Design of Terahertz Imaging Data Acquisition System Based on PCI-9812

Terahertz waves (THz) refer to electromagnetic waves with a frequency range of 0.1 to 10 THz (1 THz = 1012Hz) and a wavelength range of 30μm to 3 mm. This band is between microwaves and infrared radiation. Therefore, terahertz waves have the characteristics of both waves and light, and have shown great application prospects in object imaging, time-domain spectrum analysis, medical diagnosis, environmental monitoring, space remote sensing, and military security. The photon energy of terahertz waves is only 4.1 meV, and it does not have the ionization characteristics of X-rays, and will not cause harm to materials and the human body. Therefore, terahertz imaging technology has greater application advantages than X-rays. After the 1990s, due to the development of free electron lasers and ultrafast technology, a stable and reliable excitation light source was provided for the generation of THz pulses. Countries around the world have carried out research on terahertz wave technology in various fields. In recent years, Chinese scientific researchers have also carried out a lot of research on terahertz wave technology. The current research on terahertz imaging technology is mainly focused on the research of point-by-point scanning terahertz imaging. The basic principle of this imaging method is to use a terahertz wave of known waveform as the imaging ray, pass through the imaging sample or the intensity and phase of the terahertz electromagnetic wave reflected from the sample (including the spatial distribution of the sample's complex dielectric constant), and record the intensity or phase information of the transmitted or reflected terahertz electromagnetic wave. After digital processing and analysis, a terahertz two-dimensional image of the sample is obtained. Although the imaging time of this system is relatively long, the amount of information obtained by this imaging method is large, and two-dimensional and three-dimensional images can be constructed. Moreover, each pixel source corresponds to a terahertz time-domain spectrum, and the terahertz frequency response spectrum of each point can be obtained by Fourier transforming the time-domain spectrum.

At present, there are several main methods of generating terahertz lasers with optics: terahertz gas lasers; using ultrashort laser pulse photoconductivity or optical rectification to generate terahertz radiation; using nonlinear difference frequency process (DFG) and parametric process to generate terahertz waves. In the scanning terahertz imaging system established in China. The terahertz radiation source uses a small commercial continuous CO2 laser pumped terahertz laser, which has been promoted and applied. It is a far-infrared laser source with a compact structure, relatively stable output and high power.

The principle of the scanning terahertz imaging system is: the terahertz laser generates continuous THz laser, and the light wave first passes through the chopper to generate a series of terahertz pulses. In order to eliminate the divergence of terahertz light, a polyethylene lens is generally used to collimate the light path and focus the laser on the sample to be tested. The sample is placed on a linear stepping platform that can be scanned in the x and y directions and controlled by computer programming. The platform carries the sample to be tested and moves two-dimensionally at the focal point of the light. In order to achieve a better imaging effect, the system focuses the THz light with sample information on the detector through a polyethylene lens with high transmittance and low loss, and converts the light intensity of the light wave into a voltage value; the converted electrical signal is sampled, A/D converted, and then transmitted to the computer to construct a terahertz image.

The main work here is to use the PCI -9812 data acquisition card produced by ADLINK to design a data acquisition system for the terahertz imaging system based on the P4-42 detector produced by Coherent.

PCI-9812 is a four-channel high-speed data acquisition card based on 32-bit PCI bus launched by Taiwan ADLINK Technology Co., Ltd. It has 12-bit analog input resolution, 32 KB FIFO buffer inside the board, and the highest sampling frequency can reach 20 MHz, which ensures the high accuracy and speed of the data acquisition circuit. It has five trigger modes: software trigger, front trigger, back trigger, middle trigger and delay trigger, which can be selected through software programming. In addition, it also has three A/D clock sources that can be programmed: internal pulse, external pulse and external sine. In the acquisition system of this article, the soft trigger trigger mode is adopted and the internal pulse clock source is adopted.

The highest sampling rate that the acquisition card can achieve is related to the number of channels sampled, the amount of data collected, and the PCI bus bandwidth. When the total amount of data from all channels collected by a single trigger does not exceed the 32 KB FIFO buffer, each channel can be sampled at the highest 20 MHz A/D sampling rate. When the total amount of data exceeds the 32 KWord FIFO buffer, the data must be directly transferred to the computer memory in DMA mode. In the acquisition system of this article, one channel is used, and channel 0 is selected, which can provide FIFO up to 32 KB and a maximum data transmission capacity of 40 MB/s. It should be noted that when connecting the circuit, not only the input voltage range of PCI-9812 should be considered, but also the input impedance. PCI-9812 has four A/D converters that can simultaneously collect bipolar signals at high speed. The input voltage range is -1 to +1 V or -5 to +5 V, which can be selected through the hardware solder seal. The input impedance can also be selected through the hardware solder seal on the circuit board. The impedance values ​​that can be selected are: 15 MΩ and 50 Ω (default value) when -1 to +1 V is input; 1.25 kΩ and 50 Ω (default value) when -5 to +5 V is input. The P4-42 detector uses a 15 V voltage source, and the output resistance is 50 Ω. Its maximum output voltage value is about 12 V, so a voltage divider resistor must be connected in series before connecting the acquisition card. The acquisition card in this article has an input range of -5 V to +5 V, an input resistance of 1.25 kΩ, and a voltage divider resistor in series of 1.8 kΩ. At this time, the maximum voltage input to the acquisition card is 4.84 V.

2 Software System Design

The PCI-9812 acquisition card supports software development platforms such as VC/C++, VB, BorlandC++, Borland Delphi, Labview, etc., and mainly uses VC++6.0. It mainly includes scanning control module, data acquisition module and data processing module. The flowchart of software system design is shown in Figure 1. For the point-by-point scanning terahertz imaging system, its imaging process is a cyclic scanning process. After the platform carries the sample to be tested and moves to the position to be tested, the computer controls the THz source to emit laser, the detector receives the light signal and converts it into a voltage value, the data acquisition card collects the voltage value and transmits it to the computer, the computer processes the data, generates the pixel value of the position point (x, y) and stores it, and then controls the platform to move to the next position to be tested for scanning.

The scanning control module is used to control the optical path. It mainly includes the selection of laser frequency, the setting of chopping frequency, the setting of two-dimensional mobile platform and other functions. In order to ensure that there is no confusion in the information between each pixel point, the movement of the two-dimensional platform and the synchronization of data acquisition and recording must be ensured during the information transmission process. The computer controls the platform servo system through the serial port to set the stride and direction. In order to save scanning time, the mobile platform adopts "S" type movement. When the system is running, the movement direction of the platform is determined by the sign of the parameter z. The initial value of Z is set to 1. The accurate pixel coordinates of the acquisition point at this time can be obtained through Y+Z. This ensures that each pixel point (x, y) of the computer stored data corresponds to each projection position in the scanned sample, and there will be no pixel confusion.

The data acquisition module mainly drives the acquisition card to work. First, use the Register_Card() function to register the acquisition card. The system BIOS will return a PCI-9812 registration number. Then, you can use the AI_9812_Config() function to complete the initialization of the acquisition card, set the acquisition trigger mode, trigger source, and number of sampling points after triggering. After setting these parameters, you can use the laser to generate terahertz lasers. After scanning each transmission point, call the AI_ContReadChannel() function (when using multiple channels, you should call the AI_ContScanChannels() function) to start the A/D sampling of the detector output voltage value, and then call the AI_AsyncCheck() function to query whether the acquisition card has completed sampling. During the sampling process, the acquisition card automatically writes the collected data to the requested DMA memory in DMA mode, and the computer obtains the scanned data by reading the DMA memory. When processing data, you also need to call the data conversion function AI_ContScale() to convert the collected data volume to the corresponding voltage value. After the scanning process is completed and the acquisition is finished, use the AI_AsyncClear() function to stop the acquisition card and use the Release_Card() function to release the acquisition card.

The data processing module is mainly used to perform computer comparative analysis on the scanning data obtained at each point to construct a terahertz image of the sampled object. When the terahertz wave scans each point, a large amount of information can be obtained. The image of the sampled object can be constructed by analyzing the light intensity, phase, time delay, etc., and each parameter must be analyzed using its own algorithm. Here we mainly focus on the P4-42 pyroelectric detector, which converts the intensity of terahertz light into a voltage signal. Therefore, this article analyzes the peak-to-peak value of the voltage signal to obtain the absorption intensity of the terahertz light wave at each point on the sample as the pixel value of each pixel in the terahertz image. After the imaging system scans each position point, terahertz imaging data in the format of x*y*t is obtained, and the data of each pixel point is normalized. A terahertz grayscale image of the sample to be tested can be obtained.

3 Conclusion

The terahertz imaging data acquisition system introduced in this paper is for the P4-42 pyroelectric detector. It uses the PCI-9812 data acquisition card of ADLINK to acquire data and transmits it to the computer to reconstruct the image. It can realize the functions of scanning control, data acquisition and image reconstruction for terahertz imaging. It has certain significance for the practical application of terahertz imaging.