Multimode vertical cavity surface emitting laser having narrow laser light emitting angle

1. A vertical cavity surface emitting semiconductor laser (VCSEL) comprising a bottom distributed Bragg reflecting mirror, a top distributed Bragg reflecting mirror and an active layer that is used to generate laser light and that is positioned between the bottom distributed Bragg reflecting mirror and the top distributed Bragg reflecting mirror, the laser light being emitted from the top distributed Bragg reflecting mirror,

wherein the top distributed Bragg reflecting mirror includes high refractive index layers and low refractive index layers that are alternately stacked, one of the high refractive index layers being a top high refractive index layer positioned at a laser light emitting surface of the top distributed Bragg reflecting mirror, wherein the top high refractive index layer has a central region and a peripheral region, the central region including a projected point located on the top high refractive index layer, the projected point being obtained by projecting a center point in the active layer onto the top distributed Bragg reflecting mirror in a direction in which layers of the top distributed Bragg reflecting mirror are stacked, and the peripheral region being positioned around the central region, wherein the central region has a protrusion that projects relative to the peripheral region in a direction in which the laser light is emitted,

wherein the VCSEL satisfies relationships below:

dp×n =(1/4+ N/ 2)×λ, and

dc×n=dp×n +(1/4+ M/ 2)×λ

where

λ is a wavelength of the laser light in vacuum;

dc is a film thickness of the top high refractive index layer in the central region;

dp is a film thickness of the top high refractive index layer in the peripheral region;

n is a refractive index of the top high refractive index layer; and

N and M are zero or a natural number.

2. The vertical cavity surface emitting semiconductor laser according to claim 1 , wherein dp×n of the peripheral region is (1/4)λ, and dc×n of the central region is (1/2)λ or λ.

3. The vertical cavity surface emitting semiconductor laser according to claim 1 , wherein the top distributed Bragg reflecting mirror has a current limiting layer at an end surface that faces the active layer, the current limiting layer including an electrically conductive circular aperture that limits a path for a drive current that generates the laser light, the central region having a diameter ranging from 60% to 120% of a diameter of the electrically conductive aperture.

4. The vertical cavity surface emitting semiconductor laser according to claim 1 , further comprising a protective film that covers the top distributed Bragg reflecting mirror, the protective film having a constant film thickness.

5. The vertical cavity surface emitting semiconductor laser according to claim 4 , wherein a product of the film thickness of the protective film and the refractive index of the protective film is (1/2)λ.