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

1. A vertical cavity surface emitting semiconductor laser (VCSEL) comprising;

a semiconductor multilayer film, the semiconductor multilayer film including 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, and

a protective film that covers the top distributed Bragg reflecting mirror, the protective film allowing the laser light emitted from the top distributed Bragg reflecting mirror to pass therethrough,

the protective film has a central region and a peripheral region, the central region including a projected point located on the protective film, the projected point being obtained by projecting a center point in the active layer onto the protective film in a direction in which layers of the semiconductor multilayer film 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= ( 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 protective film in the central region;

dp is a film thickness of the protective film in the peripheral region;

n is a refractive index of the protective film (wherein a refractive index of air<the refractive index n of the protective film<a refractive index of the top distributed Bragg reflecting mirror); and

N is a natural number;

M is 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 (1/2)λ, and dc×n of the central region is (3/4)λ or (5/4)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.