Optical sampling is performed mostly in either time or frequency domain. In the former approach, sampling points are temporally taken by photo-mixing of received signals with locally generated ultrashort pulses. In the latter approach, the received signal is spectrally sliced and decomposed, which are taken as sampling points in the frequency domain. In this paper, as an alternative way, we propose and investigate an optical sampling technique called optical time-frequency-domain sampling, in which sampling points are taken in the time-frequency domain in an optoelectronic manner, by means of optoelectronic coherent matched detectors. Using this technique, it is possible to demultiplex and detect subchannels of ultrawideband multi-carrier signals at e.g. 100, 400 Gb/s, 1 Tb/s or higher, without relying on either ultrashort pulses or high-resolution optical filters necessary in the conventional time- or frequency-domain sampling approaches. Moreover, in this paper, we propose and demonstrate a loop-assisted coherent matched detector that simply and practically extends the time-frequency sampling to parallel sampling configuration just using a single coherent matched detector. Provided are some experimental proof focusing on demultiplexing and detection of orthogonal time-frequency-domain multipelxed (OTFDM) signals, a sort of ultrawideband multicarrier signals. All subchannels of 160-Gb/s QPSK-OTFDM signals are simultaneously detected and measured with the loop-assisted coherent matched detector employing a single set of low-speed 10-Gbaud photo-receivers.