Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Using radiant energy
Reexamination Certificate
1998-10-06
2001-06-26
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Using radiant energy
C324S12100R, C324S754120
Reexamination Certificate
active
06252387
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-optic sampling oscilloscope, in which an electric field generated by a signal to be measured is coupled to an electro-optic crystal, an optical pulse is caused to enter this electro-optic crystal, and the waveform of the signal to be measured is observed by means of the polarization state of the optical pulse. In particular, the present invention relates to an electro-optic sampling oscilloscope that is provided with characteristic features in the generation of the optical pulse.
This application is based on patent application No.Hei 9-273157 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
It is possible to observe the waveform of a signal to be measured by coupling the electric field generated by the signals to be measured to an electro-optic crystal, causing laser light to enter this electro-optic crystal, and using the polarization state of the laser light. Here, it is possible to use this laser light in pulse form, and to conduct measurement with extremely high time resolution when the sampling of the signal to be measured is conducted. An electro-optic sampling oscilloscope employs an electro-optic probe which takes advantage of this phenomenon.
In comparison with conventional sampling oscilloscopes which employ electrical probes, such an electro-optic sampling oscilloscope (herein below termed an “EOS” oscilloscope) has the following characteristic features:
(1) When signals are measured, a ground wire is not required, so that measurement is simplified.
(2) The metal pin which is at the lead end of the electro-optic probe is isolated from the circuit system, so that it is possible to realize a high input impedance, and as a result, the state at the point at which measurement is conducted is essentially free of fluctuations.
(3) Since optical pulses are employed, measurement is possible in a broad band up to the order of GHz.
FIG. 5
serves to explain the measurement concept of the electro-optic probe in an EOS oscilloscope.
As shown in
FIG. 5
, a metal pin
21
is provided at the lead end of the electro-optic probe, and by placing this in contact with the signal line
31
which is the subject of measurement, an electric field
23
is generated based on the measured signal. In order to couple the electric field generated with an electro-optic crystal
22
, the electro-optic crystal
22
is provided at the end of the metal pin
21
. With respect to this electro-optic crystal
22
, as a result of the Pockels effect, which is a primary electro-optical effect, the index of refraction of the electro-optic crystal changes in accordance with the coupled electric field strength, so that when an optical pulse
25
is inputted in this state, the polarization state of the optical pulse changes. The optical pulse
25
which experiences a change in polarization is reflected by reflection mirror
24
, which is a multi-layered dielectric film mirror, and is guided to the light receiver
26
, which serves as the input part of the polarization detecting optical system within the electro-optic probe (Shinagawa, et al: ““A High-Impedance Probe Based on Electo-Optic Sampling,” Proceeding of the 15
th
Meeting on Lightwave Sensing Technology, May 1995, pp 123-129).
Next, the structure of the EOS oscilloscope will be explained using FIG.
6
.
The EOS oscilloscope comprises an EOS oscilloscope main body
1
and an electro-optic probe
2
. The optical pulse
25
explained in the discussion of
FIG. 5
is generated in the optical pulse output circuit
4
on the basis of the trigger signal from a trigger circuit
3
, and is supplied to the electro-optic probe
2
. Additionally, once the optical pulse has experienced a change in polarization, the detection of the polarization thereof and the like are conducted by the polarization detecting optical system (not depicted in the figure) within the electro-optic probe
2
, and the signal thereof is inputted into the EOS oscilloscope main body
1
. Additionally, the amplification and A/D conversion of the signal are conducted by A/D circuit
5
, and processing in order to display the signal which is the subject of measurement, and the like, is conducted by processing circuit
6
.
FIG. 7
shows the details of the optical pulse output circuit
4
in FIG.
6
. From
FIG. 6
, first, timing circuit
31
generates a timing signal, which is the optical pulse generating timing, based on the trigger signal from trigger circuit
3
. Next, optical pulse generating circuit
32
generates a sample optical pulse based on the timing signal of timing circuit
31
. Since the amount of light is insufficient with this sample optical pulse, the sample optical pulse is optically amplified by optical amplifier circuit
33
, and the output of this optical amplifier circuit
33
is supplied to electro-optic probe
2
as an optical pulse.
When the sample optical pulse from the optical pulse generating circuit
32
is amplified by the optical amplifier circuit
33
, ASE (Amplified Spontaneous Emitting), which is amplified spontaneously emitted light which is unnecessary and does not contribute to sampling in the spectrum or to sampling along the time axis, is generated.
FIG. 8
shows an example of the spectral analysis of output from the optical amplifier circuit
33
in the optical pulse output circuit
4
shown in
FIG. 7. A
sample optical pulse is outputted from the optical pulse generating circuit
32
which has essentially a single frequency, and light having other frequencies, which constitutes noise, is at a sufficiently low level that it can be ignored; however, light which is unnecessary and does not contribute to the sampling in the spectra is generated by optical amplifier circuit
33
, so that light having frequencies other than the frequency outputted by the optical pulse generating circuit
32
is also outputted, and reaches a level at which it can not be ignored.
Furthermore,
FIG. 9
shows an example of output from the optical amplifier circuit
33
with respect to time. It can be seen from
FIG. 9
that, by means of optical amplifier circuit
33
, light is also outputted by optical pulse generating circuit
32
at times other than those during which the sample optical pulse is generated, so that unnecessary light which does not contribute to the sampling along the time axis is produced.
In this way, while optical amplifier circuit
33
is necessary on the one hand in order to output an optical pulse having a sufficient amount of light, this contains unnecessary light which does not contribute to the sampling in the spectrum, and light is emitted by the optical pulse generating circuit
32
at times other than those during which the sample optical pulse is generated, so that as a result of such noise, the measurement accuracy of the EOS greatly decreases.
SUMMARY OF THE INVENTION
The present invention was created in light of the above circumstances; it has as an object thereof to provide an electro-optic sampling oscilloscope which is capable of achieving a reduction in noise in the output optical pulse, and as a result, is capable of an increase in measurement accuracy.
Therefore, the present invention provides an electro-optic sampling oscilloscope in which an electric field generated by a signal to be measured is coupled with an electro-optic crystal, and an optical pulse outputted from an optical pulse output circuit is caused to enter the electro-optic crystal, and the waveform of the signal to be measured is observed using the polarization state of the optical pulse. And the optical pulse output circuit has as the input light thereof a sample optical pulse amplified by an optical amplifier circuit, and outputs, as an optical pulse, the output from an optical filter which blocks the propagation of the spontaneously emitted light of the optical amplifier circuit.
By means of this, it is possible to reduce the unnecessary light which does not contribute to the sampling in the spectra, and as a result, it is possible to
Endou Yoshio
Kikuchi Jun
Matsuhiro Kazuyoshi
Nagatsuma Tadao
Shinagawa Mitsuru
Ando Electric Co., Ltd
Darby & Darby
Snow Walter E.
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