GB2269953A

GB2269953A - Optical transmission system

Info

Publication number: GB2269953A
Application number: GB9217496A
Authority: GB
Inventors: Richard Edward Epworth
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1992-08-18
Filing date: 1992-08-18
Publication date: 1994-02-23
Anticipated expiration: 2012-08-18
Also published as: GB9217496D0; GB2269953B

Abstract

A bit stream at a bit rate N.R is shared by a distributor (31) between N sources (32, 33) each operated at a bit rate R to develop chirped optical bits. The outputs of the N sources are multiplexed (34) on to a common dispersive transmission path (35) and propagate to a receiver (36). The time-elated frequency spectrum of the chirped optical bits is such that, though the bits are multiplexed in partially overlapping relationship, the dispersion of the transmission path serves to compress them entirely removing the overlap by the time they reach the receiver. <IMAGE>

Description

OPTICAL TRANSMISSION SYSTEM This invention relates to optical digital transmission systems utilising dispersive transmission paths. When using an optical signal of finite spectral width in such systems, a consequence of the dispersion is to give rise to pulse spreading effects that set an upper limit to the maximum bit rate that can be transmitted over a given distance. Reference has been made to pulse spreading effects of dispersion, but in certain instances dispersion can have the effect of pulse compression. The present invention is directed to a transmission system which takes advantages of these pulse compression effects in the provision of a high bit rate system operating over a dispersive transmission path.

According to the present invention there is provided a method of digital transmission from an optical transmitter to an optical receiver over an optically dispersive transmission path, wherein successive bits of a data stream applied to the transmitter are distributed in the transmitter between a set of N optical sources whose outputs are multiplexed on to the transmission path, each of which sources is adapted to produce, in response to the application thereto of a high level bit, a chirped optical output of not more than N bit periods, wherein said chirped output has a time-related frequency spectrum such that it is compressed in time by its passage to the receiver over said transmission path to arrive at the receiver with a duration not exceeding a single bit period.

The invention also provides a digital transmission system having an optically dispersive transmission path from an optical transmitter to an optical receiver, wherein the optical transmitter includes a set of N optical sources optically multiplexed on to the transmission path, and a distributor adapted to distribute the bits of a digital data stream applied to the transmitter between the sources, and wherein associated with the set of N sources is a set of N drivers each adapted, in response to the input thereto of a single high-level bit, to drive its associated source as as to produce a chirped optical output of not more than N bit periods with a time-related frequency spectrum such that the output is compressed in time by its passage to the receiver over the transmission path to arrive at the receiver with a duration not exceeding a single bit period.

An embodiment of the invention will now be described together with a description of its principles of operation with reference to the accompanying drawings in which: Figure 1 is a diagram illustrating the conversion of a short-duration optical pulse of finite spectral width into a chirped pulse of extended duration by its passage through a dispersive transmission path.

Fig. 2 is a diagram illustrating the reciprocal process in which a chirped pulse of extended duration is compressed into a shorter duration pulse by its passage through the dispersive transmission path, and Figure 3 is a diagram of a transmission system embodying the invention in a preferred form.

Referring to Figure 1, when a short-duration optical pulse 10 of finite spectral width is launched into an optically dispersive transmission path, constituted for instance by a length of single mode optical fibre and represented in Figure 1 by a coil of fibre 11, the signal 12 emerging from the far end of that fibre is chirped as the result of the different frequency components of the initial pulse 10 having been propagated through the fibre with slightly different speeds. In Figure 1 this chirping is represented diagramatically by a waveform depicting the higher frequency components as arriving slightly ahead in time of the lower frequency components.

As a result of the reciprocal nature of light propagation, it is evident that a chirped pulse 20 (Figure 2), which is the inverse of the chirped pulse 12, will propagate through the fibre to produce a compressed pulse 22 which is the inverse of pulse 10. Thus although the usual effect of transmitting a pulse down a dispersive transmission path is to cause pulse spreading, with appropriate choice of a chirped pulse having the appropriate time-related frequency spectrum for the particular dispersive system involved, it is possible to arrange matters such that pulse compression takes place.The time-related spectral content of chirped pulse 12 of Figure 1 can in principle be determined theoretically from knowledge of the dispersion characteristics of fibre 11, but generally is more conveniently determined by practical analysis of what is in fact produced in response to the launching of an actual pulse 10. Once the form of chirped pulse 12 is known, the form of its inverse, chirped pulse 20, is also known. The relationship between the drive current and the emission frequency of an injection laser can readily be established, and hence the form of the chirped pulse 20 can readily be generated by an injection laser by choice of an appropriate profile of drive current pulse.

Attention is now turned to the transmission system of Figure 3. At the transmitter of this system data a bit rate N.R applied to an input terminal 30 is shared by a distributor 31 between a set of N laser drivers 32 so that each individual lase driver receives data at a bit rate R. An individual injection laser 33 is associated with each laser driver 32, and the optical outputs of all N lasers are applied to an optical multiplexer 34 which combines them all on to a single mode optical fibre dispersive transmission path 35 extending to a receiver 36.

Using the principles described above with reference to Figures 1 and 2, each laser driver 32 is arranged to provide a profile of laser drive current which has a duration not exceeding one bit period for a bit rate R, and which causes its associated laser to emit an optical output pulse with a time-related frequency spectrum such that the pulse is compressed in its passage through the fibre transmission path 35 to the detector 36 to a duration not exceeding one bit period for a bit rate N.R.

In this way the bits of the bit stream applied to the terminal 30 are divided by distributor 31 into separate components which are expanded in duration by drivers 32. The bits are next recombined in partially overlapping form by the multiplexer 34 and then, in their passage through the fibre 35, they are reduced in duration and separated so that they are no longer overlapping.

An alternative to applying the data to modulate the output of the drivers 32, is to arrange to derive a clock signal to operate the drivers 32 so that each laser 33 produces a chirped optical output at each one of its bit periods (at bit rate R), and then to employ an associated modulator (not shown) to amplitude modulate that output with the appropriate fraction of the applied data as shared out with the aid of a second distributor (not shown).

It will be appreciated that the rising and falling edges, respectively at the beginning and end of a chirped pulse, will produce their own chirp profiles. These profiles can be rendered insignificant to the system, either by arranging for the rising and falling edges to be steep enough to distribute the power widely across the spectrum so that only a very small proportion lies within the system's pass band, or by including a set of optical modulators, (not shown), one associated with each laser 33, which is operated to gate the laser's output during the rising and falling edges of its chirped pulses.

Claims

1. A method of digital transmission from an optical transmitter to an optical receiver over an optically dispersive transmission path, wherein successive bits of a data stream applied to the transmitter are distributed in the transmitter between a set of N optical sources whose outputs are multiplexed on to the transmission path, each of which sources is adapted to produce, in response to the application thereto of a high level bit, a chirped optical output of not more than N bit periods, wherein said chirped output has a time-related frequency spectrum such that it is compressed in time by its passage to the receiver over said transmission path to arrive at the receiver with a duration not exceeding a single bit period.

2. A method of digital transmission from an optical transmitter to an optical receiver which method is substantially as hereinbefore described with reference to the accompanying drawings.

3. A digital transmission system having an optically dispersive transmission path from an optical transmitter to an optical receiver, wherein the optical transmitter includes a set of N optical sources optically multiplexed on to the transmission path, and a distributor adapted to distribute the bits of a digital data stream applied to the transmitter between the sources, and wherein associated with the set of N sources is a set of N drivers each adapted, in response to the input thereto of a single high-level bit, to drive its associated source as as to produce a chirped optical output of not more than N bit periods with a time-related frequency spectrum such that the output is compressed in time by its passage to the receiver over the transmission path to arrive at the receiver with a duration not exceeding a single bit period.

4. A digital transmission system substantially as hereinbefore described with reference to the accompanying drawings.