**6. Conclusion**

**Figure 17.** Spectra of FBG filter

90 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**Figure 18.** Setup for FBG recording

germanium-silicon fiber (SMF-28).

FBG for the experiments was manufactured by a given method of continuous recording with restrictive object in the setup, shown in Fig. 18, at the R&D Institute of Applied Electrody‐ namics, Photonics and Living Systems. The setup for FBG recording was made in Novosibrsk State University on the base of amplitude-modulated ultraviolet laser with the conversion of the second harmonic on the Ar+. Laser was focused on the core of the aged in hydrogen-doped We reviewed the principle and the applications of the poly-harmonic (two-frequency or fourfrequency) cw laser systems for characterization of Mandelstam-Brillouin gain contour, Raman scattering contours and FBG reflection spectra. Investigation methods and approaches are based on the unity of resonant structures of generated fiber responses on exciting and probing radiation or external physical fields for all given effects. The main decision is based on polyharmonic probing of formed resonance responses. A variety of multiplexed measuring functions can be provided by the technique for telecom and sensing applications. As the examples, we introduced new four-frequency methods for Mandelstam-Brillouin gain contour and FBG spectra characterization classified by average and difference frequencies of probing radiation. These systems possess the advantages of the continuous-wave operation, high resolution, high accuracy, and absence of the need to measure the phase characteristics of the tested contours. We proposed for the first time the concept, approaches and methods for its implementation, which allowed reasonably formulate and solve the problem of creation of scientific and technical basis for designing software-defined down-hole fiber optic telemetry systems based on the combination of nonlinear reflectometry (inelastic Raman and Mandel‐ stam-Brillouin, elastic Rayleigh and FBG) with improved metrological characteristics of polyharmonic probing of sensing structures, including the removal of multiplicative and measurement errors caused by instability of the common source of forming radiations in wide frequency range. Universal poly-harmonic source of probing radiation, variants of scanning and multiple methods of probing, notch filters for Rayleigh scattering line suppression were demonstrated as elements of perspective systems of given class. Applications of designed methods and means in microwave range were also discussed.
