Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-11-30
2002-03-19
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S341410
Reexamination Certificate
active
06359726
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to an optical amplifier which amplifies in one batch a wavelength division multiplexed (WDM) signal light incorporating a plurality of optical signals of different wavelengths, as well as to an optical communication system which utilizes the optical amplifier and carries out repeater transmission of the WDM signal light, and in particular relates to a WDM optical amplifier and an optical communication system which display excellent noise characteristics and which will accommodate input light power level over a wide range.
(2) Description of the Related Art
The Wavelength Division Multiplexing (WDM) optical transmission system is a transmission system which, by transmitting a plurality of optical signals of different wavelengths through a single optical fiber, enables an increase in communication capacity. The WDM optical transmission system offers several advantages including low introduction costs due to the fact that existing optical fibers can be utilized, and ease of any future upgrades as the transmission path is bit rate free due to the use of optical amplifiers and the like.
In order to achieve the required transmission characteristics, an important factor for optical amplifiers for use in WDM optical transmission systems is the requirement to maintain the output light at a predetermined constant level while simultaneously suppressing the wavelength dependency of the gain in the signal light band. Specifically, the maintenance at a constant level of the output light power per single wavelength as well as the wavelength flatness of the gain is required even if the input light power varies over a wide range.
An example of an optical amplifier which meets the aforementioned requirements, in which the basic construction thereof comprises the positioning of a variable optical attenuator between the two stages of an optical amplification section of a two stage construction, has been proposed by the present applicants. In the proposed optical amplifier basic construction, automatic gain control (AGC) is carried out at both the former stage optical amplification section and the latter stage optical amplification section to control the gain at a constant level, and automatic level control (ALC) is carried out, by adjusting the amount of optical attenuation at the variable optical attenuator positioned between the two stages, to control the output light level from the optical amplifier at the required constant level. Consequently, even if the power level of the input light varies, the gain wavelength characteristic for each optical amplification section is maintained at a constant level, and moreover the output light level from the optical amplifier is also maintained at the required level.
Optical amplifiers of two stage construction have also been proposed in, for example, Japanese Unexamined Patent Publication No. 8-248455 and Japanese Unexamined Patent Publication No. 6-169122. In the optical amplifiers proposed therein, the gain for the entire optical amplifier is controlled at a constant level, and the wavelength characteristic of the gain is maintained at a constant level even if the input light power changes. Moreover, the applicant of the present invention has also proposed a technique wherein a gain equalizer (optical filter) is used for flattening the gain wavelength characteristic of the optical amplification section (refer to Japanese Patent Application No. 9-216049).
With the aforementioned conventional optical amplifiers, in the case where the input light power is comparatively small, AGC operation of each of the amplification sections is possible, but in the case where the input light power increases and the excitation light power of the former stage optical amplification section reaches an upper limit value, AGC operation of the former optical amplification section stops and the excitation light power is controlled at a constant level, resulting in a reduction in the former stage gain. Consequently, in the case where the excitation light power of the former optical amplification section reaches the upper limit value, in order to keep the gain for the entire optical amplifier at a constant value regardless of the input light power, the gain for the latter optical amplification section is controlled to be increased by an amount equivalent to the gain reduction in the former optical amplification section, thus maintaining the wavelength flatness of the gain at a constant level.
However, with the aforementioned conventional optical amplifiers, in the case where the input light power into the former optical amplification section reaches the upper limit value of the excitation light power, any increase in the input light power will result in the gain wavelength characteristic for each optical amplification section varying from the design value thereof. As a result, in those cases where compensation for the gain wavelength characteristic of the optical amplification section is made based on fixed characteristics referenced to the design value (for example, the use of a gain equalizer with a fixed loss wavelength characteristic in both the former and latter optical amplification sections), the system is unable to cope with variations in the gain wavelength characteristic when the input light power is large, and a situation arises where the signal light power is lost in excessive amounts in the former optical amplification section which has stringent noise characteristics.
Specifically, in conventional optical amplifiers of two stage construction, the gain wavelength characteristics of the former optical amplification section and the latter optical amplification section vary in accordance with the input light power as shown in FIGS.
17
(A) and
17
(B) respectively. The gain wavelength characteristics shown in
FIG. 17
are those where each of the optical amplification sections are known erbium doped optical fiber amplifiers (EDFA) and the wavelength band is the 1.55 &mgr;m band (around 1535 nm~1561 nm).
Focussing on the former optical amplification section, which has a large effect on the noise characteristics of the optical amplifier, as shown in FIG.
17
(A), when the input light power is a comparatively small −16.6 dBm/ch the gain at the short wavelength side of the 1.55 &mgr;m band is higher than the gain at the long wavelength side of the band. On the other hand, when the input light power increases to −9.6 dBm/ch there is insufficient excitation light power to achieve the required gain so that the gain decreases. In such a case the gain at the short wavelength side of the band decreases considerably, to be relatively lower than the gain at the long wavelength side of the band.
Until now, former stage optical amplification sections with gain wavelength characteristics as those described above, were fitted with a gain equalizer with loss wavelength characteristics which were previously designed to correspond with the gain wavelength characteristics for when the input light power was comparatively small (with a relatively large loss at the short wavelength side). Consequently, in the case where the input light power was increased, even though the gain at the short wavelength side of the band decreased, the gain equalizer, which has a fixed loss wavelength characteristic, caused excessive amounts of optical power to be lost at the short wavelength side, generating a problem of inferior noise characteristics for the optical amplifier at the short wavelength side.
FIG. 18
is a diagram which shows the noise characteristics (noise factor) of a conventional optical amplifier as those described above, in accordance with the input light power.
As shown in
FIG. 18
, when the input light power is comparatively small an approximately uniform noise factor is obtained for the entire width of the 1.55 &mgr;m band, but as the input light power increases the noise factor at the short wavelength side of the band becomes relatively greater, meaning the noise characteristics deteriorate f
Kinoshita Susumu
Onaka Miki
Fujitsu Limited
Hughes Deandra M.
Staas & Halsey , LLP
Tarcza Thomas H.
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