Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2003-12-22
2004-12-28
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical waveguide sensor
C372S020000
Reexamination Certificate
active
06836578
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
FIELD
The technology herein relates to systems using vertical cavity, surface emitting lasers (VCSELs), and more particularly to such VCSELs having integrated MEMS (micro-electromechanical) wavelength tuning means to interrogate optical sensors. Still more particularly, the technology herein relates to such systems for use in interrogating fiber and planar Bragg gratings and etalons sensitive to physical stimuli, and to specific system configurations for use with such Bragg grating and etalon sensing devices.
BACKGROUND AND SUMMARY
Fiber optic sensors employing measurement of the shift of wavelength position of a sensor's spectral peculiarity (maximum, minimum or some other function) under the influence of a physical stimulus are well known to those skilled in the art. Examples of such sensors include Bragg grating-based strain, pressure, temperature and current (via the associated magnetic fields) sensors and Fabry-Perot (FP) etalon pressure, temperature and strain sensors to name a few. Unfortunately, the widespread use of such sensors has generally been restricted in the marketplace because of many well known problems, including the susceptibility of simple, inexpensive sensing systems to optical noise and the great expense of most of the solutions found to overcome said susceptibility.
We have, in contrast, discovered that combining a new type of laser, a vertical cavity, surface emitting laser (VCSEL), with an integrated microelectromechanical (MEMS) tuning mechanism as an interrogating instrument with sensors of many different types will enable new, less expensive and more reliable class of optical sensor systems.
As is well known, a Bragg grating is a series of optical elements that create a periodic pattern of differing indices of refraction in the direction of propagation of a light beam. A Bragg grating is generally formed in an optical fiber by means of exposing ultraviolet sensitive glass (usually germanium doped fiber) with an ultraviolet (UV) beam that varies periodically in intensity, usually accomplished by means of an interference pattern created by a phase mask or a split beam, such as with a Lloyd's mirror apparatus. Planar Bragg gratings are created by exposing “photoresist” of any of a number of types through a phase shift or other type of mask, or holographic exposure, or they can be written directly with an electron beam. Light reflections caused by the periodic index of refraction pattern in the resulting grating interfere constructively and destructively. Since the refractive index contrast between UV-exposed and unexposed sections of fiber is small but the number of sections is very large, the reflected beam narrows its spectrum to a sharp peak, as narrow as a fraction of a nanometer in spectral width. In addition, the phase spectral dependences of the reflected and transmitted light generally exhibit some modification around the wavelength of said reflection peak.
It is known that Bragg gratings patterned into optical fibers or other waveguides may be used to detect physical stimuli caused by various physical parameters, such as, for example, strain, pressure, temperature, and current (via the associated magnetic fields) at the location of the gratings. See for example U.S. Pat. Nos. 4,806,012 and 4,761,073 both to Meltz, et al; U.S. Pat. No. 5,380,995 issued to E. Udd; U.S. Pat. No. 6,024,488 issued to J. Wu; and the publication authored by Kersey, A. D., et.al. [10
th
Optical Fiber Sensors Conference, Glasgow, October 1994, pp.53-56]. Generally, in such a sensor, the core and/or cladding of the optical fiber (or planar waveguide) is written with periodic grating patterns effective for selectively reflecting a narrow wavelength band of light from a broader wavelength band launched into the core (waveguide layer in the waveguide). The spectral positions of sharp maxima or minima in the reflected or transmitted light intensity spectra indicate the intensity of strain, temperature, pressure, electrical current, or magnetic field variations at the location of the grating. The mechanism of the spectral position variability lies in changes in either the grating period or the indices of refraction, or both, which can be affected by various environmental physical stimuli, such as, for example, temperature and pressure. Frequently, more than one stimulus or physical parameter affects the sensors at the same time, and compensation must be designed into the sensor or the measurement technique for all the variables but one (which can be accomplished by many physical, optical and electronic techniques known in the art). The typical sensitivity limits of fiber grating sensors in the current art are generally about 0.1° C. to 1° C. and/or 1 microstrain or higher (depending on the packaging and/or embedding of the sensor), respectively. Advantages of a spectral shift method of sensor interrogations include the high accuracy of wavelength determination (akin to the advantages of measuring frequency instead of magnitude) and immunity to “optical noise” due to fluctuations in fiber transmission amplitude (microbending losses, etc.). The use of Bragg gratings also allows the multiplexing of many sensors on the same fiber via wavelength dependent multiplexing techniques (WDM), e.g., dividing the total wavelength band into sections dedicated to individual sensors.
Another approach for the interrogation of fiber Bragg grating strain sensors has been disclosed by M. E. Froggatt, (U.S. Pat. Nos. 5,798,521 and 6,566,648, articles [Froggatt M., “Distributed measurement of the complex modulation of a photoinduced Bragg grating in an optical fiber”, Applied Optics, 35 (25), pp. 5162-5164, September 1996] and [Froggatt M., Moore J., “Distributed measurement of static strain in an optical fiber with multiple Bragg gratings at nominally equal wavelengths”, Applied Optics, 37 (10), pp. 1741-1746, April 1998]). This approach is based on an interferometric scheme (FP interferometer) utilizing a coherent optical source such as a continuously tunable laser with a very narrow wavelength range (over just 0.23 nm) and a discretely tunable laser (over 2.5 nm total). The Froggatt method utilizes Fourier transformation of the measured spectrum, filtering of the Fourier transform followed by inverse Fourier transformation. Such an approach permits the acquisition of phase information, which in turn permits the multiplexing of a large array of fiber Bragg gratings having the same wavelength position of their reflectance peaks (unlike WDM, where a different spectral position of the reflectance peak of each sensor is essential). Such a technique is known as Optical Frequency Domain Multiplexing (OFDM). Large numbers of multiplexed sensors (up to 22) have been demonstrated [Froggatt M., Moore J., “Distributed measurement of static strain in an optical fiber with multiple Bragg gratings at nominally equal wavelengths”, Applied Optics, 37 (10), pp. 1741-1746, April 1998]. However, such a technique also may suffer from significant limitations. First, the nature of the laser used can make the detection scheme complex due to the necessity of using complex requirements for wavelength determination. Second, the accuracy and resolution of the instrument may be far from optimal due to the limited wavelength range of the laser that was specified. Third, the update rate of such an instrument may be quite slow due to both the slow tuning speed of the laser specified and the large computational overhead from the active wavelength determination scheme used, which in turn limited the accuracy of the instrument. Fourth, the detection range of these systems may be limited by the short coherence length of the laser. Fifth, the price of such a system may be very high compared to competitive electronic techniques, due both to the laser and the active wavelength determination scheme used. Despite the attractiveness of the Froggatt approach, it may be stated that this scheme has not reach
Kochergin Vladimir
Swinehart Philip
Lake Shore Cryotronics, Inc.
Lin Tina M
Nixon & Vanderhye P.C.
LandOfFree
System and method for measuring physical stimuli using... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System and method for measuring physical stimuli using..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System and method for measuring physical stimuli using... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3287111