Semiconductor optical modulator

A semiconductor optical modulator is disclosed which is capable of high-speed modulation without the necessity of increasing the modulating voltage. The present invention has its feature in that the carrier density of the clad layer adjoining the optical waveguide layer 3 is gradually raised toward the p-n junction or Schottky junction, thereby increasing the width of the depletion layer to decrease the junction capacitance.

BACKGROUND OF THE INVENTION 
The present invention relates to a semiconductor optical modulator, and 
more particularly to improvement of the modulation speed thereof. 
Semiconductor optical modulators for optical data processing and optical 
communication use are now being developed very actively. Among them, a 
voltage-driven semiconductor optical modulator of extremely high response 
speed which utilizes the electrooptic effect or electro-absorption effect 
is attracting attention as a high-speed modulator. 
In this case, there has been a strong demand for a semiconductor optical 
modulator which is capable of high-speed modulation without the necessity 
of raising the modulating voltage, but no satisfactory proposals have been 
made up to now. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a semiconductor optical 
modulator which is capable of high-speed modulation without the necessity 
of increasing the modulating voltage. 
The present invention has its feature in that the carrier density of the 
clad layer adjoining the optical wave-guide layer is gradually raised 
toward the p-n junction or Schottky junction, thereby increasing the width 
of the depletion layer to decrease the junction capacitance.

DETAILED DESCRIPTION 
To make differences between prior art and the present invention clear, 
prior art will first be described. 
FIG. 1 is a cross-sectional view of a conventional voltage-driven 
semiconductor optical modulator. Reference numeral 1 indicates an n.sup.+ 
-type InP substrate, 2 an n-type InP lower clad layer, 3 an n.sup.- -type 
GaInAsP optical waveguide layer, 4 an n.sup.- -type InP upper clad layer, 
5 a p.sup.+ -type InP layer, 6 an n-side electrode, and 7 a p-side 
electrode. 
With this structure, a reverse voltage (V) is applied across the electrodes 
6 and 7 to increase the electric field intensity in the optical waveguide 
layer 3, and modulation is performed through utilization of variations in 
its refractive index by the electrooptic effect or in its absorption 
coefficient by the electro-absorption effect. The depth of modulation is 
dependent upon the amount of variation in the refractive index or 
absorption coefficient which is caused in accordance with the electric 
field intensity in the optical waveguide layer 3 which is produced by the 
application of the reverse voltage. 
FIG. 2 shows the electric field intensity distribution in the respective 
layers, obtained when the reverse voltage was applied to the conventional 
semiconductor optical modulator depicted in FIG. 1. The electric field 
intensity is the highest between the p-type InP layer 5 and then the 
n.sup.- -type InP upper clad layer 4 which define therebetween a p-n 
junction, and the field intensity diminishes as the distance from the p-n 
junction increases. The ordinate represents the distance from the p-n 
junction to each layer, and the width W.sub.1 of the lower clad layer 2 
exposed to the electric field means the width of a depletion layer in the 
lower clad layer 2. 
For faster modulation by such a conventional voltage-driven semiconductor 
optical modulator, it is necessary to increase the thickness of the lower 
clad layer 2 and raise the reverse voltage (hereinafter referred to as a 
"modulating voltage") which is applied across the electrodes 6 and 7. 
However, the voltage driving of the modulator is difficult since a high 
modulating voltage cannot easily be created with a high-speed pulse 
generator now available. 
For the reason given above, there has been a strong demand for a 
semiconductor optical modulator which is capable of high-speed modulation 
without the necessity of raising the modulating voltage, but no 
satisfactory proposals have been made up to now. 
The present invention will hereinafter described in detail. 
It is known, in general, to reduce the p-n junction capacitance for 
high-speed modulation without changing the modulating voltage. Based upon 
this, the present inventors considered that the relationship between the 
p-n junction capacitance C and the width W of the depletion layer adjacent 
the optical waveguide layer 3 could be expressed approximately by the 
following equation: 
EQU C=.epsilon..sub.o..epsilon./W (1) 
where .epsilon..sub.0 is the dielectric constant of vacuum and .epsilon. is 
relative dielectric constant of the optical waveguide layer 3. As is 
evident from Eq. (1), the p-n junction capacitance C is in inverse 
proportion to the depletion layer width W of the clad layer. Accordingly, 
an increase in the depletion layer width W of the clad layer without 
changing the electric field intensity in the optical waveguide layer 3 
will reduce the p-n junction capacitance, enabling the high-speed 
modulation. For this reason, according to the present invention, the 
carrier density in the clad layer is varied to thereby change the electric 
field intensity distribution in the clad layer so that the depletion layer 
width is increased, providing for reduced p-n junction capacitance. 
In the following description, the parts corresponding to those in the prior 
art example are identified by the same reference numerals and no 
description will be given of them. 
FIG. 3 illustrates, in cross section, a voltage-driven semiconductor 
optical modulator embodying the present invention. This embodiment differs 
from the conventional structure of FIG. 1 in that the n-type InP lower 
clad layer 2 of a fixed carrier density is substituted by an n-type InP 
lower clad layer 8 of varying carrier densities. The lower clad layer 8 is 
formed so that its carrier density increases toward the optical waveguide 
layer 3. With such a gradient of the carrier density in the lower clad 
layer 8, even if the clad layer 8 is formed thicker without increasing the 
modulating voltage, the electric field is applied to the lower clad layer 
8 throughout, ensuring an increase in the width W of the depletion layer. 
Since the p-n junction capacitance diminishes in inverse proportion to the 
width W of the depletion layer, as referred to previously in connection 
with Eq. (1), high-speed modulation can be achieved without the necessity 
of increasing the reverse voltage. 
FIG. 4 is diagram showing the distribution of the electric field intensity 
in the semiconductor optical modulator of the present invention. With a 
view to comparison with the prior art, in the measurement of this field 
intensity distribution, the carrier density of the lower clad layer 8 was 
varied substantially continuously from, for example, 10.sup.14 cm.sup.-3 
on the side of the substrate 1 to 5.times.10.sup.15 cm.sup.-3 on the side 
of the optical waveguide layer 3, by the same modulating voltage (9 v) as 
was used for measuring the field intensity shown in FIG. 2, and the 
thickness of the lower clad layer was changed from around 0.5 .mu.m in the 
prior art to around 1.4 .mu.m in the present invention. 
FIG. 4 reveals the following facts: 
(1) Since the modulating voltage used is equal to that in the prior art, 
the field intensity value of the optical waveguide layer 3 does not change 
either. That is, the depth of modulation remains unchanged. 
(2) The slope of the field intensity distribution is steep on the side of 
the optical waveguide layer 3 where the carrier density is high, but is 
gentle on the side where the carrier density is low; namely, the distance 
over which the electric field extends (the distance from the plane of the 
p-n junction) can be increased with a change in the carrier density. 
Accordingly, an increase in the distance over which the electric field 
extends means that the width of the depletion layer can also be increased. 
(3) The hatched region in the clad layer 2 of the conventional structure 
and the hatched region in the clad layer 8 of this invention structure 
have the same area. This means that the modulating voltage is the same. 
Accordingly, this also indicates that the width of the depletion layer 
increases although the modulating voltage remains unchanged. 
Incidentally, the difference W.sub.0 in the width of the depletion layer 
between the conventional structure and this invention structure results 
from the difference in thickness between their clad layers. Since the InP 
lower clad layer is 1.4 .mu.m in thickness, the modulation band width can 
be selected around 20 GHz. 
As described above, the present invention permits speedup of modulation 
without increasing the modulating voltage. While in the above the present 
invention has been described in connection with a case where the carrier 
density is continuously varied in the single-layered lower clad layer 8, 
it is also possible to form the lower clad layer 8 of discrete carrier 
density by laminating a plurality of layers of different carrier densities 
in the order of carrier density. Moreover, when the lower clad layer is 
formed thicker, it is necessary only to increase the carrier density. In 
the above the semiconductor layers are described to be formed of materials 
of the InP/GaAsP series, but the present invention is not limited 
specifically thereto but is applicable to semiconductor layers of other 
materials. 
Next, a description will be given of an example of application of the 
semiconductor optical modulator of the present invention. 
FIG. 5 illustrates, in cross section, an example of application of the 
present invention to a high-speed optical signal source device which is a 
monolithic integration of a distributed feedback semiconductor laser 
(hereinafter referred to as a "DFB laser") and the semiconductor optical 
modulator. 
In FIG. 5, a laser region has a structure in which an InP clad layer 8 
having periodic corrugations, a Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y 
laser-region optical waveguide layer 13, a Ga.sub.r In.sub.1-r As.sub.s 
P.sub.1-s active layer 14, and a Ga.sub.t In.sub.1-t As.sub.u P.sub.1-u 
contact layer 15 are laminated on the InP substrate 1 in that order. 
Reference numeral 11 indicates an Si.sub.3 N.sub.4 insulating film and 12 
a .ltoreq./4 shift-diffraction grating. The composition fractions of the 
above layers are 0.ltoreq.x, y, r, s, t, u.ltoreq.1, and the energy gaps 
of the optical waveguide layer 13 and the contact layer 15 are larger than 
the energy gap of the active layer 14. A p-type InP layer 9 and an n-type 
InP layer 10 constitute a window region for eliminating reflection at the 
end face of the laser region. 
On the other hand, a modulating region has a structure in which an InP clad 
layer 8 of varying carrier densitites, a Ga.sub.x In.sub.1-x As.sub.y 
P.sub.1-y modulating-region optical waveguide layer 3 of a thickness 
including the optical waveguide layer 13 and the active layer 14 of the 
laser region, the InP upper clad layer 4, the InP clad layer 5, and the 
Ga.sub.t In.sub.1-t As.sub.u P.sub.1-u contact layer 15 are laminated on 
the substrate 1 common to the laser region in that order and the Si.sub.3 
N.sub.4 insulating film 11 and a window region are added, producing the 
electroabsorption effect. Reference numerals 6 and 7 identify electrodes. 
Light lasing stably at a single wavelength in the laser region is guided by 
the modulating-region optical waveguide layer 3 connected directly to the 
optical waveguide of the laser region and is modulated in its intensity by 
a modulating voltage which is applied across the electrodes 6 and 7 of the 
modulating region. Since the lower clad layer 8 is formed so that its 
carrier density increases as the optical guide layer 3 is approached from 
the side of the substrate 1, the p-n junction capacitance is reduced. Thus 
high-speed modulation can be achieved with a high extinction ratio and a 
low modulating voltage. 
As described above, according to the present invention, the carrier density 
of the lower clad layer 8 adjoining the optical waveguide layer 3 is 
varied, by which the width of the depletion layer is increased to reduce 
the p-n junction capacitance, permitting high-speed modulation with a low 
modulating voltage. Therefore, the present invention can be applied to the 
semiconductor optical modulator for optical communications or optical data 
processing, and hence is of great utility when put to practical use.