Abstract:
A bi-directional communication assembly is provided with commonly available optoelectronic components in a compact package. Diplex functionality is achieved by orienting the receiving detector at an angle with respect to the transmitting beam. An interference coating inside the detector, on the detector surface, or on a surface in intimate contact with the detector, reflects the transmitted beam while simultaneously allowing the receiving beam to pass through the coating to the light absorbing region. The combined function of the receiving detector, providing advantages of a common beam path and close proximity of the components, enable a compact package that can be placed within the space usually occupied by the transmitter light source alone.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     There are no applications that are related to the present application. 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     There are no rights that require licensing of the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Bi-directional optical communications of arbitrary signals, including but not limited to digital communication streams or short pulses (for example), at the same or differing wavelengths, either coupled to optical fibers or propagated in free space, whereby an interference filter is disposed on or in the photodetector. 
     2. Description of Related Art 
     Optical wavelength division multiplexing is a known technique for combining a plurality of optical signals having different wavelengths and inserting the wavelengths into a single optical fiber. The multiple wavelength signal is transmitted through the fiber to a receiving end where the wavelengths are separated and de-multiplexed accordingly. Typically, the wavelengths are multiplexed and de-multiplexed by the use of diffraction gratings or thin film interference filters. These devices provide a spectral selectivity that is predetermined in accordance with the wavelengths in use. 
     A known bi-directional optical transmission and reception arrangement has an optical transmitter, which is a laser diode, and an optical receiver, which is a photodetector with an absorbing region. Such an optical delivery arrangement comprises two optical lenses. One of the lenses is for optically imaging a laser beam of the first wavelength emitting from the laser diode on a specific spatial point at a distance from the laser coincident with the end of a fiber, and the other lens is for the optical imaging of the second wavelength emitting from the fiber end onto a photodetector. The arrangement includes optical shielding means which is composed of a separate, wavelength-selective optical filter arranged obliquely in the beam path of the radiation of the two wavelengths, and this optical filter is non-transmissive for one of the two wavelengths and is only transmissive for the other of the two wavelengths. 
     One advantage that results from the use of wavelength division multiplexing is that a single optical fiber can simultaneously carry a plurality of data signals, sometimes in two directions. 
     The conventional bi-directional transmission and reception systems suffer from numerous drawbacks related to the size and separate packaging of the individual devices, the cost of manufacture, as well as the difficulty associated with alignment of the system. 
     The need therefore exists for a compact bi-directional transmission/reception system having a compact and economical design and layout. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to the object of providing an improved bi-directional optical transmission and reception arrangement which can be constructed more compactly in comparison to present-day arrangements. The arrangement of the present invention is particularly useful for communications networks and/or rangefinding devices. 
     This object is achieved by an improvement in a bi-directional optical transmission and reception arrangement which is composed of an optical transmitter having an exit pupil for the emission of an optical radiation having a first wavelength, an optical discriminator having an optical detector for the reception of optical radiation having a second wavelength, an optical delivery means for delivering the radiation having the first wavelength emitting from the transmitter to a predetermined spatial point at a distance from the transmitter and from the discriminator and for delivering the radiation having the second wavelength emitting from another co-axial spatial point to the discriminator, comprising a wavelength-selective interference filter that is non-transmissive or reflective for the radiation of the first wavelength and is only transmissive for the radiation of the second wavelength, whereby the optical filter is disposed on or within the optical detector. By virtue of this arrangement, the present invention provides a uniquely compact and efficient system that is more economical and easier to manufacture. 
     Other advantages and features of the invention will be readily apparent from the following description of the preferred embodiments, the drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary embodiment of the present invention; 
         FIG. 2  is a schematic illustration of a second embodiment of the present invention; 
         FIG. 3  is a cross sectional illustration of a bi-directional module having a third embodiment of the present invention; 
         FIGS. 4A and 4   b  illustrate different embodiments of the invention having specific lens arrangements providing special benefits; 
         FIG. 5  is a schematic illustration of a cascading array of optical discriminators in accordance with the present invention; 
         FIG. 6  illustrates the features of the present invention incorporated into a rangefinding device. 
         FIGS. 7   a – 7   c  illustrate arrangements exemplifying various designs of the discriminator of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles of the present invention are particularly useful when incorporated in a bi-directional optical transmission and reception arrangement illustrated in  FIG. 1 . The arrangement of  FIG. 1 , as well as the arrangements of  FIGS. 2–4 , each comprise an optical transmitter (e.g., a laser diode source of the edge or surface emitting type), generally indicated at  10 , having a transmission exit pupil  12  for emitting an optical radiation having the first wavelength λ 1 . The optical transmitter  10  is aimed at an optical discriminator (e.g., a PIN photodiode having a dichroic thin film coating), generally indicated at  20 , having an absorbing region  22  acting as a receiver window for the reception of an optical radiation of the second wavelength λ 2 , whereby the optical discriminator  20  is preferably disposed at an angle of 45° with respect to the axis of the optical transmitter  10 . 
     As will become apparent from this description, an important feature of the instant invention is the design and arrangement of the discriminator set forth herein. In each embodiment of this invention, the discriminator is formed from a photodetector and an interference filter disposed on or within the photodetector body.  FIGS. 7   a – 7   c  illustrate various configurations for the discriminator detector sensor and for each illustration the light receiving signal direction is generally shown with an arrow ‘A’. In  FIG. 7   a , the discriminator  2  comprises a base substrate N-contact portion  2   a , a light absorbing junction  2   b  in the p-contact layer(s)  2   d  of the diode, and a series of vacuum deposited interference filter layers (e.g., dichroic, notch, band-pass, etc.)  2   c  disposed on the surface of the p-contact layer(s)  2   d.    
     In  FIG. 7   b , the discriminator  2 ′ comprises a base substrate N-contact portion  2   a ′, a light absorbing junction  2   b ′ in the P-contact layer(s)  2   d ′, and a series of vacuum deposited interference filter layers  2   c ′ disposed on the surface of the base substrate  2   a ′. The optical cross-talk between transmitter  210  and discriminator  220  can be mitigated by using this so-called rear illumination configuration of the detector portion of the discriminator. In this way, the substrate that comprises the detector allows any leakage of the transmitter signal that passes through the filter to propagate beyond the absorbing region of the detector. 
     In  FIG. 7   c , the discriminator  2 ″ comprises a base substrate N-contact portion  2   a ″, a light absorbing junction  2   b ″ in the P-contact layer(s)  2   d ″, a series of vacuum deposited interference filter layers  2   c ″ disposed on the surface of the base substrate  2   a ″, and a series of epitaxially grown interference filter layers  2   e  disposed within the discriminator as shown. As understood by those of skill in the art, the reflectivity of the coating  2   c ″ of  FIG. 7   c  may be modified to provide an alternate arrangement that functions as a Fabry-Perot PIN detector filter. 
     In the preferred design of  FIG. 1 , the optical transmitter  10  is aimed directly at the optical discriminator  20  without any intervening elements such that the detector absorbing region  22  of the discriminator  20  can maintain a relatively small area for maximum speed performance. A lens, generally indicated at  30 , is disposed in the optical path of the optical radiation. As previously discussed, an optical filter  50  is arranged on the surface of or disposed within the optical discriminator  20  to provide maximum compactness and optical efficiency. 
     In addition, a spatial point  42  at a distance from the transmitter exit pupil  12  and the detector absorbing region  22  is allocated in common to these elements and is illustrated as being coincident with the end face  41  of an optical fiber, generally indicated at  40 . As understood by those of skill in the art, it is not necessary that the spatial point  42  be coincident with the fiber end face  41 . The spatial point  42  is not to be understood as being a mathematical point but is a specific, small spatial region whose dimensions, for example, lie on the order of magnitude of the end face  41  of a core of an optical monomode fiber  40  or an optical multimode fiber. 
     The transmitter  10  is preferably composed of a laser diode having a strip-like optical waveguide  16  integrated in or on the epitaxially grown layers integrated onto the surface of a substrate  18  and contains optical-compatible material. The waveguide  16  comprises an end face or transmission exit pupil  12  facing toward the optical discriminator  20  from which the laser emission having the first wavelength λ 1  emerges parallel to the strip-like optical waveguide  16 . The transmitter  10  may also be surface emitting such that the first wavelength λ 1  emerges from the exit pupil perpendicular to the epitaxial layers as grown. 
     The discriminator  20 , for example, comprises a photodiode having an absorbing region  22  sensitive to the radiation having the second wavelength λ 2 . As is known in the art, such absorbing region  22  may be tuned to be sensitive to some wavelengths and not others. This absorbing region  22  is constructed or fashioned either on or under a surface  23 ′ of a substrate  23 . 
     Anyone skilled in the art knows that the lens  30  may be chosen from the group consisting of: spherical lenses, aspherical lenses, gradient lenses and diffractive optical elements, and/or various combinations. 
     The filter  50 , that is disposed on or within the optical discriminator  20 , is preferably a multiple layer stack that, for example, can be produced by vapor-deposition of dielectric layers onto the surface  23 ′ of the substrate  23  and the filter characteristics are the same as that of a cut-off filter that is substantially non-transmissive or reflective for the radiation having the first wavelength λ 1  and is substantially completely transmissive for the radiation having the wavelength λ 2 . Thus, the filter  50  is a high reflector for the radiation having the first wavelength λ 1  and is, as much as possible, anti-reflecting for the radiation having the second wavelength λ 2 . 
     It is preferably, but not necessarily, established that in the present embodiment, an axial ray  11  of the laser emission having the first wavelength λ 1  emitted from the transmitter exit pupil  12  impinges the filter  50  and radiation having the first wavelength λ 1  is reflected toward lens  30 . The radiation beam  11  then passes through the lens  30  and the spatial point  42  near the end face  41  of the fiber  40 . The radiation having the second wavelength λ 2 , that is divergently emitted from the end face of the fiber  40 , follows a similar optical path as radiation having wavelength λ 1 . The radiation λ 2  passes through the lens  30 , and impinges upon the discriminator  20 . Since the filter  50  is designed to provide an anti-reflecting effect for this radiation λ 2 , it passes through the filter  50  and impinges on the absorbing region  22  where it is converted into an electrical signal. 
     In a specific example of  FIG. 1 , the thin film coating filter  50  disposed on the detector  20  is greater than 99% reflective at 1310 nm and approximately 85% transmissive at 1550 nm (or vice versa for the opposite end of the link), and is largely polarization independent at 45 degrees. Of course, this example provides only one of many possible examples. 
     The fiber  40  can be a standard monomode fiber having a diameter of 125 μm and a core size of 9 μm. As understood by those of skill in the art, one way to suppress fiber end face reflections back to the transmitter is that the end face  41  of the fiber  40  facing toward the lens  30  may be somewhat inclined relative to an axis  44  of the fiber  40 . This inclination is such that a surface normal of the end face forms an angle with the axis of the fiber  40 . To improve transmission into the fiber  40 , the axis  44  of the fiber may then be tilted with respect to the axis of the input beam  11 . 
     A second embodiment of the invention is shown in a simplified form in  FIG. 2  which differs from the arrangement of  FIG. 1  in that the lens  130  is disposed between the optical transmitter  110  and the optical discriminator  120 . As in the embodiment of  FIG. 1 , the transmitter  110  (e.g., a laser diode source of the edge or surface emitting type), has a transmission exit pupil  112  for emitting an optical radiation having the first wavelength λ 1 . The emitted radiation is aimed at an optical discriminator (e.g., a dichroic coated PIN photodiode)  120  having an absorbing region  122  for the reception of an optical radiation of the second wavelength λ 2 , whereby the optical discriminator  120  is preferably disposed at an angle of 45° with respect to the axis of the optical radiation having the first wavelength λ 1 . Unlike the design of  FIG. 1 , lens  130  is disposed between the optical transmitter  110  and the optical discriminator  120  and therefore the absorbing region  122  can maintain a relatively small area for higher speed performance. As with the embodiment of  FIG. 1 , an optical filter  150  is disposed on the surface of or within the optical discriminator  120  to provide maximum compactness and optical efficiency. 
       FIG. 2  also shows a monitor diode  160  which may, likewise, be present in the arrangement of  FIG. 1 , but which was omitted in  FIG. 1  for the sake of simplicity. The monitor diode  160  will serve the purpose of monitoring the output of laser diode  110 . The monitor diode comprises an absorbing region  162  that receives radiation having the first wavelength λ 1  from the laser diode  110 . This radiation emerges from an end face  115  of the strip-shaped waveguide  116  of the laser diode  110  that faces away from the transmitter exit pupil  112 . For transmitter systems lacking a rear end face transmission signal or incorporating an integrated monitor, such a separate monitor diode  160  may not be required. 
     For a transmitter which does not have an alternate beam that can be used for monitoring purposes, the discriminator can in some cases be used by an external circuit to monitor the output of the transmitter. 
     It can be very advantageous for construction-related reasons to package the transmitter  10 ,  110 ; discriminator  20 ,  120  and optionally the lens  30 ,  130  as a subassembly. This is true both for the arrangement of  FIG. 1  as well as for the embodiment of  FIG. 2 . 
     In  FIG. 3 , a true-to-scale arrangement or embodiment is shown in cross sectional view with reference to the axis  244  of the fiber  240  and the lens  230 . These components are separately secured relative to the housing  270 , the transmitter  210  and discriminator  220 . The end face  231  of the lens  230  faces toward the optical discriminator  220  provided with a filter  250  as described above with respect to  FIGS. 1 and 2 . The discriminator  220  is also arranged relative to the optical transmitter  210 . A radiation signal λ 1  is transmitted from the optical transmitter onto the surface of the filter  250  whereby radiation signal λ 1  is reflected by the filter  250  toward the lens  230 . The radiation signal λ 1  is then transmitted through the lens  230  and focussed by the lens  230  onto the spatial point  242  nearly coincident with the end of the fiber  240 . The bi-directional light path comprises not only radiation signal λ 1  but also return radiation signal λ 2 . The radiation signal λ 2  is transmitted from the optical fiber  240  toward the lens  230 . The lens  230  then focuses the radiation signal λ 2  onto the absorbing region of the discriminator  220 . 
       FIG. 3  also shows a monitor diode  260  which monitors the laser diode  210 . The monitor diode  260  comprises an absorbing region that receives radiation having the first wavelength λ 1  from the laser diode  210 . This radiation emerges from an end face of the strip-shaped waveguide of the laser diode  210  that faces away from the transmitter exit pupil. 
     The transmitter  210 , discriminator  220 , and diode  260  are supported by pedestals  291 ,  292  and  293  respectively, which are arranged at a distance from one another. The pedestals are supported on a housing floor  280  of the housing. The transmitter  210 , discriminator  220 , and lens  230  form the primary components of a sub-unit in the present construction. 
     The lens  230 , the transmitter  210  and the discriminator  220  are covered by a cap-shaped housing cover  270  which can be either permanently or detachably connected to the housing floor  280 . The cover  270  is preferably composed of metal and has a window opening  275  for an undisturbed passage of the radiation having the first wavelength λ 1  into and the radiation having the second wavelength λ 2  out of the optical fiber  240 . 
     The fiber  240  and lens  230  are held in a flange that lies within the housing  270  and holds the end face  241  of the fiber  240  in the region of the window opening  275  of the housing cover  270 . Thus, the radiation of the first wavelength λ 1  transmitted through the lens  230  will be focussed at the spatial point  242 , and the radiation of the second wavelength λ 2  emitted from the end face  241  will be imaged near the light absorbing region  222  of the discriminator  220  by the lens  230 . Pin-shaped, electrical terminals are referenced  296 , and these project through the housing floor  280  into the inside of the housing and serve the purpose of electrical contacting of the electrical circuits and components inside of the housing. The monitoring diode  260  having the absorbing region  262  is also included among these and is held by contact strips connected to two of the terminals  296 ; of which, three are shown. Of course, many more may be used. 
       FIGS. 4A and 4B  illustrate alternate embodiments wherein the arrangement and disposition of various lens elements provide unique benefits and advantages.  FIG. 4A  shows an arrangement whereby a transmitter  310  transmits a first radiation signal λ 1  in a manner similar to the previous embodiments. The first radiation signal then passes through a first optical lens element  330   a  that focuses the first radiation signal λ 1  onto the discriminator  320 . Radiation signal λ 1  is reflected by the filter  350  disposed on or within the discriminator  320 , and the first radiation signal λ 1  is then transmitted through a second optical lens element  330   b  which focuses the radiation signal λ 1  to the spatial point  342  on or near the end face  341  of the fiber  340 . As with the previous embodiments, the second radiation signal λ 2  passes from the end face  341  through the second optical lens element  330   b . The second optical lens element  330   b  focuses the second radiation signal λ 2  through the interference filter  350  onto the absorbing region  322  of the discriminator  320 . As with the previous embodiments, the filter  350  is designed to reflect the wavelength λ 1  and to transmit the wavelength λ 2 . 
     For the embodiment of  FIG. 4A , the first and second radiation signals λ 1  and λ 2  follow equal paths but in opposite directions. Because the laser signal is focussed onto the discriminator  320 , the best coupling of the signal λ 1  onto the end face  341  of the fiber  340  coincides with the best coupling of the signal λ 2  coming from the fiber onto the absorbing region  322 . One benefit of this common-focus arrangement is that the absorbing region  322  can be kept as small as possible for maximum high speed operation. 
     In the embodiment of  FIG. 4B , the second optical lens element is removed. For this embodiment, the end face  341  of the fiber  340  may or may not coincide with the focus  342  of the transmitter depending on the fiber coupling requirements. As illustrated by  FIG. 4B , the first radiation signal λ 1  has a focus point  342  somewhere between the discriminator  320  and the end face  341  of the fiber  340 . The advantage of this embodiment is that the fiber may be placed very close to the discriminator and only a single lens is required to keep the absorbing region  322  as small as possible for high speed performance. 
     A cascaded discriminator arrangement of this invention is schematically illustrated in  FIG. 5 . As shown, the radiation signal having wavelengths (λ 1 –λ n ) is transmitted to a series of discriminators ( 420   1 – 420   n ) whereby an interference filter is disposed on or within each optical detector comprising the discriminator. As with the previous embodiments, the interference filters selectively reflect and transmit predetermined wavelengths. For the cascaded discriminator arrangement, a first signal (λ 1 ) passes through the filter of the first discriminator  420   1  whereby the signal is converted to an electrical signal by the absorbing region, while the remaining signals (λ 2 –λ n ) are reflected. The reflected signals (λ 2 –λ n ) propagate to a mirror  430   1  and these remaining signals are then directed to a second discriminator  420   2 . Here, a second signal (λ 2 ) passes through the filter of the second discriminator  420   2 , while the remaining signals (λ 3 –λ n ) are reflected by the second discriminator  420   2 . The reflected signals (λ 3 –λ n ) then propagate to a second mirror  430   2  and these remaining signals are then directed to a third discriminator  420   3 . Likewise, a third signal (λ 3 ) passes through the filter of the third discriminator  420   3 , while the remaining signals (λ 4 –λ n ) are reflected by the third discriminator  420   3 . The reflected signals (λ 4 –λ n ) then propagate to a third mirror  430   4  and these remaining signals are then directed to any number of downstream discriminators  420   n  as deemed appropriate. 
     For the preceding cascaded arrangement, it is understood that one or more mirrors may be replaced by lenses or other spatial phase converting optical elements and/or discriminator(s) of the type described above. 
     Similarly, the photodiodes can be advantageously fashioned or constructed as linear shaped photodiode arrays that are simple to manufacture on a surface of a substrate shared in common by all the photodiodes. 
       FIG. 6  illustrates a cross sectional view of a rangefinder or free space communications device incorporating the features of the present invention. As shown in  FIG. 6 , a signal transmitter  510  transmits an electromagnetic signal through a collimating lens  530  onto a discriminator member  520 . As with the previous embodiments, the discriminator  520  comprises a photodetector and an interference filter  520   a  disposed on or within the photodetector  520   b . For these applications, the interference filter  520   a  can be designed to be polarization (and possibly also wavelength) discriminating, and the discriminator  520  should be disposed at an angle θ which in this example is set at 60°. The electromagnetic signal reflected by the discriminator  520  is preferably s-polarized. The electromagnetic signal then passes through a quarter (¼) wave plate  540  disposed at a proper angle, and is converted to generally elliptical or circular polarization. Upon reflection from the object being detected or as a result of a signal generated by another transmitter, the electromagnetic signal re-traverses the quarter (¼) wave plate  540  and is converted to p-polarization at which time it is transmitted through the filter  520   a  to the absorbing region  520   b . The absorbed radiation is then converted into an appropriate electrical signal to be processed according to known techniques. 
     The monomode fibers  40 ,  140 ,  240  employed in the exemplary embodiments can also be multimode fibers. A variety of lenses may be employed in this invention, including but not limited to one or more far-field reducing lenses, cylindrical lenses, aspherical lenses, spherical lenses, gradient index lenses, and diffractive elements. The specific dimensions and characteristic of such lenses will depend on the specific application as will be understood by those of skill in the art. 
     Apart from the monitor detector, an arrangement of the invention advantageously requires only two opto-electronic components which are the optical transmitter and an optical discriminator whose face is reflective for one of the two wavelengths and is transmissive for the other. A separate filter and detector housing, and the additional lenses required for the detector, together with the assembly and adjustment costs thereof, are eliminated. This structure can be so compactly designed that the entire module, except for the lenses and when necessary the fibers can be accommodated in a housing normally required for the transmitter alone. Moreover, the invention is advantageously capable of being arranged in an array so that multi-channel, bi-directional modules for fiber arrays can also be realized. 
     Although various minor modifications may be suggested by those versed in the art, it should be understood that I wish to embody within the scope of the patent granted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.