Abstract:
A holmium fiber laser and its ring cavity with the air path embedded inside is investigated, the path being a confocal system of two lenses. Different samples of a saturable absorber were placed at the center of the air path waist, including random single-walled carbon nanotubes (SWCNT) and those aligned along one of the directions in the SWCNT application plane. Alterations of lasing regimes and of the central radiation wavelength were demonstrated, and the self-starting characteristics of the holmium fiber laser were investigated using the intracavity z-scanning method on different SWCNT samples. In the case of a sample of randomly oriented SWCNTs in the cavity, a set of little different regimes was obtained with an average radiation power of ∼7 ± 0.1 mW and the central wavelength of ∼2075 nm. It was altered but slightly (less than 1 nm). When a sample of aligned SWCNT was used, the average radiation power varied from 7 to 7.5 mW, the central wavelength at the position of the optical waist was ∼2038 nm, and it was altered in the range of ∼2 nm. The pulse repetition rate in both cases was ∼20.7 MHz. The minimum duration of the transfer process during the transition to continuous mode locking was 25 μs in the case of the cavity with a sample of aligned SWCNTs located at the optical waist, the average power density along the beam cross-section on the sample being ∼1.1 W/mm2. The position of the SWCNT samples relative to the optical waist determined the boundaries of mode locking with or without self-starting capability and the mode locking regimes of passing into continuous lasing.
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