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Chinese Physics Letters

Dual-Wavelength Bad Cavity Laser as Potential Active Optical Frequency Standard

Zhi-Chao Xu, Duo Pan, Wei Zhuang, Jingbiao Chen

Published Sep 1, 2015• 13 citations
Publisher

Abstract

We experimentally realize the dual-wavelength bad cavity laser for the first time. As the Cs cell temperature is kept between 118°C and 144°C, both the 1359 nm and 1470 nm lasing outputs of dual-wavelength bad cavity laser are detected. The laser output power of dual-wavelength bad cavity laser is measured when changing the 455 nm pumping laser frequency and power at 127°C Cs cell temperature. Both the 1359 nm laser and the 1470 nm laser are working at the deep bad cavity regime, and the ratio between the linewidth of cavity mode and the laser gain bandwidth a ≈ 40 for 1359 nm and 1470 nm lasers. The 1470 nm laser linewidth is measured to be 407.3 Hz. The dual-wavelength bad cavity laser operating on atomic transitions demonstrated here has a potential in the application as a stable optical local oscillator, even an active optical frequency standard directly in the future.

DOI
10.1088/0256-307x/32/9/093201

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We experimentally realize the dual-wavelength bad cavity laser for the first time. As the Cs cell temperature is kept between 118°C and 144°C, both the 1359 nm and 1470 nm lasing outputs of dual-wavelength bad cavity laser are detected. The laser output power of dual-wavelength bad cavity laser is measured when changing the 455 nm pumping laser frequency and power at 127°C Cs cell temperature. Both the 1359 nm laser and the 1470 nm laser are working at the deep bad cavity regime, and the ratio between the linewidth of cavity mode and the laser gain bandwidth a ≈ 40 for 1359 nm and 1470 nm lasers. The 1470 nm laser linewidth is measured to be 407.3 Hz. The dual-wavelength bad cavity laser operating on atomic transitions demonstrated here has a potential in the application as a stable optical local oscillator, even an active optical frequency standard directly in the future.
We experimentally realize the dual-wavelength active optical clock for the first time. As the Cs cell temperature is kept between 118 ∘C^{\circ }C∘C and 144 ∘C^{\circ }C∘C, both the 1359 nm and the 1470 nm stimulated emission output of Cs four-level active optical clock are detected. The 1470 nm output linewidth of each experimental setup of Cs four-level active optical clock is measured to be 590 Hz with the main cavity length unstabilized. To stabilize the cavity length of active optical clock, the experimental scheme of 633 nm and 1359 nm good-bad cavity dual-wavelength active optical clock is proposed, where 633 nm and 1359 nm stimulated emission is working at good-cavity and bad-cavity regime respectively. The cavity length is stabilized by locking the 633 nm output frequency to a super-cavity with the Pound-Drever-Hall (PDH) technique. The frequency stability of 1359 nm bad-cavity stimulated emission output is then expected to be further improved by at least 1 order of magnitude than the 633 nm PDH system due to the suppressed cavity pulling effect of active optical clock, and the quantum limited linewidth of 1359 nm output is estimated to be 77.6 mHz.
We have experimentally demonstrated the active optical clock with a good suppression to cavity-length noise compared with the passive one. However, the frequency stability of the four-level active optical clock for thermal atoms is still limited by the residual cavity-pulling effect. In this paper, we preliminarily realized the cavity-length stabilization in 1064/1470 nm dual-wavelength good-bad cavity system. The synchronous cavity-length change between two dual-wavelength systems is realized by using the phase locking technique of 1064 nm goodcavity lasers. Therefore, the beating linewidth of 1470 nm badcavity lasers will also be reduced considering that the 1064/1470 nm dual-wavelength lasers share the one cavity. The tracking accuracy of 1064 nm good-cavity lasers is better than 3×10-16at 1 s after phase-locking, and the linewidth of 1470 nm bad-cavity laser is reduced to 77 Hz. The linewidth of active optical frequency standard is still limited by the light shift, collision shift and Zeeman shift. The beating signal of magnetic sublevels corresponding to the Cs 1470 nm clock transition is observed under Zeeman effect. Further research is needed, such as, the design of vacuum chamber and magnetic shielding, to reduce the linewidth of the continuous 1470 nm active optical frequency standard.
We realize the lasing of cesium active optical clock with 459 nm laser pumping, which operates deep in the optical bad-cavity regime. The ratio between cavity bandwidth and gain bandwidth is 55.08 theoretically. With 459 nm laser pumping, the population inversion occurs between the cesium 7S1/2state and 6P3/2state, and the stimulated radiation between the two states is realized. The 1469.9 nm output signal of cesium active optical clock can reach a power of 8 μW. The output power also shows a threshold character with pumping laser power and can be influenced by the Cs cell temperature. This active optical clock has an advantage in stability over the conventional optical clocks, due to its suppression of cavity pulling effect, which is also demonstrated experimentally.
We are setting up an experiment to investigate an active optical clock scheme in four-level configuration with Cesium atoms. The 1469 nm Cesium active optical clock output in a bad cavity laser regime is generated and the wavelength is measured to be 1469.500 nm by using spectrometer. The 1469 nm lasing threshold and output power is measured when changing 455 nm pumping laser power and frequency. The 1469 nm Cesium active optical clock output multi-threshold is observed when changing bad-cavity length. We also sweep the bad-cavity length and measure the 1469 nm Cesium active optical clock output frequency using a wavelength meter, proving the cavity-pulling reduction due to the bad-cavity effect.
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 experimentally demonstrate the collective emission behavior and suppressed cavity-pulling effect of four-level active optical clock with Cesium atoms. Thermal Cesium atoms in a glass cell velocity selective pumped with a 455.5 nm laser operating at 6S1/2_{1/2}1/2​ to 7P3/2_{3/2}3/2​ transition are used as lasing medium. Population inverted Cesium atoms between 7S1/2_{1/2}1/2​ and 6P3/2_{3/2}3/2​ levels are optical weakly coupled by a pair cavity mirrors working at deep bad-cavity regime with a finesse of 4.3, and the ratio between cavity bandwidth and gain bandwidth is approximately 45. With increased 455.5 nm pumping laser intensity, the output power of cesium active optical clock at 1469.9 nm from 7S1/2_{1/2}1/2​ level to 6P3/2_{3/2}3/2​ level shows a threshold and reach a power of 13 μμμW. Active optical clock would dramatically improve the optical clock stability since the lasing frequency does not follow the cavity length variation exactly, but in a form of suppressed cavity pulling effect. In this letter the cavity pulling effect is measured using a Fabry-Perot interferometer (FPI) to be reduced by a factor of 38.2 and 41.4 as the detuning between the 1469.9 nm cavity length of the Cs active optical clock and the Cs 1469.9 nm transition is set to be 140.8 MHz and 281.6 MHz respectively. The mechanism demonstrated here is of great significance for new generation optical clocks and can be applied to improve the stability of best optical clocks by at least two orders of magnitude.