Development of tunable continuous-wave UV laser source of high spectral purity and demonstration of high-resolution spectroscopy
Final Report Abstract
Single-frequency UV laser sources are used in a variety of research related to high-resolution spectroscopy of atoms, molecules and ions. As an example from quantum optics, the Beryllium ion, laser-cooled on the 2s → 2p transition at a wavelength of 313 nm, is a workhorse of quantum computing, optical clocks, precision spectroscopy of atomic and molecular ions via sympathetic cooling. By a convenient coincidence, the 313 nm UV wavelength, relevant for Be+ cooling, is a fifth harmonic of the 1565 nm IR wavelength, which is within the range of the Er- doped fiber lasers, and within the C-band of fiberoptic communication. Due to remarkable progress in the fiber laser technology, reliable and cost efficient near- IR fiber lasers and amplifiers with high spectral purity and high output power are available. The high fiber laser power can be combined with the performance of nonlinear materials based on quasi-phase-matching (QPM). We have developed a UV source based on quintupling of the IR laser and suitable for laser cooling of Be+ ions. A narrowband fiber laser is amplified to 15 W in a Er-doped fiber amplifier (EDFA) and then the fifth harmonic (FH) of the fundamental frequency (F) is generated in a chain of nonlinear crystals. The main advantage of the source is the compact and robust design, owing to the reliability of the fiber laser components. The footprint of the quintupling setup is 0.6 m ×0.3 m. A special feature of the source is the high power output at the intermediate second (SH) and third harmonic (TH) wavelengths: 7.5 W at 783 nm, 1.2 W at 522 nm. We used the TH output for absolute frequency stabilization and precision tuning of the source’s frequency. Frequency conversion of the fundamental wave (F) to the FH was realized in the following stages: (i) singlepass SHG in a cascade of two 50-mm long PPLN crystals (F+F→SH) [5]; (ii) single-pass SFG in the 30-mm long MgO:PPSLT crystal (F+SH→TH) (iii) SFG in a 10-mm long BBO crystal (SH+TH→FH). The BBO crystal was paced in the enhancement cavity in order to scale up the UV output power. The cavity was designed to resonate only the SH wave, while the TH wave traverses the crystal in single pass. The considerable level of 100 mW UV output has been reached. The frequency of the UV source was stabilized using molecular iodine as a reference. The Be+ cooling transition at 313.133 nm corresponds to the 521.888 nm TH wavelength. By coincidence, the molecular iodine absorption line P58 is located rather close to the 521.888 nm. This convenient line is strong enough and the obtained frequency modulation transfer spectroscopy error signals allow absolute frequency stabilization of the UV source. We determined the absolute frequencies of the hfs components of the P58 line by locking the master laser to a particular component. The laser frequency was then measured by a femtosecond frequency comb. The source was tested using our apparatus for laser cooling of Be+ ions and sympathetic cooling of molecular ions. The source was locked to the most red-shifted a1 hfs component of the P58 transition during the loading of the Be+ ions into the trap. That corresponds to 1.6 GHz red detuning from the central frequency of the cooling transition and allows more efficient cooling of hot ions. Subsequently, the UV frequency was tuned towards the cooling transition frequency. We observed the crystallization of the Be+ ions cloud when the source’s frequency was locked to the a4 – a7 hfs components. The ground state of the cooling transition has a hyperfine splitting of 1.25 GHz. The excited state can decay into either of the two hyperfine levels of the ground state. Therefore we introduced the ‘repumper’ sidebands in the source’s UV output by the phase modulation of the source’s input at the fundamental wavelength. In conclusion, we developed and tested a complete tabletop setup for generation, absolute frequency stabilization, and precision tuning of the UV laser radiation at 313 nm. The maximum output power of the source at 313 nm is 100 mW. The frequency stabilization stage allows precise adjustment of the UV output wavelength through the Be+ cooling transition. Important features of the source are compactness, robustness, and use of reliable fiber optic components. The source can be transported from one lab to another and reconnected to the fiber laser and to the electronics rack in a matter of minutes. Only very little adjustments of the optical setup are required after the transportation because all frequency conversion stages but one are single-pass. The warm-up time is short. With such properties, the source can become a workhorse for laser cooling of Be+, and similar applications. A significant amount of the source’s output at the intermediate SH and TH wavelengths remains available. The developed setup can be used as a building block for a more sophisticated multi-wavelength UV source for cooling of ions into the quantum ground state of motion in an ion trap using the Raman cooling technique.
Publications
- "Compact All-Solid-State Continuous-Wave Single-Frequency UV Source for Laser Cooling of Beryllium Ions," in Advanced Solid-State Photonics, OSA Technical Digest (CD) (Optical Society of America, 2011), paper AWA17
S. Vasilyev, A. Nevsky, I. Ernsting, M. Hansen, J. Shen, and S. Schiller
(See online at https://doi.org/10.1364/ASSP.2011.AWA17) - “Compact all-solid-state continuous-wave single-frequency UV source with frequency stabilization for laser cooling of Be+ ions”, Appl. Phys. B 103, 27-33 (2011)
S. Vasilyev, A. Nevsky, I. Ernsting, M. Hansen, J. Shen, and S. Schiller
(See online at https://doi.org/10.1007/s00340-011-4435-1)
