Optical: systems and elements – Compound lens system – Microscope
Reexamination Certificate
2000-02-07
2003-10-28
Moskowitz, Nelson (Department: 2733)
Optical: systems and elements
Compound lens system
Microscope
C359S341410, C359S341420, C359S199200
Reexamination Certificate
active
06639716
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical communication system and an optical amplifying device, for transmitting wavelength division multiplexed signal light including a plurality of optical signals of different wavelengths while amplifying the optical signals by the optical amplifying device, and more particularly to a wavelength division multiplexing optical communication system and an optical amplifying device in which signal light power per one wavelength to be output from the optical amplifying device is kept constant.
2. Related Art
It has been recently demanded to increase a capacity of optical communication system, with popularization such as of Internet and image transmission. To cope therewith, a wavelength division multiplexing (WDM) optical communication system has been put to practical use, and there has been promoted development such as increase of the number of wavelength division multiplexing.
FIG. 21
is a block diagram showing a general constitution of a WDM optical communication system for performing multi-repeating transmission, by collectively amplifying WDM signal light by an optical amplifying device.
The system in
FIG. 21
is constituted of a transmitting side terminal station
1
, a receiving side terminal station
2
, an optical fiber transmission path
3
for connecting them with each other, and a plurality of (in the figure, two) optical amplifying devices
4
(optical repeating stations) provided on the way of the optical fiber transmission path
3
.
The transmitting side terminal station
1
includes: a plurality of optical transmitters (E/O)
1
A for outputting a plurality of optical signals of different wavelengths, respectively; a multiplexer
1
B for wavelength division multiplexing the plurality of optical signals, and for outputting them as WDM signal light onto the optical fiber transmission path
3
; and a post-amplifier
1
C for amplifying the WDM signal light to a required level. The receiving side terminal station
2
includes: a preamplifier
2
A for amplifying the WDM signal light transmitted through the optical fiber transmission path
3
, to a required level; a demultiplexer
2
B for dividing light output from the pre-amplifier
2
A into a plurality of optical signals corresponding to respective wavelengths; and a plurality of optical receivers (O/E)
2
C for receiving and processing the plurality of optical signals, respectively.
At each of the optical amplifying devices
4
, the WDM signal lights transmitted through the optical fiber transmission path
3
are collectively amplified. Further, at each of the optical amplifying devices
4
, total power of output light is monitored, and there is conducted an automatic level control (ALC) for controlling the operation of optical amplifying device
4
so as to keep a monitored value constant. By rendering an output controlling method of optical amplifying device
4
to be ALC in this way, repeating gain becomes independent for each span, thereby advantageously resulting in readiness of system design.
In constituting a WDM optical communication system making use of a plurality of optical amplifying devices
4
as described above, there is generally restricted a transmission distance of WDM signal light due to “wavelength dependency of gain” (gain deviation) of each of optical amplifying devices
4
. As an optical amplifying device effective for suppressing such wavelength dependency of gain, the present applicant has proposed a constitution such as disclosed in OAA'98, WA2, pp.173-176, and OAA'98, MD1, pp.54-57.
Such an optical amplifying device effective for suppressing wavelength dependency of gain adopts a basic constitution wherein there is provided a two-stage constitution including an optical amplifying section of preceding stage and an optical amplifying section of succeeding stage, and a variable optical attenuator is inserted at a middle stage. In such an optical amplifying device, there is realized the ALC of output light, by operating the respective optical amplifying sections of preceding and succeeding stages under an automatic gain control (AGC) to thereby suppress gain deviation, and by simultaneously controlling an optical attenuation amount of the variable optical attenuator of middle stage corresponding to an output light level of the optical amplifying section of succeeding stage. Further, output setting level of ALC in the above optical amplifying device is controlled such that output light power per one wavelength is kept at a constant value even when the number of used wavelengths is changed. Concretely, the output setting level of ALC is set at m×Po, assuming that the number of used wavelengths is m and the output light power per one wavelength is Po. The number m of used wavelengths is obtained such as by a monitoring control signal to be sent from for example a monitoring system. By controlling the output setting level in this way, the WDM optical communication system becomes possible to operate as a system even when the number of used wavelengths is changed from one wavelength up to a maximum number of wavelengths.
Meanwhile, there exists an upper limit value for output light power per one wavelength which is allowed to be sent from an optical amplifying device onto an optical fiber transmission path. This upper limit value is determined due to nonlinear effect (such as self phase modulation (SPM) and cross phase modulation (XPM) of an optical fiber transmission path). In a conventional WDM optical communication system, output light power Po per one wavelength of an optical amplifying device has been designed to be a value close to such an upper limit value.
There will be now described light to be output from an optical amplifying device, in detail.
In a general optical amplifying device, there is caused spontaneous emission light (ASE light) as input signal light is amplified, and this ASE light is added to the signal light to be output. When the optical amplifying device is ALC operating such that output light power per one wavelength becomes a predetermined value Po [W], total output light power P
Tout
[W] to be output from the optical amplifying device is given by the following equation (1):
P
Tout
=M·Po=P
Tin
·G
+2
·n
sp
·h&ngr;·&Dgr;f
·(
G
−1) (1)
wherein: P
Tin
is total input light power [W] to the optical amplifying device; G is a gain of the optical amplifying device; n
sp
is a spontaneous emission coefficient of the optical amplifying device; h&ngr; is a photon energy [J]; and &Dgr;f is a bandwidth [Hz] of the optical amplifying device.
In the equation (1), the first term of right side represents a signal light component (when input light includes ASE light such as at an optical amplifying device of a preceding stage, including an amplified component of the ASE light,), and the second term represents an ASE light component caused at the above-mentioned optical amplifying device. The total output light power P
Tout
is controlled to be constant in an optical amplifying device which performs an ALC operation. Thus, when separately considering output light power Po per one wavelength by dividing it into a signal light component and an ASE light component, the ASE light component (the second term of the equation (1)) is an error component relative to a signal light component to be controlled to a constant level corresponding to the aforementioned upper limit value. Namely, the output light power Po per one wavelength can be represented by the following equation (2) derived from the equation (1):
Po=P
Tin
·G/m
+2
·n
sp
·h&ngr;·&Dgr;f
·(
G
−1)/
m=P
Tin
·G/m+&Dgr;Po
(2)
wherein &Dgr;Po=2·n
sp
·h&ngr;·&Dgr;f·(G−1)/m[W].
As described above, signal light power per one wavelength is decreased by &Dgr;Po than a predetermined level of output light power Po. Thus, in a WDM optical communication system a
Fujitsu Limited
Moskowitz Nelson
Staas & Halsey , LLP
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