Optical displacement sensor

Details Optical displacement sensor Optical displacement sensor

2001-06-28

2003-05-20

Lee, John D. (Department: 2874)

Optical waveguides

Optical waveguide sensor

C385S013000, C250S23700G

Reexamination Certificate

active

06567572

ABSTRACT:

FIELD OF INVENTION
This invention relates generally to pressure sensitive devices and optical interferometry. More particularly, it relates to diffraction-based systems and methods for optically detecting displacement of reflectors or membranes in various applications and use of displacement of reflectors or membranes for optical and ultrasonic signal transmission.
BACKGROUND
Optical interferometry is commonly used to detect small displacements over a large frequency range. Standard implementations of this method can be very bulky due to the large optical components such as beam-splitters, mirrors, lenses etc. A micro-machined phase sensitive diffraction grating can provide the same displacement sensitivity with a much simpler and compact structure. An example of such a diffraction grating formed on an atomic force microscopy (AFM) cantilever has been shown to have thermal mechanical noise limited displacement sensitivity. Examples are, for instance, provided by S. R. Manalis, S. C. Minne, A. Atalar and C. F. Quate, in a paper entitled “Interdigital cantilevers for atomic force microscopy,” published in
Appl. Phys. Lett.,
69, pp. 3944-6, 1996 and G. G. Yaralioglu, A. Atalar, S. R. Manalis and C. F. Quate, in a paper entitled “Analysis and design of an interdigital cantilever as a displacement sensor,” published in
J. Appl. Phys.,
83, pp. 7705-15, 1998 as well as U.S. Pat. No. 5,908,981 to Atalar et al.
The prior art teaches a scanning electron microscope of an interdigital AFM cantilever where the interdigital fingers form a diffraction grating. In this configuration, the interdigital fingers connected to the wide center beam form the non-moving reference. The other fingers are connected to the outer arms and to the AFM tip. A laser is used to illuminate the interdigital fingers and the intensities of the reflected diffraction orders are measured at fixed positions determined by the geometry of the grating. The grating and reflector are mechanically coupled. When a force is applied to the tip, the moving fingers connected to the tip are displaced relative to the non-moving reference fingers. This alters the phase relation between the light reflected from the adjacent fingers and the intensity of the diffraction order is changed. Monitoring the intensity of the light at those pre-determined locations gives the relative displacement of the fingers. Experimental results show that this method can be used to detect displacements from DC to 10s of MHz, membrane response being the limit as discussed by O. Solgaard, F. S. A. Sandejas and D. M. Bloom in a paper entitled “Deformable grating optical modulator,” published in
Opt. Lett.,
17, pp. 688-90, 1992. U.S. Pat. No. 5,311,360 to Bloom et al. discusses a method and apparatus for modulating a light beam. Bloom et al. teaches a modulator for modulating an incident beam of light wherein the modulator includes a plurality of equally spaced-apart elements. Each element includes a reflective planar surface. The elements are arranged parallel to each other with their light-reflective surfaces parallel to each other. The modulator includes means for supporting the elements in relation to one another and means for moving particular ones of the elements relative to others so that the moved elements transit between the first configuration wherein the modulator acts to reflect the incident beam of light as a planar mirror, and a second configuration wherein the modulator diffracts the light reflected therefrom. In operation the light-reflective surfaces of the elements remain parallel to each other in both the first and second configurations. The perpendicular spacing between the reflective surfaces of the respective elements is equal to m/4×the wavelength of the incident beam of light, wherein m equals an even whole number or zero when the elements are in the first configuration and m equals an odd whole number when the elements are in the second configuration. The apparatus and method by Bloom et al. teaches a modulator of light, but does not provide for a measurement system.
There is a need for micro-machined optical displacement sensors that are simpler and more compact than optical displacement sensors taught in the prior art to optically measure static and dynamic displacements of reflectors or membranes.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and method of an optical displacement sensor for measuring displacement of a reflector. The optical displacement sensor includes an optically transparent substrate and a reflective grating deposited on the substrate. The present invention further includes a light source to provide optical illumination on the reflector through the substrate and the reflective grating. The reflector is positioned at a vertical distance that is substantially less than half the coherence length of the light source over and with respect to the substrate and the reflective grating. In addition, the reflector is positioned substantially parallel to the substrate and the reflective grating to allow the optical illumination on the reflector to go through the substrate and reflective grating and reflect back through the reflective grating. Furthermore, the present invention includes one or more photo-detectors to monitor reflected and diffracted light from the reflector and the reflective grating.
The optical displacement sensor of the present invention includes various types of reflectors which are chosen dependent on the desired application. In a preferred embodiment of the present invention, the reflector is a flexible and optically reflective membrane. The reflective grating is formed by non-moving fingers and are not located on the same structure as the reflector. The light source is a coherent light source and can, for instance, be a semiconductor laser or from an optical fiber. Furthermore, in another embodiment of the present invention, the photodetectors can be replaced by one or more optical fibers. The substrate is transparent at a selected wavelength of the optical illumination for a particular application and light source. The position of the reflector to a desired location can be adjusted by applying at least one DC voltage between the substrate and the reflector. The DC voltage could be applied to the non-moving fingers of the reflective grating. Furthermore, the present invention includes an AC signal to be added to at least one DC bias voltage for calibration of the optical displacement sensor. The optical displacement sensor of the present invention further includes a wafer with at least one aperture for light transmission. The wafer hosts, for instance, but not limited to, one or more amplifiers and one or more photodetectors. In another embodiment of the present invention, the optical displacement sensor further includes an array of reflectors with at least one other reflector positioned at a vertical distance that is substantially less than half the coherence length of the light source over and with respect to at least one other reflective grating and the substrate. In this embodiment, the optical displacement sensor includes a transmission grating to deflect the optical illumination to the array of reflectors. Furthermore, the optical displacement sensor includes the possibility that each element in the array of reflectors is illuminated at a separate wavelength and multiplexed to a single optical fiber.
In view of that which is stated above, it is the objective of the present invention to provide an optical displacement sensor to measure static and dynamic displacement of one or more reflectors.
It is another objective of the present invention to provide an optical displacement sensor wherein the reflector and diffraction grating are not on the same structure.
It is yet another objective of the present invention to provide an optical displacement sensor that allows one to integrate different type of reflectors.
It is still another objective of the present invention to provide an optical displacement sensor amenable to integration of electronics and optics to for

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