The invention relates to lasers with axially-symmetric beam profiles.
Most lasers are designed to lase on the fundamental Hermite-Gaussian (HG) eigenmode mode of a resonant cavity, referred to as the TEM00 mode, which provides a Gaussian beam profile.
However, the generation of high-quality ring-shaped laser beams is of significant commercial interest.
Over recent years the generation of ring-shaped (doughnut) beams has been the subject of much research and for which there are a variety of techniques available.
Beam-shaping schemes, such as axicons1 or hollow-core fibres2 can be used to provide a relatively straightforward route to a ring-shaped beam, typically at the expense of a significant degradation in beam quality and brightness, thus limiting their general applicability.
Lasers designed to lase on Laguerre-Gaussian (LG) resonator eigenmodes have also been developed in order to produce ring-shaped beam profiles.
Laser beams based on LG modes have been generated in a number of different ways which can be broadly sub-classified into designs in which the LG modes are generated external to a resonant laser cavity3-12 and designs in which the LG modes are generated inside a resonant laser cavity13-23.
Several known external methods for producing LG beams exploit the fact that LG modes can be formed by the superposition of correctly phased HG modes24, alternatively a fundamental HG beam can be conditioned using polarization or phase modifications to force the appropriate conditions (e.g. radial or azimuthal polarization, or a helical phase front) as required for desired LG modes. A variety of approaches can be used, such as:
A disadvantage of the known external cavity methods is that additional optical components, typically with very precise alignment criteria, are required to achieve effective mode-conversion. The purity of the resulting LG mode is then dictated by quality of the phase control of the constituent modes, such as the resolution of the grating structure or phase converting element. Moreover scaling to high powers via this route is currently still quite challenging, particularly to produce efficient single higher-order mode TEM0m solid-state lasers, while for example in the case of spatial light modulation devices they can only be operated at modest power levels.
The known internal cavity methods for generating ring-shaped LG modes directly from a laser resonator exploit a variety of approaches:
All of these techniques, apart from references20, 21, rely upon additional cavity components or pump-power dependent processes to enforce the right phase conditions to generate a ring-shaped LG mode. The approach of the authors of22, 23 effectively aimed to reduce the threshold condition for higher-order LG mode(s) with respect to the fundamental TEM00 mode, but it is not an appropriate method for maintaining single higher order modes (HOMs) with increasing pump powers.
Another approach for generating doughnut-shaped beams relies on recent developments in specially designed optical fibre to propagate a single linearly polarized (LP) higher-order-mode (HOM)25. The ring shaped high-order-modes have similar characteristics to LG modes26. Extreme precision in the fabrication process is required to ensure exact cylindrical symmetry in the core to maintain the critical properties of the propagating mode, and ultimately the HOM fibres have limited power handling capabilities due to non-linear effects (such Stimulated Raman scattering) in the glass. Similar techniques have been also been demonstrated, using multi-mode fibres with polarization or wavelength selection of discrete HOM's in order to obtain ring-shaped and radial or azimuthal polarized beams27-29.
Laser beams propagating in Laguerre-Gaussian modes can be designated as LGpl modes12, where p and l are both integers. p+1 is the number of radial nodes and l relates to the azimuthal phase change. When p=l=0, the beam has a Gaussian transverse intensity profile. From an applications point of view, the family of Laguerre-Gaussian modes designated as LG0l (i.e. where p=0 and l>0) are of particular interest. These modes have a ring-shaped intensify profile and an intensity-null on the optical axis; they are not well matched for efficient operation when using uniform or near uniform pumping configurations, irrespective of the technique used to ensure their selection. This is purely a result of having no (or very little) stimulated emission from the excited volume along the beam axis. As such a high-purity higher-order LG mode can be difficult to generate in a power-scalable fashion as there are stringent requirements on discriminating against the fundamental TEM00 mode, which typically has the lowest threshold condition due to its intensity peak on-axis and best overlap with the excitation volume of an optimised laser. As demonstrated by the authors of 22, 23, tailoring the pump beam to provide an excitation region comparable to the desired output mode lends itself to simplified selection of single HOM's. The pump source configurations of22, 23 are limited to very short near field distances and therefore not suitable for generic gain media or power-scalable laser architectures.
The invention is based on a conventional pump laser design with a pump source operable to generate a pump beam; a waveguiding element, such as a fibre, having a first end arranged to receive the pump beam and a second end to output the pump beam after traversing the waveguiding element; and a resonant cavity in which a laser medium is arranged to receive the pump beam output from the waveguiding element and which is operable to output a laser beam. The invention is based on the waveguiding element being specially designed to re-shape the pump beam in order to excite one or more desired Laguerre-Gaussian modes in the cavity. This is achieved by the waveguiding element having a refractive index profile such that the pump beam output from the waveguiding element has an intensity distribution which spatially overlaps, and thus preferentially excites one, or more than one, desired Laguerre-Gaussian mode LG0l of the resonant cavity. The waveguiding element is thus adapted to provide beam-shaping. The LG modes of primary interest are the ring-shaped modes which have an annular or ring-shaped intensity profile. Laser oscillation can thus be realised on one or more ring-shaped LG modes.
The beam-shaping waveguiding element can be tailored to provide an intensity distribution which spatially overlaps more than one desired Laguerre-Gaussian mode of the resonant cavity, in particular more than one ring-shaped Laguerre-Gaussian mode. The output laser beam will then still have a ring shape.
The beam-shaping waveguiding element can also be tailored to provide an intensity distribution which spatially overlaps not only a ring-shaped Laguerre-Gaussian mode, but also the fundamental mode of the cavity, i.e. the TEM00 mode, so that the cavity lases both on the fundamental mode and a ring-shaped Laguerre-Gaussian mode. The laser beam will then have a profile formed of a mixture of a Gaussian profile and a ring profile, the relative strength of which can be varied, for example to create a top-hat beam profile. Top-hat profiles are desired in some materials processing applications.
A simple technique is thus provided for directly exciting very high quality ring-shaped Laguerre-Gaussian modes with radial, azimuthal or linear polarization, or a combination of one or more Laguerre-Gaussian modes in an optically-end-pumped (non-guided-wave) laser, by using an axially symmetric pump beam with a lower intensity towards the centre of the beam.
The waveguiding element can be conveniently realised as an optical fibre, e.g. a silica glass fibre. Alternatively, a rigid rod can be used, e.g. a rigid glass capillary.
To achieve the beam shaping, the fibre or rod can be fabricated to have a refractive index profile with an outer region with a higher refractive index surrounding an inner region with a lower refractive index, so that the pump beam is guided predominantly in the outer region.
One way of doing this is with a hollow fibre or hollow rod (i.e. capillary), i.e. the outer region is made of a solid material—typically a glass such as a silica glass. The hollow fibre or rod has a hole running axially along the fibre, the hole forming the inner region. In ambient conditions the hole will be filled with air. The hole could also be filled with any other gaseous or liquid medium of suitably low refractive index.
Another way of providing a suitable refractive index profile is with a micro-structured fibre. The fibre's inner region is formed of micro-structured elements that form multiple holes running along the fibre. For example, the micro-structured elements may form a ring of holes between the outer region and a core region.
The design is compatible with Q-switching and mode locking of the resonant cavity. Namely, the resonant cavity may include a Q-switch element. The Q-switch element has variable attenuation properties and may be an externally-controlled variable attenuator or utilize a saturable absorber, as is well known in the art. Moreover, the resonant cavity may include a mode locking element. The mode locking element may be an acousto-optic modulator for active mode-locking or a saturable absorber for passive mode locking, or a non-linear component, as is well known in the art.
Embodiments of the invention thus employ a fibre-based or rod-based beam shaping element with an annular waveguide to re-format the output beam from an optical pump source to yield a pump beam with a substantially axially symmetric transverse intensity distribution with a lower intensity at the centre of the beam in order to produce a population inversion distribution that spatially overlaps the desired axially-symmetric Laguerre-Gaussian mode or modes in the laser gain medium of the resonant cavity, so as to achieve preferential laser oscillation on said mode(s).
The pump source may comprise one or more diode lasers, fibre lasers, solid-state lasers or a combination of these lasers with operating wavelength(s) selected for efficient absorption of the pump laser radiation in the gain medium of the resonant cavity.
The resonant cavity may be a solid-state laser design with a rod, slab or thin disk laser medium geometry doped with a suitable active ion. The active ion may be a rare-earth ion (e.g. Nd, Yb, Er, Tm, Ho, Pr) or a combination of rare earth ions, or another active ion. Alternatively, the resonant cavity may be an optically-pumped semiconductor laser with a thin disk geometry or may be a liquid laser or a gas laser. The resonant cavity can employ a standing-wave or ring resonator architecture, and can be designed to operate in continuous-wave (CW) or high-peak-power pulsed mode of operation.
The pump beam can be coupled into the gain medium of the resonant cavity via an arrangement of one or more lenses. The pump beam can be coupled into the laser gain medium of the cavity in two or more directions to increase the absorbed pump power and hence the output power. A further increase in power may be achieved through provision of two or more laser gain media in the cavity. The output laser beam may be further amplified in power using an amplifier comprising one or more gain elements, pumped in the manner described above, and seeded by a spatially-matched signal beam. The signal beam can be derived from a laser resonator designed to operate on the desired LG mode(s), or via the use of a conventional laser resonator with an external beam shaping element.
The invention provides a laser device comprising: a pump source operable to generate a pump beam; a resonant cavity in which a laser medium is arranged to receive the pump beam and which is operable to output a laser beam; and a beam-shaping element arranged between the pump source and the resonant cavity having a refractive index profile designed to re-shape the pump beam so that the pump beam received by the resonant cavity has an intensity distribution which spatially overlaps a desired ring-shaped Laguerre-Gaussian mode of the resonant cavity sufficiently well to achieve laser oscillation on said desired Laguerre-Gaussian mode.
The invention is now described by way of example only with reference to the following drawings.
The beam conditioning element 12 comprises an optical fibre with at least one annular waveguide for the purpose of re-shaping the pump beam, an optical arrangement for coupling laser radiation from the first laser into the fibre re-shaping element and an optical arrangement for coupling the output from the fibre beam-shaper into the second laser.
The second laser 14 may be a solid-state laser in which the laser medium is a rod, slab or thin disk doped with a suitable active ion. The active on may be a rare-earth ion (e.g. Nd, Yb, Er, Tm, Ho, Pr) or a combination of rare earth ions, or another active ion, so as to produce gain at the desired operating wavelength. Alternatively, the second laser may be an optically-pumped semiconductor laser with a thin disk geometry, or, a liquid or gas laser. The second laser can employ a standing-wave or ring resonator architecture and can be designed to operate in continuous-wave (CW) or high-peak-power pulsed mode of operation. In this scheme, the pump beam provided by the first laser is spatially re-shaped by a fibre-based beam shaping element to yield an axially-symmetric beam profile with lower intensity in the centre of the beam in order to produce a population inversion distribution in the laser gain medium of the second laser that spatially-overlaps the desired Laguerre-Gaussian modes to achieve preferential lasing on these modes.
The annular waveguide 32 is preferably multimode with transverse dimensions (i.e. inner radius and outer radius) determined both by the beam parameters of the incoming pump beam (i.e. for efficiently coupling pump light into the annular waveguide 32) and by the final pump beam profile required for selective excitation of the desired Laguerre-Gaussian mode(s) in the second laser. The selective excitation can be facilitated through the choice of resonator design for the second laser and the design of the optical arrangement for coupling pump radiation from the fiber beam shaping element 18 into the second laser.
In one design, the annular waveguide 32 is fabricated from silica glass, the central region 30 is air and the outer region 34 is a low refractive index polymer or fluorine-doped silica glass. In other words, the inner region 30 is a hole and the fibre is a capillary fibre, or a solid glass capillary. The cladding region 34 may also be dispensed with in which case the waveguide would be formed solely by a capillary made of the same glass, i.e. the solid structure would solely consist of the annular glass waveguide 32. Alternatively, the central region 30 may be a low refractive index glass (e.g. fluorine doped silica). More complex axially-symmetric beam profiles as required to select different LG01 modes can be formed if required by using a fibre structure with more than one annular waveguide separated by thin regions of material (e.g. fluorine doped silica) with lower refractive index. In this case, pump light from the first laser can be distributed between the annular waveguides in the manner required by using an appropriate pump coupling scheme 16.
There are many different material and design options for the beam shaper 18, but in all cases the beam shaper has at least one annular waveguide for the purpose of re-shaping the pump beam from the first laser into an axially-symmetric beam with lower intensity at the centre of the beam to spatially overlap one or more Laguerre-Gaussian (LG0l) modes in the gain medium of the second laser in order to achieve preferential lasing on these modes.
In a variant, the glass, and thus the refractive index of, the annular region 32 ay be different from that of the central region 36—either higher or lower—but with the refractive indices of both regions 32 and 36 being greater than that of the micro-structured hole rings 38 and 39.
The central waveguide 36 and annular waveguide 32 are preferably multimode with transverse dimensions determined both by the beam parameters of the incoming pump beam (i.e. for efficiently coupling pump light into the annular waveguide 32 or, if required, the central waveguide 36 and annular waveguide 32) and by the final pump beam profile required for selective excitation of the desired Laguerre-Gaussian mode(s) in the second laser.
Coupling pump light into both the central waveguide 36 and annular waveguide 32 allows pumping of both the fundamental TEM00 (Gaussian) mode and one or more LG0m modes of the second laser respectively. The distribution of pump power between the guides can be controlled using the appropriate design of pump coupling scheme 16.
The fibre regions 32, 36 and 40 can be formed from silica or another suitable glass that has high transmission at the pump wavelength. The lower refractive index regions between 36, 32 and 40 can also be formed using one of more rings of lower refractive index rods instead of air. More complex axially-symmetric beam profiles as required to select different LG0m modes can be formed if required by using a fibre structure with more than one annular waveguide separated by thin regions with lower refractive index. In this case pump light from the first laser can be distributed between the annular waveguides in the manner required by using an appropriate pump coupling scheme 16.
In a variation of this design, the outer micro-structured ring of holes 39 and cladding 40 of refractive index n4 could be replaced by a single cladding of refractive index n3<n2, i.e. lower than that of the outer region 32, for example n4<n3<n2.
There are many other schemes for coupling pump light from the first laser 10 into the beam shaping fibre 18. The coupling methods described above represent only some examples.
Added functionality can be achieved by using a modified resonator design with additional active and/or passive components to tailor the dimension of the resonant modes and/or to Q-switch or mode-lock the second laser in order to obtain high-peak-power pulsed output with a tailored output beam profile. The second laser can also be configured as a unidirectional ring laser (e.g. for single longitudinal mode operation)
Thin-disk lasers have a greater degree of immunity to the effects of thermal loading than rod lasers, and hence offer a route to higher output power. In this embodiment, pump light 11 from the first laser 10 is re-shaped by the fibre-based beam conditioner 12 and is incident on the disk laser medium at an angle. Optionally, residual pump light (i.e. pump light not absorbed after a double-pass of the laser medium) can be retro-reflected using a mirror 82 to improve the absorption efficiency. Alternatively, a more complicated multi-pass pumping arrangement can be employed to improve the pump absorption efficiency. Otherwise, the approach for generating axially-symmetric LG0l modes (or a combination of LG0l modes) is the same as for the rod laser described in
Results from several test devices that implement the above designs are now described.
In this test device, as illustrated in
In this test device, as illustrated in
In this test device, as illustrated in
The first pump beam component 111 follows the same path as in the first test device, namely is coupled via a lens 521 into a capillary fibre 181 in which it is re-shaped and then output as pump beam component 131, coupled via a lens 221, and a plane mirrors 23, towards a further plane mirror 25.
The second pump beam component 112 is redirected by a plane mirror 53 and then coupled via a lens 522 into a conventional multimode circular-section fibre 182 from which it is output as pump beam component 132, coupled via a lens 222, and a plane mirror 232 of 50% transmissivity.
The first and second pump beam components 111 and 112 are recombined at semi-transparent mirror 232 and are then directed via plane mirror 25 and a further lens 24 into the resonator cavity formed by the input and output coupler mirrors 70 and 72 respectively which outputs a laser beam 76. The output coupler has a transmissivity of 10%. The cavity contains a laser medium formed for a rod of Erbium-doped Yttrium Aluminium Garnet (0.5% Er:YAG) as well as a lens 73. A power meter 27 is also shown adjacent mirror 232 which was used during testing to assist correct re-combination of the two pump beam components.
The purpose of splitting the pump beam into two and conditioning the two components in a capillary and circular fibre respectively is to simulate the effect of a conditioning fibre such as described in relation to
Lasers embodying the invention may be used for many applications where it is necessary to have a laser beam with a tailored intensity profile at some desired location(s), examples include hollow laser beams for manipulation of very small objects30, and top-hat or doughnut beams used in laser materials processing such as ablation, machining, drilling or welding31. Specific example applications are: optical tweezers; optical trapping, guiding and manipulation of atoms; extreme ultraviolet lithography; and LG01 beam microscopy.
The required intensity distributions can be generated through the manipulation of the laser beam phase-front, or by the superposition of selected higher-order modes, as described above. Moreover LG modes exhibit unique polarization properties, such as radial, azimuthal polarization, in addition to linear polarization states, and can be configured to have optical orbital momentum24. The combination of a tailored intensity distribution and polarization state can enhance the performance of many applications involving light-matter interaction, at the same time enabling new ones to be discovered.
In the above embodiments, the pump beam is spatially re-shaped by a fibre-based beam shaping element with at least one annular waveguide to yield an axially-symmetric beam profile with a lower intensity in the centre of the beam in order to produce a population inversion distribution in the laser gain medium of the resonant cavity that spatially-overlaps the desired Laguerre-Gaussian mode or modes, so as to yield preferential lasing or amplification of said mode(s).
Using this approach, the pump beam can be re-shaped into an axially-symmetric ring-shaped pump beam in the near-field to allow preferential excitation in the resonant cavity of a single Laguerre-Gaussian mode (e.g. LG01, LG02 or a higher-order mode) with a ring-shaped near-field and far-field intensity distribution. Additionally, the laser may be configured to operate with radial, azimuthal or linear output polarisation as required by the application.
As described, the pump beam may be re-shaped using a specially designed fibre-based beam shaping element to yield a tailored pump beam to allow preferential lasing in the second laser on two (or more) axially-symmetric transverse modes (e.g. TEM00 and LG01) for the purpose of generating an output beam with a more ‘top-hat’-like near-field and far-field beam profile with very good beam quality.
The technique is extremely simple and low cost to realise, since the only custom element is the pump beam conditioning element which can be fabricated easily out of fibre, such as silica fibre, or optionally thin rod, such as a glass capillary. References to silica fibre mean silica-based fibre, not pure silica fibre, so include the broader family of silica glasses based on alloys of silica including, for example, borosilicate, fluorosilicate and phosphosilicate glasses.
As described, various low-index-core, hollow-core, or micro-structured fibre designs are possible for achieving a sufficiently high degree of spatial overlap with the desired mode(s) in order to achieve preferential lasing on those modes.
The above approach for selective excitation of one or more axially-symmetric LG0l modes can provide low-loss, high efficiency and flexibility compared to prior art approaches. Moreover, the technique is compatible with power scalable laser architectures and hence offers a route to very high average power in continuous-wave and pulsed mode of operation serving the needs of a range of applications.
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Number | Date | Country | Kind |
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1107129.7 | Apr 2011 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2012/050868 | 4/20/2012 | WO | 00 | 10/25/2013 |