Optical Clockworks
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: devices which can phase-coherently relate optical frequencies to microwave frequencies
Alternative term: optical frequency dividers
Categories:
- optical metrology
- frequency metrology
- optical frequency standards
- optical clocks
- optical clockworks
- frequency metrology
Related: optical frequency standards optical clocks frequency metrology frequency combs
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DOI: 10.61835/gkr Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn
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What are Optical Clockworks?
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.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is an optical clockwork?
An optical clockwork is a device that phase-coherently links a high optical frequency (hundreds of terahertz) to a much lower microwave frequency. This allows the high stability of an optical frequency standard to be transferred to an electronic signal, which can be processed by electronics.
How do modern optical clockworks function?
Modern optical clockworks are based on frequency combs generated by mode-locked lasers. The comb provides a ruler of evenly spaced optical frequencies, allowing a direct comparison between an optical frequency standard and a microwave reference by measuring the laser's repetition rate and carrier-envelope offset frequency.
What technology did frequency-comb-based clockworks replace?
They replaced complex and cumbersome frequency chains, which used a cascade of many nonlinear stages to step down the frequency. Such chains were difficult and expensive to build and operate stably.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 15 suppliers for frequency comb sources. Among them:
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.
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.
RP Photonics offers competent consulting and tailored training courses on frequency comb sources. There is detailed expertise, e.g. on mode-locked lasers and noise sources determining the noise performance of frequency comb sources.
The MENHIR-1550 SERIES is the first 1-GHz turn-key femtosecond laser at 1550 nm, offering an ultra-low noise optical frequency comb with wide comb-spacing. This product is hermetically sealed and integrates both laser and electronics into a single unit, designed for low phase noise, high reliability, and robustness.
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.
The K2-1000-mini is a next-generation 1 GHz repetition rate femtosecond laser, engineered for high-power, ultra-low noise frequency comb generation. This delivers massive increase in power per comb line, significantly boosting signal-to-noise ratio for precision metrology applications.
Designed for turnkey integration, K2-1000-mini features:
- High peak power for efficient supercontinuum generation across a broad spectral range.
- Compact, quasi-monolithic design, ensuring robustness and high long-term stability.
- Dual-comb modelocking option from a single cavity.
Proprietary dual-comb modelockng technique eliminates active stabilization, simplifying system integration and enables direct dual-comb spectroscopy, reducing complexity.
With its GHz-class performance, compact form factor, and unmatched dual-comb capability, the K2-1000-mini sets a new standard for frequency comb sources in scientific and industrial applications.
The RUBRIComb® Frequency Comb is built and optimized for leading lab hero experiments and rugged field use. 20G shock tested and capable of transferring sub-hertz linewidth stability. Stays phased locked for days, weeks, months, years.
RUBRIColorTM is a modular extension of the RUBRICombTM platform that drives selectable wavelengths from 490 nm up to 2000 nm, offering a dramatic increase in leading lab hero experiments and rugged field use, thus driving reduced complexity while increasing stability for quantum computers, optical clocks, quantum sensors, and quantum networking.
Bibliography
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Questions and Comments from Users
2023年12月20日
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.
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