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Do you know any simple explanation on the reason why the turn-on delay time on a laser diode is reducing while we increase the bias current?

Turn on delay,is the time that the laser needs from the time that one applies the current until the time that the light goes out of the laser.This time is strongly depended to the input current density,the higher the bias current it is the less the turn on delay it is. That I don’t understand is the physics behind it,how that interaction occurs.

Is it something obvious, because I am trying to find a simple explanation and I can not. Best Regards, George

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The turn on delay or soft-start is by design and depends on the laser current controller. As far as I know, it has nothing to do with the diode laser itself. It is not so much as a delay as it is a slow ramping up of the current. This is done because transient currents can easily destroy an LD. –  Antillar Maximus Feb 26 '12 at 15:14
    
The power up delay is a safety feature, and it is not a current ramp. The current is ramped, but over milliseconds, while the power up delay is usually a few seconds. –  Colin K Feb 26 '12 at 15:55
    
Neither of those is what the OP is asking about though. He is talking about the latency in photon emission versus current input to the LD. –  Colin K Feb 26 '12 at 15:57
    
Turn on delay,is the time that the laser needs from the time that one applies the current until the time that the light goes out of the laser.This time is strongly depended to the input current density,the higher the bias current it is the less the turn on delay it is. That I don’t understand is the physics behind it,how that interaction occurs. –  Giwrgos Feb 26 '12 at 17:48
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There are a few factors that govern laser diode turn-on time.

The first is the junction capacitance, which is the same thing that causes turn-on delay in an ordinary junction diode. Under forward bias, the capacitance is proportional to the current and the diode transit time, just as with any pn-junction diode.

The second is unique to laser diodes.

Below a certain threshold at which lasing begins, the device acts like an ordinary LED. When the current is increased to the point that the gain of the laser is equal to the loss of the cavity and mirrors, lasing starts and the light output increases suddenly. It's at this point that the laser diode is considered to be turned on.

What causes the delay is simply the time it takes to get to the threshold current $I_{th}$ For an ideal device with no capacitance, the turn on time is $$t_d = \tau_s \ln\frac{I}{I-I_{th}}$$ where $\tau_s$ is the recombination time. (Remember that it's the recombination of electrons and holes that causes the emission of light in an LED or laser diode.)

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