Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2003-04-14
2004-11-16
Lee, John D. (Department: 2874)
Optical waveguides
Optical waveguide sensor
C372S020000
Reexamination Certificate
active
06819812
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
FIELD
This invention relates to systems using vertical cavity, surface emitting lasers (VCSELs) having integrated MEMS (micro-electromechanical) wavelength tuners to interrogate optical sensors such as fiber and planar Bragg gratings, etalons, characteristic absorption or reflection sensors such as bandgap semiconductors and surface plasmon resonance sensors sensitive to physical, chemical and biological stimuli and, more particularly, to specific system configurations for use with such Bragg grating, etalon, absorption/reflection and surface plasmon resonance sensing devices.
BACKGROUND AND SUMMARY
Fiber optic sensors employing measurements of the shift of wavelength position of a sensor spectral peculiarity (e.g., maximum, minimum, slope or some other function) under the influence of a physical stimulus are well known to those skilled in the art. The examples of such sensors include Bragg grating-based strain, pressure, temperature and current (via the associated magnetic fields) sensors, surface plasmon resonance (SPR) biological and chemical sensors, semiconductor absorption band-edge based fiber-optic sensors and Fabry-Perot (FP) etalon pressure, temperature and sensors. To date, the utilization of such sensors has been retarded in the marketplace because of many well known problems, including e.g. the susceptibility of simple, inexpensive sensing systems to optical noise and the great expense of most of the solutions found to overcome said susceptibility. It will be revealed 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 classes of optical sensor systems.
Generally, 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 can be formed in an optical fiber by exposing ultraviolet sensitive glass (usually germanium doped fiber) with an ultraviolet (UV) beam that varies periodically in intensity. This is usually accomplished by means of an interference pattern created by a phase mask or split beam, such as with a Lloyd's mirror apparatus. Planar Bragg gratings are created by exposing a “photoresist” of any of a number of types through a phase shift or other type of mask, 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 very sharp peak—as narrow as a fraction of a nanometer in spectral width. It can also be arranged by means of a phase shift design that the reflected peak can contain within it an even narrower “valley” of absorption, e.g. as narrow as a few picometers in spectral width. Conversely, the transmitted portion of the light beam exhibits complimentary spectral power characteristics, i.e., a broader valley with a narrower peak within it.
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 e.g. 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 one exemplary such 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 transmitted and reflected light intensity spectra indicate the intensity of strain, temperature, pressure, electrical current, or magnetic field variations at the location of the grating. The spectral positions change based on the grating period or the indices of refraction, or both. These can be affected by various environmental physical stimuli such as temperature and pressure.
Frequently, more than one stimulus or physical parameter affects the sensors at the same time. Compensation is often designed into the sensor or the measurement technique for all the variables but one. This 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 about 0.1° C. and/or 1 microstrain, respectively.
Exemplary advantages of a spectral shift method of sensor interrogations include the high accuracy of wavelength determination (akin to the advantages of measuring electrical frequency instead of magnitude) and immunity to “optical noise”due to fluctuations in fiber transmission amplitude (microbending losses, etc.). The spectral shift method 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.
The precision, dynamic range and multiplexing capabilities of the optical sensor interrogation techniques can be defined in part by the spectral power of the light source, especially in cases in which a broadband source is used. The LEDs, SLDs (superluminescent diodes) and various lamps usually used provide spectral power that can be too small when divided into nanometer-sized segments. This limits critical parameters such as the magnitude of the reflected peak available to the optical sensor, causing lower than desirable signal to noise ratios. Another technique, the use of a conventional laser diode tuned with a motorized external cavity, electrical current or temperature mechanisms is more effective because all the power of the laser is contained in a narrow beam as it is tuned across the spectrum. Several techniques have been proposed: see for example Froggatt, (U.S. Pat. No. 5,798,521). The use of a conventional laser diode tuned with electrical current has been proposed by Dunphy et al. (U.S. Pat. No. 5,401,956); and the use of a tunable fiber laser has been proposed by G. A. Ball et al. [J. of Lightwave Technology, vol. 12, no. 4, April 1994 p. 700].
When using a scanning laser technique, an inexpensive detector and electronics system can simply determine the wavelength at the peak (or null) of the reflected (or transmitted) light intensity against a known wavelength reference. However, past approaches are generally too expensive, too slow, too unstable or too inaccurate to have a wide range of practical applications.
Laser diodes tuned with current, while inexpensive and faster than thermal methods, generally can suffer from narrow tuning wavelength spans—which limits practical applications to only time division-multiplexed (TDM) Bragg sensors. Such lasers are completely unusable in surface plasmon or semiconductor absorption edge shift sensors. Broadband light source methods utilize inexpensive light sources, but use a spectrometer to read the signals (an optical spectrum analyzer may cost as much as $35,000). This technique is generally most practical when many sensors are multiplexed on the same fiber. Still, spectrometers are temperamental and not well suited to field use. The lasers tuned with external cavities that are now in use, on the other hand, typically are more expensive than spectrometers, but have the advantage of using an inexpensive detector. In addition, such lasers are typically sl
Kochergin Vladimir
Swinehart Philip
Lake Shore Cryotronics, Inc.
Lee John D.
Lin Tina M
Nixon & Vanderhye P.C.
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