Photonic Crystal Fiber for Medical Applications

Abstract

Optical coherence tomography (OCT) is a new technology for noninvasive cross-sectional imaging of tissue structure in biological system by directing a focused beam of light at the tissue to be image [Bouma et al., 1995; Jiang et al., 2005; Ryu et al., 2005]. The technique measures the optical pulse time delay and intensity of backscattered light using interferometry with broadband light sources or with frequency swept lasers. It is analogous to ultrasound imaging or radar, except that it uses light rather than sound or radio waves. In addition, unlike ultrasound, OCT does not require direct contact with the tissue being imaged. OCT depends on optical ranging; in other words, distances are measured by shining a beam of light onto the object, then recording the optical pulse time delay of light. Since the velocity of light is so high, it is not possible to directly measure the optical pulse time delay of reflections; therefore, a technique known as low-coherence interferometry compares reflected light from the biological tissue to that reflected from a reference path of known length. Different internal structures produce different time delays, and crosssectional images of the structures can be generated by scanning the incident optical beam. Earlier OCT systems typically required many seconds or minutes to generate a single OCT image of tissue structure, raising the likelihood of suffering from motion artifacts and patient discomfort during in vivo imaging. To counter such problems, techniques have been developed for scanning the reference arm mirror at sufficiently high speeds to enable realtime OCT imaging [Tearnery et al., 1997]. OCT can be used where excisional biopsy would be hazardous or impossible, such as imaging the retina, coronary arteries or nervous tissue. OCT has had the largest impact in ophthalmology where it can be used to create crosssectional images of retinal pathology with higher resolution than any other noninvasive imaging technique. Now a days OCT is a prospective technology which is used not only for ophthalmology but also for dermatology, dental as well as for the early detection of cancer in digestive organs. The wavelength range of the OCT light source is spread from the 0.8 to 1.6 ┤m band. This spectral region is of particular interest for OCT because it penetrates deeply into biological tissue and permits spectrally resolved imaging of water absorption bands. In this spectral region, attenuation is minimum due to absorption and scattering. It should be noted that scattering decreases at longer wavelengths in proportion to 1/┣4, indicating that the scattering magnitude at 0.8 ~ 1.6 ┤m wavelengths is lower than at the visible wavelengths [Agrawal, 1995]. Ultrahigh-resolution OCT imaging in the spectral region from 0.8 to 1.6 ┤m requires extremely broad bandwidths because longitudinal resolution depends on the coherence length. The coherence length is inversely proportional to the bandwidth and proportional to square of the light source center wavelength. This can