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1 ns red laser diode pulses by 6 ns electrical pulses

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Physikfan

Sun Dec 04 2016, 10:23AM

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

Hi

A relatively broad electrical pulse (about 6 ns) generates a relatively small laser pulse (about 1 ns).
In the following picture, the broad pulse with approximately 6ns width is the electric pulse of a pulse generator feeding a red laser diode, the slender large needle is the optical signal of the laser pulse of about 1ns width, detected with a Si-photodiode with a rise ans a fall time of 150 ps.

Lichtgeschwindigkeit SHPX001

Does anyone have an explanation for this phenomenon (my working hypothesis for the moment is a time delay caused by a slow start of the laser oscillation of the laser diode)?

Regards

Physikfan

Sulaiman

Sun Dec 04 2016, 11:59AM

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

the first half of the 6ns pulse is a classic (1-1/t) shape so
. something needs 'charging up' before it operates
. the source (sig-gen) and load (diode) impedances are not matched

e.g. 50 Ohm system, t=1ns for C = 20 pF etc.

klugesmith

Sun Dec 04 2016, 03:07PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Is this a puzzle question, where you already know the answer?
Let's assume not, and that you are looking for help to understand the behavior.

What happens to optical pulse W and H when you change the electrical PW?

As Sulaiman said, there's an impedance mismatch. 75 ohm cable with 50 ohm scope input and/or pulse generator output? We need more detail about the electrical setup, including cable lengths and other things you might think don't matter. What's the electrical pulse amplitude, in volts or amperes, taking oscilloscope and probe (?) scale factors into account?

What kind of LD do you have, and what wavelength? The ones used for optical fiber communication, and in my experience optical disk reading, are easily switched on and off (at least 20 dB power ratio) at GHz rates.

Physikfan

Sun Dec 04 2016, 05:54PM

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

The pulse generator is a HP8082A with 50 Ohm output impedance, maximum amplitude is 5V from 50Î© into 50Î©.
The voltage amplitude was about 5V plus the full postive bias from the HP8082A.
The oscilloscope was a Tektronix DSA 602 Digitizing Signal Analyzer (1 GHz) with 50 Ohm input impedance.
I was using a 50 Ohm coaxial cable but I agree, you can see a small impedance mismatch.
I will improve the impedance matching by replacing the resistor of about 38 Ohm used now by a better value.

But the interesting fact is, that a small electrical pulse width (lower than 3 ns but the same amplitude) generates almost no laser diode (visible red) intensity.
Further information (laser diode specs, cable length, etc) will follow.

In the near future I will show the laser pulse width as well as the intensity as a function of the electrical pulse width.

johnf

Sun Dec 04 2016, 06:24PM

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

That is because you are dividing the pulse energy into the junction capacitance of the laser diode and not getting above the lasing threshold

Physikfan

Sun Dec 04 2016, 06:49PM

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

Hi

What are your opinions about my working hypothesis:
time delay caused by a slow increase of the light amplitude (laser oscillation of the laser diode)?
This time delay should depend on the Q of the diode laser cavity.

Ad Klugesmith:

Please could you explain:
"What happens to optical pulse W and H when you change the electrical PW?"

Ad johnf:

Please, could you explain more details of:

"That is because you are dividing the pulse energy into the junction capacitance of the laser diode"

"not getting above the lasing threshold" below 6 ns electrical pulse width?

klugesmith

Sun Dec 04 2016, 07:38PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Physikfan wrote ...
...time delay caused by a slow increase of the light amplitude (laser oscillation of the laser diode)? This time delay should depend on the Q of the diode laser cavity.
and ...Please could you explain: "What happens to optical pulse W and H when you change the electrical PW?"

Regarding the first question, the delay you report is millions of optical cycles. Hard to imagine any real physical oscillator being that "slow" to ring up.
Regarding #2, sorry about that. I meant to abbreviate optical pulse Width and Height, and electrical Pulse Width. For the first two I picked letters from FWHM but got that wrong.

johnf

Mon Dec 05 2016, 06:34AM

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

Alright a quick reply
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
QED

Physikfan

Mon Dec 05 2016, 06:20PM

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

Hi Sulaiman

Please, what do you mean with:
"a classic (1-1/t) shape"?

the shape of the voltage of a capacitor C as a function of the time charged via a resistor R:

Uc(t) = Uo (1 - exp(-t/RC)) ?

Sulaiman

Mon Dec 05 2016, 09:17PM

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

yes, that is it, I was sloppy :(

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