Deceleration of a calcium atomic beam with a frequency-doubled diode laser

A calcium atomic beam has been decelerated by a single extended-cavity diode laser, frequency doubled to 423 nm. A potassium niobate crystal is placed in an external power buildup cavity, and the second-harmonic laser beam, counterpropagating with the atomic beam, is tuned into resonance with the strong (1)S(0)-(1)P(1) transition of calcium. For input power of 78 mW at 846 nm, we generate 22 mW at 423 nm after correction for the reflectivity of our cavity output coupler. To keep the atoms always in resonance during the deceleration process, the Zeeman tuning technique was used.     

Optimization of a beam delivery system for a short-pulse KrF laser used for material ablation

We present an optical arrangement for spatial homogenization of an UV beam carrying a short pulse (500 fs) to be used for material ablation. Conventional cylindrical fly's eye lens homogenizers (CFELH's) introduce unwanted interference effects into a beam caused by the high spatial coherence of short pulses. To prevent the disturbing effect of these intensity modulations, one can couple a low-loss distributed delay device to the CFELH. With the new design an intensity nonuniformity of <+/-5% rms can be obtained. High-resolution images of the beam profile show complete removal of the interference modulation. The pulse duration after homogenization is 12.5 ps. We performed preliminary ablation experiments in polyimide samples both by direct irradiation and by mask imaging. Uniformity and edge quality of the results are more than satisfactory, and the undesirable structure caused by interference is completely removed.     

Wavelength switching in an actively mode-locked Fabry-Perot laser diode with a highly dispersive external cavity

We demonstrate a simple and reliable scheme for wavelength switching in an actively harmonic mode-locked Fabry-Perot laser diode (FPLD) that is coupled to a highly dispersive external fiber cavity. Wavelength switching between FPLD modes is achieved by detuning of the modulation frequency applied to the FPLD. A side-mode suppression ratio of as much as 40 dB is maintained, and the supermode noise suppression ratio is more than 50 dB for the whole wavelength switching operation range of 4.7 nm when the pulse repetition rate is ~2 GHz. The wavelength-switching mechanism of our laser is based on the wavelength-dependent cavity resonance frequency shift that is due to chromatic dispersion, and we verify that our experimental results closely coincide with analytical results.     

Synthetic-aperture-radar imaging with a solid-state laser

We report the operation of an imaging Nd:YAG microchip-laser synthetic-aperture radar, with which we imaged two-dimensional (2-D) models of military targets. The images obtained showed spatial resolution significantly better than the diffraction limit of the real aperture in the along-track dimension. The signal processing is described, and the measurement sensitivity is both predicted and verified. In addition, 2-D images with high resolution in both dimensions were generated by using an asymmetric aperture to match the along-track synthetic-aperture resolution with the across-track diffraction-limited resolution.     

Power collection of a laser diode elliptical beam by a decentered circular aperture

The elliptical beam of a laser diode is collected by a circular aperture decentered with respect to the beam. The fractional optical power collected is calculated and measured as a function of the decentered distance, beam size, and aperture size. The calculation results agree well with the measurement results. An application example of the results is described.     

Phase-shift-locked interferometer with a wavelength-modulated laser diode

A phase-shift-locked interferometer has been constructed for distance measurement. A phase shift produced by sawtooth-current modulation of a laser diode is locked to a phase difference preset by polarization optics that consists of a quarter-wave plate and polarizers through an electrical feedback technique. An optical path difference can be measured from the locked sawtooth-wave current amplitude in real time. The sensitivity of the interferometer is discussed.     

Flow-velocity measurements with a laser diode array

To measure flow velocity, a new technique, laser diode array velocimetry, generates multiple laser spots (four or more) separated by short distances (20-100 mum) at the probe volume. Particles that pass through the probe volume generate a signal that is analyzed by a digital signal processor for frequency content. The product of frequency and laser-to-laser spot separation determines the velocity. Rotating wire and wind-tunnel experiments confirm the performance of the new technique. An error analysis showed that the uncertainties in the processed diode array velocimetry signal frequency were less than 0.3%, and uncertainties in the beam-to-beam separation were less than +/-0.05%.     

Temperature field of the active element of a solid-state laser with a liquid cooling system

Results of a numerical solution of a nonstationary conjugate convective heattransfer problem are presented. Temperature fields are determined in the coolant stream and in the body being cooled.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 3, pp. 520–526, September, 1980.     

Integrated atom density measurement in a zirconium atomic beam generated using a high-power strip electron beam source by diode laser absorption technique

Strip electron guns, capable of generating two-dimensional (length?width) evaporating sources, are used for the application of electron beam assisted physical vapour deposition to metallurgical coating of large substrates. The collisional relaxation processes in the atomic vapour generated using such a 2D-source differ significantly from those in a point source created using axisymmetric electron gun. In order to study the influence of collision processes, an atomic beam of zirconium was generated using a 130- mm-strip electron gun and its ground electronic state population was measured in a given optical path length using a tunable diode laser based on-line absorption spectroscopic technique. These measurements were carried out as a function of vertical height from the source (114 mm×6 mm) and for different source temperatures obtained by varying electron beam powers in the range of 40-91 kW.While the experimental values of integrated atom number density were observed to match very closely with those given by reported vapour flow model at low evaporating source temperatures, they showed significant deviation at high temperatures. The results are interpreted in terms of the collisional processes in the vapour stream and indicative of the fact that the vapour expansion from 2D source never runs out of collisions.     

半导体激光器在实时测温系统中的应用

该论文介绍了一种利用半导体激光器InGaAs I作光源 ,使用钽酸锂热释电探测器作光接收器件 ,以单片机为核心实现的中、高温度实时测量系统。该系统主要由光发射与接收系统、信号放大与处理系统及显示系统三部分组成。     

A compact extended-cavity diode laser with a Littman configuration

We have constructed a compact extended-cavity diode laser (ECDL) that is based on a Littman configuration with a grating and a reflector. The whole structure is installed in a 2-inch kinematic mount. ECDLs operating at 852 nm (Cs D/sub 2/ line), 894 nm (Cs D/sub 1/ line), 780 nm (Rb D/sub 2/ line), and 794 nm (Rb D/sub 1/ line) were fabricated and tested. As a result of the performance test, up to 9 GHz continuous tuning without mode hopping could be obtained by tuning with a piezoelectric transducer only. The linewidth from the beat-note spectrum of two ECDLs was about 200 kHz.     

Dye photodestruction in a solid-state dye laser with a polymeric gain medium

The process of dye photodestruction in a solid-state dye laser is studied, and implemented is a polymeric gain medium doped with a strongly concentrated dye. The behavior of the conversion efficiency in the polymeric gain medium pumped with different laser-pulse repetition rates and the process of dye photobleaching are analyzed. The contribution of the heating of the host material into the dye molecules' deactivation is discussed. The negative effect of high dye concentration on the dye stability under a high pump repetition rate is reported and analyzed for the first time to my knowledge. A comparison of the present results with recently published data demonstrates the major role of photodestruction, rather than direct thermodestruction, in the dye stability of the solid-state gain medium. The role of additives with low molecular weights in the polymeric matrix, for increasing the stability of the gain material, is discussed.     

Correlation-based speckle velocimeter with self-mixing interference in a semiconductor laser diode

We propose a novel self-mixing laser diode speckle velocimeter based on autocorrelation. The self-mixing laser diode launches and receives reflected light from a moving surface that has a random reflection profile. Some portion of the scattered light backcouples into the laser cavity and causes random intensity variations in the form of speckle signals. These speckle signals obtained from a self-mixing laser diode are processed by use of autocorrelation. The linear relation between the velocity and the reciprocal of the correlation time of the speckle intensity fluctuations allows us to determine the velocity easily if proper calibration is performed. The range of the investigated velocity is 20-450 mm/s. For an aluminum target that moves at a velocity of 350 mm/s, the measurement error is less than 2%.     

Linewidth reduction in a large-smile laser diode array

We present theory and simulations for a spectral narrowing scheme for laser diode arrays (LDAs) that employs optical feedback from a diffraction grating. We calculate the effect of the so-called smile of the LDA and show that it is possible to reduce the effect by using a cylindrical lens set at an angle to the beam. The scheme is implemented on a 19-element LDA with smile of 7.6 microm and yields frequency narrowing from a free-running width of 2 to 0.15 nm. The experimental results are in good agreement with the theory.     

Orthogonal linear polarization tunable-beat ring laser with a superluminescent diode

An orthogonal linear polarization operated ring laser with a superluminescent diode has been demonstrated to generate a tunable optical beat signal. The ring cavity contains a superluminescent diode as the optical gain medium, Faraday rotators, and a variable phase retarder (Babinet-Soleil compensator). By controlling the retarder, we changed the beat frequency in the range from a few tens of megahertz to 100 MHz.     

Real-time measurements of spatial velocity distribution with a laser Doppler imaging system

A new laser Doppler imaging system with a TV camera has been constructed, which brightly displays 1-D velocity distribution on a monitor. Some characteristics of this system have been experimentally investigated from measurements of simple velocity distribution on a constantly rotating ground glass disk. From an adaptation to fluid flow, it has been shown that the measurements of spatial velocity distribution can be achieved in real time.     

Development of an intracavity-summed multiple-wavelength Nd:YAG laser for a rugged, solid-state sodium lidar system

A single-cavity solid-state laser that is resonant with sodium D(2) absorption is reported. Simultaneous 1.06- and 1.32-μm emission from a Q-switched Nd:YAG laser is summed with an intracavity type II KTP crystal. A single-intracavity étalon is sufficient to provide a time-averaged linewidth of 1.7 GHz at 589 nm.     

Thermal effects in a hollow waveguide beam launch for CO(2) laser power delivery

Thermal effects caused by launching conditions in a CO(2) laser beam delivery that uses metallic hollow waveguides is investigated. It is found that front-end clipping is the main cause of thermal loading and generates a steep temperature gradient at the fiber front end while the continuous beam attenuation produces a temperature distribution declining slowly along the waveguide.     

Revising the formula of thermal focal length in a side-pumped laser rod by experiments

The correct formula of the thermal focal length in a side-pumped Nd:YAG laser rod is discussed and confirmed by experimental results. It is shown that thermally induced stresses that cause a distortion of flatness occur within the region of the whole rod. The presented calculations are in agreement with the experimental observations. The results reveal that the temperature-dependent variation of the refractive index and the distortion caused by the thermally induced stresses constitute the major contributions to thermal lensing.     

Optical levitation particle delivery system for a dual beam fiber optic trap

We combine a radiation-pressure-based levitation system with a dual fiber, laser trapping system to demonstrate the potential of delivering single particles into the fiber trap. The forces versus position and the trajectory of the particle subjected to the laser beams are examined with an enhanced ray optics model. A sequence of video images taken from the experimental apparatus demonstrates the principle of particle delivery, trapping, and further manipulation.