Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-04-21
2003-04-08
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06545780
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength allocation method using wavelength division multiplexing (WDM) technology, a transmission equipment and receiving equipment using this method, and a wavelength multiplex transmission system, and in particular, a wavelength allocation method which carries out wavelength multiplexing by allocating the carrier frequency (hereinbelow, referred to as “frequency”) of the signal beam of each channel with unequal spacing in order to mitigate the influence of nonlinear effects of the optical fiber, a transmission equipment and a receiving equipment using this method, and a WDM transmission system.
2. Background Art
In a 1.55 &mgr;m dispersion shifted fiber, the zero-dispersion wavelength is set in the region of 1.55 &mgr;m, which is around the signal frequency, so that waveform distortion is not produced by fiber chromatic dispersion, and thereby, this fiber can also be applied on high speed transmission.
However, when shifted fiber is employed to WDM systems, because the wavelength dispersion in the signal wavelength is small, there is the problem that a new optical power that is newly generated by four wave mixing, which is one of the nonlinear effects of an optical fiber, becomes large. When this newly generated four wave mixing light has the same frequency as the signal frequency, this four wave mixing light becomes noise, and transmission distance is restricted. In order to relieve this problem, a method of unequally allocating the frequency intervals of the signal frequency has been proposed.
Here, let the frequencies of the four wave mixing light generated from three signal light with frequencies f
i
, f
j
, and f
k
(k≠i, j) be expressed by f
ijk
. Next equation is satisfied,
f
ijk
=f
i
+f
j
−f
k
(1)
FIGS. 6A and 6B
shows the position where the four wave mixing light is generated. In
FIGS. 6A and 6B
, the frequencies of the signal light in Channels 1, 2, and 3 are f
1
, f
2
, and f
3
, respectively. 12 four wave mixing lights are generated.
FIG. 6A
shows the four wave mixing light when the frequencies of each signal beam are allocated with equal spacing. For example, f
223
denotes the frequencies of the four wave mixing light when f
i
=f
j
=f
2
, and f
k
=f
3
, and shows that the four wave mixing light generated by the signal beams of channels 2 and 3 overlaps the signal beam of channel 1. This is identical for f
132
, f
312
, and f
221
. Because this kind of four wave mixing light cannot be separated from the signal light using, for example, an optical filter, it generates cross-talk in the signal beam, and the S/N ratio is degraded.
FIG. 6B
shows the four wave mixing light when the frequencies of each signal beam are unequally spaced. As shown here, by unequally spacing the frequencies of each signal, the each signal frequency and the frequencies of the four wave mixing light do not coincide, it becomes possible to separate with optical filters, and the influence of the four wave mixing light can be decreased. Moreover, in the present specification, the frequency interval of any two arbitrary channels is different from the frequency intervals of all other pairs of channels, and the frequency interval of signal beams being set so that the frequencies of the signal beams and the four wave mixing light do not coincide at all is called “completely unequally spaced channel allocation”.
An algorithm that determines the frequency interval based on this type of completely unequally spaced channel allocation is disclosed in Japanese Patent Application, First Publication, No. Hei 7-264166 (Japanese Patent Application, No. Hei 7-29043; “Multiplexed Channel Optical Fiber Communication System”). Below this algorithm will be briefly explained.
The theorem of this algorithm is to assign frequency intervals such that the frequency interval i of any two arbitrary signal beams is different from all other pairs of frequency intervals. This can be understood by modifying Eq. 1 to:
f
ijk
−f
i
=f
j
−f
k
(2)
Here, if the number of channels (number of channels) is M, m
i−1
(2≦i≦M) is an integer, and the minimum frequency difference of the four wave mixing light is &Dgr;f, then the frequency f
i
of each signal beam is expressed by:
f
i
=f
i−1
m
i−1
×&Dgr;f
(3)
In this case, the frequency interval of any two arbitrary channels is the partial sum of the integers m
1
~m
M−1
multiplied by &Dgr;f, and the problem of finding a frequency interval that is a completely unequally spaced channel allocation comprises finding the M−1 integers whose arbitrary partial sums are all different. Here, partial sum means the sum of m
i−1
, which represents the frequency separation of any two arbitrary channels among the M−1 integers m
i−1
. For example, this denotes m
1
, m
2
, m
1
+m
2
, m
3
+m
4
+m
5
+m
6
, etc.
Moreover, the signal beam along with the minimum frequency difference &Dgr;f of the four wave mixing and the minimum frequency separation (the minimum value of m
i−1
×&Dgr;f) of the signal light are determined in consideration of the degree of stability of the oscillating frequency of the semi-conductor laser using as an optical source, the spectral spread of the signal light and the four wave mixing light, the transmission bandwidth of the optical filter, the optical amplifier bandwidth used as a repeater, etc., and &Dgr;f and m
i−1
are assigned the greatest lower limit.
In connection with the above, presently completely unequally spaced channel allocation has only been obtained up to 12 channels. Therefore, when the unequally spaced channel allocation is expanded, for example, to 16 channels, a method of repeatedly disposing completely unequally spaced channel allocation of 8 channels has been proposed as a alternative means (see J. S. Lee et al., “Periodic allocation of a set of unequally spaced channels for upgradable dense-WDM application using dispersion-shifted fibers”, OFC '98, FC 5, 1998).
In general, optical fiber amplifiers bandwidth is limited, for example, to 1529~1560 nm. In addition, the bandwidth is decreased by cascading. In addition, when the difference between the wavelength of the signal light and the zero dispersion wavelength becomes large, waveform degradation due to the chromatic dispersion becomes large.
Because of this, it is preferred that the frequency bandwidth that M signal beams with completely unequal spaced channel allocation occupy (referred to hereinbelow as “occupied bandwidth”) be as narrow as possible. In the above-mentioned publication, since the occupied bandwidth of these M signal beams is given as (m
1
+m
2
+ . . . m
M−1
)×&Dgr;f, a theorem is disclosed wherein the frequency interval of the completely unequally spaced channel allocation is set so that m
1
+m
2
+ . . . m
M−1
is at a minimum.
Here, one example of completely unequally spaced channel allocation for 8 channels which minimizes the occupied bandwidth when the minimum frequency difference &Dgr;f, between the signal and the four wave mixing light is 50 GHz and the minimum frequency interval (minimum channel allocation) is 3 &Dgr;f =150 GHz, is shown in Table 1.
As is clear from this Table, in the completely unequally allocated 8 channels, 2.15 (=194.45-192.30) THz is necessary for the occupied bandwidth, and it is understood that this is a great increase compared to the 1.05 (=150 GHz×7) occupied bandwidth in the case of an even allocation by minimum channel allocation.
In addition, the minimal occupied bandwidth necessary for completely unequally spaced channel allocation increases according to the number of channels as shown in
FIG. 2
, where &Dgr;f=50 GHz, and the minimum channel allocation of 3&Dgr;f=150 GHz. Moreover, in the following explanation, the occupied bandwidth is represented by wavelength. That is, the occu
Koga Masafumi
Ohteru Shoko
Takachio Noboru
Burns Doane Swecker & Mathis L.L.P.
Leung Christina Y
Nippon Telegraph and Telephone Corporation
Pascal Leslie
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