Design and Characterization of Tunable LC-VCSELs for Optical Coherence Tomography

Optical Coherence Tomography (OCT) systems are making rapid progress into various areas ranging from medicine and health science up to non-medical areas such as quality control or biometrics. Various possible configurations are present in literature, but only a few exploit liquid crystals (LCs)-based device as a light source. The strength of OCT systems, compared with other three-dimensional imaging technologies, comes from a higher spatial resolution and a fast scanning rate that allows getting high-quality images of in vivo tissue samples. These qualities make OCT the perfect candidate for ophthalmology purpose, where the human eye moves quickly and randomly. The goal of this work is to demonstrate the interest of liquid-crystals optical based devices. Particularly, in this work are discussed the potentialities of two devices for OCT applications: a tunable LC-Fabry-Perot optical micro-filter and an LC-VCSEL prototype, with the latter one that can be considered a natural technological evolution of the former one. Key advantages of this technology are large refractive index tuning for the wavelength sweep with moderate applied voltage and very low power consumption. The LC-Fabry-Perot filter is fabricated at LAAS-CNRS Toulouse and its operation is compared with a model designed by mean of Transfer Matrix Method (TMM) using Matlab. In the developed model, the grating structure, dispersion curves and Coefficient of Thermal Expansion (CTE) of the device materials are taken into account. The electrical and thermal behaviour of the LC-Fabry-Perot filter is investigated in order to define an optimal design for which the device maintains a stable tunability range. For this purpose, a Fourier-transform infrared spectroscopy (FTIR) setup coupled with a thermal and electrical controller is assembled at the laboratory of LAAS-CNRS. Measurements are carried out on two samples filled with two different nematic liquid crystals: QYDPLC-07 and QYDPLC-036 (similar Merck E7 and BL036 composition). A novel data extraction method is exploited to get a model of the refractive index variation against temperature and applied voltage. The obtained models from the LC-FPI are provided to VENUS, a computer-aided software developed by CNR-IEIIT Torino, for the design of the LC-VCSEL. The critical parameters of the LC-VCSEL design are outlined and investigated. Particularly, two phenomena are analyzed. First, the GaAs effect, neglecting the metal part of the structure, is studied and demonstrates an optimal etching radius to reduce internal losses that can compromise the proper operation of the laser. Subsequently, the effect of the Aluminum contact pad is included and demonstrates the strong influence of metal absorption inside the liquid crystals' cavity. Possibles solution, as a smaller contact pad ring and insertion of Diffractive Optical Elements (DOE) inside the LC cavity, are discussed.