Most rangefinders use sine modulation and measure phase shift, which doesn't involve picosecond timing or expensive wideband signal processing. And they report one number, not a graph. OTDRs are much more complex than rangefinders.
And the markets are different too.
Well, the OTDR needs a fiber-coupled laser, for one thing, and a decent TIA. Also as John says, it's a time-domain instrument. A laser ruler can just be a diode laser with a monitor photodiode, a collimating lens, a simple TIA and a micro with built-in ADC.
You put a small (1-3 mA) current ramp on the laser, and look at the beat signal coming out of the monitor photodiode. The frequency gives you the round-trip delay. This sort of laser feedback measurement can be pretty good if the diode stays reasonably single-mode.
I have no idea how the $16 ones do it, but if I were building one, that's the first thing I'd try.
Cheers
Phil Hobbs
Also, if a range finder fails to display an accurate result it's usually not a big deal. So it doesn't have to be hi-rel or anything. If a TDR like the one I designed fails that can make the evening news. Nobody wants to be on the evening news in a context like that.
I wonder if you even need a photodiode.
Reminds me of the 3-tube proximity fuse, where the plate current of an RF oscillator was modlated by doppler from a target airplane. Or the ground.
It’s similar. Laser feedback measurements are flaky as can be, but they have heterodyne gain and selectivity, which rejects ambient light to a very high degree.
I’ve done coherent lidars whose SNR stayed the same even looking directly into the sun. It’s all about having enough LO power and rejecting the laser noise adequately.
The monitor PD makes it fairly easy, at least for undemanding tasks such as detecting a wall. ;)
The laser may mode-hop in some range of current, but with sawtooth modulation you can just ignore the intervals where it misbehaves.
We have a very nice laser controller product, the LC120, which automates a lot of stuff like that.
Cheers
Phil Hobbs
Short distance laser rangefinders can be implemented with the integrated time-of-flight sensors now being used in cell phones.
kw
Keeping a laser diode single mode seems to mean that you keeps its junctions temperature stable to better than a degree (and they do self-heat in operation.
My 1996 millidegree thermostat was designed to stabilised the temperature of the laser's target, but the machine worked better if the laser was operating single mode, and stayed in that single mode, so my thermostat got duplicated to stabilise the temperature of the laser diode as well. The stable temperature was set up during final test to give a temperature half-way between that particular laser's single-mode switching temperatures, but that just meant writing a single word into memory.
On 12/03/2025 5:12 am, Phil Hobbs wrote: > On 2025-03-09 19:22, Tom Del Rosso wrote: >> Phil Hobbs wrote: >>> Hi, All, >>>
Keeping a laser diode single mode seems to mean that you need to keep its junctions temperature stable to better than a degree (and they do self-heat in operation).
My 1996 millidegree thermostat was designed to stabilise the temperature of the laser's target, but the machine worked better if the laser was operating single mode, and stayed in that single mode, so my thermostat later got duplicated to stabilise the temperature of the laser diode as well. The stable temperature was set up during final test to give a temperature half-way between that particular laser's single-mode switching temperatures, but that just meant writing a single word into memory.
You could detect when the laser diode changed modes,so you just ramped the laser temperature over a couple of degrees (which was enough to get you two mode hops) and you fixed a temperature halfway between the mode hops).
Have something to add? Share your thoughts — no account required.
Ask the community — no account required