THORLABS PICO D, User Manual

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27
Figure 2–14 Schematic figure illustrating the fact that the higher the optical
output power, P (P
3
> P
2
> P
1
) the narroweer the wavelength re-
gion.
λ
1
and
λ
2
corresponds to the shortest and the longest
wavelengths, respectively (see the technical specifications for
numerical values).
Note that the this is a differential input stage and that both + OPTICAL_POWER and
- OPTICAL_POWER must be connected.
At maximum optical power the laser will be in current limit and actual power will vary
considerably with wavelength.
The PICO D unit is set, at factory, to delivery 1 mW optical power when the analog
control OPTICAL POWER is set to 1V. Typical operating would be at 1V giv-
ing a guaranteed output of 1 mW over the whole wavelength range.
If the +OPTICAL_POWER is increased beyond the voltage corresponding to a
maximum allowable laser diode drive current, the _OPTICAL_POWER_ERROR will
change its logical state to TRUE - LOW (0 V). A further increase of
+OPTICAL_POWER will, for some laser units, further increase the optical output
power. The increase will vary from individual to individual. The PICO D control elec-
tronics will protect the laser diode, however it is not recommended to operate the la-
ser if the _OPTICAL_POWER_ERROR is TRUE - LOW (0 V).
Minimum optical power is difficult to define; this state is close to the threshold for the
laser and thus not a recommended working state.
NOTE THAT : In constant current mode (paragraph 2.5.3.2.5) the OPTI-
CAL_POWER control signal is re-scaled as 10 mA/V, meaning that
OPTICAL_POWER = 1 mV
→
laser diode current =10 mA
OPTICAL_POWER = 10 mV
→
laser diode current =100 mA
2.5.4.3 Analog control: MODE_HOP_ADJUST
The temperature of the laser’s mechanical structure can be fine adjusted in order to
move mode hops to non-critical wavelengths. A step in temperature will require a
settling time during which _TEMPERATURE_ERROR will be held LOW (TRUE).
The internal temperature can be adjusted by applying a voltage (0.5
°
C/V) to
+MODE_HOP_ADJUST and -MODE_HOP_ADJUST as shown in Table 2-9.