Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-08-17
2004-01-06
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
Reexamination Certificate
active
06674567
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a Raman excitation control method and optical transmission system using the same.
BACKGROUND OF THE INVENTION
In
FIG. 1
, there is shown a configuration example of optical transmission system using optical amplification by means of Raman excitation control. In the system shown in this figure, an optical amplification system
1
is connected to an oppositely located optical amplification system
2
through a bi-directional optical transmission line
3
.
For example, in
FIG. 1
, an optical signal is fed into an optical transmission line
3
through an optical amplifier
10
constituted by Erbium-doped fiber in optical amplification system
2
, reaching an optical amplifier
20
in optical amplification system
2
. Here, optical amplifier
20
is also constituted by Erbium-doped fiber.
On the contrary, an optical signal from optical amplification system
2
is fed into optical transmission line
3
through an optical amplifier
21
, reaching an optical amplifier
11
in optical amplification system
1
.
Bi-directional optical transmission line
3
connecting optical amplification system
1
with optical amplification system
2
has a transmission distance of approximately 200 km. For this purpose, optical amplification technology using Raman excitation light source has been introduced so that the optical signal is transmitted with a sufficient gain from optical amplification system
1
to optical amplification system
2
, and oppositely from optical amplification system
2
to optical amplification system
1
.
This optical amplification using Raman excitation light source is a technology in which the optical transmission line is utilized as an optical amplifier, which is similar to the Erbium-doped fiber amplification used for optical amplifiers
10
,
21
,
20
and
11
. For this purpose, as shown in
FIG. 1
, there are provided Raman excitation light sources
12
,
22
so as to perform backward excitation against optical signals for transmission.
In
FIG. 2
, there is shown a relation between the wavelengths of Raman excitation light and the gains. The gain produced by Raman amplification gradually increases from Raman excitation light wavelength I. In the case of 1550 nm band, the gain characteristic becomes maximum at the wavelength approximately 110 nm longer than the wavelength I. Accordingly, the Raman excitation light wavelength I is determined so that the optical wavelength of a main signal II is allocated in the area in which the maximum gain is produced.
In recent years, a wavelength-multiplexing optical transmission system using a plurality of main signals has been introduced. In such a system, a plurality of main signals are allocated in a main signal wavelength area. A plurality of Raman excitation light wavelengths are provided corresponding to the main signals.
In
FIG. 3
, there is shown an amplification gain characteristic produced by a plurality of Raman excitation lights. Corresponding to a plurality of main signal wavelengths II-
1
to II-
4
, a plurality of laser diodes LD
1
to LD
4
are provided for generating a plurality of Raman excitation light wavelengths I-
1
to I-
4
.
In view of the total gain characteristic in the above-mentioned case, superimposed gain of the plurality of wavelengths becomes larger as the wavelength becomes shorter. As a result, when assuming each output power of laser diodes LD
1
to LD
4
for producing excitation light is identical, superimposed total gain becomes different depending on the combinations of different wavelengths. This produces a tilt as shown ‘A’ in FIG.
3
. To cope with this problem in actual implementation, each output power of laser diodes LD
1
to LD
4
for producing excitation light is individually monitored to adjust so that the tilt may not be produced between each main signal, thus producing substantially flat gain.
Another problem is that, in case of multiple wavelengths, signal-to-noise (SN) ratio disperses depending on channels CH, resulting in deterioration of transmission quality as a whole.
In order to compensate this, a weighted power is applied to each signal in a transmission side to improve SN ratio. This control is called as the pre-emphasis control.
Also, there may be a case that the SN ratio becomes deteriorated unintentionally depending on the shape of the tilt in the above-mentioned Raman amplification. To cope with this in the conventional system, there may be introduced a method shown in
FIG. 4
to compensate the tilt produced in total gain.
In
FIG. 4
, an example of the compensation method is now assumed, which is applicable in a conventional system when the tilt is produced in total gain. Especially in
FIG. 4
, there are shown configuration examples of a unit of optical amplifier
20
(hereafter referred to as optical amplification unit
20
) of
FIG. 1 and a
unit of Raman excitation light source
22
(hereafter referred to as Raman excitation light source unit
22
).
In
FIG. 4
, in order to conduct power control of each laser diode LD
1
to LD
4
for producing excitation light, there are provided couplers (CPL)
201
,
221
to
223
and photodetectors (PD)
224
to
227
for monitoring the light power of each wavelength, in Raman excitation light source unit
22
and optical amplification unit
20
. In optical amplification unit
20
, excitation light is multiplexed into optical transmission line
3
. Also main signal light is received and extracted.
Excitation light emitted by each laser diode LD
1
to LD
4
with power control is multiplexed in coupler (CPL)
221
. Before the excitation light is output to optical transmission line
3
through optical amplification unit
20
, total power is monitored by a photodetector (PD)
220
provided in Raman excitation light source unit
22
for controlling the output.
Here, according to the assumed configuration shown in
FIG. 4
, the following problems may be pointed out.
(1) a large number of couplers CPL and photodetectors PD are required for monitoring total and individual power of excitation light, which brings about increase of equipment cost.
(2) In Raman amplification method requiring high output power for transmission, an important point is how a loss to be produced in the outputs from laser diodes LD
1
to LD
4
for producing excitation light before reaching optical transmission line
3
can be eliminated. However the assumed configuration shown in
FIG. 4
, the loss of the outputs from laser diodes LD
1
to LD
4
produced before reaching optical transmission line
3
comes to 3 to 4 dB.
If a coupler CPL of which branching ratio is approximately 100:1 (=20 dB) is used, a power of 0 to 10 dBm (=1 to 10 mW) is consumed in a photodetector (PD) for monitoring, which produces a great disadvantage.
(3) A coupler/PD portion having the ratio of 100:1 (=20 dB) produces a dispersion of ±1 dB, in the worst case, against the monitoring value a teach PD. This is caused by various dispersions in the branching ratio of the coupler, loss of the coupler itself, a splice connection loss in manufacturing, Quantum Efficiency of PD, etc. Especially, when monitoring the Raman excitation light multiplexed by a coupler (i.e. monitoring by photodetector
220
or
202
in FIG.
4
), there exists a dispersion of power in each wavelength input to photodetector
220
or
202
for monitoring.
Therefore, it is difficult to monitor total power accurately. In general, a variation of 1 dB in the excitation light power corresponds to a variation of 2 to 3 dB in the gain in the case of Raman excitation (when a backward excitation shown in
FIG. 1
is applied), although the above figure depends on excitation light power or transmission distance. As a result, in the method shown in
FIG. 4
, it is difficult to control total power value using feedback control.
(4) Each laser diode LD outputs the power of approximately 15 to 25 dBm in maximum per diode, and the power of multiplexed wave generated by a plurality of laser diodes LD reaches as much as 20 to 30 dBm. In photodetectors
22
Izumi Futoshi
Kobayashi Hideki
Mori Shota
Ohtani Toshihiro
Takahashi Tsukasa
Black Thomas G.
Fujitsu Limited
Hughes Deandra M.
Staas & Halsey , LLP
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