Semiconductor lasers using diffused quantum-well structures

We assess the relative merits and prospects of using diffused quantum-well (QW) structures in semiconductor lasers. First, different techniques to achieve interdiffusion are analyzed and compared. Second, recent development of semiconductor lasers using interdiffusion technique is also discussed. Third, the optical properties of diffused QWs are studied. In addition, novel design of diffused QWs structures to maintain stable single-mode operation in semiconductor lasers is proposed. Finally, brief discussion and conclusion are given     

Recent Advances on InAs/InP Quantum Dash Based Semiconductor Lasers and Optical Amplifiers Operating at 1.55 μm

This paper summarizes recent advances on InAs/InP quantum dash (QD) materials for lasers and amplifiers, and QD device performance with particular interest in optical communication. We investigate both InAs/InP dashes in a barrier and dashes in a well (DWELL) heterostructures operating at 1.5 mum. These two types of QDs can provide high gain and low losses. Continuous-wave (CW) room-temperature lasing operation on ground state of cavity length as short as 200 mum has been achieved, demonstrating the high modal gain of the active core. A threshold current density as low as 110 A/cm² per QD layer has been obtained for infinite-length DWELL laser. An optimized DWELL structure allows achieving of a T₀ larger than 100 K for broad-area (BA) lasers, and of 80 K for single-transverse-mode lasers in the temperature range between 25degC and 85degC. Buried ridge stripe (BRS)-type single-mode distributed feedback (DFB) lasers are also demonstrated for the first time, exhibiting a side-mode suppression ratio (SMSR) as high as 45 dB. Such DFB lasers allow the first floor-free 10-Gb/s direct modulation for back-to-back and transmission over 16-km standard optical fiber. In addition, novel results are given on gain, noise, and four-wave mixing of QD-based semiconductor optical amplifiers. Furthermore, we demonstrate that QD Fabry-Perot (FP) lasers, owing to the small confinement factor and the three-dimensional (3-D) quantification of electronic energy levels, exhibit a beating linewidth as narrow as 15 kHz. Such an extremely narrow linewidth, compared to their QW or bulk counterparts, leads to the excellent phase noise and time-jitter characteristics when QD lasers are actively mode-locked. These advances constitute a new step toward the application of QD lasers and amplifiers to the field of optical fiber communications     

Large- and small-signal dynamic behavior of high-speeddual-polarization quantum-well semiconductor lasers

The transmission-line laser model is modified to model both transverse-electric (TE) and transverse-magnetic (TM) modes so that it is applicable to quantum-well (QW) dual-polarization lasers and polarization-insensitive semiconductor optical amplifiers (SOAs). The effects of carrier transport are also included in the model. The resulting dual-polarization transmission-line laser model is used to study large- and small-signal dynamic behavior of dual-polarization lasers. We find from large-signal simulations that the polarization asymmetry (ratio of the transverse-modal powers) varies on a nanosecond time scale in dual-polarization single-quantum-well (SQW) devices. We show that unequal transverse-modal differential gains and gain nonlinearities are responsible for this temporal polarization asymmetry. In addition, our numerical simulations show that the steady-state polarization asymmetry is a strong function of the gain nonlinearity. Small-signal dynamic simulations show that the modulation response of the polarization-unresolved output of dual-polarization SQW lasers follows that of the transverse mode with the lowest gain nonlinearity coefficient, regardless of the transverse-modal differential gains     

Comprehensive modeling of diffused quantum-well vertical-cavitysurface-emitting lasers

A numerical model for investigating the thermal, electrical, and optical characteristics of vertical-cavity surface-emitting: lasers (VCSELs) with a diffused quantum-well (QW) structure is presented. In the model, the quasi-three-dimensional (quasi-3-D) distribution of temperature, voltage and optical fields as well as the quasi-two-dimensional (quasi-2-D) diffusion and recombination of carrier concentration inside the QW active layer are calculated in a self-consistent manner. In addition, the quasi-3-D distribution of implanted ions before and after thermal annealing are computed. The variation of electrical conductivity and absorption loss as well as the influence of impurity induced compositional disordering on the optical gain and refractive index of the QW active layer are also taken into consideration. Using this model, the steady-state characteristics of diffused QW VCSELs are studied theoretically. It is shown that significant improvement of stable single-mode operation can be obtained using diffused QW structure     

Modeling and measurement of the wavelength-dependent output properties of quantum-well optical amplifiers: effects of a carrier-dependent escape time

We present a model for quantum-well (QW) semiconductor optical amplifiers (SOAs) that considers bidirectional field propagation and the carrier densities in the barrier and QW regions. Carrier capture from the barriers into the QWs and carrier escape from the QWs to the barriers are included by means of effective capture and escape times. The model incorporates the wavelength dependence of the optical response of the active region and the effects of spectral hole burning via an analytical approximation to the susceptibility of the active material, which allows one to very effectively include the wavelength dependence of the output properties of the SOA. The model is used to analyze the experimental results obtained for a multiquantum-well SOA. The simulations results show a good agreement with the experimental data when a carrier-density dependent escape time from the QW to the barrier regions is considered.     

Reduction of spectral-linewidth in high power SOA integrated wavelength selectable laser

The effect of optical feedback from various reflected points on the spectral-linewidth was analyzed for a wavelength selectable laser consisting of a 12 distributed feedback (DFB) laser array, multimode interference coupler, and semiconductor optical amplifiers (SOA). For all 12 DFB lasers, a narrow linewidth of less than 2 MHz was achieved with over 50 mW by reducing facet reflectivity.     

Influences of unconfined states on the optical properties ofquantum-well structures

The population of the unconfined states, with energies above the band edge of the barrier layers, can be significant in some regions of the active volume in high power lasers and amplifiers. This paper analyzes the influences of these states on optical properties, such as gain, refractive index, differential gain, and linewidth enhancement factor, for different quantum-well (QW) structures. Our results show that at high excitation levels, the unconfined band contributions to the real part of the optical susceptibility can be significant, especially in structures with weak quantum confinement potentials. This is in agreement with recent measurements of peak gain and carrier-induced refractive index change versus carrier density, for InGaAs-GaAs QW laser structures     

1 THz Modulation in InGaAsP Multiple Quantum Wells for 40 Gb/s Applications

The terahertz (THz) rate modulation of quantum well (QW) electrooptic modulators necessitates a new way of thinking about how the modulation field modulates light; specifically, an incident narrow-band (with respect to the modulation frequency) signal once modulated acquires frequency components separated from the input signal center frequency by multiples of the modulation frequency. In this paper, we discuss the design of the QWs comprising the modulator to maximize the output at such THz sidebands of the incident optical frequency in InGaAsP QW based devices. We present a theoretical treatment of the case in which the THz modulation frequency is out of resonance between any exci- tonic levels near those exploited for the optical modulation, thus enabling an adiabatic treatment of the modulated optical susceptibility. We show that THz sideband conversion efficiencies of ~1% may be possible.     

Effect of nitrogen on the band structure and material gain of In/sub y/Ga/sub 1-y/As/sub 1-x/N/sub x/-GaAs quantum wells

The conduction subband structure of InGaAsN-GaAs quantum wells (QWs) is calculated using the band anticrossing model, and its influence on the design of long-wavelength InGaAsN-GaAs QW lasers is analyzed. A good agreement with experimental values is found for the QW zone center transition energies. In particular, a different dependence of the effective bandgap with temperature when compared to the equivalent N-free structure is predicted by the model and experimentally observed. A detailed analysis of the conduction subband structure shows that nitrogen strongly decreases the electron energies and increases the effective masses. A very small N incorporation is also found to increase the nonparabolicity, but this effect saturates for higher nitrogen contents. Both the In content and well width decrease the effective masses and nonparabolicity of the conduction subbands. Material gain as a function of the injection level is calculated for InGaAsN-GaAs QWs for moderate carrier densities. The peak gain at a fixed carrier density is found to be reduced, compared to InGaAs, for a small N content, but this reduction tends to saturate when the N content is further increased. For the gain peak energy, a monotonous strong shift to lower energies is obtained for increasing N content, supporting the feasibility of 1.55-/spl mu/m emission from InGaAsN-GaAs QW laser diodes.     

Interdiffusion induced modification of surface-acoustic-waveAlGaAs-GaAs quantum-well modulators

A theoretical study of short period AlGaAs-GaAs diffused quantum-well (QW) absorption modulators operated by using surface acoustic waves (SAWs) is carried out in this paper. The as-grown QW structure is optimized and interdiffusion is used to fine tune the modulation performance. The results show that a stack of QWs can be developed at the top surface of the modulator to utilize the steep potential induced by SAWs. The optimized structure can also produce a large absorption change and thus a fast modulation speed for the same modulation depth. In comparison to previous results, the required surface acoustic wave has a longer wavelength and a lower power so that the fabrication of the interdigital transducer can be simplified. In addition, the use of interdiffusion can provide an useful fine adjustment to the operating wavelength, further enhancement of the modulation depth and an improvement in chirping with the only drawback of an increased absorption loss     

Very low threshold current density of 1.3-/spl mu/m-range GaInNAsSb-GaNAs3 and 5 QWs lasers

The dependence of the threshold current density on the number of wells for 1.3-/spl mu/m-range edge emitting lasers using GaInNAsSb novel material, at which the incorporation of the small amount of Sb make the two-dimensional growth condition wide, is studied. The lowest record ever reported for the threshold current density per well (Jth A/cm/sup 2//well@L=900 /spl mu/m) for 3 QWs lasers was achieved. GaInNAs-based 5 QWs lasers with the very low threshold current density per well of 160 A/cm/sup 2/ were successfully grown for the first time. Therefore, no significant deterioration of Jth is observed even though the number of wells increased up to 5. Since Jth of 5 QWs doesn't increased rapidly compared to SQW and 3 QWs as decreasing the cavity length, it is considered that lower Jth can be obtained by utilizing 5 QWs in devices such VCSELs which use short cavity length.     

Non-Markovian gain of strained-layer wurtzite GaN quantum-welllasers with many-body effects

A theoretical model for the optical gain of strained-layer wurtzite GaN quantum-well (QW) lasers is developed taking into account valence-band mixing, many-body effects and non-Markovian relaxation. The valence-band structure is calculated from a 6×6 multiband effective mass Hamiltonian for the wurtzite structure taking into account built-in strain due to lattice mismatch. The theoretical foundation for the optical processes is based on the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is predicted that the Coulomb enhancement of gain is pronounced with increasing magnitude of compressive strain in the QW     

Blueshifting of InGaAsP-InP laser diodes using a low-energyion-implantation technique: comparison between strained andlattice-matched quantum-well structures

Blueshifted InGaAsP-InGaAs-InP laser diodes have been fabricated using a technique that includes a low-energy ion implantation, used to generate point defects near the surface of the structure, followed by a thermal anneal which causes the diffusion of these defects through the quantum wells (QWs). This diffusion of point defects induces a local intermixing of atoms in the QWs and barriers, which results in a decrease in the emission wavelength of the devices. Results obtained with strained and lattice-matched QW structures are compared. For lattice-matched structures, electroluminescence wavelength shifts as large as 76 nm were obtained. Strained QW structures presented a much smaller blueshift (≈10 nm). In both cases, we observed no significant change of the threshold current caused by the intermixing process     

Quantum-dot heterostructure lasers

Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough occurred when techniques for self-organized growth of QD's allowed the fabrication of dense arrays of coherent islands, uniform in shape and size, and, simultaneously, free from undesirable defects. Recently, the figure of merit of QDHS lasers surpasses some of the key characteristics of QW devices in some of the most important applications     

Silicon oxynitride planar waveguiding structures for application inoptical communication

The refractive index of silicon oxynitride (SiON), a widely used material for integrated optics devices, can be chosen in a wide range between 1.45-2.0. We describe how the consequent large design freedom can be exploited on the one hand for a “standard” polarization independent optical channel waveguide having a favorable tradeoff between efficient fiberchip coupling and small bend radii (compact devices) and on the other hand for special-purpose and hybrid components where the refractive index should be finely adjusted for obtaining the desired functionality. We illustrate the applicability of SiON by describing a few devices for optical filtering in a new architecture for wavelength multiplexing, modulation, polarization splitting and second-harmonic generation     

Single-mode distributed feedback and microlasers based on quantum-dot gain material

Quantum-dot gain material fabricated by self-organized epitaxial growth on GaAs substrates is used for the realization of 980-nm and 1.3-/spl mu/m single-mode distributed feedback (DFB) lasers and edge-emitting microlasers. Quantum-dot specific properties such as low-threshold current, broad gain spectrum, and low-temperature sensitivity could be demonstrated on ridge waveguide and DFB lasers in comparison to quantum-well-based devices. 980-nm DFB lasers exhibit stable single-mode behavior from 20/spl deg/C up to 214/spl deg/C with threshold currents < 15 mA (1-mm cavity length). Utilizing the low-bandgap absorption of quantum-dot material miniaturized monolithically integrable edge-emitting lasers could be realized by deeply etched Bragg mirrors with cavity lengths down to 12 /spl mu/m. A minimum threshold current of 1.2 mA and a continuous-wave (CW) output power of >1 mW was obtained for 30-/spl mu/m cavity length. Low-threshold currents of 4.4 mA could be obtained for 1.3-/spl mu/m emitting 400-/spl mu/m-long high-reflection coated ridge waveguide lasers. DFB lasers made from this material by laterally complex coupled feedback gratings show stable CW single-mode emission up to 80/spl deg/C with sidemode suppression ratios exceeding 40 dB.     

0.98-μm multiple-quantum-well tunneling injection laser with98-GHz intrinsic modulation bandwidth

We demonstrate GaAs-based 0.98-μm multiple-quantum-well (MQW) tunneling injection lasers with ultrahigh-modulation bandwidths. Electrons are injected into the active region via tunneling, leading to a “cold” carrier distribution in the quantum wells (QWs). The tunneling time (2 pS) measured by time resolved differential transmission spectroscopy agrees with the capture time extracted form the electrical impedance measurement. The tunneling barrier prevents electrons from going over the active region into the opposite cladding layer. The carrier escape time in tunneling injection lasers is larger than that in conventional QW lasers. Enhanced differential gain, minimized gain compression and improved high frequency performance have been achieved. The -3-dB modulation bandwidth is 48 GHz and the maximum intrinsic modulation bandwidth is as high as 98 GHz     

Semiconductor arrayed waveguide gratings for photonic integrated devices

This paper reviews recent progress of the semiconductor arrayed waveguide gratings (AWGs) and the integrated semiconductor optical devices including the semiconductor AWGs. Recent research enables us to make semiconductor AWGs with sufficiently low insertion loss and polarization dependence. Using the semiconductor AWGs as core components, the integrated semiconductor optical devices are developing by integrating with the other semiconductor devices including photodetectors, optical amplifiers, and electroabsorption modulators.     

Monolithic integration via a universal damage enhanced quantum-wellintermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO₂, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3°) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 μm are demonstrated with low loss (α=4.1 cm^-1) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration     

Techniques for Polarization-Independent Cross-Phase Modulation in Nonlinear Birefringent Fibers

We present a theoretical, numerical, and experimental investigation of the polarization dependence of cross-phase modulation in nonlinear birefringent fibers. Two new methods are described for producing a polarization-independent spectral shift through cross-phase modulation of a weak probe signal by a copropagating strong optical pulse. The birefringence of the fiber and spectral separation between the pump and probe signals are shown to play a critical role in determining the polarization dependence of the cross-phase modulation process. The methods are experimentally verified in two different highly nonlinear fibers, and are used to achieve polarization-independent optical switching at speeds of up to 160 Gb/s.