Optical communication transmission system

Optical: systems and elements – Deflection using a moving element – Using a periodically moving element

Reissue Patent

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C359S199200, C359S199200, C359S199200, C385S123000

Reissue Patent

active

RE037621

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a high-speed, long-haul communication transmission circuit by an optical fiber, and more particularly to an optical communication transmission system which is expected to be developed as a communication system for a transmission network for advanced information service and which can transmit a large amount of information with a high degree of quality over a long distance.
2. Description of the Related Art
An optical communication transmission system makes use of the broad band feasibility of light to permit high-speed, very high-capacity, high-quality communications which cannot be realized readily with conventional communications using the microwave band or the millimeter wave band. For example, the following reports have been provided with regard to elements for use with communication of, for example, 10 Gbit/s:
by T. Suzaki et al., “10-Gbit/s Optical Transmitter Model with Multiquantum Well DFB LD and Doped-channel Hetero-MISFET Driver IC,” 1990 Optical Fiber Communication Conference, Technical Digest TUI2, and
by T. Suzaki et al., “Ten-Gbit/s Optical Transmitter Module Using Modulator Driver IC and Semiconductor Modulator,” Optical Fiber Communication Conference 1992, Technical digest TUI6.
An optical communication transmission system of the optical amplifier lumped repeater system which uses erbium-doped optical fiber amplifiers will be described with reference to FIG.
1
.
An optical transmitter
3
modulates optical power outputted from a semiconductor laser source
1
by intensity modulation by an external modulator
2
of lithium niobate LiNbO3 which is driven by a signal of 10 Gbit/s outputted from a modulation signal source
5
and outputs the modulated optical power to an optical power amplifier
11
. The optical power amplifier
11
consists of an erbium-doped optical fiber amplifier and amplifies a signal light level and outputs the amplified optical signal to a first optical fiber
101
for a transmission line of an optical amplifier lumped repeater system. In this instance, when the signal light level exceeds 10 dBm, in order to avoid the influence of Brillouin scattering in the transmission fiber, the line width of the semiconductor laser is expanded in advance using the well-known technique of direct FM modulation of the semiconductor laser or a like technique. After passing the optical fiber
101
, the optical signal is amplified again by a direct optical amplifier repeater
12
which consists of an erbium-doped optical fiber amplifier and is then outputted to a second stage optical fiber
111
for transmission. The signal light inputted into the transmission line at the second stage is amplified by a second stage optical amplifier repeater
13
and outputted to a third transmission line
121
. The signal light is thereafter processed in a similar manner and transmitted finally to a last transmission line
191
. In an optical receiver
53
on the reception side, the optical signal is amplified by an optical preamplifier
21
and converted into an electric signal using a PIN photodiode
51
, which is a photoelectric transducer. The electric signal, and consequently, the signal of 10 Gbit/s transmitted from the modulation signal source
5
, is then reproduced by an equalizer amplifier regeneration circuit
52
.
In the high-speed, high-capacity communication system described above, however, it is known that waveform distortion after transmission due to such causes as chromatic dispersion of the optical fibers strongly degrades the transmission characteristic through a very long distance transmission.
Therefore, the following countermeasures are conventionally taken:
First, as a countermeasure to chromatic dispersion of an optical fiber, which is conventionally considered to be the most significant cause of degradation of the transmission characteristic, a transmission line is constructed using an optical fiber which has no chromatic dispersion in the waveband of the light source of the optical transmitter. In other words, the optical fiber employed has zero chromatic dispersion.
For example, as a communication system for a long-distance submarine cable, transmission systems wherein the dispersion value of an optical fiber for transmission is reduced substantially to zero have been proposed by:
N. S. Bergano et al., “9000 km, 5 Gbit/s NRZ Transmission Experiment Using 274 Erbium-doped Fiber-Amplifiers,” Technical Digest of Topical Meeting on Optical Amplifiers and Their Applications, Santa Fe, Jun. 24-26, 1992, postdeadline paper PD11, and
T. Imai et al., “Over 10,000 km Straight Line Transmission System Experiment at 2.5 Gbit/s Using In-Line Optical Amplifiers,” Technical Digest of Topical Meeting on Optical Amplifiers and their Applications, Santa Fe, Jun. 24-26, 1992, postdeadline paper, PDI2.
In an actual transmission line, however, the requirement for zero chromatic dispersion cannot be fully satisfied over the entire length of the optical fiber, and very small level of chromatic dispersion exists. In order to suppress the influence of the very small dispersion, several techniques for compensating for the chromatic dispersion in the transmitter side and the receiver side have been proposed, for example, in Japanese Patent Laid-open No. 1987-65529 and Japanese Patent Laid-open No. 1987-65530, and by:
A. H. Gnauck et al., “Optical Equalization of Fiber Chromatic Dispersion in a 5 Gbit/s Transmission System,” Optical Communication Conference, San Francisco, Jan. 22-26, 1990, postdeadline paper PD7, and
N. Henmi et al., “A Novel Dispersion Compensation Technique for Multigiga-bit Transmission with Normal Optical Fiber at 1.5 Micron Wavelength,” Optical Fiber Communication Conference 1990, postdeadline paper PD8.
Further, in a coherent communication system, such techniques as equalizing an electric signal in the receiver side by using a delay equalizer at the stage of an intermediate frequency of the electric signal have been reported by:
K. Iwashita et al., “Chromatic Dispersion Compensation in Coherent Optical Communications”, IEEE, Journal of Lightwave Technology, Vol. 8, NO. 3, March 1990, pp. 367-375.
It is known that the causes for degradation of the transmission characteristic of an optical amplifier lumped repeater system include, in addition to wavelength dispersion of the optical fiber described above, a noise accumulation effect caused by spontaneous emission light and a noise increase effect caused by a non-linear effect in the optical fiber through multistage optical amplifier repeaters. In order to decrease the influence of the accumulation effect of noise of spontaneous emission light, the outputs of the optical amplifier repeaters must be set high. On the other hand, in order to suppress the non-linear effect in the optical fiber, the outputs of the optical amplifier repeaters must necessarily be set low. Due to these two contradictory requirements, it is conventionally difficult to simultaneously control both the noise accumulation effect and the non-linear effect. Therefore, in order to obtain a very long-haul transmission system or achieve an increase of the repeating distance, it is necessary to increase the repeater output while decreasing the non-linear effect in the optical fiber.
However, little is known of the non-linear effect in an optical fiber, and the causes of degradation have not been specifically identified as yet.
SUMMARY OF THE INVENTION
It is believed that a self-phase modulation effect is a major factor in the non-linear effect in an optical fiber. However, as recently reported by S. Saito et al. [“2.5 Gbit/s, 80-100 km Spaced In-line Amplifier Transmission Experiments Over 2,500-4,500 km,” Technical Digest of European Conference on Optical Communication 1991, postdeadline paper 3], in addition to the self-phase modulation effect, noise is increased by the influence of a 4 wave-mixing effect between signal light and spontaneous emission light outputted from the optical amplifier, resulting in the degradation of the transmission characte

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