The present invention provides a compact, highly reliable optical module in which
a light-emitting element and optical modulator element that differ from each other
in thermal expansion coefficient are mounted in one casing. In the optical module
having a differential thermal expansion coefficient less than 510-6
[1/K] between the light-emitting element and the casing, and a differential
thermal expansion coefficient of at least 510-6 [1/K] between
the optical modulator element and the casing, a section at which the optical modulator
element and the casing are fixed is equal to or less than 10 mm wide in the direction
of an optical axis.