DEVICE AND METHOD TO ADJUST TUNABLE LASER PULSES

Abstract

The present invention relates to a device and a method for pulse modulation of laser pulses of tunable laser sources. The invention relates specifically to an arrangement for spectral and/or temporal laser beam manipulation of tunable lasers using nonlinear wave interaction. By using a variable, lens based beam forming section it is possible to manipulate a laser pulse provided by a tunable laser source (i.e. tunable in pulse energy, temporal pulse length and/or wavelength) or different laser sources (i.e. different with respect to pulse energy, temporal pulse width and/or wavelength) in such a manner, that nonlinear wave interaction can occur in the most efficient way. The beam forming section according to the invention allows for adjusting the waist of the laser beam and the focal position of the laser beam inside a cell comprising a nonlinear medium.

Claims

1. Device for modulating a laser pulse comprising at least one laser source (1), a laser beam separation or outcoupling section (3), a laser beam forming section (4) comprising at least two lenses, a cell (5) containing a nonlinear medium and optional a section of optical elements containing at least one lens (21) and/or at least one mirror positioned in the optical path behind the cell containing a nonlinear medium, wherein at least one of the at least two lenses of the laser beam forming section (4) is movable in the direction of the optical path in a way that the beam waist and the focal position of the laser beam inside the cell containing the nonlinear medium are adjusted depending on the properties of the initial laser beam and the type of nonlinear medium to adjust the beam parameters.

2. Device according to claim 1, wherein the initial laser beam is generated by a laser source (1), wherein said laser source (1) is tunable or can vary with respect to the pulse energy and/or the temporal pulse shape and/or the wavelength of the pulse and/or the divergence and/or the spatial shape of the pulse.

3. Device according to claim 2, wherein the laser source (1) comprises one tunable laser or a combination of at least two lasers, wherein in said combination the at least two lasers are tunable or non tunable or one laser is tunable and the other laser is non tunable.

4. Device according to claims 1 to 3, wherein the laser source (1) is selected from a dye laser, a solid-state laser, a gas laser or an optical parametric oscillator (OPO).

5. Device according to claims 1 to 4, wherein the device comprises at least one laser beam mirror.

6. Device according to claims 1 to 5, wherein the beam separation section (3) comprises optical active materials.

7. Device according to claim 6, wherein the optical materials are selected from a group consisting of polarization selective elements, polarizing elements, mirrors with high reflectivity for the input wavelength and high transmittance for the output wavelength and prisms.

8. Device according to claim 7, wherein the polarization selective element is a polarizer, a glassplate, a nonlinear crystal or a dichrotic mirror.

9. Device according to claim 7, wherein the polarizing element is a waveplate.

10. Device according to claims 1 to 9, wherein the nonlinear medium is encapsulated by a cell (5) comprising a length in the range of 10 cm to 3 m and a diameter in the range of 1 cm to 10 cm.

11. Device according to claims 1 to 10, wherein the cell (5) containing the nonlinear medium is equipped with removable caps on the front end (18) and/or with removable caps on the back end (19).

12. Device according to claims 1 to 11, wherein the caps on the front end (18) and/or the back end (19) of the cell containing the nonlinear medium are equipped with optical active materials.

13. Device according to claim 12, wherein the optical active materials are selected from a group consisting of a mirror, a lens, a filter (17), a flat window (13), a curved window (15), a curved window with high reflective coating on the inside, a window which is at least partially coated on the inside with a high reflective material (14).

14. Device according to claim 12, wherein the curved window with high reflective coating is a focusing mirror.

15. Device according to claim 12 wherein one optical active material is a brewster angle window.

16. Device according to claims 1 to 15, wherein the optical elements of the caps are at least partially coated by an anti-reflecting material.

17. Device according to claims 1 to 16, wherein the cell (5) is at least partially coated, preferably on the inner surface, by reflecting material.

18. Device according to claims 1 to 17, wherein the nonlinear medium is a solvent or a mixture of solvents.

19. Device according to claim 18, wherein the nonlinear medium contains a solution of non-absorbance compounds.

20. Device according to claim 19, wherein the nonlinear medium is selected from a liquid crystal or ionic liquid.

21. Method for modulating a laser pulse using the device of any of claims 1 to 20 comprising the steps: generating an initial laser beam, optionally rotating the polarization of the laser beam, shaping the laser beam waist, shaping the focal position of the laser beam, bringing the laser beam into a nonlinear medium inside a cell and coupling out an adjusted beam, wherein the laser beam is focused into the nonlinear medium for nonlinear interaction and the beam waist and the focal position of the beam inside the nonlinear medium are adjusted depending on the properties of the initial laser beam and the type of nonlinear medium.

22. Method of claim 21, wherein the shaping of beam waist and/or focal position of the laser pulse is performed by use of a lens system.

23. Method of claims 21 to 22, wherein the lens system to adjust the laser pulse comprises at least two lenses, wherein, preferably, at least one lens is movable.

24. Method according to claims 21 to 23, wherein the waist and/or focal position of the initial laser beam is modulated by adjusting the position of the lenses in the lens system of the beam forming section.

25. Method according to claims 21 to 24, wherein the spatial shape of the initial beam is adjusted by the beam forming section, which comprises at least two lenses which are selected from a group consisting of focusing and defocusing lenses, which are movable in the direction of the optical path.

26. Method according to claims 21 to 25, wherein the focal position and/or beam waist of the incident laser beam is adjusted by adjusting the position of the lenses in the lens system of the beam forming section in dependence of the wavelength of the initial laser beam.

27. Method according to claims 21 to 26, wherein the focal position and/or beam waist of the laser beam in the nonlinear medium is adjusted by adjusting the position of the lenses in the lens system of the beam forming section in dependence of the nonlinear medium in the cell.

28. Method according to claims 21 to 27, wherein the focal position of the laser beam in the nonlinear medium is adjusted by positioning of the at least one focusing lens and/or at least one focusing mirror comprised in the beam forming section.

29. Method according to claims 21 to 28, wherein the polarization of the initial laser beam is rotated by at least one spectrally tunable or non-tunable waveplate.

30. Method according to claims 21 to 29, wherein stimulated Brillouin scattering is generated due to nonlinear interactions between the laser pulse and the nonlinear medium.

31. Method according to claim 30, wherein the incident laser pulses are compressed to the optimal temporal pulse length and shape independent of their energies by adjusting the position of the lenses in the lens system of the beam forming section in dependence of the energy of the initial laser pulse.

32. Method according to claim 30 or 31, wherein initial laser pulses with different beam waist are compressed in the same setup.

33. Method according to claims 30 to 32, wherein initial laser pulses with different wavelength are compressed.

34. Method according to claims 30 to 33, wherein different nonlinear media are used to generate stimulated Brillouin scattering.

35. Method according to claims 30 to 34, wherein the position of the lenses in the lens system of the beam forming section is adjustable in dependence of the temporal shape and width of the incident laser beam.

36. Method according to claims 21 to 29, wherein stimulated Raman scattering is generated due to nonlinear interactions between the laser pulse and the nonlinear medium.

37. Method according to claim 36, wherein stimulated Raman scattering is generated by adjusting the position of the lenses in the lens system of the beam forming section in dependence of the energy of the initial laser pulse.

38. Method according to claims 36 to 37, wherein initial laser pulses with different wavelengths are used to generate stimulated Raman scattering.

39. Method according to claims 36 to 38, wherein different nonlinear media are used to generate stimulated Raman scattering.

40. Method according to claims 30 to 39, wherein the temporal pulse length of the initial laser pulse is longer in comparison to the pulse length of the adjusted laser pulse.

41. Method according to claims 30 to 40, wherein the spectral purity of the adjusted laser pulse is higher in comparison to the initial laser beam.

42. Method according to claims 30 to 42, wherein the position of the lenses in the lens system of the beam forming section is adjustable in dependence of the temporal shape of the incident laser beam.

43. Method according to claims 21 to 29, wherein a white light continuum is generated due to nonlinear interactions between the laser pulse and the nonlinear medium.

44. Method according to claim 44, wherein the position of the lenses in the lens system of the beam forming section is adjusted in dependence of the spectral band width of the initial laser pulse.

45. Method according to claims 44 to 45, wherein initial laser pulses with different energies are used to generate a white light continuum.

46. Method according to claims 44 to 46, wherein initial laser pulses with different wavelengths are used to generate a white light continuum.

47. Method according to claims 44 to 47, wherein different nonlinear media are used to generate a white light continuum.

48. Method according to claim 36 or 48, wherein the generated beam is collimated by optical elements in the optical path behind the cell containing a nonlinear medium and separated from the initial laser pulse by optical elements in the optical path behind said cell containing a nonlinear medium.

49. Use of the device according to any of claims 1 to 20 in a or with a conventional laser system, wherein the laser parameters of the initial laser pulse are adjustable.

50. Use of the device according to any of claims 1 to 20, wherein the temporal pulse length of the initial laser pulse is shortened.

51. Use of the device according to any of claims 1 to 20, wherein the spectral purity of the initial laser pulse is increased.

52. Use of the device according to any of claims 1 to 20, wherein the intensity profile of the initial laser pulse is improved.

53. Use of the device according to claims 1 to 20 in time resolved fluorescence spectroscopy or time resolved absorption spectroscopy or time-resolved emission spectroscopy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0172] FIG. 1 shows a device according to the invention.

[0173] FIG. 2 shows different embodiments of the beam forming section.

[0174] FIG. 3 shows different base caps.

[0175] FIG. 4 shows the arrangement for multiple reflections through the cell containing the nonlinear medium.

[0176] FIG. 5 shows the use of the beam forming section to account for high input energies of the incident laser pulse by varying the waist of the laser pulse.

[0177] FIG. 6 shows embodiments of a part of the invention using divergent laser beams.

[0178] FIG. 7 shows an embodiment to extract generated backward stimulated scattered Raman radiation.

[0179] FIG. 8 shows an embodiment of the invention used for white light continuum generation.

[0180] FIG. 9 shows temporal profiles of a laser pulse obtained by using stimulated Brillouin scattering as nonlinear interaction at different focal positions of the incident laser pulse for two different energies of the incident laser pulse.

[0181] FIG. 10 shows temporal profiles of a laser pulse obtained by using stimulated Brillouin scattering as nonlinear interaction for two different energies of the incident laser pulse in dependence of the positions of the lenses in the beam forming section.

[0182] FIG. 11 shows temporal profiles of a laser pulse obtained by using stimulated Brillouin scattering as nonlinear interaction in dependence of the pulse waist of the incident laser pulse.

[0183] FIG. 12 shows example spectral profiles of generated white light continuum in water using broadband input laser pulse.

[0184] FIG. 13 shows example spectral profiles of generated white light continuum in water using narrowband input laser pulse.

DETAILED DESCRIPTION OF THE DRAWINGS

[0185] In one embodiment the Invention is used to generate stimulated Brillouin scattering. FIG. 1 shows an embodiment of the invention used to generate stimulated Brillouin scattering. The embodiment comprises a tunable pulsed laser source (1). The laser source (1) generates a first laser beam which is directed to an alignment section (2) which comprises an alignment mirror. Said alignment mirror comprises two broadband laser mirrors (2a) and (2b) each having a high damage threshold. The alignment mirrors (2a) and (2b) are used to couple the first laser beam into an arrangement of a beam separation section, a beam forming section and a cuvette or cell (5). The beam separation section comprises a polarizer (i.e. cubic (Glan-Taylor), glass plate) (3a) and a tunable waveplate (3b).

[0186] The beam forming section comprises at least two lenses. In a preferred embodiment, three lenses, a concave lens (4a) (i.e. f=2 cm to 10 cm), a convex lens (4b) (i.e. f=5 cm to 50 cm), and a second convex lens (4e) (i.e. f=20 cm to 100 cm) are used. Further, the beam forming section of the embodiment consists of a lens system including at least 2 lenses with adjustable distance. In the preferred embodiment the use of at least 3 lenses with adjustable distances is recommended, due to higher degree of freedom i.e. independently adjusting focal position and beam waist. The beam forming section focuses the initial beam into the cell (5) which is filled with a pulse-compressing nonlinear medium. The generated back reflected SBS beam will be polarized again by the tunable waveplate (3b) such that the resulting polarization of the beam is polarized by 90 with regard to the first laser beam. The laser beam will leave the polarizer (3a) at a direction orthogonal to the direction of the incident laser pulse. A high-quality, tunable, shortened pulse is obtained.

[0187] The detailed function of the beam forming section (4) is depicted in FIG. 2. FIG. 2(a) illustrates one embodiment according to the invention of the beam forming section. The concave lens (4a) and the convex lens (4b) are movably mounted on a delay line (4d) and adjustable with regard to the optical path. The box (4d) around the lenses (4a) and (4b) illustrates that both lenses are moved with respect to lens (4e). Thus, the concave lens (4a) and the convex lens (4b) are moved in the direction of the optical path. The concave lens (4a) and the convex lens (4b) are mounted on the sliding delay line (4d) that can be moved in the direction of the optical path back and forth, to set distance L2. Further, the concave lens (4a) is movable with regard to the convex lens (4b), to set distance L1. For this purpose the concave lens (4a) is mounted on a separate sliding delay line which is illustrated by the box (4c) and can be moved in the direction of the optical path back and forth. By choosing proper values for L1 and L2, both the position of beam focus and beam waist at the entrance of the cell (5) containing the liquid can be set independently over a wide range. Additionally, a given range can be shifted up or down, if the beam waist is increased or decreased prior entering the beam forming section.

[0188] Furthermore the order of the lenses (4a), (4b) and (4e) can be changed according to the invention. In this case the range over which the position of the beam focus and beam waist can be set is increased. For example the order (4e), (4a) and (4b) is suitable which is illustrated in FIG. 2(b). In this embodiment convex lens (4e) is mounted on a sliding delay line and can be moved in the direction of the optical path with respect to the concave lens (4a), to set distance L1. The sliding delay line of lens (4e) is illustrated by the box (4c) around the convex lens (4e). Convex lens (4e) and concave lens (4a) are mounted on a sliding delay line illustrated by box (4d) and can be moved in the direction of the optical path with respect to the convex lens (4b) to set distance L2.

[0189] In another embodiment (shown in FIG. 2 (c)) of the beam forming section (4) the concave lens (4a) is mounted on a sliding delay line and can be moved in the direction of the optical path with respect to the convex lens (4b), to set distance L1. The sliding delay line of lens (4a) is illustrated by the box (4c) around the concave lens (4a). The convex lens (4e) is mounted on a sliding delay line too and can be moved in the direction of the optical path with respect to the convex lens (4b), to set distance L2. The sliding delay line of lens (4e) is illustrated by the box (4d) around the convex lens (4e).

[0190] According to the invention, the cell comprising the nonlinear medium has a high flexibility. At least one end cap can comprise optical elements, such as shown in FIG. 3. The outer and/or inner threads (12) of the base caps comprise an appropriate material such as Teflon. Regarding the front end cap suitable optical elements can be a window (13), a partially broadband high reflective coated window (14), a concave lens or a curved window (15), as shown in FIG. 3(a). Regarding the rear end cap as shown in FIG. 3(b) suitable optical elements are selected from the group consisting of a window (13), a broadband high reflective coated window (14), a filter (17) or a broadband high reflective coated concave lens or focusing mirror (16).

[0191] The beam forming section is also useful for larger initial pulse widths or if someone wants to use shorter cells due to limited space. Therefore, an arrangement to reflect the pulse multiple times through the cell can be used. For multiple reflections through the cell, i.e. setting very large focal positions with nods at the front and back end of the cell, the arrangements shown in FIG. 4 are used. For example in FIG. 4(a), a base cap window which is partially coated by a high reflective material (14) as front end (18) and a base cap mirror (22) as rear end (19) of the cell (5) comprising the nonlinear medium are used, to allow for multiple reflections through the cell (5). Therefore, for a cell with given length (i.e. 1 m), the focal position can be set over the whole space to nearly infinity except for focal positions close to the rear and front end of the cell (i.e. close to 1 m, 2 m, 3 m . . . ) as this would damage the optical windows. Another embodiment of this arrangement is shown in FIG. 4(b), wherein a base cap which is partially high reflectivity broadband coated in the middle (14b) is used as the front (18). A suitable base cap is shown in FIG. 4(d). The cap is partially covered with a circular high reflectivity coating, wherein the diameter of the coating is smaller in comparison to the diameter of the base cap. It is also possible that a base cap which is partially coated with a material with high reflectivity (14) is used as front end (18) and rear end (19), see FIG. 4(c). The arrangements shown in FIGS. 4(b) and 4(c) are especially useful for generation of white light continuum and/or stimulated Raman scattering. In case of white light continuum, which travels in the same direction as the incident laser beam, the arrangement shown in FIG. 4(b) is used, to couple out white light continuum which is generated after an odd number of reflections in the cell (i.e. 1, 3, 5 . . . reflections) through the front end (18). The arrangement shown in FIG. 4 (c) is used to couple out white light continuum which is generated after an even number of reflections in the cell (i.e. 2, 4, 6 . . . reflections) through the rear end (19). In case of stimulated Raman scattering, the arrangement shown in FIG. 4(b) can be used to couple out forward and backward stimulated Raman scattering which is generated after an odd number of reflections in the cell through the front end (18). The arrangement shown in FIG. 4(c) can be used to couple out stimulated Raman scattering which is generated after an even number of reflections in the cell. In the latter case, forward stimulated Raman scattering is coupled out at the rear end of the cell (19) and backward stimulated Raman scattering is coupled out at the front end of the cell (18).

[0192] In an embodiment of the invention, the beam forming section (4) is also useful for efficient compression of very high input energies >50 mJ/pulse. In this case a generator-amplifier setup is favorable. As stated by (Nori 1998), (Schiemann, Ubachs and Hogervorst 1997) and (Yoshida, et al. 2009), the temporal shape of SBS pulses also depends on the beam waist inside the amplifier part. Therefore, the lens system of the beam forming section can be used to collimate the beam at different waist over a wide range and in small steps to prevent SBS inside the amplifier part and to optimize the temporal shape of SBS beam. If the energy of the incident laser beam is changed, the beam waist can be adapted accordingly. The optimum beam waist is also dependent on the Brillouin gain of the solvent, which is dependent on wavelength of the incident laser pulse (see table 1). Thus the waist of the laser beam has to be adapted if the wavelength of the laser beam and/or the nonlinear medium is changed. FIG. 5 shows embodiments of the invention accounting for changes in the energy and/or wavelength of the initial laser pulse and/or the type of nonlinear medium. After passing a first cell (5) comprising a nonlinear medium, the laser beam is focused by a convex lens (7) into a second cell (8) containing a nonlinear medium as shown in FIG. 5(a). The nonlinear media in the first and in the second cell are equal. Another embodiment of the invention uses a focusing mirror (9) to redirect the laser pulse back into the cell (5) containing the nonlinear medium (FIG. 5(b)). Said focusing mirror (9) can also be integrated in the rear end cap of the cell (5) as shown in FIG. 5(c). Additionally said embodiments of the invention can be used to generate stimulated Brillouin scattering, stimulated Raman scattering or a combination thereof.

[0193] In an embodiment of the invention the lens system of the laser beam forming section can be used to produce a divergent beam, which is focused back into the same cell at the rear or can be focused into another cell, as illustrated in FIG. 6. Due to an often Gaussian-shaped temporal profile of the incident beam, the Stokes beam, generated in the focal region, faces different photon densities when traveling back through the cell. Especially, at the adjacent edge of the incident Gaussian-shaped beam, the Stokes beam faces a lower induced acoustic field and therefore amplification and temporal reshaping is decreased. In tapered waveguide geometry this situation is further declined due to the comparably large beam diameters in the front region of the cell. To compensate for the lower photon densities of the incident laser beam in the front region of the cell, a divergent beam is created via the beam forming section. Arrangements with different optical elements can be used as shown in FIG. 6. After passing a first cell (5) comprising a nonlinear medium, the laser beam is focused by a convex lens (7) into a second cell (8) containing a nonlinear medium as shown in FIG. 6(a). The nonlinear media in the first and in the second cell are equal. Another embodiment of the invention uses a focusing mirror (9) to redirect the laser pulse back into the cell (5) containing the nonlinear medium (FIG. 6(b)). Said focusing mirror (9) can also be integrated in the rear end cap of the cell (5) as shown in FIG. 6(c). Additionally said embodiments of the invention can be used to generate stimulated Brillouin scattering, stimulated Raman scattering or a combination thereof.

[0194] In a further embodiment the device and the method according to the invention can be used for generating stimulated Raman scattering. FIG. 7 shows the main parts of the setup for generating stimulated Raman scattering and extracting the scattered stimulated Raman radiation. The embodiment comprises a tunable pulsed laser source (1). The laser source (1) generates a first laser beam which is directed to an alignment mirror, which comprises one broadband laser mirror (2a) and one mirror with high reflectivity for the wavelength of the incident laser and low reflectivity for the Raman wavelength (2c), each having a high damage threshold. The alignment mirrors (2a) and (2c) are used to couple the first laser beam into the beam forming section and subsequently into a cuvette or cell (5). The beam forming section in the embodiment comprises 3 lenses, a concave lens (4a), a convex lens (4b) and a second convex lens (4e), wherein lens (4a) and (4b) are movable with regard to lens (4e) and lens (4a) is movable with regard to lens (4b). All 2 lenses are adjustable with regard to the optical path. Three lenses forming the beam forming section are preferred due to a higher degree of freedom i.e. independently adjusting focal position and beam waist. The beam forming section focuses the initial beam into the cell (5) which is filled with a nonlinear medium. The generation of the stimulated Raman scattering takes place in the focal region (6) inside of the cell (5). Thereby, by changing the distances of the movable lenses in the beam forming section the optimal position of the focal region inside of the cell (5) is adjustable. In FIG. 7 the backward Raman radiation leaves the cell (5) through the front end cap and is coupled out by mirror (2c).

[0195] In a further embodiment the device and the method of the invention can be used for generating a white light continuum. FIG. 8 shows the main parts of the setup for generating a white light continuum. The embodiment comprises a tunable pulsed laser source (1). The laser source (1) generates a first laser beam which is directed to an alignment mirror, which comprises two broadband laser mirrors (2a), (2b) each having a high damage threshold. The alignment mirrors (2a) and (2b) are used to couple the first laser beam into the beam forming section and subsequently into a cuvette or cell (5) which comprises the nonlinear medium. The beam forming section (4) in the embodiment comprises 3 lenses, a concave lens (4a), a convex lens (4b) and a second convex lens (4e), wherein lens (4a) and (4b) are movable with regard to lens (4e) and lens (4a) is movable with regard to lens (4b). All 2 lenses are adjustable with regard to the optical path. Three lenses forming the beam forming section are preferred due to a higher degree of freedom i.e. independently adjusting focal position and beam waist. The beam forming section focuses the initial beam into the cell (5) which is filled with a pulse-compressing nonlinear medium. The generation of the white light continuum takes place in the focal region (6) inside of the cell (5). By changing the distances of the movable lenses in the beam forming section (4) the optimal position of the focal region inside of the cell (5) is adjustable. The resulted white light is traveling in the same direction as the incident beam. Therefore, the generated white light continuum is filtered from the wavelength of the incident laser beam by a filter (20). Suitable filters are short pass, band pass or most preferred notch-filters. Spatial wavelength filtering, by using at least two prisms and blocking the wavelength of the incident laser pulse between said prisms, is also possible. Afterwards the generated white light continuum pulse is collimated by a lens/lens system (21). In another embodiment of the invention the beam which leaves the cell and consists of the generated white light continuum and the incident beam is collimated by a lens/lens system before the generated white light continuum is filtered from the wavelength of the incident beam.

EXAMPLES OF THE INVENTION

Example 1

[0196] The temporal shape of stimulated Brillouin scattering was measured in dependence of the focal position at input energies of 35 mJ/pulse (FIG. 9a) and 55 mJ/pulse (FIG. 9b). The graphs 9a and 9b show the normalized counts representing the intensity of the pulse in dependence of the time. The initial laser pulse was used with a wavelength of 570 nm and a pulse length of 5 ns. The lenses of the beam forming section were used in the order 4a, 4b, 4e with the following focal values: 4a=7.5 cm, 4b=20 cm, 4e=50 cm. Distance L2 was fixed at 20 cm and L1 was varied between 9-13 cm. Water was used as nonlinear medium, which was filtered by a 400 nm pore size filter to increase the purity of the nonlinear medium.

[0197] At this conditions the order 4a, 4b, 4e allows setting the focal position from 50 cm to nearly infinity while having a constant increased beam waist factor of 2.67 at the entrance of the cell containing the nonlinear medium. In the examples shown, the beam waist at the front window of the cell containing the nonlinear medium was approx. 1.5 cm. The optimal compression is achieved at L1=9 cm at 35 mJ/pulse input energy and L1=8 cm at 55 mJ/pulse input energy, corresponding to focal positions of 160 and 190 cm, respectively.

[0198] It can be seen that the temporal shape of the compressed pulse depends on the focal position as well as on the input energy of the laser pulse. The beam forming section is necessary to obtain optimal pulse shapes, meaning nearly Gaussian-shaped pulses. The distances of the lenses in the beam forming section have to be adjusted in dependence of the input energy of the initial pulse. The pulse length of the initial pulse was 5 ns and is clearly shortened to 2 ns.

Example 2

[0199] Temporal profiles of a laser pulse obtained by using stimulated Brillouin scattering as nonlinear interaction with input energies of the incident laser pulse of 10 mJ/pulse and 40 mJ/pulse, in dependence of the order of the lenses in the beam forming section, were measured. FIG. 10 shows the normalized counts representing the intensity of the pulse in dependence of the time. The initial laser pulse was used with a wavelength of 570 nm and a pulse length of 5 ns. On the one hand the lenses of the beam forming section were used in the order 4a, 4b, 4e with the following distances L1=10 cm and L2=20 cm and on the other hand in the order 4e, 4a, 4b with the distances L1=28 cm and L2=14 cm. The focal position is for both conditions approximately 105 cm. Water was used as nonlinear medium, which was filtered by a 400 nm pore size filter to increase the purity of the nonlinear medium.

[0200] At low energies the lens order 4a, 4b, 4e lead to a poor temporal beam profiles. To gain optimal compression the beam waist has to be decreased from 1.5 cm (dashed lines in FIG. 10) to 0.5 cm (solid lines in FIG. 10). Therefore, the order 4e, 4a, 4b is used. At these conditions pulses with higher energy are less compressed than low energy pulses.

Example 3

[0201] Temporal profiles of a laser pulse obtained by using stimulated Brillouin scattering as nonlinear interaction in dependence of the pulse waist of the initial laser pulse. The input energy of the laser pulses was 45 mJ/pulse, the distances L1 and L2 were adjusted in a way to maintain a constant focal position of 60 cm while varying the beam waist at the entrance of the cell. Water was used as nonlinear medium, which was filtered by a 400 nm pore size filter to increase the purity of the nonlinear medium. The pulse waist was varied between 0.8 cm and 2.4 cm. FIG. 11 shows the normalized counts representing the intensity of the pulse in dependence of the time. The dependence of the temporal shape of the compressed pulses on the waist of the initial laser pulse is clearly visible.

Example 4

[0202] FIG. 12 and FIG. 13 show a white light continuum obtained by the method according to the invention by using broadband 566 nm input (30 mJ, width=10 nm) and smallband 564 nm input (45 mJ, width <0.01 nm), respectively. The spectra were measured by using a Notch filter with 564 nm center wavelength and 13 nm width and normalized to the intensity at the red wing of the Notch filter. The spectra shown by the solid lines were obtained by using the lens order 4a, 4b, 4e with focal values: 4a=7.5 cm, 4b=20 cm, 4e=50 cm. For these graphs, distance L2 was set to 39 cm and L1 was set to 9 cm for both figures. The spectra shown by the dashed lines were obtained by using the lens order 4e, 4a, 4b and setting L2=18 cm and L1=31 cm for both figures. In the inset of FIG. 12 it can be seen, that in the spectral region around the pumping wavelength the lens order 4e, 4a, 4b leads to a higher signal, whereas the lens order 4a, 4b, 4e gives rise to a slightly broader spectrum, and is therefore better suitable (higher conversion efficiency of the pumping wavelength). However, when using smallband input (<0.01 nm) the same lens order 4a, 4b, 4e (L2=39 cm; L1=9 cm) leads to a very poor performance of the white light continuum, as shown by the solid lines in FIG. 13. In this case the lens order 4e, 4a, 4b (L2=18 cm; L1=31 cm) is suitable (dashed lines in FIG. 13). In the inset of FIG. 13 it can be seen that the improved white light continuum performance is accompanied by strong stimulated Raman generation around 700 nm.

REFERENCES

[0203] Brandi, F., I. Velchev, D. Neshev, W. Hogervorst, and W. Ubachs. A narrow-band wavelength tunable laser system delivering high-energy 300 ps pulses in the near-infrared. Review of Scientific Instruments, 2003. [0204] Bryce-Smith, D. Photochemistry. The chemistry society, 1979. [0205] Damzen, M. J., V. I. Vlad, V. Babin, and A. Mocofanescu. Stimulated Brillouin Scattering Fundamentals and Applications. 2003. [0206] He, G., and S. Liu. Physics in Nonlinear Optics. 1999. [0207] Kong, H. J., S. K. Lee, J. W. Yoon, J. S. Shin, and S. Park. Stimulated Brillouin scattering phase conjugate mirror and its application to coherent beam combined laser system producing a high energy, high power, high beam quality and high repetition rate output. In Advances in Lasers and Electro Optics, 838. 2010. [0208] Nori, J. Development of a laser-pulse compression device based on stimulated Brillouin scattering. University of Lund, 1998. [0209] Rikknen, E., G. Genty, O. Kimmelma, and M. Kaivola. Supercontinuum generation by nanosecond dual-wavelength pumping in microstructured optical fibres. Optics Express, 2006. [0210] Schiemann, S., W. Ubachs, and W. Hogervorst. Efficient temporal compression of coherent nanosecond pulses in a compact SBS generator-amplifier setup. Vol. 33. no. 3. 1997. IEEE Journal of quantum electronics, 1997, 33 ed. [0211] Somekawa, T., N. Manago, H. Kuze, and M. Fujita. Differential optical absorption spectroscopy measurements of CO2 using nanosecond white light continuum. Optical Letters, 2011: 4782-4784. [0212] Spaulding, D. K., R. Jeanloz, B. A. Remington, D. G. Hicks, and G. W. Collins. Nanosecond Broadband Spectroscopy for Laser-Driven Compression Experiments. [0213] Sutherland, R. L. Handbook of Nonlinear Optics. 2003. [0214] Veltchev, L. T. Stimulated Brillouin scattering pulse compression and harmonic generation: Applications to precision xuv laser spectroscopy. Vrije Universiteit Amsterdam, 2009. [0215] Wong, A. C. Experimental study of stimulated Brillouin scattering in open cells and multimode optical fibres. University of Adelaine, 2005. [0216] Xu, X. High power pulse UV source development and its applications. University of New Mexico, 2014. [0217] Yoshida, H., T. Hatae, H. Fujita, Nakatsuka N., and S. Kitamura. A high-energy 160 ps pulse generation by stimulated Brillouin scattering from heavy fluorocarbon liquid at 1064 nm wavelength. Optics Express, 2009.

REFERENCE NUMBERS

[0218] 1 laser source [0219] 2 alignment section [0220] 2a mirror [0221] 2b mirror [0222] 2c mirror [0223] 3 beam separation section [0224] 3a polarizer [0225] 3b waveplate [0226] 4 beam forming section [0227] 4a concave lens [0228] 4b convex lens [0229] 4e convex lens [0230] 5 cell [0231] 6 focal region [0232] 7 convex lens [0233] 8 second cell [0234] 9 focusing mirror [0235] 12 outer/inner thread of the base cap [0236] 13 window [0237] 14 partially broadband high reflectivity coated window [0238] 14b partially broadband high reflectivity coated window [0239] 15 curved window [0240] 16 focusing mirror [0241] 17 filter (base cap filter) [0242] 18 front end of the cell [0243] 19 rear end of the cell [0244] 20 filter [0245] 21 lens [0246] 22 fully broadband high reflective coated window (i.e. mirror)