WO2002011259A2

WO2002011259A2 - An optical contact and a method for optically connecting elements in the ir

Info

Publication number: WO2002011259A2
Application number: PCT/IL2001/000692
Authority: WO
Inventors: Edward Bormashenko; Roman Pogreb; Avigdor Sheshnev; Abraham Katzir; Yelena Bormashenko
Original assignee: Polytris Ltd.
Priority date: 2000-07-27
Filing date: 2001-07-26
Publication date: 2002-02-07
Also published as: AU2001282429A1; WO2002011259A3

Abstract

An optical contact and a method for optically connecting optical elements (11) in the IR (12) which comprises bleaching of absorbance peaks of a polymeric compound which is included in the optical contact. The bleaching is accomplished by passing a CO2 laser beam (13) having a power that exceeds a threshold of about 0.6W through the optical contact. An optical contact, which further includes a chalcognide glass, transmits a CO2 laser beam (13) having a continuous power of up to 9W at a transmittance of about 85%.

Description

AN OPTICAL CONTACT AND A METHOD FOR OPTICALLY CONNECTING ELEMENTS IN THE IR

BACKGROUND OF THE INVENTION The present invention relates generally to the optical coupling of elements in the infrared (IR), more particularly to the connecting of waveguides in the IR and even more specifically to the diminishing of transmission losses in such connections.

IR optical fibers Find use in the areas of imaging, sensing, laser welding and medical treatment, see e.g., "Infrared Optical Fibers and Their Applications" in Proceedings of the SPIE, Volume 3849, 1999.

In the patent literature; U.S. Patent No. 6,043,842 to Tomasch, et al. describes the use of an IR flexible fiber bundle in a remote surveillance device.

U.S. Patent No. 5,569,923 to Weissman, et al. discloses a fiber optic probe for IR spectroscopy which serve both for illuminating the sample and for collecting light reflected therefrom for spectral analysis.

U.S. Patent No. 5,662,712 to Pathak, et al. describes a use for a near IR optical Fiber to deliver energy in order to heat and mold an implanted polymeric structure in vivo. U.S. Patent No. 4,678,274 to Fuller describes a process for making IR optical fibers with low losses. Such IR optical fibers are now available commercially e.g., from Oxford Electronics

(http//www.oxford-electronics.com).

Frequently, waveguides (either solid core optical fibers or hollow tubes) or other optical elements in the IR have to be optically coupled to each other. E.g., in order to replace a broken segment in a fiber, to extended a fiber length, or to attached a fiber to a working handpiece.

As opposed to the splice of optical silica fibers in the visible, where fibers tips can be melted and soldered to each other, most IR optical Fibers cannot be heated to such an extend as to allow their soldering and the fibers are optically connected by attaching and gluing their respective faces.

In this case, an intermediate adhesive layer of material is introduced between the respective faces of the attached optical elements. It is desired that the index of refraction of this layer shall match as close as possible the refractive index of the attached optical elements and that this layer should not absorb any substantial part of the guided light.

The state of art clad silver halides long IR optical Fiber for the wavelength of 3-15 μm has an attenuation of less then 2 dB/m at wavelength of 10.6 μm (C0₂ laser) wherein power transmission up to 15 CW can be tolerated. In order to comply with the properties of such waveguides, a single optical contact should have a contact transmittance of over 90% at this wavelength range and at these power levels.

Most if not all adhesives now in use for optical connections are based on organic compounds that have characteristic absorbance peaks in IR (see e.g. in: Liang C, Krimm S., Sutherland G., "Infrared Spectra of Polymers ", Journal oF Chem. Phys., 25, 543, 1956 ).

These absorbance peaks in middle IR, which are inherent for organic molecules, reduce the optical transmission at the optical contact, thus the elimination of these absorbance peaks is desired.

The process of attenuation of absorbance peaks in polymers under thermal exposure was studied by many investigators. Disappearance of absorption at 2012 cm^"1, which is inherent for ketenimine groups on heating polymethacrylonitrile at 90°C in cyclohexanone solution, was studied in classical works of Grassie and McNeill (Grassie N., McNeill, J. Polymer Science, 33, 171, 1958 and Grassie N., McNeill, J. Polymer Science, 39, 211, 1959 ).

Grassie and McNeill studied different polymers and discovered the effect of disappearing of absorption peaks under heating procedure. They related the effect to the decomposition of polymer and changes in its chemical structure.

The process of peak decay in the IR is attended with changes in the polymer's color in the visible (e.g. from colorless for undegraded polymethacrylonitrile to orange-red for the same material heated during 9 hours at 100°C), so the effect is also known as polymer "coloration".

The absorbance changes due to degradation of polyethylene and epoxy resin films have been studied previously.

Fourier transform infrared (FTIR) and Raman spectroscopy were effectively used in the study of polyethylene degradation by Sammon, et al. (Sammon C, Yarwood J., Everall N., "A FTIR Study of the Effect ofHydrolytic . Degradation on the Structure of Thin PET films ", Polymer Degradation and Stability, 67, 149-158, 2000) and in the study of epoxy resin degradation by Farquhrson, et al. (Farquharson S., Bassilakis R., Ditaranto M., Haigis J., Solomon P., Smith W., Ebeling Th., "Measurement of thermal degradation in epoxy composites by Fourier transform Raman spectroscopy" , Proceedings of SPIE, vol. 2072, 319-331, 1994).

SUMMARY OF THE INVENTION We have discovered that changes in the absorbance spectra of a variety of polymeric films, similar to these which were previously disclosed, are induced by IR radiation produced by a C0₂ laser.

These changes, which are related to the intensity of the irradiating beam and to the duration of the irradiation, attenuates the absorption peaks of the polymeric layers in the mid IR (2-15 μm), and thus increase the transmission of

IR light having the wavelength of the attenuated absorption peak through the polymeric layers.

It is the object of the present invention to disclose a method for reducing the losses of an IR light beam which passes an optical contact. In accordance with the present invention there is provided a method for optically connecting elements in the IR comprising the steps of: (a) providing a First optical element and a second optical element; (b) adhering a face of the First optical element to a face of the second optical element with an adhesive layer and, (c) changing at least one optical property of the adhesive layer by irradiating a respective portion of the adhesive layer with an IR light beam.

It is another object of the present invention to disclose an optical contact in the IR which has a high contact transmittance.

It is yet another object of the present invention to disclose an optical contact in the IR which can transmit an high optical power.

In accordance with the present invention there is provided an optical contact in the IR comprising: (a) a first optical element having a polished face; (b) a layer of adhesive material adhering to the polished face of said first optical element; and (c) a second optical element having a polished face, the polished face of the second optical element adhering to the layer of adhesive material, wherein at least one optical property of the layer of adhesive material has been changed as a result of irradiating a respective portion of the layer of adhesive material with an IR beam of light.

Other objects of the invention will become apparent upon reading the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings:

FIG. 1 shows a setup for irradiating a polymeric film sandwiched between two waveguides.

FIG. 2 shows a setup for monitoring absorbance changes of a polymeric layer sandwiched between two waveguides.

FIG. 3 shows a detailed view of an optical connector. FIGs. 4A and 4B show changes in the absorption spectra of a polyethylene film at different exposures to C0₂ laser radiation at two spectral regions.

FIGs. 5 A and 5B show changes in the absorption spectra of an epoxy resin film at different exposures to C0₂ laser radiation at two spectral regions.

FIG. 6 shows the transmittance of five irradiated optical contacts vs. power of radiation.

FIG. 7 shows the temperature dependence of power absorbed in polymer layer, calculated according to Equation (8) of the Appendix.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that modiFications in the absorbance peaks of polymeric materials can be induced by IR radiation, produced by a C0₂ laser.

Namely, the absorption coefficient of the polymer at certain peak wavelengths decreases as the result of the laser irradiation in a manner that is related to the power of the laser and to the time of the exposure.

This novel optical bleaching effect is used in the present invention to increase the transmission of optical contacts in the IR.

The embodiments presented herein are not intended to be exhaustive and to limit in any way the scope of the invention, rather they are used as examples for the clarification of the invention and for enabling of other skilled in the art to utilize its teaching.

FIGURE 1 depicts a general experimental setup for irradiating a polymeric film subjected between two optical waveguides according to a first embodiment 10 of the present invention: A thin polymeric film 11 is clamped between optically flat surfaces of two IR optical waveguides 12 which are linke together with connector 17 which also helps to isolate the polymeric film from oxygen in order to prevent its oxidation. Light from an IR light source, preferably a C0₂ laser 13 is collimated by a lens 14 on the entrance face 12' of one of the waveguides 12 and is absorbed by the polymeric layer 11.

The position of the beam on entrance face 12' is located by a precise XYZ translator stage (not shown) which moves laser 13 relative to connector 17.

The light transmitted by film 11 is conducted by the second waveguide 12 to a power meter 15 which monitors the power of the transmitted laser beam. The output of the power meter 15 can be used to regulate the power of light source 13 via a feedback loop (not shown). Waveguide 12 may include hollow tubes or solid core IR optical fiber of any kind, e.g a silica optical fiber, a germanium optical fibe, a fluoride optical fiber, a chalcogenide optical fiber and an halide optical fiber.

FIGURE 2 depicts an experimental setup for observing changes in at least one of the absorption coefficients of the polymeric film 11 which were induced by the irradiation with the light of the light source 13 according to a second embodiment 20 of the present invention:

The observing mechanism includes the mounting of the physical unit having film 11 in the connector 17 with the two protruding waveguides 12, into or outside a sample compartment of an IR spectrometer (e.g. an FTIR) which produces a reading beam 28 and measuring the absorption spectrum of the irradiated film in the spectral range of 1-20 μm with a detector 25 which is sensitive to light of wavelength at this range.

The following are non-limiting examples for the effect of C0₂ laser radiation on the absorption spectrum of the polymeric layer 11 according to the present invention:

EXAMPLE 1- Bleaching absorption peaks of thermoplastic polymers

Polyethylene was chosen to represent this group. Thin polyethylene films were deposited at the ends of infrared silver halide AgBrCl fibers. Silver halide fibers were used as waveguides, being highly transparent in the middle and far infrared bands of a spectrum.

These fibers were extruded from silver halide crystals; the high quality of input and output fiber's surfaces was achieved by microtome cutting as shown by Nagli L., Bunimovich D., Shmilevich A., Kristianpoller N., and A. Katzir, in "Optical Properties of Mixed Silver Halide Crystals and Fiber ", Journal of Applied Physics, 74 (90), 1 November, 5737, 1993.

Pieces of AgBrCl fibers with diameter of 0.9 mm and with a length of 5 cm were used as waveguides 12.

The polymer was middle density polyethylene MDPE M3804RU/RUP, (manufactured by Thai Polyethylene Co. LTD). This sort of polyethylene is produced in powder form, which lowers the melting point of polymer and enables the dip-coating deposition of polymer on the end of the infrared fiber. A connector 37 for gluing the AgBrCl fiber's faces with a thin layer of polymer in between is shown in FIGURE 3. connector 37 is a block which is split into two halves 37' and 37", made of titanium alloy that preserves the chemical inertness of all the components enclosed within it. Each halve 37', 37" of the connector 37 has a polished face with a semicircular groove (not shown) along its face to accommodate about half of the thickness of an AgBrCl fiber.

In the middle of each halve 37', 37" is a cavity 36 which accommodates the spillover of the polymer. The two halves 37', 37" of connector 37 are clamped together (with the grooves of each halve facing toward each other) by Teflon screws 39.

To bond the two waveguide pieces with polyethylene, the polymer was heated up to 140 °C, and fibers tips were dip-coated by the MDPE melt, then the Fibers were guided toward each other in the groove of halve 37' of connector 37 until they contacted mechanically. Then the Fibers were clamped and hold in place by halve 37" of connector 37, which was tightened to the halve 37' of the connector 37 with the Teflon screws 39. The whole assembly was heated to 100°C to adhere the two respective faces of the optical fibers to each other.

The bulk of the polymer which was spilled over the joint and surrounded it prevented oxygen from interacting with the adhering polymer layer.

Samples were exposed to C0₂ laser radiation in the setup shown in FIGURE 1 starting from small intensities ~ 0.3W. After the exposure, the absorbance spectra of the samples were measured in the setup shown in FIGURE 2. Then the samples were removed again to the recording setup of FIGURE 1 and the amplitude of infrared radiation was increased gradually, then the samples were measured again and vice versa.

This was continued up to values of radiation that caused significant changes in absorbance peaks. The duration of exposure varied from 5 to 20 minutes.

The absorbance spectrum was measured by FTIR spectrometer (Nicolet, model 5PC), having a parabolic mirror and highly sensitive detector (EG&G Optoelectronics, J15-D16). The waveguides 12 of connector 37 was located along the path of the light of the instrument.

The spectrum of two pieces of AgBrCl fiber of a length of 10 cm was taken as background. The measured spectrum of the Fibers attached by the polymeric layer was normalized to background, so the spectrum of the polymer layer has been obtained

FIGURES 4A and 4B illustrate changes in the spectra of polyethylene layers which were exposed to different initial values of intensity of infrared radiation for different duration of time. In FIGURES 4A and 4B: Curve A is the absorbance spectrum of the non-irradiated polymer, curve B is the absorbance spectrum after irradiation with beam power of 3W for 4 minutes; curve C is the absorbance spectrum after irradiation with beam power of 4.5W for 5 minutes; curved D is the absorbance spectrum after irradiation with beam power of 4.5W for 20 minutes. Position of the peak 720 cm^"1 is inherent for the rocking vibration of

CH₂ groups. We have determined that the phenomenon of peak's disappearance is of threshold nature, and the limiting power that causes changes in the area under the absorption peak is about 2.5 W.

Exposure of polymer layers to IR radiation which is weaker than the threshold of 2.5W doesn't cause changes in peak's area (measured with a very high accuracy) even after the film was irradiated for a very long time.

Peak's position 2850-2960 cm^"1 (normally two bands as it can be seen at

FIGURE 4B) is inherent for the stretching vibration of CH₂ group. The process of disappearance of these peaks has a threshold nature as well, and the limiting energy of infrared radiation was EXAMPLE 2- Bleaching absorption peaks of thermosetting polymers

Epoxy resin DP- 125 manufactured by the 3M Corporation was chosen to represent this group. Thin Epoxy resin films were deposited at the ends of infrared silver halide AgBrCl fibers. Silver halide fibers were used as waveguides, being highly transparent in the middle and far infrared bands of a spectrum.

These Fibers were extruded from silver halide crystals; the high quality of input and output fiber's surfaces was achieved by microtome cutting as shown by Nagli L., Bunimovich D., Shmilevich A., Kristianpoller N., and A.

Katzir, in "Optical Properties of Mixed Silver Halide Crystals and Fibers ",

Journal of Applied Physics, 74 (90), 1 November, 5737, 1993.

Pieces of AgBrCl fibers with diameter of 0.9 mm and with a length of 5 cm were used as waveguides 12. A connector 37 for gluing the AgBrCl fiber's faces with a thin layer of polymer in between is shown in FIGURE 3. connector 37 is a block which is split into two halves 37' and 37", made of titanium alloy that preserves the chemical inertness of all the components enclosed within it.

Each halve 37', 37" of the connector 37 has a polished face with a semicircular groove (not shown) along its face which accommodates about half of the thickness of an AgBrCl fiber.

In the middle of each halve 37', 37" of connector 37 is a cavity 36 which accommodates the spillover of the polymer. The two halves 37', 37" of connector 37 are clamped together (with the grooves of each halve pointing toward each other) by Teflon screws 39.

To bond the two waveguide pieces with epoxy resin, the fibers tips were dip-coated by the epoxy resin mixed with curing agent, then the fibers were guided toward each other in the groove of halve 37' of connector 37 until they contacted mechanically. The fibers were clamped to hold in place by halve 37" of connector 37 which was secured to the first halve 37' of the connector 37 with the Teflon screws 39. The epoxy at the bond was left to cure, adhering the two respective faces of the optical fibers to each other.

Samples were exposed to C0₂ laser radiation in the setup shown in FIGURE 1 starting from small intensities ^~ 0.3 W. After the exposure, the absorbance spectra of the samples were measured in the setup shown in FIGURE 2. Then the samples were removed again to the recording setup of FIGURE 1 and the amplitude of infrared radiation was increased gradually, then the samples were measured again and vice versa.

This was continued up to values of radiation that caused significant changes in absorbance peaks. The duration of exposure varied from 5 to 20 minutes. The absorbance spectrum was measured by FTIR spectrometer (Nicolet, model 5PC), having a parabolic mirror and highly sensitive detector (EG&G Optoelectronics, J15-D16). The waveguides 12 of connector 37 was located along the path of the light of the instrument.

The spectrum of two pieces of AgBrCl fiber of a length of 10 cm was taken as background. The measured spectrum of the fibers attached by the polymeric layer was normalized to background, so the spectrum of the epoxy layer has been obtained

FIGURES 5A and 5B, illustrate changes in absorbance spectra of the epoxy resin induced by radiation of C0₂ laser. In FIGURES 5 A and 5B: Curve A is the absorbance spectrum of the non-irradiated epoxy resin, curve B is the absorbance spectrum after irradiation with beam power of 1W for 20 seconds ; curve C is the absorbance spectrum after irradiation with beam power of 1W for 155 seconds. The peak at 2850-2960 cm^"1 (normally two bands) corresponds to the stretching vibration of -CH₂- groups (see discussion of absorbance in polyethylene films). Such peaks are inherent to epoxy resins as well. The location the peak at 1250 cm^"1 is unambiguously inherent to the bending

vibration of the epoxides groups: ( ° ).

The threshold value of intensity, that causes the decay of absorbance peaks, was established experimentally as 0.6-1W. It is seen that epoxy resins are characterized by lesser levels of threshold powers necessary to decay absorbance peaks then the polyethylene. It is suggested that the described changes in the absorbance spectra due to the laser irradiation are caused by oxygen-free thermal degradation of the polymers and breaking of the corresponding chemical bonds. A model for such a mechanism is given in the Appendix.

Although the examples include only two specific polymers it was found that the bleaching effect of one or more absorbance peaks in the IR, is common

(at various threshold power of the C0₂ laser) to films of other kinds of polymers as well.

Among other polymers which were tested and show the effect were: polypropylene, polycarbonate, polystyrene, acrylonitrile butadiene styrene, poly vinyl ester and polysulphone.

In order to determine the maximum allowable power which can be delivered through the polymeric optical contact thus formed, the effect of increasing the power of the C0₂ laser radiation on the transmission of the contact was investigated and is shown in FIG. 6. In an experiment performed according to embodiment 10 shown in

FIGURE 1, the transmittance (I_ou Ii_n) of a C0₂ laser beam through an optical contact is plotted vs. the power of a beam (from the same C0₂ laser) which has been used to bleach the contact. In order to isolate the changes in the transmittance of the contact, the measured transmittance was normalized to the transmittance of a single optical fiber having a length as the combined length of the two pieces of the waveguide 12. Five polymeric materials were tried for gluing the fibers, none of them has an absorbance band at 10.6 μm which was the wavelength of the laser.

Curve 1 describes the behavior of acrylonitrile butadiene styrene (ABS) polymer (PA- 7475, produced by Chi Mei Corporation) which is a thermoplastic polymer. It can be seen that at beam power as low as 3W the transmission deteriorates dramatically because the absorption of chemical species within the polymeric layer which were formed as a result of its aerobic thermal decomposition.

Curve 2 describes the behavior of epoxy DP- 125 (produced by the 3M corporation) which was investigated in EXAMPLE 2. It behaves similarly to ABS but has a higher threshold for decomposition.

Curve 3 represent the behavior of polysulphone (PSU) as an irradiated adhesive, the transmission of this polymeric material (UDEL produced by Amoco) is hardly affected by the laser irradiation up to beam power of about 5 W were it suddenly degrades. Curve 4 depicts the behavior of MDPE which was investigate in

EXAMPLE 1, the contact can tolerate irradiation power up to about 4.5 W where the transmission collapses as a result of the anaerobic thermal decomposition of polyethylene.

Curve 5 demonstrates the behavior of a mixture having the composition of: chalcogenide glass (40%) and MDPE (60%), which is used as an adhesive material.

The properties of this adhesive mixture were described in PCT international application PCT/IL00/00054 filled at January 2000, wherein it has been shown that the index of refraction of such composites can be tailored to match the refractive index of the attached optical fibers.

The improvement of the transmittance, which is observed already at low levels of irradiation probably reflects some annealing of the composite, by which its refractive index increases and matches better the refractive index of the optical fibers.

However, in contrast to previous contacts, the polymer does not decompose at power above 5W and the only losses are because of reflections between the opposing faces of the two attached Fibers. These losses are small and a maximum contact transmission of 85% at wavelength of 10.6 μm is obtained. It can thus be concluded that it is possible to diminish the absorption losses of polyethylene by laser irradiation and to prevent its decomposition in the contact during the delivery of IR power of up to about 9W.

While the principle of the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made without departing from the spirit and scope of the invention.

E.g. this invention can be applied also for optically connecting IR lenses, windows, prisms, beam splitters, mirrors, polarizers and combinations thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.

APPENDIX

A thermal model for the suppression of absorbance bands of polymeric films in the IR by C0₂ laser irradiation. We supposed that the changes in the absorbance spectra were caused by the oxygen-free thermal degradation of polymers and breaking of corresponding chemical bonds. In order to prove our assumptions, we estimated the temperature in the optical contact layer. We supposed that the steady state heat conductivity problem for the long rod, which has the internal heat source and at time cooled at the lateral face, describes the experimental situation adequately.

The system fiber - polymer layer could be described by this model, and adhesive layer plays the role of a heat source. Equations, which describe the heat transport process, could be written as:

dT - T_Q) ≥ ξ dx² XR

(2)

The boundary conditions are:

— ^ = 0; = 0 dx

(3a)

T = T_g; x = ξ (3b)

λ = X ; x — ς dx dx

(3c) where, 2ξ~10 μm is the thickness of the polymer layer, and R=0.45 10^" m is the radius of the optical fiber, To = 293K is the ambient temperature, λ is the thermal conductivity coefficient, α is the effective coefficient of heat exchange,

W is the absorbed power and the index "g" relates to the adhesive layer. From equations (l)-(2) it can be seen that the characteristic length Δ could be expressed as:

Δ = J— 2a

(4)

There is good reason to believe that the main mechanism of thermal exchange in our system is a heat radiation. From the above reasoning it can be concluded that:

(5) where, ε < 1 is the reduced emissivity of the system fiber-environment, σ is the constant of radiation, T_eff is the effective temperature which gives the best approximation to the solution of the system (l)-(3). It can be seen that: T₀<T_eff <T(0).

It is felt that T_eff ~ 600°K (the characteristic temperature of oxygen-free polyethylene degradation), substituting T_eff we receive Δ ~ 3-4 10^" m; the corresponding constant τ could be expressed as:

Δ² τ ≡- (6) a

The thermal diffusivity coefficient of AgCl equals a ~ 5 10^" m Is. Substituting a and Δ in relationship (6) we receive τ ~ 20s. It can be seen that the use of the steady- state model is proper. The characteristic time of radial heat exchange τ_R could be calculated as:

(7)

So we can assure ourselves that the use of the one-dimensional approach is legitimate as well. The final result for the temperature of the polymer layer doesn't depend on parameters of the adhesive (when absorbed power is fixed) and could be expressed as:

(8) The thermal exchange coefficient depends on T_ef; which is in general

unknown. Figure (1) illustrates dependence it was assumed

that T_eff=T(0).

Now we shall summarize the explanations that have been put forward for the results obtained. According to (8), free-oxygen thermal destruction of polyethylene occurs under the temperature T~340°C. We have registered decay of peaks starting with the initial intensity of infrared radiation P~2.5-3.0 W (FIG. 4A, curve B). It is reasonable to suggest that the temperature in the polymer layer is close to the above-mentioned value. FIGURE 7 demonstrates that this temperature corresponds to the power absorbed in the polymer layer, which is something like 0.15 W. This value forms 5-6 percent of initial incident intensity of laser radiation and corresponds reasonably to our estimation obtained by experiment.

We observed changes in the color of polyethylene at the incident intensity of 4.5 W. Color was varied, from light yellow at the time lag of 5 min. up to dark brown at 20 min. As it was discussed in the introduction, such "coloration" of polymers under oxygen-free thermal destruction was discussed by Grassie and McNeil (1,2). Figures 4A, 4B illustrate the degree and rapidity of degradation processes. According to Dickens (3), such depth of degradation process occurs under the temperature 430-480°C. When the incident power equals 4.5 W, our model gives the same values of temperatures. So the developed model agrees satisfactorily with the experimental data.

For epoxy adhesive layers our treatment gives the estimation of absorbed power that is in the range 8-10%. The decay of absorbance peaks starts with incident intensity of infrared radiation close to 0.8 W, that corresponds to the temperature 200-250°C. Under the incident intensity 1 W (that corresponds to the temperature 300°C), we have registered the rapid decay of absorbance peaks. After 20 seconds of exposure to C0₂ laser radiation, the area of peak is diminished twice (Fig. 5A, 5B). When the sample was exposed to the infrared radiation for the more prolonged time lags, the degradation process was slowed.

These results are in beautiful agreement with data presented by Z.M.

Kacarevic-Popovic (4). This group revealed by thermogravimetric analysis, that epoxy films electrodeposited on metal substrates loosen 15% of mass when heated to 200-300°C. As this takes place, the maximum rate of the decomposition process fell at 250°C, when speed of heating was 10°K/min.

We obtained additional evidence of the validity of our explanation of the revealed phenomenon when spectra of exposed to IR radiation polymers were studied thoroughly. There is much evidence that thermal degradation of polymers is accompanied by making double chemical bonds (5). Vinyl tail, trans-vmyl e and vinylidene groups are formed under free-oxygen thermal destruction of polyethylene (6). We revealed the rise of peaks in polyethylene exposed to laser radiation located in the range 900-940 cm^"1. These peaks are inherent for vinyl tail double bonds -CH=CH₂ . The rise of these peaks gives the circumstantial evidence of the truth of our hypothesis.

References:

[1] Grassie N., McNeill, J. Polymer Science, 33, 171, 1958.

[2] Grassie N., McNeill, J. Polymer Science, 39, 211, 1959.

[3] Dickens B. J. Polymer Science.; Polymer Chem. Ed. V.20., 1065, 1982. [4] Kacarevic-Popovic Z.M., Miscovic-Stankovic V.B., Maksimovic M.D.,

Zotovic J.B., Kostoski D. "Study of Thermal Stability of Epoxy Coatings

Electrodeposited on Different Substrates ", Polymer Degradation and Stability,

65 (1), 91-98, 1999.

[5] Kirilova E.I., Shulgina E.S., "Aging and Stabilization of Thermoplasts ", Leningrad, 239 1988.

[6] Oakes W.G., Richards R.B., J. Chem. Soc P. 2929. 1949.

Claims

What is claimed is:

1. A method for optically connecting elements in the IR comprising the steps of:

(a) providing a first optical element and a second optical element;

(b) adhering a face of said first optical element to a face of said second optical element with an adhesive layer and,

(c) changing at least one optical property of said adhesive layer by irradiating a respective portion of said adhesive layer with an IR light.

2. The method of claim 1 wherein said first optical element is selected from the group consisting of waveguides, lenses, windows, prisms, beam splitters and polarizers.

3. The method of claim 1 wherein said second optical element is selected from the group consisting of waveguides, lenses, windows, prisms, beam splitters and polarizers.

4. The method of claim 1 wherein said first optical element includes a material which is selected from the group consisting of fluorides, chalcognides, halides, silica and germanium.

5. The method of claim 1 wherein said second optical element includes a material which is selected from the group consisting of fluorides, chalcognides, halides, silica and germanium.

6. The method of claim 1 wherein said first optical element is a waveguide and said second optical element is a waveguide, and wherein said face of said first optical element is a terminus of said first waveguide and said face of said second optical element is a terminus of said second waveguide.

7. The method of claim 6 wherein said first waveguide is selected from the group consisting of solid core optical fibers and hollow tubes.

8. The method of claim 6 wherein said second waveguide is selected from the group consisting of solid core optical fibers and hollow tubes.

9. The method of claim 1 wherein said adhesive layer includes a polymeric compound

10. The method of claim 9 wherein said polymeric compound is selected from the group consisting of a thermoplastic polymer and a thermosetting polymer.

11. The method of claim 8 wherein said thermoplastic polymer is selected from the group which consists of polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene- styrene, polycarbonate and polysulfone.

12. The method of claim 10 wherein said thermosetting polymer is selected from the group consisting of poly- vinyl-ester and epoxy resin.

13. The method of claim 9 wherein said adhesive layer further includes a chalcognide glass.

14. The method of claim 13 wherein a weight ratio percentage of said polymeric compound to said chalcognide glass is about 60% to about 40% .

15. The method of claim 1 wherein said adhesive layer is isolated from contact with oxygen.

16. The method of claim 1 wherein said IR light has a wavelength of about 3 to about 15 micrometer.

17. The method of claim 1 wherein a source of said IR light is a laser.

18. The method of claim 17 wherein said laser is a C0₂ laser.

19. the method of claim 18 wherein a power of said IR light has a threshold value.

20. The method of claim 19 wherein said value of said threshold value is at least 0.6 W.

21. The method of claim 18 wherein a continuous power of said light is between about 0.5W to about 9W.

22. The method of claim 1 wherein said at least one optical property is an absorption of the adhesive layer in the wavelength range of about 2 micrometers to about 20 micrometers.

23. An optical contact in the IR comprising:

(a) a first optical element having a polished face;

(b) a layer of adhesive material adhering to said polished face of said first optical element, wherein at least one optical property of said layer of adhesive material has been changed as a result of irradiating a respective portion of said layer of adhesive material with an IR light and,

(c) a second optical element having a polished face, said polished face of said second optical element adhering to said layer of adhesive material.

24. The optical contact of claim 23 wherein said first optical element is selected from the group consisting of waveguides, lenses, windows, prisms, beam splitters and polarizers.

25. The optical contact of claim 23 wherein said second optical element is selected from the group consisting of waveguides, lenses, windows, prisms, beam splitters and polarizers.

26. The optical contact of claim 23 wherein said first optical element includes a material which is selected from the group consisting of fluorides, chalcognides, halides, silica and germanium.

27. The optical contact of claim 23 wherein said second optical element includes a material which is selected from the group consisting of fluorides, chalcognides, halides, silica and germanium.

28. The optical contact of claim 23 wherein said first optical element is a first waveguide and said second optical element is a second waveguide, and wherein said polished face of said first optical element is a terminus of said first waveguide and said polished face of said second optical element is a terminus of said second waveguide.

29. The optical contact of claim 28 wherein said first waveguide is selected from the group consisting of solid core optical fibers and hollow tubes.

30. The optical contact of claim 28 wherein said second waveguide is selected from the group consisting of solid core optical Fibers and hollow tubes.

31. The optical contact of claim 23 wherein said layer of adhesive material includes a polymeric compound.

32. The optical contact of claim 31 wherein said polymeric compound is selected from the group consisting of a thermoplastic polymer and a thermosetting polymer.

33. The optical contact of claim 32 wherein said thermoplastic polymer is selected from the group which consists of polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene, polycarbonate and polysulfone.

34. The optical contact of claim 32 wherein said thermosetting polymer is selected from the group consisting of poly-vinyl-ester and epoxy resin.

35. The optical contact of claim 31 wherein said layer of adhesive material further includes a chalcognide glass.

36. The optical contact of claim 35 wherein a weight ratio percentage of said polymeric compound to said chalcognide glass is about 60% to about 40%.

37. The optical contact of claim 35 wherein a refractive index of said layer of adhesive material at a wavelength of about 10 micrometers is between about 1.5 to about 4.

38. The optical contact of claim 23 wherein said change of said at least one optical property of said layer of adhesive material is the attenuation of at least one absorbance peak of said layer of adhesive material in the wavelength range of about 2 micrometer to about 20 micrometer.

39. The optical contact of claim 23 wherein said layer of adhesive material is isolated from contact with oxygen.

40. The optical contact of claim 23 wherein said IR light has a wavelength of about 3 micrometer to about 15 micrometer.

41. The optical contact of claim 23 wherein a source of said IR light is a laser.

42. The optical contact of claim 41 wherein said laser is a C0₂ laser.

43. The optical contact of claim 42 wherein a power of said IR light has a threshold value.

44. The optical contact of claim 43 wherein the value of said threshold value is at least 0.6 W.

45. The optical contact of claim 42 wherein a continuous power of said light is between about 0.5W to about 9W.