A Fully Functional Ramsey-CPT Atomic Clock

Qun Zhou, Chao Guan, and Xiaosong Zhu The Application Center of New Technology,

Electronic Engineering Institute, Hefei, 230037, People’s Republic of China,

E-mail: guanchao2008@163.com

Yeqing Li School of Electronics Engineering & Computer Science,

Peking University, Beijing, 100871, People’s Republic of China,

E-mail: daphne_8896@163.com

Abstract—The abstract presents a fully functional Ramsey-CPT atomic clock, which has the characteristics of compact structure, small volume, low cost, strong adaptability, high stability, convenient for adjustment, unique design, and easy-to-commercialization. The Ramsey-CPT atomic clock combines the method of separated oscillatory fields and the phenomenon of coherent population trapping, so as to obtain narrower line width and better signal-to-noise ratio of the resonance signal for improving the frequency stability. It not only keeps the advantages of miniaturization and low power of the CPT atomic clock, but also can obtain higher frequency stability than CPT atomic clock about one order of magnitude.

I. INTRODUCTION

Coherent population trapping (CPT) phenomenon is a quantum interference generated by atoms interacting with coherent light [1]. A new chip-scale passive atomic clock implement is currently a frontier technology in the field of atomic clocks. However, the CPT atomic clock is, at present, theoretically the only one that can be realized as a chip-scale atomic clock, by making use of good coherent properties of laser to prepare atomic “CPT state”.

The scope of application of subminiature, even chip-scale atomic clock can be greatly expanded, especially used in measuring instruments, the portable communication and the navigation equipments.

In this system to be described, two coherent optical radiation fields are applied to a low-cost natural Rubidium (Rb) atoms ensemble. The fields are in resonance with the D1 transitions [2] between the two hyperfine levels of the ground state and one of the hyperfine levels of the excited state, forming a so-called Λ system.

This paper aims to do some further research on Ramsey-CPT phenomenon and to promote the study of device level and producibility of Ramsey-CPT atomic clock. The Ramsey-CPT atomic clock combines the method of separated oscillatory fields [3] and the phenomenon of coherent population trapping, so as to obtain narrower line width and better signal-to-noise ratio of the resonance signal for improving the frequency stability. Using vertical-cavity surface-emitting laser (VCSEL) [4] as optical source, we propose an appropriate physics package and design a

comprehensive servo system, forming a fully functional Ramsey-CPT atomic clock for the purpose of developing high-performance and commercialization of Ramsey-CPT atomic clock.

II. THEORETICAL ANALYSIS

A. Operating Principle

Fig. 1 shows the schematic illustration of the practical realization system of the Ramsey-CPT atomic clock. The frequency conversion circuit and the digital step attenuator are controlled by the microcontroller to generate the required microwave signal. With the digital step attenuator generating any form of microwave pulse signal, we can achieve more superior discriminator curve. Through the Bias-Tee, the signal couples with the drive current of the VCSEL to achieve two coherent lights, which phase difference is constant and the frequency difference is equal to the frequency of the microwave signal.

Figure 1. Schematic illustration of the practical Ramsey-CPT atomic clock. The bias-tee is used to couple the VCSEL injection current and the microwave modulation frequency. The λ/4 plate provides circular polarization required in the excitation process by transition selection rules. Bo is the applied small magnetic induction providing an axis of quantization for the atomic ensemble.

The form of the CPT system consists of two parts: the physics package and the servo circuit system. The physics package forms the optical path of laser beam from the VCSEL emitter to the photo receiver, and it consists of the laser used to generate the coherent light, the alkali metal atoms cell, the photodetector used to receive the radiation, and so on. The servo circuit system includes two locking loop: (1) laser frequency locking loop; and (2) microwave frequency locking loop. The first loop is to lock the laser wavelength to the atomic D1 transition line for ensuring the stability of CPT incentive. The second loop is to lock the modulated microwave frequency to the CPT hyperfine resonance frequency for providing a high-performance frequency signal.

B. Comprehensive Analysis

Based on the related data and previous studies [2], we can obtain the relationship between characteristics of Ramsey-CPT system and influencing factors as shown in Fig. 2, which can be used as an important theoretical guide for future design and optimization of the system.

Figure 2. Block scheme of comprehensive analysis of the system.

As is clearly seen in this block scheme, the influencing factors are complex with each other. The shift of CPT resonance line consists of light shift, buffer-gas shift, and magnetic field shift. The line width and the amplitude of CPT resonance signal are the main object of comprehensive analysis of the Ramsey-CPT system.

The factors to be considered are mainly in: (1) the operating temperature of VCSEL and Rb vapor cell, (2) the modulating frequency of microwave, (3) the transmitted light intensity, (4) the weak magnetic field, and (5) the driving current of VCSEL.

Therefore, in practice, we need overall consideration to develop a high-performance Ramsey-CPT atomic clock.

III. IMPLEMENTATION METHOD

A. Physics Package

Depending on the size and volume limited and considering the organic combination with servo circuit system, we realize a physics package required by the system in which the laser beam can be collimated and adjustable, both temperature and magnetic field can be digital quantization, and the light intensity can be stepping adjustments.

Fig. 3 shows the photograph of the physics package which consists of three parts. The first part includes VCSEL, lens, and λ/4 plate. We can control the diameter of the light spot by adjusting the distance from lens to VCSEL. The second part consists of two polarizers which can quantitatively change the light intensity by rolling the bearing with 32-divided scale on it. The third part includes magnetic shield, Rb vapor cell, solenoid, magnetic sensor, and photodetector. The magnetic sensor is used to monitor the weak magnetic field. The magnetic shield, which is produced to a cavity using permalloy, can reduce the effects brought by stray magnetic field and earth’s magnetic field.

( a ) ( b )

( c ) ( d )

Figure 3. Photograph of the physics package. (a) The first part; (b) The second part; (c) The third part; and (d) the whole physics package.

The beam divergence of VCSEL is about 10 ˚ – 25 ˚, so we need to collimate the beam via a collimating lens. A plano-convex lens with 10mm focal length is used here. The beam, after passing through the lens, becomes approximate parallel beam. Then, the laser beam is circularly polarized and attenuated. Finally, the photodetector is required to detect the laser beam which has interacted with Rb atoms.

In short, the physics package designing is the key for obtaining a subminiature, adjustable and measurable system. It can be further development.

B. Comprehensive Servo System

Using comprehensive design method of microwave and high precision circuit and the technology of high density printed circuit board (PCB) designing, we design and implement comprehensive servo system of Ramsey-CPT atomic clock as shown in Fig. 4.

7cm

5cm

Figure 4. Photograph of the comprehensive servo system.

The servo circuit system consists of five printed circuit boards, which are VCSEL driving circuit, temperature control circuit for VCAEL, temperature control circuit for Rb cell, photo detector and magnetic field monitor circuit, and motherboard circuit. They are connected by the four connectors on the top of the motherboard.

The small and digital servo system has been designed for realizing automatic scan and locking control, monitoring the temperature and magnetic field, and so on.

The detection signal outputted from the physics package is amplified to an appropriate range, sampled by ADC, and then sent to the microprocessor to make the phase sensitive demodulation. At this point, we can obtain the differential curve of the absorption spectrum which is regarded as correction curve for locking the laser frequency. Finally, the feedback loop can be closed and locked. Based on the same approach, the microwave laser modulation frequency can be identified and then locked to the maximum of the CPT resonance transmission line.

Figure 5. Photograph of the integral system.

In additional, based on the integral system, the chip-scale atomic clock can be developed through simplifying the structure of the physics package and the digital servo circuit, especially the physics package, because they have enough space for improvement.

IV. CONCLUSION

In conclusion, we describe the operating principle and the comprehensive analysis of the Ramsey-CPT system. Based on these theories, we have outlined the practical realization and well characteristics of the fully functional system. The whole system has the characteristics of compact structure, small volume, low cost, strong adaptability, high stability, convenient for adjustment, unique design, and easy-to-commercialization. It is believed that these characteristics of the Ramsey-CPT system can be improved in future developments.

REFERENCES [1] E. Arimondo, in Progress in Optics, edited by E. Wolf ( Elsevier,

Amsterdam, 1996) , Vol. 35, p. 257. C. The Integral System

Make the physics package connect to the servo system, and then we can obtain the overall architecture of the fully functional Ramsey-CPT atomic clock system. Its photograph can be seen in Fig. 5. This CPT atomic clock system has a volume about 60cm3, and is convenient to adjust and measure.

[2] J. Vanier, “Atomic clocks based on coherent population trapping: A review,” Appl. Phys. B, vol. 81, pp. 421–442, 2005.

[3] N. F. Ramsey, “A Molecular Beam Resonance Method with Separated Oscillating Fields”, Phys. Rev., vol. 78, p. 695, 1950.

[4] D. K. Serkland, G. M. Peake, K. M. Geib etc. “VCSELs for Atomic Clocks,” in Proc. of SPIE, Vol.6132 613208-1, 2006.