Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
1999-06-03
2001-02-13
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341430, C359S345000
Reexamination Certificate
active
06188508
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a control signal superimposer, and more specifically, to a control signal superimposer for superimposing or multiplexing a control signal on a signal light in an optical transmission system.
BACKGROUND OF THE INVENTION
In optical transmission systems, especially in optical transmission systems in which a signal light is amplification-repeated and/or branched off in the middle, it is necessary to remotely control optical repeaters or branching units. Also, in optical networks, it is necessary to transmit network control information separately from the signal light.
Therefore, an arrangement has been proposed in which the signal light whose signal is conventionally transmitted as NRZ pulses, RZ pulses or soliton waves is again intensity-fluctuated with a control signal of relatively low frequency and then propagated on optical fiber transmission lines.
As means for superimposing or multiplexing the control light in the signal light, an arrangement has been well known that uses a structure of optical amplifier consisting of an erbium doped optical fiber, an intensity of its pumping light being modulated with the control signal. That is, gain of an erbium doped optical fiber is fluctuated in accordance with the control signal and, thus, intensity level of the signal light is fluctuated according to the control signal.
However, the gain fluctuation of the erbium doped optical fiber can follow approximately 100 kHz at the most. This is not enough for superimposing the control signal.
Furthermore, in the conventional systems, when the pumping light is disappeared or the intensity is largely reduced due to trouble of a pumping light source or the like, the erbium doped optical fiber plays a role of loss medium and considerably attenuates the signal light.
An object of the invention is to solve the foregoing problems and provide a control signal superimposer for superposing the control signal on the signal light.
Namely, the object of the invention is to provide a control signal superimposer capable of responding at high speed.
An another object of the invention is to provide a control signal superimposer capable of transmitting the signal light with substantially no loss even in case of trouble of the pumping light.
A further object of the invention is to provide a control signal superimposer not causing any harmful influence such as nonlinear effect to the signal light itself.
SUMMARY OF THE INVENTION
The control signal superimposer according to the invention comprises a pumping light source for generating pumping light with intensity fluctuation in accordance with control signal; a Raman amplification medium pumped by a pumping light from the pumping light source for Raman-amplifying a signal light; a combiner for combining the pumping light output from the pumping light source and the signal light and applying them to the Raman amplifying medium; and an optical filter for extracting the signal light component in the output light from the Raman amplifying medium and terminating the pumping light component.
Since the gain of Raman amplification can fluctuate easily following up to several 10 MHz of frequency, which is the frequency generally used for the control signal, it is possible to transmit high speed control signal. By employing the Raman amplification, a wider signal wavelength band can be ensured as the modulation frequency band widens. Also, polarization dependence is hardly appeared.
While a 1.5 &mgr;m band is used for the signal light in long distance optical fiber transmission systems, a 1.4 &mgr;m band can be used for the pumping light. Therefore, it is possible to employ laser elements of satisfactory results at a low price.
To put it more precisely, the signal light belongs to the 1.55 &mgr;m band and the pumping light belongs to within the range from 1.44 to 1.49 &mgr;m band.
The pumping light source comprises a laser light source and a light source driver for driving the laser light source.
The driver applies a driving current, which amplitude changes according to the control signal, to the laser light source. Consequently, the pumping light which intensity fluctuates according to the control signal desired to be superimposed on the signal light can be easily generated.
The Raman amplification medium comprises a medium being low loss toward the signal light in case of the absence of the pumping light. Therefore, even if the pumping light source develops some trouble, the unpumped Raman amplification medium scarcely gives a harmful influence to the signal light. Consequently, a system of high reliability can be realized. The Raman amplification medium comprises for instance a silica optical fiber.
The optical filter comprises a selective transmission reflecting element for transmitting the signal light component alone in the output light from the Raman amplifying medium and reflecting the pumping light component toward another direction. The optical filter also comprises a separator for separating the signal light component and the pumping light component in the output light of the Raman amplifying medium, and an optical terminator for terminating the pumping light component separated by the separator.
This invention further comprises a modulation depth detector for detecting the modulation depth of the signal light output from the optical filter, and a comparator for comparing the modulation depth detected by the modulation depth detector with a desired modulation depth and outputting a signal to indicate the difference. According to the output from the comparator, the pumping light source adjusts amplitude of the intensity fluctuation in accordance to the control signal to be appropriate value. Consequently, the intensity modulation depth of the signal light can be constantly maintained and thus it prevents the signal light to be overly modulated.
When the control signal comprises an amplitude modulated signal, a sample-and-hold circuit should be provided for sampling and holding the data such as the detected modulation depth, the magnitude of a fluctuation component in the signal light component output from the optical filter, or the comparative result obtained by the comparator, in accordance with the data value of the control signal.
REFERENCES:
patent: 5479424 (1995-12-01), Sakuyama
patent: 5668658 (1997-09-01), Hamada
patent: 5926304 (1999-07-01), Tajima
patent: 6115174 (2000-04-01), Grubb et al.
Akiba Shigeyuki
Horiuchi Yukio
Suzuki Masatoshi
Yamamoto Shu
Christie Parker & Hale LLP
Hellner Mark
KDD Corporation
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