Perhaps Jim's laser is 'saturating'. Lowering the DC level to get more AC would seem logical in this case.
-- Paul Hovnanian mailto:Paul@Hovnanian.com ------------------------------------------------------------------ Where am I going, and what am I doing in this handbasket?
Jim, just make sure they don't change horses later and accidentally put one in that is only rated at 5V abs max. Luckily the engineer of my client told me before I designed the bias section that I had to also accommodate 5V versions.
Yawn... SNCR :-)
Mine has to go 4-1/2 orders of magnitude faster.
-- Regards, Joerg http://www.analogconsultants.com
JT,
Why not measure the "sauration you observe, mathematically and directly, INstad of terying to fool spice with circuit elements, just insert a light level dependant A/W, then the saturation will be exactly what you observe anyway!
as you indicate increasing area or decreasing the power (density?) displays a decrease in the A/w conversion.
You may be able to get away by adding a a varible impedance to the source, as this would give a linear explaination of the V va I response observed. Best regards,
marc
From what little data I've seen it looks like a MOSFET running out of hole-electron pairs.
I'm trying to cajole the client into taking careful data but he's resisting... he's sure he understands what is happening... he's a PhD ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Just curious, why do you want to spend so much effort in modeling what happens if the PD receives excessive optical energy? The docs I have seen indicate that some irreversable damage might occur. Example on page
9 of this one:I had a situation that was probably similar. The lasers for our market were mostly 5mW and above but the PD couldn't stomach more than 1mW, some of them 2mW. Called the client about it last week and we quickly agreed to use optical attenuation.
-- Regards, Joerg http://www.analogconsultants.com
Subtle usage of PD. NOT overpowering, just getting into non-linearities that cause modulation to generate harmonics.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Ok, that's different. If you are using a special low noise laser source that might matter. With normal DFBs from what I could see they seem to dominate the distortion and noise budget. This matters a lot in our case. It'll be interesting.
-- Regards, Joerg http://www.analogconsultants.com
Just a possibility: do you think maybe the effect is due to series resistance between the active diode area where the light is being collected, and the bond pads or terminals of the diode? If that series resistance multiplied by the current were great enough, then the actual junction could get forward biased over some part of the area even though it is reverse biased at the terminals. A lot of this would depend on the sheet resistivity of the diffusion which would depend on the diode design e.g. if they wanted it to be sensitive to UV etc.
See if it gets significantly worse when you drop the bias to 8V, or better if you increase the bias to 12V. This would not rule out other effects but it would be a good clue.
Chris
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