Short Pulses Generated in Mode-Locked Tm-Ho:BaY2F8 Laser

Breck Hitz

Ultrashort mode-locked pulse trains in the 2-µm spectral region are of interest for applications such as time-resolved spectroscopy and optical communications. Moreover, the short pulses enable highly efficient nonlinear conversion into the mid- and far-infrared regions, which are of intense interest for military applications such as night vision and electronic-warfare countermeasures. Now scientists in Italy have designed and demonstrated a Tm-Ho:BaY₂F₈ laser that they believe produces the shortest pulses ever obtained in this important spectral region from a holmium-doped laser.

When the acousto-optic mode-locking modulator is activated, the laser produces a 100-MHz train of sub-100-ps pulses at 2.06 µm with an average power of 20 mW.

The group included scientists from the Polytechnic Institute of Milan and from the University of Pisa. The Z-resonator laser (see figure) was modulated with an acousto-optic loss modulator and produced a 100-MHz train of output pulses whose individual duration was less than 100 ps. The pulse train's average power was in excess of 20 mW when pumped with 1.8 W from the 781-nm diode pump laser. Thulium ions in the 5.16-mm-thick, room-temperature BaY₂F₈ crystal absorbed the 781-nm radiation and transferred the energy to the holmium ions. Lasing occurred on a 2.06-µm transition in holmium.

An intracavity birefringent tuning plate acted as a Lyot filter and enabled the group to tune the laser's output across approximately 50 nm of the holmium transition's bandwidth. With the Suprasil mode-locking modulator removed from the resonator, the laser produced 190 mW; additional intracavity losses reduced the output to 90 mW when the modulator was inserted. A 50-MHz, 2.5-W RF signal injected into the modulator mode-locked the laser at 100 MHz. The laser's output dropped from 90 mW to 20 mW when the modulator was activated, mostly due to residual diffraction losses introduced by the nonoptimized mode-locker.

The scientists inferred the sub-100-ps pulse duration by frequency-doubling the 2.06-µm pulses in a periodically poled LiNbO₃ crystal and measuring the duration of the resulting 1.03-µm pulses with a 30-GHz-bandwidth optical digital acquisition system. They observed 69.9-ps pulses, which, taking into account the 15-ps resolution of the measurement system, correspond to 68.3-ps Gaussian pulses at 1.03 µm. Assuming that second-harmonic power is proportional to the square of the fundamental power, the group calculated backward to obtain the 2.06-µm pulse duration of 96.6 ps.

(On a human note, the paper describing the work is dedicated to the memory of "our beloved young friend and colleague Marcello Marano, who passionately shared with us his great talent and humaneness." Marano, an assistant professor at the Polytechnic Institute in Milan and co-author of the paper, died of a stroke in December 2002.)