The NIST prototype chip includes waveguides, a micromachined vapor cell and other components packed into a volume of less than 1 cm3 (black area at the left of the optical table shown). [Image: Hummon/NIST]
Spectroscopy of atomic vapors serves as the foundation for quantum sensors of magnetic and electric fields, time, length and other quantities. But it’s been tough to bring the scheme down to the scale of silicon-photonic chips for devices useful in field and industrial settings.
Now, researchers at the U.S. National Institute of Standards and Technology (NIST) have developed a prototype, 1-cm3 integrated photonic device that can perform precision spectroscopy on an ensemble of warm rubidium atoms (Optica, doi: 10.1364/OPTICA.5.000443). The team believes that the device could constitute a “miniature toolkit” for building compact, mass-producible quantum-based sensors for use in communications, navigation, instrument calibration and other fields.
Getting past bulk optics
Ensembles of atoms, trapped as vapors in small cells, are exquisitely sensitive to external fields or perturbations, and many sensor devices already use light, and specifically precision spectroscopy, to probe the quantum states of such ensembles. Those devices, however, tend to be built by hand with bulk optics.
Efforts to integrate precision spectroscopy of atomic vapors onto smaller-footprint devices have generally taken one of two approaches: sending light through a small waveguide, and having the evanescent light near the waveguide interact with surrounding atoms; or putting the atoms to be probed within a hollow-core waveguide itself. The limited overlap of the optical mode with the vapor, however, makes the former approach a poor fit for precision spectroscopy, according to the NIST team. The hollow-core-waveguide scheme, meanwhile, suffers from complications related to the presence of multiple modes in the fiber, which can lead to frequency shifts that muddy the signal.
Waveguide to free space
For the new device, the NIST team opted for a third way: one that packages the warm atoms to be probed in a 27-mm3 micromachined closed cell, and uses CMOS-compatible, 300-nm-wide silicon nitride (Si3N4) waveguides to deliver the light to the atoms.
A key issue the team needed to address in that geometry was how to convert the single-mode light from the narrow waveguide to a wider light field that would interact with enough of the atoms in the cell to return a good signal. The researchers solved the problem by attaching a specially designed “extreme mode converter” apodized grating to the end of the Si3N4 waveguide, where it couples the fiber light mode to free space.
The grating radically expands the mode’s diameter from 500 nm in the fiber to 120 microns in free space—while still maintaining the beam’s single-mode characteristics. The free-space beam thus passes through the cell with a large enough cross-section to probe the energy-level transitions of roughly 100 million Rb atoms, while still avoiding the potential confusion arising from overlap of multiple optical modes. After passing through the vapor cell, a portion of the beam is bounced via a reflective neutral-density filter to interact with a probe beam and create a saturated-absorption spectrometer.
Compact and mass-producible
In tests with the prototype platform, using an external distributed Bragg reflector laser, the NIST team was able to demonstrate an optical frequency reference at 780 nm, with a stability of 10−11 over a 1- to 1000-second interval, a performance that was verified by comparison with data from a separate frequency comb.
That’s not quite at the level of bulk-optic approaches involving glass-blown vapor cells, which can achieve stabilities as great as 10–13 over 100 seconds. But those larger systems have volumes of 100 cm3 or more, compared with the sub-cm3 volume of the integrated platform. Further, the lead author, Matt Hummon of NIST, noted in a press release accompanying the work that, while the chip currently uses an external laser and sits on a small optical table, “in future designs we hope to put everything on the chip.”
More important, the team leader, NIST scientist John Kitching, added, is that the setup consists of silicon-photonics-friendly components that could prove amenable to mass production. That, in turn, could make it an intriguing candidate for building a variety of cost-effective, field-ready quantum sensors for use in industrial settings, health care and more.