Abstract:
Integrated sensors are described using lasers on substrates. In one embodiment, a first sensor forms a laser beam and uses a quartz substrate to sense particle motion by interference of the particles with a diffraction beam caused by a laser beam. A second sensor uses gradings to produce an interference. In another embodiment, an integrated sensor includes a laser element, producing a diverging beam, and a single substrate which includes a first diffractive optical element placed to receive the diverging beam and produce a fringe based thereon, a scattering element which scatters said fringe beam based on particles being detected, and a second diffractive element receiving scattered light.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of U.S. Provisional application No. 60/154,486, and No. 60/154,487, both filed Sep. 17, 1999. 

   STATEMENT AS TO FEDERALLY-SPONSORED RESEARCH 
   U.S. Government may have certain rights in this invention pursuant to Darpa grant number N66001-99-1-8902 and U.S. Navy grant no. N00014-99-1-0297. 
   The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (U.S.C. 202) in which the contractor has elected to retain title. 

   BACKGROUND 
   It is often desirable to obtain different kinds of information about particles. 
   One kind of information is about shear stress. An existing method of detecting wall shear stress puts a heated wire or element in the flow to be detected. The rate of cooling of the element provides a measure of the wall shear stress. Other similar sensors, which sense other parameters, are also known. 
   However, this system by itself has certain problems. The techniques may be intrusive, meaning that they may effect the rate of flow. The techniques can be affected by contaminants in the flow. For example, certain contaminants may deposit on the heated element and cause the heated element to react differently. These techniques can also change the characteristics of the sensor; hence requiring calibration. 
   Non-intrusive optical techniques may be considered using conventional optics. However, this results in a bulky setup, and setup that is highly susceptible to vibration. Moreover, the size of such a setup may cause difficulty in allowing the system to be effectively used. 
   Other kinds of probes can be used to detect the size of particles, and may have similar drawbacks. 
   SUMMARY 
   The present application teaches integrated optical sensors for detecting particle details. 
   One aspect detects and/or measures wall shear stress in flows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will now be described in detail with reference to the accompanying drawings wherein: 
       FIG. 1  shows a schematic for a first wall stress sensor; 
       FIG. 2  shows an optical fringe pattern emitted by the sensor of  FIG. 1 ; 
       FIG. 3  shows a details of fabrication of the optical part; 
       FIG. 4  shows an assembly drawing showing the way in which the elements are held within a housing; 
       FIG. 5  shows another embodiment using a common substrate to support the laser and the optical detector. 
       FIGS. 6A and 6B  show two embodiments of integrated optical sensors. 
       FIG. 7  shows an integrated optical sensor based on a phased Doppler technique. 
   

   DETAILED DESCRIPTION 
   The present application teaches a special miniaturized and integrated optical sensor probe for measuring wall shear stress in aerodynamic and hydrodynamic flows for example. As described herein, the system also provides structure which is highly minaturizable, and which can be formed within a housing of a special type that enables the use of the sensor in harsh environments. Moreover, the system in is non-intrusive and non-invasive. The center of the probe&#39;s volume may be located very close to the surface being measured, e.g. as close as 100 microns to the surface. Moreover, the sensor as described herein may be configured in a way, as described herein, that may require less calibration. 
   A schematic of the sensor is shown in  FIG. 1. A  diode laser  100  is formed on a substrate  102 . The diode laser produces a diverging output beam  105  which diverges at a specified angle. The output beam is shaped, for example, into two, parallel very high aspect ratio ellipses. The beam is coupled toward a transparent substrate, e.g., a quartz substrate  110  which forms an optical assembly. The quartz substrate may have a size, for example, of 600 microns thick and 700 microns square on a side. The quartz substrate  110  includes a metal film  115  formed thereon, e.g., a film formed of chromium or aluminum. The metal film is processed to form specified openings therein. Optical slits are formed in an area  120  of the metal film, arranged to form a diffractive optical element. The slits can be fabricated by etching the metal from the thin film in a specified pattern. 
   The light exiting from the diffractive optical element  120  forms a two-dimensional, linearly diverging optical fringe pattern  125 . The optical fringe pattern can, for example, simply include diverging fringes. The fringe pattern may be of the type shown in  FIG. 2  where the pattern width is on the order of 25 microns, and the position is on the order of 90 microns for the main part of the fringe, with the edges of the fringe ending at 130-140 microns. 
   The fringe  125  impinges on a mask  130  which is formed on the second surface  128  of the quartz substrate  110 . 
   The second surface  128  of the quartz substrate is placed near the flow to be measured. Light is scattered off the particles crossing the fringe pattern to form reflected beam  130 . 
   Scattered light is also obtained by a second optical window  135  that is formed in the metal film  115 . The light is collected through that optical window, via another diffractive optical element  140  formed on the surface of the quartz element. The scattered light is collected by those elements and focused onto an optical fiber detector  145 . An avalanche photodiode  150  can be located at the end of the detector, receiving the light therefrom. 
   An important feature of system in  FIG. 1  is that the sensor element can be fabricated using micro-fabrication technology. The substrate  110  can be formed as shown in FIG.  3 . The surface  112  includes the laser “lens”  120 , and the collection lens  130 ,  140 . The other side  128  of the substrate includes a plurality of slits. In addition, both sides of the substrates include alignment marks.  112  include the alignment marks  312 ,  314 , which are alignment marks for the electronic beams. The front side  128  includes the alignment marks  316 ,  318  which are the alignment for the front-to-back transfer. 
   The substrate may be fabricated as follows. A quartz substrate of size 2 mm×2 mm×0.5 mm is obtained. The quartz substrate can be fused silica, for example. The substrate is first evaporatively coated with a thin film of chromium using evaporation. The result in structure is then coated with polymethylmetachrylate or PMMA. 
   Slits  130  are opened in the front side  128 . This can be formed as two different openings, e.g., a first window  320  which is 100 microns wide and 500 microns long. A plurality of slits  325  are formed to the side of that window. These can be 1-2 microns wide, and 500 microns long. The slits have 10 micron separations from one another, and may be separated by 5 microns from the window  320 . The slits and optical window pattern can be opened in the PMMA using e-beam lithography. The chromium may be subsequently wet edged in the open areas to form better openings. 
   Thereafter, the surface is coated with a thick layer of photoresist in order to protect the surface. The back side  112  is also coated with photoresist. The front side alignment marks are used to form front side holes and open holes in the photoresist using an optical mask and UV exposure. The surface is then coated with metal for liftoff. The metal is removed using E-beam alignment marks. All of the photoresist can also be removed. 
   A PMMA layer is then deposited on the bottom of substrate  112 . Two different diffractive optical elements are formed in the PMMA layer. The PMMA laser lens  120  is formed which is 200 microns wide 500 microns long. The PMMA collection lens  135  is formed that is 400 microns wide 500 microns long. These are formed using E-beam lithography and developed using acetone. 
   The sensing element is then formed and mounted in a housing  400 . The housing  400  includes all of the structure therein, including the diode laser and optical receiver. 
   This system can produce significant advantages. In addition, modifications in this system are contemplated. For example, a diffractive optical element can be used in place of the optical window  320  in order to collect the scattered light more efficiently. 
   In another embodiment, shown in  FIG. 5 , the detector is mounted directly on the substrate  102 . This avoids the use of fibers, and reduces the parts count. In this embodiment, both the laser  100 , and photodiode  500  are mounted on a single substrate  102 . A controller  502  may also be mounted on the substrate. The controller may control both the laser  100  and the photodiode  500 . For example, the controller can instruct the laser what and when to emit. It can receive information from the photodiode, and interpret it in view of timing information sent to the laser. 
   Another embodiment which forms a fiber optic particle probe is shown in  FIGS. 6A and 6B . A diode laser is used along with curved gratings and detectors.  FIG. 6A  shows a configuration with a laser  600  emitting along both sides  602  and  604 . The two-sided emission provides laser output arms  606 ,  608 . Beam  606  is reflected by mirrors  612 ,  614 , and coupled to a curved grating  616 . Beam  608  is correspondingly coupled to grating  618 . The outputs  622 ,  624  of gratings  616 ,  618  are recombined off the surface at a point  610 . The point  610 , for example, can be 3 millimeters over the surface of the substrate  600 . A fringe pattern is formed by the recombination. 
   The fringe pattern is centered on a second laser beam, called the IMAX beam, that has been created by a second laser source  635 . The IMAX beam provides information on the size of the particle and as such is a particle-sizing beam  620 . 
   Light is scattered by the particles and received by photodetectors  642 ,  644 , which are mounted on the substrates in locations to receive the scattered light from the particles at the point  610 . The phase shift of the detectors is proportional to the particle size at the point  610 . An on-chip or off processor or controller may receive the signals from the photodetectors and calculate the particle size. 
     FIG. 6B  shows an alternative embodiment in which fringes in space are formed. A single ended diode  650  produces an output  652 . The diode laser output  652  is allowed to diverge onto a curved grating  654 , which is blocked in its center shown as  656 . 
   The grating  654  redirects the light  652  into two separated light beams  660 ,  662 , which are separated by the blocked portion  656 . The two light beams  660  and  662  are directed to intersect 3 millimeters off the surface at the point  664 . A separate laser  668  produces an IMAX beam  670 . As in the  FIG. 6A  embodiment, photodetectors  680 ,  682  detect the scattered light and use the scattered light to find particle size. 
   Another embodiment shown in  FIG. 7  uses a phased Doppler technique without the technique using the IMAX beam. The same structure of the laser  650  and curved grating  654  forming the LDA beams intersecting above the surface is defined. Detectors  700 ,  710  are located on an arm extending above the surface to receive the beam. This technique works best for particle sizes close to the laser wavelength. 
   As in the other embodiments, the scattered light gathered by the two detectors exhibits a phase shift that is proportional to the phase particle size. 
   Although only a few embodiments have been defined in detail above, other modifications are possible.