Download Mode selective fiber Bragg gratings

Mode selective fiber Bragg gratings
Jens U. Thomasa , Christian Voigtländera , Stefan Noltea , Andreas Tünnermanna,b ,
Nemanja Jovanovicc , Graham D. Marshallc , Michael J. Withfordc , Michael Steelc
a Friedrich-Schiller-University,
Institute of Applied Physics,
Max-Wien-Platz 1, 07743 Jena, Germany;
b Fraunhofer
Institute for Applied Optics and Precision Engineering,
Albert-Einstein-Str. 7, 07745 Jena, Germany;
c Macquarie
University, MQ Photonics Research Centre,
North Ryde, New South Wales 2109, Australia
ABSTRACT
Focussing ultrashort laser pulses allows for inscribing fiber Bragg gratings (FBGs) directly into rare earth doped
fiber cores - without prior photosensitivity treatment. High reflective FBGs can be written into active Large
Mode Area (LMA) Fibers with 20 micron core diameter using a phase mask scanning technique. Here, we
demonstrate fiber Bragg gratings (FBGs), which cover only a fraction of the core. With this additional degree
of freedom it is possible to taylor the reflectivity of individual modes. We show for example how those FBGs
can be used in few mode LMA fibers to suppress reflections into higher order modes.
Keywords: fiber Bragg gratings, fiber laser, ultrafast applications
1. INTRODUCTION
High power fiber lasers have conquered a large market share within solid state lasers, because of their compactness
and high brilliance.1 For improved cost and stability monolithic setups become more and more important.
Therefore one strives to replace as many bulk components with fiber integrated devices. Most prominently,
fiber Bragg gratings (FBGs) compete with dielectric mirrors and volume Bragg gratings as resonator mirrors.
However, many high power fiber laser systems rely on large mode area (LMA) fibers. FBG inscription by
conventional means is challenging here, because for large, actively doped fiber cores, the necessary prior photo
sensitivity treatment comes to its limits. Additionally, fibers with larger core diameter often support more than
one transversal mode. For FBG based laser setups this results in modal instabilities, thus power scaling is
hampered. More sophisticated FBGs could filter or convert higher order modes. However, since the transversal
cross section of conventional fabricated FBG cannot be controlled, efficient elements could only be realized with
slanted gratings.2–6
These limits can be overcome by using an ultrashort laser for FBG inscription: Because of the nonlinear
absorbtion of ultrashort pulses within the femtosecond (fs) range, refractive index changes can be obtained in
non photosensitive fiber cores. FBGs have been successfully inscribed by fs pulses in standard Ge-doped fiber
cores7, 8 as well as in Er-doped9 and Yb-doped10, 11 fibers. In all cases, the rare-earth doped fibers could be
operated as one-piece-laser.9–11 In a Yb-doped, single mode LMA fiber with 10 micron diameter, 100 W could
be obtained in cw operation.11 Moreover, with a point-by-point (PbP) approach,12 it is possible to target small
subsections of the fiber core.
In the first part of this paper we apply the coupled mode theory to show, how multi mode reflections of FBGs
can be tailored by partially modified core cross sections. In the second part, we use the PbP technique (Figure
1a)) to show the impact of partial core modification on core-cladding mode coupling. We also demonstrate cross
coupling in such FBGs, which results in mode conversion. Finally, with a phase mask scanning (Figure 1b))
written FBG we obtain a cross coupling free spectrum that enables stable few mode operation.
Further author information: [email protected]
Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications X, edited by Alexander Heisterkamp,
Joseph Neev, Stefan Nolte, Rick P. Trebino, Proc. of SPIE Vol. 7589, 75890J · © 2010 SPIE · CCC code:
0277-786X/10/$18 · doi: 10.1117/12.843805
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of their transversal field components Eti with the dielectric pertubation Δ(x, y). When the coupling constants
κij are known, reflection and transmission spectra can be computed by numerically solving the coupled mode
equations.16–18
w
h
Λ
y
z
x
a1
Figure 2. Simplified model of the fs induced modification within the fiber core
In case of a FBG in a single mode fiber, evaluation of equation (3) leads to the well known Bragg reflection
at
λ = 2nef f Λ/m,
(5)
where the effective refractive index nef f of the mode is computed from its propagation constant β = 2πnef f /λ.
In a fiber that supports N modes however, a single period FBG exhibits not only N reflection peaks
λi = 2nef f,i Λ/m,
(6)
λi,j = (nef f,i + nef f,j )Λ/m.
(7)
but also N (N − 1)/2 cross coupling peaks at
Thus, a FBG in a N mode fiber has up to N (N + 1)/2 reflection peaks. The magnitude of such peaks heavily
depends on the cross section Δnmod (x, y), since it governs the coupling constant (equation 4).
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3. EXPERIMENTAL
In this paper we use two different techniques for FBG inscription with ultrashort pulses (Figure 1): while the
phase mask scanning technique is efficient for structuring large cross sections,19 the strength of the PbP method
lies in the ability to probe selected parts of the core.12, 20
3.1 Cladding mode coupling of PbP-FBG in single mode fiber
For the grating inscription we used a femtosecond laser (Spectra Hurrican) that delivers pulses of 110 fs length
at 800 nm with a repetion rate f of 1kHz. A 20× oil immersion objective (N A = 0.8) is used for focusing the
ultrashort pulses as well as for imaging the fiber core before and after inscription of the grating. This allows
for positioning the modification of the FBG within the fiber core with an error of less than one micron. Pulse
energies between 200 and 275 nJ cause a micro explosion within the focal volume, that leaves a micro void after
one pulse. Micro void chains of period Λ = v/f are written by pulling the fiber under the microscope objective
with a velocity v.20, 21 All gratings were written in second order m = 2 with the polymer coating stripped into
SMF-28e fibers (a1 = 4.5μm). The length of the gratings was 20 mm.
Transmission [dB]
The inscribed FBGs were probed in transmission: The light of a swept wavelength laser system (SWS) with a
range of λ = 1520 . . . 1560nm was launched into the fiber. Then, the signal at the end of the fiber was measured
with a photo diode that was synchronized with the SWS. The resulting cladding spectra are plotted in Figure 3
and 5. The domiant peak on the long wavelength side of the picture results from the expected core-core mode
coupling (see equation 5). The comb of transmissions dips on the short wavelength side of the Bragg peak is
caused by coupling of the core mode to cladding modes. Two envelope functions classify the cladding mode
reflections (Figure 3). While the stronger peaks result from coupling into cladding modes of azimuthal order
l = 1, the less stronger resonances in between result from coupling into cladding modes of higher azimuthal
order.
0
−10
−20
1535
1540
1545
1550
wavelength λ [nm]
1555
Figure 3. Transmission spectrum of a point by point written FBG, where the modification is located approximately 1-2
microns off the center of the fiber core.
By evalutation of equations 3 and 4 and subsequent solving of the coupled mode equation, we could compute
the transmission spectra of the micro void FBGs.17 In Figure 4 the spectra are plotted for different transversal
displacements of the micro void. The more the micro void is off center, the lower is the overall reflectivity and
the stronger the coupling to higher order modes. For a complete suppression of higher order modes, the micro
void has to be inscribed exactly in the center of the fiber core. Even small deviations cause the higher azimuthal
order comb to rise.
Therefore, in order to suppress higher order cladding modes, the modification has to be centered in the fiber
core as good as possible. The spectrum of such a grating is shown in Figure 5. Here, reflection to the higher
azimuthal cladding modes could be avoided in a range of over 10nm. Only below 1530nm, a small contribution
of the light couples into higher azimuthal order modes.
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Transmission (dB)
-20
-10
0
0
1
2
3
4
displacement (μm) 1520
1525
1530
1535
1540
1545 1550 1555
wavelength λ (nm)
Transmission [dB]
Figure 4. Location dependent evolution of the Transmission spectrum
0
−10
−20
1525
1530
1535
wavelength λ [nm]
1540
Figure 5. Transmission spectrum of a point by point written FBG, where the modification is centered at the core within
the experimental error.
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3.2 Coupling of PbP FBGs in multi mode fiber
In order to investigate for multi mode reflection, we wrote second order PbP FBGs in a two mode LMA fiber
with 15μm core diameter (Nufern LMA GSF 15/123). In order to favor higher order reflections, two parallel
lines of micro voids were written, each placed 2μm off the core center. The gratings were written for a reflection
wavelength of 1079nm. For probing we used a broad band home built Amplified Spontaneous Emission (ASE)
source, that delivered 97mW, with a center wavelength at 1060nm. This time, the transmission spectrum was
taken with a free beam grating-spectrograph (Oriel 77200). The spectrum of the grating is shown in Figure
6. The top picture shows the spectrally decomposed mode. The spatial information of the beam is maintained
vertically. With integrating different areas of the raw spectra, mode selective transmission spectra can be
obtained: While the top area (I) shows the transmission spectra of the fundamental mode, the bottom area (II)
shows contributions of both the fundamental and the higher order mode. We identify the reflection peaks as
follows: 1) is the reflection of the higher order mode into itself, 3) the fundamental Bragg peak and 2) the cross
coupling peak. At the cross coupling wavelength, mode conversion from the fundamental to the higher order
mode and backward occurs. Since the micro voids cover only a fraction of the core, the overall reflectivity is only
20 percent.
I
T
T
II
1
0.8
0.6
1
0.8
0.6
I
1078
1
2
3
1078.5
1079
Wavelength (nm)
1079.5
1078
1
2
3
1078.5
1079
Wavelength (nm)
1079.5
II
Figure 6. Spectrum of a multi mode FBG: before integration (top) the spatial information of the modes is vertically
maintained. Mode selective spectra are obtained by integrating over area I (middle) and area II (bottom)
3.3 Cross coupling suppressed LMA FBG written via phase mask scanning
Since cross coupling of the fiber mode hampers laser stability it has to be suppressed in LMA fibers for high
power lasers. One way would be to center the PbP FBG as good as possible. However, experimentally this is
not easy feasible. To overcome these limits, we chose to write a transversally homogeneous FBG. Thus, one
has to integrate over the whole core area, when computing coupling coefficients (equation 4). Cross coupling
coefficients are zero, because their integral kernel exhibits an azimuthal dependence.
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For the experimental realization we used a commercial amplified Ti Sapphire laser (Spectra physics Spitfire)
with pulse energies of up to 700μJ, a central wavelength of 800nm and a repetition rate of 1kHz. For phase
mask scanning, a phase mask of period 1.485μm is placed above the fiber. Hence, phase mask and fiber are
illuminated with the line focus of a cylindrical lens (focal length 20mm). Due to ”‘order walkoff”’ of the diffracted
ultrashort pulses, a pure two beam interference pattern modifies the core.19, 22 In order to elongate and widen
the area of modification, both fiber and phase mask are translated with respect to the beam with a velocity
of v = 0.5 mm/min.19 The FBG was inscribed into a Yb-doped LMA fiber with 20μm core diameter (Nufern
LMA-YDF-20/400); it is 10mm long and 40μm wide, thus covering the core and parts of the cladding. For
inscription, we used a pulse energy of 250μJ. The transmission spectrum is shown in Figure 7. More than 80
percent of the fundamental mode (peak 3) are reflected. The cross coupling peak 2 is much weaker in contrast
to Figure 6, thus the FBG can be used for obtaining stable laser operation.
0
transmission (dB)
−2
2
−4
1
−6
−8
3
−10
1074
1075
1076
1077 1078 1079
wavelength λ (nm)
1080
1081
1082
Figure 7. Transmission spectra of a FBG written with the phase mask scanning technique.
4. CONCLUSION
By using an ultrashort laser and the PbP inscription method, we investigated the coupling behavior of transversally inhomogeneous FBG. We described in experiment and theory, how coupling into higher order modes can be
steered with a partial modification of the core. Furthermore we accurately rendered the transmission spectra by
applying the coupled mode theory. In a few mode fiber, we demonstrated how the FBG causes mode conversion.
Since this is usually undesirable for FBGs used in a laser, we finally wrote a transversally homogeneous FBG
using a phase mask scanning technique directly into an Yb-doped LMA fiber. A monolithic fiber laser based on
this fiber delivered up to 215W of output power and is also presented at Photonic West 2010.23 Ongoing work
is on the realization of a Fabry-Perot cavity in order to also damp reflection of the higher order modes.
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ACKNOWLEDGMENTS
The authors would like to thank Ed Grace for help in probing the fiber. This work was produced with funding
from the German Federal Ministry of Education and Research (BMBF) and the Australian Research Council
under the ARC Centres of Excellence and LIEF programs. Jens Thomas acknowledges funding by the DAAD,
grant D/0846673.
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