Achromatic nulling interferometer for stellar coronography

A common-path interferometer that is intended for observation of a faint off-axis light source against the background of an intense on-axis source is suggested. The on-axis source and its replicated image acquire an achromatic phase shift of 180° and interfere in antiphase. The achromatic phase shift is due to the geometrical phase in the configuration of a 3D interferometer. Interference spatially separates the dark and bright fields of the on-axis source by directing them to different sides of a beam splitter. At the same time, interference does not attenuate the field of the off-axis source, directing it to different sides of the beam splitter with equal intensities. Such a configuration of the common-path interferometer provides mechanical stability.     

One-dimensional stellar and solar speckle interferometry

A new technique called “one-dimensional speckle-interferometry” is analysed. It appears to be a good compromise between standard speckle-interferometry and Michelson interferometry for precise, high resolution astronomical measurements.     

Simple beam combination for stellar interferometry

In stellar interferometry, image quality improves significantly with the inclusion of more telescopes and the use of phase closure. We demonstrate, using first coherent and then partially coherent white light, a compact and efficient pairwise combination of 12 or more beams. The input beams are lined up and spread through a cylindrical lens into a comb of parallel ellipses, which interferes with a perpendicular copy of itself to form a matrix of interferograms between all pairs. The diagonal elements show interference of each beam with itself, for intensity calibration. The measured white-light visibilities were high and stable.     

Physical foundations of achromatic nulling interferometry for stellar coronagraphy

We propose an achromatic interferometer for the observation of a faint off-axis light source against the background of a bright on-axis light source. The on-axis source and its copy acquire an achromatic phase shift by 180° and interfere with a phase difference of π. The achromatic phase shift is attributable to the geometric phase in the three-dimensional interferometer scheme. Interference spatially separates the nulled and bright fields of the on-axis source, redirecting them on opposite sides of the beamsplitter. Interference does not attenuate the field of the off-axis source and redirects it with an equal intensity on both sides of the beamsplitter. We consider the principle of operation of the nulling interferometer and constraints on the attenuation of an extended source due to the decrease in coherence. The laboratory breadboard and experiment are briefly described.     

Current status on the contribution of silica fibres to stellar interferometry

This paper reports on guided and integrated optics set-ups developed in our laboratory in the framework of aperture synthesis for astronomy. After summarising the function required to design a stellar interferometer, we propose guided components for these purposes. The implementation of these components is illustrated by two examples of interferometers with a view to using guided components as often as possible and testing an end-to-end imaging demonstrator.     

The stellar interferometry of a star cluster with a prominent variable

By subtracting the autocorrelation pattern of a star cluster with a prominent variable obtained by stellar interferometry at the time of variable minimum from a similar pattern taken at the time of variable maximum, all cross terms due to non-variable stars disappear and the resultant pattern greatly simplifies the interpretation of the cluster.     

Photon noise limitations on the recovery of stellar images by speckle interferometry

The signal-to-noise ratio in steliar speckle interferometry is estimated by direct application of photon counting formulae. This treatment demonstrates that the limiting observable magnitudes are dependent on object size.     

Achromatic polarimeter

A polarimeter for measuring complete state of polarization of light in a wide spectral range is investigated. The light beam to be analyzed propagates through a continuously rotating first-phase plate; then through a continuously rotating second-phase plate; and, passing through a stationary linear polarizer, is incident on a detector. The time dependence of the intensity of the light beam that propagated though the polarizer is recorded. Fourier analysis of the periodic time dependence of the intensity yields the state of polarization of the light, which is characterized by four Stokes parameters. It is shown that the relation between the rotational velocities of the two phase plates and their thicknesses can be chosen such that the working range of the polarimeter would be limited only by the transparency range of the birefringent material of the phase plates, the working range of the linear polarizer, and that of the detector.     

Speckle interferometry through small multiple apertures: Michelson stellar interferometer and aperture synthesis in optics

The use of a Michelson stellar interferometer or multiple-hole interferometers is considered as a special case of speckle interferometry. Formulas are given for the power spectrum of the interference pattern produced by the multiple apertures. The limitation in stellar magnitude is shown to increase with the number of apertures, while it is independent of the telescope size when a large single aperture is used.     

Achromatic Schmidt corrector plates

Two types of achromatic Schmidt corrector plate are described; both give an on-axis Strehl number S 0.8 over an extended wavelength region. It is found that this specification can only be achieved by combining a positive and a negative plate and by restricting the speed ratio D/N³ of the system. The theory is illustrated by numerical examples.     

Achromatic optical phase shifter-modulator

We propose and demonstrate, theoretically and experimentally, a novel achromatic optical phase shifter-modulator based on a frequency-domain optical delay line configured to maintain zero group delay as variable phase delay is generated by means of tilting a mirror. Compared with previously reported phase shifter-modulators, e.g., based on the Pancharatnam (geometric) phase, our device is high speed and polarization insensitive and produces a large, bounded phase delay that, uniquely, is one-to-one mapped to a measurable parameter, the tilt angle.     

Achromatic optical vortex lens

A vortex lens is a useful optical device having applications ranging from astronomy to microscopy. Current vortex masks operate across a narrow bandwidth. Two design schemes are proposed for creating a vortex across a bandwidth exceeding 100 nm in the visible region of the spectrum.     

Achromatic polarization-preserving beam displacer

We present a novel device based on four orthogonal reflections that displaces an optical beam while preserving the state of polarization. The principle of operation is the overall compensation of the phase shifts that s and p polarization components of the light acquire at each reflection. This compensation, which relies on the use of four identical reflectors, is independent of the actual values of the s and p reflection coefficients and thus is independent of the wavelength. Measurements of the polarization-preserving properties with different sets of reflectors and tolerances to misalignments are presented.     

Achromatic analog of a quarter-wave plate

Is it possible to achieve wavelength-independent transformation of the polarization of light on the basis of adiabatic tracking of smooth variation in the parameters of the medium by the polarization? This question is considered for conditions where circularly polarized light is converted into linearly polarized light.     

Achromatic change of circular polarization handedness

We present a new kind of achromatic device that can change very effectively the circular polarization handedness. The device is composed of a sequence of traditional half-wave plates rotated at specific angles with respect to their fast-polarization axes. The proposed apparatus is robust to variations in the parameters of both the crystal and the incoming light, these include the crystal temperature, the frequency of the incoming field, the crystal length and the angle of incidence.     

三元复合式消色差补偿器的研究与测试

光学补偿器是一种光相位可在一定范围内连续可调的延迟器件，是偏光技术中的一个重要光学元件。对于三元复合式补偿器来说，只需转动其中的一个波片便可实现延迟量在0～2π之间连续可调，而且调节方便，调整量大，精度高。本文在复合式补偿器研究基础上，进一步从理论和实验上证明了该补偿器不仅具有相位补偿性能，而且在一定的光谱范围内满足消色差性能，削弱了延迟量对波长的依赖关系，适合于复色光使用。     

Achromatic half-wave combination of birefringent plates

Half-wave retarders are primarily used in rotating the plane of polarization of polarized light. These retarders usually exhibit strong wavelength dependence. In this paper, the design and characteristics of an achromatic half-wave plate, formed by a cascaded system of birefringent plates, have been studied. Pancharatnam proposed a combination of three retarders in series and discussed the possibility of fabricating reasonably good achromatic quarter-wave plate with a suitable combination of their retardance. The system studied here is similar in construction to Pancharatnam configuration and behaves as an achromatic half-wave plate over a wide spectral range. The proposed configuration exhibits a maximum variation of only about ±1.4 degree over the entire wavelength range of 500–750 nm. In our analysis we have used Jones matrix formalism for the derivation of the general expression for the equivalent retardation and the azimuth of the combination.     

光学介质薄膜优化的宽带消色差λ/4相位延迟器

根据全反射相变理论设计了菲涅耳菱体型相位延迟器延迟量对入射角变化不敏感的结构角。根据薄膜的偏振效应,从薄膜的特征矩阵出发,通过在菱体的两个全反射面蒸镀光学介质膜的方法拓宽了菲涅耳菱体型相位延迟的消色差宽度。在ML_EB900型镀膜机上适当控制沉积条件制备了消色差相位延迟膜,实验测试结果表明:在530~700 nm的波长范围内,可以获得90°的相位延迟量,且最大偏差小于0.7°。     

Achromatic terahertz quarter-wave retarder in reflection mode

A compact achromatic quarter-wave retarder (QWR) operating in reflection mode is designed for using in terahertz region. It is a composite device utilizing form birefringence and Fabry–Pérot (FP) interference. Under illumination of plane waves with incidence angle of 45°, from 1.8 THz to 2.8 THz, the QWR achieved only ±2° variation around 90° phase retardation, enlarging the working bandwidth of ordinary QWR greatly. An analytical model combining transmission-line (TL) theory with effective medium theory (EMT) is presented and results agree well with the time-consuming numerical calculation. The 38 μm thick construction is simple and easy for fabrication by the existing lithographic technique and a promising application in terahertz or other frequency region is believed.