Patent Application
20050195402
1. Field of the Invention
The invention relates to optical sensors generally, and more particularly to interferometric optical sensors.
2. Discussion of the Background
Optical sensors are used in a wide variety of applications. They offer advantages as compared to other types of sensors, including small size, immunity to electromagnetic interference (EMI), extreme stability, long life, high temperature operation, and low cost. They are especially useful in harsh environments, including high temperature, high pressure environments.
One type of optical sensor is the diaphragm-based Fabry-Perot sensor. In such sensors, a Fabry-Perot cavity is formed between an end of an optical fiber and a reflective diaphragm. Two reflections occur in these sensors: a first reflection between the glass/air interface at the end of the fiber, and a second reflection that occurs at the surface of the diaphragm facing the end of the fiber. If the coherence length of the light source exceeds twice the length of the cavity, observable interference between the two reflections occurs. Deflections of the diaphragm due to a pressure applied to the diaphragm result in changes to the cavity length, which result in corresponding changes in the interference pattern from the two reflections. Some of these sensors are designed such that movement of the diaphragm (and corresponding changes to the cavity length) are constrained to a linear portion of a fringe. This is done to simplify the processing of the signal returned by the sensor.
Diaphragm-based sensors are often formed by attaching an optical fiber to a capillary tube or ferrule (usually glass or silica) and attaching the diaphragm to the tube or ferrule. An example of such a diaphragm based Fabry-Perot sensor is disclosed in U.S. Pat. No. 6,539,135 to Dianov et al. It is typical to use an epoxy to form the attachments between the fiber and ferrule/tube and between the ferrule/tube and the diaphragm in such sensors. However, the use of viscoelastic materials such as epoxies subjects the sensor to time dependent changes, thereby compromising the reproducibility and operation of the sensor. In addition, the use of viscoelastic materials increases the temperature dependence of the sensor.
PCT Publication No. WO 99/60341 discloses diaphragm-based Fabry-Perot sensors formed by a fiber surrounded by a ferrule/tube and a silicon wafer with a portion etched away to form a Fabry-Perot cavity. Several different methods for attaching the components of the sensors are disclosed. WO 99/60341 discloses bonding a fiber to a capillary tube using epoxy. In order to minimize thermal drifts, WO 99/60341 discloses attaching the fiber to the front end of the ferrule/tube locally by heating the capillary with a laser or local heating element and allowing the capillary to collapse along a limited section of up to a few millimeters of the fiber. In such embodiments, a flexible adhesive is used to bond the fiber to the ferrule/tube to allow for movement to alleviate stresses from thermal mismatches between the fiber and tube/ferrule. Applicants have experimented with such a procedure but the mechanical bond between the collapsed portion of the capillary tube and the fiber that results from this process has proven unsatisfactory. WO 99/60341 also discloses using solder glass to adhere the fiber to the ferrule/tube, but does not explain how thermal mismatches between the tube/ferrule and the fiber are accommodated. With respect to the bonding of the diaphragm to the ferrule/tube, WO 99/60341 discloses using adhesives, anodic bonding and diffusion bonding. The techniques disclosed in WO 99/60341 are an improvement over the use of epoxies, but are not ideal.
An additional concern when diaphragm fiber optic sensors are used in harsh environments is sensor “creep,” i.e., permanent changes in sensor geometry that occur over time and that degrade the accuracy of the sensor. Creep can occur in both directions—the sensor body (e.g., ferrule) may become elongated (due to viscous flow of the sensor body materials) or shortened (due to mechanisms, often referred to as volume consolidation, which are not well understood) under prolonged exposure of the sensor body to stress, strain and pressure.
The present invention addresses the aforementioned issues to a great extent by providing a diaphragm fiber optic sensor comprising a crystalline cylindrical ferrule including a central bore, and a diaphragm attached to the ferrule. In some embodiments, the Fabry-Perot cavity is formed by mechanically machining or chemically etching the ferrule. In other embodiments, the Fabry-Perot cavity is formed by mechanically machining or chemically etching the diaphragm. In yet other embodiments, the Fabry-Perot cavity is formed by interposing a ring between the diaphragm and the ferrule. The ring may be formed by cutting a portion of a tube. In some preferred embodiments, the ferrule is comprised of sapphire. In other preferred embodiments, the ferrule is comprised of a single crystal material, preferably single crystal sapphire. Preferably, both the ferrule and the fiber are formed from a crystal, and more preferably a single crystal, material. In preferred embodiments, the diaphragm and/or ring are also formed of crystalline material, preferably a single crystal material and preferably the same material as the ferrule.
In one aspect of the invention, bonds between the ferrule and fiber and diaphragm and ferrule are formed by welding the ferrule to the fiber and the diaphragm to the ferrule. The welding may be accomplished by any means (e.g., electric arc), but is preferably accomplished with a laser. In some embodiments, the entire ferrule is bonded to the fiber along the entire length of portion of the fiber that is within the ferrule. In other embodiments, particularly those embodiments in which there is a mismatch in the coefficients of thermal expansion of the ferrule and fiber (which may result from the presence of dopants in the fiber but not in the ferrule, or differences in the types or amounts of dopants in the fiber and ferrule), only a small portion of the fiber is welded to the ferrule to provide for small amounts of relative movement between the fiber and ferrule in the non-welded areas to accommodate movement due to thermal expansion and contraction. Using welding has the added advantage of driving air out of the cavity between the ferrule and diaphragm, which decreases the temperature dependence of the sensor.
In another aspect of the invention, both the front and rear surfaces of the diaphragm are polished and a second Fabry-Perot cavity, formed by the glass/air interfaces at the front and rear surfaces of the diaphragm, are used to measure temperature independently of pressure. The temperature reading may be used to compensate the output of the first Fabry-Perot cavity formed by the gap between the diaphragm and the end of the optical fiber.
In still another aspect of the invention, a small piece of optical fiber is spliced to an end of the main fiber to reduce or eliminate the temperature dependence of the sensor. The ferrule is laser welded to the main optical fiber, while the small piece of optical fiber is not attached to the ferrule. When the sensor is subjected to high temperatures, any air remaining in recess between the diaphragm and ferrule will expand, causing the diaphragm to deflect outward. The outward deflection of the diaphragm changes the length of the Fabry-Perot cavity between the end of the fiber and the diaphragm. The small piece of optical fiber is chosen to have a coefficient of thermal expansion such that the small piece of optical fiber will expand in an amount equal a distance that the diaphragm will deflect at elevated temperatures. Any differences between the coefficients of thermal expansion of the ferrule and the main optical fiber can also be compensated for by the small piece of optical fiber. Thus, for example, if the ferrule has a coefficient of thermal expansion that is greater than that of the main fiber, the small piece of optical fiber is chosen to have a coefficient of thermal expansion greater than both the ferrule and the main fiber. This allows the small piece of optical fiber to expand a greater amount than the ferrule in the presence of an elevated temperature to balance the lower amount of thermal expansion of the main fiber relative to the ferrule.
A more complete appreciation of the invention and many of the attendant features and advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
The present invention will be discussed with reference to preferred embodiments of diaphragm sensors. Specific details are set forth in order to provide a thorough understanding of the present invention. The preferred embodiments discussed herein should not be understood to limit the invention. Furthermore, for ease of understanding, certain method steps are delineated as separate steps; however, these steps should not be construed as necessarily distinct nor order dependent in their performance.
A cross sectional view of a diaphragm sensor 100 according to one embodiment of the invention is illustrated in
The optical fiber end 132 and the inside surface 122 of the diaphragm form a Fabry-Perot cavity with a length L. When light with a coherence length greater than twice the length L of the cavity is launched into the optical fiber 130, light reflected back into the fiber 130 at the air/glass interfaces at the fiber end 132 and at the diaphragm inside surface 122 interferes due to the phase difference resulting from the difference in optical path lengths between the two reflections. When the diaphragm 120 is deformed to due a force (e.g., a pressure) applied to the outside surface 124 of the diaphragm 120, the cavity length L changes, which results in corresponding changes in interference between the two reflections. Thus, by measuring changes in observed interference between the reflections, the corresponding force acting on the diaphragm 120 can be calculated from a knowledge of the mechanical properties of the diaphragm.
Fabrication of the sensor 100 will be discussed with reference to
This is preferably accomplished by laser welding the diaphragm 120 to the ferrule 110. Referring now to
Next, the fiber 130 is inserted into the central bore 114 of the ferrule 110 and the fiber 130 and ferrule 110 are welded together by laser 200. Relative rotation between the laser beam 200 and the ferrule 110 is provided in some embodiments as shown in
In some embodiments, the fiber 130 is welded to the ferrule 110 along only a small portion of the length of the ferrule 110 as shown in
In a preferred method of manufacturing the sensor 100, the fiber 130 is connected to a light source through a 2×2 coupler and the interference pattern of the reflections from the Fabry-Perot cavity are monitored while the fiber 130 is positioned in the ferrule prior to welding. Additionally, applicants have discovered that the length of the cavity (i.e., the distance between the inside surface 122 of diaphragm 120 and the end 132 of the fiber 130) changes during the laser welding process. In particular, applicants have discovered that the length of the cavity increases when using the laser to weld the fiber and the ferrule, and the length of the cavity decreases when welding the diaphragm to the ferrule. The reasons for these change in cavity length are not entirely clear, although it is believed that one of the factors that contributes to the change in cavity length that occurs during the process of welding the fiber 130 to the ferrule 110 results from the fact that the laser heats the ferrule 110 more rapidly than the fiber 130, thereby causing the ferrule 110 to expand more rapidly than the fiber 130.
Applicants have also learned that the amount of change in the cavity length can be controlled by adjusting the peak power, pulse width and number of pulses of the laser used for the welding process. Applicants have observed that the amount of increase in the cavity length gets larger as the peak power, pulse width, and number of pulses used when welding the fiber to the ferrule are increased. This is true regardless of what portion of the ferrule is welded to the fiber. Similarly, as the peak power, pulse width, and number of pulses used when welding the diaphragm to the ferrule are increased, the amount of decrease in the cavity length gets larger. However, for certain materials, the direction of the changes in cavity length may be opposite those described herein.
It should be noted that, in some embodiments, the fiber and ferrule are rotated during the welding process such that the laser is applied evenly around the circumference of the ferrule. In such embodiments, the rate at which the fiber and ferrule are rotated is adjusted to match changes in the pulse width (e.g., the rotation rate of the fiber/ferrule is adjusted such that the fiber/ferrule makes one complete rotation during a pulse). Of course, it is also possible to move the laser around the circumference of the fiber/ferrule during the manufacturing process.
Therefore, in one method of manufacturing a sensor, the fiber 130 is positioned at a location in the ferrule 110 (e.g., by using a micro-positioning tool) at a location at a distance from a desired final location, and the peak power, pulse width and number of pulses of the laser are controlled so as to cause the cavity length to change to the desired cavity length. In this method, either the ferrule 110 and fiber 130 or the diaphragm 120 and ferrule 110 can be welded first. In a second method, the reflections from the sensor 100 are converted to an electrical signal (e.g., by using a photodetector) and a feedback circuit is constructed to control the laser peak power and/or pulse width such that the cavity length is changed by a desired amount. Although discussed in connection with the sensor 100, it should be understood that these manufacturing techniques are applicable to all sensors discussed herein.
A sensor 400 according to an alternative embodiment of the invention is illustrated in
A sensor 500 according to yet another alternative embodiment of the invention is illustrated in
Each of the above-described sensors 100, 400, 500 is preferably fabricated using laser welding to bond components of the sensor to each other. In alternative embodiments, solder glass, glass sealants, or other materials are used in place of laser welding.
Further improvements to the above-discussed sensors are illustrated in the sensor 600 of
One improvement illustrated in
A second improvement in the sensor 600 is the provision of a fluted opening 613 in the ferrule 610 that allows a coating 638 placed over the fiber 630 to be extend into the ferrule 610. The fluted opening 613 is preferably filled with an epoxy, sol-gel, or spin-on-glass 615, which bonds to both the walls of the opening 613 of the ferrule 610 and the coating 638 on the fiber. This provides strain relief for the fiber 630, thereby making the sensor 600 more rugged. Of course, these materials must be suitable for the environment in which the sensor is to be used.
A sensor 700 that can monitor both temperature and pressure is illustrated in
The sensor 700 includes a ferrule 710 in which a pit 714 is formed. However, the temperature measurement technique using the second Fabry-Perot cavity formed by a double-polished diaphragm can be utilized in sensors in which the pit is formed in the diaphragm or created using a ring spacer between the ferrule and the diaphragm.
Those of skill in the art will recognize that the diaphragm sensors described herein are applicable to a wide variety of interferometric sensor systems including, but not limited to, linear interferometric sensor systems. In preferred embodiments of the invention, the diaphragm sensors described herein are incorporated into Self-Calibrated, Interferometric, Intensity-Based (SCIIB) sensor systems such as those disclosed in U.S. Pat. No. 6,069,686, and, more preferably still, into Q-point stabilized SCIIB sensor systems such as those disclosed in U.S. patent application Ser. No. 10/670,457, entitled “Active Q-Point Stabilization for Linear Interferometric Sensor.” The contents of both this patent and this patent application are hereby incorporated by reference herein.
While the invention has been described with respect to certain specific embodiments of diaphragm sensors, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended therefore, by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.
