Showing papers in "Optics and Laser Technology in 2001"
TL;DR: In this paper, a fiber optic acoustic sensor system is designed and tested for on-line detection of the partial discharges inside high voltage power transformers using an extrinsic Fabry-Perot interferometer.
Abstract: In this paper, a fiber optic acoustic sensor system is designed and tested for on-line detection of the partial discharges inside high voltage power transformers The fiber optic sensor uses a silica diaphragm and a single mode optical fiber encapsulated in a silica glass tube to form an extrinsic Fabry–Perot interferometer Test results indicate that the developed fiber optic sensors are capable of detecting the acoustic signals propagating inside the transformer oil with high resolution and high frequency
115 citations
TL;DR: In this paper, an analytical method for treating the problem of laser heating and melting is developed, which has been applied to aluminum, titanium, copper, silver and fused quartz and the time needed to melt and vaporize and the effects of laser power density on the melt depth for four metals are also obtained.
Abstract: An analytical method for treating the problem of laser heating and melting is developed in this paper. The analytical method has been applied to aluminum, titanium, copper, silver and fused quartz and the time needed to melt and vaporize and the effects of laser power density on the melt depth for four metals are also obtained. In addition, the depth profile and time evolution of the temperature of aluminum before melting and after melting are given, in which a discontinuity in the temperature gradient is obviously observed due to the latent heat of fusion and the increment in thermal conductivity in solid phase. Additionally, the calculated melt depth evolution of fused quartz induced by 10.6 μm laser irradiation is in good agreement with the experimental results.
102 citations
TL;DR: In this paper, the interaction phenomena of nanosecond time period Q-switched diode-pumped Nd:YAG laser pulses using 1064, 532 and 355 nm with 0.25 mm thick pure-copper foil was investigated at an incident laser intensity range of 0.5 − 57.9 GW / cm 2.
Abstract: The interaction phenomena of nanosecond time period Q-switched diode-pumped Nd:YAG laser pulses using 1064, 532 and 355 nm with 0.25 mm thick pure-copper foil was investigated at an incident laser intensity range of 0.5– 57.9 GW / cm 2 . For each sample, etch rate and surface structure were determined. Analysis of the results of the tests included scanning electron microscopy (SEM). A maximum etch rate of 13.3 μm per pulse was obtained for the etch rate tests carried out at 532 nm . The maximum etch rate obtainable for 1064 nm was 2.21 μm per pulse, and for 355 nm , 6.68 μm per pulse. The dramatic decrease in etch rate observed when processing at 1064 nm is thought to occur due the highly reflective nature of copper as the interaction wavelength is increased, plus the nature of the plasma formed above the material during the high-intensity laser–material interaction. This plasma then imparts energy to the surface of the processed area leading to surface melting of the area surrounding the hole as can be seen by the SEM photographs.
78 citations
TL;DR: In this paper, two main factors that influence the repeatability of the laser percussion drilling process are identified, and the effect of melt ejection on hole geometry repeatability is also investigated.
Abstract: In this work, the two main factors that influence the repeatability of the laser percussion drilling process are identified. Experimental parametric analysis was carried out to correlate the laser parameters with the repeatability of a laser percussion drilling process. The experiment was conducted using a flash lamp pumped Nd:YAG laser to drill 2 mm thick mild steel sheets. The relationship between the percentage standard deviation (PSD) of entrance hole diameter, hole circularity and the operating parameters is established. Thirty-five holes were drilled and analysed for each set of identical laser parameters. The PSD of entrance hole diameter ranges between 1.47% and 4.78% for an operating window of 3.5– 7 kW peak power, and 1– 3 ms pulse width. The circularity of the entrance hole (defined as the ratio between the minimum and maximum diameters of the hole) ranges from 0.94 to 0.87, and is found to correlate with repeatability. The work shows that higher peak power, and shorter pulse width gives better hole geometry repeatability. The effect of melt ejection on hole geometry repeatability is also investigated. Melt ejection and spatter formation have been found to contribute to the poor repeatability of the process.
78 citations
TL;DR: In this paper, the SMI signal is obtained by feeding the light from the target to cavity of laser diode, then it is pre-processed by an analog subtraction circuit to remove the overlapped output intensity due to the injection current modulation.
Abstract: Self-mixing interferometry (SMI) has been used to measure the distance and displacement. Although the principle of self-mixing interference is different conventional interference, we concluded that FFT analysis technique could also be used to detect the signal phase and increase the measurement precision of self-mixing interferometry. First the SMI signal is obtained by feeding the light from the target to cavity of laser diode, then it is pre-processed by an analog subtraction circuit to remove the overlapped output intensity due to the injection current modulation. Finally, SMI signal is analyzed by FFT phase detection method. Theoretical analysis and simulation calculations are presented. Experimentally, displacement of a PZT-driven target was measured with a precision of λ/50.
74 citations
TL;DR: In this article, the role of phonon-relaxation subsystem in the observed luminescence was discussed, and the substantial influence of energy transfer processes between the host and Eu3+ ions was shown experimentally through the dependences of photoluminescence on light polarization and excitation wavelength.
Abstract: Red luminescence (at wavelength about 622 nm ) from Eu3+ ions embedded in PbO–Bi2O3–Ga2O3–BaO glass hosts is reported for room and liquid helium temperatures. The substantial influence of energy transfer processes between the host and Eu3+ ions is shown experimentally through the dependences of photoluminescence on light polarization and excitation wavelength. Only polarized, excited pulsed XeII laser light (λ=714 nm ) gives substantial luminescence with efficiency up to 14.3%. The role of phonon-relaxation subsystem in the observed luminescence is discussed.
69 citations
TL;DR: In this paper, a speckle correlation technique for the determination of surface roughness, ranging from 1.6 to 50 μm, is described. But instead of moving the laser beam, the specimen is rotated to achieve angular speckles correlation (ASC) in the far field plane.
Abstract: This paper describes a speckle correlation technique for the determination of surface roughness, ranging from 1.6 to 50 μm . Instead of moving the laser beam, the specimen is rotated to achieve angular speckle correlation (ASC) in the far-field plane. The technique is simple and requires minimum optical alignment. The experimental results show a good agreement with the standard specimen of known roughness. An error analysis on the experiment has been carried out. Together with the theoretical curves, the roughness values can be easily related to the change of incidence angle at a particular visibility of the correlation fringes between two speckle patterns.
51 citations
TL;DR: In this paper, an improved electro-optic (EO) polymer light modulator based on attenuated-total internal reflection (ATR) is demonstrated, which consists of a prism-metal-polymer-metal multilayer structure.
Abstract: An improved electro-optic (EO) polymer light modulator based on attenuated-total-internal-reflection (ATR) is demonstrated. The modulator consists of a prism–metal–EO-polymer–metal multilayer structure. An applied electric field across the EO polymer layer electrically modulates the energy coupling efficiency of incoming light into guided wave resonance at fixed angles. Compared with conventional ATR modulators based on surface plasmon resonance, the driving voltage for this modulator has been greatly reduced because of the newly chosen working interior angle. It also offers advantages in terms of insertion loss and aperture size over other techniques for the amplitude modulation of a collimated light beam.
50 citations
TL;DR: In this article, a laser interferometer was proposed for determining the straightness of a moving stage, and two experiments were carried out to verify the validity of the theoretical derivation.
Abstract: This paper proposes a laser interferometer for determining the straightness of a moving stage. The optical setup is described and the theory is derived. The results show that this interferometer not only overcomes the disadvantages of the conventional interferometers but also provides a higher sensitivity. Two experiments were carried out in this research. The first experiment verifies the validity of the theoretical derivation. The second experiment demonstrates that this interferometer could be adopted for straightness measurements.
48 citations
TL;DR: In this article, two methods, scanning phase method and time sequence phase method, have been introduced for measuring the displacement caused by the deformation in temporal speckle pattern interferometry (TSPI), and experiments are performed to demonstrate their performance with a conventional ESPI system.
Abstract: If a laser beam illuminates a continual deformation object surface, it will lead to a temporal speckle pattern on the observation plane. Recording this time-dependent speckle pattern the deformation of the surface of an object can be obtained. Two methods, scanning phase method (SPM) and time sequence phase method (TSPM), have been introduced for measuring the displacement caused by the deformation in temporal speckle pattern interferometry (TSPI). Their principle is that by capturing a series of speckle interference patterns related to the object deformations, the fluctuations in the intensity of the interference patterns can be obtained. Through scanning these fluctuations and estimating both the average intensity and modulation of the temporal speckle interference patterns, the phase maps for whole-field displacements are calculated. In this way one is capable of quantitatively measuring continual displacements simply using a conventional electronic speckle pattern interferometry (ESPI) system without phase shifting or a carrier. The elaboration on the new methods is given in this paper and experiments are performed to demonstrate their performance with a conventional ESPI system.
48 citations
TL;DR: In this article, a 3D surface profilometer is described which uses a simple fiber optic coupler to form a Young's double pinhole interferometer, which is projected onto a surface, captured by a camera and analyzed using the Fourier transform method.
Abstract: A 3-D surface profilometer is described which uses a simple fiber optic coupler to form a Young's double pinhole interferometer. The Young's fringes are projected onto a surface, captured by a camera and analyzed using the Fourier transform method. The phase of the fringe pattern on the object is used to reconstruct the surface profile. System analysis, results from a simulation, and preliminary experimental results are provided which indicate a system resolution on the order of a tenth of a millimeter.
TL;DR: In this article, an inclined, cylindrical collimating lens in front of the diode bar was used to reduce the curvature of the beam of a diode's beam.
Abstract: An optical arrangement is used to characterize diode bar curvature which is known to limit the effective brightness achievable with such devices. By introducing an inclined, cylindrical collimating lens in front of the diode bar, the curvature of the diode's beam can be reduced by more than 60%. The correct inclination angle for this method is derived. Using a beam shaper together with this correction mechanism we achieve a homogeneous beam profile with low M2 values in orthogonal directions and an effective brightness increase of more than 200%.
TL;DR: A vision system is proposed to detect the degree of uniformity of light reflection using a light guide plate before the diffuser has been attached and a new bright spot search and statistical software has been designed to provide greater economy and make the device user friendly.
Abstract: A LCD light guide plate is an element of the LCD backlight module that is often used for the display of compact electronic devices. In this study, a vision system is proposed to detect the degree of uniformity of light reflection using a light guide plate before the diffuser has been attached. A new bright spot search and statistical software has been designed and the parameters for the LCD light guide plate can be adjusted before manufacture to provide greater economy and make the device user friendly. Since many different types of backlight modules had to be analyzed in this study, we used different methods to determine the nonuniform factors and the backlight module area for each type. A wrapping algorithm is presented in the searching and statistic process of bright spot. This algorithm can combine the procedures of segmentation process and nonlinear grey scale mapping. After revising carved depth of the poor brightness area, the diffuser was attached and the analysis performed again to verify the design procedure correction. When the source light density is adjusted, the lighting error for the on line inspection is retained to within 3%.
TL;DR: The compositional dependence of the optical constants, the refractive index n, and the absorption index k, of the AsSe1−xTex thin films with 0 was studied in this paper.
Abstract: The compositional dependence of the optical constants, the refractive index n, and the absorption index k, of the AsSe1−xTex thin films with 0
TL;DR: In this article, an analytical model for the laser melting of inhomogeneous workpieces with parabolic melt pool geometry was developed and the theoretical results were compared with the experimental data, and the predicted melt pool depth and the experimental values were in close correlation with the parameter P L / d v for values less than 15 W mm −1 s −1/2, in spite of simplifications introduced in the model.
Abstract: This paper reports on an investigation into the development of an analytical model for the quality control of laser marking/engraving of clay tiles using a high-power diode laser (HPDL) An analytical model for the laser melting of inhomogeneous workpieces with parabolic melt pool geometry being assumed was developed The theoretical results were compared with the experimental data The predicted melt pool depth and the experimental values were in close correlation with the parameter P L / d v for values less than 15 W mm −1 s −1/2 , in spite of simplifications introduced in the model At the relatively large values of parameter P L / d v the assumption of parabolic melt pool shape and one-dimensional heat transfer no longer holds true
TL;DR: In this paper, a novel differential double Bragg grating sensor for temperature-insensitive strain measurement is presented, which consists of two identical weak measuring and reference gratings separated by some distance.
Abstract: A novel differential double Bragg grating sensor for temperature-insensitive strain measurement is presented. The sensor consists of two identical weak measuring and reference gratings separated by some distance. The reference grating is placed inside the silica capillary that made it almost strain insensitive. The basic idea in differential double Bragg grating sensor is to measure the energy of the oscillating term in the reflection spectrum of double Bragg grating structure. The normalized energy of the interference term depends on the pitch difference of the two gratings and does not change when pitch variations of both gratings are equal with temperature variation. Therefore, the normalized energy of the interference term can be used for the temperature-insensitive strain measurement.
TL;DR: In this article, the results of micro machining of different materials (AI, Si, InP and fused silica) using a Ti:sapphire laser at the wavelength of 800 and 267 nm with variable pulse lengths in the range from 100 fs to lo ps are presented.
Abstract: This paper reports the micro machining results of different materials (AI, Si, InP and fused silica) using a Ti:sapphire laser at the wavelength of 800 and 267 nm with variable pulse lengths in the range from 100 fs to lo ps. The hole arrays with a diameter up to a few µm through microdrilling are presented. We discussed how an effective suppression of the thermal diffusion inside the ablated materials and an effective microablation could be realized. If the laser fluence is taken only slightly above the threshold, a hole array can be drilled with diameters even smaller than the wavelength of the laser. Some examples are presented in the paper.
TL;DR: In this article, the second-harmonic generation phase-matching angles of a 3mm-long ZnCd(SCN) 4 (ZCTC) crystal were reported.
Abstract: Violet 404-nm radiation is generated by type-I single-pass frequency doubling of the output of an GaAlAs diode laser in critically phase-matched ZnCd(SCN) 4 (ZCTC) crystal, a metallo-organic complex crystal. In this paper, its refractive index and second-harmonic generation phase-matching angles are reported. Using a 3-mm-long ZCTC crystal, a diode laser power of 473 mW generated 390 μW of second-harmonic radiation. More recently, 380-nm, 20 μW ultraviolet (UV) radiation is also realized by frequency doubling of the 100 mW output of a continuous-wave Ti:sapphire laser at 760 nm. ZCTC is a promising UV nonlinear optical candidate material for frequency doubling of diode lasers.
TL;DR: The role of phonon-relaxation subsystem in the observed luminescence was discussed in this article, where the influence of energy transfer processes between the host and Er3+ ions was shown experimentally through the dependences of photoluminescence on light polarization and excitation wavelength.
Abstract: Infra-red luminescence (at wavelengths about 1600 and 2500 nm ) from Er3+ ions embedded in PbO–Bi2O3–Ga2O3–BaO glass hosts is reported for room and helium liquid temperatures. The substantial influence of energy transfer processes between the host and Er3+ ions is shown experimentally through the dependences of photoluminescence on light polarization and excitation wavelength. Only the application of the polarized pumping YAG–Nd laser beam (λ=1060 nm ) stimulates substantial luminescence with quantum efficiency up to 24%. The role of phonon-relaxation subsystem in the observed luminescence is discussed.
TL;DR: This paper addresses the implementation of a bifocal range sensor which estimates the depth by measuring the relative blurring between two images captured with different focal settings, and indicates that the accuracy of this proposed sensor compares well with that offered by other methods such as stereo and motion parallax.
Abstract: Recovering the depth information derived from dynamic scenes implies real-time range estimation. This paper addresses the implementation of a bifocal range sensor which estimates the depth by measuring the relative blurring between two images captured with different focal settings. To recover the depth accurately even in cases when the scene is textureless, one possible solution is to project a structured light on the scene. As a consequence, in the scene's spectrum a spatial frequency derived from illumination pattern is evident. The resulting algorithm involves only simple local operations, this assures the possibility of computing the depth at a rate of 10 frames per second. The experimental results indicate that the accuracy of this proposed sensor compares well with that offered by other methods such as stereo and motion parallax, while avoiding the problems caused by occlusion and missing parts.
TL;DR: In this paper, the authors investigated the dynamics of repetitive pulsed laser drilling of a visually transparent media using a CO2 laser source and found that the melting velocity within each laser pulse can vary considerably from the average drilling velocity as a result of the non-uniform temporal pulse shape and the oscillation of the melt ejection rate.
Abstract: This paper presents an investigation into the dynamics of repetitive pulsed laser drilling of a visually transparent media using a CO2 laser source. This enabled the use of a high-speed imaging system for observing, in real time, the behaviour of the drilling process in the laser drilled cavity of 1.5 mm diameter holes of up to 18.5 mm in depth. The work revealed that the instantaneous drilling velocity within each laser pulse can vary considerably from the average drilling velocity as a result of the non-uniform temporal pulse shape and the oscillation of the melt ejection rate. During beam breakthrough, both upward and downward melt ejections were observed to occur inside the drilled hole for a short period of time, after which the material was ejected through the exit end of the holes. It has been shown in this work that the downward melt flow velocity increases with hole depth for a positively tapered hole (from 0.09 to 1.43 m / s ) and decreases with hole depth for a negatively tapered hole geometry (from 0.4 to 0.1 m / s ), as a result of the change in the assist gas velocity inside the drilled hole with respect to the hole taper geometry. The mechanisms of forming the positively and negatively tapered holes in the transparent media have been correlated with the hole geometry and melt flow velocity. The work has demonstrated a new method of studying the melt dynamics in laser drilling.
TL;DR: In this paper, a YAG laser delivery system based on hollow fibers with an internal polymer layer has been developed, which consists of a hollow fiber as the delivery medium, a launching coupler for effective coupling between the laser beam and the fiber, and a sealed hollow-fiber tip attached at the output end of the fiber.
Abstract: Infrared Er : YAG laser delivery systems based on hollow fibers with an internal polymer layer has been developed. The system consists of a hollow fiber as the delivery medium, a launching coupler for effective coupling between the laser beam and the fiber, and a sealed hollow-fiber tip attached at the output end of the fiber. Fabrication process of the fiber and attachment of the delivery system is reported.
TL;DR: In this paper, the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout were investigated.
Abstract: We investigate the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout. The vector synthetic gratings consist of two volume-index gratings with different grating wave vectors, which can be obtained in photorefractive materials, in optical fibers or in Bragg cells with two driving acoustic waves. This structure can realize beam splitting, beam combining, and beam cross coupling, etc. Analytic expressions for the diffraction efficiency as well as the amplitude of the waves involved are obtained. The results are presented and discussed.
TL;DR: Experimental results demonstrate that this filtering scheme using Symlet wavelet to remove the speckle noise to the large extent removes the specker noise from the fringes recorded for the surface of computer hard disk.
Abstract: In this paper we are presenting a filtering scheme using Symlet wavelet to remove the speckle noise from the time-averaged digital speckle pattern interferometry fringes. To demonstrate the potential of Symlet wavelet filtering, experiments are conducted to remove the speckle noise from the fringes recorded for the surface of computer hard disk. Experimental results demonstrate that this filtering removes the speckle noise to the large extent.
TL;DR: In this article, the absorption properties of Nd:YVO4 crystals with various doping levels and lengths are reported and the effective stimulated emission cross-sections of different doping levels are calculated with measured upper-level lifetimes.
Abstract: The absorption properties of Nd:YVO4 crystals with various doping levels and lengths are reported. The effective stimulated emission cross-sections of different doping levels are calculated with measured upper-level lifetimes. Our investigation gives good advice on the optimization of LD end-pumped Nd:YVO4 laser.
TL;DR: In this article, an in-depth study of the single pulse and multiple pulse laser (35 ps, 10 Hz and 1064 nm ) damage for threshold fluence and greater fluence of GaAs single crystal is presented.
Abstract: An in-depth study of the single pulse and multiple pulse laser ( 35 ps , 10 Hz and 1064 nm ) damage for threshold fluence and greater fluence of GaAs 〈1 0 0〉 single crystal is presented. Damage which starts at a power 2×10 11 W / cm 2 in the form of pits occurs due to accumulation of laser induced microscopic defects. Effect of multiple pulse at first makes the pits more prominent in the form of Ga emission. Then the topmost layer is removed. If the number of pulses is further increased new pits are formed in the new surface (beneath the removed surface) and the above process is repeated. The thermal model is sufficient to explain this morphology. However, for larger fluences, a large cracking and fracture and the possibility of both Ga and As emission in different ratios suggest that mechanical damage is a dominant feature for higher fluences which arises due to generation of shock waves and rapid vaporization of material. Damage threshold has been calculated with the help of the thermal model given by Meyer et al. which is in good agreement with our experimental results.
TL;DR: In this article, the fabrication and characterization of four-layer polycarbonate and polystyrene waveguides are presented. And the results show that the four layer lightguide exhibits relatively low loss values compared to the monolayer configuration.
Abstract: We report on our systematic study of fabrication and characterization of four-layer polymer waveguides. Various optical properties (such as refractive index, birefringence, and propagation loss) of polycarbonate and polystyrene waveguides are presented. The thin film structure consisting of glass/polycarbonate/polystyrene/air has been used for demonstrating polarization filter action because the two polymers are quite different with respect to their optical anisotropy. Modal electric field plots for both TE and TM are generated to support the observed behavior. It is also observed that the four-layer lightguide exhibits relatively low loss values compared to the monolayer configuration.
TL;DR: In this article, a simple process suitable for fabrication of micro-periodic structure in optical waveguide was described, and the mold was fabricated using electron beam lithography and fast atom beam etching.
Abstract: This paper describes a novel simple process suitable for fabrication of micro-periodic structure in optical waveguide. The mold was fabricated using electron beam lithography and fast atom beam etching. Sub-micron-scale patterns were transferred from silicon mold to polymer layer. Grating coupler was fabricated by the mold and normal optical mask. In the proposed method, no press which is needed for imprint lithography is required and the mold structure can be duplicated with high aspect ratio. Experimental coupling efficiency is about 25%. This technique can also be used to fabricate other nanometer-scale structures.
TL;DR: In this paper, a study of the ablation effects of an ultra-short pulsed Nd : YAG laser was carried out using a 21 μm Zn-coated carbon steel plate as the target, the relationship between ablation rate and laser fluence was investigated through computer simulation and experiment, and the optimal processing conditions were determined.
Abstract: A study of the ablation effects of an ultra-short pulsed Nd : YAG laser was carried out. Using a 21 μm Zn-coated carbon steel plate as the target, the relationship between ablation rate and laser fluence was investigated through computer simulation and experiment, and the optimal processing conditions were determined. The tendency of the scanning operation curve was confirmed and the data obtained were taken as a guide for the practical utilization of this technique. Finally, a decontamination factor was introduced and satisfactory cleaning was obtained.
TL;DR: In this paper, a new organic dye trans-4]-p-(n-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium iodide (abbreviated as HEASPI hereafter) with large two-photon absorption (TPA) cross section and excellent upconverted superradiance properties was synthesized in our group recently.
Abstract: A new organic dye trans-4-[p-(N-ethyl-N-hydroxyethylamino)styryl]-N-methylpyridinium iodide (abbreviated as HEASPI hereafter) with large two-photon absorption (TPA) cross section and excellent upconverted superradiance properties was synthesized in our group recently. The TPA cross section was measured to be σ 2 ′=7.0×10 −48 cm 4 s / photon by using an open aperture Z-scan system. Linear absorption, single-photon induced fluorescence, two-photon induced fluorescence and two-photon pumped (TPP) upconverted superradiance properties were systematically studied. The highest net upconversion efficiency from the absorbed pump energy to the output upconverted superradiance energy is as high as 19.6% at the pump energy of 2.07 mJ from a mode-locked Nd : YAG picosecond laser. The dye solution also shows a clear optical power limiting effect.