Coherent hollow-core waveguide bundles for thermal imaging

The use of lasers in many industrial or medical applications often requires the delivery of laser radiation from the laser system to the place of an exposure. One of the radiation transfer possibilities is the delivery of its by hollow glass waveguide. The fundamental advantage of this delivery media is the absence of the material through which the laser radiation is transferred. Other hollow waveguides properties are a possibility of delivery radiation in wide range of spectra, lossless delivery, non-toxicity, and low aging effect. This system is possible to use also for radiation which has strong absorption in water and therefore cannot be delivered through conventional glass fiber. Especially Er:YAG radiation which generated wavelength corresponding to maximum absorption in water and therefore it is extremely suitable for medical application. The aim of this study was to investigate radiation transmission for various parameters (repetition frequency, pulse length) of a commercially available diode-pumped Er:YAG laser. The special hollow glass waveguide with a cyclic olefin polymer coated silver layer has a length of 108 cm and an inner diameter of 700 µm. Radiation delivery as a function of laser input power and spatial distribution of output beams were investigated depending on pulse length from 100 µs to 400 µs and repetition frequency 20 Hz or 150 Hz. The transmission was achieved up to 86 % at a maximum amplitude of the output peak power of up to 406 W. The obtained results predetermine the use of this simple compact delivery laser system for further research with subsequent use in medicine.

High energy pulsed infrared lasers have many important medical and surgical applications. The authors have successfully used a hollow glass waveguide to deliver pulsed CO/sub 2/ laser energy with 0.28 MW of peak power and a 140 mJ pulse. The average delivered power has been as high as 10W. The hollow waveguides have low loss, with losses as low as 0.17 dB/m for 1000-/spl mu/m-bore fiber, and good flexibility. Therefore, these waveguides are promising candidates for pulsed CO/sub 2/ laser delivery and applications in medical science.

Hollow glass waveguides with three dielectric layers are fabricated with a chemical vapor deposition technique. The waveguides have an inner three-layer stack of aluminum oxide and titanium oxide, with the thickness optimized for the 3-μm wavelength of a Er:YAG laser. The measured attenuation spectra of the waveguides in the mid-infrared region show interference peaks that are due to the multiple dielectric layers and also exhibit a low-loss region at the design wavelength of 3 μm. The theoretical evaluation of the waveguide loss, including the inner surface roughness of the guide, shows that the roughness strongly affects the transmission losses of the multilayer-coated waveguides.

One of the alternatives for efficient delivery of infrared or powerful laser radiation is a hollow glass waveguide. The aim of the work was to investigate delivery of resonantly pumped 1645 nm Er:YAG laser radiation by this waveguide type. The Er:YAG laser system generating radiation in a free-running regime was in-band pumped by 1535 nm Er:glass laser radiation.

A compact diode pumped Er:YLF laser emitting at wavelength 2825 nm was designed and constructed. Radiation delivery from this source by special hollow glass waveguides with inner diameters of 250, 320, 540, or 700 μm was investigated. The power transmission was up to 91% for around 1 m long waveguide. Measured hollow glass waveguide with this efficient transfer could serve as an appropriate solution of diode pumped mid infrared laser radiation delivery.

The variation in properties of a fluorocarbon polymer (FCP) film during a drying-curing process is investigated for fabricating FCP-coated silver (FCP/Ag) hollow glass waveguides. A dynamic liquid-phase coating procedure is used. Through the analyses of the loss spectra of hollow waveguides made in various conditions, a relationship between the thickness of the FCP film and the coating velocity is obtained. The optimum fabrication condition is also established for producing FCP/Ag hollow glass waveguides for the mid-IR.

Imaging endoscopes have enabled the development of minimally invasive procedures in a wide range of medical applications. Flexible endoscopes have additional advantages over rigid endoscopes. Remarkably, they have an enhanced capability of being guided through the internal conduits of the human body. The development of imaging endoscopes based on coherent optical fiber bundles have made high resolution fiber endoscopy possible. In the last years, multicore fibers have been proposed as an alternative to coherent fiber bundles for high resolution applications. Both types of structures entail several limiting factors. Among them, the most critical one is the optical crosstalk that takes place between the parallel contiguous fibers of the device, which provokes a worsening in the contrast of the images. Therefore it imposes a limit to the quality of the endoscopic system that must be avoided. In this work, we present a theoretical model for the study of optical coupling in multicore fibers, which is based on an electromagnetic optics approach. This model is applied to the analysis of crosstalk within fiber imaging endoscopes. It includes the effect of core non-homogeneities and bendings. The essential equations of the model will be shown. These equations provide us with a theoretical basis that is subsequently applied to fiber endoscopes design. Therefore, we present a robust method for the adjustment of opto-geometrical parameters of the fiber in order to fulfil the quality requirements for a certain application. The key role of core diameter variations in the quality of the image will be specially highlighted.

We report on the design theory and fabrication of silver/polystyrene(PS) coated hollow glass waveguides (HGWs) for THz radiation. We find that Ag/PS coated HGWs have significantly lower attenuation for 119 mum radiation than Ag-only HGWs.

Recent developments in optical endomicroscopy (OEM) and associated fluorescent SmartProbes present a need for sensitive imaging with high detection performance. Inter-core coupling within coherent fiber bundles is a well recognized limitation, affecting the technology's imaging capabilities. Fiber cross coupling has been studied both experimentally and within a theoretical framework (coupled mode theory), providing (i) insights on the factors affecting cross talk, and (ii) recommendations for optimal fiber bundle design. However, due to physical limitations, such as the tradeoff between cross coupling and core density, cross coupling can be suppressed yet not eliminated through optimal fiber design. This study introduces a novel approach for measuring, analyzing and quantifying cross coupling within coherent fiber bundles, in a format that can be integrated into a linear model, which in turn can enable computational compensation of the associated blurring introduced to OEM images.

The CO/sub 2/ laser transmission properties of two hollow glass waveguides of different composition are reported. The results for both straight and bent guide clearly demonstrate the reductions in waveguide loss with a glass of refractive index of less than unity at 10.6 mu m wavelength. >

Using a phase-conjugate window that utilizes four-wave mixing in the photorefractive BaTiO(3) crystal, we demonstrate real-time phase-preserving optical field imaging through a coherent (ordered) fiber bundle.

Despite advances in miniaturization of CMOS sensors, current submillimeter diameter microendoscopic systems rely on coherent fiber bundles to transfer the image from its collection point to the detector. Determining the resolution of an endoscopic optical system that employs a fiber bundle is nontrivial, and there is limited published information about how to calculate the theoretical modulation transfer function (MTF) of a system that may include a distal lens, fiber bundle, relay optics, and a camera sensor. While the fiber bundle is frequently considered the resolution-limiting element, greater information may be gained from computing the full system MTF. In this paper, we describe a resolution cascade method that calculates theoretical MTFs for common components, as well as for the entire microendoscopic system. These theoretical calculations compare favorably to experimentally gathered data for four different optical configurations. The software developed for these calculations is freely available to predict the MTF of other coherent fiber bundle-based systems. Brief instructions and necessary assumptions/conditions are described.

Special hollow glass waveguides were developed for delivery of high power infrared laser radiation. Maximum peak power up to 1.5 GW and mean power up to 5 W were delivered without any breakdown. For the medical application, the sealed version of the hollow glass waveguide was used.

The differences between a contact and noncontact Er:YAG laser hard dental tissue preparation were verified. The influence of laser energy and number of pulses on a profile and depth of a drilled cavity was investigated. The delivery systems used were an articulated arm and a cyclic olefin polymer-coated silver hollow glass waveguide with or without a special sapphire tip. In the case of the noncontact preparation, the laser radiation was directed onto the dental tissue by focusing optics (CaF 2 lens) together with the cooling water spray in order to ensure that the tissues will not be burned. The water spray was also used during the preparation when the waveguide with a sapphire tip was used to deliver the radiation. For the evaluation of shapes, depth and profiles of the prepared cavities the metallographic microscope, photographs from the light microscope and scanning electron microscope were used. From the results it follows that great differences exist in the laser speed, value of energy, the profile, and depth of the cavities prepared by the contact and noncontact preparation. In the case of contact ablation the procedure is quicker, the energy fluence needed is lower and more precise cavities with larger diameters are produced.

The transmission properties of four different waveguides were investigated: (1) dielectric coated silver hollow glass waveguides, with diameters varying from 0.7 mm to 1 mm, (2) polyamide waveguides, with diameters varying from 0.7 mm to 1 mm, (3) a silica glass waveguide, with a 0.7 mm diameter and (4) a plastic waveguide, with a 0.7 mm diameter. Two different lasers were used for the evaluation of these waveguides: (1) a pulsed Er:YAG laser, emitting at 2.96 micrometers and (2) a corona preionized pulsed HF laser, emitting at 2.78 micrometers . Both straight and bent loss characteristics of the waveguides were studied.