Showing papers in "IEEE Journal of Quantum Electronics in 2000"

Multiloop optoelectronic oscillator

Abstract: We describe and demonstrate a multiloop technique for single-mode selection in an optoelectronic oscillator (OEO). We present experimental results of a dual loop OEO, free running at 10 GHz, that has the lowest phase noise (-140 dBc/Hz at 10 kHz from carrier) of all free-running room-temperature oscillators to date. Finally, we demonstrate the first fiber-optic implementation of the carrier suppression technique to further reduce the close-to-carrier phase noise of the oscillator by at least 20 dB.

Surface plasmon enhanced light-emitting diode

Abstract: A method for enhancing the emission properties of light-emitting diodes, by coupling to surface plasmons, is analyzed both theoretically and experimentally. The analyzed structure consists of a semiconductor emitter layer thinner than /spl lambda//2 sandwiched between two metal films. If a periodic pattern is defined in the top semitransparent metal layer by lithography, it is possible to efficiently couple out the light emitted from the semiconductor and to simultaneously enhance the spontaneous emission rate. For the analyzed designs, we theoretically estimate extraction efficiencies as high as 37% and Purcell factors of up to 4.5. We have experimentally measured photoluminescence intensities of up to 46 times higher in fabricated structures compared to unprocessed wafers. The increased light emission is due to an increase in the efficiency and an increase in the pumping intensity resulting from trapping of pump photons within the microcavity.

Real-time optical spectrum analysis based on the time-space duality in chirped fiber gratings

Abstract: Based on time-space duality, we deduce a time-domain equivalent to the Fraunhofer (far-field) approximation in the problem of spatial diffraction. We can use this equivalence to carry out a real-time optical spectrum analysis, which is shown to be realizable by using, as the dispersive media, filtering devices based on chirped distributed resonant coupling. In particular, we present the design of linearly chirped fiber gratings (reflection configurations) and linearly chirped intermodal couplers (transmission configurations) to work as real-time spectrum analyzers. The proposed systems are shown to work properly by means of simulation tools. Furthermore, we use joint time-frequency signal representations to get a better understanding of the physical processes that determine the behavior of these systems. In this way, we demonstrate that the propagation of a given signal through a chirped fiber grating (or a chirped intermodal coupler), under the temporal Fraunhofer conditions, translates into a temporal separation of the spectral components of the signal. The results of our study indicate potential important applications based on this effect.

An autocompensating fiber-optic quantum cryptography system based on polarization splitting of light

Abstract: We have developed a system for quantum key distribution (QKD), based on standard telecommunication lasers, detectors, and optical fiber, that passively compensates for time-dependent variations of the fiber-optic path due to stress, temperature changes, or birefringence. This approach allows information encoded in phase shifts imposed on single-photon-level pulses to be accurately read out after transmission over many kilometers of uncontrolled fiber. Cooled InGaAs avalanche photodiodes, pulse-biased using a special noise canceling circuit, are used to detect single 1.31-/spl mu/m infrared photons with a high efficiency, low dark-count rate, and subnanosecond time resolution. A single optical fiber carries both the quantum information and precise 1.55 /spl mu/m timing pulses between the two end stations. Overall synchronization of end-station activities, public discussion of basis choices, error correction, and privacy amplification have all been implemented over a local area network (LAN). The system at present generates raw, error-corrected, and privacy-amplified key data at rates of /spl sim/1000, 600, and 200 bits/s, respectively, over a 10-km single-mode fiber link.

The influence of quantum-well composition on the performance of quantum dot lasers using InAs-InGaAs dots-in-a-well (DWELL) structures

Abstract: The optical performance of quantum dot lasers with different dots-in-a-well (DWELL) structures is studied as a function of the well number and the indium composition in the InGaAs quantum well (QW) surrounding the dots. While keeping the InAs quantum dot density nearly constant, the internal quantum efficiency /spl eta//sub i/, modal gain, and characteristic temperature of 1-DWELL and 3-DWELL lasers with QW indium compositions from 10 to 20% are analyzed. Comparisons between the DWELL lasers and a conventional In/sub 0.15/Ga/sub 0.85/As strained QW laser are also made. A threshold current density as low as 16 A/cm/sup 2/ is achieved in a 1-DWELL laser, whereas the QW device has a threshold 7.5 times larger. It is found that /spl eta//sub i/ and the modal gain of the DWELL structure are significantly influenced by the quantum-well depth and the number of DWELL layers. The characteristic temperature T/sub 0/ and the maximum modal gain of the ground-state of the DWELL structure are found to improve with increasing indium in the QW It is inferred from the results that the QW around the dots is necessary to improve the DWELL laser's /spl eta//sub i/ for the dot densities studied.

Principles of parametric temporal imaging. I. System configurations

Abstract: The recently developed process of temporal imaging expands or compresses time waveforms while preserving the shapes of their envelope profiles. A key element in a temporal imaging system is a time lens which imparts a quadratic phase modulation to the waveform being imaged. Several methods, such as electrooptic modulation, can be used to produce the phase modulation. In this paper, we concentrate on the parametric mixing of a signal waveform with a linearly chirped optical pump as the time lens mechanism. We analyze all single-lens system configurations including sum- and difference-frequency mixing schemes with positive and negative group velocity dispersions using temporal ray diagrams as an aid in understanding their operation.

High-repetition-rate optical pulse generation using a rational harmonic mode-locked fiber laser

Abstract: Rational harmonic mode locking takes place in an actively mode-locked fiber laser when the modulation frequency f/sub m/=(n+1/p)f/sub c/, where n and p are both integers and f/sub c/ is the inverse of the cavity round-trip time, the 22nd order of rational harmonic mode locking has been observed when f/sub m//spl ap/1 GHz. An optical pulse train with a repetition rate of 40 GHz has been obtained using a modulation frequency f/sub m/=10 GHz. The theory of rational harmonic mode locking has also been developed. The stability of the mode-locked pulses is improved considerably when a semiconductor optical amplifier is incorporated into the fiber laser cavity. The supermode noise in the RF spectrum of a mode-locked laser is removed for a certain range of current in the semiconductor optical amplifier.

Structures for organic diode lasers and optical properties of organic semiconductors under intense optical and electrical excitations

Abstract: The challenges to realizing diode lasers based on thin films of organic semiconductors are primarily related to low charge carrier mobility in these materials. This not only limits the thickness of organic films to /spl les/100 nm in electrically pumped devices, but it also leads to changes in the optical properties of organic films induced by the large number of carriers trapped in the materials subjected to an intense electrical excitation. We describe organic waveguide laser structures composed of thin organic films and transparent indium-tin-oxide electrodes. These waveguides allow for efficient injection of an electrical current into the organic layers and provide for low optical losses required in a laser. The changes in the optical properties of organic thin films induced by electrical excitation are studied using electroluminescence and pump and probe spectroscopy. Induced transparency and absorption observed in the organic materials may be related to triplet excitons or trapped charge carriers. Pump-induced absorption is also observed in organic films under quasi-CW optical excitation. These effects must be taken into account both in the design of organic diode laser structures and in the selection of charge transporting materials.

Edge-pumped quasi-three-level slab lasers: design and power scaling

Abstract: We present a novel design for a quasi-three-level laser. The design uses a slab laser configuration with the pump light incident from the slab edge. This allows a lower threshold and better power scaling than a conventional face-pumped slab. We present an analytic description of pumping optimization, thermal distortion, and power scaling. Several point designs illustrating power scaling are also described.

Impact ionization characteristics of III-V semiconductors for a wide range of multiplication region thicknesses

Abstract: Recently, an impact ionization model, which takes the nonlocal nature of the impact ionization process into account, has been described. This model incorporates history-dependent ionization coefficients. Excellent fits to experimental gain and noise measurements for GaAs were achieved using an effective field approach and simple analytical expressions for the ionization probabilities. In the paper, we briefly review the history-dependent model and apply it to Al/sub 0.2/Ga/sub 0.8/As, In/sub 0.52/Al/sub 0.48/As and InP avalanche photodiodes. For the study, the gain and noise characteristics of a series of homojunction avalanche photodiodes with different multiplication thicknesses were measured and fit with the history-dependent model. A "size-effect" in thin (<0.5 /spl mu/m) multiplication regions, which is not adequately characterized by the local-field avalanche theory, was observed for each of these materials. The history-dependent model, on the other hand, achieved close agreement with the experimental results.

Rate-equation analysis of output efficiency and modulation rate of photonic-crystal light-emitting diodes

Abstract: The performance characteristics of photonic-crystal light-emitting diodes (LEDs) are analyzed, taking into account the effects of both nonradiative recombination and photon reabsorption processes using multimode rate equations. It is shown that, in the presence of strong photon reabsorption, the optimum output efficiency and modulation rates are achieved when the width of the photon density-of-state distribution function is comparable to the width of the spontaneous emission lineshape of the active material. On the other hand, when photon reabsorption is weak, it becomes beneficial to construct high-Q cavities. Based on this analysis, the characteristics of different photonic crystal LED configurations are discussed.

Fundamentals of stable continuum generation at high repetition rates

Abstract: A continuum generated from highly nonlinear seed pulses (N/spl Gt/1) propagating in a medium with only self-phase modulation (SPM) or with SPM and anomalous dispersion is highly sensitive to the noise of the input pump pulse. The combination of SPM and normal dispersion improves the stability. However, more efficient spectral broadening schemes are desirable for generating a broad-band continuum at gigahertz rates. The adiabatic compression of weakly nonlinear pulses (N/spl sime/1) via the soliton effect efficiently generates a broad-band continuum that is robust against noise. Detailed characterization of continuum generation in several different fibers is reported.

Open-loop chaotic synchronization of injection-locked semiconductor lasers with gigahertz range modulation

Abstract: Unidirectional chaotic synchronization between two remote injection-locked semiconductor lasers to achieve chaotic communications is investigated numerically. Different from the direct chaotic masking methods, the chaotic carrier wave is generated from different chaotic states in transience instead of a fixed chaotic state in static to prevent it from being reproduced through a reconstructed embedding phase space. The testing digital and sinusoidal message signals in the gigahertz range can be easily recovered without the use of any electronic or optical filter to filter out the synchronization error. The robustness of synchronization is examined by using the intrinsic white noise of the transmitter and the receiver as the perturbation. The effects of parameter mismatches on the quality and robustness of synchronization are analyzed in detail. The results show that different internal parameters have very different tolerances for parameter mismatch. A short discussion on the phase sensitivity of synchronization is also given.

Message encoding and decoding using chaotic external-cavity diode lasers

Abstract: Synchronization of chaotic external-cavity diode lasers has been studied in a master-slave configuration. A message is encoded into the chaotic master laser by amplitude modulation and transmitted to the slave laser. A scheme for decoding the message at the slave is demonstrated.

1.3-/spl mu/m CW lasing characteristics of self-assembled InGaAs-GaAs quantum dots

Abstract: This paper presents the lasing properties and their temperature dependence for 1.3-/spl mu/m semiconductor lasers involving self-assembled InGaAs-GaAs quantum dots as the active region. High-density 1.3-/spl mu/m emission dots were successfully grown by the combination of low-rate growth and InGaAs-layer overgrowth using molecular beam epitaxy. 1.3-/spl mu/m ground-level CW lasing occurring at a low threshold current of 5.4 mA at 25/spl deg/C with a realistic cavity length of 300 /spl mu/m and high-reflectivity coatings on both facets. The internal loss of the lasers was evaluated to be about 1.2 cm/sup -1/ from the inclination of the plots between the external quantum efficiency and the cavity length. The ground-level modal gain per dot layer was evaluated to be 1.0 cm/sup -1/, which closely agreed with the calculation taking into account the dot density, inhomogeneous broadening, and homogeneous broadening. The characteristic temperature of threshold currents T/sub 0/ was found to depend on cavity length and the number of dot layers in the active region of the lasers. A T/sub 0/ of 82 K was obtained near room temperature, and spontaneous emission intensity as a function of injection current indicated that the nonradiative channel degraded the temperature characteristics. A low-temperature study suggested that an infinite T/sub 0/ with a low threshold current (/spl sim/1 mA) is available if the nonradiative recombination process is eliminated. The investigation in this paper asserted that the improvement in surface density and radiative efficiency of quantum dots is a key to the evolution of 1.3-/spl mu/m quantum-dot lasers.

Spectro-temporal imaging of femtosecond events

Abstract: We demonstrate a new technique for the direct, real-time, femtosecond scale temporal measurement based on the conversion of temporal information to the spectral domain. The potential of the method has been experimentally investigated with an optical fiber spectral compressor device, first stretching and up-chirping the pulses in a prism dispersive delay line, and afterwards compensating the induced chirp by means of cross-phase modulation in a single-mode fiber. Spectro-temporal imaging (STI) reduces the problem of high-resolution temporal measurements to standard spectrometry.

Self-consistent analysis of high-temperature effects on strained-layer multiquantum-well InGaAsP-InP lasers

Abstract: We present a comprehensive evaluation of the temperature effects on the threshold current and the slope efficiency of 1.55 /spl mu/m Fabry-Perot ridge-waveguide lasers between 20/spl deg/C and 120/spl deg/C. Experimental results are analyzed using the commercial laser simulator PICS3D. The software self-consistently combines two-dimensional carrier transport, heat flux, strained quantum-well gain computation, and optical waveguiding with a longitudinal mode solver. All relevant physical mechanisms are considered, including their dependence on temperature and local carrier density. Careful adjustment of material parameters leads to an excellent agreement between simulation and measurements at all temperatures. At lower temperatures, Auger recombination controls the threshold current and the differential internal efficiency. At high temperatures, the vertical electron leakage from the separate confinement layer mainly limits the laser performance. The increase of internal absorption is less important. However, all these carrier and photon loss enhancements with higher temperature are mainly triggered by the reduction of the optical gain due to wider Fermi spreading of electrons.

Pulse-train uniformity in optical fiber lasers passively mode-locked by nonlinear polarization rotation

Abstract: The generation of uniform soliton pulse trains by additive pulse mode locking has been experimentally demonstrated in a birefringent fiber laser with a passive polarizer. Numerical simulations of pulse propagation around such a fiber loop are presented which reveal that this mode-locking scheme does not result in strictly uniform pulse trains. Rather, the train of output pulses exhibits periodic fluctuations in intensity and polarization. A model for the pulse dynamics is developed which shows that these fluctuations depend on the strength of the fiber birefringence and the alignment of the polarizer with the fast- and slow-polarization axes of the fiber. It is also shown that increased uniformity of pulse trains is achieved with near alignment of the polarizer with the slow axis of the birefringence.

Optical frequency domain ranging by a frequency-shifted feedback laser

Abstract: This paper describes the theoretical and experimental study of a new technique for optical frequency domain ranging (OFDR) by a frequency-shifted feedback (FSF) laser. In conventional OFDR, a frequency chirped single-mode laser is used as a light source to convert a distance into a beat frequency, and a tradeoff exists between measurement range and resolution. The FSF laser output consists of periodically generated chirped frequency components whose chirp rate is faster than 100 PHz/s (P=10/sup 15/), By use of the FSF laser, the tradeoff is removed and long-distance high-resolution OFDR is realized In the experiment, a distance of 18.5 km was measured with a resolution of 20 mm.

Optical signal processing using nonlinear distributed feedback structures

Abstract: We analyze the optical signal processing functionality of periodic structures consisting of alternating layers of materials possessing opposite Kerr nonlinearities. By elaborating an analytical model and employing numerical simulations, we explore the performance of proposed passive optical limiters and switches. We prove that the proposed limiters provide true limiting by clamping the transmitted intensity at a level which is independent of the incident intensity. We explore the response of optical switches for signal and pump beams having the same and different frequencies. We describe and quantify the performance of the proposed structures in the realization of all-optical OR gates and optical hard-limiters. In addition, we prove that, for fabrication errors as large as 10%, qualitative device functionality remains, with performance only modestly degraded.

On the dimensionality of optical absorption, gain, and recombination in quantum-confined structures

Abstract: The purpose of this paper is to explore the dimensionality of the optoelectronic properties of quantum-well and dot systems by expressing carrier distributions in the confinement directions in terms of envelope functions rather than assuming that carriers are localized to the geometrical extent of the confining potential. The conclusions apply to an ideal two-dimensional (2-D) system or a structure where only the n=1 electron and hole subbands are populated. We show that optical absorption normal to the plane of a QW cannot be expressed as an absorption coefficient but should be specified as a fraction of light transmitted or absorbed per well. The modal gain for light propagating along the plane of a QW does not scale with well width and the variation of the material gain inversely proportional to the well width is a consequence of the definition of the confinement factor and has no independent physical significance. Coupling to the optical mode can be specified as a mode width without the need to assume the gain medium is localized in the well. Optical absorption and gain by quantum dots should be expressed as a cross section per dot. The radiative recombination current should be expressed in terms of a two-dimensional recombination coefficient and use of an equivalent three-dimensional coefficient introduces an artificial dependence on well width which can lead to errors in the comparison of QW systems. We provide an analysis of experimental data for optical absorption in GaAs wells and show that, using the correct dimensional forms, it is straightforward to use this to estimate modal gain and the recombination coefficient.

An efficient split-step time-domain dynamic modeling of DFB/DBR laser diodes

Abstract: A novel and efficient approach for the numerical solution of time-dependent coupled-wave equations, which are frequently used for the modeling of distributed-feedback, distributed Bragg reflector, and Fabry-Perot laser diodes, is proposed. In this approach, the coupled-wave equations are split into two sets of equations. One of two sets of equations contains only the phase factors and time derivatives, and the other contains only the coupling terms. The separate sets of equations are solved exactly in their split form successively. This new numerical scheme, which we call the split-step time-domain model, is found to require an order of magnitude smaller number of subsections to get more accurate results than previous methods while the computation time for each time step is comparable to previous methods.

Nonlinear harmonic generation in free-electron lasers

Abstract: A three-dimensional nonlinear simulation code to treat multiple frequencies simultaneously is described and used to study nonlinear harmonic generation in free-electron lasers (FELs). Strong nonlinear harmonic gain is found where the gain length varies inversely with the harmonic number. Substantial power levels are found in the harmonics. The odd harmonics are favored with generally higher power levels since a planar wiggler geometry is employed; however, the second harmonic exhibits substantial power as well. The analysis is relevant to the emission expected from self-amplified spontaneous emission (SASE) free-electron laser schemes.

A time-domain computer simulator of the nonlinear response of semiconductor optical amplifiers

Abstract: We present a computer simulator of semiconductor optical amplifiers. The nonlinear input-output response of the device is characterized in terms of a complex gain, representing the accumulated gain and wavevector change of the propagating field across the active waveguide. We account for the gain saturation induced by stimulated recombination and by the perturbation of the carrier quasi-equilibrium distribution within the bands. A rigorous elimination of the spatial coordinate allows us to reduce the description of the amplifier dynamics to the solution of a set of ordinary differential equation for the complex gain. If the waveguide internal loss is negligible, the spatial inhomogeneity of the complex gain is implicitly yet exactly taken into account by the reduced model. The accuracy of the reduced model is the same for models based on the direct solution of the set of partial differential equations describing the interaction between the optical field and the active semiconductor waveguide, but the model is computationally much simpler. To preserve the input-output characteristics of the model, we include the amplified spontaneous emission noise in the device description by an equivalent signal applied to the device input and amplified by the saturated gain. At the expense of a minor increase of the program complexity, the waveguide internal loss may also be included. We report on the comparison between the output of the simulator and the results of four-wave mixing experiments in various pump-signal configurations. Good agreement is obtained.

Numerical modeling and optimization of cascaded CW Raman fiber lasers

Abstract: The authors solve a model which describes a nested fiber Raman cavity using Bragg reflectors by taking into account all interactions between forward and backward traveling waves. They apply the model to the cases in which a laser source at 1117 nm pumps a two-step Raman cascade leading to a 1240-nm output or a five-step cascade leading to a 1480-nm output. In order to insert into the calculations realistic fiber parameters, they have performed measurements of both Raman gain and linear losses for a few optical fibers. They derive from the numerical analysis the optimum fiber length and output coupling, and calculate, for the optimum configuration, the dependence of the output power on the pump power. The model describes with good accuracy the published experimental results.

Extended modulation bandwidth of DBR and external cavity lasers by utilizing a cavity resonance for equalization

Abstract: We have investigated the occurrence of a second resonance frequency in distributed Bragg reflector laser diodes and the high modulation bandwidth resulting from it. The influence of different laser parameters has been theoretically investigated. It is also shown that a similar behavior can be obtained in laser diodes with a passive, low-loss, and gratingless external cavity. The possibilities of large-signal digital modulation are also investigated.

Principles of parametric temporal imaging. II. System performance

Abstract: For pt. I, see ibid., vol. 36, p. 430, April 2000. The waveform manipulation technique known as temporal imaging can expand or compress signals in time while maintaining the shape of their envelope profiles. The temporal imaging system is analogous to that of its spatial counterpart, with dispersive propagation performing the role of diffraction and quadratic phase modulation in time acting as a "time lens." Recent work has concentrated on time lenses produced by the parametric mixing of the dispersed input signal with a linearly chirped optical pump pulse because of the broad bandwidth, and thus fine temporal resolution, that can be obtained. In a previous paper, we presented the numerous parametric imaging configurations that are possible and drew temporal ray diagrams to illustrate their operation. In this paper, we study the performance of these systems. Resolution, field of view, number of resolvable features, and distortions particular to this approach are discussed.

Temperature dependence of gain saturation in multilevel quantum dot lasers

Abstract: The temperature dependence of quantum dot (QD) optical gain is analyzed using a multilevel model and compared with experiment. The maximum gain is found to have a surprisingly strong temperature dependence that causes level switching and can limit laser performance in QD lasers. The model based on multiple discrete levels elucidates general design criteria that should be satisfied to obtain a stable threshold versus temperature in QD lasers. Good agreement is obtained between calculations and experiment for level switching in 1.3-/spl mu/m QD lasers.

Diode-pumped LiIO/sub 3/ intracavity Raman lasers

Abstract: We report on an all-solid-state intracavity Raman laser source operating at 1155 nm. The Raman-active medium is crystalline LiIO/sub 3/, which converts emission at 1064 nm from Nd:YAG to the first Strokes wavelength of 1155 nm. We discuss the key design principles for this Raman laser and present its operating characteristics, including output power, efficiency, and spectral, spatial, and temporal properties.

Analysis of basic four-wave mixing characteristics in a semiconductor optical amplifier by the finite-difference beam propagation method

Abstract: We have numerically analyzed nondegenerate four-wave mixing (FWM) among short optical pulses in a semiconductor optical amplifier (SOA) by the finite-difference beam propagation method (FD-BPM). We used the nonlinear propagation equation taking into account gain spectrum dynamic gain saturation which depends on carrier depression, carrier heating, and spectral hole-burning, group velocity dispersion, self-phase modulation, and two-photon absorption. To analyze FWM in an SOA, the evolution in time and spectral domain of two input optical pulses with different frequencies during propagation was calculated. From this simulation, it has become clear that the method me used here is a very useful technique for simulating FWM characteristics in SOA's. We also found that the wavelength dependence of the gain is crucial if the detuning is larger than 1 THz.