II-9. Instrumentation

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Long polarization-maintaining ﬁber link (440 m) for magneto-opticaltrapping of francium atoms
K. Nakamura,∗1 S. Nagase,∗2 T. Nakashita,∗3 T. Hayamizu,∗4 T. Aoki,∗3 H. Nagahama,∗1 N. Ozawa,∗2M. Sato,∗3,∗4 K. Yamane,∗3 M. Fukase,∗2 D. Uehara,∗2 A. Takamine,∗4 and Y. Sakemi∗1
We are currently developing an optical lattice francium (Fr) atom interferometer to determine the permanent electric dipole moment of an electron (eEDM). Fr
is the heaviest alkali atom with an eEDM enhancement
factor of approximately 799.1) The trapping of such Fr
atoms in the optical lattice leads to an ultra-sensitiveeEDM measurement system with a long interaction time
between the isolated atoms and electric ﬁeld. To load
the atoms into the optical lattice, they must be cooled
and trapped beforehand by techniques such as magnetooptical trapping (MOT). We developed a MOT systemand connected it to a 210 Fr production beamline, which
consists of a surface ionizer2) and neutralizer, installed inthe E7 vertical course. In addition, we linked the MOTsystem to laser devices in the laser spectroscopy room
(referred to as L) on the ﬁrst basement ﬂoor of the RIBF
building with 440 m-long polarization-maintaining (PM)ﬁbers. In this paper, we report the performance of thePM ﬁber link.
The ﬁber link consists of a polyvinyl chloride (PVC)coated 400 m ﬂexible stainless steel tube cable (Nippon
Steel Welding & Engineering, Picoﬂec) from L to the E7
entrance and another 40 m tube cable from the E7 entrance to the MOT system (Fig. 1(a)). Seven PM ﬁbers
are housed in each cable; among these, four are Fujikura
PM63 (cutoﬀ wavelength is 520–620 nm, mode ﬁeld diameter is 4.5 ± 0.5 μm at 630 nm), two are PM85 (650–800 nm, 5.5 ± 0.5 μm at 850 nm), and one is PM98
(870–950 nm, 6.6 ± 0.5 μm at 980 nm). Both cables
are connected using ﬁber connectors inside the opticalrack installed at the E7 entrance. The typical transmission loss due to the connector was 0.25. The optical rack was also expected to serve as a hub base for
optical transmission to other laboratories. The lightsof the trapping lasers (wavelength of 718 nm for 210 Fr,
780 nm for 87 Rb, used for oﬄine test) and repumpinglasers (817 nm for 210 Fr, 795 nm for 87 Rb), which werefrequency-stabilized within the natural linewidth of relevant transitions (several MHz) using a wavelength meter (HighFinesse, WS8-2), were transmitted to the MOT
system via ﬁbers (PM63). In general, frequency instability due to a ﬁber is low (∼3 Hz for 6 km at 778 nmin Ref. 3)), and thus, negligible for our optical transmission.
In the case of a high-power CW laser beam transmitted along a long ﬁber, there are concerns related to thelimit of transmitted power due to stimulated Brillouinscattering (SBS).4) To maximize the number of atoms
∗1
∗2
∗3
∗4

Center for Nuclear Study, University of Tokyo
Department of Physics, University of Tokyo
Graduate School of Arts and Sciences, University of Tokyo
RIKEN Nishina Center

Fig. 1. (a) Overhead view showing the path of a ﬁber link.
(b) Polarization crosstalk in the 400 m ﬁber and temperature measured at three sites. (c) Allan deviation of thecrosstalk.

to be captured, the trapping laser intensity (total of 6axes of 3D MOT) should be several times (8 times inRef. 5)) the saturation intensity of the cooling transition. The saturation intensity of 210 Fr (87 Rb) is ∼2.9(∼1.6) mW/cm2 . When using our trapping laser (SolsTiS, CW, linewidth <50 kHz), the SBS threshold output power for the 400 m ﬁber (PM63) was 30 mW at
718 nm. This value is suﬃcient for our three-axis 3DMOT system (beam diameter of 1 cm) with opposingmirrors.
The stability of polarization of the 400 m PM ﬁber
was investigated with a test laser (685 nm, 1 mW). The
polarization crosstalk calculated from the optical powerof the p- and s-polarized outputs split by the polarization beam splitter after launching from the 400 m PMﬁber and the temperature measured at three sites (L,E7, and N) are shown in Fig. 2(b). Site N is near theelevator on the second basement ﬂoor of the Nishina
Building (Fig. 1(a)). Figure 2(c) shows the Allan deviation calculated from the time series data of the polarization crosstalk. The PM ﬁbers housed in the PVC-coated
400 m tube were robust against large diel temperatureﬂuctuations along the ﬁber.
We achieved Rb MOT in July 2021 in an oﬄine test.
Online experiments are being conducted intermittently
to achieve Fr MOT, and we are getting closer to achieving it.
References
1) N. Shitara et al., J. High Energy Phys. 2021, 124 (2021).
2) N. Ozawa et al., RIKEN Accel. Prog. Rep. 54, 117 (2021).
3) B. deBeauvoir et al., Phys. Rev. Lett. 78, 440 (1997).
4) G. P. Agrawal, Nonlinear Fiber Optics, 5th Edition (Academic Press, Boston, 2013).
5) S. R. Muniz et al., Phys. Rev. A 65, 015402 (2001).

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