Hydrophone mandrel for precise placement of gratings

A method and apparatus for reducing the difficulty of controlling the length of a section of optical waveguide wrapped around a mandrel separating Bragg gratings forming an interferometric sensor are provided. The section of optical waveguide may be wrapped on a mandrel having at least two different outer diameters. The mandrel may also include one or more bores for receiving and protecting the Bragg gratings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate to optical waveguide interferometric-based hydrophones and, more specifically, to mandrels used in such hydrophones.

2. Description of the Related Art

A Bragg grating is an optical element that is reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength that is referred to as the Bragg wavelength. Bragg gratings are usually formed by photo-induced periodic modulation of the refractive index of an optical waveguide's core. A pair of Bragg gratings having a common Bragg wavelength and separated by a length of waveguide (e.g. a coiled fiber or coil) can form an interferometer that may be interrogated by sending light of the same Bragg wavelength through the interferometer. Reflections of light from the (partially-transmissive) Bragg gratings are sent back to optical detection equipment through the waveguide. By assessing the phase shift in light coincidently reflected from the two Bragg ratings, the length of the coil can be determined, as is well known.

Optical waveguide interferometers can be deployed in various ways to make the length of the coil (and hence, the phase shifts between coincidentally reflected pulses) dependent on physical parameters. For example, Bragg grating interferometers can be deployed in a number of different ways to make acoustic sensors. Reference, “A Fiber Laser Hydrophone Array,” by D. J. Hill, et al., SPIE Vol. 3860. An optical waveguide hydrophone is typically made by winding a section of an optical waveguide (e.g., an optical fiber) separating a pair of Bragg gratings around a compliant cylindrical mandrel. When acoustic pressure impinges on the mandrel, the mandrel deforms slightly, changing the length of the waveguide separating the Bragg gratings. When forming such an acoustic sensor it is beneficial to tightly wind the optical waveguide (optical fiber) around the compliant cylindrical mandrel, which makes the fiber to follow the response of the mandrel that is designed to respond to acoustic pressure wave. The sensitivity of the sensor is proportional to the number of turns (or wraps), as described below.

One issue with mandrel-based optical waveguide, Bragg grating acoustic sensors is that the Bragg gratings themselves should be protected. Strain on the Bragg gratings can cause an excessive shift in the center frequency of the Bragg wavelength such that the Bragg gratings are no longer highly reflective at the correct wavelength. One way to isolate the Bragg gratings from excessive strain is to locate them within the mandrel itself. This can be accomplished by forming bores through the mandrel, locating a Bragg grating in one bore, wrapping the optical waveguide around the mandrel, and then bringing the optical waveguide through another bore such that the other Bragg grating is located in that bore. By placing the Bragg grating loosely inside the bore will isolate the grating from excessive strain and protect it from physical damages.

The length L of an optical waveguide wrapped on a cylindrical mandrel is about:
L≈N·π·d
where N is the number of turns and d is the outer diameter of the mandrel. In order to have optimum interferometer performances in a system utilizing multiple acoustic sensors (e.g., an array), the length between the two gratings should be nearly identical between devices. However, manufacturing tolerances may lead to significant variations in length between the gratings. For example, when a mandrel is turned on a CNC machine, its outside diameter can vary by about +/−0.001 inch. If the optical waveguide is wrapped around the mandrel 70 times, the wrapped length can vary by as much as 0.14 inch. Furthermore, the process used to produce the Bragg gratings can locate the gratings only within a tolerance of about +/−0.1 inch. Thus, it is difficult to tightly wrap an optical waveguide around a cylindrical mandrel while positioning the Bragg gratings inside the mandrel (which may require a precision of +/−0.040 inch to do).

Therefore, a mandrel that reduces the difficulty of accurately controlling the length of an optical waveguide wrapped around the mandrel and allowing Bragg gratings separated by the length of optical waveguide to be accurately positioned would be useful.

SUMMARY OF THE INVENTION

One embodiment that is in accord with the principles of the present invention is a mandrel that reduces the difficulty of wrapping an optical waveguide Bragg grating interferometer such that the Bragg gratings are accurately positioned. Such a mandrel has at least two outer diameters.

Another embodiment that is in accord with the principles of the present invention is a bored mandrel that reduces the difficulty of wrapping an optical waveguide Bragg grating interferometer on the mandrel such that the Bragg gratings are accurately positioned within bores. Such a mandrel has at least two outer diameters and a bore for receiving a section of an optical waveguide that includes a Bragg grating.

Another embodiment of the present invention is an interferometric hydrophone having Bragg gratings that are physically protected in a bore or bores of a mandrel having at least two outer diameters. The mandrel enables controlled routing of the optical waveguide to prevent excessive optical loss while protecting the Bragg gratings from physical damages due to shock and vibration.

Another embodiment of the present invention is a method of controlling a length of an optical waveguide section during manufacture of an acoustic sensor. The method generally includes providing a mandrel having at least a first section with a first outer diameter and a second section with a second outer diameter, wrapping the optical waveguide section a first number of times around the first section and a second number of times around the second section, and controlling the wrapped length of the optical waveguide section by varying the first number and the second number.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The principles of the present invention provide for optical waveguide interferometric hydrophones having Bragg gratings between optical waveguide sections that are wound on mandrels having at least two different outer diameters. Some embodiments of the present invention include bored mandrels, where the bores physically protect the Bragg gratings from both physical damage and from excessive strain. The different outer diameters enable accurate control of the length between the gratings which, in turn, allows accurate positioning of the Bragg gratings.

To facilitate understanding, embodiments of the present invention are described below with reference to acoustic sensors (hydrophones) as a specific, but not limiting application example. However, it should be appreciated that the apparatus and techniques described herein may be used to control the length between (and facilitate precise placement of) reflective elements of any type of interferometric sensor device.

FIG. 1schematically illustrates a simplified optical waveguide interferometric hydrophone system100. The hydrophone system may operate in a similar manner to the hydrophone system described in the commonly owned co-pending application entitled “High Pressure And High Temperature Acoustic Sensor” Ser. No. 10/796569) filed herewith and incorporated by reference in its entirety. The hydrophone system100includes a sensing coil102comprised of a number of tightly wrapped turns of an optical waveguide104(such as an optical fiber) around a mandrel106. The mandrel106should be understood as generically representing any of the inventive mandrels that are subsequently described. The sensing coil102is bounded by a pair of Bragg gratings110and112that have the same Bragg wavelength (λB). In the illustrated configuration, the sensing coil102acts as a sensor. This is because the length of the sensing coil102depends on the diameter of the mandrel106, which, in turn, depends on the acoustic pressures impingent upon the mandrel106.

Well known interferometric interrogation techniques, such as Fabry-Perot, Michelson, or Mach-Zehnder, can determine the length of the sensing coil102. For example, a series of optical pulses from a pulse generator114can be applied to the sensing coil102through an input optical waveguide120. Reflections of optical pulses from the Bragg gratings110and112, which are partially transmissive, are detected by a detector116and analyzed by an analyzer118. By assessing the phase shift in the pulses that are reflected from the two Bragg gratings110and112, the length of the sensing coil102can be determined. An output optical waveguide130can be connected to other optical components or sensors deployed along with the hydrophone system100.

In some applications, it may not be practical to form the sensing coil102and the Bragg gratings110and112along a continuous section of optical waveguide. In that case, the individual components, such as the input and output optical waveguides120and130, the sensing coil102, and the Bragg gratings110and112can be individually formed and then spliced together.FIG. 1illustrates such splices using slash marks136.

The length L of the sensing coil102that is on the mandrel106(described in more detail below) is tightly wrapped on the outer surface of the mandrel106and, for some embodiments, is such that the Bragg gratings110and112are located in predetermined and protected positions. Acoustic energy and the compliance of the mandrel106cause the length of the mandrel106to change, which induces changes in the outer diameter of the mandrel106. This causes a change in length ΔL of the length L and a corresponding change in the round trip path of pulses reflected from the second Bragg grating112, which causes the phase relationship between the light pulses detected at the detector116to vary. The analyzer118senses the phase variance and provides an electrical output that corresponds to the acoustic energy. The compliance of the mandrel106provides the restoring force.

FIG. 2illustrates a prior art mandrel200. As shown, that mandrel has a cylindrical shape and an outer surface202formed at a diameter d. The mandrel200further includes a bore204for passing an optical waveguide back through the mandrel200after the winding is complete. As noted, positioning the Bragg gratings112in the bore204is beneficial as that enables sealing the bore204to protect the Bragg gratings112from physical damage and from external factors such as pressure. A significant problem with the mandrel200is wrapping an optical waveguide such that the Bragg gratings were both located within the bore204. As noted in the “Background” section, mandrels turned on a CNC machine have diameters that can vary by about +/−0.001 inch and Bragg grating positions can vary by as much as 0.14 inch. Thus, it is very difficult to locate both Bragg gratings within the bore204. Furthermore, the end of the optical waveguide that is brought back through the bore204after wrapping can be bent at an excessive angle. This can cause excessive optical losses.

The mandrel106ofFIG. 1generically represents a class of mandrels that can be configured in various ways. For example,FIG. 3illustrates a mandrel300that is in accord with the principles of the present invention. As shown, the mandrel300includes a generally cylindrical body304having at least two diameters, d1and d2.FIG. 3shows diameters d1and d2as being very different. However, in practice, diameters d1and d2need vary only slightly. The first diameter d1is selected to give the best acoustic response without excessive optical power loss produced by bending the optical waveguide104. The second diameter d2is selected to provide for accurate placement of the Bragg gratings112. By varying the number of turns on the surfaces of each diameter, the Bragg gratings on both ends of the optical waveguide104can be precisely placed in a bore306.

Thus, the different diameters permit small designed features that will protect and stabilize the optical characteristics of the Bragg gratings, and at the same time keeping the optical waveguide tightly wound on the sensing surface of the mandrel to give a better signal to noise ratio. The length L of the optical waveguide104for the mandrel300is determined by the following formula:
L≈N1·π·d1+N2·π·d2
Where N1is the number of turns wrapped around diameter d1, and N2is the number of turns wrapped around diameter d2. By providing a relatively small difference between d1and d2, L2can be accurately controlled by varying N1and N2to accommodate variations due to manufacturing tolerances. For example, if the circumference of the section having diameter d1is 0.010 inch smaller than the circumference of the section having diameter d2, by winding (N1−1) turns on the first section and (N2+1) turns on the second section, the total length is increased by 0.010 inch (e.g., L′=L+0.01 inch), while the total number of turns is maintained (N1+N2). Thus it can be seen how the number of wraps around each diameter (N1and N2) may be varied to precisely control the wrapped length, which may also facilitate locating the Bragg gratings110and112in bore306

Depending on the application the mandrel300can be comprised of a variety of materials, including Nylon, Teflon, or Peek. A good material for most applications will have a low coefficient of thermal expansion and will operate at high temperature.

While the mandrel300is beneficial, it may not be optimal in all applications. One drawback of the mandrel300is that one or more relatively sharp bends in the optical waveguide104is required to bring both Bragg gratings112into the bore306. Sharp bends tend to attenuate optical power in the optical waveguide104.FIG. 4illustrates another mandrel400that is in accord with the principles of the present invention. As shown, the mandrel400includes a generally cylindrical body404having at least two diameters, d1and d2. The first diameter d1is selected to give the best acoustic response without excessive optical power loss created by bending of the optical waveguide104, while the second diameter d2is selected to provide for accurate placement of the Bragg gratings112. The mandrel400includes two bores408and410. The mandrel400further includes a guide slot412in the mandrel400at the second diameter d2, a transition slot413that spans across d1and d2, a guide slot414in the mandrel400at the first diameter d1, and end slots416(one end slot on each end).

To wrap the mandrel400, the optical waveguide104is inserted into the bore408such that an optical lead extends from end428and such that a Bragg grating112is located within the bore408. The optical waveguide104is then placed in the slot416at end430and brought back through the bore410. The optical waveguide104is then located in the end slot416and wrapped so that it enters and follows the guide slot414. After the guide slot414terminates the optical waveguide104is tightly wrapped around the portion of the mandrel400having the diameter d1. Then, to assist properly locating the other Bragg grating112the optical waveguide104is placed in the transition slot413. As the optical waveguide104is wrapped further it exits the transition slot413and is tightly wrapped on the mandrel400at the portion having the diameter d2. Slightly before wrapping the second Bragg grating112, the optical waveguide104is inserted into guide slot412. Further wrapping causes the optical waveguide104to follow the guide slot412into the end slot416on the end430. That end slot416then guides the optical waveguide104into either bore408or410(depending on how the end slot416terminates). The optical waveguide104is then passed through that bore such that the Bragg grating112is located within the bore. The bores are then sealed to protect the Bragg grating.

The end result is illustrated inFIG. 5. By varying the number of turns on the surfaces having diameters d1and d2, the Bragg grating112can both be precisely located within the mandrel400. The diameter d2permits small changes in the wrapping length of the optical waveguide104so as to accurately control the length between and precisely locate the Bragg gratings, while at the same time permitting tight winding of the optical waveguide on the surfaces of the mandrel, thus improving signal to noise ratios.

While the foregoing has described inventive mandrels having two sections with different diameters, it should be understood that more than two sections are contemplated. For example,FIG. 6illustrates a mandrel600having initial and end mandrel portions602and604, respectively that have diameters d2, and a central portion606having diameter d1. Another contemplated embodiment is the mandrel700that is illustrated inFIG. 7. That mandrel has a first portion702having a diameter d1, a second portion704having a diameter d2, and a third portion706having a diameter d3. It should also be understood that some applications will use mandrels with bores, while others will not.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention exist or may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.