Photodiode, photodiode or photomultiplier?

[brdf]

I know one could write a thesis on this topic all the time....but quick answers would really help before I delve into the elaborate books like Building Electro-Optic Systems (which I'm reading slowly...)

Silicone photodiodes are easier to use and I have had very good experience with Hamamatsu silicone S8745 which gives 0.3V/nW for the wavelength I am concerned with: 633 nm (He-Ne). However, this was when my signal was around 1 nW when hitting the diode window. And I was using a SRS chopper + lock-in amplifier to enhance the SNR (which I will continue to use).

Now I will detect some weaker signals perhaps on the order of 0.1 nW (10^-10 W) or less, so I will definitely need a more sensitive detector. I get quite lost looking at Hamamatsu TE-cooled photodiode S2592 (0.3A/W@630 nm) and photomultiplier R9647 (3mA/W @630nm); how to compare their sensitivities with the previous S8745?

Thanks!

 

[IRstuff]

Sensitivity is EXTREMELY circuit dependent. Also, the "sensitivity" that you quote are actually what we would otherwise call detector responsivity, i.e., what is the output change for a corresponding input change? This is mostly a large signal transfer function value, and does not have much relevance to ultimate detectivity.

A more useful quantity to look at is the noise equivalent power (NEP), which tells you what external illumination power is the same amplitude as the rms noise of the detector. Once you have this, you know that an input signal about 6.3 times the NEP will get you about 99% probability of detection.

TTFN

FAQ731-376

 

[IRstuff]

Oh, 0.1 nW is pretty darn low, so you probably need more expert advice. This Hamamatsu APD:

http://sales.hamamatsu.com/assets/pdf/parts_S/S9717_series.pdf

has 50 pA dark current and 0.5A/W responsivity, giving it an NEP of 0.1 nW.

Since you need about 6 times that to get reliable detection, you need to figure out how to get more signal, or spend more time detecting.

TTFN

FAQ731-376

 

[DanStro]

Sorry if this is a dumb question but I am pretty new to detectors.

Is the noise equivalent power the power that matches the rms noise of a single pixel or on the entire chip? Or is it the power/pixel?

The reason that I am asking is that on some cameras there is a Noise Equivalent Irradiance specification instead.

 

[IRstuff]

NEI is usually expressed with something along the lines of W/cm^2, i.e., an area, which you would then multiply by the area of the pixel to the the NEP of the pixel.

TTFN

FAQ731-376

 

[lumenharold]

I am assuming that this detector is to be used with the system described in the 'Mechanical' forum? If so, the requirements for the detector (which are pretty demanding) would seem to dictate a faster lens than the telescope mentioned in the other post, unless 'telescope' is intended to be a generic term. The lenses I am familiar with for spectrometers and other scatter measuring devices are typically F1.2 with a large input aperture.

Harold
SW2010 SP1.0 OPW2010 SP1.0 Win XP Pro 2002 SP3
Dell 690, Xeon 5160 @3.00GHz, 3.25GB RAM
nVidia Quadro FX4600

http://www.lumenflow.com

 

[IRstuff]

Coincidentally, I've just spend a large chunk of the afternoon looking at detectors.

If you do not otherwise have volume constraints, some of Hamamatsu's other detectors might have substantially higher performance, i.e., their photomultipliers and intensifiers.

TTFN

FAQ731-376

 

[brdf]

My "telescope" is actually a radiometer, or simply put, a focal lens to focus the collected light to the detector.

Let me first improve my design by increasing the laser power and the detector aperture trying to bring up the signal level...then I'll bug the forum again

 

[lumenharold]

brdf - I would have to say that you have a very interesting project that has intrigued quite a few people judging by the amount of feedback you have rec'd on these two threads!

Harold
SW2010 SP1.0 OPW2010 SP1.0 Win XP Pro 2002 SP3
Dell 690, Xeon 5160 @3.00GHz, 3.25GB RAM
nVidia Quadro FX4600

http://www.lumenflow.com

 

[DanStro]

<quote>"NEI is usually expressed with something along the lines of W/cm^2, i.e., an area, which you would then multiply by the area of the pixel to the the NEP of the pixel. "</quote>

So the NEP listed in spec sheets is on a pixel basis then right?

 

[IRstuff]

Dan,

It SHOULD be, but obviously, I can't speak for every datasheet writer.

As mentioned earlier, sometimes neither NEP nor NEI is directly specified, and you have to infer it from other parameters.

Next time, please start a separate thread for your questions; it makes it very difficult to follow a thread when there are multiple discussions going on.

TTFN

FAQ731-376

 

[brdf]

Harold: Thanks for your comments; as Hobbs says (Building Electro-Optical Systems) "Designing and building electro-optical instruments is without a doubt one of the most interdisciplinary activities in engineering" and thus collegiality is indispensable. Unfortunately now I am alone in this "polarized goniometer" project but fortunately now I have found this site and you guys...