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| Can you excite a laser with a waveform different than square or DC? |
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| Benta:
--- Quote from: ejeffrey on December 21, 2018, 06:43:42 pm ---Most small laser diodes are roughly linear above threshold. So a 5 mA laser diode might start lasing on at 1-2 mA and then linearly increase up to 5 mA. The ranges are strongly diode dependent, so some will have a wider range of safe operation than others. Coherence and stability will be pretty poor around threshold -- that is basically making the transition from an LED to a laser. So you can do OK amplitude modulation via current drive over a decent range (maybe a modulation index of 50%), but you can't get nicely shaped pulses that go to zero. Typically you would either do on-off modulation with a square wave or AM with a modulation index less than 1. A big problem is that the threshold and linear slope depend on temperature and unit to unit variation. This is the reason for the photodiode: if you want to get reasonable power out of a cheap laser diode without damaging it, you need to servo the gain to keep the power constant. Otherwise it either won't perform well at high temperature or will destroy itself at low temperature -- especially if the device isn't temperature stabilized. Since the die temperature depends on the drive current, that adds an additional wrinkle to modulation. So a common way to do simple AM modulation is to drive the laser with a bias tee. The DC port is driven by a gain servo from the photodiode to keep the average power correct and a small AC modulation is added through a capacitor. The wavelength also depends on temperature and drive current. There are two effects here: the cavity line shifts with temperature due to index of refraction and cavity length changes. Second, the gain profile changes, causing mode hopping. So if you want single mode operation, you really don't want to be modulating at all -- you want to find a stable operating point and do external modulation. DPSS and especially FD-DPSS solid state lasers are much harder to modulate via pumping due to the extra non-linearity of the solid state gain medium and the second harmonic generation if applicable. --- End quote --- ejeffrey, very informative and thorough post. Thank You. |
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