RU2640963C1 - Method of controlling phase shift in interference systems - Google Patents

Abstract

FIELD: measuring equipment.SUBSTANCE: in the method of controlling the phase shift in interference systems, including the formation of coherent monochromatic radiation by the laser source, its separation into the reference and object light beams by a collimating system, directing them on measurement object and supporting surface to form a phase shift in them intended for the interpretation of interference pattern at their reflection on the photodetector, the phase shift Δϕ of the reference and object light beams is formed by their Bragg diffraction on the similar by number and sign orders by passing on the section between collimating system and the support surface and the measurement object, respectively, through identical acousto-optic modulators, on which the reference vibrations Uand Uare fed from the common generator so that U=Ucos[2π f×t+Δϕ] and U=Ucos[2π f×t], where Uis amplitude of reference oscillations; f is frequency of reference oscillations of acousto-optic modulators; t is the time of reference oscillations, and it is possible to supply the reference oscillations Uon the acousto-optic modulator through which the object light beam passes, and the reference oscillations U- on the acousto-optic modulator through which the reference light beam passes, or vice versa.EFFECT: increased reliability through improved accuracy and noise immunity.3 cl, 3 dwg

Description

The invention relates to measuring technique, in particular to methods for controlling the phase shift between two coherent monochromatic light waves in laser measuring information systems (LIIS).

The phase shift control method is widely used in interferometers constructed according to various optical schemes (controlled phase shift interferometers (UFS) according to the Linnik scheme [see, for example, Grigoryev S.N., Teleshevsky V.I., Andreev A.G., Ignatiev P.S., Indukaev K.V., Kolner L.S., Osipov P.A. Metrological certification of laser microscopes based on the principles of modulation interferometry with controlled phase shift // Vestnik MGTU STANKIN, 2015, No. 3 (34 ), pp. 67-75], according to the Kester's scheme [see, for example, G. Michalecki, Automatic Calibration of Gange blocks Measured by Optical In terferometry, Measurement Science Review, 2001, Vol. 1, No. 1, p. 93-96], according to the scheme of Fizeau and Twyman-Green [P. de Groot, Jim Biegen et. al. Optical Interferometry for measurement of the geometric dimensions of industrial parts // Applied Optics. 2002, vol. 41, No. 19, p. 3853-3860] and others). Note that a modulation interference microscope with a UVS provides restoration of an object with a resolution of 0.1 nm vertically and up to 10 nm in the lateral plane. Kester's UFS interferometer restores the surface of a precision end gauge with a resolution of up to 10 nm.

A known method of mechanical control of the phase shift in interference systems (RF Patent No. 2536764 C1, publ. 12/27/2014).

The essence of this method is to use a common path interferometer, which is an assembly of Dove biprisms and an inclined mirror. Biprism Dove is mounted on a linear motorized translator (in this case, a linear piezodrive), which from the corresponding programmable controller moves the biprism a certain number of steps (about 4) in the direction prependicular to the optical axis, thus realizing mechanical movement to effect controlled phase shift. Light beams emerging from the Dove biprism are reflected from a mirror tilted at a small angle, and, reflected from it, acquire a certain phase shift at the exit of the Dove prism, depending on the value of the motorized movement of the Dove prism. Thus, there is a UFS between the interfering beams, determined by the number and value of the steps of moving the Dove biprism.

The disadvantages of the known technical solutions include:

- relatively low accuracy due to the inability to realize phase shifts of high accuracy with a resolution of 1 nm or less;

- relatively low noise immunity due to the fact that the displacement of the interference pattern is achieved by mechanical movement of the reference mirror in space from an electrically excited piezoelectric element.

The closest to the claimed technical solution both in technical essence and in the achieved result - the prototype - is a method for controlling the phase shift in interference systems implemented as part of the method of obtaining a phase portrait of an object, including the formation of coherent monochromatic radiation by a source, its separation into reference and subject light beams by means of a collimation system, their direction to the measurement object and the supporting surface with the formation of a phase shift in them n Δφ, intended to interpret the interference pattern at their reflection on the photodetector (RF Patent №2463552 C1, publ. 10.10.2012).

The technical solution for the prototype is characterized in that:

- to determine the phase of the object beam at the pixel of the photodetector, at least three values of the energy perceived by the pixel of the photodetector during the exposure are determined for various values of the phase of the reference beam;

- to determine at least one value of the energy perceived by the pixel, the dependence of the pixel illumination on time is obtained when the position of the phase modulator is changed and the obtained dependence is integrated over the exposure time interval;

- for each pixel, a series of intermediate values of the phase of the object beam is obtained at a different current phase shift of the reference beam;

- the phase shift is realized by a controlled phase shift of the reference beam (possibly also an object beam), which is carried out by changing the optical path length of the reference beam during mechanical movement of the reference mirror.

An analysis of the prototype and a number of technical solutions similar to it described above allows us to identify several common shortcomings.

Firstly, the indicated technical solutions do not allow realizing high-precision phase shifts with a resolution of 1 nm or lower. So, despite the variety of applications in various interference schemes, the implementation of the phase shift itself is realized in one known way - by installing in one of the arms of a two-beam interferometer (usually in the support arm) an electromechanical engine (actuator), on which one of the mirrors of the interferometer is mounted. An electromechanical actuator under the influence of electric voltage or current displaces the mirror by a certain amount in space, i.e. shifts the spatial phase of the reference wave and, therefore, the entire interference pattern. A variety of actuators are used: piezoelectric based on piezostriction arising from the inverse piezoelectric effect, magnetostrictive ones using the giant magnetostriction effect, electrodynamic ones based on the inverse effect of electromagnetic induction. The most widely used in modern interferometry controlled phase shift are piezoelectric actuators based on piezoceramics. A wide variety of such piezoelectric motors and platforms is demonstrated by Physik Instrumente (Germany) [http://www.physikinstrumente.com/products.html], whose products include actuators with nanometer resolution for positioning and submillisecond response time (in particular, the actuator model P -602 has a range of motion of 1 mm and a minimum guaranteed sampling step of 6 nm, and P-842 has a range of motion of 90 microns, a minimum guaranteed sampling step of 1 nm). However, the positioning accuracy of piezoceramic actuators is insufficient, and the number of realized controlled shifts is usually from 3-7 to 11. This fact is also drawn to p. 159 in the above source [V. Vasiliev, I. Gurov. Computer signal processing as applied to interference systems. - St. Petersburg: BHV-Saint Petersburg, 1998 - 240 p. (chapter 5, p. 153-167)], which states that in order to increase the accuracy and noise immunity of processing the measurement results, the number of controlled phase shifts should be increased. Also known are the properties of piezoceramics such as the presence of hysteresis, aging, which also leads to additional errors in time and space. The limit on the number of shifts is an obstacle to a more accurate restoration of the surface structure of the object. An increase in the number of controlled shifts to tens of hundreds or more within the limits of the light wavelength will allow creating new algorithms for processing the interference pattern, which reduce errors associated with nonlinear distortions, noise, and other factors due to the amplitude-phase nature of the processing of interference images.

Secondly, insufficient speed is an important factor limiting the development of the controlled phase shift method. This factor in piezoelectric actuators is due to the fact that the displacement of the interference pattern is achieved by mechanical movement of the reference mirror in space from an electrically excited piezoelectric element, that is, electromechanically. In this case, the phase shift control signal is either a constant voltage (for a piezoelectric actuator) or a constant current (for magnetostrictive or electrodynamic actuators), the values of which are set by the control controller. Thus, the mechanical movement of the reference (or object) mirror of the interferometer in space is controlled by the least noise-resistant amplitude method. This circumstance is fundamental, and the possibilities of piezoceramics with its high dielectric constant are not unlimited. Inertia is characteristic of the mechanical movement of the mirror, which limits the spread of the controlled phase shift method to dynamic measurements of objects with a profile, structure, position, etc., variable in time and space, etc. The disadvantages of striction piezoelectric actuators include their characteristic hysteresis, nonlinearity of the voltage conversion function when moving, the need for high stabilization of a constant electric shear signal in a wide range of values (up to tens and hundreds of volts).

Based on the foregoing, the objective of the invention is the exclusion from the composition of the actions of mechanical displacements that control the phase shift in interference systems.

The technical result is an increase in reliability by increasing the accuracy and noise immunity.

The problem is solved, and the claimed technical result is achieved by the fact that in the method of controlling the phase shift in interference systems, including the formation of coherent monochromatic radiation by means of a laser source, its separation into reference and subject light beams by means of a collimation system, their direction to the measurement object and reference surface with the formation in them of a phase shift Δϕ, designed to interpret the interference pattern when they are reflected on the photodetector e, the phase shift reference and object light beams formed by virtue of their Bragg diffraction on the same line and sign orders by passing the section between the collimating system and the support surface and the measurement object, respectively, through identical acousto-optic modulators, which serves counterbalanced fluctuations of the total oscillator U ₁ and U ₂ so that U ₁ = U ₀ cos [2πf × t + Δϕ] and U ₂ = U ₀ cos [2π f × t]], where U ₀ is the amplitude of the reference oscillations; f is the natural frequency of the acousto-optical modulators; t is the time of carrying out the reference oscillations, while the reference oscillations U ₁ can be supplied to the acousto-optic modulator through which the object light beam passes, and the reference oscillations U ₂ to the acousto-optic modulator through which the reference light beam passes, or vice versa.

The invention is illustrated in graphic materials, where:

- in FIG. 1 shows a diagram of acousto-optical interaction implemented in the invention — light diffraction by ultrasound;

- in FIG. 2 shows an acousto-optical modulator;

- in FIG. 3 shows a phase shift control circuit.

The basis of the proposed method is the phenomenon of acousto-optical interaction - diffraction of light by ultrasound [Magdich L.N., Molchanov V.Ya. Acousto-optical devices and their use // Soviet Radio, M., 1978, p. 5 to 8]. The essence of this phenomenon is illustrated in FIG. 1. The initial radiation of the laser 1 through the collimator 2 is sent to a beam splitter 3 and a mirror 4, forming parallel light beams 5 and 6. The beams 5 and 6 are sent to acousto-optical modulators (AOM) 7 and 8, installed so that the output from them identical diffraction spectra. In this case, the light diffracts at 0 and +1 diffraction orders both on the modulator 7 and on the modulator 8. Diffraction maxima of +1 orders on the modulator 7 - a plane light wave E ₁ - and on the modulator 8 - a plane light wave E ₂ - are introduced into the optical scheme of the interferometer as interfering light waves at the LIIS. The AOM 7 and AOM 8 are excited by the generator 9, however, between the signals arriving at the AOM from the common generator 9, a phase-shifting device (FS) 10 is installed, discretely controlled electronically from the control device (BU) 11.

A feature of acousto-optical interaction is the transfer of the phase of the electric excitation of the modulator to the phase of the optical light wave in the diffraction order. We illustrate this feature (Fig. 2). With Bragg diffraction, two acousto-optic interaction schemes are possible.

возбуждается бегущая гармоническая ультразвуковая волна посредством пьезопреобразователя 12. Лазерное излучение с оптической частотой ν₀ под углом Вульфа-Брэгга

где λ - длина волны света, Λ - длина волны звука, проходит через оптически прозрачный модулятор 7 и дифрагирует на два порядка: 0-й с частотой ν₀ и +1-й с частотой

На Фиг. 2 справа представлена аналогичная картина брэгговской дифракции при падении света под отрицательным углом Вульфа-Брэгга. В этом случае, помимо 0-го порядка с частотой ν₀, формируется -1-й порядок с частотой

Важно отметить, что регулировкой амплитуды напряжения на электронном генераторе 9 можно добиться перекачки всей энергии падающего лазерного напряжения в +1 или -1 порядки дифракции, что обеспечивает эффективное использование в способе излучательной энергии.In FIG. 2 on the left is an acousto-optic modulator 7, in which from an electronic generator 9 at a frequency

a traveling harmonic ultrasonic wave is excited by means of a piezoelectric transducer 12. Laser radiation with an optical frequency of ν ₀ at a Wulf-Bragg angle

where λ is the wavelength of light, Λ is the wavelength of sound, passes through an optically transparent modulator 7 and diffracts by two orders of magnitude: 0th with a frequency ν ₀ and + 1st with a frequency

In FIG. Figure 2 on the right shows a similar picture of Bragg diffraction when light is incident at a negative Wulf-Bragg angle. In this case, in addition to the 0th order with a frequency ν ₀ , a -1th order with a frequency

It is important to note that by adjusting the voltage amplitude at the electronic generator 9, it is possible to pump all the energy of the incident laser voltage into +1 or -1 diffraction orders, which ensures efficient use of radiative energy in the method.

From the theory of coupled waves [Kogelnik N. Coupled wave theory for thick hologram gratings // Bell System Technical Journal, 1969, p. 2909-2949] implies an important property of Bragg diffraction: in the zero diffraction order E (0) = E` exp [-i (ω <sub>0</sub> t + ψ <sub>0</sub> )], where E` is the amplitude of the light wave, ω ₀ = 2πν ₀ , ψ ₀ - constant phase shift, the phase of the light wave does not depend on the phase of the exciting ultrasonic voltage. At the same time, the phase of the light wave in the +1 diffraction order

- круговая частота ультразвука, ϕ - фазовый сдвиг возбуждающего ультразвук сигнала от генератора. Аналогично и в -1 порядке дифракцииdepends on the phase of the electric harmonic oscillation U (t) = U ₀ cos (Ωt + ϕ), where U ₀ is the amplitude,

is the circular frequency of ultrasound, ϕ is the phase shift of the signal exciting the ultrasound from the generator. Similarly and in -1 diffraction order

The phase ϕ contained in (1) and (2) is the phase of the electric exciting ultrasound voltage U ₁ (object beam) and U ₂ (reference beam):

U ₀ - the amplitude of the reference oscillations.

что соответствует одному шагу фазового сдвига. Величина ϕ - разность фаз между интерферирующими в ЛИИС световыми волнами E1 и E2 определяется какThe value of the phase shift in the exciting voltage U ₁ can be changed in FS 10 (Fig. 1) in software steps from the control unit BU 11. Essentially, the FS10 device must be formed as a chain of delay lines Δt _k connected from the control system according to a given program. Then each elementary delay line creates a phase shift between the light waves E ₁ and E ₂ interfering in the LIIS (Fig. 1)

which corresponds to one step of the phase shift. The value of ϕ is the phase difference between the light waves E1 and E2 interfering in the LIIS

where k = 0, 1, ..., n is the number of phase steps taken.

The excitation of ultrasound by acousto-optical modulators is usually provided at a frequency of tens to hundreds of MHz, and the speed of ultrasound in the AOM (for example, paratellurite) is about 700 m / s, the maximum response time will be no more than 10 μs, which is not less than an order of magnitude less than the response time of known piezoelectric actuators. This means a significant increase in the speed of interferometry with an electronically controlled phase shift.

The phase shift control circuit is shown in FIG. 3.

AOM 7 and AOM 8 (Fig. 1) are identical and are excited from a common generator 9 with the only difference that a phase shifter 10 is included in the excitation circuit of one of the AOM. The main element of the phase shifter is the phase shift between the reference signal “OP”, which drives the reference AOM (for example, AOM 7 in Fig. 1), and the phase signal "ФЗ", which excites the phase-shifting AOM (in Fig. 1 - AOM 8), is a phase-locked loop 13 (PLL). The input signal to the PLL subsystem is supplied from generator 9 (GEN). PLL parameters are controlled through the dynamic reconfiguration port 14 (PDR). The microcontroller 15 (MK) acts as the phase shifter control device, which controls the PLL parameters using the dynamic reconfiguration port controller 16 (KPDR) connected to the PLL dynamic reconfiguration port (PDR). MK receives commands and gives signs of confirmation of command execution and diagnostic information from the computer through universal asynchronous transceiver 17 (UAPP).

The number of phase shifts in the proposed method is not limited, since the number of delays can be large (tens, hundreds or more). Thus, a phase shifter based on the SP601 Evaluation Kit provides the generation of two digital signals with a frequency of 40 MHz with controlled phase shift, which are used to excite acousto-optical modulators. Phase shift is controlled via USB. The phase shift resolution is Δϕ _k = 360 ° / 160 = 2.25, which in terms of wavelength for a helium-neon laser (632.8 nm) gives a discrete phase shift of the optical phases of the order of 3.96 nm, and for excimer lasers fluorine vapor (wavelength 157 nm) - phase shift resolution of 0.98 nm. This circumstance will significantly improve the accuracy of detection and processing of phase information, as well as the resolution of interference systems. In this case, the controlled phase shift is carried out not by changing the amplitude of the direct voltage of tens to hundreds of volts (as is the case in the above analogs and prototype), but by discrete-low-voltage signals with a constant amplitude from the computer, which ensures increased noise immunity.

The foregoing allows us to conclude that the task set - the exclusion from the list of actions that control the phase shift in interference systems, mechanical displacements - has been solved, and the claimed technical result - increased reliability (reliability) by improving accuracy and noise immunity - has been achieved.

The analysis of the claimed technical solution for compliance with the conditions of patentability showed that the characteristics indicated in the independent claim are interrelated with each other with the formation of a stable set of necessary attributes unknown at the priority date from the prior art sufficient to obtain the required synergistic (over-total) technical result.

Therefore, the claimed subject matter meets the requirements of the patentability conditions of “novelty”, “inventive step” and “industrial applicability” under applicable law.

Claims (8)

1. A method of controlling the phase shift in interference systems, including the formation of coherent monochromatic radiation by a laser source, its separation into reference and subject light beams by means of a collimation system, their direction to the measurement object and the reference surface with the formation of a phase shift Δϕ for interpretation interference pattern when reflected on a photodetector, characterized in that the phase shift of the reference and subject light beams form due to their Bragg diffraction into orders of the same number and sign by passing in the section between the collimation system and the reference surface and the measurement object, respectively, through identical acousto-optical modulators, to which reference oscillations U ₁ and U ₂ are supplied from the common generator so that U ₁ = U ₀ cos [2πf × t + Δϕ] and U ₂ = U ₀ cos [2πf × t], where U ₀ - the amplitude of the reference oscillations; f is the frequency of reference vibrations of acousto-optical modulators; t is the time of the implementation of the reference oscillations. 2. A method of controlling the phase shift in interference systems according to claim 1, characterized in that the reference oscillations U ₁ are supplied to an acousto-optic modulator through which the subject light beam passes, and the reference oscillations U ₂ are supplied to an acousto-optic modulator through which the reference light beam passes . 3. The method of controlling the phase shift in interference systems according to claim 1, characterized in that the reference vibrations U ₂ are fed to an acousto-optic modulator through which the subject light beam passes, and the reference vibrations U ₁ are fed to an acousto-optic modulator through which the reference light beam passes .

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