GB2179517A

GB2179517A - Wideband optical link

Info

Publication number: GB2179517A
Application number: GB8520517A
Authority: GB
Inventors: Edward Brian Butterfield
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-08-15
Filing date: 1985-08-15
Publication date: 1987-03-04
Also published as: GB2179517B; AU6085286A; GB8520517D0

Abstract

A wideband optical link, e.g. for 8 TV channels or a block of stereo audio channels uses a highly linear VCOO (1) to which the multiplex input signal is applied. The VCO output is applied to a mixer (2) where it is mixed with the signal from an oscillator (3). The mixer output goes to a filter (4) which passes the desired mixer's sideband, in this case the lower sideband, and this, via amplifiers (5) and limiter (6) controls the laser (7) whose output is applied to an optical fibre. Thus the highly linear VCO enables a laser of relatively poor linearity to be used. At the receiver (Fig. 2) the signal from an optical receiver (10) goes via a filter (11) to another mixer (12), whose output is filtered (14) to pass the desired mixer side band. This then goes via a differential demodulator (15-16-17) to an output filter (18), the output of which is the receiver's output. <IMAGE>

Description

SPECIFICATION Wideband optical link This invention relates to wideband optical links, i.e.

links in which the transmission medium is light sent via optical fibre orviafree space.

Wideband links are needed for, among other applications, multiplexed television channels and also for conveying stereo audio channels. With a reasonable number, e.g. eight, of VSB (Vestrigral Side Band) modulated television channels in a multiplex, or an equivalent number of stereo audio channels, it is at best very difficu It to obtain an adequate signal-noise ratio using direct intensity modulation of a laser.

An object of this invention is to enable an optical link to convey such multiplexed channels wherein the above disadvantages are reduced or even eliminated.

Accordingtothe invention, there is provided a wideband optical linkforconveying multi-channel information, inwhich the information is conveyed using pulse frequency modulation with the information to be conveyed modulated on to the desired carriervia a highly linearvoltage controlled oscillator, the resultofthe modulation being applied to a laser from which the modulated light beam is sent over the transmission medium ofthe link.

Such a system hasthe advantage thatthe laser used does not need to have highly linear characteristics.

While the voltage controlled oscillator (VCO) does need to be highly linear this is not a serious difficulty since highly linearVCO's are commercially available.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. lisa PFM fibre optictransmitter and Fig. 2 is a PFM fibre optic receiver, both for a system embodying the invention.

The optical link, in this case an optical fibre link, will be described as used for eightvideo (television) channel's. The system takes as its input a block of eight VSB - FIDiM (Vestigial Side Band - Pulse Frequency Modulation} signals with 8 MHz channel spacing.In one example the frequencies of the VSB carriers are as below: Channel 1 --- 10 MHz Channel 5 --- 42 MHz Channel 2 ---- 18 MHz Channel 6 --- 50 MHz Channel 3 ---- 26 MHz Channel 7 ---- 58 MKz Channel 4 --- 34 MHz Channel 8 66 MHz Such a multiplex of frequencies may be obtained in known mannerfor combining the outputs of VSB modulators andlor off-airfrequency converters, or by down-converting from an existing multiplex used on an existing co-axial VSB system.

The incoming multiplexed signal is applied to an ultra-linear 3GHz VCO 1 with a + 130 deviation.

The modulated output from the VCO is applied to a frequency mixer 2, where it is mixed with the output of a 3.75GHz oscillator3. The output of the mixer 2 therefore includes a 750MHzcarrierconveying the input multiplex and having the same deviation as the VCO.This output is applied to a 1 GHz low passfilter4 to give as its output the modulated 750MHz carrier, which is appliedvia an amplifier5 and a limiter6to a laser 7. The laser is, in the present example, a 1300nm IRW laser, and the carrier applied to it switches the laser7 on and off at a mean optical power of-3dBm.

The laser output enters the optical fibre (not shown) by which the signal reaches the receiver.

The receiver, Fig. 2, has a sensitivity estimated to be -20dBm for 56dB weighted video signal/noise ratio for eight channel capability.

The incoming light signal from the fibre is applied to an optical receiver 10, which uses as its light-receiving device an avalanche photodiode (APD). Such a receiver is betterforthe present application than one using a PIN-FET, since in the present application, unlike in digital systems the higher leakage currents of a germanium APD not significant. The optical receiver 10, in the present case, includes a series of limiting amplifiers, followed by a 1 GHz low pass filter 11,which removes any high-order harmonics which fall in the 3GHz region.

The signal which leaves the filter 11 has to be up-converted into the 3GHz region beforedemodula- tion,to obtain the required degree of linearity in frequencytovoltage conversion. Hence the filtered signal is applied to one input of a frequency mixer 12 whose other input is from a 3.75 GHz oscillator 13. The output of this mixer is applied to a 3 band-pass filter 14, which has a bandwidth of 400MHz. Thus the output from this filter is a 3.0 GHz signal, with a deviation of + 130 MHz The output of the filter 14 is applied to a demodulatorformed by a signal splitter 15, quarter-wave delay line 16 and frequency mixer 17. These items together form a delay line discriminator, and its output is applied to a 70 MHz low pass filter 18.Hence the original VSB multiplex is removed at the receiver. This eight channel multiplex is then either demodulated using commercially available demodulators or upconverters to form part of a 30 channel VSB multiplex which is compatible with a co-axial type of-VSB system.

We have mentioned above that a PFM link embodying the invention can also be used to transmit a seteo radio muliplex. This is assumed to be in the 88-108 MHz band, and is down-convertedtothe 5-25 MHz region. The multiplexthen modulates to ultra linear.

VCO as described with reference to Fig. the multiplex then is then recovered from the receiver, Fig. 2, in the manner described with reference to Fig. 2.

The multiplex is then up-converted to the 88-108 MHz band, and received using a standard FM broadcast bandtuner/radio.

In the frequency mixers 2 and 12, the desired output is the difference between the frequencies of the signals to be mixed; if the desired output is the sum of those two frequencies, the successive filters' pass characteristics would differ appropriately from those ofthefilters4and 14.

1. Awideband optical linkforconveying multichannel information, in which the information is conveyed using pulse frequency modulation with the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Wideband optical link This invention relates to wideband optical links, i.e. links in which the transmission medium is light sent via optical fibre orviafree space. Wideband links are needed for, among other applications, multiplexed television channels and also for conveying stereo audio channels. With a reasonable number, e.g. eight, of VSB (Vestrigral Side Band) modulated television channels in a multiplex, or an equivalent number of stereo audio channels, it is at best very difficu It to obtain an adequate signal-noise ratio using direct intensity modulation of a laser. An object of this invention is to enable an optical link to convey such multiplexed channels wherein the above disadvantages are reduced or even eliminated. Accordingtothe invention, there is provided a wideband optical linkforconveying multi-channel information, inwhich the information is conveyed using pulse frequency modulation with the information to be conveyed modulated on to the desired carriervia a highly linearvoltage controlled oscillator, the resultofthe modulation being applied to a laser from which the modulated light beam is sent over the transmission medium ofthe link. Such a system hasthe advantage thatthe laser used does not need to have highly linear characteristics. While the voltage controlled oscillator (VCO) does need to be highly linear this is not a serious difficulty since highly linearVCO's are commercially available. An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. lisa PFM fibre optictransmitter and Fig. 2 is a PFM fibre optic receiver, both for a system embodying the invention. The optical link, in this case an optical fibre link, will be described as used for eightvideo (television) channel's. The system takes as its input a block of eight VSB - FIDiM (Vestigial Side Band - Pulse Frequency Modulation} signals with 8 MHz channel spacing.In one example the frequencies of the VSB carriers are as below: Channel 1 --- 10 MHz Channel 5 --- 42 MHz Channel 2 ---- 18 MHz Channel 6 --- 50 MHz Channel 3 ---- 26 MHz Channel 7 ---- 58 MKz Channel 4 --- 34 MHz Channel 8 66 MHz Such a multiplex of frequencies may be obtained in known mannerfor combining the outputs of VSB modulators andlor off-airfrequency converters, or by down-converting from an existing multiplex used on an existing co-axial VSB system. The incoming multiplexed signal is applied to an ultra-linear 3GHz VCO 1 with a + 130 deviation. The modulated output from the VCO is applied to a frequency mixer 2, where it is mixed with the output of a 3.75GHz oscillator3. The output of the mixer 2 therefore includes a 750MHzcarrierconveying the input multiplex and having the same deviation as the VCO.This output is applied to a 1 GHz low passfilter4 to give as its output the modulated 750MHz carrier, which is appliedvia an amplifier5 and a limiter6to a laser 7. The laser is, in the present example, a 1300nm IRW laser, and the carrier applied to it switches the laser7 on and off at a mean optical power of-3dBm. The laser output enters the optical fibre (not shown) by which the signal reaches the receiver. The receiver, Fig. 2, has a sensitivity estimated to be -20dBm for 56dB weighted video signal/noise ratio for eight channel capability. The incoming light signal from the fibre is applied to an optical receiver 10, which uses as its light-receiving device an avalanche photodiode (APD). Such a receiver is betterforthe present application than one using a PIN-FET, since in the present application, unlike in digital systems the higher leakage currents of a germanium APD not significant. The optical receiver 10, in the present case, includes a series of limiting amplifiers, followed by a 1 GHz low pass filter 11,which removes any high-order harmonics which fall in the 3GHz region. The signal which leaves the filter 11 has to be up-converted into the 3GHz region beforedemodula- tion,to obtain the required degree of linearity in frequencytovoltage conversion. Hence the filtered signal is applied to one input of a frequency mixer 12 whose other input is from a 3.75 GHz oscillator 13. The output of this mixer is applied to a 3 band-pass filter 14, which has a bandwidth of 400MHz. Thus the output from this filter is a 3.0 GHz signal, with a deviation of + 130 MHz The output of the filter 14 is applied to a demodulatorformed by a signal splitter 15, quarter-wave delay line 16 and frequency mixer 17. These items together form a delay line discriminator, and its output is applied to a 70 MHz low pass filter 18.Hence the original VSB multiplex is removed at the receiver. This eight channel multiplex is then either demodulated using commercially available demodulators or upconverters to form part of a 30 channel VSB multiplex which is compatible with a co-axial type of-VSB system. We have mentioned above that a PFM link embodying the invention can also be used to transmit a seteo radio muliplex. This is assumed to be in the 88-108 MHz band, and is down-convertedtothe 5-25 MHz region. The multiplexthen modulates to ultra linear. VCO as described with reference to Fig. the multiplex then is then recovered from the receiver, Fig. 2, in the manner described with reference to Fig. 2. The multiplex is then up-converted to the 88-108 MHz band, and received using a standard FM broadcast bandtuner/radio. In the frequency mixers 2 and 12, the desired output is the difference between the frequencies of the signals to be mixed; if the desired output is the sum of those two frequencies, the successive filters' pass characteristics would differ appropriately from those ofthefilters4and 14. CLAIMS

1. Awideband optical linkforconveying multichannel information, in which the information is conveyed using pulse frequency modulation with the information to be conveyed modulated on tithe desired carrier via a highly linearvoltage controlled oscillator, the result ofthe modulation being applied to a laserfrom which the modulated light beam is sent over the transmission medium ofthe link.

2. A link as claimed in claim 1, in which the output from thevoltage-controlled oscillator is applied to a frequency mixerto the other input ofwhich is applied the output of an oscillator, in which the output ofthe mixer is applied to a filterwhich passes the desired side-band ofthe mixer's output, and in which the filter output is applied via amplifier/limiting circuitryto the said laser.

3. A link as claimed in claim 1 or2, in which art a receiver the light in the medium is applied to an optical receiver whose output is applied to a further mixer whose other input is a signal from an oscillator, in which the further mixer output is applied to a filter which passesthedesired side-band ofthe mixer's output, and in which said further mixer's output is applied via demodulation meansto a filterwhose output isthe signal originally applied to the voltagecontrolled oscillator.

4. Atransmitterstationforawidebandoptical link via which information is conveyed using pulse frequency modulation, in which the information as received is a multiplex of a number of information bearingchannelswhich is modulated by a highly linear voltage controlled oscillatorwhose output is applied to one input of a frequency mixer, in which a signal from an oscillator is applied to the other input of the mixer, whose output is applied to a filter, in which the filter passesthe desired sideband ofthe mixer output, which desired sideband is appliedvia ampli- fier/limiting circuitry to a laser, the modulated output ofthe laser being applied to an outgoing optical fibre.

5. A receiverstationfor awideband optical link via which information is conveyed using pulse frequency modulation,-in which a modulated light beam carrying said information as receivedoveran incoming optical fibre is applied to a light responsive diode in an optical receiver, in which the optical receiver's output is applied via afilterto an input of a frequency mixerto the other input ofwhich the signal from an oscillator is applied, in which a furtherfilterto which the output of the mixer is applied passesthe desired mixer sideband and in which said desired sideband is applied from said furtherfilterto a differential demodulator whose output, via yet a furtherfilter is the wanted information.

6. Atransmitterforawideband optical fibre link, substantially as described with reference to Fig. 1 of the accompanying drawings.

7. Areceiverforawidebandopticalfibre link, substantially as described with reference to Fig.2 of the accompanying drawings.