Laser pockels cell

Figure 9.4 Use of Pockels cell (PC) in a laser cavity to produce 0-switchmg...

Intense nanosecond pulses (Ins = 10 9s) can be produced by Q-switching. A shutter is placed between the laser rod and one of the mirrors, thus inhibiting lasing. If the shutter is suddenly opened, the excitation is dumped in one huge burst. One type of shutter is the electro-optic Pockels cell (KH2P04 crystal with a high applied potential). 

The use of fluorescence from alexandrite for temperature sensing was first reported by Augousti etal.(57,5S) using a low-power LED or a HeNe laser with a rather inefficient modulation accessory made of a bulky, high-voltage controlled Pockels cell,... 

Fig. 6. Experimental arrangement for lifetime measurements by the phase-shift method, using laser excitation. The laser beam is amplitude-modulated by a Pockel cell with analysing Nicol prism and a small part of the beam is reflected by a beam splitter B into a water cell, causing Rayleigh scattering. This Rayleigh-scattered light and the fluorescence light from the absorption cell are both focused onto the multiplier cathode PMl, where the difference in their modulation phases is detected. (From Baumgartner, G., Demtroder, W., Stock, M., ref. 122)).

There is another way to obtain giant laser pulses of a few ns duration, known as active Q-switching. The shutter is an electro-optical cell which is triggered at some preset time after the pump flash. These electro-optical shutters are Kerr cells or Pockels cells. 

Fig. 23. Setup for periodic amplitude modulation, x and y are the two axes of the Pockels cell. They are rotated by 45" with respect to the polarization of the laser beam. S = U/UK, and U is the voltage for 180" phase shift.

Figure 1. Diagram of apparatus for picosecond fluorescence studies using streak camera detection. A laser oscillator B dye cell C output reflector D polarizer E spark gap F KDP pockels cell G polarizer (crossed with D) H clear glass beamsplitter J laser amplifier K pin photodiode L transient digitizer M,N 1054 nm reflectors P 2nd harmonic generator Q 3rd or 4th harmonic generator R spectrograph S streak camera T biplanar photodiode U image...

Figure 14. Experimental apparatus for picosecond, time-resolved CD measurements using a mode-locked, Q-switched, cavity dumped pump laser. P, polarizer PC, Pockels cell Q, quarter-wave plate RHP, rotating half-wave plate S, sample cell PMT, photomultiplier tube. From ref. [42].

Recent advances, for example, replacement of the Pockels cell with a PEM system, has provided an improvement in the experimental SNR of an order-of-magnitude [44]. Further, the authors suggest that with their experimental approach, the picosecond laser system now in use could be replaced by one operating with femtosecond pulses. If successful, this would allow extension of CD measurements into a time domain where the initial structural changes which determine the outcome of a sequence of complicated events can be probed. 

Fig. 18. Schematic of apparatus used to measure fluorescence kinetics with a streak camera. The Nd glass laser emits a train of one hundred 1.06 pm pulses separated by 6 ns. A single pulse in the earlier portion of the train is selected by a Pockels cell and crossed polarizers (Pi and P2). The high voltage pulse ( 5 ns) at the Pockels cell is supplied by a laser triggered spark gap and a charged line. The single pulse ( 8 ps, 109 W) can be amplified. The second harmonic is generated from a phase matched KDP crystal. Beam splitters provide two side beams beam (1) triggers the streak camera beam (2) arriving at the streak camera at an earlier time acts as a calibrating pulse. The main 0.53 pm beam excites the sample for fluorescence measurement. The fluorescence collected with f/1.25 optics is focused into the 30 pm slit of the streak camera. The streak produced at the phosphorescent screen is recorded by an optical multichannel analyzer. (After ref. 67.)...

Figure 10. Schematics of the experimental setup for intracavity laser absorption spectroscopy (ICLAS). CD chopper driver PM power meter Mj, M2, M3, M4 spherical high reflection mirrors Mp = pump mirror MN monochromator PMT photomultiplier SP silicon photocell PC Pockels cell WF wedged filter LIA lock-in amplifier R recorder MS microscope OF optical fiber S sample (solution on BLM) IEM instruments for electrical measurements (see Figure 2).

An important development in the phase-shift technique is the use of a radiofrequency synthesizer as the driver for the Pockels cell modulator. In this way, the excitation beam can be modulated at any frequency between 1 and 200 MHz [137-139]. This approach allows use of cw lasers such as the He-Cd laser and even mode-locked lasers [139] as the excitation source. If d and M are measured at six to ten suitably spaced frequencies, least-squares curve-fitting techniques can be employed to obtain lifetimes with greatly enhanced precision. Typical data obtained by this multifrequency technique make measurement of decay times as short as 10 ps possible. Gratton and coworkers have developed other curve-fitting procedures to analyze data obtained on a multifrequency phase-shift fluorimeter. These experiments include the construction of time-resolved spectra [140], measurements of ro-... 

Fig. 7.6. Schematic diagram of a phase-modulation fluorometer using a CW light source (xenon lamp or laser) and a Pockels cell as a modulator.

Figure 1. Q-switched, mode-locked Nd YAG laser with two synchronously pumped dye lasers PC = Pockels cell POL = polarizer with escape window DLl, DL2 = cavity dumped dye lasers PMT = photomultiplier tube. (Reproduced from Ref. 7. Copyright 1986 American Chemical Society.)...

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