Polovodičový laser s externím rezonátorem pro měření vlastností povrchů

Josef Lazar, Ondrej Cíp and Bohdan Ružicka

The technology of improvement of the spectral properties of semiconductor lasers by an external cavity has been successfully tested in many applications. This technique is capable of fully exploiting the great potential of semiconductor lasers even when single-frequency operation, narrow linewidth and frequency stability are needed. Semiconductor lasers are the most common lasers these days, being mass-produced in huge numbers. They are also available in a great variety of wavelengths. Along with their small size and low price it is no wonder that we also encounter them in many everyday gadgets.

High demands posed by such applications as spectroscopy, interferometry, etc. cannot be met by most of the commercially available laser diodes. The more advanced laser diode designs with spectrally selective elements such as distributed feedback or Bragg reflectors are unfortunately available only on a handful of wavelengths mainly in the near-infrared telecommunication band. The parameters of ordinary Fabry-Perot laser diodes may be improved significantly by using the diode merely as an amplifying media and adding selective elements to the laser resonator. This technique was introduced when the first broad-range tuneable lasers - dye lasers - emerged, and was reinvented and perfected with semiconductor lasers of various wavelengths. The external cavity consists of collimating optics, a spectrally selective reflecting element - an optical grating and finally a mirror. By ganging the resonator length and the angle of the beam incident on the grating the optical feedback into the laser diode is forced to operate on a single optical frequency and can be tuned over a certain range. The resulting linewidth is reduced significantly, even below the MHz range.

We participated on a joint project with the Institute of Physical Engineering at the Faculty of Mechanical Engineering of Brno University of Technology, in which the aim was to design an interferometric profilometer for detection of surface topography and roughness. When an exposition of an interferogram with multiple - at least two - wavelengths is possible, the precision and resolution of the interferometer can also be combined with a large dynamic range. The first experiments with a tuneable argon laser were successful, and the decision was taken to put together a pair of independently tuneable and stable semiconductor lasers with a coupling of the output light into one fibre to make simultaneous exposition possible.

The result was a set-up comprised of two extended-cavity lasers operating on the wavelength 633 nm, close to the wavelength of common He-Ne lasers. This allows the use of optical elements readily available for most interferometric applications. The lasers are of a compact and robust design, the cavity body is milled from a single piece of hard Aluminium alloy ensuring mechanical stability and easy thermostatization. The external cavities are of the Littmann configuration with a grating and a tuning mirror pushed by a micrometer screw for coarse adjustment and a PZT (Piezoelectric Transducer) for fine-tuning. The configuration keeps the output laser beam at a fixed position during wavelength tuning, which is crucial for the subsequent fibre coupling.

Both lasers are fixed to one board and controlled from a single set of electronics containing two laser diode current controllers and fully digital temperature controllers. The design of the controllers ensures a high level of protection against electromagnetic interference and feature high stability of the drive current and temperature.

A pair of tuneable semiconductor lasers with control electronics