CN104614082A - TeraHertz wave line width measurement device and method - Google Patents

Abstract

The invention belongs to the technical field of terahertz wave detection. In order to realize a scanning Fabry-Perot interferometer applied to line width measurement in the terahertz wave band, the technical solution adopted by the invention is a device and method for measuring terahertz wave line width , including the following structure: the incident terahertz wave passes through the gold-plated off-axis parabolic mirror on the first surface and then exits in parallel to the cavity mirror of the Fabry-Perot interferometer; the position-fixed Fabry-Perot interferometer made of high-resistance silicon material Instrument cavity mirror a, Fabry-Perot interferometer cavity mirror b; the stepping mobile platform moves step by step along the propagation direction of the terahertz wave; the parallel incident terahertz wave from the cavity mirror b passes through the second surface of the gold-plated off-axis paraboloid The focal point of the gold-plated off-axis parabolic mirror on the second surface is provided with a chopper, and the terahertz wave output from the second surface gold-plated off-axis parabolic mirror is measured by the output of the chopper, pyroelectric detector, and AD converter result. The invention is mainly applied to terahertz wave detection.

Description

Device and method for measuring terahertz wave linewidth

technical field

The invention belongs to the technical field of terahertz wave detection; in particular, it relates to a device and method for measuring the line width of terahertz waves.

technical background

In recent years, many different types of terahertz radiation sources have been greatly developed and widely used in imaging, spectral analysis and other fields. In the field of terahertz spectral analysis, terahertz time-domain spectroscopy technology can no longer meet the actual needs in some applications because it can only achieve spectral resolution of tens of GHz. In this context, a frequency-tunable narrow-linewidth single-frequency terahertz source has been proposed for spectral detection. At present, some single-frequency terahertz radiation sources with narrow linewidth have emerged, such as terahertz radiation sources based on nonlinear difference frequency or parametric oscillation technology, etc. The output linewidth of the narrow-linewidth terahertz source can reach hundreds of MHz, which is close to the limit of Fourier transform. Using this terahertz source to build a terahertz spectrum analysis system, the accuracy of spectrum detection can reach the order of MHz. With the development of terahertz radiation sources, the performance of terahertz radiation sources has been continuously improved, and the measurement of its key performance parameters including output linewidth has shown increasing importance. In order to realize the accurate measurement of the radiation linewidth of the terahertz source, it has become the goal that people pursue to find a measurement method of the terahertz wave linewidth with high measurement accuracy and easy to use.

In the traditional laser field, using Fabry-Perot etalon to measure the line width of laser is the most widely used technique at present. By observing the fine structure of the interference ring after the laser passes through the etalon through the area array camera, the line width of the laser to be measured can be quickly and accurately obtained. In the terahertz field, the cost of an area array camera is much higher than that of an area array camera in the laser field, so this measurement method is not suitable for direct promotion in the terahertz field. As an improved form of the traditional Fabry-Perot etalon, the scanning Fabry-Perot interferometer with variable cavity length can replace the fixed-thickness Fabry-Perot etalon. The observation of the fine structure of the spectral line is realized under the premise of the camera. The improved scanning Fabry-Perot interferometer requires two additional devices to achieve the observation of interferometric fine structures, namely a high-precision displacement platform and a terahertz power detection device. With the development of terahertz technology, many different types of terahertz wave power detection devices have emerged, and the technology for high-precision detection of terahertz wave power at room temperature is quite mature. In addition, since the terahertz wavelength is relatively long, on the order of millimeters, the scanning Fabry-Perot interferometer used in the terahertz band has relatively low requirements for the movement accuracy of the displacement platform. Therefore, the realization of the scanning Fabry-Perot interferometer for linewidth measurement in the terahertz band has relatively mature conditions.

Contents of the invention

In order to overcome the deficiencies of the prior art, a scanning Fabry-Perot interferometer applied to linewidth measurement is implemented in the terahertz band. For this reason, the technical solution adopted by the present invention is that the method of terahertz wave linewidth measurement uses Fabry-Perot FP interferometer, measured power transmittance to wavelength λ, by formula:

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\mathrm{<span\; class="notranslate">\; nd</span>}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; .</span>$

Obtain the interference cavity length d, where R is the reflectivity of the two reflective surfaces, n is the refractive index of the filling medium between the reflective surfaces, and θ is the incident angle;

The expression of the free spectral range FSR according to the Fabry-Perot interferometer:

FSR=c/2nd

get FSR;

Then by the formula:

Linewidth(FWHM)＝FSR/Finesse

The Linewidth (FWHM) of the terahertz wave is obtained, where the Finesse of the fringes is approximately a constant that does not change with the length of the interference cavity.

Using the Fabry-Perot FP interferometer specifically includes the following steps:

Use a gold-plated off-axis parabolic mirror to reflect the incident terahertz wave and emit it in parallel to the cavity mirror of the Fabry-Perot interferometer;

The high-resistance silicon material is used as the Fabry-Perot interferometer cavity mirror a with a fixed position. The two surfaces of the cavity mirror a are polished, and the two polished surfaces of the cavity mirror a have a wedge angle of about 1 degree to avoid the internal formation of FP resonance. Cavity, the surface near the cavity mirror b of the Fabry-Perot interferometer is perpendicular to the incident terahertz wave;

Fabry-Perot interferometer cavity mirror b has the same material and dimensions as cavity mirror a, and the surface close to cavity mirror a is perpendicular to the incident terahertz wave. Cavity mirror b is installed on the mobile platform 4;

Stepping the mobile platform along the propagation direction of the terahertz wave;

Converge the parallel incident terahertz waves from the cavity mirror of the Fabry-Perot interferometer by using a gold-plated off-axis parabolic mirror;

Place the chopper at the focus position of the gold-plated off-axis parabolic mirror, and correct the modulation frequency of the terahertz wave to be measured to a frequency suitable for the back-end pyroelectric detector;

The pyroelectric detector measures the power of the transmitted terahertz wave, and outputs a voltage signal proportional to the detected power to the data acquisition card;

Perform AD conversion on the analog signal output by the pyroelectric detector, and output the digital signal to the computer;

Use the computer to run the measurement program, read the digital voltage value output by the data acquisition system, and issue work instructions to the stepping displacement platform; calculate the line width measurement results for the user.

The device for measuring the line width of terahertz wave includes the following structure:

The incident terahertz wave passes through the gold-plated off-axis parabolic mirror on the first surface and then exits in parallel to the cavity mirror of the Fabry-Perot interferometer;

The position-fixed Fabry-Perot interferometer cavity mirror a made of high-resistance silicon material has two polished surfaces, and the two surfaces have a wedge angle of about 1 degree. The surface close to the cavity mirror b is in contact with the incident THz wave vertical;

The materials and dimensions of cavity mirror b of the Fabry-Perot interferometer are exactly the same as those of cavity mirror a, and the surface close to cavity mirror a is perpendicular to the incident terahertz wave. Cavity mirror b is installed on the mobile platform 4;

The stepping mobile platform moves stepping along the propagation direction of the terahertz wave;

The parallel incident terahertz waves from the cavity mirror b are converged by the second gold-plated off-axis parabolic mirror;

A chopper is installed at the focal point of the gold-plated off-axis parabolic mirror on the second surface, and the terahertz wave output by the gold-plated off-axis parabolic mirror on the second surface passes through the chopper, pyroelectric detector, and AD converter to output measurement results.

Compared with prior art, technical characteristic and effect of the present invention:

The invention proposes a low-cost, miniaturized, portable terahertz linewidth measuring instrument, which has faster measurement speed, higher measurement accuracy and reliability, and can be widely used in laboratories engaged in terahertz related fields middle. The terahertz linewidth measuring instrument based on the Fabry-Perot interferometer has the following advantages: 1. The structure is simple, the volume is small, and it can be used as a miniaturized and portable instrument. 2. The manufacturing cost is low. 3. Easy to operate and fast to measure. 4. High measurement accuracy.

Description of drawings

Figure 1 Schematic diagram of line width measurement principle.

Fig. 2 is a schematic diagram of the structure of the patent of the present invention.

In the figure: 1. Gold-plated off-axis parabolic mirror; 2. Fabry-Perot interferometer cavity mirror a (fixed position); 3. Fabry-Perot interferometer cavity mirror b (moving mirror) 4. Step 5. Gold-plated off-axis parabolic mirror on the surface; 6. Chopper; 7. Pyroelectric detector; 8. Data acquisition card; 9. Computer.

Figure 3 is a schematic diagram of the workflow of the line width measurement system.

Figure 4 is a schematic diagram of a wedge-angle cavity mirror with a wedge angle of about 1 degree on both surfaces.

Detailed ways

A realization scheme of a terahertz linewidth measurement device based on a scanning Fabry-Perot interferometer is proposed. This device has a compact structure, low cost and can work at room temperature, and is suitable for terahertz research fields and Practical application fields of terahertz technology.

The purpose of the present invention is to provide a solution for monochromatic terahertz wave linewidth measurement:

According to the Airy formula of multi-beam interference, under the condition of not considering the absorption, the power transmittance of Fabry-Perot (FP) interferometer to the wavelength λ is

$\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\mathrm{<span\; class="notranslate">\; nd</span>}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; .</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 0.1</span>}<span\; class="notranslate">\; )</span>$

where d is the distance between the two reflective surfaces of the FP interferometer, R is the reflectivity of the two reflective surfaces, n is the refractive index of the filling medium between the reflective surfaces, and θ is the incident angle. Keeping other parameters constant, the transmittance T of the FP interferometer is a function of the length d of the interference cavity, and the change curve of the former and the latter is shown in Figure 1. It can be seen from the figure that the curve is periodic, which is determined by the sin square term in (0.1) formula.

The present invention introduces two parameters of fringe fineness (Finesse) and free spectral range (Free SpectralRange/FSR) of the Fabry-Perot interferometer. The size of the fringe fineness is the ratio of the distance between two adjacent interference peaks to the full width at half maximum (FWHM) of the interference curve. The full width at half maximum (FWHM) of the curve is shown in Figure 1. For a specific FP interferometer, the fringe finesse can be approximated as a constant that does not vary with the length of the interference cavity. The expression of the free spectral range FSR of the Fabry-Perot interferometer is

FSR＝c/2nd (0.2)

Wherein n is the refractive index of the filling medium in the interference cavity. For the case where the cavity is air, the expression of the free spectral range can be approximately simplified as FSR=c/2d. The unit of free spectral range is hertz (Hz).

The spectral line precision that FP interferometer can distinguish is determined by its free spectral range, and from formula (0.2), it can be seen that the size of free spectral range is determined by the cavity length of the interferometer. When the cavity length of the FP interferometer changes from 0.1mm to 1000mm, its free spectral range decreases from 1.5THz to 150MHz. In the interference curve in Figure 1, the interval corresponding to two adjacent interference peak points is the free spectral range corresponding to the current cavity length. The FWHM of the interference curve at this position is the linewidth of the terahertz wave. The expression of the line width measurement result is,

Linewidth(FWHM)＝FSR/Finesse (0.3)

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

The specific implementation scheme of the terahertz linewidth measurement device is shown in Figure 2. The names and functions of each component of the system are described as follows:

1. Gold-plated off-axis parabolic mirror. The reflective surface has a reflectivity close to 100% in the terahertz band, and the off-axis parabolic reflective surface plays the role of beam shaping, and the incident terahertz wave passes through the parabolic mirror 1 and exits in parallel.

2. Fabry-Perot interferometer cavity mirror a (fixed position). High-resistance silicon material, two surfaces are polished, and the two surfaces have a wedge angle of about 1 degree to avoid the formation of an FP resonant cavity inside. The wedge angle is shown in Figure 4. There are no special requirements, as long as the two surfaces on the inside are parallel, and the orientation of the two surfaces on the outside can be arbitrary. The surface close to cavity mirror b is perpendicular to the incident terahertz wave.

3. Fabry-Perot interferometer cavity mirror b (moving mirror). The material and external dimensions are exactly the same as those of the cavity mirror a, and the surface close to the cavity mirror a is perpendicular to the incident terahertz wave. The cavity mirror b is installed on the mobile platform 4 .

4. Step into the mobile platform. The stepping motion along the propagation direction of the terahertz wave is controlled by the computer 9 program.

5. Gold-plated off-axis parabolic mirror. Same as the parabolic mirror 1, it is used to converge the parallel incident terahertz waves.

6. Chopper. Placed at the focal point of the parabolic mirror 5, the modulation frequency of the terahertz wave to be measured is corrected to a frequency suitable for the pyroelectric detector 7 at the rear end.

7. Pyroelectric detectors. The power of the transmitted terahertz wave is measured, and a voltage signal proportional to the detected power is output to the data acquisition card 8 .

8. Data acquisition system. Perform AD conversion on the analog signal output by the pyroelectric detector, and output the digital signal to the computer 9 .

9. Computers. Run the measurement program, read the digital voltage value output by the data acquisition system, and issue work instructions to the stepping displacement platform. Calculates line width measurements for the user.

The working flow diagram of the line width measurement system is shown in Figure 3. The entire system is controlled by a computer program, first initialized and set by the user to scan step size and number of samples. Then the system is set according to the sampling number, and the original data of cavity length and transmission intensity are obtained through measurement. Finally, the linewidth of the terahertz wave to be measured is calculated according to the linewidth result expression (0.3).

Claims (3)

1. A method for terahertz wave linewidth measurement is characterized in that, utilizing a Fabry-Perot FP interferometer to measure the power transmittance to wavelength λ, by the formula: $\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; R</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; sin</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}\mathrm{<span\; class="notranslate">\; nd</span>}\mathrm{<span\; class="notranslate">\; cos</span>}\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; .</span>$ Obtain the interference cavity length d, where R is the reflectivity of the two reflective surfaces, n is the refractive index of the filling medium between the reflective surfaces, and θ is the incident angle; The expression of the free spectral range FSR according to the Fabry-Perot interferometer: FSR=c/2nd get FSR; Then by the formula: Linewidth(FWHM)＝FSR/Finesse The Linewidth (FWHM) of the terahertz wave is obtained, where the Finesse of the fringes is approximately a constant that does not change with the length of the interference cavity. 2. the method for terahertz wave linewidth measurement as claimed in claim 1, is characterized in that, utilizes Fabry-Perot FP interferometer to specifically comprise the following steps: Use a gold-plated off-axis parabolic mirror to reflect the incident terahertz wave and emit it in parallel to the cavity mirror of the Fabry-Perot interferometer; The high-resistance silicon material is used as the Fabry-Perot interferometer cavity mirror a with a fixed position. The two surfaces of the cavity mirror a are polished, and the two surfaces of the cavity mirror a have a wedge angle of about 1 degree to avoid the internal formation of an FP resonant cavity. , the surface close to the cavity mirror b of the Fabry-Perot interferometer is perpendicular to the incident terahertz wave; The materials and dimensions of cavity mirror b of the Fabry-Perot interferometer are exactly the same as those of cavity mirror a, and the surface close to cavity mirror a is perpendicular to the incident terahertz wave. Cavity mirror b is installed on the mobile platform 4; Stepping the mobile platform along the direction of terahertz wave propagation; Converge the parallel incident terahertz waves from the cavity mirror of the Fabry-Perot interferometer by using a gold-plated off-axis parabolic mirror; Place the chopper at the focus position of the gold-plated off-axis parabolic mirror, and correct the modulation frequency of the terahertz wave to be measured to a frequency suitable for the back-end pyroelectric detector; The pyroelectric detector measures the power of the transmitted terahertz wave, and outputs a voltage signal proportional to the detected power to the data acquisition card; Perform AD conversion on the analog signal output by the pyroelectric detector, and output the digital signal to the computer; Use the computer to run the measurement program, read the digital voltage value output by the data acquisition system, and issue work instructions to the stepping displacement platform; calculate the line width measurement results for the user. 3. A device for measuring terahertz wave linewidth, characterized in that the structure is: The incident terahertz wave passes through the gold-plated off-axis parabolic mirror on the first surface and then exits in parallel to the cavity mirror of the Fabry-Perot interferometer; The position-fixed Fabry-Perot interferometer cavity mirror a made of high-resistance silicon material has two polished surfaces, and the two polished surfaces have a wedge angle of about 1 degree. The surface close to the cavity mirror b and the incident terahertz wave vertical; Fabry-Perot interferometer cavity mirror b has the same material and dimensions as cavity mirror a, the surface close to cavity mirror a is perpendicular to the incident terahertz wave, and cavity mirror b is installed on the mobile platform 4; The stepping mobile platform moves stepping along the propagation direction of the terahertz wave; The parallel incident terahertz waves from the cavity mirror b are converged by the second gold-plated off-axis parabolic mirror; A chopper is installed at the focal point of the gold-plated off-axis parabolic mirror on the second surface, and the terahertz wave output by the gold-plated off-axis parabolic mirror on the second surface passes through the chopper, pyroelectric detector, and AD converter to output measurement results.