METHOD AND DEVICE FOR CAVITY-ENHANCED BROADBAND INTRAPULSE DIFFERENCE FREQUENCY GENERATION

Abstract

A method of creating difference frequency (DF) laser pulses (1) by difference frequency generation (DFG) comprises the steps of providing ultrashort laser pulses (2) having a spectral bandwidth corresponding to a Fourier limit of below 50 fs and containing first spectral components and second spectral components having larger frequencies than the first spectral components, and driving a DFG process by the ultrashort laser pulses (2) in an optically non-linear crystal (10), wherein the DF laser pulses (1) are generated in the crystal (10) by difference frequencies between the first and second spectral components, resp., said difference frequencies comprising third spectral components being lower in frequency than the first and second spectral components, wherein at least one enhancement cavity (20) with resonator mirrors (Mil to Ml4) spanning a beam path (22) is provided and the crystal (10) is placed in the beam path (22) of the enhancement cavity (20), the ultrashort laser pulses (2) are input coupled and coherently added in the at least one enhancement cavity (20), at least one circulating ultrashort laser pulse (3) is created in the at least one enhancement cavity (20), which drives the DFG process in the crystal (10) for generating the DF laser pulses (1), and the at least one enhancement cavity (20) is adapted for recycling the at least one ultrashort laser pulse (3) passing through the crystal (10). Furthermore, a photonic source (100) for creating DF laser pulses (1) is described, including one or more enhancement cavities.

Claims

1. A method of creating difference frequency (DF) laser pulses by difference frequency generation (DFG), comprising the steps of providing a first enhancement cavity with first resonator mirrors spanning a first beam path, providing ultrashort laser pulses having a spectral bandwidth corresponding to a Fourier limit of below 50 fs and simultaneously containing first spectral components and second spectral components having larger frequencies than the first spectral components, input coupling and coherently adding of the ultrashort laser pulses in the first enhancement cavity, driving a DFG process by the ultrashort laser pulses in a first optically non-linear crystal being place in the first beam path of the first enhancement cavity, wherein the DF laser pulses are created in the first optically non-linear crystal by difference frequencies between the first and second spectral components, respectively, said difference frequencies comprising third spectral components being lower in frequency than both of the first and second spectral components, wherein at least one circulating ultrashort laser pulse is created in the first enhancement cavity, which drives the DFG process in the first optically non-linear crystal for generating the DF laser pulses, and the first enhancement cavity is configured for recycling the at least one ultrashort laser pulse passing through the first optically non-linear crystal, and output coupling of the DF laser pulses from the first enhancement cavity.

2. The method according to claim 1, wherein the step of output coupling of the DF laser pulses comprises reflecting the DF laser pulses out of the first beam path at a reflecting dichroic surface on a rear side of the first optically non-linear crystal and an antireflective coating on a front side of the first optically non-linear crystal.

3. The method according to claim 1, wherein the step of output coupling of the DF laser pulses comprises transmitting the DF laser pulses of the first beam path through a dichroic surface of the first resonator mirror downstream of the first optically non-linear crystal.

4. The method according to claim 1, wherein the step of output coupling of the DF laser pulses comprises reflecting the DF laser pulses out of the first beam path at a surface of a dichroic plate arranged in the first enhancement cavity.

5. The method according to claim 1, wherein the step of output coupling of the DF laser pulses comprises relaying a divergent portion of the DF laser pulses over one of the first resonator mirrors or an auxiliary mirror with a hole transmitting the circulating ultrashort laser pulse in the first enhancement cavity.

6. The method according to claim 1, wherein the step of providing the ultrashort laser pulses comprises generating the ultrashort laser pulses with one single laser source.

7. The method according to claim 1, wherein the step of providing the ultrashort laser pulses comprises suppressing spectral components of the ultrashort laser pulses, which do not contribute to the DFG process.

8. The method according to claim 1, including a step of adjusting a polarization of the first spectral components relative to a polarization of the second spectral components of the ultrashort laser pulses, so that the polarizations of the first and second spectral components are parallel or perpendicular relative to each other.

9. The method according to claim 8, wherein the polarizations of the first and second spectral components are adjusted before input coupling the ultrashort laser pulses into the first enhancement cavity.

10. The method according to claim 1, including a step of placing a second optically non-linear crystal in the first beam path of the first enhancement cavity, the second optically non-linear crystal being configured for compensating for an ellipticity of a polarization of the light field circulating in the first enhancement cavity, said ellipticity being generated in the first optically non-linear crystal.

11. The method according to claim 10, wherein the second optically non-linear crystal is further configured for creating further difference frequencies within the third spectral component by difference frequency generation.

12. The method according to claim 1, including further steps of providing one or more further enhancement cavities spanning one or more further beam paths, wherein each of the one or more further enhancement cavities is arranged relative to the first enhancement cavity such that the first optically non-linear crystal is placed at an intersection of all of the first and the one or more further beam paths of the first and the one or more further enhancement cavities, providing two or more pulse portions by wavelength selective splitting of the ultrashort laser pulses, wherein spectral content of the first and second spectral components is distributed into the two or more pulse portions, input coupling each of the two or more pulse portions into one of the first and the one or more further enhancement cavities and coherently adding the two or more pulse portions to at least two circulating ultrashort laser pulses each of which circulating in one of the first and the one or more further enhancement cavities, respectively, wherein the polarizations of the two or more pulse portions are adjusted such that the two or more circulating ultrashort laser pulses in the first and the one or more further enhancement cavities have different polarizations at the first optically non-linear crystal, and the DFG process is driven in the first optically non-linear crystal by the two or more circulating ultrashort laser pulses.

13. The method according to claim 12, wherein two or more further enhancement cavities are provided, wherein each of the first and the two or more further enhancement cavities is arranged for coherently adding pulse portions with different spectral content and different polarizations.

14. The method according to claim 1, wherein the ultrashort laser pulses have a spectral bandwidth corresponding to a Fourier limit of below 50 fs.

15. The method according to claim 1, wherein the DF laser pulses have a center wavelength in a range of 4 μm to 20 μm.

16. The method according to claim 1, wherein the first optically nonlinear crystal has a thickness in a range of 5 μm to 5 mm.

17. The method according to claim 1, wherein the first optically nonlinear crystal is in direct contact with one of the resonator mirrors.

18. The method according to claim 1, wherein a temperature of the first optically non-linear crystal is set by one of the first resonator mirrors or a temperature-control support stage, wherein the first optically non-linear crystal is optically contacted or attached to the one of the first resonator mirrors or the temperature-control support stage.

19. The method according to claim 1, wherein an adjusting support stage of the first optically non-linear crystal is controlled for adjusting an orientation of the first optically non-linear crystal with respect to the first beam path of the first enhancement cavity.

20. The method according to claim 1, wherein the DF laser pulses are created with a spectral width of at least 500 nm.

21. The method according to claim 1, wherein the first enhancement cavity is operated in an evacuated environment.

22. A photonic pulse source, configured for creating difference frequency (DF) laser pulses by difference frequency generation (DFG), comprising a first optically non-linear crystal being arranged for creating the DF laser pulses by a DFG process driven with ultrashort laser pulses having a spectral bandwidth corresponding to a Fourier limit of below 50 fs and simultaneously containing first spectral components and second spectral components having larger frequencies than the first spectral components, wherein the DF laser pulses can be created by difference frequencies between the first and second spectral components, said difference frequencies comprising third spectral components being lower in frequency than both of the first and second spectral components, and a first enhancement cavity with first resonator mirrors spanning a first beam path, wherein the first optically non-linear crystal is placed in the first beam path, one of the first resonator mirrors is arranged for input coupling of the ultrashort laser pulses into the first enhancement cavity, and the first enhancement cavity further includes an output coupling component being arranged for output coupling of the DF laser pulses from the first enhancement cavity.

23. The photonic pulse source according to claim 22, wherein the output coupling component comprises a reflecting dichroic surface being arranged on a rear side of the first optically non-linear crystal out of the first beam path and an antireflective coating being arranged on a front side of the first optically non-linear crystal, a dichroic surface of one of the first resonator mirrors downstream of the first optically non-linear crystal, or a surface of a dichroic plate arranged in the first enhancement cavity.

24. The photonic pulse source according claim 22, further including one single laser source being arranged for providing the ultrashort laser pulses.

25. The photonic pulse source according to claim 22, further including a spectral shaping component being configured for suppressing spectral components of the ultrashort laser pulses, which do not contribute to the DFG process.

26. The photonic pulse source according to claim 22, further including an outer polarizer component being arranged for adjusting polarizations of the first and second spectral components before input coupling the ultrashort laser pulses into the first enhancement cavity.

27. The photonic pulse source according to claim 22, further including a second optically non-linear crystal being placed in the beam path of the first enhancement cavity, wherein the second optically non-linear crystal is configured for a rotation of the polarization of the light field circulating in the first enhancement cavity.

28. The photonic pulse source according to claim 22, further including one or more further enhancement cavities spanning one or more further beam paths, wherein the one or more further enhancement cavities are arranged relative to the first enhancement cavity such that the first optically non-linear crystal is placed at an intersection of all of the first and the one or more further beam paths of the first and the one or more further enhancement cavities, at least one dichroic beam splitter being arranged for providing wavelength selective splitting of the ultrashort laser pulses into two or more pulse portions, wherein spectral content of the first and second spectral components is distributed into the two or more pulse portions, and at least one polarizer component being arranged for adjusting a polarization of at least one of the two or more pulse portions, wherein the first and the one or more further enhancement cavities are arranged for input coupling of each of the two or more pulse portions into one of the first and the one or more further enhancement cavities and coherently adding the two or more pulse portions to two or more circulating ultrashort laser pulses each of which circulating in one of the first and the one or more further enhancement cavities, respectively, the at least one polarizer component is configured for adjusting polarizations of the two or more pulse portions such that the two or more circulating ultrashort laser pulses in the first and the one or more further enhancement cavities have different polarizations at the first optically non-linear crystal, and the first optically non-linear crystal is arranged such the DFG process can be driven therein by the two or more circulating ultrashort laser pulses.

29. The photonic pulse source according to claim 28, including two or more further enhancement cavities, wherein each of the first and the two or more further enhancement cavities is arranged for coherently adding pulse portions with different spectral content and different polarizations.

30. A photonic pulse source, configured for creating difference frequency (DF) laser pulses by difference frequency generation (DFG) according to the method of claim 1, comprising a first optically non-linear crystal being arranged for creating the DF laser pulses by a DFG process driven with ultrashort laser pulses having a spectral bandwidth corresponding to a Fourier limit of below 50 fs and simultaneously containing first spectral components and second spectral components having larger frequencies than the first spectral components, wherein the DF laser pulses can be created by difference frequencies between the first and second spectral components, said difference frequencies comprising third spectral components being lower in frequency than both of the first and second spectral components, and a first enhancement cavity with first resonator mirrors spanning a first beam path, wherein the first optically non-linear crystal is placed in the first beam path, one of the first resonator mirrors is arranged for input coupling of the ultrashort laser pulses into the first enhancement cavity, and the first enhancement cavity further includes an output coupling component being arranged for output coupling of the DF laser pulses from the first enhancement cavity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Further details and advantages of the invention are described in the following with reference to the attached drawings, which show in:

[0060] FIG. 1: a schematic illustration of a first embodiment of a photonic pulse source according to the invention, including one single enhancement cavity;

[0061] FIGS. 2 to 5: schematic illustrations of various techniques for output coupling MIR pulses out of the enhancement cavity;

[0062] FIG. 6: a schematic illustration of an optically non-linear crystal directly coupled with one of the resonator mirrors of the enhancement cavity; and

[0063] FIG. 7: a schematic illustration of a second embodiment of a photonic pulse source according to the invention, including two enhancement cavities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] Preferred embodiments of the invention are described in the following with particular reference to an arrangement of at least one optically non-linear crystal in one or more enhancement cavities, driving the DFG process with fs laser pulses in the enhancement cavity, controlling the polarization of the fs laser pulses circulating in the enhancement cavity and coupling the created DF laser pulses out of the beam path of the enhancement cavity. Details of creating and manipulating the driving fs laser pulses are not described as far as they are known from prior art (see e.g. [2], [4] and [12]). It is emphasized that the practical implementation of the invention is not restricted to the described examples, but rather possible with modifications, e.g. in terms of the number of enhancement cavities, the available types of optically non-linear crystals (see [13]), the number of circulating ultrashort laser pulses, and the design of the enhancement cavity. Most of the illustrated embodiments show the optically non-linear crystal with a distance from the resonator mirrors. Alternatively, the optically non-linear crystal can be positioned in direct contact with one of the resonator mirrors (see FIG. 6).

[0065] FIG. 1 illustrates a first embodiment of a photonic pulse source 100 comprising a first optically non-linear crystal 10, an enhancement cavity 20, a second optically non-linear crystal 30, and a laser source 40. Additionally, the photonic pulse source 100 is provided with a control device (not shown) and a monitoring device with at least one optical sensor (not shown), as they are known form conventional enhancement cavity techniques, e.g. for controlling the repetition frequency driving laser source, the input coupling of the driving ultrashort laser pulses and the positions of cavity mirrors and monitoring pulse parameters.

[0066] The laser source 40 comprises a fs laser 41 and optionally a spectral shaping component 42. The fs laser 41 is adapted for providing ultrashort laser pulses 2 having e.g. a centre wavelength of 1030 nm, an average power of 50 W and a repetition rate of 100 MHz. The fs laser source 41 may comprise e.g. a commercial apparatus, like a Kerr-lens modelocked Yb:YAG thin-disk laser or a chirped-pulse amplified master oscillator or an optical set-up with a combination of a thin-disk oscillator and one or two broadening stages as described in [2] or [12].

[0067] The spectral shaping component 42 is arranged for shaping the spectrum of the ultrashort laser pulses 2. Spectral components, which do not contribute to the generation of useful radiation of the third spectral component in the optically non-linear crystal 10 are attenuated or even completely suppressed. To this end, the spectral shaping component 42 comprises e.g. a notch filter (in reflection or transmission) having a preselected filter characteristic depending on the spectral components to be suppressed.

[0068] The enhancement cavity 20 comprises four resonator mirrors M.sub.11 to M.sub.14, which are arranged to span a beam path 22 for a circulating laser pulse 3. Preferably, the beam path 22 extends in a plane parallel to the drawing plane. The length of the beam path 22 is selected such that the period of the circulating laser pulse 3 is equal to the reciprocal repetition frequency of the laser source 40 or an integer multiple of the latter. The ultrashort laser pulses 2 generated with the laser source 40 are coupled into the enhancement cavity 20 at one of the resonator mirrors M.sub.13, which has a slightly reduced reflectivity, e.g. 99%, compared with the remaining resonator mirrors. The resonator mirrors M.sub.11 to M.sub.14 have plane or curved mirror surfaces, as it is known from conventional enhancement cavities. Curved mirror surfaces can be used for tailoring the field distribution of the circulating laser pulse 3, e.g. for focussing the circulating laser pulse 3 at predetermined focus positions along the beam path 22, and/or for improving the optical stability of the pulse enhancement in the cavity.

[0069] The first optically non-linear crystal 10 is arranged in a first section of the beam path 22 between two of the resonator mirrors M.sub.13 and M.sub.14, preferably at a focus position of the beam path 22. With a practical example, the optically non-linear crystal 10 is made of LiGaS.sub.2, having a thickness along the beam path direction of 100 μm. The optically non-linear crystal 10 is supported by an adjusting and/or temperature-control support stage 13, 14 (see FIGS. 2, 3), which is adapted for geometrically adjusting and/or cooling the optically non-linear crystal 10.

[0070] Additionally, a second optically non-linear crystal 30 is arranged in another section of the beam path 22, preferably also at a position between two resonator mirrors M.sub.11 and M.sub.12, where the transverse intensity profile of the circulating laser beam 22 is widened to mitigate non-linearity and thermal lensing. The second optically non-linear crystal 30 is arranged for compensating an ellipticity of the polarization introduced by the interaction of the circulating laser pulse 3 with the first optically non-linear crystal 10. Preferably, the second optically non-linear crystal 30 has a thickness like the first crystal 10 or a different thickness, e.g. 200 μm.

[0071] The DFG process is driven by the circulating laser pulse 3 in the first optically non-linear crystal 10. The DF laser pulses 1 are generated with frequency components (third frequency components) created by difference frequencies between first and second spectral components of the circulating laser pulse 3, respectively. The DFG process requires that a phase matching condition is fulfilled by the first and second frequency components in the optically non-linear crystal 10. Various types of phase matching conditions are available. According to preferred embodiments of the invention, the first and second frequency components have polarizations perpendicular to each other (type I phase matching) or parallel to each other (type II phase matching).

[0072] The two mutually perpendicular polarizations for providing the type I phase matching can be created even with the first and second frequency components being simultaneously contained in the single circulating laser pulse 3, e.g. by using elliptic polarization or by constructing the circulating laser pulse 3 from two partial pulses (see FIG. 7).

[0073] The second optically non-linear crystal 30 is preferably provided with an embodiment using the type I phase matching. Both of the first and second spectral components having mutually perpendicular polarizations are subjected to different refractive indices in the optically non-linear crystal 10, which has an effect like a quarter wave plate. This results in a circular polarization of the circulating laser pulse 3 after the passage through the optically non-linear crystal 10. By the effect of the second optically non-linear crystal 30, this effect is compensated. To this end, the second optically non-linear crystal 30 preferably is located at a position where the transverse intensity profile of the circulating laser pulse 3 has a diameter significantly larger than on the first crystal 10. Accordingly, the second optically non-linear crystal 30 is not used if the DFG is based on the type II phase matching.

[0074] Additionally, the second optically non-linear crystal 30 can be capable of creating further difference frequencies by a DFG process. In particular, the orientation and the material of the second optically non-linear crystal 30 can be selected such that the above compensation of the polarization effect of the first optically non-linear crystal 10 is incomplete, but phase matching conditions for a further DFG process are fulfilled. Accordingly, a second DF laser pulse can be generated at the second optically non-linear crystal 30 (not shown in FIG. 1).

[0075] The polarizations of the first and second spectral components can be adjusted before input coupling the ultrashort laser pulses 2 into the enhancement cavity 20, e.g. by introducing a wavelength-selective beam splitter, distributing the output of the fs laser 41 onto two different beam paths, rotating the polarization in at least one of the beam paths with at least one polarizing components and recombining both portions into a common fs laser pulse 2 (not shown in FIG. 1). Alternatively, the fs laser pulses 2 at the output of the fs laser 41 can be polarized with one single polarizing component rotating the polarization of the complete spectrum of the fs laser pulses 2 onto an intermediate angle. With this embodiment, the type I phase matching can be obtained by rotating the first and second optically non-linear crystals 10, 30, respectively.

[0076] The DF laser pulses 1 created in the first optically non-linear crystal 10 are emitted collinearly with the beam path 22 direction within the enhancement cavity 20, but with an increased divergence. Accordingly, an output coupling around one of the resonator mirrors M.sub.14 as shown in FIG. 1 or in particular according to one of the variants shown in FIGS. 2 to 5 can be provided.

[0077] With a modification of FIG. 1, only one optically non-linear crystal 10 can be provided. The second optically non-linear crystal 30 is not strictly necessary if a certain degree of the conversion efficiency of the DFG process can be tolerated in dependency on the particular application of the invention.

[0078] The intensity generated in the DFG process with input at the two high frequencies is proportional to z.sup.2 sinc.sup.2(z/z.sub.coh), where z and z.sub.coh are the crystal thickness and the coherence length respectively. The coherence length is given by z.sub.coh=2/Δk, where Δk=wavevector mismatch. Writing the above expression as z.sub.coh.sup.2 (z/z.sub.coh).sup.2 sinc.sup.2 (z/z.sub.coh) it results that for a short coherence length the DFG-efficiency becomes very low. The function (z/z.sub.coh).sup.2 sinc.sup.2(z/z.sub.coh) has a maximum of one at z=1.6 z.sub.coh and thus the optimum crystal thickness is slightly larger than the coherence length.

[0079] For monochromatic or narrowband radiation the choice of a suitable phase matching angle generates a Δk close to zero, resulting in a large coherence length. However, with broadband radiation there is no unique phase matching angle and z.sub.coh rapidly decreases with increasing bandwidth. The optimum length of the crystal depends on many parameters, such as the particular crystal, the bandwidth and the choice of phase matched frequencies.

[0080] FIG. 2 shows a portion of the enhancement cavity 20 with one of the resonator mirrors M.sub.14 and the first optically non-linear crystal 10 only. The resonator mirror M.sub.14 is a dichroic mirror that transmits the third spectral components of the DF laser pulses 1, in particular in the MIR wavelength range, and reflects the first and second spectral components of the circulating laser pulse 3 along the beam path 22. Accordingly, output coupling of the DF laser pulses 1 is obtained by passing them through one of the resonator mirrors, e.g. M.sub.14.

[0081] Additionally, FIG. 2 schematically shows an adjusting support stage 13, which accommodates the first optically non-linear crystal 10. The adjusting support stage 12 comprises e.g. a piezoelectric x-y-z- and tip-tilt drive, which is capable of adjusting an orientation and position of the first optically non-linear crystal 10 relative to the first beam path 22.

[0082] As a general important feature of the invention, the angle of incidence of the circulating laser pulse 3 on the beam path 22 relative to the surface of the optically non-linear crystal 10 influences the phase matching. As the phase matching condition depends on the crystallographic orientation of the optically non-linear crystal 10, an adjustment of the geometrical orientation and position of the optically non-linear crystal 10 relative to the beam path 22 is implemented. Preferably, the adjustment is controlled with the adjusting support 13. As an example, a feedback control can be provided. A control variable can be obtained with an optical sensor (not shown), monitoring the spectral characteristic of the DF laser pulses 1. In dependency on the control variable, the crystal adjustment can be controlled such that a certain spectral range of the emitted DF pulses 1 is obtained. Alternatively, an optimum emission spectrum within a broader spectral range can be selected. With a thicker optically non-linear crystal, which creates a narrow bandwidth of the DF laser pulses 1, the wavelength range of the DF laser pulses 1 can be tuned by the adjusting support 13.

[0083] According to FIG. 3, the DF laser pulses 1 are coupled out of the enhancement cavity 20 by using the larger divergence of the DF laser pulses 1 compared with the transverse mode size of the circulating light pulse 3 (as shown in FIG. 1). One of the cavity mirrors M.sub.14 downstream from the first optically non-linear crystal 10 has a diameter, which is comparable to the 1/e.sup.2-intensity diameter of the transverse mode of the circulating laser pulse 3 at this mirror M.sub.14. The DF laser pulses 1 can pass outside the mirror M.sub.14, so that a ring-shaped output is created.

[0084] Additionally, FIG. 3 schematically shows a temperature-control support stage 14, which is adapted for adjusting the temperature of the first optically non-linear crystal 10, e.g. cooling with Peltier elements or with water or heating with a heating coil. According to a further variant of the invention, both of the adjusting support stage 13 (FIG. 2) and the temperature-control support stage 14 (FIG. 3) can be provided by a common controllable crystal holder.

[0085] According to a further embodiment of the invention, an auxiliary mirror 21 having a through hole 23 can be arranged in the enhancement cavity as schematically illustrated in FIG. 4. The DF laser pulses 1 created in the first optically non-linear crystal 10 with a certain divergence are reflected at the auxiliary mirror 21, while the through hole 23 is adapted for passing the circulating laser pulse 3 along the beam path 22. The reflected DF laser pulses 1 (reflected MIR radiation) has a ring shape similar to the embodiment in FIG. 3.

[0086] According to a further embodiment of the invention, as shown in FIG. 5, the DF laser pulses 1 can be output coupled from the enhancement cavity by using a reflecting dichroic surface 12 on the rear side of the first optically non-linear crystal 10 relative to the travel direction of the circulating laser pulse 3 and an antireflective coating 15 for all first second and third spectral components on the front side of the first optically non-linear crystal 10. The circulating laser pulse 3 arrives at the front side 11 of the first optically non-linear crystal 10, which is provided with the antireflective coating 15 adapted for a transmission of all of the first, second and third spectral components. In the optically non-linear crystal 10, the third components are generated, which are back-reflected at the dichroic surface coating 12 at the rear side of the crystal 10. Accordingly, the DF pulses 1 are reflected out of the beam path 22.

[0087] FIGS. 2 and 3 show the first optically non-linear crystal 10 arranged on adjusting and/or temperature-control support stage 13, 14. According to an alternative embodiment of the invention the crystal 10 can be in direct contact with one of the resonator mirrors, e.g. resonator mirror M.sub.14, as shown in FIG. 6. A plane or curved surface of the crystal 10 contacts a plane or curved surface of the resonator mirror. The direct contact fulfils a double function in terms of mechanically supporting and tempering, in particular cooling, the crystal 10.

[0088] FIG. 7 schematically illustrates a second embodiment of the inventive photonic pulse source 100, which comprises the laser source 40, the first enhancement cavity 20, the first optically non-linear crystal 10 and additionally a second enhancement cavity 50. Although the first and second enhancement cavities 20, 50 are shown with plane resonator mirrors FIG. 7, a combination of plane and/or curved resonator mirrors can be used in practice.

[0089] With the embodiment of FIG. 7, each of the first and second enhancement cavities 20, 50 is arranged for coherent addition of one of the first and second spectral components of the ultrashort laser pulses, resp. Furthermore, the first and second enhancement cavities 20, 50 are coupled such that the beam path 22 (dashed line) of the first enhancement cavity 20 intersects the beam path 52 (drawn line) of the second enhancement cavity 50, wherein the first optically non-linear crystal 10 is arranged at the intersection of both paths 22, 52, and it is adapted for a DFG process with type I phase matching.

[0090] For separating the first and second spectral components of the ultrashort laser pulses 2 generated with the fs laser 41, the laser source 40 is provided with a dichroic beam splitter 43, which spatially separates e.g. the first spectral components (lower energy) from the second spectral components (higher energy) of the ultrashort laser pulses 2. On a first beam path 44, the first spectral components pass a polarizing component 45, e.g. a polarizing filter, which creates a first polarization direction (e.g. perpendicular to the first beam path 44 and parallel to the plane of drawing). This first pulse portion 2A of the ultrashort laser pulses 2 is coupled into the first enhancement cavity 20 via one of the resonator mirrors M.sub.11.

[0091] In the second beam path 46 created by the beam splitter 43, a second polarizing component 47, e.g. a periscope of half wave plate, is arranged, which creates a polarization direction of the second spectral components perpendicular to the polarization direction of the first spectral components (e.g. perpendicular to the second beam path 46 and perpendicular to the plane of drawing). Additionally, the second beam path 46 includes a delay unit 48 comprising multiple plane delay mirrors DL.sub.1 to DL.sub.4, which are movable relative to each other. The second pulse portion 2B of the ultrashort laser pulses 2 comprising the second spectral components is coupled via one of the reflector mirrors M.sub.21 into the second enhancement cavity 50. With the delay unit 48, the mutual temporal relationship of the first pulse portion 2A and the second pulse portion 2B of the ultrashort laser pulses 2 can be adjusted. The first and second beam paths 44, 46 can be modified by omitting one of the first and second polarizing components 45, 47. If the ultrashort laser pulses 2 generated by the fs laser 41 have a linear polarization, it is sufficient to provide one polarizing component, e.g. the polarizing component 47 only.

[0092] Both of the first and second pulse portions 2A, 2B of the ultrashort laser pulses 2 are coherently added in the first and second enhancement cavities 20, 50, respectively to first and second circulating pulses 3A, 3B, respectively. The temporal relationship of both circulating pulses 3A, 3B is adjusted with the delay unit 48 such that they are coherently superimposed in the first optically non-linear crystal 10 for driving the DFG process therein. The DF laser pulses 1 are emitted under a certain angle with respect to the surface of the first optically non-linear crystal 10, e.g. through a spacing between resonator mirrors.

[0093] The embodiment of FIG. 7 can be modified by coupling not only two enhancement cavities, but three or even more enhancement cavities. The ultrashort laser pulses 2 created by the fs laser 41 can be split onto three or more beam paths, where they are specifically manipulated, in particular with regard to the polarization and spectral composition thereof. For instance, three pulse portions can be coupled into the three enhancement cavities, respectively (not shown). Each of the three pulse portions comprises different spectral components of the driving ultrashort laser pulses. The three enhancement cavities can be arranged such that the pulse portions have different angles of incidence on the optically non-linear crystal. Accordingly, phase matching of different spectral components with the pulse portion in the first enhancement cavity can be improved.

[0094] The features of the invention in the above description, the drawings and the claims can be of significance both individually as well in combination or sub-combination for the realization of the invention in its various embodiments.