Single-mode Spectrum Research Articles

Theoretical calculation of relative intensity noise of multimode vertical-cavity surface-emitting lasers

An analytical calculation of relative intensity noise spectra of weakly index-guided vertical-cavity surface-emitting lasers (VCSELs) in a multimode regime is performed. We obtain analytical expressions that incorporate the spatial dependence of electrical field and carrier density profiles. Expressions are derived for single-mode and two-transverse-mode operation. Our analysis shows that single-mode noise spectrum depends on the spatial mode profile through a modal effective volume. Two-mode noise spectra also incorporate the dependence on the degree of spatial overlapping between the modes. Two resonance peaks appear in the noise spectra of the individual modes and total power of the two-mode VCSEL. We analytically show that those peaks appear at frequencies that correspond to the relaxation oscillation frequencies of the multimode laser. Simple analytical expressions are also derived for noise spectra at zero frequency.

Multimode resonances in square-shaped optical microcavities

Square-shaped two-dimensional optical microcavities (micro-cavities) were investigated for possible applications as filters for dense wavelength-division multiplexing. Multimode cavity resonances were observed in the elastic scattering of approximately 200-microm square-shaped micro-cavities in fused silica. Based on a two-dimensional k-space representation, we accounted for the multimode spectrum by different normal modes with rays confined by total internal reflection. The cavity-mode trajectories need not be closed after each round trip. Single-mode spectra are expected from smaller square-shaped micro-cavities.

Application of artificial neural networks and genetic algorithms to modeling molecular electronic spectra in solution

A novel approach is presented for finding the vibrational frequencies, Franck–Condon factors, and vibronic linewidths that best reproduce typical, poorly resolved electronic absorption (or fluorescence) spectra of molecules in condensed phases. While calculation of the theoretical spectrum from the molecular parameters is straightforward within the harmonic oscillator approximation for the vibrations, “inversion” of an experimental spectrum to deduce these parameters is not. Standard nonlinear least-squares fitting methods such as Levenberg–Marquardt are highly susceptible to becoming trapped in local minima in the error function unless very good initial guesses for the molecular parameters are made. Here we employ a genetic algorithm to force a broad search through parameter space and couple it with the Levenberg–Marquardt method to speed convergence to each local minimum. In addition, a neural network trained on a large set of synthetic spectra is used to provide an initial guess for the fitting parameters and to narrow the range searched by the genetic algorithm. The combined algorithm provides excellent fits to a variety of single-mode absorption spectra with experimentally negligible errors in the parameters. It converges more rapidly than the genetic algorithm alone and more reliably than the Levenberg–Marquardt method alone, and is robust in the presence of spectral noise. Extensions to multimode systems, and/or to include other spectroscopic data such as resonance Raman intensities, are straightforward.

Improved performance of GaAs-AlGaAs superlattice quantum cascade lasers beyond /spl lambda/=13 μm

Highly improved quantum cascade lasers based on intraband transitions in a chirped AlGaAs-GaAs superlattice have been realized. Lasing in a liquid nitrogen environment with a laser wavelength centered at 13 μm and with a peak optical power per facet exceeding 80 mW is achieved. A threshold current density of 8.6 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 80 K has been measured. Multimode lasing spectra ranging from 12.4 to 13.3 μm and single mode spectra in a short cavity at 13 μm are obtained. To improve the gain in the structure, the design of the injector is optimized. The minibands in the active region are more strongly confined beneath the conduction band edge of the barriers to decrease electron losses into the continuum. The maximum operating temperature exceeds 230 K with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> values of 170 K.

AlGaAsSb lasers emitting in the 1.6 μm region

AlGaAsSb lasers with different Al concentrations in the active and confinement regions are fabricated and investigated. The structures lase in the region ∼1.6 μm. The AlGaAsSb solid solution in the active region is a direct-gap material with a small energy separation (∼56 meV) between the direct-gap Γ minimum and the indirect-gap L minimum of the conduction band. The lasers have a single-mode spectrum with a predominant longitudinal mode in the spatial distribution of the emission. The lasers operate at room temperature in a pulsed mode.

Nonlinear mode competition between unstable drift waves in an rf heated ring discharge

The nonlinear wave-wave interaction between unstable drift modes of different wave numbers existing in an rf heated, magnetized plasma is studied experimentally by means of capacitive probe measurements during the evolution of the discharge towards steady state. The results are compared with theoretical calculations. Special interest is attached to the unstable growth of multiple modes arising out of the noise level, where strong suppressive interaction between different modes is observed, finally leading to a single mode spectrum.

Continuous wave operation of quantum cascade lasers based on vertical transitions at λ=4.6 μm

Continuous wave operation of a quantum cascade laser at λ=4.6 μm is reported above liquid nitrogen temperature. Optical powers of 15 mW at 50 K and 2 mW at 85 K are reported. The single mode spectrum is temperature tunable over 1.8 cm −1. A linewidth of 115 Mhz is measured with a Fabry–Perot on those free running devices. Gain measurements show evidence for ultra low linewidth enhancement factor α<0.1. These devices also operated in pulse mode with 20 mW peak power at 200 K.

Continuous wave operation of a vertical transition quantum cascade laser above T=80 K

Continuous wave operation of a quantum cascade laser at λ=4.6 μm is reported above liquid nitrogen temperature. Optical powers of 15 mW at 50 K and 2 mW at 85 K are reported. The single mode spectrum is temperature tunable over 1.8 cm −1. These devices also operated in pulse mode with 20 mW peak power at 200 K. Gain measurements show evidence for ultralow linewidth enhancement factor α&amp;lt;0.1.

The dependence of the output of an external-cavity laser on the relative phases of inputs from five gain elements

Five diode lasers that each have one facet antireflection (AR) coated are fiber coupled into an external-cavity and operated as a coherent ensemble. The external-cavity laser output power and spectrum are functions of the relative phases of the inputs from the optical fibers. The phases can be adjusted for an optimum single-mode spectrum simultaneous with maximum output power. In this case, the residual reflections from the AR-coated facets are also properly phased. Dephasing reduces the output power and tunes the wavelength through a series of discrete, lower-power maxima. These maxima occur at wavelengths for which the fiber inputs to the cavity are again in phase but the reflections from the AR-coated facets are not in phase. >

Linear-chain-model interpretation of Raman scattering from ZnSe/ZnTe superlattices and ZnSexTe1-x mixed crystals.

We study the vibrational dynamics of strained-layer superlattices of ZnSe/ZnTe, using Raman spectroscopy together with calculations in a linear-chain model. Our analysis suggests (1) that the superlattices are free standing, i.e., separated elastically from their substrates, whether grown on GaAs or InP, and (2) that the degree of strain within the superlattice layers depends strongly on the superlattice period \ensuremath{\Lambda}. From essentially none at \ensuremath{\Lambda}=30 \AA{}, the relaxation of the strain increases with \ensuremath{\Lambda} until full relaxation is encountered at \ensuremath{\Lambda}=90 \AA{}. Additionally we discuss briefly the evidence for interfacial disorder in the superlattices and the single-mode spectra of ${\mathrm{ZnSe}}_{\mathit{x}}$${\mathrm{Te}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ mixed-crystal alloys.

Corrugation-pitch modulated MQW-DFB lasers with narrow spectral linewidth

Narrow spectral linewidth lasers are developed for optical coherent transmission systems. To obtain a narrow spectral linewidth, a CPM (corrugation-pitch modulated) structure is introduced into 1200- mu m-long MQW-DFB (multi-quantum well distributed feedback) lasers. The CPM structure effectively suppresses the axial spatial hole burning effect and stabilized single-mode spectrum at high output powers. A spectral linewidth of 170 kHz was achieved at an output power of 25 mW. Details of corrugation fabrication method that makes the CPM structure possible are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Purely gain-coupled distributed feedback semiconductor lasers

We propose a new distributed feedback (DFB) laser structure in which almost pure gain coupling can be embodied in principle, without sacrificing low threshold operation. An analysis of coupling coefficients has revealed the condition for canceling index-coupling component. Utilizing organometallic vapor phase epitaxy, we have fabricated GaAlAs/GaAs ridge waveguide distributed feedback lasers having this structure. Excellent single longitudinal mode oscillation independent of facet reflection has been obtained along with low threshold current. The single-mode spectrum has exhibited distinctive characters of purely gain-coupled DFB lasers.

High-speed AlGaAs/GaAs multiple quantum well ridge waveguide lasers

High bandwidths (up to 14GHz) and relaxation frequencies (up to 36 GHz) have been measured for multiple quantum well ridge waveguide lasers. The single-mode optical spectra under CW operation remain nearly unchanged under highspeed modulation with up to 70% modulation depth.

A narrowband unidirectional pulsed ring dye laser

A simple unidirectional optically stable ring laser cavity has been developed for short pulse excitation. Using only one grazing incidence grating as a selective element a N2-laser pumped dye laser has been constructed having a single mode spectrum of 420 MHz, clockwise to counterclockwise wave ratio 166:1 and a conversion efficiency of 4.6%.

Liquid-phase epitaxy grown PbSnTe distributed feedback laser diodes with broad continuous single-mode tuning range

Distributed feedback (DFB) pulsed laser operation has been demonstrated in stripe geometry Pb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> Sn <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Te double-heterostructures grown by liquid-phase epitaxy. The grating structure of 0.79 μm periodicity operates in first order near 780cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (12.8 μm) and was fabricated prior to the liquid-phase epitaxial growth using holographic exposure techniques. These DFB lasers had moderate thresholds, 3.6 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and the output power versus current curves exhibited a sharp turn-on free of kinks. Clean, single-mode emission spectra, continuously tunable over a range in excess of 20 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , centered about 780 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (12.8 μm), and at an average rate of 1.2 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> from 9 to 26 K, were observed. While weaker modes could at times be seen in the spectrum, substantially single-mode operation was obtained over the entire operating range and to over 10 times threshold. These observations have been shown to be in good agreement with published lead-salt parameter values and DFB theory, and significantly increase the attractiveness of the DFB structure for tunable lead-salt laser diodes.

Two-wavelength single mode grazing incidence dye laser

Two-wavelength single mode operation of a N 2-laser pumped dye laser is described. The proper choice of a grazing incidence grating and highly selective cavity elements provided a stable operation of the laser. A single mode spectrum of 310 ± 50 MHz is measured and both wavelengths are independently tunable over the entire gain profile of the R6G DN dye.

Tunable Single Mode Operation of Gas Lasers Using Intracavity Tilted Etalons

The use of a tilted solid etalon inside the resonator of a gas laser, for the purpose of obtaining a tunable single mode output from the laser, is described and analyzed. This technique is illustrated by single mode spectra and gain profiles obtained with various argon ion laser transitions.