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Electro-optics modulator switches in 2 ns

19 Dec 2007

An ultrafast electro-optic switch achieves high repetition rates to provide short pulses of laser light for detecting atoms in an interferometer.

An electro-optical switch that can turn from off to on in just 2 ns has been made by physicists at Stanford University. The device also has a repetition rate of more than 20 MHz and low losses. It could be used in a number of applications – including highly sensitive detection of atoms in an interferometer, which may eventually lead to the most accurate measurement of the fine structure constant.

"Like any electro-optic modulator, we use an electro-optic crystal that changes the polarization of light from a laser as a function of applied voltage (the Pockels effect) and a polarizer," team leader Holger Mueller told nanotechweb.org. The device is aligned in such a way that the polarizer blocks the laser beam when the voltage across the electro-optic crystal is set to zero. When a voltage, known as the "half-wave" voltage (250 V in this case) is applied, the crystal rotates the polarization of the light by 90°, allowing all the light to pass through.

Improved design

The improved switching speed of just 2 ns is thanks to an improved modulator design, explains Mueller. "This includes electrical impedance-match and 'series compensation' of the crystal capacities."

The design is based on a grounded grid triode in the final amplifier stage driven by transmitting power transistors, which are the fastest power transistors available today. "Although transistors with sufficient voltage and current ratings for the power stage are available, our triode is superior because of its low parasitic capacities, which directly translate into higher speed," said Mueller.

Switches with fast transitions have been made before, but these devices were limited to only a few pulses per millisecond. This meant the repetition rates were restricted to below about 100 kHz, mainly due to power dissipation in the devices.

"Fibre-optic modulators can also be very fast but they have high insertion losses," added Mueller. "This means that at least 50% of the power of a free beam is lost. Moreover, the 'damage threshold' is in the order of a few milliwatts rather than a few watts. The combination of all of these features is unique for our switch."

Detecting atoms

The main application for the ultrafast switch will be to coherently drive transitions in atoms to short-lived excited states. "For example, in caesium, the lifetime of the excited state of the 'D2 line' at a wavelength of 852 nm is 30 ns, so any coherent process must happen on a timescale much shorter than this," stated Mueller.

"This could be used for highly sensitive detection of atoms, but also in atom interferometers or spectroscopy." The researchers hope that these applications will eventually lead to the most accurate measurements of the fine-structure constant, α – the dimensionless number that determines the strength of interactions between charged particles and electromagnetic fields.

The team is now busy working on a modified version of its switch, in collaboration with scientists at the Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, for use in biomedical optics. "Our colleagues Matthias Geissbuehler and Theo Lasser are developing an imaging method that employs illumination by short flashes of light," said Mueller.

"Called 'triplet state imaging by modulated excitation', the idea is to create an image contrast with different excitation patterns. This needs higher speed and power than is possible with existing electro-optical modulators."

The work will be published in Review of Scientific Instruments.

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