For almost as long as CO.sub.2 lasers have been viable tools for medical applications, the search has been on for improved modes of guiding the laser beam to the desired operating area. For the most part, lasers have been coupled with multi-section articulated arms having any number of large bore tubular sections hinged together with a reflective surface at each hinge to permit the laser light to traverse the length of the arm and to be aimed toward the desired site.
While such articulated arm laser systems have experienced wide spread acceptance lately in a variety of medical specialities, they are generally somewhat clumsy to use since the arm typically offers some "resistance" to movement by the surgeon. Such arms are inherently limited in the scope of their medical applications, because of their size and limited flexibility. Present CO.sub.2 surgical applications are essentially limited to those in which there is direct access to the area to be treated. CO.sub.2 endoscope procedures are still rare, as the present technology requires a relatively wide, but short and straight endoscopic channel to "shoot" the CO.sub.2 beam down. In addition, most articulated arms experience problems with beam alignment particularly if the surgical application calls for a small spot size. These arms also tend to be expensive, especially if precision optical alignment is required.
It is an object of the present invention to provide a small diameter, flexible fiber for carrying CO.sub.2 laser emissions, which can be threaded down a longer, narrow or flexible endoscope, or alternatively be used as a second puncture probe.
A variety of optical fibers have been proposed as the transmission medium for laser energy, but to date, not a single one has become commercially accepted for the 10.6 micron wavelength which is characteristic of CO.sub.2 lasers. Optical fibers or light pipes for the transmission of infrared light at 10.6 microns have however been proposed: in one instance a polycrystalline fiber, such as the KRS-5 fiber developed by Horiba, Ltd. of Japan; and in another, a flexible, hollow waveguide, various versions of which have been suggested by among others E. Garmire and M. Miyagi. See, for instance, M. Miyagi, et al., "Transmission Characteristics of Dielectric-Coated Metallic Waveguide for Infrared Transmission: Slab Waveguide Model", IEEE Journal of Quantum Electronics, Volume QE-19, No. 2, February 1983, and references cited therein. Recently, Miyagi, et al. suggested fabricating a dielectric-coated metallic hollow, flexible waveguide for IR transmission using a circular nickel waveguide with an inner germanium layer applied by rf-sputtering, plating and etching techniques. Miyagi, et al. predict extremely small transmission losses for a straight guide, but in fact, actual transmission degrades substantially with but nominal bending radii (20 cm). To understand this, the mechanism of transmission must be considered.
Transmission of laser light through a flexible, narrow diameter hollow waveguide is subject to losses largely due to successive reflections of the beam along the interior surface of the curved guide. For the size and curvatures contemplated for a medical fiber, rays will intersect the wall at angles of incidence ranging from, typically, 80.degree. to 90.degree.. Bending a hollow fiber increases the loss as it tends to increase the number of internal reflections and decrease the angle of incidence. In general, as the angle of incidence decreases from 90.degree. to 80.degree., the loss per reflection bounce increases. It is an object of the present invention, therefore, to provide a coating which has high reflectivity over angles of incidence ranging from 80.degree. to 90.degree..
A difficulty of curving metal walls is that at these angles of incidence, metals tend to exhibit high reflectivity for only the S polarization but low reflectivity (&lt;96%) for the P polarization. The losses for a 1 meter curved guide are of the order 10 dB. Garmire et al. attempted to avoid this problem by using a metal/dielectric guide in which the guide was oriented relative to the incoming beam such that the metal walls "saw" only the P polarization. This approach is flawed, however, because the dielectric walls show high reflectivity for only very, very high angles of incidence, typically in excess of 89.degree.-requiring, in essence, that the guide must be straight along the direction of the dielectric.
Some have suggested remedying this situation by overcoating a reflecting surface with a quarter-wave dielectric coating. Such a coating will yield high reflectivity for the P polarization, but low for the S polarization. Miyagi et al. attempt to strike a compromise by choosing a coating of thickness somewhere between those favoring the P and and those favoring the S polarization. He chose a germanium coating of approxiamately 0.4 to 5 micrometers in thickness. This coating yielded relatively good results (&gt;90%/meter transmission) for straight guides, but rather poor for bent guides.
This disparity appears to result from two factors: (1) The transmission with the He.sub.11 mode in a straight guide correlates poorly with the transmission of very high multi order modes in a bent guide; and (2) The imaginary part of the refractive index of the dielectric coating is extremely crucial in the transmission of a bent guide.
It is an object of the present invention to provide dielectric overcoated waveguides which are tuned to perform well although bent in compound curvature.