U.S. military researchers are asking industry to develop relatively simple portable photonic integrated circuits (PICs) for high-performance position, navigation, and timing (PNT) devices as an alternative to the Global Positioning System (GPS) for when GPS signals are not available.
Position, Navigation, and Timing (PNT) is a critical resource for all Department of Defense (DoD) missions, affecting areas such as communications, navigation, reconnaissance, and electronic warfare (EW). Typically, PNT needs are met using the Global Positioning System (GPS).
However, GPS signals are vulnerable to a variety of disruption methodologies, and a backup to GPS is essential. Although in the absence of GPS, tactical-grade clocks and tactical-/navigation-grade Inertial Measurement Units (IMUs) can currently provide GPS-like accuracy for the short term, longer-term, GPS-independent strategies are required.
Atomic systems are the basis for the most sensitive and accurate angle sensors and clocks demonstrated. Large, lab-based atomic physics clocks and gyroscopes are the only technology known that deliver GPS-like timing and navigation for more extended time scales. Among these atomic systems, those that use trapped atoms have the potential to be made portable while maintaining their accuracy due to the small size of the atomic trap and the inherent isolation of the atomic system from environmental disturbances provided by the trap.
However, these trapped atom systems are currently large due to their optical system’s complexity. Historical approaches to miniaturization of their hundreds to thousands of optical components has relied on removing optical elements, miniaturizing the remaining elements, and then tightly integrating them in a small package. Although this has led to more compact atomic clocks and gyroscopes, the resulting system suffers from degraded performance and a heavy reliance on maintaining very tight optical alignment which causes poor environmental robustness and tolerance to design errors. This approach also makes them difficult to manufacture at a reasonable cost.
The objective of the A-PhI program is to reduce the complexity of trapped atom-based high performance Position, Navigation, and Timing (PNT) devices using photonic integrated circuits (PIC). A-PhI aims to demonstrate that compact PICs can replace the optical bench of conventional free space optics for high-performance trapped-atom gyroscopes and trapped-atom clocks without degrading the performance of the underlying physics package.
PICs have been the subject of many recent developments, demonstrating that they can replace optical systems with readily-manufacturable and low-cost chips that have none of the alignment sensitivity of conventional free-space optics. Such examples include on-chip optical frequency combs based on microresonators, optical frequency synthesis, novel and efficient on-/off- chip coupling, wavelength demultiplexers, and on-chip phased arrays for dynamic manipulation of light fields.
DARPA seeks innovative proposals for: 1) the development of portable Photonic Integrated Circuits (PICs) to reduce the complexity of trapped atom-based high-performance Position, Navigation, and Timing (PNT) devices; and 2) proving the feasibility and advancing the development of a trapped-atom gyroscope: a matter-wave analogue of the interferometric fiber-optic gyroscope.
“If we want to have navigation and timing capabilities at a device level rather than from a satellite [from GPS], what are the things we need to do? You need timing: GPS gives you timing information, so that is one technical area,” he said. “The other technical area: a gyroscope, which goes to IMU [inertial measurement unit], would be a device-level navigation device, said John Burke, the DARPA programme manager for A-PhI.” “The capabilities that we could get out of these things would be the most beneficial in the shortest amount of time,” Burke said.
Atomic systems are the basis for the most sensitive and accurate angle sensors and clocks demonstrated. Among these atomic systems, those that use trapped atoms have the potential to be made portable while still maintaining their accuracy due to the atomic trap’s small size and the inherent isolation a trap offers an atomic system from the environment (especially from acceleration).
Currently, these systems are large due to the complexity of the optical systems used to create the trap. Historical approaches to miniaturization of the hundreds to thousands of optical components present in these benchtop systems have relied on removing optical elements, miniaturizing the remaining elements, and then tightly integrating them in a small package.
Although this has led to more compact atomic clocks and gyroscopes, the resulting systems suffer from degraded performance and a heavy reliance on maintaining very tight optical alignment, causing both poor environmental robustness and poor tolerance to design errors. The current approach makes miniaturized atomic systems difficult to manufacture at a reasonable cost.
A-PhI aims to demonstrate that a compact PIC can replace the optical bench for high performance trapped atomic gyroscopes and trapped atom clocks without degrading the performance of the underlying physics package.
PICs have been the subject of much recent development that has demonstrated that they can replace optical systems with a readily manufacturable and low cost chip that has none of the alignment sensitivity of conventional free space optics.
These PICs will replace the optical bench behind atomic physics devices while still enabling the necessary trapping, cooling, manipulation, and interrogation of underlying atoms.
A-PhI will also prove the feasibility and advance the development of the trapped atom gyroscope, a matter-wave analogue of the interferometric fiber optic gyroscope. This will not only lead to an unprecedented reduction in system size, but will also generate an order of magnitude improvement in angular sensitivity and dynamic range over its free-space based predecessor.
Subsequent work will be required to incorporate the necessary compact-and-robust lasers and electronics to achieve a high-performance, portable PNT system.
DARPA seeks innovative proposals in the following two Technical Areas (TA):
DARPA issued a two-phase Broad Agency Announcement (BAA). The 18-month Phase 1 will be focused on developing the components that will eventually be used. Phase 2 will focus on building a working prototype.
Technical Area One: Development of a photonic integrated clock prototype.
This technical area will focus on translating known trap physics to a PIC-based architecture. This will require a PIC that delivers the cooling, trapping, and clock light (as well as any re-pump light, if required). This, in turn, requires advances in larger-area on-/off-chip couplers, as well as on-chip polarization control, to allow for proper trapping, manipulation, and interrogation of atomic states. In addition, a narrow-linewidth, on-chip oscillator at the clock frequency will need to be developed.
Phase 1 will focus on the demonstration of PIC-based atom cooling and trapping, and the development of a low-phase noise local oscillator. Phase 2 will focus on integration of the components (not including electronics and laser sources) and demonstration of robust clock performance.
Technical Area Two: Development of a trapped-atom gyroscope based on a Sagnac interferometer architecture.
The focus of this technical area will be to develop the appropriate trapped-atom physics for a gyroscope. This will require the creation of an atom trapping architecture that enables a confined atomic Sagnac interferometer, an atomic analog to interferometric fiber-optic gyroscope. Success will require the advancement of atom trapping techniques and the study of detrimental loss sources and interactions.
Once the atomic Sagnac interferometers are developed, the trapping architecture will be reproduced with PICs. The development of PICs to trap and interrogate atoms for gyroscope may be the subject of a future BAA.
Phase 1 will focus on the development of the trapped-atom gyroscope architecture. Phase 2 will focus on scaling the trapped-atom physics architecture to a larger enclosed area with greater contrast. A follow-on BAA is expected to be issued during Phase 2, which will focus on the translation of the developed trap architectures into a PIC format. The optical bench developed in Phase 2 must, therefore, be amenable to replacement with a PIC device.