OPTICAL MEMORY

Abstract: An optical memory for storing information comprising an optical waveguide as an optical data store in which the information is stored as the presence or absence of one or more gratings having distinct, predetermined Bragg wavelengths and a read means for determining which, if any, gratings are present.

Description: OPTICAL MEMORY' This invention relates to optical memories, that is memories in which information is represented as a modification of an optical property of the memory. In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres. The present invention in its broadest aspect comprises the use of an optical waveguide as an optical data store for storing information in which the information is stored as the presence or absence of one or more fibre gratings having distinct, predetermined Bragg wavelengths. The present invention is exprected to find particular application with optical fibre communications networks. The optical memory in this case is conveniently formed from an optical fibre waveguide. According to a further aspect of the present invention an optical memory for storing information comprising an optical fibre waveguide as an optical data store is characterised in that the information is stored as the presence or absence of one or more fibre gratings having distinct, predetermined Bragg wavelengths and read means for determining which if any, fibre gratings are present. Other aspects of the present invention are as recited in the claims of this application and are directed to optical communications systems incorporating an optical memory according to the present invention. K. O. Hill, Y. Fujii, D. C. Johnson and B. S. Kawasaki. First reported permanent optically-induced changes of the refractive index of optical fibres in a paper entitled -1- Photosensit v ty in optical fiber waveguides: Application to reflection filter fabrication. In their experiment coherent radiation at 514.5nm, reflected from the fibre ends, generated a standing wave in the fibre which induced a periodic refractive index change along its length. This formed a high reflectivity. Bragg grating in the fibre which has a peak at the wavelength of the incident beam. Since then numerous studies into the grating growth mechanism and photosensitive fibres have been carried out. However, the mechanism which results in the perturbation to the refractive index of the fibre core is not fully understood. The spectral region where the fibre is photosensitive has been found to range from the uv to around 500nm. 3. S. Kawasaki, K.0. Hill, D. C. Johnson and Y. Fujii in an article titled "Narrow-band Bragg reflectors in optical fibres", Optics Letters Vol 3 No. 2 August 1978, pp 66 - 68 noted that an important property of the grating formation process is the extent to which the filter response can be tailored. For example, one method of forming a complex filter is to superimpose two or more simple band-stop characteristics " in the same fibre by illuminating the fibre with different wavelengths of light either simultaneously or consecutively. An alternative method forming two Bragg gratings is to apply a different longitudinal stress to the fibre before optically writing each grating, all the gratings having the same Bragg condition at the t me of writing. This method of writing two or more gratings has the advantage of obviating the need for multiple wavelength illumination. The property used in this method is to exploit the fact that optical fibre can theoretically be linearly strained by up to 20%. If a photosensitive fibre, length 1, is illuminated by a light from a laser of wavelength λO, this will result in a grating of period about λ 0 /2n. ( . where r_. (( is the fibre mode refractive index. If the fibre is now stretched by Λl, then when illuminated, a grating of similar pitch to before will be written. By allowing the fibre to relax to its normal length the pitch of this second grating will now be slightly smaller than the first grating. For the case of a reflection filter the second grating has now a peak wavelength which is smaller than the writing wavelength. It may also be possible to write reflection gratings for use at the telecommunications bandwidth of 1.3-15?m if the fibre can be stretched by approximately 10°/o. This is still within the theoretically predicted change but due to defects in the manufacture of the fibre it is not clear whether it is possible to do this. Other materials such as photorefractive materials may also be used as a data storage medium for the purposes described here. A convenient method of applying the different longitudinal stresses to the fibre to produce the different stains is to clamp one end of the fibre and apply the stress by means of a piezo-electric translation stage clamped to the other end of the fibre. Clearly, other stressing means may be used. Embodiments of these and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which, Figure 1 s a schematic diagram of a data/communication network which m this case is a network of computers; Figure 2 is a schematic diagram of an optical memory used to control access to the computer of Figure 1; Figure 3 is a diagram showing the frequency grating pattern of the optical memory of Figure 2; and Figure -4 is a diagram of a header for an information packet of the network of Figure 1. The Figures show an exemplary application of the use of an optical fibre waveguide as an optical data store for storing information in which the information is stored as the presence or absence of one or more fibre gratings having distinct, predetermined Bragg wavelengths. Referring to Figure 1, an optical fibre NXN star coupler network 2 connects computers C. to C n to a series of work stations W. to W... Each workstation ; of Figure 1 has associated with it a unique address. Each computer C.-C n can broadcast information via the network 2 to every workstation W.-W_, the signal consisting of a header frame followed by data. In this system the data has a different frequency band to that of an associated header frame. Figure 2 shows the detection system of a workstation W . The workstation has its address written in an optical fibre waveguide 4 in the form of a superposition of frequency grating patterns selected from frequencies f, to fδ as shown m Figure 3a. In this case the fibre 4 has gratings corresponding to Bragg wavelengths f χ , f 3 , f t and f s . That is, the address of the workstation W. is stored in the fibre 4 as the presence or absence of gratings having 3ragg wavelengths in the ranges f, to f s The header frame of a data packet consists of a number of time slots each of which is set aside for a particular frequency f. to f 6 , see Figure 3b. Data packets from the 3tar coupler network 2 are coupled to an input port 6 of a four-port directional coupler 8. One output port 10 of the coupler 6 is spliced to the fibre 4. The other output port 12 of the couple 6 is coupled to a pass-band filter 14 and a detector 16 in series. The filter 14 allows only the data signal to reach the detector 16 by blocking the header frequencies f, to f s . When a header frame is coupled to the fibre 4, those frequencies corresponding to the Bragg wavelengths of the gratings of the fibre 4 will be reflected back towards the coupler 5 with frequency f being reflected at time t., if oresent m the header. - s- The reflected signals are coupled into a second output port 18 of the coupler 5 and so to detector 20. The detected time sequential pulses derived from the header frame of the incoming signal is compared to the address of the workstation by a comparator 22. If they correspond a gate 24 is opened to allow the received data from the defector 16 to be output at the electrical output line 26 connected to the detector via the gate 24. The system operation has numerous operating regimes depending on the data/header frequencies and timing used in the network and the number of gratings, ie one or more may be used to store the workstation address. In this system the amount of electronic processing of an incoming optical signal is significantly reduced as the grating automatically selects the correct header frequencies. The fact that this is done in a fibre which is part of the network, also removes problems which would occur if it were to be done outside the fibre ie the alignment of optical components. The system itself is similar in some ways to wavelength division multiplexing (WDM) in that the destination of the signal is dependent on the frequency of the signal and the use of wavelength selective filters. However, unlike a WDM system the actual data itself is transmitted at the same wavelength to every workstation, as it is only the header frame which varies in frequency content. This means that receivers can potentially detect all the signals in the network, which is important if a single workstation needs to read the data for more than one user. In some networks it is necessary for the same data to be sent to a number of users, eg. from a head office to branches. It is more efficient if the information is only transmitted once. This is possible if another set of address gratings are written in the fibre but physically removed from the individual user' s address. This second, generally broadcast, address would have a different wavelength band to the one used for individual user numbers so as to avoid interference between ' signals. In this way information, which is required to go to a number of different addresses could be sent as one data stream. To give an example of the number of different workstations that could potentially be addressed using a fibre grating optical data stores, consider a network where the data and header are being transmitted at 1.3 and 1.5?m respectively. The wavelength range over which an address is written is obviously dependent on the bandwidth of each diffraction grating. This s taken in this example to be 0. Inm and the total wavelength range for the addresses is assumed to be lOnm. This gives 100 channels so the number of possible addresses is 1X10 J0 (2 i00 ), which is effectively an unlimited number of users. At present the number of gratings that can be written m a single fibre is not known but it is not thought to be realistically more than 10. If the lOnm bandwidth is split into different sections for user addresses, general broadcast addresses, parity checks, timing signal etc. , if 20 channels are set aside for personnel user addresses this mean that -«10 6 users can use the system. This figure underlines the fact that this approach has some potential use in either a telecommunications market or in an interconnection system. Another envisaged use of the grating addressed system is for the rerouting of signals in a transmission system. This has the great advantage that header frames can be read giving all the details of where the signal is to be redirected, while the actual data itself can be kept in an optical form. The data packet can be given a new header frame as it is routed through the switching system. This eliminates the need to convert the data from optics to electronics and back again which is expensive and limits the data rate. The above embodiments are exemplary of specific uses of an optical fibre waveguide as an optical data store for -1 - storing information in which the information is stored as the presence or absence of one or more fibre gratings having distinct, predetermined Bragg wavelengths can be put to use as an optical data store. Numerous alternative configurations and applications of optical data stores according to the present invention will be apparent to those skilled in the art and while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover any such alternative configurations and uses, various modifications and equivalent arrangements which within the spirit and scope of the appended claims.

Claims: - 2- CLAI MS 1. The use of an optical waveguide as an optical data store for storing information in which the information is stored as the presence or absence of one or more gratings having distinct, predetermined Bragg wavelengths. 2. An optical memory for storing information comprising an optical waveguide as an optical data store in which the information is stored as the presence or absence of one or more gratings having distinct, predetermined Bragg wavelengths and a read means for determining which, if any, gratings are present. 3. An optical memory as claimed in claim 2 in which the optical waveguide comprises a photorefractive fibre, the fibre gratings being internal grating. 4. An optical memory as claimed in either one of claims 2 or 3 in which the gratings are reflection gratings. 5. An optical communications network including a series of workstations each having associated address, the address being stored in an optical memory as claimed in any one of claims 2 to 4. 6. A communications network as claimed in claim 5 in which data signals for the workstations comprise a header frame followed by a data frame, the data frame having a different frequency band to that of the associated header frame. 7. A communications network as claimed in claim 6 in which the address of the workstation to which data is addressed is coded as a series of frequencies in the header frame. 8. A communications system as claimed in claim 6 or 7 in which the header frame consists of a number of time slots each of which is set aside for a particular distinct freσuencv. 5. A communications system as claimed m any one of claims 3 to 3 in. hich each workstation includes a passband filter which blocks the frequencies of the header frame. 10. A communications system as claimed in claim 9 in r including means for detecting which frequencies of the header frame are back reflected by the filter. 11. A communications system as claimed in claim 10 in which the reflected signals from the header frame are detected as a series of time sequential pulses, the 3 detected pulses being compared to the address of the workstation by comparator. 12. An optical communications system as claimed in claim 9 in which the data from the data frame are coupled from a detector to the workstation only if the address and the 5 data header corresponds to the address of the workstation. 13. An optical communications network as claimed in any one of claims 2 to 10 in which there are two or more broadcasting terminals, the signals from each terminal being broadcast to each of the workstations. C 14. The use of an optical fibre waveguide as an optical data store for storing information in which the information is stored as the presence or absence of one or more gratings " having distinct, predetermined Bragg wavelengths.

Inventors: CAMPBELL ROBERT JOHN, KASHYAP RAMAN, CAMPBELL, ROBERT, JOHN, KASHYAP, RAMAN
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1991
Main classification: G02B6
IPC codes: G02B6, G11C13, G11C21, H04B10