Design and Construction of an Affordable Adjustable Repetition Rate Optical Frequency Comb

TL;DR

This work tackles the need for an affordable, adjustable-$f_{rep}$ optical frequency comb suitable for dual-comb spectroscopy. It adopts a figure-9 erbium-doped fiber laser architecture and adds a free-space section, TEC-based cooling, dual PZT actuators, and an EDFA to enable precise tuning and long-term stability of $f_{rep}$. Key outcomes include a repetition rate around $f_{rep} \approx 100$ MHz with a tunable range of about $3$ MHz, long-term drift below $1$ Hz, and a demonstrable heterodyne beat indicating proper comb structure and readiness for phase-locking of $f_{rep}$ and $f_0$. The design shows that high-precision, dual-comb-capable photonic systems can be built at undergraduate scale and modest cost, broadening access to advanced spectroscopy techniques for smaller labs and liberal arts institutions.

Abstract

We designed and built an adjustable repetition rate optical frequency comb using off-the-shelf components with several improvements to an established erbium fiber design. This design, built fully by an undergraduate, was assembled at a fraction of the cost of a commercial comb. This comb has a repetition rate of roughly 100 MHz with a tuning range of 3 MHz. A stabilization control loop for $f_{rep}$ achieved long term drifts of < 1 Hz over several days. This comb will eventually be implemented for use in dual-comb spectroscopy.

Figures (2)

• Figure 1: (a) A diagram of the "figure-9" design inside a sealed aluminum box. The $f_{rep}$ of the comb is roughly 100.037 MHz, which is achieved using roughly 2 m of fiber, 58 cm of which is EDF, and 14 cm of free space. 980-nm pump diodes drive both sections of EDF. Output 1 is the output of the free-space PBS, and output 2 is the 99/1 coupler output, which in the diagram is amplified in the EDFA. Red Xs indicate splices in fiber. (b) The adjustable free-space section, where the collimator position (left) can be adjusted coarsely by sliding the translation stage along the cage rails and finely using the translation stage's micrometer. The Faraday rotator (FR) is attached to the translation stage.
• Figure 2: (a) Spectral output from the comb recorded on an optical spectrum analyzer with free-space output in blue and 99/1 coupler output in red. (b) Heterodyne beat between comb tooth and narrow-linewidth laser. The measurement was taken with a resolution bandwidth of 30 kHz. (c) $f_{rep}$ control loop performance, demonstrating the system's ability to reach a target $f_{rep}$ within minutes. (d) Long-term stabilization of $f_{rep}$, showing drifts of only 0.2 Hz over the course of 15 h. (c) and (d) were both measured using a HP 5385A frequency counter with 0.1-Hz resolution.