A heterodyne interferometer-based signal retrieval in photoacoustic/photothermal gas spectrometer using mid-infrared quantum cascade laser excitation is demonstrated. This new method for all-optical photoacoustic/photothermal signal detection allows for sensitivity enhancement using standard multi-pass cells, commonly used in absorption-based spectrometers. Two types of multi-pass cell are examined: a Herriott type with up to 33 passes and a White type with up to 46 passes. Good agreement between experimental results and numerical analysis is obtained.