Rev A Nov 2017
Page 38
Chapter 6
Software Reference
Set Laser On Button -
turns the drive current to the laser diode ON and OFF.
Note.
The laser driver output cannot be enabled unless the interlock connection is
made and the key switch is turned on.
Photodiode Current display
- sh ows the photodiode current (I
PD
).
Note
. The
maximum value is dependent upon the photodiode current range (I
PD
), set by the rear
panel micro switches (Section 3.3.7.).
Laser Voltage display
- shows the laser diode drive voltage. For grounded cathode
laser diodes, the voltage is positive, for grounded anode diodes it is negative.
Const P & Const I buttons
- the TLD001 laser diode driver can be operated in either
Constant Current or Constant Power mode.
In
Constant Current Mode (CONST I)
, a constant drive current is applied to the
laser diode. However, due to temperature fluctuations this does not result in a
constant optical power output. As th e diode warms up, the optical power will
change noticeably from the l evel at initial switch on. Amb ient 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 T TC001 TEC Controller T-Cube for su ch applications, see
www.thorlabs.com for more details.
Constant Power Mode (CONST P)
is use d to mini mize the o utput power
fluctuations described above. 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.
This feedback loop attempts to maintain constant optical power by continuously
monitoring the pho tocurrent and if necessary adjusting the output current to
maintain the same p hotocurrent. Since the photocurrent is propo rtional to the
optical power, this ensures that the optical power remains stable. The laser diode
current limit setting still applies in this 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 i s still limited by the fact that the
photocurrent is dependent on the temperature. In addition to the photodiode itself
being sensitive to temperature, if the temperature of the laser diode fluctuates, the
wavelength shifts and this also causes a change in photocurrent. For highest
accuracy, laser diodes are therefore normally operated in a tempe rature-
stabilized manner.
Safety Key LED -
lit green when the front panel key switch is turned ON, red when
the switch is OFF - see Section 4.2.
Interlock LED -
lit green when the ‘Interlock’ connection is made (short circuit), red
when open circuit - see Section 3.3.11.
Current Limit LED
- lit when the laser diode drive current limit (see Section 3.3.6.)
has been reached.
