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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

CPT pulse excitation method based on VCSEL current modulation for miniature atomic clocks

Takumi Ide, Shigeyoshi Goka, Yuichiro Yano

Published Apr 1, 2015• 10 citations
Publisher

Abstract

We propose a coherent population trapping (CPT) pulsed-excitation method based on direct drive-current modulation of a vertical-cavity surface-emitting LASER for miniature atomic clocks (MACs). Because our method needs only a simple device configuration without optical external modulators, it is suitable for MACs, including chip-scale atomic-clocks. Reducing the size, cost, and power consumption is also possible. Additionally, we report on experimental results for Ramsey-CPT fringes excited using the proposed method.

DOI
10.1109/fcs.2015.7138815

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We propose a coherent population trapping (CPT) pulsed-excitation method based on direct drive-current modulation of a vertical-cavity surface-emitting LASER for miniature atomic clocks (MACs). Because our method needs only a simple device configuration without optical external modulators, it is suitable for MACs, including chip-scale atomic-clocks. Reducing the size, cost, and power consumption is also possible. Additionally, we report on experimental results for Ramsey-CPT fringes excited using the proposed method.
We propose a pulse excitation method of coherent population trapping (CPT) with a liquid crystal optical modulator. Since liquid crystals enable reductions in size, weight, cost, and power consumption compared with acousto-optical modulators (AOMs), our method is suitable for chip-scale atomic clocks (CSACs). Experiments showed that pulse excitation with a liquid crystal modulator can narrow the CPT resonance linewidth and reduce the light shift effect using a 87Rb gas cell and the D1-line vertical-cavity surface-emitting laser (VCSEL). The CPT resonance linewidth and light shift sensitivity were less than one-eighth and one-third those for continuous excitation, respectively. They also showed that our method is comparable to that based on an AOM.
We report on the operation of a coherent population trapping (CPT) microcell atomic clock using a pulsed Ramsey-like interrogation. The Ramsey-CPT sequence, defined by two-step optical pulses separated by a free-evolution dark time, is produced by switching on and off the output power of a low-power vertical-cavity surface-emitting laser, through direct modulation of its driving current. High-contrast and narrow Ramsey-CPT fringes are detected without the use of any external optical modulator stage. We demonstrate closed-loop operation of the clock based on high-speed digital signal processing implemented in a field programmable gate array board. The clock's short-term fractional frequency stability is 1.3 × 10−10τ−1/2 until 2000 s. A power light-shift coefficient of 8 × 10−11/μW, in relative value, is obtained for a dark time of 150 μs. This value is about ten times lower than in the continuous regime. These results show the feasibility of fully integrated atomic clocks based on Ramsey spectroscopy, which could provide enhanced long-term stability.
Chip-scale atomic clocks are critical for applications requiring compact, energy-efficient, and highly stable timing, such as GPS receivers and communication systems. Coherent population trapping (CPT)-based atomic clocks, which commonly use vertical-cavity surface-emitting lasers (VCSELs), have enabled significant miniaturization. However, their precision is limited by the broad linewidth of VCSELs. Alternative laser technologies, such as distributed Bragg reflector lasers, distributed feedback lasers, and external cavity diode lasers (ECDLs), offer potential improvements but face challenges in achieving compact size and adequate modulation bandwidth. This study presents a 795 nm chip-ECDL with a direct modulation bandwidth up to 4 GHz, specifically designed for CPT chip-scale atomic clocks. Based on the chip-ECDL, a CPT atomic clock with a compact volume of 19.4 ml and power consumption of 1.5 W has been developed. Under identical operating conditions, the CPT signal produced by the chip-ECDL exhibits a 34% reduction in linewidth and a 1.14-fold increase in contrast compared to a VCSEL-based clock. Demonstrating short-term stability better than 5×10−11τ−1/2(1 s<τ<2000 s) and reaching a best stability of 6.6×10−13 at 2000 s, this work represents the application of an ECDL in a CPT chip-scale clock, advancing the development of next-generation, high-performance chip-scale atomic clocks.
The spectroscopic technique of coherent population trapping (CPT) enables an all-optical interrogation of the groundstate hyperfine splitting of cesium (or rubidium), compared to the optical-microwave double resonance technique conventionally employed in atomic frequency standards. All-optical interrogation enables the reduction of the size and power consumption of an atomic clock by two orders of magnitude, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources due to their low power consumption and circular output beam. Several research teams are currently using VCSELs for DARPA's chip-scale atomic clock (CSAC) program with the goal of producing an atomic clock having a volume < 1 cm^3, a power consumption < 30 mW, and an instability (Allan deviation) < 1x10^-11 during a 1-hour averaging interval. This paper describes the VCSEL requirements for CPT-based atomic clocks, which include single mode operation, single polarization operation, modulation bandwidth > 4 GHz, low power consumption (for the CSAC), narrow linewidth, and low relative intensity noise (RIN). A significant manufacturing challenge is to reproducibly obtain the required wavelength at the specified VCSEL operating temperature and drive current. Data are presented that show the advantage of operating at the D1 (rather than D2) resonance of the alkali atoms. Measurements of VCSEL linewidth will be discussed in particular, since atomic clock performance is especially sensitive to this parameter.
A scheme for a Ramsey-coherent population trapping (CPT) atomic clock that eliminates the acousto-optic modulator (AOM) is proposed and experimentally studied. Driven by a periodically microwave modulated current, the vertical-cavity surface-emitting laser emits a continuous beam that switches between monochromatic and multichromatic modes. Ramsey-CPT interference has been studied with this mode-switching beam. In eliminating the AOM, which is used to generate pulsed laser in conventional Ramsey-CPT atomic clock, the physics package of the proposed scheme is virtually the same as that of a conventional compact CPT atomic clock, although the resource budget for the electronics will slightly increase as a microwave switch should be added. By evaluating and comparing experimentally recorded signals from the two Ramsey-CPT schemes, the short-term frequency stability of the proposed scheme was found to be 46% better than the scheme with AOM. The experimental results suggest that the implementation of a compact Ramsey-CPT atomic clock promises better frequency stability.
Microcell CPT atomic clock using laser current-actuated power modulation with 10−12^{-12}−12 range stability at 1 day
We present a coherent-population trapping (CPT) microcell atomic clock using symmetric auto-balanced Ramsey (SABR) spectroscopy. The pulsed SABR sequence is applied through direct current-based power modulation of the vertical-cavity surface-emitting laser, eliminating the need for an external optical shutter and enabling compatibility with fully-integrated clocks. The sequence is controlled by a single FPGA-based digital electronics board. A key aspect of proper clock operation was the implementation of a real-time tracking of the atomic signal detection window. The clock frequency dependence on laser power, microwave power, laser frequency, and timing of the detection window has been measured, obtaining sensitivity coefficients lower than those obtained with Ramsey-CPT spectroscopy. The Allan deviation of the SABR-CPT clock, based on a microfabricated cell with low-permeation glass windows, is 1.1~×\times×~10−9^{-9}−9 at 1~s and averages down to the low 10−12^{-12}−12 range at 1 day integration time. These results pave the way towards the development of Ramsey-CPT chip-scale atomic clocks with enhanced timekeeping performances.