Semiconductor VSIS Laser

1. Field of the Invention

The present invention relates to a semiconductor laser and, more particularly, to a semiconductor laser structure which minimizes mode competition noise derived from unstable longitudinal mode transition caused by temperature variations.

2. Description of the Prior Art

A semiconductor laser oscillating at the single longitudinal mode is widely used as a light source in an optical information processing system such as an information recording/drawing disc system. However, in the semiconductor laser of the above-mentioned type, unstable longitudinal mode transition occurs and mode competition noise is generated when the ambient temperature rises. The thus produced mode competition noise will preclude an accurate operation of the optical information processing system.

Accordingly, an object of the present invention is to provide a semiconductor laser which ensures a stable operation even when the ambient temperature is considerably high.

Another object of the present invention is to provide a V-channeled substrate inner stripe (VSIS) semiconductor laser which minimizes the occurrence of the mode competition noise caused by the unstable longitudinal mode transition.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

To achieve the above objects, pursuant to an embodiment of the present invention, in an index guided semiconductor laser having the inner stripe structure, the mole fraction of a cladding layer and the cavity length are selected at preferable values so that the mode competition noise occurs at a temperature higher than the normal operating temperature. The mode competition noise is minimized, whereby the semiconductor laser of the present invention is suited for a light source in an optical information processing system.

The present invention will be better understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein:

FIG. 1 is a graph showing the S/N ratio of the laser beam emitted from a typical semiconductor laser, the S/N ratio varying depending on the ambient temperature;

FIG. 2 is a perspective view of an embodiment of a V-channeled substrate inner stripe (VSIS) semiconductor laser of the present invention;

FIG. 3 is a graph showing the temperature at which mode competition noise occurs when the mole fraction of a cladding layer is changed;

FIG. 4 is a graph showing the temperature at which mode competition noise occurs when the cavity length is changed; and

FIG. 5 is a graph showing the temperature at which mode competition noise occurs when the reflectivity of a cleaved facet is changed.

FIG. 1 shows the S/N ratio of the laser output varying depending on the ambient temperature. The mode competition noise occurs when the lasing longitudinal mode goes and returns between a plurality of longitudinal modes. Generally, the mode competition noise occurs when the semiconductor laser is operated at a considerably high temperature. In FIG. 1, the mode competition noise occurs when the ambient temperature rises above 45.degree. C. When the mode competition noise occurs, the S/N ratio of the laser output (center frequency 8 MHz with 10 KHz bandwidth) reduces to 70 through 80 dB. Accordingly, the laser beam having the mode competition noise is not suited for the light source in an optical information processing system.

An index guided semiconductor laser generally oscillates in a single longitudinal mode at room temperature. However, the mode competition noise occurs as the ambient temperature rises. An index guided semiconductor laser having the inner stripe structure, such as a V-channeled substrate inner stripe (VSIS) semiconductor laser wherein an optical wave guide and a current confinement structure are formed at the same time, shows a stable operation as compared with the index guided semiconductor laser of the planer structure. The index guided semiconductor laser having the inner stripe generally includes a current blocking layer formed on a substrate. A V-shaped groove is formed in the current blocking layer, which operates as a current path. A multi layered crystal is formed on the current blocking layer. The index guided semiconductor laser having the inner stripe structure ensures a stable operation because the transverse optical intensity distribution in the optical waveguide and the carrier distribution in the active layer caused by the injection current coincide with each other. Furthermore, more, the index guided semiconductor laser having the inner stripe structure does not produce the mode competition noise even at a considerably high temperature, for example, 40.degree. C. through 50.degree. C. However, when the semiconductor laser is installed in an optical information processing system, the ambient temperature becomes about 50.degree. C. through 60.degree. C. Thus, there is a great possibility that the mode competition noise occurs when the normal index guided semiconductor laser having the inner stripe structure is used as the light source in the optical information processing system.

FIG. 2 shows an embodiment of a GaAs-GaAlAs V-channeled substrate inner stripe (VSIS) semiconductor laser of the present invention. A Te doped n-GaAs current blocking layer 2 having the carrier density of 2.times.10.sup.18 cm.sup.-3 is formed, through the use of the liquid phase epitaxial growth method, to a thickness of about 0.8 .mu.m on a Zn doped p-GaAs substrate 1 having the carrier density of 1.times.10.sup.18 cm.sup.-3. A V-shaped groove 3 is formed in the current blocking layer 2 to reach the substrate 1 through the use of an etching method. The V-shaped groove 3 preferably has the width of about 3 through 4 .mu.m, and the V-shaped groove 3 functions as a current path. A plurality of V-shaped grooves 3 are formed in the current blocking layer 2 with the pitch of 300 .mu.m. A Mg doped p-Ga.sub.1-x A1.sub.x As first cladding layer 4 is formed on the current blocking layer 2 to a thickness of about 0.1 .mu.m through 0.2 .mu.m by the liquid phase epitaxial growth method. Then, a Mg doped p-Ga.sub.1-y A1.sub.y As active layer 5 of a thickness of about 0.1 .mu.m is formed on the first cladding layer 4 by the liquid phase epitaxial growth method. Further, a Te doped n-Ga.sub.1-x A1.sub.x As second cladding layer 6 of a thickness of 1 .mu.m is formed on the active layer 5 by the liquid phase epitaxial growth method. Finally, a Te doped n-GaAs cap layer 7 is formed on the second cladding layer 6 to a thickness of about 3 .mu.m by the liquid phase epitaxial growth method. A p-electrode 8 and an n-electrode 9 are secured to the p-GaAs substrate 1 and the n-GaAs cap layer 7, respectively, through the use of the evaporation method. The thus formed wafer is divided in the 300 .mu.m width so that the V-shaped groove 3 is positioned at the center of the width. Further, the cleaved facets are formed at the both ends of the laser device. A multi-layered reflection film 10 is deposited on both the cleaved facets by the evaporation method. The laser oscillation is carried out when the carrier is injected from the p-electrode 8 and the n-electrode 9.

FIG. 3 shows the temperature at which the mode competition noise occurs when the A1As mole fraction x of the first and second cladding layers 4 and 6 is changed. The measurement is carried out when the VSIS semiconductor laser of the construction of FIG. 2 emits the laser beam at 3 mW. The VSIS semiconductor laser used herein has the cavity length of 250 .mu.m, and the cleaved facets on both ends have the same reflectivity R.sub.1 and R.sub.2 of 0.32. It will be clear from FIG. 3 that the mode competition noise does not occur below 50.degree. C. when the mole fraction x is selected higher than about 0.45. On the other hand, if the mole fraction x of the cladding layers is selected higher than about 0.52, it is difficult to form the index guide by the waveguide mechanism based on the effective refractive index approximation. That is, the gain guide is formed instead of the index guide, and the laser oscillation is carried out in the multiple longitudinal mode. Therefore, the AlAs mole fraction x of the first and second cladding layers should be selected between about 0.45 and 0.52 so as to ensure the single longitudinal mode oscillation without the mode competition noise.

FIG. 4 shows the temperature at which the mode competition noise occurs when the cavity length is changed in the VSIS semiconductor laser of the above-mentioned construction wherein the A1As mole fraction x of the first and second cladding layers is selected between 0.45 and 0.52. The output power is set at 3 mW, and the cleaved facets on both ends have the same reflectivity R.sub.1 and R.sub.2 of 0.32. It will be clear from FIG. 4 that the mode competition noise does not occur when the ambient temperature is less than 60.degree. C. if the cavity length is selected longer than about 300 .mu.m even though the normal cavity length of the conventional semiconductor laser is 250 .mu.m through 300 .mu.m.

FIG. 5 shows the temperature at which the mode competition noise occurs when the reflectivities R.sub.1 and R.sub.2 at the cleaved facets are changed while the A1As mole fraction x of the first and second cladding layers is selected at 0.45 or 0.5. The reflectivity R.sub.1 represents the reflectivity on the front facet, and R.sub.2 represents the reflectivity at the rear facet. Generally, R.sub.2 is greater than or equal to R.sub.1. The laser beam output level is 3 mW, and the cavity length is 250 .mu.m. It will be clear from FIG. 5 that the mode competition noise does not occur below 75.degree. C. even in the semiconductor laser of the conventional construction if the reflectivities R.sub.1 and R.sub.2 are selected to satisfy the following condition.

On the other hand, if ln(1/R.sub.1 .multidot.R.sub.2) is less than 0.1, the laser output is not effectively emitted from the front facet. Accordingly, the reflectivities R.sub.1 and R.sub.2 should be selected to satisfy the following condition.

The concept of the present invention is applicable to the compound semiconductor laser, such as a GaP laser or a GaA1P laser in addition to the above discussed GaAs-GaA1As semiconductor laser.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.