  In this letter, we demonstrate a technique of making an ultrastable laser
referenced to a multi-cavity, corresponding to a lower thermal noise limit due
to the larger equivalent beam size. The multi-cavity consists of several pairs
of mirrors and a common spacer. We can stabilize the laser frequencies on these
cavities, and average the laser frequencies with synthesizing technique. In
comparison with a single cavity system, relative frequency instability of the
synthesized laser can be improved by a factor of the squire root of the cavity
number (n). In addition, we perform an experiment to simulate a two-cavity
system. Experimental results show that frequency instability of the synthesized
laser is improved by a factor of 1.4, and discrimination of the laser frequency
instability, introduced by the process of lasers synthesizing, is negligible,
and can reach a floor at low level 10-18 limited by noise of currently used
signal generators. This technique is comparable with other techniques; thus, it
can gain a factor of the squre root of n on the frequency instability of an
ultrastable laser to an unprecedented level.
