Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-08-20
2002-01-01
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06335814
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical transmission systems, more specifically to a system for optically transmitting an angle-modulated signal.
2. Description of the Background Art
FIG. 30
is a block diagram showing an example of the configuration of a conventional optical transmission system which transmits an angle-modulated signal. In
FIG. 30
, the optical transmission system includes an angle modulating portion
1
, an optical modulating portion
2
, an optical waveguide portion
3
, an optical/electrical converting portion
4
, an angle demodulating portion
5
and a filter F. Such optical transmission system is described, for example, in a document (K. Kikushima, et al., “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems”, IOOC'95, PD2-7, 1995, pp. 33-34.).
Next, the operation of the conventional optical transmission system structured as above will be described. As an electrical signal inputted to the angle modulating portion
1
, assumed is an analog signal such as an audio or video signal, or a digital signal such as computer data and the like. The angle modulating portion
1
converts the inputted electrical signal into an angle-modulated signal with a predetermined frequency and a predetermined angle modulation scheme to output the angle-modulated signal. The angle modulation scheme includes FM (frequency modulation) or PM (phase modulation) for an analog signal and FSK (frequency-shift keying) or PSK (phase-shift keying) for a digital signal, and is generically referred to as angle modulation hereinafter. The optical modulating portion
2
converts the inputted angle-modulated signal into an optical-modulated signal to output the optical-modulated signal. The optical/electrical converting portion
4
, which includes a photodetector having square-law-detection characteristics (a pin photo-diode, an avalanche photo-diode or the like), re-converts the optical-modulated signal transmitted by the optical waveguide portion
3
into an electrical signal to output an angle-modulated signal. The angle demodulating portion
5
converts variations in frequency (or variations in phase) of the angle-modulated signal into variations in amplitude (or variations in intensity) of an electrical signal, thereby re-generating a signal correlating with the original electrical signal. The filter F passes only a signal component corresponding to the original electrical signal (that is a signal component of the same frequency band as that of the original electrical signal) among signals outputted from the angle demodulating portion
5
.
In
FIG. 31
is shown an example of the structure of the angle demodulating portion
5
in FIG.
30
. In
FIG. 31
, the angle-modulated signal inputted from the optical/electrical converting portion
4
is branched into two signals in a branch portion
51
. One signal of the two signals obtained by the branch is provided with a predetermined delay T
p
in a delay portion
52
. A mixing portion
53
, which is generally constituted by a mixer and the like, receives the other signal outputted from the branch portion
51
and the signal outputted from the delay portion
52
to generate a product signal of these signals and output the product signal.
The conventional optical transmission system of the angle-modulated signal as described above has an advantage in the following, compared with an optical transmission system of an amplitude-modulated (AM) signal. That is, the frequency deviation (or the phase deviation) of the angle-modulated signal is set larger, so that a larger gain in angle modulation can be acquired at the optical transmission. As a result, SNR (signal-to-noise power ratio) of a demodulated signal increases, realizing transmission of a signal of good quality. Moreover, the frequency deviation (or the phase deviation) of the angle-modulated signal is increased to spread a frequency spectrum of the optical-modulated signal and suppress a peak level of the frequency spectrum, which leads to an advantage in that deterioration of signal quality due to multipath reflection on an optical transmission line is reduced.
As described above, in the conventional optical transmission system, an electrical signal to be transmitted, after being subjected to angle modulation, is converted into an optical-modulated signal to be optically transmitted, subjected to square-law-detection on a receiving side to be re-converted into an angle-modulated signal, and further subjected to angle demodulation to be the original electrical signal. Therefore, it is possible, in the conventional optical transmission system, to perform optical transmission of better quality by increasing the frequency deviation (the phase deviation) even on an optical transmission line of poor quality.
However, increasing in the frequency deviation (or the phase deviation) of the angle-modulated signal makes the frequency and band of the angle-modulated signal higher and wider. Accordingly, the conventional optical transmission system as described above, requires electrical parts for high frequencies and wide-bands in order to constitute the angle modulating portion
1
and the angle demodulating portion
5
. Connection and matching among such electrical parts for high frequencies and wide-bands are difficult and multipath reflection among the parts readily occurs. This causes deterioration of characteristics of the angle modulating portion
1
and the angle demodulating portion
5
, resulting in significant deterioration of quality of modulated/demodulated signals.
Further, in the case where an expensive electrical part for wide-bands and high frequencies (for example, the branch portion
51
and the mixing portion
53
in
FIG. 31
) is used in the angle demodulating portion
5
which is installed as a receiving terminal of an optical transmission system, when configuring an optical subscriber (optical multi-distribution) system such as a FTTH (Fiber To The Home) system, a CATV network and the like, the system cost per subscriber becomes very high to significantly degrade the system from the view point of its economy.
As explained in the above, the conventional optical transmission system is required, when optically transmitting an angle-modulated signal with a wider-band and a higher frequency, to use the electrical parts for wide-bands and high frequencies especially as constituents of the demodulating portion. Thereby, the conventional optical transmission system has a specific problem in that group delay characteristics and modulation/demodulation characteristics are easily deteriorated and economy of overall system is significantly degraded because of increase in the cost of the receiving terminal.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an optical transmission system which realizes good angle demodulation characteristics by adopting new optical signal processing and is greatly economical by constituting a receiving terminal at lower cost without electrical parts for wide-bands and high-frequencies.
The present invention has features described below in order to attain the above-mentioned object.
A first aspect of the present invention is an optical transmission system for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal into an optical-modulated signal;
an interference portion for separating the optical-modulated signal into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal,
the interference portion and the optical/electrical converting portion constituting a delayed detection system of an optical signal, and the delayed detection system performing conversion processin
Fuse Masaru
Ohya Jun
Matsushita Electric - Industrial Co., Ltd.
Negash Kinfe-Michael
Wenderoth , Lind & Ponack, L.L.P.
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