The laser doesn't "start" at a power level, it has what's known as a lasing threshold, at which point it starts producing laser light. This is generally about 30-40mA for PHR diodes, but varies from diode to diode, and with temperature. From here, the light intensity increases about linearly with current (slope efficiency), so it is possible to set a PHR diode at around 5mW - just start with the current down below lasing threshold, then slowly turn it up, until lasing threshold, then up it by just a few mA beyond, and you will be at around 5mW. Remember that this takes relatively high precision, and turning the driver too far risks killing the diode. The other option is to kill your PHR with high current, thus, a high current will produce a lower amount of output power, and more wasted heat, but also make the power slightly more easy to control. If my aim was to produce under 10mW of 405nm light, I would do as I described above.
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