Passive Q-switching of fiber lasers with saturable absorbers have attracted attention because of simplicity and high efficiency. However, the number of known materials which can be used as a saturable absorber is relatively limited due to the features of fiber lasers. While liquefying gallium mirror, semiconductor structures (SESAM), as well as Co2+:ZnSe and Co2+:ZnS crystals are recent examples of saturable absorbers that have been used in Q-switched fiber lasers, any new material which can be used for this is still very interesting.<br/><br/>Here we report, for the first time to our knowledge, a cladding-pumped passively Q-switched Er-Yb codoped fiber laser with Cr2+:ZnSe and Co2+:MgAl2O4 as saturable absorbers.<br/><br/>The fiber laser configuration consists of a 5m circular double-clad Er-Yb codoped fiber and an external cavity. Pump light at 915nm was coupled into the fiber through a perpendicularly cleaved fiber end facet. This also serves as a 4% reflecting, outcoupling, cavity mirror. In the other end of the cavity a lens system and a high-reflection (1550 nm) mirror provided external feedback. The lens system had an intermediate focus, where the saturable absorber was located. The intermediate focus waist diameter was typically 20 µm. We used the same laser configuration for both the Cr2+:ZnSe and the Co2+:MgAl2O4 crystals. The maximum average output power for both crystals was 1.4 W, for 6.5 W of absorbed pump power. The maximum obtained pulse energy was 18 pJ (45W peak power) with Cr2+:ZnSe saturable absorber. Typical pulse duration was 370 - 700 ns. The repetition rate was varied between 20 and 85 kHz.<br/><br/>The maximum pulse energy was 22 pJ (60W peak power) with Co2+:MgAl2O4 saturable absorber. The pulse duration was 370 - 600 ns. The repetition rate was varied between 15 and 65kHz.<br/><br/>Further details and comparisons of fiber lasers passively Q-switched with Cr2+:ZnSe and Co2+:MgAl2O4 crystals will be presented
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