Laser-induced Fluorescence (LIF) Based Optical Fiber Probe for Biomedical Application Jagdish P. Singh ∗ , Sunil K. Khijwania ∗∗ , Chan Kyu Kim, Fang-Yu Yueh ♣ Shane C. Burgess, Diagnostic Instrumentation and Analysis Laboratory (DIAL) ♣ Department of Basics Sciences, College of Veterinary Sciences Mississippi State University, 205 Research Boulevard, Starkville MS 39759, USA Phone: 662-325-8444, Fax: 662-325-8465 ∗ singh@dial.msstate.edu ∗∗ Department of Physics, Indian Institute of Technology-Guwahati North Guwahati, 781039, India ABSTRACT Laser induced fluorescence (LIF) spectroscopy has emerged as a potential tool for the cancer diagnosis. A comprehensive analytical study of a compact optical fiber LIF based sensor for in-vitro point monitoring of tissue auto-fluorescence as a non-invasive tool for cancer diagnosis is presented. The effect of two critically important parameters from the sensor designing aspects, namely, the excitation-collection geometry and the excitation wavelength over the tissue auto-fluorescence response and malignancy resolution is investigated. An attempt has been made to determine an optimum sensing configuration to develop a diagnostics procedure to enhance small but consistent spectral difference between the normal and malignant tissue from various parts of animal body to greatly improve the accuracy of the auto-fluorescence cancer diagnosis. Keywords: Optical fiber, Laser-induced fluorescent spectroscopy, Auto-fluorescence, cancer diagnosis. 1. INTRODUCTION Optical spectroscopy has been widely used to acquire fundamental knowledge about physical, chemical, and biological processes that occur in biomaterials 1-3 . In particular, laser-induced auto-fluorescence, which was first observed by Stokes 4 and later was recognized as a potential diagnostic tool by Stubel 5 , has been extensively studied over the past 20 years and significantly emerged as a promising technology for biomedical diagnostics. This is a process whereby molecules/atoms are excited to higher electronic energy state via laser absorption and subsequently fluoresce at emission wavelengths, which are independent and longer (red-shifted) than that of the excitation wavelength. It has been applied for the in vitro and in vivo analysis 6,7 of many different types of samples, ranging from individual biochemical species (e.g. NADH, tryptophan) to human and animal tissues as the associated signal is intense with an excellent signal to noise ratio. This leads to a greater achievable sensitivity 8, 9 . This technique has the capability to quickly, non-invasively, and quantitatively probe the biochemical and morphological changes that occur as the tissue transforms from normal to malignant. The altered tissue architecture and bio-characteristics of native fluorophores associated with malignant transformations is reflected in the spectral characteristics of the measured fluorescence 10 . Thus in recent years, laser induced fluorescence has emerged as a promising tool 6-9 and significant progress has been made in tissue fluorescence spectroscopy for cancer detection 1-17 . However, an accurate, sensitive and rapid method for the diagnosis of a normal and malignant tissue is quit challenging. Major initial studies were focused on the measurement of continuous wave (CW) fluorescence emission spectra of tissue when illuminated at a single excitation wavelength 12-14 . Differences in the emission spectra of normal and diseased tissue were then examined to attempt optical diagnosis of the disease process. These spectral differences were attributed to the variation in tissue structure and fluorescent species content. However the spectral contrast for normal and malignant tissue, which is extremely critical and a major issue for the successful diagnosis, remained extremely weak in these studies. For example, Demos et al 15 in their approach could obtain a maximum fluorescence intensity contrast of 0.4 to 0.65. Owing to the fact that the fluorescence signal from a photo-sensitizer is generally higher in intensity than that from auto-fluorescence, Nadeau et al 16 had administered aminolaevulinic acid (ALA) as a photo-sensitizer drug in order to achieve high signal contrast neglecting its own disadvantages and dangerous side effects. They could obtain a fluorescence contrast in the optical spectrum of malignant and normal tissue of not more than 5. Further, all these studies have generally incorporated bulky, expensive, complex equipments, for example pulsed nitrogen or dye lasers and time gated intensified CCD or diode array detectors. Another critically important issue related to the tissue fluorescence diagnosis is the fact that it does not depend on the concentration and