The invention relates generally to optical code generation and detection as is important for Optical Code Division Multiple Access (OCDMA) and optical packet switching, more especially but not exclusively to grating coders and decoders, and methods of fabricating grating coders and decoders for OCDMA or packet switching.
The explosive growth of the internet over recent years is placing increasing demands on both the capacity and functionality of optical transmission systems and networks. Most work to date has focussed on the use of either Wavelength Division Multiplexing (WDM), optical Time Divisional Multiplexing (OTDM) or a hybrid approach to achieve the Tbit/s aggregate channel capacity required. Now that Tbit/s systems have been demonstrated in the laboratory interest is beginning to grow in investigating alternative multiplexing schemes such as Optical Code Division Multiple Access (OCDMA) which has the potential to further enhance the functionality of optical networks [1-11]. CDMA is a spread spectrum technique that permits a large number of separate users to share the same extended transmission bandwidth but to be individually addressable through the allocation of specific address codes.
CDMA encoding can be performed either in the time domain (direct-sequence DS-CDMA) or frequency domain (frequency-hopping FH-CDMA) [12].
In DS-CDMA each data bit to be transmitted is defined by a code composed of a sequence of pulses. The individual pulses comprising the coded bit are commonly referred to as chips. The coded bits are then broadcast onto the network but are only received by users with a receiver designed to unambiguously recognize data bits of the given specific address code. Address code recognition is ordinarily achieved by simple matched filtering within the receiver.
In FH-CDMA, the carrier-frequency of the chips (or bits) is changed according to a well-defined code sequence that can once again be suitably identified by an appropriate receiver.
CDMA has been applied with great success to the field of mobile communications but has only recently generated significant interest in the optical domain. The particular attractions of OCDMA include the capacity for higher connectivity, more flexible bandwidth usage, improved cross-talk performance, asynchronous access and potential for improved system security.
CDMA for optical telecommunications, i.e. OCDMA, is still at a relatively immature stage of development. A key issue relates to how to reliably generate and recognize appropriate code sequences. (The issue of what constitutes an appropriate code sequence is described further below). To date the most common approach is to use arrays of discrete optical waveguide based delay lines to temporally, or sometimes spectrally, manipulate the individual data bits in order perform the coding and decoding process. In the earliest implementations the delay lines used were simple optical fibers of different lengths appropriately coupled together using fiber couplers [4], [5].
However this approach is not a practical solution due to its limited scalability and the difficulty in obtaining and maintaining adequate accuracy on the length of the individual delay lines.
More recently planar lightwave circuits (PLCs), such as Arrayed Waveguide Gratings (AWGs), have been used to overcome the limiting practical issues discussed above by monolithically integrating the required tunable taps, phase-shifters and combiners onto a single substrate [1,2]. While this is a more practical approach, PLCs are difficult and expensive to fabricate and therefore offer a far from ideal technical solution.
An alternative approach, and one that does not rely upon individual discrete waveguides to provide different paths through the system in order to perform the necessary pulse spreading and shaping, is to use diffractive free space optics. The standard approach is to employ a bulk grating pair to spatially separate, and then recombine, the individual frequency components of a short pulse. A spatial amplitude/phase mask can then be used to perform the necessary filtering functions and to reshape the pulse [6], [7]. However, the approach is again of somewhat limited practical value due to lack of compactness, spectral/temporal resolution and cost.
More recently, xe2x80x98single beamxe2x80x99 encoding and decoding schemes based on fiber Bragg grating (FBG) technology have been proposed and demonstrated. The most straightforward approach is to use an array of FBGs written or spliced in a sequence along a single fiber line [8]. The spatial position of the gratings and their associated reflection profile can then be used to encode both temporal and spectral information onto an incident data pulse. For example a form of fast FH-OCDMA has recently been demonstrated in which the central wavelength of sequential gratings in an encoder/decoder grating array is varied so as to define individual chips within the code [8], [9]. This particular example exploits the wavelength selectivity of the individual gratings and the positioning of the gratings within the array in only a relatively straightforward way that simply uses time-of-flight delay.
However, grating technology has progressed to the point that the optical phase of light reflected from xe2x80x98individualxe2x80x99 gratings can also be exploited, allowing the use of optical phase as a coding parameter (note that this is already possible using PLC technology [1]). Use of phase coding is significant since it is well known that bipolar codes exhibit far better cross-correlation/cross-talk characteristics than amplitude-only unipolar codes, such as those recently reported [11] where superstructured fiber Bragg gratings (SSFBGs) were used to provide an alternative approach to the discrete FBG array based pulse encoders and decoders discussed further above.
FIG. 1 of the accompanying drawings shows the general approach adopted with the unipolar OCDMA reported in the prior art [11]. At the transmitter end, an SSFBG 112 encoding a 7-chip sequence 0100111 is arranged in combination with an optical circulator 110 to receive an input signal pulse 108 and convert it into an encoded signal 116. The encoded signal 116 is conveyed through a transmission link 114 to a receiver. The receiver uses an SSFBG 120 having a 7-chip sequence 1110010 complementary to that of the transmitter-end SSFBG 112 arranged in combination with an optical circulator 118 to receive and decode the encoded signal 116. The decoded signal 120 is then output to any desired standard elements for further processing. The relatively poor performance of the unipolar decoding is schematically represented in the figure by the residual side lobes to the decoded signal 120.
The use of bipolar codes with FBG technology was first demonstrated using a segmented FBG array comprising uniform period gratings with an accurately controlled phase (path-length) between individual gratings [10], [20]. The phase mask used to xe2x80x98imprintxe2x80x99 the grating into the fiber defined the precision of the grating structure in this experiment, which places significant practical limits to the length and accuracy with which such an array could be written, as well as to the flexibility with which gratings with different codes can be written. With this approach [10], [20] a single phase mask is specially fabricated for writing a particular OCDMA signature, the signature being made up of a specific chip sequence. It is therefore necessary to fabricate one phase mask for each coding and decoding signature.
A better way of fabricating optical waveguide gratings incorporating bipolar or higher order multipolar OCDMA signatures is therefore desired.
According to one aspect of the invention there is provided a method of fabricating an optical waveguide grating for encoding or decoding an optical signal by writing a succession of grating sections into a photosensitive waveguide, each grating section representing a chip of a code signature, the method comprising:
(a) writing a first grating section into the photosensitive waveguide by repeatedly exposing an inscription beam having a periodic intensity pattern onto a first length of the waveguide and moving the inscription beam relative to the waveguide between successive exposures or groups of exposures, such that the first grating section comprises a plurality of grating lines, each of at least a majority of which is produced by multiple exposures; and
(b) writing further grating sections into further lengths of waveguide, each further grating section either being in phase with, or having a predetermined phase shift relative to, the preceding grating section, depending on whether the code signature has a change in polarity between chips.
The above-described continuous grating writing method allows essentially continuous amplitude and phase control along an individual grating structure. The technique is far more flexible from a fabrication perspective than other techniques so far demonstrated, in particular techniques which require fabrication of special phase masks incorporating structure needed to reproduce a multi-chip code, e.g. an OCDMA code, packet header code, of tens or hundreds of chips. A special phase mask would have to be fabricated for each code/decode pair, which clearly becomes increasingly expensive and inconvenient as the number of codes increases, as it will do as OCDMA and optical packet switching technology matures.
The proposed approach therefore allows for a far broader range of codes, and potential coding schemes. Most significantly it is also not bounded by the current resolution limits and device lengths imposed by phase mask technology and offers great potential for the production of low cost devices. Specific examples of codes with 63 chips are presented. Longer codes of 128 chips have also been successfully fabricated indicating that the novel fabrication method can provide the large code length gratings demanded by future applications.
In embodiments of the invention, the predetermined phase shifts are pi phase shifts. However, other phase shifts could be used. For example, the predetermined phase shifts comprise at least two different phase shifts.
The modulated refractive index profile may have a substantially constant amplitude modulation, thereby to provide multipolar (e.g. bipolar) coding purely through phase modulation, with no amplitude modulation component. Alternatively, coder and decoder gratings may be fabricated with modulation being implemented with phase and amplitude modulation.
As well as bipolar coding, higher level coding may be provided. For example, quadrupolar coding may be provided with quaternary phase shift keying (QPSK).
The photosensitive optical waveguide is preferably an optical fiber, but may be a solid state device such as a planar waveguide.
The code signature may be any number of chips to provide the desired number of independent user codes. For example, the number of chips may be at least 10, 20, 30, 40, 50, 60. Specifically, the number of chips may be at least 63 chips.
In some embodiments, the code signature is written in NRZ format with phase continuity in the refractive index modulation profile between adjoining grating sections representing adjacent chips of like polarity.
In other embodiments, the code signature is written in RZ format. This may be done by suitable manipulation of amplitude or phase in the refractive index modulation profile between adjacent chips of like polarity.
According to another aspect of the invention there is provided a grating for encoding or decoding optical signals, comprising a photosensitive optical waveguide with a modulated refractive index profile comprising a plurality of sections representing chips of a code signature, characterized in that changes in polarity between chips are implemented by pi phase shifts in the modulated refractive index profile, thereby to provide multipolar coding through phase modulation.
According to a further aspect of the invention there is provided a grating for encoding or decoding optical signals, comprising a photosensitive optical waveguide with a modulated refractive index profile comprising a plurality of sections representing chips of a code signature, characterized in that the grating has at least 10, 20, 30, 40, 50, 60 or 63 chips. The gratings may be unipolar or bipolar.
The apparatus and method can also include one or more of the following features:
1. Incorporation of both dispersion-compensating and encoding or decoding gratings into a single superstructure grating.
2. Addition of multiple codes within a single gratingxe2x80x94for example two codes at different central wavelengths.
3. Further extension of either the grating length or reduction in chip size to increase the code length to codes of greater than 5000 chips, or more, allowing rapid increases in simultaneous users.
4. More complex superstructure profiles including amplitude and phase features to shape controllably the individual chip shapes.
5. Incorporation of simultaneous additional, multiple functionality with a single grating (decoding or coding) structures e.g. loss compensation and dispersion compensation (2nd and 3rd order).
6. The apparatus may be reconfigured such that the superstructure grating as above is used in transmission mode rather than reflective mode.
7. To use higher reflectivity versions of the decoder/coder gratings designed using more advanced design algorithms (e.g. inverse scattering techniques) other than by the Fourier approach.
8. To use cascades of one or more code/decode gratings.
9. Use advanced codes such as those developed by the mobile-communications community for optimized correlation function definition e.g. M-sequences, Gold sequences or Kasami codes.
10. Use a combination of a decoder grating and nonlinear element such as a semiconductor optical amplifier or fiber-based nonlinear switch to enhance the correlation contrast and effect further enhanced processing functions such as optical routing, header removal and rewrite, data packet loading.
11. Use parallel arrays of coder-decoder gratings to enhance multi-user operation.
12. Use of coder/decoder approach to allow reduction of nonlinear optical effects by extending the bit duration in the time domain, thereby reducing optical intensities.
13. Use superstructure gratings to shape optical pulses (that may be of non-optimal form) for a given transmission technique or optical processing function to a more-desirable functional form for onward transmission or processing, e.g. chirped pulse to transform limited pulse conversion, soliton to super-Gaussian pulses, soliton to dispersion solitons, Gaussian pulses to square pulses.
14. Extend the grating bandwidths of code-decode grating to up to 200 nm or further.
15. Extend technique to other wavelength regimes in the range 700 nm to 2000 nm or further.
16. Extend the superstructure decoding technique to correlate (provide matched filtering) directly with the output from a modulated optical source. For example the source can be a directly modulated gain-switch diode, and externally modulated DFB laser, a mode-locked fiber ring laser with external modulation.
17. Addition of wavelength division multiplexers and demultiplexers such as arrayed waveguide gratings to facilitate multi-wavelength operation, with one or more wavelengths being operated under the code-division multiplexing technique described previously.
18. Operation of the system with synchronous transmitters and receivers.
19. Operation of the system with asynchronous transmitters and receivers.
20. Operation of the system with a combination of synchronous and asynchronous transmitters and receivers.
Aspects of this invention include a grating for use in code division multiplexing (CDM) system architectures, a method of using gratings in CDM systems architectures, a CDM architecture for optical communications, or a combined CDM and WDM system architecture for optical communications.
By CDM we mean not only code-division multiplexing but also include ultrafast packet-switched, or other OTDM networks or transmissions systems.
In conclusion:
Superstructure FBG technology enables high coding/decoding performance.
Flexibility in code design and device fabricationxe2x80x94code profile is determined by appropriate UV exposure, not phase mask.
Direct comparison of unipolar vs. bipolar operation.
Error-free 10 Gbit/s pulse coding/decoding over 25 km of single mode fiber (SMF) with 160 Gchip/s code.
255-chip or longer codes possible using cm-long FBG""s and shorter chip durations.
Applications: OCDMA, header recognition in packet-switched networks, etc.
In the following detailed description, results are presented from specific examples of bipolar OCDMA, which show higher data rates (10 Gbit/s), shorter chip-lengths (6.4 ps) and far longer code sequences (63 bits) than previously demonstrated. To highlight the dramatic improvements achieved with the novel fabrication process for bipolar OCDMA gratings, results are also presented for comparable unipolar structures.