GB2241128A

GB2241128A - A signal generator

Info

Publication number: GB2241128A
Application number: GB9003382A
Authority: GB
Inventors: John Martin Bagshaw; Terence Fleetwood Willats; Steven John Payne; Lionel William John Kent
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1990-02-14
Filing date: 1990-02-14
Publication date: 1991-08-21
Also published as: GB9003382D0

Abstract

A signal generator comprises means for generating band-limited noise 3, a local oscillator 2 for generating a carrier frequency, and a mixer 1 for mixing the band-limited noise and the carrier frequency to produce an output 5 consisting of a broadband radio frequency signal having a notch. The signal can be applied to an acousto-optic filter so that the filter diffracts all frequencies other than those of the notch. The acousto-optic filter can be used to tune a laser, and provides means for rapidly changing the output wavelength between pulses of a pulse laser. <IMAGE>

Description

A Signal Generator This invention relates to a signal generator and in particular, but not exclusively to a radio frequency signal generator for use with an acousto-optic tunable filter.

In certain applications it is desirable to generate a broadband radio frequency signal having a notch. One such application is disclosed in our co-pending UK application, publication number GB2207799A wherein such a radio frequency signal is applied to a piezo-electric transducer of an acousto-optic tunable filter. The transducer converts the radio frequency signal into acoustic waves which are transmitted into an acousto-optic crystal of the acousto-optic filter such that each acoustic frequency present causes Bragg diffraction of an associated optical wavelength of light passing through the crystal. The only optical frequency not diffracted is that which corresponds to the notch in the applied acoustic signal.

One application of the above described acoussto-optic tunable filter is for use in a laser where it is important that maximum light transmission through the acousto-optic tunable filter is obtained at the selected wavelength. For this to be achieved the applied acoustic signal and, therefore electrical signal, must have a very low value at that frequency associated with the wavelength of light it is wished to transmit without diffraction.

Previously the transducer of the acusto-optic tunable filter described above has been energised by a simple multi-tone radio frequency driver utilising up to twenty individually switchable oscillators, the required notch being obtained by switching one or more of the oscillators off. The problem with this technique is that the side lobes of adjacent frequency signals generated by the unswitched oscillators provide a background noise level which partially infills the notch. This often necessitates that three oscillators producing adjacent frequency signals have to be switched in order to provide a suitable notch. This of course widens the notch, and so a trade off occurs between infill and notch width, making it difficult to tune such a filter.

Another possible approach is to use a broadband noise source and a voltage tunable radio frequency band stop to remove the unwanted signal, however this system suffers from the drawback that the width of the notch is not easily controllable.

According to a first aspect, the invention provides a signal generator comprising means for generating band limited noise at baseband frequency, means for generating a carrier frequency, and means for mixing the band limited noise with the carrier frequency, the mixer being arranged such that in operation the resultant output is a broadband frequency signal having a notch.

By employing the above signal generator, a broadband signal having a notch is obtained by up converting the band limited noise in the mixer with the carrier frequency, thus eliminating the infill problems associated with other means for obtaining such a signal.

Preferably the mixer is a double balanced mixer the output of which comprises two mirror image side bands separated by a notch corresponding to the carrier frequency. The notch in the output signal occurs as a result of the base band signal containing negligible According to a second aspect, the invention provides an acousto-optical tunable filter employing a signal generator in accordance with the first aspect of the invention. The output from the signal generator when applied to a transducer of the acousto-optic tunable filter causes the transducer to generate acoustic signals within an acousto-optic medium of the filter to which it is attached. These acoustic signals interact with light within the medium such as to diffract all optical wavelengths associated with each acoustic frequency.Therefore by employing a signal generator in accordance with the first aspect of the invention the acousto-optic tunable filter permits light consisting of a narrow band of optical wavelengths to pass through the acousto-optic medium without diffraction, this band of optical wavelengths being dependant upon the shape of the notch in the output signal, from the signal generator, and may be varied by altering the characteristics of the notch. Furthermore the loss in the optical wavelengths of light transmitted without diffraction is very small as a result of the avoidance of infill in the notch.

According to a third aspect the invention provides a laser comprising an acousto-optic tunable filter in accordance with the second aspect of the invention. By employing such an acousto-optic tunable filter so that only wavelengths of light passing through the acousto-optic tunable filter without diffraction sustain lasing action, it is possible to tune the laser to a particular operating wavelength and to vary this operating wavelength by altering the characteristics of the notch in the output from the signal generator.

One embodiment of the invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 is a schematic illustration of a signal generator in accordance with the first aspect of the invention; Figure 2 is a diagrammatic representation of the signals occurring at various parts of the circuit illustrated in Figure 1; Figure 3 is a schematic illustration of an acousto-optic tunable filter in accordance with the second aspect of the invention employing the signal generator of Figure 1; and Figure 4 is a schematic illustration of a laser in accordance with the third aspect of the invention incorporating the acousto-optic tunable filter of Figure 3.

Referring to Figure 1, there is illustrated a signal generator comprising a balanced mixer 1 receiving signals from a local oscillator 2 and a radio frequency input 3. The frequency input being filtered by a high pass filter 4. The mixer includes an intermediate frequency output 5.

In operation radio frequency band limited noise at base band frequencies is applied to the input 3. This is filtered by the high pass filter 4, a third order filter with a 1 MHz pole frequency and an 18 dB/octave slope which effectively removes all frequencies below 1 MHz producing a radio frequency spectrum as illustrated in Figure 2a. This is applied to the balanced mixer 1 where it is up converted by the signal from the local oscillator 2. The output of the local oscillator is a pure signal as illustrated in Figure 2b.

The output from the mixer is as illustrated in Figure 2c and comprises two mirror image side bands about the local oscillator frequency. As the baseband signal contains negilable energy at d.c. there is correspondingly little energy at the local oscillator frequency, thereby providing a broad band radio frequency output having a notch. The frequency at which the notch occurs may be adjusted by altering the frequency of the local oscillator, whilst the width of the notch may be adjusted by altering the characteristics of the high pass filter.

In Figure 3 an acousto-optic tunable filter is schematically illustrated, which is driven by the signal generator 6 of Figure 1. The acousto-optic tunable filter comprises of an acousto-optic medium 8 of appropriate orientation to which is attached a piezo-electric transducer 7 connected electrically to the generator 6. In operation the signal generator is tuned such that the frequency at which the notch occurs in the output radio frequency noise spectrum is the frequency which, if applied to the acousto-optic tunable filter, would diffract light in the acousto-optic tunable filter having an associated optical wavelength which it is desired to transmit.Therefore because all other acoustic frequencies are present in the noise signal all wavelengths of light are diffracted by the acousto-optic tunable filter except for those associated with the acoustic frequencies at which the notch occurs.

It is therefore possible to filter one particular wavelength of light, associated with those frequencies at which the notch occurs, and this wavelength may be altered by changing the characteristics of the notch.

Figure 4 shows a laser 9 comprising the signal generator 10 and acousto-optic tunable filter 11 of Figure 3. The acousto-optic tunable filter is located within a lasing cavity defined by a cylindrical member 12, mirrored surface 13 and semi-silvered 14. Also contained within the lasing cavity is a laser amplifying element 15.

The reflective surfaces 13 and 14, and the laser amplifying element 15 are arranged such that if the acousto-optic tunable filter is inoperative then lasing action may occur with light being reflected to and fro between the surfaces 13 and 14 via the amplifying element 15 along the path indicated. A component of the light incident on the semi-silvered surface 14 being transmitted therethrough providing an output form the laser. When the acousto-optic tunable filter 11 is operative in a manner previously described with references to Figure 3, light at all wavelengths other than those associated with the notch in the output signal of the signal generator 10 is diffracted.

Thereby only light of wavelengths associated with those acoustic frequencies corresponding to the notch is transmitted to and fro between reflective surfaces 13 and 14, and therefore only light of such wavelengths is of sufficient intensity to maintain lasing action.

By altering the position of the notch in the output from the signal generator 10, as previously described, the wavelength of light generated by the laser is varied. Thus thereby providing means of rapidly changing the output wavelength of the laser such that when the laser is operated in a pulsed mode successive pulses may be of different wavelengths if so desired.

Claims

1. A signal generator comprising means for generating band limited noise at baseband frequency, means for generating a carrier frequency, and means for mixing the band limited noise with the carrier frequency, the mixer being arranged such that in operation the resultant output is a broad band frequency spectrum having a notch.

2. A signal generator as claimed in claim 1 wherein the mixer is a double balanced mixer the output of which comprises two mirror image sidebands separated by a notch corresponding to the carrier frequency.

3. A signal generator as claimed in claim 1 or 2 comprising a high pass filter arranged to filter the band limited noise.

4. A signal generator as claimed in claim 1 or 2 or 3 wherein the band limited noise at baseband frequency is a digitally derived pseudo random bit sequence.

5. A signal generator as claimed in any preceding claim wherein the carrier frequency is generated by a local oscillator.

6. An acousto-optic tunable filter employing a signal generator as claimed in any preceding claim.

7. An acousto-optic tunable filter as claimed in claim 6 comprising a acousto-optic medium and an electro-acoustic actuator for applying a signal generated by the signal generator to the acousto-optic medium such that the acousto-optic tunable filter diffracts all frequencies other than those corresponding to the notch.

8. A laser comprising an acousto-optic tunable filter as claimed in claim 6 or 7.

9. A laser as claimed in claim 8 wherein the frequency band corresponding to the notch is narrow with substantially zero infilling.

10. A laser as claimed in any of claims 8 or 9 wherein the frequency of operation may be changed.

11. A laser as claimed in any of claims 8, 9 or 10 wherein the laser is a pulsed laser.

12. A signal generator substantially as hereinbefore described with reference to Figures 1 and 2 of the drawings.

13. An acousto-optic filter substantially as hereinbefore described with reference to Figure 3 of the drawings.

14. A laser substantially as hereinbefore described with reference to Figure 4 of the drawings.