Acoustooptic modulator

As shown in Section 11.2.1.1, more details can be obtained by confocal fluorescence microscopy than by conventional fluorescence microscopy. In principle, the extension of conventional FLIM to confocal FLIM using either time- or frequency-domain methods is possible. However, the time-domain method based on singlephoton timing requires expensive lasers with high repetition rates to acquire an image in a reasonable time, because each pixel requires many photon events to generate a decay curve. In contrast, the frequency-domain method using an inexpensive CW laser coupled with an acoustooptic modulator is well suited to confocal FLIM. 

Fig. 2. Schematic diagram of the laser system to be used for a measurement of the 2Si/2-2P3/2 transition frequency in Si13+. (A/2 half wave plate, A/4 quarter wave plate, PBS polarizing beamsplitter, VCO voltage-controlled oscillator, AOM acoustooptic modulator, EOM electro-optic modulator)...

The major problem with extending these shaping methods into the mid-IR is the availability of a suitable modulator. There are many different types of phase and amplitude modulators that work in or near the visible, including liquid crystal modulators (LCM), Te02 and fused silica acoustooptic modulators... 

Intensity Modulation. The SIN ratio is improved by modulating the pump-beam intensity either by the use of a chopper or an acoustooptic modulator (with a frequency fi). By tuning the detection to this frequency with the use of a lock-in amplifier (LIA), one can selectively look at variations in the detected probe-beam intensity, caused by variations in the pump-beam intensity (leading to variations in the amount of excitation... 

A third group of methods is based on heterodyning (light beats). For this a frequency difference is introduced between the two beams in the interferometer, usually by means of a pair of acoustooptic modulators operated at sUghtly different frequencies. The output from a detector... 

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