Definition of optical frequency comb

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Photonics Dictionary

optical frequency comb: An optical frequency comb is a tool used for measuring frequencies with extremely high precision. It is essentially a spectrum consisting of a series of discrete, equally spaced frequency lines, much like the teeth of a comb, hence the name. These combs have revolutionized fields like metrology, spectroscopy, and telecommunications due to their ability to provide a precise frequency reference over a wide range of wavelengths.

Structure and generation:

Equally spaced lines: The spectrum of an optical frequency comb is made up of a series of sharp lines that are equally spaced in frequency.

Mode-locked lasers: The most common way to generate an optical frequency comb is through mode-locked lasers, which produce pulses of light at regular intervals. These pulses create a broad spectrum of light with the comb-like structure in the frequency domain.

Comb spacing and carrier envelope offset: The spacing between the comb lines is determined by the repetition rate of the mode-locked laser pulses. Another important parameter is the carrier-envelope offset frequency, which shifts the entire comb spectrum.

Applications:

Precision metrology: Optical frequency combs can be used to measure optical frequencies with extraordinary accuracy by comparing unknown frequencies to the comb lines.

Spectroscopy: They allow for the simultaneous measurement of multiple spectral lines, enabling high-resolution and rapid spectroscopic analysis of molecules and atoms.

Optical clocks: They are essential in the development of optical atomic clocks, which are the most precise timekeeping devices available.

Telecommunications: Optical frequency combs can improve data transmission rates and efficiency in fiber-optic communication systems by providing stable, evenly spaced frequency channels.

Key characteristics:

Broadband coverage: Optical frequency combs often cover a broad range of wavelengths, which makes them versatile tools for various applications.

High coherence: The lines in the frequency comb are highly coherent, which means they maintain a fixed phase relationship over time.

Stability: The accuracy and stability of the comb lines are crucial for precision measurements and applications.

Technical details:

Repetition rate: This is the frequency at which the mode-locked laser pulses are emitted and determines the spacing between the comb lines.

Carrier-envelope offset frequency: This is the frequency shift that offsets the entire comb from being exactly harmonics of the repetition rate, allowing for the precise measurement of absolute frequencies.

Phase coherence: Ensuring that the phase of each pulse is consistent, which is critical for maintaining the comb structure.

Optical frequency combs have been instrumental in advancements in fundamental science and technology, earning their inventors, John L. Hall and Theodor W. Hänsch, the Nobel Prize in Physics in 2005. Their ability to link optical frequencies with radio frequencies and provide an ultra-precise frequency standard makes them invaluable in both research and practical applications.

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