Showing papers on "Laser Doppler vibrometer published in 1991"

Optical measurement of the axial eye length by laser Doppler interferometry.

Abstract: A new technique has been developed to determine the axial length of the human eye in vivo. Based on laser interferometry in conjunction with the Doppler technique, it uses partially coherent light. This new technique complies with laser safety regulations. High accuracy is achieved, the optical length (OL) can be determined within +/- 30 microns, and the reproducibility of the geometric eye length is greater than +/- 25 microns. Possible errors are discussed. First comparisons with the ultrasound technique yield good agreement for emmetropic subjects and for subjects with a myopia of up to 10 diopters. The advantages of the laser doppler interferometry (LDI) technique are high accuracy, high transversal resolution, and more comfort for the patient (it is a noncontact method; no anesthesia is needed). Possible future applications of LDI, like measurements of fundus profiles and of retinal thickness, are mentioned.

Apparatus for direct display of an image on the retina of the eye using a scanning laser

Abstract: A direct viewing picture image display apparatus displays a good picture image with a small amount of light and low power consumption using a laser beam without producing speckle noises. The display apparatus comprises a light source for emitting a laser beam, an optical modulating system for optically modulating the laser beam from the light source in response to a video signal, a scanning system for horizontally and vertically scanning the modulated laser beam from the scanning signal in response to a scanning signal, and a projecting optical system for projecting the scanned laser beam from the scanning system to form an image of the laser beam on the retina of an eye of an observer. Depth information is provided to the laser beam at a suitable stage after optical modulation by the optical modulating system so that a stereoscopic image is displayed.

Broadband optical detection of transient motion from a scattering surface by two-wave mixing in a photorefractive crystal

Abstract: A method and apparatus for optically detecting transient motion from a scattering surface. A laser beam having a predetermined frequency is directed onto such a surface to thereby scatter the laser beam and produce a scattered laser beam defining an optical wavefront and having an optical spectrum with a central peak at the laser frequency and a sideband on either side of the central peak. The scattered laser beam is caused to interfere inside a photorefractive crystal with a pump beam derived from the laser beam so as to form an index of refraction grating diffracting the pump beam into a reference beam having an optical wavefront substantially matching the wavefront of the scattered laser beam and an optical spectrum with a single peak at the laser frequency and no sidebands, whereby to produce at an exit face of the photorefractive crystal a transmitted scattered laser beam and a diffracted reference beam interfering with one another. The interfering beams at the exit face of the photorefractive crystal, or polarization components thereof, are at least temporarily out of phase, thereby obtaining an optical signal sensitive to phase perturbations produced by the transient motion, which optical signal is detected and converted into an electrical signal representative of the transient motion. The invention is particularly useful for detecting small surface formations or displacement of a material subjected to ultrasonic energy, enabling displacements ranging from a fraction of 1Å to a few hundred Å to be detected with a large etendue or light gathering efficiency and a broad frequency bandwidth extending from frequencies as low as 1 KHz, and even below.

Variable pulse width laser and method of use

Abstract: A laser for medical includes an optically pumped laser medium, and the output of the laser is directed through an optical fiber delivery system to a tissue target. The flashlamp is driven by a pulsed power signal that is selectively variable to produce laser pulses of predetermined temporal width and pulse energy, and these values can be selected in accordance with the type of tissue being treated and the tissue effect desired. The laser may be operated to produce relatively brief pulses of high energy, which create a localized plasma at the surface of the target tissue. The plasma effect blocks any significant penetration of the laser beam into the tissue, and each laser pulse causes the ablation of a small portion of the tissue target. Thus thermal necrosis of adjacent and underlying tissue minimized. At the opposite extreme, the laser may be operated to produce relatively long pulses of low or moderate energy, so that the laser beam penetrates the tissue to create such effects as deep coagulation, deep thermal heating and necrosis, and the like. In a further aspect, the laser is provided with an automatic system to control the temporal width of the laser pulses. A photodetector apparatus is optimized to detect the optical radiation from the plasma created at the tissue target and transmitted retrograde through the optical fiber delivery system to the laser. The pulse generating circuitry of the laser terminates the laser pulse in response to the plasma signal. The laser produces tissue effects comparable to a wide range of prior art medical lasers.

Laser beam color image display apparatus

Abstract: A laser beam color image display apparatus includes a single laser beam source for emitting a laser beam from which a plurality of blue, green, and red laser beams are separated, or a plurality of laser beam sources for emitting respective blue, green, and red laser beams. The red laser beam is generated by adding a laser beam having a wavelength of 568.2 nm and a laser beam having a wavelength of 647.1 nm by way of active mixture. The laser beams are modulated in intensity with color signal components, and then deflected by a light scanner, composed of a polygon mirror and a galvanometer mirror to scan a display screen for thereby displaying a color image thereon. The laser beam having the wavelength of 568.2 nm and the laser beam having the wavelength of 647.1 nm are added at an output power ratio of 1:20, with the resultant red laser beam having a wavelength of 612 nm. The single laser beam source may comprise an argon-krypton mixed gas laser, or the laser beam source for emitting the red laser beam may comprise a krypton gas laser.

Laser detection of sound

Abstract: A differential laser Doppler velocimeter (LDV) has been assembled and tested to provide noninvasive absolute measurements of acoustic particle displacements of standing waves generated in a water‐filled tube. The principle of the technique [see K. J. Taylor, J. Acoust. Soc. Am. 59, 691–694 (1976)] is to measure the Doppler shift of laser light scattered from colloidal microparticles oscillating under the action of an acoustic field. The system tested is capable of detecting particle displacements of the order of a few nanometers with a bandwidth of several kilohertz. The performances and limitations of the system are discussed. In particular, the effect of Brownian motion is shown to produce only negligible broadening of the spectral density of the signal of interest. The sensitivity of the present LDV system is estimated to be very close to the shot noise limit of the photomultiplier tube used to detect the Doppler shift of the scattered light. Experimental results are obtained under controlled laborator...

Velocity control and cooling of an atomic beam using a modeless laser

Abstract: A newly developed continuous wave broadband laser which lacks mode structure in the frequency domain is employed together with a single mode laser beam to decelerate and cool metastable neon atoms. The velocity of the atoms in the beam can be set to a desired velocity by tuning the frequency of the single mode laser.

Long-range, noncoherent laser Doppler velocimeter.

Abstract: An experimental demonstration of a long-range, noncoherent laser Doppler velocimeter (LDV) is presented. The LDV detects incoming Doppler-shifted signal photons by using the sharp spectral absorption features in atomic or molecular vapors. The edge of the absorption feature is used to convert changes in frequency to large changes in transmission. Preliminary measurements of wind velocity using seeded aerosols showed that the LDV results agreed with mechanical anemometer measurements to within the accuracy of the LDV measurements. With optimization the LDV will provide accurate range-resolved and vibration-tolerant wind-speed measurements at large distances.

Near-IR retinal laser Doppler velocimetry and flowmetry: new delivery and detection techniques

Abstract: In laser Doppler velocimetry and flowmetry of the human ocular fundus, near infrared diode lasers, coupled with avalanche photodiode detectors, offer much improved signal-to-noise ratio compared to He–Ne laser delivery and photomultiplier tube detection. The increased signal-to-noise ratio is achieved through increased retinal irradiance and better quantum efficiency of the avalanche photodiode in the near infrared. Furthermore, the small size of the diode lasers allows delivery of two individually steerable beams for simultaneous measurements in different parts of the fundus. Applications are given as illustrations of the capabilities of the system: retinal laser Doppler velocimetry in humans without need for pupil dilatation and optic nerve head laser Doppler flowmetry in the cat using flicker as a stimulus to induce blood flow changes.

Edge technique for measurement of laser frequency shifts including the doppler shift

Abstract: A method for determining the frequency shift in a laser system by transmitting an outgoing laser beam, receiving an incoming laser beam having a frequency shift, acquiring a first signal by transmitting a portion of the incoming laser beam to an energy monitor detector and a second signal by transmitting a portion of the incoming laser beam through an "edge" filter to an edge detector, deriving a first normalized signal which is proportional to the transmission of the "edge" filter at the frequency of the incoming laser beam, deriving a second normalized signal which is proportional to the transmission of the "edge" filter at the frequency of the outgoing laser beam, and determining the frequency shift by processing the first and second normalized signals.

Signal Processing Considerations for Laser Doppler and Phase Doppler Applications

Abstract: The relative performance of the current methods for frequency and phase measurements is analyzed in this paper. These methods include the time domain processors (namely the counter and the covariance processors) and the frequency domain processor using the Fourier transform (for the actual signal or for its correlation function). It can be shown that the Fourier transform method gives the optimum frequency and phase estimation in terms of the Maximum Likelihood criteria (ML). The quantization effect is also analyzed and it is shown that 1 bit quantization provides sufficient accuracy for laser Doppler velocimeter applications and, in general, for any application where a single tone parameter estimation is of interest.

Dynamic control of laser energy output

Abstract: A laser ophthalmic surgery apparatus includes an improved control for assuring that the energy output level of the beam delivered to the eye remains within a prescribed tolerance. An energy output sensor (42) receives a small fraction of the laser beam's energy through a beam splitter (36). The sensor is located downstream of all laser pulse shaping parameters associated with the laser cavity (42). During operation of the laser the energy output sensor (42) constantly monitors laser energy as delivered to the target, and in preferred embodiment it monitors pulse-to-pulse energy deviations and calculates a moving average of the pulse energy over a relatively small number of shots. A laser beam light attenuator (22, 20) upstream of the sensor varies the intensity of the laser beam passing through, under the control of a microprocessor (46) which adjust the beam light attenuator in accordance therewith, making compensating adjustements after each shot.

Laser device with monolithically integrated frequency changer

Abstract: Disclosed is a solid state rod laser, monolithically integrated with a generator of optic frequencoes different from the emission frequency of the laser. The laser diode pumped solid state laser device has a rod, the seat of the laser effect, made of a nonlinear optic material, by doping with a rare earth, and includes an optic waveguide made in the mass of said rod of doped nonlinear optic material, parallel to the axis of the rod, said optic waveguide cooperating with means for changing the optic frequency of the laser beam. A preferred application is the obtaining of a laser frequency that is double the fundamental frequency making it possible, in the case of a neodymium laser, to obtain a green radiation, for example in digital optic disk readers.

Laser beam generation control system for optical bar code scanner

Abstract: A laser beam generation control system of an optical mark reader device including a laser generating element, a laser generation control unit, a laser scanner unit for scanning a mark, such as a bar code or the like, and a signal processor unit for photoconverting a reflected light to read thereof; wherein the laser beam generation control unit turns ON/OFF the laser generation element by a predetermined duty to carry out a generation of the laser intermittently. Further, when a reflected light from the mark is detected during an intermittent generation of the laser beam, the laser generation element is turned ON to generate a laser beam continuously; and when a reflected light from the mark is not detected for more than a predetermined first time period during a continuous generation of a laser beam, the system is switched from continuous generation to intermittent generation.

Frequency stabilized Ho:YAG laser

Abstract: A method and apparatus is presented whereby control of the frequency of pulses emitted by a resonant ring slave laser is forced to a single frequency defined by a seed laser through the control of the pathlength of the ring laser resonant path. By monitoring the time difference between turn on of a Q-switch within the laser resonant cavity and the time of occurrence of the output pulse of the slave laser, and effecting changes to the ring laser resonant path in a manner to minimize the measured time difference, the frequency of the output pulse from the slave laser is driven to a single frequency defined by the seed laser resonator.

Laser machining apparatus and method of the same

Abstract: A laser machining apparatus includes a laser beam source, such as of excimer laser, which produces a laser beam to be projected on a work piece or a sample, first and second illumination light sources which have wavelengths substantially equal to the wavelength of the laser beam and illuminate the entire image and the laser beam, respectively, a first beam splitter which guides the image produced by the illumination light to an observation unit, a second beam splitter which guides the laser beam from the laser beam source to an objective lens, and a controller which controls the machining condition including the relative positioning between the sample and the laser beam depending on the result of observation. The laser beam guide path structure from the laser beam source to the sample has its interior wall made of laser-transparent material such as glass, and the transparent material is enclosed by a laser blocking material such as a metal or water.

Laser-generation of narrow-band surface waves

Abstract: One of the factors limiting the application of laser-based ultrasonics for nondestructive evaluation, is the relatively low efficiency of ultrasound generation by laser illumination. The authors describe a method for improved narrowband surface wave generation by the use of a line focused laser and an array technique. A tone-burst-like surface wave has been generated by a diffraction grating, and a lens system. The system provides controllable center frequency by adjustment of the width and spacing of the lines of laser illumination in the array. It is demonstrated by the use of laser interferometer detection that the signal-to-noise ratio is significantly improved. >

Apparatus and method for detecting wind direction

Abstract: A laser radar system for measuring wind velocities including apparatus for permitting the system to measure the direction of the atmospheric flow field. A modulator is used to chirp the laser frequency toward increasing frequencies. The local oscillator frequency available for homodyning will be offset from the initial transmitted frequency by an amount equal to the chirp rate multiplied by the round trip transit time to the aerosols at the focal point of the converging laser beam. During the up-ramped chirps of the optical beam frequency, the Doppler shift frequency of aerosols moving toward the laser beam will be translated to a lower frequency, and the Doppler shift frequency of aerosols moving away from the laser beam will be frequency translated upward.

Phase shift device and laser apparatus utilizing the same

Abstract: A phase shift device for a laser beam has a circular first area transparent so the laser beam and a second area provided around the first area and transparent to the laser beam. The first and second areas generate a phase difference for example 180° in the passing light beams. The phase shift device is positioned for example at a beam waist of a laser scanning apparatus to form a beam spot of a diameter smaller than that defined by the numerical aperture of the imaging optical system. Structure of a laser apparatus employing such phase shift device is also disclosed.

Gap measuring device and method using frustrated internal reflection

Abstract: A narrow gap or unevenness of a surface of a specimen is measured by utilizing the tunnel effect of a light wave reflected at a boundary plane on the condition of total reflection. A laser beam emitted from a laser source is reflected at a surface of a prism on the condition of total reflection in terms of geometrical optics. If a gap between the surface of the prism and the specimen is about the wavelength of the laser beam, part of the laser beam is transmitted into the specimen, and the intensity of the transmitted beam depends on the gap width. A portion of the laser beam is reflected at the boundary plane back into the prism. Therefore, the gap can be measured by measuring the transmittance of the laser beam and comparing the same with the calculated relation between the transmittance and the gap calculated in advance. In practice, the gap width is determined by measuring intensity of the reflected laser beam in the presence of the gap and comparing it to the intensity of the reflected laser beam in the absence of the gap, i.e, when the specimen surface is more than several wavelengths away from the reflecting surface.

Frequency stabilization of a diode-laser-pumped microchip Nd:YAG laser at 1.3 μm

Abstract: We report the active frequency stabilization of a diode-pumped microchip Nd:YAG laser using a frequency-modulation sideband locking technique. The novel aspect of this scheme is that the piezo-mounted output coupler acts as both the frequency modulator and the frequency-correcting element. When the laser is locked to a reference cavity of 300-MHz free spectral range and 5-MHz linewidth, the laser frequency stability is found to be 88 kHz relative to the cavity.

Laser apparatus for measuring the velocity of a fluid

Abstract: In a laser apparatus for measuring the velocity of a fluid, a measurement laser light beam (M) fed into the fluid and scattered by a particle within the fluid is made to interfere with a reference laser light beam (R) to generate an interference signal based on the velocity of the particle; the apparatus comprises a low-coherence laser source (10) and interferometric means (11, 12) which split the light beam of the laser source (10) into the stated reference light beam (R) and measurement light beam (M), and cause the reference light beam (R) derived from the laser source (10) to interfere with the backscattered component resulting from the scattering of the measurement light beam (M)

Measurements of slider-disk contacts during dynamic load-unload

Abstract: The laser Doppler vibrometer (LDV) and acoustic emission (AE) have been used to investigate the slider dynamics and to detect possible slider-disk contacts during dynamic load-unload. One of the puzzling aspects of previous work has been the AE indication of contacts at an LDV measured spacing of about 8 mu m. The procedure of LDV spacing measurement for load-unload is investigated, and three sources of possible error are found and verified by experiment. First is the deviation between single shot and average measurements. Second is the disk motion that occurs during loading. Third is the elevation difference at the measured point and the closest point during loading, which depends on the unloaded pitch and roll angles. It is found that the jump in the velocity signal of single shot LDV measurement gives a good indicator of contacts. The electrical resistance method is applied to the detection of contacts during dynamic load-unload and compared with the results obtained by the LDV and AE. Its contact signal is in good agreement with the jump in the LDV velocity signal and more distinguishable than the AE signal. >

Multi-velocity component LDV

Abstract: A laser doppler velocimeter uses frequency shifting of a laser beam to provide signal information for each velocity component. A composite electrical signal generated by a light detector is digitized and a processor produces a discrete Fourier transform based on the digitized electrical signal. The transform includes two peak frequencies corresponding to the two velocity components.

Modulation of oxygen-iodine laser

Abstract: A method of modulating an oxygen-iodine laser comprises applying an external magnetic field having an intensity of up to 800 A.cm -1 to the active zone of the laser and, at the same time, changing the magnetic field intensity to change the output power of the laser.

Frequency locking a dye laser to the central optical Ramsey fringe in a Ca atomic beam and wavelength measurement

Abstract: The frequency of a dye laser is stabilized to the 3P1–1S0 intercombination line of 40Ca through use of the first-derivative signal of the Ramsey fringes detected in a Ca atomic beam. The long-term stability of the frequency of the Ca-stabilized dye laser is estimated to be a few kilohertz from the servo error signal. The wavelength of the Ca-stabilized dye-laser beam is measured by comparison with that for an I2-stabilized He–Ne laser, which is one of the current wavelength standards. The measured wavelength is evaluated to be 657.4594396 nm with an uncertainty of better than 1 × 10−9.

Controller for output of laser beam machine

Abstract: PURPOSE: To enable a working with the optimum laser intensity by calculating a laser intensity data uniformizing the irradiating energy density of laser beam for the work and sending the data to an output adjusting part of a laser oscillator. CONSTITUTION: The operation of a machine main body 1 is commanded and controlled by a numerical controller 2. The work is irradiated with laser beam by the laser oscillator and the intensity of the laser beam is adjusted at an output adjusting part according to the intensity data of the laser beam. Actual speed data showing the actual speed of a machine main body 1 is input by a sequencer 3 from an numerical controller 2, laser strength data always to uniformize the irradiating energy density per unit time of laser beam to the work is calculated based on this actual speed, laser intensity data is sent to an output adjusting part of the laser oscillator 4. In this way, the laser output can automatically be controlled according to the moving speed of the laser beam. COPYRIGHT: (C)1993,JPO&Japio

Coordinate measuring machine with laser interferometer and fiber optic laser delivery system

Abstract: A coordinate measuring machine (10) having laser interferometers (80, 176, 196) arranged to measure "X" and "Y" carriage and probe motions, in which fiber optic cables (66,72) are used to deliver a laser beam from a laser beam generator (60) to at least some of the interferometers (2176, 196) in order to simplify the optical system and avoid the need for precision alignment of optical components in directing the beam from the laser generator (60) to the various interferometers (176, 196). The laser generator (60) is mounted either on or off the machine base (12). A special clamp (76) and adjustment flexure (74) is used to mount and align the fiber optic cable couplings (96) to beam splitters (64) dividing the laser beam from the laser generator (60) for use along each axis.