GB2069167A - Acousto-optic deflector - Google Patents

Abstract

A Bragg cell is so arranged with respect to reflectors 16, 18, 20 and lenses 22, 24 that undiffracted light which passes directly through the cell is repeatedly redirected onto the cell at different angles of incidence. Each angle of incidence corresponds to a respective range of cell operating frequencies at which diffraction is most efficient, and the ranges of directions covered by the deflection of the respective incident beams are caused to be contiguous (the frequency may require stepwise alteration on going between incident beams) to provide a wide angle scan. <IMAGE>

Description

SPECIFICATION Acousto-optic deflector The present invention relates to acousto-optic deflectors and more particularly to deflectors utilising the Bragg cell principle of acousto-optic deflection.

A Brag cell is a device which comprises a block of optical material with acousto-optical properties such that when the device is excited in a first direction by an acoustic signal induced therein by a suitable acoustic transducer a beam of light shone through the device in a substantially orthogonal direction will be deflected by an angle which is directly related to the frequency of the acoustic signal. Such a device is extremely fast in its operation and can be used therefore, by sweeping the frequency of the acoustic signal, to cause a light beam to be deflected therefore providing controlled scanning of the beam.

Such a device therefore finds uses in optical recording and playback systems since they can provide accurate deflection of the incident light beam. In this application the light beam will normally be a laser beam and will be so referred to hereinafter. A collimated light beam could however, also be used and the term light beam is to be construed as covering such a beam.

A disadvantage of the above described deflection system is that the Brag cell operating at a given acoustic frequency can only deflect beams at a specified range of angles of incidence ("rocking curve") and only through a given angle.

This implies that the Bragg cell will diffract a light beam through the same range of angles for a given incident beam angle when the acoustic frequency is varied.

The present invention therefore seeks to provide an improved acousto-optic Bragg cell deflector particularly for scanning a laser beam.

According to the present invention there is provided an acousto-optical deflector for producing a scanning beam of light including a Bragg cell incorporating an electro-acoustic transducer, in which a single beam of light is caused to be incident on said cell and to be diffracted by an input electric signal of a first frequency to a first position and by alteration of the frequency to be swept to a second position, deflection means for the beam passing straight through said Bragg cell to cause said beam to produce a second incident beam which enters the Bragg cell at a different angle to that of the first incident beam such that as the frequency of operation of said electro-acoustic transducer is further altered the diffracted beam from said second incident beam continues the sweep of said first diffracted beam.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a light beam diagram according to the present invention.

Figure 2 shows the characteristic of the acoustic frequency applied to the Bragg cell plotted against time for a preferred embodiment, Figure 3 shows a plot of acoustic power applied to the Bragg cell against frequency, Figure 4 shows a plot of beam power applied to the Bragg cell against the output beam angle and, Figure 5 shows the rocking curves due to two adjacent incident beams plotted against output beam angle.

Figure 1 shows an arrangement of optical components external to a Bragg cell 10 to provide a means of passing a laser beam 12 several times through the Bragg cell each time with an increased angle of incidence so extending the angular range through which the emergent beam 14 may be swept.

The optical apparatus comprises a first reflecting mirror 1 6 a second reflecting mirror 1 8 and a third reflecting mirror 20. Lenses 22 and 24 are situated as shown between lenses 1 6 and 1 8 and 20 respectively, so that lenses 22 and 24 are telecentric with respect to the Bragg cell. This ensures that different light propagating directions at the Brag cell are parallel between lenses 22 and 24.

In operation the incident laser beam 12 is diffracted by the Bragg cell 10 in a conventional manner to obtain a deflection of the beam over an angular range centred about the Bragg angle of the cell by varying the acoustic frequency applied to the transducer 26 which feeds the cell 10. The useable range of deflections about the Bragg angle is characterised by the "rocking curve" of the Bragg cell.

When the acoustic frequency increases beyond the useable range the efficiency drops off rapidly and the light passes undeflected through the cell 10 producing a beam 28 which is reflected by the mirror system 16, 22 1 8, 24, 20 back to the Bragg cell 10 where it is diffracted by the cell at higher acoustic frequencies. The Bragg deflection angle is correspondingly increased to provide a useful range of deflection angles contiguous with the previous range.

The optical system shown is capable of recycling the beam back through the cell many times, without beam expansion at the Bragg cell, thus considerably extending the angular deflection capability of the acousto-optic deflection device.

Referring now to Figure 2, in order to sweep the deflected beam 14 at a uniform angular velocity over the full deflection range it is necessary to feed the electro acoustic transducer 26 of the Brag cell with a signal which varies in frequency as shown. It may also be necessary to compensate for variations in beam intensity with acoustic frequency by pre-emphasising the power of the acoustic signal. This is illustrated in Figure 3.

Variations in output beam intensity may be alternatively corrected by imposing a suitable modulation onto the intensity of the incident laser beam when the output beam is scanned as shown in Figure 4.

The deflected angle, swept out by the diffracted beam, increases smoothly with increase in acoustic frequency. While the intensity of the diffracted beam rises to a maximum at the Bragg angle, B,, and diminishes at positions either side of the Bragg angle.

The variation in diffracted beam intensity about the Bragg angle (for constant acoustic power) is characterised by the "rocking curve" and is releated to angular displacement,0, from the Bragg angle by: A preferred embodiment is to arrange that the Bragg diffraction angles, oB1, and oB2t associated with each angle of incidence in Figure 1 be separated by an angle such that the rocking curves for adjacent beam positions overlap in the manner illustrated in Figure 3. This is the condition for most uniform output beam intensity with least beam cross-talk.

It should be noted that in the preferred embodiment The Bragg angle associated with the 2nd incident beam lies mid way between the Bragg angle associated with the 1 sot incident beam and the first zero position on the rocking curve of the 1 st diffracted beam.

Claims (3)

1. An acousto-optical deflector for producing a scanning beam of light including a Bragg cell incorporating an electro-acoustic transducer, in which a single beam of light is caused to be incident on said cell and to be diffracted by an input electric signal of a first frequency to a first position and by alteration of the frequency to be swept to a second position, deflection means for the beam passing straight through said Bragg cell to cause said beam to produce a second incident beam which enters the Bragg cell at a different angle to that of the first incident beam such that as the frequency of operation of said electroacoustic transducer is further altered the diffracted beam from said second incident beam continues the sweep of said first diffracted beam.

2. A acousto-optical deflector as claimed in claim 1 in which the deflection means comprises first, second and third mirrors, telecentric lenses being disposed between said first and second and said second and third mirrors and said Bragg cell being disposed between said third and said first mirrors.

3. An acousto-optical deflector for producing a scanning beam of light substantially as described with reference to the accompanying drawings.