Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-08-24
2003-07-01
Black, Thomas G. (Department: 3663)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341400
Reexamination Certificate
active
06587260
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a band-expanding method for optical amplifiers and to an optical amplifier. More particularly, the present invention relates to a band-expanding method for optical amplifiers, which expands the transmission optical signal band by adding on a pump laser source for an optically pumped amplification medium, such as an optical fiber, as well as to an optical amplifier used for the method. In particular, the method and the optical amplifier are used for optical fiber Raman amplifiers, optical repeaters, etc. employed for optical information communications.
The wavelength-division multiplexing (WDM) optical transmission method, that is used to transmit information by multiplexing a plurality of optical signals having different wavelengths from each other in an optical fiber, is very effective for increasing the capacity of optical fiber communication. To repeat/amplify an optical signal in such a way, however, it is required to use rare-earth doped optical fiber amplifiers, such as the EDFA (Erbrium-doped Fiber Amplifier), semiconductor optical amplifiers, and optical fiber Raman amplifiers, such as the optical fiber amplifier.
The WDM transmission apparatus combines those optical amplifiers so as to repetitively amplify each optical signal that has been attenuated due to the transmission thereof in an optical fiber as long as several tens of kilometers, thereby enabling the signal to be transmitted through a distance of more than several hundreds to several thousands of kilometers. Conventionally, the EDFA has been used as an optical amplifier of the commercial WDM transmission apparatus. As the WDM transmission apparatus is getting upgraded in band width and in capacity, the conventional EDFA amplification bands (about 30 nm, i.e. from 1530 nm to 1560 nm for conventional C-band optical amplifiers) have come to be short of transmission bands. In order to solve this problem, it is now under examination to use L-band EDFAs (about 30 nm, i.e. from 1570 nm to 1600 nm) having different wavelength bands from each other that are disposed in parallel and in which optical amplifiers are used to amplify the signal in each divided band, thereby making the transmission capacity double. Using of optical amplifiers in the 1300 nm band and in the S-band (1490 nm to 1520 nm) is also under examination for some transmission systems.
As described above, the WDM transmission apparatus often employs band-upgrading, that is, a band-expanding method that adds on optical amplifiers band by band so as to reduce the initial installation cost and cope with the transmission capacity requirement that is expected to arise in the future.
FIG. 2
shows a conventional method that uses a plurality of lumped optical amplifiers having different bands from each other as optical repeaters. An optical signal
101
, after each wavelength thereof is multiplexed, is transmitted in an optical fiber transmission line
102
-
1
, amplified in an optical repeater
108
, and then output to an optical fiber transmission line
102
-
2
. Because the number of signal wavelengths is small at the initial time, the method uses no band add-on optical amplifier
107
(dark portion). The optical signal is thus amplified only in a pre-installed optical amplifier
104
. For example, a C-band EDFA or the like is usually used as the pre-installed optical amplifier
104
. A wavelength band demultiplexer
105
and a wavelength band multiplexer
106
are disposed before and after the pre-installed optical amplifier
104
, respectively, so as to cope with-bands to be added on in the future.
The above method enables both of the band add-on wavelength demultiplexer
105
and the wavelength band multiplexer
106
to be prepared beforehand, so that it is just required to provide a band add-on optical amplifier
107
in order to expand a band. As the band add-on optical amplifier, for example, any of L-band, S-band, and 1300 nm band optical amplifiers can be used. In the practical optical transmission, the configuration of any of pre-installed bands and add-on bands is arbitrary. For example, the C-band can be divided into sub-bands, each of which can be added on independently of others, or the L-band can be used as a pre-installed band.
In recent years, in addition to those optical amplifiers, it is also under examination to use the optical Raman amplifier (hereinafter to be referred to simply as the Raman amplifier) that uses an optical fiber as an amplification medium. The Raman amplification is a phenomenon in which an optical gain is caused to occur due to the Raman Effect from a pump wavelength within a range of about 110 nm at the longer wavelength side. The Raman effect is a kind of non-linear effect which is caused to appear when a strong pump laser beam (several tens of mW to several W) is entered into an optical fiber. The magnitude of the gain is not constant and a peak appears at a point spaced by about 100 nm from-the pump laser wavelength. It has a triangularly shaped gain characteristic in which the gain decreases smoothly up to the pump wavelength from the peak. Generally, the Raman amplification is employed for signal amplification, in which a Raman pump laser source is disposed in an optical repeater, an optical site, or the like so as to input the pump laser in the forward or backward direction in terms of the transmission direction of the signal light into the optical fiber transmission line, with use of the optical fiber transmission line itself as a gain medium. When a wavelength-multiplexed signal is amplified by such a Raman amplifier, a pump laser having a plurality of wavelengths is usually used, and the intensity of each pump laser is set to a proper ratio so as to flatten the gain within a wavelength range of the wavelength-multiplexed signal. It is well known that the method can obtain a flat gain up to about 100 nm.
On the other hand, while there is almost no case in which Raman amplifiers are used commercially, employment of the method is under examination. This is because the signal
oise (S/N) ratio of an optical signal can be improved and the transmission distance can be extended more than the case in which only the conventional lumped optical amplifiers are used when the Raman amplification and the lumped optical amplification are employed together. The U.S. Pat. No. 6,115,174 (document 1) discloses a method that uses the Raman gain to change signal powers and signal gain shapes.
The document
1
relates to setting of the amplification characteristic for each signal within a specific wavelength range. However, the document
1
does not mention the expanding of amplification bands and the adding-on of bands and pump laser sources so as to amplify signals in an add-on (newly-installed) wavelength range and a wavelength range (amplification range) of pre-installed amplifiers.
SUMMARY OF THE INVENTION
Unlike conventional lumped optical amplifiers, the gain band of the Raman amplifier cannot be added on band by band independently, since the optical gain of the pre-installed band and the add-on band is mutually overlapping. This is because the optical gain medium, that is, the optical fiber, is commonly used by all of the bands in the Raman amplifier.
It is well known that an increase in the number of wavelengths of Raman pump laser sources can improve the gain flatness and expand the gain bandwidth. A simple increase of pump laser sources, however, causes a problem in that the gain characteristic cannot be flattened nor shaped as desired after the add-on, since the optical gain bands-induced by the pre-installed pump-laser sources and the add-on pump laser sources are mutually overlapping. Hereinafter, this problem will be described in more detail.
FIGS.
3
(
a
) to
3
(
c
) are graphs which illustrate a synthesized gain of a Raman amplifier. FIG.
3
(
a
) shows how both wavelength and intensity of pump lasers are adjusted so as to flatten the gain at 10 dB in a pre-installed C-band, where, for example, the gain ripple is less than 1 dB. On the contrary, FIG.
Kikuchi Nobuhiko
Takeyari Ryoji
Black Thomas G.
Sommer Andrew R.
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