So what? If you're talking about a monochromatic (laser) as a source, there aren't any different colors. Is it room lighting that you would find disturbing? Scanning the angles, starting at zero degrees (zero order), your first peak is it at order #1, and isn't subject to confusion.
But if you have a monochromatic emitter you will know the wavelength to within a factor of 2 or 4 so it should be fairy obvious except possibly with the odd frequency doubler (like 1066, 533, 266 or 800, 400).
OTOH any of the visible ones you can pretty much identify by eye.
Stick to first order and interpose a long pass or high pass filter to eliminate the ambiguity then. The trick is described here:
I'm assuming for the moment that you are serious.
Semiconductor laser lines tend to be on *very( specific wavelengths. (as do all the common laser and excimer gas mixes)
We tended to be on 1066, 533, 266 ie NdYAG + doublers.
I always found the amount of visible green in the beam a bit worrying and the promise that the perspex enclosure was entirely opaque to 266nm. I never spent much time in close proximity to the systems when running.
Low pass filters plus a few detectors should be good enough to discriminate between most of them. I'd have thought that for a known part number whether or not it emits laser light or not would be good enough. I vaguely recall an amateur astronomers tunable visible wavelength dichroic filter (I know someone who bought a prototype) but it seems to have sunk without trace. I'll ask next time I see him.
Otherwise try Young's slits to figure out the wavelength.
He isn't that far off the mark though.
Your choices for discriminating between various LED laser emitters might be met by using one of each device in a 5x5 grid and illuminating them with the DUT. Sum the current that they produce with a opamp and with a bit of cunning you should be able to approximately get the wavelength from the actual current it produces. That's one device per 50nm.
Only the diodes emitting the same or lower energy photons will generate photo electrons when so illuminated.
You might get away with just one example of each of the common laser diodes. I assume the problem is to check that anonymous black plastic blobs are in fact the emitting the right wavelength ones.
The other option would be one or more photo detectors and various Schott of Hoya low pass filter glasses available from various dealers.
I recall a tunable visual band dichroic interference filter intended for amateur astronomers from a few years back that was quite impressive but I think only covered 400-700nm. I know someone who has one next time I see him I'll ask for the spec. I don't think they ever took off.
Not sure you really need to do the visible ones since there are not all that many semiconductor laser lines possible in practice.
Yeah, a cross-dispersed echelle system can get resolving powers (lambda/FWHM) up near 1E6, which is pretty impressive.
Cheers
Phil Hobbs
Test various lasers to see if they are the right wavelength, as in
1550 vs 1310 vs 850 vs 800.We can gross separation with a fiber WDM splitter and several detectors, but that's klunky.
How would you do a small spectrometer that works from 750 to, say,
1900 nm? Better yet 350 to 1900.(Mo just gave me a bybye hug. She says hi.)
Would anyone joke about that? We buy thousands of pcb mount fiber lasers per year, most of them at four wavelengths and the occasional oddball, and they all look alike. I'd like to final test our products and be sure we are shipping the correct wavelength.
Yes, a WDM splitter or two and several detectors and a bunch of opamps. We might build that in a box, with an LED to indicate each band.
Recently a customer asked for a some units with 800 nm lasers, which we hadn't done before. We offer 850 as one of our standards. At least you can see the 800, to make sure it's not one of the IRs.
Our policy is to test everything that has a spec. Mixups are possible.
I vaguely recall an amateur astronomers tunable visible
I want something that looks like a DVM... not an optical bench.
Maybe there is a linear or circular graduated bandpass filter. Rotate a marked knob for max output. Or a metal disc with multiple, selectable bandpass windows. I'd rather buy something.
We could build N boxes, each a bandpass o/e converter. Run a production source into the right one and it should light up.
Just as your eyes don't cover that whole range, so most detectors don't. This one
If you can modulate the light source, you can use a bolometer or photoacoustic sensor with wide range. The scheme will be, effectively, a lock-in amplifier.
We really need to check IR fiber lasers. LEDs are pretty obvious.
I'd need a dispersal mechanism and a bunch of detectors. If there is any wavelength ambiguity, a bunch of software might look at all the detectors and untangle things and report a single wavelength in plain sight; this would be a production test, not a research project.
Software might work from an imager, in the case of 2d dispersion, but it might be tough to find a wideband imager chip. Lotta work.
And doesn't report wavelength.
Our sources are always pulsed, so we could use an ac-coupled detector, like a few wavelength-specific SFP modules maybe.
No, bad idea, they have wide range AGC.
Fair enough.
That might be the simplest solution.
I found 800nm dim red and 400nm disappointing dim in the purple.
Indeed.
Yes that was basically it a rotating disk with a graduated shift in bandpass as you moved it. This isn't like the one I remember but a related idea based on crossed polars and an electrical tuneable cavity.
and references therein especially (1)
Xiang, J. et al. Electrically Tunable Selective Reflection of Light from Ultraviolet to Visible and Infrared by Heliconical Cholesterics. Adv. Mat. 27, 3014–3018 (2015).
If they have any working prototypes they might be worth talking to...
I was thinking more in terms of a splitter and a grid of sensors each one covered with a different bandpass (or cheaper low pass filter). You can get annealed selelenium glass low pass filters over quite a wide range of wavelengths from any of the glass companies.
For lab use with collimated beams, probably an 800 l/mm grating, a white card with a scale, and a lead salt vidicon camera. (I have a couple that I need to repair one of these times--both tubes work but there's something wrong in the black level circuitry that makes the picture disappear after a few tenths of a second.)
For barefoot diode lasers, maybe a shear plate instead of the grating--you know the radius of curvature of the wavefront, because it's just the perpendicular distance between the plate and the laser, so the fringe spacing gives you the wavelength. I have a couple of nice shear plate devices that would probably work at some level, although the coatings would be badly mistuned.
Say hi back. She's a jewel.
Cheers
Phil Hobbs
If you use a low-res grating, you'll get multiple orders from everybody. Since you know a priori that it's only one wavelength, there's no ambiguity.
Cheers
Phil Hobbs
I think your problem will be with the longer wavelengths even thinned back illuminated CCDs fall off in sensitivity steeply at around 1000nm.
If it's one source with a dominant wavelength, one can disentangle the overlapping diffraction orders with software. If the source contains a frequency doubler, there will also be some of the original un- doubled drive also present, but this approach may still work.
I would use a linear photodetector array, but silicon won't work for
1550 nm at all. There may be a detector material that will span 800 nm to 2000 nm, but it may not be suitable for a camera.More generally, if the task is simply to detect mixups in production, and we are testing a bright laser source, I'd go for simple and rugged. This approach others have mostly suggested:
Acquire a four-way passive optical power divider made of Ge-doped fused silica optical fiber. Step index is OK. This divider will provide four outputs of roughly equal power (if so designed). Attach each output to a detector for one of the possible (nominal) wavelengths, using optical filters as needed. Estimate the dominant wavelength from the combined output currents in such a way that the source brightness mostly cancels out.
If the response patterns are complex, four dedicated pattern-recognizers in parallel may be used between outputs and the decision process.
Model the decision algorithm on Receptive Fields in Biology:
.
This will give an unambiguous answer representing the best guess of the mixup-detecting box.
One can also use the four outputs to drive four green LED lights is a square, and let the human to the receptive-field processing visually.
Or both, at least initially.
This can be done in analog hardware, or in code.
Joe Gwinn
Detectors are available typically InGaAs for the NIR as are whole module solutions but I don't think he is going to like the price! eg.
Regular silicon detectors cover 900-300nm (or thereabouts) at low cost so I wonder if a non-linear crystal can be switched in ahead of the grating to double or triple 1600nm into the Si-detectable range. JL wants to verify single bright monochromatic sources so losses needn't be a worry?
piglet
A typical source is a connectorized fiber-coupled laser of a couple of milliwatts. Multiplication usually requires very high powers.
The longest wavelength lasers that we now use are 1550, and we have corresponding photodiodes. The shortest are 800, ditto.
A wavelength splitter and three photodiodes would at least identify the band of a laser: 1500ish, 1300ish, and 850ish.
If there's enough energy to unbalance a thermistor bridge, a single detector would handle the whole range. Just put one thermistor in shade, and expose the other to the 'beam'. That does require a mirror for beam forming from a small-spot source, but no wavelengths omitted if you spend the money for a reflective grating do do the wavelength selection. The replica gratings, at $1 each, don't come with a lot of detailed specs for 1500 nm.
You'd not want the operator to breathe on the detector, of course.
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