A bandwidth control method for an acousto-optic tunable filter spectral imager

Abstract

A method for bandwidth control of an acousto-optic tunable filter-type spectral imager is disclosed, which achieves spectral bandwidth control by superimposing signals from multiple spectral channels. The method includes the following steps: (1) determining the target spectral bandwidth, the number of superimposed working bands, and the center wavelength of the initial band according to actual needs; (2) calculating the center wavelength of the working bands to be superimposed based on the target spectral bandwidth, the number of superimposed working bands, the center wavelength of the initial band, and the spectral characteristics of the spectral imager; (3) determining the integration time of the first working band according to actual needs; (4) calculating the integration time of other working bands based on the diffraction efficiency, quantum efficiency, and integration time of the first working band of the spectral imager; (5) calculating the actual integration time of the detector based on the integration time of each band; (6) setting the working mode of the spectral imager's RF drive and the detector to synchronous hardware triggering; and (7) completing the bandwidth control of the acousto-optic tunable filter-type spectral imager.

Description

Technical Field

[0001] The present invention relates to a method for controlling the bandwidth of an acousto-optic tunable filter type spectral imager, which is used to control the spectral bandwidth of the acousto-optic tunable filter type spectral imager according to actual application requirements and expand the application scenarios of the acousto-optic tunable filter type spectral imager. The invention belongs to the field of spectral imaging detection. Background Art

[0002] The acousto-optic tunable filter spectroscopic imager uses an acousto-optic tunable filter (AOTF) as a spectroscopic element. It has the advantages of staring imaging, flexible band switching, fast imaging speed, and no moving parts. It is widely used in mineral resource exploration, atmospheric science, environmental monitoring, military reconnaissance and other fields. The structure of AOTF mainly includes five parts: acousto-optic crystal, piezoelectric transducer, impedance matching network, sound absorber and shell. The working process of AOTF is as follows: when a beam of non-polarized full-color light is incident on AOTF, due to the birefringence of the acousto-optic crystal, the non-polarized full-color light is converted into o-light and e-light; the RF driver generates a sine wave of a specific frequency, which acts on the piezoelectric transducer through the impedance matching network, thereby generating ultrasonic waves of a specific frequency and intensity; in the piezoelectric crystal, the ultrasonic wave interacts with the incident light to generate diffracted light. Therefore, the function of AOTF can be summarized as: by controlling the frequency and size of the input electrical signal, a monochromatic light of a specific wavelength is selected from a beam of complex light. After the AOTF spectroscopic imager has completed cutting design, processing, and assembly, the performance parameters of the spectroscopic imager are fixed, that is, the spectral bandwidth of the AOTF is fixed as the device parameters and the central wavelength are determined, and the spectral bandwidth cannot be adjusted by changing the frequency of the input electrical signal. In practical applications, the spectral bandwidth of the AOTF spectroscopic imager is narrow, so a single-frame spectral image collects very little radiation energy from the detection target, which limits the detection capability of the AOTF spectroscopic imager. Summary of the invention

[0003] The technical problem solved by the present invention is: to overcome the shortcoming that the bandwidth of the existing AOTF spectral imager cannot be adjusted, and to propose a bandwidth control method for adjusting the AOTF bandwidth by superposition of multiple spectral channel signals.

[0004] The technical solution of the present invention is: to propose a bandwidth control method for an AOTF spectral imager to achieve the widening of the bandwidth of the AOTF spectral imager. First, the working mode of the AOTF RF drive and the detector is set to synchronous hardware triggering, that is, the AOTF starts to switch to the first band, and the detector starts to integrate the signal of the band. Then, when the AOTF switches to the second band, the detector is still in the integration state. Finally, until the AOTF switches to the last band and experiences the integration time of the last band, the detector stops integrating and performs charge transfer and reading. In the whole process, the detector integrates multiple bands, so that multiple spectral channel signals are superimposed, and the bandwidth of the AOTF is controlled.

[0005] The present invention is a method for controlling the bandwidth of an acousto-optic tunable filter-type spectral imager, and the specific steps are as follows:

[0006] Step 1: Determine the target spectral bandwidth of the AOTF spectral imager as Δλ according to actual needs bw , the number of superimposed bands n and the center wavelength λ of the starting band 1

[0007] Step 2: According to the target spectral bandwidth Δλ bw , the number of superimposed bands n, the center wavelength of the starting band λ 1 and the spectral characteristics of the spectral imager, calculate the central wavelength λ of the other n-1 working bands involved in the superposition 2 , ..., λ n ;

[0008] Step 3: Determine the detector integration time t in the first working band according to actual needs 1 ;

[0009] Step 4: Based on the diffraction efficiency and quantum efficiency of the AOTF spectral imager in the n working bands in step 2 and the integration time t of the detector in the first working band in step 3 1 , calculate the detector integration time t in other working bands i , where i = 2, 3, ..., n;

[0010] Step 5: Calculate the actual detector integration time t corresponding to the target spectral bandwidth based on the detector integration time in each band 0

[0011] Step 6: Set the working mode of the AOTF RF driver and detector to synchronous hardware trigger;

[0012] Step 7: According to step 4, step 5 and step 6, the spectral bandwidth control of the acousto-optic tunable filter imaging spectrometer is completed.

[0013] The method for controlling the bandwidth of an acousto-optic tunable filter-type spectroscopic imager described in step 2 is characterized in that: the calculation of the center wavelengths λ of the other n-1 working bands involved in the superposition described in step 2 2 , ..., λ n The calculation method is as follows:

[0014] The starting wavelength λ is set by 1 , and the normalized spectral response curve of this band and bandwidth Δλ bw1 , set the distance between the central wavelength of the i-1th band and the i-th band, that is, the sampling interval is Δλ i :

[0015] Δλ i =dΔλ bwi

[0016] Where d is the sampling coefficient, and its value range is 0 <d<1,Δλ bwi is the bandwidth of the i-1th band, then i=2, 3, ..., n

[0017] The sampling interval is Δλ i A preliminary estimate of the central wavelength of the ith band can be calculated:

[0018] λ i =λ i-1 +Δλ i

[0019] Based on the preliminary estimation of the center wavelength of each band, the superposition h of the normalized spectral response curves of these n bands can be calculated. d (λ) is:

[0020]

[0021] In the formula is the normalized spectral response curve of the first band, is the normalized spectral response curve of the i-th band, i = 2, 3, ..., n

[0022] h d (λ) is the superposition spectral response function when the sampling coefficient is d, and the spectral half-height width Δλ can be determined from this curve. bwd , traverse d in the range of 0-1, so that the sampling coefficient d and the spectral half-height width Δλ are obtained bwd The corresponding relationship between , d finally takes Δλ bwd With Δλ bw Δλ at the closest approach bwd The corresponding d value; after obtaining the d value, according to Δλ i =dλ bwi and λi =λ i-1 +Δλ i , i = 2, 3, ..., n, thus finally determining the center wavelength λ of each band 1 ,λ 2 , ..., λ n .

[0023] Among them, the detector integration time t corresponding to the second working band to the nth working band described in step 4 is 2 , t 3 ,……,t n , which is calculated as follows:

[0024]

[0025] Where: η(λ 1 ) is the AOTF at wavelength λ 1 The diffraction efficiency at 1 ) is the AOTF at wavelength λ 1 The quantum efficiency at i ) is the central wavelength λ of AOTF i The diffraction efficiency at i ) is the central wavelength λ of AOTF i Quantum efficiency at i = 2, 3, ..., n, t 1 It is the integration time of the detector corresponding to the first working band set in step 3.

[0026] Among them, the actual detector integration time t corresponding to the target spectral bandwidth described in step 5 is 0 , which is calculated as follows:

[0027]

[0028] Where t i is the integration time of the detector corresponding to the i-th band, i = 2, 3, ..., n.

[0029] The advantages of the present invention compared with the prior art are:

[0030] (1) Based on the existing AOTF spectral imager, the bandwidth of the AOTF spectral imager can be adjusted to the target spectral bandwidth required in practical applications, which is suitable for detection scenarios where the imaging target spectral curve has low precision and the target energy is weak.

[0031] (2) When the AOTF bandwidth is expanded, signals from more spectral channels are received by the detector, which is beneficial to improving the imaging quality of the spectrometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1The present invention is a flowchart of a method for controlling the bandwidth of an acousto-optic tunable filter imaging spectrometer;

[0033] Figure 2 This is a schematic diagram of the AOTF radio frequency drive and detector synchronization hardware triggering principle of the present invention;

[0034] Figure 3 A schematic diagram of the effect of a method for controlling the bandwidth of an acousto-optic tunable filter imaging spectrometer according to the present invention;

[0035] The symbols in the figure are explained as follows:

[0036] Δλ bw : Target spectral bandwidth

[0037] n: number of bands involved in superposition work

[0038] λ i (i=1, 2, ..., n): center wavelength of the operating band

[0039] t i (i=1,2,……,n): integration time of the detector in the corresponding band

[0040] t 0 : Actual total integration time of the detector

[0041] Specific implementation cases

[0042] like Figure 1 As shown, the present invention comprises the following steps:

[0043] Step 1: Determine the target spectral bandwidth of the AOTF spectral imager as Δλ according to actual needs bw =50nm, number of superimposed bands n=24 and center wavelength of the starting band λ 1 =500nm

[0044] Step 2: From Δλ in step 1 bw =50nm, n=24 and λ 1 =500nm, according to Δλ i =dΔλ bwi , i =λ i-1 +Δλ i and Finding Δλ bw With Δλ bwd The d value corresponding to the closest time is used to calculate the central wavelength λ of the other 23 working bands participating in bandwidth control superposition 2 , ..., λ 24 for:

[0045] λ i =λ i-1+dΔλ bwi

[0046] Where i = 2, 3, ..., 24, λ i is the central wavelength of the i-th working band, Δλ bwi is the bandwidth of the ith band.

[0047] Step 3: Set the detector integration time t in the first working band according to actual needs 1 =40ms;

[0048] Step 4: Based on the diffraction efficiency and quantum efficiency of the AOTF spectral imager in the 24 working bands in step 2 and the integration time t of the detector in the first working band in step 3 1 , calculate the integration time t of the lower detector in other working bands 2 , t 3 ,……,t 24 for:

[0049]

[0050] Where: η(λ 1 ) is the AOTF at wavelength λ 1 The diffraction efficiency at 1 ) is the AOTF at wavelength λ 1 The quantum efficiency at i ) is the AOTF at wavelength λ i The diffraction efficiency at i ) is the AOTF at wavelength λ i Quantum efficiency at i = 2, 3, ..., 24, t 1 It is the integration time of the detector corresponding to the first working band set in step 3.

[0051] Step 5: The integration time t of the 24 bands in step 4 1 , t 2 , t 3 ,……,t 24 , calculate the actual detector integration time t corresponding to the target spectral bandwidth 0 :

[0052]

[0053] Step 6: Use FPGA to realize the working mode of AOTF RF driver and detector as synchronous hardware trigger. The timing working principle is as follows Figure 2 As shown, the detector integration time is set to t 0 .

[0054] Step 7: According to Step 4, Step 5 and Step 6, the bandwidth of the acousto-optic tunable filter spectrometer is adjusted. The final spectral bandwidth output by the AOTF spectrometer is about 50nm. See the schematic diagram of the effect. Figure 3 .

Claims

1. A method for controlling bandwidth of an acousto-optic tunable filter-type spectral imager, characterized in that: The specific steps of this method are as follows: Step 1: Determine the target spectral bandwidth of the spectral imager as Δλ according to actual needs bw , the number of superimposed bands n and the center wavelength λ1 of the starting band Step 2: According to the target spectral bandwidth Δλ bw , the number of superimposed bands n, the central wavelength λ1 of the starting band and the spectral characteristics of the spectral imager, calculate the central wavelengths λ2, ..., λ of the other n-1 working bands involved in bandwidth control superposition n ; Step 3: Set the detector integration time t1 in the first working band according to actual needs; Step 4: According to the diffraction efficiency and quantum efficiency of the spectral imager in the n working bands in step 2 and the integration time t1 of the detector in the first working band in step 3, calculate the integration time t of the detector in other working bands. i , where i = 2, 3, ..., n; Step 5: Calculate and set the actual detector integration time t0 corresponding to the target spectral bandwidth according to the detector integration time in each band Step 6: Set the working mode of the RF driver and detector of the spectrum imager to synchronous hardware trigger. The spectrum imager starts to scan the set bands. When the RF driver switches to the first band, the detector starts to integrate. The duration of each frequency of the spectrum imager is t i , the detector is set to have an integration time of t0. When the RF drive completes scanning of the nth band, the detector integration time reaches t0 and the integration ends, and the spectral imager outputs image data; Step 7: According to step 4, step 5 and step 6, the bandwidth control of the acousto-optic tunable filter spectrometer is completed.

2. The bandwidth control method of an acousto-optic tunable filter-type spectroscopic imager according to claim 1, characterized in that: The target spectral bandwidth Δλ described in step 1 bw The spectral bandwidth is greater than any single working band of the acousto-optic tunable filter spectroscopic imager.

3. The bandwidth control method of an acousto-optic tunable filter spectrometer according to claim 1, characterized in that: The spectral characteristics of the spectral imager described in step 2 refer to the normalized spectral response function and bandwidth in any band.

4. The bandwidth control method of an acousto-optic tunable filter-type spectroscopic imager according to claim 1, characterized in that: Step 2 described in accordance with the target spectral bandwidth Δλ bw , the number of superimposed bands n, the central wavelength λ1 of the starting band and the spectral characteristics of the spectral imager, calculate the central wavelengths λ2, ..., λ of the other n-1 working bands involved in bandwidth control superposition n The calculation method is as follows: The starting band λ1 is set, and the normalized spectral response curve h of this band λ1 (λ) and bandwidth Δλ bw1 , set the interval between the i-1th band and the i-th band to Δλ i : Dl i =dΔλ bwi Where d is the sampling coefficient, and its value range is 0 <d<1,Δλ bwi is the bandwidth of the i-1th band, then i = 2, 3, ..., n, and the preliminary estimate of the central wavelength of the i-th band can be calculated: l i =λ i-1 +Dl i Based on the initial estimation of each band, the superposition h of the normalized spectral response curves of these n bands can be calculated. d (λ) is: Where h λ1 (λ) is the normalized spectral response curve of the first band, h λi (λ) is the normalized spectral response curve of the i-th band, i = 2, 3, ..., n, h d (λ) is the superposition spectral response function when the sampling coefficient is d, and the spectral half-height width Δλ can be determined from this curve. bwd , traverse d in the range of 0-1, so that the sampling coefficient d and the spectral half-height width Δλ are obtained bwd The corresponding relationship, d finally takes Δλ bwd With Δλ bw Δλ at the closest approach bwd The corresponding d value; After obtaining the value of d, according to Δλ i =dΔλ bwi and λ i =λ i-1 +Δλ i , i = 2, 3, ..., n, thus determining the final center wavelength of each band: λ1, λ2, ..., λ n .

5. The bandwidth control method of an acousto-optic tunable filter spectrometer according to claim 1, characterized in that: The detector integration time t2, t3, ..., t corresponding to the second working band to the nth working band in step 4 n , which is calculated as follows: Where: η(λ1) is the diffraction efficiency of AOTF at the center wavelength λ1, Q(λ1) is the quantum efficiency of AOTF at the center wavelength λ1, η(λ i ) is the central wavelength λ of AOTF i The diffraction efficiency at i ) is the central wavelength λ of AOTF i The quantum efficiency at , i = 2, 3, ..., n, t1 is the integration time of the detector corresponding to the first working band set in step 3.

6. The bandwidth control method of an acousto-optic tunable filter-type spectroscopic imager according to claim 1, characterized in that: In step 5, the actual detector integration time t0 corresponding to the target spectral bandwidth is calculated according to the detector integration time under each band. The calculation method is as follows: Where t i is the integration time of the detector corresponding to the i-th band, i = 2, 3, ..., n.

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