Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1998-07-21
2001-05-01
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S024000, C385S037000
Reexamination Certificate
active
06226428
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical circuit device and, more particularly, to an optical circuit device having a multi/demultiplexing function suitable for wavelength division multiplexing optical communication.
2. Description of the Prior Art
With recent increases in information transmission capacity, wavelength division multiplexing (WDM) optical communication that transmits different pieces of information using different wavelengths has come to the forefront of the technology. Research institutes are engaged in vigorous research and development of devices for multiplexing and demultiplexing wavelengths as key devices in WDM.
Of such devices, arrayed-waveguide gratings (AWGS) have been developed by various research institutes, and have recently exhibited great technical advances. An AWG has satisfactory characteristics for wavelength multi/demultiplexing on a single device. However, the device size is large, and the cost is high. For this reason, the AWG can be suitably used as a device for a trunk line system, but is not suitable for a subscriber system.
A Mach-Zehnder (MZ) optical multi/demultiplexer and an optical demultiplexer using directional coupler have also been proposed. In the former device, diffraction gratings are formed on two arms of a balanced MZ interferometer to implement a multi/demultiplexing function. This device is described in detail in Japanese Unexamined Patent Publication Nos. 9-61649 and 1-172924. The latter device is described in detail in I. Baumann et al., “Compact All-Fiber Add-Drop-Multiplexer Using Fiber Bragg Gratings” (IEEE PHOTONICS TECHNOLOGY LETTERS Vol. 8, No. 10, pp. 1331-1333 (1996).
FIG. 1
is a plan view showing the schematic arrangement of a conventional optical demultiplexer using a directional coupler. In the optical device shown in
FIG. 1
, first and second fibers
68
and
69
as single-mode fibers are held nearby on a substrate
70
to form a directional coupling section
71
.
A diffraction grating
72
is formed on the directional coupling section
71
by using optically induced refractive index modulation upon ultraviolet irradiation, thereby implementing an optical demultiplexer.
When light beams having different wavelengths are inputted through an input port
73
, since a length L of the directional coupling section
71
is equal to the complete coupling length, light of the wavelength transmitted through the diffraction grating
72
is outputted from an output port
76
.
In this case, the “complete coupling length” is the minimum length that is required to completely couple light inputted to the first optical waveguide (first fiber
68
) with the second optical waveguide (second fiber
69
).
Letting &kgr;c be the coupling coefficient of the directional coupling section
71
, a complete coupling length L
1
is given by:
L
1
=&pgr;/2
/&kgr;c
(1)
Light having the wavelength reflected by the diffraction grating
72
is output from a demultiplexing port
74
.
FIGS. 2 and 3
are graphs showing light intensity/wavelength characteristics to explain the operation of the conventional optical demultiplexer.
FIGS. 2 and 3
respectively show a demultiplexing port output waveform
79
and an output port output waveform
80
at the demultiplexing port
74
and an output port
76
when broadband light is input through the input port
73
.
A Bragg wavelength &lgr;B of the diffraction grating
72
is 1,536 nm. In the conventional optical circuit device, the diffraction grating
72
is placed at a distance La from a start position
77
of the directional coupling section
71
to minimize reflection loss of light having the wavelength &lgr;B at the input port
73
.
More specifically, the position La of the diffraction grating
72
is set so that:
4×
&kgr;c×La+d&phgr;=&pgr;
(2)
where d&phgr; is the phase difference between the even and odd modes of light having the wavelength &lgr;B reflected by the diffraction grating
72
. Equation (2) indicates that the phase difference between the even and odd modes of light having the wavelength &lgr;B becomes &pgr; when the light is reflected by the diffraction grating and returns to the start position
77
.
In this case, the even mode indicates a case in which the phase difference between light beams propagating in two waveguides is 0, whereas the odd mode indicates that the phase difference between light beams propagating in two waveguides is &pgr;.
Ideally, reflection loss at the input port
73
can be perfectly suppressed by satisfying equation (2). In the prior art, L
1
=10 mm, LG=2.5 mm, &lgr;B=1536 nm, and the refractive index modulation of the diffraction grating
72
: &Dgr;n=1.3×10
−3
, and the diffraction grating is formed at La=4.7 mm.
In the conventional optical circuit device, to add a multiplexing function, light having the wavelength &lgr;B inputted through a port
75
must be multiplexed with the output from the output port
76
.
The reflection loss at the port
75
is, however, large because the phase difference between the even and odd modes of light having the wavelength &lgr;B does not become &pgr; when the light inputted through the port
75
is reflected by the diffraction grating
72
and returns to a terminal position
78
of the directional coupling section
71
.
In the prior art, a distance Lb between the terminal position
78
of the directional coupling section
71
and the diffraction grating
72
is 2.8 mm, and the phase difference between the even and odd modes of light having the wavelength &lgr;B is 0.62·&pgr; when the light returns to the terminal position
78
. In this case, 32% of the light is lost by reflection loss at the port
75
.
As described above, the conventional optical circuit device using the directional coupling section
71
has satisfactory demultiplexing characteristics for demultiplexing of light having a specific wavelength, but has difficulty in multiplexing the wavelength again. This is because the diffraction grating is shifted from the center of the directional coupling section to obtain satisfactory demultiplexing characteristics.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to provide an optical circuit device using a directional coupler and having excellent demultiplexing and multiplexing functions.
In order to achieve the above object, according to the principal aspect of the present invention, there is provided an optical circuit device comprising a directional coupling section composed of first and second optical waveguides and having a length n times (n is an integer not less than two) a minimum length required to completely couple light inputted to the first optical waveguide to the second optical waveguide, and at least one diffraction grating formed in the directional coupling section and having a specific reflection characteristic, wherein when input light beams having different wavelengths are inputted to an input side of the first optical waveguide, light having a reflection wavelength in the diffraction grating is demultiplexed/output to an input side of the second optical waveguide, and light beams having other wavelengths are outputted to an output side of the first or second optical waveguide, whereas when the light having the reflection wavelength in the diffraction grating is inputted through one of the output sides of the first and second optical waveguides from which the light beams having other wavelengths are not outputted, the light having the reflection wavelength inputted through the output side of the optical waveguide is multiplexed with the output light beams having other wavelengths.
According to another aspect of the present invention, the device according to the principal aspect further includes a mask layer having an opening in that portion on the optical waveguide in which the diffraction grating is formed.
According to the present invention, since the length of the directi
Saito Takashi
Urino Yutaka
Connelly-Cushwa Michelle R.
Lee John D.
NEC Corporation
Scully Scott Murphy & Presser
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