Distribution of an Ultrastable Frequency Reference Using Optical Frequency Combs

Frequency standards based on optical transitions in various atomic systems provide the potential for developing optical atomic clocks in the next few years that are several orders of magnitude more stable than the best existing clocks based on microwave frequency references. The excellent stability that will be provided by optical clocks, and even the stability they currently offer, has applications in several fields, ranging from studies of fundamental physics to communication and remote synchronization. However, since optical clock systems are too complex to be portable, these applications rely on the ability to transfer over several kilometers a stable frequency reference that is linked to an optical clock. In this dissertation I present my research that enables the use of optical frequency combs from mode-locked lasers for transferring over optical fibers frequency references that are linked to optical standards.

This method of transfer requires that the optical frequency comb be stabilized to the optical standard. This is most easily accomplished by first stabilizing the comb from a mode-locked Ti:sapphire laser, which is described in detail. In particular, since intensity control of the laser is used for this stabilization, an analysis is presented on the intensity-related dynamics of the laser, enabling optimization of the control scheme. To minimize loss during transmission, it is necessary to transmit a 1550-nm comb instead of the 800-nm Ti:sapphire comb. Therefore, the stabilization of a 1550-nm mode-locked laser diode to the Ti:sapphire laser is discussed. This involves both the synchronization of the two lasers and the locking of their optical phases. The lowest reported timing jitter for a mode-locked laser diode is demonstrated.

Finally, measurements of the stability for transferring a microwave frequency over several kilometers of optical fiber using an optical frequency comb are presented. Without active stabilization, the comb provides an order of magnitude higher stability than is measured for existing methods of microwave-frequency transfer over fibers. With active cancellation of the transfer noise, the lowest timing jitter reported for the transfer of a frequency reference over several kilometers using optical fibers is achieved.