Electronic devices such as radar or navigation systems may be able to employ lasers to provide precise frequency references in the future, rather than the quartz crystals normally used today, thanks to research conducted by a US-based team. This would lead to a further boost in their sensitivity.

Keeping track using lasers rather than quartz

Most electronics contain oscillators that create the precise and reliable frequencies needed to keep time (in watches for example) or transmit signals to radios. And quartz crystals are normally used as the frequency reference in these oscillators. This works in much the same way as tuning forks are used to reference – or tune – pianos: when the crystal vibrates it creates an electronic signal with a very precise frequency. However future high-end navigation systems, radar systems and consumer electronics may need more accurate references than is achievable using quartz.

Optical based oscillators such as lasers are even more stable than quartz, explains Kerry Vahala, California Institute of Technology, US. “Harnessing this stability for electronics, however, requires a way to bridge the enormous difference in frequencies between electronics and optics,” he told Materials Today.

His team has designed a compact laser source that can provide a very stable, precise frequency. They then used a device called an electro-optical frequency divider to transfer the stability of the very high frequency laser signal to the lower frequency needed by a microwave electrical oscillator. This work is published in Science [Li J., Science (2014) doi: 10.1126].

The team’s reference device contains two lasers inside a silica disc about 6mm in diameter. “Because they share the same cavity their frequencies tend to track each other very precisely. This means that the difference between the frequencies is very stable and can act as reference frequency,” he says. The difference is quite large: over a TeraHertz.

The electro-optical frequency divider is used to precisely divide down by frequency this very high reference frequency. “We divided it down in frequency by about 150 times from around 1.5 THz to10 GHz. This allowed a common electrical oscillator to be stabilized by the difference of the two lasers,” says Vahala.

This extra accuracy over quartz crystals is important because the sensitivity level possible in the detection or measurement process carried out by electrical devices depends directly on how stable the oscillators used in these devices are, explains Vahala. “For example, in a radar system, the stability of the oscillator used to fix the radar frequency can determine how sensitively the radar can pick up things like a targets speed.”

The team is now aiming to improve the stability of their oscillator even further. “Our goal is to achieve a record highest level of stability at 10GHz using this method,” he explains.