Watt-level 193 nm source generation based on compact collinear cascaded sum frequency mixing configuration

A compact, dual-stage, collinear-cascaded sum-frequency mixing configuration is presented for generating 193 nm sources. Due to the less-introduced, deep-ultraviolet optical components, the system is less prone to damage. In our proof-of-concept experiments, a 1030 nm laser and a 1553 nm laser synchronized to each other were used as drivers and an average power of ~0.7 W was obtained. For comparison, the noncollinear configuration gave an average power of 0.77 W. The difference of 0.07 W is attributed to the spatial walk-off inside the cesium lithium borate (CLBO) crystal, confirmed by indirect visualization. A possible way to overcome the small gap of 0.07 W is proposed for future work. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Generating radiation at the deep-ultraviolet (DUV) wavelength of 193 nm has attracted considerable attention for applications such as semiconductor metrology, photolithography, photoelectron spectroscopy, seeding argon fluoride (ArF) excimer lasers, and laser processing [1][2][3][4][5]. Presently, ArF excimer lasers are the only lasers that output directly at 193 nm, with average powers exceeding 100 watts. Unfortunately, the beam quality is usually poor. For example, typical M 2 values of a twin-chamber ArF laser beam are approximately 13 and 61 in the horizontal and vertical directions, respectively [6]. The use of a solid-state 193 nm seed laser has significantly improved this situation to M 2 <2 for both the horizontal and vertical directions [6]. Therefore, developing a compact and robust solid-state 193 nm seed source is critical to optimize the performance of ArF laser systems and obtain high beam quality and high coherence.
The average power of solid-state 193 nm sources is usually quite low because of the multistage nonlinear frequency conversion processes on which they are based. The two main nonlinear frequency conversion processes used are SHG and sum frequency mixing (SFM).
For second harmonic generation (SHG), the only available option is to use a potassium fluoro-beryllo-borate (KBBF) crystal pumped with a 387 nm laser, which can be obtained by frequency doubling the 774 nm output of a Ti:sapphire laser [7]. The highest average power of 1.05 W at 193 nm has been demonstrated with this type of configuration [7], but the cost of the entire system is expected to be quite high because green lasers are commonly used as pumping sources for Ti:sapphire lasers. In a much simpler scheme, a continuous wave (CW) 193 nm source was implemented by frequency quadrupling the output of amplified 772 nm diode lasers; however, the power scaling of such a system is limited [8]. Another disadvantage with KBBF crystals is that they remain commercially unavailable and the choice of thickness is restricted.
Comparatively speaking, configurations based on SFM are more complex than those based on SHG, and the need for two pump lasers increases the assembly and material costs. A major advantage of SFM systems is that they can be directly pumped by laser diodes (LD). In the aforementioned wavelength combinations, all ultraviolet (UV) beams are generated from a one-stage SFM scheme, except for 258 nm and 266 nm, for which phase matching is impossible for generating a 193 nm wavelength in a CLBO crystal. Therefore, two stages of SFM are required altogether, and they are usually carried out stage by stage. Accordingly, the setup requires not only two different dichroic mirrors (DM) to combine beam sources for SFMs, but also half-wave plates (HWP) to adjust the polarization directions. Specifically, in our previous work on a 193 nm source [11,17] This approach clearly has some drawbacks. First, additional optical elements (e.g., reflective mirrors, lenses, and DMs) must be introduced into the system for the second-stage SFM, which increases the system's complexity and instability. Second, the UV optical components are usually more expensive than those for NIR. Furthermore, the coating performance of the UV optics in terms of reflectance, transmission, and damage threshold is inferior to that for NIR optics. More so, extra efforts are required to perfectly overlap the two pump beams in the second-stage SFM temporally and spatially. Therefore, a more compact and simpler configuration is preferred, without introducing significant inefficiency into the system.
Inspired by the collinear cascaded frequency-tripling scheme [18], which is used for highpower extra-cavity 355 nm source generation, and collinear fifth-harmonic generation of 1 μm lasers in both "ω+4ω" and "2ω+3ω" configurations [19,20], we propose to extend this idea to a system to generate a 193 nm wavelength by constructing a collinear cascaded dualstage SFG configuration. Concretely, the combination of type-II CLBO and type-I CLBO in tandem, or with a dual-wavelength wave plate (DWP) introduced in between, are both possible in a collinear cascaded dual-stage SFG configuration, which can address the aforementioned problems and lead to a compact and robust DUV-generation scheme.
In this manuscript, building on our previous system, we thus use a collinear cascaded twostage SFM scheme to reduce size and cost. Two configurations were tested to verify this proposal. The first configuration involves inserting a DWP between the first and second SFM stages of our previous system. The second configuration uses a CLBO crystal cut at a different angle to enable type-II phase matching to generate a 221 nm source [1553 nm (e) + 258 nm (o) → 221 nm (e)]; a setup that avoids using a DWP. Both setups work well for generating the 193 nm sources; however, less average power is obtained compared with the more traditional setup (0.41 W and 0.7 W, versus 0.77 W). This is due to the imperfect spatial overlap, originating from their relative spatial walk-off in the first-stage SFM, of 1553 nm and 221 nm beams inside the second-stage SFM. Simple experiments were carried out to visualize the walk-off between the 1553 nm and 258 nm beams as an indirect proof for the walk-off between 1553 nm and 221 nm beams. These experiments show that, we obtain a much more compact and cost effective system by simply using CLBO crystals cut at a different angle (power at 193 nm is only ~9% less than that obtainable with the conventional system). Finally, we propose a dual-crystal system to compensate for the spatial walk-off but do not verify this approach herein. This work thus demonstrates a collinear, dual-stage cascaded SFM configuration for generating coherent radiation at 193 nm. Figure 1 shows the schematic diagram of the dual-stage collinear cascaded SFM configuration for the generation of 193 nm sources, which comprises of four parts: Part-(a) is a pulsed 1030 nm fundamental laser. It starts from a narrow-linewidth pulsed distributed feedback laser diode (LD) with a pulse duration of 10 ns and a repetition rate of 160 kHz [17]. Next, a fiber-coupled acousto-optic modulator serves to lower the pulse repetition rate down to 10 kHz. A photonic crystal fiber with a core diameter of 40 μm is used as a preamplifier and two Yb:YAG ceramic rods as the main amplifiers. This setup differs slightly from that of our previous system [17], where a single-crystal fiber (SCF) was used as the final-stage main amplifier. Replacing the SCF with the ceramic amplifiers allows us to maintain the performance of the previous system while reducing costs. Part-(b) of the system is a pulsed 1553 nm fundamental laser with a repetition rate of 10 kHz, which is synchronized, by sharing a common pulse function generator, to the 1030 nm laser system of part-(a) and delivers an average power up to 1.5 W. Part-(c) is the SHG system at 515 nm, with a noncritically phase-matched LBO crystal (5 × 5 × 30 mm 3 ) that operates at 186.5 °C, and a fourth-harmonic generation (FHG) system at 258 nm that uses a CLBO crystal (5 × 5 × 20 mm 3 ), operated at 155°C, and is sealed inside a cell purged with Ar gas to avoid hydration. Finally, part-(d) is a dual-stage collinear cascaded SFM configuration for generating a 193 nm beam and uses two 5 × 5 × 20 mm 3 CLBO crystals operated at 155°C and sealed inside a cell purged with Ar gas. A calcium fluoride (CaF 2 ) prism serves to separate the generated 193 nm beam from the other beams.

All-ceramic 1030 nm Yb:YAG amplifier and its performance for second-and fourth-harmonic generation
After replacing the SCF used in the previous system, as described in Ref [17], with a Yb:YAG ceramic thin rod as the main means of amplification for the 1030 nm laser, the overall performance was characterized using the performance of the SCF (doping concentration = 1%, diameter = 1 mm, length = 40 mm) as a benchmark. The Yb:YAG ceramic thin rods (from Konoshima Chemical Co., Ltd.) with different doping concentrations (1% and 2%), lengths (30 mm and 40 mm), and diameters (1 mm and 2 mm) were tested. The results are shown in Fig. 3 and are for a same seed laser. A fiber-coupled 940 nm LD from nLIGHT Photonics was used as pump, where the fiber core diameter was 105 μm and the NA was 0.22. Both ceramic rods (one with 1% doping, 1 mm diameter, and 40 mm long, and a second with 2% doping, 1 mm diameter, and 30 mm long) gave outputs similar to that of the SCF Taranis module. The use of the 2-mm-diameter, 30-mm-long Yb:YAG ceramic rod with 2% doping significantly improves the output power. At present, we cannot explain this result, but we note that another group has reported similar observations in SCF amplifiers [21].
Because of its noticeably improved performance, we used the 2-mm-diameter, 30-mmlong Yb:YAG ceramic rod with 2% doping for subsequent experiments. For SHG, over 22 W was obtained by using a LBO crystal, as shown in Fig. 4(a). The 1030 nm laser was not used at the maximum power. For FHG, nearly 7 W was obtained by using a CLBO crystal, as shown in Fig. 4(b). For both SHG and FHG, the conversion efficiency was not optimized to keep the system safer. Indeed, a maximum average power of 8.5 W at 258 nm beam was obtained with an average power of 22 W at 515 nm beam, which corresponds to a conversion efficiency of 38.6%. Experimentally, we observed that, when the conversion efficiency approached 40%, the CLBO crystal was more prone to damage. We deemed an average power of 7 W at 258 nm beam sufficient for subsequent measurements. During the whole experimental intentionally t     Figure 6(f) s ) and 258 nm in Fig. 6(b); th ystal was repla n the beams is condition). Fi ed. This separa nt. When the H 6(h)] because y, we conclud s limits the co of Fig. 2

Further possible improvements?
As shown herein, the efficiency of the collinear-cascaded SFM configuration is limited by the spatial walk-off between 1553 nm and 221 nm beams in the first CLBO crystal. If this could be compensated, better performance could be expected. Figure 9(a) shows a schematic diagram of the beams propagating inside the CLBO-2 crystal. Here, we propose a scheme inspired by Ref [22] and shown in Fig. 9(b). Two shorter crystals with the optical axes arranged as mirror images of each other are placed together or bonded to correct for the direction of the Poynting vector so that the 1553 nm and 221 nm beams overlap, thereby allowing a perfect spatial overlap in the following CLBO crystal for the 193 nm beam generation. In the future, we hope to improve the performance by using such a scheme. Fig. 9. Proposed method for compensating the spatial walk-off between the 1553 nm and 221 nm beams for more efficient generation of the 193 nm radiation.

Summary
In conclusion, we have developed and experimentally demonstrated a cascaded collinear SFM configuration to generate coherent radiation at the wavelength of 193 nm. The configuration is more compact and cost effective than traditional configurations, albeit at the expense of a slightly lowered conversion efficiency (~9% lower). We have also developed a 1030 nm laser amplifier based on all-ceramic Yb:YAG thin rods. Finally, we propose possible solutions to compensate for the spatial walk-off as an avenue for further improvements.

Funding
New Energy and Industrial Technology Development Organization (NEDO).