A broad emitter diode laser system for lithium spectroscopy

A laser system based on injection locking of a broad emitter diode laser operating at a wavelength of λ = 671 nm has been realized. With an injected power of 9.6 mW, 130 mW output power of the broad emitter diode laser is achieved. The broad emitter diode laser operates in a single spectral mode and its eigenmodes can be suppressed by more than 30 dB. By modulation of its operation current, sidebands of the laser frequency can be created. The laser system has been used to operate a magnetooptical trap for 7Li atoms. With 70 mW of laser power, 107 atoms have been loaded from a nearby thermal source. PACS: 42.50.Vk; 32.80.P; 42.60.By The investigation of magnetically trapped alkali atoms has become an exciting field of interest in recent years culminating in the observation of Bose–Einstein condensation [1] of rubidium, sodium, and lithium. Among the alkalies, lithium is of particular interest as there are two stable isotopes, a boson (3Li, I = 2 ) and a fermion (3Li, I = 1). To approach the regime of quantum degeneracy the sample is cooled by forced evaporation of the fast atoms in a magnetic trap. In the case of 6Li, however, evaporative cooling at low temperatures, at which interatomic collisions are dominated by swave scattering, will not work. Because of the fermionic character of 6Li, s-wave scattering of two 6Li atoms in the same magnetic substate is forbidden by the Pauli principle and elastic collisions freeze out at temperatures below 100 μK. To overcome this, 6Li can be stored simultaneously with 7Li in the same magnetic trap. Evaporative cooling of 6Li then is possible due to collisions with the 7Li background gas. Simultaneous storage of both lithium isotopes in the same magnetic trap requires a magnetooptical trap [2] for both isotopes. Thus laser radiation with two frequency components is necessary to excite the transitions 2S 1 2 (F = 2)↔ 2P3 2 (F = 3) ∗ Present address: Department of Physics, Stanford University, Stanford CA 94305-4060, USA in the case of 7Li and 2S 1 2 (F = 2 )↔ 2P32 (F = 5 2 ) in the case of 6Li (D2 lines). These transitions correspond to wavelengths of λ = 670.962 nm (7Li) and λ= 670.977 nm (6Li) and are separated by 10 GHz. The natural linewidth is 6 MHz. If the two magnetooptical traps are to be operated by the same laser, the second frequency can be generated by an electrooptic modulator as a sideband of the laser frequency. The intensity of one of the sidebands cannot be used and the situation is complicated further by the fact that in the two magnetooptical traps the Li atoms have to be repumped into the trapped hyperfine state by a suitable repumping frequency. The hyperfine splitting of the 2S 1 2 state is 803 MHz in the case of 7Li and 228 MHz in the case of 6Li. The generation of the repumping frequencies would require two additional electrooptic modulators and so the laser intensity would be reduced further. The use of two separate laser systems appears to be more favorable. Laser radiation at λ= 671 nm with power in the range of 100 mW is commonly generated by dye lasers. High power is essential to achieve a large capture range and a high number of atoms in the magnetooptical trap. An alternative to these expensive and cumbersome laser systems is the recently developed high-power diode lasers. 1 Broad emitter diode lasers as light source for spectroscopy For our spectroscopic purposes the bandwidth of the laser should be smaller than the natural linewidth of the atomic transition of interest, which is about 6 MHz. This requires the use of single-mode lasers. Moreover, Doppler cooling of Li atoms becomes more efficient with increasing laser power. With larger detuning of the cooling laser to the red, the velocity up to which atoms can be cooled increases and a higher fraction of atoms emitted from a thermal source can be caught in a magnetooptical trap. However the intensity of the laser has to be sufficiently large to saturate the transition at which the magnetooptical trap is operated even if the laser is detuned to the red. In addition high intensity allows for a larger geometric capture range with a given intensity.