Characteristics of Gain-Guided Laser Diodes with Cylindrical-Mirror Cavity

We have developed highly reliable etched-mirror laser diodes using a dry etching method. The lasers without facet-coating have been operating stably over 2500 h under automatic-power control (APC) at a power of 3 mW/facet at 50°C. The gain-guided laser diodes with a cylindrical-mirror cavity (CMC) have coaxial mirrors and a fan-shaped stripe structure. By decreasing the curvature radius of the inner facet or increasing the stripe width of the inner facet, the beam waist parallel to the junction plane can be moved outside of the laser diode, while the beam waist perpendicular to the junction plane stops at the mirror facet. A particular CMC laser has a low astigmatism of 4.1 μm and a low relative intensity of noise (RIN) less than –134 dB/Hz at 4 mW under 0–1% optical feedback without high frequency current superposition.     

Optical clock sources model based on two-section distributed feedback laser with shift-layer

We propose a numerical model, including modulation transform function and self-sustained pulsation condition, to estimate the self-sustained pulsation frequency, matched to the measured pulsation frequency, which considers the structure factors in a two-section distributed feedback laser with a thin shift-layer.     

Frequency chain towards the Ca intercombination line based on laser diodes: First step

Stable, narrow linewidth operation of red and 1.3 µm free-running laser diodes with external gratings in non-Littrow geometry is demonstrated. The resonance of the saturated fluorescence of an atomic beam with a contrast of 25% and a linewidth of 400 ± 50 kHz of the Ca intercombination line 4¹ S ₀–4³ P ₁ ( = 657 nm) is shown. A high-power (110 mW) single-mode external cavity laser diode at 1.3 µm is used for second-harmonic generation in a KTP crystal. The beat signal (signal to noise ratio about 25 dB) of 10 nW second-harmonic radiation at 1.3 µm and the radiation of a laser diode in the visible spectrum, as a step to realize a frequency chain, is observed.     

ArF-excimer-laser annealing of 3C–SiC films—diode characteristics and numerical simulation

We fabricated Schottky barrier diodes using 3C–SiC films deposited on Si(1 1 1) by lamp-assisted thermal chemical vapor deposition and annealed with an ArF excimer laser. Improvement in both the reverse current and the ideality factor was obtained with 1–3 pulses with energy densities of 1.4–1.6 J/cm² per pulse. We solved a heat equation numerically assuming a transient liquid phase of SiC. The calculated threshold energy density for melting the surface was 0.9 J/cm². The thermal effects of Si substrate on SiC film were also discussed. The experimental optimum condition was consistent the numerical simulation.     

Xe2Cl fluorescence and absorption in self-sustained discharge XeCl lasers

Fluorescence at 490 nm from the triatomic excimer Xe₂Cl^* has been investigated to determine the 308 nm absorption due to this species in an x-ray preionized, self-sustained gas discharge XeCl laser. The dependence of Xe₂Cl^* density on laser intensity (at 308 nm), buffer gas and Xe and HCl partial pressures has been determined for discharges with a peak electrical power deposition of 2.5 GWl^–1. Xe₂Cl^* absorption is estimated to reach 0.6% cm^–1 under non-lasing conditions but decreases to a non-saturable 0.2% cm^–1 for intracavity laser intensity>1 MW cm^–2. XeCl^* and Xe₂Cl^* fluorescence intensities were found to be a similar for both helium and neon buffer gases but laser output was a factor of two greater with a neon buffer.     

Measurement of low-frequency noise in tunnel diodes

Conclusions Our studies have revealed that the spectral density of low-frequency current fluctuations in GaAs tunnel diodes can be described by the relation Wi(u, F)=f² (u)mF^–, where the nonlinear function f²(u) is not proportional to the rms of excess diode current. The flicker nature of the current noise in tunnel diodes derives from conductance fluctuations on the p-n junction, which occur in tunnel diodes as well as in low-noise transistors at frequencies ranging from near zero to a few kiloheriz and produce noise of almost the same absolute intensty in both kinds of devices.Leningrad Polytechnic Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 20, No. 5, pp. 777–784, May, 1977.     

Proposal of a frequency-synthesis chain between the microwave and optical frequencies of the Ca intercombination line at 657 nm using diode lasers

We propose a frequency synthesis chain which can directly connect a microwave atomic clock with a visible laser. We design this chain for the frequency measurement of a visible laser locked on the intercombination transition of Ca at 657 nm. The proposed chain is based on both an optical difference frequency divider and an optical frequency comb generator, and it is designed to use nine visible and near-infrared diode lasers. We discuss the technical requirements to realize the frequency measurement accuracy level of 10^–14.     

Ultrafast laser ablation of indium tin oxide thin films for organic light-emitting diode application

Ultrafast laser ablation of ITO thin film coated on the glass has been investigated as a function of laser fluence as well as the number of laser pulses. The ablation threshold of ITO thin film was found to be 0.07 J/cm² that is much lower than that of glass substrate (about 1.2–1.6 J/cm²), which leads to a selective ablation of ITO film without damage on glass substrate. The changes in the electrical resistance and morphology of ablated trench of ITO electrode were found to be strongly dependent on the processing conditions. We present the performance of organic light-emitting diodes (OLED) fabricated with ITO electrode patterned by ultrafast laser ablation.     

Optical properties of Sm-doped CaF2 bismuth borate glasses

Sm³⁺-doped calcium fluoride bismuth borate glasses were prepared and characterized optically and the oscillator strengths and Judd–Ofelt parameters for the glass containing 1.5 mol% of Sm₂O₃ were calculated. Density and optical absorption, transmission and the emission spectra were measured. The values of Judd–Ofelt parameters suggested an increase in the degree of asymmetry the local ligand field at Sm³⁺ sites. The optical band gap energy, band tailing parameter and Urbach's energy were calculated for all glass samples. It was found that with increasing the concentration of Sm₂O₃ content the values of the optical band gap energy decrease whereas Urbach's energy increases. Absorption and excitation spectra indicate that commercial UV and blue laser diodes, blue and bluish-green LEDs and Ar⁺ optical laser are powerful excitation sources for Sm³⁺ visible fluorescence in the glass.     

Impurity distribution and electrical characteristics of boron-doped Si1−x Ge x /Si p +/N heterojunction diodes produced using pulsed UV-laser-induced epitaxy and Gas-immersion laser doping

A two-step pulsed UV-laser process which independently controls the metallurgical and electrical junction depth of a Si_1–x Ge _x /Si heterojunction diode has been implemented. Pulsed Laser-Induced Epitaxy (PLIE) combined with Gas-immersion Laser Doping (GILD) are used to fabricate boron-doped heteroepitaxial p ⁺/N Si_1–x Ge _x /Si layers and diodes. Borontrifluoride is used as the gaseous dopant source in the GILD process step. Boron incorporation and activation are investigated as a function of laser energy fluence and the number of laser pulses using SIMS and Halleffect measurements. The dose of incorporated dopant is on the order of 10¹³ cm^–2 per pulse. The B profiles obtained are flat except for a peak at the interface resulting from segregation effects. The B and Ge distributions are compared with shifts in the turn-on voltage of p ⁺/N Si_1–x /Si heterojunction diodes produced by the process. The GILD/PLIE process is spatially selective with the resulting diodes fabricated being quasiplanar. Hole mobilities in the heavily doped Si_1–x Ge _x films are found to be slightly lower than in comparable Si films.Presently at the Oregon Graduate Institute, Beaverton, OR 97006, USA     

Short-term frequency stabilization of diode-laser-pumped Nd:YAG lasers using double-pendulum suspended cavities

We describe the improvement of short-term frequency stability of diode-laser-pumped Nd:YAG lasers. To improve the vibrational isolation of reference cavities, the reference cavities were suspended by a double pendulum with magnetic damping. The frequency noise was reduced to lower than 1 Hz/Hz at Fourier frequencies higher than 5 Hz and the minimum noise of 7 × 10^–3 Hz/Hz was recorded. The minimum root Allan variance was about 10^–14 for the sampling time of 0.01 s. Heating of the reference cavity by absorbed laser power caused the thermal drift of cavity resonance frequencies. It resulted in the laser linewidth in the range of 30–50 Hz.     

An electrical characterization of a hetero-junction nanowire (NW) PN diode (n-GaN NW/p-Si) formed by dielectrophoresis alignment

This is a report on the electrical characterization of gallium nitride (GaN) nanowire (NW) p–n junction diodes. These diodes were formed by assembling n-GaN NWs on p-Si (1 0 0) substrates using alternating current (AC) dielectrophoresis (DEP). The AC DEP was optimized with a bias voltage of 15 V_p−p at a frequency of 1 kHz. The hetero-junction single GaN nanowire p-n diode (n-GaN NW/p-Si) showed well-defined current rectifying behavior with a forward voltage drop of 1.2–2.0 V at a current density of 10–60 A/cm². The GaN nanowire p–n diodes had a high parasite resistance in the range of >470 kΩ. We observed that these high resistances were mostly the result of the metal contacts to the n-GaN NWs. We also found that these parasite resistances were reduced by the formation of an additional capping layer on the top of the n-GaN NW as well as high temperature annealing.     

Observation of new laser transitions and saturation effects in optically pumped NO

We report investigations of an NO laser employing specially profiled magnetic fields of up to 3.4T, and F₂ pump laser intensities as great as 20 MW cm^–2. We have observed laser oscillation at 226 nm on a rotational branch of the B'-X/it(3–11) band of NO for the first time, in addition to the previously reported oscillation at 218 nm on the B'-X/it(3–10) band. We have also observed visible laser emission on a rotational branch of the B ²-B ² II(3–1) band of NO. Saturation of the NO laser pulse energy with pump intensity has been observed, the total NO laser pulse energy having been increased to 490 J. The possibility of increasing the NO laser pulse energy towards 1 mJ per transition is discussed.     

Precise force measurement over 6 decades applying the resonator-internal photoelastic effect

Mechanical forces can be measured over a range of 6 decades by applying the photoelastic effect inside the laser cavity. Our experiments with a modular test setup (633nm) demonstrate proportional force-to-frequency conversion from 1·10^–4N up to 80N with the noise equivalent resolution of 3·10^–5N. The frequency response of our test setup was established from dc up to some kilohertz. Measurement range and resolution can be extended to 10^–6N or smaller values by applying improved laser stabilization and miniaturizing cavity length and size of photoelastic material. The upper limit of the scale factor is calculated to be of the order of 25MHz/N.     

Propagation parameters of the beam extracted from a diode pumped Nd:YAG ceramic slab laser with a hybrid stable–unstable resonator

We analyze the propagation properties of the beam extracted from a diode pumped ceramic Nd:YAG slab laser adopting a hybrid stable–unstable resonator. Such a resonator produces a beam characterized by an Hermite–Gauss mixture-like distribution in one transverse direction and a hard-edge diffracted distribution on the other transverse direction. The beam propagation parameters M_x² and M_y² are measured for different values of the diodes driving current. We obtain a beam parameter product smaller than 3 mm mrad in both transverse directions and in the whole range of powers, up to an extraction of 220 W in a QCW regime.     

Experimental investigation of Schottky barrier diodes as nonlinear elements in 800-nm-wavelength region

We report the results of experimental investigations of nonlinear properties of InP, GaAs and InGaAs Schottky barrier diodes (SBD) in the near-infrared range (800 nm). The results of our previous and present work show that SBD have a unique broadband spectral sensitivity (from the millimeter to the visible range). As video detectors the GaAs-SBD proved to be the most sensitive among the other diodes, but they concede to the InP-SBD as frequency mixer–multipliers in spite of their higher cut-off frequency. The higher efficiency of the InP-SBD in a mixer–multiplier mode is supposedly connected with the bulk nonlinearity properties of indium phosphide, lower noise and lower time constants, characterizing the charge-carrier energy growth in an electric field and its relaxation. We found that the optimal operation conditions for SBD suppose not only the optimal electric regime (applying a defined bias or a defined radiation source power) but also the optimal mutual orientation of a laser radiation polarization, a contacting wire (playing the role of an antenna) and a laser beam. PACS 07.57.Kp; 42.65.Ky; 42.79.Nv     

Coherence resonance near a Hopf bifurcation

We report on the observation of coherence resonance for a semiconductor laser with short optical feedback close to Hopf bifurcations. Noise-induced self-pulsations are documented by distinct Lorentzian-like features in the power spectrum. The character of coherence is critically related to the type of the bifurcation. In the supercritical case, spectral width and height of the peak are monotonic functions of the noise level. In contrast, for the subcritical bifurcation, the width exhibits a minimum, translating into resonance behavior of the correlation time in the pulsation transients. A theoretical analysis based on the generic model of a self-sustained oscillator demonstrates that these observations are of general nature and are related to the fact that the damping depends qualitatively different on the noise intensity for the subcritical and supercritical case.     

Frequency- and wavelength-modulation spectroscopies: Comparison of experimental methods using an AlGaAs diode laser

Low-wavelength modulation (1 kHz), high-wavelength modulation (100 MHz) and two-tone frequency modulation (390±5 MHz) spectroscopies are systematically compared by measuring the minimum detectable absorption achieved using an AlGaAs diode laser tuned on a third-overtone methane transition at 886 nm. From the S/N behavior has been extrapolated a minimum relative absorption (1 Hz of bandwidth) of 4.5(1)×10^–7 for the LMW, 9.7(3)×10^–8 for the HWM and 6.4(2)×10^–8 for the TTFM. In the LWM case the detection-limit value is represented by the laser amplitude 1/f excess noise, while for the high-frequency detection techniques this contribution is negligible with respect to other noise sources. These detection limits well agree with the calculated quantum limited values based on measured laser power, modulation index, noise figure of the electronic components, and other parameters of the apparatus.     

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.     

Synthesis, luminescence and crystallographic structure of Eu-doped garnet-type yafsoanite Ca3Te2(ZnO4)3

A red-emitting phosphor of Eu³⁺-doped calcium–tellurium–zinc oxide, Ca₃Te₂(ZnO₄)₃, with a garnet-type structure was synthesized by high temperature solid-state reactions. This phosphor exhibited a strong red emission. The photoluminescence excitation spectrum showed that Ca₃Te₂(ZnO₄)₃:Eu³⁺ can be effectively excited by UV–visible light. The property of long-wavelength excitation for this material has a benefit as a red phosphor in application of white light-emitting diodes. The colour coordinates were calculated. The excitation and emission spectra and luminescence decay curves were obtained using a pulsed, tunable, narrowband dye laser. Crystallographic sites and charge compensation mechanism of Eu³⁺ ions were discussed. The emission line from Eu³⁺ in intrinsic crystallographic site in the lattice was located at 579.56 nm. The emission line from Eu³⁺ in another disturbed site, which is created by the defects created by the charge-compensation, was located at 580.88 nm. The disordered crystallographic sites of Eu³⁺ are benefit for their strong red luminescence corresponding to the ⁵D₀→⁷F₂ transition.