Optical material having periodically varying refractive index and method of making

In order to produce a periodically varying refractive index in an optical material such as optical fiber, the concentration of a refractive index varying material, for example, fluorine, is periodically varied. A mobile substance is first diffused into the optical material. A portion of this mobile substance is periodically activated by application of periodically varying radiation supplied to the optical material having the mobile substance diffused therein. Molecular hydrogen may be used as such a mobile substance. After activation of the mobile substance, inactivated mobile substance is diffused out of the optical material, this diffusion step being accelerated by application of heat. Subsequently, the optical element is heated to a predetermined level to further react the mobile substance whit a component of the optical material which may then be diffused out of the optical material to produce a periodically varying optical property such as refractive index.

FIELD OF INVENTION
 The present invention relates to an optical means and to a method of
 producing optical means in the form of material or components that has a
 spatially varying chemical composition which enables the manufacture of
 optical material or components whose optical properties vary spatially.
 The method is well suited to create refractive index variations in optical
 material for the manufacture of optical waveguides, or for creating or
 generating periodic refractive index variations in different types of
 waveguides.
 BACKGROUND OF THE INVENTION
 It is known that the refractive index of germanium-doped, SiO.sub.2 -based
 fibres (among others), can be changed by exposing the fibre to ultraviolet
 radiation within certain absorption intervals. The ultraviolet wavelengths
 used to create fraction index changes in holographic page-writing methods
 lie mainly within germanium-related absorption bands with a maximum at
 approximately 195 nm and approximately 240 nm, although other wavelength
 intervals have also been used, these latter wavelength intervals normally
 requiring much longer exposure times, however. It is possible to produce
 with holographic page-writing methods periodic refractive-index
 variations, so-called Bragg gratings, that function as wavelength
 selective mirrors or filters, with several applications within, e.g.,
 telecommunications and laser or sensor applications.
 Fibre gratings are described in the document "Fibre Gratings", Physics
 World, October 1993, Philip ST. J. Russell et al, pp. 41-46, and also in
 PCT publication WO 94/00784.
 Although the actual process lying behind these index changes has not been
 fully established, it is generally considered that germanium defects--the
 concentration of Ge.sup.2+ (c.f., for instance, U.S. Pat. No. 5,157,747,
 Atkins et al) is the main reason for the resultant photosensitivity. The
 photosensitivity of a material is, e.g., its ability to change its
 refractive index upon given exposure to electromagnetic radiation.
 Although the photosensitivity of fibre can be enhanced in many different
 ways, the method used is still highly dependent on the use of wavelengths
 of approximately 195 nm and approximately 240 nm. Sensitivity to
 ultraviolet light can be enhanced by doping with more GeO or GeO.sub.2
 and/or B.sub.2 O.sub.3.
 U.S. Pat. No. 5,500,031, Atkins et al, teaches a method of increasing the
 refractive index of glassy material, by applying heat in conjunction with
 hydrogen sensitization. Such increases in refractive index are not
 temperature-stable at temperatures above 600.degree. C. This patent
 specification teaches solely a method that is aimed at causing chemical
 reactions to take place over the space of time of some seconds and for
 temperatures higher than 500.degree. C., and not to cause diffusion of
 material that has diffused into the material or of doping substances in
 the material. In order to cause diffusion, the material is heated to
 temperatures of from 800 to 1100.degree. C. and over much longer times,
 for instance over minutes or hours.
 It has been possible to increase the photosensitivity of certain fibres or
 waveguides, by diffusing hydrogen thereinto.
 SUMMARY OF THE INVENTION
 One object of the present invention is to provide optical means, and a
 method that uses optical material that has spatially varying optical
 properties, and also a method of manufacturing such optical material. The
 optical properties of an optical material are greatly influenced by the
 chemical composition of the material, which enables a spatial change of
 its optical properties to be obtained by spatially changing its chemical
 composition. The method is well suited for generating a spatially varying
 refractive index, and also in obtaining variations in the non-linearities
 and/or the electro- or magneto-optical properties of the optical material.
 A change in the spatial chemical composition of an optical material means
 that gate writing will no longer be dependent on the wavelengths of 195 nm
 and 240 nm respectively, since the photosensitivity no longer depends on
 germanium defects that are related to these wavelengths.
 To this end, the present invention provides an optical means which has a
 spatially varying chemical composition. The means has diffused therein
 mobile substances that have taken part in at least one chemical reaction
 in said means or in parts of said means, by supplying energy through
 electromagnetic radiation, via optical writing or by subjecting said means
 to predetermined temperature changes.
 Further predetermined temperature changes in said means have caused
 diffused substances that have not taken part in the reaction to diffuse
 out from or through said means, and that predetermined temperature changes
 achieved by changing the energy supply via exposure to electromagnetic
 radiation, or temperature changes generated by some other form of energy,
 have caused the substance to diffuse out of said means or within said
 means, therewith changing the chemical structure and optical properties in
 this region. This results in a means that has a spatially varying chemical
 composition and spatially varying optical properties.
 In one embodiment of the invention, said means is produced by a combination
 of or by repetition of at least two of the steps of diffusing mobile
 substances in said means, supplying energy via optical writing, and
 predetermining temperature change for diffusion of the substances into
 said means.
 It is also a means for conducting electromagnetic radiation.
 In one embodiment of the invention, variations in refractive index have
 been achieved via the steps of diffusing mobile substances into the means,
 supplying energy by exposing said means to electromagnetic radiation, via
 optical writing or by predetermined temperature changes, and predetermined
 temperature changes for diffusion of mobile substances that have not
 reacted chemically, and predetermined temperatures for diffusing said
 substances out of said means or within said means.
 In another embodiment, spatially varying optical properties have been
 achieved in said means via the steps of diffusing mobile substances
 therein, supplying energy by exposing said means to electromagnetic
 radiation via optical writing or predetermined temperature changes,
 predetermined temperature changes for diffusion of mobile substances that
 have not reacted chemically, and predetermined temperature changes for
 diffusion of substances in said means.
 The present invention also relates to a method of producing a spatially
 varying chemical composition in optical means by
 diffusing at least one mobile substance in said means;
 inducing at least one chemical reaction between the diffused substance or
 substances and the optical means, by supplying energy through the medium
 of electromagnetic radiation via optical writing or by raising the
 temperature to a predetermined value;
 changing the temperature of the means to a predetermined temperature level,
 therewith causing diffused substances that have not participated in said
 chemical reaction to diffuse out of or within said means; and
 changing the temperature of said means to a predetermined temperature level
 by changing the energy supply via exposure of the means to electromagnetic
 radiation, or by some other form of energy supply, so that the substances
 will diffuse out of said means or within said means, therewith resulting
 in a chemically varying means having varying optical properties.
 Alternatively, the method comprises a combination of or a repetition of
 these steps.
 In one embodiment of the inventive method, the optical means includes
 fluorine, and either hydrogen, nitrogen or oxygen, or combinations
 thereof, is diffused into said optical means, therewith resulting in a
 higher concentration of hydroxyl groups that react with fluorine to form
 hydrogen fluoride, which can be readily caused to diffuse out of said
 means or within said means.
 In another embodiment of the inventive method, the optical means includes
 halogens, and hydrogen, nitrogen, oxygen or a combination thereof are
 diffused into said optical means, therewith resulting in a higher
 concentration of hydroxyl groups that react chemically with said halogens
 to form substances that consist totally or partially of hydrogen and
 halogens that can be readily caused to diffuse out of said means or within
 said means.
 In still another embodiment of the inventive method, the optical means
 includes alkali metals, and nitrogen, oxygen or combinations thereof are
 diffused into the material, therewith increasing the concentration of
 hydroxyl groups which react with the alkali metals to form substances that
 consist totally or partially of hydrogen and alkali metals, which can be
 readily caused to diffuse out of said means or within said means.
 Said means may be comprised partially of silicon dioxide (SiO.sub.2) and
 germanium oxide (GeO.sub.2) and the fluorine. It may alternatively be
 comprised partially of silicon dioxide (SiO.sub.2) and phosphorous oxide
 (P.sub.2 O.sub.5) and said fluorine.
 The inventive means is preferably a waveguide structure for conducting
 electromagnetic radiation. The waveguide structure may be an optical fibre
 or some other known waveguide.
 The method steps result in variations in refractive index. They can also
 produce spatially varying optical properties, which in one embodiment of
 the invention consist in variations in the non-linearities and/or the
 electro-magneto optical properties of said means.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 A locally or periodically varying chemical structure (composition) of an
 optical means, also referred to as material or component, is obtained by
 diffusing into said optical material one or more substances and then
 causing or inducing local or periodic chemical reactions between the
 diffused substance or substances and said optical material. Further
 chemical reactions or structural changes in the optical means or material
 are prevented, by allowing those diffused substances that do not
 participate in a reaction to diffuse out of the material or component.
 The aim of the chemical reactions is to create a spatial variation in the
 binding structure, i.e., a given atom or molecule has a spatially varying
 binding structure in the optical means. Since different molecular
 compositions exhibit different diffusion rates, depending on their
 chemical structure, it is possible to cause a given atom or molecule to
 exhibit a spatially varying diffusion rate, for instance by heating the
 optical material. Thus, it is possible to create spatially varying
 concentrations of certain atoms through the particular chemical
 composition of the material and the substance or substances diffused
 therein.
 A periodically varying refractive index can be created by, e.g.,
 periodically changing the fluorine concentration (F), which has a
 refractive index lowering effect.
 The present invention enables, e.g., periodic refractive index variations
 to be achieved in waveguides in a completely novel manner, which has the
 advantages of enabling gratings to be written--a process in which an
 optical fibre is exposed to UV laser light to produce a grating--with
 wavelengths other than those within the earlier necessary wavelength
 intervals--approximately 195 nm, approximately 240 nm, therewith enabling
 less expensive and better light sources to be used, and providing greater
 flexibility in the production of gratings, for instance. This is due to
 the fact that the inventive method is not dependent on germanium-related
 defects, which have strong absorption bands at approximately 195 nm and
 approximately 240 nm and the ability to induce chemical reactions between
 the optical material and the substance diffused therein. It is therefore
 possible, for the same reason, to use other doping materials, than, e.g.,
 germanium and boron in order to obtain high photosensitivity.
 Changes induced in the optical material in accordance with the present
 invention are also very stable, since they are caused by a variation of
 the chemical composition of said material, which results in a greater
 useful life span and periodic refractive index changes that can withstand
 very high temperatures over a long period of time, as will be illustrated
 below with reference to FIG. 9.
 A method of achieving a spatially varying chemical composition in optical
 material has been developed in accordance with the invention, this method
 comprising the steps of:
 diffusing a mobile substance in the optical material or means, i.e. a
 substance or substances that can diffuse into or out of material without
 appreciably affecting its structure;
 inducing at least one chemical reaction between the diffused substances in
 the region and the optical material, by supplying energy by exposing the
 material to electromagnetic radiation, through the medium of optical
 writing or heating;
 changing the temperature of the optical material to a predetermined
 temperature, wherewith diffused substances that do not participate in said
 chemical reaction diffuse out of or within said optical material; and
 changing the temperature of the optical material to a predetermined
 temperature level by electromagnetic radiation or by some other
 temperature changing means that accelerates varying diffusion of the
 substances (atoms/molecules) within or out of said optical material,
 therewith to obtain a varying chemical composition of said material with
 changed optical properties.
 These temperature changes can be achieved conveniently with the aid of
 ovens or other heating apparatus suitable for the purpose intended, or by
 exposing the optical means, or material, to electromagnetic radiation.
 The fundamental concept (lone) of U.S. Pat. No. 5,500,031 is to supply
 energy to the material so as to induce chemical reactions therein, which
 is only one step of the inventive method. The additional, predetermined
 temperature changes to which the material is subjected are intended to
 empty the optical means of diffused substances that have not reacted
 chemically with said means. The temperature increases are such as to allow
 unreacted substances to diffuse out of or within said optical means,
 whereas those substances that have reacted in the optical means will not
 diffuse appreciably in those regions of said means that have been
 subjected to UV radiation or heat, for instance.
 In contradistinction to the inventive method, the temperature increase
 taught by U.S. Pat. No. 5,500,031 is intended to generate as many
 heat-induced chemical reactions as possible. The reason why the patent
 mentions temperatures higher than 500.degree. C. is because the major part
 of H.sub.2 /D.sub.2 is therewith able to diffuse out of the material
 before chemical reactions take place at lower temperatures.
 The method steps will be described hereinafter in more detail with
 reference to FIGS. 6-8.
 The present invention also relates to an optical means that has a spatially
 varying chemical composition and which has mobile substances diffused
 therein. Subsequent to having been induced, the substances have undergone
 at least one chemical reaction with the optical means, by supplying energy
 to said means by exposing it to electromagnetic radiation via optical
 writing or by heating said means.
 The means has been subjected to predetermined temperature changes, such as
 to cause diffused substances that have not participated in the reaction to
 diffuse out of said means.
 Spatially varying diffusion of the substances (atoms/molecules) within said
 means or out of said means is accelerated by subjecting said means to at
 least one temperature change, by exposing said means to electromagnetic
 radiation or by changing said temperature in some other way, therewith
 obtaining an optical means of varying chemical composition and varying
 optical properties.
 The purpose of the latter temperature change or temperature changes is to
 cause diffusion, and possibly further chemical reactions, of those
 substances that have earlier reacted with the optical means at high
 temperatures and therewith change the local or spatial chemical structure
 or composition of the optical means. This latter temperature-induced
 diffusion may also include substances that were earlier present in said
 means and substances that have diffused into said means and then reacted
 chemically with the glass via diffusion and the exposure of said means to
 ultraviolet light or heat, for instance. This predetermined temperature
 change is not mentioned in U.S. Pat. No. 5,500,031 and neither is it
 relevant to the patent.
 The inventive method results in a change in the chemical structure or
 composition of the optical means, but solely at those places in said means
 into which mobile substances have diffused and a chemical reaction
 subsequently induced. In distinction, the patent U.S. Pat. No. 5,500,031
 relates solely to the creation of chemical reactions within the whole of
 the region subjected to H.sub.2 /D.sub.2 diffusion and heat treatment.
 This creates per se an index increase, which is also achieved in the case
 of the present invention, if the optical means includes germanium (even
 P-F doped glass according to the patent). However, this index increase is
 not temperature-stable. Those experiments described below with reference
 to the invention, see reference 46 in FIG. 8 below, show that index
 increases are, so to speak, "erased" during the process (prior to dip 46
 in FIG. 8) and temperature-stable index changes are created in the optical
 means upon diffusion of said substances, due to induced chemical or
 structural changes (the increase after dip 46 in FIG. 8).
 The change in the chemical composition of the optical material or component
 enables wide variations in refractive index to be achieved, therewith
 rendering the inventive method highly suitable for writing optical
 waveguide structures.
 The inventive method was applied in laboratory trials on waveguides in the
 form of an MCVD (Modified Chemical Vapor Deposition) produced SiO.sub.2
 based fibre, where the waveguide part (the core) was doped with germanium
 (Ge) and with fluorine (F). Because of its refractive index raising
 properties, germanium was used to create a waveguide and also to generate
 hydroxyl groups (-OH) together with hydrogen (H.sub.2) and/or deuterium
 diffused in the material. Fluorine was used in the trial because it has
 refractive index lowering properties and because it reacts chemically with
 hydroxyl groups (-OH) to form, among other things, hydrogen fluoride (HF),
 which is able to diffuse out of or within doped material more rapidly,
 i.e. it is essential that it diffuses out of the waveguide core.
 Chemical reactions between hydroxyls and fluorine are described, inter
 alia, in "The Properties of Glass Surfaces", L. Holland, Chaplan and Hall,
 London 1964, and in the article "Hydrogen-Induced Hydroxyl Profiles in
 Doped Silica Layers", J. Kirchof et al, OFC '95, Technical Digest, pp.
 178-179.
 Hydrogen sensitization (enhancement of photosensitivity by hydrogen
 loading) for writing germanium defect-related gratings and partial OH
 formation is documented in "Enhanced UV Photosensitivity in Fibres and
 Waveguides by High Pressure Hydrogen Loading", P. J. Lemaire, OFC '95,
 Technical Digest, pp. 162-163. Photosensitivity in germania doped glass
 and hydroxyl formation with hydrogen sensitization is also discussed in
 "Photosensitive Index Changes in Germania Doped Silica Glass Fibres and
 Waveguides", D. L. Williams et al, SPIE Vol. 2044, pp. 55-68.
 Since fluorine atoms bound solely to hydrogen have a much higher diffusion
 rate than fluorine that is bound to germanium (Ge) or silicon (Si),
 diffusion out of or within the material can be caused by HF, at the same
 time as fluorine (F) bound to GE or Si exhibits only slight diffusion,
 resulting in a spatial variation of fluorine in the core of the waveguide.
 Because fluorine has an index lowering effect, a reduction in the fluorine
 content will result in an increase in the refractive index.
 The article "Interactions of Hydrogen and Deuterium with Silica Optical
 Fibres: A Review", J. Stone, Journal of Lightwave Technology, Vol. LT-5,
 No. 5, May 1987, deals with hydrogen in glass and OH formation in
 different types of glass including Ge, P, F.
 Those optical means that can be formed by means of the inventive method
 include the type of waveguide that has a varying refractive index,
 gratings, gratings that function as sensors, light wavelength mirrors,
 filters, strain gauges, temperature sensors that withstand high
 temperatures, etc.
 Diffusion of different materials can often be described by the equation
 D=D.sub.0 e.sup.E/RT, where D.sub.0 is a constant, E is the diffusion
 process activation energy, R=1.99 cal/K-mol is the gas constant, and T is
 the absolute temperature.
 Hydrogen sensitization in combination with an induced chemical reaction
 changes the chemical structure of a material, which is used to vary the
 constants D.sub.0 and E either locally or periodically. Separation of
 certain atoms or molecules is achieved by heating the optical material,
 due to their different diffusion rates.
 In the case of doping with fluorine and the formation of hydroxyls, there
 takes place a "second" reaction which results in the formation of hydrogen
 fluoride (HF), which has a much higher diffusion rate than fluorine which
 is bound to other atoms or molecules. The HF diffusion requirement is
 coupled to the availability of fluorine and -OH. This affords greater
 flexibility when doping glass if hydroxyls are caused to form. Dopants
 other than germanium can then be used.
 Strong evidence for the occurrent reactions exist in HF formation, although
 successful attempts to write "diffusion gratings" in boron-doped germanium
 fibres and in standard telecommunications fibres, which contain solely
 germanium and, of course, SiO.sub.2, in accordance with the present
 invention.
 A common feature of all types of germanium-related gratings is that they
 disappear, are erased, at temperatures in excess of about 500-900.degree.
 C., depending on the type of fibre used. According to the present
 invention, a "diffusion grating" begins to grow at temperatures of about
 800-1000.degree. C., depending on the type of fibre used.
 FIG. 1 is a schematic, cross-sectional illustration of the construction of
 a typical optical fibre 10 including a fibre-protective coating (e.g.
 acrylates, polymers, etc.) and a cladding 14 that functions as a
 refractive medium that surrounds the fibre core 18. The interior 16 and
 the core 18 of the fibre are enlarged in FIG. 1, wherewith the rings
 indicate the deposition of silicon dioxide in accordance with the MCVD
 method, for instance. The part-area 20 is comprised of crude silicon
 dioxide, whereas the area 22 containing the rings that extend to the core
 18 is comprised of pure or refined silicon dioxide. The fibre core 18 is
 doped with germanium.
 The graphs shown immediately beneath the cross-sectional views in FIG. 1
 illustrate the variations in refractive index n along the radius r of the
 fibre.
 In order for a fibre to conduct light without significant losses the total
 reflection is utilized, which can be achieved by virtue of the wave
 conducting part of the fibre having a higher refractive index than the
 cladding. Further demands are placed on the index difference relative to
 the diameter of the fibre core, in the propagation of a single light mode
 in a fibre.
 With regard to flat waveguides (substrates), these can be manufactured and
 caused to function in accordance with the same principles as those
 applicable to fibres, i.e. with the high index core and lower index of a
 surrounding cladding. In the case of three-layer substrates, the central
 layer can be doped with Ge and F for instance, wherewith a waveguide can
 be written by slow exposure in the substrate. The other layers are doped
 with material, e.g. SiO.sub.2, that is not influenced by the writing
 process.
 FIG. 2 is a schematic illustration of an arrangement and an embodiment for
 writing a grating focused on the core 18 with the aid of a lens 24, in the
 illustrated case a cylindrical lens, while exposed to (irradiated by) two
 mutually interfering UV light beams 26 transmitted from a laser and having
 a wavelength of 240 nm, for instance. The region exposed has been doped in
 accordance with the aforegoing. Subsequent to the substances having
 diffused into the material and the grating having been written, which in
 itself induces the aforesaid reactions, the fibre is heated in accordance
 with the aforedescribed method.
 FIG. 3 is a schematic illustration of the method shown in FIG. 2, but with
 the use of interferometer-controlled movement of the fibre so as to
 continuously write a grating 28 within a desired length of the fibre core.
 The fibre is moved in the directions of the arrows whilst controlled
 interferometrically. The circular arc 30 is intended to show the
 possibility of controlling the writing process angularly, so as to obtain
 a grating 18 that has the properties desired.
 Another grating writing embodiment is illustrated schematically in FIG. 4,
 where movement of the fibre is controlled in the same manner as that in
 the FIG. 3 embodiment, but with only one UV light beam 26 focused on the
 fibre core 18 by a lens 32.
 FIG. 5 illustrates schematically a further embodiment for writing a grating
 28 with UV light 26 which directly writes a grating within a specific area
 of the fibre core 18, via a so-called phase mask 34.
 Although writing of gratings has been described in the aforegoing with the
 use of UV light, it will be understood that other electromagnetic
 radiation can also be used.
 FIG. 6 is a graph that illustrates the steps of developing a grating in a
 laboratory environment in accordance with the inventive method, with the
 temperature given on the ordinate and the time taken to write the grating
 being given on the abscissa. Prior to the temperature rises shown in the
 graph in FIG. 6, the fibre has been subjected to the diffusion of a mobile
 substance, in this case hydrogen, and then exposed to UV light in
 accordance with the aforedescribed method and with FIG. 2. The rise or
 gradient 35, the level 36, and the level 38 at which the temperature is
 held constant show the time period or step in the inventive method when
 those substances that have not participated in the chemical reaction as a
 result of exposure to UV light have diffused out of the fibre/fibre core.
 According to the method, these steps can be combined or repeated, which has
 taken place with the temperature rise or the temperature change to level
 38 at which diffusion from the core continues at a second constant
 temperature level.
 The pronounced temperature rise, which is marked with the rise 42 and a
 subsequent temperature drop 44, whereafter the temperature is held
 constant for more than forty hours, constitutes the method step in which
 the substances (atoms/molecules) diffuse out of or within the fibre,
 wherewith the written grating having optical properties in accordance with
 the present invention is formed and a chemically stable state with a
 durable and heat-resistant grating is achieved.
 FIG. 7 illustrates how the reflection of different wavelengths can be
 related directly to temperature changes.
 The graph shown in FIG. 8 is an enlarged part of the graph shown in FIG. 6
 (full line), with the graph for the reflectance of the written grating
 inserted in a broken line. The dip 46 shown at the time approximately four
 hours on the abscissa shows how a typical germanium-related grating is
 erased at high temperatures. The graph derives from a grating produced in
 accordance with the present invention and illustrates how the reflectance
 48 then grows and is recreated by spatial/periodic diffusion of the
 substances, to become constant in time despite the high temperature. This
 cannot be achieved with a conventional grating. The scale of the
 reflectance in FIG. 8 is normalized.
 FIG. 9 illustrates with the aid of a graph 50 how the reflectance with a
 percentile scale keeps constant over a period of fifty hours at a
 temperature of around 806-810.degree. C. with a grating produced in
 accordance with the present invention.
 The illustration given in FIG. 10 shows how a light waveguide 52 in an
 unloaded state is fed with a broadband light source in accordance with box
 56. The direction of the light is indicated in the core 18 of the
 waveguide with a hollow arrow. The grating 28 reflects light within a
 narrow band wavelength interval to which the grating is tuned, in
 accordance with the solid arrow in the core 18 for box 58. The original
 light propagates through the grating without the reflected light, as
 illustrated in box 60.
 In FIG. 11, the waveguide 52 in FIG. 10 has been subjected to a load, e.g.
 strain, heating, touch, etc., causing the original, reflected wavelength
 interval in box 58 to be displaced and resulting in the reflection of a
 completely different wavelength interval according to box 64 than was the
 case in FIG. 10, wherewith the light in box 66 is the light that
 propagates through the grating 28 without the light of the reflected
 wavelength.
 The grating shown in FIG. 12 may also be positioned obliquely so that the
 reflected wavelength will be directed and led out of the fibre 52 for
 processing or reading in another optical device.
 It will be understood that the invention is not restricted to the
 aforedescribed and illustrated embodiments, and that the invention is
 limited solely to the contents of the accompanying claims.