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I am not sure whether this is the correct place to ask, otherwise, please point me to the appropriate platform. I have been reading about fibre optic interferometric sensors and notice that most people use an optical spectrum analyzer (OSA) as the detector.

Therefore, I am wondering if it is possible to use a spectrometer (i.e. Ocean Optics USB4000) to observe the interference pattern in the form of a comb-like spectrum or with multiple dips.

I've tried using a 650 nm laser as the source but the spectrum appears very smooth with the peak at the corresponding wavelength. However, I am not sure whether it is due to my current setup or because the Ocean Optics USB4000 spectrometer cannot detect interference pattern.

An additional note: the entrance slit of my spectrometer is broken so I am also wondering if that is the cause.

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  • $\begingroup$ Are you interested to learn from first principles what you can or cannot expect for interference patterns from a spectrometer without a slit using a single beam source? Are you interested in understanding what is unique to the physical setup of an OSA versus the OO unit that you have? How far have you looked into the answers to these two questions? $\endgroup$ Nov 2, 2020 at 14:00
  • $\begingroup$ @JeffreyJWeimer I think I am more interested in the first question since I only have the OO unit at the moment. I have looked into the conditions for the interference pattern to be apparent and how the spectrometer receives light and plot the spectrum. I was just wondering if the OO unit I have is capable of detecting interference pattern as most papers report the use of OSA for fiber optic interferometric sensors. But then it could be due to my experimental setup that makes the interference pattern "invisible". Therefore, I just want come clarifications. $\endgroup$
    – Xenon
    Nov 3, 2020 at 6:21

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A spectrometer normally uses a diffraction grating. The interference pattern from a grating for a single wavelength concentrates the light into a single angle at each order. (The better the grating, the tighter the angle.) Try reflecting a laser beam from an unused CD or DVD (in a manner that does not put an output into your eye). (I got a pocket sized laser at a local dollar store.)

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  • $\begingroup$ Thank you for your answer! Just a follow-up question: Does the entrance slit of the spectrometer plays a role in detecting the interference pattern? From the datasheet, it says the slit is used to control the amount of light entering the spectrometer and also the spectral resolution. $\endgroup$
    – Xenon
    Nov 3, 2020 at 6:15
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    $\begingroup$ For a grating to function well, the waves arriving at any line across the grating (perpendicular to the rulings) must be in phase. To accomplish this, the slit is placed at the focal point of a lens which then directs parallel rays to the grating. A wider slit admits more light but also has a larger image in the spectrum. $\endgroup$
    – R.W. Bird
    Nov 3, 2020 at 15:23
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I've tried using a 650 nm laser as the source but the spectrum appears very smooth with the peak at the corresponding wavelength.

This sounds like what you would expect from this source. Interference patterns are not magical features where you tap on the laser and it suddenly acquires interference $-$ they come from the superposition of two or more distinct light sources which are mutually coherent. Thus, as an example, you mention

interference pattern in the form of a comb-like spectrum

which is typically the signature of a light source that produces a series of pulses with a strict periodicity and strong coherence between subsequent pulses. It does not sound like your source produces this, so there's no expectation that the spectrum would look like that.

Generally, though, if the light source has an interference pattern in its spectrum, an optical spectrum analyzer should be able to detect it, so long as its spectral range and resolution are sufficient for it.

Finally, on that note, if the entrance slit is broken then this will probably have a detrimental effect on the resolution, with the magnitude of that effect obviously depending on the details of how the entrance slit is broken. If you want to understand the effect better, look at the spectrum of a reference lamp with a sharp line spectrum with the broken slit and compare it with a non-broken one: the lines will be broader in the first case, and the amount of broadening gives you the impact on the resolution.

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