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Tunable dispersion compensation with fixed center wavelength and bandwidth using a side-polished linearly chirped fiber Bragg grating
Optical Fiber Technology 11 (2005) 159–166 www.elsevier.com/locate/yofte
Tunable dispersion compensation with fixed center wavelength and bandwidth using a side-polished linearly chirped fiber Bragg grating ✩ Jaejoong Kwon, Younghee Jeon, Byoungho Lee ∗ School of Electrical Engineering, Seoul National University, Kwanak-Gu Shinlim-Dong, Seoul 151-744, South Korea Received 13 April 2004 Available online 1 October 2004
Abstract We propose a new method for tuning the dispersion of a linearly chirped fiber Bragg grating with fixed center wavelength and bandwidth. We polished one side of the grating and attached it to a cantilever metal beam. By contacting an index matching block to the polished face of the grating, we could shorten the effective grating length through which light can propagate in the core of fiber. Decrease of grating length results in reflection bandwidth narrowing. However, by bending the grating attached on the cantilever beam, we could restore the bandwidth. Because the grating is shortened without bandwidth narrowing, the amount of dispersion is decreased. By using this method, we tuned chromatic dispersion of the linearly chirped fiber Bragg grating from −687.24 to −256.11 ps/nm for 0.75 nm fixed bandwidth without center wavelength shift. 2004 Elsevier Inc. All rights reserved. Keywords: Dispersion compensation; Fiber Bragg grating; Chirped fiber grating
✩
This work was partly supported by Novera Optics, Palo Alto, CA, USA.
* Corresponding author. Fax: +82-2-873-9953.
E-mail address: [email protected] (B. Lee). 1068-5200/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2004.08.002
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J. Kwon et al. / Optical Fiber Technology 11 (2005) 159–166
1. Introduction Chromatic dispersion, which causes different wavelengths to propagate at different group velocities in single-mode transmission fiber, is one of the critical sources of distortion of high speed signals in long-haul WDM systems. Because optical signal pulses contain a range of wavelengths, the chromatic dispersion causes the pulses to spread as they propagate. When the bit rate of the signal increases, spectral width of the signal increases and the time-interval between pulses decreases, so the transmission distance decreases inversely proportional to the square of the bit rate increment. While the dispersion limit for 10-Gb/s signals on standard single mode fiber (SMF) with a dispersion of 17 ps/nm is about 60 km, it is approximately 4 km for 40-Gb/s signal. To operate beyond this distance, dispersion compensation must be employed. Moreover, dispersion compensation should be tunable for several reasons such as inventory management, path length changes in dynamically reconfigurable networks, and environmental effects. The chirped fiber Bragg grating (CFBG) is one of the more promising technologies for tunable dispersion compensation. Various techniques for tuning the CFBG have been proposed [1–9]. The proposed methods tune the dispersion of CFBG by uniformly stretching a nonlinearly chirped FBG [1], non-uniformly stretching by use of discrete piezoelectric stretchers [2], bending a beam supporting the grating [3–5], stretching or compressing a tapered structure supporting the grating [6,7], or differential heating by use of tapered metal film deposited on the grating [8]. Because these technologies tune the dispersion of CFBG by changing the bandwidth and/or shifting the center wavelength while maintaining high reflectivity, it may reflect signals of the adjacent channel. Although the dispersion compensation unit is located at the end of the link after demultiplexing, especially in DWDM systems, non-ideal filtering in the demultiplexer retains small power of the adjacent channel and it causes a crosstalk. Therefore, the spectral change in the dispersion compensation unit may cause a variation of crosstalk. A CFBG for dispersion compensation is also a band reflection filter with sharp band edges, so it will improve the crosstalk properties if a CFBG maintains its spectral shape during dispersion tuning process. A tuning mechanism with fixed bandwidth and center wavelength has been reported previously [9]. However, there remains a reflection band originated from a uniform FBG part which has no dispersion compensation effect. This sacrificed spectral range will reduce usable bandwidth. In this paper, we propose a new method for tuning the dispersion of CFBG without center wavelength shift and bandwidth broadening or narrowing. We attached a linearly chirped fiber Bragg grating (LCFBG) on a metal cantilever beam and polished one side of LCFBG until the core mode can be perturbed by external index variation. The chirping rate of the grating is tuned by bending the beam and the effective grating length is controlled by closely contacting an index matching block on the polished face of the side polished LCFBG (SP-LCFBG). Unlike the previous work [9], there is no remained sacrificed spectral range. 2. Concept and experiment The first process to tune the dispersion of LCFBG is controlling the grating length. Figure 1 shows the schematic diagram to control the effective length of the LCFBG. We
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attached the LCFBG on a metal beam with UV epoxy (OG134, Epoxy Technology) and polished it manually with silicon carbide lapping films (1, 3, 9 µm grade, 3M Corp.). We polished one side of the LCFBG until the evanescent field of the core mode is exposed to exterior. Then, an index matching block that has slightly larger refractive index than core of the fiber is contacted on the polished face of the LCFBG. Because the evanescent field of the core mode overlaps the index matching block, the light propagating the core is coupled to the block and scattered in the block. The light cannot reach to the length of the LCFBG behind the block. Therefore, the effective grating length through which light propagates can be controlled by positioning the index matching block along the length of grating. Because the Bragg resonance wavelength varies linearly along the LCFBG, we can confirm the effective length-shortening effect by monitoring the reflection spectrum narrowing. Figure 2 shows the measured spectral responses for four values of inactive length d that is measured from the end of shorter wavelength of LCFBG. The second process is bending the cantilever beam to recover reflection bandwidth. Figure 3a shows the schematic diagram of the proposed tunable dispersion compensator with fixed bandwidth and center wavelength. To operate the device in negative dispersion regime, the signal is inserted from longer wavelength direction of the LCFBG and the index matching block is located at the opposite position. An index matching block and a
Fig. 1. Schematic diagram of tuning the effective length of an LCFBG.
Fig. 2. Spectral responses of SP-LCFBG for various inactive lengths.
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Fig. 3. (a) Schematic diagram and (b) operation principle of the proposed method for a tunable dispersion compensator with fixed bandwidth and center wavelength.
fixed block hold the grating and act as supports of a cantilever beam. Because the shorter wavelength part is interrupted by the index matching block, we bend the cantilever beam in compressive direction to generate negative strain gradient. When a cantilever beam with uniform cross-sectional area is bent in compressive direction, from the bending moment equation, the strain distribution along the fiber axis x (zero at the free end (Fig. 3), 0 x L, L is the beam length) can be described as follows: EIy (x) = M(x) = −P x, P 3 x − 3L2 x + 2L3 , y=− 6EI l P d ε(x) = = ≈ dy (x) = −d x, l ρ(x) EI
(1)
where E = Young’s modulus, I = second moment of the cross-sectional area, M = bending moment, P = concentrated force at the free end of the beam, d = distance from the neutral surface, ρ = the radius of curvature, ε(x) = local strain, y (x) = second-order derivative of y with respect to x. In the uniform cantilever beam d/EI is constant, so we can notice that the magnitude of strain is linearly increasing from the free end (x = 0) of the beam to the fixed point (x = L).
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Fig. 4. Restored spectra for various inactive lengths.
In addition, a strain-free state is obtained at the free end of the beam. Therefore, the longest wavelength part that is located at the free end of the beam undergoes no strain and its spectrum is fixed during bending process. While, the shorter wavelength part undergoes negative strain gradient so the spectrum is broadened to shorter wavelength direction. The cantilever beam is bent until the reflection bandwidth is recovered to its unperturbed state. Figure 3b shows the principle operation scheme of the proposed method. In the experiment, we used an LCFBG with 50 mm length, 1549.32 nm center wavelength, 0.75 nm 3 dB bandwidth, and 99.5% reflectivity. The refractive index of the index matching block is 1.451. Figure 4 shows measured spectra that are restored by bending for four values of inactive length d. We can notice that the reflection bands for various inactive lengths are well recovered. The center wavelength shift and bandwidth variation are less than 0.08 and 0.07 nm, respectively. In contrast to the previous work [9], the length of grating that is not bent does not reflect any light, so we can use the full reflection bandwidth of the grating without any sacrificed spectral range. However, there appears amplitude ripples (<3 dB) in the reflection spectra, and it is increased as the effective grating length is decreased. We think that the causes of the amplitude ripples are irregularities in side polishing process. By using a precisely controlled polishing machine, near uniform reflectivity may be obtained. The group delay of the SP-LCFBG was measured using the modulation phase shift method [10]. The modulation frequency, wavelength step and time resolution were 2.5 GHz, 20 pm, and 62.5 fs, respectively. Figure 5 shows the measured group delays for four values of inactive length d. We can notice that the slope of the group delay decreases as the inactive length increases. The slope decreasing appears due to the reduction of effective grating length while the reflection bandwidth is fixed. The inset of Fig. 5 shows the relationship between the inactive length and dispersion of the SP-LCFBG, and it shows that the dispersion of the SP-LCFBG linearly increases (the magnitude linearly decreases) to inactive length. We think that the difference between the theoretical dispersion line and
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Fig. 5. Measured group delays. Inset is the relationship between the inactive length and dispersion of the SP-LCFBG.
measured value comes from imprecise positioning of the index matching block or nonuniform side polishing. The proposed device has several sources of polarization dependencies. Even before side-polishing, an LCFBG has polarization dependencies due to the grating fabrication method. We used the phase mask method and a UV beam was irradiated from one direction. Thus, the index change due to photosensitivity is not uniform across the fiber core and asymmetric grating is fabricated. Figure 6a shows the polarization dependencies of an LCFBG before side-polishing. Broadband light was inserted to the LCFBG via linear polarizer and the reflected spectrum was measured using a circulator and optical spectrum analyzer. Measured maximum polarization dependent loss (PDL) and center wavelength separation was 0.20 dB and 0.013 nm, respectively. After side-polishing the polarization dependencies will be changed due to asymmetric cladding and resultant effective core index variation. However, unless the polishing surface is aligned to the birefringence axis of the LCFBG, this effect will not be simply added to the birefringence due to UV irradiation. In the experiment, we did not align the birefringence axis so the variation of polarization dependencies after side-polishing cannot be thought simply as the amount of side-polishing effect. Figure 6b shows the spectrum of a side-polished CFBG for orthogonal polarizations. The spectral shapes are changed after side-polishing due to non-ideal side-polishing. The maximum polarization dependent loss (PDL) was 0.24 dB and center wavelength separation was 0.016 nm. The PDL and center wavelength shift are slightly increased. If the birefringence axis due to side-polishing is tilted 90◦ to the birefringence axis of UV irradiation, the resultant polarization dependencies will be decreased. Figure 6c is the spectrum of a bent LCFBG after side-polishing. Bending of a grating may cause additional birefringence due to asymmetric strain distribution. The center wavelength shift was 0.014 nm and maximum PDL was 0.28 dB. We think that the decrement of center wavelength shift after bending is due to misalignment of birefringence axis. Thus, the influence
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Fig. 6. Polarization dependent spectrum of an LCFBG (a) before side-polishing, (b) after side-polishing (without bending), and (c) after side-polishing (with bending of y = 8 mm for d = 0 mm).
of bending countervailed the birefringence of side-polished LCFBG. Due to increment of bending loss, however, PDL increased.
3. Conclusion We proposed a new method for dispersion tuning of an LCFBG without center wavelength shift and bandwidth broadening. The effective grating length is controlled by evanescent field coupling to external material and the chirping rate is tuned by cantilever
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beam bending. We showed experimentally that the proposed method well operates within 0.75 nm fixed bandwidth without center wavelength shift and the dispersion is tuned from −687.24 to −256.11 ps/nm. We think that the proposed device may not be proper to be applied in the real field with current form. To be applied in the field, high performance side-polishing technique should be developed. In addition, replacement of mechanical tuning part with non-mechanical tuning such as thermal tuning may improve the durability and performance of the device. We are now studying on the methods to reduce the polarization dependencies such as wrapping the grating region with oil of which refractive index is the same as that of the cladding of the fiber.
References [1] K.-M. Feng, J.-X. Cai, V. Grubsky, D.S. Starodubov, M.I. Hayes, S. Lee, X. Jiang, A.E. Willner, J. Feinberg, Dynamic dispersion compensation in a 10-Gbit/s optical system using a novel voltage tuned nonlinearly chirped fiber Bragg grating, IEEE Photon. Technol. Lett. 11 (1999) 373–375. [2] M.M. Ohn, A.T. Alavie, R. Maaskant, M.G. Xu, F. Bilodeau, K.O. Hill, Tunable fiber grating dispersion using a piezoelectric stack, in: Technical Digest of Optical Fiber Communication Conference (OFC ’97), Dallas, TX, 1997, pp. 155–156, paper WJ3. [3] T. Imai, T. Komukai, M. Nakazawa, Dispersion tuning of a linearly shirped fiber Bragg grating without a center wavelength shift by applying a strain gradient, IEEE Photon. Technol. Lett. 10 (1998) 845–847. [4] C.S. Goh, S.Y. Set, K. Taira, S.K. Khijwania, K. Kikuchi, Nonlinearly strain-chirped fiber Bragg grating with an adjustable dispersion slope, IEEE Photon. Technol. Lett. 14 (2002) 663–665. [5] S. Chung, J. Kwon, S. Baek, B. Lee, Group delay control of super-imposed fiber gratings using a two-column system mounted on a rotatable disk, IEEE Photon. Technol. Lett. 16 (2004) 153–155. [6] J. Kwon, S. Chung, Y. Jeong, B. Lee, Group-delay-tailored chirped fiber Bragg gratings using a tapered elastic plate, IEEE Photon. Technol. Lett. 14 (2002) 1433–1435. [7] J. Kwon, B. Lee, Dispersion-order selectable chromatic dispersion compensator using strain-profile modification blocks, IEEE Photon. Technol. Lett. 15 (2003) 1564–1566. [8] B.J. Eggleton, J.A. Rogers, P.S. Westbrook, T.A. Strasser, Electrically tunable power efficient dispersion compensating fiber Bragg grating, IEEE Photon. Technol. Lett. 11 (1999) 854–856. [9] Y.W. Song, D. Starodubov, Z. Pan, Y. Xie, A.E. Willner, J. Feinberg, Tunable WDM dispersion compensation with fixed bandwidth and fixed passband center wavelength using a uniform FBG, IEEE Photon. Technol. Lett. 14 (2002) 1193–1195. [10] D. Derickson (Ed.), Fiber Optic Test and Measurement, Prentice Hall International, Englewood Cliffs, NJ, 1998.
Tunable dispersion compensation with fixed center wavelength and bandwidth using a side-polished linearly chirped fiber Bragg grating ✩ Jaejoong Kwon, Younghee Jeon, Byoungho Lee ∗ School of Electrical Engineering, Seoul National University, Kwanak-Gu Shinlim-Dong, Seoul 151-744, South Korea Received 13 April 2004 Available online 1 October 2004
Abstract We propose a new method for tuning the dispersion of a linearly chirped fiber Bragg grating with fixed center wavelength and bandwidth. We polished one side of the grating and attached it to a cantilever metal beam. By contacting an index matching block to the polished face of the grating, we could shorten the effective grating length through which light can propagate in the core of fiber. Decrease of grating length results in reflection bandwidth narrowing. However, by bending the grating attached on the cantilever beam, we could restore the bandwidth. Because the grating is shortened without bandwidth narrowing, the amount of dispersion is decreased. By using this method, we tuned chromatic dispersion of the linearly chirped fiber Bragg grating from −687.24 to −256.11 ps/nm for 0.75 nm fixed bandwidth without center wavelength shift. 2004 Elsevier Inc. All rights reserved. Keywords: Dispersion compensation; Fiber Bragg grating; Chirped fiber grating
✩
This work was partly supported by Novera Optics, Palo Alto, CA, USA.
* Corresponding author. Fax: +82-2-873-9953.
E-mail address: [email protected] (B. Lee). 1068-5200/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.yofte.2004.08.002
160
J. Kwon et al. / Optical Fiber Technology 11 (2005) 159–166
1. Introduction Chromatic dispersion, which causes different wavelengths to propagate at different group velocities in single-mode transmission fiber, is one of the critical sources of distortion of high speed signals in long-haul WDM systems. Because optical signal pulses contain a range of wavelengths, the chromatic dispersion causes the pulses to spread as they propagate. When the bit rate of the signal increases, spectral width of the signal increases and the time-interval between pulses decreases, so the transmission distance decreases inversely proportional to the square of the bit rate increment. While the dispersion limit for 10-Gb/s signals on standard single mode fiber (SMF) with a dispersion of 17 ps/nm is about 60 km, it is approximately 4 km for 40-Gb/s signal. To operate beyond this distance, dispersion compensation must be employed. Moreover, dispersion compensation should be tunable for several reasons such as inventory management, path length changes in dynamically reconfigurable networks, and environmental effects. The chirped fiber Bragg grating (CFBG) is one of the more promising technologies for tunable dispersion compensation. Various techniques for tuning the CFBG have been proposed [1–9]. The proposed methods tune the dispersion of CFBG by uniformly stretching a nonlinearly chirped FBG [1], non-uniformly stretching by use of discrete piezoelectric stretchers [2], bending a beam supporting the grating [3–5], stretching or compressing a tapered structure supporting the grating [6,7], or differential heating by use of tapered metal film deposited on the grating [8]. Because these technologies tune the dispersion of CFBG by changing the bandwidth and/or shifting the center wavelength while maintaining high reflectivity, it may reflect signals of the adjacent channel. Although the dispersion compensation unit is located at the end of the link after demultiplexing, especially in DWDM systems, non-ideal filtering in the demultiplexer retains small power of the adjacent channel and it causes a crosstalk. Therefore, the spectral change in the dispersion compensation unit may cause a variation of crosstalk. A CFBG for dispersion compensation is also a band reflection filter with sharp band edges, so it will improve the crosstalk properties if a CFBG maintains its spectral shape during dispersion tuning process. A tuning mechanism with fixed bandwidth and center wavelength has been reported previously [9]. However, there remains a reflection band originated from a uniform FBG part which has no dispersion compensation effect. This sacrificed spectral range will reduce usable bandwidth. In this paper, we propose a new method for tuning the dispersion of CFBG without center wavelength shift and bandwidth broadening or narrowing. We attached a linearly chirped fiber Bragg grating (LCFBG) on a metal cantilever beam and polished one side of LCFBG until the core mode can be perturbed by external index variation. The chirping rate of the grating is tuned by bending the beam and the effective grating length is controlled by closely contacting an index matching block on the polished face of the side polished LCFBG (SP-LCFBG). Unlike the previous work [9], there is no remained sacrificed spectral range. 2. Concept and experiment The first process to tune the dispersion of LCFBG is controlling the grating length. Figure 1 shows the schematic diagram to control the effective length of the LCFBG. We
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attached the LCFBG on a metal beam with UV epoxy (OG134, Epoxy Technology) and polished it manually with silicon carbide lapping films (1, 3, 9 µm grade, 3M Corp.). We polished one side of the LCFBG until the evanescent field of the core mode is exposed to exterior. Then, an index matching block that has slightly larger refractive index than core of the fiber is contacted on the polished face of the LCFBG. Because the evanescent field of the core mode overlaps the index matching block, the light propagating the core is coupled to the block and scattered in the block. The light cannot reach to the length of the LCFBG behind the block. Therefore, the effective grating length through which light propagates can be controlled by positioning the index matching block along the length of grating. Because the Bragg resonance wavelength varies linearly along the LCFBG, we can confirm the effective length-shortening effect by monitoring the reflection spectrum narrowing. Figure 2 shows the measured spectral responses for four values of inactive length d that is measured from the end of shorter wavelength of LCFBG. The second process is bending the cantilever beam to recover reflection bandwidth. Figure 3a shows the schematic diagram of the proposed tunable dispersion compensator with fixed bandwidth and center wavelength. To operate the device in negative dispersion regime, the signal is inserted from longer wavelength direction of the LCFBG and the index matching block is located at the opposite position. An index matching block and a
Fig. 1. Schematic diagram of tuning the effective length of an LCFBG.
Fig. 2. Spectral responses of SP-LCFBG for various inactive lengths.
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Fig. 3. (a) Schematic diagram and (b) operation principle of the proposed method for a tunable dispersion compensator with fixed bandwidth and center wavelength.
fixed block hold the grating and act as supports of a cantilever beam. Because the shorter wavelength part is interrupted by the index matching block, we bend the cantilever beam in compressive direction to generate negative strain gradient. When a cantilever beam with uniform cross-sectional area is bent in compressive direction, from the bending moment equation, the strain distribution along the fiber axis x (zero at the free end (Fig. 3), 0 x L, L is the beam length) can be described as follows: EIy (x) = M(x) = −P x, P 3 x − 3L2 x + 2L3 , y=− 6EI l P d ε(x) = = ≈ dy (x) = −d x, l ρ(x) EI
(1)
where E = Young’s modulus, I = second moment of the cross-sectional area, M = bending moment, P = concentrated force at the free end of the beam, d = distance from the neutral surface, ρ = the radius of curvature, ε(x) = local strain, y (x) = second-order derivative of y with respect to x. In the uniform cantilever beam d/EI is constant, so we can notice that the magnitude of strain is linearly increasing from the free end (x = 0) of the beam to the fixed point (x = L).
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Fig. 4. Restored spectra for various inactive lengths.
In addition, a strain-free state is obtained at the free end of the beam. Therefore, the longest wavelength part that is located at the free end of the beam undergoes no strain and its spectrum is fixed during bending process. While, the shorter wavelength part undergoes negative strain gradient so the spectrum is broadened to shorter wavelength direction. The cantilever beam is bent until the reflection bandwidth is recovered to its unperturbed state. Figure 3b shows the principle operation scheme of the proposed method. In the experiment, we used an LCFBG with 50 mm length, 1549.32 nm center wavelength, 0.75 nm 3 dB bandwidth, and 99.5% reflectivity. The refractive index of the index matching block is 1.451. Figure 4 shows measured spectra that are restored by bending for four values of inactive length d. We can notice that the reflection bands for various inactive lengths are well recovered. The center wavelength shift and bandwidth variation are less than 0.08 and 0.07 nm, respectively. In contrast to the previous work [9], the length of grating that is not bent does not reflect any light, so we can use the full reflection bandwidth of the grating without any sacrificed spectral range. However, there appears amplitude ripples (<3 dB) in the reflection spectra, and it is increased as the effective grating length is decreased. We think that the causes of the amplitude ripples are irregularities in side polishing process. By using a precisely controlled polishing machine, near uniform reflectivity may be obtained. The group delay of the SP-LCFBG was measured using the modulation phase shift method [10]. The modulation frequency, wavelength step and time resolution were 2.5 GHz, 20 pm, and 62.5 fs, respectively. Figure 5 shows the measured group delays for four values of inactive length d. We can notice that the slope of the group delay decreases as the inactive length increases. The slope decreasing appears due to the reduction of effective grating length while the reflection bandwidth is fixed. The inset of Fig. 5 shows the relationship between the inactive length and dispersion of the SP-LCFBG, and it shows that the dispersion of the SP-LCFBG linearly increases (the magnitude linearly decreases) to inactive length. We think that the difference between the theoretical dispersion line and
164
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Fig. 5. Measured group delays. Inset is the relationship between the inactive length and dispersion of the SP-LCFBG.
measured value comes from imprecise positioning of the index matching block or nonuniform side polishing. The proposed device has several sources of polarization dependencies. Even before side-polishing, an LCFBG has polarization dependencies due to the grating fabrication method. We used the phase mask method and a UV beam was irradiated from one direction. Thus, the index change due to photosensitivity is not uniform across the fiber core and asymmetric grating is fabricated. Figure 6a shows the polarization dependencies of an LCFBG before side-polishing. Broadband light was inserted to the LCFBG via linear polarizer and the reflected spectrum was measured using a circulator and optical spectrum analyzer. Measured maximum polarization dependent loss (PDL) and center wavelength separation was 0.20 dB and 0.013 nm, respectively. After side-polishing the polarization dependencies will be changed due to asymmetric cladding and resultant effective core index variation. However, unless the polishing surface is aligned to the birefringence axis of the LCFBG, this effect will not be simply added to the birefringence due to UV irradiation. In the experiment, we did not align the birefringence axis so the variation of polarization dependencies after side-polishing cannot be thought simply as the amount of side-polishing effect. Figure 6b shows the spectrum of a side-polished CFBG for orthogonal polarizations. The spectral shapes are changed after side-polishing due to non-ideal side-polishing. The maximum polarization dependent loss (PDL) was 0.24 dB and center wavelength separation was 0.016 nm. The PDL and center wavelength shift are slightly increased. If the birefringence axis due to side-polishing is tilted 90◦ to the birefringence axis of UV irradiation, the resultant polarization dependencies will be decreased. Figure 6c is the spectrum of a bent LCFBG after side-polishing. Bending of a grating may cause additional birefringence due to asymmetric strain distribution. The center wavelength shift was 0.014 nm and maximum PDL was 0.28 dB. We think that the decrement of center wavelength shift after bending is due to misalignment of birefringence axis. Thus, the influence
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Fig. 6. Polarization dependent spectrum of an LCFBG (a) before side-polishing, (b) after side-polishing (without bending), and (c) after side-polishing (with bending of y = 8 mm for d = 0 mm).
of bending countervailed the birefringence of side-polished LCFBG. Due to increment of bending loss, however, PDL increased.
3. Conclusion We proposed a new method for dispersion tuning of an LCFBG without center wavelength shift and bandwidth broadening. The effective grating length is controlled by evanescent field coupling to external material and the chirping rate is tuned by cantilever
166
J. Kwon et al. / Optical Fiber Technology 11 (2005) 159–166
beam bending. We showed experimentally that the proposed method well operates within 0.75 nm fixed bandwidth without center wavelength shift and the dispersion is tuned from −687.24 to −256.11 ps/nm. We think that the proposed device may not be proper to be applied in the real field with current form. To be applied in the field, high performance side-polishing technique should be developed. In addition, replacement of mechanical tuning part with non-mechanical tuning such as thermal tuning may improve the durability and performance of the device. We are now studying on the methods to reduce the polarization dependencies such as wrapping the grating region with oil of which refractive index is the same as that of the cladding of the fiber.
References [1] K.-M. Feng, J.-X. Cai, V. Grubsky, D.S. Starodubov, M.I. Hayes, S. Lee, X. Jiang, A.E. Willner, J. Feinberg, Dynamic dispersion compensation in a 10-Gbit/s optical system using a novel voltage tuned nonlinearly chirped fiber Bragg grating, IEEE Photon. Technol. Lett. 11 (1999) 373–375. [2] M.M. Ohn, A.T. Alavie, R. Maaskant, M.G. Xu, F. Bilodeau, K.O. Hill, Tunable fiber grating dispersion using a piezoelectric stack, in: Technical Digest of Optical Fiber Communication Conference (OFC ’97), Dallas, TX, 1997, pp. 155–156, paper WJ3. [3] T. Imai, T. Komukai, M. Nakazawa, Dispersion tuning of a linearly shirped fiber Bragg grating without a center wavelength shift by applying a strain gradient, IEEE Photon. Technol. Lett. 10 (1998) 845–847. [4] C.S. Goh, S.Y. Set, K. Taira, S.K. Khijwania, K. Kikuchi, Nonlinearly strain-chirped fiber Bragg grating with an adjustable dispersion slope, IEEE Photon. Technol. Lett. 14 (2002) 663–665. [5] S. Chung, J. Kwon, S. Baek, B. Lee, Group delay control of super-imposed fiber gratings using a two-column system mounted on a rotatable disk, IEEE Photon. Technol. Lett. 16 (2004) 153–155. [6] J. Kwon, S. Chung, Y. Jeong, B. Lee, Group-delay-tailored chirped fiber Bragg gratings using a tapered elastic plate, IEEE Photon. Technol. Lett. 14 (2002) 1433–1435. [7] J. Kwon, B. Lee, Dispersion-order selectable chromatic dispersion compensator using strain-profile modification blocks, IEEE Photon. Technol. Lett. 15 (2003) 1564–1566. [8] B.J. Eggleton, J.A. Rogers, P.S. Westbrook, T.A. Strasser, Electrically tunable power efficient dispersion compensating fiber Bragg grating, IEEE Photon. Technol. Lett. 11 (1999) 854–856. [9] Y.W. Song, D. Starodubov, Z. Pan, Y. Xie, A.E. Willner, J. Feinberg, Tunable WDM dispersion compensation with fixed bandwidth and fixed passband center wavelength using a uniform FBG, IEEE Photon. Technol. Lett. 14 (2002) 1193–1195. [10] D. Derickson (Ed.), Fiber Optic Test and Measurement, Prentice Hall International, Englewood Cliffs, NJ, 1998.