Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2002-05-08

2004-11-23

Jackson, Jerome (Department: 2815)

Coherent light generators

Particular active media

Semiconductor

C372S045013, C372S075000, C372S046012

Reexamination Certificate

active

06822990

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a high-power semiconductor laser having a current noninjected region in the vicinity of an end face (facet). In particular, the invention enhances a facet Catastrophic Optical Damage level higher and provides high reliability in long-term continuous operations.
BACKGROUND ART
A semiconductor laser device is utilized in various fields such as an excitation light source for a light amplifier used in the field of communications. These lasers are required for high power operations. However, there is a problem that it is difficult to obtain the semiconductor laser having enough life for carrying out the high power operations. In general, facet COD (Catastrophic Optical Damage) is known as the main factor of deterioration of the semiconductor laser. The COD is caused by the following steps. First, the non-radiative recombination is caused by defects produced in the vicinity of the facet. This results in an increase of the temperature. In addition, due to the temperature increase, the width of band gap is decreased and light absorption occurs, resulting in a vicious cycle of further temperature rise due to this. These steps induce melting of the facet and light output is deteriorated. Finally, a non-reversible destruction occurs
According to the above description, it becomes important to reinforce the facet for obtaining the high-power semiconductor laser, in terms of preventing the facet COD. As one means for preventing the facet COD, there is a method in which no current is injected in the vicinity of the facet. In this case, since a current injection is suppressed in the vicinity of the facet, the vicinity of the facet becomes non-excitation state. Because of this, the non-radiative recombination is suppressed, which enables to improve the facet COD level. As specified examples, there are following methods. That is, no current is injected in the facet by forming an insulating film such as SiN under an electrode formed on the facet, no current is injected in the facet by forming a current blocking layer comprising a semiconductor layer on the facet, and no current is injected in the facet by carrying out ion implantation in the vicinity of the facet.
All the above methods have complicated fabrication steps. Further, the method causes damage to the semiconductor laser device, in which no current is injected in the facet by carrying out ion implantation in the vicinity of the facet. Furthermore, all these facet current blocking structures are formed at a distance from a waveguide layer. When providing the current blocking structure, it can be considered that a current wraparound will occur. The current wraparound is considered to cause serious influence as the distance between the current blocking structure and the active layer increases. In the case where the influence of the current wraparound becomes serious even if the current blocking structure is formed on the facet, current is wrapped around the facet, resulting in function degradation of the facet current blocking structure. Consequently, in consideration of the current wraparound, it is necessary to form wide facet current blocking region. When the facet current blocking region is widely formed, an influence caused by light absorption becomes serious in the region, so that the properties (threshold current, slope efficiency, temperature characteristic, or the like) of the semiconductor laser device are deteriorated. Because of this, any conventional methods described above are not necessarily preferable when forming a current blocking structure in the vicinity of the facet.
SUMMARY OF THE INVENTION
The invention is aimed at solving the above problems. An object of the invention is to provide a semiconductor laser device having high facet COD level, and high reliability in long-term continuous operations, by providing a current blocking structure in the vicinity of the facet. According to the structure, fabrication processes become easy, no damage are caused to the semiconductor laser device, and the property deterioration can be minimized.
To achieve the above object, the invention relates to a semiconductor laser device comprising:
an active layer;
an n-type waveguide layer;
a p-type waveguide layer,
the active layer being interposed between the n-type and p-type waveguide layers;
n-type and p-type cladding layers formed so that outsides of the n-type and p-type waveguide layers are interposed therebetween;
first current blocking layers formed to define a stripe-shaped current injected region extending in a direction where a front facet of the device from which a laser light is emitted and a rear facet of the device opposing thereto are connected; and
a second current blocking layer formed to transverse the stripe-shaped current injected region in a vicinity of the front facet,
wherein the first current blocking layers and the second current blocking layer are made of the same layer.
According to the semiconductor laser device thus structured, the first current blocking layers, between which the stripe-shaped current injected region extending in the resonator direction is interposed, and the second current blocking layer, which is formed in order that no current is injected in one side or both sides of the vicinity of the facet, are made of the same layer, namely they are of the same composition and of the same film thickness. Because of this, fabrication processes become easy without increasing the number of processing steps compared with the conventional method. Consequently, the current blocking structure can be provided in the vicinity of the facet without damaging the semiconductor laser device. Accordingly, it is possible to provide the semiconductor laser device having high facet COD level and high reliability in long-term continuous operations. Not only the front facet but also the rear facet may be provided with the current blocking layer. One of both side portions of the second current blocking layer reaches the facet.
In the invention, a refractive index waveguide structure can be formed by an equivalent refractive index difference between the current injected region and the region of the current blocking layers. Further, a carrier blocking layer having larger energy gap than that of the waveguide layer is provided between the active layer and the waveguide layers, thereby a carrier is confined and a waveguide mode in the epitaxial direction can be expanded. Consequently, the facet COD level can be further improved by suppressing light intensity concentrated on the facet active layer.
In the invention, it is preferable that the first and second current blocking layers are formed inside the waveguide layer. The first and second current blocking layers may be formed in adjacent to the waveguide layer. In this case, when the width of the second current blocking layer is too large, waveguide loss is increased. Because of this, the second current blocking layer is desired to have a width within the range of 2 to 25 &mgr;m in practical use.
In the structure in which the current blocking layer in the vicinity of the facet having lower refractive index than that of the waveguide layer is formed in the waveguide layers or in adjacent to the waveguide layers, the waveguide mode profile in the vicinity of the facet can be shifted from the active layer by the use of the low refractive index layer in the vicinity of the facet. Accordingly, the beam energy density near the active layer in the vicinity of the facet can be reduced, and it is possible to provide the semiconductor laser device having substantially improved facet COD level and high reliability in long-term continuous operations. Further, since the waveguide layers are formed in the vicinity of the active layer, influence caused by the current wraparound into the active layer can be reduced by providing the facet current blocking structure in the waveguide layers. By doing this, the current blocking region necessary for ensuring improvement of the facet COD level and high reliability in long-term continuous operation

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