Enprovement of a DIY fiber optic isolated voltage probe.

[mawyatt]

Interesting idea!!

If you could get into the WDM effects, would think you could use a linear Phase or FM modulator (Lithium Niobate comes to mind, or maybe "pull" the laser diode to modulate the optical frequency) to allow a reasonably good analog bandwidth. If you could use two WDM channels, one would be fixed with no modulation while the other is modulated, this opens up many possibilities for communicating the analog signal down the fiber.

Anyway, hope someone gives this a try, seems like a interesting approach whichever way it's done

Best,

[GonzoTheGreat]

The optical Tx modues in SFP+ transceivers are sealed.  I do not know how to linearly pull their optical wavelength millions times per second.
Take a look at this video. It depicts a 1Gbps (SFP) module (not the 10Gbps SFP+) that illustrates the limited access to the optical components:
https://youtu.be/GGuqtZOurZo

[Marco]

They make ROSA/TOSA for CATV which should be somewhat linear, but I never see them sold as components.

[StillTrying]

They make ROSA/TOSA for CATV which should be somewhat linear, but I never see them sold as components.

They look too expensive and complex, at least for me to play with, and as usual I couldn't see any analogue use on the data sheets of the ones I looked at, and they seemed to start at 5kHz rather than DC.

[Marco]

The ROSA/TOSA themselves are as simple as it gets (laser) diode and wirebonded die amplifier, with pigtails and optical connector.

They are not DC coupled, but all the bare die really fast transimpedance amps aren't either. They have to be analogue and fairly linear or they would screw up the tv channels. As I said, "for CATV", not ROSA/TOSA in general.

[GonzoTheGreat]

This $30 optical module is linear but AC coupled.
https://www.mouser.com/catalog/specsheets/ELX10GDL0610710ELT_ka.pdf

[StillTrying]

"This $30 optical module is linear"

It looks digital use only to me.

[Terry Bites]

As your transmit side is mains powered why not digitise and then send data over the isolated link? Digital isolators are fast and cheap. OK the cost of a fast ADC, chipageddon, micro controller faff.
TI and AD have photodiode design wizards, well worth a look.
You can also compensate for drift using a diferential configuration . Though not with an antuique fron the TL0 series!
Other toplogies include https://www.digikey.com/en/articles/design-transimpedance-amplifiers-for-precision-opto-sensing

[Marco]

AFAICS only the SFP+ receiver on a linear module is guaranteed to be "linear" (except it can still have AGC). I see some papers where they abuse transmitters for commodity modules sold for digital modulation with PAM, so the transmitter can be linear too I guess, but don't count on it.

Here's a laser diode with fiber connector on alibaba supposedly for CATV. The price on the page is certainly very interesting, but it's alibaba so who knows.

[Terry Bites]

Ah.
I assume your measurement side is a scope of some sort, earth referrenced.
I that case, powering the HV side from mains means that all youve realy got between the scope in the way of isolation is the transformer insulation in the PSU.
Worse still youve got interwinding capacitance pasing even more leakege curent.
So that kind defeats the object?

The mains powered parts need to be on the "earthy" side of your system. A psu failure may occur because of the high voltage difference between the iso side and the earthy side. That voltage will be seen across the transformer. Shazam!

[StillTrying]

When I was doing the photodiode and linear LED light and experiments I must have read 100s of web pages and pdfs and there was nothing much out there on it.

There seemed to be not many more than JS and me that's tried it.
Some experiments here www.eevblog.com/forum/beginners/floating-probe!-for-$2-50/msg1817129/#msg1817129

To get any linearity and a least some dynamic range out of the LED light the speed seems to be stuck at not much more than around 300kHz. It seemed to me I had to get the LED current very high (X10) to get the speed up, but then they could only be used pulsed for just a few us and duty around 1%, because at high currents the LED light droops after only a few us.
You'd think there was at least one video speed linear light transmitter and receiver out there but I didn't find it.

[Marco]

I wonder what the modulation bandwidth is of those ridiculously cheap red laser diodes is (the ones with just some PCB and metal sheet stuffed into a lens housing). They aren't exactly build for low capacitance, but it's probably low enough for a couple 100 MHz.