DETERMINATION OF THE OPTICAL TRANSMITTER MODULATION TRANSMISSION FUNCTION IN THE PRESENCE OF LASERS EMITTER

Abstract

The most acceptable form of transmission of information over the fiber-optic line is its encoding in the form of rectangular pulses. In most cases, a semiconductor laser diode is used as a radiation source, and a single-mode fiber is used as a light guide. The main cause of energy losses in this node is the mismatch of the apertures of the laser diode and the fiber used to propagate the signal. In fiber-optic information systems, where the signal is modulated by the supply voltage, it is important not only to reduce energy losses, but also to transmit the signal to the fiber with minimal distortion. The most effective means of estimating the effect of the optical system of input of radiation into the optical fiber on the degree of distortion of the transformed signal is the use of Fourier transform of the signal with subsequent analysis of its frequency characteristics. Considering the node of the optical transmitter as one of the links of fiber-optic information systems, we conclude that it is necessary to study its frequency characteristics, namely its modulation transmission function. The modulation transfer function of this node depends on the design parameters of the input system. Errors in basing the radiating plane of the laser diode relative to the cardinal points of the optical system for inputting radiation into the optical fiber also have a significant effect on the modulation transmission function. Such errors include defocusing and angular and transverse decentering. Calculations of aberrations with subsequent determination of the scattering function and the use of fast Fourier transform for the calculation of FPM, as well as the calculation of FPM were performed using the program ZEMAX. The research allowed to obtain results characterizing the frequency properties of a typical optical system of laser diode radiation input into a single-mode optical fiber. It is established that the most stringent requirements (at the level of several microns) should be set before the possible defocusing of the radiating plane. Possible transverse decentering is limited to ten fractions of a millimeter, and ultimately its value is determined by the allowable vignetting of the radiation flux.