High-Precision Comparison of Time Scales Over Fiber-Optic Lines Using Satellite Modems with Additional Tone Modulation

This article discusses the application of a new type of modem with pseudo-noise coding and additional tone modulation, developed for the comparison of reference time scales over satellite communication channels. Experimental studies have compared the time scales of hydrogen masers from the composition of the State Primary Standard of Units of Time, Frequency, and the National Time Scale GET 1-2022 over an optical communication channel using the modems under consideration. As a communication channel, fiber-optic lines of different lengths were used, along which the two-way signal was transmitted. The output radio frequency signals of the modems, with the use of optical transmitters connected to the ends of the fiber lines, were transmitted counter-currently at an optical carrier frequency with a wavelength of 1.55 μm. The demodulation of optical signals into the radio frequency range was implemented at the ends of the fiber lines by optical receivers. The results of an experimental evaluation of type A indefiniteness by comparing time scales along fiber-optic lines of 50-km, 100-km, and 200-km lengths are presented. On a line 200-kmlong, the type A measurement uncertainty when comparing time scales does not exceed 3 ps in the daily measurement interval. The experiments revealed that implementing the method of comparing time scales with modems with pseudo-noise coding and additional tone modulation is possible over a 200-km-long fiber-optic line without intermediate optical amplifiers. This finding opens up the prospect of creating cascaded comparison systems for time scales of remote standards over fiber-optic communication lines with fewer intermediate optical amplifiers.