Semiconductor laser

1. A semiconductor laser comprising:

a multi-quantum well layer included in a mesa structure;

a buried layer comprising a semi-insulating semiconductor,

wherein the buried layer is in contact with a first side and a second side of the mesa structure;

a first cladding layer with a first conductivity type, under the mesa structure and the buried layer, wherein

the first cladding layer has a lower refractive index than the multi-quantum well layer,

the first cladding layer is one of a pair of first cladding layers, and

a thickness of one of the pair of first cladding layers that is adjacent to the multi-quantum well layer is greater than a thickness of another of the pair of first cladding layers;

a high refractive index layer configured to not absorb light oscillating in the multi-quantum well layer, under the mesa structure and the buried layer, below the first cladding layer, wherein:

the high refractive index layer has a higher refractive index than the first cladding layer, and

the high refractive index layer is one of a pair of high refractive index layers that are alternately stacked with the pair of first cladding layers;

a diffraction grating layer at least partially constituting a diffraction grating capable of diffracting the light oscillating in the multi-quantum well layer,

wherein the diffraction grating layer does not contact the high refractive index layer;

a substrate with the first conductivity type, below the high refractive index layer; and

a second cladding layer with a second conductivity type opposite to the first conductivity type, above the multi-quantum well layer.

2. The semiconductor laser according to claim 1 , further comprising:

a first separate confinement heterostructure layer with the first conductivity type, between the multi-quantum well layer and the first cladding layer, the first separate confinement heterostructure layer being included in the mesa structure; and

a second separate confinement heterostructure layer with the second conductivity type, between the multi-quantum well layer and the second cladding layer, the second separate confinement heterostructure layer being included in the mesa structure.

3. The semiconductor laser according to claim 1 , wherein the first cladding layer has a thickness of 500 nm or more.

4. The semiconductor laser according to claim 3 , wherein the first cladding layer has a thickness of 1500 nm or less.

5. The semiconductor laser according to claim 1 , wherein the high refractive index layer has a thickness of 50 nm or more.

6. The semiconductor laser according to claim 5 , wherein the high refractive index layer has a thickness of 100 nm or less.

7. The semiconductor laser according to claim 1 , wherein

the pair of high refractive index layers comprises two layers: an upper layer closer to the multi-quantum well layer and a lower layer farther from the multi-quantum well layer, and

the upper layer is thinner than the lower layer.

8. The semiconductor laser according to claim 1 , wherein

the pair of high refractive index layers include a first high refractive index layer that is closest to the multi-quantum well layer and a second high refractive index layer that is farther from the multi-quantum well layer than the first high refractive index layer,

the semiconductor laser further includes a third high refractive index layer that is farthest from the multi-quantum well layer relative to the first high refractive index layer and the second high refractive index layer, and

the second high refractive index layer is thinner than either the first high refractive index layer or the third high refractive index layer.

9. The semiconductor laser according to claim 1 , wherein

the mesa structure includes a spot size converter section gradually decreasing in width perpendicular to a light emission direction, and

the diffraction grating layer, at the spot size converter section, constitutes a grating configured not to diffract the light oscillating in the multi-quantum well layer.

10. The semiconductor laser according to claim 1 , wherein the second cladding layer is included in the mesa structure.

11. The semiconductor laser according to claim 10 , wherein the diffraction grating layer is inside the second cladding layer.

12. The semiconductor laser according to claim 1 , wherein the second cladding layer is on the multi-quantum well layer and the buried layer.

13. The semiconductor laser according to claim 12 , further comprising a third cladding layer between the high refractive index layer and the substrate,

wherein the diffraction grating layer is between the third cladding layer and the substrate.

14. The semiconductor laser according to claim 12 , wherein the mesa structure is between a pair of grooves with depth of being through the buried layer and the first cladding layer to the substrate.

15. The semiconductor laser according to claim 1 , wherein the first conductivity type is an n-type, and the second conductivity type is a p-type.

16. The semiconductor laser according to claim 1 , wherein the high refractive index layer comprises at least one of InGaAsP, InGaAs, or InGaAlAs.

17. The semiconductor laser according to claim 1 , wherein at least one of the substrate, the buried layer, the first cladding layer, or the second cladding layer comprises InP.

18. The semiconductor laser according to claim 1 , wherein the high refractive index layer has a lower refractive index than the multi-quantum well layer.

19. A semiconductor laser comprising:

a multi-quantum well layer included in a mesa structure;

a buried layer comprising a semi-insulating semiconductor, the buried layer being in contact with a first side and a second side of the mesa structure;

a first cladding layer with a first conductivity type, under the mesa structure and the buried layer, wherein

the first cladding layer has a lower refractive index than the multi-quantum well layer, and

the first cladding layer is one of multiple first cladding layers;

a high refractive index layer configured to not absorb light oscillating in the multi-quantum well layer, under the mesa structure and the buried layer, below the first cladding layer, wherein

the high refractive index layer has a higher refractive index than the first cladding layer,

the high refractive index layer is one of multiple high refractive index layers, and

the multiple high refractive index layers comprise:

an upper layer that is closer to the multi-quantum well layer, and

a lower layer that is farther from the multi-quantum well layer,

 wherein the lower layer is thicker than the upper layer;

a diffraction grating layer at least partially constituting a diffraction grating capable of diffracting the light oscillating in the multi-quantum well layer,

wherein the diffraction grating layer does not contact the high refractive index layer;

a substrate with the first conductivity type, below the high refractive index layer; and

a second cladding layer with a second conductivity type opposite to the first conductivity type, above the multi-quantum well layer.

20. The semiconductor laser according to claim 19 , wherein the multiple high refractive index layers further comprise:

a lowest layer that is farthest, of the multiple high refractive index layers, from the multi-quantum well layer,

wherein the lowest layer is thinnest of the multiple high refractive index layers.