KD*P Pockels cells

KD*P Pockels cells with MHV Connectors for Q-switching & Pulse-Picking

Background

The EM500 series of KD*P longitudinal Pockels cells have been designed to provide a robust package style with great connection flexibility. The through-connected type gives the lowest switching times without using true 50ohm line techniques and are normally used for pulse-picking and gating for amplifier isolation. Single connector types are more usually used for Q-switching whilst the two connector type with floating connections is best suited to use with differential amplifiers such as our M5000. These Pockels cells are manufactured from KD*P crystals that have been specially selected for their low optical loss and strain free properties. They can be used in the optical range 0.3-1.2µm and are normally supplied with fused windows optically coated at the customers specified wavelength. Along with the options for one, two or four terminal configurations these products can be supplied as either index matching fluid filled (preferred generally for low cost Q-switching solutions) or dry with or without anti-reflection coatings on the crystal as required for the application. We can also supply our specal "HHT" configuration which reduces internal surface reflections to negligible propertions (~0.05% per surface). The benefits of this are of course hugely improved transmission (as close to 100% as we can measure - limited only by the external window coatings) but also an incredible increase in static contrast, all the way to over 10,000:1 has been measured. This is a result of the reduction of multiple internal reflections and scattering.

The housing of these Pockels cells has been designed to offer good electrical shielding. The metal body which is earthed encloses the crystal with the only opening being the window required to let the light pass through. Electrical safety is also maintained by the use of special high voltage BNC (MHV) connectors. These also contribute to low radiated emissions. These features make this style of device especially suited to operation in an open environment, such as on optical tables and breadboards. For a more compact device where shielding and safety are provided by an additional enclosure or housing, see our compact electro-optic Q-switch range which offer identical optical performance in a more compact design.

Typical Specifications

Model No.	EM508	EM510	EM512	EM515	EM520	EM525
Aperture	8mm	10mm	12mm	15mm	20mm	25mm
Wavelength Range	0.3-1.2µm	0.3-1.2µm	0.3-1.2µm	0.3-1.2µm	0.3-1.2µm	0.3-1.2µm
Half-wave Voltage (DC) @ 1.06µm	6.0kV	6.0kV	6.2kV	6.3kV	6.3kV	6.3kV
Typical Rise Time^*	< 0.5ns	< 0.5ns	< 0.5ns	< 0.6ns	< 0.7ns	< 0.8ns
Maximum Voltage	10kV	10kV	10kV	10kV	10kV	10kV
Contrast Ratio @ 1.06µm	> 1000:1	> 1000:1	> 1000:1	> 1000:1	> 1000:1	> 1000:1
Crystal length	25mm	25mm	25mm	30mm	40mm	45mm
Capacitance Unterminated	10pf	10pf	12pf	15pf	20pf	20pf
Damage Threshold Q-switched	600MW/cm²	600MW/cm²	600MW/cm²	600MW/cm²	600MW/cm²	600MW/cm²
Typical Insertion Loss	2%	2%	2%	2%	2%	2%
Terminations	H.V.BNC (MHV)	H.V.BNC (MHV)	H.V.BNC (MHV)	H.V.BNC (MHV)	H.V.BNC (MHV)	H.V.BNC (MHV)
Finish	Black anodised	Black anodised	Black anodised	Black anodised	Black anodised	Black anodised
Physical Dimensions	50mm dia. 56mm long	50mm dia. 56mm long	50mm dia. 56mm long	55mm dia. 66mm long	55mm dia. 76mm long	55mm dia. 81mm long

* With through termination in 50 ohms and suitable driver.

Applications

The longitudinal field KD*P cell is one of the oldest, most well established type of Pockels cell used since the early days of laser Q-switching. It offers a large clear aperture and because of the way the field is applied to the crystal the half wave voltage (required for switching the input polarization through 90°) is essentially independent of aperture. KD*P is an extremely high quality optical material, available in large sizes so large aperture cells of high damage threshold can be offered at a much lower price than some other materials and drive voltages remain reasonable.

Although it is a hygroscopic (water-soluble) material, at Leysop we have established techniques for fabricating crystals with lambda/8 flatness and laser quality surface finish of 10-5. Our packages provide vacuum grade O-ring seals which protect the crystals from effects of atmospheric moisture and provide many years of operational use.

An obvious choice for Q-switching of YAG and ruby lasers (we supply many devices to medical and cosmetic laser manufacturers), the high damage threshold allows multi-joule pulse generatioon with average output powers in the low 10s of watts to ~50W. Another strong application area is in pulse picking where the extremely high optical quality and large clear aperture makes it a great choice for selectring of a low repetition rate pulse train from an fs mode-locked laser for input to a regenerative amplifier. We can also supply suitable driver setup and with short electrical pulses very high contrast pulse picking at >10kHz is very practical, even with large aperture cells. For applications where optical gate widths in the low-ns regime are required we can provide gating systems even up to 25mm aperture with sub-ns rise time (see below). For any applications you might have in mind, please give us a call to discuss your requirements, if it can be done, we can do it!