I saw this video - http://www.youtube.com/watch?v=yjsqrlKow44 - and I am interested in having a go at something similar. I have a little RF experience from many years ago but I’m not confident in working with RF so right now I would just like to get a feel for the scope of implementing this.

So let’s say I want to modulate a laser at 40MHz, collect the reflected laser light and use a phase detector to determine the phase difference between the reflected light and the source.

Is driving the laser as simple as a 40MHz oscillator with something like a MOSEFT to power the laser diode? Do I need a special laser diode or are most of them capable responding in the MHz range?

On the receive side I assume that an appropriate spec PIN Photodode would be up to the job? The guy in the video uses amplifiers to boost the receive signal, I assume these can be straight forward transistor amplifiers with appropriate spec transistors?

For phase detection the guy in the video is using an AD8302, which seems fairly expensive and possibly over-spec’d for the job. Could I use a PLL chip such as the TLC2933 to do the phase detection?

Sorry if this sounds like a “design my project for me” type of question but before I jump-in and then try to swim, I’d like to know have some idea deep the water is and what difficulties I may encounter.

 

I wonder why he uses a separate photo diode and related hardware to create the reference signal? Why not use the modulating signal as the reference directly?

Sure, it will have some offset that must be compensated, but that's not too hard.

 

I had wondered that, I guess it could be to keep the design "simple" in that both the reference and the reflected signals use the same circuit

 

Driving the laser diode at 40Mhz to get sharp optical output pulse it is better in terms of accuracy rather then generating a sinus wave. Integrated driver, like the iC-HG or other can simplify this task(application design - http://www.ichaus.biz/wp4_fastlaserdriver ).

 

Thanks IC-Man, interesting.

Still after thoughts on types of Laser Dioide and phase detectors...

 

Why 40 Mhz? this seems like an unnecessarily high modulation frequency to me?
Consider the concept of 'phase ambiguity' as it relates to the desired range you need to measure: at 40 Mhz light travels 7.5 meters in one period, how do you resolve longer distances? Every 7.5 meters, the phase shift will be = 0 again...
(actually the distance is 1/2- it's a round-trip for the light pulse)

 

This is just a first step. Having got it going for one frequency the next step would be to measure at a range of frequencies and then solve for longer distances (this is how the commercial units do it). But that said for >90% of what I want measurements in the 0.2m to 5m range is enough.

The trade off is resolution/accuracy. Much longer wave lengths mean either a much more accurate phase detector or lower resolution. AFAIK the commercial units use much higher frequencies for mm accuracy.

 

I find this to be a very interesting project, it's outside of my domain as far as high speed opto-electronics and the like, not a huge community of folks out there that possess this specialized knowledge. Please share what you learn!

 

I find this to be a very interesting project, it's outside of my domain as far as high speed opto-electronics and the like, not a huge community of folks out there that possess this specialized knowledge. Please share what you learn!

I'm beginning to realize that . I will share what I find if I actually get the project off the ground...

 

Driving the laser diode at 40Mhz to get sharp optical output pulse it is better in terms of accuracy rather then generating a sinus wave. Integrated driver, like the iC-HG or other can simplify this task(application design - http://www.ichaus.biz/wp4_fastlaserdriver ).

Do you have more info on this? From what I remember on commercial laser rangers they use a sine (and lots of averaging) to eliminate a lot of the innaccuracies that occur from edge delays and reflections, focus issues etc. That would be the exact opposite of what you said.

 

I wonder if the photo diode input amp has some type of AGC to keep the amps operating in a linear region, to avoid phase distortions? Also seems like tuned band band pass amplification would be necessary for long range sensing?

Where is our laser ranging guru?