JP2000298143A - Electro-optic probe - Google Patents

Abstract

(57) [Object] To provide an electro-optic probe for preventing unnecessary reflected light emitted from the surface of an optical component in a probe from entering a photodiode. SOLUTION: An optical component forming an isolator 13 is arranged at an angle with respect to an optical axis of parallel light emitted from a collimator lens 8, and unnecessary light reflected on the surface of the optical component is applied to photodiodes 10 and 11. Avoid incidence. Further, these optical components have an angle equal to or more than an angle obtained from the optical path length from the optical components to the light receiving elements in the photodiodes 10 and 11 and the light receiving diameter of the light receiving elements, and
The tilt angle is set so that the transmission characteristic of the optical component is equal to or less than the guaranteed angle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric field generated by a signal to be measured is coupled to an electro-optic crystal, light is incident on the electro-optic crystal, and the waveform of the signal to be measured is observed based on the polarization state of the incident light. More particularly, the present invention relates to an electro-optic probe having an improved optical system.

[0002]

2. Description of the Related Art An electric field generated by a signal under measurement is coupled to an electro-optic crystal, a laser beam is incident on the electro-optic crystal, and the waveform of the signal under measurement can be observed based on the polarization state of the laser beam. Here, the laser light is pulsed,
When the signal under measurement is sampled, it can be measured with a very high time resolution. An electro-optic sampling oscilloscope uses an electro-optic probe utilizing this phenomenon.

The electro-optic sampling (Electro Opt)
ic Sampling) oscilloscopes (hereinafter abbreviated as “EOS oscilloscopes”) do not require a ground line when measuring signals, as compared to conventional sampling oscilloscopes that use electrical probes. Easy 2) Since the metal pin at the tip of the electro-optic probe is insulated from the circuit system, high input impedance can be realized, and as a result, the state of the measured point is hardly disturbed. 3) Since optical pulses are used. It has attracted attention because of its feature that it can perform broadband measurement up to GHz order.

A configuration of a conventional electro-optic probe (hereinafter referred to as a probe) used for measuring a signal with an EOS oscilloscope will be described with reference to FIG. In FIG. 3, reference numeral 1 denotes a probe head made of an insulator, and a metal pin 1a is fitted in the center of the probe head. Reference numeral 2 denotes an electro-optical element, and a reflection film 2a is provided on an end face on the metal pin 1a side, and is in contact with the metal pin 1a. Reference numerals 3 and 8 are collimating lenses. Symbol 4 is 1/4
Wave plate. Reference numerals 5 and 7 are polarization beam splitters. Reference numeral 6 denotes a Faraday element that rotates the plane of polarization of incident light by 45 degrees. Reference numeral 9 denotes a laser diode that emits laser light in accordance with a control signal output from an EOS oscilloscope main body (not shown). Sign 1
Reference numerals 0 and 11 denote photodiodes, which convert the input laser light into electric signals and output the electric signals to the EOS oscilloscope main body. Reference numeral 12 is a probe main body. Code 13
Is an isolator comprising a quarter-wave plate 4, polarizing beam splitters 5 and 7, and a Faraday element 6.

Next, the optical path of the laser light emitted from the laser diode 9 will be described with reference to FIG. In FIG. 3, the optical path of the laser light is represented by reference symbol A. First, the laser light emitted from the laser diode 9 is converted into parallel light by the collimating lens 8, goes straight through the polarizing beam splitter 7, the Faraday element 6, and the polarizing beam splitter 5, and further passes through the １ ／ wavelength plate 4. The light is condensed by the collimating lens 3 and enters the electro-optical element 2.
The incident light is reflected by the reflective film 2a formed on the end face of the electro-optical element 2 on the metal pin 1a side.

[0006] The reflected laser light is applied to a collimating lens 3.
The laser beam is again converted into parallel light, passes through the quarter-wave plate 4 again, and a part of the laser light is reflected by the polarization beam splitter 5 and enters the photodiode 10. The laser light transmitted through the polarization beam splitter 5 is reflected by the polarization beam splitter 7 and enters the photodiode 11.
The quarter-wave plate 4 adjusts the intensity of the laser light incident on the photodiode 10 and the intensity of the laser light incident on the photodiode 11.

Next, the operation of measuring the signal under measurement using the electro-optic probe shown in FIG. 3 will be described. When the metal pin 1a is brought into contact with the measurement point, a voltage applied to the metal pin 1a causes the electric field of the electro-optical element 2 to propagate to the electro-optical element 2, causing a phenomenon that the refractive index changes due to the Pockels effect. Thereby, the laser light emitted from the laser diode 9 enters the electro-optical element 2, and the polarization state of the light changes when the laser light propagates through the electro-optical element 2. The laser light whose polarization state has changed is reflected by the reflection film 2a, enters the photodiodes 10 and 11, and is converted into an electric signal.

With the change in the voltage at the measurement point, the change in the polarization state is changed by the electro-optical element 2 to the photodiode 1.
It becomes the output difference between 0 and the photodiode 11, and by detecting this output difference, it is possible to measure the electric signal applied to the metal pin 1a. In the electro-optic probe described above, the electric signals obtained from the photodiodes 10 and 11 are input to the EOS oscilloscope and processed, but instead, the photodiode 1
A conventional measuring instrument such as a real-time oscilloscope may be connected to 0 and 11 via a dedicated controller to perform signal measurement. Thus, wideband measurement can be easily performed using the electro-optic probe.

[0009]

However, in the prior art electro-optical probe, the laser beam specularly reflected on the surface 4a of the quarter-wave plate, as shown by the optical path B in FIG. The light is reflected by 5 and enters the photodiode 10. This light becomes noise light and is converted into an electric signal, so that the S / N ratio is deteriorated. As a result, there is a problem that the light appears as a measurement error of the EOS oscilloscope. Also, this noise light is transmitted to the 波長 wavelength plate 4.
This occurs not only on the surface 4a but also on the surface 5a of the polarization beam splitter 5 as in the optical path C, and the same applies to other optical components.

Further, even if the surface of the optical component is subjected to an antireflection treatment, it is difficult to reduce the reflectance to "0", and there is a problem that the cost of the optical component increases.

The present invention has been made in view of such circumstances, and provides an electro-optic probe capable of reducing unnecessary reflected light in the electro-optic probe and improving the S / N ratio. With the goal.

[0012]

According to a first aspect of the present invention, there is provided a laser diode for emitting a laser beam based on a control signal of an oscilloscope main body, a first lens for converting the laser beam into a parallel beam, and the parallel lens. A second lens for condensing light, an electro-optical element having a reflective film on an end face,
And an isolator that is provided between the second lens and the second lens, passes the laser light emitted by the laser diode, and separates the reflected light reflected by the reflective film, and is separated by the isolator. And a photodiode for converting the reflected light into an electric signal, wherein the light reflected specularly on the surface of the optical component forming the isolator is incident on the optical component by an angle that does not enter the photodiode. Characterized by being arranged obliquely with respect to the optical axis.

According to a second aspect of the present invention, the photodiode and the laser diode are connected to an electro-optic sampling oscilloscope, and the laser diode outputs the laser light based on a control signal from the electro-optic sampling oscilloscope. It emits as pulsed light. The invention according to claim 3 is characterized in that the laser diode emits continuous light as the laser light. The invention according to claim 4, wherein the tilt angle of the optical component is equal to or larger than an angle obtained from an optical path length from the optical component to a light receiving element in the photodiode and a light receiving diameter of the light receiving element, and The transmission characteristic is characterized by being equal to or less than the angle at which the transmission characteristics are guaranteed.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an electro-optic probe (hereinafter, referred to as a probe) according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of the embodiment. In FIG. 1, the same portions as those of the conventional probe shown in FIG. 3 are denoted by the same reference numerals, and description thereof will be omitted. The probe shown in this figure differs from the prior art in that the mounting angles of the quarter-wave plate 4, the polarizing beam splitters 5, 7 and the Faraday element 6 are inclined. The photodiodes 10 and 11 are arranged so as to match the direction in which the laser light converted into parallel light by the collimator lens 3 is reflected on the reflection surface 5c of the polarization beam splitter 5 and the reflection surface 7c of the polarization beam splitter 7. .

Next, the optical path of the laser light emitted from the laser diode 9 will be described with reference to FIG. In FIG. 1, the optical path of the laser light is represented by reference symbol D. First, the laser light emitted from the laser diode 9 is converted into parallel light by the collimator lens 8, travels straight through the polarization beam splitter 7, the Faraday element 6, and the polarization beam splitter 5, and further passes through the 波長 wavelength plate 4.

Next, the parallel light transmitted through the quarter-wave plate 4 is condensed by the collimating lens 3 and is incident on the electro-optical element 2, and is formed on the end face of the electro-optical element 2 on the side of the metal pin 1a. The light is reflected by the reflection film 2a. Since the collimating lens 3 is arranged at a position separated from the reflecting film 2a by a focal distance from the collimating lens 3, the laser light converted into parallel light by the collimating lens 10 is
The light is focused on one point on the reflection film 2a.

The laser light reflected by the reflection film 2a is converted into parallel light again by the collimating lens 3, further passes through the quarter-wave plate 4, and is separated by the polarization beam splitters 5 and 7 to form a photodiode. 1
The light enters 0 and 11 and is converted into an electric signal.

Here, since the quarter-wave plate 4 is disposed not at right angles to the optical axis but at an angle, the laser light reflected on the surface 4a of the quarter-wave plate 4 is denoted by the symbol E The reflected light does not enter the photodiode 10 since it does not return to the optical axis of the parallel light as shown in FIG. The reflected light is reflected by the reflection surface 5c of the polarization beam splitter 5.
Does not enter the photodiode 10 because the angle is shifted.

The surface 5a of the polarizing beam splitter 5
The light reflected at the above does not enter the photodiode 11 because the polarization beam splitter 5 is tilted. Further, the reflected light does not similarly enter the photodiodes 10 and 11 on the surface 4b of the quarter-wave plate 4 and the surface 5b of the polarization beam splitter 5. This means that the reflected light does not similarly enter the photodiodes 10 and 11 in the Faraday element 6 and the polarization beam splitter 7.

Next, the angle at which the optical component is tilted will be described. FIG. 2 is an explanatory diagram showing an optical path of the reflected light on the surface 4a of the 波長 wavelength plate 4. In FIG. 2, reference numeral 10a denotes a light receiving element in the photodiode 10. Reference numerals 10b and 10c indicate positions where the photodiode 10 is rotated by 90 degrees for simplicity of description. Symbol L indicates the optical path length of the reflected light, and symbol D1 indicates the light receiving diameter of the light receiving element 10a (10c). A1 denotes an angle when the light receiving element is viewed from a point 4p on the surface 4a of the quarter wavelength plate 4.

As shown in FIG. 2, the surface 4 of the 1/4 wavelength plate 4
The light reflected at the point 4p on “a” enters the light receiving element only with respect to the reflected light within the angle A1. In addition, since the surface 4a of the quarter-wave plate 4 is a flat surface, its reflection characteristic shows a characteristic in which a regular reflection component is the strongest.

Therefore, if the light incident on the quarter-wave plate 4 is parallel light, the reflected light at the point 4p is reduced by tilting the quarter-wave plate 4 with respect to the optical axis by an angle A1 or more. Will not be incident. When this angle A1 is expressed by an equation, it is as follows. Assuming that the optical path length from a point on the surface of the target optical component to the light receiving element is L and the light receiving diameter of the light receiving element is D1, the inclination angle A1 is A1 = 2 · ta.
It is represented by n-1 ((D1 / 2) / L).

On the other hand, general optical components have different characteristics such as transmittance when used in an inclined state, so that the inclination angle is often limited.

Therefore, by setting the angle at which the optical component is inclined from the aforementioned angle A1 to the angle at which the transmittance of the used optical component is guaranteed, unnecessary reflected light is received while maintaining the characteristics of the optical component. It can be prevented from entering the element.

As described above, by tilting the optical component having a flat surface with respect to the optical axis, it is possible to prevent unnecessary reflected light from entering the photodiodes 10 and 11, thereby improving the S / N ratio. Can be.

In the configuration shown in FIG. 1, four optical components are inclined. However, the influence of unnecessary reflected light increases as the optical components having longer optical path lengths to the photodiodes 10 and 11 become larger. Alternatively, only the optical components close to the collimating lens 3 may be tilted. The tilting direction may be clockwise or counterclockwise as long as it is within the range described above. Furthermore, when tilting multiple optical components,
The tilting directions of these optical components need not be the same.
In the above embodiment, if continuous light is emitted from the laser diode 9, signal measurement by a conventional general-purpose measuring instrument such as a real-time oscilloscope, a sampling oscilloscope, and a spectrum analyzer becomes possible. In this case, the real-time oscilloscope, the sampling oscilloscope, and the photodiodes 10 and 11 are provided via dedicated controllers instead of the EOS oscilloscope.
What is necessary is just to connect a spectrumr or the like.

[0027]

As described above, according to the present invention,
The angle at which the optical component is mounted is inclined so that the reflected light from the optical component having a flat surface escapes from the optical axis, and unnecessary reflected light can be prevented from being incident on the photodiode, thereby improving the S / N ratio. The effect that it can be obtained is obtained. Further, an effect is obtained that an optical component whose surface is not treated such as an anti-reflection film can be used.

According to the fourth aspect of the present invention, the inclination angle of the optical component is set from the angle at which the reflected light does not enter the photodiode to the angle at which the performance of the component is guaranteed. It is possible to obtain the effect that the light can be sufficiently exhibited and the reflected light can be prevented from being incident on the photodiode. Further, since the inclination angle of the optical component has a range, the adjustment at the time of mounting the optical component can be easily performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an optical path of reflected light in the embodiment shown in FIG.

FIG. 3 is a configuration diagram showing a configuration of an electro-optic probe according to the related art.

[Explanation of symbols]

1 ... probe head, 1a ... metal pin, 2 ...
・ Electro-optical element, 2a ・ ・ ・ Reflection film, 3, 8 ・ ・ ・ Collimate lens, 4 ・ ・ ・ 1/4 wavelength plate, 5, 7 ・ ・ ・ Polarized beam splitter, 6 ・ ・ ・ Faraday element, 9 ・・
-Laser diodes, 10, 11 ... photodiodes, 12 ... probe bodies, 13 ... isolators.

(72) Inventor Katsushi Ota 4-19-7 Kamata, Ota-ku, Tokyo Inside Ando Denki Co., Ltd. (72) Inventor Toshiyuki Yagi 4-19-7 Kamata, Ota-ku, Tokyo Ando Denki shares Inside the company (72) Inventor Mitsuru Shinagawa 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Tadao Nagatsuma 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation Telephone Co., Ltd. (72) Inventor Junzo Yamada 2-3-1, Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation F-term (reference) 2G011 AA01 AC32 2G025 AA17 AB09 AB11 AB12 2G032 AD07 AF07 AJ04

Claims (4)

[Claims] 1. A laser diode for emitting laser light based on a control signal of an oscilloscope main body, a first lens for converting the laser light into parallel light, a second lens for condensing the parallel light, An electro-optical element having a reflective film, provided between the first lens and the second lens,
An isolator that passes laser light emitted by the laser diode and separates the reflected light reflected by the reflection film on the laser light, and a photodiode that converts the reflected light separated by the isolator into an electric signal. In the electro-optical probe provided, the optical component is arranged to be inclined with respect to the optical axis of the parallel light by an angle at which light specularly reflected on the surface of the optical component constituting the isolator does not enter the photodiode. And an electro-optic probe. 2. The method according to claim 1, wherein the photodiode and the laser diode are connected to an electro-optic sampling oscilloscope, and the laser diode emits the laser light as pulse light based on a control signal from the electro-optic sampling oscilloscope. The electro-optic probe according to claim 1, wherein 3. The electro-optic probe according to claim 1, wherein the laser diode emits continuous light as the laser light. 4. The tilt angle of the optical component is equal to or greater than an angle obtained from an optical path length from the optical component to a light receiving element in the photodiode and a light receiving diameter of the light receiving element, and the transmission characteristic of the optical component is The electro-optic probe according to any one of claims 1 to 3, wherein the angle is equal to or less than a guaranteed angle.