Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
2000-02-23
2003-04-01
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Modulation
C372S043010, C372S044010, C372S046012, C372S050121
Reexamination Certificate
active
06542525
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a modulator and a modulator-integrated semiconductor laser device, and a Manufacturing Process therefor, for use in the optical communication. In particular, the present invention relates to the modulator and the modulator-integrated semiconductor laser device with an improvement in its modulating frequency characteristics by reducing a wire capacitance of the modulator, and the Manufacturing Process therefor.
In order to expand public communication web system with optical fibers, a semiconductor laser device and peripheral devices including a modulator should be improved in their characteristics and manufactured at a reasonable cost. Particularly, for a high-density communication with the optical devices, a modulator with a high modulation rate has been demanded to process a large amount of information. For this purpose, an external modulator has been widely used to modulate a laser beam continuously emitted from the semiconductor laser device in response to the electrical signals applied to the modulator. This is because the external modulator advantageously reduces the deviation of the laser beam wavelength, which might be caused in its modulation of the laser signal, allowing the beam signal to be transmitted over the long distance.
A reverse-biased electrical field is applied on the external modulator with a beam absorption layer. By this application, a beam absorption index of the beam absorption layer is changed according to the Franz-Keldysh effect or the Stark quantum confinement effect, so that the laser beam therethrough is absorbed more effectively. Thus, the laser beam can be switched on and off by the electrical signal applied to the modulator.
As described above, the higher density communication with the optical devices requires a higher modulation rate, which in turn requires an improved modulating frequency characteristics, i.e. an improved cut-off frequency. Basically, the modulating frequency characteristics depends upon its CR time constant which has parameters such as capacitance and resistance. Therefore, reduction should be made in the capacitance and/or the resistance of the modulator for the high-density optical communication.
Disadvantageously, the incorporation of the external modulator with the semiconductor laser device brings a difficulty in optical coupling between the semiconductor laser device and the modulator and results in an increase of the additional parts, making the device costly.
In order to overcome the problems, a modulator-integrated semiconductor laser device having a modulator monolithically integrated therewith on a common semiconductor substrate has been developed. However since the modulator-integrated semiconductor laser device has the frequency characteristics dependent on the CR time constant of the modulator, it is also critical to reduce the capacitance and/or resistance of the modulator for the high-density optical communication.
Specifically, descriptions will be made to the conventional modulator. Referring to
FIGS. 11 and 12
, the modulator
200
comprises a substrate
202
of n-type InP, a modulator portion
226
, a pad portion
220
, and a channel portion
228
. In the following description, terminologies of “n-type” and “p-type” are referred to as “n-” and “p-”, respectively.
As clearly shown in
FIG. 12
, the modulator portion
226
comprises a lower cladding layer
204
, a beam absorption layer
206
, and a first upper cladding layer
208
, successively formed on the substrate
202
. The modulator portion
226
also comprises a current blocking layer
210
including a first lower embedded layer
210
a
made of InP doped with Fe, a hole-trapping layer
210
b
made of n-InP, and a second upper embedded layer
210
c
made of InP doped with Fe. The modulator portion
226
further comprises a second cladding layer
212
made of InP and a contact layer
214
made of p-InGaAs.
The pad portion
220
comprises a multi-layered structure which is similar to that of the current blocking layer
210
, the second upper cladding layer
212
, the contact layer
214
, and the insulating layer
216
.
An insulating layer
216
is disposed on the modulator portion
226
, the pad portion
220
, and the channel portion
228
, leaving an elongated opening above and opposing to the beam absorption layer
206
.
A wire layer indicated by reference numerals
220
a
,
222
, and
218
is disposed on the pad portion
220
and the channel portion
228
, and in contact with the contact layer
214
through the opening, and a back electrode
224
is formed on the back surface of the substrate
202
.
Referring again to
FIG. 11
, the operation of the modulator
200
is described hereinafter. In general, the modulator
200
receives a laser beam L
1
as indicated in
FIG. 11
at its couple-in facet and delivers a laser beam L
2
at its couple-out facet. In this regard, the reverse-biased voltage is applied between the back electrode
224
and the modulator electrode
218
through the pad electrode
220
a
, which changes the beam absorption index of the reverse-biased beam absorption layer
206
due to the Franz-Keldysh effect or the Stark quantum confinement effect. While the reverse-biased voltage is turned off, the laser beam L
2
can be delivered. But contrary to this, the reverse-biased beam absorption layer
16
absorbs the laser beam L
1
, and then prevents the laser beam L
2
from being delivered. This provides the ON-OFF switching of the laser beam L
2
generating a high-rate modulating electrical signal. In this manner, the electrical signal can be transformed into the laser beam signal in the form of pulses.
As described above, the cut-off frequency of the modulator
200
depends upon the CR time constant. Then, in order to achieve a high-rate modulator for the high-density signal communication, the CR time constant, that is, the capacitance and/or the resistance of the modulator
200
should be reduced.
It should be noted that the capacitance of the modulator
200
equals to the sum of the capacitance of the modulator portion
226
, the pad portion
220
, and the channel portion
228
. Disadvantageously, each of the capacitances of the modulator portion
226
, the pad portion
220
, and the channel portion
228
can not be readily reduced due to the structural reasons, which will be described below. Therefore, the present invention addresses to the reduction of the capacitance, especially of the channel portion
228
and the modulator portion
226
, thereby increasing the cut-off frequency for the high-density optical communication.
JP8-172242, A, discloses an another conventional semiconductor laser device. The semiconductor laser device comprises a pad portion formed on the common substrate having the same layer structure as that of an active layer (the light emitting and waveguiding layer), and a wire layer formed on an insulating layer connecting an anode electrode of the active layer with a bonding pad. However, it fails to describe the layer structure beneath the insulating layer.
Also, JP6-216464, discloses an another example of a conventional field-absorption type modulator, in which a polyimide layer is deposited on the common substrate and adjacent to the ridge-like modulator. In this structure, although a buffer layer made of undoped InP is deposited beneath the modulator and the polyimide layer, no description is made to a channel for dividing the modulator from the polyimide layer.
Examples of a manufacturing process using an etching-inhibiting layer to form a beam waveguide structure are JP11-97799, A, JP11-87836, A, and JP7-231145, A.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a modulator in which its capacitance is reduced to increase the cut-off frequency and thereby to improve the frequency-characteristics.
A second object of the present invention is to provide a modulator-integrated laser device integrating a modulator therein, in which its capacitance is reduced to increase the cut-off frequency and thereby to improve the f
Matsumoto Keisuke
Takagi Kazuhisa
Ip Paul
Leydig , Voit & Mayer, Ltd.
Rodriguez Armando
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