GB2074339A

GB2074339A - Optical Attenuator

Info

Publication number: GB2074339A
Application number: GB8012914A
Authority: GB
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1980-04-18
Filing date: 1980-04-18
Publication date: 1981-10-28
Also published as: GB2074339B; DE3113840A1; JPS56164303A; AU6936781A; FR2480949A1; AU531891B2; SE8102249L

Abstract

A continuously-variable optical attenuator comprises two optical elements each arranged to be located on the axis of a beam of radiation with a plane surface perpendicular to the axis. Each element has a constant optical density in a first direction in the plane of the surface, and a continuously-variable optical density in a second direction perpendicular to the first direction. The continuously- variable optical densities of the two elements vary at the same rate but in opposite senses along the second direction. Means are provided for moving the two optical elements relative to one another in said second direction.

Description

SPECIFICATION Continuously Variable Optical Attenuator This invention relates to continuously variable optical attenuators.

Many types of optical attenuators are known, and are used in many types of optical equipment to control the intensity of a beam of optical radiation. One commonly-used form of attenuator is the iris diaphragm, whilst others include variable density and neutral filters.

One basic disadvantage which occurs with these forms of attenuator is that their use causes the intensity of the beam to vary over its crosssection. Indeed, in the case of the iris diaphragm the effect is to reduce the cross-section of the beam. In many instances this variation of the cross-section cannot be tolerated, for example, in laser apparatus where diffraction effects could be introduced.

It is an object of the invention to provide a continuously variable optical attenuator which provides constant attenuation over the entire cross-section of a beam of radiation, and which does not affect that cross-section.

According to the present invention there is provided a continuously-variable optical attenuator which includes a pair of optical elements each arranged to be located on the axis of a beam of radiation and each having a plane surface extending perpendicular to the said axis and having a constant optical density in a first direction in the plane of the surface and a continuously-variable optical density in a second direction perpendicular to the first direction, the continuously-variable densities of said two elements varying at the same rate but in opposite senses along said second direction, and means for producing relative movement between the two optical elements in said second direction.

Preferably each optical element comprises a prism of optically-absorbing material having a constant thickness in said first direction and a continuously-variable thickness in said second direction.

The inclined faces of the two prisms may be located adjacent to one another, and each prism may be combined with a prism of optically nonabsorbing material to form a parallel-sided block.

The invention will now be described with reference to the accompanying drawings, in which: Figures 1 and 2 are drawings of a first embodiment, illustrating the operation of the invention; Figure 3 is a view of a second embodiment; Figure 4 is a view of a third embodiment; and Figures 5 and 6 illustrate methods of producing the required relative movement.

Referring now to Figures 1 and 2, the attenuator comprises two prisms 10 and 1 each made from an optically-absorbing material such as glass. The small prism 10 is fixed relative to the optical axis 1 2 of a beam of radiation 13. The prism 10 has one face 14 perpendicular to the optical axis 12, and a second face 15 inclined at an angle to the axis. The thickness of the prism in a direction perpendicular to the plane of the drawing is constant.

The second prism 11 is of the same proportions as the prism 10, but considerably larger in size. This prism also has one face 16 perpendicular to the optical axis 12 and a second face 1 7 inclined at the same angle as the face 1 5.

Again, the prism 11 is of constant thickness. The two prisms are arranged with their inclined faces, which are parallel to one another, adjacent to one another.

The larger prism 11 is arranged so that it may be moved in a direction parallel to the face 16, as indicated by the arrow. Both prisms must always intersect the whole of the beam of radiation 13, and Figure 1 shows the movable prism 11 almost at one end of its range movement. The degree of attenuation is dependent upon the length of the path of the radiation through the absorbing material. Since the faces 14 and 16, and the faces 1 5 and 17 are parallel, then it will be seen that all parts of the beam 13 pass through an equal thickness of material, and hence the attenuation is constant over the entire cross-section of the beam.

Figure 2 shows the movable prism 11 at the other end of its range of movement. Here again, the attenuation is constant over the entire crosssection of the beam, though in this case the attenuation is greater since the beam passes through a greater thickness of material.

The rate at which the attenuation may be varied is dependent upon the angle of inclination of the inclined 15 and 17, and of the attenuating properties of the absorbing material.

One of the problems associated with the arrangement shown in Figures 1 and 2 is that the path of the beam of radiation is not quite as shown, due to refraction at the various interfaces.

The result of this is to produce a lateral shift of the beam. This may be avoided by ensuring that all surfaces through which the beam passes are perpendicular to the axis of the beam. Figure 3 shows one way in which this may be done.

Each of the prisms 10 and 11 has bonded to it an identical prism, 30 and 31 respectively of optical glass of the same refractive index as the prisms 10 and 1 so as to form a parallel-sided element as shown. The effect is exactly the same as before, apart from a minimal attenuation due to the optical glass, except that there is now no refraction from the glass-air interfaces since these are now perpendicular to the beam. The two elements may be placed close together to minimise the gap between them. The length of the path of the beam through the optical glass is constant over the entire cross-section of the beam, so that no problems arise due to the presence of the additional prisms 30 and 31.

It can be shown that, in either of the embodiments described above, the power attenuation R of the beam is given by the expression R=1 OkL log,Oe dB or =4.3429 kL dB where k is the absorption coefficient of the absorbing material, and L is the path length, in the absorbing material.

The sensitivity of device, that is the charge in absorption due to a given change in relative position, may be decreased by selecting the absorption coefficient such that a larger movement is required for a given attenuation change. The overall range may be varied by providing two prisms of the same size and making both prisms movable. This is illustrated in Figure 4, in which each element is a double prism as described with reference to Figure 3.

There are many ways in which the required relative movement of the prisms may be produced. In the embodiment of Figures 1, 2 and 3 the smaller prism is fixed in position. As shown in Figure 5, the larger prism may be carried on a linear bearing (not shown) and moved by means of a micrometer adjustment 50 against a spring force shown schematically at 51. This ensures accurate and repeatable positioning. The same form of mounting may be used for each of the prisms in the embodiment of Figure 4.

Since the absorption of the material is dependent upon the wavelength of the radiation, then any calibration of the attenuator is only correct at one particular wavelength. If the attenuator is calibrated at the lowest wavelength at which it is to be used, and the material of the attenuator is such that the absorption coefficient increases as the wavelength increases, then the micrometer movement suggested above may be modified to correct for the change. This may be done, as shown in Figure 6, by displacing the micrometer 50 by a known angle P, so that a given movement of the micrometer produces less movement of the prism.The movement of the micrometer is thus the movement of the prism divided by the cosine of the angle ,B. Since the attenuation is proportional to the displacement of the prism, then the angle p is given by p=cos-' k/( k+Ak) where Ak is the amount by which the absorption coefficient k increases due to the increase in wavelength. The reading of the micrometer will then continue to correspond to the calibrated attenuation.

In place of the elements of absorbing glass described above it is possible to use a pair of variable density filters. These usually consist of a glass substrate on which is formed either an absorbing coating or a reflective coating, whose optical density is a linear function of distance. If reflective coatings are used, care must be taken to avoid unwanted effects from multiple reflections.

As before, the pair of elements are arranged so that the densities of the two vary in opposite senses. Hence, as in the embodiment described above, the attenuation is constant over the entire cross-section of the beam of radiation.

In addition, the attenuator of the variable density filter may in some cases be less wavelength-dependent than that of the variable thickness wedge.

The devices described above may be used with light of any polarisation without the need for recalibration, and any reflections which occur will remain constant regardless of the degree of attenuation. The attenuation may be changed smoothly and continuously. The wavelengthdependent nature of the attenuation may be used to advantage if it is required to provide a wavelength-dependent attenuator.

Claims

1. A continuously-variable optical attenuator which includes a pair of optical elements each arranged to be located on the axis of a beam of radiation and each having a plane surface extending perpendicular to the said axis and having a constant optical density in a first direction in the plane of the surface and a continuously-variable optical density in a second direction perpendicular to the first direction, the continuously-variable densities of said two elements varying at the same but in opposite senses along said second direction, and means for producing relative movement between the two optical elements in said second direction.

2. An attenuator as claimed in Claim 1 in which each optical elements comprises a prism of optically-absorbing material having a constant thickness in said first direction and a continuously-variable thickness in said second direction.

3. An attenuator as claimed in Claim 2 in which each prism has a face inclined at an angle to the axis, the two inclined faces being located adjacent to and parallel to one another.

4. An attenuator as claimed in Claim 2 in which each prism is combined with a prism of optically non-absorbing material so as to form a parallelsided block.

5. An attenuator as claimed in Claim 4 in which the two optical elements are arranged with the prisms of optically non-absorbing material adjacent to one another with the parallel faces of the blocks perpendicular to the axis of the beam of radiation.

6. An attenuator as claimed in Claim 1 in which each optical element comprises a substrate carrying a layer of optically absorbing material, the optical density of the layer increasing linearly in said second direction.

7. An attenuator as claimed in Claim 1 in which each optical elements comprises a substrate carrying one or more reflecting layers, the reflectivity of the layer increasing linearily in said second direction.

8. An attenuator as claimed in any one of the proceeding claims in which one of said optical elements is fixed relative to said axis, the other optical elements being movable in said second direction.

9. An attenuator as claimed in Claim 8 in which said other optical element is movable against a spring force by a micrometer.

10. An attenuator as claimed in Claim 9 in which the direction of thrust of the micrometer is adjustable.

11. A continuously-variable optical attenuator substantially as herein described with reference to the accompanying drawings.