Twelve Orders of Coherence Control

: TOPTICA’s lasers cover twelve decades of linewidth and coherence length.

Coherence length = 132 m / (linewidth in MHz)
Free-running diodes: Linewidth up to 1000 GHz
Grating-stabilized lasers: Linewidth ~ 1 MHz
Linewidth reduction down to 1 Hz with FALC 110 and DigiLock 110
Linewidth broadening from MHz to GHz possible with LCC
Advantage of diode lasers: Direct high-bandwidth modulation possible

The control of the coherence properties of laser light is of major importance in areas as diverse as spectroscopy and optical pumping of atoms, nonlinear frequency conversion, interferometry, or laser-based imaging. A controlled manipulation of the coherence properties of diode lasers requires high-bandwidth electronics – and the possibility to apply high-frequency electric fields directly to the laser emitter. This distinguishes diode lasers from other laser types, where coherence variation involves intricate external modulation schemes.

TOPTICA’s modules FALC 110, DigiLock 110 and the Laser Coherence Control LCC take direct advantage of the modulation properties of grating-stabilized diode lasers, and permit a continuous variation of the laser linewidth from GHz ranges to sub-Hertz levels. Compared to the free-running linewidth of ~1000 GHz, this is a span of no less than twelve decades.

Linewidth and coherence length. Laser linewidth and coherence length are linked via an inverse proportionality. Information about the laser’s temporal coherence may thus be equally expressed in linewidth units of MHz or pm. In case of a Gaussian spectral distribution, the coherence length is given by approx. 132 m / (linewidth in MHz), independent of the absolute laser wavelength.

In distributed feedback (DFB) or external-cavity diode lasers (ECDLs), grating feedback is used to narrow the emission profile of a diode and thus to increase its coherence length by six orders of magnitude. Expressed in frequency terms, the linewidth is reduced from approx. 1000 GHz to single-MHz levels.

Laser linewidth reduction. For a variety of applications, the laser linewidth has to be much smaller yet. Ultimately narrow laser linewidths are mandatory in precision spectroscopy, quantum optics or metrology, e.g. to match the sub-kHz linewidths of “forbidden” atomic energy levels.

Laser linewidth broadening. On the other hand, a growing number of tasks like molecular spectroscopy or optical pumping of Doppler-broadened gases require a stable and reproducible linewidth increase up to a few GHz. This in particular has been impossible to achieve with diode lasers: external-cavity systems are spectrally too narrow, free-running diodes too broad by far.

TOPTICA’s added value. Recently, dedicated locking modules and laser modulation sources, such as the FALC 110, the DigiLock 110 and the Laser Coherence Control LCC have brought about the long searched-for solution.
The LCC employs a proprietary high-bandwidth current modulation scheme to increase the spectral line profile of an external-cavity or distributed feedback diode laser. The researcher can tailor the coherence length of the laser to any value between 100 m and a few cm. At the same time, the laser line center may remain tuned to e.g. a molecular absorption line. The maximum linewidth in the GHz range, matches the Doppler-broadened absorption profiles of gases and molecules.
On the opposite side of the coherence length spectrum, high-bandwidth locking modules like the FALC 110 and DigiLock 110 have been employed for the realization of sub-kHz or even sub-Hertz laser linewidths. This translates into a coherence length of more than 250000 km, two-thirds the distance to the moon – or 12 orders of magnitude greater than the free-running values.