Knowledge Bank

The purpose of this communication is to describe the construction and performance of a small (8 cm diameter optics) experimental lamellar grating interferometer, which has been specifically designed to complement the Michelson interferometer in the very far-infrared, at frequencies below $50cm-1$, and for the main part has been constructed from commercially available Michelson interferometer modules. The instrument is operated in the `aperiodic' scanning mode to produce single sided (10 cm maximum optical path difference) or double sided ($\pm 5$ cm maximum optical path difference) interferograms, the grating movement being controlled by a precision stepping motor and micrometer screw. Examples of gaseous spectra will be presented, demonstrating optical resolutions of $\sim 0.2cm-1$ in the spectral region $5-50cm-1$, and good reproducibility at moderate resolution to frequencies as low as $3cm-1$, using a commercial room temperature pneumatic detector. Much better performance is likely to be attained at these low optical frequencies by use of the latest liquid helium cooled solid state detectors.