Pump power (W)
678 345 012
Pthresh = 48. 6 m W
= 16.4% η
20 15 10 5 01.0
polished and antireflection-coated for a
4. 5–5. 5 µm wavelength band on the
2.65 × 7 mm sides. The sample was mounted to a copper cold finger and placed into a
mostly evacuated ( 1 m Torr) dewar, where
it was cooled to 77 K with liquid nitrogen.
The x-cavity laser configuration included three mirrors with >98% transmission
at the 4 µm pump-laser wavelength and
high reflection in the 4. 5–5. 5 µm wave-
length band. The cavity output mirror
had a 65% reflection at 5. 2 µm. While
the optimum beam waist within the cavity was calculated to be 95 µm, the actual beam waist was measured to be about
190 µm—a large difference. As a result,
this first experimental prototype was not
expected to be operating near the highest possible efficiency. The pump laser, a
Fe:ZnSe fiber laser made by IPG Photonics
(Oxford, MA), emitted 220 µs pulses at
a 400 Hz repetition rate.
The output spectrum of the
Fe:CdMnTe was measured with a
mid-IR spectrum analyzer (OSA) made by
Thorlabs (Newton, NJ), showing the laser’s
5.223 µm center wavelength and a
1 nm spectral width. The researchers were
surprised at the narrowness of the laser’s
bandwidth, as ternary crystals such as
Fe:CdMn Te usually have more defects than
do binary crystals such as Fe:ZnS and so on,
and thus normally show some broadening.
The researchers attribute the narrow line-
width to the uniformity of doping achieved
by the Bridgman crystal-growth technique.
The output power, measured after the
laser’s output coupler, was determined
to be 810 m W for 8.05 W of input pump
power (see Fig. 2)—a total efficiency of
10.1%. The researchers next plan to optimize the laser-cavity setup and to cre-
ate a higher vacuum so that the laser
light is less absorbed by remaining air.
Replacing one of the laser-cavity mirrors
with a blazed grating in the Littrow configuration should allow wavelength tuning from 4. 6 to 5. 4 µm.
1. J. W. Evans et al., Opt. Mater. Express (2017);
FIGURE 2. Laser output power plotted
as a function of pump power shows a total
efficiency of 10.1% at the laser’s highest output
of 810 m W. The laser threshold was 50 m W.
At lower powers, the slope efficiency reached
16.4%. Absorption of the laser light by the
remaining air within the laser cavity, along with
an actual/theoretical beam mismatch, reduced
the first prototype’s overall efficiency; at higher
powers, thermal effects likely also contributed
to the efficiency reduction.