Scientists at Aston University in Birmingham, UK, have fabricated a microchannel across the diameter of a single-mode fiber and used the tiny device to measure the refractive index of a fluid pumped into the channel. They envision applications in biomedicine, chemistry and metrology for similar fibers with microchannels drilled across them.Figure 1. This microscope image shows the index change in the single-mode fiber when femtosecond pulses were focused directly into the fiber (A). The researchers defeated the defocusing in (A) with the arrangement in Figure 2, and the resulting index-change path was straight and uniform (B). After etching in a hydrofluoric-acid bath, the resulting microchannel passed directly through the core, as shown in photographs (C) and (D), taken from orthogonal angles. Images reprinted with permission of Optics Letters.Microfluidic devices have recently been of interest to numerous scientists, many of whom have investigated techniques of combining that technology with fiber optics. One fruitful approach has been to fill the holes of a microstructured optical fiber with a fluid that affects optical transmission in the fiber. Other investigators have butt-joined fibers to microfluidic channels, or coupled fibers to hollow-core planar waveguides. But the researchers at Aston believe that they are the first to have fabricated microchannels directly into optical fibers.They fabricated the microchannels by first writing a narrow index-changed path across the fiber with ~150-fs pulses from an ~800 nm laser operating at ~550 nJ, with the beam focused to an ~1.5-μm spot. They etched out the channel in a hydrofluoric-acid bath. (They observed that the glass whose index had been altered with the femtosecond pulses was ~200 times more susceptible to the acid etching.)Initial experiments with femtosecond writing were unsuccessful because the curved surface of the fiber defocused the beam inside it (Figure 1A). But they easily overcame this difficulty by placing the fiber on a microscope slide and using a few drops of index-matching oil (Figure 2). The resulting path of index-changed glass was narrow and uniform all the way across the fiber (Figure 1B). Microscopic images of the microchannel taken with a 40× oil-immersion microscopic lens revealed a slight narrowing of the microchannel toward the center of the fiber, apparently because the center of the channel had less exposure time to the hydrofluoric acid. This slight taper did not pose any obvious problems for microfluidic applications.Figure 2. To defeat the lensing effect of the fiber’s curved surface, the researchers mounted the fiber on a microscope slide and applied a few drops of index-matching oil.The microchannel was approximately 4 μm in diameter, and, of course, a 4-μm hole in the middle of an ~10-μm fiber core has a deleterious effect on the fiber’s transmission.The researchers used this effect to demonstrate an accurate measurement of a fluid’s refractive index. They pumped several mixtures of glycerin in water through the microchannel and observed the transmission change in the fiber (Figure 3). They calculate that this simple instrument can be used to measure refractive index with a resolution of 1.79 × 10–4.Figure 3. Transmission through the fiber provided a sensitive measurement of the refractive index of the material in the microchannel.They point out that greater resolution would be possible in a tiny device by cascading numerous densely packed microchannels in a fiber. Additionally, they believe that the technique could lead to efficient and very compact, in-fiber variable optical attenuators or, with the addition of fiber Bragg gratings to the structure, a whole range of useful photonic devices.Optics Letters, Sept. 1, 2006, pp. 2559-2561.