NL2010852C2 - Optical system for directing laser beams in ã–ffner geometry stretcher. - Google Patents

Abstract

The subject of the invention is the optical system of the laser beam running in the stretcher in Öffner geometry, containing the element returning the beam and located on a common optical axis x the following elements: diffraction grating (SD), stave mirror (L1) and convex mirror (L2), characterized by this, from the center of curvature of at least one of the mirrors (L1) or (L2) is shifted beyond the common optical axis x of the system.

Description

Optical system for directing laser beams in Öffner geometry stretcher

The subject matter of the invention is an optical system for directing laser beams in an Öffner geometry stretcher. More specifically, the invention relates to an optical system for temporal stretching of laser pulses, hereinafter referred to as the stretcher.

Typically, a stretcher system is used in various optical amplifiers of ultrashort laser pulses. In a parametric amplifier, a short laser pulse, prior to amplification, is stretched in the time domain to achieve possibly large temporal overlap with the pumping laser in the parametric amplification process - and consequently, to achieve a high efficiency of energy transfer. In traditional laser amplifier, for example a titanium-sapphire crystal based laser, the laser pulse is stretched in order to decrease the pulse peak power and to prevent damage to the amplifier optics and buildup of non-compressed non-linear spectral phase.

Following amplification, the laser pulse is recompressed, possibly to the initial pulse duration.

The context described above implies specific requirements for the stretcher system: • as for parametric amplification, the achieved efficiency of the amplifier system depends directly on the temporal overlap of the amplified and the pumping pulses, which in combination with a typical pumping pulse duration of a few nanoseconds imposes the requirement to significantly stretch the amplified pulse; • subsequent pulse compression down to the Fourier-limited time (resulting from the band width) should be possibly simple - the dispersion introduced earlier should be easy to compensate; • the system should operate in a possibly large wavelength range; • the aberrations introduced by the stretcher should be possibly low; • an obvious, but important requirement is the simplicity and possibly low cost of system fabrication.

In the present state of the art there are known 2 primary stretcher types capable of providing pulse stretching of an expected order of magnitude. These are fibre-based and diffraction grating-based solutions.

The fibre-based stretchers introduce significant material dispersion that afterwards is complicated to compensate in the compressor. Moreover, the propagation of ultrashort high peak power pulses in an optical fibre results in appearance of unfavourable nonlinear effects.

At present, grating-based stretchers are more widely used. The two most known configurations are the following systems: • Martinez stretcher - the system is, however, characterised by significant spherical aberration, and consequently the pulse compression with a grating-based stretcher is not very effective (mainly the fourth-order dispersion remains not compensated) • Öffner stretcher (with Öffner triplet) - with significantly lower or zero aberration (G. Cheriaux, P. Rousseau, F. Salin, J. Chambaret, B. Walker, and L. Dimauro, „Aberration-free stretcher design for ultrashort-pulse amplification”, Opt. Lett. 21, 414-416 (1996)) - occurs in 2 versions: 1. with two diffraction gratings - the system is aberration-free (aberration-free Öffner stretcher), but requires precise positioning of both diffraction gratings, which is difficult to carry out in practice; 2. with a single diffraction grating shifted out of the common centre of curvature of mirrors (off-centre Öffner stretcher) - the system is less complicated and displays higher resistance to inaccurate alignment, at the expense of a low spherical aberration resulting in appearance of the spectral phase - mainly of the fourth-order.

The second solution mentioned above, the stretcher with the Öffner triplet and a single diffraction grating is considered particularly advantageous. The described design provides for expected temporal stretching, allows for efficient reversing of that process with a simple grating-based stretcher, using optical elements with appropriately large apertures: a concave mirror and a diffraction grating, it can operate in a wide wavelength range.

Unfortunately, the construction of the system involves a number of considerable practical problems.

The path of laser beams in a typical off-centre Öffner stretcher system, taking into consideration the double pass through the entire system (in order to increase the temporal stretching and to return to the initial shape of the laser beam), is known from the literature (G. Cheriaux, P. Rousseau, F. Salin, J. Chambaret, B. Walker, and L. Dimauro, "Aberration-free stretcher design for ultrashort-pulse amplification," Opt. Lett. 21, 414-416 (1996)) and shown below in Fig. 1-2.

According to the above state of the art, a beam of considerable width in the plane of the optical table passes over, and subsequently under the convex mirror. This implies necessary minimisation of the height of the mirror while keeping it considerably long and with required accuracy of shape - this gives rise to large difficulties and high costs related to fabrication of such a mirror, and in addition, there is a problem with stable holder of the mirror that would allow for mirror adjustment so that the holder does not shut out the beam path.

The design problems in an off-centre Öffner stretcher could be unexpectedly solved by modifying the optical path of the beam in the vertical direction.

Therefore, the purpose of the present invention is to provide a novel, improved optical system for directing laser beams in an Öffner geometry stretcher that is free from the above drawbacks.

According to the invention, an optical system for directing laser beams in an Öffner geometry stretcher, comprising a beam turning component and the following components located on the common optical x axis: diffraction grating, concave mirror and convex mirror, characterised in that the centre of curvature of at least one of the mirrors is shifted out of the common optical x axis of the system.

Preferably, the centre of curvature of the convex mirror is located out of the optical x axis, and the centre of curvature of the concave mirror is located on the optical x axis.

Preferably, the beam turning component comprises at least one flat mirror or roof prism.

Preferably, the system according to the present invention is constructed on an optical table.

Preferably, the centre of curvature of the convex mirror is tilted vertically downward or vertically upward from the optical x axis.

As a result of the modification of the beam path in the optical system for directing laser beams in an Öffner geometry stretcher according to the present invention by tilting the concave or the convex mirror (and therefore by shifting vertically its centre of curvature), the beam can pass at one side of the convex mirror only.

The invention is now explained more in detail in a preferred embodiment, with reference to the accompanying figures, wherein:

Fig. 1 (state of the art) shows a side view of the path of laser beams in a known stretcher system;

Fig. 2 (state of the art) shows the position of the beam in consecutive passes on the L1 mirror;

Fig. 3 shows the beam path in the proposed stretcher system according to the present invention, and

Fig. 4 shows the position of the beam in consecutive passes on the L1 mirror in the system according to the present invention.

The markings used in Figures have the following meaning: L1 - concave mirror, L2 - convex mirror, L3 - flat mirror, SD - diffraction grating, L34 - beam turning component, A - incoming beam, B - outgoing beam, x - optical axis.

The solutions known from the state of the art are shown in Fig. 1 and Fig. 2.

Preferred embodiment

Reference example - from the state of the art

In the reference example known from the state of the art, as shown in Fig. 1, the optical system for directing laser beams in an Öffner geometry stretcher comprises a concave mirror L1 of the curvature radius R, a convex mirror L2 of the curvature radius -R/2, two flat mirrors L3 or roof prisms representing a beam turning component L34, and a diffraction grating SD. The mirrors L1 and L2 are positioned so that their centres of curvature are located at the same point.

The scheme of the path of the incoming beam A in the system known from the state of the art is as follows: the beam A enters the stretcher system and passes between the components L34 over the mirror L2, in the SD plane it is diffracted by a diffraction grating (at that location it is spectrally dispersed - in the plane parallel to the optical table it is divergent), then it is reflected from the concave mirror L1, the convex mirror L2, again from L1, and subsequently is diffracted by the diffraction grating (the beam has considerable width, but its spectral components are parallel to each other), then turned back and shifted vertically by the component L34 (two mirrors L3 or roof prisms), runs in the opposite direction, in the order: the grating SD, the mirrors L1, L2, L1, again the diffraction grating, respectively, and leaves the system as the beam B under the mirror L2.

Fig. 2 shows the height of the beam in consecutive passes on the L1 mirror in the solution known from the state of the art.

In the solution known from the state of the art, the incoming beam A passes over the convex mirror L2, and leaves as beam B under the convex mirror L2.

Embodiment - according to the present invention

In a preferred embodiment shown in Fig. 3, the optical system for directing laser beams in the Öffner geometry stretcher comprises a concave mirror L1 of the curvature radius R, a convex mirror L2 of the curvature radius -R/2, one flat mirror L3, a diffraction grating SD. In the system constructed according to the present invention, the centre of curvature of one of the mirrors L1 or L2 is shifted upward or downward, respectively, and as a result the beam A passes only over the mirror L2, i.e., on one side of the mirror L2.

The scheme of the path of the incoming beam A in the system according to the present invention is as follows: the incoming beam A enters the stretcher system in parallel to the optical table, it is dispersed by the diffraction grating SD, then reflected from the mirrors: the concave mirror L1, the convex mirror L2, again L1, then is reflected and collimated on the grating SD, and subsequently turned (without being shifted, at a small angle) on the flat mirror L3, and runs in the opposite direction, in the order: diffraction grating SD, mirrors L1, L2, L1, respectively, and diffracted by the diffraction grating SD has again circular shape, is collimated and as outgoing beam B leaves the system according to the present invention at a small angle in respect to the incoming beam A, which represents a substantial difference as compared with the solution presented above in the reference example, known from the state of the art.

In addition, Fig. 4 shows the height of the beam A in consecutive passes on the L1 mirror in the system according to the present invention.

In the optical system according to the present invention the following can be used: - higher convex mirror that is easier to fabricate, and much cheaper at the same time, - simplified and more stable structure of the convex mirror holder - the entire space under the mirror can be used for that purpose, the mirror is not fixed at its ends and hung over the surface of the optical table, and - approximately two times smaller (lower) diffraction grating - due to a shorter distance between the incident beams; this lowers the total cost of the system.

An experimental attempt to recompress a pulse stretched in the stretcher of proposed design showed that the required shift of the centre of curvature of the mirror L1 or L2 by a few millimetres has an insignificant effect on the phase of the pulse leaving the stretcher and the beam aberration.

Claims (5)

An optical system for directing laser beams in an Offner geometry provider, comprising a beam bending component and subsequent components disposed on the common x-axis optical: a scattering grid (SD), a concave mirror (L1) and a convex mirror (L2), characterized in that the center of the curvature of at least one of the mirrors (L1) or (L2) is offset from the common optical x-axis of the system. System according to claim 1, characterized in that the center of the curvature of the convex mirror (L2) is outside the optical x-axis and that the center of the curvature of the concave mirror (L1) on the optical x -as is located. A system according to claim 1 or 2, characterized in that the beam bending component comprises at least one flat mirror (L3) or a roof prism. A system according to claim 1, 2 or 3, characterized in that the system is built on an optical table. System according to claim 4, characterized in that the center of the curvature of the convex mirror (L2) is tilted vertically downwards or vertically upwards relative to the optical x-axis.