Spectrum light splitting imaging system light path based on volume hologram grating component light splitting

[0014] Such as figure 1 The distant target is imaged at the slit 2 by the telephoto objective lens 1, and the light entering the slit passes through the collimating objective lens 3 to reach the holographic beam splitter 4 for splitting, and the parallel beam is split by the imaging objective lens 5 and imaged on the image plane. , Received by the CCD detector 6.

[0015] The collimating objective lens and the imaging objective lens are an important part of the spectral imaging system. The collimating objective lens collimates the light passing through the slit and then enters the beam splitting device, while the imaging objective lens converges and image the dispersive light on the image plane. For imaging spectrometers, its energy utilization is an important performance indicator of the system. Therefore, when designing collimating objectives and imaging objectives, the relative aperture should be increased as much as possible.

[0016] When the size of the CCD is selected, that is, the spectral width of the imaging surface is constant, the linear dispersion rate can be determined according to the required spectral range. Linear dispersion rate of grating And imaging objective focal length f 2 The functional relationship is

[0017]

[0018] Where k is the diffraction order, d is the grating constant, Is the diffraction angle of incident light with wavelength λ, and σ is the angle between the spectral image plane and the vertical axis plane. Focal length of imaging objective f 2 After selection, the grating constant d can be determined.

[0019] The full field of view of the imaging objective can be calculated according to the following formula

[0020] FFOV = 2 arctan [ ( H λ 2 + H x 2 2 f 2 ) · cos σ ] - - - ( 2 )

[0021] Where H x , H λ They are the length of the spatial dimension and the spectral dimension on the image plane.

[0022] Considering that the collimating objective lens and the imaging objective lens have different aberration corrections, when choosing the same structure of the collimating objective lens and the imaging objective lens, only the imaging objective lens with strict aberration correction can be designed, and the collimating objective lens adopts its inversion form.

[0023] In order to ensure the direct visibility of the system, that is, the incident light is parallel to the outgoing light, the spectroscopic system adopts figure 2 The combined structure of the three volume holographic gratings shown. Such as figure 2 As shown, the parallel light enters the first holographic grating perpendicularly, and the diffracted light enters the second grating at the Bragg angle. The emitted light is diffracted by the last grating and then exits. The entire system is designed with light with a wavelength of 532nm as the center wavelength. The inclination angle θ of a holographic grating, the incident light is incident horizontally, ensuring that the beam of the center wavelength is straight in and straight out.

[0024] Compared with prism beam splitter, volume holographic grating has the following characteristics:

[0025] (1) The direction of diffraction is determined by Bragg diffraction conditions, so the choice of wavelength range can be achieved by adjusting the incident angle;

[0026] (2) The combination of high diffraction efficiency and higher grating line density can be achieved, and at the same time, high dispersion rate associated with a lower polarization angle direction can be obtained;

[0027] (3) It can be made into larger size grating elements;

[0028] (4) The volume holographic grating is small in size and light in weight, which can simplify the structure of the spectrometer and improve the design efficiency;

[0029] (5) There is a protective film and glass on the outer layer of the grating to form a closed space with high stability;

[0030] (6) The replication process is simple and suitable for mass production.

[0031] Diffraction efficiency is an important parameter of spectroscopic grating. The holographic optical element made of DCG material has high diffraction efficiency and resolution, but the post-processing process of DCG is complicated. Due to the expansion and contraction of gelatin, it is difficult to obtain stable optical performance. In addition, DCG is unstable in natural environment, which is not suitable for use. The environmental humidity is very sensitive, and it is easy to disappear under high temperature and high humidity. The invention adopts photopolymer to make volume holographic grating, after coherent exposure and simple processing, and the prepared volume holographic grating has the advantages of high diffraction efficiency, high sensitivity, high resolution, wide spectral response, stable storage, etc. .

[0032] The specific recording process of using photopolymer to make holographic grating is as follows image 3 As shown, the light emitted by the laser 1 is divided by the polarization beam splitting prism 3 into two linearly polarized lights whose vibration directions are perpendicular to each other. One of them passes through the λ/2 wave plate 8. The angle between the beam and the fast and slow axes of the wave plate is adjusted to make it The vibration direction is deflected by 90°, and then the two beams of light pass through the polarizer 9 respectively, pass through the mirror 4, the spatial filter 5, and the collimator lens 6, and finally interfere with the holographic dry plate 7 with parallel light. Pay attention to adjustment during the recording process The optical paths of the two beams are equal. Among them, the electronic shutter 2 is used to control the exposure time, and the device parameters used are shown in Table 1.

[0033] Table 1 Device parameters used in the experiment

[0034] Device name

[0035] The grating period of the recorded volume holographic grating is obtained by the following formula:

[0036] 2Λsinθ b =λ (3)

[0037] Where Λ is the grating period, θ b It is half the angle of recording light, and λ is the wavelength of recording light. When the incident beam is incident at the Bragg angle, it is satisfied:

[0038] 2Λsinθ=±λ (4)

[0039] The diffraction efficiency can be greater than 95%, where θ is the incident angle of the beam.

[0040] In order to ensure that the beam goes straight in and out, the three gratings change the angle of the two beams during the recording process.

[0041] Therefore, the grating has different grating period, grating vector, etc., and the beam is incident at the Bragg angle of 532nm light during light splitting, which improves the utilization rate of light energy.

[0042] This system has high dispersion rate, wide spectral range, no spectral stacking phenomenon, light can enter and exit straight through the holographic beam splitter, and the imaging quality in the visible light range meets the parameter requirements shown in Table 2.

[0043] Table 2 Spectral imager system parameters

[0044] Index

[0045] The holographic spectroscopic component of the present invention has the following characteristics:

[0046] (1) Compared with dispersive prisms, this beam splitting element uses volume holographic diffraction gratings, which can achieve linear dispersion and has a higher dispersion rate;

[0047] (2) Transmissive volume holographic gratings can flexibly change different wavelength ranges by changing the grating period, refractive index modulation, and grating thickness;

[0048] (3) Due to the use of transmissive volume holographic gratings, the diffraction efficiency of the beam splitting element can reach more than 75%, and the diffraction efficiency has nothing to do with the polarization characteristics of the beam;

[0049] (4) The optical path system of the entire spectrometer has a simple structure and high stability.