Efficient Ho:YAG Laser Oscillates in Single Longitudinal Mode

Breck Hitz

By minimizing thermal loading on a Ho:YAG laser rod, a research group at the University of Southampton in the UK has demonstrated an efficient high-power, single-frequency laser in the eye-safe 2-µm spectral region. Such sources are useful for remote sensing and spectroscopy and can be readily down-converted into the important 3- to 5-µm region using nonlinear optics.

The acousto-optic modulator was slightly misaligned so that it introduced more loss to the wave traveling one direction around the ring resonator than to the counterpropagating wave. The additional loss was sufficient to force unidirectional oscillation in a single longitudinal mode.

The secret is to convert the 790-nm radiation of the pump diodes to the 2.1-µm output of the Ho:YAG laser in a two-stage process and to dissipate most of the heat in the first stage (outside the rod). The diodes pump a double-clad Tm:fiber laser, whose 1921-nm output directly (in-band) pumps the Ho:YAG laser. The tiny quantum defect of the Ho laser ensures minimal heating, and most of the heat is lost in the fiber laser, whose 4.7-m length easily dissipates it. The heating of the Ho:YAG rod is so slight that the experimenters observe no detrimental effect resulting from thermal lensing.

The researchers arrange their Ho:YAG laser in a bow-tie configuration, with the pump light from the fiber laser coupled into the setup through a folding mirror (see figure). Mirrors M4, M3 and M5 reflect the pump light for another pass through the laser rod so that up to 96 percent of the pump radiation is absorbed. With 43 W of 790-nm power from the diodes launched into the fiber, the fiber laser produces 8.8 W of 790-nm pump power for the Ho:YAG laser, which in turn produces 3.7 W of single-mode output at 2.1 µm.

As with most solid-state lasers, Ho:YAG is homogeneously broadened -- that is, all of the ions in the population inversion can contribute gain to all of the longitudinal laser modes. In this situation, the modes compete with each other. Were it not for another consideration, one longitudinal mode would dominate and extinguish the others so that the laser would oscillate naturally in this longitudinal mode.

The "other consideration" is the standing wave that is produced by two counterpropagating waves in a laser resonator. The standing wave cannot interact with the population inversion at its spatial nodes, and secondary modes break into oscillation, supported by these pockets of gain. Because there is no standing wave in a unidirectional ring laser, however, it naturally oscillates in a single longitudinal mode.

In other words, a homogeneously broadened laser requires no spectrally restrictive intracavity element to force oscillation in a single longitudinal mode. All that is necessary is a unidirectional ring resonator, and the natural competition among the modes takes care of everything.

The Southampton researchers took a novel approach to forcing unidirectional oscillation around their bow-tie ring. Rather than a conventional isolator comprising a polarizer and a Faraday rotator, they inserted a simple acousto-optic modulator into the resonator. With fewer surfaces, the modulator introduced less intracavity loss than an isolator, an important advantage in a relatively low-gain CW laser.

When the modulator was slightly misaligned from the perfect Bragg angle, it introduced more loss to the wave traveling in one direction than to the one traveling in the opposite direction. This slight difference -- the researchers believe it amounted to only a fraction of a percent of additional loss -- was sufficient to allow one traveling wave to dominate.