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

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.


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 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.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];

Theoretical backgrounds
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], 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.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 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.

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 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 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.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 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.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 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.

Fig. 1 .
Fig. 1.Principle of the proposed inter-band router based on a tunable 2X2 coupler.

Fig. 2 .
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.

Fig. 3 .
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.

Fig. 4 .
Fig. 4. Variation of the center wavelength and the maximum channel isolation as a function of the rotation angle.

Fig. 5 .
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.

Fig. 6 .Fig. 7 .
Fig. 6.Variation of the center wavelength and the maximum channel isolation as a function of the rotation angle.