We report our recent progress on detection of large frequency difference (up to 1.028 THz, Deltalambda = 2.5 nm) between two laser diodes at 852 nm, using a Schottky diode as harmonic mixer/detector. Using the 11th harmonic of a klystron operating at 93.5 GHz or the 991-GHz line of an optically pumped HCOOH far-infrared laser, we were able to observe a signal-to-noise ratio of 2 dB in a 1-MHz-resolution bandwidth.