Fabrication of multiple fiber-Bragg gratings
on one SMF using a single phase-mask
Nigel Sayers, Sergio Granieri, and Azad Siahmakoun
Department of Physics and Optical Engineering
Rose-Hulman Institute of Technology
5500 Wabash Ave, Terre Haute, Indiana 47803, U.S.A.
Phone: 812 877 8400, Fax: 812 877 8023
E-mail: Azad.Siahmakoun@rose-hulman.edu
ABSTRACT
UV light from an Excimer laser operating at 248 nm with fluence of 800 mJ/cm² per pulse at 30 Hz repetition rate is
used for writing process of a fiber Bragg Grating (FBG). Multiple FBGs with different center wavelength and reflectivity
are produced by varying strain and exposure time on a single strand of a SMF where 1552.75 nm is the center
wavelength of the unstretched FBG. The in-situ measurement of the reflectivity, bandwidth and center wavelength of
each Bragg grating will be described. We will also present an RF technique for measuring the separation between two
adjacent FBGs.
1. INTRODUCTION
Since the first demonstration of side-written fiber gratings using two-interfering beams1 the fabrication technology
and applications of FBG have been developed extensively. Among others, the phase-mask technique is widely used for
fiber Bragg grating fabrication due to its simplicity and suitability for mass production. The fixed pitch of the phasemask imposes limitations over the achievable Bragg wavelength. The obvious solution consists of changing the pitch of
an interferometric pattern over the fiber by replacing the phase-mask. However, alternate methods were demonstrated for
writing FBGs at different central wavelengths using a single phase-mask. Mask image projection technique consists of
an imaging lens used to de-magnify the UV fringe pattern from the phase-mask before reaching the fiber.2 Tilting the
fiber at an angle with respect to the phase-mask also produces a change in the grating period and thus the center
wavelength of the FBG.3 A main limitation of the previously mentioned techniques is that the tuning ranges are
relatively small. An alternative approach for tuning the Bragg wavelength is by applying a strain to the fiber prior to the
exposure so that upon relaxation a smaller center wavelength is obtained.4 The last approach is particularly attractive
since the center wavelength of FBG can be selected in a wide spectral range.5 During the fabrication process the index of
refraction of the fiber core changes with the exposure time of the fiber to UV light, leading a variation in the Bragg
wavelength and reflectivity of the FBG. Effects of both strain and exposure time on Bragg wavelength must be taken into
account in order to realize a FBG at a desired location of the spectrum. However, the dynamic of the writing process
changes considerably as the fiber is strained.6
In this paper, we present a method for writing FBG at different center wavelengths using a single phase-mask.
Fabrication parameters such as applied strain and exposure time are obtained following the proposed procedure in order
to achieve FBGs with desired center wavelength and reflectivity. We demonstrate using this technique fabrication of four
Photonic Applications in Devices and Communication Systems, edited by Peter Mascher, Andrew P. Knights,
John C. Cartledge, David V. Plant, Proc. of SPIE Vol. 5970, 59701J, (2005) · 0277-786X/05/$15 · doi: 10.1117/12.628724
Proc. of SPIE Vol. 5970 59701J-1
Bragg gratings with center wavelengths at channels 30 to 33 of the ITU grid. We will also describe an RF technique for
determining the position of FBGs within a fiber delay line.
2. EXPERIMENT
When a FBG is stretched its center wavelength changes linearly as a function of the applied strain according to:
∆λ B = λ B Kε ,
(1)
where λ B and ∆λ B are the center wavelength and its shift, ε is the strain, and K is a constant that depends on fiber
material properties.5 This effect can be used to tune the Bragg wavelength during the fabrication process by controlling
the applied strain to the fiber before the writing process. Then after the fabrication of FBG, when the fiber is relaxed, the
center wavelength of the FBG drifts to a shorter value than the one that is determined for an unstrained fiber.
2.1 Fabrication of FBGs
The experimental arrangement for FBG fabrication is shown in Fig. 1. The UV light source is a krypton fluoride
Lambda-Physik Excimer laser operating at 248 nm. A mirror and a cylindrical lens of focal length of 40 mm are used to
direct and focus the laser beam onto the fiber. The cylindrical lens is placed such that its focusing line is parallel to the
optical fiber. A phase-mask of 3×10 mm2 with 1073.3 nm periodicity is located above the photosensitive fiber with a
separation distance of 170 µm. The zero-order transmission of the phase-mask at 248 nm is 9%. The FBGs are written in
Nufern germanium doped photosensitive single-mode fiber. In these experiments the fluence of the exposure is set to 600
mJ/cm2 per pulse at a repetition rate of 30 Hz. The detection system is consisted of a broadband light source (LED)
coupled to the photosensitive fiber along with an optical spectrum analyzer that is used for detecting the transmission
spectral profile of the FBG. Axial strain is applied to the fiber by using a New Focus pico-motor single-axis translation
stage. One of the fiber holders moves with the stage whereas the second fiber holder is secured in a stationary position.
The pico-motor is driven with a pulse generator which is controlled by a computer.
/
H
UV-mirror \\
KrF
Excmer
baser
Cylindrical Lens
I
•
I
Broadband
Source
I
I
OSA
I
Phase-mask
Fiber
Translator
Figure 1: Experimental setup for fabricating multiple FBGs on a single strand of SMF.
For a certain strain value applied to the fiber, a wavelength shift of ∆λB for the center wavelength is expected
according to Eq. (1). If after the writing process for a specific exposure time a FBG with a stop center wavelength of λ B
is achieved then the center wavelength of the unstrained fiber, λUS , is given by: λUS = λ B − ∆λ B . To predict the final
reflectivity for a FBG, the exposure time and the applied strain has to be taken into account in order to reach this
Proc. of SPIE Vol. 5970 59701J-2
desirable reflectivity. To determine a particular reflectivity for a FBG, a family of experimental curves is assembled to
determine the exposure time.
Figures 2(a) and (b) show reflectivity and center wavelength of four FBG versus exposure time to 248 nm UV light.
These curves correspond to four different values of applied strain to the fiber. Note that after a certain exposure period in
Figure 2(a), the reflectivity of FBG is greater for higher values of strain. Therefore, based on these results the exposure
time depends on the final desirable reflectivity. The values of central wavelength in Figure 2(b) at zero minutes are
extrapolated by fitting experimental data points to exponential functions.
(a)
1:
1
1001
80
e
U
. 1552.5j 7i''
.
• Zero strn
• O.15%strn
20
A fl )QO/
I
1
wavelet
.
flectivit
.
U
--------.
U
6O
401
(b) 1.s1
S
'
• Zero strn
• 0.15%strn
'fr-j
A fl )eO/
V O.4O%slrai
V O.4O%strn
tD
1552.
I
I
U
Exposure Time [minutes]
L
S
0
0
Exposure Time [minutes]
Figure 2: Reflectivity (a) and center wavelength (b) versus exposure time for four strain values.
Figure 3 shows the center wavelength versus strain for eight different exposure times. Each exposure time is directly
related to the reflectivity of the Bragg grating for each different value of strain applied to the fiber. Solid line represents
λ = 1552.75 Kε + 1552.75 which is related to Eq. (1) with 1552.75 nm being the desired center wavelength of the FBG.
Therefore, the intersection of the line with any curve (signifying a particular exposure time) provides the required
parameters, such as strain and stop wavelength, for producing a FBG of a desired reflectivity.
/
1553.5
• 0.5mm
• 1mm
y 2mm
• 3mm
4 mm
5mm
• 6mm
0 mm (calculated)
-c
4
:er WaveIen
D) 1Lz')C%
1552.5-/
0:2
0.3
0:4
Strain [%]
Figure 3: Center wavelength versus strain at six different exposure times. Dashed lines correspond to an
exponential fit of the experimental values. Solid line is an example plot of Eq. (1) for a final wavelength of
1552.75 nm.
Proc. of SPIE Vol. 5970 59701J-3
2.2 Making multiple FBGs with a single phase mask
Here we present a method for writing multiple FBG at different center wavelengths, bandwidth, and reflectivity using
a single phase-mask. In order to demonstrate this technique, four FBGs of different center wavelengths are recorded
using the same phase-mask. These FBGs have average reflectivity of 90% and they are centered at ITU channels 33
trough 30. The strain values applied in each case are 0.29%, 0.2%, 0.09%, and 0.03% with stop wavelengths of 1552.78
nm, 1553.01 nm, 1553.11 nm and 1553.47 nm respectively. Figure 4 shows the transmission spectrum of the relaxed
fiber with the four fabricated gratings. Experimental results show center wavelengths of 1550.94 nm, 1551.74 nm,
1552.46 nm, and 1553.26 nm that correspond to channels 33-30 of ITU grid respectively. Another set of Bragg reflectors
that is fabricated using the same technique is shown in figure 5.
Iinritsu
03-04-22 17:46
AMkr A: 1550. 94m
B: 1554. m B-A: 4. 04m
LMkr C: -54.O4dBm
D: -71.8dBm
C-D: 17.76dB
Nom
(A)
-52. 6
dBm
ivIy1:
iii
-82. 6
dBm
—
1550. Onm
1552. Snm
0. Snm/d iv
Res:0. O7nm (0. 063nm( /
VBN: 100Hz
/
Sm:Off/Intvl:Off
1555. Onm
in \/ac
Smplg: 501
Avg:Off
/
/
Aft
Off
Figure 4: Fabricated FBG of wavelengths 1550.94 nm, 1551.74 nm, 1552.46 nm, and 1553.26 nm on the same single-mode
fiber at different locations.
Iinritsu
03-02-12 14:10
AMkr A: 1551.67m
B: 1553.21m
LMkr C:
D:
B-A: 1Mm
C-D:
Non
(A)
-53.
dBm
5. 0dB
iv
-78. 0
dBm
-lO3. 0
dBm
——
1549.4mm
0.Snm'div
Res:0.O7nm(0.OSSnm) /
/
VBN:lOHz
1551.9mm
Avg:Off
Sm:Off'Intvl.Off
in \/ac
Smplg: 501
1554. 41 nm
/
Aft
Off
Figure 5: Fabricated FBG of wavelengths 1550.82 nm, 1551.67 nm, 1552.41 nm, and 1553.21 nm on the same single-mode
fiber at different locations.
Proc. of SPIE Vol. 5970 59701J-4
2.3 Determining location of a FBG
Although FBG are commercially available, but grating manufacturers often only provide an approximate position of
the grating in an optical fiber. In some applications, such as distributed FBG sensors and delay arrays, it is particularly
important to know the exact position of FBG along the fiber. In this section we will demonstrate a simple RF-modulation
technique for measuring the effective reflection point of a Bragg reflector in a SMF.
The experimental setup is shown in Figure 6. A sinusoidal signal from an RF synthesizer is divided in two by a 3-dB
splitter. One output from the splitter is used as the reference signal while the second output modulates an optical carrier
from a tunable single-mode semiconductor laser. The optical signal is sent to a FBG or a fiber reflector of known length
via an optical circulator. The reflected signal from the grating/reflector leaves the circulator and is then detected with a
photodiode. A Transition Analyzer compares the reference signal from the synthesizer with the signal from the
photodiode. Any variation of optical path length, when a FBG is replaced by a fiber reflector, can be calculated by
measuring the phase difference between these two signals. The length variation ∆l can be calculated by direct
measurement of phase difference from
∆φ = φ 0 +
2π
⎛ ∆l ⎞
n eff ⎜ ⎟ν RF ,
c
⎝ 2 ⎠
(2)
where φ0 is a constant phase difference, neff is the effective refraction index of the fiber, c is the speed of light, and νRF
is RF center frequency. Thus, the effective reflection point of a FBG can be calculated by replacing the FBG with a
reference fiber reflector of known length. The broadband fiber reflectors, used as a reference, are cleaved and polished
fibers sputtered with gold. The length of reference fiber reflectors are lR90=0.09±0.0005 m, lR1500.1505±0.0005 m, and
lR2000=2.0005±0.001 m. These reflectors exhibit a reflectivity in excess of 80%.
FBG
IFBG
¶11
TR
PD
Figure 6: Experimental setup for measuring the position of a FBG. R: reflector; OC: optical circulator; PD: photodetector.
Figure 7 shows an example of the phase difference between a FBG and three fiber reflectors versus RF center
frequency together with a linear fit. All the measurements are taken at the optical center wavelength λ1=1551.67 nm. The
slope of each line represents time difference in seconds between channels in the Transition Analyzer. Thus, by
subtracting the slope values of different data sets one is able to determine the additional time that is required for the
signal to travel through each length of fiber. Thus, using Eq. (2) the length difference can be found. This procedure is
Proc. of SPIE Vol. 5970 59701J-5
used to calculate the relative positions of four FBGs in a fabricated delay line. The spectrum of such delay line is shown
in figure 5. The measured distance between the FBGs of channels 33 and 31 results 1.47±0.10 cm meanwhile the
distance between FBGs of channels 32 and 30 is calculated to be1.41±0.10 cm.
R90
R150
R2000
FBG1
0
∆φ [radian]
-20
-40
-60
-80
-100
0.80
0.85
0.90
0.95
1.00
Frequency [GHz]
Figure 7: Phase difference vs. modulation frequency for a FBG and three broadband reflectors.
3. CONCLUSIONS
Several FBG of different central wavelengths in the range of ITU channels 30 through 33 are successfully fabricated
using a single phase mask. The center wavelength and final reflectivity of each FBG is selected by controlling the strain
applied to the fiber, the fluence of the UV light on the fiber and the exposure time.
The physical separation between Bragg reflectors is calculated by measuring the phase difference between the light
reflected from the grating a reference reflector. The results are in excellent agreement with measurements of the position
of FBGs performed during the fabrication process using a micrometer. An accuracy of better than a millimeter in
position of any FBG can be easily produced using this RF technique.
ACKNOWLEDGMENTS
The authors would like to thank the Office of Naval Research for their partial support of this project.
REFERENCES
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Electron. Lett. 29, 1614-1616 (1993).
Proc. of SPIE Vol. 5970 59701J-6
3. A. Othonos and X. Lee, “Novel and Improved Methods of Writing Bragg Gratings with Phase Masks,” IEEE Photon.
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D.C., 1997), pp. 181-183.
Proc. of SPIE Vol. 5970 59701J-7