CN101726362B - Terahertz polarization analyzer and terahertz polarization measurement method - Google Patents

Abstract

A terahertz polarization analyzer and a terahertz polarization measurement method using the terahertz polarization analyzer, the polarization analyzer includes a laser light source emitting laser light; a beam splitter is arranged behind the optical path of the laser light source to split the laser light It is the pumping light and the detection light; the terahertz generating device for generating terahertz waves is arranged on the optical path of the pumping light behind the beam splitter; the quartz crystal is arranged on the optical path behind the terahertz generating device, and the The optical axis of the quartz crystal is set at 45° to the horizontal direction; the optical path adjustment device is arranged on the optical path behind the quartz crystal and the optical path of the detection light, so that the terahertz wave and the detection light are collinear in the same direction; The crystal is arranged on the optical path behind the optical path adjustment device; the detection device is arranged on the optical path behind the detection crystal.

Description

Terahertz polarization analyzer and method for measuring terahertz polarization

technical field

The present invention relates to a measuring device and a measuring method for the polarization direction of terahertz radiation. Specifically, the present invention relates to a terahertz polarization analyzer and a terahertz polarization analyzer which can measure the polarization direction of terahertz radiation by using a quartz crystal and performing only one measurement. Measurement methods.

Background technique

Due to its unique properties such as transient, low energy and coherence, terahertz radiation has great scientific value and broad application prospects in satellite communications, nondestructive testing, and military radar. In order to have a deep understanding of the characteristics and mechanism of terahertz radiation and to extract the information of the measured sample correctly, it is necessary to measure the polarization direction of terahertz radiation.

By studying the polarization direction of the terahertz radiation generated by the dual-frequency laser excitation of air plasma, it can be known that the terahertz polarization direction of the radiation can be controlled by changing the relative phase between the fundamental frequency light and the second harmonic.

However, the existing method of measuring the polarization direction of terahertz radiation is to find the direction of maximum transmittance by rotating the polarizer. This method needs to rotate the terahertz polarizer at least 180 degrees to calculate the polarization direction of terahertz radiation. ; Or rotate the detection crystal to measure, this method needs to measure the electric field of the terahertz radiation respectively for the detection crystal in the maximum and minimum directions to calculate the polarization direction of the terahertz radiation. These two methods are not only complicated to operate, but also the measurement process will consume a lot of time.

Contents of the invention

The technical problem to be solved by the present invention is to provide a terahertz polarization analyzer and a terahertz polarization measurement method. In the analysis of the terahertz polarization direction, a quartz crystal is used to measure the polarization direction of the terahertz wave. The measurement method and device overcome the disadvantage that the traditional terahertz wire grid polarizer needs to rotate the polarizer at least 180 degrees in the process of measuring the polarization direction.

In order to solve the above-mentioned technical problems, a terahertz polarization analyzer of the present invention includes a laser light source emitting laser light; a beam splitter is arranged behind the optical path of the laser light source, and divides the laser light into pump light and probe light; The terahertz generating device of the terahertz wave is arranged on the optical path of the pumping light behind the beam splitter; the quartz crystal is arranged on the optical path behind the terahertz generating device, and the optical axis of the quartz crystal and the horizontal direction set at 45°; the optical path adjustment device is arranged on the optical path behind the quartz crystal and the optical path of the detection light, so that the terahertz wave and the detection light are in the same direction and collinear; the detection crystal is arranged on the optical path adjustment device The optical path at the rear; the detection device is arranged on the optical path at the rear of the detection crystal.

Wherein, the terahertz generating device includes a lens, a barium metaborate crystal and a polytetrafluoroethylene plate sequentially arranged on the optical path of the pumping light.

Wherein, the detection device includes a quarter-wave plate, a Wollaston prism and a differential probe arranged sequentially on the optical path.

Wherein, the detection crystal is an electro-optic crystal.

A method for measuring terahertz polarization using a terahertz polarization analyzer, comprising the following steps: (1) generating homologous terahertz waves and probe light; ° quartz crystal, so that the terahertz wave is divided into o light and e light in the time domain; (3) adjust the optical path of the probe light and the separated terahertz wave, so that the probe light and the terahertz wave are collinear in the same direction By detecting the crystal, the polarization direction of the detection light is changed; (4) detecting the light intensity difference between the horizontally polarized light and the vertically polarized light of the detection light emitted from the detection crystal; (5) obtaining the terahertz electric field according to the detection result Waveform, the polarization angle of the terahertz wave is calculated.

Wherein, the detection of the light intensity difference between the horizontally polarized light and the vertically polarized light in step (4) is to first zero-adjust the detection light, so that the light intensity of the horizontally polarized light and the vertically polarized light of the detection beam are equal at the beginning, Then the horizontally polarized light and the vertically polarized light of the probe light are separated and detected by a differential probe.

Wherein, the terahertz wave and the probe light generated in the step (1) are divided into the pump light and the probe light from the laser pulse, and the pump light is passed through the terahertz generating device to generate the terahertz wave.

Wherein, the step of generating the terahertz wave by the pump light through the terahertz generating device is to perform frequency doubling on the pump to generate frequency-doubled light, and the frequency-doubled light and the fundamental frequency light that has not been frequency-multiplied are focused to form a Phase mixing at the focal point generates plasma, excites terahertz waves, and completes the step of generating terahertz waves.

Wherein, the pumping light has most of the energy of the incident laser light, and the detection light is a low-power laser light.

Through the above-mentioned technical solution, the terahertz polarization analyzer and the terahertz polarization measurement method of the present invention make it possible to measure the polarization direction of the terahertz radiation without rotating the polarizer in the measurement process, and the measurement is simpler and more convenient, achieving beneficial technical effect.

Description of drawings

Figure 1a is the terahertz waveform in free space;

Figure 1b is a time-domain waveform diagram of a terahertz wave passing through a quartz crystal whose angle between the optical axis and the horizontal direction is fixed at 45 degrees;

Figure 2a is a schematic diagram of the direction of the quartz crystal;

Figure 2b is a schematic diagram of the direction of the incident terahertz wave;

Figure 2c is a schematic diagram of the transmitted terahertz wave direction;

3 is a schematic diagram of a terahertz polarization analyzer;

Fig. 4 is the angle at which the polarization direction of the terahertz wave changes with the change of the phase difference between the fundamental frequency wave and the double frequency wave calculated by the present invention and the prior art respectively.

Explanation of reference signs

1-lens; 2-BBO crystal; 3-PTFE plate; 4-parabolic mirror; 5-quartz crystal; 6-conductive glass; 7-ZnTe crystal; 8-quarter wave plate; Raston prism; 10-differential detector; 11-mirror.

Detailed ways

The method for calculating the polarization direction of the terahertz wave in one measurement of the present invention will be further described by describing the measurement of the polarization direction of the terahertz wave generated by the air plasma excited by the dual-frequency laser.

The principle of the terahertz polarization measurement method of the present invention is that after the terahertz wave passes through the quartz crystal whose angle between the optical axis and the horizontal direction is fixed at 45 degrees, it is decomposed into two beams of light due to birefringence: ordinary light (hereinafter referred to as For o light) and extraordinary light (hereinafter referred to as e light). The terahertz waveform in free space is shown in Figure 1a. Figure 1b shows that after the terahertz wave passes through the quartz crystal whose angle between the optical axis and the horizontal direction is fixed at 45°, two amplitude maxima appear in the waveform in the time domain, representing the separated o-light and e-light respectively.

Figure 2a is a quartz crystal in the xoy plane, the angle between the optical axis direction and the horizontal direction (ox direction) is 45°, and Figure 2b is the incident terahertz wave, and the angle between the polarization direction and the horizontal direction is θ. After the terahertz wave passes through the quartz crystal, it is decomposed into o light and e light, and the polarization directions of the two beams are along the optical axis of the quartz crystal and perpendicular to the optical axis, respectively. Since the optical axis of the quartz crystal 5 is set at 45° to the horizontal direction, the intensity of the decomposed o-light and e-light is basically equal, so when the polarization direction of the terahertz wave changes, it can be displayed more clearly.

e_in(t)cos(45°-θ)和

以

和

来表示。As shown in Figure 2c, the amplitudes of the two beams of light after decomposition are e _in (t)cos(45°-θ) and e _in (t)sin(45°-θ) respectively, then the amplitudes of the two beams of light after decomposition are The components in the horizontal direction are

e _in (t)cos(45°-θ) and

by

and

To represent.

和

故根据

则可以算出太赫兹光的偏振方向与水平方向的夹角θ。现有技术中太赫兹波的振幅方向与水平方向的夹角θ的计算方法为

其中

为太赫兹波沿水平方向的振幅，

为太赫兹波振幅的最大值。由该公式与本发明的公式

比较，并结合图2a-2c的几何关系可知，本发明与现有技术的方法测量与计算的结果是一致的。Since o light and e light travel at different speeds, they will be detected successively in the horizontal direction

and

Therefore according to

Then the angle θ between the polarization direction of the terahertz light and the horizontal direction can be calculated. In the prior art, the calculation method of the angle θ between the amplitude direction of the terahertz wave and the horizontal direction is

in

is the amplitude of the terahertz wave along the horizontal direction,

is the maximum amplitude of the terahertz wave. By this formula and the formula of the present invention

Comparing and combining with the geometric relationship of Figs. 2a-2c, it can be known that the measurement and calculation results of the method of the present invention and the prior art are consistent.

Figure 3 shows a schematic diagram of a terahertz polarization analyzer. The terahertz polarization analyzer of the present invention includes a laser light source. In this embodiment, the laser light source can be a femtosecond laser amplifier produced by American Spectrum Physics Corporation. The average output power of the laser pulse is 3.5W, the repetition rate is 1KHz, and the center wavelength 800nm, pulse width 50fs. The laser light source can also choose lasers that generate lasers with other wavelengths, which will not be repeated here.

A beam splitter is arranged coaxially behind the optical path of the laser light source, and the generated laser pulses are divided into two beams after passing through the beam splitter, pumping light L1 and probe light L2. The pump L1 has most of the energy of the incident laser, and the terahertz wave is generated by the terahertz generating device. The detection L2 is a low-power laser as the detection beam.

The terahertz wave generated by the pump light L1 through the terahertz generating device is collimated by the parabolic mirror 4 arranged on the optical path behind the terahertz generating device, and then focused on the quartz crystal 5 on the optical path behind the terahertz generating device. The optical axis of the quartz crystal 5 is set at 45° to the horizontal direction. Through the birefringence of the quartz crystal 5, the terahertz wave is divided into two o-rays and e-rays with approximately equal amplitudes and perpendicular to each other in the time domain.

The detection light L2 is used as the detection light beam and the separated terahertz waves, and after the optical paths are adjusted by the optical path adjustment devices arranged on the respective optical paths, they arrive at the detection crystal 7 in the same direction and collinearly.

In the embodiment shown in FIG. 3 , the optical path adjustment device is implemented as follows: After the separated terahertz wave is collimated and focused by the parabolic mirror 4, it is reflected by the conductive glass 6 behind the parabolic mirror 4 to the detection crystal 7. . The detection crystal 7 is an electro-optic crystal such as ZnTe, and the conductive glass 6 is ITO. After the detection beam is adjusted and focused by the mirror 11 and the lens 1, it passes through the conductive glass 6 and passes through the detection crystal 7 in the same direction as the terahertz wave passing through the quartz crystal.

Since the pump light L1 and the probe light L2 are homologous light, when the same direction collinearly passes through the probe crystal 7, the terahertz wave and the probe beam overlap on the probe crystal. In the detection crystal 7, the terahertz electric field changes the refractive index ellipsoid of the crystal, thereby changing the polarization state of the outgoing detection beam and changing the polarization direction of the originally horizontally polarized 800nm detection beam. The detection device is arranged behind the optical path of the detection crystal 7, and can detect and obtain the terahertz electric field waveform.

Specifically, the detection device is mainly composed of a quarter-wave plate 8 , a Wollaston prism 9 and a differential probe 10 sequentially arranged on the optical path. The detection beam emitted from the detection crystal 7 is zeroed through the quarter-wave plate 8, so that the intensity of the horizontally polarized light and the vertically polarized light of the detection beam are equal at the beginning. Then the horizontally polarized light and the vertically polarized light are separated by the Wollaston prism 9 , and the two separated beams are detected by the differential probe 10 .

和

即可计算得出太赫兹偏振的θ角。The light intensity difference between the separated horizontally polarized light and vertically polarized light is proportional to the terahertz electric field. Using the differential probe 10, the light intensity difference between the two beams of polarized light can be converted into a current difference, and calculated by computer software, so as to detect the terahertz electric field. From the time-domain spectrum of the electric field changing with time, the terahertz electric field waveform as shown in Figure 1b is obtained. Then according to the measured

and

The θ angle of the terahertz polarization can be calculated.

The terahertz generating device of the present invention is produced by using a dual-frequency laser to excite air to generate plasma, thereby generating terahertz waves. The terahertz generating device includes a lens 1 , a barium metaborate crystal 2 and a polytetrafluoroethylene plate 3 arranged in sequence on the optical path of the pumping light.

After the pump light L1 passes through the lens 1 set behind the beam splitter, it is frequency-doubled by the BBO (barium metaborate) crystal 2, and part of the light is frequency-doubled to 400nm frequency-doubled light, and part of the light is still transmitted through the BBO crystal. 800nm fundamental frequency light. The frequency doubled light and the fundamental frequency light are focused and mixed at a distance d from the BBO crystal to generate plasma, thereby exciting terahertz waves. There is a polytetrafluoroethylene plate 3 on the optical path behind the BBO crystal, and the generated terahertz wave can pass through the polytetrafluoroethylene plate 3, thereby filtering out other light waves and obtaining the terahertz wave required for the experiment.

In the mechanism based on dual-frequency laser excitation of air to generate plasma to generate terahertz waves, when the fundamental frequency light is linearly polarized, the excited terahertz waves are also linearly polarized. The polarization direction of the terahertz wave will change continuously with the change of the relative phase of the fundamental frequency light and the double frequency light. The relative phase of the fundamental frequency light and the double frequency light changes with the distance d from the BBO crystal to the plasma, and the change of the relative phase of the fundamental frequency light and the double frequency light is proportional to the change of the position of the BBO crystal.

When the position of the BBO crystal changes continuously, the polarization direction of the terahertz wave will also change continuously. By detecting the two pulses separated by the terahertz wave passing through the quartz crystal and performing calculations, the terahertz wave can be obtained by only one measurement. The polarization direction of the Hertzian waves. Fig. 4 shows the change of the polarization angle of the terahertz wave when the position of the BBO crystal 2 is changed. It can be seen from FIG. 4 that the results obtained by using the terahertz polarization analyzer of the present invention are in good agreement with the results obtained by direct measurement using the prior art.

The present invention adopts the terahertz polarization measurement method of the above-mentioned terahertz polarization analyzer, comprising the following steps:

(1) Generate homologous terahertz waves and probe light L2, and probe light L2 is used as a probe beam;

(2) The terahertz wave is collimated and focused by a parabolic mirror, and passes through a quartz crystal whose optical axis is 45° to the horizontal direction, so that the terahertz wave is divided into two o-rays and e-rays with roughly equal amplitudes in the time domain ;

(3) Adjust the optical path of the detection beam and the separated terahertz wave, so that the terahertz wave and the detection beam pass through the detection crystal in the same direction, and the refractive index ellipsoid of the detection crystal is changed by the terahertz electric field, so that the original horizontally polarized The polarization direction of the 800nm detection beam changes, and the detection crystal 7 can be an electro-optic crystal such as ZnTe;

(4) Using a detection device to detect the light intensity difference between the horizontally polarized light and the vertically polarized light of the detection beam emitted from the detection crystal, so as to detect and obtain a terahertz electric field waveform.

The generation of terahertz wave and probe beam in step (1) is to divide the laser pulse generated by the laser light source into two beams, pumping light L1 and probe light L2, after passing through the beam splitter. The pumping light L1 has most of the energy of the incident laser light, and the terahertz wave is generated by the terahertz generating device, and the detection light L2 is a low-power laser as the detection beam. The laser light source can be selected from the femtosecond laser amplifier produced by American Spectrophysics Corporation. The average output power of the laser pulse is 3.5W, the repetition frequency is 1KHz, the center wavelength is 800nm, and the pulse width is 50fs. In this embodiment, a laser light source of 800 nm is selected, and lasers of other wavelengths may also be selected, which will not be repeated here.

Terahertz waves can be generated by exciting air with dual-frequency lasers to generate plasma, thereby generating terahertz waves. Specifically, to double the frequency of the 800nm pump light L1 to generate light with a wavelength of 400nm, a BBO crystal can be selected for frequency doubling. The frequency-doubled light and the undoubled fundamental-frequency light are focused and mixed at the focal point to generate plasma and excite terahertz waves. The excited terahertz wave passes through the polytetrafluoroethylene plate, filters out other light waves, and obtains the terahertz wave required for the experiment. The change of the distance d between the BBO crystal and the plasma is proportional to the change of the relative phase of the fundamental frequency light and the double frequency light, so that the polarization direction of the terahertz wave also changes.

和

即可计算得出太赫兹偏振的θ角。In step (4), the detection beam is first zero-adjusted through a quarter-wave plate, so that the light intensity of the horizontally polarized light and the vertically polarized light of the detection beam are equal at the beginning. Then the optical paths of horizontally polarized light and vertically polarized light are separated by a Wollaston prism, and detected by a differential probe. The light intensity difference between the separated horizontally polarized light and vertically polarized light is proportional to the terahertz electric field. Using a differential probe, the light intensity difference between the two beams of polarized light can be converted into a current difference, and calculated by computer software to detect the terahertz electric field. From the time-domain spectrum changing with time, the terahertz electric field waveform shown in Figure 1b is obtained. Then according to the measured

and

The θ angle of the terahertz polarization can be calculated.

Using the above method, the polarization direction of the terahertz wave can be obtained by only one measurement, and the result obtained by the measurement is in good agreement with the result obtained by direct measurement using the prior art.

The above description of the present invention is illustrative rather than restrictive. Those skilled in the art understand that many modifications, changes or equivalents can be made to it within the spirit and scope of the claims, but they will all fall into within the protection scope of the present invention.

Claims (7)

1. A terahertz polarization analyzer, characterized in that, comprising a laser light source that emits laser light; A beam splitter, arranged behind the optical path of the laser light source, divides the laser light into pump light and probe light; A terahertz generating device for generating terahertz waves is arranged on the optical path of the pump light behind the beam splitter; A quartz crystal is arranged on the optical path behind the terahertz generating device, and the optical axis of the quartz crystal is arranged at 45° to the horizontal direction; The optical path adjustment device is arranged on the optical path behind the quartz crystal and the optical path of the detection light, so that the terahertz wave and the detection light are collinear in the same direction; The detection crystal is arranged on the optical path behind the optical path adjustment device; The detection device is arranged on the optical path behind the detection crystal; the detection device includes a quarter-wave plate, a Wollaston prism and a differential probe arranged in sequence on the optical path. 2 . The terahertz polarization analyzer according to claim 1 , wherein the terahertz generating device comprises a lens, a barium metaborate crystal and a polytetrafluoroethylene plate arranged in sequence on the optical path of the pumping light. 3. The terahertz polarization analyzer according to claim 1 or 2, wherein the detection crystal is an electro-optic crystal. 4. A method for measuring terahertz polarization using a terahertz polarization analyzer, comprising the steps of: (1) Generate homologous terahertz waves and probe light; (2) passing the terahertz wave through a quartz crystal whose optical axis is 45° from the horizontal direction, so that the terahertz wave is divided into o light and e light in the time domain; (3) adjusting the optical path of the detection light and the separated terahertz wave, so that the detection light and the terahertz wave pass through the detection crystal in the same direction and collinearly, so that the polarization direction of the detection light changes; (4) Detect the light intensity difference between the horizontally polarized light and the vertically polarized light of the probing light emitted from the probing crystal: the probing light is first zeroed, so that at the beginning the horizontally polarized light and the vertically polarized light of the probing beam The light intensity is equal, and then the horizontally polarized light and the vertically polarized light of the probe light are separated, and detected by the differential probe; (5) Obtain the terahertz electric field waveform according to the detection results, and calculate the polarization angle of the terahertz wave. 5. The method for measuring terahertz polarization as claimed in claim 4, wherein the generation of homologous terahertz waves and probe light in step (1) is to divide the laser pulse into pump light and probe light, and pump The Pu light passes through the terahertz generating device to generate terahertz waves. 6. The method for measuring terahertz polarization according to claim 5, wherein the step of generating terahertz waves through the terahertz generating device of the pump light is to double the frequency of the pump to generate frequency-doubled light. The frequency doubled light and the undoubled fundamental frequency light are focused and mixed at the focal point to generate plasma, excite terahertz waves, and complete the step of generating terahertz waves. 7. The terahertz polarization measurement method according to claim 5, wherein the pumping light has most of the energy of the incident laser light, and the detection light is a low-power laser.

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