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Re: HBT- wave noise

> >>> So to see this correlation/ overlap between photons, I need to meet

>>> two criteria. >>> 1.) photon flux, to have intensity (wave) noise equal to shot noise >> >> You're shooting for a fairly dramatic effect there. HB's Narrabri >> observatory worked down at the 1% level IIRC. Of course you want >> students to actually see it, but getting an effect that large is asking >> a lot. Shot noise is nearly always the ultimate limit in optical >> measurements. > Oh the equivalent to shot noise is just a measure. I'm getting > a numbers of a few percent... which looks hard. >> >>> I need I_dc = e * opt_BW,* which I guesstimate to be about 1 uA. >>> 2.) spacial 'uncertainty' I need the quantity, >>> >>> D*d/(lamda*L) > >> Source needs to be unresolved at the detector, check. >> >>> >>> Where D is detector diameter, d is source diameter, >>> L is distance from source to detector and lamda is the >>> wavelength of the light. >>> >>> So I don't know the optical BW nor the source size. >>> I can measure the Laser diode.. ~0.015" ~350 um >>> (the source size might be a lot less than this?) >> >> A single transverse mode has an etendue of lambda**2/2 in each >> polarization. With lenses you can have any tradeoff of solid angle vs >> area you like. > Yeah, but if I stick a lens in front of my detector I'm changing the > detector area.. so I don't really gain anything.. (maybe that is what you > are saying. I get confused when etendue is mentioned. )

Etendue is the product of area and projected solid angle. It's related to the thermodynamic idea of phase space volume.

> >>> >>> When I dial this all up in the lab I see about 10 nA >>> of photocurrent. (1%) The number one problem with this >>> measurement is knowing when the LD starts to lase. >> >> Stimulated emission effects start showing up well below threshold, >> unfortunately. > Hmm.. Yeah that will be a problem. The wave noise will be a function > of the det. source geometry.. so maybe can be disentangled. >> >>> (I'm thinking some real-time optical spectrum measurement >>> would help...) >>> I also dream of scraping the back reflective bit off the laser diode >>> There's a gold wire bond there and I need some good optics! >> >> You can make superluminescent diodes (SLDs) in the lab by gouging the >> back facet of a laser diode with a scriber. Of course this destroys any >> passivation back there, which doesn't help reliability. Also there are >> still important feedback effects even from a damaged back facet. > OK this is what I was thinking of. These two wavelength laser diodes have > their active element exposed. > (I can break the gold wire bonds with my finger.. or finger nail.) > Any idea How much scratching I need? Will some hunk of metal work? > Or by scribber do you mean a diamond tipped thingie?

I'd certainly use a diamond or carbide scriber to reduce the amount of force required.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs
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Cue a fav. xkcd

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It's not that I don't understand the concept...

Anyway for the HBT thing seems the smaller the PD the better.

There's a littel one from vishay VEMD1060

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And this one for fibers looks interesting. OPF470

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(more expensive.. but cheaper that other fiber coupled detectors!)

Hmm my diamond scribe is the size of a bic pen. Sand paper on the end of some tweezers/ toothpick?

George H.

Reply to
George Herold

On Sunday, February 16, 2020 at 7:36:05 PM UTC-5, George Herold

A grain of sand, glued to the end of a small dowel rod? It's left to the student to align it, point down. ;-)

(Hint: Make a depression in the center with a small knife point, or cuticle scissors. then put a drop of glue in the hole. drop the grain of sand into it, and use a thin needle or an Exacto knife point to turn it the right way.) :)

Reply to
Michael Terrell

ROSAs, fiber-coupled detectors with fast TIAs, are dirt cheap. They usually have AGC, but at low light levels they'll crank themselves to max gain.

--

John Larkin         Highland Technology, Inc 

The cork popped merrily, and Lord Peter rose to his feet.  
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply to
jlarkin

What's a ROSA? As usual I'm most likely searching for the wrong thing.

Any 'fiber coupled photodiodes' lead to spendy things ~$100 The TT OPF470 is also sold in what looks like a bnc connector in front for ~$20 each from mouser. (OPF472)?

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Reply to
George Herold

A 10 gbps SFP module typically has a ROSA, a TOSA, and an ASIC inside, for around $20 or so.

Google rosa tosa

--

John Larkin         Highland Technology, Inc 

The cork popped merrily, and Lord Peter rose to his feet.  
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply to
jlarkin

And you can get the 1.25 Gb/s ones for a buck or so on eBay.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

There must be some neat physics experiment one could do with a couple SFP modules. Or even one.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

SFPs can make pretty decent detectors for locking two lasers together at some GHz offset. Of course, that's mostly a prerequisite for other experiments rather than one by itself.

? David

Reply to
David Nadlinger

One could resolve the prop delay of a long piece of fiber to ps or fs, but I don't know how useful that could be. Maybe that experiment where acceleration drags light. Or stress. Or something.

--

John Larkin         Highland Technology, Inc 

The cork popped merrily, and Lord Peter rose to his feet.  
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Reply to
jlarkin

Yeah one problem with 'neat physics/ engineering thing' Is that I always want some good experiment to hang on the end of it. I did this little demo of locking our diode laser to a spectral line.. but really only useful if you have something 'good' to do with it once it's locked. So it was just. 'gee whiz' it's locked.

George H.

Reply to
George Herold

t

"The other way to HBT noise" So I've also built Rb lamps*. Which you can filter down to one of the ~10 GHz wide 'D' lines at 780 or 795 nm. Lamda ~0.8 um (I just use 1^-3 mm and remember to make L a bit longer)

Having measured ~100 with a bulb/source diameter of ~1/4" (6mm") and the same detector area filtered to one line. (D = d = 6 mm) With a 6" (L = 150 mm) A typical photocurrent was 2 uA, which (assuming I did the math right.) gave me ~30pA of photo current at the needed area/ distance. (d*D)/(L*lamda) ~1 (36 mm^2/0.15 mm^2 +~ 240) (2uA/ 240^2 = 34.7pA) Oh and e*10 GHz is 1.6 nA... so maybe a several percent signal.

That is out of the range of ordinary photodiodes IMO. And into PMT's or avalanche PD's.

George H.

*one problem with lamps is they can be noisy.. a lot can depend on how much Rb is put in 'em. (they also drift..)
Reply to
George Herold

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