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What is an optical frequency comb?

What is an optical frequency comb: An optical "frequency comb" is a very accurate tool for measuring different colors - or frequencies of light. Techniques made by recent advances in ultrafast laser can accurately measure many frequencies more than any other device. Optical Frequency combs are already widely used in many sectors but specially used in  metrology laboratories and physics research, and they are starting to become commercially available.

The optical frequency comb fully depends on the relation between time - obviously a familiar concept and frequency, which is less familiar to most people, but the number of oscillations per unit of time. Scientific start with  lasers that emit a continuous train of very brief, closely spaced pulses of light containing a million different colors.Over time, properties of light are converted into frequency numbers to look like comb. Time and frequency are related in contrast; That is, small units of time (or sharp oscillation of light waves) result in large frequency numbers.
detail explanation of optical frequency comb
Optical frequency comb image

Graphic of optical frequency comb

We will also see some different colors of light through the graphics, which shows how differently over time, the graphic below shows how different colors of light are visible over time. This example is very simple, and specific units are unimportant (in reality the units will be small fraction of seconds). The essential point is that the blue waves are much faster than the red waves, and the yellow and green waves are somewhere.

optical frequency comb and colors of light explanation over the time
Blue wave much faster than red

A simplified graphic of related frequency comb is shown below. Each "tooth" of the comb is a different color, which is how fast it is arranged in the time of light wave. The waves that are slowly red (red) are on the left side and the waves that fast blue (blue) are on the right side. Frequency is measured in Hertz or cycle per second. A real optical comb does not start from zero on the left, but in very large numbers, 300 trillion Hz.

Comb of frequency with different colors
tooth of the comb with different colors

A realistic optical frequency spreads to the entire visible spectrum of the comb light, and it has a very fine, evenly spreading tooth. The teeth can be used like a ruler, to measure the light emitted by exceptionally high precision lasers, atoms, stars or other objects.

Use of laser type to make comb is important for the accuracy of the ruler. Less laser pulses, broaden the range of frequencies in the comb. Scientists use "mode-lock" lasers, which emit femtosecond pulses lasting quadrillion of a second or or millionths of a billionth of a second.

As a result, the comb spreads several hundred thousand frequencies or teeth, enabling flexible and accurate measurements of various events widely and widely.Mode-locking means how laser light is made in pulses. In all lasers, the light is reflected within the reflected cavity repeatedly. In a mode-lock laser, peaks of different colors of light waves match at regular intervals, spread evenly over time. Peaks are formed by very little, bright bursts on each other, each of which has many different frequencies.

Coinciding light wave at regular interval in optical frequency comb
Colors or light wave coincide at regular interval

The time between the pulses determines the difference between the comb's tooth. Scientists use laser emitting 1 billion pulses per second. Rapid pulse repetition rate, wide differences between teeth, makes it easier to identify each individual tooth.

Finally, the stability of the laser determines the width of individual comb teeth. A highly stable laser produces very fine teeth, which enables a highly accurate measurement of changes in specific frequencies or change in frequency. Special crystals, mirrors, and other techniques are used to make light waves and comb teeth fully possible.

Whose contribution is in Optical Frequency Combs

JILA's physicist John Hall, NIST and the University of Colorado at Boulder shared a 2005 Nobel Prize in Physics in for contributing to the development of laser-based precision spectroscopy including optical frequency comb technology. For example, Hall and colleagues developed ways to stabilize lasers and a "self-referencing" technique that ensures comb teeth in the right places. It involves taking two measurements from different parts of a very wide comb and comparing the results with the exact known frequencies of an atomic clock.

NIST physicists and colleagues were about to compare the operation of many femtosecond frequency trunks, which demonstrated the ability to reproduce, and to verify that the differences between the initial position of the comb and spacing between the teeth are controlled properly.

NIST scientists have also demonstrated the most accurate synthesis of optical frequencies, which produce specific colors with the 19-digit reproduction. The experiment was an important step in the direction of next-generation "atomic clocks" based on optical rather than microwave frequencies.

NIST employees and colleagues have also increased the reach of frequency comb. A project expanded wavelength coverage to 1,000 nanometers (a measure for the wavelength of light) in infrared more than ever before, while another attempt by JILA made the world's first frequency comb in extreme ultraviolet. Apart from this, NIST has shown that very stable microwave signals can be generated from the optical frequency comb.

Frequency Combs Applications in Metrology and Other Areas

Frequency combs can be used to measure absolute optical frequencies. More precisely, this means that the optical frequency is related to the microwave frequency eg. from a cesium clock. In other words, a frequency comb can work like an optical clock.

Frequency comb can also be used to measure the proportion of optical frequencies with highly precision, which is also not limited by laser noise. In addition to Frequency Metrology, other applications in high-precision spectroscopy, optical sensing, distance measurement, laser noise characterization, telecommunication, and in fundamental physics are possible.

Others desirable Application of a frequency comb

Frequency combs improved the accuracy of frequency metrology frequency combs have dramatically simplified for the same. They are also making possible optical atomic clocks, which are expected to be 100 times more precise than today's best time-relative systems.

Better clocks, for example, will study the sustainability of nature's constraints over time, and will enable advanced technology for advanced communication and accurate navigation systems, such as the next-generation global positioning system.

Today's best atomic clocks, and the international definition of the second, are based on the natural oscillation of cesium atoms, which is a frequency in the microwave region of electromagnetic spectrum. The optical comb provides regular "Gears" located, which can be used to connect the slow "ticks" of microwave-based atomic clocks more quickly, the more accurate "tick" of optical clocks.

microwave and optical gear clock by frequency comb
Gear clock

For many other advanced areas of science, highly accurate measurements of frequencies are also required which require identification or manipulation of atoms or molecules, such as the detection of toxic biochemical agents, the study of ultrafast dynamics and quantum computing.

Scientists are constantly trying to find out how to improve frequency comb technology and made it easy to use, it can be applied in many other research areas and technologies, from medical tests in doctor's offices, to synchronization of advanced telecommunication systems, for remote detection and boundary measurements for manufacturing or defense applications.

Frequency combs in the form of "gear"of a clock

A regular, continuous, or repetitive process or action should be taken to mark all clocks to mark the equal pay increase of time. Examples include the movement of sun across the sky, a pendulum or vibration crystal, or natural vibration of atoms in atomic clock.

These days standard atomic clocks vibrate at microwave frequencies, about 9 billion cycles per second. Optical atomic clocks swell very rapidly, approximately 500,000 billion cycles per second and thus divide time into smaller units.

But there is no electronic system that can calculate these oscillations directly. A frequency comb, working like an electronics in a traditional watch, can be used to split oscillations of optical clocks into low frequencies that can be linked to microwave standards and can be counted.

Why Frequency combs a hot topic in research?

Frequency comb in research has become a hot topic, and has attracted even more attention because the Nobel Prize was awarded in Physics to Roy J. Glauber, John L. Hall and Theodor W. Hänsch in 2005. The latter two contributed pioneer. The development of optical frequency comb technology.

An optical frequency comb is an optical spectrum, which contains equidistant lines, i.e. it contains eccentric optical frequency components, while the intensity of comb lines can vary considerably.Typically, such optical spectrum is associated with a regular train of ultraviolet pulses, there is a fixed pulse recurrence rate, which determines the inverse line vacancy in the spectrum. To understand how such a spectral shape is generated, one has to consider the properties of the Fourier conversion,translating the complex amplitudes from the time domain to the frequency domain.

Frequency comb as an optical ruler

Frequency comb can be worked as optical ruler: If comb frequencies are known, then frequency comb can be used. To measure the unknown frequencies by measuring beat notes, which reveals the difference in frequency between unknown frequency and comb frequencies. For such measurement performance in a wide frequency range, a larger overall bandwidth of the frequency comb is required.

First try was that to produce the broadband frequency comb were based on the firmly operated electro-optic modulator, which could have dozens of sidebands on single-frequency input beam from single-frequency continuous-wave laser. It was then found that this process can be made more efficient (to obtain more comb lines) by keeping the modulator in a resonant cavity, especially when the intracravity dispersion was minimized. Further improvements were based on parametric amplification.

Such devices acquired a growing similarity to the mode-locked laser for ultrasearch pulse generation, and in fact it was realized that the use of a femtosecond mode-locked laser to actually create very broadband frequency comb can be done: optical spectrum of a periodic pulse. The train, as it is produced in a mode-locked laser, consists of discrete lines with absolutely constant spacing, which is equal to pulse repeat frequency.

If the duration of the pulse falls significantly below 1 ps, the optical spectrum becomes very wide, which causes a very wide frequency comb. Using strong nonlinearities outside the laser resonator, one can widen the comb. Often, optical fiber (often photonic crystal fiber) uses such supercontinum generation for such spectral broadening, and it often leads to the octave-expanded optical spectra.

Note that the generation of a frequency comb requires periodicity not only which is applicable to the pulse envelope, but also in the whole electric field of pulses, along with their optical phase, in addition to a continuous phase slip.

In other words, reconciliation between pulses is essential. Typically, the pulses from the mode-locked lasers exhibit a large amount of mutual compatibility, in which only the random phase changes during several resonator round-trip. The effects of residual noise on the comb are discussed below.

How does noise in frequency comb

The issue of noise in the lines of a frequency comb is complicated and interesting. Various vibration sources, such as mirror vibration, thermal drift, pump intensity noise and quantum noise, pulse recurrence rate and carrier-envelope offset frequency cause different and partially correlated combination of noise. In addition, all lines of frequency comb have some level of noise, which is not correlated.

To a lesser extent, the rubber band model can also be applied to noise in more general terms. In particular, quantum noise (resulting from spontaneous emission in the gain medium) causes the phase changes of acting lines in a laser with relatively long pulses (not some-cycle pulses), which ar e described almost by a certain point near the optical center can be done. The frequency of the spectrum, however, is not some additional noise described by this definite point.

The phase change timing according to the fixed point mentioned in the phase is complicated, but not the same as the length of the cavity can be due to fluctuations, because the fixed point is very different in the spectrum. One consequence of this is that the frequency of the cavity will be very strong noise of the CEO frequency due to the quantum-induced time jitter compensation.

Another important theoretical discovery is that quantum-limited CEO noise of mode-locked lasers with relatively long pulses is larger than laser with some cycle pulses, but otherwise with similar parameters. Actually, compared to titanium-amethyst lasers, there are sufficient experimental evidence for strong noise from fiber lasers, which generate small pulses.

However, there is also a significant effect of pump noise on fiber lasers. In addition, further spectral expansion of fiber laser output in a photonic crystal fiber can introduce additional noise.

Regarding the description of the noise in a frequency comb, there are some warnings related to the conception of the CEO noise. The most obvious and most harsh approach is to perceive noise as a fundamental phenomena in all the lines of spectrum. Timing jitter and CEO noise can be seen as different one-dimensional sub-space in the form of estimates of this noise.

For such applications, there is desirable properties of a frequency comb source

➥Optical frequency comb cover the optical frequency range of interest.
CEO frequency should be able to accurately measured 
Sufficiently high optical powers required for the comb lines 
The effect of noise (both quantum noise and technical noise sources) should be as weak as possible.
It is often required that the comb parameters can be rapidly adjusted e.g. within a feedback loop.

The first self-referenced frequency comb for the metrology was produced with Ti:sapphire lasers. In most cases, their output spectra is very broad, but not yet octave-spanning, as is the case with the general f-2f self-reference scheme to find out the CEO frequency.

 Extra spectral widening is used in a photonic crystal fiber. Initially, there was a concern that this method would not preserve compatibility and thus comb structure, but it was found that the comb structure is usually well preserved, even if the noise added in spectral widening processes is still the subject of investigation.

Recently, the use of erbium-doped fiber lasers has also been done with a highly nonlinear dispersion-flattened fiber for photonic crystal fiber or spectral widening. Fiber sources have the capability of more practical, robust and compact setup for real-world applications.

However, titanium-sapphire-based systems usually show better noise performance. The impact of quantum noise on the carrier-envelope offset is fundamentally strong for pulses with long duration, which generate most of the fiber lasers.In 2005, the frequency comb in the vacuum ultraviolet area was exposed by a high harmonic generation in a femtosecond enhancement cavity.

To make a frequency comb with a very large frequency spanking of several gigahertzs, one can use a micro-resonator, which has a uniformly short round-trip time. Such a device can be pumped with a single-frequency beam, and adequate number of comb lines can be generated on the basis of Kerr Nonlineity; The comma with this type of frequency is sometimes called Kerr Comb.

Combined lines are usually not mutually coherent. However, in certain regimes it is possible to achieve high level of consistency through mode-locked operation, where there is a mode-lock laser based on an effective parametric amplification, or in other words, there is an optical parametric oscillator, Where is the motivator Kerr. The fragmentation occurs with the discrepancy in an echo-causing substance.

This mode-locking regime requires careful tuning of the pump laser. Due to the many Q-factors, such devices may have very little noise. They, for example, can be applied as comb sources for optical fiber communication with wavelength division multiplexing; For example, a system that transmits over 50 Tbit / s distributed over 179 optical carriers with 100 GHz spacing (produced with two identical resonators) has been demonstrated.

Frequency comb laser sources are available commercially from various sources and have been widely used for metrology purposes. Since measurements are becoming technically more important at high-accuracy times - consider as an example on the GPS system and the European Galileo project - and other new applications also appear to be very beneficial, it is expected that the frequency comb maintain high technical importance.

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