Raman Spectroscopy Setup Not Functioning as Intended

[Bogthornder]

TL;DR

I am attempting to reuse components of an old Raman spectrometer, and cannot obtain any signals.

I am attempting to revive an old Raman spectrometer that had a few components stop working. I was left with a functioning probe head, laser system, and some optical components. Currently, I have purchased a new CCD spectrometer and a few optical components to replace those that were specific to the old CCD. Now that I have the entire set-up assembled, I am not able to obtain any sort of spectra, or any signal at all. The only signal I can obtain is noise, and even when using calibration lights there is no signal obtained. More specifications about the system are below:

Laser: Adjustable with a maximum power of 400 mW, 785nm wavelength.

Probehead: Constructed by Kaiser Optical (now defunct)
Spectrometer: ThorLabs CCD175
Laser Filter: Thorlabs FELH0800 Edgepass Filter (Cut on wavelength is 800nm)

Components are arranged in the following fashion:

Laser > Fiber optic > Probe head > Sample > Probe head > Fiber optic > Free space coupling mount > Laser Filter > Fiber optic > CCD

I have attempted to vary the exposure times, change the laser power, and use software for spectral analysis to retrieve any information, but it appears that the set-up is not actually producing any usable signals. The sample I am using to test the set-up is a polystyrene standard. Some additional steps I’ve taken in troubleshooting:

• Removing the filter from the path to ensure that the path functions as expected. When the filter was removed, light can be detected by the CCD however it is only the laser light and there appears to be no Raman signal despite the probe head’s internal filtering.
• When a white light is shone into the probe head without the filter, light can be detected including light above the 800nm cut on. However when the filter is added this light disappears. Using this light source with the filter along, there is infrared light being transmitted through the filter that can be seen with a camera.

Does anyone have any suggestions for parameters that can be tested to determine where the system is failing to work? I am happy to provide any additional information that could be useful in the diagnostics.

 

[sbrothy]

TL;DR Summary: I am attempting to reuse components of an old Raman spectrometer, and cannot obtain any signals.

I am attempting to revive an old Raman spectrometer that had a few components stop working. I was left with a functioning probe head, laser system, and some optical components. Currently, I have purchased a new CCD spectrometer and a few optical components to replace those that were specific to the old CCD. Now that I have the entire set-up assembled, I am not able to obtain any sort of spectra, or any signal at all. The only signal I can obtain is noise, and even when using calibration lights there is no signal obtained. More specifications about the system are below:

Laser: Adjustable with a maximum power of 400 mW, 785nm wavelength.

Probehead: Constructed by Kaiser Optical (now defunct)
Spectrometer: ThorLabs CCD175
Laser Filter: Thorlabs FELH0800 Edgepass Filter (Cut on wavelength is 800nm)

Components are arranged in the following fashion:

Laser > Fiber optic > Probe head > Sample > Probe head > Fiber optic > Free space coupling mount > Laser Filter > Fiber optic > CCD

I have attempted to vary the exposure times, change the laser power, and use software for spectral analysis to retrieve any information, but it appears that the set-up is not actually producing any usable signals. The sample I am using to test the set-up is a polystyrene standard. Some additional steps I’ve taken in troubleshooting:

• Removing the filter from the path to ensure that the path functions as expected. When the filter was removed, light can be detected by the CCD however it is only the laser light and there appears to be no Raman signal despite the probe head’s internal filtering.
• When a white light is shone into the probe head without the filter, light can be detected including light above the 800nm cut on. However when the filter is added this light disappears. Using this light source with the filter along, there is infrared light being transmitted through the filter that can be seen with a camera.

Does anyone have any suggestions for parameters that can be tested to determine where the system is failing to work? I am happy to provide any additional information that could be useful in the diagnostics.

I'm probably not of much help but I'm curious as to whether you have drawings or diagrams of this setup of yours that you'd be willing to share?

 

[Bogthornder]

I'm probably not of much help but I'm curious as to whether you have drawings or diagrams of this setup of yours that you'd be willing to share?

Of course! I don't know why I didn't think to attach it before.

I don't have a lot of information concerning the probe head. I know it is a Kaiser Optical Systems Inc. Mk. II, but the documentation I have doesn't seem to contain any optical diagrams that I can find, but I thought I might attach an image of the internals in case anyone other than me can see something obvious.

 

[sbrothy]

Yeah, as I suspected it's beyond me. The reason I'm interested at all is because I wanted to realize a Raman mass spec using 3D printed parts. I started a thread along those lines here:

Apropos Raspberry PI DIY Mass Spec using Raman Scattering

Well, if nothing else I got your thread bumped.

 

[PhotonMartin]

You've already done some good troubleshooting by isolating the edge filter, but at this point I'd stop looking at the system as a whole and verify each subsystem independently. Raman systems can be frustrating because a small problem anywhere in the optical path results in "no signal."

My troubleshooting order would probably be something like this:

1. Verify the laser. Confirm the output power with a power meter and, if possible, verify the wavelength. Make sure the laser is actually reaching the probe after the launch fiber.
2. Check every fiber independently. Even if they look fine externally, inspect both end faces and send a bright white light through each fiber. It's surprising how often a damaged connector or cracked fiber ends up being the culprit.
3. Verify the probe optics. Since it's a used Kaiser probe, I'd make sure the internal dichroic and clean-up/notch filters are still intact and oriented correctly. If one of those optics has shifted or degraded, you may be collecting very little Raman light.
4. Confirm the edge-pass filter. I'd verify that it really is the filter you think it is (correct part number and orientation). If you have access to another spectrometer or even a simple wavelength measurement setup, it's worth confirming that the transmission curve matches the specification.
5. Test the spectrometer completely independently of the Raman system. Before connecting everything together, make sure the CCD is producing a normal spectrum from a white-light source or calibration lamp. That verifies the detector, fiber input, software, and acquisition settings are all working correctly.
6. Use a known Raman standard. Polystyrene is a good choice, but I'd first make absolutely certain the spectrometer is functioning normally before expecting to see Raman peaks.

One thing that caught my attention is this statement:

"When the filter was removed, light can be detected by the CCD however it is only the laser light and there appears to be no Raman signal despite the probe head's internal filtering."

That makes me wonder whether the probe head itself is actually rejecting the laser correctly. If the internal notch/dichroic optics have been damaged, misaligned, or replaced at some point, the Raman signal could simply be buried under the residual Rayleigh scatter.

One resource I've found useful when thinking through Raman optical layouts and troubleshooting is this overview of Raman instrumentation:

Raman Spectroscopy Learning Center
https://www.stellarnet.us/spectrometers/raman-spectrometers/learning-center/

It covers the function of the excitation laser, collection optics, filters, spectrometer, and detector, which can be helpful when you're trying to isolate where the signal is being lost.

Hopefully you can narrow it down to one subsystem. Once you know whether the issue is the laser, probe, filters, or spectrometer, the rest of the debugging usually becomes much more straightforward.