optical frequency (original) (raw)

Definition: the oscillation frequency of the electric field of light

Categories: article belongs to category general optics general optics, article belongs to category optical resonators optical resonators, optical metrology, article belongs to category physical foundations physical foundations

optical frequency
- instantaneous frequency

Formula symbol: ($\nu$)

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DOI: 10.61835/wlo Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

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What is an Optical Frequency?

The optical frequency (for example of a quasi-monochromatic laser beam) is the oscillation frequency of the corresponding electromagnetic wave. For visible light, optical frequencies are roughly between 400 THz (1 terahertz = 1012 Hz = 1 trillion hertz) and 700 THz, corresponding to vacuum wavelengths between 700 nm and 400 nm. Infrared light has correspondingly lower optical frequencies, while ultraviolet light has higher frequencies.

Many equations in optics involve angular optical frequencies, which are ($2 \pi$) times the optical frequency: ($\omega = 2\pi \nu$).

Usually, light does not have one particular optical frequency; its optical power is distributed over some frequency range, which sometimes spends a whole octave (i.e., a factor of two in terms of frequency) or even substantially more. The optical spectrum tells one how the power is distributed over the frequencies, i.e., it specifies the power spectral density as a function of frequency or vacuum wavelength.

There are technical light sources (highly frequency-stabilized lasers) which can produce light with a very small optical bandwidth — sometimes even well below 1 Hz, which is an extremely small fraction of the mean optical frequency of hundreds of terahertz — and a very high stability of that frequency. Such light sources are called optical frequency standards and are required for optical clocks, for example.

The optical frequency can be calculated as the vacuum velocity of light divided by the vacuum wavelength: \nu = c / \lambdaForcalculatingthefrequencydifferenceoftwoquitesimilarwavelengths,onecanusethefollowingapproximateequation:For calculating the frequency difference of two quite similar wavelengths, one can use the following approximate equation:Forcalculatingthefrequencydifferenceoftwoquitesimilarwavelengths,onecanusethefollowingapproximateequation:\Delta \nu = \frac{c}{\lambda^2} \Delta \lambda$$

where the denominator contains the mean wavelength.

Measurement of Optical Frequencies

Optical frequencies can be measured far more precisely than wavelengths; see the article on optical frequency metrology. Optical frequencies cannot be directly detected like microwave frequencies; there is simply no counter which can register so fast oscillations. Nevertheless, with indirect methods, nowadays usually involving stabilized frequency combs, it has become possible to exactly relate optical frequencies (from some optical frequency standards, for example) to microwave frequencies (e.g., from cesium clocks), or to other optical frequencies. As optical frequency standards can be more precise than microwave frequency standards and also allow for much more rapid frequency comparisons, it is expected that the definition of the second is the fundamental unit for the time in the international system of units (SI system) will soon be redefined based on an optical frequency standard.

If two optical frequencies are relatively close, their difference can be measured relatively easily: a beat note is obtained by superimposing the two beams on a fast photodetector, and the photodetector's output signal can be processed with an electronic frequency counter, for example.

Frequency vs. Wavelength

The optical frequency of light is in a sense more fundamental than the optical wavelength. For example, an atom or ion exposed to light cannot “see” the wavelength since its size is only a very small fraction of it. It essentially just registers a fast oscillation, i.e., the optical frequency. If that frequency coincides with certain internal resonant frequencies, resonant excitation processes can occur. The internal resonant frequencies are not related to some distance equal to the optical wavelength. Nevertheless, for historical reasons it is more common to specify wavelengths rather than optical frequencies of light. (For example, Nd:YAG lasers operating on their standard laser transition are commonly said to emit light with 1064 nm vacuum wavelength, rather than with a frequency of 282 THz.) This is essentially because optical wavelengths could soon be determined by using various kinds of interferometers, whereas it was difficult in the early times to measure or even to estimate optical frequencies.

Frequently Asked Questions

What is an optical frequency?

The optical frequency of light is the oscillation frequency of its electromagnetic wave. For visible light, these frequencies are in the range of 400 THz to 700 THz.

The optical frequency ($\nu$) is the vacuum velocity of light ($c$) divided by the vacuum wavelength ($\lambda$): ($\nu = c / \lambda$). Therefore, a shorter wavelength corresponds to a higher optical frequency.

How can optical frequencies be measured precisely?

Since optical frequencies are too high for direct electronic counting, they are measured indirectly. Modern methods often use stabilized frequency combs to precisely relate an optical frequency to a microwave frequency standard or another optical frequency.

Why is frequency sometimes considered more fundamental than wavelength?

When light interacts with matter, for example with an atom, the atom responds to the light's oscillation frequency. Resonant excitation depends on matching the light's frequency to the atom's internal resonant frequencies, making frequency the more direct parameter in such interactions.

How can the frequency difference between two laser beams be measured?

If the frequency difference is not too large (e.g., in the gigahertz range), it can be measured by superimposing the two beams on a fast photodetector. This creates a beat note signal which can be analyzed with an electronic frequency counter.

Suppliers

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Transmission through an acousto-optic (AO) device causes the input light to experience a frequency shift equal to the RF drive frequency. Our acousto-optic frequency shifters (AOFS) are optimized for the needs of applications like interferometry, with the ability to achieve high extinction ratio between modes.

We offer standard products with frequency shifts of over 300 MHz and integrated low-power AOFS modules in which the RF driver has been built into the housing. Our team can also customize frequency shifters for specific application, including frequency shifts up to 600 MHz.

⚙ hardware

CSRayzer Optical Technology offers acousto-optic frequency shifters for 1 μm or 1.5 μm wavelength, operated with drive frequencies from 40 MHz to 300 MHz. Both free-space and fiber-coupled versions are available, as well as suitable electronic drivers.

Bibliography

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