A self-injected, diode-pumped, solid-state ring laser for laser cooling of Li atoms
Yudai Miake, Takashi Mukaiyama, Kenneth M. O'Hara, Stephen Gensemer
aa r X i v : . [ phy s i c s . a t o m - ph ] A p r A self-injected, diode-pumped, solid-state ring laser for laser cooling of Liatoms
Yudai Miake and Takashi Mukaiyama a) Institute for Laser Science, University of Electro-Communications,1-5-1 Chofugaoka, Chofu, Tokyo 182-8585,Japan
Kenneth M. O’Hara
Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802-6300,USA
Stephen Gensemer
Commonwealth Scientific and Industrial Research Organisation, Australia (Dated: 14 July 2018)
We have constructed a solid-state light source for experiments with laser cooled lithium atoms based on aNd:YVO ring laser with second-harmonic generation. Unidirectional lasing, an improved mode selection,and a high output power of the ring laser was achieved by weak coupling to an external cavity which containedthe lossy elements required for single frequency operation. Continuous frequency tuning is accomplished bycontrolling two PZTs in the internal and the external cavities simultaneously. The light source has beenutilized to trap and cool fermionic lithium atoms into the quantum degenerate regime. I. INTRODUCTION
Recent experimental advances in controlling inter-atomic interactions using Feshbach resonances offer greatopportunities to study many-body quantum physics withthe precise control of atomic physics . Lithium atoms areparticularly suitable for such study, because two isotopesof lithium with different quantum statistics (fermionic Li and bosonic Li) have relatively broad Feshbach res-onances that can be utilized to precisely control inter-atomic interactions . For Doppler cooling and an effi-cient trapping of the atoms, it is necessary to excite theirdipole transitions with a strong laser that is red-detunedfrom the atomic resonance. A dye laser would be an ap-propriate choice of light source for cooling lithium atoms.However, dye lasers require constant care to maintain thelaser condition. A combined system of a laser diode (at671 nm) and a tapered amplifier is widely used in coldatom experiments, but these laser systems typically havea poor spatial mode quality and are not as powerful andstable compared with those for cooling other alkalis atlonger resonance wavelengths.In this article, we report on the realization of a diode-pumped-solid-state (DPSS) light source consisting of aNd:YVO ring laser with second-harmonic generation(SHG) for laser cooling of lithium atoms. In previousworks, single-longitudinal-mode lasers at 671 nm havebeen produced with the combination of a Nd:YVO ringlaser and a frequency doubler . In these studies, unidi-rectional lasing was realized by placing an optical diodecomprised of a terbium gallium garnet (TGG) crystaland half-wave plate inside the ring cavity. The high ab-sorption coefficient of TGG produces both a direct op-tical loss as well as a loss due to thermal depolarization a) Electronic mail: [email protected] which limits the power scaling of such ring lasers. . Al-ternatively, one circulating mode can be favored over theother by retro-reflecting one of the two output beamsback into the cavity . However, the counter-rotatingwave can never be entirely suppressed as it is requiredto seed the stronger of the two circulating modes. In thepresent work, instead of using the previously reportedschemes, we realize unidirectional lasing by bringing apart of the output light back into the ring cavity throughthe remaining open port of the output coupler. Thus,two cavities are formed in the ring laser and an opticalisolator is placed in the weakly coupled external cavityto favor one circulating direction of oscillation over theother. By placing the optical isolator in a weakly coupledexternal cavity, a primary contributor to the intracavityloss is eliminated. This configuration also helps to makethe internal cavity length shorter to enhance the mode-hop-free tuning range. Continuous frequency scanningcan be performed by simultaneously tuning two PZTs inthe internal and external cavities. The light at 671 nm isobtained by second harmonic generation using a lithiumtriborate (LBO) crystal inside a doubling cavity. II. LASER SYSTEM
The setup of the laser system is presented in Fig. 1.A bow-tie ring cavity is formed by four mirrors (M1,M2, M3 and OC). Two commercial fiber-coupled laserdiodes (Coherent FAP-600 model) at 808 nm are used topump the Nd:YVO crystal through the dichroic mirrorsM1 and M2 (with a high-reflection coating at 1342 nmand a high-transmission coating at 808 nm). The outputfibers of the pumping laser diode have a core diameter of600 µ m, and the spot is expanded by a factor of two atthe crystal position. The Nd:YVO crystal with a size of3 × ×
10 mm is 0.15 % doped and has anti-reflectioncoating at 808 nm and 1342 nm. The crystal is held by a Nd:YVO etalon1 OC LD808nmLD808nm etalon2PZT1 PZT2
OIM1 M2M3 M4PBSLBO IC
PZT3
M7M5671nm M6 EOMHWP HWP
FIG. 1. Optical setup of the diode-pumped-solid-state laserat 1342 nm consisting of the fiber-coupled pump laser diodes,cavity mirrors (M1 to M4), and output coupler (OC). An op-tical isolator is placed together with a half-wave plate (HWP)in the external cavity path. The second cavity is used to ob-tain second-harmonic generation at 671 nm. LD, EOM andIC represent the laser diode, electro-optic modulator, and in-put coupler, respectively. copper mount and is temperature-controlled by a Peltiermodule. The OC has a reflectance of 95 %. We inten-tionally use four flat mirrors (M1, M2, M3 and OC) toconstruct the cavity so that the stability condition of thecavity is only satisfied when the Nd:YVO crystal causesa thermal lensing effect. Because the lowest transversemode has the strongest thermal lensing when the pump-ing intensity is increased, the flat-mirror cavity helps las-ing in the lowest transverse mode. Two uncoated quartzplates with different thicknesses (0.5 mm and 1 mm) areplaced inside the cavity to suppress mode hops and toachieve stable single-longitudinal-mode lasing at the de-sired wavelength.A part of the output (a few tens of mW) is picked bya polarization beam splitter (PBS) and is brought backinto the cavity to construct another cavity after pass-ing through an optical isolator (OI). This coupled-cavityconfiguration creates an asymmetry in two opposite las-ing directions and achieves the unidirectional operation.Since the lasing frequency is determined both by the in-ternal cavity (M1-M2-M3-OC-M1) and external cavity(M1-M2-M3-PBS-M4-M1) coupled to each other throughthe OC, the cavity lengths for both cavities need to betuned simultaneously to sweep the laser frequency with-out mode hops. In this setup, two PZTs are attachedto the mirrors M3 and M4 and are used to continuouslysweep the laser frequency. In order to sweep the laserfrequency, the voltage applied to PZT2 has to be pro-portional to the voltage applied to PZT1 with a factor of( L − L ) /L , where L and L are the round-trip lengthsof internal and external cavities, respectively. In our sys- (cid:18612)(cid:18608)(cid:18615)(cid:18612)(cid:18608)(cid:18610)(cid:18611)(cid:18608)(cid:18615)(cid:18611)(cid:18608)(cid:18610)(cid:18610)(cid:18608)(cid:18615)(cid:18610)(cid:18608)(cid:18610) (cid:18611)(cid:18613)(cid:18614)(cid:18612) (cid:18594) (cid:18672) (cid:18671) (cid:18594) (cid:18673) (cid:18679) (cid:18678) (cid:18674) (cid:18679) (cid:18678) (cid:18594) (cid:18674) (cid:18673) (cid:18681) (cid:18663) (cid:18676) (cid:18594) (cid:18653) (cid:18649) (cid:18655) (cid:18612)(cid:18616)(cid:18612)(cid:18614)(cid:18612)(cid:18612)(cid:18612)(cid:18610)(cid:18611)(cid:18618)(cid:18611)(cid:18616)(cid:18611)(cid:18614)(cid:18611)(cid:18612)(cid:18611)(cid:18610) (cid:18642)(cid:18679)(cid:18671)(cid:18674)(cid:18594)(cid:18674)(cid:18673)(cid:18681)(cid:18663)(cid:18676)(cid:18594)(cid:18653)(cid:18649)(cid:18655) FIG. 2. DPSS output power as a function of the incidentpump power. DPSS output increases with the pump powerwithout a remarkable saturation up to 2.7 W. Slight changeof the gradient is visible at a pump power of approximately1.7 W. tem, this factor is observed to be 0.76, which is consistentwith the ratio of the cavity lengths of our actual setup.A mode-hop-free tuning range of 8 GHz at 1342 nm hasbeen achieved, which is wider than the 3 GHz realized inthe previous work .Figure 2 shows the output power of the 1342 nm laseras a function of the total pump power at 808 nm. TheDPSS laser starts lasing at approximately 10 W of pump-ing and the output power increases with the pump powerwithout a remarkable saturation; however a slight changeof the gradient in the plot is observed at a pump power of17 W. An output of 2.7 W is obtained at the maximumpumping power, which is currently limited by the coolingcapacity of the Nd:YVO crystal in our system. (cid:18618)(cid:18610)(cid:18610)(cid:18616)(cid:18610)(cid:18610)(cid:18614)(cid:18610)(cid:18610)(cid:18612)(cid:18610)(cid:18610)(cid:18610) (cid:18645) (cid:18634) (cid:18633) (cid:18594) (cid:18674) (cid:18673) (cid:18681) (cid:18663) (cid:18676) (cid:18594) (cid:18653) (cid:18671) (cid:18649) (cid:18655) (cid:18612)(cid:18608)(cid:18615)(cid:18612)(cid:18608)(cid:18610)(cid:18611)(cid:18608)(cid:18615)(cid:18611)(cid:18608)(cid:18610)(cid:18610)(cid:18608)(cid:18615)(cid:18610)(cid:18608)(cid:18610) (cid:18632)(cid:18679)(cid:18672)(cid:18662)(cid:18659)(cid:18671)(cid:18663)(cid:18672)(cid:18678)(cid:18659)(cid:18670)(cid:18594)(cid:18670)(cid:18659)(cid:18677)(cid:18663)(cid:18676)(cid:18594)(cid:18674)(cid:18673)(cid:18681)(cid:18663)(cid:18676)(cid:18594)(cid:18653)(cid:18649)(cid:18655) FIG. 3. Closed circles show the SHG output as a function ofthe DPSS laser power. The solid curve shows the quadraticdependence. The plot starts to deviate from the quadraticfeature approximately the 1.7 W input, indicating a slightsaturation in the SHG conversion.
To obtain laser light resonant with the atomic D2 tran-sition of lithium atoms, the output light of the DPSS laserat 1342 nm is delivered to a cavity with an LBO crystalwith a size of 3 × ×
10 mm inside to obtain SHG at671 nm. The doubling cavity is frequency-locked to thefree-running DPSS laser using a conventional FM side-band technique. The input coupler (shown as IC in Fig.1) has a reflectance of 97 %; it was chosen to optimizethe SHG conversion in the present experimental condi-tion. The radius of curvature of the two concave mirrors(M6 and M7) is 50 mm. We implement the auto-relockfunction in this system so that the frequency lock of thedoubling cavity is robust against vibration . Figure 3shows the SHG output power as a function of DPSS laserpower. The SHG output increases smoothly to 800 mWwith an increase in the DPSS power. The quadratic de-pendence of the SHG output power on the DPSS laserpower (shown by the solid curve in Fig. 3) is observed upto 500 mW. The plot starts to deviate from the quadraticfeature at the higher DPSS power region, indicating asaturation of the SHG conversion.A few mW of SHG output is sent to a Li atomic va-por cell for saturation absorption spectroscopy. The twointra-cavity PZTs (PZT1 and PZT2) are simultaneouslytuned to obtain the atomic spectrum using the frequencymodulation technique (Fig. 4). Three resonance signalsappear in the spectrum. The left and right resonancescorrespond to the transitions from S / ( F = 3 /
2) and S / ( F = 1 /
2) to the excited state P / , respectively.The most prominent resonance observed at the center isthe crossover resonance between the two resonances. Welock the laser frequency at the crossover resonance whenit is used for cold atom experiments with Li atoms. Wehave been able to trap 10 atoms with an atomic tem-perature of one-tenth of the Fermi temperature, whichis deeply in the Fermi-degenerate regime, using an all-optical method .The system requires 30 minutes warmup time to reachfull power and a stable lasing condition. The system re-quires no daily adjustment except the frequency tuningto the atomic resonance. This can be done by tuning onePZT (either PZT1 or PZT2) to set up a mode hop tobring the laser frequency within the mode-hop-free tun-ing range to the atomic resonance. The laser has a rea-sonable spatial beam quality, which provides a couplingefficency of 76 % into a single-mode optical fiber.We have taken the method of the feedback cavity astep further by moving the etalons from the internal cav-ity into the external cavity, in another demonstration.We used this technique to stabilize a homemade dyelaser for laser cooling experiments at the University ofAmsterdam . In this instance, the external cavity con-tained a Faraday rotator, etalon, and a diffraction grat-ing for further mode selection, so that the internal lasercavity contains no elements aside from the gain medium.This can significantly reduce the length of the cavity andthe cost of the optics, since much more loss can be tol-erated in the external cavity. Moreover, the addition of (cid:18607)(cid:18616)(cid:18610)(cid:18682)(cid:18611)(cid:18610) (cid:18607)(cid:18613) (cid:18607)(cid:18614)(cid:18610)(cid:18607)(cid:18612)(cid:18610)(cid:18610)(cid:18612)(cid:18610)(cid:18614)(cid:18610) (cid:18645) (cid:18667) (cid:18665) (cid:18672) (cid:18659) (cid:18670) (cid:18594) (cid:18653) (cid:18659) (cid:18676) (cid:18660) (cid:18608) (cid:18594) (cid:18679)(cid:18672) (cid:18667) (cid:18678) (cid:18677) (cid:18655) (cid:18607)(cid:18614)(cid:18610)(cid:18610) (cid:18607)(cid:18612)(cid:18610)(cid:18610) (cid:18610) (cid:18612)(cid:18610)(cid:18610) (cid:18614)(cid:18610)(cid:18610)(cid:18632)(cid:18676)(cid:18663)(cid:18675)(cid:18679)(cid:18663)(cid:18672)(cid:18661)(cid:18683)(cid:18594)(cid:18653)(cid:18639)(cid:18634)(cid:18684)(cid:18655) FIG. 4. Saturation absorption spectrum of Li atoms. Twoatomic transitions (left and right resonances) and a crossoverresonance signal (center) are shown. The frequency spacingbetween the two atomic resonances corresponds to 228 MHz,which is the hyperfine splitting of the ground state of Li.The laser is locked at the crossover resonance for cooling Liatoms in the experiment. a diffraction grating or narrowband interference filter inthe feedback path can also help to control the laser wave-length. The technique could also be used for stabilizationof a Ti:Sapphire laser, or any other ring laser cavity thatnormally requires intracavity mode selection.
III. CONCLUSION
In summary, we have constructed a solid-state lightsource for an experiment in laser cooling of lithium atomsbased on a Nd:YVO ring laser followed by second-harmonic generation. Unidirectional lasing and improvedmode selection in the ring laser was achieved by weakcoupling to an external cavity containing an optical isola-tor. The light at 1342 nm is delivered to a doubling cavityto obtain laser light at 671 nm. Roughly 800 mW of theSHG output is obtained at the maximum pumping con-dition. The output power can be further improved withhigher second-harmonic-generation efficiency by prepar-ing a higher power in the fundamental light. A primarylimitation of the fundamental output power achieved todate is due to loss from the intracavity Faraday isola-tor and associated half waveplate. The coupled-cavityconfiguration presented here demonstrates a new schemeto realize unidirectional lasing while circumventing theproblem of intracavity loss. Continuous frequency tuningcan be achieved by controlling two PZTs in the internaland external cavities simultaneously and the laser fre-quency is locked to the crossover resonance of the atomicD-line transition of Li. The light source has successfullybeen utilized to trap Li in the actual setup and an ultra-cold gas of Li deep in the quantum degenerate regimecan be obtained. I. Bloch, J. Dalibard, and W. Zwerger, Rev. Mod. Phys. , 885(2008). K. M. O’Hara, S. L. Hemmer, M. E. Gehm, S. R. Granade, andJ. E. Thomas, Science , 2179 (2002). T. Bourdel, J. Cubizolles, L. Khaykovich, K. M. F. Magalh˜aes,S. J. J. M. F. Kokkelmans, G. V. Shlyapnikov, and C. Salomon,Phys. Rev. Lett. , 020402 (2003). M. W. Zwierlein, C. A. Stan, C. H. Schunck, S. M. F. Raupach,A. J. Kerman, and W. Ketterle, Phys. Rev. Lett. , 120403(2004). C. Chin, M. Bartenstein, A. Altmeyer, S. Riedl, S. Jochim, J.Hecker Denschlag, and R. Grimm, Science , 1128 (2004). F. A. Camargo, T. Z-. Willette, T. Badr, N. U. Wetter. and J-.J. Zondy, IEEE J. Quantum Electron. , 804 (2010). Y. Zheng, Y. Wang, C. Xie, and K. Peng, IEEE J. QuantumElectron. , 67 (2011). U. Eismann, F. Gerbier, C. Canalias, A. Zukauskas, G. Tr´enec, J.Vigu´e, F. Chevy, and C. Salomon, Appl. Phys. B , 25 (2012). U. Eismann, A. Bergsschneider, F. Sievers, N. Kretzschmar, C.Salomon, and F. Chevy, Opt. Express , 9091 (2013). F. R. Faxvog and A. D. Gara, Appl. Phys. Lett. , 306 (1974). F. R. Faxvog, Opt. Lett. , 285 (1980). H. Abitan, H. Bohr, and C. F. Pederson, Appl. Opt. , 7802(2005). P. C. Shardlow and M. J. Damzen, Appl. Phys. B , 257 (2009). S. Haze, S. Hata, M. Fujinaga, and T. Mukaiyama, Rev. Sci.Instrum. , 026111 (2013). Y. Inada, M. Horikoshi, S. Nakajima, M. Kuwata-Gonokami, M.Ueda, and T. Mukaiama, Phys. Rev. Lett. , 180406 (2008). T. G. Tiecke, S. D. Gensemer, A. Ludewig, and J. T. M. Wal-raven, Phys. Rev. A80