Method and device for controlling an optical delay line

Abstract

The invention relates to a method for controlling a delay obtained by an optical delay line integrated into an interferometry system (6000), the method comprising at least the following steps: • a) measuring a detected amplitude of the optical delay line, • b) comparing the detected amplitude (4) of the optical delay line with an amplitude setpoint (5), • c) generating a control signal (7) for controlling the amplitude of the optical delay line on the basis of the comparison.

Description

[0001] DESCRIPTION

[0002] TITLE: Method and device for controlling an optical delay line

[0003] Technical field.

[0004] The present invention relates to a method and device for controlling an optical delay line, particularly in an interferometry system.

[0005] Prior art

[0006] The prior art of interferometric devices for measuring optical elements has undergone continuous development. From early interferometric experiments to recent technological advances, these systems have been widely used to accurately characterize a vast array of optical elements. Classical techniques such as Michelson, Mach-Zehnder, and Fizeau interferometry paved the way for more sophisticated methods like white-light interferometry and optical coherence techniques. These approaches have enabled the measurement of properties such as shape, flatness, refractive index, thickness, and surface defects of lenses, mirrors, prisms, and other optical components.

[0007] Similarly, these techniques can be used to measure the thickness of layers or structures of structured objects.

[0008] We are familiar with low-coherence interferometric devices, particularly those implemented in the time domain, which require the use of optical delay lines capable of introducing delays or path differences between optical beams corresponding to the desired measurement ranges.

[0009] Such optical delay lines are configured to produce one or more reference peaks measured by the interferometric device, allowing the absolute distance between an interface and a reference to be determined. However, the position of the reference peak is not stable and can fall outside the measurement range of the interferometric device.

[0010] The aim of the present invention is to resolve at least one of the aforementioned drawbacks. Description of the invention

[0011] At least one of these goals is achieved with a method for controlling a delay obtained by an optical delay line integrated into an interferometry system, the method comprising at least the following steps: a) measuring a detected amplitude of the optical delay line,

[0012] b) compare said detected amplitude of the optical delay line to an amplitude setpoint,

[0013] c) generate a control signal for the amplitude of the optical delay line as a function of said comparison,

[0014] characterized in that the measurement of the detected amplitude of the optical delay line is obtained from a metrological quality signal of the position of the delay line.

[0015] It has been observed that limiting the amplitude variations of the optical delay line helps stabilize the position of the reference peak, particularly with respect to the beginning and / or end of the delay line's travel. Furthermore, estimating the detected amplitude from a metrologically accurate signal provides greater precision, resulting in better control of the delay line amplitude and thus more effective limitation of reference peak displacements.

[0016] In particular, a metrological quality signal of the position of the delay line can be obtained by the interferometry system.

[0017] A metrological quality signal of the delay line position can be used by the interferometry system as a reference signal providing the position to be taken into account, with respect to which the measurement made by the interferometry system is obtained.

[0018] A metrological quality signal of the position of the delay line can be obtained by optical interferometry.

[0019] In another embodiment, a metrologically accurate position signal for the delay line can be obtained from a displacement measuring scale. This scale has a pitch marked on a fixed portion. This pitch can be read optically. In this case, it is called an optical scale.

[0020] The interferometry system may include a measurement head that can be moved in space relative to a sample to be measured and configured to emit a measurement optical beam capable of illuminating said sample. For example, the sample to be measured may be an optical element. The reference peak can be obtained by an element reflecting the measurement beam, independently of the position of the sample. Its position is then observed when the optical delay line passes through a particular position in interferometric correspondence with that of this peak.

[0021] Alternatively, the position of the delay line can be estimated using a ruler with regular intervals, attached to the delay line. Such devices are known as rulers, particularly optical rulers. The reading of such a ruler can indeed be done optically.

[0022] According to one embodiment, step a) may include the following substeps:

[0023] a) receive data frames measured by an optical sensor of the interferometry system,

[0024] a2) analyze the data frames and determine the detected amplitude of the delay line from said analysis.

[0025] For example, the detected amplitude of the optical delay line stroke can be obtained by analyzing / counting the number of displacement steps.

[0026] For example, the control signal for the amplitude of the optical delay line can be generated by a derivative proportional-integral controller.

[0027] According to an advantageous embodiment, the optical delay line can be made by a device for generating an optical delay comprising a moving element carrying a mirror and coupled to a magnet. A spring to which the moving element can be attached, an excitation solenoid configured to generate an oscillation of the moving element, and a measuring solenoid suitable for measuring, by magnetic induction, the amplitude of the displacement of the moving element.

[0028] The device for generating an optical delay may further include a means for controlling the excitation solenoid based on the feedback from the measuring solenoid and an alternating signal setpoint, for example, an alternating voltage. The feedback from the measuring solenoid may form a first feedback loop.

[0029] The process may include varying the alternative signal setpoint as a function of the control signal generated in step c). The feedback from the analysis of the data, for example measured by an optical sensor of the interferometry system, to determine the position of the optical delay line may form a second feedback loop.

[0030] Alternatively, the data analysis feedback can also form a first feedback loop, directly modulating the variation of the alternative signal setpoint. In this case, the measurement solenoid can be omitted, or at least is not used for feedback purposes.

[0031] According to another embodiment, the process may include the following steps:

[0032] d1) detect a reference peak produced by the optical delay line, d2) compare the position of the reference peak relative to a start and / or end point of the optical delay line,

[0033] d3) generate a control signal for the position of the reference peak as a function of a position setpoint relative to the beginning and / or end of the optical delay line stroke and the comparison made in step d2).

[0034] In particular, the steps d1) to d3) of reference peak position correction can be executed when the difference between the detected amplitude of the delay line and the amplitude setpoint is less than a predetermined threshold.

[0035] In step d2), the position of the reference peak relative to a linear combination of the start position and the end position of the delay line stroke.

[0036] The control signal for the position of the reference peak can be generated by a proportional-integral-derivative controller.

[0037] According to another aspect of the invention, a device for controlling the amplitude of an optical delay line is proposed, comprising optical interferometry data reception means and processing means configured for:

[0038] a) measure a detected amplitude of the optical delay line from optical interferometry data acquired by optical interferometry data receiving means,

[0039] b) compare said detected amplitude of the delay line to an amplitude setpoint,

[0040] c) generate a control signal for the optical delay line based on said comparison. The control device may include electronic means, such as a digital / analog converter taking as input the control signal for the optical delay line and producing as output an analog signal which can be sent to the control device for the power supply of the solenoid excitation of the movement of the delay line.

[0041] The control device can be configured to implement the aforementioned process.

[0042] In particular, the control device can be configured to: d1) detect a reference peak produced by the optical delay line, d2) compare the position of the reference peak with respect to a start and / or end point of the optical delay line,

[0043] d3) generate a control signal for the position of the reference peak as a function of a position setpoint relative to the beginning and / or end of the optical delay line stroke and the comparison made in step d2).

[0044] According to yet another aspect of the invention, an interferometry system is proposed comprising a device for generating an optical delay and a control device as above configured to control the optical delay, in particular its amplitude and position, produced by the device for generating an optical delay.

[0045] The interferometry system may correspond to a full-field low-coherence interferometer.

[0046] The interferometry system may include a low-coherence source configured to illuminate a sample to be measured, beam splitter elements and optical beam focusing elements, and an optical delay line device to generate a reference beam.

[0047] Description of the figures and embodiments Other advantages and features of the invention will become apparent upon reading the detailed description of implementations and embodiments, which are by no means limiting, and the following attached drawings:

[0048] [Fig. 1] illustrates a schematic representation of an embodiment of a device for controlling the displacement amplitude of an optical delay line in an optical interferometry system.

[0049] [Fig. 2] illustrates an example of signals captured by the interferometry system shown in Figure 1. [Fig. 3] illustrates an example of the implementation of a method for controlling the displacement amplitude of an optical delay line in an optical interferometry system.

[0050] [Fig. 4] illustrates a schematic representation of another embodiment of a device for controlling the displacements of the optical delay line of the optical interferometry system.

[0051] It is understood that the embodiments described below are by no means exhaustive. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0052] In particular, all the variants and embodiments described can be combined with each other if there are no technical obstacles to this combination.

[0053] In the figures, elements common to several figures retain the same reference.

[0054] With reference to Figure 1, the 6000 interferometry system is a full-field low-coherence interferometer and is based on a Michelson or Linnik interferometer comprising a beam splitter element 604, in the form of a cube or beam splitter blade, with a measuring arm that directs a measuring beam 606 to a measuring element 1000.

[0055] The element to be measured (1000) can be an optical element such as a lens, or a volumetrically produced camera lens, or even objects or structures fabricated on the surface or within a substrate (wafer, plate). These objects or structures may have layers or specific patterns, and their thickness must be determined along a measurement axis at specific locations within the manufactured elements. This could involve measuring passage diameters in microfluidic circuits, or the wall thickness around these passage channels, for example. In the case of lenses, these objects may also correspond to an integrated optical production of one or more lenses on a wafer and / or be measured directly on a wafer substrate.

[0056] The 6000 system is illuminated by a low-coherence source 612 via a light-splitting element 603 in the form of a cube or a beam splitter. The source 612 may include, for example, a superluminescent diode (SLD), a diode, a thermal light source (halogen lamp, etc.), or a supercontinuum source. The source 612 may also include a filtering device, for example, with a grating and a slit, or interference filters, to adjust the coherence length to a few tens or a few hundred microns. The source 612 may be arranged to emit in the visible or near-infrared wavelengths, around one or more wavelengths.

[0057] Of course, the 603, 604 separating elements can be non-polarizing, or polarizing and associated with quarter-wave plates to make lossless couplers.

[0058] In order to perform measurements of the properties of the optical element 1000, a reference beam 616 is formed by a device D to generate an optical retarder which is equipped with a reference mirror 605.

[0059] The measurement beams 606 and reference beams 616, reflected respectively in the two arms of the device, are directed via the light-splitting blade 603 to a camera 601 with a sensor 602 comprising a detection matrix, for example of the CMOS or CCD type.

[0060] When the difference in optical path lengths between the measurement beam 606 and the reference beam 616 is less than the coherence length of the source 612, interference is obtained on the detector 602.

[0061] The 6000 system, as shown in the upper left of Figure 1, also includes a focusing lens 607 and a tube lens 609, arranged to define an object plane conjugate to an image plane formed on the sensor 602. The 6000 system further includes a lens 610 which, together with the tube lens 609, also defines a reference object plane conjugate to the image plane of the sensor 602. The 6000 system is a full-field imaging device that allows imaging of interfaces 703 of the optical element 1000 according to a field of view 708 that is determined by the field of view of the imaging system and by its numerical aperture at the focusing lens 607. Indeed, to obtain a measurement, the specular reflection of the measurement beam 606 on the interfaces 703 must be recoupled in the imaging system.

[0062] Typically, the 6000 system includes optical elements to focus the illumination beam into the rear focal plane of the focusing lens 607 and the lens 610. The illumination beams are not shown in the figure for clarity.

[0063] The 610 lens of the reference arm can also be adjusted to hold the reference mirror 605 in an object plane conjugate to the image plane formed by the sensor 602.

[0064] The system 6000 also includes a second displacement means 608 whose function is to move the object plane conjugate to the image plane formed by the sensor 602, so as, for example, to sequentially image successive interfaces 703 on the sensor 602. This displacement means 608 may include a system for moving the focusing lens 607 or lenses of this lens, for example, with a linear translation device. Alternatively or in addition, this displacement means 608 may include a translation device or stage for moving the system 6000 relative to the optical element 1000, or vice versa.

[0065] Device D can be implemented in different ways. For example, device D, shown at the bottom of Figure 1, includes a moving element 101 and two springs 200, the two springs 200 being fixed on the moving element 101, on either side.

[0066] In particular, the moving element 101 corresponds to a rod and is fixed to the reference mirror 605. The moving element 101 is also fixed to a magnet 102 at the level of a first side of the moving element, to a magnet 103 and to the reference mirror 605 at the level of a second side of the moving element opposite to the first side.

[0067] Device D also includes a sinusoidal source 301, an amplifier 302, a first solenoid 303, and a second solenoid 304. The moving element 101 is positioned between the two solenoids 303 and 304 to generate a controlled movement. The second solenoid 304 is connected, via an output, to a second amplifier 305, followed by an amplitude analysis circuit 306 for the signal developed by the second solenoid 304. Device D also includes a comparator 308 that takes as input the output of the analysis circuit 306 and an electronic element distributing at least one setpoint 307. The comparator 308 is connected at its output to the amplifier 302, thus forming a first feedback loop.

[0068] During operation, the sinusoidal electric generator 301 sends an electrical signal. This signal is then amplified by the amplifier 302, which supplies energy to the device D. This signal then passes into the first solenoid 303, which produces an alternating magnetic motor signal. The alternating signal is generated at a frequency equal to or close to the mechanical resonance frequency Fr of D. This alternating excitation signal sets the moving element 101 in translation via the magnet 102. The moving element 101 is displaced back and forth according to the alternating signal, which deforms the springs 200. This movement then creates an alternating signal in the second solenoid 304, specifically through the displacement of the magnet 103 within the second solenoid. This alternating signal received by the second solenoid 304 is amplified via the second amplifier 305.The amplified signal is detected by the amplitude analysis circuit 306 which deduces an observation amplitude information.

[0069] In the first feedback loop, a comparison is made via comparator 308 with a setpoint value 307, equivalent to a voltage value. The difference between the setpoint value 307 and the value of the signal measured 310 by the amplitude analysis circuit 306 can modify the gain of the amplifier 302 to maintain the oscillation amplitude as expected. The amplitude of the excitation signal can therefore be modified according to the observation signal to control the mechanical amplitude of the oscillation of the moving element 101, via the feedback 310. As an example, another type of correction can be applied to the difference between the setpoint value 307 and the value of the signal measured 310, such as a proportional-integral-derivative controller.

[0070] Figure 2 shows an example of measurements obtained by the 6000 interferometry system. The horizontal axis represents an example of the displacement of the moving element 101 forming the optical delay line. The difference in positions associated with the start of measurement DM and the end of measurement FM corresponds to the amplitude of the optical delay line. The vertical axis in the diagram in Figure 2 represents the interference amplitudes. The peaks Pi to Pn, within a useful measurement range 10, are relative to the sample being measured, while the reference peak P r is produced by a reference mirror (not shown in the figure, but which could be located on the path of beam 606 by performing a partial return of the beam, independently of the returns from sample 1000).

[0071] The difference between the positions of the reference peak Pr and a peak Pi, for example, can be used to locate the sample to be measured 1000 relative to an external reference frame, hence the usefulness of having a reference peak integrated into the interferometry system 6000.

[0072] In practice, the position of the reference peak Pr is likely to change. For example, the position of this reference peak Pr can shift towards the useful measurement range 10, making it difficult to distinguish from the other peaks Pi to Pn. This necessitates leaving a margin without measurement capability, thus reducing the extent over which the 6000 interferometry system is capable of measuring. As another example, the position of the reference peak Pr can also shift away from the useful measurement range 10 and may even fall outside the measurement range between the start of measurement DM and the end of measurement FM. In this case, the reference peak Pr is no longer detectable, rendering its associated functionalities, such as sample localization 1000, unusable.

[0073] The control device 1 shown in the upper right of Figure 1 and the process 20 of Figure 3 are designed to overcome the aforementioned drawbacks. For example, the control device 1 implements the process 20.

[0074] To this end, device 1 aims to control the amplitude of the optical delay line's displacements, integrated into the 6000 interferometry system. It has been observed that limiting the variations in the amplitude of the optical delay line, i.e., of the moving element 101 carrying the reference mirror 605, stabilizes the position of the reference peak Pr. Device 1 includes an analysis block 2 that receives interference signals captured by detector 602, or any useful signal related to the displacement of the delay line. Analysis block 2 processes the interference signals and deduces a detected amplitude 4 of the displacement of the moving element 101. For example, the amplitude of the movement of the moving element 101 is determined by analyzing / counting the number of displacement steps.

[0075] Device 1 also includes a comparator 3 which takes as input the detected amplitude 4 produced by the analysis block 2 and a desired amplitude setpoint 5 for the displacement stroke of the moving element 101. The difference between the detected amplitude 4 and the amplitude setpoint 5 is sent to a controller 6, for example, of the proportional-integral-derivative type, or alternatively, the detected amplitude 4 and the amplitude setpoint 5 can be sent directly to the proportional-integral-derivative type element 6. The correction signal 7 from the controller 6 is used to generate a voltage setpoint 307, appropriate to the correction signal 7, which serves as the setpoint in the first feedback loop based on amplitude measurement by electromagnetic fields. Device 1 thus forms a second feedback loop based on amplitude measurement by analyzing interferometry signals.Estimating the detected amplitude from optical interferometry data or another method of measuring the delay line displacement is advantageous because it is more accurate than detection by the analysis circuit 306 306 due to the quality of the optical interferometry data or other metrologically sound measurements of the delay line displacement. This provides better control of the delay line amplitude and therefore more effective limitation of reference peak displacements.

[0076] In the example in Figure 1, the optical delay line is implemented by device D, but it can be implemented by other means. Device D can be adapted to modify the control signal for the amplitude of the optical delay line directly as a function of the correction signal 7, without using the data from comparator 308 or detector 306, by applying signal 7 directly to amplifier 302. (In this case, a simple feedback loop is performed around the metrological quality data.)

[0077] Method 20 for correcting the amplitude of the optical delay line, for example of the moving element 101, comprises:

[0078] - a step 22 of measuring the detected amplitude of the optical delay line from interference signals,

[0079] - a step 24 of comparing said detected amplitude to an amplitude setpoint, - a step 26 of correcting the control signal of the amplitude of the optical delay line, i.e. of the setpoint value 307, as a function of said comparison.

[0080] Figure 4 shows another embodiment of a control device 30 for the displacements of the optical delay line, formed by the device D and integrated into the interferometry system 6000.

[0081] The control device 30 includes an amplitude control device 31 comprising the same components as the control device 1 and a position control device 32 of the reference peak P rSimilar to control device 1, position control device 32 includes an analysis block 2 that receives interference signals captured by detector 602. These signals can be synchronized to determine the position of the reference peak Pr relative to the start DM and / or end FM of the optical delay line's travel. For this purpose, a means for detecting the start or end of the delay line's travel can be implemented. This relative position of the reference peak Pr is then compared to a position setpoint, itself relative to the start DM and / or end FM of the optical delay line's travel. The difference in the comparison is modified by a proportional-integral-derivative controller, for example, to generate the control signal for the delay line's position.The signals generated respectively by the amplitude control device 31 and the position control device 32 produce the correction signal 7. The signal produced by the position control device 32 corresponds to a DC offset component of the optical delay line, in addition to the amplitude component of the optical delay line stroke. Thus, the DC component can be a DC current superimposed on the AC drive signal of the first solenoid 303 that excites the moving element 101.

[0082] The means of detecting the beginning or end of the travel of the delay line can be estimated from a ruler with regular steps, attached to the delay line, otherwise called a ruler, or optical ruler.

[0083] Of course, the invention is not limited to the examples just described and many modifications can be made to these examples without departing from the scope of the invention.

Claims

DEMANDS 1. A method (20) for controlling a delay obtained by an optical delay line integrated into an interferometry system (6000), the method comprising at least the following steps: a) measure (22) a detected amplitude of the optical delay line, b) compare (24) said detected amplitude (4) of the optical delay line to an amplitude setpoint (5), c) generate (26) a control signal (7) for the amplitude of the optical delay line as a function of said comparison, characterized in that the measurement of the detected amplitude of the optical delay line is obtained from a metrological quality signal of the position of the delay line.

2. A method according to the preceding claim, wherein the metrological quality signal is obtained by optical interferometry.

3. Method (20) according to claim 2, wherein step a) comprises the following substeps: a) receive data frames measured by an optical sensor (602) of the interferometry system (6000), a2) analyze the data frames and determine the detected amplitude (4) of the delay line from said analysis.

4. Method according to claim 1, wherein the metrological quality signal is obtained from a displacement measurement rule.

5. Method (20) according to any one of the preceding claims, wherein the control signal (7) of the amplitude of the optical delay line is generated by a derivative integral proportional compensator (6).

6. A method (20) according to any one of the preceding claims, wherein the optical delay line is made by a device (D) for generating an optical delay comprising a moving element (101) carrying a mirror (605) and coupled to a magnet (102, 103), a spring (200) to which the moving element is attached, an excitation solenoid (303) configured to generate an oscillation of the moving element, and a measuring solenoid (304) capable of measuring by magnetic induction an amplitude of displacement of the moving element, the device for generating an optical delay further comprising a control means (302) of the excitation solenoid as a function of the feedback of the measuring solenoid and a setpoint (307) of an alternating signal, the method comprising the variation of the setpoint of the alternating signal as a function of the control signal (7) generated in step c).

7. A method (20) according to any one of the preceding claims, comprising the following steps: dl) detect a reference peak (P r ) produced by the optical delay line, d2) compare the position of the reference peak relative to a start (DM) and / or end (FM) of the optical delay line's stroke, d3) generate a control signal for the position of the reference peak as a function of a position setpoint relative to the beginning and / or end of the optical delay line stroke and the comparison made in step d2).

8. Method (20) according to the preceding claim, wherein the steps d1) to d3) of correction of the position of the reference peak are executed when the difference between the detected amplitude (4) of the delay line and the amplitude setpoint (5) is less than a predetermined threshold.

9. Method (20) according to any one of claims 7 or 8, wherein the control signal for the position of the reference peak is generated by a derivative integral proportional controller.

10. Method (20) according to any one of claims 7 to 9 taken in combination with claim 6, wherein the control signal for the position of the reference peak is an offset signal added to the alternative signal setpoint.

11. Device (1,30) for controlling the amplitude of an optical delay line comprising optical interferometry data reception means (602) and processing means configured for: a) measure a detected amplitude (4) of the optical delay line from optical interferometry data acquired by optical interferometry data receiving means, b) compare said detected amplitude (4) of the delay line to a setpoint amplitude (5), c) generate a control signal (7) of the optical delay line as a function of said comparison.

12. Interferometry system (6000) comprising a device (D) for generating an optical delay and a control device (1,30) according to the preceding claim configured to control the optical delay produced by the device for generating an optical delay.

13. Interferometry system (6000) according to the preceding claim, wherein the interferometry system corresponds to a full-field low-coherence interferometer.

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