Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-08-26
2003-01-28
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06512621
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to FM modulators, and more specifically to an FM modulator for generating a wide-band frequency-modulated signal (hereinafter referred to as an FM signal) using a semiconductor laser and optical heterodyne detection.
2. Description of the Background Art
FIG. 7
is a block diagram showing the structure of a conventional FM modulator. The FM modulator with this structure is shown, for example, in a reference (K. Kikushima, et al, “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems”, IOOC '95 Technical Digest, Vol. 5, PD2-7, pp.33-34). In
FIG. 7
, the FM modulator includes a signal source
600
, a driving amplifier
602
, a frequency modulation laser (hereinafter referred to as an FM laser)
604
, a local light source
605
, and an optical-electrical converting portion
606
.
In the above structured FM modulator, the signal source
600
outputs an electrical signal which is an original signal for FM modulation and the driving amplifier
602
amplifies the electrical signal. The FM laser
604
, which is structured of a semiconductor laser element and the like, for example, oscillates light having a wavelength &lgr;
1
on condition that an injection current is constant. When the injection current is amplitude-modulated, the outputted light is modulated in an oscillation wavelength (optical frequency) as well as in intensity, and the FM laser
604
outputs an optical frequency-modulated signal having the wavelength &lgr;
1
at the center. The local light source
605
outputs unmodulated light having a wavelength &lgr;
0
which is different from the oscillation wavelength &lgr;
1
of the FM laser
604
by a prescribed amount &Dgr;&lgr;
1
. The outputted optical signal from the FM laser
604
and the outputted light from the local light source
605
are combined to be inputted to the optical-electrical converting portion
606
. The optical-electrical converting portion
606
is structured of a photodiode having a square-law detection characteristic, and the like, and generally has the properties of converting an optical intensity modulation component of the inputted light into a current amplitude modulation component (hereinafter referred to as an optical intensity modulation/direct detection component: an IM-DD component) and, when two lightwaves having different wavelengths are inputted, generating a beat component of the two lightwaves at a frequency corresponding to the wavelength difference (this operation is called an optical heterodyne detection). Accordingly, the optical-electrical converting portion
606
outputs the beat signal of the outputted optical signal from the FM laser
604
and the outputted light from the local light source
605
at a frequency corresponding to the wavelength difference &Dgr;&lgr; between the two lightwaves.
The beat signal obtained as described above is an FM signal taking the electrical signal from the signal source
600
as an original signal. Therefore, by using the appropriate FM laser
604
and local light source
605
, it is possible to easily generate a high-frequency and wide-band FM signal having a center frequency (carrier frequency) more than several GHz and frequency deviation more than several hundred MHz, which it is difficult to realize in an FM modulator with an ordinary electric circuit.
In the conventional FM modulator having the above structure, a carrier-to-noise ratio (hereinafter referred to as a CNR), which shows the quality of the FM signal, is improved as the frequency deviation in the FM laser
604
increases and as spectral line widths of the FM laser
604
and the local light source
605
become narrower. The spectral line widths of these two light sources are parameters depending on the composition and structure of each light source and cannot be changed greatly by limitations such as use conditions and the like. However, if the amplitude of the inputted signal to the FM laser
604
is increased, it is possible to increase the frequency deviation and thus improve the CNR. However, since the laser light source has a threshold characteristic, when the amplitude of the inputted signal is increased to more than a prescribed degree, a distortion characteristic is extremely deteriorated due to clipping of the signal amplitude and the like. Furthermore, the outputted signal level of the driving amplifier
602
has a limit (saturated level), and if the outputted signal level is increased over a prescribed level, the distortion characteristic is sharply deteriorated. Therefore, the amplitude increase of the inputted signal to the FM laser
604
is limited, and it is thus disadvantageously difficult to improve the CNR to more than a prescribed degree.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an FM modulator capable of further improving a CNR.
In order to achieve the above object, the present invention has the following feature.
A first aspect of the present invention is to an FM modulator for converting an electrical signal into a frequency-modulated signal by an optical heterodyne method, comprising:
a branch portion for outputting, when the electrical signal is inputted, a phase-uninverted signal (hereinafter referred to as an in-phase signal) and a phase-inverted signal (hereinafter referred to as an opposite phase signal);
a first frequency modulation light source element (hereinafter referred to as a first FM light source element) having a property of uniquely converting an amplitude change in the inputted electrical signal into an optical frequency change of outputted light, for converting the in-phase signal into a frequency-modulated first optical signal having a center wavelength &lgr;
1
;
a second frequency modulation light source element (hereinafter referred to as a second FM light source element) having a property of uniquely converting an amplitude change in the inputted electrical signal into an optical frequency change of outputted light, for converting the opposite phase signal into a frequency-modulated second optical signal having a center wavelength &lgr;
2
; and
an optical-electrical converting portion for subjecting the first and second optical signals to optical heterodyne detection and then generating a beat signal at a frequency corresponding to a wavelength difference &Dgr;&lgr;(=|&lgr;
1
−&lgr;|) between both of the optical signals.
As described above, in accordance with the first aspect, since the first and second FM light source elements perform modulating operation by electrical signals having an opposite phase relationship with each other, polarities of the frequency deviation in outputted lightwaves (the first and second optical signals) from the first and second FM light source elements also have an opposite phase relationship with each other. That is, when the first optical signal is deviated to a high frequency side, the second optical signal is deviated to a low frequency side, and on the contrary, when the first optical signal is deviated to a low frequency side, the second optical signal is deviated to a high frequency side. Therefore, in the optical-electrical converting portion, the frequency deviation of a beat signal obtained as a difference signal between these two optical signals is the sum of the frequency deviation of the first optical signal and the frequency deviation of the second optical signal. Therefore, compared to the conventional FM modulator, the frequency deviation of the outputted signal is increased, allowing great improvement in a CNR performance.
According to a second aspect of the present invention, in the first aspect, the FM modulator further comprises an amplitude adjusting portion inserted at least either between the branch portion and the first FM light source element or between the branch portion and the second FM light source element, for adjusting amplitude of the inputted electrical signal to equate frequency deviation of the first and seco
Furusawa Satoshi
Fuse Masaru
Utsumi Kuniaki
Chan Jason
Leung Christina Y
Wenderoth , Lind & Ponack, L.L.P.
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