Mitigation of atmospheric scintillation for communication

The present application is directed to an optical terminal including two linearly polarized optical transmit beams configured to exhibit a time-delay therebetween. The optical terminal may include a quarter-wave plate such that the linearly polarized transmit beam becomes circularly polarized. The optical terminal may also include a receiving ground terminal including a properly oriented quarter-wave plate for separating and directing the two recovered linearly polarized beams. The application is also directed to a method for reconstructing an originally transmitted data stream.

BACKGROUND

Over the last decade, existing optical terminals exhibit excessive power consumption and/or are large designs.

What is desired in the art is a compact/small, light-weight, and low-power consuming (SWAP) optical terminal for DTE transmission.

What is also desired in the art is a low earth orbit (LEO) optical terminal suitable for LEO-to-ground (LEOG) transmission.

What is further desired in the art is optical terminal configured to mitigate scintillation impairments resulting from free space atmospheric propagation.

SUMMARY

The foregoing needs are met, to a great extent, by the present disclosure, including an improved optical terminal and methods of using the same.

One aspect of the application is directed to an optical terminal including two linearly polarized optical transmit beams configured to exhibit a time-delay therebetween. In an embodiment, the two transmit beams exhibit the same data. The optical terminal can comprise a quarter-wave plate such that the linearly polarized transmit beam becomes circularly polarized. The optical terminal may also comprise a receiving ground terminal including a properly oriented quarter-wave plate for separating and/or directing the two recovered linearly polarized beams.

Another aspect of the application is directed to a method of comparing the quality of received beams at an optical terminal. The beams can be assessed on a packet-by-packet basis. Based on the assessment, the best packet is selected. By so doing, the originally transmitted data stream can be reconstructed.

DETAILED DESCRIPTION

Reference in this application to “one embodiment,” “an embodiment,” “one or more embodiments,” or the like means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. The appearances of, for example, the phrases “an embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by the other. Similarly, various requirements are described which may be requirements for some embodiments but not by other embodiments.

The present disclosure describes a laser communication system, terminal, and method of communication. The laser communication system can comprise a direct-to-earth (DTE) laser communication system. In particular, the DTE laser communication system can comprise an optical terminal configured to mitigate atmospheric scintillation effects.

The architecture exhibits characteristics including but not limited to data redundancy and temporal diversity, separate data channels on same wavelength, separate data channels orthogonally polarized, a circularly polarized transmit beam, separated data channels detected by individual photodetectors (e.g., APDs), and detector outputs processed (e.g., by FPGA) to determine transmitted data, or a combination thereof.

FIG. 1illustrates an example transmitter100in accordance with the present disclosure. The transmitter100can be part of a free space optical communication system. The system can comprise the transmitter100and the receiver116shown inFIG. 2and described further herein. The transmitter100can be configured to transmit a free space signal (e.g., free space optical beam). The free space signal can be transmitted via an atmosphere, such as transmission from space to a planet with an atmosphere, and/or vice versa. The transmitter100can be configured to transmit the signal such that effects due to scintillation and/or other atmospheric effects are minimized (e.g., decreased).

The transmitter100can comprise an encoder102. The encoder102can be configured to receive and encode data104. The encoder104can be configured to encode the data104by converting the data to a bit stream. The encoder104can convert the data into a plurality of blocks, segments, data chunks, data portions, packets, and/or the like. The encoder104can be configured to encode the data using forward error correction. The encoder104can addition information, such as redundant bits, bits used for error correction, bits used to verify data, and/or the like. As an example, the encoder104can be configured to apply forward error correction based on one or more standards (e.g., or algorithms thereof), such the International Telecommunications Union (ITU-T) G.709 standard, Reed-Solomon code RS (255, 239), a combination thereof, and/or the like.

The transmitter100can be configured to generate (e.g., splitting/copying the encoded data into) two separate data streams, such as a first data stream and a second data streams, based on the encoded data. The separate data streams can be the same. The first data stream can be supplied to a first laser106. The second data stream can be supplied to a delay element108. The delay element108can be configured to introduce a delay in the second data stream relative to the first data stream. The delay can be a delay for a time period. The delayed second data stream can be supplied to a second laser110. The time period can be of a time length that allows correction of transmission errors due to atmospheric scintillation. The time period can be of a time length in a range of about 5 ms to about 15 ms. In some implementations, the time period can be dynamically adjusted based on detected changes, such as changes in the atmospheric conditions, changes in interference, changes in signal intensity, and/or the like.

The first laser106can be configured to modulate the first data stream into a first optical signal. The first optical signal can comprise the data104. The first optical signal can comprise a free space optical signal. The second laser110can be configured to modulate the second data stream into a second optical signal. The second optical signal can comprise the data delayed by the time period. The second optical signal can comprise a free space optical signal. The first optical signal and the second optical signal can be modulated at the same wavelength (e.g., or different wavelengths). The first optical signal can be linearly polarized by the first laser106. The second optical signal can be linearly polarized by the second laser110.

The transmitter100can comprise a combiner112. The combiner112can be optically coupled (e.g., via one or more optical fibers) with the first laser106and the second laser110. The combiner112can be configured to combine the first optical signal and the second optical signal into a combined optical signal. The combined optical signal can comprise the first optical signal as a sub-channel and the second optical signal as another sub-channel. The combined optical signal can comprise a linearly polarized signal. The combiner112can comprise a polarization maintaining combiner. The combiner112can be configured to rotate the polarization of one of the first optical signal or the second optical signal 90 degrees. The combined optical signal can comprise the first optical signal with a polarization rotated 90 degrees from the polarization of the second optical signal as output by the combiner. The first optical signal can be rotated, in comparison to the second optical signal, 90 degrees along a direction transverse to a direction of travel of the first optical signal and second optical signal. The combined optical signal be output into free space.

The transmitter100can comprise a quarter-wave plate114(e.g., or other wave plate, wave element). The quarter-wave plate114can be optically coupled to (e.g., via a free space path) to the combiner112. The quarter-wave plate114can be configured to convert the combined signal (e.g., combined linearly polarized signal) into a circularly polarized signal. The circularly polarized signal can comprise a circularly polarized first optical signal (e.g., based on the linearly polarized first optical signal). The circularly polarized signal can comprise a circularly polarized second optical signal (e.g., based on the linearly polarized second optical signal). The quarter-wave plate112can be configured to output, into free space, the circularly polarized signal.

It should be understood that the transmitter100can comprise additional optical elements for communication, as mirrors, lens, steering motors, and/or the like. For example, the transmitter100can comprise (e.g., or be in communication with) optical elements that allow the transmitter100to perform signal acquisition, alignment, data transmission, and/or the like. Additionally, as shown inFIG. 1andFIG. 2, optical signals (e.g., in free space, along an optical transmission line) are illustrated using dotted lines, while electrical signals and/or paths are shown in solid lines.

FIG. 2illustrates an example receiver116in accordance with the present disclosure. The receiver116can be configured to communicate with the transmitter100ofFIG. 1. The receiver116can be configured to convert an optical signal received from free space into data. The receiver116can be configured to convert the optical signal to data such that effects due to scintillation and/or other atmospheric effects are minimized (e.g., decreased).

The receiver116can comprise a quarter-wave plate118. The quarter-wave plate118can be configured to receive an optical signal (e.g., optical beam) from free space. The optical signal can comprise circularly polarized optical signal. The quarter-wave plate118can be configured to convert the circularly polarized optical signal to a linearly polarized optical signal. The quarter-wave plate118can be configured to output the linearly polarized optical signal. The circularly polarized optical signal can comprise a first sub-channel (e.g., a first linearly polarized signal) comprising data. The circularly polarized optical signal can comprise a second sub-channel (e.g., a second linearly polarized signal) comprising the data delayed based on a time period. The time period can be of a time length that allows correction of transmission errors due to atmospheric scintillation. The time period can be of a time length in a range of about 5 ms to about 15 ms.

The receiver116can comprise a beam splitter120. The beam splitter120can comprise a polarization beam splitter. The beam splitter120can be optically coupled to the quarter-wave plate118(e.g., via a free space optical path). The beam splitter120can be configured to receive the linearly polarized optical signal and output a first optical signal comprising data and a second optical signal comprising the data delayed by a time period.

The receiver116can comprise at least two photodetectors, such as a first photodetector122and a second photodetector124. The at least two photodetectors can comprise one or more avalanche photodiodes. The first photodetector122can be configured to convert the first optical signal to a first data signal. The first photodetector122can be configured to output the first data signal. The second photodetector124can be configured to convert the second optical signal to a second data signal. The second photodetector124can be configured to output the second data signal.

The receiver116can comprise a delay element126. The delay element126can be communicatively coupled (e.g., electrically coupled) to the first photodetector122. The delay element126can be configured to receive the first data signal. The delay element126can be configured to cause the first data signal to be delayed based on the time period. The delay element126can be configured to remove the relative delay between the first data signal and the second data signal. For example, the second data signal can comprise data delayed by the time period with respect to the first data signal. The delay element126can cause the same or similar delay in the first data signal to remove a difference in delay between the first data signal and the second data signa.

The receiver116can comprise at least two decoders, such as a first decoder128and a second decoder130. The at least two decoders can comprise forward error correction decoders. The first decoder128can be configured to convert the first data signal to first data. The second decoder130can be configured to convert the second data signal to second data. The first data signal and/or the second data signal can comprise addition information (e.g., added by the encoder102for forward error correction), such as redundant bits, bits used for error correction, bits used to verify data, and/or the like. at least two decoders can use can. As an example, the at least two decoders can be configured to apply forward error correction based on one or more standards (e.g., or algorithms thereof), such the International Telecommunications Union (ITU-T) G.709 standard, Reed-Solomon code RS (255, 239), a combination thereof, and/or the like.

The receiver116can comprise a decision element132. The decision element can be implemented by one or more circuit elements, a controller, a field programmable gate array (FGPA), a processor, or a combination thereof. The decision element132can be configured to receive the first data and the second data. The decision element132can be configured to output the data134. The data134can comprise the first data, the second data, or a combination thereof. The decision element132can be configured to output the data134based on a determination of which of the second data or the first data has fewer errors. The data with fewer errors can be output as the data134. The data can be analyzed a data block, segment, and/or packet basis. The decision element132can be configured to determine, on a packet-by-packet basis, to output the one of the first data or the second data (e.g., or a combination thereof). Each data block, segment, and/or packet can be analyzed separately.

The decision element132can be configured to receive first decoding information from the first decoder128. The first decoding information can indicate one or more of error information (e.g., error rate), probability information (e.g., or likelihood of errors, likelihood of correctness), statistical information, noise information, and/or any other information used in decoding the first data. The decision element132can be configured to receive second decoding information from the second decoder130. The second decoding information can indicate one or more of error information (e.g., error rate), probability information (e.g., or likelihood of errors, likelihood of correctness), statistical information, and/or any other data used in decoding the second data. The decision element132can compare (e.g., or otherwise analyze) the first decoding information to the second decoding information. If the comparison indicates that the first data is likely to be more accurate than the second data, then the decision element can select the first data as the data134. If the comparison indicates that the second data is likely to be more accurate than the first data, then the decision element can select the second data as the data134.

In some implementations, the transmitter100ofFIG. 1and/or the receiver116ofFIG. 2can comprise one or more controllers. The one or more controllers can comprise a delay controller configured to control the time period of delay caused by the corresponding delay elements108,126. The delay controller can be integrated in the delay element108,126or can be a separate controller. The delay controller can be configured to update a parameter to cause a change in the time period. The delay controller can be configured to update the parameter based on a plurality of measurements of optical intensity over time.

FIG. 3is diagram showing an optical terminal of an example implementation of the receiver ofFIG. 2. The optical terminal can comprise one or more apertures (e.g., primary aperture, aperture1), one or more lenses (e.g., L1, L2, L3, L4, L5), one or more filters (e.g., solar filter, APD filters), detectors (e.g., quad detector, data detectors), mirrors (e.g., line of sight fine steering mirror), a quarter-wave plate, a beam splitter (e.g., beam splitter, polarization beam splitter), a combination thereof, and/or the like. A beam splitter can be used to supply at least a portion of an optical signal to the quad detector to allow for alignment of the receiver with the transmitter. An additional beam splitter can split the remaining portion of the optical signal to two data detectors.

The optical terminal can be configured to support at least a 25 Mb/s downlink speed. The optical terminal can comprise a reflector telescope with a 30 cm main aperture. One example includes a Schmidt/Cassegrain reflector telescope. The optical terminal can comprise narrow field of view (NFOV), avalanche photodiode (APD) detector optics, beacon transmitters and WFOV optics. The optical terminal can comprise a primary aperture and gimbal mechanism. The optical ground terminal may incorporate payload for line of sight (LOS) closed loop spatial tracking.

The optical terminal can incorporate payload for polarization-based temporal diversity. Orthogonal polarizations can be used to carry the same data with relative delay between data carried in polarization as compared to another. Generally, the delay between orthogonal data channels can be greater than the delay caused by the atmosphere (e.g., due to atmospheric scintillation). The coherence time (e.g., about 10 msec) can be selected to ensure uncorrelated atmospheric fades between identical data packets on orthogonal channels.

The optical terminal can compare (e.g., using an FPGA) received data packets between orthogonal channels. Subsequently, the optical terminal can select the best packets to reconstruct original data channels. The receive aperture can collect both orthogonal data streams. The polarizing beam splitter can separate individual data streams onto separate APD detectors. The communication system can comprise a circular polarizer on the transmitter (LEO terminal).

FIG. 4illustrates an example method400for transmitting data. The method may allow of mitigation (e.g., reduction) of transmission errors due to scintillation. At step402, a circularly polarized optical signal can be converted to a linearly polarized optical signal. The circularly polarized optical signal can comprise a first sub-channel comprising the data and a second sub-channel comprising the data delayed based on the time period. The circularly polarized optical signal can be received by a receiver from a transmitter. The circularly polarized optical signal can be transmitted through an atmosphere. The atmosphere may distort portions of circularly polarized optical signal. The receiver can be located within the atmosphere (e.g., on land, on a planet). The transmitter can be outside of the atmosphere.

At step404, the linearly polarized optical signal can be split to a first optical signal comprising data and a second optical signal comprising the data delayed by a time period. The time period can be of a time length that allows correction of transmission errors due to atmospheric scintillation. The time period can be of a time length in a range of about 5 ms to about 15 ms. The time period of delay can be introduced by the transmitter (e.g., to mitigate the effects of scintillation).

At step406, the first optical signal can be converted to a first data signal and the second optical signal to a second data signal. The first optical signal can be converted to the first data signal using a photodetector. The second optical signal can be converted to the second data signal using another photodetector.

At step408, the first data signal can be caused to be delayed based on the time period. Delay of the first data signal can result in the first data signal and the second data signal having no delay (e.g., or substantially no delay) relative to each other.

At step410, the first data signal can be converted to first data and the second data signal can be converted to second data. The first data signal can be converted to first data using a decoder, such as a forward error correction decoder. The second data signal can be converted to the second data using another decoder, such as a forward error correction decoder.

At step412, one or more of the first data or the second data can be output. One or more of the first data or the second data can be output based on analysis of the first data and the second data. The analysis of the first data and the second data can comprise a determination of which of the second data or the first data has fewer errors. The analysis can be performed on a packet-by-packet bases. The outputting, based analysis of the first data and the second data, the first data or the second data can be performed on a packet-by-packet basis.

The method can further comprise determining a plurality of measurements of optical intensity over time. A parameter can be updated to cause a change in the time period based on the plurality of measurements of optical intensity.

According to another aspect of the application, a method for testing the communication system is performed in a SV-Ground state. In particular,FIG. 5illustrates characteristics of LEO to ground downlink. The elevation angle in degrees is provided on the x-axis. The range in kilometers is provided on the y-axis.FIG. 5illustrates data for elevations beginning at 400 and 1000 km respectively. From the data, it has been determined the range varies with elevation angle. Moreover, atmospheric loss (absorption/scattering) varies with elevation angle. Further, atmospheric scintillation loss varies with elevation angle.

FIGS. 6A and 6Billustrate a plot of LEO to ground (LEOG) link observing atmospheric loss and scintillation index versus elevation angle, respectively. The calculations are performed at sea level with nominal atmospherics (Cn2=1.7×1014m2/3). The data is shown for 1550 nm. InFIG. 6A, with atmospheric transmission (dB) plotted against elevation angle (deg), an atmospheric transmission of −3.1 dB is observed at about 15 degrees.

InFIG. 6B, the scintillation index is observed at 10 cm, 20 cm, 30 cm and 40 cm. Each of the elevation angles originates at 90 degrees and reaches a final destination of about 10 degrees. The scintillation index at 10 cm and 20 cm, respectively, fell between 10-1 and 10-2. Meanwhile, the scintillation index at 30 and 40 cm fell below 10-2. From the data, it has been determined that atmospheric characteristics rapidly degrade at lower elevation angles.

FIGS. 7A and 7Billustrate plots of the communication system in SV-ground state. Characteristics of scintillation versus elevation angle are observed. Here, the calculation at sea-level with nominal atmospherics is Cn2=1.7×10−14m2/3. The observed data is for 840 nm, LEO Height of 1000 km, an Rx Aperture of 30 cm.FIG. 7Aillustrates a peak normalized irradiance (PDF) between 0.5 and 1 at a Gamma-Gamma of about 1 for zenith. Thereafter, the gamma-gamma stabilizes toward zero beginning at a normalized radiance of about 2.5-3.FIG. 7Billustrates a fade between 20-25 dB and a surge probability between 10-15 at 10−15Gamma-Gamma for zenith.

FIG. 7Arepresents a Probability Density Function for the Normalized Received optical power. This figure shows a statistical distribution of light collected by the receive aperture on the Ground station.FIG. 7Bis the related Cumulative Probability Distribution Function of Received optical power. This figure shows a Fade/Surge probability in the received optical power. The analysis shown inFIGS. 7A-Bis for the case of 90-degree elevation angle, where the transmitting satellite is directly overhead the Ground station.

FIGS. 8A and 8Billustrate plots of the communication system in SV-ground state. Similar characteristics described above inFIGS. 7A and 7Bare observed inFIGS. 8A and 8B, albeit at a 20-degree elevation.FIG. 8Aillustrates a peak normalized irradiance (PDF) between 0.5 and 1 at a Gamma-Gamma of about 1 for a 20-degree elevation. Thereafter, the Gamma-Gamma stabilizes toward zero beginning at a normalized radiance of about 4-5.FIG. 8Billustrates a fade of 25 at a Gamma-Gamma of about 10−4to 10−5, and a surge probability between about 18-20 at 10−15for a 20-degree elevation.

FIG. 8Arepresents a Probability Density Function for the Normalized Received optical power. This figure shows a statistical distribution of light collected by the receive aperture on the Ground station.FIG. 8Bis the related Cumulative Probability Distribution Function of Received optical power. This figure shows a Fade/Surge probability in the received optical power. The analysis shown inFIGS. 8A-Bis for the case of 20-degree elevation angle.

The purpose of includingFIGS. 7A-Band8A-B is to illustrate the random, statistical nature of the received light, after propagating through the atmosphere. This scintillation induced received optical power fluctuation is an inescapable result of propagating though the atmosphere. The present disclosure proposes a novel method of mitigating this impairment using temporal diversity in orthogonally polarized redundant data streams.

It is to be understood that the methods and systems are not limited to specific methods, specific components, or to particular implementations. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.