WO2009049735A1

WO2009049735A1 - Laser crystal

Info

Publication number: WO2009049735A1
Application number: PCT/EP2008/007694
Authority: WO
Inventors: Ulrich Wittrock
Original assignee: Fachhochschule Münster
Priority date: 2007-10-09
Filing date: 2008-09-16
Publication date: 2009-04-23
Also published as: DE102007048464A1

Abstract

The invention relates to a laser crystal having an end surface (E) that is convex to the interior of the crystal, said end surface being mirrored (3) in a highly reflective fashion for at least one laser wavelength, and said laser crystal being excitable only in a region near said end surface (E) to achieve a stimulated laser emission on at least one laser wavelength, and not being thus excitable in a region removed from the convex end surface.

Description

laser crystal

The invention relates to a laser crystal. Laser crystals are well known in the art and are commonly used to build laser resonators for generating laser radiation or even laser amplifiers for amplifying already generated laser radiation.

Laser crystals are characterized in that they are residual body media, sometimes with additional doping, in which a population inversion of the electrons can be generated by external excitation, for example optical excitation by so-called pump light, so that a stimulated emission of photons of one or more Laser wavelengths is generated. Depending on the desired laser wavelength, different laser crystals, i. different solid-state media can be used with or without an additional doping.

For an efficient laser process, i. efficient performance of the stimulated emission, it is essential to operate at or well above the saturation energy flux density, so that the stimulated emission dominates over other processes, in particular the enhanced spontaneous emission, otherwise the efficiency is low.

Depending on the selection of the laser crystal and the operated laser wavelength, it may happen that the required saturation energy flux densities are so high that they reach the damage thresholds of the laser material or at least any coatings, such as antireflection coatings on the exit surfaces of the laser crystals. Thus, it is often observed during efficient operation of a laser crystal that it is destroyed at the exit surfaces after a short time. This disadvantageous effect, which is observed in particular in connection with anti-reflection coatings on the exit surfaces of laser crystals, is also disadvantageous because at certain desired emission wavelengths basically provided by a laser crystal, the saturation flux density can not be achieved for effective laser operation, since the damage threshold at the Exit surfaces, especially due to anti-reflective coatings is well below the damage threshold of the crystal material and thus the practical laser operation of such a laser crystal is prevented at these wavelengths.

The object of the invention is to provide a laser crystal, by means of which an efficient laser operation, in particular a laser oscillator or laser amplifier operation is possible and with which in the region of the laser-active medium, an operation in the range or above the saturation energy flux density can be achieved, in which However, destruction of the exit surfaces, especially if they are provided with an anti-reflection coating can be effectively avoided.

In particular, the possibility should be created with a laser crystal according to the invention to develop wavelength ranges which could hitherto not be used in Q-switched operation with conventional crystals, such as e.g. the wavelength of 942.5 nanometers for Nd: GSAG crystals as well as 1030 nanometers for Yb: YAG Lasem.

According to the invention, this object is achieved in that a laser crystal has a convex end face to the crystal interior, which is highly reflective mirrored for at least one laser wavelength and the laser crystal only in a near this end face, in particular adjacent to the end surface area to achieve a stimulated laser emission at least one laser wavelength is excitable, but not in a distant from the convex end surface at a spaced area. Thus, it is an essential idea in a laser crystal according to the invention that the required population inversion to achieve a stimulated emission at a certain and desired laser wavelength, possibly even multiple wavelengths or a wavelength range can be generated only in a portion of the laser crystal according to the invention, for example by optical excitation, this region is arranged near the end face which is convex toward the inside of the crystal, in particular directly adjacent to the convex end face.

Light, in particular the laser light generated or amplified, which propagates to the convex end surface within such a laser crystal, is therefore reflected back at the convex end surface and thus runs divergently through the laser crystal due to the convex shape of the end surface, so that in principle when using such Laser crystal in a laser resonator or a laser amplifier, the minimum beam waist within the laser crystal directly on the convex end surface exists. This means that the energy flux density needed for an efficient laser process, at or well above the

Saturation energy flux density is only at that location, i. must be achieved in the laser-active the convex end surface near the area of the laser crystal.

In the region remote from the convex end surface, due to the beam geometry, and in particular due to the divergent beam path, the energy flux density will decrease as the distance from the convex end surface progresses, such that at an exit surface of the laser crystal the region remote from the convex end surface is arranged, the damage thresholds of the exit surface, in particular the damage threshold of a possible coating of this exit surface remains below and thus safe operation of such a laser crystal is possible.

It is according to the invention as mentioned above provided that in the distant region of the laser crystal not for stimulated emission on the beginning said desired wavelength or the wavelengths or the wavelength range is excitable to produce a population inversion, so that effectively prevented by a laser crystal of the type according to the invention that the laser beam generated or amplified in the near end of the convex end surface is not already in the distant area again which would be the case for three-level lasers or quasi-three-level Lasem when a laser crystal is excitable over its entire length, but in the excited far range the energy flux density is not sufficiently high to stimulate emission and thus to achieve a gain of the desired laser beam, but on the other hand reabsorption takes place or dominated by increased spontaneous emission.

A laser crystal of the type according to the invention thus combines three properties essential to the invention, namely the achievement or exceeding of the saturation energy flux density in the laser-active region of the laser crystal near the convex end surface, and furthermore the prevention of reabsorption and / or the prevention of increased spontaneous emission in that of the convex end surface Far region in which the energy flux density necessary for the laser process remains below due to the divergence of the beam and the operation of the laser crystal well below the damage threshold of the exit surface or a coating of the exit surface.

In one possible embodiment of the invention, it may be provided that the laser crystal according to the invention is formed from a one-piece crystal host having only near the convex end surface doping with a material for achieving the stimulated laser emission and the far the convex end surface either a weaker , For example, a decreasing from this convex end surface in the direction of the opposite end surface doping, no doping or doping with another material. Such one-piece crystal hosts may, for example, be garnet hosts, such as yttrium-aluminum garnet, gadolinium-scandium-aluminum garnet or other garnet hosts that, when doped with another material, such as neodymium ions or ytterbium Ions at certain laser wavelengths can be operated.

For example, it may be provided according to the invention that such a one-piece crystal host may be prepared by a known method, e.g. is pulled by the Czrochralski method, wherein during the pulling of the crystal, the doping of the crystal melt is changed, for example, the materials for doping are added only at the end of the drawing process and thus only the end of the pulled crystal has the doping, thus a doped and reach an undoped or less doped region within a one-piece laser crystal.

It is then possible, after further shaping of the crystal, to provide that region of this one-piece crystal with an end surface which is convex toward the inside of the crystal and has the desired and necessary doping for achieving a stimulated laser emission within this region.

In general, a laser crystal in the sense of the invention is also understood as meaning a laser-active solid which, according to strict crystallographic observation, does not represent a crystal, but optionally an amorphous material or also a ceramic. These laser solid-state media are understood in the context of the invention by the term of the laser crystal, even if this name is crystallographically incorrect. Accordingly, other than the above-mentioned method for producing a laser crystal of the type according to the invention with a region near the convex end surface and a laser active region and a region remote from the convex end surface and not a laser-active region are also apparent. For example, it has recently become known that laser crystals can basically not only be produced by crystallographic breeding methods, but also by sintering methods, if they are laser-active ceramics. These are also added in the sense of the invention to the said laser crystals, as well as doped glasses which are amorphous.

According to another embodiment of the invention, it can also be provided that a laser crystal according to the invention is formed as a composite crystal having a first crystal which has the convex end surface and forms the near region, which is thus laser active and in which by excitation, in particular optical excitation , a required population inversion can be generated and having a second crystal forming the far region and which can not be excited to a stimulated laser emission at the same desired wavelength or multiple wavelengths as in the first crystal, both crystals being optically coupled to each other. For the concept of crystal here the same definition as we mentioned before.

As a result, as already mentioned, the energy flux density in the first crystal with the convex end surface can be selected to be sufficiently high to achieve an efficient laser process, whereas the beam, after being widened, precludes outcoupling from the exit surface of the composite laser crystal according to the invention optically coupled crystal must propagate before it reaches the coupling surface on which, however, the diameter due to the divergent beam behavior from the convex end surface to the opposite exit surface is sufficiently large to significantly lower the damage threshold of this end face or disposed there antireflection coating.

Essentially, with such a laser crystal according to the invention, the second optically coupled crystal or, more generally, the far region has merely the task of providing sufficient propagation path within the invention To provide laser crystal to produce due to the divergence a sufficiently large cross-section on the exit surface.

In this case, it may be preferred according to the invention for the second crystal, which is optically coupled to the first crystal, to have a preferably identical, but at least similar refractive index to that of the first crystal, the refractive index difference between the crystals being therefore sufficiently small, for internal reflection effects to prevent the coupling plane of both crystals sufficiently. Accordingly, the two crystals to be coupled to each other are preferably selected such that their refractive indices preferably differ only in the second, and more preferably only in the third or later decimal places.

For the optical coupling of the two crystals to each other can be made of conventional techniques that do not relate to the subject of this invention. For example, the optical coupling can be effected by the thermal diffusion bonding, or by optical wringing. These are known in the art methods to connect crystals together without having to use a bonding material between the crystals, in particular so that the two crystals are firmly bonded together.

In a particularly preferred embodiment, it may be provided that both crystals of the composite crystal according to the invention have the same crystal host, for example a garnet host. It may then be provided that the crystal with the convex end surface is doped with a material for the purpose of obtaining the stimulated laser emission and the other crystal has no doping or with a different material. The use of the same crystal host for both crystals has the particular advantage that the refractive index of the two crystals is almost identical and at most only deviating due to the doping of the materials, which is the only difference between the two crystals. So that's one Compound crystal of this type according to the invention is particularly preferred, especially since they can also be optically coupled to one another optically, for example due to diffusion bonding.

Besides the embodiment in which the crystal having the convex end surface has a certain doping to obtain the stimulated laser emission and the other has no doping to prevent reabsorption in the region distant from the convex end surface, it may also be provided that the crystal Crystal far from the convex end surface is doped with another material. For example, doping may be selected such that the far region has absorption for unwanted parasitic modes or increased spontaneous emission of another possible laser wavelength of the crystal having the convex end faces.

Thus, using a particular doped laser material in the first crystal, i. a doped crystal material can be used with the convex end surface, which can be stimulated not only on a laser wavelength, but optionally also on a plurality of laser wavelengths or on a wavelength band to stimulated emission.

However, if it is only desired to select one of the possible laser wavelengths or a portion of a laser wavelength band, the doping of the second crystal may be chosen to result in absorption around those unwanted wavelength portions in the second crystal and thus the region remote from the convex end surface , in which only low flux densities occur, are absorbed.

In particular, in a laser resonator operation in which the absorption represents significant resonator losses, therefore, the lasing at undesired wavelengths can be effectively prevented and thus a mode selection can be operated. In a further embodiment it can also be provided that both crystals of the composite crystal have different crystal hosts and the crystal with the convex end face is doped with a material for the achievement of the stimulated laser emission and the other crystal has no doping or with a different material.

Also in this embodiment, as mentioned in the previous embodiment, it may be provided that doping in the second crystal region is selected far from the convex end surface in order to absorb unwanted parasitic modes or amplified spontaneous emission of another possible laser wavelength. Also, these crystals with different hosts are preferably selected such that the refractive indices are approximately equal in the above sense.

In all embodiments of the aforementioned type, it may be provided that the end face of the distal region opposite the convex end face is arranged either at Brewster's angle for the desired laser wavelength and / or has an antireflection coating, in which case the end face opposite the convex end face is perpendicular to the end face Spreading vector of the laser beam is arranged.

The laser crystal according to the invention is particularly preferably used in embodiments with an antireflection coating, since it is precisely the essential disadvantage of an antireflection coating that the damage threshold of the end surface on which this antireflection coating is arranged is significantly reduced.

Due to the divergence of the laser beam in the convex end surface distal region and thus the generated expansion at the exit surface, these low damage thresholds of the possible antireflection coatings are exceeded, so that the associated problem is effectively prevented. By the further fact that in this the convex end face remote area No laser action is possible at the wavelength with which the first of the convex end surface is operated near range, while still preventing beam absorption at the desired laser wavelength.

Thus, with a crystal of the type according to the invention, laser systems which are used at e.g. 942.5 nanometers work, such as neodymium-doped gadolinium-scandium-aluminum garnet or even the operation of ytterbium-doped yttrium-aluminum garnet lasers at a wavelength of 1030 nanometers, where the saturation energy flux density for optimum laser action above the damage thresholds in particular of antireflection coatings on the laser crystals.

An embodiment of the invention is shown in the following figure.

FIG. 1 shows here a composite laser crystal according to the invention, which is composed of two crystals 1 and 2. Here, the crystal 1, based on the figure on the left side to a crystal interior convex left end surface E, which is additionally provided from the outside with a highly reflective coating 3 in order to achieve for the desired laser wavelength, if necessary. Also for the pump wavelength reflection. The crystal 1 with the convex left end face E is chosen such that in this by excitation of the electrons, e.g. by optical pumping, a population inversion can be achieved, thus enabling laser action at a desired wavelength that provides the selected material of the crystal 1.

The crystal 2 relating to this embodiment has an antireflection coating 4 on its right-side exit surface A and is optically coupled with its left-side planar end surface in the connection plane VE to the right-side planar end surface of the crystal 1.

This optical coupling is preferably carried out without interposition of a further medium, for example by the technique of thermal Diffusion bonding and a cohesive connection between the crystals 1 and 2 allows.

The crystal 2 may be selected from a material which does not provide a laser transition with the same laser wavelength of the crystal 1 and accordingly in a quasi-three-level laser for lack of a corresponding laser transition for the generated and / or amplified laser radiation in the crystal 1 also non-absorbent.

The beam geometry is in the entire arrangement of the crystal according to the invention of the two partial crystals 1 and 2 preferably such that the beam waist of the laser radiation 5 and, if necessary. Also the pump radiation directly on the convex end surface in the crystal 1 has the lowest beam waist and therefore be bundled there can, that the saturation energy flux density is reached or even far exceeded, but this has no consequences, since this energy flux density is still below the damage threshold of the highly reflective coating 3 and below the damage threshold of the medium of the crystal 1.

Due to the convexity of the left end surface of the crystal 1, the laser beam propagates divergently from left to right, i. its diameter increases continuously, so that in the region of the crystal 2, which is located far away from the convex end surface of the crystal 1 and here in particular at the exit surface A in the region of the antireflection coating 4, the damage threshold, which is significantly lower than that of the highly reflective layer 3, due to the expansion of the Falls below beam.

Accordingly, with such a laser crystal according to the invention in the region of the crystal 1, an effective laser operation in the region of the saturation energy flux density and also in the area of the damage thresholds can be achieved, whereas the exit surface with the antireflection coating 4 is in a region of the radiation with a significantly reduced energy flux density, so that a Destruction risk is excluded. In this case, it is further achieved by the special selection of the crystal 2 that a reabsorption of the generated or amplified radiation of the crystal 1 is prevented, since this crystal does not provide a laser transition like the crystal 1.

In any case, it may be provided here in a special embodiment of the laser crystal, that the crystal 2 has a doping, but this is not intended to achieve laser operation by optical pumping, but rather is selected to a desired absorption at a certain To provide wavelength or even several specific wavelengths, eg In order to cause high losses in the laser crystal of the invention for a non-desired, but in the medium of the crystal 1 basically possible laser wavelength.

Thus, parasitic modes at unwanted wavelengths as well as spontaneous amplified emission at these unwanted wavelengths can be prevented, thereby increasing the efficiency of the laser crystal at the desired wavelength.

For example, in an embodiment it may be provided that the crystal 1 is formed from a neodymium-doped gadolinium-scandium-aluminum garnet, whereas the crystal 2 is formed from the same crystal host GSAG, but without doping.

The length of the crystal 2 in the direction of propagation of the laser beam can generally be adapted with respect to all embodiments of the invention such that a desired beam cross-section is achieved on the exit surface with the antireflection coating 4, ie here the damage threshold of the antireflection coating or generally the exit surface of the laser crystal according to the invention whereas the saturation flux density within crystal 1 and also at the boundary of the transition between crystals 1 and 2 is above the damage threshold of the antireflection coating, but below the damage threshold of the respective crystal materials. The length of the crystal 1 with the convex end face will preferably be less than that of the crystal 2 away from the convex end face.

With regard to all embodiments, it should be noted that the technical features mentioned in connection with one embodiment can be used not only in the specific embodiment, but also in the other embodiments. All disclosed technical features of this invention description are to be classified as essential to the invention and arbitrarily combined or used alone.

Claims

claims

1. Laser crystal, characterized in that it has a convex end face to the inside of the crystal (E), which is highly reflective mirrored for at least one laser wavelength (3) and the laser crystal only in a near this end face (E) near range to achieve a stimulated laser emission at least one laser wavelength is excitable, but not in a distant from the convex end surface area.

2. Laser crystal according to claim 1, characterized in that it comprises a one-piece crystal host, in particular a garnet host having only near the convex end surface doping with a material for achieving the stimulated laser emission and far from the convex end surface a weaker, in particular from this end face decreasing, none, or doping with another material.

3. Laser crystal according to claim 1, characterized in that it as a

Composite crystal is formed with a first crystal (1) having the convex end surface (E) and forming the near region, and a second crystal (2) forming the distal region, wherein both crystals (1, 2) to each other are optically coupled, in particular by diffusion bonding.

4. Laser crystal according to claim 3, characterized in that both crystals (1, 2) of the composite crystal have the same crystal host, in particular a garnet, and the crystal (1) with the convex end face (E) with a material for the Achieving the stimulated laser emission doped and the other crystal (2) has no or doping with another material.

5. Laser crystal according to claim 3, characterized in that both crystals (1, 2) of the composite crystal different crystal hosts, in particular a garnet, and the crystal (1) with the convex end face (E) with a material for the Achieving the stimulated laser emission is doped and the other crystal (2) has no or a doping with another material.

A laser crystal according to claim 2, 4 or 5, characterized in that doping with another material is selected such that the far region has an absorption for undesired parasitic modes or enhances spontaneous emission of another possible near wavelength laser wavelength.

7. Laser crystal according to one of the preceding claims, characterized in that the convex end face (E) opposite end face (A) of the distal region is arranged for the desired laser wavelength in the Brewster angle or has an antireflection coating.