DE102010023756A1 - Laser system with spectral filtering - Google Patents

Abstract

The invention relates to a laser system with a pulsed laser 1. Laser systems based on pulsed lasers are used to generate ultrashort laser pulses. In the prior art, only complex, mode-locked laser systems are known which can achieve a pulse duration of less than 10 ps. However, these are always complex and sensitive free jet structures. It is therefore an object of the invention to provide a laser system which generates pulse durations of less than 10 ps and which is also simple and compact to manufacture. To achieve this object, the invention proposes that the laser system have a spectrally broadening element 2, which spectrally broadened the output laser pulses of the pulsed laser 1 by self-phase modulation, and a spectrally filtering element 3, which temporally compresses the spectrally broadened laser pulses by spectral filtering.

Description

The invention relates to a laser system with a pulsed laser.

Pulsed laser-based laser systems are used to generate ultra-short laser pulses. In the prior art, only complex, mode-locked laser systems have been known to date that can achieve a pulse duration of less than 10 ps. A simple and compact solution for generating laser pulses in the sub-10 ps range therefore holds considerable market potential. Field of application is, among other things, the high-precision machining of micromaterials, since the heat input into the material, which is reduced by a short pulse duration, has quality advantages - eg. B. more precise edges during laser cutting - offers. The mode-locked solid-state lasers known in the art are heretofore used as typical sources of ps pulses. They exist in addition to the active medium of a non-linear switch, z. B. a saturable semiconductor mirror and elements for dispersion compensation. These complex and adjustment-sensitive free-jet structures deliver pulse repetition frequencies in the range from 10 MHz to more than 100 MHz. However, reasonable pulse repetition frequencies for most applications are those below 10 MHz, typically some 100 kHz. Therefore, elements which reduce the pulse repetition frequency must additionally be used in the known, mode-locked solid-state lasers. Common elements here are so-called. Resonatorverlängernde mirror arrangements, which, however, further increase the complexity of the structures and thus make them sensitive to adjustment. Alternatively, a pulse picker (eg a Pockels cell) is used before or between the amplifier stages, which reduces the pulse repetition frequency in the area required for the application. Overall, the mode-locked laser systems are usually sensitive free-jet structures, whereby they are only partially suitable for commercial use.

Also known are simple and compact passive Q-switched microchip lasers, which consist of a monolithic composite of saturable absorber, laser crystal and resonator mirror and are pumped by a simple optical system with a laser diode. In this way, pulses with pulse repetition frequencies of several 10 kHz to a few MHz can be generated at pulse durations between 50 ps and 200 ps. A pulse duration of less than 10 ps is not possible with these lasers.

It is therefore an object of the invention to provide a laser system which generates pulse durations below 10 ps and at the same time is simple and compact to manufacture.

To achieve this object, the invention proposes that the laser system has a spectrally broadening element which spectrally widens the output laser pulses of the pulsed laser by self-phase modulation, and a spectrally filtering element which temporally compresses the spectrally broadened laser pulses by spectral filtering.

By self-phase modulation (SPM) within the spectrally broadening element, the spectral width of the laser pulses can be increased so that they can be shortened in time by subsequent spectral filtering. The bandwidth of the spectrally filtering element is chosen so that it is smaller than the spectral width of the widened laser pulse to be filtered. The thus spectrally filtered laser pulse with reduced spectral width has a significantly shortened pulse duration. The effect of the temporal shortening is due to the fact that the spectrally broadening self-phase modulation acts only in the time domain.

The laser system can be a pulsed laser, a Q-switched laser, a mode-locked laser or a gain-switched laser, z. B. a diode laser having.

Likewise, the pulsed laser may be a continuously emitting laser source whose radiation is modulated by external optical components.

In the case of pulse generation by Q-switching, it is recommended that the pulsed laser is a passively Q-switched laser, in particular a passively Q-switched microchip laser. Due to their monolithic structure, microchip lasers can reach an extremely compact design and thus be easily integrated into a laser system.

In particular, a composite of a neodymium-doped vanadate crystal and a saturable semiconductor mirror is suitable as a microchip laser.

A variant embodiment provides that the pulsed laser has longitudinal single-mode laser pulses. This so-called "single-frequency" emission of the Q-switched laser, d. H. the emission of a single well-defined longitudinal mode is advantageous, but not essential. If several longitudinal modes with statistical phase relationship contribute to the emission - which corresponds to the usual situation with Q-switching - temporal compression of the spectral components newly generated by self-phase modulation would also be possible by spectral filtering. However, the time-compressed pulses would have a degraded quality.

Advantageously, the pulsed laser has a pulse duration which is less than 1 ns, less than 200 ps or less than 50 ps. A pulsed laser with this pulse duration provides a highly suitable output radiation in order subsequently to achieve a pulse duration of less than 10 ps by means of the spectral broadening and spectral filtering according to the invention.

It is proposed that the spectrally broadening element is an optical fiber, in particular a single-mode optical fiber, or a waveguide structure. In a suitable optical fiber or waveguide structure occurs with sufficiently low fiber diameter or sufficiently low waveguide thickness usually self-phase modulation, resulting in a spectral broadening of the guided radiation.

Advantageously, the spectral width of the spectrally broadened laser pulses is at least five times, ten times or twenty times the spectral width of the output laser pulses of the pulsed laser. In practice, these minimum broadening factors have proven to be optimal for the subsequent spectral filtering or the resulting pulse duration of the laser system.

It is furthermore provided that the laser system has one or more amplifier stages. In this case, one or more amplifier stages may be fiber-optic amplifier stages. In addition, it is possible for one or more amplifier stages to act as a spectrally broadening element, whereby these broaden spectrally in particular and advantageously by means of self-phase modulation. A gain is possible by a single optical amplifier or by several amplifier stages. It is also conceivable in this sense, an optical amplifier fiber, which takes on both the task of gain and the spectral broadening by self-phase modulation.

It is provided that the spectrally filtering element has a passive optical element. This element can, for. Example, a fiber optic chirped Bragg grating, a volume optical chirped Bragg grating, a conventional grating pair in transmission or reflection or even a prism structure, a Lyot filter, an etalon or an interference filter or a combination of interference filters.

Furthermore, the spectrally filtering element may comprise a nonlinear optical element. In question here are in particular elements with non-linear polarization rotation, a saturable absorber or a frequency-converting element which performs a spectral filtering or spectral trimming by phase matching.

Furthermore, the spectrally filtering element may simultaneously comprise active and passive optical elements. An active optical element, for example an active tunable filter, can be adjusted so that it optimally complements the filter characteristic of the passive optical element in the sense of the invention.

Advantageously, the spectral bandwidth of the laser pulse after the spectrally filtering element is less than 75%, 50% or 25% of the spectral bandwidth of the spectrally broadened laser pulse. The bandwidth of the laser pulse after the spectrally filtering element under these filter conditions in practice best suited to achieve laser pulses with a pulse duration less than 10 ps.

Another embodiment of the invention provides that the spectrally filtering element is an optical amplifier. Thus, the spectrally filtering element can simultaneously perform an amplification function within the laser system, so that a laser system can be produced with as few optical components as possible.

It is thereby provided that the optical amplifier has an effective amplification bandwidth which is smaller than the spectral bandwidth of the spectrally broadened laser pulse. In practice, to achieve the object according to the invention, it has proved to be advantageous that the spectral bandwidth of the laser pulse after the optical amplifier is less than 75%, 50% or 25% of the spectral bandwidth of the spectrally broadened laser pulse. The bandwidth of the active filtering is understood here as gain bandwidth including gain narrowing.

Further advantageous are optional elements which change the laser pulse with respect to its properties - such as pulse duration, pulse spacing, frequency, contrast, spectral composition - so that the characteristics and / or the quality of the output radiation of the laser system according to the invention are improved. For this purpose, the laser system may comprise a pulse stretcher, by means of which the spectrally broadened laser pulses are stretched in time. Furthermore, the laser system may have a pulse contrast enhancing element, which is arranged in the propagation direction of the laser pulse after the spectrally broadening and spectrally filtering element. Furthermore, the laser system may comprise an element which divides the laser pulse in time, or else a frequency-converting element.

Finally, the laser system according to the invention can also be traversed several times by the output laser pulses of the pulsed laser. Here, the spectrally broadened and spectrally filtered laser pulses are spectrally broadened by means of the spectrally broadening element by self-phase modulation and compressed in time by the spectrally filtering element. With such a multi-stage structure, the pulses compressed in a first stage to less than 10 ps pulse duration can be compressed by means of a second stage to a pulse duration of, for example, less than 1 ps.

Finally, it is provided that the laser system additionally has a compression element which temporally compresses the spectrally broadened laser pulses. This additional compression element can be arranged either after the spectrally broadening element or after the spectrally filtering element.

The additional compression element may be a Bragg grating, a transmissive or reflective grating pair or a prism structure. The compression element causes together with the spectral filtering element a two-stage and thus increased temporal compression. The additional compensation of the phase terms within the additional compression element results in a significantly shorter pulse duration.

Embodiments of the invention are explained in more detail below with reference to the drawings. Show it:

1 : an embodiment of the laser system according to the invention;

2 : a spectrum of a spectrally broadened laser pulse;

3 : Spectrum of the spectrally broadened laser pulse after spectral filtering;

4 : time course of the spectrally filtered laser pulse;

5 : time course of the spectrally filtered laser pulse with adjusted secondary pulses;

6 : Spectrum of the spectrally broadened and filtered laser pulse after the adjustment of the secondary pulses;

7 : a further embodiment of the laser system according to the invention.

In 1 schematically a laser system is shown, which consists of a laser 1 , a spectral widening element 2 , a spectral filtering element 3 , a saturable absorber 4 and an amplifier 5 consists. The laser 1 Here is a Q-switched laser. The spectral broadening element 2 is an optical fiber. The spectral filtering element 3 is an Nd: YAG amplifier that simultaneously amplifies the laser pulses. The amplifier 5 is also an Nd: YAG amplifier in this case.

In 7 the laser system consists of a Q-switched laser 1 , a spectral widening element 2 in the form of an optical fiber, an interference filter as a spectrally filtering element 3 and optionally also a saturable absorber 4 and an amplifier 5 ,

The laser system according to 1 works so that the passively Q-switched laser 1 emitted in the form of a Mikrochiplasers output laser pulses of an average power of 100 mW, a pulse duration of about 100 ps and a pulse repetition frequency of about 300 kHz at a wavelength of about 1064 nm. Assuming a Gaussian pulse shape in the vicinity of the transformation limit, the laser pulse has a spectral half width of 17 pm at the pulse duration of 100 ps. The laser pulses then propagate in a 3 m long single-mode fiber with a mode field diameter of 6 μm. The single-mode fiber acts as a spectrally broadening element 2 and widens the spectrum to a bandwidth of about 1 nm. The resulting spectrum is in 2 shown.

The laser pulses with a spectral bandwidth of now 1 nm are then applied to the spectrally filtering element 3 which is an Nd: YAG amplifier with a gain bandwidth of 0.4 nm and a peak gain of 400 at a center wavelength of 1064 nm. The spectral filtering by means of the opposite to the input pulse lower gain bandwidth of the amplifier takes place simultaneously with the gain.

The filtered laser pulse points to the spectrally filtering element 3 a pulse energy of 13.5 μJ and an average power of 4W. The spectral bandwidth is now only about 175 pm. The corresponding spectrum is in 3 shown.

The spectral filtering simultaneously causes a temporal compression of the laser pulse. The pulse duration after the spectral filtering in the present embodiment is 14 ps, it was thus shortened from the input 100 ps to 14 ps. The time signal is in 4 shown.

If the output laser pulse of the laser 1 is no parabolic pulse, arise in the spectral broadening in the optical fiber 2 Modulations which cause secondary pulses in the time domain - even after spectral filtering and amplification. These sub-pulses are here about 100 ps away from the main pulse and contain about 10% to 20% of the total pulse energy ( 4 ).

By a saturable absorber 4 the secondary pulses can be largely removed. The saturable absorber 4 Reduces the energy in the secondary pulse relative to the main pulse by one to two orders of magnitude when used once. 5 illustrates a type-cleaned pulse. 6 shows the associated modulation-free spectrum.

An alternative approach to avoiding the side pulses is a spectral filtering offset from the central wavelength of the spectrally broadened pulses, for example, by targeted selection of the outermost wing of the spectrum widened by self-phase modulation spectrum.

After the saturable absorber 4 the laser pulses hit an additional amplifier 5 , The amplifier 5 in the present example is identical to the spectral filtering element 3 , Alternatively, the amplifier could 5 but also an amplifier 5 with a larger gain bandwidth than that of the spectral filtering element 3 be. For example, the first run through amplifier 3 (ie the spectral filtering element) an Nd: YAG amplifier and the amplifier thereafter passed through 5 be an Nd: YVO amplifier. Such a two-stage amplification leads to no significant change in the spectral and temporal characteristics.

The laser system according to 7 works similar to the laser system according to 1 , The output laser pulses of the passively Q-switched laser 1 are also here in an optical fiber as a spectrally broadening element 2 by non-linear widening by self-phase modulation. These spectrally broadened laser pulses then pass into the spectrally filtering element 3 which is a passive filter, namely an interference filter here. The passive filter has a filter bandwidth which is less than the spectral bandwidth of the spectrally broadened laser pulse. Due to the spectral filtering within the spectrally filtering element 3 the laser pulse is compressed in time.

Subsequently, the time-compressed pulses can optionally - as in 1 - by a saturable absorber 4 , an amplifier 5 and / or further optional pulse-shaping elements (not shown) are guided.

Claims (15)

Laser system with a pulsed laser ( 1 ), characterized by - a spectrally broadening element ( 2 ), which outputs the output laser pulses of the pulsed laser ( 1 ) spectrally broadened by self-phase modulation, and - a spectrally filtering element ( 3 ), which temporally compresses the spectrally broadened laser pulses by spectral filtering. Laser system according to claim 1, characterized in that the pulsed laser ( 1 ) has longitudinal single-mode laser pulses. Laser system according to claim 1 or 2, characterized in that the spectrally broadening element ( 2 ) is an optical fiber, in particular a single-mode optical fiber. Laser system according to claim 1 or 2, characterized in that the spectrally broadening element ( 2 ) is a waveguide structure. Laser system according to one of claims 1 to 4, characterized in that the spectral width of the spectrally broadened laser pulses at least five times the spectral width of the output laser pulses of the pulsed laser ( 1 ) is. Laser system according to one of Claims 1 to 5, characterized by one or more amplifier stages ( 5 ), in particular fiber-optic amplifier stages. Laser system according to claim 6, characterized in that one or more amplifier stages ( 5 ) as a spectrally broadening element ( 2 ) Act. Laser system according to one of claims 1 to 7, characterized in that the spectrally filtering element ( 3 ) has a passive optical element. Laser system according to one of claims 1 to 8, characterized in that the spectrally filtering element ( 3 ) has a nonlinear optical element. Laser system according to one of claims 1 to 9, characterized in that the spectral bandwidth of the laser pulse after the spectrally filtering element ( 3 ) is less than 75% of the spectral bandwidth of the spectrally broadened laser pulse. Laser system according to one of claims 1 to 10, characterized in that the spectrally filtering element ( 3 ) is an optical amplifier having an effective gain bandwidth smaller than the spectral bandwidth of the spectrally broadened laser pulse. Laser system according to one of claims 1 to 11, characterized by an additional compression element, which widens the spectrally broadened Laser pulses or the spectrally filtered laser pulses compressed in time. Laser system according to claim 12, characterized in that the additional compression element is a Bragg grating, a transmitting or reflective grating pair or a prism structure. Use of a laser system according to one of Claims 1 to 13 for generating ultrashort laser pulses. Use of a laser system according to one of claims 1 to 13, wherein the spectrally broadened and spectrally filtered laser pulses by means of the spectrally broadening element ( 2 ) is spectrally broadened by self-phase modulation and by the spectral filtering element ( 3 ) are compressed in time.

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