WO2004025790A2

WO2004025790A2 - Method for producing and operating a solid polymer laser medium and arrangement for the implementation thereof in a light amplification structure

Info

Publication number: WO2004025790A2
Application number: PCT/EP2003/010057
Authority: WO
Inventors: Klaus Lips; Vladimir Kytin; Irina Kytina
Original assignee: Hahn-Meitner-Institut Berlin Gmbh
Priority date: 2002-09-10
Filing date: 2003-09-10
Publication date: 2004-03-25
Also published as: DE10243845A1; DE10243845B4; WO2004025790A3; AU2003266374A1

Abstract

The invention relates to a method for producing and operating a solid polymer laser medium and an arrangement for the implementation thereof in a light amplification structure, especially by means of a high-power dye laser with a high rate of repetition. The aim of the invention is to provide a method for producing and operating a solid polymer laser medium and an arrangement for the implementation thereof in a light amplification structure which can be used in an adjustable dye laser which is pumped by an external pulse laser, whereby the medium receiving the dye is designed to guarantee high transitional efficiency and stability in terms of the laser output while at the same time ensuring a good quality laser jet, and improved long-term stability in comparison with prior art can be obtained for the solid polymer laser medium provided with dye. To achieve this, a polymer (2) doped with dye and a transparent polymer (3) having an identical or almost identical refraction index are combined to form a discoid reinforcing medium (1) such that the transparent polymer (3) forms an outer ring and the dye-doped polymer (3) is introduced into the transparent polymer (3) in a cooled state and both bonded polymers (2,3) are placed in a special mould (13) where they are thermally treated such that optimum optical contact is established between the two polymers (2, 3) and such that, subsequently, the polymers (2, 3) which are combined to form a body are mechanically treated and polished.

Description

Process for producing and operating a solid polymer laser medium and arrangement for implementing it in a structure for light amplification

description

The invention describes a method for producing and operating a solid-polymer laser medium and an arrangement for its implementation in a structure for light amplification, in particular by means of a high-performance dye laser with a high repetition rate according to the preambles of claims 1, 4 and 7.

The principle of doping a polymer matrix with dye molecules was developed in the late 1970s.

Since then, several examples of solid-state polymer dye lasers have been developed, in most cases using an embodiment that operates in a longitudinal pump configuration (US 5,136,596).

This configuration enables an output laser beam of very good quality to be produced, since the surfaces which determine the quality can be polished very well. For this reason, diffraction losses (diffraction losses) within a laser resonator are very low. In the event that you work with a high pulse power with a low pulse repetition rate (repetition rate). (eg: with the help of an Nd: YAG laser in the low-frequency Q-switch mode (Q-switch)), a conversion efficiency of 50-70% can be achieved with a reasonable service life of the gain medium. On the other hand, the laser threshold is relatively high - around 100-200 kW / cm ² for commercially available plastics that are impregnated with dyes. For pump sources with high repetition rates, such as copper vapor lasers, the pump beam must therefore be focused on a surface with a relatively small diameter in order to achieve an adequate conversion efficiency. This leads to - a recognizable photodegradation of the laser dye, even after a single laser pulse, and in some cases even damage to the polymer matrix.

The operating time of the gain medium can be increased by periodically changing the position at which the excitation takes place, either by moving and / or rotating the gain medium with respect to the pump beam. In order to achieve this goal, the reinforcement medium was made into a thin strip (US Pat. No. 5,136,596) or into a disk (A.Costella et al: "High repitition rate solid State dye laser pumped by a copper vapor laser", Appl. Physics Letter, v .79, N.4, p. 52-454).

The longest operating time was 30 minutes.

Another procedure for increasing the operating time (stability) of the dye-doped polymer reinforcing medium can be based on the “threshold-like” dependency of the photodegradation rate of the dye as a function of the pump power density. (LK Denisov er al. “Service life of dye-impregnated polymer active elements at various energy densities and pump powers ", Quantum Electronics 1997, Vol. 27 (2) p.115-117).

The photodegradation rate decreases significantly when the pump power density falls below a certain value. However, this value is too low to achieve adequate output efficiency in the longitudinal pump configuration, but is high enough for effective lasing in the transverse pump configuration. The main disadvantage of the transverse pump configuration is the poor quality of the output beam and consequently leads to large optical losses within the resonator.

The quality of the output beam can be significantly improved by using a reinforcing medium consisting of a "sandwich" structure with a dye-doped layer and a transparent layer of comparable polymer material, both layers having to have approximately the same refractive index (RU 21 05 401) ,

Such a reinforcement medium was realized by polymerizing the transparent layer, followed by polymerizing on the dye-doped layer. However, it is difficult to implement this method if the layers are not flat.

The object of the invention is to provide a method for producing and operating a solid polymer laser medium and an arrangement for implementing it in a structure for light amplification from a suitable solid polymer for use in a tunable dye laser pumped by an external pulse laser is to be described, wherein the medium which absorbs the dye is intended to ensure both a high conversion efficiency and the stability of the laser power with a simultaneously good quality of the laser beam, and an improved long-term stability of the solid-state polymer provided with a dye compared to the known prior art -Laser medium can be reached.

This object is achieved according to the invention by the method having the features of claims 1 and 4 and by the arrangement having the features of claim 7. The process for the production is characterized in that a dye-doped polymer and a transparent polymer with the same or almost the same refractive index are combined to form a disk-shaped reinforcing medium in such a way that the transparent polymer forms an outer ring and the dye-doped polymer forms an inner surface. by cooling the dye-doped polymer into the transparent polymer and bringing both interconnected polymers into a special shape and heat-treating them in such a way that an optimal optical contact is established between the two polymers, and then the polymers assembled into a body in a known manner Way, for example mechanically processed and / or polished by turning.

The method for operating an amplification medium according to the invention is characterized in that focused radiation from a pump laser with a repetition rate v is guided onto the amplification medium rotating at a defined frequency f, as a result of which an excitation area of a length on the gain medium that corresponds to the thickness 1 of the Gain medium corresponds, and a defined width w, which determines the average pump power density, is generated, the frequency of the rotation of the gain medium being selected according to the equation f = vw / πd, in which d is the diameter of the input

Represents surface of the gain medium.

The arrangement for implementation in a structure for light amplification is characterized in that an amplification medium is coupled to a drive, preferably a motor, and can be acted upon or acted upon by the pump laser and an optical system with focused radiation, which is directed onto its rotating end face. wherein a device is connected to the focused radiation which periodically interrupts the radiation in a defined cycle. The light intensifying medium according to the invention allows the medium to be pumped transversely, which is associated with high conversion efficiency and at the same time good quality of the laser beam. As a result, it can be pumped with a significantly lower excitation power than previously, so that a clear improvement in the photodegradation of the dye used is achieved.

By choosing the operating mode according to the invention, a significantly higher long-term stability of the dye used is achieved.

All conventional laser dyes and plastics such as PMMA can be used as a medium.

Compared to known methods, it is possible to work with significantly higher repetition rates than with pump lasers available on the market.

The amplification medium can be pumped with high-power lasers with high repetition rates (e.g. copper vapor lasers). The gain medium is capable of producing laser radiation with a high conversion efficiency and with an adequate quality of the output beam.

The proposed gain medium has the longest operating time (stability) that is known to date when it is used in the operating mode proposed here.

Advantageous embodiments of the invention are described in the subclaims.

The invention is explained in more detail below in an exemplary embodiment for producing a light amplifier medium. In the accompanying drawing: 1: the schematic representation of a medium according to the invention in plan view,

2: the schematic representation of a medium according to the invention in side view,

3: the schematic representation of the essential manufacturing steps for the medium according to the Fig. 1.2

4: the schematic representation of the device configuration for light amplification by means of the medium according to FIGS. 1 and 2 in plan view,

5: the schematic representation of a part of the device configuration for light amplification by means of the medium according to FIGS. 1 and 2 in a sectional view,

Fig. 6: the schematic representation of a light chopper (chopper) in plan view to realize a defined waiting time and

Fig. 7: the graphical representation of the

Dependence of the output power of the dye laser on the operating time.

A new type of solid polymer laser medium for dye lasers with a transverse pump configuration and its manufacture is described. The proposed solid polymer laser medium 1 as shown in FIGS. 1, 2 is disc-shaped with a thickness 1. It consists of a dye-doped polymer 2 and a transparent polymer 3. Both polymers 2, 3 have an identical or almost identical refractive index. The upper and lower surfaces a, b are polished with optical quality. The area c requires a lower quality level when polishing.

The thickness k (FIG. 2) of the transparent polymer 3 must be sufficiently large to avoid the generation of interfering effects (parasitic modes) which are caused by reflections from the surface c. The thickness k is, for example, 2 mm.

The reinforcing medium 1 has a slightly conical shape, determined by the angle α, in order to prevent the generation of interference effects by the reflection from the surfaces a, b. In the case of a medium 1 designed as an example, this angle was 5 °. The smallest diameter of the dye-doped material of the reinforcing medium 1 is denoted by d.

The reinforcement medium 1 has a hole 11 in the middle. This hole 11 is required for fastening the reinforcing medium 1 to a drive 10, which is preferably formed by a motor.

3 shows a schematic representation of the most important phases in the production of the proposed solid-polymer laser medium 1.

First, the correct shape of the two polymer layers 2, 3, the dye-doped layer 2 and the transparent layer 3 is achieved by conventional mechanical or thermomechanical treatments or directly by polymerization in accordance with the corresponding requirements.

Polishing is not part of this treatment. The dye-doped polymer 2 is cooled (for example in liquid nitrogen 14) and then immediately introduced into the transparent polymer 3. The size of the two polymers 2, 3 is chosen such that only a partial introduction is possible if both parts have the same temperature, and a complete introduction is only possible after the polymer 2 has cooled.

After establishing a thermal equilibrium, the mechanically connected polymer parts 2, 3 are introduced into a special mold 13 and heated up to a certain temperature. This temperature must be high enough to bring about a slightly flexible state of the polymers 2, 3, but low enough to prevent chemical transformation of the polymers 2, 3 and also of the dye. This temperature is, for example, approximately 130-150 ° C. for copolymer MMA + 10% MMA + 10% MAH ¹ , impregnated with rhodamine B or rhodamine 6G dyes.

The shape 13 is adapted to the size of the parts 2, 3 in order to prevent deformation during the heat treatment. The inner dimensions of the form 13 correspond to the outer dimensions of the assembled polymer parts 2, 3 in the cooled state of the dye-doped polymer 2. This treatment results in high quality optical contact between the dye-doped polymer 2 and the transparent polymer 3 ,

The final mechanical treatment, for example machining by means of a lathe and polishing of the surfaces a, b and c, leads to the reinforcement medium 1 according to FIGS. 1, 2. If the outer ring 3 made of the transparent polymer were not present in a transverse pump mode, the main laser activity that occurs on the surface of the dye-doped polymer 2 would lead to a greatly reduced conversion frequency due to scattering on the free surface. However, in order to be able to guarantee laser operation with a high conversion frequency, the surface of the dye-doped polymer 2 would have to have a very high optical quality. In order to be able to achieve such an optical quality, a very complex polishing process would be necessary, which would be associated with a considerable cost disadvantage. A further disadvantage would be the occurrence of parasitic modes due to strong reflections on the surface. According to the invention, the need for a very high optical quality of the dye-doped polymer 2 is avoided by the outer ring of a transparent polymer 3. A prerequisite for this, however, is an optical contact of high quality between the polymers 2 and 3, which can be achieved with the method according to the invention.

The proposed gain medium 1 is designed for operation in a dye laser.

4, 5 show this operation in a schematic representation.

The gain medium 1 is brought into rotary motion by a rotary drive 10. An optical system 12 focuses the radiation from a pulsed pump laser 4 via a lens 15 at a repetition rate v onto the amplification medium 1, an excitation region 5 being formed (FIG. 5). This area 5 has a length which corresponds to the thickness 1 of the reinforcing medium 1 according to FIGS. 1, 2.

The width w of the excited area 5 determines the average pump power density. For this reason, the width w should be small enough to achieve adequate conversion efficiency and large enough to minimize the rate of photodegradation of the dye. The frequency of rotation f of the gain medium 1 must be selected so that each new ^• light pulse of the pump laser 4 illuminates a former non-excited region. The equation applies:

V w π d

A rear mirror 6 and an output mirror 7 form a resonator of the dye laser together with a prism 8, which is required for the selection of the wavelength. The quality factor of the resonator should be optimized with a view to minimizing the photo degradation rate but at the same time with the highest possible conversion efficiency. If the quality factor is too high, the conversion efficiency is reduced.

It has been found that the operating time of the amplification medium 1 can be increased further if periodic pauses in the illumination of the amplification medium 1 are taken with an illumination cycle of approximately 1 time unit of darkness at one point to 6 time units of the illumination. This allows the dye to recover from possible degradation during breaks. For Rhodamine B dye, the best mode of operation is 1 minute pumping with Laser 4 (lighting), followed by a 10 second pause.

Such an operating mode can be implemented by a light chopper 9 with an operating area 15 according to FIG. 6, synchronized with the rotary drive 10, in order to have a specific duration of the lighting and a specific cycle of pauses for each individual point of the lighting of the amplification medium 1 by the Ensure pump laser 4.

The desired dark time t _pause can be set via the rotation frequency f _{ch of} the light chopper 9 in FIG. 6 using the following equation (1 ^ arc length of the Choppers on the radius R _chl , R _Ch - the smallest radius in the operating range of the chopper):

ch pause f eh f 2 π R ch

The use of a light chopper 9 is provided according to the invention in order to further increase the operating time of the gain medium 1 by inserting periodic pauses in the illumination of the gain medium 1. The periodicity of the pauses does not relate to the periodicity of the pump laser 4. It has been shown that the degradation of the gain medium 1 can be prevented to a particularly advantageous extent by the fact that after a certain period of time a certain point of the gain medium 1 is periodically stressed there is a pause. This pause, which in turn should also occur periodically, is realized, for example, by the light chopper 9, which in turn prevents the periodic action of the radiation from the pump laser 4 on the gain medium 1 at intervals.

The amplification medium 1 tested as an example was made from a copolymer MMA + 10% MAA + 10% MΑH, impregnated with Rhodamine B. The concentration of Rhodamine B was 8-10 ^"4 mol / 1. For the transparent polymer 3, commercially available PMMA was used.

The medium 1 tested had the shape shown in FIGS. 1, 2.

The thickness 1 of the medium 1 was 20 mm; the diameter d was 94 mm. The thickness k of the transparent layer 3 was 2 mm. The cone angle was 5 °. The surfaces a, b of the medium 1 were polished with optical quality, the surface c being polished only to such an extent that this surface became transparent to the pump radiation. The medium 1 was tested according to the configuration in FIGS. 4, 5. A copper vapor laser (repetition rate v = 16 kHz, laser pulse length 20 ns, average output power 10 W, wavelengths 510.6 nm and 578.2 nm) served as pump source 4. The reflection of the output mirror 7 was 60%. The width w of the excited area 5 was 0.8 mm.

The radiation from the dye laser with the tested and introduced amplification medium 1 was tunable in the range 605-635 nm with a spectral bandwidth of 2.5 nm. The output beam diameter was 1.2 mm and the beam divergence was 6 mrad. The time dependencies of the output power are shown in FIG. 7 for the continuous operating mode and for the periodic operating mode (1 minute operation + 10 seconds pause per illuminated point of the medium 1). For the measurement process, the dye laser was tuned to an output wavelength of 630 nm. In continuous mode (without light chopper), the operating time of the gain medium 1 tested was approximately 2 hours, the operating time being defined as the time when the output power of the dye laser decreases to 70% of its original value. In the periodic operating mode (with light chopper), the gain medium remained stable (within an accuracy of 5%) during the 4-hour operating time. At this point, it should be noted that the duration of the breaks has been deducted from the total operating time.

It is also pointed out here that the time dependence of the output power in the periodic operating mode (FIG. 7) was measured for the same medium three days after the measurement in the continuous operating mode. Accordingly, the measured output power was already somewhat degraded in the periodic operating mode, although a small partial recovery has taken place.

With the extrapolation of the time dependence of the output power in the periodic operating mode, the operating time is 12-14 hours. 1, 2 consists of two plane-parallel planes (surfaces a, b) of a circular disk of thickness 1.

An embodiment of the reinforcing medium 1 is also possible, in which the circular disk is formed from two planes (surfaces a, b) arranged at a defined angle to one another. In this embodiment, the angle at which the laser light of the laser 4 emerges (Brewster angle) is to be determined.

LIST OF REFERENCE NUMBERS

1 gain medium

2 dye-doped polymer

3 Transparent polymer

4 pump lasers

5 Suggestion area

6 mirrors

7 mirrors

8 prism

9 chopper (shopper)

10 engine

11 holes

12 Optical system

13 form

14 coolant

15 operating area

a area b area c area d smallest diameter of the dye-doped

Material of the reinforcement medium for rotation

1 thickness of the dye-doped

Material k Thickness of the undoped material

R _ch the smallest radius of the

Operating range of the chopper l _cll arc length of the chopper on the radius R _Ch v repetition rate w width

α angle

Claims

claims

1. A method for producing a solid polymer laser medium for a high-performance solid-state dye laser with a high repetition rate, in which a dye-doped polymer (2) and a transparent polymer (3) with the same refractive index or almost the same refractive index to a disk-shaped Reinforcing medium (1) are put together in such a way that the transparent polymer (3) forms an outer ring and the dye-doped polymer (2) forms an inner surface by cooling the dye-doped polymer (2) and inserting it into the transparent polymer (3) the two polymers (2, 3) connected to one another are brought into a special shape (13) and heat-treated there in such a way that an optimal optical contact is established between the two polymers (2, 3), and then the polymers (2 , 3) machined and / or polished.

2. The method according to claim 1, characterized in that the reinforcing medium (1) is slightly conical, determined by the cone angle, α, in order to avoid the generation of interference effects by the reflection on surfaces (a, b).

3. The method according to claim 1, characterized in that the thickness of the transparent polymer (3) is chosen so large that interference effects due to reflections on a surface (c) are avoided.

4. A method for operating a gain medium produced according to at least one of claims 1 to 3 in a transversal pump mode, characterized in that focused radiation from a pump laser (4) with a repetition rate v results on the gain medium (1) rotating at a defined frequency f becomes, whereby an excitation area (5) of a length corresponding to the thickness 1 of the gain medium (1) and a defined width w, which determines the average pump power density, is generated on the gain medium (1), the frequency f of the rotation of the gain medium ( 1) is chosen according to the equation f = vw / πd, in which d represents the diameter of the input surface of the reinforcing medium (1).

5. The method according to claim 4, characterized in that the illumination of the gain medium (1) with the radiation from the pump laser (4) with the repetition rate v is additionally periodically suppressed in a defined ratio, the ratio between the illumination of the gain medium (1) and Illumination pauses are selected depending on the dye contained in the amplification medium (1).

6. The method according to claim 5, characterized in that the cycle and the duration of the lighting breaks is realized by a device such as a chopper (9), the working mode of which is synchronized with a drive (10) with which the rotary movement of the gain medium (1) is realized.

7. Arrangement for implementing a solid polymer reinforcing medium (1) produced according to at least one of claims 1 to 3 in a structure for light amplification with a pump laser (4), an optical system (12), a resonator made of mirrors (6, 7 ) and a prism (8), characterized in that the amplification medium (1) is coupled to a drive (10) and is connected via the pump laser (4) and the optical system (12) to a focused radiation which is rotating on it Is directed, with the focused radiation is connected to a device (9) which interrupts the radiation periodically in a defined cycle.

8. Arrangement according to claim 7, characterized in that the reinforcing medium (1) is formed from a circular disc from plane-parallel planes (a, b).

9. Arrangement according to claim 7 or 8, characterized in that the reinforcing medium (1) is formed from a circular disc from planes (a, b) arranged at a defined angle to one another.

10. Arrangement according to one of claims 7 to 9, characterized in that the angle at which the laser light from the pump laser (4) emerges (Brewster angle) is selected as a function of the geometry of the gain medium (1).

11. Solid-polymer laser medium (1) for a high-performance solid-state dye laser with a high repetition rate, producible by:

Joining a dye-doped polymer (2) and a transparent polymer (3) with an almost identical refractive index to form a disk-shaped reinforcing medium

(1) such that the transparent polymer (3) forms an outer ring and the dye-doped polymer (2) forms an inner surface by the dye-doped polymer

(2) cooled and inserted into the transparent polymer (3) and the two polymers (2, 3) joined together are brought into a special shape (13) and heat-treated there in such a way that optimal optical contact is established between the two polymers (2, 3) and that the polymers (2, 3) assembled into a body are then mechanically processed and polished.