Biomedical Optical Sensors

Abstract

In this dissertation, applications of optical concepts in biomedical sensing have been investigated. In particular, three biomedical optical sensors have been designed, implemented and characterized. Here, the definition of sensing is very broad and it includes imaging. First, I present the design, implementation and performance analysis of a compact multi-photon endoscope based on a piezo electric scanning tube. A miniature objective lens with a long working distance and a high numerical aperture (~ 0.5) is designed to provide a diffraction limited spot size. Furthermore, a 1700 nm wavelength femtosecond fiber laser is used as an excitation source to overcome the scattering of biological tissues and reduce water absorption. Therefore, the novel optical system along with the unique wavelength allows us to increase the imaging depth. We demonstrate that the endoscope is capable of performing third and second harmonic generation (THG/SHG) and three-photon excitation fluorescence (3PEF) imaging over a large field of view (> 400 µm) with high lateral resolution (2.2 µm). The compact and lightweight probe design makes it suitable for minimally-invasive in-vivo imaging as a potential alternative to surgical biopsies. The second device is a magnetic field sensor with a noise equivalent field of 500 nT based on a tapered fiber in a magnetic fluid (Fe3O4 nanoparticles dispersed in the deionized water). This sensitivity, which is three orders of magnitude better than the previously demonstrated tapered sensors, is achieved by biasing the fluid material with a ≈1 mT magnetic field. In addition, application of an optical modulator and an auto-balanced synchronized detector increased the sensor output signal-to-noise ratio significantly. Furthermore, a novel method is presented to employ the proposed sensor in a distributed scheme. The third biomedical optical system is a wave-front sensor. This device can measure the aberrations of human eye with 0.1 diopter accuracy using a collimated LED and a Shack-Hartman Sensor. Since the proposed designed is see-through, we can confirm the accuracy of spherical and astigmatism measurement by correcting the patient vision using fluidic lenses so that the patient’s vision becomes 20/20. In Chapter VI, a coherent anti-Stoke scattering (CARS) sensing instrument is presented. For CARS sensing we need two synchronized laser sources pump and Stoke. The laser is designed and implemented based on a single cavity mode-locked by a semiconductor saturable absorber mirror (SESAM). The Stoke beam is a wide linewidth supercontinuum generated from a highly nonlinear fiber (HNLF). This type of Stoke laser provides broadband CARS sensing which allow us to spatially resolve different materials at the same time.