Laser driver optical feedback

check the RF bypassing from -5V to ground...

my ___guess___ is RF is blasting into your circuit via the supply lines...

Mark

the tradional way of doing this is to run a injected current and use a DC block in the microwave feed. Thorlabs sells a medium cost module for this. You can still do a light loop around it with care. The module is commanly called a "Bias-T" or "Bias Tee" googling this should find you circuit examples. It should let the RF see 50 ohm impedance and keep it out of the DC loop.

good luck.

Steve roberts

Steve Roberts

this

has 3v

is

Thanks for the replies, I re aranged it and it seems to be less sensitive, also I had used a p channel mosfet for the first rransistor, (fdv302) the trouble was it has a parasitic transistor, as the gate went positive it became the emiter of a pnp transistor ! this cuased the latch up problem, and took a while to work out why.

The emiter resistor of the driver transistor is now in the collector instead, unfortunatly the laser diode itself is hardly a 50R impedance, more like 1R, the ac drive is from a phemt right close to it so there is no transmision line to match or cuase reflections.

The thing now is that as the modulation is turned up the laser apears brighter, yet the circuit reduces the laser current, does the eye react to 1ghz modulation ?

Colin =^.^=

search for the term "Bias T" or "BIAS TEE"

Hi Steve,

I did a bit of googling and found some interesting links. However, all of them seem to be designed for low power laser diodes and all seem to have a 50 Ohm input and output.

I've been thinking about the following problem for quite a while now: I want to provide fast modulation for a high power laser diode module, something like a Jenoptik 75 W fibre-coupled package (

). We want to have a modulation of a few A on top of the DC current of up to

60 A. Modulation speed would be up to a few MHz. I thought already about something like the bias-t without knowing its name. However, it seems to be difficult to implement at this type of power and current levels. I was thinking about a matching transformer for the modulation path, but don't know enough about transformers to judge whether that makes sense.

Thanks a lot for any advice,

Christoph

------------------------------------------------------------------------ CSIR - National Laser Centre Pretoria, South Africa Phone and email can be looked up on the staff list under "contact us" on the CSIR web pages:

I think that would work quite nicely but you might have to do it as a closed loop control scheme. 1 or 2 turn secondary, multiturn primary.

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...

Hi Sam,

Thanks for the advice. What do you mean by "closed loop control scheme"? Sense the current at the diode to control the voltage into the transformer? I was hoping this is not necessary.

Christoph

------------------------------------------------------------------------ CSIR - National Laser Centre Pretoria, South Africa Phone and email can be looked up on the staff list under "contact us" on the CSIR web pages:

That's sort of what I had in mind. It may not be necessary. Some careful experiments with a dying diode you don't care about may be in order. :)

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now:

module,

to

it

modulation

Ive got a few of those, only cheap pointer types though, they work worse than an led, whats wrong with putting a transistor, bjt or mosfet, accros the laser diode ? you might need to parallel a few high frequency types to get 2amps. it would have more bandwidth.

Colin =^.^=