White Light Beam Research Articles

Wide angular range operation of a dielectric multilayer film with a photonic-crystal-type defect

Light can tunnel through a high-reflectivity dielectric multilayer film when a photonic-crystal-type defect is introduced in the structure, which is useful for optical signal processing. We consider chirped structures with a defect in layer thickness for which high reflectivity is achieved over a broad wavelength range except within a narrow spectral window. The useful transmission window, while it shifts toward shorter wavelengths as the angle of incidence of the light beam is increased, does not, in general, survive; i.e., transmission disappears progressively. We show that wide angular range operation can, however, be achieved by a proper design of the chirped structure. Analytical expressions for the design parameters are derived on the basis of a semi-infinite photonic crystal model. Theoretical reflectance spectra of defect SiO2/TiO2 chirped multilayer films are presented and discussed in terms of the dispersion of the electromagnetic radiation modes of the finite photonic crystal. These devices offer a simple way to mechanically tune (through inclination of the film) the wavelength transmitted from a fixed white-light beam.

A Diffuse Reflectance Measuring Instrument to Determine Peanut Pod Brightness

For Valencia peanuts, pod brightness is determined by inspectors during the peanut grading process when peanuts are sold by the grower. Presently, inspectors make visual observation of each peanut pod, from a sample drawn from a bigger lot, and determine the percentage of discolored pods present in the sample. The percent discolored pods is one of ten grade factors that determines the price at which the Valencia peanuts are sold. The visual method seems to be reliable, but it may not be consistent among inspectors nor over time due to the human element involved. An instrument may be more consistent and objective. The design and operation of an optical system for this purpose is described here. A narrow beam of white light was collimated onto the surface of a peanut pod at four consecutive positions spaced at 90 intervals around the circumference of the peanut pod. Diffuse reflectance from the peanut surface was measured at each position using a silicon detector with UV enhanced response. The average of these four values was a good indicator of the pod brightness and was used to detect discolored peanuts. Percent discolored pods determined using this instrument was highly correlated (r2 = 0.98) with the visual evaluations made by inspectors.

The Dynamics of Self-Trapped Beams of Incoherent White Light in a Free-Radical Photopolymerizable Medium

Detailed experimental studies of the dynamics of self-trapped beams of white light (400-800 nm) in a photosensitive organosiloxane medium are presented. Self-trapped white light beams with similar spatial profiles formed in the organosiloxane at intensities ranging across an order of magnitude (2.7-22.0 W.cm-2). Beam-profiling measurements showed that these spatially and temporally incoherent wave packets propagate without diffracting (broadening) by initiating free-radical polymerization of methacrylate groups and corresponding refractive index changes in the organosiloxane medium. Analyses of their temporal evolution showed that the intensity-dependent behavior of self-trapped white light is similar to that of self-trapped laser light despite the extreme differences in their phase structure and chromaticity; the self-trapped incoherent beams even show the complementary oscillations of width and intensity that is characteristic of self-trapped coherent light. Furthermore, the dynamics of the self-trapped white light beams was found to be strongly correlated to the kinetics of free-radical polymerization and corresponding rates of refractive index changes in the organosiloxane. These studies provide accessible photochemical routes to self-trapped incoherent wave packets, which are extremely difficult to generate in conventional nonlinear optical media that owe their responses to higher-order dielectric susceptibility tensors. This could enable the experimental verification of theoretical models developed for the nonlinear propagation of white light and stimulate research into more complex self-trapping phenomena such as the interactions of self-trapped incoherent beams and spontaneous pattern formation due to modulation instability in a uniform incoherent optical field. These findings also carry potential for the development of self-induced waveguide, optical solder and interconnect technology for incoherent light emitted by incandescent sources or LEDs.

An interferometer for use with gauge blocks up to 1000 mm with laser and white-light sources

A displacement interferometer is described for measuring gauge blocks, in which a spatial filter is used to combine a laser beam with a white-light beam, and the length of the block is calculated from the phase differences of the laser signal corresponding to the centers of gravity of the achromatic fringes from the gauge and from the reference plate.

Sizing particles by backscattering spectroscopy and Fourier analysis

Optical single backscattering spectroscopy can be used for sizing particles suspended in aqueous solution. In this work, we present results of backscattering spectroscopy applied to the determination of radii of calibrated spherical latex microparticles when a beam of white light is incident on the sample. From Mie calculations and Fourier analysis, we can determine the radius of particles covering the range between 0.5 and 12 µm for monomodal samples, with a mean error of 0.7 µm. To improve the accuracy, a correlation algorithm is applied that reduces the uncertainty in more than an order of magnitude and compares to the traceable error given by the manufacturer. The method can be also applied to bimodal and trimodal samples, allowing the separation of different size components.

Multicolor Photoluminescence from Porous Silicon Using Focused, High‐Energy Helium Ions

–2 intensity. During photoirradiation, we monitored the optical texture of the blue phases with a polarizing optical microscope. Optical experiments were conducted with an optical microscope (IX71, Olympus) combined with spectrometers (USB2000, Ocean Optics). We focused the non-polarized white-light beam from a tungsten halogen light source (L7893, Hamamatsu Photonics) on the surface of the sample with a spot size of 30 lm in diameter through an objective (50, LC Plan Apo, Olympus). The spontaneous emission spectra from the blue phases were measured by using a continuouswave (CW) laser (532 nm, 0.13 mW, Edmund Optics), and lasing spectra were measured by a Q-switched Nd:YAG (yttrium aluminum garnet) laser (532 nm, 8 ns pulse, Polaris II, New Wave Research). The excitation energy was adjusted using a half-wave plate in combination with a polarizing prism. The excitation beam was conducted to the optical microscope and focused onto the sample surface through an objective (20, LC Plan Apo, Olympus) to obtain a spot size of about 50 lm. We collected the laser emission from the sample in transmission and analyzed the spectrum with a spectrometer (Spectra Pro. 150 and charge-coupled device (CCD) detector 256HB, Acton Research).

Self-Trapping of Spatially and Temporally Incoherent White Light in a Photochemical Medium

We report that a beam of spatially and temporally incoherent white light self-traps by initiating free-radical polymerization in an organosiloxane medium. Refractive index changes due to polymerization lead to the formation of a narrow channel waveguide that traps and guides the entire multimode, broadband beam without diffraction. The response time of the system, which is determined by the inherently slow rate of free-radical polymerization, exceeds by several orders of magnitude the femtosecond-scale random phase fluctuations that characterize white light. Self-trapping of incoherent light is possible in the photochemical medium because it responds to the time-averaged intensity profile of the white light beam.

Portable light transmission measuring system for preserved corneas

BackgroundThe authors have developed a small portable device for the objective measurement of the transparency of corneas stored in preservative medium, for use by eye banks in evaluation prior to transplantation.MethodsThe optical system consists of a white light, lenses, and pinholes that collimate the white light beams and illuminate the cornea in its preservative medium, and an optical filter (400–700 nm) that selects the range of the wavelength of interest. A sensor detects the light that passes through the cornea, and the average corneal transparency is displayed. In order to obtain only the tissue transparency, an electronic circuit was built to detect a baseline input of the preservative medium prior to the measurement of corneal transparency. The operation of the system involves three steps: adjusting the "0 %" transmittance of the instrument, determining the "100 %" transmittance of the system, and finally measuring the transparency of the preserved cornea inside the storage medium.ResultsFifty selected corneas were evaluated. Each cornea was submitted to three evaluation methods: subjective classification of transparency through a slit lamp, quantification of the transmittance of light using a corneal spectrophotometer previously developed, and measurement of transparency with the portable device.ConclusionBy comparing the three methods and using the expertise of eye bank trained personnel, a table for quantifying corneal transparency with the new device has been developed. The correlation factor between the corneal spectrophotometer and the new device is 0,99813, leading to a system that is able to standardize transparency measurements of preserved corneas, which is currently done subjectively.

Interference of white light in tandem configuration of birefringent crystal and sensing birefringent fiber

Spectral interference of white-light beams propagating through a tandem configuration of birefringent crystal and sensing birefringent fiber is analyzed theoretically and experimentally. The spectral interference law is expressed analytically under the condition of a Gaussian response function of a spectrometer taking into account the dispersion of birefringence in the crystal and in the fiber. Two types of spectral interferograms are modeled knowing dispersion characteristics of the sensing fiber and using a quartz crystal of the positive or a calcite crystal of the negative birefringence. The theoretical analysis is accompanied by two experiments employing a highly birefringent fiber and a birefringent quartz crystal of two suitable thicknesses. Within both experiments the spectral interference fringes are resolved in accordance with the theory with phases dependent on the fiber length.

Transmission of digital images consisting of white-light dark solitons

A distortion-free digital image is transmitted through parallel propagation of white-light photovoltaic dark solitons. The waveguide channels induced by white-light dark solitons can guide both the laser beam and the white-light beam well. We determine experimentally the critical separations of the dark solitons in the directions parallel and perpendicular to the crystalline c axis for the given crystal thickness.

Scanning white-light interferometer for measurement of the thickness of a transparent oil film on water

The thickness of a transparent layer of oil upon the surface of water is measured as the distance between the surface of oil film and the interface of the oil with the water. Two experimental results have demonstrated that the interface can reflect a white-light beam well enough to form an interferogram, even if the light is subjected to oil-film dispersion. When a beam of white light is incident vertically onto the oil-film surface, a scanning white-light interferometer in the Michelson configuration is employed to locate two serial reflections, surface reflection and interface reflection. The thickness of the transparent oil film on water is calculated based on the separation of these two interferograms. A limitation thickness, approximately 250 microm with 1.25 microm resolution, is achieved under the condition that there is 50 nW of optical power incident onto the oil-film surface with a wavelength centered at 1310 nm.

Newton, Goethe and the process of perception: an approach to design

Whereas Newton traced a beam of white light passing through a prism and fanning out into the colours of the rainbow as it was refracted, Goethe looked through a prism and was concerned with understanding what his eye subjectively saw. He created a sequence of experiments which produced what appeared to be anomalies in Newton's theory. What he was carefully illustrating concerns limitations accepted when following a scientifically objective approach. Newton was concerned with the description of ‘facts’ derived from the analysis of observations. Goethe was concerned with the synthesis of meaning. He then went on to describe subjective techniques for training ‘the mind's eye’ to work efficiently in the subjective world of the imagination. Derided as ‘not science’, what he was actually describing is the skill which is central to creative design.

Interference of white light analyzed at the output of a birefringent crystal by a fibre-optic spectrometer

Spectral interference of white-light beams propagating through a dispersive birefringent crystal alone or in a tandem configuration with a Michelson interferometer is analyzed theoretically and experimentally, including the effect of a fibre-optic spectrometer. First, the spectral interference laws are expressed analytically under the condition of a Gaussian response function of a fibre-optic spectrometer and the effect of the limiting factors is specified. Second, the theoretical analysis is accompanied by two experiments employing a Michelson interferometer, a birefringent calcite crystal of two suitable thicknesses and a fibre-optic spectrometer. Within both experiments the interference fringes are resolved only in a narrow spectral range around the so-called equalization wavelength at which the overall group optical path difference between interfering beams is zero. Knowing dispersion and thicknesses of the calcite crystal along with the bandpass of the spectrometer, the theoretical spectral interferograms and equalization wavelengths are determined and good agreement with experiment is confirmed.

Spatiotemporal coherence of white light beam trapped within dark spatial soliton

We study experimentally and theoretically temporal and spatial coherence of white light beam through its interaction with a coherent dark spatial soliton in LiNbO3:Fe noninstantaneous nonlinear self-defocusing crystal. Experimental observations show that during this process a portion of the white light beam is trapped within the dark notch of the dark spatial soliton, thus forming a sharp intensity spike. Moreover, numerical simulations show that in this region the coherence length of white light beam increases from ∼11 to 1200μm after 6mm of propagation for almost all frequencies. We find that the intensity spike of each frequency of white light increases with the increase of frequency. While the spatial coherence length of each frequency of white light decreases with the increase of frequency.

Degenerate two-photon-absorption spectral studies of highly two-photon active organic chromophores.

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (>or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.

Optical Profilometry as a Non-Destructive Technique to Quantify Engraving on Medieval Brass Astrolabes

ABSTRACTOptical profilometry has been performed on an astrolabe dated 1556 AD and attributed to the famous maker Gualterus Arsenius of Louvain, Belgium. In this non-destructive technique, a beam of white light is impinged on the sample and combined with the chromatic aberration technique to accurately measure surface features. With machine parameters set to a vertical resolution of 0.1 μm, lateral resolution to 6 μm and a 3 mm depth of field, various engraving features on the astrolabe were studied. Both hand-scribed and stamped markings are present on the astrolabe components. Inspection of the hand-scribed date on the instrument's mater allows one to quantify the shape and dimensions of the tool used, as well as the direction and inclination of the engraving tool during inscription. The results of this analysis give information about how the astrolabe was constructed in one of the Renaissance's greatest scientific instrument workshops. The results are compared to similar engravings on an undated and unsigned Islamic astrolabe tympan.

Degenerate nonlinear absorption and optical power limiting properties of asymmetrically substituted stilbenoid chromophoresElectronic supplementary information (ESI) available: Experimental details. See http://www.rsc.org/suppdata/jm/b3/b313185h/

Two-photon absorption (2PA) spectra (650–1000 nm) of a series of model chromophores were measured via a newly developed nonlinear absorption spectral technique based on a single and powerful femtosecond white-light continuum beam. The experimental results suggested that when either an electron-donor or an electron-acceptor was attached to a trans-stilbene at a para-position, an enhancement in molecular two-photon absorptivity was observed in both cases, particularly in the 650–800 nm region. However, the push–pull chromophores with both the donor and acceptor groups showed larger overall two-photon absorption cross-sections within the studied spectral region as compared to their mono-substituted analogues. The combined results of the solvent effect and the 1H-NMR studies indicated that stronger acceptors produce a more efficient intramolecular charge transfer character upon excitation, leading to increased molecular two-photon responses in this model-compound set. A fairly good 2PA based optical power limiting behavior from one of the model chromophores is also demonstrated.

Observation of chromatic effects near a white-light vortex

Using a spatial light modulator to modify the phase of a light beam we create a spatially coherent, white-light beam containing an optical vortex. All the spectral components are helically phased; hence the beam carries an orbital angular momentum that is an integer multiple of ℏ per photon. A low-dispersion prism, positioned after the modulator, ensures that the vortices associated with each spectral component are co-axial. In addition, deliberate introduction of slight spectral dispersion means that the vortices associated with each wavelength no longer overlap. Subsequent examination near the beam axis reveals the chromatic effects predicted by Berry (2002 New J. Phys.4 66; 74).

An investigation of the temporal coherence length of light

An interference filter is constructed from a pair of front-surface two-way mirrors.These partially reflecting mirrors are clamped at both ends with a small spacerplaced at one end, thus forming a thin wedge of air between their face-to-facemirrored surfaces. A series of thin film interference spectra are recorded bymoving this air wedge apparatus in incremental steps across a beam of whitelight. The mirror separation as a function of position along the wedgeis obtained from analysis of the transmitted spectra. As the separationincreases, temporal coherence is lost for successively longer wavelengths untilinterference effects are no longer observed. We define coherence length measuredin this way as twice the wedge thickness where the interference fringepattern is reduced to the noise level of the transmitted spectrum. Usingthis definition and technique, a value of 224 ± 20 µmis obtained using a white light source. This value is much larger than onewould expect for the coherence length of white light measured by morestandard techniques, e.g. Michelson interferometry. The implication of ourexperiment is that coherence length is a property of the light reaching thedetector, which in our case is the light transmitted by the interference filter.

Relevance of roughness parameters for describing and modelling machined surfaces

Describing and modelling a machined surface require the selection of relevant roughness parameters. However, this selection is difficult since a machined surface morphology can be described by a large number of roughness parameters. This investigation focuses on the roughness of metallic surfaces taking for two applications: a) the description of machined surface morphologies produced by grinding b) the relationships between machined surface morphologies (grinding or cold-rolling) and their brightness level when irradiated by the white light beam of an optical glossmeter used for industrial surface quality control. For each application, the aim is to determine, from an objective quantitative point of view, the relevance of one hundred or so surface roughness parameters. To reach this objective, a specific software program has been developed to determine a ranking of relevance thanks to the calculation of a computed statistical index of performance. The statistical results of this study show that the fractal dimension estimated by an original method is the most relevant roughness parameter to describe the surface morphology after grinding or rolling. Because of this relevance, this roughness parameter has also to be taken into consideration in models showing the interactions between machined surfaces and an optical wave. The methodology presented in this study can be a useful tool in the quality control phase to keep under control the fabrication process parameters of manufactured objects in industrial environments.