## 4 Brillouin Optical Correlation Domain Analysis: BOCDA
### Examples of Performance-Improved BOCDA Systems
#### 4.4.1 BOCDA System with Intensity Modulation Scheme
In Figure 23a, the dashed curve shows half the period of the sinusoidal FM, and the red curve shows a trial intensity modulation waveform for the apodization [51]. In Figure 23b, the red curve shows the simulation result, in which the reduced mountain-shaped background spectrum is seen. In this simulation, a 6000-\(\mu\epsilon\) strained short length of fiber portion is located at the correlation peak position. The Lorentz-shaped signal spectrum is clearly shown at the \(\Delta\
u\)= 300 MHz portion, without any background spectrum. However, at the non-strained portion, where \(\Delta\
u\) = 0, a large dip is shown [51]. This means that the apodization is too effective for the Brillouin gain spectrum, which is spread into the two dimensions. To compensate for the dip at the non-strained portion in the BOCDA system output spectrum, the IM waveform is modified as shown by the black peaks in Figure 23a. The two black peaks recover the power at both edges of the light source power spectrum. With this recovery, the dip is compensated for, as shown by the black line in Figure 23b. As shown in Figure 23c, any amount of strain applied at the correlation peak position can be clearly shown in the BOCDA output without any background spectrum. This is the function of the \"intensity modulation scheme\". Figure 24a shows the time-averaged LD power spectra without (black) and with (red) the IM modulation, respectively [51]. In Figure 24b,c, the red and black curves show the experimental results of the BOCDA output spectrum for the cases with and without the IM modulation, respectively. Figure 24b,c correspond to the BGS measurements at a strained and a non-strained position, respectively. By the IM scheme, the background spectrum is clearly reduced in the experiments. Figure 23: Simulations of the intensity modulation scheme [51]. **(a)** Two waveforms for the intensity modulation; the dashed curve is a sinusoidal FM waveform, and the red and black curves are for the IM. The red curve is a trial waveform for the IM, which is half the sinusoidal waveform to reduce the power at both edges of the time-averaged power spectrum caused by the FM. The high transmission portion that is shown by the black peak at both edges of the FM power spectrum is introduced to compensate for the over apodization; **(b)** Background spectrum reduced by the IM modulation; the red curve corresponds to the red IM waveform in **(a)**, which makes over apodization at \(\Delta\
u\) = 0. The black curve corresponds to the IM waveform with a black peak in **(a)**. **(c)** Signal spectra with reduced background spectrum by the appropriate IM waveform shown in **(a)**. Figure 25 shows the distribution of the BGS along the fiber that was obtained in experiments and simulations to show the effectiveness of the IM scheme [51]. The experimental results are shown in Figure 25a,b, and the simulation results are shown in Figure 25c,d. Figure 25a,c are for the case without the IM modulation, and Figure 25b,d are for the case with the IM modulation. The suppressed background is clearly shown at the blue circled portions in Figure 25b,d. Quite a high spatial resolution is demonstrated in the distributed BFS measurement by a BOCDA system, as shown in Figure 26[66]. In this experiment, the IM scheme is also  
Figure 24: Experimental results of the IM scheme [51]. (**a**) Time-averaged laser diode (LD) power spectrum shapes without (black) and with (red) the IM modulation. The appropriate IM waveform is used. (**b**,**c**) BOCDA output spectra for the cases with (red) and without (black) the IM modulation; (**b**,**c**) correspond to the measurement at a strained portion and a non-strained portion along the fiber, respectively. Figure 25: BGS distribution along the fiber obtained in experiments (**a**,**b**) and simulations (**c**,**d**), respectively [51]. (**a**,**c**) are for the case without the IM modulation, and (**b**,**d**) are for the case with the appropriate IM modulation.