DETAILED DESCRIPTION

[0019] Referring now to the figures where like numbers denote like elements FIG. 1 shows an example embodiment of a fiber optic pressure sensor.

[0020] FIG. 1 shows a fiber optic pressure sensor featuring a cartridge housing 150 having an end that is exposed to the atmosphere, a thin flexible membrane 140 covering the exposed end of the cartridge housing 150 such that the flexible membrane has an exposed side and a protected side. A fiber bundle 130 is disposed within the cartridge housing, featuring a transmitting fiber 110 having a first and second ends. The first end has a polished finish and the second end is coupled to a light source (not shown). The fiber bundle 130 also features a multitude of receiving fibers 120 disposed around the transmitting fiber 110 with each receiving fiber 120 having first and second ends where the first ends are also polished. The fiber bundle 130 is disposed within the cartridge housing 150 such that the first end of the transmitting fiber 110 and the first end of each receiving fiber 120 is adjacent to the protected side of the flexible membrane 140 with free space between the first fiber end and the protected side of the flexible membrane. A light sensing means (not shown) is coupled to second end of said receiving fibers wherein light launched into the transmitting fiber 110 propagates through the transmitting fiber, emerges at the polished end, propagates a very short distance in air, and is reflected by the flexible membrane 140 into the receiving fibers 120 , propagates through the receiving fibers, and is detected by light sensing means (not shown).

[0021] Upon a change in the atmospheric pressure, pressure waves cause the flexible membrane 140 to distort causing a change in the amount of light reflected by the protected side of the flexible membrane into the receiving fibers 120 . Thus the intensity of the light coupled into the receiving fibers 120 modulates in relation to the intensity of pressure wave causing the flexible membrane 140 to distort. These pressure waves can be the result of a change in the environment such as a sound, if the sensor is configure to operate as a microphone. Other modes for the pressure sensor are also possible such as an altimeter mode that senses a change in atmospheric pressure, motion sensor or etc. Other sensor modes based on an event that results in a pressure wave are also possible.

[0022] The type of fiber employed in the example embodiment shown in FIG. 1 , is generally a multimode fiber having a core that is preferably made of glass. The cladding may be plastic or some other material. In a preferred embodiment fibers with a high numerical aperture are used. Generally fibers with a numerical aperture of >0.2 are employed. A high numerical aperture provides for greater efficiency in the coupling and transmission of light. Fibers featuring high numerical apertures are not required, however. When employed in systems that have a great distance between the source and membrane a fiber having a high numerical aperture is not critical.

[0023] Generally multimode fibers with a combination of a thick core and thin clad fiber are preferred. Light incident on clad is lost, thus the core needs to be as close in proximity to the outer perimeter of the clad is possible to ensure efficient light coupling in the core. Thus light coupling within the fiber is maximized with a thick core thin clad structure. This however, does not limit the use of fibers in this device to multimode fibers with thick core thin claim structures. Varying degrees of effectiveness and light coupling are possible with other fiber configurations.

[0024] The fiber optic pressure sensor's cartridge housing is constructed of a rigid and light-weight material. A rigid structure provides the sensor with a robust design. Another advantage to the rigid structure is that the sensor is very resistant to vibration, G forces and other structural forces with could impact sensor performance.

[0025] The cartridge housing features means to adjust the distance between the polished end of the transmitting fiber and the flexible membrane.

[0026] In the preferred embodiment the system employs a screw type configuration, with a locking nut and an adhesive material applied to the nut to maintain a consistent distance. This configuration also provides the user the ability to adjust the distance from the fiber bundle to the flexible membrane by very small increments, and to lock or otherwise maintain a consistent distance in a dynamic environment.

[0027] While other attachment and adjustment methods may be applied to the sensor as an adjustment means, to extract optimum performance from the sensor the mechanism must be resistant to vibration and temperature variations. If the distance from the fiber bundle to the membrane is not maintained precisely, the accuracy and sensitivity of the sensor package will suffer.

[0028] The cartridge housing features means to adjust the tension of the flexible membrane drawn across the exposed end of the cartridge hosing. This may be accomplished via a screw type adjustment or some other means of adjustment.

[0029] The flexible membrane also features a coating on the protected side to enhance its light reflecting properties. In a preferred embodiment the flexible membrane is constructed of mylar and features at least one surface coated with thin aluminum film. The aluminum film increases the light reflecting properties of the flexible membrane, however other coatings may be applied to increase the membrane's sensitivity, or toughness.

[0030] The fiber optic pressure sensor fiber bundle features a single multimode transmitting fiber and a multitude of receiving fibers all encased in a protective tubing. In the example embodiment shown in FIG. 1 the protective tubing is a stainless steel tubing, yet other type of tubing may be used to encase the fiber bundle. One will appreciate that the type of, if any tubing that is used to encase the fiber bundle is not limiting to this invention.

[0031] The light sensing means is at least one silicon PIN diode. LEDs represent a very efficient way to launch light into the fiber. LED are generally low cost and feature low noise operation in a fiber system. LEDs are also tend to be very stable over extended periods of time. Laser diodes are also applicable, although they increase the expense and complexity of the system. Current laser diodes, also tend to introduce additional noise to the sensor package.

[0032] In another embodiment of the fiber optic pressure sensor the fiber optic pressure sensor features a cartridge housing having an end that is exposed to the atmosphere, a thin flexible membrane covering the exposed end of the cartridge housing such that the flexible membrane has an exposed side and a protected side. A fiber bundle is disposed within the cartridge housing, featuring a transmitting fiber having a first and second ends. The first end has a polished finish and the second end is coupled to a light source. The fiber bundle also features a receiving fiber disposed adjacent to the transmitting fiber. The receiving fiber has a first and second end where the first end is also polished. The fiber bundle is disposed within the cartridge housing such that the first end of the transmitting fiber and the first end of the receiving fiber is adjacent to the protected side of the flexible membrane with free space between the first fiber end and the protected side of the flexible membrane. A light sensing means is coupled to second end of the receiving fiber wherein light launched into the transmitting fiber propagates emerges at the polished end, propagates a very short distance in air, and is reflected by the flexible membrane into the receiving fiber, propagates through the receiving fiber, and is detected by light sensing means. Upon a change in the atmospheric pressure, pressure waves cause the flexible membrane to distort causing a change in the amount of light reflected by the protected side of the flexible membrane into the receiving fiber. Thus the intensity of the light coupled into the sensing fiber modulates in relation to the intensity of pressure wave causing the flexible membrane to distort.

[0033] FIG. 2 show an exemplar of a pressure sensor having the structure as described above, employed as a microphone. The device uses an LED emitting at 850 nm as the light source 280 with a silicon PIN diode as the light sensing means 290 . The fiber bundle 230 disposed within the cartridge housing, comprises 6 multimode receiving fibers 220 surrounding a single multimode transmitting fiber 210 . The cartridge housing 150 is formed with screw type adjustments for fiber to membrane distance 153 and for membrane tension 152 , and a clamping ring 154 also for setting and maintaining the membrane tension. The fiber bundle 230 is housed within a plastic cartridge made from Noryl. The first end has a highly polished finish and the second end of the transmitting fiber is coupled to the light source. The second ends of the receiving fibers are coupled to the light sensing means while the first ends also feature a highly polished finished. The optical fiber features a 200 μm glass core, and 230 μm plastic clad, a 500 μm Tefzel plastic coating, and a numerical aperture of approximately 0.37. The plastic coating is removed. The seven fiber bundle is contained within a stainless steel tube with 1.270 mm outer diameter and 838 μm inner diameter forming a probe. The probe, with includes the bundle and the protecting stainless steel tube, are housed within the plastic cartridge housing. The fiber bundle 230 is disposed within the cartridge housing 150 such that the first end of the transmitting fiber and the first end of each receiving fiber is adjacent to the protected side of the flexible membrane 240 with free space between the first fiber end and the protected side of the flexible membrane. The flexible membrane 240 is a 1.27×10 ⁻³ cm mylar (polyester) layer, having one surface that is coated with a thin aluminum film. The tension on the mylar membrane is adjusted to achieve the desired acoustic bandwidth. The membrane-probe separation is also adjusted to achieve the desired bandwidth sensitivity. A somewhat broad dynamic sensivity maximum was found for a probe-membrane separation between 180 and 250 μm.

[0034] Upon a change in the atmospheric pressure, pressure waves cause the flexible membrane to distort causing a change in the amount of light reflected by the protected side of the flexible membrane into the receiving fibers. Thus the intensity of the light coupled into the receiving fibers modulates in relation to the intensity of pressure wave causing the flexible membrane to distort.

[0035] In yet another embodiment a single fiber may be used as the transmitting and receiving fiber. In this embodiment the light source and the means for sensing the received light are both coupled to the fiber end. The light is launched from the fiber into the flexible membrane and is reflected back into the same fiber.

[0036] Although this invention has been described in relation to the exemplary embodiment's thereof, it is well understood by those skilled in the art that other variations and modifications can be affected on the preferred embodiment without departing from scope and spirit of the invention as set fourth in the claims.