JP2906285B2 - Optical sampling device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for measuring a high-speed electric signal, and more particularly to an optical sampling device with improved S / N.

<Prior Art> Recently, many high-speed electronic devices have been developed. For example, HEMT (High Electron Mobility Trans
istor), the gate delay time is about 10 ps, and the direct modulation band of a semiconductor laser used for optical communication and the like has reached several tens of GHz. For the measurement of high-speed phenomena by such a high-speed electronic device, a sampling oscilloscope is usually used. FIG. 4 shows the principle of the sampling oscilloscope. That is, the measurement is performed while gradually shifting the gate time for N consecutive signals to be measured as in (A), and the results are combined to obtain a measurement value as in (B). In this technique, the sampling width becomes the resolution of the measurement result.

However, in the sampling oscilloscope of FIG. 4, the sampling width is limited by the speed of the step recovery diode, and is limited to about 25 ps, and even an optical oscilloscope using optical pulses has a limitation of about 10 ps. .

On the other hand, since the GaAs substrate has an electro-optical effect, the plane of polarization of the return light changes according to the magnitude of the electric field. Utilizing this phenomenon, a measuring device using light has been developed.
FIG. 5 shows the configuration of a measuring apparatus using such light.
The output light of the YAG laser 1 is compressed to a pulse width of about ps by the pulse compression unit 2 and is input to the GaAs integrated circuit 5 via the polarizer 3 and the wavelength plate 4. The return light passes through the wave plate 4 and is reflected by the polarizer 3. The intensity of the returned light is measured by the light receiving element 6 and displayed on the display unit 8. When the electric field generated by the GaAs integrated circuit 5 is changed by the drive circuit 7, the plane of polarization of the returned light changes, and the intensity of light incident on the light receiving element 6 by the polarizer 3 changes. The drive circuit 7 synchronizes the timing of the output light of the YAG laser 1 with the timing of driving the GaAs integrated circuit 5 and shifts the phase difference little by little, thereby achieving the same principle as the sampling technique described in FIG. Thus, the internal voltage waveform of the GaAs integrated circuit 5 can be measured. According to this device, measurement can be performed with a sampling width of the ps order.

<Problems to be solved by the invention> However, the apparatus shown in FIG. 5 uses an Nd: YAG laser pumped by a flash lamp, which has problems in size (total length of about 2 m), power consumption, cost, and bandwidth. There is a need for improvement. Therefore, replacement with a semiconductor laser or a solid-state laser pumped by a semiconductor laser is currently considered, but there is a problem that S / N is poor due to low optical power.

<Object of the Invention> The present invention has been made in order to solve the above-mentioned problems, and has been disclosed in the art.
It is an object of the present invention to realize an optical sampling device capable of measuring a ratio.

<Means for Solving the Problems> According to the present invention, the state of the plane of polarization of an optical pulse is changed in accordance with the operating voltage of the circuit under test, and the change in the plane of polarization is detected to measure the voltage waveform of the circuit under test. The present invention relates to an optical sampling device, which is characterized by an optical pulse generating means for outputting an optical pulse having a first optical frequency, and an operating voltage of a circuit to be measured by receiving the output light of the optical pulse generating means. An electro-optical element that changes the plane of polarization by an electric field corresponding to the laser light source, a laser light source that outputs continuous light having a second optical frequency, and multiplexes return light from the electro-optical element and output light of the laser light source. Multiplexing means, a light receiving element for receiving the output light of the multiplexing means, and an arithmetic circuit for calculating the voltage waveform of the circuit to be measured based on the output of the light receiving element. Measures the voltage waveform of the measurement circuit In that it is configured to

<Operation> An optical pulse having a first optical frequency rotates a polarization plane in an electro-optical element to which an electric field of a circuit to be measured is applied, and is incident on a light receiving element together with local oscillation light having a second optical frequency. Performing heterodyne detection amplifies the signal equivalently and raises the internal voltage waveform of the circuit under test to a high S / N ratio.
It can be measured by ratio.

<Embodiment> FIG. 1 shows an embodiment of an optical sampling apparatus according to the present invention.

In FIG. 1, reference numeral 10 denotes an optical pulse generating means for outputting an optical pulse having a first optical frequency f _LD1 comprising a semiconductor laser having a high coherency, and 11 denotes a light passing through the output light of the semiconductor laser 10 and returning light. 12 is an optical isolator that blocks light, 12 is a polarizer that transmits light output from the optical isolator, 13
Is a wavelength plate that adjusts the plane of polarization of the output light of the polarizer 13, and 14 is a lens that focuses the light output from the wavelength plate 13 on a circuit under test 15 made of a GaAs integrated circuit. 16 pulse generating circuit for generating electrical pulses for driving the semiconductor laser 10, pulse generating circuit with a sinusoidal signal of the repetition frequency f _p of the light pulse 17
16 is a high-frequency oscillator for driving 16, 18 is a driving circuit for driving the circuit under test 15 at a frequency shifted by a small frequency Δf from the frequency synchronized with the oscillator 17, 20 is a local oscillation light source, and is a second optical frequency f _LD2 . A laser light source that outputs continuous light with high coherency, 21 is an optical isolator that passes output light of the semiconductor laser 20 and blocks return light, and 22 is a circuit under test 15
A half mirror constituting a multiplexing means for multiplexing the reflected light from the lens and the output light of the optical isolator 21 via the lens 14, the wavelength plate 13 and the polarizer 12, and 23 is a light receiving element on which the output light of the half mirror 22 is incident , 24 are calculation display circuits for calculating and displaying the internal voltage waveform of the circuit under test 15 based on the electrical output of the light receiving element 23.

 Next, the operation of the apparatus having the above configuration will be described.

The semiconductor laser 10 generates a narrow light pulse by the electric pulse output of the pulse generation circuit 16. The light pulse passes through the optical isolator 11, the polarizer 13, and the wave plate 13, and is focused on the circuit under measurement 15 by the lens 14. As shown in FIG. 2, the incident light enters from the back surface of the circuit under test 15 and passes through the GaAs substrate 31, and the IC pattern 32 on the surface of the circuit under test 15
The light is reflected by the back surface of the GaAs substrate 31 again, passes through the GaAs substrate 31 in the opposite direction, and is emitted from the back surface. Since the GaAs substrate of the circuit under test 15 has an electro-optic effect, the plane of polarization of this reflected light depends on the electric field strength generated corresponding to the electric signal strength on the IC pattern with respect to the plane of polarization of the incident light pulse. Change. The reflected light pulse is transmitted through the lens 14 and the wave plate 13 in the opposite direction, and then undergoes light intensity modulation according to the plane of polarization when reflected by the polarizer 12. The reflected light of the polarizer 12 is the continuous output light of the semiconductor laser 20 transmitted through the optical isolator 21 and the half mirror 22.
Are detected by the light receiving element 23, and heterodyne detection is performed in a section where both lights overlap as described later. The arithmetic display device 24 displays the internal signal waveform of the circuit under test 15 based on the output of the light receiving element 23.

The above heterodyne detection will be described below using equations. For example, when the optical output from the semiconductor laser 10 is an optical frequency to be omega _1, represented by E ₁ sinω ₁ t, the light output from the semiconductor laser 20 when the optical frequency to be ω _2, E ₂
It is represented by sinω ₂ t. When these two lights are multiplexed by the half mirror 22, the equation representing the multiplexed light on the light receiving portion of the light receiving element 23 is (E ₁ sin ω ₁ t + E ₂ sin ω ₂ t) ² (1). After it converted into an electric signal by the light receiving element 23, the signal after passing through the low-pass filter and the DC-cut _{circuit, E 1 · E 2 cos (} ω 1 -ω 2) t ... becomes (2). Here, ω ₁ −ω ₂ = 2π (f _LD1 −f _LD2 ) (3) (2) In the equation, but contain useful information corresponding to the internal voltage signal of the measuring circuit 15 to E _1, typically because it is E ₁ «E _2, E ₂ = constant, E ₁ «E a ₁ · E _2, equivalent to the signal E ₁ is that amplified _two times E,
It is clear that the S / N ratio is improved.

In order to use a sampling technique, the fundamental frequency f _d of the drive signal of the measuring circuit 15 which is output from the drive circuit 18 is expressed by _{f d = N · f p +} Δf ... (4). Here, N = 1, 2, 3,..., Δf is a minute frequency. Due to the presence of Δf in the equation (4), the signal under test in the circuit under test 15 can be sampled with an optical pulse with a slightly shifted phase, and a high-speed phenomenon can be processed as a low-speed phenomenon. . To describe this by the fundamental frequency, so that the higher frequency f _d of the object to be measured is converted to a lower frequency Delta] f.

According to the optical sampling device having such a configuration, the S / N ratio can be improved by using heterodyne detection, and the optical power is small but the size is small, as in a semiconductor laser or a semiconductor laser pumped solid-state laser. Power consumption,
It is possible to use a light source excellent in cost and bandwidth.

Further, since the optical power incident on the device under test can be reduced, it is possible to prevent the occurrence of photoelectrons and the inverse electro-optic effect caused by the input of excessive optical power, and to suppress the influence on the circuit under test.

Although a semiconductor laser is used as a light source in the above embodiment, a solid-state laser pumped by a semiconductor laser having high coherency can be used.

FIG. 3 is a block diagram of a main part showing a modification of the apparatus shown in FIG. 1, in which a detecting section has a differential structure in order to further improve the S / N ratio. 1 are given the same symbols. The light output from the semiconductor laser 10 is a polarizer
The light passes through 33, the rotator 37 and the polarizer 12, and is guided to the circuit under test 15. The output light from the circuit under test 15 has its polarization plane
The X and Y components are separated by the polarizer 12 and the polarizer 33 and output. Each split light is multiplexed with the optical output of the semiconductor laser 20 by the half mirrors 38 and 39, respectively.
The combined lights are detected by the light receiving elements 34 and 35, respectively, and are subtracted from each other by the subtractor 36. Since the polarization plane of the light pulse is modulated in the X and Y directions with opposite polarities in the circuit under test 15, if the alignment of the optical system is performed so that the opposite phase components are equally incident on the light receiving elements 34 and 35, Since the noise of the light source itself is irrelevant to the phase, it becomes in-phase at the light receiving elements 34 and 35 and is canceled out by subtraction, and the signal component of the detection light modulated by the circuit under test 15 becomes out of phase and added by subtraction. You.
Therefore, the S / N ratio is further improved.

In these examples, the case where the device under test is a GaAs integrated circuit, that is, the case where the device under test itself is made of an electro-optic material has been described, but the present invention is also applicable to a material having no electro-optic effect, such as silicon. Is done. In this case, placing a material having an electro-optic effect, such as proximity to LiTaO ₃ single crystal to be measured such as silicon, by irradiating light to the LiTaO ₃ single crystal,
An electro-optic effect may be generated by an electric field generated by a circuit to be measured made of silicon or the like.

Alternatively, transmitted light may be detected instead of detecting a change in the state of the plane of polarization of reflected light from the circuit under measurement.

<Effects of the Invention> As described above in detail with reference to the embodiments, according to the present invention, an optical sampling device capable of measuring a high-speed measurement signal at a high S / N ratio using a semiconductor laser or the like is simplified. It can be realized with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an optical sampling device according to the present invention, FIG. 2 is an explanatory diagram for explaining the operation of the device in FIG. 1, and FIG. 3 is a modification of the device in FIG. FIG. 4 is a principle diagram of a sampling technique, and FIG. 5 is a block diagram of a conventional optical sampling device. 10 …… Light pulse generating means, 20 …… Laser light source, 22,33…
... combining means, 23, 34, 35 ... light receiving element, 24 ... arithmetic circuit,
31... Electro-optical element, f _LD1 ... First optical frequency, f _LD2 .
... second optical frequency.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) G01R 19/00-19/32 H01L 21/66 G01R 31/28-31/3193

Claims (1)

(57) [Claims] An optical sampling apparatus for changing the state of the plane of polarization of an optical pulse in accordance with the operating voltage of a circuit to be measured and detecting the change in the plane of polarization to measure the voltage waveform of the circuit to be measured. An optical pulse generating means for outputting an optical pulse having an optical frequency of; an electro-optical element for receiving the output light of the optical pulse generating means and changing the plane of polarization by an electric field corresponding to the operating voltage of the circuit under test; A laser light source that outputs continuous light having an optical frequency of 2, a multiplexing unit that multiplexes return light from the electro-optical element with an output light of the laser light source, and receives an output light of the multiplexing unit. A light receiving element; and an arithmetic circuit for converting a voltage waveform of the circuit under test based on an output of the light receiving element, wherein the voltage waveform of the circuit under test is measured based on an output of the arithmetic circuit. Light sump Ring device.