【摘要】 提出了一种基于时频扫描非线性过程的激光脉冲啁啾率测量方法，并通过实验验证了其可行性。结果表明，对于一束中心波长为808 nm、光谱宽度为51.42 nm的啁啾脉冲，该方法测得其前后沿相对时间延迟量为20.63 ps，与理论值21.06 ps相比，相对偏移约为2%。进一步分析可知，该方法可以实现很高精度的啁啾率测量，其测量偏差主要由测量光谱所用的光谱仪分辨率决定。实验中采用的光谱仪分辨率为0.5 nm，理论计算表明其最大测量偏差为2.99%。目前商品化光谱仪的光学分辨率最高可达0.01 nm，因此理论上可以将最大测量偏差降低至0.04%。该方法的最大优势是支持较大的光程延迟与宽扫描时域量程，因此在数十皮秒至数纳秒脉冲的啁啾率及脉宽测量方面具有广阔的应用前景。更多还原

【Abstract】 Objective Chirp rate measurement is essential for pulse compression with chirped pulse amplification(CPA) and for chirped pulse utilization in physical experiments. Methods such as frequency-resolved optical gating(FROG), cross correlation frequency resolved optical gating(X-FROG), time-domain ptychography(TDP), two-dimensional spectral shearing interferometry(2DSI), and spectral phase interferometry for direct electric field reconstruction(SPIDER) are widely adopted for measuring pulses with sub-picosecond durations. Moreover, although a two-dimensional extension of the stationary phase point(SPP) method can be applied to the dispersion measurement of long pulses with durations exceeding tens of picoseconds, it achieves this by analyzing the phase behavior near the SPP in the interference fringes. In this paper, we propose a concept for chirp rate measurement based on a temporal-spectral scanning parametric process(TSSP).Methods The chirped pulse to be measured is beam split, with one branch providing the narrow bandwidth probe beam and the other maintaining a high fidelity with incidence. They are injected into a nonlinear crystal to realize noncollinear sum-frequency generation(NSFG). By precisely introducing a temporal delay into the probe beam, a temporal-spectral scanning between the probe beam and chirped pulse is achieved. Simultaneously, the sum-frequency generation(SFG) at different temporal delays is recorded. The chirped rate can be achieved by numerical calculation on the temporal delay and NSFG spectrum. The TSSP only requires recording the central wavelength of the SFG, rather than capturing intensity or interference fringes.Results and Discussions The time delay corresponding to different SFG spectra measured using the TSSP is shown as the dashed curve in Fig. 5(a). Correspondingly, the theoretical time delay calculated based on Eq.(5) with a preset grating separation of b0=34.3 mm is depicted as the solid curve in Fig. 5(a). A comparison of these two curves reveals that the deviation between the experimental measurements and theoretical calculations is minimal. When selecting a probe wavelength of 807.73 nm, the spectral component of the SFG signal is in one-to-one correspondence with the spectral component of the injected chirped pulse. Within a spectral width of 51.42 nm, ranging from 777 nm to 829 nm, the experimentally measured time delay is 20.63 ps, which results in a difference of 0.43 ps and a total deviation rate of 2.04% compared with the theoretical value of 21.06 ps. This demonstrates that the TSSP method achieves high accuracy in chirp rate measurements for pulses with durations on the order of tens of picoseconds. In this experiment, the primary source of deviation is the limited optical resolution of the spectrometer. Employing a spectrometer with higher optical resolution can effectively enhance the accuracy of chirp rate measurements. For instance, the spectrometer used in this study has an optical resolution of 0.5 nm, leading to a theoretical maximum measurement deviation of 2.99%. Commercially available spectrometers can achieve optical resolutions as high as 0.01 nm, theoretically reducing the deviation to 0.04%. Furthermore, if the TSSP method is applied to the measurement of chirped pulses with broader pulse durations, the measurement deviation will further decrease. In the field of picosecond pulse width measurements, the TSSP method exhibits significantly higher measurement accuracy than that based on the autocorrelation principle of large-aperture nonlinear crystals, highlighting its substantial application potential in picosecond pulse characterization. Figure 5(b) presents a comparison between the chirp rate measured using the TSSP method and the preset chirp rate. The solid curve represents the experimental measurements, calculated by substituting the SFG wavelength data and corresponding time delays obtained by the TSSP method into Eq.(4). The dashed curve is a fitted curve based on the experimental data, whereas the dotted curve represents the theoretical chirp rate derived from Eq.(6) using the preset grating separation. The fluctuations observed in the solid curve mainly originate from the derivative operation on the SFG wavelength in Eq.(4), which makes it highly sensitive to the recorded central wavelength data in the experiment. If a spectrometer with extremely high optical resolution is used for SFG spectrum measurements, ensuring more accurate recording of the central wavelength data, these fluctuations will be significantly reduced. The deviation between the theoretically preset chirp rate and the experimentally fitted chirp rate is nearly zero around 812 nm, with maximum deviations of approximately 5.9% and 4.0% in the shorter and longer wavelength regions, respectively, both significantly smaller than the fluctuations observed in the solid curve. This is because the SFG signal is independently measured and recorded during scanning, ensuring non-divergence of the measurement data in the TSSP method. Consequently, the fitted curve closely matches the theoretical curve, further validating the accuracy and reliability of the TSSP method.Conclusions This paper proposes an absolute chirp rate measurement method based on a time-frequency scanning nonlinear process. A TSSP-based measurement is implemented and experimentally studied using the front end of the SG-II 5 PW laser system. For a spectral range of 777-829 nm(pulse width Δλ=51.42 nm), the experimentally measured relative time delay is 20.63 ps, which compared with the preset value of 21.06 ps, yields a relative deviation of 2.04%. Furthermore, the chirp rate obtained from fitting the experimental data shows minimal deviation from the theoretical preset value near the central wavelength, demonstrating the high accuracy of TSSP measurements. The TSSP method is a simple and direct measurement technique that does not require algorithmic reconstruction, intensity distribution measurement, or interference fringe analysis. When combined with a high-optical-resolution spectrometer, it not only achieves higher measurement accuracy but is also applicable to chirp rate measurements for pulses with durations ranging from tens of picoseconds to nanoseconds. Additionally, by incorporating intensity information and phase reconstruction algorithms, the TSSP method holds significant potential for applications in the spectral phase and pulse width measurements.更多还原