  Optical rigidity in aLIGO gravitational-wave detector, operated on dark port
regime, is unstable. We show that the same interferometer with excluded
symmetric mechanical mode but with unbalanced arms allows to get stable optical
spring for antisymmetric mechanical mode. Arm detuning necessary to get
stability is shown to be a small one - it corresponds to small power in signal
port. We show that stable optical spring may be also obtained in
Michelson-Sagnac interferometer with both power and signal recycling mirrors
and unbalanced arms.
