Operation of diode laser pumped Tm super(3+) ZBLAN upconversion fiber laser at 482 nm

Laing at 482 nm is observed in Tm super(3+)-doped ZBLAN glass fiber pumped with single-mode InP semiconductor diode lasers. Up to 5 mW of 482 nm light is obtained with < 40 mW of absorbed pump from a single 1135 nm pump diode laser. The optimum pump wavelength is measured to be 1135-1140 nm. More efficient laser operation is observed in fiber with 2500 ppm Tm super(3+) compared to 1000 Tm super(3+) because of the reduced length of the fiber laser cavity possible with increased doping. Improved slope efficiencies are also demonstrated when the fiber laser is co-pumped with up to 5 mW from a 1220 nm diode laser. The relative intensity noise (RIN) of the fiber laser displays a maximum of -90 dB/Hz at relaxation oscillation frequencies of a few tens of kHz. The measurement of RIN is limited by shot-noise of -152 dB/Hz above 2 MHz. At higher frequencies, self mode-locking was observed in the fiber laser, which may indicate the existence of saturable absorbers in the fiber core. The presence of such bleachable absorbers is indicated by the observed increase in threshold after upconversion lasing at 482 nm.