Characteristics of intrinsic optical bistability and parameter optimization in Tm³⁺/Yb³⁺ codoped laser crystal

Intrinsic optical bistability (IOB) in Tm³⁺/Yb³⁺ codoped laser crystal under 650 nm laser excitation is theoretically studied. Based on the theory of nonlinear coupled rate equations, we numerically analyze IOB evolutions of the populations at Tm³⁺/Yb³⁺ energy levels, and the influence of system parameters on IOB and the dynamic operation of bistable hysteresis loop. The simulation result shows the fluorescence IOB in visible-infrared spectra of Tm³⁺/Yb³⁺ codoped laser crystal under photon avalanche pumping scheme. It is found that IOB phenomena can be largely enhanced by optimizing the Tm³⁺/Yb³⁺ concentration ratio, choosing low phonon-energy crystal host and using crystal cooling technology, and also by tuning the pump laser to match the resonant frequency of Tm³⁺ excited-state absorption transition. Through tuning the pump laser wavelength and adjusting the rate of change in pump intensity, the controllable IOB switching can be achieved.