Optical Transceiver Using Common Optical Path for Transmission and Reception

This invention relates to optical telecommunications systems. The invention is particularly advantageous when applied to free-space optical telecommunications.

Free-space optical telecommunications is of recently renewed interest because steps that have been taken to reduce official regulation of the telecommunications industry make it possible from a regulatory standpoint for new telecommunications services providers to enter the service areas of other existing service providers. To economically enter new areas, however, new service providers are looking for ways to avoid having to provide expensive hard-wired connections (e.g., twisted pair wire, coaxial cable, or optical fiber) to the location of each service customer. Radio communication is not a viable alternative for the bulk of the service that must be offered because the radio bandwidth available for such use is limited and still highly regulated.

Free-space optical communication has the potential to solve the problems mentioned above. Free-space optical communication means substantially unguided, point-to-point, line-of-sight communication using light (e.g., infra-red light) passing through the atmosphere. In general, such communication is bi-directional because the communications services provider must send information to and receive information from remotely located customers.

A possible limitation of free-space optical communication is that light can only be usefully transmitted through the atmosphere over relatively short distances. Typical distances might be in the range from about 500 meters to about 1,000 meters, dependent upon quality of service. Frequent regeneration of the optical information may therefore be necessary in a system that involves greater distances. In order to keep the system economically viable there is a need for low-cost and efficient optical transceivers which may be required in substantial numbers in the system.

In view of the foregoing, it is an object of this invention to improve and simplify optical transceivers.

It is a more particular object of this invention to provide improved and simplified optical transceivers that are particularly adapted for free-space optical telecommunications.

These and other objects of the invention are accomplished in accordance with the principles of the invention by providing optical transceivers having a high-speed photodetector for use in detecting the information which is transmitted by light to the transceiver, a position-sensing photodetector array (e.g., a quad sensor) for use in detecting alignment of the transceiver with the light being received, and a source of the light transmitted by the transceiver. To the greatest extent practical or reasonably possible, a common optical path is used for the light received by the transceiver and the light transmitted by the transceiver. Thus, the lens system that focuses received light on the high-speed photodetector is also used to focus received light on the position-sensing photodetector array, a beam splitter being used to direct respective portions of the received light to each of these two types of light receptors. This same lens system is also preferably used to transmit light out from the light source along the same path that light is received by the transceiver.

In a particularly preferred embodiment, the light from the light source passes out through a central aperture in the position-sensing photodetector array. By using a beam splitter to direct some of the received light to a position-sensing photodetector array which is on a different optical axis from the high-speed photodetector, interference of the position-sensing array with light reception by the high-speed photodetector is substantially reduced. This increases efficiency of light reception and information detection by the transceiver. Use of a common optical path and one lens system for most of the travel of both the received and transmitted light helps to lower the cost and increase the reliability of the transceiver.

Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.

FIG. 1 is a simplified sectional view of an illustrative embodiment of a free-space optical transceiver constructed in accordance with the principles of this invention.

FIG. 2 is a simplified elevational view of an illustrative embodiment of one portion of the apparatus shown in FIG. 1.

As viewed in FIG. 1, light directed at transceiver 10 through the atmosphere enters the transceiver from the right. At the left-hand end of the transceiver a concave mirror 20 directs incoming light back toward beam splitter 30. Beam splitter 30 allows some of the light from mirror 20 to pass through to high-speed photodetector 50 via filter 40. Elements 30, 40, and 50 are supported in the center of transceiver 10 by one or more struts 60 that extend radially out to the outer body of the transceiver. Mirror 20 focuses the light that passes through elements 30 and 40 on high-speed photodetector 50. Filter 40 is provided to reduce the amount of spurious light (e.g., ambient sunlight) that is seen by detector 50. Detector 50 converts the light it receives to an electrical signal for use by other equipment (not shown) at the location of transceiver 10.

Another portion of the light directed to beam splitter 30 by mirror 20 is reflected by the beam splitter toward filter 70 and photodetector array 80. Like filter 40, filter 70 is provided to reduce the amount of spurious light that is seen by array 80. For reasons that will become apparent as the description proceeds, array 80 preferably has a central aperture, and filter 70 also preferably has a corresponding and aligned central aperture. Array 80 may be a conventional quad sensor as shown in more detail in FIG. 2. This is a device with four, electrically isolated, photodetector segments 82a-d disposed circumferentially around a central aperture 84. Mirror 20 focuses received light on quad sensor 80 so that if transceiver 10 is properly aligned with the incoming light, each of segments 82 receives approximately the same amount of light. If transceiver 10 becomes misaligned, however, different amounts of light will be received by the different segments 82 of quad sensor 80. The output signals of the various segments will therefore also differ, and these signal differences can be used by transceiver positioning apparatus (not shown) to reposition the transceiver until it is again properly aligned with the incoming light.

Behind the aperture in photodetector array 80 is a source 90 for the light transmitted by transceiver 10. For example, source 90 may be a laser diode. Lens 100 in front of source 90 helps to focus light from the source for passage out through the apertures in photodetector array 80 and filter 70. After passing through these apertures, light from source 90 impinges on beam splitter 30. Some of this light passes through the beam splitter and is dissipated (i.e., absorbed) in absorption region 110. However, beam splitter 30 redirects another portion of the light from source 90 to mirror 20, which directs that light out of the transceiver (to the right as viewed in FIG. 1) as the light transmitted by the transceiver.

In order to prevent light from source 90 from being erroneously seen by photodetectors 50 and 80, it may be desirable to use a different frequency for the light from source 90 than is used for incoming light to the transceiver. In this way, filters 40 and 70 can be selected to pass the incoming light but to block any light from source 90 that would otherwise reach detectors 50 and 80.

From the foregoing, it will be seen that light is received and transmitted by transceiver 10 along a common optical axis. To a large degree, the same optical elements are economically used for both the received and transmitted light. This applies to mirror 20 and beam splitter 30. Near the end of its travel, the light going to detector array 80 is directed away from high-speed photodetector 50. Detector array 80, therefore, does not interfere with light reception by detector 50, as can happen in prior art arrangements in which light for the high-speed detector must pass through the aperture in the center of the detector array. Light reception by high-speed detector 50 is thereby improved.

It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. For example, instead of passing between free space and beam splitter 30 via a single convex mirror 20, light could pass between free space and the beam splitter via another type of optical system such as a Schmidt-Cassegrain telescope.