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Pulse width of a red laser diode as a function of the width of the corresponding electrical pulse

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Physikfan

Sun Dec 18 2016, 05:20PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi

In the experiment for determining the speed of light with the aid of a pulsed red diode laser,
I found out that a relatively broad electrical pulse (about 6 ns) could generate a relatively small laser pulse (about 1 ns).
In order to study this phenomenon, I have made further experiments with differently wide electrical pulses, the amplitude being 5.5 V plus 2 V bias DC at 50 Ohm.
For each electrical pulse width, the laser pulse width as well as the amplitude were determined with a Si photodiode with a rise or fall time of 150 ps.
The oscilloscope was an analog Tektronix with a 7904 frame and twice 7A19 and a 7B10, (500 MHz bandwidth).

P1050023400x300

At approximately two nanoseconds of electrical pulse width, a very small and narrow laser pulse with about 2 mV amplitude is obtained at the detector.
P1050024400x323

At approximately three nanoseconds of the electrical pulse width, a laser pulse of about 6 mV amplitude and 1 ns pulse width is obtained at the detector.
P1050025400x325

At approximately six nanoseconds of the electrical pulse width, a laser pulse with an amplitude of about 8 mV and a pulse width of 4 ns is obtained at the detector.
P1050035400x300

At approximately 15 nanoseconds of the electrical pulse width, a laser pulse of about 10 mV amplitude and 13 ns pulse width is obtained at the detector.

Please, who has ideas how to explain this phenomenon?

Sulaiman

Mon Dec 19 2016, 08:33AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

can you give a link to laser-diode and photo-diode datasheets
and an idea of the physical setup

johnf

Mon Dec 19 2016, 06:22PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

I gave you the explanation in your other post about this setup.
capacitance of the setup, source impedance of your driver. see my previous post

Physikfan

Mon Dec 19 2016, 09:49PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Sulaiman

The interesting facts are that
1. the difference between the electrical pulse width and the optical pulse width is almost constant (2ns) and
2. almost independent of the electrical pulse width.

The specs of the laser diode are:

The laser diode is a Mitsubishi ML101U29,
maximum CW power 150 mW,
maximum pulse power 400mW,
wave length 660 nm,
threshhold current 85mA,
At 120 mW CW power the operating current is 205 mA, the operating voltage is 2.35V

The Si-photodiode is a Thorlabs DET025AL/M:

Detector Si
Active Area Diameter Ã˜ 250 Î¼m
Wavelength Range 400 to 1100 nm
Peak Wavelength 730 nm (Typ.)
Diode Capacitance 1.73 pF (Max)
Cutoff Frequency 2 GHz (Max)
Rise Time 150 ps (Typ.)
Fall Time 150 ps (Typ.)
Damage Threshold 18 mW
Dark Current 35 pA
Output Voltage 0 to 2 V (50â„¦)

The experimental setup will follow.

Physikfan

Mon Dec 19 2016, 10:19PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Johnf

You wrote:

"your pulse if the area under it is intergrated that is the loule amount
now put that amount into an unknown capacitor q=CV so now you can estimate the capacitance by reducing the area under the pulse so that V or voltage does not get above the lasing threshold of your device.
dont forget the capacitance includes the cables pcb and the laser diode itself.
work backwards you do not need too much capacitance upset the diode output"

Unfortunately there is no value of the laser diode capacitance in the data sheet.

Does your explanation allow also this asymmetrical behavior between the electrical and the laser pulse, this difference of almost constant 2 ns?

johnf

Tue Dec 20 2016, 06:35PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

you can get a feel for how much the capacitance is by adding some more. ie put a 47pf cap across your diode and see what change the brings.
No change go for 470pf --if still no change 4700pf I doubt you will need to get to 4700pf. As for the assymetry does the driver source and sink current or does it only source, if it only sources then the assymetry is probably due to the stray capacitance.

Physikfan

Wed Dec 21 2016, 04:46PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Johnf

I will do these experiments.

This is my experimental setup:
Laserdiodepulse6

In the paper "Pulsing a Laser Diode" by Doug Hodgson, Kent Noonan, Bill Olsen, and Thad Orosz
they published a figure with an electrical pulse, 1V/cm, feeding the laser diode and also the laser pulse, 10mV/cm, x-axes is 50 ns/cm.
As you can see the electrical pulse width is 200 ns, whereas the laser pulse with is only 180 ns, difference is 20 ns, similar to my experiment, difference is 2 ns:

Laserdiodepulse0

There is also a new question:
Why is the difference 2 ns in my case?
Even the authors of the Newport paper did not
mention their 20 ns!

Both figures are taken from:

Please, what is your explanation?

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