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In this paper we report on the realization of a deep-UV light source using the 1.3 μm transition of neodymium as pumping wavelength. The 191.7 nm radiation was obtained by generating the seventh harmonic of a high-power Q-switched 1342 nm Nd:YVO4 laser. A cesium lithium borate crystal was used for sum frequency mixing of the sixth harmonic and the fundamental. With a total of four conversion stages, up to 240 mW were achieved, with excellent beam quality at 155 mW (M2 < 1.7) and 190 mW (M2 < 1.9).
© 2014 Optical Society of America
1. Introduction
Coherent radiation in the deep-UV spectral range below 200 nm is of great interest for applications such as spectroscopy, metrology but also for inducing refractive index changes in dielectric media. A prominent example for the latter is the writing of fiber Bragg gratings with 193 nm ArF excimer lasers [1, 2].
In the recent decades several concepts for the realization of an all solid-state deep-UV laser source have been investigated since excimer lasers lack a sufficient beam quality and have a short maintenance interval.
An early concept was the fourth harmonic generation (4HG) of Ti:Sapphire lasers in three steps: second harmonic generation (SHG), third harmonic generation (THG) and finally sum frequency mixing (SFM) of the fundamental and the third harmonic using β -bariumborate (BBO) [3]. By using potassium fluoro-beryllo-borate (KBBF), which even has lower absorption compared to BBO, 4HG below 200 nm in two steps is possible via SHG of the second harmonic [4, 5], with Kanai et al. demonstrating Watt-level power in a complex picosecond MOPA setup [5].
Another concept was the eighth harmonic generation of a narrow-band diode laser and fiber amplifier setup [6, 7]. With five conversion stages Kawai et al. generated 140 mW at 193 nm using a cesium lithium borate (CLBO) crystal for the last SFM stage [7].
Sakuma et al. presented a complex hybrid setup consisting of Q-switched single-mode Nd:YLF and Ti:sapphire MOPA systems. By cascaded SFM in CLBO up to 1.5 W at 196 nm were achieved [8]. Sakuma et al. also reported on a continuous wave (CW) source at 193.4 nm by cascaded SFM of three fiber amplifiers generating approximately 12 mW [9]. Recently, Scholz et al. presented a CW source at 193 nm which obtained over 15 mW by 4HG of a diode laser MOPA in two steps using KBBF in the last stage [10].
However, due to the reliability of neodymium doped lasers, SFM based on the 1064 nm transition was also an early concept. By mixing the fifth harmonic of a Q-switched 1064 nm laser with the 2 μm output of an optical parametric oscillator (OPO), radiation below 200 nm was generated by Borsutzky et al. using lithium triborate (LBO) [11]. Hamilton et al. reported on a single-frequency Q-switched setup which provided 46 mW at 193 nm by SFM of 2 μm from a difference frequency generation (DFG) stage and 213 nm in LBO. The DFG stage was pumped by an injection seeded, 532 nm pumped OPO [12]. Umemura et al. presented results on a setup which generated 220 mW at 193 nm based on a 1064 nm Nd:YAG laser. The output of a 532 nm pumped OPO was mixed with the residual 532 nm radiation to 236 nm in CLBO and finally mixed with the fundamental in K2Al2B2O7 [13]. Jacob et al. realized a setup generating 2.5 mW at 193.4 nm by using a frequency doubled Nd:YAG laser as pump source. The output of a 532 nm pumped OPO at 708 nm is mixed with 266 nm in BBO [14]. Subsequently, Merriam et al. used cooled BBO for SFM of 266 nm and 709 nm to 193.4 nm obtaining 35 mW [15].
In this paper we report on an all solid-state deep-UV light source at 191.7 nm by using the1.3 μm transition of neodymium as pumping wavelength for the first time to our knowledge. This allows the generation of deep-UV radiation below 200 nm by seventh harmonic generation (7HG) without a parametric conversion stage but still using reliable neodymium laser technology in a robust and straightforward setup. This is especially interesting for applications which do not require a specific deep-UV wavelength below 200 nm.
2. Experimental setup
The experimental setup, depicted in Fig. 1, consisted of a Q-switched 1342 nm laser and four conversion stages, which subsequently generated the higher harmonics.
The homemade, 888 nm pumped, 1342 nm Nd:YVO4 laser, was actively Q-switched by an acousto-optic modulator (AOM, 15 W high frequency power). The resonator consisted of four mirrors (M1 – M4) and a thin-film polarizer (TFP). A quarter-wave plate in combination with the TFP provided adjustable output coupling. The 888 nm pump radiation was focused to a radius of approximately 600 μm inside of the laser crystal which was a 0.5 at.%-doped Nd:YVO4 crystal with a length of 30 mm and an aperture of 4 × 4 mm2 (888/1064/1342 nm anti-reflection (AR) coating). The pump mirror M2 was a zero-lens convex mirror (r = 300 mm) being coated for high reflectivity (HR) at 1342 nm and high transmission (HT) at 888 nm just as mirror M3. Mirrors M1 and M4 were coated for HR at 1342 nm with M4 being a convex mirror (r = 200 mm) and M1 being a concave mirror (r = − 2000 mm). The length of the cavity was 26 cm and the highest peak power was reached for an output coupling of about 41 %. The laser operated at 10 kHz pulse repetition rate and provided an average power of 15.2 W.
After two mirrors (M5 and M6, HR 1342 nm) and a collimating lens L1, a half-wave plate and a TFP served as a variable attenuator. A telescope consisting of two lenses (L2 and L3) focused the laser radiation to a radius of 185 μm in x-direction and 174 μm in y-direction inside of the SHG crystal. After three mirrors (M7 – M9, HR 1342 nm) and a half-wave plate an average power of 13.3 W was available for SHG. The SHG crystal, a non-critically phase-matched (NCPM) bismuth triborate (BiBO) crystal (θ = 0°, ϕ = 0°, Type I), had a length of 15 mm and an aperture of 3 × 3 mm2 (671/1342 nm AR coating). The temperature for phase-matching was stabilized around 244 °C. In spite of this high temperature, we did not observe any damage to the AR coating by dilatation of the crystal. For THG, the 1342 nm and the 671 nm beam were focused with lens L4 into a NCPM LBO crystal (θ = 0°, ϕ = 0°, Type II). The radius of the 1342 nm beam was 168 μm in x-direction and 170 μm in y-direction. The radius of the 671 nm beam was 123 μm in x-direction and 129 μm in y-direction, the radii being measured at low pump power. The length of the THG crystal was 20 mm and its aperture was 3 × 3 mm2, the entrance facet being AR coated at 671/1342 nm and the exit facet at 447/671 nm. The temperature of the THG crystal was stabilized around 174 °C. Two dichroic mirrors (M10 and M11) were used to separate the 1342 nm, 671 nm and 447 nm beams. After the SHG stage up to 8.7 W at 671 nm were available. Up to 7.25 W at 447 nm were achieved after the THG stage, the temperatures of the SHG and THG crystal being optimized for maximum THG.
After a lens (L5), four mirrors (M12 – M15, HR 447 nm) and a window (W2) about 6 W at 447 nm were available for sixth harmonic generation (6HG). The 6HG crystal is a critically phase-matched BBO (θ = 64.5°, ϕ = 0°, Type I) with a length of 12 mm and an aperture of 3 × 10 mm2, the entrance facet being AR coated at 447 nm and the exit facet being cut at Brewster’s angle. The temperature of the crystal was stabilized around 65°C. The 224 nm and the 447 nm beam were separated by a CaF2 prism (P1). Since the generated power at 224 nm saturated at high pump powers, we limited the incident power at 447 nm to approximately 4.4 W by using a polarization bypass of 25 %. The bypass consisted of a half-wave plate at 1342 nm in front of the SHG crystal and a TFP after mirror M11. Hence the incident 1342 nm power at the 7HG crystal was not influenced by the SHG and THG conversion stage. The final setup used a converging beam with a radius of 307 μm in x-direction and 270 μm in y-direction inside of the 6HG crystal, measured at 4.4 W power at 447 nm. By using a bypass of 25 %, up to 525 mW at 224 nm were achieved.
The 1342 nm and the 224 nm beam were overlapped with a CaF2 prism (P2). At the 7HG crystal, which was a critically phase-match CLBO (θ = 67.5°, ϕ = 45°, Type I, uncoated) with a length of 10 mm and an aperture of 5 × 5 mm2, a power of up to 515 mW at 224 nm was available. The temperature of the CLBO was stabilized around 165°C. At full power, the 224 nm beam had a radius of 154 μm in x-direction and 165 μm in y-direction at the position of the 7HG crystal. After two lenses (L6 and L7), three mirrors (M16 – M18, HR 1342 nm), a window (W1) and a prism (P2), a bypassed power of up to 2.68 W at 1342 nm was usable for the 7HG stage with a radius of about 200 μm in both directions. Finally, the 191.7 nm, the 224 nm and the 1342 nm beam were separated with a CaF2 prism (P3). The generation of the sixth and seventh harmonic was performed in a sealed box which was purged with argon to avoid degradation of crystals and prisms by generated ozone.
3. Results and discussion
In the following subsections, detailed results concerning the fundamental laser and the SHG, THG, 6HG and 7HG conversion stages are presented.
3.1. 1342 nm fundamental laser
The output power of the fundamental 1342 nm laser depending on the absorbed pump power at 888 nm is shown in Fig. 2(a). The maximum output power of 15.2 W was reached for an absorbed pump power of approximately 98 W resulting in an efficiency of 15.5 %. The slope efficiency of the setup was 30.6 % at high pump powers. The high lasing threshold of 88.6 W is caused by the strong thermal lensing of the 1342 nm transition in Nd:YVO4 and the large difference of the thermal lens for lasing and non-lasing operation which has been investigated in detail by Lenhardt et al. in a continuous wave setup [16]. The exact emission wavelength of the laser was 1341.72 nm with a spectral width of approximately 29 GHz. The laser emitted 16.3 ns long pulses with low pulse energy fluctuations of σ < 1 %. Hence the setup provided a peak power of 87.5 kW, which promised a high conversion efficiency in the nonlinear conversion stages. The M2 measurement at full power is shown in Fig. 2(b) resulting in a beam propagation factor of M2 < 1.1 in both directions. The inset in Fig. 2(b) shows a Gaussian shaped beam profile (measured with Electrophysics MicronViewer 7290a).
Fig. 2 (a) Characteristic of the average power of the fundamental 1342 nm laser at 10 kHz pulse repetition rate. (b) M2 measurement and beam profile of the 1342 nm laser at 15.2 W output power.
3.2. 671 nm SHG stage
The generated power of the SHG stage is shown in Fig. 3(a). A power of 8.73 W at 671 nm was achieved from 13.3 W at 1342 nm, which corresponded to a conversion efficiency of 65.6 %. The pulse duration decreased to 15 ns whereas the pulse energy fluctuations increased to σ < 2 %. The results of the M2 measurement are illustrated in Fig. 3(b). Due to NCPM the beam quality stays diffraction limited with a beam propagation factor M2 = 1.1. Likewise the beam profile remains Gaussian shaped with an ellipticity of 0.99. The phase matching curve for SHG is shown in Fig. 4(a). Maximum SHG occurred for a temperature of the SHG crystal of 244.2 °C, the temperature bandwidth being 1.4 °C full width at half maximum (FWHM). The asymmetry of the phase matching curve and its deviation from the sinc2-characteristic was caused by thermal effects at high pump powers which has been theoretically investigated by Okada et al. [17, 18].
Fig. 3 (a) Power characteristics of the SHG conversion stage. (b) M2 measurement and beam profile of the second harmonic at a power of 8.73 W.
Fig. 4 (a) Phase matching curve for SHG from 1342 nm to 671 nm in BiBO. (b) Phase matching curve for THG (SFG from 1342 nm and 671 nm to 447 nm) in LBO. (c) Dependence of the generated power at 447 nm on the temperature of the SHG crystal. For details see text.
3.3. 447 nm THG stage
The phase matching curve for THG is illustrated in Fig. 4(b). The curve was asymmetric for the same reason as the SHG phase matching curve. Maximum THG was achieved for a temperature of the THG crystal of 174.44 °C. The temperature bandwidth was 0.8 °C FWHM. However, for optimum THG, not only the THG temperature has to be optimized, but also the temperature of the SHG crystal, which is shown in Fig. 4(c). It was found that when the SHG temperature was tuned for maximum power at 671 nm there was a local minimum for the power at 447 nm. By detuning the SHG temperature the efficiency in THG increased. Due to the strong asymmetry of the curve, this behavior cannot solely be explained by an improvement of the beam quality of the residual fundamental beam and the adjustment of the power ratio of the fundamental and the second harmonic. The asymmetry is caused by self-focusing and self-defocusing of the fundamental beam in the SHG crystal depending on the sign of the phase mismatch. This focusing/defocusing effect is caused by cascaded second-order effects in the phase mismatched SHG crystal resulting in an effective Kerr lens, which has been investigated by DeSalvo et al. for potassium titanyl phosphate using the Z-scan technique [19]. Optimum power and beam quality at 447 nm was achieved for a SHG temperature of 244.37 °C. A detailed analysis of this self-focusing effect in our setup is subject to current investigations.
The power characteristics of the THG stage are shown in Fig. 5(a). A power of 7.25 W at 447 nm was achieved from 13.3 W at 1342 nm, which corresponded to a conversion efficiency of 54.5 %. The pulse duration was 15.4 ns and the pulse energy fluctuations increased to σ < 3 %. The results of the M2 measurement are shown in Fig. 5(b). The THG stage provided a very good beam quality with a beam propagation factor M2 < 1.3. The inset of Fig. 5(b) shows a Gaussian shaped beam profile with an ellipticity of 0.92.
Fig. 5 (a) Characteristic of the average power of the THG conversion stage, the temperatures of the SHG and THG crystal being optimized for maximum THG. (b) M2 measurement and beam profile of the third harmonic at a power of 7.25 W.
3.4. 224 nm 6HG stage
For 6HG, a converging beam at 447 nm was used in avoidance of an additional lens for the 224 nm beam in front of the 7HG stage. However, due to the effective Kerr lens in the SHG crystal (see section 3.3) the beam radius of the 447 nm beam inside of the 6HG crystal is dependent on the pump power of the THG stage. For better visualization, the geometric mean (x- and y-direction) of the 447 nm beam radius for three different positions (A–C) of lens L5 is shown in Fig. 6(a) in dependence of the power at 447 nm. The distance between the THG crystal and lens L5 was increased from A to C. The beam radius increases from 275 μm at low power to 460 μm at high power for lens position A. Likewise the beam radii change from 225 μm to 375 μm for lens position B and from 180 μm to 285 μm for lens position C. The power characteristics and beam profiles at 224 nm are shown in Fig. 6(b). The beam profiles were measured with an UV image converter and a CCD camera. Because of absorption of the 224 nm radiation, the temperature of the 6HG crystal was optimized for each pump power separately. At high pump powers, the generated power at 224 nm saturated and in case of setup B and C decreased again. On the one hand, this saturation effect was caused by the increase of the beam size inside of the 6HG crystal at high pump powers. On the other hand, the conversion efficiency at high pump power was also affected by dephasing in the 6HG crystal caused by absorption of the 224 nm beam. The marginal increase in 224 nm power at high pump power (5–6 W) can be attributed to the latter. For lens position A, a power of 515 mW with a Gaussian shaped beam profile was achieved at 6.25 W power at 447 nm, the beam profile’s ellipticity being 0.97. This corresponded to a conversion efficiency of 8.2 % from 447 nm to 224 nm. A power of 570 mW was achieved with setup B, using a power of 4.33 W at 447 nm. The conversion efficiency was 13.1 % and the beam profile was Gaussian shaped with an ellipticity of 0.89.
Fig. 6 (a) Dependence of the beam radius of the 447 nm beam inside of the 6HG crystal on the power at 447 nm for three different positions of lens L5. The distance between the THG crystal and lens L5 was increased from A to C. (b) Average power and beam profiles at 224 nm for three different beam radii inside of the 6HG crystal.
For lens position C, a maximum power of 655 mW from 3.1 W at 447 nm was generated, which corresponded to a conversion efficiency of 21.1 %. By further increase of the pump power the beam profile at 224 nm severely degrades due to thermal stress in the crystal. The 6HG crystal had close contact to its mount with only two adjacent sides (L-shaped mount) and was pressed by springs from the other sides. Hence, the additional heat load from the 224 nm beam was not spread symmetrically in the crystal. Therefore, the transverse temperature gradient was asymmetric causing the asymmetry of the beam profile and its distortion at high pump power. The beginning of this behavior was already observable at a pump power of 2.7 W at 447 nm generating over 600 mW at 224 nm by analysing a sectional view of the beam profile. At 2.3 W at 447 nm, the 6HG stage generated 560 mW at 224 nm (24.3 % efficiency) with a Gaussian shaped beam profile, the ellipticity being 0.68.
As a compromise between efficiency and ellipticity of the 224 nm beam we chose setup B and limited the incident power at 447 nm to approximately 4.4 W by using a polarization bypass of 25 %. This also had the advantage that the 7HG stage could use the bypassed 1342 nm beam, which was unaffected by the SHG and THG stage and provided excellent beam quality instead of converting the residual 1342 nm radiation after THG. The power characteristics of the 6HG stage with polarization bypass is shown in Fig. 7(a). The temperature of the 6HG crystal was adjusted from 67.5 °C at low pump power to 63.1 °C at high pump power. A maximum power of 525 mW was obtained which corresponded to a conversion efficiency of 11.9 % from 447 nm to 224 nm. Additionally, the beam profile was Gaussian shaped over the whole power range, the ellipticity being 0.92 at full power. The results of the M2 measurement at 525 mW are shown in Fig. 7(b). There was a diffuse scattered background around the attenuated beam profile, which was focused into the actual beam nearby the focus and complicated the M2 measurement. We could remove the majority of the background with an aperture in front of the focusing lens of the M2 measurement but there was some residual influence observable around the waist. Nevertheless, an M2 fit was possible and resulted in a beam propagation factor of M2 < 1.3 for both axis. The pulse duration was 12.1 ns and the pulse energy fluctuations were σ < 7 %.
Fig. 7 (a) Average power and beam profiles at 224 nm for a polarization bypass of 25 %. (b) M2 measurement and beam profile of the sixth harmonic at power of 525 mW.
3.5. 191.7 nm 7HG stage
Since the beam diameter in the 6HG crystal depended on the pump power of the THG stage, the beam radius at the 7HG crystal also changed with the power at 224 nm. By increasing the pump power, the beam radius in x-direction decreased from 260 μm to 154 μm and in y-direction from 192 μm to 165 μm. The beam radius of the 1342 nm beam was almost constant (190–200 μm) being uninfluenced by the preceding conversion stages. For a polarization bypass of 25 %, 222 mW at 191.7 nm were obtained using a total pump power of 13.3 W. However, the beam quality suffered above a total pump power of 8 W because of thermal stress in the 7HG crystal, which resulted in an asymmetric beam profile. As explained in section 3.4, the cause for this behavior is the crystal mount having direct contact to the crystal with two facets. At a total pump power of 8 W, 155 mW at 191.7 nm were achieved with a Gaussian shaped beam profile and a beam propagation factor M2 < 1.7 in x-direction and M2 < 1.6 in y-direction.
To determine a working point as a trade-off between output power and asymmetry of the beam profile, we investigated the influence of the bypass value on the power and beam profile at 191.7 nm. We chose a total pump power of 9.45 W in order to possibly increase the power at 191.7 nm with a good beam quality. The results can be seen in Fig. 8. The generated power at 191.7 nm was almost constant for a bypass value between 22 % and 35 %. However, the shape of the beam profile was dependent on the bypass. As a compromise between power and asymmetry of the beam profile we chose a bypass value of 38 % for the further experiments.
The generated power at 191.7 nm for a polarization bypass of 38 % is shown in Fig. 9(a). The temperature of the CLBO was adjusted from 167.4 °C at low power to 165 °C at high power to compensate additional heating by absorption. By SFM of 490 mW at 224 nm and 4.06 W at 1342 nm (total pump power of 13.3 W at 1342 nm) a power of 240 mW at 191.7 nm was obtained. The overall efficiency of the conversion unit was 1.8 %. Moreover, the conversion efficiency was 49 % with respect to the incident power at 224 nm. The beam quality decreased above a total pump power of 9.45 W which becomes obvious from the insets of Fig. 9(a). At a total pump power of 9.45 W at 1342 nm (401 mW at 224 nm and 2.65 W at 1342 nm incident on the 7HG crystal), 190 mW at 191.7 nm were achieved leading to a 2 % overall efficiency of the conversion unit. With regard to the incident power at 224 nm, the conversion efficiency reached 47.4 %. The results of the M2 measurement, conducted at 190 mW, are shown in Fig. 9(b). The beam profile was Gaussian shaped and the beam propagation factor was M2 < 1.9 in x-direction and in y-direction. Therefore, the setup, operating at a bypass value of 38 %, still provided a good beam quality with an increased power of 190 mW at 191.7 nm.
Fig. 9 (a) Power and beam profiles at 191.7 nm for a polarization bypass of 38 %. (b) M2 measurement and beam profile for 190 mW at 191.7 nm (polarization bypass of 38% with 9.45 W total pump power at 1342 nm).
The pulse traces of the fundamental beam and the SHG, THG, 6HG and 7HG beams are shown in Fig. 10. The pulses were measured with a fast photo diode (rise time < 200 ps) and an oscilloscope (1 GHz bandwidth). Mode-beating peaks, which are typical for a multi-longitudinal mode Q-switched laser, are clearly visible in the pulse traces. The pulse duration at 191.7 nm was 12.1 ns and the pulse energy fluctuations were σ < 10 %.
Although our setup was constructed on an optical table and not within a sealed aluminum housing, it was very stable, requiring a realignment only once after six month including the oscillator. After a warm-up of half an hour our system provided a very good reproducibility of the output power. However, the system was sensitive to changes in ambient conditions. The temperatures of the SHG and THG crystal had to be checked after the warm-up with small adjustments (0.1 °C). A sealed housing for the whole system would avoid this behavior. During all our experiments, the CLBO crystal was in the argon filled box and additionally kept at a temperature of 165 °C to avoid hygroscopic degradation [20], which we did not observe over several months. The UV generation within the argon filled box also eliminated degradation of the facets (crystals and prisms) via generated ozone, which would be the case for normal atmosphere. During our characterization of the setup, we did not observe any degradation in the SHG and THG stage and the deep-UV stages. However, depending on the power densities and crystal qualities, degradation of the UV crystals will occur in the long term [21] and the beam positions in the crystals will have to be changed.
4. Conclusion
In conclusion, we have presented a deep-UV light source based on a high-power, Q-switched 1342 nm Nd:YVO4 laser leading to a wavelength of 191.7 nm with four conversion stages. By using a polarization bypass in front of the SHG crystal, we optimized the power ratio between the sixth harmonic and the bypassed fundamental beam, incident on the 7HG stage, with respect to the power and beam quality at 191.7 nm. For a bypass of 38 %, a power of up to 240 mW was achieved corresponding to an overall efficiency of 1.8 %. At a total pump power of 9.45 W, we obtained a power of 190 mW (2 % overall efficiency) with a Gaussian shaped beam profile and M2 < 1.9. An even better beam propagation factor M2 < 1.7 was reached with a bypass of 25 % and a total pump power of 8 W resulting in 155 mW at 191.7 nm. Finally, we have demonstrated a novel concept for generating coherent radiation in the spectral range below 200 nm using mature neodymium laser technology in a stable, direct 7HG setup without a parametric or Ti:Sapphire interstage.
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