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1 Sensors 00, 0, ; doi:0.3390/s Article OPEN ACCESS sensors ISSN Polarization Dependence Suppression of Optical Fiber Grating Sensor in a -Shifted Sagnac Loop Interferometer Jaebum Son, Min-Koung Lee, Mung Yung Jeong 3 and Chang-Seok Kim 3, * 3 School of Medicine, Korea Universit, Seoul, , Korea; jaebum.son@gmail.com Department of Phsics, Chungnam National Universit, Daejeon, , Korea; leemk@keit.re.kr W.C.U. Department of Cogno-Mechatronics, Pusan National Universit, Busan, , Korea; mjeong@pusan.ac.kr * Author to whom correspondence should be addressed; ckim@pusan.ac.kr; Tel.: ; Fa: Received: 3 March 00; in revised form: 3 April 00 / Accepted: 3 April 00 / Published: 9 April 00 Abstract: In the sensing applications of optical fiber grating, it is necessar to reduce the transmission-tpe dependence to isolate the sensing parameter. It is eperimentall shown that the -dependent spectrum of acousto-optic long-period fiber grating sensors can be suppressed in the transmission port of a -shifted Sagnac loop interferometer. General epressions for the transmittance and reflectance are derived for transmission-tpe, reflection-tpe, and partiall reflecting/transmitting-tpe -dependent optical devices. The compensation of dependence through the counter propagation in the Sagnac loop interferometer is quantitativel measured for a commercial in-line polarizer and an acousto-optic long-period fiber grating sensor. Kewords: fiber sensor; long-period fiber gratings; sagnac effect; -sensitive devices. Introduction Polarization-dependence loss (PDL) of transmission-tpe optical devices, such as long-period fiber grating (LPFG) sensor and wavelength-tunable filters, is an inherent problem that limits information

2 Sensors 00, capacit in optical sensor applications [,]. In additional to the transmission-tpe LPFG sensor, the fiber Bragg grating (FBG) sensor, which is a partiall transmission/reflection-tpe fiber device, also suffers from PDL [3]. A simple compensation method to suppress the PDL of transmission-tpe fiber gratings using a Sagnac loop interferometer has been recentl demonstrated [4,5]. However, the dependence of the Sagnac interferometer containing other tpes of devices has et to be addressed. In this work, three tpes of -s, a transmission-tpe, reflection-tpe, and mied (both transmitting and reflecting)-tpe, are analzed b use of Jones matri method. It is shown that the -dependent spectra of transmission-tpe optical devices can be suppressed in the transmission port of -shifted Sagnac loop interferometer [5,6], and that the PDL of reflection-tpe optical devices can be suppressed in the reflection port of -shifted Sagnac loop interferometer. An analsis of the PDL of a Sagnac interferometer containing a mied-tpe device is also performed. We also characterize eperimentall PDL suppression using Sagnac loop interferometer for various devices ehibiting various degrees of PDL such as an in-line polarizer (ILP) and an acousto-optic LPFG sensor. The cancellation of dependence through the counter propagation in the Sagnac loop interferometer is quantitativel measured using a commericial PDL measurement control sstem. This simple PDL suppression method can be used to minimize the various PDL effect.. Theoretical Analsis A schematic diagram of the Sagnac loop interferometer with a -, similar to the one demonstrated in [5], is shown in Figure. In this work, we also consider the cases when the - is a reflection-tpe and mied-tpe device, as well as the previous described transmission-tpe device, as shown in Table. Assuming that the principal aes of the device are along the and aes of laborator coordinates [7], the -dependent transmission, T device, between the input and transmitted field and the reflection, R device, between the input field and reflected field are described b the Jones matrices in Table. In these matrices, amplitude factors and are all real numbers and birefringence of the device is considered to be negligible. These transmission and reflection relations hold for bi-directional propagation. Figure. Schematic diagram of the Sagnac interferometer with a -dependent device and a waveplate inside the loop. Waveplate 5 6 Device E in Circulator E j Ee ER RSagnacEin :50 Coupler E T E in

3 Sensors 00, Table. Transmission and Reflection of each tpe of PDL device are presented in Jones matri form. Withtout Sagnac interferometer 0 Transmissiontpe () T Device Reflection-tpe Mied-tpe R Device T Device R Device T Device R Device 0 0 () (3) 0 0 (4) 0 0 (5) 0 0 (6) Using a procedure similar to the one shown in [5], the transmittance,, and reflectance,, of a Sagnac interferometer can be solved analticall using Jones matri analsis. In the waveplate in Figure, two parameters are considered; a retardance,, and an orientation of waveplate aes,, with respect to the laborator coordinates. (We should consider the general case.) The two cases when = 0,= 0 and = 4,= are epressed in Tables and 3, respectivel. For the waveplate with = 0,= 0 (no waveplate), the Sagnac loop interferometer works as mirror; that is, it ehibits maimum reflection and minimum transmission. Contraril, for a waveplate with = 4, = -shifted waveplate at angle 4), maimum transmission and minimum reflection can be achieved in Ports and, respectivel. We consider Port as the reflection port and Port as the transmission port of a Sagnac interferometer with a nested optical device. With l set equal to the length between Ports 3 and 5 and l set equal to the arm length between between Ports 4 and 6, l is defined as l l. Whenl = 0 or l = l, as the Sagnac interferometer is referred to as a length-smmetric Sagnac loop interferometer. The longitudinal phase constant for a traveling wave in the fiber is represented as k.

4 Sensors 00, Table. The transmittance and reflectance of a Sagnac interferometer with various nested devices ehibiting PDL when the waveplate settings are = 0 and= 0. Transmissiontpe Reflection-tpe Mied-tpe With the Sagnac loop of = 0,= 0 0 (7) E / E E E (8) E E cos kl E E E E sin kl E E E E sin kl E E E E E E E E cosk l 4E 4E sink l Table 3. The transmittance and reflectance of a Sagnac interferometer with various nested devices ehibiting PDL when the waveplate settings are = /4 and=. With the Sagnac loop of =4,= (9) (0) () () Transmissiontpe Reflection-tpe Mied-tpe 4 (3) 4 (4) (5) 4 cos k l (6) 4 cos kl 4 EE 4 coskl (7) E E cosk l cos kl cos kl 4 EE 4 coskl E E cos kl (8)

5 Sensors 00, Table shows that the reflectance,, contains -dependent terms in each of the three tpes of devices. The transmittance,, also includes -dependent terms, ecept for the case of a transmission-tpe device, in which the intensit is null at the transmission port. In comparison, the -shifted Sagnac loop interferometer shows us an interesting theoretical result. Both the reflectance,, and the transmittance,, do not have -dependent terms in both transmission-tpe and reflection-tpe devices. The case of Equation (3) has been previousl described in [5] as a specific eample in which the -dependent transmission of a transmission-tpe device is clearl averaged through the use of a -shifted Sagnac loop interferometer configuration [5]. In this research, more general cases are considered. For a length-smmetric Sagnac loop interferometer, the epression for the reflectance in Equation (6) can be further simplified to /4 ( ) (6-) meaning that the -dependent reflection of a reflection-tpe device can also be compensated. 3. Eperiment with In-Line Polarizer As a representative - having the highest dependence to the input state, a commercial in-line polarizer (ILP ) is selected in this eperiment [4] because the quantitative proof of this suppression effect has not been sufficientl investigated using commerciall available measurement tools. In this work, we measure the PDL value of a commercial in-line polarizer using a commercial PDL measurement control sstem without and with a Sagnac loop configuration, respectivel. Without a Sagnac loop, the average PDL of the ILP is measured to be ~55 db in each transmission direction. However, the average PDL value is dramaticall reduced to less than 0.45 db when it is placed within a Sagnac loop. The dependence, quantified as PDL, can be measured from a control sstem consisting of a tunable laser (Ando, AQ43B), polarizer, and polarimeter (Tektroni, PAT9000B). The intrinsic dependence of the transmission-tpe -dependent optical device can be measured b placing it between polarizer and polarimeter in an direction when the device has similar transmission characteristics in both transmission directions. Thus, the measured PDL values are similarl measured in both directions. As shown in Figure, the PDL was measured to have ver low values below 0.5 db around,550 nm, but slightl increased as the wavelength moved over,560 nm. We believe that this is caused b the wavelength dependence of intensit splitting ratio of 50:50 coupler. In general, it has been known that the C-band 50:50 coupler has the optimal splitting ratio centered around,550 nm. These can in principle be minimized b optimizing all components used to construct the Sagnac loop. A large number of the packaged optical devices have been treated as failure products at their final test stage just due to the high PDL value onl. It is epected that the intrinsic and fabrication-induced PDL of these products can be easil compensated with the incorporation of a Sagnac loop in the packaging process.

6 Sensors 00, Figure. For each configuration of in-line polarizer without and with a Sagnac loop, the average PDL is measured to be 55.6 db and 0.45 db, respectivel. PDL [db] In-line polarizer without Sagnac loop with Sagnac loop Wavelength [nm] 4. Eperiment with Acousto-Optic LPFG Sensor The dependent transmission and reflection of an acousto-optic LPFG sensor are also eperimentall eamined b placing it inside the Sagnac loop interferometer with waveplate and are compared with epressions in Equations (3) and (8), respectivel. Since acousto-optic LPFG sensors and micro-bending LPFGs are functionall based on asmmetrical cladding mode coupling, these transmission-tpe devices show intrinsicall -dependent transmission spectra and no back reflection [0,]. Spectrall-resolved and -dependent coupling to each of the cladding TE 0, TM 0, and HE modes is clearl observed for an acousto-optic LPFG sensor with an interaction region consisting of a 8-cm-long dispersion compensating fiber (OFS EHS00), [8]. The input state is varied using a polarized broadband source and an all-fiber controller [9]. The output spectra at transmission and reflection ports are measured using an optical spectrum analzer in order to analze the spectral PDL. For the case of Equation (8), the waveplate is tuned to maimize the reflected output and minimize the transmission output (and. The spectral reflectance is easil changed b tuning the input as shown in Figure 3 (a). The filtering spectra for four different input states are shown in each figure. It clearl shows that the intrinsic -dependence of the transmission of an acousto-optic LPFG sensor is transferred to the spectrum measured at the reflection port of the Sagnac loop mirror. In contrast, the -dependent mode splitting is completel eliminated when the waveplate settings are adjusted so that and /4 b maimizing the intensit in the transmission Port as shown in Figure 3 (b) and derived in Equation (3).

7 Sensors 00, Figure 3. For various input states, optical spectra of -dependent acousto-optic LPFG sensor in a Sagnac loop are measured from (a) the reflection port of Sagnac loop ( = 0,= 0) and (b) the transmission port of Sagnac loop ( = 4,= ). (a) Reflection Spectra [db] of AOTF ( (b) Transmission Spectra [db] of AOTF ( Wavelength [nm] Wavelength [nm] 4. Conclusions We have generall analzed and measured the dependence cancellation of optical fiber grating sensor using a -shifted Sagnac loop interferometer. For the feasible intrinsic PDL-free integrated optical devices, a commercial-grade in-line polarizer device was tested to verif the proposed PDL compensating method. A representative tpe of -, an acousto-optic LPFG sensor, is also eperimentall proved for the -dependence suppression. It is epected that the use of the Sagnac loop can save man optical sensor products, such as FBG sensors and micro-bending LPFG sensors which have been limited in their applications solel due to their high PDL. Acknowledgements This work was supported b National Research Foundation of Korea Grant funded b the Korean Government (KRF C008). References and Notes. Huttner, B.; Geiser, C.; Gisin, N. Polarization-induced distortions in optical fiber networks with -mode dispersion and -dependent losses. IEEE J. Sel. Top. Quant. 000, 6, Park, K.J.; Kim, H.; Lee, J.H.; Youn, C.J.; Shin, S.K.; Chung, Y.C. Polarisation-mode dispersion monitoring technique abased on polarisation scrambling. Electron. Lett. 00, 38, Bachim, B.L.; Galord, T.K. Polarization-dependent loss and birefringence in long-period fiber gratings. Appl. Opt. 003, 4,

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