1. Field of the Invention
The present invention relates to a probe for an electrooptic sampling oscilloscope that couples an electrical field generated by a measurement signal and an electrooptic crystal, inputs a beam into this electrooptic crystal, and measures the waveform of the measurement signal by the state of the polarization of the input light.
This application is based on Japanese Patent Application, No. Hei 10-288547 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
It is possible to couple an electrical field generated by a measurement signal with an electrooptic crystal, input a laser beam into this electrooptic crystal, and observe the waveform of the measurement 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 measurement signal. The electrooptic sampling oscilloscope uses an electrooptic probe exploiting this phenomenon.
When this electrooptic sampling oscilloscope (hereinbelow, referred to as an xe2x80x9cEOS oscilloscopexe2x80x9d) 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 metal pin at the end of the electrooptic probe is not connected to 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 measurement point.
3. By using an optical 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. 7. In the electrooptic probe 1 shown in FIG. 7, a probe head 3 comprising an insulator is mounted on a tip end of a metallic probe body 2, and a metallic pin 3a is fitted into the center. An electrooptic element 4 is secured to the probe head 3. A reflecting film 4a is provided on an end surface of the electrooptic element 4 on the metallic pin 3a side, and is in contact with the metallic pin 3a. Reference numeral 5 denotes a xc2xd wavelength plate, and reference numeral 6 denotes a xc2xc wavelength plate. Reference numeral 7 and 8 denote polarized beam splitters. Reference numeral 9 denotes a xc2xd wavelength plate, and reference numeral 10 denotes a Faraday element. Reference numeral 12 denotes a collimator lens, and reference numeral 13 denotes a laser diode. Reference numerals 14 and 15 denote condensing lenses, and reference numerals 16 and 17 denote photodiodes.
In addition, the two polarized beam splitters 7 and 8, the xc2xd wavelength plate 9, and the Faraday element 10 constitute an isolator 19 that transmits the light emitted by the laser diode 13, in order to separate the light reflected by the reflecting film 4a. 
Next, referring to FIG. 7, the optical path of the laser beam emitted from the laser diode 13 is explained. In FIG. 7, reference letter xe2x80x98Axe2x80x99 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 xc2xd wavelength plate 9, and the polarized light beam splitter 7, and then transits the xc2xc wavelength plate 6 and the xc2xd wavelength plate 5, and is incident on the electrooptic element 4. The incident light is reflected by the reflecting film 4a formed on the end surface of the electrooptic element 4 on the side facing the metallic pin 3a. 
The reflected laser beam then transits the xc2xd wavelength plate 5 and the xc2xc 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 impinges 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 impinges on the photodiode 17.
Moreover, the angle of rotation of the xc2xd wavelength plate 5 and the xc2xc 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. 7, the procedure for measuring the measurement signal is explained.
When the metallic pin 3a is placed in contact with a measurement point, at the electrooptic element 4 the electrical field due to the voltage applied to the metallic pin 3a is propagated to the electrooptic element 4, and a phenomenon where the refractive index is altered due to the Pockels effect occurs. Due to this, the laser beam emitted from the laser diode 13 impinges on the electrooptic element 4, and when the laser beam is propagated along the electrooptic element 4, the polarization state of the beam changes. Then, the laser beam having this changed polarization state is reflected by the reflecting film 4a, condensed and impinged 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 measure the electrical signal applied to the metallic pin 3a. 
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. However, instead, it is possible to measure the signals by connecting a conventional measuring device such as a real time oscilloscope to the photodiodes 16 and 17 via a dedicated controller. In this way, it is possible to carry out simply broadband measurement by using the electrooptic probe 1.
In the manner described above, in the signal measurement using the electrooptic probe 1, because the metallic pin 3a must contact the measurement point, the metallic pin 3a is worn by repeated measurement so that it is necessary to replace the probe head 3. In this case, since the electrooptic element 4 which is fixed to the probe head 3 is expensive, cost is increased.
Furthermore, considering the fact that in general the type of most suitable metallic pin changes depending on the characteristics of the signal of the measurement object, then since with the abovementioned electrooptic probe 1 the metallic pin 3a is secured to the probe head 3, in selecting the most suitable metallic pin 3a to match the characteristics of the signal to be measured, it is difficult to obtain a suitable match.
In consideration of the above described situation, it is an object of the present invention to provide an electrooptic probe which can facilitate replacement of the metallic pin.
In order to address the above problems, the present invention adopts the following means.
A first aspect of the present invention is an electrooptic probe wherein:
an optical path is formed within a probe body between a base end portion and a tip end portion of the probe body;
a laser diode is disposed at an end of the optical path on the base end portion side of the probe body;
an electrooptic element is disposed at an other end of the optical path on the tip end portion side of the probe body and retained in a probe head constituting the tip end portion of the probe body;
a metallic pin is provided in the probe head with a base end thereof connected to the electrooptic element, and a tip end thereof protruding from the probe head,
a laser beam emitted from the laser diode is incident on the electrooptic element via the optical path, and this incident beam is reflected by a reflecting film provided on the electrooptic element, and the reflected beam thereof is separated and converted into an electrical signal by a photodiode; and wherein
the probe head comprises a head body for retaining the electrooptic element, and a tip member detachably provided on the head body for retaining the metallic pin.
Because of this kind of construction, with this electrooptic probe, replacement of the metallic pin can be easily performed by replacing the tip member.
A second aspect of the present invention is an electrooptic probe according to the first aspect, wherein the photodiode and the laser diode are connected to an electrooptic sampling oscilloscope, and
the laser diode generates the laser beam as a pulsed beam based on a control signal from the electrooptic sampling oscilloscope.
A third aspect of the present invention is an electrooptic probe according to the second aspect, wherein a male threaded portion is provided in one of the head body and the tip member protruding towards the other of the two, while in the other of the two, a female threaded portion is formed for engaging with the male threaded portion, and the male threaded portion and the female threaded portion are able to be engaged and disengaged.
A fourth aspect of the present invention is an electrooptic probe according to the second aspect, wherein a threaded aperture is provided in the tip member, and the tip member is secured to the head body by means of a screw disposed in the threaded aperture.
A fifth aspect of the present invention is an electrooptic probe according to the second aspect, wherein the electrooptic element and the base end of the metallic pin are connected to each other through the medium of a buffer plate for absorbing shock due to contact between the electrooptic element and the metallic pin.
Because of being structured in this manner, then with the electrooptic probe, when the tip member is attached to or detached from the probe body, damage to the electrooptic element due to contact with the metallic pin can be prevented.
A sixth aspect of the present invention is an electrooptic probe according to the first aspect, wherein the laser diode generates a continuous beam as the laser beam.
In this manner, with the electrooptic probe according to the sixth aspect, a continuous beam is generated from the laser diode and thereby it is possible to obtain a continuous output from the photodiode. Consequently, it is also possible to make measurements by connecting the photodiode via a special purpose controller to a conventional general use measuring device such as a real time oscilloscope.
A seventh aspect of the present invention is an electrooptic probe according to the sixth aspect, wherein a male threaded portion is provided in one of the head body and the tip member protruding towards the other of the two, while in the other of the two, a female threaded portion is formed for engaging with the make threaded portion, and the male threaded portion and the female threaded portion are able to be engaged and disengaged.
An eighth aspect of the present invention is an electrooptic probe according to the sixth aspect, wherein a threaded aperture is provided in the tip member, and the tip member is secured to the head body by means of a screw disposed in the threaded aperture.
A ninth aspect of the present invention is an electrooptic probe according to the sixth aspect, wherein the electrooptic element and the base end of the metallic pin are connected to each other through the medium of a buffer plate for absorbing shock due to contact between the electrooptic element and the metallic pin.
Because of being structured in this manner, then with the electrooptic probe, when the tip member is attached to or detached from the probe body, damage to the electrooptic element due to contact with the metallic pin can be prevented.