Quantum communication schemes such as quantum key distribution (QKD) and superdense teleportation provide unique opportunities to communicate information securely. Increasingly, optical communication is being extended to free-space channels, but atmospheric turbulence in free-space channels requires optical receivers and measurement infrastructure to support many spatial modes. Here, we present a multimode Michelson-type time-delay interferometer using a field-widened design for the measurement of phase-encoded states in free-space communication schemes. The interferometer is constructed using glass beam paths to provide thermal stability, a field-widened angular tolerance, and a compact footprint. The performance of the interferometer is highlighted by measured visibilities of 99.02±0.05% and 98.38±0.01% for single- and multimode inputs, respectively. Additionally, high-quality multimode interference is demonstrated for arbitrary spatial-mode structures and for temperature changes of ±1.0°C. The interferometer has a measured optical-path-length drift of 130nm/C near room temperature. With this setup, we demonstrate the measurement of a two-peaked multimode single-photon state used in time-phase QKD with a visibility of 97.37±0.01%.