Optical semiconductor device

Details Optical semiconductor device Optical semiconductor device

2001-09-26

2003-07-01

Ullah, Akm Enayet (Department: 2874)

Optical waveguides

Temporal optical modulation within an optical waveguide

Electro-optic

C385S009000, C385S028000, C385S043000, C385S045000, C385S129000, C385S130000

Reexamination Certificate

active

06587604

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No. 2000-301490 filed on Sep. 28, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
This invention generally relates to semiconductor devices and especially to an optical semiconductor device that extracts optical clock signals from an optical signal.
In the field of optical telecommunication technology, it is general to superimpose an optical clock signal to optical signals that are transmitted through an optical fiber. Thus, the optical clock signals have to be reproduced from the received optical signal in repeater devices or reception devices that are connected to the optical fiber.
FIG. 1
shows the construction of a known clock-extracting optical-detection device
100
used for reproducing optical clocks from such an optical signal.
Referring to
FIG. 1
, the clock-extracting optical-detection device
100
includes an optical coupler
11
coupled optically to an optical fiber
101
that transmits an optical signal having a frequency f
0
from an input end
101
A to an output end
101
B, and the optical signal in the optical fiber
101
is branched by the optical coupler
11
and are introduced into an optical modulator
12
. The optical modulator
12
is driven by a driving signal having a frequency of f
clk
from a voltage-controlled oscillator
16
and modulates the optical signal that has been branched by the optical coupler
11
. The optical signal thus modulated by the optical modulator
12
is then detected optically in an optical detector
13
, and an output signal of frequency f
0
-mf
clk
is obtained.
The output signal of the optical detector
13
is then supplied to a phase comparator
15
for phase-comparison with a reference frequency signal supplied from a reference signal source
14
with a frequency f
1
. The phase comparator
15
thereby produces a voltage signal representing the phase difference between the output signal of the photodetection circuit
13
and the reference frequency signal, and the voltage signal thus produced is supplied to a voltage-controlled oscillator
16
. Thus, the frequency f
clk
of the driving signal is controlled so as to minimize the foregoing phase difference. In other words, the phase comparator
15
performs a feedback control of the voltage-controlled oscillator
16
.
As a result of such a feedback loop operation, the phase difference is controlled to substantially zero, and the output frequency signal
102
of the voltage-controlled oscillator
16
in this state is taken out as the clock signal that is synchronized with the optical clock in the optical signal.
Conventionally, the optical modulator
12
and the optical detector
13
have been formed as individual components in such a clock-extracting detection device
100
. The optical modulator
12
and the optical detector
13
have been connected by an optical fiber. However, such a construction is bulky and fragile, and has a problem of difficulty in realizing optical coupling without causing optical loss.
In view of the foregoing drawbacks of conventional construction, it is conceivable to form the optical modulator
12
and the optical detector monolithically on a common substrate in the form of integrated optical modulator/detector and to connect these units by a waveguide formed also monolithically on the common substrate. However, due to the inherent difference between the requirements imposed to an optical detector and the requirements imposed to an optical modulator, it is difficult to realize an efficient optical coupling between these units, in spite of the fact that both the optical modulator
12
and the optical detector
13
are formed based on a waveguide structure. Because of this reason, such a construction has not actually been attempted.
In the clock-extracting optical-detection device
100
, it should be noted that the optical modulator
12
has an active layer that forms a part of the optical waveguide. Thereby, a refractive-index change or optical absorption is induced in the active layer in response to a voltage signal, and the optical beam propagating through the active layer undergoes optical modulation. On the other hand, the optical detector
13
has an optical absorption layer and detects the optical signal by detecting the optical carriers that are produced in response to absorption of the incoming optical beam by the optical absorption layer. Thus, in the event the optical waveguide is used to connect the optical modulator
12
and optical detector
13
formed monolithically on a common substrate, it is necessary that the optical waveguide achieves an efficient optical coupling both to the active layer of the optical modulator and the optical absorption layer of the optical detector.
Meanwhile, in the case of constructing the optical modulator
12
by a high-speed optical interferometer of Y-type or Mach-Zehnder-type, it is an indispensable condition that the optical modulator
12
performs a single mode operation for realizing a satisfactory extinction ratio.
FIG. 2
shows the relationship between the thickness d and width W imposed for the active layer of the optical modulator
12
.
FIG. 2
is referred to.
Designating the curve shown in
FIG. 2
as f(W), it should be noted that the active layer of the optical modulator
12
forms a single-mode waveguide when the condition d<f(W) is met. It functions as a multi-mode waveguide when the condition d>f(W) is met. Thus, in the case of operating the optical modulator
12
in single mode, it is desirable and necessary that the thickness d is increased when the width W is small and is decreased when the width W is large. As long as the relationship of
FIG. 2
is maintained, the width W and the thickness d may be chosen appropriately so as to facilitate the fabrication process of the optical modulator
12
.
On the other hand,
FIG. 3
shows the relationship between the operational frequency band f of the optical detector and the thickness d.
As can be seen from
FIG. 3
, the distance, and hence the time, for an optically excited carrier to move through the optical absorption layer and reach an electrode is increased when the thickness of the optical absorption layer is large. Thereby, the operational frequency band f of the optical detector becomes inevitably low. Thus, it is preferable to reduce the thickness of the optical absorption layer from the viewpoint of improving the response characteristics of the optical detector. In the optical detector, it should be noted that the optical beam is not needed to be a single mode beam in the optical absorption layer.
In the event the thickness of the optical absorption layer is small like this, on the other hand, it is not possible to provide sufficient photodetection sensitivity, unless the length L of the optical absorption layer shown in
FIG. 4
is increased so that the optical beam is absorbed sufficiently. Alternatively, the width has to be increased so that the cross-section area of the optical absorption layer is increased. To increase the cross-section area of the optical absorption layer without increasing the thickness, there is no way but to increase the width of the optical absorption layer. However, such an increase of width of the optical absorption layer invites an increase of area of the optical absorption layer and associated increase of parasitic capacitance. When there occurs such an increase of parasitic capacitance, there occurs a decrease of operational frequency band f of the photodetector
13
as shown in FIG.
5
. Associated with this, the response speed is decreased. Thus, the optical absorption layer of the optical detector has to be designed in view of the relationship of
FIGS. 3-5
such that the frequency band and the photodetection sensitivity are both optimized. The restriction thus imposed on the design of the optical detector
100
is stricter than the case of optimizing the shape of the active layer of the optical modulator
12
.
Thus, the requirement impose

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