RU27950U1 - AUTOMATED INTERFEROMETER - Google Patents

Abstract

An automated interferometer containing a radiation source, an anisotropic medium, a photodetector, a processing unit, characterized in that the anisotropic medium is made in the form of two identical crystalline prisms located on moving platforms in such a way that they are displaced along the hypotenuses of the base of the prisms, it additionally contains a digital positioning system, which includes: a clock pulse generator, an electronic switch and a stepper motor, which through a gear mechanism ulation movement of the movable platforms, the block processing results comprises a personal computer that controls the positioning system and the digital processing of the measurement results.

Description

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AUTOMATED INTERFEROMETER

The proposed utility model relates to optics and measurement technology and can be used to study both femtosecond laser pulses and stationary light fields from chaotic sources with femtosecond coherence time.

Known interferometer for measuring the phase shift of light waves III, containing an optically coupled source of monochromatic radiation, an acousto-optic modulator with one electrical input and three spatially spaced different-frequency optical flows, a collimator, a measuring path consisting of series-connected optical and electric channels, a reference path is introduced into the device each of which uses a delay line.

Known optical Michelson interferometer / 2 /, which implements a change in the direction of polarization of the optical radiation passing through the light splitter in the opposite direction, orthogonal in comparison with the direction of polarization of the optical radiation passing through the light splitter in the forward direction, which is ensured by the inclusion in the measuring and reference arms of the optical interferometer corresponding polarization switches. In this case, the light splitter is made polarization sensitive.

The closest analogue is an interferometer / 3 /, containing a coherent radiation source, a radiation divider located in two channels, a radiation frequency shifter for another channel, a beam splitter, a reference photodetector, a signal photodetector, an illuminating wave divider and a frequency shift device made in the form of an optical integral schemes on single-mode strip waveguides with fiber optic end input. At the same time, the indicated -g.-r.-Ch (--- g- -1.-l1 in1 “1 circuit contains BOjmOBOMHbui except for the input of the input to

two waveguides extending open ends to the free end of the integrated circuit, and a phase modulator located on these waveguides (or one of them)

A disadvantage of known interferometers is the difficulty of recording and interpreting interferograms, which are often associated with the fact that traditional interferometers in which interacting waves travel through different geometric paths have an increased sensitivity to vibrations and the quality of optical elements.

The objective of the proposed utility model is to create an automatic interferometer designed to study both femtosecond laser pulses and stationary light fields from chaotic sources with femtosecond coherence time.

The problem is achieved in that in an interferometer containing a radiation source, an anisotropic medium, a photodetector, a processing unit, the anisotropic medium is made in the form of two identical crystalline prisms located on moving platforms in such a way that they shift along the base of the prisms, it additionally contains a digital positioning system which includes: a clock pulse generator, an electronic switch and a stepper motor, which through a transmission mechanism controls the movement of mobile platforms, the processing unit contains a personal computer that controls the digital positioning system and processes the measurement results.

The general scheme of the interferometer is shown in FIG. 1, where 1 is the radiation source; 2 - crystalline prisms; 3 - polarizer and analyzer; 4 - photodetector; 5 - analog-to-digital Converter (ADC); 6 - personal computer (PC);

2

7-shshovy elektrodvodshatel (SED); 8 - electronic switch (EC); 9 - clock generator (TTI)

The structure of the chain includes: a clock pulse generator (GTI), an electronic switch (EC), a stepper motor (SED), which through the transmission mechanism controls the movement of moving platforms. This structure of the DSP belongs to the category of followers and provides the possibility of deep frequency regulation of speed. It allows you to numerically set the path and implement reliable fixation of the final coordinates of the movement. The GTI and EC set a four-stroke control algorithm for the four-winding SED with alternately turning on the windings. The method of controlling the electric motor is changed by applying to the corresponding inputs of the EC potential signals “forward and backward” of step-by-step control.

An anisotropic medium is two identical crystalline prisms located on moving platforms in such a way that their displacement occurs along the hypotenuses of the base of the prisms.

As the FPU, either a silicon-band silicon photodiode is used, the electric signal of which is amplified in an amplifier created on the basis of an operational bottom amplifier, or, with weak light fluxes, a photoelectronic multiplier.

Using the ADC, the FPU electrical signal is converted into a 1-phase form.

In the device, a sequential approximation ADC is used, which converts an analog signal to a digital one, starting from the least significant digit to a digital 8-bit code at the output corresponding to the level of the input analog signal. The ADC is connected to the PC after multiplexing the received data via the CENTRONICS interface. To process the obtained measurement results, special software is written.

As is known, when a light wave passes through an anisO1pO1 medium, the state of its polarization depends on the properties of the substance itself and varies with distance. The nature of such changes for an infinite wave is shown in Fig. 1.

The period A of the change in polarization corresponds to a phase difference run equal to ITI between the ordinary and extraordinary waves. It is equal to:

where An, aPoIP are the refractive indices for the ordinary and extraordinary waves, respectively.

This interferometer can be used to study both femtosecond laser pulses and stationary light fields from chaotic sources with a femtosecond coherence time. When femtosecond pulses are applied to the interferometer, wave packets corresponding to the ordinary and extraordinary components scatter away from each other as they move deeper into the crystal due to different propagation velocities. When the crystal thickness y is small, leaving it, the components almost completely overlap and, therefore, are able to interfere when the directions of oscillations are reduced to one plane using the polarizer 3. At an average thickness, the rays still interfere with crystalline prisms, but the visibility of interference fringes decreases due to incomplete spatial overlap of the envelope components of the pulses. When the thickness of the prisms is much larger than c / Ap, interference is no longer there, since there is no overlap of the components of the ordinary and extraordinary pulses. Therefore, with a smooth change in the effective thickness of the crystalline medium, the visibility of the bands of the interferogram decreases from 1 to 0, as the components of the pulse scatter.

If a stationary light wave from a chaotic light source is supplied to the interferometer, then the interference process will proceed in a similar way, despite the continuity of the wave. While the displacement of light waves in the crystal

l-v

/ Up

(ordinary and extraordinary) will not exceed the length of the coherence, an interference pattern will be observed. In the general case, the intensity of the interference signal at the output, depending on the thickness of the crystal, will be determined by the reflection

J (y) lfl + Y (y), (1)

VL at

where I is the total intensity of the radiation incident on the interferometer, y (y) the degree of mutual coherence of the ordinary and extraordinary waves at a thickness of the crystal. Fuchodia y (y) can be obtained from experimental data as the envelope of the modulated part of the interferogram.

For spectrally limited femtosecond pulses, the half-width of this curve determines the pulse duration 1, and in the general case, the value of the radiation coherence time Tk.

During the tests of the described interferometer manufactured in the prototype, interferograms of radiation sources with different degrees of coherence were recorded. We used a high-pressure discharge lamp and a helium-neon laser for comparison. The radiation from the lamp was transmitted through a monochromator from an SF-4 spectrophotometer. A part of the smoothed spectrum was distinguished in the blue-green region. Due to the small dispersion in the visible spectral range of the quartz prisms used in the monochromator, the width of the absorbed spectral range could exceed 100 nm. The coherence time was changed by varying the spectral fin of monochromator slits.

It should be noted that the interferometer under consideration has a “blind zone, due to the fact that the minimum thickness of the crystalline medium with partially overlapped prisms that ensure the passage of the beam cannot be equal to zero. BejmHJUHa of this zone with the expression:

 (d, + ltga) An

(2) it was limited by the width of the light beam, in accordance with

where 1 is the geometric width of the light beam, and 8.53 ° is the smaller angle between the faces of the prisms, do 0.2 mm is the total initial thickness of the composite crystal. Therefore, an additional time shift of the ordinary and extraordinary signal components was introduced into the interferometer. This was realized by installing a crystalline plate in front of the moving prisms with an orthogonal orientation of the optical axis with a thickness of at least (do + 1 tga).

Figure 2 shows the interferograms recorded for the radiation of a high-pressure discharge lamp (a, b) and a helium-neon laser.

The test results are shown in figure 2. The interferograms for lamp radiation are shown on the upper part of the figure (curves a and b); moreover, for case (b), a narrower spectral region was cut out from the helix of the monochromator than for case (a); accordingly, the radiation coherence increased. It can be seen that the envelope width, which is a function of the mutual coherence of the ordinary and extraordinary rays Y (t), increases with increasing monochromaticity of the radiation. The coherence time Tk, defined as the difference between the maximum point of the envelope and the inflection point on its wing, is 9.2 fs for case a, and 6-18 fs for case. The interferogram of a helium-neon laser radiation recorded for comparison, having a coherence length measured in kilometers, has an adequate form.

The proposed interferometer is used both for measuring the duration of femtosecond laser pulses and stationary light fields from chaotic sources with a femtosecond coherence time, it is easy to manufacture, does not require precise adjustment and has small dimensions. Sources of information taken into account:

1. RF patent No. 2112210: G01B21 / 00 1998

2. RF patent No. 2169347 g G01B9 / 02.2001 g

3. The author's certificate of the USSR No. 1383969 G01B9 / 02 1997 (prototype)

A.I.Smirnov E.F. Martshovich V.P. Dresvyansky D.A. Ba1shsov

Claims (1)

An automated interferometer containing a radiation source, an anisotropic medium, a photodetector, a processing unit, characterized in that the anisotropic medium is made in the form of two identical crystalline prisms located on moving platforms in such a way that they shift along the hypotenuses of the base of the prisms, it additionally contains a digital positioning system, which includes: a clock pulse generator, an electronic switch and a stepper motor, which through a gear mechanism ulation movement of the movable platforms, the block processing results comprises a personal computer that controls the positioning system and the digital processing of the measurement results.