WO2002056508A1

WO2002056508A1 - Passive optical links in wireless communications systems

Info

Publication number: WO2002056508A1
Application number: PCT/IB2002/000094
Authority: WO
Inventors: Pinchas Goldstein; Moshe Goldstein; Hillel Bar-Lev
Original assignee: Roqiya Networks Inc.
Priority date: 2001-01-16
Filing date: 2002-01-16
Publication date: 2002-07-18

Abstract

A system and method for passive optical links in wireless communication networks. Multiple transmitting and receiving optical nodes can be in concurrent communication with a single service unit which processes optical signals. The optical ports of these transmitting and receiving optical nodes are in communication to the service unit by optical pathways. Many of the components, particularly many of the active components, of prior art transmitting and receiving units are removed from conventional rooftop housings and placed in remote service units, usually within the building containing the optical nodes. The modulated light beam generated by a modulated light source in the service unit is brought to a transmitting optical port by an optical pathway; similarly, received light signals are transported from a receiving optical node to a detector in the service unit.

Description

PASSIVE OPTICAL LINKS

IN WIRELESS COMMUNICATIONS SYSTEMS

FIELD OF THE INVENTION

The present invention relates to a system of passive optical links in wireless communication systems and a method for producing such links.

BACKGROUND OF THE INVENTION

Present optical telecommunication systems use optical lasers, light emitting diodes (LEDs) or other modulated light sources to transfer information. They also use detectors, usually photodiodes, as receivers of this information which is transmitted at high data rates. The use of optical wavelengths maximizes bandwidth resulting in high transmission rates. Modern wired optical communication systems operate by feeding modulated light energy into an optical fiber and transmitting that energy to a receiver. The presence or absence of modulated light energy represents a data bit. Wireless optical communication systems are also available. These systems are also known as "free-space" or fiberless optical communications systems. They generally use lasers or LEDs to transmit focused modulated light energy through space (rather than through optical fibers) to a receiving detector. The transmission through space is described as an optical link. The modulated light source and receiver are aligned so that they are in direct line-of-sight. Wireless optical systems are often used for relatively short distances and as the link between a generally wired system and the customer's transmitter or receiver. For this reason, in the telecommunications industry wireless optical systems are often referred to as "last mile" systems.

Most free-space optical systems use a network architecture containing multiple nodes with each node comprised of at least one node head. Usually each node head can communicate bi-directionally and consists of both a transmitting unit and a receiving unit, together known as a transceiver. The transmitting unit contains a modulated light source, a projecting optical system and a transmitting electronics unit, the latter usually including, but not limited to, a modulator, an amplifier, a power supply and a driver. The receiving unit contains a detector, a focussing optical system and a detecting electronics unit, the latter usually including, but not limited to, an amplifier and a filter. Node heads can also be unidirectional consisting of either a receiver or a transmitter. The node heads are usually located on the roof of a building, tower or the like. When a bi-directional transceiver receives an incoming signal it can distribute it to different locations within the building housing the node or transmit it further to other nodes in the network. Data can be exchanged between the transceivers in a network point-to-point or point-to-multipoint fashion.

Reference is now made to Fig. 3 wherein a schematic illustration of a free-space optical link 5 in a PRIOR ART wireless communication network is shown. A network node A contains a transmitting unit 10 while a node B contains a receiving unit 20. At node A, a signal is received via connection 92 from an outside line 90 by transmitting electronics unit 16 of transmitting unit 10. In this Figure and the Figures below, the presence of electronic components, such as a router or de-multiplexer, between outside line 90 and transmitting unit 10, are assumed to be well within the understanding of a person skilled in the art and are therefore not shown. Similarly, components between outside line 90 and receiving unit 20, such as a router or multiplexer, are also not shown.

Transmitting electronics unit 16 drives a modulated light source 18, the latter usually a laser or a light emitting diode (LED). Typically electronics unit 16 can include a modulator, amplifier, power supply and driver, as well as other components. The modulated light source 18 converts an electrical signal into a modulated light beam carrying information which is projected by projecting optical system 19, usually a lens, to receiving unit 20 at node B.

At node B, a focussing optical system 21, also usually a lens, directs the impinging projected modulated light beam onto detector 24, the latter usually a photodiode. Detector 24 converts the modulated light beam into an electrical signal that is delivered to detecting electronics unit 26. Typically, detecting electronics unit

26 includes an amplifier and filter, among other items. Unit 26 distributes the detected signal to various locations in the building on which node B is mounted. Alternatively, if node B also contains a transmitting unit, as is often the case, detecting electronics unit 26 can deliver the signal to the transmitting unit where processing as described at node A is repeated.

The state of the art of wireless networks can be reviewed with reference to published international patent applications: WO 99/45665, WO 00/25454, WO 00/25433, WO 00/04660, and WO 00/25456.

Transceivers used in wireless optical communications systems are complex instruments. In addition to containing electro-optical systems, a modulated light source, typically a laser or light emitting diode (LED), and a detector, usually a photodiode, they usually contain other auxiliary optics and electronics. These components must be in close proximity to each other. Moreover, the critical active components, the light source and the detector, as well as their auxiliary electro-optical and electrical components are environmentally sensitive. Accordingly, they are often physically shielded from extremes in temperature and precipitation. Maintenance functions, such as changing a detector, is not straightforward. Simple maintenance operations must be followed by realignment of the system. Because there are usually a multiplicity of transmitting and receiving units at a single node, they are expensive and easily malfunction. SUMMARY OF THE PRESENT INVENTION

In what is written herein the following terms will be used with the following meanings:

Optical node - a location in a wireless communications system through which it is desired to transmit or receive optical energy.

Service unit- a unit generally located in a building distant from the optical nodes of the building. The unit contains at least one modulated light source, at least one detector and other auxiliary electronics. The light source(s), detector(s) and other electronics may include elements typically found in the transmitting and receiving units of the prior art.

Optical pathway - any preferred pathway between a service unit and the "front end" optics at which there is defined an optical port. Optical system - "front-end" optics typically including optical hardware components only, such as a suitable lens and any auxiliary optical elements, if such are used. The lens may be a projecting lens if the port transmits an optical signal or it may be a focussing lens if it receives an optical signal.

Optical port - the entry and/or exit location of optical energy passing through an optical system located at an optical node. A pair of optical ports will be in mutual line-of-sight communication across an optical link.

There is provided in accordance with the present invention, a system having a plurality of optical nodes for facilitating optical data communications thereamong, wherein the system includes: a plurality of optical ports through which optical signals may be conveyed, each located at an optical node; a service unit, located distant from substantially all the plurality of optical ports, for processing optical signals conveyed therethrough; and a plurality of optical pathways provided between the service unit and the plurality of optical ports, for conveying optical signals therebetween.

Additionally, in accordance with a preferred embodiment of the present invention the service unit contains at least one modulated light beam source and other auxiliary electronics.

In a preferred embodiment of the present invention the modulated light beam source is chosen from a laser or a light emitting diode.

In yet another preferred embodiment of the present invention the modulated light beam source is a single source, a one-dimensional array of sources, or a two-dimensional array of sources.

Further, in accordance with a preferred embodiment of the present invention the service unit contains at least one detector and other auxiliary electronics.

In yet another preferred embodiment of the invention the detector is chosen from a group consisting of a photodiode, a photomultiplier tube, an avalanche detector and a charged coupled device.

In a further embodiment of the present invention the detector can be a single detector, a one-dimensional array of detectors, or a two-dimensional array of detectors.

In accordance with yet another preferred embodiment of the present invention, the service unit contains at least one modulated light beam source and at least one detector and other auxiliary electronics.

Additionally, in a further embodiment of the present invention, the optical pathway includes an optical fiber.

Further, in a preferred embodiment of the present invention, the optical pathway is air.

In yet another preferred embodiment of the present invention, the optical port includes an optical system.

In a further preferred embodiment of the present invention, the optical port includes an optical system which includes a focussing lens.

Additionally, in another preferred embodiment of the present invention, the optical port includes an optical system which includes a projecting lens.

In another preferred embodiment of the present invention each optical port is located at a selected optical node.

There is further provided in accordance with the present invention a method of transmitting optical signals among a plurality of optical nodes, the optical nodes having respective first and second optical ports through which optical signals may be conveyed, the method including the steps of: generating, at a location remote from the first optical port, an optical signal for transmission from the first optical port to the second optical port; and transmitting the optical signal from the remote location to the first optical port, along an optical pathway.

There is further provided in accordance in accordance with the present invention a method of receiving optical signals among a plurality of optical nodes, the optical nodes having respective first and second optical ports through which optical signals may be conveyed, the method including the steps of: receiving at the second optical port a signal transmitted over an optical link through the first optical port; and transmitting the optical signal by an optical pathway to a detector located at a location remote from the optical ports.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: Fig. 1 is a schematic illustration of a building housing a "wireless" or

"free-space" communications network with multiple transmitting optical nodes and/or multiple optical nodes constructed according to the present invention;

Fig. 2 is a schematic illustration of a "wireless" or "free-space" communications network constructed according to prior art and a wireless (free-space) communication system constructed according to the present invention;

Fig. 3 is a schematic illustration of an optical link in a "wireless" or "free-space" communications system according to PRIOR ART;

Fig. 4A is a schematic illustration of an optical link in a "wireless" or "free-space" communications system constructed and operative in accordance with a preferred embodiment of the present invention;

Fig. 4B is a schematic illustration of an optical link in a "wireless" or "free-space" communications system constructed and operative in accordance with another preferred embodiment of the present invention; Fig. 4C is a schematic illustration of optical links in a "wireless" or

"free-space" communications system constructed and operative in accordance with yet another preferred embodiment of the present invention;

Fig. 5A is a schematic illustration of a series of repeating optical links in a "wireless" or "free-space" communications system constructed and operative in accordance with still another embodiment of the present invention;

Fig. 5B is a schematic illustration of a series of repeating optical links in a "wireless" or "free-space" communications system constructed and operative in accordance with in a building another preferred embodiment of the present invention;

Similar elements in the Figures are numbered with similar reference numerals. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to Fig. 1, where a building containing a multiplicity of transmitting optical nodes and receiving optical nodes is shown; the optical nodes are deposed according to the present invention. There is an optical port serving as the entry and/or exit location of optical energy tlirough an optical system located at these optical nodes. The nodes, optical ports and optical systems are in communication with a service unit 40 by an optical pathway, usually an optical fiber. Service unit 40 generally contains at least one modulated light source, at least one detector and their auxiliary electronics with the auxiliary electronics in communication with an outside line. The present invention is particularly suited for obtaining signals via a fiber build coming off a fiber Metro ring and distributing these signals to various optical nodes in the communications system for further transmission.

The present invention requires that all modulated light sources 48, detectors 54, and electronic units 46 associated with a plurality of optical nodes, be housed in one or more service units 40. In the building shown, service unit 40 is in the basement. For clarity, only a single light source 48, detector 54 and electronics unit 46, are shown in service unit 40 but more can, and usually will, be present. The service unit 40 is generally not exposed to the elements and permits easy environmental control. Service unit 40 affords the possibility of concurrent use of a optical pathways, typically optical fibers, 42 to communicate with a plurality of optical nodes at a single site. As described below, the exact position of a given transmitting optical node or receiving optical node in the site can be chosen to minimize the distance between the optical node and its associated optical node located at a second location, usually a second building in the vicinity of the first.

By centralizing some or all of the transmitting and receiving unit components found in prior art systems at a protected centralized location, a service unit 40, distant from the transmitting optical nodes and receiving optical nodes of the site, the present invention allows for the sharing of components, resulting in a reduction in their number. For example, several lasers at the same service unit can share a single laser power source or laser driver. Similarly, a single power surge protector can be used for all the components located at one service unit 40. Reference is now made to Fig. 2 where a free-space network is schematically shown. The illustration schematically shows the advantages of the present invention where electronics and other components are kept in a service unit(s) 40 within their respective buildings vis-a-vis the positioning of transceivers (white) solely on roofs as in prior art systems. The transmitting optical nodes and receiving optical nodes according to the present invention are shown in black and are connected to service units 40 by optical pathways, here optical fibers, shown as dashed lines within the buildings. Optical links in the present invention are shown as solid lines stretching between black optical nodes. The dotted lines which start and end at transceivers (white) represent prior art optical links. It can be seen that as a result of a central service unit 40, the positioning of the optical nodes in the present invention is more flexible than that of prior art. Distances between optical nodes in different buildings can be reduced and angles of incidence lowered. For clarity, Fig. 2 does not show a service unit 40 or optical fibers 42 in every building. While not evident from this Figure and Fig. 1, there can be an optical link between nodes in the same building. The present invention, with substantially all of the modulated light sources, detectors and auxiliary electronics in a centralized service unit 40 inside a building, allows for greater flexibility in forming optical links. The advantages it provides includes

(i) Multiple transmitting optical nodes and receiving optical nodes can be used to minimize distance between receiving optical nodes and transmitting optical nodes at different locations. This reduces the effects of environmental conditions like fog. Further it allows for minimizing the angle of incidence of radiation transmitted between optical nodes. A minimal angle of incidence between the units is desirable.

(ii) There are no heat-induced effects resulting from the roof which usually acts as a radiative heat source.

(iii) The effect of reflected sunlight is reduced. (iv) There is no need to purchase roof rights.

In what has been discussed herein above, there has been described a positioning of the optical nodes within a building, specifically at its windows, containing a service unit. Notwithstanding this, it is clear to one skilled in the art that positioning transmitting and receiving optical nodes, solely or partially, on the roof is also possible.

It should be noted that because of the proximate positioning of components in the service units described herein, these systems maintain their efficiency, have low noise, and retain their high bandwidths. Moreover, these systems reduce the need for system re-alignment after maintenance operations. In order to understand more fully the details of the invention, the operation of the individual optical links will be described herein below in conjunction with Figs.

4A-5B. This should be contrasted with PRIOR ART shown in Fig. 3 and described above.

Reference is now made to Fig. 4A, 4B and 4C where schematic illustrations of preferred embodiments of the present invention are shown. Fig. 4A shows optical nodes 41 and 51 of a free-space optical link 105A and service units 40 and 50. Service unit 40 includes at least one modulated light source 48 and a transmitting electronics unit 46, with transmitting electronics unit 46 proximate to modulated light source 48.

Service unit 40 can be located at any point of the building on which transmitting optical node 41 is mounted but it is always remote from transmitting optical node

41. Transmitting electronics unit 46 receives a signal from outside line 90. As in Fig. 3 and all subsequent figures, other electronics, such as routers and de-multiplexers, well known to those skilled in the art have been omitted. Transmitting electronics unit 46 drives modulated light source 48, again usually a laser or LED, the latter converting the electrical signal into a modulated light beam.

The beam is delivered to projecting optical system 44 at the optical port of optical node 41 via the tip 42' of optical pathway, here optical fiber 42. The tip 42' of fiber 42 (fiber tip light source) is proximate to and is properly positioned with respect to projecting optical system 44. Optical system 44, usually a projecting lens, projects the modulated light beam to focussing optical system 57 at the optical port of receiving optical node 51.

At receiving optical node 51 of free-space optical link 105 A, focussing optical system 57, typically a focussing lens, directs the impinging modulated light beam received from transmitting optical node 41 onto the tip 53' of an optical pathway, here optical fiber 53 (fiber tip detector). The latter delivers the modulated light beam to a distant detector 54. Detector 54, typically a photodiode, converts the modulated light beam to an electrical signal and delivers it to detecting electronics unit 58. Unit 58 and detector 54 are located proximate to each other in a service unit 50 in the building on which receiving optical node 51 is mounted. Service unit 50 is remote from receiving optical node 51.

As is readily apparent, the tip 42' of fiber 42 must be properly aligned with projecting optical system 44 in transmitting optical node 41 and focussing optical system 57 must be properly aligned with the tip 53' of receiving optical fiber 51 of receiving optical node 51. Similarly both optical systems 57 and 44 must be aligned with each other. Methods for such alignment are well known to persons skilled in the art. The use of a fiber tip light source with a fiber tip detector in proximity to their respective optical systems can be described as a passive optical-to-optical link. It should be noted that according to the present invention transmitting electronics unit 46 and modulated light source 48 are located in service unit 40 distant from the tip 42' of optical fiber 42 (fiber tip light source) from which a modulated light beam is transmitted to the next node. The tip 42' of fiber 42 of transmitting optical node 41 can be positioned on the roof or alternatively at a window on any floor of the structure while the service unit 40 can be in another part of the building, typically the basement. Moreover, while not shown several transmitting optical nodes and receiving optical nodes can be in communication with a single service unit 40. Because a multiplicity of optical pathways, here optical fibers, can originate from service units 40 or 50, fibers can be positioned at a multiplicity of windows throughout the building. In some embodiments the fibers may each be in communication with a different modulated light source or detector, while in others they may be in communication with a single detector or modulated light source. Since a multiplicity of windows in the building can be fitted with transmitting optical nodes and receiving optical nodes, it is possible to choose a transmitting optical node and a receiving optical node which minimizes inter-nodal distance and hence the effects of optical environmental factors like fog.

Similarly, because a building containing a single service unit can contain a multiplicity of transmitting optical nodes and receiving optical nodes, each at a different window, the transmitting optical node and receiving optical node in the window with the lowest angle of incidence vis-a-vis its corresponding connecting node can be chosen. This reduces sunlight reflected from the windows of the building supporting the second node. Sunlight is known to interfere with wireless optical transmission by saturating detectors.

While service unit 40 is shown containing only transmitting electronics unit 46 and modulated light source 48, other systems associated with modulated light source 48 can be placed in site 40. Similarly, while service unit 50 contains only detector 54 and detecting electronics unit 58, additional systems associated with detector 54 can be placed in service unit 50.

Turning to Fig. 4B, another embodiment of the present invention similar to the previous embodiment is shown. The transmitting optical node 41 and auxiliary service unit 40 are identical to that shown in Fig. 4A. The receiving node B however can be any type of prior art optical receiving unit such as receiving unit 20 shown Fig. 3. It is readily understandable that an alternative embodiment to the one shown in Fig. 4B could include a prior art transmitting unit 10 shown in Fig. 3 and a receiving optical node 51 as shown in Fig. 4A with its associated auxiliary service unit 50. The optical nodes and their auxiliary service units as shown in Fig. 4A-5B do not explicitly show bi-directionality. These nodes do not contain both a modulated light source and a detector and are therefore unidirectional. It should be readily understood that bi-directionality can be included by adding the necessary transmitting optical node or receiving optical node and their auxiliary service units, whichever is lacking.

Reference is now made to Fig. 4C where a bi-directional configuration is shown. Fig. 4C contains the same components and reflects the same optical communications system as Fig. 4A. Added to Fig. 4A are projecting optical system 144, focussing optical system 157, optical fibers 142 and 153, modulated light source 148, transmitting electronics unit 146, detector 154, and receiving electronics unit 158. These components operate exactly as their corresponding components in Fig. 4A but in the reverse direction. As is shown, service units 40 and 50 contain both modulated light beams, detectors and their auxiliary electronics units. In the Figure, the newly added components are numbered similarly to their corresponding components in Fig. 4A.

Reference is now made to Figs. 5A and 5B where schematic illustrations of embodiments of the present invention containing repeated optical links are shown. Figs. 5A and 5B show optical nodes A, B, C and D of a series of repeating free-space optical links 205. Transmitting optical node 41 and service unit 40 operate as described with the corresponding parts of Figs. 4A-4C. Modulated light source 48, again usually a laser or LED, generates a modulated light beam which is delivered to transmitting optical node 41 via optical pathway, here optical fiber 42. The tip 42' of fiber 42 (fiber tip light source) is brought proximate to and is properly positioned with respect to projecting optical system 44 at the optical port at transmitting optical node

41. Projecting optical system 44 projects the modulated light beam to detecting optical system 57 at receiving optical node 51 A. As in Fig. 2, transmitting electronics unit 46 and modulated light source 48 are located in a service unit 40 distant from transmitting optical node 41 from which the modulated light beam is delivered to receiving optical node 51 A.

At receiving optical node 51A of free-space optical link 205, focussing optical system 57 directs the modulated light beam into a tip 53' of optical pathway, here optical fiber 53 that transports the signal to service unit 50. Optical fiber 53 is then joined by an optical splicer 55 to optical fiber 52 through which the modulated beam continues on its path. The tip 52' of fiber 52 (fiber tip light source) is brought proximate to, and is properly positioned with respect to, projecting optical system 59. Projecting optical system 59 of transmitting optical node 5 IB projects the modulated light beam onto focussing optical system 67 at receiving optical node 61. Focussing optical system 67 directs the impinging modulated light beam onto the tip 69' of optical pathway, here optical fiber 69 in receiving optical node 61. Optical fiber 69 delivers the modulated light beam to service unit 60.

At service unit 60, the signal can be transmitted further by repeating the process of transmission and reception at other nodes in a manner similar to the transmission and reception described for optical nodes 51A and 5 IB. This can be repeated an almost indefinite number of times provided that the signal is periodically amplified. Optical amplifiers (not shown) to amplify the beam as it is driven from optical fiber to optical fiber can be attached to the fibers, usually at the service units. At service unit 60, the signal can be delivered to another optical fiber 62 through optical splicer 65 as in service unit 50 (Fig. 5A) or detected and processed by detector 64 and detecting electronics unit 68 of service unit 63 (Fig. 5B). The processing in Fig. 5B would then proceed as in Fig. 4A.

While in Figs. 3A and 3B the embodiments use optical splicers 55 and 65 to join the fibers in repeated optical links, in other embodiments optical fiber 53 can communicate directly between optical systems 57 and 59 without the need of optical fiber 52. This communication can be effected either by passing fiber 53 through service unit 50 and bringing it directly to projecting optical system 59 of transmitting optical node 5 IB. Alternatively, fiber 53 can be brought directly from focussing optical system 57 to projecting optical system 59 without passing through service unit

50.

Again it should be noted that at service unit 40 of Fig. 5 A and service units 40 and 63 in Fig. 5B, electronics units 46 and 68, modulated light source 48 and detector 64 are distant from the locations of modulated light beam reception and transmission i.e. the optical nodes. The main components of the service units are kept proximate to each other to ensure efficiency and protect against a bandwidth reduction in protected areas, usually inside the structure housing the respective optical nodes.

Generally, there will be some additional electro-optical device(s) appended directly to the optical fibers described in Figs. 4A-5B. It is readily understood by those skilled in the art that as described above, it would be particularly beneficial to attach optical amplifiers to the optical fibers to amplify the transmitted optical beam. The embodiments described herein in Figs. 4A-5B should be deemed to include additional embodiments where other electro-optical components, such as electro-optical switches, are employed.

In what has been described herein above in Figs. 3-5B, the components of electronics units 16, 26, 46, 56, and 58 have not been explicitly defined. They are readily apparent to those skilled in the art. Moreover detailed knowledge of these units is not required for understanding the present invention. When discussing optical pathways with respect to Figs. 4A-5B mention was made of optical fibers. It should be evident to one skilled in the art, that an optical pathway can be established through air, without the need for an optical waveguide, if there is a direct line-of-sight between a modulated light source and a detector. Furthermore, it should also be evident that both optical nodes of an optical link can be in the same location, i.e. building, if a proper optical pathway is present.

Outside line 90 and connector 92 of Figs. 4A-5B can be constructed to carry optical signals. The present invention is particularly well suited for use with an outside line that is a fiber Metro ring and the connector, a fiber build. In such cases, electronics units like 46 contain electro-optic elements to deliver the signal to modulated light source 48.

In what has been written hereinabove, whenever the word "detector" or "photodiode" has been used it should be understood that charged coupled devices (CCDs), avalanche photodetectors (APD) and photomultiplier tubes (PMTs) can be used as well. Similarly, in what has been discussed above "detector" can refer to an array of detectors, the array consisting of any of the aforesaid types of detectors with the array being a one or two-dimensional array.

In what has been referred to above as a modulated light source, the source can be a single modulated light source, a one- dimensional array or two-dimensional array of modulated light sources.

It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described herein above. Rather the scope of the invention is defined by the claims that follow:

1. An optical communications system having a plurality of optical nodes for facilitating optical data communications thereamong, wherein said system includes: a plurality of optical ports through which optical signals may be conveyed, each located at an optical node; a service unit, located distant from substantially all said plurality of optical ports, for processing optical signals conveyed therethrough; and a plurality of optical pathways provided between said service unit and said plurality of optical ports, for conveying optical signals therebetween.

2. An optical communications system according to claim 1 wherein the service unit contains at least one modulated light beam source and other auxiliary electronics.

3. An optical communications system according to claim 2 wherein the modulated light beam source is chosen from a laser or a light emitting diode.

4. An optical communications system according to claim 2 wherein the modulated light beam source is a single source, a one-dimensional array of sources, or a two-dimensional array of sources.

5. An optical communications system according to claim 1 wherein the service unit contains at least one detector and other auxiliary electronics. 6. An optical communications system according to claim 5 wherein the detector is chosen from a group consisting of a photodiode, a photomultiplier tube, an avalanche detector, and a charged coupled device.

7. An optical communications system according to claim 5 wherein the detector is a single detector, a one-dimensional array of detectors, or a two-dimensional array of detectors.

8. An optical communications system according to claim 1 wherein the service unit contains at least one modulated light beam source and at least one detector and other auxiliary electronics.

9. An optical communications system according to claim 1 wherein the optical pathway includes an optical fiber.

10. An optical communications system according to claim 1 wherein the optical pathway is air.

11- An optical communications system according to claim 1 wherein the optical port includes an optical system.

12. An optical communications system according to claim 7 wherein the optical system is a focussing lens.

13. An optical communications system according to claim 7 wherein the optical system is a projecting lens. 14. An optical communications system according to claim 1 wherein each optical port is located at a selected optical node.

15. A method of transmitting optical signals among a plurality of optical nodes, said optical nodes having respective first and second optical ports through which optical signals may be conveyed, said method including the steps of: generating, at a location remote from the first optical port, an optical signal for transmission from the first optical port to the second optical port; and transmitting the optical signal from the remote location to the first optical port, along an optical pathway.

16. A method of receiving optical signals among a plurality of optical nodes, said optical nodes having respective first and second optical ports through which optical signals may be conveyed, said method including the steps of: receiving at the second optical port a signal transmitted over an optical link through the first optical port; and transmitting the optical signal by an optical pathway to a detector located at a location remote from the optical ports.