Ultralow Threshold Reported for Raman Laser

Breck Hitz

How low can you go? In the quest for ever more efficient Raman lasers, investigators have achieved thresholds that have been below a milliwatt by resonating both pump and laser wavelengths in a fused-silica microsphere.

But now scientists at California Institute of Technology’s Jet Propulsion Laboratory in Pasadena have fabricated a single-crystal fluorite whispering-gallery resonator demonstrating Raman-laser thresholds that they believe are orders of magnitude lower than any previous Raman laser. Their device reached threshold with only 3 μW of absorbed pump power, and they calculate that improvements in the experimental arrangement would reduce threshold into the submicrowatt regime.

Figure 1. A single-frequency Nd:YAG laser pumped the whispering-gallery resonator in which Raman lasing occurred. A feedback loop adjusted the Nd:YAG resonator length to lock its frequency to a mode of the whispering-gallery resonator (a). An angle-polished optical fiber provided both input and output coupling for the whispering-gallery resonator (b).

That is important because Raman lasers are valuable tools in molecular spectroscopy and in many biological studies. The tiny, low-threshold devices developed at the institute could easily be integrated into optical chips, enabling multifunction chips that would facilitate chemical and biological analysis.

Whispering-gallery resonators are ideally suited for low-threshold Raman lasers because they are compatible with optical fibers, they are compact, and they easily achieve high intracavity circulating powers. Fluorite resonators are inherently less lossy than those fabricated from fused silica and, hence, capable of microresonator Q-factors higher by at least an order of magnitude. The researchers showed that, for a 5-mm-diameter fluorite (CaF₂) whispering-gallery resonator, Q-factors greater than 10¹⁰ and Raman-laser thresholds below a microwatt should be achievable.

Figure 2. The first Stokes line reached threshold at 3 μW of absorbed 1064-nm pump power; the second Stokes, at 7.5 μW. Reprinted with permission of Optics Letters.

Experimentally, they fabricated a 5-mm-diameter whispering-gallery resonator from excimer-grade fluorite and coupled 1064-nm pump radiation into it from a single-frequency Nd:YAG laser supplied by Lightwave Electronics (Figure 1). As they increased the pump power, they observed threshold for the first Stokes component at 3 μW; at ~7.5 μW, the second Stokes line reached threshold (Figure 2). The power in the first Stokes line was clamped at about 240 nW after the second Stokes line reached threshold. The scientists believe that improved optical coupling with the whispering-gallery resonator could reduce threshold below a microwatt.

Even higher-order Stokes components reached threshold as the researchers increased the pump power into the milliwatt range, but in this case the nonlinear behavior of the mode was so pronounced that frequency stabilization of the pump laser was impossible, and the relative power of the various Stokes components fluctuated rapidly during the measurement (Figure 3).

Figure 3. When the pump power increased to nearly a milliwatt, as many as eight Stokes components reached threshold. The inset is the spectrum of the first Stokes line. Reprinted with permission of Optics Letters.

Nonetheless, they believe that this is the first observation of cascaded Raman lasing in fluorite and the first observation of eight Stokes lines in a fiber-compatible, all-solid-state crystalline laser at pump power below 1 mW.

Optics Letters, Jan. 15, 2007, pp. 166-168.