Showing papers in "Optics and Laser Technology in 2004"

A straightness measurement system using a single-mode fiber-coupled laser module

Abstract: In this paper, a quite simple and novel straightness measurement system is proposed The laser beam from a single-mode fiber-coupled laser module is highly stable, and can be used as the reference line for straightness measurements of any mechanical system A corner retro-reflector is used as the moving target in the system in order to sense the straightness error and to reflect laser beam from measurement head back to it In this way, there is no cable connection in the moving target, which is necessary for straightness measurements in workshop At the same time, relationship between voltage output from a quadrant photodiode detector and its displacement was analyzed, and the theoretical analysis shows a good consistence with practical measurement results, which gives a way to enlarge the measurement range of the system Comparative results of straightness measurement with a dual frequency laser interferometer were given, and repeatability with one micrometer at the distance of 1 m was gotten by the system

Differential FBG sensor for temperature-compensated high-pressure (or displacement) measurement

Abstract: A differential fiber Bragg grating (FBG) sensor with a free active element bulk-modulus for high-pressure (or displacement) measurement is presented. Based on the differential measurement method and an isosceles triangle cantilever structure, problems of cross-sensitivity and chirped signal in FBG sensor are improved both theoretically and experimentally. Preliminary experiments indicate that temperature-compensated measurement results agree well with the theoretical analysis. Displacement measurement sensitivity is ∼1.75 pm / μm , and the estimated pressure measurement resolution can reach 0.27 MPa in case the wavelength shift measurement resolution is 0.01 nm .

In-time motion adjustment in laser cladding manufacturing process for improving dimensional accuracy and surface finish of the formed part

Abstract: This paper presents in-time motion adjustment in laser cladding manufacturing process as a means to improve dimensional accuracy and surface finish of the built part. Defects occurring during laser cladding degrade the part quality such as dimensional accuracy and surface finish. In this paper, in-time motion adjustment strategy was presented to remedy and eliminate defects occurring during laser cladding to improve the dimensional accuracy and surface finish. Based on the relationship between the motion of laser head relative to the growing part and other parameters in effects on clad profile, the laser traverse speed, stand-off distance and laser approach orientation to the existing clad layer were adjusted by instructions from a close-loop control system in real time to remedy and eliminate defects. The results of the experiments verified the effects of in-time motion adjustment on dimensional accuracy and surface finish.

Analysis of speckle images to assess surface roughness

Abstract: Digital speckle images are photographed for different aluminum rough surfaces using a CCD camera. The obtained speckle images are fed to a PC and analyzed making use of the MATLAB program. The computerized binary images are investigated. The signal-to-noise ratio is computed from these numerical images. It is shown that the surface roughness of the examined surfaces is dependent upon the degree of agglomeration of the speckle images.

Dressing of resin-bonded superabrasive grinding wheels by means of acousto-optic Q-switched pulsed Nd:YAG laser

Abstract: The preparation of the use of resin-bonded superabrasive grinding wheels remains a problem despite their availabilities on the market in the past years and possible technological advantages. In this paper, an acousto-optic Q-switched Nd:YAG pulsed laser is applied to dressing resin-bonded superabrasive wheels in orthogonal direction. The author proposes a systematical study on the mechanism of selective removal, crater ablated by single pulse and surface topography after dressing, and consequently presents a feasible method of selecting irradiation parameters and summarizes the dressing features and disciplines of dressing effects influenced by these parameters. Experiments results indicate the surface topography dressed by acousto-optic Q-switched YAG pulsed laser dressing is much superior to mechanical dressing and CW laser dressing.

Measurement of wire diameter by optical diffraction

Abstract: A combined interference and diffraction pattern, in the form of equidistant interference fringes, resulting from illuminating a vertical metallic wire by a laser beam is analyzed to measure the diameter of four standard wires. The diameters range from 170 to 450 μm . It is found that the error in the diameter measurements increases for small metallic wires and for small distances between the wire and the screen due to scattering effects. The intensity of the incident laser beam was controlled by a pair of sheet polaroids to minimize the scattered radiation. The used technique is highly sensitive, but requires controlled environmental conditions and absence of vibration effects. The expanded uncertainty for k=2 is calculated and found to decrease from U(D)=±1.45 μm for the wire of nominal diameter 170 μm to ±0.57 μm for the diameter 450 μm .

Optical investigation of cavitation erosion by laser-induced bubble collapse

Abstract: By means of a new force sensor based on optical beam deflection (OBD), the mechanical effects of laser-matter interaction underwater at different incident laser energy are investigated in detail. The experimental results show that a target underwater is impacted in turn by laser-plasma ablation force and high-speed liquid-jet impulse induced by bubbles collapse in the vicinity of a solid boundary. Furthermore, the amplitudes of the two forces increase monotonously with laser energy. According to the ablation force detected by the experiment and the theoretical relationship between laser intensity and ablation pressure, the value of liquid-jet impact against a solid boundary can be easily obtained. In addition, based on the model of a collapsing bubble, some characteristic parameters, such as the liquid-jet impact velocity, the maximum bubble radius, the bubble energy can also be obtained at different laser energy, which are valuable in the corresponding research fields.

Phase shifting SEM moiré method

Abstract: A new phase shifting scanning electron microscope (SEM) moire method is proposed in this paper The phase shifting technique is realized in four steps from 0 to 2π by shifting electron beam in the y-axis direction controlled by the SEM system It is successfully applied to determine the residual strain of a deformed holographic grating with a frequency of 1200 lines / mm in an electronic package As a further application, it is used to measure the virtual strain of a MEMS structure with a 5000-line/mm grating and to determine the phase distribution of a SEM moire formed with a 6000-line/mm grating fabricated by electron beam lithography The experiments show the feasibility of this method It provides a new way for disposal of fringes pattern in sub-micro moire method

Giant piezooptics effect in the ZnO-Er3+ crystalline films deposited on the glasses

Abstract: An appearance of the giant photoelastic effect (up to 17×10 −13 m 2 / N ) (for the wavelength of about 450 nm ) in the ZnO films doped by erbium ions is found. For a description of the observed phenomenon, a complex approach including self-consistent band structure calculations together with the appropriate molecular dynamics simulations of the interfaces was used. The origin of the observed effect is caused by the appearance of substantial charge density redistribution within the crystalline ZnO–Er3+ films deposited on the bare glass substrate as well as due to additional charge density polarization by the Er. A possibility of enhancement of the photoelastic coefficients π2222 at λ=600 nm was proved for the first time by the simultaneous use of interface charge density redistribution and cationic doping of the films.

Propagation properties of three-dimensional flattened Gaussian beams in uniaxially anisotropic crystals

Abstract: Starting from the paraxial vectorial theory of beams propagating in uniaxially anisotropic media, the analytical propagation equations of three-dimensional flattened Gaussian beams (FGBs) in uniaxial crystals are derived and illustrated numerically. It is shown that, due to the anisotropy of crystals, the ordinary and extraordinary beams originated by incident FGBs propagate with different Rayleigh lengths, thus the linear polarization state and axial symmetry of incident FGBs do not remain during propagation in crystals.

Numerical modeling and theoretical analysis of femtosecond laser tweezers

Abstract: Optical trapping (also called optical tweezers) is a widespread technique, with a large number of applications in biology and other fields. Taking femtosecond laser pulses as a sampling of CW light, we theoretically demonstrate the feasibility of femtosecond laser tweezers and present the formulae of the induced forces which femtosecond laser pulses exert on micrometer-sized spheres. We also demonstrate the stability condition for femtosecond laser tweezers. As an example, we present the numerical results for a sphere with a radius of 10 mm .

Regimes of operation states in passively mode-locked fiber soliton ring laser

Abstract: The principal of passively mode-locked fiber soliton ring lasers is summarized, including its three output operation states: normal soliton, bound–solitons and noise-like pulse. The experimental results of the passively mode-locked fiber soliton ring lasers developed by us are given. Bound–solitons with different discrete separations and three-bound–solitons state have been observed in our fiber laser for the first time. The relationship among three operation states in fiber soliton laser is analyzed.

Time-resolved imaging of latent fingerprints with nanosecond resolution

Abstract: Imaging of latent fingerprints using time-resolved (TR) method offers a broader platform to eliminate the unwanted background emission. In this paper, a novel TR imaging technique is demonstrated and implemented, which facilitates the detection of latent fingerprints with nanosecond resolution. Simulated experiments were carried out with two overlapping fingerprints treated with two fluorescent powders having different lifetimes in nanosecond range. The dependence of the fluorescence emission intensity in nanosecond resolution of TR imaging is also revealed.

The study of chromatic dispersion and chromatic dispersion slope of WI- and WII-type triple-clad single-mode fibers

Abstract: In this paper, the chromatic dispersion and chromatic dispersion slope of two kinds of triple-clad single-mode fibers with a depressed index inner cladding named WI- and WII-type were examined. A feasible approach to calculate chromatic dispersion and higher-order dispersion was established successfully, and the influences made by the optical parameters and geometric parameters on the chromatic dispersion coefficient and its slope were analyzed in detail. The calculated results show that the optical parameter R 2 , which symbolizes the third cladding effect, has a strong impact on the chromatic dispersion coefficient and the chromatic dispersion slope, and the degrees of such impact are closely related to the other parameters.

LaCaMnO3 thin film laser energy/power meter

Abstract: A prototype laser energy/power meter was designed based on the anisotropic Seebeck effect of LaCaMnO 3 thin film grown on vicinal cut LaAlO 3 substrate. The optical response of this device to the pulsed laser and the chopped CW laser was measured from infrared to ultraviolet. The peak voltage of the measured signal shows a good linear relation to the laser energy and power in the measured range. It was shown that at 1064 nm wavelength, this prototype device demonstrates higher sensitivity than that of commercial device.

Surface roughness investigation of semi-conductor wafers

Abstract: Conventionally, surface roughness is predominantly determined through the use of stylus instruments. However, there are certain limitations involved in the method, particularly when a test specimen, such as a silicon wafer, has a smooth mirror-like surface. Hence, it is necessary to explore alternative non-contact techniques. Light scattering has recently been gaining popularity as an optical technique to provide prompt and precise inspection of surface roughness. In this paper, the total integrated scattering (TIS) model is modified to retrieve parameters on surface micro-topography through light scattering. The applicability of the proposed modified TIS model is studied and compared with an atomic force microscope. Experimental results obtained show that the proposed technique is highly accurate for measuring surface roughness in the nanometer range.

Temperature prediction for CO2 laser heating of moving glass rods

Abstract: This paper presents a heat transfer model to calculate the temperature field in moving glass rods heated by a CO 2 laser. Conduction and radiation heat transfer in radial and axial directions are taken into account in the current model. The Rosseland diffusion approximation is incorporated to analyze the radiation heat transfer in the glass rod. A two-band model is used to simulate the spectral property of the glass. Results of the simulation show that glass rods of sufficiently large optical thickness should be treated as a semitransparent medium for radiative transfer, and it is reasonably accurate to assume it to be opaque to CO 2 laser irradiation. It has been shown that the resulting temperature profile is strongly dependent on the laser parameters, i.e., the size of laser beam and the power of the laser. The diameter and speed of the moving glass rod are also important in determining the temperature field although the convective heat transfer coefficient between the glass rod and the environment has little effect.

Design of photonic band gap structures through a dual-beam multiple exposure technique

Abstract: We present a dual-beam multiple exposure technique that can generate complex 2-D and 3-D band gap template structures in a photosensitive material. The system parameters related to the planar interference pattern produced by two laser beams and reorientation effect of the sample relative to these planes is presented. Structures such as the 2-D, square and hexagonal arrays of dielectric “rods” and “holes” and the 3-D, cubic, Yablonovite and other profiles are given. We perform band gap calculations on these structures when the dielectric contrast has been increased using a backfill process and discuss techniques for increasing the band gap by sculpting the dielectric profile.

Laser-induced vibration during pulsed laser forming

Abstract: In this study, the vibration phenomenon during pulsed laser forming of thin metal plates was investigated numerically and experimentally The metal plates were made of 304 stainless steel and were heated by a CO2 Gaussian laser beam with a long pulse duration of 01 s , which incited vibration due to the elastic–plastic deformation of the specimen This uncoupled thermal–mechanical problem was solved using a three-dimensional finite element method and was subsequently satisfactorily verified with experiments Using the superposition method with multiple laser pulses, the non-linear vibration phenomenon during pulsed laser forming has also been observed experimentally for thin plates

Dispersive optical constants of a-Se100−xSbx films

Abstract: Thin films of amorphous Se100−xSbx (x=5,10 and 20 at %) system are deposited on a silicon substrate at room temperature (300 K ) by thermal evaporation technique. The optical constant such as refractive index (n) has been determined by a method based on the envelope curves of the optical transmission spectrum at normal incidence by a Swanpoel method. The oscillator energy (Eo), dispersion energy (Ed) and other parameters have been determined by the Wemple–DiDomenico method. The absorption coefficient (α) has been determined from the reflectivity and transmitivity spectrum in the range 300– 2500 nm . The optical-absorption data indicate that the absorption mechanism is a non-direct transition. We found that the optical band gap, Egopt, decreases from 1.66±0.01 to 1.35±0.01 eV with increase Sb content.

Experimental study of a high power and high efficiency CW diode-side-pumped Nd:YAG laser

Abstract: This paper reports on the characterization of a diode-side-pumped CW Nd:YAG laser. A side-pumped configuration with 9 laser diodes is used for the laser. Pump light is directly coupled into the Nd:YAG rod without focusing lenses and the pump light distribution in the Nd:YAG rod was calculated. A maximum output power of 150 W in multimode operation is obtained for a pumping power of 400 W . The optical–optical efficiency is 37%. Output power of the laser under different output couplers, resonator lengths and temperatures of the cooling water have been studied.

The measurement of refractive index profile and aberration of radial gradient index lens by using imaging method

Abstract: In this article, through the linear fit of objective distance and reciprocal of magnification in different imaging locations, we use imaging method to measure the central refractive index n (0) and focusing constant α of radial gradient index (GRIN) lens and obtain 10 −3 measurement accuracy of n (0). At the same time, under distortion, the fourth-order refractive index coefficient and aberration of GRIN lens are derived.

Real-time fingerprint identification

Abstract: The pattern matching for fingerprints requires a large amount of data and computation time. Practical fingerprint identification systems require minimal errors and ultrafast processing time to perform real-time verification and identification. By utilizing the two-dimensional processing capability, ultrafast processing speed and non-interfering communication of optical processing techniques, fingerprint identification systems can be implemented in real-time. Among the various pattern matching systems, the joint transform correlator (JTC) has been found to be inherently suitable for real-time matching applications. Among the various JTCs, the fringe-adjusted JTC has been found to yield significantly better correlation output compared to alternate JTCs. In this paper, a fingerprint identification system has been developed based on the fringe-adjusted JTC. Since all pattern matching systems suffer from high sensitivity to distortions, the synthetic discriminant function concept has been incorporated in fringe-adjusted JTC to ensure distortion-invariant fingerprint identification.

Optimization of erbium-doped waveguide amplifier

Abstract: Based on the combined model of erbium-doped waveguide amplifier with high erbium ion concentration, the rate equation and evolution equation is solved numerically. The waveguide parameters such as doping concentration, waveguide length are optimized in this paper. With pumping power of 150 mW , the optimal erbium doping concentration and waveguide length is 6×10 26 m −3 , 15 cm or so, respectively. The amplifier with these optimal parameters may reach gain more than 35 dB .

Efficient blue laser generation at 473 nm by a BIBO crystal

Abstract: It is reported that efficient blue laser generation at 473 nm in a BIBO crystal at type-I phase matching direction of (θ,ϕ)=(18.3°,90°) performed with a diode-pumped Nd:YAG laser. With incident pump power of 1.6 W , output power of 183 mW at 473 nm has been obtained using a 5.0 mm -long BIBO crystal. The optical conversion efficiency was up to 11.4%. It was found that the intracavity frequency doubling efficiency is about 50% greater than that obtained with a 10 mm -long type-I phase-matching LBO crystal.

2.23 W diode-pumped Nd:YVO4/LBO laser at 671 nm

Abstract: A fiber-coupled diode-array-pumped, intracavity frequency doubled by a LBO crystal, high-power Nd:YVO4 red laser at 671 nm has been developed in this paper. With incident pump power of 10.6 W , employing a type-I critical phase-matched LBO crystal, 2.23 W single-transverse-mode red light at 671 nm was obtained, with optical-to-optical conversion efficiency up to 21.0%.

Diode-pumped passively Q-switched intracavity frequency doubled Nd:GdVO4/KTP green laser

Abstract: A compact diode-pumped passively Q-switched intracavity frequency-doubled Nd:GdVO4/KTP green-pulse laser was demonstrated, using Cr4+:YAG as a saturable absorber in a simple flat–flat cavity. With a 5.9 W incident pump power, a passively Q-switched green laser was obtained with an average power of 397 mW , repetition rate of 40 kHz , and pulse width of 40 ns , when the initial transmission of Cr4+:YAG was 85%. The shortest pulse width of 30 ns , the highest green peak power of 696 W and the maximum pulse energy of 21 μJ were obtained when the initial transmission of Cr4+:YAG was 70%. Under CW green operation, we obtained 440 mW output power.

Group refractive index measurement by fringes of equal chromatic order

Abstract: Fringes of equal chromatic order in transmission across a thin liquid or a thin solid sample inside a wedge interferometer, followed with a grating spectrograph, are produced. A single-shot interferogram of the air and sample regions is recorded. Locations of fringes maxima in the air region are fitted in a numerical procedure based on Cauchy's dispersion function. Then it is used for measuring the interferometric gap thickness. The order of interference in the sample region is represented by a third-order polynomial in the wavenumber for deducing the sample group refractive index. An error analysis of the measured group refractive index is given. The method is applied for measuring the group refractive index of water and mica samples across the visible spectrum. The method measures both the sample thickness and its group refractive index. It is static with no moving parts and suitable for thin liquid or solid samples without immersion liquids.

Long-gauge length embedded fiber optic ultrasonic sensor for large-scale concrete structures

Abstract: A fiber optic ultrasonic sensor based on Fizeau interferometer has been developed and demonstrated. A helium–neon laser light source with wavelength 0.6328 μm is used in our experiment. A special feature is its Fizeau configuration, which enables one to eliminate much undesirable noise by combining both the reference arm and the sensing arm within the same length of fiber. The dynamic response model of photo-elastic effect of ultrasonic wave and optical fiber is established. The fiber optic ultrasonic sensor experimental results are obtained and compared with the convenient PZT transducer.

Determination of the refractive index of fibres using the modified area method: I. Homogeneous fibres

Abstract: The mathematical formulae for determining the mean refractive indices and birefringence of homogeneous fibres are modified. This modification depends on calculating the area under the fringe shift and cross-sectional area of homogeneous fibres having nearly circular cross-sectional area taking the refraction of the incident light beam by the fibre into consideration. The mathematical analysis for two-beam and multiple-beam interferometric techniques is dealt with. These formulae are applied to polypropylene fibres with draw ratios of 3.5 and 4 using the two techniques. A comparison between the results when using the new formulae and when neglecting the refraction has been studied. The diffraction of a He–Ne laser beam is used for determining the shape of the cross-sectional area of these fibres. Microinterferograms are given for illustration.