Electrooptic probe

Details Electrooptic probe Electrooptic probe

1999-08-16

2001-10-02

Karlsen, Ernest (Department: 2858)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S750010, C324S754120

Reexamination Certificate

active

06297650

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrooptic probe that couples an electrical field generated by a measured signal and an electrooptic crystal, makes light incident on this electrooptic crystal, and measures the waveform of the measured signal by the state of the polarization of the incident light. This application is based on Patent Application No. Hei 10-233351 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
It is possible to couple an electrical field generated by a measured signal with an electrooptic crystal, make a laser beam incident on this electrooptic crystal, and observe the waveform of the measured signal by the state of the polarization of the laser beam. It is possible to pulse the laser beam and observe with an extremely high time resolution when sampling the measured signal. An electrooptic sampling oscilloscope uses an electrooptic probe exploiting this phenomenon.
When this electrooptic sampling oscilloscope (hereinbelow, referred to as an “EOS oscilloscope”) is compared to a conventional sampling oscilloscope using an electrical probe, the following characteristics have received much attention:
1. It is easy to observe the signal because a ground wire is unnecessary.
2. Because the metallic pin at the end of the electrooptic probe is isolated from the circuit system, it is possible to realize high input impedance, and as a result of this, there is almost no degradation of the state of the measured point.
3. By using an optic pulse, broadband measurement up to the GHz order is possible.
The structure of a probe for an EOS oscilloscope in the conventional technology will be explained using FIG.
3
. In the electrooptic probe shown in
FIG. 3
, a probe head
3
comprising an insulator is mounted on the end terminal of the metallic probe body
2
, and a metallic pin
3
a
is fit into the center. Reference numeral
4
is an electrooptic element, a reflecting film
4
a
is provided on the end surface on the metallic pin
3
a
side, and is in contact with the metallic pin
3
a
. Reference numeral
5
is a ½ wavelength plate, and reference numeral
6
is a ¼ wavelength plate. Reference numeral
7
and
8
are polarized beam splitters. Reference numeral
9
is a ½ wavelength plate, and reference numeral
10
is a laser diode. Reference numerals
14
and
15
are condensing lenses, and reference numerals
16
and
17
are photodiodes.
In addition, the two polarized beam splitters
7
and
8
, the ½ wavelength plate
9
, and the Faraday element
10
comprise an isolator
19
that transmits the light emitted by the laser diode
13
, in order to split the light reflected by the reflecting film
4
a.
Next, referring to
FIG. 3
, the optical path of the laser beam emitted from the laser diode
13
is explained. In
FIG. 3
, reference letter “A” denotes the optical path of the laser beam.
First, the laser beam emitted from the laser diode
13
is converted by the collimator lens
12
into a parallel beam that travels straight through the polarized beam splitter
8
, the Faraday element
10
, the ½ wavelength plate
9
, and the polarized light beam splitter
7
, and then transits the ¼ wavelength plate
6
and the ½ wavelength plate
5
, and is incident on the electrooptic element
4
. The incident light is reflected by the reflecting film
4
a
formed on the end surface of the electrooptic element
4
on the side facing the metallic pin
3
a.
The reflected laser beam transits the ½ wavelength plate
5
and the ¼ wavelength plate
6
, one part of the laser beam is reflected by the polarized light beam splitter
7
, condensed by the condensing lens
14
, and incident on the photodiode
16
. The laser beam that has transited the polarized light beam splitter
7
is reflected by the polarized beam splitter
8
, condensed by the condensing lens
15
, and incident on the photodiode
17
.
Moreover, the angle of rotation of the ½ wavelength plate
5
and the ¼ wavelength plate
6
is adjusted so that the strength of the laser beam incident on the photodiode
16
and the photodiode
17
is uniform.
Next, using the electrooptic probe
1
shown in
FIG. 3
, the procedure for measuring the measured signal is explained.
When the metallic pin
3
a
is placed in contact with the measurement point, due to the voltage applied to the metallic pin
3
a
, at the electrooptic element
4
this electrical field is propagated to the electrooptic element
4
, and the phenomenon of the altering of the refractive index due to the Pockels effect occurs. Thereby, the laser beam emitted from the laser diode
13
is incident on the electrooptic element
4
, and when the laser beam is propagated along the electrooptic element
4
, the polarization state of the beam changes. Additionally, the laser beam having this changed polarization state is reflected by the reflecting film
4
a
, condensed and incident on the photodiode
16
and the photodiode
17
, and converted into an electrical signal.
Along with the change in the voltage at the measurement point, the change in the state of polarization by the electrooptic element
4
becomes the output difference between the photodiode
16
and the photodiode
17
, and by detecting this output difference, it is possible to observe the electrical signal applied to the metallic pin
3
a.
Moreover, in the above-described electrooptic probe
1
, the electrical signals obtained from the photodiodes
16
and
17
are input into an electrooptic sampling oscilloscope, and processed, but instead, it is possible to connect a conventional measuring device such as a real time oscilloscope at the photodiodes
16
and
17
via a dedicated controller. Thereby, it is possible to carry out simply broadband measurement by using the Electrooptic probe
1
.
However, in this electrooptic probe
1
, the probe head
3
is formed by an insulator, and the probe body
2
that supports the probe head
3
is formed from metal. Due to this, the change in the electrical field of the measured signal propagates as noise to the photodiodes
16
and
17
and the laser diode
13
via the probe body
2
, and there is the problem that the S/N ratio during measurement deteriorates.
In addition, in the EOS oscilloscope connected to the photodiodes
16
and
17
, there are cases of using a process in which the light from the electrooptic element
4
is converted into an electric signal, is divided and used as the desired sample rate, and because frequency of the noise generated from the display of the oscilloscope is about the same as the signal frequency of the measured signal steped down to a lower frequency by sampling, this kind of noise is detected by the photodiodes
16
and
17
, and there is the problem of causing deterioration of the measuring precision.
SUMMARY OF THE INVENTION
In consideration of the above, an object of the present invention is to prevent propagation of noise from the measured signals, display, etc., and improve the S/N ratio during measurement.
In order to resolve the above-described problems, the invention includes an electrooptic probe in which an optical path is established. The probe comprises a probe body having a base terminal, an end terminal, a probe head formed by an insulating body, and a supporting member comprising an insulating body and supporting said probe head, the optical path being established in the probe body between the base terminal and the end terminal of the probe body. The probe further comprises a laser diode disposed at one end of said optical path so as to be enclosed in a first enclosing portion of the base terminal of said probe body, an electrooptic element having a reflecting film and being disposed at the other end of said optical path so as to be enclosed in the end terminal of said probe body, and a metallic pin having a base portion and an end portion, the metallic pin being provided at the end terminal of said probe body and being supported by the

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