Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-08-30
2003-04-29
Mai, Huy (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S199200, C359S199200
Reexamination Certificate
active
06556335
ABSTRACT:
The present invention relates to an apparatus and method for optically processing optical signals for the purposes of detecting the presence of a digital bit pattern therein.
In a wide range of applications in optical data transmission it is required to detect a particular digital bit pattern within a high bit-rate optical signal. An example of this is the identification of a packet header in an optically transmitted data packet (e.g. an IP datagram), for subsequent switching or routing of that data packet. Ideally, this identification should be performed optically, that is to say, without recourse to opto-electrical conversion of the packet header.
It is well known that the act of converting an optical packet header into the electrical domain, prior to processing (e.g. routing or switching) of that optical packet, reduces the overall speed of processing. In particular the routing performance of IP routers is degraded when opto-electrical packet header identification is required and results in a transmission bottleneck at the router.
The bottleneck results at least in part from the time required to perform opto-electrical conversion of a given packet header and, as a consequence of this, the header of an optical data packet typically has to be transmitted at a lower bit rate than the payload with which it is associated. The lower bit rate of the header allows more time to be spent in reading the header data opto-electrically, but occupies a relatively large percentage of the optical packet slot thereby lowering bandwidth efficiency.
Clearly, the all-optical identification of optical packet headers would be advantageous in overcoming the aforementioned deficiencies in existing techniques.
One such all-optical technique that has been proposed employs the phase-modulation (binary shift-keying) of the optical carrier wave of a data packet to map digital routing information onto the header of that packet. In particular, the optical carrier wave associated with the header part of an optical data packet is phase-shifted by either 0 degrees or 180 degrees over a number of successive intervals (i.e. a string) of the same fixed duration (e.g. 5 ps). This encodes routing information into the optical header in terms of the pattern of the sequence of carrier phases in the aforementioned string of intervals.
A proposed optical processor is then able to recognize such header information by splitting a received phase-modulated header signal into as many copies as there are intervals in the string it seeks to identify. Each copy is then phase shifted in a predetermined manner and then correlated with every other phase-shifted copy. A correlation peak emerges when the received string (sequence of carrier phases) matches the one that the processor seeks to identify. Thus, by performing optical correlation of the phase-modulated radiation associated with a packet header, the processor may identify a predetermined header and control optical routing of the data packet accordingly. However, implementation of such a technique requires the use of optical phase-modulation in the encoding of header data into a data packet. Optical amplitude-modulation of a packet carrier wave is more typically used to encode not only the header data but also the payload data of most optical data packets in the art. Clearly, the aforementioned optical processor would therefore be unable to process such amplitude-modulated headers. Furthermore, optical phase-modulation tends to be difficult to control and the phase-shifters employed in the proposed optical processor are typically highly sensitive to bit-rate changes in the digital data that phase-modulation is being used to convey. Amplitude modulators tend to be more robust to bit-rate changes, are simple to control and are often cheaper to implement.
Consequently, a need has been identified for an optical signal processor able to recognize the presence of a digitally amplitude-modulated optical signal, in particular an amplitude-modulated optical packet header.
The present invention aims to overcome at least some of the aforementioned deficiencies in the prior art.
According to a first aspect of the present invention there is provided an optical signal processor for the optical processing of a digitally amplitude-modulated optical signal input thereto, to determine the presence or absence of a predetermined digital bit-pattern within said optical signal, the optical signal processor possessing optical processing means including;
signal duplicating means operable to derive from said input digital optical signal one or more duplicate digital optical signals which are each a duplicate of the digital bit-patterns present within the input optical signal;
signal duplicating and inverting means operable to derive from said input digital optical signal one or more polarity-inverted duplicate digital optical signals, the polarity of each bit of which is the inverse of that of the signal bit from which it is derived, wherein;
the optical processing means is operable to derive from said duplicate signals and said polarity-inverted duplicate signals an optical detection signal which indicates the presence or absence of said predetermined amplitude-modulated digital bit-pattern within the input digital signal processed thereby.
Thus, the present invention according to its first aspect provides a means via which digitally amplitude-modulated optical bit-patterns, such as the header of an optical data packet, may be detected within an optical signal without recourse to prior opto-electrical conversion thereof. Clearly, by obviating the need to convert optical signals into equivalent electrical signals prior to signal processing/detection, the present invention enables a substantial reduction in the delay associated with detection of amplitude-modulated optical bit-patterns.
Furthermore, the present invention obviates the need to employ the techniques of optical phase-modulation to optically encode digital data in an attempt to permit detection thereof without opto-electrical conversion. Consequently, the present invention provides an optical processor which aims to overcome at least some of the drawbacks and limitations inherent in employing such phase-modulation.
Preferably, the optical processing means of the present invention according to its first aspect, further includes optical transmission delay means and is operable to transmit through said delay means said duplicate signals and said polarity-inverted duplicate signals derived from said input digital optical signal, wherein;
the optical processing means is operable to derive said optical detection signal from a combination of said duplicate signals and said polarity-inverted duplicate signals output of said optical delay means.
These optical delay means may preferably comprise optical fibre delay lines through which optical signal radiation, derived from the input digital signal, may be transmitted. The delay means may alternatively or additionally include one or more fibre recirculating loops whereby signal radiation is circulated within a fibre loop until sufficient delay has been accumulated whereupon the signal is output of the loop (gated), as is known in the art. Furthermore, the delay means may employ optical memory means (write/read) such as regenerative loop memories.
Furthermore, it is preferable that the signal duplicating means and the signal duplicating and inverting means are operable to produce one or more duplicate signals (polarity-inverted or otherwise) each one of which is conveyed via optical radiation of a wavelength different from that of the radiation conveying any of the other signal duplicates produced thereby. Alternatively, two or more (e.g. all) of the duplicate signals produced by any one (or both) of the signal duplicating means and the signal duplicating and inverting means, may be of the same wavelength.
The optical processing means preferably includes a plurality of separate optical transmission delay means and is operable to transmit respective ones of said duplicate signals and said polarity
Lee Heng Loong
Simeonidou Dimitra
Alcatel
Mai Huy
Sughrue & Mion, PLLC
Tra Tuyen
LandOfFree
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