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

Active optical frequency standard based on narrow bandwidth Faraday atomic filter

Xiaogang Zhang, Jingbiao Chen, Wei Zhuang

Published Apr 1, 2015• 3 citations
Publisher

Abstract

Active optical frequency standard based on narrow bandwidth Faraday atomic filter is proposed. The output laser power achieved to 42 µW. The laser frequency is determined by narrow bandwidth Faraday atomic filter, corresponding to the Cs 62S1/2F=4 to 62P3/2F′=4, 5 crossover transition line. The transmission of the Faraday atomic filter is 23% and the bandwidth is 52.6MHz. Considering the loss in the cavity, the linewidth of the cavity mode is 345.1MHz. The bad cavity coefficient is 6.6. Working in the bad cavity scheme, the output laser signal of active Faraday optical frequency standard is insensitive to the cavity vibration and thermal noise. Besides, we redesign an integrated structure to decrease the influence of the environmental vibration. With more steady structure, hundreds of millihertz level of the active Faraday optical frequency linewidth is expected.

DOI
10.1109/fcs.2015.7138921

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Active optical frequency standard based on narrow bandwidth Faraday atomic filter is proposed. The output laser power achieved to 42 µW. The laser frequency is determined by narrow bandwidth Faraday atomic filter, corresponding to the Cs 62S1/2F=4 to 62P3/2F′=4, 5 crossover transition line. The transmission of the Faraday atomic filter is 23% and the bandwidth is 52.6MHz. Considering the loss in the cavity, the linewidth of the cavity mode is 345.1MHz. The bad cavity coefficient is 6.6. Working in the bad cavity scheme, the output laser signal of active Faraday optical frequency standard is insensitive to the cavity vibration and thermal noise. Besides, we redesign an integrated structure to decrease the influence of the environmental vibration. With more steady structure, hundreds of millihertz level of the active Faraday optical frequency linewidth is expected.
We proposed a scheme for realizing active Faraday optical frequency standards using diffuse laser cooling of alkali-metal atoms. A 1-m-long cold87Rb atomic cloud was prepared within a ϕ 2 cm ×$l$1 m vacuum glass tube, featuring a maximum optical density of 4. By utilizing this setup to construct the Faraday anomalous dispersion optical filter, we are able to achieve an ultra-narrowband laser gain close to the natural linewidth of the atomic transition and corresponding to the hyperfine transition frequency. This ultra-narrowband laser gain can directly determine the output frequency of the active Faraday optical frequency standard operating in the “bad cavity” regime, significantly suppressing the cavity-pulling effect. Furthermore, due to the suppression of atomic collision and Doppler frequency shifts by the cold atoms, it is expected to further narrow the laser linewidth of the cold-atom-based active Faraday optical frequency standard to Hz level.
We propose the mechanism of an active Faraday optical clock, and experimentally demonstrate an active Faraday optical frequency standard based on narrow bandwidth Faraday atomic filter by the method of velocity-selective optical pumping of cesium vapor. The center frequency of the active Faraday optical frequency standard is determined by the cesium 6 (2)S(1/2) F=4 to 6 (2)P(3/2) F'=4 and 5 crossover transition line. The optical heterodyne beat between two similar independent setups shows that the frequency linewidth reaches 281(23) Hz, which is 1.9×10(4) times smaller than the natural linewidth of the cesium 852-nm transition line. The maximum emitted light power reaches 75 μW. The active Faraday optical frequency standard reported here has advantages of narrow linewidth and reduced cavity pulling, which can readily be extended to other atomic transition lines of alkali and alkaline-earth metal atoms trapped in optical lattices at magic wavelengths, making it useful for new generation of optical atomic clocks.
Known as a unique optical clock, active optical clock has prior advantages to surpass the limitation of the conventional optical clock in potential stability. In this paper, we proposed an improved Cesium active Faraday optical frequency standard with 459 nm laser pumping. This scheme reduces the residual Doppler broadening in previous results with 852 nm laser pumping. The transmission is 2.4% and the bandwidth of the transmission peak is 12.3 MHz. It greatly narrows the bandwidth of Faraday anomalous dispersion optical filter and decreases the length of the bad cavity used in Cesium active Faraday optical frequency standard. This will be very helpful for further suppression of cavity pulling effect.
Two feasible schemes are proposed by laser cooling of alkali atoms to realize the narrow-linewidth, continual superradiant lasing. Utilizing the D1 gray molasses to realize a cloud of cold Cs atoms for the two-level Faraday active optical clock (AOC), the effect of Doppler broadening on the transmission bandwidth of Faraday atomic filter will be effectively suppressed to the level of natural linewidth. Thus, the influence of mechanical or thermal vibrations of cavity mirrors on the clock laser will be decreased by about 102 times, which is expected to further narrow the laser linewidth by two orders of magnitude compared with the thermal atomic Faraday AOC to Hz level. Moreover, the four-level AOC based on the narrow linewidth cooling of Cs atoms using the 6S-7P transition is proposed. The 455 nm laser at 6S1/2-7P3/2transition plays two roles simultaneously, one is the cooling laser for a magneto-optical trap (MOT), another is the pumping laser for population inversion of 1470 nm clock transition, which is helpful to improve the compactness of the system. In the four-level structure, the frequency of pumping laser is far detuning from that of clock laser, therefore, the light shift resulting from the 455 nm laser is negligible. This four-level AOC based on cold atoms will further reduce the collision shift in thermal atom system to achieve a high-precision optical frequency reference.
Abstract We demonstrate a Faraday laser using a 1.2 km fiber as an extended cavity, which provides optical feedback and obtains small free spectrum range (FSR) of 83 kHz, and have succeeded in limiting the laser frequency to a crossover transition 5 2 S 1 / 2 , F = 2 → 5 2 P 3 / 2 , F ′ = 1 , 3 of the natural 87 Rb at 780 nm. The Faraday laser is based on a Faraday anomalous dispersion optical filter (FADOF) with an ultra-narrow bandwidth and the long fiber extended cavity of 1.2 km. The peak transmission assigned to the crossover transition F = 2 → F ′ = 1 , 3 in the FADOF is 20.5% with an ultra-narrow bandwidth of 29.1 MHz. The Allan deviation of the Faraday laser is around 6.0 × 10 − 11 in 0.06 to 1 s sampling time. Laser frequency is always kept in the center of the transmitted peak assigned to F = 2 → F ′ = 1 , 3 . The Faraday laser realized here can provide light exactly resonant with an atomic transition used for atom–photon interaction experiments and is insensitive to diode temperature and injection current fluctuations.
Using laser cooling technology, we demonstrate an ultranarrow-bandwidth Faraday anomalous dispersion optical filter (FADOF) which operates on the52S1/25^{2}\text{S}_{1/2}52S1/2​(F=2F = 2F=2) to62P3/26^{2}\text{P}_{3/2}62P3/2​(F′=3F' = 3F′=3) transition at 420 nm based on cold 87Rb atoms. The filter achieves a single 2.7(2) MHz passband at a peak transmission of 3.2%. It is the narrowest bandwidth FADOF known at present, as the influence of Doppler broadening on the bandwidth narrowing of FADOF is effectively overcome by cold atoms. This ultranarrow-bandwidth FADOF has a figure of merit (FOM) value of 11.85 GHz−1, which is seven times larger than the largest FOM achieved by thermal vapor atomic optical filter published to date. Furthermore, the scheme can be extended to almost all kinds of atomic optical filters and may find applications in active Faraday optical frequency standard and self-stabilizing laser.