Modeling Optical Time and Frequency Generation and Transfer Systems

September 1, 2022

DARPA, Arlington, Virginia

As originally proposed, this research was closely tied to work that was led by Nathan Newbury at NIST, and the principal goal was to support the development of advanced laser sources.

In order to develop robust, carrier-envelope-phase-locked sources that can be transported without losing lock, the Newbury team used semiconductor saturable absorbing mirrors (SESAMs) to provide saturable absorption in their laser systems. This technology replaced nonlinear polarization rotation using the Kerr effect in optical fibers to provide the saturable absorption. As was demonstrated theoretically and the Newbury group had seen experimentally, nonlinear polarization is not a stable source of saturable absorption because the polarization state of standard optical fibers varies randomly due to environmental perturbations. The use of SESAMs, combined with polarization-preserving fibers, solved this problem. However, it led to some unexplained issues. First, it was found that it was not possible to operate too close to the zero dispersion wavelength. Second, it was found that in some cases, parasitic frequency sidebands appeared.

This necessitated explaining both of these phenomena and determining how to avoid them. This goal was accomplished within the first three years of the project. It was also proposed that atmospheric effects that would limit the performance of the free-space optical frequency transfer system that the Newbury team developed be examined. This proposed goal turned out to be superfluous because atmospheric effects were not a significant limit.

In the last two years of the project, the goals were to optimize the laser parameters to obtain higher pulse energies and to increase the wall plug efficiency, both of which were achieved. Unfortunately, this work did not have a strong impact on the experimental work of the Newbury team. At a fairly early stage in the DARPA PULSE project, the team effectively froze their laser development and focused on developing good control over their multiple laser combs using field-programmable gate arrays (FPGAs) that could be computer controlled and controlled remotely and on the application of their laser systems to free-space frequency transfer.

Nonetheless, the techniques that were developed to characterize the stable operating regimes of the lasers and their noise performance appear likely to be of significant future value. Algorithms were developed that perform many orders of magnitude faster than standard techniques that are based on brute-force solution of the evolution equations.

An additional goal of the project was to model solitons in microresonators. The goal in this case was to examine alternatives to the conventional approach to obtaining solitons, which is to start the system in a chaotic state and move to a parameter regime in which solitons will randomly appear. While engineering solutions have been found that can produce solitons with reasonable reliability, they are still randomly generated, and the search for a deterministic path to obtain solitons continues. At the suggestion of Dr. Andrew Weiner of Purdue University, the study of transverse mode interactions and soliton molecules was pursued, as was work on cnoidal waves (also known as Turing rolls). The work on cnoidal waves has been highly successful. A path was identified that can produce a broadband train of solitons deterministically.

This work was performed by Curtis R. Menyuk of the University of Maryland Baltimore County for DARPA. For more information, download the Technical Support Package (free white paper) at mobilityengineeringtech.com/tsp under the Optics, Photonics & Lasers category. DARPA-0016

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This Brief includes a Technical Support Package (TSP).

Modeling Optical Time and Frequency Generation and Transfer Systems

(reference DARPA-0016) is currently available for download from the TSP library.

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