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Hyperspectral imaging combines imaging and spectroscopy to provide detailed spectral information for each spatial point in the image. This gives a three-dimensional spatial-spatial-spectral datacube with hundreds of spectral images. Probe-based hyperspectral imaging systems have been developed so that they can be used in regions where conventional table-top platforms would find it difficult to access. A fiber bundle, which is made up of specially-arranged optical fibers, has recently been developed and integrated with a spectrograph-based hyperspectral imager. This forms a snapshot hyperspectral imaging probe, which is able to form a datacube using the information from each scan. Compared to the other configurations, which require sequential scanning to form a datacube, the snapshot configuration is preferred in real-time applications where motion artifacts and pixel misregistration can be minimized. Principal component analysis is a dimension-reducing technique that can be applied in hyperspectral imaging to convert the spectral information into uncorrelated variables known as principal components. A confidence ellipse can be used to define the region of each class in the principal component feature space and for classification. This paper demonstrates the use of the snapshot hyperspectral imaging probe to acquire data from samples of different colors. The spectral library of each sample was acquired and then analyzed using principal component analysis. Confidence ellipse was then applied to the principal components of each sample and used as the classification criteria. The results show that the applied analysis can be used to perform classification of the spectral data acquired using the snapshot hyperspectral imaging probe. MOE (Min. of Education, S’pore) Published version

In this paper, we will be initially doing review on the advancement of interference lithography (IL) which is a wellestablished technique for its facile, efficient, and economical approach to achieve large-scale patterning in a wide variety of substrates at sub-wavelength resolution. Following this, we have configured a two-beam IL patterning system to achieve patterns in a photoresist and verified against the theoretically calculated results. This work will be further extended using near-field patterning techniques to improve the resolution of the pattern as compared to the current conventional IL system. It is envisaged that the obtained initial results can be employed in a graphene substrate for further research and applications in the area of flexible electronics. MOE (Min. of Education, S’pore) Published version

A method to directly pattern nanoscale periodic features within optically transparent solid materials by means of a configurable multi-beam femtosecond laser interference is proposed. While femtosecond micromachining of optically transparent solid materials has been explored in great detail in the recent past, research in direct interference patterning on such materials has not been well established. Therefore, different design considerations such as complete void formation and flexibility of the patterned substrate are investigated as part of this study. The relationship between intensity distributions and actual void formation in optically transparent materials is also investigated to establish critical parameters for pattern formation. It is envisaged that this proposed method and data obtained may enable to explore the untapped domains of developing substrates for solid-state 3D microbattery applications. Published version

Noninvasive medical imaging techniques have generated great interest and high potential in the research and development of ocular imaging and follow up procedures. It is well known that angle closure glaucoma is one of the major ocular diseases/ conditions that causes blindness. The identification and treatment of this disease are related primarily to angle assessment techniques. In this paper, we illustrate a probe-based imaging approach to obtain the images of the angle region in eye. The proposed probe consists of a micro CCD camera and LED/NIR laser light sources and they are configured at the distal end to enable imaging of iridocorneal region inside eye. With this proposed dualmodal probe, imaging is performed in light (white visible LED ON) and dark (NIR laser light source alone) conditions and the angle region is noticeable in both cases. The imaging using NIR sources have major significance in anterior chamber imaging since it evades pupil constriction due to the bright light and thereby the artificial altering of anterior chamber angle. The proposed methodology and developed scheme are expected to find potential application in glaucoma disease detection and diagnosis.

A simple and low cost optical probe system for the high resolution imaging of the cornea is proposed, based on a Gaussian beam epi-illumination configuration. Corneal topography is obtained by moving the scanning spot across the eye in a raster fashion whereas pachymetry data is achieved by reconstructing the images obtained at different depths. The proposed prototype has been successfully tested on porcine eye samples ex vivo and subsequently on laboratory animals, such as the New Zealand White Rabbit, in vivo. This proposed system and methodology pave the way for realizing a simple and inexpensive optical configuration for pachymetry and keratometry readings, with achievable resolution up to the cellular level. This novel and non-contact high resolution imaging modality demonstrates high intraobserver reproducibility and repeatability. Together with its sophisticated data analysis strategies and safety profile, it is believed to complement existing imaging modalities in the assessment and evaluation of corneal diseases, which enable a decrease in morbidity and improvement in the effectiveness of subsequent treatment.

Optical coherence tomography (OCT) is a non-invasive, non-contact optical measurement and imaging technique that relies on low coherence interferometry. Apart from bio-imaging applications, the applicability of OCT can be extended to metrological investigations because of the inherent capability of optical interferometry to perform precise measurement with high sensitivity. In this paper, we demonstrate the feasibility of OCT for the measurement of the refractive index and thickness of bacterial biofilm structures grown in a flow cell. In OCT, the depth profiles are constructed by measuring the magnitude and time delay of back reflected light from the scattering sites by means of optical interferometry. The optical distance between scattering points can be obtained by measuring the separation between the point spread functions (PSF) at the respective points in the A-scan data. The refractive index of the biofilm is calculated by measuring the apparent shift in the position of the PSF corresponding to a reference surface, caused by the biofilm growth. In our experiment, the base layer of the flow cell is used as the reference surface. It is observed that the calculated refractive index of the biofilm is close to that of water, and agrees well with the previously reported value. Finally, the physical thickness of the biofilm is calculated by dividing the optical path length by the calculated value of refractive index. MOE (Min. of Education, S’pore) Published version

Near field optics concepts have introduced a paradigm shift in a wide variety of engineering fields in the recent past and the most significant applications of this fundamental physics concepts have been in the applied engineering problems such as improved broad band light absorption thereby enhancing the conversion efficiency of thin silicon solar cells. Also, for writing patterned structures or features using non contact optical methodologies have enabled near field optics assisted fabrication and related applications. The technology involving optics concepts and methodologies targeting energy sector have seen the impact of the same with a challenging trend to achieve smaller features or devices with micro- or nano-scale features. This demands automatically the need for achieving much smaller features beyond the forecasted sub- 30nm feature patterning methodologies. To meet such demands, a new branch of near- field optical concepts for improving patterning resolution has started developing which have been receiving considerable attention for its ability to produce high density sub-wavelength features that can find tremendous energy harvesting applications. This paper in this context mainly focuses on the review of different near field optical concepts and approaches developed for patterning by the author’s group at NTU. Different concepts were explored incorporating surface Plasmon waves ( LSPs, SPPs, LRSPs), gap modes as well as their interference in order to high resolution features and pattern dimensions at nano-scales. The absorbance of near band gap light is small and hence structuring of thin film solar cell is very important for increasing the absorbance by light trapping. The manuscript conclude by correlating the above said aspects and the challenges in achieving improved light conversion in thin film solar cells. Published version

Nanoparticles have great potential towards optical investigations in biological tissues which include imaging applications as well as therapeutic applications. However the effectiveness of these techniques is greatly governed by the optical properties of the nanoparticles. In this context this work performs computational investigations in determining the scattering and absorption efficiencies of spherical nanoparticles for a variety of nanoparticle dimensions and compositions over a wavelength band ranging from 380nm - 1064nm. The simulation results show that the optical response of the nanoparticles greatly varies with the material property, the radii of the particle as well as the laser wavelength and the results obtained aids in the selection of suitable nanoparticle for specific biomedical applications.

Achievable resolution of nano-rings, fabricated using commonly employed conventional mask based photolithography, is limited by diffraction of light. In this work conventional photolithography is modified to incorporate the phenomenon of surface plasmons to overcome the diffraction limit and thus to fabricate nano rings. Here, an embedded-amplitude mask based surface plasmon lithography is numerically investigated to conceptalize a noval methodology to fabricate the proposed nano-ring structure. Results of FDTD simulation shows sharp transmission peaks at the hole edges which could be recorded using suitable thinned photoresist to obtain nano ring structures.

In this paper, a maskless single step multiple (two and four) beams surface plasmon interference lithographic configuration is proposed and illustrated experimentally so as to obtain interference pattern with resolution several orders less than the illumination source wavelength. This technique utilizes a custom made prism layer configuration to pattern both one dimensional (grating line) and two dimensional (dot array) periodic nanostructures on the recording medium. Both aluminum and silver metal films are used for the experimental study. Large area patterns of grating lines and dot arrays with feature size ≈ 90 nm were experimentally obtained using an exposure radiation of 364 nm wavelength.

We present the implementation of a fluorescence lifetime imaging microscopy (FLIM) system for cellular characterisation. FLIM system can be used as an investigative tool to identify minor biochemical changes in cellular abnormalities. These subtle changes could possibly alter cellular fluorescence properties such as emission wavelength and lifetime. In this study, the fluorescence lifetime of haematoxylin and eosin (H&E)-stained tissues were investigated using a wide-field time-domain FLIM system. The transient response of epithelial fluorescence was investigated and the lifetime extracted using a bi-exponential model. It was found that the fluorescence lifetimes of eosin can be correlated to the tissue histology. The preliminary result suggests that tumor-associated molecules are retained in the tissues even after tissue fixation and staining. The developed FLIM system was successfully applied to detect the histological changes in the tissue samples. Optimization of system parameters is also discussed.

A flexible fiber optic endoscope system for investigations in medical cavities using specially designed probe distal and proximal ends so as to facilitate speckle correlation analysis and simultaneous cavity imaging is presented in this paper. The design of the endoscope probe and associated detection system facilitating the analysis of deformation components and their derivatives in cavity surfaces is discussed. The distal end of the probe consists of a single fiber optic port for illumination and an imaging lens-image fiber unit for imaging the reflected speckle pattern from the cavity surface. An optical component selection unit is designed and used at the probe proximal end, for switching between deformation and derivative configurations. Theoretical analysis regarding speckle fringe formation on curved surfaces, for out-of-plane and shear configurations are discussed. Preliminary experimental investigations using the developed probe on inner surfaces of curved phantom tissue specimens are carried out for the abnormal growth detection and the results are analyzed. The obtained experimental results are compared with the developed theory. This probe system and its associated concepts may later lead into the improvements of currently used surgical assisted endoscopic techniques for early cancer diagnosis.

Fluorescence lifetime imaging (FLIM) in frequency domain enables the mapping of the spatial distribution of fluorescence lifetimes of a specimen. FLIM can provide unique information about fluorophores and hence is widely used in biology and for medical diagnostics. In this paper, a theoretical analysis for the fluorescence lifetime determination of latent fingerprint samples is described, which is followed by the feasibility study of using FLIM in frequency domain for latent fingerprints detection. Experiments are carried out with fingerprint on green paper substrate and postcard substrate treated with certain fluorescent powder. The total phase lag and demodulation factor are calculated to determine the lifetimes pixel by pixel. The resulting fluorescence lifetime image of fingerprint revealed an improvement in the contrast, and was able to detect the latent fingerprint clearly.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, we present the design of an endoscope probe, which can image the inner cavity walls as well as collect fluorescence emission from the same cavity inner surfaces, for disease diagnosis in body cavities. The probe makes use of a single coherent laser illumination / excitation source for both modalities. An imaging lens at the probe end collects the fluorescence emission as well as the image from the test surface. Two types of imaging lens are used in the probe and their fluorescence collection efficiencies and imaging capabilities are compared with each other. An eyepiece at the end of the probe directs the transmitted light into a CCD camera / Monochromator through selected filters to display the image / analyze the emission spectrum. The developed probe has been tested in a phantom colon model, where cancerous growths and fluorophores are simulated, so as to illustrate the probe diagnostic efficiency.

A theoretical model, which describes both the spatial distributions of photons in fluorescence endoscopic images forthe detection of cancerous cells in the intracavities of human body, specifically gastrointestinal path is presented inthis paper. The design concept of the image probe, which was developed for collecting the low fluorescent emissionusing an excitation laser source and imaging done through a novel imaging lens scheme will be discussed with itsadvantages and limitations in comparison with the existing imaging schemes. Finally, a quantitative analysis doneby varying the different parameters affecting the tissue fluorescence is discussed in this paper.Key words : Endoscopy, Imaging lens, Fluorescent imaging. 1. INTRODUCTION Minimally invasive treatment is becoming central to medicine, both improving results and reducing recovery timesand costs. In some cases it is guided by external imaging means [1], such as fluoroscopy, ultrasound, computedtomography (CT), magnetic resonance (MR), etc ., but often it is critical to have the internal optical view providedby an endoscope. Endoscopes are long and narrow optical systems intended for the observation of technical andmedical intra cavities. Several studies have been done in the area of endoscopic imaging [1-6]. There are rigid andflexible endoscopes available for this purpose and both have found wide acceptance in medical fields [5,6]. Rigidand flexible endoscopes differ in their image-transmitting scheme, based on which most endoscopes are divided intothree distinct subsystems: the objective, the relay and the eyepiece. Of the three, the relay system, which starts fromthe tip of the endoscope to transfer the image, whose length varies according to the application is used to classify theentire endoscope system [7]. In rigid endoscopes, image is transferred by means of relay rod lenses with spherical surfaces where as in flexibleendoscopes, an ordered fiber bundle is used for this purpose [6]. For practical purposes, flexible endoscopes provideunlimited length and the demands on design and performance of endoscopic instruments, including very smalldiameters and extreme flexibility have increased over the last decade. Fiberoptic endoscopes permit visualization ofnormally accessible areas within the body and without pain or discomfort for patients. And a flexible endoscope isessential due to the fact that rigid endoscopes have limited use in digestive endoscopy [8]. A flexible endoscope isconsidered to be a more appropriate tool for minimally invasive inspection and surgery as it enters the body throughnatural openings [9]. In such cases more care should be taken in selecting appropriate biocompatible materials, asthey are supposed to interact with living tissue without any kind of conflict or collateral effect, for the endoscopeouter covering so as to ensure seldom hazards to the body tissue sites [10]*Address for correspondence

In interferometric fringe pattern analysis, specular and speckle fringe patterns are the two main divisions. While specular fringes are characterized by quality fringes with high spatial resolution, speckle (that obtains due to the diffuse scattering of the coherent radiation from an optically rough surface) fringe patterns, which contain high frequency carrier (as noise), are characterized by noisy fringes. This paper concentrates on this aspect and the matlab based filtering methods to improve quality of speckle fringe patterns by developing the appropriate software. Further, the newly developed software Macurv will be presented which can give the second order derivative (curvature) fringe information . A software with several functions is written using Matlab. The objective of the software is to provide a more effective way for post-processing of the speckle interferometric fringes. The algorithm and functions of the developed software Macurv will be explained.

Dynamic analysis of structures and machines provide valuable information on the performance and reliability of structures. In this paper, we present the development of a Fiber Optic Polarimetric Sensor (FOPS*) for dynamic and impact loading. The sensor monitors the change in the state of polarization of the light beam traversing the fiber under dynamic loading. The dynamic response of three different sensing optical fibers - high birefringence (bow-tie) fiber, low birefringence fiber and an ordinary communication grade single mode fiber are evaluated. The studies were carried out by embedding the optical fibers in a glass fiber reinforced plastic (GFRP) specimen and the performances of the three different systems were compared.

Tele-shearographic system based on bulk optics shearing and optical fiber based illumination have been used for the Non-Destructive Testing (NDT)of aircraft/composite laminates. This paper describes the application of this system and a detailed experimental analysis to assess defects in Carbon Fiber Reinforced Plastic (CFRP) composite laminates by analyzing the fringe patterns. CFRP laminates and honeycomb panels have been studied using the this system in various optical configurations. This technique provides the full-field, non-contact method for Non-Destructive Evaluation (NDE) of composite specimens. A Comparative study between digital shearography and other NDTechniques are also discussed in this paper.

Human tooth structure in the oral environment is affected by considerable thermal fluctuations while consuming hot and cold, fluids and food materials. This paper describes the use of digital moire interferometric technique and Electronic Speckle Pattern correlation Interferometry (ESPI) to study the behavior of the functionally adapted dentine materials to thermal loads in its own plane and out of plane. Further, this study also highlights the advantage of optical techniques with enhanced sensitivity and spatial resolution to examine the whole-field thermal property gradients in an anisotropic biological material such as human dentine.

With the emergence of new trends in optical communication and sensing syste, the importance and need for novel concepts such as diffractive gratings that can be implemented on a microscale has become a thrust area of research. In this paper, we compare various grating structures and their suitability for MEMS applications. The design approach, analysis and integration into optical systems are discussed in the context of both fiber and planar waveguide technology. Also, the differences and similarities among these methodologies using silicon micromachining are brought out. Parameters of interest, such as efficiency of micro-grating structures and ambient effects that influence the performance of the devices/systems are also discussed.

Studies of the effects of various parameters like pre-stress, input azimuth, fiber turns etc. on the polarimetric fiber optic sensing system for smart structure applications are presented. The presence of the smart structure modifies the output characteristics of the highly birefringent fiber due to elastic properties of the structure. Experimental procedures are repeated for Single Mode and the bow-tie HiBi fibers with different optical configurations. Both carbon fiber reinforced (CFRP) and glass fiber reinforced (GFRP) specimens are studied in detail. These experimental evaluation clearly point out that the sensitivity of such embedded polarimetric sensors are affected by the above mentioned parameters. Hence these parameters are optimized for making the smart sensing systems with specific objectives. Keeping the modulated signal at the optimum sensitivity range for the realization of a smart weighing machine and an efficient design of the sensor to increase its range of sensitivity by optimizing the parameters for defect detection in the composite laminates will be the topics of discussion in this paper.

Fiber optic phase shifting Electronic Speckle Pattern Interferometer system has been used for the Non-Destructive Testing of Composite laminates. This paper describes the application of this system to assess delamination and fiber breakage in Glass Fiber Reinforced Plastic (GFRP) laminates by analyzing the fringe patterns and by applying phase shifting techniques. GFRP laminates have been studied using the ESPI system in various optical configurations. This technique provides the full-field, non-contact method for NDE of composite specimens.

In certain fiber optic sensors and gyroscopes using birefringent fibers, it is essential to precisely align the principal axes of the fibers used, when two fibers are spliced or connected. The proper alignment of the linearly polarized input light wave with the birefringence axis of a fiber is crucial for maintaining the state of polarization of the guided wave. In this paper, a new method for the detection of the eigen axes and precise alignment of the principal axes between two birefringent fibers (HiBi) using shear interferometry is presented.

In this paper, we present the developmental details of an all fiber optic phase shifting Electronic Speckle Pattern Interferometer (ESPI) and its applications in Non-Destructive Evaluation (NDE) of Glass Fiber Reinforced Plastics (GFRP). Laminates made of GFRP are studied using the developed ESPI system in its shear configuration. This provides a full-field, non-contact method for NDE, and also observation of surface strains which are proportional to displacement derivatives. Since defects in objects usually induce strain concentrations, shear ESPI reveals defects by identifying anomalies in strain concentration. Experiments have been carried out for NDE of unidirectional GFRP specimens at different fiber orientations and having programmed defects. The experimental results are compared with the theoretical predictions.