Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-07-12
2002-07-30
Chang, Audrey (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S246000, C359S251000, C385S014000, C385S031000, C385S039000, C385S028000
Reexamination Certificate
active
06426816
ABSTRACT:
TECHNICAL FIELD
The present application relates in general to optical communications, and in specific to using a wavelength filter in wavelength division multiplex communications.
BACKGROUND
Optical wavelength division multiplexing has gradually become the standard backbone network for fiber optic communication systems. WDM systems employ signals consisting of a number of different wavelength optical signals, known as carrier signals or channels, to transmit information over optical fibers. Each carrier signal is modulated by one or more information signals. As a result, a significant number of information signals may be transmitted over a single optical fiber using WDM technology. These optical signals are repeatedly amplified by erbium-doped fiber amplifiers (EDFA) along the network to compensate for transmission losses. The amplified signals reach the receiving end and are detected using WDM filters followed by photo receivers.
Fiber optic communications networks are typically arranged with a plurality of terminals in any of a number of topological configurations. The simplest configuration is two terminals communicating data over an optical link. This can be extended to a daisy-chain configuration in which three or more terminals are connected in series by a plurality of optical links. Ring configurations are also used, as well as other two-dimensional mesh networks. In each case, the optical link between two terminals typically includes a plurality of optical fibers for bidirectional communications, to provide redundancy in the event of a fault in one or more of the optical fibers, and for future capacity.
Despite the substantially higher fiber bandwidth utilization provided by WDM technology, a number of serious problems must be overcome, for example, multiplexing, de-multiplexing, and routing optical signals, if these systems are to become commercially viable. The addition of the wavelength domain increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of combining multiple channels (each defined by its own frequency spectrum) into a single WDM signal. De-multiplexing is the opposite process in which a single WDM signal is decomposed into individual channels or sets of channels. The individual channels are spatially separated and coupled to specific output ports. Routing differs from de-multiplexing in that a router spatially separates the input optical channels to output ports and permutes these channels according to control signals to create a desired coupling between an input channel and an output port.
Note that each carrier has the potential to carry gigabits of information per second. Current technology allows for about forty channels or optical carriers, each of a slightly different wavelength, to travel on a single-mode fiber using a single WDM signal. The operating bands are limited by the EDFA amplifier (C) band, thus the increase in the number of channels has been accomplished by shrinking the spacing between the channels, and by adding new bands. The current standard is 50 and 100 GHz between optical channels, whereas older standards were 200 and 400 GHz spacings. Another characteristic of the WDM signal is the modulation rate. As the modulation rate is increased, more data can be carried. Current technology allows for a modulation rate of 10 Gigabits per second (Gbs). This has been recently increased from 2.5 Gbs. The 10 Gbs standard is SONET OC-192, wherein SONET is synchronized optical network and OC is optical carrier. The increase in the modulation rate translates into a wider signal in the spatial domain. Consequently, the wider signal and smaller spacing means that the signals are very close together (in the spatial domain), and thus are very hard to separate. As a result, crosstalk may occur from adjacent signals.
One prior art separation method is to use a Fourier based filter to pass a particular wavelength from the input signal and block the other wavelengths on the signal. Such a filter
100
is depicted in
FIG. 1A
, wherein the filter
100
receives a WDM signal
101
, which comprises &lgr;
1
, &lgr;
2
, and &lgr;
3
. The filter
100
blocks &lgr;
1
and &lgr;
3
, and passes &lgr;
2
as output signal
102
. The filter
101
has the transmission characteristics
103
shown in FIG.
1
B. Note that this filter
101
has a low peak to valley ratio, i.e. the peak is not much higher than the floor. Thus, filter will have a low signal-to-noise ratio. To provide a higher signal-to-noise ratio, several identical filters
100
a,
100
b,
100
c,
can be cascaded together as shown in FIG.
1
C. These filters also receive WDM signal
101
, which comprises &lgr;
1
, &lgr;
2
, and &lgr;
3
, and blocks &lgr;
1
and &lgr;
3
, while passing &lgr;
2
as output signal
102
. The cascaded filters
100
a,
100
b,
100
c
have the transmission characteristics
104
shown in FIG.
1
D. Note that the cascaded filters have a higher peak-to-valley ratio than the single filter of FIG.
1
A. Thus, the cascaded filters will have higher (better) signal-to-noise ratio. However, also note that this filter has a narrower width than the filter of
FIG. 1A
, thus this arrangement has better isolation but at a cost of having a narrower pass band.
Another prior art separation method is to use a Fourier based filter to divide the input signal into two periodic, inter-digitated sub-signals, each carrying an odd or even set of alternating wavelength signals, see Cohen et al. U.S. Pat. No. 5,680,490, which is hereby incorporated by reference. As shown in
FIG. 2A
, the WDM input signal
201
comprises a plurality of wavelengths, &lgr;
1
, &lgr;
2
, &lgr;
3
, and &lgr;
4
. The filter
200
separates the input signal
201
into two sub-signals, which have complementary, inter-digitated wavelengths, one signal
202
a
with the odd wavelengths, &lgr;
1
and &lgr;
3
, and the other signal
202
b
with the even wavelengths &lgr;
2
and &lgr;
4
. Note that even and odd do not literally mean even and odd numbers, but rather indicate that alternating wavelengths in the input stream are separated into two streams. This usage will become apparent in the discussion of FIG.
2
C. The filter
200
has the transmission characteristics
206
and
207
, for outputs
202
a
and
202
b
respectively, as shown in FIG.
2
B. Several filters can be cascaded to isolate single wavelengths, as shown in FIG.
2
C. The second stage filters
203
a,
203
b
have pass bands that are twice the size of first stage filter
200
, as shown in
FIG. 2D
, which depicts characteristic
205
which corresponds to signal
202
a,
and characteristic
207
which corresponds to signal
204
a
of filter
203
a.
Note that the other characteristics of filters
203
a
and
203
b
are not shown for the sake of simplicity.
These filters all suffer from several types of problems. Among the problems are polarization dependant loss (PDL), polarization mode dispersion (PMD), and chromatic dispersion within a passband. PDL occurs when light of an unknown polarization is launched into a fiber or device and changes to a different polarization at the output end, which causes noise or loss as the polarization orientation is not matched to that of the system. For example, if the light starts with horizontal polarization and changes to vertical polarization, a different insertion loss may occur. Too large of a loss will compromise system performance. PMD is a type of dispersion that occurs when the polarization components of a light beam each experience a different index of refraction. Thus, one component travels faster than the other component. Chromatic dispersion within a passband is a similar problem caused by structure of this fourier filter to achieve square-like band shape. Thus, some spectral content of the light will travel faster than other portions of the light.
These problems greatly limit the performance of the prior art filters, and thus inhibit their use in high speed, dense signal systems.
SUMMARY OF THE INVENTION
These and other objects, feat
Li Ye
Liu Jian-Yu
Wong Charles
Wu Kuang-Yi
Baker & Botts L.L.P.
Chang Audrey
Chorum Technologies LP
Curtis Craig
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