Optical semiconductor device, optical module, and method for manufacturing optical semiconductor device

1. An optical semiconductor device comprising:

an InP semiconductor substrate;

a lower mesa structure that is disposed above the InP semiconductor substrate, and includes a multiple quantum well layer;

an upper mesa structure that is disposed on the lower mesa structure, and includes a cladding layer;

a buried semiconductor layer that buries both side surfaces of the lower mesa structure; and

an insulating film that covers both side surfaces of the upper mesa structure by being in contact with both side surfaces of the upper mesa structure,

wherein the lower mesa structure includes a first semiconductor layer, above the multiple quantum well layer,

wherein the upper mesa structure includes a second semiconductor layer which is different from the cladding layer in composition and below the cladding layer,

wherein a refractive index of the second semiconductor layer is higher than the refractive index of the cladding layer, and

wherein the second semiconductor layer includes a diffraction grating layer and a separate confinement heterostructure layer.

2. The optical semiconductor device according to claim 1 ,

wherein the second semiconductor layer further comprises an electron stop layer.

3. The optical semiconductor device according to claim 1 ,

wherein the cladding layer is an InP layer, and

each of the diffraction grating layer and the separate confinement heterostructure layer is a layer of plural kinds of elements including phosphorus (P).

4. The optical semiconductor device according to claim 1 ,

wherein the first semiconductor layer is a separate confinement heterostructure layer,

including plural kinds of elements including aluminum (Al).

5. The optical semiconductor device according to claim 1 ,

wherein the multiple quantum well layer is a layer of plural kinds of elements including aluminum (Al).

6. An optical semiconductor device array, comprising:

a plurality of the optical semiconductor devices according to claim 1 ,

wherein the optical semiconductor devices are disposed on the InP semiconductor substrate side by side.

7. The optical semiconductor device array according to claim 6 , further comprising:

an isolation groove that isolates the buried semiconductor layer located between the optical semiconductor devices which are adjacent to each other.

8. A method for manufacturing an optical semiconductor device, comprising:

stacking a lower semiconductor multi-layer and an upper semiconductor multi-layer on an InP semiconductor substrate, the lower semiconductor multi-layer including a multiple quantum well layer and a first semiconductor layer in sequence, and the upper semiconductor multi-layer including a second semiconductor layer, including a separate confinement heterostructure layer and a diffraction grating layer, and a cladding layer in sequence, the second semiconductor layer being different from the cladding layer in composition, wherein a refractive index of the second semiconductor layer is higher than the refractive index of the cladding layer;

forming an upper mesa structure by forming a first mask that extends a longitudinal direction of an optical waveguide of the optical semiconductor device on the upper semiconductor multi-layer, and removing one or more portions of the upper semiconductor multi-layer which lie outside of the first mask seen from above the optical semiconductor device;

forming a lower mesa structure by removing one or more portions of the lower semiconductor multi-layer which lie outside of a second mask that surrounds an upper surface and sides of the upper mesa structure seen from above the optical semiconductor device;

covering both sides of the lower mesa structure with a buried semiconductor layer; and

covering both sides of the upper mesa structure and an upper surface of the buried semiconductor layer by being in contact with an insulating film.

9. An optical module comprising:

the optical semiconductor device according to claim 1 ; and

a lens that focuses light emitted from the optical semiconductor device.