Gratings for tunable lasers: using multidielectric coatings to improve their efficiency

Abstract

The first results of a theoretical and experimental study are presented showing that a significant enhancement of the efficiency of gratings used as wavelength selectors for cw tunable lasers may be achieved with adequate dielectric coatings. It is found that, in some cases, the energy absorbed by the grating surface can be reduced by a factor >2. The output power of a cw Rh640 dye laser was practically multiplied by the same factor. It seems that holographic gratings are more adaptable than are ruled gratings to this utilization.

Multicoated gratings: a differential formalism applicable in the entire optical region

“…Classical numerical tests show that the precision is of the order of 10-3. We have been able to reproduce the previous results of Maystre et al 4 for the improvement of the aluminum grating efficiency obtained by depositing adequate dielectric coatings on the metallic substrate. Precision problems in the evaluation of the eigenvalues and eigenvectors prevent us from achieving the computations when the total thickness of dielectric exceeds one wavelength, or when the number of layers exceeds eight, but these limitations could be modified by using more-sophisticated numerical techniques.…”

“…However, this absorption is considerably enhanced if some Wood anomaly occurs. 4 For some recent applications of gratings, this absorption must be strongly reduced. For instance, it is the case for gratings used as beam-sampling mirrors for high-power lasers.…”

“…For instance, it is the case for gratings used as beam-sampling mirrors for high-power lasers. 4 These gratings generally operate at 10.6 gm and must absorb less than about 0.1% of the energy contained in the laser beam in order to avoid destruction. We know that this goal may be reached by using adequate X/4 dielectric coatings deposited on the metallic substrate.…”

“…8 This code has been used to perform a numerical optimization of holographic and ruled gratings coated with dielectric layers, in collaboration with an experimental study by Jobin-Yvon. 4 In practice, numerical reasons prevented the authors from achieving accurate computations as soon as the number of dielectric coatings exceeded three. Nevertheless, it appeared that a significant decrease of absorption was possible, for instance by a ratio of 2 for a 3000-groove/mm holographic grating with a sinusoidal profile, a fact qualitatively confirmed by experiments made by Jobin-Yvon.…”

“…We have chosen to perform our calculations in case C of Table 1 of a previous study. 4 The first grating, Go, is an uncoated aluminum grating having 3000 grooves/mm with a sinusoidal profile of groove depth h = 0.12 gm. The second one, GI, was obtained by coating Go with a stack of two dielectrics: a firstlayer of MgF 2 on the metallic substrate, with thickness 0.106 pm, and a second layer of TiO 2 , with thickness 0.0602 Am.…”