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Lithium Niobate Q Switch

Lithium Niobate Q-Switches

The G&H electro-optic Q-switch employs the highest optical quality lithium niobate available. The electro-optic effect of lithium niobate makes it extremely useful for Pockels cell Q-switching of several laser types including Nd:YAG, Nd:YALO, Nd:YLF and Er:YLF. Lithium niobate Q-switches are used in both military and commercial applications, including target designating, range finding, and ophthalmic surgery. Our crystals offer low loss, high contrast ratio, and low wavefront distortion, and are qualified for use at 300 MW/cm2. The temperature-stable (TS and XTS) versions of our electro-optic LN Q-switches allow operation over a wide range of temperatures while maintaining excellent contrast ratio.
An electro-optic Q-switch can be used to modulate the loss in a laser cavity to generate high intensity, pulsed laser light. When an electromagnetic field is applied to an electro-optically active crystal such as lithium niobate (LN), a change in refractive index proportional to the electric field known as the Pockels effect is induced. Light passing through the active crystal experiences a phase change proportional to the interaction length, altering its polarization. By placing the crystal between a pair of cross-polarizers, light can be passed or blocked at very fast switching speeds. This allows a G&H lithium niobate crystal to be used to create a Pockels cell for modulating light intensity within a laser cavity, to achieve Q-switching and generate very short, intense laser pulses. A high quality EO Q-switch has several important characteristics:
  • Very low loss in the “off” state to maximize output intensity
  • High contrast ratio between “on” and “off” states
  • Ability to withstand very high peak laser power
At G&H, we grow and polish our high quality lithium niobate (LN) crystals in-house, applying electrodes for use in the transverse mode (i.e. electric field perpendicular to the optic or z-axis). Our LN crystals offer high extinction ratio, very low transmission loss, and low switching voltage and temperature stability throughout a wide temperature range. High-performance AR coatings are applied to each face, reducing loss to ≤0.2% per surface while withstanding high optical power (>300 MW/cm2 typical, >500 MW/cm2 upon request).

Ensuring Temperature stability

Temperature stability is of primary importance in selecting an EO Q-switch, leading to the introduction of our  TS and XTS temperature stable product series. A typical lithium niobate Q-switch suffers from pyro-electric charge buildup when operated in the cool environments often seen in aerospace, leading to premature lasing. While radioactive americium can be used to ionize the air around the crystal and bleed off the charge, handling and disposal of these materials are problematic. We use a proprietary thermo-chemical reduction technique to increase conductivity of our LN crystal faces, thus dissipating charge and eliminating the premature lasing effect. While this process increases insertion loss slightly, it allows our temperature-stable TS and XTS Q-switches to be used as they are over a wide range of operating temperatures while maintaining excellent contrast ratio.

EO vs AO Q-switching

Q-switching can also be achieved via the acousto-optic effect. An AO Q-switch operates at MHz switching speeds, while an EO Q-switch allows GHz switching. Depending on the size of the crystal face and interaction length, an EO Q-switch may require higher power than an AO Q-switch. A larger beam size requires that higher power be applied to the EO crystal, but this can be mitigated by using a longer interaction length. Support from G&H engineers ensures each customer receives the best fit to the application, from rugged range-finders to sophisticated ophthalmic surgery systems.

Applications of Lithium Niobate Q-Switches

Range-finders, target designators.

ProductWavelengthDamage ThresholdSizeExtinction RatioWavefront Distortion