We report an 830nm high power single spatial mode DFB laser design in the AlGaAs/GaAs system that offers
performance close to a Fabry-Perot design as well as manufacturing yield compatible with volume production.
Single-mode power in excess of 200mW at case temperature up to 600C is consistently obtained for current below
300mA. This performance level is enabled by use of an efficient, partially-corrugated design and a 2nd order grating
located on the p-side. Through careful design and an optimized epitaxial re-growth on the grating, promising
reliability results compatible with uncooled application are demonstrated.
The authors report on the nanowires-like and nanodots-like lasing behaviors in addition to multiple-wavelength
interband transitions from InAs/InAlGaAs quantum dash (Qdash) lasers in the range of ~1550 nm. The presence of
lasing actions simultaneously from two different dash ensembles, after postgrowth intermixing for crystalline quality
improvement, indicate the absence of optical phonon emission due to the small variation in quantized interband
transition energies. This effect is reproducible and shows different lasing characteristics from its quantum dot and
quantum wire laser counterparts. Furthermore, the small energy spacing of only 25 nm (at center lasing wavelength of
~1550 nm) and the subsequent quenching of higher energy transition states at higher bias level in Qdash lasers suggest
the absence of excited-state transition in highly inhomogeneous self-assembled Qdash structures. However, the
appearance of a second lasing line in a certain range of high injection level, which is due to the presence of different
sizes of dash assembles, corresponds to the transition from smaller size of Qdash ensembles in different planar active
medium. This unique transition mechanism will affect the carrier dynamics, relaxation process in particular and further
indicates localized finite carrier lifetime in all sizes of Qdash ensembles. These phenomena will lead to important
consequences for the ground-state lasing efficiency and frequency modulation response of Qdash devices. In addition,
these imply that proper manipulation of the Qdash ensembles will potentially result in localized nanolasers from
individual ensemble and thus contributing towards enormously large envelope lasing coverage from semiconductor
devices.
We present the development of theoretical model based on multi-population rate equation to assess the
broadband lasing emission in addition to the derivative optical gain and chirp characteristics from the supercontinuum
InGaAs/GaAs self-assembled quantum-dot (QD) interband laser. The model incorporates the peculiar characteristics
such as inhomogeneous broadening of the QD transition energies due to the size and composition fluctuation,
homogeneous broadening due to the finite carrier lifetime in each confined energy states, and the presence of continuum
states in wetting layer. We showed that the theoretical model agrees well with the experimental data of broadband QD
laser. From the model, the broadband lasing characteristics can be ascribed to the large dispersion of QD with varying
energy sub-bands and the change of de-phasing rate. These interesting characteristics can be attributed to the carrier
localization in different dots that result in a system without a global Fermi function and thus an inhomogeneously
broadened gain spectrum. Furthermore, our simulation results predict that the linewidth enhancement factor (α = 2) from
the ground state (GS) in this new class of semiconductor lasers is slightly larger but in the same order of magnitude as
the values obtained in conventional QD lasers. The calculated gain spectrum shows similar magnitude order of material
differential gain (~10-16 cm2) and material differential refractive index (~10-20 cm3) as compared to conventional QD
lasers. The comparable derivative characteristics of broadband QD laser shows its competency in providing low
frequency chirping as well as a platform for monolithic integration operation.
We demonstrate the widened broadband emission of self-assembled quantum dash laser using impurity-free vacancy
induced disordering (IFVD) technique. The 100 nm blueshifted lasers exhibit higher internal quantum efficiency and
lower threshold current densities than the as-grown devices. The laser emission from multiple groups of quantum-dash
(Qdash) families convoluted with multiple orders of subband energy levels within a single Qdash ensemble is
experimentally observed. However, the suppression of laser emission in short wavelength and the progressive redshift of
peak emission with injection current from devices with short cavity length occur. These effects have been attributed to
the nonequilibrium carrier distribution and energy exchange among different sizes of Qdash ensembles. In addition, we
perform the far-field lateral mode measurements from the fabricated as-grown Qdash laser. The analysis of mode
patterns indicate that Qdash lasers exhibit gradual broadening of beam divergence (FWHM of 3.4° to 10.8°) with
increasing injection current. However, these beam divergence angles are still narrower than the quantum well (QW) laser
(FWHM ~13°) at an injection up to 2.5 x Jth. Qdash laser exhibits an improved output beam quality, therefore reduced
filamentation, as compared to the QW laser, owing to the inherent characteristics from quantum-dot (Qdot) laser, where
injected carriers are confined by the lateral energy barriers as Qdots are disconnected laterally and are cladded by larger
bandgap materials. Our results imply a highly attractive wavelength trimming method, well suited for improved
performance, and monolithic Qdash integration of optoelectronics components.
This paper reports on the measurement and analysis of the coherence function for broadband emitters such as
superluminescent diodes (SLDs) and novel broadband laser diodes (BLDs) from self-assembled InGaAs/GaAs quantumdot
(QD) and InAs/InP quantum-dash (Qdash) structures that emit at center wavelengths of 1150nm and 1650nm,
respectively. Using the fiber-based spectral interferometry system, coherence lengths in fiber of 23 μm and 48 μm have
been measured from the QD and Qdash BLDs. Larger spectral bandwidth of 137 nm and 78 nm have been measured
from the QD and Qdash SLDs that yield coherence lengths in fiber of 3 μm and 10 μm, respectively. The coherence
function of both BLDs and SLD reveals negligible secondary coherence subpeaks and sidelobes indicating the
possibility of using these broadband sources to produce low artifacts optical coherence tomography (OCT) images.
Quantum-confined Stark effect in zero-dimensional semiconductor quantum-dot (QD) has attracted considerable interest
due to the potential applications in electro-optic modulation and quantum computing. Composition interdiffusion occurs
easily during the high temperature epitaxial growth or ex situ annealing treatment, therefore understanding the effects of
interdiffusion is essential for device design and modeling. However, relatively little attention has been devoted to a
systematic study of this effect. In this paper, the effects of isotropic interdiffusion on the optical transition energy of
self-assembled InAs/GaAs QD structure under an electric field have been investigated theoretically. Our
three-dimensional QD calculation is based on coupled QDs with different shapes arranged periodically in a tetragonal
superlattice, taking into account the finite band offset, valence-band mixing, strain, and effective mass anisotropicity.
The electron and hole Hamiltonians with the interdiffusion effect are solved in the momentum space domain. Our results
show that isotropic three-dimensional In-Ga interdiffusion will makes the Stark shift become more symmetry about F=0 in asymmetric lens-shaped and pyramidal QDs, implying the reduced build-in dipole momentum. The
interdiffusion also leads to enhanced Stark shift with more prominent effects to QDs that are under larger electric fields.
Authors report the demonstration of the emission wavelength tuning of InAs quantum-dashes within InAlGaAs
quantum-wells grown on InP substrate, that gives the initial wavelength emission at ~1.65 &mgr;m. The impurity-free
dielectric cap annealing and the nitrogen ion-implantation induced intermixing techniques have been implemented to
spatially control the group-III intermixing in the structure, which produces differential bandgap shift of 80 nm and 112
nm, respectively. Transmission electron microscopy, optical and electrical characterizations have been performed to
evaluate the quality of the intermixed QD material and bandgap tuned devices. Compared to the control (nonintermixed)
lasers, the light-current characteristics for the over 125 nm wavelength shifted QD lasers are not
significantly changed suggesting that the quality of the intermixed material is well-preserved. The intermixed lasers
exhibit the narrow linewidth as compared to the as-grown due to the improved QD homogeneity. The integrity of the QD
material is retained after intermixing suggesting the potential application for the planar integration of multiple
active/passive QD-based devices on a single InP chip.
We report the fabrication and characterization of broad emission linewidth GaAs/AlGaAs quantum-well based
superluminescent diodes. A photon absorption section and an optical amplifier sections are monolithically integrated on
the device to suppress feedback oscillation and to amplifier the optical power, respectively. The device emitters at
850 nm peak wavelength, and exhibits a broad bandwidth of 65 nm, output power > 3.5 mW, and a spectral ripple of 0.5
dB at 20oC under continuous wave operation.
We investigated the multiple cations intermixing in InAs/InGaAlAs quantum dot-in-well laser structure grown on InP substrate using impurity-free vacancy disordering (IFVD) technique. Selective control of the bandgap shifts has been achieved using SiO2 and SixNy annealing caps. A differential wavelength shift of 76 nm has been observed after a rapid thermal annealing step at 750 oC for 30 s. In contrast to most IFVD results in other materials, we observed a larger bandgap shift from the SixNy capped samples than from the SiO2 capped samples. Based on theoretical calculations, we attribute this to the different effective interdiffusion rates of group-III cations. The demonstrated intermixing process provides an effective method for fine tuning the bandgap of InAs QDs around 1.55 μm as an alternative to the growth manipulation, as well as for realizing photonics integrated circuits.
The effect of quantum nanostructure interdiffusion or intermixing using impurity free vacancy diffusion (IFVD) and impurity induced disordering (IID) techniques, on the various types of GaAs- and InP-based quantum-well (QW) and quantum-dot (QD) structures for postgrowth photonic device integration is presented Using IFVD, we have demonstrated the fabrication of photonic integrated circuits such as multi-wavelength laser chips and broadband superluminescent diodes in GaAs/AlGaAs QW structures. Postgrowth bandgap engineering using low energy neutral ion implantation technique has been developed in InGaAs/InGaAsP QW systems. Using this interdiffusion technique, 10-channel multi-wavelength lasers and 8-channel integrated optical performance monitors have been demonstrated. The effects of IFVD and IID on the interdiffusion of InGaAs/GaAs QDs have been investigated. High spatial bandgap selectively processes with differential bandgap shifts between the interdiffused and non-interdiffused sections of over 100 meV have been observed from both IFVD and IID techniques in InGaAs/GaAs QD laser structure. At theoretical modeling level, we have developed a unified three-dimensional model for the electronic states calculation of the interdiffused quantum nanostructures with arbitrary shape, which is universally applicable to various quantum structures such as QW, quantum wire, quantum dash, and QDs. This model serves as a valuable tool to gain a more physical understanding of interdiffusion or intermixing techniques suitable for the integration technology in quantum nanostructures.
A new wavelength monitor based on a multimode interference waveguide is proposed for multiwavelength communication applications. The device characteristics are studied using the beam propagation method. By adjusting the waveguide's geometric length, different wavelength ranges can be addressed. Each device can monitor up to a 50-nm range and has accuracy <5 Å.
The realization of single-mode Distributed Feedback (DFB) and Distributed Bragg Reflector (DBR) lasers, based on surface grating structures is of considerable interest. Such devices offer a relatively simple grating fabrication process without complicated multistep-epitaxial growth or regrowth, as required in more conventional devices. This simplified processing could potentially reduce the fabrication cost for these lasers. A key concern for the surface grating lasers is designing the structure to provide sufficient feedback to achieve single mode operation with high yield and high quality as compared to conventional buried-grating DFB and DBR lasers. This paper reports numerical modelling of surface grating DFB and DBR lasers based on 1.55 micrometers wavelength InGaAs/InGaAsP/InP with graded-index separate confinement heterostructure multiple quantum well (GRINSCH-MQW) structures. Bragg wavelength operation of the DFB and DBR lasers may be satisfied by deeply etching fine surface gratings on both side portions and along top of the ridge stripe respectively. Sufficiently strong optical coupling between the corrugated structure and evanescent field can be achieved by controlling various parameters such as ridge width, etch depth, and grating width. The numerical results obtained allow optimum grating geometries to be developed to provide the desired feedback effect. Furthermore, the model has been used to investigate the influence on the device performance of fabrication errors and processing effects on the grating structures, prior to fabrication.
KEYWORDS: Refractive index, Near field, Waveguides, Digital filtering, Channel waveguides, Spatial frequencies, Linear filtering, Optical filters, Cameras, Signal to noise ratio
Refractive index profile of surface channel waveguides can be determined by analyzing the near-field intensity pattern. A mathematical model, the inverse Helmholtz equation, is derived in order to use these data to reconstruct the refractive index profile under consideration. In this work, the measured near-field intensities are preprocessed by means of gamma correction, background noise subtraction, Fast Fourier Transform, and low pass finite impulse response (FIR) digital filter. Several types of FIR windows are chosen. The results are used to reconstruct the refractive index profile of the waveguide. The results show that the application of low pass FIR digital filter by using Hamming window reduces noises better than other windows. The application of this method in determination of refractive index profile of annealed proton exchange LiNbO3 channel waveguides is demonstrated.
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