Laser Ties Optical Fiber in a Knot

Breck Hitz

Minuscule microlasers have numerous potential applications in information technology and communications as well as in medical diagnostics. Scientists in several laboratories around the world have demonstrated ultracompact lasers in microdisks and other tiny structures, and now researchers at Zhejiang University in Hangzhou, China, and at the Chinese Academy of Sciences in Shanghai have demonstrated laser action in a millimeter-size knot tied in a length of microfiber.

The straightforward approach to fabricating the laser — simply micromanipulating a loop, or knot, in a piece of ~3-μm-diameter microfiber drawn directly from the bulk phosphate-glass material — may lead to inexpensive commercial devices. The fiber was doped with both erbium (1.25 molecular percent) and ytterbium (2.35 molecular percent) and was characterized by high diameter uniformity and excellent sidewall smoothness. Absorption at the 975-nm pump wavelength was ∼3.4 dB/mm.

Figure 1. A knot, or microloop, of doped phosphate-glass fiber serves as a ring resonator for the tiny laser. In the inset, the green up-converted fluorescence from erbium doping is clearly visible in a photograph of the loop. Images reprinted with permission of Applied Physics Letters.

The laser was essentially a ring resonator, with evanescent coupling both closing the ring on itself and coupling the pump light into the resonator (Figure 1). There are three fibers in this picture: the doped microfiber and two conventional silica fibers, both drawn down to ~1-μm tapers. The doped microfiber’s two ends are secured in the holders shown, and the middle of this fiber is configured into a loop. The tapered pump fiber lies alongside the microfiber loop, held against it by van der Waals and electrostatic forces, so that the 975-nm pump light is evanescently coupled into the microfiber. The second tapered fiber evanescently couples laser power out of the loop.

The ring resonator is formed by evanescent coupling of the microfiber with itself, along the length of loop overlap at the right side of Figure 1. In the absence of an isolator or some other selective element, laser light circulates in both directions around the ring.

The researchers observed lasing in loops ranging from 1 to 3 mm in diameter but not in smaller ones. Nonetheless, they expect that even tighter loops could lase, if the pump linewidth were reduced and if the pump light were more efficiently coupled into the doped microfiber.

Figure 2. Below threshold (left), broadband resonant fluorescence can be observed at the output port (port 3 in Figure 1). The spacing between the resonances corresponds to the ~6.25-mm circumference of the loop. Above threshold (right), a lasing single longitudinal mode predominates.

Laser threshold occurred at about 5 mW of pump power. Below 5 mW, the scientists saw broadband resonant luminescence from the fiber, with a spacing between the longitudinal modes of about 0.24 nm, corresponding to the ~6.25-mm circumference of the ring resonator (Figure 2). Above threshold, they observed single-longitudinal-mode oscillation centered at 1541.1 nm, with an apparently instrument-limited bandwidth of 6.3 GHz.

They measured 8 μW of output (at port 3 in Figure 1), but total power available from the laser was probably at least twice that because the laser power emerging through ports 4 and 5 was not measured.

Applied Physics Letters, Oct. 2, 2006, 143513.