GB2256761A

GB2256761A - Fsk direct detection transmission system.

Info

Publication number: GB2256761A
Application number: GB9112870A
Authority: GB
Inventors: Jonathan Paul King
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1991-06-14
Filing date: 1991-06-14
Publication date: 1992-12-16
Also published as: GB9112870D0

Abstract

An arrangement for demodulation of differentially encoded binary optical FSK signals including a path unbalanced optical interferometer (28) to which the FSK signals are applied, a multiport coupler (22) to which the output of the interferometer is applied, a plurality of optoelectronic receivers (24) to each of which the output of a respective output port of the coupler is applied, means for multiplying (26) the output of each receiver with a 1-bit delayed version of the same output and means for summing (30) the multiplied outputs. <IMAGE>

Description

FSK Direct Detection Transmission System This invention relates to the transmission and detection of digital optical signals in a form of differentially encoded (as hereinafter defined) a binary optical frequency shift Keyed (FSK) signals.

It is known that many types of semiconductor laser can be operated above the lasing threshold with a substantially constant optical power output the optical frequency of which can be changed by changing the amplitude of the laser drive current. Thus modulation of the drive current between two fixed amplitude levels can provide a frequency shift keyed optical output in which the two optical frequencies are the equivalent of two logic levels and can be used for representing binary data according to the form of encoding used.

Differential encoding of a binary digital optical FSK signal as referred to herein is defined, as encoding of a binary data signal wherein binary digits of one value, e.g. '1', are represented by a change from one logic level or optical frequency to another or vice versa, and binary digits of the other value, e.g. 'O', are represented by an absence of change from either logic level or optical frequency to the other level.

Transitions representing one value, e.g. '1' can occur either at the commencement of a bit period or at some other predetermined time during the '1' bit period. No timing is involved during '0' bit periods..

Thus a succession of '1' bits will result in an FSK signal wherein the optical frequency alternates between two optical frequencies at the bit rate while for a succession of '0' bits the optical frequency will remain constant, whichever of the two frequencies is prevailing at the commencement of the succession of '0' bits.

Coherent detection of amplitude shift keyed optical signals is known from British patent 2172766 B.

In the arrangement disclosed therein the received signals are applied to one input port of a multiport coupler, e.g. a 3x3 coupler, with a second input port receiving a local oscillator signal of substantially the same optical frequency as the received signals. The output ports of a multiport coupler provide separate output signals that are differentially related to the optical phase difference between the modulated received signal and the local oscillator, which local oscillator signal may be generated under control of a feedback signal from one of the coupler outputs. The separate output signals all carry the same baseband modulation but with different relative phase shifts. The separate outputs are individually demodulated and the demodulated signals are combined to form the receiver output.

According to the present invention there is provided an arrangement for demodulation of differentially encoded binary optical FSK signals (as hereinbefore defined) including an optical splitter which divides incoming light between two unbalanced paths to which the FSK signals are applied, a multiport optical mixer to which the outputs of the unbalanced paths are applied, a plurality of optoelectronic receivers to each of which the output of a respective output port of the optical mixer is applied, means for processing the output of each receiver and means for summing the processed outputs.

The above and other features of the invention will be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of an arrangement for transmitting differentially encoded binary optical FSK signals, and Fig. 2 is a schematic diagram of an arrangement for demodulating differentially encoded binary optical FSK signals.

Fig. 3 illustrates various waveforms in the arrangements of Figs. 1 and 2.

To transmit differentially encoded binary optical FSK signals over an optical fibre transmission system an arrangement such as that shown in Fig. 1 can be used. Conventional binary encoded data (waveform 'a' in Fig. 3) is input to a bistable toggle switch 10 to switch the drive current of a laser 12 between two levels. The laser is operated in a continuous mode, switching of the drive current between the two levels serves to cause the output optical frequency to change between two nominally fixed values (excluding any gradual changes in optical frequencies due to ageing, temperature changes etc.). Although there is a detectable difference ss f in optical frequency due to the toggle switching of the drive current the optical power output of the laser will remain substantially constant at all times, at least for practical purposes.

Thus the toggle switch 10 will operate, once for every binary value '1' in the input signal and will not operate when a binary value '0' occurs. The drive current applied to the laser will therefore change from whichever of the two values is existing to the other value each time a binary '1' occurs (waveform 'b' in Fig. 3). The laser output frequency changes accordingly (waveform 'c' in Fig. 3). Note that the toggle action is shown as effective in the centre of the '1' bit period; the timing is not crucial so long as the drive current showsa change once for each '1' bit and no change for each '0' bit. Note also that waveform '0' in Fig. 3 is drawn (with exaggeration) to show a downward drift in the laser output frequencies due to temperature drift or ageing etc.

At the receiver the transmission system fibre is coupled to one input port of a 2x2 or 50:50 optical coupler 20. The two outputs of coupler 20 are coupled to two of the inputs of a 3x3 coupler 22. One of the outputs of the coupler 20 includes an optical delay 1, the value of the delay being sufficient to cause optimum discrimination of the change A f in the optical frequency; the arrangement of the couplers and the one path delay constituting a Mach-Zehnder interferometer 28. The three output ports of the 3x3 coupler 22 are each fed to a respective optoelectric receiver 24-1, 24-2 and 24-3. Each receiver output (waveforms d, e, f in Fig. 3) is then applied to a respective multiplier 26-1, 26-2 and 26-3 and is multiplied with a version of the same receiver output subjected to a delay lr equal to 1 bit period (waveform g in Fig. 3). Finally the three multiplier outputs (waveforms h, i, j in Fig. 3) are summed in summing network 30 to form the demodulated binary data stream (waveform k in Fig. 3). It can be seen that although the data amplitude on any one of the three multiplier outputs may still fade at any given time the data on all the outputs always has the same binary value polarity, i.e. an input data '1' (waveform a) always corresponds to a negative going pulse from the multiplier output (waveform h). Because the three signals of the 3x3 coupler 22 have a relative optical phase shift of 1200 to one another the data fading from each multiplier will be 1200 out of phase relative to the other two outputs. Therefore only simple summing of the three multiplier outputs is required to reconstruct the original input data stream having a constant amplitude waveform. This arrangement using the 3x3 coupler counteracts any frequency drift of the transmitter laser (as indicated in waveform c in Fig. 3) or phase drift of the Mach-Zehnder path imbalance, since fading amplitude at any one port is compensated by increasing amplitude at the other two ports. Note that in situations where fading can lead to actual inversion of the data values at one port, the use of the 'toggled' FSK together with the 'delay and multiply' port detection processing will nullify the inversion effects caused by fading.

Claims

CLAIMS.

1. An arrangement for demodulation of differentially encoded binary optical FSK signals (as hereinbefore defined) including an optical splitter which divides incoming light between two unbalanced paths to which the FSK signals are applied, a multiport optical mixer to which the outputs of the unbalanced paths are applied, a plurality of optoelectronic receivers to each of which the output of a respective output port of the optical mixer is applied, means for processing the output of each receiver and means for summing the processed outputs.

2. An arrangement according to claim 1 wherein the processing consists of multiplying the output of each receiver with a 1 bit delayed version of the same output.

3. An arrangement according to claim 1 wherein the processing consists of subtracting the output of each receiver from a 1 bit delayed version of the same output and rectifying the resulting output.

4. An arrangement according to claim 1, 2 or 3 wherein the unbalanced paths are incorporated in an optical interferometer.

5. An arrangement according to any preceding claim wherein the path unbalance of the optical interferometer is equivalent to an optical delay of value to cause maximum discrimination between the two FSK frequencies.

6. An arrangement according to any preceding claim wherein the multiport optical mixer is a 3x3 coupler.

7. An arrangement according claim 6 wherein the coupling between the optical interferometer and the multiport coupler is effected by connection of the two paths of the interferometer to respective separate input ports of the multiport coupler.

8. An arrangement for demodulation of differentially encoded binary optical FSK signals (as hereinbefore defined) substantially as described with reference to the accompanying drawings.