Very high-aspect ratio channels may be coated using atomic layer deposition (ALD) due to the unique self-limiting nature of the process. Reactive ion-etched trenches with aspect ratios near 100:1 have been coated -. For optical and microfluidic applications, most channels are centimeters deep with diameters of tens to hundreds of micrometers. This results in a similarly high aspect ratio as etched trenches but the larger area creates more difficult problems of temperature and gas flow uniformity. To quantitatively explore the behavior, we create an air wedge between 2 square wafers of silicon approximately 7 cm on a side, with the air gap varying in thickness linearly from 0-1650 μm over 7 cm. A Fabry-Perot optical cavity composed of an ALD Al2O3/HfO2 multilayer was deposited inside the wedge, and the measured resonant wavelength only shifted by -10:3% over a distance of 6 cm along the gas flow direction and over a 118-1533 μm range of gap thickness. Our first experiments to test optical coatings inside microchannels were performed using 12 cm long glass capillary tubes with an interior dimension defined by a square 500 μm opening, a 240:1 aspect ratio. Fig. 1 shows cross-sectional SEM images at the midpoint of a capillary coated with 1000 cycles of ALD Al2O3. The inset indicates that the interior coating is near the expected 110 nm thickness for this 0.11 nm/cycle process, but it is difficult to further quantify the properties of transparent films deposited on the inside of narrow transparent tubes.