2-switching Pockels cell

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). 

Until recently a general drawback of this passive Q-switching scheme was the difficulty of obtaining an exact synchronization of the giant pulse with other events in more complex experiments. This difficulty does not exist with active Q-switching in which an electro-optic device, e.g. a Kerr-cell or Pockels-cell, is used instead of a dye cell, and one is able to determine exactly the time at which... 

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. 

The shortest integration time of the counter is currently set to 1 jls, corresponding to a maximum sampling rate of 1 MHz. The actual time resolution is limited by the high voltage driver of the Pockels cell with approximately 20 jis switching time. 

In Ref. [75], it is discussed in more detail why it is advantageous to convolute the response of the temperature grating into the excitation and how to treat systematic errors arising from this approximation and from imperfections of the components in the setup. Especially the switching properties of the Pockels cell require careful analysis, since the switching number increases from 2 in case of pulsed excitation to approximately N in case of pseudostochastic binary sequences. 

The reason for these localized perturbations stems from a slight delay and asymmetry in the switching characteristics of the Pockels cell [75]. As a consequence, x (t) in Fig. 25 must be replaced by... 

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].

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

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.)...

Fig. 23. Experimental set-up of the Gwatt photochemical iodine laser (a). The pulsecutting system after the oscillator consists of a Pockels cell and Gian prism. The Pockels cell is switched by a spark gap which is triggered by the laser light deflected from the prism. Gain can be measured by the diodes Di.Dg. The duration and sequencing of the flashlamp pulses for pumping the three stages and the switching time of the oscillator are indicated in part (b) of the figure...

Fig. 24. Pulse-sharpening in the photochemical iodine laser. The left picture shows the oscillator signal, the right one the signal shape after the first amplifier stage of Fig. 23. The series of pulses is due to multiple switching of the Pockels cell. Time scale is 20 nsec/div...

Fig. 6.5 Q-switching with a Pockels cell inside the laser resonator (a) PockePs cell between two crossed polarizers, (b) transmission T(0)ocU (c), (d) possible experimental arrangements...

The aligned mirrors that surround the active medium and permit repeated passes of laser light through the inversion, form the resonator cavity. Aside from the active medium one can also place a variety of electro-optic switches inside the cavity, including Pockels cells, acousto-optic switches. 

FIGURE 17 The basic operation of the Pockels cell is to rotate the polarization of a transmitted beam upon application of a high-voltage signal. In conjunction with one or two polarizers, Pockels cells are widely used inside of resonator cavities as Q-switches or cavity dumpers, and outside of resonators as fast switchout systems and pulse carvers. 

FIGURE 21 In the pump phase the timing diagram for the cavity-dumped case is identical to the Q-switched case. After switching the Pockels cell to enable lasing, the cavity loss now is very small since, for a cavity-dumped architecture, the reflectivity of both resonator mirrors is 100%. Once the power in the cavity has reached the maximum value, the Pockels cell is again switched and ejects the intracavity intensity from the laser in a pulse equal to the round-trip time of the resonator cavity. 

An architecture very closely related to the cavity-dumped oscillator is the regenerative amplifier. The principle difference is that the laser oscillation in a regenerative amplifier does not build up from spontaneous emission, but is initiated by a signal injected into the resonator fi om the outside, as the Pockels cell is switched to transmission. This injected signal is then trapped in the cavity and amplified until it has reached maximum intensity, at which point it is ejected (dumped) from the cavity. 

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