Actuator for micro-electromechanical system fabry-perot filter

This is a continuation-in-part application of application Ser. No. 11/447,779, entitled “MICRO-ELECTROMECHANICAL SYSTEM FABRY-PEROT FILTER CAVITY” and filed on Jun. 6, 2006. The entire contents of that application are incorporated herein by reference.

Devices may sense the presence (or absence) of particular molecules. For example, a miniature or hand-held spectrometer might be used to detect biological, chemical, and/or gas molecules. Such devices might be useful, for example, in the medical, pharmaceutical, and/or security fields. By way of example, a hand-held device might be provided to detect the presence of explosive materials at an airport.

In some sensing devices, light reflected from a sample of molecules is analyzed to determine whether or not a particular molecule is present. For example, the amount of light reflected at various wavelengths might be measured and compared to a known “signature” of values associated with that molecule. When the reflected light matches the signature, it can be determined that the sample includes that molecule.

In some sensing devices, a Fabry-Perot filter such as the one illustrated in FIG. 1 is used to analyze light reflected from a sample of molecules. The filter 100 includes a first partially reflecting mirror 110 and a second partially reflecting mirror 120 that define a resonant cavity C. Broadband light enters the filter 100 , and some photons reflect off of the first mirror 110 while others pass through the mirror 110 and enter the cavity C. While in the cavity C, the photons bounce between the first and second mirrors 110 , 120 , and eventually some of the photons pass through the second mirror 120 and exit the filter 100 .

As the photons bounce within the cavity C, interference occurs and an interference pattern is produced in light exiting the filter 100 . As a result, light having a specific wavelength may exit the filter 100 . Note that the interference occurring within the cavity C is associated with the distance d between the two mirrors 110 , 120 . Thus, the filter 100 may be “tuned” to output a particular wavelength of light by varying the distance d between the mirrors 110 , 120 (e.g., by moving at least one of the mirrors 110 , 120 ).

In some cases, one of the mirrors is formed using a diaphragm that can be flexed to change the distance d. For example, FIG. 2 is a side view of a Fabry-Perot filter 200 implemented using a flexible diaphragm mirror 210 and a fixed mirror 220 . By measuring light reflected from a sample using various distances d (i.e., at various wavelengths), and comparing the results with a known signature of values, it may be determined whether or not a particular molecule is present in a sample. The diaphragm 210 might be flexed, for example, by applying a voltage difference between the mirrors 210 , 220 .

Such an approach, however, may have disadvantages. For example, the curving of the flexible diaphragm mirror 210 may limit its usefulness as a Fabry-Perot mirror. Moreover, the use of a flexible diaphragm mirror 210 may introduce stress over time and lead to failures. The design might also require bonding materials together that have different thermal characteristics—which can lead to problems at relatively high, low, or dynamic temperature environments. In addition, as the size of the cavity C is reduced, it can be difficult to efficiently control the movement of the flexible diaphragm mirror 210 . Note that the use of piezoelectric elements to move mirrors arranged as in FIG. 2 can result in similar problems.

According to some embodiments, a bi-stable actuator may be coupled to at least one movable Fabry-Perot filter cavity mirror.

Other embodiments may include: means for routing light from a sample of molecules into a tunable Fabry-Perot cavity; means for actuating a movable Fabry-Perot filter cavity mirror between a first latched position and a second latched position, wherein the distances between the first and second latched positions are associated with a spectral range of light wavelengths; and means for detecting interference patterns across the spectral range.

Yet other embodiments may be associated with a spectrometer having a laser source and an analyte sample to reflect light from the laser source. A Fabry-Perot filter cavity to receive the reflected light may include: a bi-stable actuator, and at least one movable Fabry-Perot filter cavity mirror coupled to the bi-stable actuator. A detector may detect photons exiting the Fabry-Perot filter cavity over time as the movable mirror is moved by the actuator. A decision unit may also be provided to determine if the analyte sample is associated with at least one type of molecule based on the sensed photons.

Still other embodiments may be associated with a micro-electrical mechanical system apparatus that includes an actuator driven by a voltage and at least one movable Fabry-Perot filter cavity mirror coupled to the actuator, wherein a relationship between the voltage and an amount of displacement associated with the movable mirror is substantially linear

FIG. 1 is a side view of a Fabry-Perot filter.

FIG. 2 is a side view of a Fabry-Perot filter implemented using a flexible diaphragm.

FIG. 3A is a side view of a Fabry-Perot filter in accordance with an exemplary embodiment of the invention. FIG. 3B is a perspective view of a wafer associated with a Fabry-Perot filter in accordance with an exemplary embodiment of the invention.

FIG. 4 is a top view of a Fabry-Perot filter having a comb drive in accordance with an exemplary embodiment of the invention.

FIG. 5 illustrates how an applied voltage may be translated into mirror displacement in accordance with some exemplary embodiments of the invention.

FIG. 6 illustrates a Fabry-Perot filter drive in accordance with some exemplary embodiments of the invention.

FIGS. 7 and 8 are graphs illustrating relationships between a driving voltage and an amount of mirror displacement.

FIG. 9 illustrates a method to analyze a sample of molecules according to some embodiments.

FIG. 10 illustrates a spectrometer according to some embodiments.

FIG. 3A is a side view of a Fabry-Perot filter 300 in accordance with an exemplary embodiment of the invention. The filter 300 includes a first partially reflecting mirror 310 and a second partially reflecting mirror 320 that define a resonant cavity C. According to this embodiment, the first mirror 310 acts as a movable mirror while the second mirror 320 is fixed. Note that the movable mirror 310 may be substantially parallel to the fixed mirror 320 .

The filter 300 further includes an bi-stable structure 330 . As used herein, the phrase “bi-stable” structure may refer to an element that can rest in a first latched position or a second latched position. In this case, the bi-stable structure 330 may be snapped between the two latched positions to scan the filter 300 . The bi-stable structure 330 might be associated with, for example, a thermal device, an electrostatic device, and/or a magnetic device. According to some embodiments, a spring may be coupled to the movable mirror 310 and/or bi-stable structure 330 to improve control.

According to some embodiments, the bi-stable structure 330 is oriented within a plane, such as a plane defined by a surface of a silicon wafer. Note that the movable and/or fixed mirrors 310 , 320 may be oriented substantially normal to that plane (e.g., vertically within the wafer). According to some embodiments, the movable or fixed mirrors 310 , 320 may be associated with a crystallographic plane of silicon and the Fabry-Perot filter 300 may be associated with a Micro-electromechanical System (MEMS) device.

According to some embodiments, the bi-stable structure 330 is coupled to the movable mirror 310 via an attachment portion 340 . Note that the bi-stable structure 330 could instead be attached directly to, or be part of, the movable mirror 310 . In either case, the bi-stable structure 330 may move or “scan” the movable mirror 310 left and right in FIG. 3 to vary distance d over time.

As the movable mirror 310 is scanned, broadband light may enter the filter 300 (e.g., via fiber optic cable introducing the light through the fixed mirror 320 ) and some photons may reflect off of the fixed mirror 310 while others pass through the mirror 310 and enter the cavity C. While in the cavity C, the photons may reflect between the fixed and movable mirrors 310 , 320 , and eventually some of the photons may pass through the movable mirror 320 and exit the filter 300 .

As a result, the filter 300 may act as a narrow-band optical filter and the wavelength of light that exits the filter may vary over time (as d is varied). That is, the wavelength of light output from the filter 300 will scan back and forth across a range of the optical spectrum over time. By measuring the intensity of the light at various times (and, therefore, various distances d and wavelengths), information about the light entering the filter can be determined.

Although a single pair of mirrors 310 , 320 are illustrated in FIG. 3 , additional mirrors may be provided (e.g., to define multiple cavities). Moreover, although flat, rectangular mirrors 310 , 330 are illustrated in FIG. 3 other configurations may be provided. For example, one or both of the mirrors 310 , 320 might be curved. Similarly, one or both of the mirrors 310 , 320 might be U-shaped or I-shaped.

The bi-stable structure 330 may be any element capable of moving the movable mirror 310 . Note that, unlike the flexible diaphragm approach described with respect to FIG. 2 , the bi-stable structure 330 may be provided separate from the movable mirror 310 . That is, the activation may be decoupled from the optics (e.g., the mirrors do not act as electrodes or movable membranes). As a result, the tunability of the filter 300 may be improved. In addition, the filter 300 may be scanned over longer distances and spatial (and therefore spectral) resolution may be increased. Also note that having the light enter the Fabry-Perot filter 300 via the fixed mirror 320 (as opposed to the movable mirror 310 ) may reduce stiction issues and prevent fluctuations in any gap between a fiber optic cable and the filter 300 .

According to some embodiments, a movable or fixed mirror may be associated with a crystallographic plane of silicon and a Fabry-Perot filter may be associated with a Micro-electromechanical System (MEMS) device. For example, FIG. 3B is a perspective view of a wafer 302 that may be associated with a Fabry-Perot filter in accordance with an exemplary embodiment of the invention. As used herein, the term “wafer” refers to a structure having two, substantially parallel, planar surfaces (e.g., top and bottom surfaces larger than each side surface). In this case, portions of the wafer 302 may be etched away resulting in a pair of vertical mirrors 312 , 322 . Moreover, an actuation portion 332 may be etched onto the surface of the wafer 302 to move the movable mirror 312 . Note that the vertical orientation of the mirrors 312 , 322 might provide for taller, more thermally, mechanically, and optically stable structures as compared to horizontal ones. For example, a cavity 3 microns wide might be associated with mirrors having a height of 250 microns. Note that optical coating or Bragg reflectors (coating multi-layers and/or fine slots of air etched in the mirror wall) might be provided on one or both mirrors 312 , 322 to adjust reflection (and thereby increase resolution and contrast).

FIG. 4 is a top view of a Fabry-Perot filter 400 having a comb drive in accordance with an exemplary embodiment of the invention. In this case, a movable mirror 410 may be moved with respect to a fixed mirror 420 by a first set of conducting portions or “fingers” 430 interlaced with a second set of conducting fingers 440 . A varying voltage difference may be provided between the fingers 630 , 440 causing the fingers 430 , 640 to be pushed/pulled left or right in FIG. 4 . Note that any number of fingers may be provided for a comb drive (and that any number of comb drives may be provided for a Fabry-Perot filter 400 ).

FIG. 5 illustrates a system 500 wherein an applied or “driving” voltage applied to a drive is translated into mirror displacement in accordance with some exemplary embodiments of the invention. In particular, the driving voltage may cause rotor beams or fingers 510 to push away (or pull toward) anchored stator fingers 520 . In the case of a comb drive, the rotor fingers 510 may be pushed to pulled upwards or downwards in FIG. 5 . In the case of a parallel plate drive, the rotor fingers 510 may be pushed to pulled left or right in FIG. 5 .

The electrostatic force may, via a mechanical actuator with springs 530 , cause deflection in the springs and, as a result, a mirror may be displaced 540 from a first latched position (associated with a first voltage) to a second latched position (associated with a second voltage).

The amount of electrostatic force generated by the system 500 may depend on several factors. Consider, for example, the drive 600 illustrated in FIG. 6 . The amount of electrostatic force generated by the drive 600 may depend on, for example, a modulus of elasticity and/or a relative permittivity associated with the drive 600 ; the shapes, lengths (L 0 ), heights, widths (w), and gaps (g 1 , g 2 ) associated with rotor fingers 610 and anchored stator fingers 620 (as well as the number of fingers 610 , 620 and the overlap (L(x) between them); and stiffnesses in the actuation, orthogonal, and out of plane directions.

Typically, there is a quadratic relationship between a voltage applied to the drive 600 and an amount of mirror displacement that results from that voltage. For example, FIG. 7 is a graph 700 that illustrates a relationship between a driving voltage and an amount of mirror displacement, wherein the displacement is a function of the square of the voltage. By adjusting the physical parameters described with respect to FIG. 6 , however, the relationship between a driving voltage and mirror displacement can be altered. For example, FIG. 8 is a graph 800 that illustrates a substantially linear relationship between a driving voltage and an amount of mirror displacement. That is, the displacement is substantially a function of the voltage (as opposed to a square of the voltage). Moreover, a drive 600 may be designed to be “meta-stable.” For example, the overlap L(x) between the fingers 610 , 620 might be selected such that no force is generated at a particular voltage. Such an approach might reduce an amount of ringing associated with a latched position.

The Fabry-Perot filter drive 600 might be associated with, for example, a spectrometer. For example, FIG. 9 illustrates a method to analyze a sample of molecules according to some embodiments. At Step 902 , light is reflected from an analyte sample into a Fabry-Perot filter formed in a silicon wafer. At Step 904 , a movable mirror associated with the Fabry-Perot filter is actuated between a first latched position and a second latched position. At step 906 , light output from the Fabry-Perot filter is analyzed across an optical spectral range to determine information about the analyte sample.

FIG. 10 illustrates a spectrometer 1000 that might be associated with, for example, a Raman device, an infra-red absorption device, and/or a fluorescence spectroscopy device. According to this embodiment, the spectrometer 1000 includes a light source 1010 (e.g., a laser associated with λ L ) that provides a beam of light to an analyte sample 1020 . Photons are reflected off of the analyte sample 1020 and pass through the Fabry-Perot filter 300 as described, for example, with respect to FIG. 3 . According to some embodiments, another filter 1030 may also be provided (e.g., a Rayleigh filter to remove λ L ).

Because the Fabry-Perot filter 300 is scanning d i over time, a detector 1040 may measure light having varying wavelengths λ L over time. These values may be provided to a decision unit 1050 that compares the values with a signature of a known molecule (or sets of molecules) signatures. Based on the comparison, the decision unit 1050 may output a result (e.g., indicating whether or not any of the signatures were detected).

The following illustrates various additional embodiments of the invention. These do not constitute a definition of all possible embodiments, and those skilled in the art will understand that the present invention is applicable to many other embodiments. Further, although the following embodiments are briefly described for clarity, those skilled in the art will understand how to make any changes, if necessary, to the above-described apparatus and methods to accommodate these and other embodiments and applications.

Although a single movable mirror has been provided in some embodiments described herein, note that both mirrors associated with a Fabry-Perot cavity might be movable (and each mirror might be simultaneously moved with respect to the other mirror).

Further, although particular coatings, layouts and manufacturing techniques have been described herein, embodiments may be associated with other coatings, layouts and/or manufacturing techniques. For example, cap wafers with optical and/or electrical ports may be provided for any of the embodiments described herein. Such wafers may, for example, be used to interface with an Application Specific Integrated Circuit (ASIC) device.

Moreover, although Fabry-Perot filter designs have been described with respect to spectrometers, note that such filters may be used with any other types of devices, including telecommunication devices, meteorology devices, and/or pressure sensors.

The present invention has been described in terms of several embodiments solely for the purpose of illustration. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.