Datasheet

Chapter B: Operations for advanced users
B.4. CW modulation
To operate a laser in CW mode, an AC signal is added to the DC current. It is recom-
mended to modulate the external power supply directly, within the parameter range
specified in the ALPES LASERS datasheet. If this is not possible, an additional modu-
lated source must be used, like a waveform (ramp) generator for example.
Interface circuit
To reduce cross-talk with other system components and to improve mechanical sta-
bility, this interface circuit should be installed into a small metal box with only the
connectors accessible, as shown in figure B.2.
~
I
AC
I
DC
BNC
BNC
"box"
connector
BNC - low impedance line
678
LLH
Figure B.2.: Interface circuit for CW laser modulation.
Connectors for the LLH with a BNC plug are available for CW laser operation,
ALPES LASERS provides the CIL cable pinout, the adaptor must be made by the user.
It is center positive, shield negative, and can directly be connected to a stabilized DC
supply; make sure the polarity is correct and limits as specified in the datasheet are
set.
B.4.1. External power supply protection
To prevent current modulation to go back to the external power supply, an RLC circuit
needs to be added (see figure B.3). The values of R
AC
, L, C depend on the modula-
tion frequency, the AC source and (to some extent) on the QCL.
Rule of thumb for L
As the impedance X
L
= 2πfL of the inductivity should be much higher than the dy-
namic resistance X
Q
of the laser to block the AC component from the DC supply,
applying X
L
> 100X
Q
leads to roughly L > 16X
Q
/f.
104 QCL user’s manual v3.0