Optoelectronic component having codirectional mode coupling

An optoelectric component has two waveguide layers and a layer with a periodic structure, which layers are arranged parallel to one another and are dimensioned so that a codirectional coupling is produced between modes guided in each of the waveguide layers. In order to prevent undesirable reflections, changes in the effective refractive index in the periodic structure is gradually changed along the direction of propagation. This change can be by the boundary of the periodic structure extending at an angle other than a right angle to the direction of propagation, either in a vertical or a lateral direction. The change can also be accomplished by a gradual change of the composition at the boundary of the periodic structure and adjacent portions or sections.

BACKGROUND OF THE INVENTION 
Electronically tunable integrated laser diodes having a large tuning range, 
that is to say greater than approximately 1% of the wavelength, are key 
components for multifarious or multiple applications in optical 
communication engineering. Previously known designs of laser diodes, which 
have codirectional mode coupling require only a single tuning current, are 
advantageous. These laser diodes are described, for example, in a 
publication by I. Kim et al "Broadly tunable vertical-coupler filtered 
tensile-strained InGaAs/InGaAsP multiple quantum well laser", Applied 
Physics Letter, Vol. 64, No. 21, 23 May 1994, pp. 2764-2766 and Amann et 
al "Widely Tunable Distributed Forward Coupled (DFC) Laser", Electronics 
Letters, Vol. 29, No. 9, 29 Apr. 1993, pp. 793-794. 
In codirectional mode coupling, two waves or transverse modes running in 
the same direction are coupled to one another via a grating. This coupling 
is wavelength-selective and current- or voltage-tunable, with the result 
that the longitudinal laser modes, arranged at equidistant wavelength 
intervals, can be made to start oscillating as a function of the tuning 
current or of the tuning voltage. However, because of the periodic 
inhomogeneities which are introduced into the laser resonator by the 
grating structure, additionally disturbing retroreflections 
(contradirectional coupling) are produced. Depending on the wavelength and 
the phase angle, this can variously produce constructive or destructive 
interference for each longitudinal mode. Consequently, and because of the 
relatively weak wavelength selectivity of the codirectional mode coupling, 
it is therefore generally not possible to individually select all possible 
longitudinal laser modes, with the result that in practice, only a small 
number of discrete wavelengths are available within the tuning range. So 
far, the filter properties and spectral distribution have been 
investigated for these lasers without taking into account of any internal 
retroreflection. Consequently, so far, it has not been possible to achieve 
a theoretically calculable mode selection and sideband suppression for the 
majority of the longitudinal modes (see above-mentioned publications). 
U.S. Pat. No. 5,325,379, whose disclosure is incorporated herein by 
reference thereto and which corresponds to European 0 552 390 A1, 
describes a tunable laser diode in which codirectional mode coupling is 
produced by a periodic interrupted absorber layer arranged parallel to two 
waveguide layers. Discontinuities in the real part of the effective 
refractive index in the waveguide layers occurs in each case at the 
longitudinal edges of the individual sections of the absorber layer. The 
internal retroreflections therefore occur at periodic intervals in this 
laser diode. 
U.S. Pat. Nos. 4,932,032 and 4,944,838, whose disclosures are both 
incorporated herein by reference thereto and which correspond to European 
0 411 816 A2, describe the production of waveguide tapers, which taper in 
the vertical direction. The waveguide is formed by a multiplicity of 
InGaAsP layers with interposed InP etching stop layers. The taper is 
produced by stepwise etching of this layer structure. 
Laser diodes with a gain-coupled grid structure are disclosed in U.S. Pat. 
Nos. 5,208,824; 5,452,318; 5,143,864; 5,093,835 and 5,539,766, which 
corresponds to German DE 44 29 586 A1 and by the article by Rast et al 
"Gain-Coupled Strained Layer MQW-DFB Lasers with an Essentially Simplified 
Fabrication Process for .lambda.=1.55 .mu.m", IEEE Photonics Technology 
Letters, Vol. 7, No. 8, Aug. 1995, pp. 830-832. The disclosure of each of 
the above-mentioned U.S. Patents are incorporated herein by reference 
thereto. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optoelectric component 
having a codirectional mode coupling in which undesired internal 
retroreflections of the guided modes are suppressed, if possible. 
This object is achieved by means of an improvement in an electro-optical 
component in which two waveguide layers and a layer with a periodic 
structure are present and are arranged parallel to one another and are 
dimensioned so that codirectional coupling is produced between modes 
guided in these waveguide layers. The improvement is discontinuities, 
which are not required for the coupling and are, therefore, undesirable, 
in the real parts of the refractive index, which are decisively effective 
for the wave guidance of these modes are avoided owing to the fact that 
means for gradually changing the refractive index are present in either 
the periodic structural layer or an additional layer which is changed in 
the direction provided for the propagation of the modes. This means for 
changing may be either in a vertical plane or a lateral plane with 
reference to the direction of propagation, and the change may be in the 
boundary of the relevant layer in a direction either perpendicular to the 
direction of propagation or transverse to the direction of propagation at 
an angle in the layer plane, which is different from a right angle. It is 
also possible that the change is continuous in the vertical dimension of 
the structural layer or additional layer, owing to the fact that the 
boundary extending transverse to the direction of propagation of the modes 
of the relevant layer are aligned with respect to this direction of 
propagation at an angle perpendicular to a layer plane which differs from 
a right angle. 
The present invention solves the problem by virtue of the fact that at the 
points at which in conventional components having codirectional mode 
coupling discontinuities in the refractive index occur, tapers, that is to 
say gradual changes in the lateral or vertical dimensions, or graded 
compositions of the semiconductor mixed crystals are present in or on the 
layers or layer structures responsible for these discontinuities. These 
measures, according to the invention, replace the discontinuities in the 
refractive index by continuous or at least multiple step transitions. The 
waves partially reflected over the entire length of these continuous 
transitions interfere destructively, as a result of which no reflections, 
or only small ones, occur. The codirectional coupling in the forward 
direction is, by contrast, only slightly influenced. At the boundaries 
between the regions of the different materials, which follow one another 
in the direction of propagation of the modes in a grating producing the 
coupling, the gradual increase in the lateral dimensions of these regions 
can be achieved, for example, by virtue of the fact that these boundaries 
in the grating are arranged not at an a right angle to the direction of 
propagation of the mode, but obliquely thereto. This results in a 
structuring of the grating layer which tapers in the direction of the 
propagation in the waveguide. In a periodic absorber layer, each edge or 
boundary of a section of this absorber layer, which extends transverse to 
the direction of the propagation of the modes, can, for example, form a 
taper of this section. In the case of a grating formed by corrugations or 
periodic thickness fluctuations in a continuous layer, according to the 
present invention, these corrugations or other types of periodic 
structuring are aligned obliquely relative to the direction of propagation 
of the modes if the discontinuities in the real part of the effective 
refractive index are undesired in the direction of propagation. If the 
coupling is performed by the periodic change in the real part of the 
effective refractive index, a tapered expansion or narrowing is provided, 
for example only at the edges of the grating layer, which are at the front 
and rear of the direction of propagation of the modes. These edges extend, 
for example, obliquely relative to the direction of propagation of the 
modes, while the gating itself is structured in a conventional way. An 
alternative is the possibility of using a continuous change in the 
refractive index by means of a graded change in the composition of the 
semiconductor mixed crystal of a quaternary or ternary semiconductor 
material used for the relevant layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The principles of the present invention are particularly useful when 
incorporated into a component generally indicated at 100 in FIG. 1a. The 
component in FIG. 1a has two waveguides WG1 and WG2 and a periodically 
structured layer .DELTA.n by means of which the codirectional coupling is 
produced. Owing to the periodic change in the refractive index as an 
effect of the periodically structured layer (filter layer or absorber 
layer), the modes guided in the waveguide layers WG1 and WG2 are coupled 
at a wavelength which is equal to the product of the period .LAMBDA. and 
the difference in the real part of the effective refractive index of the 
two modes. Lacking the inventive measures represented in FIG. 1b, 
undesired discontinuities occur in the effective refractive index at the 
edges of the individual sections .DELTA.n of FIG. 1 a of the periodic 
structured layer if the propagation of the modes is performed in the 
direction of the illustrated z-axis. Because of the manufacturing 
tolerances of at least 0.1 .mu.m, the various reflections at these points 
of discontinuity exhibit statistically distributed phase differences, with 
the result that some of the longitudinal modes are not available as a 
result of randomly distributed constructive or destructive interferences. 
Therefore, according to the invention, in accordance with the 
representation of FIG. 1b, the edges 101 of the parts of the layer 
.DELTA.n are beveled or tapered in such a way that, in the longitudinal 
direction of the waveguides, these parts of the layers .DELTA.n gradually 
widen and/or narrow laterally and, in this way, a continuous change in the 
real part of the effective refractive index is produced in the waveguide 
layers. 
As illustrated in FIG. 2, the modes are propagated in the direction of the 
illustrated z-axis. On the right-hand side, a part of the periodic 
structured layer .DELTA.n is illustrated, whose edge 101 encloses an angle 
.phi. with the perpendicular to the longitudinal direction of the 
waveguide in the layer plane. The result is thus a path length .DELTA.Z, 
as illustrated, at which the real part of the effective refractive index 
experiences a continuous change. The width of the layer .DELTA.n 
corresponds, in this embodiment, to the width W of the waveguide layers 
WG1 and WG2. In FIG. 2, the codirectional coupling modes R and S are 
illustrated on the left-hand side by the amplitude of their field 
strength. In this arrangement, the overall reflection results as the 
integral along the path .DELTA.Z. If the phase difference 2k.sub.0 n.sub.z 
.DELTA.Z which occurs in a mode with an effective refractive index n.sub.z 
and the wave number k.sub.0 =2.pi./.lambda..sub.0 is greater than 2.pi., 
during integration, the reflection components are extinguished by strong 
destructive interference. On the other hand, however, there is also a 
reduction in the codirectional coupling in the case of continuous 
transitions of the refractive indices. However, this influence is 
approximately 2 orders of magnitude lower, since here it is not the 
propagation time or phase angle of the individual transverse modes R or S 
which is decisive, it is rather a question of the difference in the 
propagation time or phase of the modes upon transversal of the path length 
.DELTA.Z. There is, therefore, a large value range for .DELTA.Z within 
which, on the one hand, destructive interference of the reflection and a 
reduction in the overall reflection occur, while, on the other hand, the 
codirectional mode coupling is not yet perceptibly impaired. The result 
for the possible value .DELTA.Z is, therefore, arranged between half the 
wavelength of the laser in a vacuum, divided by the mean effective 
refractive index of the transversal modes R and S, and a quarter of the 
wavelength of the laser in a vacuum, divided by the difference between the 
effective refractive index of the modes. Typical values are, for example 
1.55 .mu.m for the vacuum wavelength, 3.3 for the mean effective 
refractive index and 0.1 for the difference between the effective 
refractive indices of the modes in the case of an InGaAsP laser diode. The 
result is a permissible interval for .DELTA.Z of 0.25 .mu.m to 4 .mu.m, as 
a typical value. For example, with a width W of 2 .mu.m for the wavelength 
strip and an angle .phi. of 30.degree. for the tapered oblique position of 
the boundaries of the parts of the structured layer .DELTA.n, the 
calculated reduction in the power reflection is a factor of 100 to 10,000, 
depending on the strength of the lateral wave guidance. Such a pronounced 
reduction in reflection produces a decisive improvement in the 
characteristics of the laser diode. 
A possible embodiment of the component is illustrated as a laser diode, 
generally indicated at 100' in FIGS. 3 and 4. In this embodiment, the 
waveguide layers 5 and 7 are arranged vertically relative to one another 
and to a periodically interrupted absorber layer 4. Located one above the 
other on a substrate 8a, whose thickness is not shown to scale in the 
drawing, are a cladding layer 8, which is doped in a p-type fashion like 
the substrate 8a. A lower waveguide 7 is arranged on the cladding layer 8, 
then an intermediate layer 6 is on the layer 7 and the upper layer 5 is on 
the intermediate layer 6. The absorber layer 4, which is arranged between 
upper and lower parts 3a and 3b of a cladding layer, which is also doped 
for p-type conduction, is disposed on the upper layer 5 and is covered by 
a conduction layer 2. A connection region 15a, is doped in a highly p-type 
fashion, and is an upper part of the lateral cladding layer 15 (see FIG. 
4). The intermediate layer 6 is oppositely doped, that is to say in an 
n-type fashion. The parts of the absorber layer of length S' together with 
the interruption adjacent thereto of the length T form a period P of the 
periodic structure. Contacts 1, 9 and 11 are used for the electrical 
connection. The lateral cladding layer 15 is doped in an n-type fashion 
and produces the conductive connection between the intermediate layer 6 
and the lateral contact 11 via a further intermediate layer 12, an etching 
stop layer 13 and a contact layer 14, which is doped in a highly n-type 
fashion. The current I.sub.a for generating laser radiation and the 
current I.sub.t for tuning are likewise illustrated. It may be seen in 
FIG. 4 that the waveguide layers 5 and 7 and the absorber layer 4 form an 
embedded web-type structure of a width W. This laser diode has the 
structure of a TTG (Tunable Twin Guide) diode, as is described in the 
above-mentioned U.S. Pat. No. 5,325,379. The respective sections of the 
absorber layer 4 are beveled here according to the invention, as 
illustrated in FIGS. 1b and 2, with the result that the boundaries or 
edges of the parts of the absorber layer 4 extend obliquely in the plan 
view relative to the longitudinal direction of the waveguide. 
An alternative possibility for the tapering ends of the periodic structure 
producing the coupling results by virtue of the fact that the tapering or 
narrowing of the sections of the layer .DELTA.n' is present in the 
vertical in component 100" (See FIG. 5a). Such a vertical taper can be 
produced, for example, by means of dry etching at a temporally variable 
angle by using a shadow mask (at a distance from the layer to be etched). 
Another possibility is described in the two U.S. Pat. Nos. 4,932,032 and 
4,944,838 and the corresponding European Published Document 0 411 816. In 
accordance with the direction of the view illustrated in FIG. 5a, FIG. 5b 
illustrates the part of the layer .DELTA.n' with the covered contours of 
the upper boundaries of these tapers (dashed lines). Thus, in this 
configuration, the boundaries of the layer .DELTA.n' are aligned at an 
angle to the direction of propagation of the modes which differs from the 
right angle perpendicular to the layer plane. In other words, the z-axis, 
shown in FIG. 5b, and a normal to such a boundary enclose an angle whose 
perpendicular projection onto a plane perpendicular to the layer plane 
differs from 0.degree. and 180.degree.. In the case of the embodiment of 
FIG. 1, the boundaries of the layer .DELTA.n are aligned at an angle to 
the direction of propagation of the mode, which differs from the right 
angle in the layer plane, that is to say the z-axis, as shown in FIG. 1b, 
and a normal to the boundary enclose an angle whose projection, 
perpendicular with respect to the layer plane, onto the layer plane is not 
90.degree.. As a result, the case is respectively included, in which 
tapers are present both in the lateral and in the vertical direction. It 
is, thus, possible to combine the embodiments of FIGS. 1 and 5 with one 
another. The boundary may not be a plane surface but can, for example, be 
cambered or rounded at the edges. The edge, shown in the top view of FIG. 
1b, of a part of the layer .DELTA.n need not be rectilinear, as 
illustrated, but can, in a suitable way, be rounded or, for example, be 
curved in the shape of an S. 
An alternative to the component according to the invention is provided by 
means of a graded composition of the semiconductor material at the point 
where no discontinuity is to occur in the refractive index, which, for 
example, in the embodiments of FIGS. 3 and 4 would be the absorber layer 
or the material surrounding this layer of the cladding layer 3. If the 
absorber layer in FIG. 3 is an InGaAs, the cladding layer can be an InP or 
at least an element arranged between the parts of this absorber layer 4, 
an InGaAsP. The composition of the semiconductor mixed crystals can then 
be undertaken so as to produce a gradual transition between the materials 
of the cladding layer 3 and the parts of the absorber layer 4 The graded 
composition of the semiconductor material produces a corresponding gradual 
change in the effective refractive index. 
A possible production method uses the different growth rates of the 
semiconductor materials of which the mixed crystal is composed and these 
growth rates are dependent on the width of the mask opening. If use is 
made of a mask having a slot-shaped opening which widens toward the edge 
of the mask, the result in the case of epitaxial growth in the region of 
the slot-shaped opening is a higher growth rate, which decreases 
continuously outward down to the growth rate on the unmasked surface. 
Apart from the vertical tapering and a lateral widening corresponding to 
the shape of the mask opening, in the region of the widening of the mask 
opening, the layer, which is grown on is given a grating in the 
composition of the semiconductor mixed crystal. 
The embodiment of FIG. 5 can, as indicated, be specifically configured so 
that the periodically structured layer .DELTA.n' comprises a plurality of 
layers, which are arranged vertically one above the other and form the 
layer .DELTA.n' in their totality. These materials can have a material 
composition differing from one another and, moreover, can be separated 
from one another by thin etching stop layers. Such a multilayered 
structure renders it possible, by means of a stepwise etching of the 
tapered parts of the layer .DELTA.n', to obtain not only the changes in 
the vertical dimension which occur in the longitudinal direction of the 
waveguide but, in addition, to obtain multiple stepped changes in the 
effective refractive index by means of a stepwise arrangement of layers 
with different material composition. The combination of the lateral 
varying dimensions of the layer .DELTA.n, as in the exemplary embodiment 
of FIG. 1, with a varying material composition in the longitudinal 
direction of the waveguide is also possible. 
The principles described here with the aid of the exemplary embodiments can 
be applied at all points in a component having codirectional coupling at 
which discontinuous changes in the effective refractive index are to be 
avoided. This can be the case, for example, where there is a transition 
between active and passive regions of the component. A configuration, such 
as described above for the part of the periodically structured layer 
(.DELTA.n or 4), can, for example, be present only at edges of a grating 
provided for the coupling. The edges of the grating, which are present in 
the direction of propagation of the modes, are then, for example, arranged 
obliquely relative to this direction of propagation. The discontinuities 
in the refractive index provided and required along the grating remain 
uninfluenced thereby. Again, it is possible in this case to undertake the 
lateral bounding of the grating conventionally, but to provide a graded 
composition of the semiconductor material between the grating and the 
semiconductor material which is adjacent in the direction of propagation 
of the modes. 
Parts or portions of the compensation layer, in which it is possible to 
keep the real part of the effective refractive index constant, can be 
provided between the parts of the absorber layer 4 in the example of FIG. 
3 or the periodical structured layer .DELTA.n (see the portions 40 in FIG. 
1a), which produces a change in the effective refractive index along the 
waveguide layers. The means according to the present invention for 
continuously adjusting the effective refractive index can then be present 
in this compensation layer or, if necessary, an additional layer which is 
arranged, for example, vertically with respect to the periodically 
structured layer .DELTA.n. 
Although various minor modifications may be suggested by those versed in 
the art, it should be understood that we wish to embody within the scope 
of the patent granted hereon all such modifications as reasonably and 
properly come within the scope of our contribution to the art.