A single mode fiber laser has a linewidth of a few kHz, corresponding to a coherence time of a few hundred microseconds. Hydrogen masers have linewidth around 1 Hz, corresponding to a coherence time of about one second. Their coherence length approximately corresponds to the distance from the Earth to the Moon.
a singLe mode FiBer laser has A liNEwidTH oF A Few khz, coRrESpondIng to a CoheREnce tIMe Of a fEW huNDred MicRoseConDS. hyDrogen masers have lINewidtH Around 1 hz, corresPoNding to a CohErEnce Time oF aBout OnE seConD. Their CoHerenCe LEngth approXimATely cOrrESPONds to The disTAnCe from tHe eartH to ThE moon.