The present invention is generally related to fiber optic sensor systems, and more particularly, an apparatus to interrogate one or more fiber optic sensors to make high-resolution measurements at long distances between the sensor and the interrogator apparatus.
U.S. patent application Ser. No. 11/105,651, titled Method and Apparatus for Continuous Readout of Fabry-Perot Fiber Optic Sensor, describes a method for readout of a Fabry-Perot fiber optic sensor. The method enables use of a Fabry-Perot fiber optic pressure transducer with signal conditioning system that includes a tunable laser. The high power, tunable laser provides rapid switching in fine increments in narrow wavelength bands with repeatability in the infrared spectral band from 1500 nm to 1600 nm. By operating in the 1500 nm to 1600 nm spectral band where attenuation in optical fiber is very low, high-resolution pressure and temperature measurements can be made using Fabry-Perot sensors at remote distances in excess of 10000 meters.
Additional information will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.
The present invention is directed to an apparatus for making high-resolution measurements at long distances between at least one sensor and the apparatus. The apparatus comprises a laser light source that is tunable over a range of frequencies, an optical switch for pulsing the laser light, and at least one sensor for reflecting the laser light. The apparatus also comprises a fiber optic cable that interconnects the sensor with the laser source, means for directing the reflected light from the sensor to a detector in order to generate a digital output, and a control logic for tuning the laser light source based on the digital output from the detector.
The invention is also directed to an apparatus for making high-resolution measurements at long distances between at least two sensors and the apparatus. The apparatus comprises a laser light source that is tunable over a range of frequencies, an optical switch for pulsing the laser light, and a first sensor and a second sensor for reflecting the laser light. The apparatus also comprises a length of fiber optic cable that interconnects the first and second sensors and the laser light source, where the cable delays the reflected light from the second sensor due to the length of the cable. In addition, the apparatus comprises means for directing the reflected light from the first sensor and the delayed reflected light from the second sensor to a detector to generate a digital output, and a control logic for tuning the laser light source based on the digital output.
Finally, the invention is directed to an apparatus for making high-resolution measurements at long distances from at least two sensors and the apparatus. The apparatus comprises a laser light source that is tunable over a range of frequencies, an optical switch for pulsing the laser light and directing the laser light into any one of N output channels, and a first sensor and a second sensor for reflecting the laser light connected via a fiber optic cable to a first output channel and a second output channel respectively. The apparatus also comprises a first length of fiber optic cable interconnecting the first sensor and the first output, and a second length of fiber optic cable interconnecting the second sensor and the second output, where the first length is greater than the second length, and the difference in length is associated with a delay of the reflected laser light of the first sensor. In addition, the apparatus comprises means for directing the reflected light from the first sensor and the delayed reflected light from the second sensor to a detector to generate a digital output, and a control logic for tuning the laser based on the digital output.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
Operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:
While the present invention is described with reference to the embodiments described herein, it should be clear that the present invention should not be limited to such embodiments. Therefore, the description of the embodiments herein is illustrative of the present invention and should not limit the scope of the invention as claimed.
The present invention relates to an embodiment for apparatus to interrogate one or more fiber optic sensors to make high-resolution temperature and/or pressure measurements at long distances between the sensor(s) and the interrogator apparatus.
A block diagram of the configuration is shown in
Numerous methods are available to turn the light on and off. Some of these include a fast optical switch, electro-optic modulator, or a simple electronic circuit to switch on and off the electric current to the laser. An optical switch 112 is used as shown in
The purpose of the control logic 126 is two-fold. First, the control logic 126 is used to tune the laser 110 to find the wavelength location of the peak of the FBG 102 (
The fiber Bragg grating (FBG) 102 is a device well known in the art. An FBG has many applications and in this embodiment, the FBG is used to measure temperature. The grating consists of a periodic series of high refractive index—low-refractive index regions within an optical fiber F. These refractive index variations are permanently embedded into the fiber using a special manufacturing process. The period of high-low index variations determines the wavelength reflected by the grating. The spectral reflectance is very well defined as shown in
A diagram of the Fabry-Perot pressure sensor 100 is shown in
Infrared light reflected from the FBG temperature sensor 102 and FP pressure sensor 100 returns to the signal conditioner (see
The pressure diaphragm 16 may be for example, a circular steel (e.g., Inconel-718) plate welded around the circumference of the plate to the steel sensor body. When external pressure is applied to the diaphragm 16 it deflects toward the end of the fiber F and the gap G decreases (see
D=0.2(Pr4)/(Et3) (1)
where:
r is the diaphragm radius
t is the diaphragm thickness
E is Young's modulus of the diaphragm material
For a typical working design:
D=8.2×10−4 inch (21•m) at P=2000 psi
r=0.3 inch t=0.105 inch
E=29×106 psi
The maximum stress S is given by:
The apparatus is compatible with other pressure sensing means in addition to a flat circular diaphragms. The alternative pressure sensing means include a corrugated diaphragm and low stress pressure sensing configurations such as a pin positioned within a cylindrical tube.
The infrared light intensity reflected back to the signal conditioner 120 from the FP sensor 10 is modulated as the diaphragm 16 deflects and the gap G changes. The ratio of the incident-to-reflected intensity IR is a function of both the laser frequency and the gap G and is given by:
where:
c=•• is the velocity of light
•=1.93×1014 Hz is the frequency of the infrared light
•=1550×10−9 m (1550 nm) is the wavelength
G is the Fabry-Perot gap distance between the diaphragm and the end of the fiber
F=4R/(1−R)2
R=(R1R2)1/2 is the composite reflectance of fiber end (R1) and diaphragm (R2)
There are two important reasons to pulse the light source and these are discussed below. First, in long distance applications, the temperature and pressure sensor may be 5 km, 10 km or 15 km away from the interrogator. To ensure that light from the tunable laser 110 reaches the sensor at the end of such long optical fiber cables, high output power is needed. An output power of 1 mW is sufficient and 10 mW is typically available from tunable laser systems. Such large power presents a fundamental problem however. When so much power is injected into the transmission fiber F, light is scattered back to the detector 120. Although the percentage of light scattered back is small, the laser power is large, and the amount of light back-scattered can cause significant detector noise. An optical time domain reflectometer (OTDR) experiences a similar problem, which is why there is a dead band for the first few meters when using an OTDR. The large scattered light signal saturates the detector. One method to minimize or reduce the effects of backscattered light noise is to pulse the light source. The time, t required for light to travel a distance L is given by:
t=nL/c (3a)
where c is the velocity of light and n is the refractive index of the fiber n • 1.5. Over a long transmission fiber length, 10 km for example, t=50 microseconds (μs). Since the FBG and FP sensors 102, 100 are reflective, the light can be can be repetitively switched on, say for 50 μs and then switched off for a longer time period (determined below). When the light is off, there is no backscattering in the fiber F to interfere with sensor signal detection. During the “light-off” interval, the reflected signals from the sensors are detected, analyzed, the light is switched on again for another 50 μs, and the process continues.
A second important reason to pulse the light source is to enable light to be transmitted and returned from the FBG sensor 102 and FP sensor 100 along the same optical fiber F. The FBG sensor 102 reflects a very narrow range of wavelengths, e.g., 1529 to 1532 nm, but the FP sensor 100 is designed to reflect all wavelengths of light emitted by the tunable laser source, e.g., 1529 to 1568 nm. A sharp step filter is not practical for high reflectance dielectric mirrors such as are used to define a Fabry-Perot sensor, which means that it is not practical to multiplex the FBG and FP sensors 102, 100 using wavelength division multiplexing methods only. Time division-multiplexing methods alone or in combination with wavelength division multiplexing can be used to assure the reflected signal from the FP sensor 100 does not interfere with the reflected signal from the FBG sensor 102.
A length of fiber F between the FBG and FP sensors 102, 100 can provide time delay and in combination with a pulsed light source, the interference between the reflected signals from the FBG and FP sensors 102, 100 is eliminated. The fiber F providing time delay may be wrapped into a coil (delay coil, 118) as shown in
After consideration of all the items above, the tunable laser 110 can be programmed to step through the tuning range at 5000 steps per second with a 200 μs time interval between steps. After the laser output has settled to a stable value at each wavelength step, the optical switch 112 is turned on to permit light to be transmitted down the fiber F to the sensors FBG, FP 102, 100 (see
To interrogate the FP sensor 100 at 10 km, the optical switch 112 is turned on for 50 μis and off for 150 μs and is synchronized to the laser 110 for wavelengths between 1532 and 1568. Similarly, when the FBG sensor 102 is interrogated, the optical switch 112 is synchronized with the laser 110. Light travels about 5 ns/m in optical fiber with refractive index n •1.5. From Equation 3a, a delay coil 118 length of 100 m provides a delay time of 1 μs=1000 μs, which accounts for two trips through the delay coil for light transmitted to and reflected from the FP pressure sensor 100 (see
Alternatively, the reflections from the FBG sensor 102 and FP sensor 100 are separated in time with use of a delay coil 118 (see
Another alternate embodiment is shown in
An alternative to separating the signal in time is to dedicate a set of wavelengths within the tuning range to pressure measurement and a different set of wavelengths to temperature measurement. At least one optical filter 24 is needed to limit the reflectance band from the one of the sensors and eliminate any cross-talk. The starting gap for each of the two sensors in this configuration must be increased in inverse proportion to the reduction in the tuning range allocated for each sensor. For example, if the tuning range for the pressure sensor is reduced from 40 nm to 15 nm after allocating bandwidth for the temperature sensor and optical filter, then the starting gap for the pressure sensor must be increased to approximately 300•m to assure the necessary number of interference fringes are observed over the 15 nm tuning range. Likewise the starting gap for the temperature sensor must also increase. Since the resolution and accuracy of the measurement is directly related to the tuning range, it may be appropriate to allocate more of the tuning range to the pressure sensor and less to the temperature sensor.
Another alternate embodiment is shown in
In general, each sensor is located at a different distance from the interrogator. The pulse duration for each channel would be a function of the actual distance from the signal conditioner unit to each sensor FP#1, FP#2, FP#3100a, 100b, 100c. Alternatively, the length of fiber used in each channel can be equalized using a separate length of optical fiber F wound into a coil 118 in each channel 201, 202, 203.
In another alternate embodiment, a 2×1 coupler at the output of the 1×1 optical switch 112 may be replaced with a 2×2 coupler. The 2×2 coupler has a second output fiber. If the end of the second fiber is cleaved perpendicular to the fiber axis, a 4% reflected signal returns to the photodiode detector 120. This reflected signal from the second coupler output fiber is detected earlier in time than the signal reflected from a sensor at the end of a long fiber cable. The reflected signal from the coupler can be used to monitor the magnitude of the laser output as a function of time. If necessary, the laser power can be controlled using the reflected signal from the coupler as the feedback signal for control.
In yet another alternate embodiment,
The invention has been described above and, obviously, modifications and alternations will occur to others upon a reading and understanding of this specification. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.
This application claims priority from U.S. Provisional Patent Application No. 60/784,881 entitled “APPARATUS FOR CONTINUOUS READOUT OF FABRY-PEROT FIBER OPTIC SENSOR” filed on Mar. 22, 2006, which is hereby incorporated by reference in its entirety. This application also claims priority from U.S. patent application Ser. No. 11/105,651 entitled “METHOD AND APPARATUS FOR CONTINUOUS READOUT OF FABRY-PEROT FIBER OPTIC SENSOR” filed on Apr. 14, 2005, which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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60784881 | Mar 2006 | US |