DE10241987B3 - Diode-pumped solid-state laser with mode diaphragms - Google Patents

Abstract

A diode-pumped solid-state laser with Q-switching and at least one intracavitary laser crystal having at least one optical lens effect and having at least one optical axis is described, along which a pump light beam emitted continuously or in a pulsed manner falls into the laser crystal, the thermal lens effect of which depends on the operating behavior of the solid-state laser takes at least two distinguishable powers D¶low¶ and D¶high¶. DOLLAR A The invention is characterized in that at least one mode aperture is provided along the at least one optical axis on both sides of the laser crystal, of which a first mode aperture M¶1¶ is a laser beam forming within the resonator at the refractive power D¶high¶ im Aperture limiting beam cross-section and of which the second mode aperture M¶2¶ has an aperture limiting the laser beam forming within the resonator at the refractive power D¶low¶ in the beam cross-section.

Description

technical area

The invention relates to a diode-pumped solid-state laser with Q-switching and at least one intracavitary laser crystal having at least one optical lens and having at least one optical axis, along which a pump light beam emitted continuously or in a pulsed manner falls into the laser crystal, the thermal lens effect of which depends on the Operating behavior of the solid-state laser takes at least two distinguishable refractive powers D _low and D _high .

State of the art

Diode-pumped solid-state lasers are powerful, compact light sources that are characterized by the progressive Development of laser diodes as pump light sources of considerable importance won. Laser diodes are now available, the above Pump light powers of 10 W and more and beyond via emission wavelength spectra feature, the laser crystals suitable for solid-state lasers in the field of optical absorption bands lie, making a supreme efficient optical excitation of the laser crystals can be achieved.

Diode-pumped solid-state lasers are basically suitable for one Many different technical fields of application, preferred they are used in areas where powerful, and small-sized laser systems are desired. For example. such monochromatic light sources are used for material processing, preferably for surface material processing, like material removal, material change or surface finishing used.

Usually is operated for targeted material processing of the laser between two operating states, namely the so-called stand-by mode, while that of the Q-switch is closed and the laser does not emit light, as well as the actual one Working fashion in which laser light is emitted when the Q-switch is open. The Q-switch is also pulsable in work mode. For a variety of material processing cases it required that the laser beam a laser beam with a constant Power and beam quality to disposal poses, and this during the total time the laser is in working mode. In particular in labeling applications where targeted light-induced Material ablation e.g. a sequence of spatially separated ones individual letters are worked into a material surface it is important that the beginning of a letter or a line or the first letters or lines of a lettering over a consistent line quality feature. With the help of high performance Scanner mirror systems it is possible to deflect a laser beam for labeling purposes in such a way that up to 500 characters per second can be "inscribed" on a material surface, which means that a character is completed within about 2 ms. going one assumes that the line of a single letter of Line start to line end over should have a constant line quality, it can be seen that the for this required laser beam over a constant light output and beam quality within should have a duration of 2 ms.

Use one for this Purpose, for example, a continuously longitudinally pumped solid-state laser with intracavitary optical quality control, so that is essentially the laser power and beam quality of the laser beam determining element of the laser crystal, its thermal equilibrium crucially depends on the operating state of the laser.

So when the Q-switch is closed, d. H. the laser is in stand-by mode, all through the pump light source heat introduced over the Laser crystal itself emitted to the environment. Because of this strong warming occurring The laser crystal forms a so-called strong positive effect thermal lens, through which the beam quality is decisively influenced becomes. However, is the quality switch open, so is a significant portion of that within the laser crystal introduced heat over the Dissipated laser radiation itself, whereby the laser crystal cools down noticeably. Although also forms in Case of light emission, i.e. with open or pulsed operated Q-switch, a positive thermal lens effect within the laser crystal out, but it is much weaker than in the previous one described stand-by mode.

If the solid-state laser from stand-by mode switched to laser mode, i.e. the quality switch is closed, so the inside of the laser crystal suddenly becomes the output power of the laser highly efficient cooled, sometimes in the first 100 - 300 ms of laser operation to a very poorly trained thermal Lens and thus to a changed beam quality leads. The laser crystal only moves after a suitable start-up time into a thermal equilibrium when the laser reaches it only over of stable light output and beam quality.

If you compare the time periods with which fast labeling systems are able to If a sign is to be written and, on the other hand, after closing the Q-switch, thermal equilibrium is reached within the laser crystal and the laser beam thus has a constant power and beam quality, it can easily be seen that labeling processes are carried out with the currently available laser systems with insufficient beam quality are, especially since the production of a character typically takes place within a period of time which is approximately an order of magnitude smaller than the period of up to 300 ms required for stable start-up behavior.

The visual impact of the transition a diode-pumped solid-state laser from stand-by mode to laser operation occur in an even clearer Dimensions in Appearance the bigger the The difference in the optical refractive power is that of the laser crystal in the two operating states accepts.

Representative of longitudinally diode-pumped solid-state lasers is the one in the US documents US 5,907,570 such as US 5,638,397 Solid-state laser systems described. In both cases, the diode-pumped solid-state lasers provide at least one laser crystal intracavitatively, which has an optical lens effect. In addition, between the laser crystal arrangement and a resonator end mirror, an aperture limiting the internal beam cross section is provided, by means of which the vibration behavior of the resonator should preferably be limited to the basic laser mode TEM _OD . Here, the opening aperture of the resonator's internal aperture is matched to a single specific vibration behavior of the resonator which is in thermal equilibrium.

The invention is based on the object of a diode-pumped solid-state laser with Q-switching and an intracavitary laser crystal having at least one optical lens and having at least one optical axis, along which a pump light beam emitted continuously or in a pulsed manner falls into the laser crystal, the thermal lens effect of which depends of the operating behavior of the solid-state laser assumes at least two distinguishable refractive powers D _low and D _high , in such a way that, regardless of the operating behavior of the solid-state laser, a largely constant light output with constant beam quality is emitted by the solid-state laser. In particular, a solid-state laser of the generic type is to be optimized in such a way that a constant light output and beam quality can be called up at any time of the laser in operation, that is to say in particular also immediately after the Q-switch from a closed to an open state. In this context, it is necessary to specify a diode-pumped solid-state laser for high-precision material processing, with which, for example, material surface inscriptions of the highest quality and quality can be carried out. Such a solid-state laser should preferably be used for material surface micro-engravings whose engraving width and depth have dimensions down to the micrometer and sub-micron range.

The solution of the invention lying task is specified in claim 1. The idea of the invention Advantageously further developing features are the subject of the subclaims as well the description with reference to the embodiments.

According to the invention, a generic diode-pumped solid-state laser according to the features of the preamble of claim 1 is designed such that at least one mode diaphragm M ₁ , M _{2 is} provided along the at least one optical axis on both sides of the laser crystal, of which one mode diaphragm M _{1 is} one within the Laser beam forming resonators at the refractive power D _high in the beam cross-section limiting aperture. In addition, the second mode diaphragm M _{2 is provided} within the resonator, which provides an aperture limiting the laser beam at the refractive power D _low in the beam cross section. In this way, the appropriate selection and arrangement of the mode diaphragms M ₁ and M ₂ ensures that the quality of the laser beam remains largely constant regardless of the operating behavior of the solid-state laser, so that the beam power and beam quality are maintained at a consistently high level even immediately after opening the intracavitary Q-switch can be accessed.

The apertures of the mode diaphragms and their arrangement within the resonator are to be selected and matched to one another in such a way that the beam intensity of the TEM ₀₀ mode is both in the operating behavior of the solid-state laser with a weakly designed thermal lens or low refractive power D _low , and in the case of a strongly pronounced thermal lens effect , ie a refractive power D _high , is largely the same. Here, the laser radiation emitted from the resonator preferably has a beam quality M ² of less than 3, particularly preferably less than 1.5, under both operating conditions.

Although the diode-pumped solid-state laser is unable to emit laser light during operation at D _low due to the closed Q-switch, which is typically designed as a Q-switch circuit, the operating situation with D _{low occurs} immediately when the laser starts up after opening the Q-Switch switch on. In this case, of the solid-state laser that is not in thermal equilibrium during the start-up period, it is necessary to intercept with the measure according to the invention and to ensure that the beam quality immediately after opening the Q-switch is almost identical to that which occurs after thermal equilibrium has been reached, thus sets D _low after reaching the thermally weaker lens effect.

In a preferred embodiment has the diode-pumped solid-state laser an asymmetrically constructed resonator, i. H. the resonator has two Resonator mirrors with different radii of curvature, either a convex-plan, convex-concave or convex-convex resonator structure correspond.

Asymmetric optical resonators with intracavitary thermal lens action in the form of a diode-pumped laser crystal can, in a manner known per se, be stabilized by the in 2 Stability diagram shown are described. The stable operating behavior of a solid-state laser is not only dependent on the pure parameters determining the geometry of the resonator, such as the radii of curvature R ₁ and R _{2 of} the resonator mirrors and their mutual distance L, but the stable vibration behavior depends to a large extent on the optical refractive power behavior D des optically pumped laser crystal. As already mentioned, the latter is, however, not a constant variable but temperature-dependent.

To facilitate the description of the optical refractive power behavior of an optically pumped laser crystal, one usually uses the notion of an optical lens which has a refractive power D which is dependent on the temperature and at least one main plane. Together with the geometric resonator sizes R ₁ , R ₂ and L, the stability criteria of such a resonator with an intracavitary thermal lens - in this context the use of the term "active resonator" is also common - using the so-called G factors formulate as follows:
0 <G ₁ ⋅G ₂ <1
with G ₁ = 1 - L * / R ₁ - D⋅d ₂
G ₂ = 1 - L * / R ₂ - D⋅d ₁
and L * = d ₁ + d ₂ - D⋅d ₁ ⋅d ₂

The above formalisms take into account the fact that the laser crystal has two main planes, d ₁ , d ₂ corresponding to the distances of the resonator mirrors from the respective main planes of the thermal lens or the laser crystal. It should be pointed to the book by W. Koechner, "Solid-State Laser Engineering", Springer 1999, 5 ^th Edition.

An asymmetrical resonator constructed in this way has two stable oscillation ranges which are dependent on the refractive power of the laser crystal, of which the first stable oscillation range is limited by the critical refractive powers D _I and D _II and the second oscillation range by the refractive powers D _III and D _IV . See the stability diagram in 2 , in which two straight lines AS1 and AS2 are entered, which describe the stability behavior of two asymmetrical resonators. This shows that, for a given resonator geometry, the stable vibration behavior of a resonator depends on the refractive power D of the laser crystal.

In this case, the first stability range, in the case of D _I and D _II, corresponds to the operating behavior of the diode-pumped solid-state laser described at the outset, in which the optical refractive power is weak, ie the state with D _low . The second stability range, between D _III and D _IV, however, corresponds to the operating behavior with strong thermal refractive power, ie D _high . If, as will be explained in detail below with the aid of a specific exemplary embodiment, if only one mode diaphragm with an aperture limiting the laser beam cross section is introduced intracavitarily, then with an appropriate arrangement of the diaphragm within the resonator, a beam quality improvement can be seen in the best case with regard to the formation of the TEM ₀₀ Modes, however, only in a single stability range. If, on the other hand, the solid-state laser is operated in the stability range not covered by the diaphragm, this shows the formation of a dramatic deterioration in the beam quality. On the other hand, if two mode diaphragms are provided according to the invention within the resonator, each of which leads to a limitation of the beam cross section of the laser beam that forms within the respective stability ranges, then with a suitable choice of the mode diaphragms and their positioning within the resonator, beam qualities M ² of less than 3, preferably less than 1.5, can be achieved can be achieved.

Using a specific embodiment should influence the invention on the beam quality of a diode-pumped asymmetric solid-state laser explained become.

The invention is hereinafter without restriction the general inventive concept based on exemplary embodiments described by way of example with reference to the drawing. It demonstrate:

1 schematic resonator arrangement of a longitudinally diode-pumped solid-state laser,

2 Stability diagram as well

3 Diagram showing the beam quality as a function of the refractive power of the laser crystal.

Ways to Execute the Invention, industrial applicability

In 1 a diode-pumped solid-state laser is shown schematically. A diode laser takes care of the details 1 as a pump light source, the laser crystal arranged within the resonator 2 optically stimulates longitudinally. The laser resonator is in the 1 Embodiment shown designed as a convex resonator and has a convex resonator mirror 3 and a flat resonator mirror 4 , It is assumed that the resonator mirror 3 a radius of curvature R ₁ of -500 mm and the resonator mirror 4 has a radius of curvature R ₂ of infinity. The resonator length L is 400 mm and the distance between the resonator mirrors 3 and the main plane H of the laser crystal 2 d ₁ = 171 mm. The laser crystal has a crystal length of 8 mm and is designed as an Nd: YVO ₄ crystal. The laser beam that can be generated with this resonator geometry has a beam cross section of 1.8 mm. Furthermore, two mode shutters M ₁ and M _{2 are} introduced within the resonator. The distance of the mode diaphragm M ₁ to the resonator mirror 3 is m ₁ = 40 mm, whereas the distance between the mode diaphragm M ₂ and the resonator mirror 4 m ₂ = 105 mm.

In a particularly preferred manner, Nd: YVO ₄ is suitable as the laser crystal. However, laser crystals that consist of the following doped crystals can also be used in the same or a similar way: Nd: YAG, Nd: YLF, Nd: GVO ₄ , Nd: YPO ₄ , Nd: BEL, Nd: YALO, Nd: LSB, Yb: YAG, Yb: FAB, Cr: LiSAF, Cr: LiCAF, Cr: LiSGAF, Cr: YAG, Tm-Ho: YAG, Tm-Ho: YLF, Er: YAG, Er: YLF or Er: GSGG.

Do you run the in 1 Diode-pumped solid-state laser shown without providing the mode diaphragms mode diaphragms M ₁ and M ₂ , so there is a beam quality in the emitted laser beam as a function of the developing thermal refractive power D of the laser crystal, which is shown by curve a in the diagram 3 is shown.

In the in 3 Diagram shown are plotted along the abscissa diopter values, along the ordinate the beam quality in M ² values. It can be clearly seen that without providing any mode diaphragms within the resonator, the beam quality M ² present in the emitted laser beam assumes values for almost the entire diopter range that are greater than 3 and thus has an insufficient beam quality for many applications. If, however, the mode diaphragm M _{1 is provided} , for example with an aperture of 1.2 mm, this affects the beam quality within the emitted laser beam in accordance with the method shown in 3 curve b shown. It can be seen that for values of refractive power D> 5 the beam quality M ² decreases to values below 3. However, the mode aperture M ₁ has an insufficient effect on the beam quality for values of the refractive power D between 0 and 5. In this range, the values M ² remain insufficiently high for the beam quality, ie by 3 and larger.

The situation is reversed when the mode diaphragm M _{2 is} only inserted into the resonator. This results in a function curve according to curve c for the beam quality in the emitted laser beam. In this case, an acceptable beam quality M ² of less than 3 is set for refractive power values D> 5. Nevertheless, the beam quality M ^{2 is} significantly reduced for refractive power values> 5.

On the other hand, according to the invention, two mode diaphragms M ₁ and M _{2 are provided} within the resonator in such a way that both mode diaphragms limit the beam cross section of the laser beam within the resonator - in the case of the resonator construction according to 1 If the beam diameter is 1.8 mm, the aperture of the mode aperture M ₁ is 1.2 mm and the aperture of the mode aperture M ₂ is 0.9 mm - then a beam quality is set depending on the refractive power D of the laser crystal through the course of the function d according to the diagram in 3 given is. It can be seen that beam qualities M ² of less than 3 can be achieved for both small and larger refractive power values. It also shows that the provision of two mode diaphragms according to the invention enables an almost constant beam quality M ^{2 to be achieved} both in the range of low refractive power values and in the range of larger refractive power values.

This property of the beam quality dependent on the refractive power in accordance with the function curve d in 3 is used according to the invention to master the problem of the starting process described at the outset immediately after opening the Q-switch in a diode-pumped solid-state laser. With the measure according to the invention, it is possible to achieve an almost constant high beam quality with values of M ² <3 at any time of the Q-switched diode-pumped solid-state laser in operation.

In the above exemplary embodiment, the mode diaphragm M ₂ can effectively reduce the beam quality M ² for low refractive powers D _low , whereas the mode diaphragm M ₁ significantly reduces the beam quality M ² for higher refractive powers D _{high to} below 3.

The mode diaphragm M _{2 also has} an additional function with regard to increasing the directional stability of the laser beam emerging from the solid-state laser. This is the mode diaphragm M ₂ as close as possible to the resonator coupling-out mirror 4 to arrange.

1

Pump light source, laser diode

2

laser crystal

3, 4

resonator

M ₁ , M ₂

mode aperture

Claims (8)

Diode-pumped solid-state laser with Q-switching and at least one intracavitary laser crystal with at least one optical axis and having at least one optical axis along which a pump light beam emitted continuously or in a pulsed manner falls into the laser crystal, the thermal lens effect of which depends on the operating behavior of the solid-state laser, at least two of them distinguishable refractive powers D _low and D _high , characterized in that at least one mode aperture is provided along the at least one optical axis on both sides of the laser crystal, of which a first mode aperture M _{1 is} a laser beam forming within the resonator at the refractive power D _high in the beam cross section has limiting aperture and of which the second mode diaphragm M _{2 has} an aperture limiting the laser beam forming within the resonator at the refractive power D _low in the beam cross section r has. Diode-pumped solid-state laser according to claim 1, characterized in that the apertures of the mode diaphragms are such are dimensioned such that at least one mode diaphragm blocks the laser beam limited in beam cross section. Diode-pumped solid-state laser according to Claim 1 or 2, characterized in that the laser beam emitted from the resonator has a beam quality M ² which is approximately the same both in the case of D _low and in the case of Dhigh and assumes values of less than 3. Diode-pumped solid-state laser according to one of Claims 1 to 3, characterized in that the laser crystal acting as a thermal lens provides two main planes, and that the following stability criteria apply to the resonator: 0 <G ₁ ⋅G ₂ <1 with G ₁ = 1 - L * / R ₁ - D⋅d ₂ G ₂ = 1 - L * / R ₂ - D⋅d ₁ and L * = d ₁ + d ₂ - D⋅d ₁ ⋅d ₂ d ₁ , d ₂ : distances of the resonator mirrors from the main planes of the thermal lens R ₁ , R ₂ radius of curvature of the resonator mirror and that the sizes d ₁ , d ₂ , R ₁ and R _{2 are selected} such that with the following critical refractive powers D _I , D _II , D _III and D _IV , for that applies

the following relationships are fulfilled: D II - D I = D IV - DIII ≤ 6 dptr. | D III - D II | ≤ 2 dptT. Diode-pumped solid-state laser according to one of claims 1 to 4, characterized in that the Q-switch is an intracavitary acousto-optical or electro-optical Q-switch switch. Diode-pumped solid-state laser according to one of claims 1 to 5, characterized in that the asymmetrical optical resonator a convex-plan, convex-concave or convex-convex resonator structure identifies. Diode-pumped solid-state laser according to one of claims 1 to 6, characterized in that the laser crystal with one or several of the following dopants is doped: Nd, Yb, Cr, Tm, Ho or Er. Diode-pumped solid-state laser according to claim 7, characterized in that the laser crystal consists of the following doped crystals: Nd: YAG, Nd: YVO ₄ , Nd: YLF, Nd: GVO ₄ , Nd: YPO ₄ , Nd: BEL, Nd: YALO, Nd : LSB, Yb: YAG, Yb: FAB, Cr: LiSAF, Cr: LiCAF, Cr: LiSGAF, Cr: YAG, Tm-Ho: YAG, Tm-Ho: YLF, Er: YAG, Er: YLF or Er: GSGG ,