Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-10-14
2002-11-26
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06486986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical transmission systems, and more particularly to a system that optically transmits a frequency-division-multiplexed signal obtained by frequency-division-multiplexing a plurality of signals.
2. Description of the Background Art
FIG. 11
is a block diagram exemplarily showing the configuration of a conventional optical transmission system for transmitting a frequency-division-multiplexed signal. As will be known from
FIG. 11
, this optical transmission system comprises a multiplexer
1100
, an FM modulator
1101
, an optical transmitter
1104
, an optical receiver
1106
, and an FM demodulator
1107
. In the optical transmission system, an electrical transmission line
1102
connects the FM modulator
1101
and the optical transmitter
1104
to each other, and an optical transmission line
1105
connects the optical transmitter
1104
and the optical receiver
1106
to each other.
The operation of the conventional optical transmission system in the above configuration will be described below. The multiplexer
1100
frequency-division-multiplexes a plurality of signals, and outputs the resultant signal to the FM modulator
1101
. The FM modulator
1101
converts the frequency-division-multiplexed signal into a frequency-modulated signal (hereinafter, referred to as “FM modulated signal”) having a predetermined frequency deviation through frequency modulation. After that, the FM modulator
1101
outputs the FM modulated signal to the electrical transmission line
1102
. The optical transmitter
1104
receives the FM modulated signal through the electrical transmission line
1102
, then converts the signal into an optical signal, and sends the optical signal to the optical transmission line
1105
. The optical receiver
1106
receives the optical signal through the optical transmission line
1105
, then converts the signal into an FM modulated signal which is an electrical signal, and outputs the FM modulated signal to the FM demodulator
1107
. The FM demodulator
1107
demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal.
The optical transmission system in the above configuration is described in detail in “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems” by K. Kikushima et al. (IOOC'95 Technical Digest, Vol. 5 PD2-7, pp.33-34), and other documents. The optical transmission system converts a frequency-division-multiplexed signal into an FM modulated signal, and then optically transmits and demodulates the FM modulated signal to reproduce the original frequency-division-multiplexed signal. The optical transmission system utilizes an FM gain in the FM transmission to improve the signal-to-noise power ratio (SNR) of the demodulated signal (i.e., the frequency-division-multiplexed signal), thereby enabling high-quality signal transmission.
Thus, the above-described optical transmission system can realize high-quality multi-channel signal transmission with an optical fiber.
However, the above-described system for optically transmitting an FM modulated signal has the following specific problems due to the properties of the FM modulated signal and the nonlinearity of an optical fiber.
SUMMARY OF THE INVENTION
An FM modulation scheme increases a frequency deviation to acquire a greater FM gain, thereby enabling signal transmission of higher quality than other modulation schemes such as amplitude modulation. On the other hand, the increased frequency deviation requires a wider signal band. In addition, in the FM modulation scheme, linear distortion tends to occur under the influence of the group delay characteristic of a transmission line and the like (the characteristic that a propagation delay varies depending on a frequency). Therefore, the transmission line must be designed with particular attention. However, as a signal band becomes wider, the group delay variations in the band become more difficult to sufficiently suppress.
In a general optical modulation scheme, the optical frequency spectrum, of an optical signal is composed of a steep-shaped optical carrier component, which has narrow spectral line-width, and upper and lower sidebands, as shown in FIG.
12
B. The upper and lower sidebands are geometrically similar to the frequency spectrum of a modulating signal. Therefore, if a wide-band signal like an FM modulated signal is used as a modulating signal in optical modulation, the optical frequency spectrum of the optical signal also becomes wider. The optical signal having such wide optical frequency spectrum becomes susceptible to the chromatic-dispersion of an optical fiber (the characteristic that a propagation delay varies depending on a wavelength). The affected optical signal component interacts with the optical carrier component to induce harmonic distortion in the FM modulated signal, resulting in waveform deterioration of the transmitted signal.
As is known from the above, the conventional optical transmission system has the specific problem that the quality of the transmitted signal is degraded due to the wide-band property of an FM modulated signal.
Therefore, an object of the present invention is to provide an optical transmission system capable of narrowing the bandwidth of an FM modulated signal while increasing the frequency deviation thereof to realize high-quality signal transmission. The present invention has the following features to attain the object above.
A first aspect of the present invention is directed to a transmission system for optically transmitting a frequency-division-multiplexed signal, which is obtained by frequency-division multiplexing a plurality of signals, from a transmitting end to a receiving end. The transmission system comprises at the transmitting end, a multiplexer for frequency-division multiplexing the plurality of signals to produce the frequency-division-multiplexed signal, an FM modulator for converting the frequency-division-multiplexed signal into a frequency-modulated signal through frequency modulation using the frequency-division-multiplexed signal as an original signal to output the frequency-modulated signal as an FM modulated signal, and an optical transmitter for converting the FM modulated signal into an optical-intensity-modulated signal whose optical carrier component is suppressed in the optical frequency spectrum through optical modulation using the FM modulated signal as an original signal to send the optical-intensity-modulated signal to the receiving end. The transmission system also comprises at the receiving end, an optical receiver for receiving the optical-intensity-modulated signal from the optical transmitter, and converting the optical-intensity-modulated signal into an electrical signal corresponding to the FM modulated signal through photodetection based on a square-law detection characteristic to output the electrical signal as a received FM modulated signal, and an FM demodulator for demodulating the received FM modulated signal to reproduce the frequency-division-multiplexed signal.
As described above, in the first aspect, the FM modulated signal is obtained through frequency modulation using a frequency-division-multiplexed signal as an original signal. The FM modulated signal is converted into an optical-intensity-modulated signal at the transmitting end. The optical-intensity-modulated signal has an optical frequency spectrum in which upper and lower sidebands distribute geometrically similarly to the frequency spectrum of the original signal for the optical modulation and in which an optical carrier component is suppressed. Then, the optical-intensity-modulated signal is photodetected based on a square-law detection characteristic at the receiving end. At the receiving end, the optical transmission system thus obtains an FM modulated signal, having a frequency deviation twice as large as the one of the original FM modulated signal produced at the transmitting end, as a received FM modulated
Matsushita Electric - Industrial Co., Ltd.
Negash Kinfe-Michael
Wenderoth , Lind & Ponack, L.L.P.
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