Miniaturized Strontium Atomic Clock using an Integrated Photonics Package

The work addresses miniaturization of strontium (Sr) optical atomic clocks by integrating all required free-space beams for laser cooling, trapping, and clock interrogation onto a bonded planar photonics platform. Conventional systems rely on bulk optics to form six-beam magneto-optical traps (MOTs) at 461 nm and 689 nm and a vertical optical lattice at 813 nm co-propagating with a 698 nm clock probe, which limits compactness and robustness. The authors propose and demonstrate a planar integrated photonics approach that generates and arranges multiple high-NA beams with prescribed polarization, tilt, and collinearity to enable compact, manufacturable photonic integrated circuit (PIC)-based atomic clocks within a (25 mm)^3 volume.

The design builds on established six-beam MOT configurations for Sr but adapts beam diameters, propagation angles, and magnetic-field orientation for compact packaging. Prior demonstrations have used discrete optics for Doppler cooling (461 nm), narrow-line cooling (689 nm), and optical lattices (813 nm) co-aligned with clock interrogation (698 nm). The authors reference grating outcouplers and metasurface (MS) optics for beam shaping, control of numerical aperture, polarization, and tilt, and note recent demonstrations of expanding-beam MOTs by the authors at NIST and Vector Atomic. These prior techniques inform the integrated, bonded PIC–MS approach enabling large, divergent beams from micron-scale waveguides.

• System architecture: Two planar photonic chips separated by ~25 mm are placed above and below a vacuum chamber with optical windows. A single bottom PIC can emit all twelve MOT beams (six blue at 461 nm and six red at 689 nm). Half of the beams emit upward and are redirected by a top metasurface reflector to achieve 3D cooling; alternatively, identical top and bottom PICs can be used instead of a top reflector. Blue and red beams are azimuthally clocked by 35° to avoid overlap at the top reflector. Magnetic-field coils are positioned to align with the left-hand circularly polarized beam pairs; the magnetic-field axis is tilted for compactness.
• Beam generation and routing: Laser light is coupled via a V-groove fiber array, one fiber per beam to allow independent power control (future designs will incorporate on-chip beam splitters for single-fiber-per-color input). Emitters are placed around a 25 mm diameter circle to form six-beam MOTs (3 emitters vertical and 3 emitters at 45° per color). A separate vertical emitter pair generates the optical lattice (813 nm) and clock probe (698 nm), designed to be collinear and near-normal incidence.
• Emitter stack (three components):
  1. Evanescent coupler (EVC): Adiabatically converts a single-mode waveguide (~100 nm width) into a collimated dielectric slab mode with tailored Gaussian intensity (y-direction) by varying the gap between waveguide and slab. The slab mode is offset in angle (TE mode) by 31° at 461 nm and 24° at 689 nm to separate TE/TM and orient the desired TE polarization for the next stage.
  2. Meta-grating (MG): Subwavelength grating elements with apodized size control outcoupling strength to shape the free-space beam along x and act as a beam expander. The EVC+MG are realized in a single silicon nitride layer with oxide cladding and together function as a single-mode beam expander and integrated polarizing beam splitter. The MG period is apodized to emit a nominally collimated free-space beam.
  3. Metasurface (MS): Fabricated on fused silica with TiO2 nanopillars and bonded atop the PIC via patterned SU8 adhesive. The MS acts as a birefringent wedge, lens, and waveplate to tilt beams (e.g., to 0° or 45° relative to normal), adjust divergence to achieve desired waists at the MOT center (~12.5 mm above chip), and set polarization (circular for MOTs; linear, cross-polarized for lattice/clock). The MS also collinearly combines the lattice and clock beams with high precision.
• Geometry and specifications (Table 1): Within a (25 mm)^3 package, 461 nm beams are designed with wo ≈ 5 mm at the MOT location using 3 emitters at 45° and 3 vertical (2 RHC, 1 LHC); 689 nm beams with wo ≈ 2 mm similarly arranged; lattice/clock vertical emitters with wo ≈ 100 µm, linear and cross-polarized. Polarization fidelities exceed 75%.
• Assembly and alignment: Flip-chip bonding achieves RMS alignment between PIC and MS of 7 µm ± 3 µm translational and 3 µm ± 2 µm rotational using alignment marks around the 25 mm circle. The system is designed so unwanted MS diffraction orders do not intersect the MOT region.

• Integrated beam generation: A compact photonic chip system generates twelve circularly polarized MOT beams (461 nm and 689 nm) with diameters up to 10 mm (at 461 nm) within a (25 mm)^3 volume, plus a vertical optical lattice (813 nm) collinear with a clock probe (698 nm). Blue and red MOT beams are clocked by 35° azimuthally to avoid overlap at the top reflector.
• Collinearity and pointing: The lattice (813 nm) and clock (698 nm) beams are collinear to within 0.1° and oriented within 1° of chip normal.
• Polarization and power: MOT beams achieve >75% polarization fidelity. Power targets: ~3 mW per blue MOT beam (461 nm), ~1 mW per red MOT beam (689 nm); lattice >100 mW; clock ≈100 µW. On-chip losses are specified to be <12 dB.
• Loss budget (measured): Fiber couplers ≈5 dB (blue) and ≈3 dB (red); propagation 2 dB/cm (461 nm) and ≈0.5 dB/cm (689 nm); grating outcouplers ≈3 dB (both); metasurfaces ≈2–3 dB excess loss. Longest paths yield ≈15 dB (blue) and ≈10 dB (red) cumulative loss from fiber to MOT center.
• Alignment: Flip-chip bonding RMS misalignment of 7 µm ± 3 µm (translational) and 3 µm ± 2 µm (rotational).
• Emission angles (designed vs measured): Blue beams designed for 33.7° (for 45° redirection) and 10.3° (for 0°); measured 32.3° ± 0.8° and 10.5° ± 0.5°. Red beams designed for 30° and 8°; measured 31.0° ± 0.3° and 7.1° ± 0.7°.
• Beam waists (wo): Blue 45° beams: x = 129 µm ± 7 µm, y = 100 µm ± 5 µm; Red 45° beams: x = 140 µm ± 4 µm, y = 83 µm ± 7 µm. Blue 0° beams: x = 65 µm ± 6 µm, y = 102 µm ± 3 µm. Red 0° beams: x = 118 µm ± 9 µm, y = 77 µm ± 8 µm. Elliptical shaping yields symmetric 45° cross-sections with wo ≈ 100 µm.
• Diffraction orders and power in unwanted modes: MS higher-order leakage measured at 0.4% ± 0.3% (blue) and 45% ± 20% (red). Layout ensures these orders avoid the MOT region.
• Imaging and trajectory mapping: Twelve beams mapped at 4 mm above the MS show intended polarizations (RHC/LHC) and trajectories consistent with design; beam divergence and radial positions vs height are consistent with Gaussian fits.

The results demonstrate that a bonded PIC–MS platform can replace complex bulk optics for Sr atomic clocks by generating multiple large, precisely arranged beams with prescribed polarization and pointing in a compact form factor. Divergent beam design allows micron-scale emitters to produce centimeter-scale beams at the atom location, enabling efficient cooling while keeping emitters small and efficient. The approach achieves precise lattice–clock collinearity and near-vertical alignment, critical for systematic control in optical clocks. Although metasurface imperfections introduce unwanted diffraction orders—especially for the red beams—the system design steers these away from the MOT region, preserving cooling performance. Independent fiber inputs per beam enable power balancing to compensate emitter variability; future on-chip splitters will simplify interconnects. Overall, the platform provides a robust, manufacturable route to miniaturized atomic technologies with capabilities beyond discrete-component systems in terms of scalability and integration complexity.

A compact, bonded planar photonics platform combining PIC outcouplers with metasurfaces successfully generates all beams required for Sr atomic clock operation: twelve MOT beams (461 nm, 689 nm) and a vertical, collinear lattice/clock pair (813/698 nm). The system achieves high-precision collinearity and pointing, adequate polarization fidelity, and targeted beam sizes within a (25 mm)^3 package. Measured loss budgets and alignment tolerances validate the feasibility of integrated, manufacturable clock optics. Future work includes integrating on-chip beam splitters to reduce fiber connectivity, improving metasurface design and fabrication to suppress unwanted orders and reduce variability (particularly at 689 nm), optimizing outcoupler efficiency to lower loss, and further co-integration of auxiliary components (e.g., magnetic coils) toward fully miniaturized, robust optical clocks.

• Excess loss: Cumulative losses up to ~15 dB (blue) and ~10 dB (red) from fiber to MOT center; metasurfaces add ~2–3 dB; grating outcouplers ~3 dB.
• Metasurface higher-order leakage: Significant for red beams (45% ± 20%), indicating fabrication variability and reduced performance relative to design.
• Beam asymmetry/scatter: Blue beams show asymmetry due to pronounced scatter at the front of the grating where fabrication limits reduction of outcoupling strength.
• Variability: Red metasurfaces exhibit larger device-to-device variability; power balancing is required at the fiber inputs.
• Polarization fidelity: >75% may be sufficient but leaves room for improvement for precision clock operation.
• Alignment tolerances: While flip-chip alignment is good (few microns RMS), residual misalignment can affect beam pointing and overlap; unwanted orders must be steered clear of the MOT region by design.