GB2200223A

GB2200223A - Apparatus for controlling the direction of a beam of optical radiation

Info

Publication number: GB2200223A
Application number: GB08602434A
Authority: GB
Inventors: Alison Brown Lessels
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1986-01-31
Filing date: 1986-01-31
Publication date: 1988-07-27
Also published as: GB8602434D0; DE3702330A1; FR2712707A1; IT8747542A0; NL8700228A; SE8700371L; GB2200223B; IT1205713B; SE8700371D0

Abstract

The apparatus includes first and second prisms (10, 11) each of small apex angle and mounted for rotation about a common optical axis (12) passing through the centres of the prisms. A third prism (13) is made from a material having a lower refractive index and has a smaller apex angle than either of the first and second prisms, and is located for rotation about the same optical axis (12). Drive means (21, 24, 27) are provided for rotating each prism independently about the common axis (12) and pick-off means (22, 25, 28) are operable to determine the angular position of each prism relative to a datum direction. Control means (29) control the positions of each prism and ensures that the third prism is positioned so that the angular deflection which it produces is opposite to the vector sum of the deflections produced by the first and second prisms. <IMAGE>

Description

APPARATUS FOR CONTROLLING THE DIRECTION OF A BEAM OF OPTICAL RADIATION This invention relates to apparatus for controlling the direction of a beam of optical radiation, and in particular to such apparatus having means for correcting for chromatic aberration.

It is frequently necessary to vary the direction of a beam of optical radiation, which might be a beam emitted by a source or a beam incident upon a receiver. In its simplest form the beam direction may be varied by moving the source or receiver itself, together with any associated optical elements. In many instances, however, this is not convenient due to the size or complexity of the source or receiver, and some means has to be provided for moving the beam independently. For example, a pair of mirrors rotatable about mutually perpendicular axes may be used to control the direction of the beam. However, the mass of the mirrors present problems due to the relatively slow response time.

It is known to provide means for varying the direction of a beam of optical-radiation using a pair of prisms of small apex angle rotatable independently of one another about an optical axis normal to the plane bisecting the apex angles of the prisms. Examples of such apparatus are to be found in British patents Nos. 1,457,116 and 1,521,931, or in U.S. patents 3,378,687 and 3,827,787. Each of these aiscloses the above arrangement of two rotatable prisms.

The dispersion introduced by the two prisms in such arrangements unfortunately causes chromatic aberration. Whilst this may not be so important in some systems it does cause problems where non-monochromatic radiation is being used, particularly if the apparatus includes a radiation receiver sensitive to a wide band of radiation frequencies. This may be the case in some tuneable laser systems and with thermal imagers.

It is an object of the invention to provide apparatus for controlling the direction of beam of optical rotation in which the problem of chromatic aberration is considerably reduced.

According to the present invention there is provided apparatus for controlling the direction of a beam of optical radiation which includes first and second prisms each of small apex angle and mountea for rotation about a common optical axis passing substantially through the centres of the major faces of the prism, a third prism made from a material having a lower refractive inaex and higher dispersion ana with a smaller apex angle than either of the first and second prisms, the third prism being mounted for rotation about said common optical axis with said axis passing substantially. through the centres of its major faces, drive means operable to rotate each prism independently about said common optical axis, pickoff means operable to determine the angular position of each prism relative to a datum direction, and control means responsive to the angular position of each prism and to signals indicating the desired direction of the beam of optical radiation to rotate the prisms to the necessary positions whilst positioning the third prism such that its angular deflection is opposite to the vector sum of the deflections produced by the said first and second prisms.

The "major faces" of the prisms referred to above are the two faces which define the apex angle of each prism.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating the operation of an embodiment of the invention; Figure 2 is a schematic diagram illustrating the main features of apparatus according to one embodiment of the invention; and Figure 3 is a schematic circuit diagram of a control circuit for controlling the movement of the third prism used in the apparatus of Figure 2.

Referring now to Figure 1, this shows first and second steering prisms 10 and 11 arranged with a common optical axis.12 passing through the centre of the major faces of the two prisms. Each prism is of small apex angle, though in the drawing the apex of each prism has been removers. Locatea between the two prisms 10 and 11 is a correction prism 13, also having the common axis 12 passing through the centre of its major faces. The prism 13 is made from a material which has a lower refractive index and a higher aispersion than either of the prisms 10 and 11. Prism 13 also has a smaller apex angle than either of prisms 10 andll.

The prisms 10 and 11 produce a certain amount of angular deflection depending upon the apex angle of the prism and upon the refractive index of the material from which the prisms are made. The dispersion is the result of the optical radiation components of lowest wavelength being refractea more than components of a higher wavelength as the radiation passes through each prism, and this causes the chromatic aberration.

Prism 13, having a lower refractive index and smaller apex angle than prisms 10 and 11, produces a lower angular deflection of the radiation but a greater amount of dispersion. If its parameters are correctly chosen there will be one angular arrangement of the three prisms which will cause complete cancellation of the .dispersion and hence of the chromatic aberration at normal incidence to the first prism.

This is shown in Figure 1, where the solid ray represents radiation of highest wavelength whilst the aottea ray represents radiation of lowest wavelength.

The technique aescribed above may be used at any optical wavelength, using prisms having the appropriate parameters. By way of example only, the arrangement may be used in the 8-12 micron thermal imaging waveband with prisms 10 and 11 being made from germanuim and each having an apex angle of 5 . The third prism 13 is made of zinc sulphide and has an apex angle of only 0.3 . Such an arrangement gives a beam steering angle of 30 with a maximum angular chromaic aberration of 0.3 milliradians. By comparison a maximum steering angle of only 15 is possible if the third prism is omitted, allowing for the same maximum angular ar chromatic aberration.

The schematic arrangement of Figure 1 shows radiation entering the optical system along the optical axis 12 ana leaving at an angle to the axis. This woula be the situation with a radiation transmitter, such as a laser, located on the optical axis. In the case of a receiver or detector located on the optical axis it is the incoming radiation which is "steered" in order to scan a field of view.

Figure 2 shows, also in schematic form, one way in which the movement of the prisms is produced and controlled.

Prism 10 is mounted in a rotatable carrier 20, only part of which is shown. The carrier is rotatable about optical axis 12 by a drive motor 21, shown as driving the carrier 20 through a toothed gear. A pickoff 22 is provided to measure the angular position of the prism 10 and carrier 20 relative to some aatum direction. In a similar manner prism 11 is mounted in a carrier 23 rotatable by a motor 24 and having a pickoff 25, anå prism 13 is mounted in a carrier 26 rotatable by a motor 27 and having a pickoff 28. Each motor and pickoff is connected to a control-circuit 29 to which may be applied a demand signal D indicating the desired direction of the beam of raaiation 15.

In order to obtain the best correction of chromatic aberration it is necessary to ensure that the angular deflection produced by the correcting prism 13 is in the opposite direction to the vector sum of the deflections produced by the two steering prisms 10 and 11. The control circuit 29 is therefore required to ensure that this relationship is maintained at all times.

The circuitry necessary to move the two steering-prisms 10 and 11, and the mathematical expressions involved, are fully described in our British patent No. 1,521,931 referred to earlier, and need not be repeated here. Also described in detail in that patent is a circuit arranged to resolve the situation arising when sin(-ss) is zero, where a and ss are the angular positions of the two steering prisms 10 and 11.

However, additional circuitry is necessary in order to control the position of the correction prism 13 in the manner specified above, and such circuitry is shown in Figure 3.

Figure 3 shows an analogue circuit of the same type as the control circuit shown in the abovenumbered British patent. In Figure 3 of that prior patent each of the two steering prisms is mounted in a rotatable carrier which may be rotated by a motor and which has an associated pick-off which generates analogue signals indicating the sine and cosine of the wangle between the position of the steering prism ana a datum direction. As has already been stated, the correcting prism 13 has to be aligned at an angle such that the deflection which it produces is in the opposite direction to the vector sum of the deflections produced by the two steering prisms.

If a and ss are the angles of the two steering prisms to a datum, as in Figure 3 of U.K. Patent 1,521,931, then the direction of the resultant deflection vector G is given by the two expressions sina+sins Sine = sina+sin (sina+sin8)2 + (cosa+coss)2 cosa+cosss Cose = - (sin +sinss)2 + (cosa+coss)2 Driving the correction prism to (o + 1800) is easily done by using a feedback device whose datum is at 180 to that of the resolvers used on the steering prisms, and by using sine and cose to drive the correction prism.

Referring now to Figure 3, two resolvers 30 and 31 measure the angular positions of the steering prisms. Their outputs are demodulated by demodulators 32 and 33 to give dc signals representing the sines and cosines of those two angles. The two sine signals are applied to a summing amplifier 34 whilst the two cosine signals are applied to a second summing amplifier 35. The output of each summing amplifier is applied to a separate squaring circuit 36 and 37 and the outputs of these are added in summing amplifier 38.

Finally the output of summing amplifier 38 is passed through a "square-root" circuit 39, the output of which represents the bottom part of each of the two mathematical expressions given above.

A divider 40 has applied to it both the output of summing amplifier 34, representing (sina+sinss) ana the output of the square-root circuit 39. The output of the divider 40 thus represents sine. Similarly a second divider 41 has applied to it the output of summing amplifier 35 and the output of the square-root circuit 39. The output of. the divider 41 thus represents cos. The outputs of the two diviaers 40 and 41 pass through separate modulators to convert them into ac signals which are converted to three-phase form to drive a synchro 44 by means of a Scott transformer 45. The synchro output is demodulated by demodulator 46 and passed through a servo amplifier 47 for application to servo motor 48 which arives both the correcting prism and the rotor of the synchro 44.The 180 correction to be applied to the angles a and ss is achieved by ensuring the necessary mechanical alignment between the correcting prism and the rotor of the synchro 44.

It will be seen that the above circuit operates in such a way that the positions of the two steering prisms control the position of the correcting prism. Some of the components of the circuit just described are already present in the circuit diagram from U.K. patent No. 1,521,931.

It will be clear that digital circuits may be used for driving all three prisms in place of the analogue circuits referred to.

In the arrangement shown in Figures 1 ana 2 the correction prism 13 has been located between the two steering prisms 10 and 11. Whilst such an arrangement may be preferable in some situations it is not essential, and the correction prism may be positioned at either end of the arrangement of three prisms.

Similarly, although the three prisms are shown as having their major faces substantially perpendicular to the optical axis of the apparatus, the prisms, or any of them, may be arranged at a different angle to the optical axis.

Other support and drive arrangements for the prisms may be used, differing from those of Figure 2, and different control circuits. In particular, digital control circuitry may be used in place of the analogue circuits of Figure 3.

Claims

What we claim is:

1. Apparatus for controlling the direction of a beam of optical radiation which includes first and second prisms each of small apex angle and mounted for rotation about a common optical axis passing substantially through the centres of the major faces of the prism, a third prism made from a material having a lower refractive index and higher dispersion and with a smaller apex angle than either of the first and second prisms, the third prism being mounted for rotation about said common optical axis with said axis passing substantially through the centres of its major faces, drive means operable to rotate each prism independently about said common optical axis, pickoff means operable to determine the angular position of each prism relative to a datum direction, and control means responsive to the angular position of each prism and to signals indicating the desired direction of the beam of optical radiation to rotate the prisms to the necessary positions whilst positioning the third prism such that its angular deflection is opposite to the vector sum of the deflections produced by the said first and second prisms.

2. Apparatus as claimed in Claim 1 in which each prism is mounted in a carrier to which is secured a toothed gear ring.

3. Apparatus as claimed in Claim 2 in which the drive means includes, for each prism, a gear pinion in engagement with the toothed gear ring, a motor arranged to drive the pinion, and pick-off means operable to determine the angular position of the prism relative to a datum direction.

4. Apparatus as claimed in Claim 3 in which each pick-off means includes a synchro resolver.

5. Apparatus as claimed in either of Claims 3 or 4 in which each pick-off means is arranged to deliver output signals representing the sine and cosine of the angle between the datum direction and the orientation of the corresponding prism.

6. Apparatus as claimed in any one of Claims 1 to 5 in which the said third prism is positioned between the said first and second prisms.

7. Apparatus for controlling the direction of a beam of optical radiation substantially as herein described with reference to the accompanying drawings.