Showing papers on "Laser linewidth published in 1992"

Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods

Abstract: A variety of frequency-modulation methods for high-sensitivity absorption detection of gas-phase species has evolved in recent years. The distinctions among these methods are mostly semantic. The mathematical derivations for wavelength-modulation spectroscopy and one- and two-tone frequency-modulation spectroscopies are presented; a common terminology is used to permit a comprehensive comparison of predicted detection sensitivities. Applying this formalism, I compare the optimum detection sensitivities of these different methods for a typical laser system, using the same parameters. As long as residual amplitude modulation is minimized by proper adjustment of the detection phase angle, high-frequency wavelength modulation and one- and two-tone frequency-modulation methods all achieve approximately the same sensitivities. The choice among techniques is most strongly driven by the individual laser tuning characteristics, the absorption linewidth, and the detection bandwidth. It is shown that excess laser noise cannot always be excluded from consideration, even at megahertz detection frequencies. Also, detection at harmonics of the modulation or beat frequency may present certain advantages in minimizing residual amplitude-modulation noise.

Optical generation of very narrow linewidth millimetre wave signals

Abstract: A novel method for the optical generation and distribution of millimetre waves is described. The method is demonstrated at 36 GHz and an electrical linewidth limited by the resolution bandwidth of the spectrum analyser is achieved. No broadening of the electrical linewidth could be observed after propagation through 8 km of standard fibre.

Resonant cavity light‐emitting diode

Abstract: A novel concept of a light‐emitting diode (LED) is proposed and demonstrated in which the active region of the device is placed in a resonant optical cavity. As a consequence, the optical emission from the active region is restricted to the modes of the cavity. Resonant cavity light‐emitting diodes (RCLED) have higher spectral purity and higher emission intensity as compared to conventional light emitting diodes. Results on a top‐emitting RCLED structure with AlAs/AlxGa1−xAs quarter wave mirrors grown by molecular beam epitaxy are presented. The experimental emission linewidth is 17 meV (0.65 kT) at room temperature. The top‐emission intensity is a factor of 1.7 higher as compared to conventional LEDs.

A wideband heterodyne optical phase-locked loop for generation of 3-18 GHz microwave carriers

Abstract: Experimental results of a wideband heterodyne second-order optical phase-locked loop with 1.5- mu m semiconductor lasers are presented. The loop has a bandwidth of 180 MHz, a gain of 181 dBHz, and a propagation delay of only 400 ps. A beat signal of 8 MHz linewidth is phase locked to become a replica of a microwave reference source close to carrier with a noise level of -125 dBc/Hz. The total phase variance of the locked carrier is 0.04 rad/sup 2/ and carriers can be generated in a continuous range from 3 to 18 GHz. The loop reliability is excellent with an average time to cycle slip of 10/sup 11/ s and an acquisition range of 640 MHz. >

Fast heterodyne optical phase-lock loop using double quantum well laser diodes

Abstract: The first experimental demonstration is reported of a heterodyne optical phase-lock loop (OPLL) using semiconductor lasers without external cavities or other linewidth narrowing methods. For a total loop propagation delay of 3 ns and a loop filter bandwidth of 700 MHz, a phase error variance of 1.02 rad2 was achieved in 500 MHz measurement bandwidth.

One-atom lasers.

Abstract: One-atom lasers are important because their governing equations can be solved exactly, even with a quantized field. We present a fully quantum-mechanical treatment of one-atom lasers modeled by quantum-optical master equations. These are solved numerically without any significant approximations. We show that laser action is possible with one atom, and that it might be achievable experimentally. Laser action is characterized by the dominance of stimulated emission over spontaneous emission. We use the one-atom laser model to investigate, without approximation, some interesting generic laser phenomena. Under certain conditions lasers produce intensity squeezed light, and then the laser linewidth increases with the pumping rate, in contrast with standard lasers. We also report ``self-quenching'' behavior: lasers with incoherent pumping out of the lower laser level turn off when the pumping is sufficiently fast because the coherence between the laser levels is destroyed.

Sub-hertz relative frequency stabilization of two-diode laser-pumped Nd:YAG lasers locked to a Fabry-Perot interferometer

Abstract: Two-diode laser-pumped Nd:YAG lasers have been frequency stabilized to a commercial 6.327-GHz free spectral range Fabry-Perot interferometer yielding a best-case beatnote linewidth of 330 mHz. In addition, a Fabry-Perot interferometer with a free spectral range of 680 MHz, a linewidth of 25 kHz, and a finesse of 27500 has been built, and when substituted in place of the commercial interferometer produced a robust and easily repeatable beatnote linewidth of 700 MHz. >

Fiber bragg reflectors prepared by a single excimer pulse

Abstract: Narrow-line, permanent Bragg reflection gratings have been created in Ge-doped silica-core optical fibers by interfering beams of a single 20-ns pulse of KrF excimer laser light. Of the fibers studied, the highest reflectance value of ~2% was observed with a linewidth (FWHM) of 0.1 nm, which corresponds to a 2-mm grating length with an index modulation of ~3 x 10(-5).

Enhanced spectral power density and reduced linewidth at 1.3 μm in an InGaAsP quantum well resonant‐cavity light‐emitting diode

Abstract: The active region of an InGaAsP single‐quantum well light‐emitting diode (LED) emitting at 1.3 μm has been placed in the antinode of a resonant cavity consisting of a 32‐period distributed Bragg reflector (DBR) and a top silver mirror, with reflectivities of 92% and 95%, respectively. The dominant feature of the 300 K electroluminescence emission at all current levels is a 3 nm (2.8 meV) wide spontaneous emission peak centered on the cavity resonance wavelength. The spectral power density of the structure is more than one order of magnitude higher as compared to a structure without cavity. The resonant‐cavity LED operates without gain yet the extremely narrow spectrum indicates that the structure is suitable for wavelength division multiplexing applications.

Pulsed laser deposition of epitaxial BaFe12O19 thin films

Abstract: Epitaxial thin films of barium hexaferrite (BaFe12O19) have been fabricated by the pulsed laser deposition technique on basal plane sapphire. Structural studies reveal the films to be predominantly single phase and crystalline, with the c axis oriented perpendicular to the film plane. The magnetic parameters deduced from vibrating sample magnetometer and ferromagnetic resonance (FMR) measurements are close to the parameters associated with bulk materials. Post annealing of the films reduced the FMR linewidth by more than a factor of 3 so that it compares reasonably well with single‐crystal films. The derivative FMR linewidth was measured to be 66 Oe at 58 GHz and 54 Oe at 86 GHz. Spin‐wave‐like modes have been observed for the first time in barium ferrite films. The deduced exchange stiffness constant of 0.5×10−6 ergs/cm is in reasonable agreement with recent calculations.

Tunable DFB laser with a striped thin-film heater

Abstract: The authors propose a very simple tunable laser with a striped thin-film heater. The wavelength of this laser changes with a sensitivity of 3.2 nm/W to the input heating power. This laser can be continuously tuned over a range of 4 nm while maintaining an optical power of 20 mW and with a linewidth of less than 2.5 MHz. When this laser is mounted on a module, its 90% response time is 6 ms, which is fast enough for use as a local oscillator in several applications. >

Broad linewidth lasers for optical fiber communication systems

Abstract: Non-linear effects in an optical fiber used for communicating AM signals at high power levels are reduced by increasing the linewidth of the pump laser output signal. The linewidth can be increased by optically broadening the laser output signal by driving an optical angle modulator with broadband electrical noise. The optical signal is then externally modulated with an AM information signal for transmission over an optical link fiber. The optical modulation can be provided using either an FM or PM optical modulator. A desired linewidth can be provided by controlling the optical modulation index during the optical modulation step and/or by controlling the bandwidth of the noise source. Other techniques for broadening a laser output signal, including modulating the signal by a periodic function such as a sine wave, or injecting spontaneous emissions into the laser cavity, are also disclosed.

Continuous-wave infrared laser spectrometer based on difference frequency generation in AgGaS 2 for high-resolution spectroscopy

Abstract: A high-resolution cw spectrometer based on difference frequency generation (DFG) in a 20-mm-long AgGaS2 crystal pumped by two stabilized single-frequency cw dye–Ti:sapphire lasers is described. Experiments performed with a Rhodamine 6G and DCM dye laser combination pumped by a 20-W argon laser are reported. Informed radiation is generated from 7 to 9 μm with an ultranarrow linewidth (<0.5 MHz) and an output power 1 μW by making use of 90° type I phase matching. The performance of the DFG laser spectrometer is evaluated by using a portion of the ν2 band of NH3 near 1177 cm−1.

On the linewidth of microcavity lasers

Abstract: In a recent letter by G. P. Agrawal and G. R. Gray [Appl. Phys. Lett. 59, 399 (1991)] the frequency and phase noise of vertical‐cavity surface‐emitting microlasers is calculated using rate equations and Langevin noise sources. In this letter we re‐examine their calculations and get different results. Specifically, we predict that an increase of the spontaneous emission coupling coefficient will not necessarily lead to an increase in linewidth. It is also found that the reported ‘‘anomalous linewidth enhancement’’ is present in all lasers with a nonzero linewidth enhancement factor.

Chirping characteristic and frequency response of MQW optical intensity modulator

Abstract: The spectral linewidth enhancement factor and frequency responses of electro-absorption-type optical-intensity modulators, especially InGaAs/InAlAs MQW modulators, are described. A method of exactly estimating the value of the alpha factor is presented under the nonlinearity of extinction-ratio characteristics. For measuring the frequency response of modulators, the sideband strength of the modulated output light with an optical spectrum analyzer, is analytically compared with the microwave power of photodiode direct detection with an electrical spectrum analyzer. >

An improved delayed self-heterodyne interferometer for linewidth measurements

Abstract: The authors demonstrated a delayed self-heterodyne interferometer with a recirculating delay, in which loss was partially compensated by an erbium-doped fiber amplifier. A resolution limit of 606 Hz was achieved with an 11-km fiber delay line, as compared to 18.2 kHz for the standard single-pass case. The possible effect of spectral broadening due to amplifier noise is considered and found to have a negligible effect on the system performance. >

Proposal of a nanometer beam size monitor for e+e− linear colliders

Abstract: A new simple method to measure the transverse beam spot size in the nanometer range is proposed for a beam diagnostic system at the collision point in the future TeV region e+e− linear colliders. A high energy electron beam is injected at a rightangle into a photon target of intense laser light, and generates high energy γ-rays by Compton scattering. Since the laser light is stored inside a cavity resonator and forms a standing wave, the γ-ray flux shows periodic variation with the period of the optical wavelength by scanning the electron beam trajectory along the laser beam axis. From this modulation depth, the transverse size of the electron beam can be easily determined by a simple relation. Using the Nd:YAG laser at wavelength 1.06 μm or 532 nm, we can measure the beam spot size of 60 nm in the FFTB experiment [9] with sufficient accuracy. The required laser peak power is about 20 MW to generate 1000 γ-rays per bunch. If we employ a shorter wavelength laser, for example Ar2 excimer laser at the wavelength of 126 nm, ot will be possible to measure the beam size down to 5 nm, which is almost close to the required beam size in future linear colliders in the TeV region.

Quantum wire lasers

Abstract: Recent progress in the development of the concept and technology of semiconductor quantum wire (QWR) lasers is reviewed. In these quasi-one-dimensional structures, optical gain is provided by charge carriers that are quantum mechanically confined in two dimensions within wire-like active regions. These devices are expected to exhibit improved laser performance, including extremely low threshold currents (in the mu A range), higher modulation bandwidth, narrower spectral linewidth, and reduced temperature sensitivity. QWR lasers would thus be particularly useful in applications involving densely packed laser arrays and monolithic integration of lasers with low-power electronics, including computer optical interconnects, optical computing, and integrated optoelectronic circuits. Approaches for fabricating these novel structures are reviewed, and recent successful demonstrations of lasing in semiconductor QWRs are described. Prospects for further progress in this area are also discussed. >

Basic analysis of AR-coated, partly gain-coupled DFB lasers: the standing wave effect

Abstract: A theoretical analysis of distributed feedback (DFB) lasers with mixed gain and index coupling (partly gain-coupled DFB) is given for perfect antireflection (AR) coatings. Analytical expressions for the threshold gain, facet loss, and the relative depth of the standing wave pattern are derived. At the same time the importance of the standing wave effect and its consideration by coupled mode equations is shown. For purely gain-coupled DFB lasers, simple expressions for the effective linewidth enhancement factor and the longitudinal spontaneous emission factor are derived. In addition, various approximations describing the performance of purely gain-coupled DFB lasers are given. >

Stable isotope analysis using tunable diode laser spectroscopy

Abstract: Measurements of ratios of stable isotopes are used in such diverse fields as petroleum prospecting, medical diagnostics, and planetary exploration. The narrow emission linewidth available from tunable diode lasers permits high-resolution infrared absorption measurements of closely spaced isotopic rovibrationallines. Our dual beam spectrometer uses the sweep integration technique in a spectral region where adjacent spectral lines are of approximately equal absorbance at the expected isotopic abundances. The experimen­ tal results reported here indicate that isotopic ratios of carbon in carbon dioxide can be measured to an accuracy of better than 0.4%. This laser spectroscopic technique offers an alternative to the mass spectrometric technique for in situ isotopic analysis in field studies, as well as flight and space applications. 13 C/ 12 C, carbon dioxide, laser spectroscopy.

Fused-silica monolithic total-internal-reflection resonator.

Abstract: We have characterized a miniature fused-silica monolithic optical ring resonator in which the Gaussian mode is confined by total internal reflection. Laser light is coupled into and out of the resonator by frustrating one of the total internal reflections by means of a prism. The resonator can be overcoupled and undercoupled by varying the resonator–prism distance. The minimum measured resonator linewidth was less than 3 MHz. This type of broadband, stable, low-loss resonator has applications in linear and nonlinear optics.

Homogeneous linewidths of Rhodamine 6G at room temperature from cavity-enhanced spontaneous emission rates

Abstract: Fluorescence lifetimes of Rhodamine 6G in levitated micron‐sized droplets have been measured using a time‐correlated photon counting technique. The coupling of emission into spherical cavity modes of the droplet results in significant emission rate enhancements which allow estimation of the homogeneous linewidth at room temperature.

Reduction of effective linewidth enhancement factor alpha /sub eff/ of DFB lasers with complex coupling coefficients

Abstract: The effective linewidth enhancement factor alpha /sub eff/ of DFB lasers which have both gain and index coupling coefficients is theoretically analyzed. The unique reduction mechanism of alpha /sub eff/ due to the optical negative feedback of modal phase and modal gain in gain coupled DFB lasers was found for the first time. The numerical result showed that alpha /sub eff/ can be reduced to almost one half of the material defined linewidth enhancement factor alpha when there exists an index coupling coefficient comparable to the gain coupling coefficient. >

Spectral signature of relaxation oscillations in semiconductor lasers

Abstract: A novel and relatively simple expression is given for the optical spectrum of a single-mode semiconductor laser which, due to the presence of relaxation oscillations, consists of a strong central line with a broad weak sideband at each side. The coupling between phase and amplitude fluctuations is included in this derivation and is shown to result in an asymmetry between the relaxation oscillation sidebands. This asymmetry can be used to determine the linewidth enhancement factor. Using optical heterodyne detection, the spectrum of a Fabry-Perot-type AlGaAs laser has been measured as a function of output power. Information on the dynamics of the relaxation oscillations was thus obtained. The power dependence of the frequency and damping of the relaxation oscillations allowed the spontaneous lifetime and the dependence of the gain on both carrier density (differential gain) and intensity (gain saturation) to be separately determined. >

Measurement of the spatial coherence of a soft-x-ray laser.

Abstract: The spatial coherence of a neonlike selenium x-ray laser operating at 206 and 210 \AA{} has been measured using a technique based on partially coherent x-ray diffraction. The time-integrated spatial coherence of the selenium x-ray laser was determined to be equivalent to that of a quasimonochromatic spatially incoherent disk source whose diameter is comparable to the line focus of the visible-light laser pumping the x-ray laser. The spatial coherence was improved by narrowing the line focus width.

Density-dependent exciton radiative lifetimes in GaAs quantum wells

Abstract: Resonant photoexcitation of a predominantly homogeneously broadened exciton line in a GaAs quantum well results in a nonexponential time-resolved photoluminescence decay transient due to the decrease in the exciton-exciton scattering contribution to the homogeneous linewidth, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{h}}$, as the exciton population decays. This result confirms the theoretically predicted relationship between the exciton radiative recombination lifetime and the homogenous linewidth (and coherence area) of the exciton state.

Phase noise of four‐wave mixing in semiconductor lasers

Abstract: A simple theoretical analysis shows that the linewidth of the conjugate wave produced in four‐wave mixing in semiconductor lasers is equal to the linewidth of the probe plus four times the linewidth of the pump. Experimental results in good agreement with the theory are presented. This result implies an enormous enhancement in the phase noise of the conjugate wave and sets a limitation on some practical applications of four‐wave mixing.

Misalignment-tolerant, grating-tuned external-cavity laser with enhanced longitudinal mode selectivity

Abstract: A grating tuned external cavity laser having improved mode selectivity and stability. A cylindrical lens is positioned between a laser amplifier and a prism pair beam expander to focus the laser beam to form a linear spot on a grating. The location of the cylindrical lens between the laser amplifier and the prism pair minimizes the optical length of the laser cavity. In this way, the wavelength separation between optical resonance modes is increased, and the mode selectivity and stability of the system is enhanced.

Intensity and linewidth measurements in the 13.7 μm fundamental bands of 12C2H2 and 12C13CH2 at planetary atmospheric temperatures

Abstract: The absolute intensities and collision-broadened half-widths of several lines in the P-,Q-, and R-branches of the nu5-fundamental band of (C-12)2H2 have been measured at various temperatures between 147 and 295 K employing the Doppler-limited spectral resolution (about 10 exp -4/cm) of a tunable diode laser spectrometer. The absolute intensities of R(5), R(7), R(9), and R(20) in the same fundamental belonging to (C-12)(C-13)H2 have also been measured at 294 K. The temperature dependence of the collision-broadened half-width has been determined for some of the lines broadened by planetary atmospheric gases, namely, He, Ar, H2, and N2. Four self-broadened linewidths of (C-12)2H2, as well as three H2-broadened linewidths and a self-broadened half-width of (C-12)(C-13)H2, have also been retrieved from the measurements.

Linewidth requirements for optical synchronous detection systems with nonnegligible loop delay time

Abstract: The spectral spreads of quadrature-phase-shift-keying (QPSK) modulation and minimum shift keying (MSK) modulation are narrower than those of other kinds of modulation with the same bit rate. Therefore, they are attractive for systems limited by electrical bandwidth and available optical frequency. The synchronous demodulation offers better sensitivity than differential demodulation. However, the propagation delay time of the phase locked loop affects the performance of synchronous detection systems. Therefore, linewidth requirements while considering the loop delay time must be considered in designing these systems. The linewidth requirements for BPSK, QPSK, and MSK homodyne/heterodyne detection systems are obtained by applying the Pade approximation in evaluating the receivers' performance. A change in the power penalty with respect to the change in these values is also obtained. >