Abstract:
A reflected light barrier has a housing with a base element. A semiconductor light emitter and a semiconductor light receiver are arranged on the base element in such a way that there is no free-standing partition between the light emitter and light receiver optically screening the light receiver from the light emitter. The light emitter has such a low beam divergence that the crosstalk from the light emitter to the light receiver is kept within operating tolerances.

Description:
[0001]    CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0002]    This application is a continuation of copending international application PCT/DE99/02591, filed Aug. 18, 1999, which designated the United States. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    The present invention relates to reflective radiation barriers and more particularly to a barrier having a housing with a base element embedded therein. The base houses a radiation emitter or transmitter and receiver or detector, all of which may be monolithic semiconductors built on a common substrate. The radiation may be light and the present invention will be discussed with respect to light emission. However, the present invention is not limited to light applications and may include other radiation wavelengths as will be discussed below.  
           [0004]    Reflective or reflexive light barriers may take many forms and applications as shown in the prior art. For example, application of the barriers may be for space detection, as well as tracking and automation systems. The barriers may frequently come in the form of miniature components and be applied in consumer and entertainment electronics. The barriers are increasingly used for monitoring mechanical device components such as video player load flap and sleds of CD drives. Essential criteria for such applications include size, degree of miniaturization as and cost.  
           [0005]    U.S. Pat. No. 3,842,263 sets out a photoelectric switching device wherein the light transmitter and the light receiver are each placed in separate, channel-shaped indentations of a common cased body. The channel-shaped indentation associated with the light transmitter serves as a directional for the light emitted by the transmitter. The partition which runs between the two indentations avoids cross-talk between emitted light and the receiver.  
           [0006]    German Patent DE 36 33 181 A1 sets out a reflected light barrier wherein the light transmitter and the light receiver are placed on a common semiconductor substrate. A free-standing screen, produced from the semiconductor substrate by an etching step, is provided for the optical separation of light transmitter and light receiver.  
           [0007]    U.S. Pat. No. 3,697,762 sets out a light scanner having a partition between an LED (luminous source) and a photo element (light receiver) in order to avoid cross talk between the source and receiver.  
           [0008]    German Patent DE 39 29 085 A1 sets out a reflex light barrier using LED&#39;s as light transmitters.  
           [0009]    Additional reflective light barriers and light scanners are described in the German patents DE 298 02 763 U1, DE 43 04 343 A1 and DE 28 24 583 C3, which set out input as well as output optics and are only marginally appropriate for miniaturization.  
           [0010]    Still additional reflective light barriers are set out in U.S. Pat. No. 5,753,929 and 3,842,263; German Patents DE 43 37 005 A1, DE 39 29 085 A1 and DE 198 28 069 A1; European Patent Applications EP 0 786 839 A1 and EP 0 751 510 A2; and Japanese Abstract 09321597.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    It is an advantage of the present invention to provide a reflective light barrier with a high degree of miniaturization potential at reduced costs. Herein, the light transmitter may be a vertically emitting semiconductor transmitter with a vertical resonator such as a vertical cavity surface emitting laser (VCSEL) or a resonant cavity light emitting diode (RCLED). Through use of a vertically emitting semiconductor transmitter with a vertical resonator, especially a VCSEL or RCLED, low beam divergence can be obtained. Accordingly, cross-talk between transmitter and receiver can be kept below a working tolerance of the present apparatus. As such, a partition is no longer necessary. In practice, a certain degree of cross-talk may be tolerated. The particular threshold may vary depending upon application and optical path length, reflectivity of the detected object, and the like. In addition, as VCSELs generally display a lower beam divergence than RCLEDs, the former may be applied where the latter divergence is unacceptably high.  
           [0012]    By avoiding use of a free-standing partition which is common in the art, the possibility now exists for closer placement of the transmitter and receiver within the present apparatus. A reflective light barrier with overall smaller dimensions is therefore possible. A preferred design of the invention includes a maximum lateral casing size equal to or less than 2,5 mm. Alternatively, the casing size may be smaller than 0,5 mm.  
           [0013]    The light transmitter can be built from a multiple number of single light transmitter elements. The elements may, for example, be arranged in an uni-dimensional row or a two-dimensional array. By optimizing the number of transmitter elements, the geometrical arrangement of the transmitters, the orientation of the transmitter-arrangement relative to the light receiver, the intensity of light, and other parameters, a minimization of cross-talking can be obtained for a select casing size and distance. Hence, cross talk may be minimized between transmitter and receiver.  
           [0014]    A further reduction of cross-talk may be obtained via use of polarizers. In particular, a polarizer may be placed in the beam path incident upon the light receiver. Because the object location, beam source and receiving point are known, it will be known in advance what effect the detected object will have on the polarity of light reflected off it. As such, the polarizer placed on the receiver may be preselected so as to pass only light reflected by the object. Other light, including that emitted by the transmitter would not pass. As such, the effectiveness of the present apparatus increases and the need for a light partition between the transmitter and receiver is obviated. A second possibility may be the inclusion of a polarizer on the transmitter as well. Herein the transmitter polarizer may be at about 90 degrees to the receiver polarizer thereby again obviating the need for a barrier.  
           [0015]    Still a further possibility includes the use of an optical filter. The filter may be a day light filter and/or a band pass filter. The filter may be placed in the incident beam path upstream from the light receiver in order to suppress stray day light. The usage of a day light filter is preferable if the emission wavelength of the light transmitter lies outside of the visible area (for example at approx. 1 μm). Selection and use of a band pass filter may be effected such that its transmitted wavelength substantially corresponds to the emission wavelength of the light transmitter.  
           [0016]    Still a further possibility includes use of a modulator to modulate the light transmitted by the light transmitter and a demodulator to distinguish between received, modulated reflected light and received, non-modulated stray light.  
           [0017]    Still a further possibility includes use of an integrated circuit (IC) coupled with the light transmitter and/or the light receiver. The IC may be located in the casing and be provided according to a functional design of the invention. The integrated circuit can either comprise a driver switching circuit for the light transmitter as well as a scoring switching circuit. If the reflected light barrier is operated with modulated emission light, the integrated circuit can be additionally equipped with a driver-equipped with a modulator stage and scoring circuit-coupled to the demodulator.  
           [0018]    The transmitter, receiver, and integrated circuit may be formed monolithically on a common semiconductor substrate thereby creating elements for an especially high degree of miniaturization.  
           [0019]    The structure of the present invention may include a circular casing embedded in the base housing. The base may include at least four internally mounted surfaces having through holes and electrical contacts. Accordingly, the transmitter and receiver may be applied on the contact surface. The application may be electrical bonding as well as possible electrical cross bonding to one of the remaining contact surfaces. Such casings are standard in the art and may be produced by large scale manufacturing for the absorption of three luminescent diodes of the elementary colors green, red, blue, to be used as color picture point in a display. These casings may therefore be economical and may further contribute to an overall cost reduction of the present invention. 
       
    
    
       [0020]    The invention is explained in greater detail below and by reference to exemplary embodiments shown in the drawings wherein like numerals refer to equivalent elements.  
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0021]    [0021]FIG. 1 depicts a perspective view of a reflected light barrier according to the present invention;  
         [0022]    [0022]FIG. 2 depicts a sectional view of the reflected light barrier of FIG. 1 along the line I-I; and  
         [0023]    [0023]FIG. 3 depicts a diagrammatic view of the structure of a VCSEL. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    As depicted in FIG. 1, the present reflected light barrier  1  comprises a plastic casing  2  with an essentially rectangular circumferential contour. The casing may be manufactured from other known materials envisioned by one skilled in the art. The casing  2  is provided with a central opening  3 . The lateral dimensions of the casing may measure about 2-3 mm in the depicted embodiment. The height of the casing may measure about 2 mm. In addition to the depicted embodiment, an essentially smaller casing  2  is possible (with an integrated reflected light barrier), wherein the casing dimensions (width, length, and height) are each less than 0,5 mm.  
         [0025]    The casing  2  includes a dropped corner portion  4  which facilitates the handling of the casing  2  during automated or manual manufacturing sequences (for example an insertion step). The casing further includes centering recesses  5   a  interspersed between and among centering projections  5   b  set out along the circumference for further alignment of the casing.  
         [0026]    The central opening  3  may be ground through the top of casing  2  to the casing bottom  7 . Four electrical metal contact surfaces (bond pads)  6   a,    6   b,    6   c ,  6   d  are placed on the surface of the casing bottom  7 . Although four pads are depicted, the number of pads is not limited to that number and may include any number envisioned by the skilled artisan. The contact surfaces  6   a  to  6   d  are spaced and are electrically insulated and otherwise not in electrical contact. As shown in FIG. 2, contacting pins  8   b,    8   d,  which project from the bottom side  9  of the casing bottom  7 , and run there through, are provided for electrical bonding of the contact surfaces  6   a  to  6   d.    
         [0027]    A light transmitter  10  is placed on contact surface  6   d.  A light receiver  11  is placed on contact surface  6   b.  Both transmitter and receiver have two electrical contacts. The first is in electrical contact with surfaces  6   b  and  6   d  respectively. The second is connected to surfaces  6   a  and  6   c  respectively. The receiver  11  includes a sickle shaped metal film  14  bonded to contact surface  6   c  via wire  12 . Likewise, transmitter  10  is connected with bond wire  13  extending from the contact surface  6   a  to a center contact  15  on the surface of the transmitter.  
         [0028]    As also depicted in FIG. 2, the distance W between transmitter  10  and receiver  11  is approx. 0,7 mm. The distance W can be less than 0,1 mm at lateral casing sizes of less than 0,5 mm.  
         [0029]    Transmitter  10  comprises four contiguous VCSELs  16  in a structured array. The number of VCSELs is a matter of design choice with other numbers and arrangements possible. Benefits of higher numbers and alternative arrangements include obtaining select and/or greater irradiance.  
         [0030]    In operation, light emitted by transmitter  17  is reflected off of an object  30  and the reflection  18  is detected by receiver  11 . In the present embodiment, no partition between the transmitter and receiver is needed. The casing bottom  7  is planar and the transmitter and receiver are likewise coplanar. However, alternate embodiments may include a non-planar casing bottom. The absence of a free-standing partition between the transmitter and receiver enables a reduction of the lateral casing size. In addition, the absence translates into one less required component and hence an overall cost savings.  
         [0031]    The possibility of cross talk normally exists absent a barrier between transmitter and receiver. To address this possibility, the present invention makes use of a low diverging light source, such as a VCSEL, RCLED or other such source. With such sources, present cross talk, if any, is within an acceptable tolerance for effective operation. In addition to low diverging light sources, it is within the scope of the present invention to make use of other means to reduce cross talk. For example, cross talk can herein be reduced by use of polarizers (not shown). The polarizers may be inserted in to the beam paths emitting from the transmitter  10  and/or incident upon the receiver  11 . Because the object location, beam source and receiving point are known, it will be known in advance what effect the detected object will have on the polarity of light reflected off of it. As such, the polarizer placed on the receiver may be preselected so as to select or pass only light of that polarity. Accordingly, other light, including that emitted by the transmitter would not pass. As such, the effectiveness of the present apparatus increases and the need for a light barrier between the transmitter and receiver is again obviated. An additional possibility may be the inclusion of a polarizer on the transmitter as well. Herein the transmitter polarizer may be at 90 degrees to the receiver polarizer thereby again obviating the need for a barrier. An additional possibility is the inclusion of polarizers with both the transmitter and receiver such that the polarizers select different polarization states and discard others. As such, light emitted by the transmitter would not pass the receiver polarizer. Rather, only light reflected off of the object for which the polarizer is already preset to select, becomes incident upon the receiver.  
         [0032]    An additional option is to model the casing bottom in a step wise manner (not shown) such that the transmitter  10  is not on the same plane as the receiver  11 . Herein, the transmitter may be either below or above the receiver. The casing bottom can be so modeled by means and methods known to one skilled in the art.  
         [0033]    An additional option arises in the use of several individual transmitters. For example, when several single VCSELs  16  in the light transmitter  10  are employed, the cross-talk can be minimized by optimizing the geometrical arrangement of the single light transmitters (row, array or overall geometry arrangement) as well as by the selection of an appropriate orientation (for the arrangement) relative to the light receiver  11  such that a minimum of light passes directly between transmitter and receiver.  
         [0034]    An additional option arises with the suppression of day light. This suppression is desired not only for the reduction of cross talk, but also for the enhancement of the signal to noise ratio as well as the possibility of using particular wavelength sources and related detectors not within the visible spectrum. Herein, for example, a day-light filter or a band-pass filter can be placed in the beam path upstream from the light receiver  11 . An additional option for improving the signal to noise ratio, distinguishing between desired and other incident light, and other benefits, arise from modulating emitted light  17  and appropriately demodulating received light at receiver  11 . Both may be driven by an appropriate IC as known to one skilled in the art.  
         [0035]    The circumferential interior walls of the mounting indentation  3  can be further developed as a reflector for the light receiver  11  as known to one skilled in the art. Reflecting walls serve to concentrate the reflected light  18  on the light receiver  11 , thereby enhancing the resulting signal. If the light receiver  11  is placed locally in the mounting indentation  3 , the reflector may be asymmetrical with respect to casing center line I. As such, light will be reflected by the reflector into the receiver. The reflector can for example be realized by a metallic mirror coating of the circumferential interior walls of the mounting indentation  3 .  
         [0036]    The mounting indentation  3  may be filled with a compound, such as transparent epoxy  19 . The epoxy  19  serves to protect transmitter  10  and receiver  11  among other components. Preferably, the epoxy  19  should have a high transparency and homogeneity so as to keep leakage of the emitted light  17  in the epoxy  19  as low as possible. Leaking or scattered light within or caused by the epoxy may be picked up by the receiver  11 , thereby increasing cross talk, decreasing the signal to noise ratio, and the like. Furthermore, such scattering decreases the strength of the emitted light  17  thereby hampering the effectiveness of the inventive apparatus. To avoid leakage produced by separation between light transmitter  10  and epoxy  19 , especially in the transition region, the epoxy  19  should preferably have a similar thermal coefficient of expansion to the semiconductor material of the light transmitter  10 .  
         [0037]    The emission surface  19   a  of the epoxy  19  may be planar or concave so that reflected emitted light  17  at the emission surface  19   a  is reflected or otherwise directed away from the direction of the light receiver. Furthermore, the epoxy surface  19   a  can be provided with an antireflective coating, for example an optical λ/2 coating.  
         [0038]    [0038]FIG. 3 shows the structure of a VCSEL  16  in a schematic view. The VCSEL  16  includes a substrate (for example GaAs)  20 , on which a first stack  21  of epitaxially applied mirror stacks  21   a,    21   b,  an active zone  22  and a second stack  23  of mirror stacks  23   a,    23   b  are placed. The layered stacks  21  and  23  may comprise alternating GaAs layers  21   a,    23   a  with low band gaps. Likewise, layers  21   b,    23   b  of AlxGa 1 -xAs with a high aluminum content (for example x=0,96) and a high band gap may also be used. The active zone  22  includes a quantum well layer  25  between two barrier layers  24 . Two optional, insulating layers may be used with a central aperture ( 26 ). These layers may serve to control the lateral mode profile of the emitted light  17 . Metallic bonding layers  27 ,  28  are placed on the bottom and top of the described layer structure. The reflectivity of the first and second mirror layer stack  21 ,  23  may be above 95% for a VCSEL so as to enable a laser-operation. A RCLED may comprise the layer structure  20 - 23  presented in FIG. 3. The essential difference to a VCSEL  16  lies in that the reflectivity of the mirror layer stack  21 ,  23  may be lower to about 90%.  
         [0039]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications would be obvious to one skilled in the art are intended to be included within the scope of the following claims.