Laser system providing a shaped intensity profile of an output beam within an unstable optical resonator layout and method thereof

Abstract

The present invention relates to a laser system and a method of generating a defined spatial mode-shaped laser beam using an unstable laser resonator layout. The laser system for mode shaping of a laser beam within an unstable optical resonator layout comprising an active medium, characterized in that, the active media comprises a pumped area, wherein the gain distribution is generated by an optical pump beam's spatially intensity profile. In a preferred embodiment, the system may further comprise an end-pumped layout to deliver the spatially shaped optical pump beam to the active medium; and/or an active element and/or a passive element for modifying the resonator losses; and/or means of output coupling of a laser beam from said unstable resonator layout. The system according to the present invention is suitable to deliver a top-hat beam profile.

Claims

1. A laser system providing a top-hat profile of an output beam within an unstable optical resonator layout comprising: a dichroic mirror for coupling an optical pump beam into the unstable optical resonator; an active medium comprising a pumped area, wherein gain distribution of the pumped area is spatially distributed within the active medium, and wherein an intra resonator beam is WIN modulated due to gain modulation, and wherein the laser system further comprises a reflective convex mirror; and a variable polarization output coupling configured to eject the output beam without introducing an aperture, and wherein the output coupling is within the unstable resonator layout.

2. The laser system according to claim 1 further comprising an end-pumped layout to deliver a spatially shaped optical pump beam to the active medium.

3. The laser system according to claim 1, wherein the active medium is made of a low gain material.

4. The laser system according to claim 1, wherein the system further comprises at least one active element and/or at least one passive element for modifying the resonator losses.

5. The laser system according to claim 1, wherein the output coupling is selected from the group consisting of: a polarizer and a polarization rotating element.

6. The system according to claim 1 for use in q-switch or cavity dump operation mode or as a regenerative amplifier or in a mode locking regime.

7. The system according to claim 1 further comprising Pockels cell within the unstable resonator for Q-switching operation.

8. The system according to claim 7, wherein the Pockels cell is switched for quarter wave operation directing a part of the intra-resonator beam into a cavity dump and a part of the intra-resonator beam is ejected as the top-hat beam.

9. A method for delivering a top-hat laser beam from an unstable optical resonator comprising steps of delivering a spatially shaped optical pump beam to an active medium; and input coupling the spatially shaped optical pump beam to the unstable optical resonator by a dichroic mirror; and WIN modulating of the spatially shaped optical pump beam and intra-resonator beam in the active medium comprising a pumped area, wherein gain distribution of the pumped area is spatially distributed within the active medium; output coupling the intra-resonator beam by a variable polarization output coupling within the unstable resonator without introducing an aperture.

10. The method according to claim 9, wherein the resonator losses are modified by using at least one active element and/or at least one passive element.

11. The method according to claim 9, wherein the laser beam is coupled by a means selected from the group consisting of: a polarizer and polarization rotating element.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 represents the basic resonator layout carried out according to the state of the art.

(2) FIG. 2 represents normalized gain distribution achieved by the system disclosed in the preferred embodiment.

(3) FIG. 3 represents output beam profiles for different output coupler reflectivity.

(4) FIG. 4 represents resonator layout with variable output coupling.

(5) FIG. 5 represents resonator layout for q-switch or cavity dump operation.

DESCRIPTION OF EMBODIMENTS

(6) The following detailed description of the embodiments is provided as a brief overview of the claimed system. It shall not limit the invention in any aspect, with a detailed and fully-enabling disclosure being set in this section. Likewise, the invention shall not be restricted to any numerical parameters, processing equipment, operational conditions, and other variables unless otherwise stated herein.

(7) According to a preferred embodiment, the laser system is based on an unstable resonator layout. The unstable resonator layout can be a positive branch cavity layout such as shown in [FIG. 1].

(8) Said layout comprises two spherical mirrors, in particular a concave mirror 1, reflective for laser and antireflective for pump radiation; and a convex output coupling mirror 2; and an active medium 3. The gain distribution in the active medium 3 is generated by an optical pump beam 6, preferably shaped. The optical pump beam 6 may be preferably provided by another laser system or by a diode laser source, wherein the layout includes means of beam shaping.

(9) In the following, the shape of optical pump beam 6 can be approximated by a quadratic top-hat shape.

(10) The active media 3 comprises a pumped area 4 in the active medium 3, into which the optical pump beam 6 is directed. From the area 4 an intra resonator beam 5 is generated so that, the profile resembles the gain distribution generated by the optical pump beam 6.

(11) As shown in FIG. 1, an input coupling of the optical pump beam 6 into the laser resonator is realized by the concave dichroic mirror 1.

(12) The output-coupling can be provided by the partially reflective convex output coupling mirror 2. The transmitted part of the intra resonator beam 5 represents the laser output beam 7.

(13) According to said layout, the distance L between the concave mirror 1 with radius R.sub.1 and the partially reflective convex output coupling mirror 2 with radius R.sub.2 is given by
2L=R.sub.1+R.sub.2

(14) The roundtrip magnification is calculated using

(15) $M=.Math.\frac{R_1}{R_2}.Math.$

(16) The 2-dimensional mode field within the laser cavity is approximately calculated via

(17) $V\left(x,y\right)=V\left(0,0\right){.Math.}_{k=1}^{?}\frac{?\left(\frac{x}{M^k},\frac{y}{M^k}\right)}{?\left(0,0\right)},$

(18) where V(x,y) is the electrical field strength of the cavity mode according to the x and y coordinates orthogonal to the optical axis. In prior art ?(x,y) is always considered a value smaller than 1, representing the spatially dependent cavity losses (concerning field strength), e.g. the reflectivity profile of a GRM. The shaping method relies on the spatial modulation of the resonator losses, hereinafter referred to as a loss modulation. ?(x,y) has a value bigger than 1, due to the gain modulation, hereinafter referred to as a win modulation.

(19) FIG. 1 represents an example, where following parameters were applied: R.sub.1=5000 mm, R.sub.2=?4000 mm, M=1.25.

(20) FIG. 2 represents the normalized gain distribution, whereas the gain distribution is a rectangular super-Gaussian distribution of order 8 with a width of 4 mm. The above mentioned normalized gain distribution was calculated for a model-system according to the one described above and shown in [FIG. 1].

(21) FIG. 3 shows an embodiment according to the present invention, wherein the normalized output shaped laser beam 7 mode profiles for different amplitude reflectivity of the partially reflective convex output coupling mirror 2. The according reflectivity R (in terms of intensity) of the partially reflective convex output coupling mirror 2 and the minimum round-trip gain g that is necessary to compensate for the resonator losses generated by magnification and output coupling is given above the individual graphs.

(22) As it can be demonstrated, that due to the win modulation of the resonator mode there is no hole generated in the middle of the shaped laser beam 7, even for very low output coupling, which allows achieve high feedback for support of low gain materials. The only visible effect for low output coupling is that the wings of the shaped laser beam 7 profile smear out. This is normally acceptable in a laser setup when the optics' size is dimensioned accordingly. Additionally, as the intensity plateau of the extracting mode is always bigger or equal compared to the pumped area 4, the extraction efficiency is always homogeneously distributed over the full pumped area 4 and therefore not geometrically limited.

(23) Furthermore, the optical pump beam profile 6 and the produced output beam profile 7 are both quadratic. In fact, the output laser beam 7 intensity profile will resemble the intra resonator beam profile 5, which is similar to the gain distribution following the optical pump beams intensity profile 6.

(24) In another embodiment, in the system as described above and with accordance to FIGS. 4 and 5, the convex output coupling mirror 2 was replaced by a convex mirror 10, which is high reflective. The output coupling is realized using a variable polarization output coupling, preferably selected form the group as mentioned in the description, preferably mentioned here a polarizer 8 and a quarter wave plate 9, which allows adjusting the output coupling to optimize a given system, such as shown in [FIG. 4].

(25) By additionally introducing a Pockels cell 11 into the resonator, shown on [FIG. 5], the layout can be used for q-switching operation as well. In contrast to GRM based systems, the output beam 7 will be emitted from the polarizer 8. In case of the Pockels cell 11 this is advantageous, as for q-switched operation mode the Pockels cell 11 can be operated at less than the quarter wave voltage. Again, by adjusting the Pockels cell 11 voltage, the output coupling per roundtrip in the switched state can be adjusted, which allows to optimize output pulse shape and energy.

(26) Furthermore, the same scheme as shown in [FIG. 4] also allows for cavity dump operation if the Pockels cell 11 is switched for quarter wave operation. Though the beam profile will have long edges in this case (output coupling embodiment in [FIG. 3]) this is still acceptable in most cases. In this operation mode, the pulse duration will be in the range of the round-trip time of the resonator, which is shorter than in q-switch mode. The actual steepness of the edges in this operation mode will be linked with the available gain in the laser system and the losses that are present in the resonator especially outside the amplifying area 4 (e.g. re-absorption).

(27) As cavity dumping is operable in this resonator scheme, direct seeding and the use as regenerative amplifier is also possible by injecting a seed pulse into the resonator counter propagating the output in the cavity dump operation mode. In principle, also the combination with sub round-trip loss modulation or mode locking techniques is possible.

INDUSTRIAL APPLICABILITY

(28) The present invention may be industrially applied in the field of optics, in particular in the field of laser technology dedicated to shape modulation thereof.

(29) More particular, the invention may be used to realize high power and/or high energy class laser systems with a wide range of parameters regarding pulsed/CW operation, pulse parameters and wavelength. Such lasers again are applicable in a wide range of scientific, medical and industrial applications like pump lasers for other laser systems, laser sintering, laser shock peening, Imaging diagnostics, surgery etc.

REFERENCE SIGNS LIST

(30) 1 concave mirror, reflective for laser and antireflective for pump radiation 2 partially reflective convex output coupling mirror 3 active medium 4 pumped area in active medium 5 intra resonator beam 6 optical pump beam 7 shaped laser beam 8 polarizer 9 quarter wave plate 10 convex mirror 11 Pockels cell