DE10327260A1 - Beam-folding optical amplification equipment for solid state laser, includes component with selective angular sensitivity - Google Patents

Abstract

At least one of the beam-folding components (2, 2') is constructed for angular sensitivity, i.e. its reflectivity for a given angle of incidence exceeds that for predetermined undesired angles of incidence.

Description

The The invention relates to an optical amplifier arrangement according to the preamble of claim 1.

Technical Fields of application of the invention are in particular diode-pumped Solid-state lasers, both oscillator and amplifier arrangements, as well as diode lasers.

State of technology

The market for high-power lasers for material processing is dominated by CO ₂ laser systems. In order for diode-pumped solid-state lasers to become competitive, various concepts for optimizing the amplifier arrangement have been proposed. However, measures for the necessary increase in the output power are usually associated with a deterioration of the beam quality of the laser beam generated.

In the US 5,271,031 describes a mode-matched, diode-pumped solid-state laser comprising a block of laser material with two opposing side surfaces and a zigzag optical optical path narrowly folded at a predetermined angle. In this case, a plurality of pump sources are present in a linear arrangement near at least one of the opposite side surfaces.

The EP 1 181 754 describes an optical amplifier arrangement with an approximately rectangular cross-section having gain medium, which is arranged between two highly reflective mirrors. These are designed and arranged such that the beam to be amplified passes through the amplification medium several times and its dimension increases after each passage in the direction defined by the long edge of the cross section of the amplification medium. By means of this arrangement, which can also be referred to as stable-unstable hybrid resonator, a high scaling of the output power is possible. However, in order to be efficient, this arrangement requires a relatively high power (several watts) input beam to achieve saturation intensity. If the arrangement is designed as a resonator, the beam quality is only limitedly good, and that is due to secondary maxima occurring in the focus, due to the one-way unstable character of the hybrid resonator.

presentation the invention

Of the Invention is based on the object, an optical amplifier arrangement to provide a high output power, the highest possible Exploitation of the reinforcing Medium and at the same time a high beam quality of the amplified beam is achieved.

The solution This technical problem is solved by the features of the independent claim 1. Advantageous embodiments and developments are by the dependent ones Claims specified.

According to the invention was recognized that the above technical problem is solved by an optical amplifier arrangement, especially for one Solid State or diode lasers in which a reinforcing medium is between at least two folding elements is arranged, and at the beam path a laser beam is folded several times by the folding elements, and at least one folding element in such an angle-selective manner is formed, that its reflectance for predetermined angles of incidence of radiation is much larger as for predetermined unwanted Incidence.

Of the The invention is based on the finding that by repeatedly Fold the laser beam path through the amplifying medium a high gain achieve and reinforcing Can use medium efficiently. To make a laser beam higher beam quality and to maintain performance, it is important to ensure that, despite repeated Wrinkles the appearance of increased spontaneous emissions (ASE) and parasitic Oscillation is prevented. This can be achieved by that one folding element or more angle selective as described above formed.

by virtue of the broad usability for the different laser types can the laser based on the invention in almost all areas used in materials handling lasers, life sciences and telecommunications are used.

The Invention will now be described with reference to embodiments and accompanying drawings explained in more detail. It demonstrate:

1 A first embodiment of the device according to the invention.

2 A second embodiment of the device according to the invention.

3 An embodiment of the invention as a resonator.

4 An embodiment of the invention as an oscillator-amplifier arrangement.

5 An embodiment of the invention as a ring resonator.

6 Addition of several placed in the laser beam path optical elements, in particular for frequency conversion, polarization and Q-switching.

7 Design of the active medium as a semiconductor medium.

8th Optimized arrangement of the amplifying areas of the semiconductor medium.

In 1 an exemplary embodiment of the device according to the invention is shown. A gain medium 1 is between two folding elements 2 . 2 ' arranged. reference numeral 3 denotes the beam path of the laser beam to be amplified. The laser beam path 3 becomes of the two folding elements 2 . 2 ' folded several times. In this exemplary embodiment, the laser beam strikes the lead element 2 respectively. 2 ' each at an angle of about 10 ° relative to its surface normal. The folding element 2 respectively. 2 ' is designed so that it has the highest possible reflectivity at this desired angle. For smaller angles near 0 °, on the other hand, the reflectivity is as low as possible. Otherwise, in this exemplary arrangement of the folding elements, undesired angles of 0 ° or angles near 0 ° could cause spontaneous emissions by multiple folding by means of the folding elements and multiple passes through the amplifying medium 1 significantly amplify and inversion of the reinforcing medium 1 dismantle. This inversion would then no longer be available for amplifying the laser beam. It is essential, therefore, that overall the reflectance for the desired incidence angle of about 10 ° is substantially greater than for the undesired angle of incidence of 0 °. In this way, the formation of ASE and parasitic lasers is suppressed and it is possible to achieve a high radiation power and good beam quality of the usable output signal.

In the 1 illustrated arrangement of the folding elements 2 . 2 ' and the beam path 3 , in particular the angle of incidence of the radiation with respect to the surface normal of the folding element 2 respectively. 2 ' , is only one possible embodiment of the inventive idea. Alternative arrangements are also conceivable.

So is in 2 For example, shown an arrangement in which the amplifying medium 1 between the folding elements 2 . 2 ' is arranged and in the two other folding elements 2 '' and 2 ''' available. In this arrangement, the folding elements 2 . 2 ' . 2 '' . 2 ''' and the laser beam path 3 are particularly favored for the formation of ASE and parasitic oscillations such spontaneous emissions, which at an angle of approximately 45 ° to the folding elements 2 . 2 ' to meet. One of the folding elements 2 . 2 ' According to the invention, this arrangement thus means to design its reflectivity such that it is particularly low at an angle of incidence of radiation of 45 ° and in a small range of 45 ° and substantially smaller than desired angles outside this range. Instead of one of the folding elements 2 . 2 ' or in addition to one of the folding elements 2 . 2 ' but it can also be one of the folding elements 2 '' . 2 ''' be executed angle selective.

The number of folds of the laser beam path 3 and passes through the amplifying medium 1 is in 1 and 2 chosen comparatively low for reasons of clarity. In order to achieve a high amplification factor, it is advantageous to provide a higher number. With the aid of the folding elements designed according to the invention, a particularly narrow folding of the laser beam path can be realized. However, the laser beam path can not be folded as narrow as desired, since from a certain tightness of the convolution, the folding elements designed according to the invention can no longer prevent the occurrence of ASE and parasitic oscillations.

So that the formation of ASE and parasitic oscillations is particularly strongly suppressed and a particularly strong increase in the beam quality and power of the usable laser beam is achieved, it is advantageous if at least one folding element 2 . 2 ' . 2 '' . 2 ''' is formed angle-selective so that its reflectance for predetermined angles of incidence of radiation by at least a factor of 5 is greater than for predetermined undesirable angle of incidence. In an even more advantageous embodiment of the invention, this factor exceeds at least the value 10.

A particularly simple embodiment of the invention is provided if at least one angle-selective folding element 2 . 2 ' . 2 '' . 2 ''' is formed as a dielectric mirror. Alternative options include at least one angle-selective folding element ment 2 . 2 ' . 2 '' . 2 ''' form as a lattice or as a photonic crystal structure. These alternatives are characterized by the fact that their angle-dependent reflectivity can be particularly sharp-edged and thus a particularly pronounced angle-selective effect can be achieved.

The device according to the invention can be as in 1 represented configured as a pure amplifier arrangement. In this case, an input beam is supplied to the amplifier arrangement and amplified there. The laser beam passes through the multiply folded laser beam just once 3 and leaves the arrangement as a greatly amplified output beam. The input beam is typically delivered by an oscillator not further described or illustrated herein and is a high beam quality, relatively low power laser beam.

As in 3 however, the device according to the invention may itself also be designed as an oscillator. Due to the high achievable with the help of the invention beam quality, this application is considered to be particularly advantageous. For this, the amplifier arrangement according to the invention is two resonator mirrors 4 . 5 complements such that a resonator is formed. The resonator mirrors 4 . 5 are arranged so that the desired multiply folded and the gain medium 1 multiple crossing laser beam path 3 can train between them. One of the resonator mirrors 5 is a highly reflective mirror, in the other resonator 4 it is a partially transmissive Auskoppelspiegel. The radiation transmitted by this mirror is a laser beam of high beam quality and is particularly suitable as an input beam or oscillator beam for a pure amplifier arrangement in which this beam is highly amplified. By suitable choice of the arrangement and dimensions of active medium and folding elements 2 . 2 ' It is possible to suppress all modes except the desired TEM00 mode. In particular, an efficient utilization of the gain range is possible, as is known from unstable resonators, but in which one or more secondary maxima form in the focus.

In a further embodiment, which in 4 is shown, the device according to the invention is designed as a two-stage amplifier arrangement. In a first region of the arrangement, in turn, two additional resonator mirrors 4 . 5 is present, and it is formed as described above, a resonator, wherein in turn one of the resonator mirror 5 a highly reflective mirror, the other resonator mirror 4 a partially transmissive Auskoppelspiegel is. In the resonator, an oscillator beam is generated by the output mirror 4 leaves the resonator. In the second region of the arrangement, this oscillator beam is highly amplified. This particular arrangement may also be referred to as an oscillator-amplifier arrangement. The oscillator and amplifier sections can use the same amplifying medium 1 or use different amplifying media from each other. It is also possible to share the same folding elements 2 . 2 ' , The advantage of the oscillator-amplifier arrangement lies in its particularly compact, space-saving design.

A further advantageous embodiment of the above, with two additional resonator mirrors 4 . 5 As a resonator, or laser oscillator designed embodiment of the invention is given by the fact that it forms the resonator as a ring resonator. Such is in 5 shown. The resonator mirrors are 4 . 5 arranged so that laser radiation, which runs from one resonator mirror starting to the other resonator, is not reflected back from there to the same way, but comes back via a different path to the starting point. The ring resonator is operated unidirectionally, that is, laser radiation runs along the cyclic laser beam path 3 only in one orientation direction. It is possible, one of the two between the two resonator mirrors 4 . 5 located sub-cycles of the laser beam path 4 outside the reinforcing medium 1 to lead along. An optical diode 10 , such as a Faraday isolator, can be placed in the laser beam path to provide unidirectional ring resonator operation. With the invention designed as a ring resonator invention, a particularly narrow-band laser beam can be generated.

The device according to the invention can be provided both for a pulsed operation as well as for a continuous wave operation. For applications using a pulsed laser beam, the arrangement may be to increase the pulse intensities and / or to shorten the pulses as in 6 represented by a Q-switch 6 be supplemented. This is arranged so that the laser beam path 3 through it, and can also be designed and dimensioned so that this happens several times. Since the amplifier arrangement according to the invention provides a laser beam of particularly high beam quality, the use of a Q-switch is also possible 6 possible with a particularly small aperture. As a Q-switch a saturable absorber or an electro-optical, acousto-optic or magneto-optical switch can be used.

To generate laser radiation of a different wavelength, a frequency-converting element may be used 7 in the laser beam path of the arrangement to be placed.

Furthermore, a mode aperture 8th be arranged in the laser beam path, are suppressed with the transverse modes of the laser radiation and thus the laser beam quality can be further increased.

If the laser radiation is to have a certain polarization, a polarizing element can also be used 9 be placed in the laser beam path of the arrangement.

The above-mentioned elements for Q-switching, frequency conversion and polarization can not only be used as in 6 represented, but be placed virtually anywhere in the beam path.

For the device according to the invention, it is sufficient if two folding elements 2 . 2 ' available. However, it may offer advantages to provide even more folding elements. These can also be suitably designed angle-selective, for example, so that overall an even more pronounced angle selection and thus suppression of parasitic modes is possible. Also, with further folding elements laser beam paths can be realized with which an even more efficient utilization of the reinforcing medium is achieved.

It is advantageous if the cross section of the gain medium 1 is made approximately rectangular and thus has two long and two short edges. It is also advantageous if the amplifying medium 1 so in the laser beam path 3 is arranged that this repeatedly passes through the long edges of the cross-section of the reinforcing medium.

It makes sense, the amplifying medium 1 , especially when using a solid state medium to pump optically, this is preferably done with diode laser radiation. Such diode pumping arrangements are characterized by high efficiency and beam quality, long life and small dimensions. It is possible, the amplifying medium 1 longitudinal or "end-on", that is to pump parallel to the laser beam to be amplified or from the side "transversal", that is perpendicular to the laser beam to be amplified. Preference is given to a longitudinal pump arrangement, since this requires less effort in beam shaping and focusing and enables a significantly higher scalability of the laser power. When using diode lasers as a pump source, the output power can be scaled almost arbitrarily, for example by arranging the diode lasers next to one another while at the same time enlarging the active medium in this direction. So that the pumping of the reinforcing medium 1 is particularly simple and allows a compact design, it is advantageous if a folding element 2 or 2 ' is designed to be transmissive to the light of the wavelength of the pump radiation and the irradiation of the pump radiation by this folding element 2 or 2 ' through. The pump radiation then hits the amplifying medium 1 irradiated from the side over one of the long edges of its approximately rectangular cross-section. It is also possible the irradiation of the pump light from two sides over both long edges, in which case two folding elements 2 . 2 ' are formed accordingly.

By providing a rectangular cross section for the reinforcing medium as described above and optically pumping it longitudinally from the side over one of the long edges, and cooling across the surfaces perpendicular to the folding plane, high beam quality can be achieved since thermal disturbances largely occur be avoided. These are generally caused by the heat loss occurring in the amplifier medium. Due to the rectangular shape and the existing cooling geometry, however, there is an approximately one-dimensional heat conduction, which means in the direction of the laser beam path 3 There are almost no thermal effects, such as birefringence, thermal lensing and Depolarisationsverlusten. This makes it possible by a close folding of the laser beam path 3 and high number of passes through the amplifying medium 1 Achieve a high gain without sacrificing beam quality significantly.

Preferably, the reinforcing medium is 1 to a solid state medium, but other media are conceivable. In general, solid state lasers are characterized by a comparatively high beam quality and a small size.

Alternatively, the amplifying medium 1 also be designed as a semiconductor medium. The advantages of such an active medium are well known, in particular special is a particularly simple excitation by an electric current by applying an external voltage possible. An arrangement of a plurality of emitting individual lasers with spaces next to one another on a base surface, as is the case with commercially available laser bars, is particularly suitable in connection with angle-selective folding elements. In general, such a reinforcing medium can be described as being reinforcing regions 11 and non-reinforcing areas 12 has, as in 7 outlined. It is particularly favorable if these areas are dimensioned and / or arranged in such a way that the area within the reinforcing medium 1 lying part of the laser beam path completely within reinforcing areas 11 runs. Preferably, the arrangement of the reinforcing regions is closely related to the beam directions of the individual sections of the laser beam path 3 correlates, as in 8th shown. In this way, a particularly high efficiency can be achieved, since there is an optimal overlap of reinforcing areas and laser beam.

The reflectivity of the folding elements 2 . 2 ' . 2 '' . 2 ''' For radiation incident at the desired folding angle, in most cases it should preferably approach the value R = 1 as closely as possible. But it can also offer advantages, the folding elements 2 . 2 ' . 2 '' . 2 ''' be designed so that they have a specifically lower reflectivity for this radiation. In particular, in the case of semiconductor lasers as an active medium, the facet of the individual emitters can thus be protected against excessive intensities. In such an embodiment, 2n beams are emitted at the folding elements, where n is the number of convolutions. The coherent coupling of the individual emitters makes it possible to realize a high beam quality with this, in contrast to the usually incoherently emitting regions of a diode laser bar.

1

gain medium

2, 2 ', 2' ', 2' ''

folding element

3

laser beam path

4

partially permeable Resonator mirror (output mirror)

5

resonator (HR mirror)

6

Q-switch

7

frequency-converting element

8th

mode aperture

9

polarizing element

10

optical Diode for unidirectional operation of the ring resonator

11

Reinforcing areas of the semiconductor medium

12

Non-reinforcing areas of the semiconductor medium

Claims (21)

Optical amplifier arrangement, in particular for a solid-state or diode laser, in which a amplifying medium ( 1 ) between at least two folding elements ( 2 . 2 ' ) is arranged, and wherein the beam path ( 3 ) of a laser beam several times from the folding elements ( 2 . 2 ' ) is folded, characterized in that at least one folding element ( 2 respectively. 2 ' ) is formed angle-selective such that its reflectance for predetermined angles of incidence of radiation is substantially greater than for predetermined undesirable angles of incidence. Optical amplifier arrangement according to claim 1, characterized in that at least one folding element ( 2 respectively. 2 ' ) is formed angle-selective such that its reflectance for predetermined angles of incidence of radiation by at least a factor of 5 is greater than for predetermined undesirable angles of incidence. Optical amplifier arrangement according to one of the preceding claims, characterized in that by means of an angle-selectively formed folding element ( 2 respectively. 2 ' ) Radiation that is not along a desired laser beam path ( 3 ), is suppressed. Optical amplifier arrangement according to one of the preceding claims, characterized in that at least one angle-selective folding element ( 2 respectively. 2 ' ) is formed as a dielectric mirror. Optical amplifier arrangement according to one of Claims 1 to 3, characterized in that at least one angle-selective folding element ( 2 respectively. 2 ' ) is designed as a grid. Optical amplifier arrangement according to one of Claims 1 to 3, characterized in that at least one angle-selective folding element ( 2 respectively. 2 ' ) is formed as a photonic crystal structure. Optical amplifier arrangement according to one of the preceding claims, characterized in that in addition at least two resonator mirrors ( 4 . 5 ) of which one of these mirrors is a coupling-out mirror which transmits part of the beam ( 4 ). Optical amplifier arrangement according to claim 7, characterized in that the amplifier arrangement with the resonator mirrors ( 4 . 5 ) is designed as a ring resonator. Optical amplifier arrangement according to one of claims 1 to 6, characterized in that in a first region of the arrangement at least two resonator mirrors ( 4 . 5 ) of which one of these mirrors is a coupling-out mirror which transmits part of the beam ( 4 ), and that the laser beam coupled out there is amplified in a second region of the arrangement. Optical amplifier arrangement according to one of the preceding claims, characterized in that at least one Q-switch ( 6 ) is arranged in the laser beam path. Optical amplifier arrangement according to one of the preceding claims, characterized gekenn records that a frequency-converting element ( 7 ) is arranged in the laser beam path. Optical amplifier arrangement according to one of the preceding claims, characterized in that at least one mode aperture ( 8th ) is arranged to limit the transverse modes in the laser beam path. Optical amplifier arrangement according to one of the preceding claims, characterized in that a polarizing element ( 9 ) is arranged in the laser beam path. Optical amplifier arrangement according to one of the preceding claims, characterized in that the amplifying medium ( 1 ) has an approximately rectangular cross-section with two long and two short edges. Optical amplifier arrangement according to the preceding claim, characterized in that the amplifying medium ( 1 ) so in the laser beam path ( 3 ) is arranged such that this multiple times the long edges of the cross-section of the reinforcing medium ( 1 ). Optical amplifier arrangement according to one of the preceding claims, characterized in that the amplifying medium ( 1 ) is pumped by means of an optical pump source. Optical amplifier arrangement according to the two preceding claims, characterized in that the pump radiation is the amplifying medium ( 1 ) irradiated from the side over at least one of the long edges of its approximately rectangular cross-section. Optical amplifier arrangement according to one of the two preceding claims, characterized in that the pump radiation before impinging on the amplifying medium ( 1 ) by at least one folding element ( 2 respectively. 2 ' ), which is designed to be transmissive for light of the wavelength of the pump radiation. Optical amplifier arrangement according to one of the preceding claims, characterized in that the amplifying medium ( 1 ) is a solid state medium. Optical amplifier arrangement according to one of Claims 1 to 18, characterized in that the amplifying medium ( 1 ) is a semiconductor medium and reinforcing regions ( 11 ) and non-reinforcing areas ( 12 ) having. Optical amplifier arrangement according to the preceding claim, characterized in that the regions ( 11 . 12 ) are dimensioned and / or arranged such that the inside of the reinforcing medium ( 1 ) lying part of the laser beam path ( 3 ) completely within reinforcing areas ( 11 ) runs.