Molecular beam epitaxial growth of strained AIGalnAs multi-quantum well lasers on InP

State of the art transparency currents as low as 41 A/cm² per well have been achieved in strained AIGalnAs multi-quantum well (MQW) 1.5 urn lasers. Grown by solid source molecular beam epitaxy, broad area lasers with seven quantum wells exhibit threshold current densities of less than 900 A/cm² for a 300 μm device length, comparable to the best results in this material system by any growth technology. The key to this threshold current density reduction is the optimization of the quantum well width. Experimentally, we found that thresh-old current densities can be reduced by a factor of two by using MQW active regions with wider wells which we attribute to a reduction in the nonradiative recombination and improved electron-hole overlap. High resolution x-ray diffraction, photoluminescence, and broad area lasers were used to characterize the MQW active regions.     

Influence of free carrier plasma effect on carrier-inducedrefractive index change for quantum-well lasers

The influence of the free carrier component due to the plasma effect on carrier-induced refractive index change and its dependency on polarization for multiple-quantum-well (MQW) and bulk lasers are experimentally studied. The ratios of the component to the total index change, R_fc, are 0.6, 0.4, and 0.1 for 1.3-μm MQW, 1.3-μm bulk, and 0.8-μm MQW lasers, respectively. The TM/TE polarization ratios of the component, R_TMTE/, are 0.8 and 0.3 for 1.3-μm MQW and 0.8-μm MQW lasers. The relationship between the index change and the carrier overflow (to barrier and separate confinement heterostructure layers) for MQW lasers is also discussed. Large R_fc and R_TMTE/ for the 1.3-μm MQW laser result from the carrier overflow     

Extremely high power 1.48 μm GaInAsP/InP GRIN-SCH strained MQWlasers

A record CW output power of 360 mW at 25°C was achieved by investigating the structure of optical confinement layer in 1.48 μm GRIN-SCH MQW lasers. It is experimentally demonstrated that the use of a wide bandgap and thin SCH layer gives a high differential quantum efficiency without expense of threshold current. Low driving currents, 195 mA for 100 mW, 450 mA for 200 mW and 890 mA for 300 mW, were obtained in the optimized cavity lengths     

Absorption loss coefficient of the active layer for 1.48-μm bulkand MQW lasers

It is shown that the absorption loss coefficient of the active layer for 1.48-μm bulk lasers is 66 cm^-1 which is between 45 and 107 cm^-1 for 1.3-μm bulk lasers and for 1.55-μm bulk lasers, respectively. It is also described that the absorption loss coefficient of the active layer for 1.48-μm multiple-quantum-well (MQW) lasers is 28 cm^-1 which is about two-fifths of that for 1.48-μm bulk lasers. Therefore, the high slope efficiency of the 1.48-μm MQW lasers is attributed not only to the small optical confinement factor but also to the small absorption loss coefficient of the active layer     

1.3-μm AlGaInAs-AlGaInAs strained multiple-quantum-well laserswith a p-AlInAs electron stopper layer

1.3-μm AlGaInAs-AlGaInAs strained multiple-quantum-well (MQW) lasers with a p-AlInAs electron stopper layer have been fabricated. The electron stopper layer was inserted between the MQW and p-side separate confinement heterostructure (SCH) layers to suppress the electron overflow from the MQW to p-SCH. The characteristic temperatures of the threshold currents and slope efficiencies were improved in the lasers with the stopper layers, especially at higher temperatures. As a result, a maximum operating temperature of 155°C was achieved, which was 20°C higher than that without the stopper layer     

Theory and experiment of a single-mode diode laser subject toexternal light injection from several lasers

A theoretical and experimental study of a graded-index separate confinement heterostructure (GRIN-SCH) distributed feedback (DFB) multiquantum-well (MQW) diode laser emitting at 1.55 μm subject to external light injection from several lasers is presented. Lang's model for the classical single master-slave configuration is extended to include light injection from several master lasers. Free carrier transport effects are taken into account. An experimental validation of the model for two master lasers is made by means of a quantitative comparison between measured and calculated optical spectra. A fiber optics experimental setup makes it possible to measure precisely the power which is injected into the slave laser from each master laser. Measurements and model are in good quantitative agreement     

High-performance λ=1.3 μm InGaAsP-InP strained-layerquantum well lasers

Compressively and tensile strained InGaAsP-InP MQW Fabry-Perot and distributed feedback lasers emitting at 1.3-μm wavelength are reported. For both signs of the strain, improved device performance over bulk InGaAsP and lattice-matched InGaAsP-InP MQW lasers was observed. Tensile strained MQW lasers show TM polarized emission, and with one facet high reflectivity (HR) coated the threshold currents are 6.4 and 12 mA at 20 and 60°C, respectively. At 100°C, over 20-mW output power is obtained from 250-μm-cavity length lasers, and HR-coated lasers show minimum thresholds as low as 6.8 mA. Compressively strained InGaAsP-InP MQW lasers show improved differential efficiencies, CW threshold currents as low as 1.3 and 2.5 mA for HR-coated single- and multiple quantum well active layers, respectively, and record CW output powers as high as 380 mW for HR-AR coated devices. For both signs of the strain, strain-compensation applied by oppositely strained barrier and separate confinement layers, results in higher intensity, narrower-linewidth photoluminescence emissions, and reduced threshold currents. Furthermore, the strain compensation is shown to be effective for improving the reliability of strained MQW structures with the quantum wells grown near the critical thickness. Linewidth enhancement factors as low as 2 at the lasing wavelength were measured for both types of strain. Distributed feedback lasers employing either compressively or tensile strained InGaAsP-InP MQW active layers both emit single-mode output powers of over 80 mW and show narrow linewidths of 500 kHz     

MBE growth and characteristics of periodic index separate confinement heterostructure InGaAs quantum-well lasers

We have used solid-source molecular beam epitaxy (MBE) to grow InGaAs quantum-well lasers emitting at 980nm in a novel configuration of periodic index separate confinement heterostructure (PINSCH). Periodic multilayers (GaAs/AlGaAs) are utilized as optical confinement layers to reduce the transverse beam divergence as well as to increase the maximum output power. The multilayers are grown by temperature modulation MBE without any shutter operation. The heterointerfaces in the multilayers are linearly graded such that the energy barrier heights are greatly decreased. This has led to a drastic reduction in the series resistance which is essential in the performance of high output power. The 5μm × 750μm device has far-field angles of 10° by 20°, a threshold current of 45 mA, an external differential quantum efficiency of 1.15 mW/mA (90%), and an output power of 620 mW, all measured at room temperature under CW operation. A record high fiber coupling efficiency of 51% has been achieved and more than 130 mW of power is coupled into a 5μm-core single mode fiber.     

Interband Cascade Lasers with Wavelengths Spanning 3.2–4.2 <Emphasis Type="Italic">μ</Emphasis>m

We report a detailed experimental investigation of five interband cascade lasers with five active stages each and emitting at wavelengths between 3.2 μm and 4.2 μm at room temperature. Pulsed threshold current densities as low as 394 A/cm² and voltage efficiencies as high as 76% are obtained at 300 K. The low pulsed threshold power densities (0.9–1.6 kW/cm² at 300 K) imply that ambient-temperature cw operation should be possible over the entire spectral band once optimized narrow ridges can be fabricated.     

Circular beam emission from 1.3 μm InGaAsPstrained-multiquantum-well lasers

The beam divergence in the vertical direction from a graded index separate confinement heterostructure (GRINSCH) multiquantum-well (MQW) laser has been studied. It is demonstrated both theoretically and experimentally that a circular beam MQW laser can be produced by choosing appropriate thicknesses for the GRINSCH layers, while maintaining other desired laser characteristics. The beam divergence is found to be more affected by the index change induced by injected carriers than by strain in the MQW active layer. Theoretical results are in good agreement with the measurements for 1.3-μm InGaAsP strained MQW lasers     

Characteristics of multistack multiquantum barrier and itsapplication to graded-index separate confinement heterostructure lasers

The enhancement of electron barrier height by multistack multiquantum barrier structure is simulated using the transfer matrix method. The validity and feasibility of this concept is verified by the experimental results on GaAs-AlAs multistack multiquantum barriers. Based on the simulated results, both 0.78 and 1.3 μm graded-index separate confinement heterostructure (GRIN-SCH) lasers with predicted enhanced carrier and optical confinements using graded multistack multiquantum barriers are designed. Lower threshold current, higher modulation bandwidth as well as higher characteristic temperature are expected for these lasers     

A novel current injected strained quantum well laser grown by MOVPE

Conventional long wavelength (1.3 and 1.55 μm emitting) GalnAsP alloy lasers suffer from two disadvantages. Firstly, carriers in the highest lying valence band have a heavy effective mass relative to carriers in the conduction band. This asymmetry leads to an increase in the carrier density required for lasing action to occur. Secondly, non-radia-tive recombination processes, such as Auger Recombination (AR) and Inter Valence Band Absorption (IVBA), which involve occupancy of the heavy-hole (HH) states, are thought to be significant in these materials. These again lead to higher thresholds and lower values ofT ₀than might otherwise be the case. Recently, there has been considerable interest in the prospect of “engineering” the band structure of a 1.5 μm emitting device so as to overcome these problems. It has been reported that for a quantum well under biaxial compression, the light-hole/heavy-hole (LH/HH) degeneracy at the gamma point will be lifted such that the highest lying valence band will be LH-like in the in-plane direction. This should reduce both the effective mass asymmetry and the thermal occupancy of the HH states, lowering the threshold carrier density and reducing the AR and IVBA rates. This paper describes MOVPE growth and characterisation of the first 1.55 μm emitting current injected strained layer laser structure. The active region contains 3.5 nm thick strained quantum wells of Ga_o.3In_o.7As situated in the central region of a quaternary waveguide and grown on InP. TEM micrographs and x-ray data demonstrate that the lattice mismatch (approximately 1%) has been accommodated elastically, without the formation of misfit dislocations. Broad area lasers have been fabricated with lengths of 200–1200 μm and threshold current densities as low as 930 Acm^-2 have been measured from the longer devices. Similar 1.55 μm emitting structures containing unstrained 7.5 nm thick Ga_o.47In_o.53As wells have also been grown and characterised for comparison. As yet, no significant improvement in either threshold current orT ₀has been observed for strained lasers over unstrained devices.     

Interband Cascade Lasers with Wavelengths Spanning 2.9 μm to 5.2 μm

We report an experimental investigation of four interband cascade lasers with wavelengths spanning the mid-infrared spectral range, i.e., 2.9 μm to 5.2 μm, near room temperature in pulsed mode. One broad-area device had a pulsed threshold current density of only 3.8 A/cm² at 78 K (λ = 3.6 μm) and 590  A/cm² at 300 K (λ = 4.1 μm). The room-temperature threshold for the shortest-wavelength device (λ = 2.6 μm to 2.9 μm) was even lower, 450 A/cm². A␣cavity-length study of the lasers emitting at 3.6 μm to 4.1 μm yielded an internal loss varying from 7.8 cm⁻¹ at 78 K to 24 cm⁻¹ at 300 K, accompanied by a decrease of the internal efficiency from 77% to 45%.     

The growth of high quality InP/InGaAs/InGaAsP interfaces by CBE for SCH multi-quantum well lasers

Bulk InGaAsP and heterointerfaces of InP/InGaAs and InGaAsP/InGaAs have been grown by chemical beam epitaxy for use in multi-quantum well separate confinement heterostructure lasers. InGaAsP has been successfully grown for λ=1.1, 1.2 and 1.4 μm. The TMI and TEG incorporation coefficients have strong dependencies on substrate temperature and also charge as the InGaAsP composition tends towards InP. InP/InGaAs and InGaAsP/InGaAs quantum wells have been grown to determine the optimum gas switching sequence to minimize the measured photoluminescence FWHM. InGaAs quantum wells as narrow as 0.6 nm have been grown with 7K FWHM of 12.3 meV. Lattice matched MQW-SCH lasers were grown using different interface switching sequences with the best laser having a threshold current density of 792A/cm² for an 800 × 90 μm broad area device.     

Dependence of differential quantum efficiency on the confinementstructure in InGaAs/InGaAsP strained-layer multiple quantum-well lasers

An internal efficiency of 91% was obtained with In_0.7Ga _0.3As/InGaAsP strained-layer multiple quantum well (MQW) lasers emitting at a wavelength of 1.5 μm. The dependence of the reciprocal differential quantum efficiency on the length of the laser cavity shows that the absorption loss in the InGaAsP (λ=1.3 μm) confinement layer caused by carrier overflowing into the confinement layer reduces the internal efficiency     

1.3-μm InP-based quantum-well lasers with n-doped separateconfinement heterostructure layers for high-temperature operation

We present a novel approach for reducing the light output power penalty in 1.3-μm InP-based strained-layer (SL) MQW lasers at elevated temperatures. It is shown that n-type doping in the separate confinement heterostructure (SCH) layers increases the barrier height in the valence band profiles effectively, which makes it possible to suppress the pile-up of holes in the SCH region under high-temperature, high-injection conditions. One significant impact of this approach is that the power penalty can be reduced to one half of that in conventional SL-MQW lasers with undoped SCH. We show that SL-MQW structures with n-doped SCH have a great potential for realizing a low power penalty as well as high efficiency in InP-based MQW lasers at elevated temperatures     

High-power operation in 0.98-μm strained-layer InGaAs-GaAssingle-quantum-well ridge waveguide lasers

High power strained-layer InGaAs-GaAs graded-index separate confinement heterostructure (GRIN-SCH) single-quantum-well (SQW) lasers at an emission wavelength of 0.98 μm have been fabricated. A light power as high as 270 mW and a maximum front power conversion efficiency of 51.5% have been obtained for the antireflective and highly-reflective coated laser with 9-μm-wide ridge and 600-μm-long cavity     

Long wavelength quantum well lasers: Synopsis of the RACE “AQUA” project: MOVPE/MBE/GSMBE for InGaAsP/InP and InGaAlAs/InP high speed MQW DFB lasers

The technological limits for ultra high speed devices are now rapidly expanding due to the use of quantum well (QW) materials. This new class of materials gives the opportunity of tailoring materials parameters by controlling geometries on an atomic scale. They look very promising as materials for lasers, detectors and transistors suitable even above 10 Gb/s. It will be demonstrated that state of the art MQW structures can be realized in both material systems, InGaAsP/InP and InGaAlAs/InP. Parallel lateral laser structures (e.g. SIBH, BRS and TBH) have been designed to take full benefit of QW technology. Ultimate reduction of parasitics, whilst using potential low cost fabrication technologies is the basis for achieving high bitrate (10 Gb/s) MQW lasers, even with the stronger damping in QW material. Using the DFB-SIBH laser structure 10 Gb/s large signal experiments are successfully performed with bulk, MQW and SLMQW lasers. Extremely low fall times of 44 ps are achieved. Additional MQW based improvements are observed such as: −3 times higher differential gain, increased output power (>110 mW), 2.5 times lower chirp (Δλ_−20dB = 0.40 nm at 10 Gb/s modulation), and 2 dB gain in power budget at 10 Gb/s digital transmission.     

Growth and characterization of continuously graded index separateconfinement heterostructure (GRIN-SCH) InGaAs-InP long wavelengthstrained layer quantum-well lasers by metalorganic vapor phase epitaxy

A report is presented on the growth and characterization of the first InGaAs-InP-based graded-index separate-confinement-heterostructure (GRIN-SCH) strained quantum-well lasers operating near 1.47 μm. The structure features linearly graded InGaAsP waveguide layers for both optical and carrier confinement in a very narrow, strained quantum-well layers. The excellent structural quality of the active and waveguide regions has been confirmed by transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS) analysis results. Strained quantum-well lasers with well widths as narrow as 5-6 nm were fabricated with threshold current densities as low as 750 A/cm². Buried-heterostructure lasers based on strained quantum-well active lasers exhibit threshold currents as low as 10-15 mA with quantum efficiency of 70-80%. With antireflection coating on one side of the sample, the laser shows threshold current of 35 mA with highest output power of 160 mW     

Analysis of electro-optic switches with series-coupled multiple microring resonators

In terms of the coupled mode theory, microring resonance and electro-optic modulation princeple, a reasonable project is proposed for designing an electro-optic switch with the series-coupled multiple microring resonators. The simulation and optimization are performed at the resonant wavelength of 1550 nm. The results are as follows: the core size of the microring is 1.6 μm×1.6 μm, the confined layer between the core and the electrode is 1.6 μm, the thickness of the electrode is 0.15 μm, the radius of the m...