## 4 Results  
### Confirmation of Torsional Mode and Flaw Detection  
Figure 4 shows a frequency scan (F-Scan) plot of data collected using ultrasound generated by the SMS collar and received by the optical fiber sensor attached on the bottom of the pipe at 3 feet from the pipe end. The F-Scan plot shows data from a series of A-scans (amplitude scans) with different pulse center frequencies vs. distance, with amplitude shown in color. The dead zone, pipe end reflections, and flaw indications are indicated. These two flaws are located at 7 feet and 10 feet from the sensor  
Figure 3: Schematic of the integrated ultrasonic-optical system setup. A servo controller was used to keep the laser wavelength locked to the slope of the FBG spectrum for each channel. The optical signal was converted to a voltage signal and recorded using conventional ultrasonic hardware.  
and on the top and right side of the pipe, respectively. The flaws are 0.5-inch diameter and 0.25-inch deep drilled holes, which represent a simulated 2% CSA (pipe cross-sectional area) corrosion flaw.  
Reflections from a reverse wave are also shown in Figure 4. Although the SMS collar was activated with directional control, in which two axially-separated coils are used with a time delay to cancel the backward-travelling wave, the cancellation is not perfect, and therefore extra reflections are shown at 3 feet and 13 feet, and to a lesser extent near 10.5 feet, from the collar location. These signals would not be as prevalent in a longer length of pipeline, since a 100% reflector (cut pipe end) would not be present. Also, these reverse wave reflections could be suppressed if two axially-separated optical fiber sensors are used and a directional control routine is applied on the received signals.  
Even though the FBG sensor is effectively sensing at only a small portion (approximately a \"point\") along the circumference, as opposed to sensing around nearly the full circumference as is achieved with a conventional long-range ultrasonic collar, it is clear from Figure 4 that positive flaw detection occurs over a range of frequencies. The ability of the \"point\" optical fiber sensor to detect a flaw reflection is dependent on the relative locations of the flaw and sensor, as will be discussed further below.  
For comparison, F-Scan data using solely the SMS collar in pulse-echo mode is displayed in Figure 5. The two 2% CSA flaws are clearly detectable at 7 and 10 feet. Also, there is a higher signal-to-noise ratio for this setup, which is typically seen for magnetostrictive sensors. Use of different optical components, such as a laser with lower noise, could achieve a similar signal-to-noise ratio. Note also that the dual axially-separated coils allow for directional control and also a suppression algorithm on the received wave, suppressing the reverse waves shown in Figure 4, which is not possible for a single fiber optic sensor but could be achieved with multiple fiber optic sensors, as noted above. The experimental pipe tested here does not highlight the advantages of the hybrid sensing method, since this application of a short pipe of low attenuation would not benefit from range extension. The overall goal of this work is to prove that detection with a hybrid system is possible and describe future applications where it could prove beneficial.  
Figure 4: F-Scan generated using an FBG sensor directly bonded to the bottom side of the pipe at 3 feet from the pipe end, collocated with SMS sending collar. Flaw indications are shown at 7 feet and 10 feet from the sensor. Pipe end reflections are shown at \(-3\) feet and 18 feet. Indications at 3 feet and 13 feet are due to waves traveling from the SMS collar in the reverse direction and bouncing off the near end of the pipe at \(-3\) feet, a result of imperfect directional control. The optical fiber sensor can clearly be used for flaw detection.