GB2421863A

GB2421863A - Using an interferometer to impose a spread spectrum code and data modulation in an optical communication system

Info

Publication number: GB2421863A
Application number: GB0500025A
Authority: GB
Inventors: Salah A Al-Chalabi
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-01-04
Filing date: 2005-01-04
Publication date: 2006-07-05
Also published as: GB0500025D0

Abstract

A secure optical interferometric communication system and network with multiple access capability where one or more transmitters have an interferometer with phase/Doppler shift difference modulation of spread spectrum type which is used for secure communication with one or more receivers on the optical network where the spread spectrum modulation is known to one or more receivers. The optical source can be of narrow optical spectrum or broad optical spectrum. In the case of the broad optical spectrum source, the path length difference of the of the transmitting interferometer can be also used to increase the security communication where the path length imbalance is known to one or more receiver, and number of users on the multiple access network. The interferometric communication system, especially with broad spectrum optical sources, can interwork with conventional wavelength division multiplexing (WDM) system and time division multiplexing (TDM) system. The interferometric system can partially tolerate jamming by WDM and TDM systems, and can be made difficult to detect by unintended listener/s.

Description

Secure Multiple Access Optical Communication This invention relates to a

secure interferometric communication system using spread spectrum techniques over an optical network that can support multiple users.

Optical communication systems can transmit information over optical fibre, free space or through the atmosphere. Interferometric optical transmission system and multiple access networks have been described in prior art: Ref [AL-CHALABI, Salah; "Optical communication device and system" patent US2004 165884; WO 02/103935 Al]. The system can be used to transmit data, voice, television and multimedia content as well as for telemetry. The system can support multiple users, i.e. multiplex, using interferometric time-delay difference, Doppler shift (frequency or phase) difference and intensity modulation techniques.

For certain applications, security of communication is an important requirement. As already known in the prior art, communication systems can be made secure by using spread spectrum (SS) encoding techniques. The spreading signal can be pseudonoise generated (PN) signals (which can include pseudo-random signal PRS, or pseudorandom binary sequence PRBS for, or pseudorandom generator PRG), frequency hopping (FH), time hopping (TH), or direct sequence (DS) type. The main advantages of spread spectrum systems and receivers are their security and their ability to reject unintentional and intentional interference, Ref [Jack K. Holmes; "Coherent Spread Spectrum System", John Wiley, 1982], Ref [Don J. Torrieri; "Principles of Secure Communication Systems", second edition, Artech House, 1992], Ref ["Handbook of Applied Cryptography"; Alfred J. Menezes, Paul C. van Oorschot, Scott A. Vanstone; 1997, CRC Press Inc.]. The spread spectrum techniques can also enable multiple users to communicate over the same communication channel (multiplexing or multiple access) by utilising orthogonal, or roughly orthogonal, signals in a common frequency band. In addition, spread spectrum modulation can produce a wideband, low power spectral density signal making the transmitted signal difficult to detect or recognise by unintended listener and resistant to jamming by intentional or unintentional interferers. A treatment of noise in digital optical transmission system is given in Ref ["Noise in digital optical communication systems", Gunner Jacobsen, 1994, Artech House Inc.] and treatment of optical receiver noise with optical preamplifiers is given in Ref [Optical fiber communication systems"; Leonid Kasovsky, Sergio Benedetto, Alan Willner; 1996, Artech House Inc.].

A secure optical spread spectrum system is conventionally implemented by modulating the optical intensity with the output of the spectrum spread signal generator. A potentially more secure optical communication system can be produced by modulating the phase/frequency, with or without amplitude modulation, of the optical field in either or both arms of an interferometer in an interferometric optical communication system.

Using spread spectrum techniques in secure interferometric system has been reported in the prior art. Ref [Lecong PHUNG patent US5 191614], Ref [Eric UDD; patent US5274488 and US56941 14] which describe a secure interferometric system using a short coherence length optical source with a Sagnac loop interferometer whose - 2- counterpropagating beams are phase/frequency modulated by a phase modulator with the same pseudonoise signal. The short coherence length of the optical source of coherence length of a few tens of microns makes it difficult to match the path length of the counterpropagating beams except at the beam splitter where the optical source is located. The system receiver is located at the point of originating and recombining the counterpropagating beams such that the scrambling sequence caused by the pseudonoise signal cancels out at the point of detection allowing the intelligence signal to be demodulated without additional electronics. A modified version of the Sagnac based system was reported by Ref[ Michael M. MORRELL et al; patent US6690890] where the counterpropagaing beams are formed by a combination of beam splitters at the optical source end, an interferometer, and a fibre with mirror together with a phase modulator driven by pseudorandom signal generator. However, these systems rely on the property of a Sagnac interfeterometer namely that the counterpropagating beams have inherent self matching path length, i.e. zero path length difference, where optical source is located. This makes the Sagnac based system most suitable for a single point-point or multi- point to a single point communication.

The current invention is a secure interferometric optical communication system using spread spectrum techniques and enabling multiple access by one or more users over an optical communication channel such as optical fibre, free space, the atmosphere etc. According to this invention the secure interferometric optical communication system comprises one or more optical sources with known coherence length and one or more transmitters where each transmitter has an interferometer, phase/Doppler shift modulator in one or both arms of the transmitter interferometer which is driven by the output of a spectrum spreading signal with spreading bandwidth B, an information source which produces one or more symbols of duration T seconds (i.e. of a bandwidth Be proportional to l/T), optical communication channel connecting one or more transmitters to one or more receivers, one or more receivers with an interferometer with a path imbalance which differs from that of one or more transmitter interferometers by less than the coherence length L of the optical source, phase/Doppler shift modulator in one or both arms of the transmitter interferometer which is driven by the output of a de-spreading signal generator similar to the spreading signal generator at one or more transmitters, one or more photodetectors at the outputs of the receiver's interferometer to convert the optical signal to an electrical signal, a signal processor to detect and acquire and lock the receiver's despreading signal generator to the transmitter's spreading signal generator and indicate locking or synchronisation, a signal processor to recover the information generated by transmitter's information source and a receiver controller to control its operation The security of the basic interferometric system and the number of users of the system is influenced by the receivers' knowledge of the values of the path imbalances assigned to the transmitters' interferometers and the properties of the chosen spectrum spreading/de-spreading signals. The security and/or number of system users can be increased by using a spreading signal generator that produces a set of orthogonal, or nearly orthogonal, signals with zero average. The system is improved when the spreading signal's bandwidth is greater than the symbol transmission rate (1 /T), i.e. B/ B (or BST) is greater than unity, and the range of phase/Doppler shift difference between the two optical signals of the two arms of the transmitter interferometer is - 3- equal to or greater than (-it, +it). The security of the interferometric system can be further improved by spreading the power of the transmitted signal over a bandwidth B, by choosing ratio of B/ B, to yield a signal power to be less than the noise power at the output of the receiver with bandwidth Be. This will also make the system more difficult to detect by unintended listener. The interferometeric receiver which has a de- spreading signal that matches spreading signal of the transmitter will achieve highest Signal- to-Noise-Ratio (SNR) in a receiver bandwidth Be. The system can be made to interwork with optical Wavelength Division Multiplexing (WDM) systems by using an optical filter at the input to the interferometric receiver to extract these wavelengths from the optical spectrum. The system has some immunity to interference byjammers of narrow optical spectrum if the spreading bandwidth B is chosen such that power spectral density due to the jamming signal is low in the detection bandwidth Be. The system can also interwork with optical Time Division Multiplexing (TDM) systems by a conventional optical intensity demodulator and using a balanced interferometeric optical detection to demodulate the interferometeric modulated signal. This type of receiver will give the system some immunity against jamming by TDM systems. The receiver can also have photocells, or thermophotovoltaic cells to convert part of the available optical energy to electrical energy to provide power to drive the system. The converted energy can be stored in a re- chargeable battery.

A specific embodiment of the invention will now be described by way of example reference to accompanying drawing in which: Figure 1: One-way secure interferometric optical communication Figure 2: Two-way secure interferometric optical communication Figure 3: Multiplexed and secure interferometric optical communication network Figure 4: Power spectral densities at the receiver of a secure and difficult to detect interferometric optical communication Figure 5: Use of optical spread spectrum system with Wavelength Division Multiplexing (WDM) signals Figure 6: Use of optical spread spectrum system with Time Division Multiplexing (TDM) signals Referring to the figure 1 the transmitter 100 consists of an interferometer 101 (which can be a Mach-Zehnder interferometer with optical paths 108 and 109 and beam splitters 111 and 112 or other type of similar interferometers) which has a path length imbalance (L1 or time delay difference ti=Lti/c where c is the speed of light) that is preferably greater than the coherence length (La) of the optical source 114. The optical source can be located either at the transmitter or in the network. The interferometer can be fabricated using either bulk optics, guided optics such as Lithium Niobate (LiNbO3) or silicon (including MEMS-Micro-Electromechanical systems) or Sol-gel based technologies. An optical source of short coherence length is preferably used (a longer coherence length optical source can be used, but shorter coherence length is preferable as they can be made more secure); such as a - 4superluminescent diode or Amplified Spontaneous Emission sources or multimode lasers. A good example is a InGaAs or InGaAlAs superluminescent diode operating with central wavelength of 850nm or 1300 nm or 1550 nm and a continuous spectrum of 40. The coherence length of such a device is 18, 42 and 60 micro-meter for central wavelengths 850, 1300 and 1550 rim respectively. It is important the optical signals in the two arms of the interferometers are the same replica of the optical signal of the optical source. The transmitter also has a spectrum spreading signal generator 116 with an output signal Sj(t) with spreading bandwidth or spreading bandwidth B and driven by the transmitter's clock 115 and controlled by the transmitter controller 139.

The spectrum spreading signal generator can be a pseudorandom signal or sequence generator. The output of the spreading signal generator 104 can be added in the adder 105 to the output 103 of the information source 102 which produces one symbol in a period of T seconds (equivalent to a bandwidth Be a l/T) to drive the phase/Doppler shift modulator 107 of the transmitter interferometer resulting in a phase/Doppler shift difference between the optical signals of the two arms 108 and 109 of the transmitter interferometer. The phase/Doppler shift modulator can be an electro-mechanical, electro-optical or other type which cause a change in the optical path length of the optical signal when an electrical signal is applied to the modulator.

The modulator can be integrated with other optical components form the interferometer by using guided optics and semiconductor based based technologies.

The Doppler shift difference 0(t) approximately equals, ignoring relativistic effects, the rate of change of the optical carrier's phase 0(t) i.e. d d L(, (t) 0(t) -O(t) = _______ Where L0(t) is the optical path difference of the two arms of the interferometer and A0 is the central wavelength of the optical source 114. An optical element 113, such as a polariser or polarisation scrambler, might be necessary to match the optical source to the transmitter interferometer' s input.

The relationship between the difference in the phase/Doppler shift and the output of the transmitter's spreading signal generator, ignoring the signal from the information source, is 01(t) = JOi(t')dt'=-[a1xS1(t)+b1] Where a1 and b1 are constants.

The output 110 of the transmitter interferometer is then transmitted over the optical channel 130 which can bean optical fibre, optical fibre network free space, the atmosphere. .etc. It can also be a network such as Passive Optical Network (PON).

An optical component, such as a lens, might be need to optimise the coupling of the optical signal to optical channel. The output of the optical channel is then fed to the receiver 117 which might have an optical components 131 such as lenses or dispersion compensators to compensate for the optical channel dispersion or a polarisation controller to match the polarisation of the optical channel to that of the - 5receiver's interferometer 140 which has two optical paths 132 and 133, two beam splitters 129 and 129 and a phase/Doppler shift modulator 134.

The receiving interferometer 140 has a path length imbalance Lri (or time delay difference rri=Lri/c). The first step for establishing communication between the transmitter and receiver is to set the difference between the path imbalance of the transmitting interferometer L1 and the path imbalance of the receiving interferometer Lri less than the coherence length L (i.e. lLriLtiI<Lc) of the optical source. The output of the de-spreading signal generator 118, which is driven by the clock 119 and controlled by the receiver's signal processor and controller 120, at the receiver has an output signal S2(t), which can be added to the information demodulating signal 138, drives the phase/Doppler shift modulator 134 of the receiver interferometer resulting in a phase/Doppler shift difference between the two arms of the receiver interferometer.

The relationship between the difference in the phase 02(t) and the Doppler shift 02(t) and the output of the receiver's spectrum de- spreading signal generator, ignoring the signal from the information demodulation signal, , is 92(t) = Jqs2(tf)d1?=.Jui[a2xs2(t) + b2] Where a2 and b2 are constants.

The optical signals at either one or both outputs 126 and 127 of the receiver's interferometer are converted to electrical current I and 2 by the photodetectors 125 and 124 respectively. The signal processor 120 at the receiver produces a signal proportional to the difference (Id) between electric current I and 12 and remove any d.c. component; i.e. Ida (11-12) If the optical field of the optical source is a waveform that can by described by the function f(t) then the optical power at one of the outputs of the receiving interferometer, when the beams from the two arms of the interferometer spatially match, can be represented by + f (t)xf (t - [r - V2])xCos(i91 [t] -02 [t]) where P0 is a constant representing the optical power at that output without the interferometric signal. The difference current Id between the receiver's two photodetectors is proportional to J(t)xf(z' - [r,1 - Tn])xCos(01 [t] -02 [t]) a Id(t) The photodetectors in the receiver is normally followed by an integrator with an integration time T (this is equivalent to using an electrical low pass filter with bandwidth B a 1/T) equals the symbol rate of the information source at the - 6- transmitter. The integrator, or low pass filter, is part of the signal processor 120. The signal at the output of the integrator (or low pass filter) Q0(t) is proportional to 1+ T / 2 Q0(t) a ff(t')xf(t'_{v,i - Tn]) xCos(91 [t'] - 2 [t'])dI' 1-7/2 When the difference between the path imbalance of the transmitter interferometer and the path imbalance of the receiver interferometer is less than the coherence length of the optical source then we can assume that tti - Zn = 0 and if a1=a2=1 (which is satisfied if the phase/Doppler shift modulators at the transmitter and the receiver have the same signal to phase/Doppler shift coefficient), and b1=b2=0, then the output of the integrator (or the low pass electrical filter) is 1+7/2 Q0(t) a Jj2 (t' )xCos(S1 [t'] - S2 [t'])dt' l- T/2 This is a maximum when the signals generated by the spectrum de- spreading signal generated by 118 at the receiver is synchronised with the spectrum spreading signal generator 116 at the transmitter, i.e. S2[t] S1[t]=0 When this condition is satisfied then the signals S1 [t'] and S2[t'] are synchronised at the receiver leading to Cos(Si[t'] - S2[t']) = I for all t' When synchronisation is achieved, the receiver becomes ready to demodulate the information signal.

The process of bringing the receiver into synchronisation is called acquisition which can be divided into two sub-steps: the initial signal acquisition (coarse acquisition or coarse synchronisation) within an uncertainty of( 1/B); and signal tracking which performs and maintains fine synchronisation between the transmitter and the receiver.

Strategies for achieving synchronisation in a conventional spread spectrum receiver is

discussed in the prior art.

When synchronisation is achieved and maintained then detection and acquisition are achieved and this is indicated by the receiver's lock and acquisition indicator 122 and communication between the transmitter and one or more synchronised receivers can start in a one-communication multiple access network such as multicasting or broadcasting.. The transmitted information can be voice, data, television multimedia, telemetry or other types of information. These operations together with demodulating the information 121 and 138 are controlled by the receiver's signal processing and receiver controller 120 and the transmitter controller 139.

The information can by imposed on the transmitted ineterferometeric signal using phase/Doppler shift difference or/and intensity modulation. The information can also be imposed on the spectrum spreading signal. Any changes in the transmitted signal can be demodulated by the receiver using the appropriate phase/Doppler shift tracking techniques, or/and amplitude/intensity demodulator, and/or demodulating the information contained in the spectrum spreading signal.

The security and multiple access features of the interferometric communication are based on the possibility that the spectrum spreading signal S1[t] and/or the value of the path imbalance of one or more of the transmitting interferometers are only known to certain receivers. The signal S1 [t] can be chosen such that the output of the receiver is as close as possible to zero if it can not achieve synchronisation by failing to match its spectrum de-spreading signal and the transmitter's spectrum spreading signal. However, the receiver's output is a maximum if it achieves synchronisation.

To satisfy these requirements better, it is preferable that the phase/Doppler shift modulator causes a phase change uniformly distributed over the range (-it,+ir) or larger and spectrum spreading bandwidth B is large enough to result in zero, or as close as possible to zero, crosscorrelation between the spectrum spreading signals except when the spectrum de-spreading signal matches the spectrum spreading signal.

The advantages and disadvantages of the different spectrum spreading signals are covered in the prior art. There are several types of spectrum spreading signal generators that can produce orthogonal, or roughly orthogonal signals with zero average value. Examples of such signal generators are: Direct Sequence, Frequency Hopping, pseudonoise generators and Time Hopping. Direct Sequence and pseudonoise generators have the best noise and anti-jamming performance and most difficult to detect (i.e. most secure), but they have long acquisition time compared to other spreading signals. Frequency Hopping has a relatively short acquisition time, but complex spreading signal generator. The choice of the pseudonoise generator will be determined by required system performance such as acquisition time, ease of implementation, cost, simplicity, .etc. If low level of security is required then the spectrum spreading signal can be a simple waveform, such as a single frequency or sawtooth waveform, known by the one or more receivers. However, if security is required then spectrum spreading signal of the pseudonoise type should used.

In all cases, detection and synchronisation must be achieved before demodulation can be performed. Detection can be achieved with either one interferometer at the receiver, or two interferomters with the difference in their path lengths as close as possible to A0/4 (i.e. in quadrature) and with identical phase/Doppler shift modulation.

In both cases, the output of the detector's receiver is monitored during the search period. In the case of one interferometer, the signal is detected when the receivers' output power increases. In the case of the two interferometers, detection is achieved when the output of the two interferometers change simultaneously.

In a two-way communication between two users each with a transmitter and receiver will require the receiver of one user to transmit the detection, acquisition and lock information to the other user so that secure communication can be established.

Referring to figure 2 a two-communication system two users 200 and 201 each have a transmitter 100 and a receiver 117 that perform detection, acquisition (synchronisation and tracking) at both ends. Once this is achieved, then this is - 8- communicated from the receiver 117 to the transmitter 100 via 202 at 200 and via 203 at 201, and a two-way communication can start by imposing the information either on the difference in the phase, Doppler shift or/and amplitude of the optical signals in one or both arms of the transmitter interferometer. Any changes in the transmitted signal can be demodulated by the receiver by either phase/Doppler shift tracking techniques or/and amplitude/intensity demodulator. The transmitted information can be voice, data, television multimedia, telemetry or other types of information.

Several users can communicate simultaneously and independently over the same optical channel forming a multiple access communication network. Referring to figure 3 transmitters 100, 301, 302 and 303 can be assigned any path length imbalance and a unique phase/Doppler shift difference spreading waveform p q(t) whose optical signals are combined using beam combiners 304, 305, 306 and 307 and the combined beam 308 is transmitted over the optical channel 130 whose output 310 is split by the beam splitters 311, 316, 317 and 318 to the receivers 117, 312, 314 and 315. Synchronisation between the spectrum dispreading at any receiver and the spectrum spreading signal can only be achieved if p0 m(t) - k q(t) = 0 which implies that I LmLrn <L i.e. only if the difference in the path imbalance of the receiving interferometer and the transmitting interferometer is less than the coherence length of the optical source, and the spectrum de-spreading signal at the receiver is synchronised with the spectrum spreading signal at the transmitter. Several transmitting interferometers can have the same path imbalance and they can be distinguished by the phase/Doppler shift spectrum spreading signal assigned to each transmitter.

An extra level of security can be introduced by modulating the optical intensity of the optical power of the transmitter interfero meter. However, intensity modulation can be easier to detect by unintended listener as conventional intensity demodulators are the easiest to construct and are most common among optical receivers. But it might be used for deception to lead the unintended listener to believe that the information is in the intensity modulation, although it is placed on the interferometric phase/Doppler shift modulation.

The interferometric optical communication can be made more difficult to detect by unintended listener, and therefore more secure, if the ratio of spectrum spreading signal's bandwidth B to the electrical bandwidth of the receiver Be (B/B or BT where T is the integration time of the receiver integrator which equals the period for transmitting information carrying symbols) is large enough such that the signal-to- noise-ratio (SNR) at the output of the integrator of integration time T (or low pass filter of bandwidth Be) is less than unity. Referring to figure 4 the receiver's noise N0 402 is assumed to have a uniform power spectral density which represents typical types of noise encountered in optical communication. The noise can be thermal or shot noise or the addition of these types of noises or any other type of noise which is always present in a receiver, It also covers the noise when an optical pre-amplifier is - 9- used before the receiver's photodetectors. The power spectral density Pb 401 represents the inetrferometeric signal power spectral density, assume uniform, over its bandwidth Be before spreading, and the power spectral density P 403 repreasenting the interferometric signal power spectral density after spreading over a bandwidth B5.

If the required SNR before spectrum spreading and de-spreading is SNR, then the transmitted interferometric signal can be made difficult to detect if the ratio of spectrum spreading signal's bandwidth B5 to the electrical bandwidth of the receiver B (B/B) is B. ____ Be SNRh This makes any receiver which has not achieved synchronisation to have an output which just looks like noise since the SNR is less than unity. However, the receiver which achieves synchronisation will achieve an SNR equal to SNRb at it output which is larger than unity. For example, if T = 100 tsec (Be = 10 kI-Iz) and the required SNRb = 100, then the spreading bandwidth should be more 1 MHz to make it difficult to detect by a receiver which does not known the spreading signal even it knows the value of the path imbalance of the transmitting interferometer.

The interferometeric communication can interwork with, and have some immunity immunity to interference or jamming by Wavelength Division Multiplexing (WDM) or Time Division Multiplexing (TDM) optical systems.

The invention can interwork with WDM systems. Referring to figure 5 discrete narrow optical spectrum wavelengths 500 can be added, by using Optical Add and Drop Multiplexer 501 (OADM), to the transmitted broad spectrum optical signal from the transmitters' interferometers. The combined optical signal can be transmitted over the same optical channel 130. At the output of the optical channel, or at the input of the receiver, an Optical Add and Drop Multiplexer 502 can be used to extract the discrete narrow spectrum wavelengths 503. The extracted WDM signal can then be demodulated in a standard maimer using a bank of optical filters and intensity demodulators. The remaining optical signal, i.e. the optical signal at the input to the minus the WDM signal, can be fed to one or more receiver's interferometer 117 and 312 to demodulate the interferometeric modulation imposed by one or more transmitter's interferometer 100 and 302.

The invented interferometeric communication has some immunity to interference or jamming by narrow optical spectrum sources such as WDM. This is due to the spreading of the WDM signal by the receiver's despreading signal. The immunity is improved by choosing the spreading bandwidth (which should be identical to the de- spreading bandwidth) such that the power of the interfering WDM signals within the receiver bandwidth B is smaller than the power due to the interferometeric signal.

Further immunity can be introduced by inserting an optical filter at the receiver input before the interferometer. The remaining broad optical spectrum signal is fed to the receiver's interferometer. - 10-

The system can also interwork with Time Division Multiplexing (TDM) systems with optical sources of either narrow optical spectrum or broad optical spectrum. In the case of a TDM system using a narrow spectralwidth source, such as a laser, an optical filter at the input of the receiver is required to extract it from the rest of the spectrum.

The remaining broad spectrum optical signal is then fed to the receiver interferometer.

Referring to figure 6 the intensity modulated optical source 601 used for TDM communication can be added using a beam splitter/combiner 602 to the broad optical spectrum signal of the transmitter interferometer 100, the receiver 117 of current invention is largely not affected by the presence such of intensity modulated signal if the receiver interferometer beam splitters/combiners 128 and 129 split the input optical signal equally; i. e. 1:1 split, where each of the two photodetectors 124 and 145 receive as close as possible one half of the input optical power. The TDM signal is then rejected by the balanced receiver which processes the difference current (Id = Ii - 2) from the two photodetectors. To extract the information of the TDM signal, the output of the optical channel can be split by the beam splitter 603 with part of the optical signal is used by a conventional optical receiver 604 to demodulate the TDM intensity modulated signal. Alternatively, the interferometeric balanced receiver with the two photodetectors 124 and 125 can be used to extract the TDM intensity modulated signal where the receiver signal processor produces a signal that represents the sum of the currents of the two photodetectors (i.e. 11+ 12) which is proportional to the intensity of the optical signal at the input to the receiver.

In certain applications power might be supplied optically from a remote optical source. The transceiver can then include a beam splitter 605, photocells/thermophotovoltaic cells 606 and possibly a battery 607 to convert and store some of the optical energy to electrical energy which can be used drive the system.

Claims

1. An optical network for carrying communication signals to or from transmitting/receiving apparatus in optical form, the transmitting/receiving apparatus employs interferometric modulation and demodulation as an information-carrying optical communication signal over the network

2. An optical network in Claim I where the path imbalance of one or more transmitter interferometer is known to one or more receivers on the network

3. An optical network in Claim I where part of the transmitter inteferometric modulation is known to one or more receivers on the network

4. An optical network in Claim I and Claim 2 or Claim 3 where the interferfermetric modulation is a phase/Doppler shift modulation

5. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 where part of the interferometric phase/Doppler shift modulation is known to one or more receivers

6. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 or Claim 5 where the path imbalance of the transmitting interferometer and part of transmitter's phase/Doppler modulation is known to one or more receivers

7. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 where the difference in the path imbalance of the interferometer in one or more transmitting apparatus and the path imbalance of the interferometer in one or more one or more receiving apparatus is less than the coherence length of the optical source

8. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 where the phase/Doppler shift modulation is a spread spectrum modulation using a spectrum spreading signal

9. An optical network in Claim I and Claim 2 or Claim 3 or Claim 4 and Claim 8 where the spectrum spreading signal of pseudo-random or random nature including pseudonoise, Frequency Hopping, Pseudo Random Signals, Pseudo Random Sequence, Time Hopping and Direct Sequence

10. An optical communication network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 5 where the receiving apparatus has spectrum de-spreading signal with one output matching the spectrum spreading signal generated in one or more of the transmitting apparatus on the network

II. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 9 or Claim 10 with a receiver having the capability to synchronise the spectrum de- spreading signal at the receiver with the spectrum spreading signal generated at the transmitter

i2 12. An optical network Claim I and Claim 2 or Claim 3 or Claim 4 and Claim 11 with a receiver having two interferometers with the same spectrum de-spreading signal and their path imbalance are in quadrature;

13. An optical network in Claim I and Claim 2 or Claim 3 or Claim 4 where the information is transmitted with the spectrum spreading signal that is used to identify the transmitting apparatus

14. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 13 where the information is transmitted using the phase/Doppler shift of the interferometric signal

15. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 13 where the information is transmitted using intensity of the interferometric signal

16. An optical network in Claim I and Claim 2 or Claim 3 or Claim 4 and Claim 13 where the spectrum spreading signal is used to transmit the information

17. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 13 and Claim 14 or Claim 15 used as a one-way communication system to broadcast or multicast the information

18. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 13 and Claim 14 or Claim 15 used as a two-way communication system

19. An optical network in Claim I and Claim 2 or Claim 3 or Claim 4 and Claim 13 and Claim 14 or Claim 15 where several users are multiplexed on the network or have multiple access to the network

20. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 5 and Claim 6 and Claim 8 where the spreading bandwidth is chosen to reduce the power spectral density of the interferometric signal making it difficult to detect by unintended receiver/listener

21. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 that interworks with Wavelength Division Multiplexing systems having channels with narrow optical spectrum by extracting the narrow spectrum optical channels from the broader optical spectrum interfermoeteric signal

22. An optical network in Claim I and Claim 2 or Claim 3 or Claim 4 and Claim 21 where an optical filter is used at input of the receiver to remove the interfering Wavelength Division Multiplexing signals

23. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 8 where the spectrum spreading or de-spreading signals make the power due to the j ammer smaller than the signal's power in the detection bandwidth.

24. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 which interworks with Time Division Multiplexing system

25. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 24 where part of the received optical signal is demodulated using a conventional optical intensity demodulator

26. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 24 where the currents from two or more photodetectors at the outputs of the receiver interferometer or interferometers are used to extract the interferometric signal and attenuates the Time Division Multiplexing system by using a difference current from the appropriate photodetectors

27. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 and Claim 24 where the currents from two or more photodetectors at the outputs of the receiver interferomer or interferometers are used to attenuate the interferometric signal and extract the Time Division Multiplexing signalling by using a sum of currents from the appropriate photodetectors

28. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 which interworks with Wavelength Division Multiplexing signal in Claim 21 and Claim 22 or Claim 23 and Time Division Multiplexing signal in Claim 24 or Claim 25 or Claim

29. An optical network in Claim I and Claim 2 or Claim 3 or Claim 4 with immunity to jamming by Wavelength Division Multiplexing signals in Claim 21 or Claim 22 or Claim 23 and immunity to jamming by Time Division Multiplexing signals in Claim 26

30. An optical network in Claim 1 and Claim 2 or Claim 3 or Claim 4 where some of the optical power is converted to electrical power

31. An optical network in Claim I and Claim 2 or Claim 3 or Claim 4 and Claim where some or all of the electrical power generated by some of the optical power is stored electrically