Abstract:
A method for determining deflection of an optical sensor having an optical cavity includes: providing an optical signal including a train of time spaced light pulses, each light pulse including a known set of wavelengths; splitting the optical signal and providing a portion of the optical signal to a reference path; detecting light pulses in the portion of the optical signal; using a remaining portion of the optical signal and interrogating the sensor; receiving a reflected optical signal from the sensor; detecting light pulses in the reflected optical signal; and analyzing the portion of the optical signal and the reflected optical signal to determine the deflection. Corresponding apparatus and computer program products are provided.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical sensing structure, and in particular, to techniques for measurement therewith, wherein the techniques are independent of optical intensity fluctuations and other similar sources of measurement error. 
         [0003]    2. Brief Description of the Related Art 
         [0004]    In optics, a Fabry-Pérot interferometer is typically made of a transparent plate with two reflecting surfaces. Fabry-Pérot interferometers are one of the most common types of optical cavity used in laser construction. Due to the versatile uses of the interferometers, the structures are sometimes referred to in generic terms as sensors. 
         [0005]    In some instances, an extrinsic Fabry-Perot interferometer may be fabricated on a silicon wafer. Such embodiments of the extrinsic Fabry-Perot interferometer includes a cavity covered by a silicon diaphragm which deflects under pressure. The structure is illuminated with visible or infrared light and a varying amount of that light is both reflected from and transmitted through the structure. Some amount of deflection occurs in the diaphragm as a result of applied pressure. The optical properties of the structure are dependent upon the deflection and the reflected or transmitted light will vary with any of the pressure, strain and stress upon the structure (where each quantity may be measured and expressed as a measurand). The reflected or transmitted optical signal may also vary due to undesired fluctuations in the incident optical power due to any number of causes (inherent fluctuations in the light source, bending of optical fibers, etc.). 
         [0006]    The structure has a certain reflectivity which is wavelength dependent. That wavelength dependence changes at different pressures. So, instead of measuring raw reflected power (which may fluctuate for reasons other than changes in the measurand), a spectral shift in the reflected optical signal may be used to determine pressure change. Comparing signals from filtered and unfiltered diodes provides one technique for evaluating spectral shift, and is known in the art. 
         [0007]    However, in order for use of the filter approach to work well, it is desirable to use a source with a rather broad spectrum (e.g., an LED). However, LEDs typically provide only a certain amount of light for a fiber. This means that optical signal levels may be lower than desired, thus resulting in lower sensitivity, higher noise, and other such factors. Although a spectrometer could be used, one skilled in the art will readily recognize that this is not practical at least for cost and complexity reasons. 
         [0008]    Therefore, what is needed is a technique for measuring output of a Fabry-Pérot interferometer that is not dependent upon optical intensity fluctuations arising form external factors such as pressure fluctuations. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    Disclosed is an apparatus for measuring deflection of an optical sensor, the apparatus including: at least one light source for providing an optical signal including a train of time spaced light pulses, each light pulse including a known set of wavelengths; a first optical coupler for receiving the optical signal, splitting the optical signal and providing a portion of the optical signal to a first optical detector on a reference path; a second optical coupler for receiving a remaining portion of the optical signal, providing an interrogation light for interrogating the sensor including an optical cavity, receiving and providing a reflected light to another optical detector; wherein each detector is coupled to electronics for analyzing the train of time spaced light pulses. 
         [0010]    Also disclosed is a method for determining deflection of a Fabry-Perot cavity, including: providing an optical signal including a train of time spaced light pulses, each light pulse including a known set of wavelengths; splitting the optical signal and providing a portion of the optical signal to a reference path; detecting light pulses in the portion of the optical signal; using a remaining portion of the optical signal and interrogating the cavity; receiving a reflected optical signal from the cavity; detecting light pulses in the reflected optical signal; and analyzing the portion of the optical signal and the reflected optical signal to determine the pressure at the sensor. 
         [0011]    Further disclosed is a computer program product stored on machine readable media, the product for determining deflection of a Fabry-Perot based sensor, the instructions including: providing an optical signal including a train of time spaced light pulses, each light pulse including a known set of wavelengths; splitting the optical signal and providing a portion of the optical signal to a reference path; detecting light pulses in the portion of the optical signal; using a remaining portion of the optical signal and interrogating the sensor; receiving a reflected optical signal from the sensor; detecting light pulses in the reflected optical signal; and analyzing the portion of the optical signal and the reflected optical signal to determine the deflection. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Referring now to the drawings wherein like elements are numbered alike in the several figures, wherein: 
           [0013]      FIG. 1  depicts an interferometer (as a sensor) optically coupled to an optical fiber; 
           [0014]      FIG. 2  depicts a physical coupler configuration; 
           [0015]      FIG. 3  depicts an interrogation scheme; 
           [0016]      FIG. 4  depicts an output of the interrogation scheme; and 
           [0017]      FIG. 5  depicts an exemplary method for analyzing pressure using the sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Referring now to  FIG. 1 , there are shown aspects of a prior art Fabry-Perot Interferometer, simply referred to as an interferometer, and more broadly, as a sensor  10 . One skilled in the art will recognize that the illustration is merely introductory and is not limiting of the teachings herein. 
         [0019]    In this exemplary embodiment, the interferometer as the sensor  10  includes a membrane  15 . In this embodiment, the membrane  15  is formed of single crystal silicon (Si) or gallium nitride (GaN). A cavity  16  of the interferometer is formed in a substrate  17  of silicon dioxide (SiO 2 ) or sapphire (Al 2 O 3 ). Exemplary designs include pairings of Si with SiO 2  and GaN with Al 2 O 3 . These pairings are preferred for various reasons. For example, a GaN membrane would perform at much higher temperatures than a Si membrane. SiO 2  and Si are compatible from a semiconductor processing standpoint, as are GaN and sapphire (Al 2 O 3 ). 
         [0020]    An optical fiber  11  for interrogating the interferometer includes a high quality quartz or sapphire multimode fiber, having a core diameter of about 50 μm. In  FIG. 1 , the optical fiber  11  includes a jacket  12  for protection of the optical portion. 
         [0021]    Typically, optical fibers  11  are coated with a polymer jacket  12  (also referred to as a “buffer”) for providing mechanical durability. However, polymer jackets  12  typically are not suitable for high temperature applications (usually greater than about 80 degrees Celsius). “High temperature” optical fiber  11  using a polyimide jacket  12  is available. Typically, high temperature optical fiber  11  is stable to about 350 degrees Celsius. Other designs of optical fibers  11  for high temperature applications are known. For example, some high temperature optical fibers  11  are coated with various metals. Metal coated optical fibers  11  are known to be stable at temperatures to over 700 degrees Celsius. However, with regard to the teachings herein, and within the package of the interferometer, uncoated optical fiber  11  may be used. 
         [0022]    Although the teachings herein are disclosed in terms of the interferometer as the sensor  10 , aspects may be useful with or relate to other types of optical sensors  10 . Accordingly, the term “sensing element” and “sensor” may be considered to be equivalent to the interferometer in at least some instances. Therefore, one skilled in the art will note that the interferometer is merely one embodiment of an optical sensor  10 . 
         [0023]    Wavelengths of light travel through the optical fiber  11  and are communicated into the sensor  10  as interrogation light (λ int ) The wavelengths are reflected by the sensor  10  and into the optical fiber  11  as reflected light (λ ref1 ) The travel of light is depicted by the directional arrows in  FIG. 1 . As properties of interferometers are known, these are generally not discussed in greater detail herein. 
         [0024]    At least one light source provides a plurality of wavelengths (λ n ). More specifically, the light source provides sets of wavelengths that are detectably distinct from each other. As used herein, “detectably distinct” refers to capabilities of electronics selected for signal analysis to reliably provide discriminations between the sets of wavelengths. A degree of reliability is typically selected by a user of a measurement apparatus to which the components disclosed herein are a part. 
         [0025]    The plurality of sets of wavelengths (λ n ) includes at least a first set of wavelengths (λ 1 ) and a second set of wavelengths (λ 2 ). The light source is capable of providing light pulses (t n ), wherein each light pulse (t n ) includes light included in a known set of wavelengths. For example, a first light pulse (t n ) may include light within the first set of wavelengths (λ 1 ), while a second light pulse (t n ) may include light within the second set of wavelengths (λ 2 ). 
         [0026]    A series (or “train”) of time spaced light pulses (t n ) provides the interrogation light (λ int ) for interrogation of the sensor  10 . Typically, the train of light pulses (t n ) includes light pulses (t n ) of alternated or varied wavelengths. For example, in one embodiment, the interrogation light (λ int ) may include a train of light pulses (t n ) wherein odd numbered light pulses (t n ) are within the first set of wavelengths (λ 1 ), while even numbered light pulses (t 2 , t 4 , t 6 , t n*2 , . . . ) are within the second set of wavelengths (λ 2 ). Reference may be had to  FIG. 4 . 
         [0027]    Referring now to  FIG. 2 , an exemplary embodiment of a measurement apparatus  20  is provided. In  FIG. 2 , at least one light source  21  provides an optical signal  26  to a first optical coupler which is referred to as a source optical coupler  310 . The source optical coupler  310  of the present embodiment is a one by two optical coupler. A second optical coupler, referred to as a sensor optical coupler  320 , is in optical communication with the source optical coupler  310  and is also is a one by two optical coupler. 
         [0028]    Each of the source optical coupler  310  and the sensor optical coupler  320  includes a common optical path, a first optical path and a second optical path. For example, the source optical coupler  310  includes a source common optical path  311 , a source primary optical path  312  and a source second optical path  313 ; while the sensor optical coupler  320  includes a sensor common optical path  321 , a sensor primary optical path  322  and a sensor second optical path  323 . 
         [0029]    Each optical coupler  310 ,  320  provides for a coupling with an optical fiber  11  for conveying an optical signal  26 . Of course, one skilled in the art will recognize that the use of optical couplers (which may be considered beamsplitters), may call for fewer or more of such devices. For example, in one embodiment, a single optical coupler or beamsplitter may be used, and may include additional paths for optical signals. Accordingly, the use of two optical couplers is merely exemplary and not limiting of the teachings herein. 
         [0030]    The common source optical path  311  is optically coupled to the light source  21 . The light source  21  provides light pulses with at least two sets of wavelengths. Typical devices for the light source  21  include two discrete light sources (individual laser diodes), either coupled directly to a single optical fiber  11  or coupled to the optical fiber  11  by use of another fiber optic coupler or splitter. 
         [0031]    Light from the light source  21  is split at a junction in the source optical coupler  310 . As depicted in  FIG. 2 , about 10% of the light from the light source  21  is directed into the second source optical path  313 , while about 90% of the light from the light source  21  is directed into the primary source optical path  312  for interrogation of the sensor  10 . 
         [0032]    In this embodiment, the second source optical path  313  provides a reference path  25  for light from the light source  21 . A first detector  281  receives the portion of the interrogation light (λ int ) on the reference path  25 , thus providing for collection of information regarding interrogation light (λ int ) prior to interrogation of the sensor  10 . 
         [0033]    The sensor optical coupler  320  communicates the interrogation light (λ int ) from the sensor primary optical path  322  to the sensor common optical path  321  and interrogates the sensor  10 . 
         [0034]    Once reflected in the sensor  10 , the interrogation light (λ int ) is received by the sensor common optical path  321 . The reflected light (λ ref1 ) is provided as the optical signal  26  which is measured by use of the sensor second optical path  323 . Typically, in this embodiment, about 50% of the reflected light (λ ref1 ) is communicated through the second sensor optical path  323 . 
         [0035]    In one embodiment, a maximum power reflectivity of the sensor  10  is about 50% of the interrogation light (λ int ) and the reflected optical power is between about 0% and about 22.5% of the incident power for the light source  21 . 
         [0036]    Detection and analysis of the various optical signals (i.e., the interrogation light (λ int ) and the reflected light (λ ref1 ) is achieved through the use of optical detectors  281 ,  282 . In typical embodiments, the light source  21  is of broad spectral width (such as through use of a light-emitting-diode LED). As discussed, the first detector  281  measures light pulses (t n ) in the interrogation light (λ int ) on the reference path  25 . A second detector  282  measures the reflected light (λ ref1 ) following interrogation of the sensor  10 . 
         [0037]    The detectors  281 ,  282  used to measure the optical signal  26  are coupled to electronics  24  as are known in the art for resolving optical signals. Accordingly, the electronics  24  are neither shown nor discussed in greater depth herein. 
         [0038]    Now with reference to  FIG. 3 , aspects of another embodiment are depicted. In  FIG. 3 , a plurality of light sources are used to provide the plurality of sets of wavelengths (λ n ) for interrogating the sensing element  10 . In this example, the plurality of light sources includes a first light source  211  providing the first set of wavelengths (λ 1 ) and a second light source  212  providing the second set of wavelengths (λ 2 ). 
         [0039]    A portion of the optical signal  26  is split off onto the reference path  25  using the first optical coupler  310 . The portion is used as a reference and measured by the first detector  281  (such as a photodiode), while the rest of the optical signal  26  is directed to the sensing element  10 . The optical signal  26  returns via the second optical coupler  320  to the second detector  282 . 
         [0040]    In typical embodiments, the spectral response for each detector  281 ,  282  has been previously characterized. In order to measure the reflected light (λ ref1 ), each of the light sources  211 ,  212  is pulsed in a predetermined sequence. Known techniques, such as the use of an integrated circuit (e.g., a microprocessor), are applied to record the response of each detector  281 ,  282  (for example, the amplitude of the current generated in each photodiode) for each of the light pulses (t n ). Various devices may be used as detectors  281 ,  282 , such as, for example, standard diodes and semiconductor diodes. 
         [0041]    Typically, a background signal is ascertained for each detector  281 ,  282  between light pulses (t n ). For example, in the case where the detectors  281 ,  282  are photodiodes, a dark current is measured. 
         [0042]    In some embodiments, and once the signal for each of the light pulses (t n ) and the background signal between the light pulses (t n ) are determined, the optical signal  26  is determined. In these embodiments, the optical signal  26  is determined by subtraction of the background signal from the optical signal  26  for each light pulse (t n ). Determinations and use of the background signal in this fashion provides for accurate determinations of the signal-to-noise ratio (SNR). In general, it is considered that the background signal is the measured optical power in the absence of a light pulse. 
         [0043]    For example, and more specifically, with regard to use of photodiodes and an integrated circuit (IC), the IC determines a voltage or current which is determined by a relationship between the optical signal  26  detected for different sets of wavelengths (λ 1 , λ 2 ), after subtracting the instantaneous background signals. The values obtained for the optical signal  26  and reference diodes are compared and a final value is output. 
         [0044]    Time division multiplexing (TDM) of discrete sources is used to relate the plurality of light sources  21  to the sensor  10  using one optical fiber  11 . Use of a plurality of sets of wavelengths (λ n ) provides for measurement of multiple optical signals  26  which exhibit a relationship that is dependent only upon the measurand. 
         [0045]    For example, and with reference to  FIG. 4 , background signals are measured prior to output of a light pulse (time t 0 ). Then, the first light source  211  is pulsed, producing a first light pulse (t 1 ) for the set of the first wavelengths (λ 1 ). The second light source  212  is then pulsed, producing a second light pulse (t 2 ) for the set of the second wavelengths (λ 2 ). Each set of wavelengths in the plurality of wavelengths is at least functionally distinct from the other sets of wavelengths. 
         [0046]    It should be noted that as used herein “distinct from the other sets of wavelengths” and similar terminology (such applications of “discrete”) refers to an ability to resolve each set of wavelengths when malting measurements. That is, the optical signal  26  associated with one set of wavelengths may be substantially resolved from the optical signal  26  associated with another set of wavelengths. Typically, the resolving is performed using commercially available equipment for producing, measuring and analyzing the light pulses (t n ). 
         [0047]    The height of each light pulse (t n ) is dependent upon, among other things, the optical power of the at least one light source  21 . Other factors include the length of the optical fiber used and optical signal loss therein. Accordingly, accurate determination of each light pulse (t n ) is desirable and of particular importance for weak signals (such as in the case of remote light sources). 
         [0048]    One technique for analysis of the optical signal  26  calls for taking a ratio of responses (i.e., time between light pulses (t n )) at the second detector  282  between the first set of wavelengths (λ 1 ) and second set of wavelengths (λ 2 ) (usually after background subtraction). The resulting ratio is divided by a ratio obtained at reference detector  281  for the first set of wavelengths (λ 1 ) and second set of wavelengths (λ 2 ) (after background subtraction). 
         [0049]    Of course, various analyses regarding the plurality of sets of wavelengths (λ n ) may be performed. That is, techniques other than determination of ratios may be used. For example, simple subtraction of the optical signal  26  for the reflected light (λ ref1 ) from the optical signal  26  for the interrogation light (λ int ) may provide useful information. 
         [0050]    Among other things, results obtained include capabilities to obtain information regarding ambient environmental conditions (i.e., environmental pressure) for the sensor  10 . In some embodiments, interrogation of the sensor  10  provides results regarding fiber-optic strain. 
         [0051]    The teachings herein provide the technical effects of various improvements over the prior art. For example, elimination of the use of optical filters and multiple diodes for both the optical signal  26  and reference signals may be realized. Fewer optical couplers (splitters) are required. Discrete laser diodes may be used to achieve the same functionality as an LED, while providing significantly more optical power when coupling to fiber optics, while consuming less electrical power and generating less heat. Further, time division multiplexing of sources provides for real-time measurement of dark currents during intervals between light pulses. The real-time measurement of dark current provides for background subtraction as well as any compensation needed for temperature of the electronics. 
         [0052]    Other advantages are also realized over the prior art. For example, filters are no longer warranted since wavelength analysis is being done on a temporal basis. Thus, the teachings provide for reducing the number of light sources (diodes) needed (when compared to the prior art, this provides for a reduction of diodes from 4 to 2). Further, splitting the signal immediately provides for a greatly improved signal to noise ratio (since the first detector  281  now sees twice as much signal). Also of note, narrow-band sources such as laser diodes may be used as the wavelength resolution of the system is not dependent upon the characteristics of a filter. 
         [0053]    In the exemplary embodiment, the techniques provide for measurement by monitoring the light reflected from the optical cavity  16  in a Fabry-Perot interferometer. The Fabry-Perot interferometer has a “flexible” surface which deforms with stress. However, one skilled in the art will recognize that the TDM approach could also be used with other optical sensors. Non-limiting examples include Fiber-Bragg-Grating (FBG) sensors and other optical sensors having an optical cavity subject to deflection induced optical variations. In the case of FBG sensors, in many cases people talk about using multiple FBGs, each with a particular wavelength response. By using TDM, one could interrogate each FBG in a sensor at different times rather than having multiple filtered diodes or WDM (wavelength division multiplexing) couplers. 
         [0054]    In an exemplary method  50  for time division multiplexing (TDM), the measuring apparatus  20  provides the train of time spaced optical signals in a first step  51 , detects the train of time spaced optical signals in a second step  52 , interrogates the sensor  10  in a third step  53 , detects the reflected light (λ ref1 ) in a fourth step  54  and analyzes the train of time spaced optical signals in a fifth step  54 . 
         [0055]    The capabilities of the present invention can be implemented in software, firmware, hardware or some combination thereof. As one example, one or more aspects of the present invention can be included in an article of manufacture (e.g., one or more computer program products) having, for instance, computer usable media. The media has embodied therein, for instance, computer readable program code means for providing and facilitating the capabilities of the present invention. The article of manufacture can be included as a part of a computer system or sold separately. 
         [0056]    Additionally, at least one program storage device readable by a machine, tangibly embodying at least one program of instructions executable by the machine to perform the capabilities of the present invention can be provided. 
         [0057]    The flow diagrams depicted herein are just examples. There may be many variations to these diagrams or the steps (or operations) described therein without departing from the spirit of the invention. For instance, aspects of the steps may be performed in a differing order, steps may be added, deleted and modified as desired. All of these variations are considered a part of the claimed invention. 
         [0058]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.