RU2532133C1 - Optical-acoustic monochromator for filtration of optical images - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to the field of applied optics and spectrometry and concerns an optical-acoustic monochromator. The monochromator contains the non-collinear optical-acoustic filter characterised by that for the dispersion compensating element the output face of the crystal of the optical-acoustic mesh of the filter is used. The slope angle of output crystal face with reference to the input face is selected so that the spectral drift of the diffraction angle in the mesh was as much as possible compensated by spectral drift of the refraction angle of diffracted beam on the slope output face.

EFFECT: increase of spectral interval of compensation of dispersion distortions.

2 dwg

Description

The invention relates to techniques for obtaining spectral images (video spectrometry).

There are various designs of acoustic video monochromators for filtering optical images containing an acousto-optical filter (US No. 5377003, publ. 1994; DA Glenar, JJ Hillman, B. Safb, J. Bergstralh. Appl. Opt, 1994, V.33, No. 33 , p.7412-24. Acousto-optic imaging spectropolarimetry for remote sensing).

The acousto-optic (AO) filter for filtering images contains an acousto-optic cell enclosed between two crossed polarizers, i.e. polarizers, the optical axes of which are mutually perpendicular, so that light in the absence of a controlling high-frequency RF signal does not pass through the system.

AO cell is a uniaxial crystal, to which a high-frequency (HF) emitter of ultrasonic waves is attached, which makes it possible to excite an ultrasonic wave of controlled frequency in the crystal. One of the components of a light beam incident on a crystal whose wavelength corresponds to the Bragg diffraction condition on a sound wave changes the direction of its propagation and its polarization as a result of diffraction.

Due to the fact that the polarization directions of the diffracted and incident light beams are mutually orthogonal, only the indicated diffracted light beam passes through the AO filter in the presence of an RF control signal, which ensures spectral filtering of light, i.e. a one-to-one correspondence between the frequency of the control RF signal and the wavelength of the diffracted light beam.

The plane defined by the optical axis of the crystal and the direction of propagation of the wave vector of the ultrasonic wave is the diffraction plane, since it is in this plane that the incident and diffracted light rays lie.

The main disadvantage of this AO video monochromator is the presence of dispersive image distortions caused by the refraction of the diffracted light beam at the exit face of the AO cell crystal, which is manifested as a function of the spatial position of the diffracted beam on the radiation wavelength. This leads to shifting and zooming at different wavelengths. The shift of the video image complicates its subsequent registration and processing with a photodetector (or other recording equipment), since it requires the use of additional adjustment to combine the recorded frames at different wavelengths, and zooming requires subsequent processing of the image to recreate the true proportions of the object.

The closest technical solution is an acousto-optic video monochromator for filtering optical images, comprising a non-collinear acousto-optic filter and a dispersion compensation element installed along the light beam behind the acousto-optical filter, the output of the acousto-optical filter being optically coupled to the optical input of the dispersion compensation element (US No. 5796512, publ. . 1998).

In this technical solution, an additional prism located after the AO filter is used as an element for compensating dispersion. However, the presence of a prism allows one to compensate for dispersion distortions only in a narrow spectral range, since the dispersion law for a prism and AOF is different.

In another close technical solution, a second AO filter identical to the first AO filter and installed with rotation of its geometric shape relative to the geometric shape of the first AO filter 180 ° around an axis perpendicular to the crystal diffraction plane of the first AO filter (RU 2258206, publ. 08/10/2005).

Prism and the second AO filter are additional elements, which leads to an increase in the cost of the entire device. The need to combine the optical output of an acousto-optical filter with the optical input of a dispersion prism or a second AO filter leads to a complication of the device setup.

The problem solved by the present invention is the simplification and cheapening of the design of a video monochromator. The technical result of the invention is the development of an acousto-optical video monochromator with the possibility of increasing the spectral interval of compensation for dispersion distortion.

To solve the problem with achieving the specified technical result in an acousto-optic monochromator for filtering optical images containing a non-collinear acousto-optic filter, according to the invention, the output face of the acousto-optic cell crystal, rotated in the diffraction plane of the acousto-optic filter by an angle φ with respect to the input crystal face of an acousto-optic filter cell.

The angle φ depends on the spectral range of the acousto-optic monochromator, the directions of propagation of ultrasonic and light beams in the crystal of the acousto-optical cell, as well as the optical properties of this crystal.

The angle φ matters at which the spectral drift of the diffraction angle in the AO cell is maximally compensated by the spectral drift of the refractive angle of the diffracted beam on the inclined output face, so that the spectral angular drift of the diffracted light beam does not exceed its diffraction divergence, or the angular size of the pixel matrix.

It should be noted that the AO filter can also be performed according to a non-polarization scheme - in this case, the diffracted (filtered) beam is spatially separated from the non-diffracted one due to the deviation of the propagation direction of the first beam from the propagation direction of the second.

For this purpose, a diaphragm can be used that transmits only the diffracted beam and blocks the non-diffracted beam.

These advantages, as well as features of the present invention are illustrated by the accompanying figures.

Figure 1 schematically depicts the claimed device, where the symbol "HF" indicates the control high-frequency input of the AO filter.

Figure 2 schematically depicts for comparison an AO video monochromator containing one AO filter for filtering an image, where there is no element for dispersion compensation.

The device (figure 1) works as follows.

The figures show the elements of a video monochromator: 1 — an acousto-optic cell, 2 — an input polarizer, 3 — an output polarizer, 4 — a lens, and 5 — a photodetector matrix of a video camera installed in the video monochromator along the rays in the usual way. Solid arrows indicate the ray path for wavelength λ ₁ , and dashed arrows indicate wavelength λ ₂ > λ ₁ .

A video monochromator eliminates dispersion distortion due to the fact that the angle between the direction of the diffracted beam in the crystal and the normal to the output face increases with increasing λ. At the same time, the angle between the direction of the diffracted beam in air and the normal to the exit face decreases with increasing λ according to the Snellius law of refraction and the dispersion dependence n (λ) of the AOI crystal. As a result, when exiting the crystal into the air, the diffracted beams are quasi-parallel and focus in the same place on the photomatrix.

In the device shown in Fig. 1, the value of the slope of the output face of the AO cell in the diffraction plane φ is such that the spectral drift of the angle of diffraction in the AO cell is maximally compensated by the spectral drift of the refractive angle of the diffracted beam on the inclined output face. As a result, the spectral angular drift of the diffracted light beam (i.e., the maximum change in the direction of this beam in the entire spectral range) can be reduced to the minimum possible value, which, like the angle φ, depends on the spectral range, the orientation of the directions of propagation of sound and light beam in the crystal and the optical properties of the crystal.

The criterion of the dispersion compensation efficiency in an AO video monochromator can be expressed as inequality:

$$\Delta {\Theta }_{dR}<Max\left\{\delta \varphi ,\Delta {\Theta }_{df}\right\},\left(\mathrm{one}\right)$$

where ΔΘ _etc. - spectral angular drift of the diffracted beam, δφ = Δφ / N - angular resolution of one pixel of the camera, which is the ratio of the angular aperture AO videomonohromatora Δφ to the number of pixels photomatrixes in the vertical direction (along the direction of displacement of the light beam when drift) N, ΔΘ _dtp - diffraction divergence of the initial collimated light beam at the aperture of the AO filter cell. The fulfillment of inequality (1) means that the amount of displacement of the spot on the photomatrix 5 (Fig. 1) is less than the size of the spot or less than one pixel. In any of these cases, drift can be neglected.

With an increase in the wavelength of the filtered radiation, the crystal dispersion decreases (i.e., the dependence of the refractive indices on the wavelength becomes weaker), and the diffraction divergence increases, therefore, the efficiency of the compensation considered also increases. Studies have shown that for most uniaxial crystals, such compensation in the red and infrared spectral regions is effective and practically feasible.

For example, for an acousto-optic video monochromator with an AO cell on paratellurite (TeO ₂ ) with a working spectral range (0.7-1.1) microns and parameters: Θ = 26.3 ° - the angle between the direction of propagation of the light beam and the Z axis of the crystal; γ = 12.3 ° - the angle between the direction of propagation of the ultrasonic beam and the axis [110] of the crystal; AO cell aperture - 12 mm, Δφ = 5.2 °; N = 1390, the following results were obtained:

_Al ΔΘ = 10 ^-3 deg ΔΘ _DF (0.7 m) = 3.3 × 10 ^-3 deg.

ΔΘ _DF (1.1 microns) = 4.6 × 10 ^-2 degrees, δφ = 3,7 · 10 ^-3 deg.

Thus, inequality (1) is fulfilled with a margin and compensation is quite effective.

Claims (1)

An acousto-optic monochromator for filtering optical images, containing a non-collinear acousto-optic filter, characterized in that the output face of the crystal of the acousto-optic filter cell having an angle of inclination φ in the diffraction plane of the acousto-optic cell with respect to the input face of the crystal of the acousto-optic filter cell is used as an element for dispersion compensation from the spectral range of the filter, the directions of propagation of ultrasonic and light beams in an acousto-optic crystal cell, as well as the optical properties of this crystal, and the angle φ matters at which the spectral drift of the diffraction angle in the AO cell is maximally compensated by the spectral drift of the refractive angle of the diffracted beam on the inclined output face, so that the spectral angular drift of the diffracted light beam does not exceed the maximum of the values of the diffraction divergence of the light beam and the angular size of one pixel of the photomatrix, and the cell used as an acousto-optical cell and on paratellurite (TeO ₂ ) with a working spectral range of 0.7-1.1 μm and parameters: the angle between the direction of propagation of the light beam and the Z axis of the crystal is 26.3 °; the angle between the direction of propagation of the ultrasonic beam and the axis [110] of the crystal is 12.3 °; the aperture of the AO cell is 12 mm; the angular aperture of the device is 5.2 °; the number of pixels of the photomatrix is 1390.

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