1. Field of the Invention
The present invention relates to a semiconductor laser with a self-sustained pulsation in which the noises attributable to the optical feedback of a laser beam are reduced.
2. Description of Related Art
The problem of conventional semiconductor lasers is that in the case where the optical feedback of a laser beam is applied repeatedly, a noise attributable to the optical feedback (called the optical feedback induced noise) is generated in the laser beam. The optical feedback induced noise is generated in such a manner that the optical feedback of the laser beam due to the reflection from the disk surface or the like reenters the semiconductor laser when it is used, for example, as a light source of an optical disk system.
An approach to a reduced optical feedback induced noise of the semiconductor laser by utilizing the self-sustained pulsation is disclosed by Japanese Patent Application Laid-Open No.63-202083(1988), for example. In this semiconductor laser, one of cladding layers sandwiching an active layer is constructed of a refractive index layer having a band gap energy considerably larger than the energy corresponding to the lasing wavelength (energy corresponding to lasing wavelength: h.nu., where h is Planck's constant and .nu. frequency) or an optical absorbing layer having a band gap energy considerably smaller than the energy corresponding to lasing wavelength to generate a self-sustained pulsation.
According to the above-mentioned conventional semiconductor laser, however, in the case where a refractive index layer is used with a band gap energy considerably larger than the energy corresponding to lasing wavelength, the astigmatic distance is increased, while the use of an optical absorbing layer with a band gap energy considerably smaller than the energy corresponding to lasing wavelength increases the operating current. With a large astigmatic distance, the laser beam is difficult to converge. In the case where the semiconductor laser is used as a light source for an optical disk system or the like, therefore, the S/N for crosstalks is deteriorated, and an increased operating current increases the power consumption.
Japanese Patent Application Laid-Open No. 61-84891 (1986), on the other hand, discloses a semiconductor laser having a thin film layer (saturable optical absorbing layer) formed only in one of the cladding layers, which has substantially the same composition as the active layer and a saturable optical absorption characteristic against a laser beam. Such a saturable optical absorbing layer can be easily formed by crystal growth, thereby making it possible to manufacture a semiconductor laser with a high volume productivity.
Nevertheless, experiments conducted by the present inventors have revealed that in the case of the above-mentioned conventional semiconductor laser, it is difficult to reduce the astigmatic distance sufficiently only by the relation between the composition ratio of the saturable optical absorbing layer and that of the active layer described.
If a high-output semiconductor laser is to be realized, a desired structure includes a first cladding layer, an active layer and a second cladding layer having a ridge section. This structure makes it necessary to control the film thickness of the flat portion of the second cladding layer at high accuracy. Japanese Journal of Applied Physics Vol.25, No.6, June 1986, pp. L498-L500, for example, discloses a semiconductor laser including a compound semiconductor containing Al in which an etching stop layer having an Al composition ratio different from the second cladding layer and absorbing no pulsated laser beam is formed on the flat portion to control the thickness of the flat portion.
Also, Nikkei Electronics, Aug. 31, 1981, pp.76-79 presents a semiconductor laser in which a very thin barrier layer for enclosing the carriers strongly without substantially affecting the light is formed on both sides of the active layer, and light is enclosed not by the refractive index difference between the active layer and the barrier layer but by the index difference between the active layer and the cladding layers thereby to improve the output beam divergence perpendicular to the junction. The disadvantage of this semiconductor laser is that thickness variations of the thin barrier layer have a great effect on the laser characteristics. It is therefore necessary to control the film thickness at high accuracy in order to avoid a low production yield.