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Vol. 56, No. 18 / June 20 2017 / Applied Optics
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Characteristics of chiral long-period fiber gratings written in the twisted two-mode fiber by CO2 laser XIBIAO CAO, YUNQI LIU,* LIANG ZHANG, YUNHE ZHAO,
AND
TINGYUN WANG
Key Lab of Specialty Fiber Optics and Optics Access Networks, School of Communication and Information Engineering, Shanghai University, Shanghai 200072, China *Corresponding author:
[email protected] Received 24 February 2017; revised 3 May 2017; accepted 23 May 2017; posted 24 May 2017 (Doc. ID 287484); published 14 June 2017
We demonstrated the fabrication of a chiral long-period grating (CLPG) by twisting a two-mode fiber (TMF) when a CO2 laser beam was sweeping along the fiber axis. The torsion, temperature, and surrounding refractive index characteristics of the fabricated TMF-CLPG were investigated experimentally. The fabricated TMF-CLPG has a high torsion sensitivity [0.7768 nm/(rad/m)], and can measure the twist rate and twist direction simultaneously. This kind of CLPG would have great potential applications in high-sensitivity optical sensors. © 2017 Optical Society of America OCIS codes: (060.2310) Fiber optics; (060.2370) Fiber optics sensors; (350.2770) Gratings. https://doi.org/10.1364/AO.56.005167
1. INTRODUCTION A long-period fiber grating (LPFG) is a passive component that consists of periodic index modulation with a pitch of hundreds of micrometers [1]. The conventional LPFG couples light from the core mode to different cladding modes at a certain resonance wavelength, and a series of attenuation bands are formed in the transmission spectrum. LPFGs have a wide range of applications, such as sensors and filters. Wang et al. measured the torsion sensitivity of a CO2 -laser-written LPFG, and the sensitivity value is 0.0645 nm/(rad/m) [2]. Different long-period gratings have been proposed for the torsion measurements [3–6]. Much attention has been paid on the helical LPFGs due to their potential application on filters, polarizers, and sensors [6–9]. The asymmetric refractive index (RI) modulation is formed along the fiber axis in the gratings due to their helical structure. We demonstrated the fabrication of a chiral long-period grating (CLPG) in a conventional single-mode fiber (SMF) for the mode coupling between fundamental core mode and cladding modes [10]. The cladding modes are found to be not only sensitive to torsion, but also sensitive to temperature and surrounding refractive index changes. In this paper, we demonstrate the fabrication of a CLPG by twisting a two-mode fiber (TMF) when a CO2 laser beam was sweeping along the fiber axis. The torsion sensitivity of the TMF-CLPG is 0.7768 nm/(rad/m), which is 1 order magnitude higher than that of conventional LPFGs. The mode coupling occurs between the core modes (LP01 and LP11 ) of the TMF-CLPG. Compared with the 1559-128X/17/185167-05 Journal © 2017 Optical Society of America
SMF-CLPG, the TMF-CLPG is much more sensitive to torsion, but insensitive to temperature and surrounding refractive index changes. Therefore, the TMF-CLPG could have great potential applications on high-sensitivity torsion measurement with lower cross sensitivity to temperature and surrounding refractive index. 2. EXPERIMENTAL RESULTS AND DISCUSSION The experimental setup of TMF-CLPG inscription with a CO2 laser is shown in Fig. 1. The CO2 laser beam was focused by a lens on the optical fiber at a light spot with a diameter of ∼100 μm. The laser beam scans repeatedly and continuously along the fiber axis to ensure the fiber can be heated uniformly and melted enough. An attenuator is used to adjust the intensity of the laser. The output energy of the CO2 laser is about 1 W. The TMF is fixed on a mobile platform with a clamping device consisting of a fixed holder and a rotated holder, and the rotated holder is controlled by the rotary engine. The transmission spectrum of the TMF-CLPG is monitored with a broadband source and an optical spectrum analyzer (OSA, AQ6370B, YOKOGAWA). The TMF is heated to melt and rotated simultaneously by the rotary holder, which will induce a spiral refractive index modulation in the fiber structure. The rotational speed of the rotated holder, and the scanning speed and energy of the CO2 laser beam are precisely controlled by a PC program. Figure 2 shows the transverse RI profile of the TMF (twomode step-index fiber, OFS) measured by a RI profiler (S14, Photon Kinetics) at the wavelength of 1550 nm. The TMF
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Laser
PC
Attenuator
Rotary engine SMF
TMF
SMF
Fixed holder BBS
Rotated holder
Mobile platform
Mobile platform
OSA
Fig. 1. Experimental setup of a TMF-CLPG inscription with a CO2 laser.
0.020 0
(a) 0.016
-5
Transmission (dB)
Index Difference
0.024
The transmission spectrum of the CLPG is shown in Fig. 3(a), and the resonance wavelength is 1603 nm. The mode pattern was measured by the CCD camera (InGaAs camera, Model C10633-23 from Hamamtsu Photonics). The resonance can be identified to be the LP11 mode. The record picture of Fig. 3(b) shows the profile and interference pattern at the resonance wavelength. The interference pattern was measured using the same experimental setup shown in Fig. 11 of [12]. The CLPG can achieve a grating contrast of more than 20 dB for a 1-cm-length grating, which means that the LP01 mode is almost completely coupled into the LP11 mode by the TMF-CLPG. Figure 3(c) shows the polarization-dependent loss (PDL) of the TMF-CLPG, measured by the optical component analyzer (N7788BD, Agilent). The maximum of the PDL is 6.7 dB, which indicates that the TMF-CLPG has higher polarization dependence. The high PDL is consistent with the asymmetric index modulation of the grating, which is necessary for the mode coupling between the LP01 and LP11 modes.
0.012 -60
-40
-20
0
20
40
60
Position (μm)
Fig. 2. Transverse RI profile of a TMF measured by S14.
-10
-15
-20
has a cladding index of 1.444 and a core index of 1.449, which can support LP01 and LP11 modes. The diameters of the fiber core and cladding are 19 μm and 125 μm, respectively. Because the TMF was exposed to the CO2 -laser beam on one side, laser energy is attenuated and gradually absorbed by the fiber along the emission direction. The asymmetrical refractive index modulation can be induced across the fiber section. The mode coupling between the LP01 and LP11 modes can be achieved by the CLPG. The phase matching conditions can be expressed as [11,5] 11 λres n01 eff − neff Λ;
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(b)
(c)
and are the effective index of the core modes of where LP01 and LP11 , respectively. Δβ β01 − β11 is the difference in propagation constant for the two core modes. LB is the beat length of the TMF. The period of the fabricated CLPG can be expressed as [4]
Transmission (dB)
(2)
n11 eff
Λ L∕N ;
8
0
6 -10 4 -20
PDL (dB)
n01 eff
1640
(1)
or 2π Λ LB ; Δβ
1600
Wavelength (nm)
2
(3)
where Λ is the period of the CLPG, L is the length of the twisted fiber section, and N is the number of rotation turns. In experiment, the length of the twisted fiber section and the number of turns was 1 cm and five turns, respectively. Therefore, the period of the CLPG is about 2000 μm.
-30
0 1560
1600
1640
Wavelength (nm)
Fig. 3. (a) Transmission spectrum of the TMF-CLPG, (b) profile and interference pattern at the resonance wavelength, (c) PDL of the TMF-CLPG.
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Vol. 56, No. 18 / June 20 2017 / Applied Optics
The torsion characteristics of the TMF-CLPG were investigated experimentally. One end of the TMF-CLPG is fixed on the stationary holder, and the other end is fixed on the rotated holder with angle used to provide torsional stress for the TMFCLPG. We define the positive and negative angles when the TMF-CLPG is twisted by clockwise (CW) and counterclockwise (CCW) directions, respectively. The twist angle range of the rotated holder is from −360° to 360°, and the OSA records a spectrum when the twist angle each additional 30°. The twist rate of the grating can be expressed as [13,14] πβ ; (4) τ 180L where τ (unit is rad/m) is the twist rate, β is the twist angle, L is the distance between the rotated holder and the fixed holder. Figure 4(a) shows the transmission spectra of the TMFCLPG with different twist rates. Figure 4(b) shows the dependence of the resonance wavelength on the twist rate. The resonance wavelength shifts linearly toward longer wavelengths when the TMF-CLPG is twisted CW, while it shifts linearly toward shorter wavelengths when the TMF-CLPG is twisted CCW. The torsion sensitivity of the TMF-CLPG is 0.7768 nm/(rad/m), which is 1 order magnitude higher than that of conventional LPFGs [2]. This is because little index perturbation is induced in the fiber and the grating period almost has no change during the twisting for conventional LPFGs due to the uniformity of the optical fiber structure. Compared with
conventional LPFGs, the TMF-LPFG also has a larger grating period when the mode coupling occurs at the same resonance wavelength [15]. When the same twist is applied, the TMFCLPG has the higher twist sensitivity due to the larger grating period. Therefore, compared with the SMF-CLPG [16], the TMF-CLPG has higher twist sensitivity. In addition, compared with the helical LPFGs written in a two-mode fiber by a CO2 laser [15], the TMF-CLPG retained higher torsional stress in the fiber structure. Hence, the TMF-CLPG has higher twist sensitivity. The axial strain characteristics of the TMF-CLPG were studied experimentally. One end of the TMF-CLPG is fixed on the mobile platform, the other end is fixed on a strain sensor. The strain gradually increased from 0 to 1116 με, and the OSA recorded the change of the transmission spectrum simultaneously. Figure 5 shows that the resonance wavelength shifts linearly toward longer wavelengths and the transmission loss gradually decreases as the strain increases. The strain sensitivity of the resonance wavelength and transmission loss is 3.76 pm/με and 0.00435 dB/με, respectively, which is an order magnitude higher than that of the conventional LPFGs [17]. Figure 6 shows linear correlation between the resonance wavelength and the curvature. The bending sensitivity of the TMF-CLPG is 12.409 nm∕m−1 . The resonance wavelength shifts linearly toward longer wavelengths with the increase of the curvature. Because the generated refractive index 0
(a) 0
-5
Transmission (dB)
-5
Transmission (dB)
-10 -15 -20 -25
-360 -120 +180
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-240 +60 +300
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-180 +120 +360
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4
1600 Wavelength (nm)
1620
1640
-17
λ=1601.8+0.7768τ
CW Twisted
1600 1590
y=0.00376x+1.29
3 2
-18
-19
y=0.00435x-20.19
1
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0
CCW Twisted -30 -25 -20 -15 -10 -5
Wavelength shift (nm)
Wavelength (nm)
1580
-16 5
1580
111.6 334.8 558.0 781.2 1004.4
(b)
Wavelength (nm)
(b) 1630
1610
0 223.2 446.4 669.6 892.8 1116
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Transmission (dB)
(a)
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-21 0
5
10 15 20 25 30
Twist rate (rad/m)
Fig. 4. (a) Transmission spectra of the TMF-CLPG with different twist rates, (b) dependence of the resonance wavelength on the twist rate.
0
200
400
600
800
1000
Strain (με)
Fig. 5. (a) Transmission spectra of the TMF-CLPG with different strains, (b) dependence of the resonance wavelength on the applied strain.
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Transmission (dB)
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0
-10
0 2.10 2.95 3.58 4.14 4.64 5.08
-15 -20 -25
1.50 2.50 3.27 3.87 4.40 4.86
-10 -15 20°C 40°C 60°C 80°C 100°C 120°C 140°C
-20 -25 -30
1560
1600
1640
1560
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Wavelength (nm)
50
(b)
20
Wavelength shift (nm)
Wavelength (nm)
(b)
30°C 50°C 70°C 90°C 110°C 130°C
10
Wavelength shift (nm)
40
y=12.409x-4.94
30 20 10
0
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0
1
2
3
4
5
-1
20
40
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80
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Temperature (°C)
Curvature (m )
Fig. 6. (a) Transmission spectra of the TMF-CLPG with different curvatures, (b) dependence of the resonance wavelength on curvature of the grating.
Fig. 7. (a) Transmission spectra of the TMF-CLPG with different temperatures, (b) dependence of the resonance wavelength on temperature.
perturbation increases with the increases of the curvature, the resonance wavelength changes with the increase of the curvature. The temperature sensitivity of the TMF-CLPG was measured experimentally by putting the fabricated gratings in the temperature-controlled heating box, whose temperature varied from 20°C to 140°C with a step of 10°C. Figure 7 shows the changes of the resonance wavelength with the increase of temperature. The experimental results show that the TMF-CLPG has lower temperature sensitivity, which can be attributed to the fact that the core modes of the TMF have lower temperature sensitivity. The refractive index sensitivity of the TMF-CLPG was also studied, and the experimental result shows that the grating is insensitive to the index change, which can be attributed to the fact that the core modes of the TMF are insensitive to the surrounding refractive index change [15].
Funding. National Natural Science Foundation of China (NSFC) (61377083).
3. CONCLUSION We demonstrate the fabrication of the CO2 -laser-written TMF-CLPG, which can be used as a high sensitive fiber torsion sensor. The experiments show that the TMF-CLPG has higher torsion sensitivity and lower temperature sensitivity. And the torsion directions can be measured by recording the drifting direction of the resonance wavelength.
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