Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-11-01
2001-09-11
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S014000, C385S046000, C385S043000
Reexamination Certificate
active
06289147
ABSTRACT:
FIELD OF THE INVENTION
The invention relates optical communications networks. In particular, the invention relates to an arrayed waveguide demultiplexing/switching element.
BACKGROUND ART
Optical wavelength-division multiplexing (WDM) elements are becoming increasingly important in advanced optical communications networks incorporating optical fiber transmission paths. Silica optical fiber has a transmission bandwidth of over 300 terahertz per second. Such an extremely large bandwidth is, however, limited by the electronics on the transmitting and receiving ends. Such electronic transmitters and receivers, typically bases on silicon electronics, are limited commercially at the present time to 2 to 10 gigabits/s (Gbs). Further increases to 40 Gbs are contemplated, but further increases will be difficult to achieved.
For these reasons, WDM has been proposed in which multiple (N) electronic data channels, as illustrated in
FIG. 1
, enter a transmitter
10
and modulate separate optical emitters such as lasers
12
having N respective output carrier wavelengths &lgr;
1
, &lgr;
2
, . . . &lgr;
N
. Conveniently, these wavelengths are arranged in a WDM wavelength comb having the neighboring wavelengths &lgr;
1
, &lgr;
2
, . . . &lgr;
N
separated by a substantially constant inter-channel spacing given by
&Dgr;&lgr;
S
=&lgr;
i+1
−&lgr;
i
. (1)
An optical wavelength-division multiplexer
14
combines the optical signals of different wavelengths and outputs the combined signal on a single optical fiber
16
. An optical receiver
20
includes a wavelength-division demultiplexer
22
which divides its received signals according to their optical wavelength to N optical detectors
24
according to the same wavelength allocation &lgr;
1
, &lgr;
2
, . . . &lgr;
N
. In view of usually experienced reciprocity in passive systems, a wavelength-division demultiplexer is usually substantially identical to a wavelength-division multiplexer with a reversal of their inputs and outputs.
Additionally, an optical add/drop multiplexer (ADM)
30
may be interposed on the optical path
16
between the transmitter and the receiver
20
. The optical add/drop circuit
30
removes from the optical channel on the fiber
16
one or more wavelength channels at wavelength &lgr;
AD
and inserts back onto the fiber
16
an optical data signal perhaps containing different information but at the same optical carrier wavelength &lgr;
AD
. The ADM
30
is typically implemented with technology closely resembling the WDMs
14
,
22
. All-optical networks have been proposed in which a distributed networks having many nodes each including a transmitter
10
and receiver
20
are linked by a functionally passive network which routes the signals between the nodes according to their wavelengths. The routing elements in such an all-optical network require switching elements similar to the ADM
30
.
In order to maximize the transmission capacity of the optical fiber
16
, the wavelength channels &lgr;
1
, &lgr;
2
, . . . &lgr;
N
should be placed as closely together as possible with a minimum channel spacing &Dgr;&lgr;
S
. In advanced systems, this inter-channel spacing &Dgr;&lgr;
S
is 1 nm or less for a signal centered around 1300 or 1550 nm, the preferred bands for silica fiber. Such closely spaced WDM networks are referred to as dense WDM networks (DWDM).
The network design described above may be subject to a problem arising from the fact that the operation of the transmitter
10
, receiver
20
and intermediate node
30
are all referenced to the same set of WDM wavelengths &lgr;
1
, &lgr;
2
, . . . &lgr;
N
. However each of the distributed elements must provide its own wavelength calibration. Due to environmental and aging effects, the wavelength calibration settings at one element are likely to differ from those at another element. In view of the close spacing of the optical channels, any miscalibration between network elements is likely to produce inter-channel interference.
For an optimized optical system, the fiber
16
, the WDMs
14
,
22
, and the ADM
30
are typically designed to be single-mode at least at their ports for the optical wavelengths being used. Although each of the lasers
12
is likely emitting light across an exceedingly narrow bandwidth, the single-mode response of the frequency sensitive elements
14
,
22
,
30
usually has a wavelength (frequency) characteristic that approximates a gaussian distribution about the center wavelength &lgr;
0
of the channel F(&lgr;)=exp(−(&lgr;−&lgr;
o
2
)/&Dgr;&lgr;
G
2
). The value of the gaussian passband &Dgr;&lgr;
G
can be fairly freely chosen for present day fabrication techniques. However, the value of the passband is subject to countervailing restraints. For dense WDM systems, the inter-channel spacing &Dgr;&lgr;
S
is made as small as possible. The gaussian passband &Dgr;&lgr;
G
must be substantially smaller than the inter-channel spacing &Dgr;&lgr;
S
to avoid interference between channels. On the other hand, the frequency characteristics of the lasers
12
and other frequency-sensitive elements are subject to permanent or temporary variations. If the passband &Dgr;&lgr;
G
is made too small, the peak is very narrow and small variations in wavelength away from the peak's wavelength &lgr;
0
causes operation to shift to the sides of the peak, thereby degrading the signal strength. That is, for a strong signal the passband &Dgr;&lgr;
G
should be made as large as possible to provide a broad top of the peak.
Amersfoort et al. have already recognized these problems, as disclosed in U.S. Pat. No. 5,629,992. These patents describe arrayed waveguide gratings, also called phasars, of the sort described by Hunsperger et al. in U.S. Pat. No. 4,773,063, and by Dragone in U.S. Pat. Nos. 5,412,744 and 5,488,680. In particular Amersfoort et al. describe a WDM phasar
40
exemplified in the schematic illustration of
FIG. 2. A
single-mode waveguide
42
is coupled to one end of a multi-mode waveguide
44
of length chosen to produce a doubled image of the radiation from the single-mode waveguide
42
at a port
46
on one side wall
47
of a first free space region
48
. The multi-mode waveguide
44
acts as a multi-mode interferometer (MMI). Multiple single-mode array waveguides
50
are coupled to ports on the other side of the first free space region
48
in the form of a star coupler. The array waveguides
50
are coupled on the other end to one side of a second free space region
52
. The array waveguides
50
have lengths with predetermined length differences between them to act as an arrayed waveguide grating (AWG), operating similarly to a planar diffraction grating. Single-mode output waveguides
54
are coupled to the other side of the second free space region
50
along an output wall
56
. The AWG causes the multi-wavelength signal from the input waveguide
42
to be wavelength demultiplexed on the respective output waveguides
54
. Because of the reciprocal nature of the device, the roles of input and output can be reversed so that the same structure can be used as a wavelength multiplexer and as a wavelength demultiplexer. The placement and number of waveguides contemplated by Amersfoort et al. are wider than the example of a single input presented below.
The gaussian wavelength distribution described above for single-mode elements is related to the gaussian spatial distribution of intensity experienced at the outputs of single-mode fibers. However, the multi-mode waveguide
44
, because it typically contains two closely spaced peaks at the port
46
, produces a spatial output pattern into the first free space region
48
that is not gaussian but is much flatter at its peak than a corresponding gaussian distribution of the same passband. The wavelength characteristic of the free space between the multi-mode waveguide
44
and the rest of the phasar
40
is therefore also flattened. As a result, with the use of the multi-mode interference filter
44
, it is possible to obtain a narrow wavelength respon
Amersfoort Martin Ronald
Bulthuis Hindrick Freerk
BBV Design BV
Guenzer Charles S.
Ngo Hung N.
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