Showing papers in "IEEE Journal of Selected Topics in Quantum Electronics in 1997"

GaInNAs: a novel material for long-wavelength semiconductor lasers

Abstract: GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 /spl mu/m and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T/sub 0/=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 /spl mu/m.

InGaAs-GaAs quantum-dot lasers

Abstract: Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A/spl middot/cm/sup -2/ at 77 K and of 62 A/spl middot/cm/sup -2/ at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of /spl sim/80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to /spl sim/50 meV, ultrahigh material gain of /spl sim/10/sup 5/ cm/sup -1/, differential gain of /spl sim/10/sup -13/ cm/sup 2/ and strong nonlinear gain effects with a gain compression coefficient of /spl sim/10/sup -16/ cm/sup 3/. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is /spl sim/0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 /spl mu/m and 1.37 /spl mu/m at 300 K.

Advances in selective wet oxidation of AlGaAs alloys

Abstract: We review the chemistry, microstructure, and processing of buried oxides converted from AlGaAs layers using wet oxidation Hydrogen is shown to have a central role in the oxidation reaction as the oxidizing agent and to reduce the intermediate predict As/sub 2/O/sub 3/ to As The stable oxide is amorphous (Al/sub x/Ga/sub 1-x/)/sub 2/O/sub 3/ which has no defects along the oxide/semiconductor interfaces but can exhibit strain at the oxide terminus due to volume shrinkage The influence of gas flow, gas composition, temperature, Al-content, and layer thickness on the oxidation rate are characterized to establish a reproducible process Linear oxidation rates with Arrhenius activation energies which strongly depend upon AlAs mole fraction are found The latter produces strong oxidation selectivity between AlGaAs layers with slightly differing Al-content Oxidation selectivity to thickness is also shown for layer thickness <60 nm Differences between the properties of buried oxides converted from AlGaAs and AlAs layers and the impact on selectively oxidized vertical cavity laser lifetime are reported

A review on fabrication technologies for the monolithic integration of tapers with III-V semiconductor devices

Abstract: The past few years a lot of effort has been put in the development and fabrication of III-V semiconductor waveguiding devices with monolithic integrated mode size converters (tapers). By integrating a taper with a waveguide device, the coupling losses and the packaging cost of OEICs in future fiber-optical networks can be much reduced. This paper gives an overview of different taper designs, the possible fabrication technologies and performances of tapered devices.

Photonic crystals and microdisk cavities based on GaInAsP-InP system

Abstract: This paper presents a preliminary guide to realize microcavity semiconductor lasers exhibiting spontaneous emission control effects. It includes: 1) theoretical consideration on the effects; 2) processing techniques for semiconductor microcavities; and 3) some demonstrations of photonic crystal and microdisk cavity. It was shown that, even with a spectral broadening of electron transition, thresholdless lasing operation and alternation of spontaneous emission rate are expected in a cavity satisfying the single mode condition that only one mode is allowed in the transition spectrum. An ideal three-dimensional (3-D) photonic crystal has the potentiality for realizing this condition. In two-dimensional (2-D) crystals and microdisk cavities, thresholdless operation is also expected, but the alternation of spontaneous emission rate may be negligible due to the insufficient optical confinement. In the experiment, some processing techniques for GaInAsP-InP system were investigated and methane-based reactive ion beam etching was selected because of the smooth sidewalls and adaptability to arbitrary structures. A GaInAsP-InP 2-D photonic crystal constructed by submicron columns was fabricated using this method. Owing to the slow surface recombination of this material, a polarized photoluminescence and peculiar transmission spectra were observed at room temperature (RT), which can be explained by a photonic band calculation. However, some technical improvement is necessary for clear demonstration of photonic bandgap, which is minimally required for device applications. In contrast to this, a GaInAsP-InP microdisk cavity of 2 /spl mu/m in diameter, which corresponds to the cavity volume 2.5 times the single-mode condition, has achieved RT lasing with threshold current as low as 0.2 mA. Further reduction of diameter and realization of continuous-wave (CW) operation will provide a significant regime for the observation of spontaneous emission control effects.

Average inversion level, modeling, and physics of erbium-doped fiber amplifiers

Abstract: We present a detailed study of a set of models for characterizing the gain, the input and output powers of single erbium-doped fiber amplifiers (EDFAs) and networks of EDFAs. The time dependent gain is described by a single ordinary differential equation for the average inversion level of an EDFA with arbitrary number of signal channels with arbitrary power levels and propagation directions. In steady state, this ordinary differential equation becomes a transcendental equation from which many important parameters are derived. Through perturbation analysis of the time dependent model, the output perturbation can be expressed explicitly in terms of the input perturbations, which is useful for tone calculations. Therefore, this set of models can be applied to the steady state, and to large- and small-signal transient states in wavelength-division multiplexed (WDM) optical communication networks with EDFAs. The models are applied to analyze fast power transients in networks of EDFAs.

Saturation effects in nondegenerate four-wave mixing between short optical pulses in semiconductor laser amplifiers

Abstract: We present a simple, yet powerful method for calculating nonlinear pulse propagation and pulse interaction in active semiconductor waveguides. The model is based on the density matrix equations, which under realistic operation conditions are shown to lead to an accurate description of the material gain, that includes the dynamics of carrier heating and spectral-hole burning (SHB). A very general and compact description of the amplifier dynamics in terms of an integral equation is derived. The model is used to analyze saturation effects in short pulse amplification and nondegenerate four-wave mixing. An analytical expression for the four-wave mixing response is derived, which extends previous results to the case of short and intense pulses. Saturation of the four-wave mixing signal is shown to be strongly pulsewidth dependent due to ultrafast gain dynamics, and self-phase modulation is shown to give excessive broadening of the conjugate pulse. Finally, the impacts on the noise characteristics are calculated and shown to explain recent experimental results.

Four-wave mixing in semiconductor optical amplifiers for frequency conversion and fast optical switching

Abstract: Four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is an important tool for frequency conversion and fast optical switching in all-optical communication networks. We review the main applications of SOAs as nonlinear optical components. Concentrating on FWM, we define general parameters that are of relevance for signal processing applications. We show, how basic experiments and general simulation procedures can be used to determine optimum operating conditions for the intended applications. Besides a comprehensive investigation of FWM among continuous waves, we present new experimental results on FWM with picosecond optical pulses. A comparison of both reveals a different behavior and demonstrates that new optimization criteria and advanced theoretical models have to be applied for the case of short optical pulses. Moreover, we discuss the possibility to extract the dynamical SOA parameters from our experiments.

Wavelength-swept fiber laser with frequency shifted feedback and resonantly swept intra-cavity acoustooptic tunable filter

Abstract: This paper concerns a wavelength-swept fiber laser (WSFL) incorporating frequency shifted feedback and an intracavity passband filter, in which the wavelength of the modeless output is linearly, continuously and repeatedly tuned (in time) over a given range by modulation of the filter peak wavelength and filter strength. We show both numerically and experimentally that amplifier noise plays a key role in determining the operation of frequency-shifted fiber laser systems and that a "noisy" amplifier can be used to suppress the natural tendency of such lasers to pulse, allowing for continuous wave, modeless operation. Furthermore, we show that significant narrowing of a WSFL instantaneous swept linewidth can be obtained if the filter peak transmission wavelength is resonantly swept so as to follow the wavelength shift per pass due to the acoustooptic frequency shift. Using these ideas we go on to demonstrate and characterize a high-power diode-driven Er/sup 3+//Yb/sup 3+/ WSFL incorporating a bulk-optic acoustooptic tunable filter (AOTF). Linewidths as narrow as 9 GHz, sweep ranges up to 38 nm and output powers as high as 100 mW are obtained. Furthermore, we demonstrate the generation of user definable average spectral output by synchronous modulation of the filter strength and multiwavelength pulsed output at higher sweep rates. Excellent agreement between the experimental results and those of the numerical modeling is obtained. Our simulations show that reduced linewidth (<0.02 nm) and improved scan linearity should be readily achievable with realistic system improvements. We believe such sources to be of considerable physical and practical interest, with applications ranging from sensor array monitoring and device characterization through to low-coherence interferometry.

Bistable laser diodes and their applications: state of the art

Abstract: Recent progresses in research on bistable laser diodes and their applications in optical communications and photonic switching are reviewed. In addition to the conventional absorptive and dispersive bistable laser diodes, bistability in two-mode lasers via gain saturation has recently attracted attention, because of its ultra high speed. On the other hand, bistable laser diodes with saturable absorbers are mainly used in the system applications because of their stable operations at present. This paper presents the theoretical analysis of the two-mode bistable laser diodes, the stripe lasers and the vertical-cavity surface-emitting lasers (VCSELs) as the two major representatives of bistable lasers, and the profound discussion of their possible applications.

Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers

Abstract: We report to our knowledge the highest to date quasi-CW output power, 600 W and pulse energy, >1 J, for an InGaAs diode-pumped Yb:YAG laser. In separate preliminary results, we have also obtained 225 W of average output power under true CW diode pumping. This performance was obtained using a laser head designed to be part of a master oscillator power amplifier (MOPA) operating at 3 kW. We summarize why the diode-pumped Yb:YAG crystal laser is ideal for scaling to high average powers and the different approaches being pursued. We also report our latest results for side-pumped rod devices.

Wafer fusion: materials issues and device results

Abstract: A large number of novel devices have been recently demonstrated using wafer fusion to integrate materials with different lattice constants. In many cases, devices created using this technique have shown dramatic improvements over those which maintain a single lattice constant. We present device results and characterizations of the fused interface between several groups of materials.

All-optical wavelength conversion at bit rates above 10 Gb/s using semiconductor optical amplifiers

Abstract: This work assesses the prospects for high-speed all-optical wavelength conversion using the simple optical interaction with the gain in semiconductor optical amplifiers (SOAs) via the interband carrier recombination. Operation and design guidelines for conversion speeds above 10 Gb/s are described and the various tradeoffs are discussed. Experiments at bit rates up to 40 Gb/s are presented for both cross-gain modulation (XGM) and cross-phase modulation (XPM) in SOAs demonstrating the high-speed capability of these techniques.

Wavelength conversion for WDM communication systems using four-wave mixing in semiconductor optical amplifiers

Abstract: Four-wave mixing (FWM) in semiconductor optical amplifiers is an attractive mechanism for wavelength conversion in wavelength-division multiplexed (WDM) systems since it provides modulation format and bit rate transparency over wide tuning ranges. A series of systems experiments evaluating several aspects of the performance of these devices at bit rates of 2.5 and 10 Gb/s are presented. Included are single-channel conversion over 18 nm of shift at 10 Gb/s, multichannel conversion, and cascaded conversions. In addition time resolved spectral analysis of wavelength conversion is presented.

Analytical and experimental studies on the characteristics of composite solid-state laser rods in diode-end-pumped geometry

Abstract: In this paper, we report analytical and experimental studies on the characteristics of end-pumped composite laser rods with undoped end, using mainly Nd:YAG rods as an example. It is found that the peak temperature rise in a composite rod decreases to <70% of that in a noncomposite crystal. Thermal stress is dramatically reduced to <60% by employing the composite rod structure, We also demonstrate high-power operation of the diode-end-pumped composite Nd:YVO/sub 4/ rod and a maximum CW output power of 9.3 W was achieved, which is about 1.5 times higher than that in the noncomposite rod. This high-power performance of the composite rod is primarily attributed to the reduction of thermal stress inside the rod.

Short cavity erbium/ytterbium fiber lasers mode-locked with a saturable Bragg reflector

Abstract: We present short cavity erbium/ytterbium fiber lasers that are passively mode-locked with a saturable Bragg reflector. The lasers produce sub-500-fs pulses at fundamental cavity repetition rates as high as 300 MHz. Stable passive harmonic operation increases the repetition rate to 2.0 GHz. The mode-locking mechanism in both the normal and anomalous group velocity dispersion regimes is investigated using complete analytical and numerical models and direct comparison with the experimental results. A simple technique for accurately measuring the total cavity dispersion is presented.

Narrow linewidth, tunable Tm/sup 3+/-doped fluoride fiber laser for optical-based hydrocarbon gas sensing

Abstract: Operation of an efficient continuous-wave (CW) thulium-doped fiber laser emitting at wavelength, /spl lambda/=2.31 /spl mu/m is reported. The fiber laser parameters are optimized with a view to ultimately producing a compact and efficient laser source for optical absorption based gas sensing. A number of fiber laser configurations are investigated to assess their suitability for narrow linewidth, tunable fiber laser operation emitting around /spl lambda/=2.3 /spl mu/m, which is a wavelength region of significant importance for hydrocarbon gas monitoring. Tuning ranges of 140 nm and linewidths of less than 210 MHz have been demonstrated with lasers with bulk external tuning grating. Preliminary hydrocarbon gas sensing investigation confirm the potential of this source for detection of ppb gas concentrations.

The differential efficiency of quantum-well lasers

Abstract: We have analyzed the internal differential efficiency of quantum-well lasers in terms of current spreading, carrier injection into the well, and the radiative efficiency within the well. We quantify the first two of these processes by extracting information from spontaneous emission measurements as a function of device length, current, and temperature. We show that the carrier injection efficiency is responsible for the temperature dependence of the external differential efficiency of GaInP quantum-well (QW) lasers by comparing values from the slope of the laser power output versus current characteristic with our experimental values for current spreading and injection efficiency.

All-optical fiber signal processing and regeneration for soliton communications

Abstract: Ultrafast all-optical signal processing techniques are expected to play a major role in future ultrafast single-carrier soliton systems, because they remove the electronics bottleneck. In this paper, two all-optical devices, the nonlinear optical loop mirror (NOLM) and the Kerr fiber modulator (KFM), are used to achieve major functions related to high bit rate soliton links. At the interface with existing networks, conversions from data at the nonreturn-to-zero (NRZ) format to return-to-zero (RZ) and soliton data, and vice-versa are required. These two conversions are demonstrated through NOLMs, and their limitations investigated. However, the main part of this paper is devoted to in-line soliton regeneration through synchronous modulation. Synchronous modulation requires both clock recovery and in-line optical modulation. In the following, all-optical approaches for these two functions are considered separately, before being associated in a true all-optical regenerator. All-optical clock recovery techniques are first reviewed. An experimental implementation of one of these techniques is described. On the other hand, all-optical modulation can be done either with intensity or phase modulators. We initially proposed the NOLM as all-optical intensity modulator. We analyze it theoretically, both from the component and the system application viewpoints. A modified configuration of the NOLM, having two optical controls, removes some limitations pertaining to the single-control configuration, yielding even higher performance. The other all-optical synchronous modulator considered here is the KFM, which is a pure phase modulator. Its potential is demonstrated in a 20-Gb/s soliton transmission experiment, when driven by an optoelectronic optical clock generation device. Issues specific to the implementation of both types of all-optical fiber-based modulators are discussed. Finally, a true all-optical synchronous regenerator, combining all-optical clock recovery circuit and KFM, is tested in an actual soliton transmission experiment at 20 Gb/s.

Wafer bonding technology and its applications in optoelectronic devices and materials

Abstract: The direct wafer bonding process has found broad applications in many critical areas including both commercial and state-of-the-art photonic devices and more recently, formation of semiconductor compliant substrates. Using the wafer bonding technology, we have demonstrated 1.3-/spl mu/m vertical-cavity surface-emitting lasers (VCSELs) with a 1-mA continuous-wave (CW) threshold current and 0.83-mA pulsed threshold current. Superior device performance has also been achieved with photodetectors and micromachined tunable devices. Applying the wafer bonding process in a novel way, we have fabricated compliant universal substrates on which largely mismatched (e.g., 15% mismatch) heteroepitaxial layers can be grown defect free.

Diode-pumped tunable Yb:YAG miniature lasers at room temperature: modeling and experiment

Abstract: We present a simple design rule for diode-laser pumped quasi-three-level lasers by using the M/sup 2/ factor. The validity of this model was demonstrated by diode-pumped Yb:YAG laser experiments. The maximum output power of 1.33 W and optical slope efficiency of 63% were obtained in a 400-/spl mu/m Yb:YAG chip miniature laser. Using a 200-/spl mu/m Yb:YAG chip, a 70% optical slope efficiency was reached. In a coupled-cavity configuration, with a quartz birefringent tuning filter, 8.2 THz (29 nm) of tuning was obtained at room temperature. By changing to a calcite birefringent filter, single-axial-mode oscillation with an output power of 500 mW was observed.

A theoretical investigation of dynamic all-optical automatic gain control in multichannel EDFA's and EDFA cascades

Abstract: We present a detailed theoretical analysis of the gain dynamics of erbium-doped fiber amplifiers (EDFA's) that have been gain-clamped using a ring laser structure and of gain-stabilized EDFA chains. We examine and analyze the effects of attenuator level in the optical feedback path, switching speed, number of channels dropped or added and the choice of lasing wavelength on the stabilization of the individual EDFA. In particular, we look at the transient power excursions and relaxation oscillations experienced by surviving channels when the number of channels passing through an EDFA changes. Using this analysis as a guide, we present and compare two different approaches to chain stabilization. We highlight the robustness of each approach, show some of their limitations and advantages, and comment on the impact on multiwavelength communication systems.

230 mW of blue light from a thulium-doped upconversion fiber laser

Abstract: We demonstrate a powerful diode-pumped blue laser source, consisting of a 7-W diode at 807 nm that pumps a Nd:YAG laser giving 1.6 W with good beam quality at 1123 nm, and a thulium-doped upconversion fiber laser. The maximum output power achieved at 481 nm is 230 mW. We also describe the behavior of a reversible loss which is generated in the fluoride fiber during high power operation.

Scattering losses from dielectric apertures in vertical-cavity lasers

Abstract: In vertical-cavity lasers (VCLs) employing oxide or airgap apertures, the lasing mode typically travels unguided throughout most of the structure. For the aperture to exactly compensate for the diffraction of the mode in these regions, it would need to have a parabolic lateral index profile (i.e. that of an ideal thin lens). Although nonparabolic aperture shapes will partially compensate diffraction losses, some light will be scattered out of the mode. These scattering losses increase as the aperture size is reduced and will limit the performance of the smallest devices. We analyze these losses first using a semianalytic approach which allows us to frame the problem in terms of two parameters of the structure: the Fresnel number and the effective optical path length across the aperture. We compare the estimate with experimental results and with an iterative numerical calculation of the actual mode and losses. Lastly, we compare the loss reduction with different amounts of tapering that provide a better approximation to the ideal parabolic lens.

The integration of III-V optoelectronics with silicon circuitry

Abstract: The integration of III-V optoelectronics with silicon circuitry provides the potential for fabricating dense parallel optical interconnects with data links capable of Terabit aggregate data rates. This paper reviews many of the current approaches used for the fabrication of integrated optoelectronic devices and then highlights the performance results. Finally, the applied method is reviewed in greater detail with recent results on VCSEL, MESFET, and photodiode integration presented.

64/spl deg/C continuous-wave operation of 1.5-/spl mu/m vertical-cavity laser

Abstract: We report on 64/spl deg/C continuous-wave (CW) operation of a 1.5-/spl mu/m vertical-cavity laser. This laser consists of two fused AlGaAs-GaAs mirrors with a strain-compensated InGaAsP-InP MQW active region. Selective lateral oxidation is used for current confinement. Minimum room-temperature threshold current is as low as 0.8 mA, and maximum CW output power is as high as 1 mW at 15/spl deg/C. Pulsed operation is achieved up to 100/spl deg/C. Current spreading losses and device heating are analyzed in detail. Dynamic parameters such as maximum 3-dB parameters such as maximum, 3-dB bandwidth (4.7 GHz), alpha factor (4.0), and linewidth (39 MHz) are also investigated.

Properties of Brillouin/erbium fiber lasers

Abstract: The unique combination of two gain media in Brillouin/erbium fiber lasers results in distinctive properties of such sources. The paper describes in detail the output and efficiency properties of Brillouin/erbium fiber lasers, from experimental results and a theoretical model. Single longitudinal mode output with linewidth <10 kHz was measured, with a 4-nm tuning range with the Brillouin pump wavelength. Frequency pulling associated with the Brillouin gain was investigated and observed to increase linearly with the Brillouin pump power.

Four-wave mixing in semiconductor optical amplifiers: a practical tool for wavelength conversion

Abstract: Four-wave mixing in semiconductor optical amplifiers is used to produce wavelength conversion. We report an extended study on the dependence of efficiency and noise on device length, pump power, operation wavelength and conversion interval. The use of longer active regions is a good way to obtain performance as good as requested by the most advanced telecommunication systems.

Low-threshold vertical-cavity surface-emitting lasers based on oxide-confinement and high contrast distributed Bragg reflectors

Abstract: The interest in low-threshold vertical-cavity surface-emitting lasers (VCSEL's) is increased by the demonstration of the small size, low loss optical mode due to oxide-confinement in the Fabry-Perot microcavity laser. Intense recent work in this area has resulted in numerous record breaking demonstrations of low-threshold current, high wall-plug efficiency, and high speed. In this paper, we discuss the impact of the dielectric cavity design to enhance control of the optical mode. We argue that high contrast dielectric mirrors can become an important design approach, especially for small apertures limited by diffraction loss. Experimental results are compared for different types of mirror and aperture designs.

InGaN-based blue laser diodes

Abstract: The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively The threshold current density was 88 kA/cm/sup 2/ The carrier lifetime and the threshold carrier density were estimated to be 35 ns and 18/spl times/10/sup 20//cm/sup 3/, respectively The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 58/spl times/10/sup -17/ cm/sup 2/, 93/spl times/10/sup 19/ cm/sup -3/, 5200 cm/sup -1/, and 43 cm/sup -1/ respectively