Coherent Population Trapping Atomic Clock Research Articles

Light shift of Ramsey coherent population trapping resonance using drive current modulation

We investigated the light shift of Ramsey coherent population trapping (CPT) resonance by drive current modulation (DCM), which can be realized with low power consumption in a small system, to improve the performance of a CPT atomic clock. The light shift makes the greatest contribution to the degradation of long-term frequency stability of a CPT atomic clock. We also devised digitizer-based DCM to reduce the uncertainty in the frequency of light shift due to wavelength variation at the time of observation. The results show that light shift with DCM is reduced compared with cw excitation and is consistent with theoretical calculations. The estimated frequency uncertainty of the light shift due to wavelength variation using the proposed method is 2.1 × 10−13, which is approximately 165 times better than that of the conventional method without a digitizer, thus demonstrating the effectiveness of the proposed method for future small atomic clocks.

Active Suppression of the Light Shift in an Atomic Clock Based on Coherent Population Trapping in 87Rb Vapor Using the Phase Jump Technique

The active suppression of light shift of the reference coherent population trapping (CPT) resonance in 87Rb vapor to improve the long-term stability of compact atomic clocks has been demonstrated experimentally. The approach is based on the phase jump technique used to synthesize an error signal, which is proportional to the light shift of the CPT resonance, from an optical transmission signal. The use of this error signal in an additional feedback loop allows one to stabilize the amplitude of a microwave signal for the phase modulation of laser radiation near a value at which the light shift of the CPT resonance is absent. This technique has made it possible to reduce the long-term instability of CPT atomic clocks in our experiments by a factor of 15 at an integration time of 10 000–20 000 s.

A low phase and amplitude noise microwave source for vapor cell atomic clocks.

A low-noise microwave source plays a key role in high-performance passive atomic clocks. Here, we propose and implement a microwave frequency synthesizer featuring a low phase and amplitude noise. With the help of a high-Q factor surface acoustic wave band-pass filter, we generate a microwave with targeted frequency by frequency multiplication of a low noise local oscillator at a radio frequency with the closest integer. At the frequency offset of 1Hz, 10Hz, 100Hz, 1kHz, and 10kHz, the absolute phase noise of the output 3.417GHz signal is -53.0, -83.3, -107.7, -119.2, and -124.0 dBc/Hz, respectively. After the microwave-to-optical conversion, the expected intermodulation effect contribution to the frequency stability of the coherent population trapping (CPT) atomic clock is 5.95 × 10-14 at an averaging time of 1s. Meanwhile, with a feature of low fluctuation of this chain's output microwave power at the level of 1.19 × 10-5W at 1s, its contribution to the frequency stability of the CPT atomic clock is 7.85 × 10-14 at the 1s integration time. Our simple and low noise microwave chain is an ideal microwave source for high-performance, compact CPT clocks and could also be applied to cold atom or ion based microwave clocks.

Microwave phase detection of coherent population trapping resonance in a Cs vapor cell

We report a phase-sensitive microwave detection setup for spectroscopy of coherent population trapping (CPT) resonance in a Cs vapor cell. In this scheme, the optically carried microwave interrogation signal that interacts with atoms is detected by a fast photodiode at the cell output and is then mixed with the local oscillator microwave signal itself. This detection scheme produces at the output of the microwave mixer a dispersive CPT signal that can be directly used for correction of the local oscillator frequency onto the atomic resonance. The evolution of the dispersive CPT resonance signal with the laser power is briefly reported. With this detection scheme, we demonstrate the operation of a CPT atomic clock with a short-term fractional frequency stability of 1 × 10 − 11 at 1 s, which is in good agreement with the slope of the frequency discriminator and the residual microwave phase noise of the setup.

A vertical cavity surface emitting laser for CPT atomic clock

A polarization-stabilized fundamental mode vertical cavity surface emitting laser (VCSEL) was developed for a coherent population trapping (CPT) Cs atomic clock. The VCSEL achieved a single stabilized polarization state through a directly-etched surface grating structure on the surface. The device polarization characteristics of different surface grating parameters were analysed using the finite-difference time-domain (FDTD) method. For the fundamental transverse mode condition, the lasing wavelength corresponded to the Cs atom’s D1 line. Owing to the polarization-dependent reflectivity of surface gratings, the VCSEL’s orthogonal polarization suppression ratio (OPSR) exceeded 20 dB. Since the surface grating eliminated the degeneracy of the two orthogonal linear polarization fundamental modes, a linewidth of approximately 53 MHz for the VCSEL could be obtained using the delayed self-homodyne method. A 7.6 GHz modulation bandwidth satisfied the atomic clock’s bandwidth requirement. We introduced the directly-etched surface grating VCSEL to the Cs atomic clock, and measured the CPT resonance signal.

Microwave Synthesizer and Self-Adaption System for High Performance Coherent Population Trapping Atomic Clocks

Microwave Synthesizer and Self-Adaption System for High Performance Coherent Population Trapping Atomic Clocks

High-performance coherent population trapping atomic clock with direct-modulation distributed Bragg reflector laser

The coherent population trapping (CPT) atomic clock is very promising for use in next-generation spaceborne applications owing to its compactness and high performance. In this paper, we propose and implement a CPT atomic clock based on the direct modulation of a large-modulation-bandwidth and narrow-linewidth distributed Bragg reflector laser, which replaces the usually used external bulk modulator in high-performance CPT clocks. Our method retains the high performance while significantly reducing the clock size. Using this highly compact bichromatic light source and simplest CPT configuration, in which a circularly polarized bichromatic laser interrogates the 87Rb atom system, a CPT signal of clock transition with a narrow linewidth and high contrast is observed. We then lock the local oscillator frequency to the CPT resonance and demonstrate an encouraging short-term frequency stability of 3.6 × 10−13 τ −1/2 (4 s ⩽ τ ⩽ 200 s). We attribute it to the ultralow laser frequency and intensity noise as well as to the high-quality-factor CPT signal. This study can pave the way for the development of compact high-performance CPT clocks based on our scheme.

Laser frequency stabilization via bichromatic Doppler-free spectroscopy of an 87Rb D1 line.

We demonstrate a bichromatic Doppler-free spectroscopy of an 87RbD1 line by using a dual-frequency, counterpropagating laser field with orthogonal linear polarizations. A reversed Doppler-free resonance dip is observed in the dual-frequency scheme, and a significant improvement of frequency discrimination curve is acquired due to the coherent population trapping (CPT) effect. The influence of the static magnetic field and laser intensity on the spectroscopy is studied in both single- and dual-frequency schemes. After locking the laser frequency to the 87RbD1 line in the dual-frequency stabilization scheme, the beat note fractional frequency stability is at the level of 7×10-12 at 1 s integration time. This technique can be used in various applications, such as CPT atomic clocks, laser spectroscopy, quantum optics, and laser-cooling experiments.

A Dual-Frequency Faraday Laser

The Faraday laser, with Faraday anomalous dispersion optical filter (FADOF) as frequency-selection element, is a natural narrow-bandwidth light source for laser physic experiments. In this work, a dual-frequency (DF) Faraday laser is demonstrated for the first time on Cs D2 line at 852 nm. The frequencies of the two modes of DF Faraday laser hinge on the peak transition frequencies of 852 nm FADOF transmittance spectrum corresponding to the ground state F=4 and F=3. The frequency difference between the two modes is tunable over a range of 1.4 GHz with the temperature of Cs vapor cell inside the FADOF. The linewidth of each laser mode is less than 33 kHz. Furthermore, the most probable linewidth of the beat-note spectra between the two modes is 902.95 Hz at different vapor-cell temperatures. Such a DF Faraday laser has many potential applications in atomic physics, such as sub-Doppler spectroscopy and coherent population trapping atomic clocks.

Magneto-optic rotation detection scheme for miniaturized coherent population trapping atomic clock.

In order to suppress the effects of carrier and high-order sidebands on the coherent population trapping (CPT) signal, a scheme of interaction between an elliptically polarized beam and alkali atoms using the magneto-optic rotation detection is examined. By canceling the spin-polarized dark state in the CPT state preparation, a CPT resonance signal with a contrast of 32.5% is obtained. Our scheme requires minor modifications of the optical path in the prevalent scheme, and the short-term frequency stability of our scheme is five times better than that of the prevalent scheme, making it ideal for miniaturized CPT atomic clock applications.

Stability properties of an Rb CPT atomic clock with buffer-gas-free cells under dynamic excitation

This work presents a study of the short-term stability properties of Rb atomic clocks based on coherent population trapping (CPT) under dynamic excitation of a CPT resonance in cells without buffer gas. It is demonstrated that in an optical cell with anti-relaxation coating of its inner walls, the best stability is achieved at the scanning frequency of the frequency difference of the bichromatic pump field equal to 2 kHz at the resonance width of 450 Hz. In cells without a wall coating, the optimal scanning frequency range is found to be 1.2–2.8 kHz at the resonance width of 29 kHz. Examination of the slope of the stabilization system’s discriminant curve at zero error signal for buffer-gas-free cells uncovered an amount of correlation between the discriminant curve slope and atomic clock stability. It is demonstrated that the highest stability of atomic clocks in dynamic excitation mode is achieved at a ratio of scanning frequency and amplitude around 1.

CPT atomic clock with cold-technology-based vapour cell

Alkali-metal vapour cells are the core of coherent population trapping (CPT) atomic clocks, devices with a multitude of applications. These cells ensure high clock stability due to an atomic reference resonance. The contemporary technologies of alkali-metal vapour cells for atomic clocks compatible with mass production are relatively complicated and require special materials, as well as sub-technologies. The present work for the first time studies atomic CPT cocks with rubidium miniature cells fabricated with a simpler direct optical bonding technology, which has never hitherto been used for this purpose. The proposed technology can be implemented both in industrial and in laboratory setting. The measured stability of CPT atomic clock with rubidium cells fabricated with this technology amounted to 4 × 10–11 over 1 s and 1.3 × 10–12 over 103 s. The generated results indicate that the direct optical bonding technology can be successfully utilised for fabrication of compact cells with alkali-metal vapours for metrological and sensor applications.

相干布局囚禁原子钟原子气室内部缓冲气体组分的优化

相干布局囚禁原子钟原子气室内部缓冲气体组分的优化

Design of PID Coprocessing Module for CPT Atomic Clock Control SoC

The CPT (Coherent Population Trapping) atomic clock is very small because there is no microwave cavity. However, the circuit control system is becoming the constraint of further miniaturization of CPT atomic clock. The CPT atomic clock control SoC integrates control related circuit into a single chip. There are many control loops in the CPT atomic clock which require PID (Proportional-Integral-Derivative) algorithm. CPU in this SoC can not afford the computation of PID algorithm. So a PID coprocessing module is integrated into this SoC. The PID algorithm can be decomposed into multiply-add operations. The coprocessing module mainly consists of a multiply-add module. Compared with SoC without PID coprocessing module, SoC with this module can reduce the time of calculating PID algorithm to 24.808%, and the area consumption increases by only 8.89%.

Noise reduction by a single photodiode in coherent population trapping atomic clocks

We present a digital differential detection scheme using a single photodiode to suppress the diode laser frequency noise and improve the short-term stability in atomic clocks based on coherent population trapping (CPT) resonance. One laser light is divided into two beams via modulation of an acousto-optic modulator. One beam induces CPT resonance with circularly polarized light, and the other beam induces only absorption with linearly polarized light. A differential algorithm counteracts common-mode noise from the two beams. A stability of 6.86×10−12 at 100 s and 2.26×10−12 at 1000 s has been demonstrated. The short-term stability is improved by a factor of 2.4 at 100 s. The scheme promises to eliminate laser-related noise and explore potential applications in chip-scale atomic clocks.

Coherent population trapping atomic clock by phase modulation for wide locking range

A method of detecting Coherent Population Trapping resonance by phase modulation (PM) was studied to expand the frequency locking range (LR). We calculated error signals by density matrix analysis using an eigenvector algorithm and verified the calculated result using an 87Rb vapor cell and a Rb-D1 vertical cavity surface-emitting laser. By comparing the error signal for PM with that for conventional frequency modulation (FM), it was found that the LR with PM is one order of magnitude wider than that with FM without degrading the frequency stability.

A polarization converting device for an interfering enhanced CPT atomic clock.

With interfering enhanced coherent population trapping (CPT) signals, a CPT atomic clock with improved frequency stability performance can be realized. We explore an optical device that converts single-polarized bichromatic light to left and right circularly polarized superposed bichromatic light to generate interfering enhanced CPT resonance with atoms. We have experimentally studied a tabletop CPT atomic clock apparatus with a microfabricated 87Rb atomic chip-scale cell, and the study results show that it is promising to realize a compact CPT atomic clock, even a chip-scale CPT atomic clock through microfabrication, with improved frequency stability performance.

Differential detection scheme for compact CPT atomic clocks

A scheme is investigated for a coherent population trapping (CPT) atomic clock, wherein the polarization of a beam produced by a vertical-cavity surface-emitting laser is converted to an elliptically polarized beam that interacts with alkali atoms, where the CPT signal is extracted by differentially detecting the magneto-optically rotated light within the transmitted beam. The scheme eliminates the spin-polarized trap state of the atoms and the unwanted background signal, and suppresses in the CPT signal the noise converted from the laser noise. This result reveals the promise of this scheme for realizing a compact CPT atomic clock possessing a significantly improved frequency stability compared to current compact CPT atomic clock devices, coupled with similar power consumption, volume, and cost therewith.

Two-step pulse observation to improve resonance contrast for coherent population trapping atomic clock

We study resonance contrast by a two-step pulse observation method to enhance the frequency stability of coherent population trapping (CPT) atomic clocks. The proposed method is a two-step Raman--Ramsey scheme with low intensity during resonance observation and high intensity after the observation. This method reduces the frequency variation in the light intensity and maintains a high signal-to-noise ratio. The resonance characteristics were calculated by density matrix analysis of a $\Lambda$-type three-level system that was modeled on the $^{133}$Cs D1 line, and the characteristics were also measured using a vertical-cavity surface-emitting laser and a Cs vapor cell.

Design of a Low Phase Noise VCO for Rubidium Atomic Frequency Standard

Compared to the size and the power consumption of the traditional atomic clocks, the ones based on coherent population trapping (CPT) can provide improvement by two orders of magnitude in both aspects above, making them needed urgently in many fields. Among different CPT atomic clocks, the one made with the rubidium atom are used widely, and their operating performance largely depends on its internal voltage-controlled oscillator (VCO) which is used to provide a proper microwave signal. Based on this, a small size and low phase noise 3.035GHz VCO is designed with a modified Clapp circuit topology using low-cost surface-mount components, including a coaxial resonant (COAX) with high quality factor. The designed VCO is simulated and optimized with the combined use of the negative resistance analysis method and the transmission analysis with a virtual-ground. In order to obtain the desired results, different values of key capacitor elements are tried according to their concrete influences on the VCO during the process of the circuit tuning. The test results show that the phase noises of the VCO are -60.49dBc/Hz@300Hz, -73.08dBc/Hz@1KHz and -97.48dBc/Hz@10KHz, the output power is -1.13dBm and the voltage-controlled tuning sensitivity is about 12MHz/V.