A high-gain free electron laser gradually became one of the most promising hard X-ray sources after its experimental demonstration in 1997. The baseline mode of operation since then remains the self-amplified spontaneous emission (SASE), which is based on the shot noise amplification. Numerous statistically independent modes emerge in the electron density modulation of the electron beam and, as a result, in the temporal structure of the pulse. In a radiation spectrum, the same number of modes would be present. In this way, SASE radiation has a poor temporal coherence. Often, only a narrow spectral bandwidth of the FEL radiation is required for the experiment, so it is passively filtered with a monochromator. In this way, only a fraction of the FEL pulse energy is effectively used during the experiments. If the FEL pulse itself was temporally coherent before reaching the monochromator, its spectral density would be significantly higher, retaining the same pulse energy; the entire energy would be concentrated within a single spectral mode. The common way to introduce temporal coherence in single-pass FEL radiation is via the interaction of the electron beam with a temporally coherent radiation provided externally. One of the methods to accomplish this task is called the “direct seeding” method. Here, the seed radiation should have the same wavelength as the resonance FEL wavelength. When overlapped in an undulator with an electron beam, the seed power is amplified . The seeding technique requires both good temporal synchronization of the seed radiation with the electron bunch and a high power of the seed signal in order to prevail over the shot noise in the electron beam (the cause of a spontaneous emission).