TEM Messtechnik presents a very compact and robust FM spectroscopy unit that can lock cw lasers to the hyperfine absorption spectrum of the iodine molecule.

For the use of cw single-frequency lasers for high precision metrology applications, in many cases an accurate determination (and sometimes even stabilization) of their oscillation frequency is prerequisite. Luckily nature provides us with ubiquitous standards to which optical frequencies can be referred: Electronic transitions in atoms and molecules often coincide with the emission or absorption of light in a narrow frequency interval (“spectral lines”). Thus, to determine an optical frequency means simply to detect emission or absorption of the light by matter (“spectroscopy”). The iodine-stabilized helium-neon laser is one of the text book examples for this method.

For visible light, a vapor of iodine molecules is a good candidate for such a frequency standard because it provides a dense line spectrum up to 583 THz (down to 514 nm) with a natural linewidth as low as ~380 kHz. Its optical properties have therefore been investigated in detail. The BIPM [Bureau International des Poids et Mesures] even recommends to use certain absorption lines for the realization of the meter (as a certain multiple of the wavelength of a laser stabilized to the iodine absorption frequency), for example the a10 component of the transition R(56)32-0 near 563 260 224 MHz.

TEM Messtechnik combines the FM spectroscopy unit with their fully digital LaseLock® control electronics to lock tunable cw laser to specific absorption lines. The system uses the FM saturation absorption technique with an amplitude modulated pump beam and an electro-optical sideband generation in the probe beam to extract the hyperfine structure of the iodine spectrum from the Doppler-broadened background.

The advantage of this technique is that just the measurement beam is modulated while the laser output beam remains unmodulated.