A ring cavity laser atomic clock

Abstract

The present invention relates to a ring cavity laser atomic clock, which is characterized by: comprising a ring traveling wave cavity, in which a semiconductor laser gain medium, a filter unit and a CPT unit are inserted, so that there are only two longitudinal mode oscillations with a frequency interval equal to the working atomic ground state hyperfine level transition frequency in the resonant cavity; the beat frequency of the two laser modes is detected and output to obtain a frequency reference signal, thereby realizing a laser atomic clock. The ring cavity laser atomic clock disclosed in this scheme uses the physical mechanism of the gain pulling effect in the process of establishing the laser field to lock the laser longitudinal mode correlation. It avoids the unstable factors caused by the detection and locking circuits in the traditional atomic clock technology, and is conducive to improving the performance of the atomic clock.

Description

Technical Field

[0001] The invention belongs to the intersecting field of atomic clocks, quantum electronics and laser technology, and in particular relates to a ring cavity laser atomic clock. Background Art

[0002] Atomic clocks are playing an increasingly important role in satellite positioning and navigation, deep space exploration, scientific research, timekeeping, and even in people's daily lives. Atomic clock technology has become a key basic technology that is related to the progress and development of world science and technology, and to the convenience and improvement of the daily life of mankind as a whole. Traditional atomic clock technology includes passive microwave clocks such as rubidium clocks and cesium clocks, active hydrogen clocks, atomic fountain clocks, and the recently developed rapidly optical frequency atomic clocks. Traditional commonly used atomic clocks are generally passive microwave clocks. The principle is to use a certain method to detect the transition frequency of the selected energy level of the selected atom, and use this as a reference to provide a standard signal output of the atomic clock. The requirements for the selected atoms and energy levels are that the lifetime of the transition energy level is as long as possible, so that the line width of the transition spectrum obtained is narrow and the uncertainty is as small as possible. Existing general passive atomic clocks use alkali metals as working atoms, typically using 87 Rb or 133 Cs atoms. 133 The ground state hyperfine level transition frequency of Cs is f = 9192631770 Hz as the definition of the second. Rubidium and cesium atoms have a Λ level system such as Figure 1 , usually the transition frequency between the very stable ground state hyperfine energy levels of atoms is selected as the frequency standard of atomic clocks. The working stability of passive microwave atomic clocks based on this working principle depends not only on the stability of the atomic energy level itself, but also on the detection of the atomic energy level and the locking of the output microwave frequency with the detected atomic transition frequency. The working stability and noise of the detection and locking circuits will also affect the working stability of such traditional atomic clock systems.

[0003] In order to prevent atomic clocks from being affected by electronic circuit problems and to improve the atomic clock and stability indicators, our research team proposed a laser atomic clock solution. This solution combines laser mode locking with the CPT principle to lock the atomic clock reference frequency by a physical mechanism, thereby avoiding the problem that the working parameters of traditional atomic clocks are affected by electronic circuits. This has opened up a new path for the future research and development of atomic clocks.

[0004] Another patent for laser atomic clocks, ZL201910634621.8, applied by this research team, proposed a solution to realize laser atomic clocks using standing wave cavities. However, one problem that cannot be avoided in standing wave cavities is the phase matching problem between the return light field and the forward transmission light field. If the optical path in the gas chamber is long and the gas chamber length is greater than the wavelength of the two longitudinal mode beat frequencies, the light field coherence will weaken the filtering effect of the CPT unit. If the gas chamber length is reduced for this purpose, the gas chamber operating temperature will be required to increase in order to ensure the number of working atoms, affecting the working stability of the entire system.

[0005] In view of the above problems, the present invention makes improvements. Summary of the invention

[0006] The present invention provides a ring cavity laser atomic clock, which solves the above problems existing in the use of the prior art.

[0007] The technical solution of the present invention is achieved in this way:

[0008] A ring cavity laser atomic clock is characterized by: comprising a ring traveling wave cavity, in which a semiconductor laser gain medium, a filter unit and a CPT unit are inserted, so that there are only two longitudinal mode oscillations in the resonant cavity with a frequency interval equal to the transition frequency of the ground state hyperfine energy level of the working atom; the beat frequency of the two laser modes is detected and output to obtain a frequency reference signal, thereby realizing a laser atomic clock.

[0009] As a preferred option, the annular traveling wave cavity is composed of three reflecting mirrors, or four or more reflecting mirrors, and an optical isolator is placed in the cavity so that the laser can only be transmitted in one direction, forming a traveling wave cavity.

[0010] The present invention discloses a ring cavity system for realizing a laser atomic clock. Figure 2 As shown: the annular traveling wave cavity is composed of three reflectors, and can also be designed as an annular cavity composed of more than three reflectors according to needs, for example Figure 3 The figure shows a ring resonator composed of four reflectors. Its advantage is that since the reflection angle of the output mirror is close to vertical, the cavity length adjustment and locking only need to adjust the position of the output mirror, avoiding the problem of two of the reflectors needing to be adjusted jointly due to the influence of the cavity length adjustment of the three-mirror ring cavity on the optical path.

[0011] The physical principle of the laser atomic clock realized by the ring cavity composed of three reflectors and four reflectors or more reflectors in the present invention is basically the same. Figure 1 and Figure 2 As shown, a semiconductor gain medium (LD in the figure) is inserted into the resonant cavity, which functions as a semiconductor laser amplifier, or the two end faces of the semiconductor laser are coated with an anti-reflection film. After the forward output of the fluorescence field, it passes through the filtering unit. The filtering unit can use a Faraday atomic filter or a coherent filter device group. The purpose is to select the Λ energy level with the working atom. Figure 1 The light field corresponding to the transition excitation light ω1 and ω2 shown in the figure, after the light field passes through the filter unit, its transmission spectrum is as follows Figure 4 shown.

[0012] As a preferred option, the semiconductor laser gain medium is collimated by a lens and outputted, and after being transmitted along the circular traveling wave cavity for one cycle, it is coupled back to the semiconductor laser gain medium through a lens.

[0013] As a preferred option, both end faces of the semiconductor gain medium are coated with anti-reflection films; a λ / 2 wave plate is placed before the light field is fed back to the gain medium, and its angle is adjusted to make the polarization plane of the feedback light consistent with the output light.

[0014] As a preferred option, the length of the ring resonator should be designed so that the longitudinal mode spacing of the resonator is a multiple of the ground state hyperfine level transition frequency of the working atom; that is, the ground state hyperfine level transition frequency of the atom is required to be n times the longitudinal mode spacing of the resonator, where n is a positive integer.

[0015] As a preferred option, the filtering unit adopts a Faraday atomic filter, or an interference filter group and system.

[0016] As a preferred option, the working atoms in the CPT unit atomic gas chamber are alkali metal atoms, and the alkali metal atoms include 87 Rb and 133 Cs, the air chamber is filled with buffer gas.

[0017] As a preferred option, piezoelectric ceramics are installed between the two reflectors and the bracket of the annular traveling wave cavity, and the two piezoelectric ceramics are linked to ensure the correctness and stability of the resonant cavity light field loop during the process of making slight adjustments to the cavity length.

[0018] As a preferred option, the two longitudinal mode light fields formed in the cavity are output through the output mirror, and the output mirror is appropriately selected with a higher transmittance to make the resonant cavity mode line width greater than the CPT transmission peak line width, so as to weaken the pulling effect of the resonant cavity on the longitudinal mode.

[0019] In summary, the technical solution proposed in the present invention is to solve the problems pointed out by the background technology. A ring traveling wave cavity system is proposed. Since the positive feedback light field is no longer transmitted in the reverse direction, the phase matching of the light field in the entire air chamber can be achieved by properly adjusting the length of the ring resonant cavity. The phase matching problem of the CPT system in the standing wave cavity is solved, the function and filtering effect of the CPT system are enhanced, and at the same time, the laser gain medium is changed from standing wave amplification to traveling wave amplification, which avoids the spatial hole burning effect of the standing wave itself in the gain medium, improves the gain competitiveness of the operating mode, and is conducive to better suppressing the oscillation of other longitudinal modes of the resonant cavity, which can make the laser atomic clock work more stably and further improve the performance.

[0020] From the above technical solutions, it can be seen that the light field enters the CPT unit after passing through the filter unit, and the transmission spectrum of the CPT unit is as follows Figure 5 As shown, there is a transmission peak formed by the CPT effect at the top of each of the two absorption peaks corresponding to ω1 and ω2. This is a very narrow transmission peak spectrum that can reach 50Hz or narrower. Figure 4 and Figure 5 The transmission and absorption cancel each other out, leaving two very narrow transmission peaks formed by the combined effect of filtering and CPT unit. Figure 6 As shown. The transmitted light field is filtered in series by the filter unit and the CPT unit, and only the light field with extremely narrow line width near ω1 and ω2 is allowed to pass through, and is reflected back to the other side of the semiconductor light amplification medium by the cavity mirror of the ring cavity, and the polarization plane is turned to be consistent with the LD by the 1 / 2λ wave plate installed in the cavity, and then coupled into the LD laser gain medium through the lens. At this time, the positive feedback light field is limited to Figure 6 Within the range shown. If the cavity length is adjusted at this time so that the light fields corresponding to ω1 and ω2 are exactly integer multiples of the wavelength after being transmitted along the ring cavity for one cycle, the light fields corresponding to the two transmission peaks will be continuously amplified to form laser oscillations, which just correspond to the two longitudinal modes of the ring cavity. The two oscillating laser modes tend to oscillate at the center frequencies corresponding to ω1 and ω2 due to the gain competition effect. The light field is amplified through multiple cycles in the ring resonator to establish two laser longitudinal mode oscillations. In fact, according to the resonant cavity theory, the resonant cavity has a series of longitudinal modes. At this time, let the frequency difference between ω1 and ω2 be equal to an integer multiple of the longitudinal mode interval. That is, let Δν = nΔv l , where the resonant cavity longitudinal mode spacing is Where C is the speed of light and L is the physical cavity length. The light field selected by the filter and CPT unit in series is consistent with the two modes of the laser longitudinal mode, while filtering out other modes, so that only the two modes corresponding to ω1 and ω2 oscillate in the cavity. According to the CPT effect, the frequency interval between the two modes is exactly equal to Figure 1 The transition frequency of the ground state hyperfine energy level of the working atom shown in the figure is the frequency signal used as the reference frequency by the traditional atomic clock. The reference frequency of the laser atomic clock can be directly obtained by detecting the beat frequency of the two laser output modes. Due to the addition of an isolator in the ring cavity, only one-way transmission is possible. Compared with the standing wave cavity, the problem of phase matching in reverse transmission is eliminated, so that the light field that meets the CPT excitation requirements in the entire gas chamber constructively interferes, further enhancing the coherent filtering effect of the CPT unit. At the same time, the CPT effect requires the coherence of the excitation light field, so that the two light fields tend to be coherently locked, and the interval between the oscillation frequencies of the two longitudinal modes is precisely equal to the transition frequency of the ground state hyperfine energy level of the working atom. Based on this mechanism, the application of a 3-cavity mirror or 4-cavity mirror ring cavity can better realize the laser atomic clock and improve the performance of the original laser atomic clock.

[0021] In summary, the beneficial effects of the present invention are:

[0022] This scheme discloses a ring cavity laser atomic clock, which uses the physical mechanism of gain pulling effect in the process of laser field establishment to lock the laser longitudinal mode correlation, avoiding the unstable factors caused by the detection and locking circuits in traditional atomic clock technology, thus helping to improve the performance of the atomic clock. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0024] Figure 1 Schematic diagram of the Λ three-level structure of an atom.

[0025] Figure 2 This is a diagram of the three-cavity mirror laser atomic clock ring resonator device of the present invention.

[0026] Figure 3 This is a diagram of the 4-cavity mirror laser atomic clock ring resonator device.

[0027] Figure 4 Schematic diagram of the transmission spectrum of a Faraday atomic filter unit or an interference filter unit.

[0028] Figure 5 Schematic diagram of the transmission spectrum of the CPT unit.

[0029] Figure 6 Schematic diagram of the transmission spectrum after an atomic filter or a coherent filter unit is connected in series with a CPT unit.

[0030] Figure 7 Interference filter assembly schematic

[0031] Figure 8 Schematic diagram of the relationship between the interference filter component and the spectra of the excitation lights ω1 and ω2. DETAILED DESCRIPTION

[0032] The following will be combined with the attached embodiment of the present invention Figure 1-7 , the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] Embodiment 1

[0034] The invention discloses a three-cavity mirror ring cavity laser atomic clock.

[0035] 3. The ring cavity of the cavity mirror laser atomic clock is as follows Figure 2 As shown, the ring cavity uses 3 reflectors, and a semiconductor laser gain diode (LD) is inserted into the cavity. The gain spectrum of the LD covers 87 The ω1 and ω2 excitation light of the Rb atom D1 transition has an output wavelength of about 795nm. The light output and light input surfaces are coupled to the resonant cavity mode using collimating lenses. The LD output light field is collimated by the lens and enters the Faraday atomic filter unit. The light field after filtering is as follows Figure 4 As shown in the figure, due to the working principle of the Faraday atomic filter, the linearly polarized light field will be rotated 90° and emitted, and the polarization plane is orthogonal to the LD output light. Then the light field enters the CPT unit and is converted by the first wave plate into two circularly polarized light fields with center frequencies of ω1 and ω2, which are connected to the air chamber. 87 The Rb atom produces a CPT effect, which makes the two coherent atoms aligned with ω1 and ω2 Figure 6 The narrow linewidth light field shown is transmitted, while other spectral components are absorbed. It is then converted into linearly polarized light orthogonal to the incident light of the CPT unit by the second wave plate. Part of the light field is output through the output mirror, while part is reflected at the same time; it enters the optical isolator unit; after passing through the isolator unit, the polarization plane of the light field is rotated 45°. A 1 / 2λ wave plate is placed in the optical path to rotate the polarization plane direction to be the same as the LD output light, and it is reflected back to the incident direction of the LD through mirrors 3 and 1, and then coupled into the LD through the incident surface lens for amplification and then output from the output surface, completing a cycle in the laser resonant cavity. After multiple cycles of amplification, two stable laser longitudinal mode oscillations will be generated, and the longitudinal mode interval is always equal to 87 The transition frequency of the hyperfine energy level of the ground state of the Rb atom. By receiving and measuring the beat frequency of the two longitudinal modes of the output laser, the reference frequency of the laser atomic clock can be obtained. In practical applications, the reference frequency can be converted to the 10MHz or 5MHz output of typical applications.

[0036] In the above design, the physical cavity length of the ring resonator needs to be adjusted to meet the requirements, and piezoelectric ceramics (PZT) are added to the ring resonator mirrors 2 and 3 to finely control the cavity length of the ring cavity. Because it is a ring cavity, the positions of the two mirrors need to be adjusted jointly to ensure the correct optical path of the resonator. PZT is controlled by an electrical feedback system. In addition, the ring cavity and the bracket are made of low expansion coefficient materials to reduce the instability of the cavity length caused by thermal effects. 87 The Rb air chamber is filled with buffer gas, which can be argon or neon to reduce the influence of temperature on the CPT resonant frequency. The air chamber is added with an axial magnetic field, temperature is controlled separately, and magnetic shielding is performed. The overall system works under temperature control and vibration reduction.

[0037] Embodiment 2

[0038] Based on the above annular cavity, the three reflectors are changed to four reflectors, such as Figure 3 As shown in the figure, a reflector is added, and the reflection of the output mirror is made as close to vertical as possible. At this time, the change of the optical path of the ring cavity can be ignored when the position is finely adjusted in a small range along the normal direction of the output mirror. At this time, the adjustment of the ring cavity length can be achieved by adjusting the position of the output mirror, and it is no longer necessary to jointly adjust other cavity mirrors, which can simplify the complexity of adjusting and controlling the cavity length, and is conducive to improving the performance of laser atomic clocks.

[0039] Embodiment 3

[0040] The working atoms in the gas chamber of the Faraday atomic filter and the CPT unit in the above embodiment 1 and embodiment 2 are changed to 133 Cs, the output light wavelength of LD is selected as 894nm, aligned with the D1 line of cesium atoms, and the same ring cavity structure as the above embodiment is applied to realize the laser atomic clock of cesium atoms.

[0041] Embodiment 4:

[0042] The Faraday atomic filter in the above embodiments is replaced by an interference filter system, such as Figure 7 As shown, the FP cavity is designed to have a free spectral region or a transmitted comb spectrum frequency interval equal to the ground state hyperfine level transition frequency of the working atom. After adjustment, the center frequencies of the two comb teeth transmission spectra are aligned with ω1 and ω2, and then an interference filter is placed, which is designed as follows: Figure 8 As shown, only two comb teeth corresponding to ω1 and ω2 are allowed to pass through, and the relative relationship of the transmission spectrum is as follows Figure 8 As shown in the figure, the unnecessary spectral components are filtered out. Its filtering effect is equivalent to that of a Faraday atomic filter. The other settings in the ring cavity remain unchanged, and the laser atomic clock function can be realized.

[0043] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A ring cavity laser atomic clock, characterized in that: The invention comprises adopting a circular traveling wave resonant cavity, into which a semiconductor laser gain medium, a filtering unit and a coherent population trapping unit are sequentially inserted along the propagation direction of the light field, the cavity length of the circular traveling wave resonant cavity is an integer multiple of the wavelength of the light field after the light field is transmitted along the circular traveling wave resonant cavity for one cycle, the light field is amplified through multiple cycles in the circular traveling wave resonant cavity, and two laser longitudinal mode oscillations are established; the cavity length of the circular traveling wave resonant cavity should be designed so that the longitudinal mode interval of the resonant cavity is in a multiple relationship with the transition frequency of the ground state hyperfine energy level of the working atom; that is, the transition frequency of the ground state hyperfine energy level of the atom is required to be n times the longitudinal mode interval of the resonant cavity, where n is a positive integer, the beat frequency of the two longitudinal modes of the resonant cavity is detected and output to obtain a frequency reference signal, and a laser atomic clock is realized.

2. The ring cavity laser atomic clock according to claim 1, characterized in that: The ring-shaped traveling wave resonant cavity is composed of three reflectors, or four or more reflectors. An optical isolator is placed in the cavity so that the laser can only be transmitted in one direction, forming a ring-shaped traveling wave resonant cavity.

3. The ring cavity laser atomic clock according to claim 2, characterized in that: The semiconductor laser gain medium is collimated by a lens and outputted, and after being transmitted along the ring traveling wave resonator for one cycle, it is coupled back to the semiconductor laser gain medium through the lens.

4. The ring cavity laser atomic clock according to claim 3, characterized in that: The two ends of the semiconductor gain medium are coated with anti-reflection films; before the light field is fed back to the gain medium, a λ / 2 wave plate, and adjust its angle to make the polarization plane of the feedback light consistent with the output light.

5. The ring cavity laser atomic clock according to claim 1, characterized in that: The filtering unit adopts a Faraday atomic filter or an interference filtering combination system.

6. A ring cavity laser atomic clock according to claim 1 or 5, characterized in that: The working atoms in the coherent population trapped unit atomic gas cell are alkali metal atoms, including 87 Rb and 133 Cs, the air chamber is filled with buffer gas.

7. The ring cavity laser atomic clock according to claim 2, characterized in that: Piezoelectric ceramics are installed between the two reflectors and the bracket of the annular traveling wave resonant cavity, and the two piezoelectric ceramics are linked to ensure the correctness and stability of the resonant cavity light field loop during the process of making slight adjustments to the cavity length.

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