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Laser Diode Drive Requirements
The following must be achieved to properly drive a laser diode:
- Absolute current limiting. This includes immunity to power line transients as well as those that may occur during power-on and power-off cycling. The parameters of many electronic components like ICs are rarely specified during periods of changing input power. Special laser diode drive chips are available which meet these requirements but a common op-amp may not be suitable without extreme care in circuit design - if at all.
- Current regulation. Efficiency and optical power output of a laser diode goes up with decreasing temperature. This means that without optical feedback, a laser diode switched on and adjusted at room temperature will have reduced output once it warms up. Conversely, if the current is set up after the laser diode has warmed up, it will likely blow out the next time it is switched on at room temperature if there is no optical feedback based regulation.
Note that the damage from improper drive is not only due to thermal effects (though overheating is also possible) but due to exceeding the maximum optical power density (E/M field gradients?) at one of the end facets (mirrors) - and thus the nearly instantaneous nature of the risk.
The optical output of a laser diode also declines as it heats up. This is reversible as long as no actual thermal damage has taken place. However, facet damage due to exceeding the optical output specifications is permanent. The result may be an expensive LED or (possibly greatly) reduced laser emission.
One kind of visible laser diode by neglecting to monitor the current really had to be cranked up well beyond the point where the brightness of the laser beam stopped increasing. It did indeed turn into a poor excuse for an LED.
Another one was blown by assuming that a particular driver circuit would work over a range of input voltages when in fact it was supposed to be powered from a regulated source. At first the degradation in brightness appeared to be reversible. However, what was probably happening was that damage to the laser diode was occurring as soon as the brightness appeared to level off. The natural tendency was then to back off and approach this same point again. Not quite as bright? Crank up the current. Finally, once it is much too late, the realization sets in that it will never be quite as bright as it was originally - ever again. This one still lases but at about 1/10th of its former brightness.
If you then try to power this damaged laser diode with a driver circuit using optical feedback, further instantaneous damage will occur as the laser diode driver attempts to maintain the normal optical output - which is now impossible to achieve and only succeeds in totally frying the device as it increases the current in a futile attempt to compensate.
Using properly laser diode drive to require something.
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Laser Diode Drive
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