Instruments nonlinear optical properties

See also Matrix Isolation Studies By IR and Raman Spectroscopies Nonlinear Optical Properties Nonlinear Raman Spectroscopy, Instruments Nonlinear Raman Spectroscopy, Theory Photoacoustic Spectroscopy, Theory Raman Optical Activity, Applications Raman Optical Activity, Theory Rayleigh Scattering and Raman Spectroscopy, Theory Surface-Enhanced Raman Scattering (SERS), Applications. 

See also Nonlinear Optical Properties Nonlinear Raman Spectroscopy, Applications Nonlinear Raman Spectroscopy, Instruments Raman Optical Activity, Applications Raman Optical Activity, Theory Raman Spectrometers. 

A comprehensive overview of frequency-domain DOT techniques is given in [88]. Particular instraments are described in [166, 347, 410]. It is commonly believed that modulation techniques are less expensive and achieve shorter acquisition times, whereas TCSPC delivers a better absolute accuracy of optical tissue properties. It must be doubted that this general statement is correct for any particular instrument. Certainly, relatively inexpensive frequency-domain instruments can be built by using sine-wave-modulated LEDs, standard avalanche photodiodes, and radio or cellphone receiver chips. Instruments of this type usually have a considerable amplitude-phase crosstalk". Amplitude-phase crosstalk is a dependence of the measured phase on the amplitude of the signal. It results from nonlinearity in the detectors, amplifiers, and mixers, and from synchronous signal pickup [6]. This makes it difficult to obtain absolute optical tissue properties. A carefully designed system [382] reached a systematic phase error of 0.5° at 100 MHz. A system that compensates the amplitude-phase crosstalk via a reference channel reached an RMS phase error of 0.2° at 100 MHz [370]. These phase errors correspond to a time shift of 14 ps and 5.5 ps RMS, respectively. 

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