Fiber-optic acceleration sensor with photoluminescent material
BACKGROUND OF THE INVENTION 
     This invention relates to a fiber-optic sensor for sensing dynamic acceleration which is arranged so that the acceleration gives rise to changes in the angle between at least one body and the optical axis of at least one lens or graded index type (a GRIN lens), at least one optical fiber being arranged to transmit optical energy to the sensor. The GRIN lens is utilized for amplifying the modulation of the optical signal. 
     The sensor is intended to be employed in a fiber-optic system such as the system described in U.S. Pat. No. 4,581,528 which is assigned to the assignee of this application. The device described herein constitutes an improvement in the sensor for acceleration sensing described in the noted patent. 
     SUMMARY OF THE INVENTION 
     A sensor according to the invention is characterized in that the optical energy reflected back into the fiber is arranged to be dependent on the angle and essentially independent of the distance between the body and the lens, and in that the sensor comprises at least one photoluminescent material arranged so that a photoluminescent signal constitutes a part of the optical output from the sensor. The improvement represented by this invention makes it possible to obtain a higher signal/noise ratio and a resultant improvement in accuracy of measurement. By utilizing a reflex signal from the movable body in combination with a control circuit, controlled by the photoluminescence, signal, a high signal level is obtained while at the same time the sensing system can be made insensitive to losses and reflections at fiber joints and branches in the optical system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described in greater detail, by way of example, with reference to the accompanying drawing, wherein 
     FIG. 1 is a sectional side elevation of a first embodiment of the inventive sensor, 
     FIG. 2 is an end view of the optical fiber of the device of FIG. 1 with the projected image shown thereon, 
     FIG. 3 shows a second embodiment of the inventive sensor using a different arrangement for achieving a photoluminescence signal, 
     FIG. 4 shiows a third embodiment of the inventive sensor, and 
     FIG. 5 shows a fourth embodiment of the inventive sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show how optical energy from a light emitting diode (LED) is supplied to a the inventive sensor via an optical fiber 1. The bundle of rays diverging from the end 9 of the fiber 1 is collimated in a lens 2 of GRaded Index type (a GRIN lens). The parallel ray bundle leaving lens 2 strikes a reflecting beam element 4, after which the optical energy is focussed back towards the end 9 of the fiber 1. However, the sensor is adjusted so that approximately only one half of the image 8 of the fiber end is reflected back to strike the fiber end 9 as can clearly be seen in FIG. 2. The remaining amount of the reflected optical energy strikes a mass 7 of photoluminescent material and gives rise to photoluminescence. When an acceleration in the direction of one of the arrows A is applied to the sensor, the beam element 4 will flex and the angle 11 between the beam element 4 and the axis 10 of the lens 2 will change, causing a shift in the image 8 so that a different amount of reflected optical energy will be returned through the fiber 1. The photoluminescence which is generated in the mass 7 is also reflected by the beam element 4 and is thus returned via the fiber 1. 
     Both the reflux signal and the photoluminescence signal are then processed by means of a system described, for example, in the above-noted U.S. patent. A body 5 is carried by the beam element 4 and provides a measure of mechanical signal amplification. The body 5 should be located symmetrically around the beam element 4 so that the sensor is insensitive to accelerations in the longitudinal direction of the beam (i.e., in the directions of the arrows B. 
     The mass 7 of photoluminescent material can be formed as epitaxial layers in a semiconductor material, for example Al x  Ga 1-x  As. The beam element 4 can also be made from a photoluminescent material, for example Al x  Ga 1-x  As and that would allow the mass 7 to be omitted. 
     FIG. 3 shows a further embodiment of accelerometer for generating a photoluminescent signal. The photoluminescence in this case is obtained from ions 12 of rare earth metals, for example, neodymium. These ions can be inserted into a glassy material, for example in the end of the fiber 1 as shown in FIG. 3. The other numbered integers shown in FIG. 3 correspond to the same numbered integers in FIG. 1. 
     FIG. 4 shows a still further embodiment. This embodiment is capable of sensing acceleration perpendicular to the optical axis 10 of the GRIN lens 2. The ray path is here changed by means of a prism 13, which could be replaced by a mirror. The angle 11 shown on the righthand side of FIG. 4 to which the beam element 4 is deflected, represents a measure of the acceleration to which the sensor has been subjected. 
     The beam element 4 contains a resilient part and a body 5 mechanically connected to the element 4. The body 5 may be distributed equally on the left- and righthand sides, respectively, of the element 4 (see FIGS. 3 and 4). As mentioned, the elment 4 may be made of a semiconductor material, and the photoluminescent material 7 may be made of a semiconductor material, such as Al x  Ga 1-x  As or In x  Ga 1-x  As y  P 1-y . 
     The mass 7 of photoluminescent material can be placed against the end surface 9 of the fiber 1 and completely (see FIG. 5) or partially covers the end surface. The photoluminescent material 12 may consist of metal ions. 
     The embodiments described above may be varied in many ways within the scope of the appended claims.