THORLABS PICO D, User Manual

Page: 33 / 48

33
terminals. Furthermore, the optical output power can be set by applying av
voltage between 0 V to +10 V to the +OPTICAL_POWER and -OPTI-
CAL_POWER (-1 V switches off the laser).
3.2 Switching OFF the PICO D
PICO D is switched OFF in reverse start-up order, see 3.1
3.3 Calibration of PICO D
PICO D does neither have internal wavelength references nor optical power refer-
ence. Hence, the PICO D must be calibrated using external instrumentation such as
wavemeter and optical power meter.
The optical length of the cavity depends on the refractive index of the laser diode,
which in turn depends on the current injected into the laser chip. Consequently,
wavelength and optical power must be calibrated simultaneously. The wavelength
shift is less than 40 pm between minimum and maximum power for a constant wave-
length control voltage, see Figure 2–1. Calibrating both power and wavelength
simultaneously will compensate the change of wavelength for different power values.
The optical output power level is controlled through measurement of light split off
from the optical fiber. Due to the non-linearity at the onset of lasing (or equivalently
at the threshold current), the output power can only be controlled above this mini-
mum level up to the maximum output power above which saturation occurs.
Using linear interpolation and a sufficiently dense grid, the unit can be calibrated to
at least a few pm absolute accuracy. Theoretically, the unit can be calibrated down
to 1 pm using 40 calibration points. Realistic calibration will require a denser mesh of
calibration points to account for irregularities. Assuming a 50% increase for a realis-
tic calibration, a 100 nm/s scan with a maximal wavelength error of 1 pm would then
require that 1.5
⋅
20/0.5/(110/100) = 66 lookups / second.
IMPORTANT
The optical power value scales linearly and is factory adjusted so that
1 mW output power at 1 V input voltage applied to the OPTICAL_POWER sig-
nal input
Finally, the sensitivity of the cavity to internal heat sources (as the laser it self)
means that the calibration data is only valid for equilibrium conditions.