Datasheet

CHAPTER 4. OPERATING QCLS IN CONTINUOUS WAVE MODE 16
Connectors for the LLH with a BNC plug are available for CW laser operation. This
is always center positive, shield negative, and can directly be connected to a stabilized
DC supply; make sure polarity is correct and limits as shown in the datasheet are set.
To modulate the QCL, an AC signal needs to be added to the DC current. To prevent
current modulation to go back to the DC source, an RLC circuit needs to be added. The
values of R
AC
, L, C depend on the modulation 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.
In general, X
Q
is of the order of 1..2 Ω. For a modulation frequency of 10kHz, L
should therefore be of the order of 3mH or larger.
rules of thumb for R and C
To prevent the AC source from dominating or reverse biasing the current through the
QCL, U
AC
< U
Q
must always be given (in absolute values). Furthermore, as the AC
voltage source together with R
AC
, C is forming a current source, to prevent variations
of QCL dynamic resistance X
Q
to influence the AC current, R
A
C + X
C
>> X
Q
must be
guaranteed. As the generator for U
AC
will have an impedance Z
AC
, impedance match-
ing demands for R
AC
+ X
C
+ X
Q
= Z
AC
. Putting these constraints together leads to
I
AC
< U
Q
/Z
AC
.
For the capacitor C to let pass the AC current, 2πR
AC
C > 1/f must be true, and with
X
C
= R
AC
/100 or smaller to make sure that the impedance of the AC current source