Achromatic Phase Retarders Research Articles

Super-achromatic Quarter-wave Phase Retarder for Visible, Near Infrared and Short Wave Infrared Region Applications-=SUP=-*-=/SUP=-

Phase retarders are essential components in optical system design as they can control the phase shifts associated with the vector components of incoming light waves and hence, decide the state of polarization. Different optical systems are designed to work at different wavelength spectrums. Most of these developments focus on the different wavelength regions between visible to infrared, of which the near infrared and short-wave infrared ranges are popular domains for defense and remote sensing applications. In this present communication, a super achromatic quarter-wave phase retarder consisting of four birefringent plates of different materials, i. e. KDP, crystalline quartz, ADP and ZnS has been proposed, to cover the visible, near infrared and short wave infrared regions from 400-2100 nm. As circularly polarized light is less affected by atmospheric conditions the use of an achromatic broadband quarter-wave phase retarder as proposed is quite useful. The proposed system shows acceptable performance over the intended range of wavelengths. Keywords: Wave plates, Super achromatic, Retarder, SWIR.

An Achromatic Quarter-Wave and Half Wave Phase Retarder Combination Operating in Near Infrared (NIR) Region

Optical phase retarders are major components in design of different optical systems. Normally these retarders exhibit retardance which varies with wavelength. But achromatic phase retardation is necessary in optical system design. These achromatic phase retarders can be designed using different waveplates, sub-wavelength grating (SWG), liquid crystals, and polymers etc in different wavelength ranges. Present communication focus on designing of quarter-wave phase retarder in the near infrared range (NIR) with waveplates of different birefringent materials having different thicknesses. This system in NIR range shows comparable performance with those developed with SWGs and it also removes some drawbacks. The proposed system deals with an achromatic combination of four birefringent plates in 800–2000 nm range using Sapphire, CdSe, Calcite, and Rutile crystals having good transmittance in the NIR range. This system can also be used as a half wave phase retarder in NIR.

Broadband achromatic phase retarder based on metal-multilayer dielectric grating**Project supported by the National Natural Science Foundation of China (Grant No. 11274188)

A new achromatic phase retarder based on a metal-multilayer dielectric grating structure is designed using the rigorous coupled wave analysis method and the genetic algorithm. The optimized phase retarder can maintain phase retardation around 90° from 900 nm to 1200 nm, and the maximum deviation is less than 4.5% while the diffraction efficiencies of TE and TM waves are both higher than 95%. Numerical analysis shows the designed phase retarder has a high fabrication tolerance of groove depth, duty cycle and incident angle. This achromatic phase retarder is simple in design and stable in performance, and can be widely used in optical systems.

Wide spectral range multiple orders and half-wave achromatic phase retarders fabricated from two lithium tantalite single crystal plates

Abstract In a broad spectral range (300–2500 nm), we report the use of channeled spectra formed from the interference of polarized white light to extract the dispersion of the phase birefringence Δ n p ( λ ) of the x - and y -cuts of lithium tantalite (LiTaO 3 :LT) plates. A new method named as wavenumber difference method is used to extract the spectral behavior of the phase birefringence of the x- and y - cuts of LT plates. The correctness of the obtained birefringence data is confirmed by using Jones vector method through recalculating the plates thicknesses. The spectral variation of the phase birefringence Δ n p ( λ ) of the x- and y -cuts of LT plates is fitted to Cauchy dispersion function with relative error for both x- and y-cuts of order 2.4×10 −4 . The group birefringence dispersion Δ n g ( λ ) of the x- and y -cuts of LT plates is also calculated and fitted to Ghosh dispersion function with relative error for both x- and y -cuts of order 2.83×10 −4 . Furthermore, the phase retardation introduced by the x- and y -cuts of LT plates is also calculated. It is found that the amount of phase retardation confirms that the x- and y -cuts of LT plates can act as a multiple order half- and quarter-wave plates working at many different wavelengths through the spectral range 300–2500 nm. For the x- and y -cuts of LT plates, a large difference between group and phase birefringence is observed at a short wavelength ( λ= 300 nm); while such difference progressively diminished at longer wavelength ( λ= 2000 nm). In the near infrared region (NIR) region (700–2500 nm), a broad spectral full width at half maximum (FWHM) is observed for either x- or y -cut of LT plate which can act as if it is working as a zero order wave plate. Finally, an achromatic half-wave plate working at 598 nm and covering a wide spectral range (300–900 nm) is demonstrated experimentally by combining both x- and y -cuts of LT plates.

Optical Compensation for Elimination of Off-Axis Light Leakage in a Homogeneously-Aligned Liquid Crystal Cell

We propose an achromatic optical compensation method using uniaxial films to eliminate the off-axis light leakage in a homogeneously-aligned liquid crystal cell. In this technique, four uniaxial films with complementary dispersion characteristics are used to compensate for one another, resulting in achromatic effective phase retardation for off-axis angles. The retardation values are optimized with the aid of the Poincaré sphere and through numerical research. The contrast ratio of the proposed configuration for white light is higher than 2400:1 at a polar angle of ±60°, irrespective of the azimuthal angle.

Achromatic wide-view circular polarizers for a high-transmittance vertically-aligned liquid crystal cell.

We propose an optical compensation scheme through which we can eliminate the off-axis light leakage in a vertically-aligned liquid crystal cell with circular polarizers. In this scheme, four uniaxial films with complementary dispersion characteristics are used to compensate one another, resulting in achromatic effective phase retardation for off-axis angles. By using the proposed optical compensation, a contrast ratio higher than 2000:1 can be realized over the entire 55° viewing cone in a multi-domain vertical-alignment liquid crystal cell with circular polarizers.

Achromatic optical compensation using dispersion of uniaxial films for elimination of off-axis light leakage in a liquid crystal cell

We propose an achromatic optical-compensation method using uniaxial films to eliminate the off-axis light leakage at the dark state in a homogeneously aligned liquid crystal cell. Three uniaxial films with different dispersion characteristics are used so that they can compensate each other to achieve achromatic effective phase retardation at off-axis. The retardation values are optimized with the aid of the Poincaré sphere and through numerical research. A contrast ratio of higher than 2000∶1 is predicted over the entire ±60° viewing cone for a homogeneously aligned LC cell with zero pretilt angle.

Design of Dual-Wavelength High Precision Achro-Matic Phase Retarder

In order to meet the demand of precise measurement and applications, based on the total-reflection phase transformation theory of the phase retardation, the principle of phase retardation with oblique incident angle is expounded. Spectral characterization of the phase retardation varing with refractive index is analyzed to high precision achromatic phase retarder to oblique incidence aided design. The result indicates that there is a maximum to dispersion curve of achromatic phase retarder with oblique incident in the center of the design refractive index department. The phase-delay drops on both sides to the center of the refractive index and can pass through a definite retardance value twice at two specified wavelength values. Accordingly, based on the detailed design of Fresnel rhomb, The new achromatic retarder with the specific incident angle and high precision is designed by choosing suitable meterial .It is shown at the achromatic property curve within the range of 500 nm to 1150 nm theoretically that the maximum deviation of the retardance from 90° is 0.006°, whereas at λR1R=532 nm and λR2R=1064 nm the retardance is approximately 90°±0.001°. and the corresponding calculation to the size of the devices indicates that the size of the devices for the same aperture will decrease for higher refractive index values.So optical materials with higher refractive indices are more convenient for this type of oblique incidence retarder .

Phase Modulator Behavior of a Wedge-Shaped POLICRYPS Diffraction Grating

Experimental characterization of a switchable wedge-shaped holographic grating involved as a tunable phase retarder is reported. The structure is made of polymer slices alternated to films of well aligned Liquid Crystals (POLICRYPS). The sample shows a birefringence that depends on the anisotropy of the composite liquid crystalline material and on the geometrical cell parameters. The phase retardation introduced by the structure on the impinging probe light can be finely tuned by varying the amplitude of the externally applied electric field or by spatially shifting the probed area on the sample. The quarter wave plate condition for a He-Ne laser probe has been obtained confirming the possible application of the structure as an achromatic phase retarder.

Achromatic phase retarder applied to MWIR & LWIR dual-band

The development of the dual-band IR imaging polarimetry creates the need for achromatic phase retarder used in dual-band. Dielectric grating with the period smaller than the illuminating wavelength presents a strong form-birefringence. With this feature, the combination of several subwavelength gratings can be used as achromatic phase retarders. We proposed a combination of 4 subwavelength structured gratings (SWGs) used as an achromatic quarter-wave plate (QWP) applied to MWIR & LWIR bandwidths. Design method using effective medium theory and optimization algorithms is described in detail. The simulation results led to the possibility of an dual-band achromatic QWP whose retardance deviates from 90 degrees by <+/-0.75 degrees with the fast axis unfixed and by <+/-1.35 degrees with the fast axis fixed over MWIR(3-5microm) & LWIR(8-12microm) bandwiths.

Optimal Design of an Achromatic Angle-Insensitive Phase Retarder Used in MWIR Imaging Polarimetry

Dielectric gratings with period in the range from λ/10 to λ/4 with λ being the illumination wavelength not only exclude higher order diffractions but also exhibit strong dispersion of effective indices which are proportional to the wavelength. Moreover, they are insensitive to the incident angle of the illumination wave. With these features, we can design a true zero-order achromatic and angle-insensitive phase retarder which can be used as the polarization state analyzer in middle wave infrared (MWIR) imaging polarimetry. A design method using effective medium theory is described, and the performance of the designed phase retarder is evaluated by rigorous coupled wave analysis theory. The calculation results demonstrate that the retardance deviates from 45° by < ±1.6° within a field of view ±10° over the MWIR bandwidth (3–5 μm).

Achromatic phase retardation by subwavelength gratings in total internal reflection

Achromatic phase retarders based on subwavelength dielectric diffraction gratings are studied numerically in total-internal-reflection configuration in both the visible and near-infrared regions. Rigorous diffraction theory is used to show that the examined gratings, with reasonable fabrication parameters, can act as quarter-wave plates over a wide range of wavelengths. The best results are obtained if the grating vector is perpendicular to the plane of incidence and the period is slightly below the cutoff period for zero-order operation. Properties of S matrices are used to demonstrate that the main contribution to the broadband effect is not the adjustment of the dispersion of effective refractive indices (like in the previously introduced achromatic diffractive elements), but phase-shift compensation effects caused by the back-boundary reflection.

Total internal reflection phase retarders constructed from prisms

It is shown that the known rhomb-type total internal reflection phase retarders could be constructed from prisms. With all the advantages of the traditional designs preserved, these prism-type devices are of smaller volumes, convenient shapes, and light traverses shorter path lengths inside the devices. Moreover, the serious reduction in the volumes of the presented devices reduces the undesired stress birefringence which affects the retardance value. Two cases will be considered, which are the 4-reflection normal-incidence AD-1 and the super achromatic 2-reflection oblique-incidence quarter-wave phase retarders.

Broadband achromatic phase retarder by subwavelength grating

A broadband achromatic phase retarder in the full visible region (wavelength λ∈[400,800 nm] ) using subwavelength grating (SWG) is designed, fabricated and tested. For the first time, it is experimentally proved that the achromatic phase retarder in the full visible region can be realized by SWG. An improved method combining enumeration with optimization for designing the achromatic phase retarder with rectangular and sinusoidal SWG is presented. Rigorous coupled wave theory (RCWT) is applied to calculate the diffraction performances. Examples show that the designed phase retarders perform broadband achromatic phase retardation successfully. Compared with the previous methods, the improved method shows the advantages of good versatility and flexibility.

Achromatic phase retarder by slanted illumination of a dielectric grating with period comparable with the wavelength

We discuss some properties of dielectric gratings with period comparable with the illuminating wavelength for slanted illumination (this illumination geometry is often referred to as concical mounting). We demonstrate the usefulness of such an illuminating geometry. We show that the threshold period (under which only the zeroth transmission and reflection orders are nonevanescent) can be significantly higher, thereby easing fabrication constraints, and that this illumination setup makes it possible to design achromatic phase retarders. Such a design, for an achromatic quarter-wave plate with lambda/60 uniformity of the retardation phase in the 0.47-0.63-mum wavelength interval, is demonstrated.

Liquid crystal phase retarder with broad spectral range

Phase retarders normally show a strong wavelength dependence of retardance. For example even without taking birefringence dispersion into account a quarterwave retarder designed for 600 nm acts as a halfwave retarder at 300 nm. Achromatic phase retarders with retardance changes of only ±5% in ≈200 nm broad spectral regions can be assembled from two layers of different birefringent materials the fast axes of which are crossed. Further improvement is possible by combining three achromatic phase retarders based on a recipe given by Pancharatnam. A quarterwave plate is realized with an inner halfwave plate the fast axis of which is positioned at a certain angle with respect to the fast axes of the outer retarders. With this design phase retardance can be kept constant within ±2% in the entire visible spectrum from 400 to 900 nm. We report on a first realization of such a broad band phase retarder using liquid crystals instead of crystals as the birefringent materials. Although the performance is not as good as calculated it is well comparable to practically realized crystalline `superachromatic' phase retarders.

A Liquid Crystal Broad Band Stokes-Meter

A device for the determination of the complete polarization state of light in a broad spectral interval is presented. Two tunable achromatic phase retarders with planar aligned nematic liquid crystals are used to modulate the polarization without moving components.

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.

Achromatic phase retarders constructed from right-angle prisms: design

The behavior of achromatic 90 and 180° phase retarder devices has been computed. These devices are made of three right-angle prisms so that the transmitted beam is not deflected or deviated from its incoming direction although the beam has suffered four internal reflections. Conventional BK-7 prisms and quartz prisms can be used as achromatic 180 and 90° phase retarders if suitable dielectric coatings are applied to the reflecting surfaces. With other glass types extremely achromatic 180° retarders can be obtained with &lt;0.2% deviation from 180° throughout the visible wavelength range. This is five times better than the so-called superachromatic retarders made of several birefringent plates of different materials.