THERMO OPTICAL CONTROL OF FOCUS POSITION OF AN ENERGY BEAM IN AN ADDITIVE MANUFACTURING APPARATUS

Abstract

A control system for thermo optical control of focus position of an energy beam in an additive manufacturing apparatus has a first doped medium and a second doped medium, each of which is optically transparent and doped with a dopant. The first doped medium has a positive thermo-optical coefficient (dn/dT) and the second doped medium has a negative thermo-optical coefficient (dn/dT) and is in series with the first doped medium. An energy beam input or coupling is configured to generate or receive an energy beam that is required to be controlled, the energy beam being within a first wavelength range and directed towards the first and second doped mediums. An absorbed beam input or coupling is configured to generate or receive at least one absorbed beam in a second wavelength range which is different from the first wavelength range, the absorbed beam being directed towards the first and second doped mediums. The first and second doped mediums have a higher beam absorption characteristic in the second wavelength range than in the first wavelength range, causing the absorbed beam to have a higher absorption than the energy beam in the first and second doped mediums and the first and second doped mediums each have a coating which allows transmission at both the first and the second wavelength ranges.

Claims

1.-20. (canceled)

21. A control system for thermo optical control of focus position of an energy beam in an additive manufacturing apparatus, the control system including: a first doped medium and a second doped medium, each of which is doped with a dopant, wherein: the first doped medium has a positive thermo-optical coefficient (dn/dT), thus configured to act like a controllable positive (converging) lens; and the second doped medium has a negative thermo-optical coefficient (dn/dT), thus configured to act like a controllable negative (diverging) lens, and is in series with the first doped medium; an energy beam input or coupling configured to generate or receive an energy beam that is required to be controlled, the energy beam being within a first wavelength range and directed towards the first and second doped mediums; and a plurality of absorbed beam inputs or couplings configured to generate or receive respective absorbed beams in a second wavelength range which is different from the first wavelength range, the absorbed beams being parallel to one another and being directed towards the first and second doped mediums; and a control unit configured to control the energy beam input or coupling and/or the absorbed beam inputs or couplings, thereby to control an intensity of the energy beam and/or the absorbed beams, wherein the first and second doped mediums have a higher beam absorption characteristic in the second wavelength range than in the first wavelength range, causing the absorbed beams to have a higher absorption than the energy beam in the first and second doped mediums and wherein the first and second doped mediums are optically transparent at the first wavelength range, and wherein the first and second doped mediums each have a coating which allows transmission at both the first and the second wavelength ranges.

22. The control system as claimed in claim 21, in which the energy beam input or coupling is any laser source/system that requires, or intermittently requires, its beam to be compensated or controlled.

23. The control system as claimed in claim 21, in which the energy beam has a power of greater than 1 kilowatt.

24. The control system as claimed in claim 21, in which the absorbed beam inputs or couplings are laser diodes, fibre-coupled diode lasers, or other homogenised diode lasers.

25. The control system as claimed in claim 21, which includes at least one beam guiding component to guide the energy beam and/or the absorbed beams.

26. The control system as claimed in claim 21, in which the absorbed beams, when absorbed, are converted to heat and causes a temperature profile within the first and second doped mediums.

27. The control system as claimed in claim 26, in which the temperature profile inside the first and second doped mediums induces a refractive index profile variation whose magnitude is primarily dependent on the thermo-optical coefficients (dn/dT) of the material.

28. The control system as claimed in claim 27, in which in which the refractive index profile variation results in formation of an optical phase change profile within the first and second doped mediums.

29. The control system as claimed in claim 28, in which the optical phase change profile modifies a radius of curvature of the energy beam and/or adds spherical aberration to it.

30. The control system as claimed in claim 28, in which the optical phase change profile in the first and second doped mediums, induced by the absorbed beams, depends on one or more of: absolute size and intensity of the absorbed beam and the energy beam; relative intensity of the absorbed beam and the energy beam; cooling/heating arrangement of the doped medium; relative size of the absorbed beam and the energy beam to each other and relative to the cooling/heating surfaces of the doped medium; intensity profile of the absorbed beam; type of doped optical medium.

31. The control system as claimed in claim 28, in which, if spherical aberrations are not dominant in thermal lensing, the absorbed beams have a flat top shape and the optical phase change profile is quadratic over the extent of the energy beam, being that of variable spherical lens which compensates for a beam focus shift.

32. The control system as claimed in claim 28, in which, if spherical aberration is significant in the thermal lensing, the shape of the absorbed beams are varied to introduce spherical aberration of opposite magnitude to that causing the thermal lensing.

33. The control system as claimed in claim 21, in in which the first and second doped mediums are a crystalline medium or a glass medium.

34. The control system as claimed in claim 21, in which the first and second doped mediums do not significantly contribute to gain at either the first or second wavelength ranges of the absorbed beam and/or the energy beam.

35. The control system as claimed in claim 21, in which: the first and second doped mediums are coated with an Anti-Reflective (AR) layer.

36. The control system as claimed in claim 35, in which the first and second doped mediums have different absorption characteristics.

37. The control system as claimed in claim 21, in which a wavelength of one of the absorbed beam inputs is such that it is not absorbed in at least one of doped mediums, but is still in the second wavelength range.

38. The control system as claimed in claim 21, which includes: at least one beam sensor configured to monitor at least one property of the energy beam directed onto a working surface by an optical arrangement; wherein the control unit is in electronic communication with the beam sensor, the control unit being configured to receive, from the sensor, a value associated with at least one monitored property, and wherein the control unit is configured to: analyse the received value or a processed received value to determine whether the received value or the processed received value is equal to a predefined value or within a predefined range of values; and if the received value or the processed received value is not equal to the predefined value or is not within the predefined range of values, cause adjustment of at least one variable parameter associated with the energy beam.

39. The control system as claimed in claim 38, in which the beam sensor is configured to monitor one or more of: at least one property at or near a monitoring point along an optical path of the energy beam; at least one property at or near a focus zone at a working plane or in a conjugated plane; the energy beam input spot size and/or the size and shape of a melt pool.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] The invention will now be further described, by way of example, with reference to the accompanying drawings.

[0087] In the drawings:

[0088] FIG. 1 illustrates a schematic side view of a first embodiment of a laser system in accordance with the invention;

[0089] FIG. 2 illustrates a control system which may form part of the laser system of FIG. 1;

[0090] FIG. 3 illustrates an additive manufacturing system incorporating the laser system of FIG. 1; and

[0091] FIG. 4 illustrates a schematic side view of a second embodiment of a laser system in accordance with the invention.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

[0092] The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiments described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

[0093] FIG. 1 illustrates a first embodiment of a laser system 100 in accordance with the invention. The laser system 100 has two optically transparent mediums 102, 103 which are doped with a dopant (hence referred to as the first doped medium 102 and the second doped medium 103). The mediums 102, 103 are crystalline or glass mediums but which are not operated as gain mediums—in other cases (in accordance with prior art techniques), the same type of doped medium could be utilised as a gain medium in a conventional laser amplifier, with different anti reflective coatings on the surfaces of the medium.

[0094] Heating or cooling elements 116 may be provided at or near the doped mediums 102, 103 (e.g., at sides of the doped mediums 102, 103) to provide additional heating or cooling characteristics.

[0095] The laser system 100 has a plurality of absorbed beam inputs 104.1, 104.2 (referred to collectively by numeral 104). The absorbed beam inputs 104 are configured to generate respective absorbed beams 106.1, 106.2 (referred to collectively by reference numeral 106) at the second wavelength. In this example, the absorbed beam inputs are simple laser diodes 104, which are relatively cheap, compact, and readily available. In this example, a laser beam generated by such laser diodes 104 has a wavelength of 792 nm.

[0096] The various absorbed beams 106 are parallel to one another. The laser system 100 has a beam guiding component 108 in the form of a dichroic mirror 108 arranged diagonally at 45° between the laser diodes 104 and the doped mediums 102, 103 to redirect the absorbed beams 106 by 90°.

[0097] FIG. 4 illustrates a slightly modified embodiment of laser system 200 compared to that of FIG. 1. The energy beam 112 is redirected by 90° by a dichroic mirror 408 and the absorbed beams 106 are transmitted, not redirected. The same or similar reference numerals in FIGS. 2 and 5 refer to the same or similar features.

[0098] The laser system 100 has an energy beam input 110 configured to generate or receive an energy beam 112 at the first wavelength. The energy beam input 110 directs the (input) energy beam towards the doped mediums 102, 103 and an output energy beam 114 exits on the other side.

[0099] In one embodiment (not illustrated), the energy beam input 110 may not itself generate the energy beam 112 but may be a coupler to receive the energy beam input 112 generated from an external laser generation device.

[0100] In this example, the first doped medium 102 is absorptive at a first wavelength range and the first laser diode 104.1 emits an absorbed beam 106.1 at the first wavelength. Similarly, the second doped medium 103 is absorptive at a second wavelength range and the second laser diode 104.2 emits the absorbed beam 106.2 at the second wavelength, the first and second wavelengths being different.

[0101] Importantly, at least some optical energy from the absorbed beams 106 is absorbed by the doped mediums 102, 103 and converted to heat. This causes the doped mediums 102, 103 to heat up in the region of the absorbed beams 106 and thereby induces a thermo-optical phase change profile.

[0102] If there is thermal lensing elsewhere (refer to system 300 of FIG. 3) and causes a focusing effect, but not a large variation in laser beam size of the energy beam input before the scanning mirrors, the power of an absorbed beam 106.1 that is absorbed in a device consisting of single negative dn/dT doped optical medium can be increased to compensate for the effect.

[0103] If there is thermal lensing elsewhere in the system 300 and causes a defocusing effect, but not a large variation in laser beam size of the energy beam input before the scanning mirrors, the power of an absorbed beam 106.2 that is absorbed in a device consisting of single positive dn/dT doped optical medium can be increased to compensate for the effect.

[0104] If there is thermal lensing elsewhere in the system 300 which causes a decrease in laser beam size of the energy beam input before the scanning mirrors, the power of both absorbed beams 106 that are absorbed in a device consisting of both a positive and a negative dn/dT doped optical mediums can be increased to compensate for the effect. The ratio of the power and distance between the two doped mediums would be such that the resultant effect would be that of a telescope.

[0105] If there is thermal lensing elsewhere in the system 300 which causes an increase in the laser beam size of the energy beam input before the scanning mirrors, 102 and 103 is interchanged and the power of two absorbed beams 106 that are absorbed in a device consisting of both a positive and a negative dn/dT doped optical mediums can be increased to compensate for the effect. The ratio of the power and distance between the two doped mediums would be such the resultant effect would be that of a telescope.

[0106] FIG. 2 illustrates a basic control system 200 which may form part of the laser system 100, 200. An electronic controller 202 can control the laser diode 104 to vary characteristics of the absorbed beams 106, e.g., their intensity. The controller 202 comprises control criteria or instructions 204 and may be embodied by a computer. Optionally, the control system 200 also includes a sensor or detector 206 to sense a characteristic of the output energy beam 114, or an effect of the output energy beam, thereby enabling the controller 202 to adjust the laser diode 104 according to a characteristic of the output energy beam 114—thus providing a feedback mechanism. This could be used to correct for transient thermal lensing in other optical elements through which the energy beam 114 is transmitted.

[0107] FIG. 3 illustrates an additive manufacturing system 300 incorporating the laser system 100. The laser system 100 has been placed at a convenient position before the two scanning mirrors 302. Controllable quadratic thermal phase change profiles in doped mediums 102 or 103 would compensate for thermal lensing in optical components 304 (a collimating lens), 306 (a focusing lens) or 304 (a protective window).

[0108] Adjustment of at least one variable parameter may include adjustment of the power of at least one of the absorbed beam inputs.

[0109] The control unit may be configured to analyse the received value and/or the processed received value (from the sensor system) by applying a compensation algorithm in order to determine the amount of power allocated to individual absorbed beam inputs, thereby compensating for the effect of the measured distortion, or by changing the energy beam input size on the working surface as required.