Angle modulator

Details Angle modulator Angle modulator

2000-05-19

2001-04-03

Dang, Hung Xuan (Department: 2872)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S238000

Reexamination Certificate

active

06211996

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a modulator for generating a wide-band angle-modulated signal (phase-modulated signal or frequency-modulated signal), more specifically, through and an optical homodyne detection technique.
2. Description of the Background Art
FIG. 26
is a block diagram showing the configuration of a conventional angle modulator. Operation of such angle modulator 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 angle modulator shown in
FIG. 26
includes an optical frequency controller
1000
, a signal source
1001
, a local light source
1004
, an optical modulator
1005
, an optical coupler
1006
, and a photo-detector
1007
.
In the above angle modulator, the signal source
1001
produces an electrical signal, which is an original signal for angle modulation. The optical modulator
1005
is implemented as, for example, a semiconductor laser. In general, the semiconductor laser emits a 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 such characteristic, the optical modulator
1005
converts the electrical signal supplied by the signal source
1001
into an optical-frequency-modulated signal for output.
FIG. 27B
is a schematic diagram illustrating a frequency spectrum of light outputted from the optical modulator
1005
.
The local light source
1004
produces an unmodulated light having a constant optical frequency f
2
.
FIG. 27A
is a schematic diagram illustrating a frequency spectrum of light outputted from the light source
1004
. The optical signal from the optical modulator
1005
and the light from the local light source
1004
are coupled by the optical coupler
1006
, and then supplied to the photo-detector
1007
.
The photo-detector
1007
is implemented as a photodiode having square-law detection characteristics, for example. The photo-detector
1007
produces a beat signal of the two input lights at a frequency fc 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 an angle-modulated signal (frequency-modulated signal) of the carrier frequency fc, and its original signal is the electrical signal from the signal source
1001
.
FIG. 27C
is a schematic diagram illustrating a frequency spectrum of the signal outputted from the optical detector
1007
.
The optical frequency controller
1000
controls one or both of the center optical frequency f
1
of the optical signal outputted from the optical modulator
1005
and the optical frequency f
2
of the light outputted from the local light source
1004
to stabilize the center frequency fc of the angular-modulated signal outputted from the optical detector
1007
.
As described above, with the use of high frequency modulation efficiency by optical signal processing (more than ten times the frequency modulation efficiency in ordinary electric circuit systems), the conventional angle modulator can easily generate an extremely high-frequency, wide-band angle-modulated signal (with large frequency deviation or phase deviation), which is difficult to be produced in the ordinary electric circuits.
However, light sources such as semiconductor lasers generally have large phase noise (oscillation spectrum line width), compared with electric oscillators. Phase noise included in the light from the local light source is represented by &Dgr;&ngr;
1
in
FIG. 27A
, while the phase noise included in the optical signal from the optical modulator is represented by &Dgr;&ngr;
2
in FIG.
27
B.
The angle-modulated signal obtained as the beat signal of these two lightwaves has, as shown in
FIG. 27C
, phase noise equal to the sum of the phase noises of the two lightwaves (&Dgr;&ngr;+&Dgr;&ngr;
2
). The phase noises are simply summed because these lightwaves from the light sources have no interrelationship in phase each other. When the angle-modulated signal is demodulated, the phase noise is also demodulated and becomes large white (intensity) noise. This white noise produces serious deterioration of a demodulated signal in quality.
Further, the conventional angle modulator has to always control and adjust the optical frequencies of the signals from the two light sources (or the difference therebetween). Therefore, the conventional angle modulator requires a control circuit for control and adjustment, for example, resulting in a complicated configuration.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an angle modulator capable of realizing extremely high-frequency, wide-band angle modulation through optical signal processing, and suppressing phase noise included in the angle-modulated signal with a simple configuration to improve noise characteristics.
The present invention has the following features to achieve the object above.
The first aspect of the present invention is directed to an angle modulator for converting an input modulating signal into an angle-modulated signal through homodyne detection, comprising:
a light source emitting a light;
an optical branching part branching the light emitted by the light source into a first light and a second light;
an optical intensity modulator performing optical intensity modulation or optical amplitude modulation on the first light with a first electrical signal having a predetermined frequency fc, and outputting an optical-intensity-modulated or optical-amplitude-modulated signal as a first optical signal;
a first optical angle modulator performing optical angle modulation on the second light with a second electrical signal, which is the input modulating signal, and outputting an optical-angle-modulated signal as a second optical signal;
an optical coupler coupling the first and second optical signals; and
a photo-detector with a square-law detection characteristic converting an optical signal outputted from the optical coupler into an electrical signal and outputting the electrical signal as the angle-modulated signal having a carrier frequency fc.
In the first aspect, in the homodyne structure that subjects two lights from the same light source to optical intensity modulation and optical angle modulation and then carries out square-law detection, one branched light is optical-intensity-modulated with the first electrical signal having the frequency fc, while the other is optical-angle-modulated with the second electrical signal, thereby down-converting the optical angle modulation spectrum and generating an angle-modulated signal at the frequency fc with the second electrical signal as the original signal. Thus, it is possible to suppress a phase noise component in the light source and achieve wide-band angle modulation with suppressed noise. Moreover, a plurality of light sources are not required, and optical frequency control over such light sources is also not required. Therefore, wide-band angle modulation can be achieved with a simple structure.
According to a second aspect, in the first aspect,
the optical intensity modulator performs single-sideband optical intensity modulation or single- sideband optical amplitude modulation.
In the second aspect, in the homodyne structure that subjects two lights from the same light source to optical intensity modulation and optical angle modulation and then carries out square-law detection, single-sideband optical modulation is used as optical intensity modulation (or optical amplitude modulation). Thus, the occurrence of an undesired component other

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