Composite form as a component for a pressure transducer

A pressure transducer using as a component for changing shape in response to a change in the pressure being measured a composite elongated body, with specially arranged reinforcing fibers. Such a pressure transducer can use various means for sensing the change in shape of the elongated body, including an optical fiber affixed to the elongated body so as to itself change in length in response to a change in pressure, and having a Bragg grating as part of the optical fiber, with the Bragg grating positioned and arranged so as to convey, in response to an optical signal, information about the change in shape of the elongated body. The elongated body is provided with at least one pair of contra-helically wound reinforcing fibers, which may even be bi-axially braided, and are wound either to amplify the effect of pressure acting on the elongated body or to insulate the elongated body from the effects of pressure and other sources of stress. The reinforcing fibers are embedded in a resin, and in some applications more than one pair of contra-helically wound reinforcing fibers are used, so that the composite body consists of more than one layer of reinforcing fibers. The resulting elongated body is a non-isotropic material and can be designed to have a Poisson's ratio significantly greater than 1/2, the maximum possible Poisson's ratio for an isotropic material, providing an enhanced axial strain accompanying any radial or circumferential strain. The elongated body can also be designed to have a small Poisson's ratio by using an inner layer and outer layer of contra-helical windings where the reinforcing fibers of the two layers intersect at different angles.

FIELD OF THE INVENTION
 The present invention pertains to the measurement of pressure, including
 both hydrostatic pressure and acoustic pressure, and in particular to a
 composite structure as a component of a pressure transducer for changing
 shape in response to pressure.
 BACKGROUND OF THE INVENTION
 As is well known in the art of pressure measurement, a pressure transducer
 typically consists of two general components: a component that responds
 mechanically to a change in pressure, i.e. by e.g. changing shape, and a
 component that senses the mechanical response of the other component and
 provides a signal that can be correlated with the mechanical response,
 i.e. a strain sensor.
 For measuring pressure, such as pressure in a fluid, the mechanically
 responding component is often a cylindrical body. One way of measuring
 pressure is to sense how a cylindrical body will lengthen, in what is
 called Poisson's effect, in response to pressure imposing a radial stress
 on the body leading to circumferential stress, also called hoop stress. In
 Poisson's effect, when pressure, such as fluid pressure, squeezes radially
 on a cylindrical body, the body tends to lengthen as it thins, i.e. it
 experiences axial strain as well as circumferential strain.
 The prior art also teaches that it is also useful to sense how a
 cylindrical body thins, instead of how it lengthens, in response to an
 increase in pressure acting on the cylindrical walls of the cylindrical
 body. Sensing either aspect of the mechanical response, either the
 lengthening or thinning response to an increase in pressure, can be done
 by the second component of a pressure transducer.
 To measure axial strain of a cylindrical body exposed to some pressure, a
 fiber optic, having a Bragg grating, is often arranged along the length of
 the body and attached so as to lengthen along with the body. A Bragg
 grating is created over a length of a fiber optic by exposing segments
 along the length to different light in the ultraviolet range causing
 different indices of refraction. The axial strain is then detected by
 interferometry, i.e. when light is passed through the fiber, the Bragg
 grating causes an interference pattern that depends on the length over
 which the Bragg grating extends; when the length changes, as a result for
 example of fluid pressure and Poisson's effect, the pattern changes and
 does so in a way that allows the change in length to be determined, which
 can then be correlated with pressure that caused the change in length.
 An alternate method of using an optical fiber Bragg grating as one
 component of a pressure transducer to sense how a cylindrical body,
 serving as the other component, strains axially in response to a change in
 pressure is to create a Bragg grating on either end of a length of optical
 fiber lengthening as a result of Poisson's effect. This method has a far
 greater sensitivity than the single Bragg grating approach, because a
 greater length of fiber is strained yielding a greater overall change in
 length.
 When a body deforms in a way that exhibits Poisson's effect, so that strain
 parallel to the applied stress is accompanied by strain orthogonal to that
 stress (e.g. squeezing circumferentially on a cylindrical body not only
 makes it thinner but also makes it longer), the ratio of the orthogonal
 strain to the parallel strain is known as Poisson's ratio and is an
 indicator of the magnitude of Poisson's effect for the particular material
 or structure composing the body.
 The use, as a temperature sensor, of an optical fiber with a Bragg grating
 is not new, and such use includes embedding an optical fiber sensor in a
 plurality of layers of resin reinforced by (non-optical) fibers. U.S. Pat.
 No. 5,399,854 to Dunphy et al. teaches embedding an optical fiber sensor
 in a plurality of layers of fiber-reinforced resin, each layer having a
 different thermal expansion coefficient, and the reinforcing fibers of
 each layer oriented to cause transverse stresses on the embedded optical
 fiber different from those caused by the other layers, the difference
 depending on the temperature being sensed. The unequal transverse stresses
 cause birefringence in the grating, which can be correlated with the
 temperature being sensed.
 In contrast, according to the prior art, in order to make a pressure sensor
 sufficiently sensitive, bare (i.e. unsheathed) optical fiber is often
 used, so that the optical fiber having a Bragg grating is exposed to the
 full pressure, undiminished by any sheathing. But bare optical fibers are
 susceptible to abrasion and chemical attack, so that in some applications,
 using ensheathed optical fibers is not practical.
 Sometimes in the prior art, bare optical fibers are sheathed in a fine
 diameter steel capillary tube filled with fluid to protect against
 chemical attack and abrasion. However, such a sheathing reduces the
 sensitivity of the optical fiber. Furthermore, steel tubing has a
 different coefficient of thermal expansion than optical fiber material,
 and this difference creates thermal-based axial strains that compound the
 pressure measurement. If one could assume that the optical fiber would
 expand with the steel capillary, one could subtract out the effect of the
 thermal strains. However, the optical fiber can slip within the metal
 capillary, so the thermally induced strains are difficult to predict and
 thus distinguish from pressure induced strain.
 In other prior art, an optical fiber is made more sensitive to pressure by
 encapsulating or jacketing the optical fiber in a soft polymer having a
 relatively low bulk modulus of elasticity and a relatively high Young's
 modulus, and using a jacket outer diameter as large as 2000 microns on an
 optical fiber with a diameter of 125 microns. A disadvantage of these
 polymer coatings is their very high sensitivity to temperature changes due
 to the very large coefficient of thermal expansion of these polymers.
 Changes in temperature cause very large expansions of the polymer
 coatings; these expansions strain the optical fiber and the Bragg grating
 giving a false indication of a pressure change, and sometimes damaging the
 optical fiber.
 Even the bare optical fiber itself will respond to temperature changes by
 undergoing thermal expansion or contraction in both length and diameter,
 but these changes in dimension can be compensated for by using a second
 grating that is not exposed to the pressure. However, with a bare optical
 fiber, even a flowing of fluid over the optical fiber can, through shear
 stresses, impart axial stresses that interfere in the pressure
 measurement.
 What is needed is a mechanical form, for use as the mechanical component of
 a pressure transducer, that will not itself experience significant thermal
 strains, but will exhibit a pronounced Poisson's effect when exposed to a
 change in pressure acting on the mechanical form, and so exhibit
 significant axial and longitudinal strains. When used with an optical
 fiber having a Bragg grating as the second component of a pressure
 transducer, the mechanical form should not reduce the sensitivity of the
 optical fiber to the pressure being measured, even if it ensheathes the
 optical fiber and so protects the optical fiber against abrasion and
 chemical attack.
 In some applications, a cylindrical body used as the mechanical component
 of a pressure sensor can extend over a distance spanning regions where
 sensitivity to pressure is not wanted, and other regions where it is.
 Because of this, an even more advantageous mechanical form would allow
 varying sensitivity to pressure along its length, so that it is more
 sensitive to pressure along some spans, and substantially insensitive
 along other spans.
 Another important advantage would be for the mechanical form to be
 producible in a continuous batch process, so that there would be no break
 between lengths of the form intended to exhibit different sensitivities to
 pressure. In other words, ideally, the manufacturing process would
 produce, as the mechanical form, a continuous material, although differing
 in its construction in different spans, according to the level of response
 to pressure wanted by the different spans.
 SUMMARY OF THE INVENTION
 In order to provide the above features, the present invention provides, as
 the mechanical component of a pressure transducer, a mechanical form that
 is an elongated body including a layer of contra-helically-wound
 reinforcing fibers, which may be bi-axially braided, arranged along the
 elongated body, the mechanical form for providing a change in shape in
 response to a change in pressure. Such a mechanical form is also referred
 to as a composite form, because it is made from both resin material as
 well as reinforcing fibers, which are a different material from the resin.
 A pressure transducer based on a mechanical form according to the present
 invention would also include a means of sensing the change in shape of the
 mechanical form and providing a signal based on the change in shape, i.e.
 a strain sensor.
 In one aspect of the invention, the means of sensing the change in shape
 (strain sensor) is based on an optical fiber affixed to the elongated body
 so as to change in length in proportion to the change in shape of at least
 a portion of the elongated body, where the optical fiber has a Bragg
 grating as part of the optical fiber. Such an optical fiber is disposed
 either lengthwise along the elongated body, or is spirally wrapped over at
 least one layer of the elongated body.
 The reinforcing fibers are embedded in a resin system, usually based on an
 elastomeric material having a low to moderate Young's modulus. In some
 embodiments, there is a second layer of contra-helically-wound reinforcing
 fibers, which may be bi-axially braided. In such embodiments, along spans
 of the elongated body intended to be sensitive to pressure, the two layers
 of reinforcing fibers are arranged so as to have a similar scissor action
 in response to pressure; along spans where the elongated body is intended
 to be insensitive, the two layers are arranged to have different scissor
 actions, a situation in which a change in length of the elongated body is
 inhibited.
 To counter thermal stress from interfering with a pressure measurement by a
 pressure transducer using an optical fiber and a mechanical form according
 to the present invention, the mechanical form uses an elastomeric
 material, for its resin, having a coefficient of thermal expansion
 substantially similar to the coefficient of thermal expansion for the
 optical fiber.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring now to FIG. 1, a mechanical form 10, according to the preferred
 embodiment, for use as a component of a pressure transducer, is shown
 having an outer annular layer of contra-helically wound and braided
 reinforcing fibers 12 on top of another, inner annular layer of
 contra-helically wound and braided reinforcing fibers 13, with both layers
 encapsulated by a resin 17 filling interstitial spaces between a core
 region 20 and a form covering 11, and also wetting the fibers 12 and 13 of
 the two layers. The two layers are distinguished by the dashed line 18.
 The resin 17 is a polymer having a low to moderate Young's modulus of
 elasticity, i.e. from approximately 0.5-50.0 ksi. Depending on how the
 reinforcing fibers 12 and 13 are arranged, the mechanical form 10 deforms
 more or less in response to pressure acting on it, as explained below;
 along some spans of the mechanical form, the reinforcing fibers are
 arranged so that the mechanical form is substantially insensitive to
 changes in pressure acting on it, and along other spans, the reinforcing
 fibers are arranged so that the mechanical form is especially sensitive to
 changes in pressure, i.e. it is made to deform in a particular way, for
 example by lengthening, more than an isotropic material would deform in
 that particular way, as explained below.
 In some embodiment, as shown in FIGS. 5a and 5b, a pressure transducer
 based on the mechanical form 10 includes in the core region 20, as the
 component that senses the change in shape of the mechanical form 10, i.e.
 the strain sensor component, at least one optical fiber 15 having a fiber
 buffer coating 14 and having at least one Bragg grating. In such an
 embodiment, in a span of the mechanical form that is made sensitive to
 pressure, as the pressure increases, the mechanical form will, preferably,
 lengthen, and the optical fibers within the core region 20 will stretch
 correspondingly, since they are coupled to the mechanical form as
 explained below. Then, when a light signal is passed down the optical
 fibers so stretched, its reflection from the Bragg grating ingrained in
 the optical fiber conveys the strain of the optical fiber, which is
 correlated to the strain of the mechanical form, which is correlated with
 the pressure being measured.
 In other embodiments, as described below, the component for sensing the
 change in shape of the mechanical form 10, of which an optical fiber
 having a Bragg grating is just an example, is not necessarily located in
 the core region 20. In those embodiments, the core region 20 is
 advantageously filled with a silicon gel, or some other compliant
 material. In other embodiments, a thin-walled, compliant, air-backed
 cylinder may be located in the core region.
 Referring now to FIG. 2, a mechanical form 10 is shown in the region where
 it is intended to be sensitive to fluid pressure. In this embodiment, the
 mechanical form 10 again includes two layers of contra-helically wound
 reinforcing fibers 12 and 13, but the reinforcing fibers 12 and 13 are not
 braided. The cutaway view shows the inner layer and outer layer. The
 reinforcing fibers 12 of the outer layer are arranged to have two
 different axes, i.e. to lie along two different directions. The
 reinforcing fibers in one direction make an angle 2.theta. relative to the
 reinforcing fibers in the other direction, or an angle .theta. with
 respect to the bisector 19 of the total angle 2.theta. between any two
 non-parallel reinforcing fibers. The angle .theta. of a layer is here
 called the polar angle of that layer. In the case of a layer that is
 braided (and so still contra-helically wound), there is also a polar angle
 according to the same description. Preferably, the mechanical form is
 constructed out of layers of reinforcing fibers with polar angles ranging
 from approximately 5.degree. to 85.degree..
 When the reinforcing fibers 12 of the outer layer are wound to have the
 same polar angle as the reinforcing fibers 13 of the inner layer, the two
 layers will tend to scissor at a single rate, under the influence of a
 circumferential stress, and so elongate the encapsulated optical fibers.
 Referring now to FIG. 4, this scissoring of reinforcing fibers 16
 amplifies Poisson's effect; as a layer is squeezed to a smaller radius, it
 elongates axially. To arrange for highest sensitivity, the polar angle
 .theta. for both the first and second layer should be close to 30.degree..
 If the two layers are wound to have different polar angles, however, there
 will be little response to pressure acting on the mechanical form. As
 described below, since the rate of elongation depends on the polar angle,
 when the two layers are wound with significantly different polar angles,
 preferably polar angles that differ by as much as 30.degree., the two
 layers will tend to elongate at significantly different rates, but because
 the two layers are coupled through the resin 17 (see FIG. 1), elongation
 of the two layers at different rates is made difficult, so the mechanical
 form tends not to elongate at all.
 Without the reinforcing fibers arranged as described to either amplify or
 reduce Poisson's effect, the mechanical form is essentially an isotropic
 material and can have at best, for purposes of sensing pressure, a
 Poisson's ratio of approximately 0.20. An optimum polymer material for a
 mechanical form will at best have a Poisson's ratio of 0.50; thus the
 actual strain can only be one-half the radial or circumferential strain.
 The contra-helically wound mechanical form of the present invention is
 non-isotropic and it is possible for it to have a Poisson's ratio of
 greater than one-half, and in fact as much as one or two.
 In other words, it is possible to provide a mechanical form that will
 experience up to twice as much axial as radial or circumferential strain
 as a result of pressure. The axial strain can then be more readily
 detected by, for example, interferometry using one or more Bragg gratings
 in an optical fiber to correlate changes in interference patterns with a
 change in length of the optical fiber affixed to the mechanical form to
 follow a change in shape of the mechanical form.
 Referring now to FIG. 3, a span of a mechanical form 10 intended to be
 insensitive to fluid pressure has an outer layer of contra-helically wound
 reinforcing fibers 12 and an inner layer of contra-helically wound
 reinforcing fibers 13, where the two layers have different polar angles.
 Preferably, in such a pressure insensitive span of the mechanical form,
 the polar angles of the two layers both differ from the single polar angle
 in a sensitive span, but by opposite amounts. In other words, if where the
 mechanical form intended to be sensitive has a polar angle of 30.degree.
 for both the inner and outer layers, then in the region where the optical
 fiber is intended to be insensitive, the outer layer is preferably wound
 with a polar angle of 30.degree.+10.degree.=40.degree., and the inner
 layer is preferably wound with a polar angle of
 30.degree.-10.degree.=20.degree..
 The basis for the control of Poisson's ratio for the mechanical form is
 that the polar angle controls the rate of axial strain per unit
 circumferential strain, because the reinforcing fibers in the
 contra-helical winding stretch very little themselves, and the elongation
 of the mechanical form results purely from scissoring of the
 contra-helical windings. The rate at which a contra-helical winding
 scissors depends on the polar angle, a larger polar angle providing a
 larger rate of elongation per change in polar angle. FIG. 4 shows two
 reinforcing fibers 16 of a contra-helical winding closing under the
 influence of a radial stress F. As the contra-helical winding scissors,
 the polar angle q decreases, and it is easy to show that the rate of
 change of the length l of the reinforcing fiber with change in q is given
 by
 dl/d.theta.=-h.multidot.sin (.theta.),
 where h is the hypotenuse formed by the reinforcing fiber used in a
 contra-helical winding; the reinforcing fiber itself not appreciably
 deforming (lengthening) in this scissoring. This result shows that in the
 case of inner and outer layers having different polar angles, the
 scissoring will be at different rates, and therefore each of the two
 layers will tend to resist scissoring by the other.
 Using a polymer for the resin 17 (see FIG. 1) can create problems stemming
 from thermal expansion. Most polymers used for a resin system have an
 inherently high and undesirable coefficient of thermal expansion (CTE).
 Thermal expansion of the resin can be controlled somewhat by using polar
 angles in some particular ranges. A polar angle of 20.degree. will
 provide, even for the overall structure of a pressure transducer using
 optical fibers, near-zero thermal expansion in the axial (lengthwise)
 direction of the mechanical form, while a polar angle of 30.degree. will
 provide, overall for the same kind of pressure transducer, a large
 negative CTE for expansion in the axial (lengthwise) direction of the
 mechanical form. The potentially different thermal and mechanical
 properties of all the materials of the pressure transducer must be
 considered in arranging for a desired strain response.
 Referring now to FIGS. 5a and 5b, in one application a pressure transducer
 22 based on a mechanical form 10 according to the present invention has a
 bundle of optical fibers 15 each having a fiber coating 14, all ensheathed
 by a low to moderate Young's modulus resin 17 reinforced by two layers of
 contra-helically wound reinforcing fibers 12 and 13 (see FIGS. 2 and 3)
 all surrounded by a form covering 11. The outer-lying of the reinforcing
 fibers 12 are placed in intimate contact with the inner lying reinforcing
 fibers 13 through their coatings 14 of the optical fibers 15. The
 reinforcing (non-optical) fibers 12 and 13 can be, for example, E-glass
 fibers. In the preferred embodiment, the reinforcing fibers occupy
 approximately 50% of the volume between the buffer coatings 14 and the
 form covering 11; the rest is filled with resin 17. Such a pressure
 transducer is useful for measuring pressure at different places using a
 single mechanical form, responding to pressure differently along different
 spans by virtue of how its reinforcing fibers 12 and 13 are arranged, as
 described above. The ensheathed optical fibers would each have one or more
 Bragg gratings arranged to provide information about the response of the
 mechanical form 10 at different spans along the mechanical form. For
 example, in case of two ensheathed optical fibers, each might have a pair
 of Bragg gratings bracketing a span of the mechanical form located in a
 region where the pressure is to be measured, or might have a single Bragg
 grating extending over such a span. Alternatively, in a multiplexing
 application, a single optical fiber has a series of Bragg gratings to
 convey information, by reflecting and altering a portion of an optical
 signal, about the pressure response of the mechanical form at various
 spans along the mechanical form.
 Referring now to FIGS. 6a and 6b, in another application a pressure
 transducer 23 based on a mechanical form 10 according to the present
 invention has an optical fiber 15, having a fiber coating 14, overwrapping
 the mechanical form 10, where the mechanical form 10 is again constructed
 from a low to moderate Young's modulus resin 17 reinforced by two layers
 of contra-helically wound reinforcing fibers 12 and 13 (see FIGS. 2 and
 3), and has a form covering 11, but does not ensheathe optical fibers and
 instead has a core region 20; the core region should be any material or
 construction that will not inhibit the desired response of the mechanical
 form 10. Thus, for example, along a span where the mechanical form is
 intended to be sensitive to pressure, the core region might be even simply
 a void (air-filled), or may be a silicone gel, which would comply with any
 tendency of the mechanical form to elongate. Even in areas where the
 mechanical form is intended to be insensitive to pressure, the core region
 can be simply a void or filled with a silicone gel, but if the pressure to
 be resisted is high, the core region is advantageously filled with a
 material that tends not to strain circumferentially. In this application,
 the reinforcing fibers occupy approximately 50% of the volume between the
 core region and the form covering 11; the rest is filled with resin 17.
 The optical fiber in this application preferably includes a series of
 Bragg gratings to provide, in a multiplexed signal, information about the
 pressure response of the mechanical form 10 at a series of span locations
 along the mechanical form.
 In constructing a pressure transducer based on a mechanical form according
 to the present invention, and using ensheathed optical fibers each having
 a Bragg grating as a means for sensing a change in shape of the mechanical
 form (see again FIGS. 5a and 5b), first the coating 14 is checked to
 ensure it is approximately 400 microns deep, and if it is less, some
 additional coating material, usually silicone, is added to build up the
 coating 14 on each optical fiber 15, until the total thickness of the
 coating amounts to approximately 400 microns. Then the first and second
 layers of reinforcing fibers are arranged over the buffer coatings 14, and
 in the preferred embodiment, these layers are both bi-axially braided,
 with the outer layer immediately on top of the inner layer. Next, the
 assembly is shrink-wrapped in a skin of some suitable material, and the
 resin 17 is caused to impregnate, under vacuum and pressure, the space
 between the buffer coatings 14 and the form covering 11. The resin moves
 between the reinforcing fibers because of the applied pressure, and
 penetrates or wets the reinforcing fibers as a combined result of the
 applied pressure and capillary forces. Finally, the shrink-wrap is
 removed. The resin can also be caused to impregnate the space between the
 buffer coatings 14 and the form covering 11 by other means, such, for
 example, pultrusion.
 In applications where a pressure transducer is exposed to high
 temperatures, the form covering 11 is advantageously a high temperature
 polymer, such as the high temperature polymers disclosed in co-pending
 U.S. patent application entitled, "Mandrel-Wound Fiber Optic Pressure
 Sensor," (WFVA/CiDRA attorney docket no. 712-2.40/CC-0067) filed on the
 same date as this application and hereby incorporated by reference. Thus,
 for example, it is preferably a polytetrafluoroethylene (PTFE),
 commercially available for example as TEFLON.RTM. from E.I. DuPont
 deNemours & Company.
 In applications where a pressure transducer based on a mechanical form
 according to the present invention has an optical fiber overwrapping at
 least a portion of the form, as in FIGS. 6a and 6b, an additional buffer
 coating (not shown) is advantageously provided. Such a buffer coating is
 based on the same high temperature polymers as the form covering 11. In
 such an application, the form covering 11 can be eliminated.
 A person skilled in the art would appreciate how the optic fiber Bragg
 grating sensors are used as sensor elements. Gratings such as those
 described in U.S. Pat. No. 4,725,110, entitled "Method for Impressing
 Gratings Within Fiber Optics", to Glenn et al may be used in the present
 invention. See also U.S. Pat. No. 4,950,883 to Glenn for a "Fiber Optic
 Sensor Arrangement Having Reflective Gratings Responsive to Particular
 Wavelengths."0 However, any wavelength tunable grating or reflective
 element embedded in an optical fiber may be used if desired.
 As described therein, a data acquisition unit has a broadband light source
 or laser diode with suitable photo optic couplers. Demodulators and
 filtering equipment can be used to monitor the shift in the wavelength of
 light reflected by a Bragg grating caused by strain undergone by the Bragg
 grating. When a Bragg grating is illuminated, it reflects a narrow band of
 light at a specified wavelength. A measurand, such as strain induced by
 pressure or temperature, will cause a change in the Bragg grating spacing,
 shifting the wavelength of the light it reflects. The value of the
 measurand is directly related to the shift in the wavelength of the light
 reflected by the Bragg grating. If more than one Bragg grating is used,
 wave division multiplexing techniques can be used to sense the shifts in
 wavelength of the light reflected from each individual Bragg grating. A
 readout device can be positioned so that a continuous reading of strain
 can be provided.
 As is well known in the art, there are various optical signal analysis
 approaches that may be utilized to analyze return signals from Bragg
 gratings, such as are disclosed in U.S. Pat. Nos. 4,996,419; 5,361,130;
 5,401,956; 5,426,297; and/or 5,493,390, all of which are hereby
 incorporated by reference. These approaches may be generally categorized
 as follows:
 a) direct spectroscopy utilizing conventional dispersive elements, such as
 line gratings or prisms, and a linear array of photodetector elements;
 b) passive optical filtering with a device having a wavelength-dependent
 transfer function;
 c) tracking using a tuneable filter such as, for example, a scanning
 Fabry-Perot filter, an acousto-optic filter such as the filter described
 in the above referenced U.S. Pat. No. 5,493,390, or fiber Bragg grating
 based filters; and
 d) interferometric detection.
 The particular technique used will depend on the Bragg wavelength shift,
 which in turn depends on the sensor design, and will also depend on the
 frequency range of the measurand to be detected. The scope of the
 invention is not intended to be limited to any particular optical signal
 analysis approach.
 Alternatively, a portion or all of an optical fiber between a pair of
 gratings (and at the gratings, if desired) may be doped with a rare earth
 dopant (such as erbium) to create a tunable fiber laser, such as is
 described in U.S. Pat. No. 5,317,576, "Continuously Tunable Single Mode
 Rare-Earth Doped Laser Arrangement", to Ball et al or U.S. Pat. No.
 5,513,913, "Active Multipoint Fiber Laser Sensor", to Ball et al, or U.S.
 Pat. No. 5,564,832, "Birefringent Active Fiber Laser Sensor", to Ball et
 al, which are incorporated herein by reference.
 In applications involving a series of pressure transducers disposed along a
 single optical fiber, the various strain sensors, each serving as a
 component of a different pressure transducer in the series of pressure
 transducers, may be multiplexed along the single optical fiber using
 wavelength division multiplexing (WDM), time division multiplexing (TDM),
 or other multiplexing techniques.
 The strain sensors may be configured using any type of optical
 grating-based measurement technique, e.g., scanning interferometric,
 scanning Fabry-Perot, acousto-optic tuned filter, optical filter, time of
 flight, etc. having sufficient sensitivity to measure the changes in the
 circumference of the pipe, such as that described in one or more of the
 following references: A. Kersey et al., "Multiplexed fiber Bragg grating
 strain-sensor system with a Fabry-Perot wavelength filter", Opt. Letters,
 Vol 18, No. 16, August 1993, U.S. Pat. No. 5,493,390, issued Feb. 20, 1996
 to Mauro Verasi, et al., U.S. Pat. No. 5,317,576, issued May 31, 1994, to
 Ball et al., U.S. Pat. No. 5,564,832, issued Oct. 15, 1996 to Ball et al.,
 U.S. Pat. No. 5,513,913, issued May 7, 1996, to Ball et al., U.S. Pat. No.
 5,426,297, issued Jun. 20, 1995, to Dunphy et al., U.S. Pat. No.
 5,401,956, issued Mar. 28, 1995 to Dunphy et al., U.S. Pat. No. 4,950,883,
 issued Aug. 21, 1990 to Glenn, U.S. Pat. No. 4,996,419, issued Feb. 26,
 1991 to Morey, all which are hereby incorporated herein by reference in
 their entirety.
 In case of wrapping a mechanical form according to the present invention
 with an optical fiber without using Bragg gratings, known interferometric
 techniques may be used to determine the length or change in length of the
 optical fiber around the mechanical form due to pressure, such as Mach
 Zehnder or Michaelson Interferometric techniques, as described in U.S.
 Pat. No. 5,218,197, entitled "Method and Apparatus for the Non-Invasive
 Measurement of Pressure Inside Pipes Using a Fiber Optic Interferometer
 Sensor" to Carroll. Interferometric sensors may be multiplexed as
 described in Dandridge, et al, "Fiber Optic Sensors for Navy
 Applications", IEEE, February 1991, or Dandridge, et al, "Multiplexed
 Interferometric Fiber Sensor Arrays", SPIE, Vol. 1586, 1991, pp176-183.
 Other techniques to determine the change in fiber length may be used.
 It is also possible to wrap an optical fiber around only a portion of the
 mechanical form in order to sense a change in circumference of the
 mechanical form because of a change in pressure, provided the length of
 optical fiber is long enough to optically detect changes to the
 circumference. Also, when a single grating is used per pressure
 transducer, the grating would be attached to the mechanical form, and the
 reflection wavelength of the grating would shift with changes in
 circumference of the mechanical form. When a pair of gratings is used per
 sensor, known Fabry-Perot, interferometric, time of flight or fiber laser
 sensing techniques may be used to measure the fiber length or change in
 fiber length due to a change in circumference, in a manner such as that
 described in the aforementioned references.
 As has been described above, a pressure transducer according to the present
 invention can use any kind of strain sensor in combination with the here
 disclosed mechanical form. For example, besides a strain sensor based on
 optical signal processing as the other component of a pressure transducer,
 a piezoelectric strain sensor could be used to sense the change in shape
 of the mechanical form. Such a strain sensor could then be arranged to
 provide either an electrical or optical signal, and such signals could be
 multiplexed in various ways known in the art, to allow for a series of
 pressure transducers arranged over a single optical fiber. Also, the
 present invention could be used in any application, including a harsh
 environment such as an oil or gas well.
 It is to be understood that the above-described arrangements are only
 illustrative of the application of the principles of the present
 invention. In particular, it is obvious that a single layer of
 contra-helically wound reinforcing fibers could be used in place of a
 two-layer sheath; then the polar angle could be made larger for greater
 pressure sensitivity, and smaller for less sensitivity. In addition, it is
 possible to practice the present invention using braid structures that are
 other than bi-axial. All that is necessary is to employ, over a span of
 the pressure transducer intended to be sensitive to pressure, a form of
 braid structure or winding, reinforcing the resin system, that imparts to
 the composite sheath a low to moderate Young's modulus in the hoop
 direction. The choice of resin in which to encase the reinforcing fibers
 is not critical, other than it have a relatively low coefficient of
 thermal expansion. Numerous other modifications and alternative
 arrangements may be devised by those skilled in the art without departing
 from the spirit and scope of the present invention, and the appended
 claims are intended to cover such modifications and arrangements.