The optical transmission module, which is configured to input differential signals including a positive-phase signal and a negative-phase signal, includes: a laser oscillator configured to output laser light; an optical modulator configured to modulate the laser light based on the positive-phase signal; a negative-phase terminating resistor configured to terminate the negative-phase signal; a Peltier device including a first region and a second region in which heat is exchanged; a substrate mounted to the first region so that heat is conducted to the first region; and a package which is hermetically sealed, the laser oscillator and the optical modulator being mounted on the substrate so that heat is conducted to the first region, the negative-phase terminating resistor being arranged outside of the substrate so as to avoid conduction of heat to the first region.

A laser diode (1) includes an AlN single crystal substrate (11), an n-type cladding layer (12) formed on the substrate and including a nitride semiconductor layer having n-type conductivity, a light-emitting layer (14) formed on the n-type cladding layer and including one or more quantum wells, a p-type cladding layer (20) formed on the light-emitting layer and including a nitride semiconductor layer having p-type conductivity, and a p-type contact layer (18) formed on the p-type cladding layer and including a nitride semiconductor that includes GaN. The p-type cladding layer includes a p-type longitudinal conduction layer (16) that includes Al.sub.sGa.sub.1-sN (0.3s1), has a composition gradient such that the Al composition s decreases with increased distance from the substrate, and has a film thickness of less than 0.5 m, and a p-type transverse conduction layer (17) that includes Al.sub.tGa.sub.1-tN (0<t1).