Spatial transmit / receive separation apparatus and method for optical communication systems

Abstract

To ensure genderless interoperability in an optical communication system, an apparatus for separating a transmit optical beam and a receive optical beam having a common wavelength includes a transmit path transmitting a transmit optical beam at a specific fundamental wavelength and a receive path propagating a receive optical beam at the same specific fundamental wavelength, at least a portion of the receive path being separated from the transmit path. The apparatus further includes an annular mirror having a receive beam area and a central aperture, the transmit path including a steering mirror, and the receive path including a steering mirror. The steering mirror has a single surface that reflects the entire transmit optical beam transmitted along the transmit path, and the single surface of the steering mirror reflects the entire receive optical beam received along the receive path. Also disclosed is a method for separating a transmit optical beam and a receive optical beam in an optical communication system.

Description

[Technical Field]

[0001] The present subject matter relates generally to optical communication systems, and more particularly to techniques for separating transmitted and received signals in an optical transceiver. [Background technology]

[0002] Conventionally, in optical communication terminals, methods for separating and isolating transmitted and received signals have been known, such as using different wavelengths or different polarization modes for the transmitted and received signals, which are called wavelength diversity and polarization diversity, respectively. Another known technique for separating signals is time diversity, in which the transmitted and received signals occupy different time slots and communication is performed in simplex mode.

[0003] A key element of the wavelength diversity approach is the dichroic beam splitter, which reflects the receive beam and transmits the transmit beam, or vice versa. The transmit beam passes through the dichroic beam splitter and travels along the transmit path. The receive beam is received along approximately the same path and reflected at a selected angle by the beam splitter. If the beam splitter is positioned at, for example, a 45° angle to the transmit path, the receive beam is reflected at 90° and can then be easily processed in the optical receiver without interfering with the transmit beam.

[0004] A similar separation of the two transmission paths can be achieved with a polarizing beam splitter, where, for example, the transmit beam is vertically polarized and the receive beam is horizontally polarized, but may be the same wavelength as the transmit beam. The vertically polarized transmit beam passes through the polarizing beam splitter and continues along the transmission path. The horizontally polarized receive beam is reflected by the polarizing beam splitter, providing the desired separation between the transmit and receive paths. Left- and right-handed circular polarization can also be used with the addition of a quarter-wave plate that converts the circular polarization to linear polarization.

[0005] In a simplex communication link, an optical switch can be used to switch between transmit and receive modes. In transmit mode, the transmit beam passes through the optical switch and travels along the transmit path. In receive mode, the receive beam enters the optical switch from the transmit path and is routed to a receive port, while the transmit signal is routed to a dump port. Again, the transmit and receive signals can have the same wavelength.

[0006] All three of these prior art approaches for optical communication terminals have significant drawbacks. To achieve a high degree of separation in the wavelength diversity approach, dichroic beam splitters must be designed with many coating layers to effectively achieve the desired wavelength separation. Such complex designs can result in higher insertion loss than dichroic beam splitters with simpler designs. Alternatively, the transmit and receive wavelengths can be selected to be widely spaced, which can significantly limit the number of wavelengths usable within the limited bandwidth of the optical amplifier. While dichroic beam splitters can be designed to trade off the complexity of such splitters and wavelength separation, wavelength diversity approaches always require a combination of design complexity and wide wavelength separation to achieve the desired high degree of separation between the transmit and receive beams. Potential problems with using wavelength diversity arise from the fact that this approach requires that the receive and transmit wavelengths be different, and that the receive and transmit wavelengths must be changed along with the wavelength-selective optics that support them in order for any terminal to be interoperable with any other terminal. By using wavelength diversity, terminals are effectively assigned a "gender" and, if the wavelength cannot be changed due to increased complexity and redundancy, terminals of the appropriate gender must be paired.

[0007] In the polarization separation approach, polarizing beam splitters typically provide approximately 20–30 decibels (dB), or a maximum of 40 dB. However, the required isolation for ultra-long-distance laser communications is greater than 110 dB. Therefore, to achieve isolation using polarization diversity, additional isolation must be achieved by other means. In-fiber filters can typically provide an additional 60 dB of isolation, but this may not be sufficient for some applications. Additionally, the use of filters can result in additional losses depending on the location of the filters relative to the optical low-noise amplifiers. Another drawback of polarization separation is that the photonics components using polarization required for space communications can be difficult to obtain and certify for use.

[0008] Finally, the simplex approach is the simplest, but of course has inherent limitations when compared to a full duplex communication system. In addition, the optical switch approach provides approximately 50 dB of isolation. Other means of isolation are required.

[0009] U.S. Patent No. 7,366,419, owned by the assignee of the present application, discloses an apparatus and method for spatially separating transmit and receive beams in a satellite communication system, the transmit beams being separated essentially by a look-ahead angle. A receive beam separation mirror is positioned in the receive beam path but out of the transmit beam path, and the receive beam is reflected along a path separated from the transmit beam path. The mirror may be annular, and the transmit beam may be directed through the center of the mirror. If the desired separation angle differs from the look-ahead angle, or if there is no look-ahead angle, the receive beam separation mirror may be used in conjunction with a dichroic beam splitter and at least one receive beam steering mirror to achieve the desired angular separation. Summary of the Invention

[0010] According to one embodiment, an apparatus for separating a transmit optical beam and a receive optical beam having a common wavelength to ensure genderless interoperability in an optical communication system includes a transmit path transmitting the transmit optical beam at a specific fundamental wavelength and a receive path propagating a receive optical beam at the same specific fundamental wavelength, at least a portion of the receive path being separated from the transmit path. The apparatus further includes an annular mirror having a receive beam area and a central aperture, the transmit path including a steering mirror, and the receive path including a steering mirror. The steering mirror has a single surface that reflects the entire transmit optical beam transmitted along the transmit path, and the single surface of the steering mirror reflects the entire receive optical beam received along the receive path.

[0011] According to another aspect, a method for separating a transmit optical beam and a receive optical beam having a common wavelength to ensure genderless interoperability in an optical communication system includes transmitting a transmit optical beam at a specific fundamental wavelength along a transmit path and receiving a receive optical beam at the same specific fundamental wavelength along a receive path, where at least a portion of the receive path is separated from the transmit path. The method further includes providing an annular mirror having a receive beam area and a central aperture, providing steering mirrors disposed in the transmit path and the receive path, the steering mirror having a single surface, and reflecting the entire transmit optical beam transmitted along the transmit path and the entire receive optical beam received along the receive path using the single surface of the steering mirror. The single surface of the steering mirror receives the transmit optical beam through the central aperture of the annular mirror and reflects the receive optical beam to the receive beam area, where the steering mirror provides a desired angular separation between the transmit optical beam and the receive optical beam.

[0012] Other aspects and advantages will become apparent from the following more detailed description taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0013] [Figure 1]FIG. 1 is a block diagram illustrating the use of wavelength diversity for separation of transmit and receive beams. [Figure 2] FIG. 1 is a block diagram illustrating the use of polarization diversity for separation of transmit and receive beams. [Figure 3] FIG. 1 is a block diagram illustrating the use of a simplex communication link for separation of transmit and receive beams. [Figure 4] FIG. 1 is a generalized block diagram illustrating the use of spatial separation to separate transmit and receive beams in accordance with the present invention. [Figure 5] FIG. 1 is a detailed block diagram of a system for achieving spatial separation of transmit and receive beams. DETAILED DESCRIPTION OF THE INVENTION

[0014] As shown in the drawings for purposes of illustration, the present invention relates to optical communication systems and the need for improved techniques for separating a transmit beam (Tx) and a receive beam (Rx). FIG. 1 shows a dichroic beam splitter (DBS) 10 used in prior art systems to separate a transmit beam 12 of wavelength λ1 from a receive beam 14 of wavelength λ2. FIG. 2 shows the use of a polarizing beam splitter (PBS) 16 to perform a similar function when the transmit beam 12 and receive beam 14 may have the same wavelength. FIG. 3 shows the use of a 2×2 optical switch 18 to provide temporal separation between the transmit beam 12 and receive beam 14 in a simplex communication link. These prior art shortcomings are discussed above in the Background section of this specification.

[0015] As seen in FIG. 4 , transmit beam 12 and receive beam 14 can be spatially separated by utilizing the “point ahead” angle associated with most optical space communications systems. It is well known that when communicating with an orbiting satellite, a ground station must point its transmit beam at a point in space ahead of the moving satellite. This is necessary because the satellite's orbital velocity is greater than the velocity of the ground station. The communication beam, of course, has a finite propagation velocity, necessitating the transmit beam being pointed at a point ahead of the satellite as it moves in orbit. In effect, this phenomenon results in a small angular separation between transmit beam 12 and receive beam 14 at any instant in time. While the point ahead angle is exaggerated in magnitude in the generalized block diagram of FIG. 4 for illustrative purposes, mirror 20 can be positioned to separate receive beam 14 without any effect on transmit beam 12. Specifically, if mirror 20 is an annular mirror, mirror 20 may be oriented and positioned such that transmit beam 12 passes through the mirror's open central aperture, while receive beam 14 enters the mirror's annular receive beam region and is reflected at a desired angle for further processing in an optical receiver (not shown). Use of an annular mirror 20 has the added advantage that the angle between transmit beam 12 and receive beam 14 may be fairly small. Furthermore, annular mirror 20 may be sized, for example, to accommodate a transmit fiber core (or core and cladding) in its central aperture. More generally, mirror 20 need not be angled but may simply be a flat mirror positioned in the path of receive beam 14 and out of the path of transmit beam 12.

[0016] In the specific embodiment of FIG. 5, both the transmit beam 12 and the receive beam 14 have the same fundamental wavelength λ1. Therefore, this communication scheme can be considered "genderless" in the sense that the transmit and receive systems are not distinguished by wavelength. That is, a particular communication scheme can operate with the transmit and receive functions relying on the same fundamental wavelength, even if it involves frequency shifting or frequency modulation. The communication system can also perform duplex communication. The first steering mirror 24 and the second steering mirror 26 provide control of the spatial isolation angle between the transmit beam 12 and the receive beam 14. The receive beam 14 is reflected from the first steering mirror 24 to the annular mirror 20, from which it is further reflected to the second steering mirror 26. The steering mirrors 24 and 26 allow for the selection of a spatial separation angle that can be larger or smaller than the line of sight angle. The second beam steering mirror 26 typically directs the receive beam 14 to a receive fiber (not shown). In this configuration, the optical components are of a relatively simple design that does not require polarization control. Alternatively, the second steering mirror 26 can be omitted and the optical components required for the receive function can be appropriately positioned. [Industrial Applicability]

[0017] Steering mirrors 24 and 26 provide the desired degree of separation between transmit beam 12 and receive beam 14. As discussed above with reference to FIG. 4, mirror 20 is preferably annular to facilitate coupling to transmit fiber 30 (FIG. 5), although the present embodiment is not limited to the use of an annular configuration for mirror 20.

[0018] From the foregoing, it will be appreciated that the present embodiments provide a simple and effective approach for separating transmit and receive beams in optical communication systems that enables interoperability of genderless terminals that use a common fundamental wavelength for both transmit and receive signals, thereby allowing any terminal to communicate with any other terminal. In particular, the illustrated embodiments provide spatial separation of transmit and receive beams having a common wavelength that enables genderless interoperability without the need for polarization or dichroic optics.

[0019] All references, including publications, patent applications, and patents, cited in this specification are incorporated by reference to the same extent as if each individual reference was individually and specifically indicated to be incorporated by reference and was set forth in its entirety herein.

[0020] The use of the terms "a," "an," and "the," and similar references in the context of describing the present invention (particularly in the context of the claims that follow) should be construed to encompass both the singular and the plural unless otherwise indicated herein or clearly contradicted by context. The recitation of numerical ranges herein is intended merely to serve as a shorthand method for individually referencing each value falling within the range, unless otherwise indicated herein, and each value is incorporated into the specification as if each value were individually set forth herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "etc.") provided herein is intended merely to further clarify the disclosure and does not limit the scope of the disclosure unless specifically claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure.

[0021] Numerous variations on the present disclosure will be apparent to those skilled in the art in light of the foregoing description. It should be understood that the illustrated embodiments are exemplary only and should not be construed as limiting the scope of the present disclosure.

Claims

1. 1. An apparatus for separating a transmit optical beam and a receive optical beam having a common wavelength to ensure genderless interoperability in an optical communication system, comprising: a transmission path for transmitting a transmitted light beam at a particular fundamental wavelength; a receive path that propagates a receive optical beam at the same particular fundamental wavelength, at least a portion of the receive path being separated from the transmit path; an annular mirror having a receive beam area and a central aperture; the annular mirror has a separation region between the central aperture and the receive beam region that separates the central aperture and the receive beam region; the transmit path includes a steering mirror, and the receive path includes the steering mirror, the steering mirror having a single surface that reflects the entirety of the transmit light beam transmitted along the transmit path, and the single surface of the steering mirror reflects the entirety of the receive light beam received along the receive path; the steering mirror receives the transmitted light beam through the central aperture of the annular mirror and reflects the received light beam only into the receive beam area due to an angle between the transmitted light beam and the received light beam, separating the received light beam incident on and reflected by the annular mirror from the transmitted light beam directed from the annular mirror to the steering mirror; Device.

2. the optical communication system communicates with an orbiting satellite; the receive path and the transmit path are separated by an angle of view; 10. The apparatus of claim 1.

3. The apparatus of claim 2 , wherein the angle between the transmitted light beam and the received light beam is equal to the angle of view.

4. 10. The apparatus of claim 1, further comprising an additional steering mirror oriented and positioned to receive the received light beam from the receive beam region of the annular mirror and reflect the received light beam along a desired path.

5. To ensure gender-neutral interoperability in optical communication systems, common wavelengths are required.

1. A method for separating a transmitted light beam and a received light beam, the method comprising: transmitting a transmit optical beam along a transmit path at a particular fundamental wavelength; receiving a receive light beam at the same particular fundamental wavelength along a receive path, at least a portion of the receive path being separated from the transmit path; providing an annular mirror having a receive beam region and a central aperture, the annular mirror having a separation region between the central aperture and the receive beam region that separates the central aperture and the receive beam region; providing steering mirrors disposed in the transmit path and the receive path, the steering mirrors having a single surface; reflecting the entire transmitted light beam transmitted along the transmit path and the entire received light beam received along the receive path using the single surface of the steering mirror; Including, the steering mirror receives the transmitted light beam through a central aperture of the annular mirror and reflects the received light beam only into the receive beam area due to an angle between the transmitted light beam and the received light beam, separating the received light beam incident on and reflected by the annular mirror from the transmitted light beam directed from the annular mirror to the steering mirror; method.

6. the optical communication system communicates with an orbiting satellite; the receive path and the transmit path are separated by an angle of view; The method of claim 5.

7. 7. The method of claim 6, further comprising providing an additional steering mirror that reflects the received light beam along a desired path after reflection from the receive beam area.

8. 7. The method of claim 6, wherein providing the steering mirror comprises orienting and positioning the steering mirror such that the angle between the transmit light beam and the receive light beam is equal to the angle of sight.

Images