RU2776279C1 - Subminiature quantum frequency standard and the method for arranging its components - Google Patents

Abstract

FIELD: electrical harmonic signal creation.

SUBSTANCE: inventions relate to means for creating an electrical harmonic signal with a given highly stable frequency. To achieve the effect, a quantum frequency standard is proposed, containing a housing, a quantum discriminator, a quartz oscillator and an electronic control system. The thermal stabilization and magnetic shielding system of a glass absorbing cell with Cs alkali metal vapors includes a pair of rectangular magnetic shields arranged in such a way that the inner magnetic shield surrounds the cell, and the outer magnetic shield surrounds the inner magnetic shield, the outer shield being airtight, and the inner shield having through holes, the axis of which coincides with the direction of propagation of laser radiation, is perpendicular to the larger plane of the rectangular screen, and the process of the cell is oriented in the direction of the long edge of the screen rectangle. The hole in the inner magnetic screen on the side of the laser radiation is provided with a window. The internal magnetic screen is provided with means for heating and thermal stabilization of the absorbing cell. Bias coils are also installed inside the inner screen.

EFFECT: reducing the size of the device and its power consumption.

5 cl, 10 dwg

Description

The inventions (device and method) relate to quantum frequency standards based on the effect of coherent population trapping (QFC-CPT), devices for generating harmonic electrical signals with high frequency stability, and can be used in electronic devices as a master oscillator in the megahertz range.

A subminiature quantum frequency standard based on CPT is known, which combines low short-term and long-term frequency instability, as well as small size and low power consumption (http://conf.laplas.mephi.ru/wp-content/uploads/2019/11/ Palchikov_min_standards23.10.19.pdf).

However, the layout with the separation of the control electronics (3 boards) and the "frequency discriminator" (the "physical part" and 2 auxiliary electronic boards) as separate modules when an external magnetic shield is placed on the surface of the "physical part" (at a short distance from the surface internal magnetic screen), in which the control electronics are assembled in the form of a “shelf”, and the magnetic screens of the “frequency discriminator” are made on the basis of cylindrical shapes enclosed in rectangular structural elements - does not allow achieving higher compactness (reaching smaller dimensions).

In addition, a subminiature quantum frequency standard https://i.moscow/patents/RU197054U1_20200326 is known, which is the prototype of the proposed inventions.

However, the layout with the separation of the electronic control system and the "quantum discriminator" ("physical part") in the form of separate blocks when the external magnetic shield is placed on the surface of the "physical part" (at a short distance from the surface of the internal magnetic shield), in which the electronic control system assembled in the form of a separate assembly unit, and the magnetic screens of the "frequency discriminator" are made on the basis of cylindrical shapes - it does not allow to achieve a higher compactness (to achieve smaller dimensions).

The objective of the proposed inventions is to eliminate the shortcomings of the prototype.

The technical result obtained from the implementation of the inventions is the creation of a subminiature quantum frequency standard of a simplified design, which has small dimensions, low power consumption and increased strength and rigidity of the structure while maintaining high stability of the 10 MHz signal at the output of the device.

The set technical result is achieved by the fact that a subminiature quantum frequency standard is proposed based on the effect of coherent population trapping (Fig. 1 - Fig. 6), containing a quantum discriminator (1), a quartz oscillator (2) and an electronic control system (3), while The quantum discriminator contains a diode laser (4) with a vertical resonator arranged in series in the optical scheme, a phase quarter-wave plate (5) for obtaining a circularly polarized laser field, a glass absorbing cell (6) with alkali metal vapor, equipped with a thermal stabilization and magnetic shielding system, including, at least a pair of magnetic screens located around the cell of the quantum discriminator in such a way that the inner magnetic screen (7) surrounds the cell, and the outer magnetic screen (8) surrounds the inner magnetic screen, and said magnetic screens are located with a gap between them, the outer magnetic screen filled with inert gas, and in the inner screen there are through holes for input into the cell and output from it of laser radiation, the axis (9) of which passes through the absorbing cell, while the inner magnetic screen is equipped with means for heating (10) and thermal stabilization (11) of the absorbing cell, inside In the inner screen, two bias coils (12) are also installed to create a uniform magnetic field in the area of the glass cell.

At the same time, the external magnetic screen (8) is at the same time a hermetically sealed housing of the standard, which reduces the dimensions and simplifies the design.

Xe was chosen as the gas filling the outer screen, which reduces power consumption both due to the low thermal conductivity of the Xe gas and due to the relatively large distances between the outer surface of the inner screen and the inner surface of the outer screen, compared with analogues and the prototype.

Cell (6) is made with a process.

The internal magnetic shield (7) has a rectangular shape, which makes it possible to simplify the design by eliminating the use of an electronic device for cooling the cell offshoot (6) and making it possible to use the natural heat sink from the offshoot of the cell.

Holes for input and output of laser radiation from the cell are made on the rectangular surfaces of the inner screen, which allows laser radiation to pass through the cell in the direction of the short side of the screen, and the cell extension to be oriented in the direction of the long side of the screen, resulting in a more compact layout.

On the surface of the inner screen facing the laser radiation, a window is placed, which is a package of a quarter-wave plate (5) and a neutral filter (13), while adding a neutral filter to the package reduces the laser radiation power in the cell, which reduces the field frequency shift and increases the stability of the standard.

A package of a quarter-wave plate and a neutral filter is supplemented with a wedge-shaped insert (14) located between the package and the surface of the inner screen, which ensures the inclination of the plate and filter, which reduces the possible negative effect of the beam reflected perpendicularly on the stability of the laser diode.

The means of heating (10) and thermal stabilization (11) of the absorbing cell are located in a separate compartment (15), which complements the inner screen containing the absorbing cell, while the separate compartment is made of the same material (permalloy) as the inner screen, using the same technology (laser welding), which simplifies the design and improves the accuracy of temperature control by transferring the hottest point (heating transistor) from the outer surface to the inner partition between the compartments.

To prevent undesirable crystallization of atoms on the cell walls, a temperature gradient is implemented with the help of a metal heat sink element connecting the cell spur containing metal atoms with the element extending beyond the screen envelope.

Natural Cs was chosen as the alkali metal of the glass cell, which reduces the cost and simplifies the manufacturing technology of the cell.

The bias coils (12) are installed in the inner screen (7), and the coils are rectangular in shape, which makes it possible to give the coils the maximum possible area for the selected screen shape, provided that the center of the coils is aligned with the axis of the holes (9) for laser radiation and with the center of the spherical cell .

Next, we consider the possibility of replacing round bias coils with rectangular ones when the coils are placed in a rectangular cavity of the magnetic screen. The possibility of improving the uniformity of the magnetic field is shown.

It is known that Helmholtz rings (coils) are two coaxially located identical radial coils, the distance between the centers of which H is equal to their average radius R. There is a zone of a uniform magnetic field in the center of the system. One coil creates a magnetic field on the axis

where I is the current through the coil, in amperes, R is the radius of the coil, in meters, x is the distance along the axis of the coil from its plane, in meters, n is the number of turns, μ ₀ is the magnetic constant (4π × 10 ^-7 T × m /BUT)

Then, taking into account that the distance along the axis from the coil to the center is chosen equal to x=R/2, the field in the center of the coil

Let us now consider a pair of infinite straight parallel wires with currents equal but opposite in sign, perpendicular to some axis, and the distances from the wires to the axis (in opposite directions) are equal to R, and the distance along the axis from the plane in which the wires lie is equal to x. Then the magnetic field on the axis is

Consequently, the second pair of wires with current, located on a plane spaced from the first by a distance H along the considered axis, will also create a zone of a uniform magnetic field near the point x=H/2.

It seems interesting to compare the size of the zones with a given tolerance for the inhomogeneity of the value K (y) \u003d B (y) / B (0), considering the magnetic field as a function of y \u003d (x-H / 2) / H, choosing first H \u003d R, and then considering other relationships.

On FIG. 7 it can be seen that the inhomogeneity of the magnetic field at H=R for a system of straight wires is greater than for Helmholtz coils, and near y=0.3 the ratio of inhomogeneities is maximum and equal to ≈2.

Therefore, for a system of straight wires (or equivalent long rectangular coils), the condition H=R is not optimal. On FIG. 8 shows that the inhomogeneity of the magnetic field at H = 1.1547? for a system of straight wires, it is noticeably less than for Helmholtz coils at H=R, and at y=0.5, the ratio of field inhomogeneities is approximately equal to 0.5. Thus, with a given dimension 2R available for conductors and a smaller distance between the coils R (for Helmholtz coils), for a system of straight wires, the optimal distance between the planes of placement of wires in the direction of the axis is H = 1.154/?.

Therefore, when placing the magnetic system in a rectangular screen with an internal cavity height of, for example, 9 mm, and choosing a system of straight wires with a height distance of 9 mm between them and a distance between the planes of pairs of wires of 5.19 mm, the field inhomogeneity at a distance from the center is ≈2 mm will be ≈1.8%. In this case, for Helmholtz coils with a radius of 4.5 mm and a distance between the coils of 4.5 mm, at the same distance of 2 mm from the center, the inhomogeneity reaches 3.6%.

In the case when the best uniformity of the field is required not exactly at the center, but in some region near the center, then other conditions become optimal. So, for example, for the cavity considered above 9 mm high, which makes it possible to place coils with a diameter of 9 mm or pairs of wires with the same distance between them, consider a zone of ±3 mm near the center along the axis. In this case, the relative deviation reaches 0.67.

On FIG. 9 it can be seen that the inhomogeneity of the magnetic field at H=1.32/R for a system of straight wires is less than for Helmholtz coils at H=1.19/R, and at y<0.7 the ratio of field inhomogeneities is approximately equal to 0.5.

Assuming that the inner volume of the absorbing cell can have a maximum diameter of 5 mm (with an outer cell diameter of 6 mm), one can try to achieve the best field uniformity in the area of ±2.5 mm from the center, which corresponds to a relative deviation y<0.56.

On FIG. 10 shows that the inhomogeneity of the magnetic field at H=1.29/R for a system of straight wires is less than for Helmholtz coils at H=1.14/R.

The relations considered above take place in the absence of the influence of an external magnetic screen. Of course, a pair of long wires can be replaced without much influence on the results by a highly elongated rectangular coil, or by a coil that is less elongated, closer in shape to a square.

In one of the latest experimental designs, square coils 9x9 mm in size were used with a distance between them of 7 mm. This distance roughly corresponds to the optimal value of 5.8 mm, taking into account the width of the coils ≈1.0 mm and the structural gap in the cavity ≈0.2 mm.

The effect of a magnetic shield can only be accurately estimated by numerical simulation in a specially designed program.

CONCLUSIONS. A pair of rectangular coils within the limited volume of a rectangular shield is capable of producing a more uniform magnetic field than a pair of round coils, if the spacing between the coils is properly chosen.

The invention is illustrated by drawings:

On FIG. 1 shows the general scheme of the claimed device,

On FIG. 2 shows the appearance of a subminiature quantum frequency standard.

On FIG. 3 shows a view of a subminiature quantum frequency standard without a case cover, without radio components on electronic boards and without a controller board installed (on top). An internal magnetic screen with a temperature control circuit is installed (on nichrome threads welded to the screen and soldered into the side boards).

On FIG. 4 shows the bottom and cover of the subminiature quantum frequency standard.

On FIG. 5 is a view of the internal magnetic shield. One of the screen walls is shown as transparent, the suspension threads with stop knots (“oyster knot”) in the holes of the screen suspension are not shown, the internal filling of the screen with a compound (silicone) is not shown. The internal rectangular magnetic screen is supplemented with a rectangular compartment for accommodating the means of heating and thermal stabilization of the absorbing cell.

On FIG. 6 shows some of the parts included in the internal magnetic screen: cells glued into auxiliary brackets; bias coils glued to the brackets; a heat sink tube glued to the cell outgrowth; variant of the cooler, put on the heat sink tube

On FIG. 7-10 are graphs illustrating text justifying the selection of rectangular bias coils. On the general scheme of the claimed device (Fig. 1), where:

1 - a group of structural elements, traditionally referred to as a "quantum frequency discriminator", consisting of a "physical part" (from a cell in a thermostat with magnetic shielding and temperature control), from a radiation source (laser), its receiver (photodiode) and from auxiliary optical elements (a package of a quarter-wave plate and a neutral light filter, supplemented by a wedge-shaped insert);

2 - voltage controlled crystal oscillator (VCO);

3 - electronic control system;

4 - diode laser with a vertical resonator;

5 - phase quarter-wave plate;

6 - glass absorbing cell with alkali metal vapor;

7 - internal magnetic screen (surrounds the cell);

8 - external magnetic shield (surrounds the internal magnetic shield);

9 - axis of through holes for input into the cell and output from it of laser radiation (passes through the absorbing cell);

10 - means for heating the absorbing cell;

11 - means of thermal stabilization of the absorbing cell;

12 - bias coils (to create a uniform magnetic field in the area of the glass cell);

13 - neutral light filter;

14 - wedge-shaped insert;

15 - a separate compartment for the means of heating and thermal stabilization of the absorbing cell;

16 - photodiode;

17 - generator of ultrahigh frequency (SHF) controlled by voltage;

18 - heater (Peltier element) with a laser diode temperature sensor (thermistor) (built into the laser diode housing);

19 - laser temperature control circuits;

20 - control circuits for cell temperature and coil bias current;

21 - laser current modulation signal (to modulate its wavelength);

22 - frequency modulation signal of the microwave generator (for capturing the CPT resonance frequency by a quantum frequency discriminator);

23 - signal modulation of the laser current by microwave frequency (for splitting the spectral line of the laser, providing observation of the CPT resonance);

24 - control signals of the phase-locked loop (PLL) microcircuit of the microwave generator (relative to the frequency of the VCO crystal oscillator);

25 - signal of a quartz oscillator (to ensure the operation of the PLL circuit of the microwave oscillator);

26 - crystal oscillator signal (for clocking the microcontroller and for generating the output signal of the quantum frequency standard, directly, or after dividing the frequency of 10 MHz by 2, up to a frequency of 5 MHz);

27 - output signal of the photodiode;

28 - lens (possible, optional, element in the optical scheme).

The device works as follows. Electronic control system:

1) converts the input supply voltage of the quantum frequency standard (for example, +5 V) into stabilized supply voltages for the control system elements (for example, +3 V);

2) generates quantum standard frequency output signals (for example, meanders with frequencies of 10 MHz and 5 MHz and a sinusoidal signal of one of these frequencies);

3) generates (if necessary) pulse time signals (for example, with a frequency of 1 Hz, or 100 Hz, or with a different frequency);

4) carries out (if necessary) information exchange with the user of the quantum standard via the RS-232 interface;

5) connects to the microcontroller via the SWD protocol, allowing you to implement debugging modes and update the microcontroller program;

6) receives (if necessary) external clock pulses (for example, GNSS pulses with a frequency of 1 Hz) to implement the standard discipline mode;

7) generates and receives signals from the laser temperature control circuit;

8) generates and receives signals from the cell temperature control circuit;

9) generates a laser current modulation signal (to modulate its wavelength);

10) generates a frequency modulation signal of the microwave generator (for capturing the CPT resonance frequency by a quantum frequency discriminator);

11) generates a signal for modulating the laser current with a microwave frequency (for splitting the spectral line of the laser, which ensures the observation of the CPT resonance);

12) receives, amplifies, digitizes and processes the output signal of the photodiode (16);

13) converts the frequency of the input clock signal from the quartz oscillator (2) into the clock frequencies necessary for the operation of the microcontroller;

14) performs interrogation of additional sensors, namely, the temperature inside the standard case, as well as 3 components of the magnetic field vector inside the standard case that shields the magnetic field, acting on the internal magnetic screen of the cell, calculation of corrections to the output frequency of the standard and their use in controlling the output standard frequency

In addition, a method is proposed for arranging the components of a subminiature quantum frequency standard, in which the boards of the crystal oscillator, laser, photodetector, electronic control system and auxiliary boards are assembled and then connected by soldering into a rectangular structure, and the contacts located along the perimeter of the boards serve both for transmission between the boards of electrical signals, and as a mechanical (soldered) connection, which allows to reduce the dimensions, as well as the power consumption of the device by increasing the distance from the outer surface of the inner screen to the inner surface of the outer screen, as well as creating an impenetrable partition between the screens, which reduces the transfer of heat behind account of turbulent gas flows.

Inside a rectangular structure formed by printed circuit boards, an internal magnetic screen with an absorbing cell, with its additional compartment, is placed, and it is attached to the auxiliary circuit boards of the structure with metal wires or synthetic threads.

The electric circuits of the bias coils and the means of heating and thermal stabilization are led to the compartment for means of heating and thermal stabilization of the absorbing cell by a flexible flat cable with low thermal conductivity. The rectangular structure of the boards is mechanically fixed to the bottom of the case, for example, by soldering.

The quantum frequency discriminator performs:

1) radiation by a laser diode of a given spectrum at the required wavelength;

2) focusing (if necessary) of laser radiation;

3) attenuation of the radiation power to a given level;

4) conversion of radiation polarization (from laser-radiated linear polarization to circular polarization);

5) ensuring the passage of laser radiation through the quantum cell;

6) receiving radiation passed through the cell and converting it into an electrical signal;

7) laser diode temperature control;

8) control of the quantum cell temperature;

9) control of the magnetic field in the cell area by controlling the bias current of the coils;

10) control of the operating temperature of the laser diode using Peltier elements and thermistor built into the laser housing.

The quantum frequency standard modulates the laser current with a low-frequency signal that modulates the operating wavelength near the observed absorption peak of the working alkali metal of the cell), after which it demodulates (synchronous detection) of the radiation received by the photodiode that has passed through the cell. As a result, a wavelength discriminator signal is generated, after software processing of which a control signal for the operating wavelength (its adjustment) is generated by changing the operating current of the laser and (or) changing its operating temperature.

The quantum frequency standard also modulates the frequency of the microwave generator (supplied to the laser and splitting its spectral line into a pair of working components) with a low-frequency signal that modulates the operating wavelength difference (radiation frequency difference) near the observed microwave transition (CPT resonance) of the working alkali metal of the cell, after which performs demodulation (synchronous detection) of the radiation received by the photodiode that has passed through the cell. As a result, a frequency discriminator signal is generated, after software processing of which a signal is generated to control the operating output frequency of the standard (its adjustment) by changing the voltage supplied to the crystal oscillator and controlling its frequency.

The quantum frequency standard performs the operation of systems for automatic tuning of working control parameters using special digital control algorithms, including those implementing the PID control algorithm (proportional, integral and differential terms) in the feedback loop during auto-tuning.

The device is implemented (performed) as follows.

The boards of the crystal oscillator, laser, photodetector, electronic control system and auxiliary boards are assembled and then soldered into a rectangular structure, and the contacts located along the perimeter of the boards serve both to transmit electrical signals between the boards and as a mechanical solder joint, the internal magnetic the screen is attached to the auxiliary boards of the structure with metal wires or synthetic threads, the electric circuits of the bias coils and the means of heating and thermal stabilization are led to the said compartment by a flexible flat cable with low thermal conductivity, the rectangular structure of the boards is mechanically fixed to the bottom of the case, for example, by soldering. The strength of a brazed rectangular structure made of printed circuit boards was evaluated experimentally on a tensile testing machine. The evaluation showed that the solder joints between the boards are able to withstand a load corresponding to an acceleration of about 14000g. At the same time, the suspension of the internal magnetic shield with an additional section for heating and temperature control on Kevlar threads is also, according to calculations, capable of withstanding high accelerations. The absorbing glass cell is located in a rigid rectangular screen and filled with a silicone compound, which ensures its mechanical unloading at high mechanical accelerations. A critical part, sensitive to accelerations and capable of irreversible failure, is a quartz oscillator, or rather, a quartz crystal suspension system in a quartz resonator. In this case, it is possible to replace the crystal oscillator with an oscillator resistant to high accelerations and strong vibrations, for example, based on a microwave oscillator controlled by voltage based on surface waves and its frequency divider up to the nominal frequency of the crystal oscillator, for example, up to a frequency of 10 MHz.

Claims (5)

1. A subminiature quantum frequency standard based on the effect of coherent population trapping, containing a quantum discriminator, a quartz oscillator and an electronic control system, while the quantum discriminator contains a diode laser with a vertical resonator arranged in series in the optical circuit, a quarter-wave phase plate for obtaining a circularly polarized laser field, a glass absorbing cell with alkali metal vapor, equipped with a magnetic shielding system, including at least a pair of magnetic shields located around the cell of the quantum discriminator in such a way that the inner magnetic shield surrounds the cell, and the outer magnetic shield surrounds the inner magnetic shield, and the named magnetic screens are located with a gap between them, the outer magnetic screen is filled with an inert gas, and in the inner screen there are through holes for input into the cell and output from it of laser radiation, the axis of which is It passes through the absorbing cell, while the inner magnetic screen is equipped with means for heating and thermal stabilization of the absorbing cell, while magnetizing coils are also installed inside the inner screen to create a uniform magnetic field in the area of the glass cell, characterized in that the outer magnetic screen is simultaneously implemented as a sealed housing standard, Xe is chosen as the gas filling the outer screen, the inner magnetic screen has a rectangular shape, while on its two long surfaces there are holes for input into the cell and output from it of laser radiation, and the cell is made with a process oriented in the direction of the long edge of the rectangle screen, on the surface of the inner screen facing the laser radiation, in front of the hole for introducing radiation, a window is placed, which is a package of a quarter-wave plate and a neutral light filter, supplemented by a wedge-shaped insert located between the package and the surface of the inner screen, the means of heating and thermal stabilization of the absorbing cell are located in a separate compartment adjacent directly to the inner magnetic screen, and the heater is located on the partition between them. 2. The frequency standard according to claim 1, in which a temperature gradient is implemented using a metal heat sink element, characterized in that the heat sink element connects the offshoot of the cell containing metal atoms with a plate extending beyond the screen. 3. The frequency standard according to claim 1, characterized in that natural Cs is selected as the alkali metal of the glass cell. 4. The frequency standard according to claim 1, characterized in that the bias coils installed in the inner screen have a rectangular shape. 5. The method of arranging the components of the subminiature quantum frequency standard according to claim 1, characterized in that the boards of the quartz oscillator, laser, photodetector, electronic control system and auxiliary boards are assembled and then connected by soldering into a rectangular structure, and the contacts located along the perimeter of the boards serve both for transferring electrical signals between the boards and as a mechanical solder joint, the internal magnetic shield is attached to the auxiliary boards of the structure using metal wires or synthetic threads, the electrical circuits of the bias coils and the means of heating and thermal stabilization are brought to the compartment for heating and thermal stabilization absorbing cell with a flexible flat cable with low thermal conductivity, a rectangular board structure is mechanically attached to the bottom of the housing.

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