Methods of fabricating flexible guides for infra-red energy

Abstract: A method of manufacturing apparatus for guiding infra-red light, comprising providing a tube of material capable of guiding infra-red light, enveloping the tube in a flexible sleeve so that the sleeve fits tightly around the tube, thereafter severing the tube at spaced intervals along its length without severing the sleeve, and aligning a severed length of the tube at such an angle to an adjacent length as to allow passage of a beam of infra-red light from one to the other.

Description: This invention relates to methods of fabricating flexible guides for infra-red energy. Infra-red radiation consists of electromagnetic radiation in a band of wavelengths extending from about 750 nanometres to 1 millimetre. Of particular interest is infra-red radiation with a wavelength of about 10.6 micrometres since such radiation can readily be produced by a carbon dioxide laser. Moreover, a typical carbon dioxide laser can generate 10.6 micrometre infra-red radiation at power levels of tens of watts on a continuous basis. A number of applications exist for such levels of continuous infra-red energy, provided the energy can be efficiently concentrated and directed. One such application of particular interest is in the field of medicine, where a concentrated and directed beam of infra-red energy can be used as a cutting instrument in place of a conventional scalpel. If the beam of infra-red energy can be reliably directed through an endoscope, internal surgery becomes possible in circumstances where a conventional scalpel could not be employed. The bulk and weight of a carbon dioxide laser for powering an infra-red "scalpel" are such that the laser must remain outside the body being operated upon, and therefore some form of infra-red energy guide is necessary to link the laser output to the location of surgical operations within the body. Fibre optic endoscopes are known for visual inspections of the interior of the human body, and require a length which may be in excess of 20 centimetres, with an approximately circular cross-section of extremely limited dimensions, usually only a few millimetres. However, equivalent fibre optic endoscopes for use at infra-red wavelengths are not available owing to the lack of optically efficient materials. Certain materials which are optically suitable, such as zinc selenide, are toxic and therefore excluded from medical applications. Proposals have been made for transmission of infra-red energy down a succession of straight hollow tubes linked by pivot systems, with each pivot system containing one or more mirrors to deflect an infra-red beam from one tube into the next, the transmission system possibly also containing one or more focussing lenses. Apart from difficulties of achieving correct alignment, optical inefficiences (particularly due to the need for multiple mirrors) generally result in the unacceptably high loss along the transmission system of about half of the initial infra-red energy. Also, no such system is available which matches the dimensional limitations or flexibility requirements of an endoscope. United States Patent No. 4,068,920 describes a flexible hollow rectangular waveguide for transmission of infra-red radiation. Although the waveguide of US Patent No. 4,068,920 is a relatively efficient transmitter of infra-red energy, its extreme width-to-thickness ratio clearly renders it impracticable for use within the circular body of an endoscope - see column 4, lines 20-47. Preferred dimensions and forms of construction also render this rectangular waveguide unsuitable for in-body surgery from the considerations of maintaining scrupulous cleanliness and an ability to be thoroughly sterilised. A very brief and undetailed reference at the end of column 2 to use for surgery therefore inevitably refers to external surgery (i.e. cutting the body open from the outside with exactly the same techniques as used with a conventional manual scalpel), since there is no teaching of internal use, nor any suggestion as to how the waveguide could be adapted to endosocopic surgery. A circular infra-red guide of suitable dimensions could be fitted inside an endoscope sheath, but lines 31-36 in column 1 of US Patent No. 4,068,920 clearly state that such circular waveguides are not flexible and cannot be used in a flexible application because of the large bending losses (of infra-red energy). The prior art thus indicated that flexible infra-red guides suitable for use in endoscopes were not known and not considered to be possible. According to the present invention there is provided a method of manufacturing apparatus for guiding infra-red light, comprising providing a tube of material capable of guiding infra-red light, enveloping the tube in a flexible sleeve so that the sleeve fits tightly around the tube, thereafter severing the tube at spaced intervals along its length without severing the sleeve, and aligning a severed length of the tube at such an angle to an adjacent length as to allow passage of a beam of infra-red light from one to the other. This application discloses furthermore an infra-red energy guide having a relatively long overall length, said guide comprising a plurality of infra-red energy guide sections which are individually short relative to said overall length, each said guide section being substantially rigid and having an energy-guiding internal section, guide section mounting means mounting said guide sections in a concatenation with each guide section aligned to project infra-red energy into the succeeding guide section in the concatenation, said guide section mounting means comprising a relatively flexible hollow sheath tightly encasing said guide sections, the flexibilty of said sheath allowing an angular deviation between adjacent guide sections in said concatenation, said angular deviation not exceeding a maximum at which the infra-red energy guide is transmissive of infra-red energy along its overall length, the infra-red energy guide having an outer tube assembly external of said sheaths, said outer tube assembly comprising a plurality of first lengths of rigid tubing in end-to-end relationships, each pair of adjacent first lengths having adjacent ends which are externally overlapped by a respective second length of rigid tubing, said second length having inner surfaces with an internal diameter slightly greater than the external dimensions of said first lengths, the lengths and internal diameter of each said second lengh of rigid tubing permitting adjacent guide sections to undergo a variable mutual angular deviation that can increase up to said maximum deviation until said first lengths of rigid tubing jam against the inner surface of the overlapping second length of rigid tubing to prevent a further increase in said mutual angular deviation beyond said maximum at which said infra-red energy guide is transmissive of infra-red energy along its overall length. This application discloses also an infra-red energy guide having a relatively long overall length, said guide comprising a plurality of infra-red energy guide sections which are individually short relative to said overall length, each said guide section being substantially rigid and having an energy-guiding internal section, guide section mounting means mounting said guide sections in a concatenation with each guide section aligned to project infra-red energy into the succeeding guide section in the concatenation, said guide section mounting means allowing an angular deviation between adjacent guide sections in said concatenation, said angular deviation not exceeding a maximum at which the infra-red energy guide is transmissive of infra-red energy along its overall length, the adjacent ends of each adjacent pair of said infra-red energy guide sections being externally overlapped by a respective length of outer tubing having an inner surface with an internal diameter slightly greater than external dimensions of said guide sections, the length and internal diameter of each said length of outer tubing permitting the respective overlapped guide sections to undergo a variable mutual angular deviation that can increase up to said maximum deviation until said guide sections jam against the inner surface of the respective length of outer tubing to prevent a further increase in said mutual angular deviation beyond said maximum at which the infra-red energy guide is transmissive of infra-red energy along its overall length. The method of this invention can provide an infra-red waveguide of a length considerably greater than its transverse dimensions with high efficiency of infra-red transmission, especially at a wavelength of about 10.6 micrometers and continuous power levels in excess of 10 watts. The infra-red waveguide can be provided with a flexible sheath which permits the waveguide to be bent, and which preferably incorporates bend limiting means to limit localised curvature of the waveguide at any point along its length to extent which prevents damage due to excessive bending. The flexible infra-red waveguide may be of simple construction using economic materials which are preferably easily cleaned and sterilised to enable safe use in a medical environment and enabling such a waveguide to be constructed with transverse dimensions which are sufficiently small as to enable the waveguide to be employed as a flexible endoscope while efficiently transmitting infra-red power of sufficient intensity for in-body surgery. Preferably the surgical endoscope is be hollow from end to end and free of internal obstructions (such as lenses) so as to enable gas or liquid to be forced down the waveguide to keep the interior of the waveguide free of body fluids, tissue, and debris. The invention derives from the surprising discovery that a collimated beam of infra-red radiation can be propagated along and out of the end of a length of infra-red waveguide, and into an adjacent further length of infra-red waveguide to be propagated therealong without substantial loss of energy, even if the adjacent waveguides are imperfectly aligned. Although the longitudinal axes of adjacent waveguides are preferably but not essentially arranged to intersect, it has been found that these longitudinal axes can have a small but distinct mutual angular divergence without significant loss of transmitted infra-red energy. Alternatively or additionally, the adjacent ends of the adjacent lengths of waveguide can be longitudinally separated without significant loss of transmitted infra-red energy. The energy-transmitting interiors of the waveguides preferably have comparable dimensions, and more preferably substantially identical dimensions; these waveguides may be circular with constant-diameter cylindrical interiors. If a plurality of such lengths of rigid waveguide are arranged end-to-end with a small angular divergence between each adjacent pair of waveguide lengths, a substantial angle can be built up between the longitudinal axes of the first and last lengths of waveguide such that the direction of transmission of the infra-red beam can be considerably altered without the energy losses associated with beam direction changes achieved by conventional mirror systems. If a large number of short sections of infra-red wave-guide are employed (together with a suitable means for keeping adjacent section ends in mutual lateral and longitudinal alignment while permitting variable mutual angular divergences), such an arrangement effectively provides a flexible infra-red waveguide with a selectively variable curvature. The preferred material for the waveguide lengths or sections is a ceramic tube which preferably has a smooth bore which is substantially totally internally reflecting to light internally incident at shallow angles. The preferred ceramic is alumina (aluminium oxide). The adjacent sections of waveguide are preferably held in mutual alignment by being encased in a continuous flexible sheath, which may be formed of a plasticised polymer or of a natural or synthetic rubber; a flexible length of waveguide assembly of this kind can be employed as a flexible endoscope with the additional advantage that since its bore is unobstructed by lenses, mirrors, or other objects, the endoscope can convey gas or liquid as well as infra-red energy. It is a further facet of the invention that an effectively flexible infra-red waveguide can be formed of large number of relatively short and mutually aligned waveguide sections by longitudinally dividing an initially relatively long and straight length of waveguide in such a way as to retain mutual alignment of the resultant short waveguide sections. In this specification and the accompanying Claims, the term "severing" is intended to signify the separation of an integral body into discrete and distinct parts. Embodiments of the present invention will now be described, by way of example, with reference to the accompaying drawings, in which: Fig. 1 schematically illustrates the generation, focussing, and launching of a beam of infra-red energy into an infra-red waveguide; Fig. 2 illustrates the transfer of a beam of infra-red energy between two lengths of infra-red waveguide which are not coaxial; Figs. 3 and 4 illustrate two successive stages in the fabrication of a flexible endoscope incorporating an infra-red waveguide; Fig. 5 illustrates part of the flexible endoscope produced by the fabrication process of Figs.3 and 4; Fig. 6 illustrates a modified form of the flexible endoscope of Fig. 5; and Fig. 7 illustrates another modified form of the flexible endoscope of Fig. 5. Referring first to Fig. 1, a carbon dioxide laser 10 produces a collimated infra-red beam 12 with a wavelength of about 10.6 micrometres. The beam 12 has a nominal diameter of approximately 5 millimetres at the ¹/e² intensity points (where "e" is the natural logarithmic base = 2.718 ...), on the basis that the beam intensity follows a Gaussian distribution measured radially outwards from the central axis of the beam. The beam 12 is focussed by lenses 14 and 16 in succession. The lens 14 has a focal length of 150 millimetres and the lens 16 has a focal length of 37.5 millimetres. The lenses 14 and 16 transform the waist of the laser output beam 12 into a narrower beam waist at the entry end of a tube 18. The tube 18 is a rigid straight cylindrical tube formed of alumina (aluminium oxide) and has a smooth bore with a diameter of 2 millimetres. The ¹/e² intensity points of the narrower waist at the entry end of the tube 18 are ideally separated by 0.64 times the tube bore diameter (of 2 millimetres) since this gives the tube 18 the best beam guiding properties with minimal loss of infra-red energy from the beam entering the tube 18. Since the wavefront curvature is zero at a beam waist, the tube 18 acts as a circular waveguide to the Gaussian collimated infra-red beam entering the tube 18. The tube 18 will also continue to function as an efficient circular waveguide for transmission of infra-red energy even if the above optical matching conditions are imperfectly satisfied, since small deviations of beam direction and some wavefront curvature can be accommodated. The grazing incidence of rays as these are deflected by the inner wall of the tube 18 still provide exceptional efficiency of infra-red energy transmission. When infra-red energy is being transmitted in the pure guided mode, the energy profile across the bore of the tube remains substantially invariant and approximately Gaussian. Thus the infra-red beam 20 emerging from the output end of the tube 18 may be considered to have a waist at the point of emergence. Near a beam waist, the cross section of a beam changes very slowly in accordance with a function of beam diameter at the waist, radiation wavelength, and distance along the beam from the waist. This property of beam propagation is beneficially employed in the present invention as detailed below to produce previously unachievable results. Figure 2 illustrates how infra-red energy can be propagated from one circular waveguide to another circular waveguide with minimal energy losses, despite a longitudinal separation and angular deviation between the waveguides. The tube or circular waveguide 18 emits the beam 20 whose bounds of ¹/e² intensity are denoted by the two parallel lines 22. Since the bore of the tube 18 is 2 millimetres, the separation of the lines 22 and hence the effective diameter of the beam 20 is 1.28 millimetres (0.64 times bore diameter, as previously explained). A second alumina tube 24 is arranged with its inlet end at a longitudinal separation "Z" from the outlet end of the first tube 18, and laterally aligned therewith (i.e. an extension of the longitudinal axis of the tube 18 intersects the longitudinal axis of the tube 24 at the inlet end of the tube 24). However, the tube 24 has its longitudinal axis angularly deviated from the longitudinal axis of the tube 18 by a small angle "A". The diameter of the bore of the tube 24 is 2 millimetres so as exactly to match the bore of the tube 18, the longitudinal separation "Z" = 5 millimetres, and the angular deviation "A" is 1 or 2 degrees. At a wavelength of 10.6 micrometres, an initial beam diameter of 1.28 millimetres, and a waveguide separation "Z" of 5 millimetres, it can be shown that the beam 20 increases its diameter in traversing the gap "Z" by a mere 0.08 per cent (only 8 parts in ten thousand), which is negligible for practical purposes. Thus virtually all the infra-red energy in the beam 20 leaving the first tube 18 is coupled into the second tube 24, and hence the direction of the infra-red beam can be altered without incurring any significant loss of energy. It is of particular significance that an energy-efficient change of beam direction between two rigid waveguides is thus accomplished without the use of any form of lens or mirror. For waveguide bore diameters in the range 0.4-2.0 millimetres, acceptable energy transmission efficiencies are possible with deflection angles "A" of up to 1 degree. If utilising a number of abutting waveguide sections which are each of short length, the deviation angle can be maximised and the section length minimised by adopting the following optimised dimensions:- (1) For a waveguide bore of 1.0 millimetre, the waveguide section length should be 5 millimetres to enable deflection of 1 degree; (2) For a waveguide bore of 0.8 millimetres, the waveguide section length should be 4 millimetres to enable deflection of 1.25 degrees. Although efficiency considerations limit the maximum value of the deviation angle "A" to a few degrees at most, the beam deflection shown in Fig. 2 can be repeated as often as is necessary to accumulate a required total angular deflection, as will be detailed below. The concatenation of lengths or sections of rigid waveguide, each angularly deviated from its neighbour, can be a chain of relatively long lengths or of relatively short sections. An advantage to be gained from using a relatively large number of short sections (as detailed in Fig. 5 below) is that a high degree of curvature can be provided without requiring an excessive overall length, and the shorter the individual rigid waveguide sections, the more nearly uniform is the achievable curvature. Also, curvature is not limited to a single plane or to a single radius. This can provide a high degree of flexibility despite the fact that the waveguide sections are individually rigid. For example, by employing alumina tubes of 1.0 millimetre bore, fewer than fifty sections are needed to give a 90 degree bend whilst transmitting in excess of 90 percent of the energy launched into the flexible guide assembly, each section being misaligned with its neighbour by no more than two degrees. The use of alumina tubes as infra-red waveguides at a wavelength of 10.6 micrometres has a significant advantage in that alumina tubes with air-filled bores give a true fibre propagation mode. This arises from the refractive index of alumina being less than 1 at a wavelength of 10.6 micrometres whereas the refractive index of the air "core" of the alumina tube is nominally 1 and hence higher than that of the alumina "cladding". A preferred method of fabricating the flexible infra-red guide of the present invention will now be described with reference to Figs. 3, 4 and 5. Referring first to Fig. 3, an alumina tube 30 of suitable overall length (for example, 20-40 centimetres) and a selected diameter (which may be in the range 0.2-2.0 millimetres) is provided with circumferential lines of weakness 32 at regular intervals along the length of the tube 30. The spacings of the lines 32 is preferably chosen to optimise infra-red energy transmission efficiency in dependence on the selected bore diameter. The circumferential lines of weakness 32 are formed by externally scoring or scribing the tube 30 by abrasion with a diamond-loaded wire or by application of a hardened cutting tool. At this stage of fabrication, the tube 30 is not penetrated by the lines 32, but the lines 32 are each of sufficient depth that subsequent bending of the tube 30 (as detailed below) will fracture the tube 30 at each of the lines 32 so as to divide the initially one-piece tube 30 into a large number of short sections. Referring now to Fig. 4, the next stage of fabrication is to encase the selectively weakened but still unbroken tube 30 within a flexible polymeric sleeve 40. The sleeve 40 is preferably a radiation-cross-linked polyolefin whose internal diameter is initially sufficiently greater than the external diameter of the alumina tube 30 that the tube 30 can easily be slid inside the sleeve 40 so that the sleeve 40 completely overlies the tube 30 from end to end. Next, the sleeve 40 is heated so as to cause it to shrink and reduce its cross-sectional dimensions until the sleeve 40 is firmly and tightly clamped around the tube 30, as shown in Fig. 4. To convert the rigid intermediate-stage assembly of Fig. 4 to the flexible final-stage assembly of Fig. 5, the Fig. 4 assembly is subjected to carefully applied bending force, either simultaneously along the whole of its length or in short localised areas progressing along its length. The bending force can be applied manually or by machinery. This application of bending force results in the tube 30 fracturing at each of the circumferential lines of weakness 32, either all more or less simultaneously, or in progressive succession along the length of the tube 30 according to the nature of the applied bending forces. The end result is the flexible waveguide structure shown in Fig. 5, in which the initially one-piece alumina tube 30 is converted to a large number of short sections 50 but without cracking the sleeve 40 or otherwise diminishing the structural integrity of the sleeve 40. The flexible sleeve 40 retains the alumina tube sections 50 in close longitudinal proximity and mutual lateral alignment, but allows a limited angular deflection between each adjacent section 50. These angular deflections can be cumulative as shown in Fig. 5 to allow a large degree of curvature, or the angular deflections can be in different planes and/or at different angles (to give different effective radii of curvature) at different portions along the overall length of the waveguide assembly. The latter facility allows the flexible waveguide to be threaded through variously curving and tortuous channels such as passages through the body of a patient undergoing investigative or surgical procedures. By way of example, the flexible waveguide structure of Fig. 5 can be formed as a 2 millimetre pediatric cystoscope which, despite its extremely limited cross-sectional dimensions, can efficiently transmit sufficient infra-red energy from a carbon dioxide laser as to perform in-body surgery without the need for an incision to gain access to the bladder since the cystoscope can be inserted through the urethra. As an alternative to the above-described fabrication procedure of Figs. 3, 4 and 5, a flexible waveguide assembly could be constructed by forming the requisite number of short individual waveguide sections prior to sheathing, aligning the sections, and then enclosing them in the flexible sheath. However, this alternative procedure is considered to involve greater difficulties in component manipulation, and a reduced accuracy of inter-sectional alignments, as well as an increased possibility bore dimension variations between sections. However, these factors need not preclude adoption of the alternative fabrication procedure in appropriate circumstances. Referring again to Fig. 5, it will be noted that only the polymer sheath 40 limits bending of the flexible waveguide assembly at the junctions between adjacent waveguide sections 50. Thus, excessive localised bending stress or fatigue cracking of the sheath 40 in use thereof may result in mechanical destruction of the waveguide assembly. Fig. 6 illustrates a simple arrangement by which excessive bending of the waveguide assembly is automatically limited in a manner which does not depend upon the bending resistance of the polymer sheath. In the Fig. 6 arrangement, the concatenation of short sections of tubular alumina waveguide 60 have each of the gaps 62 between adjacent waveguide lengths 60 encircled by a short length of large diameter metal or ceramic tube 64. The inside diameter of the tubes 64 is slightly greater than the outside diameter of the tubes 60 so that the outer tubes 64 fit loosely over the inner tubes 60. (In Fig. 6, relative dimensions are not to scale and some dimensions are exaggerated so as more clearly to show the principles involved in the Fig. 6 arrangement). This loose fit of the overlying tubes 64 allows the inner tubes 60 to undergo a variable mutual angular displacement which is ultimately limited by the inner and outer tubes 60 and 64 tilting into mutual contact, at which point their jamming together prevents further increase in angular displacement. The assembly of inner waveguide tubes 60 and outer bend-limiting tubes 64 is shrouded with a flexible shrunk-on polymer sheath 66 in the same manner as the sheath 40 is applied in the Fig. 5 arrangement. The sheath 66 holds the tubular components 60 and 64 in the requisite longitudinal and lateral alignments, and also provides a fluid-tight enclosure for the infra-red beam passage through the concatenated bores of the alumina waveguide tube sections 60. In Fig. 7 the arrangement is similar in concept to that of Fig. 6, but in order to protect the alumina lengths 60 from stress during flexing of the assembly the flex limitation is provided by overlapping inner and outer metal tubes 68, 70. The waveguide 60 is in the form shown in Fig. 5 with the alumina lengths enveloped by the polymer sheath 40, and the waveguide 60 is inserted by sliding it within the assembly of inner and outer tubes 68, 70 which is held together by the sheath 66 as described with reference to Fig. 6 Although of particular use for endoscopic surgery, the invention is not limited to such applications and may be applied to any other field requiring the guidance of infra-red energy, such as for scribing, cutting, or welding of metal or other materials, or any other application requiring concentrated application of heat along a guided path to a point away from the output of the infra-red energy source. While various alternative forms of the invention have been described above, other modifications and variations can be made without departing. from the scope of the invention as defined in the appended Claims.

Claims: A method of manufacturing apparatus for guiding infra-red light, comprising providing an infra-red waveguide in the form of a tube (30) of material capable of guiding infra-red light, enveloping the tube (30) in a flexible sheath (40) so that the sheath (40) fits tightly around the tube (30), thereafter severing the tube (30) at spaced intervals along its length producing a plurality of tube sections (50) without severing the sheath (40), said tube sections (50) being arranged in said sheath (40) to be able to undergo an angular deviation between adjacent tube sections (50). A method as claimed in Claim 1, wherein the method includes selectively weakening said tube along circumferential lines (32) at longitudinal intervals along the tube (30) before enveloping said selectively weakened tube (30) in the sheath. A method as claimed in Claim 2, wherein said tube (30) is a hollow tube (30) composed of alumina and having a uniform cylindrical bore, said alumina tube is selectively weakened along circumferential lines (32) by circumferentially scribing said tube (30) to a depth which is insufficient to penetrate to said bore, and said tube (30) is tightly encased in said sheath (50) by shrink-fitting said sheath (50) around said tube (30) following the step of selectively weakening the tube (30) but prior to the step of breaking of said tube (30). A method as claimed in Claim 2 or 3, wherein said tube (30) is broken at said circumferential lines (32) by application of bending forces to the enveloped tube (30). A method as claimed in any one of the preceding Claims, including providing a plurality of lengths of outer tubing (64) each having an inner surface with an internal diameter slightly greater than the external dimensions of said tube (60), mounting said lengths of outer tubing (64) over the tube (60) at the sites of the tube (60) to be severed, and tightly encasing the resulting assembly in the sheath (66). A method as claimed in any one of Claims 1 to 4, including the additional steps of providing a plurality of first lengths of rigid tubing (68) in end-to-end relationship around the flexible sheath (40), providing a plurality of second lengths of rigid tubing (70) and aligning each of said second lengths of rigid tubing (70) to externally overlap a respective pair of adjacent ends of adjacent first lengths of rigid tubing (68) to form an outer tube assembly for the infra-red waveguide. A method as claimed in Claim 6, including the further steps of providing a relatively flexible hollow sheath (66) initially in a form having internal dimensions greater than the external dimensions of said second lengths of rigid tubing (70), locating the sheath (66) around the assembly of first lengths (68) and second lengths (70) of rigid tubing and shrink-fitting said sheath (66) around said assembly of first lengths (68) and second lengths (70).

Inventors: COLLES MICHAEL JOHN, SMITH GEORGE RAYMOND, COLLES, MICHAEL JOHN, SMITH, GEORGE RAYMOND
Applicants: UNIV HERIOT WATT MED LASER UNI, MEDICAL LASER UNIT OF HERIOT-WATT UNIVERSITY
Application year: 1989
Main classification: G02B6
IPC codes: G02B6, G02B23