Semiconductor laser device

1. A semiconductor laser device comprising:

a compound semiconductor layer including an active layer emitting laser light, the compound semiconductor layer having a first mesa structure; and

a silicon layer bonded with the compound semiconductor layer, a diffraction grating being provided on a surface of the silicon layer facing the compound semiconductor layer, the diffraction grating including a main diffraction grating and two sub-diffraction gratings, the main diffraction grating extending in a longitudinal direction of the first mesa structure, the sub-diffraction gratings being disposed on both sides of the main diffraction grating,

wherein

each of the main diffraction grating and the two sub-diffraction gratings is entirely disposed directly under the first mesa structure

the main diffraction grating is disposed at a region containing a node of a first higher-order transverse mode of the laser light,

each of the sub-diffraction gratings is disposed at a region containing a node of a second higher-order transverse mode of the laser light,

and

each of the sub-diffraction gratings is disposed with its period and phase substantially equal to those of the main diffraction grating.

2. The device according to claim 1 , further comprising a phase shifter provided at the center of the diffraction grating in the longitudinal direction of the first mesa structure.

3. The device according to claim 2 , wherein

a product κL of a coupling coefficient κ of the diffraction grating for a fundamental transverse mode and a resonator length L defined as a length of the diffraction grating in the longitudinal direction of the first mesa structure is set within a range from 1 to 1.5 inclusive.

4. The device according to claim 1 , wherein

a width of the main diffraction grating is set to 10% or less of that of a portion of the first mesa structure facing the silicon layer.

5. The device according to claim 1 , wherein

the silicon layer has a second mesa structure,

the diffraction grating is formed on a surface of the second mesa structure, and

a width of the main diffraction grating is set to 10% or less of that of the second mesa structure.

6. The device according to claim 1 , wherein

the width of each of the sub-diffraction gratings is set to smaller than that of the main diffraction grating and greater than half that of the main diffraction grating.

7. The device according to claim 1 , wherein

the main diffraction grating is disposed below a central point of the first mesa structure in a width direction thereof, and

the sub-diffraction gratings are disposed below respective points at which the first mesa structure is divided into three equal parts in the width direction.

8. The device according to claim 1 , wherein

the silicon layer has a second mesa structure,

the diffraction grating is formed on a surface of the second mesa structure,

the main diffraction grating is disposed at a central point of the second mesa structure in a width direction thereof, and

the sub-diffraction gratings are disposed at respective points at which the second mesa structure is divided into three equal parts in the width direction.

9. The device according to claim 1 , wherein

the compound semiconductor layer is in direct contact with the silicon layer.

10. The device according to claim 1 , further comprising an insulating layer between the compound semiconductor layer and the silicon layer, the insulating layer having a thickness of 100 nm or less.

11. The device according to claim 1 , further comprising a silicon dioxide layer in contact with an opposite surface of the silicon layer from the compound semiconductor layer.

12. The device according to claim 1 , wherein

the device is a quantum cascade laser emitting light by a transition between subbands in a superlattice.