DE10107510A1 - Coupling seed laser pulses into light amplifier, involves dividing stimulation period into sections of duration greater than specific recovery time, less than twice specific recovery time - Google Patents

Abstract

The method involves dividing a laser medium (4) stimulation period into time sections whose duration is greater than a specific recovery time and less than twice the specific recovery time of the laser medium of a laser amplifier. At least two seed pulses are coupled into the light amplifier in a stimulation period so that no more than one seed laser pulse is coupled in during a time section.

Description

The invention relates to a method for coupling of seed laser pulses in light amplifiers, which ins special can be used when using resonator mirrorless laser arrangements for material processing.

Homogeneous ultra-short pulses are used for damage-free material processing with laser radiation. In order to generate powerful ultra-short pulse laser radiation, the low-energy pulses generated in special laser systems are introduced as seed pulses in laser amplifier arrangements. Such arrangements are characterized in that they lack the resonator mirrors typical of laser arrangements. You cannot emit laser radiation alone. The energy stored in the laser medium (excited laser medium) is only retained for a time and is only converted to a very small part into photon energy (ASE). The excitation period t _a represents a typical constant for the laser medium. FIG. 1 shows a typical energy-time diagram of an activated laser medium.

If one sends photons of a certain energy in such generate an excited laser medium Photons are other photons with identical properties companies. The photon current (seed pulse) is amplified. The properties of the seed beam remain receive. Such a property can be an example be a very short (ultra-short) pulse.

The energy yield from the laser medium is such Orders but limited.

Is the (constant) excitation time of the laser medium much larger than the pulse length of the seed beam, so the amplified impulse can at least one more times through the laser medium, resulting in a leads to further reinforcement. The beam return is z. B. realized via mirror.

It should be noted that the volume of the laser medium, when it was visited by a seed pulse, takes a certain time to achieve energy balance with its excited environment. Below this time, the recovery time t _e , an effective amplification of the laser beam in this volume is not possible.

In the ultrashort UV radiation range, such Ver are more known on the basis of excimer laser systems. In order to achieve the highest possible amplification of the seed pulses, up to three seed beam passes are implemented through the laser tube. This corresponds to three amplifier stages. The description of such a method and the required device is given by S. Szatmári et al. in Appl. Phys. B, Lasers and Optics, 63, (1996) 463-466. A laser beam is obtained which is stronger than the seed beam. The maximum energy of the laser beam depends on the saturation energy density of the laser medium and on the effective beam cross section. Off-axis beam guidance through the laser medium enables the spatial separation of the individual beams which follow one another in time (cf. FIG. 3).

However, all previous methods have considerable disadvantages. In particular, only one amplified output beam has so far been generated within the excitation period t _a . Of course, this extends the operating times for material processing. Another disadvantage that arises from this procedure is that a large part of the energy of the excited laser medium is not used.

The object of the present invention is to increase the Effectiveness of the seeded amplifier lasers at Application, shortening processing times as well the increase in energy yield in seeded Amplifier lasers based on volume lasers (e.g. gas lasers).

This object is achieved by the Features in the characterizing part of claim 1 in Interaction with the features in the generic term. Advantageous embodiments of the invention are in the Subclaims included.

A particular advantage of the invention is that in the method for the optimized coupling of ultrashort pulse seed laser pulses into a light amplifier comprising a laser medium 4 , the laser medium 4 being characterized by an excitation time t _a and a specific recovery time t _e and the excitation time t _{a is divided} into a number of time periods t _v in such _a way that the following applies to each time period:

t _e ≦ t _v <2.t _e ,

Within the excitation period t _a, at least two seed laser pulses are coupled into the light amplifier in such a way that at most one seed laser pulse is coupled in during a time period t _v , and amplifiers with one-sided coupling of the seed laser pulses into the light, that is to say when coupled into the side a of the light amplifier alone or if it is coupled into the side b of the light amplifier alone, at least two seed laser pulses are coupled in directly successive periods, or if the seed laser pulses are coupled on both sides into the light amplifier, i.e. if a seed laser pulse is coupled into the light amplifier side the decoupling of the seed laser pulse preceding this seed laser pulse, the coupling takes place only after a time ≧ t _{e has} elapsed after the complete decoupling of the preceding seed laser pulse.

A characteristic feature of the invention is that the overall beam guidance is arranged from the point of view of an optimized time regime. A maximum number of beam passes is thus realized. This method - increasing the number of (energy-saturated) individual pulses within the excitation period t _a - allows the total energy yield from the laser medium to be increased. At the same time, several (saturated) spatially and temporally separate output pulses are generated. Each individual output pulse can then be fed to a special purpose. The limiting factor is - in addition to the excitation time t _a (see FIG. 1) - the recovery time t _{e of} the laser medium 4 . The mandatory consideration of the recovery time t _e when generating amplified output pulses makes it necessary to clock the coupling of the seed laser pulses according to the invention.

Another criterion is the necessary spatial Separation of the individual beams outside the laser medium.

The invention is based on at least partially shown in the figures Embodiments are explained in more detail.

Show it:

Fig. 1 characteristic energy-time diagram of a laser medium

Fig. 2 energy-time diagram of a laser medium assuming an excitation time of t _a = 25 ns and a recovery time of t _e = 3 ns

Fig. 3 beam guidance for triple amplification of a seed pulse in an off-axis arrangement

Fig. 4 division of the active phase of a laser medium in time sections (sectors) on the length of recovery time t _e (excitation duration: t _a = 25 ns, relaxation time: t _e = 3 ns)

Tables 1 to 7: Representation of different possibilities of the temporal and spatial distribution in the light amplifier of the coupled seed laser pulses (within the excitation period t _a )

Using the example of an excimer laser, various Variants of the method demonstrated.  

The recovery time t _{e of} the laser medium 4 (gas mixture) in the laser tube 5 is given as an example with t _e = 3 ns. The excitation period t _a is, for example, t _a = 25 ns. This results - with the addition of FIG. 1 - in a time diagram according to FIG. 2.

The time diagram for laser medium excitation can be divided into individual sectors on the time axis. The sector length corresponds to the (energy) recovery time t _{e of} the laser medium 4 ( FIG. 4). The individual sectors can each be assigned a seed laser pulse.

In the following the respective occupancy of the sectors for individual beam routing variants are explained and shown schematically in tables. An explanation of the tables is to be given on the basis of FIG. 3:
Line two (pass) shows how often a seed laser pulse has passed through the laser medium 4 . The example shown in FIG. 3 uses only one seed laser pulse, which, however, is introduced three times into the laser medium 4 , so the corresponding runs 1 to 3 are given in the table for seed laser pulse one.

Line three (steel number) indicates the number of the seed laser pulse coupled in the corresponding interval. The example shown in FIG. 3 uses only a seed laser pulse and an output pulse (see Table 1). The attached designations mean:
V: preamp pass,
H: main gain, final gain (mostly saturated).

Line four (laser side) contains the information into which side of the light amplifier the respective seed laser pulse is coupled. In the example shown in Fig. 3, the seed laser pulse one is injected in the first interval on the side a of the light amplifier, then the seed laser pulse is deflected by a mirror system 3 and after two unused intervals in the fourth interval in the side b of the Light amplifier coupled. The second pass through the laser medium 4 now takes place. The delay of two intervals between the new coupling is realized by the mirror system 3 . After passing through the laser medium 4 , the seed laser pulse hits a mirror system 3 again and is deflected there in order to be coupled into the light amplifier (on the amplifier side a) for the third time. In this case, however, only one interval (the fifth) is left unused.

Sector shortfall for individual seed laser pulse does not make sense, a time variation upwards is permitted (extension of Dead time). It can be arranged with certain beam guidance (beam length) may be necessary. The the following time structures then shift backwards by this amount (these shifts are not shown here). Such extension is but always less than 3 ns, otherwise the sector is considered Dead time declared.

The off-axis arrangement (mentioned above and shown in FIG. 3) with quasi widening of the laser volume (increase in the beam energy of the seed laser pulse) is to be retained. Such a time regime would theoretically correspond to eight possible beam runs. Since the starting edge of the excitation energy (sector 0) cannot be used sensibly in the laser medium and because the high-voltage switch in the laser has a jitter due to technological reasons, a maximum of seven runs (in the example mentioned) are realistic.

The spatial separation of the single-seed laser pulses is ensured by different off-axis arrangements of the seed laser pulses as they pass through the laser medium 4 . The distance from the laser output, at which the spatial separation of the individual beams has been completed, depends not only on the off-axis angle but also on the beam divergence.

Depending on the required seed laser pulse energy (output beam) and available seed energy is only one here one pass (one amplifier stage) or one two pass (two amplifier stages) considered tet. Only the maximum energy yield is exceeded considered the time regime (maximum seed laser pulse Arrangement). A decrease in single-seed Laser pulses are easily possible.

An amplifier stage (one pass of the seed laser pulse through the laser medium 4 ):

• 1. Seed laser pulse inputs from one side - seed laser pulse outputs on the opposite side of the light amplifier (see Table 2).
  A maximum of six seed laser pulses make sense here (six required time intervals of 3 ns each).
• 2. Seed laser pulse inputs from two sides - seed laser pulse outputs on the opposite side of the light amplifier.
  A maximum of six seed laser pulses are possible here (seven required time intervals of 3 ns each). The change of sides (via mirror systems 3 ) of the seed laser pulses should only take place once; after half of the seed laser pulses. A page change requires at least 3 ns loss of time (see Table 3). Compared to the previous one, this arrangement has the advantage of easier spatial separation of the output pulses.

Two amplifier stages (two passes of the seed laser pulse or part of the seed laser pulse - after beam splitting - through the laser medium 4 ):

• 1. Seed laser pulse inputs from one side - seed laser pulse outputs on the same side of the light amplifier.
  A maximum of four output pulses are possible here. For the first amplifier stage, both one (six required time intervals of 3 ns; see Table 4) and two (seven required time intervals of 3 ns; see Table 5) start seed laser pulses can be used.
  In the variant with only one seed laser pulse, it is split into four individual beams after the first passage through the laser medium 4 and the necessary time regime (3 ns interval) is built up before the further passes by using different transit times between the now existing single-seed laser pulses , With two seed laser pulses, each seed laser pulse is split into two beams after the first pass. The division takes place advantageously in a square beam cross section through a beam splitter plate and in a rectangular beam cross section (aspect ratio 2: 1) by means of a beam splitter mirror.
  After beam reversal, the single-seed laser pulses now run through the laser medium 4 in the other direction.
• 2. Seed laser pulse inputs from one side - seed laser pulse outputs on both sides of the light amplifier.
  A maximum of four output pulses are also possible here. The first amplifier stage and the time intervals are to be considered as under 1. Changes can only be seen in the second amplifier stage arrangement. After beam reversal, only the first two single-seed laser pulses in terms of time pass through the laser medium 4 in the other direction, the last two seed laser pulses for the amplification are passed outside the laser medium 4 and from the side of the seed laser pulses of the first amplifier stage coupled. The total time interval is retained (see Table 6).
• 3. seed laser pulse inputs from one side - seed laser pulse outputs on the opposite side b of the light amplifier.
  Three seed laser pulses or output pulses are useful here. There is no dead sector in this variant. To do this, the seed laser pulses must be guided past the laser volume to the input side of the laser after the first pass (cf. Table 7).

To optimize the energy yield, it is necessary to adapt the seed laser pulse in terms of beam cross-section, energy density and beam divergence for the following amplifier stage. To minimize refractive elements in the beam guidance, it is advantageous to design the mirrors 3 required for the beam deflection, if necessary with a curved surface. The curvature can take place via one or two axes and can be concave or convex.

The examples relate to the maximum number of beam passes with an excitation time of t _a = 25 ns and a recovery time of t _e = 3 ns. A reduction in the beam passages compared to the examples given does not affect the patent application.

The invention is not limited to the Darge here presented embodiments. Rather, it is possible by combining and modifying the means mentioned and features to realize other design variants ren without departing from the scope of the invention.  

Tables 1-7

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

LIST OF REFERENCE NUMBERS

1

Input beam (incoming seed pulse)

2

output beam

3

deflecting

4

laser medium

5

Laser Tube
a side of the laser tube
b the side of the laser tube opposite side a

Claims (12)

1. A method for the optimized coupling of ultrashort pulse seed laser pulses into a light amplifier comprising a laser medium ( 4 ), wherein
the laser medium ( 4 ) is characterized by an excitation time t _a and a specific recovery time t _e and the excitation time t _{a is divided} into a number of time periods t _v in such _a way that the following applies for each time period: t _e ≦ t _v <2.t _e .
at least two seed laser pulses are coupled into the light amplifier within the excitation period t _a such that at most one seed laser pulse is coupled in during a period of time t _v and

• a) with one-sided coupling of the seed laser pulses into the light amplifier, ie with coupling into the side (a) of the light amplifier alone or with coupling into the side (b) of the light amplifier alone, at least two seed laser pulses in directly successive periods be coupled or
• b) when the seed laser pulses are coupled into the light amplifier on both sides, ie when a seed laser pulse is coupled into the light amplifier side of the coupling of the seed laser pulse preceding this seed laser pulse, the coupling only after a time ≧ t _{e has passed} after the complete coupling of the preceding seed laser pulse.

2. The method according to claim 1, characterized in that all time segments t _{v are} of the same length. 3. The method according to any one of claims 1 or 2, characterized in that there is no coupling in the first time period t _v . 4. The method according to any one of claims 1 to 3, characterized in that the seed laser pulses in an off-axis beam guidance be coupled. 5. The method according to any one of claims 1 to 4, characterized in that at least one of the seed laser pulses is coupled several times within the excitation period t _a . 6. The method according to any one of claims 1 to 5, characterized in that before coupling into the laser medium and / or after Leaving the laser medium a beam split of seed laser pulses occur.   7. The method according to any one of claims 1 to 6, characterized in that the beam path of the seed laser pulses through mirrors being affected. 8. The method according to any one of claims 1 to 7, characterized in that the transit time of the seed laser pulses through mirrors being affected. 9. The method according to any one of claims 7 or 8, characterized in that Mirrors have a curved surface. 10. The method according to claim 9, characterized in that the curvature over one axis or over two axes is executed. 11. The method according to one of claims 9 or 10, characterized in that it is concave or convex curved mirrors surfaces. 12. The method according to any one of claims 1 to 11, characterized in that amplified seed laser pulses leave the light amplifier within the excitation period t _a on different sides.