Variable optical filter unit and variable gain equalizing...

Details Variable optical filter unit and variable gain equalizing... Variable optical filter unit and variable gain equalizing...

2002-01-02

2003-05-06

Ullah, Akm E. (Department: 2874)

Optical waveguides

Polarization without modulation

C385S031000, C385S033000, C385S036000, C385S047000, C385S140000, C359S199200, C359S199200

Reexamination Certificate

active

06560379

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable optical filter unit for varying a wavelength characteristic of incident signal light and a variable gain equalizing system for compensating for wavelength characteristic variation in input signal light.
2. Description of the Related Art
Recently, research and development has been made on Wavelength Division Multiplexing (WDM) long-distance optical transmission in the field of optical communications. In the WDM long-distance optical transmission system, a plurality of optical amplifiers (e.g. EDFAs (Erbium-Doped Optical Fiber Amplifiers)) are inserted on the optical transmission line.
However, because the optical amplifier in a use wavelength band is different in gain depending upon a wavelength, the signal light passed the optical amplifier is different in light amount due to the wavelength. In the WDM long-distance optical transmission system, because the signal light passes a plurality of optical amplifiers, there is cumulative increase of difference in signal light amount between the wavelengths. This causes great difference in signal light amount between wavelengths on channels. This results in difference in the ratio of noise magnitude to light amount of the signal (S/N ratio) between channels (wavelengths) and hence makes impractical.
The countermeasure to the problem resulting from the optical-amplifier gain characteristic includes, for example, the means to incorporate a gain equalizer individually in each optical amplifier or insert a plurality of gain equalizers on the optical transmission line so that the difference in signal light amount between wavelength caused by the optical amplifiers can be compensated for by the gain equalizers thereby equalizing the signal light amount on every wavelength.
Conventionally, the gain equalizer has been configured with using an optical filter (e.g. dielectric multi-film filter, etalon filter, Mach-Zehnder filter) having a fixed light attenuation amount wavelength characteristic as a relationship between a light attenuation amount and a wavelength, on the assumption that the optical amplifier is steady in gain characteristic.
However, actually the optical-amplifier gain characteristic is not steady but varies due to the variation in input-signal light amount resulting from aging deterioration of the optical fiber constituting the optical transmission line or cut-working of the optical fiber upon extension of optical transmission line, or the variation in excitation state of the optical amplifier medium resulting from external disturbance such as temperature change in the environment of installing the optical amplifiers. Consequently, there are cases that the difference in input signal light amount between wavelengths cannot be satisfactorily compensated for by the gain equalizer utilizing an optical filter fixed in light attenuation amount wavelength characteristic.
Considering the above, there have been recent proposals on the variable gain equalizers capable of varying the light attenuation amount wavelength characteristic. Various variable gain equalizers have been proposed, e.g. variable gain equalizers under thermo-optical control on the principle of a waveguide-type diffraction grating and Mach-Zehnder interferometer, variable gain equalizers with mechanical means on the principle of a split-beam Fourier filter, variable gain equalizers for gain-inclination correction comprising a variable optical filter unit on the principle of a birefringence filter having a sinusoidal-like filter characteristic, variable gain equalizers comprising a plurality of such variable optical filter units arranged in series and so on.
Of the plurality of proposed variable gain equalizers, the present inventor has an attention to the variable gain equalizer using the variable optical filter unit. The reason lies in, first, that the variable gain equalizer has a high reliability in variably controlling light attenuation amount wavelength characteristic because of a structure to variably control light attenuation amount wavelength characteristic under electric control. Second, the variable optical filter unit is easy to manufacture and low in price because of the capability of manufacturing by a bulk structure that optical elements are arranged in a free space to provide action to transmission light. Third, where the plurality of variable optical filter units are arranged in series to structure a variable gain equalizer, light attenuation amount wavelength characteristic can be varied in various ways by individually controlling the variable optical filter units. Thus, light attenuation amount wavelength characteristic is high in freedom in variable setting.
FIG. 5
typically shows a system example of a variable gain equalizer using the plurality of variable optical filter units. The variable gain equalizing system
1
, shown in
FIG. 5
, is structured with a plurality of variable optical filter units
2
(
2
1
,
22
, . . .
2
n
(n is an integer equal to or greater than 2)), monitor means
3
and pattern form control means
4
. The variable optical filter units
2
each possess a sinusoidal-like light attenuation amount wavelength characteristic. The variable gain equalizing system
1
is configured for producing a compensating light attenuation amount wavelength characteristic as shown at the curve A′ in
FIG. 6B
for compensating for am input-signal-light (incident signal light) wavelength characteristic, e.g. as shown at the curve A in
FIG. 6A
, by adding together the sinusoidal-like light attenuation amount wavelength characteristics of the variable optical filter units
2
, to output signal light nearly equal in light amount at every wavelength as shown in FIG.
6
C. Note that the sinusoidal-like includes not only a sinusoidal-like form but also the waveforms approximate in form to the sinusoidal-like form throughout this specification.
In the variable gain equalizing system
1
, for example the monitor means
3
monitors a wavelength characteristic of an input signal. Depending on the result of monitor, the pattern-form control means
4
individually controls the variable optical filter units
2
to control at least one of amplitude and phase of the sinusoidal-like light attenuation amount wavelength characteristic of each variable optical filter unit
2
, thereby variably controlling the light attenuation amount wavelength characteristic for compensating the variable gain equalizing system
1
. specifically, where for example input signal light has a waveform characteristic as shown at curve A in
FIG. 6A
, produced is a light attenuation waveform characteristic for compensation as shown at the curve A′ in FIG.
6
B. Meanwhile, where the input signal light is changed to a waveform characteristic as shown at the curve B in
FIG. 6A
, the pattern-form control means
4
individually controls the variable optical filter units
2
, to variably control the amplitude or phase of the sinusoidal-like light attenuation waveform characteristic of each of the variable optical filter units
2
. By adding together the sinusoidal-like light attenuation amount wavelength characteristics of the variable optical filter units
2
, a light attenuation amount wavelength characteristic for compensation is produced as shown at the curve B′ in FIG.
6
B.
The variable gain equalizing system
1
shown in
FIG. 5
, capable of changing the light attenuation amount wavelength characteristic for compensation in various ways as in the foregoing, can output signal light nearly constant in light amount regardless of wavelength as shown in
FIG. 6C
even where the wavelength characteristic of input signal light varies.
FIG. 7
typically shows a configuration example of a variable optical filter unit
2
for constituting the variable gain equalizing system
1
. The variable optical filter unit
2
shown in
FIG. 7
is configured to have a polarizer
6
, a Faraday rotator
7
, a linear retarder
8
, a Faraday rotator
9
and a analyzer
10
arranged in the orde

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