Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-10-26
2002-10-29
Lester, Evelyn A (Department: 2873)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S015000, C257S189000, C257S622000, C257S623000, C385S002000, C385S014000, C385S131000, C372S043010, C372S068000, C372S075000, C359S240000, C359S248000
Reexamination Certificate
active
06472682
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator, a semiconductor laser device equipped with an optical modulator, and an optical communications system, and more particularly, to an optical modulator involving little distortion in the waveform of a signal after transmission thereof, a semiconductor laser device equipped with the optical modulator, and an optical communications system for transmitting an optical signal over a long distance through use of the semiconductor laser device.
2. Background Art
An improvement in the performance of a semiconductor laser and an improvement in the yield of a semiconductor laser required for inexpensively manufacturing a semiconductor laser are important in realizing widespread proliferation of a public communication network that uses an optical fiber.
High-speed modulation of a laser beam for coping with an increase in the volume of information transmitted is particularly indispensable for improving the performance of a semiconductor laser. In order to enable long-distance transmission of an optical signal while limiting variation in wavelength, which arises at the time of modulation, an external modulation system is adopted for modulating a laser beam at high speed. In this system, light emitted from a semiconductor laser usually has a predetermined intensity and is passed through an optical modulator capable of inducing and interrupting transmission of light, to thereby modulate the light.
An electroabsorption modulator (EAM) is used as an optical modulator for use with the external modulation system. EAMs can be roughly divided into two types: that is, an FAM using a single thick light-absorption layer, and an EAM employing a multiple quantum well (MQW) structure formed by means of stacking thin quantum well layers, each quantum well layer being capable of forming excitons at room temperature. The former type of EAM effects extinction by utilization of variation in an absorption spectrum due to the Franz-Keldysh effect, and the latter type of EAM effects extinction by utilization of variation in absorption spectrum due to the Stark effect.
In an optical modulator, absorption of a laser beam is changed in accordance with a voltage applied to the optical modulator. For this reason, when a modulation signal voltage is applied to a high-frequency electric circuit connected to an optical modulator, a laser beam emitted from an exit end face of an optical modulator is subjected to intensity modulation in accordance with the modulation signal voltage.
In a case where a light absorption layer of an optical modulator is constituted of an MQW structure, a high extinction ratio (the ratio between the amount of light transmitted during an ON operation of the optical modulator and the amount of light transmitted during an OFF operation of the optical modulator) is attained. Therefore, a light absorption layer having an MQW structure is usually used for high-speed transmission.
FIG. 27
is a cross-sectional view showing a conventional optical modulator. In
FIG. 27
, reference numeral
1
designates an n-type InP substrate (hereinafter an n-type is depicted as “n-,” and a p-type is depicted as “p-”);
2
designates an n-type optical confinement layer formed of n-InGaAsP;
3
designates a light absorption layer formed of InGaAsP;
4
designates a p-type optical confinement layer formed of p-InGaAsP;
5
designates an Fe-doped InP embedded layer;
6
designates an n-InP embedded layer;
7
designates a p-type cladding layer formed of p-InGaAsP;
8
designates a p-InGaAs contact layer;
9
designates a SiO
2
dielectric film;
10
designates a Ti/Au surface electrode;
11
designates an Au surface-plating layer;
12
designates an Au/Ge/Ti/Pt/Ti/Pt/Au underside electrode;
13
designates an underside plating layer; and
14
designates an optical modulator (EAM).
FIG. 28
is a cross-sectional view showing the light absorption layer
3
of conventional type. In
FIG. 28
, reference numeral
3
a
designates a quantum well layer (hereinafter referred to simply as a “well layer”), and
3
b
designates a barrier layer. In the MQW structure of the light absorption layer
3
, all ten well layers
3
a
are of equal thickness, and all nine barrier layers
3
b
are of equal thickness.
FIG. 29
is an energy diagram of the light absorption layer
3
. In
FIG. 29
, reference symbol Ec denotes the conduction band; and Ev denotes the valence band.
FIG. 30
is a graph showing an absorption spectrum of the optical modulator
14
.
In order to achieve efficient extinction by making changes in the absorption spectrum of each of the well layers
3
a
of the conventional light absorption layer
3
having an MQW structure, usually all the well layers
3
a
are made equal in band-gap wavelength and thickness. An absorption spectrum ‘A’ shown in
FIG. 30
represents a typical absorption spectrum of the optical modulator (EAM)
14
equipped with the light absorption layer
3
having an MQW structure.
If an electric field is applied to the MQW structure by application of a bias voltage to the optical modulator
14
, the absorption spectrum ‘A’ is changed to an absorption spectrum ‘B’. In a case where the wavelength of laser incident light is set to &lgr;
0
, the absorption coefficient a relative to the incident light is changed by the bias voltage. The absorption coefficient &agr; at the wavelength &lgr;
0
of the incident light is changed by application of a bias voltage, to thereby turn laser light on and off. The optical modulator (EAM)
14
operates on the basis of the principle mentioned above.
Eq. (1) of the Kramers-Kronig relation applies to the amount of changes in an absorption spectrum (&Dgr;&agr;) in response to a change in the bias voltage and the amount of changes in refractive index (&Dgr;n).
Δ
n
(
λ
0
)
=
λ
0
2
2
π
2
lim
ϵ
->
0
(
∫
0
λ0
-
ϵ
+
∫
λ0
+
ϵ
∞
)
Δ
α
λ
0
2
-
λ
2
ⅆ
λ
(
1
)
During the course of an optical modulation operation, the refractive index of the optical modulator is changed in accordance with variations in the absorption spectrum of the light absorption layer
3
of the optical modulator
14
. Eventually, the wavelength of the light emitted from the optical modulator
14
is changed. In other words, a chirping phenomenon arises.
FIGS. 31A and 31B
are graphs showing the relationship between chronological change in light intensity and a chirping phenomenon.
By reference to
FIGS. 31A and 31B
, the relationship will now be described. When optical modulation of an optical signal is actually performed, the voltage applied to the optical modulator
14
assumes a value of 0V at a point at which light of the highest intensity is transmitted (i.e., a point indicated by P
0
in FIG.
31
A). The voltage applied to the optical modulator
14
assumes a voltage of about −1V even at a point at which sufficiently low intensity of light is transmitted (i.e., a point indicated by P
1
in FIG.
31
A). Within the range between the point P
0
and the point P
1
, an &agr; parameter usually assumes a positive value; that is, variation in the refractive index of an optical modulator assumes a positive value. When the intensity of light is increased, as shown in
FIG. 31A
, a negative variation arises in the wavelength of incident laser light, as shown in FIG.
31
B. In contrast, when the intensity of light is decreased as shown in
FIG. 31A
, a positive variation arises in the wavelength of laser incident light, as shown in FIG.
31
B. Such a positive variation in wavelength is called a positive “chirp” phenomenon.
An optical fiber; which serves as a transmission line, has a wavelength dispersive characteristic such that the group velocity of light differs according to wavelength. For this reason, if a positive chirp phenomenon arises, the waveform of light is deteriorated after transmis
Lester Evelyn A
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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