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Australian Government: National Measurement InstituteAustralian Government: National Measurement Institute
National Measurement Institute

Length and Related Quantities Research

NMI is responsible for maintaining Australia's official standards of length (the metre) and angle (the radian).

A number of research activities are being undertaken to develop improved laser wavelength standards and to provide traceability for coordinate measuring machines. Recent research activities also include nanoparticle size measurements.

Laser Wavelength Standards

The vacuum wavelength of a laser may be determined by frequency comparison against NMI's primary standard lasers. At present this facility is limited to 633 nm, 612 nm and 543 nm helium-neon lasers and 780 nm diode lasers.

The reference frequencies are realised by locking the laser to iodine transitions for the He-Ne wavelengths and to rubidium transitions at 780 nm. For the 633 nm HeNe, this can be done either with an intra-cavity iodine cell system or a recently developed external cell system which is also used for the 543 nm and 612 nm HeNe lasers.

The external cell stabilisation scheme is based on locking the laser frequencies to saturated absorption transitions of iodine. The specific technique used is modulation transfer spectroscopy with the novel feature that an amplitude, rather than a frequency, modulated saturating beam is used. Amplitude modulation was chosen, as a theoretical analysis has shown that it produces optimum signals for our application, and also leads to a simpler and less expensive system.

A novel aspect of the system is that only one external iodine stabiliser is required to frequency lock all three lasers at one time. The stability of each of the lasers is better than 1 part in 1010. Consequently these three lasers can be used to calibrate the wavelength of lasers operating at any of the three wavelengths to an accuracy better than 50 kHz. Another useful feature of the system is that it provides multi-wavelength illumination which has the stability and accuracy to be used in an interferometer to calibrate end standards up to 1 m in length.