# Single-Shot Temporal Contrast Enhancement Measurement of a Plasma Mirror by a Chirped Pulse
## 3 Experiment and Discussion  
### Experiment Setup of SOCP System  
To demonstrate the validity of this method, we built up an experimental platform by using the front end of an SG-II 5 PW laser system in the National Laboratory on High Power Laser and Physics [28], which provides an OPCPA-based ultrashort high-power laser. Its wavelength ranges from 750 to 850 nm. In order to obtain the temporal contrast  
Figure 1: Schematic diagram of temporal contrast enhancement measurement by a spatiotemporally overlapped chirped pulse (SOCP): (**a**) Concept of SOCP. (**b**) Temporal diagram of SOCP and igniting pulse. (**c**) Phase 1: low reflectivity. (**d**) Phase 2: high reflectivity. (**e**) Phase 3: low reflectivity. enhancement in a hundred picosecond domain, the chirp rate is selected as 25 ps/nm, corresponding to a pulse duration of 2.5 ns. The experimental setup is shown in Figure 2. The OPCPA system provides broadband pulse at the central wavelength of 808 nm, with more than 110 mJ energy at a 1 Hz repetition rate. The laser is divided into two beams, one is compressed to 30 fs as the igniting laser with a double-pass grating pair compressor and another uncompressed OPCPA laser is used as the SOCP. In the optical path of the SOCP, a delay line is set for synchronization. By altering the delay, the igniting pulse can be covered in duration of the SOCP. Moreover, a removable mirror (M1) is inserted into the optical path to record the initial spectrum. For subsequent separation, the SOCP and igniting pulse should be in different polarization. A polarization conversion system with several mirrors is used to convert the igniting pulse from P-polarization to S-polarization while the SOCP is still P-polarized. One calorimeter is set after sampling beam splitter 1 (SBS1) to measure the energy of the igniting pulse. The SOCP and igniting pulse were combined through the polarizing beam splitter (PBS) and inserted into a vacuum container in coaxial. After that, a pair of 100 mm focal length off-axis parabolic mirrors (OAP1 and OAP2) are used for focusing and collimating, separately. The plate target of a PM using fused silica material with anti-reflection coating (cold reflectivity is \(\sim\)5%) is placed at a certain position for enough peak intensity with 45\({}^{\circ}\) of incident angle. The beam radius of the igniting pulse on the target is about 50-100 microns. All elements are placed in a vacuum container with a vacuum of 10\({}^{-3}\) Pa. The output laser reflected by the PM enters the measurement system through another window. Because the SOCP and the igniting pulse are mixed in the output laser after the PM, which would affect the spectral profile, a Glan-Taylor prism (GTP) is used to separate the igniting pulse and SOCP before the spectrometer (Oceanhood, China, XS11639). The p-polarized SOCP travels through a GTP and is collected into the spectrometer for recording the spectrum. In addition to verifying the feasibility of single-shot measurement of the SOCP, the efficiency and beam quality needs to be monitored, proving that the SOCP does not affect the physical properties of the PM. Two CCD cameras (Visiyun, China, EHD04 MB-GE) and two calorimeters (Ophir, Israel, PF50 BF-C) are used for recording the pattern and input-output energy of the separated igniting pulse. Sampling beam splitter 2 (SBS2) is used for sampling a small part of the igniting pulse into CCD cameras, and a 400 mm focal length achromatic lens for focusing the igniting pulse. Before the single-shot measurement is taken, off-line test and calibration are required. The first step is to determine the synchronization of the igniting pulse and SOCP. A photoelectric probe (Thorlabs, United states, DET025 AL/M) placed in front of the vacuum  
Figure 2: Experimental setup: BS: beam splitter; PBS: polarizing beam splitter; M1–M4: mirror; OAP: Off-axis parabolic mirror; PM: plasma mirror; GTP: Glan-Taylor prism; SBS: sampling beam splitter. A spectrometer is used for measurement of spectrum.