Optical communications – Transmitter – Having particular modulation
Reexamination Certificate
2000-03-20
2003-12-02
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter
Having particular modulation
C398S163000
Reexamination Certificate
active
06658216
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical transmission system used for optical communication, CATV, optical measurement, mobile communication and the like, and also relates to a voltage controlled oscillator used for the optical transmission system.
2. Description of the Related Art
In recent years, in video monitoring systems, CATV, subscriber systems, mobile communication systems and the like, optical transmission of multi-channel video and audio signals or data has gotten into practical use by making full use of the low loss and wideband characteristics of optical fibers. In these systems, multi-channel signals are electrically multiplexed by a plurality of subcarriers having different frequencies to obtain AM signals, and the AM signals are converted into an optical signal by directly modulating a semiconductor laser or the like and transmitted via an optical fiber. The modem for the video optical transmission of the AM signals is simple in structure and low in cost; however, the receiver requires high optical input power to obtain a C/N ratio (carrier-to-noise ratio) for ensuring desired video quality. In addition, in mobile communication, the intensity levels of audio and data signals to be transmitted change significantly depending on the movement of terminals; therefore, a high dynamic range is required to cope with the signal change. Furthermore, transmission is apt to be adversely affected by distortion due to reflected waves during optical conversion at the semiconductor laser or in the middle of transmission via the optical fiber, thereby causing a problem. Moreover, an amplifier having excellent linearity is required for an AM signal amplifier.
A system has been proposed wherein AM signals multiplexed by subcarriers so as to enhance resistance against distortion and noise are converted into an FM signal as a whole and transmitted optically. The modulation index is required to be increased to obtain a desired C/N ratio in optical transmission. Accordingly, a system has also been proposed wherein the frequency of a semiconductor laser is modulated directly to obtain an FM signal having a high modulation index.
FIG. 6
is a view showing the configuration of a conventional FM signal optical transmission system. The AM/FM converter
62
of a transmitter
61
directly modulates a semiconductor laser
41
by using multi-channel AM video signals and outputs an optical frequency-modulated signal. Optical amplitude modulation and oscillation frequency modulation are carried out simultaneously by directly modulating the semiconductor laser
41
by using AM signals
30
. By a multiplexer
43
, this optical signal is multiplexed with the light of a local oscillation light source
42
having an oscillation frequency slightly different from that of the optical signal, input to a photodiode
44
, and optically heterodyne-detected, thereby obtaining an FM-modulated signal having a wideband (1 to 6 GH for example) as a beat signal for the two lasers. An electric/optic converter
63
directly modulates a transmission semiconductor laser
69
by using a semiconductor laser drive amplifier
68
to convert the FM-modulated signal into an optical signal, and the optical signal is transmitted via an optical fiber
72
(Patent Publication No. 2700622 for example).
Hereafter, the optical signal is amplified by an optical amplifier or the like, and transmitted to individual receivers via optical branching devices. Only one receiver
73
is shown here. At the receiver
73
, an optic/electric converter
75
converts the optical signal into an electric signal by using a photo-detector
76
, amplifies the electric signal by using a preamplifier
77
, and then an FM demodulator
74
demodulates the electric signal into the original AM signals
31
. This FM demodulator
74
is a delay-type demodulation circuit comprising high-speed logic ICs
51
and
53
(AND gates for example), a delay device
52
and a low-pass filter
54
via a limiter amplifier
50
, and is capable of carrying out wideband demodulation.
However, in the above-mentioned system, when AM signals are converted into an FM signal, the phase noise of the semiconductor laser is added to the FM signal, thereby lowering the C/N ratio. Therefore, even if the intensity of optical reception is raised, it is impossible to improve the sensitivity so as to have a constant C/N ratio value or more. To obtain a desired C/N ratio, a semiconductor laser is required to have an about one-tenth line width of that of a conventional semiconductor laser, and a semiconductor laser having an external oscillator structure or the like has been used. However, the semiconductor itself is expensive, and a plurality of semiconductors must be used. In order to carry out optical heterodyne detection with low noise, the temperature and drive current of the semiconductor laser must be controlled and stabilized highly accurately, whereby the configuration becomes complicated and the whole system becomes high in cost. In addition, this causes a problem of taking a longer time until the whole system operates stably after power on.
Furthermore, it is considered that AM signals are converted directly into an electrical FM signal; however, if the modulation index at the FM demodulator is increased (the degree of modulation ≧10%), distortion occurs at the FM modulator, and this distortion deteriorates the quality of the signal, whereby it is impossible to carry out satisfactory optical transmission.
Moreover, the demodulator also uses a plurality of expensive high-speed logic ICs; therefore, this causes a problem in practical utility when it is assumed to be used for subscriber systems.
SUMMARY OF THE INVENTION
In consideration of these problems in the conventional FM signal optical transmission system, the present invention is intended to provide an FM signal optical transmission system and an FM signal optical transmitter having a simple configuration, low distortion and excellent reception sensitivity, and also to provide a voltage controlled oscillator at low cost in order to attain the FM signal optical transmission system and the FM signal optical transmitter.
The 1st invention of the present invention (corresponding to claim 1) is a voltage controlled oscillator for controlling the oscillation frequency of an output signal depending on an input voltage, being characterized to receive a modulated voltage and output an FM signal having a band including a fundamental oscillation frequency and plural higher-order harmonic components of said fundamental oscillation frequency.
The 2nd invention of the present invention (corresponding to claim 2) is a voltage controlled oscillator in accordance with said 1st invention, wherein a frequency band including only the predetermined higher-order harmonic component is filtered at the output stage.
The 3rd invention of the present invention (corresponding to claim 3) is an FM signal optical transmitter comprising a signal processor for outputting an FM signal having a band including a fundamental oscillation frequency and plural higher-order harmonic components of said fundamental oscillation frequency, a band-pass filter for taking out only the predetermined-order harmonic signal component from said FM signal output from said signal processor, a frequency converter for shifting the taken-out harmonic signal component to the lower frequency side or the higher frequency side, and an electric/optic converter for converting the output signal of said frequency converter into an optical signal.
The 4th invention of the present invention (corresponding to claim 4) is an FM signal optical transmitter comprising a signal processor for outputting an FM signal having a band including a fundamental oscillation frequency and plural higher-order harmonic components of said fundamental oscillation frequency, a band-pass filter for taking out only the predetermined-order harmonic signal component from said FM signal output from said signal processor, a frequency multiplier that directl
Adachi Hisashi
Asakura Hiroyuki
Iida Masanori
Li Shi K.
Matsushita Electric - Industrial Co., Ltd.
Pascal Leslie
Smith , Gambrell & Russell, LLP
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