## 2 Coherent Diffraction Imaging with X-ray Lasers  
During the rapid development of CDI, the X-ray light source technologies have also made great progress. Thanks to the advent of ultra-intense and ultrafast X-ray free-electron lasers [98; 99], the femtosecond pulses can outrun main radiation damage processes [100] using the so-called \"diffraction-before-destruction\" strategy [101]. Under proper conditions, XFELs can limit radiation damage and provide clear images of molecules that can otherwise be damaged using a continuous  
Figure 1: Word cloud of synchrotron-based coherent diffraction imaging published research papers. About 50 widely-read published papers were downloaded. These papers are published during the last 18 years since the first coherent diffraction imaging experiment was conducted successfully in 1999. The main text of the papers was analyzed by open python codes. The frequency of each meaningful word was calculated and then demonstrated by different size and colors. The bigger the size means the larger the frequency. The image shows that reconstruction, sample, phase and resolution are the most used terms for the past 18 years of coherent diffraction imaging research. At the same time, structure, intensity and radiation have also been key topics for biological imaging since 1999. (Original datas were collected from References [24; 31; 32; 38; 42; 43; 33; 38; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78]. Figure was generated under the help of Github open codes [89].). illumination. The state-of-the-art serial femtosecond crystallography (SFX) [102] and single particle imaging (SPI) methods [103] have proven to be promising ideas for native structure determination, with the additional benefit of time-resolution for making movies of reactions. Due to noise, detector limitation and demand of large amount dataset for atomic resolution, big data and automatic data analysis are needed to achieve the challenging goal. Owing to the ultra-intense and ultrafast X-ray pulses generated by FLASH (Hamburg, Germany) [104; 105], LCLS (Menlo Park, CA, USA) [106; 107], SACLA (Hyogo, Japan) [108; 109] and FERMI (Trieste, Italy) [110; 111], many exciting CDI results have been obtained. It is worthwhile mentioning that the European-XFEL [112] and PAL-XFEL [113] have recently been commissioned, and that Swiss-FEL [114] is under construction and soon to be commissioned. The Shanghai Soft XFEL [115; 116] is under construction, and a hard XFEL facility has also been proposed [117]. We believe that the newly-built facilities will continue to be a great impetus to ultra-small and ultrafast research. Coherent diffractive imaging experiments with X-ray free-electron lasers (XFEL-CDI) have studied a wide range of samples including man-made objects, nano-materials, clusters, and aerosol particles in the gas phase [118; 119; 120; 121; 122; 123]. Many of these were used to test the validity of XFEL-CDI and included measurements of the morphology of complex structures, functional imaging method development, and dynamic changes under a stimulus (pump-probe). On the biological side, viruses, bacteria, organelles and associated biological components have been imaged at various resolution levels [133; 134; 135; 136; 137; 138; 139; 140; 141; 142] and have helped further develop high throughput imaging methods. Figure 2 shows typical results from non-biological particles using XFEL-CDI. The first experiment demonstrating the \"diffraction-before-destruction\" principle using the FLASH soft X-ray free-electron laser by [PERSON] and colleagues [101] is shown on Figure 2a. An intense pulse with \(10^{12}\) photons at 32 nm wavelength was transported to a nanostructured non-periodic object. A speckled pattern was obtained before the beam completely destroyed the nanostructure. The diffraction pattern and the reconstructed real-space image showed no measurable effect from radiation damage.