Birefringence Measurement Research Based On Digital Holographic Microscopy

Birefringence is a description of the optical properties of materials.The anisotropy of dielectric constant at optical frequency leads to polarization-dependent phase velocity,which results in birefringence.Measuring the birefringence of substances,we can study the structural changes of substances and analyze the related properties of substances.Therefore,the measurement of birefringence has important application values in many fields such as bio-optics,optical fiber communication,optical instruments and so on.In this paper,a birefringence measurement optical system based on polarization sensitive digital holographic microscopy is constructed for birefringence measurement of biological tissues and special optical fibers.The main research contents include the following aspects:The birefringence measurement method of biological tissue using polarization sensitive digital holographic microscopy is proposed,and a birefringence measurement system of biological tissue is constructed.On the basis of Mach Zehnder interferometer,the polarization of incident light is controlled by a quarter wave plate and polarizer.The off-axis interference hologram of bladder tissue under different polarization states is recorded by CCD.To solve the spectrum aliasing problem caused by the complex structure of bladder tissue,a hybrid reconstruction algorithm combining Fresnel spatial reconstruction and frequency domain angular spectrum algorithm was used to obtain the reconstructed object light wave of the tissue to be measured.The real phase distribution of bladder tissue under different polarization states was obtained based on least square method.The data show that bladder tissue is polarization sensitive and anisotropic.The phase distributions of normal and cancerous bladder tissues were obtained by digital holograms of normal and cancerous bladder tissues under two orthogonal polarization states.The birefringence distributions of normal and cancerous bladder tissues were obtained by combining the relationship between phase difference and birefringence of orthogonal polarized light.The data show that the birefringence of cancerous bladder tissue is higher than that of normal bladder tissue,and the boundary of cancerous bladder tissue is more blurred and scattered than that of normal bladder tissue,so this technique provides a new method for characterizing the internal structure changes of biological tissue.The method for measuring birefringence of optical fibers using polarization sensitive digital holographic microscopy is proposed,and the birefringence of special optical fibers is studied.By rotating the measured optical fibers,the digital holograms of different angles of the optical fibers under two orthogonal polarization states are recorded,and the phase distribution is obtained based on the angular spectrum reconstruction algorithm.Then the refractive index of the optical fibers under two orthogonal polarization states is restored by the filter back projection algorithm,and the birefringence of the optical fibers is obtained.The birefringence of sapphire,tie,ellipse and panda polarization maintaining fibers has been measured.The results show that the birefringence at the core of sapphire fibers is larger.The birefringence of different types of polarization maintaining fibers in stress region is larger than that at the core and cladding.