Abstract:
A remote passive condition sensor apparatus such as a temperature or humidity sensor apparatus. A corner cube reflector at a remote location, where it is not feasible to have electrical connections or batteries, receives and reflects back radiant energy to an optical readout instrument. Condition responsive elements in the corner reflector affect the amount of reflected radiant energy as a function of the condition sensed.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention is directed to a remote passive condition sensor such as a temperature or humidity sensor. The invention is of an optical type having no electrical connection or source to a remotely located completely passive sensor. 
     In the prior art there has been used remotely located sensors with electrical connections extended thereto from a site where readout is desired. Other remote sensors have been electrically powered by batteries or the like and utilize radio transmission to a readout station. 
     The present invention described herein utilizes temperature or humidity sensors which require no electrical circuitry whatsoever and are read out by simple optical techniques to permit low cost sensing in remote or inaccessible areas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a system sketch of a typical installation of the invention. 
     FIG. 2 is an enlargement of a portion of FIG. 1. 
     FIG. 3 is a top view and FIG. 4 is a cross sectional view of the detail of one single element of the optical readout sensor. 
    
    
     DESCRIPTION 
     Referring now to FIG. 1, there is shown an overview of the non-electrical, optically read sensor system 10 in which an observer accessible optical readout instrument 11 comprises an optical interrogator. The optical interrogator consists preferably of a dual wavelength light source 12 which is directed towards a sensor means 13 located at a generally inaccessible location such as the top of a smokestack 14. The sensor means is completely passive having no electrical connections thereto and no electrical source. The sensor means is optical in nature and is in the form of a corner cube reflector 15 (FIG. 2), i.e. a retroreflector. This corner cube reflector 15 is one which returns a light beam, such as beam 12 in the direction of its source. This return signal beam in FIG. 1 has been shown as beam 16. The corner cube reflector 15 comprises three reflecting surfaces 20, 21 and 22 each of which is at right angles to the other two. From this construction the result is obtained that no matter how the reflector is tilted, so long as the radiant beam enters the solid angle formed by the three reflecting surfaces, the light is reflected back towards the source. The return signal beam 16 returns to the optical readout instrument 11 for detection by a sensor therein. 
     Referring to FIG. 2, the incoming interrogation light beam 12 has preferably a component λ 1  and a component λ 2 . The reflecting surfaces 20 and 21 are mirror surfaces. The reflecting surface 22 which may be a wafer of silicon has a reflective reference area 23 with a λ 2  filter, and a reflective optical sensor area 24 with a λ 1  filter. The sensor area 24 is coated with a layer 25 of silicon nitride. The thickness may be on the order of 5000-10,000 Å, for example. The reflective characteristics of the sensor area 24 are modified by condition responsive microstructure sensor elements 30 which are formed in the silicon nitride face. As the sensor temperature (or humidity for humidity sensor) changes, the reflectance of the corner cube changes such that the measured reflectance is then functionally related to the sensor temperature. To avoid absolute calibration problems of the reflectance caused by fogging or dirt on the corner cube, the cube contains the reference region 23 which is spectrally filtered from the temperature sensor region 24 such that it can be used as a reference reflector on the cube sensor. To increase reflectivity the silicon reference area 23 can be coated with a multilayer dielectric like aluminum oxide or silicon dioxide, for example. This multilayer can also be tuned to the wavelength λ 2  so that it also acts as the λ 2  filter. 
     FIGS. 3 and 4 show the detail of one single element 30 of the optical readout sensor utilized in the implementation of the corner cube reflector. In FIG. 4 a Si 3  N 4  cantilever 50 has coated thereon in strips a layer of higher thermal expansion material 51 such as by sputtering or ion beam deposition. The higher thermal expansion material 51 may be, for example, the material aluminum or platinum and of the same order of thickness as the Si 3  N 4  layer 25. For a humidity responsive sensor, the high humidity expansion material 51 may be, for example, the material polyimide. Layer 51 is applied to the layer of Si 3  N 4  before the cantilevers are formed. Thus the cantilevered bilevel film consists of two different materials which have different thermal temperature expansion coefficients (or in the case of a humidity, different moisture expansion coefficients). The coated cantilever acts like a thermally responsive bimetal and bends upwardly or downwardly with changing temperature. FIG. 4 shows a first position of the cantilever at a temperature T 1  and shows a second position of the cantilever at a temperature T 2  . This change in angle of the numerous microsensors 30 on the corner cube then deflects more or less of the return radiation away from the local ground instrument 11. 
     A silicon substrate 55, to which the Si 3  N 4  layer is grown, has an anisotropically etched cavity 56 beneath the cantilever 50. Details of fabricating microstructure cantilevers of Si 3  N 4  over cavities in a silicon substrate similar to these can be found in U.S. Pat. No. 4,472,239, assigned to the same assignee as the present invention, and the teachings are incorporated by reference. Briefly however a monocrystalline silicon substrate body 55 has its surface covered with a dielectric layer 25, such as silicon nitride which is typically 3000 to 10,000 angstroms thick. Openings are then etched through the Si 3  N 4  to delineate the cantilever structure 50 shown in FIGS. 3 and 4. Strip geometry elements 51, shown as a,b,c,d,e,f, (FIG. 3) are preferred for causing preferential bending of the cantilevered Si 3  N 4  bridge. An anisotropic etchant that does not attack the silicon nitride is used to etch out silicon in a controlled manner from beneath the cantilever structures leaving a cavity 56 (KOH plus isopropyl alcohol is a suitable etchant). Since the surface of the silicon nitride layer 50 of region 24 may not be as good a reflector as the silicon, it may be desirable to coat the surface with 100 Å of aluminum, for example, to increase the reflectability.