Re: Fast sampler

john larkin <jl@glen--canyon.com> wrote:

On Tue, 11 Mar 2025 14:12:51 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

On 2025-03-09 19:22, Tom Del Rosso wrote:

Phil Hobbs wrote:

Hi, All,
 
Late last year we did a fast sampler/TDR with nice clean 60 ps edges.
 
We're gearing up to actually sell them, so I did a short technical
writeup on the design, which may be of interest.
 
<https://electrooptical.net/News/a-high-performance-time-domain-reflectometer>
 
Cheers
 
Phil Hobbs

 
Why is a LASER ruler that can measure distances in air with 2mm accuracy
$16,
 
and an OTDR for measuring fiber $600?
 

 
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

 
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

--
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics