[SOLVED] 1GHz LED laser diode modulation

Dear All

Not being an analogue design engineer, can anybody tell me why the attached circuit could not be used to modulate a 20mA LED laser diode at 1GHz?
I do not need 100% modulation. The spice simulation shows the modulated current as 13 - 20mA with +/- 3mA p-p supplied by the oscillator.
Spice simulation attached.

View attachment Laser Driver.pdf

Thanks in advance for any support.

Because 0.1nH inductor value is too low for 1GHz, that's why the signal is shunted..It should be at least 22nH-100nH..

BigBoss said:

Because 0.1nH inductor value is too low for 1GHz, that's why the signal is shunted..It should be at least 22nH-100nH..

Click to expand...

Hi BigBoss

Thank you for your reply.
I have changed the inductor value as you suggested but the simulation results remain the same?
13mA - 20mA p-p sw through the LED. Maybe just a limitation of the simulator? I am using LTSpice IV.

Would it be better to use a high-speed op-amp with a DC offset to bias the laser LED at around 10mA?
The problem I see is that most high-speed op-amps use differential signaling. I am using a Colpitts oscillator to generate the 1GHz with a single-sided output.
Would you be able to recommend a suitable solution?

What do you want to achieve? About linear operation above the laser threshold current or on/off-modulation?

The achieved modulation index is set by your input AC voltage and resistors, so if you want a stronger modulation, you'll change the parameters. Most simple RF modulation circuits are using 50 ohm generator impedance directly AC coupled to the laser diode, you normally won't intentionally attenuate the a input signal with 330 ohm resistor.

You didn't tell laser diode parameters (threshold, maximum current), so we can hardly decide whats healthy for the diode.

Hi FvM

I would like to operate the diode in the linear region, so sinewave modulation.
I have not yet decided on the laser diode as I wanted to see if the idea was plausible before going further.
It seems from the above answers that the basis it correct so I will contruct the circuit and see what the result is.

Many thanks for the support.

Nanoprotect said:

Hi FvM

I would like to operate the diode in the linear region, so sinewave modulation.
I have not yet decided on the laser diode as I wanted to see if the idea was plausible before going further.
It seems from the above answers that the basis it correct so I will contruct the circuit and see what the result is.

Many thanks for the support.

Click to expand...

Hi,
what kind of laser diode model you used in the spice simulation?
thanks.