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Bragg grating fast tunable filter
A. Iocco, H.G. Limberger and R.P. Salathk
Indexing terms: Optical filters, Gratings in fibres
A fast tunable optical filter based on Bragg gating compression is
realised using a piezoelectric stack actuator. A setting time of
<2ms over a 15nm tuning range has been achieved. Tuning does
not affect the reflectivity, shape or bandwidth of the central Bragg
wavelength peak.
An optical fibre Bragg grating can be made by producing periodic
variations in the fibre refraction index along a short section in the
core of an optical fibre [l]. This phase grating acts as a band rejection filter reflecting wavelengths which satisfy the Bragg condition.
The sensitivity of the grating wavelength to strain and thermal
changes can be used to realise sensors and narrowband tunable
optical filters [2 - 51. However, the low temperature sensitivity of
0.0125nm/"C [2] severely limits the thermal tuning range. Compressive tuning filters [4], actuated with a linear motor, have been
realised by taking into account the good stress and strain properties of silica optical fibres and the fact that silica is 23 times
stronger under compression than under tension. Tuning ranges of
up to 32 nm have been reported, but the tuning speed was limited
to a few seconds [4]. In this Letter, we describe a Bragg grating
compressed by a piezoelectric stack actuator which allows us to
achieve a setting time in the order of ms.
Applying a strain E to an optical fibre, the wavelength shift ah
of the Bragg grating central wavelength peak h is given by
0.5nm with a central transmission peak at h = 1556.06nm. A
piezoelectric actuator, driven by an HV amplifier (loow), was
used to compress the fibre longitudinally. Metallic ferrules hold
the fibre in place and guide it. The ferrules were fKed on a
mechanical system which permits transmission of the compressive
strain from the piezoelectric actuator to the fibre. An optical low
coherence reflectometry (OLCR) measurement setup [9] was used
to exactly position the Bragg grating within the compressed zone
of 8mm in order to stress it homogeneously. Using the OLCR
technique, the grating was precisely located with an accuracy of
+500pm over a distance of 200mm. To measure the setting time,
we observed the delay between the external trigger signal which
drives the piezoelectric actuator and the signal which is backreflected by the compressed Bragg grating. The grating signal was
obtained with a tunable external-cavity laser tuned at the Bragg
shifted wavelength. The accuracy in the delay measurements was
16 p.
15
'
"
!
-
'
.
!
'
"
I
"
'
piezoelectric actuator displacement, prn
Here, eZ and E~ are the strains along the longitudinal and the
transversal axes of the fibre; p,, and p I 2are photoelastic constants
of the strain optic tensor and n is the refractive index. If the strain
is homogeneous and isotropic, eqn. 1 becomes:
AA
= (1 - p , ) ~ zY 0 . 7 8 ~ ~
I
x
(2)
where pe = 0.22 is the effective photoelastic constant [2].
A 3mm long Bragg grating has been generated by exposing a
telecommunication standard fibre to a 248 nm pulsed Excimer
laser through a phase mask with a period of 1.076 pm. The fibre
was H2 loaded for four weeks at a pressure of 120 bars in order to
increase its photosensitivity [6]. The fluence per pulse was 250mTi
cm2 and the total irradiation dose was 4.7kJ/cm2. After the UV
exposure, the grating without coating was annealed for 48h at
120°C to increase its stability [7] and then positioned in the tunable filter device. Transmission measurements of the Bragg grating
central wavelength during compression were performed by a l m
monochromator with a resolution of 2 x l(f2nm [SI. The signal
was provided by an edge emitting LED. The Bragg grating reflee
tivity after annealing was 80%, and the FWHM bandwidth was
Fig. 1 shows the transmission spectra of the tunable fdter device
for different actuator positions. The transmission curve of the
Bragg grating (solid line), corresponding to a piezoelectric actuator displacement of Ow, is composed of a main peak at
1556.06nm with an FWHM of O S n m , and a satellite peak at
1554.4nm. This satellite peak results from the particular phase
mask used in this case. The total negative wavelength shift was A?L
= -14.9nm for a piezoelectric actuator displacement of 1 0 0 ~
(dash-dotted line). The shape, the bandwidth and the transmissivity of the Bragg grating central wavelength transmission peak
remained stable during the compression. The resulting total strain
on the fibre E, was 1.22%. Fig. 2 shows the wavelength shift as a
function of the piezoelectric actuator displacement. A very high
linearity of the wavelength shift over the whole displacement
range, indicated by a regression coefficient of R Z= 0.999, has been
obtained. The slope is 0.15ndpm. For measurment of the setting
time, different displacements were imposed on the Piezoelectric
actuator by changing the amplitude of the trigger input signal that
1556.06
1536
1540
1544
1548
h,nm
1552
15356
1560
EB
0
Fig. I Transmission spectra of fibre Bragg grating under compression
___ piezoelectric actuator displacement of O p m
-~~~
piezoelectric actuator displacement of 100p.m
ELECTRONICS LETTERS
4th December 1997
m
Fig. 2 Bragg grating central wavelength shift against piezoelectric displacement
1553.16
20
h,nm
1550.20
1547.20
40
1544.26
60
80
1541.20
100
piezoelectric displacement, prn
Fig. 3 Delay between external piezoelectric driving signal and its displacement
Vol. 33
No. 25
21 47
is proportional to the shift of the central Bragg grating wavelength. Fig. 3 shows the measured setting time against the piezoelectric actuator displacement.All the measurements show a setting
time of < 2ms. Below 3 0 p q the setting time decreases with
increasing displacement; above 3 0 p it is constant. The variations
in response time depend on the constructive details of the piezoelectric actuator which was optimised for displacements of > 3 0 ~ .
This might explain the poorer performance at small displacements.
Before each measurement, the piezoelectric actuator was reset to
the initial position.
In conclusion, a fast and highly linear tunable fdter based on
fibre optic Bragg grating and a piezoelectric stack actuator has
been realised. A tuning range of 1 5 n m and a setting time of < 2ms
have been obtained at a wavelength of 15501x11. Such a device is
of potential use in telecommunication networks for wavelength
multiplexing, or as wavelength selective filter.
Acknowledgments: This work has been realised within the frame of
the European ACTS-PHOTOS project.
0 IEE 1997
Electronics Letters Online No: 19971466
22 October 1997
A. Iocco, H.G. Limberger and R.P. Salathe (Institute of Applied
Optics, Swiss Federal Institute of Technology, CH-1015 Lausanne,
Switzedandj
References
1
o , FUJII, Y.,
JOHNSON, D.c.,
and KAWASAKI, B.s.:
‘Photosensitivity in optical fibre waveguides: application to
reflection filter fabrication’, Appl. Phys. Lett., 1978, 32, (lo), pp.
647-649
2 MELTZ, c.: ‘Overview of fibre grating-based sensors’. Proc. of SPIE,
Distributed and Multiplexed Fibre Optic Sensors VI, Denver,
Colorado, 1996, Vol. 2838, pp. 1-21
3 LIMBERGER, H.G.,
NGUYEN
HONG
KY, ,
COSTANTINI, D.M.,
SALATHE, R.P., MULLER, c.A.P., and FOX, c.R.: ‘Efficient active Bragg
grating tunable filters’. OSA Tech. Dig. series, OSA topical
meeting, Williamsburg, USA, 1997
4 BALL, G.A., and MOREY, w.w.: ‘Compression-tuned single-frequency
Bragg grating fibre laser’, Opt. Lett., 1994, 19, (23), pp. 1979-1981
5 MELLE, s.M., ALAVIE, AT., KARR, s., COROY, T., LIU, K., and
MESURES, R.M.: ‘A Bragg grating-tuned fibre laser strain sensor
system’, IEEE Photonics Technol. Lett., 1993, 5, (2), pp. 263-266
6 LEMAIRE, P.J.: ‘Reliability of optical fibres exposed to hydrogen:
prediction of long-term loss increases’, Opt. Eng., 1991, 30, (6), pp.
780-789
7 LIMBERGER, H.G., VARELAS, D., and SALATHE, R.P.: ‘Reliability
aspects of fibre Bragg grating’. Proc. OFMC, Teddington, UK,
1997
8 LIMBERGER, H.G., FONJALLAZ, P.Y., and SALATHE, R.P.: ‘Spectral
characterisation of photoinduced high efficient Bragg gratings in
standard telecommunication fibres’, Electron. Lett., 1993, 29, (l),
pp. 4748
9 LAMBELET, P., FONJALLAZ, P.Y., LIMBERGER, H.c., SALATH~,,R.P.,
ZIMMER, CH., and GILGEN, H.H.: ‘Bragg grating characterisation by
optical low-coherence reflectometry’, IEEE Photonics Technol.
Lett., 1993, 5 , ( 5 ) , pp. 565-567
HILL, K
Introduction: The Landau-Placzek ratio, the ratio of the backscat-
tered Rayleigh to the spontaneous Brillouin intensities, has been
used for distributed fibre optic sensing [ l , 21. The Rayleigh signal
is insensitive to temperature changes. However, the Brillouin signal has a temperature sensitivity of approximately 0.32% K-l. The
achievable temperature resolution of this sensor not only depends
on the electrical noise of the detection system, but also on the
coherent Rayleigh noise superimposed on the Brillouin signal, due
to unsatisfactory rejection of the Rayleigh signal after fiitering.
This coherent noise is manifested as low frequency perturbations
on the Brillouin signal. Hence, an optical fdter that can provide
both high throughput of the Brillouin signal with simultaneous
high rejection of the Rayleigh signal would result in an improved
temperature resolution.
This Letter describes an innovative double-pass all-fibre Mach
Zehnder interferometer (DPMZ) used to separate the relatively
weak (-15 dBr) backscattered spontaneous Brillouin signal from
the Rayleigh signal with high throughput of the Brillouin signal
and high rejection of the Rayleigh signal. The improvement in signal-to-noise resulting from the enhanced Rayleigh rejection, is
approximately 15dB and agrees well with the supporting theoretical comparison between a single-pass all-fibre Mach-Zehnder
interferometer (SPMZ) and a DPMZ.
Theory: The antistokes to Stokes separation for Brillouin spectra
is approximately 22GHz at 1550nm. A Mach-Zehnder interferometer (MZ) having a free spectral range (FSR) equal to this freThe
quency, requires a fibre path imbalance of -9.3”.
differential birefringence of this path imbalance length in the cavity of a MZ is very much less than the beat length of fibre (typically a few cm for singlemode) and can be neglected. For a
carefully constructed MZ, the throughput and rejection will be
largely determined by the signal bandwidth rather than any imperfections (unbalanced losses, non-ideal coupling coefficients).
Consider a Brillouin signal having a Gaussian spectrum g(v), of
bandwidth AV, centred at v,, filtered by the transfer function of
the MZ, and tuned to a maximum at v,, so that maximum
Brillouin emerges from one output arm. Simultaneously, the
Rayleigh signal (also Gaussian) would be tuned to a minimum.
The throughput T, of the Brillouin signal, can be expressed as
Loo
+oo
Tn=
g(v)[0.5(1+ cos[27r(v- v,)/FSR])]”dv (I)
while the corresponding rejection of the Rayleigh signal,
given by
..=Loo
is
+oo
g(v) [0.5(1+cos[27r((v - v,)/FSR+ 1/2)])]” dv
(2)
where n = 1 and 2 for the SPMZ and the DPMZ, respectively.
Fig. l a compares the fdters’ throughput of the Brillouin signal for
various bandwidths normalised to the FSR of the MZ. Fig. 16
compares the corresponding rejection of the Rayleigh signal. As
shown, the DPMZ provides superior rejection of the Rayleigh signal over all bandwidths with a correspondingly slight reduction in
the Brillouin throughput.
Experimental demonstration: The experimental schematic diagram
re Mach-Zehnder
r for distributed
K. De Souza, P.C. Wait and T.P. Newson
Indexing terms: Light intevferorneters, Optical sensors
A double-pass configured all-fibre Mach-2hnder interferometric
optical filter has been developed and used in a distributed fibre
optic sensor for separation of Rayleigh and Bnllouin signals. Its
superior performance over a single-pass all-fibre Mach-Zehnder
interferometer is highlighted by its 15dBimproved rejection of the
Rayleigh signal and comparable throughput.
21 48
is shown in Fig. 2. The pulsed source was a Q-switched erbiumdoped fibre laser (QSL) with a centre wavelength of 1557nm,
bandwidth = 2GHz, Gaussian spectrum (see Fig. 4 inset), and
pulse duration of 100ns. The output from the QSL was monitored
at photodetector PD1 via a 9416 coupler. A 50/50 coupler coupled
300mW into a singlemode sensing fibre of length 700m.
A DPMZ with an FSR of 22.3GHz (fibre path imbalance of
9.3”)
was constructed from two continuous lengths of fibre to
avoid any splice losses. Fig. 3 shows the configuration of the
DPMZ. The backscattered light, comprising Rayleigh and spontaneous Brillouin signals, returning from the sensing fibre, entered at
port 1. The DPMZ was tuned such that the Rayleigh signal
emerged from port 3 and the Brillouin signal from port 4. A fraction of the Rayleigh signal was extracted at the coupler C1 and
used for tuning purposes by maximising the Rayleigh signal
detected on photodetector PD2. Tuning was achieved by controlling the current through a small heating coil adjacent to one of the
fibre arms with the other arm insulated to prevent thermal con-
ELECTRONICS LETTERS
4th December 1997
Vol. 33
No. 25
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