The present invention relates to an optical device comprising a first sub-chip and a second sub-chip flipped over on the first sub-chip. The first sub-chip includes a first substrate, a first lower cladding pattern on a first surface of the first substrate, and a first core layer on the first lower cladding pattern. The second sub-chip includes a second substrate, a second lower cladding pattern on a second surface of the second substrate, and a second core layer on the second lower cladding pattern. The first surface of the first substrate faces the second surface of the second substrate. The first lower cladding pattern has a first top surface parallel to the first surface and a first sidewall inclined to the first surface. The first core layer includes a first core part on the first top surface and a first side part on the first sidewall.