Abstract

We present a regenerative amplifier operating at a wavelength of 1937 nm by using a 4 at % doped Tm:YAP crystal. The system generates an uncompressed pulse energy of 709 μJ, while the pulses can be compressed to a duration below 410 fs. The seed pulse is generated by a passively mode-locked thulium-doped fiber oscillator, which is comparable to the one described in [1]. The emitted 120 fs pulses at 1937 nm are temporally stretched in 100 m of PM1950 fiber to 90 ps pulse duration and amplified in a single-stage, cladding pumped thulium-doped PM-fiber amplifier up to a pulse energy of 25 nJ. After reducing the repetition rate by pulse picking with a Pockels cell to 1 kHz, the pulses seed the regenerative amplifier (RA), which is enclosed in a housing to purge it with nitrogen. The 4 mm long Tm:YAP crystal is placed in a copper mount and electrically cooled to 17°C. The crystal is pumped by a multi-mode fiber coupled 35 W pump diode with a central wavelength of 793 nm. The seed pulses are amplified up to 700 μJ uncompressed pulse energy during 34 roundtrips (Fig. 1 (a), red curve), which is only limited by the damage threshold of the Tm:YAP crystal in this particular configuration. The beam profile is circularly shaped (Fig. 1 (a), inset), while the M² is measured to be 1.6. Due to atmospheric absorption caused by the relative humidity of 20–27 % in the laboratory, a long ps–pedestal is visible in the autocorrelation. This is also apparent in the optical spectrum due to strong H₂O absorption lines, which can be identified by the HITRAN database [2]. Therefore, purging of the RA cavity to a relative humidity below 3 % is necessary to get a clean, pedestal free autocorrelation. The negatively chirped pulses are compressed in a Martinez-Grating compressor to a pulse duration below 410 fs (Fig. 1 (b)) with a compressor efficiency of 50 %. After purging, only a satellite pulse in 3.2 ps distance to the main peak remains, whose relative amplitude decreases from 19 % to 7 % at higher pulse energies. Simultaneously, the pulse energy contained in the main peak rises from 67 % to 76 %. By replacing the RTP Pockels cell by one from a different supplier, more satellite pulses with higher amplitudes and different temporal distances appear. Therefore, the satellite pulses are caused by the Pockels cell at multiple roundtrips and cannot be compensated by additional wave plates. Any etalon effects inside and outside the RA cavity can be excluded.

© 2015 IEEE