Optical Transmitter

1 . An optical transmitter, comprising:

a distributed feedback (DFB) laser having an active region formed as a multiple quantum well that generates an optical gain by current injection and a diffraction grating;

an electro-absorption (EA) modulator having an absorption region formed as a multiple quantum well having a composition different from a composition of the DFB laser;

a semiconductor amplifier (SOA) having an active region having a same composition as the composition of the DFB laser;

a bent waveguide that rotates a light propagation direction by an angle 0 wg; and

a passive waveguide connected to the SOA and including a core having a band gap wavelength shorter than an oscillation wavelength of the DFB laser, and

wherein

the DFB laser, the EA modulator, the SOA, the bent waveguide, and the passive waveguide are monolithically integrated on one substrate,

the passive waveguide has a tapered region and a narrow waveguide region,

the tapered region converts a width W1 of the passive waveguide connected to the SOA into a width W2 of the narrow waveguide region, where W1>W2 is satisfied, and

the passive waveguide forms the angle θwg with respect to a normal line to an end surface of the substrate and is in contact with the end surface of the substrate.

2 . The optical transmitter according to claim 1 , further comprising

a second passive waveguide, wherein

the second passive waveguide

is located at a position separated from the passive waveguide by a distance D,

includes a core having a same composition as a composition of the passive waveguide,

is in contact with the end surface of the substrate at the angle θwg with respect to the normal line to the end surface of the substrate,

terminates in the substrate,

has the width W2, and

has a length L_d.

3 . The optical transmitter according to claim 2 , wherein

the substrate is an InP substrate,

the DFB laser, the EA modulator, and the SOA are formed on a (100) surface of the InP substrate,

the DFB laser has an optical axis in a direction of a substrate crystal orientation, and

the passive waveguide forms an angle θw with respect to the substrate crystal orientation and is in contact with the end surface of the substrate.

4 . The optical transmitter according to claim 3 , wherein the second passive waveguide has

the width W2 at a position in contact with the end surface of the substrate, and

has an extended waveguide portion at a terminal end,

the extended waveguide portion has a width W3, and

W2<W3 is satisfied.

5 . The optical transmitter according to claim 4 , wherein W3>3 μm is satisfied.

6 . The optical transmitter according to claim 5 , wherein a terminal end surface of the extended waveguide portion matches with a substrate crystal plane (011).

7 . The optical transmitter according to claim 2 , wherein 5 μm<L_d<25 μm is satisfied.

8 . The optical transmitter according to claim 1 , wherein 0.4 μm<W2<0.8 μm is satisfied.

9 . The optical transmitter according to claim 1 , wherein 4°<θwg<8° is satisfied.

10 . The optical transmitter according to claim 2 , wherein 30 μm<D<140 μm is satisfied.