Optical-delay-line assembly

Abstract

An optical-delay-line assembly (1) comprises a substrate (2), an optical assembly (3), and a movement device (4). The optical assembly (3) is arranged on the substrate (2) and is configured to guide an optical signal (S1) generated from a source of electromagnetic waves (5), preferably from an optical input (6) of the substrate (2) to an optical output (7) of the substrate (2), along an optical path (P1). The movement device (4), when being exposed to energy, is configured to deform at least in the region of exposure to the energy, whereby at least part of the movement device (4) is movable with respect to the substrate (2) and configured to move at least part of the optical assembly (3) with respect to the substrate (2), and whereby an optical path length (L1) of the optical path (P1) of the optical signal (S1) is changeable.

Description

[0001] TITLE OPTICAL-DELAY-LINE ASSEMBLYTECHNICAL FIELD The present invention relates to an optical-delay-line assembly according to claim 1, to a method of manufacturing such an optical-delay-line assembly according to claim 18, to amethod of changing an optical delay of optical signals with such an optical-delay-lineassembly according to claim 19, and to an optical system comprising such an optical-delay-line assembly according to claim 20.PRIOR ART Optical-delay-lines are well-known in the field of optics and serve the purpose of delaying the propagation of optical signals by a controlled amount of time. They work by introducing an optical path, i.e. a physical path or mechanism, that increases the time it takes for the optical signal to travel between two points such as an optical input and an optical output. To this end the optical-delay-line adjusts the phase or timing of the optical signals. Various optical-delay-lines exist. For instance, US 2020 / 166708 A1 discloses an optical- delay-line device comprising an optical input, an optical output, and an optical assembly that directs a beam along an optical path from said optical input to said optical output. The optical assembly comprises a retroreflector that ensures a self-alignment of the direction of the output beam of the retroreflector with the direction of the beam incident on the retroreflector and an optical element comprising several reflective surfaces from which the beam is reflected. Moreover, the retroreflector is translationally movable.The disadvantage of such optical-delay-line devices is the use of movable, disconnected orseparately formed components such as the movable retroreflector or disconnected movingstages, which reduces the precision of the optical delay line and also lead to a more complexdesign. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical-delay-line assembly that overcomes the drawbacks of the prior art. In particular, it is an object to provide an optical- delay-line assembly that allows a change of the optical path length of optical signals withhigh precision and at the same time has a simple design.This object is achieved with an optical-delay-line assembly according to claim 1. That is, anoptical-delay-line assembly is provided, wherein said assembly comprises a substrate, an optical assembly, and a movement device. The optical assembly is arranged on thesubstrate and is configured to guide an optical signal generated from a source ofelectromagnetic waves, preferably from an optical input of the substrate to an optical outputof the substrate, along an optical path extending at least partially on the substrate. Themovement device, when being exposed to energy, is configured to deform at least in theregion of exposure to the energy, whereby at least part of the movement device is movablewith respect to the substrate and configured to move at least part of the optical assembly with respect to the substrate, and whereby the optical path length of the optical path of theoptical signal is changeable. Said part of the movement device can also be referred to asdeforming part of the movement device. Regions of the movement device that are not exposed to energy and are not deforming can be referred to as non-deforming part of the movement device. That is to say, the assembly according to the invention is based on the insight that an optical path length of an optical signal can be changed via a movement device that deforms uponexposure to energy. Consequently, there is no need for additional optomechanicalcomponents and the like. The use of a deformable movement device enhances the precision of the optical-delay-line assembly, simplifies the manufacture of the assembly and also makes it simple and compact. The optical signal preferably is a beam of photons, wherein said photons can have a variety of energies such as short wavelengths to long wavelengths. For instance, the optical signalcan have a wavelength in the region of 100 nm to several microns. Additionally oralternatively, the optical signal can comprise X-ray photons up to THz waves. The movement device, upon exposure to energy, is preferably configured to deform at least in the region of exposure to the energy at least one of: permanently, plastically, temporarily, or elastically. Additionally or alternatively, the movement device, upon exposure to energy, is preferably configured to exhibit at least in the region of exposure to the energy a volume expansion and / or a volume shrinkage. Additionally or alternatively, the movement device preferably comprises or consist of at least one compliant mechanism such as a flexure. Additionally or alternatively, the movement device, upon exposure to energy, is preferably configured to undergo, at least in a region of exposure, a local change in shape such as a stretching and / or bending and / or twisting. Additionally or alternatively, the movement device, upon exposure to energy, is preferably configured to undergo at least locally, a deformation that excludes rigid-body motion.Said deformation is preferably understood in a mechanical sense of deformation. That is,the movement device being configured to plastically deform preferably means that the deformation is irreversible in the absence of an exposure of energy used to deform the movement device. In particular, the movement device is preferably configured to deformupon exposure to energy, e.g. laser energy, such that its material does not have the samestructure it had before exposure. In fact, upon a plastic deformation the material of themovement device preferably exhibits some defects that are preventing the material fromcoming back to its original state. Examples of such defects are dislocations or structuralruptures. Other examples of a permanent deformation result from a phase change, e.g. achange in the crystallographic structure of the movement device, the formation of micro- ornano- bubbles inside the material of the movement device, a densification due to atomicrearrangement (common in glass), a phase separation in a movement device being amulticomponent glass system, etc. These defects are preferably localized defects that aregenerated in the region of exposure to the energy.Consequently, the exposure to energy such as a laser exposure can be used to change amaterial structure of the movement device such that it occupies a bigger (or smaller) volume after exposure, and hence, is irreversibly deformed. In other words, the movement devicecan be said to be permanently or plastically deformed depending on the nature of thematerial structural modifications. In particular, the presence of these structural changes prevent the material of the movement device from recovering its original shape.This is in contrast to an elastic or temporary deformation. In fact, if no localized defects suchas the above mentioned dislocations or structural ruptures etc. are introduced, the movement device will return back to its original shape in the absence of energy exposureand in such case, is elastically deformed. While a temporary deformation preferably meansthat the movement device can return into its original state, which for glass for instance ispossible if the movement device is heated to a very high temperature (typically severalhundreds of degree), sufficient for annealing the defects that were introduced and that kept the material deformed. Whether a plastic or an elastic deformation of the movement device takes place preferably depends on a type of material of the movement device and / or a type of energy being applied.Hence, these capabilities enable to choose the type of deformation of the movement device.For instance, in the event of the movement device comprising or consisting of fused silicabeing exposed to femtosecond lasers, one can decide to densify the material or on thecontrary, to have it expanded, simply by choosing the pulse exposure energy and duration.Such transformations have for instance been reported in: Y. Bellouard, A. Champion, B.McMillen, S. Mukherjee, R. R. Thomson, C. Pépin, P. Gillet, and Y. Cheng, "Stress-state manipulation in fused silica via femtosecond laser irradiation," Optica 3, 1285 (2016). https: / / doi.org / 10.1364 / OPTICA.3.001285.Other types of material transformations of the movement device are likewise conceivable,such as localized crystallization event, which preferably depend on parameters of theenergy exposure such as laser-exposure parameters as well.In any case, it is preferred that the deformation of the movement device preferably corresponds to a local volume expansion of the movement device or to a local volumeshrinkage of the movement device that occurs at least in the region of exposure to energy.That is, the movement device can locally be expanded or shrank in the region of exposureto energy. In other words, it is not necessary that the entire movement device expands or shrinks but only part thereof. As such, said expanding or shrinking part of the movementdevice can act as an ‘actuator' that moves at least part of the optical assembly. Additionallyor alternatively, it is preferred that the deformation of the movement device corresponds to a local change in the shape, i.e. to changes in the relative position between internal points of the movement device that preferably exclude rigid-body motion and that cause the movement device to change shape and / or size such as the before-mentioned a stretching and / or bending and / or twisting of at least part of the movement device.That is, the movement device being configured to deform upon exposure to energy beingin the form of an applied field, such as the electrostatic field, electric field, magnetic field, or mechanical field, and / or a change thereof, is preferably configured to develop an internalstress field that causes strain within the material of the movement device and / or results inlocal changes in shape and / or displacement of the movement device. For instance, theenergy can be provided in the form of an electric field for actuating a piezoelectric material,or in the form of an electrostatic field for generating an electrostatic force between structural elements of the movement device, see also further below.The movement device preferably comprises or consists of at least one compliantmechanism such as a flexure, i.e. a deformable element configured to undergo a permanentor temporal shape modification when being exposed, and hence in a sense, is actuatableby energy. The expression "flexure" is well-known in precision engineering.The movement device preferably is transparent, although this is not mandatory.Additionally or alternatively, the movement device preferably comprises or consists of atleast one of silica such as fused silica or fused quartz, vitreous or crystalline silica, silicon,glass, ceramics, glass-ceramics such as a lithium-aluminosilicate glass-ceramic such asZerodur®, germanium oxide, sapphire, metallic glass, one or more metals, plastics, or ultra-low expansion glass (ULE®) containing, for instance, silica and titanium dioxide.That is, the movement device can comprise or consist of one or more materials of various crystallinity, such as an amorphous or non-crystalline material or a polycrystalline material.Additionally or alternatively, the movement device preferably comprises or consists of oneor more materials of a low coefficient of thermal expansion (CTE), such as fused silica orfused quartz, ULE®, Zerodur®, or any materials with a CTE preferably below 10-6 / °K.Materials having such a low CTE are robust to temperature fluctuations.Additionally or alternatively, the movement device is preferably configured to exhibit aphoto-induced deformation such as a photoelastic effect when being exposed to energy.Various effects can result in the volume expansion or shrinkage and preferably depend on the material of the movement device and / or the energy being exposed to the movementdevice. As an example, for a movement device comprising fused silica being exposed tolaser radiation, the effect is related to a laser-induced densification causing the movementdevice to shrink and a laser-induced ‘nano-gratings’ formation or the formation of‘nanopores’ causing the movement device to expand. Other conceivable effects are anamorphization in movement devices comprising crystalline materials like quartz, acrystalline form of silica, or sapphire, inducing for instance a volume expansion. Likewise,a localized crystallization induces a shrinkage.Additionally or alternatively, the movement device is preferably configured to exhibit a piezoelectric effect, i.e. a mechanical deformation, when being exposed to an electric field. As such, the movement device can comprise or consist of at least one piezoelectric material such as zirconate titanate (PZT). Although not necessary, the movement device can be transparent. Advantages of atransparent movement device are, for instance, the ability to generate complex compliantmechanisms or flexures that comprise, for instance, a 3D shape, or to modify the materialof the movement device within the volume of the movement device, i.e. in the inside of themovement device.The movement device is preferably configured to deform upon exposure to energy being inthe form of at least one of: electromagnetic radiation, an electrostatic field, an electric field,a mechanical load, thermal energy, or a magnetic field and / or a change thereof, for instancea change in an applied electrostatic field such as a change of applied voltage. For instance, the source of energy can be a laser for applying electromagnetic radiation.Additionally or alternatively, the source of energy can be a magnet for applying a magneticfield. Additionally or alternatively, at least part of the substrate can be configured to "actuate" the movement device. That is, it is conceivable that the substrate itself has at least a part being configured to act as an electrostatic actuator and to apply an electrostatic force to the movement device (see Bo Lenssen and Yves Bellouard, Applied Physics Letter, 101, 103503 (2012) "Optically transparent glass micro-actuator fabricated by femtosecond laserexposure and chemical etching", https: / / doi.org / 10.1063 / 1.4750236). Likewise, it isconceivable that at least part of the substrate is configured to provide a magnetic force inthe event that the substrate has particular magnetic properties and / or to provide anelectrostatic force and in particular exhibit a piezoelectric effect in the event that thesubstrate is piezoelectric itself - for instance a substrate comprising quartz. Additionally oralternatively, the substrate can be configured to exhibit a temperature gradient or undergoa phase-transformation such as a densification, crystallization, amorphization, phase-separation in multicomponent materials, etc. Additionally or alternatively, the substrate canbe configured to exhibit a piezoelectric effect, etc. Said energy is preferably applied to a region of the movement device. Said region can also be referred to as an actuation region that is deformed, in particular exhibits the volume expansion or volume shrinkage, as well as other forms of deformation including bending,twisting, or strain-induced shape changes as mentioned earlier, when being exposed to theenergy. Hence, the movement device is preferably configured such that only the region of the movement device that is deformed produces a movement, in particular a relative movement between different regions of the substrate, and wherein said (relative) movement results in the movement of at least part of the optical assembly being arranged on said moving region(s) of the substrate. A source of energy configured to supply energy to the movement device can be provided externally from the assembly. That is, the source of energy can be an external part of theassembly. For instance, the source of energy can be a laser that is arranged externally fromthe assembly and is arranged such that it irradiates electromagnetic radiation onto the movement device, whereupon the movement device at least in a region of irradiation deforms. In this case the source of energy is preferably not in contact with the assembly. Other conceivable sources of energy are devices that are configured to input an electrical current or a voltage passing inside certain regions of the substrate, in which case it is preferred that the substrate is in connection through the source of energy for instance via acabling, or the source of energy can be a thermal source preferably not being in contactwith the assembly either, etc. In the event of the source of energy being a laser it is conceivable that the source of energy is arranged so as to face the movement device, e.g. in the event that the movement device is provided such as formed in a surface of the substrate, it is preferred that the source of energy is arranged so as to face said surface of the substrate. In other words, the source of energy is preferably arranged so as to expose its energy from a top of the substrate andpreferably perpendicular to a plane of movement or deformation of the movement device.However, such an arrangement is not stricto senso an obligation. Other arrangements are likewise conceivable wherein the source of energy is arranged with respect to the assembly such as to expose its energy from same plane as the movement or deformation of the movement device. If the source of energy such as the laser is arranged externally from the assembly, said source of energy can be in connection with other components such as being a laser that is part of another apparatus, e.g. an optical system, a spectrometer. Said apparatus can furthermore be configured to receive the assembly. For instance, the assembly can be provided in an optical system comprising a laser and being arranged, for instance, on a motorized platform that moves the assembly under the laser beam. Other designs are of course likewise conceivable, for instance a laser beam scanning the assembly, itself being stationary and not moving. Alternatively, it is likewise conceivable that the source of energy configured to supply energyto the movement device is part of the assembly. That is, it is likewise conceivable that thesource of energy, for instance said laser, is part of the assembly. In other words, the source of energy can be attached to the assembly, in particular to the substrate. For instance, thesource of energy can be bonded, glued or welded to the substrate.The substrate is preferably monolithic. The expression “monolithic” preferably means thatthe substrate is a single-piece element, i.e. made from one single piece of material.Additionally or alternatively, the substrate preferably is transparent, although not necessary. Additionally or alternatively, the substrate preferably comprises or consists of at least oneof silica such as fused silica or fused quartz, vitreous or crystalline silica, silicon, glass,ceramics, glass-ceramics such as a lithium-aluminosilicate glass-ceramic such asZerodur®, germanium oxide, sapphire, metallic glass, one or more metals, plastics, or ultra-low expansion glass (ULE®) containing, for instance, silica and titanium dioxide.That is, the substrate can comprise or consist of one or more materials of various crystallinity, such as an amorphous or non-crystalline material or a polycrystalline material.Additionally or alternatively, the substrate preferably comprises or consists of one or morematerials of a low coefficient of thermal expansion (CTE), such as fused silica or fusedquartz, ULE®, Zerodur®, or any materials with a CTE preferably below 10-6 / °K. Materials having such a low CTE are robust to temperature fluctuations.Additionally or alternatively, the substrate is preferably configured to exhibit a photo-induceddeformation such as a photoelastic effect when being exposed to energy. Additionally or alternatively, the substrate is preferably configured to exhibit a mechanical deformation and / or a piezoelectric effect when being exposed to energy. Additionally or alternatively, the substrate comprises or consists of a piezoelectric ceramics such aszirconate titanate or piezoelectric crystal such as PMN-PT or quartz.In fact, the substrate preferably comprises or consists of the same one or more materials as the movement device and vice versa. As examples, but not limiting, the substrates can be wafers of glass (or other materials) of various diameters, like for instance 1 inch or much bigger like 20 inches, and thicknesses ranging from a few tens of microns to several millimeters, or even centimeters like for optical flats used in metrology. Alternatively, it canalso have rectangular shapes of varying dimensions and thickness, which for glass forinstance, can be cut out of large sheets of float glass. Other unconventional substrates shapes with arbitrary geometry can be cut out from these large sheets.The movement device is preferably integrated into the substrate. The movement devicebeing integrated into the substrate preferably means that the movement device is part of the substrate, in particular part of the monolithic or single-piece substrate mentioned earlier. In other words, the substrate and the movement device such as the compliant mechanismmentioned earlier can be monolithic. That is, the movement device may be mademonolithically from the same material as the substrate. Consequently, it is preferred that the movement device and the substrate are made of a same material. Additionally or alternatively, the movement device is preferably provided, in particular formed, within a volume of the substrate and / or in a surface of the substrate. The movement device being formed within a volume of the substrate preferably means that the movement device is preferably generated inside the substrate via a nonlinear absorption of electromagnetic radiation by the substrate. In contrast to linear absorption phenomenawhere the provided electromagnetic radiation is gradually depleted as it penetrates thesubstrate, nonlinear absorption is characterized by the fact that in a transparent or semi-transparent substrate to the electromagnetic radiation, the energy is barely absorbed as it penetrates the material but gets rapidly absorbed when the energy density reaches a given threshold. For instance, the substrate can be configured to exhibit non-linear absorption phenomena upon the irradiation of ultrafast laser radiation that can induce non-linear effects in the substrate and can propagate inside a material of the substrate while having some absorption taking place only at a focal point of the laser radiation inside the material, that is to say, at the location where the energy density exceeds a defined threshold. As theabsorption is taking place inside the substrate and being of a non-linear nature (i.e.,occurring only if a given energy density threshold is reached), the material is deformed, whereby the movement device is generated within a volume of the substrate. The volume affected can be smaller than the laser-wavelength, allowing the localized deformation andthus the generation of the movement device of the material within the substrate with extremeaccuracies. As an illustration, the localized deformation occurring in fused silica can be as small as 0.01 % of a volume that can be itself of submicron characteristic dimensions, and hence the induced displacement can be smaller than a nanometer. For instance, for femtosecond lasers exposure of fused silica, these displacements as a function of the laser exposure dose can be measured using micro-cantilevers as described in: A. Champion and Y. Bellouard, "Direct volume variation measurements in fused silica specimens exposed to femtosecond laser," Opt. Mater. Express 2, 789–798 (2012)https: / / doi.org / 10.1364 / OME.2.000789. This is just an illustration in the case of fused silica.As the phenomenon of nonlinear absorption is generic. Similar behavior is expected in other materials. Other formations are however likewise conceivable. For instance, the movement device can be machined or etched into the substrate. For example, the movement device can be formed into a surface of the substrate by machining or etching. It is also conceivable that the movement device is not integrated into the substrate but provided as a separate component that is, for example attached such as bonded, welded, or glued to the substrate. Hence, it is conceivable that the substrate and the movement device are made of different materials and are assembled and / or attached to one another. For instance, laser welding using a selective laser etch for fusing a portion of the movement device and of the substrate to each other via irradiation by a picosecond and / or femtosecond laser. That is, femtosecond or picosecond laser processing can be combined with etching to generate the movement device in the substrate, but other means such as Deep ReactiveIon Etching in glass, laser ablation, or additive manufacturing methods are likewiseconceivable for generating the movement device in the substrate. Consequently, the movement device can have various designs, for instance a cut-out formed in the material of the substrate, or a contour of a cut-out, etc. However, it should be noted that there are many other possible design options. A preferred design option is a movement device that comprises a lower stiffness along the direction ofmotion of the movement device, and higher stiffness along other directions. That is to say,if a Cartesian coordinate frame (X, Y, Z and the three corresponding rotations along theaxis) is assigned to the movement device to define the six possible degree-of-freedom, andsuch that one of its reference axis – say for instance X - is aligned along the linear directionof motion, a preferred design of the movement device is configured to achieve as low aspossible stiffness along this X axis that defines the designed degree of freedom, and ashigh as possible stiffnesses along all the other degrees of freedom, i.e., along the Y, Z axis,and along the three possible rotational degrees of freedom. Such conditions achieve a highstiffness contrast between the degree-of-freedom defining the movement direction and all the five other degrees of freedom.The movement device is preferably configured to perform a linear movement with respectto the substrate, whereby at least part of the optical assembly is linearly movable with respect to the substrate. The movement device being configured to perform a linear movement with respect to the substrate preferably means that the movement device can be deformed linearly, preferablyalong at least two spatial dimensions, e.g. X-Y-directions or X-Y-axis of the Cartesiancoordinate frame, and / or with respect to a (fictitious) plane, e.g. an X-Y-plane, running parallel to the substrate, in particular parallel to a surface of the substrate. However, it should be noted that the movement device preferably is configured such as to have a single degree of freedom (DOF). In the example of the movement device being a compliant mechanism as mentioned above, said single DOF is preferably defined by the axis with the lowest stiffness. According to this mechanism theory, although the compliant mechanism can be composed of multiple so-called hinges arranged in series or parallel, each of the hinge having one of multiple degrees of freedom, at the end the overall compliant mechanism may have only one degree of freedom. For instance, a four-bars linkage has a single degree of freedom, even though the motion produced can be complex and multiple joints can be used. One advantage of privileging a high stiffness contrast, that is to say, for instance a low stiffness along the linear motion axis and very high stiffnesses along orthogonal directions and along rotational axes, is to achieve a motion guidance that is immune to motion perturbations that may occur along other directions than the designed direction of motion. The movement device preferably has a linear geometry such that it deforms, in particularexpands and / or shrinks while moving linearly. Such a linear movement is preferred so thatthe optical input, i.e. a starting point, and the optical output, i.e. an exiting point, of the optical signal remain the same during the movement, thereby ensuring an alignment of the opticalsignal such as the optical signal hitting optical elements such as mirrors at the same pointsduring the movement. However, it is also conceivable that the movement device is configured such that it moves at least part of the optical assembly and thus the optical signal not only in a translatorymovement but additionally or alternatively rotates them. Said rotation of the optical signalcan be achieved through a particular geometry of the movement device, for instance with acompliant mechanism comprising a remote center of rotation as it is described inMicromachines, 2019, 10(9), 611 Saood Ibni nazir and Yves Bellouard, "A monolithic Gimal micro-mirror fabricated and remotely tuned with a femtosecond laser", https: / / doi.org / 10.3390 / mi10090611. Additionally or alternatively, the movement device and the optical assembly are preferably configured and arranged such, that a movement of the movement device by a distance ∆ causes a shortening or expansion of a distance 2∆ of the optical path of the optical signal. The movement device and the optical assembly are preferably configured and arranged such, that a movement of the movement device by a distance ∆ upon the deformation of the movement device causes a shortening or expansion of 2n∆ of the optical path length ofthe optical signal, where n is an integer corresponding to a number of passes of the opticalsignal through the movement device. For instance, an optical signal may bounce back multiple times on a set of mirrors before exiting the optical path extension assembly. For instance, if the optical signal passes through the movement device only once, n=1 and the movement of the movement device by a distance ∆ upon the deformation of the movement device causes a shortening or expansion of 2∆ of the optical path length of the optical signal. However, it is also conceivable that the optical signal passes several times through the movement device, for instance when being reflected from reflecting optical elements being provided on the substrate. In this case, it is possible through multiple reflections to multiply the beam path for a given displacement of the moving element. The number of passes of the optical signal preferably corresponds to passes of the optical signal between a deforming part of the movement device and a non-deforming, i.e. stationary part of the movement device. Additionally or alternatively, the movement of the movement device and / or of the optical assembly with respect to the substrate is preferably in the nanometer range or smaller. Thatis, the movement device is preferably configured and arranged such, that a movement ofthe movement device upon the deformation of the movement device is in the nanometer range or smaller. Consequently, the optical assembly is preferably configured and arranged such, that it is movable by the movement device upon the deformation of the movement device in the nanometer range or smaller. That is, the preferably linear expansion or shrinkage of the movement device can be as small as a few nanometers or smaller,resulting in a change of the optical path length and for instance in a time delay betweenoptical signals that can be virtually infinitely small.The optical assembly can be connected with the movement device. For instance, the opticalassembly can be in connection with the movement device via at least one recess formed inthe substrate in the region of the movement device that receives at least part of the opticalassembly. Additionally or alternatively, the optical assembly can be in connection with themovement device by being attached such as bonded, glued or welded to the substrate inthe region of the movement device.Additionally or alternatively, the optical assembly can be in connection with the substratevia one or more recesses being formed in the substrate and being configured to receive at least part of the optical assembly. Additionally or alternatively, the optical assembly can be in connection with the substrate by being attached such as bonded, glued or welded to the substrate.The movement device can comprise an actuator mounted to the substrate and configuredto undergo deformation δ upon exposure to energy. The movement device can further comprise a compliant mechanism comprising at least one flexure element connected to the substrate and a transmission flexure element connecting the actuator to the flexure element.An optical platform can be coupled to the flexure element, wherein at least part of the opticalassembly can be arranged on the optical platform. The movement device is preferablyconfigured such that the deformation δ of the actuator results in a movement of the opticalplatform, whereby the optical path length of the optical path of the optical signal is changed. That is, while the movement device can be monolithic, it can likewise comprise at least to parts such as the just described actuator and the compliant mechanism. Various kinds of actuators are conceivable, such as a piezoelectric actuator, a thermal actuator, a magnetostrictive actuator, and so on, which essentially only differ in the materialused and the type of energy required to induce deformation. For example, the actuator maybe a piezoelectric actuator made of a piezoelectric material, which deforms upon exposure to energy in the form of an applied voltage. Another example is a thermal actuator, which expands upon exposure to energy in the form of thermal energy. Another example is a shape memory alloys actuator that undergoes a phase transformation upon exposure of thermal energy, and while doing so, experiences a macroscopic deformation. Similarly, the actuator may be a magnetostrictive actuator comprising a magnetostritive material, which deforms upon exposure to a magnetic force field, etc. The flexure element of the compliant mechanism can be connected to the substrate at a first pivot point and the transmission flexure element can connect the actuator to a first point on the flexure element at a distance s from the first pivot point. The optical platform can becoupled to a second point on the flexure element at a distance l from the first pivot point.The movement device can be configured such that the deformation δ of the actuator resultsin a movement of the optical platform by a distance Δ = (l / s) • δ, whereby the optical pathlength of the optical path of the optical signal is changed by a distance of 2Δ. The actuator is preferably affixed to or embedded within the substrate, preferably via at least one adhesive element, laser welding, etc. In the following, an example is given for the actuator being a piezoelectric actuator. It should be noted that a deformation and movement for other kinds of actuators are analogous.A voltage for instance a voltage bias in the event of a piezoelectric actuator can be applied via one or more electrical wires or the like that are in connection with the source of energy and the piezoelectric actuator. Upon the application of the voltage or voltage bias, i.e. electrostatic energy, the piezoelectric actuator can deform, in particular elongate, and upon removal of the voltage or voltage bias, the piezoelectric actuator can recover, i.e. shrink back to its original state. When the piezoelectric actuator elongates / shrinks by an amount δ, the displacement is transmitted to the at least one flexure element of the compliant mechanism via the transmission flexure element. The flexure element preferably is a lever, connected at one extremity to the substrate, with the pivot point preferably acting as a fulcrum, while the other extremity of the flexure element preferably is at a distance l from the pivot point linked to the optical platform and free to move. The transmission flexure element is preferably connected to the flexure element at a distance s from the connection of the flexure element to the substrate. As a result, the deformation δ of the piezoelectric actuator is (de)amplified by a geometrical factor l / s, and produces the movement Δ = (l / s)•δ of the optical platform. The movement device is preferably designed with constraints to yield a linear translation with respect to the substrate. In other words, the movement device is preferably configured with mechanical constraints that limit the motion of the optical platform to a substantially linear translation along a defined axis relative to the substrate, thereby minimizing parasitic rotation or lateral displacement. For example, the optical platform can be a rigid optical platform that is suspended between two flexure elements, i.e. the compliant mechanism can comprise at least one secondflexure element being parallel to the flexure element and being connected to the substrateat one extremity at a second pivot point and being connected to the optical platform at the other extremity. Thus, when force is applied by the actuator such as the piezoelectric actuator, the two flexure elements bend elastically, allowing the optical platform to move straight along the actuation axis.When the optical platform moves by the distance Δ, it preferably moves the at least part ofthe optical assembly being arranged thereon by the same amount with respect to thesubstrate, whereby the optical path length of the optical path of the optical signal is modified by 2Δ. To this end it is preferred that at least one optical element, preferably at least one reflective optical element such as a semiconductor saturable absorber mirror (SESAM) is arranged on the optical platform. Said Semiconductor Saturable Absorber Mirror (SESAM) preferablyforms part of a laser resonator cavity, see further below. However, it should be noted thatthe optical assembly can comprise other optical elements such as a first and second opticalelement in the form of mirrors and configured to receive the optical signal from an opticalinput and to transmit the optical signal to an optical output as will be described further belowthat can be arranged on the optical platform, etc. The compliant mechanism is preferably preloaded with an initial mechanical offset suchthat, when the actuator such as the piezoelectric actuator is at rest, the optical path lengthof the optical path of the optical signal is increased by 2Δ_max, and under a maximum voltage being applied, is decreased by 2Δ_max, with Δ_max being a maximum achievabledistance Δ being moved under a specified voltage range.The piezoelectric actuator being at rest preferably means that no voltage is applied. Hence, the movement device is preferably configured with an offset such, that when no voltage or voltage bias is applied, the optical path length of the optical path of the opticalsignal is preferably elongated by 2Δ_max (> 0), with the piezoelectric actuator in its defaultstate at rest, and when a maximum given voltage or voltage bias is applied, the optical pathlength of the optical path of the optical signal is preferably shortened by 2Δ_max (< 0).The movement device can comprise mechanical stop elements positioned relative to the substrate and the optical platform such that the movement of the optical platform is constrained, preferably to a maximum range of ±Δ_max. Said stop elements may include rigid projections, integrated flexural limits, or gap-based interference structures that are designed to abut or engage with the optical platform when its displacement reaches the predefined movement limits as they are well-known in the art. However, it is likewise conceivable that the movement device comprises an electrostatic actuator comprising a first set of comb-drive electrodes mounted to the substrate, and a second set of comb-drive electrodes connected to an optical platform and interdigitated with the first set of comb-drive electrodes. A compliant mechanism comprising at least one flexure element is preferably connected to the substrate and mechanically coupled to the optical platform. At least part of the optical assembly is preferably arranged on the opticalplatform. The movement device is preferably configured such that, when voltage is appliedto the first and second sets of comb-drive electrodes, an electrostatic attraction is generatedbetween the interdigitated first and second set of comb-drive, thereby moving the opticalplatform by a distance Δ and changing the optical path length of the optical signal by adistance of 2Δ. Regarding the interdigitated first and second set of comb-drive electrodes it should be noted that they can be arranged parallel or perpendicular to a direction of motion of the movement device. Moreover, the movement device preferably comprises mechanical stop elements positioned relative to the substrate and the optical platform such that the movement of the opticalplatform is constrained preferably to ±Δ_max. In particular, to ensure mechanical stabilityand prevent overextension, the electrostatic actuator preferably includes physical stop gaps configured to limit the motion of the optical platform within a defined range. These stop elements are preferably positioned to prevent travel beyond ±Δ_max, thereby protecting both the electrostatic comb structures and the compliant flexure elements from mechanical damage or fatigue.The comb-drive electrodes preferably are capacitor plates in the form of so-called "teeth"as they are well-known in the art. These electrodes, plates or comb-teeth can be coated with a dielectric or metal layer, which serves to define two separate electrode regions. Thefirst set of the comb-drive electrodes is preferably mechanically coupled to the opticalplatform, which is preferably in connection with the substrate via the compliant mechanism,in particular via at least one and preferably two flexure elements that are pivotably connected to the substrate as mentioned earlier in the context of the piezoelectric actuator.That is, also the optical platform is preferably itself supported by the substrate and designedfor linear translation via the compliant mechanism. The optical platform preferably carriesat least a portion of the optical assembly. To this end it is preferred that at least one opticalelement, preferably at least one reflective optical element such as a semiconductor saturable absorber mirror (SESAM) is arranged on the optical platform. Said SemiconductorSaturable Absorber Mirror (SESAM) preferably forms part of a laser resonator cavity, seefurther below. However, it should be noted that the optical assembly can comprise otheroptical elements such as a first and second optical element in the form of mirrors andconfigured to receive the optical signal from an optical input and to transmit the optical signalto an optical output as will be described further that can be arranged on the optical platform,etc. Electrical contact pads are preferably provided on the first and second set of comb-drive electrodes. A voltage such as a voltage bias can be applied across the first and second setof comb-drive electrodes using, for instance, electrical wires that are bonded to therespective contact pads. When a voltage or voltage bias is applied between the first and second set of comb-driveelectrodes, an electrostatic force is generated between the interdigitated comb teeth ofthese comb-drive electrodes. This electrostatic attraction causes the optical platform totranslate relative to the substrate.The translation is preferably constrained by the compliant mechanism, such as the one ormore flexure elements, which allow for substantially linear motion of the optical platform along a predefined axis. The mechanical design may permit a small amount of parasitictranslation in transverse directions, although the primary motion remains linear. Themovement of the compliant mechanism and thus of the optical platform causes the part of the optical assembly arranged thereon, e.g. the optical element such as the SESAM mirror,to move by the distance Δ relative to the substrate. In the event of a double-passconfiguration of the optical path such as in a laser resonator cavity, see further below, theresulting change in optical path length is 2Δ. This configuration enables precise modulation of the optical path length in response to applied electrical signals. The optical assembly preferably comprises at least a first optical element and a second optical element. The first optical element is preferably configured to receive the optical signal from the optical input and to transmit the optical signal to the second optical element. The second optical element is preferably configured to receive the optical signal from the first optical element and to transmit the optical signal to the optical output. The first and second optical elements preferably are mirrors. To this end it is particularly preferred that said first and second optical elements such asmirrors are in connection with the movement device as described above, for instancereceived in recesses formed in the substrate and / or attached to the substrate in the region of the movement device, and whereby the deformation of the movement device upon the exposure to energy moves said first and second optical elements. Thereby, the optical pathlength of the optical signal is changed, preferably by 2n∆ as mentioned above, wherein ∆ isthe distance moved by the movement device upon its deformation and n is an integercorresponding to the number of passes of the optical signal through the movement device.The source of electromagnetic waves is preferably part of the assembly, preferably attachedsuch as bonded, glued or welded to the substrate. The source of electromagnetic waves can be a pulsed source of electromagnetic waves or a continuous source of electromagnetic waves. A pulsed source of electromagnetic waves preferably is a source of electromagnetic waves whose intensity is modulated in time. Said pulsed source of electromagnetic waves can bea coherent or non-coherent source of electromagnetic waves. A pulse duration of the pulsespreferably is in the order of nano-, pico- or femto- seconds.A continuous source of electromagnetic waves preferably is a source of electromagneticwaves that is continuously operated and continuously emits electromagnetic waves. Thecontinuous source of electromagnetic waves is particularly preferably configured to emitelectromagnetic waves being associated with an electric field which phase oscillates, butmuch faster than a detection device can detect, so that it appears constant.The source of electromagnetic waves particularly preferably is a light source such as a light emitting diode (LED), a superluminescent diode (SLED), or a laser. Various lasers areconceivable, for instance solid-state lasers such as semiconductor LDs, fiber lasers, disklasers, DPSSLs, or chemical lasers such as gas lasers or dye lasers. Additionally oralternatively, the source of electromagnetic waves may be or can comprise an optical gain medium, such as a laser crystal, that is preferably capable of generating the optical signal when being pumped, see also further below. In this case it is conceivable that a pump laser or the like is provided, and wherein said pump laser is provided externally from the assembly or is part of the assembly. For instance, in the former case a pump laser could be provided in an optical system comprising the optical-delay-line assembly such as a laser system, see also further below.The assembly is preferably configured such that a change of the optical path length of theoptical signal results in a change of the optical phase of the optical signal or in a change of an arrival time of the optical signal at the optical output. In the event that the change of the optical path length of the optical signal results in the change of the optical phase of the optical signal it is preferred that the source of electromagnetic waves is a continuous source or a pulsed source. The optical phase of the optical signal is preferably understood as the argument of the complex exponential part ofthe electric field associated with the optical signal that describes its oscillation in time at agiven pulsation. In the event of a pulsed source the movement or deformation of themovement device is preferably just enough to change the carrier phase of the optical signal,for instance within a single pulse that can be of few tens of femtoseconds, i.e. the movement or deformation of the movement device is preferably configured to perform a carrier phase modulation of the optical signal. In the event that the change of the optical path length of the optical signal results in the change of the arrival time of the optical signal at the optical output it is preferred that the source of electromagnetic waves is a pulsed source.The assembly preferably comprises a further optical assembly being configured to guide afurther optical signal from a further optical input of the substrate to a further optical outputof the substrate along a further optical path. The assembly is preferably configured such,that a deformation of the movement device changes the optical path length of the optical signal and a further optical path length of the further optical signal with respect to one another, in particular a phase delay or a time delay between the optical signal and the further optical signal. The phase delay between the optical signal and the further optical signal is preferably understood as a relative phase difference of the electric fields associated with the optical signal and the further optical signal. In this case it is preferred that the source of electromagnetic waves and, if applicable, a further source of electromagnetic waves, see below, is in each case a continuous source or a pulsed source. The time delay between the optical signal and the further optical signal is preferably understood as a relative difference between pulse arrival times associated with the optical signal and the further optical signal. In this case it is preferred that the source of electromagnetic waves and, if applicable, a further source of electromagnetic waves, see below, is in each case a pulsed source.The at least one further optical signal is preferably generated from the source ofelectromagnetic waves for instance by splitting the optical signal. That is, the optical signaland the further optical signal can be generated from a common source of electromagnetic waves. However, it is likewise conceivable that two or more sources of electromagnetic waves arepresent. That is, the assembly preferably comprises at least one further source ofelectromagnetic waves being configured to generate the at least one further optical signal.Said further source of electromagnetic waves can again be part of the assembly, forinstance attached such as bonded or welded or glued to the substrate. These sources ofelectromagnetic waves can have the same wavelengths or different wavelengths and / or can be monochromatic or not. Various sources of electromagnetic waves such as types of lasers and operating conditions are conceivable, for example: ^Split CW output from 1 laser source, optical signals being laser beams of a samewavelength. ^Split CW output from 1 laser source, optical signals being laser signals of differentwavelengths, for instance by using one or more additional optical components suchas a nonlinear crystal that frequency doubles one of the optical signals. ^Dual CW output from 1 laser source, optical signals being laser beams of samewavelength. ^Dual CW output from 1 laser source, optical signals being laser beams of differentwavelengths. ^CW output from more than 1 laser source, optical signals being laser beams of samewavelength. ^CW output from more than 1 laser source, optical signals being laser beams ofdifferent wavelengths. ^Split pulse train from 1 laser source, optical signals being laser beams of same pulseduration and same wavelength.^ Dual pulse train from 1 laser source, optical signals being laser beams of differentpulse durations and same wavelength.^ Dual pulse train from 1 laser source, optical signals being laser beams of differentpulse durations and different wavelengths.^ Pulse trains from more than 1 laser source, optical signals being laser beams ofsame pulse duration and different wavelengths.^ Pulse trains from more than 1 laser source, optical signals being laser beams ofdifferent pulse durations and same wavelength.^ Etc.It should be noted that one further optical signal and / or one further source ofelectromagnetic waves can be present or that two or more further optical signals and / or twoor more further sources of electromagnetic waves can be present. Statements made withregard to one further source of electromagnetic waves and one further optical signals preferably likewise apply to two or more further sources of electromagnetic waves and two or more further optical signals and vice versa.Any arrangement between two or more sources and / or two or more optical signals and / ortwo or more optical arrangements with respect to one are conceivable.The (further) optical arrangement can comprise further optical components such as collimating lenses, mirrors, etc., that are well-known in the art for controlling, for instance, afield of view or a collection efficiency and / or to configure a path of the optical signals, etc.Moreover, only part of the (further) optical assembly such as the first and second optical element can be arranged so as to be movable by the movement device or further optical components could be arranged so as to be movable as well.Hence, the optical-delay-line assembly according to the invention enables a change oradjustment of the relative optical paths between multiple sources of electromagnetic waves attached or otherwise connected to a single substrate by means of a movement device, forinstance comprising one or more flexures that are arranged or embedded in the substrate,and being exposed to energy such as laser radiation. However, it is likewise conceivable that the optical-delay-line assembly according to the invention can be configured for use in high-precision timing distribution and synchronization application, see also further below.That is, the optical-delay-line assembly can comprise a source of electromagnetic waves inthe form of an optical gain medium configured to generate the optical signal and the opticalassembly can comprise at least two optical reflective elements that are configured to reflect the optical signal. The optical gain medium and the two optical reflective elementspreferably define a laser resonator cavity. At least one of the optical reflective elements ispreferably mounted on the movement device, wherein a deformation of the movementdevice upon exposure to energy preferably causes a positional change of the opticalreflective element relative to the optical gain medium, thereby adjusting a cavity length ofthe laser resonator cavity.The laser resonator cavity is preferably configured to generate the optical signal as anoptical pulse train having a pulse repetition rate frep, and wherein the pule repetition ratefrep is dependent on the cavity length Lcav of the laser resonator cavity.To this end the optical-delay-line assembly comprises a source of electromagnetic wavesbeing an optical gain medium such as a laser crystal configured to generate the optical signal and that the optical assembly comprises at least one optical reflective element such as a SESAM that is configured to reflect the optical signal, wherein the optical gain medium and the optical reflective element together define a laser resonator cavity. The laser resonator cavity is preferably configured to generate the optical signal as an optical pulse train having a pulse repetition rate with very low timing jitter. To ensure the stability of the pulse repetition rate, the length of the laser resonator cavity defined by the optical assembly is preferably adjustable and stabilizes with respect to a reference signal.In particular, the optical-delay-line assembly is preferably configured such that a portion ofthe optical signal generated in the laser resonator cavity can propagate along the optical path and can exit the optical-delay-line assembly via an optical output. The pulse repetition rate of the optical signal is dependent on the effective length of the optical path, which in turn is determined by the resonator cavity length. The cavity length of the resonator cavity can be adjusted by exposing the movement device to energy, whereby the movement device at least on the region of exposure deforms, thereby causing a positional change of the optical reflective element relative to the optical gain medium. This deformation results in a change of the cavity length and thus in a corresponding change of the pulse repetition rate of the optical signal. Such a change may be permanent, for example to compensate for fabric tolerances, or dynamic and / or reversible, to compensate for environmental perturbations such as thermal expansion or contraction of the substrate, for instance.In another aspect, a method of manufacturing an optical-delay-line assembly preferably asdescribed above is provided. The method comprises the steps of i) providing a substrate,ii) providing an optical assembly, and iii) providing a movement device. The opticalassembly is arranged on the substrate and is configured to guide an optical signal generatedfrom a source of electromagnetic waves, preferably from an optical input of the substrate toan optical output of the substrate, along an optical path extending at least partially on thesubstrate. The movement device, when being exposed to energy, is configured to deformat least in the region of exposure to the energy, whereby at least part of the movement device is movable with respect to the substrate and configured to move at least part of the optical assembly with respect to the substrate, and whereby an optical path length of the optical path of the optical signal is changeable. Any statements made herein with regard to the optical-delay-line assembly preferably likewise apply to the method of manufacturing the optical-delay-line assembly and vice versa. In another aspect, a method of changing an optical delay of optical signals is provided. The method comprises the steps of i) providing an optical-delay-line assembly as described and / or as manufactured above, and ii) exposing at least part of the movement device toenergy, wherein the movement device, at least in the region of exposure to the energy, isdeformed, whereby at least part of the optical assembly is moved with respect to thesubstrate, and whereby an optical path length of the optical path of the optical signal ischanged. Any statements made herein with regard to the optical-delay-line assembly and the method of manufacturing the optical-delay-line assembly preferably likewise apply to the method of changing an optical delay of optical signals and vice versa.In another aspect, an optical system is provided, wherein the optical system comprise anoptical-delay-line assembly as described and / or as manufactured above.The optical system can be a laser system, a metrology system comprising a spectrometer,an optical time reference system, an optical telecommunication system, or a beam coherent combining system. As such, the optical-delay-line-assembly allows a variety of applications. Moreover, the optical-delay-line assembly can be tuned, i.e. beam path length is adjusted, when being mounted in the optical system. i.e. an offsite tuning can be dispensed with. This is particularly advantageous as it bypasses the risk of a misalignment during the mounting of the optical-delay-line assembly into a bigger system. For instance, it is known that simple operation such as gluing, bolting, screwing, etc. can introduce stress on the element being mounted, which, when it is a very sensitive equipment like a preset optical-delay-line can induce undesired displacement and detuning in the optical system. For instance, the optical-delay-line-assembly can be used for Time-division multiplexing (TDM), a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. To this end, the optical outputs of the optical signal and one or more further optical signals from the delay-line-assembly are preferably funneled on a same optical path i.e., spatially, with a certaindegree of timing between the optical signals that is controlled, i.e. adjusted or changed viathe movement device of the delay-line-assembly. Another conceivable application of the optical-delay-line assembly is Wavelength-divisionmultiplexing (WDM), a wherein the optical signal and one or more further optical signalsserve the purpose of optical carrier signals that are multiplexed onto the same optical pathby using different wavelengths (i.e., colors) of laser light. Another conceivable application of the optical-delay-line assembly is frequency-division multiplexing: Similar to WDM, but using optical signals of different frequencies of laser light. To this end it is preferred that frequency does not refer to the instant optical frequency of the laser light, but more to the radiofrequency of the envelope which modulates the laser light. Another conceivable application of the optical-delay-line assembly is dual frequency combs:A frequency comb can be generated by a source of electromagnetic waves in the form of acoherent pulsed laser source that is characterized by an optical spectrum and RF spectrumconsisting in discrete equally spaced lines. A dual comb consists of the interleaving of two such frequency combs. They can be generated by two sources of electromagnetic radiations, i.e. two laser sources, or within a single specially engineered laser source. Another conceivable application of the optical-delay-line assembly is pump-probe: A particular case of a two-steps process. To this end it is preferred that the optical-delay-lineassembly is used for a sequential application of laser pulses. In particular, the source ofelectromagnetic waves provides a first laser pulse to pump / excite a system, combined witha second pulse applied to probe / interrogate a target system on its physical state. Theexperiment is preferably repeated and recorded several times to scan the second pulsedelay via the optical-delay-line assembly, so that it arrives before, during and after the pumppulse. Another conceivable application of the optical-delay-line assembly is seed-write: A two-step process, sort of similar to pump-probe, wherein the first step (e.g., a pulse) prepares the physical system to be investigation and the second step (e.g., a burst of pulses) modifiessaid system. The first step favors the effect of the second step.Another conceivable application of the optical-delay-line assembly is coherent beam combining (for power scaling), wherein optical signals of N similar sources of electromagnetic waves in the form of laser sources are combined so as to obtain a single optical signal not only with correspondingly higher power (N^2 instead of simply N, due to constructive interferences) but also with approximately preserved beam quality and thus with increased radiance (brightness). Coherent combining also preserves the spectral bandwidth. Another conceivable application of the optical-delay-line assembly is spectral beamcombining (for power scaling). Similar to coherent combining, except that the N lasersources have non-overlapping optical spectra and are combined with some kind of wavelength-sensitive beam combiner (e.g., a diffraction grating). Another conceivable application of the optical-delay-line assembly is for dual frequency combs that are used in various applications, including spectroscopy. Thanks to the high- precision optical delay line principle and its ability to be made compact, it offers an accurate means to calibrate the instrument for given beat-notes and / or to achieve carrier-envelope phase stabilization. Another conceivable application of the optical-delay-line assembly is for closed-loop stabilization in a laser system. In this case, the optical system preferably comprises the delay-line-assembly as described earlier, i.e. comprising an optical gain medium and optical reflective element defining a laser resonator cavity configured to generate the optical signal as an optical pulse train. In this case the optical system preferably further comprises at least one detection device configured to receive at least a portion of the optical pulse train and to generate a detection signal indicative of the pulse repetition rate. The optical system preferably further comprises at least one reference signal source configured to generate a reference signal, a comparison device configured to compare the detection signal with the reference signal and to generate a deviation signal indicative of a difference in frequency or phase between thedetection signal and the reference signal, and a control unit configured to generate a controlsignal based on the deviation signal. A position of the at least one optical reflective elementrelative to the optical gain medium is preferably adjustable via the movement device and based on the control signal preferably such, that a frequency difference between the optical pulse train and the reference signal is minimized.That is, the optical-delay-line assembly can be in connection with a detection device suchas a photodetector configured to receive at least a portion of the optical signal and to generate a detection signal indicative of the pulse repetition rate. The optical-delay-line assembly can further be in connection with at least one comparison device configured to compare the detection signal with a reference signal generated from a reference signal source such as a maser clock or a frequency standard. The optical-delay-line assembly can be in connection with said reference signal source. The comparison device is preferably configured to generate a deviation signal indicative of a difference in phase and / or frequency between the detection signal and the reference signal. The optical-delay-line assembly can further be in connection with at least one control unit configured to generate a control signal based on the deviation signal. The control unit and the comparison device can be two distinct components or can be provided by a single, i.e. common component. The control signal is preferably used to deform the movement device to adjust the cavity length so as to minimize the deviation. For instance, the control unit can be configured to transmit its control signal to the source of energy that in turn exposes its energy to the movement device based on the control signal. The optical-delay-line assembly may thus operate as a master source, wherein the optical pulse train serves as a reference for downstream systems, or as a slave oscillator, wherein the repetition rate is actively synchronized to an external timing or frequency reference. In both cases, the optical path inside the optical assembly may be modified either statically, e.g. plastically, to set a desired cavity length, or dynamically, e.g. elastically, to track and compensate for time-varying fluctuations. This enables precise and robust stabilization of the temporal spacing between pulses within the optical signal. In other words, the optical-delay-line assembly provides a cavity length / repetition rate stabilization to adjust the repetition rate onto a given value with an internal or external time / frequency reference to make sure that an optical system such as a laser that comprises the optical-delay-line assembly can output at a fixed repetition rate against this reference and / or can actively compensate fluctuations / variations. To this end the movement device can vary the pulse repetition rate / the time periodsbetween pulses, inside the optical signal itself. In other words, the optical signal does notneed to be split in two or more parts and one part being delayed with respect to the other(s) by changing the optical path. Rather, the optical signal can be altered by the change of optical path length. BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,Fig. 1 shows a schematic top view of an optical-delay-line assembly according to theinvention;Fig. 2 shows a schematic top view of another optical-delay-line assembly according tothe invention;Fig. 3 shows a graph depicting first and second optical signals of continuous sourcesof electromagnetic waves exhibiting a relative phase difference that has been generated with an optical-delay-line assembly according to the invention;Fig. 4 shows a graph depicting first and second optical signals of pulsed sources ofelectromagnetic waves exhibiting a relative difference between their pulse arrival times generated with an optical-delay-line assembly according to theinvention;Fig. 5 shows a partial top view on a substrate comprising an optical-delay-line assembly according to the invention that comprises a moving device comprising a piezoelectric actuator, wherein different voltages are applied to the piezoelectric actuator resulting in different deformations ((a) no voltage; (b) voltage for half deformation; (c) voltage for maximum deformation);Fig. 6 shows a partial top view on a substrate comprising an optical-delay-lineassembly according to the invention that comprises a moving device comprising an electrostatic actuator, wherein different voltages are applied to the electrostatic actuator resulting in different deformations ((a) no voltage; (b) voltage for half movement; (c) voltage for maximum movement);Fig. 7 shows a schematic of an optical system comprising a delay-line-assemblyaccording to the invention, wherein the delay-line-assembly is configured as a laser resonator cavity in a closed-loop stabilization. DESCRIPTION OF PREFERRED EMBODIMENTS Aspects of the invention are now illustrated with reference to the figures. In particular, figures 1 and 2 in each case depict an optical-delay-line assembly 1 comprisinga substrate 2, an optical assembly 3, and a movement device 4. The optical assembly 3 isarranged on the substrate 2 and is configured to guide an optical signal S1 generated from a source of electromagnetic waves 5 from an optical input 6 of the substrate 2 to an optical output 7 of the substrate 2 along an optical path P1. In the depicted examples, the optical assembly 3 comprises a first optical element 9 and asecond optical element 10 in the form of mirrors. The first optical element 9 is configured toreceive the optical signal S1 from the optical input 6 and to transmit the optical signal S1 to the second optical element 10. The second optical element 10 is configured to receive the optical signal S1 from the first optical element 9 and to transmit the optical signal S1 to the optical output 7. In the depicted examples, the optical assembly 3 comprises additionaloptical components in the form of a collimating lens 15 and two mirrors 16; 17 for guidingthe optical signal S1 to the first optical element 9 and for guiding the optical signal S1received from the second optical element 19 to the optical output 7.In the depicted examples the assembly 1 comprises a further optical assembly 11 beingconfigured to guide a further optical signal S2 from a further optical input 12 of the substrate 2 to a further optical output 13 of the substrate 2 along a further optical path P2. Said further optical assembly 11 comprises an optical element 18 in the form of a collimating lens. In the depicted examples, the substrate 2 is a monolithic glass substrate, wherein the movement device 4 is integrated into the substrate 2, in particular within a volume of thesubstrate 2. Moreover, the optical assembly 3 and the further optical assembly 11 are inconnection with the movement device 4 by being attached such as bonded, glued or weldedto the substrate 2 in the region of the movement device 4. In addition, the optical assembly3 and the further optical assembly 11 are in connection with the substrate 2 by beingattached such as bonded, glued or welded to the substrate 2.The assembly 1 comprises a source of electromagnetic waves 5 in the form of a laser thatis part of the assembly 1, preferably attached such as bonded, glued or welded to the substrate 2, and wherein said source of electromagnetic waves 5 generates the opticalsignal S1. The assembly 1 comprises a further source of electromagnetic waves 14 beingconfigured to generate the further optical signal S2, wherein said further source ofelectromagnetic waves 14 is again a laser that is part of the assembly 1, preferably attachedor bonded or welded or glued to the substrate 2.The movement device 4, when being exposed to energy, is configured to deform at least inthe region of exposure to the energy, whereby at least part of the movement device 4 ismovable with respect to the substrate 2 and configured to move at least part of the opticalassembly 3 with respect to the substrate 2, and whereby an optical path length L1 of theoptical path P1 of the optical signal S1 is changeable. In the depicted examples, the sourceof energy is a laser being provided externally from the assembly 1 (not shown). Said lasercould be arranged, for instance, facing a top side 19 of the substrate 2 so as to irradiate itslaser radiation onto the top side 19 of the substrate 2.The movement device 4, upon exposure to energy, is configured to deform at least in the region of exposure to the energy at least one of: permanently, plastically, temporarily, orelastically. In particular, upon exposure to energy, the movement device 4 exhibits at leastin the region of exposure to the energy a volume expansion and / or a volume shrinkage. Asa result, the optical path length L1 associated with the optical signal S1 is either shortened or elongated by a quantity proportional to the volume exposed to the laser radiation, or in other words regions of the compliant mechanism 4 being exposed to energy are, designatedin figure 2 with the two rectangular boxes b1; b2 serving the purpose of illustration only and not being a structural element of the assembly 1. A translation of a distance ∆ of themovement device 4 causes a shortening or expansion of 2∆ of the optical path length L1.As the linear expansion can be as small as a few nanometers or smaller, the resulting time delay between the optical signal S1 and the further optical signal S2 can be virtually infinitely small.In the depicted examples, the movement device 4 consist of a compliant mechanism. Infigure 1, said compliant mechanism 4 is indicated schematically by means of the rectangularbox. Figure 2 however depicts a compliant mechanism 4 of a conceivable design. In fact,the compliant mechanism 4 comprises a deforming part 20 in the form of a U-shape beingexposed to energy here in the form of laser radiation. In response to the energy exposure,the U-shaped part 20 of the compliant mechanism 4 can either shrink or expand and thuspull or push a non-deforming part 21 of the compliant mechanism 4 here in the form of aparallelogram causing a linear translation of two to the optical elements 9; 10 in the form ofmirrors arranged in the region of said non-deforming part 21 of the movement device 4. Inthe depicted example the U-shaped part 20 of the compliant mechanism 4 permanentlydeforms upon exposure to energy, whereas the part 21 of the compliant mechanism 4 in theform of the parallelogram is non-permanently deformable but however remains deformedbecause the U-shaped part 20 of the movement device 4 remains deformed.As indicated in figure 1 and 2, the movement device 4 is configured to perform a linear movement with respect to the substrate 2, whereby at least part of the optical assembly 3 is linearly movable with respect to the substrate 2. In particular, the movement device 4 and the optical assembly 3 are configured and arranged such, that a movement of the movement device 4 by a distance ∆ causes a shortening or expansion of a distance 2n∆ of the optical path length L1 of the optical signal S1, wherein n is an integer and a number of passes ofthe optical signal through the movement device that corresponds here to n=1.In fact, the assembly 1 is configured such, that a deformation of the movement device 4changes the optical path length L1 of the optical signal S1 and a further optical path length L2 of the further optical signal S2 with respect to one another. In particular, the assembly 1 is configured such that a change of the optical path length L1 of the optical signal S1generated from a continuous laser sources results in a phase delay between the opticalsignal S1 and the further optical signal S2 as depicted in figure 3 or in a change of an arrivaltime of the optical signal S1 at the optical output 7 and thus in a time delay between theoptical signal S1 and the further optical signal S2 as depicted in figure 4. Whether a volume expansion or shrinkage is achieved depends on the desired delay oncewishes to introduce between the two optical signals S1; S2. For instance, if one wants totemporarily shift the optical signal S 1 (pulse train 1) with respect to optical signal S 2 (pulsetrain 2), so that pulses 1 arrive first, the movement device 4 is preferably deformed suchthat the optical beam path length L1 is shortened with respect to the further optical beampath length L2. If one wishes the opposite delay, i.e. pulse 2 arriving before pulse 1, themovement device 4 is deformed such that the optical path length L1 is made longer.Whether a permanent or temporary deformation of the movement device 4 is achievedpreferably depends on the final application of the optical-delay-line assembly 1.To visualize how the invention is realized step-by-step, let us consider the example illustrated in Figure 2. There, the substrate 2 is entirely monolithic and is for instance made out of fused silica. To realize such substrate, one can use a process combining non-ablative femtosecond laser exposure and chemical etching. This process is described in detail in: Y. Bellouard, A. Said, M. Dugan, and P. Bado, "Fabrication of high-aspect ratio, micro-fluidic channels and tunnels using femtosecond laser pulses and chemical etching," Opt. Express12, 2120–2129 (2004) https: / / doi.org / 10.1364 / OPEX.12.002120. In practice, the contour ofthe movement device 4 will be exposed so that after etching the volume defined by thecontour is removed. At this stage the movable part of the movement device 4, i.e. the non-deforming part of movement device 21 and its actuating means, i.e. the deforming part ofmovement device 20 are made as one element in the volume of the substrate 2 and is readyto be used, but is in its undeformed state after machining. In the illustration the movablepart, i.e. the deforming part 21 of the movement device 4 is a simple one degree-of-freedomlinear guidance made with four hinges. Other more complex design of compliant mechanism can be considered, such as a compensated linear guidance with eight hinges or other remote linear guidance such as Hocken guidance as discussed in: V. Tielen and Y. Bellouard, "Three-Dimensional Glass Monolithic Micro-Flexure Fabricated by Femtosecond Laser Exposure and Chemical Etching," Micromachines 5, 697–710 (2014) https: / / doi.org / 10.3390 / mi5030697. There are many possible design options that are known to the practitioner of precising engineering design. A usual key design objective for thedeforming part of the movement device 20 that guides the motion is to achieve a highstiffness contrast between the direction of motion and other unwanted off-axis directions.The lowest stiffness being selected for the desired direction of motion.Note that at the same time other components (not shown in the drawing) can also beexposed and etched. These components can be receptacles in the form of precision cavitiesto receive optical components, laser sources, sensors, holes for attaching the substrate onto something else, vias for allowing electrical cabling, in-volume marking for device labelling or any other useful elements that may be needed or that may ease the fabrication of the system. These steps are optional and depend on the final application and on the methods used for attaching components to the substrate. However, this ability to machine on the same substrate additional features of very different nature, and along with themovement device 4, illustrates a particular benefit of this invention.Once the substrate has been manufactured, the following step is to assemble the components on it. These components can be inserted, clamped, bolted, glued, bonded through optical-contacting process or using whatever suitable attachment method isdeemed practical in this case in order to attach the components to the substrate. Dependingon the intended function of the components, one or more of said components can beattached to the substrate in the region of the movable, i.e. deforming part, or the non-movable, i.e. non-deforming part of the movement device 4. Once all or the just the key components realizing the optical function are in place, the fine tuning of the optical-delay-line can be done. Note that this alignment step can also beperformed once the optical-delay-line-assembly 1 with all its components has itself bemounted into a larger optical system, such as a fully equipment or instrument assembly, like for instance a spectrometer instrument and a ready-to-be-used specialized laser source.In the illustration considered in Figure 2, the deforming part 21 of the movement device 4 isactuated by exposing the non-deforming part 20 of the movement device 4 to a femtosecondlaser, such as a volume expansion is observed after exposing a set of lines in the volumeof the material through nonlinear absorption phenomena b1; b2. This process is illustrated in: Y. Bellouard, "Non-contact sub-nanometer optical repositioning using femtosecondlasers," Optics Express 23, 29258 (2015). Using discrete radiation patterns such as linesused to scan the volume of the deforming part 21 of the movement device 4 to induce themotion has the advantage of fine controlling the displacement down to sub-nanometerscale, illustrating a key-advantage of the process. However, the use of discrete pattern is not a necessity, and a similar result could also be achieved by exposing the volume as a whole. Furthermore, in the example in Figure 2, one can use the same laser parameters topush or pull the deforming part of the movement device 20. The U-shaped deforming partof the movement device 20 could also consist of a single element. Back and forth motioncan then be generated by varying the laser parameters such as localized densification or expansion occurs. In practice, one would monitor how the optical signals S1 and S2 are evolving one with respect to the other, while adjusting the displacement of the movement device 4. This ensures that the system is tuned according to the function it is supposed to realize. This monitoring can for instance be done with high-speed photodetectors and for example, if it is two pulsed optical sources, by observing the pulse train on an oscilloscope. In thisexample, once the desired optical delay is achieved, the deforming part of the movementdevice 20 stays deformed as the actuating non-deforming part of the movement device 20has been permanently deformed. This is a key advantage of the invention, in particular forindustrialization, as tuned devices remain as such and permanently, without needing regularretuning steps as it is usually done with conventional delay-lines. It offers not only asubstantial gain in maintenance but is also well adapted for devices being deployed inremote locations not easily accessible for human interventions. An example of such environments are space environments.Figure 5a-c shows a partial top view on a substrate comprising an optical-delay-lineassembly 1 having a moving device 4 that comprises a piezoelectric actuator 22, whereindifferent voltages are applied to the piezoelectric actuator 22 resulting in differentdeformations δ. In particular, the movement device 4 comprises a piezoelectric actuator 22mounted to the substrate 2 and configured to undergo deformation δ upon exposure toenergy in the form of an applied voltage. The movement device 4 further comprises acompliant mechanism 23 comprising here two flexure elements 24, 24a connected in eachcase to the substrate 2 at a first pivot point 25 and a second pivot point 25a, respectively.Moreover, the movement device 4 comprises a transmission flexure element 26 connectingthe piezoelectric actuator 22 to a first point on the flexure element 24 at a distance s fromthe first pivot point 25. In addition, the movement device 4 comprises an optical platform 27coupled to a second point on the flexure elements 24, 24a at a distance l from the first pivotpoint 25 and the second pivot point 25a, wherein at least part of the optical assembly 3,here a SESAM mirror 28, is arranged on the optical platform 27. As follows from this figure,the movement device 4 is configured such that the deformation δ of the piezoelectricactuator 22 in response to an applied voltage results in a movement of the optical platform27 by a distance Δ = (l / s) • δ, whereby the optical path length L1 of the optical path P1 ofthe optical signal S1 is changed by a distance of 2Δ, wherein said deformation δ dependson the voltage being applied. In particular, when the piezoelectric actuator 22elongates / shrinks by an amount δ, the displacement is transmitted to the flexure element24 of the compliant mechanism 23 via the transmission flexure element 26. The piezoelectric actuator 22 is affixed to the substrate via an attachment 38 in the form of anadhesive element or laser welding, etc. A voltage such as a voltage bias is applied to thepiezoelectric actuator 22 via two electrical wires 39 that are in connection with the source of energy (not shown). Panel (a) depicts the piezoelectric actuator 22 in its rest state where no voltage is applied. Panel (b) depicts the piezoelectric actuator 22 upon the application of a voltage for half deformation, and panel (c) depicts the piezoelectric actuator 22 upon the application of a voltage for a maximum deformation, respectively. As follows from these panels, the compliant mechanism 23 is designed preloaded with an initial mechanical offset such that, when the piezoelectric actuator is in its rest state (panel a), the optical path length L1 of the optical path P1 of the optical signal S1 is increased by 2Δ_max, and under a maximum voltage being applied, is decreased by 2Δ_max, with Δ_max being a maximum achievabledistance Δ being moved under a specified voltage range.The movement device 4 comprises mechanical stop elements 29 positioned relative to thesubstrate 2 and the optical platform 27 such that the movement of the optical platform 27 isconstrained. Here, said stop elements 29 correspond to projections of the substrate 2towards the movement device 4 being arranged within a recess 30 formed in the substrate2. Figure 6a-c shows a partial top view on a substrate 2 comprising an optical-delay-line assembly 1 having a moving device 4 that comprises an electrostatic actuator 31, wherein different voltages are applied to the electrostatic actuator 31 resulting in different movements of the movement device 4. In particular, the electrostatic actuator 31 comprises a first set of comb-drive electrodes 32 mounted to the substrate 2, and a second set of comb-drive electrodes 33 connected to an optical platform 34 and interdigitated with the first set of comb-drive electrodes 32. The movement device 4 furthermore comprises acompliant mechanism 35 comprising here two flexure elements 36, 36a connected in eachcase via a pivot point to the substrate 2 and coupled to the optical platform 34. Part of theoptical assembly 3, here a SESAM mirror 37, is arranged on the optical platform 34. Themovement device 4 is configured such that, when voltage is applied to the first and secondsets of comb-drive electrodes 32; 33, an electrostatic attraction is generated between theinterdigitated first and second set of comb-drive electrodes 32; 33, thereby moving theoptical platform 34 by a distance Δ and changing the optical path length L1 of the opticalsignal S1 by a distance of 2Δ. Panel (a) depicts the electrostatic actuator 31 in its rest state where no voltage is applied. Panel (b) depicts the electrostatic actuator 31 upon the application of a voltage for half movement, and panel (c) depicts the electrostatic actuator 31 upon the application of a voltage for a maximum movement of the movement device 4, respectively. As follows from these panels, the compliant mechanism 35 is such that, when the electrostatic actuator 31 is in its rest state (panel a), the optical path length L1 of the optical path P1 of the opticalsignal S1 is reduced by 2Δ_max, and under a maximum voltage being applied, is increasedby 2Δ_max, with Δ_max being a maximum achievable distance Δ being moved under aspecified voltage range. In particular, when a voltage or voltage bias is applied betweenthe first and second set of comb-drive electrodes 32, 33, an electrostatic force is generatedbetween the interdigitated comb teeth of these comb-drive electrodes 32, 33 that attractsthe comb-drive electrodes 32, 33 and thereby causes the optical platform 34 to translaterelative to the substrate 2.Electrical contact pads 41 are preferably provided on the first and second set of comb-driveelectrodes 41, wherein a voltage can be applied across the first and second set of comb-drive electrodes using, for instance, electrical wires 42 that are bonded to the respectivecontact pads 41.Also in this embodiment the movement device 4 comprises mechanical stop elements 43 positioned relative to the substrate 2 and the optical platform 34 such that the movement of the optical platform 34is constrained. Here, said stop elements 29 correspond to projectionsformed on the substrate 2 that project towards the movement device 4 being arranged withina recess 44 formed in the substrate 2.Figure 7 shows a schematics of an optical system 100 in the form of a laser systemcomprising a delay-line-assembly 1 according to the invention, wherein the delay-line-assembly 1 is configured as a laser resonator cavity 103 in a closed-loop stabilization. Inparticular, the optical-delay-line assembly 1 comprises a source of electromagnetic waves5 in the form of an optical gain medium 101, here in the form of a crystal, that is configuredto generate the optical signal S1 when being pumped by a pumping laser (not shown) andthe optical assembly 3 comprises an optical reflective element 102 here in the form of aSESAM mirror that is configured to reflect the optical signal S1. The optical gain medium101 and the optical reflective element 102 define a laser resonator cavity 103. The opticalreflective element 102 is mounted on the movement device 4, wherein deformation of the movement device 4 upon exposure to energy causes a positional change of the optical reflective element 102 relative to the optical gain medium 101, thereby adjusting a cavity length Lcavof the laser resonator cavity 103.In the depicted example, the laser resonator cavity 103 is configured to generate the opticalsignal S1 as an optical pulse train having a pulse repetition rate frep, and wherein the pulserepetition rate frepis dependent on the cavity length Lcavof the laser resonator cavity 103.As furthermore follows from figure 7, the optical system 100 comprises a detection device104 configured to receive a portion of the optical pulse train and to generate a detectionsignal DS indicative of the pulse repetition rate frep.In particular, the optical signal S1 canexit the delay-line-assembly 1 via the output 7 and is then split by a beamsplitter 109,wherein part of the optical signal S1, i.e. the portion of the optical pulse train, then hits the detection device 104 in the form of an ultrafast photodiode. The optical pulse train generates a radio-frequency (RF) comb in the frequency domain,which teeth are equally spaced by a multiple of frep, i.e., frep: f_n = (n+1)•frep. The RF combcan be visualized on a monitoring device 108 e.g., oscilloscope.An envelope of the optical signal S1 is produced by the superposition of sinusoidal wavesin a Fourier transform, wherein 1 tooth in frequency domain corresponds to 1 sinusoidalwave in time domain. A frequency reference signal RS from a reference signal source 104provides another tooth, i.e. sinusoidal wave in the time domain. The goal is to lock theoptical signal S1 onto the reference signal RS, meaning no frequency difference at all,wherein the two (co)sine waves can be superimposed. The phase difference / delay isevaluated with a comparison device 105 in the form of a phase-frequency comparator, anda deviation signal DevS, in particular an error signal, is then generated that is here filteredwith a loop filter, and used as an input of a control unit 106 in the form of a driver, the outputof which is used as a control signal CS to move and control the movement device 4. Hence, and as indicated in figure 7, the optical system 100 furthermore comprises a reference signal source 104 configured to generate the reference signal RS, a comparison device 105 configured to compare the detection signal DS with the reference signal RS and to generate a deviation signal DevS indicative of a difference in frequency or phase between the detection signal DS and the reference signal RS, and a control unit 106 configured to generate a control signal CS based on the deviation signal DevS. A position of the optical reflective element 102 relative to the optical gain medium 101 is then adjusted via the movement device 4 and based on the control signal CS preferably such, that a frequency difference between the optical pulse train and the reference signal RS is minimized. This is commonly known as a «phase-locked loop».As indicated in figure 7, the movement of the movement device by the distance Δ (>0 or <0)changes the location of the SESAM mirror 102, thus modifying the span of the laserresonator cavity 103. More precisely, it shrinks or elongates the cavity length to Lcav + Δ =Lcav + δLcav, and the optical path length to L1 + 2Δ = L1 + δL1, which changes the pulserepetition rate frep’ = c / (2*(Lcav + 2Δ)) ≈ c / L1*(1 – 2Δ) ≈ frep– c / L1*2Δ ≈ frep– δfrep for Δ << Lcavand a period T’ = 2*(Lcav + Δ) / c = L1 / c + 2 Δ / c = T + δT.The other fraction of the optical signal S1 in the other channel of the beamsplitter 109 can be used for all sorts of applications.

[0002] LIST OF REFERENCE SIGNS1 optical-delay-line assembly2 substrate3 optical assembly4 movement device5 source of electromagnetic waves6 optical input7 optical output8 surface of substrate9 optical element10 optical element11 further optical assembly12 further optical input13 further optical output14 further source of electromagnetic waves15 optical element16 optical element17 optical element18 optical element19 top side of substrate20 deforming part of movement device21 non-deforming part of movement device22 piezoelectric actuator23 compliant mechanism24, 24a flexure element25, 25a pivot point26 transmission flexure element27 optical platform28 SESAM29 stop elements30 recess31 electrostatic actuator32 comb-drive electrodes33 comb-drive electrodes34 optical platform35 compliant mechanism36, 36a flexure element37 SESAM38 attachment39 electrical wires40, 40a pivot point41 contact pad42 electrical wire43 stop elements44 recess100 optical system101 optical gain medium102 optical reflective element103 laser resonator cavity104 reference signal source105 comparison device106 control unit107 loop filter108 monitoring device109 beamsplitterS1 optical signalS2 further optical signalP1 optical pathP2 further optical pathL1 optical path lengthL2 further optical path lengthb1 boxb2 boxδ deformation of piezoelectric actuators distancel distanceΔ distanceLcavcavity length freppulse repetition rateDS detection signalRS reference signalDevS deviation signalCS control signal

Claims

1. CLAIMS1. An optical-delay-line assembly (1) comprising:^ a substrate (2);^ an optical assembly (3); and^ a movement device (4),wherein the optical assembly (3) is arranged on the substrate (2) and isconfigured to guide an optical signal (S1) generated from a source ofelectromagnetic waves (5), preferably from an optical input (6) of the substrate (2)to an optical output (7) of the substrate (2), along an optical path (P1) extending atleast partially on the substrate (2), characterized in that the movement device (4), when being exposed toenergy, is configured to deform at least in the region of exposure to the energy,whereby at least part of the movement device (4) is movable with respectto the substrate (2) and configured to move at least part of the optical assembly (3)with respect to the substrate (2), andwhereby an optical path length (L1) of the optical path (P1) of the opticalsignal (S1) is changeable.

2. The assembly (1) according to claim 1, wherein the movement device (4), uponexposure to energy, is configured to deform at least in the region of exposure to the energyat least one of: permanently, plastically, temporarily, or elastically, and / orwherein the movement device (4), upon exposure to energy, is configured to exhibitat least in the region of exposure to the energy a volume expansion and / or a volume shrinkage, and / or wherein the movement device (4) comprises or consist of at least one compliantmechanism such as a flexure, and / or wherein the movement device (4), upon exposure to energy, is configured to undergo, at least in a region of exposure, a local change in shape such as at least one of a stretching, bending or twisting, and / or wherein the movement device (4), upon exposure to energy, is configured toundergo, at least locally, a deformation that excludes rigid-body motion.

3. The assembly (1) according to any one of the preceding claims, wherein themovement device (4) is transparent, and / orwherein the movement device (4) comprises or consists of at least one of silica, silicon,glass, ceramics, or glass-ceramics, germanium oxide, sapphire, metallic glass, metals,plastics, ULE glass, and / or wherein the movement device (4) is configured to exhibit a photo-induceddeformation such as a photoelastic effect when being exposed to energy, and / or wherein the movement device (4) is configured to exhibit a piezoelectric effect and / or a mechanical deformation when being exposed to energy.

4. The assembly (1) according to any one of the preceding claims, wherein themovement device (4) is configured to deform upon exposure to energy being in the form ofat least one of: electromagnetic radiation, an electrostatic field, an electric field, amechanical load, thermal energy, or a magnetic field and / or a change thereof, and / orwherein a source of energy configured to supply energy to the movement device (4)is provided externally from the assembly (1) or is part of the assembly (1).

5. The assembly (1) according to any one of the preceding claims, wherein at least oneof: the substrate (2) is monolithic, the substrate (2) is transparent, the substrate (2)comprises or consists of at least one of silica, silicon, glass, ceramics, a piezoelectric ceramics, a piezoelectric crystal, or glass-ceramics, and / or wherein at least one of: the movement device (4) is integrated into the substrate (2),or the movement device (4) is provided, in particular formed, within a volume of thesubstrate and / or in a surface (8) of the substrate (2).

6. The assembly (1) according to any one of the preceding claims, wherein themovement device (4) is configured to perform a linear movement with respect to thesubstrate (2), whereby at least part of the optical assembly (3) is linearly movable withrespect to the substrate (2), and / orwherein the movement device (4) and the optical assembly (3) are configured andarranged such, that a movement of the movement device (4) by a distance ∆ causes ashortening or expansion of a distance 2n∆ of the optical path length (L1) of the optical signal(S1), wherein n is an integer corresponding to a number of passes of the optical signal through the movement device, and / or wherein the movement of the movement device (4) and / or of the optical assembly(3) with respect to the substrate (2) is in the nanometer range or smaller.

7. The assembly (1) according to any one of the preceding claims, wherein the opticalassembly (3) is in connection with the movement device (4) preferably at least one of: viaone of recesses formed in the substrate in the region of the movement device that receiveat least part of the optical assembly, by being attached such as bonded, glued or welded to the substrate in the region of the movement device, and / or wherein the optical assembly (3) is in connection with the substrate (2) via one ormore recesses being formed in the substrate and / or by being attached such as bonded, glued or welded to the substrate.

8. The assembly (1) according to any one of the preceding claims, wherein the sourceof electromagnetic waves (5) is part of the assembly (1), preferably attached such asbonded, glued or welded to the substrate (2).

9. The assembly (1) according to any one of the preceding claims, wherein theassembly (1) is configured such that a change of the optical path length (L1) of the opticalsignal (S1) results in a change of the optical phase of the optical signal (S1) or in a changeof an arrival time of the optical signal (S1) at the optical output (7).

10. The assembly (1) according to any one of the preceding claims, wherein themovement device (4) comprises: ^an actuator (22) mounted to the substrate (2) and configured to undergo deformationδ upon exposure to energy,^ a compliant mechanism (23) comprising at least one flexure element (24) connectedto the substrate (2) and a transmission flexure element (26) connecting the actuator(22) to the flexure element (24), and^ an optical platform (27) coupled to the flexure element (24),wherein at least part of the optical assembly (3; 28) is arranged on the optical platform (27), and wherein the movement device (4) is configured such that the deformation δ of theactuator (22) results in a movement of the optical platform (27), whereby the optical path length (L1) of the optical path (P1) of the optical signal (S1) is changed.

11. The assembly (1) according to claim 10, wherein the flexure element (24) of thecompliant mechanism (23) is connected to the substrate (2) at a first pivot point (25) andthe transmission flexure element (26) connects the actuator (22) to a first point on the flexureelement (24) at a distance s from the first pivot point (25), wherein the optical platform (27) is coupled to a second point on the flexure element(24) at a distance l from the first pivot point (25), and wherein the movement device (4) is configured such that the deformation δ of theactuator (22) results in a movement of the optical platform (27) by a distance Δ = (l / s) • δ,whereby the optical path length (L1) of the optical path (P1) of the optical signal (S1) is changed by a distance of 2Δ.

12. The assembly (1) according to any one of claims 1 to 9, wherein the movementdevice (4) comprises: ^an electrostatic actuator (31) comprising:o a first set of comb-drive electrodes (32) mounted to the substrate (2), ando a second set of comb-drive electrodes (33) connected to an optical platform(34) and interdigitated with the first set of comb-drive electrodes (32), and^ a compliant mechanism (35) comprising at least one flexure element (36) connectedto the substrate (2) and coupled to the optical platform (34),wherein at least part of the optical assembly (3; 37) is arranged on the optical platform (34), andwherein the movement device (4) is configured such that, when voltage isapplied to the first and second sets of comb-drive electrodes (32; 33), an electrostaticattraction is generated between the interdigitated first and second set of comb-drive electrodes (32; 33), thereby moving the optical platform (34) by a distance Δ andchanging the optical path length (L1) of the optical signal (S1) by a distance of 2Δ.

13. The assembly (1) according to any one of the preceding claims, wherein the opticalassembly (3) comprises at least a first optical element (9) and a second optical element(10), wherein the first optical element (9) is configured to receive the optical signal (S1)from the optical input (6) and to transmit the optical signal (S1) to the second optical element(10), wherein the second optical element (10) is configured to receive the optical signal(S1) from the first optical element (9) and to transmit the optical signal (S1) to the opticaloutput (7), and wherein the first and second optical elements (9; 10) preferably are mirrors.

14. The assembly (1) according to any one of the preceding claims, wherein theassembly (1) comprises a further optical assembly (11) being configured to guide a furtheroptical signal (S2) from a further optical input (12) of the substrate) (2) to a further opticaloutput (13) of the substrate (2) along a further optical path (P2), andwherein the assembly (1) is configured such, that a deformation of the movementdevice (4) changes the optical path length (L1) of the optical signal and a further opticalpath length (L2) of the further optical signal with respect to one another.

15. The assembly (1) according to any one of the preceding claims, wherein at least onefurther optical signal (S2) is generated from the source of electromagnetic waves (5)preferably by splitting the optical signal (S1), orwherein the assembly (1) comprises at least one further source of electromagneticwaves (14) being configured to generate at least one further optical signal (S2), the furthersource of electromagnetic waves (14) preferably being part of the assembly (1), particularlypreferably attached or bonded or welded or glued to the substrate (2).

16. The assembly (1) according to any one of claims 1 to 12, wherein the assembly (1)comprises a source of electromagnetic waves (5) in the form of an optical gain medium (101) configured to generate the optical signal (S1) and the optical assembly (3) comprisesat least two optical reflective elements (102) that are configured to reflect the optical signal(S1), wherein the optical gain medium (101) and the two optical reflective elements (102)define a laser resonator cavity (103),wherein at least one of the two optical reflective elements (102) is mounted on themovement device (4), wherein deformation of the movement device (4) upon exposure to energy causesa positional change of the at least one optical reflective element (102) relative to the opticalgain medium (101), thereby adjusting a cavity length (Lcav) of the laser resonator cavity(103).

17. The assembly (1) according to claim 16, wherein the laser resonator cavity (103) isconfigured to generate the optical signal (S1) as an optical pulse train having a pulserepetition rate (frep), and wherein the pule repetition rate (frep) is dependent on the cavity length (Lcav) of thelaser resonator cavity (103).

18. A method of manufacturing an optical-delay-line assembly (1) preferably as claimedin any one of the preceding claims, the method comprising the steps of: ^Providing a substrate (2);^ Providing an optical assembly (3); and^ Providing a movement device (4),wherein the optical assembly (3) is arranged on the substrate (2) and isconfigured to guide an optical signal (S1) generated from a source ofelectromagnetic waves (5), preferably from an optical input (6) of the substrate (2)to an optical output (7) of the substrate (2), along an optical path (P1) extending atleast partially on the substrate (2), characterized in that the movement device (4) is configured to deform whenbeing exposed to energy, whereby at least part of the movement device (4) is moved with respect tothe substrate (2) and moves at least part of the optical assembly (3) with respect tothe substrate (2), andwhereby an optical path length (L1) of the optical path (P1) of the opticalsignal (S1) is changed.

19. A method of changing an optical delay of optical signals (S1), the method comprisingthe steps of: ^Providing an optical-delay-line assembly (1) as claimed in any one of claims 1 to 17and / or as manufactured in claim 18, and ^Exposing at least part of the movement device (4) to energy,wherein the movement device (4), at least in the region of exposure to theenergy, is deformed, whereby at least part of the optical assembly (3) is moved with respect tothe substrate (2), andwhereby the optical path length (L1) of the optical path (P1) of the opticalsignal (S1) is changed.

20. An optical system (100) comprising an optical-delay-line assembly (1) as claimed inany one of claims 1 to 17 and / or as manufactured in claim 18, the optical system preferablybeing a laser system, a metrology system comprising a spectrometer, an optical time reference system, an optical telecommunication system, or a beam coherent combining system.

21. The optical system (100) according to claim 20 comprising the assembly (1) asclaimed in claim 17 and further comprising:^ at least one detection device (104) configured to receive at least a portion of theoptical pulse train and to generate a detection signal (DS) indicative of the pulserepetition rate (frep), ^at least one reference signal source (104) configured to generate a reference signal(RS), ^a comparison device (105) configured to compare the detection signal (DS) with thereference signal (RS) and to generate a deviation signal (DevS) indicative of a difference in frequency or phase between the detection signal (DS) and the reference signal (RS), and^ a control unit (106) configured to generate a control signal (CS) based on thedeviation signal (DevS), andwherein a position of the at least one optical reflective element (102) relative to theoptical gain medium (101) is adjustable via the movement device (4) and based on the control signal (CS) preferably such, that a frequency difference between the optical pulse train and the reference signal (RS) is minimized.

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