RP Photonics logo
RP Photonics
Encyclopedia
Consulting Software Encyclopedia Buyer's Guide

Short address: rpp-con.com

Dr. Paschotta, the founder of RP Photonics, supports your R & D with his deep expertise. Save time and money with efficient support!

Short address: rpp-soft.com

Powerful simulation software for fiber lasers and amplifiers, resonator design, pulse propagation and multilayer coating design.

Short address: rpp-enc.com

The famous Encyclopedia of Laser Physics and Technology provides a wealth of high-quality scientific and technical information.

Short address: rpp-bg.com

In the RP Photonics Buyer's Guide, you easily find suppliers for photo­nics products. As a supp­lier, you can profit from enhanced entries!

Learn on lasers and photonics every day!
VL logo part of the
Virtual
Library

Beat Note

<<<  |  >>>  |  Feedback

Buyer's Guide

Use the RP Photonics Buyer's Guide to find suppliers for photonics products! You will hardly find a more convenient resource.

Ask RP Photonics for advice on how to use beat notes to measure optical frequencies, characterize laser noise, etc.

Definition: an oscillation of the optical intensity arising from the superposition of light with different optical frequencies

German: Schwebung

Category: optical metrology

How to cite the article; suggest additional literature

If two laser beams with different optical frequencies are superimposed on a photodetector measuring the optical intensity, a beat note – i.e., a signal with the difference of the optical frequencies – can usually be observed, if some conditions are met:

beat note

Figure 1: Superposition of two optical oscillations with a frequency difference of 25 THz. The bottom two curves show the electric field strengths of the isolated oscillations, and the to curve the additive superposition. A sufficiently fast intensity detector would record an oscillation of the power with the difference frequency.

As a fast photodetector can have a bandwidth of tens of gigahertz (or even higher), optical frequency differences of this order of magnitude can be measured e.g. by analyzing the photodetector output with an electronic frequency counter or an electronic spectrum analyzer. An important application of this is in frequency metrology. For example, the frequency of some laser can be measured by recording a beat note between that laser and a close-by optical signal with known optical frequency. Such measurements are greatly facilitated by an optical frequency comb which can cover a wide range of well-defined optical frequencies, so that a sufficiently nearby reference frequency for a beat measurement can be found for any frequency in this large range.

The linewidth of a beat note of two free-running lasers with uncorrelated laser noise is larger than the linewidth of each laser separately. However, the beat linewidth can be smaller if the phase noise of both lasers is at least partially correlated. In an extreme case, one of the lasers may be phase-stabilized so as to obtain a constant beat frequency, as defined e.g. by some electronic oscillator. The linewidth of the beat note, measured against a clock in synchronism with the electronic oscillator, can then be exactly zero, if the phase difference exhibits only small stationary fluctuations.

Optical beat notes are essential for the technique of optical heterodyne detection.

See also: frequency metrology, photodetectors, optical heterodyne detection, Spotlight article 2008-07-26, Spotlight article 2009-07-29

How do you rate this article?

Your general impression: don't know poor satisfactory good excellent
Technical quality: don't know poor satisfactory good excellent
Usefulness: don't know poor satisfactory good excellent
Readability: don't know poor satisfactory good excellent
Comments:

Found any errors? Suggestions for improvements? Do you know a better web page on this topic?

Spam protection: (enter the value of 5 + 8 in this field!)

If you want a response, you may leave your e-mail address in the comments field, or directly send an e-mail.

If you like our website, you may also want to get our newsletters!

arrow

Photonics Newsletters --
get high quality information,
not the usual advertisements!

You have too little time for reading all kinds of newsletters? You are tired of the common ad distribution? Sure – but what we offer is different:

e-mail

We won't flood you with e-mails!

Our newsletters appear only about once per month, and they carry valuable information. You will not regret to register!

– Show all banners –

– Get your own banner! –