What is an Optical Frequency Comb?

Optical frequency combs, also known as optical frequency comb generators, are specialized lasers or devices that generate a series of precisely spaced and accurately known optical frequencies. These frequencies are often arranged in a "comb" pattern, hence they are called frequency combs. These evenly spaced spectral lines have a well-defined frequency, and they are produced based on the principles of nonlinear optics. 

The first optical frequency comb was developed in the 1970s by Theodor W. Hänsch of the Max Planck Institute for Quantum Optics, Germany. The approach involved using a mode-locked laser, which produces a series of ultrashort pulses at regular intervals. The spectrum of these pulses is a series of equally spaced lines, with a spacing that is determined by the repetition rate of the laser pulses.

Figure1: Representation of a frequency comb spectrum

As seen in the figure 1, the optical frequency comb spectrum looks like a comb's teeth, dividing each frequency into a separate spike. They act like ticks on a ruler to measure light emitted by stars, atoms, other lasers, etc. with exceptional precision and accuracy because of their extremely small and uniform tooth spacing.

Optical frequency combs can provide timing and frequency measurements with an accuracy of 1 part in 10¹⁵ which makes them a valuable tool in fields such as precision spectroscopy, metrology, and telecommunications, and researchers continue to explore new ways to utilize this technology.

Structure and working of an optical frequency comb

Optical frequency combs are produced by a mode-locked laser, which generates a series of ultrashort pulses at regular intervals. The frequency spectrum of these pulses produced is a series of equally spaced lines and the spacing is determined by the repetition rate of the laser pulses. 

The mode-locked laser consists of a laser cavity, which is typically made of a solid-state or fiber material, and a mode-locking element, which is used to create the ultrashort pulses. A semiconductor saturable absorber, an acousto-optic modulator, or a nonlinear optical crystal is used as the mode-locking element. Ti:sapphire solid-state lasers or Er:fiber lasers with repetition rates ranging between 100 MHz and 1 GHz or even going as high as 10 GHz are the most popular lasers utilised for frequency-comb creation.

When a laser is mode-locked, the laser cavity is designed to resonate at multiple frequencies simultaneously. These frequencies are combined to create a single pulse of light with a duration that is much shorter than the period of any of the individual frequencies. This pulse of light then enters a nonlinear optical medium and undergoes harmonic generation. i.e., it is converted into a series of sidebands with frequencies that are integer multiples of the original frequency. The resulting spectrum of the mode-locked laser is a series of equally spaced spectral lines with a well-defined frequency, which is the frequency comb. This spacing can be as small as a few hundred gigahertz or as large as several terahertz, depending on the laser used. The generated frequency combs can be further refined using a process called frequency stabilization, which involves locking the laser to a reference frequency using a feedback loop. This feedback loop compares the frequency of the laser to the reference frequency and adjusts the laser frequency to match it. This results in a highly stable and precise frequency comb that can be used for a wide range of applications.

Figure 2: Frequency comb generation using mode-locked laser

The process of frequency comb generation is shown in figure 2. Here a continuous wave laser pulse is passed through a mode-locked fiber oscillator which produces a series of optical pulses separated by the time limit (in ns). This sharp spectral line series is called a frequency comb. 

At first, the mode-locked laser emits a spectrally narrow and unstabilized frequency comb. The amplifier then increases the pulse energy and shortens the pulse time necessary for offset frequency detection. At this point, the equally spaced frequency comb structure is formed, but the exact optical frequency of each comb tooth is unknown and unstabilized. 

To stabilize the comb, using a detector, the offset frequency, f₀ is measured and the comb tooth spacing is given by the repetition rate, f_r, which stabilizes the comb to a radio frequency reference. To stabilize the comb to an optical reference the offset frequency, f₀, and the frequency of a single optical comb tooth, f_n0, are also measured using the detector. Any noise in the frequency comb is cancelled through the control electronics and the signal processing unit as shown in figure 2.

Advantages

• High accuracy and precision
• Broad bandwidth
• Versatile
• Compact and portable

Disadvantages

• Complex system
• High cost 
• Sensitive to environmental fluctuations 
• Limited frequency range

Applications

Optical frequency combs are used to measure time and frequency with high accuracy and precision which makes them an essential tool for a wide range of scientific and technological applications. They are used to measure the spectral lines of molecules and enable the development of new materials and chemical compounds with properties that were previously unknown or difficult to measure.

These frequency combs are used in the development of new atomic clocks, which are used in GPS systems, communication networks, and scientific research, among other applications. They can also be used to generate microwave signals with ultra-high precision that has the potential to revolutionize fields such as radar, telecommunications, and satellite communication.

Optical frequency combs are also used to synthesize and stabilize the frequency of other lasers and optical sources enabling the development of new technologies and applications.