[SOLVED] 1GHz LED laser diode modulation

[Nanoprotect]

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.

 

[BigBoss]

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

 

[Nanoprotect]

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

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?

 

[FvM]

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.

 

[Nanoprotect]

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.

 

[Gigan]

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.

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