high dynamic optical power meter

[bene-tam]

Dear all

I am currently using photodiode + TI log102 amplifier + ADC0804 +
8bitMCU to measure optical power, sampling rate is < 100Hz, range is
40dB only

is there any good suggestion so that the application can be improved
to measure a larger range and a faster speed?? any reference to go
from??

thx for advice

 

[Rene Tschaggelar]

Dear all

I am currently using photodiode + TI log102 amplifier + ADC0804 +
8bitMCU to measure optical power, sampling rate is < 100Hz, range is
40dB only

is there any good suggestion so that the application can be improved
to measure a larger range and a faster speed?? any reference to go
from??

Without more information I have to stay generic.
The dynamic range is limited at the upper end by how much
you want do shine on the diode before is evaporates or otherwise
becomes damaged. The limit on the lower end is the noise level
of the circuit. And yes, you cannot go lower than one bit.
Can you give us some figures, on power and voltages ?

Rene

 

[Rick]

What power level? I just measured a 650mJ pulse from a q-switched ND:YAG
laser with a Molectron J25LP-0696 sensor head I scavaged out of a medical
laser device. It is rated for 5Volts/Joule at 1064nm, and the pulse I hit
it with probably lasted about 40ns (at many kilowatts). I am pretty sure I
was at least getting several hundred millijoules since the air sparked with
a snap and flash of light at the focal point of the lens I was using to
spread the laser beam out so I didn't damage the sensor surface...again,
that is :-( I am not sure what the sensor is made of or if you can find
something to homebrew your own. The surface is some black carbon type
material in appearance.

If you are doing low power stuff, then you can get a Hamamatsu photodiode
with response time so fast you can measure events in the gigahertz range. I
was thinking of trying to score one of these as a sample to do some tests
for single longitudinal mode operation of a green laser I built.


Rick

 

[bene-tam]

Currently the power level is between -10dBm to -50dBm, @1550nm

what i need is a O/E converter that has fast response, and a wider
range, say 0dBm to -60dBm(extends on both ends)

seems the current design using R and C doesnt not give fast enough
response.
should you would like to know more, i may send a schematic

 

[Product developer]

Try http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&group=sci.optics.

 

[Rene Tschaggelar]

The optical level is -10dBm to -50dBm ?
What is the corresponding electrical power level ?
I'm sure you can shine with 10mW on a standard photo diode.
Then the lower end of the range is limited by
- the dark current
- the noise
- one bit

There are log amps doing 160dB, see the AD8304 of AnalogDevices.

Rene