Avalanche photodiode detectors

This chapter will concentrate on the very high quality detectors that are needed in scientific imagers and spectrographs, and other applications that require high sensitivity, such as acquisition and guiding, adaptive optics and interferometry. We limit our discussion to focal plane arrays - large two-dimensional arrays of pixels - as opposed to single pixel detectors (e.g., avalanche photodiodes). 

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). 

A laser beam highly focused by a microscope into a solution of fluorescent molecules defines the open illuminated sample volume in a typical FCS experiment. The microscope collects the fluorescence emitted by the molecules in the small illuminated region and transmits it to a sensitive detector such as a photomultiplier or an avalanche photodiode. The detected intensity fluctuates as molecules diffuse into or out of the illuminated volume or as the molecules within the volume undergo chemical reactions that enhance or diminish their fluorescence (Fig. 1). The measured fluorescence at time t,F(t), is proportional to the number of molecules in the illuminated volume weighted by the... 

The experiment is performed with a spectrofluorometer similar to the ones used for linear fluorescence and quantum yield measurements (Sect. 2.1). The excitation, instead of a regular lamp, is done using femtosecond pulses, and the detector (usually a photomultiplier tube or an avalanche photodiode) must either have a very low dark current (usually true for UV-VIS detectors but not for the NIR), or to be gated at the laser repetition rate. Figure 11 shows a simplified schematic for the 2PF technique. 

Not only PMTs and other detectors such as avalanche photodiodes suffer from dead-time effects also the detection electronics may have significant dead-times. Typical dead-times of TCSPC electronics are in the range 125-350 ns. This may seriously impair the efficiency of detection at high count rates. The dead-time effects of the electronics in time-gated single photon detection are usually negligible. 

Often, experiments are carried out on specimens that emit only very weak fluorescence. For these cases, the most sensitive detectors should be used, for instance fast avalanche photodiodes or high quantum yield PMTs. These detectors may have somewhat longer dead-times causing longer exposure times but maximal sensitivity. 

The alignment of discrete detectors for each input is no less a difficult task than the source assembly problem. The extra traces associated with connecting discrete field effect transistors (FETs), PIN diodes, or avalanche photodiodes leads to degradation of signal, lower reliability, and greater cost. The integration of photo FETs onto the IC does provide a way to simplify the detector side of the problem, unfortunately at the... 

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. 

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