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The potential of 1.3-/spl mu/m AlGaInAs multiple quantum-well (MQW) laser diodes for uncooled operation in high-speed optical communication systems is experimentally evaluated by characterizing the temperature dependence of key parameters such as the threshold current, transparency current density, optical gain and carrier lifetime. Detailed measurements performed in the 20/spl deg/C-100/spl deg/C temperature range indicate a localized T/sub 0/ value of 68 K at 98/spl deg/C for a device with a 2.8 /spl mu/m ridge width and 700-/spl mu/m cavity length. The transparency current density is measured for temperatures from 20/spl deg/C to 60/spl deg/C and found to increase at a rate of 7.7 A/spl middot/cm/sup -2//spl middot/ /spl deg/C/sup -1/. Optical gain characterizations show that the peak modal gain at threshold is independent of temperature, whereas the differential gain decreases linearly with temperature at a rate of 3/spl times/10/sup -4/ A/sup -1//spl middot//spl deg/C/sup -1/. The differential carrier lifetime is determined from electrical impedance measurements and found to decrease with temperature. From the measured carrier lifetime we derive the monomolecular ( A), radiative (B), and nonradiative Auger (C) recombination coefficients and determine their temperature dependence in the 20/spl deg/C-80/spl deg/C range. Our study shows that A is temperature independent, B decreases with temperature, and C exhibits a less pronounced increase with temperature. The experimental observations are discussed and compared with theoretical predictions and measurements performed on other material systems.

We report optical signal amplification in a solid-state dye-doped polymer with a rib waveguide structure. A 625nm pulsed signal and a collinear 575nm pump are coupled into a 1μm×120μm poly(methyl methacrylate) waveguide doped with 1% by weight Rhodamine 640 dye. Depending on the signal intensity, a maximum optical gain in the 21–26dB range is obtained from a 1.2-cm-long device, accompanied by a signal-to-noise ratio in the 9–16dB range.

35GHz passive mode-locking of 1.3μm (InGa)As∕GaAs quantum dot lasers is reported. Hybrid mode-locking was achieved at frequencies up to 20GHz. The minimum pulse width of the Fourier-limited pulses was 7ps with a peak power of 6mW. Low uncorrelated timing jitter below 1ps was found in cross correlation experiments. High-frequency operation of the lasers was eased by a ridge waveguide design that includes etching through the active layer.

The key parameters in the fabrication of deep-etch high-order λ/4 Bragg gratings for short-wavelength nitride-based lasers are investigated. Calculations indicate that, for an air-gap thickness of 1.73 μm and single-spot Gaussian beam profile, the reduction in grating reflectivity due to light diffraction in the air gaps is only 17% with respect to a first-order structure with 0.1 μm air gaps. Scanning electron microscopy and microphotoluminescence characterizations confirm the validity of the numerical predictions and show that the 28%–38% reflectivity obtained from prototype focused-ion-beam-etched air/nitride gratings is mainly limited by imperfections and material disorder due to etching. Improving the etching technique would, therefore, allow standard lithographic fabrication of reduced-threshold GaN lasers.

Colliding pulse mode-locking is demonstrated for the first time in quantum-dot lasers. Close to transform limited, 7 ps, 20 GHz pulses are achieved using an absorber length considerably longer than typically used in similar quantum-well lasers.

Ultra-short pulse, high power mode-locking is demonstrated in InGaAs quantum dot lasers using a flared waveguide design. 24 GHz mode-locking with 790 fs wide pulses and 500 mW peak powers suitable for telecommunications applications are presented. (2 pages)

Extensive mode-locking investigations are performed in InGaAs/InAs/GaAs quantum dot (QD) lasers. Monolithic mode-locked lasers are fabricated using QD material systems grown by MOCVD and MBE techniques and emitting at 1.1μm and 1.3μm, respectively. The mode-locking performance is evaluated using a variety of laser designs, with various ridge waveguide geometries, cavity and absorber lengths. Passive and hybrid mode-locking are studied and compared in 3.9mm long devices emitting at 1.1μm and operating at a repetition rate of 10GHz. Using 2.1mm long devices emitting at 1.3μm, 18GHz passive mode locking with 10ps Fourier transform limited pulses is demonstrated. This confirms the potential of quantum dot laser for low chirp, short optical pulse generation. Preliminary investigation of the timing jitter of QD passively mode-locked lasers and the behaviour of the QD absorber are also presented. Finally, we report 36GHz passive mode-locking with 6ps optical pulse obtained using 1.1mm long QD lasers emitting at 1.3μm.

Universal self-organisation on surfaces of semiconductors upon deposition of a few non-lattice-matched monolayers using MOCVD or MBE lead to the formation of quantum dots. Their electronic and optical properties are closer to those of atoms than of solids. We have demonstrated for QD-lasers a record low transparency current density of 6A/cm2 per dot layer at 1.16 μm, high-power of 12W, an internal quantum efficiency of 98%, and an internal loss below 1.5 cm-1. Relaxation oscillations indicate the potential for cut-off frequencies larger than 10 GHz. GaAs-based QD-lasers emitting at 1.3 μm exhibit output power of 5 W and single transverse mode operation up to 300 mW. At 1.5 μm again an output power of 5 W has been obtained for first devices showing a transparency current of 700 A/cm2. Single mode lasers at 1.16 and 1.3 μm show no beam filamentation, reduced M2, sensitivity to optical feedback by 30 db and α-parameter as compared to quantum well lasers. Passive mode locking of 1.3 μm lasers up to 20 GHz is obtained. Thus GaAs-lasers can now replace InP-based ones at least in the range up to 1.3 ´m, probably up to 1.55 μm.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this work we present a detailed study of picosecond optical pulse generation using high-repetition rate mode-locked quantum dot lasers. MOCVD-grown quantum dot lasers emitting at 1.1μm and MBE-grown quantum dot lasers emitting at 1.3μm are investigated. Passive mode-locking at 10GHz, 18GHz and 36GHz with pulse widths in the 6-12ps range are reported. Hybrid mode-locking is demonstrated at 10GHz, showing a significant improvement in the RF spectral characteristics when compared with passive mode-locking. A timing jitter of 600fs (2.5MHz to 50MHz) is measured in the 18GHz passively mode-locked laser. Autocorrelation techniques are used to characterise the high repetition rate mode-locked lasers as well as the time-bandwidth product of the optical pulses. Fourier-transform limited pulses are obtained from passively mode-locked QD lasers.

Summary form only given. To study the potential of this material system, results of threshold current variation, transparency current density and optical gain over temperature as well as the effective carrier lifetime of a 1.3 /spl mu/m ridge waveguide grating (RWG) AlGaInAs MQW laser are presented.

Single mode operation over an extended current range and reduced threshold current is obtained from InGaN MQW lasers by introducing two pairs of air/nitride quarter wavelength layers at one end of the cavity.

Threshold reduction and enhanced mode selectivity are demonstrated in pulsed GaN-based lasers upon the introduction of 5 /4air/nitride Bragg gratings defined by focused ion beam (FIB) etching. A 13% reduction in threshold current is obtained from a laser witha 5 m wide ridge by introducing a deep-etch air/nitride mirror. The presence of a reduced-depth Bragg grating, etched across 4 m wideridge structure using a lower FIB dose, results in single-peak spectral characteristics for currents up to 1.14 I Th . The introduction of theBragg mirrors always results in a broadening of the near field parallel to the epitaxial planes. Keywords: laser diode, InGaN, air/semiconductor, Bragg grating, transverse mode 1. INTRODUCTION. In recent years, the technology for blue/ultra-violet emitting diode lasers, based on wide-bandgap nitride compounds, hasundergone major developments [1]. However, achieving long-lifetime devices with sufficiently high output power(>30mW) and precise control over the spectral and spatial characteristics of the lasing emission has proven quite difficult.At present, a full exploitation of this new class of devices is not yet possioble. Long lifetime nitride devices with lowthreshold currents and high-quality single-spot beam profile are required for high-density optical data storage, laser printingand scanning. Stable single-frequency operation is desirable for applications such as spectroscopy and gas analysis.Improving the optical confinement in the transverse direction has proven essential in order to prevent lasing in a high ordertransverse mode and subsequent multi-spot far field patterns [2,3]. The use of long scale distributed feedback (DFB) [4,5]and distributed Bragg reflector (DBR) [6] designs has been crucial for obtaining stable single-frequency emission. High-reflectivity dielectric coatings have been widely used to improve the low facet reflectivity and fabricate low-threshold nitrid elasers. However, most of these approaches to enhancing the performance of GaN lasers diodes (LDs) require additionalgrowth and fabrication steps, which can affect the complexity and cost of manufacturing. In this work, we demonstrate a simple etching technique for improving the threshold current and the spectral and spatialemission characteristics of nitride lasers. The approach is based on the introduction of a short stack of air/semiconductor /4Bragg reflectors, as previously demonstrated on other material systems [7,8]. This technique offers the advantage that itcould be integrated with the etching of the laser facets and mesa structure.

Summary form only given. Blue/ultraviolet emitting diode lasers, based on nitride compounds, are suitable for spectroscopy, gas analysis, optical data storage, laser printing and scanning. Many of these applications require low-threshold devices with high output powers, high quality beam profile, and stable single-frequency emission. We have recently demonstrated a simple post-processing technique which improves some of these device characteristics. A significant reduction in both the threshold current and spectral linewidth of InGaN MQW lasers has been achieved by introducing a 5/spl lambda//4 air/nitride Bragg grating defined by focused ion beam (FIB) etching. In this paper, some of the design and fabrication parameters which determine the characteristics of the air/nitride reflectors are theoretically and experimentally investigated. It is found that the etch depth of the grating structure plays an important role in controlling both the degree of threshold reduction and the mode selectivity introduced by the mirror structure. Decreasing the etch depth, by lowering the FIB ion dose from 5000 to 4000 pC//spl mu/m/sup 2/, reduces the threshold reduction from 13% to 7%; however, it introduces a 63% reduction in spectral linewidth. These observations are modeled in terms of spatial and spectral mode selectivity.

A study of the gain-switching process in GaInN MQW laser diodes is reported. Single peak gain-switched optical pulses with pulse widths less than or equal to 40 ps and optical powers equal to 100 mW are observed when electrical pulses with duration of 800 ps are applied. Sub-nanosecond optical pulses with peak powers in excess of 450 mW are also obtained and degradation mechanisms are analyzed. The transient response characteristics of the laser diodes are studied in both the time and spectral domains.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Schottky barrier height at Al/n-In x Ga 1 -x As contacts (0.19 < x < 0.38) was engineered by fabricating Si bilayers at the metal-semiconductor interface. Si interlayers grown under As flux led to the suppression of the barrier, while interlayer deposition under Al flux yielded an increase in the barrier height. In situ X-ray photoemission spectroscopy results and ex situ current-voltage measurements indicated a barrier tunability range of about 0.8 eV, with ideal ohmic behavior of diodes engineered for low barrier height, and barrier heights as high as 0.7 eV in diodes engineered for high barrier height.

We have investigated the use of focused ion beam (FIB) etching for the fabrication of GaN-based devices. Although work has shown that conventional reactive ion etching (RME) is in most cases appropriate for the GaN device fabrication, the direct write facility of FIB etching - a well-established technique for optical mask repair and for IC failure analysis and repair - without the requirement for depositing an etch mask is invaluable. A gallium ion beam of about 20nm diameter was used to sputter GaN material. The etching rate depends linearly on the ion dose per area with a slope of 3.5 × 10-4 μm3/pC. At a current of 3nA, for example, this corresponds to an etch rate of 1.05μm3/s. Good etching qualities have been achieved with a side wall roughness significantly below 0.1μm. Changes in the roughness of the etched surface plane stay below 8nm.

Ultrashort pulse, high-power mode locking is demonstrated in InGaAs quantum dot lasers using a flared waveguide laser incorporating a narrow waveguide sections acting as both a mode filter and saturable absorber. 24GHz mode locking with 780fs pulses and 500mW peak powers are demonstrated at an emissions wavelength of 1.3μm.

We report a study of the optical signal amplification properties of Rhodamine 640 in solution, in thin-film format dispersed within a poly(methyl methacrylate) (PMMA) polymer matrix, and in a waveguiding structure created by coating polymer-dispersed Rhodamine 640 onto a patterned quartz substrate. The optical signal amplification experiments were carried out using a collinear pump and signal setup. Depending on both pump and signal energies, gains of up to 41 dB were achieved in solution, while 1% by weight dye-doped PMMA films showed net gains of 54cm−1, and associated losses of 16cm−1. Using the same concentration dye/PMMA dispersions, waveguide structures were fabricated with a 120×1μm2 cross section and a length of 1.2 cm. The internal optical gain values measured at 625 and 650 nm were found to be as high as 26 and 8 dB, respectively. The corresponding signal-to-noise ratio was in the 7–21-dB range at 625 nm and in the 3–21-dB range at 650 nm. Both in solution and in the waveguide structures we foun...

Colliding pulse modelocking is demonstrated for the first time in quantum dot lasers. Using 3.9 mm-long devices with a 245 pm-long central absorber, 7 ps pulses at a repetition rate of 20 GHz is obtained. For Gaussian pulses a time-bandwidth product close to the Fourier transform limit is determined. These results confirm the potential of quantum dot lasers for high repetition rate harmonic modelocking.

18 GHz passive modelocking with Fourier-transform-limited optical pulses in InGaAs/GaAs quantum dot lasers emitting at /spl sim/1.3 /spl mu/m has been achieved for the first time. An upper limit of 600 fs is estimated for the root-mean-square timing jitter (2.5-50 MHz).

Hybrid modelocking of monolithic quantum dot lasers is reported for the first time. A marked performance improvement over passive modelocking is demonstrated. Both hybrid and passive modelocking at /spl sim/10 GHz are achieved for the first time in quantum dot lasers.

A 13% reduction in the threshold current density of InGaN laser diodes is demonstrated upon the introduction of two 5/spl lambda//4 air/nitride Bragg reflectors. These are defined at one end of the laser cavity by means of focused ion beam etching.

Al/InxGa1−xAs(001) diodes incorporating Si bilayers deposited under As or Al flux were fabricated by molecular-beam epitaxy on GaAs(001) wafers for 0.2

An investigation into the emission and degradation characteristics of GaInN multiquantum well lasers driven by short, high current pulses for both the time and spectral domains is presented. Sub-nanosecond optical pulses with peak powers exceeding 450 mW and gain-switched pulses shorter than 50 ps have been generated.

Saturable absorbers properties are characterised in monolithic mode-locked InGaAs quantum dot lasers. We analyse the impact of weak quantum confined Stark effect, fast absorber recovery time and low absorber saturation power on the mode-locking performance.