JPH02285688A - Saturable absorption filter for krf laser - Google Patents

Abstract

PURPOSE:To make a saturable absorption filter transparent in a low energy density to a short-pulse laser beam and to contrive to enable the filter to possess a large attenuation factor to ASE by using an acridine solution for the saturable absorption filter. CONSTITUTION:An acridine solution is used for a filter. This acridine is shown by a molecular formula, C13H9N, and has a structure shown by the formula. At this time, a short pulse of 44ps (100muJ), for example, which is outputted from a dye laser 2, is converted its wavelength from a visible wavelength to an ultraviolet length by a wavelength converter 3 to turn into a pulse (3muJ) of 30ps and this pulse of the ultraviolet wavelength of 30ps is made to pass through a filter SA when being amplified two times by a KrF laser amplifier 4, whereby a short pulse (20mJ) of 10ps or lower can be generated. Moreover, if the filter is inserted in a multistage amplifying path, noise light ASE generated at the preceding-stage part of the amplifier 4 is inflicted an absorption loss more than a gain, which is amplified by each amplifier, by the filter.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an excimer laser having a wavelength of 248 nm.
This invention relates to a saturable absorption filter for KrF lasers.

[Conventional technology]

Excimer lasers have a wide gain band and operate effectively as a short-pulse laser amplification medium, and because they are gas lasers, they are less prone to self-phase modulation, which is a problem with solid-state lasers and dye lasers. , the laser medium has advantages such as large-sized laser medium and easy cooling.

Among these excimer lasers, KrF laser and ArF laser have a ground state (lower level) that exhibits a dissociated state, and emit highly efficient energy up to an ultra-short pulse region with a pulse width of femtoseconds (fs). It is possible to
In particular, KrF lasers, like dye lasers, have a high gain but a low saturation energy density, so the Radey amplifier easily reaches saturation and can easily extract energy even in the low input energy region. .

In such a KrF laser, it takes more than 10 nanoseconds (ns) to follow a complicated collision reaction and generate a KrF excimer in the laser excited state (upper level). The lifetime is short, a few nanoseconds, and the excitation takes much longer than this effective lifetime (absorption recovery time).

Therefore, in short-pulse laser amplifiers, spontaneous emission light is emitted when the excited state reaches the end of its lifetime and transitions to the ground state before the short-pulse laser light enters, and noise light originating from this spontaneous emission light is generated. is amplified, and under high gain conditions A
S E (Amplified 5 pontaneou
This results in the generation of strong optical noise called s emission.

Generally, in short-pulse laser amplifiers, the amplification gain is a function of the input light intensity and is a small signal gain during most of the pumping time, so laser light with high light intensity easily shows saturation, but ASE does not Since the pulse width is sufficiently longer than the effective life of the KrF excimer, the laser beam is constantly amplified with a small signal gain, and the amplification gain of the laser beam with high optical intensity is lower than that of ASE with relatively low optical intensity (see Figure 10). ).

For this reason, during the amplification process, the laser light and the AS, which is noise light,
The ratio with E, that is, the S/N ratio of laser amplifiers, continues to decrease, and especially in high-gain KrF lasers, even if the ASE component at the input part is at an extremely low level, the ASE increases rapidly. becomes.

In order to eliminate such a decrease in the S/N ratio in the amplification process, a medium that absorbs the loss that cannot be solved by a spatial filter and compensates for the unequal amplification, that is, the unequal absorption, is required. As an absorption medium, there is a saturable absorption filter (saturable absorber) that causes a large absorption loss for noise light ASH with low incident light intensity and becomes substantially transparent for strong light such as laser light. Traditionally used.

[Problem to be solved by the invention]

However, although saturable absorption filters with good characteristics have been used for pigmented lasers in the visible region, in the ultraviolet region the residual absorption of the saturable absorption medium is either very large or small. However, the laser energy required for saturation is large (therefore, it does not function as a filter for those with small input optical energy), and a saturable absorption filter that can be put to practical use for KrF lasers has not yet been developed.

Therefore, the current situation is to reduce the ASE level by lowering the gain of the laser amplifier.

For this reason, there is still a few percent of ASE for laser light.
remained, and the high gain characteristic, which is the original advantage of the KrF radar, could not be utilized.

Furthermore, if a filter that requires a large amount of energy for absorption saturation is used for a KrF laser, the light intensity required for absorption saturation will result in, for example, two-photon absorption, which may cause the normally used optical elements or solvent itself to become opaque. , there were practical problems.

Therefore, it is an object of the present invention to realize a saturable absorption filter for a KrF laser that is transparent to short-pulse laser light at a low energy density and has a large attenuation rate for ASE.

[Means and actions for solving the problem] First, noise light A
Consider the conditions for a saturable absorption filter necessary to suppress S and E and amplify the laser pulse.

ASE is amplified with small signal gain, but if a saturable absorber with a small signal attenuation rate equal to the small signal gain of the amplifier is inserted at the input of the laser amplifier, the laser pulse can be amplified without allowing any amplification of ASE. It is possible to do so. That is, go ·I, -α.・L, (+). Here, go, α. are the small signal gain coefficient of the amplifier and the small signal absorption coefficient of the saturable absorption filter, and 7, L are the gain length of the amplifier and the absorption length of the saturable absorption filter.

On the other hand, a saturable absorption filter needs to be transparent in a light intensity range that can actually be irradiated with a laser pulse.

Assuming that the excited state relaxation time of the saturable absorption filter is sufficiently short with respect to the laser pulse, the energy density E required to make the saturable absorption filter transparent is equal to the saturation energy density (fluence) of the saturable absorber. ) and the small signal transmittance as E s a and α, respectively.・■ When set to 7, Eb=E, ・α0−LIl (2) E s a
It is given by 3hν/σ3(3). Here, σa is the absorption cross section of the saturable absorption filter.

Also, due to limitations such as two-photon absorption and nonlinear refractive index, the maximum laser light intensity that can be irradiated to the saturable absorption filter is 10G.
Since it is on the order of "W/cm", the pulse width is Ip
Assuming that s, the energy density that can be irradiated to the saturable absorption filter is 10 mJ/cm2.

Typical small signal gain g of a KrF laser amplifier.

Since L is about 4 to 10, αo−L=10, E,
By substituting -10 mJ/cm2 into equation (2), the saturation energy density E sm that can be practically used as a saturable absorption filter for KrF laser can be calculated as F+. The absorption cross section that satisfies this saturation energy density is given by formula (3) as m = 8 x 10-1
It is calculated as 6cm2. This is a large value equivalent to more than three times the stimulated emission cross section of KrF excimer.

On the other hand, in the case of aromatic compounds, the residual absorption that determines the quality of a saturable absorption filter is dominated by absorption from the excited level, and the absorption cross section of the ground state of the compound σ. The quality of saturation characteristics is evaluated by the ratio of absorption cross section σ17 from the excited state to 5σ02/σ11. It is a measurable constant, and it is difficult to determine it directly from calculations or the absorption cross section of the ground state.

For this reason, in order to expect a large value for the unknown σI11, an effective means is to select a medium with a large ground state absorption cross section σ.2 and actually measure the r value. The method is also consistent with the conditions for obtaining low saturation energy mentioned above.

Based on the above considerations, a medium having a large absorption cross section of lQ-''cm2 or more at the KrF excimer laser wavelength was selected as a candidate and its characteristics were evaluated.

Here, three-ring aromatics have a strong absorption band around 25 Onm and are promising candidates for saturable absorbers for KrF lasers. In particular, the linear series three-ring aromatic represented by anthracene has a higher and sharper absorption peak than the nonlinear series (phenanthrene), as shown in Figure 1, but the absorption peak of anthracene has a long wavelength of 252 nm. Therefore, the absorber shown in Table 1 below, which has an absorption peak near the laser wavelength of 248.5 nm, was used as a promising candidate for measurement.

Table 1 The absorption cross-sectional area of each of these absorbers was measured and the results are shown in Table 2 below.

Table 2 It was found that the medium with a large r value and a large absorption cross section was an acridine solution.

The absorption spectra of such an acridine solution (methanol solution) are shown in Figure 2, and include the saturable dye DODCI (ethanol solution) for rhodamine 6G dye lasers and the saturable absorber BBQ (dioxane solution) for XeCl lasers. Compared to the acridine solution, the BB
Ground state absorption cross section is significantly larger than Q6. 8 X
10-16 cm” and its absorption characteristics are DODCI
It has been shown that it is almost comparable to

This acridine is represented by the molecular formula Cl5H9N and has the following structure.

From Table 1.2, it can be seen that the KrF laser wavelength of 248.5 nm has the smallest saturation energy and is actually the pulse K.
The saturable absorption filter of the acridine solution was irradiated with rF laser light to measure the saturation energy density and effective lifetime.

This measurement system is shown in Figure 3, where ■ indicates XeCl! which oscillates at a wavelength of 308 nm! , laser, 2 is this 308
3 is a wavelength converter (SHG - nonlinear optical crystal β-B) that converts the output wavelength in the visible region from the dye laser 2 to a wavelength in the ultraviolet region of 248 nm. a B2
04) 4 is a KrF laser amplifier that amplifies the wavelength of 248 nm, 5 is a spatial filter consisting of two lenses and an aperture and evacuated to avoid plasma generation, S
A is a saturable absorption filter, 6.7 is a calorimeter for measuring output laser energy, and 8 is a photodiode for measuring transmitted energy.

The change in energy transmittance (saturation characteristic) with respect to the input energy density of the acridine methanol solution obtained using such a measurement system is shown in Figure 4. On the other hand, the dotted line portion B shows that the transmittance is poor and almost opaque, and the dotted line portion B shows a saturated state where the transmittance is good and almost transparent for a large energy density.

That is, significant saturation is exhibited at an input energy density of 2 mJ/cm2 or higher, and the transmittance increases to 0.21 at 20 mJ/cm2. However, if the input energy density is higher than this, the saturation becomes dull and the maximum transmittance becomes 0.42.

When calculating the saturation energy based on this saturation characteristic, the saturation energy of the acridine solution with the largest absorption cross section is found to be 1.2 mJ by analyzing the experimental data shown in Figure 4.
/cm2 was obtained. This was found to be a very small and excellent saturable absorber. As mentioned above, assuming that the upper limit of the focused energy is 10 mJ/cm2, this means that the small signal gain of the amplifier per stage is g. -L=8.3
This means that the laser pulse can be amplified without any growth of ASE even if it is increased to . In conventional amplification systems, g is used to suppress ASH amplification. -L
Considering that the gain is limited to <4 and that an ASE of several percent relative to the laser energy is allowed, this saturable absorption filter is shown to be very effective.

Next, the effective lifetime (absorption recovery time) is an important parameter that determines the ASE suppression characteristics after laser pulse transmission.

FIG. 5 shows absorption recovery of acridine/methanol solution. Absorption recovery is 0.4 ns shown in (a) and (b) ((
b) shows an extension of (a)) ] 15 ns 2
It has two time constants. The lifetime of 0.4 ns is sufficiently smaller than the gain recovery time of KrF, and the attenuation rate can be recovered along with the gain recovery to effectively suppress ASE.

Such an effective life is approximately equal to the life of a KrF excimer, and as the gain is restored, the attenuation rate is restored and ASE can be effectively suppressed.

On the other hand, an ideal saturable absorption filter becomes completely transparent when it absorbs energy corresponding to equation (2).

However, in the saturable absorbers of these dyes, reabsorption occurs from the excited level, but generally the absorption cross section from the excited level σ1a is equal to the absorption cross section from the ground state σ. 2 and the relaxation time of the excitation unit is also short, so absorption from this excited level is often non-saturated absorption.

Here, the unsaturated component of absorption was actually measured, and the pulse width was 20p.
A KrF laser beam of 100 mJ/cm2 was irradiated with an energy density of 100 mJ/cm2, and the change in transmittance with respect to the small signal transmittance was measured.

The measurement results are shown in Figure 6, where the small signal transmittance on the horizontal axis is equivalent to the concentration of acridine, and the small signal transmittance -
10° indicates only the solvent.

The unsaturated characteristic of the ground state has a slope of 1 in the case of steady light, and the slope of the saturated characteristic of the excited state by laser light is 0.1.
It was 6.

The pulse width used in the measurement was determined by the effective lifetime of the excited level (0,
4 ns), so non-saturated absorption can be regarded as absorption from an excited level.

The ratio of the absorption cross sections of the ground state and excited state is expressed as ``
, this " is the logarithmic ratio of the small signal transmittance T. and the transmittance T., so "Miσ.2/σI, l-110,16=6.25 (
5). It was found that this value is larger than -3.5 of the absorber developed for XeC1, and that the non-saturated absorption component is also sufficiently small.

〔Effect of the invention〕

As described below, the measurement results of the acridine solution used in the saturable absorption filter for KrF lasers according to the present invention are summarized as shown in Table 3 below.

Table 3 In addition to these solvents, acridine is also well soluble in ethylene glycol, cyclohexane, etc. as mentioned above, and both exhibit similar excellent saturable absorption properties.

In addition, the saturable absorption filter for KrF laser using acridine solution is the saturated dye DO for rhodamine 6G dye laser.
Since it has the same characteristics as DCI, it makes it possible to apply saturable absorber application technology that has been used with conventional dye lasers to KrF lasers.

Examples include generation of long and short repetitive pulses using a mode rotor, pulse compression amplification in a saturating amplifier, and optical noise removal in short pulse amplification.

In addition, the ultrashort pulse that prevents pre-pulses caused by noise light ASE has high practical value, such as being able to perform ultraprecision machining in polymer machining and the like without thermal damage.

〔Example〕

FIG. 7 shows a short pulse amplification device formed by incorporating the saturable absorption filter according to the present invention into a KrF laser device, and the ASE suppression characteristics were measured using this device.

In this measurement, in order to generate short pulse ultraviolet rays as in the measurement system shown in Fig. 3, a short cavity type XeCl laser 1 and a mode rotator dye laser 2 are used, and 248 nm ultraviolet light is obtained by a nonlinear optical crystal 3. Ta. The laser amplifier includes two discharge-pumped KrF laser amplifiers.
Used as a pass amplifier.

An example of the saturable absorption filter SA used here is shown in Part 8, and in the case of FIG. 8(a), for example, 9. A transparent cell was prepared by adding 56 x 10-6 mol/l of acridine/methanol solution, as shown in Figure (b).
In case ', this acridine methanol solution is used as a nozzle jet injector with a thickness of 150 μm.

The former is used for low-concentration, large-diameter beams, and the latter is used when using high-concentration solutions.

Figure 9 shows the change in pulse waveform due to amplification actually measured using a streak camera (not shown).
Waveforms a) to (e) show pulse waveforms at locations indicated by the same reference numerals in FIG.

As can be seen from this waveform diagram, the wavelength converter 3 converts the short pulse of 44 ps (100 μJ) output from the dye laser 2 from visible wavelength to ultraviolet wavelength, converting it into a 5 ops pulse (3 μJ). When a 30 ps pulse is amplified twice by the KrF laser amplifier 4, the 10 ps pulse is passed through the saturable absorption filter SA of the present invention.
The following short pulses (20 mJ) can be generated.

In this way, in such a multistage amplification system that does not include a saturable absorption filter, the noise light ASB, which perceives a higher gain as the amplification progresses due to gain saturation, grows faster than the laser pulse, and the laser Although the quality is decreasing,
When a saturable absorption filter is inserted into a multistage amplification path, the optical noise ASE generated in the front stage of the laser amplifier receives an absorption loss greater than the gain amplified by each amplifier by the saturable absorption filter, and the KrF in the ps region Mode-locked oscillation and pulse compression of the laser become possible, and a system configuration that uses a high-gain amplifier, which is an inherent feature of KrF lasers, can be expected to lead to higher output and shorter pulses in short-pulse KrF laser systems.

[Brief explanation of drawings]

Figure 1 is a graph showing the absorption cross section characteristics of three-ring aromatics with respect to wavelength. Figure 2 is a graph showing the absorption spectrum of a saturated dye containing an acridine solution used in the saturable absorption filter of the present invention. , Fig. 3 is a block diagram showing the measurement system of the saturable absorption filter of the present invention, Fig. 4 is a graph of the input energy density versus transmittance of the acridine solution, and Fig. 5 is a graph of the transmittance of the acridine solution. Figure 6 is a graph showing changes in transmittance over time; Figure 6 is a graph showing changes in transmittance with respect to small signal transmittance; Figure 7 is a graph showing changes in transmittance with respect to small signal transmittance;
FIG. 8 is a diagram showing an embodiment of the rF laser device, FIG. 8 is a diagram showing an embodiment of the saturable absorption filter used in the present invention, and FIG. 9 is a waveform diagram showing the pulse compression process according to the present invention in FIG. 7. , FIG. 10 is a waveform diagram for explaining the relationship between the laser pulse and the noise light ASE. In the figure, SA indicates a saturable absorption filter, 4 indicates a KrF laser amplifier, and the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] A saturable absorption filter for KrF laser, characterized by using an acridine solution.