Optical Clockworks
Author: the photonics expert Dr. Rüdiger Paschotta
Definition: devices which can phase-coherently relate optical frequencies to microwave frequencies
Categories: photonic devices, optical metrology
DOI: 10.61835/gkr Cite the article: BibTex plain textHTML Link to this page LinkedIn
In analogy with a mechanical clockwork, an optical clockwork is a device which phase-coherently relates a high and a low frequency and can serve as a central ingredient of an optical clock. The higher frequency is an optical frequency, i.e., typically in the range of hundreds of terahertz, whereas the lower frequency is typically in the microwave region (e.g. between 1 and 100 GHz), so that it can be processed with fast electronics and easily related to even lower frequencies. An optical clockwork can thus relate an optical frequency standard to an electronic one, the latter being based on, for example, a cesium atomic clock.
Early optical clockworks have been frequency chains, which involved a complicated combination of many nonlinear stages. Each of these stages related some frequency to a certain multiple of that frequency, often just two times the lower frequency, or with some known frequency offset. Such frequency chains were rather difficult and expensive to set up and stably operate over long times.
The clockwork serves to compare the optical and microwave frequency, and thus to transfer the superior long-term stability of the optical standard to the electronic time signal.
The advent of very broadband mode-locked lasers has made it possible to realize by far simpler optical clockworks, as the output of such a laser is a frequency comb, where all frequencies occurring are determined by just two parameters: the pulse repetition frequency and the carrier–envelope offset frequency. An optical frequency from some frequency standard (e.g. a single ion in a Paul trap) can then be expressed by the sum of the carrier–envelope offset frequency, a certain integer multiple of the pulse repetition frequency, and a beat note frequency, which can all be measured and processed with fast electronics. It is thus possible to phase-coherently compare the frequencies of the optical standard and a cesium clock and correct the timing signal of the latter, using the superior stability of the optical frequency standard. Figure 1 shows the setup of an optical clock which can be realized in that way. Note that the frequency comb does not necessarily have to be stabilized itself to fulfill its function [3].
See the articles on frequency metrology and frequency combs for more details.
More to Learn
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Suppliers
The RP Photonics Buyer's Guide contains 14 suppliers for frequency comb sources. Among them:
Octave Photonics
Octave Photonics offers electro-optic frequency combs at 5 to 30 GHz repetition rates. These combs can be fully stabilized using Octave's nanophotonic chip technology.
TOPTICA Photonics
TOPTICA’s Difference Frequency Comb (DFC) is a compact, robust and high-end solution featuring turn-key operation in a 19 inch format. The patented CERO technology uses difference frequency generation to intrinsically fix the νCEO at 0 Hz. This allows for one control loop less compared to standard f-2f-approaches resulting in lowest CEP noise and a decoupling of νCEO and frep. Thus, the DFC is the number one choice for anyone looking for high-end performance combined with a high level of robustness.
Menhir Photonics
The MENHIR-1550 SERIES is the first 1-GHz turn-key femtosecond laser at 1550 nm allowing for ultra-low noise optical frequency comb with wide comb-spacing. Hermetically sealed and all-in-one (laser and electronic is one box), Menhir Photonics’ products have been designed to achieve lowest phase noise combined with high-reliability and robustness.
Alpes Lasers
Alpes Lasers offers mid-IR frequency combs centred around 6 μm or 8 μm. The QCL comb is a stand alone device as it integrates the pump laser and the microcavity in its waveguide contrarily to other comb technologies. This makes it a very compact comb source. Being based on QCL technology, comb devices can be manufactured over all the MWIR and LWIR.
Menlo Systems
As the pioneer in the optical frequency comb technology, Menlo Systems offers a full product line from the compact and fully automated SmartComb to the ultra-low noise optical frequency comb FC1500-ULNplus. Our patented figure 9® mode locking technology ensures lowest phase noise and long-term reliable operation.
Bibliography
[1] | S. A. Diddams et al., “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb”, Phys. Rev. Lett. 82 (18), 3568 (1999); https://doi.org/10.1103/PhysRevLett.82.3568 |
[2] | R. Holzwarth et al., “Optical clockworks and the measurement of laser frequencies with a mode-locked frequency comb”, IEEE J. Quantum Electron. 37 (12), 1493 (2001); https://doi.org/10.1109/3.970894 |
[3] | H. R. Telle et al., “Kerr-lens mode-locked lasers as transfer oscillators for optical frequency measurements”, Appl. Phys. B 74, 1 (2002); https://doi.org/10.1007/s003400100735 |
[4] | S. T. Cundiff and Jun Ye, “Colloquium: Femtosecond optical frequency combs”, Rev. Mod. Phys. 75, 325 (2003); https://doi.org/10.1103/RevModPhys.75.325 |
[5] | J. Ye et al., “Optical frequency combs: from frequency metrology to optical phase control”, J. Sel. Top. Quantum Electron. 9 (4), 1041 (2003); https://doi.org/10.1109/JSTQE.2003.819109 |
[6] | J. E. Stalnaker et al., “Optical-to-microwave frequency comparison with fractional uncertainty of 10−15”, Appl. Phys. B 89, 167 (2008); https://doi.org/10.1007/s00340-007-2762-z |
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2023-12-20
Why use a frequency comb for obtaining the beat note instead of just using two frequency-stabilized lasers that are separated by some RF frequency? I am assuming that generating a comb would be more tedious than having another frequency-stabilized laser.
The author's answer:
Making two highly frequency-stabilized lasers is also not exactly convenient. Note that the explained scheme requires a mode-locked laser which does not need to be as precisely frequency-stabilized as the other laser, which serves as the actual frequency standard.