A high power and repetition rate wavelength tunable actively mode-locked Holmium-doped fiber laser for bidirectional transmission between two HAPS

The optical transmission window around 2000 nm is getting significant attention for terrestrial and deep-space communication due to lowest absorption. Moreover, it is also being considered as an alternate optical window for spectral expansion beyond C-, L-, and U-bands to resolve the capacity related issues of future optical networks. Holmium-doped gain materials have the capacity to emit light in the 2000–2200 nm wavelength range where Thulium-doped gain materials are comparatively inefficient. In this paper, the design of high power and repetition rate tunable actively mode-locked Holmium-doped fiber laser (AML-HDFL) is demonstrated through numerical simulations. The design is based on a single ring cavity for bidirectional transmission of data over optical wireless channel (OWC) at the rate of 25 Gb/s between two high-altitude platform stations (HAPS). A wide wavelength tuning range from 2000–2150 nm at a side-mode suppression ratio (SMSR) of 35 dB is observed. Train of mode-locked pulses having peak power of 21 W, full-width at half maximum (FWHM) of 7.4 ps, pulse energy of 0.148 nJ and signal to noise ratio (SNR) of 40.6 dB is generated. Moreover, the performance of tunable AML-HDFL using 1950 nm pumping is compared with 1840 nm pumping. Performance of the downlink (DL) and uplink (UL) channels are observed using Gamma-Gamma channel model for medium and strong turbulence where forward error correction (FEC) limit of 3.8\(\times\)10\(^{-3}\) is achieved between two HAPS that are separated by a distance of 200 km.