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I'm looking for a specific type of laser for interferometry experiments. There seem to be a lot of 1550nm fiber-coupled DFB laser diodes on websites like Alibaba. This seems to be because they are commonly used for optical communications purposes. On the surface, these lasers would seem to be perfect for my experiments. However, since they are used for optical communications, I noticed that they list a "modulation rate"/"bandwidth", such as 2.5 Gb/s or something similar. For example:

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I'm not knowledgeable enough about lasers to understand what the ramifications of this is, or whether this changes their suitability for my experiments. What are the ramifications of the existence of this "modulation rate" for the use of these lasers for interferometry? Does this mean that these lasers are no longer suitable for interferometry experiments, compared to "normal" 1550nm fiber-coupled DFB laser diodes? Or does the existence of this "modulation rate" not affect the suitability of these lasers for interferometry?

The Pointer
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  • "Modulation rate" is how rapidly the laser intensity can be varied. In this case, 2.5 billion times per second. Do you even want to vary the intensity for interferometry purposes? – rdtsc Oct 26 '21 at 18:55
  • @rdtsc No, I do not. Is this more of a potential capability, rather than something that must be done? Can I just choose not to modulate the device (and instead just operate it in normal continuous wave mode)? – The Pointer Oct 26 '21 at 19:10
  • @rdtsc My understanding is that modulation depends on the laser diode driver, so I suspect that the "modulation rate"/"bandwidth" of 2.5 Gb/s is just a potential capability that the laser is built to withstand, if you want to utilise it. Therefore, I suspect that you can also just drive the laser normally (without modulation) as well. Am I correct in my thinking? – The Pointer Oct 26 '21 at 19:58
  • Yes. Modulation is intended to create a data stream by varying the photon intensity over time - but nothing says you *must* use modulation. If you can find a CW laser diode, it might cost less though. – rdtsc Oct 26 '21 at 20:14
  • @rdtsc Ok, thanks for the clarification. – The Pointer Oct 26 '21 at 20:17
  • Clicking though a few of the listings, some actually give seemingly identical specs but list "2.5 GHz" rather than 2.5 Gb/s, so I suspect the vendor is simply using the terms interchangeably. Usually for interferometry you don't care about GHz bandwidth either way. – user1850479 Oct 26 '21 at 23:24

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