[Newbie] >1W Blue/Green diode collimated for small spot size 50-100m away

[Rocket]

Hi there. I'm toying with the idea of using a cheap blue/green diode to check my homemade submersible isn't heading towards anything that isn't water. Not a Lidar system, just measuring the reflected light. My back-of-envelope calculations suggest a 1-2W laser will give me a detectable photocurrent out to 50-100m BUT I have two problems....

1) what should i be looking for in an LD for getting a great collimation (445/450 or 532 - don't really mind - although I would get a stronger photocurrent from the green per watt of laser power) is there an obvious candidate?

2) the diode is going to be in a vacuum and I want to run it continuously. Can you do that with a copper host? I'd mount it on a TEC and get the heat elsewhere but are the hosts sufficiently efficient to keep lasing continuously (for an hour or more)?

Bit of a random first post but there seems to be a depth of wisdom on these forums that is worth tapping

Many thanks for any insights/tips you guys may have....

 

[diachi]

Is there a reason you can't go with some sort of sonar?

 

[Rocket]

I've currently got everything in a single vacuum glass tube so a sonar unit would need to be attached to the front of the submersible and linked to the brain of the unit in the pressure housing. a sonar unit is also heavier, costs more and requires more signal processing than a relatively cheap LD+photodiode.

it just feels the right design choice if i can get it to work.

The operating depth is a few 1000m so having a single pressure vessel is heaven.

 

[diachi]

I've currently got everything in a single vacuum glass tube so a sonar unit would need to be attached to the front of the submersible and linked to the brain of the unit in the pressure housing. a sonar unit is also heavier, costs more and requires more signal processing than a relatively cheap LD+photodiode.

it just feels the right design choice if i can get it to work.

The operating depth is a few 1000m so having a single pressure vessel is heaven.

My concern would be reflections/low transmittance in the water. If there's a lot of silt or other particles in the water your photodiode may detect reflections from those, and depending on on how 'murky' the water is, the beam may not even make it as far as you need - and back. The diffuse reflection has to make it all the way back to the source too and it's not going to be a nice tight beam. If you're using returned intensity to figure out the distance that's definitely not going to work as the intensity will vary depending on transmittance.

I've never used a laser under water like that so I'm really not entirely sure, maybe someone else here has - truth be told I don't much like the water!

For sonar - I'm not talking of some sonar imaging system, just a basic sonar rangefinder - that shouldn't require a ton of signal processing and should be a hell of a lot cheaper than a proper sonar imaging system. I imagine a cheap sonar transducer and something like an arduino might be able to take care of it. That'll certainly be lighter, less power hungry and cheaper than 1W of 520nm, a heatsink for the 520, driver for the 520, TEC, TEC driver and power source for the TEC + driver. One problem with that approach may be a lack of "directivity" - you may just know that something is within "X distance" - but not where it is.