1.5  μm Lasers with Sub-10 mHz Linewidth

Silicon-Based Optical Mirror Coatings for Ultrahigh Precision Metrology and Sensing

“…Future detectors such as the Einstein Telescope [8,9] and upgrades to Advanced LIGO [10] are planned to be operated at cryogenic temperatures to reduce thermal noise. Optical resonators for laser stabilization, e.g., for optical atomic clocks [11][12][13], are another important application for HR mirror coatings with low thermal noise at low temperatures. However, due to a temperature dependence of material properties [14], cooling alone does not solve the thermal-noise problem.…”

“…HR coatings consist of alternating layers of materials with a high and a low refractive index n. In the simplest case, the layers are a quarter of the coating's design wavelength in optical thickness, which is n multiplied by the geometrical thickness t. Silica (SiO 2 ) and tantala (Ta 2 O 5 x), which are widely used coating materials [13,15], meet the requirements on the optical absorption at the ppm (10 −6 ) level [16]. However, the mechanical loss of these materials limits the thermal noise performance of mirrors at room temperature and shows evidence of a significant increase on cooling [17,18].…”

Dynamics of bad-cavity-enhanced interaction with cold Sr atoms for laser stabilization

“…We show that this distortion originates from the transfer function of the cavity itself, and thus cannot be circumvented. We have realized a system with a theoretically attainable shot-noise limited (SNL) laser linewidth of ν ≈ 40 mHz, possibly allowing laser performance at the level of the state-of-the-art reported values [5][6][7][8]. We use the system to map out the dynamical range and investigate the consequences of an imperfect cavity servo, which causes a mismatch of the cavity resonance with respect to the laser frequency.…”

“…The ability to reach exceedingly high accuracies within a reasonable time is made possible by the correspondingly huge effort to bring down the frequency noise in ultrastable laser sources [5][6][7][8].…”

“…Traditionally such interrogation lasers are stabilized to highly isolated optical reference cavities. This stabilization method is mainly limited by thermal fluctuations in the optical coating, mirror substrate, and cavity spacer [8][9][10][11], demanding considerable experimental effort in order to construct cryogenically cooled monocrystalline cavities and crystalline mirror coatings [5,8]. Several new approaches are being pursued in the so-called bad-cavity regime [12], in order to significantly suppress thermally induced length fluctuations.…”