Competing radiative and nonradiative decay of embedded ions states in dielectric crystals: theory, and application to Co2+:AgCl0.5Br0.5.

We present a generally applicable theoretical model describing excited-state decay lifetime analysis of metal ions in a host crystal matrix. In contrast to common practice, we include multi-phonon non-radiative transitions competitively to the radiative one. We have applied our theory to Co2+ ions in a mixed AgCl0.5Br0.5 crystal, and as opposed to a previous analysis, find excellent agreement between theory and experiment over the entire measured temperature range. The fit predicts a zero absolute temperature radiative lifetime τrad(0) = 5.5 ms, more than three times longer than the measured effective low-temperature one τeff(0) = 1.48 ms. Furthermore, the fit configuration potential dissociation energy has been estimated as D = 2500 cm-1 and the lattice vibrational cutoff frequency as hωco = 180 cm-1. We have experimentally verified the latter by optical reflection measurement in the far-IR.

Mid-infrared fiber-optic evanescent field spectroscopy for in situ monitoring of tetrahydrofuran hydrate formation and dissociation.

Tetrahydrofuran is a relevant auxiliary molecule when storing carbon dioxide or hydrocarbons as gas hydrates. The present study demonstrates the application of in situ mid-infrared fiber-optic evanescent field absorption spectroscopy for studying the formation and dissociation of THF hydrates. Thereby, the utility of this analytical technique for providing unique molecular-level insight even under harsh environmental conditions is evidenced.

Optimization of fiber-optic evanescent wave spectroscopy: a Monte Carlo approach.

The absorbance of the evanescent waves of infrared radiation transmitted through an optical fiber depends on the geometry of the fiber in addition to the wavelength of the electromagnetic radiation. The signal can thus be enhanced by flattening the midsection of the fiber. While the dependence of the absorbance on the thickness of the midsection has already been studied and experimented upon, we demonstrate that similar results are obtained using Monte Carlo methods based simply on geometrical optics, given the dimensions of the fiber and the power distribution of the fired rays. The optimization can be extended to fibers with more complex geometries of the sensor.

Modal filtering for midinfrared nulling interferometry using single mode silver halide fibers.

We demonstrate the modal filtering properties of newly developed single mode silver halide fibers for use at midinfrared wavelengths, centered at 10.5 microm. The goal was to achieve a suppression of nonfundamental modes greater than a factor of 300 to enable the detection and characterization of Earthlike exoplanets with a space-based nulling interferometer. Fiber segments of 4.5 cm, 10.5 cm, 15 cm, and 20 cm lengths were tested. We find that the performance of the fiber was limited not by the modal filtering properties of the core but by the unsuppressed cladding modes present at the output of the fiber. In 10.5 cm and longer sections, this effect can be alleviated by properly aperturing the output. Exclusive of coupling losses, the fiber segments of 10.5-20 cm length can provide power suppression of undesirable components of the input field by a factor of 15,000 at least. The demonstrated performance thus far surpasses our requirements, such that even very short sections of fiber provide adequate modal filtering for exoplanet characterization.

Potential of 'flat' fibre evanescent wave spectroscopy to discriminate between normal and malignant cells in vitro.

The present study focuses on evaluating the potential of flattened AgClBr fibre-optic evanescent wave spectroscopy (FTIR-FEWS) technique for detection and identification of cancer cells in vitro using cell culture as a model system. The FTIR-FEWS results are compared to those from FTIR-microspectroscopy (FTIR-MSP) method extensively used to identify spectral properties of intact cells. Ten different samples of control and malignant cells were measured in parallel by the above two methods. Our results show a significant similarity between the results obtained by the two methodologies. The absorbance level of Amide I/Amide II, phosphates and carbohydrates were significantly altered in malignant compared to the normal cells using both systems. Thus, common biomarkers such as Amide I/Amide II, phosphate and carbohydrate levels can be derived to discern between normal and cancer cells. However, marked differences are also noted between the two methodologies in the protein bands due to CH3 bending vibration (1480-1350 cm(-1)). The spectral differences may be attributed to the variation in the penetration depth of the two methodologies. The use of flattened fibre rather than the standard cylindrical fibre has several practical advantages and is considered as an important step towards in vivo measurements in real time, such as that of skin nevi and melanoma using special designs of fibre-optic-based sensors.

Investigations of the structure of water using mid-IR fiberoptic evanescent wave spectroscopy.

Liquid water is one of the most studied but still one of the least understood substances. The absorption spectra of water in the mid-IR were measured from -10 and up to 90 degrees C using fiberoptic evanescent wave spectroscopy. The changes in the spectrum and the existence of an isosbestic point during the ice-liquid phase transition were studied. Based on the spectroscopic data we propose a multispecies model for the structure of liquid water. The new model provides an explanation for the results of the measurements reported in this work. It also provides an explanation for some of the unique thermodynamic and kinetic properties of liquid water and for many of the anomalies of water.

Evanescent-wave infrared spectroscopy with flattened fibers as sensing elements.

Fiber-optic evanescent-wave spectroscopy (FEWS) is a novel method for measuring the absorption spectra of samples in contact with a segment of an optical fiber that serves as a sensing element. We used a cylindrical IR-transmitting AgClBr fiber whose central section, of length L, was flattened to a thickness d. This section was used as the FEWS sensing element. Our theoretical work predicted that the signals obtained in FEWS measurements should be linearly dependent on L and inversely proportional to d. Decreasing the thickness can significantly increase its sensitivity of the sensor. These theoretical results were verified experimentally by measurements of methanol and water.

In situ sensing of volatile organic compounds in groundwater: first field tests of a mid-infrared fiber-optic sensing system.

A prototype mid-infrared sensor system for the determination of volatile organic pollutants in groundwater was developed and tested under real-world conditions. The sensor comprises a portable Fourier transform infrared spectrometer, coupled to the sensor head via mid-infrared transparent silver halide fiber-optic cables. A 10 cm unclad middle section of the 6-m-long fiber is coated with ethylene propylene copolymer in order to enrich the analytes within the penetration depth of the evanescent field protruding from the fiber sensor head. A mixture of tetrachloroethylene, dichlorobenzene, diethyl phthalate, and xylene isomers at concentrations in the low ppm region was investigated qualitatively and quantitatively in an artificial aquifer system filled with Munich gravel. This simulated real-world site at a pilot scale enables in situ studies of the sensor response and spreading of the pollutants injected into the system with controlled groundwater flow. The sensor head was immersed into a monitoring well of the aquifer system at a distance of 1 m downstream of the sample inlet and at a depth of 30 cm. Within one hour, the analytes were clearly identified in the fingerprint region of the IR spectrum (1300 to 700 cm(-1)). The results have been validated by head-space gas chromatography, using samples collected during the field measurement. Five out of six analytes could be discriminated simultaneously; for two of the analytes the quantitative results are in agreement with the reference analysis.

Direct monitoring of soil and water nitrate by FTIR based FEWS or membrane systems.

The development of silver halide fibers that transmit with minimal loss into the mid-IR (MIR) paved the way to their successful utilization as effective ATR (Attenuated Total Reflectance) elements, promoting the implementation of Fiberoptic Evanescent Wave Spectroscopy (FEWS) for direct monitoring of nitrate in environmental systems. Samples containing nitrate in water, soil extracts, and pastes were used for the determination of nitrate concentration with a common ATR ZnSe crystal and with silver halide fibers (FEWS). Spectra of soil pastes and suspensions and those of phosphate, carbonate, sulfate, ammonium, and soil organic constituents were collected to study possible interference with nitrate determination. The standard error of estimate (SEE) and R2 values obtained with flat fibers, using the simple single-point correlation method, were superior to those obtained for cylindrical FEWS and ZnSe ATR crystals in pure water. A significant improvement in the SEE and R2 was achieved in most soil pastes by applying the simple mode of the Cross Correlation method. Direct transmission of MIR radiation through ion-exchange membranes, partially loaded with nitrate or carbonate, was found an effective alternative for MIR-FTIR determination of these ions. Further development and modification of the FEWS devices should allow in-situ and online determination of nitrate in soil and environmental systems.

Online sensing of volatile organic compounds in groundwater using mid-infrared fibre optic evanescent wave spectroscopy: a pilot scale test.

A prototype sensing system for in-situ monitoring of volatile organic compounds in contaminated groundwater was tested at a pilot scale plant. The sensor consists of a commercially available Fourier transform infrared spectrometer, connected to a 6 m long infrared transparent silver halide fibre optic cable. A 10 cm long core-only section at the centre of the fibre is mounted on a sensor head and coated with a hydrophobic polymer layer, while the remaining fibre is protected by Teflon tubing and thus not in contact with the surrounding media. The sensor head was immersed into the monitoring wells of the pilot plant testing the sensor system under circumstances close to field conditions and typical for in-situ measurements. The pilot plant consists of a 1 m3 cubic tank filled with gravel. A pump is used to circulate water horizontally through the tank, simulating a natural aquifer. The evolution of the concentration of analytes injected into the system is monitored with time using the developed prototype sensing system. The results are validated by corresponding sampling and analysis with headspace gas chromatography.

Laser soldering of rat skin, using fiberoptic temperature controlled system.

BACKGROUND AND OBJECTIVE: Laser soldering of tissues is based on the application of a biological solder on the approximated edges of a cut. Our goal was to use laser soldering for sealing cuts in skin under temperature feedback control and compare the results with ones obtained using standard sutures. STUDY DESIGN/MATERIALS AND METHODS: Albumin solder was applied onto the approximated edges of cuts created in rat skin. A fiberoptic system was used to deliver the radiation of a CO(2) laser, to heat a spot near the cut edges, and to control the temperature. Laser soldering was carried out, spot by spot, where the temperature at each spot was kept at 65-70 degrees C for 10 sec. RESULTS: The tensile strength of laser-soldered cuts was measured after 3-28 days postoperatively and was found comparable to that of sutured cuts. Histopathological studies showed no thermal damage and less inflammatory reaction than that caused by standard sutures (P = 0.04). CONCLUSIONS: Temperature controlled laser soldering of cuts in rat skin gave strong bonding. The cosmetic and histological results were very good, in comparison to those of standard sutures.

Carbon dioxide laser and silver halide infrared transmitting fibers for tympanoplasty: an experimental animal model.

OBJECTIVE: This study evaluates the effectiveness and safety of fiberoptic carbon dioxide (CO2) laser welding for graft closure of tympanic membrane perforations in an animal model. STUDY DESIGN AND SETTING: Tympanic membrane perforation was surgically induced in 11 eardrums of 7 given pigs. A lumbar facial graft was placed over the wound, and albumin drops served as a biologic solder. CO2 laser energy, transmitted through silver halide infrared transmitting fibers, was used for "spot-welding" along the circumference of the graft. The welded sites were evaluated by using a surgical microscope as well as by evaluating the sites histologically. RESULTS: Healing started 3 to 4 days after surgery and was completed within 3 weeks with the formation of a neotympanum. Some inflammation with granulation tissue was noted in 5 eardrums. CONCLUSIONS AND SIGNIFICANCE: These preliminary results indicate that CO2 laser tympanoplasty with a fiberoptic delivery system may be a promising new technique for the clinical setting.

Monitoring of the infrared radiation emitted from skin layers during CO2 laser resurfacing: a possible basis for a depth navigation device.

CO2 laser resurfacing is very accurate, but it is not free of complications such as scarring. The objectives of the present study were to evaluate the pattern of the infrared (IR) radiation emitted from skin layers and to use this pattern to distinguish between these layers during resurfacing. A CO2-resurfacing laser (Sharplan SilkTouch) was used for the de-epithelialisation of skin. A silver halide optical fibre delivered the radiation emitted from the skin during resurfacing to an IR photonic detector. Time-dependent curves of the signals emitted from the skin layers were statistically evaluated and showed significant differences between the epidermis and the dermal layers. Similar results were obtained during in-vivo and ex-vivo measurements. The difference between the skin layers emission may be used for depth navigation during laser resurfacing.

Fiberoptic infrared radiometer for real time in situ thermometry inside an MRI system.

A warm body emits radiation whose intensity I is dependent on the surface temperature T. For T near room temperature this radiation is mostly in the mid-IR. Radiometric measurement of I is often used for non contact thermometry. Temperature measurements in situ and in remote locations can be carried out using optical fibers, but one needs fibers that are highly transparent in the mid-IR. Our group has developed fibers made of silver halides. These are flexible, nontoxic, not hygroscopic and highly transparent in the mid IR. These fibers served for the development of a novel fiberoptic radiometer. Using this radiometer we have measured the temperatures of samples while being imaged in an MRI system. The presence of the fibers inside the MRI system did not interfere with the operation of the MRI nor did not the MRI system affect the radiometer. The temperature measurements were made with accuracy of 0.3 degrees C, response time of 1 s and spatial resolution of 1 mm. Fiberoptic radiometers would be highly suitable for temperature measurement of human tissues, for example during surgical procedures done inside an MRI system.

Thermal-feedback-controlled coagulation of egg white by the CO2 laser.

Temperature feedback control during laser-assisted tissue coagulation was investigated and demonstrated with the egg-white model. We observed the dynamics of photothermal denaturation during CO2 laser irradiation by simultaneously controlling surface temperature and monitoring He-Ne laser transmission of egg-white samples. Once a quasi-constant surface temperature was established, transmission of egg white tended to decrease linearly with time. Analysis of experimental data strongly suggested a first-order rate process. Since transmission was primarily affected by heat-induced increase in the scattering coefficient and depth of coagulation, we speculated that changes in transmission were reliable indicators of accumulating photothermal damage. Our experiments demonstrated that thermal feedback can effectively control or limit photothermal damage.

Theoretical evaluation of a four-band fiber-optic radiometer.

A theoretical simulation of a four-band fiber-optic radiometric technique is presented. This is a technique for remote, noncontact temperature measurement of a sample near room temperature, under conditions of unknown emissivity and ambient temperature. A realistic setup of a broadband IR detector, a set of three filters, an IR fiber, and a MATLAB software package for the calculations, is simulated in two steps: a calibration process and a measurement process. The results of the simulation show the limitations and advantages of the four-band radiometric technique and show the expected resolution of the sample temperature and emissivity and of the ambient temperature measurement. The theoretical resolution of the sample temperature measured by the four-band radiometric setup comes close to the resolution achieved in an equivalent single-band radiometric setup. The four-band method has an additional advantage of making it possible to calculate values of emissivity and ambient temperature.

Graded-index mid-infrared planar optical waveguides made from silver halides.

Since the index of refraction of AgCl(x)Br(1-x) (x<1) is higher than that of AgCl, by diffusing Br(-) ions into AgCl it was possible to control the index and thus obtain planar waveguides made from silver chlorobromide (AgClBr) on a AgCl substrate. Silver halides are transparent in the mid IR, and it was therefore possible to characterize the waveguides by transmission of 10.6-mum CO(2)-laser radiation through them. In a typical case, the thickness was optically measured and was found to be 65mum , and the propagation loss was 16 dB/cm. The output-beam profile distribution was determined experimentally and found to be well correlated with a numerical analysis simulation based on a ray-tracing model of the eikonal equation. Planar waveguides that are transparent in the mid IR will likely be useful in numerous applications.

Temperature controlled CO(2) laser welding of soft tissues: urinary bladder welding in different animal models (rats, rabbits, and cats).

BACKGROUND AND OBJECTIVE: Laser welding of tissues is a method of closure of surgical incisions that, in principle, may have advantages over conventional closure methods. It is a noncontact technique that introduces no foreign body, the closure is continuous and watertight, and the procedure is faster and requires less skill to master. However, in practice, there have been difficulties in obtaining strong and reliable welding. We assumed that the quality of the weld depends on the ability to monitor and control the surface temperature of the welded zone during the procedure. Our objective was to develop a "smart" fiberoptic laser system for controlled temperature welding. STUDY DESIGN/MATERIALS AND METHODS: We have developed a welding system based on a CO(2) laser and on infrared transmitting AgClBr fibers. This fiberoptic system plays a double role: transmitting laser power for tissue heating and noncontact (radiometric) temperature monitoring and control. The "true" temperature of the heated tissue was determined by using an improved calibration method. We carried out long-studies of CO(2) laser welding of urinary bladders in various animal models. Cystotomies were performed on the animals, and complete closure of the bladder was obtained with a surface temperature of 55 +/- 5 degrees C at the welding site. RESULTS: In early experiments on 31 rats, the success rate was 73%. In later experiments with 10 rabbits and 3 cats, there was an 80% and a 100% success rate, respectively. CONCLUSION: The success rate in these preliminary experiments and the quality of the weld, as determined histologically, demonstrate that temperature controlled CO(2) laser welding can produce effective welding of tissues. The fiberoptic system can be adapted for endoscopic laser welding.

Ordered bundles of infrared-transmitting AgClBr fibers: optical characterization of individual fibers.

Silver halide (AgCl(x) Br(1-x)) crystals were extruded to form polycrystalline fibers that are highly transparent in the spectral range 3-30 mum. Ordered bundles consisting of as many as 9000 fibers were fabricated by multiple extrusion steps. The transmission loss of an individual fiber in the 100-fiber bundles was 0.12 dB/cm, and the cross talk between neighboring fibers in the 900-fiber bundles was 25%. Thermal images of bodies at room temperature have been transmitted through the bundles. Such ordered bundles provide a solution for the problem of thermal imaging in regions where there is no line of sight between a thermal camera and a warm object.