**5. Conclusions**

fiber delay line made using SMF-28 fiber (*L*d=20 km in our case). As the result, optical frequency was shifted by *f*AOM=111 MHz at each pass along the delay line. After that, the signals (undelayed and delayed) were combined and registered by a fast PD, connected to a RF spectrum analyzer (RFSA). To increase the method's sensitivity, an EDFA was included into the DSHI scheme at the fiber delay line's exit. We should note that the multi-pass self-heterodyne scheme used for estimation the EDFL's line width was chosen because, for correct measurements, the path difference should be much higher that the light source's coherence length (~ 20 km, see

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

**Figure 22.** DSHI setup. AOM serves as a 111 MHz frequency shifter (always is open); 100%-FBG filters ASE produced

**Figure 23.** (a) An example of the CW EDFL RF-spectrum obtained from DSHI at 111 MHz; experimental data are shown by circles, solid line is a Lorentzian fit that gives 9.8-kHz width of the spectrum. (b) Spectrum width of CW EDFL measured using the DSHI technique at RF frequencies multiplied by AOM's frequency shift (111 MHz) when the cavity is blocked (circles). Solid line is a fit obtained using the theory presented in [37] (equation 16). The point at zerodelay was obtained with a 111-MHz frequency shift in the absence of delay line, which gives the RFSA's resolution (1

Figure 23 shows the spectral width of the signals at the frequencies multiplied by the AOM's frequency shift, measured after fitting the DSHI signal by the Lorentzian law, in function of

by EDFA (reflects light only at the wavelength of the EDFL's "bad" cavity).

below).

kHz).

In this Chapter, we reported some of the important nonlinear-optic features of EDFs, which, on one hand, impact efficiency of CW EDFLs on their base and, on the other hand, underlie the operation regimes established in EDFLs Q-switched using AOMs.

In particular, we showed that strong ESA transitions inherent in the Er3+ system at both the pump (~978 nm) and the laser (~1550 nm) wavelengths and ions' clustering inherent in the EDFs heavily doped with Er3+ cause unavoidable nonlinear losses that, in turn, strongly reduce efficiency of EDFLs, as compared to efficiency of Ytterbium-doped fiber lasers. We demon‐ strated as well that for making a correct numerical modeling of an EDFL one needs to consider all kinds of the nonlinear losses intrinsic in EDFs.

We also discussed in details the peculiarities of EDFLs operated in actively Q-switched regime using AOM. We demonstrated that the operation regimes of these lasers strongly depend on EDF length and AOM's repetition frequency. Specifically, at short EDF length or at high AOM's repetition frequency the laser operates in "conventional" Q-switching regime (being in fact multi-pass amplification of Er3+ SE) where pulses with relatively moderate power and relatively long in duration are composed of several, stable in time, sub-pulses, separated by a photon's round-trip time in the cavity. Furthermore, if EDF length is long enough and AOM's repetition frequency is not too high, the laser turns to the completely different pulsing regime, characterized by much shorter and much powerful pulses; however, pulses of this type are subjected to noticeable timing and amplitude jitters, originated from the stochastic in nature SBS process, ignited by spurious narrow-line CW lasing in "bad" (at closed AOM) cavity.
