  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
$\mu$W. 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.
