Optical free-space communication device using polarized light

Abstract

An optical communication system includes two communication terminals that communicate with each other using optical signals having the same wavelength. Both terminals include a half-wave plate polarizer that rotates the linearly polarized signal and a quarter-wave plate polarizer that circularly polarizes the optical signal. The quarter-wave plate polarizers are oriented 90° to each other so that the circularly polarized signal sent from one terminal to the other is linearly polarized at 90° to the transmit polarization orientation and can be separated from the transmitted optical signal by a beam splitter.

Description

[Technical Field]

[0001] Cross-reference to related applications

[0001] This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 63 / 115,764, filed November 19, 2020, and entitled Optical Free Space Communication Apparatus Using Polarization.

[0002] Background Field

[0002] The present disclosure relates generally to optical communication systems that use polarized beams to enable transmission between terminals at the same beam wavelength, and more particularly to optical communication systems that use polarized beams to enable transmission between terminals at the same beam wavelength, with both terminals circularly polarizing their transmit beams at 90° to each other to separate linearly polarized transmit and receive beams. Summary of the Invention [Problem to be solved by the invention]

[0003] Optical communication systems, such as those employed by some satellites, transmit optical signals containing data and other information over communication links between communication terminals. The optical signals transmitted by a communication terminal must be isolated from the optical signals received by the terminal to prevent crosstalk. Furthermore, it is often desirable to limit the number of terminals used in a particular system. Typically, this is accomplished by transmitting optical signals from a terminal at one frequency and receiving optical signals from other terminals at a different frequency, along a common boresight between the terminals. Filters and other optical components are employed at these terminals to separate the signals. However, these components required to separate the signals add size, cost, and weight to the overall system. Furthermore, these filters and components require each terminal to always transmit and receive signals at a set wavelength, preventing more than two communication terminals at more than two different locations from communicating with each other. [Brief explanation of the drawings]

[0004] [Figure 1] 1 is a schematic block diagram of an optical communication system including two communication terminals communicating with each other using beams having the same wavelength but different polarizations, the same wavelength beam being used for both terminal acquisition and information extraction. [Figure 2] 1 is a schematic block diagram of an optical communication system including two communication terminals communicating with each other using beams having the same wavelength but different polarizations, and using different wavelength beams for terminal acquisition and information extraction. DETAILED DESCRIPTION OF THE INVENTION

[0005]

[0006] The following discussion of embodiments of the present disclosure is directed to optical communication systems that use polarized beams to enable transmission between communication terminals at the same beam wavelength, and is merely exemplary in nature and is in no way intended to limit the present disclosure or its application or uses.

[0006]

[0007] FIG. 1 is a schematic block diagram of an optical communication system 10. The optical communication system 10 includes a first communication terminal 12 at one location and a second communication terminal 14 at another location, which are in optical communication with each other. The terminals 12 and 14 may be ground-based terminals or may reside on any suitable platform, such as a satellite, aircraft, ship, etc. The terminals 12 and 14 are intended to represent any optical communication terminals that transmit and receive optical signals for any communication purpose. It should be noted that none of these components are shown because of the signal modulation and processing at the terminals 12 and 14. These components may be any suitable components, as will be readily understood by those skilled in the art.

[0007]

[0008] Terminal 12 includes a light source 20. Light source 20 generates a linearly polarized beam having a wavelength λ1 to be transmitted and provides this beam to a transmit fiber collimator 22. The light beam from transmit fiber collimator 22 is sent to a polarizer 24, such as a half-wave plate (HWP) polarizer. Polarizer 24 rotates the polarized beam from fiber collimator 22, for example, to a P-polarized direction. The linearly polarized beam is sent to a beam separator 26, such as a polarizing beam splitter (PBS) 26, for example, a crystal Brewster angle prism. Beam separator 26 splits the light into one linear polarization direction, here, the P-polarized direction, and reflects the linearly polarized light into an orthogonal direction, such as the S-polarized direction. The linearly polarized P-polarized beam passing through beam separator 26 is rotated by a circular polarizing element 28, such as a quarter-wave plate (QWP) polarizer, to become circularly polarized, for example, right-hand circularly polarized. The circularly polarized beam is then transmitted through free space to terminal 14. Note that the propagation of the beam to terminal 12 can occur entirely within fiber, and beam separator 26 can be an optical circulator. That is, all or part of the polarization control and isolation can be performed within the fiber or free-space components.

[0008]

[0009] Similarly, terminal 14 includes a light source 30. Light source 30 generates a light beam having a similarly transmitted wavelength λ1 and provides this beam to a transmit fiber collimator 32. The light beam from transmit fiber collimator 32 is sent to a polarizer 34, such as a HWP polarizer. Polarizer 34 again rotates the polarized beam from collimator 32, for example, to a P-polarized direction. The linearly polarized beam is sent to a beam separator 36, such as a polarizing beam splitter, for example, a crystal Brewster angle prism. Beam separator 36 splits the light into one linear polarization direction, here, the P-polarized direction, and reflects the linearly polarized light into an orthogonal direction, such as a S-polarized direction. The linearly polarized P-polarized beam passing through beam separator 36 is rotated by a circular polarizing element 38, such as a QWP polarizer, to become circularly polarized, for example, left-handed. The circularly polarized beam is then transmitted through free space to terminal 12.

[0009]

[0010] The optical beam transmitted by terminal 12 is received by circular polarizing element 38, and the optical beam transmitted by terminal 14 is received by circular polarizing element 28. The polarization axis of circular polarizing element 38 is set at 90° to the polarization axis of polarizing element 28, and the polarization axes of both circular polarizing elements 28 and 38 are switchable between these two orthogonal switching states. Therefore, when circular polarizing element 28 or 38 receives the optical beam from transmitting terminal 14 or 12, the optical beam is converted into a linearly polarized beam. This linearly polarized beam is polarized at 90° to the linear polarization it had before being converted into a circularly polarized beam at transmitting terminal 12 or 14. In other words, the switchable orientation state of circular polarizing elements 28 and 38, which sets the orientation of circular polarizing elements 28 and 38, must be rotated 90° at receiving terminal 12 or 14 relative to transmitting terminal 12 or 14. Furthermore, when the receiving terminal 12 or 14 becomes a transmitting terminal, this 90° orientation must be switched back to the original transmit orientation to allow its transmit beam to be received by the receiving terminal 12 or 14. This allows the inbound polarization of the light beam to be orthogonal to the outgoing polarization of the light beam. This can be done using either a mechanically rotated QWP polarizer or a liquid crystal variable retarder to achieve the required phase retardation. The fact that the circular polarizing elements 28 and 38 can be rotated without affecting their function in the terminal 12 or 14 and that they have a relatively high tolerance for small angular changes in the incident light passing through the circular polarizing elements 28 or 38 advantageously makes them suitable as gender-switchable elements; furthermore, both mechanical rotation and electronically adjusted liquid crystal retardation can be used to perform this switching function.

[0010]

[0011] The left-handed circularly polarized beam transmitted by terminal 14 is received by terminal 12 and linearly polarized by circular polarizing element 28. Because the polarization axis of circular polarizing element 38 is set at 90° to the polarization axis of circular polarizing element 28, polarizer 28 polarizes the received beam in a south-pole direction, orthogonal to the north-pole direction. The south-pole beam is reflected by beam separator 26 and directed through linear polarizing filter 42 to partial beam splitter 40. Linear polarizing filter 42 filters out any remaining light that is not linearly polarized in the south-pole direction. Beam splitter 40 splits this beam, and one split beam is sent to communication sensor 44 for processing, and information is extracted from this split beam. The other split beam is sent to acquisition sensor 46 for processing for purposes of alignment between terminals 12 and 14.

[0011]

[0012] Similarly, the right-hand circularly polarized beam transmitted by terminal 12 is received by terminal 14 and linearly polarized by circular polarizing element 38. Because the polarization axis of circular polarizing element 38 is set at 90° to the polarization axis of circular polarizing element 28, circular polarizing element 38 polarizes the received beam in a south-polarized direction, orthogonal to the north-polarized direction. The south-polarized beam is reflected by beam separator 36, passes through linear polarizing filter 52, and is directed to partial beam splitter 50. Linear polarizing filter 52 filters out any residual light that is not linearly polarized in the south-polarized direction. Beam splitter 50 splits the beam, and one split beam is sent to communication sensor 54 for processing, and information is extracted from the split beam. The other split beam is sent to acquisition sensor 56 for processing for purposes of alignment between terminals 12 and 14.

[0012]

[0013] In system 10, the same wavelength beam was used for both signal processing and acquisition purposes. However, in some communication systems, signal strength may be too low to use the same optical beam for both terminal acquisition and information processing. Furthermore, certain situations may arise where terminals are in close proximity and one terminal may receive a beam intended for the other terminal. In these and other situations, it may be desirable to use beams having different wavelengths for signal processing and terminal acquisition.

[0013]

[0014] FIG. 2 is a schematic block diagram of an optical communication system 60. The optical communication system 60 includes a first terminal 62 and a second terminal 64, which are in optical communication with each other. Here, different wavelength beams are used for terminal acquisition and information extraction, or for signal redundancy purposes, for terminal acquisition and information processing. Similar elements to system 10 are identified with the same reference numerals. In this system 60, a second light source 66 at terminal 62 and a second light source 68 at terminal 64 provide optical beams of wavelength λ2 to fiber collimators 22 and 32, respectively. Both optical beams are processed by polarizers 24 and 34, beam separators 26 and 36, and circular polarization elements 28 and 38 in the same manner as discussed above. However, the beam received by terminal 62 and reflected by beam separator 26 is sent to a dichroic beam splitter 70, which splits the beams based on their wavelengths. The beam having wavelength λ1 is sent to communication sensor 44, and the beam having wavelength λ2 is sent to acquisition sensor 46. Similarly, the beam received by terminal 64 and reflected by beam separator 36 is sent to dichroic beam splitter 72, which splits the beams based on their wavelengths. The beam having wavelength λ1 is sent to communication sensor 54, and the beam having wavelength λ2 is sent to acquisition sensor 56.

[0014]

[0015] The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. Those skilled in the art will readily appreciate from such discussion, along with the accompanying drawings and claims, that various changes, modifications, and variations can be made without departing from the spirit and scope of the present disclosure, as defined in the following claims.

Claims

1. 1. An optical communication system, comprising: a first terminal operable to transmit and receive optical signals, the first terminal including: a first half-wave plate (HWP) polarizer responsive to a first linearly polarized beam having a first wavelength and rotating the first optical beam in a first linear direction; a first beam splitter responsive to and passing the first linearly polarized beam; and a first circular polarization element responsive to the first linearly deformed beam from the first beam splitter and circularly polarizing the first beam for transmission, the first circular polarization element being switchable between two orthogonal switching states; and a first terminal operable to transmit and receive optical signals, the first terminal including: a first half-wave plate (HWP) polarizer responsive to a first linearly polarized beam having a first wavelength and rotating the first optical beam in a first linear direction; a first beam splitter responsive to and passing the first linearly polarized beam; and a first circular polarization element responsive to the first linearly deformed beam from the first beam splitter and circularly polarizing the first beam for transmission, the first circular polarization element being switchable between two orthogonal switching states; a second terminal operable to transmit and receive optical signals, the second terminal including: a second HWP polarizer responsive to a second linearly polarized beam having the first wavelength and rotating the second optical beam in the first linear direction; a third beam splitter responsive to and passing the second linearly polarized beam; and a second circular polarization element responsive to the second linearly polarized beam from the third beam splitter and circularly polarizing the second beam for transmission, the second circular polarization element being switchable between the two orthogonal switching states; and a second terminal further including a fourth beam splitter, a second acquisition sensor, and a second communication sensor. the first terminal receives the second circularly polarized beam from the second terminal, the first circular polarizing element linearly polarizing the second beam in a second linear direction orthogonal to the first linear direction, the first beam splitter directing the second beam to the second beam splitter, the second terminal receives the first circularly polarized beam from the first terminal, the second circular polarizing element linearly polarizing the first beam in the second linear direction, and the third beam splitter directing the first beam to the fourth beam splitter; the first HWP polarizer is responsive to a third linearly polarized beam having a second wavelength and rotates the third linearly polarized beam to the first linear direction; the second HWP polarizer is responsive to a fourth linearly polarized beam having the second wavelength and rotates the fourth linearly polarized beam to the first linear direction; the second beam splitter and the fourth beam splitter are dichroic beam splitters that direct a light beam having the first wavelength to the communication sensor and a light beam having the second wavelength to the acquisition sensor.

2. 10. The system of claim 1, wherein the second beam splitter and the fourth beam splitter are partial beam splitters that split the first light beam or the second light beam and direct one split beam to the communication sensor and the other split beam to the acquisition sensor.

3. 2. The system of claim 1, wherein the first terminal further includes a first linear polarizing filter that receives the second beam from the first beam splitter prior to the second beam splitter, and the second terminal further includes a second linear polarizing filter that receives the first beam from the third beam splitter prior to the fourth beam splitter.

4. 2. The system of claim 1, wherein the first beam splitter is responsive to and passes the third linearly polarized beam, the first circular polarizing element is responsive to the third linearly polarized beam from the first beam splitter and circularly polarizes the third beam for transmission, the third beam splitter is responsive to and passes the fourth linearly polarized beam, the second circular polarizing element is responsive to the fourth linearly polarized beam from the third beam splitter and circularly polarizes the third beam for transmission, the first terminal receives the fourth circularly polarized beam from the second terminal and the first circular polarizing element linearly polarizes the fourth beam in the second linear direction, the first beam splitter directs the fourth beam to the second beam splitter, the second terminal receives the third circularly polarized beam from the first terminal and the second circular polarizing element linearly polarizes the third beam in the second linear direction, and the third beam splitter directs the third beam to the fourth beam splitter.

5. 5. The system of claim 4, wherein the first terminal further includes a first linear polarizing filter that receives the second beam and the fourth beam from the first beam splitter prior to the second beam splitter, and the second terminal further includes a second linear polarizing filter that receives the first beam and the third beam from the third beam splitter prior to the fourth beam splitter.

6. 10. The system of claim 1, wherein the first and second circular polarizing elements mechanically rotate quarter wave plate (QWP) polarizers.

7. 10. The system of claim 1, wherein the first circular polarizing element and the second circular polarizing element are liquid crystal variable retarders.

8. 10. The system of claim 1, wherein the first beam splitter and the second beam splitter are quartz Brewster angle prisms.

9. 2. The system of claim 1, wherein the first linear direction is a north pole direction and the second linear direction is a south pole direction.

10. 2. The system of claim 1, wherein the two orthogonal switching states are right-hand circular polarization and left-hand circular polarization.

11. 1. An optical communication system, comprising: a first terminal operable to transmit and receive optical signals, the first terminal including: a first polarizing element responsive to a first linearly polarized beam having a first wavelength and rotating the first optical beam in a first linear direction; a first beam separator responsive to and passing the first linearly polarized beam; and a first circular polarizing element responsive to the first linearly polarized beam from the first beam separator and circularly polarizing the first beam for transmission, the first circular polarizing element being switchable between two orthogonal switching states; and a first terminal operable to transmit and receive optical signals, the first terminal including: a first beam separator; a first acquisition sensor; and a first communication sensor. a second terminal operable to transmit and receive optical signals, the second terminal including: a second polarizing element responsive to a second linearly polarized beam having the first wavelength and rotating the second optical beam in the first linear direction; a third beam separator responsive to and passing the second linearly polarized beam; and a second circular polarizing element responsive to the second linearly polarized beam from the third beam separator and circularly polarizing the second beam for transmission, the second circular polarizing element being switchable between two orthogonal switching states; and a second terminal operable to transmit and receive optical signals, the second terminal including: a fourth beam separator; a second acquisition sensor; and a second communication sensor. wherein the first terminal receives the second circularly polarized beam from the second terminal, the first circular polarizing element linearly polarizing the second beam in a second linear direction orthogonal to the first linear direction, the first beam separator directing the second beam away from the first polarizing element, the second terminal receives the first circularly polarized beam from the first terminal, the second circular polarizing element linearly polarizing the first beam in the second linear direction, and the third beam separator directing the first beam away from the second polarizing element; the first polarizing element is responsive to a third linearly polarized beam having a second wavelength and rotates the third linearly polarized beam to the first linear orientation; the second polarizing element is responsive to a fourth linearly polarized beam having the second wavelength and rotates the fourth linearly polarized beam toward the first linear direction; the second beam separator and the fourth beam separator are dichroic beam splitters that direct a light beam having the first wavelength to the communication sensor and a light beam having the second wavelength to the acquisition sensor.

12. 12. The system of claim 11, wherein the first polarizing element and the second polarizing element are half-wave plate (HWP) polarizers.

13. 12. The system of claim 11, wherein the first and second circular polarizing elements are mechanically rotating quarter wave plate (QWP) polarizers.

14. 12. The system of claim 11, wherein the first circular polarizing element and the second circular polarizing element are liquid crystal variable retarders.

15. 12. The system of claim 11, wherein the first beam separator and the third beam separator are polarizing beam splitters.

16. 16. The system of claim 15, wherein the polarizing beam splitter is a quartz Brewster angle prism.

17. 12. The system of claim 11, wherein the first beam separator and the third beam separator are optical circulators.

18. 12. The system of claim 11, wherein the first linear direction is a north pole direction and the second linear direction is a south pole direction.

19. 12. The system of claim 11, wherein the two orthogonal switching states are right-hand circular polarization and left-hand circular polarization.

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