frequency metrology (original) (raw)

Definition: the field of technology dealing with precise frequency measurements

Category: optical metrology

• optical metrology
  • autocollimators
  • beam profilers
  • colorimeters
  • colorimetry
  • frequency metrology
    * optical frequency standards
    * optical clocks
    * optical clockworks
  • laser beam characterization
  • optical energy meters
  • optical frequency standards
  • optical power meters
  • optical power monitors
  • optical profilometers
  • optical spectrum analyzers
  • optical time-domain reflectometers
  • powermeters
  • photometry
  • polarimeters
  • refractometers
  • spectrographs
  • spectrometers
  • spectrophotometers
  • wavemeters
  • (more topics)

Related: optical frequencyfrequency combsoptical frequency standardsoptical clocksoptical clockworksbeat notestabilization of laserssynchronization of laserslaser absorption spectroscopytiming electronics for photonics

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DOI: 10.61835/jut Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Frequency Metrology?

This article is focused on optical frequency metrology, even though the connection from optical frequencies to microwave frequencies is also of central importance. For a range of applications it is necessary to determine accurately the absolute frequencies of optical signals. For the highest precision, it is not sufficient to measure a wavelength and convert it into a frequency by using the vacuum velocity of light as defined within the International System of Units (SI system): the accuracy of wavelength measurements (e.g. with wavemeters) is limited by effects such as wavefront distortions. Much higher accuracy can be achieved with real frequency measurements, where an optical frequency, or a difference between two optical frequencies, is related to a microwave reference. Note that the second as the basic time unit is currently defined in the SI system of units via a 9.192631770-GHz microwave frequency of a certain transition between hyperfine levels of the cesium-133 atom.

Frequency Differences and Absolute Frequencies

A relatively easy task is to compare the difference between two optical frequencies with a microwave reference, using a beat note, if that frequency difference is of the order of some tens of gigahertz or less. One simply superimposes the two beams on a fast photodetector and obtains an electronic beat signal. The latter can then be compared with a microwave reference either by counting the cycles or by monitoring an electronic beat between the two microwave signals.

A much more difficult task is to measure absolute optical frequencies. An early approach, taken by several metrology laboratories in the world, was based on a frequency chain which started with a stable microwave reference (linked to a cesium atomic clock) and generated exactly known higher frequencies with a number of other oscillators. The frequencies of the latter were connected to the lower frequencies by recording beat signals with harmonics of the lower-frequency signals, and automatically adjusting the oscillator frequencies so as to maintain not only given beat frequencies, but a phase-coherent connection. Various kinds of nonlinear devices (Schottky diodes, metal–insulator–metal diodes, and nonlinear crystals) were used for generating harmonics in different spectral regions. By operating the intermediate oscillators as so-called fly-wheel oscillators, good temporal coherence was achieved all the way from the RF region to microwaves, the far- and mid-infrared regions and further to visible light. However, the technology was very demanding, and having the whole system operate perfectly for long periods was a substantial challenge.

Frequency Comb Techniques

In the late 1990s, a new technique based on frequency combs from mode-locked lasers revolutionized optical frequency metrology. It is based on the fact that the optical spectrum of the output of a mode-locked laser consists of a comb of exactly equidistant lines (disregarding noise influences). This means that such a frequency comb is determined by only two parameters: the frequency spacing (which equals the laser's pulse repetition rate) and the absolute position, specified as the carrier–envelope offset frequency. If these two parameters can be related to a microwave reference, all optical frequencies of the comb are known. Subsequently, any optical frequency within the range of the comb can be measured by determining beat frequencies with comb lines.

Obviously, the frequency comb technique is much simpler than that based on a traditional frequency chain, and it makes it possible to construct very compact frequency reference sources and frequency measurement devices. Moreover, it delivers closely spaced lines of known frequencies in a wide spectral range, allowing for frequency measurements in this wide range, rather than only around a single optical frequency as for a frequency chain. Nowadays, frequency comb laser sources are commercially available and are beginning to be widely used for metrology purposes.

Technological and Scientific Applications

There can be no doubt about the technological and scientific importance of frequency metrology. The next generation of atomic clocks will be based on optical frequency standards, combined with optical clockworks. Such optical clocks would allow time or frequency measurements with a precision beyond that of the currently used cesium atomic clocks, which already makes the frequency physical quantity which can be measured with by far the highest precision.

Extreme precision in time measurements has many technological implications, as is obvious even considering only the many existing or envisaged applications of the American GPS system and the European Galileo system.

The measurement of other physical quantities such as electrical voltages and currents and also magnetic field strengths can also strongly profit from accurate and precise frequency standards. Furthermore, the clarification of fundamental scientific questions depends on ultraprecise time or frequency measurements; for example, such measurements are vital for checking whether there might be any time dependence of certain quantities (e.g. the fine structure constant ($\alpha$)) which are so far considered as physical constants. If any changes in such quantities could be detected, this would have a profound impact on future theoretical descriptions of most fundamental phenomena.

Also, some kinds of scientific devices, such as free-electron lasers for the generation of ultrashort pulses and arrays of radio telescopes for astronomy, require extremely precise timing synchronization of different parts, which can e.g. be done with actively stabilized fiber-based timing links.

Frequently Asked Questions

What is optical frequency metrology?

Optical frequency metrology is the field concerned with the highly accurate measurement of the absolute frequencies of optical signals, typically by relating them to a microwave frequency reference standard.

Why is direct frequency measurement more precise than deriving frequency from a wavelength measurement?

Direct frequency measurements achieve much higher accuracy because wavelength measurements are limited by effects like wavefront distortions. In contrast, an optical frequency can be directly compared with an extremely stable microwave reference defined by the SI second.

What was the traditional method for measuring absolute optical frequencies?

Before modern techniques, absolute optical frequencies were measured using complex frequency chains. These started with a stable microwave reference and used a series of phase-locked oscillators and harmonic generators to create a phase-coherent link to the optical domain.

How do frequency combs from mode-locked lasers simplify optical frequency measurements?

A frequency comb provides a wide spectrum of precisely known, evenly spaced optical frequencies. It allows any optical frequency within its range to be measured by a simple beat note comparison, replacing the complex, single-frequency-oriented traditional frequency chains.

What two parameters define all the frequencies of a frequency comb?

The entire set of optical frequencies in a frequency comb is determined by just two radio or microwave frequencies: the frequency spacing, which equals the laser's pulse repetition rate, and the absolute position, defined by the carrier–envelope offset frequency.

What are the most important applications of optical frequency metrology?

Key applications include the development of next-generation optical clocks that surpass current atomic clocks, improving navigation systems like GPS, enabling tests of fundamental physical constants, and providing precise timing for large scientific instruments like radio telescopes.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains five suppliers for frequency metrology. Among them:

⚙ hardware

The optical frequency discriminator (OFD) system of SILENTSYS smartly delivers a voltage signal that is proportional to the fluctuations of the optical frequency of the input laser beam. This turn-key module is suitable for laser frequency noise characterization and/or for laser frequency stabilization to drastically reduce its optical linewidth. The OFD features ultralow noise performances being successful in achieving frequency noise level as low as 0.01 Hz²/Hz with >60 dB noise reduction, and that is achieved in a compact and user-friendly package. This product is available in a huge wavelength range from UV, VIS to NIR, with one or two optical modules inside to be a very versatile tool.

The optical frequency correlator (OFC) system contains a common 2-input optical frequency discriminator (OFD). This makes it possible to frequency.stabilize two wavelength distant lasers onto the same optical reference in order to reduce their frequency fluctuations and to correlate them precisely.

Based on this fact, the optical beat frequency between the two stabilized lasers generates THz or GHz signals that reach a very low frequency noise level and are easily frequency tunable. Moreover, as a standard OFD, it smartly delivers a voltage signal that is proportional to the frequency fluctuations of the input laser beam. This turn-key device is suitable for laser frequency noise characterization and/or for laser frequency stabilization.

⚙ hardware

TOPTICA’s frequency metrology product line uses TOPTICA proprietary CERO-technology which is based on difference frequency generation (DFG). It is inherently _f_CEO-stable and is characterized by a high robustness combined with high-end performance.

Bibliography

(Suggest additional literature!)

(See also the references in the articles on frequency combs and optical frequency standards.)

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