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Journal of the Korean Physical Society

Optoelectronic oscillator stabilized to an intra-loop atomic filter with a coherent population trapping resonance

Sin Hyuk Yim, D. Cho

Published Jul 1, 2012• 3 citations
Publisher

Abstract

We report the construction and characterization of an optoelectronic oscillator stabilized to an intra-loop 85Rb cell with a 3.036-GHz coherent population trapping (CPT) resonance. We use an extended-cavity diode laser (ECDL) as a light source and directly modulate its injection current by using the beating signal filtered by the CPT resonance. Use of the ECDL guarantees stability of the carrier frequency and eliminates a bulky and power-consuming external modulator. We also adjust the cavity length and operate the ECDL in multimode in order to achieve a high modulation efficiency. To further improve the oscillator’s frequency stability, we incorporate a negative feedback system based on a CPT dispersion. The Allan deviation of the oscillator frequency is 3 × 10 −10 at a 100-s integration time.

DOI
10.3938/jkps.61.17

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Journal of the Optical Society of America B•Feb 1, 2003•131 citations
We report the construction and characterization of an optoelectronic oscillator stabilized to an intra-loop 85Rb cell with a 3.036-GHz coherent population trapping (CPT) resonance. We use an extended-cavity diode laser (ECDL) as a light source and directly modulate its injection current by using the beating signal filtered by the CPT resonance. Use of the ECDL guarantees stability of the carrier frequency and eliminates a bulky and power-consuming external modulator. We also adjust the cavity length and operate the ECDL in multimode in order to achieve a high modulation efficiency. To further improve the oscillator’s frequency stability, we incorporate a negative feedback system based on a CPT dispersion. The Allan deviation of the oscillator frequency is 3 × 10 −10 at a 100-s integration time.
The frequency pulling effect in an optoelectronic oscillator (OEO) due to the high dispersion produced by coherent population trapping (CPT) resonance in an intraloop Rb85 cell has been experimentally measured. With the direct modulation of the injection current of an extended-cavity diode laser, we construct the OEO and a beating signal with a frequency of 3.036 GHz is filtered by CPT resonance. The effective total loop length of the OEO under the CPT resonance condition can be significantly increased. As the optical path length of the OEO is changed, the microwave-frequency shift of the OEO corresponding to the effective total loop length is measured and reported.
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.
We present a microwave frequency standard based on coherent population trapping (CPT) in the 87Rb D1 line. The CPT spectrum is obtained using two Raman lasers with a 6.8 GHz frequency offset by injection locking of a master laser to a slave laser. We have constructed an atomic clock employing a 5 cm long Rb vapor cell confined with 6.67 kPa neon buffer gas at 70 °C. Using this system, we improve the CPT contrast through the elimination of undesired off-resonant fields created by the direct modulation method. We measured the frequency shift of the CPT signal as a function of the temperature of the Rb cell and estimated it to be approximately 1.3 ×10−9/K. The frequency of a 10 MHz crystal oscillator has been stabilized to the CPT spectrum between the two ground states in 87Rb. The relative frequency stability is approximately 2.3×10−12 for an average time of 68 s.
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.
An all-optical microwave frequency standard based on coherent population trapping (CPT) in 85Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at <20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two mF=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10-11 τ-1/2(1 s<τ<2000 s). The best stability of 6.4×10-13 is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.