Single-mode Laser Diode Research Articles

Cavity ring-down measurements in flames using a single-mode tunable laser system

High-resolution pulsed cavity ring-down spectroscopy (CRDS) of OH in a flame has been demonstrated using a novel single-mode tunable laser (STL). This system operates by pulse amplification of the output of a single-mode diode laser in a modeless dye laser. Ring-down curves obtained using the narrow-bandwidth STL, for both strong and weak transitions, are shown to be well fitted by single exponentials. These results are demonstrated to be in direct contrast with those obtained using a standard dye laser, for which the bandwidth is comparable to the transition linewidths and ring-down curves require multi-exponential fits. Accurate lineshape analysis is thus made possible using the STL, allowing the temperature to be derived from the measured Doppler width. The resulting measurement is in good agreement with the value derived from a Boltzmann plot of data obtained using a conventional laser in a similar flame. The advantages of using the STL system for quantitative CRDS measurements are discussed, together with a suggestion for quantitative measurements of the ASE content of narrowband lasers using CRDS.

Two-color coherent population trapping in a single Cs hyperfine transition, with application in magnetometry

We investigate the coherent population trapping effect occurring between the Zeeman sublevels of a given hyperfine state of Cs when excited by a single-mode diode laser, which is frequency modulated in the kHz–MHz range. In the presence of a dc magnetic field, simultaneous excitation of pairs of ground-state magnetic sublevels to common excited magnetic sublevels is performed. As a result, coherent population trapping resonance is detected at only a given modulation frequency, whose value gives a measure of the magnetic field. The parameters of the resonances are measured in order to determine the ultimate accuracy of the magnetic field measurement.

Time-dependent gas density and temperature measurements in pulsed helicon discharges in argon

Time-dependent measurements of the temperature and density of the Ar*(3P0) metastable atoms in two high-density pulsed helicon discharges, deduced from the absorption profile of the 772.42 nm argon line, are presented. A single-mode tuneable diode laser is used for recording these profiles, and temperatures up to 1000 K are obtained from their Doppler width. As in high-density plasmas the metastable and ground-state atoms are strongly coupled by electron impact collisions, the temperature of the metastable atoms reflects the gas temperature. From the time variation of the Ar*(3P0) density during the discharge pulse we were able to deduce the density of the ground-state argon atoms and found for the helicon regime that the neutral atoms can be depleted by more than a factor of 10. Over a wide range of plasma parameters, we examined the axial asymmetry that is characteristic for helicon discharges with helical antenna coupling. In particular, we analysed the plasma-induced emission of the Ar (750 nm) and Ar+ (461 nm) lines as well as the electron density in the beginning of the plasma pulse to better understand the evolution of the plasma and the transition to the helicon regime. The measurements of the gas temperature and the metastable density reveal the asymmetry to become pronounced when the discharge changes from inductively coupled plasma to the helicon discharge. In the helicon regime, the argon atom depletion of up to 90% and ionization degrees up to 65% were achieved in the antenna zone. In the afterglow, all excited states of argon, including the Ar*(3P0) state, are fed by electron–Ar+ ion recombination. The measured very high temperature of Ar*(3P0) atoms is partially related to the temperature of the Ar+ ions. Production of Ar+ in its excited states is also observed during several hundreds of microseconds in the afterglow. It is related to the electron–Ar2+ ion recombination and indicates the presence of a significant amount of doubly charged argon ions in the present helicon plasmas.

High-power 1.7–1.8 μm single-mode laser diodes

Separate confinement InGaAsP/InP laser heterostructures were grown by metalorganic-hydride vapor-phase epitaxy. High-power single-mode laser diodes of mesastripe design based on these heterostructures operate in a wavelength interval of 1.7–1.8 μm with a maximum continuous room temperature output power of 150 mW. The single-mode lasing regime is maintained up to an output power level of 100 mW.

System theory of the single-mode laser-diode

The system theory of the single-mode laser-diode is developed in this paper. Limits of the mathematical validity of the rate equations are analyzed and discussed. The theoretical model of the single-mode laser-diode obtained is shown to be a stable low-pass system whose pole positions are injection current dependent. A Volterra model of the laser diode with electronic feedback is obtained. Conditions for stability are obtained from the model and stability is shown to be dependent on the injection current. It is shown that the system is a bandpass filter whose center frequency and bandwidth can be specified by the feedback parameters. The Volterra model also allows the determination of the nonlinear distortion and, in contrast to optical feedback, methods for reducing it by modifying the feedback operator.

Applications of the single-mode laser-diode system theory

In this paper, we present a numerical analysis of the system theory of the single-mode laser-diode developed in [Opt. Commun. (Submitted for consideration with this paper)] to obtain numerical results of the gain, pulse response, and the harmonic distortion for electronic feedback. The dependence of the system gain on the feedback gain and delay are presented. The pulse response is studied and it is shown that a transmission rate over 1 Gbyte/s can be achieved. Finally, the harmonic distortion and its dependence on frequency, input amplitude, and feedback delay is presented. Some sources of error and their avoidance in the measurement of harmonic distortion are discussed. Finally, it is shown how, with electronic feedback, distortion can be reduced or even eliminated.

Precise Beam Shaping of Diode Laser Direct Application of LD Light for Material Processing

Current techniques developed for beam shaping of multi-mode laser diode (LD) output are reviewed and a new approach for the direct micro-processing with a single-mode LD is reported. The advantage of single-mode LD for high brightness focusing, the source of aberration in LD light, and the method to improve the brightness are described. The experiment on the laser ablative shaping (LAS) for single-mode laser light indicated that the initial wavefront distortion of 0.9 λ was improved to 0.4 λ. Possible applications of LD micro-marking system are described.

Swept-Cavity Ringdown Absorption Spectroscopy: Put Your Laser Light In and Shake It All About

Cavity ringdown (CRD) absorption spectroscopy with continuous-wave (cw) tunable coherent light sources provides a convenient, sensitive, and precise way to measure very weak optical absorption spectra and to analyze trace gas concentrations. Several recently introduced innovations in cw-CRD spectroscopy are described. Our approach involves one or more single-mode tunable diode laser sources, a rapidly swept ringdown cavity, and a simple optical heterodyne detected approach (based on the dynamics of coherent light reflected from such an active optical cavity). This offers high performance in a relatively simple, low-cost, compact instrument that is facilitated by fibre optic and other standard photonics or telecommunications components. New experimental results are reported, comprising CRD spectra of water vapour in the 0.815 µm wavelength region and of mixtures of CO2 and CO in the range of 1.56–1.59 µm. Promising applications of such techniques include chemical analysis of trace gases in medicine, agriculture, industry, and the environment.

Higher-order photon statistics of single-mode laser diodes and microchip solid-state lasers

We examine higher-order photon statisitics of laser diodes (LDs) and microchip solid-state lasers (SSLs). When the lasers are operated above lasing threshold, significantly different features of fluctuations are exhibited, which are the result of the asymmetric non-Gaussian distribution characteristics that are always present in SSLs, whereas, in contrast, Gaussian fluctuation persists in LDs. It is found that the deterministic dynamic trajectory in phase space, which is controlled by the ratio between the fluorescence and the photon lifetimes, is crucial to the corresponding behavior of higher-order statistics.

All-optical clock recovery at line rate by narrow-band resonant modulation of a single-mode laser diode

All-optical clock recovery at line rate (34 GHz) was demonstrated by narrow-band resonant modulation of a single-mode monolithic laser diode. The dependence of timing jitter of the recovered clock on frequency detuning and injected optical signal power has been investigated. This scheme does not require an intracavity saturable absorber and the recovered clock signal exhibits a large locking range.

High power single-mode (λ=1.3–1.6 μm) laser diodes based on quantum well InGaAsP/InP heterostructures

The possibility of achieving maximal optical output power in the single-mode lasing for mesa-stripe laser diodes fabricated on the basis of InGaAsP/InP quantum-well heterostructures with separate confinement have been studied both experimentally and theoretically. The basic condition for the single-mode lasing of laser diodes in a wide range of driving currents is shown to be the precise choice of the effective refractive index ΔnL discontinuity in the plane parallel to the p-n junction. A InGaAsP/InP separate confinement heterostructure with a step waveguide, with a threshold current density of 180 A/cm2 and an internal quantum efficiency of stimulated emission of 93–99%, has been manufactured via the MOCVD method. The optimization of the mesa-stripe diode design for the developed InGaAsP/InP heterostructure is carried out with the aim of achieving maximal optical output power in the case of single-mode lasing. An output power of 185 mW is attained in the laser diode with the mesa-stripe width W=4.5 μm (λ=1480 nm). The maximal continuous output power was as high as 300 mW. The full width at half-maximum (FWHM) of the lateral far-field pattern increased by 1° relative to the threshold value.

Off-axis continuous-wave cavity-enhanced absorption spectroscopy of narrow-band and broadband absorbers using red diode lasers

We present an application of continuous-wave (cw) cavity-enhanced absorption spectroscopy (CEAS) with off-axis alignment geometry of the cavity and with time integration of the cavity output intensity for detection of narrow-band and broadband absorbers using single-mode red diode lasers at λ=687.1 nm and λ=662 nm, respectively. Off-axis cw CEAS was applied to kinetic studies of the nitrate radical using a broadband absorption line at λ=662 nm. A rate constant for the reaction between the nitrate radical and E-but-2-eneof (3.78±0.17)×10-13 cm3 molecule-1 s-1 was measured using a discharge-flow system. A nitrate-radical noise-equivalent (1σ≡ root-mean-square variation of the signal) detection sensitivity of 5.5×109 molecule cm-3 was achieved in a flow tube with a diameter of 4 cm and for a mirror reflectivity of ∼99.9% and a lock-in amplifier time constant of 3 s. In this case, a noise-equivalent fractional absorption per one optical pass of 1.6×10-6 was demonstrated at a detection bandwidth of 1 Hz. A wavelength-modulation technique (modulation frequency of 10 kHz) in conjunction with off-axis cw CEAS has also been used for recording 1f- and 2f-harmonic spectra of the RR(15) absorption of the b1Σg+-X3Σg- (1,0) band of molecular oxygen at \(\overline{\nu}\)=14553.947 cm-1. Noise-equivalent fractional absorptions per one optical pass of 1.35×10-5, 6.9×10-7 and 1.9×10-6 were obtained for direct detection of the time-integrated cavity output intensity, 1f- and 2f-harmonic detection, respectively, with a mirror reflectivity of ∼99.8%, a cavity length of 0.22 m and a detection bandwidth of 1 Hz.

Amplification of high-bandwidth phase-modulated signals at 793 nm

Amplification of high-bandwidth phase-modulated optical signals from integrated-optics phase modulators at 793 nm is experimentally demonstrated using an injection-locking technique. Off-the-shelf wide-bandwidth integrated-optics modulators are power limited at 793 nm owing to photorefractive damage of the LiNbO3 waveguides. Typical optical input powers for these devices at this wavelength are less than 10 mW with optical output powers typically less than 1 mW. To amplify the outputs of these modulators, we injected the phase-modulated light into an antireflection-coated 100-mW single-mode diode laser. With the injection-locking technique, small-signal gains of 23 dB are demonstrated with good signal fidelity up to bandwidths of 3 GHz. A bandwidth limitation is found at approximately 3 GHz for sinusoidal phase-modulated signals, above which signal fidelity is seriously degraded. This limitation is significantly less than the measured relaxation oscillations of ∼5.6 GHz, suggesting a new limitation to injection locking of phase-modulated signals. Amplification of binary-phase-shift-keyed-modulated signals to 6 Gbit/s is also demonstrated with no bit errors over the 256-bit test sequences.

Measurement of the populations of metastable levels in gadolinium vapor by diode laser-based UV and near-IR absorption spectroscopy

The metastable populations of gadolinium atoms, which were generated by an electron beam gun, were measured by diode laser-based absorption spectroscopy. Transitions originating out of the ground and low-lying metastable levels were investigated in the spectral range of 390–394 nm. A tapered semiconductor power-amplifier which was seeded by a single-mode external-cavity diode laser was used as a fundamental light source for the second-harmonic generation in the ultraviolet wavelength range. The population appears to be frozen at an atomic excitation temperature of ∼600 K, which is about 1/4 of the heated surface temperature. The results obtained from the ultraviolet absorption spectra are compared to those obtained from the near-IR absorption spectra with wavelength ranges of 673–692 and 773–775 nm.

High-power narrowed-bandwidth output of a broad-area multiple-stripe diode laser with photorefractive phase-conjugated injection

By using a new mutually pumped phase-conjugation setup consisting of two photorefractive crystals, a broad-area three-stripe diode laser is locked to a single-mode diode laser at a drive current up to 2.25 times the threshold current. The bandwidth is narrowed from about 2 to 0.034 nm, with an available output power of 1.5 W.

Influence of features of QW InGaAs/(Al)GaAs heterostructures grown by MOCVD on the emission spectrum of single-mode laser diodes

Laser heterostructures with an active layer consisted of two strained InGaAs quantum wells are fabricated by the MOCVD epitaxy method. Single-mode laser diodes with a 3 — 4-μm wide stripe contact are made of the heterostructures and their spectral characteristics are studied. It is found that the spectrum of the diodes consists of two groups of longitudinal modes, which cause the spectral broadening. The power distribution between these groups of modes depends on time, the pump current and the temperature of the active region. It is assumed that such a behavior of the diode spectra is explained by the asymmetry of the composition of the diode active region, which arises due to the segregation of In because of the complicated mechanism of its penetrating in the solid solution during the epitaxial growth. This assumption was confirmed by the study of the active region using a modified Auger spectroscopy method.

Fiber-optic evanescent-field laser sensor for in-situ gas diagnostics

A compact, rugged and portable fiber-optic evanescent-field laser sensor is developed for the detection of gaseous species in harsh environments such as volcano fumaroles or industrial combustion of glass furnaces. The sensor consists of an optical multi-mode fused silica fiber with jacket and cladding removed and the bare fiber core in direct contact with the surrounding molecules. The beam of a single-mode DFB diode laser with an emission wavelength centered at 1.5705 μm is coupled into the fiber. At the other end of the fiber an infrared detector is used to record the transmitted infrared laser light intensity. Due to the frustrated total reflection (FTR) and the attenuated total reflection (ATR) the laser intensity is attenuated when passing through the fiber. The FTR is related to a change of the index of refraction while the latter one is related to a change of the absorption coefficient. While tuning the DFB laser wavelength across absorption lines of molecules surrounding the fiber a spectral intensity profile is measured. Voigt functions are fitted to the recorded intensity profiles to estimate relative molecule concentrations. In this paper results from first field measurements at the volcano site ‘Solfatara’ in Italy are reported that use such a sensor device for simultaneous detection of H 2S, CO 2 and H 2O directly in the gas stream of a volcano fumarole.

Nonlocal electron kinetics and excited state densities in a magnetron discharge in argon.

The densities of argon metastable (3)P(2), and resonance (1)P(1), (3)P(1) states were measured along a cylindrical magnetron discharge radius by absorption spectroscopy using a narrow bandwidth single mode diode laser. The theoretical treatment includes calculations of the rates of numerous excitation and decay processes based on nonlocal electron kinetics, and analysis of the transport equations for the resonance and metastable atoms. The solution technique of the Biberman-Holstein equation of radiation transport is developed in conformity with magnetron discharge geometry. The radial profile of the effective lifetime is obtained, taking into account radiation escape on the inner and outer electrodes. The distinction in formations of the radial profiles of the resonance and metastable atoms caused by specifics of radiation transport and diffusion is demonstrated. The results of experiments and calculations are compared.

Practical low-noise stretched-pulse Yb^3+-doped fiber laser

We report on the development of what we consider to be a practical and highly stable stretched-pulse laser based on Yb(3+) -doped silica fiber. The Fabry-Perot cavity uses nonlinear polarization rotation as the mode-locking mechanism, and a semiconductor saturable-absorber mirror to ensure robust self-starting and incorporates a diffraction grating pair to compensate for the normal dispersion of the fiber. Use of a single-mode grating-stabilized telecommunications-qualified pump laser diode ensures reliable, low-noise operation (~0.05% amplitude fluctuations at 10-Hz measurement bandwidth). The laser generates high-quality, 60-pJ pulses of <110-fs duration at a repetition rate of ~54 MHz (3-mW average power).

Single-mode laser diode at 778-nm wavelength: effect of p-doping

An AlGaAs-GaAs double quantum well structure has been grown by metal-organic chemical vapor deposition. High p-doping of the cladding layer (4/spl times/10/sup 18/ cm/sup -3/) is used to optimize the performance of ridge waveguide laser diodes. A stable single-mode operation is obtained with output power of 120 mW at room temperature. The temperature constant T/sub 0/ is /spl sim/230 K. Improvements are explained by reduction of electron leakage and by the current self-distribution effect.