Photomechanical Positioning and Stabilization Method and Devices Using Optical Fibers and Feedback

This invention concerns a method for positioning and/or stabilizing movable objects using optical fibers and the photomechanical transducer effect, and to devices that employ the method.

The absorption of thermal energy by a material having a positive thermal expansion coefficient causes the material to expand, which is an important consideration when manufacturing a device. For example, the changes that occur in the physical dimensions of circuit-board components upon heating can stress the board, thereby reducing its efficiency and lifetime. Many published documents refer to this problem. For instance, Williams etal. state that in "a magnetic field sensor . . . the temperature dependence of the Verdet constant and the thermal expansion of the material set a fundamental limit to the precision of the measurement." See "Compensation for Temperature Dependence of Faraday Effect in Diamagnetic Materials: Application to Optical Fibre Sensors," Electronics Letters, 27:1131-1132 (1991).

Electromechanical devices have been used to compensate for the changes that occur as a result of temperature-induced structural changes. For instance, in an electrical-mechanical positioner, a computer instructs a motor controller to move a motor shaft to induce corrections in the position of a movable object. The controller sends an electrical impulse that steps the motor shaft through electromagnetic induction. An encoder returns an electrical signal to the computer which determines the position of the movable object. The computer then decides whether another adjustment is necessary. Electrical-mechanical positioners require using these additional components to monitor and correct the position of the movable object. As a result, compensating for temperature-induced structural changes adds to the cost of manufacturing and the bulk of such devices.

Light transmission through an optical fiber heats the fiber through optical thermal absorption. For fibers having positive thermal expansion coefficients, this causes an increase in fiber length. This effect also has been considered a nuisance, primarily because the change in dimensions is associated with reduced performance for the apparatus in question.

The negative attributes associated with thermal expansion have been well documented. However, until the present invention, no one apparently appreciated that the photomechanical transducer effect (i.e., transducing light energy into mechanical energy, referred to hereinafter as the photomechanical effect) that occurs in optic fibers can be used to accurately and quickly position a movable object, such as movable components of electronic devices that are attached to such fibers, and to stabilize the position of such objects (i.e., compensate for gradual position changes or drifts that occur in such objects).

The present invention exploits the change in fiber length that occurs upon optical heating, and provides a novel photomechanical positioning method and apparatuses that employ the method. A number of materials may be used to form optic fibers useful for the present invention, and the invention is directed to using any such fibers in any application wherein a change in length of the fiber by optical heating is used to bring about a positive result, such as the positioning of a movable object. The selection of an appropriate optic material will best be decided by considering various factors, including the application in question, the ability of the material to transmit light at various wavelengths, the ability to form fibers of suitable dimensions (which will vary from device to device), the expansion coefficients of the material, particularly the linear expansion coefficient, the toxicity of the material and the cost of the material.

Currently, polymer materials are preferred compounds for forming optic fibers for the present invention. In general, polymer materials useful for the present invention may be selected from the group consisting of poly(alkenoic acids), poly(alkenes), poly(vinyl pyrrolidones), poly(vinyl halides), poly(vinyl alcohols), and combinations thereof. Presently preferred compounds are selected from the group consisting of poly(alkyl acrylates), including combinations of different poly(alkyl acrylates). Especially preferred polymers, as presently contemplated for the present invention, are selected from the group consisting of poly(alkyl methacrylates), with poly(methyl methacrylate) being a presently preferred compound. The polymeric material also may comprise a co-polymer of two or more different monomeric subunits. For instance, the polymeric material may comprise a co-polymer of the materials listed above, or a co-polymer of an olefin, such as ethylene, and one of the materials listed above.

The optic fiber also may include a dye material. Typically, the dye material is selected to absorb light having similar wavelengths as emitted by the light source. Preferably, the dye is selected to absorb an appropriate amount of light for performing the task in question, and may be selected to have a maximum light absorption corresponding to the wavelength of light emitted by the light source. Solely by way of example, azo dyes and squarylium dyes have been shown to be useful dyes for the present invention. The polymer material generally, but not necessarily, satisfies either the formula (--R--Y--R'--).sub.n or (--R--Y--Y'--R'--).sub.n. R and R' may be the same or different polymeric materials that contract or expand in response to the transmission of light through the fiber. Y and Y' may be the same or different dye compounds. The determination of n is made by considering, inter alia, whether a compound having a particular n value transmits light and whether it can be formulated into fibers suitable for practicing the method. The resulting mechanical properties of the fibers, such as flexibility and strength, also must be considered. R and R' generally are selected from the groups discussed above. Currently, poly(methyl methacrylate)-doped optic fibers having about 1 weight percent (based on the initial weight of the monomer used to form the polymer) of an azo or squarylium dye are presently preferred polymer materials.

The present invention provides a method for positioning an object. The method comprises providing an optic fiber that changes length in response to the transmission of a light through the fiber. The fiber is operably connected to an object capable of movement in response to a change in the fiber's length. A light source generates a beam of light, which is transmitted through the fiber. This causes the length of the fiber to change, as a result of photothermal heating, from a first length to a second length. The optic fibers typically have a plurality of fiber lengths corresponding to stable states of length between which the length of the fiber may be changed by the application of a force. Each stable state is maintained by photothermal heating of the fiber. The method may comprise selecting a particular stable length position for a particular application by transmitting light through the optic fiber.

The method also may involve the step of providing an optical feedback path for transmitting a portion of the light beam into the end of the fiber opposite the end of the fiber to which the object is attached. By providing feedback, the object may be continuously repositioned, i.e. stabilized, when its position changes in response to the application of an external force.

One specific embodiment of an apparatus that employs the method of the present invention is directed to an interferometer. Using optical feedback, an optic fiber continuously positions the mirror without using motors, encoders or computers. In this specific embodiment, an optical system directs light from a light source into the end of the fiber. The absorption of light heats the fiber. The optical system also provides optical feedback for transmitting a portion of the light into the end of the fiber opposite the end of the fiber to which the mirror is attached. This helps stabilize the position of the mirror when its position is changed in response to the application of an external force, such as air vibrations, or bumping the instrument.

Accordingly, an object of the present invention is to provide a method, and devices employing the method, for continuously or discretely positioning and/or stabilizing a movable object using optic fibers and the photomechanical effect.

An advantage of the present invention is that the optic fiber is capable of making minute adjustments to the position of the movable object, either continuously or discretely, with a fast response time, thereby eliminating the need to use electrical-mechanical positioning devices.

FIG. 1 is a schematic diagram of an interferometer, which is one embodiment of an apparatus that utilizes optic fibers to position a movable object according to the method of the present invention.

FIG. 2 is a side elevational view of the apparatus represented schematically in FIG. 1.

FIG. 3 is an exploded front view of the device illustrated in FIG. 2.

FIG. 4 is an exploded view of a fiber holding apparatus used in the device of FIG. 2.

FIG. 5 is a graph of the change in fiber length that occurs as the intensity of the light output changes from the apparatus of FIG. 2.

FIG. 5A is a graph of the change in fiber length versus intensity for an optic fiber used in the apparatus of FIG. 2.

FIG. 6 is a graph showing the determination of equilibrium points for an optic fiber used in the apparatus of FIG. 2.

FIG. 7 is a graph of the change in output intensity over time for the apparatus of FIG. 2.

FIG. 8 is a graph of the change in output intensity over time for the apparatus of FIG. 2 which illustrates that optic fibers of the present invention can adopt plural lengths corresponding to plural fiber-length equilibrium points.

This invention is directed to a method for positioning movable objects, and stabilizing the position of such objects, using the photomechanical effect. This is accomplished by attaching a light-transmitting fiber to a movable object, directing a beam of light at the object to sense its position, and then using optical feedback through the fiber to change its length through optical heating. A detailed discussion is provided below concerning how to select and make suitable optic fibers, including polymeric materials doped with a dye to increase the absorption efficiency of the polymer, and to impart greater effects on the bulk mechanical properties of the fiber in response to the transmission of light. Thereafter, a discussion is provided concerning how to make and use a specific embodiment of an apparatus that utilizes the fibers and the photomechanical effect to continuously monitor the position of a movable mirror. It should be understood by one skilled in the art that the present invention is not limited to the use of polymeric organic optic fibers, nor to such fibers in the described interferometer.

There are virtually unlimited examples of materials that are suitable for use as optic fibers for positioning movable objects, including both inorganic and organic materials. Moreover, it will be appreciated that the proper selection of such fibers primarily will be determined by considering both the properties of the fiber, as well as the requirements imposed by the particular application of the fibers for stabilizing and/or positioning movable objects. For instance, the wave length and intensity of the light transmitted through the fiber must be considered, as should the cooling rate and ambient conditions surrounding the fiber. Furthermore, it has been determined that the increase in the length of the fiber is proportional to the optical absorption coefficient and the linear thermal expansion coefficient of the fiber, as well as the intensity of the light. And, the change in the length of the fiber is inversely proportional to the mass of the fiber, and the specific heat of the material used to form the material. Thus, one skilled in the art will understand that the selection of a fiber for a particular application is determined by considering these and other factors.

Additional information concerning materials for forming optic fibers that are useful for practicing the present invention may be obtained by referring to, inter alia: Brandrup and Immergut's Polymer Handbook, 3rd Ed. (Wiley-Interscience, New York 1989); Musikant's Optical Materials: Volume 1, (1990); and "Materials for Nonlinear Optics: Chemical Perspectives," American Chemical Society, ACS Symposium Series, Vol. 455, S. R. Marder et al. (1991). Each of these references is hereby incorporated by reference.

Polymer materials currently are preferred compounds for forming optic fibers useful for the present invention. To reiterate, the selection of an appropriate polymeric material will best be decided by considering the factors listed above, as well as the toxicity and cost of the material. However, solely to provide more specific guidance concerning the selection of an appropriate polymeric optic material, polymers of the lower alkenoic acids, such as poly(alkyl acrylates) have been used as suitable optic materials for the present invention. As used herein, the term "lower" describes alkyl or alkenyl straight or branched carbon chains having from about 1 to about 10 carbon atoms, and more generally from about 3 to about 10 carbon atoms. Examples of suitable lower alkenoic acids include acrylic acid, butanoic acid, etc. Suitable materials also include olefinic position isomers, optical isomers, and other derivatives of such materials.

With specific reference to the polyacrylate derivatives, suitable polymeric materials include polymeric materials made from lower alkyl ester derivatives of acrylic acid, such as methyl acrylate, ethyl acrylate, propyl acrylate, etc. Moreover, the lower alkyl derivatives of methacrylic acid also are useful polymeric compounds. These compounds would include poly(methyl methacrylate), poly(ethyl methacrylate), poly(propyl methacrylate), and additional lower alkyl derivatives.

Other examples of suitable light transparent materials include polyethylene, cellulose, Poly(vinylpyrrolidone), poly(vinylhalides), such as poly(vinyl chloride), and poly(vinyl alcohols).

Thus, in general, materials useful for forming optic fibers for the present invention may be selected from the group consisting of poly(alkenoic acids), poly(alkenes), poly(vinyl pyrrolidones), poly(vinyl halides), poly(vinyl alcohols), and combinations thereof. Presently preferred compounds are selected from the group consisting of poly(alkenoic acids), particularly poly(acrylates), including combinations of poly(acrylates). Especially preferred polymers, as presently contemplated for the present invention, are selected from the group consisting of poly(alkyl methacrylates), with poly(methyl methacrylate) being a presently preferred compound.

In general, long, thin materials are most efficient for exploiting the photomechanical effect in optic fibers. This is because, for a given strain, the light-induced change in the length of a material is proportional to its length. The strain is proportional to the light intensity within the material. Optic fibers therefore provide at least two advantages. First, they provide a way of confining light of high intensity. Second, optic fibers can be made both long and thin so that the change in the length of the fiber that accompanies optical heating also can be significant.

The optic fibers of the present invention also may be formed from materials that are doped with a light-absorbing dye. Where polymers are used, the polymeric portion of these optic materials acts as a host for the dye molecules. For certain polymers, the bulk mechanical properties of the polymer change in the presence of light. This change in the bulk mechanical properties may be mediated by using molecular dopants. Typically, the polymer acts as a fairly rigid support for the dye molecules.

U.S. Pat. No. 5,252,494 describes a number of dye materials that potentially are useful for forming optic fibers for the present invention. The disclosure of U.S. Pat. No. 5,252,494 is hereby incorporated by reference. However, solely by way of example, the following dyes are currently considered dyes useful for forming polymer/dye optic fibers.

This example describes the preparation of a poly(methyl methacrylate) (PMAA) optic fiber. Standard test tubes were filled with methyl methacrylate (MMA) monomer, which can be purchased commercially from such companies as Aldrich Chemical Company. A minor portion of PMAA can be added to the MMA to increase the viscosity thereof. A small portion of AIBN, a well-known polymerization initiator, was then added to the composition in the preforms. The composition in the test tube was then allowed to slowly polymerize over a period of several weeks at room temperature. The slow polymerization helped reduce the formation of bubbles in the core region of the fiber. The preform was removed from the test tube and then heated in an oven at about 90.degree. C. to remove any absorbed water.

A fiber drawing apparatus was then used to form long thin fibers. The fiber drawing apparatus included a preform feeding system, a heater and a fiber take-up wheel. This apparatus is shown schematically in Kuzyk et al.'s "Guest Host Polymer Fiber for Nonlinear Optics," Appl. Phys. Lett., pp. 902-904, Vol 59, No. 8 (1991). This reference is hereby incorporated by reference. The drawing apparatus was used to form a PMAA fiber having a length of about 30 cm and a diameter of about 400 .mu.m. The end-face of the polymer fiber was rendered smooth by contacting it with a hot plate.

This example describes the preparation of a poly(methyl methacrylate) (PMAA) optic fiber doped with a squarylium dye material. Standard test tubes were filled with a solution comprising methyl methacrylate (MMA) monomer and about 1 weight percent of a squarylium dye as shown above. A minor portion of PMAA also can be added to the MAA to increase the viscosity of the composition. A small portion of AIBN was then added to the composition, and the composition was allowed to slowly polymerize over a period of several weeks at room temperature. The preform was removed from the test tube and then placed in an oven and baked at about 90.degree. C. to remove any absorbed water.

The fiber drawing apparatus was then used to form long thin fibers, such as fibers having a length of about 30 cm and a diameter of about 400 .mu.m. The end-face of the polymer fiber was then rendered smooth by contacting it with a hot plate.

In a manner similar to that described above for Example 2, additional fibers have been made from PMAA that were doped with from about 0.001 to about 1 weight percent, based on the weight of the MAA monomer, of each of the dyes shown above.

This example describes an alternative process for the production of a dye-doped fiber according to the present invention. Ten milliliters of MMA were placed in a test tube and then mixed with 2 drops of t-butyl peroxide (a polymerization initiator), two drops of a butanethiol (a chain transfer agent), and 35 mg of a squarylium dye as shown above. The mixture was then thoroughly agitated using a mechanical mixer for about 15 minutes, and thereafter filtered through a 0.2 micron syringe filter. The mixture was then placed in a capped test tube and heated in an oven at 95.degree. C. for a period of about one week to polymerize the MMA. The cap of the test tube was then removed, and the polymerized mixture allowed to degas for about a week. The test tube was then broken, the core material was removed and then squeezed between two half-round clamps until closed. Once the half-round clamps were closed, the core material was then squeezed from either end using a rail squeezer at a temperature of about 120.degree. C. The plug was then removed, and the polymer degassed for a week at 95.degree. C. The material was then pulled into a fiber using the fiber drawing apparatus.

FIGS. 1-4 illustrate one embodiment of a photomechanical positioning apparatus 2 that employs optic fibers for positioning a movable object. The device depicted in these figures includes an interferometer. An interferometer is capable of making various measurements, such as the precise determination of wavelengths, spectral fine structure and indices of refraction. The interferometer operates by separating light into two beams using a beam splitter. The light beams are then directed along different optical paths having different path distances using a system of prisms and mirrors, including a fixed mirror and a movable mirror. The light beams are then reunited and interfere with each other. The nature of the interference (i.e., destructive or constructive interference) determines the output intensity of the apparatus.

Interferometers are not new per se. However, the illustrated embodiment of apparatus 2 includes the first known interferometer to use an optic fiber and the photomechanical effect to continuously position a movable mirror as described in more detail below.

With reference to FIG. 1, it can be seen that the operation of apparatus 2 is initiated by a light source 10. An optical isolator 12 also may be used to prevent back reflection from entering the light source, although isolator 12 is not necessary to the function of apparatus 2. When used, optical isolator 12 includes a polarizing beam splitter 14 and a wave plate 16. The apparatus further includes a beam splitter 26 for splitting the light emitted from the light source into a two light beams which travel along different light paths 34A, 34B to a movable mirror 36 and fixed mirror 46. The two light beams reflected by these mirrors 36 and 46 are then recombined at the beam splitter 26. A portion of the combined beam is directed to a detector 100 by mirror 18 (FIG. 1), and another portion is directed along light path 34C to an optic fiber 52 as described above. Fiber 52 is located in an evacuated chamber 48. The upper end of the fiber 52 is fixed by attachment to a fiber holder 54 (See FIG. 4). The other end of the fiber 52 is attached to suspended, movable mirror 36.

More specifically, and with reference to FIGS. 2-4, the apparatus 2 is assembled on a standard optical table 20 that was purchased from the Newport Company. The apparatus 2 generates a light beam using a light source 10, such as a laser. Although any light source likely will work for this invention, an apparatus 2 has been built using a 250 mW continuous krypton laser of conventional type (not shown) which emits light having a wavelength of about 647.1 nm. The light source 10 focuses light on a reflecting prism 22, which is supported by a conventional optic mount 24. For instance, the mount may be an MM-1 mount from the Newport Company. Prism 22 may be attached to the mount 24 using any conventional technique. However, for the illustrated embodiment of apparatus 2, prism 22, and all subsequent prisms, are attached to the holders using a transparent adhesive tape (not shown).

Prism 22 directs the light from light source 10 to a beam splitter 26, which is held securely in place by beam splitter mount 28. Mount 28 includes any structural device capable of holding the beam splitter 26 securely in place, such as a beam-splitter clamp. The illustrated embodiment of the mount 28 was purchased from the Newport Company. The mount 28 may be movably mounted to device 2 so that the position of the beam splitter 26 can be changed. For instance, beam splitter 26 may be attached to a plate that is free to move relative to the device 2. Such a plate is illustrated as plate 30 (FIG. 3) to which mirror 46 is attached. Plate 30 is free to move along rails 32. The position of sliding plate 30 along rails 32 can be fixed by tightening set screws (not shown).

Optical isolator 12 is shown schematically in FIG. 1, but the structure thereof is not illustrated in FIGS. 2-4. An optical isolator, such as isolator 12, may be used to prevent light from being transmitted from beam splitter 26 back into the light beam emitted by the laser 10. However, it has been found that the isolator 12 is not essential for the operation of the apparatus 2, and hence its incorporation into the device 2 is optional. If an optical isolator 12 is desired, it may comprise a polarizing beam splitter 14 and a wave plate, such as a .lambda./4 wave plate 16.

The beam splitter 26 typically is a broad band, 50:50 beam splitter which is designed to operate in the visible wavelengths. Beam splitter 26 is of a conventional design and may be purchased from such companies as Newport Company or Melles Griot. Beam splitter 26 splits the light directed thereto by the prism 22 into two light beams. Fifty percent of the light entering the splitter 26 is reflected and directed along light path 34A, and fifty percent is transmitted and directed along light path 34B.

As can best be seen in FIG. 3, light emitted by beam splitter 26 is directed through a quartz window 38 along a first light path 34A to a hanging, movable mirror 36. Hanging mirror 36 has a diameter of about 0.5 inch, weighs about 9 grams, and was purchased from the Newport Company. The distance light travels along path 34A may vary, but the illustrated embodiment of apparatus 2 had a light path distance between the beam splitter 26 and the movable mirror 36 of about 10 cm. Window 38 is mounted using conventional techniques in an attachment plate 40, which is attached to housing plate 42B using attachment screws 44. Quartz window 38 allows light to enter vacuum chamber 38 while maintaining the vacuum in this chamber. Attachment plate 40 may be sealed against vacuum leaks using conventional means, such as O-rings (not shown).

The second light beam from beam splitter 26 is directed to a fixed mirror 46 along light path 34B. Path 34B had a fixed light path distance of about 10 cm. Thus, the first arm of the interferometer in apparatus 2 is defined by the position of the hanging mirror 36. The second arm is defined by the position of the fixed mirror 46.

Mirror 36 is suspended inside chamber 48 using an optic fiber 52. Chamber 48 is defined by a cylindrical plexiglass housing member 50, which had a thickness of about 3/8 inch. Mirror 36 is attached to a lower end of optic fiber 52. The upper end of fiber 52 is fixed to a fiber holder 54. With reference to FIG. 4, it can be seen that the fiber holder 54 is attached to support ring 56A using attachment screws 58A, 58B. Support ring 56A defines a bore 60 therethrough. Fiber holder 54 also defines a bore 62 therethrough. Bores 60 and 62 are aligned when member 54 is attached to support ring 56A. Bore 62 typically has a diameter of about 0.5 inch. Fiber holder 54 also includes plural set screws 64A, 64B, which are inserted through fiber holder 54 and contact an insert sleeve 66, such as an aluminum insert, having a bore 68 therethrough. When tightened, set screws 64A, 64B hold insert 66 securely in place. Insert 66 is about 1 inch long and has an outer diameter of about 0.5 inch so that the insert 66 can be inserted into bore 62 of fiber holder 54. Insert sleeve 66 also includes an inner flange 70 at its lower end.

A second fiber-contacting insert 72 is dimensioned to be received in the bore 68, which has a diameter of about 0.25 inch. Insert 72 is placed inside insert 66 and contacts flange 70. In this manner, insert 72 can be held inside of insert 66. Although any material likely will work for forming insert 72, the illustrated embodiment of insert 72 was made from brass. Insert 72 is not a closed cylindrical member, as is insert 66. Rather, insert 72 is a split sleeve that has a thin slice of material removed from the cylinder wall to form a groove which runs the entire length of the cylinder in a direction parallel to the longitudinal axis of insert 72. Fiber 52 is inserted into split sleeve member 72, which in turn is placed in insert 66. Set screws 64 are then tightened to hold the inserts 66 and 72 inside the fiber holder 54.

The position of the movable mirror 36 is determined by the length of fiber 52. For this particular embodiment, optic fiber 52 was about 30 centimeters long, and had a radius of about 150 .mu.m. The fiber 52 is attached to the mirror 36 by fiber attachment member 74. The illustrated embodiment of fiber attachment member 74 may be structurally identical to the fiber holder 54 described above, and hence also includes plural inserts and set screws. However, one skilled in the art will realize that attachment member 74 may be any structural mechanism that allows the fiber 52 to be securely attached to the mirror 36.

Attachment member 74 is situated inside the vacuum chamber 48. Plural support rods 80A, 80B and 80C, and plural support rings 56A, 56B, provide a framework for supporting the fiber receiving members 54 and 74. The plural support rods 80A-80C pass through plural orifices in plural support rings 56A, 56B, and are thereafter firmly attached to support plates 84A, 84B. Plates 84A, 84B are attached to housing plates 42A, 42B using any suitable means, such as attachment screws 44. Support rings 56A, 56B are movably attached to the plural support rods 80A-80C using plural set screws 86 as shown in FIG. 4. In this manner, plates 56A, 56B can be moved along support rods 80A-80C to adjust the vertical position of mirror 36. In the illustrated embodiment, support rods 80A-80C were threadedly attached to plates 84a, 84b.

Hanging mirror 36 is suspended from the fiber 52 and can move in a vertical direction parallel to the longitudinal axis of chamber 48 as a result of a change in length of fiber 52. Hanging mirror 36 was secured against lateral motion using plural radial threads 90 as shown in FIG. 2. The tension of these threads was relatively small compared to the tension of the fiber 52. Threads 90 may be made of any suitable material, such as a nylon monofilament. Threads 90 are attached to the lower support ring 56B and the attachment member 74 at attachment points which are oriented at about 120.degree. intervals about the circumference of support ring 56B.

As stated above, beam splitter 26 splits the beam of light emitted by light source 10 into a first and second beam. The second beam emerging from the beam splitter 26 is directed along path 34B towards a fixed mirror 46. Mirror 46 is held securely in place by high precision optic mount 94, which includes fine adjustment screws. Optic mount 94 is movably attached to housing plate 88. Optic mount 94 is illustrated as being attached to plate 30, which is free to move along rails 32.

The first beam of light is transmitted along path 34A from beam splitter 26 to the hanging mirror 36 and is reflected back through the beam splitter 26 by the hanging mirror 36 as shown in FIG. 3. The second beam of light emitted by beam splitter 26 is reflected back through the beam splitter 26 by fixed mirror 46 so that the two beams of light are recombined. About 50 percent of the recombined light then is directed back towards the light source 10, and about 50 percent of the recombined light is directed towards prism 98 and reflected along light path 34C. A small portion of the combined light beam emerging from the beam splitter 26 is directed to a detector 100 (see FIG. 1).

The detector 100 used to make the illustrated embodiment of the apparatus 2 was a silicon photoelectric detector. This detector was interfaced with a 486 IBM compatible digital computer equipped with a LAB-PC board, and LABWINDOWS software. FIG. 1 shows the detector 100 placed to monitor the combined light beam as it emerges from the beam splitter 26. This portion of the combined light is not directed to the fiber 52. Positioning the detector 100 as shown in FIG. 1 does not divert any portion of the combined light intended to be absorbed by the fiber 52 to the detector 100. One skilled in the art will realize that the detector 100 also may be placed at different locations and still have apparatus 2 function properly, although the position of the detector 100 as illustrated in FIG. 1 currently is believed to be the best position for efficient operation of apparatus 2. This is because it is best to put as much of the combined light emerging from beam splitter 26 into fiber 52. If the detector 100 were positioned elsewhere, for instance along light path 34C, then a certain amount of the combined light intended to absorbed by fiber 52 would have to be diverted to the detector 100.

The combined light beam is reflected from the prism 98 along third light path 34C. The combined light beam includes position information concerning the position of the hanging mirror 36. This is because the intensity of the combined light emerging from the beam splitter 26 depends on the amount of destructive and constructive phase interference that is produced when the light beams are recombined. The amount of interference depends upon the distance each light beam travels along light paths 34A, 34B.

Prism 98 is held securely in place by a high-precision optic mount 102, having fine adjustment screws 104. Optic mount 102 is secured to housing plate 88 by conventional means, such as a clamp. Prism 98 directs the combined light beam to a third prism 106. Prism 106 is held securely in place by a high precision optic mount 108, which also includes fine adjustment screws 110. As with optic mount 102, mount 108 is secured to housing plate 88 using a clamp or any conventional means. The combined light beam is directed from third prism 106 through a shutter 112 (see FIG. 1) to a fourth prism 114. Prism 114 is held securely in place using a high-precision optic mount 116, having fine adjustment screws. The shutter 112 used to make the illustrated embodiment of the apparatus 2 was a standard camera shutter, which can be opened continuously, or for a brief period of time. Optical mount 116 is secured to upper housing plate 42A as shown in FIG. 2. The combined light beam from prism 106 to prism 114 may be directed through a shield tube 120 (FIG. 3), although this is not critical to the operation of the device. Shield tube 120 is used to protect the eyes of a person while operating the apparatus 2.

The light beam reflected by prism 114 is then directed through a pin-hole plate 122, which was purchased from Thorlabs, and towards a focusing lens 124. The pin-hole plate 122 has an adjustable aperture that adjusts from a maximum aperture of about 0.5 inch to a minimum aperture of about 1 mm. Plate 122 is threadedly attached to the housing plate 42A. Plate 122 is used to limit the amount of "light noise" that enters fiber 52. In other words, pin-hole plate 122 limits the light entering fiber 52 to the center-fringe portion of the combined light from apparatus 2 that travels along light path 34C.

The focusing lens 124 has a diameter of about 1 inch and a focal length of about 15 cm. These characteristics of lens 124 are not critical to the operation of the apparatus 2 because the position of the lens 124 can be changed relative to the other components of the apparatus 2. Lens 124 is held securely in place using a standard optical clamp 126.

Lens 124 focuses the light onto a fifth prism 128 (FIG. 1). As with the Previous prisms, prism 128 is held in place using a high-precision optic mount 130 having fine adjustment screws. This illustrated embodiment of the apparatus 2 uses a 5-way high precision optic mount 130, model number PB-3 by MRC, as shown in FIG. 2. Optic mount 130 is secured to housing plate 42A using attachment screws (not shown). The position of optic mount 130 can be changed by loosening the screws and moving the mount along a mounting rail (also not shown).

Prism 128 directs the light beam into an upper end of optic fiber 52. About 10-30 mW of the light are absorbed by the fiber 52. It should be noted that the light entering the fiber 52 does not re-enter the apparatus 2. Fiber 52 is located in chamber 48. Chamber 48 generally is evacuated by attaching a vacuum pump to vacuum nozzle 134. Evacuating chamber 48 helps minimize external disturbances that alter the position of the mirror 36. Such disturbances may result, for example, from sound vibrations, thermal gradients and air currents.

Because the position of the mirror 36 is determined by the length of fiber 52, the output light intensity of the interferometer also is determined by the length of the fiber 52. As stated above, the light output of the apparatus 2 is directed through the shutter 112 as shown in FIG. 1, and to the upper end of fiber 52. A portion of the light entering the fiber 52 is partially absorbed by the fiber 52, which in turn causes photothermal heating. If the length of the fiber 52 changes as a result of photothermal heating, then the position of the hanging mirror 36 changes. This alters the output light intensity of apparatus 2. In other words, the output of the apparatus 2 is determined by the length of the fiber 52, and the length of the fiber 52 is determined by the output of the apparatus 2. Thus, the optical system described above provides an optical-feedback system which allows for continuous adjustment in the position of the mirror 36.

The apparatus described above has been used to evaluate the positioning ability of optical fibers which change their length as a result of photothermal heating. In the described embodiment of apparatus 2, heat energy produced by optical heating is used to control the length of the fiber 52. By using feedback, the length of the fiber 52 can be controlled in three ways, namely stabilization, discrete positioning and continuous positioning.

FIG. 5 depicts the change in fiber length that occurred in a light-transmitting PMAA-squarylium dye doped optic fiber as a function of the intensity of the light output from the apparatus 2. FIG. 5 shows that the relationship between the fiber length and the interferometer output light intensity describes a sinusoidal curve. FIG. 5 shows that if the fiber length of the polymeric fiber is L.sub.0 and the light intensity of the fiber is I.sub.0, and if the conditions are such that the slope at point A (L.sub.0 and I.sub.0) is negative, then an external impulse that acts to shorten the fiber length will lead to an increase in the light intensity of the fiber. If the thermal expansion coefficient of the optic fiber is positive, then thermal heating due to such increased light intensity causes the fiber length to increase. This helps to negate the effect of the external impulse.

If the conditions are such that the slope at the point B (L.sub.0 and I.sub.0) is positive, then a material having a negative thermal expansion coefficient also will counteract any external impulse that causes the fiber length to lengthen. Thus, it will be understood that if the apparatus is tuned so that the slope of the curve at particular intensity and length values is in the negative slope domain, then active stabilization of the fiber will require a material having a positive thermal expansion coefficient. Conversely, if the apparatus is tuned so that the slope is positive at the point corresponding to particular intensity and fiber-length values, then active stabilization will require that the material used to make the optic fiber 52 have a negative thermal expansion coefficient.

For any set of conditions provided the apparatus interferometer (e.g., thermal expansion coefficient of the fiber, the length of the fiber, the wavelength of light emitted by the light source, the heat capacity of the fiber, the mass of the fiber, etc.) there are several intensities at which stabilization of the fibers occurs. These points are referred to herein as stable or equilibrium points. The motion of the fiber is described by Equation 1 below:

where .alpha. is the coefficient of thermal expansion, c is the heat capacity of the fiber, .tau..sub.c is the thermal cooling time constant, .lambda. is the wavelength of light, l is the fiber length, l.sub.0 is the initial length of the fiber, m is the mass of the fiber, .phi. is the ambient phase difference between the two arms of the interferometer, P.sub.0 is the maximum laser power input to the fiber at the constructive interference condition of the interferometer, a is the fraction of laser power absorbed by the fiber, and t is time. Using this formula, the stable lengths of the fiber can be determined by setting the velocity of the end of the fiber to zero. This results in the following Equation 2:

The term (l-l.sub.0) is small compared to l.sub.0. As a result, this term can be ignored without incurring significant error in solving Equation 2. The stable points can be determined by adjusting the phase difference so that there is a maximum power input at l=l.sub.0, and by plotting the two sides of Equation 2 as shown in FIG. 6. The left hand side of this equation describes a straight line, whereas the right hand side of the equation defines a cosine-squared function. Each point where the line intersects the cosine-squared curve is a stable point that can be adopted by the fiber. It is important to realize that the determination of the equilibrium points, and hence the intersection points on the cosine-squared curve, depends upon the thermal expansion coefficient of the fiber. More specifically, FIG. 6 represents the situation where the fiber has a positive thermal expansion coefficient because the point of intersection shown on the cosine-squared curve is on the negative slope domain of the curve. However, if the material used to make the optic fiber has a negative thermal expansion coefficient, then the stable points would be determined by the points of intersection on the positive slope regions of the cosine-squared curve.

It has been found that if an optic fiber is tuned to some equilibrium point (L.sub.n, I.sub.n) then an external impulse that causes the fiber length to increase past the minimum of the cosine squared curve will cause the fiber to settle to the new equilibrium point (L.sub.n+1, I.sub.n+1). This is referred to as discrete positioning because there is a change in the length of the fiber from one length to the next, rather than a continuous change in length over the lengths intermediate L.sub.n and L.sub.n+1. However, the change in the length of the fiber that corresponds to each of these equilibrium points differs. For instance, the length of the fiber change that corresponds to a transition from a first to a second equilibrium point is not necessarily the same length change that corresponds to a transition from the second equilibrium point to a third equilibrium point.

There are numerous applications for a device capable of discrete positioning. For instance, discrete positioning could be used to position optics to focus a light beam. Another example would be to use a laser pulse to position a laser beam for cutting materials in a manufacturing process.

FIGS. 7 and 8 present experimental data obtained using apparatus 2. FIG. 7 shows the output intensity as it changes over time. When the shutter of the apparatus 2 is closed (from T=0, to T=about 270 seconds), the drift in the output intensity of the apparatus is apparent. When the shutter is closed, there is no optical feedback because the fiber 52 is not absorbing any of the light output from the apparatus 2. As a result, with the shutter closed there is no correction in the length of the fiber 52 as the output drifts.

At a time t equal to about 270 seconds, the shutter was opened and the apparatus 2 began operating using optical feedback. It can be seen from FIG. 7 that the present invention provides a means for stabilizing the length of the optic fiber within a very short period of time. More specifically, FIG. 7 shows that the fiber length adopted a stable equilibrium point (N=1) in a period of only about 250 milliseconds. Thereafter, the fiber length spontaneously changed from a first stable point N=1, to a second stable point N=2. This transition occurred after a period of about 40 seconds. It is not surprising that the length of the fiber underwent a transition from the first equilibrium point to the second equilibrium point. This is because the first equilibrium point generally is the least stable of all the equilibrium points. After the transition from the first equilibrium point to the second equilibrium point, the fiber length was stable at the second equilibrium point to within plus or minus 2 nm for a period of at least about 30 minutes. It also is important to note that the fiber of the apparatus had a length of about 30 cm, and was able to be stabilized within about plus or minus 2 nm, which translates to about 1 part in 10.sup.8 stability.

Thus, FIG. 7 clearly demonstrates that the apparatus light intensity output, and therefore the fiber's length, can be stabilized using optical feedback and the photomechanical effect. Furthermore, FIG. 7 shows that the optic fiber will adopt an equilibrium length that allows the fiber to maintain the output intensity virtually constant, even over a period of greater than about 30 minutes. By testing the equilibrium points of a PMAA fiber, it has been determined that such fibers adopt a length and an intensity relationship that generally allows the fibers to adopt a stable equilibrium point. The fibers maintain the length corresponding to that equilibrium point until the application of some critical external perturbing force, such as mechanical agitation.

If the fiber is to change its length from one equilibrium point to a higher equilibrium point, then the length of the fiber must increase to a length that is greater than that corresponding to the minimum intensity output of the apparatus, which corresponds to the minimum of the cosine-squared curve. Increasing the fiber length can be accomplished by applying an external force to the fiber. This external force can by virtually any force, such as a mechanical force or an acoustical force. Hence, if the position of the mirror is altered by being jarred, or from air currents, then the length of the fiber may change from a first equilibrium point to a second point, thereby repositioning and stabilizing the movable mirror.

This situation is depicted in FIG. 8, which presents experimental data showing that the fiber can adopt multiple stable lengths. More specifically, FIG. 8 illustrates the change in apparatus output over time. At a time t=0 to about t=5 seconds, the apparatus was operating with the fiber at a first equilibrium length. Thereafter, the apparatus was mechanically agitated, which induced the fiber to undergo a change in fiber length from a first equilibrium point to a second equilibrium point. This mechanical agitation was then repeated, so that the fiber length adopted plural fiber equilibrium lengths. However, after the fiber adopted a sixth equilibrium length, further agitation could not induce a subsequent change in fiber length to a seventh equilibrium point. Thus, FIG. 8 shows that the fiber can adopt plural equilibrium lengths until a fiber length is obtained which resists perturbations that tend to change the position of the mirror At the sixth stable length, further mechanical agitation could not induce a subsequent change in the length of the fiber. At t=about 40 seconds, the shutter was closed, and the drift in the apparatus output is apparent.

Alternatively, the thermal energy accompanying the transmission of a light beam can be used to induce a change in the length of the fiber from a first equilibrium point to a second equilibrium point. This provides a means for positioning a movable object at discrete positions, such as moving a focusing lens from a first position to a second position by applying an external force sufficient to cause a transition from a first equilibrium point to a second equilibrium point.

An estimate of the relative stabilities of each equilibrium point can be determined from the critical distance. This is because the two major factors influencing the stability of such points are the critical distance and the slope of the cosine-squared curve. A large critical distance will allow larger perturbations to occur without causing the fiber to progress to another stable point. Moreover, the greater the slope of the cosine-squared curve, the greater will be the stabilization effect because of the sensitivity of the apparatus to changes in the length of the fiber. By plotting the critical distance versus the slope of the cosine-squared curve, it has been determined that the least stable of the equilibrium points is the first equilibrium point. The most stable point corresponds to N/2. Thus, for the N=10 line of FIG. 6, N=5 or N=6 are the most stable equilibrium points for the fiber.

It also is apparent that the amount of time it takes a fiber to adopt an equilibrium point (response time) will vary. The main parameters that determine the response time for a particular fiber are the radius of the fiber, the absorbance of the fiber, and the power of the light source. For PMAA fibers, it has been determined that thinner fibers will have a faster response time. More specifically, for a PMAA fiber having a radius of about 40 .mu.m, the response time is on the order of about 8 ms.

FIG. 6 also indicates that the length of the fiber can be continuously changed, as opposed to undergoing discrete fiber length changes, by using a variable power source. One means of accomplishing this would be to vary the output intensity of the light source. For example, assume that a device is tuned so that the fiber length corresponds to the N=6 equilibrium point. By increasing the laser power, the length of the fiber can be changed continuously from the point corresponding to N=6 on the line having a slope with six equilibrium points, to the N=6 point on the line having a slope with ten equilibrium points.

The intensity of the light leaving the interferometer is given by Equation 3 with the shutter closed:

where .tau. is a time constant. The cooling constant for the fibers can be determined by plotting ln(COS.sup.-1 (l l.sub.max).sup.1/2) versus time. It has been determined that the poly(methyl methacrylate) fibers described above have cooling constants on the order of about 2 seconds, although this depends to a large extent on the extent of the vacuum in the chamber 48.

The previous paragraphs presented a detailed description concerning one embodiment of an apparatus that utilizes the method associated with positioning and stabilizing movable objects using optic fibers and the photomechanical effect. However, the scope of this invention is not limited to this particular application, as will be readily appreciated by those skilled in the art. For instance, instead of the Michelson-Morley type interferometer described above, the method could be used with a Fabry-Perot interferometer. The invention also could be used in an optical-speaker arrangement.

The present invention has been described with reference to preferred embodiments. However, it should be understood that the invention can be modified without departing from such principles. We claim as our invention all such modifications as fall within the scope of the following claims.