Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-05-13
2004-07-06
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S015000, C385S024000, C385S027000, C385S037000, C385S048000, C385S052000
Reexamination Certificate
active
06760521
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communication technique and, more particularly, to a wavelength correction method and apparatus, wavelength check method and apparatus, which are used to correct the center wavelength output from an optical element, e.g., an arrayed waveguide diffraction grating having a slab waveguide, to a target value, when light is input to the optical element, a wavelength-corrected arrayed waveguide diffraction grating, and an interleaver.
2. Description of the Prior Art
A demand has arisen for an increase in transmission capacity in an optical fiber communication system as well as an increase in the volume of data transmitted. For this reason, a great deal of attention has been paid to DWDM (Dense Waveguide Division Multiplexing), and greater importance has been attached to optical elements such as an optical waveguide filter serving as a multiplexing/demultiplexing device for dividing/combining wavelengths.
Optical wavelength filters take various forms. Among these filters, an arrayed waveguide diffraction grating has wavelength characteristics represented by a narrow band and high extinction ratio, and also has characteristics as a multi-input/multi-output filter device. Therefore, this device allows demultiplexing a multiplexed signal or reverse operation, and hence allows easy formation of a wavelength multiplexing/demultiplexing device. In addition, if an arrayed waveguide diffraction grating is formed by using a quartz waveguide, good coupling to an optical fiber is ensured, and low insertion loss operation with an insertion loss of about several dB (decibel) can be realized. Owing to these advantages, among optical wavelength filters, an arrayed waveguide diffraction grating has attracted a great deal of attention as an important device and has been vigorously studied worldwide.
FIG. 1
shows the overall arrangement of a conventional arrayed waveguide diffraction grating. An arrayed waveguide diffraction grating
11
is comprised of one or a plurality of input waveguides
12
formed on a substrate (not shown), a plurality of output waveguides
13
, a channel waveguide array
14
having waveguides bent at different curvatures, an input-side slab waveguide
15
which connects the input waveguides
12
to the channel waveguide array
14
, and an output-side slab waveguide
16
which connects the channel waveguide array
14
to the output waveguides
13
. The traveling path of multiplexed signal light incident from the input waveguides
12
is expanded by the input-side slab waveguide
15
. The respective light components are then incident on the channel waveguide array
14
in equiphase. These incident light components vary in intensity at the respective incident positions in the input-side slab waveguide
15
; the intensities increase toward the center, exhibiting an almost Gaussian distribution.
Predetermined optical path length differences are set among the respective arrayed waveguides constituting the channel waveguide array
14
such that the optical path lengths sequentially increase or decrease. Therefore, light components guided along the respective waveguides reach the output-side slab waveguide
16
with phase differences at predetermined intervals. In practice, owing to wavelength dispersion, equiphase planes tilt depending on the wavelengths. As a result, light components are imaged (focused) at different positions on the interfaces between the output-side slab waveguide
16
and the output waveguides
13
depending on the wavelengths. Since the output waveguides
13
are arranged at the respective positions corresponding to the wavelengths, arbitrary wavelength components can be extracted from the output waveguides
13
.
The center wavelength of the arrayed waveguide diffraction grating
11
is very sensitive to a change in the refractive index of a waveguide material. In some case, therefore, the center wavelength varies due to variations in a film formation process as a manufacturing process, and the design value cannot be obtained. If the center wavelength varies, a high optical loss occurs at the wavelength used.
According to Japanese Unexamined Patent Publication No. 9-49936, therefore, input/output waveguides for wavelength correction are provided in addition to general input/output waveguides formed from AWGs (arrayed waveguides). The input/output waveguides are changed in accordance with the correction amount of wavelength.
If an angle difference in a demultiplexing direction with respect to a wavelength difference &dgr;&lgr; is represented by &dgr;&thgr;, a center wavelength &lgr;n can be corrected by the value given by equation (1) by changing the positions of the input waveguides
12
in the arrayed waveguide diffraction grating, i.e., a slab incident angle &thgr; in.
&dgr;&lgr;in=(&dgr;&lgr;/&dgr;&thgr;)·&thgr;in (1)
These input/output waveguides for wavelength correction are, however, discretely arranged, resulting in discrete wavelength correction amounts. This makes it impossible to correct the wavelength to an arbitrary wavelength. In order to obtain an arbitrary wavelength correction amount, the slab incident angle &thgr;in must be set to an arbitrary value.
FIG. 2
shows the arrangement of an arrayed waveguide diffraction grating designed to solve such a problem. According to, for example, the technique disclosed in “P. CPU. Clements et al., IEEE, Photon, Tech, lett, Vol. 7, No. 10, pp. 1040-1041, 1995”, a substrate is cut at a slab incident portion
22
on the input side of an AWG (arrayed waveguide) wafer
21
. An input fiber
24
clamped between glass members
23
is bonded (fixed) to the slab incident portion
22
reinforced by a glass member. At the time of this bonding operation, centering is directly performed to arbitrarily change the position of the input fiber
24
in accordance with a wavelength correction amount.
FIG. 3
shows how slab centering is performed by this proposed arrayed waveguide diffraction grating. An ASE (Amplified Spontaneous Emission) light source
31
is connected to the input side of an input waveguide
12
. The ASE light source
31
has wide-band wavelength characteristics like a white light source. The light output from the ASE light source
31
is incident from the input waveguide
12
onto the input-side slab waveguide
15
. The input-side slab waveguide
15
is cut in a direction almost perpendicular to the optical axis and separated into a first input-side waveguide component
15
A and second input-side waveguide component
15
B.
A spectrum analyzer
32
is connected to the output side of the output waveguide
13
to measure a wavelength. Prior to this measurement, the spectrum analyzer
32
is directly connected to the ASE light source
31
without the mediacy of the arrayed waveguide diffraction grating to measure the amounts of light output the light source at the respective wavelengths in advance. In this state, measurement is started on the light output from a port of output waveguides
13
to which the spectrum analyzer
32
is connected. Measurement is performed while the relative position of the first input-side waveguide component
15
A and second input-side waveguide component
15
B is moved little by little in the direction as indicated by an arrow
33
in FIG.
3
. The measured values are compared with the measurement results obtained while the arrayed waveguide diffraction grating is not connected to the ASE light source
31
, and the differences are taken into consideration. The respective output light amounts are discriminated as specific values for the respective wavelengths to obtain a center wavelength, thereby correcting a wavelength shift.
Since the spectrum analyzer
32
is relatively expensive, such measurement is repeated while different ports are connected one by one. Theoretically, if the wavelength is corrected at one port, correction is done in all the channels of the arrayed waveguide diffraction grating. In practice, however, even if the center wavelength is corrected
Healy Brian
McGinn & Gibb PLLC
NEC Corporation
Petkovsek Daniel
LandOfFree
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