Optical amplifier

Details Optical amplifier Optical amplifier

1999-06-23

2001-05-22

Buczinski, Stephen C. (Department: 3662)

Optical: systems and elements

Optical amplifier

Optical fiber

C359S199200

Reexamination Certificate

active

06236500

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical amplifier designed to compensate for the optical power loss and/or wavelength dependency output power difference occurring during transmission of a WDM (Wavelength Division Multiplexing) signal.
2. Description of Related Art
When a WDM signal is transmitted over long distances, optical amplifiers are introduced into transmission lines in order to compensate for the optical power loss in the transmission lines. Systems obtained by the multistage connection of optical amplifier units are often used as such optical amplifiers.
On the other hand, when a WDM signal is transmitted over a transmission line, nonlinear optical effects will occur in the transmission line if the WDM signal is too powerful.
DCFs (Dispersion compensating fibers) or FBGs (Fiber Bragg gratings) are commonly introduced into transmission lines in order to inhibit the formation of such nonlinear optical effects.
Both DCFs and FBGs have high optical power loss, however. Consequently, neither DCFs nor FBGs are introduced on the input or output side of an optical amplifier. The reason is that the NF (Noise Figure) increases when a DCF or FBG is introduced on the input side of an optical amplifier, and output decreases when these are introduced on the output side thereof. In view of this, DCFs or FBGs are commonly introduced between an optical amplifier unit and another optical amplifier unit (between stages). For example, in the case of a two-stage optical amplifier comprising a pre(preliminary)-stage optical amplifier unit and a post(subsequent)-stage optical amplifier unit, DCFs or FBGs are introduced between the pre-stage optical amplifier unit and the post-stage optical amplifier unit.
In the case of optical amplifiers for wavelength division multiplexing (WDM) signals, it is important that powerful output be achieved across a wide wavelength band, the optical output power difference be kept small across a wide wavelength band, a low NF be obtained, and the like. However, some of the aspects of conventional optical amplifiers still need improvement because of the following, for example.
A DCF has a wide compensation wavelength bandwidth, but it also has a nonlinear optical limit. As used herein, the term “nonlinear optical limit” refers to the light input conditions under which a nonlinear optical effect is created in a transmission line.
FIG. 12
is a diagram depicting a system comprising a conventional two-stage optical amplifier
15
composed of a pre-stage optical amplifier unit
11
and a post-stage optical amplifier unit
13
. When the aforementioned DCF
17
is introduced between the pre-stage optical amplifier unit
11
and the post-stage optical amplifier unit
13
, the optical input power for the DCF
17
, while varying with the system, must be kept at a maximum level of no more than about +3 dBm per channel. This is the reason that the gain of the pre-stage optical amplifier unit
11
must limited, the output of the prestage optical amplifier unit
11
must be lowered by means of an attenuator, or the like.
In addition, the DCF
17
creates substantial signal transmission loss (optical power loss, approximately equal to several decibels), making it impossible for the optical amplifier
15
to generate high total output. Specifically, high output is difficult to achieve with an optical amplifier
15
obtained using DCF.
Furthermore, the total NF (NF total) of a two-stage optical amplifier
15
can be expressed as NF total=NF1+(NF2/G1), where NF1 is the NF of the pre-stage optical amplifier unit
11
, NF2 is the NF of the post-stage optical amplifier unit
13
, and G1 is the gain of the pre-stage optical amplifier unit
11
. When, however, a DCF
17
is introduced between the pre-stage optical amplifier unit
11
and the post-stage optical amplifier unit
13
, the gain of the pre-stage optical amplifier unit
11
becomes (G1−Ld), where Ld is the optical power loss of the DCF
17
, and the result becomes NF total=NF1+{NF2/(G1−Ld)}.
Ultimately, introducing a DCF into the intermediate stage of a two-stage optical amplifier
15
limits the gain of the pre-stage optical amplifier unit
11
because of the nonlinear optical limit of the DCF, and appears to further reduce the gain of the pre-stage optical amplifier unit
11
because of the optical power loss caused by the DCF
17
. Consequently, the NF of the post-stage optical amplifier unit
13
affects the total NF of the optical amplifier
15
.
The performance of the post-stage optical amplifier unit
13
is important for compensating the factors (limited optical input power for the DCF, and the optical power loss originating in the DCF itself) that reduce the output of the optical amplifier
15
depicted in FIG.
12
. With the conventional practice of amplifying the entire wavelength bandwidth of WDM signals by a single post-stage optical amplifier unit
13
, however, inherent limitations are encountered when attempts are made to achieve high output and low optical output power difference across a broad wavelength band.
On the other hand, an FBG has lower signal transmission loss (optical power loss) than a DCF. In addition, an FBG is free from nonlinear optical limits. Consequently, introducing an FBG (not shown) instead of a DCF between the pre-stage optical amplifier unit
11
and the post-stage optical amplifier unit
13
of a two-stage optical amplifier
15
yields an optical amplifier capable of chromatic dispersion compensation and makes it possible to obtain an optical amplifier in which the signal transmission loss originating in the FBG itself is reduced and in which no limitations are imposed on the optical input power for the FBG.
In the case of an FBG, however, the wavelength bandwidth within which the chromatic dispersion can be compensated for is narrow (for example, about 7 nm), limiting the bandwidth that can be transmitted by an optical amplifier when the FBG is introduced between the stages of the optical amplifier.
A need therefore existed for an optical amplifier in which higher output and lower wavelength dependency output power difference could be achieved across a wider wavelength bandwidth than in the past.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide an optical amplifier designed to compensate for the high output power across a wide wavelength band.
A second object of the present invention is to provide an optical amplifier for outputting a WDM signal having low wavelength dependency output power difference across a wide wavelength band.
Aimed at attaining the stated objects, the optical amplifier of the present invention (hereinafter occasionally referred to as “the optical amplifier of the first invention”) comprises an optical amplifier unit including a first optical component of M inputs and N outputs that has an optical-demultiplexing function, and P units of optical amplification means for the individual amplification of light that is output by P output terminals selected from among the N output terminals of the first optical component.
In addition, the optical amplifier of the second invention comprises an optical amplifier unit including a first optical component of N inputs and M outputs that has an optical multiplexing function, and P units of optical amplification means for the individual amplification of light to be input to P input terminals selected from the N input terminals of the first optical component, and for the individual input of this light to the corresponding input terminals.
In either amplifier, M is an integer of 1 or greater, N is an integer of 2 or greater, and P is an integer from 1 to N. M typically is 1, but is not limited to this value alone.
Since it is provided with a first optical component having an optical demultiplxing function, the optical amplifier of the first invention can, for example, separate wavelength-multiplexed signal light by wavelength or bandwidth. Out of the N types of light obtained by such opt

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