Datasheet

Chapter 4: QCL Operation
4.3. QCL spectra
A QCL is characterized by its emission frequency (spectral domain), wavelength or
wavenumber (spatial domain); all three are equivalent. The following table gives typi-
cal values for ALPES LASERS QCLs.
wavelength (λ) frequency (f) wavenumber (ν)
4 - 20 µm 15 - 75 THz 500 - 2500 cm
−1
4000 - 20 000 nm
4.3.1. Temperature and current tuning
QCL emission can be precisely tuned with temperature (slow tuning) or current (quick
tuning), but the later is in fact just apparent, as it results from the increase of temper-
ature of the active region
1
.
DFB QCL
Figure 4.1 shows the temperature and current tuning on a DFB laser operated in CW
and pulsed modes. The wavelength increases (or the wavenumber decreases) by a
factor varying between 7 · 10
−5
and 9 · 10
−5
for a temperature increase of 1 degree.
For example, the total tuning for a 2000 cm
−1
QCL operating over a range of -30 to
+30
◦
C will vary between 8.4 and 10.8 cm
−1
, depending on the laser.
FP QCL
The spectrum of a FP laser of figure 4.2 shows an interference structure caused by
the Fabry-Perot effect of the reflections at both ends of the laser chip, superimposed
on a wide gain envelope. A temperature change will modify both these structures:
the FP spectrum evolution is driven by the length of the laser cavity, which can vary,
and the gain envelope evolves in unpredictable ways due to mode competition. The
ALPES LASERS datasheet provides spectra taken at regular temperature intervals;
additional requests on the spectral shape should be discussed with ALPES LASERS.
Figure 4.3 shows the spectrum of broadgain lasers.
1
The temperature changes the apparent optical length of the grating, tuning the waveguide refraction
index.
44 QCL user’s manual v3.0