Cut-off filter for integrated optics

A cut-off filter for integrated optics, which filter has a form of a relief-like surface grating on a film or strip waveguide, is formed in a portion of the strip waveguide having a reduced layer thickness and is formed with a constant corrugation depth. Preferably, the formation of the grating is done in a simple way by simultaneously etching a depression in the strip waveguide and also etching the grating structure onto the floor of the depression with the assistance of anisotropically acting etchants.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention is directed to a cut-off filter for integrated 
optics, which filter is formed as a relief-like surface grating on a film 
or strip waveguide and to a method of forming the filter. 
What is to be understood hereinafter by the term "a cut-off filter" of this 
type is that it is a filter that exhibits a better sidelobe suppression 
when compared to a filter in the form of a relief-like surface grating 
having a constant corrugation depth and period on a film or strip 
waveguide of a uniform layer thickness. 
A cut-off filter of this type is disclosed in an article by P. S. Cross and 
H. Kogelink, "Sidelobe Suppression in Corrugated Waveguide Filters", 
Optics Letter, 1 (1977), pp. 43 ff. In this filter, the improved sidelobe 
suppression is achieved by a variation of the corrugation depth and period 
of the grating in accordance with the prescribed mathematical function. 
Suitable functions are also specified. 
Cut-off filters of this type come into consideration for integrated optical 
waveguide division multiplex circuits for optical communications (see page 
43, first column, paragraph 1 of the above-cited article). 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an easily manufactured 
cut-off filter of the type comprising a relief-like surface grating on a 
film or strip waveguide. The object of the present invention is achieved 
by an improvement in the cut-off filter for integrated optics in the form 
of a relief-like surface grating on a film or strip waveguide. The 
improvements are that the grating is fashioned in the region of a reduced 
layer thickness of the waveguide and comprises a constant corrugation 
depth. 
In the cut-off filter of the present invention, the cutoff filter 
properties recited in greater detail above are achieved in that the 
grating is formed in a region of reduced layer thickness of the waveguide. 
It is known from the theory of interference filters that cut-off filters 
can be realized by raising and/or lowering the refractive index of the 
surrounding media with respect to the filter layers (see Optica Acta 28 
(1981) 29). The structure of the filter, however, is thereby also of great 
significance. The invention is based on the perception that due to the 
formation of the grating in a region of reduced layer thickness of the 
waveguide, it can be achieved that the effective refractive indexes in the 
region of the grating are smaller than in the regions proceeding and 
following the grating, even when it is a filter of the type initially 
mentioned. 
A simple manufacture of the cut-off filter of the invention resides in the 
fact that a grating having a constant corrugation depth and a region of 
reduced layer thickness of the waveguide can be manufactured in an 
especially easy manner and even in one method step as shall be discussed 
hereinafter. 
The grating is preferably formed in a plane and/or in a region of the 
waveguide having a constant, reduced layer thickness. 
It is expedient when the grating extends in the region of reduced layer 
thickness that is defined by a depression fashioned in the waveguide. The 
grating is preferably formed on the floor of this depression and the 
depression preferably comprises an essentially trapezoidal or box-like 
profile. 
In the preferred embodiment of the filter of the invention, the region of 
the reduced layer thickness which contains the grating in a strip or film 
waveguide applied on a substrate, is covered with a cover layer of a 
material whose index of refraction like that of the material of the 
substrate is lower than the refractive index of the material of the 
waveguide. 
An especially preferred embodiment of the development is fashioned so that 
the substrate and layer covering the strip or film waveguide is composed 
of InP and the strip or film waveguide itself is composed of InGaAsP. 
In the manufacture of the filter, one advantageously proceeds so that the 
film or strip waveguide is covered with an etching mask housing a window, 
a grating structure is formed in the window and is subsequently exposed to 
an anisotropically acting etchant until the layer thickness of the 
waveguide has been reduced to the desired value. The grating and the 
reduction in layer thickness are thereby simultaneously formed and are 
formed on their own. 
The etching mask with the window and the grating structure in the window 
are preferably generated so that the thin metal layer with the window is 
applied to the waveguide and a layer of photoresist is then applied to the 
metal layer, the layer photoresist in the region of the window of the 
metal layer is exposed with a light pattern corresponding to the grating 
structure of the grating to be generated and the photoresist is 
subsequently developed. 
The rectangular window of the etching mask is preferably oriented so that 
the grating lines that occur due to the anisotropic etching process 
proceed parallel to the lateral edges of the rectangle. 
Other advantages and features of the invention will be readily apparent 
from the following descriptions, drawings and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The principals of the present invention are particularly useful when 
incorporated into a cut-off filter of FIG. 1 for a film or strip waveguide 
2 which has a reduced layer thickness in certain areas or regions. As 
illustrated, a waveguide layer 2 of InGaAsP is epitaxially grown on a 
surface 11 of a substrate 1 of InP. The waveguide is covered by a cover 
layer 3 which is epitaxially grown InP on a surface 21 of the waveguide. 
In the region L, the waveguide 2 has a depression 5 in which a normal 
thickness D of the waveguide is diminished to a small thickness d. This 
small thickness d in FIG. 1 is equal to a distance between the substrate 
surface 11 and a plane E which extends parallel to the surface 11. This 
plane E defines a floor of the depression 5 in which a relief-like surface 
grating 4 is fashioned. The grid lines of the grating 4 are defined by 
furrows 41 formed in the floor of the depression which extend 
perpendicular to the plane of the drawing and have an essentially 
triangular profile. The furrows or corrugations 41 have a depth t which is 
defined by the distance between the lowest point of the furrow from the 
plane E and is the same for all furrows of the grating. 
In a plan view of FIG. 2 of the cut-off filter, a grating axis A proceeds 
perpendicular to the furrows 41 and parallel to a plane E of FIG. 1. Rays 
of light having a defined wavelength .lambda. are guided in the waveguide 
2 in a direction R in the plane E and impinge on the grating 4 at an angle 
of incidence .alpha.. Broken line 51 shows a rim of the depression 5 
whereas the dot-dash lines 610 indicate an edge of a window 61 FIG. 3) 
which determines the spatial extent of the grating 4. This window 61 will 
be discussed in greater detail hereinafter. 
As an example, let us consider a grating 4 having triangular furrow 
profiles given an angle incidence of .alpha.=60.degree. for a wavelength 
.lambda.=1500 nm. The thickness D.sub.A of the cover layer 3 of the InP is 
selected to be 2 .mu.m. Given this thickness D.sub.A, the maximum value of 
the thickness D of the waveguide 2 at which the latter is still 
transversely monomode is 0.9 .mu.m. The reduced thickness d of the 
waveguide 2 in the region of the grating 4 is selected to be 0.25 .mu.m. 
The corrugation depth t is 0.1 .mu.m and the grating constant b is 
selected to be b=0.5 .mu.m. The refractive index n.sub.1 of the InP 
material amounts to 3.175 and the refractive index n.sub.2 of the InGaAsP 
material is 3.29. The gap wavelength of the waveguide 2 amounts to 1100 
nm. 
Given these materials, values and dimensions, the effective refractive 
index n.sub.A for the TE.sub.o mode assumes the value 3.2513 in the region 
of the waveguide 2 having the thickness 0.9 .mu.m, which is outside of the 
grating 4. Whereas in the region of the grating 4, the effective 
refractive index n.sub.A ranges between 3.1827 and 3.1938. 
The insertion losses that occur due to the depression 5 in the waveguide 2 
can be generally kept within bounds by a cover layer 3 having a suitable 
refractive index. A computational investigation shows that insertion 
losses of 1 to 2 dB can be anticipated given the examples set forth. This 
investigation also shows that the reflectivity of the grating 4 in the 
depression 5 for the wavelength .lambda. below 1500 nm decreases more 
quickly than a comparative filter of the type initially cited wherein the 
grating is fashioned on a waveguide having a uniform thickness selected 
equal to the thickness d=0.25 .mu.m. Accordingly, the exemplary cut-off 
filter realizes a low pass with respect to the wavelengths .lambda.=1500 
nm. 
In addition, the investigation shows that given a required cross-talk 
attenuation of 20 dB with respect to the optical power, the minimum 
channel spacing for the exemplary cutoff filter amounts to 30 nm instead 
of amounting to 45 nm for a comparison filter. Whereas, the width of the 
stop band for the cut-off filter is even somewhat greater. 
The stated results were acquired for TE modes. As a consequence of the 
cover layer 3 of InP, however, the difference between the effective 
refractive index for a TM and TE modes are slight so that polarization 
independent filters are possible. 
The cut-off filters of FIGS. 1 and 2 can be manufactured in the following 
way. The starting point is a substrate 1 of InP on whose surface 11 the 
film or strip waveguide 2 is grown in the form of a layer of InGaAsP 
having a thickness D, i.e., a thickness that is desired later outside of 
the grating 4. A thin metal layer 6 of, for example, gold, is applied to 
an exposed surface 21 of the waveguide 2 to a thickness of, for example, 
about 5 nm. This application can be by sputtering. A photoresist is then 
whirled onto the metal layer 6. A mask exposure is subsequently carried 
out and the afore-mentioned window 61 is then etched into the metal layer 
by a wet chemical etching, for example, with a mixture of 90 g iodine, 200 
g potassium iodide and 200 g water. 
The grating 4 is later formed in the region of this window 61. The 
rectangular window 61 is aligned on the surface 21 of the waveguide 2 that 
is indicated in FIG. 2 with the furrows 41 having triangular profiles and 
the grating 4, which is formed by a later anisotropic etching process 
wherein the grating 4 is etched by exposing the (111)-surfaces of InGaAsP 
crystal, proceed parallel to an edge side of the window 61. In the region 
of the window 61, this photoresist layer 7 is whirled onto the metal layer 
and is then exposed with an interference pattern in the form of a grating 
that is generated, for example, by a laser emission. This grating is 
composed of parallel like streaks whose grating constant is selected equal 
to the grating constant b of the later formed grating. After this, the 
photoresist layer 7 is developed whereupon the structure shown in FIG. 4 
will occur. Narrow photoresist strips 71 have remained in the region B of 
the exposure. These narrow photoresist strips are separated from one 
another by narrow, strip-shaped interstices 72. Depending on the type of 
photoresist employed, either the interstices 72 or the photoresist strips 
71 can then arise under the light streaks of the interference pattern. The 
grating constant of the grating structure composed of the photoresist 
strips 71 is the same as the grating constant b of the grating 4 to be 
produced. 
The surface 21 of the waveguide 2 in the interstices 72 in FIG. 4 is 
subjected in the region of the window 61 with an anisotropically acting 
etchant and the (111)-surfaces of the crystal structure of the InGaAsP 
material are laid free. For example, a mixture of ten parts 48% HBr, one 
part saturated bromine water and forty parts H.sub.2 O is suitable. 
The grating 40 shown in FIG. 5 will occur in the surface 1 of the waveguide 
2 as a result of this etching process. The furrows 401 have a V-shaped or 
triangular profile and the grating constant thereof is equal to b. The 
strips 71 of the photoresist and the photoresist layer 7 itself must be 
imagined as having been removed from FIG. 5 even though this is not yet 
necessary at this point and time. 
The grating 40 in FIG. 5 is now transferred unmodified into the depth by 
allowing the etchant to act for a longer period of time. This process 
occurs on its own and has already been set forth in greater detail in an 
earlier filed co-pending allowed U.S. patent application, Ser. No. 
853,946, filed Apr. 21, 1986, which issued as U.S. Pat. No. 4,670,093 on 
June 2, 1987 and whose disclosure is incorporated by reference. The metal 
layer 6 with a window 61 acts as an etching mask during this process. The 
window 61 determines the spatial extent of the grating to be produced and 
of the depression that occurs in the waveguide 2. The etchant must act 
until the grating 40 is lowered into to the predetermined, reduced 
thickness d where it then forms the grating 4 of FIG. 6, which has 
V-shaped furrows 41 and this grating is then formed on the floor of the 
depression 5. 
After the metal layer 6 is removed, the layer 3 of InP is epitaxially grown 
on the waveguide 2 so the structure shown in FIG. 1 will occur. The 
constant corrugation depth t of the grating 4 decreases slightly in this 
step. 
Although various minor modifications may be suggested by those versed in 
the art, it should be understood that I wish to employ within the scope of 
the patent granted hereon all such modifications as reasonably and 
properly come within the scope of my contribution to the art.