Semiconductor Laser Element With a Single Longitudinal Oscillation Mode

The present invention relates to a distributed feedback type semiconductor laser that is capable of oscillating in a single longitudinal mode by changing the gain of the laser resonator in the same direction as the propagation of the laser light at almost the same interval as the laser light wavelength.

It has been verified recently that a distributed feedback type semiconductor laser device whose angle of refraction is changed in the same direction as the laser beam propagation at almost the same interval as the laser light wavelength is effective for realizing single longitudinal mode oscillation. The distributed feedback type semiconductor laser element, having its angle of refraction changed, provides two longitudinal modes of basically equal gain. However, in a steady state operation where constant current is allowed to flow at all times, oscillation is assumed to take place in either of the two longitudinal modes due to an inherent asymmetry of the resonator. In most distributed feedback type semiconductor laser elements, one of the two cleavage planes is etched obliquely to prevent the two cleavage planes from constituting a Fabry-Perot resonator. Therefore, the asymmetry of the resonator increases significantly and a stable single longitudinal mode oscillation is realized.

However, when the above conventional distributed feedback type semiconductor laser element is used as a light source for optical communication, it is operated in a non-steady state with the amount of exciting current modulating rapidly. In such a non-steady state operation, the asymmetry of the resonator is considered to change continuously so that the laser element does not always oscillate in a single mode. Actually, oscillation spectrum expansions and discontinuous oscillation mode changes have been observed.

In view of the above, the object of the present invention is to provide a distributed feedback type semiconductor laser element which oscillates in a single longitudinal mode even in a non-steady state operation without causing oscillation spectrum expansions and discontinuous oscillation mode changes.

According to the present invention, a semiconductor laser element of a double hetero junction structure having two clad layers and an active layer formed between the two clad layers. High resistance regions are formed in at least one of the two clad layers, at almost the same interval as the laser light wavelength in a vertical direction to the light propagating direction.

Since the upper or lower clad layer has high resistance regions formed vertically to the light propagating direction at the same period as the light wavelength to constitute a periodic current blocking structure, the amount of injected current changes periodically according to the light wavelength which allows the gain in the resonator, with respect to the laser light to vary periodically. Accordingly, with only one longitudinal oscillation mode of the largest gain, the semiconductor laser element of the present invention oscillates in a single longitudinal mode even in a non-steady state operation.

According to the present invention, as mentioned above, a periodic current blocking structure is formed by the high resistance regions which permits the gain in the resonator to change periodically, and the oscillation of the semiconductor laser element in a single longitudinal mode can be achieved even in a non-steady state operation. As the laser element oscillates in a single longitudinal mode, an oscillation spectrum does not expand and an oscillation mode does not change intermittently even in a non-steady state operation.

The present invention will become more fully 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 side view of a distributed feedback type semiconductor laser element of an embodiment of the present invention; and

FIG. 2 is a side view of a distributed feedback type semiconductor laser element of another embodiment of the present invention.

Embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is not our intention for the present invention to be limited to the following embodiments.

FIG. 1 is a side view of a distributed feedback type semiconductor laser element of one embodiment of the present invention. A 2 .mu.m thick p-type InP clad layer 3, a 0.15 .mu.m thick undoped In GaAsP active layer 4 and a 0.3 .mu.m thick n-type InP clad layer 5 are sequentially grown on a p-type InP substrate 2 in the normal liquid phase growth method. Fe ions focused into a width of about 0.15 .mu.m are shot in the n-type InP clad layer 5 from above the upper side in a linear direction vertical to the light propagating direction by using a Focused Iron Beam (FIB) method, to form a high resistance region 9 which is different in resistance from the rest of the n-type InP clad layer 5. The Fe ions are shot in the 3-n-type InP clad layer 5 to a depth of 0.2 to 0.3 .mu.m to be close to the InGaAsP active layer 4. This operation is repeated to produce high resistance regions 9 at intervals of about 0.23 .mu.m or almost the same period as the laser light wavelength in a vertical direction to the light propagating direction.

Another 2 .mu.m thick n-type InP clad layer 6 and a 0.7 .mu.m thick n-type InGaAsP contact layer 7 are grown by the liquid phase growth method on the n-type InP clad layer 5 in which the high resistance regions 9 have been formed. Thereafter, an Au/Zn electrode 1 and an Au/Ge/Ni electrode 8 are deposited by evaporation on the bottom side of the p-type InP substrate 2 and on the n-type InGaAsP contact layer 7, respectively, and alloyed to obtain ohmic electrodes. The resultant wafer is cleaved to produce semiconductor laser elements.

FIG. 2 is a side view of a distributed feedback type semiconductor laser element of a second embodiment of the present invention. According to the second embodiment, an n-type clad layer 12 is grown on an n-type InP substrate 11 that has been carefully cleaned, and Fe ions are shot in the n-type clad layer 12 by the FIB method to form high resistance regions 17 vertically to the light propagating direction at almost the same interval as the laser light wavelength. The Fe ions are shot to a depth of 0.1 to 0.2 .mu.m so that the Fe ions are not diffused. An undoped InGaAsP active layer 13, a p-type InP clad layer 14 and a p-type InGaAsP contact layer 15 are sequentially grown on the n-type clad layer 12 on which the high resistance regions 17 have been formed in the normal liquid phase growth method. Electrodes 10 and 16 are formed and a semiconductor laser element is obtained.

In the above two embodiments, the invention is applied to an InGaAsP/InP type semiconductor laser element. Needless to say, the present invention is applicable to other types of semiconductor laser elements such as GaAlAs/GaAs type and InGaP/InGaAlP/GaAs type. Further, the above embodiments of the present invention use Fe ions that the injected into the clad layer. Other ions may be used if they can form a high resistance region and if they can be sufficiently focused. The high resistance regions may be grown selectively on the clad layer either by using an oxide film or by a combination of light irradiation and an organic metal gas phase growth method in place of the FIB method.

While only certain embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the present invention as claimed.