Question
With the WLD, I discovered a normal delay between the SHD signal and power to the Laser Diode (slow start). I need to produce pulses of zero to set power (which is set in VSET), and 100 to 1000 ms width. I thought to use SHD control, but the delay is a problem. Which is the best way to put the LD on and totally off with no (or little) delay? Or, how much is this delay, in msec, in order to modify the control software? Eventually, I can use constant power or constant current mode.
Answer
The WLD is very well suited to an application where the laser needs to be square-wave modulated.
The ¼ second delay is a CDRH safety requirement, and it allows the WLD time to stabilize before current is driven to the laser. We recommend against modulating the laser using the SHD input (pin 1) because better results are obtained by modulating a square wave signal on the VSET input (pin 2). There are two different ways to configure the modulation function.
Modulation Method A: The input voltage setpoint is adjusted to set the desired laser drive current (or photodiode current) and the drive current limit is set 5% to 10% above the output current level. This method takes advantage of the fast 460 nsec rise-time and 320 nsec fall-time of the WLD3343.
Modulation Method B: Set the output current limit to the desired laser drive current, and then modulate with a signal that drives the WLD3343 into current limit, and then off. This method may result in longer rise- and fall times.
Before proceeding we strongly recommend you use a test load while configuring the WLD for external modulation.
Modulation Method A – Constant Current or Constant Power mode
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Follow the directions in the datasheet to configure the sense and limit resistors for a drive current that is 5% – 10% above the desired drive current level.
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Connect a modulation source (eg. function generator) to VSET, and set the output to 0 V.
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Enable the laser output, and wait at least ¼ second for the WLD3343 output circuit to come on.
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Set the frequency and duty cycle of of the square wave.
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Set the amplitude of the square wave signal to control the WLD3343 output current or photodiode current.
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Modulation Method B – Constant Current mode
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Follow the directions in the datasheet to configure the sense and limit resistors so the output will be limited at the current level you wish to drive your laser.
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Connect a modulation source (eg. function generator) to VSET, and set the output to 0 V.
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Enable the laser output, and wait at least ¼ second for the WLD3343 output circuit to come on.
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Configure the modulation source to provide a square wave signal at the desired frequency and duty cycle, and with an amplitude that modulates from 0 to 2 volts.
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Keep in mind that the voltage noise characteristics of the modulation source will affect the shape of the modulating signal. For the best results select a low-noise modulation source.