In a fiber-based terahertz-time domain spectroscopy (THz-TDS) system, the femtosecond pulse emitted by the femtosecond laser as the pump source has a wide spectrum and high peak power. When transmitted through the fiber, dispersion and nonlinear effects occur, which results in the broadening of the output optical pulse and distortion of the waveform. However, due to the low peak power required by the transmitter and receiver (usually less than 10 mW), the nonlinear effects are weakly influential. Therefore, when building the system, the primary consideration is the dispersion compensation of the femtosecond laser in the optical path to ensure the optimal performance of the THz-TDS system. In the 1560 nm wavelength band, both positive and negative values of dispersion can be controlled by designing the waveguide dispersion of the fiber. Additionally, the coupling efficiency of the fiber connection is higher than that of the spatial dispersion compensation module. To further improve the stability and compactness of the system, dispersion compensating fiber (DCF) is usually used to compensate for the dispersion in the antenna tail fiber. However, there are two issues when using DCF for dispersion compensation in THz-TDS systems. On the one hand, DCF cannot compensate for dispersion in real time. In the fiber-optic terahertz time-domain spectroscopy system, all components, except for the photoconductive antenna, are usually packaged as a whole. In practical engineering applications, the length of the tail fiber after the photoconductive antenna is changed according to different usage requirements. However, DCF can only accurately compensate for fibers of a specific length. When the fiber length changes, it is necessary to redesign and replace the DCF with one of the corresponding length. This replacement process is complex and time-consuming. On the other hand, due to the high nonlinear coefficient of DCF, it can also lead to distortion of the femtosecond laser pulse waveform output to the antenna after compensation. The autocorrelation curve will produce side lobes, reduce the pump pulse energy, and affect system performance. To address the issue that DCF cannot compensate for dispersion in real time, we use grating pairs to realize adaptive dispersion compensation for tail fibers of different lengths. Considering that it is difficult for grating pairs with a conventional parallel structure to provide normal dispersion, a Martinez-type stretcher is adopted based on normal dispersion. Ray tracing methods are used to analyze and calculate the influence of parameters such as the equivalent spacing of the Martinez stretcher grating, the incident angle, and the number of grating lines on the dispersion compensation effect. The selection of the number of grating lines and the incident angle is determined based on the analysis results. The dispersion compensation module based on the Martinez stretcher is then constructed using this grating and lens. The adaptive compensation ability of this structure for optical fiber is verified experimentally. When compared with the compensation results of DCF, the superiority of the compensation effect of the Martinez stretcher structure is highlighted.

In this paper, we utilize a dispersion compensation module based on a Martinez stretcher, capable of generating normal dispersion, to pre-chirp a femtosecond laser with a central wavelength of 1560 nm in an optical-type THz-TDS system. First, we analyze the influence of dispersion introduced by femtosecond laser transmission in standard single-mode fiber, the structure of the Martinez stretcher, and then calculate the dispersion introduced. Then, we analyze and calculate the effects of the number of grating lines, the equivalent spacing of the grating, and the incident angle on the dispersion compensation effect in the Martinez stretcher using ray tracing and numerical simulation. Based on the analysis results, we select the number of grating lines and the incident angle. Finally, the dispersion compensation module based on the Martinez stretcher is built using the determined grating and lens. The dispersion compensation is carried out for a 10.0 m fiber, and the dispersion compensation ability of the module is verified through experiments. In addition, the DCF and dispersion compensation module are used to compare the dispersion compensation of a 4.3 m fiber, and the dispersion compensation effects of the two are compared and analyzed.

The optical path simulation is carried out using ray tracing software, and the working structure of four types of line gratings is determined: the working structure of the gratings with 1200 lp/mm and 1000 lp/mm is an “eight” shape, while the working structure of the gratings with 600 lp/mm and 300 lp/mm is an inverted “eight” shape (Fig. 2). The incident angle range of the four types of line gratings is determined as follows: the working angle of the grating with 1200 lp/mm is 82°?85°, the working angle of the grating with 1000 lp/mm is 63°?67°, the working angle of the grating with 600 lp/mm is 11°?15°, and the working angle of the grating with 300 lp/mm is 0°?5°. The distance between the grating and the lens is also determined. A dispersion compensation module is built and used for dispersion compensation. When the fiber length is 10.0 m, the femtosecond laser with an initial pulse width of 100 fs is broadened to 7.2 ps and then compressed to 63 fs under the action of the dispersion compensation module (Fig. 7). When the fiber length is 4.3 m, the femtosecond laser with an initial pulse width of 100 fs is broadened to 3.1 ps and then compressed to 60 fs under the action of the dispersion compensation module (Fig. 8). Under the action of the DCF, it is compressed to 59 fs (Fig. 9). By comparing and analyzing the two results, it is found that the dispersion compensation module provides better compensation.

Aiming at the problem of pulse broadening caused by negative second-order dispersion when the central wavelength 1560 nm femtosecond pulse is transmitted in standard single-mode fiber, numerical analysis and experimental construction are carried out based on the positive second-order dispersion Martinez-type stretcher that can be generated. Firstly, the working structure, incident angle, and the range of equivalent adjustment spacing of four common line gratings—1200, 1000, 600, and 300 lp/mm—are analyzed using the ray tracing method. After that, the influence of the structural parameters of the stretcher on the dispersion value is analyzed by establishing a mathematical model. In the case of the same beam incident angle, the larger the equivalent grating spacing, the greater the dispersion provided by the stretcher structure; in the case of the same equivalent grating spacing, the smaller the incident angle, the greater the dispersion provided by the stretcher structure. Next, by comprehensively analyzing the dispersion and adjustment accuracy provided by the four gratings, the 600 lp/mm grating and a 15° incident angle are selected. Finally, a dispersion compensation module is built based on a one-way Martinez stretcher. The spatial light after the module is coupled into the single-mode fiber, and then the fiber of a certain length is connected. The autocorrelation instrument is then connected to measure the pulse width, and the dispersion compensation effect is verified. At the same time, the compensation results of the dispersion compensation fiber and the Martinez stretcher for the same length of fiber are compared. In summary, the structural parameters of the stretcher—such as the number of grating lines, the incident angle, and the equivalent grating spacing—affect the dispersion value of the DCM. Through real-time regulation of the equivalent grating spacing, adaptive compensation for single-mode fibers of different lengths is achieved. Finally, an ultra-short pulse output with a pulse width of less than 100 fs can be realized, meeting the needs of the THz system. Compared with DCF, its flexibility is more prominent, and the compensated femtosecond laser pulse has better quality and no baseline.