MULTI-CRYSTAL ACHROMATIC RETARDER FOR VISIBLE REGION APPLICATIONS

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MULTI-CRYSTAL ACHROMATIC RETARDER FOR VISIBLE REGION APPLICATIONS Nilanjan Mukhopadhyay 1 and Saswati De 2 1,2 Department of Electronics & Communication Engineering, Global Institute of Management

1 MULTI-CRYSTAL ACHROMATIC RETARDER FOR VISIBLE REGION APPLICATIONS Nilanjan Mukhopadhyay 1 and Saswati De 2 1,2 Department of Electronics & Communication Engineering, Global Institute of Management and Technology Abstract A numerous number of techniques have been studied to design achromatic phase retarders. Popular choices of designing materials are birefringent materials, thin films etc. This retarders are designed to be used in different wavelength regions. In this present study we have proposed the design of an achromatic quarter wave retarder using four different birefringent materials for the use in visible region i.e nm. This system shows excellent achromatic nature compare to other designed systems in visible region. The common application includes biomedical imaging, spectroscopy and others. Keywords Achromatic, birefringent material, retardance, visible region, quarter wave plate I. INTRODUCTION Achromatic retarders are widely used in visible, near and mid infrared regions for various applications like thermal imaging, spectroscopy etc. With the development of new optical instruments this wavelength region is exploring different new areas of research. Earlier various achromatic combinations were designed in different ways with different materials like birefringent materials, thin films etc. A combination of three plates fabricated from the same material, proposed by Pancharatnam shows a good achromatism over a range of visible wavelength [1]. But the system shows a wavelength dependency on the orientation of the principal axis. Hariharan [2] also proposed an achromatic system with two plates of different materials for the visible region. Nicolas Passily et al [3] studied achromatic phase retardation based on sub-wavelength dielectric diffraction grating in both the visible and near-infrared regions. Retarders can be designed with thin films and liquid crystal also [4]. By increasing the number of wave plates a system can also be designed for better performance like one proposed by Masson and Gallot [5]. Recent works includes bio-medical applications that demands achromatic waveplates in the visible region [6]. In this present communication, a four plate achromatic quarter-wave retarder has been proposed in the nm wavelength spectrum. Theory and the computation of thickness of the plates are discussed in section 2 and 3 respectively followed by the simulation results and material selection criterion in section 4 and 5. II. THEORETICAL BACKGROUND As the light wave passes through the birefringent plates the corresponding path difference will be responsible for the retardation calculation. Here the thickness of three plates are denoted by,, and for which the combination will act as quarter wave plate (i.e. phase difference of π/2), then thicknesses can be calculated by solving the following equation [7], (1) DOI: /IJMTER DDQGL 232

2 Birefringence Birefringence Here,, and are the four selected design wavelengths which are taken to be 0.45 µm, 0.55 µm,0.65 µm and 0.7 µm respectively. The values of the birefringence of the four materials at the design wavelengths are,,, (Plate1),,,, (Plate2), (Plate3),,,, (Plate4).The overall phase difference between two orthogonal directions of light may be either +π/2 or π/2, which means that if the input is a linearly polarized light at 45 then the output light will be either left or right circularly polarized [7]. In this case positive values are considered which can be satisfied with the combination of KDP (negative), quartz (positive),adp (negative) and CdSe (positive) Y: Figure 1. Birefringence variation of KDP in visible region 9.45 x Y: Figure 2. Birefringence variation of crystalline Quartz in visible All rights Reserved 233

3 Birefringence Birefringence Y: Figure 3. Birefringence variation of ADP in visible region Y: Figure 4. Birefringence variation of CdSe in visible region III. THICKNESSES CALCULATION OF THE PLATES For the computation of the thickness of the plates when the system is designed for quarter wave plate four wavelengths λ X =0.45µm, λ Y =0.55 µm, λ Z =0.65 µm and λ K =0.7 µm are considered from the µm spectrum with the birefringence values of the three crystals taken from table 8 in [8]. Using the values of birefringence for those designing wavelengths in the eqn.(1) and solving it by using Cramer s rule we get the thickness values as =176.4µm for KDP, =35.7µm for crystalline quartz, =174.2 µm for ADP crystal and µm for All rights Reserved 234

4 Percentage deviation Overall Retardance in degree IV. SIMULATION RESULTS AND DISCUSSION The variation of birefringence with the wavelength for KDP, crystalline quartz, ADP and CdSe are shown in figures 1, 2, 3 and 4 respectively. From the figure 5 which shows the retardation of the combination the maximum deviation of retardation is within ±0.33 which shows its excellent achromatic property. We can see from the percentage deviation plot (shown in figure 6) that the maximum deviation is within +0.5% and -0.1% throughout the spectrum. So the total deviation is 0.6% over the wavelength region of µm X: Y: X: Y: Figure 5. Wavelength dependency of retardation of 4 plate system in visible region X: Y: X: Y: Figure 6. Wavelength dependency of percentage deviation of retardation of 4 plate system in visible region This four plate system shows good achromatism i.e variation of retardation (90 o ±0.33 o ) over other designed systems [6] in the same wavelength region. So it can be used for visible region All rights Reserved 235

5 V. MATERIAL SELECTION The optical materials selected for an optical system depends upon the application, the required system performance and the environment in which the system is to perform [9]. All the four materials chosen for the present system shows good transmittance over the intended wavelength band. The next factor to check is hygroscopic property i.e. the ability of the materials to absorb water from surroundings. So less hygroscopic value of the materials is desired. Some materials are hygroscopic up to certain temperature and often it will impose a limitation on the operating temperature. The third factor we have to consider is cost effectiveness and large scale availability. All these properties are well supported by KDP, crystalline quartz and ADP and CdSe. Among them KDP and ADP crystals are little hygroscopic which can lead to surface degradation. This can be avoided by maintaining the component temperature higher than the temperature of its immediate surroundings or by coating the surfaces with a transparent, water-impermeable barrier coating [10]. A new process for reducing the surface degradation due to water vapour is reported [11]. VI. CONCLUSION An achromatic quarter wave combination of 4 plates has been studied here. It shows good achromatism over the entire visible region and the variation of retardation is very less i.e 90 o ±0.33 o. This four plate system can be used with bio medical imaging system for better performance. All the four materials are less hygroscopic and the system does not show a wavelength dependency on the orientation of the principal axis. Among the four materials ADP shows some hygroscopicity but that can be removed by the process mentioned above. REFERENCES [1] S Pancharatnam,Achromatic combinations of birefringent plates.part II.An achromatic quarter wave plate,proceedings of the Indian Academy of Sciences XLI (4), Sec. A, 137(1955) [2] P Hariharan, Achromatic retarder using quartz and mica Meas. Sci. Technol. 6, 1078 (1995). [3] Nicolas Passily et al, J. Opt. A: Pure Appl. Opt. 10, (6pp) (2008). [4] Hubert Seiberle, Thomas Bachels, Carsten Benecke and Mohammed Ibn-elhaj, Volume photo aligned retarders IEICE Transactions 90-C(9), 2088 (2007). [5] Jean-Baptiste Masson and Guilhem Gallot,Terahertz achromatic quarter wave plate : Opt. Lett. 31(2), 265 (2006). [6] Yi-Jun Jen et al, Biologically inspired achromatic waveplates for visible light Nature Communications volume2, Article number: 363, doi: /ncomms1358 (2011) [7] Arijit saha, Kallol Bhattacharya and Ajoy Kumar Chakraborty, Pramana J, A near-infrared zero-order achromatic retarder Phys., Vol. 77, No. 4, October 2011 [8] J M Bennet, Polarizers, in: Handbook of optics edited by M Bass (McGraw-Hill, New York) Vol. 2, Chap. 3, pp , 1995 [9] Jeffrey L. Tosi, Kumar M. Khajurivala, Janos Technology LLC: Photonics handbook, Common Infrared Optical Materials and Coatings: A Guide to Properties, Performance and Applications. [10] Matthew J. Penn, National Solar Observatory, 950 N Cherry Av, Tucson, AZ USA, Infrared Solar Physics, Living reviews in solar physics:2014;11:2, doi: Mar 21. [11] Arash Ghorbani, Peter Frank, Ian Edmond, VACUUM CELL FOR OPTICAL COMPONENTS, United States Patent, Patent No.: US 7,724,453 B2, May 25, All rights Reserved 236

Symmetrically coated pellicle beam splitters for dual quarter-wave retardation in reflection and transmission

University of New Orleans ScholarWorks@UNO Electrical Engineering Faculty Publications Department of Electrical Engineering 1-1-2002 Symmetrically coated pellicle beam splitters for dual quarter-wave retardation