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Problem with Saturating Photodiode

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Jim Thompson

Jan 1, 1970
0
I just ran onto a situation where a modulated laser impinging upon a
photodiode had such a high light level that the photodiode was
"saturated"... no more available carriers.

Thus "clipping" of the modulation was occurring in the photodiode
itself, creating harmonics.

And lowering the DC bias on the laser raised the AC signal level...
really strange lab results to ponder when you're tired ;-)

Any ideas how one might *model* the photodiode in Spice?

...Jim Thompson
 
J

John Larkin

Jan 1, 1970
0
I just ran onto a situation where a modulated laser impinging upon a
photodiode had such a high light level that the photodiode was
"saturated"... no more available carriers.

Thus "clipping" of the modulation was occurring in the photodiode
itself, creating harmonics.

And lowering the DC bias on the laser raised the AC signal level...
really strange lab results to ponder when you're tired ;-)

Any ideas how one might *model* the photodiode in Spice?

...Jim Thompson

How can it run out of carriers? Photons make h-e pairs, and more
photons should make more, right? Does high pair density increase
recombination before the charges can be collected? Is it possible that
the photocurrent is limited by ohmic effects? The DC effect does need
pondering.

One of the makers of GaAs photodiodes told us that their devices would
limit above about a milliwatt optical input, so we got access to a
multi-watt pulsed fiber source, and it turns out they were wrong.

John
 
J

Joerg

Jan 1, 1970
0
Jim said:
I just ran onto a situation where a modulated laser impinging upon a
photodiode had such a high light level that the photodiode was
"saturated"... no more available carriers.

Thus "clipping" of the modulation was occurring in the photodiode
itself, creating harmonics.

And lowering the DC bias on the laser raised the AC signal level...
really strange lab results to ponder when you're tired ;-)

Any ideas how one might *model* the photodiode in Spice?

I don't, didn't have to use SPICE on that section in my current design.
But the optical folks have attenuators for that case.

This had me puzzled as well. The common DFB modules these days pump out
5mW-20mW but photodiodes tend to saturate around 1mW-2mW. So we'll
attenuate in order to be able to run the DFB at reasonable current
levels. I guess it's all for the telco market where they assume a mile
or more of fiber.

If you come across a DFB that is really, really low in phase noise let
me know ;-)
 
J

Jim Thompson

Jan 1, 1970
0
How can it run out of carriers? Photons make h-e pairs, and more
photons should make more, right?

Hell! I don't know, I'm a circuit guy, not a physicist ;-)
Does high pair density increase
recombination before the charges can be collected? Is it possible that
the photocurrent is limited by ohmic effects? The DC effect does need
pondering.

10V reverse bias.
One of the makers of GaAs photodiodes told us that their devices would
limit above about a milliwatt optical input, so we got access to a
multi-watt pulsed fiber source, and it turns out they were wrong.

John

Problem cured by going to a larger photodiode.

This data is all remote provided... lab in Atlanta, me in Phoenix.

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
John said:
How can it run out of carriers? Photons make h-e pairs, and more
photons should make more, right? Does high pair density increase
recombination before the charges can be collected? Is it possible that
the photocurrent is limited by ohmic effects? The DC effect does need
pondering.

One of the makers of GaAs photodiodes told us that their devices would
limit above about a milliwatt optical input, so we got access to a
multi-watt pulsed fiber source, and it turns out they were wrong.

I was told that besides non-linearity there comes a point beyond which
there would be irreversable damage. Also, the datasheet for the Sumitomo
diode I am using here states 2mA Ir as the abs max limit.

BTW I found this to be a pretty good paper:
http://sales.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pdf
 
J

Joerg

Jan 1, 1970
0
Jim said:
Hell! I don't know, I'm a circuit guy, not a physicist ;-)




10V reverse bias.

That can reduce your selection. Some are spec'd at 5V max, so I went
from 12V to 5V.
Problem cured by going to a larger photodiode.

This data is all remote provided... lab in Atlanta, me in Phoenix.

Paper said it was 22F at night. Ask them to scrape the ice off the PD's
front and measure again :)))
 
L

LVMarc

Jan 1, 1970
0
Jim said:
I just ran onto a situation where a modulated laser impinging upon a
photodiode had such a high light level that the photodiode was
"saturated"... no more available carriers.

Thus "clipping" of the modulation was occurring in the photodiode
itself, creating harmonics.

And lowering the DC bias on the laser raised the AC signal level...
really strange lab results to ponder when you're tired ;-)

Any ideas how one might *model* the photodiode in Spice?

...Jim Thompson


Jim,

The model would be a light controlled current source. PD's have about
0.8A/W conversion ratio. Double the light POWER and double the current
output. his is a log convertsion ratio. so it is never really "linear"
but always log. This continues till the voltage acrros the diode (the
signal) is about 1/10 the reverse bias and then you get somne cross
prodcut distortion. adign more power and then you saturate the amplifer
front end, add more power and there is a power density/max current which
is athe peak power limit.

If you send the schematic Ill give it a lokk

Marc Popek
 
J

Jim Thompson

Jan 1, 1970
0
Jim,

The model would be a light controlled current source. PD's have about
0.8A/W conversion ratio. Double the light POWER and double the current
output. his is a log convertsion ratio. so it is never really "linear"
but always log. This continues till the voltage acrros the diode (the
signal) is about 1/10 the reverse bias and then you get somne cross
prodcut distortion. adign more power and then you saturate the amplifer
front end, add more power and there is a power density/max current which
is athe peak power limit.

I'm not saturating the amplifier... that was the first thoughts, given
peculiarities in a SwitCap filter output (harmonics) that followed the
TIA.

But something is happening in carrier generation... reducing light, or
going to a different PD (larger) solves the issue.

I'm just looking for some direction on how running out of carriers
might be modeled. It's sort of like channel saturation in a MOSFET or
maybe current-crowding in a bipolar device.
If you send the schematic Ill give it a lokk

Marc Popek

Can't do that... proprietary... start-up with VC money ;-)

...Jim Thompson
 
J

Joel Kolstad

Jan 1, 1970
0
Jim Thompson said:
Can't do that... proprietary... start-up with VC money ;-)

You'll have to give us your take on whether or not the company will make it.
:)

I have a friend who worked for a number of years as a consultant (these days
he's an FAE at a big distributor) who said that of all the consulting jobs he
worked one, although in the end the widget always worked and the schedules
were generally close to being on time, only something like 1 in 10 ever ended
up with a profitable product!
 
J

Jim Thompson

Jan 1, 1970
0
You'll have to give us your take on whether or not the company will make it.
:)

I have a friend who worked for a number of years as a consultant (these days
he's an FAE at a big distributor) who said that of all the consulting jobs he
worked one, although in the end the widget always worked and the schedules
were generally close to being on time, only something like 1 in 10 ever ended
up with a profitable product!

This is high-end medical. I think it has a very good chance of
success. After all, they hired me to convert their concept into
working schematics ;-)

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
Joel said:
You'll have to give us your take on whether or not the company will make it.
:)

I have a friend who worked for a number of years as a consultant (these days
he's an FAE at a big distributor) who said that of all the consulting jobs he
worked one, although in the end the widget always worked and the schedules
were generally close to being on time, only something like 1 in 10 ever ended
up with a profitable product!

Hmm, that's very different in med electronics. It's pretty much 100% to
production there, with many of the designs being produced longer than
many car models are.
 
M

Mike Monett

Jan 1, 1970
0
Joerg said:
Hmm, that's very different in med electronics. It's pretty much 100% to
production there, with many of the designs being produced longer than
many car models are.
Regards, Joerg

I often wonder why you don't start a manufacturing company. With your
skills and background, you should do very well, especially since you
already have the experience running that kind of operation.

You are probably doing very well consulting, but boy, you could make a mint
making and selling your own product line. For you, the entire field is wide
open.

There's always the Chinese to think about. But you could start in niche
areas that require an excellent technical background for support, and where
sales are largely word-of-mouth. By the time they catch on, you could
dominate the market and it would be difficult or impossible for them to
break into it.

It may take a year or so, but I'd bet you would be a great success.

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm
 
J

Joerg

Jan 1, 1970
0
Mike said:
I often wonder why you don't start a manufacturing company. With your
skills and background, you should do very well, especially since you
already have the experience running that kind of operation.

Thanks for the kudos. Well, what can I say, we live in California and
when you do production or sales of OEM'd goods the bureaucrats can get
in the way really fast.

I did run a division of a company for a few years until we were bought.
This included manufacturing and it was great fun. But we had our bouts
with strange rules being imposed (that was before the previous governor
was recalled). Example: The whopper was a new law that required overtime
pay after 8hrs/day instead of after 40hrs/week. Keeping 10hr shifts
suddenly became a real financial problem. But we had people who needed
that so they could take 4day/week turns and care for frail relatives. I
had people burst into tears in my office. That might have all made sense
to some union boss but it sure didn't to the workers.

You are probably doing very well consulting, but boy, you could make a mint
making and selling your own product line. For you, the entire field is wide
open.

There's always the Chinese to think about. But you could start in niche
areas that require an excellent technical background for support, and where
sales are largely word-of-mouth. By the time they catch on, you could
dominate the market and it would be difficult or impossible for them to
break into it.

Much of the stuff I designed is being produced in China. Works great.

It may take a year or so, but I'd bet you would be a great success.

Maybe in another location :)
 
J

Jim Thompson

Jan 1, 1970
0
Thanks for the kudos. Well, what can I say, we live in California and
when you do production or sales of OEM'd goods the bureaucrats can get
in the way really fast.

I did run a division of a company for a few years until we were bought.
This included manufacturing and it was great fun. But we had our bouts
with strange rules being imposed (that was before the previous governor
was recalled). Example: The whopper was a new law that required overtime
pay after 8hrs/day instead of after 40hrs/week. Keeping 10hr shifts
suddenly became a real financial problem. But we had people who needed
that so they could take 4day/week turns and care for frail relatives. I
had people burst into tears in my office. That might have all made sense
to some union boss but it sure didn't to the workers.



Much of the stuff I designed is being produced in China. Works great.



Maybe in another location :)

You'd love Arizona. I can even point you to a location very similar
in weather to where you live now ;-)

...Jim Thompson
 
J

Joerg

Jan 1, 1970
0
Jim said:
You'd love Arizona. I can even point you to a location very similar
in weather to where you live now ;-)

Oh, I do love it there. Spent quite some time in the PHX area and north
of it in the 90's. But when you said something about "being
californicated" a while ago that had we worried.
 
M

Mike Monett

Jan 1, 1970
0
Joerg said:
Much of the stuff I designed is being produced in China. Works great.

There you go. You already have your manufacturing facilities and necessary
contacts. You know who to deal with and who to avoid. That information is
priceless.

Just send them your new designs and get them to put your name on it. It's
handed to you on a platter - you couldn't ask for a better opportunity.

Make sure you tell your current customers you would never go into
competition with them. Then keep that promise until after you make your
first billion:)
Regards, Joerg

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:
http://silversol.freewebpage.org/index.htm
SPICE Analysis of Crystal Oscillators:
http://silversol.freewebpage.org/spice/xtal/clapp.htm
Noise-Rejecting Wideband Sampler:
http://www3.sympatico.ca/add.automation/sampler/intro.htm
 
J

Jim Thompson

Jan 1, 1970
0
Oh, I do love it there. Spent quite some time in the PHX area and north
of it in the 90's. But when you said something about "being
californicated" a while ago that had we worried.

Just the cost of housing in the PHX metro area.

...Jim Thompson
 
P

Phil Hobbs

Jan 1, 1970
0
Jim said:
I just ran onto a situation where a modulated laser impinging upon a
photodiode had such a high light level that the photodiode was
"saturated"... no more available carriers.

Thus "clipping" of the modulation was occurring in the photodiode
itself, creating harmonics.

And lowering the DC bias on the laser raised the AC signal level...
really strange lab results to ponder when you're tired ;-)

Any ideas how one might *model* the photodiode in Spice?

...Jim Thompson

I'm not a big Spice guy--I use it about twice a year--but the physics of
what's going on is probably local forward bias caused by big lateral
voltage drops in the (very thin) epi layer. That's usually the
situation with modulated CW beams. Femtoseond beams can do truly evil
things to photodiodes, messing up the populations etc, but for
reasonable duty cycles the diode generally melts first.

Your first line of defense is a big fat reverse bias.

Cheers,

Phil Hobbs
 
J

Jim Thompson

Jan 1, 1970
0
I'm not a big Spice guy--I use it about twice a year--but the physics of
what's going on is probably local forward bias caused by big lateral
voltage drops in the (very thin) epi layer. That's usually the
situation with modulated CW beams. Femtoseond beams can do truly evil
things to photodiodes, messing up the populations etc, but for
reasonable duty cycles the diode generally melts first.

Your first line of defense is a big fat reverse bias.

Cheers,

Phil Hobbs

Hi Phil,

I have 10V reverse bias, but it's behaving as if the PD is running out
of carriers.

A larger PD behaves itself.

Modulation is ~2KHz

If running out of inducible carriers is a possibility I can probably
figure out how to behaviorally model it. I'm just trying to match
theory to lab.

Pretty durn close except for this anomaly ;-)

...Jim Thompson
 
M

Martine Riddle

Jan 1, 1970
0
What color laser?

I'm sure the photodiode is sensitive to a certain wavelength. This is
probably part of the problem. Increasing the die size is masking it.

I think I may be familiar with a few companines that might be involved in
this. Were they at this years RSNA?

Cheers
 
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