SU1221504A1 - Laser-interferometric system of measuring extralow vibrations - Google Patents

Abstract

The invention relates to optoacoustic measurements and can be used to measure amplitudes and phases of oscillations of a sample surface. The aim of the invention is to increase the sensitivity, noise immunity and enhancement of functionality. In the laser-interferometric system for measuring ultra-small oscillations, the second generator excites oscillations of the object surface. The bias module under the action of the voltage of the first generator causes vertical, low-frequency piston oscillations of the object surface. The laser interferometer converts the phase modulation of the reflected light beam into an interference signal. The interference signal from the output of the photodetector is divided by high-frequency and low-frequency filters into high-frequency and low-frequency Ui; Compiled by yupche. The amplitude of the signal U i is proportional to the amplitude of the oscillations A (k, y) and the reflection coefficient R, and the amplitude of the signal U depends on R. The signal from the output of the first attenuator depends only on A (x, y) and does not depend on a randomly varying reflection coefficient. The high-frequency {th signal after the high-frequency filter enters the mixer, where it is modulated by the voltage of the same frequency f. The output signal of the analog multiplier is a fast oscillating or alternating function. The polarity switches, together with the operational amplifier, eliminate low-frequency signal oscillations. Inverter provides antiphase switching. At the output of the integrating amplifier, a constant component of the signal is extracted. When scanning a selected sample cross section, the output voltage tracks the amplitude and phase of the oscillations of the sample surface. 1 il. § SO ju sd about 4

Description

The invention relates to optical measurements and can be used to measure the amplitudes and phases of the oscillation surface of a sample.

The aim of the invention is to increase the sensitivity, noise immunity and enhancement of functionality.

The drawing shows a laser interferometric system for measuring super-1 Al X surface oscillations.

The laser-interferometric system for measuring ultra-small oscillations contains a laser interferometer 1, a photodetector 2 installed at the output of the laser interferometer 1, the first generator 3 connected in series, and the piezomodulator 4, the second generator 5 designed to excite the surface of an object 6 made in the form of a quartz piezoresonator, which is fastened to the piezomodulator 4, and the recorder 7 made in the form of a two-coordinate recorder; connected in series the first phase-shifting unit 8 whose input is designed to receive oscillations of the surface of the object 6 phase switch 9, the second input of which is connected to the output of the second generator 5, mixer 10, low-frequency amplifier 11, first attenuator 12, first amplifier 13 and analog multiplier 14 - high-frequency filter 15, the input of which is connected to the output of the photodetector 2, and the output with the second input of the mixer l 10j connected in series the first low-pass filter 16, the input of which is connected to the output of the photodetector 2, the second attenuator, 17, the second amplifier 18. and the second phase shifting unit 19, the output of which is connected to the second input of the analog multiplier 14s serially connected amplitude detector 20, the input of which is connected to the output of the second amplifier 18, and the second low-pass filter 21, whose output connected to the second input of the second attenuator 17 and to the second input of the first attenuator 12, the third phase-shifting unit 22 connected in series, the input of which is connected to the output of the first generator 3, the comparator 23j inverter 24 and the first cable mutator 25 floor r215042

the second input is connected to the output of the analog multiplier 14, serially connected to the second switch 26 of the field, the input of which is connected to the output of the analog multiplier 14, operational amplifier 27, the second input of which is connected to the output of the first switch 25 polarity, and 0 an integrating amplifier 28, the output of which is connected to the recorder 7, and the output of the comparator 23 is connected to the second input of the second polarity switch 26.

15

The surface of the object 6 is set so that it is located in the focus of the focusing optics 29.

A piezomodulator 4 is rigidly connected to the object table 30, which is driven by an electric actuator 31. The second input of the recorder 7 is designed to be connected to a displacement sensor 32, whose junction is proportional to the displacement of the object table 30.

The laser-interferometric system for measuring ultramal oscillations operates as follows.

The second generator 5 excites oscillations of the surface of the object 6, made in the form of a quartz piezoresonator. The oscillations of the surface of object 6 can be described by the dependence of

20

thirty

35

a (x, y, t) A (x, y) sin 2 Ht -f- Q (x, y), where)

f (x, yJ

sin distribution of the amplitude of the surface of the object 6;

the oscillation frequency of the surface of the object 6, excited by the second generator 5;

phase distribution of the oscillations of the surface of the object 6; object surface coordinates, 6, time coordinate; sine function.

The amplitude of oscillation of the surface of the object 6, due to the excitation

those. the transmission coefficient of the second attenuator 17 is inversely proportional to the signal and or the light reflectance. The first and second attenuators 12 and 17 are technically complete (i.e., they have the same parameters and characteristics), as a result of which the law of variation of the transmission coefficient K o of the first attenuator 12 in the beat of the signal Uf changes synchronously with

TO.

0 - K, o and “/ and ,.

four

(9)

The signal Ur at the output of the first attenuator 12 is equal to

Uf

and

Jk

and,

(ten)

five

i.e. does not depend on a randomly varying reflection coefficient R.

Informative high frequency signal-uj. after filtering in the high-frequency filter 15 enters the mixer 10, where it is modulated by mixing with a voltage signal of the same frequency f, flowing through the phase switch 9 directly from the second generator 5 and rt through the first phase-shifting unit 8, the low-frequency amplifier 11 selects low frequency signal

BO

h "tx (

xsin2irFt + (p (t) q) (x, y) + b, (C

where 9 is the phase shift of the signal,

carried out by phase shifting unit 8.

After this signal is normalized by the first attenuator 12 and amplified by the first amplifier 13, the informative signal arrives at the first input of the analog multiplier 14, the second input of which is differentiated in the second phase shifter 19, the signal from the output of the second amplifier 18. The output signal of the analog multiplier 14 is a rapidly oscillating function at Q (Г / 2 and the alternating function at 0 0.

The first and second switches 25 and 26 are polar with operation

five

0

five

0

five

By using the amplifier 27, the low-frequency signal oscillations associated with the multiplier are eliminated, since the operation of the switches 25 and 26 polarity with the operational amplifier 27 is equivalent to a synchronous rectification (detection of a signal with a clock frequency F), and the switching times of the first and second switches 25 and 26 the polarities coincide with the moment of change of the sign (zoning) of the multiplier cos2 uFt. The required switching mode is provided by serially connected to the third phase-shifting unit 22, providing phase shift, and the switch 23.

The phase switching of the first and second polarity switches 25 and 26 is provided by the inverter 24. At the output of the operational amplifier (differential amplifier) 27, a signal is observed

you., AU, y) / cos2i; Ft.

t-87: os (cpU, y) + e) + cos -; j - BjSin2 HFt +

.t) + h (x, y)) (12)

On the integrating amplifier 28 npo-j, a constant component is released (correlation voltage)

t L

one

and

kOi

f

out

(13)

40

where o is the integration time

signal. Moreover, as can be seen from (13), with

Fulfillment of conditions fg “output

with

the voltage of the integrating amplifier is 28

tcop y) cos (c | (x, y) +

so + e ;,

(14)

where K c. - constant installation,

depending on Us and the integrator transfer ratio.

Thus, when scanning a selected section of the sample 6, the output voltage tracks the amplitude

second generator 5, much less than the light wavelength

The piezoelectric modulator 4, under the action of the voltage of the first generator 3, causes vertical low-frequency Hf) i.e., the piston oscillations of the surface of the object 6, described by the expression

B (t) sin2fFt,

where in,

F is the amplitude of the vertical low-frequency piston oscillations of the object surface 6, the frequency of the low-frequency vertical piston oscillations of the surface of the object 6.

The amplitude of the low-frequency vertical piston oscillations is determined by the expression

AT.

(3 - 4) l / 2.

The laser interferometer 1 converts the phase modulation of the reflected light reflected by the oscillating surface of the light wave into an interference signal. At the output of the photodetector 2, the interference signal is determined by the expression

On

Ying (-%, -sin2m.

On

A (x, y), SpCt)

.

where Urt (R) is the amplitude of the interference signal at the output of the photodetector 2, the value of which is proportional to the amplitude coefficient R of light reflection from the object 6j (p c (t)) are random changes in the phase of the operating point of the interferometer associated with the vibrational background and the drift of the interferometer The signal (3) at A (x, yJ "ii d / 2 and f F by the high-pass filter 15 and the first low-pass filter 16 is divided into a high-frequency (informative) component U and a low-frequency auxiliary signal IL, which as can be seen from (3 ) have the form

four

U, it) U (R) A .sin 2irFt + VgU.) - si, n2-ilftj; (four)

about

Q Ha (4) shows that the amplitude of the signal U is proportional to the amplitude of the oscillations A (, x, y) and the reflection coefficient R, while the amplitude of the auxiliary signal depends

15 only from R (. And directly proportional to R),

The automatic normalization of the signal Ur by the level of the reflection coefficient is produced by the first attenuator 12Q 12, whose transmission coefficient C is inversely proportional to the coefficient R of light reflection at each point on the surface. As a result, the signal at the output of the first attenuator 12 has the form

2S

and

  K (R), A),

6)

.

where UARJA) corresponds to U (t}.

The signal from the output of the first attenuator 30 pa | 2 is dependent only on the amplitude of the studied oscillations of the surface A (x, y). The required control mode of the first attenuator 12 is provided by the following sequential closed-loop communication: the second attenuator 17 | second amplifier 18, amplitude detector 20 | the second low-pass filter 21, which operates at a low-pass;

40 auxiliary signal U / and in

the operating mode is stabilized by the voltage at the output of the second amplification;

l 18 around some constant U o.

45 At the output of the second amplifier 18, the signal is determined from the expression

and

L

 to, „(t) -u ,,

ten

(7)

where Kjjj (t). is the transmission coefficient of the second attenuator 17 at the current moment of scanning the surface of the object 6. From expression (7) it follows that

TO

ten

 uv / u.

(eight)

The phase of the surface of the sample 6, the information about which is output to the recorder 7, made in the form of a two-coordinate recorder. The second coordinate of the recorder is supplied with voltage from the displacement sensor 32 proportional to the linear displacement of the object table 30. Scanning the section of the sample 6 with a probing light beam is provided by the movement of the subject table 30, which is driven by the actuator 31. As a result, the light beam senses the points of the selected section of the sample surface consistently .

With fluctuations of the surface points of sample 6 with an arbitrary initial phase (measurement is performed near resonance, but not with fine tuning for it), measurements are performed twice in each cross section: when S is 0 and 2, as a result, recorder 7 draws two curves

and (x) / 0, O n Uj (x) and

(x) / b UgU),

whereupon the amplitudes and phases of the ultra-small oscillations of the surface of the sample 6 are determined from the expressions:

Claims (1)

A (x, y)) + and | (k), (15) C | (x, y) arctgUg (x) / Ug (x). () Invention Formula A laser-interferometric system for measuring ultra-small oscillations, containing a laser interferometer, a photodetector installed at the output of the laser interferometer, hence the connected first generator and a piezomodulator, a second generator designed to excite the object's surface oscillations, and a recorder, TL and .215048 due to the fact that, in order to increase sensitivity, noise immunity and enhance functionality, it is equipped with 5 are connected in series by a first phase-shifting unit, the input of which is intended for receiving oscillations of the object surface, a phase switch, the second input of which is connected to 10 by the output of the second generator, and by a mixer, a low-frequency amplifier, a first attenuator, a first amplifier and an analog multiplier, a high-frequency filter, which is connected to the output of the photodetector, and the output is connected to the second input of the mixer, serially connected by the first low-frequency filter, input which is connected to a photodetector output, a second attenuator, a second amplifier, and a second phase-shifting unit, the output of which is connected to the input of an analog multiplier, successively connected It is a common detector whose input is connected to. the output of the second amplifier, and the second low-pass filter, the output of which is connected to the second input of the second; attenuator and the second input per30 35 40 45 a third attenuator connected in series by a third phase-shifting unit, the output of which is connected to the output of the first generator, a comparator, an inverter and the first polarity switch, the second input of which is connected to the output of an analog multiplier, connected in series with a second polarity switch, the input of which is connected to the output of an analog retransmitter, operating amplifier. the second input of which is connected to the output of the first switch of the field, and an integrating amplifier, the output of which is connected to the recorder, and the output of the comparator to the second input of the second switch of polarity. Sh .ZHL 22 -