Optical code generation and detection

Details Optical code generation and detection Optical code generation and detection

2001-03-08

2003-09-30

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S024000, C385S001000, C385S002000, C385S003000, C385S031000, C359S199200, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06628864

ABSTRACT:

BACKGROUND OF THE INVENTION
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, ‘single beam’ 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 ‘individual’ 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 ‘imprint’ 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.
SUMMARY OF THE INVENTION
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

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