Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2002-05-23
2004-01-20
Dunn, Drew (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S566000, C359S584000, C372S027000, C372S102000, C372S106000, C372S019000, C257S098000
Reexamination Certificate
active
06680799
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to multidielectric mirrors and to coupling devices, in particular for use in a laser device. It also concerns optical devices comprising a multilayer mirror and a grating, said device being of high polarization selectivity, of particularly large structural tolerance, and of particularly large wavelength range.
A microchip laser polarization device is known from the market (Nanolase, Grenoble, France) whereby a mechanical stress induced transversally on a Nd:YAG microchip laser favours laser emission with the electric field polarized along the applied external force. The disadvantage of such solution is that it is a one by one solution applied to an otherwise batch production process of microlasers. Such solution is also limited because it can practically only lead to a uniform distribution of the electric field.
Another device is known from the scientific literature (V. N. Bel'tyugov, S. G. Protsenko, Y. V. Troitsky, “Polarizing laser mirrors for normal light incidence ”, Proc. SPIE, Vol. 1782, 1992, p. 206) comprising a multilayer mirror composed of at least one corrugated interface between layers whereby the grating couples the undesired polarization of a gas laser into a guided mode of the multilayer, and induces a differential loss in the laser cavity between the coupled undesired polarization and the uncoupled desired polarization.
The practical limitations of this device are following.
In case one interface only is corrugated, the coupling efficiency into a guided mode of the multilayer is too weak for the device to be applied to microchip lasers where the beam diameter is 100 &mgr;m or less; furthermore, the grating would in such case diffract the lasing polarization into diffraction orders propagating into the high index active crystal substrate. This results in non-acceptable losses. Moreover, the weakness of the coupling efficiency has the first consequence that the linewidth of the coupling phenomenon is very narrow and prevents the polarizing effect to be effective over a wide wavelength range, as for instance over the full gain bandwidth of about one nanometer of Nd:YAG lasers; a second consequence is that the spectral position of the narrow line at which the desired polarising effect occurs is highly dependent on the multilayer characteristics, thus on the fluctuations of its manufacturing conditions, and on the environmental dependence of the refractive index of the layers especially on humidity and temperature. This renders the device of the state of the art unusable in practice since it would require individual post-trimming and temperature control. In case all, or a large number of interfaces are corrugated, the coupling efficiency is increased but these corrugations lead to a perturbation of the layer deposition conditions which will be even less reproducible and provoke scattering losses on the lasing polarization.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful grating device such as a polarizing mirror or a polarizing coupling device in which the above described problems are eliminated.
Another, more specific object of the invention is to provide tolerant coupling means comprizing a grating capable of damping one polarization of a laser beam over a large wavelength range while maintaining the resulting scattering at a low level.
A first embodiment of a device according to the invention is an optical device comprising a substrate, a multilayer mirror, a pair of low and high refractive index layers, and a corrugation grating in said high refractive index layer.
Such a device can be used as a laser coupler or as a coupler in a laser device.
The combination of the pair of low and high refractive index layer, and of the corrugation grating in said high refractive index layer, results in a reduction of the reflection coefficient of a first polarization, by means of a destructive interference in the multilayer mirror for this first polarization, with essentially no change of the reflection coefficient for the other (second) polarization.
A beam of light is directed toward the optical device according to the invention through said substrate. In other words, the light beam is incident from the substrate side. The beam then successively traverses the multilayer mirror and the pair of low and high refractive index layers. One polarisation of the beam is reflected as in the absence of the grating. The other polarisation is reflected differently due to the grating, which is placed or made on, or in, the last high index layer, for example at the air side.
The multilayer mirror reflects both polarizations equally. The polarizing function is performed by the grating substructure comprizing the said corrugated pair of low and high refractive index layers.
For a corrugation grating on the last layer to have a non-negligible effect on the lasing condition in the cavity, the polarization selection effect can not simply be the coupling of one polarization into a guided mode of the multilayer as disclosed in the state of the art, for example in the article by Bel'tyugov cited above. The device according to the invention teaches the use of an abnormal reflection from the last high index waveguide grating under normal incidence: that is, when the incident polarization is coupled into the last high index layer mode, the optical power is reflected back from the grating with a phaseshift of &pgr;. Thus, the reflection of the damped polarization is not damped by its coupling into a waveguide mode, but it is efficiently reflected back into the cavity with a &pgr; phaseshift by virtue of the said abnormal reflection, thus giving rise to an appreciable degree of destructive interference in the multilayer mirror for the coupled polarization, and consequently inducing on the latter a significantly more efficient damping.
Those familiar to the art will not be tempted to place the grating on the side of the last layer of the multidielectric mirror, for example at the air side, because the field is much weaker there than in the first layers, and because, in case of use in a laser device it is believed that the coupling of the polarization to be damped into a waveguide mode nearly outside the laser resonator will hardly have an influence on the intra-cavity polarization lasing conditions.
The substrate can be a laser active material substrate, for example the active material of a microchip laser.
The first embodiment of the device according to the invention can be advantageously used at one side of a laser cavity. Its efficiency is so high that the necessary damping of the coupled polarization can be obtained with a grating of very small depth, thus leading to a reduced scattering.
The efficiency of the grating is high because the last high index layer, acting as a waveguide, concentrates the modal field in the said layer, and in particular in the grating zone. The high radiation efficiency of the grating reduces the quality factor of the wavelength resonance and gives rise to a large wavelength tolerance of the grating mode coupling.
The first embodiment of the invention being substantially lossless, the damped polarization is not necessarily filtered out. It can thus be a lossless polarization filter exhibiting two different reflection coefficients for the two incident polarisations.
The polarization device according to the above first embodiment offers the following specific advantages:
it prevents the diffraction of the uncoupled polarization into the substrate, in particular in the case of a microchip laser,
the grating causes little scattering since it is not at the substrate side; in the case of a laser, it is not used at an active medium side within the laser resonator but at the air side,
it can provide a substantial and controllable difference between the reflection coefficient of the two polarizations without inducing any power loss on the polarization having the smaller reflection coefficient.
it can induce the polarization selection of very narrow bea
Parriaux Olivier
Pigeon Florent
Tishchenko Alexander V.
Boutsikaris Leo
Dunn Drew
Universite Jean Monnet
Weingarten Schurgin, Gagnebin & Lebovici LLP
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