GB2237468A - Optical transmission process and system - Google Patents

Abstract

In a frequency shift keyed direct detection (FSKDD) optical (e.g. infra-red) transmission system, a first light source 21 is intensity modulated with a lower frequency part 25 of an information signal and a second light source 22 is frequency modulated with a higher frequency part 24 of that signal. The frequency modulation is then converted to intensity modulation to provide a composite intensity modulated signal carrying both the high and low frequency parts. The technique overcomes the problem of poor low frequency response of solid state light sources. The conversion between modulation types may be effected in the high frequency branch of the transmitter or at the receiver (Fig.2). <IMAGE>

Description

OPTICAL TRANSMISSION PROCESS AND SYSTEM This invention relates to the transmission of signals on an optical carrier, and in particular to transmission via frequency shift keying (FSK) modulation of the carrier. Conveniently, the modulated carrier may be transmitted via a fibre optic transmission path.

Frequency shift keying is a form of frequency modulation in which the modulating wave shifts a carrier frequency between predetermined values. The technique is finding increasing use in fibre optic transmission systems, particularly in submarine applications. A preferred form of frequency shift keying is that using direct detection of the modulation to regenerate an intensity modulated carrier. Such a technique is described for example in our UK specification No. 2 107 147B. In a frequency shift keying direct detection (FSKDD) system, signals are transmitted on a carrier by frequency modulation of that carrier. A frequency discriminator is used to convert the frequency modulation to a corresponding intensity modulated signal which is then demodulated to recover the transmitted signal.Conventionally, the frequency modulated signal is generated via a solid state injection laser whose output spectrum is determined by the level of the signal drive applied thereto. Problems have been experienced with many semiconductor lasers as their low frequency FM response is unstable. This has restricted transmission of the lower frequency part of the information spectrum.

The object of the present invention is to minimise or to overcome this disadvantage, According to one aspect of the invention there is provided an optical frequency shift keying (FSK) transmission system, including a transmitter having first and second light sources whereby an input signal is modulated on to optical carrier signals, a transmission medium for the modulated signals, a receiver, and optical frequency discrimination means intermediate the transmitter and receiver whereby to convert frequency modulated signals to intensity modulated signals, wherein, in use, the first light source is intensity modulated with the low frequency part of the input signal and the second light source is frequency modulated with the high frequency part of the input signal, both modulated signals being transmitted via the transmission medium and the discrimination means to provide a composite intensity modulated signal at the receiver.

According to another aspect of the invention there is provided a method of transmitting an optical carrier and information signal having a higher frequency part and a lower frequency part, the method including encoding the higher frequency part as a frequency shift keying modulated signal and encoding the lower frequency part as an intensity modulated signal, both modulated signals being transmitted over a common channel.

According to a further aspect of the invention there is provided a method of transmitting on an optical carrier an information signal having a higher frequency part and a lower frequency part, the method including frequency modulating a first light source with the higher frequency part of the information signal, intensity modulating a second light source with the lower frequency part of the information signal, transmitting both modulated signals over a common channel to a receiver, and providing, intermediate the light sources and the receiver, a frequency discriminator whereby to convert the frequency modulated signal to a corresponding intensity modulated signal.

The transmission path may comprise a repeatered or unrepeatered fibre optic link e.g. a submarine fibre optic cable.

It will be understood that the term 'light source' includes devices operating in the visible and infra-red regions of the spectrum. We prefer to operate in the infra-red region of the spectrum where silica optical fibres exhibit minimum dispersion.

The lower frequency parts of the input signal are intensity modulated on one light source thus overcoming the problem of low frequency FM response.

The higher frequency part of the input signal provides FSK modulation of the other light source. When the FSK component of the composite signal is converted or demodulated to an intensity modulated signal, a composite intensity modulated signal containing both the high and low frequency component is provided for detection at the receiver whereby to regenerate the original input signal.

Reference is directed to our co-pending application No. (R.E. Epworth 46) of even date which relates to an FSK transmission system in which a single light source is modulated with the higher frequency part of the input signal and in which the lower frequency part of the input signal intensity modulates the high frequency modulated signal path.

Embodiments of the invention will now be determined with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of an optical transmission system employing both frequency shift keying and intensity modulation, and Fig. 2 shows a modified optical transmission system.

Referring to Fig. 1, the transmission system includes a transmitter station 11, a receiver station 12 having a transmission path, e.g. a fibre optic link 13, therebetween. The transmitter 11 includes first and second light sources 21, 22, typically semiconductor injection lasers, both of which are coupled to the optical fibre transmission path 13 via a Y coupler 23.

The transmitter station 11 also includes a high pass filter 24 and a low pass filter 25, the output of the filters being coupled to the first light source and second light source respectively. In use, an input signal is applied to both filters 24, 25 so that the lower frequency part of the signal is fed to the first light source 21 and the higher frequency part of the signal is fed to the second light source 22. Typically, in a 1GHz bandwidth system, the cut-off between the higher and lower frequency part of the signal is about 1MHz.

The first light source, to which the lower frequency part of the input signal is applied, is intensity or amplitude modulated by that part of the signal, whilst the second light source is frequency modulated by the other part of the signal, the frequency modulation being converted to intensity modulation. The two intensity modulated optical signals are then fed via the Y-coupler 23 to the fibre optic link and thence to the receiver station 12. Advantageously the coupler 23 is a polarisation splitting coupler. This allows polarised sources to be combined in a single fibre without incurring splitting loss.

An optical frequency discriminator 26 is disposed in the light path between the second light source 22 and that arm of the Y-coupler 23 corresponding to that light source. The discriminator 26 may comprise a Michelson interferometer, a Mach Zehnder interferometer or a Fabry Perot resonant cavity, and is tuned to allow light of one wavelength to pass through.

This converts the frequency modulated signal to a corresponding intensity modulated signal which is then transmitted, simultaneously with the other intensity modulated signal, to the receiver station. The two signals are added incoherently. At the receiver both intensity modulated signals are demodulated together to recover the original broad band input signal.

In some applications the transmitter station including the Y-coupler and discriminator may be provided as a self-contained unit for coupling to an existing fibre optic system.

Fig. 2 shows an alternative transmission system in which conversion of the frequency modulated signal to the corresponding intensity modulated signal is effected at the receiver end of the system. In this arrangement it is important that the discriminator 26 does not corrupt the intensity modulated component of the transmitted signal. This can be prevented by ensuring that the intensity modulated light source is not sufficiently coherent to be demodulated by the discriminator. This condition may be provided by employing a light emitting diode (LED) for the intensity modulated source, or by operating that source below its lasing threshold.

It will be appreciated that where the two light sources have different output frequencies, the two optical carriers will have different propagation delays along a fibre optic transmission path. Compensation for this effect may be provided by introducing a corresponding delay at the input of the appropriate light source in the transmitter. Alternatively, the two transmitted carriers may be provided with synchronising or timing pulses to allow appropriate correction to the effected at the receiver station.

Claims (8)

1. A method of transmitting on an optical carrier an information signal having a higher frequency part and a lower frequency part, the method including encoding the higher frequency part as a frequency shift keying modulated signal and encoding the lower frequency part as an intensity modulated signal, both modulated signals being transmitted over a common channel.

2. A method of transmitting on an optical carrier an information signal having a higher frequency part and a lower frequency part, the method including frequency modulating a first light source with the higher frequency part of the information signal, intensity modulating a second light source with the lower frequency part of the information signal, transmitting both modulated signals over a common channel to a receiver, and providing, intermediate the light sources and the receiver, a frequency discriminator whereby to convert the frequency modulated signal to a corresponding intensity modulated signal.

3. A method of transmitting an information signal substantially as described herein with reference to Fig.

1 or Fig. 2 of the accompanying drawings.

4. An optical frequency shift keying (FSK) transmission system, including a transmitter having first and second light sources whereby input signals are modulated on to optical carrier signals, a transmission medium for the modulated signals, a receiver, and optical frequency discrimination means intermediate the transmitter and receiver whereby to convert frequency modulated signals to intensity modulated signals, wherein, in use, the first light source is intensity modulated with the low frequency part of the input signal and the second light source is frequency modulated with the high frequency part of the input signal, both signals being transmitted via the transmission medium and the discrimination means to provide a composite intensity modulated signal at the receiver.

5. An optical transmission system as claimed in claim 4, wherein said first light source comprises a light emitting diode and said second light source comprises an injection laser.

6. An optical transmission system as claimed in claim 4 or 5, wherein said transmitter includes means for compensating for propagation time differences between the optical carrier signals.

7. An optical transmission system substantially as described herein with reference to and as shown in Fig.

1 or Fig. 2 of the accompanying drawings.

8. An optical transmitter for an information signal having a high frequency part and a lower frequency part, the transmitter including first and second light sources, means for intensity modulating the first source with the lower frequency part of the information signal, means for frequency modulating the second light source with the higher frequency part of the information signal, frequency discrimination means for correcting the frequency modulation of intensity modulation, and means for transmitting both intensity modulated signals to a remote station.