CN103454902B - A kind of atomic clock - Google Patents

Abstract

The invention discloses an atomic clock, which is composed of an electronic system and a physical package, and the physical package includes a laser, a conversion optical path, a 1/4 wave plate, an alkaline atomic bubble gas chamber and a photoelectric detector, and is characterized in that: the The conversion optical path includes a four-port fiber coupler, four optical fibers connected to the fiber coupler, a self-focusing lens and an AR coating, the output end of the laser is connected to the first optical fiber, the second optical fiber is connected to the self-focusing lens, and the third optical fiber is connected to the photoelectric Detector; the anti-reflection film is located on the exit side of the basic atomic bubble gas chamber, and reflects the outgoing light back to the basic atomic bubble gas chamber. The invention can keep the laser and the photodetector away from the alkaline atomic bubble chamber, reduce the influence of temperature and magnetic field on it, and improve the stability of the atomic clock.

Description

an atomic clock

technical field

The invention relates to an atomic clock and belongs to the field of frequency standard devices.

Background technique

The history of the development of atomic clocks can be traced back to around the Second World War, which mainly benefited from the rapid development of quantum mechanics and microwave spectroscopy at that time. Early microwave clocks used incoherent light sources for pumping and probing light. Later, with the development of lasers, laser state selection and detection methods were applied to the research of atomic clocks, and better results were obtained.

With the rapid development of electronic technology and control technology, research on atomic clocks mainly focuses on two aspects: one is to explore and develop atomic clocks with higher accuracy and stability. In recent years, many different types of new atomic clocks with higher accuracy and stability have been successfully developed, such as cold atomic fountain clocks, ion trap clocks, optical clocks, etc.; on the other hand, we are actively looking for small-scale engineering atomic clocks that achieve high precision The way to meet the development needs of various engineering technologies, such as the development of small space-borne atomic clocks, and the development of miniaturized coherent population trapping (CPT, Coherent Population Trapping) atomic clocks using the principle of coherent population trapping.

Coherent population pinning is a quantum interference phenomenon produced by the interaction between atoms and coherent light. Using the good coherence characteristics of lasers to prepare coherent population pinning states in atomic systems, a new chip-type CPT atomic clock can be realized. This is The current cutting-edge technology in the field of atomic clocks and navigation. Its advantages are: on the one hand, no microwave cavity is required, and the volume can be significantly reduced; on the other hand, the coherent dichromatic light can be prepared by using a laser modulated by microwave frequency, which can reduce the optical frequency shift. Although the CPT atomic clock has not been proposed since 1998, it has developed rapidly and has shown superior performance, and there is still a lot of room for improvement.

In the prior art, a CPT atomic clock includes an electronic system and a physical package, wherein the physical package is usually composed of a laser, a conversion optical path, a 1/4 wave plate, an alkaline atomic bubble chamber and a photodetector, and the laser light emitted by the laser is converted The optical path is converted into parallel light, which is circularly polarized by a 1/4 wave plate and then enters the basic atomic bubble cell, and is finally received by the photoelectric detector. Among them, the conversion optical path is usually a lens group.

The performance index for evaluating atomic clocks is mainly Allan variance. According to the definition of Allan variance, it can be seen that the performance of an atomic clock mainly depends on the contrast of absorption lines.

In practice, the general CPT desktop experiment system only pursues the aspect of commissioning, and does not consider the issue of size and power consumption, while the miniature or even chip-level CPT atomic clock does pay attention to the reduction of size and power consumption, but it is not convenient for commissioning. The indicators of CPT atomic clocks realized so far are not very high, mainly due to serious temperature drift, high power consumption caused by temperature control, and low stability of atomic clock output signals. The main reason is that the above-mentioned parts are packaged together in the prior art, and the distance between the laser and the photodetector and the alkaline atomic bubble chamber is usually kept at a working temperature of 70-90°C. Closer, will be affected by its temperature,

Therefore, it is necessary to improve the structure to reduce the temperature drift phenomenon, thereby reducing the error of the atomic clock.

Contents of the invention

The object of the present invention is to provide an atomic clock, which reduces the influence of temperature on the atomic clock through structural improvement.

In order to achieve the above-mentioned purpose of the invention, the technical solution adopted in the present invention is: an atomic clock, which is composed of an electronic system and a physical package, and the physical package includes a laser, a conversion optical path, a 1/4 wave plate, an alkaline atomic bubble air chamber and Photodetector; the conversion optical path includes a four-port fiber coupler, four optical fibers connected to the fiber coupler, a self-focusing lens and an antireflection film, the output end of the laser is connected to the first optical fiber, and the second optical fiber is connected to the self-focusing lens , the third optical fiber is connected to the photodetector; the antireflection film is located on the exit side of the basic atomic bubble gas chamber, and reflects the outgoing light back to the basic atomic bubble gas chamber.

In the above technical solution, the laser uses a direct bandgap semiconductor material as an optical gain medium, injects carriers through a pn junction to achieve particle population inversion, and uses a Fabry-Perot cavity or a distributed Bragg grating as a resonant cavity to perform Stimulated emission light amplified diode laser.

The four-port fiber coupler is a 2×2 single-mode fiber coupler.

The self-focusing lens is a cylindrical lens whose internal refractive index distribution gradually decreases along the radial direction.

The thickness of the antireflection film is equal to a quarter of the laser wavelength.

In a further technical solution, a second photodetector is provided, and the second photodetector is connected to the fourth optical fiber.

The preferred technical solution is that the basic atomic bubble chamber is arranged in different independent packages from the laser and the photodetector.

In the above technical solution, the electronic system includes a phase-locked loop, an electronically adjustable attenuator, a micro-control system and a digital-to-analog conversion chip to control the wavelength and frequency of the laser.

Taking the cesium cavity as an example for the basic atomic bubble gas chamber, the working principle of the present invention is explained as follows: the laser emits laser light, and modulates the 4.596 GHz microwave signal generated by the phase-locked loop to the left and right sidebands of the laser light, and the laser light is irradiated on the cesium On the cavity; a 2×2 fiber coupler is located between the laser and the cesium cavity, and four optical fibers are connected to it. The second optical fiber is connected to the self-focusing lens, and a quarter-wave plate is placed behind the self-focusing lens to convert the laser beam into circularly polarized light and irradiate it into the cesium cavity; the cesium cavity is connected with an anti-reflection film, and the optical path passes through the anti-reflection film reflection, enters the fiber coupler again, and is output by the third optical fiber; a photodetector is connected to the output port, and the photodetector converts the optical signal into a current signal, which is extracted and processed by the micro-control system, and the micro-control system will further generate control signals. Until the whole atomic clock system is locked.

In a further technical solution, a second photodetector is connected to the fourth fiber port of the fiber coupler to detect the incident light intensity of the laser.

In the above scheme, the laser modulation method adopts half-bandwidth modulation, that is, the RF modulation frequency is equal to half of the difference between the two ground state energy levels, and the difference between the left and right first sideband frequencies is just equal to the ground state hyperfine energy level splitting difference. The CPT resonance is realized by excitation of the first-order sideband pump. This modulation method is defined as half-width modulation.

Due to the use of the above-mentioned technical solutions, the present invention has the following advantages compared with the prior art:

1. The present invention is provided with a four-port optical fiber coupler and an anti-reflection film, so that the laser and the photodetector are connected to the fiber coupler respectively, so that the laser and the photodetector can be kept away from the alkaline atomic bubble chamber, reducing the temperature and The influence of the magnetic field on it.

2. Since the conversion optical path is composed of optical fiber and optical fiber coupler, the loss of light can be reduced.

3. The setting of the anti-reflection film, on the one hand, enables the photodetector to be connected to the fiber coupler together with the laser, and on the other hand, by reflecting the outgoing light into the alkaline atomic bubble gas chamber again, the optical path can be increased, so that the light follows the The action time of the cesium atom or the rubidium atom becomes longer, which improves the stability of the atomic clock.

4. The setting of the second photodetector can detect the incident light intensity of the laser, so as to observe the change of the light intensity of the laser in time.

Description of drawings

Fig. 1 is a block diagram of the overall system of the atomic clock in the embodiment.

Fig. 2 is a block diagram of the PLL system in the embodiment.

Fig. 3 is an optical path diagram of a laser collimator composed of a self-focusing lens and an optical fiber in the embodiment.

Fig. 4 is a diagram of the atomic energy level transition of the cesium/rubidium cavity in the physical package of the atomic clock.

Fig. 5 is a circuit diagram of the loop filter in the embodiment.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing and embodiment:

Example 1:

As shown in Figure 1, an atomic clock with a self-focusing lens mainly includes a laser 170, a fiber coupler 120 and four optical fibers 121, 122, 123, 124 connected thereto, a self-focusing lens 130, a quarter-wave Chip 140, cesium cavity 150, AR film 160, photodetector 110 and second photodetector 180, phase-locked loop 200, electronically adjustable attenuator 300, micro-control system 500, digital-to-analog converter 400 and 600. Wherein the laser 170, the fiber coupler 120 and four optical fibers 121, 122, 123, 124 connected thereto, the self-focusing lens 130, the quarter wave plate 140, the cesium cavity 150, the antireflection film 160, the photodetector 110 and the second photodetector 180 constitute the physical package 100 of the atomic clock of the present invention.

The working principle of the whole system: the micro-control system 500 first initializes the phase-locked loop 200, so that the phase-locked loop is locked to the frequency of the atomic hyperfine energy level transition (half-bandwidth modulation is used here, and the phase-locked loop The generated frequency is 4.596GHz), and the hyperfine energy level difference of cesium atoms is 9.2GHz. The micro-control system 500 controls the microwave power entering the physical package 100 by controlling the current entering the electronically adjustable attenuator 300, so that the CPT resonance peak reaches the maximum. The most direct way to determine whether the resonance peak reaches the maximum is to extract the current signal fed back by the photodetector inside the physical package, and feed back the error signal to the micro-control system 500 after passing through the peripheral lock-in amplifier circuit. At this time, the micro-control system 500 is locked The phase loop 200 is programmed and adjusted, and at the same time, the current of the laser is adjusted until the CPT resonance peak of the system reaches the maximum, so that the entire system is locked.

As shown in FIG. 2 , the phase-locked loop 200 involved in the present invention mainly includes a temperature-compensated crystal oscillator 210 , a frequency synthesizer 220 , a third-order loop filter 230 , and a voltage-controlled oscillator 240 . The main working mode is as follows: the temperature-compensated crystal oscillator 210 provides a reference frequency of 10 MHz, the initialized frequency synthesizer 220 is a fractional frequency division multiplier, and performs frequency and phase discrimination based on the frequency after the frequency division of the feedback signal of the voltage-controlled oscillator 240 and the reference frequency The error current is generated, and then the error signal is output to the voltage-controlled oscillator through the charge pump until the phase-locked loop system locks to the required frequency (4.596GHz). Among them, the loop filter 230 is particularly important because it will affect the phase noise of the radio frequency output, so the design of the loop filter 230 is very important, and the specific circuit design is shown in FIG. 5 .

As shown in FIG. 3 , the collimator involved in the present invention mainly includes an optical fiber 300 and a self-focusing lens 310 . The function of the self-focusing lens 310 is to obtain the required parallel light beam, and at the same time, recombine the light reflected from the AR coating 160 into the fiber coupler 120 and output it to the photodetector 110 from the fiber 122 .

As shown in Figure 4, under the action of a weak magnetic field, the energy levels of cesium atoms split into hyperfine energy levels. For cesium, the energy level difference is 9.2 GHz. The present invention relates to half-wave modulation, ie the frequency generated by the peripheral radio frequency loop is equal to 4.596 GHz. After modulation, the laser will generate two sidebands, and the frequency difference between the two sidebands is exactly 9.2GHz. At this time, the cesium atoms will be trapped in the ultrafine energy level, and will no longer absorb light. The light intensity of the device will peak. For the rubidium element, only the energy level difference of the hyperfine energy level is different, and the energy level difference of rubidium is 6.8GHz.

Claims (7)

1. an atomic clock, be made up of electronic system and physical package, described physical package comprises laser instrument, conversion light path, quarter wave plate, basic atom bubble air chamber and photodetector, it is characterized in that: four anti-films of optical fiber, GRIN Lens and increasing that described conversion light path comprises four fiber port coupling mechanisms, is connected with fiber coupler, the output terminal of described laser instrument connects the first optical fiber, second Fiber connection GRIN Lens, the 3rd Fiber connection photodetector; The anti-film of described increasing is positioned at the exiting side of basic atom bubble air chamber, emergent light is reflected back basic atom bubble air chamber.

2. atomic clock according to claim 1, it is characterized in that: described laser instrument is for optical gain medium with direct band-gap semicondictor material, inject charge carrier by pn knot and realize population inversion, with Fabry-Perot-type cavity or distribution bragg grating for resonator cavity, carry out the diode laser of stimulated emission light amplification.

3. atomic clock according to claim 1, is characterized in that: described four fiber port coupling mechanisms are 2 × 2 single-mode optical-fibre couplers.

4. atomic clock according to claim 1, is characterized in that: described GRIN Lens is that inner refractive index distributes the lens pillar radially reduced gradually.

5. atomic clock according to claim 1, is characterized in that: the thickness of the anti-film of described increasing equals 1/4th optical maser wavelengths.

6. atomic clock according to claim 1, is characterized in that: be provided with the second photodetector, and described second photodetector connects the 4th optical fiber.

7. the atomic clock according to claim 1 or 6, is characterized in that: described basic atom bubble air chamber is arranged in different individual packages from described laser instrument, photodetector.