Optical semiconductor device

1. An optical semiconductor device which comprises plural semiconductor lasers, plural light detectors, plural waveguides and an optical multiplexer circuit that are formed on a semiconductor substrate,

said plural semiconductor lasers each serving to output first light from a front-end side thereof and also to output second light from a rear-end side thereof that is a side opposite to the front-end side;

said optical multiplexer circuit serving to multiplex respective rays of the first light outputted from the plural semiconductor lasers, to thereby send out output light;

said plural waveguides serving to guide respective rays of the second light toward one end face of the optical semiconductor device; and

said plural light detectors serving to receive respective rays of reflected light that are due to reflection of the respective rays of the second light after being guided by the waveguides, on the one end face or on respective inclined end faces in plural concave portions formed on the one end face;

wherein the light detectors are each located between the rear-end side of each of the semiconductor lasers and the one end face or each of the inclined end faces; and

wherein the second light to be outputted from each of the waveguides is outputted diagonally relative to a perpendicular line with respect to the one end face or the inclined end face.

2. The optical semiconductor device of claim 1 , further comprising mesa grooves each provided between the light detector and the semiconductor laser that outputs the second light to be received by that light detector.

3. The optical semiconductor device of claim 2 , wherein the mesa groove is formed nearer to the light detector than to the semiconductor laser.

4. The optical semiconductor device of claim 3 , wherein the light detector extends in a manner inclined relative to an extending direction of the semiconductor laser.

5. The optical semiconductor device of claim 3 , further comprising an optical amplifier for amplifying the output light sent out from the optical multiplexer circuit.

6. The optical semiconductor device of claim 3 , wherein the waveguide has, in its end region located on a side where the second light is outputted, a tapered portion that becomes wider in a traveling direction of that light.

7. The optical semiconductor device of claim 3 , wherein the light detector comprises:

a first cladding layer formed on a front surface of the semiconductor substrate;

a light absorption layer formed on a front surface of the first cladding layer;

current blocking layers each formed on a front surface of the first cladding layer and connected to each of side surfaces of the light absorption layer;

a second cladding layer formed on front surfaces of the light absorption layer and the current blocking layers;

an anode electrode formed on a front-surface side of the second cladding layer; and

a cathode electrode formed on a back surface of the semiconductor substrate.

8. The optical semiconductor device of claim 2 , wherein the light detector extends in a manner inclined relative to an extending direction of the semiconductor laser.

9. The optical semiconductor device of claim 2 , further comprising an optical amplifier for amplifying the output light sent out from the optical multiplexer circuit.

10. The optical semiconductor device of claim 2 , wherein the waveguide has, in its end region located on a side where the second light is outputted, a tapered portion that becomes wider in a traveling direction of that light.

11. The optical semiconductor device of claim 2 , wherein the light detector comprises:

a first cladding layer formed on a front surface of the semiconductor substrate;

a light absorption layer formed on a front surface of the first cladding layer;

current blocking layers each formed on a front surface of the first cladding layer and connected to each of side surfaces of the light absorption layer;

a second cladding layer formed on front surfaces of the light absorption layer and the current blocking layers;

an anode electrode formed on a front-surface side of the second cladding layer; and

a cathode electrode formed on a back surface of the semiconductor substrate.

12. The optical semiconductor device of claim 1 , wherein the light detector extends in a manner inclined relative to an extending direction of the semiconductor laser.

13. The optical semiconductor device of claim 1 , wherein the light detector extends in a manner inclined relative to an extending direction of the semiconductor laser, and

wherein mesa grooves are provided each being formed so as to surround one of long sides of the light detector being located nearer to the semiconductor laser that outputs the second light to be received by that light detector, and one of short sides thereof being located farther from the one end face or the inclined end face.

14. The optical semiconductor device of claim 1 , wherein the light detector extends in a manner inclined relative to an extending direction of the semiconductor laser, and

wherein mesa grooves are provided each being formed so as to surround both long sides of the light detector and one of short sides thereof being located farther from the one end face or the inclined end face.

15. The optical semiconductor device of claim 1 , further comprising an optical amplifier for amplifying the output light sent out from the optical multiplexer circuit.

16. The optical semiconductor device of claim 1 , wherein the waveguide has, in its end region located on a side where the second light is outputted, a tapered portion that becomes wider in a traveling direction of that light.

17. The optical semiconductor device of claim 1 , wherein the light detector comprises:

a first cladding layer formed on a front surface of the semiconductor substrate;

a light absorption layer formed on a front surface of the first cladding layer;

current blocking layers each formed on a front surface of the first cladding layer and connected to each of side surfaces of the light absorption layer;

a second cladding layer formed on front surfaces of the light absorption layer and the current blocking layers;

an anode electrode formed on a front-surface side of the second cladding layer; and

a cathode electrode formed on a back surface of the semiconductor substrate.

18. The optical semiconductor device of claim 17 , wherein the semiconductor laser comprises:

a first cladding layer formed on a front surface of the semiconductor substrate;

an active layer formed on a front surface of the first cladding layer;

current blocking layers each formed on a front surface of the first cladding layer and connected to each of side surfaces of the active layer;

a second cladding layer formed on front surfaces of the active layer and the current blocking layers;

an anode electrode formed on a front-surface side of the second cladding layer; and

a cathode electrode formed on a back surface of the semiconductor substrate,

wherein the active layer and the light absorption layer of the light detector are formed on the front surfaces of the first cladding layers that are provided as a common layer.

19. The optical semiconductor device of claim 18 , wherein the anode electrodes of the plural light detectors are connected to each other.

20. The optical semiconductor device of claim 17 , wherein the anode electrodes of the plural light detectors are connected to each other.

21. The optical semiconductor device of claim 1 , wherein

each of the light detectors is offset from an axis of the semiconductor laser in an extending direction of the semiconductor laser toward the light detector.