Improve SNR for Raman Spectroscopy with Photoresistor: Tips and Tricks"

[engineer23]

Homework Statement

We are using Raman spectroscopy to detect the presence of chemicals from emitted light.

We have a light transistor that behaves as a photo-resistor of 50k-ohm resistance. The light signal generates 2 microvolts of 2 kHz signal at normal Raman levels. We want at least a 20 to 1 SNR (Signal to noise ratio).

Cool the circuit to 77 K. What is the SNR?
What two things can be done to improve SNR without changing the sensor?

Homework Equations

noise for a resistor = sqrt(4KTRf), where K is the Boltzmann constant, T is the temperature, R is the resistance, and f is the bandwidth

The Attempt at a Solution

Substituting directly into the equation for noise with T = 77 K, R = 50000 ohms, and f = 2000 Hz, I get sqrt (4*1.38*10E-23*77*50000*2000) = .651 microvolts.

The SNR is then signal/noise = 2 microvolts/.651 microvolts = 3 This is much less than the 20 SNR we need.

But shouldn't the resistance of the photoresistor change with temperature as well? Intuitively, it seems that resistance would increase with decreasing temperature? Is there an equation I should be aware of which governs this behavior?

Also, I suppose that we could cool the circuit more to meet spec? But is there something else that can be done without changing the device itself?

 

[mgb_phys]

You are assuming that all the noise is thermal.
There is also power supply ripple, 1/f noise and shot noise.

If you can find a copy "Building electro optical systems" - Philip Hobbs, is excellent for this sort of thing

 

[piercas]

There are a few ways to improve the SNR for Raman spectroscopy with a photoresistor. The first step would be to cool the circuit to 77 K as mentioned in the prompt. This will reduce the noise level and improve the SNR. However, as you have pointed out, the resistance of the photoresistor may also change with temperature and this should be taken into account when calculating the noise level.

To improve the SNR without changing the sensor, you can try the following tips and tricks:

1. Increase the light intensity: By increasing the intensity of the light source, you can increase the signal level and improve the SNR. This can be achieved by using a more powerful light source or by increasing the exposure time.

2. Use a narrowband filter: A narrowband filter can be used to filter out unwanted background noise and improve the SNR. This is especially useful if there are other sources of light in the environment.

3. Optimize the detector: Make sure that the detector is properly aligned and positioned to receive the maximum amount of light from the sample. This will help to improve the signal level and therefore, the SNR.

4. Increase the integration time: By increasing the integration time, you can increase the amount of light collected and improve the SNR. However, this may also increase the risk of thermal noise, so it is important to find the right balance.

5. Use a lock-in amplifier: A lock-in amplifier can be used to improve the SNR by filtering out noise that is not at the same frequency as the signal. This is particularly useful for low-level signals.

Overall, it is important to carefully optimize the experimental setup and consider all possible sources of noise in order to improve the SNR for Raman spectroscopy with a photoresistor.