A tunable and traceable spectral calibration device

Abstract

The present invention discloses a tunable and traceable spectral calibration device, which includes a tunable laser, an optical frequency comb frequency stabilization module, a power monitoring module, an integrating sphere, a DC motor, a computer, and a spectrometer to be calibrated. The tunable laser provides a continuous light source for spectral calibration; the optical frequency comb frequency stabilization module locks the output optical frequency of the tunable laser to the optical frequency comb frequency; the power monitoring module is used to monitor the output optical power of the tunable laser in real time; the integrating sphere and the DC motor are used to eliminate the speckle effect of the laser during spectral calibration; the computer is used to record spectral calibration data and algorithm processing; the spectrometer to be calibrated is used to provide a measured object. The spectral calibration device is suitable for high-precision, high-spectral resolution, and traceable fine spectral calibration measurement applications. The advantage is that the optical frequency comb frequency locking module is used to trace the light source frequency of the spectral calibration to the atomic time scale reference, thereby ensuring the high precision and traceability of spectral calibration from the source.

Description

Technical Field

[0001] The present invention relates to high-precision fine spectrum measurement technology, continuous spectrum measurement technology within a certain spectral range, traceable spectrum measurement technology, etc., and specifically refers to a high-precision tunable and traceable spectrum calibration device, which can be widely used in the spectrum measurement field of high spectral resolution, high precision, and traceable spectrum instruments. Background Art

[0002] With the development of aerospace technology, the spectral resolution and signal-to-noise ratio of detection payloads are getting higher and higher. In order to achieve highly quantitative remote sensing, it is necessary to perform high-precision spectral calibration on the payload. At present, the laboratory spectral calibration device cannot meet the requirements of ultra-fine spectral calibration. For example, the wavelength calibration accuracy of the gas monitor with sub-nanometer spectral resolution is better than 10pm, and the ILS (Instrument Line Shape) calibration accuracy is better than 1% to meet the requirements of preliminary research on atmospheric fine components and climate detection. The realization of this spectral calibration accuracy index puts forward new requirements for the precision, accuracy and stability of the laboratory spectral calibration system. At present, the spectral calibration methods in the laboratory include characteristic spectral line method, monochromatic collimation method, gas absorption cell method, tunable laser method, etc. When the characteristic spectral line method is used for spectral calibration, the wavelength is affected by the characteristics of the characteristic substance, the characteristic wavelength is limited, and the full spectrum spectral calibration cannot be achieved; the monochromatic collimation method can have multiple spectral lines, but the output light is uneven, the tuning resolution is low, and it does not meet the high-precision requirements. The gas absorption cell method can theoretically achieve very high accuracy, but it has high environmental requirements, many sources of error, and requires comparison with a database, which is relatively complicated. The tunable laser method can achieve spectral calibration with high spectral resolution, but the frequency stability of the light source is easily affected by the environment and cannot be traced. In recent years, with the development of optical frequency comb technology, optical frequency comb has received widespread attention in the field of precision measurement. At present, no technology similar to the present invention has been found in publicly available research literature. Summary of the invention

[0003] The main purpose of the present invention is to overcome the defects of the above-mentioned technology, solve the problems of low frequency stability of the light source in the fine spectrum test of the hyperspectral instrument and untraceable spectrum calibration, and use the optical frequency comb stabilization module as a bridge to provide a high-precision, tunable and traceable spectrum calibration device.

[0004] In order to achieve the above tasks, the present invention adopts the following technical solutions:

[0005] A traceable and tunable spectrum calibration device comprises a tunable laser 1, an optical frequency comb stabilization module 2, a power monitoring module 3, a rotating integrating sphere 4, a DC motor 5, an instrument to be calibrated 6, and a computer 7.

[0006] Features:

[0007] The optical frequency comb stabilization module 2 includes a pump source amplifier 8, a 1×3 polarization-maintaining fiber coupler 9, a frequency doubling crystal 10, a beat frequency device 11, an optical frequency comb 12, an optical filtering device 13, a photodetector 14, an amplifier and filter device 15, a frequency reference source 16 (rubidium clock, hydrogen clock, etc.), a frequency synthesizer 17, a proportional-integral controller 18, and a wavelength meter 19. The optical frequency comb stabilization module 2 is specifically as follows Figure 2 shown.

[0008] Figure 2 The pump source amplifier 8 and the frequency doubling crystal 10 are used to solve the problem of the 2.1-2.5μm band, when the wavelength range of the optical frequency comb 12 is not covered, and the light output by the tunable laser 1 is frequency-doubled to the coverage range of the optical frequency comb 12 by using the method of amplification and frequency doubling. The laser output by the semiconductor laser enters the 1×3 polarization-maintaining fiber coupler 9 after passing through the optical isolator ISO and the fiber collimator C, and is divided into 3 paths through the 1x3 polarization-maintaining fiber coupler 9. The first path is directly output for spectrum calibration; the second path enters the wavelength meter 19 to measure the wavelength of the output laser; the third path enters the beat frequency device 11 after being frequency-doubled by the frequency doubling crystal 10.

[0009] In order to improve the signal-to-noise ratio of the beat signal of the laser and the optical frequency comb, a beat device 11 composed of a half-wave plate HWP, a polarization beam splitter cube PBS and a polarizer P is used. The direction of rotating the half-wave plate can change the output optical power of the tunable laser 1 and the optical frequency comb 12 for the beat. The photosynthetic light output by the tunable laser 1 and the optical frequency comb 12 enters the optical filtering device 13, which can use a grating or a Glan Taylor prism. The comb teeth of the optical frequency comb adjacent to its frequency are incident on the photodetector 14. The photodetector 14 can use an avalanche photodiode, so that the obtained beat signal has a signal-to-noise ratio of about 30dB. The beat signal of the laser and the optical frequency comb is processed by the amplification and filtering device 15, and then the error signal is formed after phase detection with the reference signal output by the frequency synthesizer 17. The error signal is fed back to the laser drive control port to control the current I and the piezoelectric ceramic PZT after the proportional integral controller 18, thereby realizing the frequency locking of the output light of the tunable laser 1 and the output light of the optical frequency comb 12. The optical frequency comb 12 and the frequency synthesizer 17 are connected to the same frequency reference source 16. The frequency reference source 16 can be a rubidium clock or a hydrogen clock. Therefore, after locking, the stability of the optical frequency comb teeth frequency is basically consistent with the stability of the rubidium clock, that is, 10 -12 This ensures that the light source for spectral calibration can achieve high frequency stability.

[0010] The output laser average power of the optical frequency comb 12 is greater than 30mW, the repetition frequency is greater than 100MHz, and the output spectrum range should cover the wavelength range of the output light of the tunable laser 1 after frequency doubling. If the output spectrum of the optical frequency comb 12 cannot fully cover the output range of the tunable laser 1, the output light of the tunable laser 1 can be processed by amplification-frequency doubling. First, the output wavelength of the tunable laser, such as 2.3μm, is frequency-doubled to 1.15μm using the pump source amplifier 8 and the frequency doubling crystal 10. This method solves the problem that the range of the optical frequency comb 12 cannot be covered. The output pulse width of the optical frequency comb 12 is generally in the order of femtoseconds.

[0011] In order to tune the frequency of the laser, the frequency of the laser is locked to different optical frequency comb teeth by using the "open-tune-lock" method to achieve the tuning of the laser frequency. Here, the integer of the comb teeth can be measured by a high-precision wavelength meter 19. The wavelength measurement accuracy of the wavelength meter 19 should generally be less than 1 / 2 of the repetition frequency of the optical frequency comb, and 1 / 3 can be used as a margin. For example, if the repetition frequency of the optical frequency comb is 150MHz, the accuracy of the wavelength meter can be selected to be 50MHz.

[0012] When the tunable laser 1 is used for spectrum calibration, its output power should generally be greater than 30mW. If the power is lower, the light source should be amplified to more than 30mW by the pump source amplifier 8. The output light is divided into three paths, the first path is directly output for spectrum calibration; the second path enters the wavelength meter 19 to measure the wavelength of the output laser; the third path enters the beat frequency device 11 after being multiplied by 10 times by the frequency doubling crystal.

[0013] The rotating integrating sphere 4 is used to eliminate the speckle effect of the laser output by the frequency-stabilized tunable laser 1. The general method is to open a hole in a part of the integrating sphere 4, install a DC motor 5, and load the internal diffuse reflection surface of the integrating sphere hole size. When the laser is incident, turn on the DC motor 5 and adjust the DC motor to a reasonable speed to eliminate the laser speckle.

[0014] The power monitoring module 3 uses a power meter to monitor the output optical power of the tunable laser in real time.

[0015] The spectrometer 6 to be calibrated is generally a high spectral resolution spectrometer and is the object to be measured.

[0016] The computer 7 is used for data collection, control and processing. The collected calibration data is generally obtained by using Gaussian fitting or Lorentz fitting method.

[0017] It should be noted that the entire device runs on an optical vibration isolation platform in the laboratory.

[0018] The traceability chain of the present invention is implemented as follows Figure 3 shown.

[0019] The advantages of the present invention are:

[0020] 1) In the currently available spectral calibration technologies, traceability is not achieved during the calibration process using the characteristic spectral line method, the monochromatic collimation method, the gas absorption cell method, the tunable laser method and other technologies. The present invention is based on the traditional tunable laser calibration technology and innovatively introduces an optical frequency comb stabilization module to achieve traceability of the calibration source, thereby ensuring the accuracy requirements of the inversion data from the source.

[0021] 2) Break through the current limitation of the optical frequency comb to 2.1μm wavelength, and use the amplification-frequency doubling method to double the continuous laser source (such as 2.3μm) to the coverage range of the optical comb, so as to achieve traceable calibration in the field of short-wave infrared spectrum calibration at home and abroad.

[0022] 3) Realize frequency stabilization of a tunable continuous laser source. The literature currently surveyed does not explain how to achieve frequency stabilization of a tunable continuous laser source to an optical frequency comb. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the system device and a structural diagram of a high-precision tunable and traceable spectral calibration device.

[0024] Figure 2 Schematic diagram of the optical frequency comb stabilization module.

[0025] Figure 3 Schematic diagram of the spectrum calibration traceability to the National Institute of Standards and Technology (NIST). DETAILED DESCRIPTION

[0026] The features and other related features of the present invention are further described in detail below through embodiments in conjunction with the accompanying drawings to facilitate understanding by those skilled in the art. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0027] Figure 1 This is a structural diagram of a high-precision tunable and traceable spectrum calibration device according to an example of the invention. It includes: a tunable laser 1, an optical frequency comb stabilization module 2, a power monitoring module 3, a rotating integrating sphere 4, a DC motor 5, a spectrometer to be calibrated 6, and a computer 7:

[0028] Among them, the specific structure diagram of the optical frequency comb stabilization module 2 Figure 2 As shown, I represents the output current of the tunable laser, PZT represents piezoelectric ceramics, ISO represents optical isolator, C represents fiber collimator, HWP represents half-wave plate, PBS represents polarization splitter cube, P represents polarization, A / D represents analog-to-digital converter, and the photodetector 14 can be an avalanche photodiode detector. Figure 2 The solid black line represents the light propagation route, and the dashed black line represents the electrical signal transmission route.

[0029] like Figure 2 As shown, the tunable laser 1 may be a semiconductor external cavity tunable laser with an output wavelength of 2250-2450 nm. Since the optical frequency comb 12 does not cover 2200-2400nm at present, its output light is amplified and frequency-doubled to the coverage range of the optical comb, and then its output laser is divided into three paths through a 1×3 polarization-maintaining fiber coupler 8, one of which is used for beat frequency. The specific beat frequency optical path is a beat frequency device 11 composed of a fiber collimator, a half-wave plate, a polarization cube and a polarizer. One end input is the output laser of the tunable laser 1, and the other end is the output light of the optical frequency comb 12. After the beat frequency, optical filtering is performed. The optical filtering device 13 is generally a Glan Taylor prism. After optical filtering, a beat frequency signal is obtained. The beat frequency signal enters the photodetector 14, and electronic amplification, filtering and other signal processing are performed. It is compared with a reference frequency reference source 16, such as a rubidium clock, to obtain an error signal, and then the proportional integral controller 18 is used to control the current I and piezoelectric ceramic PZT of the tunable laser, where the current I is coarse adjustment and the piezoelectric ceramic PZT is fine adjustment. Finally, the output laser frequency of the tunable laser 1 is locked to the frequency of the output laser of the optical frequency comb 12. Since the optical frequency comb 12 and the frequency synthesizer 17 are connected to the same frequency reference source 16, the frequency of the rubidium clock, the tunable laser 1 is locked to a comb tooth of the optical frequency comb 12. In this way, the output frequency of the tunable laser 1 is locked to the output frequency of the rubidium clock of the reference signal source 16, reaching 10 -12 Order of magnitude. Thus, the stability and traceability of the calibration source are guaranteed during spectral calibration. It should be noted here that when a wavelength point is locked and the next wavelength is locked, it is necessary to first lose the lock for about 10 seconds, then tune the tunable laser 1 to the next wavelength, and then lock to different optical frequency comb 12 teeth.

[0030] like Figure 1 As shown, the output light of the tunable laser after frequency doubling is outputted using spatial light or optical fiber, one of which is directly coupled into an integrating sphere. The integrating sphere uses a rotating integrating sphere 4, and the diffuse reflection surface of the laser incident is driven to rotate by a DC motor 5 to eliminate the influence of laser speckle on the calibration system. The light output by the rotating integrating sphere 4 enters the entrance pupil of the spectrometer to be calibrated 6. By tuning the step length of the laser output by the tunable laser 1, the response of each pixel point on the focal plane to different wavelengths is obtained, and finally the instrument linear function of the point is obtained. The power monitoring module 3 uses a power meter to monitor the optical power of the tunable laser output light in real time. Figure 1 The computer 7 is used for the control of the whole device, data recording and processing, etc. The calibration data is processed by Gaussian fitting algorithm to obtain the linear function of the instrument to be calibrated 6, and then the spectral resolution and central wavelength of the spectrometer to be calibrated 6 are obtained.

[0031] like Figure 3As shown in the figure, the traceability link of the light source of the tunable and traceable spectrum calibration device is realized. The uncertainty of spectrum calibration includes the frequency deviation of the rubidium clock signal output, the repetition frequency of the femtosecond optical comb, the uncertainty of the carrier envelope bias frequency, the uncertainty of the laser light source, the uncertainty of data fitting, etc. Then, the evaluation is carried out according to the uncertainty evaluation method, and finally the traceability of spectrum calibration is realized. Figure 3 As shown, a schematic diagram of the spectrum calibration device 20 traceable to the National Institute of Standards and Technology (NIST) 22 of the United States. The light source of the spectrum calibration device 20 is a tunable laser 1, and the output frequency of the tunable laser 1 is traceable to the comb frequency of the optical frequency comb 12. The output frequency of the optical frequency comb 12 is locked to the output frequency of the reference signal source 16, which is the rubidium clock frequency. The rubidium clock frequency can be traced back to the atomic time scale reference 21, and the atomic time scale reference 21 can be traced back to the National Institute of Standards and Technology (NIST) 22 of the United States, thereby ensuring the traceability of the spectrum calibration device from the source.

Claims

1. A tunable and traceable spectral calibration device, comprising a tunable laser (1), an optical frequency comb stabilization module (2), a power monitoring module (3), a rotating integrating sphere (4), a DC motor (5), a spectrometer to be calibrated (6), and a computer (7), characterized in that: The spectrum calibration device is centered on a rotating integrating sphere (4), with a tunable laser (1) and an optical frequency comb stabilization module (2) placed in sequence on the left side, a spectrometer to be calibrated (6) placed on the right side, a power monitoring module (3) placed on the top, a computer (7) placed on the bottom, and a DC motor (5) installed on the rotating integrating sphere; the entire device is placed on an optical platform with a vibration isolation function; light output by the tunable laser (1) passes through the optical frequency comb stabilization module (2), and then passes through an optical fiber beam splitter, a portion of which enters the rotating integrating sphere (4) with a DC motor (5), the output light is used for spectrum calibration of the spectrometer to be calibrated (6), and the other portion enters the power monitoring module (3) for power monitoring; The optical frequency comb stabilization module (2) is used to lock the output optical frequency of the tunable laser (1) to the comb teeth of the optical frequency comb; the optical frequency comb stabilization module (2) comprises a pump source amplifier (8), a 1×3 polarization-maintaining fiber coupler (9), a frequency doubling crystal (10), a beat frequency device (11), an optical frequency comb (12), an optical filtering device (13), a photodetector (14), an amplifier and filter device (15), a frequency reference source (16), a frequency synthesizer (17), a proportional-integral controller (18), and a wavelength meter (19); the laser light output by the tunable laser (1) is amplified by the pump source amplifier (8) and then divided into three paths by the 1×3 polarization-maintaining fiber coupler (9), the first path of which is directly output for spectral calibration; the second path of which enters the wavelength meter (19) for measuring the wavelength of the output laser light; The third path thereof enters the beat frequency device (11) after being frequency-doubled by the frequency-doubling crystal (10); the laser light output by the tunable laser (1) and the laser light output by the optical frequency comb (12) enter the beat frequency device (11) and combine them, then enter the optical filtering device (13) and enter the photodetector (14) to obtain a beat frequency signal; the beat frequency signal is processed by the amplifying and filtering device (15), and then formed into an error signal after phase detection with the reference signal output by the frequency synthesizer (17); the error signal is fed back to the driving control port of the laser to control the current and the piezoelectric ceramic after passing through the proportional integral controller (18), thereby realizing the frequency locking of the output light of the tunable laser (1) and the comb teeth output light of the optical frequency comb (12); the optical frequency comb (12) and the frequency synthesizer (17) are both connected to the same frequency reference source (16); the frequency of the tunable laser (1) is locked to different comb teeth frequencies of the optical frequency comb (12) by using the open-tune-lock method, thereby realizing the tunability of the output light frequency of the tunable laser (1); The computer (7) controls the controller of the tunable laser (1), rotates the speed of the DC motor (5) on the integrating sphere (4), records the output response data of the spectrometer to be calibrated (6), processes the calibration data with a Gaussian fitting algorithm to obtain the linear function of the spectrometer to be calibrated (6), and further obtains the spectral resolution and central wavelength of the spectrometer to be calibrated (6); The traceability of the spectrum calibration device is achieved by locking the laser frequency output by the calibration source to the comb tooth frequency of the optical frequency comb, thereby achieving the traceability of the spectrum calibration result.

2. A tunable and traceable spectral calibration device according to claim 1, characterized in that: The tunable laser (1) is a semiconductor laser or an all-solid-state laser, the output laser power should generally be greater than 30 mW, and the wavelength range can cover 1-2.5 μm.

3. A tunable and traceable spectral calibration device according to claim 1, characterized in that: The beat frequency device (11) is composed of a half-wave plate, a polarization beam splitting cube and a polarizing plate.

4. A tunable and traceable spectral calibration device according to claim 1, characterized in that: The optical frequency comb (12) is a femtosecond fiber oscillator, and the wavelength range may not completely cover the wavelength output range of the tunable laser.

5. The tunable and traceable spectral calibration device according to claim 1, characterized in that: The optical filtering device (13) adopts a Glan Taylor prism or a grating.

6. The tunable and traceable spectral calibration device according to claim 1, characterized in that: The photoelectric detector (14) adopts an avalanche photodiode.

7. A tunable and traceable spectral calibration device according to claim 1, characterized in that: The frequency reference source (16) is a rubidium clock or a hydrogen clock.

8. The tunable and traceable spectral calibration device according to claim 1, characterized in that: The accuracy of the wavelength meter (19) is 1 / 3 of the repetition frequency of the optical frequency comb (12).

9. The tunable and traceable spectral calibration device according to claim 1, characterized in that: The rotating integrating sphere (4) has a DC motor (5) mounted thereon and can rotate 360 ​​degrees.

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