Semiconductor laser capable of coupling with single mode optical fiber at high coupling efficiency
A semiconductor laser includes a substrate made of InP, an active layer including a multiquantum well structure, which is formed in a width of 7 to 14 μm on the substrate, and an n-type cladding layer made of InGaAsP and a p-type cladding layer made of InP, which are formed on the substrate with the active layer interposed therebetween. The semiconductor laser oscillates only in the fundamental lateral mode, and light emitted from an exit facet can be optically coupled with an external single mode optical fiber.
1. A semiconductor laser comprising:
a substrate made of InP;
an active layer including a multiquantum well structure formed above the substrate, the active layer having a width and a thickness which are constant in a direction of a resonator length L, the width of the active layer being in a range of 7 to 14 μm; and
an n-type cladding layer made of InGaAsP and a p-type cladding layer made of InP which are formed above the substrate with the active layer interposed therebetween, a bandgap wavelength of the InGaAsP of the n-type cladding layer being in a range between 0.96 μm and 0.98 μm,
wherein the n-type cladding layer made of InGaAsP has a refractive index that is higher than a refractive index of the p-type cladding layer made of InP, so that distribution of light generated in the active layer is biased to the n-type cladding layer to suppress an increase of optical loss due to inter valance band absorption in the p-type cladding layer, and
wherein light which oscillates only in a fundamental lateral mode to be emitted from an exit facet is configured to be optically coupled with an external single mode optical fiber without generation of a higher lateral mode, and the light emitted from the exit facet has a spot size such that a ratio of a horizontal size to a vertical size of the spot size is in a range of 1:0.65 to 1:1.35.
2. The semiconductor laser according to claim 1 , wherein the light emitted from the exit facet is optically coupled with the external single mode optical fiber without using a lens.
3. The semiconductor laser according to claim 1 , wherein the light emitted from the exit facet is optically coupled with the external single mode optical fiber by a butt joint.
4. The semiconductor laser according to claim 1 , further comprising:
a first separate confinement heterostructure (SCH) layer made of InGaAsP, which is formed between the active layer and the n-type cladding layer; and
a second SCH layer made of InGaAsP, which is formed between the active layer and the p-type cladding layer.
5. The semiconductor laser according to claim 4 , wherein
the first SCH layer includes a multilayer structure formed of a plurality of layers, and
the second SCH layer includes a multilayer structure formed of a plurality of layers.
6. The semiconductor laser according to claim 5 , wherein:
a refractive index of a plurality of partition layers in the active layer is ns;
respective refractive indices of said plurality of layers in the first SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the first SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
respective refractive idices of said plurality of layers in the second SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the second SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
a refractive index of the n-type cladding layer is na;
a refractive index of the p-type cladding layer in nb; and
the following relationships are satisfied:
t1=t2=t3=, . . . , =tN,
ns>n1>n2>n3>, . . . , >nN>na>nb, and
ns− n 1> n 1− n 2> n 2− n 3>, . . . , >nN−nb>nN−na.
7. The semiconductor laser according to claim 5 , wherein:
a refractive index of a plurality of partition layers in the active layer is ns;
respective refractive indices of said plurality of layers in the first SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the first SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
respective refractive indices of said plurality of layers in the second SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the second SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
a refractive index of the n-type cladding layer is na;
a refractive index of the p-type cladding layer is nb; and
the following relationships are satisfied:
ns>n1>n2>n3>, . . . , nN>na>nb,
nN−nb>nN−na,
ns− n 1= n 1− n 2= n 2− n 3=, . . . , =nN−nb, and
t1<t2<t3, . . . , <tN.
8. The semiconductor laser according to claim 5 , wherein:
a refractive index of a plurality of partition layers in the active layer is ns;
respective refractive indices of said plurality of layers in the first SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the first SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
respective refractive indices of said plurality of layers in the second SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the second SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
a refractive index of the n-type cladding layer is na;
a refractive index of the p-type cladding layer in nb; and
the following relationships are satisfied:
ns>n1>n2>n3>, . . . , >nN>na>nb,
ns− n 1> n 1− n 2> n 2− n 3>, . . . , >nN−nb >nN−na, and
t1<t2<t3<, . . . , <tN.
9. The semiconductor laser according to claim 5 , wherein:
a refractive index of a layer having a lowest refractive index among the layers forming the active layer is ns;
respective refractive indices of said plurality of layers in the first SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the first SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
respective refractive indices of said plurality of layers in the second SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thichness of said plurality of layers in the second SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from the side closer to the active layer;
a refractive index of the n-type cladding layer is na;
a refractive index of the p-type cladding layer is nb; and
the following relationships are satisfied:
t1=t2=t3=, . . . , =tN,
na>nN, and
ns−n1 >n 1 −n 2 >n 2 −n 3 >, . . . , >n ( N− 1)−nN.
10. The semiconductor laser according to claim 5 , wherein:
a refractive index of a layer having a lowest refractive index among the layers forming the active layer is ns;
respective refractive indices of said plurality of layers in the first SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the first SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
respective refractive indices of said plurality of layers in the second SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the second SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
a refractive index of the n-type cladding layer is na;
a refractive index of the p-type cladding layer is nb; and
the following relationships are satisfied:
ns>n1>n2>n3>, . . . , >nN>nb,
na>nN,
ns− n 1 =n 1 −n 2 =n 2 −n 3=, . . . , =nN−nb, and
t1>t2>t3>, . . . , >tN.
11. The semiconductor laser according to claim 5 , wherein:
a refractive index of a layer having a lowest refractive index among the layers forming the active layer is ns;
respective refractive indices of said plurality of layers in the first SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the first SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
respective refractive indices of said plurality of layers in the second SCH layer are n 1 , n 2 , n 3 , . . . , nN, respectively, and respective thickness of said plurality of layers in the second SCH layer are t 1 , t 2 , t 3 , . . . , tN, respectively, sequentially from a side closer to the active layer;
a refractive index of the n-type cladding layer is na;
a refractive index of the p-type cladding layer is nb; and
the following relationships are satisfied:
ns>n1>n2>n3>, . . . , >nN>nb,
na>nN,
ns− n 1 >n 1 −n 2 >n 2 −n 3 >, . . . , >n ( N −1)−nN, and
t1<t2<t3<, . . . , <tN.
12. The semiconductor laser according to claim 4 , wherein the semiconductor laser is formed in a buried heterostructure.
13. The semiconductor laser according to claim 12 , wherein part of the n-type cladding layer, the first SCH layer, the active layer, the second SCH layer, and the p-type cladding layer are formed in a mesa shape, and
wherein the semiconductor laser further comprises:
a first buried layer made of p-type InP, which has a first side that is in contact with the semiconductor substrate or the n-type cladding layer at both sides of the mesa shape; and
a second buried layer made of n-type InP, which has a first side that is in contact with the p-type cladding layer, and a second side that is in contact with a second side of the first buried layer at both sides of the mesa shape.
14. The semiconductor laser according to claim 1 , wherein the semiconductor laser is formed in a ridge structure.
15. The semiconductor laser according to claim 14 , wherein the semiconductor substrate is n-type, the p-type cladding layer comprises a ridge structure portion at which substantially a center of an outer side of the p-type cladding layer is raised upward, and
wherein semiconductor laser further comprises:
a contact layer formed on an upper side of the ridge structure portion of the p-type cladding layer;
an insulating layer which has an opening portion above a center of the contact layer, and which is formed to cover the p-type cladding layer including the ridge structure portion; and
an electrode formed on a top of the insulating layer with a part thereof connected to the contact layer.
16. The semiconductor laser according to claim 1 , wherein, the semiconductor substrate is n-type, the n-type cladding layer is formed beneath the active layer, and the p-type cladding layer is formed above the active layer.
17. The semiconductor laser according to claim 1 , wherein, the semiconductor substrate is p-type, the n-type cladding layer is formed above the active layer, and the p-type cladding layer is formed beneath the active layer.