Laser Doppler Flowmetry: Measuring Blood Flow with Light


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I had a technical question about how the BLOOD FLOW METER works, can you help me?

What questions do you have about the flow meter?

Thank you very much for giving us your time
We are a group working on making this device, we encountered a problem during the work process.
We used a red rotating plate for the blood simulator and a thin translucent plate for the skin simulator.
We shined the laser light obliquely on the semi-transparent plate, part of the light was reflected and part passed, hit the rotating plate and was reflected from it; We changed the optical instrument so that both reflections fall at the same point and placed the detector at that point. The detector is connected to an amplification circuit, which according to our calculations at a frequency of 70kHz, the gain is 2500, and the output of the amplification circuit is connected to the oscilloscope.
In the oscilloscope, no other signal was displayed except the noise signal, which was still there even after the laser was cut off. But when we hit the LED light on the detector, the signal was observed. What is the problem with our work that the LED light gives a signal, the interference of the laser reflection does not?!

Based on your description, there are several possibilities that could lead to this issue. Let’s break them down one by one:

1. Laser Intensity and Detection Sensitivity: It’s possible that the intensity of the laser light being reflected off both the skin simulator and the rotating plate is too low to be detected above the noise floor of your detector and amplification setup. The LED might be giving off a higher or more direct intensity on the detector.

2. Wavelength Sensitivity: Detectors have specific sensitivity curves, meaning they’re more sensitive to certain wavelengths (colors) of light. If your laser has a wavelength outside of the detector’s most sensitive range, and the LED falls within that range, then the detector will more readily pick up the LED light.

3. Interference & Coherence: Lasers are coherent light sources, meaning their waves are in phase. When you’re trying to capture interference, it’s crucial to ensure that the phase difference between the light reflecting off the two plates is consistent. If the phase difference is shifting too rapidly (because of, say, slight wobbles in the rotating plate), you might not see a consistent interference pattern.

4. Amplification Circuit Frequency: Your amplification circuit is designed for a gain of 2500 at 70kHz. If the signal you’re trying to detect from the laser is not near this frequency, it might be getting attenuated or not amplified enough.

5. Ambient Light & Noise: Ambient light can introduce noise. If your setup isn’t shielded from other light sources, the detector might be picking up additional light, masking the signal you’re looking for. Ensure you’re working in controlled lighting conditions.

6. Polarization: Lasers often produce polarized light. If there’s any element in your setup that affects the polarization (like certain types of plates or reflections), this could interfere with the detection.

7. Alignment: Make sure the alignment of your optical setup is precise. Any slight misalignment could mean that the light doesn’t reflect as intended or doesn’t hit the detector correctly.

What to try next:

1. Test Laser Output: Ensure that the laser is producing the intended output and is not malfunctioning.

2. Adjust Laser Intensity: If possible, try increasing the intensity or focusing the laser beam to get a stronger reflection.

3. Frequency Analysis: If you have the capability, perform a frequency analysis on the output to see if there’s any signal around 70kHz (or elsewhere) that might be getting drowned out by noise or not being amplified correctly.

4. Re-check Setup: Go through your optical setup again, ensuring all elements are correctly aligned and at the intended angles.

5. Shielding: Add shields to block out external light sources.

6. Check for Coherence: Ensure that the laser’s interference pattern is stable. You could test this by removing the rotating plate and just reflecting the laser off the skin simulator to see if the interference pattern appears.

Troubleshooting often requires a systematic approach, eliminating one potential issue at a time until you find the root cause.