An optical apparatus includes a cooling device with a lower clad disposed thereon; a waveguide disposed on the lower clad and including an active waveguide to define a gain section and a passive waveguide to define a wavelength-tunable section; gratings disposed in the lower clad of the wavelength-tunable section; an upper clad disposed on the waveguide; a first upper electrode disposed on the upper clad of the gain section; and a second upper electrode disposed on the upper clad of the wavelength-tunable section. The lower clad of the wavelength-tunable section has a recess region to expose an upper surface of the cooling device, the recess region forming an air gap-having a height of 10 μm to 80 μm from the upper surface of the cooling device. The gratings are formed in a depth of at least 5 μm from a bottom surface of the lower clad of the recess region.