Modelocked femtosecond laser emits at 2420 nm

A room-temperature, long-wavelength (2 to 3 µm) femtosecond laser would be useful for high-harmonic generation in the deep-UV or soft-x-ray regions, or as a pump source to create even-longer-wavelength IR femtosecond pulses via parametric or Raman conversion. Researchers at Koç University and Teknofil (both in Istanbul, Turkey) have created such a laser, using chromium-doped zinc selenide (Cr2+:ZnSe) as a gain medium. The laser produces 95 fs pulses at a pulse-repetition rate of 94.3 MHz and an average power of 40 mW.

A 5 W thulium fiber laser operating at 1800 nm end-pumps the laser cavity, which contains a 2.4-mm-long Cr2+:ZnSe crystal. Two magnesium fluoride prisms were inserted in the cavity to create a Kerr-lens modelocking configuration with a cavity dispersion of -1700 fs2 at 2420 nm. The full-width half-maximum pulse width was 95.2 fs, assuming a sech2 pulse profile, while the wavelength spectrum had a width of 69 nm; the resulting time-bandwidth product of 0.335 was almost identical to what would be expected for a transform-limited sech2 pulse (0.315). The laser operated continuously and stably for up to 15 minutes; longer periods will be achieved with better optical mounts, say the researchers. Contact Alphan Sennaroglu at asennar@ku.edu.tr.

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