Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-05-29
2001-01-23
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200
Reexamination Certificate
active
06178026
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an analog optical transmission apparatus for optical fiber communication and, more particularly, to an analog optical transmission apparatus suitable for SCM (Sub Carrier Multiplex) transfer.
In a CATV (CAble TeleVision) system or the like, an SCM scheme is used for data transfer. When data transfer based on the SCM scheme is used for optical transfer, an intermodulation distortion due to the nonlinearity of the I-L (current vs. optical output) characteristics of a semiconductor laser often poses a problem.
An analog optical transmission apparatus having a function of compensating for such an intermodulation distortion is disclosed in Japanese Patent Laid-Open No. 5-27582 (reference
1
).
FIG. 2
shows the schematic arrangement of the analog optical transmission apparatus disclosed in reference
1
. Referring to
FIG. 2
, reference numeral
11
denotes an analog signal source;
12
, a semiconductor laser connected to the analog signal source
11
;
13
, an optical fiber for transmitting an optical signal output from the semiconductor laser
12
;
14
, an optical fiber coupler optically coupled to the optical fiber
13
;
15
, a light-receiving element for receiving the output light from the optical fiber coupler and converting the light into an electrical signal;
16
, an amplifier for amplifying the output signal from the light-receiving element
15
;
17
, a delay line for delaying the output from the analog signal source; and
18
, a differential amplifier for differentially amplifying the output from the amplifier
16
and the output from the analog signal source
11
through the delay line.
The semiconductor laser
12
is intensity-modulated by the analog signal source
11
and outputs an optical signal to the optical fiber
13
. At this time, part of the optical signal passing through the optical fiber
13
is extracted by the optical fiber coupler
14
. The optical signal extracted by the optical fiber coupler
14
is converted into an electrical signal by the light-receiving element
15
. The electrical signal converted by the light-receiving element
15
contains an intermodulation distortion (cross modulation distortion) generated by the semiconductor laser
12
. The electrical signal containing the distortion component is amplified by the amplifier
16
and input to the first input terminal of the differential amplifier
18
. The signal from the analog signal source
11
is input to the second input terminal of the differential amplifier
18
through the delay line
17
.
When the amplitude and phase of the signal output from the analog signal source
11
match those of the signal output from the light-receiving element
15
, the carrier wave (carrier) contained in the signal is canceled as a common mode signal of the differential amplifier
18
, so only the distortion component is extracted. At this time, the delay line
17
adjusts the phase of the signal output from the analog signal source
11
and the phase of the signal output from the light-receiving element
15
such that the two phases match. The signal corresponding to only the distortion component, which is output from the differential amplifier
18
, is superposed on the signal to be output from the analog signal source
11
to the semiconductor laser
12
. When the amplitude and phase of the distortion generated by the semiconductor laser
12
are inverted to those of the distortion component output from the differential amplifier
18
, the distortion can be canceled.
However, in the conventional analog optical transmission apparatus, distortion compensation in a broadband is impossible in principle. More specifically, in the analog optical transmission apparatus shown in
FIG. 2
, the light emitted by the semiconductor laser
12
is monitored by the optical fiber coupler
14
inserted midway in the optical fiber
13
and by the light-receiving element
15
. The distortion component generated by the semiconductor laser
12
, which is detected upon monitoring, is superposed on the signal from the analog signal source
11
to the semiconductor laser
12
. At this time, a time lag occurs between the timing when the distortion is generated by the semiconductor laser
12
and the timing when the distortion component is superposed on the signal output from the analog signal source
11
to the semiconductor laser
12
. That is, the phases of the distortion components cannot be made to match in principle. In this case, the distortion cannot be compensated for in a broadband.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an analog optical transmission apparatus capable of compensating for a distortion in a broadband.
It is another object of the present invention to provide an analog optical transmission apparatus capable of obtaining a stable distortion compensation effect independently of a change in ambient temperature and a change over time.
In order to achieve the above objects, according to the present invention, there is provided an analog optical transmission apparatus comprising a semiconductor laser module including a semiconductor laser for converting a multi-carrier analog signal into an optical signal and outputting the optical signal and a light-receiving element for receiving the optical signal from the semiconductor laser and converting the optical signal into an electrical signal, filter means for extracting only a specific distortion component from the electrical signal output from the light-receiving element, distortion level detection means for detecting a level of the distortion component output from the filter means, control signal generation means for generating a control signal corresponding to the distortion component level output from the distortion level detection means, and distortion erasure means for generating a distortion signal having the same level and phase as those of a distortion from the semiconductor laser on the basis of the control signal from the control signal generation means and canceling the distortion from the semiconductor laser using the generated distortion signal.
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patent: 4611352 (1986-09-01), Fujito et al.
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patent: 5012475 (1991-04-01), Campbell
patent: 5034334 (1991-07-01), Flynn et al.
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patent: 5077619 (1991-12-01), Toms
patent: 5161044 (1992-11-01), Nazarathy et al.
patent: 5243613 (1993-09-01), Gysel et al.
patent: 5257124 (1993-10-01), Glaab et al.
patent: 5680238 (1997-10-01), Masuda
patent: 5745275 (1998-04-01), Giles et al.
patent: 5850305 (1998-12-01), Pidgeon
patent: 56-4948 (1981-01-01), None
patent: 59-36446 (1984-02-01), None
patent: 59-207756 (1984-11-01), None
patent: 4-69988 (1992-03-01), None
patent: 5-167161 (1993-07-01), None
patent: 5-275782 (1993-10-01), None
patent: 5-299775 (1993-11-01), None
Chan Jason
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
Patel Sanjay
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