Compact, Portable Fabry-Pérot Reference Cavities

Kelleher, Megan L.

Graduate Thesis Or Dissertation

Compact, Portable Fabry-Pérot Reference Cavities Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/pk02cc191

Abstract

Lasers locked to optical cavities produce electromagnetic waves with exceptional frequency stability. The optical signals from optical-cavity stabilized lasers have applications in precision spectroscopy and optical atomic frequency standards. These signals can be used in gravitational wave detection and tests of fundamental physics, such as models for weakly interacting dark matter candidates. Using femtosecond frequency combs, the stability of the optical cavities can be transferred to the microwave regime. These microwave sources have some of the lowest phase noise of any microwave source. These microwave signals can be used for radar, better communications technology, and GPS.

The pursuit of portable vacuum-gap reference cavities arises from the need for rigid, compact, and robust laser frequency stabilization solutions in demanding and unpredictable environments. Many applications, such as portable optical atomic clocks, earthquake detection using undersea optical fiber, and low phase noise microwave generation, require sub- 10-13 instability in the optical domain, but the size, weight, and infrastructure demands of large or cryogenic cavity systems are incompatible with these applications. To address these challenges, I designed and developed three compact optical cavities. These designs represent promising steps towards achieving high stability performance while overcoming the limitations of traditional cavity systems, thereby opening up new possibilities for practical applications that require precise and portable laser frequency references.

The first design involves a series of two cavities with 6.3 mm long spacers made of ULE. These cavities were specifically tailored for low phase noise microwave generation. These cavities offer compactness while aiming to maintain high stability levels (1-2 x 10-14). One of the two cavities has 25.4 mm diameter ULE mirrors and is referred to as the ULE-ULE cavity. The other cavity has 12.7 mm diameter FS mirrors and is referred to as the FS-ULE cavity. High-bandwidth locking of the FS-ULE cavity demonstrates thermal noise limited laser noise to nearly 10 kHz. The ULE-ULE cavity was brought to a telecom fiber launch site and demonstrated remote operation. Both the FS-ULE cavity and the ULE-ULE cavity were used in a novel measurement of the cavity holding force sensitivity. The acceleration sensitivity of the FS-ULE cavity is better than 6 x 10-10g-1 along all mechanical axes.

The second design targets a portable Yb lattice clock. The spacer is made of ultra-low expansion (ULE) glass and is 25 mm long and 50 mm in diameter. The fused silica (FS) mirrors are 25.4 mm in diameter and 6.35 mm long with 10.2 m radius of curvature and crystalline coatings. This cavity has a thermal noise limited fractional frequency instability of ≈ 10-15. The design is highly symmetric, and the acceleration sensitivity is better than 2x10-10 per g along all mechanical axes. Preliminary phase noise measurements of a laser locked to the cavity show more than 90dB suppression of the free running laser noise, and thermal noise limited performance between 1 and 10 Hz. Preliminary measurements of the ADEV show that the laser lock is likely suffering from residual amplitude modulation (RAM) noise and drifting due to temperature. Further efforts are expected to improve the long-term stability of the cavity.

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