GB2288247A

GB2288247A - Beam sampling mirror having regular array of secondary reflecting surfaces

Info

Publication number: GB2288247A
Application number: GB9406189A
Authority: GB
Inventors: Brooke Armitage Ward
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-03-29
Filing date: 1994-03-29
Publication date: 1995-10-11
Also published as: GB9406189D0

Abstract

A beam sampling device consisting of a mirror 21 having a regular array of identical depressions 24 in the surface 23. The bottoms 25 of the depressions form mirrors adapted to reflect a portion 27 of an incident beam of radiation 26 in a direction different to that in which the remainder of the incident beam is reflected by the mirror as a whole. The device is used to sample a laser beam. <IMAGE>

Description

Beam Samnlinq The present invention relates to the sampling of beams of radiation and, more specifically, to the monitoring of a beam of laser radiation to determine its propagation mode or optical quality and/or its power, polarization state or pointing direction.

The parameters of a beam which can be of prime importance to a process which employs a beam of intense radiation, such -as that from a laser, as a processing tool include the optical mode quality, the power, the state of polarization and pointing direction. In order for any such process to be carried out on a repetitive basis it is necessary to be able to measure these parameters, monitor them and control the operation of the beam generation and delivery system so as to maintain the beam quality, power, polarization and pointing direction within predetermined limits. In an industrial environment it is not really practicable to make the desired measurements upon a workpiece directly and beam sampling techniques are used.

One way in which this is done is to use an optical element, the reflectivity of which is controlled so that either a major, known proportion of the incident radiation is reflected and used for processing while the remainder is transmitted and used for monitoring the quality, power, polarization or pointing direction of the incident beam of radiation, or vice versa. Such devices are not suitable for use with high power beams of radiation both because of the inevitable absorption of some of the radiation by the body of the device leads to thermal distortion of the device and hence to the degradation of quality of the major part of the beam of radiation, and because the multilayered coatings which are used to achieve the controlled reflectivity can be damaged, particularly when the beam power is greater than about 3 kilowatts.

Another approach to beam sampling is to use in the beam delivery system a mirror which has an array of holes in it so that part of the beam of radiation is reflected and part transmitted as before.

In this case however, the portion of the beam that is transmitted is determined by the ratio of the total area of the holes to that of the reflecting surface of the mirror. Such a hole matrix mirror is shown in Figure 1 of the accompanying drawings and consists of a substrate 1 which has a reflecting surface 2 and an array of parallel holes drilled in it. Each hole 3 is provided with a counterbore 4. In the example shown, the hole matrix mirror is intended to be used as a 450 beam-folding mirror and the holes 3 and counterbores 4 are drilled at this angle to the reflecting surface 2.

An array of holes in a mirror will act as a diffraction grating and if the radiation emanating from the holes is brought to a focus by a lens, an array of spots is formed in the focal plane of the lens.

The spots are the results of interference between the radiation passing through and diffracting from each of the holes in the mirror and recombining in the focal plane. Each spot has an intensity distribution that is a close approximation to the farfield diffraction pattern of the original full beam of radiation.

The envelope of the peak intensities of all the focused spots in the diffraction pattern is equivalent to that from a single hole in the hole matrix mirror.

The zero order diffraction pattern in the centre of the array of focused spots is that which approximates most closely to the farfield diffraction pattern of the original beam. This is partly because the position of the undeflected order is insensitive to the wavelength of the radiation. Furthermore, if the holes in the reflecting surface of the mirror present a circular aperture to the input beam, then the transmitted beam diffraction pattern and power is virtually insensitive to the state of polarization of the incident beam.

In practice, hole matrix mirrors have a hole spacing that is between 5 and 20 times the diameter of the holes, with at least eight holes across the diameter of the incident beam. When the beam of laser radiation is that from a CO2 laser , which is one of the lasers most commonly used for material processing, then the above requirements lead to hole diameters in the range 0.05 mm to 0.3 mm.

Not only are such holes difficult to form by conventional drilling processes, but the portion of the original beam emanating from each hole has a large divergence. This is the reason why each hole 3 in the hole matrix mirror described above has a counterbore 4. The length of the holes 3 must be sufficiently short and the counterbores 4 must have a sufficient diameter at the exit from the rear face of the mirror so that the beam emanating and diverging from the entrance to each hole 3 does not impinge on the inner surface of the counterbore.

The net effect of the holes 3 and their counterbores 4 is that a hole matrix mirror is restricted in the range of diameters of the incident beam with which it can be used and, as with the controlled reflectivity beam sampling device, a hole matrix mirror is susceptible to distortion by thermal stressing if it is used with high power laser beams because of the interference of the holes 3 and counterbores 4 with the thermal flux within the mirror substrate 1.

It is an object of the present invention to provide an improved beam sampling device for use in the monitoring of beams of radiation.

According to the present invention there is provided a beam sampling device comprising, a mirror including a main reflecting surface and a regular array of identical secondary surfaces adapted to direct a pre-determined portion of a beam of radiation incident upon the device in a direction different to that in which the remainder of the incident beam of radiation is reflected by the main reflecting surface of the mirror.

Preferably the secondary reflecting surfaces are formed by flatbottomed depressions in the main reflecting surface of the mirror and the bottoms of the depressions are aligned so as to produce the sample beam of radiation which is directed in a direction different to that in which the remainder of the incident beam of radiation is reflected by the main reflecting surface of the mirror.

The invention will now be explained, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-section of a portion of a hole matrix mirror.

Figure 2 is a cross-section of a portion of a mirror embodying the invention Figure 3 is a representation of a section of the mirror of Figure 2 on an enlarged scale, and Figure 4 is a representation of the mirror of Figure 2 as presented to an incident beam of radiation.

Referring to the drawings, Figure 1 has already been described in the introduction to this specification and will not be described further.

Figure 2 illustrates the action of a dimple matrix mirror embodying the invention. The dimple matrix mirror 21 consists of a body 22 of material, which may include cooling channels, if desired, although none is shown in the drawing, which has a main reflecting surface 23. The reflecting surface 23 has formed in it a regular array of depressions or dimples 24 the bottoms 25 of which are plane and aligned with one another. The depressions 24 are identical to one another and so oriented as to reflect a known proportion of a beam 26 of radiation, which is incident upon the mirror 21 at an angle of 450, through an angle of 1350 to form a beam 27 of radiation which is a sample of the beam of radiation 26.

In effect, the bottoms 25 of the depressions 24 act as a blazed diffraction grating. If desired, another angle of deflection can be chosen so long as it is such that interference between the beam 26 and the beam 27 does not occur at a position at which analysing equipment could be placed.

The cross-section of the depressions 24, as seen from a direction normal to the main reflecting surface 23 of the mirror 21, can be made to be such as to present a circular cross-section to the incident beam 26 of radiation. Similarly, the spacing of the depressions 24 can be made to present a square or other regular array to the incident beam as shown in Figure 3.

The optical design criteria for a dimple matrix mirror according to the invention are similar to those for a hole matrix mirror, but one is no longer constrained to such an extent with regard to the size of the depressions 24, their spacing or the optical aperture covered by them.

The depressions 24 can be formed by pressing a suitably shaped punch a controlled distance into a substrate body in a step-andrepeat process to form the desired matrix of dimples. The most important requirement is that the bottoms 25 of the depressions 24 should all be at the same depth and orientation with respect to the reflecting surface 23 of the mirror 21. This is to ensure that the relative phases of the wavelets emerging from the depressions do not vary.

Because the beams 26 and 27 of radiation do not pass through the body 21 of material which forms the dimple matrix mirror, and in any case such cooling channels or fins as one desires can be incorporated into the mirror body 21, the mirror is far less susceptible to thermal stress or damage than is either a controlled reflectivity sampling device or a hole matrix mirror.

The uses of a dimple matrix mirror such as that described above are not limited to sampling a beam of radiation to determine its farfield diffraction pattern. Such mirrors can also be used to extract a calibrated fraction of the power of a beam of radiation. Also, collection of all the diffracted orders of the sample beam 26 onto a suitable detector will enable the combination to act as a power meter for recording the power of the beam 26 during a processing operation. Alternatively, any single order of the diffracted beam can be selected by means of a suitably sized pinhole and the power transmitted through it can be monitored. A reduction in the measured power more than a pre-determined value could be used to indicate that a laser system had drifted into a state which was unsuitable for a process being undertaken. Remedial action could then be taken.

The sampling mirror described is not limited to having a planar reflecting surface. The depressions could be formed in the reflecting surface of a spherical surface or paraboloidal surface of mirrors in a beam line used for beam expanding or final focusing duties. In the latter case the quality or power in a beam could be examined immediately prior to impinging on a workpiece. Also, it is not necessary for the dimples to be identical to one another.

The dimples could be of more than one type, each type being spread uniformly over the mirror so that each group of dimples could produce a separate beam sample directed at different diffraction angles.

Claims

1. A beam-sampling device comprising, a mirror including a main reflecting surface and a regular array of identical secondary reflecting surfaces adapted to direct a predetermined portion of a beam of radiation incident upon the device in a direction different to that in which the remainder of the incident beam is reflected by the main reflecting surface of the mirror.

2. A beam-sampling device according to claim 1 wherein each of the secondary reflecting surfaces is situated at the bottom of a localised depression in the main reflecting surface of the mirror.

3. A beam-sampling device according to claim 1 or claim 2 wherein the secondary reflecting surfaces have a cross-section such as to present a circular aspect to the incident beam of radiation.

4. A beam-sampling device according to any preceding claim wherein the array of secondary reflecting surfaces is a square array.

5. A beam-sampling device according to any preceding claim adapted to act as a beam-folding mirror in a beam delivery system.

6. A beam-sampling device according to any of the claims 1 to 4 adapted to act as a focusing, collimating or diverging mirror in a beam delivery system.

7. A beam-sampling device according to any preceding claim in association with means for measuring the total power in the portion of the incident beam reflected by the secondary surfaces thereby to provide a measure of the power in the remainder of the incident beam.

8. A beam-sampling device according to any of claims 1 to 6 in association with means for measuring the power in any single order of the diffracted beam produced by the secondary reflectors, means for producing a signal when the power in the selected order of the diffracted beam falls below a predetermined level and means responsive to the said signal to alter the operating conditions of a source of the said radiation to rectify the operation of the source of radiation to an optimum condition.

9. A beam-sampling device according to any preceding claim in association with means for measuring the polarization state in the portion of the incident beam reflected by the secondary surfaces thereby to provide a measure of the state of polarisation in the remainder of the incident beam.

10. A beam-sampling device according to any preceding claim in association with means for measuring the transverse position of any single order of the diffracted beam thereby to provide a measure of the pointing direction of the incident beam.

11. A beam-sampling device substantially as hereinbefore described and with reference to Figures 2, 3 and 4 of the accompanying drawings.