GB2040549A

GB2040549A - Improvements Relating to Laser Apparatus

Info

Publication number: GB2040549A
Application number: GB7942276A
Authority: GB
Original assignee: Crosfield Electronics Ltd
Current assignee: Crosfield Electronics Ltd
Priority date: 1979-01-08
Filing date: 1979-12-07
Publication date: 1980-08-28

Abstract

A laser apparatus comprises a laser oscillator including a laser medium (1), positioned between a pair of oscillator mirrors (2, 3) defining an optical cavity, a laser amplifier including laser media (8, 14) positioned downstream from the output of the laser oscillator, an optical isolator (7, 12) downstream of the laser oscillator, and a narrow band transmission filter (10) in the laser amplifier tuned to the output frequency of the laser oscillator. The combination of the optical isolator (7, 12) to prevent the laser radiation resulting from the laser oscillator and the majority of any radiation of the preferred wavelength setting up oscillations in the amplifier, and the narrow band transmission filter (10) to prevent the transmission of laser radiation at any frequency other than the preferred wavelength in the laser amplifier, results in the suppression of all oscillation in the laser amplifier as a result of reflection from a work piece (16). <IMAGE>

Description

SPECIFICATION Improvements Relating to Laser Apparatus This invention relates to laser apparatus and in particular to frequency selection in high energy laser apparatus.

Such laser apparatus may be used to cut into and engrave the surface of material, for example to engrave an image into the surface of a printing member. It is common for lasers to have more than one operating frequency and it is a well known requirement that the output energy of the laser should be concentrated into a particular, preferred output frequency. One example of this is where the laser medium is carbon dioxide and this produces two basic groups of output radiation one centred at a wavelength of around 10.6 microns and the other centred at a wavelength of around 9.6 microns. In this case, the radiation centred at around 10.6 microns is usually the preferred radiation.

In high energy laser apparatus it is also common to have at least two distinct parts of a laser apparatus, a laser oscillator portion and a laser amplifier portion arranged downstream of the laser oscillator. The laser oscillator includes an optical cavity comprising a pair of mirrors with a laser medium between them arranged to generate and enhance the preferred wavelength of radiation and the output of the oscillator is usually applied to a modulator cell and then to the laser amplifier which includes another region filled with laser medium but with no resonant cavity. The output from the laser oscillator and modulator is merely amplified by the laser amplifier. When the high energy laser apparatus is used to cut or engrave a workpiece the output from the laser amplifier is brought to a focus on the workpiece.

In such a system, oscillations can occur in the amplifier section as a result of reflections from the workpiece. Thus, a secondary optical cavity is set up using the laser medium of the amplifier with the work surface as one of its mirrors and one of the mirrors of the optical cavity of the laser oscillator or another reflecting surface in the amplifier portion as its other mirror. Any such secondary cavity is most unlikely to be tuned to the preferred wavelength and this gives rise to the generation of laser radiation at other than the preferred wavelength. The generation of this laser radiation constitutes an unacceptable noise on the modulated laser output, reduces the intensity of the radiation at the preferred wavelength and may reduce the overall intensity of the output of the laser beam.Also, the introduction of these other wavelengths leads to a smearing of the focus of the laser beam in relation to a work surface as a result of the non-monochromaticity of the laser beam.

To reduce this effect an isolator has been included in the optical system, the isolator consisting of a polariser followed by a quarter wavelength plate which is matched to the wavelength of the preferred radiation. Thus, light, after leaving the laser oscillator is plane polarised and then passes through the quarter wavelength plate which results in the light being circularly polarised. Thus, the output from the quarter wavelength plate is circularly polarised and this circularly polarised light is amplified during its passage through the laser amplifier. Any light reflected from the work piece passes through the laser amplifier in the opposite direction and then through the quarter wavelength plate, which converts it into plane polarised light.As a result of the reflection from the work piece and then the second passage through the quarter wavelength plate the light returning to the polariser is polarised in a direction perpendicular to that passed by the polariser and therefore is filtered out. Hence, the isolator prevents light produced by or resulting from the laser oscillator from reentering the optical cavity of the laser oscillator after reflection from the work piece. The inclusion of such an isolator has been entirely successful in achieving this result but light, particularly light not at the preferred frequency, may be spontaneously emitted in the laser amplifier. Such light even if it is at the preferred frequency does not necessarily have the same state of polarisation as the light entering the laser amplifier and may well be at a frequency other than the preferred frequency.Any such spontaneously emitted radiation from the laser amplifier may also be reflected from the work piece. This radiation is amplified on its return path through the laser amplifier and then, since its wavelength may or may not be matched to that of the quarter wavelength plate and, as it may have any initial state of polarisation, a significant part of this unpreferred radiation passes through the polariser and is reflected from the mirror at the output end of the optical cavity, some other reflecting surface in the apparatus, or even possibly enter the optical cavity of the oscillator and be reflected from the mirror at the other end of the optical cavity of the oscillator.

Thus, radiation which does not result from the laser oscillator and particularly that which is not at the preferred laser frequency can oscillate between the work surface and some other reflecting surface and result in oscillation being produced in the lasing medium of the laser amplifier. These oscillations result in the generation of laser radiation which is not at the preferred wavelength and this laser radiation which is not at the preferred wavelength is enhanced by further passes through the amplifier.

This introduces a considerable element of noise on to the modulated laser beam at the preferred wavelength and reduces the pumping available for the preferred wavelength of radiation.

According to this invention a laser apparatus comprises a laser oscillator including a laser medium positioned between a pair of oscillator mirrors defining an optical cavity, a laser amplifier including a laser medium positioned downstream from the output of the laser oscillator, an optical isolator downstream of the laser oscillator, and a narrow band transmission filter in the laser amplifier tuned to the output frequency of the laser oscillator.

The combination of the optical isolator to prevent the laser radiation resulting from the laser oscillator and the majority of any radiation of the preferred wavelength setting up oscillations in the amplifier, and the narrow band transmission filter to prevent the transmission of laser radiation at any frequency other than the preferred wavelength in the laser amplifier results in the suppression of all oscillation in the laser amplifier as a result of reflection from a work piece.

Preferably the narrow band transmission filter is positioned between the laser oscillator output mirror and the laser amplifier, although it may be positioned downstream from the output of the laser amplifier. The laser amplifier may be formed by a multi-stage arrangement and, in this case, the narrow band transmission filter may be included in some point in the path of the multistage laser amplifier.

In high power systems, it is preferred that the narrow band transmission filter is not positioned at the final output of the amplifier since losses of up to 15% occur in the narrow band transmission filter even at the preferred frequency. This loss would result in a loss of output of the same amount but, if the narrow band transmission filter is positioned upstream from the final stage of the amplifier, a loss can be tolerated since it can readily be arranged for the final stage of the amplifier to result in saturation and hence maximum intensity. Further, the power density of the output from the high power amplifier may be sufficient to "burn-off" the narrow band transmission filter since this is usually formed by a dielectric multi-layer filter.

A particular example of a laser apparatus in accordance with this invention for use in engraving a printing member will now be described with reference to the accompanying drawing which is a diagram of the optical path of the apparatus.

The apparatus includes a laser oscillator formed by a laser medium 1 and mirrors 2 and 3 defining an optical cavity tuned to an output of 10.6 microns. The output from the laser oscillator is fed to a plane polariser 4 through an electrooptic modulator cell 5 and an analysing polariser 6. Electrical signals are applied to the electrooptic cell 5 to change the axis of polarisation of the light passing through it and, since the axes of the polarisers 4 and 6 are mutually perpendicular the light output from the analyser 6 is intensity modulated in dependence upon the signal applied to the electro-pptic cell 5. In practice, polarisers 4 and 6 are both formed by plates inclined at Brewster angle so that all the light reflected from the plates is plane polarised.At present, there is no suitable polariser of the Glan prism type available which operates at 10.6 microns but if there were, such prisms would be preferred for the polarisers 4 and 6.

The light then passes through a plate 7 inclined at the Brewster angle and to maximise the light transmission of the system the plate is oriented so that all the plane polarised light from the analyser 6 passes through the Brewster plate 7 and into a laser medium 8 forming the first stage of a laser amplifier. The light is amplified in the laser medium 8 and then passes through a quarter wavelength plate 9 and a narrow band transmission filter 10 formed by a dielectric multilayer filter. After passage through the narrow band transmission filter 10, all of the light is reflected by a mirror 11 and the light returns through the narrow band transmission filter 10, the quarter wavelength plate 9 and through the laser medium 8. The light is further amplified by its second pass through the laser medium 8 and then the majority of the returning light is reflected from the Brewster plate 7.The reflected component is completely plane polarised and this then passes through a second quarter wavelength plate 12, is reflected from a mirror 13 and passes through a laser medium 14 forming the final stage of the amplifier. The output from the laser medium 14 is focused by a lens assembly 15 onto the surface of a work piece 1 6.

The output from the laser oscillator is generally plane polarised but this is "cleaned up" by the polariser 4 and the output of the polariser 4 is entirely plane polarised. The electro-optic cell 5 changes the axis of polarisation of the light and the analyser polariser 6 removes all light having its plane of polarisation the same as the output of the polariser 4. The output beam from the analyser polariser 6 is plane polarised and hence the light after passing through the Brewster plate 7 is still plane polarised. The plane polarised light is amplified by stimulated emission in passage through the laser medium 8 and then, on passage through the quarter wavelength plate 9 this plane polarised light is circularly polarised.Passage through the narrow band transmission filter 10 removes any light that has been generated in the laser medium 8 by spontaneous emission and which does not have the preferred wavelength of 10.6 microns. On reflection from the mirror 11 the direction of circular polarisation is reversed and then upon return through the quarter wavelength plate 9 the polarisation of the light beam is converted to plane polarisation but, its plane of polarisation is now at right angles to that on the forward passage through the laser medium 8. The plane polarised light returns through the laser medium 8 and impinges upon the Brewster plate 7. The light reflected from the Brewster plate 7 to the quarter wavelength plate 12 is plane polarised. Light that is transmitted through the Brewster plate 7 is filtered out by the polariser 4 and anaylser polariser 6. The plane polarised light reflected from the Brewster plate 7 passes through the quarter wave plate 12 and is once again circularly polarised. The direction of polarisation of the circularly polarised light is reversed by the mirror 13 and then this circularly polarised light is amplified by stimulated emission in the laser medium 14. The output from the laser beam 14 is then focused on the work piece 16.

Any light reflected from the work piece 16 is circularly polarised in the same sense as that leaving the quarter wave plate 12 and, upon returning through the lens and the laser medium 14, is amplified to some extent by stimulated emission. The sense of polarisation is reversed by the mirror 13 and hence when this returning light passes through the quarter wavelength plate 12 it is once again plane polarised but, with its plane of polarisation normal to that reflected from the Brewster plate 7 on the forward passage of the light beam so that it is not reflected by the plate 7 but transmitted through it. Thus, the combination of the Brewster plate 7 and the quarter wave plate 12 forms an optical isolator which prevents any light emanating from the laser oscillator being returned to the laser oscillator after reflection from the surface of the work piece 1 6.

However, light may be emitted from both the laser medium 8 and the laser medium 14 by spontaneous emission and this may be at a wavelength which is not the preferred wavelength of the 10.6 microns. Any light emitted by spontaneous emission in the laser medium 8 may be at least partially reflected by the Brewster plate 7, pass through the quarter wavelength plate 12, be reflected from the mirror 13 and then pass through the laser medium 14. Here it may be enhanced by stimulated emission and then focused by the lens 15 onto the work surface 16.

If this non-preferred radiation is then reflected by the work piece 16 it returns through the laser medium 14, is amplified further, passes through the quarter wave plate 12 and, since the quarter wave plate 12 is not matched to the frequency of this radiation, may then be at least partially reflected from the Brewster plate 7 and returned to the laser medium 8. Once again, more of the non-preferred wavelength may be generated by stimulated emission in the medium 8 but, on leaving the laser medium 8 the light passes through the narrow band transmission filter 10 which effectively filters out this non-preferred radiation.Thus, the non-preferred radiation cannot be returned by the mirror 1 1 or either of the mirrors 2 or 3 after passage through all the various other components of the assembly and so set up an oscillation between say the mirror 11 and the work piece 16 and so define an optical cavity using the mirror 11, the work surface 16 and the laser mediums 8 and 14. Thus, any light emitted by spontaneous emission is rapidly damped by the narrow band transmission filter 10 and the generation of oscillation in the amplifier part of the apparatus is thereby prevented.

This results in substantially all of the light reaching the work piece being of the preferred wavelength and results in a low noise level within the apparatus even though, for example, the mirror 11 and the work piece 16 make up an ideal situation whereby at least part of the amplifier could be turned into an oscillator.

When the apparatus is not arranged to operate with a preferred wavelength of 1 0.6 microns it is preferred that the polariser 4 and the analyser polariser 6 are formed by Glan prism type polarisers. In this case the separate Brewster plate 7 is not required since the Glan prism forming the anaylser polariser 6 serves as the polariser of the isolator as well as the analyser polariser of the modulator.

Claims

1. A laser apparatus comprising a laser oscillator including a laser medium positioned between a pair of oscillator mirrors defining an optical cavity, a laser amplifier including a laser medium positioned downstream from the output of the laser oscillator, an optical isolator downstream of the laser oscillator, and a narrow band transmission filter in the laser amplifier tuned to the output frequency of the laser oscillator.

2. A laser apparatus according to claim 1, in which the narrow band transmission filter is positioned between the output mirror of the laser oscillator and the laser amplifier.

3. A laser apparatus according to claim 1, in which the laser amplifier is formed by a multistage arrangement and, in which the narrow band transmission filter is included in some point in the path of the multi-stage laser amplifier.

4. A laser apparatus according to claim 3, in which the narrow band transmission filter is positioned upstream from the final stage of the amplifier.

5. A laser apparatus according to any one of the preceding claims, in which the optical isolator comprises a polariser followed by a quarter wavelength plate.

6. A laser apparatus according to any one of the preceding claims, further comprising an optical modulator positioned immediately downstream from the laser oscillator.

7. A laser apparatus according to claim 6, when dependent upon claim 5, in which the polariser in the optical isolator and the analyser in the optical modulator are formed by the same Glan prism type polariser.

8. A laser apparatus according to any one of claims 1 to 6, in which the optical isolator comprises a plate inclined at the Brewster angle, followed by a quarter wavelength plate.

9. A laser apparatus according to claim 1, constructed substantially as described with reference to the accompanying drawings.

10. An apparatus for preparing printing members by laser gravure including a laser apparatus in accordance with any one of the preceding claims.