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drop channel filter based on two parallel long-period fiber gratings coupler
ARTICLE IN PRESS
Optik
Optics
Optik 120 (2009) 855–859 www.elsevier.de/ijleo
Add/drop channel filter based on two parallel long-period fiber gratings coupler Xiaowei Dong, Suchun Feng, Ou Xu, Shaohua Lu, Li Pei State Key Laboratory of All Optical Network & Advanced Telecommunication Network, Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China Received 23 October 2007; accepted 13 February 2008
Abstract A wavelength-selective coupler constructed with two parallel long-period fiber gratings (LPFGs) was investigated thoroughly. Both the numerical and experimental results verified that the filter response could be improved by adjusting the UV refractive modulation of the gratings and by controlling the external surrounding. And an add/drop channel filter with high efficiency and good spectral characteristic was achieved even though the claddings of the two gratings were not processed. This feature is very favorable to simplify the fabrication process. r 2008 Elsevier GmbH. All rights reserved. PACS: 42.79.Dj; 42.81.Qb Keywords: Fiber optics components; Long-period fiber grating (LPFG); Add/drop channel filter
1. Introduction Optical add/drop filter, which locally extracts, terminates, and reinserts the wavelengths, without preventing the transit of throughput traffic to remote nodes, is one of the most critical components to enhance network transparence and flexibility [1,2]. In order to ensure cost effectiveness, an add/drop filter with features such as simplicity and easy fabrication should be chosen. Compared with fiber-Bragg grating, long-period fiber grating (LPFG) shows much lower back reflection and better isolation [3]. Recently, a novel add/drop channel filter based on two parallel LPFGs coupler has been proposed. Due to the stronger evanescent-field coupling between the cladding modes induced by the LPFGs, the Corresponding author. Fax: +86 10 516 83625.
E-mail address: [email protected] (X. Dong). 0030-4026/$ - see front matter r 2008 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2008.02.026
add/drop channel efficiency can be improved significantly [4–6]. In this paper, based on the general model of two parallel LPFGs, the influences of UV refractive modulation in the gratings and the refractive index of external surroundings are investigated thoroughly. A method to improve the drop channel efficiency and filtering spectral response is proposed and the corresponding experimental results are demonstrated.
2. Theoretical modeling Fig. 1 is the general model of two parallel LPFGs coupler with the gratings having same pitch L and length L [7]. To transfer the input signal from the first fiber core to the second fiber core, three coupling processes are
ARTICLE IN PRESS 856
X. Dong et al. / Optik 120 (2009) 855–859 L
Fiber 1 Input
LPG 1
Transmission d
LPG 2
Drop
Add Fiber 2 L z=0
L1
L2
K0 and K1 are the modified Bessel functions of the second kind; D is the relative index difference; W ¼ ð2p=lÞrðn20m n22 Þ1=2 , U ¼ ð2p=lÞrðn22 n20m Þ1=2 , and V ¼ ð2p=lÞrðn22 n23 Þ1=2 are the normalized parameters with n0m being the effective index of the LP0m mode; and r is the fiber radius.
L3
Fig. 1. Add/drop channel filter based on two parallel LPFGs coupler.
involved: (I) core-mode/cladding-mode coupling induced by LPFG1; (II) cladding-mode/cladding-mode coupling due to the evanescent-field effect; (III) cladding-mode/core-mode coupling induced by LPFG2. Therefore, the two outputs are complementary with the first fiber showing band rejection and the second fiber showing band-pass filtering characteristics. In order to investigate the filtering performance and stability of the device, two cases are discussed.
2.1. Two LPFGs are aligned perfectly When the two LPFGs are aligned (offset distance L1 ¼ 0), the three coupling processes take place simultaneously. If the core-mode and cladding-mode fields of the first and the second fibers are expressed as d d ¯ ¯ A1 ðzÞ ¼ A1 ðzÞ exp j2z , B1 ðzÞ ¼ B1 ðzÞ exp j2z , A2 ðzÞ ¼ ¯ 2 ðzÞ exp jdz , and B2 ðzÞ ¼ B ¯ 2 ðzÞ exp jdz , the coupledA 2 2 mode equation can be written in the matrix form as dH 1 ¼ jM 1 H 1 (1) dz ¯ 1 ðzÞ B¯ 1 ðzÞ B¯ 2 ðzÞ A ¯ 2 ðzÞ T ; Here H 1 ¼ A 2 d 3 2 k 0 0 6 k d 0 C 7 6 7 2 7 M1 ¼ 6 (2) d 7 6 0 C k 4 2 5 0 0 d2 k d ¼ ð2p=LÞ ðl0 =lÞ 1 is the phase-detuning factor with l0 the center resonance wavelength; k the grating coupling coefficient between the core-mode and the copropagating cladding-mode induced by LPFG with the refractive index modulation Dn [3]: ZZ o 2n1 DnðzÞE t01 ðx; yÞE tov ðx; yÞ dx dy (3) k 4 1
C is the evanescent field-coupling coefficient between the cladding modes of the two fibers, which can be expressed approximately [8] as pffiffiffiffiffiffi 2 2D U K 0 ½W ð2 þ d=rÞ (4) C¼ r V3 K 21 ðW Þ
2.2. Two LPFGs are offset by distance L1 (0oL1oL) When an offset distance L1 (0oL1oL) is introduced in the lognitudinal, three regions should be discussed. In the region [0 L1], only I and II coupling processes occur. And the coupled-mode equation can be written as dH 2 ¼ jM 2 H 2 dz ¯ 1 ðzÞ B ¯ 1 ðzÞ B ¯ 2 ðzÞT ; with H 2 ¼ ½A 2 d 3 0 2 k 6 7 d k C 7 M2 ¼ 6 2 4 5 d 0 C 2
(5)
(6)
In the region of [L1 L2], the coupled-mode equation has the same form as (1) and (2) because both the gratings exist. In the region of [L2 L3], the coupling effect is similar to the region of [0 L1]] and its matrix M3 ¼ M2 if the vector H3 is expressed as ¯ 2 ðzÞ B ¯ 2 ðzÞ B ¯ 1 ðzÞ T . H3 ¼ A
3. Numerical analyses and discussions In the following, single-mode fibers with core index n1 ¼ 1.462, cladding index n2 ¼ 1.457, core radius a ¼ 4 mm, and cladding radius r ¼ 62.5 mm are used. Runge–Kutta numerical integration is used to investigate the performances of the two parallel LPFGs coupler. The transmission spectra of a single LPFG are shown in Fig. 2. When the LPFG is placed in air (n3 ¼ 1), the grating coupling coefficient and the evanescent-field coupling coefficient corresponding to the LP03 mode (l3 ¼ 1358.54 nm) and the LP07 mode (l7 ¼ 1548.35 nm) are calculated as k ¼ 10.81 m1, C ¼ 0.1720 m1 (LP03) and k ¼ 19.537 m1, C ¼ 0.9927 m1 (LP07), respectively. Compared with the lower-order resonance mode, both the grating coupling efficiency and the evanescent field coupling efficiency are enhanced if the higher-order resonance mode is used. When two LPFGs are placed side by side and are aligned perfectly, Fig. 3 depicts the power exchange ¯ 2 ðzÞ, and ¯ 1 ðzÞ, B ¯ 1 ðzÞ, A actions of the normalized modes A B¯ 2 ðzÞ along the device length. From the figure we can seen that, in order to obtain 100% power transfer
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Fig. 4. (a) Power exchange actions of normalized modes when surrounding index is adjusted; (b) transmission and drop filtering responses with L ¼ 145 mm.
Wavelength /µm
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Fig. 2. Transmission characteristics of a single LPFG.
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Fig. 5. (a) Power exchange with simultaneously enhanced grating coupling and evanescent-field coupling; (b) transmission and drop filtering responses.
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¯ 1 ðzÞ and A ¯ 2 ðzÞ Fig. 3. Power exchange of normalized modes A along the device length.
¯ 1 ðzÞ and the output mode between the input mode A ¯ 2 ðzÞ, the length of gratings must be L ¼ 3054 mm even A if the higher-order resonance LP07 mode is utilized. The gratings are too long to be fabricated by the amplitude mask method. Therefore, some measures must be taken to reduce the device size.
3.1. External surrounding is adjusted to enhance the evanescent field coupling effect It is well known that the cladding-mode field spreads significantly if the external index n3 is close to the refractive index of fiber cladding n2 [9,10]. Therefore, the evanescent field coupling between the cladding modes of the two fibers can be enhanced if a suitable surrounding such as polymeric material or index-matching liquid with refractive index similar to the cladding material is used. Compared with the case of n3 ¼ 1, Fig. 4(a) demonstrates that 100% power transfer can be achieved by LPFGs with much shorter length L ¼ 145 mm. This is due to the stronger evanescent-field coupling effect, which is calculated as C ¼ 21.7 m1. At the same time, from Fig. 4(b) it can be seen that the drop channel filter
response shows very low side-lobes characteristic. This feature is very favorable to avoid the crosstalk interference.
3.2. Suitable UV refractive modulation is chosen to increase the grating coupling effect Besides the evanescent field coupling, the gratings induced by the UV refractive modulation play a very important role in the drop filter efficiency. According to formula (3), the grating coupling coefficient k is approximately proportional to the UV refractive modulation Dn. If the refractive modulation is changed from Dn ¼ 5 105 (used in Figs. 2–4) to Dn ¼ 5 104, the grating coupling coefficient will reach k ¼ 187 m1. Fig. 5 depicts the power exchange actions and the output spectral responses. When both the grating coupling and evanescent-field coupling are enhanced by adjusting the surrounding and by controlling UV refractive modulation, the device size with 100% power transfer is reduced to L ¼ 145 mm, and the drop filtering characteristic is improved, which appears to be more step-like. Furthermore, the device demonstrates enough stability. Even if the two LPFGs are not aligned, 100% power transfer can still be realized by adjusting the offset distance L1 ¼ 5 mm, and the drop channel filter characteristics are quite good, as shown in Fig. 6.
ARTICLE IN PRESS 858
X. Dong et al. / Optik 120 (2009) 855–859 -20 Original spectrum of LED
Output power /dB
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Fig. 6. Stability of the device to the longitudinal offset distance L1. (a) Impact of longitudinal offset L1 on the power exchange action; (b) transmission and drop filtering response for 100% power transfer by adjusting L1 ¼ 5 mm.
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(b)
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We design and fabricate a two parallel LPFGs coupler to verify the reliability of numerical analyses given above. The two LPFGs are fabricated by periodic UV refractive modulation in SMF-28 hydrogen-loaded single-mode fibers using Bragg-Star laser, whose output wavelength is 248 nm, frequency is 30 Hz, and energy is 9 mJ. A precise stress adjustment is used to match the resonance wavelengths of the two LPFGs. Then, unlike the traditional fabrication of the coupler, the two fibers are not processed by HF acid etching or side polishing to reduce their cladding thicknesses. The two LPFGs are placed only side by side and twisted slightly to increase their evanescent field coupling. The filtering characteristics are measured by a LED broad light source (wavelength range is 1380–1680 nm) and the ANDO6319 spectrum analyzer (resolution is 0.01 nm). From the measured result in Fig. 7, it can be seen that the higher-order resonance (in 1630 nm) show 5 dB higher output power in the drop port compared with the lower-order resonance. In the second experiment, the output energy of the laser is doubled and another two LPFGs with stronger refractive modulation are fabricated. As can be seen from Fig. 8, the higher-order resonance in 1630 nm is eliminated due to the over-modulation effect (kL4p/2), and the highest resonance in the wavelength range of LED is only in 1510 nm. However, when the two LPFGs
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4. Experimental results
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Fig. 8. Transmission of a single LPFG with stronger refractive modulation.
0
Fig. 7. Measured filtering spectra of the two parallel LPFGs coupler. (a) Transmission response; (b) drop response.
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Fig. 9. Measured spectra of a coupler constructed by two LPFGs with stronger UV modulation. (a) Transmission; (b) drop.
are used to form the coupler, from the measured results in Fig. 9, it can be seen that transmission and drop filtering characteristics are not deteriorated. And the well-shaped step-like filter response is very advantageous to decrease the drop signal distortion.
5. Conclusions We have proposed and fabricated an add/drop filter based on a two parallel LPFGs coupler. Influences of UV refractive modulation in the gratings and the refractive index of external surroundings and the longitudinal offset between the two gratings are analyzed in detail, and the methods to enhance the drop efficiency are discussed. Both the numerical and experimental results demonstrate that a very good drop filtering characteristic can be achieved even if the claddings of the two LPFGs are not processed. This will simplify the fabrication and reduce the device cost in the future.
Acknowledgments This work is jointly supported by the National Natural Science Foundation of China (60607001), the
ARTICLE IN PRESS X. Dong et al. / Optik 120 (2009) 855–859
Beijing Natural Science Foundation (4052023), and the Science Foundation of Beijing Jiaotong University (2007XM003).
References [1] S. Park, R. Leavitt, R. Enck, et al., Semiconductor optical amplifier for CWDM operating over 1540–1620 nm, IEEE Photon. Technol. Lett. 17 (5) (2005) 980–982. [2] S. Rotolo, A. Tanzi, S. Brunazzi, et al., Integrated optic tunable add-drop filters for WDM ring networks, J. Lightwave Technol. 18 (4) (2000) 569–578. [3] T. Erdogan, Cladding-mode resonances in short- and long- period grating filters, J. Opt. Soc. Am. 14 (8) (1997) 1760–1773. [4] Myoung Jin Kim, Florence Y.M. Chan, Un-Chul Paek, et al. Tunable add-drop filter for CWDM system using
[5]
[6]
[7]
[8] [9]
[10]
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cladding mode coupling assisted by long-period fiber gratings, OFC2006, OtuM6. P.K. Lam, A.J. Stevenson, J.D. Love, Bandpass spectra of evanescent couplers with long period gratings, Electron. Lett. 36 (11) (2000) 967–969. Kin Seng Chiang, Yunqi Liu, Mei Nar Ng, et al., Coupling between two parallel long-period fiber gratings, Electron. Lett. 36 (16) (2000) 1408–1409. Kin Seng Chiang, Florence Yuen Ming Chan, Mei Nar Ng, Analysis of two parallel long-period fiber gratings, J. Lightwave Technol. 22 (5) (2004) 1358–1366. A.W. Snyder, J.D. Love, Optical Waveguide Theory, Chapman and Hall, London, New York, 1983. D.B. Stegall, T. Erdogan, Leaky cladding mode propagation in long-period fiber grating devices, IEEE Photon. Technol. Lett. 11 (3) (1999) 343–345. Xuewen Shu, Lin Zhang, Ian Bennion, et al., Sensitivity characteristics of long-period fiber gratings, J. Lightwave Technol. 20 (2) (2002) 255–266.
Optik
Optics
Optik 120 (2009) 855–859 www.elsevier.de/ijleo
Add/drop channel filter based on two parallel long-period fiber gratings coupler Xiaowei Dong, Suchun Feng, Ou Xu, Shaohua Lu, Li Pei State Key Laboratory of All Optical Network & Advanced Telecommunication Network, Institute of Lightwave Technology, Beijing Jiaotong University, Beijing 100044, China Received 23 October 2007; accepted 13 February 2008
Abstract A wavelength-selective coupler constructed with two parallel long-period fiber gratings (LPFGs) was investigated thoroughly. Both the numerical and experimental results verified that the filter response could be improved by adjusting the UV refractive modulation of the gratings and by controlling the external surrounding. And an add/drop channel filter with high efficiency and good spectral characteristic was achieved even though the claddings of the two gratings were not processed. This feature is very favorable to simplify the fabrication process. r 2008 Elsevier GmbH. All rights reserved. PACS: 42.79.Dj; 42.81.Qb Keywords: Fiber optics components; Long-period fiber grating (LPFG); Add/drop channel filter
1. Introduction Optical add/drop filter, which locally extracts, terminates, and reinserts the wavelengths, without preventing the transit of throughput traffic to remote nodes, is one of the most critical components to enhance network transparence and flexibility [1,2]. In order to ensure cost effectiveness, an add/drop filter with features such as simplicity and easy fabrication should be chosen. Compared with fiber-Bragg grating, long-period fiber grating (LPFG) shows much lower back reflection and better isolation [3]. Recently, a novel add/drop channel filter based on two parallel LPFGs coupler has been proposed. Due to the stronger evanescent-field coupling between the cladding modes induced by the LPFGs, the Corresponding author. Fax: +86 10 516 83625.
E-mail address: [email protected] (X. Dong). 0030-4026/$ - see front matter r 2008 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2008.02.026
add/drop channel efficiency can be improved significantly [4–6]. In this paper, based on the general model of two parallel LPFGs, the influences of UV refractive modulation in the gratings and the refractive index of external surroundings are investigated thoroughly. A method to improve the drop channel efficiency and filtering spectral response is proposed and the corresponding experimental results are demonstrated.
2. Theoretical modeling Fig. 1 is the general model of two parallel LPFGs coupler with the gratings having same pitch L and length L [7]. To transfer the input signal from the first fiber core to the second fiber core, three coupling processes are
ARTICLE IN PRESS 856
X. Dong et al. / Optik 120 (2009) 855–859 L
Fiber 1 Input
LPG 1
Transmission d
LPG 2
Drop
Add Fiber 2 L z=0
L1
L2
K0 and K1 are the modified Bessel functions of the second kind; D is the relative index difference; W ¼ ð2p=lÞrðn20m n22 Þ1=2 , U ¼ ð2p=lÞrðn22 n20m Þ1=2 , and V ¼ ð2p=lÞrðn22 n23 Þ1=2 are the normalized parameters with n0m being the effective index of the LP0m mode; and r is the fiber radius.
L3
Fig. 1. Add/drop channel filter based on two parallel LPFGs coupler.
involved: (I) core-mode/cladding-mode coupling induced by LPFG1; (II) cladding-mode/cladding-mode coupling due to the evanescent-field effect; (III) cladding-mode/core-mode coupling induced by LPFG2. Therefore, the two outputs are complementary with the first fiber showing band rejection and the second fiber showing band-pass filtering characteristics. In order to investigate the filtering performance and stability of the device, two cases are discussed.
2.1. Two LPFGs are aligned perfectly When the two LPFGs are aligned (offset distance L1 ¼ 0), the three coupling processes take place simultaneously. If the core-mode and cladding-mode fields of the first and the second fibers are expressed as d d ¯ ¯ A1 ðzÞ ¼ A1 ðzÞ exp j2z , B1 ðzÞ ¼ B1 ðzÞ exp j2z , A2 ðzÞ ¼ ¯ 2 ðzÞ exp jdz , and B2 ðzÞ ¼ B ¯ 2 ðzÞ exp jdz , the coupledA 2 2 mode equation can be written in the matrix form as dH 1 ¼ jM 1 H 1 (1) dz ¯ 1 ðzÞ B¯ 1 ðzÞ B¯ 2 ðzÞ A ¯ 2 ðzÞ T ; Here H 1 ¼ A 2 d 3 2 k 0 0 6 k d 0 C 7 6 7 2 7 M1 ¼ 6 (2) d 7 6 0 C k 4 2 5 0 0 d2 k d ¼ ð2p=LÞ ðl0 =lÞ 1 is the phase-detuning factor with l0 the center resonance wavelength; k the grating coupling coefficient between the core-mode and the copropagating cladding-mode induced by LPFG with the refractive index modulation Dn [3]: ZZ o 2n1 DnðzÞE t01 ðx; yÞE tov ðx; yÞ dx dy (3) k 4 1
C is the evanescent field-coupling coefficient between the cladding modes of the two fibers, which can be expressed approximately [8] as pffiffiffiffiffiffi 2 2D U K 0 ½W ð2 þ d=rÞ (4) C¼ r V3 K 21 ðW Þ
2.2. Two LPFGs are offset by distance L1 (0oL1oL) When an offset distance L1 (0oL1oL) is introduced in the lognitudinal, three regions should be discussed. In the region [0 L1], only I and II coupling processes occur. And the coupled-mode equation can be written as dH 2 ¼ jM 2 H 2 dz ¯ 1 ðzÞ B ¯ 1 ðzÞ B ¯ 2 ðzÞT ; with H 2 ¼ ½A 2 d 3 0 2 k 6 7 d k C 7 M2 ¼ 6 2 4 5 d 0 C 2
(5)
(6)
In the region of [L1 L2], the coupled-mode equation has the same form as (1) and (2) because both the gratings exist. In the region of [L2 L3], the coupling effect is similar to the region of [0 L1]] and its matrix M3 ¼ M2 if the vector H3 is expressed as ¯ 2 ðzÞ B ¯ 2 ðzÞ B ¯ 1 ðzÞ T . H3 ¼ A
3. Numerical analyses and discussions In the following, single-mode fibers with core index n1 ¼ 1.462, cladding index n2 ¼ 1.457, core radius a ¼ 4 mm, and cladding radius r ¼ 62.5 mm are used. Runge–Kutta numerical integration is used to investigate the performances of the two parallel LPFGs coupler. The transmission spectra of a single LPFG are shown in Fig. 2. When the LPFG is placed in air (n3 ¼ 1), the grating coupling coefficient and the evanescent-field coupling coefficient corresponding to the LP03 mode (l3 ¼ 1358.54 nm) and the LP07 mode (l7 ¼ 1548.35 nm) are calculated as k ¼ 10.81 m1, C ¼ 0.1720 m1 (LP03) and k ¼ 19.537 m1, C ¼ 0.9927 m1 (LP07), respectively. Compared with the lower-order resonance mode, both the grating coupling efficiency and the evanescent field coupling efficiency are enhanced if the higher-order resonance mode is used. When two LPFGs are placed side by side and are aligned perfectly, Fig. 3 depicts the power exchange ¯ 2 ðzÞ, and ¯ 1 ðzÞ, B ¯ 1 ðzÞ, A actions of the normalized modes A B¯ 2 ðzÞ along the device length. From the figure we can seen that, in order to obtain 100% power transfer
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Fig. 4. (a) Power exchange actions of normalized modes when surrounding index is adjusted; (b) transmission and drop filtering responses with L ¼ 145 mm.
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Fig. 5. (a) Power exchange with simultaneously enhanced grating coupling and evanescent-field coupling; (b) transmission and drop filtering responses.
0.4 0.2 0 0
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¯ 1 ðzÞ and A ¯ 2 ðzÞ Fig. 3. Power exchange of normalized modes A along the device length.
¯ 1 ðzÞ and the output mode between the input mode A ¯ 2 ðzÞ, the length of gratings must be L ¼ 3054 mm even A if the higher-order resonance LP07 mode is utilized. The gratings are too long to be fabricated by the amplitude mask method. Therefore, some measures must be taken to reduce the device size.
3.1. External surrounding is adjusted to enhance the evanescent field coupling effect It is well known that the cladding-mode field spreads significantly if the external index n3 is close to the refractive index of fiber cladding n2 [9,10]. Therefore, the evanescent field coupling between the cladding modes of the two fibers can be enhanced if a suitable surrounding such as polymeric material or index-matching liquid with refractive index similar to the cladding material is used. Compared with the case of n3 ¼ 1, Fig. 4(a) demonstrates that 100% power transfer can be achieved by LPFGs with much shorter length L ¼ 145 mm. This is due to the stronger evanescent-field coupling effect, which is calculated as C ¼ 21.7 m1. At the same time, from Fig. 4(b) it can be seen that the drop channel filter
response shows very low side-lobes characteristic. This feature is very favorable to avoid the crosstalk interference.
3.2. Suitable UV refractive modulation is chosen to increase the grating coupling effect Besides the evanescent field coupling, the gratings induced by the UV refractive modulation play a very important role in the drop filter efficiency. According to formula (3), the grating coupling coefficient k is approximately proportional to the UV refractive modulation Dn. If the refractive modulation is changed from Dn ¼ 5 105 (used in Figs. 2–4) to Dn ¼ 5 104, the grating coupling coefficient will reach k ¼ 187 m1. Fig. 5 depicts the power exchange actions and the output spectral responses. When both the grating coupling and evanescent-field coupling are enhanced by adjusting the surrounding and by controlling UV refractive modulation, the device size with 100% power transfer is reduced to L ¼ 145 mm, and the drop filtering characteristic is improved, which appears to be more step-like. Furthermore, the device demonstrates enough stability. Even if the two LPFGs are not aligned, 100% power transfer can still be realized by adjusting the offset distance L1 ¼ 5 mm, and the drop channel filter characteristics are quite good, as shown in Fig. 6.
ARTICLE IN PRESS 858
X. Dong et al. / Optik 120 (2009) 855–859 -20 Original spectrum of LED
Output power /dB
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Fig. 6. Stability of the device to the longitudinal offset distance L1. (a) Impact of longitudinal offset L1 on the power exchange action; (b) transmission and drop filtering response for 100% power transfer by adjusting L1 ¼ 5 mm.
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We design and fabricate a two parallel LPFGs coupler to verify the reliability of numerical analyses given above. The two LPFGs are fabricated by periodic UV refractive modulation in SMF-28 hydrogen-loaded single-mode fibers using Bragg-Star laser, whose output wavelength is 248 nm, frequency is 30 Hz, and energy is 9 mJ. A precise stress adjustment is used to match the resonance wavelengths of the two LPFGs. Then, unlike the traditional fabrication of the coupler, the two fibers are not processed by HF acid etching or side polishing to reduce their cladding thicknesses. The two LPFGs are placed only side by side and twisted slightly to increase their evanescent field coupling. The filtering characteristics are measured by a LED broad light source (wavelength range is 1380–1680 nm) and the ANDO6319 spectrum analyzer (resolution is 0.01 nm). From the measured result in Fig. 7, it can be seen that the higher-order resonance (in 1630 nm) show 5 dB higher output power in the drop port compared with the lower-order resonance. In the second experiment, the output energy of the laser is doubled and another two LPFGs with stronger refractive modulation are fabricated. As can be seen from Fig. 8, the higher-order resonance in 1630 nm is eliminated due to the over-modulation effect (kL4p/2), and the highest resonance in the wavelength range of LED is only in 1510 nm. However, when the two LPFGs
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4. Experimental results
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Fig. 8. Transmission of a single LPFG with stronger refractive modulation.
0
Fig. 7. Measured filtering spectra of the two parallel LPFGs coupler. (a) Transmission response; (b) drop response.
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Fig. 9. Measured spectra of a coupler constructed by two LPFGs with stronger UV modulation. (a) Transmission; (b) drop.
are used to form the coupler, from the measured results in Fig. 9, it can be seen that transmission and drop filtering characteristics are not deteriorated. And the well-shaped step-like filter response is very advantageous to decrease the drop signal distortion.
5. Conclusions We have proposed and fabricated an add/drop filter based on a two parallel LPFGs coupler. Influences of UV refractive modulation in the gratings and the refractive index of external surroundings and the longitudinal offset between the two gratings are analyzed in detail, and the methods to enhance the drop efficiency are discussed. Both the numerical and experimental results demonstrate that a very good drop filtering characteristic can be achieved even if the claddings of the two LPFGs are not processed. This will simplify the fabrication and reduce the device cost in the future.
Acknowledgments This work is jointly supported by the National Natural Science Foundation of China (60607001), the
ARTICLE IN PRESS X. Dong et al. / Optik 120 (2009) 855–859
Beijing Natural Science Foundation (4052023), and the Science Foundation of Beijing Jiaotong University (2007XM003).
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