DE4101522C2 - - Google Patents

Description

The invention relates to a solid-state laser, the Nd: YAG Kri stall via collimation and focusing optics from a Hochlei Stungsdiode is pumped, according to the preamble of claim 1.

Through the technology of diode-pumped solid-state lasers, where as Replacement of conventional flash or arc lamps as an excitation source now Semiconductor laser diodes are used today as solid state lasers feasible due to their compactness and typical dimensions of a few mm to cm as well as their efficiency and long service life are suitable for miniaturized measuring systems. Because of this tech completely new applications have become possible were not marketable due to the inefficiency. To develop Example of the large market for laser measurement systems and their Spreading over a wide area is absolutely necessary Lich to operate such laser measurement systems eye-safe. It means that the emitted laser light power even in the worst case no ge may cause health damage and, for example, none Eye retinal injury. For the devices according to the status In technology, this is achieved by beam expansion, which is achieved by an increase in the image of the laser beam spot the power density of the laser radiation is reduced to a value that such a ver excludes last. Here, however, the laser radiation is on one expanded to such a large diameter that one is necessary for many measurements agile, spatially resolved measurement becomes impossible.

Another way to achieve eye safety is by using the laser beam to use such a wavelength at which the human Au ge has little or no transmittance. Laser radiation Such wavelength is already on the cornea or in the so-called The vitreous of the eye is completely absorbed and can affect the retina  no longer reach. Firstly, the cornea is essentially insensitive The second is laser radiation, which affects the eye ge can penetrate, focused on the retina, so that there are large Power densities arise. Laser radiation of such a wavelength that be is already absorbed in the cornea, but still shows a lot there larger beam diameter and thus a much lower lei density. So far, only for a single laser wavelength the so-called eye safety quantified, so that from this Limits for the radiation exposure of the human eye are set could be, for example, in the military laser protection ordinance (Stanag 3606). This single wavelength is the 1.54 µm line of the He: glass laser, which - however in previously very voluminous systems - Is used to a limited extent in measurement technology. The too casual radiation exposure at this wavelength is by several sizes orders higher than the permissible radiation exposure of for example the 1.06 µm line of the Nd: YAG laser.

From this state of the art results that an Er: glass laser the ideal le laser light source for eye-safe measuring systems would be if possible of a diode pump from Er: Glas could be given. This was but not yet feasible.

Er-doped solid-state lasers and lasers constructed with them are well known in the prior art. In most known cases, the Er ³⁺ ion is pumped with laser radiation in the range between 1047 and 1064 nm, generally with Nd: YAG laser radiation, the use of Nd: fibers as indicated in the prior art of the documents cited below , is equivalent to using Nd: YAG lasers. For example, B. Hanna in Optics Communications, Vol. 63, No. 6, 1987, pages 417 to 420, an Er laser rod, which is optically excited with an Nd solid-state laser. Hanna also points out in particular that it would be advantageous to arrange the Er rod within the Nd laser resonator, and that both diode-pumped Nd: YAG lasers and diode-pumped Nd: YLF lasers are provided as suitable pump sources for the Er material can be.

A corresponding arrangement is also by Gapontsev et al. in Optics and Laser Technology, August 1982, pages 189 ff. That one Er laser shown is in a so-called folded resonator configuration constructed, the Er laser rod within an Nd laser resonator is arranged and has a resonator which is independent and is not collinear to the Nd laser resonator.

A similar arrangement is described by Espie, Hanna et al. in Optics Communications, Vol. 69, No. 2, Dec. 15, 1988, pages 153 to 155, also using an Er laser rod is arranged in a folded resonator, which, however, here is excited by an external Nd laser.

The arrangements according to the prior art described here have however, in particular for a miniaturizable structure, which with Semiconductor laser diodes can be excited, significant disadvantages.

On the one hand, arrangements according to Gapontsev et al. on the one hand in the present Configuration is difficult with semiconductor laser diodes pumpable, on the other hand the arrangement is based on the relative use long Er crystals, which act as absorbers in the Nd laser resonator high laser threshold, which contradicts the use of semiconductor laser diodes exists as a pump source.

Furthermore, neither Hanna nor Gapontsev or Espie is suitable Resonator configuration presented, which on the one hand high miniaturizability of the laser, on the other hand, for high efficiency and leads to a low laser threshold.

The invention is based on solid-state lasers, which use semiconductor laser diodes are pumped; these are well known and exemplary for a simple Nd: YAG laser from Berger et al. in Appl. Phys. Lett., Vol. 51, No. 16, October 19, 1987, pages 1212 ff.  

The publication "Soviet Journal of Quantum Electronics ", Vol. 6, No. 10, 1976, p. 1190 ff. - Articles by Galant et al. - in Considered, as is apparent from the description below.

State-of-the-art high-power laser diodes all emit at approximately 800 nm, whereas Er: glass has no or only insignificant absorption at this wavelength, as can be seen from the diagram in FIG. 5. In contrast to this is, for example, the Nd: YAG crystal, which has a sharp absorption line here and can therefore be pumped very well with laser diodes.

The present invention is based on the object; To design a laser that can be optically excited by high-power semiconductor laser diodes at around 800 nm and emits at 1.54 µm in the eye-safe area without a high output power of the Nd: YAG laser for a sufficient population inversion in the Er ³⁺ require.

This object is reliably achieved by the in claims 1 and 2 Measures identified resolved. In the description below Exemplary embodiments are explained and in the figures of the Sketched drawing. It shows

Fig. 1 shows the structure of a construction of an embodiment of a semiconductor laser diode with He pumped: glass laser according to the invention,

Fig. 2 shows the structure of a diode-pumped Nd: YAG laser according to the state in turn a He for the optical excitation of the art: Glass laser is used

FIG. 3 shows the construction of another embodiment of an Er: glass laser according to the invention, which can be pumped by semiconductor laser diodes,

Fig. 4 is a schematic diagram of the pump light and a resonator volume Anord voltage according to Fig. 3,

Fig. 5 is a diagram of a Adsorptionsspektrums an Er: glass laser crystal.

Figs. 5 of the drawings illustrates the typical absorption spectrum of an Er: glass laser crystal. If ytterbium is coded with such a solid-state laser, a strong absorption caused by the Yb results in the range between 850 nm and 1.1 μm, compared to an absorption of the erbium⁴I _11/2 level of only 980, which is only slight in the dashed course of the spectrum nm. If optically excited in the range between 850 nm and 1.1 µm, with a suitable choice of the doping conditions, the excited Yb ^{3+ is} able to release its energy to the Er ⁺³ quite completely by means of collision processes of the second kind. This leads to a population of the upper laser level of Er ⁺³ , so that a population inversion necessary for stimulated emission can be built up. Here, too, the coded crystal does not have a much stronger absorption at 800 nm than the non-codot crystal, but the falling edge of the Yb absorption is just around 1.06 µm, i.e. the emission wavelength of the Nd: YAG crystal laser . Such a codotized crystal can in turn be pumped by means of a diode-pumped Nd: YAG laser, as can also be seen from the above-mentioned article by Galant, the laser structure of which is schematically outlined in FIG. 2.

Fig. 2 illustrates how the radiation 31 of a Hochleistungsla serdiode 11 by means of collimation and focusing optics 12 in a laser resonator, which is formed from the mirrors 13 and 14 , so focused and that the pump light radiation in the Nd: YAG crystal 15 is as possible is fully absorbed all the time. The crystal end faces 20 are planpo lined and anti-reflective. With suitable shaping of the resonator, this laser radiation emits at 1.064 μm, which is focused via a second collimation and focusing optics 16 in a second resonator formed from the two mirrors 17 and 19 into an Er: glass rod 18 . Its end faces 21 are also polished and anti-reflective. With suitable shaping of this second resonator, this eye-safe laser radiation 35 emits at 1.54 μm. The Er laser formed from the laser rod 18 and the resonator mirrors 17 and 19 is optically excited via a laser diode pumped Nd: YAG laser.

The major disadvantage of this prior art design is the high output power of the Nd: YAG laser required, which is necessary to achieve a sufficient population inversion in the Er ³⁺ .

Fig. 3 shows an embodiment of the invention, in which the pump light radiation 31 of a high-power laser diode 11 via a collimation and focusing optics 12 is focused into a Nd: YAG crystal 15 , which is, however, between two resonator mirrors 22 and 23rd located. Within this resonator formed from the mirrors 22 , 23 there is now an Er: glass rod 18 . The mirrors and the end faces of the Nd: YAG crystal 15 as well as of the Er: glass crystal 18 are now coated as follows:

The mirror 22 has a high reflection for 1.06 μm, as does the rod end surface 26 . The Nd: YAG end faces 24 , 24 'are coated with an anti-reflective coating for 1.064 µm. The end surface 25 of the Er: glass rod 18 is coated with an anti-reflective coating for 1.064 µm and at the same time highly reflective for 1.54 µm. The mirror 23 is partially reflective coated for 1.54 microns. This results in a mode profile within this resonator, as is illustrated in FIG. 4. For the sake of clarity, the two laser rods 15 and 18 are only shown in broken lines in FIG. 4. The resonator volume 33 of the Nd: YAG laser is built up inside the resonator, consisting of mirror layers 22 and 26 . The Er: glass rod is located inside the Nd: YAG resonator and absorbs part of the high photon flux density there. If the Er: glass rod is now suitably codotated, the edge of the absorption according to FIG. 5 is just such that it has a partial absorption at 1.064 μm. The photon flow in the resonator 22-26 is not completely prevented in this way, stimulated emission of the Nd takes place, but a large part of it is absorbed in the Er: glass rod 18 . If a population inversion is built up in this, it can in turn emit stimulated by the resonator formed from the mirror layers 25 and 23 . With a suitable tell coating of the mirror 23 , laser radiation 35 emerges at 1.54 μm. It can therefore be said that the Er: glass rod is pumped "intra-cavity" in the Nd: YAG resonator. The larger flux density of the photons that occurs enables a population inversion, which is almost two orders of magnitude higher than in the prior art (see FIG. 2). A decisive factor for the functioning of such a laser is a suitable shift in the Yb absorption by appropriate choice of the codoping.

The resonator arrangement presented here, which is nested within itself Includes resonators, thus goes beyond that known from US Pat. No. 4,956,843 State of the art, in which only two of each other independent laser resonators irradiated by one and the same laser diode to generate two different wavelengths.

In FIG. 5, an embodiment of a pumped semiconductor laser diodes gensicheren au laser is outlined at 1.54 microns. An Nd: YAG crystal 15 and an Er: glass rod 18 are processed as follows: the surface 27 is convex and has a high-transmission coating for 810 nm and a highly reflective coating for 1.064 μm. The opposite Kri stallendfläche 29 is ground and contacted with a flat surface 18 a of Er: glass rod 18 . The flat end surface 29 of the Nd: YAG crystal 15 is coated with an anti-reflective coating for 1.064 μm, the flat end surface 18 a of the Er: glass rod 18 is coated with an anti-reflective coating at 1.064 μm and at the same time highly reflective coated at 1.54 μm. The Er: glass end surface 28 is machined and partially reflective coated at 1.54 microns. At a suitable distance, a mirror 30 is positioned, which is concave and has a partially reflective coating for 1.54 μm.

In a special embodiment, this mirror 30 can be replaced by an equivalent coating on the crystal end face 28 . The Nd: YAG crystal 15 is now optically excited via the emission 31 of a high-performance laser diode 11 , which is suitably shaped via a collimation and focusing optics 12 , so that a photon flow stimulated emission is produced between the end face 27 and the end face 28 . Here, an inversion density is generated in the Er: glass rod 18 in such a way that a photon flow stimulated emission at 1.54 μm arises between the crystal end face 18 a and the mirror 30 or the crystal end face 28 . In the latter case of a further exemplary embodiment, the mirror 30 is omitted. Part of this photon flux is coupled out as a laser beam 35 at 1.54 µm from the resonator.

This shows lasers that can be pumped with semiconductor diodes and emit at the same time at 1.54 µm, using a so-called "intra-cavity pumping". Both laser rods 15 and 18 can in this case be firmly connected to one another, so that an extremely stable and rigid structure is obtained, in particular with the omission of the mirror 30 - due to the corresponding coating of the end face 28 of the crystal 18 . Here, dimensions of the solid-state laser 10 of typically a few mm to a few cm as well as a high conversion efficiency from pump light radiation to laser output radiation are given. Such miniaturized lasers are ideally suited for integration into laser measuring systems.

Claims (3)

1. Solid-state laser, the Nd: YAG crystal is pumped via a collimation and focusing optics ( 12 ) by a high-power laser diode ( 11 ) and the laser radiation generated is focused in an Er: glass rod ( 18 ), the at λ1 emitting Nd: YAG crystal ( 15 ) and the Er: glass rod ( 18 ) emitting at λ2 are coupled to one another within the coupling-in and coupling-out mirrors ( 27 and 28 ) of a first resonator such that the Er: emitting in the eye-safe area λ2: Glass rod ( 18 ) has an absorption at the wavelength λ1 by ytterbium codotation, so that the Nd: YAG crystal ( 15 ) emitting at λ1 can barely oscillate and the Er: glass rod ( 18 ) is in a second resonator, which is formed by the one end face ( 18 a) of the ER: glass rod ( 18 ) and a corresponding coating ( 28 ) on the other side of the Er: glass rod ( 18 ), the Solid state laser ( 15, 18 ) are firmly connected to one another, and the coating ( 28 ) on the other side of the Er: glass rod ( 18 ) acts as a decoupling mirror. 2. Solid-state laser, the Nd: YAG crystal ( 15 ) is pumped via a collimation and focusing optics ( 12 ) by a high-power laser diode ( 11 ) and the laser radiation generated is focused into an ER: glass rod ( 18 ), wherein the Nd: YAG crystal ( 15 ) emitting at λ1 and the Er: glass rod ( 18 ) emitting at λ2 within the coupling-in and coupling-out mirrors ( 27, 28; 22, 26 ) of a first resonator are coupled to one another such that the Er: glass rod ( 18 ) emitting in the eye-safe area λ2 has an absorption at the wavelength λ1 due to ytterbium codoping, so that the Nd: YAG crystal ( 15 ) emitting at λ1 can barely oscillate and the one emitting in the eye-safe area λ2 Er: glass rod ( 18 ) is located in a second resonator, which is formed by the end face ( 18 a; 25 ) of the Er: glass rod ( 18 ) and a second mirror ( 30; 23 ), which is outside the first Resonators is arranged. 3. Solid-state laser according to claim 1, characterized in that one side ( 27 ) of the Nd: YAG crystal emitting at λ1 ( 15 ) is antireflective for the pump light wavelength and highly reflective for λ1 and the other side ( 29 ) of the Nd: YAG -Crystal ( 15 ) is coated anti-reflective for λ1, while one end face ( 18 a) of the Er: glass rod ( 18 ) is anti-reflective for the wavelength λ1 (not eye-safe) and highly reflective for the wavelength λ2 and the other side is partially reflective for the Wavelength λ2 is coated.