WO2004004080A1

WO2004004080A1 - Resonator, regenerative amplifier for ultrashort laser pulses, and a multi-layer mirror

Info

Publication number: WO2004004080A1
Application number: PCT/EP2002/007058
Authority: WO
Inventors: Carsten Fallnich
Original assignee: Lzh Laserzentrum Hannover E.V.
Priority date: 2002-06-26
Filing date: 2002-06-26
Publication date: 2004-01-08
Also published as: AU2002328304A1

Abstract

The invention relates to a resonator (4) for amplifying laser pulses, especially ultrashort laser pulses. Said resonator comprises at least two mirrors (6, 8, 12) for reflecting the laser pulses, an optical amplifying medium which is arranged between said mirrors (6, 8, 12) and is used to amplify the laser pulses, and means for temporally stretching the laser pulses. According to the invention, the means for temporally stretching the laser pulses are formed by at least one mirror, the group velocity dispersion of said mirror being selected such that the laser pulses are temporally stretched for the reduction of the maximum output. The mirror in the inventive regenerative amplifier preferably has a positive group velocity dispersion.

Description

Resonator, regenerative amplifier for ultrashort laser pulses and multilayer mirrors

The invention relates to a resonator of the type mentioned in the preamble of claims 1 and 2, a regenerative amplifier of the type mentioned in the preamble of claim 13 for ultrashort laser pulses and a multilayer mirror of the type mentioned in the preamble of claim 15.

A resonator of the type in question is generally known. It serves to amplify ultrashort laser pulses in a regenerative amplifier and has at least two mirrors for reflecting the laser pulses, between which an optical amplification medium, in particular a laser medium, is arranged for amplifying the laser pulses. During operation of the known resonator, laser pulses are injected into the resonator by a seed laser, reflected back and forth between the mirrors and amplified as they pass through the laser medium. Since the pulses experience a high amplification, there is a risk that the optical components of the resonator or the amplifier will be damaged by the high peak powers of the laser pulses that occur.

For this reason, it is necessary to temporally stretch the ultrashort laser pulses in order to ensure that the peak powers occurring during the amplification do not damage the optical components of the laser resonator and the regenerative amplifier. To For this purpose, the known resonator has means for the temporal expansion of the laser pulses, which are formed, for example, by a grating stretcher arranged outside the resonator. A disadvantage of an amplifier for the use of the known laser resonator for ultra-short laser pulses is that it is complex and therefore expensive to manufacture and takes up a lot of space.

The invention has for its object to provide a laser resonator of the type mentioned in the preamble of claim 1, which does not have the disadvantages of the known resonator, which therefore enables the construction of an amplifier for ultrashort laser pulses, which is simpler and easier than the known amplifiers so that it can be manufactured more cost-effectively and is therefore more compact. The invention is also based on the object of specifying an amplifier for ultrashort laser pulses which is simple and inexpensive to produce and is compact. This object is achieved with regard to the resonator by the teaching of claims 1 and 2 and with regard to the amplifier by the teaching of claim 13.

The invention is based on the finding that complex optical components, for example a grating stretcher arranged outside the resonator, are not required in order to achieve a sufficient temporal expansion of the laser pulses, but rather that simple and inexpensive optical components, such as those found in resonators or amplifiers are present anyway, can be used for this purpose, provided that their group speed dispersion is selected so that a time required to reduce the peak power to the extent necessary before the laser pulses are stretched by these components. The group velocity dispersion (GVD) is defined as follows:

GVD = ß ₂ λ ³ d ² n

2πc ² dλ ²

where λ is the wavelength of the laser radiation used, c is the speed of light and n = n (λ) is the wavelength-dependent refractive index. A GVD> 0 is called normal dispersion and a GVD <0 is called anomalous dispersion.

According to the teaching of claim 1, the means for temporal expansion are formed by at least one mirror, the group velocity dispersion of which is selected such that the temporal expansion of the laser pulses required to reduce the peak power is achieved. Through a targeted choice of the group velocity dispersion of the mirror, for example in the case of a multilayer mirror by choice of the material or the thickness of successive layers of the mirror in the beam direction, the required temporal expansion of the laser pulses can be achieved with simple and inexpensive means.

The resonator and the amplifier according to the invention are thus simple and therefore inexpensive to manufacture. According to the teaching of claim 2, the group speed dispersion of the internal resonator optical components of the resonator is chosen so that a time expansion of the laser pulses required to reduce the peak power is achieved exclusively by the optical component of the resonator is achieved. The resonator-internal optical components thus form the means for temporal expansion, so that additional components are not required and the resonator is particularly simple and therefore inexpensive. An extremely advantageous development of the teaching of claim 1 provides that the mirror is formed by an internal resonator mirror. In this embodiment, a separate mirror is not required to achieve the desired stretching of the laser pulses, so that a particularly compact construction results.

According to the respective requirements, however, the mirror used to stretch the laser pulses can also be formed by a separate mirror arranged outside the resonator, as is provided in one embodiment. At least one mirror, which is used for pulse stretching, can also be provided both within the resonator and outside the resonator. If the resonator-internal optical components of the resonator have a positive group velocity dispersion and thus cause a certain temporal expansion of the laser pulses anyway, it is to achieve an additional temporal expansion, which may be necessary to reduce the peak power to the required extent, depending on the respective requirements. expedient that the mirror has a positive group velocity dispersion. In this way, the temporal expansion of the laser pulses introduced by the optical components of the resonator is increased to the required extent.

In contrast, if the optical components of the resonator have a negative group velocity dispersion, for example when using certain ones Laser media as optical amplification media, it is expedient to achieve the required temporal expansion of the laser pulses that the mirror has a negative group velocity dispersion. In order to be able to select the group velocity dispersion in accordance with the respective requirements, the mirror is expediently designed as a multilayer mirror, the thickness and / or the material of successive layers of the dielectric material applied to a substrate being selected such that the mirror has the desired group velocity dispersion. If the mirror is to have a negative group velocity dispersion, for example a "chirped" or "double chirped" mirror can be used, in which the thickness of successive layers applied to a substrate increases from the free side of the mirror towards the substrate. Such mirrors to achieve a negative group velocity dispersion are described, for example, in the essay "Theory of Double-chirped Mirrors" by Nicolai Matuschek, Franz X. Kärtner, Ursula Keller in the IEEE Journal of Selected Topics in Quant. Electron. 4, 197/1998. In order to achieve a positive group velocity dispersion, the invention proposes for the first time a chirped mirror in which the layer sequence is inverted compared to the known chirped mirrors, so that the thickness of successive layers decreases from the free side of the mirror to the substrate. In the case of such a mirror, which is also referred to below as an inverted-chirped or inverted double-chirped mirror, the long-wave frequency components of the laser pulse are preferably in the front, further away from the substrate in the beam direction Layers of the mirror reflected, since their greater layer thickness is adapted to the longer wavelength. Accordingly, the shorter-wave frequency components are preferably reflected in the thinner layers closer to the substrate, since only there does the reflectivity increase for shorter wavelengths due to the decreasing layer thickness of the layers. The different reflection locations of the different spectral components of the laser pulses lead to time delays or phase shifts, so that the pulse leading edge is reflected earlier and the pulse trailing edge is reflected later. As a result, the pulse is stretched in time and phase-modulated at the same time, so that the pulse peak power decreases with increasing pulse duration. Accordingly, in order to achieve a positive group velocity dispersion, it is expedient for the thickness of the layers of the mirror to decrease from its free side towards the substrate.

Another extremely advantageous development of the teaching according to the invention provides that the resonator has means which spatially separate different spectral components of the laser pulses from one another. In this way, the peak power of the laser pulses can be reduced further, in particular in combination with the temporal expansion of the laser pulses provided according to the invention. In this way it is ensured that the pulse peak power does not exceed the destruction threshold of the optical components of the resonator. Due to the spatial separation of different spectral components of the laser radiation, these spectral components are amplified spatially separated. In this way, the spectral gain narrowing is significantly reduced, so that particularly short pulses can be amplified. Another The advantage of this embodiment is that, due to the spatial separation of different spectral components of the laser radiation from one another, the temporal pulse profile can be controlled further. A further development of the aforementioned embodiment provides that the means which spatially separate different spectral components of the laser pulses from one another, at least one refractive optic, for example a prism, and / or at least one diffractive optic, for example a diffraction grating, and / or at least one refractive-diffractive Optics, for example a so-called GRISM (granting on prism). Such optics are available as simple and inexpensive standard components, so that the structure of the resonator according to the invention or of the amplifier according to the invention is simple and inexpensive.

To compensate for phase errors of the resonator components, at least one amplitude mask and / or at least one phase mask can be provided, which is or are arranged in the Fourier plane, as is provided by a further development.

The gain medium may be a laser medium or an optically parametric gain medium, as embodiments provide. In particular in the embodiment with the spatial separation of different spectral components of the laser pulses from one another, the amplification medium can be formed, for example, by a flat laser crystal which can be cooled particularly easily. A high amplification of the laser pulses can be achieved by active laser media, for example Nd.YVO, Yb: glass or Cr: LiCAF crystals.

A development of the amplifier according to the invention is specified in claim 14. According to this Education is followed by at least one optically parametric amplifying medium or a periodically poled crystal. In the case of a periodically polarized crystal, for example a crystal in which inverted domains are arranged in a fan-like manner, with spatial separation of the spectral components of the laser pulses, as is achieved in the embodiment of the resonator according to the invention, a broadband frequency conversion of the laser pulses is involved - For example, by generating the second harmonic. In addition, the crystal can be provided with electrodes running in the beam direction. The phase position of the different wavelengths contained in the laser pulses can then be influenced separately from one another by applying a high voltage to these electrodes, so that an adaptive optic is formed. In this way, amplification with a high bandwidth and at the same time control of the spatially separated spectral components of the laser pulses, for example with regard to their phase position, is made possible.

A multilayer mirror according to the invention with a positive group velocity dispersion is specified in claim 15. To achieve a positive group velocity dispersion, it is expedient if the thickness of the layers of the mirror decreases from its free side towards the substrate, as is provided by a further development. The invention is explained in more detail below with reference to the accompanying drawing, in which exemplary embodiments are shown.

It shows: 1 is a schematic block diagram of a first embodiment of a regenerative amplifier according to the invention for ultrashort laser pulses, FIG. 2 is a highly schematic sectional view of a mirror used in the resonator of the amplifier according to FIG. 1,

3 shows an enlarged view of a detail in the area of the laser crystal in the embodiment according to FIG.

1, FIG. 4 in the same representation as FIG. 1, a second exemplary embodiment of an amplifier according to the invention and FIG. 5 a highly schematic sectional view of a periodically polarized crystal for broadband frequency conversion.

In the figures of the drawing, identical or corresponding components are provided with the same reference symbols.

1 shows an amplifier 2 for ultrashort laser pulses, which has a resonator 4 which has a first mirror 6 in the form of an end mirror and a second mirror 8 in the form of an end mirror, between which a laser medium in the form of a laser crystal 10 is arranged. The resonator also has a further mirror 12 and a pair of prisms 14, 16. For pumping the laser crystal 10, a diode laser 18 is provided, which irradiates laser light into the laser crystal 10 via two lenses 20, 22 serving in this exemplary embodiment for mode adjustment.

To extract amplified laser pulses from the Resonator 4 are provided with optical switch means, which in this exemplary embodiment are formed by a Pockels cell 24 and a thin-film polarizer 26. The optical switch means are generally known per se and are therefore not explained in more detail here.

The amplifier 2 serves to amplify ultrashort laser pulses which are radiated into the amplifier 2 by a seed laser 28 via a coupling mirror 30. For mode matching, a pair of lenses 32, 34 is provided between the seed laser 28 and the mirror 30 in this embodiment.

An optical isolator 36 is provided for decoupling the seed beam from amplified laser radiation in the form of laser pulses, which is coupled out of the resonator 4 and is generally known per se and is therefore not explained in more detail here.

According to the invention, the resonator 4 has means which temporally expand the laser pulses injected into the resonator 4 by the seed laser 28. In the exemplary embodiment according to FIG. 1, these means are formed by mirrors 6, 8, 12, which are designed as inverted double-chirped mirrors. In order to achieve a temporal expansion of the laser pulses, the thickness of the successive layer is applied to a substrate

Layers of the mirrors 6, 8, 12 are selected such that the mirrors 6, 8, 12 have a positive group velocity dispersion (GVD) for the laser radiation of the wavelength used, which is essentially constant over the spectral bandwidth of the laser pulses. FIG. 2 schematically shows a sectional view through the mirror 6, which has a substrate 38 to which a plurality of successive layers of dielectric material are applied, of which in FIG. 2 only two layers are provided with the reference symbols 40, 42. The thickness of the layers 40, 42 is shown exaggerated for illustration. The thickness of the successive layers 40, 42 of the mirror 6 decreases from a free side 43 of the mirror 6 towards the substrate, so that the mirror 6 has a positive group velocity dispersion for the laser radiation of the wavelength used. As a result, the laser pulses radiated into the resonator 4 by the seed laser 28 are expanded in time when they are reflected on the mirror 6. A pulse 44 is shown on the left in FIG. 2, which is expanded in time when reflected at the mirror 6, as is indicated in FIG. 2 at 44 ′. The expansion of the pulse is based on the fact that the spectral components of longer wavelength are preferably reflected on the front layers of mirror 6, which are further away from substrate 38 in the beam direction, since the thickness of these layers is adapted to the reflection of the spectral components of longer wavelength. In contrast, the spectral components of a lower wavelength are preferably reflected on the thinner layers in the beam direction, which are arranged closer to the substrate 38, since the thickness of these layers is adapted to the reflection of the spectral components of a lower wavelength. Due to the different reflection locations of different spectral components, the pulse leading edge is reflected earlier and the pulse trailing edge later, so that the pulse is stretched in time. The mirrors 8, 12 of the resonator 4 are correspondingly constructed as inverted chirped mirrors with a positive group velocity dispersion.

When the regenerative amplifier according to the invention is operated, the laser pulses radiated into the resonator 4 by the seed laser 28 become in the resonator 4 amplified, the laser pulses being temporally stretched when reflected at the mirrors 6, 8, 12 having a positive group velocity dispersion. The pulse peak power is reduced by the temporal expansion of the laser pulses, so that damage to the components of the amplifier 2 is avoided and at the same time a high amplification of the laser pulses is made possible.

The pair of prisms 14, 16 brings about a spatial separation of different spectral components of the laser pulses from one another, as is indicated in FIG. 3. Due to the spatial separation of different spectral components, the pulse peak power of the laser pulses is further reduced. On the other hand, the different spectral components of the laser pulses are amplified in a spatially separated manner in this way, so that the spectral gain narrowing is significantly reduced.

Because the required temporal expansion of the laser pulses is achieved by the mirrors 6, 8, 12 already present in the resonator 4, additional means for the temporal expansion of the pulses are not required, so that the amplifier 2 is simple in construction and thus inexpensive and compact is. It also enables high amplification of the laser pulses.

4 shows a second exemplary embodiment of a regenerative amplifier 2 according to the invention for ultrashort laser pulses, which differs from the exemplary embodiment according to FIG. 1 in that the Pockels cell 24 is arranged between the laser crystal 10 and the prism 16 and in that the thin-film polarizer 26 is not formed as a separate component, but is vapor-deposited onto the prism 16. FIG. 5 shows a highly schematic sectional view of a crystal 46 which has inverted domains in a manner which is generally known to the person skilled in the art, of which only one domain is provided with the reference symbol 48 in FIG. 5. The domains 48 run essentially transversely to the beam direction of the laser pulses and are arranged in a fan-like manner. During operation, the crystal 46 is arranged after an amplifier according to the invention, for example the amplifier 2 according to FIG. 1, at the output of which the different spectral components of the laser pulses are spatially separated from one another. These different spectral components thus enter the crystal 46 spatially separated from one another, as is indicated in FIG. 5 for three wavelengths WL_1, WL_2 and WL_3. By means of the

Crystal 46 enables a broadband frequency conversion of the laser pulses, for example by generating the second harmonic or by optical parametric amplification. In addition, the crystal 46 can be arranged in the direction of the beam

Drawing not shown electrodes are provided. By applying a high voltage to these electrodes, the phase position of the different wavelengths contained in the laser pulses can then be influenced separately from one another, so that adaptive optics are formed. In this way, amplification with a high bandwidth and, at the same time, control of the spatially separated spectral components of the laser pulses, for example with regard to their phase position, is made possible. The way the electrodes on the

Crystal 46 attached are generally known to the person skilled in the art and are therefore not explained in more detail here.

Claims

claims

1. resonator for amplifying laser pulses, in particular ultrashort laser pulses,

with at least two mirrors for reflecting the laser pulses,

with an optical amplification medium arranged between the mirrors for amplifying the laser pulses and

with means for the temporal expansion of the laser pulses

characterized,

that the means for temporal expansion are formed by at least one mirror (6, 8, 12), the group velocity dispersion of which is selected such that the temporal expansion of the laser pulses required to reduce the peak power is achieved.

2. resonator for amplifying laser pulses, in particular ultrashort laser pulses,

with at least two mirrors for reflecting the laser pulses,

with means for the temporal expansion of the laser pulses, characterized,

the group velocity dispersion of the internal resonator optical components of the resonator (4) is selected such that the temporal expansion of the laser pulses required to reduce the peak power is achieved in such a way that the means for the temporal expansion of the laser pulses through the internal resonator optical components of the resonator (4 ) are formed.

3. Resonator according to claim 1, characterized in that the mirror or mirrors for the temporal expansion of the laser pulses is or are formed by an internal resonator mirror or internal resonator mirror (6, 8, 12).

4. Resonator according to claim 1, characterized in that the mirror for the temporal expansion of the laser pulses is formed by a separate mirror arranged outside the resonator.

5. Resonator according to claim 1, characterized in that the mirror (6, 8, 12) has a positive group velocity dispersion (GVD).

6. Resonator according to claim 1, characterized in that the mirror has a negative group velocity dispersion (GVD).

7. Resonator according to claim 3, characterized in that at least one of the mirrors (6, 8, 12) is designed as a multilayer mirror, the thickness and / or the material of successive layers (40, 42) applied to a substrate (38) ) of Mirror (6, 8, 12) is selected such that the mirror (6, 8, 12) has a positive group velocity dispersion.

8. Resonator according to claim 7, characterized in that the thickness of the layers (40, 42) of the mirror decreases from its free side (43) to the substrate (38).

9. Resonator according to claim 1, characterized in that the resonator (4) has means which spatially separate different spectral components of the laser pulses from one another.

10. Resonator according to claim 5, characterized in that the means which spatially separate different spectral components of the laser pulses from one another, at least one refractive optic, for example a prism (14, 16), and / or at least one diffractive optic, for example a diffraction grating, and / or have at least one refractive-diffractive optic, for example a so-called GRISM (grating on prism)

11. Resonator according to claim 9, characterized in that at least one amplitude mask and / or at least one phase mask is provided, which is or are arranged in the Fourier plane.

12. Resonator according to claim 1, characterized in that the gain medium is a laser medium (10) or an optically parametrically amplifying medium.

13. Regenerative amplifier for ultra-short laser pulses, characterized,

that it has a resonator (4) according to one of the preceding claims.

14. Amplifier according to claim 13, characterized in that the amplifier (2) is followed by at least one optically parametrically amplifying medium or a periodically polarized crystal (46).

15. A multilayer mirror for reflecting laser pulses, in particular for use in a laser resonator according to one of the preceding claims and / or an amplifier according to one of the preceding claims,

with several layers of dielectric material applied to a substrate,

characterized,

that the thickness and / or the material of successive layers (40, 42) is selected such that the mirror has a positive group velocity dispersion (GVD).

16. Mirror according to claim 15, characterized in that the thickness of successive layers (40, 42) of the mirror (6) decreases from its free side (43) to the substrate (38).