Method for amplifying a forward-moving laser pulse and less amplifying a backward-moving laser pulse

Abstract

The invention relates to 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 method steps: 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.

Description

The invention relates to a method for amplifying a laser pulse.It is known to amplify a laser pulse in a laser amplifier. Reflections of such an amplified laser pulse at a target are problematic, since the reflected laser pulse is likewise amplified again in the laser amplifier and can damage a laser source of the laser pulse. Therefore, it is typically attempted to block such reflected laser pulses or to emit them in front of the laser source. Often, however, a certain portion of reflected laser radiation remains with comparatively high power.It is therefore an object of the invention to provide a method which substantially better protects the laser source compared to methods known from the prior art.This object is achieved by a method according to independent claim 1.The object according to the invention is thus achieved by a method for amplifying a forward-running laser pulse and a lower amplification of a backward-running laser pulse, wherein the laser pulses pass through a laser amplifier, having the method steps: 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 B) operating the laser amplifier with such a high low-signal amplification and / or such a low saturation power that the forward-running laser pulse substantially reduces the population inversion of the laser amplifier for the backward-running laser pulse.According to the invention, the population inversion of the laser amplifier is significantly lower when the reverse-running laser pulse is passed through than when the forward-running laser pulse is passed through. As a result, both the forward laser pulse can be amplified more efficiently and the backward laser pulse can be reduced, so that the backward laser pulse represents a lower risk for the laser source.The term population inversion is understood to mean the excitation state of a lasant medium in which it is capable of generating and / or amplifying laser radiation by stimulated emission.In addition, the term small signal amplification is understood to mean a measure of the ability of the active laser medium to increase the intensity of laser radiation.Furthermore, the term saturation power in the present context in the broader sense means the absolute total amount of energy achieved by the lasant medium in the state of population inversion. The saturation power is consequently dependent on the pump power applied to the lasant medium on the one hand and on the total amount of lasant medium within a certain volume on the other hand. The total amount of the lasant medium within the specific volume can be deduced, for example, by means of a pressure prevailing in this specific volume, provided that the lasant medium is a gas or gas mixture.In other words, the invention is based on the realization that the small signal gain and the saturation power are dependent on one another, namely in such a way that the small signal gain, i.e. the degree of amplification of the laser radiation, can only be increased up to a specific threshold of the saturation power. In yet other words, the laser radiation cannot be increased significantly further from a specific reached value of the saturation power, but instead the amplification of the laser radiation transitions into a stagnation starting from this specific value of the saturation power.Methods known from the prior art do not take this circumstance into account, so that the lasant medium is essentially in a state of "supersaturation" for example on account of the total amount of lasant medium used and / or the pump power applied, when the forward pulse passes through the laser amplifier. The laser pulse passing through the laser amplifier then does not absorb all the excitation energy present in the laser amplifier because of the limited low-signal amplification, but leaves a substantial portion of the population inversion in the lasant medium. In the known methods, the reverse-running 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.The low-signal amplification is therefore preferably greater than 0.14 1 / m, in particular greater than 0.17 1 / m, particularly preferably greater than 0.20 1 / m.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 CO 2- laser amplifier.The laser amplifier may be pumped by RF radiation.In method step A), the pump power can be switched on or increased for more than 1 μs and / or less than 2 μs can be switched off or decreased. The pump power is preferably switched on or increased for more than 5 μs, in particular for more than 15 μs. Alternatively or additionally, the pump power is preferably switched off or reduced for less than 1 μs, in particular less than 0.5 μs.In method step B), the low-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. A standard CO 2- laser gas with the components He, N 2, CO 2 and Xe in the ratio 3He:1N 2:1 CO 2+ 3% Xe, or a composition optimized for the laser amplifier can be used as the gas mixture.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.Analogously to the previous embodiments, in method step B) the low saturation power can be achieved by reduced gas pressure and optimized pump power.Generally, in a preferred embodiment, low pump power is achieved by a short on-time of the pumped laser amplifier. The on-time (duty cycle) is preferably less than 80%, in particular less than 70%, particularly preferably less than 60%, of a continuous on-time (CW mode). The pump power during the switch-on time is preferably 100% of the maximum possible pump power, in particular between 8 kW and 12 kW.The forward laser pulse can comprise a plurality of partial pulses; in particular, the forward laser pulse can consist of a plurality of partial pulses. By dividing the laser pulse into a plurality of partial pulses, EUV radiation can be generated particularly effectively. In this case, the partial pulses can have at least one main pulse and / or at least one prepulse.The laser pulse running backwards can be generated by reflecting the laser pulse running forwards on a target. The target can be designed in the form of a tin droplet in order to generate EUV radiation.The invention further relates to a laser arrangement for carrying out a method described here.Further advantages of the invention are evident from the drawing and its description. In this case, the features mentioned above and those set out further below can be used according to the invention in each case individually or together in any desired combinations.In the drawing, the following are shown: FIG. 1 shows a schematic side view of a laser arrangement with a laser amplifier for carrying out the method according to the invention; FIG. 2 shows Frantz-Nodvik gain curves for different small signal gains and saturation powers; FIG. 3a shows the input pulse and amplified pulse in the forward direction; FIG. 3b shows the effective amplification along the laser amplifier before and after the passing of the forward laser pulse; FIG. 4a shows the output power, i.e. amplified power, of a laser amplifier after passing through the forward laser pulse as a function of small signal amplification and saturation power, normalized to the input power; FIG. 4 bshows the effective gain of a laser amplifier integrated along the laser amplifier after passing through the forward-running laser pulse as a function of small signal gain and saturation power; FIG. 4 cshows the amplification of a reverse-running laser pulse after passing through a forward-running laser pulse as a function of small signal amplification and saturation power, wherein it is assumed by way of example that the input power of the reverse-running laser pulse is equal to the input power of the forward-running laser pulse.FIG. 1 shows a laser arrangement 10 having a laser source 12 and a laser amplifier 14. The amplified forward laser pulse 16 impinges on a target 18, The target 18 can be designed, for example, in the form of a tin droplet in order to generate EUV radiation. A reverse-running laser pulse 20 can emerge from the target 18, which could damage the laser source 12.In order to reduce the reverse-running laser pulse 18, the pump power of the laser amplifier 14 can be at least reduced before the reverse-running laser pulse passes through. Preferably, the pump power of the laser amplifier 14 is completely reduced before the passing through of the reverse-running laser pulse.Alternatively or additionally, the laser amplifier 14 can be operated with such a high low-signal amplification and / or such a low saturation power that-as will be explained below-the population inversion of the laser amplifier 14 by the forward-running laser pulse 16 is reduced to the greatest possible extent.For population inversion:A laser amplifier 14 amplifies incident optical radiation by stimulated emission in a laser transition between a higher energy level (excited state) and a lower energy level (ground state). By excitation (pumping) of the lasant medium, population inversion can be achieved, i.e. the particle density of the lasant 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 lasant medium.For small-signal gain (small-signal gain) and saturation power (saturation power) according to the Frantz-Nodvik theory:The gain of a laser can be described approximately by the Frantz-Nodvik theory: where G 0 denotes the small signal gain, P sat denotes the saturation power, P in denotes the input power and P out denotes the output power. The small signal gain is proportional to the population inversion of the lasant medium and refers to the state before an incident pulse has been amplified. The saturation power is indirectly proportional to the effective cross section of the laser transition. According to the Frantz-Nodvik theory, the following applies approximately to small input powers:As input power increases, the effective gain (the energy stored in the lasant medium is finite) decreases. For very large input powers, the maximum extractable power is approximately G 0 P sat.FIG. 2 shows, by way of example, the output power of a laser amplifier 14 as a function of the input power in accordance with the Frantz-Nodvik model. For the three cases illustrated, G 0 P sat= is constant, but small signal gain and saturation power differ. Increasing the small signal gain and decreasing the saturation power allow for an increase in output power (given finite input power) without increasing the energy stored in laser amplifier 14.FIGS. 3 a, 3 b show, by way of example, the amplification of a pulse in a laser amplifier 14 in accordance with the Frantz-Nodvik model. Before passing through the forward laser pulse 16, the initial effective gain is equal to the small signal gain. The gain of the forward laser pulse 16 reduces the population inversion and thus the effective gain experienced by a subsequent reverse laser pulse 20 in the laser amplifier 14. In FIGS. 3 aand 3 b, the forward laser pulse 16 is marked with a solid line and an amplified laser pulse 22 is marked with a dashed line.FIGS. 4 a, 4 b show the output power (=amplified power, normalized to the input power) of the forward laser pulse 16 and the effective gain of the laser amplifier 14 (integrated along the laser amplifier) after passing through the forward laser pulse 16 as a function of the small signal gain and saturation power (normalized to the input power). The gain of the backward-going laser pulse 20 is dependent on the still present effective gain or "residual population inversion" after the forward-going laser pulse 16 has passed through the laser amplifier 14.FIG. 4 cshows the output power (=amplified power, normalized to the input power) of the backward-running laser pulse 20 after the forward-running laser pulse 16 has passed through the laser amplifier 14 (it is assumed by way of example that the input power of the backward-running laser pulse 20 is equal to the input power of the forward-running laser pulse 16). Optimizing small signal gain and saturation power for a target output power makes it possible to reduce the gain of the reverse-running laser pulse 20 by setting a correspondingly high small signal gain and a correspondingly low saturation power.In a real laser amplifier 14, the amplification of the reverse-running laser pulse 20 also depends on a time difference between the forward-running laser pulse 16 and the reverse-running laser pulse 20 when the laser amplifier 14 continues to be pumped and the population inversion increases again after passing through the forward-running laser pulse 16.In a laser amplifier 14 in the form of a CO 2- laser amplifier, the small signal amplification and the saturation power can be adjusted for a given geometry (length and volume of the laser amplifier). This can be implemented by means of setting a laser gas (composition and / or pressure) and RF excitation of the laser gas. Starting from an operating point of the laser amplifier 14 known from the prior art, the small signal amplification increases and the saturation power decreases, for example, when a gas pressure is reduced. A strong influence on the saturation power is an average RF pump power, wherein a reduction in the average RF pump power reduces the saturation power, for example. At the same time, in such a case, with reduced RF pump power, the small signal amplification increases until a maximum is reached.In summary, small signal gain can be increased and saturation power can be reduced by reducing (starting from the typical operating point) the gas pressure and the average RF pump power. If the gas pressure is too low and the RF amplitude is too low, an RF discharge and thus the laser amplifier 14 becomes unstable. In order to reduce the average RF pump power, it is therefore expedient to feed the RF pump power into the laser amplifier 14 in a pulsed mode, wherein an amplitude is then kept at a maximum, but the average RF pump power can be reduced over the duty cycle.

Claims

Method for amplifying a forward-running laser pulse (16) and a lower amplification of a reverse-running laser pulse (20), wherein the laser pulses (16, 20) pass through a pumped laser amplifier (14), having the method steps: 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 (16) and switching off or decreasing the pump power provided by the pump source for the laser amplifier (14) before passing through the reverse-running laser pulse (20); and / or B) operating the laser amplifier (14) with such a high low-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 reverse-running laser pulse (20) to the greatest possible extent.Method according to Claim 1, wherein the laser amplifier (14) is designed in the form of a CO 2- laser amplifier.The method of any preceding claim, wherein the laser amplifier (14) is pumped by RF radiation.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 μs and / or less than 2 μs is switched off or decreased.Method according to one of the preceding claims, wherein in method step B) the small signal amplification and / or the saturation power is 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.Method according to one of the preceding claims, wherein the forward laser pulse (16) comprises a plurality of partial pulses.Method according to Claim 6, wherein the partial pulses have at least one main pulse and / or at least one prepulse.Method according to one of the preceding claims, wherein the back-running laser pulse (20) is produced by reflection of the back-running laser pulse (16) at a target (18).Method according to claim 8, wherein the target (18) is designed in the form of a tin droplet for generating EUV radiation.

Images