GB2257319A

GB2257319A - Direct demodulation of optical binary fsk signals.

Info

Publication number: GB2257319A
Application number: GB9113938A
Authority: GB
Inventors: Jonathan Paul King
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1991-06-27
Filing date: 1991-06-27
Publication date: 1993-01-06
Anticipated expiration: 2011-06-27
Also published as: GB9113938D0; GB2257319B

Abstract

A self heterodyne direct demodulation arrangement for differentially encoded optical binary FSK signals comprising a two-path optical interferometer (23) having a path unbalance optical delay (21) not exceeding FSK bit period, means for applying the FSK signals to both paths of the interferometer, and means for detecting (26-1, 26-2, 28) the optical beat frequency of the interferometer output. <IMAGE>

Description

DIRECT DEMODULATION OF OPTICAL BINARY FSK SIGNALS This invention relates to direct demodulation of differentially encoded optical binary frequency shift keyed (FSK) signals in transmission system.

It is known that many types of semiconductor laser can be operated continuously above the lasing threshold with substantially constant output power 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 (FSK) 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.

The term 'differential encoded optical binary FSK' as used herein defines encoding arrangements in which binary data of one value, e.g. '1', is represented by a change from one logic level or optical frequency to another while the other binary data value, i.e. 'O', is represented by an absence of change in logic level or optical frequency. For example, if non-return-to-zero (NRZ) binary data is clocked with a square waveform clock running at the bit frequency but, say, 900 out of phase therewith, a form of differentially encoded FSK signal is produced, wherein 'l's are represented by a step change from either logic level to the other logic level at the mid-point of each bit period while 'O's are represented by the absence of change from either logic level, whichever logic level is prevailing at the commencement of the bit period.Likewise, if the binary NRZ waveform then binary '1' is represented by ramping up or down from one logic level to the other while binary '0' is again represented by an absence of change from whichever logic level prevailed at the commencement of the bit period. Alternatively, a well known form of encoding is that known as "return-to-zero" (TZ) in which binary 'l's are represented by a square wave pulse of duration e.g. 1/2 bit period, of high logic level followed by 1/2 bit period of low logic level, whereas binary 'O's are at all times represented by a period of full bit duration of low logic level.

Hitherto self heterodyne demodulation of FSK optical signals has required either a phase locked optical discriminator or absolute frequency control of the transmitter laser. This latter option in turn requires complex feedback circuitry to compensate for temperature dependent frequency drift.

The present invention seeks to provide a self heterodyne direct demodulation arrangement for FSK modulated signals which does not require a phase locked optical discriminator or absolute frequency control of the transmitter laser.

According to the invention there is provided a self heterodyne direct demodulation arrangement for differentially encoded optical binary FSK signals (as hereinbefore defined) comprising a two-path optical interferometer having a path unbalance optical delay not exceeding FSK bit period, means for applying the FSK signals to both paths of the interferometer, and means for detecting the optical beat frequency of the interferometer output.

The above and other features of the invention and embodiments thereof will now be described with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of a transmitter arrangement for producing differentially encoded optical binary FSK signals, Fig. 2 is a schematic diagram of a receiver for the signals produced by the transmitter of Fig. 1, Fig 3 illustrates various waveforms associated with the transmitter and receiver of Figs. 1 and 2, Fig. 4 is a schematic diagram of an alternative transmitter arrangement to that of Fig. 1 for producing differentially encoded signals, Fig. 5 is a schematic diagram of a receiver for the signals produced by the transmitter of Fig.

4, Fig. 6 illustrates various waveforms associated with the transmitter and receiver of Figs. 4 and 5, Fig. 7 is a schematic diagram of a transmitter arrangement for producing return-to-zero encoded optical FSK signals, Fig. 8 is a schematic diagram of a receiver for the signals produced by the transmitter of Fig.

7, and Fig. 9 illustrates various waveforms associated with the transmitter and receiver of Figs. 7 and 8.

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 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 shows a 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 change.

At the receiver the transmission system fibre 20 carrying the incoming differentially encoded FSK signal is connected to one input port of a first 2x2 optical coupler 22. The two output ports of coupler 22 are connected to respective input ports of a second 2x2 coupler 24. The two output ports of coupler 24 are coupled to respective photodetectors 26-1, 26-2 connected in a push-pull or balanced detection arrangement, the output of which feeds, via bandpass filter 28, conventional detector arrangement (not shown). One of the connection paths between coupler 22 and coupler 24 includes an optical delay 21 of l-bit (nominal) duration. Thus couplers 22 and 24 and differing length paths connecting them together form a Mach-Zehnder interferometer 23 having a path imbalance of one bit period.The incoming signal will therefore interfere with a l-bit delayed version of itself (waveform d in Fig. 3), so that the frequency shift resulting from the encoding process at the transmitter is converted to a beat frequency at the output of the photodetectors (waveform e in Fig. 3). This beat frequency (or IF) is then detected in known manner to reconstitute the original data stream. The balanced photodetectors followed by a bandpass filter in effect form a conventional tuned receiver.

The absolute optical frequency of the transmitter laser is not critical, because it is the relative change in optical frequency between the delayed and undelayed signal in the receiver which determines whether the IF falls within the receiver passband.

Similarly, transmitter laser frequency drift is not critical provided it does not drift so much that it forces the IF totally outside the receiver passband.

In the alternative arrangement shown in Fig. 4, the raw data is not toggled with a square wave as in Fig. 1, instead it is used to drive an alternate positive/negative slope ramp waveform generator 40.

Thus the data stream (waveform a in Fig. 6) causes the laser drive current, and hence the output optical frequency of the laser 42, to ramp up and ramp down on alternate binary 'l's, while for binary '0's there is no change in the drive current level existing at the end of the previous bit period (waveform b in Fig. 6). In the receiver, Fig. 5, configuration is basically the same as that in Fig. 2 except that the optical path delay in this case is much less than 1 bit period (waveform c in Fig. 6). This results in the IF st the photodetector output (waveform d in Fig. 6) being smaller than the optical frequency excursion in the transmitter laser.

This is advantageous where the laser linewidth is a significant fraction of the IF.

In the further alternative arrangement of Fig.

7 no toggling or ramping of the laser drive current is used. Instead, if the original binary data is presented in the well-known return-to-zero (RZ) format code (waveform a in Fig. 9) the laser drive current of laser 70 is directly modulated by the data such that binary 'l's cause the current to have a high level for the first half of the bit period while binary '0's cause the current to remain at a low level throughout the bit period. The laser output frequency will thus be high for the first half of each '1' bit period and low for the rest of the time (waveform b in Fig. 9). The receiver configuration is again the same as that of Fig.

1 but the optical path delay is now equal to approximately one half of a bit period. Greater bandwidth in the receiver is required than in the receiver of Fig. 3 or Fig. 6, otherwise the configuration is the same. However, RZ-FSK does allow a higher linewidth laser to be used in the transmitter.

The delayed signal (waveform c in Fig. 9) is again interfered with the undelayed signal (waveform b in Fig.

9) and beat signal output at the photodetectors will appear as full bit-period twists of IF (waveform d in Fig. 9). After normal IF detection, demodulation and baseband filtering the original data will be reconstituted as conventional binary full bit-period 'l's and '0's (waveform e in Fig. 9).

Claims

CLAIMS:

1. A self heterodyne direct demodulation arrangement for differentially encoded optical binary FSK signals (as hereinbefore defined) comprising a two-path optical interferometer having a path unbalance optical delay not exceeding FSK bit period, means for applying the FSK signals to both paths of the interferometer, and means for detecting the optical beat frequency of the interferometer output.

2. An arrangement according to claim 1 for demodulation of FSK signals in which binary data of the one value is represented by a step change from either logic level to the other which data of the other value is represented by an absence of change from either logic level, wherein the path unbalance optical delay is of l-bit period (nominal) duration.

3. An arrangement according to claim 1 for demodulation of FSK signals in which binary data of the one value is represented by a ramped change from either logic level to the other while data of the other value is represented by an absence of change from either logic level, wherein the path unbalanced optical delay is of less than l-bit duration.

4. An arrangement according to claim 1 for demodulation of FSK signals in which binary data of the one value is represented by a square wave pulse of high logic level of duration of 1/2-bit period followed by 1/2-bit period of low logic level while data of the other value is represented by a l-bit period of low logic level, wherein the path unbalance optical delay is of 1/2-bit period (nominal) duration.

5. An arrangement according to any one of claims 1-4 wherein the two-path optical interferometer comprises a first 2 x 2 optical fibre coupler, means for applying to one input of the first coupler the FSK signals to be demodulated, a second 2 x 2 optical fibre coupler, the two output ports of the first coupler being connected to respective input ports of the second coupler, a pair of photodetectors connected in a push-pull balanced detection arrangement, the photodetectors being coupled to respective output ports of the second coupler and tuned receiver means to which the output of the balanced detection arrangement is applied, one of the connections between the first and second couplers including said optical delay.

6. A self heterodyne direct demodulation arrangement substantially as described with reference to Fig. 2 or Fig. 5 or Fig. 8 of the accompanying drawings.