Mike Noone wrote...
Hi - can anybody lend me any guidance as to how to build a laser
diode driver that can be modulated? I would like to be able to
modulate it at 1-100Mhz or so. Most importantly, I need it's
rising edge to be as sharp as possible. Are there any
inexpensive ICs that can do this?
You are talking about modulation, and not 100% on/off driving,
right? The way I do that is quite simple. I use a standard
moderate-speed light-sensor feedback laser-diode current-source
controller, and add a modulation network at the laser diode.
I operate the laser at about half current, as appropriate.
The laser impedance is very low at these currents, a few ohms.
The low impedance allows me to treat this node like a summing
junction, so that adding a 50-ohm resistor in series with an
ac-coupling cap from the coax cable carrying the RF modulation
provides both the RF signal to the diode, and well terminates
the 50-ohm coax. Using small dimensions, I have been able to
modulate low-cost CD-ROM lasers up to 200MHz in this fashion,
and done a bit differently in a quick lash up, up to 1.5GHz
(I may have modulated the laser successfully well above that,
but the photodiode I was using wimped out).
From my Jan 10 2005 s.e.d. post,
"The 664nm red DVD laser was an Hitachi HL6504FM and my detector
was an Optek OPF480 PIN diode, biased at -100V, measured into a
25-ohm load (double-end termination) with an HP network analyzer.
"Components were 1206 SMT hand-soldered with zero-distance
spacing. Bias-tees were 12GHz-bandwidth Picosecond Pulse Labs.
A first attempt at a PCB stripline replacement only goes to
600MHz so far."
.. bias-tee laser
.. ________| |______,---------,
.. ________|-||-+--|______(-50R-|>|-'
.. 50-ohm |____X__| \\ mirror PD bias
.. coax | \\| optics 100V
.. 50R //| & etc |
.. | // __|____
.. laser // ________| X |___50-ohm
.. current ,-|>|----+-)_______|--+-||-|____ coax
.. supply +-||-50R-' | coax |_______| term.
.. '----------' bias-tee
Jamie Morken answered, "Cool! That looks like a neat circuit, I
am buying that same photodiode from Digikey (part#: 365-1029-ND)"
Comment: In the latter approach, the bias-T isolates the 50-ohm
coax RF signal from the low-frequency current source, all on the
50-ohm line. But, making a good bias-T can be tricky, and using
the low-Z at the laser diode instead should be an easier approach,
perhaps not good to 1.5GHz, but certainly to 200-400MHz.