The present invention relates to an evanescent optical sensor and, more particularly, to an evanescent optical sensor in the form of a fiber coil.
Optical waveguides and, in particular, optical fibers, are often used for sensing changes in an ambient medium. Optical sensors have been used to measure changes in various parameters such as temperature, pressure, sound, refractive index and the like. In many cases, these changes are detected by monitoring the transmission (or reflection) spectrum of light as it propagates along an optical waveguide disposed within the ambient. Some optical sensors function as evanescent sensors based on the detection of changes in light propagating through an optical waveguide due to the optical mode that evanescently penetrates into the surrounding ambient.
Indeed, evanescent wave absorption is an effective technique for performing various types of environmental sensing. When a beam of light propagates along an optical fiber, the electromagnetic field does not abruptly fall to zero at the core/cladding interface. Instead, the overlap of an incoming beam and the internally reflected beam leads to a field that penetrates into the medium adjacent to the core region of the fiber. This electromagnetic field, which tails into the adjacent medium, is defined as the “evanescent field”.
In order to enhance the response of the transmission (or reflection) spectrum to variations of ambient medium parameters, an optical sensor is typically configured as a Mach-Zehnder interferometer (MZI) having at least two separate arms along which an optical signal will propagate. At a given wavelength λ, the output power of an N-arm MZI is determined by the following equation:
where Ln is defined as the length of waveguide n and An and βn are the amplitude and propagation constants of the particular optical signal propagating along waveguide n. In the simplified case where n=2 and each arm has the same length L, the above equation reduces to the following relation:
P=|A|2{1+cos [L(β1−β2)]}.
In the analysis of an exemplary measured parameter q (where q may be, for example, temperature, refractive index, etc.), a variation in q causes variation in at least one of the propagation constants, say β1(q). From the above, it is clear that the sensitivity of the sensor is proportional to the following:
Thus, it is shown that the sensitivity grows proportionally to the length L of the MZI arm. For this reason, it is desirable to make the interferometer arm as long as possible. On the other hand, increasing the length L results in increasing the overall physical size of the sensor. The latter is undesirable for at least two reasons. First, this causes spatial delocalization of the measurement since the ambient may change over the length of the interferometer arm. Second, many applications require the use of a “miniature” sensor (for example, in a “lab on a chip” application).
It has previously been suggested to fabricate miniature MZI sensors based on photonic wires that are folded or spirally bent to be used as a planar photonic circuit. However, these devices are known to experience relatively high losses and cannot provide the degree of sensitivity required for many applications. Input/output coupling to/from these photonic wire devices is also problematic and introduces unwanted optical losses into the system.
Thus, a need remains in the art for a “miniature” optical sensor that exhibits the sensitivity generally associated with larger, multi-component arrangements.
The needs remaining in the prior art are addressed by the present invention, which relates to an evanescent optical sensor and, more particularly, to an evanescent optical sensor in the form of a fiber coil.
In accordance with the present invention, an evanescent optical sensor is formed as a coil of either optical fiber or microfiber. By coiling the fiber/microfiber, the size of the sensor is significantly reduced when compared to “straight path” fiber sensors, yet exhibits a similar degree of sensitivity. For example, a prior art sensor formed of a section of optical fiber having a length of 15 cm can now be formed as coil of dimensions 3 mm×3 mm×4 mm.
In operation, an optical signal is coupled into a coil that has been immersed in an ambient to be analyzed. The use of a coil configuration results in creating a plurality of whispering gallery modes (WGMs) that will propagate along the coil by reflecting from the surface of the curved fiber/microfiber forming the coil. The interference between these modes (i.e., at least two modes) is modified as a function of the properties of the ambient environment within which the coil is immersed. That is, environmental changes cause variations in the optical length of the coil as “seen” by the various modes, with the interference between/among the modes analyzed by studying the transmission spectrum at the output of the coil.
It is an advantage of the compact nature of the coiled structure of the present invention that spatial delocalization of the measurement, associated with relatively “long” prior art sensors, is essentially eliminated.
Other and further advantages and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
Referring now to the drawings,
a) and (b) are photographs of exemplary coiled fiber evanescent optical sensors formed in accordance with the present invention;
a) and (b) are diagrams of transmission power as a function of wavelength for the coiled fiber evanescent optical sensors of
a) and (b) show the transmission spectra of exemplary coiled microfiber evanescent optical sensors formed in accordance with the present invention.
An evanescent optical sensor useful for analyzing various parameters of the ambient environment (e.g., temperature, pressure, acoustic, refractive index, etc.) is formed from a coiled configuration of an optical fiber or microfiber. For the purposes of the present invention, the term “microfiber” is defined as a fiber with a diameter on the order of one micron (or less than/or on the order of the wavelength of an optical signal propagating through the fiber). It is contemplated that the coiled sensor of the present invention may be constructed of either conventional optical fiber (having a diameter on the order of ten to a hundred microns) or microfiber, where differences in sensitivity of various embodiments can be attributed, in part, to the selection of either fiber or microfiber.
It is preferred that the radius of curvature R of the coil is gradually transitioned between infinity (within each coupler 12, 14) and the selected value as the signal is coupled into and out of the coil itself (that is, it is preferred that couplers 12 and 14 take the form of adiabatic couplers and mode converters). However, it is not necessary that output coupler 14 perform a mode conversion function. As compared to conventional “planar” optical sensors, the coiled configuration of the present invention creates a long optical path length (providing increased sensitivity) within a relatively compact area.
In operation, an incoming optical signal is propagating as a fundamental mode signal at the entrance of input optical coupler 12. Optical coupler 12 introduces the signal into coil 10, and functions as a mode converter so as to split the propagating optical signal into a plurality of modes, particularly in the form of whispering gallery modes (WGMs) that will thereafter propagate along coil 10, where at least two modes are required to be exited. The mode(s) nearer the outer surface of coil 10 will necessarily interact with a larger amount of the ambient, as shown in
a) and (h) illustrate the results of performing numerical modeling of coiled fiber sensor 10 of
For the purposes of this analysis, the refractive index of the ambient medium surrounding coil 10 was set to the value of 1.000 for curve A and to 1.001 for curve B. It is seen that the characteristic oscillations of the transmission power are relatively frequent than those of the transmission associated with a prior art fiber loop sensor, as shown in
a) and (b) are digital camera pictures of exemplary coiled fiber sensors, where the coil of
The exemplary coiled fiber sensors of
It was found that the coil as shown in
As mentioned above, evanescent coiled optical sensors of the present invention may be formed from either standard optical fiber or optical microfiber.
Various modifications can be utilized with the coiled evanescent optical sensor to further improve its sensitivity. For example, using relatively thin fibers (or microfibers), as well as tapered and/or coiled input and output couplers, will significantly reduce insertion losses in the system. Optimization of the input and output connections, which may also function as mode converters between the fundamental mode and other created modes, can allow for excitation of interfering WGMs, which generate larger sensitivity. The sensitivity of the coiled sensor will also increase with decreasing diameter of both the fiber forming the coil and the coil itself.
While the present invention has been particularly described and shown with reference to particular embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made thereto without departing from the spirit and scope of the present invention as defined by the claims appended hereto.
This application claims the benefit of US Provisional Application No. 61/234,834, filed Aug. 18, 2009 and herein incorporated by reference.
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Number | Date | Country | |
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20110043818 A1 | Feb 2011 | US |
Number | Date | Country | |
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61234834 | Aug 2009 | US |