OPTICAL METROLOGY: Fiber-laser-based frequency comb nears vibration immunity
With applications in distance metrology, microwave generation, spectroscopy, and optical-waveform measurement, frequency combs have moved well beyond their initial application as a calibration tool for optical clocks.
With applications in distance metrology, microwave generation, spectroscopy, and optical-waveform measurement, frequency combs have moved well beyond their initial application as a calibration tool for optical clocks. Even though ytterbium- and erbium-based fiber-laser frequency combs are more robust than solid-state-laser frequency combs and can achieve a frequency stability of 10-18 or below, there is still excess phase noise at close-in Fourier frequencies; that is, the comb “teeth” still do not precisely follow the optical reference frequency. In the presence of vibrations, this noise is further increased, causing the comb to lose its phase lock with the reference oscillator (at high enough vibrations, some laser systems can even stop mode locking altogether).
Recognizing that a vibration-immune frequency comb could have numerous applications outside the laboratory environment, researchers at the National Institute of Standards and Technology (NIST; Boulder, CO) and OFS Laboratories (Somerset, NJ) have developed a figure-eight fiber-laser frequency comb that remains phase-locked even when subjected to mechanical vibrations of more than 1 g (as in g-force, not grams).1
A robust figure eight
The experimental setup for the frequency comb consists of a figure-eight laser that uses polarization-maintaining (PM) fiber for environmental robustness (see figure) and incorporates a fast intracavity electro-optic phase modulator (EOM) to achieve vibration immunity. The figure-eight laser produces output pulses that are amplified by an erbium-doped fiber amplifier (EDFA), compressed in approximately 47 cm of standard single-mode fiber to a full-width half-maximum of 53 fs, and then launched into 45 cm of highly nonlinear fiber (HNLF) to generate an octave-spanning continuum.
As always, stabilization of the frequency-comb output requires stabilizing two degrees of freedom of the comb. Here, the comb is stabilized by locking the carrier-envelope-offset (CEO) frequency to a radio-frequency source and an optical-comb tooth to a narrow-linewidth cavity-stabilized laser. Ideally, each frequency-comb tooth would then follow the reference cavity-stabilized laser regardless of vibrations. In this work, the comb is indeed very tightly phase-locked to this cavity-stabilized reference laser through feedback to both a slower piezoelectric transducer (PZT) and the fast EOM, which enables an optical lock with a bandwidth of 1.6 MHz—an order of magnitude wider than for previously demonstrated frequency combs. This results in excess phase noise equal to the shot-noise limit of –94 dBc/Hz at Fourier frequencies between 20 Hz and 200 kHz.
When the figure-eight cavity was placed on a breadboard connected to a speaker, an integrated acceleration of 1.6 g produced by a white-noise signal was not enough to cause the frequency comb to “unlock” from the reference laser. The all-PM figure-eight fiber-laser cavity is intrinsically insensitive to mechanical vibrations through its design, while the high-bandwidth EOM actuator can actively correct for vibration-induced length changes.
The residual vibration sensitivity of the comb was determined by driving the speaker with a swept sine-wave input ranging from 40 Hz to 3 kHz, staying below 10-13/g for frequencies up to 500 Hz and below 2.2 × 10-12/g for frequencies up to 3 kHz, well below the typical 10-10/g measured for the best cavity-stabilized lasers. In other words, the comb will not add excess noise beyond that of the underlying reference laser. With 1 g vibration immunity, the figure-eight frequency comb would be suitable for operation in buildings (vibrations to approximately 0.02 g), on spacecraft (vibrations to approximately 0.2 g), and possibly, on moving platforms (several-g vibrations).
Esther Baumann, NIST guest researcher, says, “This comb’s extraordinary vibration robustness not only made its handling really easy, it also stayed locked while we played music on the vibration setup’s speaker.” Fabrizio Giorgetta, NIST guest researcher, adds, “After it stayed phase locked during the overture of Bizet’s opera ‘Carmen,’ we are confident that this comb can handle the vibrational challenges of various field applications.”
- E. Baumann et al., Optics Lett. 34(5) p. 638 (March 1, 2009).