CPT PHASE MODULATION AND DEMODULATION METHOD AND SYSTEM

Abstract

The invention relates to a coherent population trapping (CPT) phase modulation and demodulation method and a system for implementing the method of this invention. The method comprises the following steps: Generating a coherent bichromatic light, in which the relative phase between the two frequency components is modulated with proper modulation depth. The phase modulated coherent bichromatic light interacts with a quantum resonance system, and prepares it alternately into two inverted CPT states. Detecting the transmitted light with a photodetector, two inverted dispersive CPT signals in two detection windows are observed. With synchronous phase demodulation, a CPT error signal is obtained, which is used for locking the local oscillator to implement a CPT atomic clock.

Claims

1. A CPT phase modulation and demodulation method is characterized by comprising the following steps: 1) providing a coherent bichromatic light. 2) modulating the relative phase of the two frequency components in the coherent bichromatic light, wherein the modulation depth is π/2, i.e. the relative phase is switched between ϕ+0 and ϕ+π/2, in which ϕ is an initial arbitrary phase. 3) interacting the phase modulated coherent bichromatic light with a quantum resonance system, then the quantum resonance system is prepared alternately into two inverted dispersive CPT states. 4) converting the coherent bichromatic light transmitted from the quantum resonance system into an electric signal by a photodetector, demodulating synchronously the electric signal to obtain an error signal, feeding back and locking the local oscillation frequency, and realizing an atomic clock.

2. A CPT phase modulation and demodulation method of claim 1, wherein the quantum resonance system comprises a CPT resonance energy level structure from two ground states to the same excited state, and adopts hydrogen atoms (H), alkali metals (Li, Na, K, Rb, Cs), Hg.sup.+, Ca.sup.+, Yb.sup.+, Ba.sup.+ or fullerene (C-60) particles. The particles are in a gaseous hot atom, gaseous cold atom, gaseous atomic beam, ion, molecular or plasma state. The quantum resonance system adopts an active CPT or passive CPT configuration.

3. A CPT phase modulation and demodulation method of claim 1, wherein the modulation period of the relative phase modulation in the step 2) is at the same order of the ground states coherence time, for maximizing the contrast of CPT signal.

4. A CPT phase modulation and demodulation method of claim 1, where in step 4) the error signal can be obtained by two methods of synchronous demodulation: digital or analog demodulation. For the digital demodulation, an analog-to-digital converter (ADC) converts the photodetector signal into a digital signal. Signals in two windows during phase switching in each phase modulation period are collected, averaged respectively. Then, an error signal is obtained by subtracting the signals in two windows. For the analog demodulation, the photodetector electric signal is multiplied by a square wave signal which is synchronous with the phase modulation. After filtering out a high-frequency component by a low-pass filter, an error signal is obtained.

5. A CPT phase modulation and demodulation method of claim 1, wherein the obtained clock transitions, i.e. |m.sub.F=0.fwdarw.|m′.sub.F=0, wherein m′.sub.F and flip are magnetic quantum numbers of two ground-state quantum levels, the CPT error signal of which is applied to the CPT atomic clock; non-clock transitions, i.e. |m.sub.F≠0.fwdarw.|m′.sub.F≠0, the CPT error signal is applied to the atomic magnetometer; both the CPT error signals of clock and non-clock transition can be applied to atomic precision spectroscopy.

6. A CPT phase modulation and demodulation system for implementing the method of claim 1, comprising a direct current source, a microwave coupler (bias-tee), a phase modulator, a microwave signal source, a laser diode, a quarter-wave plate, a quantum resonance system, a photodetector and a signal processor, wherein: the direct current source supplies power to the laser diode; a microwave signal source sends phase modulated microwave signal by a phase modulator to the laser diode through a microwave coupler; coherent bichromatic light emitted by the laser diode passes through a quarter-wave plate to obtain circularly polarized coherent bichromatic light. After it interacts with the quantum resonance system, transmitted light is detected by a photodetector, thus an optical signal is converted into an electric signal, and finally a CPT error signal of phase modulation and demodulation is obtained through a signal processor.

7. A CPT phase modulation and demodulation system of claim 6, wherein the coherent bichromatic light can be circularly polarized laser light (σσ CPT), a pair of parallel and linearly polarized laser light (lin//lin CPT), a pair of orthogonally and linearly polarized laser light (lin⊥lin CPT), push-pull optical pumping CPT (PPOP CPT) or double modulation CPT (DM CPT).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 is a schematic diagram of the principle of the present invention.

[0028] FIG. 2 is a schematic view of the system of the present invention.

[0029] FIG. 3 is a schematic view of new designed microwave source for CPT atomic clock based on the present invention.

[0030] FIG. 4 is a timing diagram illustrating the operation of the present invention.

[0031] FIG. 5 illustrates a conventional frequency modulation and demodulation method.

[0032] FIG. 6 is a plot of typical dispersive CPT signals in two detection windows obtained by the present invention.

[0033] FIG. 7 is a plot of a typical CPT error signal obtained by phase modulation and demodulation in accordance with the present invention. For comparison, the figure also shows the error signal obtained by the conventional frequency modulation and demodulation method under the same conditions.

[0034] In FIG. 2, 1-a direct current source; 2-a microwave coupler (bias-tee); 3-a phase modulator; 4-a microwave signal source; 5-a high modulation bandwidth (>GHz) laser diode; 6-a quarter-wave plate (λ/4); 7-a quantum resonance systems; 8-a photodetector; 9-a signal processing system.

DETAILED DESCRIPTION

[0035] The present invention will be further described with reference to the accompanying drawings and embodiments, which include, but are not limited to, the following embodiments.

[0036] CPT is a quantum interference effect. In the field of atomic clocks application, it includes both active CPT and passive CPT. The quantum resonance system comprises a CPT resonance energy level structure from two ground states to the same excited state, and adapts to hydrogen atoms (H), alkali metals (Li, Na, K, Rb, Cs), Hg.sup.+, Ca.sup.+, Yb.sup.+, Ba.sup.+ or fullerene (C-60) particles, the particles being in a gaseous hot atom, gaseous cold atom, gaseous atomic beam, ion, molecular or plasma state. The invention takes the passive CPT based vapor cell atomic clock, in which a continuous laser interacts with .sup.87Rb atom ensemble, as an example to describe the specific implementation mode of the invention. But the present invention is not limited thereto, and the applicable scope covers all the configurations described above.

[0037] The invention discloses a CPT phase modulation and demodulation method, as shown in FIG. 1, which comprises the following steps: Generating a coherent bichromatic light, in which the relative phase between the two frequency components is modulated with modulation depth equal to π/2. The phase modulated coherent bichromatic light interacts with a quantum resonance system, and prepares it alternately into two inverted CPT states. Detected the transmitted light with a photodetector, two inverted dispersive CPT signals are observed in two successive detection windows. With synchronous phase demodulation, a CPT error signal is obtained, which is used for locking the local oscillator to realize a CPT atomic clock.

[0038] The CPT phase modulation and demodulation method provided by the embodiment of the invention comprises the following steps: [0039] 1) providing a coherent bichromatic light. [0040] 2) modulating the relative phase between the two frequency components in the coherent bichromatic light, wherein the modulation depth is π/2 and the modulation period is at the same order of the ground states coherence time, for maximizing the contrast of CPT signal. [0041] 3) interacting the phase modulated coherent bichromatic light with a quantum resonance system, then the quantum resonance system is prepared alternately into two inverted dispersive CPT states. [0042] 4) converting coherent bichromatic light transmitted from the quantum resonance system into an electric signal by a photodetector, demodulating the electric signal to obtain an error signal by two methods: Digital or analog demodulation.

[0043] For the digital demodulation, the photodetector electric signal is converted into digital signals through an analog-to-digital converter (ADC), signals in two successive detection windows during phase switching in each phase modulation period are collected, averaged respectively and subtracted to obtain an error signal. The duration of the two successive detection windows is t.sub.w1 and t.sub.w2 respectively, the t.sub.w1 window is located at the initial moment of CPT state before phase switching, the t.sub.w2 window is located at the moment of phase switching, the duration of t.sub.w1 and t.sub.w2 is usually not more than 1 ms for a high CPT signal contrast. After the error signal is processed through a proportional-integral-derivative (PID) algorithm, an analog error signal is generated by a digital-to-analog converter (DAC) and used to feedback and lock the frequency of local oscillation, and finally an atomic clock is realized.

[0044] For the analog demodulation, which is similar to the conventional frequency modulation and demodulation method shown in FIG. 5, the photodetector electric signal is multiplied by a square wave which is synchronous to the phase modulation, this multiplication can be implemented by an analog mixer. A low-pass filter is used for filtering the high-frequency component, then an error signal is obtained.

[0045] As shown in FIG. 2, the embodiment of the present invention further discloses a physical system for CPT phase modulation and demodulation.

[0046] In the system, a direct current source 1 drives a laser diode system 5 and tunes its output light wavelength to a proper position of atomic resonance. A microwave signal source 4 is coupled to the laser diode through a microwave coupler 2 and modulates its output light frequency. Then a multicolor light is generated by the laser diode 5. The ±1st order sidebands of the multicolor light form the desired coherent bichromatic light, which is used for CPT states preparation and detection, while other frequency components do not interact with an atomic system obviously and exist only as the detection background.

[0047] The phase modulator 3 implements phase modulation on the microwave signal source 4, i.e., switches phase between φ+0 and φ+π/4 according to design rule, where φ is the initial arbitrary phase. Correspondingly, the relative phase of the coherent bichromatic light is switched between ϕ+0 and ϕ+π/2, where ϕ is the initial arbitrary phase.

[0048] The phase-modulated coherent bichromatic light passes through a λ/4 wave plate 6 and converts its polarization to a circular polarization. Then the phase-modulated coherent bichromatic light interacts with a quantum resonance system 7 and prepares it alternately into two inverted CPT states.

[0049] Converted the transmitted light into electric signal by a photodetector 8 and sent it to a signal processing system 9, an error signal can be obtained through synchronous phase demodulation with a digital or analog method. Take digital phase demodulation method for example, as shown in FIG. 4, the photodetector electric signals are converted into digital signals through an analog-to-digital converter (ADC). Signals in two successive detection windows during phase switching in each phase modulation period are collected, averaged respectively, then two CPT signals with inverted dispersive shape are observed, as shown in FIG. 6. An error signal is obtained by subtracting the two CPT signals, as shown by the solid line in FIG. 7. After the error signal is processed through a proportional-integral-derivative (PID) algorithm, the analog error signal is generated by a digital-to-analog converter (DAC) and used to feedback and lock the frequency of local oscillation, and finally an atomic clock is realized.

[0050] The invention uses a phase rather than a frequency modulation, thus it offers a new design concept for microwave source, as shown in FIG. 3, in which the LO, ×34, BPF and DDS are abbreviations for local oscillator, frequency multiplier, band-pass filter and direct digital frequency synthesis, respectively. This design gives an example of a low phase noise and simple structure microwave source for CPT clock.

[0051] The coherent bichromatic light source can be obtained by means of frequency locking of two independent lasers, external modulation (such as EOM, AOM and the like), internal modulation (direct-modulated lasers) and the like, wherein the microwave frequency used for the modulation is ν.sub.hf/n, ν.sub.hf for the clock transition frequency, n for a positive integer. The invention takes the half-wave modulation for example, in which the coherent bichromatic light is generated by the direct modulation of a high modulation bandwidth laser diode with microwave frequency around ν.sub.hf/2.

[0052] In the invention, the phase modulator dynamically controls the relative phase of the two frequency components of bichromatic light with the modulation depth is π/2. This can be realized by a phase shifter, a direct digital frequency synthesizer (DDS) or a phase-locked loop (PLL).

[0053] In the invention, the phase modulation of coherent bichromatic light can be implemented by a square-wave or sine-wave modulation.