Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-03-30
2001-05-15
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200, C359S199200, C359S341430
Reexamination Certificate
active
06233090
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical amplifier and in particular to an optical amplifier using an optical fiber amplifier pumped by a light source.
2. Description of the Related Art
The noise characteristic of an optical fiber amplifier is one of the important performance items. To reduce in noise level, there has been disclosed an optical amplifier having two rare-earth-doped fibers connected in series through an optical isolator (see Japanese Patent Unexamined Publication No. 7-30178). The optical isolator can block ASE (Amplified Spontaneous Emission) propagating from the second rare-earth-doped fiber at the output stage, resulting in reduced noise at the first stage.
According to the conventional optical circuit of an optical fiber amplifier, however, ASE generated in the first rare-earth-doped fiber at the input state passes through the optical isolator into the second rare-earth-doped fiber. Therefore, the second rare-earth-doped fiber amplifies the ASE as well as the signal light, resulting in reduced efficiency and output power. In other words, the pumping energy of the light pumping the second rare-earth-doped fiber is consumed to amplify not only the signal light but also the ASE.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an optical amplifier, which can achieve high output power and low noise.
According to the present invention, an optical circuit includes a first optical fiber amplifier pumped by a first pumping light, a second optical fiber amplifier pumped by a second pumping light, and a direction-dependent filter connected between the first and second optical fiber amplifiers, which has a first filtering characteristic in a direction and a second filtering characteristic in an opposite direction.
The direction-dependent filter may be comprised of an optical circulator having four ports, wherein a first port is connected to an output end of the first optical fiber amplifier, a third port is connected to an input end of the second optical fiber amplifier. Further, a first filter having the first filtering characteristic is connected to a second port of the optical circulator, and a second filter having the second filtering characteristic is connected to a fourth port of the optical circulator. The first filter filters out light components other than an input signal light and the first pumping light. The second filter filters out light components other than the second pumping light.
As described above, according to the present invention, the direction-dependent filter is connected between the first and second optical fiber amplifiers. Therefore, noise light generated at the first stage is filtered out and only the signal light and the first pumping light are transferred from the first optical fiber amplifier to the second optical fiber amplifier. Therefore, efficient amplification can be achieved at the second stage.
Further, noise light generated at the second stage is also filtered out and only the second pumping light is transferred from the second optical fiber amplifier to the first optical fiber amplifier. Therefore, a reduction in noise can be achieved at the first stage.
REFERENCES:
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patent: 5698615 (1997-12-01), Alexander
patent: 5710659 (1998-01-01), Cline
patent: 5812712 (1998-09-01), Pan
patent: 5815308 (1998-09-01), Kim et al.
patent: 5822113 (1998-10-01), Delavaux et al.
patent: 5867306 (1999-02-01), Isshiki
patent: 5978131 (1999-11-01), Lauzon et al.
patent: 7-30178 (1995-01-01), None
McGuireWoods LLP
Moskowitz Nelson
NEC Corporation
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