Super-Tiny Mach-Zehnders Assembled with Nanofibers

Breck Hitz

Mach-Zehnder interferometers find application in fields ranging from telecommunications to medical diagnostics to spectroscopy. Investigators have constructed the devices using free-space mirrors, optical fibers, photonic crystals and microfluidic circuits. Recently, Yuhang Li and Limin Tong of Zhejiang University in Hangzhou, China, reported what they believe is the first Mach-Zehnder constructed from nanofibers.

Figure 1. The scientists assembled the tiny Mach-Zehnder by nudging together the two nanofibers to form a pair of evanescent couplers (a). With silica fibers, as shown in the micro-photograph, the coupler had to be tens of micrometers long to achieve 3-dB coupling (b). Images reprinted with permission of Optics Letters.

Nanofibers are tiny optical fibers, 1 μm or less in diameter, drawn from conventional optical fibers or glass rods. As any fiber does, they confine and guide light, but because they are so small, they have high evanescent fields that facilitate strong coupling with their environment. Thus, sensors based on nanofiber Mach-Zehnders could be very responsive, and they also could be very small.

The researchers built several Mach-Zehnders, the first using silica nanofibers taper-drawn down to a 1-μm diameter from a single-mode fiber. They simply placed a second nanofiber alongside the first on an MgF₂ substrate and nudged them together at two points, forming two evanescent couplers (Figure 1). The electrostatic and van der Waals forces held the tiny fibers in place on the substrate, and the researchers permanently affixed the connecting single-mode fibers to the substrate, as indicated by the blue spots in Figure 1.

Figure 2. The spectral fringes created with the interferometer of Figure 1 showed a contrast ratio of ∼10 dB.

They illuminated the interferometer with broadband light and examined the transmission spectrum with an optical spectrum analyzer. The spectral fringes showed 10-dB contrast and indicated that the path length difference between the two arms was ~87 μm, in good agreement with the 85 μm measured directly from Figure 1.

Because the nanofibers were not attached permanently to the substrate, the scientists easily could adjust the interferometer’s geometry by micromanipulating the nanofibers. Working with a Mach-Zehnder very similar to the one described above, they adjusted the path difference from 8.3 μm to 58 μm and observed the corresponding spectral fringes. The contrast ratio of ~10 dB, as indicated in Figure 2, was relatively unchanged in the various geometries.

Figure 3. A Mach-Zehnder constructed with 480-nm tellurite nanofibers was only 50 μm on a side (a). The interferometer’s spectral fringes showed a contrast ratio of ∼8 dB (b).

To shrink the interferometer into an even smaller package, the scientists went to smaller fibers drawn from a higher-index material. Their tellurite (refractive index ~1.76) nanofibers were only 480 nm in diameter, and the assembled Mach-Zehnder was barely 50 μm on an edge (Figure 3). The interference fringes with this interferometer showed a contrast ratio of ∼8 dB, and the distance between the fringes indicated a path difference of 31 μm, in good agreement with the ~29 μm measured directly from Figure 3.

Optics Letters, Feb. 15, 2008, pp. 303-305.