Abstract:
An electrooptic probe which can facilitate replacement of a metallic pin. A probe head constituting a tip end portion of a probe body including a head body for retaining an electrooptic element and a tip member detachably provided on the head body for retaining the metallic pin.

Description:
BACKGROUND OF THE INVENTION 
     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 “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 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  3   a  is fitted into the center. An electrooptic element  4  is secured to the probe head  3 . A reflecting film  4   a  is provided on an end surface of the electrooptic element  4  on the metallic pin  3   a  side, and is in contact with the metallic pin  3   a.  Reference numeral  5  denotes a ½ wavelength plate, and reference numeral  6  denotes a ¼ wavelength plate. Reference numeral  7  and  8  denote polarized beam splitters. Reference numeral  9  denotes a ½ 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 ½ 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  4   a.    
     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 ‘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 then 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 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 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. 7, the procedure for measuring the measurement signal is explained. 
     When the metallic pin  3   a  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  3   a  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  4   a,  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  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. 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  3   a  must contact the measurement point, the metallic pin  3   a  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  3   a  is secured to the probe head  3 , in selecting the most suitable metallic pin  3   a  to match the characteristics of the signal to be measured, it is difficult to obtain a suitable match. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section view of an electrooptic probe schematically showing an embodiment of the present invention. 
     FIG. 2 is a plan view of the electrooptic probe shown in FIG.  1 . 
     FIG. 3 is a cross-section view of a probe head showing an example for the case where a buffer plate is disposed between a metallic pin and an electrooptic element in the electrooptic probe of FIG.  1 . 
     FIG. 4 is a cross-section view of a probe head showing a condition when a tip member and a head body are secured by a different means to that of FIG.  1 . 
     FIG. 5 is a cross-section view showing an example for the case where a buffer plate is disposed between a metallic pin and an electrooptic element in the probe head of FIG.  4 . 
     FIG. 6 is a cross-section view showing an example for the case where the metallic pin is divided into a base end portion and a tip end portion, in the probe head of FIG.  4 . 
     FIG. 7 is a simplified diagram of the electrooptic probe schematically showing the conventional technology of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be explained with reference to the drawings. 
     FIG.  1  and FIG. 2 are respectively cross-sectional and plan views of an electrooptic probe  21  illustrating an embodiment of the present invention. This electrooptic probe  21  shows the schematic structure of an optical path  23  formed inside a probe body  22 . 
     A tip end portion  22   a  of the probe body  22  is formed as a probe head  24 , and in a base end  22   b  of the probe body  22  a laser diode  25  is accommodated. The laser diode  25  is positioned at one end  23   a  on the base end  22   b  side of the probe body  22  in the optical path  23 , and connected to an EOS oscilloscope, omitted from the drawings. 
     At an other end  23   b  on the tip end portion  22   a  side of the probe body  22  in the optical path  23 , an electrooptic element  26  is disposed. The electrooptic element  26  is retained by the probe head  24 , and on an end surface  26   a  of the electrooptic element  26  on the tip end portion  22   a  side of the probe body  22 , a reflecting film  28  is formed. 
     Moreover, the probe head  24  comprises a head body  30  for securing to the electrooptic element  26 , and a tip member  31  further provided on a tip end of the head body  30 . As shown in the figure, a male threaded portion  30   a  is provided on the head body  30  protruding towards the tip member  31  side, while in the tip member  31 , a female threaded portion  31   a  is provided so as to be engagable with the male threaded portion  30   a.  By means of the male threaded portion  30   a  and the female threaded portion  31   a,  the tip member  31  can be attached to and detached from the head body  30 . 
     Furthermore, a metallic pin  32  is secured to the tip member  31 . With the metallic pin  32 , a base end  32   a  thereof is connected to the electrooptic element  26 . Moreover, a tip end  32   b  thereof protrudes from the tip member  31 . 
     As shown in FIG. 1, on the optical path  23 , in order from the right in the figure, a collimator lens  33 , a polarized beam splitter  34 , a Faraday element  35 , a polarized beam splitter  37 , and a ¼ wavelength plate  38  are disposed. In addition, at the positions corresponding to the polarized beam splitters  34  and  37  on the side of the optical path  23 , photodiodes  41  and  42  are respectively installed. These photodiodes  41  and  42  are connected to an EOS oscilloscope, and convert the incident beam into an electrical signal, and can send the signal to the EOS oscilloscope. 
     In addition, the polarized beam splitters  34  and  37  can function as an isolator that separates a part of the light transiting the optical path  23 , and makes this incident on the photodiodes  41  and  42 . 
     When the electrooptic probe  21  is used in signal measurement, the tip end  32   b  of the metallic pin  32  is placed in contact with the measurement point, and the EOS oscilloscope is activated. Thereby, based on the control signal generated from the EOS oscilloscope, a laser beam is emitted from the laser diode  25 , and this laser beam is converted into a parallel beam by the collimator lens  33 , transits the optical path  23 , and arrives at the electrooptic element  26 . 
     The laser beam that has arrived at the electrooptic element  26  impinges on the reflecting film  28 , and is reflected and progresses along the optical path  23  to the laser diode  25  side. At this time, because the refractive index of the electrooptic element  26  fluctuates due to the fluctuation in the electrical field of the measurement point propagated via the metallic pin  32 , the polarization state of the laser beam fluctuates when propagating through the electrooptic element  26 , and the reflected light with fluctuations in the polarization is separated by the polarized beam splitters  34  and  37 , focused and impinged on the photodiodes  41  and  42 , and converted into an electrical signal. Thereby, the fluctuation in the polarization state of the laser beam is detected as an output difference of photodiodes  41  and  42 , so that it is possible to measure the electrical signal of the measurement point. 
     In the case of repeatedly performing signal measurement in this manner, the metallic pin  32  wears from the tip end  32   b  side thereof. Consequently it is necessary to replace the metallic pin  32 . However, in this case, since with the electrooptic probe  21 , the tip member  31  can be attached to and detached from the head body  30 , then if the metallic pin  32  is replaced together with the tip member  31 , replacement of the metallic pin  32  can be easily performed. As a result, with replacement of the metallic pin  32 , in contrast to the conventional situation, it is not necessary to replace the whole probe head  24 . Hence, replacement of the high cost electrooptic element  26  becomes unnecessary so that there is a cost benefit. 
     Moreover, with this electrooptic probe  21 , since the tip member  31  can be easily replaced with one fitted with the most suitable metallic pin  32  to match the characteristics of the signal of the measurement object, then compared to heretofore, measurement accuracy can be improved. 
     An example of the embodiment of the present invention has been explained above. However the present invention is not limited thereby, and it is possible to alter the shape and structure without departing from the gist of the invention. 
     For example, as shown in FIG. 3, a buffer plate  44  is provided between the electrooptic element  26  and the base end  32   a  of the metallic pin  32  so that at the time of attaching and detaching the tip member  31 , shock occurring between the metallic pin  32  and the electrooptic element  26  can be absorbed. In this case, the danger of occurrence of damage to the electrooptic element  26  is minimized so that the durability of the electrooptic probe  21  can be improved. 
     Moreover, the means for securing the tip member  31  to the head body  30  is not limited to that of the above embodiment. For example, a male threaded portion may be provided on the tip member  31  side and a female threaded portion may be provided on the head body  30  side. Furthermore, as shown in FIG. 4, a threaded aperture  45  may be provided in the tip member  31  and a screw  46  may be disposed in the threaded aperture  45 . In this way, the tip member  31  may be secured to the head body  30 . Of course, in this case also, as shown in FIG. 5, a buffer plate  44  may be provided between the electrooptic element  26  and the base end  32   a  of the metallic pin  32 . 
     Furthermore, with a different arrangement, as shown in FIG. 6, the metallic pin  32  may be divided into a base end portion  32   c  and a tip end portion  32   d,  with the base end portion  32   c  secured to the electrooptic element  26  and the tip end portion  32   d  secured to the tip member  31 , and when the tip member  31  is attached to the head body  30 , the base end portion  32   c  and the tip end portion  32   d  are connected together as one. 
     By means of the above, the positional relation between the base end  32   a  of the metallic pin  32  and the electrooptic element  26  can be kept constant so that highly accurate measurement can be realized. Moreover, in this case, if silver paste is disposed between the base end portion  32   c  and the tip end portion  32   d,  then these can be better connected as one. 
     Moreover, in the above-described embodiment, if a continuous beam is emitted from the laser diode  25 , signal measurement by a conventional general measuring device such as a real time oscilloscope, a sampling oscilloscope, or a spectrum analyzer is possible. In this case, in place of the EOS oscilloscope, a real time oscilloscope, a sampling oscilloscope, or a spectrum analyzer can be connected to the photodiodes  41  and  42 , via a dedicated controller.