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
I'm pretty much with Phil on this one. I saw this same effect way back when in grad school at that little joint in Cambridge. The working theory at the time was that the photocurrent and the diode's bulk resistance reduced the reverse bias to nothing. It would be interesting if your customer was able to increase the magnitude of the reverse-bias voltage just at the saturation point to test this hypothesis. And yes, a bigger diode solved my problem as well, although we ultimately used a beamsplitter (made of a simple microscope slide, about
4% reflection from each surface) so we could keep the bandwidth that the smaller diode allowed. In our case the PD was inside a feedback loop so we couldn't afford the lower BW.I never thought about local forward-biasing, I'm not sure how this would work in physics-land, or if it's testable just from the leads.
Steve
Did you have that larger PD hooked up to a cascoded TIA to squeeze more BW?
Steve, your post only made it into the CAD newsgroup. Check your newsreader to make sure it posts to all groups in threads that are cross-posted.
-- Regards, Joerg http://www.analogconsultants.com
We were using an op-amp-based TIA with rather low feedback resistance, just a few kohms. We could get away with this because we had a laser with quite a bit of intensity (something like a few hundred mW single-frequency at 5145A).
The opamp was the 9826 from Optical Electronics Inc in Arizona. It had huge GBW, around 1GHz. I learned just a couple of years ago that a now-coworker actually designed those OEI amplifiers!
BTW, that was quite a laser. Spectra-Physics Argon-Ion laser, the biggest one they made. It was good for about 8W CW output on all lines (before we put the modulators and etalon in the cavity for single-frequency locked operation). It required about 75KVA, delivered from 3-phase 440V. Not very efficient, but very impressive.
Right you are. Fixed.
Steve
Check out TI's OPA847. GBW almost 4GHz and under 1nv/sqrtHz. But I am lucky since I'll have enough receive level at all times.
Wow, you could probably burn a hole through a boulder with that one.
-- 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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