March 2006

EEEL Researchers Narrow Up Fiber Laser Frequency Comb

Optical frequency combs have been vigorously developed over the past seven years. Combs based on Ti:Sapphire lasers have been perfected at NIST in the Time and Frequency division and elsewhere and are now a tool used in high accuracy frequency measurement and in evaluating the stability of the optical clocks. In recent years, a second comb technology has emerged based on optical fiber laser frequency combs. These combs are similar in principle to the Ti:Sapphire laser-based combs but have several important distinctions. Fiber laser-based frequency combs are potentially more compact, less expensive and cover the near infrared region from 1 to 2 microns in wavelength. This region of the spectrum is particularly important as it contains the telecommunication band around 1.5 microns. Therefore, fiber laser-based frequency combs have the potential to impact telecommunication and sensing applications that use fiber optics and fiber optic devices. Unfortunately, the performance of fiber laser frequency combs has lagged considerably behind that of the original Ti:Sapphire laser-based frequency combs. Specifically, the width of the individual modes of the comb has been quite large. This broad linewidth will be a limitation on their application to the highest precision metrology needed to test optical clocks and for any time-domain experiments that exploit the carrier-envelope phase. In order to further the applications of fiber laser frequency combs in metrology, remote sensing, and time-domain experiments, it is important to improve the performance of fiber laser frequency combs, specifically to narrow the modes of the frequency comb.

Using both experimental data and our previously developed theory, we have recently identified a major cause of the broad optical linewidths as arising from white noise on the laser that pumps the fiber laser. By passively and actively removing this noise, we are able to narrow the beat note characterizing the offset frequency of the fiber laser comb to Hertz levels. The comb lines are still broadened by vibration and temperature effects, but these effects should be more easily removed by inserting the appropriate fiber stretcher or modulator in the laser cavity. Based on this work, there appears to be no fundamental reason why the fiber laser-based frequency comb cannot reach levels of performance close to that of the Ti:Sapphire laser-based frequency comb, while simultaneously providing the other distinct advantages of compactness, ease-of-use, and compatibility with fiber optic technologies.

Contact: Nathan R. Newbury, phone: 303-497-4227