GB2103384A

GB2103384A - Beam deflecting apparatus for servo-controlled beam stabilisation

Info

Publication number: GB2103384A
Application number: GB08124249A
Authority: GB
Inventors: Robert John Cormack
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1981-08-07
Filing date: 1981-08-07
Publication date: 1983-02-16
Also published as: FR2511163A1; IT1149033B; IT8248934A0; FR2511163B1; GB2103384B; AU8676182A; DE3228839A1; AU545405B2

Abstract

The apparatus includes first and second lenses, 10, 11 each having a plane surface and a spherically-curved surface. The lenses are arranged with their curved surfaces adjacent to one another and separated by a gap of uniform width which is small compared with the radius of curvature of each curved surface. Means are provided for producing relative movement between the two lenses about axes which are perpendicular to the optical axis of the apparatus and at an angle to one another. These axes pass through the centres of curvature of the two lenses. An automatic control system may be provided to control the lens position. <IMAGE>

Description

SPECIFICATION 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 without the need to move the source of the radiation.

It is frequently necessary to control the direction of a beam of radiation, and the simplest method is to move the source itself together with any associated optical elements such as lenses. In some instances however, this is not always convenient, especially of the source is large. It is known to use a pair of mirrors rotatable about perpendicular axes, or a single gimbal led mirror. However, these solutions themselves tend to be rather bulky. Our British patent No. 1,521,931 describes an alternative technique which uses two optical wedges rotatable about a common axis along which the radiation enters the device. This allows the beam to be displaced in any direction within a cone. However, small movements of the beam near to the centre of the field of view require quite large angular movements of the wedges.Since the speed at which the wedges may be moved is limited, this results in long response times to demanded beam movements. This makes it unsuitable as a servo-controlled beam stabilising device, though it may still be suitable for pointing a beam in a required direction where speed of response is not of paramount importance.

It is an object of the invention to provide optical apparatus for controlling the direction of a beam of optical radiation which does not suffer from these disadvantages.

According to the present invention there is provided optical apparatus for controlling the direction of a beam of optical radiation incident upon the apparatus along the optical axis thereof, which includes first and second lenses each having a plane surface and a spherically-curved surface and arranged with their curved surfaces adjacent to one another and separated by a gap of substantially uniform width which is small compared with the radius of curvature of each curved surface, and means for producing relative movement between the two lenses about axes passing through the centres of curvature of the curved surfaces perpendicular to the optical axis and at an angle to one another.

The means for producing relative movement may be such as to respond automatically to a demand input defining a required beam direction.

The term "optical radiation" is used to inciude not only radiation in the visible part of the spectrum, but also radiation in adjacent parts of the spectrum which obeys the laws of optics. This includes infra-red and ultra-violet radiation.

The invention will now be described with reference to the accompanying drawings, in which Figure 1 is a perspective view showing the arrangement of the two lenses; Figures 2 and 3 are sectional views showing the lenses in different relative positions; Figures 4 and 5 illustrate the effect of moving one or other of the lenses; Figures 6and 7illustrate a practical form of the apparatus; and Figure 8 is a schematic diagram of a control circuit.

Referring now to Figure 1, this shows a first lens 10 and a second lens 11 positioned about an optical axis 12. Lens 10 has a plane surface and a convex surface, whilst lens 11 has a plane surface and a concave surface. The curved surfaces of the lenses are positioned adjacent to one another. The lenses are movable relative to one another, each being rotatable about an axis perpendicular to the optical axis 12. Hence lens 10 is rotatable about an axis 13, whilst lens 11 is rotatable about an axis 14. The axes 13 and 14 are preferably, but not necessarily, perpendicular to one another.

Figure 2 is a sectional view of the two lenses in their normal position, in which a beam of light passing through the lenses is not deflected. It will be seen from the Figures that the two lenses are arranged with a small gap 15 of uniform width between their curved surfaces. The gap should be small in relation to the radius of curvature of each curved surface. In order to produce a gap of uniform width the two lenses should have the same centre of curvature, and to maintain the uniform gap as the lenses move relative to one another the axes of rotation 13 and 14 must pass through this centre of curvature. It will be appreciated, however, that small departures from a perfectly uniform gap may be tolerated, depending upon the dimensions involved.The departure from a uniform gap which may be tolerated depends upon a number of factors, including the required maximum angle of deflection, the refractive index of the lens material and the wavelength of the radiation. If the gap departs too much from uniform width, then the lenses will tend to act as such. Too large a gap may cause problems due to spherical aberration.

It will be seen from Figure 2 that with the lenses positioned symmetrically about the optical axis 10 the effect is that of a parallel-sided glass block. Hence a light beam passing through the two lenses will not be deflected from that direction.

Figure 3 shows the lens 10 rotated about the axis 13so that the plane faces of the lenses are no longer parallel. This results in deflection of a light beam as will be described below. The other lens 11 may, at the same time, be rotated aboutthe axis 14 to produce deflection in a second plane.

Figure 4 illustrates the situation which exists if the plano-convex lens 10 is rotated about the axis 13. The effect is to produce a prism with its sides formed by the plane surfaces of the two lenses. The plane face of lens 11 is perpendicular to the optical axis 12, and hence no refraction takes place at this surface. Lens 10 is shown deflected so that the angle between the two plane surfaces is a. The light beam strikes the other face at an angle of incidence i, and leaves at an angle of refraction r. It will be seen that the angle of deflection of the light beam is thus (r-i). The angle of incidence i is equal to the angle a of the effective prism, and the angles of incidence and refraction are reflected by the expression sin r/sin i = y where y is the refractive index of the lens material relative to air.It can therefore be shown that the angle of deflection D of the light beam is given by the expression D = sin sin a) -a If the deflection of lens 10, and hence the angle a, is small, then, approximately.

D=a( The deflection is thus directly related to the movement of the lens 10.

If the plano-concave lens 11 is moved, the situation is slightly different, as shown in FigureS. The effect again is to produce a prism formed by the plane surfaces of the lenses. However, in this case it is the plane surface of the lens 10 which is perpendicular to the optical axis 12. Hence refraction occurs at the plane surface of lens 11 as shown in the drawing. Since the light beam strikes the plane surface of lens 10 at an angle, refraction also occurs at this surface. However, it will be seen that the deflection of the light beam is determined only by the angle of refraction r2 at the second surface.

The angle of incidence ii at the first surface is equal to the prism angle a, and the angle of incidence i2 at the second surface is given by i2 = ii -ra Where r1 is the angle of refraction at the second surface.

Therefore i2 = a - r1 Now sin r2 = sin i2 = Where p is the air-to-glass refractive index, therefore sin r2 = lisin i2 = sin (α - r1) Also, sin ii sin r therefore sin r1 = sin il !1 Hence sin r2 = sin[a - sin1(sin a/ > )] Therefore the deflection angle D is given by the expression D = sin-'sin[a - sin-'(sin CI/CL)I When all angles are small, an approximation is D=a(ii-1) This approximation is the same as that resulting from movement of the other lens.

So far, nothing has been said about the means for moving the two lenses to provide a required deflection of the light beam. It is, as suggested above, possible to rotate each lens relative to the optical axis about an axis perpendicular to the optical axis. Preferably the two axis of rotation are perpendicular to one another.

Alternatively it is possible to fix one lens relative to the optical axis and to rotate the other lens about two mutually perpendicular axes. Figures 6 and 7 illustrates an arrangement which may be used in either way.

Referring now to Figures 6 and 7, these show an outer carrier 60, which is a generally tubular structure aligned about the optical axis 12 of the apparatus. The outer carrier carries one of the lenses, in this case the plano-concave lens 11, which is retained in position by a retaining ring 61. Supported within the outer carrier 60, coaxial therewith, is an intermediate member 62 in the form of a tube. This is supported for rotation about an axis 63 passing through and perpendicular to the optical axis 12, and intersecting the optical axis at the centre of curvature of the curved surface lens 11. The supports for the intermediate member 62 are provided by a torque motor 64 and an angle pick-off 65.

The intermediate member itself supports an inner carrier 66which carries the plano-convex lens 10. The inner carrier 66 is supported for rotation about an axis 67 passing through the optical axis 12 and the centre of curvature of the curved surfaces of the lenses, and perpendicular to the optical axis 12 and to axis 63. The supports for the inner carrier are provided by a torque motor 68 and an angle pick-off 69.

If it is required to be able to rotate both of the lenses 10 and 11, then the intermediate member 62 is secured to a supporting structure, leaving carriers 60 and 66 able to rotate about the perpendicular axes 63 and 67. Alternatively, the outer carrier 60 may be fixed, along with lens 11. The inner carrier 62 and lens 10 is then able to move about both of the axis 63 and 67, in a gimbal-like structure.

The torque motors and angular pick-offs on the two axes are provided to enable the apparatus to be controlled automatically. Since it is possible to relate beam deflection to the angle of deflection of the or each lens, an automatic control system may readily be provided.

As stated earlier, for movement of either lens, the deflection angle of the light beam D is given, approximately, by D = α ( - 1), where a is the angle of the prism formed by the plane surfaces of the two lenses. This angle a is the angle through which the lens is rotated from the datum position in which the plane surface is perpendicular to the optical axis of the apparatus. In a controlled system it is this angle a which has to be varied, and this is given by the expression a=(D/(ii- 1) Hence any control circuit has to apply the ratio 1/(ii - 1) to the demand input to obtain the required lens position about one axis. The apparatus of Figures 6 and 7 enables a servo system to be used, and a diagram of one such system is shown in Figure 8.

Figure 8 shows a simple system applied to one axis of the apparatus which makes use of the above approximation. The demand input D is applied through a resistor R1 to the input of the servo amplifier 81, the output of which drives the servo motor 82 and hence the appropriate one of the lenses 10 and 11. The pickoff 83 is mechanically coupled to the lens and provides a feedback signal which is applied through a resistor R2 to the input of amplifier 81.

Itwill be seen that D a R1 - R2 butD = a(y1) Therefore: a(-1) ~ a R1 R2 or R1 R2 = Thus if the values of the resistors R2 and R2 are chosen to satisfy the relationship given above, then the system will automaticallyfollowthe required law.

If the approximation is not satisfactory, either for accuracy or because larger angular excursions are required, then the simple system of Figure 8 is no longer adequate. Figure 9 shows an alternative servo system which may then be used.

Figure 9 shows a servo system which has a modified feedback loop to take account of the non-linear relationship between the demanded light beam angle and the lens position. As is usual, the demand input D is applied to a difference circuit 91 where the feedback signal is subtracted to form the error signal. This error signal passes to a servo amplifier 92 and then to the torque motor 93 which drives the lens. The pick-off 94 is coupled mechanically to the lens, as shown in Figures 6 and 7. If the pick-off gives an analog output, then this is passed to an analog-to-digital converter 95. The output of the converter 95 is applied to a read-only memory 96 which has previously been given the correction factor corresponding to any actual lens position, for each increment of position. The correction factor is converted back to analog form by a digital-to-analog converter 97. The correction factor and the original pick-off output are added by a summing amplifier 98 which applies its output to the difference circuit 91.

The control circuit may be varied. For example, the correction may be applied to the demand input signal before its application to the difference circuit 91, in which case the read-only-memory 96 need only convert demand input angle directly to the lens position necessary to achieve this angle.

It is, of course, necessary to provide a separate control circuit for each axis of movement of the lens system, regardless of whether one lens is moved about two axes, or each lens about its own axis of rotation.

It will be appreciated that the apparatus may be controlled manually, without the need for any form of control system.

Claims

1. Optical apparatus for controlling the direction of a beam of optical radiation incident upon the apparatus along the optical axis thereof, which includes first and second lenses each having a plane surface and a spherically-curved surface and arranged with their curved surfaces adjacent to one another and separated by a gap of substantially uniform width which is small compared with the radius of curvature of each curved surface, and means for producing relative movement between the two lenses about axes passing through the centres of curvature of the curved surfaces perpendicular to the optical axis and at an angle to one another.

2. Apparatus as claimed in Claim 1 in which the means for producing relative movement between the two lenses includes drive means for producing the relative movement and pick-off means indicating the extent of such movement.

3. Apparatus as claimed in Claim 2 which includes control means responsive to the output of the pick-off means to produce signals for application to the drive means.

4. Apparatus as claimed in Claim 3 in which the control means include a servo loop having a read-only memory in the feedback path.

5. Apparatus as claimed in any one of claims 1 to 4 in which the two axes about which relative movement between the lenses is produced are mutually perpendicular.

6. Apparatus as claimed in Claim 5 in which each lens is movable about a separate axis perpendicular to the optical axis.

7. Apparatus as claimed Claim 5 in which one lens is fixed relative to the optical axis, the other lens being movable about two mutually perpendicular axes.

8. Optical apparatus for controlling the direction of a beam of optical radiation, substantially as herein described with reference to the accompanying drawings.