Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-04-10
2002-08-27
Sanghavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S031000, C385S015000, C385S014000, C385S024000
Reexamination Certificate
active
06442314
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an arrayed waveguide grating to be used in the field of optical transmissions.
BACKGROUND ART
Recently, in the field of optical transmissions, as a method of significantly increasing transmission capacity, optical wavelength division multiplexing transmission has been successfully researched, developed, and made practicable. As an example of an optical transmission element for optical wavelength division multiplexing transmissions, there is an arrayed waveguide grating (AWG) as shown in FIG.
9
. The arrayed waveguide grating is formed so that waveguide forming part
10
having a waveguide composition as shown in the same figure is provided on substrate
11
. The composition of the waveguide is as follows.
That is, first slab waveguide
13
is connected to the exit side of one or more optical input waveguides
12
which are disposed in parallel. A plurality of arrayed waveguides
14
disposed in parallel are connected to the exit side of the first slab waveguide
13
, and second slab waveguide
15
is connected to the exit sides of the plurality of arrayed waveguides
14
. A plurality of optical output waveguides
16
disposed in parallel are connected to the exit side of the second slab waveguide
15
. The arrayed waveguides
14
propagate light led out from the first slab waveguide
13
, and are formed so as to have lengths which are different from each other.
The optical input waveguides
12
and optical output waveguides
14
are provided so as to accord to the number of, for example, signal light beams which have varying wavelengths to be demultiplexed by the arrayed waveguide grating. Normally, the arrayed waveguides
16
are provided by a large number, for example, 100. However, in this figure, for simplification of the figure, the number of the waveguides
12
,
14
, and
16
are simplified.
An optical fiber at the transmission side, for example, is connected to the optical input waveguides
12
, whereby wavelength multiplexed light is led therein. Light which has passed through the optical input waveguides
12
and has been led into the first slab waveguide
13
is spread by the diffraction effect, made incident onto the plurality of arrayed waveguides
14
, and propagated in the arrayed waveguides
14
.
Light propagated in the arrayed waveguides
14
reaches the second slab waveguide
15
, and furthermore, is condensed by the optical output waveguides
16
and outputted. Since the lengths of the arrayed waveguides
14
are different from each other, phase differences occur in the light beams after being propagated in the arrayed waveguides
14
. In accordance with the phase differences, the wavefront of the converged light inclines, and in accordance with the angle of this inclination, the light condensation position is determined. Therefore, the condensation positions of the light beams with varying wavelengths are different from each other, and output waveguides
16
are formed at these positions, whereby light beams with varying wavelengths can be outputted from the different optical output waveguides
16
for each wavelength.
For example, as shown in this figure, if wavelength multiplexed light beams with wavelengths of &lgr;
1
, &lgr;
2
, &lgr;
3
, . . . &lgr;n (n is an integer of 2 or above) are inputted from one optical input waveguide
12
, these light beams are expanded in the first slab waveguide
13
. Then, the light beams reach the arrayed waveguides
14
, pass through the second slab waveguide
15
, and as mentioned above, are condensed on different positions at the exit end of the second slab waveguide
15
for each wavelength. Thereafter, the light beams which are different in wavelength from each other are made incident onto different optical output waveguides
16
, pass through the respective optical output waveguides
16
, and are outputted from the exit ends of the optical output waveguides
16
. When an optical fiber for outputting light is connected to the exit ends of the optical output waveguides
16
, the light beams with varying wavelengths are taken out via this optical fiber.
In the arrayed waveguide grating, the wavelength resolution of the grating is in proportion to the differences (&Dgr;L) in length between the arrayed waveguides
14
which comprise the grating. Therefore, by properly setting the &Dgr;L, wavelength multiplexed light can be demultiplexed at narrow wavelength intervals. As an example of such an arrayed waveguide grating, an arrayed waveguide grating is proposed which is arranged so that the difference in optical path lengths between the arrayed waveguides
14
is set to 65.2 &mgr;m, the order of diffraction is set to 61, the total number of arrayed waveguides
14
is set to 100, and wavelength multiplexed light is demultiplexed into 32 waves at intervals of 100 GHz.
When manufacturing an arrayed waveguide grating, first, flame hydrolysis deposition is used to deposit and form a lower cladding layer on silicon substrate
11
, and then the layer is consolidate. Next, flame hydrolysis deposition is used to deposit and form a core layer on the consolidate lower cladding layer, and then the core layer is consolidate. Thereafter, an arrayed waveguide grating pattern is transferred onto the core layer by means of photolithography and the reactive ion etching method via a photomask on which the arrayed waveguide grating is drawn.
Thereafter, the core is etched, and the waveguide composition (waveguide pattern) of the arrayed waveguide grating is formed. Thereafter, at the upper side of this waveguide composition, an upper cladding layer is formed by means of flame hydrolysis deposition and consolidate, whereby the arrayed waveguide grating is formed.
The number of deposited layers of the core is generally
6
. In
FIG. 10
, an example of the transmission spectrum of the arrayed waveguide grating is shown. The arrayed waveguide grating having this transmission spectrum is manufactured so that the number of deposited layers of the core is 6, and the optical path length difference between the arrayed waveguides
14
, the total number of the waveguides, and the order of diffraction are set to the abovementioned numerical values. In addition, regarding this transmission spectrum, the transmission bandwidth is standardized by the FSR (Free Spectral Range: 25 nm herein), and the transmittance is standardized by means of minimum loss. As can be clearly understood from this figure, in this prior-art arrayed waveguide grating manufactured as mentioned above, the isolation (hereinafter, referred to as cross talk) which is the gap between the transmission loss of the transmission wavelength (A of the figure) and the background transmission loss (B of the figure) is 30 dB.
In the abovementioned dense wavelength division multiplexing transmission system (hereinafter, referred to as the D-WDM transmission system), a crosstalk of approximately 40 dB is required for the arrayed waveguide grating to be applied to this system. However, in the abovementioned prior-art arrayed waveguide grating, since the cross talk is only 30 dB, the characteristics required from the D-WDM transmission system side cannot be satisfied.
Furthermore, the present inventor considers that the deterioration in the cross talk to a degree of 30 dB in the prior-art arrayed waveguide grating is caused by fluctuations in the propagation constant of the core comprising the arrayed waveguide
14
. When the amount of deviation of propagated light within each arrayed waveguide from the equiphase wave surface is defined as a phase error, fluctuations in the propagation constant are the phase errors of the propagated light, which causes deterioration of the cross talk of the arrayed waveguide
14
. That is, originally, light is condensed to a predetermined one point at the output end of the second slab waveguide
15
for each wavelength. However, if the light deviates due to the phase errors, the light is not condensed to the predetermined one point for each wavelength, but leaks to an adjacent channel, and the cross talk deteriorates
Kashihara Kazuhisa
Nara Kazutaka
Pak Sung
Sanghavi Hemang
The Furukawa Electric Co. Ltd.
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