Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-01-27
2002-09-17
Chan, Jason (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06452706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an FM signal optical transmission apparatus and an FM signal optical reception I! apparatus used for, for example, optical communication, CATV, optical measurement, or mobile communication.
2. Description of the Related Art
In recent years, it has been practically used to perform optical transmission of video or audio data for many channels by making the most use of the small loss and wide-band characteristic of an optical fiber for a video monitoring system, CATV, subscriber system, or mobile communication. The above optical transmission system electrically multiplexes signals of many channels by a plurality of subcarriers having frequencies different from each other and converts them into AM signals and then, directly modulates a semiconductor laser beam or the like by the AM signals to convert the semiconductor laser into an optical signal and transmits the optical signal through an optical fiber. Optical transmission of an AM signal is characterized in that the structure of a MODEM can be simplified particularly for transmission of a video signal and the cost of it can be decreased.
However, the above optical transmission system has the following problems. That is, in the case of video optical transmission, it is necessary to secure a high C/N (carrier-to-noise ratio) in order to secure a desired signal characteristic (such as video quality). Moreover, to obtain a high C/N for video optical transmission of an AM signal, the receiver side indispensably requires a high optical input power.
Moreover, in the case of mobile communication, because the intensity level of an audio or data signal to be transmitted is greatly fluctuated due to movement of a terminal, a high dynamic range is necessary for signal fluctuation. Furthermore, mobile communication is subject to a distortion due to a reflected wave at the time of optical conversion by a semiconductor laser or under transmission through an optical fiber. Furthermore, an AM-signal amplifier requires an amplifier having a high linearity.
To solve the above problems and improve the distortion resistance and noise resistance, an optical transmission system has been proposed so far which simultaneously converts subcarrier-multiplexed AM signals into FM signals and optically transmits them. Moreover, to obtain a desired C/N by increasing the modulation index of the proposed optical transmission system, it is also proposed to obtain an FM signal having a high modulation index by directly modulating the frequency of a semiconductor laser.
FIG. 17
shows the structure of an optical transmission system improved as described above.
This optical transmission system outputs an optical-frequency modulated signal by directly modulating a semiconductor laser
41
with a multichannel AM signal (e.g. AM video signals)
30
in an AM-to-FM conversion section
82
of an optical transmitter
81
. In this case, by modulating the semiconductor laser
41
into the AM signal
30
, not only the amplitude of light is modulated but also the oscillation frequency of the light is modulated. By generating the light having an oscillation frequency slightly different from that of the optical-frequency-modulated signal thus generated by a local-oscillation light source
42
and multiplexing the light and the above optical-frequency-modulated signal by a multiplexer
43
and thereafter, inputting the multiplexed light to a photodiode
44
and heterodyne-detecting the light, a wide-band (e.g. 1 to 6GHz) FM-modulated signal is generated as a beat signal of two lasers and output to an electricity-to-light conversion section
83
.
The electricity-to-light conversion section
83
inputs the FM-modulated signal to a semiconductor-laser driving amplifier
88
, directly modulates a semiconductor laser
89
for transmission in accordance with the output of the semiconductor-laser driving amplifier
88
to generate an FM optical signal, and transmits the FM optical signal to an optical-fiber cable
92
. (The above structure is disclosed in, for example, Japan Patent No. 2700622.)
The FM optical signal transmitted to the optical-fiber cable
92
is amplified by an amplifier (not illustrated) set to the middle of the optical-fiber cable
92
or the like and thereafter, optical-fiber-transmitted to each light-receiving section
93
through an optical turnout (not illustrated) set to the middle of the optical-fiber cable
92
.
The light-receiving section
93
first converts an FM optical signal into an electric signal and amplifies the electric signal by a light-to-electricity converter
96
and a preamplifier
97
constituting a light-to-electricity conversion section
95
and thereafter, demodulates the electric signal to an AM signal
31
by an FM-to-AM demodulation section
94
. The FM-to-AM demodulation section
94
is a delay-type demodulation circuit which is constituted with high-speed logic ICs
51
and
53
(e.g. AND gate), a delay section
52
, and a low-pass filter
54
through a limiter amplifier
50
and realizes wide-band demodulation.
In the case of the above conventional FM transmission system, however, when converting an AM video multiplexed signal into an FM optical signal by the semiconductor laser
41
, the CNR (carrier-to-noise ratio) is greatly deteriorated because phase noises of the semiconductor layer
41
are added to the FM optical signal. Therefore, even if the light-receiving intensity of the optical receiver
93
is raised, the sensitivity is only improved up to a certain CNR value. To obtain a desired CNR from the optical receiver
93
, it is necessary to use a semiconductor laser having a line width of approx. {fraction (1/10)} the line width of the above conventional system and a semiconductor laser having an external resonator structure. Therefore, there are problems that these semiconductor lasers are expensive and moreover, a plurality of semiconductor lasers must be used.
Moreover, a method is considered which directly converts an AM signal into an electric FM signal in a low frequency band. However, when increasing the modulation index of an FM modulator (modulation factor ≧10%), problems occur that a distortion occurs in an FM modulator, the signal quality is deteriorated due to the distortion, and thereby, preferable optical transmission cannot be realized.
Moreover, a conventional optical transmission system has problems that, because of transmitting wide-band FM signals of 1 to 6 GHz, the uniformity of frequency bands of signals is broken due to the delay characteristic of parts of an amplifier in the optical transmitter
81
or those of the optical-fiber cable
92
and a distortion due to a phase delay occurs in the AM signal
31
demodulated by the light-receiving section
93
.
The present invention is made to solve the above problems and its object is to provide an FM signal optical transmission apparatus and an FM signal optical reception apparatus having a simple structure, a low cost, and a high reception sensitivity compared to conventional ones.
SUMMARY OF THE INVENTION
The 1st invention of the present invention is an FM signal optical transmission apparatus comprising
modulation means for converting a multiple signal obtained by subcarrier-multiplexing a plurality of signals into an FM signal having a predetermined carrier frequency;
frequency conversion means for shifting an FM signal converted by said modulation means to a frequency lower than said carrier frequency; and
optical modulation means for converting an optical signal into an FM optical signal by modulating said optical signal in accordance with an FM signal output from said frequency conversion means and transmitting said FM optical signal through an optical fiber cable.
According to the above structure, an optical signal to be transmitted is obtained by performing narrow-band FM modulation and low-frequency conversion, and then intensity modulation. So neither additional circuit nor optical heterodyne detection circuit are necessary. Therefore, the circuit structu
Asakura Hiroyuki
Iida Masanori
Chan Jason
Tran Dzung
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