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If I have a monochromatic light source of wavelength λ, that is quite feeble, say 1nW, is there an equipment that will let me amplify this signal with an output at precisely the same wavelength?

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    $\begingroup$ Sure. This is what an optical amplifier does (en.wikipedia.org/wiki/Optical_amplifier). But as with all amplifications this will not only amplify signal and the existing noise but also adds some additional noise (and this noise is typically not monochromatic). $\endgroup$ – Andreas H. Jul 30 '18 at 13:18
  • $\begingroup$ This begs the question why you don't simply create a light source with the required power to start with, particularly if you require a specific wavelength. $\endgroup$ – StephenG Jul 30 '18 at 13:32
  • $\begingroup$ @StephenG I'm thinking in the lines of electrical amplification. The source is weak, I want to amplify it so I can sense it more accurately. $\endgroup$ – user1155386 Jul 30 '18 at 13:42
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    $\begingroup$ @user1155386, normally you only use the word "amplify" when you are talking about amplifying a signal. If all you want is steady state optical power, build a bigger laser. If all you want is steady state electrical power, build a bigger power supply. $\endgroup$ – Solomon Slow Jul 30 '18 at 14:42
  • $\begingroup$ Amplification in a precise way (for the purposes of detection and accuracy) implies that the device amplifying must itself be measuring - i.e. a sensor (it must effectively be sensing the wavelength even if it's not explicitly reported - it's an implicit measurement). What you probably need to do is work on improving the the sensor and/or the error sources. For example in some applications you would apply cooling to e.g. a CCD array. Changing methodology is sometimes better - e.g. using a time average over a long a time base. It might help to know the source of the light. $\endgroup$ – StephenG Jul 30 '18 at 18:06
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In most of the optical setups for detecting weak signals

1st) use a chopper to modulate the signals

2nd) use a lock-in amplifier to amplify the signal without amplifying the noises.

enter image description here

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