Selection Criteria

The selection of the correct Q-switch for a given appli­cation is determined by the excitation of the type of laser; the required pulse parameters, the switching vol­tage, the switching speed of the Pockels cell, the wave­length, polarization state and degree of coherence of the light.

1. Type of Excitation

Basically, both off and on Q-switching are equivalent in physical terms for both cw and for pulse pumped lasers. On Q-switching is, however, recommended in cw opera­tion because a high voltage pulse and not a rapid high voltage switch-off is necessary to generate a laser pulse. This method also extends the life time of the cell. Over a long period of time, the continuous appli­cation of a high voltage would lead to electro­chemical degradation effects in the KD*P crystal. We advice the use of an on Q-swit­ching driver.

Off Q-switching is more advantageous for lasers stimula­ted with flash lamps because the λ/4 plate is not required. In order to prevent the electrochemical degradation of the KD*P crystal in the off Q-switching mode we recommend a trigger scheme in which the high voltage is turned off between the flashlamp pulses and turned on to close the laser cavity before the onset of the pump pulse.

The cell CPC and SPC series are recommended for diode pumped solid state lasers. These cells are ultra compact and will operate in a short length resonator: this is necessary to achieve very short laser pulses.

2. Pulse Parameters

The series LM n, LM n IM, and LM n SG cells are recom­mended for lasers with a power density of up to 500 MW/cm2.

The LM n and LM n SG cells are used for lasers with very high amplification. The SG cells with sol-gel technology have the same transmission as the immer­sion cells and both are typically used when a higher trans­mission is required. At high pulse energies LMxx cells are prefered.

Brewster Pockels cells are recommended for lasers with low amplification, such as Alexandrite lasers. The passive resonator losses are minimal due to a high trans­mission of 99%.

The CPC and SPC series cells are suitable for small, compact lasers and especially for OEM applications. They are available as dry cells and immersion cells.

The level of deuterium content in an electro-optic crystal influences the spectral position of the infrared edge. The higher the deuterium level the further the absorption edge is shifted into the infrared spectral region: for Nd: YAG at 1064nm, the laser absorption decreases. Crystals, which are deuterated to >98%, are available for lasers with a high repetition rate or a high average output power.

3. Pockels Cell Switching Voltage

Using double Pockels cells can half the switching voltage.  This is achieved by switching two crystals electrically in parallel and optically in series. The damage threshold is very high and the cells are mainly used outside the resonator.

4. Pockels Cell Switching Speed

Cells with an impedance of 50Ω are available both as a single and as a double Pockels cell. These cells are used for rapid switching in the sub-nanosecond range, such as coupling laser pulses into regenerative amplifier units. The cells are available with four connectors which permit control of the timing by means of a delay line.

5. Wavelength

The selection of the electro-optic material depends on its transmission range. For wavelengths from 0.25µm to 1.1µm, longitudinal Pockels cells made of KD*P and a deuterium content of 95% should be considered. If the deuterium content is higher the absorption edge of the material is shifted farther into the infrared. KD*P crystal cells with a deuterium content >98% can be used up to 1.3µm. For wavelengths up to 3µm we recommend LiNbO3 and LiTaO3.

6. State of Polarization

The IQS and CIQS series cells are supplied with an integrated polarizer: the alignment of the Pockels cell relative to the polarizer thus becomes unnecessary. The rotational position of the cell relative to the resonator axis can be chosen at will. However, should the polari­zation state of the light in the resonator be de­termined by other components, such as anisotropic amplification of the laser crystal or Brewster surfaces of the laser rod, then the rotational position of the cell will be determined by these factors. Thin film polarizers are used and the substrate is mounted at the Brewster angle. A parallel beam displacement of 1mm results from this configu­ration and can be compensated for by adjus­ting the resonator.