Terahertz grating characterization system based on self-mixing technology

Abstract

The invention relates to a terahertz grating characterization system based on a self-mixing technology, and the system comprises a terahertz quantum cascade laser which is disposed in a constant-temperature environment and is used for generating multi-mode laser; the optical collimation system is used for collimating the multi-mode laser and forming parallel light beams; the displacement platform is arranged on an irradiation light path of the parallel light beams; the rotating platform is arranged on the displacement platform, is used for mounting a blazed grating to be measured, and can change the incident angle of the parallel light beams by adjusting the angle; the control system is used for controlling the displacement platform, so that the parallel light beams can irradiate the blazed grating to be detected on the rotating platform; and the oscilloscope system is used for observing a signal which is reflected by the blazed grating to be measured and returns to the laser cavity of the terahertz quantum cascade laser, and the signal is acquired by a data acquisition card. According to the invention, the key structure parameters of the blazed grating can be rapidly and accurately measured.

Description

Technical Field

[0001] The present invention relates to the technical field of optical element characterization, and particularly to a terahertz grating characterization system based on self-mixing technology. Background Art

[0002] As an important optical diffraction element, the blazed grating has been widely used in the fields of spectral analysis, laser technology, optical communication, and optical sensing due to its unique optical properties. The structural parameters of the blazed grating, such as the blaze angle, groove depth, period, and groove shape, directly affect the key performance indicators of the grating, such as diffraction efficiency, resolution, and polarization characteristics. Therefore, accurate characterization of the blazed grating is crucial for optimizing its design, improving its performance, and ensuring its reliability in practical applications. However, traditional methods for characterizing the structure of blazed gratings, such as optical microscopes, atomic force microscopes (AFMs), and scanning electron microscopes (SEMs), are used to observe the surface morphology and groove structure of the grating, but they often have limitations such as limited measurement resolution, slow measurement speed, high requirements for the sample surface, and difficulty in achieving in-situ measurement. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a terahertz grating characterization system based on self-mixing technology, which can quickly and accurately measure the key structural parameters of the blazed grating.

[0004] The technical solution adopted by the present invention to solve its technical problems is: providing a terahertz grating characterization system based on self-mixing technology, including:

[0005] A terahertz quantum cascade laser, placed in a constant temperature environment, for generating multimode laser;

[0006] An optical collimation system, for collimating the multimode laser and forming a parallel light beam;

[0007] A displacement platform, arranged on the irradiation light path of the parallel light beam;

[0008] A rotation platform, arranged on the displacement platform, for installing the blazed grating to be measured and capable of changing the incident angle of the parallel light beam by adjusting the angle;

[0009] A control system, for controlling the displacement platform so that the parallel light beam can irradiate the blazed grating to be measured on the rotation platform;

[0010] An oscilloscope system, for observing the signal that is reflected by the blazed grating to be measured and returns to the laser cavity of the terahertz quantum cascade laser, and collecting it with a data acquisition card.

[0011] The operating frequency range of the terahertz quantum cascade laser is 4.1 - 4.3 THz, and the frequency interval is 10 - 10.05 GHz.

[0012] The terahertz quantum cascade laser operates in continuous wave mode.

[0013] The surface of the blazed grating to be measured is aluminized.

[0014] The optical collimation system is an off-axis parabolic mirror.

[0015] The selection resolution of the rotary stage is ±1'.

[0016] Advantageous Effects

[0017] Due to the adoption of the above technical solutions, compared with the prior art, the present invention has the following advantages and positive effects: The terahertz grating characterization system based on the self-mixing technology of the present invention can accurately measure the key structural parameters of the grating. Compared with the traditional characterization methods, the present invention has achieved significant improvements in both measurement accuracy and measurement speed, providing an efficient technical means for the rapid evaluation of the grating performance. Description of the Drawings

[0018] Figure 1 is a schematic structural diagram of the terahertz grating characterization system based on the self-mixing technology in the embodiment of the present invention;

[0019] Figure 2 is an emission spectrum diagram of the terahertz quantum cascade laser under different injection currents in the embodiment of the present invention;

[0020] Figure 3 is a schematic structural diagram of the blazed grating to be measured in the embodiment of the present invention;

[0021] Figure 4 is an error analysis diagram between the theory and experiment of the grating constant;

[0022] Figure 5 is a diagram showing the change of the diffraction efficiency of the grating with the emission frequency at a specific angle. Detailed Embodiments

[0023] The following further elaborates the present invention in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0024] The embodiment of the present invention relates to a terahertz grating characterization system based on the self-mixing technology, as Figure 1 shown, including:

[0025] A terahertz quantum cascade laser (THz QCL) 1 is placed in a constant temperature environment and is used to generate multimode laser light;

[0026] An optical collimation system 2 is used to collimate the multimode laser light and form a parallel light beam;

[0027] A displacement platform 3 is arranged on the illumination light path of the parallel light beam;

[0028] A rotary platform 4 is arranged on the displacement platform 3 and is used to mount the blazed grating 5 to be measured, and can change the incident angle of the parallel light beam by adjusting the angle;

[0029] A control system 6 is used to control the displacement platform 3 so that the parallel light beam can irradiate the blazed grating 5 to be measured on the rotary platform 4;

[0030] An oscilloscope system 7 is used to observe the signal that is reflected by the blazed grating to be measured and returns to the laser cavity of the terahertz quantum cascade laser, and is collected by a data acquisition card.

[0031] In this embodiment, the THz QCL is the core device of the entire characterization system. Its spectral width directly determines the measurement coverage range and is a key parameter affecting the grating characterization ability. Compared with single-mode THz QCLs, the spectral width of traditional single-mode devices is usually only a few GHz, and the spectral coverage range is limited, which severely restricts the ability to identify multi-dimensional parameters of blazed gratings. To solve this problem, a wide-spectrum multimode THz QCL is used in this embodiment to significantly expand the spectral range. Each independent mode of the wide-spectrum THz QCL usually has a tuning range of several GHz. By superimposing the tuning ranges of multiple modes, the overall spectral coverage range can be doubled, thereby significantly improving the resolution and analysis ability of grating characterization. The development of wide-spectrum THz QCLs involves multiple key technical links, including active region design, molecular beam epitaxy (MBE) material growth, and device fabrication processes, etc. In terms of active region design, a multi-active region heterostack structure can be adopted. By superimposing active region designs with different center frequencies, effective spectral expansion can be achieved. In addition, optimizing the waveguide structure and thermal management design are also important factors to ensure the high-performance operation of wide-spectrum THz QCLs. In this embodiment, a multimode THz QCL device with a working frequency range of 4.1 - 4.3 THz is used, and its active region is designed as a hybrid of bound state - continuum and resonant phonon structures. The laser provides multimode emission when the driving current exceeds the threshold current, and the frequency of the laser can be tuned by changing the driving current or the temperature of the radiator. The frequency interval of this device is 10.042 GHz, the overall spectral coverage range reaches approximately 200 GHz, and the current tuning rate for each frequency is approximately 2.3 MHz / mA.Figure 2 The spectral diagrams of the device under different injection current conditions are shown, clearly reflecting its multimode characteristics and wide spectral coverage ability. The use of such a wide-spectrum THz QCL not only improves the accuracy and efficiency of grating characterization but also provides strong technical support for the rapid identification and analysis of complex grating structures.

[0032] A reflective blazed grating is a specially designed grating structure. Compared with traditional gratings, its uniqueness lies in being able to concentrate energy on a specific diffraction order, thus achieving efficient spectral splitting. The diffraction energy of traditional gratings is usually concentrated on the zero-order spectrum. However, the blazed grating redistributes and concentrates the energy of the zero-order spectrum onto a certain specific order (usually the first or second order) by optimizing the groove structure. This phenomenon is called the "blaze phenomenon". The core design of the blazed grating lies in the fact that the groove surface and the grating plane are distributed at a fixed angle, as Figure 3 shown. This design separates the diffraction main maximum direction of a single groove from the zero-order main maximum direction of the overall grating interference. Specifically, the diffraction main maximum direction of each groove is determined by the inclination angle of the groove surface, while the zero-order main maximum direction of the overall grating interference is determined by the grating period. By precisely designing the groove angle and the grating period, the diffraction main maximum direction of a single groove can be made to coincide with the interference main maximum direction of a certain order, thereby transferring the energy originally concentrated on the zero-order spectrum to that order and achieving efficient energy concentration. This energy concentration characteristic gives the blazed grating significant advantages in fields such as spectral analysis, laser tuning, and optical sensing. For example, in a spectrometer, the blazed grating can concentrate more energy within the effective receiving range of the detector, thus increasing the signal intensity and the signal-to-noise ratio. In addition, the design of the blazed grating can also achieve efficient diffraction of a specific wavelength range by adjusting the groove angle and grating parameters, further expanding its application range. In short, the reflective blazed grating provides an important technical means for optimizing the performance of optical systems through its unique blaze phenomenon.

[0033] The terahertz grating characterization system based on the self-mixing technique in this embodiment can characterize a reflective blazed grating operating in the terahertz band. The size of the reflective blazed grating is 50 mm × 50 mm, and the substrate material is selected as K9 glass. To improve the reflection efficiency of the blazed grating, an aluminum plating treatment is performed on its surface to enhance the reflection ability of terahertz waves. The overall structure of the grating characterization system in this embodiment is as Figure 1As shown, its core components include a THz QCL, an optical collimation system, a high-precision displacement platform, and a rotary platform, etc. The THz QCL, as the light source, is placed in a thermostat and operates in continuous wave (cw) mode at a heat dissipation temperature of 20K. The drive current of the THz QCL is provided by the QCL2000LAB to ensure the stable output of the laser. The terahertz wave emitted by the THz QCL is transmitted through the exit window made of high-density polyethylene (HDPE) material, and then collimated by an optical collimation system composed of a two-inch off-axis parabolic mirror to form a parallel light beam. The collimated parallel light beam irradiates the blazed grating to be measured, and the blazed grating to be measured is installed on the rotary platform, and the rotary platform is installed on the high-precision displacement platform. The position of the high-precision displacement platform can be controlled through the control system to achieve precise adjustment of the position. In order to precisely control the incident angle, the resolution of the rotary platform in this embodiment is ±1′. By adjusting the angle of the rotary platform, the incident angle of the terahertz parallel light beam can be precisely changed, so as to comprehensively characterize the diffraction characteristics of the blazed grating. This characterization system combines high-precision mechanical control and a stable terahertz light source, providing a reliable experimental platform for the performance evaluation of the blazed grating. Through this system, the key parameters such as diffraction efficiency, blaze angle, and grating constant of the blazed grating in the terahertz band can be deeply studied, providing an important basis for the design optimization and application of the grating.

[0034] The structural parameters of the blazed grating can be experimentally measured and error analyzed using this characterization system. During the test, first fix a working frequency, then adjust the incident angle by adjusting the rotary platform, find the optimal incident angle at this frequency through the curve shown by the oscilloscope system, and then adjust to the next working frequency and repeat the above steps until the optimal incident angle corresponding to each working frequency is found. As Figure 4 shown, it reveals the relationship between the grating incident angle and the working frequency. From Figure 4It can be clearly seen that as the working frequency increases, the optimal incident angle shows a downward trend, which is completely consistent with the theoretical expectation, verifying the superiority of the test system in terms of measurement accuracy and reliability. Based on the optimal incident angles corresponding to each measured working frequency above, fitting them can obtain a grating constant of approximately 84.89 μm. Through the confirmation of the grating constant, the blaze angle can be calculated to be 24.9°, and there is only a small difference of 0.1° between this value and the designed value. The occurrence of this difference is mainly attributed to the parameter errors in the grating manufacturing process, which provides an important reference for further optimizing the manufacturing process. During the grating manufacturing process, the error ranges of the blaze angle and the grating constant are ±0.5° and ±2% respectively. These errors are typical phenomena during the manufacturing process, mainly caused by process fluctuations and material properties. The structural parameters measured using this characterization system are in agreement with the designed values, within its error range, fully demonstrating the high accuracy of this measurement method. Despite these minor errors, the consistency between the experimental data and the designed values indicates that this characterization system can effectively overcome the uncertainties during the manufacturing process and provide reliable technical support for the precise evaluation of the grating performance. Through the measurement and error analysis of the grating constant, not only the accuracy of this characterization method is verified, but also an important reference is provided for the optimization of the grating manufacturing process, which helps to further improve the performance and application potential of the grating.

[0035] After obtaining the structural parameters of the grating using this characterization system, the performance parameters of the grating can also be analyzed. First is its angular resolution, which is one of the key parameters for measuring the grating performance and directly reflects the ability of the grating to distinguish different frequencies. Through the analysis of the above experimental data, the angular resolution ability of this grating for different frequencies can be estimated to be approximately 0.117 rad / THz. Secondly, the variation of the diffraction intensity of the grating with frequency at a specific diffraction angle is measured in detail, and high-resolution spectral information such as Figure 5 is shown. The experimental results show that each frequency can reach its maximum diffraction efficiency at a specific incident angle, which fully demonstrates that the grating has significant blazing characteristics at different frequencies. At a frequency of 4.26205 THz, the suppression ability between two adjacent modes of the grating reaches 4.316 dB. Figure 5 The inset in shows the spectral information of the grating after splitting light directly obtained using this system. By precisely controlling the incident angle and frequency, the diffraction characteristics of the grating can be comprehensively evaluated, providing an important experimental basis for the design optimization and application of the grating, and offering new possibilities for the optical system design in the terahertz band.

[0036] In summary, through the analysis of the structural parameters and performance parameters of the blazed grating, the high accuracy and reliability of the terahertz grating characterization method based on the self-mixing technology in this embodiment in grating characterization are fully verified. The experimental results show that the characterization system can accurately measure the key structural parameters of the grating, such as the blaze angle and the grating constant, and the measurement results are in good agreement with the design values. By determining the structural parameters, the performance parameters of the grating can be accurately analyzed. Compared with the traditional characterization methods, the characterization system in this embodiment has achieved significant improvements in both measurement accuracy and measurement speed, providing an efficient technical means for the rapid evaluation of the grating performance. This efficient and accurate characterization method not only provides strong technical support for the design optimization and manufacturing process improvement of the blazed grating, but also opens up a new way for the performance evaluation of optical components in the terahertz band.

Claims

1. A terahertz grating characterization system based on self-mixing technology, characterized in that, Including: A terahertz quantum cascade laser, placed in a constant temperature environment, for generating multimode laser; An optical collimation system, for collimating the multimode laser and forming a parallel light beam; A displacement platform, arranged on the illumination optical path of the parallel light beam; A rotary platform, arranged on the displacement platform, for mounting the blazed grating to be measured and capable of changing the incident angle of the parallel light beam by adjusting the angle; A control system, for controlling the displacement platform so that the parallel light beam can irradiate the blazed grating to be measured on the rotary platform; An oscilloscope system, for observing the signal that is reflected by the blazed grating to be measured and returns to the laser cavity of the terahertz quantum cascade laser, and collecting it with a data acquisition card.

2. The terahertz grating characterization system based on the self-mixing technique according to claim 1, wherein The working frequency range of the terahertz quantum cascade laser is 4.1 - 4.3 THz, and the frequency interval is 10 - 10.05 GHz.

3. The terahertz grating characterization system based on the self-mixing technique according to claim 1, wherein The terahertz quantum cascade laser operates in a continuous wave mode.

4. The terahertz grating characterization system based on self-mixing technology according to claim 1, characterized in that, The surface of the blazed grating to be measured is treated by aluminizing.

5. The terahertz grating characterization system based on the self-mixing technique according to claim 1, wherein, The optical collimation system is an off-axis paraboloid mirror.

6. The terahertz grating characterization system based on self-mixing technology according to claim 1, wherein The selection resolution of the rotary platform is ±1′.

Images