GB2117511A

GB2117511A - Laser beam alignment detection

Info

Publication number: GB2117511A
Application number: GB08304736A
Authority: GB
Inventors: Dr Paul Derek Cook
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-02-19
Filing date: 1983-02-21
Publication date: 1983-10-12
Also published as: GB8304736D0

Abstract

A laser beam detector for use in an alignment system comprises an array of four contiguous photo-sensitive elements 4a, 4b, 5a and 5b arranged so that an incident beam 7 passes out through an aperture at the centre of the array and is incident on a mirror 8 attached to a surface whose misalignment is to be measured the reflected beam 9 returning and striking the photo-detector to indicate any misalignment. <IMAGE>

Description

SPECIFICATION A laser beam detector and method of alignment using a laser The invention relates to a laser beam detector and to a method of alignment using a laser.

Laser beam alignment systems have been proposed where a laser beam is directed at a mirror on a surface to be aligned and the position of the reflected beam is detected by means of a lightsensitive detector arrangement. Prior systems have required beam-splitting arrangements to separate the incident and reflected beams. The introduction of further optical elements in the laser beam path not only detracts from the optical efficiency of the system but also introduces potential errors because of imperfections or misalignment or vibration in the further optical elements. The present invention seeks to provide an improvement.

According to one aspect of the invention there is provided a laser beam detector comprising a planar array of photo-sensitive elements with an aperture at the centre of the array, the detector being arranged in use so that an incident laser beam passes through the aperture and, after reflection, returns to the detector, misalignment of the incident and reflected beams being thus detected and indicated by instruments connected to the detector.

According to another aspect of the invention there is provided a method of alignment using a laser, the method consisting in directing a laser beam as an incident beam to a mirror on a surface to be aligned with reference to the laser beam, the incident laser beam being arranged to pass through the aperture of a laser beam detector as described above, and the method consisting in observing the output of instruments connected to the photo-sensitive elements to give an indication of misalignment between the incident and reflected laser beams.

Preferably the detector comprises a quadrant of opposed pairs of elements and the instruments comprise amplifiers and meters connected to amplify and indicate differences in laser beam incident on the members of the respective pairs and coincidence of the incident and reflected beams is detected by reference to periodic fluctuations of the meter readings. This technique takes advantage of the discovery that when the beams are coincident there is developed, apparently by an interference effect, a regularly fluctuating signal from the opposed pairs of detectors. Typically, the period of this signal is about one cycle per second, which is well within the response time of the meters. Thus, with the arrangement of the invention, interference effects can be used to advantage and are not introduced by added optical elements in the laser path.

The invention will further be described with reference to the accompanying drawings, of which Figure 1 is a perspective view of a laser alignment system using a detector in accordance with the invention; and Figure 2 is a front elevation of the detector of Figure 1.

Referring to Figure 1 the system comprises a helium-neon laser 1 to the end of which is screwthreaded a cylindrical mount 2. Mount 2 has an end face on which is mounted a quadrant array 3 of four photo-sensitive elements.

The array 3 is shown in Figure 2 and comprises four contiguous semi-conductor crystals 4a, 4b, 5a, and 5b. The crystals are impurity-doped silicon crystals and the members of the opposed pairs are matched in their photo-sensitive responses.

The inner corner of each crystal is cut away so as to leave an aperture 6 at the centre of the array.

Reverting to Figure 1, the incident beam 7 from the laser passes through aperture 6 and strikes a mirror 8 which is mounted on a surface (not shown) of which the alignment with respect to beam 7 is to be determined. The mirror is shown misaligned, with the consequence that the reflected beam 9 strikes the detector off-centre. In this example the beam strikes the crystal 4a immediately above the aperture 6, so there is vertical misalignment but no horizontal misalignment.

The crystals 4a and 4b are connected to the input of a first amplifier, which amplifies differences in the currents therefrom and gives a corresponding output to a vertical meter 10. Similarly, the crystals 5a and 5b are connected to another amplifier to cause a horizontal meter 11 to respond to any differences in output from the crystals. A switch 12 allows the gain of the amplifier to be adjusted.

It is found that precise alignment of the incident and reflected beams causes the two meter pointers, which nominally read zero, to oscillate from side to side. The period of oscillation is typically of the order of one second. It is thought that this is due to an interference effect. In practice it is found that this effect considerably facilitates precise determination of exact alignment.

It will be seen that the detector of the present invention provides a simple arrangement which may be retro-actively fitted to an existing laser. The end surface of the detector housing in which the crystals are mounted may be of any suitable material, including glass or quartz. The aperture 6 will be sited over a hole in the end surface if the surface is opaque. There may or may not be a hole if the surface is transparent.

1. A laser beam detector comprising a planar array of photo-sensitive elements with an aperture at the centre of the array, the detector being arranged in use so that an incident laser beam passes through the aperture and, after reflection, returns to the detector, misalignment of the incident and reflected beams being thus detected and indicated by instruments connected to the detector.

2. A laser beam detector as claimed in claim 1 wherein the array of photo-sensitive elements is carried on a mount which is adapted to be fitted directly to the end of a laser.

3. A laser beam detector as claimed in either of the preceding claims wherein the array consists of a quadrant of four elements arranged as matched

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION A laser beam detector and method of alignment using a laser The invention relates to a laser beam detector and to a method of alignment using a laser. Laser beam alignment systems have been proposed where a laser beam is directed at a mirror on a surface to be aligned and the position of the reflected beam is detected by means of a lightsensitive detector arrangement. Prior systems have required beam-splitting arrangements to separate the incident and reflected beams. The introduction of further optical elements in the laser beam path not only detracts from the optical efficiency of the system but also introduces potential errors because of imperfections or misalignment or vibration in the further optical elements. The present invention seeks to provide an improvement. According to one aspect of the invention there is provided a laser beam detector comprising a planar array of photo-sensitive elements with an aperture at the centre of the array, the detector being arranged in use so that an incident laser beam passes through the aperture and, after reflection, returns to the detector, misalignment of the incident and reflected beams being thus detected and indicated by instruments connected to the detector. According to another aspect of the invention there is provided a method of alignment using a laser, the method consisting in directing a laser beam as an incident beam to a mirror on a surface to be aligned with reference to the laser beam, the incident laser beam being arranged to pass through the aperture of a laser beam detector as described above, and the method consisting in observing the output of instruments connected to the photo-sensitive elements to give an indication of misalignment between the incident and reflected laser beams. Preferably the detector comprises a quadrant of opposed pairs of elements and the instruments comprise amplifiers and meters connected to amplify and indicate differences in laser beam incident on the members of the respective pairs and coincidence of the incident and reflected beams is detected by reference to periodic fluctuations of the meter readings. This technique takes advantage of the discovery that when the beams are coincident there is developed, apparently by an interference effect, a regularly fluctuating signal from the opposed pairs of detectors. Typically, the period of this signal is about one cycle per second, which is well within the response time of the meters. Thus, with the arrangement of the invention, interference effects can be used to advantage and are not introduced by added optical elements in the laser path. The invention will further be described with reference to the accompanying drawings, of which Figure 1 is a perspective view of a laser alignment system using a detector in accordance with the invention; and Figure 2 is a front elevation of the detector of Figure 1. Referring to Figure 1 the system comprises a helium-neon laser 1 to the end of which is screwthreaded a cylindrical mount 2. Mount 2 has an end face on which is mounted a quadrant array 3 of four photo-sensitive elements. The array 3 is shown in Figure 2 and comprises four contiguous semi-conductor crystals 4a, 4b, 5a, and 5b. The crystals are impurity-doped silicon crystals and the members of the opposed pairs are matched in their photo-sensitive responses. The inner corner of each crystal is cut away so as to leave an aperture 6 at the centre of the array. Reverting to Figure 1, the incident beam 7 from the laser passes through aperture 6 and strikes a mirror 8 which is mounted on a surface (not shown) of which the alignment with respect to beam 7 is to be determined. The mirror is shown misaligned, with the consequence that the reflected beam 9 strikes the detector off-centre. In this example the beam strikes the crystal 4a immediately above the aperture 6, so there is vertical misalignment but no horizontal misalignment. The crystals 4a and 4b are connected to the input of a first amplifier, which amplifies differences in the currents therefrom and gives a corresponding output to a vertical meter 10. Similarly, the crystals 5a and 5b are connected to another amplifier to cause a horizontal meter 11 to respond to any differences in output from the crystals. A switch 12 allows the gain of the amplifier to be adjusted. It is found that precise alignment of the incident and reflected beams causes the two meter pointers, which nominally read zero, to oscillate from side to side. The period of oscillation is typically of the order of one second. It is thought that this is due to an interference effect. In practice it is found that this effect considerably facilitates precise determination of exact alignment. It will be seen that the detector of the present invention provides a simple arrangement which may be retro-actively fitted to an existing laser. The end surface of the detector housing in which the crystals are mounted may be of any suitable material, including glass or quartz. The aperture 6 will be sited over a hole in the end surface if the surface is opaque. There may or may not be a hole if the surface is transparent. CLAIMS

3. A laser beam detector as claimed in either of the preceding claims wherein the array consists of a quadrant of four elements arranged as matched pairs, the members of each pair being on opposite sides of the aperture.

4. A laser beam detector as claimed in claim 3 wherein the photo-sensitive elements are semiconductor crystals.

5. A method of alignment using a laser, the method consisting in directing a laser beam as an incident beam to a mirror on a surface to be aligned with reference to the laser beam, the incident laser beam being arranged to pass through the aperture of a laser beam detector as claimed in any of the preceding claims, and the method consisting in observing the output of instruments connected to the photo-sensitive elements to give an indication of misalignment between the incident and reflected laser beams.

6. A method of alignment as claimed in claim 5 wherein the detector comprises a quadrant of opposed pairs of elements and the instruments comprise amplifiers and meters connected to amplify and indicate differences in laser beam incidence on the members of the respective pairs,

7. A method of alignment as claimed in claim 6 wherein coincidence of the incident and reflected beams is detected by reference to periodic fluctuations of the meter readings.

8. A laser beam detector substantially as hereinbefore described with reference to the accompanying drawings.