The development of hollow‐core photonic crystal fiber (HC‐PCF) technology over the past decade has opened up a vast array of possibilities for new applications. When the hollow core is filled with gas, the HC‐PCF is ideal for molecular spectroscopy applications that require long path length interaction. When light is coupled into the HC‐PCF, the overlap between light and the molecules inside the hollow core is excellent all along the length of the fiber, which can be hundreds of meters long. Coiling the fiber up provides a compact, low‐weight gas cell at the same time featuring a high level of interaction between laser light coupled through the fiber and the molecules inside
