Automatic laser power control in an optical communication system

A system and method for use with an optical communication beam of light is disclosed. The system allows the beam of light to operate at an adequate power level that provides a robust optical link while minimizing any safety risk to humans. Such a system includes multiple operating modes which control the power output of the beam of light. In the normal mode, the beam of light operates at a selected power level which provides a desired signal to noise ratio. Once a blocking occurs, the beam of light enters a power reduction mode to prevent harm to the blocking object. An acquisition and recovery mode is then employed to reestablish the blocked communication link.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for free-space optical communication networks and to a system and method for controlling the power of a laser used in such a network.

2. Description of the Related Technology

Currently, the primary method for data transmission between remote locations utilizes wired lines or fiber optic cables. Some of the costs associated with this method are due to the expense in obtaining rights-of-way for the cable runs as well as installing the cables by burying or hanging. While this method has proven successful where great distances separate two locations, it is prohibitively expensive between locations that are within close proximity to one another.

The dramatic growth in the demand for broadband services and the time and expense associated with deploying traditional wired lines or fiber optic cables have led to the development of new wireless broadband access technologies. One of these new wireless technologies employs a Light Amplification Stimulated Emission of Radiation (laser) beam to transmit information. Such a system may consist of at least 2 optical transceivers accurately aligned to each other with a clear line-of-sight to deliver the information using such a laser beam.

However, when the communication laser beams are present in a location accessible by people, laser safety becomes an important issue. Unlike light produced by a common lamp or the sun, laser light is not divergent and often emits radiation within a narrow band of wavelengths to form a monochromatic light. Furthermore, because this laser light is coherent and non-divergent, it is easily focused by the lens of a human eye to produce images on the retina with greater intensity than is possible with these other common sources of light.

Safety guidelines do exist for the use of lasers. For example, such guidelines are promulgated by the International Electrotechnical Commission (IEC) based on a maximum permissible exposure (MPE) level. If one were to apply such a standard, a maximum power level could be predicted (known as an Accessible Emission Limit (AEL)) that would make the communication laser beam eye-safe to a viewer, known as a class 1 laser system in the IEC standard. However, to establish and maintain a high-bandwidth connection, the lasers used in such systems may transmit at power levels that exceed the class 1-power levels designated by these laser safety guidelines.

Therefore, there is a need for a system and a method that allows the use of optical communication beams of light with adequate power to provide a robust optical link between communication terminals while minimizing safety risks to either users or a passerby. Such a system and method may maintain a signal-to-noise ratio above a desired value at the distant receiving communication terminal and under various environmental conditions that tend to degrade the signal, such as fog, smog, rain, or snow. Moreover, such a system and method could expand the permissible locations for placement of such optical transceivers to places that are accessible to humans.

SUMMARY OF THE EMBODIMENTS

The systems and methods have several features, no single one of which is solely responsible for its desirable attributes. Without limiting the scope as expressed by the claims which follow, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of the Preferred Embodiments” one will understand how the features of the system and methods provide several advantages over traditional communication systems.

One aspect is a method for controlling laser power in a communication system which includes a first node and a second node. The first node transmits a first beam to the second node and the second node transmits a second beam to the first node and the first and second beams maintain a safe exposure level to a blocking object. The method comprises maintaining power of a first beam transmitted by a first node to the second node at a first level when the power of the second beam transmitted by the second node and received by the first node is above a minimum value. The method further comprises reducing the power of the first beam to a second level when power from the second beam falls below the minimum value to limit an object's radiation exposure to a safe level when the object blocks the first beam. The power of the first beam is pulsed to limit the radiation exposure of the blocking object to the safe level. The method further comprises transmitting information during the pulsing of the first beam to reestablish communication with the second node. Finally, the power of the first beam is increased to the first level.

Another aspect is a system configured for controlling laser power in a communication system which includes a first node and a second node. The first node transmits a first beam to the second node and the second node transmits a second beam to the first node and the first and second beams maintain a safe exposure level to a blocking object. The system comprises a first node having a first transceiver configured to transmit a first beam at a first power level and configured to receive a second beam, a second node having a second transceiver configured to transmit the second beam at a second power level to the first transceiver and configured to receive the first beam transmitted by the first transceiver. The system further comprises a first control module configured to control the first transceiver to maintain a safe exposure level to a blocking object by changing the first power level of the first beam based on the power level of the received second beam. The system still further comprises a second control module configured to control the second transceiver to maintain the safe exposure to the blocking object by changing the second power level of the second beam based on the power level of the received first beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A free-space communication network may consist of at least two pairs of optical receivers and transmitters accurately aligned with each other with a clear line-of-sight to deliver high-bandwidth access over the air using beams of optical radiation, commonly called light. The light's wavelength is a function of a selected laser medium. Such laser mediums include, for example, solids, gases or liquids. The wavelengths form a continuous range but are often broken into specific regions, for example, infrared radiation (800 nanometer–3 millimeters), visible light (400 nm–700 nm), ultraviolet radiation (300 nm–3 nm), x-rays and gamma rays (<3 nm). In one embodiment, the optical receiver and transmitter are combined into an optical transceiver. Each optical transceiver can include at least one Light Amplification Stimulated Emission of Radiation (laser) and an optical detector. Embedded within the beams of radiation from the transmitter is information, for example, in the form of data, voice, and video. The corresponding receiver, which has an optical detector and associated signal processing circuit may convert the information into an electrical signal for further routing or processing.

FIG. 1is a diagram illustrating an exemplary communication network100. The communication network100includes a plurality of nodes108, interconnected by communication links110. Each communication link110includes two opposing beams of radiation between two nodes (i.e. incoming and outgoing beams). Certain of the communication links110may be radio links or microwave links under appropriate circumstances. According to one embodiment, the nodes108are disposed on facilities104. Although only one node108is provided per facility in the example illustrated inFIG. 1, more than one node108can be provided at one or more of facilities104, depending on the communication requirements, and also, perhaps, depending on the particular facility. Facilities104can be buildings, towers, or other structures, premises, or locations.

Nodes108are interconnected with one another by optical communication links110. Nodes108include one or more optical transmitters and receivers to provide the communication links110among the plurality of nodes108. The transmitters and receivers at nodes108can be implemented using, for example, lasers or light emitting diodes (LEDs) as the optical transmitters and charge-coupled devices (CCDs), photomultiplier tubes (PMTs), photodiode detectors (PDDs) or other photodetectors as the receivers. Although the network100illustrated inFIG. 1is illustrated as a mesh network structure, other network structures or geometries can be implemented. For example, in one embodiment, a branching tree architecture is used. In one embodiment, the nodes108include the capability to interface with up to four separate communication links110.

Still referring toFIG. 1, network100provides a two-way connection between one or more users in one or more facilities104and with a provider network116via a root node114. The root node114connects with the provider network116via another communication link112. In one embodiment, the provider network116is a high bandwidth copper or fiber service provider. Although only one provider network116is illustrated inFIG. 1, one or more root nodes114can be used to interface to more than one provider network116.

FIG. 2is a diagram illustrating an example implementation of a node108which is generally cylindrical in shape and can include four node heads200and a node base202. Node heads200each include a transceiver (not shown) to facilitate communication with one or more other nodes108in a network100(seeFIG. 1). Each node head200provides a two-way communication link110with one other node head in the network100at a given time. Thus, where each node head200has a single transceiver, node108communicates with up to four other nodes108at four separate locations. Alternatively, two node heads can provide parallel links to a single node. Other numbers of node heads200can be included, depending on the fan-out capability desired for the node108. Node108further includes a drop204for connecting to a user. In one embodiment, the drop is hardwired between the node base202and into a facility104(seeFIG. 1).

Node base202includes electronics and mechanics to provide a communication interface between, for example, a provider network116and the one or more node heads200via a communication link112(seeFIG. 1). A communications interface to perform protocol or format conversions can be included in the node base202as well as mechanics to drive the pointing of one or more node heads200.

One embodiment of the communication network100uses an optical transmission and multiplexing scheme for transferring data between the nodes108and the provider network112. Such schemes use a physical layer technology to handle the actual transmission and reception of data. In one embodiment, synchronous optical network (SONET) is used which the American National Standards Institute standardizes. In another embodiment, synchronous digital hierarchy (SDH) is used which the International Telecommunications Union standardizes. The basic SONET channel transmits 52 Mbps or OC-1. Higher transfer rates are obtained with the use of multiplexing. For example, a transfer rate of 155 Mbps, or OC-3, is achieved where three OC-1 channels are byte-interleaved.

FIG. 3is a block diagram illustrating a blocked communication link between two node heads200(a),200(b) of two nodes108(a),108(b). Node108(a) includes a node base202(a) coupled to at least one node head200(a) via communication electronics300. Node108(b) includes a node base202(b) coupled to at least one node200(b) via communication electronics300. Communication electronics300interface each node head200(a),200(b) to node base202(a),202(b). In one embodiment, the communication electronics300includes a bus which connects the node heads200(a),200(b) to their respective node bases202(a),200(b). In embodiments where each node108(a),108(b) includes multiple node heads, a multiplexer can be provided as part of the communication electronics300to allow communications among the various elements over a shared bus.

Each node head200can include a pointing mechanism such that it can be rotated to point to a designated other node108. Such pointing can be performed in both azimuth and elevation. Ideally, each node head200can be independently pointed to a designated node108.

Node head200(a) includes a transmitter304(a) and a receiver306(a), thereby providing two-way communications. However, in alternate embodiments, the node head200(a) has only the transmitter304(a) or the receiver306(a), thereby providing one-way communication. In another embodiment, the transmitter304(a) and the receiver306(a) are combined into a transceiver308(a). Additionally, it is possible that node head200(a) include more than one transceiver, or an additional receiver or transmitter to provide additional capabilities. Node head200(b) includes a transmitter304(b) and a receiver306(b), thereby providing two-way communications. In one embodiment, the transmitter304(b) and the receiver306(b) are combined into a transceiver308(b).

Node base202(a) includes a control module310(a). Similarly, node base202(b) includes a control module310(b). Each control module310(a),310(b) receives signals from the receiver306(a),306(b) and controls the operation of its respective transmitter304(a),304(b) based on the received signal. More specifically, the control module310(a),310(b) interrupts or reestablishes the transmission of the transmitter304(a),304(b). Thus, each control module310(a),310(b) controls its portion of the communication link with another node. The communication link is illustrated inFIG. 3as including two communication beams110(a),110(b).

The term “module,” as used herein, means, but is not limited to, a software or hardware component, such as a FPGA or ASIC, which performs certain tasks. A module may advantageously be configured to reside on the addressable storage medium and configured to execute on one or more processors. Thus, a module may include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. The functionality provided for in the components and modules may be combined into fewer components and modules or further separated into additional components and modules. Additionally, the components and modules may advantageously be implemented to execute on one or more computers.

In operation, data that is transferred from node108(a) to node108(b) is modulated onto the communication beam110(a) emitted by the transmitter304(a). Receiver306(b) processes the received modulated signal in the communication beam10(a) such that it can be repeated or forwarded to another node108in the network100. Alternatively, the processed signal can be passed either to an end user at a facility104or to a provider network116(seeFIG. 1).

As mentioned above, the transmitter304(b) can be interrupted due to an object312being present in the optical communication beam110(b). The object may be any opaque matter that sufficiently attenuates the transmitted signal to a level such that the associated data is not detectable by the receiver306(a). In one embodiment, the object reduces the power level of the communication beam110(b) which is detected by the receiver306(a). For example, a bird, a baseball, smog, fog, or an airplane could block the beam of radiation. In one embodiment, the lower bound signal-to-noise ratio that defines the block is selected based on the error rate associated with the received data. In another embodiment, the block is defined based on the duration of the interruption.

FIG. 4illustrates three different operating modes at different times that may be implemented by the control module310(a),310(b) depending on the status of the communication beams110(a),110(b).FIG. 4depicts the average power of a communication beam over time. Referring toFIGS. 3 and 4, for example, when the communication beams110(a),110(b) are not blocked and are properly targeted, the control modules310(a),310(b) operate in a “normal operation” mode (Mode1). In Mode1, nodes108(a),108(b) modulate data on their respective communication beams110(a),110(b). The power levels of the communication beams110(a),110(b) are set to a high level to achieve desired signal-to-noise ratios at the respective receiver306(a),306(b), for example, 9.5 mW.

Assume, however, at a time Tb, the object312blocks one or both of the communication beams110(a),110(b) between the nodes108(a),108(b). For example, inFIG. 3, communication beam110(b) is blocked by object312. The power level of the communication beam110(b) received by the receiver306(a) suddenly drops. The control module310(a) responds to this event by beginning the power reduction mode (Mode2).

In the power reduction mode, the power level of the signal being transmitted by the transmitter304(a) is immediately reduced to a low level or zero after a short period T of delay. In one embodiment, period T is 800 msec. The duration of T can be selected such that the total energy of the radiation transmitted by the transmitter304(a) during period T is below a level that would present a safety hazard to humans. For example, if the transmitter304(a) was transmitting at an initial power level of 9.5 mW during Mode1, the maximum value of T is 0.85 seconds. The control module310(a) stops sending data on communication beam110(a). Instead, the data received by node108(a) that would have been sent to node108(b) can be re-routed to an alternate node108(not shown) via one of the other node heads.

In response to the drop in power by node108(a), the control module310(b) of node108(b) can operate in a similar manner. Alternatively, the unblocked beam110(a) can be left transmitting while a signal is sent, via a network management system (not shown), to alert node108(b) that beam110(b) is not being received. When the second beam is forced to fail, the control module310(b) reduces the power of the communication beam110(b) and stops sending data to node108(a). Hence, blocking of a single communication beam110(b) between two nodes108(a),108(b) results in an interruption and failure of the two-way communication. However, this response may have a delay since the node108(b) is responding to the actions of node108(a). By stopping the transmission of the unblocked beam110(a), an immediate signal, in the form of a lack of signal, is sent to the node transmitting the blocked beam thus minimizing the complexity of notifying the blocked node and the associated delay in such notification. The value of T is selected to account for this delay so that the radiation transmitted by the transmitter304(b) during T is also below a level that would present a safety hazard to humans.

Still referring toFIG. 4, once Mode2is executed and the output of the communication beam110(b) is reduced to a safe level or shut off, the control module310(b) begins an acquisition and recovery mode (Mode3). Mode3will continue until the communication beam110(b) is no longer blocked. As shown inFIG. 4, in one embodiment the control module310(b) operates the transmitter304(b) in a pulsed transmission mode by intermittently raising its power to a high level for a short pulse duration, Td, with a time interval of Tp. The power level during each pulse duration, Td, is sufficiently high so that the signal-to-noise ratio at receiver306(a) is acceptable for the purpose of reestablishing optical communication. In one embodiment, the power level in each pulse is the same as the power level during the normal operation mode (Mode1). In another embodiment, the pulsed power level is at a lower level. The communication beam110(b) is modulated during each pulse duration, Td, with acquisition data for establishing optical communication and is not modulated to carry data between pulses. The acquisition data may include, for example, a node ID, position, and orientation information. In another embodiment, the communication beam110(b) sends out other data along with the acquisition data during the pulse duration. In still another embodiment, the control module310(b) alternates between the acquisition data and other data between each pulse duration. The pulse duration Tdand the period Tpare selected so that the total radiation is below a level that would present an unacceptable hazard to humans. Thus, during mode3, the object312is not exposed to a radiation level that would present a hazard to humans.

FIG. 5is a graph of the power levels of an interrupted communication beam over time.FIG. 5depicts an embodiment where Mode3includes at least two different power levels, Tdand Td2. Using different power levels can improve reestablishing optimal communication between nodes108(a),108(b) even during adverse weather conditions. For example, on a clear day when visibility is good and the communication beam110(b) is not blocked, the transmitter304(b) operates at a high power level, Td1. However, such a high power level may saturate receiver306(a). To prevent this, the transmitter304(b) transmits at a lower power level during Td2so that the receiver306(a) will properly detect the communication beam110(b) and be able to extract the transmitted data. Conversely, the communication beam110(b) transmitted at the low power level, Td2, may be too weak on a foggy day to achieve a desired signal-to-noise ratio at the receiver306(a). By transmitting at the high power level during Td1the receiver306(a) will properly detect the communication beam110(b) and be able to extract the transmitted data. Thus, this pulse structure allows two communicating nodes108(a),108(b) to reestablish optical communication at local environmental and weather conditions throughout the year.

Still referring toFIG. 5, in one embodiment, the pulse durations Td1and Td2are of equal duration and last for Td/2. In another embodiment, both the high and low power levels, Td1and Td2, are sufficiently high for communicating data to node108(a). In still another embodiment, Td1and Td2are modulated to carry the same data. In this embodiment, the data on the first half of the pulse, Td1, is at one power level (e.g., the high level) while the same data is replicated on the second half of the pulse, Td2, at a different power level (e.g., the low level). This dual-level pulse technique may also be used to accommodate communication links within the network100architecture that have different node108distances. The pulse durations Td1and Td2and the period Tpcan be selected so that the total radiation exposure is below a level that would present an unacceptable hazard to humans.

The acquisition and recovery mode (Mode3) is completed when both nodes108(a),108(b) reestablish optical communication. In one embodiment, node108(b) sends a “ping” to node108(a) and expects an “echo” back. If node108(a) returns this “echo” through communication beam110(a), node108(b) knows it has made a connection and that both communication beams310(a),310(b) are not blocked. Alternatively, transmitter304(a) sends a “ping” to receiver306(b). If receiver306(b) receives the “ping,” control module310(b) sends an “echo” through transmitter304(b) back to node108(a).

At this point, the control modules310(a),310(b) of each node108(a),108(b) terminate Mode3and begin the normal operating mode (Mode1) as discussed above. As obvious to one skilled in the art, the control sequence is not limited by the order of the modes discussed above. For example, the modes disclosed could be repeated in various orders without disturbing the scope.

FIG. 6is a block diagram of a control module310(a) and/or310(b) coupled to its associated transmitter304and receiver306fromFIG. 3. The control module310includes a turret control module600, a processor602, and a switch604.

The transmitter304includes a power supply switch914, a driver circuit916, and a laser672. The power supply switch914drives power through laser672. In one embodiment, the power switch914is a field effect transistor (FET). The driver circuit916controls the output power and data modulation of the laser672and can be independently controlled. Hence, in an event of blocking by an object, the output power of the laser672is independently controlled from the power switch914and/or the driver circuit916.

The receiver306includes processing circuit elements921and an optical detector704. The beam of a communication link that is transmitted by the laser672is focused onto the optical detector704. In one embodiment, the optical detector704is a high-speed optical detector such as, for example, a PIN photodiode detector or avalanche photodiode detector (APD). The optical detector704is coupled to the processing circuit elements921. The processing circuit elements921generate two different output signals922and924from the input signal received from the optical detector704. The first signal922is the high-speed data extracted from the received beam of radiation and sent to the switch604.

In one embodiment, the switch604is an ATM switch. ATM switches are generally well known in the art. Generally speaking, the ATM switch detects an arriving cell, aligns boundaries of cells arriving on multiple input lines, inspects the virtual path identifiers to determine the routing for a cell, converts the serial stream into a word parallel format, and time multiplexes the words onto time slots on a shared bus. A routing controller provides routing translation instructions to routing tables or accepts arriving virtual path identifiers from line interfaces to provide the correct routing instruction. A plurality of routing elements can be provided for each output. The routing element inspects the routing instruction associated with each word appearing on the shared bus, and delivers to its corresponding output cue only those cell segments intended for that output. In the ATM embodiment, each output cue reassembles the arriving word into ATM cells and delivers each ATM cell to the corresponding output port in serial format.

The second signal924is a received signal strength indicator (RSSI) which indicates whether an incoming beam of radiation is blocked by an object. The RSSI signal924is forwarded to the turret control module600. In one embodiment, the RSSI signal924is in analog form.

One embodiment of the turret control module600includes a programmable logic device (PLD)934, a digital multiplexer931, a timer933, and a digital pot935. The PLD934provides local control intelligence for the turret control module600and includes a counter936. The RSSI signal924sent by the receiver306is received by the PLD934and an analog to digital (“A/D”) converter942. When the RSSI signal924indicates a blocking has occurred at time Tb(seeFIG. 4), the PLD934initiates Mode2operation after the delay time T to reduce or turn of the power to the laser672in the transmitter304. The delay time T in Mode1, as illustrated inFIG. 4, is controlled by a timing signal from the timer933. Thus, once the RSSI signal924is lost, the counter936within the PLD934begins counting down the time. Once the counter936counts to the end of the delay T, a signal934ais sent to turn off the laser672or reduce its power via the driver circuit916. The resulting power level of the laser672is selected to limit the exposure of the object to the beam of radiation. In one embodiment, the PLD934generates a second signal934bthat is coupled to the power switch914to turn off the laser672or reduce its power, providing a single level of redundancy.

Still referring toFIG. 6, the processor602includes the A/D converter942which also receives the RSSI signal924. The processor602controls the operations of the modules described above and is programmed with software (not shown) to perform the power control sequence illustrated inFIG. 4. The turret control module600interfaces with and receives commands from the processor602via the digital multiplexer931. In response to commands from the processor602, the digital multiplexer931generates control signals931a,931b,931c. Signal931ais sent to the PLD934to reset the counter936. The signal931ais toggled periodically, for example, every500msec or less, to continually reset the counter936within the PLD934. By continually resetting the counter936, the PLD signal934ais maintained at a value that keeps the laser672at a desired power level during the acquisition and recovery mode (Mode3). During Modes1and2, the signal931ais not generated. In one embodiment, the signal931ais left on during Modes1and2to allow continuous power to the laser672.

The second control signal generated by the digital multiplexer931is signal931b. Signal931bcontrols both the PLD934and the power switch914in the transmitter306. For example, if the processor602receives the RSSI signal924, via the A/D converter942, and determines that the beam of radiation is blocked by an object, signal931b is set to a value that either turns off the power switch914or controls the power switch914so that the power of the laser672is reduced to a safe level. The signal931bis also fed to the PLD934instructing the PLD934to set the value of the signal934ato turn off or reduce the power of the laser672via the driver circuit916. In another embodiment, the PLD934also sends signal934bto control the power switch914. Besides receiving the RSSI signal924, the processor602is also notified that a block has occurred through a “loss of data” signal951. The “loss of data” signal951is generated by the switch604when the high speed data signal922is lost.

The third control signal generated by the digital multiplexer931is signal931c. Signal931ccontrols the digital pot935. In response to signal931c, the digital pot935controls the modulation power level of the driver circuit916of the transmitter304.

Table A shows one example of the logic status of different signals in the control module310for the control sequence described above.

METHOD OF OPERATION

Operation of a communication network100in accordance with one embodiment is described below with reference toFIGS. 7 and 8along with reference toFIG. 3. For convenience of description, the following text describes the communication network100where a single communication beam110(b) has been blocked by an object312. However, the following method can be used when both communication beams110(a),110(b) between nodes108(a),108(b) are blocked.

The process begins at a start state1000. Next, at a state1002, an object312blocks the communication beam110(b). This may occur due to weather or an object, for example, a human or flying bird, entering the communication beam110(b). Continuing to a state1004, the control module310(a), through receiver306(a), detects a power drop in the communication beam110(b) from a transmitter304(b). Next, at a state1006, in response to the drop in power, the control module310(a) drops the power in a communication beam110(a) sent by a transmitter304(a) and stops sending data through transmitter304(a) to node108(b). Flow proceeds to state1008where the control module310(a) re-routes the data that was earmarked for receiver306(b) through an alternate node (not shown). Next, at a state1010, the control module310(b), through receiver306(b), detects a power drop in the communication beam110(a) from transmitter304(a). Flow continues to a state1012where, in response to the drop in power, the control module310(b) drops the transmission power of its communication beam110(b) being sent by the transmitter304(b) to node108(a). Next, at a state1014, the control module310(b) stops sending data through transmitter304(b) to receiver302(a). Flow moves to state1016where the control module310(b) re-routes the data that was earmarked for receiver306(a) through an alternate node (not shown).

The acquisition and recovery process (Mode3) performed by the free-space optical communication system100will now be described with reference toFIG. 8. For convenience of description, the following text describes a free-space optical communication system100where a single communication beam110(b) is recovered. However, the acquisition and recovery process can also be used when both communication beams110(a),110(b) need to be recovered.

The free-space optical communication system100begins at a start state1100. Next, at a state1102, a control module310(b) transmits the acquisition information during Td1through transmitter304(b). Flow proceeds to a decision state1104to determine if a receiver306(a) of node108(a) receives the transmission. In one embodiment, the control module310(b) sends a “ping” through transmitter304(b) along communication beam110(b) and expects an “echo” back. If the “echo” is received by receiver306(b) along communication beam110(a), the control module310(b) knows it has made a connection. The free-space optical communication system100then proceeds to an end state1112where the process terminates. Once Mode3terminates, Mode1is initiated. Referring again to decision state1104, if the receiver306(b) does not receive the “echo” transmission, the free-space optical communication system100continues to a state1106where transmitter304(b) transmits the acquisition information during Td2. Flow moves to decision state1108to determine if the receiver receiving node received the information during Td2. If the receiving node receives the transmission, the free-space optical communication system100continues to the end state1112. Referring again to decision state1108, if receiver304(a) does not receive the transmission, the free-space optical communication system100continues to a state1110where the acquisition and recovery process waits for the duration of Tp–Td1–Td2. Flow then proceeds to state1102as described above to repeat the transmissions.

While the above detailed description has shown, described, and pointed out novel features as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit. The scope is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.