U.S. patent application number 16/856953 was filed with the patent office on 2021-08-12 for free space optical terminal with dither based alignment.
The applicant listed for this patent is SA Photonics, Inc.. Invention is credited to William C. Dickson.
Application Number | 20210250092 16/856953 |
Document ID | / |
Family ID | 1000005735999 |
Filed Date | 2021-08-12 |
United States Patent
Application |
20210250092 |
Kind Code |
A1 |
Dickson; William C. |
August 12, 2021 |
FREE SPACE OPTICAL TERMINAL WITH DITHER BASED ALIGNMENT
Abstract
Embodiments relate to a bidirectional free space optical (FSO)
communications system. Specifically, data-encoded FSO beams are
transmitted and received between two terminals. A transmit (Tx)
direction of a beam transmitted from the first terminal is dithered
by a beam steering unit (BSU). As the dithered beam is received by
the second terminal, the power levels of the beam are measured. The
power levels are then encoded in a data-encoded FSO beam
transmitted to the first terminal. This allows the first terminal
to decode the received FSO beam and determine the power levels. The
power levels allow the first terminal to determine Tx direction
misalignments and adjust the Tx direction for the Tx beam sent to
the second terminal. This process may be repeated to reduce Tx
misalignments and may be performed by both terminals such that each
terminal sends power level information to the opposite
terminal.
Inventors: |
Dickson; William C.;
(Granville, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SA Photonics, Inc. |
Los Gatos |
CA |
US |
|
|
Family ID: |
1000005735999 |
Appl. No.: |
16/856953 |
Filed: |
April 23, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62972570 |
Feb 10, 2020 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 10/116 20130101;
H04B 10/0775 20130101; H04B 10/676 20130101; H04B 10/1123 20130101;
H04B 10/1143 20130101 |
International
Class: |
H04B 10/112 20060101
H04B010/112; H04B 10/67 20060101 H04B010/67; H04B 10/077 20060101
H04B010/077; H04B 10/116 20060101 H04B010/116; H04B 10/114 20060101
H04B010/114 |
Claims
1. A bidirectional free space optical communications system
comprising: two terminals communicating by free space optical (FSO)
communications links, each terminal acting as a transmitter for one
of the FSO communications links and as a receiver for the other of
the FSO communications links; wherein each FSO communications link
comprises: a beam steering unit (BSU) in the transmitter that
steers a data-encoded FSO beam in a direction towards the receiver;
a motion generator coupled to the BSU that controls the BSU to
dither the direction of at least a portion of the transmitted FSO
beam, wherein a dither amplitude A.sub.d is based on an amount of
FSO communication interference; a detector in the receiver that
detects a received power of at least the portion of the transmitted
FSO beam; and a feedback loop from the detector in the receiver to
the BSU in the transmitter, the feedback loop including a return
data-encoded FSO beam from the receiver to the transmitter, the
feedback loop controlling the direction of the transmitted FSO beam
based on the received power of at least the portion of the
transmitted FSO beam.
2. The communications system of claim 1, wherein the motion
generator in one of the terminals dithers at a frequency f.sub.1
and the motion generator in the other of the terminals dithers at a
frequency f.sub.2, and misalignment of the transmitted FSO beams is
determined by demodulating the received powers using frequencies
f.sub.1 and f.sub.2 and time averaging over a time period that is
an integer number of cycles of the frequencies f.sub.1 and
f.sub.2.
3. The communications system of claim 1, wherein the motion
generator dithers the direction of the at least the portion of the
transmitted FSO beam in two dimensions.
4. The communications system of claim 3, wherein the motion
generator dithers the direction of at least the portion of the
transmitted FSO beam in a circular scan of amplitude A.sub.d and
frequency f, and alignment errors X.sub.10 and Y.sub.10 in the X-
and Y-directions of the transmitted FSO beam are described by: X 1
.times. 0 = R g .times. 1 2 .times. A d .times. log .function. ( P
2 .function. ( t ) ) .times. cos .function. ( .omega. 1 .times. t )
##EQU00005## Y 1 .times. 0 = R g .times. 1 2 .times. A d .times.
log .function. ( P 2 .function. ( t ) ) .times. sin .function. (
.omega. 1 .times. t ) ##EQU00005.2## where Rg.sub.1 is a beam
radius of the transmitted FSO beam that overfills an aperture of
the receiver, P.sub.2 is the received power of the transmitted FSO
beam, .omega..sub.1=2.pi.f.sub.1, and angle brackets indicate time
average over multiple cycles at rate .omega..sub.1.
5. The communications system of claim 4, wherein the amplitude
A.sub.d is not more than 1/5 of the beam "1/e" radius Rg.sub.1.
6. The communications system of claim 3, wherein the motion
generator dithers the direction of at least a portion of the
transmitted FSO beam in a conical scan.
7. The communications system of claim 1, wherein received power
data representative of the received power of at least the portion
of the transmitted FSO beam is encoded onto the return FSO beam of
the feedback loop along with other data.
8. The communications system of claim 7, wherein the transmitted
FSO beam is coupled into a demodulation path for demodulating the
transmitted FSO beam, and the received power data is representative
of the power coupled into the demodulation path.
9. The communications system of claim 7, wherein the transmitted
FSO beam is coupled into a wavefront sensing path, and the received
power data is representative of the power coupled into the
wavefront sensing path.
10. The communications system of claim 7, wherein the received
power data includes at least two different measures of the received
power of the transmitted FSO beam.
11. The communications system of claim 10, wherein the transmitted
FSO beam is coupled into a demodulation path for demodulating the
transmitted FSO beam and into a wavefront sensing path, and the
received power data includes (a) data representative of the power
coupled into the demodulation path, and (b) data representative of
the power coupled into the wavefront sensing path.
12. The communications system of claim 11, wherein each feedback
loop further comprises: a selector that selects either (a) the data
representative of the power coupled into the demodulation path, or
(b) the data representative of the power coupled into the wavefront
sensing path, to control the direction of the transmitted FSO
beam.
13. The communications system of claim 1, wherein data for
correcting the direction of the transmitted FSO beam is included in
the return FSO beam of the feedback loop.
14. The communications system of claim 1, wherein each receiver
comprises: an optical fiber leading to a demodulation path, the
transmitted FSO beam coupled into the optical fiber and demodulated
by the demodulation path; and an optical tap that taps a portion of
the FSO beam coupled into the optical fiber, the optical tap
coupled to the detector.
15. The communication system of claim 1, wherein the BSU includes
an optical component that steers the FSO beam in a direction
towards the receiver and that dithers the direction of at least the
portion of the FSO beam.
16. The communication system of claim 1, wherein the BSU includes a
first set of one or more optical components that steers the FSO
beam and a second set of one or more optical components that
dithers the direction of at least the portion of the FSO beam,
wherein the second set of optical components operates independently
from the first set of optical components.
17. (canceled)
18. The communication system of claim 1, wherein the motion
generator is configured to increase the dither amplitude A.sub.d
responsive to the FSO communication interference exceeding a
threshold level.
19. The communication system of claim 1, wherein the motion
generator is configured to decrease the dither amplitude A.sub.d
responsive to the FSO communication interference being below a
threshold level.
20. (canceled)
21. A bidirectional free space optical communications system
comprising: two terminals communicating by free space optical (FSO)
communications links, each terminal acting as a transmitter for one
of the FSO communications links and as a receiver for the other of
the FSO communications links; wherein each FSO communications link
comprises: a beam steering unit (BSU) in the transmitter that
steers a data-encoded FSO beam in a direction towards the receiver;
a motion generator coupled to the BSU that controls the BSU to
dither the direction of at least a portion of the transmitted FSO
beam; a detector in the receiver that detects a received power of
at least the portion of the transmitted FSO beam; and a feedback
loop from the detector in the receiver to the BSU in the
transmitter, the feedback loop including a return data-encoded FSO
beam from the receiver to the transmitter, the feedback loop
controlling the direction of the transmitted FSO beam based on the
received power of at least the portion of the transmitted FSO beam,
wherein the motion generator in one of the terminals dithers at a
frequency f.sub.1 and the motion generator in the other of the
terminals dithers at a frequency f.sub.2, and misalignment of the
transmitted FSO beams is determined by demodulating the received
powers using frequencies f.sub.1 and f.sub.2 and time averaging
over a time period that is an integer number of cycles of the
frequencies f.sub.1 and f.sub.2.
22. A bidirectional free space optical communications system
comprising: two terminals communicating by free space optical (FSO)
communications links, each terminal acting as a transmitter for one
of the FSO communications links and as a receiver for the other of
the FSO communications links; wherein each FSO communications link
comprises: a beam steering unit (BSU) in the transmitter that
steers a data-encoded FSO beam in a direction towards the receiver;
a motion generator coupled to the BSU that controls the BSU to
dither the direction of at least a portion of the transmitted FSO
beam, wherein: the motion generator dithers the direction of the at
least the portion of the transmitted FSO beam in two dimensions,
and the motion generator dithers the direction of at least the
portion of the transmitted FSO beam in a circular scan of amplitude
A.sub.d and frequency f, and alignment errors X.sub.10 and Y.sub.10
in the X- and Y-directions of the transmitted FSO beam are
described by: X 1 .times. 0 = R g .times. 1 2 .times. A d .times.
log .function. ( P 2 .function. ( t ) ) .times. cos .function. (
.omega. 1 .times. t ) ##EQU00006## Y 1 .times. 0 = R g .times. 1 2
.times. A d .times. log .function. ( P 2 .function. ( t ) ) .times.
sin .function. ( .omega. 1 .times. t ) ##EQU00006.2## where
Rg.sub.1 is a beam radius of the transmitted FSO beam that
overfills an aperture of the receiver, P.sub.2 is the received
power of the transmitted FSO beam, .omega..sub.1=2.pi.f.sub.1, and
angle brackets indicate time-average over multiple cycles at rate
.omega..sub.1; a detector in the receiver that detects a received
power of at least the portion of the transmitted FSO beam; and a
feedback loop from the detector in the receiver to the BSU in the
transmitter, the feedback loop including a return data-encoded FSO
beam from the receiver to the transmitter, the feedback loop
controlling the direction of the transmitted FSO beam based on the
received power of at least the portion of the transmitted FSO beam.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e) to U.S. Provisional Patent Application Ser. No. 62/972,570,
"Free Space Optical Terminal with Dither Based Alignment," filed on
Feb. 10, 2020, the content of which is incorporated herein by
reference in its entirety.
BACKGROUND
1. Technical Field
[0002] This disclosure relates generally to free space optical
(FSO) communications and, more particularly, to reducing alignment
errors between FSO terminals.
2. Description of Related Art
[0003] Free space optical (FSO) communications is a communications
technology that uses light propagating in free space to wirelessly
transmit data, for example, for telecommunications or computer
networking. Here, "free space" is a medium wherein light
propagates; it can include air, water, outer space, or vacuum. This
contrasts with guided wave communications over media such as
coaxial cable or optical fibers. FSO technology is useful where
physical connections are impractical due to high costs or other
considerations. In contrast with other free-space electromagnetic
communication media, FSO signals are more directional. This confers
benefits both for communications capacity and for communications
privacy.
[0004] The high directionality of FSO signals, however, requires
more accurate pointing alignment between systems to maintain the
benefit of the directionality. The requirement for accuracy is so
demanding that mechanical movement or flexing of the terminal
mounting structure, or even the optical effects of atmospheric
turbulence can degrade communications performance. Pointing
accuracy benefits generally accrue when the transmission beam's
electromagnetic wavelength is short (or equivalently, when the
electromagnetic frequency is high); this can apply not only FSO
systems, but other communications that depend on accurate
propagation alignment as well. When FSO communications terminals
operate in unpredictable or rapidly changing conditions, measures
may be required to maintain alignment between stations. For
example, if an FSO node is mounted on a tower, strong winds may
move the tower such that the FSO terminal sways with the tower. In
another example, an FSO terminal is mounted on a moving vehicle
that communicates with a stationary FSO terminal. In these and
similar situations, the high directionality of FSO technology may
require rapid adjustment and accurate pointing to maintain a viable
FSO communications link.
SUMMARY
[0005] A bidirectional free space optical (F SO) communications
system is described herein. The system includes data-encoded FSO
beams that are transmitted and received between two terminals,
establishing bidirectional communication. Each terminal acts as a
transmit (Tx) terminal for one direction of the link and as a
receive (Rx) terminal for the other direction of the link. A
beam-steering unit (BSU) is associated with the transmitter and
this steers the data-encoded FSO beam toward the receiver. To
improve (e.g., optimize) its pointing angle, the BSU may dither the
Tx beam angle about its currently-estimated Tx direction. The Rx
terminal of each link measures the received power of the incoming
dithered FSO beam; it encodes the receive power measurements into
its own Tx FSO beam that is propagated in the reverse direction,
making this information available to the first terminal. The first
terminal uses the power measurements from the far terminal to
reduce Tx beam steering errors. The data path for the power
measurements is advantageously the data-encoded FSO beam
transmitted from the Rx terminal to the Tx terminal, which in this
context will be referred to as the return FSO beam, however, other
data paths such as radio frequency (RF) communication may be used
instead. A control system on the Tx terminal can then use the power
measurements as a feedback signal to adjust the direction of its
FSO beam. This process may be performed once or repeated on a
regular basis as needed for a specific operating environment.
[0006] Dithering is characterized by a pair of periodic basis
functions {circumflex over (X)}(t) and (t) that describe the
dithering "path" in angular deflection. The actual beam deflection
is then a scaled version of these basis functions. Although we
consider the simple trigonometric functions sine and cosine for
{circumflex over (X)}(t) and (t), embodiments are not limited to
these functions. Alternate implementations may take advantage of
other periodic forms of {circumflex over (X)}(t) and (t) that cover
a non-uniform beam perturbation pattern. For example, applications
of embodiments to other environments such as onboard a ship or an
airplane could entail substantially different disturbance patterns
and amplitudes that might motivate different preferred functions
for of {circumflex over (X)}(t) and (t). Alternate forms of
{circumflex over (X)}(t) and (t) might also be motivated by ease of
implementation, especially when dithering is implemented through
mechanical system displacement.
[0007] In some embodiments, both terminals use this dither approach
for alignment detection. Terminal 1 transmits an FSO beam to
terminal 2, and terminal 2 transmits an FSO beam to terminal 1. In
one approach, the motion generator in terminal 1 dithers the
direction of its transmitted FSO beam at a frequency f.sub.1 and
the motion generator in terminal 2 dithers the direction of the FSO
beam at frequency f.sub.2. For the FSO beam transmitted from
terminal 1 to terminal 2, the power received at terminal 2 will
depend on the f.sub.1 dither imparted at terminal 1 and may also be
affected by the f.sub.2 dither imparted by terminal 2. In some
embodiments, the effect of the f.sub.2 dither is reduced or
separated by selection of the frequencies f.sub.1 and f.sub.2 in
conjunction with a given signal processing algorithm.
[0008] In some embodiments, the BSU dithers the direction of the
transmitted FSO beam in two dimensions. For example, it may impart
a conical (elliptical or circular) scan in the transmitted beam. If
the dither is a circular scan of amplitude A.sub.d and frequency
f.sub.1, then alignment errors X.sub.10 and Y.sub.10 in the X- and
Y-directions of the transmitted FSO beam may be described by:
X 1 .times. 0 = R g .times. 1 2 .times. A d .times. log .function.
( P 2 .function. ( t ) ) .times. cos .function. ( .omega. 1 .times.
t ) .times. .times. Y 1 .times. 0 = R g .times. 1 2 .times. A d
.times. log .function. ( P 2 .function. ( t ) ) .times. sin
.function. ( .omega. 1 .times. t ) ( 1 ) ##EQU00001##
In these expressions, R.sub.g1 is the beam radius of the
transmitted FSO beam, P.sub.2 is the received power of the
transmitted FSO beam, .omega..sub.1=2.pi.f.sub.1, and angle
brackets denote time average. The sine and cosine functions as
written in the equation apply to the case of a circular dithering
pattern. Alternate dithering patterns (as noted earlier) would
require a different pair of appropriately selected periodic
functions to express the dither pattern. The equation assumes that
the FSO beam overfills the aperture at the receiver. The unitless
amplitude scale factor value A.sub.d is not critical, but
preferably is a small fraction of 1.0, such as 0.2.
[0009] The data path from the receiver back to the transmitter for
the measured power is advantageously in the form of the return FSO
beam from the receiver. In some embodiments, the return FSO beam
includes data (received power data) that represents the received
power of the transmitted FSO beam at the receiver. In some
embodiments, this data is included in a packet header of the return
FSO beam. In some embodiments, the received power data includes
data for at least two different measures of the received power. For
example, the Rx terminal may include a demodulation path and a
wavefront sensing path, and one measure of received power may be
based on power in the demodulation path while another measure is
based on power in the wavefront sensing path. A selector may
determine which power measure to use.
[0010] In some embodiments, the demodulation path is implemented as
follows at the receiver. The BSU directs the incoming FSO beam onto
the terminal's input port, where it is coupled into an optical
fiber which then leads to the rest of the demodulation path. An
optical tap from the optical fiber leads to a detector, which
measures the power in the demodulation path. In some embodiments,
the wavefront sensing path may use a multi-cell sensor (such as a
quad-cell sensor). The total power received by the multi-cell
sensor is a measure of the power in the wavefront sensing path.
[0011] In other embodiments, the alignment approach described above
may be used in only one of the terminals. Alternatively, the FSO
communications system may not be directional and the data path may
be implemented by a communications channel other than a return FSO
beam, such as a radio frequency (RF) link.
[0012] Other aspects include components, devices, systems,
improvements, methods, processes, applications, computer readable
mediums, and other technologies related to any of the above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Embodiments of the disclosure have other advantages and
features that will be more readily apparent from the following
detailed description and the appended claims, when taken in
conjunction with the examples in the accompanying drawings, in
which:
[0014] FIGS. 1A and 1B are block diagrams of two terminals
communicating via FSO communications links, according to some
embodiments.
[0015] FIG. 2A is a block diagram of a single terminal, according
to an embodiment.
[0016] FIGS. 2B-2E are block diagrams of a single FSO
communications link with feedback loop, according to an
embodiment.
[0017] FIG. 3 is a plot that illustrates the effect of dithering on
received power, according to an embodiment.
[0018] FIG. 4 is a diagram showing detail of the feedback loop,
according to an embodiment.
DETAILED DESCRIPTION
[0019] The figures and the following description relate to
preferred embodiments by way of illustration only. It should be
noted that from the following discussion, alternative embodiments
of the structures and methods disclosed herein will be readily
recognized as viable alternatives that may be employed without
departing from the principles of what is claimed.
[0020] The technology relates to measuring system response as it
makes small, controlled changes in pointing angle. This is used to
improve the pointing direction. We use the term "dither" to refer
to these directional pointing variations. In some technical fields,
"dither" refers to the addition of "white" or indeterminate noise;
our application of "dither" is determinate and operates to improve
the optical configuration by applying known variations and
correlating the resulting changes to system performance. Many
different approaches may be used to implement the pointing
variations (i.e., to generate the dither). For example, a mirror or
reflective surface in front of the terminal might be tilted
mechanically to implement pointing variation. Alternatively, a
rotating prism or series of prisms might be used. Lenses, internal
or external to the rest of the optical train, may be shifted
laterally across the optical axis to implement pointing variation.
A detection device may also be shifted relative to the optical axis
to implement pointing variations; this is an example of pointing
variation without the optical elements being re-oriented at
all.
[0021] FIGS. 1A and 1B are block diagrams of terminals 100A and
100B communicating via free space optical (FSO) communications
links, according to some embodiments. Specifically, the terminals
100 are communicating by transmitting and receiving data-encoded
FSO beams 105. In FIG. 1A, each terminal 100 receives and transmits
FSO beams 105 through different apertures, while in FIG. 1B, the
terminals are co-boresighted so that beams are received and
transmitted though the same aperture. As described herein, if
terminal 100A is referenced, terminal 100B may be referred to as a
remote terminal, beam 105A may be referred to as a transmit (Tx)
beam, and beam 105B may be referred to as a receive (Rx) beam.
Intermediate beam configurations between single- and multi-aperture
are also possible, wherein the beam is split into dithered and
undithered beams by a partially reflective surface. These are not
illustrated.
[0022] As further described below, each terminal 100 dithers the
direction of its Tx beam. The opposite terminal detects the
received power of the incoming FSO beam 105 and can transmit this
power information back to the transmitting terminal 100. The
transmitting terminal 100 can use this information to detect
alignment errors and adjust the direction of its Tx beam to reduce
(e.g., minimize) the detected alignment errors.
[0023] FIG. 2A is a block diagram of a single terminal 100,
according to an embodiment. The terminal 100 includes a data I/O
interface (not shown), modem 235, Tx source 225, Rx detector 230, a
circulator 220, a Tx/Rx fiber 215, a wavefront sensor 210, a BSU
205, a telescope 200, a motion generator 255, a power detector 245,
and a controller 250. In FIG. 2A, electrical signals (both analog
and digital) are indicated by the solid arrows and optical signals
(both guided and free space) are indicated by the line arrows.
[0024] The components are optically coupled as follows. The
telescope 200 is optically coupled to the BSU 205. The BSU 205 is
optically coupled to the wavefront sensor 210. It is also optically
coupled to the circulator 220 via the fiber 215. The ports of the
optical circular 220 are optically coupled to the Tx source 225,
the Tx/Rx fiber 215 and the Rx detector 230. The power detector 245
detects light that is tapped from the light to the Rx detector
230.
[0025] The components are electrically coupled as follows. The
motion generator 255 and controller 250 are electrically coupled to
the BSU 205. The modem 235 is electrically coupled to the Tx source
225 and the Rx detector 230. It also receives data from the power
detector 245 and wavefront sensor 210 (after conversion to digital
form) and provides data to the controller 250.
[0026] FIGS. 2B-2E illustrate one FSO communications link from a Tx
terminal to a Rx terminal. FIGS. 2B and 2C show the data path from
Tx terminal 100A to Rx terminal 100B. FIG. 2B shows the data path
in the Tx terminal 105A from incoming digital data to outgoing FSO
transmit beam 105A. Modem 235 modulates the Tx source 225 based on
the incoming data. The data-encoded light is transmitted by
circulator 220, fiber 215, BSU 205 to telescope 200, which directs
the data-encoded FSO beam 105A to the Rx terminal 105B. The motion
generator 255 adds a dither to the direction of the Tx beam 105A,
as will be described in more detail below. FIG. 2C shows the data
path at the Rx terminal 105B. Relative to the Rx terminal 100B, the
incoming beam is its Rx beam, but it is labelled as transmit beam
105A in FIG. 2C to maintain consistency throughout the figures. The
transmit beam 105A is optically coupled from telescope 200, to BSU
205, fiber 215 and circulator 220 to the Rx detector 230. The
output of the Rx detector 230 is demodulated by modem 235 to
produce the outgoing digital data.
[0027] FIGS. 2D and 2E show the feedback loop from the Rx terminal
100B back to the Tx terminal 100A. FIG. 2D shows the feedback path
in the Rx terminal 100B. The received power may be measured at two
locations in this example. One is the sum power received by the
wavefront sensor 210. The other is power detector 245, which takes
an optical tap from the main data path used to demodulate the
incoming data. These measurements may be processed (not shown in
FIG. 2D) and the resulting data will be referred to as receive
power data 260. The receive power data 260 is returned to the Tx
terminal 100A via the data-encoded FSO beam 105B produced by the Rx
terminal 100B. With respect to the Rx terminal 100B, the beam 105B
is its transmit beam but it is labelled as return beam 105B to
maintain consistency through the figures. FIG. 2E shows the
feedback path in the Tx terminal 100A. The return FSO beam 105B is
demodulated and the recovered receive power data 260 is used by
controller 250 to control the BSU 205 to maintain alignment.
[0028] FIGS. 2B-2E unroll the transmit data path and return
feedback loop for an FSO communications link which involves two
terminals. FIG. 2A shows both aspects for a single terminal. The
terminal 100 includes at least two optical paths: an Rx beam path
and a Tx beam path. In the Rx beam path, a Rx beam 105B propagates
through the telescope 200 and is directed towards the BSU 205. The
BSU 205 steers the Rx beam to the wavefront sensor 210. A portion
of the beam is detected by the wavefront sensor 210 and another
portion of the beam is coupled into the fiber 215. Light in the
fiber 215 is directed by the circulator 220 to the Rx detector 230.
In the Tx beam path, a Tx beam from the Tx source 225 is directed
to the fiber 215 by the circulator 220. The Tx beam is emitted from
the fiber 215 and towards the BSU 205. The Tx beam is directed by
the BSU 205 towards the telescope 200. The Tx beam 105A propagates
through the telescope 200 and into free space.
[0029] The telescope 200 and BSU 205 are optical components that
direct Rx beams to the wavefront sensor 210 and fiber 215, and
direct Tx beams to the remote terminal. The telescope 200 includes
components that can spread, focus, redirect, and otherwise modify
the beams 105 passing through it. The position of the telescope 200
relative to the terminal 100 is typically fixed. The telescope 200
may be as simple as a single lens or it may include additional
optical components, such as diffusers, phase screens, beam
expanders, mirrors, and lenses.
[0030] The BSU 205 can take many different forms. The BSU 205 can
be a mechanically-driven reflective or refractive device. Examples
of such devices include mirrors, Fresnel devices, lenslet arrays
and more. A mechanical driver for any one of these examples can
consist of voice-coil actuators, piezoelectric actuators,
servo-motor driven positioners, and many other approaches. In
another example, a series of wedge-shaped prisms may be put in
continuous rotation at different rates to produce a complex
dithering pattern through the series. Microelectronic arrays (MEMS)
devices can also be used to steer a beam. Opto-acoustic devices
that exploit acoustic waves in reflective or refractive materials
can also be used. The BSU 205 may operate in different modes, such
as a beam acquisition mode or a beam tracking mode. For example, an
initial Tx direction can be established through a beam acquisition
mode. The Tx direction may be determined or updated based on
feedback signals (e.g., alignment errors) from the controller 250
and the wavefront sensor 210 (this feedback path not shown in FIG.
2). In some cases, the Tx beam 105A is transmitted by the telescope
200 along the same direction as the Rx beam 105B is received (the
Rx direction may be determined from the wavefront sensor 210). In
some cases, the Tx direction is not parallel to the Rx direction.
For example, atmospheric conditions between terminals 100 can
affect beams differently depending on their propagation direction.
In these cases, Tx and Rx beams may travel different optical paths
between terminals 100. In another example, if a remote terminal is
moving, the BSU 205 may direct a Tx beam with an angular bias
(referred to as point-ahead bias) to account for travel time of the
Tx beam.
[0031] Although the figures illustrate an implementation concept
with a dithering device deflecting the entire Tx and Rx beams,
other embodiments are possible, in which only part of the beam (Tx
or Rx) is dithered. The beam could be divided, for example, with a
partially reflective mirror extending across the entire beam, or a
small mirror might dither only a spatially selected portion of the
beam. Such an embodiment, where only part of the beam is dithered
while most of the communication beam is not dithered, benefits by
reducing or eliminating the variable power of dithering onto the
communication channel.
[0032] While steering Tx beams in a Tx direction towards a remote
terminal, the BSU 205 may dither the Tx direction. Specifically,
the motion generator 255 can generate control signals to dither the
Tx direction. The Tx direction can be dithered along one or more
axes. For example, conical scans (circular and elliptical) are
two-dimensional dither patterns that may be used. The dither
frequency is preferably at least 5.times.-10.times. larger than the
desired bandwidth of the closed-loop system while observing any
limitations related to the data packet broadcast repetition rate or
other FSO frequencies. In some embodiments, increased performance
occurs when the closed-loop system bandwidth is greater than the
beam deflection being countered. The amplitude of the dither should
be high enough to yield sufficiently low noise in the detected
alignment errors, but low enough to reduce (e.g., minimize) the
coupling loss due to the intentionally mis-pointed Tx beam. An
example preferred dither amplitude of about 1/5 of the Tx Gaussian
beam divergence radius (1/e radius) may provide a suitable
compromise between detection noise and coupling. During periods of
low interference (e.g., little terminal movement), the dither may
be set to a small fraction of the Tx beam size to reduce the
likelihood of (e.g., prevent) the dither from causing data
transmission errors. During periods of high interference (for
example, a tower-mounted unit during periods of high wind), the
dither amplitude may be increased to reduce the likelihood (e.g.,
ensure) that uncontrolled terminal motion or variability does not
overcome and invalidate the system's estimate of
position-to-performance correlations. During periods of extreme
interference, the system may abandon data throughput objectives in
favor of maintaining or recovering system-to-system alignment.
Dithering is further described with reference to FIG. 3.
[0033] The wavefront sensor 210 is a component used to measure the
incidence angle of the Rx beam relative to the Tx direction. The
wavefront sensor 210 may be a quad-cell (or other multi-cell)
sensor. The detectors of the wavefront sensor 210 can be
photodetectors or other electromagnetic-wave detectors that convert
the incoming electromagnetic waves into electrical current. The
wavefront sensor 210 can include light detectors capable of
detecting different types of light signals, e.g., low and high
light intensities, specific wavelengths, etc. This allows the
terminal 100 to operate in low light (e.g., at night) and high
light situations (e.g., at mid-day). The wavefront sensor 210 may
include a hole filled by an end of the fiber 215. In one
implementation, light that does not enter the fiber falls on other
light sensors near the hole; the relative amplitudes of signal in
each of these nearby sensors indicates the wavefront alignment
relative to the fiber 215. In another example, the wavefront sensor
210 includes a fiber bundle connected to detectors, and light
detected in the non-central fibers similarly indicates wavefront
alignment. These example wavefront sensors 210 and fiber
combinations 215 are described in U.S. Pat. No. 10,389,442 "Free
Space Optical (FSO) System" and U.S. Pat. No. 10,411,797 "Free
Space Optical Node with Fiber Bundle" which are incorporated herein
by reference in their entirety.
[0034] The Tx/Rx fiber 215 is an optical fiber, such as a
multi-mode fiber (MMF), dual core fiber, or double clad fiber. If
the fiber 215 is a double clad fiber, Tx beams may propagate
through the core while Rx beams propagate predominantly through the
inner cladding. The circulator 220 can be a single-mode or
multi-mode circulator. Example circulators are described in patent
application Ser. No. 16/259,899 "Optical Circulator with
Double-Clad Fiber" which is incorporated herein by reference in its
entirety. In most FSO applications, the configuration described in
this application works better than single-mode circulator
configurations. The Rx detector 230 is a photodetector that
converts Rx beams from the circulator 220 into electrical signals.
For example, the Rx detector 230 is an avalanche photodiode (APD).
The Tx source 225 converts transmit data from the modem 235 into Tx
beams. The Tx source 225 can include a laser.
[0035] The power detector 245 determines power levels of an Rx beam
received by the terminal 100. The power detector 245 can determine
the power levels of an Rx beam coupled into the fiber 215 (referred
to as the received signal strength indicator (RSSI) signal). This
is one measure of the power of the incoming beam. Another measure
is the power of the Rx beam incident on the wavefront sensor 210
(referred to as the P.sub.QC signal). The P.sub.QC signal may be
determined by summing the power received by each of the detectors
of the wavefront sensor 210. As described earlier, an
implementation may have independently-steered dithered and
undithered beams. This may be accomplished, for example, with
separate apertures or by diverting part of a single beam with a
part-reflecting mirror. Such a configuration may exploit power
measurements taken with the dithered and non-dithered beams. If the
wavefront sensor 210 is a quad cell, the signal strength from the
four detectors are added together to determine P.sub.QC. In another
example, if the wavefront sensor 210 includes a fiber bundle, the
signals detected in each of the fibers are summed. To determine the
RSSI signal, an optical tap or an optic splitter may be used to
sample a portion of light or direct a portion of light in the fiber
215 (or the fiber to the Rx detector 230, as shown in FIG. 2) to
the power detector 245. An avalanche photodiode (ADP) and an
analogue to digital converter (ADC) may be used to determine the
amount of light coupled into the fiber 215. Since less light may be
coupled into the fiber 215 compared to the amount of light incident
on the wavefront sensor 210, the P.sub.QC signal may be a better
indicator of received power than the RSSI signal. Depending on
details of system architecture, the RSSI indicator may be more
sensitive to intermittent impairments (such as in heavy rain).
After the P.sub.QC signal and the RSSI signal are determined, the
signals are transmitted to the modem 235 to be encoded in a Tx beam
and transmitted to the remote terminal. This allows the remote
terminal to determine the received power levels of beams received
by the terminal 100. Note that the term "power" as used herein is
used for simplicity. In some embodiments, the determined power
levels are the light energy received over time (e.g., the radiant
flux). In other embodiments, the determined power levels are
indicators of the received power, such as signals that represent,
are proportional to, or approximate to the power received by the
terminal 100.
[0036] The modem 235 modulates data to be transmitted in Tx beams.
The data includes header information from the I/O interface 240
that may include received beam power and terminal status
information. The modem 235 combines and converts this and the data
"payload" that is to be delivered to the network beyond the FSO
link itself into a single data stream as a modulated electrical
signal. In some embodiments, power information as well as terminal
status and BSU status (as opposed to raw power measurements) may be
processed to compress them and occupy less of the final data
stream. The modulated electrical signal is sent to the Tx source
225 and imparted on the Tx beam as a modulation. The modem can also
demodulate data encoded in Rx beams. Specifically, the modem 235
decodes information in the electrical signals from the Rx detector
230. The decoded information can include received power data 260,
which may be separate from the payload data. This received power
data 260 is transmitted to the controller 250. The remaining
decoded information may be transmitted to I/O interface (e.g., to
be transmitted to another terminal). The modem 235 can include any
electronics and/or computer instructions that modulate or
demodulate signals, including physical (PHY) layer or medium access
control (MAC) related processes (such as error correction).
[0037] The controller 250 receives received power data 260 from the
modem 235 that was previously encoded in a Rx beam. The received
power data is representative of the received power of a previous
beam transmitted by the terminal 100 and detected by the remote
terminal. The received power data can include the P.sub.QC and RSSI
signals as determined by the remote terminal and the time and date
of the power measurements. If the remote terminal was also
dithering its Tx beam while the beam was received, the received
power data can include dithering information of the remote terminal
(e.g., in the header), such as the frequency, amplitude, and
direction of the dithering. In a preferred embodiment, the header
information is structured in a way that received power, dithering
information, and other remote terminal information are readily
extracted from the header. In some FSO implementations, the
transmitted power changes adaptively (for example, to conserve
power, to reduce (e.g., minimize) likelihood of intercept, and
other concerns). In such cases, the remote terminal may transmit
information concerning its transmitted power or its beam-pointing
state and the receiving terminal may elect to incorporate this
information into its own pointing control system. The controller
250 uses the received power data to determine alignment errors
between the transmitting terminal 100 and the remote terminal and
to adjust a Tx direction of the Tx FSO beam to correct the
alignment errors. Determining the alignment errors is further
described below with respect to Eqs. 2-6 below.
[0038] FIG. 3 is a plot that illustrates the effect of dithering on
received power, according to an embodiment. The main plot 310 plots
received power at the Rx terminal as a function of the X- and
Y-misalignment of the Tx beam. The Z-axis is the coupled beam power
normalized by the power with no misalignment, measured in decibels
(dB). The X- and Y-axes are the misalignment of the Tx beam,
normalized by the beam radius Rg. Here, the misalignments X.sub.ang
and Y.sub.ang are angular misalignments, as is the beam radius Rg.
In addition, the Tx beam has a Gaussian profile and overfills the
aperture of the Rx terminal.
[0039] If the Tx beam is perfectly aligned with the aperture (point
312, with X.sub.ang/Rg=0 and Y.sub.ang/Rg=0), then the Rx aperture
couples maximum power (0 dB). If the beam is misaligned with the
aperture (e.g., point 314, with X.sub.ang/Rg=+0.35 and
Y.sub.ang/Rg=-0.55), the aperture receives less power (less than 0
dB). Thus, the received power can indicate the alignment of the Tx
beam with the Rx aperture. However, if the peak power level is not
known, it may be difficult to determine if the aperture is aligned
with the peak of the curve. For example, the maximum power level
may change based on weather conditions between the terminals
100.
[0040] To determine the location of the current Tx beam on plot
310, the Tx direction of the beam may be dithered. By dithering the
Tx direction, changes in received power can indicate the location
on the curve 310. In the embodiment of FIG. 3, the Tx beam is
dithered using a circular scan. For the Tx beam that is nominally
aligned (point 312), the circular dithering produces the trajectory
322. For the Tx beam that is misaligned (point 314), the circular
dithering produces the trajectory 324. If an aperture receives more
power when the direction of the Tx beam is dithered along an axis
(e.g., in the -y direction), this can indicate the alignment of the
Tx beam relative to the Rx aperture. Thus, by communicating the
received power to the transmitter terminal, alignment errors can be
calculated and corrected. This calculation is explained below.
[0041] Alignment errors (also referred to as misalignment vectors
or direction misalignments) are calculated by the controller 250
using the following approach. The terminal 100 transmitting the
beam will be referred to as Terminal 1 and the terminal receiving
the beam will be referred to as Terminal 2. If the shape of the
received beam is assumed to be Gaussian and overfills Terminal
2'aperture, then the power P.sub.2 coupled by the aperture of
Terminal 2 is
P 2 = P 2 .times. 0 .times. e - 2 .times. ( X 1 2 + Y 1 2 ) / R g
.times. .times. 1 2 , ( 2 ) ##EQU00002##
where P.sub.20 is the power coupled when there is no misalignment,
X.sub.1 and Y.sub.1 are the angle errors of the Tx direction (of
Terminal 1) and Rg.sub.1 is the angle radius of the received beam.
Taking the natural logarithm of both sides of Eq. 2 yields:
log P.sub.2=log
P.sub.20-2(X.sub.1.sup.2+Y.sub.1.sup.2)/Rg.sub.1.sup.2. (3)
If Terminal 1 dithers the beam in a circular motion at frequency
f.sub.1, then the angle errors X.sub.1 and Y.sub.1 are the sum of
unknown alignment errors X.sub.10 and Y.sub.10 and the circular
dither components:
X.sub.1=X.sub.10+A.sub.dRg.sub.1 cos(.omega..sub.1t)
Y.sub.1=Y.sub.10+A.sub.dRg.sub.1 sin(.omega..sub.1t), (4)
where A.sub.d is the amplitude of the circular dither and
.omega..sub.1=2.pi.f.sub.1. Substituting Eq. 4 into Eq. 3
yields:
log .function. ( P 2 ) = log .function. ( P 2 .times. 0 ) - 2
.times. X 1 .times. 0 2 + Y 1 .times. 0 2 R g .times. .times. 1 2 -
A d 2 - 4 .times. A d .times. cos .function. ( .omega. 1 .times. t
) R g .times. .times. 1 .times. X 1 .times. 0 - 4 .times. A d
.times. sin .function. ( .omega. 1 .times. t ) R g .times. .times.
1 .times. Y 1 .times. 0 ( 5 ) ##EQU00003##
[0042] Compared to the dither reference terms cos(.omega..sub.1t)
and sin(.omega..sub.1t), the remaining terms in Eq. 5 are assumed
to be quasi-static (having a much lower rate of change). With this
assumption, the unknown alignment errors X.sub.10 and Y.sub.10 can
be detected via demodulation of the log P.sub.2 measurement with
the orthogonal cos(.omega..sub.1t) and sin(.OMEGA..sub.1t)
reference signals. This is equivalent to a single-frequency Fast
Fourier Transform (FFT):
X 1 .times. 0 = R g .times. 1 2 .times. A d .times. log .function.
( P 2 .function. ( t ) ) .times. cos .function. ( .omega. 1 .times.
t ) .times. .times. Y 1 .times. 0 = R g .times. 1 2 .times. A d
.times. log .function. ( P 2 .function. ( t ) ) .times. sin
.function. ( .omega. 1 .times. t ) ( 6 ) ##EQU00004##
In the equation, angle brackets indicate time-average over multiple
cycles at rate .omega..sub.1. Thus, the alignment errors of
Terminal 1 (X.sub.10 and Y.sub.10) can be calculated after
receiving log P.sub.2 from Terminal 2. Similarly, the alignment
errors of Terminal 2 (X.sub.20 and Y.sub.20) can be calculated by
using the known dither frequency of Terminal 2 (f.sub.2) and
receiving log P.sub.1 from Terminal 1.
[0043] Note that the power term detected by Terminal 2 (log P.sub.2
in Eq. 5) only includes terms associated with alignment errors of
Terminal 1 (X.sub.10 and Y.sub.10) and dither components at
frequency f.sub.1. In some cases, the power detected by Terminal 2
has similar loss terms associated with the alignment error terms of
Terminal 2 (X.sub.20 and Y.sub.20) and dither reference signals at
frequency f.sub.2. Cross-coupling of the parallel detection
processes in the presence of signal content at both frequencies
f.sub.1 and f.sub.2 may be reduced (and possibly eliminated for DC
errors) by taking advantage of the Fourier orthogonality of
sinusoidal basis functions. Fourier orthogonality ensures detection
orthogonality when the averages in Eq. 6 are carried out over an
integer number of cycles of both f.sub.1 and f.sub.2. For example,
if f.sub.1=200 Hz and f.sub.2=300 Hz, averaging over 0.01 seconds
covers 2 cycles of f.sub.1 and 3 cycles of f.sub.2. Thus, the
detection of the respective alignment error terms can be
decoupled.
[0044] FIG. 4 is a diagram showing detail of the feedback loop,
according to an embodiment. For simplicity, some components of the
terminals 100A and 100B are omitted compared to FIG. 2 (e.g.,
telescopes 200). In this example, terminal 100A acts as the
transmitter and terminal 100B acts as the receiver. Terminal 100A
transmits a Tx FSO beam 105A to terminal 100A via the BSU 205A. In
this case, 205A is a fast steering mirror. Motion generator 255
adds a circular dither to the Tx beam 105A. The motion generator
255 applies voltage driving signals V.sub.x and V.sub.y to the BSU
205A. The driving signals are determined based on reference
oscillators (cos(.OMEGA..sub.1t) and sin(.OMEGA..sub.1t)), the
amplitude A.sub.d of the dither, Rg.sub.1 (from the Tx beam
geometry), and the inverse dynamics of the BSU 205A.
[0045] The power of Tx beam 105A received by terminal 100B is
measured in two ways: by the quad cell in the wavefront sensor
(P.sub.QC) and from an optical tap along the demodulation data path
(received signal strength indicator or RSSI), as described above.
This received power data 260A is then encoded into the header part
of the FSO modulated beam 105B, then transmitted to terminal 100A.
At terminal 100A, the received power data 260A is decoded by the
modem 235 and transmitted to the controller 250. In this example
case, since the header 400 includes received power data 260A, the
header 400 of return beam 105B is transmitted to the controller
250. The received power data 260B in the header 400 includes the
P.sub.QC and RSSI signals. The controller 250 selects either the
P.sub.QC or the RSSI signal via the selector 405. In some
embodiments, the controller 250 selects both signals (e.g., the
signals are summed).
[0046] As illustrated in the controller and described by Eq. 6, the
alignment errors X.sub.10 and Y.sub.10 can be obtained by
performing various calculations on the selected received power
data, such as taking the natural logarithm of the received power
data, multiplying it by a reference oscillation
(cos(.omega..sub.1t) or sin(.omega..sub.1t)), and multiplying it by
various constants such as 1/2 A.sub.d and Rg.sub.1 (e.g.,
determined from the Tx Beam geometry). Additional processing steps
may be performed to determine the alignment errors. For example, a
band-pass filter and a low-pass filter (e.g., to determine an
average) may be applied to the calculated signals. The calculated
alignment errors X.sub.10 and Y.sub.10 are transmitted to the BSU
205A.
[0047] Although this detailed description contains many specifics,
these should not be construed as limiting the scope of the
invention but merely as illustrating different examples. It should
be appreciated that the scope of the disclosure includes other
embodiments not discussed in detail above. The dither pattern may
be something other than circular, for example, a path selected for
ease of mechanical implementation or that covers the specific types
of motion and scintillation that may occur between the Tx and Rx.
Various other modifications, changes and variations which will be
apparent to those skilled in the art may be made in the
arrangement, operation and details of the method and apparatus
disclosed herein without departing from the spirit and scope as
defined in the appended claims. Therefore, the scope of the
invention should be determined by the appended claims and their
legal equivalents.
[0048] Note that the components and terminals illustrated and
described can include any electronics and/or computer instructions
that may be embodied in digital or analog circuits. This may be
implemented using any one or more of Application Specific
Integrated Circuits (ASICs), field-programmable gate arrays
(FPGAs), and general-purpose computing circuits, along with
corresponding memories and computer program instructions for
carrying out the described operations. The specifics of these
components are not shown for clarity and compactness of
description.
[0049] Depending on the form of the components, the "coupling"
between components may take different forms. For example, dedicated
circuitry can be coupled to each other by hardwiring or by
accessing a common register or memory location, for example.
Software "coupling" can occur by any number of ways to pass
information between software components (or between software and
hardware, if that is the case). The term "coupling" is meant to
include these examples and is not meant to be limited to a
hardwired permanent connection between two components. In addition,
there may be intervening elements. For example, when two elements
are described as being coupled to each other, this does not imply
that the elements are directly coupled to each other nor does it
preclude the use of other elements between the two.
[0050] In the claims, reference to an element in the singular is
not intended to mean "one and only one" unless explicitly stated,
but rather is meant to mean "one or more." In addition, it is not
necessary for a device or method to address every problem that is
solvable by different embodiments of the invention in order to be
encompassed by the claims.
* * * * *