Ultrashort pulse laser

Abstract

An ultrashort pulse laser having a multipass cell (2) which has at least two mirrors (3) arranged such that the at least two mirrors (3) repeatedly reflect laser pulses, wherein the multipass cell (2) has a partial beam density of at least 0.2 / cm3.

Description

[0001] 33577-32

[0002] 1

[0003] Ultrashort pulse laser

[0004] This disclosure concerns ultrashort pulse lasers. Ultrashort pulse lasers are defined as lasers capable of producing laser pulses with a pulse duration – also known as pulse width – of less than IO. - 9 to generate pulses of less than 1 nanosecond. Ultrashort pulse lasers are also commonly referred to as femtosecond lasers or picosecond lasers.

[0005] Modern ultrashort pulse lasers exist primarily in two variants: either as solid-state laser systems or as fiber laser systems. Fiber laser systems are assumed to be inherently smaller, since the laser light is guided in small fiber cores and no bulky optics such as mirrors are required. However, although the fibers have a small diameter, they require a minimum bending diameter, which for a true polarization-preserving single-mode fiber like the PM-980 XP is on the order of 26 mm. Furthermore, every fiber laser oscillator requires fiber-coupled diodes and so-called fiber-coupled bulk elements (such as WDMs, or Wavelength Division Multiplexers), ultimately resulting in minimum total area for such oscillators on the order of approximately 100 mm x 150 mm.

[0006] With an ultrashort pulse laser, a distinction must be made between the aforementioned pulse duration of less than IO. - 9s and the repetition frequency, i.e., the number of ultrashort laser pulses that are generated or emitted per second.

[0007] Virtually every laser of the prior art, and preferably also the ultrashort pulse lasers of the present disclosure, employs a laser resonator bounded by resonator mirrors – hereinafter referred to simply as a resonator – in which light of certain defined wavelengths is reflected back and forth and amplified, for example, by stimulated emission in a laser medium. An ultrashort pulse laser is essentially determined by its repetition frequency (among various other parameters). This repetition frequency is directly related to the total length of the corresponding resonator according to the following equation.

[0008] Here, frepetition rate is the repetition frequency (sometimes also called the repetition rate), c is the speed of light in a vacuum, n is the refractive index of the material with which the resonator is filled (usually air), and L is the total length of the resonator, i.e., the length from one resonator mirror to the other. The product of refractive index and length is also called the optical path length (OPL). to t denotes .

[0009] Typical short-pulsed laser oscillators have a repetition rate of, for example, about 50 MHz, which, according to the formula given above, corresponds to a total physical resonator length L of about 3 meters. Miniaturizing such a laser is therefore obviously a challenge.

[0010] Furthermore, so-called multipass cells, which have at least two mirrors to reflect light back and forth, are generally known, for example from US 2008 / 0212217 Al. These are primarily used in spectroscopy for measuring low concentrations of components or for observing weak spectra in gases or liquids. It is advantageous if the light travels a long optical path through the gas or liquid so that the effects of the gas or liquid on the light can be measured. Miniaturization of any kind is not intended. It also appears to be known to implement an ultrashort pulse laser using a multipass cell as a resonator. For example, a laser called "FemtoTrain™ High-energy Fixed-Wavelength Femtosecond Oscillators" is available, which apparently uses a multipass cell.Due to the size of this device, it is obvious that miniaturization was not intended here either.

[0011] It is known that multipass cells are also used in ultrashort pulse laser resonators. This primarily involves generating low repetition rates, since according to at the same average output power P avg The pulse energy Epui se becomes greater the lower the repetition frequency fr. Rep The emission rate is... The size of such systems plays a subordinate role. This basic idea is sometimes used nowadays to generate high pulse energies directly from ultrashort pulse lasers.

[0012] The present disclosure aims to provide the smallest possible ultrashort pulse laser, with a particular focus on reducing its volume and / or footprint. This should result in no, or at least only acceptable, reductions in the laser's performance. Furthermore, the manufacturing effort for the ultrashort pulse laser is preferably kept within reasonable limits.

[0013] This problem is solved by an ultrashort pulse laser according to claim 1, namely an ultrashort pulse laser with a multipass cell which has at least two mirrors arranged such that the at least two mirrors repeatedly reflect laser pulses, characterized in that the multipass cell has a partial radiance of at least 0.2 / cm² 3 exhibits , where the partial radiance is given by the equation

[0014] OPL tot TSD = - -

[0015] “MPC ■ d maxis defined and

[0016] - TSD is the partial radiance ,

[0017] OPLtot is the total optical path length within the multipass cell.

[0018] - V M PC, the volume of the multipass cell is as well as

[0019] - d max the longest distance within the volume of the multipass cell is .

[0020] A fundamental aspect of this disclosure is the use of a multipass cell, primarily known from spectroscopic applications, for the miniaturization of an ultrashort pulse laser. The multipass cell is to be miniaturized to such an extent that the partial radiance TSD is greater than 0.2 µS / cm². 3 lies .

[0021] The partial radiance is calculated according to the formula given above, using the total optical path length (OPL) required for the specific ultrashort pulse laser. tot within the multipass cell through the volume of the multipass cell V MPC itself as well as through the longest distance within this volume d max The volume is divided. This formula ensures that small volumes with a small area but a large length are not given preferential treatment.

[0022] A small volume V MPC the multipass cell and – contrary to previous applications of multipass cells – a short distance d max In the multipass cell, the partial beam density (TSD) is increased and therefore improved. The ultrashort pulse laser is sometimes also referred to in technical language as a laser oscillator or simply oscillator.

[0023] As mentioned, the term ultrashort pulse laser refers to lasers capable of emitting laser pulses with a pulse duration – also known as pulse width – of less than IO. - 9to generate s (or less than a nanosecond).

[0024] In preferred embodiments, the ultrashort pulse laser can be used to generate or emit laser pulses with a pulse duration of less than 100 picoseconds (10 -10 s ), less than 10 picoseconds ( 10- 11 s ) or less than 1 picosecond ( 10- 12 s ).

[0025] Since the medium between the at least two mirrors can differ from air, the term optical path length (OPL) is commonly used to account for the fact that the medium may differ from air. This means that, due to the different speeds of light in various media, a substitute quantity can be defined that would correspond to the length of the multipass cell if it contained no medium but a vacuum with the corresponding speed of light c in a vacuum, and if the light took the same amount of time to travel from one end to the other.

[0026] The total optical path length traveled by a pulse in the multipass cell with multiple reflections at the at least two mirrors is referred to in this sense as OPL. to t denotes . A complete revolution within the resonator would thus be, for example, 2 ■ OPL totIn short, the entirety or at least a large part of the total optical path length required for the respective ultrashort pulse laser is folded into the smallest possible space by the repeated reflections of the laser pulses at the at least two mirrors of the multipass cell, according to the present disclosure.

[0027] The maximum length d max is defined in this disclosure as the longest possible distance within the volume V MPC the multipass cell.

[0028] In a particularly simple embodiment, the multipass cell could be formed by two mirrors facing each other, between which the laser pulses are reflected back and forth in a plane with a small offset at each reflection.

[0029] In this document, light is defined as electromagnetic waves of all frequencies or wavelengths, regardless of whether the frequency or wavelength is within the visible spectrum. Laser light, in particular, exhibits coherence, meaning that the electromagnetic waves in laser light oscillate in phase to a certain degree—ideally a high degree—which can be specified by the coherence length.

[0030] As an alternative to or in addition to the partial radiance, the radiance according to claim 16 can be used to quantify the miniaturization of the ultrashort pulse laser by means of a multipass cell. The radiance is given by the equation

[0031] OPL tot SD = - —

[0032] VMPC defines , where

[0033] SD is the radiance, OPLtot is the total optical path length within the multipass cell, and

[0034] - VM pc is the volume of the multipass cell.

[0035] According to the present disclosure, the radiance can preferably be at least 5 / cm². 2 exhibit .

[0036] It should be noted that while the partial radiance and the radiance are formulated with reference to rays, in the context of this disclosure, light rays are not necessarily reflected in the multipass cell. Rather, in an ultrashort pulse laser, laser pulses with pulse durations of less than IO are used. - 9 The light is reflected back and forth within the multipass cell. The beams can therefore be understood as imaginary paths along which the laser pulses propagate.

[0037] Radiance could also be described as the quality factor.

[0038] The volume of the multipass cell is not necessarily understood to be an external volume of the device, which depends, for example, on the physical design of the mirrors. Within the scope of the invention, the volume of the multipass cell can alternatively or additionally, and particularly preferably, be understood as the volume spanned by the aforementioned rays; that is, it is the smallest convex volume that encompasses all rays within the multipass cell.

[0039] Preferred further developments of the disclosure are defined in the dependent claims.

[0040] The multipass cell can have at least one flat mirror and one curved mirror, the curved mirror preferably being a spherical mirror. However, as experts know, it is in principle possible to replace any curved mirror with a suitable lens, and possibly also a flat mirror for beam deflection.

[0041] A crucial aspect of multipass cells is the pattern in which the laser pulses are reflected back and forth. Reflecting back and forth between two identical points on the at least two mirrors would obviously be counterproductive, because then the laser pulses entering the multipass cell could not be separated from those exiting.

[0042] It has been found that particularly dense reflection patterns can be generated in a relatively simple way by a combination of curved, especially spherically curved, mirrors and flat mirrors.

[0043] The use of cylindrically curved mirrors instead of spherically curved mirrors is also conceivable.

[0044] Spherically curved mirrors may be preferred due to their spherical symmetry, because reflections from spherical surfaces introduce less beam astigmatism and beam radius variations.

[0045] Multipass cells may be particularly preferred, where the points at which the imaginary rays reflect form Lissaj ous patterns, as these promise a particularly high beam partial density.

[0046] From a mathematical perspective, Lissa jous patterns arise from the superposition of two harmonic oscillations of different frequencies, perpendicular to each other. The multipass cell can preferably have a first mirror, a second mirror, and a third mirror, and the second and third mirrors can be positioned opposite the first mirror.

[0047] Put another way, the second mirror and the third mirror can face the first mirror and be arranged, for example, next to each other or one above the other.

[0048] By using a third mirror, an increase in the partial radiance can be achieved by creating an additional degree of freedom through the precise alignment of the third mirror (or the second mirror), which increases the reflection pattern with respect to the density of the imaginary partial rays.

[0049] Of course, the use of a fourth or fifth element in the form of mirrors is not excluded in this way.

[0050] Regarding the alignment of the mirrors, it may be provided that

[0051] - the first mirror has a first optical axis, the second mirror a second optical axis, and the third mirror a third optical axis.

[0052] - the first optical axis and the second optical axis are parallel to each other and

[0053] - the third optical axis is rotated relative to the first optical axis and the second optical axis.

[0054] Rotating the third optical axis relative to the first and second optical axes can be a particularly simple way of creating an additional degree of freedom for optimizing the partial radiance. For example, in multipass cells of this type, the first mirror can be curved, preferably spherically curved, and the second and third mirrors can be planar, or vice versa. Of course, other combinations are also easily conceivable.

[0055] Designs in which the first mirror, the second mirror and the third mirror are curved, preferably spherically curved, are also conceivable.

[0056] In particular, the points at which the imaginary rays reflect form the preferred Lissa ous patterns in multipass cells of this type.

[0057] This type of multipass cell was proposed by C. Robert in "Simple, stable, and compact multi-reflection optical cell for very long optical paths" (Einfache, stabile und kompakte Optische Mehrreflectionzelle für sehr lange Optische Pfade), Appl. Opt. 46, pp. 5408-5418, 2007, and in US 2008 / 0212217 Al, but in connection with applications in spectroscopy, not for ultrashort pulse lasers.

[0058] The third optical axis can be rotated relative to the second optical axis along an offset direction between the second and third mirrors by less than 20°, preferably less than 10°, and particularly preferably less than 5°. These values ​​have proven optimal for the miniaturization of ultrashort pulse lasers in the applicant's investigations.

[0059] This twist angle can also be called the yaw angle. As mentioned, the twist angle is not zero in preferred design configurations.

[0060] In preferred embodiments, the angle of twist can be between 0.1° and 2°, particularly preferably between 0.2° and 0.4°.

[0061] In particularly preferred embodiments, the angle of twist can be 0.25°.

[0062] Particularly advantageous are designs in which the second mirror is laterally offset relative to the third mirror. This creates spaces next to both the second and third mirrors, which are used for incoming and / or outgoing beams. These spaces allow incoming and outgoing laser pulses to enter and exit the multipass cell. This avoids the need for complex openings in the mirrors or similar components for the laser pulses entering or exiting the multipass cell, thus minimizing the need for valuable installation space.

[0063] It is important to emphasize that the incoming and outgoing imagined rays can also use the same space that is freed up.

[0064] Other particularly preferred embodiments may provide that the second and third mirrors are dimensioned such that the input and output beams lie on the same side of the two mirrors.

[0065] In principle, the positions of the input and output beams can be located anywhere within the cell. It is also conceivable that the input and / or output beam lies on the side of the first mirror, or that, with the help of so-called pick-off mirrors, the input beam is guided into the multipass cell and / or the output beam is guided out of the multipass cell.

[0066] For a simple determination, the longest extent of the multipass cell d can be used. max essentially with the value of a mirror distance d mirrOThe distance between the at least two mirrors is preferably substantially approximated to a distance of the first mirror from the second mirror and / or substantially approximated to a distance of the first mirror from the third mirror.

[0067] It should be noted that the partial radiance in this case can be expressed as follows:

[0068] Furthermore, if one defines an effective cross-section A e ff for the multipass cell The partial radiance can be expressed as follows:

[0069] If we further simplify by assuming that the total optical path length OPL to t an integer multiple n of the mirror distance d mirrO rs corresponds to , OPL tot = n ■ d mirrors , where the natural number n>0 describes the number of beams within the multipass cell, the partial radiance can be simplified as follows:

[0070] This nicely demonstrates that the partial radiance is a measure of the number of partial beams in the multipass cell volume.

[0071] Incidentally, with this definition of A e The radiance can be expressed as follows.

[0072] In preferred embodiments, the second and third mirrors can be bonded together, preferably by gluing, laser welding, soldering, and / or manufacturing from a single piece. This also allows for a simple construction of the multipass cell.

[0073] It should be mentioned that separate storage solutions for the second and third mirrors are certainly conceivable.

[0074] In particularly preferred embodiments, the multipass cell exhibits a partial radiance of at least 1 / cm². 3 or 10 / cm 3 or 20 / cm 3According to the present revelation, the miniaturization of ultrashort pulse lasers can not only begin, but also progress by several orders of magnitude. The higher the partial beam density, the smaller the ultrashort pulse laser can obviously be made.

[0075] In particularly preferred versions, the

[0076] Partial radiance in the range of 23 / cm 3 amount to .

[0077] In highly preferred configurations, the multipass cell exhibits a radiance of at least 20 / cm². 2 or 40 / cm 2 or 60 / cm 2These designs can offer similar size reductions, whereby designs with minimum radiation density may have a greater extent in one direction and a smaller extent in another compared to designs with minimum partial radiation density. In other words, designs with minimum radiation density can be particularly long compared to designs with minimum partial radiation density.

[0078] In particularly preferred embodiments, the radiance can be in the range of 65 / cm². 2 amount to .

[0079] In particularly preferred embodiments, the physical size of the ultrashort pulse laser, especially the housing, can be optimized by choosing the size of the at least two mirrors such that the volume spanned by the at least two mirrors of the multipass cell is only so much larger than the volume of the multipass cell that the desired quality of the laser pulses is achieved.

[0080] Other multipass cells besides those proposed by Robert above can also be used according to the present disclosure. The following discusses some different known types of multipass cells that can, in principle, also be used with the present disclosure.

[0081] An early multipass cell was developed by John U. White in 1942 (J. U. White, "Long Optical Path of Large Aperture", J. Opt. Soc. Am., Vol. 32, pp. 285-288, 1942). The White cell consists of three spherical, concave mirrors with the same radius of curvature. The distance between the mirrors corresponds to their radii of curvature. The beam then passes between these three mirrors in a single plane, with the number of passes adjustable by slight rotation of the mirrors.

[0082] The Herriott cell is probably the best-known multipass cell and consists of only two concave spherical mirrors. It was proposed by Donald R. Herriott in 1964 (DR Herriott, "Off-axis paths in spherical mirror interferometers", Appl. Opt., Vol. 3, pp. 523ff, 1964). The light beam enters and exits the cell through a hole in one of the mirrors. The path of the light within the Herriott cell is no longer confined to a single plane; instead, the reflections from the mirrors describe ellipses. The number of possible paths is determined by the distance between the mirrors. Because the reflections from the mirrors describe ellipses, most of the mirror surface is unused due to the multiple reflections. Therefore, large mirrors are required for long optical path lengths, as the light occupies only a small portion of the cell's volume. In this sense, these cells are not optimized for space efficiency.Denser patterns can be generated with astigmatic mirrors (DR Herriott, "Folded optical delay lines", Appl. Opt., Vol. 4, pp. 883-889, 1965), but this requires custom-made mirrors with exceptionally well-defined focal lengths. Furthermore, the laser beam in such cells is highly astigmatic and varies considerably in size. Therefore, while such cells are suitable for external beam folding, their use in laser resonators is limited. However, as mentioned, their use in connection with the present disclosure is certainly conceivable.

[0083] An improved method for generating dense patterns was proposed by Hoa et al. in 2002. Their cell consists of two cylindrical mirrors with different focal lengths and orthogonally opposite curvatures (L.-Y. Hao, "Cylindrical mirror multipass Lissajous system for laser photoacoustic spectroscopy," Rev. Sei. Instrum., vol. 73, pp. 2079-2085, 2002). This is essentially the folding of an astigmatic cell as derived by Herriott et al., using a plane mirror and an astigmatic mirror, and then transferring a curved axis to the plane mirror. Consequently, this design still requires very tight tolerances for the corresponding focal lengths. In addition to all the disadvantages of astigmatic cells for laser resonators (such as beam astigmatism and large beam radius variations), the use of cylindrical optics introduces an additional layer of complexity.

[0084] In 2005, Joel Silver (JA Silver, "Simple dense-pattern optical multipass cells", Appl. Opt., Vol. 44, pp. 6545-6556, 2005; JA Silver, "Dense pattern optical multipass cell". Patent EP 1 621 867 Al, July 21, 2005; US 2006158644 Al) introduced a new, simpler, and less demanding mirror system for achieving a multipass optical cell with dense dot patterns. This new cell consisted of either a pair of cylindrical mirrors or one cylindrical and one spherical mirror. In principle, however, this cell behaves like a single astigmatic Herriott cell and exhibits all the disadvantages of this type of cell for use in laser resonators. Dense Lissa ous patterns only appear when one of the mirrors is rotated around its optical axis.

[0085] In 2006, Claude Robert introduced the aforementioned advanced type of multipass cell, which combines White and Herriott cells. His main idea was to combine two Herriott cells with slightly different optical axes into a single cell consisting of three mirrors (as in a White cell). This allows the light beam to circulate along several different ellipses within the cell, utilizing most of the available mirror area. Robert's idea was primarily aimed at measuring very small

[0086] Gas concentrations in industrial processes and in the ambient air (air pollution) decrease because the corresponding absorption signals - and thus also the sensitivity - scale exponentially with the length of the optical path.

[0087] The multipass cell can preferably be integrated into or form a resonator of the ultrashort pulse laser.

[0088] The integration of the multipass cell into the resonator can be understood to mean that the multipass cell is integrated into the resonator's imaginary beam path in such a way that the laser pulses enter the multipass cell, are repeatedly reflected by at least two mirrors according to the present disclosure, and exit the multipass cell again in the resonator's beam path. In a certain sense, according to this description, the multipass cell is thus arranged between the resonator mirrors of the resonator. However, it is also conceivable that the multipass cell itself functions as a resonator mirror of the resonator.

[0089] The fact that the multipass cell forms the resonator can be understood to mean that the multipass cell is the resonator, that is, that the at least two mirrors of the multipass cell are the resonator mirrors of the resonator.

[0090] Of course, hybrid forms are also conceivable, where the at least two mirrors are a subset of the resonator mirrors and additional resonator mirrors complete the resonator. A mode-locking device, particularly preferably integrated into the resonator, is especially preferred. Mode-locking is a well-known group of techniques for generating ultrashort laser pulses, whereby it is enforced in various ways that not a uniformly distributed electromagnetic wave is formed in the resonator, but rather one or more pulses are generated that are coupled out of the laser (usually with the aid of a partially transparent coupling mirror) and thus emitted.

[0091] Mode locking is sometimes also referred to as mode coupling.

[0092] A distinction is made between active and passive mode-locking.

[0093] In active mode-locking, an active modulator is present, which, for example, via an acousto-optic or an electro-optic, actively modulates the electromagnetic field in the resonator. In these implementations, the active modulator thus constitutes the mode-locking device.

[0094] In passive mode-locking, for example, a saturable absorber is present whose absorption coefficient decreases with increasing light intensity, so that it becomes transparent at high intensities and can therefore serve as a passive switch in laser resonators to generate short, powerful pulses. Here, the saturable absorber can be considered a mode-locking device.

[0095] At least one of the mirrors – preferably several or all mirrors – of the multipass cell can preferably have a second-order dispersion coating. The dispersion of the laser pulses resulting from reflection at the at least two mirrors ensures that the ultrashort laser pulses do not "disperse" but are balanced and thus do not lengthen.

[0096] In particular, second-order dispersion coatings can be used to control the total group velocity dispersion (GVD) present in the resonator. This group velocity dispersion is also called second-order dispersion. The group velocity dispersion is independent of the length L. mate riai of the respective optical medium, while the group delay dispersion (GDD) takes into account the length of the medium:

[0097] GDD = GVD ■ L materkl |

[0098] By applying and adjusting the coating to at least one mirror, the GDD of the entire resonator can be adapted to the GDD necessary for mode-locking; this prevents the ultrashort pulses in the resonator from broadening on average.

[0099] Other coatings can also be used as an alternative or in addition, such as at least one of the following:

[0100] - dispersive layers (for example, GTI layers and / or so-called "chirped mirrors")

[0101] - dichroic coatings (highly transparent for at least one wavelength and highly reflective for at least one other wavelength)

[0102] - partially permeable coatings

[0103] According to the present disclosure, ultrashort pulse lasers can have a width of less than 120 mm, preferably less than 100 mm, and particularly preferably less than 80 mm. According to the present disclosure, ultrashort pulse lasers can have a depth of less than 100 mm, preferably less than 80 mm, and particularly preferably less than 60 mm.

[0104] According to the present disclosure, ultrashort pulse laser housings can have a height of less than 80 mm, preferably less than 60 mm and particularly preferably less than 40 mm.

[0105] The dimensions of the ultrashort pulse laser mentioned here are preferably the dimensions of a housing for the ultrashort pulse laser.

[0106] It should be mentioned here that within the aforementioned volume, particularly within the housing, preferably all parts necessary for the operation of an ultrashort pulse laser are located; these include, but are not limited to:

[0107] - all electronics for both the operation of the laser system and for any diagnostics

[0108] - the entire cooling system for the operation of the laser system

[0109] Therefore, only an external power supply, preferably 24V or 48V, should be required to operate the laser. However, other DC or AC voltages are also possible.

[0110] The ultrashort pulse laser can preferably have a repetition frequency between 5 MHz and 200 MHz.

[0111] Further details of the present disclosure will become apparent from the figures and the accompanying figure description. Figure 1 schematically shows an embodiment of a

[0112] Multipass cell,

[0113] Fig. 2 schematically shows an embodiment of a

[0114] Multipass cell,

[0115] Figs. 3a and 3b show diagrams of possible reflection patterns in a multipass cell.

[0116] Fig. 4 schematically shows a multipass cell from Fig. 2 in a view along an offset direction between the second mirror and the third mirror.

[0117] Figs. 5 and 6 Diagrams showing the course of beam radii in a multipass cell,

[0118] Fig. 7 shows a ray-tracing simulation of the multipass cell from

[0119] Fig. 2,

[0120] Fig. 8 a diagram of reflection points in a

[0121] Multipass cell according to Fig. 2,

[0122] Figs. 9a and 9b schematically show exemplary embodiments of a

[0123] Ultrashort pulse lasers,

[0124] Fig. 10 is a rendered representation of a

[0125] Exemplary embodiment of an ultrashort pulse laser, as well as

[0126] Fig. 11 shows a particularly simple embodiment of a

[0127] Multipass cell with the associated variables for calculating the multipass cell volume.

[0128] Fig. 1 schematically shows an embodiment of a multipass cell 2. In a first embodiment, it consists of two mirrors 3 arranged opposite each other, which are positioned on the left and right outer edges of the drawing and between which laser pulses can be reflected. The mirrors 3 positioned on the left and right outer edges of the drawing are spherically curved mirrors 3, although this is not apparent in the schematic drawing, partly because the radii of the spherical curvatures are very large.

[0129] Preferably, the radii of the spherical curvatures can be the same. However, it would of course be conceivable if the spherical curvature of mirror 3 differed between the leftmost and rightmost parts of the drawing.

[0130] In a second embodiment, a planar mirror 3 could be provided in the center instead of the mirror 3 shown on the far right in Fig. 1. The laser pulses would then be reflected back and forth between the spherically curvature mirror 3 located on the far left of the drawing and the central planar mirror 3. This is indicated in the drawing by the dashed line representing the right part of the beam path.

[0131] As can be seen, even in this simple embodiment a reduction in the size of the multipass cell 2 can be achieved simply by changing the geometry of the mirror 3 and moving the mirror 3 closer to the other mirror 3.

[0132] Fig. 2 schematically shows another embodiment of a multipass cell 2. In this embodiment, a first mirror 4, a second mirror 5, and a third mirror 6 are provided, and the second mirror 5 and the third mirror 6 are arranged opposite the first mirror 4, so that the laser pulses can be reflected back and forth between the first mirror 4 on one side and the second mirror 5 and the third mirror 6 on the other side. In this embodiment, the second mirror 5 and the third mirror 6 could also be considered a split mirror 3.

[0133] The first mirror 4 has a first optical axis 7, the second mirror 5 has a second optical axis 8 and the third mirror 6 has a third optical axis 9.

[0134] The optical axis of a mirror is preferably understood as an imaginary line which passes through the center of this mirror and is perpendicular to its surface.

[0135] In this exemplary embodiment, the first mirror 4 and the second mirror 5 are arranged such that the first optical axis 7 and the second optical axis 8 are parallel to each other, as can also be seen in Fig. 4.

[0136] The third optical axis 9 is rotated by approximately 5° relative to the second optical axis 8 along an offset direction between the second mirror 5 and the third mirror 6 (yaw angle). Reference should also be made to Fig. 4, which schematically shows the implementation of the multipass cell 2 from Fig. 2 along an offset direction between the second mirror 5 and the third mirror 6. This offset direction is indicated in Fig. 2 by an upward-pointing arrow.

[0137] In this exemplary embodiment, the offset direction is identical to the axis of rotation about which the third mirror 6 is rotated relative to the second mirror 5. To illustrate how the laser pulses are reflected back and forth between the different mirrors 3, reference is made to Fig. 7.

[0138] Fig. 7 shows a ray-tracing simulation, in which the path of a laser pulse entering the multipass cell 2 (“Input”) was calculated and drawn in the figure, up to the point where it leaves the multipass cell 2 (“Exit”).

[0139] As can be clearly seen in Fig. 7, the imaginary beams are packed very densely along which the laser pulses are reflected back and forth between the mirrors 3. This makes it possible to "fold" the entire optical path length required for the ultrashort pulse laser 1 into a small volume, thereby achieving a small footprint for the ultrashort pulse laser 1. This intuitively illustrates why the multipass cell 2 according to Fig. 2 allows very high partial beam densities and beam densities.

[0140] The multipass cell 2 according to Fig. 2 has a partial radiance TSD of 20 / cm². 3 on .

[0141] The multipass cell 2 according to Fig. 2 has a radiance SD of 60 / cm². 2 on .

[0142] To illustrate possible patterns that can be achieved by the reflection of the laser pulse at the mirrors 3, reference is made to Fig. 3. Fig. 3 shows representations of two possible paths of the reflection points on the second mirror 5 and the third mirror 6 of the embodiment according to Fig. 2, specifically in a plane that lies directly on the second mirror 5. It can be seen from Fig. 2 and Fig. 7 that the second mirror 5 and the third mirror 6 are laterally displaced relative to each other.

[0143] In this case, the lateral displacement is oriented perpendicular to the offset direction indicated by the upward-pointing arrow in Fig. 2.

[0144] As mentioned, there are preferred embodiments in which the offset direction is simultaneously an axis of rotation for a third mirror 6 rotated relative to the second mirror 5.

[0145] It is evident that incoming laser pulses ("input") and outgoing laser pulses ("exit") can each pass through the free space created by the lateral offset above the second mirror 5 and below the third mirror 6, respectively. The laser pulses can thus enter and exit the multipass cell 2 in an elegant manner, without the need for, for example, difficult-to-manufacture openings in the mirrors.

[0146] In the exemplary embodiment according to Fig. 2, the second mirror 5 and the third mirror 6 are materially bonded to each other, for example by gluing, laser welding, soldering and / or manufacturing from one piece.

[0147] The configuration of the multipass cell 2 according to Fig. 2 can also be varied. For example, the first mirror 4 could be a planar mirror, and the second mirror 5 and the third mirror 6 could have a curvature, preferably a spherical curvature. The second mirror 5 and the third mirror 6 of the multipass cell 2 according to Fig. 1 or Fig. 2 preferably have a second-order dispersion coating.

[0148] Investigations by the applicant have shown that it can be particularly advantageous to also consider the radial structure of the individual laser pulses, i.e., their area as seen along the beam path. The beam radius is used as the parameter for this purpose.

[0149] It should be noted that the laser pulses are preferably rotationally symmetric with respect to the direction of propagation, so that the beam radius is indeed a radius. However, this is not strictly necessary. For example, if mirrors 3 with a cylindrical curvature are used instead of mirrors 3 with a spherical curvature, which is quite possible, the laser pulses can lose their rotational symmetry about the direction of propagation. Instead of a single beam radius, two beam radii can then be defined, for example, on two axes at 90° to each other. It is also possible to use an average value, or another statistical measure, over the individual azimuthal amplitudes of the laser pulses.

[0150] Two opposing influences come into play when modulating the beam radius of the individual laser pulses. On the one hand, as many reflections at the mirrors 3 as possible should be accommodated in the multipass cell 2. On the other hand, excessively large beam radii can lead to losses if the laser pulses no longer fit entirely on the reflecting mirror and are therefore laterally clipped.

[0151] Fig. 5 shows the beam radius along an arbitrary pattern of

[0152] Reflections in the multipass cell 2 according to Fig. 2. It is evident that a fairly high variation in the beam radii occurs. For example, where the beam radius has a maximum at the reflection from the first mirror 4, it is quite possible that the laser pulses no longer fit entirely onto the first mirror 4 and are therefore laterally clipped.

[0153] However, optimized reflection patterns can be found to avoid high beam radius values. An example of this is shown in Fig. 6. The reflection pattern used to achieve the beam radius shown in Fig. 6 is, incidentally, the same as that shown in Fig. 7 and is further illustrated in Fig. 8.

[0154] As can be seen in Fig. 8, in a preferred embodiment, the reflection points are controlled such that they lie at one of several heights on the second mirror 5 and the third mirror 6. Compare also the alignment lines 14, which illustrate two of the heights.

[0155] The multipass cell according to the embodiment shown in Figures 7 and 8 folds a path length of approximately 2.6 meters into a cell with a total volume of only 4.0 cm³. 3 2.6 meters corresponds to a repetition frequency of approximately 58 MHz.

[0156] Fig. 9a schematically shows an ultrashort pulse laser 1. In this embodiment, a strong feedback is generated by a resonator 10 to maintain a continuous oscillation.

[0157] The resonator 10 of the ultrashort pulse laser 1 is formed by one or more resonator mirrors 12 and the multipass cell 2, wherein at least one of the resonator mirrors 12 is, for example, a semi-transparent mirror, so that individual laser pulses can be coupled out through this resonator mirror 12 and thus emitted.

[0158] The multipass cell 2 can be designed, for example, as shown in Fig. 1 or 2, or in the described variants.

[0159] Within the resonator 10 is the amplification medium 13, which generates the laser light by stimulated emission. Amplification media 13 are known per se in the prior art. They can exist in solid, liquid and / or gaseous phases.

[0160] Furthermore, a mode-locking device 11 is provided, which is known in the prior art. Even though the mode-locking device 11 is shown inside the resonator in Fig. 9a.

[0161] While 10 are shown, this is not necessarily required. For example, fashion locking devices can

[0162] 11 may also be integrated into one of the resonator mirrors 12.

[0163] For example, alternatively or additionally, a semiconductor mirror with a saturable absorber or another means of mode-locking the oscillator may be provided.

[0164] The ultrashort pulse laser 1 according to Fig. 9a, for example, has a repetition frequency of 50 MHz.

[0165] The ultrashort pulse laser 1 also features a pumping device known in the prior art (not shown).

[0166] Fig. 9b shows an embodiment of an ultrashort pulse laser 1, which is essentially analogous to that of Fig. 9a. In contrast to the embodiment of Fig. 9a, the embodiment according to Fig. 9b has a second resonator mirror 12 and the multipass cell 2 is optically arranged between the resonator mirrors 12.

[0167] Fig. 10 shows a rendered representation of an ultrashort pulse laser 1 according to the present disclosure in comparison to a two-euro coin. As can be seen, the ultrashort pulse laser 1 fits into a volume approximately equivalent to two matchboxes. The footprint of the ultrashort pulse laser 1 is considerably reduced compared to the prior art.

[0168] In fact, the ultrashort pulse laser 1 according to Fig. 10 allows ultrashort laser pulses in the femtosecond range to be generated and sent out, with high output powers of more than 1 W on a footprint of only 50 x 70 mm. 2 can be achieved.

[0169] Fig. 11 shows a particularly simple example of a multipass cell 2, in which two mirrors 3 are present between which the laser pulses are reflected back and forth multiple times.

[0170] The angle of incidence a indicates the angle at which the laser pulses enter the multipass cell 2 (bottom left) in relation to its longitudinal axis.

[0171] The longitudinal axis is parallel to the optical axes of the mirrors 3.

[0172] Figure 11 illustrates what, according to the present disclosure, is called volume V. MPC The volume is defined as the volume spanned by the rays reflected back and forth within the multipass cell 2. As mentioned, this is the volume spanned by the rays reflected back and forth within the multipass cell 2. This volume is shaded gray in Fig. 11.

[0173] It is essentially the volume between the incident ray, the outgoing ray and the planes bounded by the mirrors 3.

[0174] In the exemplary embodiment according to Fig. 11, the laser pulses are reflected back and forth only in one plane, namely the plane of the drawing in Fig. 11. The third dimension of the volume V MPC In this exemplary embodiment, the multipass cell 2 is therefore essentially defined by twice the beam radius, which serves as a measure of the diameter of the laser pulses and thus the thickness of the volume V. MPC The multipass cell 2 can be used.

[0175] The maximum length d max In this example, the distance d would be more or less the same. M The reflections of the two mirrors correspond, at least as long as the lateral extent of the at The length of the multipass cell is much smaller compared to its length. To determine the length of the cell... max To calculate it exactly, one would also have to take into account the lateral extent of the multipass cell; in this example, this would be:

[0176] It should be mentioned that, even in the very simple embodiment shown in Fig. 11, it is obviously possible to achieve a relatively large total optical path length (OPL) in a small volume. to to accommodate . Reference list

[0177] 1 ultrashort pulse laser

[0178] 2 Multipass cell

[0179] 3 mirrors

[0180] 4 first mirror

[0181] 5 second mirror

[0182] 6 third mirror

[0183] 7 first optical axis

[0184] 8 second optical axis

[0185] 9 third optical axis

[0186] 10 Resonator

[0187] 11 Mode-Locking Device

[0188] 12 resonator mirrors

[0189] 13 Amplification medium

[0190] 14 alignment lines

Claims

33577-32 Patent claims 1. Ultrashort pulse laser with a multipass cell (2) which has at least two mirrors (3) arranged such that the at least two mirrors (3) repeatedly reflect laser pulses, characterized in that the multipass cell (2) has a partial radiance of at least 0.2 / cm² 3 exhibits, where the partial radiance is determined by the equation OPL tot TSD = - - “MPC ■ d max is defined and TSD is the partial radiance OPLtot is the total optical path length within the multipass cell (2), V M pc is the volume of the multipass cell (2) as well as d max the longest distance within the volume of the multipass cell (2).

2. Ultrashort pulse laser according to claim 1, wherein the multipass cell (2) comprises at least one planar mirror (3) and one curved mirror (3), wherein the curved mirror (3) is preferably a spherical mirror (3).

3. Ultrashort pulse laser according to one of the preceding claims, wherein the multipass cell (2) has a first mirror (4), a second mirror (5) and a third mirror (6) and the second mirror (5) and the third mirror (6) are arranged opposite the first mirror (4). 33577-32 33 4. Ultrashort pulse laser according to claim 3, wherein the first mirror (4) has a first optical axis (7), the second mirror (5) has a second optical axis (8) and the third mirror (6) has a third optical axis (9), the first optical axis (7) and the second optical axis (8) are parallel to each other and the third optical axis (9) is rotated relative to the first optical axis (7) and the second optical axis (8).

5. Ultrashort pulse laser according to claim 4, wherein the third optical axis (9) is rotated relative to the second optical axis (8) along an offset direction between the second mirror (5) and the third mirror (6) by less than 20°, preferably less than 10° and particularly preferably less than 5°.

6. Ultrashort pulse laser according to one of claims 3 to 5, wherein the second mirror (5) is laterally displaced relative to the third mirror (6).

7. Ultrashort pulse laser according to one of claims 3 to 6, wherein the second mirror (5) and the third mirror (6) are materially bonded to each other, preferably by gluing, laser welding, soldering and / or manufacturing from one piece.

8. Ultrashort pulse laser according to one of the preceding claims, wherein the multipass cell (2) has a partial radiance of at least 1 / cm² 3 or 10 / cm 3 or 20 / cm 3 exhibits. 33577-32 34 9. Ultrashort pulse laser according to one of the preceding claims, wherein the multipass cell (2) is integrated into and / or forms a resonator (10) of the ultrashort pulse laser (1).

10. Ultrashort pulse laser according to one of the preceding claims, wherein a mode-locking device (11) is provided.

11. Ultrashort pulse laser according to one of the preceding claims, wherein at least one of the mirrors (3) - preferably all mirrors (3) - of the multipass cell (2) is of a 2nd order have a dispersion coating.

12. Ultrashort pulse laser according to one of the preceding claims, wherein the ultrashort pulse laser (1) has a width of less than 120 mm, preferably less than 100 mm and particularly preferably less than 80 mm.

13. Ultrashort pulse laser according to any of the preceding claims, wherein the ultrashort pulse laser (1) has a depth of less than 100 mm, preferably less than 80 mm and particularly preferably less than 60 mm.

14. Ultrashort pulse laser according to any of the preceding claims, wherein the ultrashort pulse laser (1) has a height of less than 80 mm, preferably less than 60 mm and particularly preferably less than 40 mm.

15. Ultrashort pulse laser according to one of the preceding claims, wherein the ultrashort pulse laser (1) has a repetition frequency between 5 MHz and 200 MHz. 33577-32 35 16. Ultrashort pulse laser, in particular according to one of the preceding claims, with a multipass cell (2) which has at least two mirrors (3) arranged such that the at least two mirrors (3) repeatedly reflect laser pulses, characterized in that the multipass cell (2) has a radiance of at least 5 / cm²2 exhibits, where the radiance is determined by the equation OPL tot SD = - — VMPC is defined and SD is the radiance. OPLtot is the total optical path length within the multipass cell (2) as well as V M pc is the volume of the multipass cell (2).

17. Ultrashort pulse laser according to claim 16, wherein the multipass cell (2) a radiance of at least 20 / cm² 2 or 40 / cm 2 or 60 / cm 2 exhibits.

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