Interferometer for measuring properties of diffractive optics

Abstract

An interferometer instrument for measuring properties of diffractive optics includes a quantum cascade laser configured to emit light at a wavelength centered at approximately 4.2 microns, a beam expander configured to expand a beam emitted by the quantum cascade laser, a beamsplitter configured to split the expanded beam into a reference beam and a test beam, a reference mirror configured to reflect the reference beam, a test path configured to direct the test beam to a diffractive optical element under test, and a detector configured to capture an interference pattern formed by the combination of the reflected reference beam and the test beam after interaction with the diffractive optical element. The instrument enables improved measurement accuracy for diffractive optics designed for operation in the Mid-Wave Infrared wavelength range.

Description

Interferometer for Measuring Properties of Diffractive Optics FIELD OF INVENTION

[0001] The present disclosure relates to interferometric measurement systems, and more particularly to an interferometer instrument utilizing a quantum cascade laser for measuring properties of diffractive optics in the Mid-Wave Infrared (MWIR) regime.BACKGROUND

[0002] Interferometry is a widely used technique in optical testing and metrology. It allows for precise measurements of surface profiles, wavefront distortions, and optical path differences by analyzing interference patterns created when light waves are combined. Traditional interferometers typically employ monochromatic light sources, such as helium-neon lasers operating at 632.8 nanometers wavelength, to generate interference fringes.

[0003] In the field of optics manufacturing and testing, there is an ongoing trend towards developing more complex optical components with diffractive and aspheric surfaces. These advanced optical components can provide improved performance and enable more compact optical systems. However, they also present challenges for conventional interferometric measurement techniques.

[0004] Diffractive optical elements in particular can be difficult to measure accurately using standard interferometry approaches. This is because the diffractive structures are often designed to operate at specific wavelengths that may differ from the wavelength of the interferometer's light source. When measured at a non-design wavelength, diffractive optics can produce complex fringe patterns that are challenging to interpret.

[0005] Additionally, the surface profiles of diffractive optics frequently contain steep local slopes and discontinuities. These features can lead to fringe densities that exceed the resolution capabilities of typical interferometer detectors and analysis algorithms. As a result, conventional interferometry may provide incomplete or inaccurate measurements of diffractive optical surfaces.

[0006] There is an increasing demand for metrology tools and techniques that can provide high-precision, full-aperture measurements of diffractive and othercomplex optical surfaces. Improved measurement capabilities could enable tighter manufacturing tolerances, more effective quality control processes, and ultimately lead to higher performance optical systems incorporating diffractive elements.

[0007] Advancements in light sources, detectors, and computational techniques offer opportunities to enhance the capabilities of interferometric systems for measuring diffractive optics. New approaches that can overcome the limitations of traditional monochromatic interferometry when applied to diffractive elements would be beneficial to the optics industry.SUMMARY

[0008] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0009] According to an aspect of the present disclosure, an interferometer instrument for measuring properties of diffractive optics is provided. The interferometer instrument includes a quantum cascade laser (QCL) configured to emit light at a wavelength centered at approximately 4.2 microns. The interferometer instrument is configured to perform interferometric measurements on diffractive optics designed for operation in the Mid-Wave Infrared (MWIR) wavelength range. The 4.2 micron wavelength is often chosen because it is common to design the diffractive surfaces for a central wavelength around 4.2 microns. For diffractive optics designed to operate within a sub-spectrum in the MWIR or any other spectral range, a QCL with an appropriate central wavelength within that sub-spectrum is chosen. For example, for diffractive optics designed to operate in the range 3.40 microns to 4.10 microns, a QCL having a center wavelength of around 3.70 microns may be chosen.

[0010] According to other aspects of the present disclosure, the interferometer instrument may include one or more of the following features. The QCL may be configured to emit light at a wavelength that matches a design wavelength of the diffractive optics being measured. The interferometer instrument may be configured to eliminate or reduce broken fringes in interference patterns produced whenmeasuring the diffractive optics. The interferometer instrument may be configured to perform measurements on individual optical elements or on optical assemblies containing diffractive optics. The MWIR wavelength range may be approximately 3.6 microns to approximately 4.9 microns.

[0011] According to another aspect of the present disclosure, a method of measuring properties of diffractive optics is provided. The method includes emitting light from a quantum cascade laser (QCL) at a wavelength centered at approximately 4.2 microns, and performing interferometric measurements on diffractive optics designed for operation in the MWIR wavelength range using the emitted light from the QCL.

[0012] According to other aspects of the present disclosure, the method may include one or more of the following features. The method may include selecting the wavelength of the QCL to match a design wavelength of the diffractive optics being measured. The method may include eliminating or reducing broken fringes in interference patterns produced when measuring the diffractive optics. The method may include performing measurements on individual optical elements or on optical assemblies containing diffractive optics. The MWIR wavelength range may be approximately 3.6 microns to 4.9 microns.

[0013] The foregoing general description of the illustrative embodiments and the following detailed description thereof are merely exemplary aspects of the teachings of this disclosure and are not restrictive.BRIEF DESCRIPTION OF FIGURES

[0014] Non-limiting and non-exhaustive examples are described with reference to the following FIGS.

[0015] FIG. 1 shows an interference image from a prior art interferometer instrument, showing distortions in the interference pattern, and a list of measured properties of an optical assembly measured by the prior art interferometer instrument.

[0016] FIG. 2 shows a schematic of an embodiment of an improved interferometer instrument used for measuring the properties of optical elements or optical assemblies.

[0017] FIG. 3 shows an image of the display of the embodiment of an improved interferometer instrument, showing multiple measured properties of the optical assembly referred to in FIG. 1.

[0018] FIG. 4 shows a detailed view of the interference image shown in FIG. 3, showing a corrected image of the interference pattern and measured properties of using the improved interferometer instrument.

[0019] FIG. 5 is a table comparing measurement results from the same optical assembly measured by the prior art interferometer instrument and by the improved interferometer instrument.DETAILED DESCRIPTION

[0020] The following description sets forth exemplary aspects of the present disclosure. It should be recognized, however, that such description is not intended as a limitation on the scope of the present disclosure. Rather, the description also encompasses combinations and modifications to those exemplary aspects described herein.

[0021] Unless otherwise expressly stated, in the drawings the sizes, positions, etc., of components, features, elements, etc., as well as any distances therebetween, are not necessarily to scale, and may be exaggerated for clarity. In the drawings, like numbers refer to like elements throughout. Thus, the same or similar numbers may be described with reference to other drawings even if they are neither mentioned nor described in the corresponding drawing. Also, even elements that are not denoted by reference numbers may be described with reference to other drawings.

[0022] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Also, the terms “at least one”, “at least a”, and “one or more” may are intended to include both the singular and plural forms, depending on the context. It should be recognized that the terms “comprises” and / or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Unless indicated otherwise, terms such as “first,” “second,” etc., are only used to distinguish one element from another. For example, one member could be termed a “first member” and similarly, another member could be termed a “second member”, or vice versa.

[0023] Unless indicated otherwise, spatially relative terms, such as “below,” “beneath,” “lower,” “above,” and “upper,” “opposing,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element or feature, as illustrated in the FIGS. It should be recognized that the spatially relative terms are intended to encompass different orientations in addition to the orientation depicted in the FIGS. For example, if an object in the FIGS, is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. An object may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may be interpreted accordingly. A set of reference axes (e.g., X, Y, Z), directions, or coordinates, and the rotation around them (e.g., 9X, 0Y, 0Z) may be included in the FIGS, for the purpose of orienting the reader to facilitate understanding of the FIGS, and the specification, and do not necessarily indicate that any particular feature or element is aligned with, or is orthogonal to, any other feature or element.

[0024] The paragraph numbers used herein are for organizational purposes only, and, unless explicitly stated otherwise, are not to be construed as limiting the subject matter described. It will be appreciated that many different forms, embodiments and combinations are possible without deviating from the spirit and teachings of this disclosure and so this disclosure should not be construed as limited to the example embodiments set forth herein. Rather, these examples and embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the disclosure to those skilled in the art.

[0025] The present disclosure relates to an interferometer instrument for measuring properties of diffractive optics in the MWIR range. Interferometry is awidely used technique for characterizing optical components and systems by analyzing interference patterns created when light waves interact. In the context of MWIR optics, particularly diffractive elements, traditional interferometric methods may encounter challenges due to wavelength mismatches between the measurement light source and the design wavelength of the optics under test. As used herein, the terms “optical element”, “optic under test”, or “optic” may refer to a single optical component (e.g., a single lens) or an assembly of more than one optical component (e.g., a lens assembly such as a zoom lens). In various embodiments, a diffractive optical element may contain one or a plurality of more diffractive surfaces having the same optical properties or different optical properties.

[0026] This disclosure presents an approach that addresses these challenges by utilizing a quantum cascade laser as the light source in an interferometric measurement system. The quantum cascade laser may be configured to emit light at a wavelength that closely matches the design wavelength of MWIR diffractive optics, typically centered at approximately 4.2 microns. This wavelength matching may allow for more accurate and representative measurements of diffractive optical elements designed for operation in the MWIR range. In some embodiments, the MWIR wavelength range is between approximately 3.0 microns to about 8.0 microns. In some embodiments, other laser sources having broadly tunable wavelengths (e.g., external cavity lasers) may be used. In other embodiments, the quantum cascade laser may be configured to operate in Long-Wave Infrared (LWIR) wavelength range (typically between approximately 8.0 and 14.0 microns).

[0027] The diffractive surface(s) of the optical element / assembly / system is / are designed to work across the entire wide wavelength spectrum of the camera, but is / are most effective at the central wavelength for which the diffractive steps (i.e., diffractive features) are designed. In a wide wavelength spectrum, the benefit of the diffractive surfaces is significantly greater than any loss of fidelity, but when performing a measurement with a monochromatic light source that is not at the central wavelength, measurements can contain errors that are not easily corrected. Because an optical system or assembly often has several diffractive surfaces, wherein each diffractive surface have a different slit frequency and a different effect on the interferometric measurement result, there is currently no way to completely take into account and offset the total effect on the measurement result.

[0028] Quantum cascade lasers can be designed to operate at any wavelength within a typical MWIR range (e.g., about 3.0 microns to about 8.0 microns), and may be configured to be tunable over a smaller wavelength range within that overall range. In the embodiments described herein, the terms “about” and “approximately” when used to describe an operating wavelength or the lower and upper limits of an operating wavelength range, can be within 1%, 5%, or 10% of those values. For example, in one embodiment, the QCL operating at “about” or “approximately” a 4.2 micron wavelength may emit light at a wavelength from between 4.16 microns and 4.24 microns (i.e., within ±1% of the nominal 4.2 micron wavelength). In another example embodiment, the QCL operating at “about” or “approximately” a 4.2 micron wavelength may emit light at a wavelength from between 3.99 microns and 4.41 microns (i.e., within ±5% of the nominal 4.2 micron wavelength). In another example embodiment, the QCL operating at “about” or “approximately” a 4.2 micron wavelength may emit light at a wavelength from between 3.78 microns and 4.62 microns (i.e., within ±10% of the nominal 4.2 micron wavelength).

[0029] The interferometer instrument described herein may incorporate various optical components to manipulate and direct the laser light, creating reference and test beam paths. These paths may be recombined to produce interference patterns that can be captured and analyzed to derive information about the optical properties of the diffractive element under test.

[0030] By employing a quantum cascade laser tuned to an appropriate MWIR wavelength, the interferometer instrument may overcome limitations associated with traditional light sources used in MWIR interferometry. This approach may potentially improve measurement accuracy and provide clearer, more interpretable interference patterns for diffractive optics designed for the MWIR spectral region.

[0031] The following sections will describe in greater detail the components, configuration, and operation of the interferometer instrument, as well as the methods for analyzing the resulting interference patterns to characterize MWIR diffractive optics.

[0032] The interferometer instrument may utilize a quantum cascade laser as its light source. Quantum cascade lasers are semiconductor lasers that emit light in the mid-infrared to far-infrared portion of the electromagnetic spectrum. In the context ofmeasuring MWIR diffractive optics, a quantum cascade laser may be configured to emit light centered at a wavelength of approximately 4.2 microns.

[0033] The selection of a 4.2 micron wavelength for the quantum cascade laser may provide several advantages for measuring MWIR diffractive optics. This wavelength falls within the typical operating range of MWIR optical systems, which spans from approximately 3.6 microns to approximately 4.9 microns. In some cases, the operating range of the MWIR optical systems spans from approximately 3.4 microns to approximately 5.0 microns. By using a light source that closely matches the design wavelength (or has a wavelength that is close to the middle of the operating wavelength range) of the diffractive optics under test, the interferometer instrument may produce more accurate and representative measurements. In other embodiments, this wavelength falls within a wider operating range of MWIR optical systems, that spans from approximately 3.0 microns to approximately 8.0 microns.

[0034] In some cases, the quantum cascade laser may be tuned or selected to emit at wavelengths other than 4.2 microns. This flexibility allows the interferometer instrument to be adapted for measuring diffractive optics designed for different specific wavelengths within the MWIR range. The ability to match the measurement wavelength to the design wavelength of various diffractive optical elements may enhance the versatility and applicability of the interferometer instrument.

[0035] A notable characteristic of quantum cascade lasers is their relatively narrow emission linewidth. This property results in a relatively long coherence length for the emitted light. The long coherence length may be beneficial for interferometric measurements, as coherence length directly impacts the maximum optical path difference that can be measured effectively. This feature may allow for precise measurements of optical elements with larger physical dimensions or more complex geometries.

[0036] The quantum cascade laser 210 may be integrated into the interferometer instrument as shown in FIG. 2. In this configuration, the laser 210 serves as the primary light source for the interferometric measurements. The emitted beam 212 from the quantum cascade laser 210 may be expanded and collimated (becoming the collimated beam 216) before being split into a reference beam (as shown by thebidirectional arrows 220) and a test beam (as shown by the bidirectional arrows 224) for the interferometric analysis of the diffractive optical element 10 under test.

[0037] By employing a quantum cascade laser 210 tuned to an appropriate MWIR wavelength, the interferometer instrument 200 may overcome limitations associated with traditional light sources used in MWIR interferometry. This approach may potentially improve measurement accuracy and provide clearer, more interpretable interference patterns for diffractive optics designed for the MWIR spectral region.

[0038] The interferometer instrument 200 may incorporate various optical components to manipulate and direct the laser light for precise measurements of diffractive optics. These components may include a beam expander 214, a beamsplitter 218, and reflectors (e.g., mirrors 222 and 230), each playing a specific role in the measurement process. FIG 2 presents a schematic of the key components and setup of a Twyman-Green interferometer. In other embodiments, a Fizeau interferometer may be used.

[0039] As illustrated in FIG. 2, the interferometer instrument 200 may include a beam expander 214 positioned after the quantum cascade laser 210. The beam expander 214 may serve to increase the diameter of the laser beam 212, creating a wider, collimated beam 216. This beam expansion may be beneficial for several reasons. In one case, the beam expansion is done to match the aperture diameter of the optical element being tested. A wider beam may allow for more uniform illumination of the optical element under test, potentially improving measurement accuracy. Additionally, expanding the beam may reduce its divergence, which may be advantageous for maintaining beam quality over longer optical paths within the instrument 200.

[0040] Following the beam expander 214, the interferometer instrument 200 may utilize a beamsplitter 218. The beamsplitter 218 may be a crucial component in the interferometer, as it may divide the collimated laser beam 216 into two separate paths, such as a reference beam 220 and a test beam 224. In some cases, the beamsplitter 218 may be a partially reflective optical element, allowing a portion of the incident light to pass through while reflecting the remainder. This division of thebeam collimated beam 216 may enable the creation of two coherent light paths that can later be recombined to produce interference patterns.

[0041] The interferometer instrument 200 may also incorporate various mirrors to guide and redirect the light beams. A reference mirror 222 may be positioned in the path of the reference beam 220. This mirror may reflect the reference beam 220 back towards the beamsplitter 218, maintaining a consistent optical path length for comparison with the test beam 220.

[0042] In the test beam path, a flat mirror 230 may be used to redirect the test beam 224 towards the diffractive optical element 10 under test. This arrangement may allow for flexibility in the positioning of the optical element 10 within the instrument, potentially accommodating various sizes and configurations of test samples. In other embodiments, a spherical mirror may be used. Before entering the lens under test, additional optical systems are sometimes also incorporated to adjust further properties of the beam for measurement of the lens under test.

[0043] After interacting with the diffractive optical element 10 under test, the test beam 224 may be reflected or transmitted back towards the beamsplitter 218. At the beamsplitter 218, the reference beam 220 and the test beam 224 may recombine, creating an interference pattern (e.g., shown as 290 in FIG. 4) that contains information about the optical properties of the tested element 10.

[0044] The specific configuration and arrangement of these optical components may vary depending on the design of the interferometer instrument 200 and the requirements of the measurements being performed. In some cases, additional optical elements such as lenses or waveplates may be incorporated to further manipulate the light beams or enhance measurement capabilities.

[0045] By carefully selecting and arranging these optical components, the interferometer instrument 200 may be capable of producing high-quality interference patterns 290 that can be analyzed to characterize the properties of MWIR diffractive optics with improved accuracy and clarity compared to traditional measurement methods.

[0046] The interferometer instrument 200 may include a detector 240 configured to capture the interference patterns produced by the recombination of the reference beam 220 and the test beam 224. This detector 240 may be a specialized imagingsensor sensitive to MWIR wavelengths, particularly around 4.2 microns. In some cases, the detector 240 may be an array of photosensitive elements capable of recording the spatial distribution of light intensity across the interference pattern 290.

[0047] The captured interference pattern 290 may contain valuable information about the optical properties of the diffractive element 10 under test. To extract and interpret this information, the interferometer instrument 200 may incorporate a processor 250 coupled to the detector 240. This processor 250 may be a dedicated computing unit or a general-purpose computer equipped with specialized software for interferometric analysis.

[0048] As illustrated in FIG. 2, the detector 240 may be positioned to receive the combined beams after they have interacted with the optical element 10 under test and the reference mirror 222. The processor 250 may be connected to both the detector 240 and a display 260, forming an integrated system for measurement and analysis.

[0049] The processor 250 may perform various functions to analyze the captured interference patterns. In some cases, these functions may include:

[0050] 1. Image processing: The processor 250 may apply algorithms to enhance the quality of the captured interference pattern image 290, potentially reducing noise and improving contrast.

[0051] 2. Fringe analysis: The processor 250 may identify and characterize the fringes 292 in the interference pattern 290, extracting information about the phase differences between the reference beam 220 and the test beam 224.

[0052] 3. Wavefront reconstruction: Based on the fringe analysis, the processor 250 may reconstruct the wavefront of the light that has passed through or reflected from the diffractive optical element 10 under test.

[0053] 4. Optical parameter calculation: The processor 250 may compute various optical parameters from the reconstructed wavefront, such as peak-to-valley measurements (PV), root mean square (RMS) wavefront error, defocus, coma, astigmatism, field curvature, image distortion, and spherical aberration.

[0054] 5. Modulation Transfer Function (MTF) estimation: In some cases, the processor 250 may calculate the MTF, which provides information about the optical system's ability to transfer contrast at various spatial frequencies.

[0055] The results of these analyses may be presented through a graphical user interface 270 on the display 250. As shown in FIG. 3, this interface 270 may include multiple panels (e.g., 280, 300, and 310) displaying different aspects of the measurement results. The interference pattern image 290 may be presented alongside numerical data and graphical representations of the calculated optical parameters.

[0056] In some cases, the processor 250 may also control other aspects of the measurement process, such as adjusting the quantum cascade laser's wavelength or managing the positioning of optical components within the interferometer instrument. This integration may allow for automated measurement sequences and real-time adjustment of measurement parameters based on the captured interference patterns.

[0057] By combining a sensitive MWIR detector 240 with powerful processing capabilities, the interferometer instrument 200 may provide comprehensive and accurate characterization of diffractive optical elements designed for operation in the MWIR spectral range.

[0058] The interferometer instrument may produce interference patterns that provide valuable information about the optical properties of diffractive elements under test. These patterns may be analyzed to assess the quality and performance of the optical components.

[0059] FIG. 1 illustrates an example of an interference pattern 100 obtained using a traditional 3.39 micron wavelength light source for measuring a diffractive optical element designed for operation in the MWIR range. The interference pattern 100 in FIG. 1 exhibits noticeable distortions, discontinuities, and irregularities 103 in the fringe structure 102, particularly in the upper right portion 103 of the image 100. This phenomenon, often referred to as "broken fringes," may occur when the measurement wavelength does not closely match the design wavelength of the diffractive optics under test.

[0060] In contrast, FIG. 4 demonstrates an interference pattern 290 produced by the interferometer instrument 200 using the quantum cascade laser (QCL) light source 210 centered at approximately 4.2 microns. The interference pattern 290 in FIG. 4 that belongs to the same specific optics under test displays more uniform and continuous fringes 292 across the entire measurement area. The improved fringe quality may be attributed to the closer match between the QCL wavelength and the typical design wavelength of MWIR diffractive optics.

[0061] FIG. 5 provides a side-by-side comparison of interference patterns and measurement results obtained using the 3.39 micron and 4.2 micron light sources. The visual difference in fringe quality between the two light source wavelengths may be readily apparent, with the 4.2 micron measurement showing more consistent and well-defined fringes.

[0062] The quantitative measurements presented in FIG. 5 further illustrate the potential advantages of using a wavelength-matched QCL source for MWIR diffractive optics measurements. The peak-to-valley, root mean square, and spherical aberration values obtained with the 4.2 micron source are all improved compared to those from the 3.39 micron source. This reduction in measured aberrations may indicate improved measurement accuracy when using a light source that more closely matches the design wavelength of the optics under test.

[0063] In some cases, the modulation transfer function (MTF) results may also differ between the two measurement approaches. The MTF data obtained using the 4.2 micron QCL source may indicate performance closer to the diffraction limit, potentially providing a more accurate representation of the diffractive optic's true performance in its intended MWIR operating range.

[0064] By analyzing these interference patterns and the associated quantitative measurements, the interferometer instrument 200 may provide more reliable and representative characterization of MWIR diffractive optics compared to traditional measurement methods using non-matched wavelength sources.

[0065] The interferometer instrument 200 may be capable of performing measurements on both individual optical elements and complete optical assemblies containing diffractive optics designed for operation in the MWIR wavelength range.This versatility allows for comprehensive characterization of MWIR optical components at various stages of integration.

[0066] When measuring individual optical elements, the interferometer instrument 200 may be configured to isolate and analyze the specific properties of a single diffractive optical component. In some cases, this may involve positioning the individual element in the test beam path of the interferometer, as illustrated in FIG. 2. The resulting interference pattern may provide detailed information about the optical characteristics of the standalone diffractive optical component.

[0067] The measurement process may typically begin with the alignment of the optical element or assembly under test within the interferometer instrument 200. Once properly positioned, the quantum cascade laser 210 may emit light at the specified wavelength, which may be directed through the optical element, assembly, or system under test. The resulting interference pattern may be captured by the detector 240 and processed for analysis.

[0068] FIG. 4 illustrates an example of an interference pattern 290 (also referred to herein as the “fringe pattern 290”) obtained from a measurement of a diffractive optical element. The fringe pattern 290 displayed in the image may provide visual indications of the optical quality and performance of the measured diffractive optical element. In some cases, continuous and evenly spaced fringes 292 may suggest good optical quality, while distortions or discontinuities in the fringe pattern may indicate potential aberrations or manufacturing defects.

[0069] The interpretation of measurement results may involve both qualitative assessment of the interference patterns and quantitative analysis of derived optical parameters. As shown in FIG. 4, the measurement results may include numerical values for various optical characteristics, such as peak-to-valley measurement, root mean square (RMS) wavefront error, and spherical aberration.

[0070] In some cases, the modulation transfer function (MTF) may be calculated as part of the measurement results. The MTF may provide information about the optical system's ability to transfer contrast at various spatial frequencies, which may be particularly relevant for assessing the performance of diffractive optics in imaging applications.

[0071] FIG. 5 demonstrates a comparative analysis of measurement results obtained using different light source wavelengths. This type of comparison may be useful for evaluating the impact of wavelength matching on measurement accuracy and for validating the performance of MWIR diffractive optics at their intended operating wavelength.

[0072] By analyzing both the visual interference patterns and the quantitative measurements, engineers and researchers may gain comprehensive insights into the optical properties and performance of MWIR diffractive optics. This information may be valuable for quality control, design verification, and optimization of optical systems operating in the MWIR spectral range.

[0073] The interferometer instrument 200 may integrate various components to perform accurate measurements of diffractive optics designed for operation in the MWIR range. As illustrated in FIG. 2, the system may incorporate a QCL 210 configured to emit light at a wavelength centered around 4.2 microns. This wavelength selection may provide advantages for measuring MWIR diffractive optics compared to traditional light sources.

[0074] The QCL may emit a beam 212 that passes through a beam expander 214, creating a wider, collimated beam 216. This expanded beam 216 may then encounter a beamsplitter 218, which may divide the light into a reference path and a test path. The reference path may direct light to a reference mirror 222, while the test path may guide light towards the diffractive optical element 10 or other optics under test.

[0075] In some cases, the test path may include a flat (or spherical) mirror 230 to redirect the beam, allowing for flexible positioning of the optical element 10 within the instrument 200. After interacting with the diffractive optics, the test beam may recombine with the reference beam 220 at the beamsplitter 218, creating an interference pattern (e.g., the interference pattern 290).

[0076] A detector 240 may capture this interference pattern 290, which may contain information about the optical properties of the tested element 10. The detector 240 may be coupled to a processor 250 that may analyze the captured pattern 290 and generate measurement results.

[0077] By utilizing a QCL source at about 4.2 microns, the interferometer instrument 200 may produce clearer and more representative interference patterns for MWIR diffractive optics. This approach may potentially reduce the occurrence of "broken fringes" (e.g., those shown in FIG. 1 ) that can arise when using nonmatched wavelength sources.

[0078] The integrated system may allow for comprehensive characterization of MWIR diffractive optics, including measurements of wavefront error, spherical aberration, and modulation transfer function (MTF). In some cases, the processor may also control other aspects of the measurement process, such as adjusting the QCL wavelength or managing the positioning of optical components.

[0079] The combination of a wavelength-matched QCL source, precise optical components, and advanced processing capabilities may enable the interferometer instrument to provide accurate and reliable measurements of MWIR diffractive optics. This integrated approach may offer potential improvements in measurement quality and interpretation compared to traditional methods using non-matched wavelength sources.

[0080] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.

Claims

CLAIMS1. An interferometer instrument for measuring properties of diffractive optics, comprising:a quantum cascade laser configured to emit light at a wavelength centered at approximately 4.2 microns;a beam expander configured to expand a beam emitted by the quantum cascade laser;a beamsplitter configured to split the expanded beam into a reference beam and a test beam;a reference mirror configured to reflect the reference beam;a test path configured to direct the test beam to a diffractive optical element under test; anda detector configured to capture an interference pattern formed by the combination of the reflected reference beam and the test beam after interaction with the diffractive optical element.

2. The interferometer instrument of claim 1 , wherein the quantum cascade laser is configured to emit light at a wavelength that matches a design wavelength of the diffractive optical element under test.

3. The interferometer instrument of claim 1 , further comprising a processor configured to analyze the interference pattern captured by the detector.

4. The interferometer instrument of claim 3, wherein the processor is further configured to generate measurement results based on the analysis of the interference pattern.

5. The interferometer instrument of claim 4, further comprising a display configured to present the measurement results and the interference pattern.

6. The interferometer instrument of claim 1 , wherein the test path includes a reflector configured to redirect the test beam towards the diffractive optical element under test.

7. The interferometer instrument of claim 1 , wherein the interferometer instrument is configured to perform measurements on individual optical elements or on optical assemblies containing diffractive optics designed for operation in a MidWave Infrared (MWIR) wavelength range of approximately 3.6 microns to 4.9 microns.

8. A method of measuring properties of diffractive optics, comprising:emitting light from a quantum cascade laser at a wavelength centered at approximately 4.2 microns;expanding the emitted light using a beam expander;splitting the expanded light into a reference beam and a test beam; directing the test beam to a diffractive optical element under test; combining the reference beam and the test beam after interaction with the diffractive optical element to form an interference pattern; andcapturing the interference pattern using a detector.

9. The method of claim 8, further comprising analyzing the interference pattern using a processor to generate measurement results.

10. The method of claim 9, further comprising displaying the measurement results and the interference pattern on a display.

11. The method of claim 8, wherein the quantum cascade laser emits light at a wavelength that matches a design wavelength of the diffractive optical element under test.

12. The method of claim 8, wherein directing the test beam to the diffractive optical element under test comprises redirecting the test beam using a reflector.

13. The method of claim 8, wherein the diffractive optical element under test is designed for operation in a MWIR wavelength range of approximately 3.6 microns to 4.9 microns.

14. The method of claim 9, wherein analyzing the interference pattern comprises determining at least one of peak-to-valley measurement, root mean square measurement, defocus measurement, coma measurement, astigmatism measurement, field curvature measurement, image distortion measurement, and spherical aberration measurement, and modulation transfer function (MTF) measurement.

15. An optical measurement system, comprising:an interferometer instrument including a quantum cascade laser configured to emit light at a wavelength centered at approximately 4.2 microns;a processor coupled to the interferometer instrument and configured to analyze interference patterns produced by the interferometer instrument; anda display coupled to the processor and configured to present measurement results based on the analyzed interference patterns.

16. The optical measurement system of claim 15, wherein the interferometer instrument further comprises:a beam expander configured to expand a beam emitted by the quantum cascade laser;a beamsplitter configured to split the expanded beam into a reference beam and a test beam;a reference mirror configured to reflect the reference beam; anda test path configured to direct the test beam to a diffractive optical element under test.

17. The optical measurement system of claim 16, wherein the interferometer instrument further comprises a reflector in the test path configured to redirect the test beam towards the diffractive optical element under test.

18. The optical measurement system of claim 15, wherein the quantum cascade laser is configured to emit light at a wavelength that matches a design wavelength of a diffractive optical element under test.

19. The optical measurement system of claim 15, wherein the processor is configured to analyze the interference patterns to determine at least one of: peak-to-valley measurement, root mean square measurement, spherical aberration measurement, and modulation transfer function (MTF) measurement.

20. The optical measurement system of claim 19, wherein the display is configured to present the interference patterns alongside the measurement results in a graphical user interface.

21. An interferometer instrument configured to measure a plurality of properties of an optical element, comprising:a first laser source configured to emit a first measurement beam in a first wavelength range of approximately 4.1 microns to approximately 4.3 microns;a detector; anda processor having a display,wherein the detector is configured to output data representative of a first plurality of interference fringes to the processor, wherein the first plurality ofinterference fringes has a reduced fringe distortion relative to an original fringe distortion created by a second laser source,wherein the processor processes the data representative of the first plurality of interference fringes, computes the plurality of properties of the optical element and outputs images representative of the plurality of properties of the optical element to the display.

22. The interferometer instrument of claim 21 , wherein the first laser source is a quantum cascade laser having a first wavelength of approximately 4.2 pm.

23. The interferometer instrument of claim 21 , wherein the second laser source is a helium-neon laser having a second wavelength of approximately 3.39 pm.

24. The interferometer instrument of claim 21 wherein the plurality of properties of the optical element includes a modulation transfer function.

25. The interferometer instrument of claim 21 , wherein the plurality of properties of the optical element includes a peak-to-valley measurement of a deviation of a surface of the optical element from an ideal optical surface.

26. The interferometer instrument of claim 21 , wherein the plurality of properties of the optical element includes a spherical aberration.

27. The interferometer instrument of claim 1 , wherein the optical element is an optical assembly.

28. The interferometer instrument of claim 8, wherein the optical assembly is a lens assembly.

29. The interferometer instrument of claim 8, wherein the optical assembly includes a diffractive optical element having one or more diffractive surfaces.

30. The interferometer instrument of claim 1 , wherein the optical element is a diffractive optical element.

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