2. 7–3.0 µm
1. 9–2. 1 µm
Anticipated tuning range for various Stokes orders
( 1–6)RFLs based on MIR glasses
2nd 3rd 4th 5th 6th
Technical advances from around the globe
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15 Laser Focus World www.laserfocusworld.com June 2017
See page 22
Ultranarrow-linewidth mid-IR laser
family could span 2. 5 to 9. 5 µm
A group at the University of New Mexico
(Albuquerque, NM) has developed—in
theory, at least—a family of narrow-linewidth
mid-infrared (mid-IR) lasers of novel design:
they are distributed-feedback Raman fiber
lasers (DFB RFLs) based on π-phase-shifted
fiber Bragg gratings (PPS-FBGs) written in a
glass fiber called a low-phonon-energy fiber.
The family of fiber lasers spans the spectral
region of 2. 5 to 9. 5 µm with no gaps. 1
Pumped with either a thulium-doped silica
(Tm:silica) fiber laser emitting at 1. 9 to 2. 1
µm or an erbium-doped fluoride (Er:ZBLAN)
fiber laser emitting at 2. 7 to 3.0 µm, the PPS-DFB-RFLs are made of arsenic sulfide (AsS),
arsenic selenide (As2Se3), or tellurite (TeO2);
the choice of material and pump laser determines the laser’s tuning range (see figure).
Proof of concept
The purpose of the theoretical study was
threefold, the authors say. First, they
wanted to demonstrate feasibility of the
new Raman laser type; second, they aimed to specify the
optimum design for single-mode or single-frequency lasers of
this type; and third, they wished to sketch out approaches for
future experimental efforts.
The lasers are potentially viable because stimulated Raman scattering with high Raman gains can be achieved in low-loss optical
fibers at most wavelengths in the transparency windows of the
fibers by using moderate pump powers at pump wavelengths corresponding to a broad range of Stokes shifts. In addition, cascading multi-Stokes processes open up new wavelength possibilities.
The choice of glass for a Raman fiber laser for a specific wavelength range is based on the availability of near-single-mode fibers
in that wavelength region, the magnitudes of the peak Raman
gain coefficients, the needed values and ranges of the Raman
shifts, and whether or not PPS-FBGs with high-enough efficiencies can be created in the fibers.
In one example of their calculations, a laser output wavelength of 3.596 µm was chosen,
which is the wavelength used
for sensing of formaldehyde
via absorption spectroscopy.
The researchers modeled two
lasers—one based on TeO2
(core refractive index of 2. 7;
nonlinear index of 1. 1 × 10-17
m2/W) and the other based
on As2Se3 glass (core index of
2. 1; nonlinear index of 5.0 ×
10-19 m2/W). While commercial fibers of these types have
mode areas of around 100
µm2, newly developed pho-tonic-crystal fibers have much
smaller mode areas of around
10 µm2—the researchers used
these for their models.
The calculations showed
threshold pump powers below
The model shows that ultranarrow linewidths of < 1 MHz are
achievable. The researchers believe that optimizations could lead
to the ability to pump these fiber lasers with more than 50 W
of power, resulting in an ultranarrow-linewidth output power in
the watt-level range for some wavelength ranges. The ultimate
power is limited by factors such as thermal or other changes in
the refractive index of the fiber, causing phase anomalies in the
PPS-FBGs, and possibly by material-damage thresholds.
The lasers can be tuned by mechanically stretching the FBGs,
for example, by using piezoelectric actuators, achieving continuous tunabilities on the order of 1 nm.—John Wallace
1. B. Behzadi et al., arXiv:1705.02535v1 [physics.optics] (May 6, 2017).
The possible output wavelengths for a family of
Raman fiber lasers pumped with either a Tm:silica
fiber laser or an Er:ZBLAN laser are shown; the fibers
are made of AsS, As2Se3, or TeO2. The ranges of
efficient cascaded Raman wavelengths (1st through
6th) are shown in different colors. The bottom portion
shows the use of both TeO2- and As2Se3-based
lasers, with nested TeO2 fiber-based lasers followed
by three orders of nested cascaded As2Se3 lasers
(green rectangle at bottom right), yielding ultranarrow-linewidth wavelengths between 7 and 9. 5 µm.