Quarter-wave Retardation Plate Research Articles

A METHOD TO CALIBRATE THE HIGH-RESOLUTION CATANIA ASTROPHYSICAL OBSERVATORY SPECTROPOLARIMETER

ABSTRACT The Catania Astrophysical Observatory Spectropolarimeter (CAOS) is a white-pupil cross-dispersed échelle spectrograph with a spectral resolution of up to R = 55,000 in the 375–1100 nm range in a single exposure, with complete coverage up to 856 nm. CAOS is linked to the 36-inch telescope, at Mount Etna Observatory, with a couple of 100 μm optical fibers and it achieves a signal-to-noise ratio better than 60 for a V = 10 mag star in one hour. CAOS is thermally stabilized in temperature within a 0.01 K rms, so that radial velocities are measured with a precision better than 100 m s−1 from a single spectral line. Linear and circular spectropolarimetric observations are possible by means of a Savart plate working in series with a half-wave and a quarter-wave retarder plate in the 376–850 nm range. As is usual for high-resolution spectropolarimeters, CAOS is suitable to measure all Stokes parameters across spectral lines and it cannot measure the absolute degree of polarization. Observations of unpolarized standard stars show that instrumental polarization is generally zero at 550 nm and can increase up to 3% at the other wavelengths. Since polarized and unpolarized standard stars are useless, we suggest a method to calibrate a high-resolution spectropolarimeter on the basis of the polarimetric properties of spectral lines formed in the presence of a magnetic field. As applied to CAOS, observations of magnetic chemically peculiar stars of the main sequence show that the cross-talk from linear to circular polarization is smaller than 0.4% and that conversion from circular to linear is less than 2.7%. Strength and wavelength dependences of cross-talk can be entirely ascribed, via numerical simulations, to the incorrect retardance of achromatic wave plates.

Synchronization of Two Photoelastic Light Modulators to Obtain Müeller Matrix

We report a method for the temporal synchronization of two photoelastic light modulators. For synchronizing, we used the transistor-transistor logic output signals from each modulator, which contain the information on the light polarization. These signals were introduced in a phase-detector circuit, which provided the phase difference value between both modulators. Three optical devices were used to test the synchronization method proposed: a polarizer, a half-wave, and a quarter-wave retarder plate. The value of each of the elements of the Mueller matrix for these devices was obtained using the method of the 36 measurements. The results show a high correlation between the theoretical and experimental values obtained. The results are also in accordance with other values obtained by different methods reported in the literature. Errors calculated by the proposed method are approximately in the order of a thousandth. The advantages and disadvantages of the method are discussed.

Orientation of a bacteriorhodopsin thin film deposited by dip coating technique and its chiral SHG as studied by SHG interference technique

We show that by observing SHG interference bR thin films prepared by a simple dip coating technique have a polar orientation with C∞ symmetry. The SHG interference measurements were performed under various input and output polarization combinations at different incident angles or under the rotation of the quarter-wave retardation plate at specific incident angles. The interference patterns provide us with insight into the characteristics of non-vanishing nonlinear optical coefficients including chiral components. Abundant information can be obtained by observing SHG interference by using two chiral SH active films.

Linearly-polarized White-light-emitting OLEDs Using Photon Recycling

We herein report on the production of efficient polarized white electroluminescence (EL) from an organic light-emitting diode (OLED), which was fabricated using an achromatic quarter-wave retardation plate (QWP) film and a giant birefringent optical (GBO) photonic reflective polarizer. Luminous polarized white EL light of 14,600 cd/m, with a high peak efficiency (>18 cd/A), was produced from a polarized white OLED that was fabricated using this method. These values are almost double those achieved by using a similar white OLED with only a polarizer film. Furthermore, the ratio of linear polarization obtained was about 35:1 over the whole range of brightness of the white EL emission.

Effect of chromaticity on quarter-wave retarder performance

When a polarized polychromatic beam passes through a birefringent medium, the constituent spectral components suffer different change of state of polarization. As a result when the beam passes through an analyzer, the intensity of the resultant beam changes depending on the orientation of the analyzer, state of polarization of the input beam, spectral and spatial intensity distribution of the source and the polarizing properties of the birefringent medium. In the present study the intensity variation of the resulting beam passing through a quarter-wave retarder plate is observed theoretically with the variation of the azimuthal angle of the analyzer for monochromatic and polychromatic source of light. A simple experiment is conducted to observe the intensity variation for an incandescent lamp and HeNe laser as polychromatic and monochromatic source of light passing through a birefringent polymer-based quarter-wave retarder plate. The experimentally obtained values are compared with theoretical values and a good agreement is observed.

Design and testing of low intensity laser biostimulator

BackgroundThe non-invasive nature of laser biostimulation has made lasers an attractive alternative in Medical Acupuncture at the last 25 years. However, there is still an uncertainty as to whether they work or their effect is just placebo. Although a plethora of scientific papers published about the topic showing positive clinical results, there is still a lack of objective scientific proofs about the biostimulation effect of lasers in Medical Acupuncture. The objective of this work was to design and build a low cost portable laser device for stimulation of acupuncture points, considered here as small localized biosources (SLB), without stimulating any sensory nerves via shock or heat and to find out a suitable method for objectively evaluating its stimulating effect. The design is aimed for studying SLB potentials provoked by laser stimulus, in search for objective proofs of the biostimulation effect of lasers used in Medical Acupuncture.MethodsThe proposed biostimulator features two operational modes: program mode and stimulation mode and two output polarization modes: linearly and circularly polarized laser emission. In program mode, different user-defined stimulation protocols can be created and memorized. The laser output can be either continuous or pulse modulated. Each stimulation session consists of a pre-defined number of successive continuous or square pulse modulated sequences of laser emission. The variable parameters of the laser output are: average output power, pulse width, pulse period, and continuous or pulsed sequence duration and repetition period. In stimulation mode the stimulus is automatically applied according to the pre-programmed protocol. The laser source is 30 mW AlGaInP laser diode with an emission wavelength of 685 nm, driven by a highly integrated driver. The optical system designed for beam collimation and polarization change uses single collimating lens with large numerical aperture, linear polarizer and a quarter-wave retardation plate. The proposed method for testing the device efficiency employs a biofeedback from the subject by recording the biopotentials evoked by the laser stimulus at related distant SLB sites. Therefore measuring of SLB biopotentials caused by the stimulus would indicate that a biopotential has been evoked at the irradiated site and has propagated to the measurement sites, rather than being caused by local changes of the electrical skin conductivity.ResultsA prototype device was built according to the proposed design using relatively inexpensive and commercially available components. The laser output can be pulse modulated from 0.1 to 1000 Hz with a duty factor from 10 to 90 %. The average output power density can be adjusted in the range 24 – 480 mW/cm2, where the total irradiation is limited to 2 Joule per stimulation session. The device is controlled by an 8-bit RISC Flash microcontroller with internal RAM and EEPROM memory, which allows for a wide range of different stimulation protocols to be implemented and memorized. The integrated laser diode driver with its onboard light power control loop provides safe and consistent laser modulation. The prototype was tested on the right Tri-Heater (TH) acupuncture meridian according to the proposed method. Laser evoked potentials were recorded from most of the easily accessible SLB along the meridian under study. They appear like periodical spikes with a repetition rate from 0.05 to 10 Hz and amplitude range 0.1 – 1 mV.ConclusionThe prototype's specifications were found to be better or comparable to those of other existing devices. It features low component count, small size and low power consumption. Because of the low power levels used the possibility of sensory nerve stimulation via the phenomenon of shock or heat is excluded. Thus senseless optical stimulation is achieved. The optical system presented offers simple and cost effective way for beam collimation and polarization change. The novel method proposed for testing the device efficiency allows for objectively recording of SLB potentials evoked by laser stimulus. Based on the biopotential records obtained with this method, a scientifically based conclusion can be drawn about the effectiveness of the commercially available devices for low-level laser therapy used in Medical Acupuncture. The prototype tests showed that with the biostimulator presented, SLB could be effectively stimulated at low power levels. However more studies are needed to derive a general conclusion about the SLB biostimulation mechanism of lasers and their most effective power and optical settings.

Fiber optical loop mirror with a symmetrical coupler and a quarter-wave retarder plate in the loop

In this paper we report an experimental method to vary the transmission through a fiber optical loop mirror with a symmetrical coupler, highly twisted fiber and a quarter-wave (QW) retarder plate in the loop. We demonstrate that as the QW retarder plate is rotated it is possible to adjust the transmission over a wide range (from around 2% to around 82%) and that by adjusting the twist in the loop nearly ideal operation can be achieved. We provide a detailed theoretical analysis of the operating characteristics.

Second Harmonic Optical Activity of Tryptophan Derivatives Adsorbed at the Air/Water Interface

Optical activity in second harmonic reflection from the air/water interface of solutions of simple tryptophan (trp) derivatives has been investigated. The derivatives include d- and l-optical isomers of Boc-trp and dd- and ll-isomers of the dipeptide Boc-trp-trp, where Boc is tert-butyloxycarbonyl. An experimental method based on polarization modulation of fundamental radiation by rotation of a quarter-wave retardation plate has been used to completely characterize the second-order nonlinear susceptibilities of the chiral surfaces, for the fundamental wavelength λ = 564 nm. Boc-trp and Boc-trp-trp show similar chiral contributions to the nonlinear susceptibilities, which indicates that the chiral response does not depend on intramolecular interactions between trp residues. The efficiency of second harmonic reflection has been shown to be resonantly enhanced at the two-photon level, in the region of the near-UV bands (∼280 nm) of the indole chromophore of tryptophan. The origin of the chiral response is discussed in terms of simple microscopic models, and a comparison is made with optical activity of tryptophanyl compounds in linear spectroscopy. The peptide gramicidin, incorporating several trp residues, showed no evidence of optical activity in second harmonic reflection from the air/water interface. This indicates that second harmonic optical activity is sensitive to structural effects such as conformational and orientational distributions of trp residues at the interface.

3M PP2500™ film as quarter wave retarder for light at [formula omitted] nm

In this work we introduce the use of the 3M PP2500™ film as a quarter wave retarder plate for light at λ=632.8 nm. This film is made of polyester and distributed by the 3M company. Although this film behaves as other commercial retarder devices, it has several advantages that are a low cost and availability.

Transmission-mode perpendicular incidence ellipsometry of anisotropic thin films

We show that transmission-mode perpendicular incidence ellipsometry, which yields the trivial values Psi =45 degrees , Delta =0 for an isotropic thin film, can be used to provide useful data during the vacuum deposition of anisotropic coatings. Thus the number of quarter-wave film thicknesses necessary for a quarter-wave retardation plate can be determined simply by counting interference modulations on a Delta versus Psi graph, and the material property Delta n, the difference between the slow and fast axis refractive indices, is available from the gradient of a smoothed Delta versus thickness graph. The influences of differential absorption, differential refractive index inhomogeneity, and instrumental errors on the ellipsometric parameters are discussed.

Accuracy of Polarization Attenuators

The accuracy of various polarization attenuators is discussed, including systematic errors due to imperfections of the polarizers, setting errors of the rotating elements, alignment errors of fixed elements, and errors caused by oblique incidence and partial polarization of the incident light. Whereas the accuracy of conventional three-polarizer attenuators is limited to 0.001 transmittance unit, at least ten times more accuracy can be achieved with two new types of attenuators, which employ either a half-wave or a quarter-wave retardation plate in conjunction with two sheet polarizers.