**3. Experimental results**

**Figure 8** shows the experimental setup, the setup used as a source of pump pulse is a diode laser with *λ* = 1528 nm. This diode laser is directly modulated by the pulse generator to gener‐ ate pulses with duration of 2 ns. The pulses from the diode laser are amplified by an Erbium Doped Fiber Amplifier (EDFA) for power of several tens of Watts [21]. The pump pulses are introduced to the Coupler 1 (85/15), whose 85% port is spliced with the first stage comprising Fiber 1 + Fiber 2. In Fiber 1 we used the Corning SMF‐28 fiber with anomalous GVD equal to 20 ps/nm‐km, in the Fiber 2 we used the Corning SMF‐LS dispersion shifted fiber with GVD equal to −6 ps/nm‐km (normal dispersion at pump wavelength). The CW radiation with wavelength equal to 1620 nm is also introduced using the coupler 3 for two stages, the sig‐ nal power introduced into Fiber 1 was 0.5 mW. The polarization controller inserted after the EDFA allows adjusting the polarization of the pump to provide maximum Raman amplifica‐ tion in the fibers because Raman amplification depends on polarization states of the pump and Stokes [22, 23]. The SRS in the first stage causes the signal amplification of 1620 nm and depletion pump pulse. In the experiment, two filters were used to reject the 1620 nm radiation at the end of the first stage. The Fabry‐Perot (FP) filter and a broadband filter were used. For the launch the 1528 nm pump pulses and 1620 nm CW signal to the second stage of the experi‐ mental setup was used, Coupler 2 (90/10). The signal power of 1620 nm launched into fiber 3 was of 0.5 mW. We used 4.5 km of OFS True Wave (RS) fiber as fiber 3. With this configuration, we can measure simultaneously the pump pulse at the input of the first stage (pump monitor‐ ing, output 1), the pump output of the first stage (output 3), and pulses Stokes at the output of the second stage (output 2). We use the laser at this wavelength because it was what was available, but as the displacement of the wavelength between the signal and pumping must be very close to the maximum gain Raman (about 110 nm for the band of 1550 nm), thought we could use other lasers of around 1550 nm and can get a good operation of the device.

**Figure 8.** Experimental setup of Raman circuit.

From the above results, we can say that the setup of Raman circuit allows effective optical switching, logic operation, and noise reduction with signal of low power of the pump power. As we show above, to improve the operation of the Raman circuit we proposed the connection in series of fibers with normal and anomalous dispersion. We must consider that the power required for strong Raman amplification is lower than that required for the effects of MI and

**Figure 6.** The input signal "off" (a) and the input signal "on" (b) in the presence of random noise.

188 Raman Spectroscopy and Applications

**Figure 7.** Output Stokes pulses for input pulses "ON" and "OFF."

To measure the Raman gains were launched to test fibers 0.5 mW of power with CW diode at 1620 nm. Stokes signal at output 3 of **Figure 8** was measured with a monochromator. The fiber used was SMF‐28 with 100, 200, 300, 600, and 4500 m of length. The polarization of the pump was adjusted with the PC for maximum Raman amplification. The dependencies of powers Stokes with pump powers fit well with an exponential dependence, which cor‐ responds to the Raman amplification given by exp (*gP*<sup>b</sup> *L*/*A*eff), where *g* is the coefficient of Raman amplification, *P*<sup>b</sup> is the pumping power, *A*eff is the effective area of the fiber core, and *L* is fiber length. In the results are also shown the critical power for breaking pulses. From these results we can say that the breaking of the pulse always starts with lower power compared to the Raman effect threshold. This can affect the efficiency of the switch since the breakup of the pulse starts before Raman amplification reaches significant values. The Raman gain average experimental value = 0.72 W−<sup>1</sup> /km and the theoretical value for linearly polarized pump and Stokes = 0.72 W−<sup>1</sup> /km (see **Figure 9**). Theoretical calculations agree well with the experiment.

**Figure 9.** Raman gain.

The experimental setup is shown in **Figure 8**. We used the composed fiber, which consists of different spans of fiber 1 and fiber 2 for the investigated pump saturation in the first stage. **Figure 10** presents the results of waveforms of pump pulses at the output of the filter for differ‐ ent pump powers when only fiber 1 was used in the first stage. In the experiment, we used dif‐ ferent span of fiber SMF‐28. **Figure 10(a)** was obtained with a fiber length of 300 m and the inset is for the fiber with a length of 600 m. Strong depletion of the pump pulse was observed for the 300 m y 600 m of fiber even if the 1620 nm radiation was not applied. In this case, the effect of the 1620 nm radiation on the pulse depletion was not detected. The maximum power at the FP filter output was measured to be equal to 7 W for 300 m y 3 w for 600 m. The pump depletion is caused by the pulse breakup process followed by the soliton self‐frequency shift that results in broadening of the spectrum and the decrease of the power at the output of the FP filter.

To measure the Raman gains were launched to test fibers 0.5 mW of power with CW diode at 1620 nm. Stokes signal at output 3 of **Figure 8** was measured with a monochromator. The fiber used was SMF‐28 with 100, 200, 300, 600, and 4500 m of length. The polarization of the pump was adjusted with the PC for maximum Raman amplification. The dependencies of powers Stokes with pump powers fit well with an exponential dependence, which cor‐

is fiber length. In the results are also shown the critical power for breaking pulses. From these results we can say that the breaking of the pulse always starts with lower power compared to the Raman effect threshold. This can affect the efficiency of the switch since the breakup of the pulse starts before Raman amplification reaches significant values. The Raman gain average

The experimental setup is shown in **Figure 8**. We used the composed fiber, which consists of different spans of fiber 1 and fiber 2 for the investigated pump saturation in the first stage. **Figure 10** presents the results of waveforms of pump pulses at the output of the filter for differ‐ ent pump powers when only fiber 1 was used in the first stage. In the experiment, we used dif‐ ferent span of fiber SMF‐28. **Figure 10(a)** was obtained with a fiber length of 300 m and the inset is for the fiber with a length of 600 m. Strong depletion of the pump pulse was observed for the 300 m y 600 m of fiber even if the 1620 nm radiation was not applied. In this case, the effect of

is the pumping power, *A*eff is the effective area of the fiber core, and *L*

/km (see **Figure 9**). Theoretical calculations agree well with the experiment.

/km and the theoretical value for linearly polarized pump and

*L*/*A*eff), where *g* is the coefficient of

responds to the Raman amplification given by exp (*gP*<sup>b</sup>

Raman amplification, *P*<sup>b</sup>

190 Raman Spectroscopy and Applications

Stokes = 0.72 W−<sup>1</sup>

**Figure 9.** Raman gain.

experimental value = 0.72 W−<sup>1</sup>

**Figure 10.** Pump pulses at the output of the FP filter when the SMF‐28 fiber was used as the Fiber 1; (a) fiber length is equal to 300 m and the inset 600 m; (b) fiber length is equal to 1 km.

According to previous results, it can be concluded that for fiber lengths long critical power required for breaking pulses decays slower compared to the power required for Raman ampli‐ fication. Therefore we assume that the effect of input radiation 1620 nm may be larger for larger fiber lengths. The pump saturation for 1 km of fiber SMF‐28 is shown in **Figure 10(b)**, the dashed line shows the pulse when radiation 1620 nm is off, and the solid line shows when we have 0.5 mW of 1620 nm radiation. The delay between pump pulses of 1528 nm and Stokes pulse 1620 nm for SMF‐28 fiber is 2 ns for 1 km fiber, for this reason only the second half of the pulse is reduced. Depletion of the pump pulse when there was no input signal revealed that an effect of breaking pulse appears. The pump power for **Figure 10(b)** is 26 W.

**Figure 11.** (a) Saturation of the pump by the input signal with 350 m of SMF‐LS fiber for 20 W and the inset for 33 W pump power and (b) for 550 m SMF‐Ls for 17 W and the inset for 30 W pump power.

Also tested in the experiment is the SMF‐LS fiber, this fiber has normal dispersion for a wave‐ length of 1528 nm and so the effect of MI and breaking pulses are suppressed. The fiber shows conventional saturation of the pump and corroborates well with the simulations based on Eq. (1). **Figure 11** presents the results for a fiber of 350 and 550 m of length. **Figure 11(a)** shows pump power equal to 20 W and the inset of 33 W for 350 m of SMF‐LS; **Figure 11(b)** shows pump power equal to 17 W and the inset of 30 W for 550 m of SMF‐LS. Without input signal (solid line), no change in the waveform of the pump pulse is observed for **Figure 11(a)** and **(b)**. However, at 0.5 mW strong depletion of signal input power (dashed line) can be seen in **Figure 11(a)** and **(b)**. At 33 W for 350 m of SMF‐LS and 30 W for 550 m of SMF‐LS pumps power the depletion (solid line) is observed even without input signal, see insets of **Figure 11**.

the dashed line shows the pulse when radiation 1620 nm is off, and the solid line shows when we have 0.5 mW of 1620 nm radiation. The delay between pump pulses of 1528 nm and Stokes pulse 1620 nm for SMF‐28 fiber is 2 ns for 1 km fiber, for this reason only the second half of the pulse is reduced. Depletion of the pump pulse when there was no input signal revealed that

**Figure 11.** (a) Saturation of the pump by the input signal with 350 m of SMF‐LS fiber for 20 W and the inset for 33 W

pump power and (b) for 550 m SMF‐Ls for 17 W and the inset for 30 W pump power.

an effect of breaking pulse appears. The pump power for **Figure 10(b)** is 26 W.

192 Raman Spectroscopy and Applications

We use a special fiber, consisting of a span of SMF‐LS fiber spliced with another span of SMF‐28 fiber, which was also tested in the experiment. Several tests were done and found that if the span of the fiber SMF‐28 was respectively shorter than approximately 300 m, the effect of this span of fiber in low powers was negligible and depletion of the pump was determined by the fiber SMF‐LS. For the case when we used 600 m of SMF‐28 fiber spliced with 550 m fiber SMF‐LS the effect of SMF‐28 fiber was significant (with pump powers of 18 and 23 W). From these results, we can conclude that the effect of saturation of the pump by input signal was observed if the length of SMF‐LS fiber is at least two times longer than the length of the SMF‐28 fiber. For this case, the power required for MI effect is higher than that required for effective pump depletion. **Figure 12** presents an example of the output pump pulses for the first stage and the composed fiber consisted of a 550 m span of SMF‐LS spliced with 300 m span of SMF‐28, with pump power of 17 W and the inset for 20 W [17].

**Figure 12.** Saturation of the pump by the input signal if the first stage comprises a 550 m SMF‐LS fiber connected to a 300 m SMF‐28; pump power is 17 W and the inset for 20 W.

We can say that using the broadband filter obtain a considerable improvement in pump satura‐ tion, and with this result, we obtain a considerable improvement to the efficiency of the switch. The saturation measured for the composed fibers, which consists of a span of SMF‐28 fiber spliced with another span of SMF‐LS fiber, is shown in **Figure 13** and it presents the results of saturation for these fibers. **Figure 13(a)** presents results for 350 m SMF‐LS fiber spliced with 300 m SMF‐28 fiber, and **Figure 13(b)** presents results for 350 m SMF‐LS fiber spliced with 600 m SMF‐28 fiber. These graphs were obtained when using the broadband filter and spectral filter. We can see that when connecting the SMF‐LS fiber, SMF‐28 fiber can increase the saturation of the pump.

**Figure 13.** Saturation of peak power pump for variety of pump powers for two configurations: (a) 350 m SMF‐LS fiber + 350 m SMF‐28 fiber and (b) 350 m SMF‐LS + 600 m SMF‐28.

If the pump depletion in the first state of the SMF‐28 fiber is determined by the MI, the deple‐ tion has to be dependent on the bandwidth of the filter inserted between the first and second stage. To show this, we use a broadband filter which is made with a spam of SMF‐28 fiber. The difference between losses is sufficient to measure the depletion of pump of the broadband filter. **Figure 14** shows the results of the depletion measured with the broadband filter and the inset shows the results of the depletion measured with Fabry‐Perot filter. We can see from the figure that with the narrow band filter pump depletion occurs in low power pump (about 5 W) and for the case with the broadband filter pump depletion occurs in about 30 W of pump power. We can conclude that the problem connected with the MI could be overcome using wideband filter between the first and the second stage. We can consider, for example, a filter based on a liquid filled photonic crystal fiber filter [24].

We can say that using the broadband filter obtain a considerable improvement in pump satura‐ tion, and with this result, we obtain a considerable improvement to the efficiency of the switch. The saturation measured for the composed fibers, which consists of a span of SMF‐28 fiber spliced with another span of SMF‐LS fiber, is shown in **Figure 13** and it presents the results of saturation for these fibers. **Figure 13(a)** presents results for 350 m SMF‐LS fiber spliced with 300 m SMF‐28 fiber, and **Figure 13(b)** presents results for 350 m SMF‐LS fiber spliced with 600 m SMF‐28 fiber. These graphs were obtained when using the broadband filter and spectral filter. We can see that when connecting the SMF‐LS fiber, SMF‐28 fiber can increase the saturation of the pump.

194 Raman Spectroscopy and Applications

**Figure 13.** Saturation of peak power pump for variety of pump powers for two configurations: (a) 350 m SMF‐LS fiber +

350 m SMF‐28 fiber and (b) 350 m SMF‐LS + 600 m SMF‐28.

**Figure 14.** Saturation of pump measured with broadband and narrowband filter.

We investigate the experimental setup of the Raman circuit composed of two stages. In stage 1, we used different combinations of the SMF‐LS fiber and SMF‐28 fiber, and for stage 2, we used OFS True Wave (RS) fiber with 4.5 km of length. The results show the waveforms of the signal pulses at the output 2 of the experimental setup. We used for stage 1: SMF‐LS with different length; the composed fibers, which consists of a span of SMF‐LS fiber spliced with another span of SMF‐28 fiber. The results of **Figure 15(a)** were obtained with 350 m of SMF‐LS as the first stage at 37 W pump power and the inset is for 550 m of SMF‐LS at 23 W; the results of **Figure 15(b)** were obtained with 550 m of SMF‐LS spliced with 600 m of SMF‐28 used as the first stage at 18 W pump power. The inset of **Figure 15(b)** show magnifications of the output signal when input signal is applied.

**Figure 15.** The waveforms of the output signal at 1620 nm at the output 2 of the experimental setup; dashed line with input signal (OFF), solid line without input signal (ON). The first stage consists of the 350 m of the SMF‐LS fiber and the inset for 550 m of SMF‐LS fiber (a); the first stage consists or of the 350 m SMF‐LS connected to the 600 m SMF‐28 the inset show the magnification of the when input signal is applied (b).

Finally, we measured the ratio of output energy without input signal ("OFF") and with input signal ("ON") for different pump powers. **Figure 16** shows the dependencies of the contrast on the pump power. The dependencies are shown for the first stage consisting of the 350 m SMF‐ LS fiber, **Figure 16(a)**, and for the first stage consisting of the 350‐m SMF‐LS and the 600 m SMF‐28 fiber, **Figure 16(b)**. We can see that the best results from our configuration are for 350 m from the SMF‐LS fiber in the first stage. For this configuration, the contrast OFF/ON came up to 27. With 500 m of SMF‐LS in the first stage maximum contrast was equal to 7 and for configuration of 600 m of SMF‐28 m spliced with 500 m of SMF‐LS increase the contrast to 13.

pulses at the output 2 of the experimental setup. We used for stage 1: SMF‐LS with different length; the composed fibers, which consists of a span of SMF‐LS fiber spliced with another span of SMF‐28 fiber. The results of **Figure 15(a)** were obtained with 350 m of SMF‐LS as the first stage at 37 W pump power and the inset is for 550 m of SMF‐LS at 23 W; the results of **Figure 15(b)** were obtained with 550 m of SMF‐LS spliced with 600 m of SMF‐28 used as the first stage at 18 W pump power. The inset of **Figure 15(b)** show magnifications of the output

**Figure 15.** The waveforms of the output signal at 1620 nm at the output 2 of the experimental setup; dashed line with input signal (OFF), solid line without input signal (ON). The first stage consists of the 350 m of the SMF‐LS fiber and the inset for 550 m of SMF‐LS fiber (a); the first stage consists or of the 350 m SMF‐LS connected to the 600 m SMF‐28 the

inset show the magnification of the when input signal is applied (b).

signal when input signal is applied.

196 Raman Spectroscopy and Applications

**Figure 16.** Energy of Stokes pulses, (a) for 350 m SMF‐LS in the first stage; (b) for 550 m SMF‐LS + 600 m SMF‐28 in the first stage.
