Common mode noise and balanced detectors

[matheyrichs]

I'm working on a laser interferometry system that uses a pair of balanced photodetectors to record the signal. Each detector is on a separate port of a beamsplitter cube, and as a result receives (in theory) the SAME signal. The rationale for using the balanced detectors is that "they reject common-mode noise on the signal".

What exactly is this common-mode noise in the first place? If it is something present at the detector in each signal, then why does that specific piece of the signal get filtered? It seems to me if this is really as simple as subtracting what's common at each detector, then the desired signal would also be lost (seeing that it is the same at each point...).

Can anyone help me understand how a balanced detector scheme works? Thanks!

 

[dlgoff]

...if both inputs fluctuate by the same amount (while remaining constant relative to each other), this change will have no bearing on the output.

http://en.wikipedia.org/wiki/Common-mode_rejection_ratio"

You may have seen this wikipedia discription of Optical Hybrids. But I think it will answer your question about balanced detectors.

http://en.wikipedia.org/wiki/Optical_hybrid"

Regards
Don

 

[oswaler]

Common mode noise refers to any unwanted interference or signal that is present in both detectors in a balanced detection system. This type of noise can come from various sources such as electrical interference, thermal noise, or environmental factors. The reason for using balanced detectors is that they are designed to cancel out this common mode noise, resulting in a cleaner and more accurate signal.

The way this works is by taking the difference between the two signals from the balanced detectors. Since the common mode noise is present in both signals, it will be subtracted out, leaving only the desired signal. This is possible because the common mode noise is typically not correlated with the signal, meaning it is random and can be cancelled out through subtraction.

It is important to note that while the desired signal may be the same at each detector, it is not affected by the subtraction process because it is present in both signals equally. Only the noise, which is present in one signal but not the other, is cancelled out.

In order for this method to work effectively, it is important to ensure that the two detectors are as identical as possible and that they are receiving the same level of input signal. Any differences between the two detectors can result in incomplete cancellation of the common mode noise.

Overall, using balanced detectors in a laser interferometry system is a common and effective way to reduce unwanted noise and improve the accuracy of the signal. I hope this helps to clarify how the balanced detector scheme works.