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We experimentally demonstrate the collective emission behavior and suppressedcavity-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 nmlaser operating at 6S$_{1/2}$ to 7P$_{3/2}$ transition are used as lasingmedium. Population inverted Cesium atoms between 7S$_{1/2}$ and 6P$_{3/2}$levels are optical weakly coupled by a pair cavity mirrors working at deepbad-cavity regime with a finesse of 4.3, and the ratio between cavity bandwidthand gain bandwidth is approximately 45. With increased 455.5 nm pumping laserintensity, the output power of cesium active optical clock at 1469.9 nm from7S$_{1/2}$ level to 6P$_{3/2}$ level shows a threshold and reach a power of 13$\mu$W. Active optical clock would dramatically improve the optical clockstability since the lasing frequency does not follow the cavity lengthvariation exactly, but in a form of suppressed cavity pulling effect. In thisletter 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 the1469.9 nm cavity length of the Cs active optical clock and the Cs 1469.9 nmtransition is set to be 140.8 MHz and 281.6 MHz respectively. The mechanismdemonstrated here is of great significance for new generation optical clocksand can be applied to improve the stability of best optical clocks by at leasttwo orders of magnitude.

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