Method and system for generating astronomical spectrum calibration light source

Abstract

The invention discloses a method and a system for generating an astronomical spectrum calibration light source. According to the invention, double optical frequency combs with similar repetition frequencies are adopted, most output light of one optical frequency comb passes through an FP cavity to obtain pulses with high repetition frequencies, the pulses and most output light of the other optical frequency comb are collinearly interfered and incident to a balanced photoelectric detector, and data are collected and subjected to Fourier transform to be used as a signal spectrum; a small part of output light of the two optical frequency combs is subjected to beam combination and interference, is received by a photoelectric detector, is subjected to data acquisition, is subjected to Fourier transform and then serves as a reference spectrum, a spectrum transmission function of the FP cavity is obtained, a calibration light source filtered by the FP cavity is corrected, and a calibration light source of the astronomical spectrometer is obtained; the method has the advantages of wide spectrum coverage, ultrahigh precision and long-term stability; the FP cavity does not need to be absolutely locked, and only the frequency change trend needs to be measured; high-stability output is obtained through a low-cost and low-stability device, and the actual spectral response of the FP cavity is accurately solved.

Description

Technical Field

[0001] This invention relates to astronomical observation instrument calibration technology, specifically to a method and system for generating astronomical spectral calibration light sources. Background Technology

[0002] Traditional astronomical spectrometer calibration relies on thorium-argon lamps or iodine absorption cells, which suffer from problems such as sparse spectral lines, inconsistent intensities, narrow wavelength coverage, and poor long-term stability.

[0003] Existing ultrawideband light sources output comb-shaped calibration light sources through Fabry-Perot cavity (FP cavity) filtering. The comb tooth intensity is basically uniform, but the comb tooth spacing is non-uniform and cannot self-calibrate. At the same time, thermal drift and aging of the FP cavity cause the comb tooth spacing to change over time, making it difficult to meet the calibration requirements of extremely high resolution spectrometers (such as ELT / HIRES).

[0004] Laser frequency combs can theoretically provide millions of equally spaced calibration spectral lines traceable to atomic frequencies; however, their frequency spacing (laser repetition frequency) is much smaller than the resolution of astronomical spectrometers, for example, 1 GHz: 30 GHz; frequency combs that directly generate frequency spacing of tens of GHz, such as electro-optical combs (EO combs) or microcavity optical combs, are difficult to spread to hundreds of THz or convert to visible light.

[0005] Low-frequency spacing (100MHz-1GHz) optical frequency combs can have their frequency spacing multiplied to a frequency spacing that can be resolved by astronomical spectrometers through FP cavity filtering. It is envisioned that as long as the frequency spacing of the FP cavity is designed to be an integer multiple of the frequency spacing of the optical frequency comb and aligned with the transmission peak of the FP cavity, the frequency stability and accuracy of the optical frequency comb can be transferred to the filtered astronomical spectral calibration comb teeth.

[0006] Its drawbacks are also obvious. The FP cavity filtering method is essentially the frequency domain modulation of the optical frequency comb teeth by a series of transmission peak functions (Lorentz type) of the FP cavity. If the FP cavity has no dispersion, the spacing of these transmission peaks is equal. If the optical frequency comb teeth are aligned with the transmission peaks of the FP cavity according to multiples of the frequency, once the frequency of one transmission peak is determined, the frequencies of all transmission peaks can be extrapolated.

[0007] FP cavities generally exhibit dispersion, meaning their resonant frequencies (extinction range) are not uniformly distributed in the frequency domain. The comb teeth transmitting light from an FP cavity are not necessarily located at the transmission peaks. For spectrometers with resolutions far lower than the transmission peak width of FP cavities, the frequency value of each transmission peak is not determined by the position of the transmitted optical comb teeth, but rather by the peak value of each transmission function of the FP cavity. Furthermore, even FP cavities operating under vacuum and temperature control can only provide short-term stability. Due to the aging of the cavity mirror film system and ultra-low expansion (ULE) glass, FP cavities cannot provide long-term frequency stability.

[0008] Therefore, it is necessary to perform precise calibration on the spectrum of each transmission.

[0009] Calibration is typically achieved using a dual-frequency comb method, which requires two frequency combs with similar frequency spacing and identical spectral envelopes. To meet the sampling rate of the data acquisition card, the frequency difference between the two frequency combs should be on the order of kHz.

[0010] However, since the high-frequency optical frequency comb is filtered from the low-frequency optical frequency comb through the FP cavity, it is extremely difficult to fabricate two FP cavities with spectral ranges differing by kHz if the dual-frequency-comb method is used. Furthermore, ensuring that the spectral envelopes of the two ultra-wideband optical frequency combs are identical is also very challenging. Therefore, in practical measurements, calibrating the FP cavity using two optical frequency combs of tens of GHz is impractical.

[0011] In summary: Existing technologies for directly generating large-frequency-spaced optical frequency combs are not only difficult but also cannot cover visible light. Using the FP cavity filtering method, regardless of whether the light source is supercontinuum white light or white light spread by the frequency comb, the position of the transmission frequency is determined by the FP cavity. The difficulties of the dual-frequency-comb calibration method are as follows: fabricating two FP cavities with a spectral difference on the order of kHz is extremely challenging. Even if such two FP cavities are fabricated, because broadband optical frequency combs are usually generated through nonlinear fiber spreading, the spectral envelopes of the two frequency combs are unlikely to be completely identical, thus requiring new calibration methods. Summary of the Invention

[0012] To address the problems existing in the prior art, this invention proposes a method and system for generating an astronomical spectral calibration light source, which involves using a low-repetition-frequency optical comb that does not pass through the FP cavity to perform high-speed asynchronous optical sampling calibration with an optical comb that passes through the FP cavity and has a repetition frequency tens of times higher.

[0013] One object of the present invention is to provide a method for generating an astronomical spectral calibration light source.

[0014] The method for generating an astronomical spectral calibration light source according to the present invention includes the following steps:

[0015] 1) The first optical frequency comb outputs a low repetition frequency pulse, which is split into two beams by the first beam splitter. At this time, the first flip mirror in front of the FP cavity flips down and does not block the optical path. Most of the light passes through the light guide system and enters the FP cavity. After being filtered by the FP cavity, a high repetition frequency pulse is output as the first output light.

[0016] 2) The first output light with a high repetition frequency output from the FP cavity is flipped up by the second flip mirror and totally reflected by the second flip mirror. It is then split into a reflected beam and a transmitted beam by the first semi-transparent mirror.

[0017] 3) The second optical frequency comb outputs a low repetition frequency pulse as the second output light, which is split into two beams by the second beam splitter. Most of the light is transmitted to the first semi-transparent and semi-reflective mirror and split into a transmitted beam and a reflected beam. These beams interfere collinearly with the reflected beam and the transmitted beam split by the first output light through the first semi-transparent and semi-reflective mirror, respectively, and are incident on the balanced photodetector.

[0018] 4) The interference signal obtained by subtracting the two beams of light from the output of the balanced photodetector is sampled and recorded by the data acquisition card, transmitted to the computer, and then subjected to Fourier transform by the computer as the signal spectrum S. sample,raw (ν);

[0019] 5) Simultaneously, a small portion of the output light from the first optical frequency comb, split by the first beam splitter, and a small portion of the output light from the second optical frequency comb, split by the second beam splitter, are combined and interfered with. This interference is received by a photodetector and recorded by a data acquisition card, then transmitted to a computer. The computer performs a Fourier transform and uses this as the reference spectrum S. ref (ν);

[0020] 6) Obtain the spectral transmission function |T(v)| of the FP cavity based on the signal spectrum and the reference spectrum. 2 :

[0021]

[0022] Where T(v) is the transmission peak function and v is the frequency;

[0023] 7) Flip down the second flip mirror and introduce the light filtered by the FP cavity into the astronomical spectrometer. Based on the spectral transmission function of the FP cavity, obtain the frequency position of the transmission peak of each white light source and obtain the calibration source of the astronomical spectrometer.

[0024] In step 1), the light guiding system employs two collimating plane mirrors and a collimator matched to the FP cavity mode. At least two of the collimating plane mirrors are finely adjusted to ensure the light path is coaxial with the FP cavity. The repetition frequency f of the first optical frequency comb... A The frequency ranges from 250MHz to 1GHz. The repetition frequency after FP cavity filtering is from 10GHz to 50GHz.

[0025] In step 3), the repetition frequency f of the second optical frequency comb B The repetition frequency f of the first optical frequency comb A The two are similar, and the difference in their repetition frequencies is Δf = |f A -f B |≤1kHz, determined by the data acquisition card's rate and time resolution. The higher the rate, the greater the repetition frequency difference; the higher the time resolution, the smaller the repetition frequency difference.

[0026] In step 7), the first flip mirror is flipped up to introduce a white light source as a calibration source, or the first optical frequency comb is used as a calibration source, and the second optical frequency comb is turned off. The white light source can be a laser-driven white light source, which has high brightness, a wide spectral coverage (170nm~2100nm), uniform spectral distribution, and long lifetime; or a supercontinuum true white light source generated by a semiconductor diode, instead of three-color synthesis; or a broadband laser optical frequency comb with a low repetition frequency and a frequency interval of 50MHz~1GHz, which, due to its low repetition frequency, is easier to broaden the spectrum, and due to the small frequency interval, multiple comb teeth can pass through within the transmission linewidth of a single FP cavity, allowing for precise position determination without fitting; or a high repetition frequency (>1GHz) laser optical frequency comb with a large frequency interval, where the comb tooth spacing is far, effectively filtering out side modes and highlighting the position of individual comb teeth. The light filtered by the FP cavity is then guided into the coupling fiber of the astronomical spectrometer through a light guiding system, which includes two collimating plane mirrors and a collimator.

[0027] The method for preparing the FP cavity includes the following steps:

[0028] i. Preparation of the vacuum core cavity:

[0029] a) Provide an annular gasket made of ultra-low expansion glass with a light-transmitting hole in the middle;

[0030] b) Provide two reflective lenses, the material of which is ultra-low expansion glass. A high reflectivity film is coated on the center part of one surface of each reflective lens, and the area of ​​the high reflectivity film is the same as the area of ​​the light-transmitting hole of the gasket.

[0031] c) The surfaces of the two mirrors coated with high reflectivity films face each other, with an annular gasket sandwiched in the middle. The annular gasket is used to fix the gap between the two mirrors. Pressure is applied in a vacuum environment to compact them, and they are bonded directly by van der Waals forces to form a permanently sealed vacuum core cavity that is in a vacuum in the air.

[0032] ii. Place the bonded vacuum core cavity in a glass sleeve and add a getter to the glass sleeve; the side wall of the glass sleeve is provided with a vacuum tube, one end of which is connected to the inside of the glass sleeve and the other end is connected to the outside.

[0033] iii. A fixing structure is installed inside the glass sleeve to fix the vacuum core cavity inside the glass sleeve;

[0034] iv. Optical windows are placed at both ends of the glass sleeve, and the two ends of the glass sleeve are bonded to the glass sleeve using a UV bonding method at room temperature.

[0035] The interface of the optical window is closed;

[0036] v. To evacuate the glass sleeve through a vacuum tube;

[0037] vi. After the glass sleeve is evacuated, the opening of the evacuation pipe is locally heated and sintered to seal the glass sleeve, forming a double-stage vacuum barrier.

[0038] Another object of the present invention is to provide a system for generating astronomical spectral calibration light sources.

[0039] The system for generating an astronomical spectral calibration light source according to the present invention includes: first and second optical frequency combs, first and second beam splitters, first and second flip mirrors, a photomultiplier-stage (FP) cavity, first and second semi-transparent mirrors, a photodetector, a balanced photodetector, a data acquisition card, a computer, and a calibration light source; wherein, first and second flip mirrors are respectively arranged before and after the FP cavity; the first flip mirror is flipped down and the second flip mirror is flipped up: the first optical frequency comb outputs a low repetition frequency pulse, which is split into two beams by the first beam splitter, and most of the light enters the FP cavity through the first light guide system; after being filtered by the FP cavity, a high repetition frequency pulse is output as the first output light, which is split into a reflected beam and a transmitted beam by the first semi-transparent mirror through the second light guide system; the second optical frequency comb outputs a low repetition frequency pulse as the second output light, which is split into two beams by the second beam splitter. The light beam is split into a transmitted beam and a reflected beam by the first semi-transparent mirror, which interfere with the first output light in a collinear manner and are incident on the balanced photodetector. The balanced photodetector outputs an interference signal, which is sampled and recorded by the data acquisition card. A small portion of the output light from the first optical frequency comb is split by the first beam splitter and then combined with a small portion of the output light from the second optical frequency comb, which is split by the second beam splitter. This combined beam interferes with the signal, which is received by the photodetector and sampled and recorded by the data acquisition card. The signal is then transmitted to the computer for Fourier transform to obtain the transmission function of the FP cavity, which is used as the calibration function. The second flip mirror is then flipped down to guide the light filtered by the FP cavity into the astronomical spectrometer. Based on the spectral transmission function of the FP cavity, the frequency position of the transmission peak of each white light source is obtained, thus obtaining the calibration source for the astronomical spectrometer.

[0040] The calibration light source can be either the first optical frequency comb or white light. If the first optical frequency comb is used as the calibration light source, the second optical frequency comb can be turned off and the second flip mirror can be flipped down to introduce the light filtered by the FP cavity into the astronomical spectrometer, thus obtaining the calibration light source of the astronomical spectrometer. If white light is used as the light source, the first flip mirror can be flipped up and the second flip mirror can be flipped down to introduce the white light source. The light filtered by the FP cavity can then be introduced into the astronomical spectrometer, thus obtaining the calibration light source of the astronomical spectrometer.

[0041] The FP cavity comprises: an annular gasket, reflective mirrors, a glass sleeve, a suction tube, a getter, and an optical window. The annular gasket is made of glass and has a central aperture. The reflective mirrors are made of ultra-low expansion glass, with a high-reflectivity film coated on the center of one surface of each mirror; the area of ​​the high-reflectivity film is the same as the area of ​​the aperture. 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 together using van der Waals forces to form a permanently sealed vacuum core cavity that remains vacuum in air. The bonded vacuum core cavity is placed within the glass sleeve. Inside the tube, a vacuum tube is installed 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. A getter is placed inside the glass sleeve. The inner diameter of the glass sleeve is larger than the outer diameter of the vacuum core cavity. The vacuum core cavity is fixedly installed inside the glass sleeve by a fixing structure. Optical windows are set at both ends of the glass sleeve. 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 vacuum tube. After the glass sleeve is evacuated, the opening of the vacuum tube is locally heated and sintered to seal the glass sleeve, forming a double-stage vacuum barrier.

[0042] The reflecting mirror uses a zero-dispersion film or a double-mirror composite film system with zero dispersion, a reflectivity ≥99%, and a reflection bandwidth of visible light, near-infrared, ultraviolet, or other wavelengths; the spectral ablation range is three times the resolution of the astronomical spectrometer at the center wavelength requiring calibration; the annular gasket uses ultra-low thermal expansion glass with a thermal expansion coefficient ≤3×10⁻⁶. -8 / K, the thickness L of the annular gasket is set at a frequency interval c / (2L), where c is the speed of light. The surface of the annular gasket is polished to a smoothness of λ / 40~λ / 20, where λ is the wavelength. The FP cavity maintains a vacuum state to suppress thermal noise, and a temperature control device suppresses thermal drift. The diameter of the light-transmitting hole of the annular gasket is 10~15mm. The vacuum level of the FP cavity depends on the calibration strategy of the FP cavity: if calibration is performed daily, a vacuum level <10Pa is not required; if it is necessary to reduce the number of calibrations, a vacuum level maintenance device is added.

[0043] Furthermore, the present invention also includes a spectrum spreading device and a filter, disposed between the first optical frequency comb and the FP cavity, to spread the output light of the first optical frequency comb before filtering, so that the bandwidth of the wavelength of the output light of the first optical frequency comb is consistent with the reflection bandwidth of the reflecting mirror of the FP cavity. The spectrum spreading device uses a highly nonlinear dielectric waveguide or a photonic crystal fiber; the highly nonlinear dielectric waveguide uses a silicon nitride waveguide, an aluminum nitride waveguide, an aluminum oxide waveguide, or a lithium niobate waveguide.

[0044] Advantages of this invention:

[0045] 1. Wide spectral coverage: Supercontinuum light source + broadband FP cavity cover the ultraviolet to infrared band;

[0046] 2. Ultra-high precision: Dual optical frequency comb sampling achieves 10 -12 Level optical frequency positioning error;

[0047] 3. Long-term stability: Online calibration is possible, and the frequency drift trend of all transmission peaks can be tracked in real time;

[0048] 4. It is not necessary to absolutely lock the FP cavity; only its frequency change trend needs to be measured.

[0049] 5. Achieve high-stability output with low-cost and low-stability components;

[0050] 6. Accurately solve for the actual spectral response of the FP cavity.

[0051] The key indicators of this invention compared with those of existing technologies are shown in the table below:

[0052]

[0053] Attached Figure Description

[0054] Figure 1 This is a schematic diagram of an embodiment of the system for generating an astronomical spectral calibration light source according to the present invention;

[0055] Figure 2 This is a structural diagram of an FP cavity, representing an embodiment of the system for generating an astronomical spectral calibration light source according to the present invention.

[0056] Figure 3 A linear sampling interferogram of a dual-frequency comb is shown as an embodiment of the system for generating an astronomical spectral calibration light source according to the present invention.

[0057] Figure 4 The Fourier transform result is shown in one embodiment of the system for generating astronomical spectral calibration light sources according to the present invention. Detailed Implementation

[0058] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0059] like Figure 1 As shown, the method for generating an astronomical spectral calibration light source in this embodiment includes the following steps:

[0060] 1) The first optical frequency comb output repetition frequency f A A pulse of 0.9665 GHz is split into two beams by a first beam splitter with an R (reflection):T (transmission) ratio of 1:9. At this point, the first flip mirror in front of the FP cavity flips down, not obstructing the light path. Most of the light passes through a light guide system consisting of two collimating plane mirrors and a collimator matched to the FP cavity mode, and enters the FP cavity. The FP cavity has a resonant frequency of 30 GHz, is compatible with an astronomical spectrometer with a resolution of 450,000 and a precision of 300. The cavity length is 50 mm, and it uses an annular gasket made of ultra-low expansion glass. It is placed in a vacuum chamber with a vacuum level ≤10. -5 Pa; reflectivity 99.0%, reflection bandwidth 560nm~900nm; after filtering by FP cavity, the output pulse with a repetition frequency of 30GHz is used as the first output light;

[0061] 2) The first output light with a high repetition frequency output from the FP cavity is flipped up by the second flip mirror and totally reflected by the second flip mirror. It is then split into a reflected beam and a transmitted beam by the first semi-transparent mirror.

[0062] 3) The second optical frequency comb outputs a low repetition frequency f B A 1GHz pulse is used as the second output light, Δf = 1kHz (relative difference 10). -9 The light is split into two beams by the second beam splitter with a ratio of R:T = 1:9. Most of the light is transmitted to the first semi-transparent and semi-reflective mirror, where it is split into a transmitted beam and a reflected beam. These beams interfere collinearly with the reflected beam and the transmitted beam split by the first output light through the first semi-transparent and semi-reflective mirror, respectively, and are incident on the balanced photodetector.

[0063] The first and second optical frequency combs are respectively spread by photonic crystal fiber, and then filtered by long-pass and long-stop filters to cut out light with wavelengths of 560nm to 900nm, matching the reflection bandwidth of the FP cavity; the repetition frequency and initial frequency of the dual optical frequency combs are locked: a rubidium atomic clock is used as a reference, and the relative instability of the lock is <10. -12 @1s;

[0064] 4) The interference signal obtained by subtracting the two beams of light from the output of the balanced photodetector is sampled and recorded by the data acquisition card, transmitted to the computer, and then subjected to Fourier transform by the computer as the signal spectrum S. sample,raw (ν);

[0065] 5) Simultaneously, a small portion of the output light from the first optical frequency comb, split by the first beam splitter, and a small portion of the output light from the second optical frequency comb, split by the second beam splitter, are combined and interfered with. This interference is received by a photodetector and recorded by a data acquisition card, then transmitted to a computer. The computer performs a Fourier transform and uses this as the reference spectrum S. ref (ν);

[0066] 6) Obtain the spectral transmission function |T(v)| of the FP cavity based on the signal spectrum and the reference spectrum. 2 :

[0067]

[0068] Where T(v) is the transmission peak function and v is the frequency;

[0069] 7) Flip up the first flip mirror to introduce a white light source as a calibration source; flip down the second flip mirror to introduce the light filtered by the FP cavity into the astronomical spectrometer. Based on the spectral transmission function of the FP cavity, obtain the frequency position of the transmission peak of each white light source and obtain the calibration source of the astronomical spectrometer.

[0070] Fabrication of FP cavity, such as Figure 2 As shown, it includes the following steps:

[0071] i. Preparation of the vacuum core cavity:

[0072] a) Provides an annular gasket 101 of ultra-low expansion glass with a diameter of 25 mm and a thickness of 5 mm, with a light-transmitting hole in the middle, a surface flatness polished to λ / 20, a light-transmitting hole diameter of 10 mm, and λ=632.8nm;

[0073] b) Provide ultra-low expansion glass as the first reflective lens 102 and the second reflective lens 103, the bonding surface surface flatness is polished to λ / 20, the material of the first and second reflective lenses 102 and 103 is glass, and a high reflectivity film with a diameter of 10 mm is deposited on the central portion of one surface of the first and second reflective lenses 102 and 103.

[0074] c) The surfaces of the first and second reflecting mirrors 102 and 103 with high-reflectivity films face each other, with an annular spacer 101 sandwiched between them. The annular spacer 101 is used to fix the gap between the two reflecting mirrors. The contact surfaces of the reflecting mirrors and the annular spacer 101 are thoroughly cleaned with deionized water and placed in a 5×10⁻⁶ ohmmeter. -4In the vacuum chamber, the first reflecting mirror 102 is placed horizontally, and the horizontal annular gasket 101 falls onto the first reflecting mirror 102 by gravity. The horizontal second reflecting mirror 103 falls onto the annular gasket 101 by gravity. Pressure is then applied from top to bottom to compact the three components. The reflective mirror and the gasket are bonded together by van der Waals forces to form a vacuum core cavity 1 that is permanently sealed and can maintain a vacuum for a short time in air. ii. 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. A suction pipe 3 is provided on the side wall of the glass sleeve 2, with one end of the suction pipe 3 connected to the inside of the glass sleeve 2 and the other end connected to the outside.

[0075] iii. 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, fixing the vacuum core cavity to the center of the glass sleeve 2. The outer diameter 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 tube = (length of glass sleeve 2 - length of vacuum core cavity) / 2. Irregular holes may be present on the first and second glass short tubes 501 and 502 to ensure smooth evacuation. iv. Set the first and second optical windows 201 and 202 at both ends of the glass sleeve 2. Apply ultraviolet adhesive to the interface between the transparent window and the glass sleeve 2. The ultraviolet adhesive is a cationic ultraviolet adhesive with a wavelength of 365nm and an intensity of 1W / cm. 2 The ultraviolet light source irradiates the two interfaces for 0.5s to 3s respectively, with the time being shorter for higher power; after ultraviolet light irradiation, the ultraviolet adhesive hardens, sealing the interfaces between the two ends of the glass sleeve 2 and the first and second optical windows 201 and 202.

[0076] v. Evacuate the glass sleeve 2 to a pressure of 3 × 10⁻⁶ through the evacuation pipe 3. -4 Pa;

[0077] vi. After the glass sleeve 2 is evacuated, the port 4 of the evacuation pipe 3 is locally heated and sintered using a miniature oxyhydrogen flame gun to seal the glass sleeve 2 and form a double-stage vacuum barrier.

[0078] Dual-frequency comb linear sampling interferogram as shown Figure 3 As shown. Figure 3 The image shows the fundamental frequency period (966.5MHz) of two optical frequency combs, with 31 interference peaks in each period, indicating that 31 pulses are coherent with 1 pulse in one period.

[0079] Figure 3 The Fourier transform of the interference pattern is as follows Figure 4 As shown. Figure 4Only five calibration comb teeth are shown, each with a calibration frequency. It should be noted that this calibration value is not strictly equal to the value of the corresponding fundamental frequency optical comb.

[0080] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand 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 content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A method for generating an astronomical spectral calibration light source, characterized in that, The method includes the following steps: 1) The first optical frequency comb outputs a low repetition frequency pulse, which is split into two beams by the first beam splitter. Most of the light is incident on the FP cavity; after being filtered by the FP cavity, a high repetition frequency pulse is output as the first output light. 2) The first output light with a high repetition frequency output from the FP cavity is flipped up by the second flip mirror and totally reflected by the second flip mirror. It is then split into a reflected beam and a transmitted beam by the first semi-transparent mirror. 3) The second optical frequency comb outputs a low repetition frequency pulse as the second output light, which is split into two beams by the second beam splitter. Most of the light is transmitted to the first semi-transparent and semi-reflective mirror and split into a transmitted beam and a reflected beam. These beams interfere collinearly with the reflected beam and the transmitted beam split by the first output light through the first semi-transparent and semi-reflective mirror, respectively, and are incident on the balanced photodetector. 4) The interference signal after the two beams of light are subtracted from the output of the balanced photodetector is sampled and recorded by the data acquisition card and transmitted to the computer. The computer performs a Fourier transform and uses it as the signal spectrum. 5) At the same time, a small portion of the output light from the first optical frequency comb is split by the first beam splitter and a small portion of the output light from the second optical frequency comb is split by the second beam splitter. The two beams are combined and interfered, and the light is received by the photodetector. The light is recorded by the data acquisition card and transmitted to the computer. The computer performs a Fourier transform and uses the result as a reference spectrum. 6) Obtain the spectral transmission function of the FP cavity based on the signal spectrum and the reference spectrum; 7) Flip down the second flip mirror and introduce the light filtered by the FP cavity into the astronomical spectrometer. Based on the spectral transmission function of the FP cavity, obtain the frequency position of the transmission peak of each white light source and obtain the calibration source of the astronomical spectrometer.

2. The method as described in claim 1, characterized in that, In step 1), a spectrum spreading device and a filter are set after the first optical frequency comb to spread the output light of the first optical frequency comb and then filter it, so that the bandwidth of the wavelength of the output light of the first optical frequency comb is consistent with the reflection bandwidth of the reflective mirror of the FP cavity.

3. The method as described in claim 1, characterized in that, In step 1), a light guiding system is set in front of the FP cavity, the light guiding system employing two collimating plane mirrors and a collimator.

4. The method as described in claim 1, characterized in that, In step 6), based on the signal spectrum S sample,raw (ν) and reference spectrum S ref (ν) Obtain the spectral transmission function |T(v)| of the FP cavity. 2 ; Where T(v) is the transmission peak function and v is the frequency.

5. The method as described in claim 1, characterized in that, In step 7), the first flip mirror is flipped up to introduce a white light source as a calibration light source, or an optical frequency comb is used as a calibration light source.

6. The method as described in claim 1, characterized in that, The method for preparing the FP cavity includes the following steps: i. Preparation of the vacuum core cavity: a) Provide an annular gasket made of ultra-low expansion glass with a light-transmitting hole in the middle; b) Provide two reflective lenses, the material of which is ultra-low expansion glass. A high reflectivity film is coated on the center part of one surface of each reflective lens, and the area of ​​the high reflectivity film is the same as the area of ​​the light-transmitting hole of the gasket. c) The surfaces of the two mirrors coated with high reflectivity films face each other, with an annular gasket sandwiched in the middle. The annular gasket is used to fix the gap between the two mirrors. Pressure is applied in a vacuum environment to compact them, and they are bonded directly by van der Waals forces to form a permanently sealed vacuum core cavity that is in a vacuum in the air. ii. Place the bonded vacuum core cavity in a glass sleeve and add a getter to the glass sleeve; the side wall of the glass sleeve is provided with a vacuum tube, one end of which is connected to the inside of the glass sleeve and the other end is connected to the outside. iii. A fixing structure is installed inside the glass sleeve to fix the vacuum core cavity inside the glass sleeve; iv. Optical windows are set at both ends of the glass sleeve, and the interfaces between the two ends of the glass sleeve and the optical windows are sealed at room temperature by ultraviolet light bonding method; v. To evacuate the glass sleeve through a vacuum tube; vi. After the glass sleeve is evacuated, the opening of the evacuation pipe is locally heated and sintered to seal the glass sleeve, forming a double-stage vacuum barrier.

7. A system for generating an astronomical spectral calibration light source, characterized in that, The system includes: first and second optical frequency combs, first and second beam splitters, first and second flip mirrors, a photomultiplier (FP) cavity, first and second semi-transparent mirrors, a photodetector, a balanced photodetector, a data acquisition card, a computer, and a calibration light source; wherein, first and second flip mirrors are respectively arranged before and after the FP cavity; the first flip mirror is flipped down and the second flip mirror is flipped up: the first optical frequency comb outputs low repetition frequency pulses, which are split into two beams by the first beam splitter, and most of the light passes through the first light guide system and enters the FP cavity; after being filtered by the FP cavity, high repetition frequency pulses are output as the first output light, which passes through the second light guide system to the first semi-transparent mirror and is split into a reflected beam and a transmitted beam; the second optical frequency comb outputs low repetition frequency pulses as the second output light, which are split into two beams by the second beam splitter, and most of the light passes through the second beam splitter and is split into two beams, and most of the light passes through the second beam splitter and enters the first beam. The light is transmitted to the first semi-transparent and semi-reflective mirror, split into a transmitted beam and a reflected beam, which interfere collinearly with the first output light and are incident on a balanced photodetector. The balanced photodetector outputs an interference signal, which is sampled and recorded by a data acquisition card. A small portion of the output light from the first optical frequency comb is split by the first beam splitter and then interferes with a small portion of the output light from the second optical frequency comb, which is split by the second beam splitter. This interference is received by the photodetector, sampled and recorded by the data acquisition card, and transmitted to a computer for Fourier transform to obtain the transmission function of the FP cavity, which is used as the calibration function. The second flip mirror is flipped down, and the light filtered by the FP cavity from the calibration source is introduced into the astronomical spectrometer. Based on the spectral transmission function of the FP cavity, the frequency position of the transmission peak of each white light source is obtained, thus obtaining the calibration source of the astronomical spectrometer.

8. The system as described in claim 7, characterized in that, The calibration light source uses an optical frequency comb or white light as the light source; for white light as the light source, the first flip mirror flips up and the second flip mirror flips down to introduce the white light source.

9. The system as described in claim 7, characterized in that, The FP cavity includes: an annular gasket, a reflective mirror, a glass sleeve, a suction tube, a getter, and an optical window; wherein, the annular gasket is made of glass and has a light-transmitting hole in the center; the reflective mirror is made of ultra-low expansion glass, and a high-reflectivity film is coated on the central part of one surface of each reflective mirror, the area of ​​the high-reflectivity film being the same as the area of ​​the light-transmitting hole; the surfaces of the two reflective mirrors coated with the high-reflectivity film face each other, with the annular gasket sandwiched in between, and are directly bonded by van der Waals forces to form a permanently sealed vacuum core cavity that is in a vacuum in air; the bonded vacuum core cavity is placed... The vacuum core cavity is housed within a glass sleeve, with a vacuum tube on its side wall. One end of the vacuum tube connects to the inside of the glass sleeve, while the other end connects to the outside. A getter is placed inside the glass sleeve. The inner diameter of the glass sleeve is larger than the outer diameter of the vacuum core cavity, which is fixedly installed inside the glass sleeve by a fixing structure. Optical windows are installed 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 an ultraviolet bonding method. The glass sleeve is evacuated through the vacuum tube. After the glass sleeve is evacuated, the opening of the vacuum tube is locally heated and sintered to seal the glass sleeve.

10. The system as described in claim 7, characterized in that, It also includes a spectrum spreading device and a filter, which are set between the first optical frequency comb and the FP cavity. The output light of the first optical frequency comb is spread and then filtered, so that the bandwidth of the wavelength of the output light of the first optical frequency comb is consistent with the reflection bandwidth of the reflective mirror of the FP cavity.

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