Wavelength-multiplexed computational temporal ghost imaging

Summary form only given. Ghost imaging is a novel imaging technique based on correlation measurements between a structured illumination pattern (the reference) and the total intensity transmitted or reflected by an object. The reference illumination patterns may be either randomly generated by a spatially incoherent light source, or pre-programmed e.g. with a spatial light modulator. Light transmitted (or reflected) by the object is measured by a single-pixel “bucket” detector with null spatial resolution. A unique feature associated with ghost imaging is that (i) neither the bucket detector nor the reference measurement caries enough information to retrieve the object shape and (ii) it is insensitive to distortions placed after the object. Recently, the concept of ghost imaging was transposed into the time domain to retrieve ultrafast waveforms using a temporally incoherent light source and dispersive propagation in optical fibre. However, the technique requires a large number of distinct measurements corresponding to different probing patterns to construct the correlation with a high signal to noise ratio, and it is therefore not real time (just as in the spatial case). Here, we solve the problem of multiple measurements in time-domain ghost imaging by using wavelength-multiplexing of mutually orthogonal time-varying patterns. Time-structured illuminations based on Hadamard matrices are encoded onto different frequency bands of a broadband stationary light source and simultaneously transmitted through the object. The spectrum of the transmitted light is then recorded by a line-camera based spectrometer with a long acquisition time that works as a wavelength-multiplexed bucket detector. The image of the probed object can then be reconstructed in real time by correlating the measured spectrum with the different wavelength-multiplexed Hadamard patterns.The experimental setup uses a fiber-coupled superluminescent diode centered at 1550 nm with a 40 nm bandwidth. The sequence of Hadamard matrix rows is encoded in equal time periods onto the source spectrum by a waveshaper acting as a tunable spectral filter. The temporal object consists in the time-dependent transmission function of an electro-optic modulator driven by a programed arbitrary function generator. The transmitted spectrum is recorded by a spectrometer integrating light over the measurement window duration. The retrieved ghost image of the temporal object calculated from the correlation of the multiplexed patterns with the recorded spectrum is illustrated to be in excellent agreement with the original time-varying object. Note that the technique is also insensitive to any distortion that may occur after the object. The temporal resolution of our measurement is currently limited to 500 ms by the slow response time of the waveshaper but scalability to much higher speed is possible using e.g. multiple electro-optic modulators. We believe that this new proof-of-concept technique opens up new perspectives for real time waveform measurement.