# Fiber-Optic System for Monitoring Pit Collapse Prevention
## 2 Materials and Methods
### Experimental Installation  
The hypothesis assumes the application of a G.652 type single-mode OF as a sensor and data transmission system. Taking into account the length of mine workings, it is necessary to ensure the installation of a FOS at a considerable distance from the data processing unit. The only option to build a distributed monitoring system with low power consumption and signal attenuation is possible only on the basis of OF. To test the theory, a laboratory bench was developed (Figures 1 and 2). The displacement measurements were made by two methods to ensure reliability of the results. Figure 1 shows the first method of controlling the change in the pixels of a light spot that is formed at the exit from the OF and falls on the surface of the television matrix. The second method shown in Figure 2 is based on controlling the additional losses of light waves that appear in the OF during the occurrence of the mechanical impact or the bending. Figure 1: Laboratory bench with the use of the developed HSC. Figure 2: Laboratory bench with the use of a reflectometer. In the first case, a solid-state laser from a visual fault locator in an OF was used as a radiation source. An important point was selecting this light source according to the parameters of radiation coherence and its stability, because even radiation frequency deviations of more than 5% lead to a failure in the monitoring system. On the other hand, the sensor was connected to a light source at one end and to a television matrix at the other. Data processing as carried out using a hardware-software complex; the visualization of measurements as represented by graphs on a computer screen. A VIAVI Smart Pocket OLP-38 optical power meter (JDSU) (Viavi Solutions, San-Jose, CA, USA) was used to determine optical power and loss values. It operates in the dynamic range from \(-50\) to \(+26\) dBm with a wavelength range of \(780\div 1650\) nm and has the ability to automatically estimate the level of optical power without switching the measurement range, as well as to measure the insertion losses formed in the optical conductor when a light wave passes through its core. In addition, a Smart Pocket OLS-34 cogeneration radiation (Viavi Solutions, San-Jose, CA, USA) source based on a semiconductor laser was used. This source has a number of necessary automatically supported measurement functions, including automatic stabilization of light wave radiation in the range of 900-1625 nm, as well as the Auto-\(\lambda\) and Multi-\(\lambda\) functions, which are essential for achieving measurement accuracy. The instruments were used to set up and calibrate the system. The connection to the OF was made through a universal UPP 2.5 mm adapter and optical connectors (OC) of the SC type. Figure 3 shows a photograph of the used tension measurement instrument (TMI) with an accuracy of 0.01 mm. An experimental setup was developed for the experiment (Figure 4). The experiment was carried out as follows. The displacement was performed by tightening the turnbuckles (TB) and nut bolt (NB) and measured by TMI. When displaced, the ends of the ferrules into which the OF was inserted diverged by up to 30 mm; if needed, the measurement range can be extended to 100 mm. So far, there are limitations associated with the parameters of the used ferrules, which edged the ends of the OF through OC and the tube in which the ferrules move. Several sensors can be connected in series or used as a transmission mechanism to increase the divergence distance of the OF. Changes in the properties of light occur in relation to the intensity and phase of propagation when the ends of the OF move away from each other. It is reflected in changing the shape of the light spot. The farther the OF ends diverge from each other, the greater the level of additional losses and loss of optical power are. All the changes are recorded by the hardware-software complex and displayed on the screen. Figure 3: Device for measuring displacements (minimum one division value).