A preliminary study of the distribution of plankton using hologrammetry

The distribution and dynamics of particles in the aquatic environment play an important role in the modelling of bio-geochemical processes. Previous work on the measurement of such particles, which vary in size from tens of micrometres (individual cells) to several centimetres (aggregates such as ‘marine snow’), has mainly used electronic counting or conventional photography coupled with image analysis. Here we report on an initial study of the use of holographic mensuration, otherwise known as hologrammetry, for the enumeration, sizing and spatial distribution determination of plankton. We present results on imaging plankton in water tanks using both in-line and off-axis pulsed-laser holography. In this work, we have recorded in-line holograms in a volume of 2400 cm³ of water with a resolution of better than 20 μm and off-axis holograms in a volume of 36000 cm³ with a resolution of 140 μm. In both cases, identifiable images of plankton were obtained and precise spatial coordinates determined from the in-line holograms.     

Dynamic moiré patterns produced by sinusoidal gratings superposition in the Bi12 TiO20 crystal

In the present note a real-time holographic moire-like pattern is shown. It is produced by the superposition of two rotated sinusoidal phase gratings and the result represents a promising technique for possible metrological applications. The experiments were performed in a diffusion-only recording mechanism in a photorefractive Bi₁₂TiO₂₀ crystal illuminated by λ = 0.633 μm from a HeNe laser light source.     

Microscopic 3-D displacement field measurements using digital speckle photography

A technique to measure object shape and 3-D displacement fields in micro-scale is offered by microscopic stereo digital speckle photography. The displacement of the random features that are often present on many engineering surfaces when viewed in a microscope is measured with the system, using image correlation. In this paper the equipment, physical model and calibration routines are described. The technique can be applied for sub-mm sized objects of arbitrary shape for small deformation fields. As a verifying experiment, an in-plane rotation of a flat calibration plate is presented. The expected in-plane errors are shown to be less than 0.1 μm and the corresponding out-of-plane errors about three times larger. As a pilot experiment, micro-structural paper expansion is studied, when exposed to humidity. The scaling properties of the microscope as well as the sampling criteria and reliability of the system are discussed in detail.     

Echo-Planar Imaging of Porous Media with Spatial Resolution below 100 μm

A modified version of the echo-planar imaging technique incorporating a Carr–Purcell train of 180° rf pulses (PEPI) has been implemented on a standard spectrometer. It is demonstrated that artifacts in the image due to cumulative errors in the rf field can be reduced by replacing each 180° pulse by a composite sequence of three rf pulses. Artifact-free 3D images at 94 μm voxel resolution are obtained within 15 min. This technique has been applied to study the drying process in an initially water-saturated model porous medium with characteristicT^*₂of order 700 μs.     

Application of ESPI to NDT

Interest in optical methods of NDT increased rapidly with the advent of holographic interferometry which made it possible to observe surface increments of less than 1 μm on practically any surface texture. Invariably the optical method is coupled with a mechanical loading cycle which tests the component directly in terms of its strength, a claim which cannot be made for most other NDT methods. Holographic interferometry is unfortunately slow and expensive to apply and other methods of optical examination based on coherent optical scattering have been investigated. Of particular interest in the author's laboratories is the technique of electronic speckle pattern interferometry. This technique enables a process directly comparable with holographic interferometry to be carried out with a specially adapted closed circuit television system, giving on line information. This work is discussed together with some other relevant processes and methods of optical NDT.     

Measurement of wire diameter by optical diffraction

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.     

A high-precision optical metrology system for determining the thickness of birefringent wave plates

This study develops a non-destructive measurement system for determining the thickness and refractive indices of birefringent optical wave plates. Compared to previous methods presented in the literature, the proposed metrology system provides the ability to measure the thickness of the birefringent optical plate in high-precision. The results show that for a commercially available birefringent optical wave plate with refractive indices of n_e=1.5518, n₀=1.5427 and a thickness of 452.1428 μm, the experimentally determined value for the error in the wave plate thickness measurement is just 0.046 μm. The measurement resolution of the proposed system exceeds that of the interferometer hardware itself. The proposed method provides a simple yet highly accurate means of measuring the principal optical parameters of birefringent glass wave plates.     

Optical particle sizers for on-line applications in industrial plants

We present three optical particle sizers properly conceived for different on-line applications in industrial plants. The first one is a diffraction-based particle sizer utilising an innovative optical scheme that allows the instrument to operate at very low particle concentration regimes (i.e. at extinction values as small as 10⁻⁵) in the size range 0.9–90 μm. The second one is based on the same principle of operation, but has been designed with a different optical configuration that makes it suitable for the characterisation of high concentration particle laden flows (like for e.g. pulverised coal downstream of the grinding mills) in the size range 3–300 μm. The third instrument is based on a multi-wavelength extinction technique and provides measurements over long optical paths (up to 10 m) in the 0.1–3 μm size range.     

A compact 1.06/1.32/2.94 μm pulsed laser for dentistry

A three-wavelength pulsed laser for dental application is developed. The laser houses the Nd:YAG resonator (1.06/1.32 μm) for soft-tissue treatment and Er:YAG resonator (2.94 μm) for caries removal and fits and fissure treatment. Two heads share the cooling unit and two identical high-voltage power supply modules in order to achieve compactness. The Nd:YAG laser has 10 W at 1.06 μm and 7 W at 1.32 μm with a pulse duration of 100 μs. An Er:YAG laser of 2.94 μm has 3.5 W, 20 Hz and a pulse duration of 250 μs. The beams are delivered through fibers and the laser size is 75×55×32.5 cm.     

Simultaneous spectral calibration of two phase plates

Based on a new model for the spectral calibration of phase plates in pairs, experimental results are presented for three mica plates between 430 and 684 nm. One of the plates is 36.4 μm thick and the remaining two are identical, each being 28.6 μm thick. In the studied spectral range, the retardances varied from 123.5° to 86.8° for the first plate and from 96.9° to 68° for each of the two other plates. Different procedures for performing the calibration of the plates and the special case of two identical plates are discussed.     

Single exposure photography applied to a slow flow field

A speckle photography technique has been applied to measuring the velocity profiles of laminar and turbulent flow through pipes of circular cross-section. A single exposure photography technique has been used to visualize slow fluid flow. The fluids used were seeded with tracer particles whose size range from 20 to about 120 μm in diameter. A mathematical expression representing the recorded intensity is presented. A single exposure with a long exposure time was taken on a holographic film material. The recorded intensity was represented by the convolution product of the image formed and a rectangular function representing a one-dimensional continuous motion. The analysis shows that the intensity distribution of the Fourier spectrum of the diffracted image follows a sinc² function on a noise background representing the scattered light of the tracer particles.     

High resolution Moiré contouring by a hybrid technique combining light and electron optics

A technique for measuring the form of small objects is presented. The process is similar to conventional Moiré contouring but, by combining the advantages of light and electron optics, the problem of obtaining a large depth of field and high resolution is overcome. The technique is applied to the measurement of a Rockwell diamond indenter, using a scanning electron microscope to form height contours over a field diameter of 800 μm and to a depth of 200 μm. Qualitative results are presented and the expected accuracy of subsequent microprocessor aided, quantitative metrology is predicted.     

Efficient frequency upconversion of coherent radiation from 10.26 to 1.187 μm in a GaSe crystal

Frequency upconversion of laser pulses at 10.26 μm to those at 1.187 μm was achieved in the presence of Nd:YAG laser pulses based on difference-frequency generation in a 10 mm-long GaSe crystal. The highest power conversion efficiency for the parametric conversion was determined to be 20.9%, corresponding to the photon conversion efficiency of 2.42%. This value is two orders of magnitude higher than the highest value reported on GaSe in the literature. The saturation of the output power at 1.187 μm as the input power at 10.26 μm was increased, due to the back conversion, i.e. 1.187 μm + 10.26 μm → 1.064 μm, was clearly evidenced. Such a parametric process has potential for achieving sensitive detections of mid-infrared radiation.     

An Advanced Signal Processing Technique Designed for Multilevel Signal Optical Disc System

We propose a novel signal processing technique by which we can increase the effective signal to noise ratio of optical discs. This technique is capable of reducing the influence of random noise even when the playback signal contains a distortion. We apply this signal processing technique to single carrier independent pit edge recording optical disc system. When a linear recording density of 0.2 μm/bit and a track pitch of 0.8 μm for an optical pickup is used in a DVD pickup (γ =635 nm, NA=0.6), the experimental results show that the system can obtain tangential and radial skew margins of ±0.5° and ±0.75°, respectively. From the experimental results, we estimate that the proposed method is capable of increasing the capacity of DVD by a factor of about 1.2.     

Short wave pass and long wave pass dichroic coating at 45° on zinc selenide substrate for dual band thermal imager

In dual band thermal imager dichroic coating plays a vital role in separating 3–5 μm and 7.5–10.5 μm wavelength region for observing better image quality from two different channels. In this work a study has been carried out on the design and fabrication of short and long wave pass dichroic coating at 45° on zinc selenide flat substrate. These dichroic coated optics can be used to separate 3–5 μm (in reflection or transmission channel) and 7.5–10.5 μm (in transmission or reflection channel) wavelength region. An inhomogeneous refractive index profile which is a polynomial of 5th order was considered to design the high and low wave pass dichroic coating on zinc selenide substrate. The inhomogeneous profile was then approximated with five steps from substrate to air medium. These steps were then converted in terms of durable coating materials of six and seven layer stack for short and long wave pass dichroic coating respectively. The coating material combination used was germanium as high index material and IR-F625 as low index material. Result achieved for short wave pass dichroic filter was 94% average transmission in 3–5 μm region and 95% average reflection in 7.5–10.5 μm region. Similarly, result achieved for long wave pass dichroic filter was 95% average reflection in 3–5 μm region and 94% average transmission in 7.5–10.5 μm.     

Simulation study and experiment on laser-ablation surface cleaning

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.     

Holographic imaging of side-scattering particles

An off-axis holographic system in the side-scattering mode has been studied, and the requirements of fringe resolution and film sensitivity associated with holographic imaging of side-scattering particles analysed. By assuming the Mie scattering model, the fringe distribution about the y-axis (perpendicular to the plane of incidence of the reference light) was found to be asymmetrical, with unevenly distributed spacing, that is narrower in one x direction but greater in the side that is close to the origin. It was demonstrated that commercially available holographic film was sufficient to resolve the fringe frequency over the entire area of the holographic plate, at 60 ° for z = 120 mm, and that images of particles as small as 20 μm could be recorded. Astigmatism was found to occur and be responsible for the distortions of the particle images.     

Tuning of long-wavelength emission in InxGa1−xAs quantum dot structures

This work explores the conditions to obtain the extension of the PL emission beyond 1.3 μm in InGaAs quantum dot (QD) structures growth by MOCVD. We found that, by controlling the In incorporation in the barrier embedding the QDs, the wavelength emission can be continuously tuned from 1.25 μm up to 1.4 μm at room temperature. However, the increase in the overall strain of the structures limits the possibility to increase the maximum gain in the QD active device, where an optical density as high as possible is required. By exploring the kinetics of QD surface reconstruction during the GaAs overgrowth, we are able to obtain, for the first time, emission beyond 1.3 μm from InGaAs QDs grown on GaAs matrix. The wavelength is tuned from 1.26 μm up to 1.33 μm and significant improvements in terms of line shape narrowing and room temperature efficiency are obtained. The temperature-dependent quenching of the emission efficiency is reduced down to a factor of 3, the best value ever reported for QD structures emitting at 1.3 μm.     

Spectrophotometric calibration of a quarterwave plate

A new photometric method for calibrating a quarterwave phase plate is presented and the underlying physical phenomenon is explained. A mica sheet of thickness about 30 μm was found to introduce quarterwave phase retardance at 515 nm. The accuracy of the method and the sources of error are discussed.