation of ∼16 Å between adjacent emitters, is described. The four-wavelength array requires only a single MOCVD growth step. Lateral optical confinement is achieved with dry-etched lateral third-order gratings. Each element operates in the fundamental lateral mode with a near-field FWHM of 2.6±0.1 μm and the entire width of the near-field pattern is <80 μm.

Resonance and switching in a native-oxide-defined AlxGa1−xAs-GaAs quantum-well heterostructure laser array

A twin linear-array coupled-cavity AlxGa1−xAs-GaAs quantum-well heterostructure laser, a stripe-geometry coupled two-dimensional array, is described that, unlike the usual stripe laser, exhibits mode switching and multiple switching (ON–OFF twice) in the light power (L) versus current (I) characteristics (L-I) with increasing current.

Effects of substrate heating on the spatial uniformity of threshold current and emission wavelength in GaAs and InGaAs graded-index separate-confinement heterostructure quantum-well lasers grown by molecular-beam epitaxy

We show that if the active region of a quantum-well (QW) laser is grown at a high temperature such that the sticking coefficients of group III species are less than unity, the emission wavelength λ and threshold current density Jth are sensitive to the spatial variations in substrate growth temperature Ts despite the time average flux variations of less than 1% across the surface of a rotating 7.5 cm diam substrate in our molecular-beam epitaxy system. InGaAs strained QW lasers, in which the active region is grown at Ts < 580 °C at which In and Ga have unity sticking coefficients, have uniform Jth (212±4 A cm−2) and λ(989±1 nm) on a 5 cm diam substrate. These lasers were grown with In mounting. In contrast, GaAs QW lasers, which are grown at ∼700 °C with In mounting, have less uniform Jth (395±20 A cm−2) and λ(850±9 nm). This is the result of nonuniform substrate heating which is partly due to voids in the In back coating. This causes variations in GaAs growth rate because of the high temperature dependence of the Ga sticking coefficient at ∼700 °C. Uniformity of GaAs QW lasers deteriorates further if direct radiation heating of the substrate with In free mounting is used. With In free mounting, the Jth is minimum (245 A cm−2) in the center and increases monotonically to maximum (365 A cm−2) towards the edge of the wafer, suggesting the degradation of material quality towards the edge. Variation of λ in GaAs QW lasers is random or systematic depending upon whether they are grown with In mounting or In free mounting, respectively.

Coupled-stripe in-phase operation of planar native-oxide index-guided AlyGa1−yAs-GaAs-InxGa1−xAs quantum-well heterostructure laser arrays

High-performance, coupled-stripe, planar, index-guided AlyGa1−yAs-GaAs-InxGa1−xAs quantum-well heterostructure (QWH) laser arrays are fabricated by the formation of a relatively thick, current-blocking, native oxide from the high-gap AlyGa1−yAs upper confining layer between active stripes. Precise control of the thickness of the native oxide layer between emitters provides a means of varying the index step between stripes, and permits tailoring of the optical profile to produce in-phase operation. The 10-stripe coupled QWH laser arrays (∼3-μm-wide stripes on 4 μm centers) exhibit near-diffraction-limited, single-lobed, far-field patterns with low continuous (cw) thresholds (∼45 mA) and cw output powers (total external differential quantum efficiency ≳50%) of over 100 mW per uncoated facet.