what components to choose for my laser project

[BraveWart]

I recently took up a rather ambitious project for my skill level as I have never worked with almost all involved components before.
I would be very grateful if you could help me find the best components to hopefully make this work.
What I am trying to achieve is this:

A laser beam is shining at a spinning polygon motor, creating a laser scan field α.
Within this I want to be able to detect if a small object enters the field and be able to deduce the angle of detection β.
The distance of the object will be anything from a few cm to up to 2m.

What I am expecting to get in the end is a scope reading that looks something like this:

where at 300Hz (50Hz motor spin x 6 sides of the mirror) I get sort of a constant signal landscape and a peak to whenever an object enters the scan field.

• the polygon motor spins at 50Hz precisely
• the laser beam is at 650nm. I use this laser module
• as part of this project, I am currently I am trying to get a useful reading out of this photodiode: SGPD3027C. But I already suspect this is not a good choice for this project at all.
• further down the project, I would like to also modulate the laser, so within that peak in the second sketch should be also a visible modulation waveform.
• Eventually, given that I get all this working, I would like to repeat the test with an infrared laser.

My biggest question right now is about how do I choose the best suitable and affordable photodiode to measure the reflected light in this setup.
I have never worked with photodiodes before, but I read about the an avalange photodiode would be best for this.
I have also found some APD modules, which sound like that's what I need, but they seem to go for around 300$ - definitely not in the scope of this project.

But then there is the question about sensor size, with or without preamp and also where and how do I actually place the photodiode to get the return signal? In my sketch i placed it next to the laser as I had seen modules where this is done that way. But is this suitable to my setup?
I am a hobbyist and I am doing all of this by trial and error, so apologies if some of my questions have obvious answers. Thank you.

 

[rjenkinsgb]

That should work to give you the angle of the detected object.

For distance as well, you either need ultra-high speed timing or a modulated laser so the modulation phase of the reflected signal can be compared to the transmitted signal.

I played around with a similar system, just for distance, many years ago. The detector needed a large, short focal distance lens to get a reasonable signal from the photodiode I used, for just to a few metres range, with a laser sine modulated at 25MHz. I can't remember what the diode was, but I think it was working on around 150V bias.

Such as relatively cheap commercial laser distance measuring devices can work to 60m om more, so there are obviously far better approaches than I tried back then!

 

[BraveWart]

I played around with a similar system, just for distance, many years ago. The detector needed a large, short focal distance lens to get a reasonable signal from the photodiode I used, for just to a few metres range, with a laser sine modulated at 25MHz. I can't remember what the diode was, but I think it was working on around 150V bias.

That is great to hear you had a similar project going on in the past. I hope you don't mind if I ask you a couple of questions. I think it would help me greatly in making informed decision

• did you place the PD in line of sight with the LD or perpendicular to it? I am wondering if the PD will receive the correctly timed return as it is offset from the line of sight of the LD to the spinning mirror. I do have a beam splitter, but I am wondering if that would be a good idea as it further reduces the already weak return signal from the object even further.
• Did you use a PD with a TIA as me in my other post you also replied to, or did you use a PD with built in preamp? I read that these two systems must be very much balanced to each other to function properly and a PD with preamp built might be the better option.
• Did you do TOF readings with your setup? If I get to it, I would like to read vibration frequencies of the detected object. I am thinking this might be possible to read the modulation frequency from the object in 300Hz intervals as soon as it gets detected. And I assume that would be done with something similar to TOF readings.
• I am a bit confused by the hug bias Voltage you mentioned. The TIA I am using says to use a bias Voltage 0f 1.7V in the circuit diagram:
  
  
  the SGPD3027C PD I am currently using has a reverse Voltage rating of 30V. Do I understand this as a maximum reverse Voltage but the 1.7V in the TIA is fine too? Do I need a TIA with 30V reverse Voltage?
  

thank you in advance

 

[rjenkinsgb]

The bias is down to the diode type; they have advanced a lot in the last 20 years or so since then!

I just used a generic high speed opamp.
Ranging was done via the modualtion, comparing the phase of the received signal to the phase of the transmitted signal.

Phase shift is proportional to the modulating frequency and round trip distance.

I was interested in short range accuracy & repeatable to 1 - 2mm, so time of flight was impractical.