What are different ways to 'elongate' an electrical pulse?

Question

I'm designing a low-cost system to record femtosecond laser pulses and characterize the average power and shot-to-shot deviations (of the laser pulses). A photodiode seems to be the economic solution, but I'm running into issues on the ADC side: the laser-induced pulse coming out of the photodiode is on the order of 10's of nanoseconds, requiring expensive ADC solutions to properly record. So I'm searching for analog circuits that might somehow temporally elongate these electrical pulses, allowing me to sample more points across the signal and accurately measure it. Simply adding some capacitance might work, but I'm looking into other solutions (peak/envelope detectors, etc.)

Currently, the pulses I get from my photodetector are about 30-40 ns wide and a few volts (up to 4V) in amplitude.

I'd like to be able to measure RMS/Average power of the laser (which is essentially measuring RMS/Average of the photodiode output) as well as shot-to-shot deviations (this is the real tough one, not sure its possible with my requirements). I want the lowest possible sampling rate (slowest ADC) to save on money. It will probably be in the 10-100 MS/s range

Thanks for your thoughts.

Answer 1

I'm sceptical that you'll be able to retain the pulse energy information through excessive amounts of low pass filtering. The more you slow down the pulse, the smaller the amplitude will get, and the lower the SNR of your sampling process will be.

As for how to measure this signal remember that the fastest scopes available actually have very low sample rates (on the order of 40 kHz). The trick is to use a fast sample-and-hold or track-and-hold circuit.

For a 1 ns aperture, you should be able to make a T/H circuit with just a few dollars worth of parts for a reasonable price. The challenge will be to syncronize the T/H circuit with your laser pulse. Pretty much any ECL logic family still available will have the timing performance needed for this, but the details of how to do it depend on what signals your laser produces to syncronize with.

Answer 2

For a crazy idea on catching super-fast pulses look up the (very old) Tektronix 545 analogue sampling scope that fires a pulse "backwards" up a delay line against the input signal coming the other way, it's mad genius. It's a 30MHz scope that can display GHz signals.

You could use a similar idea to fire a chain of cheap ADC's from a delay line or somesuch.

Edit: Can't find detail on the 545 but here's a link to Jim Williams explaining why "old [Tek] scopes are better" and a few specs: Reading Jim Williams - 3.9GHz bandwidth & 10uV per division sounds pretty groovy to me.

Answer 3

Phosphorescence is your friend (maybe). Wiki says: -

Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours after the original excitation.

The important thing here is that the energy taken in from the laser becomes translated into an "output" that lasts a lot longer - this should give you bags of time to sample it with a really slow photodiode and ADC. Think about old CRT screens that had long-persistance phosphur coatings.

Answer 4

If your pulses are 30/40ns wide, then what about using quick ADCs, use this type of detect-peak-and-hold solution -- and resetting this one after you made your measurement?

Sure, circuit would still need to be adapted, but with quick enough ADCs, you may have an original (and flexible) answer.