Optical discriminator for transmitting and receiving in both optical fiber and free space applications

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.

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.

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 inFIG. 1. The arrangement ofFIG. 1, as well as the arrangements ofFIGS. 2–4, each comprise an optical transmitter (e.g., a laser diode source of the edge or surface emitting type), generally indicated at10, having a transmission exit pupil12for emitting an optical radiation having the first wavelength λ1. The optical transmitter10is aimed at an optical discriminator (e.g., a PIN photodiode having a dichroic thin film coating), generally indicated at20, having an absorbing region22acting as a receiver window for the reception of an optical radiation of the second wavelength λ2, whereby the optical discriminator20is preferably disposed at an angle of 45° with respect to the axis of the optical transmitter10.

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. 7a–7cillustrate various configurations for the discriminator detector sensor and for each illustration the light receiving signal direction is generally shown with an arrow ‘A’. InFIG. 7a, the discriminator2comprises a base substrate N-contact portion2a, a light absorbing junction2bin the p-contact layer(s)2dof the diode, and a series of vacuum deposited interference filter layers (e.g., dichroic, notch, band-pass, etc.)2cdisposed on the surface of the p-contact layer(s)2d.

InFIG. 7b, the discriminator2′ comprises a base substrate N-contact portion2a′, a light absorbing junction2b′ in the P-contact layer(s)2d′, and a series of vacuum deposited interference filter layers2c′ disposed on the surface of the base substrate2a′. The optical cross-talk between transmitter210and discriminator220can 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.

InFIG. 7c, the discriminator2″ comprises a base substrate N-contact portion2a″, a light absorbing junction2b″ in the P-contact layer(s)2d″, a series of vacuum deposited interference filter layers2c″ disposed on the surface of the base substrate2a″, and a series of epitaxially grown interference filter layers2edisposed within the discriminator as shown. As understood by those of skill in the art, the reflectivity of the coating2c″ ofFIG. 7cmay be modified to provide an alternate arrangement that functions as a Fabry-Perot PIN detector filter.

In the preferred design ofFIG. 1, the optical transmitter10is aimed directly at the optical discriminator20without any intervening elements such that the detector absorbing region22of the discriminator20can maintain a relatively small area for maximum speed performance. A lens, generally indicated at30, is disposed in the optical path of the optical radiation. As previously discussed, an optical filter50is arranged on the surface of or disposed within the optical discriminator20to provide maximum compactness and optical efficiency.

In addition, a spatial point42at a distance from the transmitter exit pupil12and the detector absorbing region22is allocated in common to these elements and is illustrated as being coincident with the end face41of an optical fiber, generally indicated at40. As understood by those of skill in the art, it is not necessary that the spatial point42be coincident with the fiber end face41. The spatial point42is 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 face41of a core of an optical monomode fiber40or an optical multimode fiber.

The transmitter10is preferably composed of a laser diode having a strip-like optical waveguide16integrated in or on the epitaxially grown layers integrated onto the surface of a substrate18and contains optical-compatible material. The waveguide16comprises an end face or transmission exit pupil12facing toward the optical discriminator20from which the laser emission having the first wavelength λ1emerges parallel to the strip-like optical waveguide16. The transmitter10may also be surface emitting such that the first wavelength λ1emerges from the exit pupil perpendicular to the epitaxial layers as grown.

The discriminator20, for example, comprises a photodiode having an absorbing region22sensitive to the radiation having the second wavelength λ2. As is known in the art, such absorbing region22may be tuned to be sensitive to some wavelengths and not others. This absorbing region22is constructed or fashioned either on or under a surface23′ of a substrate23.

Anyone skilled in the art knows that the lens30may be chosen from the group consisting of: spherical lenses, aspherical lenses, gradient lenses and diffractive optical elements, and/or various combinations.

The filter50, that is disposed on or within the optical discriminator20, is preferably a multiple layer stack that, for example, can be produced by vapor-deposition of dielectric layers onto the surface23′ of the substrate23and 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 λ1and is substantially completely transmissive for the radiation having the wavelength λ2. Thus, the filter50is a high reflector for the radiation having the first wavelength λ1and is, as much as possible, anti-reflecting for the radiation having the second wavelength λ2.

It is preferably, but not necessarily, established that in the present embodiment, an axial ray11of the laser emission having the first wavelength λ1emitted from the transmitter exit pupil12impinges the filter50and radiation having the first wavelength λ1is reflected toward lens30. The radiation beam11then passes through the lens30and the spatial point42near the end face41of the fiber40. The radiation having the second wavelength λ2, that is divergently emitted from the end face of the fiber40, follows a similar optical path as radiation having wavelength λ1. The radiation λ2passes through the lens30, and impinges upon the discriminator20. Since the filter50is designed to provide an anti-reflecting effect for this radiation λ2, it passes through the filter50and impinges on the absorbing region22where it is converted into an electrical signal.

In a specific example ofFIG. 1, the thin film coating filter50disposed on the detector20is 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 fiber40can 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 face41of the fiber40facing toward the lens30may be somewhat inclined relative to an axis44of the fiber40. This inclination is such that a surface normal of the end face forms an angle with the axis of the fiber40. To improve transmission into the fiber40, the axis44of the fiber may then be tilted with respect to the axis of the input beam11.

A second embodiment of the invention is shown in a simplified form inFIG. 2which differs from the arrangement ofFIG. 1in that the lens130is disposed between the optical transmitter110and the optical discriminator120. As in the embodiment ofFIG. 1, the transmitter110(e.g., a laser diode source of the edge or surface emitting type), has a transmission exit pupil112for emitting an optical radiation having the first wavelength λ1. The emitted radiation is aimed at an optical discriminator (e.g., a dichroic coated PIN photodiode)120having an absorbing region122for the reception of an optical radiation of the second wavelength λ2, whereby the optical discriminator120is preferably disposed at an angle of 45° with respect to the axis of the optical radiation having the first wavelength λ1. Unlike the design ofFIG. 1, lens130is disposed between the optical transmitter110and the optical discriminator120and therefore the absorbing region122can maintain a relatively small area for higher speed performance. As with the embodiment ofFIG. 1, an optical filter150is disposed on the surface of or within the optical discriminator120to provide maximum compactness and optical efficiency.

FIG. 2also shows a monitor diode160which may, likewise, be present in the arrangement ofFIG. 1, but which was omitted inFIG. 1for the sake of simplicity. The monitor diode160will serve the purpose of monitoring the output of laser diode110. The monitor diode comprises an absorbing region162that receives radiation having the first wavelength λ1from the laser diode110. This radiation emerges from an end face115of the strip-shaped waveguide116of the laser diode110that faces away from the transmitter exit pupil112. For transmitter systems lacking a rear end face transmission signal or incorporating an integrated monitor, such a separate monitor diode160may 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 transmitter10,110; discriminator20,120and optionally the lens30,130as a subassembly. This is true both for the arrangement ofFIG. 1as well as for the embodiment ofFIG. 2.

InFIG. 3, a true-to-scale arrangement or embodiment is shown in cross sectional view with reference to the axis244of the fiber240and the lens230. These components are separately secured relative to the housing270, the transmitter210and discriminator220. The end face231of the lens230faces toward the optical discriminator220provided with a filter250as described above with respect toFIGS. 1 and 2. The discriminator220is also arranged relative to the optical transmitter210. A radiation signal λ1is transmitted from the optical transmitter onto the surface of the filter250whereby radiation signal λ1is reflected by the filter250toward the lens230. The radiation signal λ1is then transmitted through the lens230and focussed by the lens230onto the spatial point242nearly coincident with the end of the fiber240. The bi-directional light path comprises not only radiation signal λ1but also return radiation signal λ2. The radiation signal λ2is transmitted from the optical fiber240toward the lens230. The lens230then focuses the radiation signal λ2onto the absorbing region of the discriminator220.

FIG. 3also shows a monitor diode260which monitors the laser diode210. The monitor diode260comprises an absorbing region that receives radiation having the first wavelength λ1from the laser diode210. This radiation emerges from an end face of the strip-shaped waveguide of the laser diode210that faces away from the transmitter exit pupil.

The transmitter210, discriminator220, and diode260are supported by pedestals291,292and293respectively, which are arranged at a distance from one another. The pedestals are supported on a housing floor280of the housing. The transmitter210, discriminator220, and lens230form the primary components of a sub-unit in the present construction.

The lens230, the transmitter210and the discriminator220are covered by a cap-shaped housing cover270which can be either permanently or detachably connected to the housing floor280. The cover270is preferably composed of metal and has a window opening275for an undisturbed passage of the radiation having the first wavelength λ1into and the radiation having the second wavelength λ2out of the optical fiber240.

The fiber240and lens230are held in a flange that lies within the housing270and holds the end face241of the fiber240in the region of the window opening275of the housing cover270. Thus, the radiation of the first wavelength λ1transmitted through the lens230will be focussed at the spatial point242, and the radiation of the second wavelength λ2emitted from the end face241will be imaged near the light absorbing region222of the discriminator220by the lens230. Pin-shaped, electrical terminals are referenced296, and these project through the housing floor280into the inside of the housing and serve the purpose of electrical contacting of the electrical circuits and components inside of the housing. The monitoring diode260having the absorbing region262is also included among these and is held by contact strips connected to two of the terminals296; of which, three are shown. Of course, many more may be used.

FIGS. 4A and 4Billustrate alternate embodiments wherein the arrangement and disposition of various lens elements provide unique benefits and advantages.FIG. 4Ashows an arrangement whereby a transmitter310transmits a first radiation signal λ1in a manner similar to the previous embodiments. The first radiation signal then passes through a first optical lens element330athat focuses the first radiation signal λ1onto the discriminator320. Radiation signal λ1is reflected by the filter350disposed on or within the discriminator320, and the first radiation signal λ1is then transmitted through a second optical lens element330bwhich focuses the radiation signal λ1to the spatial point342on or near the end face341of the fiber340. As with the previous embodiments, the second radiation signal λ2passes from the end face341through the second optical lens element330b. The second optical lens element330bfocuses the second radiation signal λ2through the interference filter350onto the absorbing region322of the discriminator320. As with the previous embodiments, the filter350is designed to reflect the wavelength λ1and to transmit the wavelength λ2.

For the embodiment ofFIG. 4A, the first and second radiation signals λ1and λ2follow equal paths but in opposite directions. Because the laser signal is focussed onto the discriminator320, the best coupling of the signal λ1onto the end face341of the fiber340coincides with the best coupling of the signal λ2coming from the fiber onto the absorbing region322. One benefit of this common-focus arrangement is that the absorbing region322can be kept as small as possible for maximum high speed operation.

In the embodiment ofFIG. 4B, the second optical lens element is removed. For this embodiment, the end face341of the fiber340may or may not coincide with the focus342of the transmitter depending on the fiber coupling requirements. As illustrated byFIG. 4B, the first radiation signal λ1has a focus point342somewhere between the discriminator320and the end face341of the fiber340. 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 region322as small as possible for high speed performance.

A cascaded discriminator arrangement of this invention is schematically illustrated inFIG. 5. As shown, the radiation signal having wavelengths (λ1–λn) is transmitted to a series of discriminators (4201–420n) 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 (λ1) passes through the filter of the first discriminator4201whereby the signal is converted to an electrical signal by the absorbing region, while the remaining signals (λ2–λn) are reflected. The reflected signals (λ2–λn) propagate to a mirror4301and these remaining signals are then directed to a second discriminator4202. Here, a second signal (λ2) passes through the filter of the second discriminator4202, while the remaining signals (λ3–λn) are reflected by the second discriminator4202. The reflected signals (λ3–λn) then propagate to a second mirror4302and these remaining signals are then directed to a third discriminator4203. Likewise, a third signal (λ3) passes through the filter of the third discriminator4203, while the remaining signals (λ4–λn) are reflected by the third discriminator4203. The reflected signals (λ4–λn) then propagate to a third mirror4304and these remaining signals are then directed to any number of downstream discriminators420nas 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. 6illustrates a cross sectional view of a rangefinder or free space communications device incorporating the features of the present invention. As shown inFIG. 6, a signal transmitter510transmits an electromagnetic signal through a collimating lens530onto a discriminator member520. As with the previous embodiments, the discriminator520comprises a photodetector and an interference filter520adisposed on or within the photodetector520b. For these applications, the interference filter520acan be designed to be polarization (and possibly also wavelength) discriminating, and the discriminator520should be disposed at an angle θ which in this example is set at 60°. The electromagnetic signal reflected by the discriminator520is preferably s-polarized. The electromagnetic signal then passes through a quarter (¼) wave plate540disposed 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 plate540and is converted to p-polarization at which time it is transmitted through the filter520ato the absorbing region520b. The absorbed radiation is then converted into an appropriate electrical signal to be processed according to known techniques.

The monomode fibers40,140,240employed 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.