980nm laser diode is probably the most expensive component in an optical fiber laser system or a fiber amplifier. Back reflection of light back into the laser diode is not desirable since it affects the operating of the laser diode. More severely, back reflection of high optical power can damage the laser diode. This is an important issue for optical amplifiers for high peak power pulses or high power Q-switch optical fiber laser.

980nm pump laser diode is connected to the doped fiber through WDM couplers. 980nm laser power is mostly absorbed by the doped fiber, converted into amplified optical signals and ASE power. Both amplified optical signal and ASE are at longer wavelengths than 980nm (usually 1060nm for Ytterbium doped fiber system or 1550nm for Erbium doped fiber system). WDM couplers filter out wavelength other than 980nm, and provide certain degree of isolation to the pump laser diode. However, the isolation may not be enough for high peak power pulses.

To increase the isolation of 980nm, isolator can be used. But 980nm isolators are expensive, very bulky and heavy. At the same time the extra insertion loss and optical power handling limit introduced by the isolator can not be neglected. Comparably, thin film based 980nm protection filter is in most circumstances a better alternative. The 980nm protection filter has pass band at 980nm, while at the same time filters out light at either 1060nm or 1550nm bands. The filter costs only a fraction of the 980nm isolator, very compact and light in weight (similar to a standard fused fiber coupler). Please note to work in high peak power optical pulse system, specially designed filters should be used.

It is necessary to know that 980nm protection filter only eliminate the optical power other than 980nm. There is no isolation for back reflected 980nm power.