RESEARCH ARTICLE

Fabrication and characterization of ultralong period reversiblefiber gratings

Sunita Pandit Ugale & Vivekanand Mishra

Received: 13 August 2012 /Accepted: 30 January 2014# The Optical Society of India 2014

Abstract We report here for the first time the characterizationof mechanically induced ultralong period fiber reversible grat-ings (MULPFG) with period size up to several millimeters. Inthese gratings the coupling of the fundamental guided coremode takes place with cladding modes of high diffractionorders. The transmission characteristic of grating with differ-ent periods and different external applied pressure has beenexperimentally verified.

Keywords MULPFG . Reversible grating . Pressure sensing

Introduction

Long period gratings have emerged as important componentin a variety of light wave applications such as band rejectionfilter [1], wavelength selective attenuator, dispersion compen-sator, multichannel filters in WDM applications and gainflatteners for Erbium doped fiber amplifiers [2]. These grat-ings are also very suitable for various sensing applications.Sharp filtering characteristics, ease of fabrication, direct con-nectivity to fiber, high sensitivity to external parameters, andeasiness in adjusting the resonant wavelength by simplyadjusting the grating period are the strong points to pushtowards the detail study of this device. The period of a typicallong period fiber grating (LPFG) ranges from 100 μm to1,000 μm [1]. The LPFGs with periods exceeding one milli-meter are called ultralong period fiber gratings (ULPFG). Thelong period size makes fabrication of ULPFG very easy aswell as cheap.

ULPFG can be induced optically or mechanically [3–5].Optically induced gratings are permanent, whereas mechani-cally induced gratings are reversible. Xuewen Shu et al. re-ported fabrication and characterization of ULPFG for the firsttime in 2002 by using point-by-point writing technique with244 nm UV beam from a frequency doubled Argon ion laser[3]. An ultralong period fiber grating with periodic groovestructure (G-ULPFG) fabricated by using an edge-writtenmethod with high-frequency CO2 laser pulses is reported byTao Zhu and co-workers in 2009 [4].

We report here, for the first time to our knowledge, thefabrication and characterization of mechanically inducedULPFGs (MULPFG) with periods up to several millimeters.Mechanically induced long period fiber gratings (MLPFG)andMULPFG induced by pressure need neither a special fibernor an expensive writing device for fabrication. These grat-ings also offer advantages of being simple, inexpensive, eras-able, and reconfigurable and also gives flexible control oftransmission spectrum,

Theory

ULPFG is a special case of LPFG. In LPFG the core LP01mode is coupled with cladding modes having same symmetry,namely LP0m modes [6]. Whereas in ULPFG the coupling ofthe fundamental guided core mode to the cladding modes ofhigh diffraction orders takes place [7]. The phase matchingcondition for a high diffraction order grating is given by (1).

λres ¼ ncoeff −ncl;meff

� � ΛN

ð1Þ

Where λres is the resonant wavelength, neffco and neff

cl,m areeffective indexes of fundamental core mode and mth cladding

S. P. Ugale (*) :V. MishraS. V. National Institute of Technology, Surat, Gujarat, Indiae-mail: spu.eltx@gmail.com

V. Mishrae-mail: vive@eced.svnit.ac.in

J OptDOI 10.1007/s12596-014-0201-1

mode of Nth diffraction order respectively. Λ is the gratingperiod and N is the diffraction order. N=1 for LPFG.

The resonant wavelength with the variation in the effectiveindexes of the core and cladding ignoring the dispersion effectis given by (2).

λ0res ¼ ncoeff −n

cl;meff

� � ΛN

� 1þδncoeff −δn

cl;meff

� �� dλres

dΛ

ncoeff −ncl;meff

� �2

264

375 ð2Þ

Where λ'res is the resonant wavelength with variation in theeffective indexes of core and cladding, δneff

co and δneffcl,m are the

effective index changes of the fundamental core mode andmth cladding mode of the Nth diffraction order

Experiment

Reversible MLPFG and MULPFG of different periods rang-ing from several hundred microns to several millimeters wereinduced and characterized.

The gratings were induced in single mode fiber SMF28with core diameter of 9 μm and cladding diameter of 125 μm.A special probe with unjacketed fiber of 10 cm length at thecenter and APC connectors at both ends was prepared.

The experimental set up as shown in Fig. 1 was prepared.The light from broadband superluminacent LED with out-

put power of −8dBm, center wavelength 1,530 nm and

bandwidth of 69 nm was passed through the grating undertest and the transmitted signal was analyzed with the help ofoptical spectrum analyzer covering the wavelength range from1250 to 1650 nm.MLPFGwith period 600 μm andMULPFGwith period 1200 μm and 1800 μm were induced andcharacterize.

The response of these gratings to external applied pressurewas also studied.

Results

The transmission characteristics for MLPFG with period600 μm and MULPFG with period 1,200 μm and 1,800 μmare shown in Fig. 2. and the results are summarized in Table 1.

Fig. 1 Experimental set up forinducing MULPFG and itscharacterization

Fig. 2 Transmission curve for MLPFG and MULPFG

Table 1 Resonance wavelength for gratings with different periods

Grating Period in μm Resonance wavelength for different periods innm

Mode1 Mode2 Mode3

1 600 1,466 1,510 1,595

2 1,200 1,475 1,521 1,605

3 1,800 1,500 1,555 –

Fig. 3 Transmission loss to external applied pressure for grating havingperiod of 600 μm

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The experimental results showed that the resonance wave-lengths are mainly affected by the period of grooved plate.Thus important information regarding the required gratingperiod for a particular application can be extracted fromFig. 2. Several resonance wavelengths fall in the third tele-communication window, this technique offers a straightfor-ward way to produce filters in an important wavelength rangefor communication applications.

The transmission spectra of MLPFG of period 600 μmwith different pressure applied on it is shown in Fig. 3 and

the results are summarized in Table 2. High pressure givesdeep notch in the transmission spectrum.

Similarly the transmission spectra ofMULPFGwith period1,200 μm and 1,800 μm with different pressure applied on itare shown in Figs. 4 and 5 respectively..

In MULPFG the transmission loss increases with appliedpressure similar to grating 1 (MLPFG) but here we alsoobserved the shift in resonance wavelength. The results aresummarized in Tables 3 and 4.

Fig. 5 Transmission loss to external applied pressure for grating havingperiod of 1,800 μm

Table 3 Transmission loss to applied pressure for grating 2

External appliedpressure (P) inN/mm2

Transmission loss in dBm forgrating1 (period Λ=600 μm)

Resonancewavelength in nmfor mode 2

0.028 54.5 1,512

0.035 55.5 1,514

0.042 56.0 1,515

0.049 57.0 1,517

0.052 57.5 1,519

0.053 58.0 1,520

0.054 58.4 1,521

Table 4 Transmission loss to applied pressure for grating 3

External appliedpressure (P) inN/mm2

Transmission loss in dBm forgrating1 (period Λ=600 μm)

Resonancewavelength in nmfor mode 2

0.028 53.8 1,534

0.035 55.0 1,538

0.042 56.2 1,546

0.049 57.2 1,550

0.054 58.4 1,555

Fig. 6 Relative change in resonance wavelength with change in externalapplied pressure

Table 2 Transmission loss to applied pressure for grating 1

External applied pressure(P) in N/mm2

Transmission loss in dBm for grating1 (periodΛ=600 μm)

Mode1 Mode2 Mode3

0.035 62.0 52.5 44.5

0.042 63.0 55.0 45.7

0.049 63.8 57.5 48.5

0.056 64.4 59.0 49.0

Fig. 4 Transmission loss to external applied pressure for grating havingperiod of 1,200 μm

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Relative change in resonance wavelength with changein applied pressure for grating 2 and 3 are plotted inFig. 6.

Conclusion

MLPFG of 600 μm period and MULPFG with1,200 μm and 1,800 μm were induced successfully inSMF28 fiber. For these gratings it has been experimen-tally verified that the resonance wavelength is mainlydefined by the period of the grooves. In the transmis-sion spectra of MLPFG of 600 μm period, resonant losspeaks with strengths of upto 7 dB have been observed.Whereas for MULPFG with periods of 1,200 μm and1,800 μm, we observed significant shift of 20 nm inresonance wavelength. MULPFG with greater period ismore sensitive to external applied pressure. ThusMLPFG can be used as band rejection filter, wavelengthselective attenuator, gain flatterers for optical amplifier

and MULPFG can be used as a sensor for pressure,temperature or RI.

References

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2. A. Sohn, J. Song, Gain flattened and improved double pass two stageEDFA using microbending longperiod fiber gratings”. Opt. Commun.236(1–3), 141–144 (2004)

3. X. Shu, L. Zhang, I. Bennion, Fabrication and characterization ofultralong period fiber gratings. Opt. Commun. 203, 277–281 (2002)

4. T. Zhu, Y. Song, Y. Rao, Y. Zhu, Highly sensitive optical refractometerbased on edge-written ultralong period fiber grating formed by peri-odic grooves, IEEE Sensors J. 9(6) (2009)

5. S. Savin, M. Digonnet, G. Kino, H. Shaw, Tunable mechanicallyinduced long-period fiber gratings. Opt. Lett. 25(10), 710–712 (2000)

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