FREE SPACE OPTICAL (FSO) SYSTEM
A detector configuration for use in a free space optical (FSO) node for transmitting and/or receiving optical signals has a plurality of sensors for detecting received optical signals. The plurality of sensors is configured along a common optical path and are used for separate functions. According, the detectors may be optimized for the respective function.
This application claims priority to U.S. Application No. 62/208,561, filed Aug. 21, 2015, which is incorporated by reference in its entirety into this application.BACKGROUND
In a two-node bi-directional Free Space Optical (FSO) communication system, the two FSO nodes exchange data encoded on optical carrier beams sent across an unobstructed line of sight (LOS) between the two nodes. As shown in
The communication system only works if the transmit path of the first node is aligned with the receiving components of the second node. In order to optimize tracking, conventional systems have split the received beam into two paths: one for detection and one for alignment. As shown in
A free space optical terminal is disclosed including a wave front sensor comprising a free space in an interior region of the wave front sensor; and a receiver within the free space of the wave front sensor. The resulting free space optical (FSO) terminal therefore may have a wave front sensor used for aligning the system and a detector used to receive a data transmission received on an optical beam. In an exemplary embodiment, the wave front sensor and the detector are different optical components, and the terminal may be configured such that a first portion of the received light is received at the wave front sensor and a second portion of the received light source is received at the detector, where a beam splitter is not used to separate the first portion from the second portion. Therefore, the first portion of light and the second portion of light may follow the same optical beam path along an entire length or along a portion of a length at the sensors within the system. In an exemplary embodiment, the first portion circumscribes the second portion.
The following detailed description illustrates by way of example, not by way of limitation, the principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. It should be understood that the drawings are diagrammatic and schematic representations of exemplary embodiments of the invention, and are not limiting of the present invention nor are they necessarily drawn to scale.
Exemplary embodiments may be used to greatly simplify the complexity of a free space optical (FSO) terminal, while maintaining the benefit achieved by separate alignment and detection sensors. Accordingly, exemplary FSO terminals according to embodiments described herein include separate detection sensor(s) and alignment sensor(s) configured or positioned such that the received optical path is maintained as a single received optical path. Accordingly, exemplary embodiments may reduce misalignment into the system by not subdividing the paths to the separate detectors. An exemplary FSO terminal may be capable of unidirectional or bi-directional high bandwidth optical communications.
Although embodiments of the invention may be described and illustrated herein in terms of an alignment sensor and detection sensor, it should be understood that embodiments of this invention are not so limited, but are additionally applicable to functional components of the system. For example, the respective sensors may be used for other purposes. Accordingly, exemplary embodiments may be used when it is desired to have two system components using portions of the same free space signal and it is desired to keep the components along the same signal path and not split the signal into separate paths. Accordingly, the detector and alignment sensors described herein may be used for any system function. Moreover, exemplary embodiments may be adapted to other free space systems, not necessarily limited to optical applications or communication systems.
As shown, the detection sensor is positioned out of plane from the alignment sensor. However, such configuration is not necessary. In an exemplary embodiment, the system includes a lens 16 or other optics for directing and/or focusing the received light 18 toward the sensor(s). The detection sensor 12 is shown positioned approximately at the focal point of the received path as set by lens 16. The alignment detector is shown positioned in a plane after the lens 16 and before the focal point at the detection sensor 12, relative to the received optical path 18 (or between the focal point and the optics defining the focal point). The detection sensor 12 may be in plane with the alignment sensor 14, out of plane with the alignment sensor 14, or before or after the alignment sensor 14. The purpose of the sensors may also be swapped such that sensor 14 is the detection sensor and detector 12 is the alignment sensor. For example, the alignment sensor is a central sensor, while the detection sensor is the annular sensor. Multiple annular sensors may be incorporated for different purposes to permit two or more detector functions on the same optical path. The component shown as the detection sensor 12 may also be any combination of optical components. For example, the detection sensor may be replaced with other components, such as mirrors, lenses, splitters, optical fibers, etc. that is used to direct the light before the detection sensor. Exemplary configurations of such additional component combinations are described with respect to
At step 64, the alignment sensor can detect the horizontal and vertical displacement of the beam 18 on the detector face. At step 66, the displacements may be determined or calculated based on a comparison of the detected signals from step 64. For example, the ratio of the difference of the light on each half of the detector divided by the whole by be used to determine a percentage offset from the center of the detector in orthogonal (x-y) directions. In this case, the x displacement will be the signal difference from the total of the first and second quadrants minus the total from of the third and fourth quadrants divided by the total signal: [(14a+14b)−(14c+14d)]/(14a+14b+14c+14d). Similarly, the y displacement can be determined by comparing the signal from the upper quadrants to that of the lower quadrants [(14a+14d)−(14b+14c)]/(14a+14b+14c+14d). At step 68, the system may be manually or automatically adjusted to realign the node such that the received beam 18 is centered on the detection sensor 14. After the system is aligned, at step 70, the detection sensor may be used to detect the incoming light, which is decoded by the system.
Exemplary embodiments of the alignment sensor comprise a sensor portion defining an outer section and an aperture through a central section. The central section may be coaxial with the center of the optic or may be off-center from the optic. The alignment sensor therefore includes a central aperture 13 circumscribed by a plurality of sensors. In an exemplary embodiment, the central aperture is surrounded by two or more and preferable three to six detectors. The detectors may circumscribe the aperture and substantially fill a perimeter around or substantially surround the aperture, where substantially can be understood by a person of skill in the art to include more than a majority and is approximately the entire perimeter but accounts for dead space between sensors and positioning tolerances required between components. The detection sensor 12 (either an outer perimeter or the working surface of the detector area) can be larger, smaller, or approximately equal to the aperture. The detection sensor 12 may be positioned in front of, flush with, or behind the alignment sensor surface 14.
As seen in
For a bi-directional link between two FSO nodes, exemplary embodiments may be used such that the incoming data beam can also be used for tracking. In an exemplary embodiment, an exemplary free space optical node may include any combination of:
a common objective lens for transmit (Tx) and receive (Rx);
an annular area around the Tx is captured by the quadcell with a hole for guiding; for a uni-directional link, the quadcell can be chosen to match the wavelength of the guide beacon, or use nearly the same wavelength as the Tx.
for a bidirectional link, the Tx and Rx can be separated with a beam splitter or optical circulator;
for use with a Tx/Rx fiber, the quadcell and Tx/Rx fiber could be integrated into a structure that eliminates the need for beamsplitters, simplifying the mechanical design; and/or an internal baffle cone could serve to provide isolation between Tx and Rx if a similar wavelength is used.
The above features are exemplary only, and may be used in any combination or sub-combination as is desired for the application. Other features may be added or the above features may also be modified to achieve the objective of a user. For example, ranges, such as for wavelengths, may be redefined for particular applications, distances, environments, etc. Also, features may be removed and others redefined to accommodate the removal of a feature, such as the added or removed baffle cone of exemplary
Exemplary embodiments may be used to align and use (send/receive signals) the terminal while reducing system complexity. Exemplary embodiments use an increased lens size and use the light with a higher numerical aperture (NA) on a wave front sensor (WFS), such as a quadcell with a hole. An annular ring from this section of the objective will be seen on the quadcell. The inside annulus comes from the hole in the quad while the outside comes from the edges of the objective. Any angular change moves this outside edge shifting the balance of light on the quadrants.
Part of the tuning parameters of the system include the position of the optics, such as the sensors and/or optical fibers (see, e.g.
In an exemplary embodiment, the detection sensor and alignment sensor may be along an optical path, but used with different optical beams. Specifically, if the two sensors detect different wavelengths and transparent to the wavelength of the other sensor, then the backscatter or other interference between the sensors is reduced or eliminated. However, two light sources are necessary at the sending node to be received. For example, if an FSO system is using 1550 nm wavelength light for transmitting data (the detection sensor), a silicon quadcell could be used as an alignment sensor as it is transparent at the data wavelength. In this embodiment, a guide beacon would be chosen for Silicon and the data transmitted on 1550 nm. Therefore, an extremely high isolation can be achieved without the baffle cone.
Exemplary embodiments described herein include using a free space optical terminal in which a portion of the received beam is used for aligning the system and a separate portion of the beam is used for receiving, transmitting, and any combination thereof for a data signal. The exemplary method can be used without a beam splitter that separates the beam into separate paths. In an exemplary embodiment, the first portion of the beam used for alignment is an exterior portion circumscribing the second portion used for transmitting and/or receiving the data signal. Therefore, the first portion may be a central portion, while the second portion may be a circumferential exterior portion of the same beam. Exemplary embodiments may also capture the entire beam for communication by positioning the detection sensor before the alignment sensor and focusing the light on the detection sensor. In exemplary embodiments, a controller is coupled to one or more optical components to adjust or control the position of the components and are able to position, aligned, or alter the working components of the system.
The method may include receiving an optical beam at the FSO terminal. The method includes positioning the beam such that a first portion falls on one or more detector(s) for aligning the terminal with the received beam (or any first system function), and a second portion falls on one or more receiver(s), such as a fiber optic or detection sensor, for detecting and/or directing the signal for analyzing a data signal carried on the received light. The system is configured such that the detector(s) circumferentially surround the receiver(s).
As shown and described, a quadcell is used to illustrate the plurality of detectors around the optical fiber or detection sensor. However, it should be understood that any combination of detectors may be positioned around the common receive/transmit path. The detectors may be of the same kind, or may be different. There are variants for the tilt sensor choice and a quadcell is not exclusive. Anything from normal quads to custom multi pixel detectors including focal plane arrays with random sub array read out may be used. Exemplary embodiments permit the light to pass through a hole, aperture, or space between detectors, or have a material that transmits the light used for data transmission.
As shown and described, the receiver for receiving and transmitting the light for data transmission is shown and described interior the detectors for alignment or other system function, such that the detectors circumferentially surround the receiver. However, these functions and/or components may be switched and is not limited to the exemplary embodiment described.
“Substantially fill” or “substantial” is intended to mean greater than a majority, such as more than 75%. A majority is intended to mean 50%. Numerical ranges are also used herein and are approximations only. Approximations are understood to be within the person of skill in the art. For example, when a series of detectors approximately fully surround or circumscribe an optical fiber, it is understood that natural dead space or gaps must accompany the areas between the detectors. These approximations are within the skill of the art to determine and may depend on system components, tolerances, wavelengths, system size, etc. Therefore, approximately fully surround is understood to have detectors positioned around the detectors to minimize the dead space, but would be dependent upon the kind and quantity of detectors selected. An optical beam path is understood to be the linear longitudinal direction of a propagated beam.
Exemplary embodiments may be incorporated into a free space optical terminal used for both transmitting and receiving data signals. In an exemplary embodiment, the FSO terminal may use common optics for transmitting and receiving a data signal therefrom/thereto. For example, U.S. application Ser. No. 14/608,166, titled “Data Retransmission for Atmospheric Free Space Optical Communication System,” owned by the present applicant, and incorporated by reference in its entirety herein, discloses an FSO unit that may use a common aperture and optics for transmitting and receiving a data signal. Exemplary embodiments described herein may be used in conjunction with or replace the components for alignment and detecting. For example, the components labeled 20, 22, and 24 of FIG. 2 of the Data Retransmission application may be replaced by embodiments described herein. Other exemplary systems that may inform alternative configurations of the instant invention include, but are not limited to those disclosed by U.S. application Ser. No. 14/608,133, filed Jan. 28, 2015, titled “Free Space Optical Communication Tracking with Electronic Boresight Compensation . . . ”, U.S. Provisional Application No. 62/238,637, filed Oct. 7, 2015, titled “Fast Tracking Free Space Optical Module,” and U.S. Provisional Application 62/266,710, filed Dec. 14, 2015, titled “Free Space Optical System with Common Transmit and Receive Paths,” both filed concurrently herewith, and incorporated in their entirety herein.
Although embodiments of this invention have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of embodiments of this invention as defined by the appended claims.
1. A free space optical terminal, comprising:
- a wave front sensor comprising a free space in an interior region of the wave front sensor;
- a receiver concentric with the free space of the wave front sensor.
2. The free space optical terminal of claim 1, wherein the receiver comprises an optical fiber.
3. The free space optical terminal of claim 2, wherein the optical fiber is a bi-directional fiber.
4. The free space optical terminal of claim 2, wherein the optical fiber is a uni-directional fiber.
5. The free space optical terminal of claim 1, wherein the wave front sensor is a plurality of sensors circumscribing a perimeter of the receiver as seen from a front view normal to a receiving surface of the receiver.
6. The free space optical terminal of claim 5, wherein the plurality of sensors substantially completes a perimeter around the receiver as seen from the front view.
7. The free space optical terminal of claim 6, wherein the plurality of sensors approximately completely completes the perimeter around the receiver as seen from the front view.
8. The free space optical terminal of claim 5, wherein a terminal surface of the optical fiber is positioned in plane with a working surface of the plurality of sensors as oriented with respect to an incoming received beam.
9. The free space optical terminal of claim 5, wherein the plurality of sensors comprises a quadcell.
10. The free space optical terminal of claim 9, wherein the free space is defined by a hole in the center of the quadcell.
11. A free space optical (FSO) terminal, comprising: a wave front sensor used for aligning the system and a detector used to receive a data transmission received on an optical beam, wherein the wave front sensor and the detector are different optical components, the system being configured such that a first portion of the received light is received at the wave front sensor and a second portion of the received light source is received at the detector, where a beam splitter is not used to separate the first portion from the second portion.
12. The free space optical terminal of claim 11, wherein the FSO terminal is configured to transmit and receive with common optics.
13. The free space optical terminal of claim 11, wherein the first portion and second portion follow the same optical beam path along an entire length within the system.
14. The free space optical terminal of claim 12, wherein the first portion circumscribes the second portion.