Long-term vacuum maintenance type Fabry-Perot cavity and preparation method thereof

Abstract

The invention discloses a long-term vacuum maintenance type Fabry-Perot cavity and a preparation method thereof. An annular gasket is clamped between two reflecting lenses to directly bond in a vacuum environment through Van der Waals force, a vacuum core cavity which is permanently sealed and is vacuum in the air is formed, the vacuum core cavity is fixedly placed in a glass sleeve, and the two ends of the glass sleeve are sealed through an ultraviolet light adhesive bonding interface by adopting optical window pieces. After the interior of the glass sleeve is vacuumized through the exhaust pipe, the glass sleeve is sealed through local heating sintering, and a two-stage vacuum barrier is formed; the Fabry-Perot cavity is long in vacuum holding time and small in size, and does not need to be externally connected with a vacuum pump to maintain vacuum; the device is simple, and the vacuum degree of the Fabry-Perot cavity can be maintained for more than two years under the condition that the processing difficulty is not improved.

Description

Technical Field

[0001] The present invention relates to optical device preparation technology, and in particular to a long-term vacuum-maintaining Fabry-Perot cavity and a preparation method thereof. Background Art

[0002] The highly stable Fabry-Perot (FP) cavity is an important reference device for laser frequency stabilization and filtering. One of the main factors affecting frequency stability is the fluctuation of the refractive index caused by gas movement and collisions, that is, thermal noise. In order to reduce thermal noise, the number of gas molecules in the FP cavity should be as small as possible. The usual practice is to place the FP cavity in a large stainless steel vacuum chamber and continuously evacuate it with an ion pump, with the gas pressure reaching 10 -4 Below Pa. This FP cavity is not only bulky but also has high maintenance costs.

[0003] In reality, this high vacuum isn't essential. Microscopic models must account for the effects of individual gas molecules passing through the optical path. This means that the molecules collide with each other within the beam's path, which implies that the mean free path of the molecules should be significantly greater than the beam diameter within the cavity. For a beam diameter of 0.2 mm, a wavelength of 500 nm, and a temperature of 300 K, maintaining a pressure of 1 Pa reduces the thermal noise power spectral density to approximately -44 dBc / Hz at 1 Hz and approximately -104 dBc at 1 kHz.

[0004] Against this backdrop, a FP cavity with an internal vacuum structure has been proposed. This involves sealing the FP cavity's mirrors and gaskets in a vacuum environment using van der Waals forces (so-called "optical adhesive"), and then placing them in air, eliminating the need for continuous vacuuming. However, when manufacturing this vacuum cavity, the surface profile requirements for the contact surfaces between the mirrors and the gasket are very high: the surface profile is on the order of λ / 100, and the surface roughness is on the order of 0.1nm, where λ is the wavelength, typically 632.8nm. This level of precision processing is very expensive. Using a conventional surface profile of λ / 20 and a polishing accuracy of 1nm for a surface roughness of 1nm, it would be difficult to maintain a continuous internal vacuum state in an atmospheric environment. Even with high-precision grinding and polishing processes, a pressure difference between the inside and outside of the internal vacuum sealed cavity persists. As the external air pressure changes, the mirrors will also undergo slight deformations, affecting the cavity's resonant frequency.

[0005] In summary, a typical high-stability FP chamber must be placed within a bulky stainless steel vacuum chamber and continuously pumped to maintain a vacuum. Even with a simplified internal vacuum FP chamber, the bonding surface requirements are too high, resulting in high processing costs. Using conventional machining precision also carries the risk of long-term air leakage. Furthermore, the pressure differential between the inside and outside of the chamber mechanically affects the long-term stability of the FP chamber. Therefore, designing a simple device that can maintain a vacuum level in the FP chamber for more than two years without increasing the processing complexity remains a pressing technical challenge in this field. Summary of the Invention

[0006] To address the problems existing in the above-mentioned prior art, the present invention proposes a long-term vacuum-maintaining Fabry-Perot cavity and a preparation method thereof. The cavity adopts a sleeve-type FP cavity structure, that is, a second glass sleeve is added to the outside of the inner vacuum FP cavity, which is sleeved outside the inner vacuum FP cavity; the sleeve-type glass sleeve is sealed with a transparent optical window, and the sleeve-type glass sleeve is also evacuated, providing dual protection for the vacuum FP cavity and capable of maintaining the long-term vacuum of the FP cavity within an economical price range.

[0007] One object of the present invention is to provide a method for preparing a long-term vacuum-maintaining Fabry-Perot cavity.

[0008] The method for preparing a long-term vacuum-maintaining Fabry-Perot cavity of the present invention comprises the following steps:

[0009] 1) Preparation of vacuum core cavity:

[0010] a) Provide an annular gasket made of ultra-low expansion glass with a light hole in the middle;

[0011] b) providing two reflective lenses, the reflective lenses being made of ultra-low expansion glass, with a high reflectivity film being applied to the center portion of one surface of each reflective lens, with the area of ​​the high reflective film being consistent with the area of ​​the light hole of the gasket;

[0012] c) The surfaces of the two reflectors coated with high reflectivity films face each other, with an annular gasket sandwiched between them. The annular gasket is used to fix the distance between the two reflectors. Pressure is applied in a vacuum environment to compact the two reflectors, and the two reflectors are directly bonded by van der Waals forces to form a permanently sealed vacuum core cavity that is vacuum-proof in air.

[0013] 2) placing the bonded vacuum core cavity in a glass sleeve, and adding a getter into the glass sleeve; a vacuum tube is provided on the side wall of the glass sleeve, one end of the vacuum tube is connected to the inside of the glass sleeve, and the other end is connected to the outside;

[0014] 3) Setting a fixing structure in the glass sleeve and fixing the vacuum core cavity in the glass sleeve;

[0015] 4) Optical windows are respectively provided at both ends of the glass sleeve, and the interfaces between the two ends of the glass sleeve and the optical windows are sealed by UV adhesive bonding at room temperature;

[0016] 5) Evacuate the glass casing through the vacuum tube;

[0017] 6) After the glass sleeve is vacuumed, the mouth of the exhaust pipe is locally heated and sintered to seal the glass sleeve to form a double-stage vacuum barrier.

[0018] Wherein, in step 1) a), the material of the annular gasket is ultra-low expansion glass, with a thermal expansion coefficient of ≤3×10 -8 / K. The frequency interval c / (2L) is set by the thickness L of the annular gasket, where c is the speed of light. The surface of the annular gasket is polished to a flatness of λ / 40 to λ / 20, where λ = 632.8nm. The diameter of the aperture is 10 to 15mm.

[0019] In step 1) b), the reflective lens is made of ultra-low expansion glass, and the bonding surface of the reflective lens is polished to a flatness of λ / 40 to λ / 20, where λ=632.8 nm.

[0020] In step 1) c), the pressure of the vacuum environment is ≤10-3Pa.

[0021] In step 2), the getter is made of a zirconium-aluminum alloy, a zirconium-nickel alloy, a zirconium-iron-vanadium alloy, or zirconium-graphite. The inner diameter of the glass sleeve is 0.1 mm to 0.5 mm larger than the outer diameter of the vacuum core cavity, and a D9 / h9 sliding fit tolerance is adopted. A D9 / h9 sliding fit tolerance is adopted between the outer wall of the vacuum core cavity and the inner wall of the glass sleeve. The thermal expansion coefficient of the glass sleeve is consistent with that of the vacuum core cavity.

[0022] In step 3), the fixing structure uses two short glass tubes. The vacuum core cavity is located in the center of the glass sleeve. The two short glass tubes are inserted into the glass sleeve from both ends. The two short glass tubes clamp the vacuum core cavity from both ends, fixing the vacuum core cavity in the center of the glass sleeve. The outer diameter of the two short glass tubes is 0.1mm to 0.5mm smaller than the inner diameter of the glass sleeve, or adopts an H9 / d9 sliding fit tolerance. The length of the short glass tubes = (length of the glass sleeve - length of the vacuum core cavity) / 2.

[0023] In step 4), the optical window is made of transparent glass. Both sides of the optical window are coated with anti-reflection film. UV glue is applied to the interface between the transparent window and the glass sleeve, and the interface between the transparent window and the glass sleeve is irradiated with UV laser to achieve UV glue bonding. The UV glue is cationic UV glue, using common UVA bands such as 365nm, 385nm and 395nm wavelengths and 0.1W / cm 2 ~1W / cm 2 After irradiating with high-intensity ultraviolet light for 0.5s to 3s, the ultraviolet glue hardens and achieves airtightness.

[0024] In step 5), the vacuum tube is made of a glass capillary tube with an inner diameter of 2 mm to 4 mm, and the vacuum is pumped to a pressure less than 10 -4 Pa.

[0025] In step 6), the sealed exhaust pipe is locally heated and sintered at 300-400° C. using an oxyhydrogen flame.

[0026] It also includes low temperature accelerated aging (85°C / 85%RH).

[0027] Another object of the present invention is to provide a long-term vacuum-maintaining Fabry-Perot cavity.

[0028] The long-term vacuum-maintaining Fabry-Perot cavity of the present invention comprises: an annular gasket, a reflective lens, a glass sleeve, an exhaust pipe, a getter and an optical window; wherein the annular gasket is made of glass and has a light-through hole in the middle; the reflective lens is made of ultra-low expansion glass, and a high-reflectivity film is plated on the central part of one surface of each reflective lens, and the area of ​​the high-reflectivity film is consistent with the area of ​​the light-through hole; the surfaces of the two reflective mirrors plated with the high-reflectivity film face each other, with the annular gasket sandwiched between them, and are directly bonded by van der Waals force to form a vacuum core cavity that is permanently sealed and vacuumed in air; the vacuum core cavity formed by bonding The body is placed in a glass sleeve, and an exhaust pipe is provided on the side wall of the glass sleeve, one end of the exhaust pipe is connected to the glass sleeve, and the other end is connected to the outside, and a getter is placed in the glass sleeve; the inner diameter of the glass sleeve is larger than the outer diameter of the vacuum core cavity, and the vacuum core cavity is fixedly installed in the glass sleeve through a fixed structure; optical windows are respectively provided at both ends of the glass sleeve, and the interfaces between the two ends of the glass sleeve and the optical windows are sealed by ultraviolet bonding; the glass sleeve is evacuated through the exhaust pipe; after the glass sleeve is evacuated, the pipe mouth of the exhaust pipe is locally heated and sintered to seal the glass sleeve to form a double-stage vacuum barrier.

[0029] Advantages of the present invention:

[0030] The FP chamber of the present invention has a long vacuum retention time, a small size and does not require an external vacuum pump to maintain vacuum; the device of the present invention is simple and can maintain the vacuum degree of the FP chamber for more than two years without increasing the difficulty of processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a schematic diagram of an embodiment of a long-term vacuum-maintaining Fabry-Perot cavity according to the present invention;

[0032] Figure 2 The figure is a flow chart of the method for preparing the long-term vacuum-maintaining Fabry-Perot cavity of the present invention. DETAILED DESCRIPTION

[0033] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0034] This example uses a long-term vacuum-maintained Fabry-Perot cavity as a multiplier for the calibration comb spacing in an astronomical spectrometer. The input light is a 1 GHz-spaced visible light frequency comb. This broadband zero-delay Fabry-Perot cavity multiplies the 1 GHz comb spacing to a 30 GHz spacing. The FP cavity has a 5 mm spacing between its two mirrors, a resonant frequency of 30 GHz, and a spectral range of 560 nm to 900 nm.

[0035] like Figure 1 As shown, the long-term vacuum-maintaining Fabry-Perot cavity of this embodiment comprises: an annular gasket 101, a first reflective lens 102, a second reflective lens 103, a glass sleeve 2, an exhaust pipe 3, a getter, a first optical window 201, a second optical window 202, a first glass short tube 501 and a second glass short tube 502; wherein the annular gasket 101 is made of glass with a light hole in the middle; the first and second reflective lenses 102 and 103 are made of ultra-low expansion glass, and the first and second reflective lenses 102 and 103 are made of ultra-low expansion glass. The central part of one surface of 03 is coated with a high reflectivity film, and the area of ​​the high reflectivity film is consistent with the area of ​​the light hole; the surfaces of the first and second reflectors coated with the high reflectivity film are opposite to each other, and a ring-shaped gasket 101 is sandwiched between them. They are directly bonded by van der Waals force to form a vacuum core cavity 1 that is permanently sealed and vacuumed in the air; the vacuum core cavity formed by bonding is placed in a glass sleeve 2, and a vacuum pipe 3 is provided on the side wall of the glass sleeve 2. One end of the vacuum pipe 3 is connected to the inside of the glass sleeve 2, and the other end is connected to the outside, and a vacuum pipe 3 is placed in the glass sleeve 2. Getter; The inner diameter of the glass sleeve 2 is larger than the outer diameter of the vacuum core cavity by 0.1 mm to 0.5 mm, or adopts a D9 / h9 sliding fit tolerance. The first glass short tube 501 and the second glass short tube 502 are installed in the glass sleeve 2 from both ends. The outer diameters of the two glass short tubes are smaller than the inner diameter of the glass sleeve 2 by 0.1 mm to 0.5 mm, or adopt an H9 / d9 sliding fit tolerance. The length of the glass short tube = (the length of the glass sleeve 2 - the length of the vacuum core cavity) / 2. The two glass short tubes clamp the vacuum core cavity from both ends. The vacuum core cavity is fixed in the center of the glass sleeve 2; the first and second optical windows 201 and 202 are respectively arranged at the two ends of the glass sleeve 2, and both sides are coated with anti-reflection film. The interfaces between the two ends of the glass sleeve 2 and the first and second optical windows 201 and 202 are sealed by ultraviolet bonding at room temperature, and the positions of the short tubes 501 and 502 are also fixed; the glass sleeve 2 is evacuated through the exhaust pipe 3; after the glass sleeve 2 is evacuated, the pipe mouth 4 of the exhaust pipe 3 is locally heated and sintered to seal the glass sleeve 2, forming a double-stage vacuum barrier.

[0036] The method for preparing the long-term vacuum-maintaining Fabry-Perot cavity of this embodiment comprises the following steps:

[0037] 1) Preparation of vacuum core cavity:

[0038] a) providing an annular spacer 101 of ultra-low expansion glass with a diameter of 25 mm and a thickness of 5 mm, with a light hole in the middle, a surface polished to λ / 20, a central aperture of 10 mm, and λ = 632.8 nm;

[0039] b) providing ultra-low expansion glass as the first reflector 102 and the second reflector 103, with the bonding surface surface polished to λ / 20, the first and second reflectors 102 and 103 being made of glass, and coating a high reflectivity film on a central portion of one surface of the first and second reflectors 102 and 103 with a diameter of 10 mm;

[0040] c) The surfaces of the first and second reflective mirrors 102 and 103 with high reflectivity films are opposite to each other, with an annular gasket 101 sandwiched between them. The annular gasket 101 is used to fix the distance between the two reflective mirrors. The contact surfaces of the reflective mirrors and the annular gasket 101 are strictly cleaned with deionized water and placed on a 5×10 -4 In the Pa vacuum chamber, the first reflector lens 102 is placed horizontally, and the horizontal annular gasket 101 falls onto the first reflector lens 102 by gravity. The horizontal second reflector lens 103 also falls onto the annular gasket 101 by gravity. Then, pressure is applied from top to bottom to compact the three. The reflector lens and the gasket are bonded by the van der Waals force between them, forming a permanently sealed vacuum core cavity that can maintain vacuum for a short period of time even in air.

[0041] 2) The bonded vacuum core cavity is placed in a borosilicate glass sleeve 2 with an inner diameter of 25.2 mm, and a getter is added to the glass sleeve 2; an exhaust pipe 3 is provided on the side wall of the glass sleeve 2, one end of the exhaust pipe 3 is connected to the inside of the glass sleeve 2, and the other end is connected to the outside;

[0042] 3) Insert the first and second glass short tubes 501 and 502 into the glass sleeve 2 from both ends. The two glass short tubes clamp the vacuum core cavity from both ends, and the vacuum core cavity is fixed to the center of the glass sleeve 2. The outer diameters of the two glass short tubes and the inner diameter of the glass sleeve 2 adopt a sliding fit tolerance. The length of the glass short tubes = (length of the glass sleeve 2 - length of the vacuum core cavity) / 2. Irregular holes may be formed on the first and second glass short tubes 501 and 502 to ensure smooth vacuuming.

[0043] 4) Set the first and second optical windows 201 and 202 at both ends of the glass sleeve 2 respectively, and apply UV glue on the interface between the transparent window and the glass sleeve 2. The UV glue adopts cationic UV glue with a wavelength of 365nm and an intensity of 1W / cm 2 The UV light source is used to irradiate the two interfaces for 0.5s to 3s respectively, with the higher the power, the shorter the time; after the UV light irradiation, the UV glue is hardened to seal the interfaces between the two ends of the glass sleeve 2 and the first and second optical windows 201 and 202;

[0044] 5) Evacuate the glass sleeve 2 to a pressure of 3×10 -4 Pa;

[0045] 6) After the glass sleeve 2 is vacuumed, the nozzle of the exhaust pipe 3 is locally heated and sintered using a miniature hydrogen-oxygen flame gun to seal the glass sleeve 2 and form a double-stage vacuum barrier;

[0046] 7) Further includes low temperature accelerated aging (85°C / 85%RH).

[0047] The comparison of the indicators of this embodiment and the traditional cavity is shown in the following table:

[0048]

[0049] Finally, it should be noted that the purpose of disclosing the embodiments is to facilitate a further understanding of the present invention. However, those skilled in the art will appreciate that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the contents disclosed in the embodiments; the scope of protection claimed by the present invention shall be determined by the scope defined in the claims.

Claims

1. A method for preparing a long-term vacuum-maintaining Fabry-Perot cavity, characterized in that: The preparation method comprises the following steps: 1) Preparation of vacuum core cavity: a) Provide an annular gasket made of ultra-low expansion glass with a light hole in the middle; b) providing two reflective lenses, the reflective lenses being made of ultra-low expansion glass, with a high reflectivity film being applied to the center portion of one surface of each reflective lens, with the area of ​​the high reflective film being consistent with the area of ​​the light hole of the gasket; c) The surfaces of the two reflectors coated with high reflectivity films face each other, with an annular gasket sandwiched between them. The annular gasket is used to fix the distance between the two reflectors. Pressure is applied in a vacuum environment to compact the two reflectors, and the two reflectors are directly bonded by van der Waals forces to form a permanently sealed vacuum core cavity that is vacuum-proof in air. 2) placing the bonded vacuum core cavity in a glass sleeve, and adding a getter into the glass sleeve; a vacuum tube is provided on the side wall of the glass sleeve, one end of the vacuum tube is connected to the inside of the glass sleeve, and the other end is connected to the outside; 3) Setting a fixing structure in the glass sleeve and fixing the vacuum core cavity in the glass sleeve; 4) Optical windows are respectively provided at both ends of the glass sleeve, and the interfaces between the two ends of the glass sleeve and the optical windows are sealed by UV adhesive bonding at room temperature; 5) Evacuate the glass casing through the vacuum tube; 6) After the glass sleeve is vacuumed, the mouth of the exhaust pipe is locally heated and sintered to seal the glass sleeve to form a double-stage vacuum barrier.

2. The preparation method according to claim 1, wherein In step 1) a), the thermal expansion coefficient of the annular gasket is ≤3×10 -8 / K.

3. The preparation method according to claim 1, wherein In step 1) b), the thermal expansion coefficient of the reflective lens is ≤3×10 -8 / K.

4. The preparation method according to claim 1, wherein In step 1) c), the pressure of the vacuum environment is ≤10 - 3 Pa.

5. The preparation method according to claim 1, wherein In step 2), the getter is zirconium-aluminum alloy, zirconium-nickel alloy, zirconium-iron-vanadium alloy, or zirconium graphite.

6. The preparation method according to claim 1, wherein In step 3), the fixing structure uses two short glass tubes, and the vacuum core cavity is located in the center of the glass sleeve. The two short glass tubes are inserted into the glass sleeve from both ends. The two short glass tubes clamp the vacuum core cavity from both ends respectively, and the vacuum core cavity is fixed in the center of the glass sleeve.

7. The preparation method according to claim 1, wherein In step 4), UV adhesive is applied to the interface between the transparent window and the glass sleeve, and UV laser is used to irradiate the interface between the transparent window and the glass sleeve to achieve UV adhesive bonding.

8. A long-term vacuum-maintaining Fabry-Perot cavity, characterized in that: The long-term vacuum-maintaining Fabry-Perot cavity comprises: an annular gasket, a reflective lens, a glass sleeve, an exhaust pipe, a getter and an optical window; wherein the annular gasket is made of glass with a light-through hole in the middle; the reflective lens is made of ultra-low expansion glass, and a high-reflectivity film is coated on the central part of one surface of each reflective lens, and the area of ​​the high-reflectivity film is consistent with the area of ​​the light-through hole; the surfaces of the two reflective mirrors coated with the high-reflectivity film face each other, with the annular gasket sandwiched between them, and are directly bonded by van der Waals force to form a vacuum core cavity that is permanently sealed and vacuumed in air; the vacuum core cavity formed by bonding It is placed in a glass sleeve, and a vacuum pipe is provided on the side wall of the glass sleeve. One end of the vacuum pipe is connected to the inside of the glass sleeve, and the other end is connected to the outside, and a getter is placed in the glass sleeve; the inner diameter of the glass sleeve is larger than the outer diameter of the vacuum core cavity, and the vacuum core cavity is fixedly installed in the glass sleeve through a fixed structure; optical windows are respectively provided at both ends of the glass sleeve, and the interfaces between the two ends of the glass sleeve and the optical windows are sealed by ultraviolet bonding; the glass sleeve is vacuumed through the vacuum pipe; after the glass sleeve is vacuumed, the pipe mouth of the vacuum pipe is locally heated and sintered to seal the glass sleeve to form a double-stage vacuum barrier.

9. The long-term vacuum-maintaining Fabry-Perot cavity according to claim 1, wherein: The frequency interval c / (2L) is set by the thickness L of the annular spacer, where c is the speed of light.

10. The long-term vacuum-maintaining Fabry-Perot cavity according to claim 1, wherein: Both sides of the optical window are coated with anti-reflection films.

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