Laser Interference Fringes Research Articles

Measurement of Shock Waves Using an Acousto-Optic Laser Deflector

Laser interference fringe patterns and shadowgraphs were obtained with a minimum exposure time of 200 ns using an acousto-optic laser deflector. Experimental results showed that this method has sufficient spatial and temporal resolution to image the wavefront of shock waves generated by laser-induced breakdown in air. The shock waves propagated at a speed of over 103 m/s immediately after breakdown, and at a nearly constant speed of about 510 m/s for 3-9 μs after breakdown. The width of the wavefront of the shock wave was measured to be 0.17 mm, and the neutral density variation at the wavefront was estimated to be 9×1024 m-3.

Relationship between visual field loss and contrast threshold elevation in glaucoma.

BackgroundThere is a considerable body of literature which indicates that contrast thresholds for the detection of sinusoidal grating patterns are abnormally high in glaucoma, though just how these elevations are related to the location of visual field loss remains unknown. Our aim, therefore, has been to determine the relationship between contrast threshold elevation and visual field loss in corresponding regions of the peripheral visual field in glaucoma patients.MethodsContrast thresholds were measured in arcuate regions of the superior, inferior, nasal and temporal visual field in response to laser interference fringes presented in the Maxwellian view. The display consisted of vertical green stationary laser interference fringes of spatial frequency 1.0 c deg-1 which appeared in a rotatable viewing area in the form of a truncated quadrant extending from 10 to 20° from fixation which was marked with a central fixation light. Results were obtained from 36 normal control subjects in order to provide a normal reference for 21 glaucoma patients and 5 OHT (ocular hypertensive) patients for whom full clinical data, including Friedmann visual fields, had been obtained.ResultsAbnormally high contrast thresholds were identified in 20 out of 21 glaucoma patients and in 2 out of 5 OHT patients when compared with the 95% upper prediction limit for normal values from one eye of the 36 normal age-matched control subjects. Additionally, inter-ocular differences in contrast threshold were also abnormally high in 18 out of 20 glaucoma patients who had vision in both eyes compared with the 95% upper prediction limit. Correspondence between abnormally high contrast thresholds and visual field loss in the truncated quadrants was significant in 5 patients, borderline in 4 patients and absent in 9 patients.ConclusionWhile the glaucoma patients tested in our study invariably had abnormally high contrast thresholds in one or more of the truncated quadrants in at least one eye, reasonable correspondence with the location of the visual field loss only occurred in half the patients studied. Hence, while contrast threshold elevations are indicative of glaucomatous damage to vision, they are providing a different assessment of visual function from conventional visual field tests.

Effect of myopia on visual acuity measured with laser interference fringes

The aim was to determine how visual acuity is affected by myopia when optical factors of the eye are controlled. Grating acuity was measured with interference fringes to avoid the effects of aberrations, and ocular biometry was used to compensate for differences in retinal image size among subjects. Distance spectacle refractions ranged from +2.25 to −14.75 D. The retinal magnification factor (RMF) in mm/deg was computed for each eye from the distance refraction, central corneal power and ultrasound biometry. A forced-choice orientation discrimination method was used to measure acuity for high-contrast 543 nm laser interference fringes in three retinal locations: the fovea, and at 4 deg and 10 deg eccentricity in the temporal retina. Acuity, expressed in c/deg and adjusted for spectacle magnification, was not significantly correlated with refraction at any of the three retinal locations. When acuity was converted to retinal spatial frequency units (c/mm) via the RMF, acuity decreased with increasing myopia at all three retinal locations (significantly at the fovea and at 10 deg eccentricity). Retinal acuity values in highly myopic subjects (>6 D) are consistent with retinal sampling distances that are larger than published values of human cone or ganglion cell spacing. The results imply that a highly myopic eye has retinal neurons that are more widely spaced than normal, but the increased axial length enlarges the retinal image enough to compensate for the retinal stretching. The data are consistent with a retinal stretching model that primarily affects the posterior pole.

Direct grating writing using femtosecond laser interference fringes formed at the focal point

Af emtosecond pulse laser has been used for the machining of gratings primarily due to its ability of direct ablation and its capability to fabricate sub-wavelength structures. In this paper, we present a direct ablation technique for the fabrication of gratings by interfering femtosecond pulses in a specially designed optical configuration. This technique ensures that gratings are formed only at the focal point. The grating line-width can be varied with minor adjustment to the optical configuration. This configuration not only simplifies the optical set-up, but also immunizes the system to extraneous and inherent vibrations, thus enabling gratings of good edge acuity. With this technique, we have successfully fabricated planar gratings of different line-widths on a silicon substrate. Methods of adjusting the grating period are demonstrated. The effects of pulse number and the laser threshold on the grating quality have been qualitatively studied using SEM analysis. This method offers a new approach of the fabrication of a grating on a surface by direct ablation. Also, the feasibility of effectively writing internal gratings in transparent materials is discussed.

Development of a high‐speed laser interferometer using an acousto‐optic deflector

AbstractAn acousto‐optic laser deflector was used as a shutter for high‐speed imaging of laser interference fringes using an ordinary CCD camera. The exposure duration was set by the pulse width of the high‐frequency signal applied to the acousto‐optic deflector. Changes in laser interference fringes due to an impulse discharge in air were obtained at an exposure duration of 4 µs. By applying a sequence of high‐frequency signals with different frequency, the beam was deflected to four different angles at different times, allowing four interference images to be captured on a single video frame. This was used for time‐resolved imaging of the interference fringe pattern. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(2): 76–83, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20011

Application of an Acousto-optic Laser Deflector to Interferometric Measurement of Discharges in Air

An acousto-optic laser deflector was applied to laser interferometric measurement of impulse discharges in air as a high-speed shutter. Laser interference fringes were obtained at exposure times of about 1 μs. The deflector was also used for time-resolved imaging of laser interference fringes by using a sequence of high frequency signals with different frequencies to deflect the laser beam to different angles at different times, allowing multiple interference images to be captured on a single video frame. Changes in laser interference fringes due to an impulse discharge in air showed electron densities of about 2x1024 m-3 and perturbation in the neutral density expanding at speeds of about 103 m/s.

Development of a High-Speed Laser Interferometer Using an Acousto-Optic Deflector

An acousto-optic laser deflector was used as a shutter for high-speed imaging of laser interference fringes using an ordinary CCD camera. The exposure duration was set by the pulse width of the high-frequency signal applied to the acousto-optic deflector. Changes in laser interference fringes due to an impulse discharge in air were obtained at an exposure duration of 4 µs. By applying a sequence of high-frequency signals with different frequency, the beam was deflected to four different angles at different times, allowing four interference images to be captured on a single video frame. This was used for time-resolved imaging of the interference fringe pattern. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(2): 76–83, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20011

Acoustic Fields of Bulk Acoustic Waves Excited by Phase Velocity Scanning of Laser Interference Fringes

Directivity and generation efficiency of transverse and longitudinal bulk acoustic waves (BAWs) excited by phase velocity scanning (PVS) of laser interference fringes were theoretically investigated. Formulas were derived for the BAW generation based on scanning interference fringes (SIF) which were produced by intersecting two coherent laser beams with different frequencies on an opaque specimen. It was deduced that the amplitude of the BAW was proportional to the product of the scanning length of the SIF and the directivity pattern due to the thermoelastic line source. Unidirectionality of the BAW generated by the SIF was theoretically verified in an ideal state.

Retinal contrast losses and visual resolution with obliquely incident light.

To determine whether vision with obliquely incident light is degraded by contrast losses originating in the retina, laser interference fringe patterns were produced on the retina for various directions of incidence of the two interfering beams. Contrast-modulation flicker [Vision Res. 38, 985 (1998)] was used as a psychophysical measure of contrast at the level of the photoreceptors. Fringe contrast was shown to be maximal when the interfering beams were equal in perceived brightness, not in physical intensity. The effective fringe contrast was slightly reduced with oblique incidence for high spatial frequencies, but the reduction was too slight to be an important factor in visual resolution. The loss was similar whether the incident beams were displaced from the pupil center in a direction parallel or perpendicular to the grating bars.

Diffraction efficiency increase by corona discharge in photoinduced surface-relief gratings on an azo polymer film

The diffraction efficiency of a surface-relief grating (SRG) was dramatically increased by corona discharge. We fabricated this SRG by applying surface modulation to azo polymer films, using laser interference fringes. The electric charge was deposited upon the SRG by a corona discharge in an oven. The first-order diffraction efficiency measured before and after corona discharge above the glass-transition temperature increased from ~2% to ~40% in a SRG written with a circularly polarized beam. The relief depth of the SRG as measured with an atomic-force microscope also increased, from ~130 nm to ~450 nm. The increase in diffraction efficiency was independent of the writing polarization.

Detection of Defects in Micro-Machine Elements by Using Acoustic Waves Generated by Phase Velocity Scanning of Laser Interference Fringes

High aspect ratio microstructuring is a key process for fabricating microelectromechanical systems (MEMSs). Many microfabrication methods have been developed. Among them, inductively coupled plasma (ICP) source etching is especially attractive because it can fabricate high aspect ratio microstructures, at high speeds with a dry etching process. However, in such microstructures, it is difficult to nondestructively evaluate the quality of high aspect ratio etched bottoms. Therefore, we developed a technique for detecting defects in high aspect ratio microstructures on Si wafers machined by ICP source etching. Using acoustic waves generated by phase velocity scanning of laser interference fringes, we nondestructively detected 13-µm-high defects located on the bottom of narrow and deep grooves from the other side of the Si wafer, which could not be detected by means of optical techniques or a scanning electron microscope.

Feasibility Study of Grain Size Estimation of S45C by Attenuation Measurement of 40 MHz Surface Acoustic Wave Generated by Phase Velocity Scanning of Laser Interference Fringes

Structural change in 0.45 mass % carbon steel during tempering was measured nondestructively as a change in the attenuation coefficient of 40 MHz surface acoustic waves (SAW) generated by scanning interference fringes (SIF). The SIF selectively generate the SAW at an adequate center frequency, which is useful to distinguish the attenuation coefficients between slightly different structures. The attenuation coefficient at 40 MHz increased linearly with heating time (30, 60 and 90 min) at 750°C, representing a minute change in the structure of ferrite and pearlite grains. The method revealed a minute structural change in the ferrite-pearlite ratio and in grain size growth due to the heat treatment.

Increasing the frequency of surface acoustic waves generated by phase velocity scanning of laser interference fringes

To nondestructively detect surface defects that reduce the strength of ceramics and structural materials of micromachines, we previously developed the phase velocity scanning method. By using scanning interference fringes generated by two laser beams whose frequencies are shifted, the method can generate a 110 MHz surface acoustic wave (SAW). However, the frequency of the SAW depends on the frequency of the Bragg cell used, thus making the upper-limit frequency about 110 MHz. In this study, we propose multiple use of Bragg cells in series, and successfully generated a 200 MHz SAW.

Profile measurement taken with liquid-crystal gratings

Profile measurement taken with liquid-crystal gratings and a phase-shifting technique is proposed, and its effectiveness is verified by experiment. The surface profile is obtained by measurement of the phase distributions of the sinusoidal gratings deformed by an object's surface. The liquid-crystal grating gives an accurate phase shift, an arbitrary projection pitch, and a constant surface brightness compared with conventional gratings such as a laser interference fringe grating and a Ronchi grating. Therefore a flexible measuring system may be developed with it. Two gratings with different pitches are used to measure an object with large steps. A two-color projection system can be used to produce such gratings simultaneously. Locally varying reflectivity on a surface can also be compensated by adjustment of the color component of the projected grating with a liquid-crystal grating. Thus the contrast in the projected grating can be made uniform, and a good profile measurement can be accomplished.

Gratinglike modulation of GaAs/AlGaAs quantum well intermixing fabricated with laser interference

We report the observation of a μm range periodical modulation of a band gap caused by gratinglike quantum well intermixing in an intrinsic GaAs/AlGaAs quantum well structure. The intermixing grating was formed with the irradiation of the interference fringe of the second harmonic of a Q-switched Nd:YAG laser followed by a rapid thermal annealing process. The intermixing grating periods were measured with an optical setup for simultaneously monitoring the photoluminescence intensities of two closely spaced wavelengths with a sub-μm spatially scanning resolution. The measured periods included a cluster of features around 2 μm which was consistent with that of the laser interference fringe.

The optical quality of the human eye revisited

A subjective assessment of the optical quality of the human eye may be derived from a comparison between the contrast sensitivities obtained in response to a refracted grating display and a non-refracted grating display. The former may be generated on a cathode ray tube (CRT) while the latter may be obtained with laser interference fringes observed in the Maxwellian view. With this method, the rate of decline of the contrast ratio of the optical media with increasing spatial frequency has been shown to be appreciably less than the rate of decline of the neural performance represented by the laser contrast sensitivities. However, this comparison has been based on results in response to a green CRT display and red (Helium-Neon) interference fringes. In the present study, we have confirmed this result with respect to comparisons between a green CRT display and a green laser display, although the absolute value of the contrast ratio of the optical media for the green laser display was reduced compared with values for the red laser display.

Evaluation of Standard Defects Using Surface Acoustic Waves Generated by Phase Velocity Scanning of Laser Interference Fringes

In order to maintain the reliability of structural materials such as ceramics and micromachines, it is important to nondestructively evaluate micrometer-order surface defects that reduce the strength of these materials. Standard defects whose size and shape are precisely defined are needed but until now, only Vickers and Knoop indentation cracks were available. However, since indentation cracks are created stochastically, it is difficult to define their precise size and shape. Alternatively, we tried to make standard defects on a Si (001) wafer using the focused ion beam machining process. Four grooves of 200 µm length and 6.8, 7.8, 12.4 and 28.7 µm depth were formed by a 90-nm-diameter Ga ion beam on Si surface. We used iodine gas for making fine edges. We then tried to evaluate these defects by using the phase velocity scanning (PVS) method which generates surface acoustic waves (SAW) with excellent directivity. We used 60–110 MHz SAW generated by the PVS method and the distance from defects to probe of SAW was about 2 mm. We succeeded in detecting the scattered wave from the 6.8-µm-deep slit (d/λ∼0.08) and confirmed the usefulness of these standard defects. Next, we compared the theoretical reflection coefficient of an infinite-length crack with our experimental result and confirmed that they agree qualitatively.

Evaluation of standard defects using surface acoustic waves generated by phase velocity scanning of laser interference fringes

In order to maintain the reliability of structural materials such as ceramics, it is important to nondestructively evaluate micrometer-order surface defects that reduce the strength of materials. The standard defects are needed whose size and shape are precisely defined, but only a Vickers and Knoop indentation crack can be used traditionally. However, since indentation cracks are created stochastically, it is difficult to precisely define the size and shape. Alternatively creation of standard defects on Si(100) wafers was attempted by using the focused ion beam (FIB) machining process, and three grooves of 3-μm depth and 50-, 100-, and 200-μm length were made by a 90-nm-diam Ga ion beam on a Si surface. Iodine gas was used for making fine edges. Then these defects were evaluated by using the phase velocity scanning (PVS) method which generates excellently directive surface acoustic waves (SAWs). A 60-MHz SAW generated by the PVS method was used and the distances from detects to the probe of SAW were about 4 mm. The reflection coefficients of 200-, 100-, and 50-μm defects were evaluated to be about 9.6%, 3.5%, and 1%, respectively, and the usefulness of these standard defects was confirmed.

Generation of focused bulk acoustic waves by phase velocity scanning of laser interference fringes

In conventional methods of laser ultrasound, the efficiency, the mode selectivity and the directionality were insufficient for precise measurements. In order to overcome these difficulties, the phase velocity scanning (PVS) method [K. Yamanaka et al., Appl. Phys. Lett. 58, 1591 (1991)] of the scanning interference fringe (SIF) approach was proposed, and a single-mode, high-frequency unidirectional ultrasound was efficiently generated [H. Nishino et al., Appl. Phys. Lett. 62, 2036 (1993)]. But in this approach, the amplitude of the bulk acoustic wave (BAW) was not as large as that of the surface acoustic wave, because the interaction length between the fringe and the generated ultrasounds was shorter (Nishino et al., 1993). In this paper, as a novel extension of the SIF approach, a point focusing type of SIF (PFT-SIF) is proposed to focus the BAW with a large amplitude. By intersecting a focused laser beam and a collimated beam with different frequencies on a specimen, a PFT-SIF is formed, which consists of concentric circles scanned into the center with scanning velocities faster for inner circles. Verification is carried out using second-harmonic waves of a Q-switched Nd:YAG laser and an Al specimen, and profiles of focused acoustic fields for 50- to 80-MHz BAWs are studied.

Acoustic waves generated by phase velocity scanning of laser interference fringes

A method for noncontact and selective generation of surface acoustic waves (SAWs), and bulk acoustic waves (BAWs) is discussed. To generate SAWs, the thermoelastic sources of scanning interference fringes (SIFs) are scanned on an opaque solid surface with a velocity equal to the phase velocity of SAWs, and to generate BAWs the SIFs are scanned at a higher velocity. SAW velocity within the laser beam spot is measured as the ratio of observed frequency of SAWs and wave number of the SIF. The frequency measurement is precise because of the large number of generated carriers, amplitude enhancement of about 10 times, and suppression of other spurious modes (back-propagation SAWs and BAWs). This method was applied to evaluate thin films of SiO2, Si3N4, and porous silicon layers, with a precision around 0.1%. In addition, since a large-amplitude SAW (e.g., 1.4 nm) was realized at high frequencies (e.g., 110 MHz), a large acceleration (up to 109 m/s2) of the surface is achieved, with a moderate peak laser power of 710 kW/cm2. This feature may be attractive for applications in particle manipulation, such as removing dust particles from LSI wafers, in a dry process.