Design of superachromatic quarter-wave retarders in a broad spectral range

Tunable broadband polarization retarders

We theoretically propose a type of tunable polarization retarder, which is composed of sequences of half-wave and quarter-wave polarization retarders, allowing operation at broad spectral bandwidth. The constituent retarders are composed of stacked standard half-wave retarders and quarter-wave retarders rotated at designated angles relative to their fast polarization axes. The proposed composite retarder (CR) can be tuned to an arbitrary value of the retardance by varying the middle retarder alone while maintaining its broadband spectral bandwidth intact.

Customized retarders based on waveplates

Phase control is a critical parameter in polarization measurements. It is well known that a proper combination of wave plates allows to obtain achromatic phase shift, i.e., a constant retardation in certain spectral ranges. This paper is focused on a different, but more useful, goal, as it is to achieve customized variable retarders in broad spectral ranges. To do that, a merit function was used to measure the similarity between the overall phase shift of the wave plate combinations and the desired target. The control variables are the thicknesses and orientations of the wave plates. All possible combinations with four and five wave plates of quartz and MgF₂ were analyzed, but our approach can be perfectly extended to deal with more wave plates. The result of an optimization process determines the thicknesses and orientations of the wave plates, which results in the closest retarder to the desired one. Numerical results show deviations below 10% between the target and the obtained retardation. These systems are of special interest in those fields and instruments in which polarization control plays a fundamental role.

Superachromatic polarization modulator for stable and complete polarization measurement over an ultra-wide spectral range

The polarization measurement system deals with polarized light-matter interactions, and has been a kind of powerful optical metrology applied in wide fields of physics and material. In this paper, we address several general theoretical aspects related to the system model and optimization for linear polarization systems from a view of the matrix algebra. Based on these theories, we propose a new framework of superachromatic polarization modulator (PM) by combining a linear polarizer and a sequence of parallel linear retarders (LRs) for a typical kind of linear polarization system based on the rotating compensator (RC) principle. In the proposed PM, the LRs are made of quarter-wave plates and as a whole act as the RC. Compared with conventional achromatic/superachromatic composite waveplates, the LR sequence has general axis orientations and is optimized by the condition number of the instrument matrix of the PM, which thereby provide much more flexibility to achieve uniform, stable and complete polarization modulation over ultra-wide spectral range. The intrinsic mechanisms, including the working principle, optimization strategy and in-situ calibration method of the proposed PM, are presented and revealed mathematically by the matrix algebra. Results on several prototypes of the PM demonstrate the validity and capability of the proposed methods for applications in broadband polarization measurement systems. The fabricated PM is further applied to a home-made dual RC Mueller matrix ellipsometer, and the accuracy and precision in the full Mueller matrix measurement are better than 2‰ and 0.6‰ respectively over the ultra-wide spectral range of 200∼1000 nm. Compared with existing techniques, the proposed PM has advantages due to superachromatic performances over ultra-wide spectral ranges, stable and complete modulation of the polarized light, and convenience for adjustment and calibration.

Maximum bandwidth of a composite achromatic quartz half-wave plate

To investigate the characteristics of the achromatic bandwidth of composite achromatic quartz half-wave plates (QHWPs), three kinds of multi-element achromatic QHWPs with central wavelengths at 1000-3000 nm and relative deviation of the maximum phase retardation (Δδ_max) of 1%-5% are discussed. Based on the particle swarm optimization algorithm, the maximum bandwidth of the composite achromatic QHWPs at room temperature is obtained. The results show that the achromatic bandwidth increases with Δδ_max. At the same Δδ_max, the achromatic bandwidth of four-element achromatic QHWPs is larger than that of two- and three-element achromatic QHWPs. The maximum achromatic bandwidth of four-element achromatic QHWPs can reach 2229 nm when Δδ_max is 5%. In addition, the temperature effect on bandwidth in the wavelength range of 300-1500 nm is analyzed, and the maximum bandwidth of temperature insensitive composite achromatic QHWPs is 840 nm. The results provide a great reference for designing achromatic wave plates with broad bandwidths.

Patterned achromatic elliptical polarizer for short-wave infrared imaging polarimetry

Short-wave infrared (SWIR) imaging polarimetry has widespread applications in telecommunication, medical imaging, surveillance, remote-sensing, and industrial metrology. In this work, we design, fabricate, and test an achromatic SWIR elliptical polarizer, which is a key component of SWIR imaging polarimetry. The elliptical polarizer is made of a patterned linear polarizer and a patterned optical elliptical retarder. The linear polarizer is a wire grid polarizer. The elliptical retarder is constructed with three layers of nematic phase A-plate liquid crystal polymer (LCP) films with different fast axis orientations and physical film thicknesses. For each LCP layer, four arrays of hexagonal patterns with individual fast-axis orientations are realized utilizing selective linearly polarized ultraviolet (UV) irradiation on a photo-alignment polymer film. The Mueller matrices of the optical filters were measured in the wavelength range 1000 nm to 1600 nm and compared with theory. Our results demonstrate the functionality and quality of the patterned retarders with normalized analyzer vector parameter deviation below 7% over this wavelength range. To the best of our knowledge, this work represents the first polymer-based patterned elliptical polarizer for SWIR polarimetry imaging applications.

Composite achromatic quartz wave plate with adjustable retardation and temperature insensitivity

In order to obtain a composite achromatic wave plate with adjustable retardation, temperature insensitivity, and achromatic bandwidth of 500 nm, a five-element composite achromatic quartz wave plate is designed based on particle swarm optimization. The total phase retardation can be adjusted by rotating the azimuth angle of the central wave plate. The results show that the total phase retardation is adjustable with the range of 90°-180° when the range of temperature and the achromatic wavelength is -20-80^∘C and 750-1250 nm, respectively. The absolute value of the relative deviation of the maximum phase retardation is less than 3.6%, which meets the design requirements of the wave plate. The temperature insensitivity of the five-element wave plate is better than the three- and four-element wave plates. This method is of great reference value for designing this kind of composite achromatic wave plate.

Wavelength-adaptable effective q-plates with passively tunable retardance

Wave retarders having spatially varying optical axes orientations, called q-plates are extremely efficient devices for converting spin to orbital angular momentum of light and for the generation of optical vortices. Most often, these plates are designed for a specific wavelength and have a homogeneous constant retardance. The present work provides a polarimetric approach for overcoming both these limitations. We theoretically propose and experimentally demonstrate q-plates with tunable retardance, employing a combination of only standard q-plates and waveplates. A clear prescription is provided for realizing wavelength indepedent q-plates for a desired retardance, with a potential for ultrafast switching. Apart from the potential commercial value of the proposed devices, our results may find applications in quantum communication protocols, astronomical coronography, angular momentum sorting and in schemes that leverage optical vortices and spin to orbital angular momentum conversion.

Optical wave retarder based on metal-nanostripe metamaterial

Wave retarders, including quarter- and half-wave plates, are used in many optical systems for polarization conversion. They are usually realized with anisotropic crystalline materials. However, much thinner and possibly also less expensive wave plates can be made of micro- and nanostructures. We present a new way to create thin-film optical retarders based on a highly birefringent metamaterial. The wave plate is capable of low-loss, broadband operation, which we verify both numerically and experimentally. Owing to the remarkable simplicity of our design, the wave plates operating on the proposed principle can meet the requirements of large-scale production and find widespread application in optics and photonics.

Comprehensive characterization of a general composite waveplate by spectroscopic Mueller matrix polarimetry

Composite waveplates (CWs) consisting of multiple single waveplates are basic polarization elements and widely used to manipulate the polarized light in optical systems, and their performances affect the final accuracy and precision significantly. This research proposes a method for the comprehensive characterization of an arbitrary CW based on spectroscopic Mueller matrix polarimetry. An analytical model is established to describe a general CW by extending Jones' equivalent theorem with Mueller matrix calculus. In this model, an arbitrary CW is optically equivalent to a cascaded system consisting of a linear retarder with slight diattenuation followed by an optical rotator, and its polarization properties are completely described by four polarization parameters, including the retardance, the fast axis azimuth, the rotation angle, and the diattenuation angle. Analytical relations between the polarization properties, the structure, and the Mueller matrix of the CW are then derived from the established model. By the proposed method, the polarization parameters of an arbitrary CW can be comprehensively characterized over an ultra-wide spectral range via only one measurement. Moreover, the actual structure of the CW, including the thicknesses and fast axis azimuths of the single waveplates, as well as the axis alignment errors, can be completely reconstructed from the polarization spectra. Experiments performed with a house-developed broadband Mueller matrix polarimeter on three typical CWs including a compound zero-order waveplate, an achromatic waveplate and a specially designed biplate have demonstrated the capability of the proposed method.

Achromatic linear retarder with tunable retardance

We present a universal design and proof-of-concept of a tunable linear retarder of uniform wavelength response in a broad spectral range. It consists of two half-wave retarders (HWR) between two quarter-wave retarders (QWRs), where the uniform retardance can be tuned continuously by simply rotating one of the HWRs. A proof-of-concept of this design is built by using commercially available Fresnel rhomb retarders that provide retardation with almost wavelength uniformity in the visible and near infrared from 450 to 1550 nm. The design is universal, since other achromatic QWRs and HWRs could also be employed. The system is experimentally demonstrated to control the state of polarization of a supercontinuum laser.

Design, fabrication and testing of achromatic elliptical polarizer

A method of designing achromatic elliptical polarizers using a combination of multiple birefringent waveplates is demonstrated. This approach has a simple geometric interpretation and simplifies the problem of designing an achromatic elliptical polarizer to find overlapping arcs on the Poincaré sphere. The technique is applied to the design of achromatic elliptical polarizers for a broadband division-of-focal-plane full-Stokes imaging polarimeter for visible wavelength band (λ = 450nm to 650nm). An achromatic elliptical polarizer sample with a two-layer retarder is fabricated using liquid crystal polymer. The performance of the polarizer sample is measured and compared with the theoretical calculation. For comparison, a superachromatic polarizer design (λ = 400nm to 1μm) is also presented by using three-layer and four-layer retarder configurations.