Abstract:
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. Here, in the probe body  22 , the probe head  23  and the supporting member  44  positioned between the end terminal  22   a  and the part that encloses the laser diode  25  and the photodiodes  38  and  39  are formed by an insulating body (polyacetal resin). Furthermore, the photodiodes  38  and  39  and the laser diode  25  are covered by electromagnetic shield members  41  and  42  that are separated from each other.

Description:
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 probe head so that the base portion of the metallic pin is connected to said electrooptic element and the end portion of the metallic pin protrudes from said probe body. A photodiode is enclosed in a second enclosing portion of said probe body and an isolator is disposed in the optical path, wherein a laser beam generated from said laser diode is incident the electrooptic element via the optical path, the laser beam being reflected by the reflecting film provided on said electrooptic element and being separated by the isolator so as to impinge the photodiode and thereby be converted into an electric signal, and wherein the probe head, the first enclosing portion and the second enclosing portion are disposed so as to be separated from each other, and wherein said supporting member is disposed between the probe head, the first enclosing portion and the second enclosing portion. 
     By making this kind of structure, in this electrooptic probe, the fluctuation in the field by the measured signal can be prevented from being transmitted via the probe body to the laser diode and the photodiode by an insulating body. 
     Further in accordance with the invention, an electrooptic probe is provided in which an optical path is established, the probe comprising 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 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 the probe body so that the base portion of the metallic pin is connected to said electrooptic element and the end portion protrudes from said probe body. An isolator is mounted in the optical path, a photodiode enclosed in the probe body, and an electromagnetic shield member provided so as to surround the photodiode and the laser diode, wherein a laser beam generated from said laser diode is incident on said electrooptic element via said optical path, and is reflected by the reflecting film to be split by the isolator and then converted by the photodiode into an electric signal. 
     By making this kind of structure, in this electrooptic probe, it is possible to prevent detection of external noise by the laser diode and the photodiode. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cut-away drawing of an electrooptic probe schematically showing an embodiment of the present invention. 
     FIG. 2 is a planar drawing of the same. 
     FIG. 3 is a simplified drawing of the electrooptic probe schematically showing the conventional technology of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Below, an embodiment of the present invention is explained referring to the drawings. FIG.  1  and FIG. 2 are a cut-away drawing and a planar drawing of the electrooptic probe  21  showing an embodiment of the present invention. In this electrooptic probe  21 , the end terminal  22   a  of the probe body  22  is formed by a probe head  23 , and at the same time the laser diode  25  connected to the EOS oscilloscope (not shown) is enclosed in the base terminal  22   b  of the probe body  22 . In addition, the electrooptic element  26  is enclosed in the probe head  23 . 
     In addition, at the end of the probe head  23 , a metallic pin  27  is provided. This metallic pin  27  is supported by the probe head  23 , and at the same time this base terminal  27   a  connects to the electrooptic element  26 , and this end terminal  27   b  protrudes from the probe head  23 . In addition, a reflecting film  26   a  is formed on the end surface of the electrooptic element  26 . 
     From the right in the figure, a collimator lens  29 , a polarized beam splitter  30 , a Faraday element  31 , a polarized beam splitter  33 , a ¼ wavelength plate  34 , and a condensing lens  36  are disposed in the optical path  28  to form an optical path  28  between the laser diode  25  and the electrooptic element  26 . In addition, at positions corresponding to the polarized beam splitters  30  and  33  on the side of the optical path  28  are provided photodiodes  38  and  39 . These photodiodes  38  and  39  are connected to an EOS oscilloscope, and convert the incident light into an electrical signal, and can transmit this to an EOS oscilloscope. 
     In addition, the polarized beam splitters  30  and  33  can function as an isolator that splits the reflected beam from the electrooptic element  26  transiting the optical path  28  and reflects it to the photodiodes  38  and  39 . 
     In addition, as shown in the figure, electromagnetic shield members  41  and  42  are provided so as to be separated from each other and respectively surrounding the neighborhood of the photodiodes  38  and  39  and the polarized beam splitters  30  and  33 , and the neighborhood of the laser diode  25 . 
     Among these, the electromagnetic shield member  42  that covers the neighborhood of the photodiodes  38  and  39  and the polarized beam splitters  30  and  33  is formed by aluminum foil, while in contrast, the electromagnetic shield member  41  covering the laser diode  25  is formed by a copper foil that has been silver-plated so that it has favorable electro-conductivity. 
     Furthermore, in the electrooptic probe  21 , the probe head  23  and the support member  44  that forms one part of the probe body  22  and supports the probe head  23  are formed by polyacetal resin. 
     Next, the operation and effect will be explained. 
     Moreover, here the frequency of the signal that is the object of measurement is in the order of several GHz. 
     In the case that the electrooptic probe  21  is used in signal measurement, the EOS oscilloscope is activated with the end terminal  27   b  of the metallic pin  27  in contact with the measurement point. 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 to a parallel beam by the collimator lens  29 , travels straight along the optical path  28 , is converged by the condensing lens  36 , and arrives at the electrooptic element  26 . 
     Because the condensing lens  36  is disposed at a position separated only by the focal distance of the condensing lens  36  from the reflecting film  26   a , the laser beam converged by the condensing lens  36  is converged at one point on the reflecting film  26   a . Furthermore, this laser beam is reflected by reflecting film  26   a , converted to a parallel beam by the condensing lens  36 , and at the same time progresses along the optical path  28  to the laser diode  25  side. 
     At this time, because the state of the refractive index of the electrooptic element  26  changes due to the change in the field at the measuring point, when propagating along the electrooptic element  26 , the polarization state of the light changes. With the state of its polarization changed, the light is separated by the polarized beam splitters  30  and  33 , converged and incident on the photodiodes  38  and  39 , and converted into an electrical signal. Thereby, the change in the polarization state of the laser beam is detected as an output difference between photodiodes  38  and  39 , and the electrical signal of the measured point is measured. 
     In this case, the probe head  23  and the supporting member  44  function as insulators because they are formed by polyacetal resin, and therefore it is possible to prevent the detection of the change in the field due to the measured signal by photodiodes  38  and  39  as noise. In addition, because the probe head  23  and the supporting member  44  act as an insulator between the metallic pin  27  and the laser diode  25 , it is possible to avoid the result that the change in field due to the measure signal acts as noise to the laser diode  25 , that is, avoid the noise being included in the laser beam input into the electrooptic element  26 . 
     Moreover, the probe head  23  and the supporting member  44  that are formed from polyacetal resin in this manner have superior workability, and in addition, can be formed inexpensively in comparison to ceramic. Furthermore, the polyacetal resin is light, and in addition, in comparison to other resins, is very strong, has a high heat-deformation temperature, and thus can be applied favorably to the probe head  23  and the supporting member  44 . 
     In addition, because the laser diode  25  is shielded by the electromagnetic shield member  42 , it is possible to decrease further the propagation of the field due to the measured signal. In this case, because the electromagnetic shield member  42  is formed by a copper foil that has been silver-plated, and the surface conductivity is good, it is appropriate in particular for shielding noise of high frequency waves (several GHz), and when the electric signal which is the object of measurement, as in the present embodiment, consists of high frequency waves of several GHz, it has a striking shielding effect. 
     In addition, because the electromagnetic shield member  41  shields the photodiodes  38  and  39 , it is possible to decrease further the propagation of noise to the photodiodes  38  and  39 . In addition, because the electromagnetic shield member  41  is formed by an aluminum foil, it can be particularly appropriate for shielding the electric signal of the measured object from low frequency noise of several MHz to several tens of MHz. Therefore, when detecting a measured signal by sampling it after being converting down to a lower frequency, concern about noise decreasing the measuring precision from the display, etc, can be ameliorated. 
     Moreover, in the above embodiment, it is possible to use other structures and still be within the gist of the present invention. 
     For example, in the above embodiment, the electromagnetic shield member  41  is formed by aluminum foil, but instead, it is possible to use an aluminum tube. 
     In addition, in the present embodiment, if a continuous beam is generated from the laser diode  25 , it is possible to carry out signal measurement by conventional general-use measuring devices such as a real time oscilloscope, a sampling oscilloscope, or spectrum analyzer. In this case, instead of an EOS oscilloscope, it is possible to connect a real time oscilloscope, a sampling oscilloscope, or spectrum analyzer to photodiodes  38  and  39  via a dedicated controller.