Thermal tuning of mechanically induced
long-period fiber grating
Takashi Yokouchi, Yoshifumi Suzaki, Kiyoshi Nakagawa, Makoto Yamauchi,
Masanori Kimura, Yasuo Mizutani, Susumu Kimura, and Seiki Ejima

We have developed a wideband tunable optical filter that uses a long-period fiber grating (LPFG) in
which both resonance wavelength and its signal attenuation can be adjusted. We create the grating
mechanically by pressing a spring coil to an optical fiber. We achieve continuous fine tuning ofwavelength and attenuation by varying the temperature of the LPFG. The adjustable ranges of theLPFG are more than 200 nm in resonance wavelength and more than 10 dB in signal attenuation. ©
2005 Optical Society of America
OCIS codes: 050.2770, 060.2310, 060.2340, 120.2440.

1. Introduction

A long-period fiber grating (LPFG) is a loss filter thatuses mode coupling between the fundamental guided
mode and forward-propagating cladding modes in an
optical fiber.1 Owing to its simple structure and excellent characteristics, the device is used for manyapplications, such as amplitude equalizing devices
for optical amplifiers and various types of optical fibersensor.
The LPFG has been conventionally formed in an
optical fiber by inducing periodic changes in the fiber’s refractive index with period lengths of typicallyseveral hundred micrometers. The index variation
was achieved by irradiation with UV light through anamplitude mask.1 The method of periodic local heating of the optical fiber by CO2 laser irradiation wasalso developed.2,3 These fabrication methods give permanently fixed LPFG parameters, such as grating
period, modulation amplitude, and total length. Recently, in a search for flexibility in applications, interest has shifted to tunable LPFGs. For thispurpose, a mechanical method of inducing refractiveindex changes along the fiber was also proposed. BeT. Yokouchi, Y. Suzaki, K. Nakagawa, and S. Ejima (ejima@eng.
kagawa-u.ac.jp) are with the Faculty of Engineering, Kagawa University, 2217-20 Hayashi-cho, Takamatsu, Kagawa, 761-0396
Japan. M. Yamauchi, M. Kimura, Y. Mizutani, and S. Kimura arewith the Shinko Electric Wire Company, Ltd., 1298-12 Shido,Shido-cho, Sanuki, Kagawa, 769-2101 Japan.
Received 16 December 2004; revised manuscript received 3March 2005; accepted 25 March 2005.
0003-6935/05/245024-05$15.00/0© 2005 Optical Society of America
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APPLIED OPTICS 兾 Vol. 44, No. 24 兾 20 August 2005

cause the period is as large as several hundred
micrometers, one can induce the index modulation
mechanically, for example, by pressing the fiber between a periodically grooved plate and a flat plate4 orby winding a nylon string around the fiber periodically.5
Besides these, other methods were also reported,for example, tuning resonance wavelength by periodically spaced electrical heating of the metal film
coated on the LPFG,6 adjusting signal attenuation
by temperature control of a periodically corrugatedsubstrate, which induces thermal strains on theLPFG,7 and controlling the spectral shape by adjusting the temperature of the LPFG section, using
divided coil heaters.8 Fine tuning of resonancewavelength by bending the LPFG with a piezoelectric actuator was reported,9 but high voltage andfine adjustment are needed there. Each method has
its strengths and weaknesses in actual applications.
A tunable optical filter requires control of the resonance wavelength and signal attenuation independently. The resonance wavelength should beadjustable without affecting the signal attenuation,and the signal attenuation should be adjustable without affecting the resonance wavelength. In search ofsuch a tunable LPFG device, we developed a methodof fabricating a mechanically tunable LPFG by using
an impressing spring.10 Recently, by adding temperature control to this mechanical LPFG, we made it
tunable in a range of 200 nm in wavelength and adjustable in signal attenuation by more than 10 dB.
We describe the details of this tunable LPFG in thefollowing sections.

Fig. 1. Setup for mechanically tuning a LPFG with a coiled spring.

2. Mechanically Induced Tunable Long-Period Fiber
Grating

In a LPFG, the fundamental mode of incident light
couples with cladding modes. The cladding mode islost to outside the fiber; thus the amplitude of thefundamental mode is decreased. Resonance wavelength ␭m of the LPFG is given by␭m ⫽ ⌳(ncore ⫺ ncladm),

the first, the second, and the third attenuation bands
from the short-wavelength side.
Figure 3 shows how the resonant wavelengths of
the first, second, and third attenuation bands areproportional to the coil pitch of the spring, that is, tothe grating period ⌳. Figure 4 shows transmission(1)

where ⌳ is the grating period of the LPFG, ncore is therefractive index of the LP01 fundamental mode, andncladm is the index of LP0m 共m ⫽ 2, 3, 4. . .兲 cladding
modes. Resonance wavelength ␭m can be adjusted byproper selection of grating period ⌳.
The setup for mechanically tuning the LPFG byusing a coiled spring is shown in Fig. 1. We can adjust
grating period ⌳ by expanding the length of the coiledspring. An optical fiber is placed on the bottom of thefiber mount, i.e., the U-shaped metal base. The springis inserted there, and a weight is placed on the spring
to press it against the fiber. The spring is made fromstainless steel with a diameter of 0.5 mm, a coil diameter of 6 mm, and a coil section length of 40 mm.
The length of the LPFG is the same as that ofU-shaped base. The optical fiber that we used is standard single-mode fiber (SMF-28, Corning, Inc.).
Wideband optical light was injected into the fiberfrom an amplified spontaneous emission light source(Fiber Lab., ASE-FL7701), and the light that passed
through the LPFG was measured by an optical spectrum analyzer (Ando, AQ6317C). The transmission
spectra in the wavelength range 1400–1650 nm areshown in Fig. 2 for two coil pitches of the spring, 0.6
and 0.625 mm. As shown in the figure, three attenuation bands appeared in this wavelength range.
They are apparently the same as those of mechanical
LPFGs reported in Refs. 4 and 5. Here we call themFig. 2. Measured transmission spectra of LPFGs with gratingperiods ⌳ ⫽ 0.6 and 0.625 mm.

Fig. 3. Resonance wavelength for several grating periods.
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Fig. 4. Measured transmission spectra of LPFG with ⌳⫽ 0.593 mm and several weights from 1.5 to 4.0 kg.

spectra of the LPFG with grating period ⌳ fixed at0.593 mm and with several impressing weights from1.5 to 4.0 kg on the fiber. The measured spectra arefor the third attenuation band. The amount of attenuation increases as the impressing weight is increased. It should be noted that the central value ofthe resonance wavelength does not change for different weights.
3. Thermal Control of Resonance Wavelength

We studied, first, the dependence of the resonancewavelength on the ambient temperature by placing
the device shown in Fig. 1 into a thermostatic oven.
Coil pitch, that is, grating period ⌳, was set to0.58 mm. The light from the amplified spontaneousemission light source was passed through the LPFG,
and the transmission spectra were monitored in situ
by the optical spectrum analyzer. The LPFG device’stemperature in the thermostatic oven was changed inincrements of 20 °C from 0 °C to ⫹100 °C, and theevolution of the third attenuation band was measured. The temperature dependence of the LPFG resonance wavelength is shown in Fig. 5. The amount ofthe shift of the resonance wavelength is 6 nm for thetemperature change from 0 °C to ⫹100 °C, giving
0.060 nm兾°C, which is close to the amounts of shift
reported previously.1,11,12,13 Our results show that weFig. 6. Measured transmission spectra of LPFG for several ambient temperatures with the fiber mount temperature fixed at60 °C.

can fine tune the wavelength as much as 6 nm bytemperature control of the LPFG, making it continuously tunable over 200 nm. Next we studied theeffects of ambient temperature on the LPFG maintained at a constant temperature by local heating. We
heated the LPFG to a given fixed temperature,changed the ambient temperature, and measured thespectra to see whether the resonance wavelength andsignal attenuation remained the same. The LPFG ofFig. 1 was placed on a small hot plate, and the hotplate was put into a thermostatic oven. A temperature sensor was attached to the fiber mount of theLPFG for temperature monitoring. The grating period ⌳ was 0.58 mm, the same as in the experimentdescribed above.
The temperature in the thermostatic oven waschanged in increments of 20 °C from 0 °C to ⫹60 °C,
and the voltage supplied to the heater was adjusted
such that the temperature of the LPFG might be fixed
to 60 °C regardless of any ambient temperature. Thetransmission spectra are shown in Fig. 6 for ambienttemperatures. The resonant wavelengths were1560.16, 1560.16, 1560.24, and 1560.08 nm at 0 °C,20 °C, 40 °C, and 60 °C, respectively. The shapes of
all these spectra were nearly identical. This result
indicates that it is possible to control the temperatureof the LPFG section locally by use of an external
heating source independently of the ambient temperature.
4. Mechanically Induced Tunable Long-Period Fiber
Grating with Thermal Control

Fig. 5. Temperature dependence of LPFG with ⌳ ⫽ 0.58 mm.
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To change the resonance wavelength of the LPFG
mechanically by adjusting the spring, first we release
the weight pressing on the optical fiber and then wereadjust grating period ⌳ by changing the coil spacing of the spring. The weight is then reapplied. This
process gives discrete values of the resonance wavelengths. To ensure continuous rather than discrete
resonance wavelengths, we added the thermal control described in Section 3. It is possible to do thetuning thermally over 6 nm when the temperature of

Fig. 7. Range of resonance wavelengths for several ambienttemperatures.
Fig. 9. Measured transmission spectra for four ambient temperatures with adjustable weights.

the fiber mount is changed from 0 to 100 °C. The
LPFG that uses the spring can control the tuning
mechanically for more than 200 nm as describedabove, and its accuracy in setting the wavelength is
1 nm or greater. Hence the combination of the mechanical tuning with thermal tuning will give finecontinuous tuning for more than 200 nm.
As shown in Fig. 5, when grating period ⌳ is0.58 mm, we can control the resonance wavelengthfrom 1556.5 to 1562.5 nm by changing the temperature of the fiber mount from 0 to 100 °C. For therange of ambient temperature from 0 to 60 °C, therange of tunable control of the resonance wavelengthis shown in Fig. 7 by the hatched area. To confirm thisrange of resonance wavelengths, we changed the temperature of the fiber mount to as much as 100 °C fromthermostatic oven temperatures of initially 0, 20°,40°, and 60 °C and monitored ways in which thewavelength might vary. The results are shown by thethick lines in Fig. 7. For the oven temperature of⫹60 °C the wavelength has been controlled from1560 to 1562.5 nm, exactly as expected. Here thetemperature of the fiber mount was increased fromambient temperature by heating. If a Peltier semiconductor cooling element is used, the controllable
range should also be easily extendable to the opposite
direction, i.e., a colder temperature covering the entire range from 0 to 100 °C.
Figure 8 shows measured transmission spectrawith grating period ⌳ ⫽ 0.58 mm with the weight on

the fiber fixed at 3.2 kg and at ambient temperaturesof 0, 20°, 40°, and 60 °C. As the temperature ischanged, not only the resonance wavelength but also
the amount of attenuation has changed. The amountby which the attenuation has decreased as the temperature is raised is shown in the figure. The temperature dependence of resonance wavelength andattenuation varies with experimental conditions, asreported in Refs. 7, 8, and 14. The variations areattributed to the differences in fiber doping duringfiber fabrication and preparation and to the methodby which the LPFG is made. It is important to control
the amount of attenuation without affecting the resonance wavelength.
Figure 9 shows the results after the weights wereadjusted to have the same amount of attenuation fordifferent temperatures: The weights added to that at0 °C are ⫹40, ⫹90, and ⫹160 g at 20 °C, 40 °C, and
60 °C, respectively. These values of weights are inagreement with the relation that the amount of attenuation is a quadratic function of weight.10 If thedependence of the amount of attenuation on temperature is already known, the pressure on the fiber canbe programmed and applied automatically.
Our LPFG has an inherently asymmetric structure, which can be sensitive to polarization. This sensitivity may affect some applications of the device. We
intend to investigate these potential effects.
5. Discussion and SummaryFig. 8. Measured transmission spectra for four ambient temperatures with a fixed weight.

It has been demonstrated that the tunable longperiod fiber gratings can control both resonancewavelength and signal attenuation. The unique feature of this LPFG is that it is formed only by the forceapplied to an optical fiber by a spring, unlike theconventional LPFG, for example, which is fabricatedby exposure to a UV laser. The spring method has twomajor advantages: One is that the resonance wavelength can easily be selected and readjusted by thespring and the other is that the signal attenuationlevel can be adjusted by a change in the pressure of
the optical fiber. In the conventional LPFG, the modulation period and the modulation amplitude of thegrating are both permanently inscribed in the fiber,
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so it is difficult to change both the resonance wavelength and the signal attenuation level. Furthermore, our LPFG has both an extremely wide tunable
range of more than 200 nm in the resonance wavelength and the capability of continuous tuning overthat wavelength range. These ranges were achievedby a combination of mechanical tuning and thermaltuning.
In an actual implementation of this LPFG in various applications, we propose the following device
packaging: In the device configuration of Fig. 1, theweight pressing on the optical fiber may be replaced
by a metal plate, which is tightened by screws to thefiber mount. The amount of signal attenuation levelcan be controlled by adjustment of the tightening
force by screws. Grating period ⌳ can be controlledeasily by a small fixture on the fiber mount that
adjusts the length of the spring.
Although temperature control was done in thepresent experiment by use of a hot plate, a small
heater can be installed in the fiber mount and, if
desired, a cooling plate can be added underneath thefiber mount. This combination with a temperature
sensor attached to the fiber mount may permit automatic computer control.
We thank W. D. L. Finn, Faculty of Engineering,
Kagawa University, for his careful proofreading ofthe manuscript. We also thank the reviewers for theirvaluable suggestions for revising this manuscript.
References
1. A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T.
Erdogan, and J. E. Sipe, “Long-period fiber gratings as bandrejection filters,” J. Lightwave Technol. 14, 58 – 65 (1996).
2. B. H. Kim, T.-J. Ahn, D. Y. Kim, B. H. Lee, Y. Chung, U.-C.
Paek, and W.-T. Han, “Effect of CO2 laser irradiation on therefractive-index change in optical fibers,” Appl. Opt. 41, 3809 –3815 (2002).
3. Y. J. Rao, T. Zhu, Z. L. Ran, Y. P. Wang, J. Jiang, and A. Z. Hu,

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APPLIED OPTICS 兾 Vol. 44, No. 24 兾 20 August 2005

“Novel long-period fiber gratings written by high-frequency
CO2 laser pulses and applications in optical fiber communication,” Opt. Commun. 229, 209 –221 (2004).
4. S. Savin, M. J. F. Digonnet, G. S. Kino, and H. J. Shaw,“Tunable mechanically induced long-period fiber gratings,”
Opt. Lett. 25, 710 –712 (2000).
5. G. Rego, J. R. A. Fernandes, J. L. Santos, H. M. Salgado, andP. V. S. Marques, “New technique to mechanically induce longperiod fibre gratings,” Opt. Commun. 220, 111–118 (2003).
6. D. M. Costantini, C. A. P. Muller, S. A. Vasiliev, H. G. Limberger, and R. P. Salathe, “Tunable loss filter based on metalcoated long-period fiber grating,” IEEE Photon. Technol. Lett.
11, 1458 –1460 (1999).
7. Y. Jiang, Q. Li, C.-H. Lin, E. Lyons, I. Tomov, and H. P. Lee, “Anovel strain-induced thermally tuned long-period fiber grating
fabricated on a periodic corrugated silicon fixture,” IEEE Photon. Technol. Lett. 14, 941–943 (2002).
8. J. K. Bae, S. H. Kim, J. H. Kim, J. Bae, S. B. Lee, and J.-M.
Jeong, “Spectral shape tunable band-rejection filter using along-period fiber grating with divided coil heaters,” IEEE Photon. Technol. Lett. 15, 407– 409 (2003).
9. M. I. Braiwish, B. L. Bachim, and T. K. Gaylord, “PrototypeCO2 laser-induced long-period fiber grating variable opticalattenuators and optical tunable filters,” Appl. Opt. 43, 1789 –1793 (2004).
10. T. Yokouchi, Y. Suzaki, K. Nakagawa, S. Ejima, M. Yamauchi,
M. Kimura, Y. Mizutani, and S. Kimura, “Fabrication andestimation of mechanically induced tunable long-period fibergrating,” IEICE Trans. Electron. J88-C 1, 22–27 (2005).
11. V. Bhatia and A. M. Vengsarkar, “Optical fiber long-periodgrating sensors,” Opt. Lett. 21, 692– 694 (1996).
12. L. Qin, Z. X. Wei, Q. Y. Wang, H. P. Li, W. Zheng, Y. S. Zhang,and D. S. Gao, “Compact temperature-compensating package
for long-period fiber gratings,” Opt. Mater. 14, 239 –242 (2000).
13. C.-H. Lin, Q. Li, A. A. Au, Y. Jiang, E. Wu, and H. P. Lee,
“Strain-induced thermally tuned long-period fiber gratings
fabricated on a periodically corrugated substrate,” J. Lightwave Technol. 22, 1818 –1827 (2004).
14. M. N. Ng and K. S. Chiang, “Thermal effects on the transmission spectra of long-period fiber gratings,” Opt. Commun. 208,
321–327 (2002).