Optical communications – Multiplex – Wavelength division or frequency division
Reexamination Certificate
2000-02-23
2004-01-06
Pascal, Leslie (Department: 2633)
Optical communications
Multiplex
Wavelength division or frequency division
C398S091000
Reexamination Certificate
active
06674969
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a two-optical signal generator for use in an optical fiber link system or the like, and in particular, to two-optical signal generator for generating two optical signals, where a difference between optical frequencies or optical wavelengths of the two optical signals can be adjusted. In the specification, the difference between the optical frequencies is referred to as an optical frequency difference hereinafter, and the difference between the optical wavelength is referred to as an optical wavelength difference hereinafter.
2. Description of the Related Art
An optical fiber link system modulates a digital data signal into an optical signal, transmits a modulated optical signal to a radio base station, performs photoelectric conversion for a received optical signal to output a radio signal, which is then power-amplified and radio-transmitted from an antenna of a radio base station.
FIG. 11
is a block diagram showing a configuration of an optical fiber link system of a prior art.
Referring to
FIG. 11
, a light source
1
such as a semiconductor laser or the like modulates an optical signal according to an inputted digital data signal, and outputs a modulated optical signal as a first optical signal (optical frequency f
1
) via an optical combining circuit
3
and an optical branch circuit
4
to an optical amplifier
5
. On the other hand, a light source
2
such as a semiconductor laser or the like has its optical frequency controlled by an optical frequency controller
10
, and generates and outputs an optical signal as a second optical signal (optical frequency f
2
) via the optical combining circuit
3
and the optical branch circuit
4
to the optical amplifier
5
. In this case, a difference |f
1
−f
2
| in the optical frequency is set to, for example, a radio frequency in a millimeter-wave band of several tens to several hundreds GHz, as shown in FIG.
13
. The optical amplifier
5
amplifies a power of an inputted optical signal, and then, transmits a power-amplified optical signal to an optical receiver
200
via an optical fiber cable
300
for connecting an optical transmitter
101
and an optical receiver
200
located in the radio base station.
On the other hand, a mixture optical signal obtained by mixing the first and second optical signal branched by the optical branch circuit
4
is photoelectrically converted by a photoelectric converter
6
which comprises a high-speed photodiode with a nonlinear photoelectric conversion characteristic, and then, the photoelectrically converted signal is frequency-converted into a high-frequency signal having a frequency lower than that of the photoelectrically converted signal by a frequency converter which consists of a millimeter wave signal oscillator
7
and a mixer
8
. Then, from the components of the thus converted high-frequency signal, a high-frequency signal, which is in proportion to an optical frequency difference |f
1
−f
2
| and which has been generated by the above-mentioned nonlinear photoelectric conversion characteristic, is taken out by a band-pass filter
9
, and then, is outputted to an optical frequency controller
10
. In such an optical frequency loop circuit as configured above, based on the inputted high-frequency signal, the optical frequency controller
10
controls the optical frequency f
2
of the second optical signal generated from the light source
2
so that the above-mentioned optical frequency difference |f
1
−f
2
| becomes the constant. That is, an interference component between the two optical signals is taken out by the photoelectric converter
6
so that an oscillation frequency difference between the oscillation frequency of the light source
1
and that of the light source
2
becomes a millimeter wave frequency, the taken interference component is compared in frequency with the millimeter-wave frequency of the millimeter-wave signal generator
7
, and then, the optical frequency of the light source
2
is controlled in accordance with its error signal. The optical transmitter
101
is disclosed in, for example, a first prior art document of, R. P. Braun, et al., “Optical millimeter-wave generation and transmission experiments for mobile 60 GHz band Communications,” Electronics Letters, Vol. 32, pp. 626-627, 1996 (hereinafter referred to as a first prior art).
In the optical receiver
200
, an optical amplifier
11
receives an optical signal through the optical fiber cable
300
, and then, outputs the same optical signal to a photoelectric converter
12
. The photoelectric converter
12
comprises a high-speed photodiode having a nonlinear photoelectric conversion characteristic, photoelectrically converts the inputted optical signal into an electric signal, and outputs the same electric signal to a band-pass filter
13
. From the signal components of the photoelectrically converted signal, the band-pass filter
13
takes out a radio signal of a millimeter-wave band corresponding to the optical frequency difference f
0
=|f
1
−f
2
| which has been generated by the above-mentioned nonlinear photoelectric conversion characteristic, and then, outputs the same radio signal to a radio transmitter
14
. The radio transmitter
14
comprises a power amplifier which power-amplify the inputted radio signal, and transmits the same radio signal via an antenna
15
toward, for example, an antenna
91
connected to a radio receiver
210
shown in FIG.
12
.
FIG. 12
is a block diagram showing a configuration of the radio receiver
210
according to the first prior art.
Referring to
FIG. 12
, the radio signal received by the antenna
91
is amplified by a low-noise amplifier
92
, which then outputs the received radio signal to a mixer
94
via a band-pass filter
93
which passes therethrough only a radio signal having a frequency f
0
of the millimeter-wave band. The mixer
94
mixes the inputted radio signal with a local oscillation signal having a local oscillation frequency equal to an addition result of the above-mentioned millimeter-wave frequency f
0
generated by a millimeter-wave signal oscillator
95
to a predetermined intermediate frequency, so as to generate a received base-band signal having an intermediate frequency of a frequency difference between these two signals, and then, outputs the received base-band signal, via a band-pass filter
96
which passes therethrough only the signal component of the intermediate frequency band, and via a signal amplifier
97
to a demodulator (not shown). Then, the demodulator demodulates the received base-band signal into the original digital data signal.
Also, a second prior art document of, D. S. George et al., “Further Observations on the Optical Generation of Millimeter-wave Signals by Master/Slave Laser Side-band Injection Locking,” MWP' 97, Post-Deadline Papers, PDP-2, 1997, discloses a constitution of a two-optical signal generator (hereinafter referred to as a second prior art) utilizing a heterodyne interference of two light waves in such a configuration provided with two single-mode semiconductor lasers that an optical signal from a slave laser is intensity-modulated according to a sine-wave signal, and the resultant higher-order mode frequency of the intensity-modulated optical signal is locked into a frequency of a master laser.
Also, the following optical transmission system has been proposed as a system for transmitting optical signals using three distributed feedback semiconductor lasers.
Further, a third prior art document of, Z. Ahmed, et al., “Low phase noise millimeter-wave signal generation using a passively mode-locked monolithic DBR laser injection locked by an optical DSBSC signal,” Electronics Letters, Vol. 31, No. 15, pp.1254, 1995, discloses a two-optical signal generator (hereinafter referred to as a third prior art) utilizing a heterodyne interference of two light waves, in such a configuration that a distributed Bragg reflection-type semico
Pascal Leslie
Singh Dalzid
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