Achromatic polarization rotator

What we believe to be a new optical element consisting of a single piece isotropic material with three internally reflecting faces for polarization plane rotation by the angle phi=pi/2 is proposed. The principle of operation is based on the geometrical phase effect, which is different in its origin but produces a similar effect to the circular birefringence. It is shown that the rotator made of polymer as well as silica glasses is not affected by the chromatic dispersion in a visible range.     

Multichannel polarization interferometers

Measurement Techniques -     

Achromatic interfero-coronagraph with two common-path interferometers in tandem

To attain deeper nulling for an extended incoherent star disk a scheme for an achromatic interfero-coronagraph, incorporating two common-path interferometers in tandem, is proposed. Analytical and numerical predictions of the performance, which are in reasonably good agreement, are presented. The predicted performance improvement, by using two interferometers in tandem, is demonstrated by a preliminary experiment. A star coronagraph based on the proposed technique has the possibility to reach a 10(-10) achromatic nulling contrast for an almost 10(-2) lambda/D effective source size.     

Self-referenced rectangular path cyclic interferometer with polarization phase shifting

A polarization phase shifting interferometer using a cyclic path configuration for measurement of phase nonuniformities in transparent samples is presented. A cube beam splitter masked by two linear polarizers is used to split the source wavefront into two counter propagating linearly polarized beams that pass through the sample. At the output of the interferometer, the two orthogonally polarized beams are rendered circularly polarized in the opposite sense through the use of a quarter wave plate. Finally, phase shifting is achieved by rotating a linear polarizer before the recording plane. In a rectangular path interferometer, although the two counter propagating wavefronts are laterally folded with respect to each other in the interferometer arms, the beams finally emerge mutually unfolded at the output of the interferometer. This phenomenon is utilized to create a reference if the sample is introduced in one lateral half of the beam in any one of the interferometer arms. The polarization phase shifting technique is used to generate four phase-shifted interferograms, which are utilized to evaluate the phase profile of the phase sample. Experimental results presented validate the proposed technique.     

Achromatic wave retarder by phase subtraction

We analyze the method of phase subtraction in two identical optical structures to build an achromatic phase retarder. The two structures are made of right-angle prisms and are aligned orthogonal to each other. They are also made of materials of different refractive indices so that dispersion compensation can be taken advantage of. Essentially the phase retardation between the s and p waves in the first structure is subtracted from the phase retardation in the second structure. This can be done by reversing the roles of the s and p waves. By choosing the materials of the prisms properly, the phase retardation can be made to be constant over a broad spectral range. Indeed, calculations made with commercial optical glasses show that phase errors in the visible and near-infrared regions can be rather small. For example, for a 90° phase retarder (quarter-wave plate), a phase error of 0.35° can be obtained from 0.35 to 0.81 μm and from 0.59 to 1.26 μm.     

Fiber Fabry-Perot interferometers with very low polarization sensitivity

The sensitivity to input polarization in long-cavity (1 cm to 5 m) and high-finesse (~ 100) fiber Fabry-Perot interferometers (FFPI's) is reduced from greater than 20 dB to between 0.2 and 0.6 dB in passband amplitude variation by the use of very-low-birefringence fibers. Furthermore the scanning FFPI's could allow very-high-resolution measurement of polarization-mode dispersion in short-length low-birefringence fibers.     

Real-time phase tracking in single-photon interferometers

A new technique for phase tracking in quantum cryptography systems is proposed that adjusts phase in an optimal way, using only as many photon counts as necessary. We derive an upper bound on the number of photons that need to be registered during phase adjustment to achieve a given phase accuracy. It turns out that most quantum cryptosystems can successfully track phase on a single-photon level, entirely with software, without any additional hardware components or extensive phase-stabilization measures. The technique is tested experimentally on a quantum cryptosystem.     

Inhomogeneous phase shifting: an algorithm for nonconstant phase displacements

In this work, we have developed a different algorithm than the classical one on phase-shifting interferometry. These algorithms typically use constant or homogeneous phase displacements and they can be quite accurate and insensitive to detuning, taking appropriate weight factors in the formula to recover the wrapped phase. However, these algorithms have not been considered with variable or inhomogeneous displacements. We have generalized these formulas and obtained some expressions for an implementation with variable displacements and ways to get partially insensitive algorithms with respect to these arbitrary error shifts.     

Signal-to-noise ratio of phase sensing telescope interferometers

The paper described is the third part of a trilogy dealing with the principles, performance, and limitations of what the author named "telescope-interferometers" (TIs). The basic idea consists in transforming one telescope into a wavefront error (WFE) sensing device. This can be achieved in two different ways, namely, off-axis and phase-shifting TIs. In both cases the point-spread function measured in the focal plane of the telescope carries information about the transmitted WFE, which is retrieved by fast and simple algorithms suitable to an adaptive optics (AO) regime. The uncertainties of both types of TIs are evaluated in terms of noise and systematic errors. Numerical models are developed to establish the dependence of driving parameters such as useful spectral range, angular size of the observed star, or detector noise on the total WFE measurement error. The latter is found particularly sensitive to photon noise, which rapidly governs the achieved accuracy for telescope diameters higher than 10 m. A few practical examples are studied, showing that the TI method is applicable to AO systems for telescope diameters ranging from 10 to 50 m, depending on seeing conditions and magnitude of the observed stars. Also discussed is the case of a space-borne coronagraph, where the TI technique provides high sampling of the input WFE map.     

Opaque optics thickness measurement using a cyclic path optical configuration setup and polarization phase shifting interferometry

Thickness measurement of an opaque optics using a cyclic path optical configuration (CPOC) setup and polarization phase shifting interferometry (PPSI) is presented. The CPOC setup is used to simultaneously focus two orthogonally polarized counterpropagating converging beams at its hypotenuse arm. The opaque optics is placed at the hypotenuse arm of the CPOC setup such that one of its surfaces reflects back one of the counterpropagating focusing beams. Because of the thickness of the opaque optics, the other focusing beam suffers a longitudinal shift in the beam focus. Applying PPSI, the longitudinal shift in the beam focus which is twice the thickness of the opaque optics is determined. The results obtained for a silicon plate of thickness 0.660 mm with a measurement uncertainty of 0.013 mm are presented.     

基于传输相位和几何相位协同调控的消色差超透镜

超透镜是超表面在成像领域中具有较大应用潜力的平面光学器件,但限于色差和较窄的工作带宽,通常难以应用于彩色成像及显示技术。本文设计了一种相位调控型透射式超透镜,实现了400 nm~650 nm波段的宽带消色差聚焦功能,带宽范围内焦平面处的平均聚焦效率约为29%。该方法利用具有低损耗、高折射率优势的二氧化钛(TiO2)介质柱结构,在可见光波段内获得了类似截断波导产生的传输相位响应。同时分析了几何相位和传输相位相结合的色散调控机制,并使用粒子群算法对构建的相位响应仿真数据库进行优化,完成了实际波面与理想聚焦波面的相位匹配。设计的宽带消色差器件有望在显微成像、计算机视觉和机器视觉等领域发挥作用。     

Polarization phase shifting cyclic interferometer for surface profilometry of non-birefringent phase samples

A rectangular path cyclic interferometer has the unique property that the counter-propagating wavefronts travelling in the interferometer arms are folded with respect to each other in the plane of the interferometer although the two wavefronts finally emerge from the interferometer unfolded. A phase disturbance introduced in one lateral half of the interferometer arm is therefore manifested in complementary lateral halves of the observed interference pattern. This phenomenon is utilized to evaluate the surface profile of a reflecting sample placed on one of the interferometer mirrors. The sample phase is retrieved using polarization phase shifting. Experimental results showing three-dimensional surface morphology of a small scale integrated circuitry directly etched on silicon are presented.     

Pseudorandom phase masks for superresolution imaging from subpixel shifting

We present a method for overcoming the pixel-limited resolution of digital imagers. Our method combines optical point-spread function engineering with subpixel image shifting. We place an optimized pseudorandom phase mask in the aperture stop of a conventional imager and demonstrate the improved performance that can be achieved by combining multiple subpixel shifted images. Simulation results show that the pseudorandom phase-enhanced lens (PRPEL) imager achieves as much as 50% resolution improvement over a conventional multiframe imager. The PRPEL imager also enhances reconstruction root-mean-squared error by as much as 20%. We present experimental results that validate the predicted PRPEL imager performance.     

Investigation of polarization-induced fading in fiber-optic interferometers with polarizer-based polarization diversity receivers

A technique is described to eliminate polarization-induced fading in fiber-optic interferometers with two or three polarizers. Modeling and simulation show that two polarizers should be angularly spaced by 90 degrees to alleviate polarization-induced fading. A test system is set up with two 6.5 km fiber coils to link the data. A bistate polarization diversity receiver is tested for reducing polarization-induced fading. The signal is demodulated using a digital phase-generated carrier and the system demonstrates self-noise up to 7.7 microrad/square root Hz (at 1 kHz).     

Two mechanisms of phase modulation in multimode fiber-optic interferometers

The mechanisms of phase modulation of optical radiation in multimode light guides are treated, namely, the first mechanism with equal relative phase increments for all modes and the second mechanism including the mode interaction and characterized by different relative phase increments for each mode. It is demonstrated theoretically that, in the case of several modulating effects on the light guide, the behavior of signals of modal interference from each one of those effects upon variation of the parameters of the medium surrounding the fiber depends on the type of effect. The variations of signals from the effects of the first type are in full agreement. Signals from effects of the second type or of different types may vary differently. In experiments with an intermode fiber-optic interferometer 500 m long, modulators associated with the mechanism of the first type exhibited a high correlation (97%) of the behavior of signals upon drift of the light guide temperature. The variations of signals from modulators of the second type and of different types were almost uncorrelated (6 and 1%).     

Achromatic phase-shifting for white-light interferometry

The geometric (Pancharatnam) phase provides a method of introducing a variable phase shift that is almost independent of the wavelength and opens up new possibilities in broadband interferometry.