Efficient mid-IR spectral generation via
spontaneous fifth-order cascaded-Raman
amplification in silica fibers
Peter T. Rakich,* Yoel Fink, and Marin Solja~ic ´
Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,
Massachusetts 02139, USA
* Corresponding author: firstname.lastname@example.org
Received April 2, 2008; revised June 9, 2008; accepted June 10, 2008;
posted June 13, 2008 (Doc. ID 94589); published July 24, 2008
Spontaneous cascaded Raman amplification is demonstrated as a practical and efficient means of power
transfer from telecommunications wavelengths to mid-IR wavelength bands through use of conventional
silica fibers and amplifiers. We show that silica fibers possessing normal dispersion over all near-IR and
mid-IR wavelengths can facilitate 37% and 16% efficient Raman power conversion from 1.53 ?m to 2.15 and
2.41 ?m wavelength bands, respectively, using nanosecond pulses from an all-fiber laser source. In contrast
to supercontinuum-based techniques for long-wavelength generation, the high levels of Raman gain gener-
ated at these wavelength bands could produce useful optical amplification necessary for the development of
numerous mid-IR laser sources. © 2008 Optical Society of America
OCIS codes: 140.3550, 140.3538, 190.5650, 190.5890, 140.4480.
Light sources spanning mid-IR wavelengths are the
subject of intense interest for such applications as
semiconductor processing, coherent x-ray generation,
chemical sensing, and cancer detection . To this
end, sources spanning mid-IR wavelengths have been
developed in the form of quantum-cascade lasers ,
rare-earth-doped fiber-gain media , and fiber-
based supercontinuum sources . In this Letter, we
present a simple and practical means of generating
mid-IR wavelengths through spontaneous cascaded-
Raman (CR) amplification, demonstrating efficient
and controlled spectral transfer from telecommunica-
tions wavelengths to mid-IR wavelength bands
through spontaneously seeded Raman amplification
in normal dispersion silica fibers. Through use of a
low-noise all-fiber nanosecond pulsed source employ-
ing erbium amplifiers, as much as 37% (16%) of the
incident pump light (centered at 1.53 ?m) is trans-
fered to 2.14 ?2.41? ?m wavelength bands.
It has been demonstrated that at high powers, sub-
stantial levels of Raman gain can be achieved
through the use of conventional silica fibers to pro-
duce significant spectral redshifts through spontane-
ous CR processes [5–8]. Knowing this, the possibility
of constructing simple and efficient Raman-based
sources and gain blocks for long-wavelength spectral
generation seems promising. However, one’s ability
to continually redshift through CR does suffer some
practical limitations. Spectral broadening and pulse
distortions owing to both spontaneous Raman emis-
sion and four-wave mixing (FWM) generally limit the
efficiency of this process.
For instance, at high powers, anomalous dispersion
and Kerr nonlinearities can give rise to modulation
instabilities, yielding significant spectral broadening
and pulse distortion and unnecessarily stifling the ef-
ficiency of the CR process. For this reason, normal
dispersion fibers are more desirable for controlled
and maximally efficient CR at the nanosecond time
anomalous dispersion from 1.3 to 2.4 ?m owing to
the presence of a strong mid-IR absorption resonance
. However, we illustrate that normal fiber disper-
sion can be obtained over the entire silica transpar-
ency window through proper choice of fiber cutoff
wavelength ??c? and NA. Since the waveguide disper-
sion of a step-index fiber is normal (in sign), and
monotonically increasing in magnitude for wave-
lengths ?cto 2.2?c, the waveguide dispersion can be
made to match the material dispersion for suffi-
ciently high NA, making the total chromatic disper-
sion of the fiber normal for all wavelengths . This
can be seen in Fig. 1(b), which shows the regions of
normal and anomalous dispersion for a silica step-
index fiber with a cutoff wavelength of 1.45 ?m. For
an NA of greater than 0.4, the zero-dispersion wave-
length is pushed to 2.6 ?m, enabling normal disper-
sion for the entire transparency window of silica.
In what follows, we examine spontaneous Raman
amplification using a germanium-doped (?58 wt.%
Ge doping) step-index fiber (Nufern UHNA7) with a
?cof 1.45 ?m and an NA of 0.41. The computed dis-
persion and measured loss profiles for this fiber can
be seen in Figs. 1(b) and 1(c), showing normal disper-
sion over the entire transparency window of the fiber.
Note that although the dispersive effects of dopants
are not included in the computations of Fig. 1(a),
high doping concentrations can result in nonnegli-
gible redshifts of zero of dispersion wavelength of
silica (i.e., up to 3%).
Maximally efficient spontaneous Raman transfer
requires a spectrally narrow laser source, with a low
amplified spontaneous emission (ASE) pedestal,
since the extent of FWM-induced spectral broadening
is generally very sensitive to initial conditions. To
this end, we developed the fiber-based laser source
seen in Fig. 2(a), which produces 2 ns pulses at a
variable repetition rate, making output powers of
several kilowatts possible [4,11]. The source amplifies
OPTICS LETTERS / Vol. 33, No. 15 / August 1, 2008
0146-9592/08/151690-3/$15.00© 2008 Optical Society of America
seed pulses from an electrically gain-modulated
distributed-feedback (DFB) laser diode through two
stages of erbium gain. In the first stage, the seed
pulses of 1531.12 nm wavelength are copropagated
with a cw seed signal from a tunable laser source (de-
tuned from the DFB wavelength by ?100 GHz). The
cw signal produces saturation of the amplifiers, en-
suring a very low ASE component to the amplified
seed signal. Once the seed pulses reach 10 mW aver-
age power levels, a fiber Bragg grating with a
10-GHz-wide stopband is used, in conjunction with a
circulator, to spectrally filter the seed pulses from the
amplified cw laserline and ASE background signals.
With sufficient power to saturate the second stage of
erbium gain, the pulses are boosted to average pow-
ers of 240 mW, yielding an ASE component that is
typically less than 1% of the total output power.
To produce spontaneous Raman energy transfer,
the output of the high-power amplifier is spliced to
50 m of small-core normal-dispersion silica fibers
(Nufern UHNA7). It should be noted that splice
losses ?0.5 dB? and linear propagation losses ?1.5 dB?
reduce the laser output power from 240 to 150 mW at
1.53 ?m wavelengths. For a repetition rate of
680 kHz, the laser output is measured with spec-
trometer and PbSe detector as a function of laser
power. The spectral evolution of the resulting cas-
caded Raman process is summarized by the intensity
map of Fig. 3(a) as the laser power is increased,
showing significant and controlled spectral redshifts
through higher-order CR power transfer. As the laser
power is increased we see that the fundamental
?1531 nms? pump wavelength is shifted in 14.7 THz
(or 490 cm−1) steps [8,12] to 1.64, 1.78, 1.94, 2.14, and
2.41 ?m wavelength bands. While the spectral width
of each successive order does broaden, the generated
spectral bands are very clean, showing negligible
power shedding to continuum. Despite the high ma-
terial losses of silica at 2.41 ?m, a strong fifth Raman
order is formed, producing significant power transfer.
The efficiency of power transfer to long wave-
lengths can be more precisely examined by integra-
tion of the laser power measured in each Raman or-
der as the average laser power is increased. The
measured fraction of laser power found in each order
silica fiber ??c=1.45 ?m?. (b) Computed dispersion and (c)
measured loss for NUFERN UHNA7 silica fiber.
(a) Dispersion versus NA and wavelength for a
Fig. 2.Schematic of fiber-based pulsed laser source.
??m? and laser power. (b) Fraction of laser power in each
order versus incident laser power. (c) Measured (circles)
and estimated (solid curve) total power efficiency.
(a) Measured spectral intensity versus wavelength
August 1, 2008 / Vol. 33, No. 15 / OPTICS LETTERS
can be seen in Fig. 3(b), revealing that as much as
68% of the output power is transferred to the
2.14 ?m wavelengths, while 30% is transferred to
2.41 ?m wavelengths. Accounting for the splice and
propagation losses of the fiber, we see that these per-
centages correspond to 37% and 16% total power con-
version efficiencies to 2.14 ?m and 2.41 ?m wave-
length bands, respectively. Remarkably, at maximum
output power, less than 2% of the incident laser
power remains at 1.53 ?m.
As seen in Fig. 3(c), the total power efficiency of the
Raman conversion process was directly obtained by
measuring the power exiting the fiber with a thermal
power meter. For increasing spectral redshifts (or in-
creasing powers) one finds that the total power effi-
ciency monotonically decreases from 62% (consistent
with linear losses) to 35%, indicating that nonlin-
early induced power dissipation occurs at higher
powers. Note that some component of the losses is a
fundamental consequence of the Raman process,
which requires that each Stokes-shifted photon coin-
cides with the production of a (dissipative) phonon.
An estimate of the power efficiency attributable to
Raman process (or phonon-induced losses) can be
made using the known quantum efficiency of the Ra-
man process and the measured power fraction in
each Raman order, as is shown by a solid curve in
Fig. 3(c), revealing reasonable agreement with mea-
surement. The discrepancy between the estimated
and measured efficiencies is most pronounced at high
powers, indicating that high silica absorption losses
at 2.41 ?m wavelengths are limiting the efficiency of
the power transfer at long wavelengths.
Finally, we examine the temporal evolution of the
laser pulse through the spontaneous Raman process.
The measured initial pulse profile (before entering
the nonlinear fiber) is seen in Fig. 4(a), while the
pulse profiles found by spectrally resolving the pump
(solid curve) and first Raman order (dashed curve)
exiting the nonlinear fiber can be seen in Fig. 4(b), re-
vealing that the central portion of the pulse is trans-
ferred from 1.53 to 1.64 ?m, leaving only the wings
of the pulse behind. Pulse apodization of this form is
well understood in the context of spontaneous Raman
buildup [5,6] and results from the intensity depen-
dence of the Raman gain. These measurements agree
well with temporal simulations, found through
coupled amplitude equations incorporating disper-
sion, Kerr nonlinearities, and Raman terms , seen
in Figs. 4(c) and 4(d). Interestingly, the peak pulse in-
tensity and pulse shape are essentially preserved, al-
lowing the same process to occur numerous times. Fi-
nally, we note that the walk-off length for the pump
and Stokes wavelengths is ?300 m for 2 ns pulse du-
rations, meaning that dispersion has little impact on
the temporal evolution at these length scales.
In conclusion, we have shown that, through use of
near-IR and mid-IR wavelengths, 37% (16%) efficient
?2.41? ?m wavelength bands can be obtained using
nanosecond pulses from an all-fiber laser source. In
this case, the total conversion efficiencies were un-
necessarily limited by fiber losses. However, efficien-
cies approaching 70% can, in principle, be achieved
at these wavelengths if fiber losses are made negli-
gible. Finally, in contrast to supercontinuum tech-
niques for long-wavelength generation, we note that
high levels of Raman gain generated at these wave-
length bands could produce useful optical amplifica-
tion necessary for the development of numerous
mid-IR laser sources.
from 1.53 ?mto2.15
We thank E. P. Ippen and J. W. Sickler for helpful
discussions. This work was supported by the Air
Force Research Laboratories (grant FA8650-05-C-
5426) and the Army Research Office via The Institute
for Soldier Nanotechnologies (contract W911NF-07-
1. J. S. Sanghera, I. D. Aggarwal, L. B. Shaw, L. E.
Busse, P. Thielen, V. Nguyen, P. Pureza, S. Bayya, and
F. Kung, J. Optoelectron. Adv. Mater. 3, 627 (2001).
2. P. Werlea, F. Slemra, K. Maurera, R. Kormannb, R.
Mucke, and B. Janker, Appl. Phys. B 67, 307 (1998).
3. L. Esterowitz, R. Allen, and I. Aggarwal, Electron.
Lett. 24, 1104 (1988).
4. C. A. Xia, M. Kumar, O. R. Kulkarni, M. N. Islam, F. L.
Terry, and M. J. Freeman, Opt. Lett. 31, 2553 (2006).
5. R. H. Stolen, C. Lee, and R. K. Jain, J. Opt. Soc. Am. B
1, 652 (1984).
6. C. Lin, L. G. Cohen, R. H. Stolen, G. W. Tasker, and W.
G. French, Opt. Commun. 20, 426 (1977).
7. S. V. Chernikov, Y. Zhu, J. R. Taylor, and V. P.
Gapontsev, Opt. Lett. 22, 298 (1997).
8. G. P. Agrawal, Nonlinear Fiber Optics (Academic,
1989), Chap. 8.
9. H. Xie, Z. C. Wang, and J. X. Fang, Phys. Status Solidi
A 96, 483 (1986).
10. K. Okamoto, Fundamentals of Optical Waveguides
11. O. P. Kulkarni, C. Xia, D. J. Lee, M. Kumar, A.
Kuditcher, M. N. Islam, F. L. Terry, M. J. Freeman, B.
G. Aitken, S. C. Currie, J. E. McCarthy, M. L. Powley,
and D. A. Nolan, Opt. Express 14, 7924 (2006).
12. R. H. Stolen, C. Lee, and R. K. Jain, J. Opt. Soc. Am. B
1, 652 (1984).
Simulated pulse profiles for comparison.
(a) and (b) Measured pulse profiles. (c) and (d)
OPTICS LETTERS / Vol. 33, No. 15 / August 1, 2008