Diode lasers for optical metrology

Helium-neon lasers with a wavelength of 633 nm have been used for a long time as wavelength references for industrial interferometric length measurements. With comparatively little effort, they can achieve a relative accuracy of 10^–8, which corresponds to an uncertainty of 10 nm per meter and is absolutely sufficient for most applications. This technology is, however, obsolete, and the number of manufacturers has been constantly decreasing. Moreover, compared to modern diode lasers, these gas lasers are bulky, they need high voltage, and they exhibit rather poor efficiency as well as a low output power.

Alternative solutions must keep the wavelength of 633 nm to make it possible to continue using the large number of existing interferometers for length measurement seamlessly. For this reason, diode lasers are a suitable solution, although their inherent wavelength accuracy is not sufficient. This is where stabilization with iodine comes into play: Iodine molecules have numerous absorption lines in the relevant wavelength range. These absorption lines can serve as a wavelength reference.

A special laser diode chip (with internal optical wavelength selection at 633 nm) has been combined with an iodine cell of only 3.3 cm in length in a housing of 27 cm × 15 cm. This has been undertaken by Toptica Photonics AG, a laser manufacturer, within the scope of a project funded by the German Federal Ministry of Education and Research. The laser frequency is automatically stabilized at a defined Doppler-broadened iodine absorption line. A comparatively high power of approx. 5 mW is available at the output of an optical fiber. The device was evaluated with an optical frequency comb against atomic clocks of PTB. This evaluation yielded a relative instability of less than 10^–10 for averaging times of more than 10 s. This is considerably less than the values provided by commercially available helium-neon lasers with simple stabilization. The absolute frequency obtained was in agreement with expected values. The line shape and the stabilization were modelled to be able to easily predict the absolute frequency and stability when other iodine lines are selected.

Integrated with micro-optical elements into a small housing (only a few centimeters in size), the prototype has the potential to enable very compact and accurate interferometers in the future.