**4. Effect of SBS upon operation of actively Q-switched EDFL**

Actively Q-switched (AQS) EDFLs based on acousto-optics modulator (AOM), implemented in the Fabry-Perot geometry, usually produce Q-switch (QS) pulses with duration from a few to hundreds ns [1]. The QS pulses are normally composed of a few sub-pulses separated by round-trip time of photon inside the cavity [31, 32]; this AQS regime will be called further "conventional" (CQS). In the meantime, it is known that in certain conditions FLs demonstrate stochastic QS pulsing, which stems, as it will be clearly demonstrated below, from intra-cavity stimulated Brillouin scattering (SBS) [33]. Such pulses, referred further to as SBS-QS ones, are characterized by dramatic increasing of power as compared to CQS pulses but, at the same time, by perceptible jitters [34]. In this section, we show that in certain circumstances SBS-QS pulsing is inherent in AOM-based AQS EDFLs. We also demonstrate that the areas (basins) where CQS and SBS-QS regimes exist are defined by definite values of EDF length and AOM's repetition rate and that the most important condition for turning of the laser to one or another pulsing regime is absence or presence of spurious narrow-line continuous wave (CW) lasing in the intervals where the laser cavity is blocked (AOM is switched OFF).

#### **4.1. Experimental setup**

An experimental setup of the QS-EDFL is sketched in Figure 17. The laser cavity consists of a piece of a standard low-doped "M" EDF (*Thorlabs*, M5-980-125), two FBGs (1 and 2) centered at ~1549.4 nm (laser wavelength), which form Fabry-Perot cavity, and a standard downfrequency shifting AOM with fiber outputs (*Gooch & Housego*, operation frequency – 111 MHz), placed nearby FBG2. The full AOM's rise time was measured to be 50 ns, AOM gate was fixed at 2 μs in experiments. FBGs' reflection coefficients were ~ 30% (FBG1) and ~ 100% (FBG2). A long period grating (LPG) tuned to ~1533 nm was used as in-line stop-band filter for neutral‐ izing fiber gain at Er3+ SE peak and thus avoiding spurious CW lasing at this wavelength, which might otherwise discharge EDF and thereby reduce QS pulse energy and, via interfering with targeted pulsed lasing at the wavelength selected by FBGs, produce instability of pulsing. The EDF was pumped by a fiber-coupled 976-nm LD through a 980/1550-nm WDM. To decrease the cavity loss, FBG1 and LPG were written in the EDF core after preliminary hydrogenation. The laser signal was registered by OSA with a 50-pm resolution or by 1.2-GHz PD used in-line with a 2.5-GHz oscilloscope. In experiments, pump power was fixed at 500 mW and AOM's repetition rate (*f*AOM) was varied within a 0...30-kHz range.

**Figure 17.** Experimental setup of the QS-EDFL (crosses indicate fiber splices).

#### **4.2. Properties of CQS and SBS-QS regimes**

As well-known, AQS FLs operated in CQS regime usually generate pulses consisting of train of sub-pulses (ripples), separated by a time interval equal to a photon's round-trip in the cavity. Such kind operation is fully described by the model of two contra-propagated waves in Fabry-Perot cavity, once considering the laser as a multi-pass amplifier of SE reflected several times by selective mirrors (FBGs) [32]. CQS is observed at any *f*AOM when EDF length (*L*EDF) is shorter than some specific value and at larger EDF when *f*AOM is high. The common features of CQS pulsing observed experimentally and also modeled are as follows: Delay of a QS pulse with respect to the moment of AOM opening increases while its energy and power decrease with increasing AOM's repetition rate. Usually, the first detectable sub-pulse arises in a few roundtrips of ASE after AOM got opened. For example, when *L*EDF=8.8 m and *f*AOM=8 kHz, the first visible sub-pulse appears at ~250 ns (~2.5 photon round trips; see Figure 18(a)). The RF (FFT) spectrum of pulse train measured at *f*AOM=16 kHz (see Figure 18(b)) has three peaks centered at 0 MHz, ~10 MHz and ~20 MHz. Width of peak 0 relates to QS pulses width, whereas peaks 1 and 2 correspond to the first and the second harmonics of the round-trip frequency (an inverted interval between sub-peaks, or round-trip time), respectively.

time, by perceptible jitters [34]. In this section, we show that in certain circumstances SBS-QS pulsing is inherent in AOM-based AQS EDFLs. We also demonstrate that the areas (basins) where CQS and SBS-QS regimes exist are defined by definite values of EDF length and AOM's repetition rate and that the most important condition for turning of the laser to one or another pulsing regime is absence or presence of spurious narrow-line continuous wave (CW) lasing

An experimental setup of the QS-EDFL is sketched in Figure 17. The laser cavity consists of a piece of a standard low-doped "M" EDF (*Thorlabs*, M5-980-125), two FBGs (1 and 2) centered at ~1549.4 nm (laser wavelength), which form Fabry-Perot cavity, and a standard downfrequency shifting AOM with fiber outputs (*Gooch & Housego*, operation frequency – 111 MHz), placed nearby FBG2. The full AOM's rise time was measured to be 50 ns, AOM gate was fixed at 2 μs in experiments. FBGs' reflection coefficients were ~ 30% (FBG1) and ~ 100% (FBG2). A long period grating (LPG) tuned to ~1533 nm was used as in-line stop-band filter for neutral‐ izing fiber gain at Er3+ SE peak and thus avoiding spurious CW lasing at this wavelength, which might otherwise discharge EDF and thereby reduce QS pulse energy and, via interfering with targeted pulsed lasing at the wavelength selected by FBGs, produce instability of pulsing. The EDF was pumped by a fiber-coupled 976-nm LD through a 980/1550-nm WDM. To decrease the cavity loss, FBG1 and LPG were written in the EDF core after preliminary hydrogenation. The laser signal was registered by OSA with a 50-pm resolution or by 1.2-GHz PD used in-line with a 2.5-GHz oscilloscope. In experiments, pump power was fixed at 500 mW and AOM's

As well-known, AQS FLs operated in CQS regime usually generate pulses consisting of train of sub-pulses (ripples), separated by a time interval equal to a photon's round-trip in the cavity. Such kind operation is fully described by the model of two contra-propagated waves in Fabry-Perot cavity, once considering the laser as a multi-pass amplifier of SE reflected several times by selective mirrors (FBGs) [32]. CQS is observed at any *f*AOM when EDF length (*L*EDF) is shorter than some specific value and at larger EDF when *f*AOM is high. The common features of CQS pulsing observed experimentally and also modeled are as follows: Delay of a QS pulse with respect to the moment of AOM opening increases while its energy and power decrease with

in the intervals where the laser cavity is blocked (AOM is switched OFF).

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

repetition rate (*f*AOM) was varied within a 0...30-kHz range.

**Figure 17.** Experimental setup of the QS-EDFL (crosses indicate fiber splices).

**4.2. Properties of CQS and SBS-QS regimes**

**4.1. Experimental setup**

**Figure 18.** (a) CQS pulses registered on the EDFL output at LEDF=8.8 m and at various fAOM-values. Zero-time in both snapshots corresponds to the moment when AOM gets opened. (b) Averaged RF spectrum of CQS pulsing at *f*AOM=16 kHz.

If the active fiber is long enough and AOM's repetition rate is not too high the QS EDFL turns into the regime of SBS-induced pulsing. This kind of pulsing is quite different as compared with CQS. Typical SBS-QS pulses are shown in Figure 19(a) for *L*EDF=8.8 m and *f*AOM=1 kHz. These pulses, as compared with CQS ones, arise earlier, approximately in ~180...280 ns after the moment of AOM's switching on; they are much narrower (~2.5...10 ns at 3-dB level); the pulses amplitude is more than by 10 dB higher as compared with the one at CQS while their envelop is apparently irregular. Emphasize that no SBS-QS pulses arisen within the intervals between the adjacent AOM's windows, in contrast to SBS-QS pulsing in an ytterbium-doped FL [34].

**Figure 19.** (a) CQS pulses registered on the EDFL output at *L*EDF=8.8 m and *f*AOM=1 kHz. (b) Averaged RF spectrum of CQS pulsing at *f*AOM=1 kHz.

One more detail of SBS-QS is that pulses released in this regime suffer strong amplitude and timing jitters. Apparently, the presence of jittering is an indication of the stochastic nature of the SBS process involved. Furthermore, since the SBS-QS pulses are not composed of subpulses spaced by photon round-trip time, their RF (FFT) spectrum does not have peaks at the round-trip frequency (~10 MHz) and its harmonics (see Figure 19(b)).

#### **4.3. Basins of CQS and SBS-QS regimes**

To find basins of CQS and SBS-QS regimes existence, we measured the value of *f*AOM at which the laser transits from one to another QS regime, for different *L*EDF. The experimental points in the space (*f*AOM, *L*EDF) segregating CQS and SBS-QS operations were easily fixed since QS pulses captured at the laser output in these two regimes differ drastically in pulse amplitude, duration and shape (see snapshots in Figures 18(a) and 19(a)). We found that if *L*EDF is less than or equal to a certain value (5.4 m in our arrangement) the laser operates in CQS regime, at any *f*AOM. But if EDF is longer than 5.4 m, an operation regime depends on AOM's repetition rate: At low *f*AOM the laser generates SBS-QS pulses while at high *f*AOM it turns to CQS operation. The basins of CQS and SBS-QS regimes are illustrated in Figure 20. As seen from the figure, the laser operates in CQS and SBS-QS above and below the border line, respectively (this line schemat‐ ically marks a transition between the regimes).

**Figure 20.** Basins of CQS and SBS-QS regimes; symbols label the experimental points corresponding to the border (sol‐ id line) between the basins.

The reason of the AQS-EDFL's switching to CQS or SBS-QS regime is the existence or the absence of spurious narrow-line CW lasing when AOM is blocked. In the last case the cavity is formed by the output reflector FBG1 and by a small reflection from closed AOM (~ –40 dB) ("bad" cavity). The overall loss of this cavity is estimated to be ~45-47 dB, revealing its very low Q-factor. At long EDF and low AOM repetition rates (the area below the border line in Figure 20) the EDF charge is sufficient to provide fiber gain capable of overcoming the cavity loss, which results in arising CW lasing. After switching AOM on, the CW wave starts to propagate along the main laser cavity (formed by FBG1 and FBG2) with simultaneous amplification of its power by the EDF until the latter reaches SBS-threshold and produces a "giant" SBS-QS pulse. Thus, CW spurious lasing arising in the "bad" cavity is a startup mechanism for SBS-QS pulsing. In the area above the border line in Figure 20 the spurious CW lasing is not established since the EDF cannot accumulate gain sufficient to overcome the "bad" cavity's loss.

One more detail of SBS-QS is that pulses released in this regime suffer strong amplitude and timing jitters. Apparently, the presence of jittering is an indication of the stochastic nature of the SBS process involved. Furthermore, since the SBS-QS pulses are not composed of subpulses spaced by photon round-trip time, their RF (FFT) spectrum does not have peaks at the

To find basins of CQS and SBS-QS regimes existence, we measured the value of *f*AOM at which the laser transits from one to another QS regime, for different *L*EDF. The experimental points in the space (*f*AOM, *L*EDF) segregating CQS and SBS-QS operations were easily fixed since QS pulses captured at the laser output in these two regimes differ drastically in pulse amplitude, duration and shape (see snapshots in Figures 18(a) and 19(a)). We found that if *L*EDF is less than or equal to a certain value (5.4 m in our arrangement) the laser operates in CQS regime, at any *f*AOM. But if EDF is longer than 5.4 m, an operation regime depends on AOM's repetition rate: At low *f*AOM the laser generates SBS-QS pulses while at high *f*AOM it turns to CQS operation. The basins of CQS and SBS-QS regimes are illustrated in Figure 20. As seen from the figure, the laser operates in CQS and SBS-QS above and below the border line, respectively (this line schemat‐

**Figure 20.** Basins of CQS and SBS-QS regimes; symbols label the experimental points corresponding to the border (sol‐

The reason of the AQS-EDFL's switching to CQS or SBS-QS regime is the existence or the absence of spurious narrow-line CW lasing when AOM is blocked. In the last case the cavity is formed by the output reflector FBG1 and by a small reflection from closed AOM (~ –40 dB) ("bad" cavity). The overall loss of this cavity is estimated to be ~45-47 dB, revealing its very low Q-factor. At long EDF and low AOM repetition rates (the area below the border line in Figure 20) the EDF charge is sufficient to provide fiber gain capable of overcoming the cavity

round-trip frequency (~10 MHz) and its harmonics (see Figure 19(b)).

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

**4.3. Basins of CQS and SBS-QS regimes**

ically marks a transition between the regimes).

id line) between the basins.

To confirm the hypothesis that the mechanism "igniting" SBS-QS pulsing relates to arising of the narrow-line CW lasing in "bad" cavity (when AOM is closed), we fulfilled the experiments on measuring the laser's optical spectra.

**Figure 21.** (a) Normalized AQS EDFL spectrum measured at *f*AOM=8 kHz (open circles, left scale) and FBG1 spectrum (solid line, right scale). (b) Normalized AQS EDFL spectra measured at *f*AOM=4 kHz (black crossed circles) and at AOM being always blocked (CW lasing, blue circles). In both cases *L*EDF=7.6 m. OSA resolution is 50 pm.

Firstly, we compared the optical spectra of the laser operated in CQS and SBS-QS regimes; see Figure 21. Comparing the lasing spectra at *f*AOM=8 kHz (CQS) and *f*AOM=4 kHz (SBS-QS), one sees that in the former case (Figure 21(a)) the spectrum virtually repeats the reflection spectrum of FBG1 (the one of FBG2 is broader), whereas in the latter case (Figure 21(b)) it consists of two spectral lines A and B, spaced by ~90 pm (~11 GHz, a Brillouin shift at 1550 nm). Both laser lines A and B are narrower (~60 pm) than the lasing (in fact ASE) spectrum at CQS, ~160 pm. Supposedly, line A, centered at the FBG1's peak, corresponds to CW lasing arisen when AOM is in OFF state (between the adjacent AOM's gates) whereas line B – to SBS-QS pulsing. To provide more arguments in favor of this hypothesis, we plot in the same figure (Figure 21(b)) the spectrum of CW lasing, when the main cavity is always blocked (i.e. AOM is continuously in OFF state). It is seen that line A vastly reproduces the CW lasing spectrum. This allows us to reveal that the narrow line A is the signature of CW lasing and that it "ignites" the SBS-process; accordingly, the narrow line B, shifted by ~90 pm to the Stokes side, is the signature of SBS-lasing.

To shade more light on the scenario drawn above, we also measured the spectral width of spurious CW lasing arising when AOM is closed by employing another technique that utilizes a modified delayed self-heterodyne interferometer (DSHI, see Figure 22), described in details in [35-37]. To carry out this, the laser output signal was split into two beams, one of which being passed through an optical frequency shifter (AOM) and then through a long recirculating 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 below).

**Figure 22.** DSHI setup. AOM serves as a 111 MHz frequency shifter (always is open); 100%-FBG filters ASE produced by EDFA (reflects light only at the wavelength of the EDFL's "bad" cavity).

**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 kHz).

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 the delay-line length *L*d × *N* (*N* is the number of passes through the line). As shown in [35-37], at high *N* a signal's width approaches the real value of an EDFL's optical width; in our case, this value is ~30 kHz, which corresponds to coherence length of ~20 km. The earlier reports [38, 39] also revealed a very narrow EDFL's line.

Consequently, a narrow-line CW laser wave developing in "bad" cavity and being a prereq‐ uisite of SBS-QS pulsing is highly coherent. This explains why SBS is unavoidably boosted by spurious CW lasing in the "blocked" cavity after the moment of AOM's opening, when the EDF is strongly inverted and thus strongly amplifying. Indeed, "unlimited" in length and therefore "uniform" Brillouin dynamic grating [40], induced in intra-cavity EDF (the fiber is always shorter than the estimated coherence length, ~20 km), is the main cause of SBS-QS pulsing.
