We present a study of high-power quantum-cascade lasers (QCL) for 8 μm spectral range with active regions of latticematched to InP substrate and strain-balanced designs. The use of the strained quantum well/barrier pairs made it possible to increase the energy barrier between the upper laser level and continuum by ~ 200 meV. Our experiments show that utilization of the strain-balanced design of the active region makes it possible to more than double the characteristic temperature T0 to 253 K from 125 K for the lattice-matched design. In pulsed mode, QCLs with strain-balanced active region demonstrated high efficiency of 12% and high output optical power of 21 W (over 10 W per facet). This is the highest value of the optical power demonstrated to date in 8 μm spectral region to the best of our knowledge.
We present a study of quantum cascade laser dynamical properties accounting for the Joule heating released in the active region. In particular, we study the QCL emitting at 8 μm in the pulsed pumping mode and present experimental measurements, as well as a theoretical description of the QCL build-up time, showing the features appearing due to the Joule heating released inside the active region.
We study high-power high bit rate single-mode 1550 nm vertical-cavity surface-emitting lasers fabricated using wafer-fusion. The optical cavity was grown on an InP wafer, and the two AlGaAs/GaAs distributed Bragg reflectors were grown on GaAs wafers, all three by molecular-beam epitaxy. The active region is based on thin InGaAs/InAlGaAs quantum wells and a composite InAlGaAs tunnel junction. To confine current and optical radiation, we use a lateral-structured buried tunnel junction with ≈ 6 µm diameter and an etching depth of ≈ 20 nm. These VCSELs demonstrate up to 5 mW single-mode continuous-wave output power and a threshold current of ≈ 2 mA at 25 °C. Even at an ambient temperature of 85 °C, the maximum optical output power is larger than 1 mW. The lasers demonstrate a 34 Gbps non-return-to-zero data transfer rate and 42 Gbps (21 GBaud) using 4-level pulse amplitude modulation at 25 °C back-to-back conditions with ≈ 934 fJ/bit power consumption per bit, which is amongst the lowest values reported for this wavelength range and bit rate.
Sergei Blokhin, Andrey Babichev, Andrey Gladyshev, Innokenty Novikov, Alexey A. Blokhin, Mikhail A. Bobrov, Nikolay Maleev, Vladislav Andryushkin, Dmitrii Denisov, Kirill Voropaev, Victor Ustinov, Vladislav E. Bougrov, Anton Egorov, Leonid Karachinsky
1300-nm vertical-cavity surface-emitting lasers (VCSELs) were fabricated by wafer fusion (WF) technique and studied. The active region based on InGaAs/InAlGaAs superlattice was grown by molecular-beam epitaxy (MBE). Current and optical confinement was provided by composite n ++ -InGaAs / p ++ -InGaAs / p ++ -InAlGaAs buried tunnel junction (BTJ) realized by selective etching and overgrowth by n-InP. AlGaAs/GaAs distributed Bragg reflectors grown by MBE were applied on both sides of the cavity by WF and substrate removal techniques. The devices with BTJ diameter of 5 μm demonstrated a stable single-mode lasing with threshold current <1.3 mA and output optical power >6 mW and operation in a wide temperature range. The measured −3 dB bandwidth was more than 8 GHz at 20°C and about 5.5 GHz at 85°C, the eye diagrams were open with a bit rate up to 20 Gbps using nonreturn-to-zero (NRZ) modulation standard at 20°C. Using 5-tap feedforward equalizer, the NRZ transmission at 25 Gbps was demonstrated up to 5 km single-mode fiber at 20°C. The developed VCSELs represent a platform for further significant performance improvement.
The paper presents the results of the research and development of 1300-nm vertical-cavity surface-emitting lasers, fabricated by wafer fusion technique for hybrid integration of an InAlGaAs/InP optical cavity with two AlGaAs/GaAs distributed Bragg reflectors using molecular-beam epitaxy. The active region is based on InGaAs/InAlGaAs superlattice, while current and optical confinement is provided by n++-InGaAs/p++-InGaAs/p++-InAlGaAs buried tunnel junction (BTJ). The proposed device design results in low internal loss (about 3.2 cm-1 at 20 °C). The devices with BTJ diameter of 5 μm demonstrate a stable single-mode lasing with threshold current less than 1.3 mA and output optical power more than 6 mW and operation in a wide temperature range. The measured -3 dB bandwidth is more than 8 GHz at 20 °C and about 5.5 GHz at 85 °C, the eye diagrams are open with a bit rate up to 20 Gbps using nonreturn-to-zero (NRZ) modulation standard. Using 5-tap feedforward equalizer, the NRZ transmission at 25 Gbps has been demonstrated up to 5km single-mode fiber.
We report for the first time on wafer-fused InGaAs-InP/AlGaAs-GaAs 1550 nm vertical-cavity surface-emitting lasers (VCSELs) incorporating a InAlGaAs/InP MQW active region with re-grown tunnel junction sandwiched between top and bottom undoped AlGaAs/GaAs distributed Bragg reflectors (DBRs) all grown by molecular beam epitaxy. InP-based active region includes seven compressively strained quantum wells (2.8 nm) optimized to provide high differential gain. Devices with this active region demonstrate lasing threshold current < 2.5 mA and output optical power > 2 mW in the temperature range of 10-70°C. The wall-plug efficiency (WPE) value-reaches 20 %. Lasing spectra show single mode CW operation with a longitudinal side mode suppression ratio (SMSR) up to 45 dB at > 2 mW output power. Small signal modulation response measurements show a 3-dB modulation bandwidth of ~ 9 GHz at pump current of 10 mA and a D-factor value of 3 GHz/(mA)1/2. Open-eye diagram at 30 Gb/s of standard NRZ is demonstrated. Achieved CW and modulation performance is quite sufficient for fiber to the home (FTTH) applications where very large volumes of low-cost lasers are required.
The ability to create metamorphic hybrid heterostructure of 1300 nm spectral band VCSEL is demonstrated. Metamorphic semiconductor part of heterostructure with GaAs/AlGaAs DBR and InAlGaAs/InGaAs QW active region has been grown by molecular beam epitaxy (MBE) on GaAs (100). Top dielectric SiO2/Ta2O5 DBR is made by the magnetron sputtering method. VCSEL has been studied under optical pumping (λ = 532 nm, diameter of the focused laser beam of ~ 1 μm) by using micro-PL setup in the range of optical pump power 0 – 70 mW at room temperature. Presence of the superlinear PL intensity growth having threshold-like dependence of PL integral intensity together with the PL peaks narrowing and mode composition modification with the pumping density increasing could be attributed to lasing behavior of the structure. Obtained results indicate the opportunity to use metamorphic growth on GaAs substrates for the 1300 nm range VCSEL manufacturing.
G. Sokolovskii, V. Dudelev, E. Kolykhalova, Ksenya Soboleva, A. Deryagin, Innokenty Novikov, Mikhail Maximov, A. Zhukov, V. Ustinov, V. Kuchinskii, W. Sibbett, E. Rafailov, Evgeny Viktorov, T. Erneux
We study InGaAs QD laser operating simultaneously at ground (GS) and excited (ES) states under 30ns pulsed-pumping and distinguish three regimes of operation depending on the pump current and the carrier relaxation pathways. An increased current leads to an increase in ES intensity and to a decrease in GS intensity (or saturation) for low pump range, as typical for the cascade-like pathway. Both the GS and ES intensities are steadily increased for high current ranges, which prove the dominance of the direct capture pathway. The relaxation oscillations are not pronounced for these ranges. For the mediate currents, the interplay between the both pathways leads to the damped large amplitude relaxation oscillations with significant deviation of the relaxation oscillation frequency from the initial value during the pulse.
In recent years, quantum-dot (QD) semiconductor lasers attract significant interest in many practical applications due to their advantages such as high-power pulse generation because to the high gain efficiency. In this work, the pulse shape of an electrically pumped QD-laser under high current is analyzed. We find that the slow rise time of the pulsed pump may significantly affect the high intensity output pulse. It results in sharp power dropouts and deformation of the pulse profile. We address the effect to dynamical change of the phase-amplitude coupling in the proximity of the excited state (ES) threshold. Under 30ns pulse pumping, the output pulse shape strongly depends on pumping amplitude. At lower currents, which correspond to lasing in the ground state (GS), the pulse shape mimics that of the pump pulse. However, at higher currents the pulse shape becomes progressively unstable. The instability is greatest when in proximity to the secondary threshold which corresponds to the beginning of the ES lasing. After the slow rise stage, the output power sharply drops out. It is followed by a long-time power-off stage and large-scale amplitude fluctuations. We explain these observations by the dynamical change of the alpha-factor in the QD-laser and reveal the role of the slowly rising pumping processes in the pulse shaping and power dropouts at higher currents. The modeling is in very good agreement with the experimental observations.
Single mode (SM) 850 nm vertical-cavity surface-emitting lasers (VCSELs) are suitable for error-free (bit error ratio
<10-12) data transmission at 17-25 Gb/s at distances ~2-0.6 km over 50μm-core multimode fiber (MMF). Reduced
chromatic dispersion due to ultralow chirp of SM VCSELs under high speed modulation (up to 40 Gb/s) are responsible
for the dramatic length extension. Good coupling tolerances of the SM devices to the MMF manifest their applicability
for low cost optical interconnects. As the higher resonance frequency (up to 30 GHz) is reached at lower current
densities in small aperture (3 μm -diameter) devices the SM devices are also preferable due to reliability considerations.
We report on the development of 25Gb/s 850nm VCSEL and PD components for efficient short-reach optical fiber
communication systems. VCSELs with the aperture size 6-7μm show the highest -3dB bandwidth (~20GHz) and Dfactor
(~8Ghz/mA1/2). K-factor is less than 0.25ns for VCSEL with 6 μm aperture. Eye diagrams are clearly open at 25C
up to 35Gb/s. The dark current of PDs remain below 1nA at T < 50°C and below 10nA when T < 90°C out to -10V. The
extracted PD capacity is linearly proportional to the detector area and less than 200fF even for 45μm PD diameter. Due
to elimination of contribution of diffusion process and quite small capacitance of the depletion region eye diagrams are
opened at 28Gb/s, even for the PDs with the largest active diameters. Using 35μm PD and 6μm VCSEL error-free
25Gb/s optical fiber communication links were tested over lengths of 203m and 103m at 25°C and 85°C, respectively.
Received optical power for the lowest BER is at both temperatures smaller than -4dBm. Obtained results indicate that
from the speed and power dissipation perspective developed high-speed CSELs and PDs are suitable for applications in
the next generation of short-reach multimode optical fiber interconnects.
As the density of transistors in CMOS integrated circuits continues to roughly double each two years the processor
computational power also roughly doubles. Since the number of input/output (I/O) devices can not increase without
bound I/O speed must analogously approximately double each two years. In the Infiniband EDR standard (2011) a single
channel bit rate of 26 Gb/s is foreseen. The maximum reliable and efficient copper link length shrinks at bit rates above
10 Gb/s to a few meters at best. At higher bit rates the length of a given multimode fiber link must also shrink, due to
both modal and wavelength dispersions. Although the modal dispersion in modern multimode OM3 and OM4 fibers that
are optimized for 850 nm vertical-cavity surface-emitting lasers (VCSELs) is reduced, the wavelength dispersion
remains a serious issue for standard multimode VCSELs. An ultimate solution to overcome this problem is to apply
single-mode VCSELs to extend and ultimately maximize the link length. In this paper we demonstrate recent results for
single-mode VCSELs with very high relaxation resonance frequencies. Quantum well 850 nm VCSELs with record high
30 GHz resonance frequencies are demonstrated. Additionally single-mode data transmission at 35 Gb/s over multimode
fiber is demonstrated. For comparison we also present specific device modeling parameters and performance
characteristics of 850 nm single-mode quantum dot (QD) VCSELs. Despite a significant spectral broadening of the QD
photoluminescence and gain due to QD size dispersion we obtain relaxation resonance frequencies as high as 17 GHz.
KEYWORDS: Waveguides, Semiconductor lasers, Photonic crystals, Active optics, Cladding, Near field, Near field optics, High power lasers, Crystals, Phase matching
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.
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°.
We report on the modeling, epitaxial growth, fabrication, and characterization of 830-845 nm vertical cavity surface
emitting lasers (VCSELs) that employ InAs-GaAs quantum dot (QD) gain elements. The GaAs-based VCSELs are
essentially conventional in design, grown by solid-source molecular beam epitaxy, and include top and bottom gradedheterointerface
AlGaAs distributed Bragg reflectors, a single selectively-oxidized AlAs waveguiding/current funneling
aperture layer, and a quasi-antiwaveguiding microcavity. The active region consists of three sheets of InAs-GaAs
submonolayer insertions separated by AlGaAs matrix layers. Compared to QWs the InAs-GaAs insertions are expected
to offer higher exciton-dominated modal gain and improved carrier capture and retention, thus resulting in superior
temperature stability and resilience to degradation caused by operating at the larger switching currents commonly
employed to increase the data rates of modern optical communication systems. We investigate the robustness and
temperature performance of our QD VCSEL design by fabricating prototype devices in a high-frequency ground-sourceground
contact pad configuration suitable for on-wafer probing. Arrays of VCSELs are produced with precise variations
in top mesa diameter from 24 to 36 μm and oxide aperture diameter from 1 to 12 μm resulting in VCSELs that operate in
full single-mode, single-mode to multi-mode, and full multi-mode regimes. The single-mode QD VCSELs have room
temperature threshold currents below 0.5 mA and peak output powers near 1 mW, whereas the corresponding values for
full multi-mode devices range from about 0.5 to 1.5 mA and 2.5 to 5 mW. At 20°C we observe optical transmission at 20
Gb/s through 150 m of OM3 fiber with a bit error ratio better than 10-12, thus demonstrating the great potential of our QD
VCSELs for applications in next-generation short-distance optical data communications and interconnect systems.
Presently VCSELs covering a significant spectral range (840-1300 nm) can be produced based on quantum dot (QD)
active elements. Herein we report progress on selected QD based vertical-cavity surface-emitting lasers (VCSELs)
suitable for high-speed operation. An open eye diagram at 20 Gb/s with error-free transmission (a bit-error-rate < 10-15)
is achieved at 850 nm. The 850 nm QD VCSELs also achieve error-free 20 Gb/s single mode transmission operation through multimode fiber without the use of optical isolation. Our 980 nm-range QD VCSELs achieve error free transmission at 25 Gb/s at up to 150°C. These 980 nm devices operate in a temperature range of 25-85°C without current or modulation voltage adjustment. We anticipate that the primary application areas of QD VCSELs are those that require
degradation-robust operation under extremely high current densities. Temperature stability at ultrahigh current densities,
a forte of QDs, is needed for ultrahigh-speed (> 40 Gb/s) current-modulated VCSELs for a new generation of local and storage area networks. Finally we discuss aspects of QD vertical extended-cavity surface emitting lasers with ultra high power density per emitting surface for high power (material processing) and frequency conversion (display) applications.
We discuss wavelength stabilized all-epitaxial Tilted Cavity Lasers (TCLs). Optical cavity of a TCL favors propagation of only one tilted optical mode ensuring wavelength-selective operation. The possibility of full control of the thermal shift of the lasing wavelength d λ/dT in TCL
including positive, zero or negative shift, is proved theoretically. Broad-area
(100 μm) 970-nm-range devices have been fabricated showing a high temperature stability of the lasing wavelength
(0.13 nm/K), a high power operation (> 7 W in pulsed mode and > 1.5 W in continuous wave (cw) mode), and a narrow
vertical far-field beam divergence (FWHM ~ 20°). Single transverse mode edge-emitting 4 μm-wide-ridge TCLs
demonstrated high-power spatial and spectral single mode cw operation with a longitudinal side mode suppression ratio
(SMSR) up to 41.3 dB at 93 mW output power. Such a result is similar to the best values achieved for DFB lasers in the
same spectral range, while no etching and overgrowth is used in present case.
KEYWORDS: Photonic crystals, Laser crystals, Waveguides, Semiconductor lasers, Refractive index, Crystals, Reflectivity, High power lasers, Gallium arsenide, Near field optics
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.
We report on lasers and light emitting diodes based on the longitudinal photonic bandgap crystal (PBC) concept. The PBC design allows achieving a robust and controllable extension of the fundamental mode over a thick multi-layer waveguide region to obtain a very large vertical optical mode spot size and a very narrow vertical beam divergence. An efficient suppression of high order modes can be realized either by the optical confinement factor selection of the fundamental mode, which is localized at the "optical defect" region and has a higher overlap with the gain region. All the other modes spread across the thicker PBC waveguide. In another approach leakage loss selection can be used to suppress excited modes in case of absorbing substrate or the substrate with a higher-refractive index. In this paper we concentrate on growth and performance of high power single mode visible (650 nm) GaInP/AlGaInP PBC lasers, giving a comprehensive example. The devices show narrow far field pattern (full width at half maximum of vertical beam divergence of about 7°), which is stable up to the highest output powers. Differential efficiency up to 85% is demonstrated. Total continuous wave single mode output power as high as 120 mW is achieved in 4 micrometer-wide stripes. Infrared (980 nm) InGaAs/AlGaAs PBC lasers with a beam divergence down to 4.2 degrees and a high temperature stability of the threshold current are also demonstrated.
We have performed a systematic study of structural and optical properties of Quantum dot (QDs) lasers based on InAs/InGaAs quantum dots grown on GaAs substrates emitting in the 1.3 - 1.5 μm range. 1.3 μm range QD lasers are grown using GaAs as matrix material. It is shown that the lasers, grown with large number of QD stacks are metamorphic, with plastic relaxation occurring through the formation of misfit dislocations. Thus, 1.3 μm QD lasers with large number of stacks grown without strain compensation are metamorphic. Another type of defects is related to local dislocated clusters, which are the most dangerous. When proper optimization of the growth conditions is carried out, including a selective thermal etching off of statistically formed dislocated clusters through the defect-reduction technique (DRT), no significant impact of misfit dislocations on the degradation robustness is observed. In uncoated devices a high cw single mode power of ~700 mW is realized limited by thermal roll-over, which is not affected by 500 h ageing at room temperature. At elevated temperatures the main degradation mechanism revealed is catastrophic optical mirror damage (COMD). When the facet are passivated, the devices show the extrapolated operation lifetime in excess of 106 h at 40°C at ~100 mW cw single mode output power. Longer wavelength (1.4 - 1.5 μm) devices are grown on metamorphic (In,Ga,Al)As layers deposited on GaAs substrates. In this case, the plastic relaxation occurs through formation of both misfit and threading dislocations. The latter kill the device performance. Using DRT in this case enables blocking of threading dislocation with growth of QDs in defect-free upper layers. DRT is realized by selective capping of the defect-free areas and high-temperature etching of nano-holes at the non-capped regions near the dislocation. The procedure results in etching of holes and is followed by fast lateral overgrowth with merger of the growth fronts. If the defect does not propagate into the upper layer when the hole is capped, the upper layers become defect-free. Lasers based on this approach exhibited emission wavelength in the 1.4 -1.5 μm range with a differential quantum efficiency of about ~50%. The narrow-stripe lasers operate in a single transverse mode and withstand continuous current density above 20 kA cm-2 without degradation. A maximum continuous-wave output power of 220 mW limited by thermal roll-over is obtained. No beam filamentation was observed up to the highest pumping levels. Narrow stripe devices with as-cleaved facets are tested for 60°C (800 h) and 70°C (200 h) on-chip temperature. No noticeable degradation has been observed at 50 mW cw single mode output power. This shows the possibility of degradation-robust devices on foreign substrates. The technology opens a way for integration of various III-V materials and may target degradation-free lasers on silicon for further convergence of computing and communications.
The Tilted Cavity (TC) concept has been proposed to combine advantages of edge- and surface-emitting lasers (detectors, amplifiers, switches, etc.). Tilted Cavity Lasers (TCL) enable wavelength-stabilized high-power edge and surface emitters (TCSEL) in low-cost single-epitaxial step design. The concept covers numerous applications including mode-locked TCL for light speed control, dispersion and linewidth engineering, GaN-based light-emitters, electrooptic wavelength tunable devices, and other applications. Presently, wavelength stabilized TC operation is realized between -200°C and 70°C in broad TCL diodes with cleaved facets based on quantum dots (QDs). The spectral width is below 0.6 nm in broad area 100 μm-wide-stipe devices. The far fields are: 4° (lateral) and 42° (vertical). Wavelength-stabilized 1.16 μm and 1.27 μm edge-emitting QD TCL lasers are demonstrated. Quantum well TCL demonstrate high-temperature operation up to 240°C with a low threshold, high temperature stability and improved wavelength stability. The tilted cavity approach can also be applied in wavelength-optimized photodetectors, switches, semiconductor optical amplifiers, including multi-channel devices, in optical fibers, in photodetectors, in light-emitting diodes and in many other applications. Moreover, microelectronic devices based on similar tilted angle resonance phenomena in quantum wells and superlattices can be realized in electron- or hole-wavefunction-engineered structures, thus, merging the fields of nanophotonics and nanoelectronics. The tilted cavity concept can be further complimented by lateral patterning and (or) processing of three-dimensional photonic crystal structures further extending horizons of modern optoelectronics.
Quantum dot (QDs) heterostructures structurally represent tiny 3D insertions of a narrow bandgap material, coherently embedded in a wide-bandgap single-crystalline matrix. The QDs are produced by conventional epitaxial techniques applying self-organized growth and behave electronically as artificial atoms. Strain-induced attraction of QDs in different rows enables vertically-coupled structures for polarization, lifetime and wavelength control. Overgrowth with ternary or quaternary alloy materials allows controllable increase in the QD volume via the island-activated alloy phase separation. Repulsive forces during overgrowth of QDs by a matrix material enable selective capping of coherent QDs, keeping the defect regions uncapped for their subsequent selective evaporation. Low-threshold injection lasing is achieved up to 1350 nm wavelength at 300K using InAs-GaAs QDs. 8 mW VCSELs at 1.3 μm with doped DBRs are realized. Edge-emitters demonstrate 10 GHz bandwidth up to 70°C without current adjustment. VCSELs show ~4 GHz relaxation oscillation frequency. QD lasers demonstrate above 3000 h of CW operation at 1.5 W at 45°C heat sink temperature without degradation. The defect reduction technique (DRT) applied to thick layers enables realization of defect-free structures on top of dislocated templates. Using of DRT metamorphic buffer layers allowed 7W GaAs-based QD lasers at 1500 nm.
Tilted Cavity Laser (TCL) is developed that combines advantages of a high power operation of an edge-emitting semiconductor diode laser and wavelength-stabilized operation of a surface emitting laser. A TCL emits laser light in a tilted optical mode that propagates effectively at a certain tilt angle to the p-n junction. Designed TCL comprises a high-finesse cavity into which an active region is placed and at least one multilayer interference reflector (MIR). The cavity and the MIR are designed such that the spectral position of the reflectivity dip of the cavity and the position of the stopband reflectivity maximum of the MIR coincides at one tilt angle of a tilted optical mode, and draw apart as the angle deviates from the optimum value. As a result, the leakage loss of the optical modes to the substrate is minimum at the optimum wavelength and increases dramatically as the wavelength deviates from the optimum one. This ensures the stabilization of the wavelength of the emitted laser light. Both quantum well (QW) and quantum dot (QD) TCLs have been fabricated on the basis of GaAs/GaAlAs waveguides. QW TCL using InGaAs QW as the active region and operating at 1000-1100 nm reveals the temperature shift of the lasing wavelength 0.2 nm/K. QW TCL operates up to and above 210°C with the differential efficiency 20%. QD TCL using InAs QD overgrown by InGaAs alloy as the active region and operating at 1100-1200 nm reveals the temperature shift of the lasing wavelength 0.165 nm/K. These shifts are significantly slower than the shift for a conventional edge-emitting semiconductor diode laser. The QD TCL shows an output power 2W in a pulsed mode. Characteristic temperature of the threshold current measured at and below room temperature (T0) is 150 K.
Two-photon absorption of 1.55 μm light in quantum well InGaAsP/InP laser heterostructures has been measured. Nonlinear response as high as 0.78 nA/mW2 has been found. Minimal detectable peak power 60 μW allows using this kind of semiconductor waveguide as a detector in optical autocorrelator to investigate low-power signal.
Electroluminescent study of heterolasers based on vertically coupled self-assembled quantum dots has been done. Luminescent parameters were measured in the 77 ÷ 300 K temperature range. Lasing via ground state of quantum dots up to room temperature has been shown. Temperature independence of the electoluminescent peak position, which corresponds to the second excited state in quantum dots, has been explained.
Dicke superradiance mechanism is suggested as a transition phase from spontaneous to stimulated emission in semiconductor laser heterostructures. Model, which describes “macrodipoles” formation in the active layer of heterostructures is proposed. Estimated characteristic radiation time of these “microdipoles” was obtained in sup-picosecond range, which is in a good agreement with our previous experimental results.
The concept of resonant carrier many body interaction during radiative recombination was applied to explain spectra of quantum well electroluminescence at 77 K. Extremely good agreement of the calculated and experimental spectra in the entire range of emission has been achieved. Estimations give a sub-picosecond characteristic time of such radiation process.
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