Benchtop Power Supply Options

Ouch, pico amps with a PD. That's hard. I assume the wavelenghts were too long for a PMT? (Maybe a job for a APD?) I might have tried eating the PD capacitance and running it at zero bias... at least there is no leakage current then.

George H.

One good method for that is to use a multimode fibre bundle on the Rx side--you spread one end out into a line to form the exit slit, and bunch the other end into a disc to match the photodiode shape. Then you focus it down onto the PD to reduce the required area.

If the spectrometer runs at f/4 (0.125 NA), which is common, you can match that with catalogue fibres. Then you can save a factor of 16 in detector area just by going to 0.5 NA on the detector, and another (maybe bigger) factor due to illuminating the whole detector area.

You can typically reduce the PD area by a factor of 100 to 1000 compared with a square PD the size of the exit slit.

(I did a SWIR spectrometer last year that did that--worked great except for some X-axis instability problems that I'm working on now.)

Cheers

Phil Hobbs

Good guess on wavelength; we did eventually borrow a red-sensitive PMT, and chopper for the illuminator, and phase-lock amp - the results were ... excellent. Two days later, we gave those all back, though.

It was running at zero bias (we thought the signal cable, a plain old RG-58 BNC , would have been sensitive otherwise). An off-the-shelf picoammeter was the preamp we used, and it probably had low bandwidth (10 Hz?) which helped the noise situation. The diode(and cable) capacitance just didn't hurt us at all at the scan speeds we were using.

Yeah, I think you mentioned this trick. Do you loose much light getting it into the fibers? (I'll guess ~ 10% at each face, and then some other filling factor due to the round fibers and long square slit... I guess you have to match the fiber to the slit width too...)

And what do you gain in a low light situation? (I guess the smaller PD means less reverse bias leakage... so you can do the measurements faster and average a bit.

Hmm, lets see Z axis is the light direction, and I'll guess y is up and down, (Along the long slit axis) so x is the short slit dimension?

You'll want accurate angular alignment of your fiber bundle and slit, as well as getting it in the right x-position.

George H.

Oh, a PD on the end of coax, only physicists are allowed to do that :^)

George H.

You lose about 4% at each face to Fresnel reflection, and the fill factor for 200/240 um fibres is about 0.8. (You don't care about the cladding on the slit edges, and the little bit of scalloping of the effective slit area doesn't reduce the resolution much.)

Right. 100x less area means 100x less crap in the data, and more than that if you're in the omega*eN*Cd limit.

Yes. If you have a wavelength uncertainty of 0.01 nm and a 1 AU/100 nm slope, you get 1e-4 AU of absorption uncertainty.

That isn't hard at all at f/4 with multimode fibre. The fibre bundle _is_ the exit slit, so you get that for free, and eyeball accuracy is better than good enough for the angle.

Cheers

Phil Hobbs

I've seen this recommendation before (and it makes sense) but I've also seen long, thin photosensors (the CCD in most flatbed scanners). Why doesn't someone make slit-shape photodiodes?

Light enters low-index media most effectively. Does that mean a plastic light fiber would be preferred? Cladding has high refrective index, would reflect much of the illumination.

One recommendation was to use epoxy to hold the fibers in position, which would be a light-directing nightmare (can one dismiss the light leaking into the epoxy?). Come to that, why not just use an acrylic lightpipe with a slot-to-spot shape?

The use of index-matching cement at the photodiode was not raised (perhaps because that contaminates the photodiode chemically?).

Finally, if one wants an efficient coupling of diode to slit, what's wrong with using a V-assembly with a mirror opposite the photodiode? The multiple reflections in such a structure would result in most of the slit's light hitting the diode, and any reflected light would get a few more passes at the diode on its way out.

You can get them made if you have the volume, or can afford the NRE. Of course surface leakage would tend to go like the perimeter rather than the area, but in big enough quantity that would be cheaper than a fibre bundle.

POF is generally much larger than glass, but it would work OK over the spectral range where it's transparent.

Basically because it modifies the angular spectrum of the light in an undesirable way. Rearranging fibres gives you a pretty well circular light cone coming out.

You can coat the fibre ends if you care about the 4%.

The light emerging from the slit is fairly far from collimated, though. Using fibres allows you to slice and dice your etendue (area-solid angle product) in a fine-grained way that is a big win in this application.

Cheers

Phil Hobbs