I have an RF input signal in the frequency range of 1000MHz to 2000MHz with the power level of 0dBm+/-1dBm throughout the band, this signal needs to be converted to optic signal using RF to optic converter.

I have studied that, based on the input RF signal the RF to optic converter laser diode bias current will be adjusted and accordingly the optical signal intensity will be varied.

Now the question here is how does the frequency information will converted into optical form, Means how the frequency information will be modulated and taken through the converted optical signal?

  • 2
    \$\begingroup\$ if the time varying optical signal intensity follows that time variation of the RF input signal then it encodes the frequency information of the original RF signal more or less faithfully; this is basically the same question of how standard (non-optical) amplitude modulation preserves frequency information \$\endgroup\$
    – obataku
    Jul 20, 2016 at 12:35

2 Answers 2


This is done with amplitude modulation. The intensity of the laser is adjusted linearly with the amplitude of the input signal. At the other end, a wideband optical detector converts this back into a electrical signal. This works even up to relatively high frequencies (many GHz) because the frequency of the light used is around 200 THz.

  • \$\begingroup\$ You also get frequency modulation when you modulate the current of a laser diode. Are you sure that it is AM that is detected? (Not doubting you, just asking.) \$\endgroup\$ Jul 20, 2016 at 14:51
  • 1
    \$\begingroup\$ Frequency modulation as in changing the wavelength? Yeah, it's possible, but the detector is insensitive to this as it really only looks at the overall optical power. Now, it is possible to build interferometers that are sensitive to the phase of the light, but these are really only used for coherent digital communications. \$\endgroup\$ Jul 20, 2016 at 14:54
  • \$\begingroup\$ I think that chirp is more of an issue if you want to do DWDM. Also, it is relatively easy to avoid by using an external modulator (external to the laser cavity, it could be physically on the same substrate) \$\endgroup\$ Jul 20, 2016 at 14:55
  • \$\begingroup\$ "detector only sensitive to optical power". Duh.. (sorry brain freeze.) I modulate the current in diode lasers to change the wavelength (or add side bands) the concurrent AM is (mostly) not needed. \$\endgroup\$ Jul 20, 2016 at 19:14

As you say, the optical intensity is varied. The light varies in brightness. It gets brighter and dimmer at the same frequency as the signal sent to the laser diode - in your case, 1000 Mhz to 2000 MHz.


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