We buy fiber-coupled lasers (850, 1310, 1550) and corresponding photodiodes. They all look alike. We'd like to test the incoming parts and our final products to be sure they are for the correct wavelength.
The lasers are both singlemode and multimode. The photodiodes don't much care.
I tweaked my beautiful GHz o/e converter and later discovered that the wrong photodiode was installed. I'm concerned that we don't verify wavelengths on our products.
Most OSAs are narrowband, high resolution.
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
I've had the same problem, lasers and diodes with wrong wavelength being placed (a very subtle difference in type number). In the company junk box I found a coarse WDM multiplexer, several fibers in, one combined out, all with SC connectors. Added a set of adapters to it. It was easy to check lasers 850 / 1300 / 1550 with it, using 2 detectors and a DMM. The reverse, differentiate detectors with a set of known lasers, is also easy. Hookup a few 'scrap' lasers (rejected for bad moding, etc) to a battery as a set of known sources. A detector quickly shows its type with that.
When you are using SFP's with AC coupling a set of modulated sources is needed, a 50 MHz squarewave is enough at the TX side.
I was going to say a tungsten lamp into a spectrometer/ monochromator is pretty broad band. Some NIR/IR grating. (in a past life I had access to these McPherson spectrometers with ~2" curved diffraction gratings, the gratings were treated as gold.. better.)
Yup. But for this sort of use the etendue of a 25-mm filter is hard to beat for the price. Also monochromators can be misleading if you don't guard against order overlap (aka aliasing). A tungsten bulb is quite a bit brighter at 750 nm than at 1500.
Delta filters in Denmark make very nice circular variable-frequency filters--sometime when I've got $3k burning a hole in my pocket I might buy one to play with........nah. Cute though.
For my photoreceivers, I could get a tungsten source, a motorized filter wheel, a bunch of filters, and some sort of lens to focus the filtered light into a multimode fiber. Spin the filters and look at the PD output on a scope. That's not totally horrible, but I probably wouldn't get much optical power. I'd like some decent fraction of a milliwatt.
Or I could get a bunch of diode lasers of different wavelengths, scan their drive, and find some way to concentrate them all into one fiber. Crude: I could get three fiber lasers, 850, 1310, 1550, and connect them one at a time to my receiver and note the levels. That would be a pain in production.
In the other direction, checking lasers, this looked great:
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until I read the fine print. None of the heads will do wavelength measurement over my range.
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
I can imagine some number of fiber lasers, at least three, that go into a combiner of some sort and come out on one fiber. I can pulse the lasers sequentially, drive my photoreceiver, and sync it on a scope; a 3-point spectrum analyzer.
Do people make fiber combiners that work from 850 to 1550?
Or an electrically-operable N-to-1 switch, ditto?
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John Larkin Highland Technology, Inc
picosecond timing precision measurement
Could you just get an Ethernet switch with SFP sockets, and get three (850, 1310, 1550) transceivers? Plug in the right source, send a broadcast, and watch your diodes' output?
Not cheap not single mode. I know of 1200-1700nm tunable lasers and similar spectrum 'white' lasers. In ten years you'll find them from scrap yard, but at the moment you'll need deep pockets and event that may not be enough.
I've got HP70950B, which does 600-1700nm. Shorter wavelengths are second order from monochromator, but that is now a problem with narrowband sources. It's not very sensitive (compared to cooled InGaAs spectrometer with long exposures), but you won't need that with lasers.
A black-and-white camera with a 0.3 mm thick Si filter sees light from a 1550 nm communication laser. Is this a two-photon absorption or an random short-wave laser light ?
Good point. I guess turning the current down won't help much. :^) I recall these nice bulbs we would use. They had a filament that was a coil in a line. You set the filament to be vertical and focus that onto the spectrometer slits.. It worked OK.
Huh, neat. I never heard of those. What's inside? Some variable interference filters?
I think they must use a rotating chuck with a shield, and change the rotation speed cyclically so that the layers come out with varying thicknesses.
It would use quite a bit of tool time, which probably explains the high prices. It's not a complete solution for JL, because the tuning ranges don't cover both 850 and 1500 nm.
We have 3 wavelengths of fiber-connectorized laser diodes in stock, and can get more. And we have power meters. The issue is whether we have to connect them one at a time to the DUT, note the output, and do some math, in production. Of course we'd have to trust about the wavelengths of the test set lasers, or send them to some lab to be checked.
The power meters want you to tell them what the wavelength is. They don't measure it themselves. Optical instrumentation is barbaric.
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John Larkin Highland Technology, Inc
Science teaches us to doubt.
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