WO2015059630A1

WO2015059630A1 - A beam-shaping amplifier containing a crystalline gain medium

Info

Publication number: WO2015059630A1
Application number: PCT/IB2014/065498
Authority: WO
Inventors: Ihar Anatolievich Litvin; Oliver J P COLLET
Original assignee: Csir
Priority date: 2013-10-23
Filing date: 2014-10-21
Publication date: 2015-04-30

Abstract

A laser beam-shaping amplifier (100) has a doped crystalline gain medium (102) having a longitudinal axis and a radially-varying doping profile (404; 04). The amplifier (100) also has a coupler (104) for coupling a laser beam (110) outputted from a laser (108) to the gain medium (102) and an excitation source (106; 206) operable to excite the gain medium (102). In use, a laser beam (110) having an input beam profile (402; 502) generated by the laser (108) is out-coupled to the gain medium (102) and amplified in accordance with the radially-varying doping profile (404; 504), thereby changing the beam profile to produce an output laser beam (112) having an output beam profile (406; 506) which is different from the input beam profile (402; 502).

Description

A beam-shaping amplifier containing a crystalline gain medium

FIELD OF INVENTION

This invention relates broadly to optics and lasers and specifically to a beam- shaping amplifier containing a crystalline gain medium, and an associated method.

BACKGROUND OF INVENTION

A laser is operable to generate a laser beam having a particular beam profile. The particular beam profile generated by a laser depends on the configuration of the laser, e.g. the optical cavity, the gain medium, the optical elements at either end of the optical cavity, etc. It is often required to change the shape of the beam profile for a particular application. For example, the laser may be configured to generate and emit a laser beam with a Gaussian profile, but an application may require a laser beam having a top-hat profile. In such case, beam shaping techniques are used to change the shape of the beam profile.

The Applicant is aware of at least one method of doing this using external beam-shaping optics. Typically, an output from a laser is out-coupled to (external) optical elements such as mirrors or lenses. This technique is fairly widely employed because proper selection of the external optical elements can produce the desired beam profile. Changing the external optical elements, while retaining the same laser, can produce another, different beam profile. Thus, it is not necessary to change the laser itself merely to achieve a desired profile.

However, the Applicant notes that there are also drawbacks involved in the use of external beam-shaping optics. The external optics can be complicated and bulky. The external optics generally require a coherent laser beam from the laser for the optical elements to function as intended.

The Applicant wishes to overcome these drawbacks. Thus, it is an object of the invention to provide beam-shaping without using external beam-shaping optics. Advantageously, the Applicant wishes to provide simultaneous amplification and beam-shaping.

SUMMARY OF INVENTION

According to one aspect of the invention, there is provided a beam-shaping amplifier which includes: a doped crystalline gain medium having a longitudinal axis and a radially-varying doping profile; a coupler for coupling a laser beam outputted from a laser to the gain medium; and an excitation source operable to excite the gain medium, such that, in use, a laser beam having an input beam profile generated by the laser is out-coupled to the gain medium and amplified in accordance with the radially-varying doping profile, thereby changing the beam profile to produce an output laser beam having an output beam profile which is different from the input beam profile. The radially-varying doping profile may yield non-constant, or radially varying, amplification. Thus, the output beam profile may be a function of the input beam profile and the doping profile.

It will be appreciated that, although the term "amplifier" is used, the output laser beam may not necessarily be more powerful than the input laser beam. For example, an amplification factor realised by the gain medium, may be greater than 1 , about 1 , or less than 1 . In one example, the amplification factor is greater than 1 and the output laser beam will therefore indeed be amplified relative to the input laser beam.

The Applicant notes that a doped crystalline gain medium as such is not new; it has been used in optical cavities for generating the laser beam itself. Further, the Applicant has noted that doping a crystalline gain medium to have a non-constant doping profile is also not new (see references below).

However, a beam-shaping amplifier using such a doped crystalline gain medium with a radially-varying doping profile is new and inventive (to the best of the Applicant's knowledge).

The gain medium may be a poly-crystalline ceramic medium. The radially- varying doping profile may be realised by varying a concentration of a dopant as a function of radius. Instead, or in addition, radially-varying doping profile may be realised by varying the type of dopant as a function of radius. The dopants may be optically active. One existing method of which the Applicant is aware of creating a radially-varying doping profile may be to arrange a powder or particulate form of the dopant in a desired profile and sinter the powder to form the ceramic medium.

The excitation source may be end-pumped or side-pumped. The excitation source may be a flash lamp or a laser diode (e.g. a fibre-coupled laser diode, laser diode stacks and bars, etc.). The excitation source may generate an optical field(s) that transfers energy to dopant ions within the gain medium. The gain medium may then amplify the input laser beam in accordance with the doping profile provided by the energised/excited dopant ions.

The crystalline gain medium may comprise uniaxial or biaxial crystals. The crystalline gain medium may comprise one or more of the following:

YAG;

Sc₂O₃;

Y₂O₃; Lu₂O₃;

CaF₂;

SrF₂; and/or YbF₃.

The Applicant notes that ceramic manufacturing is still improving and that the above list of materials is therefore not exhaustive. Other materials may be used without departing from the spirit of the invention. The Applicant acknowledges the following sources as possibly providing enabling teachings for ceramic gain media as such: Ikesue, A., et al; Ceramic Laser Materials, Nature Photonics 2, p 721 -

727, 2008;

Wisdom, J., et al, Ceramic Lasers: Ready for Action, Photonics Spectra, P 4-8, Feb 2004; and

Richardson, M.; Transparent Ceramics for Lasers - A Game Changer, American Ceramic Society Bulletin, Vol. 91 , No. 4, p 30-33, 2012.

One or more of the following dopants may be used to dope the crystalline gain medium: trivalent lanthanides, e.g. neodymium (Nd³⁺), ytterbium (Yb³⁺), holmium (Ho³⁺), thulium (Tm³⁺), erbium (Er³⁺); and/or chromium ions (Cr²⁺, Cr³⁺, Cr⁴⁺).

Chromium ions may have multiple functions in crystals, being co-sensitizers, and saturable absorbers. Co-sensitizers absorbed pump light and transfer the energy to other ion species which interact with the seed beam.

The doping profile may be calculated based on the anticipated input beam profile and the desired output beam profile.

In one embodiment, the doping profile may be longitudinally constant. In other words, the doping profile may be the same or similar along the length of the gain medium.

In another embodiment, the doping profile may be longitudinally varying. In other words, the doping profile may be different at different points along the length of the gain medium. For example, the doping profile may vary along the length by increasing or decreasing, converging or diverging, or may have a more complicated function of concentration vs. longitudinal position.

According to another aspect of the invention, there is provided a method of beam shaping, the method including: providing a beam-shaping amplifier as defined above; out-coupling an input laser beam having an input beam profile generated by the laser to the gain medium; and amplifying the input laser beam in accordance with the radially-varying doping profile, thereby to change the beam profile to produce an output laser beam having an output beam profile which is different from the input beam profile.

The method may include the previous step of calculating the doping profile based on the anticipated input beam profile and the desired output beam profile, and configuring the beam-shaping amplifier accordingly.

BRIEF DESCRIPTION OF DRAWINGS

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

In the drawings:

FIGURE 1 shows a schematic view of one embodiment of a beam- shaping amplifier, in accordance with the invention;

FIGURE 2 shows a schematic view of another embodiment of a beam- shaping amplifier, in accordance with the invention;

FIGURE 3 shows a flow diagram of a method of beam shaping, in accordance with the invention;

FIGURES 4-5 shows schematic views of beam profiles shaped by the amplifier of FIGURES 1 -2;

FIGURE 6 shows a schematic view of a first embodiment of a doping profile of the amplifier of FIGURE 2; and

FIGURES 7-8 show schematic views of a second embodiment of a doping profile of the amplifier of FIGURE 2. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

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.

FIGURE 1 shows a beam-shaping amplifier 100 in accordance with the invention. The amplifier 100 has a doped ceramic poly-crystalline gain medium 102 having a longitudinal axis. Importantly, the gain medium 102 has a radially-varying doping profile provided by radially-varying or radially- dependent distribution of a dopant 1 14. For example, the dopant 1 14 may be more concentrated closer to a centre.

It will be appreciated by one skilled in the art that amplification provided by the gain medium 1 14 is dependent on the doping profile. Thus, a higher doping concentration results in higher amplification. Accordingly, if the dopant 1 14 is more concentrated at the centre, then amplification will be greater at the centre, proportional to the doping concentration.

The amplifier 100 includes a coupler 104 to couple the laser 108 output to the gain medium 102. The laser 108 may be a conventional laser and need not be of any particular type. The coupler 104 may simply be a spacing element to space the gain medium 102 from, and align it with, the laser 108 The laser 108 generates a laser beam 1 10 (referred to, relative to the gain medium 102, as an input laser beam 1 10). The input laser beam 1 10 has an input beam profile (refer to FIGURES 4-5) which is dependent on the characteristics of the laser 108.

The amplifier 100 includes an excitation source 106 operable to generate an optical field 1 16 to excite the gain medium 102, and more specifically to excite the dopant 1 14 within the gain medium 102. In this example, the excitation source 106 is an end-pumped source that is not necessarily co-linear with the input laser beam 1 10.

FIGURE 2 shows a variation in the amplifier 200 which includes an excitation source 200 to the side of the gain medium 102, i.e. side-pumped, such that the optical field is distributed from the side. Side-pumping may minimise thermal lensing problems, improve gain volume filling, increase amplifier efficiency, and improve M² values.

FIGURE 3 shows a flow diagram of a method 300 which illustrates the amplifier 100, 200 in use. The amplifier 100, 200 is coupled to the laser 108 such that the laser beam 1 10 (having an input beam profile) generated by the laser 108 is coupled (at block 302) to the gain medium 102 and amplified (at block 304) in accordance with the radially-varying doping profile, thereby changing the beam profile to produce an output laser beam 1 12 having an output beam profile which is different from the input beam profile.

In other words, the non-constant amplification characteristics of the gain medium 102 do not provide constant amplification of the input laser beam 1 10, but rather provided radially-varying amplification dependent on the doping profile. Thus, by anticipating the input beam profile of the laser beam 1 10 from the laser 108, the doping profile can be configured to produce an output laser beam 1 12 having both a different and amplified output beam profile.

FIGURE 4 shows example profiles. It will be appreciated that the profiles in FIGURES 4-5 have been roughly drawn merely to illustrate the inventive principle simply, and are not necessarily to scale or mathematically accurate. The input laser beam 1 10 has a Gaussian profile 402 - the laser 108 is configured to output a Gaussian laser beam 1 10. The dopant 1 14 has accordingly been distributed to produce a doping profile 404 with a double peak thereby to produce an output beam 1 12 with a top-hat (or flattop) beam profile 406. In addition to altering the out beam profile 406, the gain medium 102 has also amplified the output laser beam 1 12.

FIGURE 5 shows another series of example profiles. In this example, a top- hat beam profile 502 is coupled to the gain medium 102 and the doping profile

504 has a single peak, Gaussian-like profile. The output beam profile 506 is thus near Gaussian and the output laser beam 1 12 is amplified relative to the input laser beam 1 10.

The doping profile 504 is constant along the length of the gain medium 102.

Thus, the Gaussian doping profile 504 is identical at most points along the length of the gain medium 102. FIGURE 6 shows a schematic axial-sectional view of the gain medium 102 further illustrating that the doping profile is radially varying, but longitudinally constant (the shading is representative of doping concentration).

In an alternate embodiment, the doping profile may, in addition to being radially varying, also be longitudinally varying. FIGURES 7-8 illustrate such an embodiment. In this embodiment, and with reference to FIGURE 7, the doping profile 504.1 at a proximate end of the gain medium 102 is Gaussian with a very high concentration of dopants at a radially-inner centre and a low concentration at a radially-outer periphery. Moving about half way along the gain medium 102, the doping concentration generally diminishes with the doping profile 504.2 still being Gaussian but being moderately doped at a radially-inner centre and having a low concentration at the radially-outer periphery. Towards a distal end of the gain medium, the doping profile 504.3 is flatter still. FIGURE 8 shows a schematic axial-sectional view, in which the shading represents that the doping concentration changes (e.g. diminishes) both in a radially outwardly direction and in a longitudinal direction.

It is noted that the longitudinal variation need not be linear, but may be a more complicated variation depending on what amplification characteristics are required. A possible use for such a longitudinal variation is to reduce amplification towards the distal end of the amplifier to minimise any saturation effect.

The Applicant believes that the invention as exemplified has a number of advantages. Beam shaping and amplification can be achieved simultaneously. Thus, two discrete components are not required. The amplifier 100, 200 is relatively compact as optical elements (e.g. mirrors or lenses) are not required. The gain medium 102 may be designed to be applied to a wide range of lasers, without any modification to the lasers being needed. Importantly, coherent and incoherent beams may be shaped. With use of prior art beam-shaping optics, only coherent beams could be shaped.

Claims

A laser beam-shaping amplifier which includes: a doped crystalline gain medium having a longitudinal axis and a radially-varying doping profile; a coupler for coupling a laser beam outputted from a laser to the gain medium; and an excitation source operable to excite the gain medium, such that, in use, a laser beam having an input beam profile generated by the laser is out-coupled to the gain medium and amplified in accordance with the radially-varying doping profile, thereby changing the beam profile to produce an output laser beam having an output beam profile which is different from the input beam profile.

The laser beam-shaping amplifier as claimed in claim 1 , in which the output beam profile is a function of the input beam profile and the doping profile.

The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which an amplification factor is greater than 1 and the output laser beam is amplified relative to the input laser beam.

The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the gain medium is a poly-crystalline ceramic medium.

The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the radially-varying doping profile is realised by varying a concentration of a dopant as a function of radius.

6. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the radially-varying doping profile is realised by varying the type of dopant as a function of radius.

7. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the dopants are optically active.

8. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the excitation source is end-pumped or side-pumped.

9. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the excitation source is a flash lamp or a laser diode.

10. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the crystalline gain medium comprises uniaxial or biaxial crystals.

11. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the crystalline gain medium comprises one or more of the following:

YAG; Sc2O3; Y2O3; Lu2O3; CaF2;

SrF2; and/or YbF3.

12. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which one or more of the following dopants may be used to dope the crystalline gain medium: trivalent lanthanides; and/or chromium ions.

13. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the doping profile is calculated based on an anticipated input beam profile and the desired output beam profile.

14. The laser beam-shaping amplifier as claimed in any one of the preceding claims, in which the doping profile is longitudinally constant.

15. The laser beam-shaping amplifier as claimed in any one of claims 1 -13, in which the doping profile is longitudinally varying.

16. A method of laser beam shaping, the method including: providing a beam-shaping amplifier as claimed in any one of the preceding claims; out-coupling an input laser beam having an input beam profile generated by the laser to the gain medium; and amplifying the input laser beam in accordance with the radially- varying doping profile, thereby to change the beam profile to produce an output laser beam having an output beam profile which is different from the input beam profile.

17. The method as claimed in claim 16, which includes the preceding step of calculating the doping profile based on an anticipated input beam profile and the desired output beam profile, and configuring the beam-shaping amplifier accordingly.