Fabrication of semiconductor Mach-Zehnder modulator

Details Fabrication of semiconductor Mach-Zehnder modulator Fabrication of semiconductor Mach-Zehnder modulator

2000-05-30

2001-08-28

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Including integrally formed optical element

Reexamination Certificate

active

06281030

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a semiconductor Mach-Zehnder modulator and, more particularly, to a semiconductor Mach-Zehnder modulator for use in an optical communication systems or optical data processing systems and driven with a single driver in a push-pull modulation scheme.
(b) Description of the Related Art
Recently, optical communication techniques are directed more and more to high bit-rate transmission. In the optical communication systems, most optical fibers installed in the world, especially around the North America, are 1.3 &mgr;m zero dispersion fiber, and provides minimum loss in 1.55 &mgr;m range in these fibers. Conventionally, a semiconductor laser direct modulation technique has been generally used in the optical communication. This technique, however, involves wavelength chirping due to dispersion when a high bit-rate 1.55 &mgr;m range optical signal is transmitted through a 1.3 &mgr;m zero dispersion fibers, thereby causing a signal distortion. The level of the distortion is generally proportional to a (bit rate)
2
×(transmission distance) product.
The problem wavelength chirping can be solved to some extent by employing an external modulation technique. Among other external modulators, an absorption type modulator exhibits smaller chirping compared to semiconductor lasers; however, not zero. On the other hand, if a Mach-Zehnder modulator, which uses optical interference as its operational principle, is used as an external modulator operating in a push-pull modulation scheme, the wavelength chirping can be entirely removed theoretically. Accordingly, Mach-Zehnder modulators are expected to be key external modulators for use in ultra high-speed and long distance optical communication systems.
Some known Mach-Zehnder modulators have dielectric substances such as LiNbO
3
. On the other hand, semiconductor Mach-Zehnder modulators are considered to be advantageous over the dielectric type Mach-Zehnder modulators, in view of the integration capability with optical elements such as semiconductor lasers or semiconductor optical amplifiers and electric elements such as FETs, as well as in view of their smaller dimensions and lower power consumption.
FIG. 1A
shows a conventional semiconductor Mach-Zehnder modulator in a perspective view, and
FIG. 1B
is a cross-sectional view taken along X—X in FIG.
1
A.
The semiconductor Mach-Zehnder modulator of
FIG. 1A
comprises an input waveguide
6
, a pair of input branch waveguides
7
-
1
and
7
-
2
branching off input waveguide
6
, a pair of phase modulators
8
-
1
and
8
-
2
receiving inputs from respective branch waveguides
7
-
1
and
7
-
2
, a pair of output branch waveguides
9
-
1
and
9
-
2
receiving outputs from respective phase modulators
8
-
1
and
8
-
2
, and an output waveguide
10
receiving combined output from output branch waveguides
9
-
1
and
9
-
2
.
The Mach-Zehnder modulator of
FIG. 1A
is fabricated by depositing consecutively undoped InP layer
102
, undoped In
x
Ga
1−x
As
y
P
1−y
layer
103
(&lgr;
PL
=1.3 &mgr;m), p-type InP layer
104
on an n-type InP substrate
101
, patterning specified deposited layers to form a combined mesa structure, and forming independent drive electrodes
105
-
1
and
105
-
2
and a common electrode
106
of the phase modulators, as shown in FIG.
1
B.
The semiconductor Mach-Zehnder modulator generally uses changes in the refractive index generated upon a reverse-bias voltages applied to a p-n junction. The optical characteristic of the semiconductor Mach-Zehnder modulator is shown in
FIG. 2
, wherein the optical output thereof is plotted against the drive voltage (reverse bias voltage). The curve denoted by “V1” shows a single arm drive wherein one of the modulators is driven, whereas the curve denoted by “V1 & V2” shows a double arm drive wherein both the modulators are driven for a push-pull modulation.
FIG. 3
shows a timing chart for the push-pull modulation of the modulator such as shown in
FIGS. 1A and 1B
, wherein modulator
8
-
1
is applied with a reverse bias voltage V1 through electrode
105
-
1
changing between 0 and V
&pgr;/2
whereas modulator
8
-
2
is applied with a reverse bias voltage V2 through electrode
105
-
2
changing between V
&pgr;/2
and V
&pgr;
in opposite phase with respect to the voltage V1, wherein V
&pgr;
provides a phase shift of &pgr; to the phase modulator whereas V
&pgr;/2
provides a phase shift of &pgr;/2. As shown in
FIG. 2
, drive voltage for the double arm modulation (V1 & V2), i.e., push-pull modulation, is about a half of that for the single arm modulation (V1) for a specified optical output.
John C. Cartledge et al. report that a double arm modulation scheme achieves a transmission distance which is double the transmission distance obtained by a single arm modulation scheme, in an article “Dispersion Compensation for 10 Gb/s Lightwave Systems Based on a Semiconductor Mach-Zehnder Modulator”, IEEE Photonics Technology. Letters, 1995 February, Vol. 7, No. 2, pp 224-226.
FIG. 4
shows full pulse width (ps) at half maximum for a Gauss pulse plotted against fiber length (km) for a single arm modulation and a double arm (push-pull) modulation of the Mach-Zehnder modulator, obtained in our experiments. As understood from
FIG. 4
, push-pull modulation achieves a small waveform distortion due to pulse compression and thus maintains a half-value width better than a single arm modulation. From the results, it is considered that the push-pull modulation can provide a double or triple transmission distance compared to the single arm modulation.
A push-pull driven semiconductor Mach-Zehnder modulator, such as
301
shown in
FIG. 5
, generally requires a pair of drivers
200
-
1
and
200
-
2
for applying drive voltages to electrodes
302
and
303
of the respective phase modulators and a timing generator
203
for driving the phase modulators
301
exactly in opposite phases. It is difficult to accurately adjust the timing by the timing generator
203
, especially at higher frequencies, for example, over 2.5 Gb/s, which fact renders the operation of the phase modulator arms to be difficult at such high frequencies.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a semiconductor Mach-Zehnder modulator which is capable of operating in a push-pull modulation by using a single driver at higher frequencies over 2.5 Gb/s.
It is another object of the present invention to provide a method for manufacturing such a semiconductor Mach-Zehnder modulator.
The present invention provides a semiconductor Mach-Zehnder modulator comprising a substrate and a combination waveguide overlying the substrate, the combination waveguide including an input optical waveguide, first and second input branch optical waveguides branching off the input optical waveguide, first and second modulator arm waveguides optically coupled to the first and second input branch optical waveguides, respectively, first and second output branch optical waveguides optically coupled to the first and second modulator arm waveguides, respectively, and an output optical waveguide optically coupled to both the first and second output branch optical waveguides, each of the waveguides having a first cladding layer of a first conductivity type, a second cladding layer of a second conductivity type and an undoped optical guide layer sandwiched between the first cladding layer and the second cladding layer, the first modulator arm waveguide having a first electrode electrically connected to the second cladding layer thereof and a second electrode electrically connected to the first cladding layer thereof and to the second cladding layer of the second modulator arm waveguide, the second modulator arm waveguide having a third electrode electrically connected to the first cladding layer thereof.
The present invention also provides a method for manufacturing a semiconductor Mach-Zehnder modulator comprising the steps of forming a combination mesa struct

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