GB2376825A

GB2376825A - Coherence multiplexed optical telephone network using optical power supply

Info

Publication number: GB2376825A
Application number: GB0114649A
Authority: GB
Inventors: Salah A Al-Chalabi
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-06-15
Filing date: 2001-06-15
Publication date: 2002-12-24
Also published as: ES2601235T3; GB0114649D0

Abstract

An optical telephone and communication system with interferometers at the transmitter and receiver using optical time-delay difference and differential frequency shift modulation and multiplexing techniques. The power to the system is provided from either a remote optical source through the optical network or a local optical source. The optical network provides one or two optical connections to the sets. Part of the optical signal from the network is converted to electrical energy stored in a rechargeable battery. The optical sources used must have a defined coherence function, wavelength and coherence length Lc. The transmitter converts the information to phase variation in one arm of the transmitter interferometer and the receiver recovers this signal by using a matched interferometer. The path imbalance between the two arms of either interferometer is greater than Lc. Each set is a assigned a particular time delay difference and or differential frequency shift for addressing.

Description

Optical Telephone and Communication System Description This invention relates to an optical telephone and communication system with interferometers at the transmitter and receiver using optical time-delay difference and differential frequency shift modulation and multiplexing techniques for communication and assigning addresses to the optical telephone and communication sets. The power used by the optical telephone and communication system is provided from either a remote optical source connected to the optical network or a local optical source. In the case of a remote optical source, the communication set can convert part of the received optical signal to electrical energy to be stored in a rechargeable battery.

Today's public telephone system provides electrical power to the telephone apparatus from the telephone exchange or local switching board to enable the apparatus to function without the need for an additional local power supply. This remote power provision makes it possible for the apparatus to be used in emergencies, such as fire or failure of the electricity supply from the electricity grid. These similar requirements are expected to be met by telephones and communications systems using optical communication systems deployed in the public or private networks. Most telephone systems use wires, mainly copper, to conduct the power to the telephone apparatus. However, it is desirable to supply the required power over the optical connection/s between the optical network and the optical telephone or communication sets to conduct voice and/or data communication over the optical network without the need for any wires. In recent years, the power levels of optical sources and amplifiers for optical fibre systems have increased to the level where it is possible to provide the required power over an optical connection to drive the opto-electronic and the generate ringing sound (the telephone bell). These optical power levels can also be chosen to be within the safe limit as specified by the recent international standards on safe optical power levels in optical communication channels.

To mininise energy losses, this invention uses interferometric modulation and addressing techniques for facilitating voice and data communications.

It might be necessary to identify some of the technical terms used in this application. The term"time delay difference"between the two arms of an interferometer refers to the difference in time between taken by each optical signal to travel through the arm of the interferometer. The"time delay difference"equals the difference in optical lengths of the two arms of the interferometer divided by the speed of light. The other terms that might need clarification is"differential frequency shift"which refers to the change in optical frequency of the optical signal in one arm of the interferometer with respect to the optical frequency of the signal travelling in the other arm of the interferometer. The"frequency shift"equals the rate of change of the phase of the optical carrier which can be caused by a change in the optical path length in time.

According to the present invention the communication system comprising of one or more optical telephone and/or communication sets, one or more optical sources with known coherence function and coherence length Lc and wvelength, and an optical network which connects one or more optical telephones and communications sets with either one or two optical connections. The optical signal provided over the optical connection is used partly to communicate voice and data between one or more optical telephone and communication sets with interferometers in their transmitter and receiver modules, and partly to supply power to the optical telephone and communication sets. Part of the optical power supplied by the network is converted to electrical current by a photocell and is partly stored in a re-chargeable battery and another part directly supplies current to the electronic system in the optical telephone and communication set. The transmitter uses either the optical signal provided by the network or a local optical source with a known coherence function and a coherence length less than Lc. Each optical telephone and/or communication set is identified on the network by assigning the interferometers in the transmitter and/or receiver a specific time delay difference and a specific differential frequency shift between the two arms of the relevant interferometer. The receiver module of each set extracts the voice and communication channel and generates the necessary signals for the satisfactory operation of the telephone set and communication system, such as ringing signal and voice signal.

The optical sources used to provide the power and optical signal to the network should produce as much power as possible and have a well defined wavelength and coherence functions. The sources used for communication and providing power should have a well defined coherence function with a coherence length less than a specified value, Lc say.

To ensure that the coherence function of the source is well behaved and independent of the reflections from the optical network, optical isolators and polarisation controllers might be required to reduce the effects of such reflections. These types of sources can be combined with optical sources of narrow linewidth and specified wavelength to provide power to the optical communication set. The latter sources are used only to provide power at a specific wavelength and are filtered by the transmitter and/or receiver optical system to be converted to electrical power.

The optical telephone and communication set consists of a transmitter module and a receiver module. The transmitter module can use either the optical signal generated by the remote optical source with coherence length less than Lc, or an optical signal generated by a local optical source with similar coherence length. The transmitter module

which uses the remote optical source as a power and signal source consists of : an optical power splitter to divide the incoming optical signal from the remote optical source into two parts. The first is fed to one or more photocells which convert it to electric power that can be used to supply the electric current to drive directly the electronics and to charge a rechargeable battery. The other part of the optical signal is fed to the transmitter optical system which has an interferometer with a time delay difference between the two arms of the interferometer greater than the coherence correlation time of the optical source. This requirement is equivalent to having the difference between the optical path lengths of the two arms

of the interferometer to be greater than the coherence length Lc of the optical source. The time-delay difference of the two arms of the transmitter interferometer should match the time-delay difference between the two arms of the interferometer of the receiver/s to which the voice and data are to be communicated. The phase and/or frequency shift and/or time delay of one, or both arms of the interferometer are modulated by the voice transducer and/or the data transducer. when an optical power source of a specific wavelength is used only to provide power, an optical filter is added before the power splitter to extract the power at this wavelength and direct it the photocells. one or more photocells for converting part of the optical power to electrical power.

Part of the generated photocurrent is stored in a rechargeable battery, and part is used to drive directly the opto-electronic systems of the transmitter and receiver. a rechargeable battery for storing the electrical power generated by the photocells.

The battery is used to supply electric power to drive the systems and circuits of the optical telephone and communications set. a voice transducer to convert the acoustic signal generated by the user to a phase or frequency shift modulation of the optical signal in one arm of the transmitter interferometer. The voice transducer can be a diaphragm which is displaced by the acoustic signal and acts as the reflecting mirror of one arm of the interferometer.

The motion of the diaphragm due to the acoustic signal will cause phase modulation of the optical signal. This phase modulation can also be referred to as a Doppler shift in the optical frequency due to the motion of the reflector. The voice transducer can have many other implementations including those using guided optics. However all implementations perform the same function of converting the sound pressure to optical phase and/or frequency modulation. a data transducer which converts the information in the electric signal at its input to a modulation of the phase and/or frequency of the optical signal in one arm of the transmitter interferometer. A specified frequency shift of the optical signal in one arm of the interferometer with respect to the other arm can be used to assign each receiver a particular part of the spectrum of the differential frequency shifted signal.

In this scheme the difference in the time delays of the transmitter and the receiver interferometers must be equal, and each receiver with this particular time delay difference can be assigned a particular part of the spectrum of the differential frequency shifted signal. The data transducer can have many implementations including guided Lithium Niobate devices, electro-optic crystals, microelectromechanical systems (MEMS), vibrating mirrors. However, all implementations perform the same function of converting the sound pressure to optical phase and/or frequency modulation. the transmitter also includes an opto-electronic system to process the voice and data signals and generate suitable electrical signal to drive the voice and data transducers.

There are two types of transmitter modules covered by this invention. The first type of transmitter module uses the remote optical sources as the only optical sources for communications. However, the second type of transmitter modules use a local optical source for communication. The optical source should have a coherence length less than

Lc and well defined coherence function. The output power of the transmitter should be sufficient to give a good signal to noise ratio at the output of the receiver for adequate voice quality and data communication whether digital or analogue.

The optical telephone and communication set has also a receiver module which consists of : an interferometer with path difference greater than the coherence length Lc of the optical sources. The path imbalance of the receiver interferometer should match the path imbalance of the transmitter interferometer sending the voice and/or data. The preferred type of interferometers in the receiver are those with two outputs such as an off-set Michelson or a Mach-Zehnder, however a resonator type such as Fabry- Perot can be used but it is not recommended. The interferometer can also have other optical components such as polarisation controllers or optical dispersion compensators to maximise the optical signal. each the two outputs off the interferometer is collected and converted to electrical signal by a photodetector. The two photodetectors are connected in series to generate difference current proportional to the difference in intensities of the output of the interferometer. The difference between the two photocurrents, as well as each photocurrent, are generated and fed to a signal processor. the signal processor conditions the electrical signals and extracts the transmitted voice and data information. It also generates the required electrical signals to drive the acoustic ringer of the telephone (telephone bell), the electrical signal corresponding to the voice signal to drive the telephone's ear piece or speaker, and the electrical signal corresponding to the demodulated data. The voice channel will occupy the frequency spectrum which was allocated to it by the transmitter while the data channel will occupy the other frequencies of the demodulated signal. There are several ways to process the signal from the photodetectors. However the optimum way will be the one which generates a matching between the time delay and differential frequency shifts of the transmitter interferometer and the receiver interferometer to yield a maximum difference between the photocurrents of the two photodetectors. A sub-optimal method is to match the path imbalances of the receiver interferometer to the transmitter interferometer and then pass the difference current between the two photodetectors through a bank of electrical filters to recover the data information. The optimal and sub-optimal methods might require the signal processor to generate signals to drive feedback circuits for modulating the phase and/or difference frequency shift of the optical signals in either arm of the receiver interferometer.

- a feedback circuit from the output of the signal processor to drive the phase and/or frequency modulator might be used to extract the transmitted voice or/and data signals. The exact feedback signals and circuit will depend on the signal processing technique used to extract the voice and data information as well as the time delay, phase and differential frequency modulators and compensators.

In addition to the transmitter and receiver and optical sources, the communication system contains an optical network to connect one or more optical sources, and two or more optical telephone and/or data communication sets. The optical connections can be either

one or two optical connections providing one or two way communication channels. The network can be configured in a ring, star and/or mesh topology or a combination of these configurations. The network can be built from passive optical components; such optical fibres and couplers and active optical components such as optical amplifiers and switches. The network used in this invention should provide an optical path between two or more optical telephones and communications modules with sufficient optical bandwidth to enable the use of optical sources with a coherence length less than Lc.

Several optical telephone and communication systems can be connected to the optical network. Each system is assigned a specific time delay difference between the two arms of either its receiver or transmitter interferometer depending on whether it is used for one-to-one, many-to-one, one-to-many or many-to-many communication. In addition to the specific time delay difference, each system can also be assigned a specific differential frequency shift between the two arms of either its receiver interferometer or its transmitter interferometer. This specific time delay difference and differential frequency shift multiplexing techniques can be used to address one or more receivers which can receive information from one or more specific transmitters. In the case of one-to-one communication, the receiver interferometer of each set is assigned a unique time delay difference and differential frequency shift. The transmitter interferometer is adjusted to match that of the receiver to establish the communication channel. In the case of the oneto-many communication system, the transmitter interferometer of one set is assigned a unique time delay difference and differential frequency shift. The time delay difference and differential frequency shift of the receivers'interferometers of the other sets must be adjusted to match that of the transmitting interferometer to establish a one-to-many or broadcasting communication. In the case of many-to-many communication, a specific time delay difference is assigned to the transmitter interferometer of all members of the group and each set is assigned a differential frequency shift for transmission. The time delay difference of all interferometers in the receivers in the group are set to match the time delay difference assigned to all transmitting interferometers. The transmitting interferometers can then transmit at the same differential frequency shift. Alternatively, each transmitter is assigned a specific differential frequency shift to avoid interference, however, the receiver should be able to demodulate the differential frequency shifts of all transmitters.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which :- Figure I shows the general optical telephone and communication system together with an optical network Figure 2 illustrates the transmitter opto-electronic system with interferometer, photocells, a transducer for converting the voice pressure to optical time delay, phase and/or frequency shift modulation, and data transducer converting the electrical signal representing the data to optical time delay, phase and/or frequency shift modulation.

Figure 3 illustrates a transmitter module with a local optical source, interferometer and phase, frequency or amplitude modulation for data communications system Figure 4 illustrates the receiver for the optical telephone and data communication system for recovering voice and data information Figure 5 illustrates an optical network built using passive optical components; such as optical fibres and optical couplers, and active optical components such as optical amplifiers.

Referring to the drawing the optical telephone and communication system comprises of one or more an optical telephone and communication sets 1,2, 3 and 4 together with one or more optical sources 5A and 5B and an optical network 6. The optical network can include passive and active optical components such as optical power splitters and optical amplifiers. The optical sources and optical telephone and communication systems are connected to the optical network through the optical connections 7,7A, 7B and 7C. The connection type 7 is one connection for two communication, 7A connects the transmitter module in a two way two connection communication, 7B connects the receiver module in a two way two connection communication, and 7C connects the optical source to the network.

The optical telephone and communications sets 1,2, 3 and 4 can be one of two types.

The first type, such as 1,2 and 4, does not have an optical source and derives most if not all its power from the optical sources 5A and 5B and other optical sources connected to the network. The second type of optical telephone and communications sets, such as 3, contains an optical source, such as 3T3, and can derive some of its power from a local electrical power source. All optical sources must have a well specified optical power spectral density, or a coherence function, and those that are used for communication rather than just power with a coherence length less than a specified value Lc which should be as short as possible. The optical sources, such as 5A and 5B, should be powerful enough to provide enough optical energy to operate the telephone and communication sets 1,2 and 4 of type one. The main factors that limit the power levels of these sources are safety, and performance quality of the voice and communication channels. The optical power received by the optical telephone and communications and converted to photoelectric current should be sufficient to provide, either directly or through a re-chargeable battery, to drive the electronic, electro-mechanical and magnetomechnical systems and devices to affect voice and/or data communication system.

Each optical telephone and communication set consists of a transmitter module IT, 2T, 3T, 4T and a receiver module 1R, 2R, 3R and 4R. The transmitter module, such as 1 T, consists of an optical system part IT1, 2T1, 3T1, 4T1 with an interferometer of a specified path length imbalance greater than Lc, and an opto-electronic part 1 T2, 2T2, 3T2 and 4T2 with electronic components and transducers to convert the acoustic signal and data to modulation of the phase and/or frequency shift of the optical signal in one or both arms of the interferometer in the transmitter. Figure 2 shows a specific embodiment of the transmitter module of the first type which derives all its power from the optical

network through the optical connections 7 or 7A. Figure 2 shows a Michelson interferometer 9 where the optical power splitter 10 splits the optical signal coming from the network through connections 7 or 7A to two paths 13A and 13B. The optical path imbalance of the two arms 13A and 13B is greater than the coherence length Lc and matches the optical path imbalance of the two arms of the receiver interferometer to with which the communication is to established. The mirror 11 in the Michelson interferometer is static, while the mirror 12, which can be reflective diaphragm, is coupled to the transducer 14A which converts the acoustical signal and pressure to phase and/or frequency modulation of optical signal in arm 13B. The optical signal in each arm can be travelling in either free space or guided media such as optical fibre or integrated optical device. The data signal is imposed on the phase and/or frequency of the optical signal in arm 13A or 13 B by the transducer 14B. The reflected signals from the two arms of the interferometer are combined by the optical power splitter 10 with one part directed to the optical network 6 through the optical connection 7 or 7A. The other part of the signal 17 is directed to photocells 15 which convert the incident optical power to electrical power which ca be partly or completely stored in the re-chargeable battery 16 which also provides power to the receiver 18 of optical telephone and communications set. When an optical source of a specified wavelength is used to provide power only, an optical filter can be inserted at the input to the interferometer and guide the filtered optical signal to the potocels. The embodiment of the second type of the optical telephone and data communications set is shown in figure 3. This transmitter type contains an optical source with a known coherence function and with a coherence length less than Lc.

The output of the optical source 3T3 is fed to a Mach-Zehnder interferometer 19 with a path imbalance between the two arms of the interferometer 19A and 19B greater than the coherence length Lc of the source and equals the path imbalance (or time-delay difference) of the two arms of the receiver interferometer with which communication is to be established. The data information is imposed on the phase and/or frequency of the optical signal in one or both arms by the data transducer/modulator 20.

Figure 4 shows an embodiment of the receiver modules 1R, 2R, 3R and 3R of the optical telephone and communications sets 1,2, 3 and 4 together with the photocells 15 and the rechargeable battery 16. The optical signal is received from the optical network through connection 7 or 7B, and it consists of at least two signals with a time-delay difference equals the time delay difference between the two arms 21 A and 21B of the receiver interferometer 21. The optical outputs 22A and 22B of the interferometer are guided or focused on the photodetectors 23A and 23B which are connected in series. The difference between the two currents of the two photodetectors is fed through the connection 24 to a signal processor 26. The signal processor receives the difference current and a signal proportional to the current of photodetector 23B from the circuit 25 to extract the voice and data information. There are several processing techniques for recovering the information from the photocurrents. Some of those techniques require the signal processor to generate a signal fed to the frequency and/or phase modulator 28 through the connection 29A and a signal to the reflector 21C through the feedback system 27 and connection 29B of the receiver interferometer. These signals can be used to demodulate and recover the information from the phase and/or frequency difference between the two components of the input to interferometer with a time delay-difference equals the time-

delay difference between the two arms of the receiver interferometer. The signal processor also generates the electrical current to drive the telephone ringer 30, the voice signal to the ear piece or speaker 31 of the optical telephone and the recovered data to the data channel 32 of the optical communication set.

An embodiment of the optical network which provides two way two connections is shown in figure 5 which consists of optical fibres 33, optical couplers or power splitter 34, and optical amplifiers either unidirectional or bi-directional 35. The connection to the receiver module 7BN of the Nth optical telephone and communication, for example the receiver for set 1 is 7B 1, and the connection to the transmitter of the same Nth set, for example the transmitter of set 1 is 7 AI. Similarly the connection to the receiver module of the optical telephone and communication set 2 is 7B2 and the connection to the transmitter of the same set is 7A2 The assignment of addresses to identify the optical telephone and communication sets in this invention is based on assigning a particular time delay difference and differential frequency shift for the optical interferometers in the receiver and transmitter modules.

The assignment of a particular time-delay and/or differential frequency shift to the optical interferometers depends on the use of the communications system. The following communication schemes are covered by this invention: one-to-one, one-to-many or broadcasting, many-to-one, and many-to-many. In the case of a one-to-one communication system, the interferometer of each receiver set is assigned a specific timedelay difference and a specific differential frequency shift. The path imbalance of the transmitter interferometer is adjusted to match the path imbalance of the receiver interferometer with which the voice or/and communication channels are to be established.

The voice and data channels for that specific time delay difference can be allocated to one or more specified differential frequency shifts. In a data communication system, several sets can be assigned the same time delay difference, however, they have to be assigned different differential frequency shifts to facilitate one-to-one communication.

The second addressing schemes enables one-to-many communication, and in this case the transmitter interferometer is assigned a specific time-delay difference and a specific differential frequency shift. The listening receivers will then have to adjust their receiver interferometer to match that specific time-delay and differential frequency shift. The third assignment scheme is that of many-to-one communication. In this case the receiver interferometer is assigned a specific time delay difference and one or more differential frequency shift. The transmitting sets have to adjust their transmitter interferometer to match this time delay and differential frequency shift. The fourth assignment scheme is the many-to-many case where the receiver interferometer and the transmitter interferometer of all communication sets in the group are set to the same time delay difference and differential frequency shift. Alternatively the transmitter and receiver interferometers are set to the same time delay difference and each transmitter is assigned a specific differential frequency shift, the receivers however must be able to demodulate all the differential frequency shifts of the group.

In addition to the voice and data communication channels, several interferometers with a specified optical path imbalance can be connected to the network to sense environmental

parameters. These parameters, such as pressure, temperature, vibration, magnetic field, electric field can be detected if they cause a change in the time delay difference, phase and/or a differential shift in the optical sensing interferometer. The receiver used for such telemetry systems has the same structure as the one used for communication as described in this invention but with different signal processing techniques. For such systems, the signal processor has to estimate the time delay difference, phase and/or differential frequency shift of the sensing interferometer accurately and without ambiguity.

Claims

Claims 1. A communication system comprising of one or more optical telephone and/or communication sets, one or more optical sources with known coherence function and wavelength and coherence length Lc, and an optical network which connects one or more optical telephones and communications sets with either one or two optical connections and the optical sources. The optical signal provided over the optical connection is used partly to communicate voice and data between one or more optical telephone and communication sets which have interferometers in their transmitter and receiver modules, and partly to supply power to the optical telephone and communication sets. Part of the optical power supplied by the network is converted to electrical current by a photocell and is partly stored in a re-chargeable battery and partly used directly to supply current to the electronic system in the optical telephone and communication sets. The transmitter of the optical telephone and/or communications set uses either the optical signal provided by the network or a local optical source with a known coherence function and a coherence length less than Lc to carry the voice and/or the data information. Each optical telephone and/or communication set is identified on the network by assigning the interferometers in the transmitter and/or receiver a specific time delay difference and/or a specific differential frequency shift between the two arms of either receiver interferometer and/or the transmitter interferometer. The receiver module of each set extracts the voice and communication channel and generates the necessary signals for the satisfactory operation of the telephone set and communication system.
2. An optical telephone and/or communication systems as in Claim 1, wherein some or all of the power used by one or more of the telephone and/or communication sets in Claim 1 is derived from the optical source/s connected to the optical network and supplied through optical connections.
3. An optical telephone and/or communication systems as in Claim 1 and Claim 2, wherein part of the optical power to one or more of the optical telephone and/or communications sets is supplied on a specific optical wavelength.
4. An optical telephone and/or communication systems as in Claim 1, wherein one or more optical sources as in Claim 1, Claim 2 and Claim 3 are connected to the optical network to supply power and optical signal for the purpose of communication over the optical network to one or more optical telephone and/or communication system and/or telemetry system.
5. An optical telephone system and/or communications system as in Claim 1, wherein a photocell is used to convert part of the supplied optical power to electrical energy to operate the telephone and/or communications set.
6. An optical telephone system and/or communications system as in Claim 1 and Claim 5, wherein part or all of the optical power converted to electric power is stored in a re-

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chargeable battery which provides electric energy to the optical telephone and/or communication set.
7. An optical telephone and/or communication systems as in Claim 1 and Claim 2 and Claim 3, wherein an optical filter is used to separate the optical power from a source with a specified wavelength and narrow linewidth from the optical signal of short coherence length Lc used for communication and power provision.
8. An optical telephone system and/or communications system as in Claim 1, wherein one or two optical connection are provided to facilitate a two way communication between one or more optical telephone system and/or communications systems.
9. An optical telephone system and/or communications system as in Claim 1; wherein each transmitter and receiver has an interferometer with a time delay difference between the two arms of the interferometer greater than the coherence time of the optical sources in the network. This is equivalent to the statement that the path imbalance between the two arms of the interferometer to be greater than the coherence length Lc of the source.
10. An optical telephone system and/or communications system as in Claim 1, Claim 9, wherein the voice signal is converted to phase modulation and/or differential frequency modulation of the optical signal in one arm of the transmitter interferometer.
11. An optical telephone system and/or communications system as in Claim 1, Claim 9 and Claim 10, wherein the voice signal is recovered from the phase and/or differential frequency shift between the optical signals in the two arms of its receiver interferometer which has an optical path imbalance matching that of the transmitter interferometer of the transmitting optical telephone and/or communication set.
12. An optical telephone system and/or communications system as in Claim 1, Claim 9, Claim 10, and Claim 11, wherein the data information is recovered from the phase and/or differential frequency shift between the two arms of its receiver interferometer which has an optical path imbalance matching that of the transmitter interferometer of the transmitting optical telephone and/or communication set.
13. An optical telephone system and/or communications system as in Claim 1, wherein the transmitter interferometer and/or receiver interferometer are assigned a specific time delay difference and differential frequency shift between the arms of either one or both interferometers.
14. An optical telephone system and/or communications system as in Claim 1 and Claim 13, wherein the voice and the communication channels are multiplexed as different parts of the spectrum of the differential frequency shifts.

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15. An optical telephone system and/or communications system as in Claim 1 and Claim 14, wherein the time delay differences and differential frequency shift are used to facilitate one-to-one, one-to-many, many-to-one and many-to-many communication.
16. An optical telephone system and/or communications system as in Claim 1; wherein the receiver uses two photodetectors to recover voice and data information from the phase and/or time delay and/or frequency shift between the two arms of the receiver interferometer.
17. An optical telephone system and/or communications system as in Claim 1 ; wherein the receiver uses the difference of the intensities between the two outputs of the receiver together with the intensity in each output to recover voice and data information from the phase and/or time delay and/or frequency shift between the two arms of the receiver interferometer.
18. An optical telephone system and/or communications system as in Claim 1; wherein the receiver provides the drive current to the telephone ringer and to the telephone ear piece or speaker.
19. An optical telephone and/or communication systems as in Claim 1, wherein some or all of the power transmitted by one or more of the telephone and/or communication sets in Claim 1 is derived from an optical source/s inside the transmitter. The optical source have a well defined coherence function and a coherence length less than Lc.