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Blocking Capacitor

Hi all,

I am running a laser diode off a varialbe pulsed current source with a variable current output and pulse width. I want to run my laser at 1mA or less, however the current driver is designed for high power current applications and on switch on with no selected current output it has a 6mA d.c. offset. In other words there is a 6mA floor to the lowest current I can drive the laser. Is it possible to put a blocking capacitor in series with the +'ve output lead inorder to only have the a.c. component of the current? Will this likely damage the current driver or cause it any loading issues if the dc offset blocked?

Pulse frequencies necessary for me are ~1MHz down to 500KHz. Am I right in thinking that using the formula:

C=1/(2*PI*Res*Freq)

with system impedence of 50ohms and using 500KHz as lowest frequency, the capacitor I would need is 30pF, providing its power rating is high enough?

Thank you in advance,

Ryan

Reply to
Ryan
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You need to select a capacitor value with consideration to the load into which it will work. Just off the top, 30 pF is way too small. Remember, it will be part of a voltage divider along with the load R or Z, so it can seriously impact the drive level. For a start try making it's Xc at 500 kHz about 1/4 or less, of the load R or Z.

You will probably receive more definitive answers, but this can get you started if they are slow coming.

Don

Reply to
Don Bowey

Laser diodes are typically run with pulsed DC sources. Putting a blocking cap on such a source to limit current won't work. The cap will pass the first pulse, or several pulses then it will be charged to the supply voltage and sit there. Think in terms of battery, switch and load resistor.

Your choices are to modify the current set point (assuming the diode will still lase at the current you want) with the circuitry you already have, use a cap and provide a way to discharge the cap between pulses (similar to a single pole, double throw switch - and diode around the laser to protect the laser diode from reverse polarity), or add a current limiting resistance..

Another possibility is to use a current limiting resistor with a cap paralleling it - initial current pulse goes through the cap and some through the resistance - the resistor discharges the cap between pulses. Assuming the discharge time frame is sufficient to discharge the cap - short "on" duty cycle, long "off" time.

No mention of average power/current in your post - usually that's what one really cares about in duty cycle applications..

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Thank you for that. You are right it is run off a pulsed dc source and not a sin wave source so the cap will charge and not discharge on each pulse. For this experiment I need a very small pulsed current into the laser so the injected carrier density is very low. However the 6mA dc bias floor provides too high an initial carrier density and swamps the effect I am looking for. I will try the cap and resistor parallel combination to see if it removes the 6mA floor.

Ryan

Reply to
Ryan

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