Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2000-10-27
2003-09-02
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06614511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a light wavelength dispersion measuring apparatus and a light wavelength dispersion measuring method for measuring a wavelength dispersion characteristic of an optical fiber.
2. Description of the Related Art
Hitherto, the wavelength dispersion characteristic of the optical fiber has been found from degradation of propagation time relative to the wavelength of an optical signal, namely, the group delay characteristic; generally, the wavelength dispersion characteristic of the optical fiber is represented by propagation time difference per unit length. In optical fiber communications using the optical fiber as a transmission line, distortion occurs in a signal waveform after transmission because of a relationship between the wavelength spread of an optical signal and the wavelength dispersion characteristic of the optical fiber, and the reception characteristic is degraded; this is a problem.
Further, in an optical amplification relay system, the wavelength dispersion characteristic of each optical fiber forming a part of the system is accumulated, thus the effect of a nonlinear phenomenon of the optical fiber caused by the wavelength dispersion characteristic on the transmission characteristic is extremely large. Therefore, to construct a light communication system, it is indispensable to keep track, of the wavelength dispersion characteristic in detail.
FIG. 5
is a block diagram to show the main configuration of a light wavelength dispersion measuring apparatus
20
in a related art. In
FIG. 5
, the light wavelength dispersion measuring apparatus
20
comprises an electrical oscillator
21
for supplying a single frequency signal, a tunable wavelength Laser diode source
22
provided by placing a plurality of light sources having different light wavelengths, respectively, or a tunable wavelength Laser diode source
22
of a single light source capable of oscillating a plurality of light wavelength signals, an optical fiber directional coupler
23
for branching output of the tunable wavelength Laser diode source
22
, a reference optical fiber
24
, a measured optical communication line
25
of an optical amplification relay system, etc., connecting one stage or multiple stages of optical fiber using an optical fiber or an optical amplifier, a photoelectric converter
26
for receiving output of the reference optical fiber
24
and converting the optical signal into an electric signal, a photoelectric converter
27
for receiving output of the measured optical communication line
25
and converting the optical signal into an electric signal, and a phase comparator
28
for making a comparison between phases of the electric signals output by the photoelectric converter
26
and
27
.
The electrical oscillator
21
supplies a single frequency signal for strength-modulating the optical signal output of the tunable wavelength Laser diode source
22
to the tunable wavelength Laser diode source
22
. The strength-modulated optical signal from the tunable wavelength Laser diode source
22
is branched through the optical fiber directional coupler
23
. The optical signal branched to one, which is used as a reference signal in the phase comparison, is applied to reference input of the phase comparator
28
through the reference optical fiber
24
which is short and the photoelectric converter
27
. The optical signal branched to the other is input to the phase comparator
28
through the measured optical communication line
25
and the photoelectric converter
26
. The phase comparator
28
detects the phase difference between the two optical signals. If the phase comparison is made for each wavelength, the group delay characteristic can be obtained.
That is, relative propagation time &tgr; (&lgr;) of the measured optical communication line
25
at light wavelength &lgr; is found according to the following expression (1) from measured phase difference&thgr; (&lgr;):
&tgr;(&lgr;)=&thgr;(&lgr;)/2&pgr;
f
(1)
where f is the oscillation frequency of the electrical oscillator
21
.
If the relative propagating time &tgr; (&lgr;) is converted into unit distance, kilometers (km) and the wavelength &lgr; is used to enter the horizontal axis and the relative propagation time &tgr; (&lgr;) is used to enter the vertical axis, the group delay characteristic is found and further the &tgr; (&lgr;) characteristic is differentiated by the wavelength &lgr;, whereby the wavelength dispersion characteristic can be obtained.
However, the above described light wavelength dispersion measuring apparatus
20
in the related art is adapted to change the wavelength and measure the wavelength dispersion characteristic, and thus involves a problem of prolonging the measurement time. The length of the optical fiber used with the measured optical communication line
25
changes due to change in ambient temperature during measurement, thus causing a measurement error to occur in the relative propagation time &tgr; (&lgr;); this is also a problem.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a light wavelength dispersion measuring apparatus and light wavelength dispersion measuring method for making it possible to shorten measurement time of measuring a wavelength dispersion characteristic and to correct a measurement error of relative propagation time &tgr; (&lgr;) caused by ambient temperature change.
A light wavelength dispersion measuring apparatus comprising:
a short pulse light generator for generating short pulse light;
a first photoelectric conversion unit for executing photoelectric conversion of measured pulse light input from the short pulse light generator through a device under test (DUT) and for outputting a measured pulse signal;
a second photoelectric conversion unit for executing photoelectric conversion of reference pulse light branched and input from the short pulse light generator and for outputting a reference pulse signal;
a first band-pass filter for allowing only an arbitrary frequency component to pass through from the measured pulse signal output from the first photoelectric conversion unit and for outputting a measured frequency signal;
a second band-pass filter for allowing only the same arbitrary frequency component to pass through from the reference pulse signal output from the second photoelectric conversion unit and for outputting a reference frequency signal;
a phase comparison unit for detecting a phase difference between the measured frequency signal output from the first band-pass filter and the reference frequency signal output from the second band-pass filter and for outputting a phase difference signal; and
a wavelength dispersion calculation unit for measuring a group delay amount based on the phase difference signal output from the phase comparison unit and for calculating a wavelength dispersion value.
According to the first aspect of the invention, the first photoelectric conversion unit executes photoelectric conversion of measured pulse light input from the short pulse light generator for emitting short pulse light through the device under test (DUT) and outputs a measured pulse signal, the second photoelectric conversion unit executes photoelectric conversion of reference pulse light branched and input from the short pulse light generator and outputs a reference pulse signal, the first band-pass filter allows only an arbitrary frequency component to pass through from the measured pulse signal output from the first photoelectric conversion unit and outputs a measured frequency signal, the second band-pass filter allows only the same arbitrary frequency component to pass through from the reference pulse signal output from the second photoelectric conversion unit and outputs a reference frequency signal, and the phase comparison unit detects the phase difference between the measured frequency signal output from the first band-pass filter and the reference frequency signal output from the second band-pass filter and outputs a phase difference signal, a
Minami Takao
Sakairi Yoshiyuki
Ando Electric Co. Ltd.
Fish & Richardson P.C.
Font Frank G.
Nguyen Tu T.
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