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17874-D03
T-Cube Laser Diode Driver
4.8
Mode Button Operation
The TLD001 laser diode driver can b e operated in either C onstant Current or
Constant Power mode.
4.8.1 Constant Current Mode (CONST I)
As the name implies, this mode applies a constant drive current to the laser diode.
However, due to temperature fluctuations this does not result in a co nstant optical
power output. As the diode warms up, the optical power will increase noticeably from
the level at initial switch on. Ambient temperature changes will also effect the output.
This mode is used when the lowest noise and highest response speed is required.
Most applications in this mode will also require the temperature to be stabilized by an
additional temperature controller. Thorlabs offers the TTC001 TEC Controller T-Cube
for such applications, see www.thorlabs.com for more details.
4.8.2 Constant Power Mode (CONST P)
To minimize the output power fluctuations described above, the laser diode can be
operated in Constant Power mode . This involves a signal from the internal
photodiode, integrated into most laser diode packages, being fed back to the TLD001
unit in order to monitor and correct the power output.
In constant power mode, the feedback loop inside the TLD001 attempts to maintain
constant optical power by continuously monitoring the photocurrent and if necessary
adjusting the output current to maintain the same photocurrent. As the photocurrent
is proportional to the optical power, the scheme en sures that the op tical power
remains stable. The laser diode current limit setting still applies in th is mode; the
output current will not be increased above the limit.
Note that whilst in constant power mode the optical power is normally more stable
than in constant current mode , the scheme is still limited b y the fact that the
photocurrent generated by the photodiode is dependent on both the temperature and
the wavelength. If the temperature of the laser diode fluctuates, the wavelength shifts
and this in itself also causes a chan ge in photocurrent. For highest accuracy, laser
diodes are therefore normally operated in a temperature-stabilized manner. Thorlabs
offers the TTC001 TEC Con troller T-Cube for su ch applications, see
www.thorlabs.com for more details.
An adjustment of the full scale photodiode current in CONST P mode is provided in
order to compensate for the differences in the photodiode currents between different
laser diodes - see Section 3.3.7.
