# Single-Shot Temporal Contrast Enhancement Measurement of a Plasma Mirror by a Chirped Pulse
## 3 Experiment and Discussion
### Experiment Setup of SOCP System
A far-field CCD, a near-field CCD, and two calorimeters are used for monitoring the output beam and energy of igniting pulse. container is used to obtain the temporal signal of two pulses, which are displayed on the oscilloscope (Tektronix, United states, TD5684C). For raising the accuracy, we introduced another femtosecond pulse as the reference to avoid signal crosstalk near synchronization in the experiment. The inborn temporal difference was 3.76 ns. By deducting an extra 3.76 ns of the measured reference from the SOCP, the synchronization of the igniting pulse and SOCP was obtained. We selected the appropriate attenuation ratio for the two channels of laser respectively, so that two signals appear on the oscilloscope with appropriate amplitude. The wide signal is the SOCP and the narrow one is the igniting pulse. Regarding the signal image on the oscilloscope, constantly and carefully adjust the delay until two signals overlap. According to the test results, the accuracy should be within 0.05 ns, and adjusting the delay line so that the 30 fs igniting pulse is in the range of 2.5 ns chirped pulse can satisfy the needs of the measurement. The second step is to ensure the total coaxiality, as any spatial deviation at the PM position may lead to experimental failure. The coaxiality is adjusted by judging the patterns on the far-field and near-field CCD cameras with two pulses. The third step is to accurately rotate the GTP, matching the polarization direction and completely separating the igniting pulse and SOCP. Similarly, the adjusting of GTP is realized by judging the intensity variation of two pulses on a CCD camera. The last step is to calibrate the energy measurement. The key is to calibrate the system efficiency between the two energy measurement positions of the calorimeters in Figure 2. The AR-coated target of the PM is replaced by a silver mirror considered to be close to 100% high reflectivity. Without amplification, a power meter (Thorlabs, United states, PM100 USB) is used to calibrate the efficiency, which is considered to be the same as under amplification. The two calorimeters have the same configuration and test performance. In this way, the PM efficiency is equal to the output energy divided by the input energy, and then divided by the system efficiency. After all of the above offline tests are completed, the temporal contrast enhancement measurement experiment can be carried out. According to the Equation (1), the initial spectrum \(S_{0}(\lambda)\) and background spectrum \(S_{b}(\lambda)\) of the SOCP should be recorded for spectral data processing. Using the same spectrometer after the background filtering, we first recorded the initial spectrum \(S_{0}(\lambda)\) at the input spectrometer position in Figure 2. Then, we blocked the optical path of the SOCP, and recorded the background spectrum \(S_{b}(\lambda)\) by the same spectrometer placed at the output spectrometer position in Figure 2. After that, we recorded the spectrum \(S_{1}(\lambda)\) of the reflected SOCP. Moreover, considering the possible spectral modulation by optical components in the system, we tested the spectra of the two measurement positions without amplification. The results showed no spectral modulation, indicating that all the spectral modulation of the SOCP in an online experiment should only come from the PM, rather than the optical components in the system.