Beck, T.

Rein, B.

Walther, T.

GSI Repository

An all-solid-state based laser system for laser cooling of relativistic ion beams∗Tobias Beck1, Benjamin Rein1, and Thomas Walther†11Institute of Applied Physics, TU Darmstadt, GermanyIn 2004 and 2006 laser cooling experiments of relativis-ticC3+ beams were performed at GSI using frequency dou-bled Argon-ion lasers [1]. The results of these experimentswere promising, but were limited by the large linewidth andthe severely limited tunability of the Ar-ion lasers. WhileAr-ion lasers are still quite common with applications in ar-eas such as spectroscopy, laser pumping, medical care andeven light shows, they have a relatively low efficiency andhigh maintenance costs [2].Within this research project, we successfully developedan alternative laser system far superior to the existing Ar-ion laser: it is a rugged, efficient all-solid-state based sys-tem with output wavelengths of 1028 nm, 514 nm and257 nm. It provides high output power, narrow linewidth,wide and fast tunability and a near perfect Gaussian beamprofile.ECDL 2ECDL 1 FA SHG FHGWLM10 W1028 nm5 W514 nmmin 100 mW257 nm200 mW1028 nmWavemeter-LockY- fiber25 GHz PD to experimentFVC+ ~PIDFigure 1: Schematic diagram of the system. ECDL: ex-ternal cavity diode laser, FA: fiber amplifier, SHG: sec-ond harmonic generation, FHG: fourth harmonic genera-tion, WLM: wavelength meter, FVC: frequency to voltageconverter, PD: photo detector, PID: proportional-integral-derivative controller, +: signal adder,∼: function generatorA schematic overview of our system is shown in Fig. 1.It mainly consists of a fiber amplifier seeded by an exter-nal cavity diode laser (ECDL). The output of this amplifieris frequency doubled and quadrupled in bow-tie built-upcavities using LBO and BBO crystals, respectively. Thefiber amplifier delivers up to 15.3 W of optical power at1028 nm. In Fig. 2 the output of the first cavity is plot-ted over the input power. We achieved nearly 5 W with aconversion efficiency of 57 % at 514 nm. The second cav-ity delivered up to 180 mW of UV light with a conversionefficiency of 12 %.The system is stabilized to an absolute frequency usingan offset lock on another identical ECDL (master), which∗Work supported by BMBF Grant Nr. 06DA9045I and 06DA7053I† thomas.walther@physik.tu-darmstadt.deFigure 2: Optical power at 514 nm and conversion effi-ciency over infrared power in the first build-up cavity. Theerror bars result from the uncertainties of the detectors.itself is locked to a high-precision wavelength meter. Byadding an arbitrary ramp from the function generator tothe error signal of the offset lock, the seed ECDL of thefiber amplifier can be scanned with respect to the masterECDL. In the UV it was possible to scan 12 GHz in 10 ms.Mode hops were suppressed by a novel locking scheme de-veloped in our group [3]. The offset lock is achieved bymixing small amounts of light of both ECDLs in a Y-fiberand observing the resulting beat signal with a fast photodetector. The beat frequency is divided by a factor of 1000and fed into a frequency to voltage converter whose outputserves as the error signal. By observing the beat signal ofthe two identical ECDLs, their linewidth was determinedto 890 kHz in a 60 s time interval. This corresponds toΔff ≈ 3 ∗ 10−06.In conclusion, we were able to replace the Argon-ionlaser with a more versatile light source. During a beamtime in August 2012 stable long term operation was suc-cessfully demonstrated. Another beam time in Lanzhou(China) is planned in 2013 and further experiments at FAIRare possible.References[1] M. Bussmann et al., “All-optical ion beam cooling and on-line diagnostics at relativistic energies”, Cool 2009, Lanzhou,http://cool09.impcas.ac.cn/.[2] M. H. Dunn and J. N. Ross, “The Argon ion laser,” Progr.Quant. Electr. 4, 233–269 (1976).[3] T. Fu¨hrer, D. Stang and T. Walther, “Actively controlled tun-ing of an external cavity diode laser by polarization spec-troscopy,” Opt. Expr. 17, 4991–4996 (2009).PNI-IONS-EXP-06 GSI SCIENTIFIC REPORT 2012314

An all solid-state based laser system for laser cooling of  relativistic ion beams

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An all solid-state based laser system for laser cooling of relativistic ion beams

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