DE19532440C2 - Solid state laser arrangement for frequency conversion - Google Patents

Description

The present invention relates to a solid-state laser arrangement for frequency conversion, which has at least one optically pumped solid-state laser, the laser-active one Medium is a birefringent prism and at least one nonlinear crystal holds, the birefringent prism has an end face which is oriented so that it meets the conditions for total internal reflection for radiation of a frequency.

Such an arrangement, referred to as a quantum generator, is known from DE-OS 15 89 968 known.

The further development of the high-performance diode laser also shows its effect on the Development of solid-state lasers, since solid-state lasers are increasingly Diode lasers are pumped. Diode-pumped solid-state lasers can be very compact and be built efficiently. In particular, solid-state lasers with a Output power from a few watts to a few tens of watts the laser medium with one Diode lasers or pumped with diode laser field arrangements or arrays. It can Here, sizes are achieved that a Pac with output powers up to 500 mW size smaller than one cubic centimeter.

Due to the increasingly compact structure and the ho in relation to the size Such output is opened up by such diode laser-pumped solid-state lasers increasingly new areas of application. One such field of application lies in the Be  range of displays built using solid-state lasers become. Such display screens are currently mainly called monochromatic Screens built using the respective frequency-doubled solid perlaser. Solid state lasers with wavelength are used to build up chromatic screens blue, green and red colors. These required wavelength ranges can be based on different lines of neodymium: YAG, which can be generated in a stable basic mode, by Frequency doubling can be achieved.

In the case of the longitudinally pumped solid-state lasers mentioned at the beginning, there is under different implementation concepts, such as the monolithic single-frequency laser and the frequency-doubled microchip laser. With the monolithic single frequency Lasers do not allow frequency doubling within the resonator. With the microchip A dichroic coupler is required for lasers, which significantly increases the complexity and the resonator quality and thus the efficiency is reduced.

The quantum generator known from the aforementioned DE-OS 15 89 968 for the Generation and amplification of light containing wavelengths from red to ultraviolet except one preferably consisting of an optical transmitter or amplifier Pump light source for generating a primary beam, an optical means, in particular a crystal with an optically highly non-linear effect, to produce a secondary beam, the frequency of which is a harmonic of the frequency of the primary beam, where in this quantum generator is characterized in that another optical Mit tei, in particular a prism consisting of a strong birefringent crystal, the contains a reflective surface for the purpose of separating primary and secondary beam is ground by reflection on the thinner medium so that its normal with the optical axis of this crystal encloses an angle and that mirrors continue to optional amplification of the beams which are spatially separated out of the crystal are provided by optical feedback.

From US 5,249,196 a laser arrangement is known in which a solid as Penta prism resonator is used. The prismatic solid medium serves in  essential to increase the number of pumping surfaces used. In an out the frequency of a laser is doubled with a non-linear crystal.

In LASER MAGAZIN 6/88, pages 35-39, "Optical problems of diode-pumped solids perlaser ", by P. Grelle, among other things, thin coating layers are combined addressed with diode-pumped solid-state lasers. It is stated that layer packets are required for an intracavity frequency doubling, where λ / 4- Layers at the fundamental wavelength simultaneously as λ / 2 layers for frequency doubling radiation can act.

The present invention is now based on the above State of the art, the object of a solid-state laser arrangement, which is preferred is pumped longitudinally by means of diode laser radiation, in such a way that in ei nem stable operating mode other wavelengths of the output radiation, with a structurally simple structure can be obtained.

The above object is achieved by the features of claims 1 or 2.

Two alternative measures are given, based on the fundamental wave, with which the solid is excited to achieve a frequency doubling and thereby compared to the basic mode a changed, usable wavelength in a stable Be to get the solid-state laser. Of the two Al Alternatively, the structure of the solid-state laser arrangement is preferred in which the Face is totally reflective for a fundamental wave and none for the converted wave Total reflection fulfilled. With such an arrangement, there are no dichroic outcouplers, as required by the prior art, the resonator losses are low and it high conversion efficiencies can be achieved.

Preferred developments of the solid-state arrangement, as stated above, are specified in the respective subclaims to which reference is made.

In certain cases it may be necessary to double the second harmonic in order to for example to get the fourth harmonic wave (4ω). This will be a second Doubler crystal is provided, the first doubler crystal and the second ver doppler crystal are assigned to different end faces of the solid, the  decoupled second harmonic wave (2ω) from one end face into the second Doubler crystal is irradiated and doubled, the resonator for that second harmonic wave (2ω) between two highly reflective surfaces connected to the both, opposite the respective irradiation surfaces of the two doubler crystals end faces are formed or on two mirrors behind the crystals are formed, is built.

In a further embodiment of the solid-state laser arrangement, a further solid body is provided which, in comparison with the prism-shaped solid body which oscillates with a first fundamental wave (ω ₁ ), oscillates in a different fundamental wave (ω ₂ ), the two solid-state lasers being arranged in relation to one another, that at one end face a total reflection for the first fundamental wave (ω ₁ ) occurs and which is transmissive for the second fundamental wave (ω ₂ ), so that the two fundamental waves (ω ₁ , ω ₂ ) have a common resonator path, and that in the common resonator path is a non-linear Kri stall for sum frequency generation or difference frequency generation is arranged.

The various embodiments are described below with reference to FIG Drawings explained in more detail. In the drawings:

Fig. 1 shows a schematic structure for intra-cavity Summenfrequenzerzeu supply based on a prism laser according to an embodiment of the invention,

Fig. 2 shows a laser arrangement for doubling the second harmonic wave accelerator as a further embodiment of the invention,

Fig. 3 shows a solid-state laser arrangement for frequency doubling with a solid body in the form of a birefringent prism and a doubler crystal associated with an end face or interface of the prism, and

Fig. 4 shows the arrangement of FIG. 3, wherein the doubler crystal is contacted directly with the solid.

It should be noted that in the respective figures, components of the individual embodiments Men, which are comparable to each other, are designated by the same reference numerals. If individual components are described in detail in relation to an embodiment, these descriptions and explanations are based on comparable components of the other Embodiments not made again, but they can be analog to the other embodiments.

The solid-state fiber arrangement, as shown in FIG. 3, has as its solid body a birefringent prism 1 , for example made of Nd: YVO ₄ , which has a first end face 2 , a second end face 3 and a fourth end face 4 . The end face 3 is assigned a doubler crystal 5 .

The first end face 2 is not coated. The solid body 1 is, although this is not shown, pumped longitudinally by means of diode laser radiation 9 , the radiation being radiated into the prism-shaped solid body 1 via the third end face 4, for example. The third end face 4 is designed to be highly reflective for the fundamental wave (ω), while the second end face, at least in the region opposite the doubler crystal 5 , for the fundamental wave (ω) and the converted wave (the second harmonic, 2ω) is coated with an anti-reflective coating. The same applies to the entry surface 6 of the doubler crystal 5 . In contrast, the end face 7 of the doubler crystal 5 for the fundamental wave (ω) and the second harmonic (2ω) is coated with a highly reflective coating.

The doubler crystal 5 is a non-linear crystal, for example of potassium titanyl phosphate (KTiOPO ₄ ) or a beta barium borate (BBO-BaB ₂ O ₄ ). The solid-state medium used Nd: YVO ₄ is a birefringent laser medium.

As shown in FIG. 3, a laser beam with frequency ω is irradiated into the doubler crystal 5 . Within the doubler crystal 5 , the non-linearity of the doubler generates the laser beam with twice the frequency (2ω) (the second harmonic). In the case of phase matching, as used in FIG. 1, the polarization of the fundamental wave (ω) and the second harmonic (2ω) are perpendicular to one another. This fact is used in conjunction with the birefringent laser medium (solid body 1 ) to decouple the second harmonic wave (frequency 2ω). The stimulated emission cross section of the π polarization (the E field runs parallel to the c crystal axis) is significantly larger than the σ polarization (the E field is perpendicular to the c crystal axis). Under this condition, the laser swings with the π polarization. The second harmonic (Fre quenz 2ω) has, provided that the polarization of the fundamental wave and the second harmonic wave are perpendicular to each other, the σ polarization (E field) perpendicular to the c-axis. Since with the solid material Nd: YVO ₄ the refractive index n _π with respect to the π polarization is greater than the refractive index n _σ for the σ polarization, the following applies to the total reflection angle at the unrefined, first end face 2 : α _π = sin ^-1 (1 / n _π ) <α _σ = sin ^{- 1} (1 / n _σ )
in which
α _π (ω): total reflection angle of the polarization on the end face
α: angle of incidence of the fundamental wave on the end face
α _σ (2ω): total reflection angle of the σ polarization

If the angle of incidence α is chosen so that α _π (ω) <α <α _σ (2ω) is satisfied, the total reflection occurs for the fundamental wave with the π polarization, while the second harmonic with the σ polarization the resonator, which is defined between the third end face 4 and the end face 7 , is coupled out, as indicated by the beam 8 in FIG. 3.

With the structure given above is a simple solid-state laser arrangement given the second harmonic, with a frequency of 2ω from the resonator can be uncoupled and used.

The structure of the solid-state laser arrangement, as shown in FIG. 4, corresponds essentially to the structure according to FIG. 3. However, in this case the entry surface 6 is optically directly with the second end face 3 of the solid body 1 , for example by wringing, contacted. In this way, the antireflection coating of the two interfaces, ie the second end face 3 of the solid body 1 and the entry surface 6 of the doubler crystal 5 , can be dispensed with, and the structure of FIG. 3 can be made even more compact while reducing the losses arise at the two interfaces of the embodiment of FIG. 3.

For self-frequency doubling crystals, the laser medium 1 and the frequency doubler 5 are combined. In this case the loss of the resonator is minimal.

It should be noted at this point that in the individual figures the points that surrounded by a circle, represent arrows that are perpendicular to the Paper plane and extend out of this and the π polarization or s Polarization mean while the arrows in the paper plane are each perpendicular extend to the radiation lines that represent E-polarization that are parallel to the Pa pier plane (σ polarization or p polarization).

As already mentioned above, the solid body 1 is preferably pumped longitudinally by means of diode laser radiation 8 , as is indicated, for example, by the arrow 9 , in the case of FIG. 3 via the third end face 4 .

While arrangements with which the frequency of the fundamental wave (ω) was doubled (2ω) were explained with reference to FIGS . 3 and 4, an arrangement is schematically shown in FIG. 1, with which a sum-frequency generation within the resonator is achieved can, again using the basic structure of the prism-shaped solid and a sum-forming crystal, in the embodiment of FIG. 1 with the reference numeral 25 . The prismatic solid body consists, for example, of an Nd: YVO ₄ crystal that vibrates at the frequency ω ₁ . The orientations of the polarization and the crystal axis result from the arrows shown in FIG. 1 in an analogous manner to the previously described embodiments. In the example shown, the laser field has the polarization that is perpendicular to the paper plane. The solid body is in turn pumped over an end face, in this case over the upper end face 2 . With the lower, second end face 3 is accordingly the embodiment of FIG. 4, the sum frequency formation crystal 25 immediately contacted with the end face 6 bar. Another solid-state laser, which is preferably pumped with diode laser radiation, which is designated by the reference symbol 22 , oscillates at a second frequency (ω ₂ ) and is p-polarized (parallel polarized). The rear end surface 26 seen in the beam direction of the solid-state laser 22 is highly reflective for ω ₂ and highly transmissive for the pump frequency ω _p , while the front coupling-out surface 27 is designed for ω _{2 to be} anti-reflective. The radiation 23 (ω ₂ ) of the solid-state laser 22 is radiated via the end face 2 of the prismatic solid-state laser 21 . Since the angle of incidence with respect to the end face 2 deviates only a few degrees from the Brewster angle, this structure is distinguished by low losses. The sum frequency formation crystal 25 used has an end surface 7 which for the fundamental wave (ω ₁ ) with which the solid-state laser 21 vibrates and for the second fundamental wave (ω ₂ ) with which the solid-state laser 22 emits, and for the sum frequency ω ₁ + ω ₂ , is highly reflective. As can be seen in the figure, the two solid-state lasers are arranged such that the two fundamental waves (ω ₁ + ω ₂ ) form a common resonator path between the end face 7 of the summation crystal 25 and the first end face 2 . With the sum frequency formation crystal 25 , the sum men frequency (ω ₁ + ω ₂ ) is generated within the common resonator path. The third end face 4 is designed to be highly reflective for the fundamental wave with the frequency ω ₁ , while being highly transmissive for the wave with the sum frequency ω ₁ + ω ₂ , so that radiation with the sum frequency ω via the third end face 4 ₁ + ω _{2 is} hidden. With such an arrangement, for example, output radiation 28 can be generated with a red color by the prismatic solid-state laser 21 oscillating with a wavelength λ ₁ = 1123 nm and the solid-state laser 22 oscillating with a wavelength λ ₂ = 1342 nm, which results in λ = 611 nm .

A structure similar to the embodiment of FIG. 1 can be used for the explained sum frequency generation by using a negative, uniaxial solid-state laser prism 21 .

A further embodiment of the solid-state laser arrangement, based on the principle according to the invention, is shown in FIG. 2. The basic structure of the prism-shaped solid 31 and the doubler crystal 35 are comparable with the solid body 1 and the doubler crystal 5 of FIG. 4. In addition, a second doubler crystal 36 is provided for the second harmonic wave (2ω). The Nd: YVO ₄ crystal is arranged in such a way that a folding takes place on the end face 2 for the fundamental wave (ω). The c-axis of the prism-shaped solid-state laser 31 is parallel to the folding plane. The polarization runs parallel to the c-crystal axis and is perpendicular to the paper plane. The Nd: YVO ₄ crystal is pumped with a diode laser from the third end face 4 , as is again indicated by the beam 8 . The emitted fundamental wave (ω) is totally reflected on the first end face 2 and directed towards the first doubler crystal 35 , where the frequency is doubled (2ω). The end face 7 of the first doubler crystal is again highly reflective for the fundamental wave (ω) and the second harmonic cal (2ω). At the first end face 2 , the total reflection is not fulfilled for the second harmonic (2ω) and the angle of incidence is approximately the same as the Brewster angle of the second harmonic, so that a beam 37 with the frequency 2 ω is coupled out, which is transmitted via a dichroic beam splitter 38 in the second doubler crystal 36 enters. This second doubler crystal 36 is designed for frequency doubling of the second harmonic (2ω). The end surface 39 is highly reflective for both the second harmonic (2ω) and the fourth harmonic (4ω). The fourth harmonic (4ω), which is generated internally in the second doubler crystal 36 , emerges from the exit surface 40 and is coupled out at the dichroic beam splitter 38 , as indicated by the beam 41 .

Based on the different options based on the different, example liable embodiments have been explained above, based on waves, different wavelengths of usable radiation producing a Fre frequency doubling and / or a sum frequency formation can be obtained. In addition to egg nem compact construction very low resonator losses can be achieved or it can high conversion efficiency can be achieved.

Claims (7)

1. Solid-state laser arrangement for frequency conversion, which has an optically pumped solid-state laser, the laser-active medium ( 21 ) of which is a birefringent prism, and contains at least one nonlinear crystal ( 25 ), the double-refractive prism having an end face ( 2 ) which is oriented in this way that it fulfills the conditions for the total internal reflection for radiation of a frequency (ω1), characterized in that the prismatic, laser-active medium ( 21 ) oscillates at a first fundamental frequency (ω1), a second optically pumped solid-state laser with a laser-active medium ( 22 ) is provided, which vibrates with a second fundamental frequency (ω2), which differs from the first fundamental frequency (ω1), the two laser-active media ( 21 , 22 ) being arranged in relation to one another in such a way that on the end face ( 2 ) of the prism-shaped , laser-active medium ( 21 ) total reflection for radiation of the first fundamental frequency (ω1) occurs and that the end face ( 2 ) for radiation of the second fundamental frequency (ω2) is transmissive, so that the radiations of the two fundamental frequencies (ω1, ω2) pass through a common resonator path in which the non-linear crystal ( 5 ) is used to form the sum frequency or to form the difference frequency two fundamental frequencies (ω1, ω2) is arranged. 2. Solid-state laser arrangement for frequency conversion, which has an optically pumped solid-state laser, the laser-active medium ( 31 ) of which is a birefringent prism, and contains at least one nonlinear crystal ( 35 , 36 ), the birefringent prism having an end face ( 2 ) which is oriented in this way is that it fulfills the conditions for total internal reflection for radiation of a frequency (ω), characterized in that the prismatic, laser-active medium ( 31 ) vibrates at a first fundamental frequency (ω), a second non-linear crystal ( 36 ) for frequency doubling The first doubler crystal ( 35 ) and the second doubler crystal ( 36 ) are assigned to different end faces ( 2 , 3 ) of the laser-active medium ( 31 ), the radiation coupled out of one end face ( 2 ) of the second Harmonic African (2ω) of the fundamental frequency (ω) in the second doubler crystal ( 36 ) is radiated and freq is doubled, a resonator for radiation of the second harmonic (2ω) being formed between two highly reflecting surfaces ( 7 , 39 ) which lie opposite the two incident surfaces ( 6 , 40 ) of the two doubler crystals ( 35 , 36 ). 3. Solid state laser arrangement according to claim 1 or 2, characterized in that the laser-active medium ( 21 ; 31 ) is longitudinally pumped by means of diode laser radiation. 4. Solid-state laser arrangement according to claim 1 or 2, characterized in that the laser-active medium ( 21 ; 31 ) is an Nd: YVO ₄ crystal, the one end face ( 2 ) being oriented parallel to its c-axis. 5. Solid-state laser arrangement according to claim 2, characterized in that the one beam surface ( 6 ) of the doubler crystal ( 35 ) for the fundamental frequency (ω) and for the second harmonic (2ω) has an anti-reflection coating. 6. Solid state laser arrangement according to one of claims 1 to 5, characterized in that the non-linear crystal ( 25 ; 35 ) and the prismatic, laser-active medium ( 21 ; 31 ) on an end face ( 3 ) of the laser-active medium ( 21 ; 31 ) optically are contacted. 7. Solid-state laser arrangement according to one of claims 1 to 6, characterized in that the prism-shaped, laser-active medium ( 21 ) generates a first fundamental wave with a wavelength of 1123 nm, while the second laser-active medium ( 22 ) a fundamental wave with a wavelength of 1342 nm is generated, so that a wavelength of 611 nm is generated and coupled out when the sum frequency is formed.