Abstract:
An electro-optic sampling oscilloscope facilitates adjustment of signal-to-noise ratio caused by electrical, optical and temperature factors. The instrument includes: a light generation section for generation a reference laser beam; a crystal, exhibiting an electro-optic response behavior so as to receive the reference laser beam and to result in changing a refractive index in accordance with electrical field strength generated by target signal; a reflection mirror formed on a rear surface of the crystal for reflecting the reference laser beam that has passed through the crystal; an optical circuit for separating the reference laser beam reflected from the reflection mirror into a first signal light and a second signal light; a first photo-electric conversion section for converting the first signal light into first electrical signals; a second photo-electric conversion section for converting the second signal light into second electrical signals; a differential amplification section for differentially amplifying the first electrical signals and the second electrical signals, and for outputting differentially amplified signals as detection signals of the target signal; and a gain adjustment section for varying gains to adjust a signal-to-noise ratio while the reference laser beam is not being radiated into the crystal, so as to match strength levels of the first electrical signals and the second electrical signals in association with feedback signals of the target signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electro-optic sampling oscilloscope used in various signal measurements. 
     This application is based on patent appellation No. Hei 10-122514 filed in Japan, the content of which is incorporated herein by reference. 
     2. Description of the Related Art 
     In recent times, electro-optic sampling oscilloscopes are favored that can measure ultra-fast bit-rate signals of the order of 2.4 Gbps on a target circuit, without disturbing the operation of the target circuit. 
     In such an electro-optic sampling oscilloscope, an electro-optic probe based on electro-optic effects is used to detect signals in the target circuit. 
     Such electro-optic sampling oscilloscopes are favored for measurements in communication technologies that are constantly evolving into ultra-fast systems, because of the following features of the device. 
     (1) Measurement process is facilitated because the technique does not required a ground line. 
     (2) Metal pin placed at the end of an electro-optic probe is insulated from the test circuit resulting an extremely high input impedance, so that measurement process hardly affects the performance of the target circuit. 
     (3) Because optical pulses are used, measurements are possible over a wide bandwidth of the order of GHz. 
     FIG. 5 is a schematic side view showing the components of an electro-optic probe used in a conventional electro-optic sampling oscilloscope. An electro-optic probe  4  is based on a principle that when an electro-optic crystal, that is being subjected to an electrical field generated by the target signal, is irradiated with a laser beam, the polarization state of the laser beam is altered. 
     In the electro-optic probe  4 , a metal probe  5  with a tapered tip touches a signal line in a circuit. The metal pin  5  serves the purpose of facilitating the electrical field of the target signal to affect the condition of the electro-optic (e-o) crystal  7 . A circular shaped insulator  6  is provided to contact the end of the metal pin  5  in the center of the rear surface of the insulator. In other words, the metal pin  5  is surrounded by the insulator  6 . The e-o crystal  7  is a cylindrical crystal of BSO (B 12 SiO 20 ), and has a property, known as the Pockels&#39; effect, that the primary opto-electric effect, which is its refraction index, is altered in response to an electrical field coupled through the metal pin  5 . 
     A reflection mirror  8  is a dielectric film laminated mirror and is made by vapor deposition of a reflecting substance on the rear surface of the e-o crystal  7 . A reference laser beam La 0  transmitted through the e-o crystal  7  is reflected by the reflection mirror  8 , which is bonded to the front surface of the insulator  6 . 
     A cylindrical casing  9  is comprised by a tube section  9   b  and an end piece  9   a  of a tapered-shape integrally formed at one end of the tube section  9   b  having a hole through the axial center. The end piece  9   a  houses the metal pin  5 , insulator  6 , e-o crystal  7  and the reflection mirror  8 . 
     An optical fiber  10  is a polarization-maintaining optical fiber, and connects a connector  11  and a laser generator (not shown). The laser generator generates a linearly polarized reference laser beam La 0 . The reference laser beam La 0  is comprised by base band component signal that does not contain signal components in the measurement band. The connector  11  is disposed so that the reference laser beam La 0  output from the ejection end  11   a  will be injected at right angles to the e-o crystal  7  and the reflection mirror  8 . A collimator lens  12  is disposed on the left of the connector  11 , and converts the reference laser beam La 0  to a parallel beam of light. 
     A polarized beam splitter  13  is disposed on the left of the collimator lens  12 , and transmits a polarized component of the reference laser beam La 0  parallel to the plane of the paper in a straight line, while the polarized component of the reference laser beam La 0  is bent at 90 degrees to the plane of the paper, and the bent beam is transmitted as the second signal light La 2  in a straight line. A Faraday element  14  is disposed on the left of the polarized beam splitter  13 , and rotates the polarized component of the reference beam La 0 , transmitted through the polarized beam splitter  13 , at 45 degrees to the plane of the paper. 
     A half-wave plate  15  is disposed on the left of the Faraday element  14  in such a way that the orientation of its crystal axis in inclined at 22.5 degrees, and re-directs the polarized beam rotated by the Faraday element  14  in a direction parallel to the plane of the paper. A polarized beam splitter  16  is disposed on the left of the half-wave plate  15 , and has the same structure as the polarized beam splitter  13 , and splits a portion of the reference laser beam La 0  reflected from the reflection mirror  8  as the first signal light La 1 . A full-wave plate  17  is disposed on the left of the polarized beam splitter  16 , and adjusts the S/N (signal to noise) ratio of the output signals ultimately obtained from the e-o probe  4 , by adjusting the intensity balance of the reference laser beam La 0  transmitted through the polarized beam splitter  16 . Adjustment of S/N ratio is performed by varying the angle between the reference laser beam La 0  and the wave plate  17  by rotating the wave plate  17 . 
     A first photo-diode  18  is disposed above the polarized beam splitter  16 , and converts the first signal light La 1  (a portion of the reference laser beam La 0  split by the polarized beam splitter  16 ) into first electrical signals and outputs the electrical signals to a positive (+) terminal of a differential amplifier  30 . A second photo-diode  19  is disposed above the polarized beam splitter  13 , and converts the second signal light La 2  (a portion of the reference laser beam La 0  split by the polarized beam splitter  13 ) into second electrical signals and outputs the electrical signals to a negative (−) terminal of the differential amplifier  30 . 
     In such an apparatus, when the metal pin  5  shown in FIG. 5 is made to contact a signal line (not shown), an electrical field of a magnitude, corresponding to the level of the signal in the target circuit, propagating in the signal line, and couples with the e-o crystal  7 . Accordingly, refraction index of the e-o crystal  7  changes with the strength of the electrical field. In this condition, a reference laser beam La 0  is injected into the front surface of the e-o crystal  7 , through the output end  11   a  of the connector  11 , collimator lens  12 , polarized beam splitter  13 , Faraday element  14 , ½ wave plate  15 , polarized beam splitter  16  and the wave plate  17 . 
     Under this condition, the polarization state of the reference laser beam La 0  propagated through the e-o crystal  7  is changed. Polarization-affected reference laser beam La 0  is reflected from the reflection mirror  8 , and is output from the front surface of the e-o crystal  7 , and is separated in the polarized beam splitter  16 . The first signal light La 1  produced by this splitting process is converted into first electrical signals in the first photo-diode  18 , and the first electrical signal S 1  are input in the (+) terminal of the differential amplifier  30 . 
     In the meantime, the second signal light La 2  produced by the polarized beam splitter  16  is diverted by the polarized beam splitter  13  to the second photo-diode  19 , and is converted into second electrical signal S 2  in the second photo-diode  19 , and the second electrical signal S 2  are input in the (−) terminal of the differential amplifier  30 . 
     Accordingly, the first and second electrical signals S 1 , S 2  are amplified in the differential amplifier  30 , in such a way that the in-phase noise components contained in the reference laser beam La 0  generated by fluctuation and other factors are canceled. 
     Differentially amplified signals are input as detection signal SO of the e-o probe  4  into the input terminal of the sampling oscilloscope. 
     The result is a display of the waveform of the signals transmitting in the signal line on the display section of the sampling oscilloscope. 
     It was mentioned above that in the conventional e-o sampling oscilloscope, noise components are canceled in the differential amplifier  30 . However, it presupposes that the noises in the first electrical signal S 1  and that in the second electrical signal S 2  are at the same level. 
     In practice, however, the noise level contained in the electrical signal S 1  (first signal light La 1 ), and the noise level contained in the second electrical signal S 2  (second signal light La 2 ) are different because of the following reasons, so that the S/N ratio in the detection signal S 0  can be rather high. Some of the reasons are: 
     (1) Optical properties of the optical components and electrical properties of electrical components are altered due to external factors such as temperature variations, 
     (2) Optical paths for propagation of the first signal light La 1  and the second signal light La 2  are affected by vibration and pressure and the like, and 
     (3) Electrical properties of the first photo-diode  18  and second photo-diode  19  are not uniform. 
     To counter such problems, conventional e-o sampling oscilloscopes provide a manual adjustment device to adjust the orientation of the wave plate  17  to improve the S/N ratio in the detection signal S 0 . However, this process is delicate and requires considerable experience for proper adjustment, and furthermore, a special adjustment device is required for the wave plate  17 . 
     Additionally, in the conventional e-o sampling oscilloscopes, even if the S/N ratio of the detection signal S 0  is improved, it still leaves the problem of degrading S/N ratio caused by external factors. 
     Therefore, the conventional e-o sampling oscilloscope present a cumbersome problem that whenever S/N ratio is degraded, it is necessary to manually adjust the wave plate  17 . 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an electro-optic sampling oscilloscope that can easily modify S/N ratio of detection signals. 
     The object has been achieved in an electro-optic sampling oscilloscope comprising: 
     a light generation section for generation a reference laser beam; 
     a crystal, exhibiting an electro-optic response behavior so as to receive the reference laser beam and to result in changing a refractive index in accordance with electrical field strength generated by a target signal; 
     a reflection mirror formed on a rear surface of the crystal for reflecting the reference laser beam that has passed through the crystal; 
     an optical circuit for separating the reference laser beam reflected from the reflection mirror into a first signal light and a second signal light; 
     a first photo-electric conversion section for converting the first signal light into first electrical signals; 
     a second photo-electric conversion section for converting the second signal light into second electrical signals; 
     a differential amplification section for differentially amplifying the first electrical signals and the second electrical signals, and for outputting differentially amplified signals as detection signals of the target signal; and 
     a gain adjustment section for varying gains to adjust a signal-to-noise ratio while the reference laser beam is not being radiated into the crystal, so as to match strength levels of the first electrical signals and the second electrical signals in association with feedback signals of the target signal. 
     The object is also achieved in an electro-optic sampling oscilloscope comprising: 
     a light generation section for generation a reference laser beam; 
     a crystal, exhibiting an electro-optic response behavior so as to receive the reference laser beam and to result in changing a refractive index in accordance with electrical field strength generated by a target signal; 
     a reflection mirror formed on a rear surface of the crystal for reflecting the reference laser beam that has passed through the crystal; 
     an optical circuit for separating the reference laser beam reflected from the reflection mirror into a first signal light and a second signal light; 
     a first photo-electric conversion section for converting the first signal light into first electrical signals; 
     a second photo-electric conversion section for converting the second signal light into second electrical signals; 
     a first amplifier for amplifying the first electrical signals to a first gain; 
     a second amplifier for amplifying the second electrical signals to a second gain; 
     a differential amplification section for differentially amplifying first amplified electrical signals amplified by the first amplifier and second amplified electrical signals amplified by the second amplifier, and for outputting differentially amplified signals as detection signals of the target signal; and 
     a gain adjustment section for varying gains to adjust a signal-to-noise ratio while the reference laser beam is not being radiated into the crystal, so as to match strength levels of the first electrical signals and the second electrical signals by adjusting either the first gain for the first amplifier or the second gain for the second amplifier in association with feedback signals of the target signal. 
     Additionally, in the present electro-optic sampling oscilloscope, the gain of first electrical signals and the gain of second electrical signals are adjusted by using the gain adjustment section to match each other when the reference beam is not being radiated into the crystal, the detection signals output from the differential amplification section become zero. 
     Therefore, unlike the conventional electro-optic sampling oscilloscope that requires adjustment of wave plate, the present electro-optic sampling oscilloscope enables to cancel noise components caused by electrical factors, so that S/N ratio of detection signals can be optimized quickly as well as to retain this optimum condition. 
     The object is also achieved in an electro-optic sampling oscilloscope comprising: 
     a light generation section for generation a reference laser beam; 
     a crystal, exhibiting an electro-optic response behavior so as to receive the reference laser beam and to result in changing a refractive index in accordance with electrical field strength generated by a target signal; 
     a reflection mirror formed on a rear surface of the crystal for reflecting the reference laser beam that has passed through the crystal; 
     an optical circuit for separating the reference laser beam reflected from the reflection mirror into a first signal light and a second signal light; 
     a first photo-electric conversion section for converting the first signal light into first electrical signals; 
     a second photo-electric conversion section for converting the second signal light into second electrical signals; 
     a differential amplification section for differentially amplifying the first electrical signals and the second electrical signals, and for outputting differentially amplified signals as detection signals of the target signal; 
     a filter section for transmitting only base band component as base band signals of the reference laser beam while blocking transmission of the target signal contained in the reference laser beam; and 
     a gain adjustment section for varying gains to adjust a signal-to-noise ratio while the reference laser beam is being radiated into the crystal and the target signal are being output at a given level, so as to match strength levels of the first electrical signals and the second electrical signals by adjusting either the first gain for the first amplifier or the second gain for the second amplifier in association with feedback signals of the base band signal. 
     Additionally, because the present electro-optic sampling oscilloscope uses a filter section to transmit only the base band signals as the base band components from the target signal contained in the detection signals containing a target signal and base band signals of the reference laser beam, it presents an advantage that S/N ratio adjustment can be performed even during the process of measuring the target signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit diagram of a first embodiment of the electro-optic sampling oscilloscope of the present invention. 
     FIG. 2 is a schematic circuit diagram of a second embodiment of the electro-optic sampling oscilloscope of the present invention. 
     FIG. 3 is a schematic circuit diagram of a third embodiment of the electro-optic sampling oscilloscope of the present invention. 
     FIG. 4 is a variation of the electro-optic sampling oscilloscope presented in the first to third embodiments. 
     FIG. 5 is a schematic diagram of the electro-optic probe in a conventional electro-optic sampling oscilloscope. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following embodiments do not affect the scope of the claims. It is not necessary that all the combinations of the features disclosed in the embodiments are required to achieve the objectives. 
     [Embodiment 1] 
     FIG. 1 shows a block diagram of the key parts of the electro-optic sampling oscilloscope, which includes an amplifier of the present invention to replace the differential amplifier shown in FIG.  5 . 
     A first amplifier  20  amplifies first electrical signal S 1  output from a first photo-diode (refer to FIG. 5) and outputs amplified signals as first electrical signal S 1 ′. The gain of the first amplifier  20  is designated as G 1 . 
     A variable-gain amplifier  100  is a second amplifier, whose gain is designated as G 2 , and is comprised by an amplification section  21  and a gain adjustment section  22 . The second amplifier  100  amplifies second electrical signal S 2  output from a first photo-diode (refer to FIG. 5) and outputs amplified signals as second electrical signal S 2 ′. 
     The gain adjustment section  22  adjust gain G 2  of the second amplifier  100  to a value specified by a gain control signal SG. The gain adjustment section  22  is comprised by fixed resistors R 1 , R 2  and an electron volume Rx as a variable resistor. The fixed resistor R 1  is connected between the second photo-diode  19  and the amplification section  21 . The fixed resistor R 2  and the electron volume Rx are connected in series and constitute a series circuit, which is connected in parallel with the amplification section  21 . Also, the second fixed resistor R 2  and the electron volume Rx perform the role of negative feedback of output signals from the amplification section  21 . The resistance of the electron volume Rx is altered by the gain signal SG. 
     The gain G 2  of the second amplifier  100  is given by the following equation where r 2  is the resistance value of the fixed resistor R 2 , and the resistance value of the electron volume Rx is given by rx: 
     
       
           G   2 =( r   2 + rx )/ r   1   (1) 
       
     
     A differential amplifier  23  differentially amplifies first electrical signal S 1 ′ received in its (+) input terminal and second electrical signal S 2 ′ received in its (−) input terminal, and outputs detection signal S 0 . The gain of the differential amplifier  23  is designated as G 3 . An analogue-to-digital (A/D) conversion section  24  converts analogue detection signal S 0  to digital detection signal S 0 ′. 
     A control section  25  controls the gain G 2  of the second amplifier  100  so that the level (absolute value) of the detection signal S 0 ′ (detection signal S 0 ) will be a minimum or zero. Specifically, the control section  25  computes a resistance value rx of the electron volume Rx so that the level of the detection signal S 0 ′ will be a minimum or zero using an equation (to be described later), and outputs a gain control signal SG, varying with the resistance value rx, to the electron volume Rx. 
     In this case, achieving a minimum or zero for the detection signal S 0 ′ means that the levels of the first and second electrical signals S 1 ′ and S 2 ′ are matched (balanced state). In more basic terms, it means that the in-phase noise components contained in the first and second electrical signals S 1 ′, S 2 ′, i.e., first and second signal light La 1  and La 2 , are canceled. The operation of the control section  25  will be explained in more detail later. 
     The process of S/N ratio adjustment in the first embodiment will be explained below. 
     Here, the differences in the electrical characteristics of the first and second photo-diodes  18 ,  19  are defined as electrical factors. 
     Reviewing the e-o probe  4  shown in FIG. 5, this probe is in the non-measuring state, therefore, there is no effect of signals in the target line, and no S/N ratio adjustment is being made. 
     When the probe is to carry out S/N adjustments, the laser generator is stopped. It follows that there will be no reference laser light La 0  in FIG. 5 so that there will be no input of first and second signal light La 1  and La 2  into the respective photo-diodes  18 ,  19  shown in FIG.  1 . Therefore, in principle, there is no output of first and second electrical signals S 1 , S 2  from the respective first and second photo-diodes  18 ,  19 . 
     However, in practice, first and second signals S 1 , S 2  are still being output as noise signals caused by the effects of dark current and external noises and the like on the first and second photo-diodes  18 ,  19 . 
     Also, it is supposed that the electrical properties of the first amplifier  20 , second amplifier  100  and differential amplifiers are not uniform. 
     Thus, the first electrical signal S 1  is amplified in the first amplifier  20  with a gain of G 1 , and the amplified first electrical signal S 1 ′ is input in the (+) terminal of the differential amplifier  23 . 
     In the meantime, the second electrical signal S 2  is amplified in the second amplifier  100  with a gain of G 2 , and the amplified first electrical signal S 2 ′ is input in the (−) terminal of the differential amplifier  23 . 
     Accordingly, the differential amplifier  23  differentially amplifies first and second amplified signals S 1 ′, S 2 ′. Because the levels of the first and second amplified signals S 1 ′, S 2 ′ are different, the detection signal S 0  output from the differential amplifier  23  is not zero. The detection signal S 0  is converted in the A/D conversion section  24  into digital detection signal S 0 ′, and input in the control section  25 . 
     Accordingly, the control section  25  adjusts the gain control signal SG, in association with feedback signals of S 0 ′, so as to make the detection signal S 0 ′ zero. This results in changing the resistance value rx of the electron volume Rx to lead to changes in the gain G 2  of the second amplifier  100 , expressed in equation (1). 
     When the detection signal S 0 ′ becomes zero, the control section  25  maintains the level of gain control signal SG given by the computation. 
     As explained above, the e-o sampling oscilloscope of the first embodiment is able to match apparent electrical properties of the first and second photo-diodes  18 ,  19  by adjusting the gain G 2  of the second amplifier  100 . 
     Further, in addition to correcting noise caused by electrical factors, noises caused by optical and temperature factors can also be corrected by undertaking the following procedure to further improve the S/N ratio. 
     It is again assumed that the starting point of the procedure is as shown in FIG. 5 so that the e-o probe  4  is in the non-measuring state, and is not affected by signals flowing in the target line. 
     In this state, as mentioned earlier, the reference laser beam La 0  shown in FIG. 5 is injected into the front surface of the e-o crystal  7 , through the output end  11   a  of the connector  11 , collimator lens  12 , polarized beam splitter  13 , Faraday element  14 , ½ wave plate  15 , polarized beam splitter  16  and the wave plate  17 . It is assumed that fluctuation noise components are contained in the reference laser beam La 0 . 
     In this case, because the field effect of the target signal is not coupled with the crystal  7 , the polarization state of the reference laser beam La 0  propagating in the crystal  7  does not change. 
     Then, the reference laser beam La 0  is reflected by the reflection mirror  8  and is output from the front surface of the crystal  7 , and is separated into first signal light La 1  and second signal light La 2 , respectively by the polarizing beam splitters  16 ,  13 . In this case, both the first and second signal lights La 1 , La 2  contain only the base band signal components, and no target signal component is included. 
     At this time, first signal light La 1  from the beam splitter  16  is input in the first photo-diode  18  shown in FIG. 5, and is converted into first electrical signal S 1 . 
     In the meantime, second signal light La 2  from the polarized beam splitter  13  is input in the second photo-diode  19  shown in FIG. 5, and is converted into second electrical signal S 2 . 
     It is supposed that the intensity of first signal light La 1  is lower than that of the second signal light La 2  caused by factors mentioned above such as optical and temperature factors. It is further supposed that the electrical properties of the first and second photo-diodes  18 ,  19  are different such that the level of the first electrical signal S 1  is lower that the level of the second electrical signal S 2 . 
     Thus, the first electrical signal S 1  is amplified in the first amplifier  20  with a gain of G 1 , and the amplified first electrical signal S 1 ′ is input in the (+) terminal of the differential amplifier  23 . 
     In the meantime, the second electrical signal S 2  is amplified in the second amplifier  100  with a gain of G 2 , and the amplified first electrical signal S 2 ′ is input in the (−) terminal of the differential amplifier  23 . 
     Accordingly, the differential amplifier  23  differentially amplifies first and second amplified signals S 1 ′, S 2 ′. Because the levels of the first and second amplified signals S 1 ′, S 2 ′ are different, the detection signal S 0  output from the differential amplifier  23  is not zero. 
     In other words, although signal level differences caused by electrical factors have been corrected in the e-o sampling oscilloscope, there are signal level differences caused by optical and temperature factors. 
     The detection signal S 0  are converted in the A/D conversion section  24  into digitized detection signal S 0 ′, and are input in the control section  25 . 
     Thus, the control section  25  adjusts the gain control signal GS, in association with the feedback signals of S 0 ′, so as to make the detection signal S 0 ′ zero. 
     That is, the control section  25  obtains an error level ε of the current detection signal S 0 ′ (detection signal S 0 ). In this case, suppose that the error level of ε is ε 1 . 
     Next, the control section  25  obtains a resistance value rx of the electron volume Rx from the gain control signal SG, and the resistance value rx, known values of fixed resistance R 1 , resistance values r 1  and r 2  are input in equation (1) to obtain a value of the gain G 2  for the second amplifier  100 . 
     Next, the control section  25  varies the value of the gain G 2  of the second amplifier  100  to G 2 ′ by adjusting the gain control signal GS. This operation changes the level of second electrical signal S 2 , thereby changing the level ε of the detection signal S 0  (detection signal S 0 ′) from ε 1  to ε 2 . 
     Here, the signal level ε 1  of the detection signal S 0  (detection signal S 0 ′) at the gain G 2  of the second amplifier  100  is given by the following equation (2), while the signal level ε 2  of the detection signal S 0  (detection signal S 0 ′) at the gain G 2 ′ of the second amplifier  100  is given by the following equation (3): 
     
       
         ε 1 =( S   1 · G   1 − s   2 · G   2 ) G   3   (2) and 
       
     
     
       
         ε 2 =( s   1 · G   1 − s   2 · G   2 ′) G   3   (3) 
       
     
     where s 1  and s 2  are signal levels for the first and second electrical signals S 1 , S 2 , and are unknown, G 1 , G 3  are, respectively, values of the gain of the first amplifier  20  and the differential amplifier  23 . 
     By solving simultaneous equations including the above equations (2), (3), the unknown signal levels can be expressed as in equations (4), (5) shown below. 
     
       
           s   1 =(ε 1 −ε 2 )/ G   3 ( G   2 ′− G   2 )  (4) 
       
     
     
       
           s   2 =( 1 / G   1 · G   3 )(ε 1 + G   2 (ε 1 −ε 2 )/( G   2 ′− G   2 ))  (5). 
       
     
     Further, the gain G 2 ″ (ε=0) of the second amplifier  100 , for reducing the signal level ε for the detection signal S 0  (detection signal S 0 ′) to zero, is given by the signal levels s 1 , s 2 , for the first and second electrical signals S 1 , S 2 , according to the following equation (6). 
     
       
           G   2 ″(ε=0)=( s   1 / s   2 )· G   1   (6) 
       
     
     Next, in order to reduce the error level ε 1  of the detection signal S 0  at the gain G 2  of the second amplifier  100  to zero, the control section  25  computes gain G 2 ″ by substituting the values of signal levels s 1  and s 2  obtained from equations (4) and (5) for s 1  and s 2 , together with the known value of G 1  of the differential amplifier  23  for G 1 , in equation (6). 
     Next, the control section  25  obtains equation (7) from equation (1) by solving for the resistance value rx of the electron volume Rx. 
     
       
           rx=r   1 · G   2 − r   2   (7) 
       
     
     Next, the control section  25  computes the resistance value rx of the electron volume Rx, by substituting in equation (7), the known value of r 1  of the fixed resistor R 1  for r 1 , the value of G 2 ″ obtained from equation (6) for G 2 , and the known resistance value r 2  of the fixed resistor R 2  for r 2 . 
     At this time, the control section  25  outputs a gain control signal SG to correspond with the resistance value rx of the electron volume Rx obtain in equation (7). The resistance value rx of the electron volume Rx is thus altered, resulting in changing the gain of the second amplifier  100  from G 2 ′ to G 2 ″. 
     Therefore, because the signal level (s 1 ·G 1 ) of the first electrical signal S 1 ′ is now matched with the signal level (s 2 ·G 2 ″) of the second electrical signal S 2 ′, the detection signal S 0  output from the differential amplifier  23  becomes zero. 
     That is, the differential amplifier  23  is completely freed of all noise components, contained in the reference laser beam La 0 , caused by fluctuations due to such factors as electrical, optical and temperature effects. 
     Also, in this condition, the properties of the systems (optical and electrical) involved in the path through the first photo-diode  18  and the second photo-diode  19  are matched, so that noise components generated by electrical factors are canceled, without adjusting the wave plate  17 . 
     As explained above, the first embodiment of the e-o sampling oscilloscope is able to nullify all the noise components contained in the reference laser beam La 0  without having to adjust the wave plate  17  (refer to FIG.  5 ), or the adverse effects caused by electrical, optical and temperature factors, and therefore, it is able to provide a significantly low S/N ratio in the detection signal S 0 . 
     Further, it is reminded that the wave plate  17  can be omitted from the present sampling oscilloscope without affecting the instrument sensitivity. 
     [Embodiment 2] 
     Next, a second embodiment of the e-o sampling oscilloscope will be presented. FIG. 2 shows a block diagram of the key parts of the instrument. Those parts that are the same as those in FIG. 1 are given the same reference numerals, and their explanations are omitted. In this instrument, a high-pass filter  300 , an A/D conversion section  301  and a display section  302  have been added. 
     The feature of the sampling oscilloscope of the second embodiment is that S/N adjustments can be performed even during the measuring operation. 
     The high-pass filter  300  shown in FIG. 2 is inserted between the differential amplifier  23  and the A/D conversion section  24 , and transmits base band signal components of the reference laser beam La 0 , but blocks target signal component. Specifically, the high-pass filter  300  selects only the base band signals from the detection signal S 0  containing the base band signal components and target signal component, and transmits only the base band signal component as the base band signals Sf. The A/D conversion section  24  converts the base band signals Sf into digitized base band signals Sf′. The A/D conversion section  301  converts detection signal S 0  into digitized detection signal S 0 ′. The display section  302  displays the results of measurements according to the detection signal S 0 ′. 
     Next, S/N adjustment operation during measurements of the target signal using the second embodiment instrument will be explained. 
     When the metal pin  5  shown in FIG. 5 is made to contact a target signal line (not shown), electrical field of a strength corresponding to the target signal level is coupled to the e-o crystal  7 . Accordingly, the refractive index of the crystal  7  changes according to the strength level of the electrical field. In this state, a reference laser beam La 0  is injected into the front surface of the e-o crystal  7 , through the output end  11   a  of the connector  11 , collimator lens  12 , polarized beam splitter  13 , Faraday element  14 , ½ wave plate  15 , polarized beam splitter  16  and the wave plate  17 , as described in the first embodiment. 
     Polarization state of the reference laser beam La 0  propagating in the e-o crystal  7  is changed. Polarization-affected reference laser beam La 0  is reflected from the reflection mirror  8 , and is output from the front surface of the e-o crystal  7 , and is separated in the polarized beam splitter  16 . The first signal light La 1  produced by this splitting process is received by the first photo-diode  18  shown in FIG.  2 . 
     In the meantime, the second signal light La 2  produced by the polarized beam splitter  16  is diverted by the polarized beam splitter  13  to the second photo-diode  19  shown in FIG.  2 . 
     Here, the first and second signal light La 1  and La 2  contains base band signal components as well as target signal component. 
     First signal light La 1  shown in FIG. 2 is converted into first electrical signal S 1  in the photo-diode  18 , and the first electrical signal S 1  are amplified to a gain G 1  by the firs amplifier  20 , and the amplified first electrical signal S 1 ′ are input in the (+) terminal of the differential amplifier  23 . 
     In the meantime, the second electrical signal S 2  are amplified to a gain G 2 , and the amplified second electrical signal S 2 ′ are input in the (−) terminal of the differential amplifier  23 . 
     Accordingly, differential amplifier  23  amplifies first and second electrical signals S 1 ′ and S 2 ′, and the results are output as detection signal S 0 . The detection signal S 0  contains the base band signal components as well as the target signal component, and it is supposed that noise components due to electrical, optical and temperature factors are contained in the base band signal components. 
     When the detection signal S 0  is input in the high-pass filter  300 , which outputs only the base band signal Sf containing the base band signal components to be input in the A/D conversion section  24 . Following the process described earlier, the base band signal Sf is converted into digitized base band signals Sf′, and are input in the control section  25 . 
     At this time, the control section  25  adjusts the gain control signal SG so as to reduce the error level ε of the base band signals Sf′ to zero according to feedback signals of base band signals Sf′. 
     The result is that, in the differential amplifier  23 , the noise components contained in the detection signal S 0 , due to fluctuations caused by electrical, optical and temperature factors, are completely nullified. 
     In the meantime, the detection signal S 0  input in the A/D conversion section  301  is converted into digitized detection signal S 0 ′, and are input in the display section  302 . The detection signal S 0 ′ is constituted only by target signal, because the noise components are not contained in the analogue detection signal S 0 . Therefore, only the target signal with no noise components is displayed on the display section  302 . 
     As explained above, because of the inclusion of the high-pass filter  300  in the e-o sampling oscilloscope of the second embodiment to extract the base band signals Sf, S/N adjustments can be performed even during measurement of target signal. 
     Also, according to the present instrument, the display section  302  displays detection signal S 0  that have been subjected to S/N adjustment, so that waveforms of the target signal can be displayed with high precision. 
     [Embodiment 3] 
     Next, a third embodiment of the e-o sampling oscilloscope will be presented. FIG. 3 shows a block diagram of the key parts of the instrument. Those parts that are the same as those in FIG. 1 are given the same reference numerals, and their explanations are omitted. In this instrument, a computation section  400  has been added. 
     As in the second embodiment, the feature of the sampling oscilloscope of the third embodiment is that S/N adjustments can be performed even during the sampling operation. 
     The computation section  400  is a digital filter inserted between the differential amplifier  23  and the control section  25 , and transmits base band signal components of the reference laser beam La 0 , but blocks target signal component. The computation section  400  transmits only base band signal Sf containing only the base band signals by digital filtering of detection signal S 0 ′ containing digitized base band signal components and target signal component. 
     Next, S/N adjustment operation during measurement operation of target signal using the third embodiment instrument will be explained. 
     When the metal pin  5  shown in FIG. 5 is made to contact a target signal line (not shown), electrical field of a strength corresponding to the target signal level is coupled to the e-o crystal  7 . Subsequently, by following the process described earlier, first signal light La 1  is converted into first electrical signal S 1  in the first photo-diode  18 , and the first electrical signal S 1  is amplified in the first amplifier  20  to a gain of G 1 , and the amplified electrical signal S 1 ′ is input in the (+) terminal of the differential amplifier  23 . 
     In the meantime, second electrical signal S 2  is amplified in the second amplifier  100  to a gain of G 2 , and the amplified second electrical signal S 2 ′ is input in the (−) terminal of the differential amplifier  23 . 
     Accordingly, differential amplifier  23  amplifies first and second electrical signals S 1 ′ and S 2 ′, and the results are output as detection signal S 0 . The detection signal S 0  contains the base band signal components as well as target signal component, and it is supposed that noise components due to electrical, optical and temperature factors are contained in the base band signal components. 
     The detection signal S 0  is converted to digitized detection signal S 0 ′ in the A/D conversion section  24 , and is input in the computation section  400 . Accordingly, the computation section  400  transmits, through digital filtering, only the base band signal components contained in the detection signal S 0 ′ and base band signal Sf is output to the control section  25 . 
     At this time, the control section  25  adjusts the gain control signal SG so as to reduce the error level ε of the base band signal Sf′ to zero according to feedback signals of base band signal Sf′. 
     The result is that, in the differential amplifier  23 , the noise components contained in the detection signal S 0 , due to fluctuations caused by electrical, optical and temperature factors, are completely nullified. 
     As explained above, because of the inclusion of the computation section  400  in the e-o sampling oscilloscope of the third embodiment to extract the base band signal Sf, S/N adjustments can be performed even during measurement of target signal. 
     In the various foregoing embodiments, it should be noted that the specific configurations are not limited only to those presented, and design modifications can be made within the scope of the principles demonstrated. 
     For example, in the e-o sampling oscilloscopes presented in first to third embodiments, the examples related to adjusting the gain G 2  of the second amplifier  100  shown in FIGS.  1 ˜ 3 , but the same effects can be achieved by adjusting the gain G 1  of the first amplifier  20 . In this case, a gain adjustment section  22  is provided in the first amplifier  20 . 
     Also, in the e-o sampling oscilloscopes presented in first to third embodiments, the second amplifier  100  shown in FIGS.  1 ˜ 3  may be replaced with the second amplifier  200  shown in FIG.  4 . The second amplifier  200 , similar to the second amplifier  100 , provides variable gain G 2  to the second electrical signal S 2 . The second amplifier  200  is comprised by an amplification section  26  and a variable attenuator  27 . The variable attenuator  27  has the same capability as the gain adjustment section  22  (refer to FIG.  1 ), and attenuates output signals from the amplification section  26  according to the gain control signal SG.