WO2021148840A1

WO2021148840A1 - Optical arrangement

Info

Publication number: WO2021148840A1
Application number: PCT/IB2020/050422
Authority: WO
Inventors: Sargis HAKOBYAN; Yves Bellouard
Original assignee: Ecole Polytechnique Federale De Lausanne (Epfl)
Priority date: 2020-01-20
Filing date: 2020-01-20
Publication date: 2021-07-29

Abstract

In an optical arrangement (A) for use in an optical circuit, wherein the optical arrangement (A) comprises a substrate plate (1), wherein the substrate plate (1) is configured to receive a multitude of optical components (12.2, 12.5), the substrate plate (1) is configured such that once the optical components (12.2, 12.5) are received by the substrate plate (1), an essentially fully aligned optical circuit is formed.

Description

Title:

Optical arrangement”

Technical Field

The invention relates to an optical arrangement according to the preamble of claim 1. The invention furthermore relates to a laser assembly according to claim 15.

Background Art

An optical cavity (also referred to as resonating cavity or optical resonator) is an arrangement of optical components such as mirrors or lenses that is capable of creating standing waves in optical systems. Optical cavities are for example typically used in laser systems. In such laser systems, light is introduced into the cavity by means of a light pumping source and reflects multiple times inside the cavity so as to produce standing waves for certain resonance frequencies. Such optical cavities typically comprise a multitude of passive optical components, such as mirrors, lenses and the like.

In order for optical systems (and laser systems in particular) to function properly, it is very important that the optical cavities are properly aligned in such systems. The term “alignment” basically refers to the orientations of the elements of the optical cavities with respect to each other and/or with respect to an optical axis as defined as the axis along the k-vector that defines the light propagation direction. For example, a proper alignment (also referred to as "full alignment”) is typically such that a path followed by a reflected light beam inside the cavity is centered through each optical element of the cavity. Already very small deviations from the proper alignment can have important negative influences on the functioning of a laser system, for example. Alignment of optical cavities can be a time-consuming and complicated procedure.

Another problem in optical systems is the fact that different components of the optical system, for example components of the optical cavity and other components like placeholders, fixtures and the like, react to heat and/or heat up or cooldown in different ways. Such differences in heat dissipation can lead to alignment problems in optical systems in general, and in optical cavities in particular.

Problem to be Solved

It is the object of the invention to solve or to at least diminish the above-mentioned disadvantages. Solution to the Problem

This problem is solved by an optical arrangement for use in an optical circuit, wherein the optical arrangement comprises a substrate plate, wherein the substrate plate is configured to receive a multitude of optical components, wherein the substrate plate is configured such that once the optical components are received by the substrate plate, an essentially fully aligned optical circuit is formed.

In typical embodiments, the essentially fully aligned optical circuit comprises an essentially fully aligned optical cavity, wherein the essentially fully aligned optical cavity preferably is the essentially fully aligned optical circuit.

The expression “optical circuit” shall be understood as referring to any optical device, in particular any optical device that can comprise an optical cavity, such as a laser assembly, a laser system, an optical parametric oscillator or an interferometer for example. In typical embodiments, the expression “optical circuit”, shall be understood as referring to any multi-components device fulfilling an optical function, e.g. a device processing, analysing or transforming light, in particular any optical device that can comprise an optical resonating cavity, such as a laser system. The expression “optical component” shall be understood as referring to components that typically form part of an optical circuit and/or an optical cavity, such as passive optical components, like for example mirrors and/or lenses or active optical components like for example crystals. In typical embodiments, the optical cavity is a laser cavity. In typical embodiments, the optical circuit is a laser cavity. The expression “essentially fully aligned” is to be understood such that when the optical components are received by the substrate plate (or in other words: are placed on the substrate plate), their positions and orientations with respect to each other and with respect to the substrate plate are precisely predefined by the configuration of the substrate plate. Like this, the optical components can only be placed in a certain, predefined manner on the substrate plate. This certain, predefined manner is chosen such that once the optical components are in place, they are fully aligned with respect to each other and form an optical circuit, for example an optical cavity, in particular a properly (or in other words: fully) aligned optical circuit and/or optical cavity. All that needs to be done to align such a combination of the optical arrangement with the optical components received within the substrate plate, is to align the substrate plate with respect to an optical axis and/or a reference point of the optical arrangement and/or of the optical circuit, for example with respect to an active medium of a laser assembly. Like this, the alignment of the optical circuit, in particular of the optical cavity, is simplified, because the optical circuit and/or optical cavity as such is already pre aligned on the substrate plate and all that needs to be done for properly aligning the optical arrangement is to carry out a fine-alignment of the substrate plate carrying the optical components with respect to a particular reference, for example with an optical axis and/or an optical centre of the optical circuit.

Furthermore, the expression “essentially fully aligned” is to be understood such that extremely small deviations from a theoretical perfect alignment are acceptable, for example deviations of up to 1000 nm from a given axis, and/or up to 10mrad (10 milliradians) in angular deviations. In other words: “fully aligned” refers to a theoretical perfect alignment of the optical circuit and/or the optical cavity and “essentially” introduces room for minimal deviations from such a perfect alignment. More generally, the word “essentially” when used in this specification introduces room for minimal deviations from a certain perfect state, condition or the like. In typical embodiments, the substrate plate comprises a multitude of placement holes, wherein each placement hole is configured to receive at least one optical component, wherein each placement hole has a particular position and/or orientation with respect to at least one reference axis of the optical arrangement and/or with respect to the other placement holes and/or with respect to a geometric centre of the substrate plate.

In typical embodiments, each placement hole is configured such that it allows for an optical component to only be placed in the placement hole in one predefined, precise way. This helps to ensure a proper placement of the optical components in the placement holes and therefore a precise alignment of the optical cavity and/or the optical circuit. Placement holes are a simple and yet effective way of guaranteeing a precise positioning of the optical components on the substrate plate. However, alternative ways of fixing the optical components to the substrate plate are also possible, for example fixtures that are installed on top of the substrate plate, in particular fixtures which are protruding from the substrate plate. In general, a placement hole shall be understood as referring to any type of hole, for example a complete recess that runs through the entire substrate plate (in other words: an opening) or yet to a well-like opening, that only runs through the substrate plate in part.

In typical embodiments, the substrate plate comprises four placement holes, in particular four placement holes wherein at least one placement hole, preferably at least two placement holes, has/have different dimensions than the rest of the placement holes.

In typical embodiments, the placement holes typically have dimensions ranging from a few millimeters, approximately at least 3 mm, to a few centimeters, for example up to 5 cm, wherein the longest dimension typically corresponds to a longitudinal axis of the placement hole.

In typical embodiments, the placement holes are positioned and/or oriented such that, when the optical components are received by the substrate plate and/or placed in the placement holes, the essentially fully aligned optical cavity is at least partly formed by the optical components.

In typical embodiments, the reference axis is typically defined by one of the components of the optical circuit in which the optical arrangement is employed. This component typically has an active function and/or may heat up and/or has a particular shape that makes it difficult to precisely position this particular component in space (e.g. cuboids). In such an embodiment, the reference axis of the optical arrangement is then chosen as one of the principle axis of this particular component. The principle axis of this particular component can for example correspond to a crystallographic physical axis of this component or, as another example, could also be linked with a predefined angle to a geometrical surface of this component.

In typical embodiment, at least one of the placement holes, preferably all of the placement holes, comprise(s) a three-point support on a first internal side and/or a beam flexure on a second internal side, wherein the first internal side and the second internal side are preferably arranged opposite to each other. This particular configuration of the placement holes makes it possible to create a kinematic mount for the optical components, which allows a precise, safe and simple positioning of the optical components inside the placement holes. In particular, the beam flexure can push the optical component on one side so that the optical component is kept in contact with the three-point support of the placement hole on its other side. In typical embodiments, the beam flexure has a protruding contact point for establishing contact with an optical component, wherein the protruding contact point typically has the form of a half-cylinder. In typical embodiments, the three-point support comprises one rectangular contact point and two triangular contact points, wherein the rectangular contact point is typically arranged in the middle between the two triangular contact points. In preferred embodiments, the protruding contact point of the flexure and the rectangular contact point of the three-point support are aligned with each other.

In typical embodiments, the combination of the beam flexure and the three-point support defines a set of contact points on the optical component placed inside the placement hole, such that the elastic force from the beam flexure and the reaction forces from the three-point support are balanced to maximize the mechanical stability of the optical component placed inside the placement hole.

In typical embodiments, at least one of the placement holes, preferably all of the placement holes, comprise(s) a blocking mechanism for preventing permanent damage to the placement hole(s). In particular embodiments, the blocking mechanism is at least partly established by a beam flexure comprising a flexure portion and a stop portion, wherein stop portion is typically thicker than the flexure portion. In particular embodiments, the blocking mechanism is at least partly established by an auxiliary chamber comprised in the placement hole. In typical embodiments, the blocking mechanism is configured such that when the flexure beam is bent in order to allow for an optical component to be inserted into the placement hole, the stop portion touches a side wall of the auxiliary chamber before the flexure portion reaches a state of permanent deformation and/or a state of rupture.

In typical embodiments, the substrate plate comprises a recess for placing an active medium inside the optical circuit and/or inside the optical cavity without establishing a direct thermal contact between the active medium on one hand and the substrate plate and/or the optical component(s) on the other hand. Such a configuration makes it for example possible to place the substrate plate on top of a holding means, which is configured to hold the active medium, in such a way that the active medium protrudes through the recess. Like this, the active medium can be positioned inside the optical cavity, but there is no direct contact between the substrate plate and the active medium. Like this, heat dissipation for the active medium can be simplified and alignment problems due to heat gradients can be minimised. The recess is typically placed around a geometric centre of the substrate plate. In typical embodiments, the substrate plate is essentially circular and/or the recess has a rectangular form. In typical embodiments, placement holes are oriented in such a manner with respect to the recess and/or the geometric centre of the substrate plate that the essentially fully aligned optical cavity is formed when a corresponding optical component is placed in each placement hole. In typical embodiments, the recess is positioned precisely in the focus position of a pump laser beam when the optical arrangement is mounted in a laser assembly comprising a pump optics and the optical arrangement. This has the advantage of achieving a maximum efficiency of pumping. The recess typically has dimensions at least as large as an active medium, which is to be placed inside the recess, or larger. Typically, these dimensions of the recess range from a few tens of microns, e.g. at least 10 pm, to a few centimeters, e.g. 3, 5 or 7 cm.

In typical embodiments, the substrate plate comprises a multitude of reference surfaces, preferably V-grooves, preferably three such reference surfaces, for fine- aligning the substrate plate, in particular with respect to the at least one reference axis of the optical arrangement and/or with respect to the optical axis of an optical circuit in which the optical arrangement is mounted.

In typical embodiments, the reference surfaces are arranged around an edge of the substrate plate, preferably in equal intervals, preferably with equal distances between each other. In typical embodiments, three V-grooves are arranged circumferentially near an outer edge of an essentially round substrate plate, preferably with angular distances of 120 degrees between each other. One possibility is that a first V- groove is arranged at a first position referred to as 0-degree position, a second V-groove is arranged at a second position referred to as 120-degree position and a third V-groove is arranged at a third position referred to as 240-degree position. In typical embodiments, the V-groves are aligned such that their longitudinal axes all point towards the geometric centre of the substrate plate. In general, the term “V-groove” refers to a groove with a cross-section in the form of a “V”. In principle, other types of grooves are also possible, for example grooves with cross-sections in the form of a “U” or any other types of grooves. It is also possible that different grooves have different cross-sections.

The size of the reference surfaces can vary from a few tens of microns, e.g. at least 10 pm, to a few millimeters, e.g. 3, 5 or 7 mm.

In typical embodiments, the reference axis of the optical arrangement is an axis defined by a pump laser beam and an active medium, in particular a crystal. In typical embodiments, the reference surfaces, in particular the V-grooves are positioned and oriented such that respective extrapolations of longitudinal axes of the reference surfaces, intersect at the centre of the substrate plate where the active medium is typically positioned. In a laser assembly comprising the optical arrangement, this active medium is considered as reference point for fine adjustment of the optical circuit and/or the optical cavity.

In typical embodiments, the V-grooves (or the grooves/reference surfaces in general) are configured to each interact with one adjusting means, wherein the adjusting means are configured to be actuated such as to modify an inclination of the substrate plate. Such a configuration makes it possible to fine-align the substrate plate (and therefore the entire optical cavity when the optical components are mounted in/on the substrate plate) with respect to a given reference, for example the optical axis of the optical arrangement and/or of the optical circuit.

In typical embodiments, the substrate plate comprises one or more flexure element(s) configured to apply a force and/or forces onto one or more components, preferably optical component(s), placed in one or more place holders, preferably placement holes.

In typical embodiments, the optical arrangement comprises a free-space optical circuit. In this context, the expression “free-space optical circuit” is to be understood as referring to an optical circuit in which light beams at least partly run freely without running through hard matter, such as glass or the like. In other words, a free-space optical circuit is characterised in that it always comprises parts, where light beams run through air, vacuum or the like. In typical embodiments, the optical components at least partly form a free-space optical circuit.

In typical embodiments, the substrate plate comprises a silica wafer. In typical embodiments, the substrate plate is a silica wafer.

In typical embodiments, the silica comprises or is a fused or vitreous silica. In typical embodiments, the substrate plate comprises a fused quartz.

In typical embodiments, the substrate plate has a diameter between 0,5 cm and 50 cm, typically between 0,8 cm and 20 cm, preferably between 1 cm and 10 cm. In typical embodiments, the substrate plate has a thickness between 100 pm and 10 mm, typically between 200 pm and 8 mm, preferably between 300 pm and 5 mm. In typical embodiments, the substrate plate comprises silica of any grade or equivalent fused quartz.

In typical embodiments, the optical arrangement comprises one or more embedded optical waveguide(s) in the substrate plate.

In typical embodiments, the substrate plate is essentially thermally stable and/or the substrate plate has a thermal expansion coefficient below 10⁵/K, typically below 10⁶/K, preferably below 10⁷/K.

In typical embodiments, the substrate plate is essentially chemically stable to moisture.

In typical embodiments, the substrate plate comprises one or more of the following materials: fused silica, sapphire, alumina, calcium fluoride, barium fluoride, lithium-aluminosilicate glass-ceramic (like Zerodur™), glass or ceramic families based on chalcogenide, aluminosilicate, phosphate, telluride,, photosensitive glass, ruby, crystals displaying optical activity. In typical embodiments, the substrate plate is made of one or more of the following materials: fused silica, sapphire, alumina, calcium fluoride, barium fluoride, Zerodur, photosensitive glass, ruby, glass or ceramic families based on chalcogenide, aluminosilicate, phosphate or telluride.

In typical embodiments, the optical arrangement comprises optical components inserted into the placement holes and/or an active medium arranged inside the recess. In typical embodiments, the active medium is an ytterbium-KYW crystal.

In typical embodiments, the optical components have the form of round cylinders. In typical embodiments, the active medium is physically separated from the optical arrangement, in particular from the optical cavity and/or the substrate plate.

In typical embodiments, the active medium is placed by means of a medium holder wherein the medium holder is fixed to a base mount, wherein the base mount is not forming part of the optical arrangement. Like this, the active medium does not have any thermal contact to the substrate plate and the optical cavity, hence preventing possible heat accumulation in the optical cavity.

In typical embodiments, the active medium is force-fitted in the recess and/or welded in the recess and/or glued in place and/or held in place by means of an elastic element.

In typical embodiments, an incident pump beam surface of the active medium defines an optical alignment axis and or a reference axis of the optical arrangement.

In typical embodiments, the active medium comprises any lasing crystals like for instance Nd-YAG, Ytterbium, Ti-Sapphire.

In typical embodiments, the active medium forms a reference point for fine- aligning the substrate plate and/or the optical components and/or the optical cavity and/or the optical circuit.

In typical embodiments, the substrate plate, when being installed in an optical circuit, is being held by actuators, also referred to as actuating adjustment means, through the V-grooves. Hence, by varying the actuators, the entire substrate plate can be aligned with respect to the active medium and/or a pump laser beam. In typical embodiments, one or more placement holes comprise(s) a special beam for fine alignment. This special beam is configured to be activated by means of laser exposure and/or other means and to carry out thermal expansion when activated such as to thereby carry out fine-alignment of the placement hole.

Atypical alignment procedure of the optical arrangement typically involves direct observations of beam positions on the optical elements of the optical arrangement and/or it involves monitoring an optical circuit efficiency, for instance, by measuring the output power in the case of a laser cavity. In other words: a method for fine-aligning an optical arrangement according to any of the above-mentioned embodiments typically comprises a step of observing beam positions on the optical elements of the optical arrangement and/or a step of monitoring an optical circuit efficiency.

A laser assembly according to one embodiment of the invention comprises a pump optics, an optical arrangement according to one embodiment of the invention and an adjustment means for fine-aligning the optical arrangement. In this context, the expression “pump optics” shall be understood as referring to an optical system that is capable of creating a light beam that can serve as pumping beam for the optical cavity inside the optical arrangement and for the active medium, such that a laser beam of a particular type can be created.

In typical embodiments, the adjustment means are configured to interact with the V-grooves of the optical arrangement.

In typical embodiments, each adjustment means comprises at least one, preferably at least two, spheres, which are configured to at least partly fit into the respective V-grooves. In typical embodiments, the adjustment means comprises one or more screwing mechanisms for changing a position of the spheres and thereby changing an inclination of the substrate plate. FIGURES

In the following, the invention is described in detail by means of drawings, wherein show:

Figure 1 : a schematic top view of an optical arrangement according to one embodiment of the invention,

Figure 2: the optical arrangement already shown in Figure 1 , also in top view, with a multitude of dashed lines visualizing the orientation and positions of V- grooves,

Figure 3: three schematic views (one top view, two perspective views) of a placement hole according to one embodiment of the invention,

Figure 4: three schematic views (one top view, two perspective views) of another placement hole according to one embodiment of the invention.

Description of Preferred Embodiments

Figure 1 shows a schematic top view of an optical arrangement A according to one embodiment of the invention. The optical arrangement A comprises a substrate plate 1 , which is made from a silica wafer. The substrate plate 1 is essentially round, meaning that it has almost the shape of a circle. In the embodiment shown in Figure 1 , the substrate plate 1 is, however, not completely round but comprises a straight edge 5 at the bottom in the view shown in Figure 1 . In other embodiments, the substrate plate 1 could be completely round or could also have another shape. The substrate plate 1 comprises four placement holes 2.1 , 2.2, 2.3, 2.4. The substrate plate 1 furthermore comprises a recess 3, which is arranged around a geometric centre of the substrate plate 1 (supposing that the shape of the substrate plate is a complete circle). The placement holes 2.1 -2.4 are configured to each receive an optical component, in particular a passive optical component, for example a round cylindrical mirror (passive optical components not shown in Figure 1). The placement holes 2.1-2.4 are positioned and oriented such that, when the passive optical components are placed in the placement holes 2.1 -2.4, an essentially fully aligned optical cavity is created.

The substrate plate 1 shown in Figure 1 furthermore comprises three V- grooves 4.1 , 4.2, 4.3. Each of these V-grooves 4.1 -4.3 is configured to interact with an adjusting means (not shown in Figure 1). By means of these adjusting means which interact with the V-grooves 4.1 -4.3, it becomes possible to amend for example the orientation and inclination of the substrate plate 1 , and therefore also the optical cavity which is formed when passive optical components are placed in the placement holes 2.1-2.4. Like this, it becomes possible to carry out a fine-aligning of the optical arrangement 1 , for example with respect to an optical axis of an optical system or optical circuit, in which the optical arrangement A is installed. Neither this optical axis, nor an optical system or an optical circuit, in which the optical arrangement A is integrated, are shown in Figure 1. However, it becomes clear from Figure 1 , that when for example cylindrical mirrors are placed in the placement holes 2.1 -2.4, these cylindrical mirrors are already aligned with respect to each other and with respect to the substrate plate 1 and the recess 3 and will therefore form an essentially fully aligned optical cavity. All that needs to be done to align this optical cavity in a larger optical system, such as a laser assembly, is to fine-align the optical arrangement A, in particular the substrate plate 1 , with respect to a reference, for example an optical axis of the overall optical system, to achieve a complete alignment of the optical arrangement A. Like this, it is especially not necessary to align each passive optical component, for example each cylindrical mirror, of the optical cavity separately, because their positions and orientations are already given by the positions and orientations of the placement holes 2.1 -2.4 which are integrated in the substrate plate 1 .

Figure 2 shows the optical arrangement already shown in Figure 1 , also in top view, with a multitude of dashed lines 6 visualising the orientation and positions of the three V-grooves 4.1-4.3. The dashed lines 6 in Figure 2 in particular visualise that the V-grooves 4.1-4.3 are arranged in such a manner around the edge of the essentially circular substrate plate 1 such that an equilateral triangle can be drawn between the three V-grooves 4.1-4.3. In other words, the distances between each V-groove are equal. Furthermore, the V-grooves 4.1 -4.3 are arranged around the substrate plate 1 with angular distances of 120 degrees between each other. Furthermore, each of the V-grooves 4.1-4.3 is arranged such that it is pointing towards a geometric centre 17 of the substrate plate 1. From Figure 2, it becomes obvious that for example an inclination of the substrate plate 1 can be modified by actuating adjustment means (not shown) interacting with any of the V-grooves 4.1-4.3.

Figure 3 shows three schematic views of a placement hole 2.2 according to one embodiment of the invention. In particular, part a) of Figure 3 shows a top view of the placement hole 2.2 and parts b) and c) of Figure 3 each show perspective views of the same placement hole 2.2. In part c), a passive optical component, namely a round-cylindrical mirror 12.2, is placed inside the placement hole 2.2. Referring now to part a) of Figure 3, it can be seen that a first internal side of the placement hole 2.2 comprises a three-point support with three contact points, namely a first triangular contact point 7.2, a rectangular contact point 8.2 and a second triangular contact point 9.2. The placement hole 2.2 furthermore comprises a beam flexure 10.2 and this beam flexure 10.2 comprises a protruding contact point 11.2. In part b) of Figure 3, it can clearly be observed, that the first triangular contact point 7.2 has the form of a triangular prism, the second triangular contact point 9.2 has also the form of a triangular prism and the rectangular contact point 8.2 has the form of a rectangular prism. From part c) of Figure 3, it becomes clear how a passive optical component 12.2 can be received by the placement hole 2.2. In particular, this passive optical component 12.2 is inserted from above into the placement hole 2.2 until it gets in touch with the three-point support which comprises the contact points 7.2, 8.2 and 9.2. The protruding contact point 11.2 of the beam flexure 10.2 pushes the passive optical component 12.2 against this three-point support and therefore, the passive optical component 12.2 is securely fixed and positioned in the placement hole 2.2.

Figure 4 shows three schematic views, namely one top view and two perspective views, of another placement hole 2.5 according to another embodiment of the invention. Similar to the embodiment already shown in Figure 3, the placement hole 2.5 comprises a three-point support comprising the three contact points 7.5, 8.5, 9.5, which essentially have the same shape as the contact points 7.2, 8.2, 9.2 already shown in Figure 3. Furthermore, the placement hole 2.5 also comprises a beam flexure 10.5 with a protruding contact point 11.5. One difference between the placement hole 2.5 in Figure 4 and the placement hole 2.2 in Figure 3 is the fact that the beam flexure 10.5 is shaped differently than the beam flexure 10.2 shown in Figure 3. In particular, the beam flexure 10.5 comprises a flexure portion 13 and a stop portion 14. The stop portion 14 is much thicker than the flexure portion 13. In particular, a thickness of the stop portion 14 equals approximately three times the thickness of the flexure portion 13. Another difference between the embodiment in Figure 4 and the embodiment in Figure 3 is the fact that the placement hole 2.5 comprises a main chamber 15 and an auxiliary chamber 16. As can be seen from part e) of Figure 4, the main chamber 15 is configured to receive a passive optical component 12.5. The purpose of the auxiliary chamber 16 is to define a room for movement for the stop portion 14 of the flexure beam 10.5. In particular, the combination of the auxiliary chamber 16 and the stop portion 14 of the placement hole 2.5 makes it impossible for the flexure beam 10.5 to break when the passive optical component 12.5 is placed in the main chamber 15. In particular, the flexure portion 13 is prevented from being bent too strongly because the stop portion 14 will hit the lower interior wall of the auxiliary chamber 16 before the flexure portion 13 can be bent so much that it breaks.

In typical embodiments, the optical arrangement comprises a single substrate plate with low-thermal expansion, with placement holes onto which optical components forming a laser cavity are positioned relatively to an active media, and such that the substrate plate can be oriented, according to three axes defined at the active media center of mass that is used as a center point for rotations. The placement holes for the optical components are typically designed to approach theoretical conditions of isostatic positioning, i.e. an optical component can have only one stable spatial orientation and position in space.

In general, the placement holes are typically machined with high precision (e.g. < 1 micron absolute accuracy and <100 nm resolution). The placement holes are typically machined by means of a process combining non-ablative femtosecond laser exposure and chemical etching. Hence, the entire system can typically be considered to be already aligned sufficiently accurately so that lasing operation can occur, and so that only fine adjustments may be required for optimizing the output efficiency of the cavity.

In general, the fine-aligning of the optical arrangement typically comprises a step of adjusting the entire substrate plate (and hence the optical cavity as a whole) with respect to a reference point such as the one(s) defined above, rather than changing orientations or positions of individual optical components of the cavity. The active medium is typically attached to a separate placeholder also referred to as holding means, itself detached from the rest of the optical cavity. This placeholder allows for efficient heat dissipation. Separating the active medium from the rest of the components is advantageous as it prevents thermal perturbations that could affect the cavity alignment. On the other hand, this results in having all passive optical components self-aligned and sharing a common, thermally stable, substrate plate made of a material with a low coefficient of thermal expansion, e.g. below 10⁵/K.

In one particular embodiment of a laser assembly according to the invention, the laser assembly comprises a femtosecond laser configured to emit pulses at a GHz rate. In particular, the laser assembly comprises for example a 980 nm-Fiber Bragg grating stabilized laser for optical pumping of an active medium that consists for example of an ytterbium-KYW crystal. The laser can be operated in continuous wave or pulsed operation at GHz repetition rate. For femtosecond pulses operation, traditional methods can be considered such as Kerr-lens modelocking, semiconductor saturable absorber modelocking (SESAM), or modelocking through any other types of passive modulators. The pump beam defines the optical reference axis of the laser assembly, and the entire optical cavity, which comprises the passive optical components mounted on the substrate plate, is aligned with respect to this axis.

In this laser assembly, since the mirrors of the optical cavity are already pre aligned in the substrate plate, there will be only a need for a fine-alignment. This is typically done by not individually altering the position or orientation of single components of the cavity, but rather by aligning the entire optical cavity as one body with respect to the pump laser and the active medium. This fine adjustment is done by means of three adjusters that are holding and precisely positioning the entire substrate plate (which can for example be a silica wafer), wherein the adjusters are themselves positioned in the respective three V-grooves defining a kinematic mount. In typical embodiments, the substrate plate is machined with a femtosecond laser combined with chemical etching.

In typical embodiments, the optical cavity is geometrically referenced with respect to the active medium (also referred to as gain crystal) and the pump beam, with the help of the three positioning V-grooves.

In typical embodiments, the active medium is positioned at the center of the substrate plate, which preferably has the shape of a circle. Preferably, the active medium is mounted on a separate holding means (or placeholder or mount), wherein the recess 3, shown in Figure 1 , is typically configured to leave enough space for the active medium to protrude through the substrate plate from below in order to interact with the optical cavity, but without being in contact with the substrate plate.

In typical embodiments, the passive optical components are mirrors and are precisely positioned in the placement holes by means of three unique contact points (for example the ones shown in Figures 3 and 4) and a cantilever beam flexure (for example like the one shown in Figures 3 and 4), that applies a force, so that the mirror position is defined uniquely. In one particular embodiment, the mirrors have cylindrical shapes with heights (also referred to as thicknesses) between 1 mm and 6 mm, preferably between 3 mm and 4 mm, and diameters between 5 mm and 7 mm, preferably with diameters of 6 mm +/- 10%. In typical embodiments, at least some of the passive optical components, in particular mirrors, have different shapes.

Typical applications of the optical arrangement and/or the laser assembly according to the invention and/or their respective components are ultrafast spectroscopy, machining, telecommunication and fundamental research. The invention is not limited to the preferred embodiments described here. The scope of protection is defined by the claims.

Furthermore, the following claims are hereby incorporated into the Description of Preferred Embodiments, where each claim may stand on its own as a separate embodiment. While each claim may stand on its own as a separate embodiment, it is to be noted that - although a dependent claim may refer in the claims to a specific combination with one or more other claims - other embodiments may also include a combination of the dependent claim with the subject matter of each other dependent or independent claim. Such combinations are proposed herein unless it is stated that a specific combination is not intended. Furthermore, it is intended to include also features of a claim to any other independent claim even if this claim is not directly made dependent to the independent claim.

It is further to be noted that methods disclosed in the specification or in the claims may be implemented by a device having means for performing each of the respective acts of these methods.

Reference list

1 Substrate plate

2.1 - 2.4 Placement holes 3 Recess

4.1 - 4.3 V-grooves

5 straight edge

6 multitude of dashed lines

7.2, 7.5 first triangular contact points

8.2, 8.5 rectangular contact points

9.2, 9.5 second triangular contact points

10.2, 10.5 beam flexures 11.2, 11.5 protruding contact points

12.2, 12.5 cylindrical passive optical components / cylindrical mirrors

13 flexure portion

14 stop portion

15 main chamber

16 auxiliary chamber 17 geometric centre

Claims

Patent Claims

1 . Optical arrangement (A) for use in an optical circuit,

- wherein the optical arrangement (A) comprises a substrate plate (1),

- wherein the substrate plate (1) is configured to receive a multitude of optical components (12.2, 12.5), characterized in that

- the substrate plate (1) is configured such that once the optical components (12.2, 12.5) are received by the substrate plate (1), an essentially fully aligned optical circuit is formed.

2. Optical arrangement (A) according to claim 1 , characterized in that the essentially fully aligned optical circuit comprises an essentially fully aligned optical cavity, wherein the essentially fully aligned optical cavity preferably is the essentially fully aligned optical circuit.

3. Optical arrangement (A) according to any of the previous claims, characterized in that the substrate plate (1) comprises a multitude of placement holes (2.1 to 2.5), wherein each placement hole (2.1 to 2.5) is configured to receive at least one optical component (12.1 , 12.5), wherein each placement hole (2.1 to 2.5) has a particular position and/or orientation with respect to at least one reference axis of the optical arrangement (A) and/or with respect to the other placement holes (2.1 to 2.5) and/or with respect to a geometric centre (17) of the substrate plate (1).

4. Optical arrangement (A) according to claim 3, characterized in that at least one of the placement holes (2.1 to 2.5), preferably all of the placement holes (2.1 to 2.5), comprise(s) a three-point support on a first internal side and/or a beam flexure (10.2, 10.5) on a second internal side, wherein the first internal side and the second internal side are preferably arranged opposite to each other.

5. Optical arrangement (A) according to any of the previous claims, characterized in that the substrate plate (1) comprises a recess (3) for placing an active medium inside the optical circuit and/or inside the optical cavity without establishing a direct thermal contact between the active medium on one hand and the substrate plate (1) and/or the optical component(s) (12.2, 12.5) on the other hand.

6. Optical arrangement (A) according to any of the previous claims, characterized in that the substrate plate (1) comprises a multitude of reference surfaces, preferably V-grooves (4.1 to 4.3), preferably three such reference surfaces, for fine-aligning the substrate plate (1), in particular with respect to the at least one reference axis of the optical arrangement (A) and/or with respect to the optical axis of an optical circuit in which the optical arrangement (A) is mounted.

7. Optical arrangement (A) according to any of the previous claims, characterized in that the substrate plate (1) comprises one or more flexure element(s) configured to apply a force and/or forces onto one or more components, preferably optical component(s) (12.1, 12.5), placed in one or more place holders, preferably placement holes (2.1 to 2.5).

8. Optical arrangement (A) according to any of the previous claims, characterized in that the optical arrangement (A) comprises a free-space optical circuit and/or the substrate plate (1) comprises a silica wafer.

9. Optical arrangement (A) according to any of the previous claims, characterized in that the optical arrangement (A) comprises one or more embedded optical waveguide(s) in the substrate plate (1).

10. Optical arrangement (A) according to any of the previous claims, characterized in that the substrate plate (1) is essentially thermally stable and/or the substrate plate (1) has a thermal expansion coefficient below 10⁵/K, typically below 10⁶/K, preferably below 10⁷/K.

11. Optical arrangement (A) according to any of the previous claims, characterized in that the substrate plate (1) is essentially chemically stable to moisture.

12. Optical arrangement (A) according to any of the previous claims, characterized in that the substrate plate (1) comprises one or more of the following materials: fused silica, sapphire, alumina, calcium fluoride, barium fluoride, lithium-aluminosilicate glass-ceramic (like Zerodur™), glass or ceramic families based on chalcogenide, aluminosilicate, phosphate, telluride, photosensitive glass, ruby, crystals displaying optical activity.

13. Optical arrangement (A) according to any of the previous claims, characterized in that the optical arrangement (A) comprises optical components (12.2, 12.5) inserted into the placement holes (2.1 to 2.5) and/or an active medium arranged inside the recess (3).

14. Optical arrangement (A) according to any of the previous claims, characterized in that the active medium forms a reference point for fine-aligning the substrate plate (1) and/or the optical components (12.2, 12.5) and/or the optical cavity and/or the optical circuit.

15. Laser assembly, comprising a pump optics, an optical arrangement (A) according to any of the previous claims, and an adjustment means for fine-aligning the optical arrangement (A).