GB2241078A

GB2241078A - An acousto-optic tunable filter

Info

Publication number: GB2241078A
Application number: GB9003364A
Authority: GB
Inventors: Terence Fleetwood Willats
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
Anticipated expiration: 2010-02-14
Also published as: GB2241078B; GB9003364D0

Abstract

This invention provides an acousto-optic tunable filter 11 comprising an acousto-optic medium 12 and an electro-acoustic transducer 13 attached to a signal source 25 for applying acoustic signals to the medium 12 such that light from a source A at a frequency associated with that of the applied acoustic signal from source 25 and properties of the medium 12 is diffracted by the cell. The transducer 13 comprising a plurality of piezo-electric elements 14, 15, 16 and 17 connected electrically in series so as to provide the transducer 13 with a high impedance for impedance matching with the signal source 25. The elements are arranged parallel to the interaction plane of the acousto-optic medium 12 such that the gaps between adjacent elements do not detrimentally effect the wave vector distribution 27 of the output light. <IMAGE>

Description

An Acousto-Optic Tunable Filter This invention relates to an acousto-optic tunable filter and in particular to such a filter including an electro-acoustic transducer.

In our co-pending UK patent application publication number GB 2215074 there is disclosed an acousto-optic tunable filter whose properties are modified by applying an acoustic signal to one face of an a acousto-optic medium by means of a transducer. The transducer converts an electrical signal into an acoustic signal within the cell such that the shear (transverse) acoustic waves generated by the transducer interact with the optical wave causing diffraction of a optical light beam through the medium if the frequency of the light is appropriate for the frequency of the acoustic signal applied and the structure of the medium. The apparatus is normally arranged as illustrated schematically in Figure 1. Light from a source A is incident upon a acousto-optic medium 2 of filter 1 such that it is diffracted in a direction 3.

The diffracted output corresponding to a single tone applied acoustic signal will be as illustrated at 4, having a standard sinc2 distribution. The wavelength at which the peak occurs is dependent upon the frequency applied to the transducer 5, which normally comprises a piezo-electric element fed with an electrical signal input from a signal generator, this typically operating in the radio frequency band.

In a practical device each side of the filter may be several centimetres long and therefore the piezo-electric element will have a low impedance due to its relatively large size. It is important, especially at radio frequencies, that the impedance of the transducer is matched to that of the signal source often 50 ohms, this being far in excess of the impedance of the transducer which would be of the order of a few ohms.

One way in which the impedance matching problem may be overcome is by employing apparatus as illustrated in Figure 2, whereby the transducer 5 is vertically separated into five elements 6 to 10, wherein each adjacent element is reversed through 180 degrees to its neighbours, and connected in series. This arrangement ensures that the acoustic waves emitted from the individual elements are in phase. It will be appreciated that forming the transducer from a plurality of smaller elements greatly increases the impedance or the transducer. If for example the transducer of Figure 1 has an impedance of 2 ohms, then the impedance of each element 6 to 10 will be 5 times this value, 10 ohms, giving a total impedance of the five elements connected in series of 50 ohms, as required for matching to a 50 ohm signal source.Unfortunately the output from apparatus as illustrated in Figure 2 has been found to exhibit a distorted wave vector distribution with further generated fifth order sidelobes as shown. This is particularly a problem when a broad spectrum output is required having a notch, as the additional sidelobes infill the notch.

This invention provides an acoustic optic tunable filter including an acousto-optic medium and an electro-acoustic transducer, the transducer comprising a plurality of elements, the elements being arranged in a plane orthogonal to a plane in which acoustic waves generated by the transducer interact in use with optical waves propagating through the medium, and such that an elongate gap between adjacent elements extends in a direction parallel to the interaction plane.

As the above acousto-optic filter employs a transducer having a plurality of elements arranged parallel to the interaction plane, the gap between each pair of adjacent elements extends in the direction of light propagation within the medium such that the transducer appears as a continuous element in the interaction plane. Therefore the sound field in the interaction plane typically has a simple sinc2 angular distribution as would be associated with a single transducer element, and thus reduces the sidelobe structure in the interaction plane. However, the advantages of a multi-element transducer to increase impedance are retained.

Preferably the transducer elements comprise piezo-electric material thereby providing a simple means of converting the electrical signal into an acoustic signal within the medium.

Advantageously the transducer elements are electrically connected in series, thereby enabling the plurality of elements to have a higher impedance than would be the case with a single element of the same dimensions as the combined plurality of elements.

Preferably the impedance of the transducer elements is matched to a signal source ensuring maximum power transmission from the source to the transducer.

A Preferred embodiment of the invention will now be described by way of example only with reference to Figures 3 and 4 of the accompanying drawings of which: Figure 3A illustrates schematically an acousto-optic tunable filter in accordance with the invention; Figure 3B is a cross-section through the line I-I of Figure 3A; and Figure 4 is a schematic cross-section through a tunable laser embodying an acoustic filter in accordance with the invention.

Referring to Figure 3A there is illustrated an acousto-optic tunable filter 11 comprising an acousto-optic medium 12 and a transducer 13 comprising piezo-electric elements 14, 15, 16 and 17. These elements are mounted on a conductive layer which has been etched to form regions 18 and 19 seen in Figure 3B, and serve to connect elements 14 and 15 and 16 and 17 respectively. Elements 15 and 16 are connected by fine wires 20 which in conjunction with wires 21 and 22 connected to power supply lines 23 and 24 respectively form a series circuit to a power supply 25.

In operation light from source A is incident on the filter in a plane orthogonal to that on which the transducer lies, the said plane being the interaction plane 26 seen in Figure 3B. Light waves interact with the acoustic waves generated by the transducer element 14, 15, 16 and 17 such that a wavelength of light associated with the applied acoustic frequency and properties of the medium is diffracted, the output .2 having a simple sine angular distribution as illustrated at 27. It should 'be noted however that this is achieved at the expense of increasing the sidelobe structure in the much less important orthogonal plane.

An acousto-optic filter producing a diffraction pattern approximating to a sinc2 distribution with reduced sidelobes is particularly advantageous when embodied in a tunable laser as illustrated in Figure 4.

This apparatus comprises a laser cavity 28 containing a laser amplifying element 29. The ends of the cavity 28 comprise a mirrored surface 30 and a semi-silvered surface 31 respectively. In operation the semi-silvered surface 31 lets a component of radiation incident on it to pass through it but reflects a large enough component to maintain lasing action within the amplifying element 29.

The acousto-optic tunable filter of Figure 3, 32, is positioned in the light path 33 within the laser cavity such that light passing without diffraction through the medium 34 is unaffected by the acousto-optic tunable filter 32 and continues to pass to and fro between the reflective surfaces 30 and 31.

When an acoustic signal is applied via the signal generator 35 and radio frequency signal notch filter 36 all wavelengths of light are diffracted by the acousto-otpic tunable filter except for those corresponding to the acoustic frequency band stopped by the notch filter. Light diffracted by the acousto-optic tunable filter will no longer be reflected to and fro within the cavity between surfaces 30 and 31 but instead will be diffracted away from the centre of the cavity and subsequently absorbed by the walls 37 and 38. By this process lasing action will only occur at the optical wavelength which corresponds to the acoustic frequency which is stopped by the notch filter for only light of this frequency will pass through the acousto-optic tunable filter without diffraction, and therefore only light at this frequency will be incident on the amplifying element sufficient enough to generate lasing action.

The acoustic frequency may be varied by control 39 enabling the frequency of operation of the laser to be rapidly changed such that when operating in pulsed mode the frequency of successive pulses may be varied.

Claims

CbAIMS

1. An acousto-optic tunable filter including an acousto-optic medium and an electro-acoustic transducer, the transducer comprising a plurality of elements, the elements being arranged in a plane orthogonal to a plane in which acoustic waves generated by the transducer interact in use with optical waves propagating through the m(dium, and such that an elongate gap between adjacent elements extends in a direction parallel to the inteaaction plane.

2. A filter as claimed in claim 1 wherein the elements comprise piezo-electric material.

3. A filter as claimed in claim 1 or 2 wherein the elements are electrically connected in series.

4. A filter as claimed in claim 1, 2 or 3 wherein the impedance of the transducer is matched to a signal source.

5. A filter as claimed in any preceding claim wherein the frequency at which diffraction of incident light occurs is dependent upon the frequency of the applied acoustic signal.

6. A tunable laser including a laser cavity containing a laser amplifying element and an acousto-optic tunable filter as claimed in any preceding claim arranged so that, in operation, only light which passes through the filter without diffraction is returned to the laser amplifying element.

7. A tunable laser as claimed in claim 6 comprising electronic means for tuning the tunable filter such as to select one of the operating frequencies of the laser amplifying element from a range of frequencies.

8. A tunable laser according to claim 7, wherein the electronic means drives the tunable filter to produce acoustic waves at a plurality of frequencies such that light from the laser amplifying element other than that selected frequency undergo diffraction.

9. An acousto-optic tunable filter, substantially as hereinbefore described with reference to Figures 3A and 3B of the drawings.

10. A tunable laser including an acousto-optic tunable filter, substantially as hereinbefore described with reference to Figure 4 of the drawings.