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I found some interesting applications for modulation for lasers. They always talk about some sort of frequency modulation (ex. FMCW)

AFAIK the frequency of a laser is pretty much stable (for all affordable lasers). So the only way to modulatate the laser would be changing the current wich results in a amplitude modulation of the emitted light. Right?

Knowing from RF one would normally use FM to always stay on 100% of power emission. So to clear up the confusion, how can you run a laser diode in AM and FM mode?

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You are confusing wavelength modulation with frequency modulation. The FM that you are referring to is the signal frequency, the distance between the peaks of the signal that you are modulating.

There are small effects of wavelength shift that is due to driving level that ultimately limit the power/distance and frequency of transmission as this wavelength shift causes symbol confusion.

Think of wavelength as a color. You can have a red laser blink fast or slow (FM) or bright and dim (AM) or any combination between and more. And the same thing can happen for a Blue laser. In actual fact telecommunication lasers are typically in the Near- Infrared and the wavelength separation is much tighter. And it's not correct to refer to colors.

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  • \$\begingroup\$ The time between like features of a recurring signal is the period of that signal, and the reciprocal of that period is the signal's frequency. The wavelength of a signal is the spatial distance it must travel for like features to repeat, therefore, if the frequency of a signal is modulated, its period and its wavelength will follow that change, making wavelength modulation equivalent to frequency modulation. \$\endgroup\$
    – EM Fields
    Jul 17, 2014 at 23:17
  • \$\begingroup\$ @EMFields - the use of wavelength in optical systems is to prevent this exact confusion, which the OP is having. The wavelength refers to the photons and the frequency refers to the modulations scheme. KEEP them separate. Yes you can talk about the wavelength of the modulation, but that has no meaning in context. \$\endgroup\$ Jul 17, 2014 at 23:38
  • \$\begingroup\$ As far as I've been able to tell, "wavelength" means the same thing when referring to optical systems as it does anywhere else. That is, if the light from an LED - or anything else - is specified as having a wavelength of umpty-ump nanometers, then its corresponding period will be the reciprocal of the LED's output frequency. Ergo, if the wavelength is changed by, say, increasing or decreasing the current through the LED, then - VOILA! - the LED's frequency will also change and will have been changed by modulating the LED's wavelength. \$\endgroup\$
    – EM Fields
    Jul 18, 2014 at 10:01
  • \$\begingroup\$ @EMFields Oh my, you're very confused. I can see why, the classic equation of \$ c=f\lambda \$ might apply after all, but it doesn't. Further more you're conflating the fact that some diodes/lasers shift wavelength (slightly) with drive level to the wavelength implied by their modulation frequency. This actually should be turned into a separate question. If you know classical communications theory it is accurate to consider the laser frequency to be the carrier frequency. \$\endgroup\$ Jul 18, 2014 at 12:16
  • \$\begingroup\$ Well, then, knowing that we agree with the classical definitions of wavelength, period, and frequency, that the laser's frequency can be considered a carrier, and that its frequency can be shifted back and forth about its center frequency by varying the laser's drive, it shouldn't take a great leap to realize that the carrier's deviation from its center frequency will vary as a function of the amplitude of the modulating signal, and its frequency of undulation about the center frequency will be precisely the modulating signal's frequency; a classical example of Frequency Modulation. \$\endgroup\$
    – EM Fields
    Jul 18, 2014 at 13:32
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Not the only way - you could use a Pockels or acousto-optical Bragg cell modulator to phase modulate the light from a laser. Phase modulation is FM using the time derivative of the signal.

Some types of laser diodes are more frequency variable with current than others- eg. VCEL types.

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  • \$\begingroup\$ Yeah, you can totally frequency modulate diode lasers with the current. Even "ordinary" consumer grade stuff. Up to ~1MHz or so it's a thermal effect and pretty strong. Above that modulation of the current causes a change in the index of refraction (I think, I'd have to look it up.) which is weaker. And then at really high frequency ~GHz. you can couple to these relaxation oscillations in the laser and things get stronger again.. over some limited bandwidth. Here's some sidebands. teachspin.com/instruments/fpc/index.shtml \$\endgroup\$ Jul 17, 2014 at 23:45
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Optical phase modulation is generally done with an external device using the electro-optic effect, rather than by manipulating the laser itself.

This falls into the realm of coherent optical communications. Coherent systems were widely studied in the 1980's because they have an inherent sensitivity advantage over incoherent (amplitude-modulated) systems, but only found limited use for economic reasons. Recently, coherent communications is re-emerging as the fiber optics industry develops systems for 40 Gb/s and higher data rates.

For long-distance systems, even amplitude modulation is done with an external Mach-Zehnder interferometer. The reason is that directly modulating the laser current causes an unwanted frequency modulation as well (also known as "chirp"). This unwanted frequency modulation is much larger than what would be used for deliberate phase modulation and it interacts with the dispersion properties of the optical fiber to limit the achievable transmission distance (for a given baud rate).

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