A branch portion 101 branches an inputted electrical signal into an in-phase
signal and an opposite phase signal which have an opposite relation as to a phase.
A first FM laser 104 converts the in-phase signal into an optical frequency-modulated
signal (a first optical signal) having a center wavelength 1 and
then outputs the resultant signal. A second FM laser 105 converts the opposite
phase signal into an optical frequency-modulated signal (a second signal) having
a center wavelength 2 and then outputs the resultant signal. The
two optical signals are combined and then inputted into an optical-electrical converting
portion 106. The optical-electrical converting portion 106 subjects
the inputted optical signals to optical heterodyne detection by its square-law
detection characteristic, and outputs a beat signal between the two optical signals
which is a wide-band FM signal at a frequency corresponding to a wavelength difference
(=|1-;2|) between the first optical signal and
the second optical signal. It is thus possible to increase frequency deviation
of the outputted FM signal and thus greatly improve a CNR.
