# Comparing Optical Signal Power with RMS Voltage

• daniel1211
I am still not entirely sure how to properly measure the power in an analog output but I am working on it.In summary, you can measure the signal power in an analog output using an oscilloscope and the 10 log(Vout / Vnoise).

#### daniel1211

I have a CW laser system, the optical detector receives a pulse width modulated signal and converts it to an analog signal. Both the source and detector have 32 channels and I would like to know what the analog signal power is, I thought maybe I could use the RMS voltage received on the oscilloscope and convert that to dB. I can then compare the input and output signals, is this possible?

Good question!

In most applications, you wish to understand what your signal to noise ratio is which is simple if your modulation is simple. Mulitplexed and pulsed modulation schemes become very difficult as the RMS voltage is dependent upon your modulation.

You may wish to look at the RMS background noise and multiply it by 6.6 to convert that to a peak-peak background noise.
Then, measure your peak-to-peak from modulation.

SNR (dB) = 20 * log (peak-peak_mod / peak-peak_noise)

For a part to do monitor your output strength, you may wish to look into the LTC5507 low frequency peak power detector.

Fun project. I used to mux signals in high school, and acquired a fiber optic link in college to send stereo using PWM :)

• daniel1211
daniel1211 said:
I have a CW laser system, the optical detector receives a pulse width modulated signal and converts it to an analog signal. Both the source and detector have 32 channels and I would like to know what the analog signal power is, I thought maybe I could use the RMS voltage received on the oscilloscope and convert that to dB. I can then compare the input and output signals, is this possible?
I think you mean the signal power of one analogue output. If the modulation is sinusoidal, you can find the RMS voltage using an oscilloscope. You must have the correct load resistor in place. Then the power in the load (Pr) is Vrms^2 / R. If wanted, you can then measure the analogue signal power into the TX (Pt) in a similar way (you need to know the input resistance). If you want overall loss in decibels it is 10 log (Pt/Pr).

• daniel1211
This type of signal is an asymmetric pulse rather than sine wave. Essentially, you cannot have a "negative" amount of light. Peak-Peak measurements are more applicable.
However, if your sensor's output reflects cable power, then the 10 log (Vout / Vnoise) reflects the signal to noise in terms of cable power.
If your addressing the noise power in terms of output voltage, then the 20 log(Vout / Vnoise).
Tech99's suggestion holds valid for computing the effects of fiber changes (i.e. more cable), so I suspect it is most valid.
Mike

• daniel1211
Thanks everyone for your help, I was able to reproduce the results while testing my systems which allowed me to easily determine the quality of the analog output signal. One issue that I ran into was that some of the channels on the laser detectors were sending out inverse signals but when looking at the RMS on the oscilloscope based off my calculations the signal should of been correct. I decided to take the ratio of the RMS and the amplitude which allowed me to correct this issue.

## 1. What is the relationship between optical signal power and RMS voltage?

The relationship between optical signal power and RMS voltage is that they are two different ways of measuring the same physical quantity, which is the strength or intensity of an optical signal. Optical signal power is measured in units of power, such as watts or milliwatts, while RMS voltage is measured in units of voltage, such as volts or millivolts. They are related through a conversion factor that takes into account the characteristics of the detector being used to measure the signal.

## 2. How do you convert between optical signal power and RMS voltage?

To convert between optical signal power and RMS voltage, you need to know the conversion factor for the specific detector being used. This conversion factor can be found in the detector's datasheet or can be calculated using the detector's sensitivity and responsivity values. Once you have the conversion factor, you can multiply the optical signal power by it to obtain the corresponding RMS voltage, or divide the RMS voltage by the conversion factor to obtain the optical signal power.

## 3. Which is a more accurate way to measure an optical signal, optical signal power or RMS voltage?

Both optical signal power and RMS voltage are accurate ways to measure an optical signal, but they serve different purposes. Optical signal power is a better measure of the overall strength or intensity of the signal, while RMS voltage is a better measure of the electrical energy carried by the signal. Therefore, the more appropriate measurement to use depends on the specific application and what information is needed.

## 4. Can you compare optical signal power and RMS voltage for different types of signals?

Yes, you can compare optical signal power and RMS voltage for different types of signals as long as the same detector is used to measure both. This is because the conversion factor used to convert between the two quantities is specific to the detector being used and does not depend on the type of signal being measured.

## 5. How does the comparison between optical signal power and RMS voltage affect the performance of a photodetector?

The comparison between optical signal power and RMS voltage does not directly affect the performance of a photodetector. However, it is important to understand the relationship between the two quantities when choosing a photodetector for a specific application, as the detector's sensitivity and responsivity values will affect the conversion factor and therefore the accuracy of the measurements. Additionally, understanding this relationship can aid in troubleshooting and optimizing the performance of a photodetector in a given system.