Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-01-18
2003-01-28
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S199200, C359S237000, C359S278000
Reexamination Certificate
active
06512619
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a modulation apparatus for generating a wide-band frequency modulated signal (hereinafter referred to as FM signal) through an optical frequency modulation scheme of a semiconductor laser and an optical heterodyne detection technique.
2. Description of the Background Art
FIG. 14
is a block diagram showing the configuration of a conventional modulation apparatus. Operation of such frequency modulation apparatus is described in detail in documents such as “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems”, K. Kikushima, et al, IOOC 1995 Technical Digest, Vol. 5 PD2-7, pp. 33-34, which is incorporated herein by reference. The frequency modulation apparatus shown in
FIG. 14
includes a signal source
700
, an optical modulator
702
, a local light source
703
, a first optical waveguide
706
, a second optical waveguide
708
, and an optical receiver
714
.
In the above frequency modulation apparatus, the signal source
700
produces an electrical signal Si, which is an original signal for FM modulation. The optical modulator
702
is constructed of, for example, a semiconductor laser. In general, the semiconductor laser emits light having a constant optical frequency f
1
, provided that an injection current is constant. When the injection current is amplitude-modulated, the optical frequency is also subjected to modulation, and the semiconductor laser emits an optical-frequency-modulated signal centering on the optical frequency f
1
. With this characteristic, the optical modulator
702
converts the electrical signal Si supplied from the signal source
700
into an optical-frequency-modulated signal L
1
. The local light source
703
produces an unmodulated light L
2
having a constant optical frequency f
2
. The lights L
1
and L
2
are supplied to the optical receiver
714
through the first and second optical waveguides
706
and
708
, respectively. The optical receiver
714
is constructed of a photodiode having square-law detection characteristics, for example, producing a beat signal of the supplied two lights L
1
and L
2
at a frequency f
s
equal to the optical frequency difference |f
1
−f
2
| between the two optical signals L
1
and L
2
(this operation is called optical heterodyne detection). The beat signal obtained in the above described manner is outputted as a frequency-modulated signal Sfm whose original signal is the electrical signal Si from the signal source
700
.
As described above, with the use of high frequency modulation efficiency of the semiconductor laser (more than ten times the frequency modulation efficiency in ordinary electric circuit systems), the conventional frequency modulation apparatus shown in
FIG. 14
can easily generate an extremely high-frequency, wide-band FM signal with large frequency deviation, which is difficult to be produced in the ordinary electric circuit.
However, light sources such as semiconductor lasers generally have large phase noise, compared with electric oscillators. Therefore, the above conventional modulation apparatus has a unique problem that white noise components increase at demodulation of the FM signal. More specifically, when the optical-frequency-modulated signal L
1
from the optical modulator
702
and the unmodulated light L
2
from the local light source
703
have frequency spectrums as shown in
FIG. 15A
, the frequency spectrum of the FM signal Sfm outputted from the optical receiver
714
becomes as such shown in FIG.
15
B. As shown in
FIGS. 15A and 15B
, the phase noise included in the FM signal Sfm becomes the sum of the phase noises included in the optical-frequency-modulated signal L
1
and the unmodulated light L
2
. Therefore, the white noise components increase when the FM signal is demodulated.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a frequency modulation apparatus capable of realizing high-frequency, wide-band frequency modulation and suppressing phase noise included in the frequency-modulated signal, with a combination of optical frequency modulation of a semiconductor laser and optical heterodyne detection.
The present invention has the following features to achieve the object above.
A first aspect of the present invention is directed to a frequency modulation apparatus for converting an input electrical signal into an FM signal through optical frequency modulation and optical heterodyne detection using a first light source emitting first light and a second light source emitting second light, the first light and the second light having different optical frequencies from each other; the apparatus comprising:
a first optical modulator for outputting the first light frequency-modulated with the input electrical signal as a first optical signal;
a beat signal generating part for generating an unmodulated beat signal corresponding to a carrier component of a beat signal obtained from the first and second lights through optical detection based on a square-law detection characteristic;
a frequency converter for generating the frequency-converted signal by converting frequency of the unmodulated beat signal;
a second optical modulator for generating a second optical signal by optical-amplitude-modulating or optical-intensity-modulating any one of the first optical signal and the second light with the frequency-converted signal; and
an optical receiver for receiving one of the first optical signal and the second light, which is not subjected to optical-amplitude-modulation or optical-intensity-modulation by the second optical modulator, and the second optical signal, and generating the FM signal through optical detection based on the square-law detection characteristic.
In the first aspect, two light sources (the first and second light sources) and two square-law detectors (the beat signal generating part and the optical receiver) constitute two optical heterodyne systems. A first optical heterodyne system is constructed of the first optical modulator corresponding to the first light source, the second light source as a local light source, and the beat signal generating part. A second optical heterodyne system is constructed of the first optical modulator, the second light source (local light source), and the optical receiver. With the frequency-converted signal obtained by converting the frequency of the carrier component (center frequency component) of the beat signal generated in the first optical heterodyne system, one of the two lights in the second optical heterodyne system is amplitude-modulated or intensity-modulated. Thus, the frequency modulation apparatus of the first aspect can suppress the phase noise of the FM signal, which is a beat signal generated in the second optical heterodyne system, realizing frequency modulation with low noise.
According to a second aspect, in the first aspect,
the first optical modulator generates the first optical signal through direct modulation; and
the beat signal generating part comprises
a photo-detection device for receiving the first optical signal and the second light, and generating a modulated beat signal, which is a modulated electrical signal having a center frequency equal to a difference of optical frequencies between the first optical signal and the second light, through optical detection based on the square-law detection characteristic; and
a filter for extracting a carrier component from the modulated beat signal, and outputting the carrier component as the unmodulated beat signal.
In the second aspect, the first optical signal is generated through direct modulation. Then, an unmodulated beat signal, which is the carrier component, is extracted by the filter from the modulated beat signal generated from the first optical signal and second light. With the frequency-convert signal obtained by converting the frequency of the unmodulated beat signal, one of two lights in the second optical heterodyne system is amplitude-modulat
Matsushita Electric - Industrial Co., Ltd.
Pascal Leslie
Phan Hanh
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