Method for the amplification of a forward-running laser pulse and the lower amplification of a backward-running laser pulse

Abstract

The invention relates to a method for the amplification of a forward-running laser pulse (16) and the lower amplification of a backward-running laser pulse (20), wherein the laser pulses (16, 20) pass through a pumped laser amplifier (14), said method having the following method steps: A) switching on or increasing a pump power provided by a pump source for the laser amplifier (14) before the forward-running laser pulse (16) passes through, and switching off or reducing the pump power provided by the pump source for the laser amplifier (14) before the backward-running laser pulse (20) passes through; and / or B) operating the laser amplifier (14) with such a high small-signal amplification and / or such a low saturation power that the forward-running laser pulse (16) largely reduces the population inversion of the laser amplifier (14) for the backward-running laser pulse (20).

Description

[0001] Method for amplifying a forward-moving laser pulse and reducing the amplification of a backward-moving laser pulse

[0002] The invention relates to a method for amplifying a laser pulse.

[0003] It is known to amplify a laser pulse in a laser amplifier. Reflections of such an amplified laser pulse from a target are problematic, as the reflected laser pulse is also amplified in the laser amplifier and can damage the laser source of the laser pulse. Therefore, attempts are typically made to block such reflected laser pulses or divert them away from the laser source. However, a certain amount of reflected laser radiation with comparatively high power often remains.

[0004] It is therefore an object of the invention to provide a method which protects the laser source significantly better than methods known from the prior art.

[0005] This object is achieved by a method according to independent patent claim 1. The dependent patent claims represent preferred embodiments of the method.

[0006] The object of the invention is thus achieved by a method for amplifying a forward-running laser pulse and less amplifying a backward-running laser pulse, wherein the laser pulses pass through a laser amplifier, with the method steps:

[0007] A) switching on or increasing a pump power provided by a pump source for the laser amplifier (14) before passing through the forward-running laser pulse and switching off or decreasing the pump power provided by the pump source for the laser amplifier (14) before passing through the backward-running laser pulse; and / or

[0008] B) Operation of the laser amplifier with such a high small-signal gain and / or such a low saturation power that the forward-running laser pulse reduces the population inversion of the laser amplifier for the backward-running laser pulse as much as possible.

[0009] According to the invention, the population inversion of the laser amplifier is significantly lower when passing through the backward-propagating laser pulse than when passing through the forward-propagating laser pulse. This allows both the forward-propagating laser pulse to be amplified more efficiently and the backward-propagating laser pulse to be attenuated, so that the backward-propagating laser pulse poses less of a threat to the laser source.

[0010] The term population inversion refers to the excited state of a laser-active medium in which it is capable of generating and / or amplifying laser radiation through stimulated emission.

[0011] In addition, the term small-signal amplification is understood as a measure of the ability of the active laser medium to increase the intensity of laser radiation.

[0012] Furthermore, the term "saturation power" in this context refers, in a broader sense, to the absolute total energy amount achieved by the laser-active medium in the state of population inversion. The saturation power therefore depends, on the one hand, on the pump power applied to the laser-active medium and, on the other hand, on the total amount of the laser-active medium within a specific volume. The total amount of the laser-active medium within a specific volume can be determined, for example, from the pressure prevailing in this specific volume, provided the laser-active medium is a gas or gas mixture.

[0013] In other words, the invention is based on the finding that small-signal amplification and saturation power are interdependent, in the sense that small-signal amplification, i.e., the degree of amplification of the laser radiation, can only be increased up to a certain threshold of saturation power. In other words, once a certain saturation power value is reached, the laser radiation cannot be significantly increased any further; instead, the amplification of the laser radiation stagnates beyond this certain saturation power value.

[0014] Prior art methods do not take this circumstance into account, so that the laser-active medium is essentially in a state of "supersaturation" when the forward-propagating pulse passes through the laser amplifier, for example due to the total amount of laser-active medium used and / or the applied pump power. Due to the limited small-signal amplification, the laser pulse passing through the laser amplifier does not absorb all of the excitation energy present in the laser amplifier, but instead leaves a significant portion of the population inversion in the laser-active medium. In the prior art methods, the backward-propagating laser pulse is then amplified in the reverse direction by this "residual population inversion," which can lead to damage to or destruction of the laser source.

[0015] Preferably, the small signal amplification is therefore greater than 0.14 1 / m, in particular greater than 0.17 1 / m, particularly preferably greater than 0.20 1 / m.

[0016] The saturation power is preferably less than 30 W, in particular less than 25 W, particularly preferably less than 20 W. In a particularly preferred embodiment of the invention, the laser amplifier is designed in the form of a CO2 laser amplifier.

[0017] The laser amplifier can be pumped by RF radiation.

[0018] In method step A), the pump power can be switched on or increased for more than 1 ps and / or switched off or decreased for less than 2 ps. The pump power is preferably switched on or increased for more than 5 ps, in particular for more than 15 ps. Alternatively or additionally, the pump power is preferably switched off or decreased for less than 1 ps, in particular less than 0.5 ps.

[0019] In process step B), the small-signal amplification and / or the saturation power can be achieved by a gas pressure in a range from 80 hPa to 115 hPa, in particular 90 hPa to 110 hPa, in particular 100 hPa, and / or a pump power in a range from 4.5 kW to 7 kW, in particular 4.8 kW to 6.7 kW. The gas mixture used can be a standard CC laser gas with the components He, N2, CO2, and Xe in a ratio of 3He: lN2: lCO2 + 3% Xe, or a composition optimized for the laser amplifier.

[0020] The pump power is preferably less than 100%, in particular less than 80%, particularly preferably less than 60% of the pump power at which the maximum temperature of the laser gas reaches a value of approximately 200°C. Alternatively or additionally, the pump power is preferably less than 10 kW, in particular less than 8 kW, preferably 4.5 kW to 7 kW, more preferably 4.8 kW to 6.7 kW, and particularly preferably less than 6 kW.

[0021] Analogous to the previous explanations, in process step B) the low saturation power can be achieved by reduced gas pressure and optimized pump power.

[0022] In general, in a preferred embodiment, a low pump power is achieved by a short turn-on time of the pumped laser amplifier. The duty cycle is preferably less than 80%, in particular less than 70%, and particularly preferably less than 60% of a continuous duty cycle (CW mode). The pump power during the turn-on time is preferably 100% of the maximum possible pump power, in particular between 8 kW and 12 kW.

[0023] The forward-moving laser pulse can comprise several sub-pulses; in particular, the forward-moving laser pulse can consist of several sub-pulses. By splitting the laser pulse into several sub-pulses, EUV radiation can be generated particularly effectively. The sub-pulses can comprise at least one main pulse and / or at least one pre-pulse.

[0024] The backward-traveling laser pulse can be generated by reflection of the forward-traveling laser pulse from a target. The target can be in the form of a tin droplet to generate EUV radiation.

[0025] The invention further relates to a laser arrangement for carrying out a method described here.

[0026] Further advantages of the invention will become apparent from the drawings and their description. The above-mentioned and further described features can be used individually or in combinations according to the invention.

[0027] The drawing shows:

[0028] Fig. 1 is a schematic side view of a laser arrangement with a laser amplifier for carrying out the method according to the invention;

[0029] Fig. 2 Frantz-Nodvik gain curves for different small-signal gains and saturation powers;

[0030] Fig. 3a Input pulse and amplified pulse in forward direction;

[0031] Fig. 3b shows the effective gain along the laser amplifier before and after passing through the forward-moving laser pulse; Fig. 4a shows the output power, ie, amplified power, of a laser amplifier after passing through the forward-moving laser pulse as a function of small-signal gain and saturation power, normalized to the input power;

[0032] Fig. 4b shows the effective gain of a laser amplifier, integrated along the laser amplifier, after passing through the forward laser pulse as a function of small-signal gain and saturation power;

[0033] Fig. 4c shows the amplification of a backward-running laser pulse after passing through a forward-running laser pulse as a function of small-signal amplification and saturation power, assuming, for example, that the input power of the backward-running laser pulse is equal to the input power of the forward-running laser pulse.

[0034] Fig. 1 shows a laser arrangement 10 with a laser source 12 and a laser amplifier 14. A forward-moving laser pulse 16 is generated in the laser source 12 and amplified in the laser amplifier 14. The amplified forward-moving laser pulse 16 impinges on a target 18. The target 18 can be in the form of a tin droplet, for example, to generate EUV radiation. A backward-moving laser pulse 20 can emanate from the target 18, which could damage the laser source 12.

[0035] To reduce the backward-traveling laser pulse 18, the pump power of the laser amplifier 14 can be at least reduced before the backward-traveling laser pulse passes through. Preferably, the pump power of the laser amplifier 14 is completely reduced before the backward-traveling laser pulse passes through.

[0036] Alternatively or additionally, the laser amplifier 14 can be operated with such a high small-signal amplification and / or such a low saturation power that - as will be explained below - the population inversion of the laser amplifier 14 is largely reduced by the forward-moving laser pulse 16.

[0037] On occupation inversion:

[0038] A laser amplifier 14 amplifies incident optical radiation through stimulated emission in a laser transition between a higher energy level (excited state) and a lower energy level (ground state). Population inversion can be achieved by exciting (pumping) the laser-active medium; that is, the particle density of the laser-active particles in the excited state is higher than that of the particles in the ground state. In this state, stimulated emission and thus amplification of an incident optical beam can be achieved in the laser amplifier. The higher the population inversion, the higher the gain and the energy stored in the laser-active medium.

[0039] On small-signal gain and saturation power according to the Frantz-Nodvik theory:

[0040] The amplification of a laser can be approximately described using the Frantz-Nodvik theory: where Go is the small-signal gain, P sa t is the saturation power, P in the input power and P ou t denotes the output power. The small-signal gain is proportional to the population inversion of the laser-active medium and refers to the state before an incident pulse is amplified. The saturation power is indirectly proportional to the cross section of the laser transition. According to the Frantz-Nodvik theory, for small input powers, the following approximately applies:

[0041] F'out ~ ex p (G0)P in

[0042] As the input power increases, the effective gain decreases (the energy stored in the laser-active medium is finite). For very high input powers, the following approximately applies:

[0043] F'out ~ Pin + GoF'sat the maximum extractable power is approximately GoPsat.

[0044] Fig. 2 shows an example of the output power of a laser amplifier 14 as a function of the input power according to the Frantz-Nodvik model. For the three cases shown, GoPsat is constant, but the small-signal gain and saturation power differ. Increasing the small-signal gain and decreasing the saturation power allow for an increase in the output power (for a given, finite input power) without increasing the energy stored in the laser amplifier 14.

[0045] Figures 3a and 3b show, by way of example, the amplification of a pulse in a laser amplifier 14 according to the Frantz-Nodvik model. Before passing through the forward-propagating laser pulse 16, the initial effective amplification is equal to the small-signal amplification. The amplification of the forward-propagating laser pulse 16 reduces the population inversion and thus the effective amplification experienced by a subsequent backward-propagating laser pulse 20 in the laser amplifier 14. In Figures 3a and 3b, the forward-propagating laser pulse 16 is indicated by a solid line, and an amplified laser pulse 22 by a dashed line.

[0046] Figures 4a and 4b show the output power (= amplified power, normalized to the input power) of the forward-propagating laser pulse 16 and the effective gain of the laser amplifier 14 (integrated along the laser amplifier) ​​after the forward-propagating laser pulse 16 has passed through, as a function of small-signal amplification and saturation power (normalized to the input power). The gain of the backward-propagating laser pulse 20 depends on the remaining effective gain or "residual population inversion" after the forward-propagating laser pulse 16 has passed through the laser amplifier 14.

[0047] Fig. 4c shows the output power (= amplified power, normalized to the input power) of the backward-propagating laser pulse 20 after the forward-propagating laser pulse 16 has passed through the laser amplifier 14 (it is assumed, for example, that the input power of the backward-propagating laser pulse 20 is equal to the input power of the forward-propagating laser pulse 16). Optimizing the small-signal gain and saturation power for a target output power makes it possible to reduce the gain of the backward-propagating laser pulse 20 by setting a correspondingly high small-signal gain and a correspondingly low saturation power.

[0048] In a real laser amplifier 14, the gain of the backward-running laser pulse 20 also depends on a time difference between the forward-running laser pulse 16 and the backward-running laser pulse 20 if the laser amplifier 14 continues to be pumped and the population inversion increases again after passing through the forward-running laser pulse 16.

[0049] In a laser amplifier 14 in the form of a CO2 laser amplifier, the small-signal gain and the saturation power can be adjusted for a given geometry (length and volume of the laser amplifier). This can be achieved by adjusting a laser gas (composition and / or pressure) and by RF excitation of the laser gas. Starting from an operating point of the laser amplifier 14 known from the prior art, the small-signal gain increases and the saturation power decreases, for example, when the gas pressure is reduced. The average RF pump power has a strong influence on the saturation power, with a reduction in the average RF pump power, for example, reducing the saturation power. At the same time, with a reduced RF pump power, the small-signal gain increases until a maximum is reached.

[0050] In summary, small-signal amplification can be increased and saturation power reduced by reducing the gas pressure and the average RF pump power (starting from the typical operating point). Experimental results are summarized in Figures 6a and 6b. If the gas pressure and the RF amplitude are too low, an RF discharge and thus the laser amplifier 14 become unstable. To reduce the average RF pump power, it is therefore advisable to feed the RF pump power into the laser amplifier 14 in a pulsed mode, whereby the amplitude is then maintained at a maximum, but the average RF pump power can be reduced via the duty cycle.

Claims

Patent claims 1. A method for amplifying a forward-running laser pulse (16) and less amplifying a backward-running laser pulse (20), wherein the laser pulses (16, 20) pass through a pumped laser amplifier (14), comprising the steps of: A) switching on or increasing a pump power provided by a pump source for the laser amplifier (14) before passing through the forward-moving laser pulse (16) and switching off or decreasing the pump power provided by the pump source for the laser amplifier (14) before passing through the backward-moving laser pulse (20); and / or B) Operating the laser amplifier (14) with such a high small-signal amplification and / or such a low saturation power that the forward-running laser pulse (16) reduces the population inversion of the laser amplifier (14) for the backward-running laser pulse (20) to the greatest possible extent.

2. The method according to claim 1, wherein the laser amplifier (14) is in the form of a CO2 laser amplifier.

3. Method according to one of the preceding claims, wherein the laser amplifier (14) is pumped by RF radiation.

4. Method according to one of the preceding claims, wherein in method step A) the pump power is switched on or increased for more than 1 ps and / or switched off or decreased for less than 2 ps.

5. Method according to one of the preceding claims, wherein in method step B) the small-signal amplification and / or the saturation power is determined by a gas pressure in a range from 80 hPa to 115 hPa, in particular 90 hPa to 110 hPa, in particular 100 hPa, and / or a pumping power in a range of 4.5 kW to 7 kW, in particular 4.8 kW to 6.7 kW, is achieved.

6. Method according to one of the preceding claims, wherein the forward-moving laser pulse (16) comprises a plurality of partial pulses.

7. The method according to claim 6, wherein the partial pulses comprise at least one main pulse and / or at least one pre-pulse.

8. Method according to one of the preceding claims, wherein the backward-moving laser pulse (20) is produced by reflection of the forward-moving laser pulse (16) at a target (18).

9. The method according to claim 8, wherein the target (18) is in the form of a tin droplet for generating EUV radiation.

Images