Can an IR Laser diode salvaged from CD burner be used for ranging?

[kapilp]

Can IR diodes like the ones shown in the following link (salvaged from CD burners) be used for ranging applications?
LucidScience - Build the LASER NIGHT VISION - Page 3 of 13

If yes, will a simple IR photo-diode work or will I need a specialized sensor?
also what king of protection and focusing optics will be needed to prevent harm to other people/objects and produce a more or less uniform thin beam.

Also has someone attempted something of this kind?

Thanks

 

[keith1200rs]

They aren't the right type. Time of flight laser rangefinders usually use high power pulsed lasers. The ones in domestic equipment are low power, continuous ones. Also, I would recommend playing with such devices unless you have the appropriate safety equipment and the means to measure the power and ensure you are driving them safely. You WILL go blind.

Keith

 

[kapilp]

@keith thanks for the reply and the word of caution. I do not have any safety equipment as of now, so will not be fiddling around with them ....

are any other low power 'safe' lasers available which can be salvaged out of old electronic parts ?

Thanks

 

[keith1200rs]

Visible lasers are 'safer' because you can see the light and blink/look away. With IR ones you don't know of the damage to your eyes until it is too late.

You can make a short range device with visible lasers by using continuous modulation and heterodyne demodulation.
You can also do it with LEDs.

Keith

 

[kapilp]

You can make a short range device with visible lasers by using continuous modulation and heterodyne demodulation.
Keith

what kind of a range do you mean? do you mean the 20-30 cm max range available with typical IR leds or somewhere greater than a meter ?

 

[keith1200rs]

I have designed a laser rangefinder with 30m range for non-cooperative targets with a 1mW laser, but that isn't easy. Something with a few metres range should be ok.

Keith

 

[kapilp]

WOW!! .... 30m with salvaged electronic parts sounds awesome to me ... actually anything above 1.5m from salvaged parts is awesome!!
Can you share which transmitter and receiver pair you used ... should be enough to atleast get me started ...
You used continuous modulation and hetrodyne demodulation as you stated earlier right ?

 

[keith1200rs]

I didn't use salvaged parts - these are professional designs, although the target cost was low so I used the cheapest visible lasers available and a cheap 1mm PIN diode.

I cannot share circuits designed in the course of my work for customers. However, the principle is straightforward - modulated the laser with a frequency and mix the returned signal with a similar but slightly different frequency to get a low frequency signal. The phase shift is preserved so now you only need to measure the phase of a low frequency which is a lot easier than doing it for a high frequency. Mind you, there are some phase measuring chips from Analog Devices which allow you to do the job without mixing but you need to be good as GHz design.

Keith

 

[kapilp]

If i get this correctly, you are performing a kind of 'time-of-flight' measurement by calculating the phase difference.... ??
If this is the case, can the same be achieved by using a pulsed laser?
And if i plan to use this laser in an open environment with a lot of people, do i still risk any damage (to them)?

Thanks

 

[keith1200rs]

Yes, measuring the phase shift of a modulated signal is the same as measuring the time of flight - just in a different way. It is not without its problems, such as what to do when the phase shift is >360 degrees. With a pulsed laser you have to actually measure the time. As you are dealing with times which you need to resolve to maybe tens of picoseconds or less, it is not trivial.

You absolutely MUST obtain the relevant standards for laser safety and stick to them. All the laser rangefinders I have designed are "eye safe" - Class I or Class II.

Keith.

 

[kapilp]

'picoseconds' !!?? beyond my scope. thanks though ... and i will check the standards out for sure.

Is there a way to measure reflectance of laser in a way we do it with normal IR? .. reflectance using a photodiode maybe ... ??

 

[keith1200rs]

Triangulation would be an easier method to consider. I wouldn't get too hung up on using lasers. LEDs can do the job in a lot of cases (I have even seen a thesis on using LEDs for a time of flight rangefinder).

Keith.

 

[kapilp]

triangulation will require a closely packed linear array of photodiodes (i guess they are called cmos arrays) to work with ... i've no idea where to find them ... are they salvageable ?
I've already worked with triangulation sensors from sharp and know their power, but they are our of my budget ...

 

[keith1200rs]

You could use two photodiodes close together and measure the ratio of the reflected light to triangulate. Hamamatsu make some linear photodiodes with a connection at each end where you can measure the position of the spot by the ratio of the currents but you would have to buy those. I don't know what devices you will be able to find in salvage.

Keith

 

[kapilp]

Nice idea! ... will work on it ... Getting Hamamtsu parts will be difficult ... so will have to stick to photodiodes instead.

This will require high power LEDs wont it? .... normal ones give distances upto 30cms max.

Can the 1mW commercially available laser pointers be used as a replacement .... ?

(my reasoning is that the intensity of lasers>led hence more signal is reflected, hence longer distances)

Thanks ...


-------------------------

just checked out the price for a 1D psd by hamamatsu .... way out of my budget .... sticking to photodiodes

 

[keith1200rs]

Lasers aren't brighter than LEDs. They are coherent light sources and are usually a very small source so can be made into a very small spot even at a reasonable distance. However, spot size also depends on the optics. With sensible optics you should be able to go several metres with LEDs. As I mentioned there is a thesis which used LEDs for rangefinding and that was for mapping out the profile of a room, I think. You need to modulate your light source to remove ambient light and you will need a lot of gain. The receiving optics are important. Also, the receiving optics diameter has a large effect on the amount of signal you will get. Old camera lenses can be useful for making the optics.

Keith.

 

[kapilp]

here's what i will do .... I will first try reflection with high brightness LEDs on high gain. Then move to triangulation with the same setup. If I dont get the desired results, will move onto optics (should be fun) ....
btw can you share any info about the thesis that you are talking about ... should help a lot ...

 

[keith1200rs]

Unfortunately I only have a paper copy - I never managed to find an online version.It is "A time-of-flight optical range sensor for mobile robot navigation" by Michael James Brownlow, University of Oxford 1993.

Keith.

 

[kapilp]

thanks for the info