  Higher-order mode locking has been proposed to reduce the thermal noise limit
of reference cavities. By locking a laser to the HG02 mode of a 10-cm long all
ULE cavity, and measure its performance with the three-cornered-hat method
among three independently stabilized lasers, we demonstrate a thermal noise
limited performance of a fractional frequency instability of 4.9E-16. The
results match the theoretical models with higher-order optical modes. The
achieved laser instability improves the all ULE short cavity results to a new
low level.
