FIELD OF INVENTION

This invention relates broadly to optics and lasers and specifically to a laser including a doped 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.

In order to sustain and amplify the laser beam, the gain medium has to be pumped or energised, usually by means of an excitation source. Two primary excitation configurations exist: end-pumped and side-pumped.

In the end-pumped configuration, the excitation source is arranged at one end

(usually an inward end) of the gain medium and directs an excitation optical field generally parallel to, although not co-axial with, the laser beam. The end- pumping excitation source may in fact be in the form of an additional laser fitted to the fundamental mode of the main laser. In the side-pumped configuration, one or more excitation sources are arranged on the sides of the gain medium and direct an excitation optical field into the gain medium from a direction which is roughly perpendicular to that of the laser beam.

In the end-pumped configuration the crystalline gain medium typically has high absorption characteristics, e.g. Nd:YAG with high doping concentration can absorbs more than 50% of the pump energy within 2 mm. This causes high temperature gradients which can limit the power output of the laser and even damage the gain medium. The high absorption coefficient of the crystalline gain medium necessitates large input energy which creates cooling difficulties. An internal aperture may be needed which decreases the output energy and M ² of the laser.

Side-pumped excitation sources overcome some, but not all of these problems. The high absorption issue is still present and radially outer regions of the gain medium absorb most of the energy. An internal aperture is required to achieve a Gaussian output which improves M ² but decreases output energy. Thermal management issues are improved.

The Applicant desires a laser with a side-pumped gain medium which further overcomes drawbacks associated with energising the gain medium.

SUMMARY OF INVENTION According to one aspect of the invention, there is provided a laser which includes an optical cavity having a longitudinal axis, an optical element at each end of the optical cavity, a doped crystalline gain medium provided in the optical cavity, and at least one excitation source operable to excite the gain medium, the laser being operable to generate a laser beam having a predefined fundamental mode and beam profile, characterised in that: the gain medium has a radially-varying doping profile corresponding to the beam profile of the fundamental mode of the laser; and the, or each, excitation source is arranged laterally relative to the gain medium therefore being operable to side-pump the gain medium.

"Corresponding to the beam profile" may mean matched to the beam profile, e.g., an exact match. Instead, the doping profile may be a function of, or mathematical relation to, the beam profile of the fundamental mode, without being an exact match.

The fundamental mode may be, for example, Gaussian, tophat, etc.

One skilled in the art will know that the gain of the gain medium is influenced by, or even dictated by, the doping profile. The excitation source provides an optical field which excites dopant ions in the doped gain medium. Thus, by providing the doping profile corresponding to the beam profile of the laser, optimum gain can be extracted from the gain medium, with more gain being available where the beam profile of the laser beam is higher/more intense.

Having a radially-varying doping profile may effectively render some portions of the gain medium more transmissive in some places (e.g. those places with lower dopant concentration) and more absorptive in other places (e.g. those places with a higher concentration) from the perspective of the excitation source. Accordingly, if the doping profile is larger in the centre, e.g. in the case of a Gaussian profile or a tophat profile, then radially outer regions of the gain medium will be more lightly doped and more transmissive, allowing light from the excitation source to be more freely transmitted to the centre of the gain medium. Prior art lasers of which the Applicant is aware include a radially-constant gain medium (doped or un-doped). This has the drawback that all regions of the gain medium are equally absorptive, specifically radially outer regions where very little amplification may be needed. Thus, by relating or matching the doping profile of the gain medium to the beam profile of the laser, in accordance with the invention, the gain medium may be excited proportionally to its use, e.g. regions where the beam profile is higher will have a higher doping profile and thus have more excited dopants than areas with a lower beam profile. Accordingly, the laser provides optimised gain volume utilisation and small (or at least smaller) temperature gradients throughout the gain medium.

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, the 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 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; Sc ₂ O ₃ ;

Lu ₂ O ₃ ;

CaF ₂ ;

SrF ₂ ; and/or YbF ₃ .

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 (Nd ³⁺ ), ytterbium (Yb ³⁺ ), holmium (Ho ³⁺ ), thulium (Tm ³⁺ ), erbium (Er ³⁺ ); and/or chromium ions (Cr ²⁺ , Cr ³⁺ , Cr ⁴⁺ ).

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. 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 operating a laser, the method including: providing a laser as defined above; and side-pumping the gain medium thereby to excite the gain medium.

The method may include the previous step of calculating the doping profile based on the fundamental mode of the laser.

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 a laser, in accordance with the invention; FIGURE 2 shows a flow diagram of a method of operating a laser, in accordance with the invention;

FIGURE 3 shows a schematic view of beam and doping profiles of the laser of FIGURE 1 ; FIGURE 4 shows a schematic axial-sectional view of a doping profile of a gain medium of the laser of FIGURE 1 ; and

FIGURES 5-6 show schematic views of a second embodiment of a doping profile of the laser of FIGURE 1 .

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 laser 100 in accordance with the invention. The laser 100 includes some aspects conventionally associated with lasers, such as an optical cavity 102 defining a longitudinal or lengthwise axis bounded at either end by an optical element 108.1 and 108.2 such as a mirror or lens/mirror combination. A gain medium 104, in accordance with the invention, is arranged within the optical cavity 102. The gain medium 104 has a radially- varying doping concentration, thereby to provide a radially-varying doping profile. In this example, the fundamental mode of the laser 100 provides a Gaussian beam profile and therefore the doping profile is similarly Gaussian (refer to FIGURES 3 and 4).

The laser 100 includes an excitation source 106 operable to side-pump the gain medium 104. Other than the important aspect that the excitation source

106 provides side-pumping excitation, the specifics of the excitation source 106 are not germane to this invention.

A laser beam 1 10 generated and sustained by the laser 100 has a beam profile in accordance with the fundamental mode of the laser 100. Various components and specifically the optical elements 108 can be configured to provide a specific, calculated fundamental mode, which may be Gaussian, tophat, etc. The laser beam 1 10 is represented in the drawings by a clearly defined bar but this is for ease of illustration only and practically an intensity of the beam profile is usually more gradually distributed and complex.

A coupler 1 12 is provided adjacent one of the optical elements 108.2 (usually a partially-transmissive optical element) to out-couple the laser beam 1 10 for use.

FIGURE 2 shows a flow diagram of a method 200 which illustrates the laser 100 in use. The laser 100 is configured, using conventional laser design techniques, to have a particular fundamental mode which will give rise to a particular beam profile. The gain medium 104 is provided (at block 202) to have a doping profile to match the beam profile. The gain medium 104 is side-pumped (at block 204) to optimise transfer of the optical field from the excitation source to the dopant in the gain medium 104.

FIGURE 3 shows a schematic view of the matched beam profile 302 and the doping profile 304 both having, for example, roughly Gaussian profiles. FIGURE 4 shows the doping concentration of the gain medium 104 schematically. Continuing the Gaussian profile example, the doping concentration is higher at the centre, e.g. nearer to the longitudinal axis, of the gain medium 104. The doping concentration is lower towards a radially outer periphery. The dopant - specifically the doped ions - is receptive to the light emitted by the excitation source. Thus, the lower the doping concentration, the less absorptive / more transmissive the gain medium 104 will be from the perspective of the excitation source 106.

Accordingly, the light field generated by the excitation source 106 tends to be only minimally absorbed in the radially outer region with a lower doping concentration and more substantially absorbed in the radially inner region with a higher doping concentration. (This applies with the necessary changes to other doping profiles.) Thus, the gain medium 104 is energised in accordance with the beam profile 302. Stated differently, areas where low amplification is required (e.g. radially outer regions) are energised less than areas (e.g. radially inner regions) where greater amplification is required. This provides for a more optimised pumping of a gain medium 104.

In an alternate embodiment, the doping profile may, in addition to being radially varying, also be longitudinally varying. FIGURES 5-6 illustrate such an embodiment. In this embodiment, and with reference to FIGURE 5, the doping profile 304.1 at one end of the gain medium 104 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 104, the doping concentration generally diminishes with the doping profile 304.2 still being Gaussian but being moderately doped at a radially-inner centre and having a low concentration at the radially-outer periphery. Towards the other end of the gain medium 104, the doping profile 304.3 is flatter still. FIGURE 6 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 should be noted that although the doping profile 304.1 -304.3 may vary, it is still Gaussian and thus still matches the beam profile 302.

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 a distal end of the gain medium 104 to minimise any saturation effect.

The Applicant believes that the invention as exemplified has a number of advantages:

No spatially fitted pump laser is required;

Use of the gain medium volume is optimised and maximised;

Smaller temperature gradients occur due to uniform absorption of pump energy from the excitation source 106 throughout the entire gain medium 104;

No internal aperture is required; and

Lower M ² values are provided for the output laser beam.