Long-focal-depth focusing optical system

Abstract

The invention discloses a long-focal-depth focusing optical system, which comprises an axicon, a diffractive optical element (DOE) and a lens which are sequentially clung to one another along an optical axis and cooperate with phase modulation to expand the focal depth. Before linear modulation of the axicon generates a conical wave, sinusoidal phase modulation is carried out on the DOE by means of a microstructure, and secondary modulation focusing is carried out on the lens. Through overall phase function cooperation, in combination with the cone angle of an axicon, DOE parameters and lens focal length optimization, multiple focuses are formed by using multiple diffraction orders of DOE, the Bessel function components are uniform by regulating and controlling the parameters, and the focal depth is expanded by multi-focus interference. The system can adapt to scenes such as photoetching, biological imaging and laser processing, breaks through the limitation of traditional focal depth, improves the focusing stability and the light spot quality, guarantees the system performance through assembly and debugging, and meets the requirements of multiple fields for long focal depth.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical system design, and in particular relates to a long-focus deep-focus optical system. Background Art

[0002] In modern optical applications such as photolithography, optical imaging, and laser processing, the demand for optical system depth of focus is becoming increasingly stringent. Depth of focus (DOF) refers to the range of axial movement of the image plane that can be achieved while maintaining a certain level of imaging quality. Traditional focusing optical systems are limited by the diffraction limit and geometric optical aberrations, resulting in a typically shallow depth of focus, making them difficult to meet in applications requiring a high axial working range.

[0003] Take photolithography technology as an example. As chip manufacturing processes continue to shrink, the exposure system needs to achieve high-precision pattern transfer on larger wafer surfaces. Shallow depth of focus can easily cause focal plane offset due to uneven wafer surface and vibration of the photolithography machine, reducing pattern accuracy and yield. In biomedical imaging, when imaging thick tissue samples, shallow depth of focus makes it difficult to clearly obtain information at different depth levels, requiring frequent focus adjustments, increasing imaging time and complexity. In the field of laser processing, such as processing three-dimensional structures inside glass, precise focusing at different axial depths is required. Insufficient depth of focus will limit processing efficiency and structural complexity.

[0004] To solve the problem of insufficient depth of focus, existing technologies have proposed a variety of methods. Axicon is one of the commonly used methods, which can extend the depth of focus of the optical system to a certain extent by generating a conical wavefront. However, the depth of focus distribution formed by a single axicon still has problems such as poor energy concentration and insufficient flexibility of the axial distribution of the focus, making it difficult to adapt to complex application scenarios. Diffractive optical elements (DOEs) can achieve flexible wavefront control through phase modulation and use multiple diffraction orders to generate multiple focal points or special light field distributions. However, when using DOE alone to expand the depth of focus, it is easily restricted by processing accuracy and energy utilization, and it is difficult to control the quality of the focused light spot.

[0005] There are also attempts to combine axicon mirrors with simple lenses, or to pair DOEs with other optical components. However, when designing multiple components together, there is a lack of in-depth exploration of the phase modulation synergy mechanism, and the phase control advantages of different components are not fully utilized to achieve complementarity. For example, if the linear phase modulation of the axicon mirror, the sinusoidal phase modulation of the DOE, and the focusing phase modulation of the lens are simply superimposed, it is impossible to accurately control the multi-focal axial distribution and energy uniformity, resulting in limited depth of focus extension and the introduction of additional aberrations, which affects the quality of the focused spot. Summary of the Invention

[0006] To this end, the present invention provides a long-focus deep-focus optical system to solve the problems of poor component coordination, difficult phase control, and insufficient multi-focus optimization in existing long-focus deep-focus optical systems.

[0007] To achieve the above-mentioned object, the present invention provides the following technical solution: a long focal depth focusing optical system, comprising an axicon, a diffractive optical element, and a lens arranged closely in sequence along an optical axis, wherein the axicon, the diffractive optical element, and the lens cooperate to phase modulate incident light to extend the focal depth;

[0008] The axicon performs linear phase modulation on the incident light to generate a conical wavefront, the diffractive optical element provides sinusoidal phase modulation for the light beam through a surface microstructure, and the lens focuses the sinusoidally phase modulated light beam to form a long focal depth energy distribution.

[0009] As a preferred solution for the long-focus deep-focus optical system, the phase modulation function of the axicon is:

[0010]

[0011] Where r is the radial coordinate, λ is the wavelength of the incident light, n is the refractive index of the axicon material, and α is the cone angle parameter, which is used to linearly phase modulate the incident light to generate a conical wavefront.

[0012] As a preferred solution for a long-focus deep-focus optical system, the phase modulation function of the diffractive optical element is:

[0013]

[0014] Where A is the amplitude of sinusoidal phase modulation, and Λ is the spatial period. Sinusoidal phase modulation is provided to the light beam by etching the microstructure.

[0015] As a preferred solution for a telephoto deep-focus optical system, the phase modulation function of the lens is:

[0016]

[0017] Where k is the wave number and f is the focal length, which is used to focus the modulated beam and form a long focal depth energy distribution.

[0018] As a preferred solution for a telephoto deep-focus optical system, the overall phase modulation function of the optical system is:

[0019]

[0020] Through cooperative modulation, the light beam forms a stable focused spot in the axial range.

[0021] As a preferred solution for a long focal depth focusing optical system, the cone angle α of the axicon, the modulation amplitude A and period Λ of the diffractive optical element, and the focal length f of the lens need to be optimized according to the focal depth formula:

[0022]

[0023] Where D is the incident light spot diameter.

[0024] As a preferred solution for a long-focus deep-focus optical system, the sinusoidal phase modulation of the diffractive optical element is expanded by Fourier:

[0025]

[0026] Where, J m (A) is the mth-order Bessel function. By adjusting A, the energy of different diffraction orders can be evenly distributed.

[0027] As a preferred solution for a telephoto deep focusing optical system, the wave vector component k corresponding to the diffraction order m generated by the diffraction optical element is r (m) Satisfy:

[0028]

[0029] Based on the wave vector conservation condition The axial wave vector is derived Light waves of different diffraction orders are focused in turn at different axial positions to form multiple focal points;

[0030] The phase difference between adjacent foci reaches 2π, which causes interference to form new foci. The focal distance Δz f satisfy By adjusting the parameters of the diffractive optical element, multi-focal distribution control can be achieved to extend the focal depth.

[0031] As a preferred solution for a long-focus deep-focus optical system, the axicon is made of quartz glass; the diffraction optical element is prepared by photolithography or electron beam etching to form a microstructure, and the lens is optimized in curvature and material according to the long focal length requirement to reduce aberrations.

[0032] As a preferred solution for a telephoto deep-focus optical system, the assembly method of the optical system includes:

[0033] The axicon, diffractive optical element and lens are mounted closely on the optical bracket along the optical axis. The center is ensured to coincide with the optical axis through a translation stage. The focused light spot and energy distribution are debugged using a laser light source and a spot analyzer.

[0034] The beneficial effects of the present invention are as follows:

[0035] First, multiple elements collaboratively modulate the light field, combining the conical wavefront of the axicon and the multi-focus effect of DOE to break through the focal depth limitation of traditional systems and adapt to large axial range requirements such as lithography and imaging.

[0036] Second, the DOE parameters are adjusted to make the Bessel function components uniform and the multi-focus energy distribution balanced, thereby improving the quality of the focused light spot and ensuring the processing and imaging accuracy.

[0037] Third, component parameters (axis conic angle, DOE modulation amplitude, etc.) can be optimized on demand according to the scene to adapt to the differentiated requirements of different applications for focal depth and light spot.

[0038] Fourth, collaborative design compensates for component aberrations, and lens curvature and material optimization further suppress aberrations to ensure clear focusing even at long focal depths.

[0039] Fifth, component preparation relies on mature photolithography and etching processes, which makes assembly and debugging simple, facilitates industrial application, and promotes the upgrading of optical systems in multiple fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are merely exemplary, and those skilled in the art can, without inventive effort, derive other implementation drawings based on the provided drawings.

[0041] The structures, proportions, sizes, etc. illustrated in this specification are intended solely to complement the contents disclosed herein and to facilitate understanding and reading by persons skilled in the art. They are not intended to limit the conditions under which the present invention may be implemented and therefore have no substantive technical significance. Any structural modifications, changes in proportions, or adjustments in sizes, without affecting the efficacy and objectives of the present invention, shall remain within the scope of the technical contents disclosed herein.

[0042] Figure 1 A schematic diagram of a telephoto deep-focus optical system provided by an embodiment of the present invention;

[0043] Figure 2 Schematic diagram of phase modulation of a telephoto deep-focus optical system provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0044] The following describes the implementation of the present invention using specific embodiments. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. Obviously, the embodiments described are only a portion of the present invention, not all of it. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0045] See also Figure 1 and Figure 2An embodiment of the present invention provides a long focal depth focusing optical system, comprising an axicon, a diffractive optical element, and a lens arranged closely in sequence along an optical axis, wherein the axicon, the diffractive optical element, and the lens cooperate to phase modulate the incident light to extend the depth of focus; the depth of focus of the optical system is determined by the propagation characteristics of the light beam after phase modulation. The axicon, the diffractive optical element (DOE), and the lens sequentially change the wavefront phase of the light, so that the light beam maintains focusing ability at different axial positions. The close arrangement can avoid phase modulation deviation caused by the optical path spacing, ensuring that the collaborative modulation effect is directly superimposed.

[0046] The axicon performs linear phase modulation on the incident light to generate a conical wavefront, the diffractive optical element provides sinusoidal phase modulation for the light beam through a surface microstructure, and the lens focuses the sinusoidally phase modulated light beam to form a long focal depth energy distribution;

[0047] Specifically, the linear phase modulation of the axicon mirror causes the phase of the light field to change linearly with the radial distance, and the outgoing light forms a conical wavefront. This wavefront maintains the Bessel beam characteristics during propagation and naturally has the potential for expanding the depth of focus; the microstructure of the diffractive optical element forms periodic fluctuations through an etching process, applying sinusoidal phase modulation to the light, changing the spatial frequency component of the wavefront; the secondary phase modulation of the lens converges the modulated light beam, and at the same time cooperates with the first two to regulate the axial energy distribution of the light field, avoiding the depth of focus limitation of traditional lens focusing.

[0048] In this embodiment, the phase modulation function of the axicon is:

[0049]

[0050] Where r is the radial coordinate, λ is the wavelength of the incident light, n is the refractive index of the axicon material, and α is the cone angle parameter, which is used to linearly phase modulate the incident light to produce a conical wavefront. This function shows that the amount of phase modulation is proportional to the radial distance r, while the cone angle α and the refractive index n determine the slope of the linear modulation. When parallel light is incident, the axicon creates a phase difference between light beams at different radial positions. The outgoing light beams are superimposed to form a conical wavefront with a radial component. This wave vector maintains a constant central intensity during beam propagation (Bessel beam characteristics), thereby extending the depth of focus.

[0051] In this embodiment, the phase modulation function of the diffractive optical element is:

[0052]

[0053] Where A is the sinusoidal phase modulation amplitude, and Λ is the spatial period. Sinusoidal phase modulation is applied to the light beam by etching the microstructure. This periodic modulation of the sine function introduces multiple spatial frequency components into the light field. The modulation amplitude A determines the intensity distribution of each frequency component, while the spatial period Λ determines the frequency spacing. The height difference of the etched microstructure corresponds to the phase modulation amount. When light passes through, the optical path difference at different locations varies sinusoidally, effectively frequency encoding the wavefront.

[0054] In this embodiment, the phase modulation function of the lens is:

[0055]

[0056] Where k is the wave number and f is the focal length, which is used to focus the modulated beam and form a long-depth energy distribution. The quadratic phase function causes the light field phase to vary with r2, corresponding to the focusing characteristics of the lens (spherical wavefront). The wave number k = 2π / λ, and the focal length f determines the focusing strength. When this phase is superimposed with the phase of the axicon and diffractive optical element, the axial focus position and energy distribution of the beam can be manipulated, avoiding the depth of focus limitation imposed by a single focal point of a traditional lens.

[0057] In this embodiment, the overall phase modulation function of the optical system is:

[0058]

[0059] Through coordinated modulation, the light beam forms a stable focused spot within the axial range; the phase superposition principle determines the final wavefront morphology of the light field. The combination of linear phase (axicon), sinusoidal phase (DOE), and quadratic phase (lens) gives the light field the focal depth extension, multi-focal distribution, and focusing capability of a conical wavefront. Through parameter optimization, the synthesized wavefront can meet focusing conditions (e.g., phase difference less than π / 2) over a large axial range, thus forming a stable spot.

[0060] In this embodiment, the cone angle α of the axicon, the modulation amplitude A and period Λ of the diffractive optical element, and the focal length f of the lens need to be optimized according to the depth of focus formula:

[0061]

[0062] Where D is the incident spot diameter; this formula is derived from the phase matching conditions of the axicon-lens system. In the numerator, f2λ reflects the magnification effect of focal length and wavelength on the depth of focus. In the denominator, the first term accounts for the phase modulation introduced by the axicon's cone angle, and the second term accounts for the traditional diffraction limit effect. When optimizing these parameters, a balance must be struck between the depth of focus extension achieved by the axicon and the focusing capability of the lens. For example, increasing α enhances depth of focus extension but may result in energy dispersion.

[0063] In this embodiment, the sinusoidal phase modulation of the diffractive optical element is expanded by Fourier transform:

[0064]

[0065] Where, J m (A) is the mth order Bessel function. By adjusting A, the energy of different diffraction orders is evenly distributed. According to Fourier optics, the exponential form of the sinusoidal phase can be decomposed into the superposition of infinitely many orders of Bessel functions, each order corresponding to a diffraction order m. The Bessel function value J m (A) determines the energy proportion of the order. When A takes a specific value (such as 2.4), J0(2.4)≈J1(2.4)≈J2(2.4), which balances the energy of each diffraction order and avoids the depth of focus limitation caused by the energy concentration of a single focus.

[0066] In this embodiment, the wave vector component k corresponding to the diffraction order m generated by the diffraction optical element is r (m) Satisfy:

[0067]

[0068] Based on the wave vector conservation condition The axial wave vector is derived Different diffraction order light waves are focused in turn at different axial positions to form multiple focal points; wave vector conservation requires that the square of the total wave vector is equal to the sum of the squares of the radial component and the axial component. r (m) is determined by the diffraction order and spatial period, and the axial wave vector k z (m) with k r increases and decreases. z Determines the propagation speed of light. Light waves corresponding to different m have different axial propagation speeds, resulting in the separation of the focus position along the axial direction, forming a multi-focal array and extending the focal depth range.

[0069] Among them, the phase accumulation difference between adjacent foci reaches 2π, which causes interference to form a new focus. The focal distance Δz f satisfy By adjusting the parameters of the diffractive optical element, the multi-focal distribution can be adjusted to extend the focal depth. When the phase difference between the two light waves in the axial direction reaches 2π, constructive interference will occur, forming a new focal point. The focal distance is inversely proportional to the square of the radial wave vector, so reducing the spatial period Λ or increasing the diffraction order m can make k r Increase, thereby reducing the focal distance, increasing the axial focus density, and achieving fine control of energy distribution within the focal depth range.

[0070] In a possible embodiment, the axicon is made of quartz glass; the diffractive optical element is prepared by photolithography or electron beam etching to form a microstructure; and the lens is optimized in curvature and material according to long focal length requirements to reduce aberrations.

[0071] Specifically, quartz glass has high transmittance and stable optical properties, making it suitable as an axicon mirror material to avoid interference of material dispersion on phase modulation; photolithography / electron beam etching can precisely control the height and period of the microstructure of the diffraction optical element (with an accuracy of nanometers), ensuring the accuracy of sinusoidal phase modulation; long-focal-length lenses can reduce aberrations such as spherical aberration and chromatic aberration by optimizing curvature (such as using aspheric surfaces) and materials (such as low-dispersion glass), thereby avoiding the influence of aberrations on the quality of the long-focus deep-focus light spot.

[0072] In one possible embodiment, a correlation model including diffractive optical element parameters, axicon parameters and focal depth extension is constructed. By inputting the target focal depth requirement, the element parameter combination is automatically optimized and calculated, thereby realizing the rapid design of a long focal depth focusing optical system adapted to different scenarios.

[0073] Specifically, a mathematical model (e.g., neural network or analytical expression) for the parameter-depth of focus is established based on the depth of focus formula and phase modulation theory. After inputting conditions such as the target focal depth, wavelength, and spot diameter, the model automatically optimizes the axicon cone angle, DOE modulation amplitude / period, and lens focal length through iterative calculations (e.g., genetic algorithms or gradient descent), balancing focal depth extension with energy efficiency, enabling rapid system design.

[0074] In one possible embodiment, the method for assembling an optical system includes:

[0075] The axicon, diffractive optical element and lens are mounted closely on the optical bracket along the optical axis. The center is ensured to coincide with the optical axis through a translation stage. The focused light spot and energy distribution are debugged using a laser light source and a spot analyzer.

[0076] Specifically, component coaxiality errors can cause wavefront tilt and introduce aberrations, necessitating a high-precision translation stage (with micron-level accuracy) to adjust component position and ensure strict alignment between the optical axis and component center. During debugging, a laser light source simulates actual incident light, and a spot analyzer measures the size, energy distribution, and axial extension of the focused spot. Fine-tuning component parameters (such as tilt angle) optimizes depth of focus to ensure system performance meets specifications.

[0077] The long-focus deep-focus optical system of the present invention achieves extended depth of focus by changing the wavefront propagation characteristics of the light beam through the coordinated phase modulation of the axicon, diffractive optical element (DOE), and lens. The specific principles are as follows:

[0078] Element-independent phase modulation principle:

[0079] Axicon: Utilizing linear phase modulation, the phase of the incident light varies linearly with radial distance r, resulting in a conical wavefront in the outgoing light. This wavefront imparts Bessel-like properties to the beam, resulting in stable central light intensity during propagation and the potential for extended depth of focus.

[0080] Diffractive optical elements: Surface microstructures apply sinusoidal phase modulation to light, decomposing the light field into multiple diffraction components. Different diffraction orders correspond to different spatial frequencies, which disperse the beam propagation direction and lay the foundation for subsequent multi-focal formation.

[0081] Lens: The focusing function is achieved through secondary phase modulation, which converges the previously modulated light beam and regulates the axial energy distribution of the light field, avoiding the limitation of focal depth due to the single focus of traditional lenses.

[0082] Multi-element cooperative modulation mechanism:

[0083] The axicon, diffractive optical element (DOE), and lens phase superposition combine to create a light field that simultaneously exhibits: the axicon's conical wavefront focal depth extension (extending the effective focusing range of a single focal point); the DOE's multi-diffraction-order, multi-focal properties (axially dispersing the focus position to increase the effective focusing range); and the lens's focus control properties (constraining the light field energy to ensure focused spot quality). By optimizing parameters such as the axicon's cone angle α, the DOE's modulation amplitude A / period Λ, and the lens' focal length f, the synthesized wavefront maintains a phase difference less than the imaging threshold (e.g., π / 2) over a wide axial range, forming a stable, long-depth-of-focus focused spot.

[0084] The principle of multi-focus and extended depth of focus:

[0085] The sinusoidal phase of the DOE is decomposed into multi-order Bessel function components through Fourier expansion. Different diffraction orders m correspond to wave vector components. Combining the wave vector conservation, the axial wave vector is derived. Due to the difference in axial wave vectors, the axial propagation speed of light waves of different orders is different, and the focus position is separated along the axial direction to form a multi-focus array. Adjacent focuses interfere because the phase accumulation difference reaches 2π, and the spacing between the new focuses satisfies By adjusting the DOE parameters (e.g., when A=2.4, J0(2.4)≈J1(2.4)≈J2(2.4)), the multi-focal energy can be evenly distributed, and combined with the axicon effect, a significant extension of the depth of focus can be achieved.

[0086] The application scenarios of the present invention are as follows:

[0087] 1. Photolithography manufacturing field

[0088] Chip Exposure System: In integrated circuit chip manufacturing, photolithography machines must precisely transfer nanometer-scale circuit patterns onto the wafer surface. Wafers have surface irregularities (such as thickness deviation and warpage), and traditional shallow-depth-of-focus systems are prone to pattern distortion and linewidth errors due to focal plane offset. The long-depth-of-focus optical system of this invention maintains sharp focus over a wide axial range (e.g., ± several microns), meeting the high-precision exposure requirements of processes at 5nm and below, improving chip yield and performance.

[0089] Mask manufacturing and repair: High-precision mask (photolithography template) fabrication and repair require laser direct writing or correction of micro-nanostructures (e.g., defect repair) on the mask surface. The long depth of focus accommodates the thickness tolerance of the mask substrate (e.g., ±10μm for quartz substrates), ensuring stable focused energy delivery to the target layer and avoiding reduced repair accuracy and structural damage due to insufficient depth of focus.

[0090] 2. Biomedical Imaging

[0091] Thick Tissue 3D Imaging: Traditional multiphoton microscopy systems for biological tissue (such as brain slices and tumor tissue blocks) require frequent focus adjustments (scanning layer by layer), which is time-consuming and prone to motion artifacts. The long focal depth of this invention enables clear imaging within an axial range of several millimeters. A single focus captures structural information, such as cells and blood vessels, at varying depths in thick tissue, accelerating pathological analysis and neuroscience research.

[0092] Endoscopic imaging systems: Medical endoscopes (such as gastroscopes and bronchoscopes) must image within curved tubes and irregular cavities. Due to the undulating surface of the cavity and variations in distance, a shallow depth of focus can easily lead to partial loss of focus. Integrating a long-depth-of-focus system maintains clarity within ± several millimeters in the axial direction, improving the detection rate of early-stage lesions (such as polyps and precancerous lesions) and assisting in minimally invasive diagnosis and treatment.

[0093] 3. Laser Advanced Manufacturing

[0094] Machining 3D structures within glass: In the manufacturing of consumer electronics (such as mobile phone covers) and optical devices (such as optical communication glass waveguides), micro-nanostructures (such as micropores and waveguide channels) must be fabricated within the glass. The glass thickness (e.g., 0.5-2mm) and the required depth of internal processing require a long-focus, deep-field focusing system. This invention enables precise focusing within the submillimeter to millimeter range along the glass axis, enabling simultaneous processing of structures at multiple depths, improving efficiency and yield.

[0095] Metal Additive Manufacturing (3D Printing): In metal powder bed fusion 3D printing, the laser must maintain stable focus on the dynamically changing powder surface (which varies in height due to layer-by-layer sintering) to melt the powder. Conventional systems suffer from insufficient depth of focus, which can lead to fluctuating energy density (overmelting / undermelting the surface). The long depth of focus of the proposed system can accommodate variations in powder layer thickness (e.g., ±50μm), ensuring melt pool stability and improving the density and mechanical properties of printed parts.

[0096] 4. Space Optics and Remote Sensing

[0097] Onboard optical payloads: Satellite remote sensing cameras are required to image the Earth's complex terrain (mountains, canyons, oceans), where elevation differences can reach hundreds of meters. Long-focus depth systems maintain sharp focus across a wide axial range, reducing image blur caused by terrain undulations and improving resolution for Earth observations (e.g., forest monitoring and urban mapping).

[0098] Laser communication terminals: In intersatellite and satellite-to-ground laser communications, the beam must maintain focus over long distances (tens of thousands of kilometers) and despite dynamic pointing deviations (such as satellite vibration and orbital drift). The long focal depth of this invention can tolerate a certain degree of axial alignment error (e.g., ± several meters of optical path difference), ensuring stable energy reception within the communication link and improving link reliability.

[0099] 5. Precision testing and measurement field

[0100] Surface topography measurement: White light interferometers, laser confocal microscopes, and other equipment are required to measure the microscopic topography of a workpiece surface (e.g., roughness, step height). Traditional systems have a shallow depth of focus and require point-by-point scanning, resulting in low efficiency. The long focal depth of our invention can cover height differences on the workpiece surface ranging from a few microns to tens of microns, allowing for more topographic information to be obtained in a single measurement, accelerating quality inspection of semiconductor wafers and optical components.

[0101] Micro- and nano-dimensional metrology: When measuring the dimensions of micro- and nano-structures (such as MEMS devices and photonic crystals), the three-dimensional morphology of the structure (such as high-aspect-ratio trenches) requires clear axial imaging. A long-depth-of-focus system can fully capture structural details in the axial direction, assisting with precise metrology and failure analysis.

[0102] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made thereto. Therefore, such modifications and improvements, without departing from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A telephoto deep focus optical system, characterized in that: The invention comprises an axicon, a diffractive optical element and a lens which are sequentially and closely arranged along the optical axis, wherein the axicon, the diffractive optical element and the lens cooperate to perform phase modulation on the incident light to extend the focal depth; The axicon performs linear phase modulation on the incident light to generate a conical wavefront, the diffractive optical element provides sinusoidal phase modulation for the light beam through a surface microstructure, and the lens focuses the sinusoidally phase modulated light beam to form a long focal depth energy distribution.

2. The telephoto deep-focus optical system according to claim 1, wherein: The phase modulation function of the axicon is: Where r is the radial coordinate, λ is the wavelength of the incident light, n is the refractive index of the axicon material, and α is the cone angle parameter, which is used to linearly phase modulate the incident light to generate a conical wavefront.

3. The telephoto deep-focus optical system according to claim 2, wherein: The phase modulation function of the diffractive optical element is: Where A is the amplitude of sinusoidal phase modulation, and Λ is the spatial period. Sinusoidal phase modulation is provided to the light beam by etching the microstructure.

4. The telephoto deep-focus optical system according to claim 3, wherein: The phase modulation function of the lens is: Where k is the wave number and f is the focal length, which is used to focus the modulated beam and form a long focal depth energy distribution.

5. The telephoto deep-focus optical system according to claim 4, characterized in that: The overall phase modulation function of the optical system is: Through cooperative modulation, the light beam forms a stable focused spot in the axial range.

6. The telephoto deep-focus optical system according to claim 5, characterized in that: The cone angle α of the axicon, the modulation amplitude A and period Λ of the diffractive optical element, and the focal length f of the lens need to be optimized according to the depth of focus formula: Where D is the incident light spot diameter.

7. The telephoto deep-focus optical system according to claim 3, wherein: The sinusoidal phase modulation of the diffractive optical element is expanded by Fourier: Where, J m (A) is the mth-order Bessel function. By adjusting A, the energy of different diffraction orders can be evenly distributed.

8. The telephoto deep-focus optical system according to claim 7, wherein: The wave vector component k corresponding to the diffraction order m generated by the diffraction optical element r (m) Satisfy: Based on the wave vector conservation condition The axial wave vector is derived Light waves of different diffraction orders are focused in turn at different axial positions to form multiple focal points; The phase difference between adjacent foci reaches 2π, which causes interference to form new foci. The focal distance Δz f satisfy By adjusting the parameters of the diffractive optical element, multi-focal distribution control can be achieved to extend the focal depth.

9. The long-focus deep-focus optical system according to claim 1, characterized in that: The axicon is made of quartz glass; the diffraction optical element is prepared by photolithography or electron beam etching to form a microstructure; and the lens is optimized in curvature and material according to the requirement of long focal length to reduce aberration.

10. The telephoto deep-focus optical system according to any one of claims 1 to 9, characterized in that: Optical system assembly methods include: The axicon, diffractive optical element and lens are mounted closely on the optical bracket along the optical axis. The center is ensured to coincide with the optical axis through a translation stage. The focused light spot and energy distribution are debugged using a laser light source and a spot analyzer.

Images