All fiber optical inter-band router for broadband wavelength division multiplexing

All fiber optical inter-band router for broadband wavelength division multiplexing W. Shin, S.W. Han C. S. Park and K. Oh Department of information and communications, Kwangju Institute of Science and Technology (K-JIST), 1 Oryong-dong, Buk-gu,Kwangju, 500-712, KOREA Tel: +82-62-970-2213, Fax:+82-62-970-2204 koh@kjist.ac.kr Abstract: We propose a new type of all-fiber device for inter-band router using a novel hybrid waveguide-mems technology. Both continuous and discrete band-routing functions are realized by precise twist control over the fused coupling region, which results in π phase shift between the output ports. Experimentally we demonstrate inter-band routing functions between O and C-band as well as between E and L-band with a low insertion loss, wide bandwidth of operation, high channel isolation and fast response. 2004 Optical Society of America OCIS codes: (060.0060) Fiber Optics and Optical communications; (060.2310) Fiber Optics. References 1. R. Li, Service-integrated selective DWDM in metro area, in Proceedings of World Computer Congress- International Conference on Communication Technology (WCC - ICCT 2000), (Beijing, China, 2000), 1184-1187. 2. T. Pfeiffer, J. Kissing, J.-P. Elbers, B. Deppisch, M. Witte, H. Schmuck and E. Voges, Coarse WDM/ CDM/TDM concept for optical packet transmission in metropolitan and access networks supporting 400 channels at 2.5 Gb/s peak rate, J. Lightwave Technol. 18, 1928-1938 (2000). 3. IEEE standard 802.3 (2000 edition), 1039-1041. 4. P. M. Hagelin, Scalable optical cross-connect switch using micromachined mirrors, IEEE Photon. Technol. Lett. 12, 882-884 (2000). 5. J. E. Fouquet, Compact optical cross-connect switch based on total internal reflection in a fluid-containing planar lightwave circuit, in Proceedings of Optical Fiber Communication Conference and Exhibit (OFC2000), (Optical Society America, Washington, D.C., 2000), 204-206. 6. M. B. J. Diemeer, W. J. De Vries and K.W. Benoist, Fused coupler switch using a thermo-optic cladding, Electon. Lett. 24, 457 (1988). 7. T. A. Birks, Power equalization in 3x3 fibre couplers by twisting tuning, Electron. Lett. 27, 1087 (1991). 8. W. Shin and K. Oh, Re-configurable 2x2 wavelength selective cross connect switch based on a fused optical fiber coupler, in Proceedings of Optical Fiber Communication Conference and Exhibit (OFC2002), (Optical Society America, Washington, D.C., 2002), paper ThGG57. 9. W. Shin and K. Oh, Re-configurable all fiber wavelength selective optical switch based on tunable fused fiber couplers, in Proceedings of The 16th International Conference on Optical Fiber Sensors (OFS-16), (Noh-Drama Theater, Nara, Japan, 2003), paper ThP-14. 10. W. Shin, Y. Jeong, K. Oh, Broadband high power fiber variable optical attenuator for gain control of raman amplifier, in Proceedings of 28 th European Conference on Optical Communication (ECOC2002), (September 8-12, Bella Center, Copenhagen, Denmark, 2002), paper P1.24. 11. F. Gonthier, D. Ricard, S. Lacroix and J. Bures, Wavelength-insensitive couplers in form of all-fiber Mach-Zehnder interferometer, Electron. Lett. 27, 1829 (1991). 12. G. W. Scherer, Stress of fibers:cladding, Appl. Opt. 19, 2000-2006 (1980). 1. Introduction As optical communication bands for wavelength division multiplexing (WDM) systems continues to extend in silica optical fibers, various technologies are being developed to fully utilize the optical bandwidth. Dense WDM technologies in conventional band (C-band) from (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1815

1530nm to 1565nm are being deployed and serviced in the commercial long haul systems. Analogous technologies are being applied to transmission bands from 1200nm to 1700nm including original band (O-band), extended band (E-band), short wavelength band (S-band), long wavelength band (L-band), and ultra-long wavelength band (U-band), whose individual bandwidths are over 30nm [1]. As an economic WDM solution for access networks, coarse WDM systems in the above wavelength ranges have been also proposed with much less requirements over MUX/DEMUX and light sources to alleviate system costs [2]. On the while, Giga-bit Ethernet systems are being actively developed for LAN and WAN applications using optical signals in 0.8, and 1.3µm [3]. As these techniques further develop, wavelength selective routing and switching systems will be in great demand to control data traffic with a high compatibility. Especially signal routing among those optical communications bands would make the optical network highly re-configurable and flexible for seamless transport of data. For example, in a DWDM metro network in C-band, it would be very useful to have an optical solution to route the signals to Giga-bit Ethernet system at 1.3 µm and or CWDM signals in other optical bands, and vice versa. Current routing technologies, however, have concentrated on manipulation of individual optical channels narrowly spaced within a band, especially within C, and L bands. Switches based on micro-machined mirrors are being developed taking advantages of integration capabilities and scalability [4]. Fluid containing planar light wave circuits have also demonstrated similar switching performances using total internal reflection in the fluid droplets [5]. These switches, however, are based on free-space beam steering within optical interconnects, which are sensitive to wavelength separation among optical signals as well as environmental perturbations. For a wide spacing operation such as between O and C-band, waveguide type switches would be preferred for high channel isolations and versatile wavelength selectivity. Thermo-optic switches have been demonstrated in a 2 2 coupler by thermal index change of clad in the coupling region but with limited channel isolation and switching time [6]. Mechanical tuning of fused taper coupler has been attempted to adjust the peak transmission position and power splitting ratio [7] and recently the authors have experimentally demonstrated a 2 2 cross connect switch using a mechanically tunable fiber fused coupler that can be applied to inter-band routing between 1.3 µm Gigabit Ethernet signal and 1.5 µm C-band WDM signals [8]. In recent works [9-10] we have proposed a new concept waveguide-mems where a micron or sub-micron size coupler waveguide is mounted on a MEMS platform such that the coupling constant of the waveguide is controlled by mechanical perturbation to result in flexible control over spectral selectivity, power splitting ratio, and polarization extiction ratio, which have not been achieved in prior optical MEMS. In this paper, we further develop the technology to demonstrate a new type of inter-band router among optical bands such as O/C band and E/L band so that the device could re-route the whole channels in the corresponding bands, for the first time to the best knowledge of the authors. Operation details and device characteristics are described for the devices. 2. Theoretical backgrounds Fig. 1. Principle of the proposed inter-band router based on a tunable 2X2 coupler. (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1816

The principle of the proposed device is schematically shown in Fig. 1. A wavelength selective coupler is made to separate two bands centered at λ 1 and λ 2 to port 1 and 2, respectively. The coupling constants for fused taper couplers are given as [11]; 2 δu ( z) K0( W ) C z) = (1) 3 bv K ( W ) ( 2 1 where δ= 1+(n cladding +n air ) 2 and U, V, W(z) are the normalized frequencies in a circular waveguide. K i is the modified Bessel function of the second kind of order i. b is the radius of the cylindrical waveguide in the coupling zone. Over the coupling zone, torsional stresses are then deliberately applied by precise twist-control. The relation between torsional stress and refractive index change is known as Eq. (2) [10], τ r nr = Cbσ z = Cb ρ 3 dρ (2) Aρ 0 where τ is external torque. ρ is radius and r is diameter of fiber. n r is relative refractive index in radial direction and C b is stress optic coefficient (in silica 4.2x10-12 Pa -1 ) and σ z is axial stress. 0.0 Refractive Index ( n r ) -2.0x10-5 -4.0x10-5 -6.0x10-5 -8.0x10-5 -1.0x10-4 -1.2x10-4 -1.4x10-4 0.0 5.0M 10.0M 15.0M 20.0M 25.0M 30.0M Axial Stress (σ z ) [Pa] (a) (b) Fig. 2. (a) Refractive index change along the applied axial stress. (b) Coupling characteristic change due to the width variation of coupling region of coupler. Βy twisting a fused coupler, the axial stress is introduced to coupling region and it alters the coupling constant to result in π phase shift routing the outputs between the ports [8]. The relative refractive index change, n r, due to the axial stress, σ z, is shown in Fig. 2(a). The twisting process also affects the geometrical dimension such as the waist width of coupling (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1817

region, which can bring the further change of coupling coefficient in fused coupler. The Fig. 2(b) shows the change of coupling characteristics induced by the variation in the waist width at the coupling region. Therefore, by applying torsional stress on fused coupler, its pristine coupling characteristic can be changed by either of the-stress optic effect or geometrical deformation of coupling region. 3. Experiments and results In order to design inter-band routers for O/C band and E/L band, 2 2 fused fiber couplers whose peak wavelengths at the output ports coincided at the band centers were fabricated by optimization of tapering and fusion conditions. The location of the bands were confirmed by measuring the transmission spectra through the output ports as shown in Fig. 3(a) and Fig. 5(a) for O/C and E/L bands, respectively. Note that the transmission peaks at the output ports in Fig. 3(a) are located at 1310 and 1545 nm for O and C bands, respectively with the peak-topeak channel isolation over 25 db and insertion loss less than 0.15 db. (a) (b) Fig. 3. Transmission characteristic of O/C-band router : (a) Shift of transmission spectra under torsion stress, (b) Transmission spectra of output port before and after routing. Transmission spectra through one of output ports were continuously monitored as a torsional stress was applied over the coupling region using a specially designed fiber rotating apparatus [8]. The spectrum of the C-band output port shifts continuously toward O-band as (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1818

shown in Fig. 3(a) such that the ports initially set for C-band showed the maximum transmission for O-band after switching, which verifies π phase shift induced by the torsional stress. In order to analyze the bandwidth of operation, the output port transmission was carefully measured before and after routing and the results are shown in Fig. 3(b). Here we used a power stabilized white light source and the power levels were measured using an optical spectrum analyzer. For the O/C band router in Fig. 3(b), bandwidths of operation more than 32, and 59 nm were obtained for channel isolation of 20, and 15 db, respectively. The routed ports showed very low insertion loss of 0.15 db and the bandwidths of operation for 10 db and 20dB isolation was 99.81 nm and 32.43 nm, respectively. Note that the proposed devices can be cascaded in serial such that higher channel isolation could be achieved. In fact by cascading two identical router in serial channel isolation over 40 db was achieved for 30 nm bandwidth in both O and C bands. The device performances were furthermore assessed by quantifying the tunability and the change in channel isolation. As the torsional stress was applied over the coupling region as shown in Fig. 1, the transmission through one of the ports of the device was monitored to trace the center wavelength and the maximum channel isolation. The results are plotted in Fig. 4. For O/C inter-band router, complete routing was achieved by less than two turns of the coupling region with a rotation angle of ~560 o, which corresponds to the shift of the center wavelength from 1310nm to 1550nm as shown in Fig. 4. The center wavelength could be tuned within a broad range of 230nm and a linear fitting showed the tuning slope of 0.448 nm/rotation angle ( o ). The maximum channel isolation was found to be stable below 30dB in the entire routing process. Wavelength (nm) 1550 1500 1450 1400 1350 1300 Center wavelength variation Center wavelength isolation variation Linear Fitting λ/ θ θ= 0.448 nm/ o 0-5 -10-15 -20-25 -30-35 Maximum Isolation (db) 1250-40 0 100 200 300 400 500 600 Rotation angle (degree) Fig. 4. Variation of the center wavelength and the maximum channel isolation as a function of the rotation angle. We have also fabricated an inter-band router for E/L bands. Similar measurements and characterization processes have been followed as in the case of O/C inter-band router. The results are shown in Figs. 5 and 6. The transmission peaks of the pristine coupler s outputs were located at 1410 and 1590 nm, for E and L-bands, respectively with the channel isolation over 25 db and insertion loss below 0.15 db. As we apply the torsional stress over the coupling region by the micro rotational platfom, we could achieve successfully the E/L interband routing as shown in Fig. 5(a). The transmission peak from the L band output port continuously shifted toward the E band, confirming the π phase shift in the proposed device once again. The bandwidth of operation was measured for the routed E and L band ports and the results are shown in Fig. 5(b). Bandwidths of more than 75, and 26 nm were obtained for channel isolation of 10 and 20 db, respectively for E/L bands. These bandwidths are narrower than those of O/C inter-band router, which is attributed to the coupling characteristics in E (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1819

band near the LP 11 mode cut-off wavelength and relatively weak guidance in the longer wavelength of the L-band. Improvements in bandwidth can be made with a special cut-off wavelength optimized single mode fibers. Note that those bandwidths, nevertheless, do cover the major portions of individual optical bands and we could confirm that the device could be of practical use for broadband routing applications. (a) (b) Fig. 5. Transmission characteristic of O/C-band router : (a) Shift of transmission spectra under torsion stress, (b) Transmission spectra of output port before and after routing. In Fig. 6, the tuning performance of the E/L router is characterized in terms of the rotation angle. The E-band port is switched to the L-band port by a rotation angle of ~760 o. The tuning range of the center wavelength was 180nm and the linear tuning slope was 0.249 nm/rotation angle ( o ). The maximum channel isolation was kept under 27dB. Due to its monotonic and continuous nature of the response to rotation as shown in Figs. 4 and 6, simultaneous automation of power splitting and inter-band routing could be realized with a proper electronic circuitry. In the experiments, we have designed a preliminary rotational platform for the coupler that gave switching time less than 1 second, which could be further reduced by optimization of electronic circuits. The characteristics of the other port in the devices showed the exactly reciprocal response to Fig. 3(a) and Fig. 5(a), which confirms the proposed routing principle shown in Fig. 1. The spectrum regains its initial shape when the (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1820

torsional stress was relieved and no abnormal hysteresis was observed during repeated cycles. By varying the torsional stress, it is also found that the power splitting ratio between the two ports could be continuously adjusted. The traces of the transmission spectra are shown in the Fig. 3(a) and Fig. 5(a). The results indicate that the device can simultaneously function as a variable power divider between the two ports such that optical signals in O/C bands, or E/L bands, can be routed to either of two output ports with a re-configurable power splitting ratio. The routing performance in 10Gb optical signals at 1550nm in C-band was measured by observing eye-patterns and BER curves. The results are summarized in Fig. 7. Wavelength (nm) 1600 1575 1550 1525 1500 1475 1450 1425 1400 Center wavelength variation Center wavlength isolation variation Linear Fitting λ/ angle =0.249nm/ o 0 100 200 300 400 500 600 700 Rotation Angle (degree) 0-5 -10-15 -20-25 -30-35 -40 Maximum Isolation (db) Fig. 6. Variation of the center wavelength and the maximum channel isolation as a function of the rotation angle. (a) Bit Error Rate(BER) 10-1 10-3 10-5 10-7 10-9 10-11 10-13 10Gb signal at 1550nm port Befor switching After switching -22-21 -20-19 -18-17 -16 Reciever Power[dBm] (b) (c) Fig. 7. Band routing performance in 1550nm 10Gb signal:(a)bit error rate measurement setup, (b)eye diagram, (c)ber vs Reciever power. (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1821

The clearly opened eye pattern at 17dBm receiver power after switching in port 2 is shown in Fig. 7(b). The bit error rate (BER) of 10Gbit signal at 1550nm was measured along the receiver power variation. In Fig. 7(c), the BER value changes from 10-12 to 10-2 along receiver power. This results show that the proposed device could be applied for more than 10Gbps transmission system with a low data loss. 4. Conclusion We presented a novel 2 2 inter-band router using unique monotonic change of coupling constant by torsional stress over the coupling region of a broad band WDM coupler provided by a micro-mechanical rotational platform. Within the rotation angle range less than 560 o for O/C band and 760 o for E/L band, inter-band routing and continuous control of power splitting ratio from 0 to 100% were achieved. The device showed an insertion loss less than 0.16dB, maximum channel isolation over 27 db and switching time less than 1 sec. Effective bandwidth of operation for 20dB channel isolation was found to be over 32nm and 26nm for O/C and E/L band routers, respectively. The proposed device could be used in band-to-band cross-connect optical switches for broadband WDM applications. Acknowledgments This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) through the Ultrafast Fiber-Optic Networks Research Center at Kwangju Institute of Science and Technology. (C) 2004 OSA 3 May 2004 / Vol. 12 No. 9 / OPTICS EXPRESS 1822