**1. Introduction**

The chapter is devoted to applications and construction principles of poly-harmonic (twofrequency or four-frequency) cw laser systems for characterization of different nonlinear scattering effects in fibers and reflection of devices based on fiber Bragg gratings (FBG) in telecommunication lines and sensor nets. In particular, we'll speak about evaluation of Mandelstam-Brillouin gain contour, Raman scattering contours and FBG reflection spectra characterization. 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 poly-harmonic probing of formed resonance responses.

At a certain level of the laser power, exciting the optical fiber, resonant contours of Mandel‐ stam-Brillouin and Raman scattering are formed [1]. The first are based on periodic photonphonon interactions, the second – the photon-photon ones. Similarly, spectral reflection characteristics of Bragg gratings, based on periodic variation of the refractive index in the fiber core, can be described by the resonant contour [2]. In telecommunication lines Mandelstam-Brillouin and Raman scatterings are undesirable effects, but in sensor nets there are the main sources of measuring information. FBG, as known, is the powerful instruments as for tele‐ communication lines and so on sensor nets design. Thus, characterization of Mandelstam-Brillouin gain and Raman scattering contours and FBG reflection spectra is the actual and important task for scientists and designers. The typical characteristics of given effects are discussed in the first part of the chapter.

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To convert the information about the spectral contours characteristics are classically used broadband or tunable over a wide range optoelectronic means (optical spectrum analyzers, tunable lasers, optical reflectometers in time (OTDR) and frequency (OFDR) domains, scanning Fabry-Perot interferometers, diffraction gratings with CCD, and etc.) and complex "fit" algorithms for determining a desired value of accuracy of the central wavelength of the contours [3]. In recent years, significant progress in terms of accuracy and resolution of measurements, as well as practicality, was registered in the use of narrowband poly-harmonic technology [4] for characterization of contour spectrums that makes them competitive for the above mentioned methods in metrological characteristics, ease and cost of implementation. Their main advantage is that no measurements in the resonance region of spectral character‐ istics are necessary, that allows eliminating the influence of power instability of probing laser radiation and to detect information on the difference inter-harmonic frequency in the region with small noise level of photo-detector.

Second part of the chapter is devoted to poly-harmonic characterization of Mandelstam-Brillouin gain contours, Raman scattering contours and FBG reflection spectra for telecom‐ munication applications. For example, Mandelstam-Brillouin frequency shift for silica fibers is about 10-20 GHz, and Mandelstam-Brillouin gain is observed in a bandwidth of 20-100 MHz [1]. The main parameters are the central frequency of a gain spectrum, its Q-factor and gain coefficient. The classical method for characterization of stimulated Mandelstam-Brillouin scattering (SMBS) gain spectrum is based on use of two lasers: one – for SMBS pumping, and another – for probing of generated gain spectrum [5]. The disadvantage of this method is need in strong control of a frequencies difference of two sources. Not long ago the measurement system which is free from this drawback was presented [6]. It is based on SMBS gain spectrum conversion from optical to the electrical field by single-sideband amplitude modulated radiation, in which the upper sideband is suppressed. Despite advantages, realization of this method is not always effective; because relevant low sensitivity of measurements is remained, similar to measurements by double-band amplitude modulated probing radiation in a wide bandwidth. A new method for SMBS gain spectrum characterization in single-mode optical fiber was presented in our papers [7-8]. It is based on use of advantages of single-sideband modulation and two-frequency probing radiation, which gives possibility of transfer the data signal's spectrum in the low noise region of a photo-detector. Also this radiation is character‐ ized by effective procedure of received spectral information processing by the envelope's characteristics (phase and amplitude) of two spectral components beats. In the end of second part we also go back to the issues of poly-harmonic analysis of the FBG reflection spectra affected by us in [9-10].The modernization of previously used methods [4] will be presented, which based on the four-frequency probing radiation and only amplitude analysis of the envelopes [11]. Consideration of stimulated Raman scattering (SRS) in this part will be carried out only in general terms because of its negligible impact on the performance of telecommu‐ nication none-WDM lines [1].

Third part of the chapter is devoted to poly-harmonic characterization of Mandelstam-Brillouin gain contours, Raman scattering contours and FBG reflection spectra for sensor nets applications. An illustrative example of the relevance of considered issues is the use of all three given physical mechanisms in distributed and the quasi-distributed sensor nets for down-hole telemetry [12,13]. If Mandelstam-Brillouin and Raman scatterings are carrying distributed information of the measured parameters (temperature, pressure), the FBG gratings allows to receive its point localization and flow velocity data [14,15]. For more information OTDR with Rayleigh scattering can be used and characterized to the Mandelstam-Brillouin ones by Landau-Placzek relation [16]. So way it seems reasonable to use a single radiation source for getting fiber-forming response to external stimuli, forming its special signal shape or spectra, optimized for recording spectrally separated responses from various nonlinear effects and reflections, and monitoring their bound to the central wavelength of the radiation. Some pair wise effects of such an implementation are known in practice of Weatherford, Schlumberger, Halliburton and other companies. Comprehensive option to generate and use of responses from the three types of scattering and reflection from the Bragg gratings not yet been studied. Its implementation could put information redundancy in the process of measuring, the use of which would improve the metrological characteristics systems being developed.

To convert the information about the spectral contours characteristics are classically used broadband or tunable over a wide range optoelectronic means (optical spectrum analyzers, tunable lasers, optical reflectometers in time (OTDR) and frequency (OFDR) domains, scanning Fabry-Perot interferometers, diffraction gratings with CCD, and etc.) and complex "fit" algorithms for determining a desired value of accuracy of the central wavelength of the contours [3]. In recent years, significant progress in terms of accuracy and resolution of measurements, as well as practicality, was registered in the use of narrowband poly-harmonic technology [4] for characterization of contour spectrums that makes them competitive for the above mentioned methods in metrological characteristics, ease and cost of implementation. Their main advantage is that no measurements in the resonance region of spectral character‐ istics are necessary, that allows eliminating the influence of power instability of probing laser radiation and to detect information on the difference inter-harmonic frequency in the region

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

Second part of the chapter is devoted to poly-harmonic characterization of Mandelstam-Brillouin gain contours, Raman scattering contours and FBG reflection spectra for telecom‐ munication applications. For example, Mandelstam-Brillouin frequency shift for silica fibers is about 10-20 GHz, and Mandelstam-Brillouin gain is observed in a bandwidth of 20-100 MHz [1]. The main parameters are the central frequency of a gain spectrum, its Q-factor and gain coefficient. The classical method for characterization of stimulated Mandelstam-Brillouin scattering (SMBS) gain spectrum is based on use of two lasers: one – for SMBS pumping, and another – for probing of generated gain spectrum [5]. The disadvantage of this method is need in strong control of a frequencies difference of two sources. Not long ago the measurement system which is free from this drawback was presented [6]. It is based on SMBS gain spectrum conversion from optical to the electrical field by single-sideband amplitude modulated radiation, in which the upper sideband is suppressed. Despite advantages, realization of this method is not always effective; because relevant low sensitivity of measurements is remained, similar to measurements by double-band amplitude modulated probing radiation in a wide bandwidth. A new method for SMBS gain spectrum characterization in single-mode optical fiber was presented in our papers [7-8]. It is based on use of advantages of single-sideband modulation and two-frequency probing radiation, which gives possibility of transfer the data signal's spectrum in the low noise region of a photo-detector. Also this radiation is character‐ ized by effective procedure of received spectral information processing by the envelope's characteristics (phase and amplitude) of two spectral components beats. In the end of second part we also go back to the issues of poly-harmonic analysis of the FBG reflection spectra affected by us in [9-10].The modernization of previously used methods [4] will be presented, which based on the four-frequency probing radiation and only amplitude analysis of the envelopes [11]. Consideration of stimulated Raman scattering (SRS) in this part will be carried out only in general terms because of its negligible impact on the performance of telecommu‐

Third part of the chapter is devoted to poly-harmonic characterization of Mandelstam-Brillouin gain contours, Raman scattering contours and FBG reflection spectra for sensor nets applications. An illustrative example of the relevance of considered issues is the use of all three

with small noise level of photo-detector.

nication none-WDM lines [1].

In fourth part perspective systems and their elements are presented, describing and discussing the methods, tools and systems parameters of means to get poly-harmonic radiation on the base of dual-drive MZM, scanning and poly-harmonic (more than four harmonics) methods for SMBS characterization based on technology transfer from LIDAR systems, designing of notch filters for blocking of elastic Rayleigh scattering in SRS and SMBS measurements. Additionally Raman and Mandelstam-Brillouin scattering in sensor nets so as a FBG reflection carry vast amount information about fiber conditions but sometime have low energy level. That's why it's one more cause to detect these types of scattering with high SNR and determine their properties. Applying of photo mixing allows significantly increase the reflectometeric system sensitivity under the condition of weak signals and receives information from fre‐ quency pushing of backscattered signal spectrum [56]. We offer in [4] to use two-frequency heterodyne and the second nonlinear receiver in the structure of reflectometers and now discuss its advantages comparatively to other methods of SNR increasing in the end of the fourth part.

In conclusion we'll resume results of above mentioned researches, mark it practical imple‐ mentation and show new tasks in Mandelstam-Brillouin gain contours, Raman scattering contours and FBG reflection spectra characterization.
