Multi-drop optical communication

An optical communication system including an optical communication fiber and a plurality of modules. Each of the modules has an optical transceiver that is optically coupled to the optical communication fiber by a corresponding optical drop. And each of the transceivers is configured for transmitting and/or receiving one or more optical signals via the optical communication fiber. The optical signals represent a plurality of individual data streams formatted according to one or more different communication protocols. In this manner, optical communication is enabled among the modules via the optical communication fiber.

BACKGROUND

Optical communication involves propagating a light signal through an optical fiber. While a variety of encoding and/or modulating techniques may be employed, a simple optical signal may represent binary values by its on and off states. For example, the presence of light during a time interval may indicate a ‘1’ value and the absence of light during a time interval may indicate a ‘0’ value.

Conventional optical communication systems comprise optical fiber and nodes. A node receives an optical signal via an optical fiber, converts the optical signal to an electrical representation, and then processes the electrical signal in some manner in the electrical domain. The node then converts the electrical representation back to an optical signal and re-transmits the optical signal via the same or another optical fiber. For example, a node may act as a relay that restores the amplitude of the optical signal if it is undesirably attenuated when transmitted through an extended length of optical fiber. Alternatively, a node may provide some kind of signal processing functionality and/or bridging between disparate communications protocols, such as between an optical fiber-based telephony system and a copper wire-based telephony system.

Modern industrial process control systems rely on a highly architected suite of devices and systems for the proper transport of data from, for example, one plant location to another. In the electrical realm, this had led to including switches, hubs and routers. There is a need for a system that reduces the number of IT electrical switching equipment and allows for multiple devices to be easily connected and disconnected from the transport network's physical medium while remaining inherently secure. Preferably, the system should also allow for the transport of multiple protocols among all devices connected to the network.

SUMMARY

Briefly described, an optical communication system embodying aspects of the invention includes an optical communication fiber and a plurality of modules. Each of the modules has an optical transceiver that is optically coupled to the optical communication fiber by a corresponding drop. And each of the transceivers is configured for transmitting and/or receiving one or more optical signals via the optical communication fiber. The optical signals represent a plurality of individual data streams formatted according to one or more different communication protocols. In this manner, optical communication is enabled among the modules via the optical communication fiber.

An optical communication system according to another aspect comprises an optical communication fiber and three modules. The first module comprises a first optical transceiver, wherein the first module is optically coupled to the optical communication fiber by a first drop. The second module comprises a second optical transceiver, wherein the second module is optically coupled to the optical communication fiber by a second drop. The third module comprises a third optical transceiver, wherein the third module is optically coupled to the optical communication fiber by a third drop. Optical communication is enabled between the first module, the second module, and the third module via the optical communication fiber.

In an aspect, an optical communication system comprises a polymer optical fiber (POF) and two modules. The first module comprises a first optical transceiver, wherein the first module transmits a first optical signal having a first wavelength on the POF. The second module comprises a second optical transceiver, wherein the second module transmits a second optical signal having a second wavelength on the POF and wherein the second wavelength is different from the first wavelength.

A method of communication embodying aspects of the invention comprises transmitting a first optical signal having a first wavelength on a first POF, wherein the first wavelength is allocated for a first category of communication and transmitting a second optical signal having a second wavelength on the first POF, wherein the second wavelength is allocated for a second category of communication. The method further comprises separating the first optical signal from the second optical signal based on the first wavelength, and, after separating the first optical signal, receiving the first optical signal.

According to yet another aspect, an optical communication system comprises a polymer optical fiber, a plurality of optical transceivers, a plurality of optical drops, and a plurality of processors. Each of the optical transceivers is configured for transmitting and/or receiving one or more optical signals via the optical fiber. The optical signals represent a plurality of individual data streams each formatted according to a different communication protocol. The plurality of optical drops couple the optical transceivers to the optical fiber, each of the optical drops corresponding to one of the optical transceivers. And each of the processors is configured for executing a control application to control one or more process control devices in an industrial plant and corresponds to one of the optical transceivers. In this manner, optical communication is enabled among the process control devices via the optical fiber.

DETAILED DESCRIPTION

An optical communication system is taught herein. While the optical communication system is described below in the context of an industrial control system environment, it will be appreciated by one of ordinary skill in the art that the optical communication system may be used in a wide variety of other applications.

One embodiment of the present invention is an optical communication system that includes an optical communication fiber and a plurality of modules. A typical optical fiber is made from glass and has a transparent core surrounded by a cladding material. Total internal reflection keeps light within the core. According to aspects of the invention, the optical communication fiber is a polymer optical fiber (POF).

Referring now toFIG. 1, a “drop” architecture embodying aspects of the invention implements multiple communication path physical connections. InFIG. 1, an optical communication system100comprises a plurality of modules102that are optically coupled to an optical line104. Each module102in the illustrated embodiment comprises an optical transceiver106that couples the module102to the optical line104via a drop108. For example, a first module102acomprises a first optical transceiver106athat couples the first module102ato the optical line104by a first drop108a; a second module102bcomprises a second optical transceiver106bthat couples the second module102bto the optical line104by a second drop108b; and a third module102ccomprises a third optical transceiver106cthat couples the third module102cto the optical line104by a third drop108c. It is to be understood that system100could include any number of modules and drops implemented in this manner.

According to aspects of the invention, drop108couples each of the modules102to the optical fiber104. Once the first module102ais optically coupled to the optical fiber104, the corresponding optical transceiver106acan receive optical signals transmitted via the optical fiber. In addition, optical transceiver106acan introduce optical signals onto optical fiber104for transmission. Second and third modules, for example, may be configured in the same manner, where the drop associated with each module couples it to the same optical fiber. In this configuration, the system enables optical communication among the modules via the optical fiber.

In the embodiments described herein, the optical fiber104may include a cladding (seeFIG. 2) affixed to its exterior at a point opposed to the drop used to optically couple the module to the optical communication fiber. The cladding generally promotes better coupling between the first drop and the optical communication fiber, so that the light transmitted by the optical transceiver is optimally introduced onto the optical communication fiber. The cladding is, for example, a reflective material.

According to aspects of the invention, each module102can transmit different optical signals having different wavelengths as well as receive different optical signals having different wavelengths using, for example, wavelength division multiplexing (WDM). WDM can be used in a distributed control system to control multiple field devices, with each optical signal of a different wavelength providing information regarding a particular device or set of devices. In other words, WDM permits the available bandwidth to be subdivided into several channels. This is especially valuable in an industrial process control systems, which transports data from, for example, one plant location to another and from one device to another using a variety of different communication protocols.

In this embodiment, the optical transceivers106of the various modules102transmit optical signals having different wavelengths. And the different wavelengths may be allocated for different categories of communication according to WDM technologies. For example, a dispersive element such as a wavelength selective device, such as a filter, prism, diffraction grating, or the like can be used to separate optical signals having differing wavelengths.

Alternatively, a form of Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA) may be used to allow the optical fiber to simultaneously carry a plurality of signals.

The optical communication system embodying aspects of the invention provides at least a portion of the communications among process control equipment housed within an equipment cabinet. In an embodiment, different wavelengths of light are used to transmit and receive different categories of process control information. For example, safety information may be communicated using a first wavelength of light signal, security information may be communicated using a second wavelength of light signal, control command information may be communicated using a third wavelength of light signal, and so with other categories of information. An optical communication bus or optical line may be implemented having a plurality of optical fibers where different portions of the optical communication bus or optical line are segregated by optical filters. For example, a first segment of the optical communication bus promotes communication with six different light wavelengths including a sixth wavelength associated with security information and a second segment of the optical communication bus is coupled to the first segment by a filter that blocks propagation of the sixth wavelength of light from the first segment into the second segment.

Turning now toFIG. 1in greater detail, in an embodiment, the optical line104comprises a plurality of segments implemented by optical fibers that are coupled to each other. For example, the optical line104comprises a first optical line segment104aand a second optical line segment104boptically coupled to each other by a filter110. The optical line segments104a,104bmay be any optical fiber. In an embodiment, the segments104a,104bare glass optical fibers. In an alternative embodiment, the segments104a,104bare polymer optical fibers. In yet another embodiment, one of the segments104a,104bis a glass optical fiber and the other segment104a,104bis a POF. Light in the first segment104apropagates in one direction (i.e., downward with reference to the orientation illustrated inFIG. 1), passes through the filter110, and continues to propagate in the second segment104b.

In various control applications contemplated for some embodiments of the system100, the optical communication line104is of relatively limited length, for example less than 100 meters. In this application, polymer optical fibers may provide cost savings and ease of deployment with reference to glass optical fibers. The procedures for splicing polymer optical fibers entail less exacting alignment than splicing glass optical fibers. Additionally, polymer optical fibers tolerate tighter turns than glass optical fibers. While a single optical line104is illustrated inFIG. 1, it is understood that the system100may have a plurality of optical lines104. In an embodiment, one optical line104may be dedicated for one direction of communication, for example from a distributed control system (DCS) to field devices and a second optical line104may be dedicated for a second direction of communication, for example from field devices to the distributed control system.

The first optical transceiver106ain the illustrated embodiment ofFIG. 1transmits a first optical signal via the first drop108aonto the first segment104a. The first optical signal propagates (e.g., downward) in the first segment104a, passes through the filter110, and continues to propagate in the second segment104b. Part of the first optical signal may enter the third drop108cand be received by the third optical transceiver106c. It is understood that at least a portion of the first optical signal may also propagate in the other direction (e.g., upward) in the first segment104a. The third optical transceiver106cmay transmit a second optical signal via the third drop108conto the second segment104b. The second optical signal in this embodiment propagates (e.g., upward) in the second segment104b, passes through the filter110, and continues to propagate in the first segment104a. Part of the second optical signal enters the first drop108aand is received by the first optical transceiver106a. It is understood that at least a portion of the second optical signal may also propagate in the other direction (e.g., downward) in the second segment104b.

In a similar manner, the second optical transceiver106bis capable of transmitting a third optical signal via the second drop108bonto the first segment104a. The third optical signal in one embodiment propagates in both directions (i.e., upward and downward) in the first segment104a. Thus, the third optical signal may propagate upward in the first segment104a, and part of the third optical signal may enter the first drop108aand be received by the first optical transceiver106a. At the same time, the third optical signal may propagate downward in the first segment104a, pass through the filter110, and continue to propagate downward in the second segment104b. Part of the third optical signal may enter the third drop108cand be received by the third optical transceiver106c.

Communication on the optical line104preferably comprises optical signals at different wavelengths, for example using WDM techniques. In an embodiment, different categories of information may be transmitted and received on different wavelengths. For example, security type of information may be transmitted on the optical line104in a first optical signal having a first wavelength, safety information may be transmitted on the optical line104in a second optical signal having a second wavelength, control command information may be transmitted on the optical line104in a third optical signal having a third wavelength, and sensor data may be transmitted on the optical line104in a fourth optical signal having a fourth wavelength. It is to be understood that other categories of information may be communicated over the optical line104. Additionally, the example categories of information may be further subdivided, and communication associated to each subdivided information category may be allocated to individual wavelengths. For example, different priorities may be associated with control command information, and a first optical wavelength may be allocated for communicating priority1control commands, a second optical wavelength may be allocated for communicating priority2control commands, and a third optical wavelength may be allocated for communicating priority3control commands. Any number of different optical wavelengths may be used for communicating over the optical line104.

Some optical transceivers106and/or some drops108such as those shown inFIG. 1may not be configured to receive or transmit on some optical wavelengths that are used for communicating over the optical line104, for example in the case that a category of information associated with the subject optical wavelength is irrelevant to the subject module102. As a result, the drop108and/or the optical transceiver106may be a lower cost unit. Alternatively, the optical transceiver106and/or the module102may experience a reduced processing load by not receiving and/or processing unwanted optical signals.

In some cases it may be desirable to isolate a module102from a category of information, for example to isolate a module102from privileged security information. The filter110may block or attenuate propagation of an optical wavelength associated with security type information, and the subject module102may be coupled to the second segment104b. The filter110is used, for instance, to subdivide the optical line104to promote increased bandwidth. As an example, if the filter110blocks propagation of a first optical wavelength, some modules102coupled to the first segment104amay communicate using the first optical wavelength on the first segment104aat the same time that other modules102coupled to the second segment104bmay communicate using the same first optical wavelength on the second segment104b.

Referring further toFIG. 1, in an embodiment, one or more of the modules102further comprises a control application112that executes on a processor of the module102. For example, the first module102acomprises a first control application112a, the second module102bcomprises a second control application112b, and the third module102ccomprises a third control application112c. The control application112may execute instructions that promote monitoring and/or controlling process control devices in an industrial plant such as an oil refinery, a chemical processing plant, an electrical power generation plant, a brewery, a food processing plant, a glass manufacturing plant, or other industrial plant. The process control devices may include motors, valves, heaters, conveyors, agitators, and other devices. The control application112in this example executes, at least partly, as an iterative, repeating control loop. The control applications112associated with different modules102may execute different sets of instructions and may be associated with different levels of responsibility. For example, the third control application112cmay execute instructions to control a turbine speed while the first control application112amay execute instructions to control a plurality of modules102, such as the third module102c.

One or more of the modules102may be located in an industrial environment that experiences elevated temperatures, for example as a result of proximate industrial processes. As such, some of the components of the modules102or of the drops108are preferably selected for use in such elevated temperature environments. Additionally, portions of the system100may be used in hazardous environments where the non-sparking property of optical communications and the optical line104may provide safety benefits.

Turning now toFIG. 2, a drop150embodying aspects of the invention is described. The drops are preferably butted up against the optical fiber in a non-intrusive fashion such that light received by the drop is light that is normally radiated from the optical fiber. The drops may be butted up against the optical fiber so as to maintain an acute angle between the axis of the drop and the axis of the optical fiber below a maximum angle associated with effective introduction of light onto the optical fiber. The drop150comprises a separator152and one or more optical detectors154. In an embodiment, the drop further comprises one or more emitters156. In some embodiments, the drop150may comprise each of the separator152, the optical detectors154, and the emitter156. In other embodiments, the drop150may comprise the separator152and one or more optical detectors154and may not comprise the emitter156. Alternatively, in another embodiment, the drop may comprise the emitter156and may not comprise either the separator152or the optical detectors154.

The separator152comprises, for example, a diffraction grating, a prism, or other optical component that is effective for separating light according to wavelength. In other words, drop150preferably includes a component for separating the WDM optical signal, which includes a plurality of wavelengths, into its individual wavelength components. As illustrated inFIG. 2, optical communication is carried over five different wavelengths on the optical line104, but it is to be understood that either fewer or greater number of wavelengths may be used to communication on the optical line104. Light that is propagating in the optical line104may radiate from the optical line104at any point along its entire length. This passively radiated light propagates to the separator152which directs different wavelengths of light at different angles with reference to the surface of the separator152.

Each of the optical detectors154in the illustrated embodiment is located at a position so as to detect one of the wavelengths of light. Preferably, the light detectors154are each aligned with the path of one of the separated wavelength signals. The output of the detectors154are electrical signals that may be processed by the optical transceiver106to receive the communication encoded on the subject optical wavelength. The detectors154may be implemented by photodetectors or the like. It is to be understood that either fewer or more detectors154may be present in the drop150depending on the number of wavelengths of light used for communicating on the optical line104. In an embodiment, there may be other passive optical components in the path between the optical line104and the separator152and/or between the separator152and the optical detectors154, for example a lens structure.

As described above, separator152in one embodiment butts up against the optical line104. It is to be understood that the separator152may be coupled to the optical line104by an appropriate interfacing material such as a light coupling gel or other material. Alternatively, the separator152may be coupled to a short stub of optical fiber, and the optical fiber may be coupled to the optical line104, for example the optical fiber stub may be fused or glued to the optical line104.

The emitter156ofFIG. 2emits light that is introduced into the optical line104in response to an electrical signal input provided to the emitter156from the optical transceiver106. The emitter156may be implemented by a laser diode, a photodiode, or another light emitting device. In an embodiment, emitter156emits light158in a beam that is somewhat directional. The emitter156may be butted up against the optical line104or it may be coupled to an optical fiber stub, and the optical fiber stub may be coupled to the optical line104, for example fused to the optical line104. The emitter156may be coupled to the optical line104by an appropriate interfacing material such as a light coupling gel or other material.

As shown inFIG. 2, a portion of cladding material160is positioned on the surface of the optical line104opposed to the location of the emitter156. The cladding material160is, for example, a reflective or mirror-like material. The cladding material160in one embodiment also has wavelength selective behavior, for example reflecting one or more specific wavelengths of light and absorbing other wavelengths of light. Alternatively, the cladding material160is an absorptive material such as a black colored material. Various cladding materials160have been observed to improve the coupling of the emitted light158into the optical line104.

Referring further toFIG. 2, each emitter156emits light primarily at a single wavelength or within a narrow band of frequencies, which may be referred to in some contexts as monochromatic light. Hence, for the optical transceiver106to transmit on a plurality of optical wavelengths, the drop150comprises a plurality of emitters156, each emitter156tuned to emit at the appropriate wavelength. The drop150may be conceptualized as an optical to electrical interface and/or transducer because it converts signals from the optical domain to the electrical domain and from the electrical domain to the optical domain.

In an embodiment, the drop150may be affected at least in part by a mechanical assembly (not shown) that clamps the drop to the optical fiber104at the desired angle. The mechanical assembly comprises in one embodiment a segment of cladding that engages the optical fiber opposed to the drop, where the segment of cladding increases the effectiveness of introduction of light from the drop into the optical fiber. In an alternative embodiment, drop150may be affected at least in part by a mechanical assembly that clamps the drop to the optical fiber150as a circuit module and/or circuit board is plugged into an equipment cabinet backplane, where the optical fiber may be a component of the backplane and may link a plurality of circuit boards plugged into the same equipment cabinet.

Turning now toFIG. 3, an angle made by the junction of the emitter156with the optical line104is described. Depending on the method of coupling the emitter156to the optical line104, a maximum coupling angle θ is expected to provide acceptable coupling of the emitted light158into the optical line104. In other words, acceptable coupling of the emitter156to the optical line104is achieved if the angle formed by the emitter156and the axis of the optical line104, i.e., the launch angle, is less than the maximum coupling angle θ that depends on the method of attaching the emitter156to the optical line104. In an embodiment, the maximum coupling angle θ is in the range of about 10° to about 15°. In combination with the present disclosure, one of ordinary skill in the art will be able to determine the maximum coupling angle θ without undue experimentation. In some contexts, the angle made between the emitter156and the axis of the optical line104may be referred to as an insertion angle.

It is to be understood that while the majority of the emitted light158propagates in the direction most aligned with the insertion angle, some of the emitted light158propagates in the opposite direction. These directions are illustrated inFIG. 3by the heavy arrow, aligned with the insertion angle, and the light arrow, directed oppositely to the heavy arrow. Also, the light propagating in the optical line104may reflect off an end or termination of the optical line104and propagate back in the reverse direction.

In an embodiment, a mechanical structure (not shown) is used to couple the drop150to the optical line104. The mechanical structure may comprise the separator152, the detectors154, the emitters156, and/or the cladding material160. Preferably, the mechanical structure promotes maintaining appropriate positions and angles of the several optical components of the drop150. In some contexts, this mechanical structure may be referred to as an optical clamp or as a vampire clamp.

Turning now toFIG. 4, a method200embodying aspects of the invention is described. As described above, optical communication fiber104preferably comprises a POF. At block202, a first optical signal having a first wavelength is transmitted on a first POF, wherein the first wavelength is allocated for a first category of communication. For example, a first optical signal, such as a control command, is transmitted on the optical line104by the first optical transceiver106a. At block204, a second optical signal having a second wavelength is transmitted on the first POF, wherein the second wavelength is allocated for a second category of communication. For example, a second optical signal, such as a safety-related message, is transmitted on the optical line104by the second optical transceiver106b. At block206, the first optical signal is separated from the second optical signal based on the first wavelength. At block208, after separating the first optical signal, the first optical signal is received. For example, the third drop108cseparates the first optical signal from the second optical signal, and the third optical transceiver106creceives the first optical signal and the second optical signal.

In an embodiment, the third control application112cprocesses the second optical signal at a higher priority than the first optical signal due to the communication category associated with the second optical signal versus the communication category associated with the first optical signal. For example, the third control application112cmay be configured to process and respond to a safety-related message before processing a control command message.

As described above, aspects of the invention provide a system for transporting multiple forms of data traffic along a POF. Several protocols, which are co-resident with each other on the POF, use different wavelengths for signal separation. Devices are readily added or removed from the POF through the use of a multi-drop signal injection/extraction technique. The active photonic sources and detectors are within the devices and are used in conjunction with optical devices and techniques for efficient data transport.

The following non-limiting examples are provided to further illustrate aspects of the present invention relating to distinguishing signals and protocols co-residing on the optical fiber104. For example, the optical field emanating from a photonic transmitter (e.g., laser, laser diode, light emitting diode, etc.) is described in Equation 1.
I(t)=I0(t)e[jωt+φ(t)]Eq. 1.where I0(t) is the time varying intensity modulation of the light source,ω is the radian frequency of the emitted light,t is time, andφ(t) is the phase of the emitted light.

In terms of wavelength, rather than the frequency notation of Equation 1, the field is as described in Equation 2.
I(t)=I0(t)e[j(2πλ/c)t+φ(t)]Eq.2.where λ is the wavelength of the emitted light, andc is the speed of light.

According to an embodiment of the invention, only intensity modulation, i.e., manipulation of the time-varying I0(t) term, is used. A description of the variety of optical intensity modulation schemes applicable for embodiments of the invention is shown inFIG. 5.

A traditional use of WDM fiber optic communications is for parallel data streams to co-reside within optical fiber104. Aspects of this invention are similar to traditional WDM in that multiple data streams, each with its unique wavelength, are within an optical fiber. Advantageously, however, aspects of the invention improve upon traditional WDM (and conventional industrial process communications) in that individual data streams carry different communication protocols. The isolation of the protocols eliminates interference and prioritization conflicts between information conveyed through different protocols.

Current Ethernet protocols, on the other hand, rely on point-to-point connectivity; both for copper-based media and glass-based optical media. Point-to-point connectivity requires the use of switches to allow nodes in the network to communicate. The use of switches increases both the cost and complexity of a system as well as reduces reliability. The use of polymer optical fiber for the transport media allows nodes to connect into the communication fiber through side mount connections without interruption to the communication fiber. The side mount connectivity supports a multi-drop environment and, thus, reduces the number of switches required by the system.

With respect to signal transmission, the multiple data streams, with potentially unique protocols, rely on intensity modulation of the optical transmitter, i.e., subcarrier intensity modulation. In an embodiment of the invention, amplitude modulated subcarrier intensity modulation is used. The analog amplitude modulation time-varying signal, I0(t), of Equation 2 becomes Equation 3 (written in cosine versus exponential notation).
I0(t)=Is(1+mcos ωmt)cos [(2πλsc/c)t]Eq.3.where m is the modulation extinction level (between 0 and 1),ωmis the mean radian frequency of the message, andλscis the optical carrier wavelength.

Modulation of the light source, such as the instance in which laser diodes (LDs) or light emitting diodes (LEDs) are used as described above, relies on a standard bias insertion circuit shown inFIG. 6for blocking DC (bias) from the RF/pulse signal source. Similarly, an inductor blocks the RF/pulse from the DC source. This allows the LD/LED to be biased into its higher optical output operating region. The bias current also acts as a form of protection for the data sequence. This is because applying AC (about 0V) directly to the LD/LED, would place the device into a reverse-bias operation on each half cycle. If the pulse source is designed for a 50Ω impedance, then the bias insertion circuit shown inFIG. 6can be augmented by the addition of 47Ω chip resistor in series with the LD.

FIG. 7illustrates multiple communication path physical connections in communication system100in which parallel data streams have individual wavelengths and protocols. Digital signals may be conveyed as an amplitude-shift keying (ASK) stream with the modulation extinction factor, m, in Equation 1, which is a time varying value m(t) that varies as the 1/0 data varies. In this manner, multiple intensity-based protocols (data streams) may be applied to sources with different light wavelengths, λ, or in radian frequency notation, ω, with the light injected into the POF. The result is a virtual set of parallel channels of data traffic within the POF as illustrated inFIG. 7.

FIG. 8illustrates a plurality of users (e.g., six) sharing a single channel via time division multiplexing. The individual messages (e.g., Comm 3/Protocol 3) may be multiplexed prior to modulation of the optical source. TDMA is particularly useful where there is a single frequency (or band) available for use. The multiple users each share this resource, with the conceptually simplest method being to take turns using the transmission channel. An illustration of TDMA is presented inFIG. 8where six users share a single frequency interval, ωm. And when any individual user has control of the channel, the actual transmission does not have to mimic that shown in this figure (although it does fit into the allocated temporal period). In an embodiment of the invention, a number of synchronization signals placed at the leading positions (header) of the message indicate to whom the message is bound, etc. This header information is followed by the actual data which is then followed by some form of error control checking (a cyclic redundancy checksum is illustrated in this figure).

Multiple users, six inFIG. 8, with differing data packet structures yields a message stream, I0(t), that may be highly complicated, yet easily used to modulate the output from a photonic source at a single wavelength, λn. By using this structure, aspects of the invention allow for parallel channels of multiplexed TDMA traffic such as Ethernet, Profibus, Modbus, and many other industrial protocols to be all co-resident on the optical fiber104.

FIG. 9AandFIG. 9Billustrate exemplary Ethernet and ModBus/TCP frame structures, respectively, embodying aspects of the invention. In an embodiment, an Ethernet frame and a ModBus/TCP frame may be used to modulate the light source. The data sequence modulates the light source with the resultant signal being coupled into the POF. TDMA requires each device on the network to have a fairly accurate clock synchronization for allowing the node to know when to access the channel. The TDMA data field typically includes node ID and start/stop bits. Examples of TDMA data frames for Ethernet and ModBus are presented inFIGS. 9A and 9B.

Frequency modulation intensity modulation of the light signal may also be achieved for distinguishing signals and protocols co-residing on the optical fiber104. The situation is described by Equation 4.
I0(t)=Iscos [(2πλsc/c)t+β sin ωmt)]]  Eq.4.where β is the modulation index (between 0 and 1).

Digital 1/0 information may be conveyed as frequency-shift keying (FSK) with set frequencies for 1's and 0's. In this regard, frequency division multiplexing permits the available bandwidth to be subdivided into several channels. FDMA data traffic may also be used to modulate the light source. For systems using FDMA, the available bandwidth that the optical carrier light source can provide is subdivided into a number of narrower band channels. Each user is allocated a unique frequency band in which to transmit and receive on.

With respect to cellular telephony, for example, during a call, no other user can use the same frequency band. Each user is allocated a forward link channel (from the base station to the mobile phone) and a reverse channel (back to the base station), each being a single way link. The transmitted signal on each of the channels is continuous allowing analog transmissions. The channel bandwidth used in most FDMA systems is typically low (e.g., 30 kHz for most telephony applications) as each channel only needs to support a single user. According to embodiments of the present invention, data signals are transmitted. The bandwidth of each user (source of data) is indeterminate with an approximate value of 200 MHz (100 Mbps) each. The limit is determined by the electrical modulation bandwidth of the photonic transmitter and detector. According to an embodiment of the invention, FDMA is used as the primary subdivision of large allocated frequency bands and is used as part of most multi-channel systems.

Digital modulation requires a wider bandwidth than FSK or ASK. For this reason, an adequate bandwidth transmitter is used in the embodiment described above. Return-to-zero, non-return-to-zero, pulse-position modulation, and other forms of digital modulation are variants on amplitude modulation and can be performed with the multi-drop optical communication system embodying aspects of the invention. Therefore data formats such as Ethernet, Profibus, ModBus, and other industrial bus protocols may be transported across the POF as isolated or multiplexed data streams. Each protocol is transmitted in its own data stream providing isolation from interference and supporting message prioritization.

With respect to signal detection,FIG. 10illustrates placing a photodetector in the “detector plane” to allow the multiplexed signals to be recovered. This embodiment achieves the multi-wavelength signal detection by placing a photodetector at the correction spatial (angular) position in the dispersed optical beam, as shown inFIG. 10. The spatial/angular position corresponds to a specific wavelength. A photodetector placed at that location converts the photons to an electrical signal and the message (see Equation 2) is retrieved.

WhileFIG. 10shows the color separation as the light emerges from the end of the fiber, the same demultiplexing principle is used if the light is emitted from the side of the fiber.

FIG. 11shows a serial-to-parallel converter with data encoding embodying aspects of the invention. An additional level of security may be used by taking the input data (serial) stream and converting it bit-by-bit, byte-by-byte, or other data frame lengths into parallel data. In the case of a bit-by-bit decomposition of the serial data stream, as exemplified inFIG. 11, each bit is converted to its own (radian) frequency ωnc. The series of frequencies are then combined into a single data stream as a combination of multiple frequency signals. This signal is then applied to the optical source, as shown inFIG. 7, with the output light coupled into the POF. This situation leads to numerous encryption possibilities, such as with the frequencies of each bit in the data frame being pseudo-randomized following, for example, a Fibonacci-based PN code, or with the frequencies of the data bits being non-sequential (shuffled). Multiple information streams may also be thought of as serial-to-parallel conversion followed by a combination of the data streams back into a “serial stream” in a frequency sense (not necessarily temporal bit sequence sense).

FIG. 12shows a parallel-to-serial converter with decoding embodying aspects of the invention. The situation is reversed at the receiver where the frequency-varying optical data stream is emitted from the POF and photodetected. The reverse steps of the frequency serial-to-parallel “encryption” steps are then undertaken with the associated parallel-to-serial converter at the receiver end of the system. An exemplary parallel-to-serial converter for use with embodiments of the present invention is shown inFIG. 12.

The optical signal, PT, is coupled into the POF and transmitted. At the receiving location, the optical signal is converted back to an electrical signal with the parallel-to-serial conversion process executed in an embodiment as shown inFIG. 12. The resultant data stream is shown on the right hand side ofFIG. 12.

Embodiments of the invention may be implemented with computer-executable instructions. The computer-executable instructions may be organized into one or more computer-executable components or modules. Aspects of the invention may be implemented with any number and organization of such components or modules. For example, aspects of the invention are not limited to the specific computer-executable instructions or the specific components or modules illustrated in the figures and described herein. Other embodiments of the invention may include different computer-executable instructions or components having more or less functionality than illustrated and described herein.