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Abstract
This paper reports on the generation of a 100 MHz repetition rate, 1.7 mW average power and femtosecond deep-ultraviolet (DUV) 243 nm laser source. The infra-red output of a broadband Titanium-Sapphire (TiSa) laser containing 729 nm light is mixed with its second harmonic in a β-barium borate (BBO) crystal. By manipulating the group delay dispersion (GDD), we customize the spectral shape of TiSa resonator to improve conversion efficiency. This DUV laser is employed for direct frequency comb spectroscopy of hydrogen.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Precision spectroscopy of hydrogen plays a central role in tasks such as testing Quantum Electrodynamics (QED), measuring the values of fundamental constants, and solving the proton charge radius puzzle [1]. Researches in this field have been carried out since 1980s [2,3]. One of the difficulties in these researches is to generate enough power in DUV band. Frequency up-conversion of continuous wave (CW) with second harmonic generation (SHG) and sum frequency generation (SFG) in nonlinear crystals is an optional choice [4–6]. However, restricted by poor conversion efficiency, the continuous-wave schemes require laser source of high power and subtle cavities. Additional amplifier is required. The narrow coverage and modulation bandwidth also restrict their application to measurement of different transitions.
Mode-locked lasers with high peak power and corresponding efficient nonlinear conversion make it easier to generate source in hard-to-reach spectral regions. The use of pulsed lasers can also avoid the photo-refractive effect [7], which is one of the main limiting effects in DUV generation. Pulsed-laser-drived precision spectroscopy in DUV becomes a good choice.
Output of a mode-locked laser can be understood as regularly spaced narrow frequency modes in frequency domain, which is vividly named frequency comb. The comb teeth contribute pairwise to a two-photon transition. In a two-photon transition drived by a pulse train, the population of upper level in atomic has a ladder-shape increase along with the number of accumulated pulses and can reach a top comparable to that of two-photon transitions drived by CW laser with same average power [8]. Direct frequency comb spectroscopy of hydrogen has recently been demonstrated [9,10]. It is shown that the accuracy of the 1S–3S transition frequency measured with pulsed laser is comparable to that measured with CW laser.
The 1S–2S 2 photon transition frequency is most accurate of any in hydrogen [11] and has been measured using CW or quasi-CW laser system. This transition excited by picosecond (ps) or femtosecond (fs) mode-locked lasers has not yet been reported. In recent years, mode-locked lasers are progressing towards ultra-broadband, extremely high power, high repetition rate, low phase noise and programmable spectral shape [12–16]. These new techniques make it possible to generate light of particular wavelength from ultraviolet radiation to extreme ultraviolet radiation [17,18].
In this paper, we demonstrate a fs mode-locked laser source at 243 nm for hydrogen 1S–2S spectroscopy. Considering gain of TiSa crystal at 729 nm is higher than that at 972 nm, we choose a SHG-SFG process rather than a SHG-SHG process. Compared with former realization of 243 nm laser, this configuration can cover more frequency in DUV region and needs no additional amplifier, making the system simpler and more flexible.
2. Experimental setup
2.1 TiSa resonator
We build a fs TiSa mode-locked laser as the fundamental light source. As shown in Fig. 1(a), the TiSa laser is formed by a standard x-folded resonator. The gain medium is a Brewster-cut TiSa crystal that is set between two concave mirrors with a radius of curvature of 75 mm placed at the astigmatism compensating angle. The crystal is mounted on a chunk of gold-plated copper that is cooled to 20°C by circulating water. The thickness of the TiSa crystal is 1.8 mm and the absorption coefficient is 5.0 cm−1. The reflectivity of output coupler is 95%. The cavity of the resonator is 150 cm long, corresponding to 100 MHz repetition rate.
Fig. 1. (a) Setup of TiSa fs laser used to generate fundamental pulses. M3 and M4 are concave chirped mirror pairs with −50 fs2 per bounce. M5, chirped mirror with −50 fs2 per bounce. M6, reflector with low group delay dispersion. OC, output coupler. PZT, piezoelectric transducer. PBS, polarization beam splitter. λ/2, half wave plate. AOM, acousto-optic modulator. (b) Relationship between spectrum and arm length ratio of the laser. Long arm corresponds to M3–M5–OC. Short arm corresponds to M4–M6.
It has been proven, that an octave-spanning spectrum can be generated directly from a prismless TiSa laser with precise dispersion compensation over a wide bandwidth [19,20]. To generate enough power at 729 nm, the spectrum of resonator’s output should be optimized. This can be achieved by manipulating the value and distribution of the GDD in the cavity.
In this experiment, the group delay dispersion is suppressed to −21 fs2 by selected chirped mirror (M5) and double-chirped mirror pairs (M3, M4). All mirrors in the cavity are standard products from Layertec. The dispersion can be further compensated by inserting a pair of thin fused-silica wedges. However, the reflecting interface of wedges will reduce the output power distinctly, which is unfavorable for our goal.
The distribution of GDD in the cavity can be adjusted by changing the arm length ratio of the cavity. Figure 1(b) shows that the power of the output spreads out among the spectrum as the long-arm to short-arm length ratio decreases. There is a configuration that comb lines near 729 nm get the largest power, with an arm length ratio near 1.71.
2.2 Generation of 243 nm pulses
The setup to generate 243 nm pulses is shown in Fig. 2. For the frequency doubling stage (729 nm → 364.5 nm), we couple the output of TiSa laser into a 6-mirror ring enhancement cavity. A 5 mm × 5 mm × 2 mm BBO crystal is placed between two concave mirrors with a radius of curvature of r = 75 mm. The phase matching angle for SHG at 364.5 nm in the crystal is θBBO = 32.2°. The BBO crystal is held in a copper mount that is attached to a Peltier element to cool the crystal to 20°C. The input coupler has 75% reflectivity from 700 nm to 900 nm. The remaining mirrors are high reflective for the fundamental pulses (R > 99.8%).
Fig. 2. Setup for SHG and SFG. M1, M3, M6–M12, reflector with low GDD. M4, M5, concave mirrors. M2, input coupler. M13, dichroic mirror. L1, L2, mode-matching lens. L3–L5, focus lens. λ/4, quarter wave plate. DPD, difference photo diode. PID, proportional-integral-derivative controller. BS, Beam splitter.
The output of our TiSa fs laser contains up to 106 modes in frequency domain. To ensure an efficient buildup in the cavity, all modes of optical frequency comb should be enhanced. In time domain, this requires that the pulse after travelling one round trip in the cavity must be overlapped with the next pulse exactly. The envelope and the phase of the overlapped pulses should be identical [21]. In our setup, the length of the enhancement cavity is set to be 3 m so that the free spectral range (FSR) of the cavity is the same as the repetition rate of the TiSa laser. The cavity is locked to resonate with the modes of the laser by using the polarization method [22] and feeding back on a piezo-mounted plane folding mirror (M6). To reduce the phase shift introduced by the cavity, all mirrors have low GDD.
The generated 364.5 nm UV light pass through one of the curved mirrors (T > 90%) and is focused by a pair of lenses. Ninety percent of the fundamental laser is directed by low GDD mirrors M8–M12 and is focused by lens L5 to participate the SFG process. M10 and M11 are mounted on a translation stage so that the length of light path is tunable. The polarization is adjusted to be the same with the UV light by a half wave plate. A dichroic mirror (M13) is used to combine two beams together. The BBO used here is 2 mm long and the phase matching angle is θBBO = 50.8°. The SFG stage occurs when pulses containing 364.5 nm and 729 nm light overlap inside the BBO crystal both in time and in space.
3. Experimental results
3.1 Output of the laser system
The whole output power of TiSa fs laser is 1.2 W, with a 210 mW light around 729 nm integrated in a 10 nm bandwidth. The pulse full width at half-maximum (FWHM) duration is 27 fs. This corresponds to the configuration where the power around 729 nm is maximized. Fifty mW of the output is used for frequency locking. The repetition rate is locked to a 100 MHz RF signal and a comb tooth is stabilized on a rubidium saturated absorption frequency. The remaining power is used for DUV laser generation. We obtain 58 mW average power at 364.5 nm through one of the curved mirrors (T > 90%) for about half an hour. Influenced by the heating effect of the crystal and photochemical reactions at the surface, the power drops obviously in the long run [23]. This phenomenon is more severe in SFG stage. We finally obtained 1.7 mW at 243 nm light for about 12 min.
Figure 3 shows the spectral range of the laser system. Coverage of the final output is estimated to be 10 nm. By changing the configuration of TiSa laser and phase matching angle of BBO crystal, the output of the system can be easily shifted to other wavelength wanted in DUV [24]. The power and duration of the DUV light can be further improved in future with the following steps: (i) setting the SHG-SFG unit into a vacuum chamber can reduce photo chemical reactions and GDD introduced by air, (ii) a dynamic regulation of the temperature of BBO crystals is useful to fully compensate the phase mismatch caused by heating effect, (iii) the reflectivity of the input coupler M2 should be optimized for optimal SHG conversion efficiency and (iv) an fs enhancement cavity can be employed in the SFG stage.
Fig. 3. Spectral range of the laser system. The amplitude of the curve does not represent the actual power, for the coupling efficiency to spectrograph changes with wavelength.
3.2 Hydrogen spectroscopy
This 243 nm pulsed laser source can be used to perform the two-photon spectroscopy of the 1S–2S transition in atomic hydrogen. A top view of the apparatus for hydrogen spectroscopy is represented in Fig. 4. The UV light is coupled into a low finesse cavity in a vacuum chamber, which is evacuated with a dry pump and a turbo pump. A background pressure as low as 10−5 Pa can be achieved. The enhancement cavity is comprised of a high reflective mirror and an input coupling mirror with a reflectivity of 99.5% and 96% respectively. The length between M2 and M3 is 60 cm, restricted by the dimension of vacuum chamber. The length can be fine-tuned by a translation stage according to the repetition rate of laser. By measuring the transmission light from M3, an enhancement factor of 18 is estimated in the cavity. A steady H2 flux of 2 sccm is produced with the help of a gas mass flow controller (AST10-DL). The H2 gas is dissociated in a thermal gas cracker to produce an ion-free atomic hydrogen beam. The atomic beam is then horizontally directed into an oxygen-free copper nozzle which is cooled to 7.8 K by a G-M refrigerator (KDE420SA). The nozzle is shielded by a gold-plated shell cooled to 40 K to eliminate outer thermal radiation. The newly designed cooling system can offer up to 4 W cooling power at 4 K and consume no helium. When leaving the nozzle vertically, the atoms interact with DUV laser. The fluorescence from 2S level to 1S level at 121.6 nm is collected by a photomultiplier (R10454, Hamamatsu).
Fig. 4. Setup for hydrogen 1S–2S spectroscopy. M1, reflector. M2, concave input coupler. M3, concave mirror. PMT, photomultiplier tube. GMFC, gas mass flow controller. PC, personal computer.
We record the transition by scanning the frequency of the TiSa laser using an acousto-optic modulator. The resonance signal obtained versus the radio frequency of the acousto-optic modulator is shown in Fig. 5. In this experiment, the repetition rate is simply locked to a known 100 MHz reference but carrier envelope frequency is unknown. Although the center frequency of the signal is not determined now, we can make sure that the signal comes from the 1S-2S transition, considering that the spectral response of the photomultiplier (115 nm to 195 nm) and the spectral range of DUV laser make it impossible to record other transitions.
Fig. 5. 1S–2S resonance in hydrogen. The fitted solid curve presents the fluorescence decay in the 2S level versus the frequency of the AOM scanning the TiSa laser. Integration over 500 s of recording. The dash lines show the 95% prediction bounds.
4. Conclusion
In summary, we demonstrate an all-solid state DUV laser operating at 243 nm suitable for driving the 1S–2S two-photon resonance in atomic hydrogen. Output of a 100 MHz fs TiSa laser with 1.2 W average power (210 mW around 729 nm) is frequency doubled in an fs enhancement cavity and then mixed with its second harmonic in a BBO crystal. An average power of 1.7 mW at 243 nm is achieved. Hydrogen 1S–2S resonance excited by this DUV source is observable. The large gain of the TiSa laser ensures a wide tunability of this UV source.
Funding
National Natural Science Foundation of China (61535001, 91836301).
Acknowledgments
We thank Zhigang Zhang for suggestion on the TiSa fs laser and Dong Hou for frequency locking.
Disclosures
The authors declare no conflicts of interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. A. Beyer, L. Maisenbacher, A. Matveev, R. Pohl, K. Khabarova, A. Grinin, T. Lamour, D. C. Yost, T. W. Hänsch, N. Kolachevsky, and T. Udem, “The Rydberg constant and proton size from atomic hydrogen,” Science 358(6359), 79–85 (2017). [CrossRef]
2. C. Wieman and T. W. Hänsch, “Precision measurement of the 1S Lamb shift and of the 1S–2S isotope shift of hydrogen and deuterium,” Phys. Rev. A 22(1), 192–205 (1980). [CrossRef]
3. E. A. Hildum, U. Boesl, D. H. McIntyre, R. G. Beausoleil, and T. W. Hänsch, “Measurement of the 1S–2S frequency in atomic hydrogen,” Phys. Rev. Lett. 56(6), 576–579 (1986). [CrossRef]
4. S. Galtier, F. Nez, L. Julien, and F. Biraben, “Ultraviolet continuous-wave laser source at 205 nm for hydrogen spectroscopy,” Opt. Commun. 324, 34–37 (2014). [CrossRef]
5. O. Arnoult, F. Nez, L. Julien, and F. Biraben, “Optical frequency measurement of the 1S–3S two-photon transition in hydrogen,” Eur. Phys. J. D 60(2), 243–256 (2010). [CrossRef]
6. H. Fleurbaey, S. Galtier, S. Thomas, M. Bonnaud, L. Julien, F. Biraben, F. Nez, M. Abgrall, and J. Guéna, “New measurement of the 1S–3S transition frequency of hydrogen: contribution to the proton charge radius puzzle,” Phys. Rev. Lett. 120(18), 183001 (2018). [CrossRef]
7. S. Bourzeix, B. de Beauvoir, F. Nez, F. de Tomasi, L. Julien, and F. Biraben, “Ultra-violet light generation at 205 nm by two frequency doubling steps of a cw titanium-sapphire laser,” Opt. Commun. 133(1-6), 239–244 (1997). [CrossRef]
8. A. Marian, M. C. Stowe, J. R. Lawall, D. Felinto, and J. Ye, “United Time-Frequency Spectroscopy for Dynamics and Global Structure,” Science 306(5704), 2063–2068 (2004). [CrossRef]
9. E. Peters, D. C. Yost, A. Matveev, T. W. Hänsch, and T. Udem, “Frequency-comb spectroscopy of the hydrogen 1S–3S and 1S–3D transitions,” Ann. Phys. (Berlin) 525(7), L29–L34 (2013). [CrossRef]
10. A. Grinin, A. Matveev, D. C. Yost, L. Maisenbacher, V. Wirthl, R. Pohl, T. W. Hänsch, and T. Udem, “Two-photon frequency comb spectroscopy of atomic hydrogen,” Science 370(6520), 1061–1066 (2020). [CrossRef]
11. C. G. Parthey, A. Matveev, J. Alnis, B. Bernhardt, A. Beyer, R. Holzwarth, A. Maistrou, R. Pohl, K. Predehl, T. Udem, T. Wilken, N. Kolachevsky, M. Abgrall, D. Rovera, C. Salomon, P. Laurent, and T. W. Hänsch, “Improved measurement of the hydrogen 1S–2S transition frequency,” Phys. Rev. Lett. 107(20), 203001 (2011). [CrossRef]
12. L. Chen, A. J. Benedick, J. R. Birge, M. Y. Sander, and F. X. Kärtner, “Octave-spanning, dual-output 2.166 GHz Ti:sapphire laser,” Opt. Express 16(25), 20699–20705 (2008). [CrossRef]
13. W. Li, Z. Gan, L. Yu, C. Wang, Y. Liu, Z. Guo, L. Xu, M. Xu, Y. Hang, Y. Xu, J. Wang, P. Huang, H. Cao, B. Yao, X. Zhang, L. Chen, Y. Tang, S. Li, X. Liu, S. Li, M. He, D. Yin, X. Liang, Y. Leng, R. Li, and Z. Xu, “339J high-energy Ti:sapphire chirped-pulse amplifier for 10 PW laser facility,” Opt. Lett. 43(22), 5681–5684 (2018). [CrossRef]
14. A. J. Benedick, J. G. Fujimoto, and F. X. Kärtner, “Optical flywheels with attosecond jitter,” Nat. Photonics 6(2), 97–100 (2012). [CrossRef]
15. G. Pu, L. Yi, L. Zhang, C. Luo, Z. Li, and W. Hu, “Intelligent control of mode-locked femtosecond pulses by time-stretch-assisted real-time spectral analysis,” Light: Sci. Appl. 9(1), 13 (2020). [CrossRef]
16. H. Fattahi, H. G. Barros, M. Gorjan, T. Nubbemeyer, B. Alsaif, C. Y. Teisset, M. Schultze, S. Prinz, M. Haefner, M. Ueffing, A. Alismail, L. Vámos, A. Schwarz, O. Pronin, J. Brons, X. Geng, G. Arisholm, M. Ciappina, V. S. Yakovlev, D. Kim, A. M. Azzeer, N. Karpowicz, D. Sutter, Z. Major, T. Metzger, and F. Krausz, “Third-generation femtosecond technology,” Optica 1(1), 45–63 (2014). [CrossRef]
17. M. Galli, V. Wanie, D. P. Lopes, E. P. Månsson, A. Trabattoni, L. Colaizzi, K. Saraswathula, A. Cartella, F. Frassetto, L. Poletto, F. Légaré, S. Stagira, M. Nisoli, R. M. Vázquez, R. Osellame, and F. Calegari, “Generation of deep ultraviolet sub-2-fs pulses,” Opt. Lett. 44(6), 1308–1311 (2019). [CrossRef]
18. G. Porat, C. M. Heyl, S. B. Schoun, C. Benko, N. Dörre, K. L. Corwin, and J. Ye, “Phase-matched extreme-ultraviolet frequency-comb generation,” Nat. Photonics 12(7), 387–391 (2018). [CrossRef]
19. L. Matos, D. Kleppner, O. Kuzucu, T. R. Schibli, J. Kim, E. P. Ippen, and F. X. Kaertner, “Direct frequency comb generation from an octave-spanning, prismless Ti:sapphire laser,” Opt. Lett. 29(14), 1683–1685 (2004). [CrossRef]
20. T. Fuji, A. Unterhuber, V. S. Yakovlev, G. Tempea, A. Stingl, F. Krausz, and W. Drexler, “Generation of smooth, ultra-broadband spectra directly from a prism-less Ti:sapphire laser,” Appl. Phys. B 77(1), 125–128 (2003). [CrossRef]
21. J. Zhang, L. Hua, S. Yu, Z. Chen, and X. Liu, “Femtosecond enhancement cavity with kilowatt average power,” Chin. Phys. B 28(4), 044206 (2019). [CrossRef]
22. T. W. Hansch and B. Couillaud, “Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity,” Opt. Commun. 35(3), 441–444 (1980). [CrossRef]
23. E. Peters, S. A. Diddams, P. Fendel, S. Reinhardt, T. W. Hänsch, and T. Udem, “A deep-UV optical frequency comb at 205 nm,” Opt. Express 17(11), 9183–9190 (2009). [CrossRef]
24. H. Lu, H. Xu, J. Zhao, and D. Hou, “A Deep Ultraviolet Mode-locked Laser Based on a Neural Network,” Sci. Rep. 10(1), 116 (2020). [CrossRef]
