Showing papers in "IEEE Journal of Quantum Electronics in 2009"

Self-Consistent Analysis of Strain-Compensated InGaN–AlGaN Quantum Wells for Lasers and Light-Emitting Diodes

Abstract: Strain-compensated InGaN-AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band kmiddotp formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN-AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN-AlGaN QW as active regions for diode lasers.

Quantum Cascade Detectors

Abstract: This paper gives an overview on the design, fabrication, and characterization of quantum cascade detectors. They are tailorable infrared photodetectors based on intersubband transitions in semiconductor quantum wells that do not require an external bias voltage due to their asymmetric conduction band profile. They thus profit from favorable noise behavior, reduced thermal load, and simpler readout circuits. This was demonstrated at wavelengths from the near infrared at 2 mum to THz radiation at 87 mum using different semiconductor material systems.

Time-Delay Identification in a Chaotic Semiconductor Laser With Optical Feedback: A Dynamical Point of View

Abstract: A critical issue in optical chaos-based communications is the possibility to identify the parameters of the chaotic emitter and, hence, to break the security. In this paper, we study theoretically the identification of a chaotic emitter that consists of a semiconductor laser with an optical feedback. The identification of a critical security parameter, the external-cavity round-trip time (the time delay in the laser dynamics), is performed using both the auto-correlation function and delayed mutual information methods applied to the chaotic time-series. The influence on the time-delay identification of the experimentally tunable parameters, i.e., the feedback rate, the pumping current, and the time-delay value, is carefully studied. We show that difficult time-delay-identification scenarios strongly depend on the time-scales of the system dynamics as it undergoes a route to chaos, in particular on how close the relaxation oscillation period is from the external-cavity round-trip time.

Cost-Effective Multimode Polymer Waveguides for High-Speed On-Board Optical Interconnects

Abstract: Cost-effective multimode polymer waveguides, suitable for use in high-speed on-board optical interconnections, are presented. The fundamental light transmission properties of the fabricated waveguides are studied under different launch conditions and in the presence of input misalignments. Low loss (~0.04 dB/cm at 850 nm) and low crosstalk (<-30 dB) performance, relaxed alignment tolerances (plusmn20 mum) and high-speed operation at a 10-Gb/s data rate are achieved. No degradation in the high-speed link performance is observed when offset input launches are employed. Moreover, a range of useful waveguide components that add functionality and enable complex on-board topologies are presented. The optical transmission characteristics of the fabricated components are investigated and it is shown that excellent performance is achieved. Excess losses as low as 0.01 dB per waveguide crossing, the lowest reported value for such components, and bending losses below 1 dB for 90-degree and S-shaped bends are obtained even with multimode fiber launches. Moreover, high-uniformity power splitting and low-loss signal combining are achieved with Y-shaped splitter/combiners while a variable splitting ratio between 30%-75% is demonstrated with the use of multimode couplers. Overall, the devices presented are attractive potential candidates for use in on-board optical links.

Reliable Low-Cost Fabrication of Low-Loss $\hbox{Al}_{2}\hbox{O} _{3}{:}\hbox{Er}^{3+}$ Waveguides With 5.4-dB Optical Gain

Abstract: A reliable and reproducible deposition process for the fabrication of Al2O3 waveguides with losses as low as 0.1 dB/cm has been developed. The thin films are grown at ~ 5 nm/min deposition rate and exhibit excellent thickness uniformity within 1% over 50times50 mm2 area and no detectable OH- incorporation. For applications of the Al2O3 films in compact, integrated optical devices, a high-quality channel waveguide fabrication process is utilized. Planar and channel propagation losses as low as 0.1 and 0.2 dB/cm, respectively, are demonstrated. For the development of active integrated optical functions, the implementation of rare-earth-ion doping is investigated by cosputtering of erbium during the Al2O3 layer growth. Dopant levels between 0.2-5times1020 cm-3 are studied. At Er3+ concentrations of interest for optical amplification, a lifetime of the 4I13/2 level as long as 7 ms is measured. Gain measurements over 6.4-cm propagation length in a 700-nm-thick Al2O3:Er3+ channel waveguide result in net optical gain over a 41-nm-wide wavelength range between 1526-1567 nm with a maximum of 5.4 dB at 1533 nm.

Incoherent Combining and Atmospheric Propagation of High-Power Fiber Lasers for Directed-Energy Applications

Abstract: High-power fiber lasers can be incoherently combined to form the basis of a directed high-energy laser system which is highly efficient, compact, robust, low-maintenance and has a long operating lifetime. This approach has a number of advantages over other beam combining methods. We present results of the first field demonstration of incoherent beam combining using kilowatt-class, single-mode fiber lasers. The experiment combined four fiber lasers using a beam director consisting of individually controlled steering mirrors. Propagation efficiencies of ~90%, at a range of 1.2 km, with transmitted continious-wave power levels of 3 kW were demonstrated in moderate atmospheric turbulence. We analyze the propagation of combined single-mode and multimode beams in atmospheric turbulence and find good agreement between theory, simulations and experiments.

Thin-Disk Yb:YAG Oscillator-Amplifier Laser, ASE, and Effective Yb:YAG Lifetime

Abstract: We report on a thin-disk Yb:YAG laser made from a Q-switched oscillator and a multipass amplifier delivering pulses of 48 mJ at 1030 nm. The peculiar requirements for this laser are the short delay time (< 500 ns) between electronic trigger and optical output pulse and the time randomness with which these triggers occur (with trigger to next trigger delay ges 1.5 ms). Details concerning the oscillator dynamics (-switching cycle, intensity stabilization), and the peculiar amplifier layout are given. Simulations of the beam propagation in the amplifier based on the Collins integral and the measured aspherical components of the disk reproduce well the measured beam intensity profiles (with higher order intensity moments) and gains. Measurements of the thermal lens and ASE effects of the disk are also presented. A novel method to deduce the effective Yb:YAG upper state lifetime (under real laser operation and including ASE effects) is presented. That knowledge is necessary to determine gain and stored energy in the active medium and to understand the limiting factors for energy scaling of thin-disk lasers.

Fundamental Formulation for Plasmonic Nanolasers

Abstract: We develop a fundamental formulation of plasmonic nanolasers from Fermi's Golden rule, taking into account the inhomogeneity and dispersion of the cavity media. We demonstrate that the electromagnetic energy confinement factor and the corresponding effective volume lead to a more consistent formulation for the rate equations of nanolasers. We also use a Fabry-Perot nanolaser based on the plasmonic slab waveguide as an example and compare the energy confinement factor with other counterparts. In particular, we show that the power confinement factor is inappropriate for plasmonic cavity modes.

Quantum-Dot Lasers—Desynchronized Nonlinear Dynamics of Electrons and Holes

Abstract: We analyze the complex turn-on behavior of semiconductor quantum-dot (QD) lasers in terms of a nonlinear rate equation model for the electron and hole densities in the QDs and the wetting layer, and the photons. A basic ingredient of the model is the nonlinearity of the microscopic carrier-carrier scattering rates. With the framework of detailed balance, we analytically relate the microscopic in- and out-scattering rates. We gain insight into the anomalous nonlinear dynamics of QD lasers by a detailed analysis of various sections of the 5-D phase space, accounting for density-dependent carrier scattering times. We show that the strongly damped relaxation oscillations are characterized by a desynchronization of electron and hole dynamics in the dots. Analytic approximations for the steady-state characteristics are also derived.

DFB Quantum Cascade Laser Arrays

Abstract: DFB quantum cascade laser (DFB-QCL) arrays operating between 8.7 and 9.4 mum are investigated for their performance characteristics-single-mode selection of the DFB grating, and variability in threshold, slope efficiency, and output power of different lasers in the array. Single-mode selection refers to the ability to choose a desired mode/frequency of laser emission with a DFB grating. We apply a theoretical framework developed for general DFB gratings to analyze DFB-QCL arrays. We calculate how the performance characteristics of DFB-QCLs are affected by the coupling strength kappaL of the grating, and the relative position of the mirror facets at the ends of the laser cavity with respect to the grating. We discuss how single-mode selection can be improved by design. Several DFB-QCL arrays are fabricated and their performance examined. We achieve desired improvements in single-mode selection, and we observe the predicted variability in the threshold, slope efficiency, and output power of the DFB-QCLs. As a demonstration of potential applications, the DFB-QCL arrays are used to perform infrared absorption spectroscopy with fluids.

Theoretical and Experimental Study of High-Speed Small-Signal Cross-Gain Modulation of Quantum-Dot Semiconductor Optical Amplifiers

Abstract: We numerically and experimentally investigate the high-speed small-signal cross-gain modulation (XGM) characteristics of a quantum-dot (QD) semiconductor optical amplifier (SOA). From a p-doped QD SOA operating at 1.3 mum, high-speed small-signal XGM responses up to 40 GHz are measured from low to high injection currents and improve at high injection currents. In the numerical model, we set up about six hundred coupled rate equations, where the carrier dynamics of QD electron and hole states are considered separately and the enhanced hole occupation due to p-type doping is included. The high-speed small-signal XGM spectra are calculated at various modulation frequencies and pump-probe detunings. We identify how the two separate XGM mechanisms of total carrier density depletion (TCDD) at low injection current and spectral hole burning (SHB) at high injection current affect the high-speed small-signal XGM behavior. From the measured and calculated results, we show that high-speed small-signal XGM responses of QD SOAs can be improved by injecting more carriers to the QD excited states, which enhances high-speed XGM induced by SHB rather than by TCDD.

Cerenkov-Type Second-Harmonic Generation in Two-Dimensional Nonlinear Photonic Structures

Abstract: We study the Cerenkov-type second-harmonic generation in several different two-dimensional nonlinear photonic structures formed in birefringent crystals with the 3 m symmetry. Depending on the degree of birefringence, we observe either single or double Cerenkov-like second-harmonic rings. We discuss the properties of these parametrically generated rings and show that their sixfold azimuthal modulation is associated with the hexagonal symmetry of the individual ferroelectric domains.

Measurement and Simulation of Distributed-Feedback Tapered Master-Oscillator Power Amplifiers

Abstract: The spectral and spatial behavior of monolithically integrated distributed-feedback tapered master-oscillator power amplifiers emitting around 973 nm is experimentally and theoretically investigated We demonstrate a good agreement between experiments and theory and analyze peculiarities of the observed dynamical regimes

Optical Parabolic Pulse Generation and Applications

Abstract: Parabolic pulses in optical fibers have stimulated an increasing number of applications. We review here the physics underlying the generation of such pulses as well as the results obtained in a wide range of experimental configurations.

Incoherent Method of Optical Interference Cancellation for Radio-Frequency Communications

Abstract: In this paper, we describe a system for cancelling radio-frequency (RF) interference using optical techniques. Specifically, we attempt to receive a weak RF signal-which we assume to be of the order of microwatts-in the presence of high-power local RF interference. This local interference is a signal whose power is of the order of 100 W and is generated in close proximity to the receiver. We wish to emphasize that the nature of the interfering signal is completely known to us in practice, since we are generating it for communications purposes. This knowledge of the interfering signal will prove to be useful in our attempts to cancel it, as will be shown. We refer to this technique as optical interference cancellation, or opto-cancellation. We have demonstrated that this opto-cancellation system can cancel a simple sinusoid at 3 GHz, as well as broadband interference of approximately 100-MHz bandwidth centered at 3 GHz. We have also demonstrated cancellation of sinusoids and broadband signals at other center frequencies as well. In the case of sinusoidal signals, we have demonstrated optical cancellation over 70 dB; and in the case of the ~ 100-MHz signal, we have demonstrated optical cancellation over 30 dB.

Comprehensive Characterization of InGaAs–InP Avalanche Photodiodes at 1550 nm With an Active Quenching ASIC

Abstract: We present an active quenching application-specific integrated circuit (ASIC), for use in conjunction with InGaAs-InP avalanche photodiodes (APDs), for 1550-nm single-photon detection. To evaluate its performance, we first compare its operation with that of standard quenching electronics. We then test four InGaAs-InP APDs using the ASIC, operating both in the free-running and gated modes, to study more general behavior. We investigate not only the standard parameters under different working conditions but also parameters such as charge persistence and quenching time. We also use the multiple trapping model to account for the afterpulsing behavior in the gated mode, and further propose a model to take account of the afterpulsing effects in the free-running mode. Our results clearly indicate that the performance of APDs with an on-chip quenching circuit significantly surpasses the conventional quenching electronics and makes them suitable for practical applications, e.g., quantum cryptography.

Ultrafast All-Optical Processor Based on Quantum-Dot Semiconductor Optical Amplifiers

Abstract: We have developed a theoretical model of an ultra- fast all-optical signal processor based on the Mach-Zehnder interferometer with quantum-dot semiconductor optical amplifiers in both its arms. It is shown that such a processor under different conditions may realize wavelength conversion, XOR logic operation, and optical 3R regeneration.

Emission Directionality of Semiconductor Ring Lasers: A Traveling-Wave Description

Abstract: We use a traveling-wave model for explaining the experimentally observed changes in the directionality of the emission of semiconductor ring lasers and its different behavior when current is increased or decreased. The modulation of the cavity losses imposed by the light extraction sections together with the thermal shift of the gain spectrum and spatial hole burning in the carrier density play a crucial role in the directionality of the emission and its changes with operation current. The differences as the current is increased or decreased correspond to the different role played by spatial hole burning.

Determination of Phase Noise Spectra in Optoelectronic Microwave Oscillators: A Langevin Approach

Abstract: We introduce a stochastic model for the determination of phase noise in optoelectronic oscillators. After a short overview of the main results for the phase diffusion approach in autonomous oscillators, an extension is proposed for the case of optoelectronic oscillators where the microwave is a limit-cycle originated from a bifurcation induced by nonlinearity and time-delay. This Langevin approach based on stochastic calculus is also successfully confronted with experimental measurements.

Phase-Noise-Compensated Optical Frequency-Domain Reflectometry

Abstract: The theory of phase-noise-compensated optical frequency-domain reflectometry (PNC-OFDR), a novel type of optical frequency-domain reflectometry (OFDR) with a measurement range much longer than the laser coherence length, is described, and the signal and noise spectral densities are deduced for a discussion of signal-to-noise ratio (SNR). The analysis of PNC-OFDR shows the possibility of obtaining a high SNR by using many reference signals for phase-noise compensation. By using a ldquoconcatenately generated phaserdquo (CGP), only a single auxiliary interferometer is needed for phase-noise compensation, and other reference signals can be easily generated by performing a calculation based on signal use obtained from the single auxiliary interferometer. An experimental investigation shows the feasibility of using CGPs for PNC-OFDR by dividing the fiber under test into several sections for phase-noise compensation. Moreover, the influence of strong reflection events on Rayleigh backscattering is discussed by considering the dead zone caused by a fiber/air Fresnel reflection. It is shown theoretically that a dead zone that has no influence on the neighboring section can be achieved by using suitable parameters in an actual system.

Spectral Analysis of 1.55- $\mu$ m InAs–InP(113)B Quantum-Dot Lasers Based on a Multipopulation Rate Equations Model

Abstract: In this paper, a theoretical model is used to investigate the lasing spectrum properties of InAs-InP(113)B quantum dot (QD) lasers emitting at 1.55 mum. The numerical model is based on a multipopulation rate equations analysis. Calculations take into account the QD size dispersion as well as the temperature dependence through both the inhomogeneous and the homogeneous broadenings. This paper demonstrates that the model is capable of reproducing the spectral behavior of InAs-InP QD lasers. Especially, this study aims to highlight the transition of the lasing wavelength from the ground state (GS) to the excited state (ES). In order to understand how the QD laser turns on, calculated optical spectra are determined for different cavity lengths and compared to experimental ones. Unlike InAs-GaAs QD lasers emitting at 1.3 mum, it is shown that a continuous transition from the GS to the ES is exhibited because of the large inhomogeneous broadening comparable to the GS and ES lasing energy difference.

Accuracy of Transfer Matrix Approaches for Solving the Effective Mass SchrÖdinger Equation

Abstract: The accuracy of different transfer matrix approaches, widely used to solve the stationary effective mass Schrodinger equation for arbitrary one-dimensional potentials, is investigated analytically and numerically. Both the case of a constant and a position-dependent effective mass are considered. Comparisons with a finite difference method are also performed. Based on analytical model potentials as well as self-consistent Schrodinger-Poisson simulations of a heterostructure device, it is shown that a symmetrized transfer matrix approach yields a similar accuracy as the Airy function method at a significantly reduced numerical cost, moreover avoiding the numerical problems associated with Airy functions.

Experimental Demonstration of Multi-Bit Optical Buffer Memory Using 1.55- $\mu{\hbox {m}}$ Polarization Bistable Vertical-Cavity Surface-Emitting Lasers

Abstract: We demonstrate a novel all-optical buffer memory using 1.55-μm polarization bistable vertical-cavity surface-emitting lasers (VCSELs). A one-bit data is stored as one of two orthogonal polarization states of a VCSEL in this memory. The polarization state is transferred from the VCSEL to another VCSEL which is optically connected in cascade as a shift register. A 4-bit optical buffer memory is constructed using two sets of the shift-register memory connected in parallel. These results show the technical feasibility of multi-bit optical buffer memory.

Background Limited Performance of Long Wavelength Infrared Focal Plane Arrays Fabricated From M-Structure InAs–GaSb Superlattices

Abstract: The recent introduction of a M-structure design improved both the dark current and R0 A performances of type-II InAs-GaSb photodiodes. A focal plane array fabricated with this design was characterized at 81 K. The dark current of individual pixels was measured between 1.1 and 1.6 nA, 7 times lower than previous superlattice FPAs. This led to a higher dynamic range and longer integration times. The quantum efficiency of detectors without antireflective coating was 74%. The noise equivalent temperature difference reached 23 mK, limited only by the performance of the testing system and the read out integrated circuit. Background limited performances were demonstrated at 81 K for a 300 K background.

Modeling of the Transient Characteristics of Heterojunction Bipolar Transistor Lasers

Abstract: A charge control model for the transient analysis of the bipolar transistor laser in forward active mode is developed to describe the dynamics of electron, photon and charge densities. From three coupled-rate equations, analytical expressions are obtained for the threshold base current density, the steady-state electron and photon densities, and small-signal frequency response. We find that the threshold current density decreases with increasing spontaneous emission lifetime of electrons in the quantum well as well as a linear dependence of the photon density on the base current. By optimizing the model parameters in their physical range, we show that a -3-dB bandwidth of 55 nGHz can be achieved for small-signal modulation of the laser. By numerically solving the rate equations, we investigate the -3-dB bandwidth under large-signal modulation to show that it remains nearly unchanged from the small-signal case. We further perform analysis of laser switching and find that the turn-on time is reduced by increasing the base current or decreasing the spontaneous emission lifetime of electrons in the quantum well.

Optical Feedback Self-Mixing Interferometry With a Large Feedback Factor $C$ : Behavior Studies

Abstract: This paper studies the behavior of optical feedback self-mixing interferometric (OFSMI) systems, where the semiconductor lasers operate at a single mode (perturbed external cavity mode) with a large optical feedback factor C. Based on analysis of the spectral linewidth associated with all the possible lasing modes at different C values, a set of mode jumping rules are proposed following the minimum linewidth mode competition principle proposed in . According to the rules, the C factor can be classified into different regions, on which an OFSMI system will exhibit distinct phenomena. In particular, for the same amount of displacement associated with the external cavity, the fringe number reduction on the OFSMI signal should be observed when C increases from one region to the next. An experimental setup with a laser diode HL7851G was implemented and employed to verify the proposed rules. The behavior of the OFSMI predicted by the paper has been confirmed by the experiments with C value up to 8.0.

Characteristics of Fast Physical Random Bit Generation Using Chaotic Semiconductor Lasers

Abstract: We investigate the characteristics of fast random bit generation using chaotic semiconductor lasers. The optical amplitudes of two lasers with chaotic oscillations induced by optical feedback are each sampled at a fixed rate to extract binary bit sequences which are then combined by an exclusive-OR operation to obtain a single random bit sequence. Bit sequences generated at rate of 1 Giga bit per second are verified to pass statistical tests of randomness. We describe the dependence of randomness on laser parameters, in particular the injection current, the external cavity length and the feedback strength. The results provide clear empirical guidelines for tuning the chaotic laser parameters to achieve random bit sequences. This study shows that chaotic laser devices can be fast and reliable sources of physical entropy for computing and communication applications.

Numerical Study of Dynamical Regimes in a Monolithic Passively Mode-Locked Semiconductor Laser

Abstract: Bifurcation mechanisms of the development and break up of different operation regimes in a passively mode-locked monolithic semiconductor laser are studied by solving numerically partial differential equations for amplitudes of two counterpropagating waves and carrier densities in gain and absorber sections. It is shown that mode-locking regimes with different repetition rates can be multistable for a wide range of laser parameters and that the harmonic mode-locking regime with two counterpropagating pulses in the cavity can exhibit a period-doubling bifurcation leading to different amplitudes and separations of the pulses. The effect of linewidth enhancement factors in gain and absorber sections on the laser dynamics is discussed.

Dynamics and Spectra of Monolithic Mode-Locked Laser Diodes Under External Optical Feedback

Abstract: In this paper, the effect of external optical reflection on the dynamic regimes and spectral properties of Fabry-Perot and distributed Bragg reflector monolithic mode-locked laser diodes is investigated numerically. The relation of the findings to experimental results is discussed, and optimizing the laser construction for reducing the feedback effects is also assessed.

Semiconductor Double-Ring-Resonator-Coupled Tunable Laser for Wavelength Routing

Abstract: A monolithic widely tunable semiconductor laser based on a double-ring resonator is developed for use in a wavelength-routing switch. By using the double-ring resonator as a wavelength-selective filter, operation over a wide wavelength tuning range is achieved with a low tuning current. This low-tuning-current operation makes the laser very promising as a high-speed tunable light source for a wavelength-routing-based switch by effectively suppressing the thermal wavelength drift induced by current injection. In addition, the laser fabrication process is simpler compared to conventional distributed Bragg reflector tunable lasers. A tuning range of 50.0 nm, covering the entire C-band, is successfully demonstrated with an injection current of less than 5.2 mA. The wavelength drift caused by the thermal transients is less than 5 GHz.