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Review of Scientific Instruments

Quasi-bichromatic laser for a lin⊥lin coherent population trapping clock produced by vertical-cavity surface-emitting lasers

Peter Yun, Bozhong Tan, Wei Deng, Jing Yang, Sihong Gu

Published Sep 1, 2012• 17 citations
Publisher

Abstract

With two vertical-cavity surface-emitting lasers working under the master-slave sideband injection-locking configuration, we have realized a quasi-bichromatic laser beam with residual phase noise Δφ(2) < 0.282 rad(2). The two wanted frequency components share more than 96% power of the beam. With the realized beam, we have carried out coherent population trapping (CPT) resonance experiment with (87)Rb in the lin[perpendicular]lin CPT scheme, and recorded CPT resonance signal with contrast of 60%. Such laser system is promising to realize a lin[perpendicular]lin CPT clock with high performance and low power consumption.

DOI
10.1063/1.4754629

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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.

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arXiv:2103.00707•Metrologia•May 11, 2021•22 citations
With two vertical-cavity surface-emitting lasers working under the master-slave sideband injection-locking configuration, we have realized a quasi-bichromatic laser beam with residual phase noise Δφ(2) < 0.282 rad(2). The two wanted frequency components share more than 96% power of the beam. With the realized beam, we have carried out coherent population trapping (CPT) resonance experiment with (87)Rb in the lin[perpendicular]lin CPT scheme, and recorded CPT resonance signal with contrast of 60%. Such laser system is promising to realize a lin[perpendicular]lin CPT clock with high performance and low power consumption.
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.
We present a chip scale lin⊥lin quasi-bichromatic laser beam generator scheme and our experimental study results. Through modulating two vertical-cavity surface-emitting lasers (VCSELs) with the same microwave, both VCSELs output multichromatic lasers and every frequency component in their outputs contributes for the frequency mutual injection locking between VCSELs. The system outputs a lin⊥lin quasi-bichromatic laser beam in which the two wanted frequency components share more than 90% of the total laser power. It is promising to apply the chip device in the chip scale coherent population trapping (CPT) atomic clock and the chip scale CPT magnetometer.
Coherent population trapping (CPT) atomic clocks are prevailingly realized by exciting CPT resonance with circularly polarized bichromatic coherent light (σ+-σ+ scheme), while atom utilization efficiency is relatively low in the scheme. For improving efficiency, we experimentally study the scheme of realizing CPT resonance through the interaction between atoms and parallel linearly polarized bichromatic coherent light field (lin//lin scheme). In the same experimental conditions the experimental results show that the signal-noise ratio of the CPT resonance signal is twice, and the differential slope is 1.65 time higher than those of σ+-σ+ scheme. The experimental results reveal that the scheme is an ideal candidate for low power consuming atomic clock and magnetometer.
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.
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.
Abstract 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 87 Rb 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.