A decoherence device for short-wavelength light

Abstract

This invention discloses a decoherence device for short-wavelength light. The decoherence device has the following structure: a fiber core with a vacuum medium, and an inner wall coated with a metal or carbon layer; the cross-sectional shape of the fiber core is polygonal, specifically a square, hexagon, or octagon; the diameter of the fiber core is the diameter of the circumcircle of the polygon, ranging from 10µm to 100µm; and the length L of the decoherence device ranges from 50 mm to 200 mm. The coherence detection method is Young's double-slit interferometry, which quantitatively reflects the decoherence effect by comparing the changes in fringe contrast before and after decoherence. This invention can be applied to short-wavelength light sources in the extreme ultraviolet (EUV) band. By exciting a large number of higher-order transverse modes and increasing mode coupling during EUV light transmission, it can effectively reduce EUV light coherence over shorter distances with lower transmission loss.

Description

Technical Field

[0001] This utility model belongs to the field of optical technology, specifically relating to a decoherence device for short-wavelength light and a method for detecting its coherence. Background Technology

[0002] Free-electron lasers are a technology that uses free electrons as the working medium to generate strongly coherent light through stimulated emission. Among them, short-wavelength free-electron lasers in the extreme ultraviolet (EUV) band, with their potential high-power laser output of kilowatts or even megawatts, are expected to provide stable light source support for several or even dozens of lithography systems simultaneously. Therefore, they have shown considerable potential in the technological competition for next-generation EUV light sources.

[0003] Because free-electron laser sources have strong coherence, the beam will exhibit a significant speckle effect during the imaging process, which in turn affects the imaging quality of the lithography system.

[0004] Currently, commonly used coherence modulation methods can be divided into two categories: one is to suppress the coherence of the output light by changing the structure of the laser, but this method may reduce the output power of the laser; the other is to introduce optical devices (such as rotating frosted glass, spatial light modulators, multimode optical fibers, etc.) to modulate the phase of the output light.

[0005] Patent application CN114556224A discloses a decoherence method that combines an engineered diffuser with multimode fiber, employing the synergistic effect of the engineered diffuser and multimode fiber for coherence modulation. However, the introduction of this engineered diffuser greatly increases the complexity of the decoherence system and significantly raises its cost in engineering applications.

[0006] The academic paper "Acta Physica Sinica, 2024, 73(11):114201" describes the design of a homogenizer for modulating 13.5nm wavelength laser light. For the design of this homogenizer, its length needs to be more than 400mm and it must have a complex structure with specific tilting and bending to significantly disrupt the phase relationship within the optical waveguide.

[0007] Unlike previous reports, this invention proposes a decoherence device for short-wavelength light and a method for detecting its coherence. This device, by exciting a large number of higher-order transverse modes and increasing mode coupling between fiber transmission modes, averages the energy distribution between different modes, improving the decoherence performance of multimode fibers. It enables low-loss decoherence modulation of short-wavelength light with shorter fiber lengths, lower maintenance costs, and in a static state where the fiber structure does not require tilting or bending. Summary of the Invention

[0008] To overcome the shortcomings of the prior art, this invention provides a decoherence device for short-wavelength light and a method for detecting its coherence. This decoherence device, by exciting a large number of transverse mode fields and possessing effective near-field mode scrambling capability, can significantly reduce the coherence of short-wavelength light within a relatively short device length and with high transmission efficiency.

[0009] The objective of this utility model is achieved through the following technical solution:

[0010] On the one hand, this utility model provides a decoherence device for short-wavelength light, characterized in that it includes: Vacuum fiber cores have a regular polygonal cross-section; A reflective layer is uniformly coated on the inner wall surface of the vacuum fiber core; The outer cladding layer encloses the vacuum fiber core; Wherein, the diameter D of the vacuum fiber core is the diameter of the circumcircle of the polygon.

[0011] Preferably, the length L of the decoherent device is the straight length of the optical fiber, ranging from 50mm to 200mm. Preferably, the diameter D of the vacuum fiber core is in the range of 10μm to 100μm.

[0012] Preferably, during the decoherence process, the optical fiber device does not require any bending, coiling, vibration, or squeezing of the optical fiber; that is, it can effectively decohere short-wavelength light while the device is in a static state.

[0013] Preferably, the material of the coating on the inner wall of the fiber core is gold, ruthenium, molybdenum or carbon, and the thickness d of the coating is 10nm to 60nm.

[0014] Preferably, the short-wavelength light has a wavelength range of 5nm to 100nm, and the optical fiber device can perform decoherent modulation on narrowband light with any wavelength as the center wavelength within the wavelength range.

[0015] The decoherence device is equipped with collimating lens groups at both its input and output ends to control the incident and exit angles of short-wavelength light.

[0016] The collimating lens assembly can be achieved by a combination of one or more of the following: mirror array, hyperboloid mirror, precision pinhole, photon sieve, KB (Kirkpatrick-Baez) mirror, Fresnel zone plate, and lobster eye mirror.

[0017] On the other hand, this invention also provides a method for detecting coherence in short-wavelength light. This method employs Young's double-slit interference and quantifies the decoherence effect of the decoherence device by comparing the changes in fringe contrast before and after decoherence. The interference fringe contrast is expressed by a formula. Obtained through calculation.

[0018] In the formula, C represents the contrast of the interference fringes, and I... MAX I is the average of the maximum coherent irradiance of all interference bright fringes. MIN This is the average of the minimum coherent irradiance for all dark fringes.

[0019] The degree of change in interference fringe contrast can be calculated by assessing the degree of decoherence of short-wavelength light using a decoherence device. The formula for calculating the degree of change in interference fringe contrast is as follows:

[0020] In the formula, V represents the degree of change in the contrast of the interference fringes, and C... input The contrast of the interference fringes of undecohered short-wavelength light; C output The contrast of the interference fringes of short-wavelength light after decoherence by the decoherence device of this utility model.

[0021] The coherence of laser light decreases with the excitation of higher-order transverse modes, and multimode fiber helps to reduce the coherence of incident laser light. Furthermore, for the same length, polygonal fiber has more propagation modes than circular fiber with the same core diameter.

[0022] Meanwhile, the coherence of fiber-embedded lasers is highly dependent on the power weight distribution of spatial characteristic modes. Hollow-core polygonal multimode fibers can alter the distribution and relative amplitude of fiber modes through mode scrambling, making the energy of different modes more even, thereby reducing the coherence of fiber-embedded lasers.

[0023] The coherence of fiber-embedded laser light is determined by the complex coherence μ. 12 Quantitative evaluation. Young's double-slit interference is a typical method for detecting coherence. The contrast of the double-slit interference fringes is related to the complex coherence μ of the laser. 12 The contrast is directly proportional to the coherence of the laser emitted from the fiber, so the contrast of the interference fringes can be used as a parameter to measure the coherence of the laser emitted from the fiber.

[0024] Since the decoherence device involved in this invention can be applied to short-wavelength light sources in the extreme ultraviolet band (such as free-electron lasers with a center wavelength of 13.5nm or 6.7nm), its diffraction effect will be homogenized during optical fiber transmission. Therefore, the decoherence effect of optical fiber on short-wavelength light can be analyzed by geometric optics methods.

[0025] When a Gaussian beam with a certain divergence angle is coupled into an optical fiber, the light undergoes multiple reflections during transmission. Since different modes correspond to different light propagation paths, these modes have different optical path lengths during transmission, creating an optical path difference. When a laser beam exits the fiber, due to the optical path difference between different modes, if this difference exceeds the coherence length of the laser beam, the coherence of the exited beam will decrease.

[0026] Compared with the prior art, the present invention has at least the following beneficial effects;

[0027] 1) Compared with the "uniform tube" with a specific tilting structure, the decoherence device of this invention can effectively and with low loss achieve decoherence control of short-wavelength light with a shorter device length and without the need for a tilting design. 2) This utility model eliminates the need for additional complex optical systems such as "engineering diffusers", effectively reducing the complexity of decoherent devices and their manufacturing, use and engineering maintenance costs. 3) This utility model does not require any additional special operations such as bending, coiling, vibration, or squeezing of the decoherent device in the transmission optical path. It can effectively decohere short-wavelength light when the device is in a static state. Therefore, it can reduce the additional losses of short-wavelength light during the above-mentioned special operations, so as to further maintain a high transmission efficiency. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of the decoherence device for short-wavelength light according to this utility model.

[0029] Figure 2 This is a schematic diagram of the cross-sectional shape of the fiber core and the fiber core diameter D of the decoherent device of this utility model.

[0030] Figure 3 This is a schematic diagram of a method for detecting the coherence of short-wavelength light in the extreme ultraviolet (EUV) band, based on this invention. The detection method is Young's double-slit interferometry, which forms bright and dark interference fringes on a screen through light interference. The contrast of the interference fringes can be used to quantify the coherence of the EUV light.

[0031] Figure 4 This is the result obtained by detecting undecohered extreme ultraviolet short-wavelength light using Young's double-slit interferometry. Among them, Figure 4 (a) shows the interference fringes obtained by Young's double-slit interferometry, which are clearly visible periodic alternating bright and dark fringes; Figure 4 (b) Corresponding Figure 4 (a) shows the coherent irradiance distribution when the Y-coordinate value is 0.

[0032] Figure 5 The result, obtained by decohering short-wavelength light in the extreme ultraviolet band using the decoherent device of this invention and then detecting it using Young's double-slit interferometry, shows no visible fringes. Detailed Implementation

[0033] The present invention will be further described below with reference to the embodiments and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0034] Please refer to the following first. Figure 1 , Figure 1 This is a schematic diagram of the structure of the decoherence device for short-wavelength light according to this utility model. As shown in the figure, a decoherence device for short-wavelength light includes: a vacuum fiber core 1 with a regular polygonal cross-section; a reflective layer 2 uniformly coated on the inner wall surface of the vacuum fiber core 1 to reduce the transmission loss of short-wavelength light in the extreme ultraviolet band, the material being gold, ruthenium, molybdenum, or carbon; and an outer cladding layer 3 enclosing the vacuum fiber core 1; wherein, the diameter D of the vacuum fiber core 1 is the diameter of the circumcircle of the polygon.

[0035] Please see Figure 2 , Figure 2 This is a structural schematic diagram of the cross-sectional shape of the fiber core and the diameter D of the fiber core in the decoherence device of this utility model. The cross-sectional shape of the fiber core is polygonal, and the polygon is any one of square, regular hexagon, and regular octagon (the cross-sectional shape of the fiber core in this schematic diagram is a regular octagon); the diameter D of the fiber core is the diameter of the circumcircle of the polygon.

[0036] Example 1: The decoherence effect of this invention on short-wavelength light in the 13.5nm extreme ultraviolet band.

[0037] Implementation method 1, a decoherence device for short-wavelength light in the 13.5nm extreme ultraviolet band, has a fiber core diameter D of 20μm; the cross-sectional shape of the fiber core is a regular octagon; the material of the coating on the inner wall of the fiber core is gold; the length L of the decoherence device is 100mm.

[0038] Implementation method 2, see details below. Figure 3 Young's double-slit interference detection method.

[0039] Figure 3 The coherence detection described above reflects the coherence of extreme ultraviolet light by the clarity of the interference fringes at the screen.

[0040] In Implementation Method 3, the decoherence effect of the decoherence device is quantitatively reflected by calculating the contrast of the interference fringes at the screen.

[0041] The formula for calculating the contrast of interference fringes on a screen is:

[0042]

[0043] Among them, I MAX I represents the average of the maximum coherent irradiance of all bright fringes at the screen. MIN It represents the average of the minimum coherent irradiance of all dark stripes on the screen.

[0044] In implementation method 4, the extreme ultraviolet light source is a Gaussian point light source with a certain divergence angle. The extreme ultraviolet light source is a narrowband light with a wavelength of 13.5 nm and a divergence angle of 20 mrad.

[0045] Implementation method 5, see Figure 4 The results of detecting undecohered extreme ultraviolet light were obtained using Young's double-slit interferometry. Figure 4 (a) shows the interference fringes obtained by Young's double-slit interferometry, which are clearly visible periodic alternating bright and dark fringes. Figure 4 (b) Corresponding Figure 4 (a) shows the coherent irradiance distribution when the Y-coordinate value is 0.

[0046] Among them, the average value I of the maximum coherent irradiance of the bright fringes at the screen. MAX 1.3685×10 -17 The average value of the minimum coherent irradiance of the dark fringes at the screen, I MIN 1.338×10 -18 Using the calculation formula described in

[0040] , the value of its interference fringe contrast is 0.8219.

[0047] Implementation method 6, see Figure 5 This study describes the detection results of extreme ultraviolet light after decoherence through the decoherence device of this invention, using Young's double-slit interference method.

[0048] in, Figure 5 There are no visible fringes, so the value of its interference fringe contrast is considered to be 0.

[0049] Implementation method 7: The method for calculating the degree of change in interference fringe contrast is as follows:

[0050]

[0051] Among them, C input The contrast of the interference fringes of the undecoherent extreme ultraviolet light; C output The contrast of the interference fringes of extreme ultraviolet light after decoherence by the decoherence device of this invention.

[0052] In this embodiment, C input The value is 0.8219, C output The value is 0. Implementation method 7 calculates that the value of the change in interference fringe contrast is negative 100%, thus achieving a near-complete decoherence effect.

[0053] As can be seen from the results in this embodiment, compared with the uniform light tube described in the paper "Acta Physica Sinica, 2024, 73(11):114201", the decoherence device of this invention does not require the design of a complex tilting structure; and the length range of the decoherence device is only 50mm to 200mm. Compared with the uniform light tube with a length of 400mm to 600mm, this invention can achieve a significantly effective decoherence effect on short-wavelength light in the extreme ultraviolet band with a wavelength of 13.5nm by using a shorter device and in a static state without bending.

[0054] The above description is only a specific embodiment and implementation method of this utility model, and is not intended to limit this application. Any modifications, combinations of implementation methods, equivalent substitutions and improvements made within the spirit and principle scope of this application should be included within the protection scope of this application.

Claims

1. A decoherence device for short-wavelength light, characterized in that, include: Vacuum fiber core (1) has a regular polygonal cross-section; A reflective layer (2) is uniformly coated on the inner wall surface of the vacuum fiber core (1); The outer cladding layer (3) wraps around the vacuum fiber core (1); Wherein, the diameter D of the vacuum fiber core (1) is the diameter of the circumcircle of the polygon.

2. The decoherence device for short-wavelength light according to claim 1, characterized in that, The length L of the decoherent device is the straight length of the optical fiber, ranging from 50 mm to 200 mm.

3. The decoherence device for short-wavelength light according to claim 1, characterized in that, The diameter D of the vacuum fiber core (1) ranges from 10 µm to 100 µm.

4. The decoherence device for short-wavelength light according to claim 1, characterized in that, The polygon can be any one of a square, a regular hexagon, or a regular octagon.

5. The decoherence device for short-wavelength light according to claim 1, characterized in that, The material of the reflective layer (2) is selected from at least one of gold, ruthenium, molybdenum or carbon, and its thickness is kept uniform along the fiber core axis.

6. The decoherence device for short-wavelength light according to claim 1, characterized in that, The short-wavelength light has a wavelength range of 5 nm to 100 nm, and the device can perform decoherent modulation on narrowband light with any wavelength as the center wavelength within the wavelength range.

7. The decoherence device for short-wavelength light according to any one of claims 1-6, characterized in that, The decoherence device is equipped with collimating lens groups at both its input and output ends to control the incident and exit angles of short-wavelength light.

8. The decoherence device for short-wavelength light according to claim 7, characterized in that, The collimating lens assembly is achieved by a combination of one or more of the following: a mirror array, a hyperboloid mirror, a precision pinhole, a photon sieve, a KB mirror, a Fresnel zone plate, and a lobster-eye mirror.

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