Frequency-Shift Feedback Mode-Locked Deep-Red/Green Fiber Lasers at 717 nm and 544 nm

Visible-wavelength ultrafast fiber lasers, particularly those operating in the green and deep-red spectral regions, hold significant promise for a wide range of scientific and technological applications. However, the direct emission of true-green (540–550 nm) wavelength from laser diodes (LDs) remains underdeveloped, leading to the well-known “green gap”. The development of true-green fiber laser at 544 nm addresses this critical spectral deficiency, enabling advanced applications in biomedicine, spectroscopy, and laser display technologies. In parallel, the 717 nm deep-red fiber laser offers superior tissue penetration with minimal photo-induced damage, making it highly suitable for the treatment of deep-seated tissue disorders. This paper demonstrates frequency-shifted feedback (FSF) mode-locked fiber lasers operating at 717 nm (deep red) and 544 nm (ture green). The Pr<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3+}$</tex-math></inline-formula>- and Ho<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3+}$</tex-math></inline-formula> -doped fluoride fiber systems pumped by blue laser diodes have become the preferred architecture due to the high quantum efficiency of direct down-conversion, generating emissions at 717nm and 544nm, respectively. A visible tellurium dioxide (TeO<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula>) acousto-optic modulator (AOM) is employed to enable stable FSF mode-locking. The deep-red and green lasers deliver pulse durations of 25ps and 134ps, repetition rates of 37.1MHz and 19.47MHz, and average output powers of 42mW and 30mW, respectively. This study establishes a controllable and compact platform for visible-wavelength ultrafast fiber lasers, advancing the development of practical sources in the visible spectral domain.