Emitting Laser Diodes Research Articles

Optical polarization anisotropy of tensile strained InGaN/AlInN quantum wells for TM mode lasers

In this paper, we discuss the optical characteristics and polarization anisotropy of a tensile strained polar c-plane InGaN/AlInN quantum well. We found that if the quantum well is under the tensile strain, the |Z⟩-like state will be lifted up so that the emitted light will be TM mode. In addition, with a particular aluminum composition of the AlInN alloy as the barrier for the tensile strained InGaN quantum well, it is possible to reduce quantum-confined Stark effect. The self-consistent Poisson and 6×6 k⋅p Schrödinger solver has been used for studying light emitting characteristics. Our results show that the tensile strained InGaN quantum well on AlInN barrier has much larger optical gain and lower threshold carrier density compared to the conventional InGaN/GaN system, and it has a potential to be TM light source for edge emitting laser diodes with the photonic crystal cavity made by nanorod arrays.

Absolute Measurements of Total Peroxy Nitrate Mixing Ratios by Thermal Dissociation Blue Diode Laser Cavity Ring-Down Spectroscopy

Peroxycarboxylic nitric anhydrides (PANs) have long been recognized as important trace gas constituents of the troposphere. Here, we describe a blue diode laser thermal dissociation cavity ring-down spectrometer for rapid and absolute measurements of total peroxyacyl nitrate (SigmaPAN) abundances at ambient concentration levels. The PANs are thermally dissociated and detected as NO2, whose mixing ratios are quantified by optical absorption at 405 nm relative to a reference channel kept at ambient temperature. The effective NO2 absorption cross-section at the diode laser emission wavelength was measured to be 6.1 x 10(-19) cm2 molecule(-1), in excellent agreement with a prediction based on a projection of a high-resolution literature absorption spectrum onto the laser line width. The performance, i.e., accuracy and precision of measurement and matrix effects, of the new 405 nm thermal dissociation cavity ring-down spectrometer was evaluated and compared to that of a 532 nm thermal dissociation cavity ring-down spectrometer using laboratory-generated air samples. The new 405 nm spectrometer was considerably more sensitive and compact than the previously constructed version. The key advantage of laser thermal dissociation cavity ring-down spectroscopy is that the measurement can be considered absolute and does not need to rely on external calibration.

Efficient quasi-three-level Nd:YAG laser at 946 nm pumped by a tunable external cavity tapered diode laser

Using a tunable external cavity tapered diode laser (ECDL) pumped quasi-three-level Nd:YAG laser, a fivefold reduction in threshold and twofold increase in slope efficiency is demonstrated when compared to a traditional broad area diode laser pump source. A TEM 00 power of 800 mW with 65% slope efficiency is obtained, the highest reported TEM 00 power from any 946 nm Nd:YAG laser pumped by a single emitter diode laser pump source. A quantum efficiency of 0.85 has been estimated from experimental data using a simple quasi-three-level model. The reported value is in good agreement with published values, suggesting that the model is adequate. Improvement of the 946 nm laser due to the ECDL's narrow spectrum proves to be less significant when compared to its spatial quality, inferring a broad spectrum tapered diode laser pump source may be most practical. Experimental confirmation of such setup is given.

Physical limits of semiconductor laser operation: A time-resolved analysis of catastrophic optical damage

The early stages of catastrophic optical damage (COD) in 808 nm emitting diode lasers are mapped by simultaneously monitoring the optical emission with a 1 ns time resolution and deriving the device temperature from thermal images. COD occurs in highly localized damage regions on a 30 to 400 ns time scale which is determined by the accumulation of excess energy absorbed from the optical output. We identify regimes in which COD is avoided by the proper choice of operation parameters.

Time-resolved analysis of catastrophic optical damage in 975 nm emitting diode lasers

Catastrophic optical damage (COD) is analyzed during single current pulse excitation of 975 nm emitting diode lasers. Power transients and thermal images are monitored during each pulse. The COD process is unambiguously related to the occurrence of a “thermal flash” of Planck’s radiation. We observe COD to ignite multiple times in subsequent pulses. Thermography allows for tracing a spatial motion of the COD site on the front facet of the devices. The time constant of power decay after the onset of COD has values from 400 to 2000 ns, i.e., an order of magnitude longer than observed for shorter-wavelength devices.

Confocal Laser Endomicroscopy: Technical Advances and Clinical Applications

Since its introduction in 2004, confocal laser endomicroscopy (CLE) has emerged as a valuable tool for gastrointestinal endoscopic imaging. Endomicroscopy enables the endoscopist to obtain real time in vivo histology during ongoing endoscopy thereby creating “optical biopsies.” To date, numerous studies have shown potential applications of endomicroscopy in the clinical setting, including in vivo diagnosis of esophageal squamous cell carcinoma, Barrett’s esophagus, celiac disease, and colonic polyps. Moreover, recent data suggest the potential application of endomicroscopy in the field of molecular imaging. Additionally, in recent months new applications and developments in the field of confocal imaging were introduced including endomicroscopy of the liver, pancreatic, and bile ducts. Furthermore, by introducing a new needle-based confocal imaging system, which is small enough to be introduced through a 22gauge puncture needle, a wide field for new applications of endomicroscopic imaging has been opened. Currently, 2 CE- and US Food and Drug Administration (FDA)approved devices for endomicroscopy are available (Figures 1 and 2; Supplementary Table 1). In this review, we introduce both systems and discuss new technical advances and clinical applications of CLE.

Blue‐green light generation using high brilliance edge emitting diode lasers

AbstractA review about second harmonic generation using edge emitting diode lasers and nonlinear crystals to obtain laser radiation in the blue‐green spectral range is presented. Therefore, pump laser radiation with high brightness and narrow bandwidth is necessary. Thus, this review gives an overview of the advances made with distributed feedback and Bragg reflector lasers, tapered lasers and amplifiers as well as external cavity diode lasers and master oscillator power amplifier schemes to achieve high brilliance emission. Since periodically poled materials have enabled high second harmonic conversion efficiencies with low and moderate pump powers, the review is focused on frequency doubling using those materials. The most commonly used materials, their properties and limitations are discussed briefly. Single pass and resonant SHG setups with waveguide and bulk nonlinear crystals are discussed and an emphasis on building compact and integrated devices is made.

Waveguide design of green InGaN laser diodes

AbstractIn this paper we investigate the waveguiding (WG) of direct green InGaN laser diodes grown on c‐plane GaN substrates. The problem of parasitic modes emerges due to the reduced refractive index difference between the GaN waveguide and AlGaN cladding layers for green compared to blue emitting laser diodes. We discuss several approaches to avoid substrate modes. We investigate different materials and designs for optimized WG of green InGaN laser diodes using a 1D transfer matrix simulation tool.

Highly Uniform Tm3+-Doped NaYbF4Microtubes: Controlled Synthesis and Intense Ultraviolet Photoluminescence

Here, we present a facile and controlled synthesis of high-quality and uniform hexagonal phase NaYbF4 microtubes by a rational hydrothermal method. In addition, highly uniform and monodisperse NaYbF4 nanospheres and nanocubes are synthesized by simply tuning the reaction parameters, including sodium fluoride content, reaction temperature and reaction time. Under excitation with a 980 nm laser diode, intense ultraviolet (UV) and blue upconversion emissions were achieved from a single dopant ion (Tm3+) in NaYbF4. The effect of the crystal phase on the upconversion emission properties is also investigated. The results demonstrate that upconversion emission intensities from hexagonal phase NaYbF4 microtubes are much more efficient than the nanocubes by 2 orders of magnitude. These microtubes with unique upconvertion emissions demonstrate that NaYbF4 is an excellent host material, as compared with some of its other fluoride and oxide counterparts, and that it may find applications in UV and visible solid-state lasers or the fiber lasers.

Violet–green laser converter based on MBE grown II–VI green lasers with multiple CdSe quantum dot sheets, pumped by InGaN laser diode

AbstractThe violet–green laser converter based on a molecular‐beam‐epitaxy (MBE) grown CdSe quantum dot (QD) laser heterostructure pumped by a commercial InGaN laser diode (LD) emission has been fabricated and studied in detail. The optimized II–VI laser heterostructure consists of asymmetrical ZnSe/ZnSSe superlattice (SL) waveguide and active region comprising five CdSe QD sheets (QDS) placed in the centre of 2‐nm‐thick ZnSe quantum wells. The new laser structure design provides both a high homogeneity of optical pumping of the CdSe QDS due to tunnelling of charge carriers between the QDS separated by 5‐nm‐thick ZnSe/ZnSSe/ZnSe barriers and high optical confinement factor. Optimization of both cavity length of the II–VI laser and parameters of optical focusing system to obtain a narrow stripe with the length slightly exceeding the cavity length has been performed. As a result, the maximum achieved quantum efficiency and pulse output power in green have been as high as 8% and 65 mW, respectively.

Efficient second-harmonic generation using a semiconductor tapered amplifier in a coupled ring-resonator geometry

A new approach for efficient second-harmonic generation using diode lasers is presented. The experimental setup is based on a tapered amplifier operated in a ring resonator that is coupled to a miniaturized enhancement ring resonator containing a periodically poled lithium niobate crystal. Frequency locking of the diode laser emission to the resonance frequency of the enhancement cavity is realized purely optically, resulting in stable, single-frequency operation. Blue light at 488 nm with an output power of 310 mW is generated with an optical-to-optical conversion efficiency of 18%.

Lanthanide doped Y6O5F8/YF3 microcrystals: phase-tunable synthesis and bright white upconversion photoluminescence properties

High-quality Y(6)O(5)F(8)/YF(3) microcrystals have been synthesised by using a hydrothermal and subsequent calcination route. Upon changing the initial solution pH value, the as-prepared microcrystal can be well tuned from YF(3) octahedron microcrystals to YF(3) hollow spheres and finally to Y(6)O(5)F(8) microtubes. The as-obtained microcrystals have been characterised by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence (PL) spectra. When the Y(6)O(5)F(8):Ln(3+) microtubes are excited by a 980 nm continual wave laser diode, bright red, green, and blue room temperature upconversion PL emissions have been observed. A series of white light emissions have been obtained by precisely adjusting dopants concentration in Y(6)O(5)F(8) microtubes.

Microsystem Light Source at 488 nm for Shifted Excitation Resonance Raman Difference Spectroscopy

A microsystem light source emitting at 488 nm was tested and applied as a light source for shifted excitation resonance Raman difference spectroscopy (SERRDS). A nonlinear frequency conversion using a distributed feedback (DFB) diode laser emission at 976 nm and a periodically poled lithium niobate (PPLN) waveguide crystal was realized on a micro-optical bench with a footprint of 25 mm x 5 mm. Joint temperature management via the microbench is used for wavelength tuning. Two emission lines at 487.61 nm and 487.91 nm are used for the SERRDS experiments. The Raman spectra of the test sample polystyrene demonstrate that a laser bandpass filter did not need to be implemented. Resonance Raman spectra of Tartrazine (FD&C Yellow 5, E 102) in distilled water are presented to demonstrate the suitability of this light source for SERRDS in, e.g., food safety control.

Atomic clocks and coherent population trapping: Experiments for undergraduate laboratories

We demonstrate how to construct and operate a simple and affordable apparatus for producing coherent effects in atomic vapor and for investigating their applications in time-keeping and magnetometry. The apparatus consists of a vertical cavity surface emitting diode laser directly current-modulated using a tunable microwave oscillator to produce multiple optical fields needed for the observation of coherent population trapping. This effect allows very accurate measurement of the transition frequency between two ground state hyperfine sublevels, which can be used to construct a coherent population trapping-based atomic clock.

Quasi-monolithic ring resonator for efficient frequency doubling of an external cavity diode laser

A quasi-monolithic second-harmonic-generation ring resonator assembled with miniaturized components is presented. The ring contains a 10-mm-long bulk periodically poled lithium niobate crystal for second-harmonic generation, four plane mirrors and two gradient-index lenses. All parts are mounted on a glass substrate with an overall size of 19.5 mm×8.5 mm×4 mm. As pump source a broad-area laser diode operated in an external resonator with Littrow arrangement is utilized. This external cavity diode laser provides near diffraction limited, narrow-bandwidth emission with an optical output power of 450 mW at a wavelength of 976 nm. Locking of the diode laser emission to the resonance frequency of the ring cavity was achieved by an optical self-injection locking technique. With this setup more than 126 mW of diffraction-limited blue light at 488 nm could be generated. The opto–optical conversion efficiency was 28% and a wall plug efficiency better than 5.5% could be achieved.

Study of a low power dissipation, miniature laser-pumped rubidium frequency standard

This paper studies a miniature low power consumption laser-pumped atom vapour cell clock scheme. Pumping 87Rb with a vertical cavity surface emitting laser diode pump and locking the laser frequency on a Doppler-broadened spectral line, it records a 5 × 10−11τ−1/2 (τ < 500 s) frequency stability with a table-top system in a primary experiment. The study reveals that the evaluated scheme is at the level of 2.7 watts power consumption, 90 cm3 volume and 10−12τ−1/2 short-term frequency stability.

67% CW power conversion efficiency from Al-free 1060 nm emitting diode lasers

1060 nm emitting broad-waveguide-type aluminium-free diode lasers have been optimised for maximum power conversion efficiency (PCE). PCE at 67% is achieved at continuous-wave operation and 25°C heatsink temperature from a 1 mm cavity length, 100 µm stripe diode laser. The improvement is mainly the result of an increase in injection efficiency via optimisation of the bandgap structure, and reductions in operating voltage via doping optimisation of the broad-waveguide structure.

Spectroscopic study of erbium-activated crystals of double calcium yttrium fluoride Ca0.89Y0.11F2.11:Er3+: I. Intensity of spectra and luminescence kinetics

Ca0.89Y0.11F2.11:Er3+ (CYF:Er) crystals with an erbium content of 1–15 at % have been grown. The optical spectra and luminescence kinetics of CYF:Er crystals have been investigated at low (∼5 K) and room temperatures. Based on an analysis of the absorption spectra at low temperature, the structure of Stark splitting of erbium levels in CYF:Er crystals is determined. Room-temperature absorption spectra are used to calculate the spectra of absorption cross sections and oscillator strengths of transitions from the erbium ground state to excited multiplets. It is shown that the absorption spectrum of CYF:Er crystals contains broad bands in the ranges of 790–815 and 965–980 nm, which correspond to the range of emission of laser diodes. For the band peaking near 967 nm, the peak absorption cross section is σ abs max = 2.7 × 10−21 cm2. The intensity parameters are determined by the Judd-Ofelt method to be Ω2 = 1.39 × 10−20, Ω4 = 1.34 × 10−20, and Ω6 = 2.24 × 10−20 cm2. The radiative transition probabilities, radiative lifetimes, and branching ratios are calculated with these values. The luminescence decay kinetics from excited erbium levels upon selective excitation is investigated and the experimental lifetimes of the 4F 9/2, 4 S 3/2, and 4 G 11/2 radiative erbium levels are determined. The dependences of multiphonon relaxation rates on the energy gap in CYF:Er crystals are obtained. The rates of nonradiative multiphonon relaxation from radiative erbium levels are determined.

Epitaxial design of 475 nm InGaN laser diodes with reduced wavelength shift

AbstractWe have analyzed the quantum well stability of blue‐green emitting laser diodes grown on c ‐plane GaN substrates. Different parameters such as the material composition of the surrounding layers of the quantum well and the number of quantum wells were varied. We observed a strong influence of the barrier material composition (AlGaN, GaN, InGaN) on the quality of In rich quantum wells. Our preferred barrier material is GaN. In contrast InGaN barriers result in a higher defect density of 9.6 × 106 cm–2 in the quantum well whereas GaN barriers show a defect density of 1.8 × 106 cm–2. High indium content quantum wells with good crystalline quality were grown by improving the structure of the complete active region. Further enhancement could be achieved by using a double quantum well structure instead of a single quantum well structure. This offers the advantage that the wavelength shift is strongly reduced as a result of the lower current density in each quantum well. With our improvements we could realize laser diodes emitting at 475 nm with threshold current densities of 12 kA/cm2 and a slope efficiency of 500 mW/A. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Towards green lasing: ingredients for a green laser diode based on GaInN

AbstractLaser structures based on GaInN quantum wells were investigated using the variable stripe length technique. Two major problems in designing a green emitting laser diode were identified. The first one is the decrease of the optical confinement for increasing wavelength. To avoid this limitation and to simultaneously maintain good optical confinement we grew samples with lattice‐matched AlInN lower cladding layers.The second major issue is to push the emission of the structures to longer wavelength by increasing the indium concentration in the quantum well. We demonstrate optical gain up to a wavelength of 480 nm (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)