SUPERLUMINESCENT DIODE

1 . A superluminescent diode having, above a substrate, a layered portion including at least a first cladding layer, a luminescent layer, and a second cladding layer in this order, the superluminescent diode comprising

an optical waveguide provided in the layered portion, the optical waveguide having a refractive-index guiding structure,

wherein the optical waveguide includes:

a first mesa portion formed by processing the second cladding layer into the first mesa portion having a first width; and

a second mesa portion formed by processing the first cladding layer, the luminescent layer, and the second cladding layer into the second mesa portion having a second width greater than the first width.

2 . The superluminescent diode according to claim 1 ,

wherein a distance between a side surface of the first mesa portion and a side surface of the second mesa portion ranges from 0.1 μm to 2.0 μm.

3 . The superluminescent diode according to claim 1 ,

wherein a distance between a side surface of the first mesa portion and a side surface of the second mesa portion is constant throughout the optical waveguide.

4 . The superluminescent diode according to claim 1 ,

wherein the first width and the second width vary gradually in a direction of light propagation through the optical waveguide.

5 . The superluminescent diode according to claim 1 ,

wherein the optical waveguide is linear, and

the optical waveguide has a front end face and a rear end face whose normals are tilted relative to a longitudinally extending axis of the optical waveguide.

6 . The superluminescent diode according to claim 1 ,

wherein the optical waveguide is linear,

the optical waveguide has a front end face whose normal is tilted relative to a longitudinally extending axis of the optical waveguide, and

the optical waveguide has a rear end face whose normal is parallel to the longitudinally extending axis of the optical waveguide.

7 . The superluminescent diode according to claim 1 ,

wherein the optical waveguide includes a linear waveguide portion and a curved waveguide portion,

the curved waveguide portion has an end face that is a front end face of the optical waveguide, and

the linear waveguide portion has an end face that is a rear end face of the optical waveguide.

8 . The superluminescent diode according to claim 7 ,

wherein the curved waveguide portion has a radius of curvature of 1000 μm or more.

9 . The superluminescent diode according to claim 6 ,

wherein a high reflectivity layer including a dielectric multilayer film is formed on the rear end face.

10 . The superluminescent diode according to claim 6 ,

wherein a low reflectivity layer including a dielectric single-layer film or a dielectric multilayer film is formed on the front end face.

11 . The superluminescent diode according to claim 5 ,

wherein a low reflectivity layer including a dielectric single-layer film or a dielectric multilayer film is formed on each of the front end face and the rear end face.

12 . The superluminescent diode according to claim 1 ,

wherein the second mesa portion has a protrusion spaced apart from the first mesa portion.

13 . The superluminescent diode according to claim 1 ,

wherein a first portion of the luminescent layer located under the first mesa portion has a bandgap smaller than a bandgap of a second portion of the luminescent layer in the second mesa portion located between a side surface of the first mesa portion and a side surface of the second mesa portion.

14 . The superluminescent diode according to claim 1 ,

wherein a first portion of the luminescent layer located under the first mesa portion and a second portion of the luminescent layer in the second mesa portion are located at different levels in a stacking direction of the layered portion.

15 . The superluminescent diode according to claim 1 ,

wherein the layered portion comprises a III-nitride semiconductor represented as Al x Ga y In 1-x-y N, where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1.

16 . The superluminescent diode according to claim 1 ,

wherein the layered portion comprises a III-V compound semiconductor represented as Al x Ga y In 1-x-y As z P 1-z , where 0≦x≦1, 0≦y≦1, 0≦z≦1, and 0≦x+y≦1.

17 . The superluminescent diode according to claim 1 ,

wherein the second cladding layer comprises a conductive transparent material having a refractive index of 2.5 or less, and

the conductive transparent material also acts as an electrode.

18 . The superluminescent diode according to claim 17 ,

wherein the first mesa portion has a height of 150 nm or less.

19 . The superluminescent diode according to claim 17 ,

wherein the first cladding layer comprises Al x In 1-x N, where 0<x<1, and

the first cladding layer has a refractive index of 2.4 or less.