DE102011121559B3 - Method for performing multi-photon excitation with e.g. polarization, at output end of optical transmission system i.e. endoscopic device, involves applying laser pulse withdrawn from end of transmission system for multi-photon excitation - Google Patents

Abstract

The method involves computing a control signal for a pulse shaper (120) for formation of a laser pulse with back-computed parameters, where the shaper is located in front of an input end in a beam path. The control signal is applied (130) to the shaper, and amplitude, phase and polarization are modulated for the laser pulse. The laser pulse is preformed based on the parameters. The preformed laser pulse is introduced (140) into an input end of an optical transmission system. The laser pulse withdrawn from an output end of the transmission system is applied (150) for multi-photon excitation. An independent claim is also included for a computer program product having a set of instructions for executing a method for performing multi-photon excitation.

Description

Field of Technology:

The invention relates to a method for carrying out multiphoton excitation with at least one generated laser pulse having predefined amplitude, phase and polarization at the output end of an optical transmission system with at least one optical fiber. Such methods are particularly applicable in medicine especially in endoscopic two-photon microscopy.

State of the art:

The formation of short laser pulses by means of a pulse shaper in amplitude, phase and polarization is known. Such a pulse shaper is in DE 10 2006 029 035 A1 disclosed. Reference is made to the description of pulse shapers there. An embodiment of a suitable pulse shaper is an in 1 shown 4-f spatial pulse shaper, which has near the Fourierbene four liquid crystal arrays with suitably aligned optical axes such as 45 °, -45 °, 45 °, 0 ° and a polarizer behind the second array, wherein the liquid crystals with respect to the change of their optical Properties are individually controllable. Methods are known in which a very short laser pulse with a pulse shaper shaped in amplitude, phase and polarization is guided into an optical fiber and used at the output end of the fiber-containing optical transmission system. Such a method using femtosecond laser pulses is disclosed for a microstructured hollow core fiber in F. Weise et al., Optics Communications 284 (2011) 3759-3771.

Multi-photon excitations are excitations of atoms or molecules caused by laser pulses, which require more than one photon, as in the two-photon excitation, where two usually different frequency photons are required for the desired excitation, that is, when excited, an intermediate virtual energy level the electron is considered as an intermediate step. For example, in medical imaging, multi-photonic excitations in dye-marker substances are exploited. From V.V. Lozovoy et al., J. Chem. Phys. 118 (2003) 3187-3196 it is known that it is possible to use a suitably shaped phase of the laser pulse, such as antisymmetric, for selective multi-photon excitation. In particular, for endoscopic two-photon microscopy to be performed by means of optical fiber, it is important because of the necessary spatial narrowness that the laser pulse is already formed before entry into and not out of the optical fiber. In the case of the multiphotonic excitations, high laser light intensities are advantageous, above all because of the then better signal yield. However, at high laser light intensities in the optical fiber, the laser pulse will experience significantly deforming non-linear processes.

Such nonlinear processes are, for example, self-phase modulation, cross-phase modulation of the pulse and self-division of the laser pulse profile. For example, the spectral shift increased by nonlinear processes can increase the efficiency of multiphoton excitation. In the methods known to date for multiphoton excitation, these non-linear processes have either not been considered in pulse shaping, or rudimentary, as in F. Weise et al., Journal of Optics 13 (2011) 075301, by experimental observation of the influence of a simple temporal pre-chirp of the laser pulse on the spectral width of the pulse to characterize the optical fiber examined. Targeted control of the overall pre-shaping of the amplitude, phase and polarization of the laser pulse, which compensates for nonlinear processes in the optical fiber or even makes advantageous use of multiphoton excitation, is not possible with the currently known multiphoton excitation methods. True, is in US020110299558A1 discloses a method of pulse shaping in which parameters of a nonlinear element in an optical fiber transmission system are taken into account in the calculation of the pulse shaper control signal, but neither the calculation method nor an application to multiphoton excitation is mentioned. For multi-photon excitation, the previously known pulse shaping methods using optical fibers reach their limits at particularly advantageous high laser light intensities.

The invention has for its object to provide an improved method for performing multiphoton excitation with pre-defined in amplitude, phase and polarization guided by optical fiber laser pulse.

Summary of the invention:

The object is achieved by a method for carrying out multiphoton excitation with at least one generated laser pulse having predefined amplitude, phase and polarization at the output end of an optical transmission system which contains at least one optical fiber, the method comprising the steps of first determining parameters of the desired shape of the laser pulse in terms of phase, polarization and amplitude for the Mehrphotonenanregung second, the back calculation of the parameters of the shape of the laser pulse from the output end along the Fiber to the input end of the optical transmission system using a nonlinear Schrödinger equation, taking into account at least one nonlinear physical parameter of the optical fiber; third, calculating a control signal for the pulse shaper located in the beam path in front of the input end to form a laser pulse with the parameters back-calculated; the control signal to the pulse shaper and modulating the amplitude, phase and polarization for a laser pulse guided into the pulse shaper thereby preforming the laser pulse according to the recalculated shape, fifthly the introduction of the preformed laser pulse into the input end of the optical transmission system and, sixthly, the application of containing the output end of the optical transmission system laser pulse for multiphoton excitation contains. By first determining the parameters of the laser pulse proper for multiphoton excitation, and then using these particular parameters to back-calculate along the optical fiber using a nonlinear Schödinger equation, taking into account nonlinear parameters of the fiber, it is possible to proceed from the parameters determined for the desired multiphoton excitation of the laser pulse to determine the correct parameters of the appropriate laser pulse before the input end of the optical transmission system completely in phase, amplitude and polarization and preforming the laser pulse in the pulse shaper with the calculated control signals accordingly in this complex designed form. The nonlinear properties of the optical fiber are thus taken into account in the complete preforming of the pulse before the fiber already by the back calculation of the parameters of the complex design of the pulse, which is particularly good with the nonlinear Schrödinger equation. Deformations of the laser pulse due to nonlinearities at high laser light intensities can thus be compensated effectively and even the nonlinear parameters of the optical fiber can be advantageously exploited. At the output end of the optical transmission system is space-saving only the optical coupling of the laser pulse from the fiber. Advantageously, laser pulses can be selectively formed, which are then formed with very high intensities at the output end of the optical transmission system suitable for multiphoton excitation. High efficiency for the multi-photon excitation is achieved in this way, which increases the signal yield, for example in multiphoton microscopy. The spectral shift increased by non-linear processes also increases the effectiveness of multiphoton excitation when the laser pulse is preformed according to the invention.

In the subclaims advantageous refinements and improvements of the respective subject of the invention are given.

According to an advantageous development of the method according to the invention, the non-linear part of the refractive index is taken into account in the calculation of the laser pulse parameters along the optical fiber. This nonlinear parameter of the optical fiber, which is very important at high laser light intensities, is decisive for the nonlinear processes, such as self-phase modulation, cross-phase modulation of the pulse and the self-division of the laser pulse profile. Thus, the consideration of the nonlinear part of the refractive index is advantageous in the calculation of the laser pulse parameters.

It is advantageous to use a split-step Fourier method in the evaluation of the nonlinear Schrödinger equation.

According to an advantageous development of the method according to the invention, it contains the step of determining the parameter space of all possible with the technical system pulse shapes at the output end of the optical transmission system. Besides the optical transmission system, the technical system includes the pulse laser and the pulse shaping unit with pulse shaper and controller. By determining the parameter space of the possible pulse shapes at the output end of the optical system, the selection of laser pulse parameters that can not be realized with the technical system is advantageously already prevented when determining the laser pulse parameters at the output end of the optical system.

The optical transmission along the optical fiber is preferably set to a sufficiently low value for the recalculation, so that the optical transmission does not become greater than 1 during recalculation. In non-linear processes, for example, new frequencies for the laser pulse can arise. For example, unrecoverable optical transmission of more than 1 can normally occur during recalculation. Nevertheless, in order to be able to calculate back the laser pulse parameters, the optical transmission for the back calculation is reduced sufficiently. This has the advantage that otherwise not rückrückbare laser pulse shapes are rückrechenbar with the method and thus selectively preformed.

According to an advantageous development of the method, the pulse shaping on the pulse shaper contains a specific polarization-dependent incorporation calculation of the nonlinear parameters of the optical fiber. Thus, the different influence of the non-linear parameters on the different polarization directions in the preforming of the laser pulse which improves the accuracy to the desired pulse shape at the output end of the optical transmission system.

A further improvement is the targeted preformability of the laser pulse on the pulse shaper in such a way that at the output end of the optical transmission system, the laser pulse is shaped differently in two different polarization directions and shifted spectrally to each other. Thereby, in the multi-photonic excitation, both polarization directions can be individually shaped and shifted from each other, for example with the laser pulse part of one polarization direction for a two-photon excitation of one substance and with the laser pulse part of the other polarization direction for a two-photon excitation of another substance. Alternatively, the first intermediate step may also be performed with the laser pulse part of one polarization direction and the second intermediate step of the two-photon excitation with the other polarization direction.

According to an advantageous development of the method, the determination of the parameters of the laser pulse contains an iterative optimization of the method for multiphoton excitation by changing at least one parameter of the shaped laser pulse emerging from the optical transmission system back to the control signal of the pulse shaper. Thereby, the parameters of the laser pulse at the output end of the optical system can be selectively varied for the optimization of the multi-photon excitation, which is a much more meaningful and systematic change, than an optimization of individual parameters of the laser pulse at the input end of the optical transmission system without knowing how this Pulse changed at the output end. In particular, the laser pulse is significantly deformed by the nonlinear parameters of the optical fiber.

In an advantageous variant of the method, the optical fiber is a singlemode glass fiber. The method makes good use of the obvious effects of the nonlinear parameters of such an optical fiber.

In a further advantageous variant of the method, the optical fiber is a microstructured hollow-core fiber. This laser pulses can be transmitted with very high light intensities with little dispersion.

According to an advantageous development of the method, a parametric laser pulse shaping method is used. This makes the laser pulses particularly easy and easy to shape.

According to an advantageous variant, the method is suitable for a targeted molecular multi-photonic excitation after the optical fiber. A molecular multi-photonic excitation is thus possible even at high fiber light intensities at the output end of the optical transmission system with specifically shaped laser pulses.

According to a further advantageous variant, the method is suitable for the specific modification and displacement of multiphoton spectra after the optical fiber. As a result, multiphoton spectra can be changed and shifted particularly well.

According to a further advantageous variant, the method is suitable for determining the molecular orientation and the anisotropy of a sample, which significantly improves the analysis of a sample. In particular, the contrast with respect to a different molecular orientation can be increased by suitable polarization-dependent laser pulse shaping.

According to an advantageous development of the method, the optical transmission system is part of a medical device, such as an endoscopy device. In medicine, the method can be used particularly well, because in particular high laser light intensities can be used in targeted shaped pulses in the body of the living to save space for Mehrphotonenanregungen.

The object is also achieved by an alternative method for performing multiphoton excitation with at least one generated laser pulse having predefined amplitude, phase and polarization at the output end of an optical transmission system containing at least one optical fiber, characterized by the steps of first determining the parameter space of possible Secondly, from the determined parameter space at the input end of the forward optical transmission system along the fiber using a nonlinear Schrödinger equation, taking into account at least one of its nonlinear physical parameters, calculates the parameter space of the possible laser pulse shapes at the output end of the laser beam optical transmission system for assigning possible laser pulse forms at the input end to then laser pulse forms at the output end d Third, determine the shape of the laser pulse in terms of phase, polarization and amplitude for multiphoton excitation, matching parameters suitable for multiphoton excitation Fifth, determine the associated pulse shape control signal of the laser pulse at the input end of the optical transmission system from the assignment calculated in the second step, apply the control signal to the pulse shaper and modulate the amplitude, phase and polarization for seventh introducing the preformed laser pulse into the input end of the optical transmission system, and eighth applying the laser pulse emerging from the output end of the optical transmission system for multiphoton excitation. This alternative solution of the task has, in addition to the improvements listed for the first alternative, the advantage that, after selection of the output pulse shape, a time-saving insertion of the assigned pulse shaping can be carried out without further calculation.

Also for this alternative solution of the problem as sub-claims, the same advantageous refinements and improvements of the respective subject of the invention as in the first alternative of the invention with the same benefits realized, except the advantageous for the back calculation advantageous reduction of the transmission for the back calculation.

The invention also relates to a computer program product stored on a computer-readable medium comprising instructions for configuring a computer to calculate the back-calculation along the optical fiber and the control signals for the pulse shaper respectively according to one of the methods of the invention.

Brief description of the drawings:

With reference to the drawings, embodiments of the invention will be explained.

Show it

1 a schematic representation of a pulse shaper from the prior art in longitudinal section;

2 a schematic representation of a technical system with which the method is feasible;

3 in the representation of a flowchart, an embodiment of the method;

4 in the representation of a flowchart, another embodiment of the method with iterative optimization,

5 in the representation of a flowchart, another embodiment of the method with previous parameter space allocation,

6 Spatial representation of a polarization-shaped laser pulse after an optical fiber having antisymmetric phases;

7 a diagram representation of a transmission adaptation;

8th a diagram of a comparison of the laser pulse shape at the output end of the optical transmission system in processes once without and once taking into account nonlinear parameters of the optical fiber.

Detailed description of the invention:

In the figures, the same reference numerals designate the same or functionally identical components. All drawings are to be understood schematically. Scale-true images have been omitted for clarity of illustration.

In 1 is a pulse shaper 1 from the prior art shown in longitudinal section schematic. The pulse shaper 1 contains two cylindrical lenses 3 and 4 with a focal length of for example 200 mm and two optical gratings 6 and 7 with, for example, 1200 grooves per mm, which may have a polarization-dependent difference in reflectivity for horizontally and vertically polarized light. The pulse shaper 1 also contains three common double liquid crystal array modulators 9 . 10 and 11 such as two SLM1280 and one SLi-256 from CRi. The optical axes of the three double liquid crystal array modulators 9 . 10 and 11 have an orientation of +/- 45 degrees. Between the first 9 and the second double liquid crystal array modulator 10 there is a polarizer 13 and in front of and behind the third double liquid crystal array modulator 11 one half-wave plate each 15 and 16 , Each of the modulators 9 . 10 and 11 are precisely mounted to each other on a bracket. The pixels of the modulators are aligned with each other for the same frequency band ωn. The wavelength calibration coefficient for the liquid crystal arrays is in the range of 0.3415 nm / pixel. The laser pulses are modulated in the Fourier plane of the 4-f arrangement.

In 2 is schematically mainly in block diagram an embodiment of a technical system 20 shown for carrying out the method according to the invention. With a femtosecond laser 22 a femtosecond laser pulse is generated and this with a steel divider 24 divided into a first pulse for the application and a second pulse as a reference for the detection. The first pulse is placed on it in a pulse shaper 1 according to the specification of the computing and control unit 26 preformed in amplitude, phase and polarization. In this specification are the nonlinear parameters of the optical fiber 28 , such as the nonlinear component of the refractive index, is taken into account. The in the pulse shaper 1 preformed laser pulse is then at the input end of the optical transmission system 30 by means of a conventional collimation optics 32 into the optical fiber 28 coupled and occurs at the output end of the optical transmission system 30 from the fiber 28 through another collimation optics 33 out. With the help of another beam splitter 28 the laser pulse is divided again, wherein one part is used for the desired multi-photon excitation and the other much weaker part for the analysis of the laser pulse shape together with the reference pulse in the usual laser pulse shape used for the detector 37 is used. Such a detector 37 contains for example a FROG, XFROG, PG-FROG, SEA-TADPOL, autocorrelator or cross-correlator. The detection result is for comparison to the computing and control unit 26 directed. This calculates, as far as corrections of the laser pulse shape are necessary, parameters of the laser pulse back along the fiber with a nonlinear Schrödinger equation in the split-step Fourier method taking into account non-linear parameters of the fiber, such as the nonlinear component of the refractive index of the fiber. The recalculated parameters for a desired laser pulse are stored in the arithmetic and control unit 26 in the control signal for the pulse shaper 1 converted and the pulse shaper 1 controlled accordingly.

3 shows a flowchart of an embodiment of the method. In the first step 100 For example, the parameters of the shape of the laser pulse are determined in terms of phase, polarization and amplitude for the desired multiphoton excitation. Such parameters include, for example, subpulse intensities, spacings, chirps, polarizations, antisymmetric phase functions. From previous measurements and / or for physical considerations, the desired shape of the laser pulse is determined for good effectiveness of the desired multi-photon excitation, such as two-photon excitation. In this case, a parametric laser pulse shaping method is used to model the desired laser pulse shape with its parameters. The user selects pulse parameters such. B. subpulse distances, intensities, chirps, phase differences, and ellipticity, major axis angle, direction of rotation (helicity), etc .. For each subpulse then the respective intensity in both components of the electric field from the total intensity of the subpulse and the ellipticity is calculated. The individual subpulses are now combined separately for both components in succession. From this, the electric fields of both components of the entire pulse can thus be produced. However, other methods are also conceivable for creating a desired laser pulse shape in the model. At the second step 110 In the method of multiphoton excitation, the parameters of the laser pulse shape determined for the particular multiphoton excitation are calculated along the fiber back to the input end of the optical fiber from the output end of the optical transmission system consisting essentially of an optical fiber. In this calculation, in particular the non-linear part of the refractive index of the fiber is taken into account. In this case, a conventional singlemode optical fiber is used. Also conceivable are other optical fibers such as a microstructured hollow core fiber, Kagome fiber, Bragg fiber or hybrid fiber. Depending on which fiber is used, the parameters are mainly different from the material and the shape of the fiber. These parameters, including the nonlinear parameters of the fiber, are known, for example, from previous measurements and calculations.

The mathematical description of the recalculation in the process step 110 is as follows:
For the reverse propagation the corresponding inverse nonlinear Schrödinger equation has to be solved: ∂E / ∂z = α / 2E + β ₁ ∂E / ∂t + iβ₂ / 2 ∂²E / ∂t² - β₃ / 6 ∂³E / ∂t³ - iγ | E | ² E + γ / ω₀ ∂ / ∂t (| E | ² E) with the absorption coefficient α, the dispersion coefficients β ₁ , β ₂ and β ₃ and the nonlinear parameter γ. This equation can be obtained by simply conjugating the parameters for the forward nonlinear Schrödinger equation. It determines the backpropagation of the total electric field E including the first, second and third dispersion order, the self-phase modulation, the cross-phase modulation and the four-wave mixing. These non-linear effects, which are determined by the third-order susceptibility, do not involve an energy exchange with the medium and can therefore be reversed in time. For a simplified calculation, the following transformation is performed. T = t - β ₁ z

With that follows: ∂E / ∂z = α / 2E + iβ₂ / 2 ∂²E / ∂T² - β₃ / 6 ∂³E / ∂T³ - iγ | E | ² E + γω / ₀ ∂ / ∂T (| E | ² E)

This equation can also be written in the following form: ∂E / ∂z = (D ^ + N ^) E with the differential operator: D ^ = - iβ₂ / 2 ∂² / ∂T² + β₃ / 6 ∂³ / ∂T³ - α / 2 and the nonlinear operator: N ^ = -iγ | E | ² + γω / ₀ ∂ / ∂T (| E | ² E)

Individual terms can be taken out for a simplified calculation.

In particular, the equation can be simplified for pulses longer than about 1 ps to: ∂E / ∂z = α / 2E + iβ₂ / 2 ∂²E / ∂T² - β₃ / 6 ∂³E / ∂T³ - iγ | E | ² E

The inverse nonlinear Schrödinger equation can advantageously be solved numerically by the split-step Fourier method. In this method, the fiber is divided into small segments and for each segment, the linear and non-linear calculation separately performed separately. In order to avoid the derivatives of the dispersive operator, the dispersion step is carried out in the frequency domain. Given sufficient small-scale segmentation, this method provides a reliable solution to the nonlinear Schrödinger equation. This calculation can be carried out with a corresponding computer program on a standard PC in the seconds range with sufficiently high resolution. The use of the non-linear Schrödinger equation with a split-step Fourier method for recalculating laser light along nonlinear optical media is a well known computational method, but its use is not known for a multi-photon excitation with phase, amplitude and polarization shaped laser pulses.

In the following process step 120 the electric field of the laser pulse thus obtained for the input end of the optical transmission system is converted into a control signal for the pulse shaper located in the beam path in front of the input end for shaping a laser pulse with the parameters calculated back. Then this calculated control signal in the process step 130 applied to the pulse shaper and the amplitude, phase and polarization modulated for a guided in the pulse shaper laser pulse. In this way, the laser pulse is preformed according to the back-calculated shape. This preformed laser pulse is in the process step 140 introduced into the input end of the optical transmission system. In this case, the laser pulse is coupled into the optical fiber by means of precise collimation optics. As the last procedural step 150 the application of the emerging from the output end of the optical transmission system laser pulse with or near the desired pulse shape for the Mehrphotonenanregung. The optical transmission system is conceivable, for example, as part of a medical Endoskopiegeräts. The multiphoton excitation can be a targeted molecular multi-photon excitation. A targeted change and shift of multiphoton spectra is conceivable. An example of the use of such multi-photon excitation is the determination of the molecular orientation and anisotopy of a sample.

In 4 is shown as a flowchart, another embodiment of the method with iterative optimization. In the first process step 200 the determination of the parameter space of all possible with the technical system pulse shapes at the output end of the optical transmission system. In this case, a parameter scan of the pulse shape parameters is carried out at the output end with determination of the parameter limits for the realizable input pulse energies. At the second step 210 the parameters of the laser pulse shape optimized for the desired multiphoton excitation are determined from the determined parameter space. This is done by an iterative optimization method, which is the one from step 210 goes through the following process steps, then again when not sufficiently good result for the step 210 come back. At least one parameter is kept variable in the case of a laser pulse shape selected from the determined parameter space for the multiphoton excitation. With the parameters of an output laser pulse shape takes place in the step 220 a recalculation of the pulse parameters along the fiber according to step 110 in 3 with a nonlinear Schrödinger equation and the split-step Fourier method. As in step 120 in 3 will then step in 230 a control signal for the located in the beam path in front of the input end pulse shaper for the formation of a laser pulse with the calculated back parameters calculated. Then this calculated control signal in the process step 240 applied to the pulse shaper and the amplitude, phase and polarization modulated for a guided in the pulse shaper laser pulse. In this way, the laser pulse is preformed according to the back-calculated shape. This preformed laser pulse is in the process step 250 introduced into the input end of the optical transmission system. In this case, the laser pulse is coupled into the optical fiber by means of precise collimation optics. As a further process step 260 the application of the output from the output end of the optical transmission system laser pulse with or near the selected pulse shape for the Mehrphotonenanregung. In the next step 270 the result of multiphoton excitation is determined. Meets that If the result obtained does not meet the expectations, the variable parameter in the pulse shape to be selected becomes the multiphoton excitation according to the optimization algorithm in step 210 changed and again with this altered pulse shape the steps 220 to 270 carried out until finally, after sufficiently many such iteration steps, the optimal or sufficiently good pulse shape of the laser pulse for the result of Mehrphotonenanregung is found. Then the iteration process terminates to determine the laser pulse shape.

In 5 In the form of a flow chart, another embodiment of the method with prior parameter space allocation is shown. The method for performing multiphoton excitation with at least one generated laser pulse having predefined amplitude, phase and polarization at the output end of an optical transmission system with an optical fiber is determined in advance in step 300 the possible parameter space of possible laser pulse forms at the input end of the optical transmission system and their associated control signal for the pulse shaper. In particular, the lower and upper limits of the variable parameters are determined for this purpose. Above all, the limits of the laser pulse intensity and the parameter limits to the respective laser pulse energy are determined. In the next step 310 is calculated from the determined parameter space at the input end of the optical transmission system in the forward direction along the optical fiber, the parameter space of the possible laser pulse shapes at the output end of the optical transmission system. The calculation is done again by a nonlinear Schrödinger equation using the split-step Fourier method as in step backwards 110 in 3 is described. As a result, an unambiguous assignment of possible laser pulse shapes at the input end to then laser pulse shapes at the output end of the optical transmission system is obtained depending on the pulse intensity. The calculation takes into account nonlinear physical parameters of the fiber, such as its non-linear part of the refractive index. With this calculated assignment of the parameter space of the possible laser pulse forms at the input end to that at the output end of the optical transmission system, a direct assignment of control signals for the pulse shaper before the input end of the optical transmission system to possible pulse shapes at the output end so the laser pulse shape in the multiphoton excitation is achieved. This eliminates the back calculation along the fiber when selecting the pulse shape at the output end of the optical transmission system, because of the in step 300 certain assignment, the control signal for the pulse shaper can be read directly without calculation. In process step 310 Thus, suitable parameters of the shape of the laser pulse with regard to phase, polarization and amplitude are determined for the multiphoton excitation and in step 320 these are selected from the calculated parameter space of the possible laser pulse shapes at the output end of the optical transmission system. Out of step 300 calculated assignment will be in the next step 330 determines the associated Pulse Former Control signal of the laser pulse at the input end of the optical transmission system. This Kotrollsignal is in step 340 is applied to the pulse shaper and modulates the amplitude, phase and polarization for a laser pulse guided into the pulse shaper, thereby preforming the laser pulse according to the shape intended for the input end of the optical transmission system. In step 350 The preformed laser pulse is introduced into the input end of the optical transmission system and comes out at the output end in the desired or almost desired shape. In the last step 360 the laser pulse for multiphoton excitation emerging from the output end of the optical transmission system is applied.

6 shows a spatial representation of a polarization-shaped laser pulse after an optical fiber having antisymmetric phases. The laser pulse with a length of approximately three picoseconds has different shapes in the two polarization planes, with their maxima being shifted significantly in time relative to one another. This double pulse after the fiber has linearly polarized square chirped subpulses perpendicular to each other. A subpulse has an antisymmetric cubic phase for the two-photon transition at 388 nm and the other one for the two-photon transition at 392 nm.

7 shows a diagram representation of a transmission adaptation. Since the spectral components are changed by non-linear effects in the fiber, a change in the incoming spectrum is required. This is realized by amplitude modulation in the pulse shaper. The transmission obtained by recalculation is entered into the pulse shaper so that the predefined laser pulse is obtained on the decoupling side. The transmission can not be greater than 1, however, the recalculation provides some larger values. To circumvent this problem is scaled so that one specifies a total transmission of less than 1 so that higher transmission values than this means are possible. Thus, for the recalculation, a fixed average transmission is entered so small that the maximum transmission does not become greater than 1, which is accompanied by correspondingly intensity-adjusted nonlinear effects. The 7 FIG. 4 shows an example of the transmission to be written onto the pixels of the modulator of the pulse shaper as a result of a recalculation for a specific laser pulse form for the multiphoton excitation, which is carried out with the aid of a transmission mean value set sufficiently below one.

In 8th 2, two examples of a comparison of the laser pulse shape at the output end of the optical transmission system in methods once without and once taking into account nonlinear parameters of the optical fiber are shown diagrammatically. These experimentally measured laser pulses agree well with the pulse shapes predetermined by the method according to the invention and are clearly better than those obtained without calculation of the non-linear behavior only dispersion-compensated pulses. In the determination, the non-linear processes include the self-phase modulation and self-division, for not too high pulse intensities these are the leading terms, so that there is already a good match with the experimentally found pulse shapes.

Claims (19)

Method for carrying out multiphoton excitation with at least one generated laser pulse, the predefined amplitude, phase and polarization at the output end of an optical transmission system ( 30 ) comprising at least one optical fiber ( 28 ), characterized in that the method comprises the following steps: - determine ( 100 ) of parameters of the desired shape of the laser pulse in terms of phase, polarization and amplitude for the multi-photon excitation; - to calculate ( 110 ) the parameter of the shape of the laser pulse from the output end along the at least one fiber ( 28 ) to the input end of the optical transmission system ( 30 ) using a nonlinear Schrödinger equation, wherein at least one nonlinear physical parameter of the at least one optical fiber ( 28 ) is taken into account; - to calculate ( 120 ) a control signal for the pulse generator located in the beam path in front of the input end ( 1 ) for shaping a laser pulse with the parameters calculated back; - apply ( 130 ) of the control signal on the pulse shaper ( 1 ) and modulate the amplitude, phase and polarization for one in the pulse shaper ( 1 ), whereby the laser pulse is preformed according to the back-calculated shape; - initiate ( 140 ) of the preformed laser pulse into the input end of the optical transmission system ( 30 ), and - apply ( 150 ) from the output end of the optical transmission system ( 30 ) exiting laser pulse for multiphoton excitation. Method according to claim 1, wherein in the calculation of the laser pulse parameters along the at least one optical fiber ( 28 ) the non-linear part of the refractive index is taken into account. The method of claim 2, wherein in the evaluation of the non-linear Schrödinger equation a split-step Fourier method is used. Method according to one of claims 1 to 3, wherein it comprises the step ( 200 ) the determination of the parameter space of all possible with the technical system pulse shapes at the output end of the optical transmission system ( 30 ) contains. Method according to one of claims 1 to 4, wherein the optical transmission along the at least one optical fiber ( 28 ) for the calculation along the at least one fiber ( 28 ) is set to a sufficiently low value so that the optical transmission does not become greater than 1 during the recalculation. Method according to one of claims 1 to 5, wherein the pulse shaping on the pulse shaper ( 1 ) a specific polarization-dependent incorporation of the non-linear parameters of the at least one optical fiber ( 28 ) contains. Method according to one of claims 1 to 6, wherein at the pulse shaper ( 1 ) the laser pulse is preformable in such a way that at the output end of the optical transmission system ( 30 ) The laser pulse is shaped differently in two different directions of polarization and spectrally shifted from each other. Method according to one of claims 1 to 7, wherein the determination of the parameters of the laser pulse, an iterative optimization of the method on the Mehrphotonenanregung by changing at least one parameter of the optical transmission system ( 30 ) exiting formed laser pulse back to the control signal of the pulse shaper ( 1 ) contains. Method according to one of claims 1 to 8, wherein the at least one optical fiber ( 28 ) is a single-mode optical fiber. Method according to one of claims 1 to 8, wherein the at least one optical fiber ( 28 ) is a microstructured hollow core fiber. A method according to any one of claims 1 to 10, wherein a parametric laser pulse shaping method is used. Method according to one of claims 1 to 11, wherein it is for a targeted molecular multi-photonic excitation after the at least one optical fiber ( 28 ) suitable is. Method according to one of claims 1 to 12, wherein it is used for the targeted modification and displacement of multiphoton spectra after the at least one optical fiber ( 28 ) suitable is. A method according to any one of claims 1 to 13, wherein it is suitable for determining the molecular orientation and anisotopy of a sample. Method according to one of claims 1 to 14, wherein the optical transmission system ( 30 ) Is part of a medical device, such as an endoscopy device. A computer program product stored on a computer-readable medium, comprising instructions for configuring a computer to calculate the recalculation along the at least one optical fiber ( 28 ) and the control signals for the pulse shaper ( 1 ) according to a method according to any one of claims 1 to 15. Method for carrying out multiphoton excitation with at least one generated laser pulse, the predefined amplitude, phase and polarization at the output end of an optical transmission system ( 30 ) comprising at least one optical fiber ( 28 ), characterized in that the method comprises the following steps: 300 ) of the parameter space of possible laser pulse forms at the input end of the optical transmission system ( 30 ) and its associated control signals for the pulse shaper ( 1 ); - to calculate ( 310 ) - from the determined parameter space at the input end of the optical transmission system ( 30 ) in the forward direction along the at least one fiber ( 28 ) using a nonlinear Schrödinger equation considering at least one of the non-linear physical parameters of the at least one fiber ( 28 ) - the parameter space of the possible laser pulse forms at the output end of the optical transmission system ( 30 ) for assigning possible laser pulse shapes at the input end to laser pulse shapes at the output end of the optical transmission system ( 30 ); - determine ( 310 ) parameters of the shape of the laser pulse with respect to phase, polarization and amplitude suitable for multiphoton excitation, for multiphoton excitation; - choose ( 320 ) of the determined parameter of the laser pulse from the calculated parameter space of the possible laser pulse shapes at the output end of the optical transmission system ( 30 ); - determine ( 330 ) of the associated pulse-shaping control signal for the laser pulse at the input end of the optical transmission system ( 30 ) from the assignment calculated in the second method step; - apply ( 340 ) of the control signal on the pulse shaper ( 1 ) and modulate the amplitude, phase and polarization for one in the pulse shaper ( 1 ) laser pulse, whereby the laser pulse according to that for the input end of the optical transmission system ( 30 ) preformed in certain shape; - initiate ( 350 ) of the preformed laser pulse into the input end of the optical transmission system ( 30 ); - apply ( 360 ) from the output end of the optical transmission system ( 30 ) exiting laser pulse for multiphoton excitation. A method according to claim 17, comprising one or more of the features of claims 2, 3 and 6 to 15. A computer program product stored on a computer-readable medium, comprising instructions for configuring a computer to calculate the computation along the at least one optical fiber ( 28 ) and the control signals for the pulse shaper ( 1 ) according to a method according to one of claims 17 and 18.

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