Acousto-optical modulator

In frequency-domain FLIM, the optics and detection system (MCP image intensifier and slow scan CCD camera) are similar to that of time-domain FLIM, except for the light source, which consists of a CW laser and an acousto-optical modulator instead of a pulsed laser. The principle of lifetime measurement is the same as that described in Chapter 6 (Section 6.2.3.1). The phase shift and modulation depth are measured relative to a known fluorescence standard or to scattering of the excitation light. There are two possible modes of detection heterodyne and homodyne detection. 

Figure 9.12. Block diagrams of fiberoptic sensing instrumentation Top externally modulated laser with an acousto-optic modulator. (From Ref. 20 with permission.) Bottom internally modulated laser, (From Ref. 21 with permission.)...

Consider first the simpler case of acousto-optic interaction in an isotropic medium (i.e. a standard acousto-optic modulator). If the wavevector of the incident light is ko, that of the scattered light k+ or k and that of the phonon K, we have for conservation of momentum... 

Gies, D. and Poon, T.-C., Measurement of Acoustic Radiation Pattern in an Acousto-Optic Modulator Proceedings of IEEE SoutheastCon 2002, pp. 441-445. 

Figure 3.19 Intensity-modulated photocurrent spectroscopy, showing (a) the layout of a typical spectrometer, and (b) the response obtained AOM, acousto-optic modulator RE, reference electrode WE, working electrode CE, counter electrode FRA, frequency response analyzer...

Fig. 6. Laser system in the muonium ls-2s experiment. A cw laser at 732 nm is locked to a molecular I2 resonance. Its light is amplified in an alexandrite ring amplifier and then frequency tripled. The fight frequency is scanned using acousto-optic modulators...

Fig. 1. Set-up of the PTB laser system. The Nd YAG laser is frequency stabilized onto a selected iodine absorption line using the phase modulation method. The probe beam is modulated at 2.05 MHz by an electro-optic modulator (EOM), the pumb beam is frequency shifted by an acousto-optical modulator (AOM). The driving AOM rf power is chopped in order to cancel frequency offsets introduced by the Doppler background using a lock-in detection scheme. The transmitted probe beam signal is detected by a photodiode (PD) and mixed with the EOM rf in a double balanced mixer (DBM)...

Fig. 2. Set-up of the ILP laser system. Intracavity frequency-doubling is realized with a KTP crystal which, together with a Brewster plate, serves as a Lyot filter. This allows to frequency time the laser by more than 500 GHz by changing the temperature of the KTP crystal. The 532 nm laser radiation, after passing an acousto-optical modulator (AOM), is directed into an external I2 fluorescence cell. A photomultiplier (PM) detects the fluorescence signal over a solid angle of almost 0.2 n. The photodiode D is used to detect a fraction of the 532 nm laser beam to power stabilize the 532 nm light via the AOM...

The power outputs of the lasers will be actively stabilized and matched using a system of thermopile power meters (PM) and acousto-optic modulators (AOM s). Spatial filters (SF) will be used to ensure a well defined laser mode at the interaction region. A system of beam scanners will be used to accurately characterize the laser and ion beams at the interaction region and to measure the intersection angles 61. ... 

He-Ne laser. The frequency of the shifted infrared beam is locked to this reference Fabry-Perot cavity whose length is fixed. By changing the acousto-optic modulation frequency, which is provided by a computer-controlled frequency synthesizer, we can therefore precisely control the dye laser frequency over a range of 250 MHz centered at any desired frequency. 

Figure 1 Apparatus of Oxford experiment [6]. LI, L2 tunable dye lasers. UV ultra violet radiation (243 nm). RF radiofrequency dissociation of flowing molecular hydrogen. PI signal photomultiplier (Lyman-a detector). P2 photomultiplier for cavity locking and signal normalisation. SI cavity length servo-control. C conrouter. AOM acousto-optic modulator. T heated quartz cell containing tellurium. S2 laser frequency servo-control. D fast photodiode...

Figure 2 The stronger component of the 1S-2S two photon transition in deuterium. The signal is the normalised Lyman-a fluorescence observed as a function of the frequency difference between lasers LI and L2 (fig. 1) when LI is locked to the appropriate transition (b2) in 13°Te2. The measured offset frequency is 20 MHz greater than the true value because of the shift introduced by the acousto-optic modulator. The pressure in the deuterium cell was 270 mtorr...

The novel aspect of this experiment was that the confocal cavity was locked to continuous-wave radiation which was frequency shifted by an acousto-optic modulator such as to centre the filtering cavity onto the chirped amplified radiation. This reduced the residual amplifier shift to -2(1 MHz. The dominant contribution to this shift resulted from the cw light being injected off-axis into the cavity. Because the filter cavity had a high finesse we used a phase modulation scheme for locking. Indeed, we normally locked the dye laser to the filtering cavity and scanned the spectrum by scanning the filter cavity. 

Figure 7 Schematic of the laser system used in the Raman FID and echo experiments. PC = Pulse compressor AOM = acousto-optic modulator PD = photodiode FB = feedback electronics PBS = polarizing beamsplitter 3PBF = 3-plate birefringent filter SDL/LDL = Stokes/Laser dye laser P = pellicle AC = autocorrelator OC = output coupler LBO/KDP = doubling crystals. Final pulses have widths of 0.5-1 ps and energies of 0.3-1 mJ (From Ref. 6.)... 

Q switching of the fibre lasers by lasers using an acousto-optic modulator/rotating chopper results in peak powers of several watts in pulses in the range of 50 ns to 1 ps. 

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