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We report and characterize an amplifier which provides gain to a record six spatial modes simultaneously. Two sections of erbium-doped fibre with different mode-dependent gain characteristics are concatenated to equalize gain between modes. The overall gain excursion does not exceed 4dB across all modes. The noise figure is found lower than 7dB.

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A wavelength-switchable single-longitudinal-mode thulium-doped fiber laser using a sampled fiber Bragg grating is proposed in this study. The laser can be switched between three single-wavelength and three dual-wavelength modes of operation. For the single-wavelength mode of operation, the maximum power and wavelength fluctuations in 50 min were ±0.404 dB and ±0.01 nm, and the linewidths at three wavelengths were 0.39, 1.88, and 0.89 kHz, respectively. For the dual-wavelength mode of operation, the maximum power and wavelength fluctuations in 20 min were ±0.926 dB and ±0.03 nm, respectively. The proposed thulium-doped fiber laser will find useful application in optical communication and free-space optical detection and sensing.INDEX TERMS thulium-doped fiber laser, wavelength-switchable, narrow linewidth, sampled fiber Bragg grating.