Polarimetric multi-mode tilted fiber grating sensors

Tuan Guo,1 Fu Liu,1 Bai-Ou Guan,1,*and Jacques Albert2 1Institute of Photonics Technology, Jinan University, Guangzhou 510632, China

2Department of Electronics, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada *tguanbo@jnu.edu.cn

Abstract: The polarimetric sensing characteristics of multi-mode-fiber based tilted fiber Bragg gratings (MMF-TFBG) have been analyzed and experimentally demonstrated. The larger diameter fiber core and graded index core/cladding profile enable the tilted gratings to excite multiple high-order core modes with significantly different polarization dependence and to form a well-defined “comb” of spectrally separated resonances at different wavelengths. Orientation-recognized twist/rotation measurements (−90° to 90°) have been achieved with sensitivity of 0.075 dB/deg by using intensity monitoring of two orthogonally polarized odd core-modes (LP11 and LP12). The proposed sensor is compact, works in reflection (a short section of MMF-TFBG spliced with a leading-in single mode fiber without any offset or tapering), is insensitive to temperature (intensity detection instead of wavelength monitoring) and is immune to unwanted intensity fluctuations (differential intensity measurement). Other TFBG sensing modalities, such as lateral pressure and surrounding refractive index are demonstrated separately with the same device configuration and interrogation principles.

©2014 Optical Society of America

OCIS codes: (060.2370) Fiber optics sensors; (060.0060) Fiber optics and optical communications.

References and links

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#203621 - $15.00 USD Received 26 Dec 2013; revised 12 Mar 2014; accepted 12 Mar 2014; published 21 Mar 2014(C) 2014 OSA 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007330 | OPTICS EXPRESS 7330

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1. Introduction

The tilted fiber Bragg grating (TFBG) is a device that possesses all the advantages of the well- established fiber Bragg grating (FBG) technology in addition to being able to excite cladding modes resonantly. TFBGs have been around almost as long as FBGs and used in many applications beyond sensing [1–3]. For more than one decade, research on TFBG has been mainly focused on single mode fiber (SMF) based configurations, including the in-fiber polarizer [4], vibroscope [5,6], accelerometer [7], bending sensor [8], inclinometer [9], refractometer [10–13] and surface plasmon resonance applications [14–16]. The grating tilt has the effect of locally breaking the cylindrical symmetry of the fiber in a way that allows strong coupling between the core guided light and a large number of (hundreds of) cladding modes. Since the cladding modes each have a unique mode field shape and effective index, they show strong polarization dependence and react differently to perturbations inside and outside of the fiber, and therefore open up a multitude of opportunities for single-point sensing in hard-to-reach spaces. Early research on the polarization dependence of TFBG was mainly focused on cladding modes. To interrogate such cladding modes in reflection, an additional cladding-to-core recoupling device (offset splicing [5,11] or fiber tapering [7,9]) should be added, which makes the sensor complex and reduces its mechanical strength. So it is worthy to transfer such polarization dependence from cladding modes to core modes, which simplifies the sensor configuration (making the cladding-to-core recoupling unnecessary), improves the signal quality (improved signal intensity and signal-to-noise ratio) and reduces the sensor in size (working in reflection to facilitate the insertion into materials to be sensed). A TFBG inscribed in multi-mode fiber (MMF) provides a potential solution due to the larger diameter fiber core, which permits multiple guided core modes with different polarization dependence and low transmission loss.

In contrast with the early reports of MMF-TFBG sensors that focused on the polarimetric analysis of cladding modes [17,18] and refractive index (RI) measurements [19–21], here in this paper, we transfer the above “polarimetric” sensing characteristics from the cladding modes to the core modes. Multiple high-order core modes with different polarization dependence can be excited by the tilted gratings inscribed in a MMF and they show a well-defined “comb” of resonances which are spectrally well separated from each other (this provides an easy way to isolate them at different wavelengths). Orientation-recognized twist/rotation measurement (−90° to 90°) has been achieved with a sensitivity of 0.075 dB/deg

#203621 - $15.00 USD Received 26 Dec 2013; revised 12 Mar 2014; accepted 12 Mar 2014; published 21 Mar 2014(C) 2014 OSA 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007330 | OPTICS EXPRESS 7331

by using power detection of two orthogonally polarized odd core-modes (LP11 and LP12) of 4° MMF-TFBG. Meanwhile, RI measurement with sensitivity higher than 500 RIU/nm has also been achieved by monitoring the “cut-off” resonance of the cladding modes of 12° MMF-TFBG. It is the combination of the physical sensing properties (curvature, twist/rotation, lateral pressure) with the clearly distinct biochemical (RI) response that opens up a multitude of opportunities for “multiparameter” sensing with a single kind of MMF-TFBG instead of trying to extract the same information from the complex mode structure of the SMF-TFBG.

2. Comparison between SMF-TFBG and MMF-TFBG

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Fig. 1. Comparison between SMF-TFBG and MMF-TFBG: (I) schematic diagram of SMF-TFBG (Δ = 0.36% step index profile with core/cladding diameters of 9/125 μm, tilted angle 4°) and (II) MMF-TFBG (Δ = 2% graded index profile with core/cladding diameters of 62.5/125 μm, tilted angle 4°); (a) experimental transmission spectra of 4° SMF-TFBG (with single core mode and strong claddings modes) and (b) 4° MMF-TFBG (with multiple core modes and weak cladding modes); (c) simulation spectra of ghost modes and core mode of SMF-TFBG with component azimuthal mode families LP0m and LP1n and that of (d) multiple core modes of MMF-TFBG, insets show the transverse electric field amplitude distributions of each mode.

Figure 1 gives a brief comparison between the SMF-TFBG and MMF-TFBG. Due to the introduction of a tilt angle between the UV laser beam and the fiber axis, TFBG provides an effective way couple the input light from the forward-propagating core mode to backward-propagating modes (or forward-propagating modes for the case when the tilt angle is more than 45° [22]). For fibers with different core diameters and refractive index profiles (SMF: Δ = 0.36% step index profile with core/cladding diameters of 9/125 μm; MMF: Δ = 2% graded index profile with core/cladding diameters of 62.5/125 μm), we experimentally get different transmission spectra for the grating inscription with the same tilted angle (4°). That is, the SMF-TFBG transmission consists of only single core mode (LP01) and a large number of strong cladding modes, while the MMF-TFBG transmission consists of several strong core modes and much compressed cladding modes (as shown in Fig. 1(a) and 1(b)). Early reports

#203621 - $15.00 USD Received 26 Dec 2013; revised 12 Mar 2014; accepted 12 Mar 2014; published 21 Mar 2014(C) 2014 OSA 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007330 | OPTICS EXPRESS 7332

[5,7] have demonstrated that the low order cladding modes in SMF-TFBG can be used for vibration or acceleration measurement, these modes are usually called the “ghost modes” (a group of low-order strongly guided cladding modes that interact much with the fiber core but little with the cladding boundary). However, one key limitation of previous work is that it is hard to indentify the vibration orientation due to the reason that the “ghost modes” are constituted by a complex spectral superposition of mode components that may have different polarization dependence, as the numerical mode simulation results show in Fig. 1(c). The “ghost modes” in fact are an envelope of low order cladding modes. On the contrary, the MMF-TFBG allows several high-order core-modes with “well-defined” (i.e. non-overlapping) resonances. Simulation results also show that these core modes are polarization-dependent, with a spectral distribution of odd and even modes separated from each other, as the insets (transverse electric field amplitude distributions of each core modes) show in Fig. 1(d). Modelling of the SMF-TFBG and MMF-TFBG with Optigrating (from Optiwave) shown in Fig. 1 (c) and 1(d) use the fiber parameters given above for the SMF and MMF, and the same TFBG inscription condition for both (4 degree tilted grating with a grating period of 537 nm over 10 mm and with a sinusoidal refractive index modulation, the grating is inscribed by well-established phase-mask technique using a 193 nm UV light with 3mJ per pulse at a frequency of 200Hz). Excellent agreement is obtained and allows for the identification of the LP0m and LP1n resonances. The mode field profiles associated with these resonances clearly show that the LP1n modes do not possess cylindrical symmetry and that their coupling response to changes in input polarization and twist/rotation will be greatest.

3. MMF-TFBG characteristics

The MMF-TFBG sensor was manufactured using the techniques reviewed in [3], i.e. in a commercial multi-mode fiber (Corning, OM1 type) with a TFBG inscribed in the fiber core. The TFBG was manufactured using the phase-mask technique by using 193 nm UV light. The grating planes were written with a certain tilt relatively to the longitudinal axis of the fiber (Fig. 1). The tilt of the grating is an important parameter that can be used to choose which set of modes (core or cladding modes, as shown in Fig. 2(a)) is going to be excited. As a result, it makes it possible to adjust the operating range of the sensor in order to optimize the response for certain RI measurement (using large tilt angle to excite high-order cladding modes with strong evanescent field) or mechanical measurement (using small tilt angle to achieve multi-core-modes with good polarization dependence). The separation between core modes and cladding modes of MMF-TFBG in transmission can be clearly indentified, as the red dotted line marked in Fig. 2(a). It is important to note that the mode distribution in the MMF is controlled by the upstream splice with the SMF and therefore very stable, as long as the MMF section is short enough (here about 12 mm) and the splice point undisturbed (in the lateral pressure experiments for instance).

Figure 2(b) presents the RI response of 12° MMF-TFBG over a set of sucrose solutions with RI range from 1.41 to 1.44. By monitoring the wavelength shift of the “cut-off” resonance (the boundary between guided and leaky modes which is characterized by a sharp decrease in transmission loss, as the red arrows marked in Fig. 2(b), a linear sensitivity of ~570 nm/RIU (refractive index unit) has been achieved. Figure 2(c) presents the lateral pressure response of 4° MMF-TFBG. Two orthogonal core resonances, i.e. LP11 and LP12, only change when the fiber is pressed along a specific direction relative to the X- and Y-axis respectively, which agrees well with the theoretical considerations provided earlier and provides evidence of their potential for orientation-recognized measurement. Meanwhile, the LP0m modes are really insensitive to the fiber lateral pressure (and its orientation) and can be used as references to remove the intensity fluctuations.

#203621 - $15.00 USD Received 26 Dec 2013; revised 12 Mar 2014; accepted 12 Mar 2014; published 21 Mar 2014(C) 2014 OSA 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007330 | OPTICS EXPRESS 7333

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Fig. 2. MMF-TFBG characteristics: (a) experimental transmission spectra of MMF-TFBG with the tilt angle range from 2° to 16°, (b) spectral response (cladding modes in transmission) of 12° MMF-TFBG versus surrounding RI, (c) spectral response (core modes in reflection) of 4° MMF-TFBG versus two orthogonal lateral pressure (X- and Y-axis).

4. Vector rotator sensor

Figure 3 shows the schematic diagram of an orthogonal-polarimetric vector rotation sensing system based on a MMF-TFBG. The implementation is somewhat similar to that used in earlier devices based on a high-birefringence fiber [23] or a regular FBG in a polarization-maintaining fiber [24]. The advantage here is that the multiple core modes provide stronger reflected resonances that are more widely separated and hence result in increased signal to noise ratio, and also that a polarization splitter with dual detectors are not required. The reflective sensor probe here is made up of a short section of MMF (12 mm in length, inscribed with a TFBG) spliced back to a 1m long piece of SMF without any offset. Light from a broadband source (BBS) was linearly polarized via polarizer and polarization control (PC) and then launched into the MMF-TFBG sensor through a 3 dB coupler, and the reflection was monitored by an optical spectrum analyzer (OSA) with a resolution of 0.02 nm.

Figure 4 (a) shows the reflected spectral response of MMF-TFBG versus rotation over 0° to 90°, with the polarization control system lined up to the strongest LP11 or LP12 (always found at 90 degrees from each other). Clearly, the twist angle can be accurately measured by

#203621 - $15.00 USD Received 26 Dec 2013; revised 12 Mar 2014; accepted 12 Mar 2014; published 21 Mar 2014(C) 2014 OSA 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007330 | OPTICS EXPRESS 7334

Fig. 3. Schematic diagram of orthogonal-polarimetric vector rotation sensing system.

monitoring the intensity (reflection intensity) of the two orthogonal-polarimetric core modes (LP11 and LP12), as the two resonances colored in green and yellow. By calculating the peak to peak differential intensity (Ip-p) of these two orthogonal-polarimetric core modes, as presented in Fig. 4(b), rotations from 0° to 90° (clockwise) and 0° to −90° (anticlockwise) can be unambiguously measured, with minimized intensity fluctuations at other resonances. Linear response for clockwise rotation (increased Ip-p) and for anticlockwise rotation (decreased Ip-p) has also been achieved.

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Fig. 4. Spectral response of orthogonal-polarimetric MMF-TFBG core-modes: (a) reflection spectra of rotation over 0° to 90°, (b) differential spectral response (Ip-p) versus rotation, (c) differential intensity response of orthogonal-polarimetric core modes (LP11 and LP12) versus clockwise and anticlockwise rotation.

5. Conclusion

The polarimetric characteristics of a MMF-TFBG sensor were demonstrated and its applications for sensing lateral pressure magnitude and direction, as well as unambiguous twist/rotation measurements have been experimentally demonstrated. The additional presence of cladding modes further allows high RI sensitivity (similar to that of the SMF-TFBG). The proposed MMF-TFBG sensor is compact (with a simple configuration), works in reflection (a short section of MMF-TFBG spliced with a leading-in single mode fiber without any offset or tapering), is insensitive to temperature (intensity detection instead of wavelength monitoring)

#203621 - $15.00 USD Received 26 Dec 2013; revised 12 Mar 2014; accepted 12 Mar 2014; published 21 Mar 2014(C) 2014 OSA 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007330 | OPTICS EXPRESS 7335

and is immune to unwanted intensity fluctuations (because of the differential intensity measurement).

Acknowledgments

This work was funded by the National Natural Science Foundation of China (No. 61205080, No. 61235005), Guangdong Natural Science Foundation of China (No. S2012010008385, No. S2013030013302), Doctoral Program of Higher Education of China (No. 20114401120006), Pearl River Scholar for Distinguished Young Scientist (No. 2011J2200014). J. Albert is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chairs Program.

#203621 - $15.00 USD Received 26 Dec 2013; revised 12 Mar 2014; accepted 12 Mar 2014; published 21 Mar 2014(C) 2014 OSA 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007330 | OPTICS EXPRESS 7336