Optical measurement system and method for measuring optical grating

Abstract

An optical measurement system for measuring an optical grating (102 comprises a light source (100) that outputs collimated light beam (200) that is monochromatic and coherent. An interferometer arrangement (5) splits the collimated light beam (200) and guides the first and second light beams (104, 106) to the optical grating (102) for back-diffraction to the interferometer (5). The system causes interference between each of grating light beams (122G, 122G') diffracted from the optical grating (102) set in said two positions (G, G'), and a reference light beam (122M) reflected from a reference (118). The system also causes at least four predetermined relative wavefront phase shifts between the first and second light beams. A camera (124) forms interferograms of the interference in the two different positions (G, G') of the optical grating (102) with the at least two predetermined wavefront shifts. And a data processing unit (140) processes the interferograms, and present data on the optical grating (102) based on the interferograms.

Description

[0001] Optical measurement system and method for measuring optical grating

[0002] Field

[0003] The invention relates to optical measurement system for measuring an optical grating and an optical measurement method of measuring an optical grating.

[0004] Background

[0005] Diffraction gratings have a variety of applications. They can be used in spectroscopy for dispersing light into a spectrum of different wavelengths and in augmented reality (AR) devices for in- and out-coupling light of a lightguide, for example. The quality of the diffraction gratings is important for a proper operation, and the diffraction gratings can be characterized based on optical metrology. However, the optical measurement is undesirably slow and spatially inaccurate for larger surfaces such as those of optical gratings. Hence, an improvement would be welcome.

[0006] Brief description

[0007] The present invention seeks to provide an improvement in the measurements.

[0008] The invention is defined by the independent claims. Embodiments are defined in the dependent claims.

[0009] If one or more of the embodiments is considered not to fall under the scope of the independent claims, such an embodiment is or such embodiments are still useful for understanding features of the invention.

[0010] List of drawings

[0011] Example embodiments of the present invention are described below, by way of example only, with reference to the accompanying drawings, in which Figure 1 illustrates an example of an optical measurement system

[0012] Figure 2 illustrates an example of an optical measurement system that utilizes polarization;

[0013] Figure 3 illustrates an example of an optical grating with an optical measurement beam on it;

[0014] Figure 4 illustrates an example of the detector of the camera of the measurement system;

[0015] Figure 5 illustrates an example of a turntable polarizer of the camera;

[0016] Figure 6 illustrates an example of polarization directions of light beams;

[0017] Figure 7 illustrates an example of the optical grating and a few degrees of diffraction;

[0018] Figure 8 illustrates an example of a data processing unit; and

[0019] Figure 9 illustrates of an example of a flow chart of a measuring method.

[0020] Description of embodiments

[0021] The following embodiments are only examples. Although the specification may refer to "an" embodiment in several locations, this does not necessarily mean that each such reference is to the same embodiment's), or that the feature only applies to a single embodiment.

[0022] Single features of different embodiments may also be combined to provide other embodiments. Furthermore, words "comprising" and "including" should be understood as not limiting the described embodiments to consist of only those features that have been mentioned and such embodiments may also contain features / structures that have not been specifically mentioned. All combinations of the embodiments are considered possible if their combination does not lead to structural or logical contradiction.

[0023] The measurement of an optical grating 102 may be performed without utilization of polarization or with utilizing polarization. Utilization of polarization enables the measurement without mechanical movement for predetermined relative wavefront phase shifts between the optical paths related to the reference 116 and the optical grating 102. Fig.l illustrates an example of an optical measurement system for measuring an optical grating 102. The optical measurement system comprises a light source 100 that outputs a collimated light beam 200. The collimated light beam 200 is monochromatic and coherent. The collimated light beam 200 can be defined to have rays of light that are aligned with the optical axis of the light beam 200. Monochromatic light has only one wavelength in principle, but in practice monochromatic light has electromagnetic waves of varying wavelengths within a narrow bandwidth, where the bandwidth can be considered tolerance. Coherent light means that the wavy motions of the light are synchronized. For interference measurement coherence means a possibility to distinguish phase shifts between waves of two beams. A coherent length of the measurement system must be large enough for the measurement, which is easily achieved with a laser, for example. A person skilled in the art is familiar with collimation, monochromatism and coherence.

[0024] In an embodiment, the optical light source 100 may output visible light, for example. In an embodiment, the optical light source 100 may output infrared light, for example.

[0025] The optical measurement system also comprises an interferometer arrangement 5. The interferometer arrangement 5 may comprise an interferometer of a Michelson design or the like. A person skilled in the art is familiar with the interferometers, per se. The interferometer comprises a first optical path 202 and a second optical path 204 which are utilized for forming the required interference and the predetermined relative wavefront phase shifts.

[0026] The interferometer 5 splits the collimated light beam 200 into a first light beam 104 and a second light beam 106. The interferometer 5 guides the first light beam 104 to a first optical path 202, which leads the first light beam 104 to the optical grating 102.

[0027] The optical grating 102, an example of which is illustrated in Figs 3 and 7, is a latticework that has spatially arranged diffraction structures 260 that have regular distance D therebetween (only one structure is shown as a line and is marked with reference number in Figs 3 and 7 for clarity). The optical grating causes interference between incident and reflected beams which results in angles at which monochromatic light is diffracted from the optical grating 102. The angles represent diffraction or spectral orders. A person skilled in the art is familiar with optical gratings, per se.

[0028] The optical grating 102 is put in two positions G, G’ for the measurement. The optical grating 102 can be turned such that the opposite orders of diffraction are toward the same direction i.e. toward the interferometer 5. In both positions the optical grating 102 diffracts the beam of light directed from the interferometer 5 to it back to the interferometer 5. In more detail, the first position G and the second position G’ cause opposite orders of diffractions back into the direction of the incident beam (Fig. 7 shows neither of such positions). That kind of measurement principle is called a Littrow configuration. A person skilled in the art is familiar with the Littrow configuration, per se. In an embodiment, the opposite orders of diffraction are the first order (1) and the negative first order (- 1)-

[0029] The interferometer 5 guides the second light beam 106 to the second optical path 204, which directs the second light beam 106 to a reference 116. The reference 116 may comprise a flat mirror. The reference 116 may have a surface which is polished optically smooth such that it has no optical defect. That means it neither deforms the optical wavefront of the optical beam interacting with it nor spoils coherence. A person skilled in the art is familiar with optical mirrors for an interference measurement, perse.

[0030] The interferometer 5 provides the camera 124 with at least two predetermined relative wavefront phase shifts between the grating light beam 122G of the first position G of the optical grating 102 and a reference light beam 122M. The interferometer 5 also provides the camera 124 with at least two predetermined relative wavefront phase shifts between the grating light beam 122G’ of a second position G’ of the optical grating 102 and the reference light beam 122M. The predetermined relative wavefront phase shifts are realized by manipulating optically the light beams within the first optical and the second optical path 202, 204. In an embodiment, the first and / or the second optical paths 202, 204 are configured to make it possible to capture interferograms with four predetermined relative wavefront phase shifts between the grating light beam 122G of a first and second positions G and G’ of the optical grating 102 and the reference light beam 122M. The predetermined relative wavefront phase shifts for the camera 124 may be realized by light beam propagation length manipulation or by polarization type manipulation. The four predetermined relative wavefront phase shifts may be one-quarter phase shift. That makes it possible to detect surface errors of the optical grating 102 at different phases of the wavefront, which gives reliable information on the quality of the optical grating 102.

[0031] The interferometer 5 splits the collimated light beam 200 into a first light beam 104 and a second light beam 106. The interferometer 5 guides the first light beam 104 through the first optical path 202 onto the periodic diffraction structures 260 of the optical grating 102. The interferometer 5 guides the second light beam 106 through the second optical path 204 to the reference 116. The interferometer 5 keeps the collimation and the cross section of said first and second light beams 104, 106 constant. The interferometer 5 causes interference between each of grating light beams 122G, 122G’ diffracted from the optical grating 102 set in said two positions G, G’, and a reference light beam 122M reflected from the reference 116. Finally, the interferometer 5 causes at least two predetermined relative wavefront phase shifts between light beams propagated through the first optical and the second optical path 202, 204.

[0032] The first beam 104 makes an illuminated spot 300 on the optical grating 102, an example of which is illustrated in Fig. 3. A cross sectional area of the first beam 104 i.e. the illuminated spot 300 on the optical grating 102 covers a plurality of periodic diffraction structures 260 of the optical grating 102, see the horizontal lines in Fig. 3. In an embodiment, a diameter of the illuminated spot 300 may be about 5 mm, for example. In an embodiment, the illuminated spot 300 may cover the whole area of the optical grating 102, for example. In an embodiment, a diameter of the illuminated spot 300 may cover a major part of the optical grating 102, for example. The interferometer 5 causes interference between a grating light beam 122G, 122G’ diffracted from the optical grating 102 and a reference light beam 122M reflected from the reference 116. The grating light beams 122G, 122G’ may be similar to those directly diffracted from the optical grating 102 i.e. third light beams 104’, 104" or the third light beams 104’, 104" may be optically manipulated within the interferometer 5. Correspondingly, the reference light beam 122M may be similar to that reflected from the reference 116 i.e. a fourth light beam 106’ or the fourth light beam 106’ may be optically manipulated within the interferometer 5. In any case, the grating light beams 122G, 122G’ and the reference light beam 122M are those propagating from the interferometer 5 to the camera 124. The interference enables detection of relative phase shifts of wavefronts of the grating and reference light beams 122G, 122G’, 122M.

[0033] A camera 124 of the optical measurement system forms interferograms of relative phase shifts of the wavefronts caused by the interference of the grating light beams 122G, 122G’ and the reference light beam 122M in two different positions G, G’ of the optical grating 102. That means, interference is formed between the grating light beam 122G and the reference light beam 122M during one measurement stage. Interference is formed between the grating light beam 122G’ and the reference light beam 122M during another measurement stage. In both measurement stages, the predetermined relative wavefront phase shifts are formed and utilized.

[0034] The at least two predetermined relative wavefront phase shifts between the first light beam 104 and the second light beam 106 are naturally observable in the grating and reference light beams 122G, 122G’, 122M. The at least two predetermined wavefront phase shifts may be enabled by manipulation of the optical paths 202, 204. Additionally or alternatively, the optical grating 102 may be moved for causing at least partially the predetermined relative wavefront phase shifts.

[0035] A data processing unit 140 of the optical measurement system receives and processes the interferograms of both of the positions of the optical grating 102 where the at least two predetermined relative wavefront phase shifts have also been applied. The data processing unit 140 presents data on the optical grating 102 based on the interferograms.

[0036] The interferogram is a pattern formed by the interference. The interferogram has typically fringes caused by the interference. The shape of the fringes may depend on the optical grating 102, or more accurately it may depend on a difference between the optical surfaces of the optical grating 102 and the reference 116. The at least two predetermined wavefront phase shifts enable detection and analysis of the interference with at least two phase shifts of the wavefronts. In an embodiment, at least four predetermined wavefront shifts are formed and utilized. In an embodiment, four predetermined wavefront shifts are formed and utilized. Different predetermined relative wavefront phase shifts cause different formation of the fringes and that is why they give more information of the optical surface of the optical grating 102 than an interferogram with only one predetermined relative wavefront phase shift. Detection of errors in the optical grating 102 is efficient with more than one predetermined relative wavefront phase shift.

[0037] In an embodiment, four predetermined wavefront shifts are formed and utilized. By utilizing four predetermined wavefront shifts, no analysis or calibration at a pixel level is needed because it is always possible to make sure if a measured intensity of a pixel is above or below the middle line of a sine wave. That is, accurate information on the optical grating 102 is possible obtain in that manner. The same is true for a larger number of predetermined wavefront shifts. Utilizing a smaller number of the predetermined wavefront shifts may require some additional work for accurate information. Some information on the optical grating 102 is possible to obtain with only two predetermined wavefront shifts if detailed information is not required.

[0038] When the optical grating 102 is turned from one position G to the other position G’, the wavefront shift changes. The algorithm calculating the grating constant or the like utilizes different wavefront shifts. Some require a larger number of them, some require a smaller number of them. A person skilled in the art is familiar with the algorithms calculating information on the optical grating 102, such as the grating constant, perse. A person skilled in the art can decide how many predetermined wavefront shifts are needed or are suitable for the measurement. The person skilled in the art is familiar with the algorithms, per se, and the necessary number of the predetermined wavefront shifts, per se. This method based on at least two predetermined wavefront shifts requires only a few captured images of the for having information on the optical grating 102. Using the four predetermined wavefront shifts for both of the Littrow positions of the optical grating allow a single shot phase interferometry. That can be used for holistic augmented reality glass grating quality control, for example.

[0039] Because the light spot 300 is large and covers a plurality of diffraction structures 260 on the optical grating 102, the optical grating 102 can be characterized quickly.

[0040] The measurement taught in this document is particularly meant to measure back-diffraction of the optical grating 102 and not transmission of light through the optical grating 102.

[0041] In an embodiment, the optical grating 102 is set in a first position G and a second position G’ at different moments for the formation of the interferograms with at least two predetermined relative wavefront phases in both of the positions G, G’. That can be done by turning the optical grating 102.

[0042] In an embodiment an example of which is illustrated in Figs 1 and 2, the optical measurement system comprises an illumination arm 10, a reference arm 12, a grating arm 14, reception arm 16 and a beamsplitter arrangement 18.

[0043] The illumination arm 10 may receive the collimated light beam 200 from the optical light source 100. In an embodiment, the optical light source 100 may comprise a laser. The laser may be a stabilized single frequency laser. The laser may be a helium-neon laser or a semiconductor laser, for example.

[0044] The beamsplitter arrangement 18 may receive the collimated light beam 200, and split the collimated light beam 200 to the first light beam 104 and the second light beam 106.

[0045] The grating arm 14 may receive the first light beam 104 and direct the first light beam 104 to the optical grating 102. The optical grating 102 may direct the third light beams 104’, 104", which are caused by diffraction in each of the two positions G, G’, back to the beamsplitter arrangement 18.

[0046] The reference arm 12 may comprise the reference 116, which directs the fourth light beam 106’ to the beamsplitter arrangement 18 as a reflection of the second light beam 106.

[0047] The beamsplitter arrangement 18 may interact with the third and fourth light beams 104’, 104”, 106 and may direct, on the basis of an interaction between the beamsplitter arrangement 18 and the third and fourth light beams 104’, 104", 106’, the grating light beams 122G, 122G’ and a reference light beam 122M to the reception arm 16. The reception arm 16 may enable interference between the grating light beam 122G, 122G’ and the reference light beam 122M.

[0048] The camera 124 captures interferograms of the interference in both of the two different positions G, G’ of the optical grating 102. Additionally, the interferograms are captured with at least two relative wavefront shifts based on optical manipulation performed within the grating arm 14 and / or the reference arm 12. If four relative wavefront shifts at one position of the optical grating 102 are utilized, a total number of interferograms will be eight. That is, four interferograms per position G, G’ of the optical grating 102.

[0049] The data processing unit 140 forms and presents information on at least one of the following based on the data on interferograms: a grating constant, variation of the grating constant and deviation from a desired grating constant.

[0050] In an embodiment, the beamsplitter arrangement 18 comprises a beamsplitter 130 that splits the collimated light beam 200 into the first light beam 104 and the second light beam 106. The beamsplitter 130 may divide the power of the incident light into two at least approximately equal powers. The optical power division may be based on partial reflection. The partial reflection may be caused by a partially transparent layer of metal such as aluminum, for example, particularly if polarization is not utilized. A person skilled in the art is familiar with the beamsplitters, per se.

[0051] In an embodiment, the reference arm 12 may comprise the reference 116 that receives the second light beam 106. The reference 116 directs the fourth light beam 106’ to the beamsplitter 130 as a reflection of the second light beam 106. In principle, the fourth light beam 106’ is very similar to the second light beam 106. However, for clarity and the fact that a reflection causes more or less power losses and scattering, the incident beam and the reflected beam have a different reference number.

[0052] In an embodiment, the beamsplitter 130 may direct the first light beam 104 to the optical grating 102. The optical grating 102, in turn, back-diffracts the first light beam 104 because in both measurement positions G, G’ the first order diffraction and the corresponding diffraction at the opposite angle are set to do that. That is, the optical grating 102 directs the third light beams 112’, 112” to the beamsplitter 130 caused by diffraction of the first light beam 104 in the two different positions G, G’ of the optical grating 102. The grating light beams 122G, 122G’ are then formed in the interaction between the beamsplitter 130 and the third light beams 112’, 112" in embodiments where polarization is not utilized.

[0053] In principle, the third light beams 112’, 112” are similar to the first light beam 104 depending on the quality of the optical grating 102. However, for clarity and the fact that a diffraction causes more less optical deviations, power losses and scattering, the incident beam and the reflected beam have different reference numbers and names. Note also that the grating light beams 122G, 122G’ are similar to the third light beams 112’, 112" but different reference numbers are used because of the interaction between the third light beams 112’, 112" and the beamsplitter 130.

[0054] In an embodiment, at least one of the optical grating 102 and the reference 116 may be configured to be mechanically shifted in direction parallel to the propagation of light for causing the predetermined relative wavefront shifts. The predetermined relative shifts of the wavefronts may alternatively be realized as an optical shift in the optical paths 202, 204. The mechanical shift means physical movement of either of the optical grating 102 or the reference 116 or both of them. The optical shifting means making the optical path of the light shorter or longer by changing the refractive index of the medium through which the optical beams propagate. In an embodiment, the reference 116 may move for causing the at least two predetermined wavefront shifts. The predetermined wavefront shifts may be one-quarter wavelength shifts. The number of the predetermined relative wavefront shifts may be four, for example. Then the reference 116 may have four different distances from the beamsplitter arrangement 18 or the beamsplitter 130. That more than one relative wavefront shift is utilized by movements of the reference 116 is illustrated in Fig. 1 by dashed rectangles of the reference 116.

[0055] The mechanical shift or the optical shifting may be replaced by varying the wavelength of the monochromatic light in order to cause the predetermined wavefront shifts. The wavelength of a laser, for example, can be tuned by a few nanometers without limiting to this, for example.

[0056] In an embodiment, the beamsplitter 130 may combine the third light beams 112’, 112" and the fourth light beam 106’, which after the combination are the grating light beams 122G, 122G’ and the reference light beam 122M, for causing the interference therebetween. The beamsplitter 130 may in this manner direct the interference of the grating light beams 122G, 122G’ and the reference light beam 122M to the camera 124. In Fig. 2, the third light beams 112’, 112" pass through the beamsplitter 130 and become, on the basis of the interaction between the beamsplitter 130 and the third light beams 112’, 112", the grating light beams 122G, 122G’. The fourth light beam 106’ reflects from the beamsplitter 130 and becomes, on the basis of the interaction between the beamsplitter 130 and the fourth light beam 106’, the reference light beam 122M.

[0057] In an embodiment, the data processing unit 140 may compute at least one of the following based on the data on interferograms: a grating constant, variation of the grating constant and deviation from a desired grating constant.

[0058] In an embodiment, data processing unit 140 may present data on the variation of the grating constant and / or the deviation from a desired grating constant as a function of a location on the grating 102. The location may be expressed in a two-dimensional manner using a two-dimensional coordinate system. The two-dimensional coordinate system may be Cartesian, for example.

[0059] Examine now how polarization can be utilized. In an embodiment an example of which is illustrated in Fig. 2, the measurement system may utilize polarization in order to cause the optical path length shift for the measurement. In this embodiment, the camera 124 may be based on polarization, and the camera 124 may comprise at least one linear polarizer 126.

[0060] The interferometer 5 may direct the first light beam 104 in a form of a first circularly polarized light beam 112 to the optical grating 102, and direct the second light beam 106 in a form of a second circularly polarized light beam 114 toward the reference 116. The first circularly polarized light beam 112 is similar to the first light beam 104 but in this embodiment polarization is utilized. The corresponding applies to the second polarized light beam 114 and the second light beam 106.

[0061] The polarization directions of the first circularly polarized light beam 112 and the second circularly polarized light beam 114 may then have 90° difference as illustrated in Fig 6. That is, the polarization directions of the circularly polarized light beams 112, 114 may be orthogonal. Both polarization directions rotate as the light beams propagate (in example of Fig. 6, in direction of curved arrow) but they keep 90° difference.

[0062] The circularly polarized light has electric vectors of equal amplitudes in two perpendicular directions (or dimensions) and they have a one-quarter wavelength phase difference. It can also be considered the polarization direction rotates 360° per wavelength. In this document, also elliptic polarization is considered a possible form of circular polarization in some embodiment(s).

[0063] The interferometer 5 may then receive the third circularly polarized light beams 112’, 112" from the optical grating 102 as diffraction of the first circularly polarized light beam 112 in response to the two positions G, G’ of the optical grating 102. The first third beam 112’ may refer to a first position G of the optical grating 102, and the second third beam 112" may refer to the second position G’ of the optical grating 102. The diffraction to the interferometer 5 is based on interaction between the optical grating 102 and the first circularly polarized light beam 112. Correspondingly, a fourth circularly polarized light beam 114’ is formed as a reflection of the second circularly polarized light beam 114 from the reference 116. In an embodiment, the interferometer 5 may convert the third circularly polarized light beams 112’, 112" into third linearly polarized light beams 104’, 104”, and the fourth circularly polarized light beam 114’ into a fourth linearly polarized light beam 106’. Then the interferometer 5 may combine the third linearly polarized light beams 104’, 104", which has interacted with the optical grating 102 in the circularly polarized form, and the fourth linearly polarized light beam 106’ which has interacted with the reference 116 in the circularly polarized form. The polarization directions of the third linearly polarized light beam 104’ and the fourth linearly polarized light beam 106’ may be considered to be orthogonal i.e. 90° at this stage assuming the optical grating 102 is perfect. A perfect optical grating 102 can be considered to mean that its surface corresponds to that of an optically flat reference 116. The optically flat reference 116 has a surface that is polished to have variation of its surface only a few tens of nanometres at maximum. As the optical grating 102 may deviate from the perfect, optically flat surface, the orthogonality may not be fully satisfied, and the potential deviation may be detected by the camera 124 and analyzed by the data processing unit 140. However, the computer program determining the grating constant may, in an embodiment, compensate for the surface non-idealities such that the computer program defines the grating constant that is non-disturbed by the surface deviations.

[0064] In an embodiment, the interferometer 5 may convert the third linearly polarized light beams 104’, 104” and the fourth linearly polarized beam 106’ into the grating light beams 122G, 122G’ of circular polarization and the reference light beam 122M of circular polarization, respectively. The conversion is performed for enabling and emphasizing the interference between the grating light beam 122G and the reference light beam 122M in the first positions G of the optical grating 102. The conversion is also performed for fully enabling or emphasizing the interference between the grating light beam 122G’ and the reference light beam 122M in the second positions G’ of the optical grating 102. These grating light beams 122G and 122G’ are formed separately one after another in response to setting or turning the optical grating 102 in the first and second positions G, G’ of the back-diffraction.

[0065] The polarization camera 116 captures the interferograms of the interferences. The interferograms include information on spatial wavefront phase distributions relating to the area of the spot 300 of the first circularly polarized beam 112 on the optical grating 102 that is measured at at least two positions of the at least one polarizer 118 and the two different positions G, G’ of the optical grading 102. The at least two directions of the at least one polarizer 118 may enable detections at the two predetermined relative wavefront shifts between the grating light beams 122G, 122G’ and the reference light beam 122M in a similar manner to the mechanical movement of the reference 116 (or the optical grating 102) in the embodiment(s) not utilizing polarization.

[0066] In an embodiment an example of which is illustrated in Fig. 2 and which utilizes polarization, the optical light source 100 may output collimated light beam 200 of linear polarization to the interferometer 5. In an embodiment, the optical light source 100 may comprise a source polarizer 250 that is configured to polarize the collimated light beam 200 in a linear manner.

[0067] In an embodiment, a beam splitter 130 of the interferometer 5 may have the polarization direction that deviates from the polarization direction of the linear polarization of the collimated light beam 200 by 45° or at least approximately 45°. The beamsplitter 130, that may be a polarizing beamsplitter, may split, based on polarization, the linearly polarized optical source beam 200 into the first light beam 104 of linear polarization and the second light beam 106 of linear polarization. The polarizing beamsplitter 130 causes, in this manner, the polarization directions of the beam reflected from the beamsplitter 130 and the beam passed through the beamsplitter 130 to deviate by 90° from each other. A person skilled in the art is familiar with polarizing beamsplitters, perse.

[0068] The beam splitter 130 may direct the first linearly polarized light beam 104 to a first waveplate 108 of the interferometer 5. The first waveplate 108 may convert the first linearly polarized beam 104 into the first circularly polarized light beam 112. The first circularly polarized light beam 112 of the embodiment(s) utilizing polarization is similar to the first light beam 104 of the embodiment(s) not utilizing polarization.

[0069] The first waveplate 108 may direct the first circularly polarized light beam 112 to the optical grating 102, which may the diffract third circularly polarized light beams 112’, 112" to the first waveplate 108 as the back-diffraction of the first circularly polarized light beam 104. That is performed in response to the two different positions G, G’ of the optical grating 102.

[0070] The first waveplate 108 may convert the third circularly polarized light beams 112’, 112" into the third polarized light beams 104’, 104" of linear polarization. Then the third linearly polarized light beams 104’, 104" may propagate to the beam splitter 102.

[0071] The beam splitter 102 may direct the second linearly polarized light beam 106 to a second waveplate 110 of the interferometer 5. Then the second waveplate 110 may convert the second linearly polarized light beam 106 into a second circularly polarized light beam 114.

[0072] The reference 116 may receive the second circularly polarized beam 114 and reflect it as a fourth circularly polarized beam 114’ back to the second waveplate 110, which may convert the fourth circularly polarized beam 114’ into a fourth polarized beam 106’ of linear polarization. The second waveplate 110 may forward the fourth linearly polarized beam 106’ to the beamsplitter 130. An amount of the rotation of the polarization direction of the fourth linearly polarized light beam 106’ and the second linearly polarized light beam 106 may deviate by 90° from each other. The first waveplate 108 may make corresponding rotations of the polarization direction to the first and the third linearly polarized light beams 104, 104’ and 104". In this manner, optical power loss of the combination is minimized in the beamsplitter 130.

[0073] The beam splitter 130 may combine, based on polarization, the third and fourth linearly polarized light beams 104’, 104", 106’, and direct them to a third waveplate 120 of the interferometer 5. The third waveplate 120 may convert them to the grating light beams 122G, 122G’ of circular polarization and the reference light beam 122M of circular polarization, respectively, for causing or emphasizing the interference therebetween.

[0074] In an embodiment an example of which is illustrated in Fig. 4, the polarization camera 124 may comprise a matrix 400 of groups 402 of detecting elements 404. At least two detecting elements 404 of each group 402 have linear polarizer elements 118’ in front of the detecting elements 404. The linear polarizer elements 118’ of each detecting element 404 within each groups 402 have different linear polarization directions. The different linear polarizations directions correspond to different relative wavefront shifts between the reference arm 12 and the grating arm 14 or the first and second optical paths 202, 204. The wavefront shifts may be enabled by the polarization type manipulation. The polarization type manipulation is performed by the polarizing beamsplitter 130 and the waveplates 108, 110 and 120 in the reference arm 12 and the grating arm 14.

[0075] The different polarization directions of the polarizer elements 118’ may be arranged such that any two polarizer elements 118’ have a difference of 90° between them. In that manner, the first polarizer element may be set to angle 0° (freely chosen angle), the second polarizer element may be set to angle 90° with respect to the first polarizer element, the third polarizer element may have an angle 180° with respect to the first polarizer element and the fourth polarizer element may have an angle 270° with respect to the first polarizer element. These polarization direction angles may correspond to one-quarter wavelength shifts of the wavefronts.

[0076] In an embodiment an example of which is illustrated in Fig. 4, the matrix comprises groups of four detecting elements 404 each detecting element 404 of each group having a linear polarizer elements of different polarization directions for forming four interferograms of each position of the optical grating 102. The detecting elements may be pixels of the detecting cell of the camera 124, for example.

[0077] In an embodiment an example of which is illustrated in Fig. 5, the linear polarizer 118 comprises a linear polarizer component 118" that may be turned in a plurality of positions 1, 2, 3, 4 in front of the camera 124 or in front of the detecting elements 404 of the camera 124. The polarizer component 118" enables different polarization directions of the grating light beam 122G, 122G’ and the reference light beam 122M to propagate to the detecting elements of the camera 124 one after another in response to a turn of the linear polarization component 118".

[0078] As to the third light beams, the third light beam 104’, 104" may be polarized or non-polarized in embodiments not utilizing polarization. The third light beams have reference signs 112’, 112" in embodiments utilizing polarization, and in those embodiments the third light beams 112’, 112" are circularly polarized.

[0079] In an embodiment, the waveplates 108, 110 and 120 can also be called retardation plates. They may cause one-quarter wave retardations. The retardation corresponds to mechanical or optical shifts in the reference arm 12 and / or the grating arm 14. The retardation may be caused by mechanical movement the optical grating 102 or the reference 116, or shifts of both of them. The movement may be performed in quarter-wave steps. The movement may be caused by a piezoelectric translator 500 which can cause a precision movement of the reference 116. Correspondingly, the optical grating 102 may be moved in a corresponding manner. A person skilled in the art is familiar with piezoelectric translators, per se.

[0080] Fig. 7 illustrates an example of the optical grating 102 and a first degree diffraction, a zeroth degree of diffraction and a first negative degree diffraction. Different order diffractions are directed to different angles. When the optical grating 102 is turned suitably, the first order diffraction points to the direction of the incident light beam. When the optical grating 102 is turned to another suitable position, the first negative degree diffraction points to the direction of the incident light beam.

[0081] Fig. 8 illustrates an example of the data processing unit 140. The data processing unit 140 comprises one or more processors 800, and one or more memories 802 including computer program code within the computer 142. The one or more memories 802 and the computer program code may, with the one or more processors 800, cause apparatus at least to receive and process the interferograms of both of the positions of the optical grating 102 and the at least two relative wavefront shifts, and present data on the optical grating 102 based on the interferograms.

[0082] The term "computer" includes a computational device that performs logical and arithmetic operations. For example, a "computer" may comprise an electronic computational device, such as an integrated circuit, a microprocessor, a mobile computing device, a laptop computer, a tablet computer, a personal computer, or a mainframe computer. A "computer" may comprise a central processing unit, an ALU (arithmetic logic unit), a memory unit, and a control unit that controls actions of other components of the computer so that steps of a computer program are executed in a desired sequence. A "computer" may also include at least one peripheral unit that may include an auxiliary memory (such as a disk drive or flash memory), and / or may include data processing circuitry.

[0083] A user interface 144 means an input / output device and / or unit. Nonlimiting examples of a user interface include a touch screen, other electronic display screen, keyboard, mouse, microphone, handheld electronic controller, digital stylus, display screen, speaker, and / or projector for projecting a visual display.

[0084] However, the data processing unit 140 determining the grating constant may, in an embodiment, compensate for the surface non-idealities such that the computer program defines the grating constant that is non-disturbed by the surface deviations.

[0085] Figure 9 is a flow chart of the measurement method. In step 900, a light source 100 outputs collimated light beam 200 that is monochromatic and coherent. The interferometer 5 performs the following steps.

[0086] In step 902, the collimated light beam 200 is split into a first light beam 104 and a second light beam 106.

[0087] In step 904, the first light beam 104 is guided through the first optical path 202 to periodic diffraction structures 260 of the optical grating 102, the optical grating 102 being measured in two different positions G, G’, a light spot 300 of the first light beam 104 covering a plurality of the periodic diffraction structures 260 of the optical grating 102, the two positions G, G’ of the optical grating 102 causing back-diffraction to the interferometer 5.

[0088] In step 906, the second light beam 106 is guided through the second optical path 204 to the reference 116,

[0089] In step 908, the collimation and the cross section of said first and second light beams 104, 106 is kept constant.

[0090] In step 910, interference is caused between each of grating light beams 122G, 122G’ diffracted from the optical grating 102 set in said two positions G, G’, and a reference light beam 122M reflected from a reference 116.

[0091] In step 912, at least two predetermined relative wavefront shifts are caused between light beams propagated through the first optical and the second optical path 202, 204.

[0092] In step 914, the camera 124 forms interferograms of the interferences of the two different positions G, G’ of the optical grating 102 with the at least two predetermined wavefront shifts.

[0093] In step 916, the data processing unit 140 receives and processes the interferograms of both of the positions of the optical grating 102 and the at least two relative wavefront shifts.

[0094] In step 918, the user interface 144 presents data on the optical grating 102 based on the interferograms.

[0095] It will be obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the example embodiments described above but may vary within the scope of the claims.

Claims

What is claimed is:

1. An optical measurement system for measuring an optical grating (102), c h a r a c t e r i z e d in that the optical measurement system comprises a light source (100) configured to output collimated light beam (200) that is monochromatic and coherent; an interferometer arrangement (5) comprising a first optical path (202) to and from an optical grating (102) and a second optical path (204) to and from a reference (116), the interferometer (5) being configured to split the collimated light beam (200) into a first light beam (104) and a second light beam (106), guide the first light beam (104) through the first optical path (202) periodic diffraction structures (260) of the optical grating (102), the optical grating (102) being measured in two different positions (G, G’), a light spot (300) of the first light beam (104) being configured to cover a plurality of periodic diffraction structures (260) of the optical grating (102), the two positions (G, G’) of the optical grating (102) being configured to cause back-diffraction to the interferometer (5), guide the second light beam (106) through the second optical path (204) to the reference (116), maintain the collimation and the cross section of said first and second light beams (104, 106) constant, cause interference between each of grating light beams (122G, 122G’) diffracted from the optical grating (102) set in said two positions (G, G’), and a reference light beam (122M) reflected from a reference (118), and cause at least two predetermined relative wavefront shifts between light beams propagated through the first optical and the second optical path (202, 204), and / or the light source (100) is configured to cause the at least two predetermined relative wavefront shifts;a camera (124) configured to form interferograms of the interference in the two different positions (G, G’) of the optical grating (102) with the at least two predetermined wavefront shifts; and a data processing unit (140) is configured to receive and process the interferograms of the two positions (G, G’) of the optical grating (102) and the at least two relative wavefront shifts, and present data on the optical grating (102) based on the interferograms.

2. The optical measurement system of claim 1, c h a r a c t e r i z e d in that the optical grating (102) is set in a first position (G) and a second position (G’) of the two positions (G, G’) for the formation of the interferograms based on at least two predetermined relative wavefront shifts in both of the positions (G, G’), the first position (G) and the second position (G’) being Littrow positions causing opposite orders of diffractions back into the direction of the incident beam.

3. The optical measurement system of claim 1, c h a r a c t e r i z e d in that the optical measurement system comprises an illumination arm (10), a reference arm (12), a grating arm (14), reception arm (16) and a beamsplitter arrangement (18); the illumination arm (10) is configured to receive the collimated light beam (200) from the optical light source (100); the beamsplitter arrangement (18) is configured to receive the collimated light beam (200), split the collimated light beam (200) to the first light beam (104) and the second light beam (106); the grating arm (14) is configured to receive the first light beam (104) and direct the first light beam (104) to the optical grating (102), and the optical grating (102) is configured to direct third light beams (104’, 104"), which are caused by diffraction in each of the two positions (G, G’) of the optical grating (102), to the beamsplitter arrangement (18); the reference arm (12) comprises the reference (116), which is configured to direct a fourth light beam (106’) to the beamsplitter arrangement (18) as a reflection of the second light beam (106);the beamsplitter arrangement (18) is configured to direct, on the basis of an interaction between the beamsplitter arrangement (18) and the third and fourth light beams (104’, 104”, 106’), the grating light beam (122G, 122G’) and a reference light beam (122M) to the reception arm (16), which is configured to enable interference between the grating light beam (122G, 122G’) and the reference light beam (122M); a camera (124) is configured to capture interferograms of the interference in both of the two different positions (G, G’) of the optical grating (102); and a data processing unit (140) is configured to form and present information on at least one of the following based on data on interferograms: a grating constant, variation of the grating constant and deviation from a desired grating constant.

4. The optical measurement system of claim 3, c h a r a c t e r i z e d in that the measurement system comprises the beamsplitter arrangement (18) comprises a beamsplitter (130) configured to split the collimated light beam (200) into the first light beam (104) and the second light beam (106); the reference arm (12) comprises the reference (116) configured to receive the second light beam (106) and direct the fourth light beam (106’) to the beamsplitter (130) as a reflection of the second light beam (106); the beamsplitter (130) is configured to direct the first light beam (104) to the optical grating (102) that is configured to direct the third light beams (104’, 104") to the beamsplitter (130) caused by diffraction of the first light beam (104) in the two different positions (G, G’) of the optical grating (102); at least one of the optical grating (102) and the reference (116) is configured to be shifted in direction of the propagation of light for causing the predetermined relative phase wavefront shifts; and the beamsplitter (130) is configured to interact with the third light beam (104’, 104") and the fourth light beam (106’) for forming the interference ofthe grating light beam (122G, 122G’) and the reference light beam (122M) and directing them to the camera (124).

5. The optical measurement system of claim 1, characterized in that the data processing unit (140) is configured to compute at least one of the following based on data on interferograms: a grating constant, variation of the grating constant and deviation from a desired grating constant.

6. The optical measurement system of claim 1, characterized in that data processing unit (140) configured to present data on the variation of the grating constant and / or the deviation from a desired grating constant as a function of a location on the grating (102).

7. The optical measurement system of claim 1, characterized in that the reference (116) is configured to move for causing the at least two predetermined wavefront shifts.

8. The optical measurement system of claim 1, characterized in that the camera (124) is based on polarization and comprises at least one linear polarizer (126); the interferometer (5) is configured to direct the first light beam (104) in a form of a first circularly polarized light beam (112) to the optical grating (102), and direct the second light beam (106) in a form of a second circularly polarized light beam (114) toward the reference (116), the polarization directions of the first circularly polarized light beam (112) and the second circularly polarized light beam (114) having 90° difference; and the interferometer (5) is configured to receive third circularly polarized light beams (112’, 112") from the optical grating (102) as diffraction of the first circularly polarized light beam (112) in response to the two positions (G, G’) of the optical grating (102), receive a fourth circularly polarized light beam (114’) as a reflection of the second circularly polarized light beam (114) from the reference (106),convert the third circularly polarized light beams (112’, 112") into third linearly polarized light beams (104’, 104"), and the fourth circularly polarized light beam (114’) into a fourth linearly polarized light beam (106’), combine the third linearly polarized light beams (104’, 104") and the fourth linearly polarized light beam (106’); convert the third linearly polarized light beams (104’, 104") and the fourth linearly polarized beam (106’) into the grating light beams (122G, 122G’) of circular polarization and the reference light beam (122M) of circular polarization, respectively, for causing the interference between the grating light beams (122G, 122G’) and the reference light beam (122M) separately in the two positions (G, G’) of the optical grating (102); and the polarization camera (124) is configured to capture the interferograms of the interference, the interferograms including information on relative spatial wavefront phase distributions relating to the cross section of the first circularly polarized beam (112) on the optical grating (102) in at least two directions of the at least one polarizer (126) and the two different positions (G, G’) of the optical grading (102), the at least two directions of the at least one polarizer (126) being configured to cause the predetermined wavefront shifts between the grating light beams (122G, 122G’) and the reference light beam (122M).

9. The optical measurement system of claim 8, c h a r a c t e r i z e d in that the optical light source (100) is configured output collimated light beam (200) of linear polarization to the interferometer (5), a beam splitter (130) of the interferometer (5) is configured to split, based on polarization, the linearly polarized optical source beam (200) into the first light beam (104) of linear polarization and the second light beam (106) of linear polarization, the polarization direction of the beam splitter (102) and the polarization direction of the light beam (200) being configured to deviate by 45° from each other; the beam splitter (130) is configured to direct the first linearly polarized light beam (104) to a first waveplate (108) of the interferometer (5), the firstwaveplate (108) being configured to convert the first linearly polarized beam (104) into the first circularly polarized light beam (112); the first waveplate (108) is configured to direct the first circularly polarized light beam (112) to the optical grating (102), which is configured to diffract the first circularly polarized light beams (112’, 112") to the first waveplate (108) as the back-diffraction in response to the two different positions (G, G’) of the optical grating (102); the first waveplate (108) is configured to convert the first circularly polarized light beams (112’, 112") into the third polarized light beams (104’, 104") of linear polarization, the third linearly polarized light beams (104’, 104") being configured to propagate to the beam splitter (102); the beam splitter (102) is configured to direct the second linearly polarized light beam (106) to a second waveplate (110) of the interferometer (5), the second waveplate (110) being configured to convert second linearly polarized light beam (106) into a second circularly polarized light beam (114); the reference (116) is configured to receive the second circularly polarized beam (114) and reflect it as a fourth circularly polarized beam (114’) back to the second waveplate (110), which is configured to convert the fourth circularly polarized beam (114’) into a fourth polarized beam (106’) of linear polarization, and forward the fourth linearly polarized beam (106’) to the beam splitter (102), an amount of the rotation of a polarization direction of the fourth linearly polarized light beam (106’) and the second linearly polarized light beam (106) deviating by 90° from each other; and the beam splitter (102) is configured to combine, based on polarization, the third and fourth linearly polarized light beams (104’, 104", 106’), and direct them to a third waveplate (120) of the interferometer (5), the third waveplate (120) being configured to convert them to the grating light beams (122G, 122G’) of circular polarization and the reference light beam (122M) of circular polarization, respectively, for causing the interference therebetween.

10. The optical measurement system of claim 8 or 9, c h a r a c t e r i z e d in that the polarization camera (124) comprises a matrix(400) of groups (402) of detecting elements (404), at least two detecting elements (404) of each group (402) having linear polarizer elements (126’) of the at least one linear polarizer (126) that are configured to have different polarization directions.

11. The optical measurement system of claim 9 or 10, characterized in that the matrix (400) comprises groups (402) of four detecting elements (404) each detecting element of each group (402) having linear polarizer elements (126’) of different polarization directions for forming four interferograms related to each position of the optical grating (102).

12. The optical measurement system of claim 10, characterized in that the linear polarizer (126) comprises a linear polarizer component (126") that is configured to be turned in a plurality of positions that are configured to cause different polarization directions of the grating light beam (122G, 122G’) and the reference light beam (122M) that are configured to propagate to the detection elements (404) of the camera (124).

13. An optical measurement system of claim 1, characterized in that the data processing unit (140) comprises one or more processors (800), and one or more memories (802) including computer program code; the one or more memories (802) and the computer program code configured to, with the one or more processors (800), cause apparatus at least to: receive and process the interferograms of both of the positions (G, G’) of the optical grating (102) and the at least two predetermined relative wavefront shifts; and present data on the optical grating (102) based on the interferograms.

14. An optical method of measuring an optical grating (102), characterized by outputting (900), by a light source (100), collimated light beam (200) that is monochromatic and coherent; performing the following by an interferometer (5)splitting (902) the collimated light beam (200) into a first light beam (104) and a second light beam (106), guiding (904) the first light beam (104) through the first optical path (202) to periodic diffraction structures (260) of the optical grating (102), the optical grating (102) being measured in two different positions (G, G’), a light spot (300) of the first light beam (104) covering a plurality of the periodic diffraction structures (260) of the optical grating (102), the two positions (G, G’) of the optical grating (102) causing back-diffraction to the interferometer (5), guiding (906) the second light beam (106) through the second optical path (204) to the reference (116), keeping (908) the collimation and the cross section of said first and second light beams (104, 106) constant, and causing (910) interference between each of grating light beams (122G, 122G’) diffracted from the optical grating (102) set in said two positions (G, G’), and a reference light beam (122M) reflected from a reference (118); and causing (912) at least four predetermined relative wavefront phase shifts between light beams propagated through the first optical and the second optical path (202, 204); forming (914), by a camera (124), interferograms of the interferences of the two different positions (G, G’) of the optical grating (102) with the at least two predetermined wavefront shifts; and receiving and processing (916), by a data processing unit (140), the interferograms of both of the positions of the optical grating (102) and the at least two relative wavefront shifts; and presenting (918), by a user interface (144), data on the optical grating (102) based on the interferograms.

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