Advanced Technologies for Optical Frequency Control and Optical Clocks

An interdisciplinary group of investigators at the Research Laboratory of Electronics at MIT have joined with collaborators from industry and government laboratories to develop a set of key technologies for advanced optical metrology and ultraprecise optical clocks. This project, sponsored by the MURI program of the Department of Defense, seeks to seize the current opportunity to greatly increase the precision of time and frequency standards that arises from recent advances in ultrashort-pulse modelocked lasers and in ultracold atom and ion physics.

A major goal of the program is the extend the accuracy of optical metrology and frequency standards by application of femtosecond technology to fundamental studies of ultracold hydrogen. As a primary optical frequency standard, ultracold hydrogen offers truly exciting possibilities because of the narrow linewidth and high signal rates. This program will seek to develop a new source of ultracold hydrogen with improved optical access, perform precision 2S -nS spectroscopy in the range of 720-800 nm, and develop the design and apparatus needed for optical clock applications.

The MIT team is led by Professor Erich P. Ippen, and senior investigators include professors Yoel Fink, Thomas J. Greytak, Franz Kärtner, Leslie Kolodziejski, Daniel Kleppner, and Jeffrey Shapiro, as well as Dr. Franco N. C. Wong. Collaborating organizations include the Office of Naval Research, the Air Force Research Laboratory, Lincoln Laboratory, Lucent Technologies, Spectra-Physics, Omniguide Communications, and Nanolayers.

Femtosecond Lasers -- The objective of this project is to demonstrate the operation of a variety of prismless and compact femtosecond lasers for optical comb generation. Some of these will be diode pumped. Laser systems will be created that generate optical combs spanning 2/3 of an octave, one octave, and even 1.5 octaves. Diode-pumped 10-30 fs lasers for optical comb generation will also be constructed in a variety of microstructured fibers an din novel highly nonlinear omniguides to be developed in this program. In addition, the comb will be extended using nonlinear optical crystals, highly efficient PPLN-structures, and a 3:1 self-phase-locked optical frequency divider, also to be developed in this program. These techniques will extend the comb into the visible and permit self-referencing. Finally, these combs generated directly from the laser will be stabilized and locked, to the 1S-2S hydrogen transition as well as to a cesium clock in order to use them for hydrogen spectroscopy and to study the ultimate capabilities and limitations of the different technologies.

Optical Frequency Metrology with Ultracold Hydrogen -- The objective of this project is the implementation of an optical comb and the demonstration of a hydrogen clock. In the first phase, an optical comb will be created in the range of 650 - 1000 nm. The sideband spacing will be controlled by an atomic clock. This phase of the project will also include demonstration of spectroscopy of trapped hydrogen from the 2S state. The second phase of the project will involve increasing the resolution of the 1S-2S signal and locking a self referencing comb to it. The final phase will be the development of an ultracold hydrogen apparatus with high collection efficiency, suitable for optical clock applications.

Nonlinear Optical Frequency Conversion -- Quasi-phase-matched nonlinear optics can play a significant role in improving and extending the performance of octave-wide optical frequency combs (OOCs) by efficiently generating harmonic, subharmonic, and auxiliary comb frequencies for OOC phase locking and frequency referencing. Periodically-poled lithium niobate is a highly nonlinear crystal that can be wavelength tailored, via quasi-phase matching, for efficient second harmonic generation, difference-frequency generation, and parametric oscillation. This project will involve the design, fabrication, and implementation of three such nonlinear optical devices: a chirped-grating second harmonic generator; a chirped-grating difference-frequency generator; and a self-phase-locked optical frequency divider.

Novel Nonlinear Mirrors -- Nonlinear mirrors will be important components in robust femtosecond laser systems. They can be designed to assure self-starting of the modelocking process, they can play a role in improving amplitude stability, and they can be used to lock two different lasers together. This project will involve the design, engineering, and fabrication of a variety of saturable absorber mirrors for implementation in short pulse mode-locked laser systems. Graded layers of AlGaAs, in which the aluminum fraction varies at the interfaces, will be used in an approach that will provide saturable absorber mirror structures with very wide bandwidth and high reflectivity and with the appropriately designed absorbing region. The focus of the project will involve high index contrast mirrors composed of GaAs and oxidized AlAs.

Microstructured Fibers -- This project will investigate spectrally broadened combs produced by propagation in microstructured fibers. Limits on comb frequency range, limits on comb intensity and the influence of fiber structure on comb stability will be examined. A variety of microstructured fibers will be obtained, in addition to the design, manufacture, and investigation of novel, highly nonlinear, hollow core fiber fabricated at a new facility. The objective will be to construct transmission fibers that will have strong nonlinear properties.