We are interested in the use of hollow-core photonic crystal fibers (PCFs) for gas refractive index sensing. Traditional Mach-Zehnder interferometers consist of two separate waveguiding 'arms' of equal length, one reference and one sample arm. In an in-fiber MZ, these two arms exist in the same length of fiber, but occupy two different fiber modes. By filling the hollow core of a PCF with a specific gasses, it is possible to accurately measure the gas index to six decimal places through the interference of fiber modes.