Since long, optical intensity modulation/demodulation techniques have encountered numerous applications in telemetry, free-space communications or optical characterization of scattering media. Upgrading those techniques to a full-field, real-time imaging modality can allow massive multiplexing, an essential asset not only for 3D imaging or optical communications, but also for imaging in turbid media (medical diagnosis, underwater vision, imaging in colloids, or navigational aid for safe transports). In this context, we have recently proposed a new concept of Full-field All-optical Snapshot Technique for QUADrature demodulation imaging (FAST-QUAD), whose capacity in real-time image demodulation have been demonstrated up to frequencies of 500 kHz, without requiring any synchronization between the receiver and the intensity-modulated source(s) in the imaged scene.
This technique relies on an all-optical architecture, at the heart of which is an electro-optical crystal and appropriate polarization optics components, making it possible to spatially multiplex four transmission « gates » in quadrature to each other (0°, 90°, 180°, 270°), addressing four sub-images detected on the same single standard sensor (CCD/CMOS). This setup behaves as a quadrature lock-in detection circuit, well-known in the electronics field, but in the optical domain and in a massively spatially multiplexed way, using the acquisition time of the camera as a low-pass integrator. This optical module can therefore be inserted in front of any camera, and allows the number of electronics components to be minimized. This property provides FAST-QUAD with a major asset, as its operating frequency is fully and continuously tunable in the RF range, which allowed us to establish an experimental proof-of-concept between 0 Hz (DC) and 500 kHz on the first prototype built in the laboratory. We will detail the instrumental conception of this prototype as well as the calibration/processing pipeline developed. Experimental validation results and examples of application of the FAST-QUAD approach will also be presented.