DE102012209625B3 - Laser array for laser scanning microscope for treatment of maculate generation during photodynamic therapy of eye conditions, has mirror designed in reflective manner for wavelength range around fundamental wave length with high threshold - Google Patents

Abstract

The array has an end mirror (2), and a wavelength-selective element for a fundamental wave length with a low laser threshold, where multi-mode operation is performed with the fundamental wave length with a high laser threshold. A transmissive wavelength-selective element is arranged within a laser resonator such that spectral selective effect does not exhibit for the fundamental wave length with high laser threshold. Another end mirror is designed in a complete reflective manner for a wavelength range around fundamental wave length with high laser threshold.

Description

The invention relates to a laser arrangement. The laser arrangement comprises a laser resonator in which an active laser medium for generating homogeneously diffused radiation of a first fundamental wavelength λ ₁ by stimulated emission from an upper laser level to a first base level and for generating homogeneously diffused radiation of a second fundamental wavelength λ ₂ by stimulated emission of the upper laser level is arranged at a second base level. The laser threshold or pumping threshold indicates the minimum required pumping power of a laser, in which the gain is balanced by energy supply on the one hand and decoupling of the generated emission as well as losses due to absorption in the laser medium on the other. At higher pump powers the laser starts to work.

The laser assembly also includes an end mirror and an output mirror that confine the laser resonator. In the laser resonator, finally, a non-linear crystal is arranged, which is designed to mix radiation with the wavelengths of the two fundamental wavelengths λ ₁ and λ ₂ to the radiation of a sum frequency v _S. Finally, the laser arrangement also includes a transmissive or reflective wavelength-selective element.

A number of applications are known in the art which require coherent light in the visible yellow-orange spectral range of about 560 nm to 630 nm. These include, for example, fluorescence spectroscopic methods, but also ophthalmological methods for the treatment of macular degeneration, for example in photodynamic therapy of eye conditions characterized by neovascularization. The generation of wavelengths in this spectral range by second harmonic generation (SHG) of solid-state lasers is not possible, since there are no commercially available laser media that emit light in the required spectral range around 1200 nm. Moreover, in contrast to the blue, red and green spectral range, there are no laser diodes emitting directly in the yellow spectral range. This "gap" is referred to in the English-language literature as "yellow-gap".

Therefore, coherent light in this wavelength range is usually produced by the use of special laser media that allow simultaneous operation at two wavelengths and where light of the desired wavelength by sum frequency mixing (SFM) - also referred to as sum frequency generation (SFG) - belongs to the corresponding wavelengths Frequencies is generated.

For example, to generate light in the spectral range between 589 nm and 594 nm, neodymium-doped yttrium aluminum garnet (Nd: YAG), yttrium orthovanadate (Nd: YVO) or yttrium aluminum perovskite (Nd: YAP) in crystal form. With Nd: YAG, for example, a simultaneous two-wavelength operation at the wavelength of 1064 nm and at the wavelength of 1319 nm or alternatively at 1338 nm is possible. The radiation emitted by stimulated emission is spectrally homogeneously line broadened in this crystal, without additional measures, the stronger laser transition - in the example of the Nd: YAG crystal at the wavelength of 1064 nm - displace the weaker laser transitions. In order to suppress this displacement, for example, resonators with two independent active media are used in the prior art, whereby a competition of the two wavelengths can be avoided by the same inversion. Such an arrangement is for example in the EP 1 695 421 B1 described. Such resonators only have a common resonator load, in which the two wavelengths are superimposed for the purpose of summation frequency generation.

Alternatively, in the DE 44 22 077 A1 proposed to use two resonators whose optical axes are superimposed in the neodymium-doped crystal. Only in the one branch is a nonlinear crystal introduced for frequency doubling. However, this crystal doubles only the frequency of one wavelength, in this case the wavelength of 1319 nm. The frequency doubling results in a non-linear dependence of the inversion density on the pump power, which is exploited to reach the threshold inversion density for the second laser transition in the other resonator with increasing pump power this way, ultimately, to create a simultaneous oscillation on two different laser transitions. However, the generation of the sum frequency plays no role in this application.

In the prior art resonators are known in addition to split resonators, which allow at least nominal stimulated emission at both wavelengths of interest. It is used for this purpose that the saturation of the inversion is not the same at all spatial points in the active medium, but there at the strongest, where the field strength of the standing light waves in the resonator has maxima. If the laser oscillates only on one resonator mode, a spatially modulated inversion occurs in the active medium with a modulation period equal to half the wavelength of the corresponding mode, also referred to as spatial hole burning. At the nodes of the oscillating mode there is an inversion which is higher than that Threshold inversion and therefore allows the gain of another mode whose wavelength is just offset so that it has maxima in the field strength at the points where the other mode has nodes. Even in a spectrally homogeneous line-broadened active medium, two wavelengths can partially coexist in this way. Experience shows that such lasers provide significant output power at the wavelength of the sum frequency over time. As a result of the nonlinear coupling of the two wavelengths involved by the nonlinear crystal, periodic or even chaotic fluctuations or oscillations may occur, which are also known from conventional intracavity frequency-doubled lasers, as already described in an article by T. Baer, published in 1986 in J. Opt. Soc. At the. B., Vol. 3, no. 9, has been described. Since these strong power fluctuations were first observed in lasers in the green spectral range, the term "green problem" has been used in the literature - regardless of the wavelength - for natural phenomenological description. For this reason, such systems are rather less suitable for applications where high output power stability is required, for example for laser scanning microscopy.

In the DE 195 10 423 C2 a laser arrangement for resonator-internal sum frequency mixing is described. Two Nd: YAG laser crystals are arranged in two resonators. Each of the two laser crystals is pumped separately, wherein in each resonator a different frequency is generated. Each of the resonators has two end mirrors. One of the two end mirrors transmits radiation from the pump source. The other two end mirrors are arranged so that the two resonators are interleaved, so that the area between these two end mirrors is shared by both resonators. In this area, an LBO crystal is arranged for the sum frequency mixing of the two radiation of different wavelengths generated by the laser crystals. The radiation generated by sum frequency mixing is coupled out by a deflecting mirror arranged in the beam path, which is permeable for this wavelength.

In the DE 198 22 065 C1 describes a diode-pumped laser arrangement in which by means of a diode pump laser by sum frequency mixing laser radiation is generated in the red visible spectral range. For this purpose, a pump diode laser, two laser crystals and an optically nonlinear crystal are arranged in series. The pump diode laser generates laser radiation in the first laser crystal, the laser radiation emitted by this crystal is used to optically pump the second laser crystal. The optically nonlinear crystal is the sum frequency mixing.

In the DE 10 2005 025 128 A1 a laser arrangement for generating a multi-mode operation is described with resonator-internal frequency doubling. The laser crystal is formed here as a flat disc, the pump radiation strikes at an oblique angle to the crystal, where it is coupled into the resonator beam path. In the resonator a non-linear crystal for frequency doubling and an etalon are arranged. In the resonator standing waves of two wavelengths are formed in two spectral ranges. In order to prevent power fluctuations due to the non-linear crystal frequency mixing of the longitudinal modes of the two spectral regions and to obtain a stable frequency-doubled output, an etalon with a free spectral range approximately twice the distance of the two spectral regions resulting from spatial hole burning is used.

The object of the invention is to further develop a laser arrangement of the type described above in such a way that the two fundamental wavelengths are decoupled more strongly, so that they do not tend to reciprocally oscillate and an improved constancy of the respective powers and thus the power at the sum frequency is achieved.

This object is achieved in an arrangement of the type described above in that the wavelength-selective element for the fundamental wavelength with the lower laser threshold has such a spectral selectivity that allows a single-mode operation at this fundamental wavelength. In the case of the fundamental wavelength with the higher laser threshold, on the other hand, multimode operation is to be enabled. In the event that the wavelength-selective element is designed as a transmissive element, this has for this purpose therefore for the fundamental wavelength with the higher laser threshold no spectrally selective effect. Alternatively, the wavelength-selective element is designed as a reflective element, in this case it is arranged outside the laser resonator on the side of the end mirror, wherein the end mirror for the fundamental wavelength with the lower laser threshold is designed to be transmissive and completely reflective for a wavelength range around the fundamental wavelength with the higher laser threshold is designed so that some of the modes from the homogeneously broadened excitation spectrum of this fundamental wavelength are also reflected to allow a multi-mode operation.

The transmissive or reflective wavelength-selective element thus has with respect to Fundamental wavelength with the lower lasing threshold, the effect of a filter, which passes in the region of the homogeneously broadened fundamental wavelength only a single mode. If the pump power is ramped up, the fundamental wavelength starts to oscillate with the lower laser threshold, for example the wavelength λ ₁ , due to the lower laser threshold. Due to the spectrally selective filtering effect of the wavelength-selective element for this fundamental wavelength, it oscillates only in a longitudinal mode, so that along the propagation direction of the light in the active medium, a spatially modulated inversion or saturation of the gain forms by the resulting standing wave in the laser resonator, the so-called spatial hole burning.

With a further increase in the pump power beyond the higher laser threshold, the laser can also begin to oscillate and stimulate at fundamental wavelength with the higher laser threshold, in this case at the wavelength λ ₂ . This is only possible because the inversion is degraded spatially modulated incomplete. Unlike the fundamental wavelength with the lower laser threshold, the transmissive wavelength-selective element for the fundamental wavelength with the higher laser threshold has no spectrally selecting effect, or the end mirror reflects a whole wavelength range in the case of a reflective wavelength-selective element, so that at this wavelength homogeneous broadening will set a multi-mode operation.

Due to the spatial modulation of the inversion and the monomode operation at the fundamental wavelength λ ₁ , the laser will automatically adapt its wavelength and the number of longitudinal modes so that in spatial and temporal mean the antinodes at the wavelength λ ₂ in the troughs at the wavelength λ ₁ lie. As a result, the radiation at the two wavelengths no longer competes energetically for the same inversion, which results in an at least partial decoupling and thus a reduced tendency to oscillations in intensity relative to each other.

The filtering function of the wavelength-selective element can be realized in various ways, either by a transmissive or by a reflective element. In the case of a transmissive element arranged in the laser resonator, this preferably has a transmission maximum in the region of the fundamental wavelength with the lower laser threshold, but otherwise drops off immediately, so that the next modes can no longer penetrate.

As transmissive element, for example, an etalon can be used, the transmission profile is then designed so that one of the transmission maxima in the wavelength range with the lower laser threshold, the other transmission maxima outside the homogeneously widened gain profile. This adjustment can be achieved by adjusting the degree of reflection of the etalon and its thickness in the production as well as the arrangement with a corresponding tilting against the optical axis in the laser resonator during assembly.

For the fundamental wavelength with the higher lasing threshold, the transmissive wavelength-selective element has no spectrally selective effect, meaning that it transmits radiation at this wavelength and in an area comprising at least part of the homogeneous broadening unhindered or at least almost unhindered. This can usually be controlled by an appropriate choice of material. Suitably, the transmissive elements, in particular also the etalon, can additionally be coated with an antireflective layer acting for the wavelength with the higher laser threshold.

If the wavelength-selective element is alternatively designed as a reflective element, it has, equivalent to the transmissive element, preferably in the region of the fundamental wavelength with the lower laser threshold, a reflection maximum. The end mirror is designed to be transparent to the fundamental wavelength with the lower laser threshold, for the fundamental wavelength with the higher laser threshold or a multi-mode operation permitting range around the fundamental wavelength he is accordingly designed - except for the usual losses - as completely as possible reflective, so that the light of this Wavelength is not coupled out of the laser resonator, but contributes to sum frequency generation.

The reflective element can be designed, for example, as an optical grating, in particular as a line or volume Bragg grating.

Analogous to the use of a transmissive element, the reflexive element, in the case of a grating by varying the lattice constant and / or its orientation or tilting relative to the optical axis, can also be set such that the effective cross-section, that is to be actually achieved, for the stimulated emission at the wavelength with the lower laser threshold is in the region of the maximum of the wavelength-dependent stimulated emission cross section at the wavelength with the higher laser threshold.

If the cross sections for the emission of the two fundamental wavelengths are strong differ, in particular if the cross section for the fundamental wavelength with the lower laser threshold is substantially greater than for the fundamental wavelength with the higher laser threshold, the transmissive or equivalent, the reflective element is made or arranged in the laser resonator that the value of the wavelength-dependent cross section for the stimulated emission at the fundamental wavelength with the lower lasing threshold is in the region of the maximum of the wavelength-dependent stimulated emission cross section at the higher lasing threshold fundamental wavelength.

The emission cross section for the stimulated emission is, inter alia, a function dependent on the wavelength and has in each case a relative maximum at the two fundamental wavelengths, ie. H. the cross section is high compared to the environment, but the maxima can have different values. In the case of Nd: YAG lasers, for example, the value of the aforementioned cross section at a fundamental wavelength of 1064 nm is substantially higher than at a fundamental wavelength of 1038 nm. At adjacent wavelengths, the value of the cross section then drops sharply. If the frequency-selective element is designed accordingly or arranged with a corresponding inclination against the optical axis, then the transmission or reflection maximum of the relevant element, which is usually exactly at the respective fundamental wavelength, can be shifted so that it still does is in the region of the fundamental wavelength with the lower laser threshold, but on an edge of the cross-section as a function of the wavelength, ie spectrally outside the maximum. The transmission or reflection maximum is then set so that the value of the cross section at this approximately corresponds to the value of the cross section for the stimulated emission at the fundamental wavelength with the higher laser threshold, this is thus approximated or in the range.

In this way, the stimulated emission cross-section at the lower lasing threshold fundamental wavelength is attenuated to such an extent that both cross sections are approximately equal to each other and the lower laser threshold transition does not dominate, which is a significant advantage in summation frequency generation.

Usually, the output mirror for the two fundamental wavelengths λ ₁ and λ _{2 is designed to be} reflective and transmissive for the wavelength of the sum frequency, this construction is particularly space-saving and comes with a minimum number of optical elements. In an alternative embodiment, at least one deflection mirror is arranged in the laser resonator, which is transmissive to the sum frequency, the transmission here should be maximum with minimum losses, and which is at least partially reflective designed for at least one of the fundamental wavelengths. In this way it is for example possible to decouple the wavelength with the sum frequency laterally from the laser resonator, the output mirror can then the same reflective properties as the end mirror. This arrangement has the advantage that a double pass of the laser radiation can be realized by the non-linear crystal, which leads to a more efficient sum frequency generation.

In addition to the variation of the cross-section for the stimulated emission for determining the laser threshold by tilting the wavelength-selective element as described above, there are further means for determining and / or varying the laser thresholds, which may additionally or alternatively optionally comprise the laser arrangement. The laser threshold for a wavelength λ _i , at two fundamental wavelengths with i = 1, 2, is calculated according to the condition

R _i is the outcoupling reflectance, Δn _i is the population inversion, σ _{i is} the emission cross section for the stimulated emission, and l is the length of active amplifying medium to be traversed by the light. Finally, V _i denotes the loss factor for the respective laser transition.

A simple way of determining or varying the laser thresholds is therefore to vary the Auskoppelreflexionsgrade and the loss factors. In order to vary the laser thresholds of the two transitions of interest for the stimulated emission or to set them to a desired value, for example, the end mirror and / or the coupling-out mirror can have different reflectivities for the two fundamental wavelengths λ ₁ and λ ₂ . Also, the at least one deflecting mirror can have different degrees of reflection for the two fundamental wavelengths, and this can also be combined with the different degrees of reflection for the end and / or outcoupling mirror.

Alternatively or in addition, an optical attenuation element may also be arranged in the laser resonator, which radiation is embodied differently attenuating by absorption and / or reflection for each of the two fundamental wavelengths. This optical attenuation element can also be designed to be controllable. As such a weakening element can for example. serve a dielectrically coated plane-parallel plate, which in each case has a defined residual reflection at the two wavelengths. Furthermore, it is possible to use the tilt angle of the attenuation element to fine-tune the residual reflection, as long as the dielectric layer design is designed accordingly.

It is also possible to design at least one of the optical boundary surfaces of the active medium and / or the nonlinear crystal to define the laser thresholds such that they attenuate each of the two fundamental wavelengths λ ₁ and λ ₂ differently by absorption and / or reflection. This can be achieved for example by means of a corresponding coating.

In a preferred embodiment, which allows a compact design, at least two components are connected directly in contact standing by the components end mirror, active laser medium, non-linear crystal, wavelength-selective element and Auskoppelspiegel. This can be achieved for example by the technique of so-called "diffusion bonding". This has the advantage that the distance which the laser radiation in the resonator has to propagate through air can be shortened in order to minimize refractive index changes of the air and thus of the resonator length due to external influences such as air pressure, temperature or air humidity.

In a further preferred embodiment, which also allows a compact design with few elements to be adjusted, end mirror and / or coupling-out mirror are each as a dielectric coating on one of the other components, d. H. here on the laser medium or on the nonlinear crystal.

In a particularly preferred embodiment, a further non-linear crystal for power-dependent attenuation of the fundamental wavelength with the low laser threshold in single-mode operation by frequency doubling (second harmonic generation, SHG) is arranged in the laser resonator.

By means of this additional damping, the already mentioned intensity fluctuations with respect to a laser resonator with only one nonlinear crystal for summation frequency generation can be further stabilized in terms of time than is already the case in the case of the arrangement according to the invention. The two non-linear crystals can be designed, for example, by the respective cutting angle or the temperature so that they only double in their frequency one of the two fundamental waves, or else mix both fundamental wavelengths to the sum frequency. By the choice of crystal type and / or the crystal length, the respective strength of the non-linearity can be adjusted independently.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the specified combinations but also in other combinations or alone, without departing from the scope of the present invention.

The invention will be explained in more detail for example with reference to the accompanying drawings, which also disclose characteristics essential to the invention. Show it:

1 a first laser arrangement,

2 an alternative embodiment of a laser arrangement,

3 an intensity distribution of different modes in the amplifying medium of this laser arrangement,

4 a third embodiment of a laser array and

5 a fourth embodiment of a laser arrangement.

In the 1 The laser arrangement shown comprises a laser resonator in which an active laser medium 1 for generating homogeneously broadened radiation of a first fundamental wavelength λ ₁ by stimulated emission from an upper laser level to a first base level and for generating homogeneously diffused radiation of a second fundamental wavelength λ ₂ by stimulated emission from the upper laser level to a second fundamental level. The laser thresholds of the two fundamental wavelengths λ ₁ and λ ₁ differ. For the active laser medium 1 For example, it may be a neodymium-doped crystal. The host crystal is preferably yttrium aluminum garnet (YAG) or yttrium vanadate (YVO ₄ ). The host crystals can also be doped with other elements such as erbium or ytterbium. Titanium-doped sapphire crystals or fluorides such as yttrium lithium fluoride are also suitable as active lasing media, to name just a few examples.

wobei in der Regel ohne Steuerung anderer Parameter mindestens die Wirkungsquerschnitte σ; der verschiedenen Laserübergänge verschieden sind.The laser threshold, or the threshold conditions above which the laser begins to operate, is calculated for a fundamental wavelength λ _i , with i = 1.2 indicating the fundamental wavelength, according to the formula

usually without control of other parameters at least the cross sections σ; the different laser transitions are different.

The laser resonator is on the left side by an end mirror 2 and on the right side through a Auskoppelspiegel 3 limited. In the in 1 The arrangement shown, the two mirrors for both fundamental wavelengths λ ₁ and λ _{2 are} highly reflective, ie they reflect both wavelengths with the lowest possible losses. For the wavelength λ _{S of} the sum frequency v _S is the output mirror 3 however transparent. The laser resonator also has a light linear crystal 4 arranged to mix the radiation of the two fundamental wavelengths λ ₁ and λ ₂ in radiation of a sum frequency v _S. The frequencies v ₁ and v _{2 of} the fundamental wavelengths λ ₁ and λ ₂ are here mixed to the sum frequency v _S , which corresponds to an addition of the reciprocals of the fundamental wavelengths to the reciprocal of the sum wavelength. For example, without limitation in terms of usable, active laser media 1 means - a neodymium-doped yttrium-aluminum garnet crystal as a laser medium 1 , ie a Nd: YAG laser, with the two fundamental wavelengths λ ₁ = 1064 nm and λ ₂ = 1319 or 1338 nm, the result is a sum wavelength λ _S = 589 or 593 nm Measures, as described below, are set in a wide range of variation, so that depending on the definition of both the smaller and the larger of the two fundamental wavelengths, the lower laser threshold can be assigned, and the designation with λ ₁ and λ ₂ so no conclusion a priori allows for the ratio of laser thresholds.

Finally, the laser arrangement also comprises a wavelength-selective element which can be embodied either transmissively or reflexively and in which 1 example shown as a transmissive element - here in the form of an etalon 5 - trained and arranges in the laser resonator. In another, in 2 the arrangement shown, the wavelength-selective element is designed as a reflective element, there as an optical grating 6 For example, as a line or volume Bragg grating. At the in 2 shown arrangement is another end mirror 7 used, which is permeable to the fundamental wavelength with the lower laser threshold - in the example shown λ ₁ - and reflective as possible for light of the fundamental wavelength with the higher laser threshold. The optical grid 6 is outside the laser resonator on the side of this end mirror 7 arranges. This in 2 shown optical grating 6 has in the range of the fundamental wavelength with the lower laser threshold on a reflection maximum, accordingly, the in 1 shown etalon 5 in the region of the fundamental wavelength with the lower laser threshold, a transmission maximum.

The characteristics of the wavelength selective element and its interaction with the remaining components are essential to successful operation of the laser assembly to achieve the desired functionality. In the case of a transmissive wavelength-selective element, the properties for the two fundamental wavelengths λ ₁ and λ ₂ differ depending on the laser threshold. For the fundamental wavelength with the lower laser threshold, the wavelength-selective element - even for a reflective wavelength-selective element - such a spectral selectivity, which allows a single-mode operation at this fundamental wavelength. The spread of the transmission maxima at the etalon 5 or the reflection maxima in the optical grating 6 must be so large that only one mode of the homogeneous line-broadened spectrum is excited about the central wavelength and excitations in the other modes in the homogeneously broadened line are suppressed. At the Etalon 5 This can be done, for example, by a suitable choice of material, thickness and angle in which the etalon 5 is tilted relative to the optical axis of the laser resonator can be adjusted. Similar setting options also result in the optical grating 6 ,

On the other hand, for the fundamental wavelength with the higher lasing threshold, the transmissive wavelength-selective element must not exhibit a spectrally selective effect, ie it leaves this fundamental wavelength and also at least a region around this fundamental wavelength comprising some of the modes of the homogeneously broadened line unhindered, ie with the lowest possible attenuation , At this fundamental wavelength, therefore, multimode operation is possible and even desirable. For this purpose is the etalon 5 for radiation having the fundamental wavelength with the higher laser threshold, preferably completely transmissive, for which purpose it may, for example, be coated with an antireflective layer which acts only around this fundamental wavelength for this region.

In case of using an optical grating 6 as a wavelength-selective element, this is arranged outside the laser resonator, light of the fundamental wavelength with the higher laser threshold is then usually already from the end mirror 7 reflects why the properties of the optical grating 6 with respect to this fundamental wavelength play no role .. In the case of using an optical grating 6 So it does not depend on the spectral selective effect with respect to the fundamental wavelengths with the higher laser threshold, but on the end mirror 7 , The end mirror 7 can in this case, for example, with only for the Basic wavelength can be coated with the lower laser threshold antireflective layer to maximize the transmissivity. For the fundamental wavelength with the higher laser threshold, the end mirror is 7 in a highly homogeneous at least the homogeneous broadening comprehensive wavelength range designed. When using an etalon 5 owns the end mirror 2 usually the same reflectance for both fundamental wavelengths, in contrast, has the output mirror 3 generally different reflectivities for the two fundamental wavelengths. Regardless of this is the Auskoppelspiegel 3 is highly transmissive for the wavelength of the sum frequency, so that this wavelength λ _{S is} essentially completely decoupled independently of the reflectivities for the fundamental wavelengths.

The following is based on 3 be explained, which situation arises under the set conditions in operation. Both laser transitions of the active medium 1 are homogeneously line-broadened and have a common upper laser level, so that both laser transitions compete for equal population inversion. In addition, both fundamental wavelengths λ ₁ and λ _{2 have} different cross sections for the stimulated emission, σ ₁ and σ ₂ . As a result, different reinforcements and different Schwellpumpleistungen come about.

With a continuous increase in pump power starting at zero, due to the lower lasing threshold, the stimulated emission first begins at the fundamental wavelength with the lower lasing threshold. In the laser resonator and in particular in the active laser medium 1 Then a standing wave is formed only for the longitudinal fashion, at which the etalon 5 a transmission maximum or the optical grating 6 has a reflection maximum, off. This intensity distribution of the oscillating longitudinal mode in the laser medium 1 in the form of a standing wave is in 3 symbolized by the thick, solid line. Shown is the intensity over the location coordinate along the optical axis, both magnitudes in arbitrary units.

This intensity distribution corresponds directly to the saturation of the population inversion, so this is particularly high at the maxima of the intensity distribution, while it is very low or not present in the minima. The saturation of the gain or inversion is thus spatially modulated with a modulation period that is equal to half the wavelength of the mode (spatial hole burning). Thus, while the saturation has already been reached at the maxima, the inversion or saturation in the valleys of the intensity distribution is only incomplete.

Therefore, if the pump power is further increased, due to this incompletely saturated inversion in the valleys of the modulated saturation of the gain for the fundamental wavelength with the lower lasing threshold, the laser may now also pass above the lasing threshold for the fundamental wavelength with the higher lasing threshold. Im in 1 Example shown has the etalon 5 For the fundamental wavelength with the higher laser threshold no spectral selectivity, so that a multi-mode operation is possible here. When using an optical grating 6 , as in 2 shown, corresponding to the end mirror 7 no spectrally selective effect for the fundamental wavelength with the higher laser threshold, which in the case of the mirror means that it is designed to reflect at least a larger region of the homogeneous line broadening by this fundamental wavelength.

Therefore, at the fundamental wavelength with the higher lasing threshold, the laser resonator readily adapts to the number of longitudinal modes and their wavelengths such that in the spatial average and in multimode operation at the second fundamental wavelength also in the time average - the antinodes at the fundamental wavelength with the higher Laser threshold are optimal in the troughs at the fundamental wavelength with the lower laser threshold. In 3 For example, two such modes are shown with dashed and dot-dashed lines. In this way it is achieved that both fundamental wavelengths are spatially decoupled and thus can coexist despite competition for inversion. As a result, they do not tend to reciprocate, ie to amplify their own fundamental wavelength at the expense of others, and vice versa alternately.

As an example, two cases will be described below in which the active laser medium 1 is a Nd: YAG crystal and the fundamental wavelengths are at 1064 nm and 1319 nm. The sum frequency mixing of the two wavelengths gives the sum wavelength λ _S = 598 nm in the yellow-orange spectral range, which can only extremely difficult or impossible to generate in direct frequency doubling.

The natural cross section for stimulated emission is significantly larger at 1064 nm than at 1319 nm. However, there are various possibilities for determining the laser thresholds, whereby only the variation of the coupling-out reflectance R _i in the formula for the threshold condition is to be considered here by way of example. Because the Auskoppelspiegel 3 necessarily only the wavelength λ _{S has} to decouple at the sum frequency, one has with respect to the adjustment of the reflectance for the two fundamental wavelengths λ ₁ and λ ₂ just at this mirror a large margin. In the Use of an etalon 5 you have this scope also at the end mirror 2 when using an optical grating 6 These are the options for the end mirror 7 highly limited.

For example, if the reflectance for both wavelengths, 1064 nm and 1319 nm of the output mirror 3 At nearly 100%, the lasing threshold for the 1064 nm wavelength is typically lower than the lasing threshold for the 1319 nm wavelength due to the higher stimulated emission cross section at 1064 nm. The fundamental wavelength at the lower lasing threshold would be λ in this case ₁ = 1064 nm and the fundamental wavelength with the higher laser threshold λ ₂ = 1319 nm. The advantage of this embodiment is that a high efficiency results for the resonator-internal sum frequency mixing, since the power at both wavelengths greatly increased by the high reflectance of the coupling-out mirror becomes. However, it is disadvantageous that the dominant wavelength of 1064 nm extracts the majority of the inversion, so that a very high pump power is necessary to achieve oscillation at the wavelengths of 1319 nm.

In a second example, the coupling-out reflectance R ₂ for the wavelength λ ₂ = 1319 nm is left at 100%. The decoupling reflectance R ₁ for the wavelength λ ₁ = 1064 nm, however, is chosen so that it is substantially smaller than the Auskoppelreflexionsgrad for the wavelength λ ₂ . Here then exactly the opposite picture results from the previously considered case, namely that the laser threshold at λ ₁ = 1064 nm is higher than the laser threshold for λ ₂ = 1319 nm. The fundamental wavelength with the higher laser threshold is here λ ₁ , the fundamental wavelength with the lower laser threshold, the wavelength λ ₂ . In this embodiment, a stable single-mode operation can initially be set at λ ₂ = 1319 nm. Only when the pumping power continues to rise is the remaining inversion extracted by laser operation at λ ₁ = 1064 nm with a low resonator quality, which represents only a weak perturbation with respect to λ ₂ . The tendency to reciprocate between the two wavelengths is therefore low. A disadvantage of this setting, however, is that the wavelength λ ₁ oscillates with high Auskoppelverlusten, therefore, the power increase in the resonator is low and overall reduces the sum frequency mixing.

The determination of the laser thresholds by different reflectivities of the end mirror and / or the coupling-out mirror for the two fundamental wavelengths λ ₁ and λ ₂ is only one possibility for defining or varying the laser thresholds. Another possibility is to arrange an optical attenuation element in the laser resonator, which may optionally be controllable. This attenuation element attenuates radiation for each of the two fundamental wavelengths λ ₁ and λ ₂ differently by absorption and / or reflection. Also, at least one of the optical interfaces of the active laser medium 1 or the nonlinear crystal 4 be configured so that, for example, by means of a corresponding coating radiation for each of the two fundamental wavelengths λ ₁ and λ ₂ attenuates differently by absorption and / or reflection.

Instead of influencing the reflectance or absorption levels of the mirrors, an adjustment of the laser thresholds can also be carried out by means of the wavelength-selective element, wherein this setting additionally or alternatively to the above-mentioned. Options can be done. In the case of a transmissive element, the transmission maximum, or in the case of a reflective element the reflection maximum, is at a wavelength at which the value of the wavelength-dependent cross section for the stimulated emission at the wavelength is at the wavelength spectral spectrally outside the maximum value and thus tends to approximate the maximum possible cross section at the wavelength with the higher laser threshold. In other words, the value of the wavelength-dependent stimulated emission cross section at the lower laser threshold wavelength is in the region of the maximum of the wavelength dependent stimulated emission cross section at the higher lasing threshold wavelength, namely at one edge of the wavelength cross-section curve higher laser threshold.

The value of the stimulated emission cross-section depends, among other things, on the material and on the wavelength and, as a rule, naturally differs for both fundamental wavelengths and varies greatly with the wavelength. In the actual fundamental wavelength, in the example above, this would be the fundamental wavelength λ ₁ = 1064 nm, the cross section has a maximum, which is much higher here, than the maximum of the cross section for the fundamental wavelength λ ₂ = 1319 nm with the higher natural laser threshold. The wavelength-selective element is now designed so that the transmission or reflection maximum is not exactly at this maximum of the cross-section, but at an edge of the cross-section, that is still in the range of the fundamental wavelength λ ₁ , but at a different wavelength, so that the value of the cross section at this wavelength at the edge has a value which corresponds approximately to the maximum of the - natural or otherwise adjusted - cross section for the fundamental wavelength λ ₂ = 1319 nm. Thus, it can be easily prevented that the fundamental wavelength λ ₁ dominated too much, by setting a tilt of the etalon 5 or of the optical grating 6 The position of the transmission maximum or reflection maximum with respect to the fundamental wavelength λ ₁ can be set specifically.

In the 4 The laser arrangement shown has a deflection mirror in the laser resonator 8th which is transmissive to the sum frequency and is at least partially reflective for at least one of the fundamental wavelengths. Also conceivable are arrangements with more than one deflection mirror 8th , for example, to couple a plurality of resonator branches. By introducing such a deflection mirror 8th can also set the laser thresholds by the at least one deflection mirror 8th has different reflectivities for the two fundamental wavelengths λ ₁ and λ ₂ . Here are also several deflecting mirrors 8th each of which has a different selectivity, in order to influence the reflectance of the wavelengths as precisely as possible. The deflection mirror 8th but can also be used simply to constructively advantageous manner, the wavelength of the sum frequency λ _S not via a Auskoppelspiegel, but on the deflection mirror 8th decouple. The Auskoppelspiegel used here 9 therefore has the same function as an end mirror, ie it is not used for decoupling.

In other embodiments, which can also be combined with each other, the laser resonator can also be designed so that of the components end mirror 2 . 7 , active laser medium 1 , nonlinear crystal 4 , wavelength-selective element and output mirror 3 . 9 at least two components are connected directly to each other in contact standing, the compound can be prepared for example by the so-called "diffusion bonding". end mirror 2 . 7 and / or output mirror 3 . 9 can also be designed in each case as a dielectric coating of one of the components. Im in 2 example shown, for example, the end mirror 7 as a dielectric coating on the active laser medium 1 and the Auskoppelspiegel 3 as a dielectric coating on the nonlinear crystal 4 be upset. Are then still nonlinear crystal 4 and laser medium 1 directly connected, creates a very compact component.

In 5 Finally, a further embodiment of the invention is shown on the in 1 is shown in the arrangement and in the laser resonator another nonlinear crystal 10 is arranged. The arrangement of the other nonlinear crystal 10 can be readily transferred to the other embodiments. This other nonlinear crystal 10 serves to attenuate a frequency doubling of the fundamental wavelength with the lower laser threshold in single-mode operation. In crystals used for sum frequency mixing, unavoidable frequency doubling also generally occurs. By the further nonlinear crystal 10 With damping effect, the intensity of the laser transitions that compete with each other for the population inversion can be stabilized on average over time by suppressing larger fluctuations.

With the above-described laser arrangement, a temporally high constant of the sum frequency of two fundamental wavelengths λ ₁ and λ _{2 can be} achieved, the oscillation of the two wavelengths relative to one another can be largely suppressed, so that the laser arrangement readily permits the application of sum frequency mixing in laser scanning microscopy. The laser arrangement requires only one resonator load and can therefore be made very compact.

LIST OF REFERENCE NUMBERS

1

laser medium

2

end mirror

3

output mirror

4

nonlinear crystal

5

etalon

6

optical grating

7

end mirror

8th

deflecting

9

end mirror

10

another nonlinear crystal

λ ₁

first fundamental wavelength

λ ₂

second fundamental wavelength

v _p

sum frequency

λ _S

Wavelength of the sum frequency

Claims (12)

Laser arrangement comprising - a laser resonator in which an active laser medium ( 1 ) for generating homogeneously broadened radiation of a first fundamental wavelength (λ ₁ ) by stimulated emission from an upper laser level to a first baseline level and generating homogeneously broadened radiation of a second fundamental wavelength (λ ₂ ) by stimulated emission from the upper laser level to a second basal level is arranged, wherein the laser thresholds of the two fundamental wavelengths (λ ₁ , λ ₂ ) differ, - an end mirror ( 2 . 7 ) and a Auskoppelspiegel ( 3 . 9 ), which limit the laser resonator, - a laser resonator arranged in the non-linear crystal ( 4 ) for mixing the radiation of the two fundamental wavelengths (λ ₁ , λ ₂ ) into radiation of a sum frequency (v _S ), - And a transmissive or reflective wavelength-selective element, characterized in that - the wavelength-selective element for the fundamental wavelength with the lower laser threshold has such a spectral selectivity that allows a single mode operation at this fundamental wavelength, and at the fundamental wavelength with the higher laser threshold, a multi-mode operation is possible for which - in the case of a transmissive wavelength-selective element, this is arranged inside the laser resonator and has no spectrally selective effect for the fundamental wavelength with the higher laser threshold, or - in the case of a reflective wavelength-selective element this outside the laser resonator on the side of the end mirror ( 7 ), wherein the end mirror ( 7 ) is designed to be transmissive for the fundamental wavelength with the lower laser threshold and is designed to be completely reflective for a wavelength range around the fundamental wavelength with the higher laser threshold. Laser arrangement according to claim 1, characterized in that the wavelength-selective element, if it is designed as a transmissive element, has a transmission maximum in the region of the fundamental wavelength with the lower laser threshold or, if it is designed as a reflective element, in the region of the fundamental wavelength with the lower Laser threshold has a reflection maximum. Laser arrangement according to claim 2, characterized in that in the case of a transmissive element, the transmission maximum and in the case of a reflective element, the reflection maximum at a wavelength at which the value of the wavelength-dependent stimulated emission cross section at the fundamental wavelength with the lower laser threshold in the range of the maximum of the wavelength-dependent stimulated emission cross-section at the higher lasing threshold fundamental wavelength. Laser arrangement according to claim 2 or 3, characterized in that the wavelength-selective element in the case of a transmissive element as etalon ( 5 ), which is preferably coated with an antireflective layer acting for the wavelength with the higher laser threshold and, in the case of a reflective element, as an optical grating ( 6 ), preferably designed as a line or volume Bragg grating. Laser arrangement according to one of claims 1 to 4, characterized in that in the laser resonator at least one deflection mirror ( 8th ) is arranged, which is transmissive to the sum frequency and for at least one of the fundamental wavelengths (λ ₁ , λ ₂ ) is designed to be at least partially reflective. Laser arrangement according to claim 5, characterized in that for fixing the laser thresholds, the at least one deflecting mirror ( 8th ) has different reflectivities for the two fundamental wavelengths (λ ₁ , λ ₂ ). Laser arrangement according to one of claims 1 to 6, characterized in that for fixing the laser thresholds of the end mirror ( 2 . 7 ) and / or the Auskoppelspiegel ( 3 . 9 ) have different reflectivities for the two fundamental wavelengths (λ ₁ , λ ₂ ). Laser arrangement according to one of claims 1 to 7, characterized in that for the definition and / or variation of the laser thresholds at least one preferably controllable optical attenuation element is arranged in the laser resonator, which radiation for each of the two fundamental wavelengths (λ ₁ , λ ₂ ) by absorption and / or reflection is configured differently attenuating. Laser arrangement according to one of claims 1 to 8, characterized in that for fixing the laser thresholds at least one of the optical interfaces of the active medium and / or the nonlinear crystal ( 4 ) Radiation for each of the two fundamental wavelengths (λ ₁ , λ ₂ ), preferably by means of a corresponding coating, by absorbing and / or reflection differently attenuating, is configured. Laser arrangement according to one of claims 1 to 9, characterized in that of the components end mirror ( 2 . 7 ), active laser medium ( 1 ), nonlinear crystal ( 4 ), wavelength-selective element and output mirror ( 3 . 9 ) at least two components are connected directly in contact with each other. Laser arrangement according to claim 10, characterized in that the end mirror ( 2 . 7 ) and / or the Auskoppelspiegel ( 3 . 9 ) each as a dielectric coating on one of the components active laser medium ( 1 ) and / or nonlinear crystal ( 4 ) are executed. Laser arrangement according to one of claims 1 to 11, characterized in that in the laser resonator another non-linear crystal ( 10 ) is arranged for frequency doubling of the fundamental wavelength with the lower laser threshold in the single-mode operation for the purpose of power-dependent vibration damping of this fundamental wavelength.

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