Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-07-08
2003-03-25
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06538786
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communication system and an optical reception apparatus using a synchronous polarization scrambler for scrambling the polarized state of a signal light according to a polarization modulation signal of a repetition frequency which coincides with the bit rate of the signal light.
2. Description of the Related Art
Heretofore, in optical transmission systems for long distances of over several thousands of kilometers and across the ocean, the optical signal has been converted to an electric signal, and transmitted using an optical regenerative repeater for retiming, reshaping and regenerating. Recently, however, optical amplifiers have been put to practical use, and an optical amplifier-repeated transmission method which uses an optical amplifier as a linear repeater has been studied. By substituting an optical regenerative repeater for an optical amplifying repeater, the number of parts in the repeater is greatly reduced, and the reliability is secured while the manufacturing cost can be greatly reduced.
In 1993, M. G. Taylor pointed out a phenomenon where noise light generated by a light repeating amplifier depends on the polarized state of a signal light, and the excessive noise light increases (polarization hole burning). Since due to the polarization hole burning, the mean value of the signal light to noise light ratio (optical SNR) decreases while the fluctuation of the optical SNR increases, this becomes a big problem in performing optical amplifier-repeated transmission.
As a measure against this, for example, polarization scrambling which can positively vary the polarized state of a signal light on the transmitting side has been proposed.
FIG. 21
shows a schematic construction of a conventional optical communication system using polarization scrambler.
The conventional system shown in
FIG. 21
comprises an optical transmitter
100
for sending a polarization-scrambled signal light to a transmission system
200
, and an optical receiver
300
for receiving the signal light from the transmission system
200
and performing decision processing and the like. With the optical transmitter
100
, a signal light having a predetermined bit rate which has been modulated according to the transmission data is projected from a signal light generator (E/O)
101
, and the signal light is polarization-scrambled by a polarization scrambler (PS)
102
according to the polarization modulation signal and output to the transmission system
200
. As a method for the polarization scrambling, there are, for example, a method using a phase modulator, a method for imparting a stress from a side of an optical fiber, and a method using two light sources. In the optical receiver
300
, the signal light from the transmission system
200
is converted into an electric signal by a light receiving device (O/E)
301
, and is subjected to the decision processing with a decision circuit (DEC)
302
.
For example, in an experiment with a bit rate of 5.33 Gb/s and a transmission distance of 8100 km, in 1994, F. Heismann et al. attained a Q value improvement by 4 dB in 40 kHz and by 5 dB in 10.66 GHz for the repetition frequency in the polarized state, with a polarization scrambler using a phase modulator of lithium niobate (LiNbO
3
) with a designated input polarized state of 45 degrees. The former repetition frequency is lower than the bit rate, and is referred to as low-speed polarization scrambling, while the latter is equal to or higher than the bit rate, and is therefore referred to as high-speed polarization scrambling. High-speed polarization scrambling has the effect of suppressing fluctuations in the optical SNR due to the polarization dependence loss of the optical transmission path and the optical amplifying repeater, and hence the improved amount thereof is large.
A polarization scrambler using a phase modulator will now be briefly described. The phase difference &Dgr;Ø(t) of light in the TM mode and the TE mode generated by the phase modulator can be expressed by the following expression:
&Dgr;Ø(
t
)=&pgr;/&lgr;[(
ne
3
&ggr;
33
−no
3
&ggr;
13
)
V
(
t
)
L&Ggr;]
where, &lgr; denotes the optical wavelength, &ggr;
33
and &ggr;
13
denote the electrooptical constant of the TM mode and the TE mode, ne and no denote the optical refractive index of the TM mode and the TE mode, V(t) denotes the applied voltage, L denotes the length of an electrode, and &Ggr; denotes a reduction coefficient of the applied voltage. By using this phase difference &Dgr;Ø(t) of light in the TM mode and the TE mode, the polarization scrambler using the phase modulator can change the polarized state of the signal light.
Furthermore, as one method for realizing a large capacity of the optical transmission system, a wavelength division-multiplexing (WDM) optical transmission system is noted, which multiplexes and transmits light signals having two or more different wavelengths in a single optical transmission path.
With the WDM optical amplifier-repeated transmission method combining the WDM optical transmission method and the above-mentioned optical amplifier-repeated transmission method, it is possible to amplify light signals having two or more different wavelengths in the block using an optical amplifier, hence a large-capacity and long-distance transmission can be realized with a simple (economical) construction.
With the WDM optical amplifier-repeated transmission method, it is important to reduce the deterioration of the transmission characteristics due to the nonlinear effects of the optical transmission path. The incidence efficiency of, for example, the four-wave mixing (hereinafter referred to as “FWM”) which is one of the nonlinear effects becomes maximum, when the polarized state of some signal lights coincide. Therefore, since it can be so set that the coincidence of the polarized state of some signal lights cannot be positively maintained by, for example, carrying out high-speed polarization scrambling, it is possible to reduce the incidence of the four-wave mixing.
As one example, the present inventors confirmed in 1996 in an experiment with four-wave multiplexing, a bit rate of 5.33 Gb/s, and a transmission distance of 4800 km, that by performing high-speed polarization scrambling in which the repetition frequency in the polarized state was twice as high as the bit rate, the incidence of the four-wave mixing was reduced thus improving the transmission characteristics. Furthermore, the WDM transmission method in which polarization scrambling is performed is described in detail in Japanese Unexamined Patent Publication No. 9-149006 which is the prior application of the present applicant.
Moreover, as one of the other important problems with the WDM optical amplifier-repeated transmission method, there can be mentioned the reduction of the channel interval, that is, the increase in the number of wavelength multiplexing. However since the signal light subjected to the high-speed polarization scrambling has an expanded spectrum, this becomes a hindrance when the high-density wavelength multiplexing is realized.
Therefore, N. S. Bergano et al. proposed in 1995 polarization scrambling in which the spectrum expansion in the signal light is relatively small and the repetition frequency is the same as the bit rate. Such polarization scrambling is referred to herein as synchronous polarization scrambling. In this proposal however, there is required a construction such that the intensity modulation and the polarization scrambling of a signal are synchronized in order to improve the transmission characteristics.
As the optical transmitter for performing the synchronous polarization scrambling, for example, there is a transmitter, as shown in
FIG. 22
, comprising a light source (LD)
101
A for generating continuous light, an intensity modulator (IM)
101
B for modulating the intensity of the light from the light source
101
A, a first driving circuit (DRV)
101
C for driving the intensity modulator
101
B
Fujitsu Limited
Pascal Leslie
Singh Dalzid
Staas & Halsey , LLP
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