Avalanche photodiode detectors APDs

The avalanche photodiode detector (APD), which is able to detect conversion electrons with very high intensity can also be usefiil in special applications (Weyer 1976 Gruverman 1976). 

Figure 2. Diagram of a confocal Raman detection system. A Helium-Neon laser is focused onto a sample through a microscope objective. Raman signals are epi-detected and sent to either an avalanche photodiode detector (APD) for imaging or dispersed onto a CCD camera in the spectrometer.

Fig. 12. The figure at the left is the schematic illustration of laser induced total internal reflection fluorescence microscopy for the single molecule detection at the liquid-liquid interface. Abbreviations ND ND filter, 1/2 1/2 plate, M mirror, L lens, C microcell, O objective (60 x), F band path filter, P pinhole, APD avalanche photodiode detector. The figure at the right shows the composition of the microcell.

FIGURE 10.5. The left figure shows the schematic illustration of laser-induced fluorescence microscopy under the total internal reflection for the detection of single Dil molecules at the dodecane-water interface. Abbreviations ND, ND filter XJ2, kl2 plate M, mirror L, lens C, microcell containing dodecane and aqueous phases O, objective (60 x ) F, bandpath filter P, pinhole APD, avalanche photodiode detector. The right portion of the figure shows the composition of the tnicrocell. 

APD = avalanche photodiode detector APS = advanced photon source DFT = density functional theory ESRF = European synchrotron radiation facility HOPE = high-density polyethylene IR = infrared INS = inelastic neutron scattering KED = kinetic energy distribution Mb = myoglobin NIS = nuclear inelastic scattering NRVS = nuclear resonance vibrational spectroscopy NRIXS = nuclear resonant inelastic X-ray scattering OEP = octaethylporphyrin sGC = soluble guanylate cyclase VDOS = vibrational density of states. 

Fig. 1. Experimental setup. PBS, polarizing cubic beam splitters M1-M3, cavity mirrors APD, avalanche photodiode detector PZT, piezo-electric transducer, (b) Three-level A -type atomic system.

In real curvature sensors, a vibrating membrane mirror is placed at the telescope focus, followed by a collimating lens, and a lens array. At the extremes of the membrane throw, the lens array is conjugate to the required planes. The defocus distance can be chosen by adjusting the vibration amplitude. The advantage of the collimated beam is that the beam size does not depend on the defocus distance. Optical fibers are attached to the individual lenses of the lens array, and each fiber leads to an avalanche photodiode (APD). These detectors are employed because they have zero readout noise. This wavefront sensor is practically insensitive to errors in the wavefront amplitude (by virtue of normahzing the intensity difference). 

Measurements at low light levels are routinely performed with photon-counting techniques. The development of ultrasensitive optical detectors has made great progress in the last couple of years. Integrated photon-counting modules with cooled avalanche photodiodes (APD) have been available for some years [31]. These detectors can have quantum efficiencies of 50% with less than 10 dark counts per second. The light sensitive area of such a device has a diameter of about 200 (im and can serve directly as a pinhole in a confocal detection channel. 

Currently available single photon avalanche photodiodes (SPADs) are not applicable to optical tomography. Although the efficiency in the NIR can be up to 80%, the detector area is only of the order of 0.01 mm. Diffusely emitted light cannot be concentrated on such a small area. A simple calculation shows that SPADs carmot compete with PMTs unless their active area is increased considerably. Another obstacle is the large IRF count-rate dependence sometimes found in single-photon APDs. 

Single photon avalanche photodiodes (SPADs) achieve the highest radiant sensitivity of all detectors in the NIR. Currently available APD detectors have ex-... 

FIGURE 45.9 Schematic of a confocal LIP detection system with source, optics, and detector shown. The optics include mirrors (M), laser line filter (LF), half-wave plate (X/2), polarizer (pol), dichroic beamsplitter (DB), microscope objective (MO), pinhole (ph), filter, and achromat lenses (achr). The source shown is an argon ion laser, and the detector is an avalanche photodiode (APD). While the electrophoresis channel shown here is in a capillary (CE), the system could be readily applied to a microchip. (Reprinted from Johnson, M.E. and Landers, J.P., Electrophoresis, 25, 3515, 2004. With permission.)... 

Wide selection of different detectors can be used in fluorometers. Most fluorom-eters rely on PMTs, but modem detection technology offer other choices like avalanche photodiodes (APDs). 

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