RU2699921C1 - Method of determining mutual position of overlapping optical beams - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to measurement optics. Disclosed method of determining the mutual position of overlapping optical beams involves modulating optical beams and detecting light fluxes of the beams using a photodetector. Optical beams are modulated by passing each n-th optical beam from a set of N optical beams through a separate light modulator, performing change in the intensity of the optical beam according to law 1+Un(t), where Un(t) is an orthogonal Walsh function. Optical beams are then collected in a focal plane using an optical system. Opaque screen is moved along the focal plane along one or two coordinates from the position when all optical beams are open to a position when all optical beams are closed to allow gradual overlapping of the optical beams. Recording the light flux of each of the N optical beams is carried out in the form of an orthogonal function by means of a photodetector installed behind the screen near the focal plane, with a multichannel receiver of orthogonal signals connected to it. One or two coordinates corresponding to the position of said screen in the focal plane are simultaneously recorded. According to the obtained data for each of the N beams, the dependence of the light flux on the coordinates of the screen is determined, numerical differentiation of optical beam light flux dependences from screen position coordinates is performed to obtain intensity distribution of optical beams on the screen and mutual arrangement of overlapping beams is determined from the distance between centers of masses of optical beams. Providing submicron accuracy of measurement of mutual position of optical beams by distribution of intensity of each beam in focal plane, and also reduced time of measurements.

EFFECT: provision of submicron accuracy of measurement of mutual position of optical beams with determination of intensity distribution of each beam in focal plane with considerable reduction of measurement time.

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Description

The invention relates to measuring optics, and in particular, to a method for determining the relative position in space of several optical beams, partially or completely overlapping beams. The invention may find application in the optical measurement of thermal, mechanical, and acoustic parameters of nanostructures and bulk materials using photothermal, picoacoustic techniques requiring control of combining focal spots with sizes less than 1 μm with an accuracy of 100 nm, as well as for adjusting optical devices containing lenses with large numerical aperture.

Of all known measurement methods, optical measurements are among the most accurate. The threshold sensitivity and accuracy of classical methods of optical measurements is at the level of the radiation wavelength, which for visible radiation is about 0.5 μm. The task of modern technologies is to increase the accuracy and sensitivity by a factor of ten (Kirillovsky V.K. et al. Optical Measurements, part 2, St. Petersburg, ITMO University, 2017, p. 3 // http: //aco.ifmo. com / upload / publications / book_opt_mes_part_2_2017.pdf).

A known method of measuring the parameters of the laser beam (SU 1791788 Al, G02B 26/00, H01S 3/00, 01/30/1993), based on the periodic transmission of parts of the optical beam using a system of slots located on two rotating coaxial disks. One of the disks is located before the waist of the optical beam, and the other after, directly in front of the photodetector. The values of defocusing and astigmatism parameters of the light beam are expressed in terms of the parameters of the ellipses (the length of the semiaxes, the angle of inclination of one of the axes of the ellipse to the plane of the device), which are formed when the light beam intersects the disks. When the disks rotate through the system of slots, the projections of the ellipses are counted, according to which, using the Radon transform, it is possible to restore the parameters of the ellipses and then the position of the focus and astigmatism of the optical beam. The main disadvantage of this method is the impossibility of simultaneously measuring the intensity distribution of two or more beams that partially overlap in space. To determine the mutual position of the beams, it is necessary to measure the intensity distributions separately for each beam, and high mechanical stability of the disk drive is required, since the intensity distributions are compared in absolute coordinates.

The closest in technical essence to the proposed method is a method of measuring the parameters of the laser beam (US 5459565 A, G01J 1/04, 10.17.1995), based on the periodic shading of the measured beam with the edges of knives or slots located in the side wall of the rotating drum, inside which there is a photodetector , recording the dependence of the intensity on the angle of rotation of the drum. Due to the presence of the edges of the knives and / or slots located at different angles relative to the axis of the drum, it is possible to obtain enough information in one revolution of the drum to determine the intensity distribution in the optical beam. The disadvantage of this method is the impossibility of simultaneously measuring the distribution of intensity and the relative position of two or more rays, partially coinciding in space. Carrying out such measurements using the described method requires its consistent application to each beam, as a result of which the measurement time increases by a factor of N (N is the number of rays), while the instability of measuring the absolute coordinates of the rays should be less than 100 nm.

An object of the invention is to increase the accuracy and simplicity of measuring the intensity distribution and determining the relative position of N overlapping (including partially or completely overlapping) optical beams in the focal plane.

The technical result of the invention is the provision of submicron accuracy of measuring the relative position of optical beams with determining the intensity distribution of each beam in the focal plane with a significant reduction in measurement time.

The method for determining the mutual position of overlapping optical beams includes the following sequence of stages:

- each n-th optical beam from a set of N beams is passed through a separate light modulator, which changes the beam intensity according to the law 1 + Un (t), where Un (t) is the Walsh orthogonal function,

- using an optical system, optical beams are collected in the focal plane,

- along the focal plane, one or two coordinates move an opaque screen made in the form of a knife, or two knives with mutually perpendicular edges, or a diaphragm with a round or rectangular hole, from the position when all the optical beams are open, to the position when all the optical beams closed, providing gradual overlapping of the optical beams, while simultaneously registering one or two screen coordinates in the focal plane and the corresponding values of the light fluxes of each of the N beams s as the set of orthogonal functions by the screen near the focal plane and photodiode connected thereto multichannel orthogonal signal receiver,

- according to the data obtained for each of the N optical beams, the dependence of the light flux on the coordinates of the screen is determined,

- carry out a numerical differentiation of the dependences of the light flux of the beams on the coordinates of the screen to obtain the distribution of the intensity of the optical beams on the screen and, by the distance between the centers of mass of the optical beams, determine the relative position of the overlapping beams.

The invention is illustrated by the following drawings:

FIG. 1. The modulation and registration scheme of the optical flux of optical beams

FIG. 2. Possible forms of execution of an opaque screen

FIG. 3. The modulation and registration scheme of the light flux of three overlapping optical beams

FIG. 4. The dependence of the light flux of three optical beams on time at the outputs of the modulators

FIG. 5. Knife overlap pattern of three overlapping optical beams

FIG. 6. The time dependence of the light flux on the photodiode

FIG. 7. The modulation and registration scheme of two overlapping optical beams

FIG. 8. The dependence of the light flux of two overlapping optical beams on time at the outputs of the modulators

FIG. 9. Measurement results in two coordinates for two overlapping optical beams

Optical beams B ₁ , B2, ... B _N (Fig. 1), generally not parallel (in the case when the beams are parallel, their spots in the focal plane coincide) of the optical axis of the system, with light fluxes P ₁ , P ₂ , ... P _N , where N is the number of beams, pass through modulators 1, where they are modulated by the functions 1 + U ₁ (t), 1 + U ₂ (t), l + U _N (t), and the functions Un (t), where n = 1,2 ... N, are the Walsh orthogonal functions wal (n, t / T) (Trakhtman AM et al. Fundamentals of the theory of discrete signals at finite intervals, M., Soviet Radio, 1975, p. 207. / [1]). Here P ₁ , P ₂ , ... P _N is the value of light fluxes (W), U ₁ (t), U ₂ (t), ... U _N (t) are the dimensionless values of the Walsh functions with parameters n = 1,2 ... N and t / T, t is time (s), T is period (s). After passing through the modulators, the light fluxes of the optical beams become time-dependent according to the law: P ₁ ⋅ (1 + U ₁ (t)), P ₂ ⋅ (1 + U ₂ (t)), ..., P _N ⋅ (1 + U _N (t)).

As modulators, any known modulators can be used, such as mechanical beam choppers, electro-optical (based on the Pockels effect) and acousto-optical. The modulated beams are focused using a lens or objective 2 in the focal plane 3. On the right in FIG. Figure 1 shows an example of a possible distribution of the beam intensity in the focal plane 3 when the screen 4 does not intersect the beams. Along the focal plane, one or two coordinates move the screen 4 using the XY micrometric drive. Possible screen shapes are shown in FIG. 2, while the circles schematically show the possible position of the beam sections in the plane of the screen. The shape of the screen in the form of a knife (see Fig. 2a) allows measurements of the intensity distribution and the relative position of the beams along one coordinate. The screen forms in the form of a double knife (see Fig. 2b)) and diaphragms (see Fig. 2c), d)), allow consecutive measurements in two coordinates. Behind the screen 4, a photodiode 5 is installed, which records the light flux P _Σ = P ₁ ⋅S ₁ (x, y) ⋅ (1 + U ₁ (t)) + P ₂ ⋅S ₂ (x, y) ⋅ ( 1 + U ₂ (t)) + ... + P _N ⋅S _N (x, y) ⋅ (1 + U _N (t)), which changes as the screen moves. The functions S ₁ (x, y), S ₂ (x, y), ..., S _N (x, y) show which part of the light flux is not obscured by the screen in the x, y position. If the beam intensities are small, we can consider the photodiode operating mode to be linear, with the output current I _D + k⋅P _Σ , where k is the sensitivity of the photodiode.

The signal from the output of the photodiode is fed to the input of the receiver of orthogonal signals 6. Each of the outputs of the receiver of orthogonal signals is configured for one and only one function from the set Un (t), n = 1 ... N. At the corresponding outputs, we obtain P ₁ ⋅ S ₁ (x, y), P ₂ ⋅ S ₂ (x, y), ..., P _N ⋅S _N (x, y). Thus, moving the screen from the position when all the beams are open, to the position when all the beams are closed, simultaneously registering the coordinate and outputs of the multi-channel receiver 6 get the functions S ₁ (x, y), S ₂ (x, y), ..., S _N (x, y). Differentiating these functions with respect to the coordinates, we obtain the intensity distributions of the beams in plane 3, and the distance between the beams, as the distance between the centers of mass of the beams.

Due to the fact that the dependences of light fluxes on the screen position for all beams are determined simultaneously (at a certain coordinate value, the fluxes of all beams are measured at once), there are no errors in measuring the relative positions of the intensity distributions of individual beams. Such errors inevitably arise due to mechanical instabilities (backlashes) in the screen drive during the measurement separately for each beam.

The use of orthogonal modulation of the intensities of optical beams gives the proposed method a new quality - multichannel, significantly expanding its capabilities, and absent in existing methods for measuring the parameters of optical beams.

Example 1

In FIG. 3 shows a modulation scheme for three overlapping beams. Suppose that along the optical axis Z three optical beams B ₁ , B ₂ , B ₃ propagate with the values of the light fluxes P ₁ = 2 mW, P ₂ = 1 mW, P ₃ = 0.5 mW at small angles ≤10 ^-4 ÷ 10 ^-3 radians to the Z axis (Fig. 3). We note immediately that such angles with respect to the optical axis of an objective with a focal length of ≈5 mm lead to a shift of the focal spot by 0.5–5 μm. With the diameter of the focal spot 1.5 ÷ 2 μm, this can lead to the fact that the beams will not overlap in the focal plane.

Modulators 1, modulate light beams by the functions 1 + wal (1, t / T), 1 + wal (2, t / T), 1 + wal (3, t / T), where wal (m, t / T) - Walsh functions. The time dependence of the light fluxes at the outputs of the modulators is shown in FIG. 4a), b), c). As an example, we consider measuring the distribution of the beam intensity along one coordinate, in this case, a knife is used as a screen.

In FIG. 5a) does not overlap the knife beams, the beams of light flux in the photodiode is equal to P _a (t) = 2⋅ (1 + wal (1, t)) + 1⋅ (1 + wal (2, t)) + 0.5⋅ (1 + wal (3, t)) and has the form shown in Fig. 6a). Since each output (channel) of the receiver (see Fig. 3, where the receiver circuit is shown, low-pass filters - low-pass filters) responds to a signal caused only by its “own” modulating function, we have P ₁ at the outputs of channels 1, 2, 3, P ₂ , P ₃ , since S ₁ (x ₀ ) = S ₂ (x ₀ ) = S ₃ (x ₀ ) = 1.

In the position of FIG. 5b) the beam P _{1 is} partially blocked by a knife, while its flow is P ₁ ⋅ S ₁ (x ₁ ). On the photodiode, the sum of the light fluxes is P _b (t) = 0.7⋅2⋅ (l + wal (1, t)) + 1⋅ (1 + wal (2, t)) + 0.5⋅ (1 + wal (3, t )) and has the form of FIG. 6b). At the outputs of channels 1, 2, 3 of the receiver, we have P1⋅S ₁ (x ₁ ), P ₂ , P ₃ .

In the position of FIG. 5c) all three beams are partially blocked, the sum of the light fluxes on the photodiode is equal to P _in (t) = 0.1⋅2⋅ (1 + wal (1, t)) + 0.6⋅1⋅ (1 + wal (2, t)) + 0.7⋅0.5⋅ (1 + wal (3, t)) and has the form of FIG. 6c), and at the outputs of channels 1, 2, 3 of the receiver we have P ₁ ⋅ S ₁ (x ₂ ), P ₂ ⋅ S ₂ (x ₂ ), P ₃ ⋅ S ₃ (x ₂ ). Thus, an independent measurement of the functions S ₁ (x), S ₂ (x), S ₃ (x) takes place, differentiating which, they find the beam intensity distribution in the focal plane.

Obtaining a technical result - submicron accuracy of measuring the mutual position of three beams, is ensured by the simultaneous measurement of light fluxes at each position of the knife, which allows to determine all functions S ₁ (x), S ₂ (x), S ₃ (x) in one pass of the knife. This eliminates the requirement of high absolute accuracy (~ 100 nm) for measuring the coordinate of the knife, which is the main one in sequential measurement of the functions S ₁ (x), S ₂ (x), S ₃ (x). All that is needed is the relative accuracy of measuring knife coordinates ~ 100 nm, which is realized in medium-precision mechanical drives.

Example 2

In FIG. 7 shows a modulation scheme for two overlapping beams. Laser beams B ₁ and B ₂ with luminous flux values P ₁ = 2 mW, P ₂ = 2 mW pass through a mechanical chopper (modulator) 1, shaper 7 and electro-optical modulator 8, which modulate the beam intensities with the functions 1 + wal (1, t / T) and 1 + wal (2, t / T) (see Fig. 8), after which they are combined using mirror 9 and then fall into lens 2 with a numerical aperture of 0.4. A double knife 4 (Fig. 2b)) is moved along the focal plane along two coordinates from the position when both beams are open to the position when both beams are closed. Measurements are carried out first along the X coordinate, then along the Y coordinate. Using a photodiode 5, installed behind the focal plane 3, connected to the vector synchronous amplifier 6, two orthogonal functions are recorded, and, at the same time, the coordinates of the double knife. The XY aperture micrometric drive and amplifier 6 are controlled by computer 10. The measurement results in two coordinates are shown in FIG. 9a) and b). In FIG. 9a) solid curves 1 and 2 show the results of simultaneous measurements of beam fluxes B ₁ and B ₂ when the knife moves along the coordinate X. Dotted curves 1 and 2 show the corresponding beam intensity distributions obtained by numerical differentiation. The arrows indicate the calculated positions of the centers of mass of the beams 4.9 μm and 5.4 μm. Thus, the distance between the beams along the X coordinate is 0.5 μm. The beam width at half maximum along this coordinate is 1.9 μm and 4.2 μm. In FIG. 9b) shows the results of similar measurements for the case when the knife moved along the Y coordinate. In this case, the position of the centers of mass of the beams was 8.2 μm and 8.6 μm, so the distance between the beams along the Y coordinate is 0.4 μm. The beam width at half maximum along this coordinate is 2.5 μm and 5 μm. Here, as in Example 1, the technical result - submicron accuracy of measuring the relative position of two beams, is ensured by simultaneous measurement of light fluxes at each position of the knife, which allows determining the dependence of light fluxes on the X coordinate of the knife for both beams for the first pass and the second pass dependences of the light fluxes of both beams on the Y coordinate. The absolute values of the knife coordinates are not used in determining the mutual position of the beams.

Thus, the above example clearly demonstrates the possibility of simultaneously determining the intensity distribution and relative position, including partially coincident beams in space.

Claims (1)

A method for determining the mutual position of overlapping optical beams, including modulating the optical beams and registering the light fluxes of the beams with a photodetector, characterized in that the modulation of the optical beams is carried out by passing each n-th optical beam from a set of N optical beams through a separate light modulator that changes the intensity of the optical beam according to the law 1 + Un (t), where Un (t) is the Walsh orthogonal function, then the optical beams are collected in the focal plane using the optical system, along the opaque screen made in the form of a knife, or two knives with mutually perpendicular edges, or a diaphragm with a round or rectangular hole, from the position when all the optical beams are open, to the position when all the optical beams are closed, with the gradual overlapping of the optical beams, while the registration of the light fluxes of each of the N optical beams is carried out in the form of an orthogonal function by means of a photodetector made in the form of a photodiode, it is established about behind the screen near the focal plane, with a multi-channel receiver of orthogonal signals connected to it, and provide simultaneous registration of one or two coordinates corresponding to the position of the said screen in the focal plane, after which the dependence of the light flux on the position coordinates is determined for each of the N beams screen, carry out numerical differentiation of the dependences of the optical flux of optical beams on the coordinates of the position of the screen to obtain the distribution of the optical intensity beams on the screen and the distance between the centers of mass of the optical beams determine the relative position of the overlapping beams.

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