Optical crosstalk

The probability of getting a crosstalk count in one detector is proportional to the eount rate in the other. Consequently, the total crosstalk count rate is proportional to the sum of the count rates of both detectors, while the coincidence rate is proportional to the product. Therefore, optical crosstalk becomes noticeable espe-eially at low deteetor count rates. 

The empty-chamber counting rate of 12.00 cpm indicates an optical crosstalk between the two photomultipliers, which apparently constitutes the major part of the 0 channel background. 

As discussed in component 1, the dominant background in the LSS is caused by optical crosstalk. It was established that the effect of cosmic events interacting with the liquid scintillating solution is negligible. Noise pulses from photomultiplier tubes are usually cut down to less than 1 cpm by electronic means (Packard Instrument s manual, 1977). Radioactive impurities in carefully-selected materials and surroundings should not contribute more than... 

The most important appHcation of fiber-optic laser-based communication is in long-distance telecommunications (92,93). Fiber-optic systems offer very high capacity, low cost-per-channel, light weight, small size, and immunity to crosstalk and electrical interference. 

Use fiber optic cable in preference to wires to reduce interference, crosstalk, and difference in ground potential at different location... 

Another promising alternative to the rather restrictive MBLL model is the layer model using diffusion equation. Diffuse optical imaging provides a better model for photon migration where approximation of path length is possible. This would provide a much improved focal change information and potentially eliminate crosstalk noise. 

N. Okui and E. Okada. Wavelength dependence of crosstalk in dual-wavelength measurement of oxy- and deoxy-hemoglobin. Journal of Biomedical Optics, 10(l) 011015-l-011015-8, 2005. 

Today fiber-optic transmission systems offer several advantages over conventional copper wire and coaxial cable systems. Among these are increased bandwidth, smaller size, lower weight, lack of crosstalk, and a very low susceptibility to electromagnetic interference. It is to be expected that these advantages will open widespread application of fiber-optic transmission systems in the future. This seems to be supported by the fact that a great number of public and in-house trial systems are under test all over the world. 

Two-photon excitation is preferable in 3D optical memory because the crosstalk between two adjacent layers is much reduced. Another advantage of two-photon excitation is reduction in multiple scattering. This reduction occurs because of the use of an illumination beam at infrared wavelength. 

C. Fiher-optic cable can span distances of several kilometers, because it has higher bandwidth and much lower crosstalk and interference in comparison to copper cables. 

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. 

Amplitude-phase crosstalk is intrinsically low in frequency-domain instraments that use gain-modulated PMTs as detectors and mixers [166]. Results presented in [98, 346] show that optical properties can be obtained with an accuracy comparable to that of TCSPC-based instraments. The modulated-PMT technique is somewhat less efficient than TCSPC and does not work well at extremely low photon rates. Nevertheless, the sensitivity is well within the range required for fluorescence detection in DOT. 

Fig. 5.105 crosstalk between actively quenched APDs in a Hanbury-Brown-Twiss experiment. Time scale 5 ns/div., optical configuration of Fig. 5.106, left... 

The data sheet of the R5900-L16 specifies a channel crosstalk of only 3%. There is certainly no reason to doubt this value. However, in a real optical setup it is almost impossible to reach a crosstalk this small. If crosstalk is an issue, the solution is to use only each second channel of the R5900-L16. If the PML-16 is used with only 8 channels, the data of the unused channels should simply remain unused. If the R5900-L16 is used with an external router, the unused anodes should be grounded via 50 Q resistors. 

---