Proceedings Volume 7597

Physics and Simulation of Optoelectronic Devices XVIII

Bernd Witzigmann, Fritz Henneberger, Yasuhiko Arakawa, et al.
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Proceedings Volume 7597

Physics and Simulation of Optoelectronic Devices XVIII

Bernd Witzigmann, Fritz Henneberger, Yasuhiko Arakawa, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 15 February 2010
Contents: 16 Sessions, 57 Papers, 0 Presentations
Conference: SPIE OPTO 2010
Volume Number: 7597

Table of Contents

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Table of Contents

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  • Front Matter: Volume 7597
  • Photovoltaic Devices I
  • Photovoltaic Devices II
  • Light Emitting Diodes
  • Heterostructures and Novel Materials
  • Dynamics and Nonlinearities I
  • Surface Emitting Lasers
  • Passive Optics Modeling
  • Detectors, Modulators and Amplifiers
  • Random Semiconductor Lasers
  • Quantum Dot Lasers
  • Laser Mode Stabilization and Noise
  • Edge Emitting Light Sources
  • Dynamics and Nonlinearities II
  • Quantum Dots and Microcavities
  • Poster Session
Front Matter: Volume 7597
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Front Matter: Volume 7597
This PDF file contains the front matter associated with SPIE Proceedings Volume 7597, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
Photovoltaic Devices I
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Microscopic theory and numerical simulation of quantum well solar cells
Quantum well solar cells have been introduced as high efficiency photovoltaic energy conversion devices nearly twenty years ago. Since then, their capability to outperform bulk devices under concentrated illumination was established and efficiencies close to the single gap Shockley-Queisser limit for the corresponding bulk materials were reached. In order to further increase the efficiency, the mechanisms behind the extraordinary performance need to be analyzed and understood. To this end, novel theoretical approaches to the simulation of quantum optoelectronic devices are required, since the device behavior depends on complex processes between localized and extended states, such as carrier capture and escape into and from quantum wells, which cannot be consistently described by the combination of macroscopic semiconductor transport equations with detailed balance rate models conventionally used in photovoltaics. This paper presents a suitable theoretical framework based on the nonequilibrium Green's function formalism that allows the unification of quantum optics and dissipative quantum transport on the quantum kinetic level and is thus able to provide insight into the microscopic mechanisms of quantum photovoltaic device operation.
Zonal efficiency limit calculation for nanostructured solar cells
We extend the well-known Shockley-Queisser detailed balance calculation for determining the efficiency limit of a solar cell to the case of strong local deviations of the optical power absorption as present in nano-structured photovoltaic devices. In addition, the simple assumption of perfect absorption of all incident light exceeding the bandgap is refined. We present a modified Shockley-Queisser efficiency limit calculation for nano-structured photovoltaic devices, it incorporates a rigorous wave optics calculation and spatially resolved generation of electron-hole pairs. We apply this method to core-shell single-junction InP nanowire array for the use in concentrator solar cells. We investigate the efficiency limits regarding the arrangement of the active regions within the wire. Our results indicate that in a nanowire array solar cell with low volume fill factor the efficiency limit can approach the values of planar thin-film devices. This observation indicates the occurrence of micro-concentration and underlines the necessity of a wave optics approach. The spatially and spectrally resolved analysis shows that generation on the surface of the nanowires is considerable, particularly with regard to high energy photons. Therefore, it is necessary to efficiently extract those carriers.
Higher limiting efficiencies for nanostructured solar cells
Jessica G. J. Adams, Warren Elder, Geoff Hill, et al.
It is possible to tailor the band gap of the strain-balanced quantum well solar cell to match the local solar spectral conditions by altering the quantum well depth. This has led to a recent single-junction world-record efficiency of 28.3%, as well as giving advantages for current matching in multi-junction solar cells. Radiative recombination is the dominant loss mechanism for the strain-balanced quantum well solar cell, so practical improvements focus on techniques for light management in the cell, such as enhancing the optical path length with epitaxial mirrors. Furthermore, the compressive strain in the quantum wells suppresses emission into TM-propagating modes, reducing the overall optical loss and increasing the cell efficiency. As biaxial strain can only be engineered into a cell on the nanoscale, quantum well solar cells are seen to have a fundamental efficiency advantage over bulk semiconductor cells.
FDTD simulation of metallic gratings for enhancement of electromagnetic field by surface plasmon resonance
Enhancement of electromagnetic field by two dimensional arrays of rectangular and cylindrical nanopillars of both gold and silver metals arranged in either square or triangular lattices was investigated. We simulated these gratings by 3D Finite Difference Time Domain (3D-FDTD) method in visible and near infrared (NIR) wavelengths regime and investigated field enhancement by exciting surface plasmon polaritons (SPPs) as a function of geometrical parameters of grating. It was found that the geometrical grating parameters such as period, shape, thickness and size can be tuned for excitation of SPPs at particular frequency of interest. The tuned grating would lead to an electric field intensity enhancement by greater than 100× near the grating surface due to excitation of SPPs. Cylindrical gratings tuned for 750 nm at zero degree incident angle showed that the thickness of grating is the most sensitive geometrical parameter of resonance. Furthermore, triangular lattice gratings have wider bandwidth of resonance than square lattice gratings. Meanwhile, wavelength versus incident angle diagram showed that the enhancement was highly sensitive with angle of incidence.
Photovoltaic Devices II
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Analysis of changes in efficiency by simulating dye-sensitized solar cells varying the characteristics of TiO2
Alessio Gagliardi, Desiree Gentilini, Fabrizio Giordano, et al.
Dye Sensitized solar cells (DSC) are an interesting alternative to conventional silicon based solar cells. Although DSCs are very close to be commercialized, still many issues need to be addressed. Part of the problem is related to the lack of a reliable and consistent simulator able to catch the physics and the chemistry underlining the functioning of the cell. The need of a reliable simulator and modelling is particularly important for the engineering of the cell and to define trends not only in the component characteristics, but also in the building of the device. Among the different parts which compone a DSC the relevance of semiconductor titanium oxide substrate can hardly be underestimated. TiO2 is where the dye molecule is chemisorbed and where the recombination occurs. Moreover, changes in the topology of the semiconductor paste can lead to other smaller effects in the total efficiency. In this paper we investigate the effects of changing working parameters for the titanium oxide and varying its topology. The simulations are performed using a finite element code based on TiberCAD software1 to describe in details the electrical properties of the cell. The CAD allows to calculate steady-state properties and ideal I-V characteristics of the cell solving a set of differential equations on meshes in 1, 2 and 3 dimensions.
Light Emitting Diodes
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Simulation and design of core-shell GaN nanowire LEDs
B. Connors, M. Povolotskyi, R. Hicks, et al.
The high crystalline quality, large junction surface area, and insensitivity to c-axis oriented polarization fields make core-shell doped GaN nanowire p-n junctions exciting prospects for use as LEDs. The LED external efficiency depends upon the spatial distribution of optical recombination within the device, which may be controlled through the use of radial heterojunctions, such as quantum wells and electron blocking layers. In this work, we explore the impact of an axially varying doping profile on the spatial distribution of optical recombination in a GaN nanowire LED. The numerical simulation of the nanowire LED is carried out using the TiberCAD simulation package. This package provides a finite-element-based solution to the drift-diffusion model of a nanowire. Simulations of core-shell nanowire LEDs are performed with various doping profiles to produce variations in the optical recombination distribution throughout the device. In a core-shell device with a uniformly doped n-type core, the current density tends to travel primarily along the core, as the mobility of electrons is much greater than that of holes in GaN devices. The optical recombination is concentrated beneath the p-contact, where most of the current crosses the p-n junction. By properly setting a tapered doping profile in the n-type core, it is possible to increase the uniformity of the optical recombination along the junction. In certain geometries this will increase the emission efficiency of the nanowire LED.
Modeling of AlN/GaN superlattices for integration in near-UV distributed Bragg reflectors
A. Zorila, J. Jacquet, A. Ougazzaden, et al.
One of the main problem for the realization of high reflectivity GaN-based Bragg mirrors operating in the near-UV wavelength range is to limit the crack formation due to the lattice mismatch between the different nitride compounds while keeping a large refractive index contrast. Recent works have demonstrated that the introduction of several AlN/GaN superlattices (SLs) in a classical AlN/GaN quarter wavelength layers mirror structure strongly improved the crystalline quality and therefore the optical properties of such a mirror. In this work, several AlN/GaN SLs were studied for their direct use as pseudo-alloy layers pair material in a Bragg mirror. Such a configuration should allow combining the limitation of cracks by SLs with the improvement of the index contrast. First, the band structure of different AlN/GaN SLs was simulated using a self-consistent 8-band-k.p Schrödinger-Poisson solver. Then, the influence of surrounding layers such as AlN bulk ones on the band structure were considered. Using miniband-to-miniband transitions deduced from these calculations, refractive indices of these SLs were finally estimated for the design of an optimized high reflective Bragg mirror at 450 nm.
Optoelectronic and transport properties of nanocolumnar InGaN/GaN quantum disk LEDs
F. Sacconi, G. Penazzi, A. Pecchia, et al.
In this work we use the multi-scale software tool TiberCAD to study the transport and optical properties of InGaN quantum disk (QD) - based GaN nanocolumn p-i-n diode structures. IV characteristics have been calculated for several values of In concentration in the QD and of nanocolumn width. Strain maps show a clear relaxation effect close to the column boundaries, which tends to vanish for the larger columns. Effects of strain and polarization fields on the electron and hole states in the QD are shown, together with the dependence of optical emission spectra on geometrical and material parameters.
Role of the electron blocking layer in the current transport of efficient III-N light-emitting diodes
Oskari Heikkilä, Jani Oksanen, Jukka Tulkki
Electron blocking layers (EBLs) are commonly used to reduce the leakage current in modern multi-quantum well (MQW) InGaN light-emitting diodes (LEDs). We study the effect of the EBL and doping on the operation and efficiency of LEDs. We simulate both conventional MQW LEDs with AlGaN EBL, LEDs with quaternary AlInGaN EBL and LEDs without EBL. We show that the elimination of the polarization charges at the EBL interface greatly enhances the injection efficiency and that the hole injection in MQW lattice can be optimized by doping. The efficiency droop limiting the high power operation is also analyzed to determine the underlying mechanisms in the simulated MQW structures. Based on these results, we discuss the measures to increase the overall efficiency MQW structures.
Heterostructures and Novel Materials
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Properties of mid-IR diodes with n-InAsSbP/n-InAs interface
B. A. Matveev, A. V. Ankudinov, N. V. Zotova, et al.
IR imaging (2D radiation mapping) of forward biased point contact LEDs based on p-InAsSbP/n-InAs/n-InAsSbP DHs and p-InAsSbP/n-InAs SHs revealed differences in current crowding effect that was attributed to the properties of isotype n-InAsSbP/n-InAs barrier. 2D radiation distribution has been used for determination of L-I and I-V characteristics that are not distorted by the current crowding. The existence of the n-InAsSbP/n-InAs barrier was also traced in AFM and C-V measurements.
Band structure calculation of dilute-As GaNAs by first principle
Xiao-Hang Li, Hua Tong, Hongping Zhao, et al.
We present the band structure calculation of dilute-As GaNAs alloys (from 0% to 6.25% As) by employing the densityfunctional theory that adopts the local density approximation. Our studies indicate that the GaNAs shows a direct bandgap property. A small incorporation of As into the GaN alloy leads to the a significant decrease in the energy gap, which allows direct band gap transition covering from 3.47 eV (0% As) down to 1.93eV (6.25% As). The finding implies the dilute-As GaNAs alloy as an excellent candidate for the active material for optoelectronics that covers the entire visible spectral regime. The carrier effective masses of dilute-As GaNAs alloys are also presented.
Dynamics and Nonlinearities I
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Physical-random number generation using laser diodes' inherent noises
Hiroki Nishimura, Kohei Doi, Tetsurou Ushiki, et al.
Random numbers can be classified as either pseudo- or physical-random in character. This work demonstrates how laser diodes' inherent noise can be exploited for use in generating physical-random numbers in cryptographic applications. In the initial stages of the experiment, we measured a laser diode's output, at a fast photo detector and generated physical-random numbers from intensity noises. We then identified and evaluated the binary-number-line's statistical properties. Our preliminary results show that fast physical-random numbers are obtainable, using the laser diode's frequency noise characteristics.
Complex low energy gain switching pulse processing using a highly nonlinear optical loop mirror
This experimental work presented in this paper investigates the quality improvement of pulsed diode laser Gain- Switched (GS) optical pulses. GS is a straightforward technique, however, these pulses exhibit long duration (10ps - 100ps), low power (in the mW range), are asymmetric and often come accompanied by pedestals or subpulses. Simultaneous compression and reshaping of these input low quality pulses has been observed experimentally for several input power values. These effects have been achieved using a Highly Nonlinear Optical Loop Mirror (HNOLM) designed to directly process these long duration complex pulses thus eliminating the requirement for pre-pulse conditioning. The NOLM is based on a Microstructured Optical Fiber and a Highly Nonlinear Semiconductor Optical Amplifier. This device is compact, and offers overall benefits in terms of reduced system complexity.
Surface Emitting Lasers
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Quantum design and experimental realization of high-power VECSELs
S. W. Koch, J. Hader, J. V. Moloney, et al.
The quantum design of VECSEL structures is discusssed using a commercially available design tool. Examples of realized structures are presented and comparisons between experimental results and modelling predictions are shown.
Ultrafast circular polarization oscillations in spin-polarized vertical-cavity surface-emitting laser devices
N. C. Gerhardt, M. Li, H. Jaehme, et al.
Spin-polarized lasers offer new encouraging possibilities for future devices. We investigate the polarization dynamics of electrically pumped vertical-cavity surface-emitting lasers after additional spin injection at room temperature. We find that the circular polarization degree exhibits faster dynamics than the emitted light. Moreover the experimental results demonstrate a strongly damped ultrafast circular polarization oscillation due to spin injection with an oscillation frequency of approximately 11GHz depending on the birefringence in the VCSEL device. We compare our experimental results with theoretical calculations based on rate-equations. This allows us to predict undamped long persisting ultrafast polarization oscillations, which reveal the potential of spin-VCSELs for ultrafast modulation applications.
Passive Optics Modeling
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Micro diffraction lenses with subwavelength structures designed by the genetic algorithm
Tatsuya Shirakawa, Kenichi L. Ishikawa, Shuichi Suzuki, et al.
We design subwavelength structure (SWS) micro diffraction lenses by a method based on the finite-difference timedomain (FDTD) method and the genetic algorithm (GA). By using this GA-FDTD method, we designed lenses with binary subwavelength structures whose grating width is 100 nm at least and grating aspect ratio is 5 at most. The material refractive index, and lens radius are 2.1466 at 660 nm and 4.0 ìm, respectively. The focusing efficiency (59.3 %) of the obtained structure at 660 nm at a focal distance of 8.3 ìm is close to that of the Fresnel lens (70.4 %) and is more than twice as high as that (28.0 %) previously reported based on direct binarization of the Fresnel lens. The optimized grating height (400 nm) is different from that of the usual Fresnel lens (576 nm), and the relation between the widths of neighboring gratings cannot be expressed in a simple way. Also, the designed lens turns out to be robust against fabrication error. Furthermore, we have designed a lens with equal focal length of 8.3 ìm at three different wavelengths (660, 532, and 445 nm). The structures, which are not intuitive, are hard to deduce from experiences. These results indicate the effectiveness of the GA-FDTD method in the design of SWS diffractive optical elements.
Study of propagation modes of bent waveguides and micro-ring resonators by means of the aperiodic Fourier modal method
Davide Bucci, Bruno Martin, Alain Morand
In the last years, several numerical methods have been studied and applied to the analysis of high index contrast bent waveguides. Very often, the problem is treated using a conformal mapping, which translates the bending into an equivalent graded index profile and a straight waveguide. In this article, we discuss the implementation of a full vectorial 2D mode solver by means of the Aperiodic Fourier Modal Method, developed directly in cylindrical coordinates. This does not require the conformal mapping technique. In the first part of our work, we develop a shorthand notation and the mathematical rules useful to describe the problem in a matrix form. The calculation of propagation modes is then reconducted to the search of eigenvectors of a matrix. We will at first confront our formulation in 1D with results described in the literature. In a second time, we will use the complete 2D solver to determine the resonance frequencies and the quality factors of micro-ring resonators made on silicon surrounded by silica. These characteristics are indeed related to the real and imaginary parts of the propagation constants. By comparison with 3D-FDTD analysis, we will show that our implementation can be used to accurately describe the behavior of micro-rings having a bending radius as low as 1.1 μm in the near infrared region. This technique is general and can be applied to any micro-ring having an arbitrary cross-section and a quality factor which is less than 10000. Perspectives of this work include the study of the field propagation in a bent structure, as well as the coupling between micro-ring resonators and straight waveguides.
Synthesis of titanium indiffused LiNbO3 waveguides with desired modal fields
Geetika Jain, Enakshi K. Sharma
In this paper we report a procedure based on variational approximation, to obtain the refractive index profile and process parameters for fabrication of waveguide from the desired modal field. We have illustrated our procedure by the design of a diffused channel waveguide for optimum coupling to a standard communication grade fiber.
Detectors, Modulators and Amplifiers
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Thermal crosstalk reduction in IR thermo-electric photodetectors by lock-in method: 4D numerical simulations and experimental verification
W. Vandermeiren, J. Stiens, C. De Tandt, et al.
Laser induced temperature distributions inside doped semiconductor materials are used to derive laser beam profiles by means of the thermo-electric Seebeck effect. Thermal diffusion will lead to a discrepancy between the optical intensity profile of the laser beam and the measured temperature distribution inside the semiconductor. An advanced numerical 4D finite element model describing the laser induced spatial temperature distribution in function of time in a layered GaAs based structure was developed in Comsol Multiphysics. Non-linearities as the temperature dependence of the absorption coefficient, the thermal conductivity and the Seebeck coefficient were taken into account. This model was used to investigate the optical chopper frequency dependence on the spatial thermal cross-talk level and the responsivity near the illuminated surface of the detector structure. It was shown that the frequency dependent cross-talk level can be reduced significantly by applying short chopping periods due to the dependence of the thermal diffusion length on the frequency. The thermal cross-talk is reduced to -21 dB and -38.6 dB for the first and second neighboring pixel respectively for a lock-in frequency of 140 Hz. Experimental results of the spatial thermal cross-talk level and the responsivity were compared with simulations and satisfactory agreements between both were achieved. High power CO2 laser profile measurements obtained with our thermo-electric detector and a commercially available Primes detector were compared.
Ultrafast compact silicon-based ring resonator modulators using metal-insulator switching of vanadium dioxide
Joyeeta Nag, Judson D. Ryckman, Michael T. Hertkorn, et al.
We present an optical modulator based on a silicon ring resonator coated with vanadium-dioxide (VO2) motivated by the need for compact silicon-compatible optical switches operating at THz speeds. VO2 is a functional oxide undergoing metal-insulator transition (MIT) near 67°C, with huge changes in electrical resistivity and near-infrared transmission. The MIT can be induced thermally, optically (by ultra-fast laser excitation in less than 100 fs), and possibly with electric field. VO2 is easily deposited on silicon and its ultrafast switching properties in the near-infrared can be used to tune the effective index of ring resonators in the telecommunication frequencies instead of depending on the weak electro-optic properties of silicon. The VO2-silicon hybrid ring resonator is expected to operate at speeds up to 10 THz at low Q-factor and with shorter cavity lifetimes, thus enabling compact, faster, more robust devices. We have made ring resonator structures on SOI substrates with rings varying in diameter from 3-10 μm coupled to 5 mm-long nanotapered waveguides at separations of 200 nm. Rings were coated with 80 nm of VO2 by pulsed laser deposition. As proof-of-concept, by switching the VO2 top layer thermally, we were able to modulate the resonance frequency of the ring to match with the predictions from our FDTD simulations.
Evanescent wave modulator for medium infrared wavelengths (8-12 µm)
J. Stiens, W. Vandermeiren, G. Shkerdin, et al.
We present a new modulation concept for medium infrared (8 - 12 μm) wavelengths. The operation principle of the presented modulator is based on evanescent wave absorption by means of a bulk, single or multiple quantum well structure. A sub-wavelength grating ensures efficient coupling of the optical field to the absorption medium. Modulation is then achieved by depletion of this absorption medium. We present an analysis of concept parameters and point out their respective advantages and disadvantages with respect to the modulation performance. In this context, we investigated the impact of different absorption media as bulk, single and multiple quantum well structures and found that single quantum well structures are best suited for modulation purposes. Simulations pointed out that an absolute modulation depth of the order of 60% can be achieved. We also investigated the impact of the diffraction order on the modulation performance. Furthermore, some preliminary experimental results on this modulation concept are presented and compared with simulations.
Random Semiconductor Lasers
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Random lasing in nanocrystalline ZnO powders
Heinz Kalt, Janos Sartor, Felix Eilers, et al.
We review the random-lasing properties of nanocrystalline ZnO powders. The lowest threshold for lasing occurs for average particle diameters of about 260nm. Reproducible lasing features are achieved for reduced ensemble sizes. Spatially resolved luminescence spectroscopy is used to probe directly the degree of localization of random laser modes. We find that strongly confined and extended modes can coexist in the same spatial area. However, localized modes appear for small optical gain while extended modes are only supported in the presence of large optical gain, as is expected from theory. Uniform line spacing is found in the case of extended random laser modes resulting from strong modal interactions.
Control random laser modes by local pumping
Jonathan Andreasen, Christian Vanneste, Li Ge, et al.
A study on the effects of optical gain nonuniformly distributed in one-dimensional random systems is presented. It is demonstrated numerically that even without gain saturation and mode competition, the spatial nonuniformity of gain can cause dramatic and complicated changes to lasing modes. Lasing modes are decomposed in terms of the quasi modes of the passive system to monitor the changes. As the gain distribution changes gradually from uniform to nonuniform, the amount of mode mixing increases. Furthermore, we investigate new lasing modes created by nonuniform gain distributions. We find that new lasing modes may disappear together with existing lasing modes, thereby causing fluctuations in the local density of lasing states.
Quantum Dot Lasers
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Radiative efficiency of MOCVD grown QD lasers
Luke Mawst, Gene Tsvid, Peter Dudley, et al.
The optical spectral gain characteristics and overall radiative efficiency of MOCVD grown InGaAs quantum dot lasers have been evaluated. Single-pass, multi-segmented amplified spontaneous emission measurements are used to obtain the gain, absorption, and spontaneous emission spectra in real units. Integration of the calibrated spontaneous emission spectra then allows for determining the overall radiative efficiency, which gives important insights into the role which nonradiative recombination plays in the active region under study. We use single pass, multi-segmented edge-emitting in which electrically isolated segments allow to vary the length of a pumped region. In this study we used 8 section devices (the size of a segment is 50x300 μm) with only the first 5 segments used for varying the pump length. The remaining unpumped segments and scribed back facet minimize round trip feedback. Measured gain spectra for different pump currents allow for extraction of the peak gain vs. current density, which is fitted to a logarithmic dependence and directly compared to conventional cavity length analysis, (CLA). The extracted spontaneous emission spectrum is calibrated and integrated over all frequencies and modes to obtain total spontaneous radiation current density and radiative efficiency, ηr. We find ηr values of approximately 17% at RT for 5 stack QD active regions. By contrast, high performance InGaAs QW lasers exhibit ηr ~50% at RT.
Inhomogeneous quantum dot gain medium for improved spatial coherence in wide-aperture semiconductor lasers
To analyze the problem of modal filamentation and beam-instabilities in wide-aperture semiconductor lasers, we have developed a sophisticated opto-electro-thermal model based on Maxwell-Bloch formalism, to describe frequency-, carrier- and temperature-dependent gain and dispersion. Effects of both homogeneous and inhomogeneous gain broadening are analyzed. It is shown, via linear stability analysis and high resolution space-time adaptive FEM simulations that inhomogeneous gain broadening in quantum dot (QD) lasers enhances spatial coherence and leads to suppressed filamentation and stable far-fields in both thermal and non-thermal regimes even when the phase-amplitude coupling is comparable to that in quantum well (QW) gain medium.
Bandwidth enhancement in an injection-locked quantum dot laser operating at 1.31-μm
N. A. Naderi, M. Pochet, V. Kovanis, et al.
The high-speed modulation characteristics of an injection-locked quantum dot Fabry-Perot (FP) semiconductor laser operating at 1310-nm under strong injection are investigated experimentally with a focus on the enhancement of the modulation bandwidth. The coupled system consists of a directly-modulated Quantum Dot (QD) slave injected-locked by a distributed feedback (DFB) laser as the master. At particular injection strengths and zero detuning cases, a unique modulation response is observed that differs from the typical modulation response observed in injection-locked systems. This unique response is characterized by a rapid low-frequency rise along with a slow high-frequency roll-off that enhances the 3-dB bandwidth of the injection-locked system at the expense of losing modulation efficiency of about 20 dB at frequencies below 1 GHz. Such behavior has been previously observed both experimentally and theoretically in the high-frequency response characteristic of an injection-locked system using an externally-modulated master; however, the results shown here differ in that the slave laser is directly-modulated. The benefit of the observed response is that it takes advantage of the enhancement of the resonance frequency achieved through injection-locking without experiencing the low frequency dip that significantly limits the useful bandwidth in the conventional injection-locked response. The second benefit of this unique response is the improvement in the high frequency roll-off that extends the bandwidth. Finally a 3-dB bandwidth improvement of greater than 8 times compared to the free-running slave laser has been achieved.
Laser Mode Stabilization and Noise
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Modeling of photonic-crystal-based high-power high-brightness semiconductor lasers
Vitaly Shchukin, Nikolai Ledentsov, Vladimir Kalosha, et al.
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.
Applying the joint Wigner time-frequency distribution to characterization of ultra-short optical dissipative solitary pulses in the actively mode-locked semiconductor laser with an external single-mode fiber cavity
Alexandre S. Shcherbakov, P. Moreno Zarate, Joaquin Campos Acosta, et al.
We discuss specifically elaborated technique for characterizing the train-average parameters of low-power picosecond optical pulses with the frequency chirp, arranged in high-repetition-frequency trains, in both time and frequency domains. This technique is applied to rather important case of pulse generation when a single-mode semiconductor heterolaser operates in a multi-pulse regime of the active mode-locking. In fact, the trains of optical dissipative solitary pulses, which appear under a double balance between mutually compensating actions of dispersion and nonlinearity as well as gain and optical losses, are under characterization. The presented approach involves the joint Wigner time-frequency distributions, which can be found for those picosecond optical dissipative solitary pulses due to the exploitation of a novel interferometric technique. Practically, the semiconductor InGaAsP/InP-heterolaser generating at the wavelength 1320 nm was exploited during the illustrating experiments carried out and the possibility of evaluating the corresponding joint Wigner time-frequency distributions has been obviously demonstrated.
Semiconductor laser oscillation-frequency stabilization using the Faraday effect
The semiconductor lasers in use today are on one hand, prized, and highly praised, for their small size, light weight, longevity and energy-efficiency, -and on the other, criticized for their susceptibility to frequency-fluctuations brought about by changes in temperature and driving current. Once this "wrinkle" is ironed out, semiconductor lasers will become the default light-sources, for satellites' onboard interferometers. Our studies have been directed at stabilizing oscillation frequency to the atomic absorption line, and using negative electrical feedback to the injection current. Frequency stabilization is accomplished, by either; a) applying direct modulation to the semiconductor laser's driving current, or b) modulating the reference frequency, to obtain the error signal needed for stabilization. In this instance, Faraday effect-based stabilization was used. This indirect oscillation frequency stabilization has no discernable effect on spectra width, but, stability was no better than that observed in the system using the direct modulation. When we compared Faraday effect- and direct modulation-based methods of stabilization, in order to uncover the root-cause of the discrepancy, sensors picked up system noise, the source of which was heat generated by the heavy current applied to a magnetic coil used to apply the Faraday effect. We also substituted a permanent magnet for the electromagnet.
Accurate source simulation in modern optical modeling and analysis software
Modern optical modeling and analysis programs allow users to create and analyze accurate optical and opto-mechanical systems in the software environment prior to building actual hardware based systems. The resultant accuracy of these models depends on the accuracy of the components that make up the model including the light source characteristics, surface and material properties, and the model geometry. In this paper we will consider factors that lead to improved modeling of the light source such as spectral and angular properties, the spatial distribution of light within the source, and the interaction of the light with the structure of the source. These factors are extremely important for near field modeling, especially for fiber and light pipe coupling. Several options will be discussed including simple source models such as point sources, ray files, surface properties that define optical parameters such as spectral and angular distribution, and detailed 3D solid models of the source. Simulated results for spectral, angular, and spatial distributions will be compared to actual measurements. Discussion will also include the appropriateness of each modeling approach with respect to different applications.
Edge Emitting Light Sources
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Optical components for very short reach applications at 40 Gb/s and beyond
Nikolay N. Ledentsov, James A. Lott, Vitaly A. Shchukin, et al.
Just as the density of transistors on a silicon chip about doubles with each new generation, processor bandwidth also about doubles. Consequently the speed of input-output (I/O) devices must grow and today we find processor I/O speed approaching or slightly surpassing 10 Gb/s (G) per channel for 100G Ethernet server applications. Similarly Storage Area Networks are supported by Fibre Channel FC16G transceivers operating at the newly standardized serial signaling rate of 14 Gbaud. Further upgrades will require within only a few years links at 25, 28 and 40 Gbaud, speeds that are barely feasible with copper cabling, even for very short reach distances. Thus the role of optical interconnects will increase dramatically as the data transfer rates increase. Furthermore an increased bandwidth demand necessitates an equal or greater demand for low cost and highly power efficient micro-laser and -detector components along with their associated driver and transimpedance amplifier (TIA) integrated circuits (ICs). We summarize our recent achievements in vertical cavity surface emitting lasers (VCSELs) and PIN photodetectors suitable for very short reach multimode fiber links that enable bit rates up to and beyond 40 Gb/s. We address achievements in current modulated VCSELs, electrooptically modulated VCSELs, top illuminated PIN photodiodes, TIA and driver ICs, and packaging solutions.
Simulation of facet heating in high-power red lasers
J. M. G. Tijero, H. Odriozola, I. Esquivias, et al.
A two-dimensional self-consistent laser model has been used for the simulation of the facet heating of red emitting AlGaInP lasers. It solves in the steady-state the complete semiconductor optoelectronic and thermal equations in the epitaxial and longitudinal directions and takes into account the population of different conduction band valleys. The model considers the possibility of two independent mechanisms contributing to the facet heating: recombination at surface traps and optical absorption at the facet. The simulation parameters have been calibrated by comparison with measurements of the temperature dependence of the threshold current and slope efficiency of broad-area lasers. Facet temperature has been measured by micro-Raman spectrometry in devices with standard and non absorbing mirrors evidencing an effective decrease of the facet heating due to the non absorbing mirrors. A good agreement between experimental values and calculations is obtained for both devices when a certain amount of surface traps and optical absorption is assumed. A simulation analysis of the effect of non absorbing mirrors in the reduction of facet heating in terms of temperature, carrier density, material gain and Shockly-Read-Hall recombination rate profiles is provided.
Saturation characteristics of InGaAsP-InP bulk SOA
Amita Kapoor, Enakshi K. Sharma, Wolfgang Freude, et al.
Semiconductor optical amplifiers (SOAs) can be used as linear in-line amplifiers for extended-reach passive optical networks, or as gain/phase-switchable devices. For these applications, gain, bandwidth and saturation power are important. The saturation power can be increased by decreasing the confinement factor and by increasing the length such that the overall gain remains constant. In this paper we investigate the saturation characteristics of 1.55μm InGaAsP-InP bulk SOA. We do so by using the physically based simulation tool ATLAS. The simulation tool ATLAS supports simulation of semiconductor lasers only, however making the mirror reflectivities small, the lasing threshold is increased such that lasers are essentially reduced to amplifiers. Next, for investigating the saturation characteristics of SOA, the amplifier gain should be influenced by injecting an optical light power. However, ATLAS cannot simulate the required source directly. Instead, we use in the electron rate equation simultaneously two competing independent models for spontaneous radiative recombination, namely the so-called general model (total recombination rate BnT p with bimolecular recombination coefficient B, electron and hole concentrations nT and p) and the standard model for recombination due to amplified spontaneous emission into the mode under consideration (determined by the product of Fermi functions for electrons and holes). In the photon rate equation, only the standard model is used. We then increase B, and thus simulate a decrease of the carrier concentration that would physically result from an external optical signal. We show that under conditions of constant injection current and device length an n-doping (p-doping) of the active layer increases (decreases) the input saturation power. In addition we observe that for constant injection current and amplifier gain, a p-doping (n-doping) of the active layer increases (decreases) both the input and output saturation powers because of an reduced (slightly increased) Auger-dominated carrier lifetime.
Vertically stacked InAs quantum dots for polarization-independent semiconductor optical amplifiers
Tomoya Inoue, Masaki Asada, Osamu Kojima, et al.
This paper describes a technique to control the polarization property in quantum dot (QD)-semiconductor optical amplifiers (SOAs) using vertical stacking of self-assembled InAs QDs. QD-SOAs have been expected to realize high saturation power, multi-channel processing, and high-speed response. However, in conventional QDs, the significant polarization dependence in the optical gain caused by the flattened QD shape has been a serious problem. One of the well-known approaches to realize the polarization-independent gain relies on columnar QDs, in which InAs QDs layers are closely stacked with very thin (several monolayers) intermediate layers. The isotropic shape of columnar QDs realizes a polarization-independent gain. On the other hand, in this paper, we propose a different approach, where QDs are vertically stacked with moderately thick intermediate layers. Therefore each QDs layer is well separated geometrically and high precision control of overall QD shape is expected. Vertically aligned InAs QDs are known to create the electronically coupled states, where we expect the enhancement of the optical transition probability along the vertical direction. We have achieved such vertical stacking of QDs up to 9 layers by optimizing the amount of GaAs and InAs deposition. The 9-stacked QDs have shown transverse-magnetic-mode dominant emission in edge photoluminescence in the 1.3 μm telecommunication wavelength region. Our results have suggested that the electronically coupled QDs can be a powerful tool to realize the polarization-independent QD-SOAs
Electron-photon and electron-LO phonon intersubband scattering rates in GaN/AlN quantum wells
Guangyu Liu, Hongping Zhao, Nelson Tansu
In this work, we analyze and optimize the GaN / AlN coupled quantum well design with and without polarization for achieving intersubband transition wavelength at 1.55 μm to serve as quantum cascade lasers (QCL) active region. The computations of the electron-LO phonon and electron-photon scattering rates were carried out to optimize the gain media design for intersubband quantum well (QW) lasers. The AlN / GaN coupled QW structure leads to improved design in optimizing the intersubband transition, in comparison to that of single stage GaN / AlN QW structure. The comparison between polar and non-polar coupled QW results in different characteristics in various scattering rates, which in turn leads to different intersubband gain.
Dynamics and Nonlinearities II
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Experimental stability maps of a 1550nm-VCSEL subject to polarized optical injection
A. Hurtado, K. Schires, N. Khan, et al.
We report an experimental study of the injection locking properties and nonlinear dynamics of a 1550nm-Vertical Cavity Surface Emitting Laser (VCSEL) subject to parallel and to orthogonal polarized optical injection into the two orthogonal polarizations of the fundamental transverse mode of the device. A rich variety of nonlinear dynamics have been observed for both parallel and orthogonal optical injection outside the stable locking range, including limit cycle, period doubling, chaos and polarization switching. Additionally, for the case of orthogonal optical injection, we report a first experimental observation of the change in the shape of the stability map as the applied bias current is increased well above threshold.
Experimental observation of the locking regimes and chaotic dynamics in laterally coupled diode lasers
The analysis of the locking, unlocking and non linear dynamics observed in lateral couple diode lasers (LCDL) is a key issue on the study and understanding of these devices. In this work an analysis of the different nonlinear regimes observed in these devices is made. By the observation of both the RIN spectrum and the high resolution Fabry Perot optical spectrum, a clear identification of the different nonlinear regimes is made. From these measurements a mapping of the nonlinear regimes is presented.
Frequency stabilization of a laser diode using Rb saturated absorption lines
We have succeeded, thus far, in stabilizing laser diode (LD) frequencies to Rb absorption lines, by means of negative electrical feedback. While the absorption lines were stable over the long term, the Doppler Effect's influence was evident, in broadened spectrum linewidth. To avoid the problem in subsequent tests, we used Rb-saturated absorption signals. In this work, we demonstrated potentials of two applications; one was as a reference signal source for estimation of other LD's frequency stabilization system, and the other was a light source for generation of THz radiation.­¬«W
Locking of three coupled lasers
We investigate the stability of an array of three laterally coupled semiconductor lasers. This study of the simplest system with an underlying structure that is also found in larger arrays constitutes a first step towards understanding the stability properties of large arrays. We use a composite-cavity model, where the individual lasers are described by the transverse modes of the entire composite-cavity system. Specifically, we analyze the stable locking region, where the laser array exhibits continuous wave emission for different detunings and coupling strengths between the individual lasers. We find that the optical fields in the outer lasers are out of phase with the middle laser.
Quantum Dots and Microcavities
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Theory of few photon dynamics in electrically pumped light emitting quantum dot devices
Alexander Carmele, Matthias-Rene Dachner, Janik Wolters, et al.
Quantum dot based light emitters can be used as sources of nonclassical light. We focus on the theory of InAs/GaAs quantum dots embedded in a two dimensional wetting layer at low electrical pump currents but strongly correlated electron-photon dynamics. For this purpose a self-consistent theory of transport and emission is developed. A substantial carrier heating in such devices is predicted and the relation of electrical pumping and single photon emission is analyzed within the photon-probability-cluster-expansion.
Polarization conservation and dephasing in InAs quantum dot ensembles
Alexej Chernikov, Swantje Horst, Stephan W. Koch, et al.
We present time-resolved photoluminescence measurements performed on an ensemble of InAs quantum dots with density of 1011 dots/cm2 and ground state transition energies centered at 1.216 eV. The wavelength of the 100fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon assisted) photoluminescence (PL). The PL was detected with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal's cleave axes). The decay of the PL was time-resolved with a streak camera in the interval 1.5 - 3ns to avoid scattered laser light. A strong polarization dependence was observed. Considerable amount of the resonant fluorescence signal and even of the non-resonant PL signatures remained linearly polarized on a nanosecond time scale. A phenomenological rate equation analysis is made.
Multiscale thermal modeling of GaN/AlGaN quantum dot LEDs
G. Romano, G. Penazzi, A. Di Carlo
In this work we develop a multiscale model to investigate the self-heating in nanodevices. The scheme splits up the simulation region in two domains: the micro domain, modeled by the phonon Boltzmann Transport Equation (BTE) and the macro domain, where the heat transport is calculated within the Fourier model. Appropriate boundary conditions match the two domains. The multiscale method is applied to a GaN/AlGaN quantum dot LED. We find out that the maximum temperature is about 334 K. A comparison with the temperature profile given by the BTE and Fourier model is provided. Finally, the effect of the temperature on the optical spectrum is investigated.
Poster Session
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Blue-emitting ZnSe random laser
Toru Takahashi, Toshihiro Nakamura, Sadao Adachi
We demonstrate a blue-emitting semiconductor random laser using a thin ZnSe powered film at room and low temperatures. Below the threshold excitation, we observe a broad spontaneous emission band centered at ~470 nm due to the excitonic transitions in ZnSe particles. Above the threshold excitation, several discrete lasing lines caused by the light amplification in closed-loop paths and emitted in all directions were observed near the center of the spontaneous emission band (~475 nm). The linewidth of each discrete lasing line is less than 0.4 nm. Lasing threshold power density is determined to be ~700 kW/cm2. The effective cavity diameter Deff is also estimated to be ~6 μm from the experimental lasing spectrum by performing Fourier transform analysis. The temperature dependence of the lasing characteristics in ZnSe random laser is also investigated and found that decreasing temperature shifts the discrete lasing lines toward higher-energy side with increasing their intensities.
All optical logic gates using active plasmonic device block
We have proposed a novel all-optical logic gates based on active plasmonics that may control the electron-photon coupling through an external effect. The phenomenon of surface plasmon resonance (SPR) is basically appeared on attenuated total reflection mirror block. The waveguide-type Kretchmann-Raether configuration with high sensitivity to the metal surface was used for all-optical OR and NAND gates. Here, the double thin metal layers can enhance the confinement of plasmon waves and can be utilized as an output. When the external light source is injected into the thin ZnO film deposited on the facet of a GaAs waveguide, the total refractive index of the thin ZnO layer is changed by the nonlinear refractive index. The proposed waveguide-type configuration was analyzed and optimized using finite-difference time-domain method for all-optical OR and NAND gates. When the external light is injected on the metal layer, the intensity of SPW is decreased by 10.76 dB. However, the reflected light into the waveguide is increased by 2.23 dB.
Formulation of differential transfer matrix method in cylindrical geometry
Mohsen Jiani, Sina Khorasani, Bizhan Rashidian, et al.
Transfer and scattering matrix methods are widely in use for description of the propagation of waves in multilayered media. When the profile of refractive index is continuous, however, a modified formulation of transfer matrices does exist, which provides a complete analytical solution of the wave phenomena in such structures. Previously reported variations of the so-called Differential Transfer Matrix Method (DTMM) had been limited to Cartesian geometry where layered media form one-dimensional structures and plane waves are used as basis functions. In this work, we extend the formalism to cylindrical geometry with radial symmetry, in which Bessel functions need to be employed as basis functions. Hence, complete analytical formulation of the DTMM under radial and axial symmetry is described and derived. This work could have applications in the analysis of propagation in optical fibers and motion of electrons in nanowires and nanotubes.
Passive fiber rings as a basic part of fiber optic sensors
The article deals with topics about fiber optic loop, evaluation of the signals delay in the loop and in case of the first designed arrangement measurement of the fiber length. The second task is to detect the effective refractive index n1eff of the fiber core at the given length of loop. Author's team gradually discusses how configurations for the tasks are created. From configuration designs is possible to build up optical fiber sensor.
Propagation of light in Schwarzschild geometry
In this paper, the equivalent medium of Schwarzschild metric is discussed. The corresponding ray-tracing equations are integrated for the equivalent medium of the Schwarzschild geometry, which describes the curved space around a spherically symmetric, irrotational, and uncharged blackhole. We make comparison to the well-known expression by Einstein. While Einstein's estimate is reasonably good for large closest distances of approach to the star, it disregards the optical anisotropy of space. Instead, Virbhadra's estimate which takes the effects of anisotropy of Schwarzschild metric is shown to be more consistent with numerical simulations. Hence, a true physical anisotropy in the velocity of light under gravitational field does exist. We argue that the existence of such an optical anisotropy could be revealed exactly in the same way that the optical interferometry is expected to detect gravitational waves. Therefore, if no optical anisotropy under gravitational fields could be observed, then the possibility of interferometric detection of gravitational waves is automatically ruled out, and vice versa.
Role of electron blocking layer in III-nitride laser diodes and light-emitting diodes
Yen-Kuang Kuo, Jih-Yuan Chang, Mei-Ling Chen
A high energy bandgap electron blocking layer (EBL) just behind the active region is conventionally used in the nitride-based laser diodes (LDs) and light-emitting diodes (LEDs) to improve the confinement capability of electrons within the quantum wells. Nevertheless, the EBL may also act as a potential barrier for the holes and cause non-uniform distribution of holes among quantum wells. A most recent study by Han et al. (Appl. Phys. Lett. 94, 231123, 2009) reported that, because of the blocking effect for holes, the InGaN LED device without an EBL has slighter efficiency droop and higher light output at high level of current injection when compared with the LED device with an EBL. This result seems to contradict with the original intention of using the EBL. Furthermore, findings from our previous studies (IEEE J. Lightwave Technol. 26, 329, 2008; J. Appl. Phys. 103, 103115, 2008; Appl. Phys. Lett. 91, 201118, 2007) indicated that the utilization of EBL is essential for the InGaN laser diodes. Thus, in this work, the optical properties of the InGaN LDs and LEDs are explored numerically with the LASTIP simulation program and APSYS simulation program, respectively. The analyses focus particularly on the light output power, energy band diagrams, recombination rates, distribution of electrons and holes in the active region, and electron overflow. This study will then conclude with a discussion of the effect of EBL on the optical properties of the InGaN LDs and LEDs.
Numerical simulation on high-efficiency GaInP/GaAs/InGaAs triple-junction solar cells
Shu-Hsuan Chang, Miao-Chan Tsai, Sheng-Horng Yen, et al.
In this paper, the high-efficiency GaInP/GaAs/InGaAs triple-junction solar cells are investigated numerically by using the APSYS simulation program. The solar cell structure used as a reference was based on a published article by Geisz et al. (Appl. Phys. Lett. 91, 023502, 2007). By optimizing the layer thickness of the top and middle cells, the appropriate solar cell structure which possesses high sunlight-to-energy conversion efficiency is recommended. At AM1.5G and one sun, the conversion efficiency is improved by 2.3%. At AM0 and one sun, the conversion efficiency is improved by 4.2%. At AM1.5D and one sun, the conversion efficiency is improved by 1.3%. Furthermore, based on the optimized structures, this device can achieve efficiencies of more than 40% at high concentrations. For the triple-junction solar cell under AM1.5G solar spectrum, the conversion efficiency reaches 40.2% at 40 suns. For the device under AM0 solar spectrum, the conversion efficiency reaches 36.2% at 30 suns. For the device under AM1.5D solar spectrum, the conversion efficiency reaches 40.2% at 50 suns.
Study on temperature characteristic of green photodetector on Si substrate
Xiansong Fu, Suying Yao, Shengcai Zhang, et al.
In the paper, a silicon PN junction photodetector is developed on the basis of n-type single-crystal (100) silicon substrate. The properties of this semiconductor photodetctor depend on the temperature to certain extent. We emphasize on the study on temperature characteristic of the photodetector: Firstly, the temperature behavior of dark current at zero bias voltage and wide temperature range was investigated. Results show that dark current increases exponentially with temperature over room temperature. Secondly, the temperature behavior of photo current at zero bias voltage and wide temperature range was studied. The temperature characteristic is analysesed in the theory and optimized.
Modeling and simulation of AlGaAs/GaAs QW DBR silicon dual junction photovoltaic devices
Hamid Z. Fardi, Bart Van Zeghbroeck
This paper investigates the validation and design of a new high efficiency photovoltaic cell through modeling and simulation. The goal of this article is present the absorption and recombination-generation model of the simulation program for the current-voltage analysis and optimization of MQW's AlGaAs/GaAs solar cells. In an ideal device structure, an efficiency as high as 45% can be achieved by combining a standard silicon single-crystalline cell with a GaAs/AlGaAs multi-QW structure enclosed in a light-confining structure such AlGaAs/GaAs DBR. The predicted efficiency and the analysis is for an ideal crystalline cell, which does not include shadowing effects, reflection and recombination of minority carriers. This combination makes maximum use of the absorption in the silicon and the addition of GaAs QW and selective reflector between the two junction devices boost the efficiency A possible practical design implementation is the use of a transparent contact between the two cells such as ITO and the wafer bonding of the two cells.
Precise frequency stabilization technique for 850nm vertical cavity surface emitting lasers by controlling their optical beat frequency
Takao Ohhara, Tomoaki Yoshimi, Wakao Sasaki
We have demonstrated a compact and inexpensive frequency stabilization technique for commercially available 1mW, 850nm Vertical Cavity Surface Emitting Laser (VCSEL) using a Fabry-Perrot Resonator (FPR) as frequency standard. We have performed frequency discrimination using a transmitted light from FPR, and frequency stabilization has been carried out by electrical negative feedback to injection current. Optical frequency fluctuation of VCSEL is estimated by error signal, and its stability is evaluated by Allan variance. We have achieved to detect optical beat frequency signal of 850nm type VCSEL for the first time, by fabricating two sets of frequency stabilized VCSEL, which are quite similar with each other by controlling each locking frequency. We have estimated VCSEL's frequency accurately fluctuations from the beat signal. As a result, we have successfully suppressed the amount of frequency fluctuations for the free-running VCSEL of as much as 600MHz to be within 80MHz. In this paper, we propose compact, inexpensive and precise frequency stabilization for 850nm VCSEL, and describe an accurate method for estimating its fluctuations.
Linewidth reduction of a 30mW-405nm GaN violet laser diode by optical-electrical double feedback method
Kouki Mizutani, Toshiki Kuromori, Wakao Sasaki
We have demonstrated a compact and efficient frequency stabilization system based on Pound-Drever-Hall method, along with optical feedback. The frequency of a 30mW 405nm GaN violet laser diode (LD) was stabilized to a reference confocal Fabry-Perot cavity (CFP cavity) by negative electrical feedback to the injection current of the LD based on Pound-Drever-Hall technique. Moreover, by employing optical feedback from another tilted CFP cavity, the residual frequency noise has been efficiently suppressed. The minimum square root of the Allan variance was 1.65×10-11 at the integration time of 0.5s under the optical-electrical double feedback condition. We have achieved the stabilization of visible violet LD by optical feedback method for the first time.
Study of beam propagation in finite photonic crystals
Amin Khavasi, Khashayar Mehrany, Mehdi Miri, et al.
Many authors simply use band structure of infinite photonic crystals to predict the beam's direction in a finite structure. The validity of this approximation for high frequencies has been questioned by Felbacq (PRL 92, 193902) and instead a dressed (by evanescent waves) transfer matrix has been suggested. In this work, we show through numerical examples that the direction obtained by conventional band structure is more accurate than that of dressed transfer matrix of Felbacq et. al. We also demonstrate that this approximation can be improved by taking the effect of evanescent Bloch modes into consideration. The effect of these modes leads to a constant shift of beam's center inside and far enough from the PC's interface.
VCSEL's frequency stabilization of an external cavity diode laser: countermeasures against atmospheric temperature variations
We introduced the vertical cavity surface emitting laser (VCSEL) as the laser diode in tour external cavity system. Because VCSELs are now commercially available, and the External cavity diode laser (ECDL) systems using them are expected to improve their frequency stability, we have replaced a Fabry-Perot type laser diode with a VCSEL, and examined its oscillation-frequency stability. Therefore we were able to expect that the VCSELs with our double optical feedback system have good oscillation frequency stability. The obtained VCSEL's oscillation-frequency stability, i.e., the square root of Allan variance σ was 4×10-10, at an averaging time of τ=1 sec.
Analysis and characterization of the small-signal modulation of a vertical external cavity surface emitting laser
N. Terry, M. Walton, R. Bedford
The small signal modulation of a vertical external cavity surface emitting laser (VECSEL) is examined. The modulation transfer function (MTF) of the cavity is measured for multiple photon lifetimes operating between Class A and Class B regimes, where the photon and carrier lifetimes are of the same order. Three coupled ordinary differential equations with similarities to an electrically-injected quantum-well laser with a separate confinement heterostructure are used to mathematically describe the time-dependant VECSEL response. We present a series of measurements that provide important laser parameters such as internal device losses and differential gain. The VECSEL operating in this regime is an overdamped oscillator and has free-running characteristics that are not unlike quantum-dot and quantum-cascade lasers.