NL2028161B1

NL2028161B1 - Device for moving an optical element

Info

Publication number: NL2028161B1
Application number: NL2028161A
Authority: NL
Inventors: Maarten Schneider Ronald
Original assignee: Mi Partners Bv
Priority date: 2021-05-06
Filing date: 2021-05-06
Publication date: 2022-11-23

Abstract

A device for moving an optical element, comprises a base, as well as a main frame rotatably connected to the base. The device further comprises a mounting frame on which an optical element is mounted and moving means for moving the mounting frame with respect to the main frame. The device further comprises a sub frame connected to the main frame via first springs and being displaceable with respect to the main frame in only one direction at right angle to the axis of rotation of the main frame, wherein the mounting frame is connected to the sub frame via second springs and being movable with respect to the sub frame. The device further comprises a balance mass connected to the sub frame via third springs and being movable with respect to the sub frame in the same directions of the degrees of freedom as the mounting frame, wherein the balance mass is connected to the mounting frame via the moving means.

Description

Device for moving an optical element DESCRIPTION:

Technical filed of the invention The invention relates to a device for moving an optical element, comprising: - a base, - a mounting frame on which the optical element is mounted and which is movably connected to the base, and - moving means for moving the mounting frame relative to the base.

Background of the invention Such a device is generally known, for example as a monochromator.

A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input.

A device that can produce monochromatic light has many uses in science and in optics because many optical characteristics of a material are dependent on wavelength.

Although there are a number of useful ways to select a narrow band of wavelengths (which, in the visible range, is perceived as a pure color), there are not as many other ways to easily select any wavelength band from a wide range.

See below for a discussion of some of the uses of monochromators.

In hard X-ray and neutron optics, crystal monochromators are used to define wave conditions on the instruments.

A monochromator can use either the phenomenon of optical dispersion in a prism, or that of diffraction using a diffraction grating, to spatially separate the colors of light.

It usually has a mechanism for directing the selected color to an exit slit.

Usually the grating or the prism is used in a reflective mode.

A reflective prism is made by making a right triangle prism (typically, half of an equilateral prism) with one side mirrored.

The light enters through the hypotenuse face and is reflected back through it,

being refracted twice at the same surface. The total refraction, and the total dispersion, is the same as would occur if an equilateral prism were used in transmission mode. Studies with x-rays allow for the investigation of properties of matter that may be inaccessible by other means. Methods using absorption, transmission, fluorescence, scattering and diffraction provide information of composition and structure of matter, as well as images and tomographies. These methods are often non- destructive and make an important set of tools for research, complementing many other methods of chemical analyses and conventional microscopy. The synchrotron light sources are research facilities in which radiation of particular properties is generated, of which broad spectra and high flux can be emphasized. A synchrotron light source is a source of electromagnetic radiation (EM) usually produced by a storage ring, for scientific and technical purposes. First observed in synchrotrons, synchrotron light is now produced by storage rings and other specialized particle accelerators, typically accelerating electrons. Once the high-energy electron beam has been generated, it is directed into auxiliary components such as bending magnets and insertion devices (undulators or wigglers) in storage rings and free electron lasers. These supply the strong magnetic fields perpendicular to the beam which are needed to convert high energy electrons into photons.

The major applications of synchrotron light are in condensed matter physics, materials science, biology and medicine. A large fraction of experiments using synchrotron light involve probing the structure of matter from the sub-nanometer level of electronic structure to the micrometer and millimeter level important in medical imaging. An example of a practical industrial application is the manufacturing of microstructures.

From 2016 on the new generation of light sources start to operate. These new sources are characterized by ultralow emittance. in which the diffraction limit is reached by the photon beams of low energies. The low emittance is related to very small sources, in the order of micrometers, and highly collimated beams. Due to those high demands the limits of stability of both the accelerator of electrons and the experimental stations, the so called beamlines, are pushed to extreme levels.

Summary of the invention

It is an object of the present invention to provide a beamlight device which meets the high demands due to the use of the new generation of light sources. To this end the device according to the invention is characterized in that it further comprises: - at least one balance mass movably connected to the mounting frame and being movable with respect to the base in one or more of the directions of the degrees of freedom as the mounting frame, wherein the moving means move the balance mass relative to the mounting means, - a beam positioning monitor for monitoring a beam leaving the optical element, and - a main feedback loop comprising the beam positioning monitor and the moving means wherein the information from the beam positioning monitor is used for controlling the moving means.

With the device according to the invention active feedback can be improved (larger bandwidth) using the balance mass. In the device according to the invention the critical degrees of freedom are controlled by a feedback loop with a sufficiently high close loop bandwidth with a correspondingly high disturbance rejection and set point (Bragg angle) tracking performance. To achieve the necessary control bandwidth (typical > 100 Hz) a dynamic architecture is used in which a dynamic low-pass filter is applied in the reaction path, such that only the forward path of the control-loop is limiting the reachable close loop bandwidth. And as such only the dynamics of the mechanical components in the forward path are critical in the mechanical design of the overall device. The dynamic low-pass filter is established by a balance mass (also called reaction mass) and actuators with a low stiffness (inherent compliant actuators).

An embodiment of the device according to the invention is characterized in that the device further comprises a main frame movably connected to the base and main frame moving means for moving the main frame with respect to the base. Preferably the main frame being rotatable around a horizontal axis relative to the base and the main frame moving means are rotating means for rotating the main frame.

A further embodiment of the device according to the invention is characterized in that it comprises a sub frame movably connected to the main frame, wherein the mounting frame and the balance mass are movably connected to the sub frame. Preferably the maximum distance the sub frame can be displaced relative to the main frame is greater than the maximum distance the mounting frame and the balance mass can be displaced relative to the sub frame.

In yet a further embodiment of the device according to the invention the sub frame is connected to the main frame via first spring means which are such that the sub frame being displaceable with respect to the main frame in only one direction at right angle to the axis of rotation of the main frame.

Preferably, the mounting frame is connected to the sub frame via second spring means.

The balance mass is, preferably, connected to the sub frame via third spring means. The composite stiffness of the second leaf spring means in the displacement direction is, preferably, equal to the composite stiffness of the third leaf spring means in the displacement direction.

The first, second and/or third spring means, preferably, each comprises at least one leaf spring and, preferably, the moving means are voice coils.

A further embodiment of the device according to the invention is characterized in that the sub frame being displaceable with respect to the main frame in only one direction at right angle to the axis of rotation of the main frame, said first leaf springs are weak in the displacement direction and stiff in two other directions perpendicular to the displacement direction and perpendicular to each other.

The balance mass preferably being displaceable with respect to the sub frame in only one direction parallel to the displacement direction of the sub frame, and said second leaf springs are weak in the displacement direction and stiff in two other directions perpendicular to the displacement direction and perpendicular to each other.

Preferably, the mounting frame being displaceable with respect to the sub frame in only one direction parallel to the displacement direction of the sub frame, and said third leaf springs are weak in the displacement direction and stiff in two other directions perpendicular to the displacement direction and perpendicular to each other, and being rotatable around two rotating axis perpendicular to said displacement direction.

The main disadvantage of elastic elements (second and third leaf springs) is the small stroke that is typically allowed, typically limited to a few millimeters. Less elastic elements (first leaf springs) do not present such limitations. Therefore, considering that extreme performances are possible only with elastic elements, it 1s suggested that the translation stages should be made of a two-level system when more than a few millimeters is necessary. For long strokes, elastic systems with low stiffness leaf springs with different characteristics can be coupled to each other, so that fine adjustment can be associated to coarser motion systems. Thus it is preferred to have a two-level system of elastic guides (leaf springs).

5 Yet a further embodiment of the device according to the invention is characterized in that the device further comprises a further mounting frame movably connected to the main frame and a further optical element mounted on the further mounting frame, wherein the light beam also passes this further optical element.

Preferably, the device comprises a number of interferometers measuring the position of the mounting frame relative to the further mounting frame, and a local feedback loop comprising the interferometer and the moving means to control the relative position of the mounting frames relative to each other.

The device according to the invention is especially advantageous in the embodiment of a double crystal monochromator wherein the optical elements are crystals, one of which is rigidly attached to the mounting frame and the other is rigidly attached to the further mounting frame. It is common for two crystals to be connected in series, with their mechanical systems operating in tandem so that they both select the same color. This arrangement is not intended to improve the narrowness of the spectrum, but rather to lower the cutoff level. A double crystal monochromator may have a cutoff about one millionth of the peak value, the product of the two cutoffs of the individual sections. The intensity of the light of other colors in the exit beam is referred to as the stray light level and is the most critical specification of a monochromator for many uses. Achieving low stray light is a large part of the art of making a practical monochromator.

Double crystal monochromators (DCMs) are typically used at X-rays beamlines of energy above a few keV. The principle is that a given condition of Bragg diffraction is achieved in two subsequent crystals, so that a narrow band of energy is selected from a multi-energetic incident beam and the outgoing monochromatic beam is maintained at a constant position. The schematic is shown in Figure 1.

In hard X-rays monochromators the energy selection is related to the incidence angle of the photon beam on the diffraction crystals (so called Bragg angle). So, the change in the selected energy of an experiment is given by the rotation of the crystals with respect to the incident beam. The basic concept of a DCM is allowing that the outgoing monochromatic beam be kept at a constant position regardless the angle of the DCM. This means that the separation, or gap, between the two crystals must be correspondently changed as a function of the rotation angle, as depicted in Figures 2A and 2B which show an example of an DCM where to axis of rotation (Bragg angle rotation) is in the face of the first crystal and crossing the incident beam. To achieve a constant position of the outgoing beam, the distance between first and the second crystal need to change as function of the rotation (e.g. in Fig. 2A, where higher energies are selected then in Fig. 2B, the crystals are more close).

As any mechanical motion is inevitably contaminated by positioning errors and parasitic motion, the DCMs typically rely on additional mechanisms for angular correction in two axes, called pitch and roll (after aviation). These mechanisms must guarantee the diffraction condition on both crystals and a stable parallelism between them. They are typically based on micromotors and/or piezoelectric actuators and use feedback signals to compensate assembly errors, thermal effects and imperfections in the translation mechanism (Fig. 4). So, the basic DCM has at least three relative degrees of freedom between the two crystals, namely: gap, pitch and roll, which may be concentrated in one of the crystals or separate according to any desired combination.

The present technologies of the DCMs are limited to keep the average stability of parallelism between crystals in the range of 150 nrad A few instruments have proven to achieve 50 nrad, but only while the rotating system is kept steady, i.e., for a determined energy selection. In addition, due to gravity effects, horizontal deflecting monochromators typically show superior stability performance.

For the new light sources, however, owing to the exceptional characteristics of the photon source, this stability in the parallelism between the two crystals must be improved to levels below 10 nrad, even during energy change to allow the so called flyscan experiments.

During the past years, much effort in incremental progress with respect to the existing technology has brought about only limited evolution towards the ever higher stability demands.

Indeed, the present technology is based in designs with focus on maximum mechanical stiffness. Typical a layout of stacked axes is used: individual translational and rotational stages stacked together to achieve the necessary motion degrees of freedom (gap, pitch and roll).

This approach is, however, intrinsically limited by the stiffness of each part and connection, which cannot be infinite. Besides, the levels of stability are so demanding that the capacity of disturbance (noise) rejection must be very high. Typically a close loop bandwidth of more than 100 Hz is required. These disturbances come from the ground (floor vibrations) or vibration sources nearby, as vacuum pumps, but may also be generated inside the monochromator by the cooling subsystem, motors and actuators, and linear guides and bearings.

The embodiment of the device according to the invention as a double crystal monochomator having active feedback meets all above requirements.

Brief description of the drawings The invention will be further elucidated below on the basis of drawings. In the drawings: Figure 1 illustrates the principle that a given condition of Bragg diffraction is achieved in two subsequent crystals; Figures 2A and 2B illustrates that the gap between the two crystals changes as a function of the rotation angle; Figure 3 shows an embodiment of the device according to the present invention; and Figure 4 shows the feedback loop of the device shown in figure 3. Detailed description of the drawings Figure 3 shows schematically an embodiment of the device according to the present invention constituted as a vertical DCM. The photon beam is in the z-axis, coplanar with the rotation axis 11 (x-axis) of the rotating frame 2, which is coupled to a base 1 (inertial reference) through a double bearing system I; (on both sides of the rotating frame). The device has main frame moving means 10 for rotating the main frame around the rotation axis.

The first crystal mounting frame 3 is also one of the metrology references, rigidly coupled to the rotating frame and with no relative degrees of freedom. The rotating frame is also linked to the first level of gap adjustment 4 by the 5 (or 6) folded leaf springs /;. Finally, both the second crystal mounting frame 5 (the second metrology reference) and the balance mass 6 are coupled to the first level of the gap adjustment by two sets of three leaf spring /> e /;3. So, the three voice coils 7 have the forces acting between the second crystal mounting frame and the balance mass, assuring an internal dynamics of forces for the fine adjustment of the gap and the control of the angles of the second crystal 9, so that the disturbances are not propagated to 1, 2, 3 or 4. The position of the crystals can be adjusted by crystal moving means 12, for example piezoelectric actuators.

By a proper design a few elements (folded leaf springs) can be combined to provide the desired degrees of freedom of the guiding with the required stiffness to suppress the non-controlled degrees of freedom. The feedback is given by the optical interferometers 8, that measure the distance and the two angles between the two metrology references. It is necessary that the number of interferometers is at least the same as the number of degrees of freedom, i.e., three for this setup. The use of distance measurement interferometers allow for an arrangement and design of the mechanical parts such that high stability and high dynamic performance can be reached.

Figure 4 shows the feedback loop of the device according to the invention. The device has an active feedback loop between a beam position monitor (which may be part of the device or a separate part) and the moving means and a local feedback loop between the interferometer(s) which measures the distance between the two crystals and the moving means. These feedback loops form additional mechanisms for angular correction in two axes: pitch and roll, and guarantee the diffraction condition on both crystals and a stable parallelism between them. The moving means are typically based on micromotors and/or piezoelectric actuators and may be concentrated in one of the crystals or in both crystals according to any desired combination.

Although the present invention is elucidated above on the basis of the given drawings, it should be noted that this invention is not limited whatsoever to the embodiments shown in the drawings. The invention also extends to all embodiments deviating from the embodiments shown in the drawings within the context defined by the claims. The device can also be a device for moving mirrors, sample manipulation stages, slits, detectors, undulators and other mechatronics high-end systems where fast and/or accurate positioning is required.

Claims

Claims: I. Device for moving an optical element, comprising: - a base (1), - a mounting frame (5) on which an optical element (9) is mounted and which is movably connected to the base, and - moving means (7) for moving the mounting frame relative to the base. characterized in that the device further comprises: - at least one balance mass (6) movably connected to the mounting frame (5) and movable relative to the base (1) in one or more of the directions of the degrees of freedom when the mounting frame, the moving means (7) displacing the balance mass relative to the mounting means, and - a beam positioning monitor for monitoring a beam leaving the optical element (9), and - a main feedback loop comprising the beam positioning monitor and the moving means, wherein the information from the beam positioning monitor is used to control the moving means.

A device according to claim 1, characterized in that the device further comprises a main frame (2) movably connected to the base (1) and main frame moving means (10) for moving the main frame relative to the base.

The device according to claim 2, characterized in that the device further comprises a sub-frame (4) movably connected to the main frame (2), the mounting frame (5) and the balance mass (6) being movably connected to the sub-frame .

A device according to claim 3, characterized in that the main frame (2) is rotatable about a horizontal axis (11) relative to the base (1) and the main frame moving means (10) are rotation means for rotating the main frame.

A device according to claim 4, characterized in that the subframe (4) is connected to the main frame (2) via first spring means (11) which are such that the subframe (4) is movable relative to the main frame (2) in only one direction perpendicular to the axis of rotation of the main frame.

A device according to claim 5, characterized in that the mounting frame (5) is connected to the subframe (4) via second spring means (12).

A device according to claim 6, characterized in that the balancing mass (6) is connected to the subframe (4) via third spring means (13).

A device according to claim 7, characterized in that the composite stiffness of the second leaf spring means (12) in the direction of displacement is equal to the composite stiffness of the third leaf spring means (13) in the direction of displacement.

A device according to claim 5, 6, 7 or 8, characterized in that the first, second and/or third spring means (11, 12, 13) each comprise at least one leaf spring.

A device according to claim 9, characterized in that the subframe (4) is displaceable relative to the main frame (2) only in one direction perpendicular to the axis of rotation (11) of the main frame (2), wherein said first leaf springs be slack in the direction of displacement and stiff in two other directions perpendicular to the direction of displacement and perpendicular to each other.

A device according to claim 9 or 10, characterized in that the balancing mass (6) is movable relative to the subframe (4) in only one direction parallel to the direction of displacement of the subframe, and said second leaf springs (12) are slack are in the direction of displacement and are rigid in two other directions perpendicular to the direction of displacement and perpendicular to each other.

A device according to claim 9, 10 or 11, characterized in that the mounting frame (5) is movable relative to the subframe (4) in only one direction parallel to the direction of displacement of the subframe, and said third leaf springs (13 ) be slack in the direction of displacement and rigid in two other directions perpendicular to the direction of displacement and perpendicular to each other, and rotatable about two axes of rotation perpendicular to the direction of displacement.

A device according to any one of the preceding claims, characterized in that the moving means (7) are voice coils.

A device according to any one of the preceding claims, characterized in that the device further comprises a further mounting frame (3) which is movably connected to the main frame (2), and a further optical element (9) mounted on the further mounting frame.

A device according to claim 14, characterized in that the device comprises a number of interferometers (8) that measure the position of the mounting frame (5) relative to the further mounting frame (3), as well as a local feedback loop that measures the

I interferometer and the moving means for controlling the relative position of the mounting frames to each other.

A device according to claim 14 or 15, characterized in that the device is a double crystalline monochromator and the optical elements (9) are crystals, one of which is rigidly attached to the mounting frame (5) and the other one is rigidly attached to the further mounting frame (3).