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We demonstrate an optical quadrature phase-shift keying (QPSK) signal transmitter and an optical receiver for demodulating optical QPSK signal with homodyne detection and digital signal processing (DSP). DSP on the homodyne detection scheme is employed without locking the phase of the local oscillator (LO). In this paper, we present an extracting one-dimensional array of down-sampling method for reducing unwanted samples of constellation diagram measurement. Such a novel scheme embodies the following major advantages over the other conventional optical QPSK signal detection methods. First, this homodyne detection scheme does not need strict requirement on LO in comparison with linear optical sampling, such as having a flat spectral density and phase over the spectral support of the source under test. Second, the LabVIEW software is directly used for recovering the QPSK signal constellation without employing complex DSP circuit. Third, this scheme is applicable to multilevel modulation formats such as M-ary PSK and quadrature amplitude modulation (QAM) or higher speed signals by making minor changes.

We analyze a simple model of a scalar optical wave with partial coherence. The model is devised to describe the influence on the coherence of the wave, of the statistical properties of its random phase, including both the second-order statistics (phase correlation) — which is classic, but also the first-order statistics (phase distribution) — which is nonclassic. Expectedly, upon increasing the disorder of the fluctuating phase through a reduction of its correlation duration, the model shows that the coherence of the wave is always reduced. By contrast, upon increasing the disorder of the fluctuating phase through an increase of its dispersion, the model reveals that the coherence of the wave can sometimes be enhanced. This beneficial consequence of an increase in disorder is related to the phenomenon of stochastic resonance or improvement by noise in signal processing.

We consider the problem of characterizing the spatial extent of a composite light source using the super-resolution imaging technique based on mode demultiplexing when the centroid of the source is not known precisely. We show that the essential features of this problem can be mapped onto a simple qubit model for joint estimation of a phase shift and a dephasing strength.

Studies of non-invasive glucose measurement with optical coherence tomography (OCT) in tissue-simulating phantoms and biological tissues show that glucose has an effect on the OCT signal slope. Choosing an efficient fitting range to calculate the OCT signal slope is important because it helps to improve the precision of glucose measurement. In this paper, we study the problem in two ways: (1) scattering-induced change of OCT signal slope versus depth in intralipid suspensions with different concentrations based on Monte Carlo (MC) simulations and experiments and (2) efficient fitting range for glucose measurement in 3% and 10% intralipid. The results show that the OCT signal slope expresses a contrary change with scattering coefficient below a certain depth in high intralipid concentrations, so that there is an effective fitting depth. With an efficient fitting range from 100 μm to the effective fitting depth, the precision of glucose measurement can be 4.4 mM for 10% intralipid and 2.2 mM for 3% intralipid.