Lasing and suppressed cavity-pulling effect of Cesium active optical clock

Abstract

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 6S 1/2 to 7P 3/2 transition are used as lasing medium. Population inverted Cesium atoms between 7S 1/2 and 6P 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 7S 1/2 level to 6P 3/2 level shows a threshold and reach a power of 13 µW. 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.

Cold-Strontium Laser in the Superradiant Crossover Regime

“…The bad-cavity regime of laser physics has been explored in the optical domain in gas lasers [15], with homogeneous and inhomogeneous transition linewidths of hundreds of MHz, and in a 4-level system in Cesium with an Doppler-broadened gain bandwidth of 9 MHz [16]. In the microwave domain, masers operate deep in the bad-cavity regime [17].…”

Dual-wavelength active optical clock

“…According to our previous experimental results, the maximum output power of the dualwavelength Cs four-level active optical clock is obtained when the 455 nm pumping laser frequency is locked to the Cs 455 nm 6S 1/2 (F=4) and 7P 3/2 (F'=5) transition [50]. The 455 nm pumping laser power introduced into the main cavity is measured to be 11.77 mW.…”

“…The 455 nm pumping laser power introduced into the main cavity is measured to be 11.77 mW. The velocity selective pumping scheme where the 455 nm pumping laser beam is aligned parallel to the 1359 nm/1470 nm main cavity mode is employed to reduce the laser gain bandwidth to about 9 MHz, while the linewidth of the main cavity mode is measured to be more than 400 MHz [50]. The laser gain bandwidth is then much narrower than the main cavity mode linewidth as shown in Fig.…”

“…Currently the 1470 nm lasing of Cs four-level active optical clock has been realized [49,50,51]. The cavity pulling effect is measured to be reduced by a factor of about 40 [49,51] and a linewidth of sub-kHz is observed.…”

“…Unlike the traditional passive optical clocks, active optical clocks [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54] utilize the stimulated emission output of the unperturbed clock transition as optical frequency standards, and the super-cavity stabilized probing lasers are thus unnecessary. Population inversion is established between the two levels of the clock transition and a cavity (the main cavity) is employed to provide weak optical feedback to the stimulated emission of active optical clock.…”