AR coatings for laser diodes (VIS and NIR)

Recently Laser Components has introduced antireflection (AR)-coatings for the facets of laser diodes. AR-coated laser diodes provide tuneable sources, without mode-hopping, for applications in spectroscopy.

To effectively use semiconductor lasers in spectroscopy, the spectral emission bandwidth typically needs to be as narrow as possible. This may be accomplished by using a frequency-selecting element, e. g. a grating, which picks the desired wavelength from the resonator modes within the gain profile.

lt is advantageous to place the grating directly inside the resonator, so that modes that do not meet the grating-equation are suppressed.

The supplied pump energy concentrates in only one mode, consequently a singlemode laser emits with much less noise than a multimode version, where the modes are "competing" for the excitation energy.

A regular laser diode resonator is formed by the reflecting surfaces of a semiconductor crystal. Printing or etching a grating structure to the semiconductor material itself is a complicated and expensive procedure (e.g. DFB-lasers, DBR-lasers, figure 1a and b).

Fig 1: Commonly used resonators for single mode-semiconductor lasers:
a) Distributed Feedback (DFB)
b) Distributed Bragg Reflection (DBR)
c) External resonator with grating

Moreover the emission wavelength is fixed by the separation of the grating lines, which can only be altered thermally.

Single mode emission by use of an external resonator

An external resonator (figure 1c) provides more freedom to the spectroscopist, as far as wavelength selection is concerned.

The existing Fabry-Perot-resonator of the semiconductor crystal is enhanced by

an external grating serving as a coupling mirror. Turning the mirror changes the emission wavelength.

However the laser diode facet nearest to the grating must be suitable treated, in order for this to work.

An adequate antireflection coating on the facet decreases the reflectivity to approximately zero and therefore eliminates the influence of that surface to the crystal resonator. lf the external resonator is skilfully arranged the laser diode may be tuned over a wide spectral range without mode hopping.

Fig. 2: Littman Resonator:
The laser diode may be tuned over a wide spectral range without mode hopping, if turning point P of the tuning mirror S2 coincides exactly with the point of intersection of the grating plane G and planes S1 and S2.

The Littman Resonator (figure 2; see for example Demtröder: Laserspectroscopy, Springer) is an example of a straightforward mechanical configuration. Turning the tuning mirror about point P, the angle between S2 and G changes and the wavelength is tuned. At the same time the length of the resonator changes by an amount fitting the new wavelength.

For more information please contact:

Mark Day: 01245 491 499