Frequency comb

The frequency comb generator is a measuring device for high-precision frequency measurement; This also enables highly precise distance measurements to be made indirectly . This instrument generates a light beam, the frequency comb , with which the oscillation frequency of another light beam can be determined five orders of magnitude more precisely than with the methods known up to then.

With a frequency comb, the frequency of electromagnetic radiation (including light ) can be measured very precisely. The device was invented in 1998 in the working group of Theodor W. Hänsch at the Max Planck Institute for Quantum Optics , who received the Nobel Prize in Physics for it in 2005 .

The frequency spectrum of the light from a frequency comb generator consists of discrete and strictly periodic lines (shown here in color), the "prongs" of the frequency comb. (Details on the symbols under carrier-envelope phase )

The researchers faced the problem of  wanting to measure a frequency of almost 10 ¹⁵ Hz. Up to now this was impossible with the usual electronics, which only allowed frequency measurements up to about 10 ¹¹  Hz. The frequency comb works according to the optical analogy of a differential gear : the frequency to be measured is translated into a lower frequency, such as radio waves . The centerpiece is a laser that delivers light waves of a very precisely known frequency that interfere with the light beam to be measured. An interference pattern (a so-called beat ) forms with a frequency in the radio range, from which the unknown frequency can then be deduced. A frequency comb works not only with a single frequency, but with several sharp lines in the visible area, the “prongs of a comb”, hence the name.

construction

The frequency comb generator is composed of a femtosecond laser , the carrier envelope phase of which is measured and kept constant with the aid of a non-linear interferometer (f-2f interferometer, frequency doubling ).

The relatively broad optical spectrum of this laser is made up of several very sharp lines at exactly constant frequency spacing. The distance between these lines is usually in the megahertz or gigahertz range and can be measured and stabilized with relatively simple means. The f-2f interferometer also measures the absolute position of the entire comb. By comparing with an atomic clock , both quantities can now be determined very precisely. This means that the absolute frequency of each individual frequency needle in the spectrum of this laser is known exactly. By measuring a beat , the frequency difference between a frequency needle calibrated in this way and a not so precisely known frequency of another light beam can now be determined.

The handy size of the device is also significant; it's no bigger than a shoebox. Previous experiments for exact frequency measurement (the “frequency chain”) took up several rooms.

Applications

The most important areas of application are:

• several orders of magnitude higher data transmission rates in fiber optics with less interference to adjacent channels and improved security against eavesdropping, so that z. B. more phone calls can be transmitted simultaneously with an overseas light cable
• cheaper and probably several orders of magnitude more accurate replacement for mobile atomic clocks, which u. a. are important for satellite navigation
• highly sensitive chemical detectors
• Expansion of the possibilities of “designer chemistry” in the field of ultra-cold chemical reactions.
• Improvement of distance measurement systems based on lidar technology by several orders of magnitude. With this device, for example, the very small Doppler shifts in the spectrum of stars revolving around one another can be measured so precisely that it can even be detected when planets orbit distant stars; because when a small planet revolves around a heavy star, both revolve around a common center of gravity, which is not far from the center of the star. This results in a slight oscillation of the main star, which causes a small Doppler shift. The frequency comb can be used to measure the extent of the Doppler shift, from which the planetary orbit can be calculated.

swell

1. ↑ Steven Cundiff, Jun Ye and John Hall: Rulers of Light . In: Spectrum of Science . tape 8/2009 ( online ).
2. ↑ Frequency comb in astronomical observations

Web links

• Frequency Combs: High Precision Measurements for Fundamental Physics
• See Chapter 7: The Frequency Comb (PDF file; 11.6 MB)
• Worldwide installations of frequency combs (illustration by Menlo Systems, only readable with Flash)