Shock and detonation velocities are today measured continuously using long silica chirped fiber Bragg gratings (CFBGs). These thin probes can be directly inserted into high-explosive samples. The use of a polymer fiber increases the sensitivity at low pressure levels when studying, for instance, shock-to-detonation transitions in wedge tests. The 22-mm-long multimode polymer CFBGs have, therefore, been manufactured and characterized. A first detonation experiment was realized on a narrow Formex strip using such a sensor. The feasibility is demonstrated, and the associated uncertainties, mostly coming from the use of a multimode fiber, are discussed.