The optical spectrum analyzer gives the opportunity to obtain information about the waveform / quality / state of the spectrum of the optical signal after passing through the passive and active elements of the optical line. The changed, deteriorated image of the spectrum affects the quality and distance of data transmission. Thanks to the device, we can determine the influence of the optical line element on the number of channels, compliance of the distance between channels, attenuation of individual channels, inter-channel crosstalk. Properly made and secured fiber optic splices cannot deteriorate the optical spectrum. In order to be sure of this, it should be investigated. In our company, we use the ACTERN ONT-50 network tester for this purpose, equipped with the OSA-160 optical spectrum analysis module. The device allows us to analyze the optical spectrum in the wavelength range from 1250 to 1650nm with a measurement resolution of 0.001nm and power measurement from -75 to + 23dBm.