The potential of mid-infrared optical sensing technology for liquid phase applications is tightly related to the implementation of appropriate chemical modifications of the sensing surface. Besides recognition and enrichment of analyte molecules, suppression of interfering water absorptions is of substantial interest. Utilizing the principle of evanescent wave spectroscopy for the signal generation is the basis for the realization of powerful spectroscopic sensing systems, which provide access to molecule specific vibrations of organic analytes. With the availability of mid-infrared transparent fiberoptic materials, access to the mid-infrared spectral region from 220 micrometers is gained, enabling remote qualitative and quantitative analysis. Besides polymeric molecular recognition and enrichment layers, sol-gel films are among the most promising surface modification. This highly reproducible process allows detailed control on the surface properties of the sol-gel layer formed at the waveguide surface. Due to the inertness and robustness of sol-gel coatings they can be considered among the most promising surface modification materials for optical sensors applied in the biomedical and biological field.

In many bio-medical optical imaging applications such as monitoring cell and tissue dynamics and imaging of phase objects, ultra-small changes in refractive index must be detected. We describe a fiber-based optical biosensor, which is capable of detecting ultra-small refractive index changes in highly scattering media with high lateral and longitudinal spatial resolution. The system is a dual channel phase-sensitive optical low coherence tomography system that measures relative optical path length differences between the orthogonal modes of the polarization-maintaining fiber.

We report the application of an optical fiber-based humidity sensor to the problem of breathing diagnostics. The sensor is fabricated by molecularly self-assembling selected polymers and functionalized inorganic nanoclusters into multilayered optical thin films on the cleaved and polished flat end of a singlemode optical fiber. Prior work has studied the synthesis process and the fundamental mechanisms responsible for the change in optical reflection from the film that occurs as a function of humidity. We will briefly review that prior work as a way to introduce more recent developments. This paper will then discuss the application of these sensors to the analysis of air flow. We have designed the sensor thin film materials for the detection of relative humidity over a wide range, from approximately 10 to 95%, and for response times as short as several tens of milliseconds. This very fast response time allows the near real-time analysis of air flow and humidity during a single breath, with the advantage of very small size.

A simple technique for the optical characterization and detection of early stages of biofilm is presented. A bacterial biofilm was developed in a microcell, consisting of of two glass slides and the biofilm between the slides. This technique allowed the measurement of biofilm thickness and refractive index of the medium as a function of position and time. Using a sensititive broad bandgap foil as surface, the early stage of biofilm formation was investigated in a simple reflectivity measurement using a fiber optic spectrometer. The optical results were compared with the measurements of confocal laser scanning microscope (CLSM). The fiber optic spectrometer technique was suitable for the detection of low bacterial densities (10⁴ cm^-2).

Evaluation of tissue O₂ balance (Supply/Demand) could be done by monitoring in real-time 2 out of the 3 components of the tissue O₂ balance equation. In our previous publication (Mayevsky et al, SPIE Vol. 4255:33-39, 2001) we had shown the use of the multiparametric monitoring approach in the neurosurgical operating room, using a device combined of laser Doppler flowmeter (LDF) and surface fluorometer reflectometer. The two instruments having two different light sources, were connected to the tissue via a combined bundle of optical fibers. In order to improve the correlation between tissue blood flow and mitochondrial NADH redox state, the new Tissue Spectroscope (TiSpec) that was designed has a single light source and a single bundle of optical fibers. Preliminary results show very clear correlation between TBF and NADH redox state. In addition, the reflected light at the excitation wavelength could be used as an indication for blood volume changes. The results obtained by the TiSpec enabled us to compare tissue O₂ delivery (TBF) with O₂ balance (NADH redox state) in the brain of gerbils and rats exposed to ischemia, anoxia and spreading depression. Real-time monitoring of the metabolic state of the tissue has immense potential during surgical procedures.

The use of several silver halide and chalcogenide infrared transmitting fibers in the detection of cancer is investigated. As a test sample for all types of fibers we used a thin section of an entire rat brain with glioblastoma. Moving the sample with an XY stage maps across the whole tissue section with more than 200 spectra were recorded. Data evaluation was performed using Principal Components Analysis (PCA). The silver halide fibers have provided excellent results. The tumor was clearly differentiable from the normal tissue. It wasn't possible to identify the tumor region using chalcogenide fibers because the fiber has a very low transmittance in the important fingerprint region.

We report a novel approach to the fabrication of multiplexed optical fiber-based humidity and chemical sensors based on the molecular-level self-assembly of multilayered thin films that consist of several discrete segments through their thickness. As part of previous work, our groups have cooperatively demonstrated the formation of such self- assembled sensor thin films on optical fibers for the detection of humidity, and for the detection of chemical species. The layer-by-layer deposition of the alternating polyelectrolyte species used for each of these sensors independently has been discussed in several related papers. Here we have investigated the formation of multiple thin film sensor segments, one on top of the other, on the end of a singlemode optical fiber. The polymers and nanoclusters in each of the segments make them react differently to different chemical species. Together the segments form a spectral filter on the end of the fiber. We have investigated multiplexing and report initial experimental results.

Novel sensor chips for evanescent wave sensing have been developed and investigated for various (bio-) chemical applications. A preferred integrated optical sensing scheme requires an array of independent sensing pads being present on the chip, each one of them having two different regions for in- and out coupling of the optical readout beam. We present a novel chip type, where this goal is achieved by thickness variations of the waveguiding film and one single grating period. The sensor chips consisted of a 300 nm thick Ta₂O₅ waveguide deposited on a glass substrate structured with a uniform grating of 360 nm period. By this optical structure a first coupling angle was defined. Sensing pads with a different coupling angle were realized by etching the film at selected regions down to a thickness of about 150 nm. The performance of the novel chips was demonstrated in various refractometric sensing applications. The experiments included cover medium refractive index variation as well as monitoring of affinity binding of small molecules. A very high resolution of 3x10^-8 in the effective refractive index was achieved. The emphasis of this paper is on describing this approach and on presenting optical chip characterization methods together with modeling results.

A common-path, heterodyne interferometric system for studying the phase variations under surface plasmon resonance (SPR) is presented. The reflected beam from SPR is further going into a total internal reflection device (TIR) for increasing the sensitivity by the phase shift between TE-wave and TM-wave, as described by the Fresneli's equation. With the combination of a SPR prism and a TIR prism, the system can avoid the change of direction in the output light, which is always happened when only a SPR prism has been used. An unaltered output light is convenient for the detection devices. The system utilizes a pair of orthogonally linearly polarized beams with heterodyne frequency of 60 kHz as the light source. They are perfectly collinear so that the noises resulting from the ambient conditions are greatly reduced. Compared with the technique of reflectivity variation measurement, which is widely used in traditional SPR, the phase variation measurement using common-path, heterodyne techniques is estimated to be higher in sensitivity and thus can be used as a high-sensitivity-demanded biosensor.

Photonic crystal fibers having a microstructured air-silica cross section offer new optical properties compared to conventional fibers. These include novel guiding mechanisms, new group velocity dispersion properties and new non-linear possibilities.

Within the past few years, the new class of photonic crystal fibres has attracted significant attention in the international research community. In these fibres, the cladding structure typically is a system of air holes in a matrix of undoped silica. By properly designing this cladding structure unique properties can be achieved. Due to the complexity of such structures, modelling these fibres is cumbersome and time consuming. Thus, the particularities of the model used are of great interest. We use a variant of the localised function method to enable modelling of real fibre structures with finite claddings. In this presentation, we discuss the advantages and challenges of this approach. We further present a number of improvements of the method and some results obtained therewith. With the rapid improvement of fabrication techniques the last few years have seen, precise control of the cladding structures has become possible. The ability to model these structures is, therefore, becoming increasingly important. It is, also therefore, increasingly important to precisely model the properties of these structures.

An all-dielectric hollow waveguide structure consisting of radially alternating dielectric layers and exhibiting a photonic bandgap is proposed. Fabrication involves the extrusion of a stack-of-plates into a hollow structure composed of alternating high/low index pairs. Specifically, a billet consisting of alternating glass layers is forced through a die such that laminar flow forces the periodicity from an axial to a radial orientation. The concept is demonstrated using lead-borosilicate and As₂Se₃ glasses.

Hollow waveguides are very sensitive to bending and coupling conditions, which cause large losses. In order to overcome these limitations we suggest to develop a multilayer hollow waveguide based on one-dimensional photonic crystal. Photonic crystals have been investigated for many years. The most common photonic crystal structure is the one- dimensional photonic crystal. For over a decade many companies and research groups have been manufacturing 'perfect mirrors' made of alternating pairs of dielectric materials with different index of refraction. These mirrors are made of a large number (approximately 10) of pairs. Applying the same type of coating techniques, with large number of pairs, to tubular shapes is very difficult and hollow waveguides based on this technology cannot be manufactured. We suggest an alternative method of coating flat surfaces with a pairs of layers of high ratio of index of refraction that can be applied later with minimal difficulties for tubes (hollow waveguides). We used a thin transparent metal layer (silver) as one of the dielectric materials of the pair. The thin metal layers have a large index of refraction in the MIR and the dielectric layer (silver iodine) has lower refractive index. Using these materials enables us to achieve a large ration of index of refraction, which is required for creating photonic crystal properties with a low number of pairs. We developed a mirror from alternating pairs of silver and silver iodine using an electroless chemical method. A mirror made of 4 pairs has reflectance close to 100% and omnidirectional behavior over a wide spectral region (6 - 10 micrometers ). This experimental result is in agreement with our theoretical model as well as other approaches. Using a ray model we have shown that a hollow waveguide based on the same structure of layers will have negligible attenuation when bent and will not be sensitive to the focal length of the coupling lens (omnidirectional).

For delivery of femtosecond laser pulses, a flexible, hollow glass fiber with inner coating of silver and polymer is employed. By using the hollow fibers, ultrashort pulses with a pulse width of 196 fsec, energy of 700 mJ, and repetition rate of 1 kHz are transmitted with no damage on the fiber. The transmission loss is 0.13 dB in the 1.0-mm bore fiber and pulse broadening after transmission in 1-m long fiber is 17 fsec. Theoretical calculation shows that the numbers of transmitted modes in the fiber is estimated from the pulse broadening.

We have investigated the transmission characteristics of an alternative all-optical-waveguide system for x-ray delivery to a precise tissue area. The delivery system includes two basic optical elements: a funnel-shaped uncoated hollow glass taper and a flexible hollow delivery waveguide. The hollow taper provides direct launching of the input x-ray radiation into a delivery waveguide. It is an uncoated glass taper whose operating principle is based on the grazing-incidence effect. We investigated both experimentally and theoretically how the transmission properties of the hollow taper depend on its geometrical parameters such as cone shape, length, input and output core diameters. The x-ray-source-to-taper coupling efficiency obtained was about 20-25%. That is relatively low in comparison with typical laser-to-taper coupling efficiencies due to the poorly collimated x-ray beam. Furthermore, we have studied the x-ray beam profile conversion by the grazing-incidence-based hollow taper. The x-ray radiation was launched into the delivery waveguide by a direct taper-to-waveguide coupling. In our experiment, we used both uncoated and metal-coated hollow waveguides with various geometrical parameters. The waveguide transmission characteristics, including the coupling efficiencies and beam profile conversion, were investigated for both straight and bent delivery waveguides. The results obtained as presented in this report give considerable confidence for successful application of the all-waveguide system as an alternative x-ray delivery technique for biomedical use.

Flexible polycarbonate tubing with bore sizes ranging from 500 to 2000 micron was used to make hollow waveguides. Silver/silver iodide and copper/copper iodide were chosen as the metal and dielectric layers respectively. The Ag/AgI waveguide loss at lambda equals 10.6 micron is below 0.1 dB/m for bore sizes over 1200 micron. The small bore size waveguides are single mode even on bending, while the larger bore size waveguides are multimode. The measured loss at 10.6 micron for a 1000 micron bore size Cu/CuI waveguide is 0.33 dB/m. An approximately flat transmission window was obtained for Cu/CuI waveguides in the wavelength region from 2 to 16 micron.

For the last years, several types of hollow waveguides have been under development and, as proved, these waveguides can deliver radiation to the target with high transmittances. The problem appears in the case of the so-called 'contact mode' when, in medical applications, the end of the waveguide is in contact with the soft or hard tissues. The irradiated substance is melted or ablated and a part of this material usually destroys the inner surface of the waveguide end. To overcome these problems, the end of the hollow waveguide must be sealed .In this paper, the results with two types of sealed hollow waveguides prepared for the delivery of Er:YAG laser radiation were investigated. Sealing was conducted by a cap of short Pyrex or fused silica glass tube whose inner diameter was just slightly larger than the outer diameter of the hollow waveguide to be sealed. The transmittance of the fused silica cap connected with the hollow glass waveguide was measured to be 94%, Pyrex glass cap transmittance was found to be slightly smaller (78%). The damage output energy threshold of the cap material was found 750 mJ. This value was sufficiently high for using this sealed waveguides in the ophthalmology application.

Different dielectric thin films are being studied in order to facilitate the processing of low loss multiple metallic dielectric hollow glass waveguides (HGWs). In this paper, we present our results for high index lead sulfide (PbS) and low index optical polymers, especially, polystyrene (PS) and cyclic olefin polymer (COP). We have observed that the morphology of the thin film is dependent on the processing conditions. The morphology of the metal and dielectric thin film affects the attenuation of the hollow glass waveguides. We have been successful in depositing and controlling the morphology of high n PbS thin films and low n polymer films. In order to aid in thin film characterization we have also deposited on planar substrates.

In order to change the output direction of laser light in a small operation space, two types of hollow bent output devices are proposed for medical laser light delivery system. One is the hollow fiber insert-type, which is of the bending radius of 40 mm. The other type, based on a fixed- bent pyrex glass tube, is of the minimum bending radius of 2 mm. Both types of devices are inner-coated with a silver layer and a cyclic olefin polymer (COP) layer. Loss properties for CO₂ laser light are clarified for the insert-type devices, called laryngo-tip. And the transmission properties for pilot beams are experimentally discussed. For the fixed-bent tips, the losses are around 0.7 dB, which are almost independent on the bending angles and radii. Transmissions of the tips remain undamaged after 1-hour-delivery of 5 W output CO₂ laser light.

Pure silica core fibers with a high OH content are believed to be the best fiber type for the visible and ultaviolet region concerning low attenuation and high power stability. Here we present systematic investigations of silica core - fluorine doped cladding fibers made by MCVD, where the composition of the fiber core was slightly modified by doping with germanium, phosphorus, boron, aluminium, and fluorine at a low level of typically 0.5 mol%. Moreover, the influence of hydrogen treatment was studied. The attenuation level in the visible region, the defect band at 330 nm and the oxygen deficiency bands around 250 nm could be correlated with the preform modifications in a systematic manner. Interaction mechanisms between the defect bands and with the OH groups are discussed. The stability of the fiber against high power blue light in the Watt range was investigated. By suitably modified fibers, stability times could be realized much longer than in conventional high OH fibers.

Commercial optical endoscopes rely on image transfer and acquisition based on an array of photon detectors, such as a coherent fiberoptic bundle, a video camera, and/or the human retina. An alternative approach uses a resonantly vibrating optical fiber that scans laser illumination. However, the limitation of laser-scanning endoscopic development has been the technological challenge of fabricating a small diameter, opto-mechanical scanner. A proof-of-concept micro-optical scanner has been built using a 2.3 mm diameter piezoelectric actuator and 4 mm diameter lenses. Images are generated using resonant spiral scanning of the fiber, projecting monochromatic laser light to an illumination plane. A single photodetector is used to acquire grayscale images one pixel at a time. In vitro, the acquired images of test targets have 10 to 20 micrometers maximum spatial resolution and a field-of-view that can be electronically varied.

Key to many laser and sensor applications, in the medical area, is the desire to maintain high core to clad ratios for minimum penetration and maximum flexibility. The transmission of laser beams through optical fibers in a stable, uniform manner is a critical need and assumption for many surgical and sensing medical applications. Cladding thickness has been found to affect the transmission of signals across the electromagnetic spectrum in an uneven manner, especially when typical jacketing materials are used to protect the optical fibers against mechanical/environmental degradation. Experimental data and analysis of the effect of cladding thickness of the spectral transmission of optical fibers having core diameters below 300 micrometers are presented. Particularly for fibers with below 100 micrometers core diameters, fibers with cladding/core ratios below 1.2 are shown to have altered transmission spectra at wavelengths above 600 nm. The sensitivity is more pronounced for 'water-free,' low-OH optical fibers, which have significant transmission through the near infrared [NIR] region.

Optical imaging of objects within highly scattering media requires the detection of ballistic/quasiballistic photons through these media. Recent works have used Phase/Coherence Domain or Time Domain Tomography (femtosecond pulses) to detect the shortest path photons through scattering media. Our collimation detection uses Small Acceptance Angle Devices to extract photons emitted within a small source angle. This work employs a high aspect, micromachined collimating detector array fabricated by high-resolution silicon surface micromachining. Consider a linear collimating array of very high aspect ratio (200:1) containing 51x1000 micrometers etched channels with 102 micrometers spacing over a 10 mm silicon width. With precise array alignment to a laser source, unscattered light passes directly through the channels to the CCD detector and the channel walls absorb the scattered light at angles >0.29 degree(s) Objects within a scattering medium were scanned quickly with a computer-controlled Z-axis table. High-resolution images of 100 micrometers wide lines and spaces were detected at scattered-to-ballistic ratios of 500,000:1. At >5,000,000:1 ratios, a uniform background of scattered illumination degrades the image contrast unless recovered by background subtraction. Simulations suggest smaller channels and longer arrays could enhance detection by factors greater than 100. Detection using Silicon Micromachined Collimating Arrays are also nearly wavelength independent.

Many potential microsurgical applications of UV laser radiation need a flexible beam guiding system. Especially for the argon fluorine excimer laser ((lambda) equals 193 nm) and for the 5th harmonic of the Nd:YAG laser ((lambda) equals 213 nm) the use of optical fused silica fibers is difficult. In this work we designed and tested a laboratory prototype of a scalpel for surgical treatments of the retina based on UV laser ablation. To achieve the necessary flexibility and to provide laser fluences above the ablation threshold of retina we developed a new type of beam guiding device. A hollow core waveguide is used in combination with a short length of a special fused silica optical fiber to guide the laser beam. To increase the laser fluence at the distal scalpel tip and to achieve a very small cut width a fused silica fiber (core diameter 600 micrometers ) has been tapered down to a diameter of about 150 micrometers .

A COP (cyclic olefin polymer)-coated silver hollow waveguide was used to transmit 1064-nm nanosecond laser pulses that irradiated a metal target to produce an ultrasound wave (stress wave) through plasma formation on the target. Porcine gastric and myocardium tissues, on which a solution of rhodamine-B isothiocyanate-dextran (10 kDa) was dropped, were exposed to the stress wave. Much deeper drug penetration was observed for the tissues exposed the stress wave than for the tissues without stress wave exposure.

The sample analysis with Thin Layer Chromatography is an off-line method, in contrast to other separation methods. In addition to the sample preparation, the key-step is the in- situ densitometry for qualitative or quantitative purposes by measuring the optical density of the separated spots directly on the plate. With a newly developed HPTLC-system using fiber-optic and diode-array detection, simultaneous measurements at different wavelengths can be performed. The fiber-optic assembly consists of UV-improved silica-based fibers transporting the light and illuminating the surface of the TLC-plate. The light is scattered and reflected from the surface and finally collected by the reading fibers, carrying the relevant information. Up to now, only the absorption mode was used. However, using fluorescent substances, the light-power in a well-defined wavelength- region can be increased: the collected light from the illuminated spot of a substance on the TLC-plate is higher than the reference probe without substances. Fluorescent measurements are possible without filters or special lamps and the improvement of signal-to-noise ratios is significant. Due to the detailed structure of the PAH's absorption- and fluorescence spectra, diode-array HPTLC makes quantification of 16 PAH, recommended for monitoring by EPA, on one track possible. There is an additional advantage: although the spatial resolution of the TLC-separation of PAH is not sufficient for in situ quantitative densitometric analysis each of the 16 compounds can be quantified with the new method using suitable wavelengths.

Optical features of a novel technique for fiber-optic-based laser delivery into a precise tissue area are investigated. As a key optical element, the technique includes a single delivery fiber with a specially angle shaped tip. Because of the frustrated-total-internal-reflectance caused by the refractive-index change of the surrounding medium, the angled fiber tip acts as a smart, tissue-activated probe. It provides a safe way for laser delivery that includes only two states of tissue illumination: (1) off-state (no tissue illumination), when the fiber tip is out of the tissue area and the laser emission is backreflected due to total-internal-reflection; and (2) on-state (maximum tissue illumination), when the fiber tip is on the absorbing tissue area and becomes transparent because of the frustrated-total-internal-reflectance and the laser energy is coupled into the absorber. Here, optical properties of tissue-activated fiber probes used for precise laser delivery are investigated both experimentally and theoretically by analyzing the backreflectance signal power. Optical fibers with various geometrical parameters are used and a spatial resolution of 2 micrometers is achieved when the fiber tip is moved toward the absorption tissue surface. The results confirm the system potential for on-the-spot laser delivery applicable to precise laser treatment, tissue diagnostics, and micro-scale surgical procedures.

An optical technique based on the surface plasmon resonance of thin was used to study the influence of aqueous organic subtances on thin aluminum films. In a quasi continous flow experiment over several days it was found that the aluminium was dissolved and the thickness decrease could be monitored very accurately. In a series of experiments with different organic components the corrosion could be attributed to the nutrient Alginate promomoting medium and the extracellular polymeric substances produced by the bacteria pseudomonas aeruginosa. In comparison with inorganic substances the same behaviour could not be observed. Regardless of the particular chemical reactions involved this method seems to be suitable to monitor low aluminium corrosion rates in the order of nm/h quite precisely.

Microstructured optical fibres (MOFs) have aroused great interest in recent years because of their unusual optical properties. These include their ability to be effectively single moded over a very large range of wavelengths, tailorisable dispersion, high or low non-linearity(depending on the hole design) and large core single mode fibres. We have recently fabricated the first Microstructured Polymer Optical Fibres (MPOFs), which further extend the range of possibilities in MOFs. The properties of polymers can be tailored to specific applications (eg:made highly non-linear or having gain) in a way that is not possible in glass. Further, the large range of fabrication methods available in polymers, including casting and extrusion, mean that the structures that can be obtained are very difficult to make by capillary stacking- the method used in glass MOFs. Here we present the latest results from our group using MPOFs, including single mode fibre and Bragg fibres.