Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
2000-01-24
2002-07-30
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341410, C359S349000
Reexamination Certificate
active
06426832
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength-multiplexed light amplifying apparatus, an optical amplifier, and further to an optical add-and-drop apparatus (optical ADM: Optical Add-Drop Multiplexer) using a wavelength-multiplexed light amplifying basic unit.
2. Description of the Related Art
In recent years, the improvement of the transmission technology using an optical fiber has developed enterprises related to point-to-point wavelength division multiplexing (WDM), where the technical development for a photonic network has taken place extensively. This photonic network signifies a network which uses optical wavelengths as identification information for multiplexing and non-multiplexing. As one of the features the photonic network has, there is an optical add-and-drop function. The dropping function of an optical add-and-drop unit in this photonic network is realizable by a broadcast function. This broadcast function means a function of distributing the directions multiplexed signals advance in, but not different from a function of demultiplexing an optical signal power. Referring to
FIG. 18
, a description will be given hereinbelow of the broadcast function according to a conventional technique.
FIG. 18
is an illustration of one example of configuration of an optical add-and-drop multiplexer. For instance, let it be assumed that, when an optical signal comprising a plurality of wavelengths (ch
1
to ch
64
) multiplexed is transmitted from a city A positioned on the left side in
FIG. 18
to a city B positioned on the right side in the same illustration, in a city C lying between these cities A and B, of these wavelengths, ch
1
to ch
32
are dropped, whereas different ch
1
to ch
32
are instead added thereto. In this instance, ch
33
to ch
64
are passed through. A wavelength-multiplexed optical signal coming from the left side in
FIG. 18
is amplified in an optical amplifier
70
a
while another optical signal coming through an adding section
70
d
is then added thereto in an AOTF (Acoustooptical Tunable Filter)
70
b
. Furthermore, a portion of the first-mentioned optical signal is dropped and inputted to a branching secti on
71
, while the second-mentioned optical signal is amplified in an optical amplifier
70
c
and then transmitted to the right side in FIG.
18
. This AOTF
70
b
is a convenient device while it is difficult to make it. AOTF
70
b
can drop ch
1
to ch
32
and newly add ch
1
to ch
32
. Herein, the channel number is equivalent to the assigned wavelength. For example, when ch
1
is dropped in the AOTF
70
b
, ch
1
becomes totally empty so that another information can be assigned to ch
1
and added thereto. On the other hand, after the optical signal dropped in the AOTF
70
b
is amplified in an EDF optical amplifier (Erbium-doped Fiber Optical Amplifier)
71
a
, its optical signal power is divided or split in an optical coupler
71
b
and subsequently inputted to tunable filters
72
so that the wavelength-multiplexed optical signal appears at each port. In this case, the reason for the amplification in the EDF optical amplifier
71
a
is that, for example, if an optical signal is divided into 1000, its power reduces to {fraction (1/1000)}. That is, a need for the amplification exists for maintaining the original power. In addition, the optical signal is distributed through the use of a splitter not having a wavelength characteristic or an AWG (Arrayed Waveguide Grating) having a wavelength characteristic. That is, a component for realizing this optical add-and-drop function utilizes a property of changing its advancing direction in accordance with an optical wavelength. When a wavelength-multiplexed light is distributed in relation to each desired wavelength in this way, since that distribution is made after it is amplified up to an extremely high output level in an optical amplifying system, the loss of the optical signal in the latter section of the EDF optical amplifier
71
a
has great influence on the efficiency. This efficiency signifies the efficiency of conversion from an excitation optical power into a signal optical power.
In
FIG. 18
, let it be assumed that the dropped 32ch-wavelength-multiplexed signal is distributed or branched into 16 ports. Additionally, let it be assumed that 0 dBm per Ich (which will be referred to hereinafter as 0 dBm/ch) is kept as an optical signal quality at each branch (division) port. In this case, for maintaining 0 dBm/ch at the optical signal quality at each branch port, an output power at each branch port requires 15 dBm (32 mW for 32ch), and a value forming the sum of 15 dBm and a theoretical limit value on the division into 16 becomes necessary at an output terminal of the EDF optical amplifier
71
a
. This theoretical limit value signifies the value of the original optical signal needed for the optical signal to have a given quality after the division. As well known, when the original optical signal is divided into two, each of the optical signals after the division results in the loss of 3 dB as compared with the original value, and when being divided into 16, each of the optical signals after the division suffers a loss of 12 dB as compared with the original value. Accordingly, when one optical signal is divided into 16, the original optical signal is required to have a value larger by at least 12 dB than the power value of each of the divided optical signals. In other words, in the case that one optical signal is divided into 16, the theoretical limit value of the output power of the EDF optical amplifier
71
a
becomes 12 dB, and in this instance, the needed value at the output terminal of the EDF optical amplifier
71
a
comes to 15 dBm+12 dBm=27 dBm. Additionally, taking into consideration an excess loss of the distribution (branch) optical coupler
71
b
, the output power of (27+&agr;) dBm becomes necessary at the output terminal of the EDF optical amplifier
71
a
. In the following description, this value a will be taken as 2 dB, for example. That is, the output power of 29 dBm becomes necessary at the output terminal of the EDF optical amplifier
71
a.
On the other hand, looking at a system of input to this EDF optical amplifier
7
a
, since 0 dBm/ch is needed while 32ch is dropped, the total input power to the EDF optical amplifier
71
a
comes to 15 dBm. Besides, this EDF optical amplifier
71
a
is made up of optical parts such as an optical isolator and an optical coupler, which causes a loss of approximately 2 dB. In consequence, a large output of 31 dBm (=27+2+2=1260 mW) develops at the output terminal of an EDFA (Erbium-Doped Fiber Amplifier) (not shown) in the EDF optical amplifier
71
a
, while the output terminal of the EDF optical amplifier
71
a
produces 29 dBm (=790 mW). Additionally, assuming that the conversion efficiency from an excitation light to a signal light in the EDF optical amplifier
71
a
is 50%, the required excitation power reaches 1580 mW.
Thus, the operating conditions of this optical amplifying section
7
a
are as follows.
1) input power: 15 dBm (0 dBm(ch, 32 waves)
2) output power: 29 dBm (31 dBm at the output terminal of EDFA)
3) gain: 14 dB
4) required excitation power: 1580 mW
From this, it is found that a large signal optical power of 1260 mW×0.37=470 mW (1260 mW=31 dBm) disappears between the output terminal of the EDF optical amplifier
71
a
and the output terminal of the optical amplifying section
71
a
. That is, since an excess loss (0.37; corresponding to −2 dB) occurs at the time when the signal power rises, the useless signal power increases. Additionally, even if the divisions is small in number, because a high-output optical amplifier becomes necessary at the initial introduction, the initial investment becomes higher.
For this reason, there is a need to realize the broadcast function efficiently. That is, if a broadcast system with higher efficiency is realized, a signal with a desired wavelength becomes selectab
Fujitsu Limited
Hellner Mark
Staas & Halsey , LLP
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