Photonic Crystal Fiber Enables Optical Clock
Atomic clocks are already the most accurate timepieces on the planet, but a new version promises accuracy 1000 times better than the current cesium-based ones. It employs a narrow optical transition in a single, laser-cooled, trapped 199Hg+ ion, and a photonic crystal fiber introduced in 1999 helped make it possible.
Atomic clocks excite a narrow transition in a reference atom with an oscillator, such as a microwave oscillator in cesium clocks. When the oscillator's frequency matches the resonance of the atomic transition frequency, the cycles of the oscillator or the radiation field can be counted to generate accurate time.
Scott A. Diddams, a researcher on the project from the National Institute of Standards and Technology in Boulder, Colo., explained that the microwave source in a cesium clock oscillates roughly 9 billion times per second, not so fast that conventional electronics cannot keep up Atomic transitions excited by an optical source, however, are at a much higher frequency, roughly 1015 times per second. This offers a far more stable and accurate clock, but the optical oscillations are much too fast for electronic detection. To detect the pulse rate of the optical oscillator, the frequency must be stepped down. Until recently, researchers attempted this with complicated chains of phase-locked lasers.
To excite the mercury ion, the researchers at NIST and at the University of Colorado in Boulder frequency-doubled the output of a continuous-wave 564-nm dye laser. Next they locked the frequency of the 282-nm radiation to the 1.064-pHz "clock" transition in the Hg+ ion and heterodyned the fundamental with the output of a femtosecond Ti:sapphire laser operating at a gigahertz repetition rate.
The short pulse width of a femtosecond laser spectrally broadens the emitted beam. The researchers fed this broad beam into a photonic crystal fiber, which further broadened it. In the new clock, the spectrum of the light from the fiber stretches from 1170 to 520 nm.
Each component of this source can be phase-locked to the atomic-stabilized laser beam, effectively phase-locking the repetition rate of the femtosecond laser to the same atomic-stabilized source. This allows the researchers to gear down the frequency of the optical clock to a detectable oscillation rate.
Before the photonic crystal fiber was introduced in 1999, researchers did not know how to effectively reduce the rate of oscillation, Diddams said. Used in conjunction with the femtosecond laser, however, the fiber acts like a reduction gear.
Diddam acknowledged that, because today's atomic clocks lose approximately 1 second every 20 million years, there is no immediate practical application for the increase in stability offered by the new ones. However, they may find application in deep-space communication and navigation, in much the same way that today's clocks have made the global positioning system possible.
"We're exploring advanced concepts, which are valuable for the future," he said. "We need to be here now rather than trying to catch up later."
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