IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 7, JULY 2008 1357 Laser Noise Cancellation in Single-Cell CPT Clocks Vladislav Gerginov, Svenja Knappe, Vishal Shah, Leo Hollberg, and John Kitching Abstract—We demonstrate a new technique for the suppression of noise associated with the laser source in atomic clocks based on coherent population trapping (CPT). The technique uses dif- ferential detection of the transmission of linearly and circularly polarized beams that propagate through different parts of a single rubidium vapor cell filled with a buffer gas mixture. The common- mode noise associated with the laser frequency and amplitude noise is suppressed by the differential detection of the two laser beams. The CPT signal, which is present only in the circularly polarized laser beam, is unaffected. The implementation of the technique requires only a change of the polarization of part of the laser beam and an additional photodiode. The technique is simple and applicable to CPT frequency references where a major source of noise is the laser, such as compact and chip-scale devices. Index Terms—Atomic clocks, coherent population trapping (CPT), compact frequency references, diode lasers, noise reduc- tion, vertical-cavity surface-emitting lasers (VCSELs). I. I NTRODUCTION T HE INCREASING demand for precise timing in such areas as communications, network synchronization, and navigation has led to the development of small and robust lamp-pumped atomic clocks. Significant advances in the field of coherent population trapping (CPT) have made it possible to reduce the size of these devices even further [1]–[3]. In such devices, a laser replaces the lamp, and no RF cavity is required, because the atomic microwave transition is excited by light components that are present in the laser spectrum that have a frequency difference equal to the ground state splitting. The use of vertical-cavity surface-emitting lasers (VCSELs) with high microwave modulation efficiencies and low threshold currents, combined with the advantages of small size enabled by CPT, has reduced the power consumed by the physics package [4] to the point where these devices are expected to be powered by batteries. The properties of the light assembly in any laser-pumped atomic reference are of great importance in determining the frequency stability of the system. Slow changes in the laser intensity, optical frequency, or modulation properties may Manuscript received February 28, 2007; revised July 24, 2007. This work was supported by the Microsystems Technology Office of the U.S. Defense Advanced Research Projects Agency. This paper is a contribution of the National Institute of Standards and Technology (NIST), which is an agency of the U.S. Government, and is not subject to copyright. Some of the data in this manuscript were previously submitted to [15]. V. Gerginov is with Cymer, Inc., San Diego, CA 92127 USA. S. Knappe, L. Hollberg, and J. Kitching are with the Time and Fre- quency Division, National Institute of Standards and Technology, Boulder, CO 80305 USA. V. Shah is with the Department of Physics, University of Colorado, Boulder, CO 80309 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2007.915123 degrade the performance of the atomic reference over long time scales. AM and FM noise on the laser light largely determines the short-term performance. In particular, the partitioning of the optical power between transverse and polarization modes in VCSELs can lead to intensity noise significantly above the shot noise level on the laser output. Such noise increases if a polarizer is placed in the beam path to purify the polarization state of the light. If the microwave modulation of the laser injection current with a high FM modulation index is used, the intensity noise increases further. The conversion of frequency noise into amplitude noise, due to the frequency-dependent absorption of the atomic media (FM–AM conversion) [5], [6], is another important source of noise. For VCSELs, the FM noise is particularly high because of their relatively large laser linewidth (often around 50 MHz) and the sensitivity of the optical frequency to variations in the injection current (300 GHz/mA) and temperature (30 GHz/K). In this paper, we present a method for reducing the sensitivity of CPT-based atomic references to laser noise. Similar tech- niques have been developed for optically pumped microwave clocks [7]–[10]. In contrast with the work in [7], here, a single cell is used to create the CPT signal and cancel the noise, which allows for a smaller overall size and builds on the original advantage of CPT over microwave excitation with respect to miniaturization. In [9], two spatial regions of the same cell were used, and noise cancellation was achieved through the use of different laser and microwave intensities in each region. Al- though this method eliminates the complexity inherent in using two cells, the size of the device is still set by the wavelength of the microwave radiation. In [10], an optical delay was required, which may limit the size of the device to a centimeter scale. The scheme presented in this paper allows simplification of the physics package miniaturization to a length scale set by the diffusion length of the atoms and is compatible with the chip- scale atomic clock architectures [2]. It is especially valuable in VCSEL-based devices where the noise due to the laser signifi- cantly contributes to the overall instability of the instrument. The basic idea presented in this paper aims at illuminating spatially distinct parts of an alkali vapor cell with the light of two different polarizations: one linear and one circular. If a dc magnetic field is oriented along the direction of propagation of the light field, only the light beam with a circular polarization excites a CPT resonance on the m g1 =0 → m g2 =0 transi- tion. In the case of the linearly polarized beam, each of the two circularly polarized components forms its own Λ-system, cor- responding to m g1 =0, m g2 =0 and m e = ±1 [see Fig. 1(a)]. Because of the signs of the Clebsh–Gordan coefficients, the transition amplitudes of these Λ-systems interfere destructively [11], and no superposition of the ground state components exists that is uncoupled from all light fields. Therefore, no CPT 0018-9456/$25.00 © 2008 IEEE