Watt-level narrow-linewidth fibered laser source at 852 nm for FIB application
Résumé
We realize a 1 Watt all-fibered polarized compact and robust laser source at 852 nm for laser cooling of cesium atoms. The architecture is based on the sum-frequency generation of 1540 nm and 1908 nm lasers, realized through a periodically-poled lithium niobate waveguide with a conversion efficiency of 40%. A linewidth of 20 kHz is achieved with the development of a distributed feedback fiber laser at 1908 nm. The operation of this laser source is demonstrated on a focused ion beam experiment based on cold cesium atoms. For several decades, there has been significant interest in single-frequency narrow-linewidth polarized light at 852 nm for laser cooling on the D 2-line of atomic cesium. In studies of fundamental physics and metrology, cesium laser cooling is the basis of precision measurements using atomic clocks [1-5] and atom interferometry for inertial sensing [6-12]. Nowadays, this cooling process is also attractive for the industrial development of a new focused ion beam (FIB) generation [13, 14]. For this application, a 2D blue molasses [15] is utilized to realize a collimated atomic beam. Most applications in research or industry require robust and transportable experiments that can operate in harsh environments [16, 17]. However, current laser architectures at this wavelength are primarily based on free-space external-cavity laser diodes (ECDLs) and tapered semiconductor amplifiers as laser sources, which are prone to misalignments caused by vibrations and temperature fluctuations. In this context, the development of an all-fibered laser source at 852 nm based on a sum-frequency generation (SFG) was recently presented as a more robust solution [18]. In this work, Diboune et al. realized a frequency-stabilized, polarized laser source (with a linewidth below 5 MHz and output power of 210 mW) by using lasers at 1560 nm and 1878 nm as a basis for SFG. This method benefits from off-the-shelf fiber components in the telecom domain, which are efficient, compact and robust, and fiber components at 2 µm, which have been highly developed over several years for a variety of applications such as medical physics, material processing and LIDAR [19]. However, most cold-atom experiments require sub-MHz linewidth to limit detection noise and phase noise, and a high output power to increase the atom number and signal-to-noise ratio (SNR). In this letter, we present a compact, stable and robust fiber architecture to produce a Watt-level narrow-linewidth polarized laser at 852 nm. Wavelength conversion via SFG is realized through a fiber-pigtailed periodically-poled lithium nio-bate (PPLN) waveguide, from the combination of lasers at 1540 nm and 1908 nm. The choice of these wavelengths optimizes the power efficiency in the thulium-doped fiber, and keeps the second wavelength in the telecom domain. Moreover, the linewidth of the source is reduced by the development of a single-frequency polarized distributed feedback (DFB) thulium-doped fiber laser at 1908 nm. Indeed, single-frequency DFB laser diodes near 2 µm with a linewidth below 100 kHz are presently not widely available. Hence, there are few quality laser sources adapted for applications at this wavelength, such as spec-troscopy or remote sensing. Fiber laser sources are attractive for their narrow linewidth and compactness. Thulium-doped fiber lasers with different configurations have already been demonstrated , such as with DFBs [20, 21], distributed Bragg reflectors (DBRs) [22, 23], or ring-shaped cavities [24], and they have achieved linewidths below 3 kHz [22] and slope efficiencies with respect to the launched power up to 36% [25]. However, few ro