Optical frequency combs have been vigorously developed over the past seven years at NIST and elsewhere. They provide a uniquely broadband and coherent optical output and are finding ever-increasing applications from frequency metrology to low phase noise microwave generation to spectroscopy. As the applications increase, it is becoming increasingly important to develop combs that can provide a high level of performance outside of the laboratory in the "field." Fiber laser-based frequency combs are an attractive candidate for field use since they can be compact, inexpensive, and power efficient. However, typical fiber laser designs will not work well in the presence of strong vibrations since the entire comb will not remain phase-locked to a reference; as a result, the comb will lose its coherence. Furthermore, the underlying laser may even stop mode-locking, causing the broadband frequency comb to cease to exist as the laser then emits only at a few frequencies.
Optoelectronics Division researchers Esther Baumann and Fabrizio Giorgetta, in collaboration with other researchers at NIST, built a fiber-laser frequency comb based on a very robust "figure-8" polarization-maintaining fiber laser built by Jeff Nicholson at OFS. This laser can be vibrated very intensely, and it never loses mode-locking. Furthermore, it incorporates a highbandwidth phase modulator so that it can remain tightly phase-locked even at greater than one g of acceleration. This work represents a first important step towards a robust, field-deployable frequency comb that can successfully operate out of the laboratory and even on a moving platform.