  We report a velocity selective bi-polarization spectroscopy (VS-BPS)
technique to generate a background-free, dispersion-like reference signal which
is tunable over a wide range of frequency. In this technique, a pair of
linearly polarized weak probe beams passing through a gas cell of metastable
Krypton (Kr*) atoms, overlaps with a pair of counter-propagating circularly
polarized strong pump beams derived from an independently tunable control
laser. The polarization spectroscopy signals from the two probe beams, after
subtraction, result in VS-BPS signal. The spectral shifting in VS-BPS signal
can be achieved by tuning the frequency of the control laser. The dependence of
the amplitude and slope of the VS-BPS signal on the RF power used for
excitation of Kr atoms in the gas cell and on the power of pump beams has been
studied. The frequency stability of a diode laser locked with VS-BPS signal has
been found to be better than the frequency stability of the laser locked with a
saturated absorption spectroscopy (SAS) signal. The VS-BPS signal is finally
used for stabilization and tuning of the cooling laser frequency for a
magneto-optical trap (MOT) for Kr* atoms.
