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Ternary-crystal mid-IR laser
shows power-scaling promise
JOHN WALLACE, Senior Editor
Because the mid-infrared (mid-IR) spec-
tral region coincides with the molecu-
lar fingerprint region, which is important for the spectroscopic detection and
identification of many organic and inorganic molecules, the development of
new mid-IR laser sources is a perennial hot topic.
One traditional type of coherent mid-
IR source takes advantage of nonlinear
optical frequency conversion to produce
mid-IR light. Two well-known exam-
ples are the optical parametric oscillator
(OPO) and its sister, the optical para-
metric amplifier (OPA). However, these
sources, which use a nonlinear optical
crystal to convert a shorter-wavelength
laser input beam to a coherent
mid-IR beam, are complicat-
ed, inefficient, and expensive.
The development of the
quantum-cascade laser (QCL)
and interband-cascade laser (ICL) has
resulted in small and rugged commer-
cially available mid-IR sources that
serve well for low-power and energy
applications, but fall short when more
output is needed.
Another contender for a mid-IR la-
ser-gain material is transition-metal
(TM) doped II-VI semiconductors such
as iron-doped zinc sulfide and zinc selenide (Fe:ZnS and Fe:ZnSe) and chro-mium-doped ZnS and ZnSe. In fact,
these materials can be used in fiber lasers with output powers of up to 20 W.
Another mid-IR laser of this type is
based on tellurides, using iron as the dopant and either cadmium or zinc telluride
as the base (Fe:Cd Te and Fe:Zn Te). These
materials produce longer wavelengths
than Fe:ZnS and Fe:ZnSe and thus ex-
pand the wavelength range reachable
by such lasers. However, these telluride
materials have been difficult to produce.
A group of researchers from the Air
Force Research Laboratory (Wright-
Patterson Air Force Base, OH), Berrie-
Hill Research Corporation (Dayton,
OH), and NAS/NRC Research
Associates Programs (Washington,
DC) has now produced working laser
crystals of Fe:CdMn Te (which is simi-
lar to Fe:Cd Te, but contains some man-
ganese). 1 The output wavelength of the
first prototype was 5.223 µm and had
an 810 m W output power, and demon-
strated a slope efficiency of 16.4%.
The researchers had Brimrose (Sparks
Glencoe, MD) grow a boule of
Fe:CdMnTe from melt using the so-
called Bridgman technique, which pro-
duces directional solidification of the
melt by moving it gradually from a hotter to a cooler portion of the furnace.
The Fe: Cd
Mn1-x Te material had an alloy fraction x of 0.91, as determined by
The absorption of the fabricated
sample was mapped using a Fourier-
transform interferometer (FTIR), and
the sample was then diced into rectangular pieces. One of the pieces was
cryogenically cooled and its transmission spectrum measured via FTIR. In
addition, the mid-IR fluorescence of
the piece was characterized by exciting
it with laser pulses at a 3. 45 µm wavelength from an OPO.
The fluorescence lifetime was determined by fitting an exponential curve
to the measurements—it was found to
be 111 µs at 80 K and 78.5 µs at 10 K.
The fluorescence spectrum was mea-
sured using a spectrometer by Princeton
Instruments (Trenton, NJ). From this
information, the emission cross-section
was calculated (see Fig. 1).
The slab-shaped experimental
sample of Fe:CdMnTe measured
2.65 × 4 × 7 mm, and was
Specially grown Fe:CdMn Te crystal
that emits in the 5 µm spectral
region could be made wavelength-
tunable in the future.