THERMO-OPTIC INTRACAVITY BEAM SHAPING AND MODE CONTROL WITH DOPED OPTICAL MATERIALS

Abstract

A laser beam shaping system which has a laser resonator configured to operate at a resonating, first wavelength range to produce an intracavity resonating beam and a laser gain material, configured to produce gain and to amplify the first wavelength range within the laser resonator. The system has at least one doped medium, which is optically transparent at the first wavelength range, which is doped with a dopant, and which is provided intracavity in the laser resonator and at least one absorbed beam input or coupling configured to generate or receive at least one absorbed beam at a second wavelength range which is different from the first wavelength range and which is directed towards the doped medium. The doped medium has a higher absorption characteristic at the second wavelength range than at the first wavelength range, causing the absorbed beam to have a higher absorption than the resonating beam in the doped medium, but which does not provide gain in the first wavelength range. Optical surfaces of the doped medium are coated to be anti-reflective at the first wavelength range and highly transmissive at the second wavelength range.

Claims

1. A laser beam shaping system which includes: a laser resonator configured to operate at a resonating, first wavelength range to produce an intracavity resonating beam; a laser gain material, configured to produce gain and to amplify the first wavelength range within the laser resonator; at least one doped medium, which is optically transparent at the first wavelength range, which is doped with a dopant, and which is provided intracavity in the laser resonator; at least one absorbed beam input or coupling configured to generate or receive at least one absorbed beam at a second wavelength range which is different from the first wavelength range and which is directed towards the doped medium, wherein the doped medium has a higher absorption characteristic at the second wavelength range than at the first wavelength range, causing the absorbed beam to have a higher absorption than the resonating beam in the doped medium, but which does not provide gain in the first wavelength range, and wherein optical surfaces of the doped medium are coated to be anti-reflective at the first wavelength range and highly transmissive at the second wavelength range.

2. The laser beam shaping system as claimed in claim 1, wherein the absorbed beam input has at least one of a beam profile, shape, size, and/or position to cause a specific transformation via a thermo-optical phase change profile of a phase of the resonating beam at the first wavelength range, thereby modifying an output of the resonator at the first wavelength range.

3. The laser beam shaping system as claimed in claim 1, in which: the laser resonator is a high average power laser resonator (>1W average output); the laser resonator is a high peak power laser resonator (>1 kW output); or the laser resonator is a high energy laser resonator (>1 mJ output).

4. The laser beam shaping system as claimed in claim 1, which is configured to be used in: laser material processing applications; or high power communications and lidar applications.

5. The laser beam shaping system as claimed in claim 1, in which the absorbed beam input or coupling is one or more laser diode, fibre-coupled diode laser, or other homogenised diode laser.

6. The laser beam shaping system as claimed in claim 1, which is configured to provide the absorbed beam parallel to the resonating beam.

7. The laser beam shaping system as claimed in claim 1, which is configured to provide the absorbed beam with an angular offset (i.e., not parallel) to resonating beam.

8. The laser beam shaping system as claimed in claim 1, which includes at least one beam guiding component to guide the resonating beam and/or the absorbed beam.

9. The laser beam shaping system as claimed in claim 1, in which the absorbed beam, when absorbed, is converted to heat and causes a temperature profile within the doped medium.

10. The laser beam shaping system as claimed in claim 9, in which the temperature profile inside the doped medium induces a refractive index profile variation whose magnitude is primarily dependent on a thermo-optical coefficient or coefficients (dn/dT) of the material.

11. The laser beam shaping system as claimed in claim 10, in which the refractive index profile variation results in formation of an optical phase change profile within the doped medium.

12. The laser beam shaping system as claimed in claim 11, in which the optical phase change profile inside the doped medium modifies the resonating beam.

13. The laser beam shaping system as claimed in claim 12, in which the resonating beam is modified by either controlling the modes inside a laser resonator or by non-quadratically changing the phase inside the laser resonator.

14. The laser beam shaping system as claimed in claim 11, in which the optical phase change profile within the doped medium, induced by the absorbed beam, depends one or more of: absolute intensity of the absorbed beam and the resonating beam; relative intensity of the absorbed beam and the resonating beam; cooling/heating arrangement of the doped optical medium; relative size of the absorbed beam and the resonating beam to each other and relative to the cooling surfaces of the doped optical medium; position of the absorbed and resonating beams relative to each other and relative to the cooling surfaces of the doped optical medium; intensity profile of the absorbed beam; and/or type of doped optical medium.

15. The laser beam shaping system as claimed in claim 1, in which the resonating beam has higher power than the absorbed beam.

16. The laser beam shaping system as claimed in claim 1, in which the doped medium is a crystalline medium or a glass medium.

17. The laser beam shaping system as claimed in claim 16, where the function of the dopant in the doped medium is to selectively absorb only the absorbed beam in the second wavelength range and to convert at least some of the absorbed beam to heat, while providing no gain to the resonating beam at the first wavelength range.

18. The laser beam shaping system as claimed in claim 1, in which: the doped medium is coated with an Anti-Reflective (AR) layer at the first wavelength and High Transmissive (HT) at the second wavelength.

19. The laser beam shaping system as claimed in claim 1, in which the doped medium is provided in series with the gain material in the laser resonator, the doped medium having inverse dn/dT from that found in the gain material.

20. The laser beam shaping system as claimed in claim 1, which includes a controller configured to control the absorbed beam input, thereby to control the absorbed beam.

21. (canceled)

22. The laser beam shaping system as claimed in claim 20, in which the controller is configured to control an electronic tip and tilt of at least one cavity end mirror in the laser resonator in order to keep a cavity aligned for different outputs of absorbed beams.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0066] The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings.

[0067] In the drawings:

[0068] FIG. 1 shows a schematic side view of a first embodiment of a laser system in accordance with the invention;

[0069] FIG. 2 shows a schematic front view of an example absorbed beam configuration of the laser system of FIG. 1;

[0070] FIG. 3 shows a schematic three-dimensional view of part of a second embodiment of a laser system in accordance with the invention including only a doped medium and absorption beams;

[0071] FIG. 4 shows a schematic front view of an example absorbed beam configuration of the laser system of FIG. 3;

[0072] FIG. 5 illustrates a control system which may form part of the laser system of FIG. 1 or 3; and

[0073] FIG. 6 shows a schematic side view of a third embodiment of a laser system in accordance with the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENT

[0074] 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 embodiment 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.

[0075] FIG. 1 illustrates a first embodiment of a laser system 100 in accordance with the invention. The laser system 100 comprises two resonating end mirrors 118 which define a laser resonator 120 there between. The laser resonator 120 is configured to resonate at a first wavelength range. One of the end mirrors is fully reflective and the other partially reflective at the first wavelength range.

[0076] The laser system 100 has a pump input 110 configured to generate a pump beam 112 at a third wavelength range different from the first wavelength range. The laser system 100 has two types of doped optically transparent media 102, 114 which are doped with different dopants provided within the laser resonator 120. The first doped medium 114 is to provide gain to the laser cavity 120 (hence referred to as the gain medium 114). It absorbs energy from a pump source (in the third wavelength range) in order to provide gain to the laser resonator 120. This can either be longitudinally/end pumped (as illustrated) or from transverse/side pumped (not illustrated). In some embodiments the gain material can also be pumped by non-optical means such as electrical or chemical pumping. In this example, the doped gain medium 114 is a standard Nd:YAG crystal coated to be end-pumped at the third wavelength range near 808 nm. It is coated to be anti-reflective at 1 μm and highly transmissive at 808 nm.

[0077] The second doped medium 102 is to provide variable and controllable thermo-optical phase change profiles to the laser resonator 120. The second doped medium 102 absorbs light at a second wavelength range which is different from the first wavelength range. The second doped medium 102 itself is a bulk crystalline or glass medium but which is not operated as a gain medium in other cases (in accordance with prior art techniques), the same type of crystalline medium could be utilised as a gain medium in a conventional laser amplifier, with different anti-reflective coatings at wavelength ranges other than first wavelength range. The second doped medium 102 is optically transparent at the first wavelength range. This is the wavelength range that is amplified inside the first doped medium 114 and which is resonant within the laser resonator 120. In this example, the first wavelength is near 1 μm. The second doped medium 102 is at least partially optically absorptive at the second wavelength near 792 nm. In this example, the absorptive properties are due to the presence of the dopant Thulium in the second doped medium 102.

[0078] The laser system 100 has a plurality of absorbed beam inputs 104.1, 104.2, 104.n (referred to collectively by numeral 104). The absorbed beam inputs 104 are configured to generate respective absorbed beams 106.1, 106.2, 106.n (referred to collectively by reference numeral 106) at the second wavelength. In this example, the absorbed beam inputs 104 are fibre coupled laser diodes, which are relatively cheap, compact, and readily available. In this example, a laser beam generated by such laser diodes 104 has a wavelength near 792 nm.

[0079] 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 is arranged diagonally at 45° between the absorbed beams 106 and both doped media 114, 102. The beam guiding component 108 transmits both the pump (at the third wavelength range) 112 and absorbed beams (at the second wavelength range) 106 (transmits 95 to 99.9%) and reflects a resonating beam (at the first wavelength range) 122 (by more than 99.9%).

[0080] In this example, the second doped medium 102 is coated with a layer of material that is Anti Reflective (AR) at resonating wavelength ranges near 1 μm and Highly Transmissive (HT) at absorbed wavelength ranges near 792 nm.

[0081] The second doped medium 102 may also have one surface that is coated highly reflective at the resonating wavelength range. In such cases the face that is thus coated acts as one of the resonating end mirrors (118) of the laser resonator 120.

[0082] Importantly, at least some optical energy from the absorbed beams 106 is absorbed by the second doped medium 102 and converted to heat. This causes the second doped medium 102 to heat up in the region of the absorbed beams 106 and thereby induces a thermo-optical phase change profile.

[0083] The precise type of thermo-optical phase change profile may vary depending on a number of factors, including the substance of the medium, the dopant, the absorbed beam(s) 106, the resonating beam 122, the relative arrangement of the absorbed beams 106, cooling configuration of the medium, etc.

[0084] The thermo-optical phase change profile acts as a variable phase element within the cavity. The effect can either be to act as a variable lens or as a variable beam transforming element. A profile of an output beam 115 is thus controllable.

[0085] Heating or cooling elements 124 and 125 may be provided at or near the doped medium 102 (e.g., at sides of the doped medium 102) to provide additional heating or cooling characteristics. Applying a controllable temperature difference on two opposite placed elements causes a tilt thermo optical phase change aberration. This can be used to compensate for any unwanted tilt aberrations caused by the second absorptive beams (104), keeping the resonator aligned.

[0086] FIG. 2 illustrates an input beam configuration of the absorbed beams 106 and the resonating beam 122 provided by the laser system 100 (with four orthogonally placed heating or cooling elements 124, 125). In this example, the resonating beam 122 is arranged centrally and the absorbed beams 106 are arranged linearly. The absorbed beams in this example have a uniform circular (top hat) intensity profile.

[0087] FIG. 3 illustrates a second embodiment of part of a laser system 200 in accordance with the invention, showing only the doped medium and absorption beams (without the heating or cooling elements). The same numerals in different FIGS refer to the same or similar features. The laser system 200 has laser diodes 104 but instead of providing parallel absorbed beams, they provide converging absorbed beams 206 and thus dispense with the need for the dichroic mirror 108 of the laser system 100 of FIG. 1. Here the resonating beam 122 is transformed by the doped medium 102.

[0088] FIG. 4 illustrates the arrangement of beams 206 provided by the laser system 200. In this case, the flat-top shaped absorbed beams 206 are arranged in a cross. It will be appreciated that FIGS. 2 and 4 illustrate but two of a large number of potential input beam arrangements, which may vary considerably based on design requirements and preferences.

[0089] An advantage of the laser system 100, 200 is that relatively lower power absorbed beam(s) can be used to control a relatively higher power resonating beam.

[0090] An advantage is that the magnitude and distribution of the phase change profile can be changed by changing the amount of optical energy in the absorbed beam(s) (from the laser diodes 104. The result is the ability to easily vary the effect and/or the 3o magnitude of the phase change element(s).

[0091] Another advantage may be that the laser diodes 104 are easily electronically controllable. FIG. 5 illustrates a basic control system 300 which may form part of the laser system 100, 200. An electronic controller 302 can control the laser diode 104 to vary characteristics of the absorbed beams 106, e.g., their intensity. The controller 302 comprises control criteria or instructions 304 and may be embodied by a computer. Optionally, the control system 300 also includes a sensor or detector 306 to sense a characteristic of the output resonating beam 115 or any other relevant sensor, thereby enabling the controller 302 to adjust the laser diode(s) 104 according to a characteristic of the output resonating beam thus providing a feedback mechanism. This could be used to correct/control the characteristics of the output of the laser beam from the resonator 120, e.g., changing its mode content, beam shape, BPP and divergence etc. The electronic controller 302 can also control the heating or cooling elements (124, 125) or electronic tip and tilt of one of the two cavity end mirrors in order to keep the cavity aligned for different outputs of the laser diode(s) 104.

[0092] As illustrated in FIG. 6, in some embodiments, a laser system 400 may be provided with two (or more) second doped elements 102 in the laser resonator 120 or cavity. Traditionally, this has been very difficult to implement with conventional beam shaping techniques. In such cases, two or more beam directing dichroic mirrors 108 may be added to the system 400.

[0093] The Applicant envisages that the inventive principle may enable a range of new laser products. These products would enable variable BPP output, variable beam shapes and higher brightens than is currently available from bulk solid-state lasers.

[0094] These could be applied to more efficient laser material processing or high-power communications and lidar applications, etc.