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Optics Letters

Dual microwave frequency modulation of the VCSEL injection current for a CPT-based atomic clock

Maria I. Vaskovskaya, Dmitry S. Chuchelov, Eugene A. Tsygankov, Vitali V. Vassiliev, Sergey A. Zibrov, Vladimir L Velichansky

Published Jul 31, 2024• 5 citations
Publisher

Abstract

Previously, we have proposed a method to control the emission spectrum of the vertical-cavity surface-emitting laser (VCSEL) with the synchronized modulation of the injection current at single and doubled frequencies. In this work, the above method is used to improve the metrological characteristics of the coherent population trapping (CPT) resonance in 87 Rb. The dual-frequency (DF) modulation reduces the carrier power and suppresses the light shift of the resonance frequency, if it is unattainable with the single-frequency modulation. In addition, a higher resonance contrast is achieved by equalizing the powers of the resonant components of the spectrum.

DOI
10.1364/ol.532983

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Applied Physics Letters•Jun 2, 2008•11 citations
Previously, we have proposed a method to control the emission spectrum of the vertical-cavity surface-emitting laser (VCSEL) with the synchronized modulation of the injection current at single and doubled frequencies. In this work, the above method is used to improve the metrological characteristics of the coherent population trapping (CPT) resonance in 87 Rb. The dual-frequency (DF) modulation reduces the carrier power and suppresses the light shift of the resonance frequency, if it is unattainable with the single-frequency modulation. In addition, a higher resonance contrast is achieved by equalizing the powers of the resonant components of the spectrum.
A lin⊥lin quasi-bichromatic light can be employed to realize a double-Λ coherent population trapping (CPT) resonance configuration, thereby enabling a CPT atomic clock with improved performance. A method for generating such light is proposed, in which two lasers with different linewidths are used to achieve a microwave frequency separation of several gigahertz. The effects of the optical injection ratio from the master extended-cavity diode laser into the slave vertical-cavity surface-emitting laser and the microwave power modulating the slave laser on the locking range were systematically investigated. The influence of both the injection ratio and microwave power on the contrast, linewidth, and the ratio of contrast to linewidth of the CPT signal is also studied. To enhance the trapping efficiency, single-pass and multi-pass interactions between the lin⊥lin quasi-bichromatic light and 87Rb atoms are investigated, with near-purely coherent states achieved under multi-pass interactions. Our scheme offers a promising solution for high-performance atomic clock applications.
Abstract In this study, the light shift of coherent population trapping (CPT) resonances is studied in a miniature glass cell (~0.1 cm3) filled with 133Cs vapor. The atoms exposed to the radiation from a vertical-cavity surface-emitting laser (VCSEL). The laser current is modulated at a microwave frequency (≈4.6 GHz), which leads to the frequency modulation (FM) of output radiation. In addition, the light beam is transmitted through an electro-optic modulator (EOM), which is assembled as a Mach–Zehnder interferometer, acquiring amplitude modulation (AM). This dual (FM–AM) modulation leads to essentially nonlinear behavior of the function describing the light shift of the CPT resonance versus the total optical power in the cell. In particular, this function has an extremum that can be used to suppress the influence of small optical power variations in the cell on the stability of the CPT-based quantum frequency standard (QFS). The proposed scheme is characterized by a number of parameters such as the phase difference between the FM and AM signals as well as the bias voltage at EOM, which are additional degrees of freedom for controlling the behavior of the light shift of the CPT resonance. This can be used for the optimization of the QFS operation regime.
We report on the performance of the first table-top prototype atomic clock based on coherent population trapping (CPT) resonances with parallel linearly polarized optical fields (lin‖lin configuration). Our apparatus uses a vertical-cavity surface-emitting laser (VCSEL) tuned to the D1 line of 87Rb with the current modulation at the 87Rb hyperfine frequency. We demonstrate cancellation of the first-order light shift by the proper choice of rf modulation power and further improve our prototype clock stability by optimizing the parameters of the microwave lock loop. Operating in these optimal conditions, we measured a short-term fractional frequency stability (Allan deviation) 2×10−11τ−1/2 for observation times 1 s ≤ τ ≤ 20 s. This value is limited by large VCSEL phase noise and environmental temperature fluctuation. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated lin‖lin CPT resonance clock with environmental impacts minimized.
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.
We investigate a versatile vertical cavity surface-emitting laser (VCSEL) with a slight optical feedback from a simple wedge positioned at a well determined distance from the VCSEL. We use such a laser in conjunction with a miniature Rb atomic cell to detect coherent population trapping (CPT) when the external cavity mode spacing frequency corresponds to half the hyperfine frequency of Rb. The feedback greatly enhances the rf modulation efficiency of the VCSEL and as a result we obtain high contrast CPT signals at an input of −16dBm of rf modulation power compared to +1.25dBm needed in the absence of the external cavity.