Red-emitting Lasers Research Articles

96 mW longitudinal single mode red-emitting distributed Bragg reflector ridge waveguide laser with tenth order surface gratings

Red-emitting ridge waveguide lasers with integrated tenth order surface distributed Bragg reflector gratings were developed. The grating was implemented by the use of a BCl3-Ar-plasma, while the shape of the grating trench was controlled by additional He-backside cooling of the wafer. The devices exhibit longitudinal single mode operation up to 96 mW at 635.3 nm with a side mode suppression ratio of 18 dB and a good beam quality of M2<3. The spectrum is free of mode hops for a span of more than 55 pm.

Wavelength tunable ultraviolet laser emission via intra-cavity frequency doubling of an AlGaInP vertical external-cavity surface-emitting laser down to 328 nm

We demonstrate an optically pumped vertical external-cavity surface-emitting laser in a compact v-shaped cavity configuration for frequency doubling to the ultraviolet (UV) spectral range at ∼330 nm. The fundamental red laser emission is realized with a metal-organic vapor-phase epitaxy grown (GaxIn1−x)0.5P0.5/[(AlxGa1−x)yIn1−y]0.5P0.5 multi-quantum-well structure. Second harmonic generation is accomplished by using a beta barium borate non-linear crystal to generate maximum UV output powers exceeding 100 mW. By using a birefringent filter, we are able to tune the fundamental laser resonance to realize a maximum tuning range of 7.5 nm of the second harmonic.

Efficient intracavity frequency-doubled Nd:YLiF4-LiB3O6 red laser emission under diode pumping into the emitting level

We report for the first time (to our knowledge) an efficient compact red laser at 660.5 nm generation by intracavity frequency doubling of a continuous wave laser operation of a diode direct pumped Nd:YLiF4 (Nd:YLF) laser on the 4 F 3/2 → 4 I 13/2 transition at 1321 nm. A LiB3O6 (LBO) crystal, cut for critical type I phase matching at room temperature is used for second harmonic generation of the laser. At an absorbed pump power of 16.2 W, as high as 1.5 W of continuous wave output power at 660.5 nm is achieved with 10-mm-long LBO. The optical-to-optical conversion efficiency is up to 9.3%. Comparative results obtained for the pump with diode laser at 806 nm, into the highly-absorbing 4 F 5/2 level, are given in order to prove the advantages of the 880 nm wavelength pumping.

650-nm laser emission under upconversion pumping at 1.064 μm in Tm+3: ZBLAN fiber laser

We report for the first time continuous wave (CW) red laser emission in Tm+3-doped ZBLAN fiber laser, operated at 650 nm (1G4 → 3F4 transition of Tm+3). The excitation uses a three step upconversion scheme. The pump source is a Nd:YAG laser operated at 1.064 μm. A laser output power of CW 80 mW was obtained for 1.42 W of launched pump power. The slope efficiency with respect to launched pump power was measured to be 7.7%. The temporal behavior of the emitted laser is also addressed.

Time resolved studies of catastrophic optical mirror damage in red-emitting laser diodes

We have observed the changing light intensity during catastrophic optical mirror damage (COMD) on the timescale of tens of nanoseconds using red-emitting AlGaInP quantum well based laser diodes. Using as-cleaved facets and this material system, which is susceptible to COMD, we recorded the drop in light intensity and the area of damage to the facet, as a function of current, for single, high current pulses. We found that in the current range up to 40 A, the total COMD process up to the drop of light intensity to nonlasing levels takes place on a timescale of hundreds of nanoseconds, approaching a limiting value of 200 ns, and that the measured area of facet damage showed a clear increase with drive current. Using a straightforward thermal model, we propose an explanation for the limiting time at high currents and the relationship between the time to COMD and the area of damaged facet material.

Chip‐based immunoaffinity CE: Application to the measurement of brain‐derived neurotrophic factor in skin biopsies

A chip-based immunoaffinity CE system has been employed to measure the concentrations of brain-derived neurotrophic factor in human skin biopsies, taken during atopic inflammatory events. The device employs a replaceable immunoaffinity disk to which capture antibodies have been chemically immobilized. Homogenates obtained from micro-dissected human skin samples were injected into the system, where the analyte of interest was captured in the immunoextraction port, thus allowing non-reactive materials to be removed prior to analysis. The captured analyte was labeled in situ with a red-emitting laser dye before being released from the capture antibody, separated by electrophoresis, and the resolved peaks detected by online LIF. Comparison of this chip-based system with conventional immunoassay demonstrated good correlation when analyzing both standards and patient samples. The system was semi-automated resulting in a CE analysis within 1.5 min and a total of circa 5 min. Intra- and inter-assay CV's of 3.85 and 4.19 were achieved with circa 98.8% recovery of brain-derived neurotrophic factor at a concentration of 100 pg/mL. The assay demonstrated clear differences between clinical stages of atopic dermatitis in human patients and could run 10-15 samples per hour. This system holds the potential for being modified to be a portable unit that could be used in clinics and other biomedical screening studies.

A cavity ring down/cavity enhanced absorption device for measurement of ambient NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt;

Abstract. An inexpensive, compact instrument for the sensitive measurement of NO3 and N2O5 in ambient air at high time resolution is described. Light from a red-emitting laser diode (≈662 nm) is coupled off-axis into an optical resonator defined by two highly reflective mirrors to achieve effective absorption paths exceeding 20 km. Light leaking from the cavity is detected either as single ring-down events (time constant of ≈100 μs) following rapid switching of the laser intensity at 200 Hz (Cavity Ring Down mode), or as an integrated intensity (Cavity Enhanced Absorption mode). The operational conditions, detection limits (2 pptv in 5 s) and total uncertainty (&lt;15% for NO3&lt;10 pptv) for the prototype device for NO3 and N2O5 detection/monitoring are assessed and the first measurements in ambient air and from an environmental chamber are described.

Analysis of thermal performance of InGaP/InGaAlP quantum wells for high-power red laser diodes

The effect of quantum well (QW) strain on the thermal performance of InGaP/InGaAlP lasers emitting at around 635 nm is investigated theoretically. It is found that compressively-strained QWs offer superior thermal performance due to an increased barrier height for electrons, leading to a reduced electron leakage. The theoretical predictions are supported by experimental data from red-emitting laser diodes with an improved characteristic temperature T 0.

Defect investigation and temperature analysis of high-power AlGaInP laser diodes during catastrophic optical damage

A variety of applications, such as photodynamic therapy, require efficient red-emitting semiconductor lasers with high output power and continuous wave life times >1,000 h. AlGaInP lasers have shown to be the best candidates in this spectral range. However, compared to infrared emitters, high-power performance is still limited by major degradation effects, especially by catastrophic optical damage (COD). COD in AlGaInP lasers has hardly been researched in the past. We employed an innovative combination of concepts, namely microphotoluminescence (μPL) mapping and focused ion beam (FIB) microscopy to investigate the COD induced defects in AlGaInP broad-area lasers. High resolution μPL demonstrated that those defects are composed of highly nonradiative complex dislocation networks, which start from the output facet and propagate in form of branches inside the cavity. Moreover, FIB analysis showed that those dark line defects are confined to the active region, containing the quantum wells and the waveguide. In addition, facet temperature changes during COD were analyzed by means of micro-Raman spectroscopy. Although no visible damage at the coated output facet could be observed, an extreme temperature increase in the immediate vicinity of the COD starting point at the laser output facet was detected. This was followed by a sudden temperature decrease afterward. After analyzing several lasers with emission wavelengths between 635 nm and 650 nm, we concluded that in AlGaInP lasers, absorption of stimulated photons at the laser facet is the major source of facet heating, and that a critical facet temperature must be reached in order for COD to occur. This understanding of nature and behavior of COD is a key element for further improvement in efficiency of high-power diode lasers.

Laser emission from spiral-shaped microdisc with waveguide of conducting polymer

Spiral microdiscs with waveguide based on a conducting polymer have been fabricated on quartz substrates and the emission properties have been studied. The directivity and the polarization properties of laser emission from the spiral microcavities with waveguide based on a poly(p-phenylenevinylene) derivative have been investigated by pulsed photo-pumping. The red laser emission from the spiral microdisc with waveguide has been observed, and the directivity of this microdisc geometry has been discussed taking the mathematical simulation into consideration.

Modeling multiple quantum barrier effects and reduced electron leakage in red-emitting laser diodes

Severe electron leakage impedes the full exploitation of AlGaInP laser diodes in the 630nm regime. Such thermally activated currents are attributed to inherently small conduction band offsets and intervalley transfer between the Γ and X conduction band minima. To negate the detrimental effect of these two intrinsic material issues a theoretical model is proposed. A multi-quantum-barrier (MQB) structure able to inhibit both Γ- and X-band transmissions is inserted in the p-doped region adjacent to the active region of the device, allowing a greater percentage of injected electrons to be reflected back within the active region. The design of the MQB follows a strict optimization procedure that takes into account fluctuations of superlattice layer width and composition. This model is used in conjunction with a dual conduction band drift-diffusion simulator to enable the design of the MQB at an operating voltage and hence account for nonlinear charge distribution across it. Initial results indicate strong agreement between experimentally determined effective enhancements and those predicted theoretically.

Continuous-wave ultraviolet generation at 320 nm by intracavity frequency doubling of red-emitting Praseodymium lasers

We describe a new approach for the generation of coherent ultraviolet radiation. Continuous-wave ultraviolet light at 320 nm has been obtained by intracavity frequency doubling of red-emitting Praseodymium lasers. Lasing at the 640-nm fundamental wavelength in Pr:LiYF(4) and Pr:BaY(2)F(8) was realized by employing an optically pumped semiconductor laser at 480 nm as pump source.Using LiB(3)O(5) as nonlinear medium, ~19 mW of ultraviolet radiation with ~9% optical efficiency with respect to absorbed power was reached for both laser crystals; the visible-to-ultraviolet conversion efficiency was 26% and 35% for Pr:LiYF(4) and Pr:BaY(2)F(8), respectively.

ULTRASTRUCTURAL ANALYSIS OF RABBIT RETINA IRRADIATED WITH A NEW 670-NM DIODE RED LASER AT DIFFERENT POWERS

Diode lasers have compact dimensions, efficient electric-optical conversion, absence of major cooling requirements, long useful life, and minimal maintenance. We verified the structural and ultrastructural effects of diode red laser emissions at 670 nm on pigmented rabbit retina. Transpupillary laser pulses were applied to rabbit retinas at variable powers with a spot size of 500 microm ( approximately 1,000 microm at the retina level) and duration of 200 milliseconds. Light and electron microscopic analyses were performed. Subthreshold non-visible lesions were produced using a power of 200 mW. Ophthalmoscopically visible lesions were produced using a power of 400 mW. More prominent lesions were obtained using a higher power (600 mW). Structural and peculiar ultrastructural changes correlated with the power used. In general, damage was incurred by the retinal pigment epithelium with extension into the superficial choroid posteriorly and into the outer retina anteriorly. The inner retina was involved at higher power levels. Ultrastructural analysis of rabbit retina treated with the 670-nm diode laser adds newer insights on tissue changes after thermal irradiation. The effects observed are consistent with those reported for the krypton red laser. The 670-nm diode laser is an economic small-size photocoagulator that appears to be a promising modality for transpupillary laser photocoagulation of the retina.

Controlled Growth of III-V Compound Semiconductor Nano-Structures and Their Application in Quantum-Devices

This paper reviews our work on controlled growth of self-assembled semiconductor nanostructures, and their application in light-emission devices. High-power, long-life quantum dots (QD) lasers emitting at ~1 µm, red-emitting QD lasers, and long-wavelength QD lasers on GaAs substrates have successfully been achieved by optimizing the growth conditions of QDs.

Photopumped red-emitting InP/In0.5Al0.3Ga0.2P self-assembled quantum dot heterostructure lasers grown by metalorganic chemical vapor deposition

We report the 300 K operation of optically pumped red-emitting lasers fabricated from InP self-assembled quantum dots embedded in In0.5Al0.3Ga0.2P layers on GaAs (100) substrates grown by metalorganic chemical vapor deposition. Quantum dots grown at 650 °C on In0.5Al0.3Ga0.2P layers have a high density on the order of 1010 cm−2 and the dominant size of individual quantum dots ranges from ∼5 to ∼10 nm for 7.5 monolayer “equivalent growth.” These InP/In0.5Al0.3Ga0.2P quantum dot heterostructures are characterized by atomic force microscopy, high-resolution transmission electron microscopy, and photoluminescence. Laser structures are prepared from wafers having two vertically stacked InP quantum dot active layers within a 100-nm-thick In0.5Al0.3Ga0.2P waveguide and upper and lower 600 nm InAlP cladding layers. We observe lasing at λ∼680 nm at room temperature in optically pumped samples.

Material issues in AlGaInP red-emitting laser diodes

Red-emitting AlGaInP quantum well lasers are now a well-established commercial product with a range of applications, many in high-volume, low-cost market sectors. Although the operation of these devices is generally understood, there are a number of critical aspects of the device structure, determined by the crystal growth process, which have an influence on key performance characteristics. A number of these are considered in this paper, particularly strain limits and non-planar growth, the effect of interface roughness on optimisation of the well width for minimum threshold current, and the influence of the doping density of the p-cladding layer on the temperature sensitivity of threshold current. These factors are examined in terms of the mechanisms by which they affect the operation of the device.

Electron transport in AlGaInP quantum well lasers

By placing a direct-gap monitor layer (or collector) in the p-cladding layer of red-emitting AlGaInP laser diode structures, we have studied the transport of electrons through this layer by observation of spontaneous emission. Pulsed optical excitation superimposed on cw electrical injection has been used to determine the delay time between optical injection of carriers into the well and radiative recombination from the monitor pit. Computer simulations using measured values of minority carrier lifetime for the well and monitor layer show that the transit time for electrons through the p-cladding layer correspond to an electron mobility of 160 cm2/V s.

Intermixing induced changes in the radiative emission from III–V quantum dots

We have examined the effect of thermally induced interdiffusion on the luminescence emission from red and infrared emitting self-assembled III–V quantum dots. Three different combinations of dot/barrier materials have been investigated: InAlAs/AlGaAs, InGaAs/AlGaAs and InGaAs/GaAs. In all cases, thermal intermixing was found to result in significant blueshifts of the photoluminescence (PL) emission. In addition, narrowing of the linewidth of the inhomogeneously broadened PL peak was observed. Both effects were found to be strongly dependent on the material system and average dot size. InAlAs/AlGaAs quantum dots exhibited the greatest linewidth reduction after intermixing, indicating this to be a promising method of achieving narrower luminescence lines for devices such as red-emitting zero-dimensional lasers.

Temperature effects in semiconductor quantum dot lasers

Self-assembled quantum dots (QDs) of highly strained InAlAs have been grown by molecular beam epitaxy (MBE) in separate confinement p-i-n heterostructres on (001) GaAs substrates. At low temperatures, the lasing threshold currents for red-emitting QD lasers are found to be more temperature-independent than for quantum well (QW) lasers. At higher temperatures, the temperature dependence of the threshold currents is governed mainly by the depth of the separate confinement region which was designed to obtain QD lasers capable of room temperature emission with simple broad area laser devices having external efficiencies of ∼13% at low temperatures.

Reliable red-emitting laser diodes grown by solid source molecular beam epitaxy

We achieved over 1000 h stable continuous wave (CW) operation of AlGaInP laser diodes (LDs) with output power of 30 mW at 50°C by using all solid source molecular beam epitaxy (MBE). In this paper the essential points for reliability are described. If sufficient outgassing of source materials was performed, secondary ion mass spectrometry (SIMS) signals of hydrogen and oxygen from the as-grown layers became small. Reduction of these impurities dramatically increased photoluminescence (PL) intensity and decreased the threshold currents of LDs. Growth condition dependence of device performance was also investigated. Whilst the acceptable ranges of V/III ratio and phosphorus cracking temperature needed to obtain reliable operation were extensive, the suitable growth temperature range was limited.