RU176011U1 - Optical design of an ultra-precision holographic linear displacement sensor with a controlled phase modulator - Google Patents

Abstract

The optical scheme of an ultra-precision holographic linear displacement transducer consists of a coherent optical radiation source, a collimating system that transmits a diffraction grating, a multi-section phase element, which is a single optical element with several zones, made end-to-end with each other with the possibility of phase-shifting these zones into diffracting optical beams , a block of receivers of the indicated recorded optical signals. The multi-sectional phase element is made in the form of a controlled phase modulator with the possibility of controlled change of phase shifts. The technical result is to increase the accuracy of the sensor by obtaining in real time the maximum signal increment with a minimum change in the controlled parameter. 7 ill.

Description

Technical field

The utility model relates to the field of precision linear displacement sensors using optical measuring instruments by monitoring the parameters of light beams that are diffracted by various combinations of diffraction gratings operating according to the interference principle (pairwise interference of the diffracted beams with each other) and finally detected by photocells (photodetectors).

State of the art

Known sensor company Heidenhain (Germany), operating on the interference principle. The principle of operation and the optical design of this sensor (Fig. 1) are described in US Pat. No. 4,776,701 DISPLACEMENT MEASURING APPARATUS AND METHOD (IPC G01B 11/00; G01D 5/38; (IPC1-7): G01B 9/02, publ. 1988- 10-11) and can be taken as one of the analogues of the proposed utility model. This patent also had another patent analogue - USSR patent SU 1560068 (IPC G01B 11/00; publ. 04/23/1990) a device for measuring displacements. The objective of the invention of this device was to increase the resolution due to the use of diffraction gratings with a pitch of 40 microns. When illuminating gratings, one of which is transparent (transmissive), is attached to the object and made with a toothed profile, and the other is reflective and attached to another object, an interference pattern is formed, recorded using detectors located respectively in the zone of formation of zero, positive and negative first orders of diffraction image. The electrical signals from the detectors judge the direction and magnitude of the movement. In other words, this sensor contains a moving reflective grating and a non-moving optical head assembly consisting of a radiation source, an optical system, transmission gratings and four radiation receivers (detectors). Thus, this system is multi-channel - with 2 axial and 2 inclined channels.

The disadvantage of this optical sensor circuit is the increased dimensions of the optical head assembly along the length (along the long side of the reflective array). This leads to the fact that when the reflective grating is displaced in such a way that displacement is measured at the edge of the reflective grating, half of the optical head assembly extends beyond the reflective grating of length L (Fig. 2, notes on the notation of Fig. 2: - for measuring the same displacements in the analog sensor, the length of the moving reflective array 4 is required to be ΔL longer than in the proposed optical sensor circuit; The optical head in the analog sensor has a size ΔL1 larger than in the proposed optical sensor circuit). This can complicate the use of the sensor on some types of objects (for example, in machines), since it requires the presence of free space (in length) around the moving reflective grating. Another disadvantage is that for measuring displacements of magnitude L, it is necessary to make the reflective grating longer, since its symmetric illumination relative to the axis of the optical head is required (Fig. 1, 2).

The same drawback of the increased dimensions is also inherent in a number of other optical schemes of linear displacement sensors, patented in several other US patents.

US 5,430,546 OPTICAL DEVICE FOR MEASURING RELATIVE POSITION OF OR ANGLE BETWEEN TWO OBJECTS (IPC G01B 11/00; G01B 11/26; G01D 5/38; (IPC1-7): G01D 9/02, publ. 1995-07-04 ) - this system is multi-channel, but, like most devices of this type, it uses several diffraction directions, which leads to significant dimensions of the device.

US 5,120,132 POSITION MEASURING APPARATUS UTILIZING TWO-BEAM INTERFERENCES TO CREATE PHASE DISPLACED SIGNALS (IPC: G01D 5/38; (IPC1-7): G01B 9/02, publ. 1992-06-09) - also a multi-channel system. Its main drawback is the use of additional massive optical components — prisms — to produce beams, which leads to significant design dimensions.

периода по нарастающей, то есть

и

Таким образом, данная система является четырехканальной. Данные каналы являются осевыми (то есть направления излучения, падающего на приемники излучения (G3a, G3b, G3c, G3d) параллельны направлению излучения, сформированного после коллимирующей системы (объектива) (2). Принцип работы данной оптической схемы датчика заключается в том, что определение координаты происходит за счет получения и логической обработки оптических сигналов, продифрагировавших несколько раз на решетках и попарно проинтерферировавших друг с другом, на четырех приемниках излучения (PD1…PD4). На каждом из приемников сигнал формируется за счет интерференции двух пучков (интерференционный способ оценки перемещения) несколько раз продифрагировавших сигналов. Синусоидальное изменение интенсивности сигналов (при равномерном перемещении) на каждом из приемников происходит при перемещении отражающей решетки (G2) относительно пропускающей дифракционной решетки (G1). В данной схеме минимизированы габариты неперемещаемого узла оптической головки (она состоит из источника излучения (1), коллимирующей системы (объектив) (2), пропускающей дифракционной решетки (G1), четырех вторичных дифракционных решеток (G3a, G3b, G3c, G3d) и четырех приемников излучения (PD1, PD2, PD3, PD4).The closest analogue (prototype) of the proposed optical scheme of an ultra-precision holographic linear displacement sensor can be recognized as a sensor scheme according to US patent US 005569913 A (B2) OPTICAL DISPLACEMENT SENSOR (IPC: H01J 3/14, publ. 1996-10-29) (Fig. 3 ) The optical scheme of this high-precision linear displacement transducer contains an unmovable optical head assembly consisting of a coherent optical radiation source (laser) (1), a collimating system (lens) (2), a transmission diffraction grating (G1), and a composite component (as a multi-section phase element in the form of a single optical element with several (four) zones, made end-to-end with each other with the possibility of these phases introducing phase shifts into diffracting optical beams and transmitting radiation reflection reflected from a movable reflecting grating) in the form of four secondary diffraction gratings (G3a, G3b, G3c, G3d) and four radiation detectors (photodetectors) (PD1, PD2, PD3, PD4), as well as a moving reflective grating (G2). Moreover, the period of all gratings is constant and unified, but four secondary diffraction gratings (G3a, G3b, G3c, G3d) are shifted relative to each other by

growing period, i.e.

and

Thus, this system is four-channel. These channels are axial (that is, the directions of the radiation incident on the radiation receivers (G3a, G3b, G3c, G3d) are parallel to the direction of the radiation formed after the collimating system (lens) (2). The principle of operation of this optical sensor circuit is that the definition the coordinates are due to the receipt and logical processing of optical signals that diffracted several times on the gratings and pairwise interfered with each other, on four radiation detectors (PD1 ... PD4). They are due to the interference of two beams (an interference method for estimating the displacement) of several times diffracted signals.The sinusoidal change in the signal intensity (with uniform displacement) at each of the receivers occurs when the reflective grating (G2) is moved relative to the transmission diffraction grating (G1). dimensions of the non-moving optical head assembly (it consists of a radiation source (1), a collimating system (lens) (2), a transmission diffraction grating (G1), four second egg diffraction gratings (G3a, G3b, G3c, G3d) and four radiation detectors (PD1, PD2, PD3, PD4).

However, a significant drawback of this implementation is that a composite element in the form of four secondary diffraction gratings (G3a, G3b, G3c, G3d) is difficult to manufacture, since the necessary accuracy of the displacement of the secondary diffraction gratings relative to each other (in their manufacture) to determine the displacement with accuracy of the order of 1 nm should be of the order of fractions of a nanometer. This can only be achieved by lithographic production (but with limitations) or the use of complex optical schemes.

Also a common drawback of all the above sensor implementation schemes is that the phase relationships introduced into the interfering beams (by measuring the intensity of which the movement is determined) are implemented using optical components with parameters that are constant over time. This does not allow the use of interchangeable diffraction gratings in the sensors, and also does not allow to dynamically (in time) change the phase relationships introduced into the interfering beams during the operation of the sensor.

Utility Model Disclosure

The use of an optical component that changes the phase relations in the beams during the operation of the sensor allows one to increase the accuracy of position determination (with the same degree of signal sampling), since it allows one to obtain in real time the maximum (or correspondingly large) signal increment (changes in the beam intensity at the receiver) with (or, accordingly, small) change in the controlled parameter (movement of the reflecting grating).

The technical result achieved by the utility model consists in increasing the accuracy of the sensor by using a multisection phase element in its optical design in the form of a controlled phase modulator, which allows one to obtain a set of optical signals propagating in one direction and with controlled changes in the phase shifts of these signals from the control of the modulator zones at moving a movable reflective diffraction grating in order to obtain osektsionnogo large precast receiver (to improve accuracy) of the increment of the recorded signal at a low moving movable reflective grating.

The result is achieved in that the optical scheme of the ultra-precision holographic linear displacement sensor contains an unmovable optical head assembly consisting of a source of coherent optical radiation, a collimating system, a transmission diffraction grating, transmission radiation in the direction of a moving reflective diffraction grating, and a multi-section phase element mounted in a light beam diffracted on a movable reflective diffraction grating, and representing a single the first optical element with several zones made end-to-end with each other with the possibility of introducing phase shifts by these zones into diffracting optical beams and transmitting radiation reflected from the indicated movable reflecting array, which is not part of the optical head, and the receiver unit of the indicated recorded optical signals as a combination of pairwise interfering beams. The period of all diffraction gratings is constant and uniform. Moreover, the specified multi-sectional phase element is made in the form of a controlled phase modulator with the possibility of a controlled change in phase shifts from the action of its zones when moving a movable reflective diffraction grating to obtain a large increment of the recorded signal at a corresponding receiver or in the corresponding zone of a multi-section prefabricated receiver with a small movement of the moving reflective diffraction lattice.

The main advantage of the proposed optical sensor circuit compared to analogues and prototype is to increase the determination of the accuracy of the position of the movable reflective diffraction grating due to the use of a multi-section element in the sensor in the form of a phase modulator with the number of zones (n × m). An increase in the accuracy of position determination (with an identical degree of signal discretization) is achieved due to the fact that the dynamic change in phase relations in the corresponding zones of the phase modulator allows receiving the maximum signal increments (changes in intensity receiver) with a minimum change in the controlled parameter (movement of the reflecting grating).

The transfer of a multi-sectional element in the form of a phase modulator into one of the beams diffracted by a movable reflective grating allows you to save the minimum dimensions of the circuit proposed in the prototype.

Diffraction gratings 3 and 4 (Fig. 4) can best be performed by holographic technology due to registration of the interference field created in the zone of intersection of two radiation beams with plane wave fronts. That is, these lattices are holographic by the method of their preparation.

List of figures

FIG. 1 is an optical diagram of an analog of a holographic linear displacement transducer from Heidenhain (Germany);

FIG. 2 is an optical diagram of a prototype sensor explaining the principle of operation;

FIG. 3 is an optical diagram of a prototype linear displacement sensor;

FIG. 4 is a proposed optical design of an ultra-precision holographic linear displacement sensor;

FIG. 5 is an illustration explaining the structure of a multi-sectional element in the form of a multi-pixel liquid crystal spatio-temporal light modulator, divided into zones that are adjacent to each other;

FIG. 6 is an illustration explaining the structure of a multi-sectional element in the form of a set of their simple electro-optical cells filled with a liquid crystal and controlled electrically, which are located end-to-end to each other.

FIG. 7 is a diagram of a change in the signals at the receivers during the operation of the sensor

Utility Model Implementation

In FIG. 4 of the proposed optical scheme, the following general elements are indicated: 1 - a source of coherent radiation (for example, a semiconductor laser); 2 - collimating optical system (lens); transmissive diffraction grating 3, a movable reflecting diffraction grating 4, 5 — a set of receivers located end-to-end to each other, or a single composite signal receiver, phase modulator 6 divided into adjacent zones (parts). All receivers in the proposed scheme are located in the same diffraction order of the transmission grating 3 - in a single direction (channel). In FIG. 4 the small triangles show the course of interference beams of rays diffracted on gratings 3 and 4 with the passage and reflection of radiation from these gratings until the resulting beams arrive at the respective signal receivers 5. With this implementation, the sensor is multi-channel, but the total radiation of all channels propagates when incident on the block receivers 5 in one direction (as in one channel). The zones of the phase modulator 6 are introduced into the phase delays in the diffracting beam α (1,1) ... α (n ... m). When the sensor is initially set up when implementing a phase modulator with four zones, the starting phase delays in the zones can be set, for example, as 0, π / 2, π and 3π / 2 (taking into account the parameters of diffraction gratings 3 and 4 - in particular, the diffraction efficiency of the grating 4 in the order in which the phase modulator is installed). Also, the starting phase delays in the zones of the modulator can be adjusted based on maximizing the difference between the signals received at the receiver 5 in different zones. When the sensor starts to work, depending on the signal change rate, the phase delays in the corresponding zones of the modulator are changed in order to obtain at least one receiver (or in one zone of the multi-section receiver) the maximum signal increment with minimal movement of the reflecting array 4.

In the proposed optical sensor circuit (Fig. 4), one direction is used in which four or more channels are combined. All receivers 5 (or a multi-section receiver) in the proposed scheme are located in the same diffraction order of the grating 3. A multi-section phase element in the form of a phase modulator 6 is divided into parts (zones), the number of which is equal to the number of radiation receivers (or the number of zones of a multi-section receiver). Zones of the phase modulator contribute to the phase delays α (1,1) ... α (n ... m) in the light beam diffracted by the moving reflective diffraction grating.

A multi-section phase element in the form of a phase modulator can be a multi-pixel liquid-crystal spatio-temporal light modulator, described, for example, in the article (I.N. Kompanets, A.L. Andreev “Micro-displays for digital holography”, abstract of the International Conference “ GoloExpo-2016 ", pp. 114-121) and divided into zones that are located end-to-end to each other (Fig. 5). Each zone has its own phase delay, which can dynamically change during the operation of the sensor.

Also, a multisectional phase element in the form of a phase modulator can be a set of their simple electro-optical cells filled with a liquid crystal and electrically controlled, described in the article (A.L. Andreev, T. B. Andreeva, I.N. Kompanets, N.V. Zalyapin, “Prospects for the suppression of speckles using cells with a ferroelectric liquid crystal”, Abstracts of the International Conference “GoloExpo-2015”, pp. 94-102). At the same time, these cells are also located end-to-end to each other, FIG. 6 (a, b, c, d): FIG. 6a) - division into zones of a phase modulator in case there are four such zones; FIG. 6b) - designation of refractive indices n in the corresponding zones of the phase modulator in case there are four such zones; FIG. 6c) —division of the phase modulator into zones if such zones in the matrix (n × m) are pieces; FIG. 6d) - designation of the refractive indices n in the corresponding zones of the phase modulator if there are such zones in the matrix (n × m)), each cell represents a separate zone, and in each cell its own phase delay α (1,1 ) ... α (n ... m) due to the application of a constant electric field of the required power to it. Phase delays α (1,1) ... α (n ... m) are determined by a change in the refractive indices n _1,1 ... n _{n, m} , which are created in the zones of the phase modulator when a constant electric field of the required power is applied to them. Phase delays in cells (zones) can dynamically change during sensor operation.

In more detail, the scheme of changing the phase difference in the zones of the phase modulator and the corresponding signal change at the receivers to obtain at least one of them maximum signal increment (change in intensity at the receiver) with a minimum change in the controlled parameter (movement of the reflecting grating) will be explained using a phase modulator with two zones (6 ₁ and 6 ₂ ) and two receivers (5 ₁ and 5 ₂ ). The initial phase relations in the beams are set so that the signals at the receivers 5 ₁ and 5 ₂ have the form shown in FIG. 7 (step 1).

In this case, during the first measurement of the movement of the reflecting array (obtaining the first measurement reference) at the first receiver, the maximum signal increment (change in intensity at the receiver) will be measured with a minimum change in the parameter being monitored (movement of the reflecting array). At the second receiver 5 _{2, the} signal should differ from the signal at the receiver 5 ₁ , since the direction of movement of the reflecting grating is determined by their mutual change.

In the second measurement of the movement of the reflecting array (obtaining a second measurement sample), the phase zone of the modulator 6 ₂ introduces a phase delay so that the signal at the receiver 5 ₂ takes the form shown in FIG. 7 (step 2). In this case, the signal at the receiver 5 ₂ will satisfy the condition for the maximum increment of the signal (change in intensity at the receiver) with a minimum change in the controlled parameter (movement of the reflecting grating).

In the third measurement of the movement of the reflecting array (obtaining the third measurement sample), the phase zone of the modulator 6 ₁ introduces a phase delay so that the signal at the receiver 5 ₁ takes the form shown in FIG. 7 (step 3). In this case, for the signal at the receiver 5 _{1, the} condition for the maximum increment of the signal (change in intensity at the receiver) will be satisfied with a minimum change in the controlled parameter (movement of the reflecting grating).

Then there is a sequential change in the phase delays in the zone 6 ₁ , then in the zone 6 ₂ for receiving signals at the respective receivers 5 ₁ and 5 ₂ , for which the condition of maximum signal increment would be fulfilled alternately when measuring the corresponding step (obtaining the next measurement reference) with a minimum change in the controlled parameter (movement of the reflecting grating).

Since the speed of movement of the reflecting grating can be significant, the frequency of changes in phase relationships introduced by the zone of the phase modulator 6 will also be high. Currently, in order to reduce the requirements for the frequency of phase delay changes for a particular zone, it makes sense to use a phase modulator with more than 2 zones. Then the frequency of phase delays in the corresponding zone will decrease by a multiple of the number of zones used in the phase modulator. In this case, the determination of the final movement of the reflecting grating occurs due to the summation of the samples. These samples are obtained after digitizing the analog signal. To simplify the scheme of digitizing signals and summing the samples, it is necessary to use additional zones on the phase modulator. In one of these zones, the current position measurement sets the phase delay with the addition of the π / 2 phase to the phase at the receiver, which provides a change with a maximum signal increment with a minimum change in the controlled parameter. With such an implementation, displacement is calculated using the standard sin - cos model.

Implementation example

This utility model was developed as part of the theme “Research and development of an experimental sample of an ultra-precision holographic linear displacement sensor” under agreement of October 14, 2015 No. 14.577.21.0197 of MSTU named after N.E. Bauman with the Ministry of Education and Science of the Russian Federation in the framework of the federal target program “Research and Development in Priority Directions for the Development of the Scientific and Technological Complex of Russia for 2014-2020”. A prototype of the proposed optical scheme of a high-precision linear sensor was designed. The period of all gratings is T = 1 μm. In the scheme, a variant with 4 radiation detectors was used. The phase modulator was a 4-zone multisection element (a set of their simple electro-optical cells filled with a liquid crystal, controlled electrically and located end-to-end to each other). During the operation of the sensor, the measurement of the position of the moving reflective grating was worked out to speeds of 200 mm / min.

The dimensions of the optical head assembly with a grating 3, a phase modulator 6, and receivers 5 can be approximately 30 × 70 × 40 mm. In this case, the resolution of the sensor when measuring linear displacements is higher than that of similar sensors with identical parameters and using static components of the change in phase delays (phase masks; composite diffraction gratings displaced relative to each other), namely: the resolution when measuring displacement is 0.5 nm (this accuracy corresponds to the concept of "ultra-precision" of the sensor) when moving the reflecting grating by 1 μm (when shifting by one period of the grating equal to 1 μm, we have two periods of change of the meter Nogo signal).

Claims (1)

The optical scheme of an ultra-precision holographic linear displacement sensor in the form of an optical head, consisting of a coherent optical radiation source, a collimating system, transmitting a diffraction grating, transmitting radiation in the direction of a moving reflective diffraction grating, and a multi-section phase element mounted in a light beam diffracted by a moving reflective diffraction lattice, and representing a single optical element with several zones, made end-to-end with each other with the possibility of introducing phase shifts by these zones into diffracting optical beams, and transmitting radiation reflected from the indicated movable reflecting grating, and the receiver unit of said recorded optical signals as a set of diffracted and pairwise interfering beams, while the period of the diffraction gratings is constant and single, characterized in that the specified multi-phase phase element is made in the form of a controlled phase modulator with the possibility of controllable changing phase shifts from its zone of action when moving the movable reflective grating for obtaining a large increment corresponding receiver detected signal with a small moving movable reflective grating.

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