The authors use a surface-relief fiber Bragg grating with a polydimethylsiloxane (PDMS) layer as a volatile organic compound chemical sensor. A PDMS layer is used because it is compatible with the optical properties of the grating and exhibits good chemical selectivity. As the analyte is absorbed the refractive index of the PDMS changes, causing the Bragg wavelength to shift, and this shift is correlated to chemical type and concentration. The direction and amount of the Bragg wavelength shift is dependent on the absorbed chemical. The authors demonstrate chemical differentiation between dichloromethane and acetone in gaseous states.