RU2817826C1 - Polarized radiation electro-optical modulator - Google Patents

Abstract

FIELD: optical instrument making.

SUBSTANCE: invention relates to optical instrument-making and relates to an electro-optical modulator of polarized radiation based on a longitudinal electro-optical effect. Electrooptical modulator comprises an electrooptic crystal having a longitudinal electrooptic effect. Two transparent film coatings are successively deposited on the working surfaces of the crystal: a protective (niobium oxide) and a current-conducting (indium-tin oxide). Modulating voltage is supplied through contact rings diffusely bonded with indium along the entire perimeter of the current-conducting coating of the crystal surfaces. Working surfaces of crystals inside rings are connected to optical windows on immersion. Side surface of the crystal between the rings and, independently, the gaps between the rings and the optical windows are protected by a sealant.

EFFECT: protection of modulator from unfavourable ambient atmospheric conditions, increase of its service life, improved operational characteristics of the modulator when measuring radiation polarization parameters in a wide frequency range at any form of control voltages.

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Description

The invention relates to optical instrumentation, namely to systems for regulating the intensity and polarization of optical radiation.

In scientific research and experiment, an electro-optical modulator (EOM) containing a Pockels cell is widely used, the action of which is based on the longitudinal electro-optical effect in solid uniaxial crystals, for example KDP and DKDP, despite the need to apply a high control voltage to them. As a polarization modulator as a voltage-controlled wave plate in combination with other polarization elements, the electro-optical modulator is used to effectively analyze the polarization state - Stokes parameters of radiation, such as measurements of magnetic fields on the Sun.

The requirements for the operation of a modulator with a Pockels cell relate to the following parameters:

- Control from a sinusoidal voltage source or from a constant, alternating pulse voltage, ensuring a given slope of the optical signal front shape;

- Uniformity of the phase shift from the control voltage across the field of view of the modulator;

- Distortion of the wavefront of optical radiation across the field of view;

- Losses in the modulator due to reflection on surfaces and absorption;

- Induced birefringence due to mechanical stress in the modulator;

- Angular aperture;

- Ability to work in humid atmospheres and low temperatures;

- Stability of parameters.

These requirements in their extreme value must be satisfied by electro-optical polarization modulators used, for example, for astrophysical observations. When observing solar magnetic fields with a telescope, polarimetric measurements may be encountered in which both long exposures with a high steepness of the time front and a high speed of obtaining sequences of frames may be needed. To do this, it is necessary that the EOMs cover a wide frequency range from zero to several kilohertz and above. In the linear field of the optical window of the modulator, the diffraction quality of the wavefront of optical radiation and the uniformity of the phase shift must be preserved in order to measure the degree of polarization with an accuracy of 10 ^-4. In a high-altitude astrophysical observatory, EOMs can be operated in conditions of a humid atmosphere, low atmospheric pressure and changes in ambient temperature from - 30 to +30 degrees Celsius. The KDP and DKDP crystals used in the EOM are very hygroscopic and require a high control voltage. Therefore, EOMs must be protected from moisture and high voltage breakdown.

In the known solution, the phase modulator [1] includes a DKDP crystal, protective glasses with applied transparent electrodes, and a sealed housing in which an electro-optical crystal is placed between the protective glasses in transparent liquid dielectric immersion. The disadvantage of this modulator is the discrepancy between the applied voltage and the effective internal field, as a manifestation of the effect of electrical polarizability of the electro-optical crystal [2]. Charges accumulate on the surfaces of the crystal, creating a field inside the crystal directed towards the applied one. As a result, the modulation depth at operating frequencies decreases, and at low frequencies it approaches zero. This makes it impossible to use such a modulator when working with modern multi-element CCD photodetectors, for which the readout time is quite long. And in astronomy, this is also supplemented by the low illumination of the photodetector, which requires an increase in exposure time and thereby forces a transition to low modulation frequencies.

In device [3], an electro-optical cell with a DKDP crystal, which has a longitudinal electro-optical effect, is assembled in a housing made of electrically insulating material with light windows. The voltage is applied to the working surfaces of the crystal (perpendicular to the Z axis) using opaque ring electrodes with a central hole pressed to the crystal, which do not intersect the light beam. The crystal-electrode assembly is installed in a housing, the windows of which are sealed using transparent portholes. The body is filled with immersion with a refractive index close to the refractive indices of the windows and crystal to reduce reflection losses. This immersion also serves as an electrically insulating liquid, since it bathes almost the entire crystal and is held in the housing by sealing joints. The control voltage is supplied to the electrodes by busbars, which are also sealed in the housing. The advantages of the modulator are its reliable protection against electrical breakdown at high voltage and the absence of transparent or mesh electrodes in the path of the light beam, which ensures high transmission. However, the use of ring electrodes leads to inhomogeneities in the electric field across the open aperture. The field strength across the aperture varies from a maximum around the inner edge of the rings to a minimum at their geometric center. With such “ringing” it is necessary to apply a voltage 10-15 percent higher than with a uniform field. Partial compensation of inhomogeneities is achieved by increasing the length of the crystal and making it approximately 30 percent larger than the diameter of the open aperture. But in this case, unacceptable inhomogeneities of the phase shift arise across the angular field of view of the cell. In addition, the deformation of the windows, inevitable when the housing is sealed, leads to deformation of the wavefront of the radiation passing through the modulator, and the pressure of the pressed electrodes on the crystal causes double refraction, distorting the electro-optical effect.

A modulator is also known [4], in which the DKDP electro-optical crystal is made in the shape of a cylinder, the axis of which coincides with the optical axis of the crystal. A conductive coating is sprayed onto the ends of the side wall of the cylinder. Two electrodes in the form of flat thin metal rings provide soft contacts with conductive coatings so as not to deform the crystal and avoid double refraction caused by this deformation. Cable connectors are attached to flat rings. For protection from moisture and electrical breakdown, the entire assembly is placed in an immersion-filled sealed housing with attached adjustable end windows. The parallelism of the windows and their inclination to the optical axis can be adjusted with differential screws to reduce reflection from surfaces. Compared to the previous modulator, Pockels cell electrodes in the form of cylinders and connecting cables placed as close as possible to the surface of the cylinders reduce the configuration to a minimum capacitance and inductance, which improves the uniformity of the applied field across the cell aperture by an order of magnitude and increases the switching speed. Applying voltage to the side surface of the crystal with ring electrodes gives the advantage of achieving maximum optical transparency of the Pockels cell (there are no absorbing conductive coatings in the path of the light beam), but it still does not provide a uniform electric field. The transmission inhomogeneity for all cell sizes does not exceed 8% when the ratio of the crystal length to its diameter is in the range from 0.8 to 2.0 [5]. In addition, it is necessary that the ratio of the distances between the electrodes and the aperture of the electrode itself be ~ 1.2. Therefore, to fulfill these conditions at large transverse cell sizes, the length of the electro-optical crystal is increased, which leads to an increase in the dependence of the induced phase shift on the angle of incidence of the beam, i.e. to a decrease in the angular aperture of the electro-optical modulator, and it cannot be used in polarization measurements of the radiation of extended objects. In addition, failure to make a “soft” electrical contact due to non-compliance with strict requirements for diameter in the manufacture of electrode rings leads to their pressure on the crystal and, as a consequence, to uncontrolled double refraction.

A solution for increasing the angular field of the modulator while maintaining the phase shift across the field of view is provided by a modulator in which the control voltage is supplied by liquid transparent electrodes. They have high electrical conductivity, which ensures field uniformity combined with low optical losses. When using voltage supply using liquid electrodes, it is possible to reduce the length of the crystal cell and increase the angular aperture of the EOM. It has been shown [6] that the use of liquid electrodes increases the dynamic range of electro-optical modulators compared to the use of ring electrodes. However, DCDR crystals are highly soluble and can be easily destroyed. The application of thick films cannot be used for chemical protection of the crystal: although they are quite transparent, they have a high resistivity. This leads to an increase in capacitance and resistance in the power circuit and requires a significant increase in the field required for half-wave rotation of the plane of polarization.

The closest technical solution adopted for the prototype is a polarization modulator [7], in which conductive transparent coatings of indium tin oxide (ITO) are applied directly to the working surfaces of the electro-optical crystal, and the control signal is supplied to the conductive transparent coatings along the entire perimeter through slip rings , which are diffusely bonded by indium to conductive coatings on the crystal. The working surfaces of the electro-optical crystal are protected from the influence of the external atmosphere by optical windows installed on immersion inside the contact rings. The assembly - an electro-optical crystal with contact rings and protective optical windows - is placed in a frame with sealant.

The advantages of this modulator are as follows:

If the electrodes are applied directly to the surface of the crystal, the polarization effect, which reduces and distorts the control voltage, is completely absent (there is no adhesive or immersion connecting layer between the transparent electrode and the crystal), and this makes it possible to control the polarization and measure the Stokes parameters of radiation in a wide range of modulation frequencies - from a fraction of a Hz to several MHz for any form of control voltage (sinusoidal or rectangular).

When the control voltage is supplied in a ring along the entire perimeter of the conductive coating, the ring contacts withstand a significant amount of supplied currents that arise when using voltages with high edge slopes.

The contact ring itself on the crystal side has the same conductive coating as the electro-optical crystal. This increases the strength of the diffusion bond between the indium ring and the crystal. The working surfaces of the electro-optical crystal are protected from the influence of the external atmosphere by optical windows located on immersion inside the slip rings

Disadvantages of the modulator adopted as a prototype:

During operation of the modulator, formations of a crystalline sector and layered structure appear in amorphous films of transparent conductive coatings on both surfaces of the electro-optical KDP crystal (DKDP), scattering the transmitted radiation. Scattering changes the polarization state of an object and introduces error into measurements.

The reason for this shortcoming is as follows. A conductive transparent ITO film is deposited in vacuum by magnetron sputtering of an indium-tin alloy target in an atmosphere of a mixture of argon and oxygen at room temperature, so as not to destroy the electro-optical crystal, for example DKDP [8]. The amorphous ITO films obtained in this process provide the specified parameters of resistance and transparency of the conductive coatings of the modulator. However, when the modulator is operated under the influence of heating, for example, in a solar beam and vibrations from the piezoelectric effect, crystallization and heteroepitaxial orientation of the atoms [9] of the resulting crystals of the conductive transparent ITO coating deposited on the electro-optical crystal begin over time. Upon crystallization, the amorphous ITO film reproduces the Z-cut crystal structure of the DKDP substrate. The dislocations present in the crystal [10, 11] transfer to the ITO layer: the atoms of the crystallizing amorphous layer are pulled together around the traces of the boundaries of growth pyramids, dislocations, growth layers, and slip planes. The materials of the crystallizing conductive transparent film ITO and the substrate - DKDP crystal - are different and rigidly connected. Due to the mismatch between the coefficients of thermal expansion and the crystal lattice constants of these materials, mechanical stresses arise. At their critical value, the mechanism that allows elastic stress relaxation is the disintegration of the crystalline film into individual islands and the formation of a quasiperiodic relief. Immersion, which is used to attach the modulator's protective windows to the surfaces of the DKDP electro-optical crystal, fills the gaps in the relief of the conductive transparent coating layer. This layer becomes an inhomogeneous optical medium due to differences in the refractive indices of the ITO film and immersion. Inhomogeneities (which are themselves small in size), with distances between them greater than the wavelength of light, behave as independent secondary light sources. The waves they emit are not coherent and do not interfere when superimposed. As a result, the inhomogeneous medium scatters light in all directions. This reduces the accuracy of measurements with a modulator of polarized light parameters due to signal leakage caused by scattered light, reducing the performance characteristics and service life of the modulator.

To protect against moisture, an electro-optical crystal with conductive coatings and contact rings with optical windows is located in a frame. The side surface of the electro-optical crystal is sealed with a compound hardened in the frame. When used in open atmospheric conditions, for example on a telescope, under the influence of changes in ambient temperature and heating in the solar beam, the frame with sealant is deformed and puts pressure on the electro-optical crystal due to different expansion coefficients. As a result, uncontrolled polarization occurs in the crystal, distorting the useful signal.

In addition, the gaps between the side surfaces of the optical windows and the inner surfaces of the slip rings are not sealed. Immersion in unsealed gaps absorbs atmospheric moisture, which destroys the water-soluble electro-optical crystal. This reduces the performance characteristics and service life of the modulators.

The purpose of the invention is to increase the service life and improve the performance characteristics of the modulator when measuring radiation polarization parameters in a wide frequency range for any form of control voltage (sinusoidal or rectangular).

The problem is solved due to the fact that in an electro-optical modulator of polarized radiation, in which conductive transparent coatings of indium-tin oxide are deposited on the working surfaces of the electro-optical crystal, the modulating voltage is supplied to the crystal along the perimeter of the coatings by slip rings bonded with indium to the conductive coatings of the crystal, the crystal is protected optical windows on immersion and frame, the following differences are provided.

Before applying a transparent coated ITO, a transparent coating is first applied to the working surfaces of the DKDP crystal to protect the ITO coating from crystallization and heteroepitaxial orientation of the atoms of the resulting crystals of the ITO coating and its disintegration into separate islands on the surface of the DKDP crystal. The crystal lattice of this first transparent coating should not be conjugated with the crystal lattice of the DKDP electro-optical crystal - the coating substrate: heteroepitaxy does not take place in this case. For example, a coating of niobium oxide Nb2O5, obtained by reactive magnetron sputtering at room temperature, has an amorphous structure and lattice parameters a = 3.60, b = 3.61 [12]. while the lattice parameter of the amorphous ITO coating a=10.2 [13] is closer to the lattice parameters of the DKDP crystal (a=7.45, b=7.45, c=6.97) [14]. The niobium oxide coating is stable in air and water, resistant to acids and alkalis. The niobium oxide film not only protects ITO from crystallization and destruction, but also prevents moisture damage to the electro-optical crystal. For maximum light transmission by the modulator, the thickness d of this protective film must satisfy the condition d = λ _slave /4n, where λ _slave is the wavelength of the middle of the operating range of the modulator, n is the refractive index of the protective layer substance. For niobium oxide n = 2.34 (λ _slave =500 nm), the film thickness is 52 nm. With such a film thickness, an increase in the control voltage is not required.

In the proposed modulator, not only the side surface of the electro-optical crystal between the conductive rings is sealed, but also the gaps between the conductive rings and the optical windows are separately sealed. The hardening compositions of these areas are not associated with the frame in which the modulator is elastically fixed (for example, polyurethane foam). When operating under conditions of sudden temperature changes, the elastic mount does not transfer the deformation of the frame to the crystal, preventing squeezing of the crystal and the occurrence of uncontrolled polarization. The sealant in the gap between the optical windows and the conductive rings does not allow moisture to enter the immersion and protects the water-soluble electro-optical crystal from destruction.

The essence of the invention is illustrated by drawings:

Fig. 1 is a block diagram of an embodiment of an electro-optical modulator.

In fig. 2 shows the test results of the electro-optical modulator.

A schematic representation of an electro-optical modulator in a frame is presented in Figure 1. Electro-optical crystal 1, for example DKDP, has two transparent coatings on two working surfaces: niobium oxide 2 and a conductive indium tin oxide coating 3. The control signal is connected to the modulator using terminals 4, and then, voltage is applied to the conductive transparent coatings of both surfaces of the electro-optical crystal along the entire perimeter through electrically conductive slip rings 5. The slip rings are made of a material whose expansion coefficient is close to the expansion coefficient of the electro-optical crystal (in the direction perpendicular to the optical axis of the crystal), for example, aluminum, so that deformations and stresses do not occur in the crystal when the ambient temperature changes. Indium poorly wets metallic aluminum, but aluminum can be activated [15] using eutectic, which contains indium, tin, etc. Therefore, the contact rings on the crystal side also have a conductive coating 6. The rings are attached to the conductive surfaces of the electro-optical crystal using an annular spacer made of indium 7 due to the diffusion of indium into both the slip rings and the crystal during connection under pressure. The working surfaces of the electro-optical crystal are protected from the influence of the external atmosphere by optical windows 8 located on the immersion 9 inside the slip rings. The sealant 10 in the gap between the windows and the rings protects the immersion and working surfaces of the electro-optical crystal from moisture, and the sealant 11 protects the side surface of the crystal between the contact rings. The entire assembly is mounted on elastic supports 12 in a protective frame 13.

Tests of the modulator have shown its performance in the modes of constant and alternating control signals, while the electrical strength of the modulator allows it to achieve half-wave voltages in the visible region of the spectrum. Figure 2a shows an interferogram of the radiation wavefront across the aperture of a modulator with a diameter of 50 mm, which is under a pulsed control voltage with a frequency of 0.6 Hz. The interferogram shows that the uniformity of the induced birefringence across the aperture and the wavefront distortion are within 0.1 λ Charge spreading when applying a pulse voltage of ±1.9 kV is less than 1% (Fig. 2b).

In the proposed device, the linear dimensions of the light window of the modulator are not fundamentally limited. The angular field of the modulator increases with decreasing crystal thickness. When the thickness of the modulator crystal is 2 mm, electrical breakdown of the crystal does not yet occur from the applied control voltage, and such a modulator can be used to measure the polarization of extended objects.

Literature

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10. De-Gao Zhong, Teng Bing, Zheng-He Yu, Shu-Hua Wang, Xue-Jun Jiang, Lin-Xiang He, and Wan-Xia Huangjnvestigation on the regeneration of Z-cut KDP crystals Cryst. Res. Technol. 46, No. 9, 911-916 (2011) / https://www.researchgate.net/publication/264649530 _Investigation_on_the_regeneration_of_Z-cut_KDP_crystals

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Claims (1)

Electro-optical modulator of polarized radiation based on the longitudinal electro-optical effect, containing a crystal with a transparent conductive coating of indium-tin oxide on the working surfaces, to which the modulating voltage is supplied through contact rings, diffusely bonded with indium along the entire perimeter of the coating, optical windows installed on the crystal inside the contact rings rings on immersion in the capillary gap, and a frame in which the side surface of the electro-optical crystal is sealed, characterized in that two transparent coatings are sequentially applied to the working surfaces of the electro-optical crystal, the first is a protective coating of niobium oxide and the second is a conductive coating of indium-tin oxide, sealed the gaps between the contact rings and the optical windows and the side surface of the electro-optical modulator crystal, which is elastically fixed in the frame, is separately sealed.