Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
1999-03-18
2001-03-20
Healy, Brian (Department: 2874)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S043010, C372S050121, C372S101000, C372S102000, 43
Reexamination Certificate
active
06205165
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a gain-coupling type distributed feedback semiconductor laser device, and a method for producing the same.
2. Description of the Related Art
Distributed feedback (DFB) semiconductor laser devices are utilized as coherent light sources having variable or stabilized wavelengths and capable of operating in a monoaxial mode, and as such are increasingly in demand in the field of optical measurement, optical communications/transfer, optical recording, laser beam printers, and the like. In a DFB laser device, a diffraction grating is provided in an active layer or a guide layer for causing optical feedback so as to provide laser oscillation. DFB laser devices are generally classified into two categories: refractive index-coupling (IC) DFB laser devices, in which distributed feedback is attained by periodically varying the refractive index provided by a diffraction grating; and gain-coupling (GC) DFB laser devices, in which distributed feedback is attained by periodically varying the gain provided by a diffraction grating.
Due to their operation principles, IC-DFB laser devices are likely to laser oscillate in two modes because a stop band which exists in the vicinity of the Bragg wavelength is interposed between two symmetrically-positioned longitudinal modes having minimum threshold gains. On the other hand, GC-DFB laser devices have been theoretically confirmed to yield a single longitudinal mode with a probability of substantially 100%, irrespective of the end face phase.
GC-DFB laser devices can be further classified into two subcategories, with respect to the kind of structure that is employed to provide a periodically varying gain: a gain diffraction grating structure, in which the active layer provides a periodical varying gain; and an absorptive diffraction grating structure, which incorporate periodic regions for absorbing part of the induced emission that is generated from the active layer, thereby effectively providing a periodically varying gain.
Since the absorptive diffraction grating includes regions which are periodically located in the vicinity of the active layer for absorbing part of the induced emission generated from the active layer, a large absorption loss of energy may result, thereby increasing the oscillation threshold. However, the gain diffraction grating structure is free from the problem of increased oscillation thresholds because it does not incorporate any regions for absorbing a portion of the induced emission generated from the active layer.
Some conventional DFB laser devices incorporate a quantum well structure in their active layers. A quantum well structure features light-emitting layers which are composed of very thin well layers (e.g., several nanometers), with a view to enhancing the carrier density within the active layer and improving the laser characteristics of the device.
FIG. 8
is a cross-sectional view illustrating a conventional gain diffraction grating type GC-DFB laser device incorporating a quantum well layer structure as an active layer (taken along the direction of its laser cavity).
This conventional GC-DFB laser device is constructed as follows: First, an n-cladding layer
81
and an undoped quantum well layer
82
are formed on an n-substrate
80
by MOCVD (metal organic chemical vapor deposition). The undoped quantum well layer
82
includes well layers
82
a
and barrier layers
82
b
. Next, a diffraction grating is impressed on the uppermost layer among the growth layers by a two-beam interference exposition method and etching. Then, a further growth (regrowth) process is performed by MOCVD to form a p-cladding layer
83
, and p-contact layer
84
. Thereafter, electrodes
851
and
852
are deposited. Finally, the wafer is cleaved, whereby the semiconductor laser device is completed.
This conventional laser device provides a periodic gain distribution by directly etching the active layer, thereby realizing a GC-DFB laser device.
However, in accordance with the above conventional example, it is necessary to first perform a growth process up to the active layer, temporarily suspend the growth to allow the impression of a diffraction grating thereupon, and then resume the growth (regrowth) process to further form a cladding layer. Generally, during the period of suspended growth which proceeds a regrowth process, the substrate surface is exposed to the atmosphere and therefore degraded; as a result, the crystallinity of the material at the regrowth interface is greatly deteriorated. In the case of the above conventional example, in which a relatively large area within the active layer defines a regrowth interface, the crystallinity at the regrowth interface within the active layer undergoes severe deterioration. As a result, the semiconductor crystalline structure of the active layer suffers from defects.
Such crystal defects increase a recombination component which is unrelated to light emission, thereby greatly reducing the induced emission. This degrades the characteristics of the semiconductor laser and greatly reduces the reliability of the device.
SUMMARY OF THE INVENTION
A gain-coupling type distributed feedback semiconductor laser device having a periodically varying gain along a cavity direction according to the present invention includes: a first periodic undulation formed along the cavity direction, the first periodic undulation including convex and concave portions; a semiconductor embedded layer formed on the first periodic undulation; and at least one active layer formed within the semiconductor embedded layer so as to extend along a direction which is perpendicular to the surface of a substrate, the at least one active layer being present on the concave portions of the first periodic undulation with a period which is substantially equal to a period of the periodic undulation.
In one embodiment of the invention, the semiconductor embedded layer and the at least one active layer include a III-V group compound semiconductor containing at least one III group element and at least one V group element, the semiconductor embedded layer including the at least one III group element at a component ratio not equal to a component ratio at which the at least one active layer includes the at least one III group element.
In another embodiment of the invention, the at least one active layer includes a vertical quantum well structure.
In still another embodiment of the invention, the at least one active layer is formed of a quantum well wire structure, at least one layer of the quantum well wire structure being provided along a direction which is perpendicular to the surface of the substrate.
In still another embodiment of the invention, at least two periodic undulations including the first periodic undulation are provided, the at least two periodic undulations comprise convex and concave portions which are formed In a same period and positions as that of the first periodic undulation.
In still another embodiment of the invention, the at least two periodic undulations have substantially a same shape.
In still another embodiment of the invention, each of the concave portions of the periodic undulation includes a flat portion having a width along the cavity direction which is in the range of about 0 nm to about 30 nm.
In still another embodiment of the invention, each of the concave portions of the periodic undulation includes lateral surfaces formed of a (n
11
) plane and a bottom formed of the (
100
) plane.
In still another embodiment of the invention, each of the concave portions has a depth equal to or greater than about 40 nm.
In still another embodiment of the invention, the semiconductor embedded layer includes a III-V group compound semiconductor containing at least one III group element and at least one V group element, wherein the at least one III group element is selected from Al and Ga.
In still another embodiment of the invention, the semiconductor embedded layer includes Al at a component ratio in the range of about 0.1 to about
Kawanishi Hidenori
Yamamoto Kei
Healy Brian
Morrison & Foerster / LLP
Sharp Kabushiki Kaisha
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