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NPL narrows Nd:YAG linewidth

04 Aug 2004

A team from the UK's National Physical Laboratory reduces the linewidth of a Nd:YAG laser to unprecedented levels.

Scientists from the National Physical Laboratory (NPL) in the UK have built the world's first Nd:YAG laser system with a subhertz emission linewidth. The laser, which is locked to a high finesse Fabry-Perot (FP) etalon, has a linewidth of just 0.46 Hz and a frequency stability of about 1 part in 1015 (Optics Letters 29 1497).

To put this into context, most solid-state or semiconductor lasers have a linewidth of several kilohertz or megazhertz. Potentially, the NPL locked-laser could become a valuable reference source for optical frequency standards which require ultrastable, precise wavelengths for calibration purposes.

To date, the only other subhertz laser that's been demonstrated is a FP-locked dye laser at the US National Institute of Standards and Technology (NIST). It boasts the world's narrowest linewidth of 0.16 Hz.

"This is the second narrowest linewidth that has been reported but there are some other groups that are close behind at the 1 Hz region," Stephen Webster, a senior researcher at NPL told Optics.org. "We're hoping to transfer the techniques that we've learnt to other sources such as diodes and Ti:Sapphire lasers."

In the NPL system light from a 1064 nm Nd:YAG laser with a free-running linewidth of about 1 kHz is coupled into two identical FP etalons which have a free spectral range of 1.5 GHz and a finesse of about 180,000. The etalons consist of 10 cm long, 6 cm wide cylinders of ultralow expansion glass sandwiched between two highly reflective (99.998%) mirrors.

One etalon serves to lock the laser's wavelength while the second is used to monitor the stability of the locking and allow a measurement of the linewidth. In order for the locking to be as stable as possible it is of vital importance that the etalons are isolated from any disturbances as even the slightest change in their length would degrade the stability.

As a result, Webster and his colleagues Mark Oxborrow and Patrick Gill have taken elaborate measures to protect the etalons from the unwanted temperature fluctuations and vibrations of the outside world. The etalons are housed within temperature controlled, vacuum chambers which sit on platforms with active vibration isolation.

The entire experiment is located within a double-walled acoustic measurement chamber which is sealed by doors weighing more than 1.5 tonnes. Thanks to these extraordinary precautions, the NPL team estimate that the temperature fluctuations in etalon's glass are reduced to less than 1 mK and that vibrations between 10 and 100 Hz are reduced to around 10-6 ms-2Hz-1/2.

Despite its impressive result, the NPL team believe that it is still possible to improve the set-up by adding a further layer of vibration isolation and performing the experiment at a temperature where the thermal expansion of the etalons' glass is negligible.

"The thing we're tackling now is to reduce the drift which is mainly due to the temperature stability," said Webster. "At the moment the system is stable at the 10-15 level for 5 seconds and then after that the drift kicks in. We're really looking to improve that."

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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