阅读说明：本技术 光钟间歇运行驾驭微波钟产生高精度时间的方法 (Method for generating high-precision time by driving microwave clock by intermittent operation of light clock ) 是由 徐琴芳 常宏 于 2021-07-27 设计创作，主要内容包括：本发明提供了一种光钟间歇运行驾驭微波钟产生高精度时间的方法,包括光钟频率下转换及与微波钟的频差测定、频差模型建立、光钟驾驭及光频原子时输出三个步骤,通过间歇性运行光钟驾驭微波钟实现连续的高精度时间尺度输出。本发明与现有时间频率系统相比,其时间精度和稳定度更高,系统更加简单。本发明具有方法简单、实用、可靠性高等优点,可在光频标中推广使用。(The invention provides a method for generating high-precision time by driving a microwave clock by intermittently operating a light clock, which comprises three steps of down-conversion of the frequency of the light clock, frequency difference measurement with the microwave clock, frequency difference model establishment, driving of the light clock and output of light frequency atoms, and realizes continuous high-precision time scale output by intermittently operating the light clock to drive the microwave clock. Compared with the existing time frequency system, the time frequency system has higher time precision and stability and is simpler. The method has the advantages of simplicity, practicability, high reliability and the like, and can be popularized and used in the optical frequency standard.)

1. A method for driving a microwave clock to generate high-precision time by intermittently operating the light clock is characterized by comprising the following steps:

1) converting the frequency of the output signal of the optical clock to a microwave frequency band, and then measuring the frequency difference with the output signal of the microwave clock;

2) establishing a frequency difference model of a microwave clock relative to an optical clockWhere y (t) is the frequency difference of the microwave clock relative to the optical clock at time t, t_sIs the starting time of the model and is,is a constant term of the model, k is the slope of the frequency difference model,and k are both obtained by fitting, w (t) consists of various noises and measurement errors;

using N fractional frequency differences y based on the past time T_iThe values are subjected to linear fitting to obtain the frequency difference of the microwave clock relative to the optical clockWill t_sThe value is assigned as the last measuring time T in the window T time_NPredicting a future period of time t according to the above linear fit equation_PMSThe frequency difference of the internal microwave clock relative to the optical clock;

3) and (3) realizing control of the output signal of the free-running microwave clock through the phase microswitches, and inputting the frequency difference predicted in the step 2) into the phase microswitches as a frequency control quantity so as to correct the real-time physical signal output of the phase microswitches.

2. The method for generating high precision time for an optical clock intermittently operating a microwave clock as claimed in claim 1, wherein said optical clock output signal is frequency converted by a femtosecond optical frequency comb.

3. The method of claim 1, wherein said frequency difference between said optical clock and said microwave clock is measured over a period of time Δ_opMeasuring delta using a phase detector_opThe relative phase of the optical clock and the microwave clock at the ending and starting time are respectively recorded as phi_FAnd phi_IThen the optical clock and the microwave clock are compared for N times to obtain N fractional frequency differencesWherein f is_HMIs the frequency of a microwave clock, f_SrIs the frequency of the optical clock.

4. The method for generating high precision time by intermittently operating a microwave clock according to claim 1, wherein said step 2) of establishing a frequency difference model comprises the steps of:

step1, initialization parameters T, t_PMSAnd an initial linear fitting equation established according to the two frequency difference measurement models;

step2, judging whether new frequency difference measurement data are generated, if so, updating t_sAnd fitting the linear equation again according to the new data in the T, and entering Step 4; if not, entering Step 3;

step3, judging whether the measured data in the current time window T changes, if so, fitting a linear equation again according to the new data in T; if not, keeping the fitting equation unchanged;

step4, calculating future t according to a fitting equation_PMSPredicting the frequency difference in time according to the time length t_PMSTo move the time window T back to Step 2.

5. A method for generating high precision time for an intermittent clock in a microwave clock as recited in claim 1, wherein said accumulated phase offset between two clock runs is

Wherein σ_PIs to evaluate the statistical uncertainty, sigma, in the process of driving microwave frequency when the light clock runs_FIs the shot noise floor, sigma, of the Hadamard deviation of a microwave clock_PAnd σ_FThe contribution to the total phase error is expressed as ε_PAnd ε_F。

6. A method for generating high precision time by intermittently operating a microwave clock with a light clock as claimed in claim 1, wherein said window T is 30 days, the light clock is operated N-15 times, and each operation has a duration of 10⁴And second, driving the microwave clock to output the microwave clock with optical frequency atomic time.

Technical Field

The invention belongs to the technical field of optical frequency standards, and particularly relates to a method for generating high-precision time.

Background

The atomic clock uses the atomic resonance frequency standard as the time frequency reference, and uses the precise microwave or optical signal released when the electronic energy level is changed as the clock signal, which is the most accurate time measurement tool and frequency standard in the world. For time standards, a device that can accurately implement a defined unit is called a benchmark standard. By atomic-second definition, the atomic clock with high performance index can be used as the reference standard of time, namely the reference atomic clock.

The reference atomic clock is the highest time frequency standard with self-evaluation capability, and is the most accurate atomic clock for reproducing international atomic hour-second long units. The optical clock is a time frequency reference based on the optical band transition of an atom, takes the optical band transition of the atom as a clock frequency to carry out accurate measurement as an implementation means, and is the latest international generation reference atomic clock at present. Since the optical frequency is 5 orders of magnitude higher than the microwave frequency, using the optical transition frequency of atoms as a time frequency standard has a higher accuracy and stability than the microwave frequency standard. The accuracy and stability of the current atomic light clock are both in E-18 magnitude, which is two magnitudes higher than that of the current reference clock, namely a cesium microwave fountain clock. In view of the ultra-high performance of the optical clock, the optical clock has been a research hotspot of a high-precision time frequency reference and is expected to be very promising, on one hand, the optical clock is very likely to become a time reference of the next generation, and the second is newly defined; on the other hand, the light clock is used as a reference clock to drive the clock keeping group, multiple weighted averages are not needed, and the performance of the clock keeping group can be effectively improved. However, one challenge faced by this technology is that these experimental clocks have not been operated continuously for a long time, because they are a complex set of large scientific devices integrating light collection, electricity, vacuum, atomic system and automatic control, and their complexity makes it difficult to maintain the whole optical clock system in an optimal state, so that the optical clocks generally operate intermittently, and it is difficult to incorporate them into a conventional time keeping system. Therefore, how to generate and maintain a stable and continuous high-precision time scale by using the optical clock which runs intermittently is one of the key problems to be solved by the optical clock which becomes the next generation time reference and is the difficulty of the optical clock facing specific applications.

At present, a stable and reliable comprehensive time scale is obtained by utilizing a plurality of high-precision atomic clocks through a control scale algorithm, so that local time is controlled, and a control clock or a control clock group used for controlling is basically continuously operated. The time keeping clock group needs a plurality of hydrogen clocks and cesium clocks to keep and output time scales in a weighted average mode, the system is large and complex, and the accuracy can only be maintained at the E-15 level. The microwave watch clock (hydrogen clock or cesium clock group) is driven by using the light clock as a reference clock, a mode of weighted averaging of a plurality of microwave watch clocks adopted at present is not needed, and the working mode of keeping time or serving as a system reference clock by adopting a plurality of microwave clock groups at present is completely expected to be replaced. Two or three microwave watch clocks are driven by one light clock, and even a single watch clock can ensure high-precision system time output. Therefore, the complexity of the system can be greatly reduced, and the performance of the clock keeping group is effectively improved.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a method for generating high-precision time by driving a microwave clock by intermittently operating an optical clock, which utilizes the intermittently operating optical clock to generate and maintain a stable and continuous high-precision time scale, improves the time precision, simplifies a time-frequency system, and replaces the existing working mode of using a plurality of microwave clock groups to keep time or serve as a system reference clock.

The technical scheme adopted by the invention for solving the technical problem comprises the following steps:

1) converting the frequency of the output signal of the optical clock to a microwave frequency band, and then measuring the frequency difference with the output signal of the microwave clock;

2) establishing a frequency difference model of a microwave clock relative to an optical clockWhere y (t) is the frequency difference of the microwave clock relative to the optical clock at time t, t_sIs the starting time of the model and is,is a constant term of the model, k is the slope of the frequency difference model,and k are both obtained by fitting, w (t) consists of various noises and measurement errors;

using N fractional frequency differences y based on the past time T_iThe values are subjected to linear fitting to obtain the frequency difference of the microwave clock relative to the optical clockWill t_sThe value is assigned as the last measuring time T in the window T time_NPredicting a future period of time t according to the above linear fit equation_PMSThe frequency difference of the internal microwave clock relative to the optical clock;

3) and (3) realizing control of the output signal of the free-running microwave clock through the phase microswitches, and inputting the frequency difference predicted in the step 2) into the phase microswitches as a frequency control quantity so as to correct the real-time physical signal output of the phase microswitches.

And the output signal of the optical clock is subjected to frequency conversion through a femtosecond optical frequency comb.

The frequency difference measurement time length of the optical clock and the microwave clock is delta_opMeasuring delta using a phase detector_opThe relative phase of the optical clock and the microwave clock at the ending and starting time are respectively recorded as phi_FAnd phi_IThen the optical clock and the microwave clock are compared for N times to obtain N fractional frequency differencesWherein f is_HMIs the frequency of a microwave clock, f_SrIs the frequency of the optical clock.

In the step 2), the step of establishing the frequency difference model comprises the following steps:

step1, initialization parameters T, t_PMSAnd an initial linear fitting equation established according to the two frequency difference measurement models;

step2, judging whether new frequency difference measurement data are generated, if so, updating t_sAnd fitting the linear equation again according to the new data in the T, and entering Step 4; if not, entering Step 3;

step3, judging whether the measured data in the current time window T changes, if so, fitting a linear equation again according to the new data in T; if not, keeping the fitting equation unchanged;

step4, calculating future t according to a fitting equation_PMSPredicting the frequency difference in time according to the time length t_PMSTo move the time window T back to Step 2.

The accumulated phase deviation between two running of the optical clock

Wherein σ_PIs to evaluate the statistical uncertainty, sigma, in the process of driving microwave frequency when the light clock runs_FIs a Hadamard deviation of a microwave clockNoise floor of shot noise, σ_PAnd σ_FThe contribution to the total phase error is expressed as ε_PAnd ε_F。

The window T is 30 days, the optical clock runs for N15 times, and the running time of each time is 10⁴And second, driving the microwave clock to output the microwave clock with optical frequency atomic time.

The invention has the beneficial effects that: the single light clock is utilized to drive the microwave clock, the frequency drift of the microwave clock can be accurately evaluated in a short time, and the linear frequency drift and random phase fluctuation of the microwave clock can be eliminated to a great extent. Because the stability and accuracy of the light clock are far superior to those of the microwave clock, the light clock can achieve the effect which is comparable to the continuous operation of the cesium fountain clock when the cesium fountain clock drives the hydrogen clock to watch. The light clock is operated intermittently, the light clock is not occupied, and the light clock can also be applied to other aspects. The method has the advantages of simplicity, practicability, high reliability and the like, and can be popularized and used in time scale output based on the optical clock.

Drawings

FIG. 1 is a flow chart of the present invention.

Fig. 2 is a flow chart of frequency offset model establishment according to embodiment 1 of the present invention.

Detailed Description

The present invention will be further described with reference to the following drawings and examples, which include, but are not limited to, the following examples.

The invention uses a microwave clock which operates continuously but has lower precision as a flywheel, a light clock with high stability and high accuracy as a reference clock, and the light clock operates intermittently to guide time scale, thereby realizing the continuous time scale output of the light clock control.

The invention comprises the following steps:

1. frequency down-conversion of optical clock and frequency difference measurement with microwave clock

The frequency signal output by the optical clock is an optical frequency band signal, and in order to measure the frequency difference with the microwave clock, the frequency needs to be down-converted by a femtosecond optical frequency comb, the optical clock signal in the optical frequency band is converted to a microwave frequency band, and then the frequency difference is measured and compared with the microwave clock signal. The down conversion of the optical frequency is to obtain a microwave signal by a harmonic wave and frequency division technology, and then to accurately measure the frequency difference between the frequency value and the frequency output by the microwave clock. The frequency difference measurement adopts a phase comparison frequency difference measurement method, and the phase comparison frequency difference measurement is converted into voltage output in a linear relation with the linear phase discriminator through the linear phase discriminator, so that the frequency difference of two frequency signals is calculated by adopting more accurate phase change difference to obtain a high-precision measurement result.

Suppose that the frequency difference measurement time length of the microwave clock and the optical clock is delta_opMeasuring delta using a phase detector_opThe relative phase of the optical clock and the microwave clock at the ending and starting time are respectively recorded as phi_FAnd phi_I. Then, the fractional frequency difference y of the microwave clock and the optical clock_i(i 1.., N) is

Wherein f is_HMIs the frequency of a microwave clock, f_SrIs the frequency of the optical clock. The frequency difference between the optical clock and the microwave clock can be indirectly obtained by measuring the relative phase difference between the two clocks.

2. Frequency difference model establishment

The frequency difference model of the microwave clock relative to the optical clock is

Where y (t) is the frequency difference of the microwave clock relative to the optical clock at time t, t_sIs the starting time of the model and is,is a constant term of the model, k is the slope of the frequency difference model,and k are both obtained by fitting, w (t) is composed of various noises and measurement errors.

Suppose that the optical clock operates N times within the time window T, i.e. the optical clock and the microwave clock advanceObtaining N fractional frequency differences y after N times of phase difference comparison_iThe value is obtained. At this time, N fractional frequency differences y obtained based on the past time T are used_iThe values are linearly fitted, and it is considered that within the time window T, w (T) changes little, i.e. the error influence on the linear fitting is small, w (T) is approximately zero, so that the frequency difference y (T) of the microwave clock relative to the optical clock is obtained:

in the formula t_sIs assigned as the last measurement instant within the time of the window T, i.e. T_N. The future period of time t can be predicted according to the linear fitting equation_PMSThe frequency difference of the microwave clock relative to the optical clock (i.e., the frequency compensation duration) is used to compensate the frequency of the microwave clock.

3. Frequency control and optical frequency atomic time output

The frequency control means that compensation quantity of a frequency standard is obtained through a certain algorithm, the free trend of the frequency standard is interfered, and the accuracy and the long-term stability of the frequency standard are improved on the premise that the short-term stability of the frequency standard is guaranteed to the greatest extent. The frequency control aims to improve the accuracy on the premise of not influencing the frequency stability. The light clock drives the microwave clock and realizes the control of the output signal of the free running microwave clock through the phase jump device. Frequency-driving implementation: and estimating the frequency offset value of the microwave relative to the optical clock through the established frequency difference model, and inputting the value into the phase micro-jump device as a frequency control quantity so as to correct the real-time physical signal output of the phase micro-jump device. The frequency of the microwave clock is controlled not by the microwave clock, the microwave clock runs freely and only frequency signals input to the phase microstepper from the microwave clock are controlled, and 1PPS and 5MHz/10MHz signals output after precise control are time frequency signals based on the light clock after control.

The light clock drives the microwave clock, the light clock is operated discontinuously, but how long the light clock is operated once and how long the light clock is operated each time is mainly determined by the stability of the microwave clock, and the higher the stability of the microwave clock can be reached and the shorter the required integration time is, the lower the time and frequency of the required light clock driving the microwave clock are. The uncertainty of the time scale is mainly due to the prediction error of the linear trend and the random phase of the microwave clock. It is possible to calculate which uncertainties are possible by taking into account the error of the least squares fit and the noise characteristics of the microwave clock. The phase offset that is estimated to cause accumulation between two optical clock runs is therefore:

where it is assumed that N +1 clock runs are evenly distributed over time T, σ_PIs to evaluate the statistical uncertainty, sigma, in the process of driving microwave frequency when the light clock runs_FIs the shot noise floor, sigma, of the Hadamard deviation of a microwave clock_PAnd σ_FThe contribution to the total phase error is expressed as ε_PAnd ε_F. Suppose σ_PAnd σ_FAre respectively 4X 10^-16And 3X 10^-16As can be seen from the simulation, the optical clock is operated 15 times in 30 days, and each time the operation time is 10⁴And second, the microwave clock is driven to output the optical frequency atomic time, so that the output optical frequency atomic time stability enters the E-17 order.

In the step2 of establishing the frequency difference model, the process of establishing the frequency difference model is shown in fig. 2 and comprises four steps.

Step1, initialization, including parameters T, T_PMSAnd an initial linear fitting equation established according to the two frequency difference measurement models.

Step2, judging whether new frequency difference measurement data are generated, if so, updating t_sAnd fitting the linear equation again according to the new data in the T, and entering Step 4; if not, go to Step 3.

Step3, judging whether the measured data in the current time window T changes, if so, fitting a linear equation again according to the new data in T; if not, the fitting equation is kept unchanged.

Step4, calculating future t according to a fitting equation_PMSThe predicted value of frequency deviation in time is used to compensate the frequency of microwave clockRate and in time length t_PMSTo move the time window T back to Step 2.

Taking the example of the strontium atom light clock for driving the hydrogen clock, the method for indirectly operating the light clock to drive the microwave clock to generate high-precision time is described as follows.

1. Measuring frequency difference between strontium atom optical clock and hydrogen clock

The output frequency of the hydrogen clock comprises frequency signals of 5MHz, 10MHz, 100MHz and the like, and after the strontium atomic optical clock is locked in a closed loop, an optical wave with a wavelength of 698nm, namely an optical frequency signal with a frequency of 429THz, is output, so that the optical frequency is down-converted to the microwave frequency by a femtosecond optical frequency comb with a repetition frequency of 250MHz, the down-converted microwave signal with the frequency of just 1GHz is derived through the fourth harmonic of the optical frequency comb, and the 1GHz microwave signal is divided to 100MHz through 10 frequency, namely, the strontium atomic optical clock is down-converted to the microwave frequency band to obtain the frequency signal with the same frequency as the hydrogen clock. The relative phase of the strontium atomic optical clock and the hydrogen clock is measured by using the phase discriminator, and the fractional frequency difference between the strontium atomic optical clock and the hydrogen clock can be indirectly obtained by substituting the relative phase value into the following formula.

Wherein Δ_opMeasuring the time duration, phi, for the frequency difference between a microwave clock and an optical clock_FAnd phi_IRespectively at the measuring time period delta_opThe relative phase of the optical clock and the microwave clock at the end and start times.

2. Frequency difference model based on windowed linear fitting method

The frequency difference model of the microwave clock relative to the optical clock is as follows:

wherein y (t) is the frequency difference of the microwave clock relative to the optical clock at time t;is a constant term of the model, and k is the slope of the frequency difference model, mainly composed of microThe frequency drift of the wave clock is introduced,and k are obtained by fitting; t is t_sIs the starting time of the model; w (t) is composed of various noises and measurement errors.

Assuming that the strontium atomic optical clock operates N times in the time window T, i.e. the strontium atomic optical clock and the hydrogen clock perform N times of phase difference comparison to obtain N fractional frequency differences y_iThe value is obtained. At this time, N fractional frequency differences y obtained based on the past time T are used_iThe values are subjected to linear fitting, and it is considered that within the time window T, w (T) changes little, namely, the error influence on the linear fitting is small, w (T) is approximate to zero, so that the frequency difference y (T) of the hydrogen clock relative to the strontium atom optical clock is obtained:

in the formula t_sIs assigned as the last measurement instant within the time of the window T, i.e. T_N. The future period of time t can be predicted according to the linear fitting equation_PMSThe frequency difference of the hydrogen clock relative to the strontium atom optical clock in the frequency compensation duration is used for compensating the frequency of the hydrogen clock.

A specific process for establishing the frequency offset model based on the windowed linear fitting method is shown in fig. 2.

Step1, initialization, including parameters T, T_PMSAnd an initial linear fitting equation established according to the two frequency difference measurement models.

Step2, judging whether new frequency difference measurement data are generated, if so, updating t_sAnd fitting the linear equation again according to the new data in the T, and entering Step 4; if not, go to Step 3.

Step3, judging whether the measured data in the current time window T changes, if so, fitting a linear equation again according to the new data in T; if not, the fitting equation is kept unchanged.

Step4, calculating future t according to a fitting equation_PMSThe predicted value of frequency deviation in time is used to compensate the frequency of hydrogen clockRate and in time length t_PMSTo move the time window T back to Step 2.

3. Frequency control and optical frequency atomic time 1PPS output

And estimating the frequency offset value of the hydrogen clock relative to the strontium atomic optical clock through the established frequency difference model, and inputting the value into the phase microstepper as a frequency control quantity so as to correct the real-time physical signal output of the phase microstepper. The frequency of the hydrogen clock is controlled not by the hydrogen clock, the hydrogen clock runs freely and only frequency signals input from the hydrogen clock to the phase jump device are controlled, and 1PPS and 5MHz/10MHz signals output after precise control are time frequency signals based on the strontium atom light clock, namely light frequency atomic time after control.