Showing papers in "IEEE Journal of Quantum Electronics in 1998"
TL;DR: In this article, an approximate approach is used showing the importance of small cavity order m/sub c/ and of the m/ sub c/n/sup 2/ ratio.
Abstract: We address the long-standing issue of extracting light as efficiently as possible from a high-index material, n/spl ges/2, where as little as 2%-10% of light not suffering total internal reflection is extracted at standard plane faces due to the small critical angle /spl sim/1/n. Using a planar microcavity to redirect spontaneous emission toward the surface, constructive interferences can bring 15%-50% of the light out, enhancing brightness and efficiency. In this first of two papers, an approximate approach is used showing the importance of small cavity order m/sub c/ and of the m/sub c//n/sup 2/ ratio. We define a condition for microcavity regime as m/sub c/<2n/sup 2/. It is shown that most of light extraction is usually attained for moderate mirror reflectivities /spl sim/1-m/sub c//n/sup 2/ typically below 90%, and without strong directionality. Balance between emission directionality, radiance (brightness), and spectral narrowing is discussed. We define a brightness enhancement factor B given by Bm/sub c//spl Delta//spl Omega/=4/spl pi/ where /spl Delta//spl Omega/ is the largest internal solid angle of either the cavity mode or that deduced from the material emission linewidth. Design rules are applied to distributed dielectric mirrors yielding an optimal number of periods. The underlying physical competition between emission into guided modes, Fabry-Perot modes and the so-called "leaky modes" is analyzed.
459 citations
TL;DR: In this article, an equation for the current density based on a tight-binding approximation is proposed to obtain a quasi-equilibrium between the population of the injector ground state and that of the excited state of the laser transition characterized by a common quasi-Fermi level.
Abstract: Experimental evidence that in quantum cascade lasers electron injection into the active region is controlled by resonant tunneling between two-dimensional subbands is discussed. A quantitative analysis is carried out using an equation for the current density based on a tight-binding approximation. Electron injection into the active region is optimized when the current density is limited by the lifetime of the excited state of the laser transition. In this regime, quasi-equilibrium is reached between the population of the injector ground state and that of the excited state of the laser transition characterized by a common quasi-Fermi level. The design of the injector depends on the selected laser active region; in particular, the choice of physical parameters, such as doping concentration and injection barrier thicknesses, is in general different for vertical or diagonal transition lasers. The paper concludes with an investigation of the transport properties at threshold and its dependence on stimulated emission; a relationship between the differential resistance above threshold and the value of the slope efficiency is deduced.
294 citations
TL;DR: The latest 400-Gbit/s transmission experiments based on optical signal processing are presented and the possibility of terabit/second TDM transmission is discussed.
Abstract: Recent progress in optical time-division-multiplexed (TDM) transmission technologies is reviewed including optical short pulse generation, time-division multiplexing/demultiplexing, timing extraction, and waveform measurement. The latest 400-Gbit/s transmission experiments based on optical signal processing are presented and the possibility of terabit/second TDM transmission is discussed.
223 citations
TL;DR: The phase response and resulting dispersion of optical filters in general and their impact on WDM system performance are considered and possible ways of linearizing the phase response without affecting the amplitude response are considered in an attempt to approximate the ideal filter and achieve the highest bandwidth utilization.
Abstract: Wavelength division multiplexing (WDM) communication systems invariably require good optical filters meeting stringent requirements on their amplitude response, the ideal being a perfectly rectangular filter. To achieve high bandwidth utilization, the phase response of these filters is of equal importance, with the ideal filter having perfectly linear phase and therefore constant time delay and no dispersion. This aspect of optical filters for WDM systems has not received much attention until very recently. It is the objective of this paper to consider the phase response and resulting dispersion of optical filters in general and their impact on WDM system performance. To this end we use general concepts from linear systems, in particular, minimum and nonminimum phase response and the applicability of Hilbert transforms (also known as Kramers-Kronig relations). We analyze three different classes of optical filters, which are currently being used in WDM systems and compare their performance in terms of their phase response. Finally, we consider possible ways of linearizing the phase response without affecting the amplitude response, in an attempt to approximate the ideal filter and achieve the highest bandwidth utilization.
215 citations
TL;DR: In this paper, the authors analyzed strongly pumped fiber laser systems based on a rate equation model and showed that approximate analytical and quasi-analytical expressions are in excellent agreement with the exact numerical solution of the rate equations, and both agree well with recently published experimental data.
Abstract: Strongly pumped fiber lasers are analyzed, based on a rate equation model. Examples include Nd/sup 3+/-doped and Yb/sup 3+/-doped fiber lasers, with distributed Bragg reflector mirrors at either end. Approximate analytical and quasi-analytical expressions are shown to be in excellent agreement with the exact numerical solution of the rate equations, and both agree well with recently published experimental data.
204 citations
TL;DR: The two-dimensional (2D) quantum-well (QW) laser diode simulator Minilase-II is presented in detail in this paper, which contains a complete treatment of carrier dynamics including bulk transport, quantum carrier capture, spectral hole burning, and quantum carrier heating.
Abstract: The two-dimensional (2-D) quantum-well (QW) laser diode simulator Minilase-II is presented in detail. This simulator contains a complete treatment of carrier dynamics including bulk transport, quantum carrier capture, spectral hole burning, and quantum carrier heating. The models used in the simulator and their connectivity are first presented. Then the simulator is used to demonstrate the effects of various nonlinear processes occurring in QW lasers. Finally, modulation responses produced by Minilase-II are compared directly with experimental data, showing good quantitative agreement.
198 citations
TL;DR: In this paper, the interpulse dynamics in harmonic passively mode-locked soliton lasers were investigated and it was shown that gain depletion in conjunction with its recovery provided an effective repulsion force between adjacent solitons by imparting a group-velocity drift proportional to the intra-pulse spacings.
Abstract: We consider the interpulse dynamics in harmonic passively mode-locked soliton lasers and find that gain depletion in conjunction with its recovery provides an effective repulsion force between adjacent solitons by imparting a group-velocity drift proportional to the interpulse spacings. Analytic descriptions for both the two-pulse and N-pulse per round-trip configurations demonstrate the stability of the equally spaced condition. These theoretical findings, which are supported by experimental results, hold in the limit where cavity losses and perturbations are small per round trip so that the weak gain depletion and recovery mechanisms can dominate the interpulse interactions.
179 citations
TL;DR: In this paper, the dispersion characteristics of apodized, linearly chirped fiber Bragg gratings and their potential as dispersion compensators have been studied systematically, and it is shown that the positive hyperbolic-tangent profile results in an overall superior performance, as it provides highly linearized time-delay characteristics with minimum reduction in the linear dispersion.
Abstract: The dispersion characteristics of apodized, linearly chirped fiber Bragg gratings and their potential as dispersion compensators have been studied systematically. It is shown that the positive hyperbolic-tangent profile results in an overall superior performance, as it provides highly linearized time-delay characteristics with minimum reduction in the linear dispersion. To compensate for the linear dispersion of 100 km of standard telecom fiber over certain bandwidth (in nanometers), the required grating length is 19.24 cm/nm.
156 citations
TL;DR: In this paper, an experimental study of excited-state absorption (ESA) in several inorganic saturable absorbers, Cr/sup 4+/:YAG (YAG-YAG, YAG-GGG, Crsup 4 +/:GGG), and YAG +/spl tau/*=(0.55/spl plusmn/0.1) ns.
Abstract: An experimental study of saturable absorption and excited-state absorption (ESA) in several inorganic saturable absorbers, Cr/sup 4+/:YAG, Cr/sup 4+/:GGG, and Cr/sup 4+/:YSGG, is presented. We provide the theoretical background of absorption characteristics in saturable absorbers that exhibit ESA, with some new results: approximate analytical solutions are proposed for the optical transmission in the case of a slow absorber, and for various light intensity conditions of spatially or temporally Gaussian beams in fast and slow absorbers. Experimentally, partial bleaching of the first excited state itself could be observed in Cr/sup 4+/:YAG at /spl lambda/=1064 nm, yielding the higher excited-state lifetime as /spl tau/*=(0.55/spl plusmn/0.1) ns. The regular transmission bleaching curve was measured in Cr/sup 4+/:GGG, for the first time in this material, yielding /spl sigma//sub ga/=(58/spl plusmn/5)/spl times/10/sup -1/ cm/sup 2/, and /spl sigma/e/sub s/=(13/spl plusmn/2)/spl times/10/sup -19/ cm/sup 2/ at /spl lambda/=1064 nm, ESA spectra were measured for the three materials between /spl sim/700 and 900 nm. All three exhibit crossing between saturable absorption at longer wavelengths and inverse saturable absorption at shorter wavelengths.
155 citations
TL;DR: In this article, a GaInAs-AlInAs quantum cascade structure operating in the mid-infrared has been reported for high power continuous-wave (CW) laser action.
Abstract: High-power continuous-wave (CW) laser action is reported for a GaInAs-AlInAs quantum cascade structure operating in the mid-infrared (/spl lambda//spl sime/5 /spl mu/m). Gain optimization and reduced heating effects have been achieved by employing a modulation-doped funnel injector with a three-well vertical-transition active region and by adopting InP as the waveguide cladding material to improve thermal dissipation and lateral conductance. A CW optical power as high as 0.7 W per facet has been obtained at 20 K with a slope efficiency of 582 mW/A, which corresponds to a value of the differential quantum efficiency /spl eta//sub d/=4.78 much larger than unity, proving that each electron injected above threshold contributes to the optical field a number of photons equal to the number of periods in the structure. The lasers have been operated CW up to 110 K and more than 200 mW per facet have still been measured at liquid nitrogen temperature. The high overall performance of the lasers is also attested by the large "wall plug" efficiency, which, for the best device, has been computed to be more than 8.5% at 20 K. The spectral analysis has shown finally that the emission is single-mode for some devices up to more than 300 mW at low temperature.
153 citations
TL;DR: In this article, the exact calculation of dipole emission modifications in an arbitrary multilayer structure was used to obtain the extraction efficiency from realistic planar microcavities, additional insights gained through this exact approach compared to the simplified one of Part I of this paper were first discussed in the case of a dielectric slab.
Abstract: In this paper we use an exact calculation of dipole emission modifications in an arbitrary multilayer structure to obtain the extraction efficiency from realistic planar microcavities, additional insights gained through this exact approach compared to the simplified one of Part I of this paper [see ibid., p. 1612, 1998] are first discussed in the case of a dielectric slab. We next optimize for the extraction purpose asymmetric cavities bounded by metal on one side and dielectric mirrors on the output side for any pair of material indices in a broad range (n=1.4-4). The decrease of extraction when taking into account relative linewidths of the source of a few percent is shown to be moderate, allowing the large enhancements of monochromatic light to be maintained in many useful cases. The fractions of power emitted into guided modes, leaky modes, etc., are detailed. The beneficial role of possible photon recycling (reabsorption of emitted photons by the active layer) on extraction efficiency is evaluated using the fractions of power in guided and leaky modes. Extraction efficiencies in the 50% range are predicted for optimized, hybrid, planar metal-semiconductor structures for a wide range of active materials and wavelengths. We show that exact calculations justify the simple model used in Part I evaluating the extraction efficiency of a microcavity-based light-emitting diode as 1/m/sub c/ where m/sub c/ is the effective cavity order.
TL;DR: In this paper, the effect of the thermal load on the mechanical and optical properties is significantly influenced by the method used for cooling the pumped surface, and four methods for longitudinally pumped Nd:YAG rods were compared experimentally and with finite-element (FE) analysis.
Abstract: Longitudinal diode laser pumping causes pronounced inhomogeneous heating of the laser material. The effect of the thermal load on the mechanical and optical properties is significantly influenced by the method used for cooling the pumped surface. Four methods for longitudinally pumped Nd:YAG rods were compared experimentally and with finite-element (FE) analysis. The pumped surface exposed to air or to flowing water, a sapphire plate pressed onto the pumped surface and composite rods, having an undoped pumped end, were investigated. The comparison includes temperature and stress distributions with the emphasis on thermal lensing. Careful validation of the FE code with experiments allowed to numerically comparing the cooling methods under identical conditions.
TL;DR: In this paper, the propagation of strong picosecond optical pulses in semiconductor optical amplifiers (SOAs) is investigated numerically by taking into account carrier heating, spectral hole-burning, and two-photon absorption, as well as ultrafast nonlinear refraction.
Abstract: The propagation of strong picosecond optical pulses in semiconductor optical amplifiers (SOAs) is investigated numerically by taking into account carrier heating, spectral hole-burning, and two-photon absorption, as well as ultrafast nonlinear refraction. Very good agreement with published experimental results are observed in both the time and frequency domains. It is shown that the effects of two-photon absorption and ultrafast nonlinear refraction are very important in determining the output pulse properties for pulse energy larger than 1 pJ.
TL;DR: In this article, a transfer matrix analysis is presented that can describe the general class of grating-guided waveguides with either finite and/or infinite grating boundaries, and the theory is applied to describe the new angled-grating distributed feedback laser (/spl alpha/-DFB), predicting a broad near-Gaussian near field and a collimated diffraction-limited far field.
Abstract: Angled-distributed-feedback semiconductor lasers have demonstrated 1-W diffraction-limited collimated output from a broad-area aperture. We present the first theoretical analysis of the modes of these devices, explaining their principle of operation and the reasons for their high spatial mode quality. A transfer matrix analysis is presented that can describe the general class of grating-guided waveguides with either finite and/or infinite grating boundaries. Lateral grating-confined broad-area waveguides are shown to have extremely high spatial mode selectivity. The theory is applied to describe the new angled-grating distributed feedback laser (/spl alpha/-DFB), predicting a broad near-Gaussian near field and a collimated diffraction-limited far field, in good agreement with experiment.
TL;DR: In this article, a monomode semiconductor laser is subjected to external injection from another laser and the system stability is investigated as a function of detuning and of the relative injected power.
Abstract: In this paper, we analyze the phenomena arising when a monomode semiconductor laser is subjected to external injection from another laser. The system stability is investigated as a function of detuning and of the relative injected power. Different regimes, spanning from phase locking to chaos and coherence collapse, are described by analytical and numerical methods for weak and moderate injection. Previous theoretical studies are extended by describing the inverse transition from chaos to stability and by deriving the final locking condition. Also, further investigation on the coherence collapse regime has been performed. Besides contributing to the exploration of an interesting fundamental phenomenon, the results of this analysis are useful for different applications, including coherent detection and chaotic cryptography.
TL;DR: In this paper, the width of an optical frequency comb (OFC) was increased to 30 THz by using self-phase modulation (SPM) in an optical fiber, which is 2.7 times larger than the maximum OFC span obtained by the generator alone.
Abstract: The width of an optical frequency comb (OFC) was increased to 30 THz by using self-phase modulation (SPM) in an optical fiber. This value is 2.7 times larger than the maximum OFC span obtained by the OFC generator alone. We compare the resulting spectrum to numerical simulations to confirm that the SPM and the higher order dispersion of the fiber contribute to broaden the spectral profile.
TL;DR: In this paper, the authors investigated the upconversion in Nd:YAG and its implications for intensely pumped devices and found that 1 at.% Nd-doped YAG amplifiers give an Auger up-conversion rate of 7/spl times/10/sup 3/ s/sup -1/
Abstract: We report on the investigation of upconversion in Nd:YAG and its implications for intensely pumped devices. Analysis of lifetime measurements and the performance of a 1 at.% Nd-doped YAG amplifier give an Auger upconversion rate of 7/spl times/10/sup 3/ s/sup -1/. This is significantly smaller than previously reported, but modeling of the performance of Nd:YAG devices with high-inversion densities shows that even this rate can still seriously degrade the small-signal gain and significantly increase the thermal load. The variation of cross-relaxation and upconversion rates with doping level is also described. Finally, it is found that the effect of bleaching of the Nd:YAG absorption can lead to a reduced spatial overlap between the signal and inversion profiles and thus can also significantly reduce the gain.
TL;DR: In this paper, the temporal behavior of the optical frequency emitted by several DFB lasers under direct square wave modulation was measured using an all-fiber implementation of a Mach-Zender interferometer with an imbalance of 30 ps.
Abstract: The temporal behavior of the optical frequency emitted by several DFB lasers under direct square wave modulation was measured using an all-fiber implementation of a Mach-Zender interferometer with an imbalance of 30 ps. The impulse response of the optical frequency to injection current modulation was found to contain a time constant as short as 10-20 ns, together with a few longer ones. The existence of such a short time constant is consistent with a thermal analysis of a laser structure with finite thermal impedance of the active region and should be taken into consideration in various wide bandwidth applications of direct modulated semiconductor lasers.
TL;DR: In this article, a poly(2,5-dialkoxy-p-phenylene ethynylene) derivative (EBO-OPPE) was used for power limiting and stabilization in a polymer solution excited by /spl sim/810 nm and /splsim/7-ns laser pulses.
Abstract: Optical power limiting and stabilization based on the two-photon absorption (TPA) mechanism is performed in a polymer solution excited by /spl sim/810 nm and /spl sim/7-ns laser pulses. The solute is a novel polymer, a poly(2,5-dialkoxy-p-phenylene ethynylene) derivative (EBO-OPPE). Using 1-cm path-length EHO-OPPE solution in chloroform of d/sub 0/=0.03 mol of repeat unit/liter as the nonlinear absorptive medium, the dynamic transmission changes from T=0.92 to 0.28 when the input intensity of the /spl sim/810-nm laser beam is increased from I/sub 0/=15 to 600 MW/cm/sup 2/. The measured nonlinear absorption coefficient is 14.5 cm/GW. Optical power stabilization is demonstrated at an average input intensity level of I/sub 0//spl ap/400 MW/cm/sup 2/ with a /spl Delta//spl ap//spl plusmn/25% peak-power fluctuation of the laser pulse. After passing through the nonlinear medium, the output peak-power fluctuation is reduced to /spl Delta//spl ap//spl plusmn/8%. The spectral-width effect of the input laser beam on the nonlinear absorption of the EHO-OPPE solution is investigated. For three different spectral structures of the input laser beam (single narrow spectral line, multiple spectral lines, and broad spectral band), measured values of TPA cross section for EHO-OPPE are /spl sigma//sub 2/=66, 80, and 101/spl times/10/sup -20/ cm/sup 4//GW, respectively. This means that EHO-OPPE is one of the best known nonlinear absorptive materials for power limiting purposes.
TL;DR: In this paper, the authors achieved a stability of 3/spl middot/10/sup -13/ /spl tau//sup -1/2/ for 3/pl tau/<30 s with a laser-pumped rubidium gas-cell frequency standard by reducing the effects due to noise in the microwave and laser sources.
Abstract: We have achieved a stability of 3/spl middot/10/sup -13/ /spl tau//sup -1/2/ for 3
TL;DR: In this article, the authors present a comprehensive theory describing the CW balance of heat and fluorescence powers in Nd:YAG and discuss the individual and total heat, fluorescence and stimulated-emission fractions of the absorbed power, with or without stimulated emission, and as a function of the Nd doping density.
Abstract: We present a comprehensive theory describing the CW balance of heat and fluorescence powers in Nd:YAG and discuss the individual and total heat, fluorescence, and stimulated-emission fractions of the absorbed power, with or without stimulated emission, and as a function of the Nd doping density. /spl chi/ values are presented for both CW and pulsed diode pumping. It is shown that heating due to multiphonon emission and concentration quenching alone cannot account for recent CW measurements of the heat fraction, but that upconversion appears to be a negligible effect.
TL;DR: In this paper, a simple circuit model for a metal-semiconductor-metal (MSM) electrode structure was proposed, and the injected current depends on MSM parameters as well as optical coherence, power, and polarization.
Abstract: In an earlier publication, preliminary observations of the generation of electrical currents were reported in GaAs and low-temperature-grown GaAs (LT-GaAs) at 295 K using quantum interference control of single- and two-photon band-band absorption of 1.55- and 0.775-/spl mu/m ultrashort optical pulses. Time-integrated currents were measured via charge collection in a metal-semiconductor-metal (MSM) electrode structure. Here we present detailed characteristics of this novel effect in terms of a simple circuit model for the MSM device and show how the injected current depends on MSM parameters as well as optical coherence, power, and polarization. For picosecond pulse excitation with peak irradiance of only 30 MW/cm/sup -2/ (1.55 /spl mu/m) and 9 kW/cm/sup -2/ (0.775 /spl mu/m), peak current densities of /spl sim/10 A/cm/sup -2/ at peak carrier densities of 10/sup 15/ cm/sup -3/ are inferred from the steady-state signals. This compares with 50 A/cm/sup -2/ predicted theoretically; the discrepancy mainly reflects inefficient charge collection at the MSM electrodes.
TL;DR: In this paper, three asymmetries in asymmetric Mach-Zehnder interferometer (MZI) configurations with SOA's on their arms are discussed. And the theoretical predictions are verified and found to be in excellent agreement with experiments for a switch with symmetric MZI splitters in a monolithically integrated InP waveguide version.
Abstract: Asymmetric Mach-Zehnder interferometer (MZI) configurations are proposed to build all-optical space switches with gain and principally ideal extinction ratios. Actually, three asymmetries in MZI configurations with semiconductor optical amplifiers (SOA's) on their arms are discussed. The asymmetries in the all-optical switches are necessary to overcome the extinction ratio limitations that are due to the disturbing gain changes that arise when control signals are introduced into the SOA's to induce the necessary refractive index change for switching. Starting from a generic MZI configuration with SOA's on the arms, a description in terms of transmission matrices is used and applied to identify 1/spl times/2 and 2/spl times/2 all-optical switch configurations with high on-state transmissions and close to ideally large extinction ratios. The theoretical predictions are verified and found to be in excellent agreement with experiments for a switch with symmetric MZI splitters in a monolithically integrated InP waveguide version that allows operation with equally or unequally biased SOA's.
TL;DR: In this article, the temperature dependence of threshold current density of a semiconductor quantum dot (QD) laser is analyzed and the temperature is calculated as a function of the parameters of the structure.
Abstract: Detailed theoretical analysis of the temperature dependence of threshold current density of a semiconductor quantum dot (QD) laser is given. Temperature dependences of the threshold current density components associated with the radiative recombination in QDs and in the optical confinement layer (OCL) are calculated. Violation of the charge neutrality in QDs is shown to give rise to the slight temperature dependence of the current density component associated with the recombination in QD's. The temperature is calculated (as a function of the parameters of the structure) at which the components of threshold current density become equal to each other. Temperature dependences of the optimum surface density of QD's and the optimum thickness of the OCL, minimizing the threshold current density, are obtained. The characteristic temperature of QD laser T/sub o/ is calculated for the first time considering carrier recombination in the OCL (barrier regions) and violation of the charge neutrality in QDs. The inclusion of violation of the charge neutrality is shown to be critical for the correct calculation of T/sub o/. The characteristic temperature is shown to fall off profoundly with increasing temperature. A drastic decrease in T/sub o/ is shown to occur in passing from temperature conditions wherein the threshold current density is controlled by radiative recombination in QD's to temperature conditions wherein the threshold current density is controlled by radiative recombination in the OCL. The dependences of T/sub o/ on the root mean square of relative QD size fluctuations, total losses, and surface density of QDs are obtained.
TL;DR: In this article, a unified study of the noise characteristics of semiconductor lasers with optical feedback and short external cavity length is presented. And the effect of the variation of some of the laser diode parameters on the relative intensity noise (RIN) is calculated by using a numerical solution of these equations.
Abstract: A unified study of the noise characteristics of semiconductor lasers with optical feedback and short external cavity length is presented. A new set of nonlinear rate equations that can describe a laser diode with any amount of optical feedback is proposed. The relative intensity noise (RIN) is calculated by using a numerical solution of these equations. This paper concentrates mainly on the moderate and strong feedback regimes. The spectral phenomena observed during the transition from the weak feedback to the "coherence collapse" regime and then to the strong feedback regime are studied and explained. The effect of the variation of some of the laser diode parameters on the RIN characteristics is also investigated.
TL;DR: In this article, a Yb:YAG laser capable of delivering up to 434 W of CW power has been demonstrated using a unique composite rod design which allows for high average power operation while simultaneously suppressing parasitic oscillations.
Abstract: Using a diode-end-pumped technology, a Yb:YAG laser capable of delivering up to 434 W of CW power has been demonstrated. The system incorporates a unique composite rod design which allows for high-average-power operation while simultaneously suppressing parasitic oscillations. Modeling and experimental data to support the quenching of parasitics are discussed. Beam quality measurements for CW operation with several cavity configurations are presented. In particular, beam quality measurements at 340-W CW yielded a beam quality factor of M/sup 2/=21. Predictions of a quasi-three-level model are compared with the experimental data for several output coupler reflectivities. An observed dependence of the cavity mode fill as a function of output coupler reflectivity is discussed. Employing a single acoustooptical switch, the system was Q-switched at 10 kHz and generated output powers up to 280 W with a measured beam quality of M/sup 2/=6.8 at 212 W, With an external dual-KTP crystal configuration, the Q-switched output was frequency converted to 515 nm and produced up to 76 W at 10 kHz in a 30-ns pulse length.
TL;DR: In this paper, the authors examined the strain tensor in self-assembled quantum dots using a valence force-field model and discussed the electron-phonon interactions in the quantum dot structures.
Abstract: Strained epitaxy has been shown to produce pyramidal-shaped semiconductor dot structures by single-step epitaxy. The very high density of these dots (approaching per wafer) and their ever improving uniformity suggest that these features could have important applications in future microelectronics. Understanding the structural and electronic properties of these quantum dots is therefore of great importance. In this paper, we examine some of the physics controlling the performance of devices that could be made from such structures. The self-assembled quantum dots are highly strained and we will examine the strain tensor in these quantum dots using a valence force-field model. In this paper we will address the following issues: (1) What is the general nature of the strain tensor in self assembled quantum dots? (2) What are the electron and hole spectra for InAs-GaAs quantum dots? (a) What are the important intersubband radiative and nonradiative scattering processes in the self assembled quantum dots? In particular, we will discuss how the electron-phonon interactions are modified in the quantum dot structures. Consequences for uncooled intersubband devices such as lasers, detectors, and quantum transistors will be briefly discussed.
TL;DR: In this article, the authors studied the problem of normalizing the nonlinear Brillouin signal and presented a new technique that solved this problem by mathematically combining the values of the Stokes and anti-Stokes powers to produce a linear effective power.
Abstract: The simultaneous determination of strain and temperature distributions from the measurement of noise-initiated Brillouin scattering (NIBS) power and frequency shift in optical fibers is discussed. Equations governing the growth of the NIBS signal are derived and from these, we calculate the dependence of the Brillouin power on temperature and strain. We study the potential problem given by the need to normalize the nonlinear Brillouin signal and present a new technique that solves this problem by mathematically combining the values of the Stokes and anti-Stokes powers to produce a linear effective power. Experimental results are presented that support this theory and allow the verification of the coefficients governing the dependence of the Brillouin power and frequency shift on temperature and strain. The signal-to-noise ratio of the sensor is discussed, and it is found that the noise associated with the field statistics plays a limiting role in the sensor performance and that an optimum value for the Brillouin gain factor can be determined. A simultaneous distributed temperature and strain sensor is demonstrated; preliminary results show a strain resolution of 100-/spl mu/m strain, a temperature resolution of 4/spl deg/C, and a spatial resolution of 40 m, over a sensing length of 1200 m.
TL;DR: In this paper, a theoretical proposal to double the pulse repetition frequency by alternating current modulation of two orthogonal high-order transverse modes of a vertical-cavity surface emitting laser (VCSEL) is presented.
Abstract: A theoretical proposal to double the pulse repetition frequency by alternating current modulation of two orthogonal high-order transverse modes of a vertical-cavity surface-emitting laser (VCSEL) is presented. Static and dynamic characteristics of weakly index-guided VCSELs in a multitransverse mode regime are analyzed. An efficient model that takes into account all the transverse modes supported by the waveguide is developed. Mode partition noise in a current modulated VCSEL is studied, taking into account the azimuthal degree of freedom. Different transverse modes can be excited with a probability that is numerically calculated. For high injection currents, modes that are not favored in the steady state can be excited with higher probability. The excitation probability is similar for azimuthally orthogonal modes. This symmetry can be broken by selecting a particular high-order transverse mode by using azimuthal-dependent current profiling. This selection can be achieved over current ranges as wide as ten times the threshold current. Current modulation of this transverse mode is then analyzed. Alternate current modulation of two orthogonal high-order transverse modes is also studied. Alternating modulation of these modes can double the pulse repetition frequency obtained by modulating just one high-order transverse mode, without increasing injected current density levels. This current-induced spatial switching leads to high-frequency beam steering in the laser azimuthal direction.
TL;DR: In this article, the authors show that the initial set of five coupled differential equations can be reduced to a simple set of two coupled equations for the inversion density and photon flux for a Q-switched Nd:YAG laser.
Abstract: The conventional rate equations for a Q-switched laser are augmented to explicitly include the effects of time- and level-dependent pumping, thermalization among the sublevels in the upper and lower multiplets, and multiplet relaxation in a homogeneously broadened four-level laser medium. To make the numerical computations more generally valid, we introduce a number of dimensionless variables. We show that the initial set of five coupled differential equations can be reduced to a simple set of two coupled equations for the inversion density and photon flux. Via numerical modeling, we have investigated the manner in which both thermalization and lower multiplet relaxation affect Nd:YAG laser characteristics such as output energy and temporal waveform. Our numerical results confirm earlier predictions that the Q-switched Nd:YAG laser output energy increases monotonically by a factor of 3.33 as one progresses from the assumption of slow to rapid thermalization and by an additional factor of 1.46 if one further assumes a terminal multiplet relaxation which is fast relative to the resonator photon decay time. We also find that the laser pulsewidth is substantially broadened when the resonator photon decay time is comparable to the thermalization, and to a lesser extent, the terminal multiplet relaxation times.