A frequency comb is a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines that provides a coherent link between the optical and microwave domains. Frequency combs have seen rapid development since their inception in 2005 due to their wide range of applications. The next generation of frequency combs, which are targeted for miniaturization and field deployment, are still under development but have the potential to revolutionize a wide range of industries, including telecommunications, aerospace, and healthcare.
Despite their demand, frequency combs are difficult to manufacture and their performance requires careful validation. Challenges include:
Stability
Frequency combs require very high levels of stability in order to transfer spectral purity from one frequency domain to another. This is because small differences in the comb architecture can introduce non-common environmental noise into the system, which can degrade the performance of the comb. Additionally, insufficient bandwidth in the comb stabilization can potentially limit the applications of the comb.
Accuracy
Frequency combs are used as a ruler for measuring optical frequencies. This can be done by observing molecular absorption lines in spectroscopy or by comparing an optical clock against the primary caesium fountain frequency standard. Therefore, it is important to verify that the frequency comb can measure these features accurately and does not introduce any systematic offset. The required accuracy will depend on the specific application.
Miniaturisation
A full frequency comb system is typically a large, complex device that requires a lot of power and space (for example, a full-height 19-inch rack, large optical breadboard and hundreds of watts of electrical power to run the optical amplifiers etc). This makes it difficult to deploy a frequency comb beyond laboratories and into in real world industrial situations.
Despite the challenges of manufacturing and validating frequency combs, this technology is promising and has several commercial embodiments supporting a wide range of applications. As the technology continues to develop, we can expect to see frequency combs playing an increasingly important role in our day-to-day lives outside of the research environment. Here are a few examples of the applications of frequency combs:
Precision timekeeping
Frequency combs are already being used in research laboratories to develop the most accurate optical atomic clocks in the world. These clocks are about one hundred times more accurate than the current caesium primary frequency standard, which is the basis for the SI second. This has led to calls to redefine the SI second based on these optical clocks, which would revolutionise international timekeeping and the systems that rely on it.
Compact clocks
The miniaturization of frequency comb technologies is making it possible to integrate them into compact atomic clock systems that operate at novel optical frequencies. These compact systems could be deployed on mobile platforms for navigation, into remote data centres for synchronizing telecommunications networks, or placed in strategic locations for geopotential monitoring.
Frequency comb spectroscopy
Frequency combs are used for spectroscopic applications where the absorption of molecules and atoms can be observed within the broad spectral bandwidth of the comb. This knowledge could be used to develop new drugs and materials, to monitor pollution in the atmosphere or even in rapid detection of viruses such as COVID-19. The use of ultrashort optical pulses for spectroscopy can then be used to study the ultrafast dynamics of these atoms and molecules to further understand how they interact over time.
How NPL can support the development of Frequency Combs?
NPL has access to several state-of-the-art optical and microwave frequency references and commercial frequency combs that can be used to evaluate frequency combs on a variety of parameters, including their accuracy and stability.
To measure the comb instability, the repetition rate of the comb under test can be optically locked to one of NPL's optical ultrastable reference lasers. This transfers the spectral purity of the comb to another frequency domain, whose stability can then be measured via an out-of-loop comparison with one of NPL's optical frequency combs.
The frequency comb accuracy can be determined by measuring the same optical reference on both the comb under test and NPL's optical frequency comb to check for agreement between the combs.
NPL offers a valuable resource for researchers and developers of frequency combs and atomic clocks who require traceable optical frequency measurements and stable frequency references for performance verification. NPL's test and evaluation capabilities can also be applied to optical microresonator-based frequency combs, which have the potential to perform the same job as combs on a single chip with far less power. NPL can collaborate to evaluate the performance of these devices in a controlled environment and make sure that they are meeting the requirements of their target applications.
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