JP7350296B2 - Optical alignment based on spectral control interferometry - Google Patents

Description

FIELD OF THE INVENTION This invention relates generally to interferometry and, more particularly, to a new method of aligning components of optical instruments using spectrally controlled interferometry.

Optical instruments are very complex and their performance depends on the precise positioning of each optical element within the assembly. Each element must be placed and aligned within the optical axis of the system. Accordingly, various techniques have been developed to address alignment issues for manufacturing and maintenance purposes. A common approach is to use a laser beam as the reference axis. The optical elements of the instrument are placed in the appropriate locations along the axis by observing the interference patterns caused by reflections from the surfaces of the various elements. However, as the number of elements in the system increases, the analysis of these interference patterns becomes increasingly complex.

A typical alignment setup using a laser beam for alignment is shown in FIG. Laser 10 emits a laser beam 12 that is used to illuminate the optical system through a small hole in screen 14. As the highly coherent beam 12 propagates through the optical axis 16 of the system, a portion of the beam reflects off each refractive element in the system (e.g., beams 18, 20 from lens 22, beam 24 from lens 28, , 26). The reflected light propagates in a direction opposite to the irradiation direction of the beam 12 and can be observed on the screen 14. Due to the high temporal coherence of the laser beam, the reflected light interferes and forms characteristic so-called bullseye patterns that are used to guide the alignment process. The first optical element 22 of the system is typically used to align the beam with its optical axis and establish a coordinate system. This is done by manipulating the elements such that a bullseye with a symmetrical configuration is centered within the image formed on the screen 14, thereby creating a single Form the typical interference pattern produced by a lens.

Analysis of such a bull's eye interference pattern is straightforward when there are only two optical interfaces, as in the case of a single lens. However, as more optical elements are added to the alignment path, coherent reflections from each interface in the system interfere pairwise, producing a pattern that is difficult to determine the source of misalignment; becomes difficult. As a result, this alignment method is also difficult to automate and instead requires skilled personnel. All of these things are not suitable for use in large scale production. The present invention addresses this problem by applying the properties of Spectrally Controlled Interferometry (SCI).

US Patent No. 8,422,026 US Patent No. 8,810,884 U.S. Patent No. 8,675,205 US Patent No. 5,398,113

The present invention uses spectrally controlled interferometry for different alignment processes that allow alignment of each component based on interference fringes created independently of other components of the optical train of the instrument under alignment. It is recognized that the characteristics of (SCI) can be used advantageously. Additionally, SCI allows introducing phase shifts in the interference fringes without physically scanning the object. These two features are therefore particularly suitable for automating the alignment process.

Another important advantage of the SCI-based approach is that a well-defined coordinate system can be generated in space that can be easily established by the optical axis of the first optical element. Prior art methods such as autocollimation require mechanical means for reference, typically a high precision air bearing spindle, the axis of which is used for alignment. Such components are expensive and introduce errors into the alignment process.

According to the most general approach, the invention uses a spectrally modifiable light source to produce a light beam with a varying spectral distribution. For all SCI applications, such a light source is modulated such that the interference fringes produce a beam with detectable temporal coherence within the measurement space. The beam generated by the light source is first aligned with the optical axis of the instrument. The first optical element to be aligned is placed at its predetermined location along the optical axis. The spectral modulation of the light source is then configured such that one surface of that element is used as a reference surface for the interfering beam produced by the light source, and reflections from the reference surface and other surfaces of the optical element produce a correlogram. The spectrum of the light source is modulated such that . The correct position of the optical element is determined by adjusting the position of the optical element such that the correlogram matches a bull's eye configuration that meets predetermined design parameters. Finally, the process is repeated with other components of the instrument, using the same reference plane or a more suitable plane from those already placed along the optical train of the instrument. It will be done. The spectral modulation required to generate the bull's eye correlogram used to align each optical component is preferably performed with a sinusoidal modulation of the phase of the light produced by the light source.

According to another aspect of the invention, the process of placing an optical element aligned at a suitable predetermined location along the optical axis of the instrument is such that a correlogram of maximum contrast is formed at that predetermined location. This can be done by changing the modulation period of the light source.

According to yet another aspect of the invention, the beam produced by the SCI light source is expanded by suitable optics to obtain a higher resolution interference correlogram. As a result, the surface under alignment can be captured and detailed information about the optical system can be provided, such as the presence of aberrations that affect the alignment process.

Various other advantages of the invention will become apparent from the novel features particularly pointed out in the following written description and appended claims. To the accomplishment of the above objects, the invention therefore consists of the features illustrated in the following drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and descriptions disclose only some of the various ways in which the invention may be practiced.

FIG. 1 is a schematic diagram of a conventional alignment system that uses a laser beam as a reference to align the various elements of the system with its optical axis. FIG. 2 shows a typical bull's eye interference pattern formed by a prior art alignment process. FIG. 3 shows a spectrum-controlled interferometry-based alignment system according to the invention. FIG. 4 is a flowchart of the essential steps necessary to implement the invention.

As used in this disclosure, "white light" typically has a bandwidth on the order of a few nanometers and refers to any type of light used in the field of white light interferometry ("WLI"). It refers to broadband light. WLI and CSI (coherence scanning interferometry) are used interchangeably. With respect to light in general, the terms "frequency" and "wavelength" are used interchangeably, as is common practice in the art, due to their well-known inverse relationship. Because of the space/time relationship in interferometric measurements, "optical path difference" or "OPD" and "time delay" can be used instead. As commonly done in the art with reference to interferometric devices, "optical path difference" and "OPD" are also used to refer to the difference in optical path length between the test and reference arms of the device. Similarly, unless otherwise specified, "sine" and "cosine" and related terms are used instead. The terms "reference surface" and "reference optics" may be used alternatively, as is common practice in the field of interferometry. Similarly, the terms "test surface", "measuring surface", "test article", "test object", "test lens" and optical "element" or "component" all refer to the article that is the subject of measurement. used for. "Optical path difference" or "OPD" and "time delay" may be used alternatively for the space/time relationship in interferometric measurements. With respect to the alignment process of optical elements within the optical train of an optical system, the terms "placed" and "intended (or predetermined) location" refer to the initial placement of each optical element along the optical axis of the system. used for pointing. On the other hand, "position", "positioned", and "positioned" are general in relation to the various positions of an optical element in the process of changing its X/Y, tip/tilt, Z placement in the alignment process. used for.

The terms "modulate" and "modulate" with respect to a light source shall include, in the broadest sense, any change in the frequency, amplitude, or phase distribution of the energy produced by the light source. ,Also. It is implied that optical signals having a desired frequency, amplitude, or phase distribution are synthesized by any means. When used in connection with interference fringes, the term "modulation" refers to the envelope of the interference fringes. For spectrally controlled light sources or multi-wavelength light sources, "localized interference fringes" is intended to mean clearly distinguishable interference fringe patterns formed at a predetermined distance from a reference plane. . Localized interference fringes are described as being located on the surface on which they are generated to show how they relate to the surface and surface geometry that generates them. However, such physically localized interference fringes are merely virtual interference fringes, and actual interference fringes are formed only on the surface of the detector. Also, although the expression "generates localized interference fringes at a predetermined position in space" and related expressions are used for convenience, the precise meaning intended is "to generate an environment for interferometric measurements relative to a reference plane." It is understood that the objective is to create a pattern of interference fringes that is clearly distinguishable when the test surface is brought into position in space. The terms "fringes," "fringe pattern," "fringes," and "correlogram" are used interchangeably within the meanings commonly assigned in the art. The general term "interferometry" and related terms are to be interpreted broadly as used in the art and are not limited to shape measurements using imaging interferometers. Interferometry is therefore intended to include the measurement of changes in the position of an object, or the measurement of the thickness of an optical element, using any known interferometry technique. Finally, the term "spectrally controllable light source" refers to a light source that is capable of spectrally modulation. For example, single-component spectrally controllable light sources, such as currently used spectrally modulated lasers, or multi-component light sources, such as light sources that include a broadband light source and a modulator as separate components.

The invention is based on the use of spectrally controlled interferometry to facilitate alignment of optical systems. As described in numerous shared publications (see, e.g., U.S. Pat. WLI) is an interferometric technique that allows the implementation of measurement schemes. WLI (coherence scanning interferometry, also defined as CSI) eliminates the spurious interference problem (coherent noise) that exists in traditional laser interferometry because the coherence length of light is typically on the order of a few micrometers. It is a feature. Therefore, the multiple interference fringe patterns produced by coherent light used in conventional alignment systems are not formed in CSI systems.

SCI successfully combines the advantages of both common path interferometry and WLI. To generate localized interference fringes in an unbalanced OPD interferometer, it allows surface measurements in a large space in front of the interferometer. Therefore, it is suitable for axial alignment of various optical elements of optical instruments. As described in many SCI-related disclosures and further detailed herein, by manipulating only the spectral characteristics of a spectrally controllable light source according to SCI principles, many different interferometric techniques for analysis can be performed. .

Essentially, spectrally controlled interferometry is based on the ability to use an interferometer under unbalanced OPD conditions to form localized interference fringes at a predetermined distance from a reference plane. For example, by modulating the spectrum of the light source, it is possible to form such localized interference fringes, and by changing the mode of modulation, it is also possible to phase shift the interference fringes, This allows measurements of the test article using modern fringe analysis methods without physically scanning the object or reference surface. Therefore, in addition to the implementation of interferometry in the WLI and the traditional laser interferometer mode with the aforementioned advantages, SCI allows the measurement of isolated surfaces and direct measurement of the distance from the reference plane of the interferometer. enable.

The distribution and phase of interference fringes produced by an interferometer are governed by the Wiener-Khinchin theorem (Born M, Wolf E., Principles of optics: electromagnetic theory of propagation, interference and diffraction of light, 7th Expand Ed. ., Cambridge, New York: Cambridge University Press; 1999) and is expressed as the Fourier transform of the spectral power distribution of the light source. SCI teaches that by modulating the spectrum of a light source, it is possible to define the position and distribution of interference fringes in space. For a light source with an average wavelength λ ₀ , Equation 1 below describes the distance of the position L of fringe formation (i.e. the peak of fringe contrast) from the reference plane as a function of the period Δλ of the spectral modulation.

For example, to form interference fringes at a distance of 1 meter from the reference plane with a light source operating at an average wavelength of 500 nm, the modulation period must be 0.125 pm. This property, along with techniques used in the art for analyzing interference fringes, such as white light scanning and phase-shifting interferometry algorithms, is the basis for implementing SCI.

In most cases, it is convenient to modulate the spectrum using a sine function. This function produces a single location in the measurement space where the interference fringes are visible (the other locations are at the conjugate location outside the measurement space and at the zero OPD of the reference plane). In such cases, the phase of the interference fringes is tied to the phase of the modulated signal. Equation 2 below shows the interference pattern resulting from sinusoidal modulation of the source spectrum.

where λ ₀ is the wavelength, z is the distance from the zero OPD point (relative to the reference plane), Λ is the total bandwidth of the light source, and φ is the phase of the spectral modulation. Therefore, given a known light source with a fixed bandwidth and average wavelength, determine the position of the test surface relative to the reference plane by determining the wavelength modulation period required to produce the highest contrast fringes. be able to.

Based on these advantages provided by SCI, FIG. 3 schematically depicts an exemplary system suitable for alignment of optical instruments. As in conventional approaches, a spectrally controllable light source 30 is used to generate a narrow alignment beam 32 along the axis of the instrument under alignment. However, instead of the screen 14 shown in FIG. 1, a beam splitter 34 is introduced. Beam splitter 34 directs the light reflected from the optics to camera 36, which is used to view and record the interferometric image. The SCI light source 30 is controlled to produce changes in both the phase and period of modulation to selectively interfere with reflections from various interfaces of the optical system. Because SCI can form low-coherence interference fringes at desired locations in space in front of the light source, the present invention allows each optical component of the instrument to be analyzed and adjusted individually without interference from other elements of the system. I can do it.

The alignment process therefore relies on interfering beams selectively reflected only from each of the various optical elements along the optical axis of the instrument. We begin by placing the first optical element 38 in its intended location and aligning it with the beam 32 to establish a frame of reference. Typically, the direction of light propagation along the optical axis of the device is chosen as the Z direction. X and Y are orthogonal directions in the plane perpendicular to Z, and one vertex, preferably the first vertex, of the surface of element 38 is used as the origin of the coordinate system. Reflections 42 and 44 from the two surfaces of the first optical element 38 are interfered by modulating the light source according to Equation 1 such that the distance L is equal to the optical thickness of the element 38 along the optical axis of the beam 32. . For this purpose, the two surfaces of element 38 are considered as reference surfaces and test interference surfaces, and the interference fringes (bull's eyes) produced by reflections thereon are monitored for alignment purposes. The position of the elements is adjusted until the alignment criterion is met by the bullseye pattern satisfying a predetermined configuration measured with optical fit parameters (position and symmetry of the fringe intensity pattern registered with the camera or (e.g. by measuring phase).

Next, another element, such as element 40 of FIG. 3, is added to the intended location of the system and the light source is adjusted in the same way to create interference between the front and back surfaces of the element. The elements are then aligned using conventional methods to reduce the resulting interference fringes into as nearly a perfect bull's eye as possible. And it can be done by visually monitoring the interference fringes or by automated processing based on fringe pattern analysis. After the second element is aligned, another element can be introduced. A similar process is then repeated for the elements being aligned until all elements in the system are positioned according to predetermined quality control parameters. SCI has the flexibility to form (and thus analyze) correlograms produced by interference between any two surfaces along beam 32, so that only the interference fringes produced by the selected surfaces are considered for alignment. This contributes to the bull's eye pattern used for the purpose, thereby avoiding spurious interference fringes present in conventional coherent optical systems.

Furthermore, in addition to the alignment in terms of lateral position It is possible to arrange each element at the correct position (predetermined design position) along the Z direction. Furthermore, by changing the period of spectral modulation, interference fringes can be placed at any position in the space within the optical system during alignment, thereby satisfying the need for absolute distance measurement. When used with white light, such a modulation method allows precise detection of optical elements along the beam path, allowing adjustment of the Z configuration to the required design parameters. This is another unique feature not found in other alignment methods.

Additionally, SCI allows the formation of selective interference fringes on the two surfaces at any practical location along the optical axis of the instrument measured for element alignment, so It is also important to note that each new element taken can be placed anywhere on the instrument, either in front of or behind the Z position of the last measured element. Each alignment step involves a computer algorithm analyzing the initial correlogram and using feedback signals to drive a positioning mechanism to reposition the elements until the final correlogram is a bull's eye pattern that meets the predetermined alignment parameters. This can be done automatically by applying conventional automated processes. It can also be performed visually by a skilled operator viewing the shape of the bull's eye pattern generated by the elements of interest on a monitor.

Another advantage of the invention lies in the option provided by the SCI light source to generate an expanded beam using suitable optics instead of the narrow beam described with reference to FIG. By doing so, high-resolution interferometric images of the aligned surfaces can be captured and provide more information about the optical system, such as the presence of aberrations that affect the alignment of the optical system. Furthermore, under these conditions, other methods in addition to the bull's eye pattern can be used to find the correct alignment position of each optical element. For example, the modulation content of the wavefront can provide additional information regarding the location of elements within the system.

Thus, a new method has been disclosed for aligning elements of an optical system in terms of Z position as well as X, Y and tip/tilt alignment. This method has two important advantages over traditional approaches. One is the ability to selectively interfere with only two beams at a time by appropriately choosing the period of the spectral modulation, and the other is the ability to introduce phase shifts in the interference fringes. These enable automated analysis methods. Therefore, the analysis and automation of interference fringes in the alignment process is greatly simplified. FIG. 4 is a flowchart overview of the important steps involved in implementing the invention. Although actual solutions will likely be optimized in a more complex manner to address a particular problem, the description provided herein will help those skilled in the art to understand the important aspects of the invention. It provides a sufficient foundation to be able to understand and build upon.

Although the invention has been shown and described herein in what is considered to be the most practical and preferred embodiment, it will be recognized that deviations therefrom are possible. For example, although a reference surface is described throughout this disclosure as one of the surfaces on one optical element that is aligned according to the invention, other surfaces that are located along the optical axis of the interfering wavefront during measurement are Surfaces can also be used in an equivalent manner. Similarly, it is stated that correct Z positioning of the optical element is preferably carried out by changing the modulation period of the SCI light source in order to identify the position corresponding to maximum contrast. However, those skilled in the art will readily appreciate that other techniques for identifying the correct Z position of a test surface can be used as well, such as the method taught in US Pat. Therefore, the invention is not to be limited to the details disclosed, but is to be accorded the full scope of the appended claims, including all equivalent apparatus and methods.

12, 32... Beam, 16... Optical axis, 30... Light source, 38, 40... Optical element, 42, 44... Reflection

Claims (15)

Spectral control interferometry for aligning optical elements along an optical axis, the method comprising:
providing a spectrally modulated light source that produces a light beam with a varying spectral distribution, the interference fringes having detectable temporal coherence in a measurement space along the optical axis;
modulating the light source to produce a correlogram formed by reflections from a reference surface and a surface of an optical element located at a predetermined location along the optical axis;
adjusting the position of the optical element such that the correlogram satisfies a predetermined configuration indicative of optical alignment of the optical element;
method including. 2. The method of claim 1, wherein the predetermined location along the optical axis is determined by a change in the modulation period of the light source. 3. The method of claim 2, wherein the modulation period of the light source is varied so as to form a correlogram of maximum contrast when the optical element is located at the predetermined location. 2. The method of claim 1, wherein the correlogram is generated by changing the phase of modulation of a light source. 2. The method of claim 1, wherein the correlogram is generated by sinusoidal spectral modulation of a light source. 2. The method of claim 1, wherein the predetermined configuration is a bull's eye configuration. 2. The method of claim 1, wherein the reference surface is another surface of the optical element. 2. The method of claim 1, further comprising repeating the modulating and adjusting steps for each additional optical element positioned at corresponding additional predetermined locations along the optical axis. 9. The method of claim 8, wherein the corresponding additional predetermined location along the optical axis is determined by a change in the modulation period of the light source. 10. The modulation period of the light source is modified so as to form a correlogram of maximum contrast when each of the additional optical elements is located at the corresponding additional predetermined location. Method described. 10. The method of claim 9, wherein the correlogram is generated by changing the phase of modulation of the light source. 10. The method of claim 9, wherein the correlogram is generated by sinusoidal spectral modulation of the light source. 10. The method of claim 9, wherein the predetermined configuration is a bull's eye configuration. 10. The method of claim 9, wherein the reference surface is the other surface of each of the additional optical elements. Spectral control interferometry for aligning optical elements along an optical axis, the method comprising:
providing a spectrally modulated light source that produces a light beam with a varying spectral distribution, the interference fringes having detectable temporal coherence in a measurement space along the optical axis;
placing an optical element at a predetermined location along the optical axis;
changing the modulation period of the light source so as to form a correlogram of maximum contrast when the optical element is in place;
sinusoidally modulating the phase of the light source to generate a correlogram formed by reflections from the test surface and the surface of the optical element;
adjusting the position of the optical element such that the correlogram satisfies a predetermined bull's eye configuration indicative of optical alignment of the optical element;
placing additional optical elements at additional predetermined locations along the optical axis; and
repeating the modifying, modulating, and adjusting steps using the additional optical element;
method including.

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