  Standard Quantum Limit (SQL) of a classical mechanical force detection
results from quantum back action impinged by the meter on a probe mechanical
transducer perturbed by the force of interest. In this paper we introduce a
technique of continuous \vy{broadband} back action avoiding measurements for
the case of a resonant signal force acting on a linear mechanical oscillator
supporting one of mirrors of an optical Michelson-Sagnac Interferometer (MSI).
The interferometer with the movable mirror is an opto-mechanical transducer
able to support polychromatic probe field. The method involves a dichromatic
optical probe resonant with the MSI modes and having frequency separation equal
to the mechanical frequency. We show that analyzing each of the harmonics of
the probe reflected from the mechanical system separately and postprocessing
the measurement results allows excluding the back action in a broad frequency
band and measuring the force with sensitivity better than SQL.
