# Laser pulse width measurement

## Main Question or Discussion Point

For a pin photodiode, risetime=bandwidth/0.35
i get ~3ns for s5973
will it be affecting the measurement of 10ns laser pulse?
How can i reduce the rise time?

Related Electrical Engineering News on Phys.org
Bobbywhy
Gold Member
apuz, Welcome to Physics Forums!

The response speed of a photodiode is a measure of the time required for the accumulated charge to become an external current and is generally expressed as the rise time or cut-off frequency. The rise time is the time required for the output signal to change from 10 % to 90 % of the peak output value and is determined by the following factors:

1) Terminal capacitance Ct and time constant t1 of load resistance RL
2) Diffusion time t2 of carriers generated outside the depletion layer
3) Carrier transit time t3 in the depletion layer

For exact details see section 3-2 of this application note:

http://sales.hamamatsu.com/assets/html/ssd/si-photodiode/index.htm

Hope this helps.

hi Bobby, thank you 4 your interest.

the link gives me a rise time of 0.35ns.Which seems not to create problems of broadening.But when we try to measure a 10ns pulse using this s5973, it is giving a broader pulse of 20ns. We did ensure the following:
1)1 Ghz oscilloscope
2)50 ohms coaxial cable
3)50 ohms dc coupling on oscilloscope
4)3.3 supply@ which terminal impedance is min

any help?

Last edited by a moderator:
Bobbywhy
Gold Member
apuz, the first question I have is: How do you know your laser pulse input is 10nS? It may actually be 20 nS!

If you can be sure the input is 10 nS, you might try adjusting the reverse bias voltage on the APD. You might try to raise it up a bit while watching that pulse on the oscilloscope for any changes. Stray capacitances (unplanned) will spread out high risetime signals...check your circuit layout/groundplane. One more thing...you may get a new (different) s5973 and see what happens.

Sorry not much help. Last thought: contact Hammatsu applications engineers. They will be interested in solving this, I think.

the pulse width was measured by the people who installed it. They used a black box kind of thing and didn't reveal what it is.

sophiecentaur