# Simultaneous Transmission of Photonic Services over One Fiber with an ITU 100 GHz Grid
## 3 Measurement Setup  
### \(\Phi\)-Otdr  
\(\Phi\)-OTDR is a technique that enables the detection and localization of acoustic vibrations acting on fiber. Compared to other techniques such as Brillouin OTDR, it is not necessary to generate so many high-energy pulses, and the detection is not so complicated. On the other hand, it is not possible to distinguish local temperature changes and acoustic vibrations (there is a possibility, but it is based on a very precise classification of the event). Since we have designed and developed our own (\(\Phi\)-OTDR), we have also developed interferometric distributed systems based on Mach-Zehnder or Michelson interferometers, and we can compare their benefits. A sizable advantage of interferometers is their usage of low power/energy signals and hence a lower probability of interference with other signals. Even though we would consider the use of a special setup allowing the use of only one fiber and event localization, paradoxically, the greatest disadvantage of interferometers is their high sensitivity. In the case of a real network (which is considered for use), all optical fibers are terminated in rack rooms containing active devices with fans and other sources of noise that can degrade the detected signal. Based on the abovementioned information, \(\Phi\)-OTDR seems to be a suitable solution for fiber infrastructure protection.  
Our own developed system is based on an ultranarrow linewidth laser source from NKT Photonics that has a wavelength of 1550.92 nm and is optimized for use in fibers with active data transmission. Compared to other commercial/research solutions, we amplify the continuous wave (CW) signal using an erbium-doped fiber amplifier (EDFA) booster type so that the maximum output power is 0.5 Watts. Another advantage of CW signal amplification is a signal with lower fluctuations compared to a situation where the pulse signal is amplified. The amplified CW signal is then modulated in the acousto-optic modulator (AOM) to 200 ns pulses, with a repetition rate given by the total length of fiber under test. If we consider the total length of approximately 40 km, then the repetition rate is given as follows:  
\[\tau=\frac{2\times L}{c/n}=\frac{2\times 40\times 10^{3}}{3\times 10^{8}/1.46} =3.89\times 10^{-4}\quad\text{(s)}, \tag{1}\]  
and the repetition rate is then as follows:\[f=\frac{1}{\tau}=\frac{1}{3.89\times 10^{-4}}=2.57\quad\text{(kHz)}. \tag{2}\]  
Based on definition of peak energy,  
\[P_{peak}=\frac{E}{\Delta t}, \tag{3}\]  
we can calculate the total pulse energy, from which average power is given by the following equation:  
\[P_{avg}=\frac{E}{T}=Ef\approx 2.5\times 10^{-4}\quad[W]. \tag{4}\]  
Before the pulse signal is coupled through the 3 rd port of the circulator to the optical fiber, the amplified spontaneous emission (ASE) is filtered out using the fiber Bragg grating (FBG) mirror on port 2 of the circulator. The backscattered signal is detected on the 4 th port of the circulator, and before the opto-electric conversion in the photodetector, the signal is amplified in the low-sensitivity EDFA preamplifier. Our measurement setup is shown in Figure 1.