SHORT-PULSE LASER SYSTEM

Abstract

A short-pulse laser system includes a first and a second resonator, and an amplification means for amplifying the electromagnetic pulses both in the first and in the second resonator. The first resonator supports precisely one first linear polarization state, and the second resonator supports precisely one second linear polarization state perpendicular to the first polarization state. The short-pulse laser system has first and second birefringent material sections. The first birefringent material section and/or the second birefringent material section is designed in such a way that a difference between the sum of the optical path length of the first resonator in the first birefringent material section and the optical path length of the first resonator in the second birefringent material section and the sum of the optical path length of the second resonator in the first birefringent material section and the optical path length of the second resonator in the second birefringent material section can be changed in an adjustable manner.

Claims

1. A short-pulse laser system for generating electromagnetic pulses, comprising a first resonator, which has a first beam path, a second resonator, which has a second beam path, and an amplification means, wherein the amplification means is arranged both in the first beam path, so that it amplifies electromagnetic pulses in the first resonator during operation of the short-pulse laser system, and in the second beam path, so that it amplifies electromagnetic pulses in the second resonator during operation of the short-pulse laser system, wherein the first beam path and the second beam path are spatially superimposed at least in sections in the amplification means and wherein the first resonator is set up in such a way that it supports precisely one first linear polarization state of the electromagnetic pulses and the second resonator is set up in such a way that it supports precisely one second linear polarization state of the electromagnetic pulses, wherein the first and the second polarization states are perpendicular to each other, wherein the short-pulse laser system has a first birefringent material section and a second birefringent material section, wherein the first beam path and the second beam path run collinearly in the first birefringent material section and in the second birefringent material section, wherein the first birefringent material section and the second birefringent material section are designed and set up in such a way that in one state of the first birefringent material section and of the second birefringent material section the sum of the optical path length of the first beam path in the first birefringent material section and the optical path length of the first beam path in the second birefringent material section is equal to the sum of the optical path length of the second beam path in the first birefringent material section and the optical path length of the second beam path in the second birefringent material section, and wherein the first birefringent material section or the second birefringent material section is designed in such a way that a difference between the sum of the optical path length of the first beam path in the first birefringent material section and the optical path length of the first beam path in the second birefringent material section and the sum of the optical path length of the second beam path in the first birefringent material section and the optical path length of the second beam path in the second birefringent material section can be changed in an adjustable manner.

2. The short-pulse laser system according to claim 1, wherein the first beam path and the second beam path are collinear over the entire length of the first and of the second resonator.

3. The short-pulse laser system according to claim 1, wherein the first birefringent material section and the second birefringent material section each have a fast axis and a slow axis, wherein the fast axis of the first birefringent material section is rotated by 90 relative to the fast axis of the second birefringent material section.

4. The short-pulse laser system according to claim 3, wherein the first polarization state is parallel to the fast axis of the first or of the second birefringent material section.

5. The short-pulse laser system according to claim 1, wherein the short-pulse laser system has a device for altering the geometric length of the first and/or of the second birefringent material section and/or a device for altering a difference between a first refractive index and a second refractive index of the first and/or of the second birefringent material section.

6. The short-pulse laser system according to claim 1, wherein the first birefringent material section and the second birefringent material section are polarization-maintaining optical fibres.

7. The short-pulse laser system according to claim 6, wherein the short-pulse laser system has a device for mechanically stretching or compressing the first birefringent material section or the second birefringent material section.

8. The short-pulse laser system according to claim 1, wherein the short-pulse laser system is a fibre laser.

9. The short-pulse laser system according to claim 8, wherein the short-pulse laser system has a mode coupler.

10. An optical pump/probe arrangement comprising a short-pulse laser system according to claim 1.

11. The optical pump/probe arrangement according to claim 10, wherein the optical pump/probe arrangement is set up such that pulses which were generated in the first resonator are directed onto a physical system to excite it, and pulses which were generated in the second resonator are directed onto the physical system to probe it.

12. The optical pump/probe arrangement according to claim 10, wherein the optical pump/probe arrangement is set up such that pulses which were generated in the first resonator are directed onto a generator for electromagnetic radiation in the THz frequency range, and pulses which were generated in the second resonator are directed onto a detector for electromagnetic radiation in the THz frequency range.

Description

[0055] Further advantages, features and possible applications of the present invention are made clear with reference to the following description of embodiments of it and the associated figures.

[0056] FIGS. 1a and 1b show schematic representations of a first embodiment of a short-pulse laser system according to the present invention with linear resonators.

[0057] FIGS. 2a to 2c show embodiments of polarization-maintaining optical fibres.

[0058] FIG. 3 shows a schematic representation of a second embodiment of a short-pulse laser system according to the present invention with ring resonators.

[0059] FIG. 4 shows a schematic representation of a further embodiment of a short-pulse laser system according to the present invention with ring resonators.

[0060] In the figures identical elements are given identical reference numbers.

[0061] The short-pulse laser systems shown in FIGS. 1, 3 and 4 are fibre lasers based on optical fibres which are designed for operation at a wavelength of 1.55 m.

[0062] The optical fibres used are so-called polarization-maintaining fibres 1, 1, 1 with a core 4, to which stresses are applied in a targeted manner in one direction by a special design of the cladding of the fibres. In this way, electromagnetic radiation which is coupled into these fibres with a linear polarization parallel or perpendicular to the marking direction propagates without noteworthy proportions of the radiation being transferred from one polarization state into the other during the propagation through the fibre. In other words, in such polarization-maintaining fibres there is no crosstalk between the two channels formed by the linear polarization states perpendicular to each other.

[0063] FIGS. 2a to 2c show examples of such polarization-maintaining fibres 1, 1, 1, such as can be alternatively used to construct the fibre laser system from FIGS. 1, 3 and 4. FIG. 2a shows a so-called Bow-Tie fibre 1, in which two structures 3 which together with the core 4 resemble a bow-tie in the sectional view are introduced into the fibre cladding 2. The two structures 3 in the cladding 2 of the fibre 1 result in the core 4, which is embedded in the centre of the cladding 2, having a marked axis, into which for example linearly polarized electromagnetic radiation can be coupled in a polarization-maintaining manner.

[0064] FIG. 2b shows an alternative embodiment of such a polarization-maintaining optical fibre 1, which is called a PANDA fibre. In order to build up a corresponding stress in the core 4, two glass strands 5, which have approximately the same effect as the bow-tie structures 3 of the fibre 1 from FIG. 2a, are run in the cladding 2 of the optical fibre 1.

[0065] FIG. 2c shows a third embodiment of a polarization-maintaining optical fibre 1, in which within the cladding 2 the core 4 is embedded in an elliptical structure 6, which impresses the necessary anisotropic stress on the core 4. Such a polarization-maintaining optical fibre 1 is also called an elliptical clad fibre.

[0066] Due to the formation of all fibre components of the short-pulse laser systems 10, 10, 10 from FIGS. 1, 3 and 4 as polarization-maintaining fibres 1, the short-pulse laser systems 10, 10, 10 have two completely collinear channels or resonators, which support two linear polarizations perpendicular to each other. Although they are completely collinear, these two resonators are separated from each other in such a way that they experience as little mutual influencing as possible and no crosstalk. This means that both channels form resonators that are largely independent of each other in a single system. In particular, there is an at least reduced mode competition between the two channels in the amplification means 11.

[0067] The amplification means is formed by an erbium-doped fibre section 11. The latter is excited with the aid of an optical pump 12 in order to be able to provide the necessary amplification of the radiation oscillating in the lasers 10, 10, 10. The pump radiation 12 is coupled into the amplifying fibre section 11 with the aid of a wavelength-division multiplexing fibre coupler 13.

[0068] While both polarization modes of the first and of the second resonator in the same fibre of the laser 10, 10, 10 propagate collinearly within the fibre laser 10, 10, 10, the two polarization channels behind the output coupler 14, 23 can be spatially separated from each other with the aid of a polarization beam splitter (not shown in the figures).

[0069] As the electromagnetic radiation both in the linear fibre laser 10 from FIGS. 1a and 1b and in the ring resonators of the fibre lasers 10, 10 from FIGS. 3 and 4 is completely collinear, in all embodiments shown the geometric path lengths of the first and second beam paths of the first and second resonators are exactly the same length. In order nevertheless to be able to introduce an optical wavelength difference, all three lasers 10, 10, 10 have two birefringent fibre sections 16, 17.

[0070] In all embodiments these two fibre sections are given the reference numbers 16 and 17, wherein here reference is made first to the embodiment according to FIGS. 1a and 1b, as in particular the schematic representation from FIG. 1b makes it easier to understand the basic idea of the present invention. Both polarization-maintaining fibre sections 16, 17 are realized by PANDA fibres as birefringent material sections within the meaning of the present application. However, the fibre sections at the splices 18 connecting them are rotated relative to each other by 90 about their longitudinal axes, as is represented schematically in FIG. 1b and in the inserts of FIGS. 1a, 3 and 4.

[0071] While a linear polarization in the fibre section 16 initially propagates along the fast axis of the fibre 1, it propagates along the slow axis of the fibre section 17 rotated relative thereto by 90. If the geometric length in one fibre section 16 is now altered, this results in the same alteration of the geometric length for both channels, but in a difference between the optical path lengths due to the birefringent property of the fibre 1. However, as the optical path length is decisive for the repetition rate of the pulses in the resonators, a stretching of the fibre section 16 relative to the fibre section 17 results in an alteration of the difference between the optical path lengths and thus in an alteration of the repetition rates of the pulses in the two resonators.

[0072] In order to achieve a stretching of the fibre section 16, this fibre section has a fibre stretcher 19. This fibre stretcher 19 consists of two support posts 20, the spacing of which can be adjusted and varied with the aid of a piezo element. Several fibre loops of the fibre section 16 are laid around the two support posts 20, with the result that a movement of the two support posts 20 away from each other results in a noteworthy length alteration of the fibre section 16 and thus in an alteration of the difference between the repetition rates of the two resonators.

[0073] Here the fibre sections 16, 17 have exactly the same geometric length in one state and thus the two linear polarization states of the electromagnetic radiation in the two resonators or channels also have the same optical path lengths in this state. In the embodiment shown the state of exactly the same geometric length is specifically chosen such that an extending of the fibre section 16 with the aid of the fibre stretcher 19 is necessary to achieve it. During a stretching of the fibre section 16 with the fibre stretcher 19, the fibre section 16 is therefore extended from a state in which the fibre section 16 has a shorter geometric length than the fibre section 17 such that it has the same geometric length as the fibre section 17, and beyond this such that it has a greater geometric length than the fibre section 17. In other words, the repetition rate of the first resonator is initially greater than that of the second resonator, and in the course of the stretching of the fibre section 16 the repetition rates of the two resonators equal each other, before the repetition rate of the first resonator becomes smaller than the repetition rate of the second resonator.

[0074] This variation of the repetition rates results in a shifting of the phasings of the pulses from the two resonators and thus in a variation of the time offset between the pulses from the first resonator and the pulses from the second resonator.

[0075] In the linear design of the resonators of FIGS. 1a and 1b a small proportion of the power of the laser pulses oscillating in the resonators is coupled out of the fibre laser 10 by means of the output mirror 14. The majority of the power remains in the resonators and is reflected back by the end mirror 21.

[0076] Here the end mirror 21 also acts as a mode coupler, as it is designed as a saturable absorber. The saturable absorber 21 acts as a passive optical switching element and thus serves for passive Q-switching of the two laser resonators. The saturable absorber consists of a material with an intensity-dependent absorption coefficient. In the embodiment represented the saturable absorber 21 is a semiconductor component, namely a SESAM (Semiconductor Saturable Absorber Mirror), which acts both as a saturable absorber and as an end mirror. The saturable absorber results in a boundary condition of the resonators, which in turn has the effect that the laser generates modes phase-coupled to each other.

[0077] The ring laser 10 from FIG. 3 is also constructed from two polarization-maintaining fibre sections 16, 17, wherein these are connected to each other at two splices 18. Again, the fast axes of the two fibre sections 16, 17 at the splices 18 are rotated relative to each other by 90, with the result that an alteration of the geometric path length in one fibre section 16 with the aid of the fibre stretcher 19 results in an optical path length difference between the two collinear resonators. The ring laser 10 additionally has an optical diode 22 in the beam paths of the two collinear resonators, in order to prevent the formation of standing waves in the resonators. The coupling out of the ring laser is effected via a fused fibre coupler 23, which couples part of the power out of the ring resonators. A saturable absorber 24 likewise serves as a mode coupler in the ring laser 10.

[0078] The laser 10 from FIG. 4, on the other hand, combines a linear resonator structure with a ring structure. Again, two polarization-maintaining fibre sections 16, 17 are connected to each other with their fast axes rotated relative to each other by 90 at a splice 18. The fibre section 17 is connected to the linear part of the laser 10 with the aid of a 21 fused fibre coupler 25. In order to achieve the complete return of the power propagating in the ring 24 back into the linear part of the laser system 10, a non-reciprocal element 27 is provided in the ring 26.

[0079] For the purpose of original disclosure, it is pointed out that all features, as revealed to a person skilled in the art from the present description, the drawings and the claims, even if they have been described specifically only in connection with particular further features, can be combined both individually and in any combinations with others of the features or groups of features disclosed here, unless this has been explicitly ruled out or technical circumstances make such combinations impossible or meaningless. The comprehensive, explicit representation of all conceivable combinations of features is dispensed with here merely for the sake of the brevity and readability of the description.

[0080] While the invention has been represented and described in detail in the drawings and the above description, this representation and description is done merely by way of example and is not intended to limit the scope of protection as defined by the claims. The invention is not limited to the embodiments disclosed.

[0081] Modifications of the disclosed embodiments are obvious for a person skilled in the art from the drawings, the description and the attached claims. In the claims the word to have does not rule out other elements or steps, and the indefinite article a or an does not rule out a plurality. The mere fact that particular features are claimed in different claims does not rule out the combination thereof. Reference numbers in the claims are not intended to limit the scope of protection.

LIST OF REFERENCE NUMBERS

[0082] 1, 1, 1 polarization-maintaining fibres [0083] 2 cladding [0084] 3 structures [0085] 4 core [0086] 5 glass strands [0087] 6 elliptical structure [0088] 10, 10, 10 short-pulse laser system [0089] 11 amplification means [0090] 12 fibre section guiding pump radiation [0091] 13 wavelength-division multiplexing fibre coupler [0092] 14, 23 output coupler [0093] 16, 17 fibre sections [0094] 18 splice [0095] 19 fibre stretcher [0096] 20 support posts [0097] 21 end mirror [0098] 22 diode [0099] 24 saturable absorber [0100] 25 21 fused coupler [0101] 26 ring [0102] 27 non-reciprocal element