Abstract:
A sampling device that samples a first signal by modulating a second signal using the first signal, comprising: a holding circuit that holds a value of the first signal; a modulator that modulates the second signal using a difference between the value held by the holding circuit and a present value of the first signal to produce a third signal indicating the difference; and an adder that adds the difference indicated by the third signal to the value held by the holding circuit. Preferably, the first signal is an electric signal, the second signal is an optical signal, and the modulator modulates the optical signal by applying to the optical signal an electric field formed by the electric signal.

Description:
This patent application claims priority based on a Japanese patent application, H10-191276 filed on Jul. 7, 1998, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical sampler that produces a low speed signal based on a high speed electric signal so that the high speed electric signal can be analyzed using the low speed electric signal. 
     2. Description of the Related Art 
     FIG. 1 schematically shows the structure of a typical optical sampler. In the figure, the signal source  1000  generates: a signal Vin, an electric signal changing at high speed, which is measured by the optical sampler. The optical sampler has an optical modulator  2000 , a laser pulse source  3000 , a polarizer  4000 , an analyzer  5000 , an optical detector  6000 , a detecting circuit  7000 , ad a holding circuit  6000 . The optical modulator  2000  is of a bulk type, including, for example, an electrooptical effect crystal  2100 , which can be made from LiNbO 3 , and a pair of electrodes  2200 A and  2200 B opposing to each other. 
     FIG. 2A shows a waveform of an optical pulse PF output by the laser pulse source  3000 , FIG. 2B shows a waveform of the signal Vin output by the signal source  1000 , and FIG. 2C shows a waveform of the low speed signal LO output by the holding circuit  8000 . 
     The optical pulse PF provided by the laser pulse source  3000  is polarized by the polarizer  4000 , which feeds a polarized optical signal Pin to the optical modulator  2000 . The sampling frequency of the optical pulse PF is set to be higher than the signal frequency of the signal Vin. Meanwhile, the signal Vin generated by the signal source  1000  is fed to the optical modulator  2000 , whereby an electric field is applied to the electrooptical effect crystal  2100  using the pair of electrodes  2200 A and  2200 B. 
     The direction of the electric field is right-angled to the direction in which the optical signal Pin advances. Hence, by providing the polarization plane electric field to the electrooptical effect crystal  2100 , the polarization plane of the optical signal Pin advancing through the electrooptical effect crystal  2100  is rotated according to the electric field. In other words, the angle of the polarization plane of the optical signal Pin is defined by the electric field, that is to say, by the signal Vin. In this way, the signal Vin is sampled using the polarized optical signal Pin, that is, the polarized optical signal Pin is modulated by the signal Vin. After being rotated, the optical signal Pin is input to the analyzer  5000  which subsequently outputs an optical signal Pout. 
     FIG. 3 shows a relationship between the signal Vin and the a ratio of the signal Pin to the signal Pout. The relationship is represented by a sine curve. In FIG. 1, it is assumed that the polarizer  4000  and the analyzer  5000  are set so that the difference between the angle of the polarization plane of the polarizer  4000  and that of the analyzer  5000  is 45 degrees. If there is no an electric field, that is to say, there is no signal Vin, a signal Pout corresponding to Vin=0 is fed from the analyzer  5000 . 
     In FIG. 3, the characteristic near Vin=0 is as follows. If the signal Vin is positively applied, the polarization plane of the optical signal Pin is rotated clockwise by +θ. Thus, the optical signal Pout is increased according to the sine curve. For example, an optical signal Pout corresponding to Vin=Vp is output. On the contrary, if the signal Vin is negatively applied, the polarization plane of the optical signal Pin is rotated counterclockwise by θ (clockwise by −θ) . Thus, the optical signal Pout is decreased according to the sine curve. For example, the optical signal Pout corresponding to Vin=Vn is output. In summary, the ratio of the optical signal Pin to the optical signal Pout depends upon the angle of the polarization plans given by the optical signal Pin, and therefore depends upon the signal Vin. 
     The optical signal Pout fed by the analyzer  5000  undergoes optical/electric conversion in the optical detector  6000 , whereby an electric signal corresponding to the optical signal Pout is produced and fed into the detecting circuit  7000 . After receiving the electric signal, the detecting circuit  7000  amplifies it to output the amplified electric signal to the holding circuit  8000 . The holding circuit  8000  carries out sampling/holding on the electric signal to provide the low speed signal LO. The frequency of the low speed signal LO is a beat frequency. In other words, the frequency is equal to the difference between the frequency of the signal Vin and the frequency of the optical pulse PF or the optical signal Pin. Accordingly, the change in the signal Vin can be represented by the low signal LO. 
     FIG. 4 shows the structure of an optical interferometer type optical modulator  9000 . Unlike the optical modulator  2000  of FIG. 1, the optical modulator  9000  comprises a plate  9100  made of electrooptical effect crystal such as LiNbO 3 . The optical modulator  9000  further includes an input port  9000 A, an output port  9000 B, a division port  9000 C, a combination port  9000 D, and optical paths  9000 E- 1  and  9000 E- 2 . The input port  9000  is formed on a side of the plate  9100  while the output port  9000 B is formed on the opposite side thereof. The division port  9000 C and the combination port  9000 D are formed between the input port  9000 A and the output port  9000 B, wherein the optical paths  9000 E- 1  and  9000 E- 2  are formed in parallel with each other therebetween. 
     An electrode  9200 A is formed along the optical path  9000 E- 1 , an electrode  9200 B is formed between the optical path  9000 E- 1  and the optical path  9000 E- 2 , and an electrode  9200 C is formed along the optical path  9000 E- 2 . The signal Vin generated by the signal source  1000  is applied across the electrodes  9200 A and  9200 B, while both the electrodes  9200 B and  9200 C are grounded. The laser pulse source  3000  is connected to the input port  9000 A, and the optical detector  6000  is connected to the output port  9000 B. 
     The optical pulse PF generated by the laser pulse source  3000  Is fed into the input port  9000  A to be divided into two components at the division port  9000 C. One component advances along the optical path  9000 E- 1 , while the other component advances along the optical path  9000 E- 2 . The former component changes in propagation velocity through the electric field formed by the signal Vin, while the velocity of the latter component remains unchanged. Hence, the components interfere with each other according to the change of the former component in the propagation velocity at the combination port  9000 D. The light that is phase-modulated along the optical path  9000 E- 1  is combined at the combination port  9000 D to be an amplitude modulation light by interference. Consequently, the amplitude modulation light, that is, an intensity modulation light is output from the output port  9000 B. 
     FIG. 5 shows a relationship between the signal Vin and the ratio of the signal Pout to the signal Pin. Here, unlike the above bulk-type optical modulator accompanied by the polarizer, the optical signal PF is identical with the optical signal Pin. The relationship is given on assumption that the length of the optical path  9000 E- 1  and that of the optical path  9000 E- 2  are equivalent to each other. In the figure, with respect to the characteristic when the signal Vin is zero or near zero, Vin=0 provides the maximum ratio while both Vin&gt;0 and Vin&lt;0 provides other ratios smaller than the maximum ratio. 
     FIG. 6 shows another relationship between the signal Vin and the ratio of the signal Pout to the signal Pin. The relationship is given based on an assumption that the length of the optical path  9000 E- 1  differs from that of the optical path  9000 E- 2  by λ/4 where λ denotes the wavelength of light. The characteristic around Vin=0 in FIG. 6 is widely and sharply linear similar to the relationship in FIG. 3 concerning the optical modulator  2000 . Accordingly, this characteristic is more useful than that in FIG.  5 . 
     As described above, the signal Vin can be sampled using either the bulk-type optical modulator  2000  or the interference-type optical modulator  9000 . However, with respect to the characteristic of the signal Vin—the optical signal Pout, the linear range of the sine curve around Vin=0 is rather small. Therefore, it the amplitude of the signal Vin excesses the linear range, the optical signal Pout or the low speed signal Lo distorted, which disables accurate measurement of the high speed signal Vin using the low speed signal LO. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an optical sampler which overcomes the above issues in the related set. This object is achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention. 
     According to an aspect of the present invention, there is provided a sampling device that samples a first signal by modulating a second signal using the first signal, comprising: a holding circuit that holds a value of the first signal; a modulator that modulates the second signal using a difference between the value held by the holding circuit and a present value of the first signal to produce a third signal indicating the difference; and an adder that adds the difference indicated by the third signal to the value held by the holding circuit. 
     According to another aspect of the present invention, there is provided a sampling method of sampling a first signal by modulating an optical signal using the first signal, comprising: holding a value of the first signal: modulating the optical signal according to a difference between the first signal and the value held by the holding circuit to produce the third signal including the difference; and adding the difference in the third signal to the value held by the holding circuit. 
     According to still another aspect of the present invention, there is provided a sampling device that measures an electric signal produced by a semiconductor device by modulating an optical signal using the electric signal, comprising: a holding circuit that holds a value of the electric signal; a modulator that modulates the optical signal by forming an electric field using the value held by the holding circuit and the electric signal to produce a difference signal indicating a difference between the value held by the holding circuit and a present value of the electric signal; and an adder that adds a value indicated by the difference signal to the value held by the holding circuit. 
     This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the structure of a conventional optical sampler: 
     FIGS. 2A-2C show waveforms of signals in FIG. 1; 
     FIG. 3 shows a characteristic of an optical modulator of a bulk type; 
     FIG. 4 shows the structure of an optical modulator of an interference type; 
     FIG. 5 shows a characteristic of the optical modulator of FIG. 4; 
     FIG. 6 shows another characteristic of the optical modulator of FIG. 4; 
     FIG. 7 shows the structure of an optical sampler of the first embodiment; 
     FIGS. 8A-8D show waveforms of signals in FIG. 7; 
     FIGS. 9A-9D show waveforms of signals in FIG. 7; 
     FIG. 10 shows the structure of an optical sampler of the second embodiment; 
     FIG. 11 shows the structure of an optical sampler of the third embodiment; 
     FIG. 12 shows the structure of an optical sampler of the fourth embodiment; and 
     FIG. 13 shows the structure of another optical sampler of the third embodiment; 
     FIG. 14 shows the structure of another optical sampler of the fourth embodiment; 
     FIG. 15 shows the structure of an optical sampler of the fifth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described based on preferred embodiments, which do not intend to limit the scope of the present invention, but rather to exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention. 
     &lt;First Embodiment&gt; 
     FIG. 7 shows the structure of the optical sampler of the first embodiment. To sample the signal Vin being measured, the optical sampler has an optical modulator  20 , a laser pulse source  30 , a polarizer  40 , an analyzer  50 , an optical detector  60 , a detecting circuit  70 , a holding circuit  80 , and an adder  90 . The optical modulator  20  further comprises a first optical modulator  20 A and a second optical Adulator  20 B. Both the first and second optical modulators  20 A and  20 B are of a bulk type. As shown in FIG. 3, the modulator  20  modulates the optical signal Vin more strongly as the signal is larger. 
     More specifically, the signal Pout changes almost linearly in the area ranging from −λ/2 to λ/2, and in particular the linearity is sharper closer to Vin=0. Further, the electrode of the first modulator  20 A to which the signal Vin is applied opposes to the electrode of the second modulator  20 B to which the signal HD is applied, with respect to the direction in which the optical signal Pin advances. The modulator  20  rotates the optical signal Pin according to the electric field corresponding to the difference between the usual Vin and the signal HD. Both the directions of the electric fields in the first optical modulator  20 A and the second optical modulator  20 B are right-angled to the direction in which the optical signal Pin advances. 
     The signal Vin generated by the signal source  10  is applied to the first optical modulator  20 A and the sampling/holding signal HD is applied to the second optical modulator  20 B so that the electric field formed by the signal Vin is inverse to the electric field formed by the sampling/holding signal HD. The holding circuit  80  holds the previous value of the first signal Vin as the sampling/holding signal HD. By applying the current sampled signal Vin to the first optical modulator  20 A and applying the previous value of the signal Vin to the second optical modulator  20 B, the difference between the current value and the previous value is provided. The difference lies in the optical signal fed from the optical modulator  20 . 
     FIG. 8A shows a waveform of the optical pulse PF output by the laser pulse source  30 , FIG. 8B shows a waveform of the signal Vin generated by the signal source  10 , FIG. 8C shows a waveform of the low speed signal LO provided by the holding circuit  80 , and FIG. 8D shows a waveform of the sampling/holding signal HD fed into both the second optical modulator  20 B and the adder  90 . Because the operation of the circuits other than the optical modulator  20 , the holding circuit  80 , and the adder  90  are almost the same as those of the related set, the detailed explanation below will focus on the improved features of this embodiment. 
     The first optical modulator  20 A is provided with the signal Vin from the signal source  10  shown in FIG.  8 B. The present value of the signal Vin generates an electric field as outlined above in the first optical modulator  20 A. Meanwhile, as shown in FIG. 8A, the laser pulse source  30  generates an optical pulse PF, which is polarized in the polarizer  40 . The polarized optical signal Pin is fed into the first optical modulator  20 A. In this way, the polarization plane of the optical signal Pin is rotated clockwise according to this electric field, whereby the rotated or modulated optical signal Pin is fed into the second optical modulator  20 B. In short, the first modulator  20 A modulates the optical signal Pin according to a present value of the signal Vin. 
     The second optical modulator  20 B is applied the sampling/holding signal HD by the holding circuit  80 . The electric field generated by the sampling/holding signal HD, that is, the electric field generated by the value of the signal Vin held by holding circuit  80  is inverse to the electric field in the first optical modulator  20 A. Therefore, the polarization plane of the received optical signal Pin is rotated counterclockwise according to the electric field in the second optical modulator  20 B. 
     That is, the second modulator  20 B modulates the optical signal Pin according to the value of the signal Vin held by the holding circuit  80 . Consequently, the optical signal Pout output by the second optical modulator  20 B indicates an angle corresponding to the difference between the signal Vin that is sampled this time and the signal that has been sampled last time. In other words, the modulator  20  modulates the optical signal Pin according to the difference between the value of the signal Vin held by the holding circuit  80  and the present value of the signal Vin. This optical signal Pout is fed into the analyzer  50  for analysis. The optical signal Pout undergoes an optical/electric conversion in the optical detector  60  and further the electric signal experiences amplification in the detecting circuit  70 , thus being an electric signal ΔHD denoting the above difference. 
     The adder  90  adds the difference ΔHD to the sampling/holding value HD held by the holding circuit  80 . In other words, the adder  90  adds the difference ΔHD, which is the difference between the value of the signal Vin held by the holding circuit  80  and the present value of the signal Vin, to the value of the signal Vin held by the holding circuit  80 . Here, the sampling/holding value HD denotes the signal Vin sampled last time. Thus, the adder  90  provides a new sampling/holding value HD denoting the signal Vin sampled this time. The new sampling/holding value HD is fed into the holding circuit  80 . The holding circuit  80  provides the new sampling/holding value HD to both the second optical modulator  20 B and the adder  90 , as shown in FIG.  8 D. Simultaneously, the holding circuit  80  outputs the new sampling/holding value HD as the low speed signal LO, as shown in FIG.  8 C. In this way, the sampling/holding value HD is continually updated. 
     Consequently, the second optical modulator  20 B rotates the polarization plane of the optical signal Pin counterclockwise corresponding to the signal Vin sampled last time while the first optical modulator  20 A rotates the polarization plane of the optical signal Pin clockwise corresponding to the signal Vin sampled this time. Therefore, the optical modulator  20  can always provide the optical signal Pout that is polarized by an angle according to the difference between the signal Vin sampled this time and the immediately preceding signal Vin, that is, according to the difference between the present value of the signal Vin and the value of the signal Vin which is held by the holding circuit  80 . 
     Because this angle is smaller than the angle that the polarization plane of the optical signal Pout is rotated according to the signal Vin being sampled this time, it is possible to use a linear range closer to Vin=0 in FIG. 3 than in the related set. Hence, the distortion accompanying the optical signal Pout or the low speed signal LO becomes reduced. In short, even though the optical modulator  20  has the characteristic of FIG. 3, a linear range closer to Vin=0 is consistently available because the maximum of the difference between the signal Vin sampled this time and the previously sampled signal Yin sampled last time is always smaller than the maximum of the signal Vin sampled this time. 
     FIG. 9A shows another waveform of the optical pulse PF, FIG.  9 B shows another waveform of the signal Vin, FIG. 9C shows another waveform of the low speed signal, and FIG. 9D shows another waveform of the sampling/holding signal HD. In FIG. 8B, the signal Vin is an alternating current; however, the signal Vin may be a combination of an alternating current and a direct current Vdc. Similar to above, the noise accompanying such a signal Vin can also be reduced. 
     &lt;Second Embodiment&gt; 
     An optical sampler of a second embodiment of the present invention will now be discussed with reference to FIG.  10 . FIG. 10 shows the structure of the second embodiment. As this structure is almost the same as that of the first embodiment, the following explanation will focus on the features of the second embodiment. 
     While in the first embodiment the optical modulator  20  incorporates the first optical modulator  20 A that rotates the optical signal Pin clockwise and the second optical modulator  20 B that rotates the optical signal Pin counterclockwise, in the second embodiment, the optical modulator  20  incorporates only an optical modulator  20 C. This optical modulator  20 C has the functions of both the first and second optical modulators  20 A and  20 B. Specifically, the signal Vin generated by the signal source  10  is applied to an electrode of the optical modulator  20 C while the sampling/holding signal HD held by the holding circuit  80  is applied to the opposite electrode. 
     Hence, the optical modulator  20 C rotates the polarization plane of the optical signal Pin clockwise according to the signal Vin and also rotates it counterclockwise according to the sampling/holding signal HD. In short, the optical modulator  20 C rotates the polarization plane of the optical signal Pin clockwise or counterclockwise according to a electric field corresponding to the difference between the signal Vin and the sampling/holding signal HD. Since the sampling/holding signal HD indicates the previously sampled signal Vin, the optical modulator  20 C outputs the optical signal Pout having an angle corresponding to the difference between the signal Vin being sampled and the previous signal Vin. In this way, analogously to the first embodiment, the optical modulator  20 C serves to prepare a low speed signal LO with less distortion. 
     &lt;Third Embodiment&gt; 
     A third embodiment of the optical sampler according to the present invention will now be described with reference to FIG.  11 . FIG. 11 shows the structure of the optical sampler employing an optical modulator of optical interference type. In the figure, the optical modulator  100 ,includes a first optical modulator  100 A and a second optical modulator  100 B. The optical modulator  100  further includes a first optical path  101 E- 1  and a second optical path  101 E- 2 , both of which pass through both the first and second modulators  100 A and  100 B. Specifically, in the first optical modulator  100 A, an electrode  110 C is formed along the first optical path  101 E- 1 , an electrode  110 C is formed along the second optical path  101 E- 2 , and an electrode  110 B is formed along both the first and second optical paths  101 E- 1  and  101 E- 2 . Similar to in the first optical modulator  100 A, an electrode  120 A is formed along the first optical path  101 E- 1 , an electrode  120 C is formed along the second optical path  101 E- 2 , and an electrode  120 B is formed along both the first and second optical paths  10 lE- 1  and  101 E- 2 . 
     The operation of the optical modulator  100  will now be explained. The signal Vin generated by the signal source  10  is fed into the first optical modulator  100 A while the sampling/holding signal RD held by the holding circuit  80  is fed into the second optical modulator  100 B. More exactly, the signal Vin is applied across the electrodes  110 A and  110 B with the electrodes  110 B and  110 C being grounded as a reference voltage; the sampling/holding signal HD is applied across the electrodes  120 B and  120 C with the electrode  120 A and  120 B being grounded as a reference voltage. Thus, an electric field formed by the electrodes  110 A and  110 B based upon the signal Vin is applied to the path  101 E- 1 ; an electric field formed by the electrodes  120 B and  120 C based upon the sampling/holding signal HD is applied to the path  101 E- 2 . 
     Meanwhile, the optical signal Pin generated by the laser pulse source  30  is input into the optical modulator  100  via the input port  101 A. The optical signal Pin is divided into two components at the division port  101 C. One component advances along the first optical path  101 E- 1  whereby the velocity thereof is decreased according to the signal Vin in the first optical modulator  100 A. That is, the first component is delayed by the electric field formed by the signal Vin. The reason why velocity is changed is that the signal Vin changes the refractive index of the first path  101 E- 1 . Similarly, the other component advances along the second optical path  101 E- 2  whereby the velocity thereof is decreased according to the sampling/holding signal HD. That is, the second component is delayed by the electric field formed by the sampling/holding signal HD. Here, whether the velocities of the those components are increased or decreased depends upon the material of the optical modulator  100 ; Accordingly, both the velocities may be increased if the optical modulator  100  is made of other material. 
     After passing through the first and second optical modulators  100 A and  100 B, the two components are combined at the combination port  101 D, the combined optical signal Pout, the optical signal Pout modulated by the difference between the velocity of the former component and the velocity of the latter components is fed from the output port  101 B. That is, the first component delayed by the signal Vin and the second component delayed by the sampling/holding signal HD are combined at the combination port  101 D. Hence, analogous to the optical signal Pout of the first embodiment, the optical signal Pout of this embodiment is produced with less distortion based upon the sine curve of FIG. 5 or FIG.  6 . 
     Further, if the difference between the length of the first optical path  101 E- 1  and that of the second optical path  101 E- 2  is set to be λ/4, the sine curve of FIG. 6 can be available. Such a setting can give a better optical signal Pout because the linear range of FIG. 6 is wider than that of FIG.  5 . 
     &lt;Fourth Embodiment&gt; 
     A fourth embodiment of the optical sampler according to the present invention will now be described with reference to FIG.  12 . FIG. 12 shows the structure of the fourth embodiment. This optical sampler employs an optical modulator  200  of a traveling wave type. Because the structure of this embodiment is almost identical to that of the third embodiment, the following explanation will principally focus on the features of the fourth embodiment. 
     The optical modulator  200  incorporates a first optical modulator  200 A and a second optical modulator  200 B. The structures of the first and second optical modulators  200 A and  200 B are almost the same as those of the first and second optical modulator  100 A and  100 B in the third embodiment. However, unlike the first optical modulator  100 A, the first optical modulator  200 A includes a microstrip line  210  instead of the electrode  110 A. The microstrip line  210  is formed along the optical path  201 E- 1 . More specifically, the microstrip line  210  is formed in parallel with optical path  201 E- 1  so that the direction in which the signal Vin flows and the direction in which the optical signal Pin flows are the same. Thereby, the signal Vin flowing along the microstrip line  210  can modulate the optical signal Pin advancing along the optical path  201 E- 1 . Consequently, similar to the case in the third embodiment, an optical signal Pout is produced that is the difference between the sampled signal Vin and the previous signal Vin. 
     FIG. 13 shows the structure of another optical modulator of the third embodiment while FIG. 14 shows the structure of another optical modulator of the fourth embodiment. In FIG. 13, the optical sampler  100  incorporates electrodes  130 A,  130 B, and  130 C, wherein the electrode  130 A in formed wholly along the optical path  101 E- 1 , the electrode  130 C is formed wholly along the optical path  101 E- 2 , and the electrode  130 B is formed wholly along both the optical paths  101 E- 1  and  101 E- 2 . The signal Vin is applied across the electrodes  130 A and  130 B like the electrodes  110 A and  110 B, and the electrode  110 B is grounded as reference voltage. However, the signal HD is; applied to the electrode  130 C directly. Such a compact configuration can also provide the same effect as that of the embodiment of FIG.  11 . 
     Similarly, in FIG. 14, the optical sampler  100  incorporates a microstrip line  220  and electrodes  230 A and  230 B. The microstrip line  220  is formed wholly along the optical path  200 E- 1 , to be applied the signal Vin. Meanwhile, the electrode  230 A is formed wholly along the optical path  200 E- 2  and the electrode  230 B is formed wholly along both the optical paths  200 E- 1  and  200 E- 2  with the electrode  230 B grounded as reference voltage. Such a structure can also give the same effect as that of the embodiment of FIG.  12 . 
     &lt;Fifth Embodiment&gt; 
     A fifth embodiment of the optical sampler according to the present invention will now be described referring to FIG.  15 . FIG. 15 shows the structure of the fifth embodiment. For ease of explanation and understanding, mainly the unique components of this embodiment will be discussed below. The optical sampler samples a signal Vin produced in a semiconductor device  390 . In the figure, the optical sampler incorporates an optical modulator  300  of a reflection type and a beam splitter  370 . The optical modulator  300  incorporates a first optical modulator  300 A and a second optical modulator  300 B. The first optical modulator  300 A includes an electrooptical effect crystal  310 , a transparent electrode  320 , and a reflection electrode  330 , while the second optical modulator  300 B includes an electrooptical effect crystal  340 , a transparent electrode  350 , and a transparent electrode  360 . 
     More specifically, in the first optical modulator  300 A, the transparent electrode  320  and reflection electrode  330  are so placed that the direction in which the optical pulse PF advances is at right angle thereto. In other words, the direction of the optical pulse PF and the direction of the electric field formed by the electrodes  320  and  330  are parallel to each other. Further, the transparent electrode  320  is grounded whereas on the outer surface of the reflection electrode  330  is deposited a probe  380  that establishes or keeps contact with the semiconductor device  390 . The signal Vin generated by the semiconductor device  390  is applied across the transparent electrode  320  and the reflection electrode  330  via the probe  380 . 
     In the second optical modulator  300 B, the transparent electrode  350  and the transparent electrode  360  are placed on opposite surfaces, so that the direction in which the incoming optical pulse PF advances is at a right angle to both the surfaces. Further, the sampling/holding signal HD is applied to the transparent electrode  350  while the transparent electrode  360  is grounded. 
     The operation in the fifth embodiment is as follows. The laser pulse source  30  outputs the optical pulse PF to the first optical modulator  300 A via the polarizer  40  and the beam splitter  370 . The optical pulse PF is rotated clockwise according to the electric field of the electrodes  320  and  330 , that is, according to the signal Vin. Thereafter, the rotated optical pulse PF is reflected by the reflection electrode  330  to reach the second optical modulator  300 B via the tranasparent electrode  320  and the beam splitter  370 . In the second optical modulator  300 B, the optical pulse PF is rotated counterclockwise according to the electric field of the electrodes  350  and  360 , that is, according to the sampling/holding signal HD. In this way, similar to the above embodiments, the optical modulator  300  provides the optical pulse PF indicative of the difference between the sampled signal Vin s and the preceding signal Vin. A low speed signal LO with less distortion is thereby provided. 
     As described above, the optical sampler according to the present invention provides the difference between the latest signal Vin and a preceding signal Vin. Since the maximum value of the difference is smaller than the maximum value of the signal Vin itself, a linear range closer to Vin=0 in the sine curve is available, which can provide a low speed signal LO with less distortion. 
     While the optical samplers described in the above embodiments are of an electrooptical effect type, the optical sampler may also be of a magnetoopical type, which can provide the same effects of electrooptical effect type sampler. 
     Although the present invention has been described by way of exemplary embodiments, it should be understood that many changes and substitutions may be made by those skilled in the art without departing from the spirit and the scope of the present invention which is defined only by the appended claims.