Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-03-30
2002-07-30
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S017000, C438S022000
Reexamination Certificate
active
06426515
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims priority of Japanese Patent Application No. 2000-121436, filed on Apr. 21, 2000, the contents being incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the structures of DFB lasers capable of single-wavelength operation among semiconductor light-emitting devices principally used in optical communication systems. As the communication rate of optical communication systems is increased, semiconductor light-emitting devices having high wavelength stability are demanded. In particular, such semiconductor light-emitting devices are required to induce no mode hop even during modulation with high output power or even if external optical feedback exists.
2. Description of the Related Art
Conventionally, gain-coupled DFB lasers having periodic gain modulation in the direction of resonant cavity are proposed as a single-wavelength laser having high mode stability (such gain-coupled DFB lasers include complex-coupled DFB lasers characterized by having both of index coupling and gain coupling). Several methods are available for realizing gain modulation. Examples are a method of periodically modulating the thickness of active layers or guide layers, a method of forming periodic current-blocking layers adjacent to active layers, and a method of forming periodic light absorption layers adjacent to active layers. In particular, a structure (to be referred to as an MQW diffraction grating structure hereinafter) in which the number of multiple quantum well layers (MQW layers) as active layers is periodically changed has the advantages that a relatively large gain coupling coefficient can be ensured, the phases of gain coupling and index coupling match, and no extra absorption occurs.
FIGS. 1A
to
1
C show a conventional gain-coupled DFB laser using this MQW diffraction grating structure.
FIG. 1A
is a schematic sectional view of the main components of a semiconductor light-emitting device.
FIG. 1B
is a schematic sectional view of a region C in FIG.
1
A.
FIG. 1C
shows a bandgap along a broken line D in FIG.
1
B.
An MQW diffraction grating
102
includes periodically divided MQW layers (MQW-A) and flat MQW layers (MQW-B), and is formed between an n-InP substrate
101
and a p-InP cladding layer
103
. The film characteristics and film thicknesses of well layers and barrier layers in MQW-A are the same as in MQW-B.
Referring to
FIG. 1
, of the total of six MQW layers, three upper layers are periodically divided to form MQW-A, and three lower layers are flattened to form MQW-B. In the MQW diffraction grating structure, a gain coupling coefficient, an index coupling coefficient, and the gain of the whole active layers can be controlled by changing the number of layers in each of MQW-A and MQW-B. Basically, the gain coupling coefficient and the index coupling coefficient increase when the number of layers in MQW-A is increased; the total gain of the active layers increases with no large changes in the coupling coefficients when the number of layers in MQW-B is increased.
A gain-coupled DFB laser having this MQW diffraction grating structure has the following several problems.
The first problem will be described below. In a gain-coupled DFB laser, the ratio of the gain coupling coefficient to the index coupling coefficient greatly contributes to mode stability. For example, in a uniform diffraction grating containing no phase shift in a resonant cavity, it is generally desirable that the ratio of the gain coupling coefficient to the index coupling coefficient be large. The values of these two coupling coefficients have influence on other various characteristics. For example, if these coupling coefficients are large, generally the lasing threshold current becomes small and the resistance to external optical feedback becomes high, but the slope efficiency lowers and the influence of the spatial hole-burning effect increases. Accordingly, it is desirable to control appropriately both the index coupling coefficient and the gain coupling coefficient in accordance with required device characteristics.
Unfortunately, in the conventional MQW diffraction grating structure shown in
FIG. 1
, the number of layers in MQW-A contributes to both the gain coupling coefficient and the index coupling coefficient. This makes it difficult to control these parameters independently. For example, if the number of layers in MQW-A is increased to increase the gain coupling coefficient, the index coupling coefficient increases at the same time. Consequently, the slope efficiency lowers, or the spatial hole-burning effect becomes strong.
The second problem of the conventional MQW diffraction grating structure shown in
FIG. 1
is as follows. That is, when a differential gain (a change in gain with a change in carrier density) is increased to increase a modulation bandwidth, the dependence of the gain on the carrier density, i.e., the dependence of the gain coupling coefficient on the carrier density increases in MQW-A. This increases variations in the gain coupling coefficient during modulation.
The third problem of the conventional diffraction grating structure shown in
FIG. 1
will be described below. When strain is introduced in MQW layers, this strain enters in different ways into layers of MQW-A and MQW-B (see FIG.
2
), even if these layers have the same composition (i.e., the same lattice constant). This produces a difference between the gain spectra in MQW-A and MQW-B. For example, when compressive strain is introduced, the gain peak in MQW-A shifts to a shorter wavelength than in MQW-B; when tensile strain is introduced, the gain peak in MQW-A shifts to a longer wavelength than in MQW-B. When this is the case, it is difficult to set properly the gain peak position with respect to the lasing wavelength that is determined by the diffraction grating period. As an example, if the gain peak wavelength is too far from the lasing wavelength, the lasing threshold current increases, or the temperature characteristics deteriorate.
SUMMARY OF THE INVENTION
It is an object of the present invention to control independently the index coupling coefficient and the gain coupling coefficient and improve the mode stability without deteriorating the characteristics such as the lasing threshold current, slope efficiency, and the resistance to external optical feedback, in a semiconductor light-emitting device of a gain-coupled DFB laser using the MQW diffraction grating structure.
It is another object of the present invention to provide a semiconductor light-emitting device having the MQW diffraction grating structure, which has less dependence of the gain coupling coefficient on the carrier density while keeping the total differential gain of the active layers large, so as to achieve a large modulation bandwidth, and less wavelength variation during modulation.
It is still another object of the present invention to provide a semiconductor light-emitting device which, even if strain is introduced in multiple quantum well layers, can match the gain peak wavelength of first multiple quantum well layers (MQW-A) with that of second multiple quantum well layers (MQW-B), and which can appropriately set the gain peak position with respect to the lasing wavelength that is determined by the diffraction grating period, thereby preventing an increase in the lasing threshold current or deterioration of the temperature characteristics.
The present invention provides a semiconductor light-emitting device having a multiple quantum well structure (MQW structure) in which well layers are stacked via barrier layers, and which amplifies light by current injection, characterized in that the number of barrier layers and the number of well layers periodically change in the propagation direction of light in a partial region or the whole region of the multiple quantum well structure, and that the multiple quantum well structure comprises, in the region, first multiple quantum well layers (MQW-A) divided
Ishikawa Tsutomu
Kobayashi Hirohiko
Shoji Hajime
Yamamoto Tsuyoshi
Armstrong Westerman & Hattori, LLP
Fujitsu Limited
Huynh Andy
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