Intracavity Raman Laser Is Continuous-Wave

Breck Hitz

Although pulsed Raman lasers have been thoroughly investigated during the past two decades, only a few continuous-wave devices have been built. While such non-pulsed lasers would be useful in many applications, the drawback has been that the high powers required for efficient Raman conversion cannot easily be obtained in CW devices. But several breakthroughs have been announced during the past two years, including CW Raman lasing in silicon, which may lead to important advances in the integration of photonics and electronics.

Recently, Helen M. Pask at Macquarie University in Sydney, Australia, demonstrated a CW Raman laser capable of generating 800 mW at 1176 nm, with a 4 percent overall conversion efficiency from diode laser pump light to Raman output.

Figure 1. Mirrors M1 and M2 formed the resonator for both the 1.06-µm Nd:YAG laser and the 1176-nm KGd(WO₄)₂ Raman laser. ©OSA.

The Raman laser was an intracavity device, designed to take advantage of the fact that the power that circulates inside a CW laser always is much greater than the output power. Both the diode-pumped Nd:YAG laser and the Raman laser utilized the same resonator (Figure 1). Pask selected a 5 × 5 × 50-mm crystal of KGd(WO₄)₂ as the Raman material because of its good thermal properties, high damage threshold, and high Raman gain coefficient.

She pumped the Nd:YAG crystal with a fiber-coupled diode laser from Jenoptik AG of Jena, Germany, capable of producing up to 30 W at 808 nm. Operating the laser without the intracavity KGd(WO₄)₂ crystal and with an output mirror that transmitted 5 percent of the 1.06-µm light, she observed up to 8 W of output in a multimode beam. From measurements of the laser resonator stability, she estimated that the YAG’s thermally induced back focal distance when pumped with 20 W was approximately 10 cm.

Using LasCAD, a commercial computer program from Las-Cad GmbH of Munich, Germany, Pask then computed the intracavity beam size at the nonlinear crystal for both multimode (TEM₂₂) and single-mode oscillation. The beam sizes suggested that she could obtain Raman threshold at an intracavity power of several hundred watts. Since the 8-W output had been achieved with a 5-percent mirror (i.e., ~160 W intracavity power) and since the intracavity power would be boosted if the 5 percent mirror were replaced with a maximum reflector, she was optimistic about achieving Raman threshold.

Figure 2. The Raman output was stable at up to 20 W of pump power (800 mW output), but it was very sensitive to misalignment and fluctuations in cooling water temperature at higher pump powers.

Pask first inserted the KGd(WO₄)₂ crystal into the resonator while the 1.06-µm output mirror was in place and observed no Raman lasing. But when she replaced that mirror with a maximum 1.06-µm reflector (which had 0.25 percent reflectivity at the Raman wavelength of 1176 nm), she immediately saw the 1176-nm output, corresponding to a 901^cm–1 Stokes shift in the KGd(WO₄)₂.

Laser threshold occurred at 4 W of pump power from the diode, and the laser was stable at pump powers of up to 20 W, from which it generated 800 mW (Figure 2). At higher pump powers, however, the output was very sensitive to optical alignment and to temperature changes in the cooling water.