WO2020261209A1

WO2020261209A1 - Optical element

Info

Publication number: WO2020261209A1
Application number: PCT/IB2020/056075
Authority: WO
Inventors: Gergely HUSZKA; Niels QUACK; Marcell KISS
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl)
Priority date: 2019-06-27
Filing date: 2020-06-26
Publication date: 2020-12-30

Abstract

The present invention concerns an optical grating comprising a body defining a first external surface and a second external surface. The first external surface defines a periodic structure comprising a plurality of elongated protrusions and a plurality of elongated recesses or depressions. The periodic structure is configured to receive an incident first electromagnetic wave propagating in an input propagation direction and to transmit the first electromagnetic wave to the second external surface for transmission through the second external surface. The periodic structure is configured to reflect, back into the input propagation direction, an incident second electromagnetic wave propagating in an opposite direction. The first electromagnetic wave has a wavelength lower that of the second electromagnetic wave.

Description

OPTICAL ELEMENT

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to International Patent Application PCT/IB2019/055441 filed on June 27th, 2019, the entire contents thereof being herewith incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to an optical element, and more particularly to an optical grating providing a low reflection and high transmission therethrough at a first wavelength and a high reflection at a different second longer wavelength. The optical grating may be used as a component of an optical or laser cavity, in particular, an optically pumped optical or laser cavity.

BACKGROUND

US2012/0128019 discloses a high-contrast grating providing a reflection approaching 100% and low transmission. A grating bar width and grating period are set at different values to obtain a high reflection at different wavelengths. The high-contrast grating forms one of the mirrors of an optical cavity of an electrically driven laser.

Such an optical grating is, however, not suitable for optically pumped lasers.

SUMMARY OF THE INVENTION

The present disclosure addresses the above-mentioned limitation by providing an optical grating, the optical grating comprising a body defining a first external surface and a second external surface, the first external surface being located opposite the second external surface, the first external surface defining a periodic structure comprising a plurality of elongated protrusions and a plurality of elongated recesses or depressions, the periodic structure being configured to receive an incident first electromagnetic wave propagating in an input propagation direction and configured to transmit the first electromagnetic wave to the second external surface for transmission through the second external surface. The periodic structure is further configured to reflect, back into the input propagation direction, an incident second electromagnetic wave propagating in an opposite direction to the input propagation direction, the incident first electromagnetic wave having a wavelength lower than a wavelength of the second electromagnetic wave.

Alternatively or additionally, the second external surface is configured to receive an incident first electromagnetic wave propagating in an input propagation direction, the first external surface defining a periodic structure comprising a plurality of elongated protrusions and a plurality of elongated recesses or depressions, the periodic structure being configured to receive the incident first electromagnetic wave propagating in the input propagation direction from the second external surface and configured to transmit the first electromagnetic wave through the first external surface, the periodic structure being further configured to reflect, back into the input propagation direction, an incident second electromagnetic wave propagating in an opposite direction to the input propagation direction, the incident first electromagnetic wave having a wavelength lower than a wavelength of the second electromagnetic wave.

The optical grating advantageously assures dual operation or is a multiple function grating in that it efficiently supplies or transfers optical energy through the optical grating at a first wavelength in an incidence direction, and precisely controls the amount of optical energy, arriving from an opposite direction at a second longer wavelength, that can escape or pass out through the grating. The optical grating thus assures the provision of more energy efficient devices or systems when used in these devices or systems.

The optical grating functions on-axis or at normal incidence thus assuring more compact and less complex devices or optical systems.

The optical grating is particularly advantageous for optically pumped lasers. The optical grating may be used in other devices such as those included in optical communication networks and is not limited to use in optical pumped lasers.

Other advantageous features can be found in the dependent claims.

The present disclosure also concerns an optically pumped laser or vertical external cavity surface emitting laser (VECSEL) including at least one or a plurality of such optical gratings.

The present disclosure additionally concerns a power-beaming system or data communication system including such an optically pumped laser or the vertical external cavity surface emitting laser (VECSEL). The above and other objects, features, and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description with reference to the attached drawings showing some preferred embodiments of the invention.

A BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figures 1A, IB and 1C are schematic representations showing embodiments of an optical grating according to the present disclosure.

Figures 2(a), 2(b). 3(a) and 3(b) are schematic representations showing embodiments of an optical grating according to the present disclosure and different operation modes. Figure 4 shows a numerical analysis of the reflection efficiency of the grating of the present disclosure for various wavelengths in operation mode 1 of Figures 2 and 3. The solid line represents reflectance into all orders, while the dashed line shows the reflected light into 0th order.

Figure 5 shows a numerical analysis of the reflectance efficiency of the grating of the present disclosure for a narrowed wavelength region around a target wavelength (in this example, l₂=1550 nm) in operation mode 1 of Figures 2 and 3. The solid line represents reflectance into all orders, while the dashed line shows the reflected light into 0^th order.

Figure 6 shows a numerical analysis of the transmittance efficiency in the region of a second target wavelength (in this example, li=980 nm) in operation mode 1 of Figures 2 and 3. The solid line represents transmittance into all orders, while the dashed line shows the transmitted light into the 0^th order.

Figure 7 is a scanning electron microscope photgraph of an exemplary fabricated optical grating of the present disclosure.

Figure 8 shows a measured deviation from the target dimensions of elements of an exemplary fabricated optical grating of the present disclosure.

Figure 9(a) is a schematic view of a known design concerning a typical off-axis pumped structure with a bottom DBR (Source: V. Iakovlev et al., "Double-diamond high-contrast-gratings vertical external cavity surface emitting laser," J. Phys. Appl. Phys., vol. 47, no. 6, p. 065104, Jan. 2014).

Figure 9(b) is a schematic view of a new design that is an "in-line"-pumped design with an optical grating of the present disclosure deposited or formed in diamond. The white and grey arrows show the direction of pumping and emitting light, respectively, OC represents an output coupler, In represents, for example, an indium solder.

Herein, identical reference numerals are used, where possible, to designate identical elements that are common to the Figures.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

Figures 1 to 3 show exemplary optical gratings 1 according to the present disclosure.

The gratings 1 are designed or configured for operation with transverse-magnetic TM polarized light (or P polarized light), and more particularly TM linearly polarized light. The values shown in Figures 4 to 6 are for TM linearly polarized light. The gratings 1 also operate with other pollination states, such as TE linearly polarized light, but optimum operation is attained for TM linearly polarized light.

The optical grating 1 comprises or consists of a body or support 3 defining a first external surface B and a second external surface A. The first external surface B is located opposite the second external surface A. The body 3 includes or defines a periodic structure PS (see, for example, Figure 7) consisting of or comprising a plurality of structures or formations 5 (Figures 1 to 3). The structure 5 continuously repeats or reoccurs across the body 3. For comprehension reasons, Figures 1 to 3 only show one structure 5 having a periodic distance D. It should be understood that the body 3 preferably includes a plurality of such structures 5 that repeats itself periodically at each periodic distance D to form the periodic structure PS. The periodic structure PS may extend fully or partially across the body 3.

Figure 7 shows multiple structures 5 of the periodic structure PS extending across the body 3. The structure 5 and the periodic structure PS include a plurality of elongated protrusions Bari, Bar2 and a plurality of elongated recesses or depressions (or trenches) Gap 1, Gap 2.

In an alternative embodiment, the body 3 may include only one structure 5.

The body 3 has a thickness t (extending for example in a X-direction (Figure 7), a width W extending in a direction (substantially) perpendicular to that of the thickness t (extending for example in a Y- direction (Figure 7), and a length L extending in a direction (substantially) perpendicular to that of the thickness t and the width W (extending for example in a Z-direction (Figure 7).

The body 3 may, for example, define a thickness t extending between the first external surface B and the second external surface A having a value between 2.0pm and 1000pm, or between 2.0pm and 5000pm, or between 2.0pm and 10000pm.

The body 3 defines, for example, a planar construction as for example shown in Figure 7.

The body 3 preferably consists solely of a single layer or a single substrate. The body 3 preferably consists solely of a single material. The periodic structure 5 and the body 3 are preferably of unitary construction.

Alternatively, the body 3 may comprise a plurality of layers, or at least one layer and a substrate upon which the at least one layer is located.

The body 3 comprises or consists solely of a material having a higher refractive index to that of the surrounding air. The body 3 may comprise or consists solely of a semiconductor material, for example, a material selected from the group of lll-V compounds, ll-VI, compounds, Si, ZnOx, GaN, or sapphire. In a preferred embodiment, the body 3 comprises or consists solely of a single crystal diamond layer or substrate. The body 3 may comprise or consist solely of a synthetic single crystal diamond layer or substrate. Such an optical grating is particularly advantageous for heat management in high-power optical devices.

In a preferred embodiment, the body 3 is surrounded or enclosed by air, or at least the periodic structure PS is surrounded or enclosed by air. In an alternative embodiment, the body 3 or at least the periodic structure PS may be surrounded or enclosed by a material having a lower refractive index (at the wavelength or wavelengths of interest) to that of the material of the body 3 and/or structure 5. The first external surface B of the body 3 defines the periodic structure PS. The periodic structure PS is configured to receive an incident first electromagnetic wave W1 (Figures 1 to 3) propagating in an input propagation direction ID. The first electromagnetic wave W1 is, for example, incident at normal incidence (perpendicular or substantially perpendicular) to the first external surface B. The periodic structure PS is configured to transmit the first electromagnetic wave W1 to the second external surface A for transmission through the second external surface A.

The first external surface B comprises at least one period, or a plurality of periods of the structure 5 to define the periodic structure PS.

The first electromagnetic wave W1 is transmitted through the second external surface A and outside the body 3 for normal light incidence. The first electromagnetic wave W1 is transmitted through the second external surface A and outside the body 3, for example, for an air/second external surface A interface, or for an interface with the second external surface A formed by a material that is of higher or lower refractive index, or that is preferably non-metallic.

The periodic structure PS is configured to reflect, back into the input propagation direction ID, an incident second electromagnetic wave W2 (Figures 1 to 3) propagating in an opposite direction OD to the input propagation direction ID. The incident first electromagnetic wave W1 has a wavelength lΐ lower than a wavelength X2 of the second electromagnetic wave W2.

The second electromagnetic wave W2 propagating in the opposite direction OD to the input propagation direction ID is incident on the second external surface A.

The first wavelength lΐ and/or the second wavelength X2 are wavelengths of interest or central wavelengths for which a high transmission and high reflection by the grating 1 are respectively desired.

In one exemplary embodiment of the optical grating 1 of the present disclosure, the first electromagnetic wave W1 has a wavelength lΐ of 980nm and the second electromagnetic wave W2 has a wavelength X2 of 1550nm. These wavelengths were chosen to demonstrate a working prototype and are non-limiting examples of first and second wavelengths lΐ, X2 and it should be understood that the optical grating of the present disclosure operates at other first and second wavelengths lΐ, X2. The incident second electromagnetic wave W2 propagating in an opposite direction OD is also, for example, incident at normal incidence on the second external surface A and transmitted to the first external surface B.

The second external surface A defines, for example, a planar or flat surface (or a substantially planar or flat surface). The external surface B and the external surface A may, for example, both extend to define parallel or substantially parallel external surfaces of the body 3. As shown in Figures 1 to 3 and 7, the periodic structure PS and the structure 5 comprises at least one first elongated protrusion Bari, at least one second elongated protrusion Bar2, at least one first elongated recess or depression Gapl and at least one second elongated recess or depression Gap2. The first and second elongated protrusions Bari, Bar 2 are defined by the elongated recesses Gapl, Gap2 of the body 3; or defined by one elongated recess and an edge of the body 3.

The first and second elongated protrusions Bari, Bar 2 define fingers or strips extending across the body 3 (Figure 7). The first and second elongated protrusions Bari, Bar 2 extend in the same direction, or extend (substantially) parallel to each other. The first and second elongated protrusions Bari, Bar 2 and the elongated recesses Gap 1, Gap2 extend in the same direction, or extend (substantially) parallel to each other.

The protrusions and recesses extend in an elongated manner in a direction (for, example direction Z or length direction L of the body shown in Figure 7) perpendicular to their depth d/height FI and width w.

The elongated recesses Gap 1, Gap2 may have the same depth d and the elongated protrusions Bari, Bar 2 may have the same height FI as shown, for example, in Figures 1 to 3. The structure 5 can thus be defined in an outer layer or section of the body 3. The body 3 may, for example, comprise or consist of the outer layer defining the periodic structure PS and a supporting substrate as shown, for example, in Figure 1.

Alternatively, the depths d and heights FI may be different, as it is not necessary for these to be of the same or similar value.

The recesses extend a depth d into the body 3 in the direction of the second external surface A. The recesses include a floor F extending to or between at least one or a plurality of side walls sw to define the recess in the body 3. The side walls sw and a connecting ceiling c (or a side wall and an outer edge of the body 3) define the elongated protrusions Bari, Bar 2.

The floors F and the ceilings c may, for example, extend (substantially) parallel to each other to define (substantially) parallel outer surfaces of the body 3 that are flat or planar. Alternatively, the floors F may be sloped (using side walls sw of different heights) and define an angled surface relative to the ceiling c. Alternatively, both the floors F and the ceilings c may define sloped surfaces.

The elongated recesses Gap 1, Gap2 and/or the elongated protrusions Bari, Bar 2 have or delimit a width w extending in a direction perpendicular to the depth d or height FI. As the elongated recesses Gap 1, Gap2 and/or the elongated protrusions Bari, Bar 2 extend in the elongated direction along the length L of the body 3, the width w of elongated recesses Gap 1, Gap2 and/or the elongated protrusions Bari, Bar 2 remains (substantially) the same, as seen for example in Figure 7. The elongated recesses Gap 1, Gap2 and/or the elongated protrusions Bari, Bar 2, may extend linearly to define straight-line strips or cavities as shown in Figure 7, or alternatively may extend to define curved strips or cavities, for example circular/annular strips or cavities.

The at least one first elongated protrusion Bari, the at least one second elongated protrusion Bar2, the at least one first elongated recess or depression Gapl and the at least one second elongated recess or depression Gap2 have different widths. The heights H and depths d may, for example, be the same. In a preferred embodiment, the structure 5 comprises or consists solely of the following sequence of elements: the first elongated protrusion Bari, the first elongated recess Gapl, a further first elongated protrusion Bari, the second elongated recess Gap2, the second elongated protrusion Bar2, and a further second elongated recess Gap2. This sequence is shown, for example, in Figure 1A.

As mentioned, this structure 5 is periodically repeated across the body 3 (as shown in Figure 7). In view of the periodicity of the structure 5 of the body 3, the sequence can thus start at any of the elements of the structure 5 and does not necessarily start with the first elongated protrusion Bari as shown in Figure 1 A. Figure IB shows, for example, the structure 5 that comprises or consists solely of the following sequence of elements: the further second elongated recess Gap2, the first elongated protrusion Bari, the first elongated recess Gapl, the further first elongated protrusion Bari, the second elongated recess Gap2, and the second elongated protrusion Bar2.

The sequence of elements of the structure 5 can thus begin with any one of the above-mentioned constituent elements, the full sequence of structure 5 then being periodically repeated in the body 3. In one embodiment, where the first and second wavelengths lΐ, l2 of interest are 980nm and 1550nm respectively (or surrounding these wavelengths), the first elongated protrusion Bari and the further first elongated protrusion Bari may have a width w between 95nm and 115nm (for example, 105nm), the first elongated recess Gapl may have a width w between 210nm and 310nm (for example, 259nm), the second elongated recess Gap2 and the further second elongated recess Gap2 may have a width between 155nm and 220nm (for example, 187nm), and the second elongated protrusion Bar2 may have a width between 510nm and 570nm (for example, 541nm).

The first elongated protrusion Bari, the further first elongated protrusion Bari, and/or the second elongated protrusion Bar 2 may for example have a height FI between 805nm and 865nm (for example, 834nm) relative to a neighboring recess.

The first elongated recess or depression Gapl, and/or the second elongated recess or depression Gap2 and the further second elongated recess Gap2 may have a depth d between 805nm and 865nm (for example 834nm).

The structure 5 may, for example, define a periodic distance D between 1220nm and 1550nm, for example 1384nm. The periodic structure PS and the optical grating 1 configured in the above-described manner permits to transmit, for example, between (a) 50% and (b) 70%, or 74% or 80% or 85%; or at least 50% or at least 70% or at least 74% or at least 80% or at least 85% of the first electromagnetic wave W1 to the second external surface A, and to simultaneously reflect, in the input propagation direction ID, for example, between (a) 90 % and (b) 95%, or 97% or 98% or 99%, or 99.5% or 99.9%; or at least 95% or at least 99% or at least 99.99999% of the incident second electromagnetic wave W2 propagating in an opposite direction OD to the input propagation direction ID.

The periodic structure PS is configured to transmit at least 50% or at least 70% or at least 74% of the first electromagnetic wave W1 into the 0^th diffraction order of the optical grating 1, and at least 95% or at least 99% or at least 99.99999% of the second electromagnetic wave W2 into the 0^th diffraction order of the optical grating 1.

The above values being for TM linearly polarized light.

A further manner of describing or presenting the optical grating 1 of the present disclosure is shown in Figure 1C. As shown in Figure 1C, the structure 5 may, for example, comprise or consists solely of one elongated protrusion Bar2, one elongated recess or depression Gp, and a gate structure or inserted structure GS located or positioned in the elongated recess or depression Gp. The one elongated recess or depression Gp is illustrated in a dashed-line frame in Figure 1C.

The gate or inserted structure GS assures the dual operation or multiple functionality of the grating that provides both high reflectivity and transmission at the first and second wavelengths lΐ, l2 of interest.

The gate structure GS comprises or consists of a first elongated recess or depression Gap2, a first elongated protrusion Bari, a second elongated recess or depression Gapl, a second elongated protrusion Bari and a third elongated recess or depression Gap2.

The one elongated protrusion Bar2 defines a width w larger than that of the one elongated recess or depression Gp.

The first elongated recess or depression Gap2 and the third elongated recess or depression Gap2 may for example have (substantially) identical widths, the first elongated protrusion Bari and the second elongated protrusion Bari may have (substantially) identical widths, and the first elongated recess or depression Gap2 may have a width less than that of the second elongated recess or depression Gapl. In one embodiment, where the first and second wavelengths lΐ, l2 of interest are 980nm and 1550nm respectively, the first elongated recess or depression Gap2 and the third elongated recess or depression Gap2 may for example have a width between 155nm and 220nm, the first elongated protrusion Bari and the second elongated protrusion Bari may for example have a width between 95nm and 115nm, and the second elongated recess or depression Gapl may have for example a width between 210nm and 310nm.

The optical grating 1 also provides the same advantages in an orientation in which the grating 1 is flipped or rotated by 180° as shown in Figures 2(b) and 3(b) and indicated as Operation mode 2' in these Figures. Figures 2(b) and 3(b) show the same optical grating 1, the grating 1 in Figure 3(b) is presented so that the incident first electromagnetic wave W1 enters from the top, in comparison, the incident first electromagnetic wave W1 enters from the bottom in Figure 2(b).

In this Operation mode 2' (see Figures 2(b) and 3(b)), the second external surface A is configured to receive the incident first electromagnetic wave W1 propagating in the input propagation direction I D. The first external surface B defines the periodic structure PS that is configured to receive the incident first electromagnetic wave W1 propagating in the input propagation direction ID from the second external surface A and also configured to transmit the first electromagnetic wave W1 through the first external surface B and out of the body 3.

The periodic structure PS is configured to reflect, back into the input propagation direction ID, the incident second electromagnetic wave W2 propagating in an opposite direction OD to the input propagation direction I D. As mentioned before, the incident first electromagnetic wave W1 has a wavelength (or central wavelength) lΐ lower than the wavelength X2 of the second electromagnetic wave W2.

The optical grating 1 and the periodic structure PS are, for example, configured to transmit, for example, between (a) 50% and (b) 70%, or 74% or 80% or 85%; or at least 50% or at least 70% or at least 74% or at least 80% or at least 85% of the first electromagnetic wave W1 incident on the second external surface A to the first external surface B, and the periodic structure PS is simultaneously configured to reflect, back into the input propagation direction I D, for example, between (a) 90 % and (b) 95%, or 97% or 98% or 99%, or 99.5% or 99.9%; or at least 95% or at least 99% or at least 99.99999% of the incident second electromagnetic wave W2 incident onto the first external surface B and propagating in the opposite direction OD to the input propagation direction ID. As mentioned, the above values being for TM linearly polarized light.

The optical grating 1 and the periodic structure PS in this Operation mode 2' is similarly able to provide the previous mentioned advantageous. Furthermore, this faculty to operate in multiple orientations and operation modes facilitates integration of the optical grating 1 into multiple different optical devices and systems.

The grating 1 can be, for example, a non-focusing grating.

The grating 1 can be produced using known grating fabrication methods, for example, using material etching and lithography. The grating 1 can be produced in diamond material using known diamond etching and patterning methods. For example, a metal layer (for example, Titanium) can be used as a hard mask material for etching diamond. Lithography, such as e-beam lithography, can be used to define a desired structure pattern in a further mask layer (for example, Hydrogen silsesquioxane HSQ) deposited on the metal mask layer. Etching can be carried out on the metal layer using a Reactive Ion Etcher, and for the case of a Titanium hard mask, by using chlorine chemistry to open the metal mask and reveal the diamond material. Reactive Ion Etching of diamond can be carried out using oxygen plasma. Hard mask removal, for example using Hydrofluoric acid, then reveals the diamond grating. Such processing of diamond is well known (see, for example, references 18 to 22).

In this description, the Inventors now present the concept and a specific exemplary prototype confirming the concept of the optical grating 1 of the present disclosure that combines the advantageous properties of two traditional optical gratings.

In the exemplary optical grating 1 produced to confirm the innovative concept of the present disclosure, one period of the structure 5 is composed of a bar (for example 541 nm), followed by a gap (for example 187 nm), then a second bar (for example 105 nm) and a second gap (for example 259 nm) with another copy of the second bar and the first gap. It can reflect the target wavelength (l₂=1550 nm) that approaches the grating from one side with >99.99999% efficiency and simultaneously transmit a secondary target wavelength (li=980 nm) approaching from the other side of the grating 1 with >74%. Such complex tasks are typically fulfilled by a multi-layer design that increases the complexity and cost of the fabrication, however, the solution provided by the Inventors can be fabricated in a less complex and less costly manner, for example, by a single-layer microfabrication process.

The inventors present the concept of the innovative optical grating 1 that combines the properties of two traditional optical gratings. The problem undertaken by the Inventors was to design a mirror (i.e. high reflectance) by an optical grating for a target wavelength (for example, l₂=1550 nm) arriving from one side of the grating and simultaneously allow to transmit another target wavelength (for example, li=980 nm) from the other side of the grating. In the state-of-the-art, gratings which can reflect a target wavelength are known. Similarly, gratings that can transmit a different target wavelength are also known. However, gratings that can do the operation simultaneously when both wavelengths are arriving from different sides of the grating are unknown.

Furthermore, advantageously the structure 5 of the grating 1 proposed by the Inventors in the present disclosure is periodic and is a single layer structure, which to the knowledge of the Inventors is also unknown in the state of the art. The Inventor's design brings multiple advantageous compared to state of the art gratings because (i) it reflects the target wavelength (l₂) arriving from one side and transmits the other target wavelength (li) arriving from the other side simultaneously, (ii) is periodic and (iii) is formed in a single layer (Figure 1).

Figure 1 shows a schematic of the grating design. The grating 1 can be fabricated into a single layer on top of the substrate. It consists of or contains a particular geometrical combination of four building blocks of recesses/trenches and protrusions/projections (bar 1, gap 1, bar 2, gap 2). The grating 1 is periodic and can reflect significantly (for example, >99.99999%) significantly the first target wavelength approaching from one side and simultaneously transmit significantly (for example, >74%) the second target wavelength arriving from the other side.

Furthermore, the optical grating 1 allows two operation modes, as illustrated in Figures 2 and 3. Figures 2 and 3 concern two operation modes as previously mentioned. The main difference between the operation modes is the direction of the reflected and the transmitted wavelengths. In mode 1, the wavelength to be reflected (A2) approaches from side "A" and the wavelength to be transmitted (Al) from side "B". While in mode 2, the wavelength to be reflected (A2) approaches from side "B" and the wavelength to be transmitted (Al) from side "A".

The fact that the period structure of a combination sequence of bars and gaps can solve this problem was discovered in a non-trivial way by the Inventors. In particular, the grating of the present disclosure is not simply a combination of two individual gratings, the simple combination of two gratings by adding one bar and one gap with a second bar (of different size than bar one) and a second gap (of different size than gap one) does not permit the results shown herein be achieved. The Inventors investigated numerous geometrical combinations of several building blocks (bar 1, gap 1, bar 2, gap2) which resulted in the presented combined grating 1 of the present disclosure. A numerical optimization routine in close vicinity of the target dimensions allowed the Inventors to further optimize the performance and converge to the final grating design described in the present disclosure. The advantage of this grating design is that it is compact (due to periodicity and single layer structure) and has such a high performance that enables its implementation into optical applications such as laser applications.

The starting idea of the Inventors behind the grating design and operation is that instead of having simply two periodic gratings combined together, the Inventors separately developed a structure for reflection and another, much smaller one for transmission. Then the smaller structure GS was placed into the gap Gp of the bigger structure and functions as a gate for transmission at a different wavelength of interest. It can provide the required high level of transmission (for example, >74%), while simultaneously being small enough not to lower the high level (>99.99999%) reflection. This is illustrated in operation mode 1 in Figures 4 to 6. In operation mode 2, the grating can produce similar results, as mentioned above. Such a task is typically solved by a double-layer structure, where each subtask is solved by a single layer structure.

The Inventors fabricated a prototype grating 1 in order to confirm their numerical analysis and approach previously described. Numerical analysis showed that the grating 1 can be fabricated with the following dimensions and tolerances without degrading its performance. The exemplary grating 1 was fabricated for a wavelength lΐ of 980nm and a wavelength A2 of 1550nm.

TABLE 1

The first prototype fabricated is shown in Figure 7.

The fabricated grating 1 was analysed by atomic force microscope (AFM) to measure the deviation from the target dimensions. As shown in Figure 8, it was observed that the dimensions of Bar 1 and the Height of the grating were slightly off the target

The grating 1 can be used in many different applications.

The grating 1 can, for example, be a key component in a novel laser device. It can act as a gain mirror of the laser, that has high transmission at a first wavelength from one side, and high reflection at a second wavelength from the other side.

Such elements are highly sought after in lasing structures, that are optically pumped. The optical pump wavelength is shorter (of higher energy) to excite the active layer, and high transmission is required that the pump can reach the active layer situated between the two cavity mirrors. The emission wavelength (of lower energy for the longer wavelength) needs to be reflected at the two cavity mirrors. Moreover, by integrating the reflector directly in, for example, single crystal diamond, the bulk material with the highest thermal conductivity, excess heat generated in the active layer can be evacuated more efficiently, which leads to the possibility to increase the emission power of the laser. The grating 1 can simultaneously provide the role of a gain mirror and a heatsink.

In addition, the grating 1 allows for very compact optically pumped VCSELs or VECSELs, with high power output and high beam quality.

The optical grating 1 is particularly beneficial in a vertical external cavity surface emitting laser (VECSEL). VECSELs are semiconductor light sources that provide high optical output power (in the range of Watts), simultaneously with good optical beam quality. However, there is still much room for improvement in terms of efficiency, thermal management and compactness.

Addressing these issues, the Inventors propose a design (Figure 9), where the active region is in direct contact with a grating 1, and in particular, a grating 1 of diamond plates for optimized cooling and the pumping is arranged in the so-called "in-line" pumping scheme to increase compactness.

It is also important to note that integrating or replacing the distributed Bragg reflector (DBR) into the bottom diamond plate as a HCG improves simultaneously the efficiency and the compactness of the system. This integrated element of the VECSEL is the newly designed grating 1 component described in the present disclosure.

The grating 1 is an important part regarding the final VECSEL device, as it will enable the implementation of a new pumping scheme, a more compact design and higher power output due its superior thermal management. Generally, the proposed grating can have superior performance as a gain mirror and allow to build lasers with higher power output. Such high-power lasers have widespread of applications (i.e. power beaming, military, communication).

The optical grating 1 may thus be, for example, or define a high-contrast optical grating (HCG).

The optical grating 1 may be, for example, or define a laser cavity optical element or a laser gain mirror or a collinear optical pumping element or an in-line optical pumping element.

At least one or a plurality of optical gratings 1 may be included in an optically pumped laser, or in a Vertical external cavity surface emitting laser (VECSEL). The optically pumped laser or Vertical external cavity surface emitting laser (VECSEL) includes an active layer, wherein the at least one or the plurality of optical gratings 1 contact or directly contact the active layer of the optically pumped laser or the Vertical external cavity surface emitting laser. While such known lasers using DBR mirrors required off-axis pumping as shown in Figure 9a, the inclusion of one or more gratings 1 to optically enclose the optically pumped active region allows for in-line or vertical pumping and a more compact device also facilitating system designs containing such devices.

A reduced or simplified cooling system is allowed, as seen in Figure 9b.

The optical gratings 1 advantageously allow in-line operation and optical pumping, and more efficient optical energy conversion. Higher optical powers can be manipulated by the devices. The use of gratings consisting or comprising diamond in particular is advantageous for reliable operation at high optical powers.

The optically pumped laser or the vertical external cavity surface emitting laser (VECSEL) may be advantageously be included in a power-beaming system or data communication system to improve the performance of such systems.

While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments, and equivalents thereof, are possible without departing from the sphere and scope of the invention. Accordingly, it is intended that the invention not be limited to the described embodiments and be given the broadest reasonable interpretation in accordance with the language of the appended claims. The features of any one of the above described embodiments may be included in any other embodiment described herein.

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[21] WO2019043432

[22] WO2019043570

[23] US 8,755, 118

[24] US20120128019

[25] https://ieeexplore.ieee.org/document/6480776l FILING DATE: NA|PRIORITY DATE:

NA|PUBLICATION DATE: APR, 2013; TITLE: A CHIRPED SUBWAVELENGTH GRATING WITH BOTH REFLECTION AND TRANSMISSION FOCUSING; XIAOMIN LV, WEIBIN QIU, JIA-XIAN WANG, YUHUI MA, JING ZHAO, MENGKE LI, HONGYAN YU, AND JIAOQING PAN

[26] WO2016012084

[27] US 7,420,735 B2

[28] CN 104914510 A

[29] Double high refractive-index contrast grating VCSEL in Proceedings of SPIE - The International Society for Optical Engineering 9381 March 2015 DOI: 10.1117/12.2079328

The content of each of the above references being fully incorporated herein by reference.

Claims

1. Optical grating (1) comprising

a body (3) defining a first external surface (B) and a second external surface (A), the first external surface (B) being located opposite the second external surface (A),

wherein

the first external surface (B) defines a periodic structure (PS) comprising a plurality of elongated protrusions (Bari, Bar2) and a plurality of elongated recesses or depressions (Gap 1, Gap 2), the periodic structure (PS) being configured to receive an incident first electromagnetic wave (Wl) propagating in an input propagation direction (ID) and configured to transmit the first electromagnetic wave (Wl) to the second external surface (A) for transmission through the second external surface (A),

the periodic structure (PS) being further configured to reflect, back into the input propagation direction (ID), an incident second electromagnetic wave (W2) propagating in an opposite direction (OD) to the input propagation direction (ID),

the incident first electromagnetic wave (Wl) having a wavelength (lΐ) lower than a wavelength (A2) of the second electromagnetic wave (W2);

or wherein

the second external surface (A) is configured to receive an incident first electromagnetic wave (Wl) propagating in an input propagation direction (ID),

the first external surface (B) defining a periodic structure (PS) comprising a plurality of elongated protrusions (Bari, Bar2) and a plurality of elongated recesses or depressions (Gap 1, Gap 2), the periodic structure (PS) being configured to receive the incident first electromagnetic wave (Wl) propagating in the input propagation direction (ID) from the second external surface (A) and configured to transmit the first electromagnetic wave (Wl) through the first external surface (B), the periodic structure (PS) being further configured to reflect, back into the input propagation direction (ID), an incident second electromagnetic wave (W2) propagating in an opposite direction (OD) to the input propagation direction (ID),

the incident first electromagnetic wave (Wl) having a wavelength (Al) lower than a wavelength (A2) of the second electromagnetic wave (W2).

2. Optical grating according to the previous claim, wherein the periodic structure (PS) comprises a plurality of structures (5) each consisting of one elongated protrusion (Bar2) and one elongated recess or depression (Gp); and the structure (5) includes a gate structure (GS) located in said elongated recess or depression (Gp), the gate structure (GS) comprising or consisting of a first elongated recess or depression (Gap2), a first elongated protrusion (Bari), a second elongated recess or depression (Gapl), a second elongated protrusion (Bari) and a third elongated recess or depression (Gap2).

3. Optical grating according to the previous claim, wherein the one elongated protrusion (Bar2) defines a width (w) larger than that of the one elongated recess or depression (Gp), the width extending in a direction perpendicular to the direction of elongation of the one elongated protrusion (Bar2).

4. Optical grating according to previous claims 2 or 3, wherein the first elongated recess or depression (Gap2) and the third elongated recess or depression (Gap2) have identical widths, the first elongated protrusion (Bari) and the second elongated protrusion (Bari) have identical widths, and the first elongated recess or depression (Gap2) has a width less than that of the second elongated recess or depression (Gapl).

5. Optical grating according to any one of claims 2 to 4, wherein the first elongated recess or depression (Gap2) and the third elongated recess or depression (Gap2) have a width between 155nm and 220nm, the first elongated protrusion (Bari) and the second elongated protrusion (Bari) have a width between 95nm and 115nm, and the second elongated recess or depression (Gapl) has a width between 210nm and 310nm.

6. Optical grating (1) according to any one of the previous claims, wherein the second electromagnetic wave (W2) propagating in the opposite direction (OD) to the input propagation direction (ID) is incident on the second external surface (A).

7. Optical grating (1) according to any one of the previous claims, wherein the second electromagnetic wave (W2) propagating in the opposite direction (OD) to the input propagation direction (ID) is incident on the first external surface (B).

8. Optical grating according to any one of the previous claims, wherein the body (3) consists solely of a single layer or a single substrate.

9. Optical grating (1) according to any one of the previous claims, wherein the body consists solely of a single material, or the periodic structure and the body are of unitary construction.

10. Optical grating (1) according to any one of the previous claims, wherein the body comprises or consists solely of a single crystal diamond layer or substrate; or a synthetic single crystal diamond layer or substrate.

11. Optical grating (1) according to any one of the previous claims, wherein the periodic structure (PS) is configured to transmit, for TM linearly polarized light, at least 50% or at least 70% or at least 74% or at least 80% or at least 85% of the first electromagnetic wave (Wl) to the second external surface (A), and the periodic structure (PS) is simultaneously configured to reflect, for TM linearly polarized light, in the input propagation direction (ID), at least 95% or at least 99% or at least

99.99999% of the incident second electromagnetic wave (W2) propagating in an opposite direction (OD) to the input propagation direction (ID).

12. Optical grating (1) according to any one of the previous claims 1 to 10, wherein the periodic structure (PS) is configured to transmit, for TM linearly polarized light, at least 50% or at least 70% or at least 74% or at least 80% or at least 85% of the first electromagnetic wave (Wl) incident on the second external surface (A) to the first external surface (B), and the periodic structure (PS) is simultaneously configured to reflect, for TM linearly polarized light, back into the input propagation direction (ID), at least 95% or at least 99% or at least 99.99999% of the incident second electromagnetic wave (W2) incident onto the first external surface (B) and propagating in an opposite direction (OD) to the input propagation direction (ID).

13. Optical grating (1) according to any one of the previous claims, wherein the periodic structure (PS) is configured to transmit at least 50% or at least 70% or at least 74% of the first electromagnetic wave (Wl) into the 0^th diffraction order of the optical grating, and at least 95% or at least 99% or at least 99.99999% of the second electromagnetic wave (W2) into the 0^th diffraction order of the optical grating.

14. Optical grating (1) according to any one of the previous claims, wherein the first electromagnetic wave (Wl) has a wavelength (lΐ) of 980nm, and the second electromagnetic wave (W2) has a wavelength (A2) of 1550nm.

15. Optical grating (1) according to any one of the previous claims, wherein the periodic structure (PS) comprises a plurality of structures (5) comprising at least one first elongated protrusion (Bari), at least one second elongated protrusion (Bar2), at least one first elongated recess or depression (Gapl) and at least one second elongated recess or depression (Gap2).

16. Optical grating (1) according to claim 15, wherein the at least one first elongated protrusion (Bari), the at least one second elongated protrusion (Bar2), the at least one first elongated recess or depression (Gapl) and the at least one second elongated recess or depression (Gap2) have different widths.

17. Optical grating (1) according to claims 15 or 16, wherein the structure (5) comprises or consists solely of the following sequence of elements:

the first elongated protrusion (Bari),

the first elongated recess (Gapl),

a further first elongated protrusion (Bari),

the second elongated recess (Gap2),

the second elongated protrusion (Bar2), and

a further second elongated recess (Gap2).

18. Optical grating (1) according to any one of claims 15 to 17, wherein:

(a) the first elongated protrusion (Bari) and the further first elongated protrusion (Bari) have a width between 95nm and 115nm,

(b) the first elongated recess (Gapl) has a width between 210nm and 310nm,

(c) the second elongated recess (Gap2) and the further second elongated recess (Gap2) have a width between 155nm and 220nm,

(d) the second elongated protrusion (Bar2) has a width between 510nm and 570nm.

19. Optical grating (1) according to any one of claims 15 to 18, wherein:

(a) the first elongated protrusion (Bari) and the further first elongated protrusion (Bari) have a width of 105nm,

(b) the first elongated recess (Gapl) has a width of 259nm,

(c) the second elongated recess (Gap2) and the further second elongated recess (Gap2) have a width of 187nm,

(d) the second elongated protrusion (Bar2) has a width of 541nm.

20. Optical grating (1) according to any one of the previous claims 15 to 19, wherein the first elongated protrusion (Bari), the further first elongated protrusion (Bari), and/or the second elongated protrusion (Bar 2) have a height (H) between 805nm and 865nm relative to a neighboring recess.

21. Optical grating (1) according to any one of the previous claims 15 to 20, wherein the first elongated protrusion (Bari), the further first elongated protrusion (Bari), and/or the second elongated protrusion (Bar 2) have a height (H) of 834nm relative to a neighboring recess.

22. Optical grating (1) according to any one of the previous claims 15 to 21, wherein the first elongated recess or depression (Gapl); and/or the second elongated recess or depression (Gap2) and the further second81 elongated recess (Gap2) have a depth (d) between 805nm and 865nm.

23. Optical grating (1) according to any one of the previous claims 15 to 22, wherein the first elongated recess or depression (Gapl); and/or the second elongated recess or depression (Gap2) and the further second elongated recess (Gap2) have a depth (d) of 834nm.

24. Optical grating (1) according to any one of the previous claims, wherein the structure (5) defines a periodic distance (D) between 1220nm and 1550nm.

25. Optical grating (1) according to any one of the previous claims, wherein the structure (5) defines a periodic distance (D) of 1384nm.

26. Optical grating (1) according to any one of the previous claims, wherein the second external surface (A) defines a planar surface.

27. Optical grating (1) according to any one of the previous claims, wherein the first external surface (B) comprises at least one period or a plurality of periods of the structure (5).

28. Optical grating (1) according to any one of the previous claims, wherein the body (3) defines a thickness (t) extending between the first external surface (B) and the second external surface (A) having a value between 2.0pm and 1000pm, or between 2.0pm and 5000pm, or between 2.0pm and 10000pm.

29. Optical grating (1) according to any one of the previous claims, wherein the optical grating (1) is or defines a high-contrast optical grating (HCG).

30. Optical grating (1) according to any one of the previous claims, wherein the optical grating (l)is or defines a laser cavity optical element or a laser gain mirror or a collinear optical pumping element or an in-line optical pumping element.

31. Optically pumped laser or Vertical external cavity surface emitting laser (VECSEL) including at least one or a plurality of optical grating (1) according to any one of the previous claims.

32. Optically pumped laser or Vertical external cavity surface emitting laser (VECSEL) according to the previous claim further including an active layer, wherein the at least one or the plurality of optical grating (1) contact or directly contact the active layer of the Optically pumped laser or the Vertical external cavity surface emitting laser.

33. Power-beaming system or data communication system including the optically pumped laser or the vertical external cavity surface emitting laser (VECSEL) according to the previous claims 31 or

32.