WO2008029187A2

WO2008029187A2 - Bandwidth-independent method and setup for detecting and stabilizing carrier-envelope phase drift of laser pulses by means of spectrally and spatially resolved interferometry

Info

Publication number: WO2008029187A2
Application number: PCT/HU2007/000081
Authority: WO
Inventors: Károly OSVAY; Mihály Görbe
Original assignee: Szegedi Tudományegyetem
Priority date: 2006-09-05
Filing date: 2007-09-05
Publication date: 2008-03-13
Also published as: HU0600709D0; WO2008029187A3

Abstract

The invention relates to a method and to a setup for detecting and stabilizing a carrier-envelope phase drift of laser pulses by means of spectrally and spatially resolved interferometry. According to the method, a laser pulse train of at least two pulses (1, 2, 3) is directed along a first light path and a second light path that extend between a starting point and an end point and are characterized by different ray paths. One of the pulses travelling in the first light path is taken as reference pulse (2R). A closed light resonator ring (46) is inserted into the second light path, whereby a certain portion of the pulses (1, 2, 3) is circulated back from a second point of the second light path along a light path segment differing from the second light path into a first point of the second light path, wherein the second point is located in the direction of light propagation farther from the starting point and the first point is located in the direction of light propagation nearer to the starting point. Spectrally resolved interference patterns are generated at the end point via interference of the reference pulse (2R) with the whole of the individual pulses (1, 2, 3) travelling through the second point of the second light path towards the end point. The thus obtained interference patterns are imaged by an optical imaging means that is connected to an output of a unit for performing resolution with respect to wavelength arranged at the end point. On the basis of a superimposed image obtained as the superposition of interference images captured, a spectral visibility value is determined. Finally, from a change in the spectral visibility value, the carrier-envelope phase drift of subsequent laser pulses (1, 2, 3) is deduced.

Description

BANDWIDTH-INDEPENDENT METHOD AND SETUP FOR DETECTING AND

STABILIZING CARRIER-ENVELOPE PHASE DRIFT OF LASER PULSES BY MEANS OF

SPECTRALLY AND SPATIALLY RESOLVED INTERFEROMETRY

The present invention relates to a method for a band-width independent detection and then stabilization of the carrier-envelope phase drift of laser pulses by means of spectrally and spatially resolved interferometry (SSRI). The present invention also relates to a setup for accomplishing the method.

A large portion of the basic physical, biological and chemical processes of the real world surrounding us takes place on the atto- and/or femtosecond time scale (1fJ¹⁸ to 10^"15 s). The resolving power of electronical scientific-technical devices falls behind this with several orders of magnitude. As a result of recent developments in the field of laser techniques, however, such laser pulses can be generated nowadays which also enables the experimental study of said elementary phenomena. Such a laser pulse can be considered as a wave packet, it disperses during propagation and in accordance with Fourier's theorem, the shorter in time it is, the larger bandwidth it possesses, i.e. the more frequencies are present therein. In parallel, for such a laser pulse the coherence length is also getting shorter and shorter. Due to the large bandwidth and the dispersive behaviour of materials and various optical devices, the approach of slowly changing carrier-envelope applied generally does not hold for said short laser pulses or it holds but with essential restraints.

Nowadays, the so-called ,,f-to-2f" technique is used to detect a carrier- envelope phase drift and to control the phase stabilization of laser pulse trains. The basis for this technique is that if a carrier-envelope phase drift develops, the spectrum of the laser is shifted relative to the longitudinal modes of the resonator. If the carrier-envelope phase changes by ΔφcE from pulse to pulse, the offset of frequencies present in the spectrum (that is, of the so-called frequency comb) relative to the longitudinal modes (that is, the so-called carrier-envelope offset, Δfc_Eo) will be constant in time and is directly proportional to Δφcε, as it is shown in Figure 1. The basic idea behind the ,,f-to-2f" technique is that the interaction of the original pulses and the second harmonics thereof (that is, the pulses of doubled frequencies) should be studied. Namely, in this case a beating with frequency Δfcεo can be observed between the low-frequency portion of the second harmonics spectrum and the high-frequency portion of the original spectrum, as it is shown in Figure 2.

If the beating frequency is kept at a constant value (for instance, either by rotating the closing mirror of the laser source or by modulating the power of the pumping laser, as it is shown in Figure 3), Δφcε, i.e. the amount of carrier- envelope phase drift will also be constant. If said constant phase drift is equal to 2π/m, wherein m is a positive integer, every m-th member of the pulse train will have got the same carrier-envelope phase. A pulse train with this feature is called a phase stabilized pulse train. A more detailed discussion of the ,,f-to-2f" technique can be found e.g. in Few cycle laser pulse generation and its applications (Topics in Applied Physics, Vol. 95; editor: F. X. Kartner; published by Springer [2004]).

Such an ,,f-to-2f" type method and an apparatus for effecting the method are disclosed e.g. by U.S. Pat. No. 6,850,543 B2.

A major drawback of carrier-envelope phase stabilizing methods based on the ,,f-to-2f" technique comes from the fact that the wavelength range (or bandwidth) of the laser light source to be stabilized should be at least of an octave in width; if this condition does not hold, the bandwidth must be widened by a suitable non-linear optical technique (e.g. in a photonic fiber). For this purpose, however, (due to the losses occurring) it is a must to operate the laser source to be stabilized with a high light intensity on the one hand and to make use of (generally non-linear) optical elements capable of widening the wavelength range on the other hand, which in certain cases considerably increases the manufacturing costs and complexity of the appropriate setup.

International Publication Pamphlet No. WO 2006/008135 A2 discloses a method for stabilizing the carrier-envelope phase of ultrafast laser pulses and a setup that is suitable for carrying out the method. According to the method concerned, the laser pulses are coupled into a single peculiar non-linear optical medium, wherein they undergo self phase modulation and difference frequency generation taking place simultaneously. The laser pulses leaving the non-linear optical medium have got a widened frequency range and experience group delay dispersion. To compensate the latter, the laser pulses have to be led through a series of suitable optical elements, the application of which also considerably increases the manufacturing costs and complexity of the suitable setup.

The object of the present invention is to elaborate a novel technique - as a possible alternative to the prior art phase stabilization methods discussed above - for detecting and then for stabilizing a carrier-envelope phase drift which also eliminates the drawbacks of the existing solutions. In particular, the object of the present invention is to develop a method that can be successfully applied for both the detection and then preferentially for the stabilization of the carrier- envelope phase drift of a pulse train, wherein the pulse train is generated by a laser source operating at an arbitrary central wavelength, with arbitrary bandwidth, as well as with arbitrary light intensity. Moreover, it is also an object of the invention to provide a setup for completing the method according to the invention. A yet further object of the invention is to provide a method and also a setup for accomplishing the method that are both free of any nonlinear optical conversions (i.e. frequency mixing, white-light generation, etc.), relatively simple and hence the production costs are relatively low.

The object related to the implementation of a method for detecting the carrier-envelope phase drift of a pulse train is achieved by the provision of the method in accordance with Claim 1. Possible preferred embodiments of the inventive method are set forth in Claims 2 to 8. The object related to the accomplishment of a setup for the detection of the carrier-envelope phase drift of a pulse train is achieved by elaborating the setup in accordance with Claim 9. Possible preferred embodiments of the inventive setup are set forth in Claims 10 to 18.

The subject-matter of the invention is a method and also a setup - both based on spectrally resolved interferometry - that are capable (i) of detecting the carrier-envelope phase drift for successive laser pulses, as well as (ii) of stabilizing the carrier-envelope phase of the pulse train by controlling suitable optical elements on basis of a physical quantity (the so-called spectral visibility) derived by exploiting the interference phenomenon of laser pulses, and thereby of main- taining the carrier-envelope phase of the laser pulses having initially a varying carrier-envelope phase (from now on, laser pulses with no phase stabilization) at a constant value, that is of generating a phase stabilized pulse train.

In what follows, the invention is discussed in more detail with reference to the attached drawings, wherein

- Figures 1A and 1 B illustrate the basic concept of the ,,f-to-2f" technique applied widespread;

- Figure 2 shows a possible setup for accomplishing the ,,f-to-2f technique;

- Figure 3 shows a spectrally and spatially resolved interferometer in a perspective view which comprises a phase-shifting optical element in its sampling arm;

- Figure 4 shows the schematic diagram of a possible embodiment of the measuring setup according to the inventive concept realized by making use of an asymmetrical Mach-Zehnder type interferometer;

- Figure 5A represents the (spectral) visibility of the fringes of the obtained interference pattern as a function of the carrier-envelope phase drift;

- Figure 5B represents the (spectral) visibility of the fringes of the interference pattern as a function of the laser pulse duration for a slightly detuned (Δτ = 0.1667 ps) light resonator ring;

- Figure 6 illustrates the block diagram of a possible embodiment of the system capable of phase stabilization based on the inventive concept; and

- Figure 7 shows the interference pattern obtained by the experimental setup according to the Example, as well as the wavelength-dependence of the interference pattern's spectral visibility value in such a case, wherein the light resonator ring is correctly tuned [plot (a)] and in two further cases wherein it is detuned to various degrees [plot (b) - 10 μm; and plot (c) - 20 μm].

Figure 1A illustrates the basic concept underlying the ,,f-to-2f" technique, wherein the spectrum (or the frequency comb) 11 of a laser pulse 10 is offset relative to the longitudinal modes 12 of the laser resonator to a degree of Δfcεo that is proportional to the migration of the carrier-envelope phase. Figure 1B schematically shows the ,,f-to-2f" technique itself: at the overlapping 13 between the original and the doubled spectra 11 , 11' of the laser pulse 10 the difference between the frequencies of the ,,quills" is equal to Δf_CEo, and hence a beating _ K _

with the same frequency can be observed there (when monitored e.g. by a pho- todiode).

Figure 2 illustrates a prior art measuring setup 20 for accomplishing the J- to-2f" technique. Here, the laser pulses 22 leaving the laser source (not shown) enter an (f-to-2f) interferometer 21 known by the person skilled in the art, wherein the pulses 22 are decomposed into a farther infrared beam 22b and a nearer infrared beam 22a by a dichroic mirror DM. A beam 22c characterized by the second harmonic frequency comb representing the base of the ,,f-to-2f technique is generated by a frequency doubling crystal BBO arranged in the path of the beam 22b. Then, beams 22a, 22c strike onto a diffraction grating DG through suitable optical elements, are spectrally decomposed there by means of diffraction and then, by being absorbed by a suitable device, the interference of their overlapping spectral portions creates photoelectrons with the beating frequency of Δfc_Eo- The photoelectrons then enter a photomultiplier PMT and generate an electrical signal. The thus obtained electrical signal drives an electro-optical modulator EOM by means of a servo electronics SE. During their propagation along appropriate light paths, beams 22a, 22b, 22c pass through lenses L1 , L2, beam deflecting mirrors SM, wedges W3, W4, a beam splitter BSP and a polarizer P.

Figure 3 illustrates schematically a spectrally and spatially resolved interferometer 30. The term ..spectrally and spatially resolved interferometer" refers to a combination of a division-of-amplitude interferometer and a resolution unit (in a basic configuration typically a spectrograph) that decomposes the interference image appearing at the output of the former with respect to wavelength. Accordingly, the interferometer 30 is formed by a two-beam interferometer 32 equipped with a spectrograph 34 capable of optical mapping. The two-beam interferometer 32 contains two various light paths defined by a reference arm 35 and a sampling arm 36, respectively. Guiding of a laser beam 31 coupled into the two- beam interferometer 32 over the light paths is ensured by suitable optical elements (for instance, beam splitters BSi and BS₂ of appropriate spectral characteristics and beam deflectors SMi and SM₂ (e.g. mirrors)). A phase-shifting element 37 to be studied is preferably also arranged in the sampling arm 36 of the interferometer 30 shown in Figure 3. The optical mapping is completed by an imaging device preferentially in the form of a CCD chip 38 or camera or a photodi- ode located after a dispersive optical element (i.e. an element that executes a decomposition with respect to wavelength, such as e.g. a diffraction grating 39) which is arranged in the light path constituting part of the spectrograph 34. Naturally, if needed, the interferometer 30 may also comprise other optical elements and/or it can be realized with a structural build-up differing from the embodiment shown here in detail. Spectrally and spatially resolved interferometers are well known by the person skilled in the relevant art, and hence are not discussed in the present application in more detail.

A measuring setup 40 of Figure 4 constructed by applying the inventive concept is formed by a spectrally resolved interferometer (see e.g. the interferometer 30 shown in Figure 3) and a light resonator ring 46 built into the sampling arm of the interferometer. Here, the light resonator ring 46 is provided with a given loop time (i.e. the time required by a pulse to complete one single round within the ring) that is equal to the separation time between subsequent pulses (or is detuned from that by at most a slight amount compared to the laser pulse's coherence length). For the setup 40 shown in Figure 4, the line segments with arrows represent the light paths travelled by the laser pulses - here, a laser light 41 introduced into the setup 40 as laser pulses enters at the starting point on the left-hand side. In the light path travelled by the laser light 41 , various optical elements for guiding the laser light 41 are arranged, such as e.g. beam splitters BS₄ and BS₃ and light reflecting mirrors M₁, M₂, M₃ and M₄ each provided with a light reflecting coating on the side which faces to the direction of incidence of the laser light 41. The beam splitter BS₄ effecting a branch-off of the light paths after the starting point directs the laser pulses that strike it, divided as required, into the reference and sampling arms of the interferometer forming part of the setup 40. The actual technical implementation of this measure is apparent to the person skilled in the art. An interferometer of any type, e.g. a Michelson or a Mach- Zehnder type interferometer and/or a symmetric or an asymmetric one can equally be used. Here, and from now on, the term ..symmetric interferometer" refers to an interferometer with the optical path lengths of the reference arm and of the sampling arm being equal to one another. Here, and from now on, the term ..asymmetric interferometer" refers to an interferometer with a difference between the optical path lengths of the reference arm and of the sampling arm being equal to an integer multiple of the separation distance of two subsequent laser pulses emitted by the laser oscillator to be used, wherein said integer is typically equal to one.

The light resonator ring 46 of the setup 40 according to the invention is formed essentially by an entrance beam splitter BSi and an exit beam splitter BS₂, both arranged in the light path of the sampling arm and having a reflectivity chosen/set as desired, as well as optical elements defining a light path segment 48 that returns the light resonator ring 46 between the beam splitters BSi, BS₂, in the embodiment depicted in Figure 4, for instance, light reflecting mirrors M₅, Me, M₇ and M₈. Further optical elements (typically e.g. phase-shifting mirrors or pair(s) of prisms or any suitable combination of said elements) can also be arranged in the light resonator ring 46; their function is to diminish or to eliminate the phase shift due to spectral dispersion of the laser pulses entering the light resonator ring 46. Characteristics/properties of the beam splitters BSi to BS₄ and light reflecting mirrors M-i to M₈ forming the optical elements used in the setup 40 constructed in accordance with the inventive concept are known by the person skilled in the art and, hence, not discussed here in more detail.

Part of the laser pulses gets stored in the light resonator ring 46 inserted into the sampling arm, that is, a lower-energy copy of each of the laser pulses entering via the starting point will circulate in the light resonator ring 46; for instance, the pulses 1 , 2, 3 of Figure 4 to be phase stabilized just leave the sampling arm of the interferometer, while a copy each of the pulses 1 , 2, 3 is also present in the light resonator ring 46. The stored pulses 1 , 2, 3 are continuously circulated back into the sampling arm, which means that a great deal of pulses originating from different time instants is present at the output of the sampling arm. The earlier a certain pulse has entered the ring, the less energy it stores. If the carrier-envelope phase changes along the pulse train, a great number of pulses of different carrier-envelope phases will be present at the output of the sampling arm, too.

When laser pulses of different carrier-envelope phases of the sampling arm meet the only reference pulse, denoted by 2R in Figure 4, of the reference arm, an interference pattern, in particular an array of interference fringes forms in the optical imaging device, provided preferably as a CCD camera, of a spectro- graph 44 forming part of the setup 40. The spatial position of fringes varies linearly with the carrier-envelope phase offset (in a suitably chosen coordinate system it might even be proportional to the offset). As said arrays of interference fringes generated by a plurality of sampling pulses and the same single reference pulse are typically imaged by the optical imaging device simultaneously and together (that is, as a single image), the larger the amount of the carrier- envelope phase drift from pulse to pulse is, the more blurred the interference fringes of the superimposed interference pattern will be. In the sense of physics, a degree of blurring can be unambiguously described by the spectral visibility, which is, by definition, the absolute value of the difference between the maximum and minimum intensities at a given wavelength divided by the sum of the intensities taken into account (see Figure 5A). It is noted that as the energy of the pulses 1 , 2, 3 present in the light resonator ring 46 is decreasing (to a degree that depends on the light reflectivity of the beam splitters BSi and BS₂) from round to round, their contributions to the superimposed interference pattern captured also become more and more negligible. To put this in another way, the number of pulses present in the light resonator ring 46 and hence contributing to the superimposed interference pattern can be controlled through the degree of reflectivity of the beam splitters BSi and BS₂.

Figure 5A shows the visibility of the fringes of the obtained interference pattern as a function of the carrier-envelope phase drift. The curves drawn in different line types belong to feedback factors of different magnitude. Here, the feedback factor is defined by a product of light reflectivities of the beam splitters BSi and BS₂. It is a general tendency that a larger phase drift results in reduced visibility, moreover, except for large feedback factors, a well-defined visibility maximum belongs to a phase drift of zero value.

Figure 5B illustrates the visibility of the fringes of the interference pattern as a function of the wavelength of the laser pulses for a light resonator ring 46 which is slightly detuned (Δτ = 0.1667 ps) compared to the pulse separation time of the pulses. The curves drawn in different line types belong to different carrier- envelope phase drifts.

In light of the above, the method according to the invention unambiguously detects the rate of change of the carrier-envelope phase through a de- _ Q _

crease in spectral visibility. Consequently, it can be applied to control the optical elements that stabilize the carrier-envelope phase.

If the visibility of the interference fringes is maximized by said optical elements stabilizing the carrier-envelope phase, stabilization of the laser pulses' carrier-envelope phase is achieved. This allows further investigations of processes with a length of several hundreds of attoseconds (~10^"16 s), experiments of high efficiency and of high-precision reproducibility in the fields of high-harmonics and X-ray pulse generations, as well as synchronization of the carrier-envelope phase of laser pulses generated by a laser oscillator with no phase stabilization to pulses of an external and independent, however, phase stabilized oscillator.

If the visibility of the interference fringes is kept at a certain level by said optical elements stabilizing the carrier-envelope phase, the carrier-envelope phase drift from pulse to pulse is kept constant which enables frequency measurements of high precision within the optical range. The same result is achieved if the loop time of the light resonator ring 46 is slightly detuned relative to the time interval between the subsequent pulses (see the double arrow of Figure 4) and simultaneously the visibility maxima and minima of the array of interference fringes generated are kept in the same position, as it is shown in Figure 5B already mentioned.

Figure 6 illustrates the block diagram of a possible embodiment of a system 50 for phase stabilization in accordance with the inventive concept. The system 50 comprises a laser beam 51 emitted by a laser (system) 52, a beam splitter 54, known per se, that splits the laser beam 51 into parts 53a and 53b of pre-determined intensities, a spectrally resolved interferometer 56, discussed with reference to Figure 4, that receives and processes part 53a of the laser beam 51 , as well as a control unit 58 that receives an electrical signal 55 appearing on the output of the interferometer 56. The control unit 58 is preferably provided by a computer. The control unit 58 controls/regulates the laser (system) 52 by an electric signal 57. The control/regulation is achieved by the piece of information (the spectral visibility) carried by part 53a of the laser beam 51 processed by the interferometer 56. Part 53b of the laser beam 51 exiting the beam splitter 54 forms a phase stabilized laser light beam that is coupled out for further applications. The bandwidth of the laser pulses plays role in case of neither the method according to the inventive concept nor the setup for accomplishing the method. Therefore, the method and the setup according to the invention can equally be applied for laser light sources of narrow bandwidth and/or of small intensity.

EXAMPLE

Detectability of the carrier-envelope phase drift of a laser pulse train was studied. For this, pulses 1 to 5 of 21 fs in duration, emitted by a Kerr-lens mode- locked Ti:S oscillator at a repetition rate of 80 MHz with a central wavelength of 800 nm are introduced into the setup 40 illustrated in Figure 4. In the light resonator ring 46, the light reflectivity of the entrance beam splitter BS₁ and the light reflectivity of the exit beam splitter BS₂ were set to 50% and 98%, respectively. Figure 7 shows the superimposed interference patterns (top series of plots) and the wavelength-dependence of the spectral visibility value of the interference pattern (bottom series of plots) for a light resonator ring 46 without detuning [plot (a)] and with two different detunings [plot (b) - 10 μm; plot (c) - 20 μm]. Here, the detuning is effected by increasing the length of the light resonator ring 46 by the given values. It is apparent from the curves of Figure 7 that (spectral) visibility of the wavelength-resolved interference pattern varies with the amount of detuning of the light resonator ring 46; in a portion of the wavelength range, the fringes are getting more and more blurred, while at certain definite wavelengths they keep the original sharpness. The greater the amount of detuning is, the more times portions of smaller and larger visibility alternate with one another in the pattern.

Claims

1. A method for detecting a carrier-envelope phase drift of laser pulses by means of spectrally and spatially resolved interferometry, characterized by the steps of

- directing a laser pulse train comprised of at least two pulses (1 , 2, 3) along a first light path and a second light path, both extending between a starting point and an end point and being characterized by different ray paths, meanwhile taking one of the pulses travelling in the first light path as reference pulse (2R);

- inserting a closed light resonator ring (46) into the second light path, whereby circulating back a certain portion of the pulses (1 , 2, 3) from a second point of the second light path along a light path segment differing from the second light path into a first point of the second light path, wherein the second point is located in the direction of light propagation farther from the starting point and the first point is located in the direction of light propagation nearer to the starting point;

- generating spectrally resolved interference patterns at the end point via interference of the reference pulse (2_R) with the whole of the individual pulses (1 , 2, 3) travelling through the second point of the second light path towards the end point;

- imaging the thus obtained interference patterns by an optical imaging means connected to the output of a unit for performing resolution with respect to wavelength arranged at the end point;

- determining a spectral visibility value on the basis of a superimposed image obtained as a superposition of the interference images obtained by said imaging;

- deducing the carrier-envelope phase drift of subsequent laser pulses (1 , 2, 3) from a change in the spectral visibility value.

2. The method according to Claim 1 , further comprising the step of stabilizing the laser pulses by employing the spectral visibility value as a control parameter for controlling the laser pulse source.

3. The method according to Claim 2, wherein the carrier-envelope phase of subsequent laser pulses is made constant via maximizing the spectral visibility value of the superimposed image obtained by superposing the interference patterns.

4. The method according to Claim 2, wherein the carrier-envelope phase drift of subsequent laser pulses is made constant via maintaining the spectral visibility value of the superimposed image obtained by superposing the interference patterns at a given level.

5. The method according to any of Claims 1 to 4, wherein imaging and superposing the interference patterns takes place simultaneously.

6. The method according to any of Claims 1 to 5, wherein a phase offset of laser pulses due to spectral dispersion in the light resonator ring (46) is subjected to compensation.

7. The method according to any of Claims 1 to 6, wherein the laser pulses are generated by a laser source operated at an arbitrary central wavelength, with arbitrary bandwidth and with arbitrary light intensity.

8. The method according to any of Claims 1 to 7, wherein the reference pulse (2R) and the pulses (1 , 2, 3) fed into the first and second light paths are generated by different laser sources.

9. A setup for detecting a carrier-envelope phase drift of laser pulses by means of spectrally and spatially resolved interferometry, characterized in that the setup (40) comprises

- first and second light paths extending between a starting point and an end point and being characterized by different ray paths;

- first and second optical elements (BSi, BS₂) capable of effecting beam splitting, said elements being arranged in the second light path;

- a closed light resonant ring (46) inserted into the second light path, said light resonant ring (46) being formed by a first light path segment and a second light path segment, wherein said first light path segment being identical to a section of the second light path extending between said first and second optical elements (BSi, BS₂), and wherein said second light path segment being formed by a light path section differing from the first light path segment;

- at least one pulsed laser source being arranged at the starting point; - a unit for performing resolution with respect to wavelength being arranged at the end point, said unit being electrically connected via its output to an optical imaging means.

10. The setup according to Claim 9, wherein the first light path is formed by the reference arm (35) of a division-of-amplitude interferometer (30) and the second light path is formed by the sampling arm (36) of the same division-of- amplitude interferometer (30).

11. The setup according to Claim 9 or 10, wherein the interferometer (30) is a symmetric interferometer or an asymmetric interferometer.

12. The setup according to Claim 11, wherein the interferometer (30) is a Michelson type interferometer.

13. The setup according to Claim 11, wherein the interferometer (30) is a Mach-Zehnder type interferometer.

14. The setup according to any of Claims 9 to 13, wherein the optical imaging means is a CCD chip or camera (38) or a photodiode.

15. The setup according to any of Claims 9 to 14, wherein the unit for performing resolution with respect to wavelength is a spectrograph (44).

16. The setup according to any of Claims 9 to 15, wherein at least one optical element for compensating spectral phase shift due to the phenomenon of dispersion is arranged in the second light path segment of the light resonant ring (46).

17. The setup according to Claim 16, wherein the at least one optical element for compensating spectral phase shift is provided by phase-shifting mirrors, at least one pair of prisms or a combination of said optical element.

18. The setup according to any of Claims 9 to 17, further comprising a control unit (58) electrically connected to the unit for performing resolution with respect to wavelength, said control unit (58) being electrically connected to the laser source, wherein by determining a spectral visibility value on the basis of a superimposed interference pattern imaged by the optical imaging means and by employing said visibility value as a control parameter said laser source is controlled by said control unit (58) in such a way that said laser source, by means of suitable optical elements, emits pulse trains of phase stabilized or phase drift stabilized laser pulses.