Optical filter and optical device using same

An optical filter is formed by providing a plurality of gaps, which have a desired width and such a depth that is larger than the thickness of the waveguide layer, in a slab or a waveguide layer in a three-dimensional optical waveguide so as to extend in the light propagating direction at desired period intervals. These gaps are filled with a film of a material, the refractive index of which is different from that of the waveguide layer, to complete the optical filter. A multiplex wavelength transmission device is formed monolithically by providing at least one optical filter, which is formed in the above-mentioned manner, in an optical waveguide, and arranging one or both of a light-emitting semiconductor element and a photodetector on the side of an optical signal which has passed through the optical filter, and on the side of an optical signal which has been reflected on the same optical filter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a waveguide type optical filter having wavelength 
selectivity, and more particularly to an optical module for wavelength 
division multiplexing using a waveguide type optical filter. 
2. Description of the Prior Art 
The techniques for the multi-wavelength transmission for optical fiber 
communication is very important for economizing the same communication. In 
the multi-wavelength transmission, an optical multi/demultiplexer is an 
essential device. 
The conventional optical multi/demultiplexers include an optical 
multi/demultiplexer constructed by using an optical interference film 
filter ("Optical Communication Handbook" published by the Asakura Shoten, 
pages 325-331). The optical multi/demultiplexer using this optical 
interference film filter has excellent loss characteristics in a pass band 
and a loss band, and excellent pass band characteristics, and is going to 
be widely used. In this optical multi/demultiplexer, an optical 
interference film filter is vacuumevaporated on a glass plate, and this 
optical interference film filter-carrying glass plate is bonded to a glass 
block with an adhesive agent. However, it is necessary that an accurate 
axis-aligning operation be carried out during the bonding of such a 
filter-carrying glass plate to the glass block with an adhesive agent. 
When the adhesive agent is applied to the glass plate to a certain 
thickness, the angle of the glass plate bonded to the glass block becomes 
different. Therefore, it is necessary to adjust the position and angle of 
the glass plate while exciting the light. Moreover, the 
assembly-processing time is required excessively, and this makes it 
difficult to reduce the manufacturing cost. In addition, this optical 
multi/demultiplexer also requires the glass block to be lustered, and the 
size accuracy and the angle accuracy to be improved, so that the device 
becomes very expensive. The low mass productivity of the device is also a 
cause of its high price. If a light-emitting semiconductor device and a 
photo-detector are combined with this optical multi/demultiplexer so as to 
obtain a hybrid module for bidirectional transmission, much more time is 
required for assembly-processing the parts and adjusting the optical axis 
since the object module is a combination of separate parts. The price of 
such a product also becomes high. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve these problems, i.e., 
provide a one-chip monolithic optical filter capable of being manufactured 
more simply and more economically by using a process for forming a 
conventional lightemitting semiconductor device and a conventional 
photodetector on an optical waveguide, and an optical module using this 
optical filter. 
The optical filter according to the present invention is obtained by 
forming in an optical waveguide a plurality of gaps of a desired period, a 
desired width and such a depth that is larger than the thickness of the 
waveguide so that the gaps are arranged in the light propagation 
direction. An optically permeable material having a refractive index 
different from that of the waveguide is packed in these gaps to complete 
the optical filter. An optical device according to the present invention 
is formed monolithically by providing at least one optical filter referred 
to above, on an optical waveguide, and setting one or both of an optical 
element, i.e. a light-emitting semiconductor device or a photo-detector on 
the portions of the waveguide which are on the side of an optical signal 
which permeates through the optical filter and on the side of an optical 
signal reflected on the same optical filter. 
The optical filter according to the present invention is also formed by 
packing an optically permeable material having a refractive index (nL) 
different from that nH of the waveguide in the gaps mentioned above. In 
the case of an optical band rejection filter, the width of the gap is set 
to 
##EQU1## 
(wherein m1 is an odd number) with respect to the propagation wavelength 
.lambda.0, and the pitch of the gaps to 
##EQU2## 
(wherein m2=1, 3, 5, 7, . . . ). In the case or an optical band-pass 
filter, at least one gap of a width of 
##EQU3## 
(m3 is an odd number), or at least one inter-gap portion of a width of 
##EQU4## 
or a combination of these is provided at an intermediate section of a row 
of gaps of 
##EQU5## 
These optical waveguide type filters are designed so as to reflect or 
transmit the light thereon or therethrough owing to the wavelength 
characteristics thereof, and an optical element, i.e. a light-emitting 
semiconductor device, or a photo-detector are formed monolithically on the 
light-transmitting and reflecting sides of the waveguide to thereby obtain 
an optical device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing the embodiments of the present invention, a general 
description of an optical filter will now be given. An optical filter 
employs the well-known construction of an optical interference film filter 
as shown in FIG. 1. In an optical interference film filter, layers of a 
higher refractive indexes (for example, n.sub.2, n.sub.4, n.sub.6 . . . ) 
and layers of a lower refractive indexes (for example, n3, n5, n7 . . . ) 
are formed alternately to a width (in this case, d.sub.2, d.sub.3 . . . 
d.sub.n-1) of substantially 
##EQU6## 
(m=1, 3 . . . ), wherein .lambda.0 represents the wavelength of the light. 
In contrast to this optical interference film filter, an embodiment of the 
optical filter according to the present invention is as shown in FIGS. 2A 
and 2B. FIG. 2A is a plane view of the optical filter, and FIG. 2B a 
sectional view taken along the line A--A' in FIG. 2A. In this embodiment, 
a plurality of gaps having a width of substantially 
##EQU7## 
are formed at substatially m/2 wavelength intervals waveguide layer of a 
high refractive index (or a low refractive index), and films of a material 
having a low refractive index (or high refractive index) is packed in 
these gaps. When a required optical filter has certain wavelength 
characteristics, at least one gap 4-3 having a width of about 
##EQU8## 
may be formed at an intermediate portion of the waveguide as shown in 
FIGS. 2C and 2D, to pack films of a low (or high) refractive index in this 
gap. Conversely, as shown in FIGS. 2E and 2F, at least one waveguide 3-1 
having a width of about 
##EQU9## 
may be provided in the waveshort guide layer. Namely, a cavity layer is 
provided so that a short wavelength pass type, long wavelength pass type, 
band pass type or band rejection type optical filter can be formed. The 
waveguide layer is formed of a slab waveguide type or three-dimensional 
waveguide type layer. 
The optical filter shown in FIG. 2 is a slab waveguide type optical filter, 
in which a substrate 1 consists of a semiconductor, a dielectric, a 
magnetic substance or glass, a waveguide layer 3 being formed via a 
cladding layer 2. The cladding layer 2 has a refractive index lower than 
that of the waveguide layer 3. The layers 4-1.about.4-m are low refractive 
index layers the refractive index of which is lower than that of the 
waveguide layer 3, and they are formed at about m/2 wavelength intervals. 
It is desirable that the refractive index of the layers 4-1.about.4-m be 
higher than that of the cladding layer so as to reduce the radiation loss 
therein. The width of these low refractive index layers 4-1.about.4-m is 
set to about m/4 wavelength. In this embodiment, InP was used for the 
substrate 1, InP for the cladding layer 2, InGaAsP for the waveguide layer 
3, and oxide films, which was obtained by doping SiO.sub.2 with TiO.sub.2, 
for the low refractive index layers 4-1.about.4-m. The results of 
calculations about an optical filter provided with two cavity layers of a 
high refractive index and two cavity layers of a low refractive index at 
an intermediate portion thereof, in which the refractive index of a 
waveguide layer 3 is set to 3.2; the refractive index of a low refractive 
index layers 4-1.about. 4-m to 1.8; and the number of layers m to 23, are 
shown in FIG. 3. The longitudinal axis of FIG. 3 represents the 
transmission loss (dB) of light, and the lateral axis the wavelength 
(.mu.m). As may be understood from the drawing, the optical filter works 
as a band-pass filter when the wavelength .lambda.0 of the light is 1.2 
.mu.m, 1.3 .mu.m and 1.55 .mu.m. 
FIGS. 4A-4D show the steps of manufacturing this optical filter. FIG. 4A 
shows a step of forming a cladding layer 2 and a waveguide layer 3 on a 
substrate 1 by the CVD technique, for example, and FIG. 4B a step of 
forming gaps 5-1.about.5-5 by the dry etching. These gaps can be formed by 
the dry etching in practice with an accuracy of not more than 0.05 .mu.m 
with respect to a width of 1 .mu.m. Accordingly, when the gap width and 
gap interval are set to, for example, 3/4 wavelength and 3/2 wavelength, 
respectively, the characteristics shown in FIG. 3 can be obtained. FIG. 4C 
shows a step of packing oxide films 6 (in this case, films of TiO.sub.2 
-containing SiO.sub.2) in the gaps 5-1.about.5-5 by the plasma CVD 
technique, for example. FIG. 4D shows a step of etching the oxide film 6 
on the waveguide layer 3. The oxide film 6 on this waveguide layer 3 may 
be left as it is without being etched, for preventing variations in the 
characteristics, which may occur due to the humidity of the optical 
filter, and the absorption and scattering loss of light, which may occur 
due to the contamination of the surface of the optical filter. It is 
important that the oxide film formed on the waveguide layer 3 has a 
refractive index lower than that of the low refractive index layers 
4-1.about.4-5, for the purpose of reducing the radiation loss in the 
waveguide. If a semiconductor material and glass, such as SiO.sub.2 are 
used for the substrate 1 and low refractive index layer 6, respectively, 
in the optical filter according to the present invention, a refractive 
index difference can be set large. This enables the pass-band width of the 
band-pass filter to be increased, and a rejection band to be set large. 
Accordingly, the number of layers m may be half or not more than a half of 
that of a conventional optical filter. 
FIGS. 5A and 5B show an embodiment of a multiplex wavelength transmission 
device according to the present invention, wherein FIG. 5A is a front 
elevation; and FIG. 5B a side elevation. A cladding layer 2 is formed on a 
substrate 1, and a slab waveguide layer 3, the refractive index of which 
is higher than that of the cladding layer 2, on the cladding layer 2, 
optical filters 7, 8 according to the present invention being formed on 
the slab waveguide layer 3. Reference numerals 13, 14, 15, 16 denote 
lenses, which are formed of concave recesses made in the slab waveguide 
layer 3, and which are used to collimate the light projected from an 
optical fiber 17, and the light from a light-emitting semiconductor 
element, or focus the light advancing to a photo-detector. The 
light-emitting semiconductor elements or photo-detectors 9, 10, 11, 12 are 
fixed to side end surfaces 20, 21 of the slab waveguide layer 3. 
The operation of the multiplex wavelength transmission device of FIG. 5 
will now be described. A semiconductor laser having a wavelength .lambda.1 
(1.55 .mu.m in this case) was used as the light-emitting semiconductor 
element 9, a Ge-APD (germanium avalanche photodiode) for receiving an 
optical signal of a wavelength of 1.2 .mu.m as the photo-detector 11, and 
a Ge-APD for receiving an optical signal of a wavelength of 1.3 .mu.m as 
the photo-detector 12. A along-wavelength passing type filter, which is 
adapted to pass optical signals of wavelengths of 1.3 .mu.m and 1.55 .mu.m 
therethrough and reflect an optical signal of a wavelength of 1.2 .mu.m 
thereon, was used as the optical filter 7, and an optical band rejection 
filter, which is adapted to pass optical signals of wavelengths of 1.2 
.mu.m and 1.55 .mu.m therethrough and reflect an optical signal of a 
wavelength of 1.3 .mu.m thereon, as the optical filter 8. An optical 
signal of 1.55 .mu.m is propagated in an optical fiber 17 in the direction 
of an arrow 19, and the optical signals of wavelengths of 1.2 .mu.m and 
1.3 .mu.m propagated in the direction of an arrow 18 are received by the 
photo-detectors 11, 12. Reference numeral 10 denotes a photo-detector for 
monitoring a signal of the light from the light-emitting semiconductor 
element 9. The light-emitting semiconductor element and photo-detector may 
be grown in a liquid phase epitaxyonthe substrate 1, or fixed thereto from 
the outside. 
FIGS. 6A and 6B show a multiplex wavelength transmission device, another 
embodiment of the present invention, wherein FIG. 6A is a front elevation; 
and FIG. 6B a side elevation. This embodiment is a device having a 
threedimensional waveguide structure, in which the parts designated by the 
same reference numerals as in FIG. 5 have the functions identical with 
those of the parts of these reference numerals shown in FIG. 5. Referring 
to FIG. 6, reference numeral 22 denotes a waveguide layer, which has a 
raised strip type structure in this embodiment, and which may have a ridge 
type or diffusion type structure, and 23, 24 band-pass filters, the 
band-pass filter 23 being adapted to pass an optical signal of a 
wavelength of 1.2 .mu.m alone therethrough, the band-pass filter 24 being 
adapted to pass an optical signal of a wavelength of 1.3 .mu.m alone 
therethrough. 
FIG. 7 shows still another embodiment of the multiplex wavelength 
transmission device according to the present invention, in which tapering 
portions 25, 26 are provided at the parts of a waveguide layer which are 
in front of the photo-detectors 11, 12, to adiate unnecessary optical 
signals to the outside of the waveguide and thereby reduce such optical 
signals. This embodiment is identical with the embodiment of FIG. 6 except 
that these tapering portions 25, 26 are provided. 
FIG. 8 shows a further embodiment of the multiplex wavelength transmission 
device according to the present invention. In this embodiment, a 
semiconductor laser having a wavelength of 1.3 .mu.m was used as a 
light-emitting semiconductor element 9, a light-receiving element adapted 
to receive an optical signal having a wavelength of 1.2 .mu.m as a 
photo-detector 11, and a light-receiving element adapted to receive an 
optical signal having a wavelength of 1.55 .mu.m as a photo-detector 12. 
Band-pass filters having the characteristics shown in FIG. 3 were used as 
optical filters 27, 28, 29. Namely, in the embodiment of FIG. 3, a 1.3 
.mu.m bandpass filter (BPF) adapted to pass a 1.3 .mu.m optical signal 
therethrough and reflect 1.2 .mu.m and 1.55 .mu.m optical signals was used 
as the optical filter 27, a 1.2 .mu.m band-pass filter as the optical 
filter 28, and a 1.55 .mu.m band-pass filter as the optical filter 29. 
FIG. 9 shows a further embodiment of the multiplex wavelength transmission 
device according to the present invention. This embodiment is an 
embodiment of a so-called multiplex three-wavelength transmission device, 
in which 1.2 .mu.m and 1.3 .mu.m optical signals are transmitted in the 
direction of an arrow 38 with a 1.55 .mu.m optical signal transmitted in 
the opposite direction of an arrow 39. Referring to FIG. 9, reference 
numerals 31, 32 denote a semiconductor laser having a wavelength of 1.2 
.mu.m and a semiconductor laser having a wavelength of 1.3 .mu.m, 33, 34 
photo-detectors for monitoring the light from these semiconductor lasers 
31, 32, and 30 elements for multiplexing the emitted 1.2 .mu.m and 1.3 
.mu.m laser beams. An optical filter 36 is a short-wavelength pass type or 
band rejection type filter adapted to pass 1.2 and 1.3 .mu.m optical 
signals therethrough and reflect a 1.55 .mu.m optical signal. The optical 
filter 37 is a longwave-length pass type filter adapted to reflect 1.2 
.mu.m and 1.3 .mu.m optical signals. 
The present invention is not limited to the above-described embodiments. 
First, the number of wavelengths to be multiplexed is not limited to three 
in the above embodiments. Namely, it can be increased to any number which 
is not less than two. The light-emitting semiconductor elements may by 
substituted by semiconductor lasers or light-emitting diodes. In the 
embodiment of FIG. 5, the semiconductor lasers may consist of surface 
emission type lasers which are adapted to emit a laser beam at right 
angles to the substrate. The photo-detectors may consist of InGaAs-APD or 
InGaAs-PINPD (PIN photodiodes) besides Ge-APD. The films 6 packed in the 
gaps 5-1.about.5-5 may consist of a material containing a refractive 
index-controlling dopant, such as TiO.sub.2, GeO.sub.2, PeO.sub.5, ZnO and 
Al.sub.2 O.sub.3 besides a single-component material SiO.sub.2. These 
films may also consist of a semiconductor material, such as InGaAsP, 
AlGaAs and GaAs, a dielectric or a magnetic material. In addition to the 
bidirectional transmission system, an optical multiplex device or an 
optical demultiplex device, which constitutes a single directional 
transmission system, may also by used as the optical transmission system. 
These systems can be formed by using optical elements consisting of 
light-emitting semiconductor elements alone or photo-detectors alone. If, 
in the embodiment of FIG. 4 , the substrate 1, cladding layer 2 and 
waveguide layers 3 are formed of, for example, Si, a layer of B.sub.2 
O.sub.3 -containing SiO.sub.2 and SiO.sub.2, respectively, and, if films 
of a high refractive index, which consist of TiO.sub.2 or SiO.sub.2 
containing a refractive index-increasing dopant, are then formed in the 
gaps 5-1.about.5-5, a glass waveguide type optical filter can be obtained. 
It is possible that this optical filter has the characteristics of an 
extremely low loss as compared with an optical filter using a 
semiconductor material. The gaps 5-1.about.5-5 shown in FIG. 4B can be 
formed by the dry etching, such as the ion beam etching, high-frequency 
sputter etching, reactive high-frequency sputter etching, plasma etching 
and ion-applying acceleration etching, or equivalent etching techniques. 
As described above, the optical filter and the multiplex wavelength 
transmission device using this optical filter according to the present 
invention can be obtained by forming in a slab or a waveguide layer in a 
three-dimensional optical waveguide a plurality of gaps, each of which has 
a desired width and a depth larger than the thickness of the waveguide 
layer, so that the gaps are arranged at desired period intervals along the 
light-propagating direction; filling these gaps with films of a material 
having a refractive index different from that of the waveguide layer, to 
form an optical filter; and providing one or both of a light-emitting 
semiconductor element and a photo-detector at the side of an optical 
signal which has passed through the optical filter, and at the side of an 
optical signal which has been reflected on the optical filter to form the 
filter monolithically. Thus, an optical filter of novel construction can 
be formed by using a conventional process for forming light-emitting 
semiconductor elements and optical detectors. Furthermore, a simpler and 
more economical multiplex wavelength transmission device can be obtained 
by using this optical filter. 
FIG. 10 shows a further embodiment of the optical waveguide type filter 
according to the present invention. FIG. 10A is a top view, FIG. 10B a 
front elevation, and FIG. 10C a left side elevation. Reference numeral 1 
denotes a substrate of a semiconductor, a high-degree dielectric, a 
magnetic material, or glass, 3 a waveguide layer having a refractive index 
of nH, 2 a cladding layer the refractive index nc of which is lower than 
that nH of the waveguide layer 3, and 4-1.about.4-4 gaps. The width dL of 
each gap is set to 
##EQU10## 
(m1=any one of 1, 3, 5, 7, . . . ) with respect to the center wavelength 
.lambda.0, and the depth of the gap equal to or larger than the thickness 
of the waveguide layer 3. The nL represents the refractive index of the 
gap. In this case, the gap consists of air, so that nL=1. Both side 
surfaces of the gap are made substantially perpendicular. These gaps 
4-1.about.4-4 are formed by, for example, the dry etching. According to 
the currently-available dry etching techniques, a gap of 0.5 .mu.m in 
width and 7 .mu.m in width can be formed satisfactorily. For example, if 
the wavelength .lambda.0=1.2 .mu.m, the refractive index of the gap nL=1, 
m1=3, dL is 0.9 .mu.m in accordance with the above equation 
##EQU11## 
A gap of this width can be formed satisfactoril the m1 is, the more easily 
the gap can be formed. However, when m1 becomes larger, the specific band 
for the filter becomes narrow. Therefore, it is necessary that m1 be 
determined so that the manufacturing accuracy and specific band are 
balanced with each other. Reference numerals 5-1.about.5-3 denote the 
portions of the waveguide layer which are left after the dry etching of 
the layer, and the width dH of each of these portions is set to 
##EQU12## 
(m2=any one of 3, 5, 7, . . . ). If a compound semiconductor, for example, 
InP is used for the substrate, the refractive index nH of the waveguide 
layer 3, which consists of InGaAsP, is about 3.2. If .lambda.0=1.2 .mu.m, 
dH is about 0.28 .mu.m when m2 is 3; about 0.47 .mu.m when m2 is 5; and 
about 0.66 .mu.m when m2 is 7. Therefore, m2 is preferably set to 5 or 7 
taking the current dry etching techniques into consideration. 
FIG. 11 shows an example of the wavelength characteristics of the optical 
waveguide type filter of FIG. 10. It shows the results of calculations 
about the filter, in which the substrate 1 consists of InP; the waveguide 
layer 3 consists of InGaAsP (refractive index nH=3.2); nL=1 (air); 
.lambda.0=1.2 .mu.m; and m1=m2=7. L=7 denotes the characteristics of this 
type of filter in which four gaps are formed, and L=5 the characteristics 
of a similar filter in which three gaps are formed. These characteristics 
are expressed on the basis of the results of proximate calculations of the 
plane wave propagation. They become slightly different when the 
propagation modes are taken into consideration but substantially identical 
when they are determined on the basis of the proximate calculation 
results. As may be understood from the above results, when around three to 
four gaps are provided, the attenuation rate of the rejection band becomes 
very high, and the attenuation gradient from the pass band to the 
rejection band becomes large. The reasons reside in that a difference 
between the refractive indexes nH, nL is very large as compared with that 
between the refractive indexes of a high refractive index layer and a low 
refractive index layer of a regular optical interference film filter. 
Moreover, even when m1=m2=7, a band rejection filter having a large 
rejection band width can be advantageously formed. In a conventional 
interference film filter, a film of only 1/4 wavelength can be formed, 
while, in the filter according to the present invention, a film of not 
less than 3/4 wavelength can be obtained. 
FIG. 12 shows the results of calculations of the wavelength characteristics 
of the embodiment of FIG. 10, in which .lambda.0 is set to 1.2 .mu.m, 1.3 
.mu.m and 1.55 .mu.m. 
FIG. 13 shows the results of an example of calculations of the wavelength 
characteristics of the optical waveguide type filter in the embodiment of 
FIG. 10, in which the refractive indexes are set to m1=5 and m2=7. As may 
be understood from a comparison between FIGS. 13 and 11, the wavelength 
characteristics vary. Namely, in the wavelength characteristics shown in 
FIG. 11, the center wavelengths of rejection band are 1.2 .mu.m and about 
1.7 .mu.m, while, in the wavelength characteristics shown in FIG. 13, the 
center wavelengths of rejection band are 1.2 .mu.m and 1.8 .mu.m. The 
filter of the wavelength characteristics of FIG. 13, which has lowloss 
pass characteristics in the vicinity of 1.55 .mu.m, can be used as a 
filter having a 1.55 .mu.m pass band and a 1.2 .mu.m rejection band. Thus, 
if m1 and m2 are set to levels selected from 1, 3, 5, 7, . . . and 
combined arbitrarily, filters having various filter characteristics can be 
obtained. 
FIG. 14 shows a further embodiment of the optical waveguide type filter 
according to the present invention. FIG. 14A is a top view, FIG. 14B a 
front elevation, and FIG. 14C a left side elevation. The parts of this 
embodiment which have the same reference numerals as those of the 
embodiment of FIG. 10 have the same functions. Reference numerals 
6-1.about.6-4 denote gaps formed by packing films of a low refractive 
index nL in the gaps similar to those shown in FIG. 10. These films may 
consist of SiO.sub.2 or SiO.sub.2 containing a refractive index 
controlling dopant (TiO.sub.2, PaO.sub.5, GeO.sub.2, B.sub.2 O.sub.3). 
FIG. 15 shows the results of calculations about the wavelength 
characteristics of the embodiment of FIG. 14. This drawing shows the 
characteristics of optical filters, which have a substrate 1 of InP, a 
waveguide layer 3 of InGaAsP (refractive index nH=3.2), rectangular groove 
fillings 6-1.about.6-4 of SiO.sub.2 (nL=1.46), and m1=m2=7, and which use 
four groove fillings (corresponding to L=7), five groove fillings 
(corresponding to L=9) and six groove fillings (corresponding to L=11). 
The calculation results relative to the rejection band center wavelength 
of 1.2 .mu.m show that these simply-constructed filters can obtain a very 
high rejection band attenuation rate. 
FIG. 16 shows the results of calculations about the embodiment of FIG. 15, 
in which L=11; and m1=m2=3, 5 and 7. It is understood that the specific 
band in the rejection band decreases in inverse proportion to the values 
of m1 and m2, and that, however, in order to carry out the bidirectional 
multiplex wavelength transmission using wavelengths of 1.2 .mu.m, 1.3 
.mu.m and 1.55 .mu.m, m1 and m2 should preferably have a value of around 7 
in contrast to the above case. 
FIG. 17 shows the characteristics of the embodiment of FIG. 15, in which 
L=9; and dL and dH deviate by several percent from predetermined levels 
##EQU13## 
In this optical waveguide type filter, when dL becomes small, dH becomes 
large correspondingly, and, conversely, when dL becomes large, dH becomes 
small correspondingly. In a gap-forming process, the above-mentioned 
phenomena necessarily occur. The characteristic curves in FIG. 17 show 
such cases. Referring to this drawing, the solid curve shows the 
characteristics of a filter in normal condition 
##EQU14## 
the broken line the characteristics of a filter in which the width of the 
gaps is too large 
##EQU15## 
and the one-dot chain line the characteristics of a filter in which the 
width of the gaps is too small in contrast to the preceding case. As may 
be understood from this drawing, even when there are any dimensional 
errors in the widths of the gaps formed, they offset each other and 
substantially do not have influence upon the wavelength characteristics. 
These constitute the unique features of the present invention. In a 
conventional optical interference film filter, layers are laminated one by 
one, and, moreover, a refractive index difference cannot be set large. 
Therefore, such an extremely high manufacturing accuracy was demanded that 
an error of the thickness of each layer has to be controlled to be not 
more than 1%. On the other hand, in the case of the optical waveguide type 
filter according to the present invention, the gaps are formed after the 
waveguide layer has been formed. During this time, if dL is small, dH 
necessarily becomes large correspondingly, and, conversely, if dL becomes 
large, dH necessarily becomes small correspondingly. Consequently, 
nLdL+nHdH is maintained at a level, 
##EQU16## 
Therefore, the characteristics substantially identical with the normal 
wavelength characteristics can be obtained as shown in FIG. 17. 
FIG. 18 shows a further example of calculations of the wavelength 
characteristics of the optical waveguide type filter according to the 
present invention. This drawing shows the characteristics of the 
embodiment of FIG. 15, in which L=7; nLdL=nHdH=3/4.lambda.0; and 
.lambda.0=1.2 .mu.m. The solid curve A represents the characteristics of 
the filter which has not yet been improved, and the broken curve B the 
results in a case where the pass loss on the larger wavelength side of 
1.35 .mu.m is reduced. In order to reduce this pass loss value, the gap 
width dL on the input and output sides may be in this case set, for 
example, slightly smaller than 
##EQU17## 
In order to further reduce the pass loss, at least one ot dL and dH may be 
shifted from a predetermined levels. Changing nL and nH instead of dL and 
dH also enables the same characteristics to be obtained. 
FIG. 19 shows a further embodiment of the optical waveguide type filter 
according to the present invention. This embodiment uses a ridge waveguide 
layer (a portion designated by the reference numerals 3 and 7). The depth 
of gaps is set equal to or larger than the thickness of the layer 3, 7. 
FIGS. 20A-20D shows the steps of manufacturing the optical waveguide type 
filter according to the present invention. FIG. 20A shows a step of 
forming a cladding layer 2 and a waveguide layer 3 on a substrate 1. FIG. 
20B shows a step of forming gaps 4-1.about.4-6 by an etching means, such 
as the dry etching. FIG. 20C shows a step of packing a film 8 in the gaps. 
FIG. 20D shows a step of etching the film on the waveguide layer 3. When 
the refractive index of the film is lower than that of the waveguide layer 
3, the film 8 on the waveguide layer may be left as it is to obtain a 
higher optical stability of the filter. Namely, it is recommended that the 
film 8 be left as it is without being etched, for the purpose of 
preventing the humidity of the filter from causing the fluctuations of the 
characteristics thereof, and the contamination of the surface of the 
filter from causing an increase in the absorption and scattering of the 
light. 
FIG. 21 shows an example of the application of the optical waveguide type 
filter according to the present invention. In this example, an optical 
waveguide type filter 49 is formed on a portion of a Y-shaped waveguide. 
The optical signals of wavelengths .lambda.1, .lambda.2 entering a 
waveguide layer 3 in the direction of an arrow 41 propagate in the 
interior thereof and enters the optical waveguide type filter 49. This 
filter has the characteristics (identical with those shown in FIG. 13, in 
the filter, which has these characteristics, .lambda.1 and .lambda.2 being 
1.2 .mu.m and 1.55 .mu.m, respectively) of rejecting and reflecting an 
optical signal of a wavelength .lambda.1 and passing an optical signal of 
a wavelength .lambda.2 therethrough. Accordingly, an optical signal of a 
wavelength .lambda.2 (1.55 .mu.m) passes through the filter in the 
direction of an arrow 42, and an optical signal of a wavelength .lambda.1 
(1.2 .mu.m) is reflected thereon. Since the filter is formed diagonally 
with respect to the incident wave, the reflected optical signal propagates 
in the direction of an arrow 43 in the interior of a branch waveguide 
layer 40. Namely, a filter for demultiplexing optical signals of 
wavelengths .lambda.1, .lambda.2 is formed. 
The embodiments, which have described so far, of the optical waveguide type 
filter relate to band rejection type filters. A band-pass type filter can 
also be formed. In a band-pass type filter, at least one gap having a 
width of 
##EQU18## 
(m3=1, 3, 5, . . . ), i.e. a cavity layer may be provided in an 
intermediate position in the periodic gaps, or at least one portion in 
which the distance between the centers of two adjacent gaps is 
##EQU19## 
may be provided, or a combination of such a cavity and such a portion may 
be provided. An example of such a filter provided with one cavity is shown 
in FIG. 22. FIG. 22A is a top view, and FIG. 22B a side elevation, in 
which reference numeral 26 denotes a cavity, the width of a waveguide 
layer being 
##EQU20## 
An embodiment of the optical module for wavelength division multiplexing 
using the above-described optical waveguide type filter will now be 
described. 
FIG. 23 shows an embodiment of an optical module for multiplex transmission 
of three wavelengths. Optical signals .lambda.1, .lambda.3 (.lambda.1=1.2 
.mu.m, .lambda.3=1.55 .mu.m) enter a waveguide layer 3 in the direction of 
an arrow 44. Reference numeral 90 denotes a light-emitting semiconductor 
element having a wavelength of .lambda.2, 50 a photo-detector for 
monitoring an optical signal of .lambda.2, 51 a photo-detector for 
receiving an optical signal having a wavelength of .lambda.1,52a 
photo-detector for receiving an optical signal having a wavelength of 
.lambda.3, and 53, 54, 55 waveguide type filters. The reference numeral 53 
denotes a filter (the characteristics of which are shown by a one-dot 
chain line in FIG. 12) having a center wavelength of 1.55 .mu.m, 54 a 
bandpass filter adapted to pass only an optical signal of a wavelength of 
.lambda.1 therethrough, and 55 a band-pass filter adapted to pass only an 
optical signal of a wavelength of .lambda.3 therethrough. When the module 
is formed in this manner, an optical signal (of a wavelength of .lambda.2) 
from the light-emitting semiconductor element 90 enters the waveguide 
layer 3, passes through the optical filter 53, and is transmitted from the 
module to the interior of an optical fiber (not shown) in the direction of 
an arrow 44. On the other hand, the optical signals having wavelengths of 
.lambda.1, .lambda.3 transmitted in the direction of an arrow 41 enters 
the interior of the waveguide layer 3 to reach the optical filter 53. 
These optical signals of .lambda.1, .lambda.3 are then reflected on this 
optical filter 53 to propagate into the interior of a branch waveguide 
layer 40 in the direction of an arrow 45 and enter the optical filter 54. 
The optical signal of the wavelength .lambda.1 passes through this optical 
filter 54 to be received by a photodetector 51. The optical signal of the 
wavelength .lambda.3 is reflected on the optical filter 54 to propagate in 
the direction of an arrow 47, pass through the optical filter 55, and be 
received by a photo-detector 52. 
The radiation loss and scattering loss from the waveguide layer 3 may be 
reduced by covering the waveguide layer 3 with a film 57 of a low 
refractive index nP (nP.ltoreq.nL) as shown in FIG. 24. FIG. 24A is a top 
view, FIG. 24B a sectional view taken along the line A--A' in FIG. 24A, 
and FIG. 24C a left side elevation. The film 57 of a low refractive index 
is formed of a film of SiO.sub.2 or a film of SiO.sub.2 doped with B.sub.2 
O.sub.3 or F, for example, when the rectangular groove fillings 
6-1.about.6-4 consist of SiO.sub.2. 
FIG. 25 shows a further embodiment of the optical filter according to the 
present invention. FIG. 25A is a top view, FIG. 25B a sectional view taken 
along the line A--A' in FIG. 25A, and FIG. 25C a side elevation. Reference 
numeral 1 denotes a substrate, which may be formed of an 
arbitrarily-selected material, such as a semiconductor, a dielectric or a 
magnetic substance, 3 a waveguide layer having a refractive index of nH, 
62 a first cladding layer having a refractive index of nB which is lower 
than nH, 64 a layer of a low refractive index nL which is lower than nH, 
and 65 a second cladding layer the refractive index of which has to be 
selectively set lower than nL. A structure in which a second cladding 
layer 65 is provided under a layer 64 of a low refractive index 
constitutes the important characteristics of the optical filter according 
to the present invention, and providing this second cladding layer 65 
makes it possible to set the refractive index nL of the layer of a low 
refractive index irrespective of the refractive index nS of the substrate 
1. When nL is set lower than nS in a structure, in which a second cladding 
layer 65 is not provided, the light leaks into the substrate to cause a 
great loss. Therefore, it is difficult to determine nL and nS so that they 
have a large difference. FIG. 26 shows calculation values of a loss of a 
waveguide which is provided with the second cladding layer 65. As may be 
understood from this drawing, the loss can be reduced exponentially by 
increasing the thickness of the second cladding layer 65. Accordingly, 
even when nL is set lower than nS, the loss can be reduced to a 
satisfactorily low level by increasing the thickness of the second 
cladding layer 65, so that a difference between nH and nL can be increased 
to an arbitrary extent. The difference between nH and nL is inversely 
proportional to the number of the filters. When the difference between nH 
and nL can be increased, the number of filters can be reduced, so that the 
device can be miniaturized to a great extent. The examples of calculation 
values of the characteristics of filters having an InP substrate 1, an InP 
layer 62 (doped with a certain substance at a low ratio), an InGaAsP layer 
3, and layers 64, 65 consisting of SiO.sub.2 doped with TiO.sub.2 (the 
concentration of the dopant in the layer 64 is higher than that of the 
dopant in the layer 65) are shown in FIGS. 27 and 28. The value of dH is 
determined selectively so that the waveguide has a single mode. The values 
of lL, lH may be selectively set to 
##EQU21## 
(m1, m2=1, 3, 5, 7 . . . ), wherein .lambda.0 center wavelength of a 
filter. It is understood from these drawings that the filters have a 
structure of a band rejection filter having a center wavelength of 1.175 
.mu.m. The attenuation rate of a rejection band increases in proportion to 
the number of filters. Since the difference between nH and nL is set 
large, a satisfactory attenuation rate is obtained by using such a number 
of filters that is not more than a half of that of regular interference 
film filters. As may be understood from FIG. 28, the band width can be 
reduced arbitrarily by increasing m1, m2 when lL, lH are set. 
FIG. 29 shows a further embodiment of the optical filter according to the 
present invention. FIG. 29A is a top view, FIG. 29B a sectional view taken 
along the line A--A' in FIG. 29A, and FIG. 29C a side elevation. Among the 
reference numerals and letters in these drawings, the same numerals and 
letters as in FIG. 1 designate parts having the same functions. The 
embodiment of FIG. 29 having a waveguide of so-called rib-type 
construction, and is characterized in that slab portions 66 having a 
thickness dR are provided at both sides of a waveguide layer 3. Since this 
rib type waveguide is capable of lessening the manufacturing accuracy as 
compared with the rectangular waveguide of FIG. 25, it has a higher 
practicality. It is regarded that the waveguide shown in FIG. 25 is formed 
by reducing the thickness dR of the slab portions of the embodiment of 
FIG. 29 to zero. If the thickness dH of the slab portions 66 in the 
structure of FIG. 29 is reduced to zero, the structure is changed to a 
two-dimensional slab type waveguide structure. 
FIG. 30 shows a further embodiment of the optical filter according to the 
present invention, which is formed by providing a cover layer 67 on the 
structure of FIG. 29. In the structures of FIGS. 25 and 29, in which the 
waveguide layers are in direct contact with the outside air, a part of the 
light leaks to and propagates through the outside air, so that these 
structures are apt to be influenced by the disturbance. The cover layer 67 
is provided for the purpose of preventing this inconvenience. This cover 
layer 67 also serves to prevent the material of the waveguide layer from 
being degenerated by an active gas in the air, such as oxygen. It is 
necessary that the refractive index nC of the cover layer 67 be set 
selectively to a level lower than those of nH and nL. 
The greatest characteristics of the optical filter according to the present 
invention reside in that reflective parts 64, 65 having a large refractive 
index difference are provided. Owing to these characteristics, the filter 
can be formed of a small number of filter elements. This simultaneously 
means that it is possible that the radiation loss per reflective part 
becomes large. The greater part of the radiation loss of around 2-3 dB, 
which appears in the calculation values in FIGS. 27 and 28, consists of 
the radiation loss occurring on the discontinuous surface of the 
waveguide. In order to reduce the loss of the filter, it is necessary to 
minimize this radiation loss to as great an extent as possible. In order 
to minimize the radiation loss, it is necessary that the electromagnetic 
field distribution in a guided mode in the waveguide layer 3 and first 
cladding layer 62 and that in a guided mode in the reflective parts be 
matched. However, carrying out this matching operation on the basis of a 
thorough analysis of the electromagnetic fields is troublesome, and, 
moreover, does not necessarily give a clear solution. Therefore, 
parameters consisting of normalized frequencies of the waveguide are used, 
and a method of proximately carrying out the matching of the 
electromagnetic distribution of guided light by mating these parameters is 
employed. A normalized frequency V is a parameter obtained by the 
following equation and generally used to indicate the condition of the 
light guided in a waveguide. 
V=(nH.sup.2 (L)-nB.sup.2 (L))dH(L)k (wherein k is a free space wave number 
of the light) 
FIG. 31 is a graph in which a radiation loss per filter is taken in the 
direction of the longitudinal axis with a normalized frequency difference 
taken in the direction of the lateral axis. The graph shows that a point 
at which the normalized frequency difference becomes zero and a point at 
which the radiation loss becomes minimal substantially agree with each 
other, and it serves to ascertain that employing in this case a method 
using normalized frequencies is proper. 
FIG. 32 shows an example of a process for manufacturing an optical filter. 
FIG. 32A shows a step of forming a first cladding layer 62 and a waveguide 
layer 3 on a substrate 1, and FIG. 32B a step of forming gaps in which 
reflective parts are to be packed, this packing operation being to be 
carried out by dry etching. If the thickness of each layer is determined 
so that the waveguide has a single mode, the depth of the gaps becomes 
around 2-3 .mu.m. FIG. 32C shows a step of packing second cladding layers 
65 in the gaps, and FIG. 32D a step of packing low-refractive-index layers 
64 in the same gaps. Using CVD for this process is considered most 
suitable in view of the present state of development of the relative 
techniques. 
FIG. 33 shows an embodiment of the optical integrated circuit according to 
the present invention, which is a multiplex wavelength transmission device 
having a multiplexing number of three (receiving the light of 1.2 .mu.m 
and 1.3 .mu.m and emitting the light of 1.55 .mu.m). Reference numeral 78 
denotes a waveguide, and 79, 80 optical filters having rejection 
characteristics with respect to the bands having center wavelengths of 1.2 
.mu.m and 1.33 .mu.m. These optical filters 79, 80 are provided at an 
angle to the waveguide, whereby the directions of advancement of incident, 
light and reflected light are separated to carry out the, dimultiplexing 
of wavelength. Accordingly, out of the rays of light entering the 
waveguide in the direction of an arrow 77, only the rays of light having a 
wavelength of 1.2 .mu.m are reflected on the optical filter 79 and guided 
to a photo-detector 82. The rays of light having a wavelength of 1.3 .mu.m 
passes through the optical filter 79 and are reflected on the optical 
filter 80 to be guided to a photo-detector 83. When the incident light 
includes rays of light of a wavelength other than 1.2 .mu.m and 1.3 .mu.m, 
these rays of light are guided to a light-emitting element 81, and do not 
reach the photo-detectors. The light of a wavelength of 1.55 .mu.m from 
the light-emitting element 81 passes through the optical filters 79, 80 
and advances outward in the direction of an arrow 76. A part of the light 
of a wavelength of 1.55 .mu.m leaks at the branching portions of the 
waveguide toward the photo-detectors 82, 83 but this light turns into 
cut-off waves due to the influence of the low-refractive index portions 
84, 85 provided in the waveguide, so that the light advances to the 
outside of the waveguide and does not reach the photo-detectors. The 
low-refractive-index portions 84, 85 are to be made by a method, such as 
the ion implantation. They can also be made collectively by the steps 
similar to those of manufacturing the optical filter. Referring to FIG. 
33, the element 81 may consist of a photo-detector, and the elements 82, 
83 light-emitting elements. The present invention is not limited to the 
above-described embodiments. For example, the layer of nH may be used as a 
low-refractive-index layer, and the layer of nL as a high-refractive-index 
layer. In this case, the refractive indexes are set selectively so that 
nH&gt;nB; and nL&gt;nI. 
According to the present invention, a waveguide structure can be optimized 
so that the radiation loss in the film-packed portions thereof becomes 
minimal. Consequently, a wide-band optical film of a low radiation loss 
and a multiplex optical wavelength transmission device using this optical 
filter can be obtained. Owing to the achievement of the production of a 
one-chip module, the manufacturing cost can be reduced greatly, and the 
reliability of such filter and device can be improved greatly.