Patent Description:
Grating mirrors showing the aforementioned features are known from the prior art.

Prior art documents include <CIT>, <CIT> and <CIT>.

These known grating mirrors are difficult to manufacture if a high diffraction efficiency shall be obtained.

In particular known grating mirrors are very sensitive to variations in design parameters.

It is the object of the invention to define a grating mirror providing a high diffraction efficiency and showing a good design tolerance in order to allow cost efficient manufacturing of such grating mirror.

This object is solved by a grating mirror as defined above according to which the grating layers have grooves etched from a top surface into said stack of grating layers with an etching depth between <NUM> or more and <NUM> or less in order to obtain a waveguide grating structure.

The advantage of the present invention has to be seen in the fact that such a grating mirror shows a high diffraction efficiency in particular when used under Littrow conditions and also shows a high design tolerance in particular with respect to the etching depth.

According to one advantageous embodiment said stack of grating layers comprises at least two grating layers. These grating layers are designed according to an optimum diffraction efficiency.

In order to maintain the high design tolerance it is of advantage if said stack of grating layers comprises three grating layers at maximum.

However, an advantageous embodiment of the grating mirror comprises a stack of grating layers having only two grating layers.

With respect to the etching of the grating layers no further details have been defined.

One advantageous solution provides that said stack of grating layers has at least a top layer etched said top layer being arranged on a side of said stack opposite to said reflector.

This solution comprises a design according to which said top layer is partially etched.

A further advantageous solution provides that said stack of grating layers has said top layer fully etched so that the grooves fully penetrate the top layer.

With respect to the extension of the grooves in the layer below the top layer no further definitions have been given.

A further advantageous solution provides that said stack of grating layers has said top layer fully etched and said layer below said top layer at least partially etched.

However, the etching can also be done in accordance with another advantageous embodiment such that said stack of grating layers has said top layer and said layer below said top layer fully etched.

It is even possible in another embodiment that said grooves extent into an uppermost layer of said reflector.

In order to obtain the advantageous reflection behavior, in particular the high diffraction efficiency, a feature defined in claim <NUM> provides that an evanescent radiation field of incident radiation extents into said grating layers and upper layers of said reflector forming a waveguide for said incident radiation field with a direction of guidance extending parallel to said top surface of said stack of grating layers so that the grating mirror acts as a waveguide grating.

The upper layers of said reflector are primarily the upper three or four layers arranged close to said stack grating layers.

According to a further advantageous solution said grating has an aspect ratio between <NUM>,<NUM> and <NUM>,<NUM> in order to obtain the desired high design tolerances of said grating mirror.

The aspect ratio is defined by the lateral width of the grooves of said grating divided by the depth of the respected grooves.

In connection with the embodiments explained before the thickness of the grating layers has been not defined further.

One advantageous solution provides that the grating layers have a thickness between <NUM> and <NUM>.

In particular it is advantageous if the top grating layer has a thickness between <NUM> and <NUM> depending on the order of diffraction at a certain wavelength for which the grating is designed, the duty cycle and the grating period.

Further it is of advantage if the bottom grating layer has a thickness between <NUM> and <NUM>.

Specific adaptions of the thickness of the top grating layer and the other grating layers comprising the bottom grating layer have been made in accordance with the Littrow conditions under which the grating is used, in particular in accordance with the order of diffraction of the Littrow conditions under which the grating is used, the grating period and the duty cycle of the gratings.

One advantageous solution provides adaption of the grating layers for the minus first order of diffraction of the Littrow conditions.

Further according to the present invention the periodicity of the grating has not been specified further.

One advantageous solution provides that the grating has a periodicity between <NUM> and <NUM>.

Further the duty cycle has not been specified. One advantageous solution provides that the grating has a duty cycle between <NUM>,<NUM> and <NUM>,<NUM>.

In connection with the various embodiments explained before no further details have been given with respect to the thickness of the reflector layers.

One embodiment provides that the reflector layers have a thickness between <NUM> and <NUM>.

In particular the reflector layers are dimensioned to have a thickness corresponding essentially to a quarter of the wavelength to be reflected, taking into consideration the angle of incidence of the wavelength to be reflected with respect to the planer extension of the reflector layers.

Till now the material, from which the grating layers and the reflector layers have been made has not been specified in detail.

According to an advantageous solution the grating layers and the reflecting layers comprise Ta<NUM>O<NUM>, HfO<NUM>, Nb<NUM>O<NUM>, TiO<NUM>, Al<NUM>O<NUM>, Si<NUM>N<NUM> and a:Si as high refractive index material and SiO<NUM>, MgF<NUM> as low refractive index materials.

The grating mirror design can be transferred to any wavelength region where resonant waveguide gratings can be realized such as the <NUM> domain for CO2 lasers by choosing suitable materials for quarterwave layers and grating layers and adapting the grating profiled.

The invention also relates to a laser system having a laser cavity defining a cavity radiation field extending along an optical axis between cavity end mirrors of said cavity and a laser active material arranged to be penetrated by said cavity radiation field.

It is the object of such a laser system to have the cavity radiation field stabilized with respect to its polarization and its wavelength.

This object is solved by a laser system showing the aforementioned features and having at least one of said cavity mirrors being a grating mirror according to one or more the aforementioned features and arranged in Littrow mount with respect to the optical axis of said cavity radiation field.

Such a cavity mirror provides the advantageous features such that it stabilizes the polarization and stabilizes the wavelength of said radiation field in a cost effective design.

In particular said cavity mirror can be an end mirror.

However, said cavity mirror can also be an output coupler.

Another advantageous solution provides that a second harmonic generator is arranged in said cavity radiation field and so that a second harmonic radiation field is generated in said laser cavity.

In particular such second harmonic radiation field can be easily coupled out from said laser cavity so that the laser system enables easy generation of a second harmonic radiation field in addition to said cavity radiation field.

The invention also relates to a pulse stretching device and/or a pulse compression device using at least one diffraction grating for separating the wavelengths within a laser pulse.

It is the object of the present invention to provide such a pulse stretching device and/or such a pulse compression device with minimum loss of the laser pulse intensity.

This object is solved by a pulse stretching device using at least one diffraction grating for separating the wavelengths within a laser pulse wherein according to the present invention the at least one diffraction grating is a grating mirror according to the features outlined before.

This object is further solved by a pulse compression device using at least one diffraction grating for separating the wavelengths within a laser pulse wherein according to the present invention the at least one diffraction grating is a diffraction mirror according to one or more of the features of such a grating mirror outlined before.

The advantage of using a grating mirror as outlined before in a pulse stretching device and/or in a pulse compression device is to be seen in the fact that the high diffraction efficiency provides minimum intensity losses for the stretched laser pulse and/or the compressed laser pulse.

In particular an advantageous solution provides that this pulse stretching device and/or the pulse compression device use two grating mirrors for separating the wavelengths of a laser pulse to be stretched or compressed.

In such a device the high diffraction efficiency provides minimum intensity losses for the stretched light pulse or the compressed light pulse compared to use of conventional diffraction gratings.

Further features and advantages are explained in connection with the subsequent detailed description and the drawings.

A first embodiment of a grating mirror <NUM> according to the present invention as shown in <FIG> comprises a substrate <NUM> having a first surface <NUM> and a second surface <NUM>, the first surface <NUM> and the second surface <NUM> being arranged on opposite sides of said substrate <NUM>.

Further the grating mirror <NUM> comprises a reflector <NUM> arranged on the first surface <NUM> of said substrate <NUM> and comprising a stack <NUM> of reflector layers <NUM> and <NUM> arranged in said stack <NUM> in an alternating arrangement.

For example reflector layer <NUM> is a layer of a high refractive index material whereas reflector layer <NUM> is a layer of a low refractive index material.

The reflector layers <NUM> and <NUM> have a thickness WHR, WLR in the range between <NUM> or more and <NUM> or less and are adapted to the wavelength to be reflected.

In particular the thickness WHR and WLR of layers <NUM> and <NUM> corresponds to a quarter of the wavelength to be reflected taking into consideration the angle of incidence.

The specific values of WHR and WLR can be calculated according to the following formulas.

Quarter wave layer thickness WH/\L for high (H) and low (L) refractive index layer: <MAT> with <MAT> and.

WL, WH, θL and θH depend on the operating wavelength for which the grating mirror is designed, as well as the refractive indices nH and nL set by the coating technology chosen.

However a grating mirror as explained be can adapted to any other wavelength by adapting the quarter wave layers and the grating layers.

According to the present invention reflector <NUM> can be arranged with a high refractive reflector index layer <NUM> or with a low refractive index reflector layer <NUM> directly abutting on the first surface <NUM> of substrate <NUM>.

In an advantageous version of the design as shown in <FIG> reflector <NUM> has a high refractive index reflector layer <NUM> being directly abutting on first surface <NUM> of substrate <NUM>.

In an advantageous embodiment as shown in <FIG> the uppermost layer <NUM> of reflector <NUM> is a reflector layer <NUM> of a high refractive index material.

Uppermost layer <NUM> of reflector <NUM> which is arranged opposite to substrate <NUM> supports a grating <NUM> which in the advantageous embodiment according to <FIG> comprises a stack <NUM> of two layers <NUM> and <NUM> one being a high refractive index grating layer and the other being a low refractive index grating layer.

In the advantageous embodiment according to which uppermost layer <NUM> of reflector <NUM> is of high refractive index material grating <NUM> comprises low refractive index grating layer <NUM> as bottom layer which is directly arranged on uppermost layer <NUM> and high refractive index grating layer <NUM> as top layer extending from low refractive index layer <NUM> up to a top surface <NUM> of grating <NUM>.

Grating <NUM> is designed as a waveguide grating and comprises a plurality of grooves <NUM> each groove <NUM> extending in a longitudinal direction <NUM> with all longitudinal directions <NUM> of all grooves <NUM> extending parallel to each other and all grooves being arranged at a certain distance from each other.

Since grating <NUM> is arranged on stack <NUM> of reflector layers <NUM>, <NUM> the electromagnetic field of an incident radiation extends into grating <NUM> by extending into and penetrating grating layers <NUM>, <NUM> and also at least partially into the upper two to five layers <NUM>, <NUM> of reflector <NUM> arranged closest to grating <NUM> in the form of an evanescent field so that a waveguide effect with a direction of guidance extending parallel to said top surface <NUM> is obtained by excitation of said evanescent field in said grating layers <NUM>, <NUM> and said upper layers <NUM>, <NUM> of reflector <NUM>.

In particular each groove <NUM> comprises two side surfaces <NUM>, <NUM> extending essentially parallel to longitudinal direction <NUM> and also extending from top surface <NUM> and transverse to top surface <NUM> into grating <NUM> down to a bottom surface <NUM> of the respective groove <NUM>.

The distance, at which bottom surface <NUM> is arranged inside grating <NUM> and apart from top surface <NUM> defines a depth D of the respective groove <NUM> and the distance between the side surfaces <NUM> and <NUM> in a direction transverse to longitudinal directions <NUM> defines a lateral extension L of the respective groove <NUM>.

Further the distance S between adjacent but separated grooves <NUM> defines the extension of the section of the top layer <NUM> of grating <NUM> arranged between two adjacent grooves <NUM> in a direction transverse to longitudinal directions <NUM>, said section of said top layer <NUM> carrying a section of top surface <NUM>.

The sum of the distance S between two adjacent grooves <NUM> and the lateral extension L of each groove <NUM> results in a periocity P of grating <NUM> and this periocity P together with the lateral extension L of each groove <NUM> and the lateral distance S between adjacent grooves <NUM> as well as the depth D of said grooves <NUM> define the optical behavior of grating <NUM>.

According to one embodiment of the invention shown in <FIG> all grooves <NUM> show essentially the same depth D as well as essentially the same lateral width L and all grooves <NUM> are arranged essentially at the same lateral distance S from each other.

According to the first embodiment the present invention depth D of grooves <NUM> is selected such that grooves <NUM> extend exclusively in uppermost layer <NUM> which according to the first embodiment is the high refractive index layer grating <NUM> of grating <NUM>.

Grooves <NUM> are preferably obtained by etching top layer <NUM> and thereby removing material of top layer <NUM> in the areas where grooves <NUM> appear during the etching process.

The duty cyle F of grating <NUM> is defined to be <MAT>.

Grating mirror <NUM> can be used as diffraction grating in Littrow configuration, which Littrow configuration is explained in <FIG>.

The Littrow configuration is defined by the formula <MAT> with.

Littrow configuration means that an incident beam is reflected back into itself by grating <NUM> with maximum diffraction efficiency in one specific order of diffraction of grating <NUM>.

This means that an incident beam I hits grating <NUM> of grating mirror <NUM> in the specific Littrow angle Θ which is a Littrow angle of the -nth diffraction order so that under this angle Θ the incident beam is counter propagating in itself with maximum diffraction efficiency.

For example grating mirror <NUM> is used in the minus first order Littrow mount in which a diffraction efficiency of more than <NUM>,<NUM> is obtained at the respective Littrow wavelengths within a broad spectral bandwidth in particular in a TE polarization mode as can be seen from <FIG> and <FIG>.

A further advantage of the grating mirror according to the present invention when used in Littrow mount is shown in table <NUM> which demonstrates that the grating mirror according to the first embodiment <NUM> is highly design tolerant so that manufacturing of these grating mirrors can be done in a cost efficient manner.

In order to obtain a maximum diffraction efficiency in the respective Littrow mount the thickness of grating layers <NUM> and <NUM> has to be adapted.

The thickness WHG of grating layer <NUM> with a high refractive index can be extracted from <FIG> and the thickness WLG of grating layer <NUM> with a low refractive index can be extracted from <FIG>. <FIG> and <FIG> show the dependency of the thickness of layers <NUM>, <NUM> (values for the respective thickness indicated in nm in the curved lines) from duty cycle F (vertical axis) and grating period P (horizontal axis) for a Littrow mount with maximum diffraction efficiency in the minus first order at <NUM>.

The optimization can also be performed for a maximum of diffraction efficiency at other wavelengths.

High refractive index materials used for the respective layers of reflector <NUM> and grating <NUM> in the near infrared and infrared region are materials having a refractive index between <NUM>,<NUM> and <NUM>,<NUM>. In particular these materials comprise Ta<NUM>O<NUM>, HfO<NUM>, Nb<NUM>O<NUM>, TiO<NUM>, Al<NUM>O<NUM>, Si<NUM>Ni<NUM> and a:Si.

Low refractive index materials used for the respective layers of reflector <NUM> and grating <NUM> are materials having a refractive index in the range between <NUM>,<NUM> and <NUM>,<NUM>. In particular these materials comprise SiO<NUM>, MgF<NUM>.

In a second embodiment of grating mirror <NUM>', shown in <FIG>, substrate <NUM> and reflector <NUM> are identical with the first embodiment.

According to the second embodiment grating <NUM>' comprises grating layer <NUM> as high refractive index layer and grating layer <NUM> as low refractive index layer made of the same material as disclosed in connection with the first embodiment.

However in contrast to the first embodiment of grating mirror <NUM> depth D of grooves <NUM>' is enhanced in the second embodiment of grating mirror <NUM>'. According to the second embodiment depth D extends from top surface <NUM> into grating <NUM>' to an extend that grooves <NUM>' fully extend through layer <NUM> and partially into layer <NUM> so that the bottom surface <NUM>' of the grooves <NUM>' is arranged in layer <NUM> having a low refractive index.

The other parameters of reflective grating <NUM>' are defined in the same manner as disclosed in connection with the first embodiment.

In particular the periodicity P the lateral extension L of grooves <NUM>' and the distance S between adjacent grooves <NUM>' are selected as defined in connection with the first embodiment.

Grating mirror <NUM>' according to the second embodiment shows the same behavior in Littrow mount as grating mirror <NUM> according to the first embodiment.

In a third embodiment of the grating mirror <NUM>" as shown in <FIG> the thicknesses of reflector layers <NUM> and <NUM>, the arrangement of reflector layers <NUM> and <NUM> and the materials of reflector layers <NUM> and <NUM> are selected as disclosed in connection with the first embodiment.

According to the third embodiment of grating mirror <NUM>" shown in <FIG>, grating <NUM>" comprises still two grating layers <NUM>, <NUM> with grating layer <NUM> made of high refractive index material being the top layer, and grating <NUM> arranged below grating layer <NUM> being of low refractive index material. According to the third embodiment grating <NUM>" comprises grooves <NUM>" extending from top surface <NUM> into grating <NUM>" with a depth D" having such a dimension that bottom surface <NUM>" of grooves <NUM>" is arranged within uppermost layer <NUM>" of reflector <NUM>" so that grooves <NUM>" extend through top layer <NUM> made of high refractive index material, through grating layer <NUM> made of low refractive index material and into uppermost layer <NUM>" of reflector <NUM> made of high refractive index material.

Further parameters such as periocity P of grating <NUM>", duty cycle F of grating <NUM>" as well as lateral extension L of grooves <NUM>" and distance S between adjacent grooves <NUM>" are selected as explained in connection with the first embodiment.

All elements of the second embodiment and the third embodiment which are not explained explicitly and which are provided with the same reference signs as in the first embodiment correspond to those of the first embodiment so that with respect to these elements reference is made to the explanations given in connection with the first embodiment.

One characteristic feature of all three embodiments <NUM>, <NUM>' and <NUM>" of grating mirror as explained before is to be seen in the fact that the depth D of grooves <NUM>, <NUM>' and <NUM>" is within the range from <NUM> or more up to <NUM> or less and further all gratings <NUM>, <NUM>' and <NUM>" are provided with aspect ratios between <NUM>,<NUM> or more and <NUM>,<NUM> or less, whereas the aspect ratio is defined by the lateral width L of grooves <NUM>, <NUM>' and <NUM>" divided by the depth D, D' or D" of the respective grooves <NUM>, <NUM>', <NUM>".

Such aspect ratios contributing to the good design parameter tolerance of the grating mirrors according to the present invention.

Grating mirrors according to the present invention can be used in various applications.

For example one of grating mirrors <NUM>, <NUM>' and <NUM>" can be used in a laser system.

In such a laser system a laser cavity <NUM> comprises for example one cavity end mirror <NUM> and grating mirror <NUM> according to the present invention arranged as the other cavity end mirror <NUM> with grating mirror <NUM> being arranged in Littrow mount with respect to an optical axis <NUM> of a cavity radiation field RF extending between cavity end mirrors <NUM> and <NUM> within cavity <NUM>.

Further cavity <NUM> is provided with a laser active material <NUM>, for example a thin disk used as a folding mirror, and an output coupler <NUM> also used a folding mirror for cavity radiation field RF within cavity <NUM>.

Grating mirror <NUM> used as second cavity end mirror <NUM> with its high diffraction efficiency above <NUM>,<NUM>% and its polarization efficiency for the TE polarization enables a high amplification with stabilized polarization and stabilized narrow laser emission bandwidth of the cavity radiation field RF within cavity <NUM>.

If a second harmonic generation crystal <NUM> is placed between cavity end mirror <NUM> and output coupler <NUM> which is transparent for a second harmonic radiation field the second harmonic radiation field generated by second harmonic generation crystal <NUM> can leave cavity <NUM> through output coupler <NUM> as output beam <NUM> so that an efficient laser source for green light laser at high power is provided.

If for example the laser active material <NUM> is pumped by a pumping beam <NUM> of a pumping light source <NUM> it is possible to obtain a conversion efficiency of up to <NUM>% or in particular even up to <NUM>% or higher which means that the power of output laser beam <NUM> comprising the second harmonic radiation field can amount to up to <NUM>% or more of the power of pumping radiation beam <NUM>.

Grating mirrors according to the present invention can be also used for laser pulse stretching and laser pulse compression due to their high diffraction efficiency.

For pulse stretching and pulse compression the propagation of a laser pulse in a dispersive medium, in the present case represented by a diffraction grating mirror, introduces a phase modulation of its field components, e.g. the E-field of the light pulse.

Using a diffraction grating enables a spectral modification of the optical path length for each spectral component of the light pulse and the possibility to modify the optical path length for the various spectral components of a light pulse can be used to stretch or compress a light pulse.

The theoretical explanations for laser pulse stretching and laser pulse compression are outlined in "<NPL>", which book is herewith incorporated by reference.

For laser pulse stretching and laser pulse compression grating mirrors according to the present invention and optimized according to Littrow conditions can be used as diffraction gratings but then the grating mirrors have to be mounted such that the incident laser pulse deviates for the Littrow angle of incidence by some degrees, such as for example <NUM>° to <NUM>°, so that as a consequence the maximum of diffraction of the minus first order does not coincide with the incident beam.

However it is also possible to optimize grating mirrors according to the present invention such that the maximum of diffraction appears under a certain angle Θc with a diffraction efficiency essentially identical with the diffraction efficiency outlined before. In this case the thickness of layers <NUM> and <NUM> has to be adjusted and is in the range between <NUM> and <NUM>. The duty cycle F and grating period P are in the same dimension as defined earlier.

<FIG> shows one example of a laser pulse stretching device <NUM> using grating mirrors <NUM> and <NUM> as diffraction gratings which are designed by use of the concepts for gratings mirrors <NUM>, <NUM>' and <NUM>" according to the present invention.

The design of grating mirrors <NUM> and <NUM> concerning the angle Θc at which a maximum of diffraction appears with respect to a spectral component represented by a central wavelength Wc of an incident laser pulse <NUM> is based on the design concepts as outlined before and the angle Θc is the same at both grating mirrors <NUM>, <NUM>.

The grating mirrors <NUM> and <NUM> are arranged antiparallel with respect to each other and a laser pulse <NUM> incident along a path <NUM> is diffracted by grating mirror <NUM> under various angles which are different for different spectral components of laser pulse <NUM> having different wavelengths, so that for example the central wavelength Wc of laser pulse <NUM> is diffracted under the angle Θc and a longer wavelength WL in laser pulse <NUM> is diffracted with an angle ΘL greater than Θc whereas a shorter wavelength Ws of laser pulse <NUM> is diffracted with an angle Θs smaller than Θc.

All exemplary wavelengths Wc, WL, Ws are imaged by a confocal imaging system <NUM> onto diffraction grating <NUM> which diffracts all wavelengths Wc, WL, Ws such that all wavelengths Wc, WL, Ws propagate in parallel towards a reflector <NUM> which reflects back all wavelengths Wc, WL, Ws so that all wavelengths Wc, WL, Ws travel back the same path to diffraction gratings <NUM> and <NUM> and back on path <NUM>, forming a stretched laser pulse <NUM> which can be separated from the incident laser pulse <NUM> by a decoupling device <NUM>.

Stretching of laser pulse <NUM> to laser pulse <NUM> occurs due to the difference in the light paths for the central wavelength Wc the longer wavelengths WL and the shorter wavelengths Ws when travelling from grating mirror <NUM> to grating mirror <NUM> to reflector <NUM> and back to grating mirror <NUM> and grating mirror <NUM>.

Based on similar concepts pulse compression can be obtained by a pulse compression device <NUM> shown in <FIG>.

This pulse compression device <NUM> comprises two diffraction gratings designed by use of the concepts for grating mirrors <NUM>, <NUM>' and <NUM>" according to the present invention.

The design of grating mirrors <NUM> and <NUM> concerning the angles Θ at which a maximum of diffraction appears with respect to a central wavelength WL of an incident laser pulse <NUM> is based on the design concepts as outlined before and the angle Θc is the same at both grating mirrors <NUM>, <NUM>.

The grating mirrors <NUM> and <NUM> are arranged parallel to each other and the laser pulse <NUM> along path <NUM> hits diffraction grating <NUM> which diffracts a central wavelength Wc of laser pulse <NUM> at an angle Θc but a longer wavelength WL at an angle ΘL and a shorter wavelength at an angle ΘS so that the paths of the various wavelengths Wc, WL, Ws are separated.

These separated wavelengths Wc, WL, Ws hit diffraction grating <NUM> and are again diffracted at separate angles Θc, ΘL, Θs such that the diffracted wavelengths Wc, WL, Ws propagate from grating mirror <NUM> on parallel paths to a reflector <NUM> and are reflected back so that they propagate on the identical paths back to diffraction grating <NUM> and diffraction grating <NUM> and back on path <NUM>.

Due to the difference in the propagating paths from diffraction grating <NUM> to diffraction grating <NUM> to mirror <NUM> and back to diffraction gratings <NUM> and <NUM> on path <NUM> a compressed light pulse <NUM> is formed which is decoupled by a decoupling device <NUM>.

The advantage of using grating mirrors <NUM>, <NUM>' and <NUM>" according to the present invention as diffraction gratings has to be seen in the high diffraction efficiency of more than <NUM>,<NUM> which is important due to the fact that the laser pulses <NUM> and <NUM> in the stretching device <NUM> and in the compression device <NUM> respectively are diffracted several times, e.g. four times, so that the obtained pulse intensity is significantly affected by the diffraction efficiency.

Claim 1:
Grating mirror comprising a substrate (<NUM>) having a first (<NUM>) and a second (<NUM>) surface on opposite sides thereof,
a reflector (<NUM>) arranged on the first surface (<NUM>) of said substrate (<NUM>) and comprising a stack (<NUM>) of reflector layers (<NUM>) with high refractive index layers (<NUM>) and low refractive index layers (<NUM>) stacked in alternating arrangement,
a grating (<NUM>) arranged on a side of said reflector (<NUM>) opposite to said substrate (<NUM>),
said grating (<NUM>) comprising a stack (<NUM>) of grating layers (<NUM>, <NUM>) with at least one high refractive index grating layer (<NUM>) and at least one low refractive index grating layer (<NUM>) stacked in alternating arrangement, said stack (<NUM>) of grating layers (<NUM>, <NUM>) having grooves (<NUM>) etched from a top surface (<NUM>) into said stack (<NUM>) of grating layers (<NUM>, <NUM>) with an etching depth (D) between <NUM> or more and <NUM> or less, characterized in that said grating is designed to obtain a waveguide grating structure, and that said grating (<NUM>) is arranged on an uppermost layer (<NUM>) of said reflector (<NUM>), and that an evanescent radiation field of incident radiation extends into said grating layers (<NUM>, <NUM>) and upper layers (<NUM>, <NUM>) of said reflector (<NUM>) forming a waveguide for said incident radiation field with a direction of guidance extending parallel to said top surface (<NUM>) of said stack (<NUM>) of grating layers (<NUM>, <NUM>) so that the grating mirror acts as a waveguide grating and each of the reflector layers (<NUM>, <NUM>) have a thickness between <NUM>
and <NUM>.