Gain narrowing and output behavior of InP-InGaAlP tunneling injection quantum-dot-well laser

Polarization-resolved amplified spontaneous emission (ASE) and gain from tensile-strained multiple quantum wells (QWs) coupled to a single layer of compressively strained quantum dots (QDs) show interesting output characteristics. Low current injection reveals transverse electric polarized ASE from the QD ground state and QD-coupled-QW state. Additionally, transverse magnetic ASE from the QW state is observed. The modal gain of this laser shows coupled active state activation which is evident by spectral narrowing and change from QW-like to QD-like spectrum.     

Influence of nonuniform carrier distribution on the polarizationdependence of modal gain in multiquantum-well lasers and semiconductoroptical amplifiers

We investigate the modal gain seen by transverse-electric (TE) and transverse-magnetic (TM) modes of bulk and multiquantum-well (MQW) lasers given a nonuniform distribution of active region carriers. We find that the dependence of modal gain on the nonuniformity of carrier profile differs for TE and TM modes. This experimentally observable phenomenon is proposed as a measure of carrier density nonuniformity. We discuss the importance of the confinement picture for TE and TM modes, in the generalized presence of some asymmetry, in assuring injection-level independent polarization insensitivity in semiconductor lasers and optical amplifiers     

Time-Resolved Linewidth Enhancement Factors in Quantum Dot and Higher-Dimensional Semiconductor Amplifiers Operating at 1.55 $mu{hbox{m}}$

We report time-resolved measurements of the linewidth enhancement factors (-factors) , and , associated with the adiabatic carrier recovery, carrier heating, and two-photon absorption dynamical processes, respectively, in semiconductor optical amplifiers (SOAs) with different degrees of dimensionality-one InAs/InGaAsP/InP quantum dot (0-D), one InAs/InAlGaAs/InP quantum dash (1-D), and a matching InGaAsP/InGaAsP/InP quantum well (2-D)-all operating near 1.55- wavelengths. We find the lowest values in the QD SOA, 2-10, compared to 8-16 in the QW, and values of and that are also lower than in the QW. In the QD SOA, the -factors exhibit little wavelength dependence over the gain bandwidth, promising for wide-bandwidth all-optical applications. We also find significant differences in the -factors of lasers with the same structure, due to the differences between gain changes that are induced optically or through the electrical bias. For the lasers we find the QW structure instead has the lower -factor, having implications for directly modulated laser applications.     

Design and ASE characteristics of 1550-nm polarization-insensitivesemiconductor optical amplifiers containing tensile and compressivewells

The polarization dependence of 1550-nm semiconductor optical amplifiers (SOA's) containing tensile and compressive wells has been investigated both theoretically and experimentally. Our model to predict the polarization-resolved (TE and TM) gain spectra of these structures has been confirmed by amplified spontaneous emission measurements. It is found that there can be appreciable carrier redistribution between the two types of wells when the tensile layers have the large thickness (greater than 100 Å) needed for gain at wavelengths around 1550 nm. This carrier redistribution can significantly modify the ratio of the gains for different polarizations, in particular, decreasing the TM gain with respect to the TE gain, and, hence, is an important design consideration. We use our model and experimental data to explore design criteria for 1550-nm polarization-independent SOA's     

Saturation characteristics ofGa0.68In0.32As/GaInAsP/InP tensile-strainedquantum-well semiconductor laser amplifiers with tapered-waveguidestructures

With an aim toward a high-saturation power operation, a tapered-waveguide traveling-wave semiconductor laser amplifier (SLA) with a Ga_0.68In_0.32As/GaInAsP/InP tensile-strained quantum-well (QW) structure was investigated and realized for the first time. In spite of the high differential gain in the tensile-strained QW active layer, a high-saturation-output power of 18.5 dB (1 mW) (71 mW) and a high-maximum-output power of 20.4 dB(1 mW) (110 mW) were obtained with a narrow single-lobed beam-divergence property. These results indicate that the saturation performances of SLA's with tensile-strained active layers can be improved by the tapered-waveguide structures     

First digital optical switch based on InP/GaInAsP double heterostructure waveguides

A Y-junction digital optical switch based on InP/GaInAsP has been realised and operated using either carrier injection or carrier depletion. the switch functions with both TE and TM polarised light. In the case of current injection, 30 mA is enough to obtain a crosstalk up to 20 dB for TM polarisation and 10 dB for TE polarisation.<>     

Suppression of TE polarization by a multiquantum-barrier structure in sub-630-nm tensile-strained GaInP-AlGaInP QW lasers

Many applications require laser emission with stable single polarization. While stable transverse-electric (TE) emission is favored by several mechanisms and, consequently, is relatively easy to obtain, achieving stable transverse-magnetic (TM) emission is more difficult. This letter proposes a method for suppressing the TE polarization by using a multiquantum-barrier. In our experiments, the multiquantum-barrier structure was inserted into the p-type cladding of tensile-strained GaInP-AlGaInP quantum-well lasers emitting at wavelengths shorter than 630 nm. As a result, the TE emission was suppressed over a wide range of injection levels, operating temperatures, and device lengths, proving that the method is effective for achieving stable TM emission.     

Polarization-independent field-induced absorption-coefficientvariation spectrum in an InGaAs/InP tensile-strained quantum well

The authors report the absorption-coefficient variation of a tensile-strained InGaAs/InP quantum well measured throughout the spectral range near and away from the bandgap. They discuss the spectral absorption-coefficient variation spectra of an unstrained, a 0.15%, a 0.30%, and a 0.45% tensile-strained quantum well and show that the difference between the wavelengths of absorption-coefficient variation peaks for TE and TM modes becomes zero with 0.3% tensile strain. It is shown that this wavelength difference varies linearly with the magnitude of the strain     

Analysis and optimization of polarization-insensitive semiconductoroptical amplifiers with delta-strained quantum wells

Polarization sensitivity of semiconductor optical amplifiers (SOAs) with delta-strained quantum-well (QW) structures is investigated. The valence band structures and TE, TM optical gain spectra are calculated for the various delta-strained QW structures. It is shown that the number and location of the delta layers affect the polarization dependence of the delta-strained quantum well SOA signal gains. The optimal delta-strained QW structure for the SOA application is identified and its theoretical verification is provided     

Polarization dependence of linewidth enhancement factor inInGaAs/InGaAsP MQW material

Measurements and calculations on the differential gain, the differential refractive index, and the linewidth enhancement factor have been performed for unstrained quantum-well (QW) material. The differential refractive index is considerably lower for the transverse magnetic (TM) polarization than for the transverse electric (TE) polarization, which is ascribed to absence of the plasma effect for the TM polarization. This has implications for the linewidth enhancement factor and thus linewidth and chirp in QW lasers     

Polarization dependence of a 1.52 μm InGaAs/InP multiple quantumwell waveguide electroabsorption modulator

A ridge-waveguide In_0.53Ga_0.47As/InP multiple-quantum-well (MQW) electroabsorption modulator operating at a wavelength of 1.52 μm is demonstrated. The modulator exhibits large polarization-dependent electroabsorption behavior which favors the modulation of the TM mode. At a reverse bias of 10 V, the modulator has a 24.5-dB extinction ratio for the TM mode, whereas that for the TE mode is only 11.1 dB. Polarization-dependent saturation of absorption has been observed in this device for incident optical power levels of less than 1 mW     

Gain saturation properties of a semiconductor gain medium withtensile and compressive strain quantum wells

Gain saturation properties of a multiple-quantum-well structure with both tensile and compressively strained quantum wells are investigated analytically. This type of structure has recently been experimentally demonstrated to serve as a basis for the implementation of a two-polarization/two-frequency laser and polarization insensitive travelling wave(TW) amplifier. The performance of these devices strongly depends on the interaction between the TE and TM gains of the structure. The gain medium model appropriate for this type of structure is developed and the rate equation approach is used to describe the saturation properties of TE/TM gains and the coupling between the TE and TM gains due to gain saturation. The minimum amount of coupling between the two is governed by the basic symmetry of the light-hole wavefunction which interacts with photons of both polarization: photon cross-coupling. The finite rate of carrier escape from the quantum wells provides for carrier induced coupling between the populations of the two well types and therefore also couples TE and TM gains: carrier cross-coupling. The performance of a polarization insensitive amplifier, laser, and polarization control element is evaluated as a function of the amount of carrier cross-coupling, which is a structure dependent parameter. A structure with high degree of cross-coupling is desirable for polarization insensitive TW amplifier, while two-polarization lasers and polarization control elements require minimum cross-coupling     

Measurements of the polarization dependence of the gain of strained multiple quantum well InGaAs-InP lasers

The polarization-dependent gain spectra of both tensile and compressive strain multiple-quantum-well (MQW) In/sub x/Ga/sub 1-x/As-InP lasers in a relatively large strain regime are presented. The results show that MQW lasers with tensile strain and an In concentration as low as 43% in the wells lase in a pure transverse magnetic (TM) mode rather than a transverse electric (TE) mode with a gain difference of 60-70 cm/sup -1/ at all the injection currents investigated. The peak gain for the TE mode is shifted toward shorter wavelengths from that of the TM mode, indicating that the emission is principally due to light hole-electron transition. The differential gain of the TM mode is about 1.5 times higher than that of the TE mode operation. Opposite phenomena were observed in the compressive strained MQW lasers.<>     

Nano-engineering approaches to self-assembled InAs quantum dot laser medium

A number of nano-engineering methods are proposed and tested to improve optical properties of a laser gain medium using the self-assembled InAs quantum dot (QD) ensemble. The laser characteristics of concern include higher gain, larger modulation bandwidth, higher efficiency at elevated temperatures, higher thermal stability, and enhanced reliability. The focus of this paper is on the management of QD properties through design and molecular beam epitaxial growth and modification of QD heterostructures. This includes digital alloys as high-quality wide-bandgap barrier; under- and overlayers with various compositions to control the dynamics of QD formation and evolution on the surface; shape engineering of QDs to improve electron-hole overlap and reduce inhomogeneous broadening; band engineering of QD heterostructures to enhance the carrier localization by reduction of thermal escape from dots; as well as tunnel injection from quantum wells (QWs) to accelerate carrier transfer to the lasing state. Beneficial properties of the developed QD media are demonstrated at room temperature in laser diodes with unsurpassed thermal stability with a characteristic temperature of 380 K, high waveguide modal gain >50 cm⁻¹, unsurpassed defect tolerance over two orders of magnitude higher than that of QWs typically used in lasers, and efficient emission from a two-dimensional (2-D) photonic crystal nanocavity.     

Self‐Assembled InAs Quantum Dots on InP(001) for Long‐Wavelength Laser Applications

Self‐assembled InAs quantum dots (QDs) embedded in an InAlGaAs matrix were grown on an InP (001) using a solid‐source molecular beam epitaxy and investigated using transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. TEM images indicated that the QD formation was strongly dependent on the growth behaviors of group III elements during the deposition of InAlGaAs barriers. We achieved a lasing operation of around 1.5 µm at room temperature from uncoated QD lasers based on the InAlGaAs‐InAlAs material system on the InP (001). The lasing wavelengths of the ridge‐waveguide QD lasers were also dependent upon the cavity lengths due mainly to the gain required for the lasing operation.     

Envelope function calculations of linear and nonlinear opticalgains in a strained-layer quantum-well laser

The linear and nonlinear optical gain of strained-layer InGaAs-AlGaAs quantum well (QW) lasers are studied theoretically, with band mixing effects taken into account. Effects of the biaxial compressive strain of the InGaAs-AlGaAs QW on the band structure are investigated by solving for the Pikus-Bir Hamiltonian. The biaxial compressive strain separates the HH and the LH subbands by pulling down the HH subbands and pushing the LH subbands away from the valence band edge. Since the C-HH transition is dominated by the TE polarization, it is expected that the TE mode gain would be substantially larger than the TM mode gain. The gain and the gain-suppression coefficient are calculated from the complex optical susceptibility obtained by the density matrix formalism. Optical output power is calculated by solving the rate equations for the stationary states with nonlinear gain suppression. The calculated L-I characteristics shows reasonable agreement with the experimental data     

Rate equation model for nonequilibrium operating conditions in aself-organized quantum-dot laser

A nonequilibrium rate equation model is presented and analyzed for the self-organized quantum dot (QD) laser. The model assumes the QD zero dimensional levels are coupled to a thermal electron distribution in the wetting layer through reservoir rate equations. By including the energy dependence of the wetting layer reservoir versus temperature, the model accounts for the spectral narrowing of the gain with increasing temperature, the negative temperature coefficient of the lasing threshold, and a reduction of the spectral hole burning with increasing temperature, all found experimentally in QD lasers     

InP-based 10-GHz bandwidth polarization diversity heterodynephotoreceiver with electrooptical adjustability

A polarization diversity optical receiver, integrated with two pairs of balanced photodiodes in the InP/InGaAsP material system, is described. This circuit includes two polarization splitters based on modal birefringence and, for the first time, adjustable 3-dB TE and TM directional couplers (relaxing fabrication tolerances). On-chip losses are below 2.5 dB (TE) and 5.5 dB (TM). Waveguide to PIN coupling efficiency is >95%. Polarization crosstalk is in the 9-10-dB range, 3-dB couplers balance can be recovered, and common mode rejection ratio (CMRR) lower than -30 dB is obtained and remains below -20 dB over 6 GHz. Balanced receiver circuit 3-dB bandwidth is in excess of 10 GHz     

High-performance strain-compensated InGaAs-GaAsP-GaAs(λ=1.17 μm) quantum well diode lasers

This letter reports studies on highly strained and strain-compensated InGaAs quantum-well (QW) active diode lasers on GaAs substrates, fabricated by low-temperature (550°C) metal-organic chemical vapor deposition (MOCVD) growth. Strain compensation of the (compressively strained) InGaAs QW is investigated by using either InGaP (tensile-strained) cladding layer or GaAsP (tensile-strained) barrier layers. High-performance λ=1.165 μm laser emission is achieved from InGaAs-GaAsP strain-compensated QW laser structures, with threshold current densities of 65 A/cm² for 1500-μm-cavity devices and transparency current densities of 50 A/cm². The use of GaAsP-barrier layers are also shown to significantly improve the internal quantum efficiency of the highly strained InGaAs-active laser structure. As a result, external differential quantum efficiencies of 56% are achieved for 500-μm-cavity length diode lasers     

Incoupling waveguide hologram with reduced polarization sensitivity

An incoupling waveguide hologram (IWGH) with significantly reduced polarization sensitivity was designed and fabricated in InP for 1550 nm wavelength. The IWGH couples the light from an optical fiber, irrespectively of the state of polarization, into the InP waveguide and simultaneously focuses it to a desired position in the waveguide. Conventional IWGHs are strongly polarization sensitive with a measured 19 dB difference in the incoupling efficiency between the TE and TM mode. In contrast, although some design parameters turned out to be slightly in error, the fabricated IWGHs designed for reduced polarization sensitivity exhibited a 3.1 dB difference in the incoupling efficiency between the TE and TM modes