In this contribution, we present the development of a passively demodulated interferometer based on 3 × 3 waveguide couplers to measure light absorption of trace gases and aerosol particles via the photothermal effect. In contrast to a “classical” interferometer with two outputs, active quadrature control is not required to ensure a high sensitivity of the system. An algorithm for the evaluation of the photothermal interferometry signal from the outputs of asymmetric 3 × 3 couplers is detailed. The performance of the algorithm is demonstrated with N O 2 calibration experiments using couplers with different working principles (i.e., fused-fiber and planar-waveguide based). The results of a laboratory measurement campaign using aerosolized nigrosin are discussed, and the measured aerosol absorption is compared to a reference instrument. A noise analysis shows interferometer phase noise to be the primary noise component. Improvements to the setup are recommended, which should improve the current instrumental detection limit in terms of absorption coefficient to below the current value of 100 M m − 1 ( 1 σ , 60 s). This corresponds to mass concentrations of about 10 µ g / m 3 for submicrometer-size black carbon particles.