Compact Module for Frequency Modulation (FM) Spectroscopy

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.


Compact Module for Frequency Modulation (FM) Spectroscopy

A suitable light source to realize an iodine stabilized cw laser is the Coherent Prometheus diode-pumped Nd:YAG laser because its monolithic design of a nonplanar ring oscillator (NPRO) ensures an extremely low frequency noise of the emitted light wave. The laser emits infrared light near 1064 nm which is subsequently converted into green light (532nm) using a nonlinear optical effect (SHG) in a crystal.

The accuracy and stability of TEM Messtechnik’s FM spectroscopy system was assessed using the optical frequency comb generator at PTB where the reference frequency is derived from a hydrogen maser that is in turn referenced to a caesium fountain clock.

Figure: Modified Allan deviation of the stabilized laser frequency from its mean value over 20h

In a more than 20h recording, the LaseLock® feedback loop kept the laser locked to the center of the absorption line with only -20.1 kHz absolute deviation from the BIPM recommended value. The relative instability was ~10-13 (Allan deviation at  1s averaging time) down to 3·10-14 (at 1000s averaging time). The latter value corresponds to 17Hz in the green and 8.5Hz in the infrared wavelength range.

Pricing (EU-Countries):

Iodine FM spectroscopy unit – on request

8 % surcharge for non-European countries plus shipment and possible import duties/taxes in the destination country.