Free-space optical terminal

A device includes an optical fiber bundle having at least one optical data fiber and at least three optical tracking fibers, a mirror package configured to direct an incoming optical beam to the optical fiber bundle, at least three detectors, each detector corresponding to one of the at least three optical tracking fibers, the at least three detectors configured to receive portions of the incoming optical beam from the corresponding optical tracking fibers and convert the portions of the incoming beam to electrical tracking signals, and a controller configured to receive the electrical tracking signals from the at least three detectors and generate a feedback control based on the electrical tracking signals to control a position of the mirror package.

BACKGROUND

The disclosure relates generally to free-space optical (FSO) terminals, and more particularly, to a simplified FSO terminal architecture.

Conventionally, two monostatic FSO terminals in conjunction with two corresponding optical modems establish and utilize a data link to send and receive optical data signals. Due to a high directionality of the data link, very high precision beam steering is required. Thus, the two monostatic FSO terminals include and utilize some sort of beam steering element (e.g., a tip/tilt mirror) to actively point, send, and receive, therebetween, and the optical data signals generated by the corresponding optical modems. For closed loop tracking, portions of those optical data signals are used for position information to achieve an optical alignment for the data link, while data of these signals is parsed and processed. In this regard, the two monostatic FSO terminals utilize separate quadrant/position sensing detectors to track the incoming optical data signal and generate an error signal for controlling the beam steering element.

For example, a first optical modem generates and provides an outgoing optical data signal to an optical fiber of a first monostatic FSO device. The optical fiber directs the outgoing optical data signal to a first beam steering element of the first monostatic FSO device, which projects the outgoing optical data signal as a beam to a second monostatic FSO device. The second monostatic FSO device receives the beam, as an incoming optical data signal, through its aperture. In conventional monostatic FSO terminals, an additional, second beam steering element is needed in the second monostatic FSO device (in this example) to direct, through a passive beam splitter, a portion of the beam to a quadrant/position sensing detector in the second monostatic FSO device. The quadrant/position sensing detector provides position information to a controller of the second monostatic FSO device that adjusts the first beam steering element as needed to achieve an optical alignment between the first and second monostatic FSO terminals. A remainder of the beam, which includes the data, is received and passed by an optical fiber of the second monostatic FSO device to a second optical modem for processing. Note that, at the same time, the second monostatic FSO device is also sending an outgoing optical data signal that is received and processed by the first monostatic FSO device in a similar manner.

Optical alignment between the two monostatic FSO terminals is a key consideration that introduces significant complexity and cost to the design of these monostatic FSO terminals. In particular, any drift in a relative optical axis between the optical fibers and the quadrant/position sensing detectors can result in highly degraded acquisition and tracking of the optical data signals. Further, any significant misalignment before an initial acquisition of the optical alignment could prevent ever acquiring the data link between the monostatic FSO terminals.

Additionally, the quadrant/position sensing detectors include at least three (e.g., four) individual detectors and a common cathode. The common cathode is shared by and, in turn, sets a noise floor for the at least four individual detectors. Thus, the common cathode limits the noise floor to higher levels, which furthers limits the acquisition and tracking link margin for the monostatic FSO terminals.

Thus, there is a need for an improved FSO device/system that overcomes at least these deficiencies of conventional FSO terminals/systems.

BRIEF DESCRIPTION

According to one or more embodiments, a device includes an optical fiber bundle having at least one optical data fiber and at least three optical tracking fibers, a mirror package configured to direct an incoming optical beam to the optical fiber bundle, at least three detectors, each detector corresponding to one of the at least three optical tracking fibers, the at least three detectors configured to receive portions of the incoming optical beam from the corresponding optical tracking fibers and convert the portions of the incoming beam to electrical tracking signals, and a controller configured to receive the electrical tracking signals from the at least three detectors and generate a feedback control based on the electrical tracking signals to control a position of the mirror package.

According to one or more embodiments, a system includes a first network comprising a first optical terminal, a second network comprising a second optical terminal, and an optical data link established between the first and second optical terminals. The first optical terminal includes an optical fiber bundle having at least one optical data fiber and at least three optical tracking fibers, a mirror package configured to direct an incoming optical beam to the optical fiber bundle, at least three detectors, each detector corresponding to one of the at least three optical tracking fibers, the at least three detectors configured to receive portions of the incoming optical beam from the corresponding optical tracking fibers and convert the portions of the incoming optical beam to electrical tracking signals, and a controller configured to receive the electrical tracking signals from the at least three detectors and generate a feedback control based on the electrical tracking signals to control a position of the mirror package.

According to one or more embodiments, a method includes capturing, by a mirror package and an optical fiber bundle of a first optical terminal, an incoming beam from a second optical terminal, converting, by a plurality of detectors coupled to the optical fiber bundle, the incoming beam into a tracking signal, processing, by a controller of the optical terminal, the tracking signal to generate alignment information and determine, based on the alignment information, whether an alignment of the incoming beam is correct, and articulating, by the controller, the mirror package based on the alignment information, when the alignment of the incoming beam is not correct.

DETAILED DESCRIPTION

A simplified FSO terminal architecture is provided herein. This architecture at least utilizes an optical fiber bundle in contrast to conventional monostatic FSO terminals. The optical fiber bundle can include three or more tracking fibers to act as the quadrant/position sensing detector and include a data fiber to transmit and receive data.

Turning now toFIG. 1, a system100is depicted according to one or more embodiments. The system100can include at least two networks (e.g., a first network101and a second network102), with a plurality of devices105and106included respectively therein.

The networks101and102can be any type of network, for example, a local area network, a wide area network, a wireless network, and/or the Internet, including copper transmission cables, optical transmission fibers, wireless transmissions, routers, firewalls, switches, gateway computers, edge servers, and/or the like. The devices105and106can be any electronic or computing devices and components, such as desktops, laptops, servers, tablets, phones, digital assistants, e-readers, and the like.

The networks101and102reside with respect to stationary locations and/or mobile objects, such as a ship, a ground vehicle, an aircraft, a satellite, a building, a spaceship, a tower, a light house, a buoy, and the like, where a cost of running physical cables there between is prohibitive and/or impractical. As shown inFIG. 1, the networks101and102reside in separate locations or structures, such as structures107and108. In accordance with one or more embodiments, the structures107and108can be high-rise buildings in a location where a cost of running physical cables therebetween is prohibitive and/or impractical.

The networks101and102support communications respectively between the devices105and106. Further, the networks101and102are connected by at least two optical terminals110and112and corresponding optical modems120and122(e.g., the first network101includes a first optical terminal110, and the second network102includes a second optical terminal112). In this way, the networks101and102may be in any location so long as a line of sight (LOS)130is present between the two optical terminals110and112. In turn, the two optical terminals110and112can establish an optical data link, over-the-air across/along the LOS130(e.g., a free-space between the structures107and108), so that at least one device105of the network101can communicate with at least one device106of the network102, and vice versa.

The optical terminals110and112can be any free-space optical electronic, computer framework including and/or employing any number and combination of computing devices and components utilizing various communication technologies, as described herein. The optical terminals110and112can be easily scalable, extensible, and modular, with the ability to change to different services or reconfigure some features independently of others. The optical terminals110and112interface with the optical modems120and122via optical fibers A and B that capture incoming signals and/or that launch outgoing signals into the free-space between the structures107and108(e.g., over-the-air across/along the LOS130). The optical fibers A and B and the optical modems120and122can utilize single-mode communications for higher data rates, e.g., greater than 10 Gigabits per second (Gbps), or multimode communications for lower data rates (e.g., less than 1 Gbps), as the optical terminals110and112permit universal interfacing with different hardware and alternative configurations. Note that the optical terminals110and112leverage a reciprocity of the incoming/outgoing signals, in that the optical terminals110and112launch the outgoing signal out of a same aperture as the incoming signal is received and adjust a pointing of the outgoing signal based on an angle of the incoming signal.

FIG. 2depicts an optical terminal200according to one or more embodiments. The optical terminal200is an example of the simplified FSO terminal architecture described herein and/or the optical terminals110and112ofFIG. 1.

The optical terminal200captures or receives a beam201(e.g., an incoming optical beam201or an optical data signal201; note that, while an incoming portion of the beam201is described for ease of explanation, the illustrated beam201can be further representative of an incoming/outgoing optical beam(s) or data signal(s)). The beam201, for example, can be an emission of light through optical amplification via a stimulated emission of electromagnetic radiation, sometimes referred to as a laser beam or laser. In accordance with one or more embodiments, the beam201includes data sent by the opposite terminal.

The beam201is received through front-end optics, such as a lens202. The lens202(e.g., a telescope/lens assembly) can be any transmissive optical device (e.g., a single piece of transparent material or a compound unit including several pieces arranged along a common axis) that focuses or disperses the beam201by means of refraction. The lens202directs the beam201to a mirror package210, which further steers or directs the beam201to an optical fiber bundle220. For example, the lens202captures and the mirror package210steers the beam201onto the optical fiber bundle220.

The mirror package210includes at least an actuator211, a mount212, and a mirror213, e.g., a steering mirror213. Examples of the mirror package210include a single tip/tilt mirror configuration and a fast steering mirror (FSM) configuration. The actuator211can be any electric device that converts electrical energy into mechanical torque by using the electrical energy to articulate the steering mirror213based on control signals. Examples of the actuator211include an electric motor, a hydraulic cylinder, a piezoelectric element, a solenoid, etc. The mount212can be any gimbal or dynamic two-axis mount that permits movement by the actuator211of angles of the steering mirror213. The steering mirror213can be any reflective surface fixed to the mount212that directs the beam201to the optical fiber bundle220. The actuator211, in response to control signals, can move the steering mirror213(as supported by the mount212) to any desired position so as to direct the beam201to the optical fiber bundle220and achieve alignment,

The optical fiber bundle220is a dual use bundle (e.g., it is used for both data and position) that receives the beam201from the mirror package210. The optical fiber bundle220can include at least one optical data fiber (e.g., for a first use) and at least three optical tracking fibers (e.g., for a second use) to transmit and receive data. In this regard, the technical effect and benefit of the optical terminal200is to provide the optical fiber bundle220as a single optical element that combines the separate optical fiber and quadrant/position sensing detector elements of conventional monostatic FSO terminals.

In accordance with one or more embodiments, in an example configuration221, the optical fiber bundle220includes six optical tracking fibers222and one optical data fiber223. The six optical tracking fibers222can be arranged in a ring around the optical data fiber223. As shown inFIG. 2, for example, the optical fiber bundle220can include these seven fibers arranged in a packed hexagonal configuration, e.g., the six optical tracking fibers222form an outer hexagon around the optical data fiber223. The optical data fiber223can be a single mode fiber, a multimode fiber, or a double clad fiber (DCF; also referred to herein as a “double clad data fiber”). A DCF can be used as the data fiber to provide mode diversity and universality in interfacing with a variety of optical modems (e.g., the optical modems120and122ofFIG. 1).

For example, as shown inFIG. 3, an example configuration300of the optical fiber bundle200ofFIG. 2includes an optical data fiber310and six optical tracking fibers320. The six optical tracking fibers320can be arranged in a ring around the optical data fiber310and used to track a position of the beam201ofFIG. 2. In accordance with one or more embodiments, the optical data fiber310can also be used to generate tracking information as well. The optical data fiber310(e.g., a center data transmit/receive fiber) can be the DCF including a core322, a first cladding324, a second cladding326, and a coating328. The double clad data fiber supports both single mode and multimode operations simultaneously via the use of two claddings (e.g., the first and second claddings324and326). Thus, a technical effect and benefit of using the double clad data fiber in the example configuration300includes enabling the optical terminal200to be interchangeable with either single mode or multimode fibers (e.g., the optical fibers A and B and the optical modems120and122ofFIG. 1.

The six optical tracking fibers222are used to track a position of the beam201. In accordance with one or more embodiments, the six optical tracking fibers222are multimode to provide a widest acceptance angle, but alternative example embodiments are not limited thereto. In accordance with one or more embodiments, the optical data fiber223can also be used to generate tracking information as well. A technical effect and benefit of using the optical fiber bundle220includes eliminating a susceptibility to misalignment between the separate optical fiber and quadrant/position sensing detector elements of the conventional monostatic FSO terminals because a relative alignment of the seven fibers in the optical fiber bundle220is fixed.

Regarding tracking detection230(FIG. 2), the at least three optical tracking fibers (e.g., six optical tracking fibers222) of the optical fiber bundle220communicate the beam201or portions thereof to at least three detectors231. The at least three detectors231can be any transducer, sensor, photo-sensor, or photo-detector that converts light photons of the beam201into current or other electromagnetic radiation. Examples of the detectors231include, but are not limited to photodiodes and phototransistors. The at least three detectors231receive the beam201from the optical fiber bundle220, where each detector231corresponds to one of the at least three optical tracking fibers (e.g., six optical tracking fibers222) of the optical fiber bundle220. As shown inFIG. 2in the tracking detection230block, six detectors231can be employed in accordance with one or more embodiments. The at least three detectors231then converts the beam201to an electrical signal (e.g., the beam201is focused through the lens assembly202such that when it hits the optical fiber bundle220, light that is not captured by the optical data fiber223spills over onto the optical tracking fibers222and is converted to multiple electrical signals).

In accordance with one or more embodiments, the optical terminal200can optionally utilize wavelength filtering. In this regard, the optical terminal200can include one or more filters232, such as fiber based bandpass filters232, (e.g., optical filters) arranged in front of the detectors231so that only expected incoming wavelengths are tracked. In an example embodiment, the filters232are placed directly in front of corresponding detectors231. A technical effect and benefit of the fiber based bandpass filters232before the detectors231, in contrast to the free space bandpass filters required for conventional position detection using the quadrant/position sensing detector, includes being able to utilize fiber based switches to swap in filter banks to allow for rapid red/blue terminal reconfiguration.

Further, the optical data fiber223communicates the beam201or portions thereof to a data detection235block/portion. The data detection235block is representative of multimode receivers237and/or single mode receivers238that convert the optical data signals of the optical data fiber223to electrical signals.

Regarding the data detection235block, a data portion of the beam201can be directed to the multimode receiver237and/or the single mode receiver238through a coupler239. The multimode and single mode receivers237and238can be any device that receives information as light (e.g., an optical signal or the beam201). Note that single-mode refers to an optical signal designed to carry only one or single ray of light, while multimode refers to an optical signal able to transmit multiple modes or light rays simultaneously, each at a different reflection angle. The coupler239is further described with respect toFIG. 4.

FIG. 4depicts a double clad coupler400according to one or more embodiments. The double clad coupler400(an example of the coupler239ofFIG. 2) provides mode diversity (e.g., for the optical data fiber310ofFIG. 3), as it supports both multimode and single mode receivers237and238. The double clad coupler400connects to a data fiber to the terminal402, on which a signal404is conducted, via a port S406. From the port S406, the double clad coupler400divides into a double clad fiber407and a multimode fiber408. The double clad fiber407leads to a port A410, and the multimode fiber408leads to a port B412.

The signal404is both single mode and multimode signal. In this regard, the signal404is divided at a multimode inner cladding light collection414, so that a multimode signal416can proceed to a data fiber418connected to a multimode receiver (e.g.,237). Further, a single mode core signal420can proceed to an optical circulator422, where data fibers424and426are connected to and from a single mode receiver (e.g.,238) and a transmitter429. The optical circulator422is a fiber-optic component used to separate optical signals that travel in opposite directions. In this regard, the optical circulator422can be a three-port device that circulates between signals from the transmitter429and to the single mode receiver (e.g.,238). The transmitter429is any device that sends information as light (e.g., an optical signal or the beam201).

The technical effects and benefits of the double clad coupler400ofFIG. 4include permitting an implementation of a diversity scheme for mitigating deleterious effects (e.g., intensity fades and wave front aberrations introduced by atmospheric turbulence) by allowing the simultaneous use of both single mode and multimode receiver architectures. Intensity fades reduce an amount of link margin available and can introduce burst errors in the link when a power momentarily drops below a receiver sensitivity. Wave front aberrations decrease a coupling efficiency of a received signal into a single mode fiber. Thus, embodiments herein provide mitigation techniques, such as utilizing both single mode and multimode receiver architectures that monitor incoming data and ingest valid packets received on either path. In such a design, the double clad coupler400ofFIG. 4maximizes the benefits of both approaches, as the single mode receiver architecture (with its superior sensitivity) enables long range operation under low to moderate turbulence conditions and as the multimode receiver enables operation at moderate ranges (even under very strong turbulence conditions). Additionally, the double clad coupler400ofFIG. 4can add fade redundancy for when the single mode receiver experiences a deep fade because of poor coupling and not because of lack of power at an aperture.

Referring again toFIG. 2, the controller240can be any electrical or electronic circuitry (e.g., field-programmable gate arrays (FPGA) or programmable logic arrays (PLA)) that is configured to, e.g., is physically and/or electrically specifically arranged to, execute computer readable program instructions by utilizing state information therein to personalize the electronic circuitry. In accordance with one or more embodiments and as shown inFIG. 2, the controller240has a processor241, which can include one or more central processing units (CPUs), and be referred to as a processing circuit, microprocessor, and/or computing unit. The processor241is coupled via a system bus242to a system memory243and various other components. The system memory243, which is an example of a tangible storage medium readable executable by the processor241, can include read only memory (ROM) and random access memory (RAM). The system memory243stores software244and data245. The software244is stored as instructions for execution on the controller240by the processor241(to perform processes, such as the process flow500ofFIG. 5). The data245includes a set of values of qualitative or quantitative variables organized in various data structures to support and be used by operations of the software244. The controller240ofFIG. 2includes one or more interfaces247(e.g., one or more adapters, controller, network, or graphics adapters) that interconnect and support communications between the processor241, the system memory243, and other components of the optical terminal200(e.g., peripheral and external devices). Thus, as configured inFIG. 2, the operations of the software244and the data245(e.g., the controller240) are necessarily rooted in the computational ability of the processor241and to overcome and address the herein-described shortcomings of the conventional monostatic FSO terminals. In this regard, the software244and the data245improve computational operations of the processor241and/or the controller240by receiving the electrical tracking signals from the detectors231and performing a feedback control based on the electrical tracking signals to control the mirror package210.

Note that a consideration during a build process and for maintenance of the optical terminal200is that the optical alignment between the beam201going to the mirror package210and the optical fiber bundle220is monitored by the controller240. In accordance with one or more embodiments, the controller240implements a control loop to optimize alignments. In turn, the controller240of the optical terminal200can account for build and maintenance complexity, along with the effect of vibration or temperature variations. Further, any drift in a relative optical axis between the mirror package210and the optical fiber bundle220can be detected by the controller240to prevent degraded acquisition and tracking. Note that a technical effect and benefit of coupling the electrical tracking signals to the optical tracking fibers is that individual fiber coupled detectors with adaptive biasing circuits can be utilized for dramatically improved noise floors. For example, better than −90 decibel-milliwatts (dBm) has been demonstrated in the lab as compared to quadrant detectors that have about a −70 dBm noise floor due to the shared biasing scheme.

FIG. 5depicts a process flow500according to one or more embodiments. The process flow500is described with respect to the optical terminal200ofFIG. 2. The process flow500begins at block510, when incoming light (e.g., the beam201) is captured by the mirror package210and the optical fiber bundle220, which includes at least three tracking fibers and one optical data fiber, as described herein.

At block520, the at least three detectors231corresponding to the least three tracking fibers (e.g., six optical tracking fibers222) receive the incoming light (e.g., the beam201). At block525, the at least three detectors231convert the incoming light to tracking/positioning information. At block540, the controller240determines alignment information by processing the tracking/positioning information. At block545, the mirror package210steers the steering mirror213based on the alignment information (e.g., the actuator211articulate the steering mirror213based on control signals generated by the controller240in view of the alignment information).

At decision block560, the optical terminal200determines whether the alignment is correct. Determining whether the alignment is correct can include determining whether data received by the optical data fiber is readable or unreadable (e.g., lost, corrupted, or otherwise rendered useless or unusable). If the data is not readable, then the alignment is not correct. If the alignment is not correct, the process flow500returns to block510(as shown by the NO arrow). If the data is readable, then the alignment is correct. If the alignment is correct, the process flow500proceeds to block570(as shown by the YES arrow). At block570, the optical data fiber provides the incoming light (e.g., the beam201) to any corresponding detectors (e.g., the receivers237and238).

The Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments. In this regard, each block in the Figures may represent one or more components, units, modules, segments, or portions of instructions, which comprise one or more executable instructions for implementing the specified logical function(s). The functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the Figures, and combinations of blocks in the Figures, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.