Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2000-10-11
2002-05-14
Sugarman, Scott J. (Department: 2873)
Optical waveguides
With optical coupler
Particular coupling structure
Reexamination Certificate
active
06389201
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an arrayed waveguide grating used, for example, for a wavelength-division multiplexer/demultiplexer in the field of optical wavelength-division multiplex communication or the like.
2. Description of the Related Art
Recently, optical wavelength-division multiplex communication has been practically used, in which a number of optical signals having different wavelengths are multiplexed and transmitted in a single optical fiber. In systems realizing such multiplex communication, an optical multiplexer/demultiplexer for multiplexing/demultiplexing optical signals according to their wavelengths is an important element.
As such an optical multiplexer/demultiplexer, a bulk-type diffraction grating, a dielectric multiplayer element, and the like are known. However, these known devices have problems such as (i) difficulty of determining each selected wavelength, (ii) high manufacturing cost due to complicated manufacturing processes, (iii) large optical loss, and the like; thus, it has been difficult to apply these devices to wavelength-division multiplex communication for multiplexing/demultiplexing a number of different wavelengths.
Recently, the arrayed waveguide grating, disclosed in H. Yamada, et al, “10 GHz-spacing arrayed-waveguide grating with phase-error-compensating a-Si film”, Proceedings of the 1996 Electronics Society Conference of IEICE, C-162, p. 162, 1996, has become the focus of attention.
FIG. 7
is a plan showing an example of the arrayed waveguide grating disclosed in the above document.
The shown arrayed waveguide grating consists of one or more input waveguides
71
, an input-side slab waveguide
72
connected to the input waveguides
71
for receiving signal(s) from the input waveguides
71
, an arrayed waveguide
73
composed of a number of waveguides, connected to the other side of the slab waveguide
72
, an output-side slab waveguide
74
connected to the other side of the arrayed waveguide
73
, and one or more output waveguides
75
connected to the other side of the slab waveguide
74
.
An optical signal incident on the input waveguide
71
is input into the input-side slab waveguide
72
, and the optical signal is further input into the arrayed waveguide
73
(composed of a number of waveguides) at the same phase. The input end of the arrayed waveguide
73
and the output end of the input waveguides
71
are respectively arranged to form circles, where the radius of the relevant circle of the arrayed waveguide
73
is twice as much as the radius of the relevant circle of the input waveguides
71
, and the positional relationship is such that the center of the relevant circle of the arrayed waveguide
73
corresponds to a position on the relevant circle of the input waveguides
71
.
In the arrayed waveguide
73
, each waveguide is adjusted so as to provide the same phase difference between any two adjacent waveguides, and the output-side slab waveguide
74
is connected to the other end of the arrayed waveguide, as explained above. Similar to the input side, in the arrangement of the arrayed waveguide
73
, the output-side slab waveguide
74
, and the output waveguides
75
, the output end of the arrayed waveguide
73
and the input end of the output waveguides
75
are respectively arranged to form circles, where the radius of the relevant circle of the arrayed waveguide
73
is twice as much as the radius of the relevant circle of the output waveguides
75
, and the positional relationship is such that the center of the relevant circle of the arrayed waveguide
73
corresponds to a position on the relevant circle of the output waveguides
75
.
According to the above structure, a wavelength-division multiplexed optical signal incident on the input waveguide
71
is divided into signals having different wavelengths, and each signal is output from an output waveguide
75
corresponding to the relevant wavelength, thereby realizing the wavelength multiplexing/demultiplexing function.
Generally, in optical wavelength division multiplexing (WDM) communication, the ON/OFF state, light intensity, wavelength, or the like of the optical signal should be monitored for each wavelength at some points in the transmission path. In order to execute such a monitoring operation, the optical signal is divided into signals having different wavelengths by using an arrayed waveguide or the like, and the optical signal of a target wavelength is further divided into a main optical signal and a monitored optical signal by using an (optical) fiber coupler or the like, and then the monitored optical signal is monitored using a photodetector or the like.
However, in the above monitoring method, the same number of fiber couplers as the number of the wavelength channels are necessary, and thus the system is complicated and the system cost is increased due to the necessary cost and space for providing the fiber couplers. Additionally, in this case, after the division of an optical signal using a wavelength multiplexer/demultiplexer, the optical power of the main optical signal is again decreased by further dividing the signal using fiber couplers or the like. Therefore, the power loss of the main optical signal is large.
In order to solve the above problem, Japanese Unexamined Patent Application, First Publication, No. Hei 10-303815 discloses an optical wavelength-division multiplexer/demultiplexer having a monitor function explained later in detail, in which a function of monitoring the optical signal of each wavelength is added to the wavelength-division multiplexing/demultiplexing function necessary for the WDM communication, thereby omitting fiber couplers or the like, and decreasing the cost, size, and optical loss of the WDM communication system.
FIG. 8
is a plan showing the optical wavelength-division multiplexer/demultiplexer having the monitor function disclosed in the above publication. The disclosed system comprises an input waveguide
81
, an input-side slab waveguide
82
connected to the input waveguide
81
for receiving a signal from the input waveguide
81
, an arrayed waveguide
83
composed of a number of waveguides, connected to the other side of the slab waveguide
82
, an output-side slab waveguide
84
connected to the other side of the arrayed waveguide
83
, N output waveguides
85
connected to the other side of the slab waveguide
84
, and N monitoring waveguides
86
used for the monitoring operation. The optical signal is wavelength-divided by the arrayed waveguide, and transmitted through the output-side slab waveguide
84
and converged onto the output waveguides
85
. Simultaneously, the optical signal is wavelength-divided by the arrayed waveguide due to interference of the next order to the main order of interference (i.e., order of diffraction) of the arrayed waveguide, and the N monitoring waveguides
86
are positioned where these wavelength-divided optical signals (related to the next order of interference) converge.
Here, the difference &Dgr;&thgr; between diffracted optical signals (i.e., diffracted light beams) having a difference “i” of the diffraction order (i.e., order of diffraction) therebetween can be defined as follows while &Dgr;&thgr; is sufficiently small:
&Dgr;&thgr;=
i
&lgr;/(
nd
) (1)
where n indicates the effective refractive index in the output-side slab waveguide, &lgr; indicates the wavelength of the optical signal, and d indicates the pitch of the arrayed waveguide at the joint (portion) of the arrayed waveguide and the output-side slab waveguide.
In
FIG. 8
, if it is assumed that (i) “a” is the center point of the arrayed waveguide
83
at the joint of the arrayed waveguide
83
and the output-side slab waveguide
84
, (ii) “b” is an end of the output waveguide
85
, at which an optical signal of a target wavelength is converged via the output-side slab waveguide
84
, and (iii) “c” is an end of the monitoring waveguide
86
, at which the corresponding monitored optical signal is converged via the output-side slab waveg
Hasan Mohammed
NEC Corporation
Sugarman Scott J.
Sughrue & Mion, PLLC
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