Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-09-15
2003-07-15
Tweel, John (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06594054
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to elimination of dispersion caused by filtering of optical signals in optical systems. In particular, the invention relates to elimination of dispersion in systems using wavelength division multiplexing or dense wavelength division multiplexing.
BACKGROUND OF THE INVENTION
The use of wavelength division multiplexing (“WDM”) in optical systems is well known for transmitting optical signals along a fiber in optical networks. The information capacity of optical networks has been steadily increasing. The number of wavelength channels transmitted in each fiber typically varies between 8 and 20 and it is believed that it will soon exceed 30, and may eventually approach 50. This requires the use of special filters, known as wavelength routers, in order to separate and combine the various wavelength channels. At the present time, the frequency spacing between channels, or “channel separation” is typically in the range of 50-200 GHz. Due to the limited usable bandwidth of a fiber, as more channels are used, the channel separation decreases. When the channel separation is small, filters with the required sharp cutoff characteristics are difficult to realize with sufficient accuracy. In particular, it is difficult to keep crosstalk from adjacent channels below a satisfactory level, and to produce the desired passband characteristics such as center wavelength and passband flatness. Moreover, a signal transmitted over an optical network typically must pass through several filters, for instance, if the transmission path for a particular signal includes several nodes. For this reason, tighter tolerances must be imposed on the filters because signal distortion, e.g., dispersion, caused by each filter is additive and a signal is further degraded by each filter through which it passes.
To reduce the level of crosstalk between adjacent channels, it is advantageous to separate an incoming signal containing multiple channels into two separate outgoing signals, one containing the channels having the first, third, fifth, etc. longest wavelengths (the “odd” channels) and a second group containing the channels having the second, fourth, sixth, etc. longest wavelengths (the “even” channels). Such an arrangement produces two separate outgoing signals, each having double the channel separation and half the number of channels of the incoming signal. In this manner, the individual channels/signals may be discerned with less crosstalk. This separation into two outgoing signals may be achieved by a slicer.
A slicer is essentially a periodic device having a period equal to the incoming signal's channel separation. The slicer has two output ports, each respectively producing an outgoing signal carrying either the even or the odd channels. Each output port transmits only every other channel and, therefore the channels transmitted to each output port have twice the channel separation of the incoming signal's channels. The output channels may be separated by connecting each output port to suitable channel dropping filters as is known in the art. In most cases, it is desirable that the two output ports have identical responses displaced from each other by half a period. A slicer having excellent passband flatness and low crosstalk may be realized using a set of birefringent plates.
FIG. 1
a
shows an example of a birefringent filter
20
having a set of four birefringent plates
22
,
24
,
26
,
28
of the same properties and thickness. Each plate is characterized by two orthogonal planes of input polarization producing different delays through the plate. The delay difference between the two polarization choices is denoted by a. Assuming the same delay a for all plates, the periodic frequency response has a period (channel separation) determined by a.
Angles &thgr;
1
, &thgr;
2
, . . . , &thgr;
N
denote the various angles of rotation of the plates. Each angle specifies the differences between principal planes of polarization of adjacent plates or the input or output reference axes. For example, &thgr;
1
is the angle of the principal plane of the first plate relative to the input reference axis, &thgr;
2
is the angle of the principal plane of the second plate relative to that of the first, and &thgr;
N
specifies the output reference axis relative to the principal plane of the last plate.
Once these angles are selected, the transmission coefficients for each plate, neglecting losses and ignoring a constant delay, are given by the matrix
[
exp
(
j
φ
/
2
)
0
0
exp
(
-
j
φ
/
2
)
]
(
Eq
.
1
)
where the phase factor exp(j&phgr;/2) is determined by the optical frequency f and the delay difference a, where
φ
=
2
π
f
a
(
Eq
.
2
)
Each relative rotation of the plates is specified by the matrix
[
cos
θ
i
sin
θ
i
-
sin
θ
i
cos
θ
i
]
(
Eq
.
3
)
The product of the matrices given in Eqs. 1 and 3 above gives a matrix having coefficients which are the transfer functions between input and output polarizations of the birefringent plate assembly as shown in
FIG. 1
c
. This matrix has the form
[
C
(
φ
)
-
D
*
(
φ
)
D
(
φ
)
C
*
(
φ
)
]
(
Eq
.
4
)
where the first column gives the two transmission coefficients C(&phgr;), D(&phgr;) of interest here. The two transmission coefficients in the second column are the complex conjugates of C(&phgr;) and D(&phgr;). A filter
20
using birefringent plates is shown in
FIG. 1
b
. Such a filter transforms a linearly polarized input beam into a single output beam containing two orthogonal components that are linearly polarized. These components can be separated into the component beams using a beam displacer
30
.
Typically, the input signal includes different wavelength channels that are to be separated and transferred to different fibers. A linearly polarized (e.g., vertically) input signal (e.g., as shown at
21
) is transformed by the filter
20
into two linearly polarized signals having orthogonal polarizations (e.g., horizontal and vertical). Using well known methods, the filter can be designed to act as a slicer to cause some of the channels (e.g., the even channels) to be produced in one of the two output polarizations (e.g., vertical) and, the remaining channels to be produced in the other output polarization (e.g., horizontal).
By separating the two polarizations with a beam displacer
30
, two separate beams
27
,
29
are created that can be transmitted to separate fibers. Here, C and D denote the filter transmission coefficients corresponding to the output polarizations of the separate beams
27
,
29
. These two coefficients are functions of the signal wavelength and designed to yield approximately unity magnitude in the passbands of C and approximately zero magnitude in the stopbands, neglecting losses. Once C is specified, the magnitude of the complementary coefficient D is also specified, since |C|
2
+|D|
2
=1, because of power conservation neglecting losses. Thus, each passband of C corresponds to a stopband of D, and vice versa. In other words, the channels that are not transmitted by C will be transmitted by D, and vice versa. In such an arrangement, the filter acts as a slicer.
As shown symbolically in
FIG. 1
c
, a birefringent filter characterized by transmission coefficient C for particular input and output polarizations is characterized by coefficients D, C*, −D* for all other polarization states obtained by 90° rotations of the input and output polarizations. Also shown in
FIG. 1
c
are the amplitudes (pC−qD* and pD+qC*) of the output polarizations produced by input amplitudes p, q.
An example of a slicer
10
of the type known in the prior art is shown in FIG.
2
. The slicer
10
combines a birefringent filter
20
such as that shown in
FIG. 1
a
with suitable input and output stages. The input s
Lucent Technologies - Inc.
Tweel John
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