  We demonstrated a frequency offset locking between two laser sources using a
waveguide-type electro-optic modulator (EOM) with 10th-order sidebands for
magneto-optical trapping of Fr atoms. The frequency locking error signal was
successfully obtained by performing delayed self-homodyne detection of the beat
signal between the repumping frequency and the 10th-order sideband component of
the trapping light. Sweeping the trapping-light and repumping-light frequencies
with keeping its frequency difference of 46 GHz was confirmed over 1 GHz by
monitoring the Doppler absorption profile of I2. This technique enables us to
search for a resonance frequency of magneto-optical trapping of Fr.
