DE102007021915A1 - Laser device and operating method for this - Google Patents

Abstract

The invention relates to a method for operating a laser device (26), which has a laser-active solid (44) with a passive Q-switching (46). In accordance with the invention, pumping light (28a, 28b) of different wavelengths (lambda <SUB> a </ SUB>, lambda <SUB> b </ SUB>) is applied to the laser device (26) in order to produce a more homogeneous intensity distribution of the pumping light (28a, 28b). to achieve in the laser-active solid (44).

Description

State of the art

The Invention relates to a method for operating a laser device, the one laser-active solid with a passive quality circuit having.

The The invention further relates to a laser device with a passive one Q-switching having laser-active solid.

such Operating methods and devices are known and, in particular also for a future one Use within ignition devices of Internal combustion engine provided.

adversely on the known passively Q-switched Laser systems is the fact that the pulse energy of means These systems did not produce laser pulses by a simple one Magnification of the in the laser-active solid radiated optical pump power can be increased. The maximum pulse energy of a single generated in this way Rather, the laser pulse is determined by the size of one with the pump light acted upon, d. H. optically pumped, volume of the laser active Solid.

A known proposal to increase the pulse energy of the laser pulses, the magnification of a beam cross section of a laser beam used as a pumping light to the object. Another known proposal for solving the problem provides, not longitudinally but transversely excite the laser-active solid to increase the pumping volume. However, both solutions do not lead to a satisfactory solution, because on the one hand, the magnification of the beam cross section of a pumping light beam is limited by the size of the desired oscillating laser mode and the beam quality of the generated laser pulse decreases with increasing radius of the pumping light beam. The beam quality of a laser beam corresponding to the laser pulse can be described, for example, by the diffraction factor M ² , which indicates how many times the laser beam is diffraction-limited, or by the so-called beam propagation factor, which represents the reciprocal of the diffraction factor M ² .

on the other hand offers the transverse excitation of the laser-active solid Pumplicht a much lower efficiency than a longitudinal Stimulation and leads also to a reduced beam quality of the laser pulse.

Disclosure of the invention

Accordingly It is an object of the present invention, an operating method for one Laser device and a laser device of the aforementioned Art to improve that the pulse energy generated Laser pulses are increased flexibly and without a significant reduction of the beam quality or at all can be varied, and that overall a more versatile operation the laser device possible is.

These Task is in the operating method of the type mentioned according to the invention thereby solved, that the laser device is exposed to pump light of different wavelengths.

By the inventive use of Pump light of different wavelengths to excite the laser-active solid is a particularly flexible Operation of the laser device according to the invention allows because the laser-active solid for the different wavelength of the pumping light has different absorption coefficients and accordingly, depending on the wavelength of the pump light used a different pumping volume is available to laser pulses generate, so in a simple way by an appropriate choice the pumping light wavelength the pulse energy of the laser pulses can be varied.

laser pulses with relatively low pulse energy can in the operating method according to the invention be obtained that the laser device or the laser-active solid is operated with pumping light of such a first wavelength for which the Absorption coefficient in the laser-active solid is relatively large, so that due to the low penetration depth of the pumping light into the laser-active solid correspondingly small pump volume results. Laser pulses with greater pulse energy can in the operating method according to the invention be obtained that the laser device or the laser-active solid operated with pumping light of a second wavelength, for the Absorption coefficient in the laser-active solid is relatively small, so due to the greater penetration depth of the pumping light of the second wavelength in the laser-active solid correspondingly larger pump volume results.

Particularly preferred is an embodiment of the method according to the invention, in which the laser device or its laser-active solid body is simultaneously applied with pump light of different wavelengths, whereby the available material of the laser-active solid Body is optimally used in terms of maximizing the pumping volume.

A special spatial intensity distribution in the laser-active solid irradiated pumping light can a further advantageous embodiment the operating method according to the invention According to be achieved by the intensity or the time course of the intensity of the pump light of a first wavelength dependent on the intensity or the time course of the intensity of the pump light a second wavelength chosen becomes. By tuning the intensities of the pump light different wavelength The possibility is also particularly advantageous to one another given, the intensity of the total irradiated pump light over a maximum possible Area of the laser-active solid to keep it constant, d. H. a homogeneous spatial Pumping light distribution, which makes a particularly efficient Operation of the laser device according to the invention possible is.

One Another degree of freedom in the operation of the laser device according to the invention is given in another advantageous embodiment by that the laser device temporally successively with pump light different wavelength is charged. For example, if laser pulses with a lower as the maximum possible Pulse power to be generated, it is conceivable, the laser-active solid only with pump light to act on a single wavelength, while the Generation of laser pulses with the maximum possible pulse power again Pump light of several wavelengths is used. A combination of successive times Exposure to pump light of different wavelengths with the simultaneous loading described above and possibly the setting of the respective intensities is also conceivable.

in principle can the inventive method According to a further advantageous embodiment, both a transversal as well as a longitudinal excitation of the laser-active Solid and / or the passive quality circuit Provide with pumping light and a combination thereof. That's it furthermore possible, to perform the longitudinal excitation at a first wavelength, and to perform the transverse excitation at a second wavelength, or also different pumping light wavelengths for the transverse and / or longitudinal To use suggestion.

Particularly preferred, however, is the purely longitudinal excitation of the laser-active solid and / or the passive Q-switching with pump light of different wavelengths, because this avoids the aforementioned disadvantages of the transverse excitation and a correspondingly high beam quality of the generated laser pulse, ie, for example, a smallest possible diffraction factor M ² , yields.

A Particularly efficient supply of the laser device with pump light different wavelengths is according to the invention possible that the laser device pump light of different wavelengths over at least a common optical fiber is supplied. Here are both or several pumping light wavelengths via suitable, the expert known means such. B. a beam shaping optics, coupled into the same optical fiber, and that of the optical fiber guided, several wavelengths If necessary, the pump light will be replaced by another Beam shaping optics from the optical fiber irradiated in the laser-active solid.

alternative or in addition can according to a further advantageous embodiment of the invention be provided that the laser device pump light different Wavelengths over at least a fiber bundle having a plurality of optical fibers is supplied, wherein over each Optical fiber only pump light of a wavelength is guided, so that a combination of pump light of different wavelengths in an optical fiber is not necessary and a corresponding optics formed easier can be.

Further Options, Pump light of different wavelengths supply, provide for the use of beam splitters, for example, highly reflective for one first used pump light wavelength and non-reflective for one further pump light wavelength used are formed, and over the The different pump light beams can be combined and combined accordingly an optionally arranged in front of the laser-active solid optics are fed.

at the use of a plurality of optical fibers having fiber bundle a further very advantageous embodiment of the present invention According to the invention be provided that the laser device facing End portions of the plurality of optical fibers leading the pumping light in dependence a specifiable spatial intensity distribution the pumping light in the laser device or in their arranged laser active solid become. As a result, in particular the quality of a laser device according to the invention generated laser pulse, in particular its beam quality, optimized.

The operating method according to the invention is particularly advantageous for generating laser pulses within an ignition device an internal combustion engine. The operating method according to the invention can advantageously be used both in internal combustion engines of motor vehicles and in stationary engines.

When another solution The object of the present invention is a laser device according to claim 11 indicated. Further advantageous embodiments of the present invention Invention are the subject of the dependent claims.

Further Features, applications and advantages of the invention will become apparent from the following description of exemplary embodiments of the invention, which are illustrated in the figures of the drawing. All described or illustrated features form for themselves or in any combination, the subject matter of the invention, regardless of their summary in the claims or their dependency as well as independently from their formulation or presentation in the description or in the drawing.

Short description of the drawing

In the drawing shows:

1 an embodiment of the laser device according to the invention,

2 by way of example a spatial intensity distribution of the pump light in a laser-active solid of the laser device according to the invention, as it results when using the operating method according to the invention,

3a and 3b different variants for supplying the laser device according to the invention with pump light of different wavelengths,

4a to 4c Further variants of optical fiber arrangements for supplying the laser device according to the invention with pump light of different wavelengths, and

5 an ignition device for an internal combustion engine with the laser device according to the invention according to 1 ,

Embodiments of the invention

1 schematically shows a detailed view of a first embodiment of the laser device according to the invention 26 , The laser device 26 has a laser-active solid 44 on, the one referred to as Q-switch passive Q-switch 46 is optically subordinate. The laser-active solid 44 forms here together with the passive Q-switching 46 as well as in 1 On the left of this arranged Einkoppelspiegel 42 and the output mirror 48 a laser oscillator, whose oscillatory behavior of the passive Q-circuit 46 depends and thus at least indirectly controllable in a conventional manner.

At the in 1 the illustrated configuration is the laser device according to the invention 26 or the laser-active solid 44 through the coupling mirror 42 through with pump light 28a . 28b applied to the electrons in the laser-active solid 44 stimulates and thus leads to a known population inversion. The coupling mirror 42 owns the pump light 28a . 28b a relatively large transmission coefficient.

While passive Q-switching 46 has its resting state in which it has a relatively low transmission coefficient, a laser operation in the laser-active solid 44 or in the by the coupling mirror 42 and the Auskoppelspiegel 48 limited solids 44 . 46 avoided. With increasing pumping duration, that is, during continued application of the pumping light 28a . 28b However, the radiation intensity in the laser oscillator also increases 42 . 44 . 46 . 48 so that the passive Q-switching 46 finally fading. That is, their transmission coefficient increases, and laser operation in the laser oscillator 42 . 44 . 46 . 48 starts. This condition is indicated by the double arrow 24 ' symbolizes.

In the manner described above, a laser pulse, also referred to as a giant pulse, is produced 24 which has a relatively high peak power. The laser pulse 24 is then replaced by the in 1 right arranged Auskoppelspiegel 48 from the laser oscillator 42 . 44 . 46 . 48 decoupled and, for example, in a laser-based ignition device for an internal combustion engine for igniting a located in a combustion chamber of the internal combustion engine air / fuel mixture usable. For this purpose, the laser pulse 24 for example, by a corresponding optical fiber device or directly by a Auskoppelspiegel 48 downstream combustion chamber windows are coupled into the combustion chamber of the internal combustion engine.

According to the invention, it is provided that the laser device 26 with pump light 28a . 28b different wavelengths λ _a , λ _{b is} applied. This advantageously provides the possibility of a more homogeneous spatial intensity distribution of the laser-active solid 44 irradiated pump light 28a . 28b than conventional systems using pump light of a single wavelength. This is there through, that to the training of the laser-active solid 44 Material used a wavelength-dependent absorption coefficient for the pump light 28a . 28b and thus pump light 28a . 28b different wavelength λ _a , λ _b - starting from the in 1 by way of example from the left, shown longitudinal excitation - different distances in the laser-active solid 44 penetrates.

2 shows an example of a spatial distribution of the intensity of the laser-active solid 44 ( 1 ) radiated pump light 28a . 28b plotted over a location coordinate z, which is a length of the laser-active solid 44 out 1 as seen from its left end.

The spatial intensity distribution for the pump light 28a ( 1 ) of the first used wavelength λ _a is in 2 marked with the reference numeral I _a (z), and the spatial intensity distribution of the pump light 28b ( 1 ) of the second used wavelength λ _b is in 2 marked accordingly by the reference I _b (z). Both pump light wavelengths λ _a , λ _b are present with the same intensity - based on the spatial coordinate z = 0, ie the in 1 left edge of the laser-active solid 44 , fed. The corresponding pump light rays 28a . 28b are present in the same way slightly in the laser-active solid 44 into focus, ie to a location z> 0.

Due to different absorption coefficients of the laser-active solid 44 for the two wavelengths λ _a , λ _b can in the example 2 the pump light 28b with the wavelength λ _b in the present case considered longitudinal excitation further into the laser-active solid 44 penetrate, as the pump light 28a , whereby the pump volume of the laser-active solid available for establishing the desired population inversion 44 is enlarged.

Another advantage of the inventive combination of pumping light 28a . 28b different wavelengths λ _a , λ _b is that in the laser-active solid 44 a relatively homogeneous spatial distribution of the intensity of the total irradiated pump light, in contrast to conventional systems 28a . 28b compare the aggregated pumping light intensity I (z) 2 , which result in a simultaneous exposure of the laser device 26 with the pump light 28a . 28b both wavelengths λ _a , λ _b yields.

The homogeneous spatial distribution of the pump intensity allows the saturation of the passive Q-switching 46 or the time at which this saturation is reached, shifting in time. Due to the longer-lasting optical excitation thereby achieved, more energy can advantageously be present in the laser-active material of the components 44 . 46 get saved. A laser pulse generated in this configuration thus has a higher pulse energy.

A significant advantage of the operating method according to the invention therefore consists in the fact that an effective enlargement of the pumping volume can be realized without relying on a transverse excitation of the laser-active solid 44 must be resorted to with pump light. As a result, using the operating method according to the invention, a high-energy laser pulse 24 ( 1 ) are produced with a particularly high beam quality, ie with a particularly low diffraction factor M ² .

In addition to the simultaneous application of the laser-active solid 44 with pump light 28a . 28b different wavelengths λ _a , λ _b , by the laser pulses as described above 24 can be generated with maximum pulse energy, allows the inventive use of pump light 28a . 28b In addition, different wavelengths λ _a , λ _b further degrees of freedom in the pumping, leading to the structure of the population inversion in the laser-active solid 44 serves. This makes it possible, with a suitable specification of the pumping light intensity laser pulses 24 to produce with different pulse energy. For example, laser pulses 24 be generated with relatively low pulse energy, characterized in that the laser-active solid 44 in a manner comparable to conventional systems alone with pump light 28a a first wavelength λ _{a is} excited. As soon as laser pulses 24 to be generated, which have a maximum possible pulse energy, the laser-active solid 44 again simultaneously with pump light 28a . 28b different wavelengths λ _a , λ _{b are} acted upon.

Although the excitation of the laser-active solid 44 Due to the better beam quality in the preferred embodiment is carried out solely by longitudinal pumping, the inventive method, the combination of pumped light 28a . 28b of different wavelengths λ _a , λ _b , also for transversely exciting the laser-active solid 44 or used for a combined longitudinal and transverse excitation.

In a particularly advantageous further embodiment of the present invention, the laser device 26 the pump light 28a . 28b different wavelengths λ _a , λ _b via a common optical fiber 280a supplied, compare 3a , The pump light 28a . 28b can via an unillustrated beam shaping optics in the optical fiber 280a initiated and by the in

3a pictured and unspecified further optics finally in the laser-active solid 44 be coupled.

Alternatively to the use of a common optical fiber 280a for the several wavelengths λ _a , λ _{b of} the pump light 28a . 28b can be an example in 3b imaged beam splitter configuration may be provided, wherein the pump light 28a . 28b via a beam splitter, which is highly reflective for the wavelength λ _{b of} the pump light 28b and not reflective for the wavelength λ _a , of the pump light 28a is formed, merged and by an optionally existing downstream optics on the laser-active solid 44 the laser device 26 is shown.

In a further very advantageous embodiment of the laser device according to the invention 26 Proceed that the laser device 26 pump light 28a . 28b different wavelengths λ _a , λ _b via at least one fiber optic having a plurality of fiber bundles 280b is supplied, compare 4a , Everyone in 4a unspecified individual optical fibers is only pump light 28a . 28b one of the wavelengths used, wherein preferably a total of a plurality of optical fibers for guiding the pumping light 28a a first wavelength λ _a and for guiding the pump light 28b a second wavelength λ _{b are} provided. To a predefinable spatial intensity distribution of the pump light 28a . 28b in the laser device 26 or in their laser-active solid 44 to achieve that of the laser device 26 facing end portions of the plurality of pump light 28a . 28b leading optical fibers are arranged in different ways to each other, cf. 4a . 4b . 4c ,

The laser device according to the invention 26 ( 1 ) can be used as a laser-active medium 44 For example, have a Nd: YAG-based material, and passive Q-switching 46 For example, it may comprise a Cr ⁴⁺ : YAG-based material. As a pump light 28a . 28b For example, light, in particular laser light, of the wavelength λ _a = 808 nm and λ _b = 885 nm can be used. The Nd: YAG material containing laser-active solids 44 with a doping of z. B. has about 1 at.% At the wavelength λ _{a has} an absorption coefficient of about 10 cm ^-1 , and at the wavelength λ _b an absorption coefficient of about 2 cm ^-1 .

Overall, the inventive use of different wavelengths λ _a , λ _b allows for the pump light 28a . 28b in a purely longitudinal excitation of the laser-active solid 44 the generation of laser pulses 24 ( 1 ) with a comparatively large pulse energy, which is not achievable in a comparable longitudinal excitation using conventional operating methods with only one pumping light wavelength.

At the same time, the laser pulse generated according to the invention 24 a relatively high beam quality due to the purely longitudinal excitation on.

5 shows schematically an ignition device 27 for an internal combustion engine 10 in which the laser device according to the invention 26 and the above-described method of operation according to the invention are used to generate laser pulses 24 to generate, for the ignition of one in a combustion chamber of the internal combustion engine 10 serve air / fuel mixture.

The internal combustion engine 10 includes several cylinders, one of which is in 5 only one with the reference numeral 12 is designated. A combustion chamber 14 of the cylinder 12 is from a piston 16 limited. Fuel gets into the combustion chamber 14 directly through an injector 18 , to a designated as a rail or common rail fuel pressure accumulator 20 connected.

In the combustion chamber 14 injected fuel 22 is by means of the laser pulse described above 24 ignited, that of the laser device according to the invention 26 the ignition device 27 in the combustion chamber 14 is emitted.

The laser device 26 is according to the invention via a light guide device 28 with pump light 28a . 28b ( 1 ) of different wavelengths λ _a , λ _b supplied by the pump light source 30 provided. The pump light source 30 is controlled by a control device 32 which also controls the injector 18 controls.

For example, the pump light source 30 Have one or more non-illustrated semiconductor laser diodes, the pump light in response to a control current 28a . 28b corresponding intensity over the light guide device 28 to the laser device 26 output. Although semiconductor laser diodes and other small-sized pumping light sources are preferably used for use in the automotive field, for the operation of the ignition device according to the invention 27 in principle, any type of pumping light source usable, which provides a plurality of different wavelengths λ _a , λ _b .

It is also conceivable to use pump light with more than two different wavelengths λ _a , λ _b .

Claims (13)

Method for operating a laser device ( 26 ) containing a laser-active solid ( 44 ) with a passive quality circuit ( 46 ), characterized in that the laser device ( 26 ) with pump light ( 28a . 28b ) of different wavelengths (λ _a , λ _b ) is applied. Method according to claim 1, characterized in that the laser device ( 26 ) simultaneously with pump light ( 28a . 28b ) of different wavelengths (λ _a , λ _b ) is applied. Method according to one of the preceding claims, characterized in that the intensity or the temporal course of the intensity of the pump light ( 28a ) of a first wavelength (λ _a ) as a function of the intensity or the time profile of the intensity of the pump light ( 28b ) of a second wavelength (λ _b ) is selected. Method according to one of the preceding claims, characterized in that the intensity of the pumping light ( 28a . 28b ) of different wavelengths (λ _a , λ _b ) is selected so that the most homogeneous possible distribution of the total in the laser-active solid ( 44 ) and / or passive Q-switching ( 46 ) irradiated pump light ( 28a . 28b ). Method according to one of the preceding claims, characterized in that the laser device ( 26 ) in chronological order with pump light ( 28a . 28b ) of different wavelengths (λ _a , λ _b ) is applied. Method according to one of the preceding claims, characterized in that the laser-active solid ( 44 ) and / or passive Q-switching ( 46 ) of the laser device ( 26 ) longitudinally and / or transversally with pump light ( 28a . 28b ) is excited at least one wavelength (λ _a , λ _b ). Method according to one of the preceding claims, characterized in that the laser device ( 26 ) Pump light ( 28a . 28b ) of different wavelengths (λ _a , λ _b ) via at least one common optical fiber ( 280a ) is supplied. Method according to one of the preceding claims, characterized in that the laser device ( 26 ) Pump light ( 28a . 28b ) of different wavelengths (λ _a , λ _b ) via at least one fiber bundle having a plurality of optical fibers (US Pat. 280b ) is supplied, wherein over each optical fiber only pump light ( 28a . 28b ) of a wavelength (λ _a , λ _b ) is guided. Method according to claim 8, characterized in that the laser device ( 26 ) facing end portions of the plurality of the pumping light ( 28a . 28b ) leading optical fibers as a function of a predefinable spatial intensity distribution of the pump light ( 28a . 28b ) in the laser device ( 26 ) or in their laser-active solid ( 44 ) to be ordered. Use of the method according to one of the preceding claims for generating laser pulses ( 24 ) for an ignition device ( 27 ) an internal combustion engine ( 10 ). Laser device ( 26 ), with a passive circuit ( 46 ) having laser-active solid ( 44 ), characterized in that the laser device ( 26 ) a pump light source ( 30 ), via which the laser device ( 26 ) with pump light ( 28a . 28b ) of different wavelengths (λ _a , λ _b ) can be acted upon. Laser device ( 26 ) according to claim 11, characterized in that it is designed for carrying out the method according to one of claims 1 to 10. Ignition device ( 27 ) for an internal combustion engine ( 10 ), in particular of a motor vehicle, with a laser device ( 26 ) according to one of claims 11 or 12.