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Super-Separation Thin Film Filtering for Coexistence-Type Colorless WDM-PON Networks Jarmila Müllerová, Dušan Korček* University of Žilina, Faculty of Electrical Engineering, Workplace Liptovský Mikuláš, ul. kpt. J. Nálepku 1390, 031 01 Liptovský Mikuláš, Slovakia * Alcatel – Lucent Slovakia, a.s., Apollo BC II – B Block, Prievozská 4/4, 821 09 Bratislava, Slovakia Tel: (+421)41-513-1740, Fax: (+421)44-532397, e-mail:
[email protected] ABSTRACT Two wavelength blocking filters (WBF) are designed to separate wavelength regions allocated for the Gigabit passive optical network (GPON) downstream and upstream signals. The designed WBFs guarantee high filter contrast factors achieved by steep transmission curves in the vicinity of cut-on/cut-off wavelengths of 1290 nm / 1330 nm and 1480 nm / 1500 nm. FBFs have pass passbands of less than ~5 dB ripple and reject wavelengths outside the widths of the passband at ~32 dB. The deployment of both WBFs could not only prevent the allocated wavelength bands from the undesirable interference with the future next-generations PON such as XG-PON, but it could also vacate wavelength space for future WDM or additional services re-allocation. Keywords: coexistence, GPON, XG-PON, band-pass filter, thin film. 1. INTRODUCTION Passive optical networks (PON) are the most attractive and extensively studied solutions for optical access technologies. Legacy PONs (GPON = Gigabit-capable PON and EPON = Ethernet-based PON) built with power splitter technologies are currently deployed as high-speed and high-capacity optical access technologies using time division multiplexing (TDM). All signals are distributed through this optical network from the optical line terminal (OLT) to every end user’s optical network termination unit (ONT) connected on the same PON branch. The next-generation (NG) PON should satisfy demands for increasing traffic and higher bandwidths [1-4]. All future strategies for NG-PON evolution are expected to deploy wavelength division multiplexing (WDM). The candidate systems are based on evolutionary growth (the so-called NG-PON1) or revolutionary change (NG-PON2). Current GPONs systems are intended to migrate to NG-PON1 applying identical colorless ONTs. The migration would occur in the same optical distribution network which implies coexistence. The advantage appears in cost saving, easier planning, maintaining and expanding of this network. The evolutionary growth under NG-PON1 is based on the demand of the minimal equipment investments [1]. NG-PON1 should operate with the same infrastructure as GPON. However, coexistence should be supported by some preventative measures to avoid interferences between downstream and upstream channels of GPON and NG-PON1. Therefore, current GPON and NG-PON1 coexistence requires wavelength separation of signals assigned for ONTs of GPON, 10 Gbit PON (termed XG-PON) and WDM-PON. Low-cost wavelength blocking filters (WBF) are required for the wavelength band separation to ensure that GPON ONTs can operate undisturbed alongside NG-PON1 deployments. Thin-film passive filters (TFF) are suitable low-cost, ONTindependent (i.e. coexisting) and simple operation candidates [5, 6]. In this article two WBFs based on TFF are designed to separate wavelength regions allocated for the GPON downstream and upstream signals. The deployment could not only prevent the allocated wavelength bands from interference with XG-PONs but also vacate certain wavelength bands for re-usage, e.g. for course WDM (CWDM). 2. GPON AND XG-PON COEXISTENCE ITU-T standardization provides framework for the coexistence and defines reserved wavelength ranges. GPON is specified by ITU-T G.984 series [7]. The wavelength allocation meets this standard and synergies with IEEE (Fig. 1). The band allocations for GPON are: 1260 – 1360 nm for upstream and 1480 – 1500 nm for downstream. The neighboring bands are referred to as guard bands separating basic and enhancement bands and preventing interference. G.984.5 standard recommends pre-installing commercially available low-cost WBFs in GPON ONTs to obtain the required isolation outside the guard bands [7]. In Fig. 2 we propose the coexistence of GPON and XG-PON by implementing two super-separating WBFs in the GPON ONTs (Fig. 2). We suppose that these filters inserted into the legacy PON deployments can significantly decrease the migration costs. WBFs of steep spectral characteristics should separate the allocated passbands to avoid the interference and moreover to narrow the guard bands. An ideal band-pass has a completely flat passband and completely rejects all wavelengths outside the widths of the passband. Accordingly, the requirements for the performance of the promising optical band super-separation filters are:
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− at least 32 dB (according to ITU-T Recommendation G.984.2) insertion loss outside the required passband width and maximum 5 dB insertion loss within the required passband width of 1290 – 1330 nm for WBF upstream and 1480 – 1500 nm for WBF downstream, − guarantee of the filter contrast factor (dB/nm) as high as possible achieved by steep transmission curves in the vicinity of cut-on/cut-off wavelengths of 1290 nm / 1330 nm and 1480 nm / 1500 nm, − minimizing severity of production by designing both band-pass WBFs as combinations of short-pass and a long-pass filters in series to achieve the required passbands. O band
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Figure 2. Coexistence of GPON and XG-PON using original ONTs supplemented by WBFs. 3. WBF DESIGN The thin-film interference filters (TFF) are suitable candidates for WBFs. TFF is a sequence of non-absorbing thin films of the thickness that is comparable to the wavelength of light and of materials of high and low refractive index. They should have optical characteristics resistant against temperature changes. The interference of light entering a multilayer structure of a specific number of alternating thin films causes the spectral dependent transfer characteristics of the filter (transmittance or reflectance of light). The number of layers, layer materials, optical properties and thicknesses influence significantly TFF transmission characteristics. Therefore, the spectral transmittance or reflectance of a desired filter can be tailored for a specific application by the number and optical properties of layer used in the design. The spectral bands generally arise from the n-fold periodic repetition of high (H) and low (L) refractive index layers. Steep transmission or reflection characteristics can be achieved by structures consisting of tens of layers.
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Let a TFF structure consist of M dielectric layers of refractive indices N i and optical thicknesses di (i =1, ..., M). The geometrical thickness of the layer is h i = di /Ni. For nearly normal light incidence and non-absorbing media, the elementary reflection coefficients ri of each interface between two adjacent layers defined as the ratio of N − N i −1 reflected and incident electric fields can be written as ri = i , i = 1, 2,… , M + 1 , where N0 is the N i + N i −1 refractive index of the semi-infinite substrate and NM+1 is the refractive index of semi-infinite ambient medium. The overall reflectance R defined as the ratio of reflected and incident light intensities can be obtained recursively in a variety of ways, e.g. by the propagation matrices or the propagation of the reflection responses [8]. The spectral transmittance T of a multilayer non-absorbing structure can be calculated as T = 1 – R. We used a Delphi-based program with the recursive algorithm for the simulation of spectral transmittance of various structures with SiO2 as L-layer and TiO2 as H-layer. The simulations were performed to separate allocated GPON and XG-PON spectral bands at (1290 – 1330) nm and (1480 – 1500) nm. Schott NBK 7 glass was simulated as the substrate; air as ambient medium. The values of refractive indices of TiO2, SiO2 and Schott NBK 7 glass were taken from [9].
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4. RESULTS Large numbers of thin films may cause difficulties in the WBF fabrication connected with the necessity to control the thickness and optical properties of individual thin films (the random deviations of film thickness and refractive indices may significantly affect resulting transmittance and reflectance). Therefore, both WBFs have been designed as combinations of two filters. The most effective structures meeting the requirements for WBF characteristics are combinations of long-pass (LP) and short-pass (SP) filters. Therefore, LP and SP filters should be designed for resulting WBF(up) for GPON upstream at (1290 – 1330) nm and WBF(dn) for GPON downstream at (1480 – 1500) nm. H H The following sequence: substrate/( )(LH) n /L/( )/ambient written in the commonly used notification, 2 2 where L(H) are layers of low (high) refractive index of the specific thickness, was found to be satisfactory. The repetition number n of (LH) elements inside the structure was equal to 26. The geometrical thicknesses of the L, H layers were fixed at 345 nm for SP filter of WBF(up), 590 nm for LP filter of WBF(up), 391 nm for SP filter of WBF(dn) and 681 nm for LP filter of WBF(dn). The simulations show that the deviations of the thickness by ~ 1 nm cause the undesirable shift of cut-on/cut-off wavelengths. The filter contrast at the vicinity of the cut-on/cut-off wavelengths is achieved at the expense of the passband or stop-band ripple (Fig. 3, 4). However, both WBFs succeed to keep the ripple under ~ 5 dB in agreement with the starting requirements. The WBFs are designed to have zero roll-offs within the stop bands at the level of 32 dB what means that the transmittance outside the desired spectral range is completely rejected.
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Figure 3. Transmittance of the designed WBF(up) for Figure 4. Transmittance of the designed WBF(dn) for GPON upstream. LP (SP) are long-pass (short-pass) GPON downstream. LP (SP) are long-pass (shortfilters, WBF (up) is a LP and SP combination. pass) filters, WBF (dn) is a LP and SP combination. A closer look at the spectral responses of both WBFs is in Fig. 5, 6. We characterize the filter spectral width response by the abruptness coefficient η = Δλ5dB / Δλ32dB determining the abruptness of the transition from the passband to the stopband. The coefficient η as close as 100% represents the ideal super-separation filter. For the designed WBFs, η was determined to be: WBF(up) η = 84%, WBF(dn) η = 5%. We see that WBF(up) filter was designed much steeper than the WBF(dn). Integrated transmittance (filled areas in Fig. 5, 6) represents the
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difference in the energies of light on the ONT receivers/transmitters through WBFs. WBF(up) manifests the attenuation against the ideal (completely flat and steep passband) filter of ~18%, WBF(dn) of ~ 35%, which is acceptable for common GPON ONTs and in agreement with the requirement of maximum insertion loss of ~5 dB within the allocated spectral bands. Therefore, we conclude that both filters satisfy the proposed conditions being suitable for the coexistence and vacating spectral bands for future WDM PONs.
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Figure 5. Transmittance of WBF (up) (blue line). Integrated transmittance (filled area) attenuated by 18% against the ideal (completely flat and steep passband) filter (violet solid line).
Figure 6. Transmittance of WBF (dn) (blue line). Integrated transmittance (filled area) attenuated by 35% against the ideal (completely flat and steep passband) filter (violet solid line).
5. CONCLUSIONS Thin-film filters as WBFs have been designed as steep as possible to reject the wavelength bands outside the designed bands of GPON downstream and upstream to coexist with XG-PON. Although the primary mission is to prevent the interference, the proper design and steep filter characteristics are expected to be promising for using WBFs in the first migration phase for the downstream and upstream band and vacate partly the guard bands for future legacy WDM channels. ACKNOWLEDGEMENTS This work was partly supported by the Slovak Grant Agency under the project No. 1/0411/10. REFERENCES [1] M. De Andrade, et al.: Evaluating strategies for evolution of passive optical networks, IEEE Communication Magazine, to appear 2011. [2] J. Kani, et al.: Next-generation PON – Part I: Technology roadmap and general requirements, IEEE Communication Magazine, pp. 43 - 49, Nov. 2009. [3] F. Effenberger, et al.: Next-generation PON – Part II: Candidate systems for next-generation PON, IEEE Communication Magazine, pp. 50 - 56, Nov. 2009. [4] F. Effenberger: The XG-PON System: Cost effective 10 Gb/s access, J. Lightwave Technol., vol. 29, pp. 403-409, Feb. 2011. [5] A.N. Uehara, R. Otowa, R. Okuda: Advanced band separation thin-film filters for coexistence-type colorless WDM-PON, in Proc. Optical Fiber Communication / National Fiber Optic Engineers Conference 2008 (OFC/NFOEC 2008), 2008, pp.1 - 3. [6] D. Korček, J. Müllerová: On influence of optical accuracy of the band-pass thin-film filter design for coexistence-type colorless WDM-PON, in: Proc. SPIE Wave and Quantum Aspects of Contemporary Optics, vol. 7746, 2010, p. 77461X-1 – 7. [7] ITU-T Rec. G.984 series: Gigabit-capable passive optical networks (G-PON). [8] A. Thelen: Design of Optical Interference Coatings, New York: McGraw-Hill Book Company, 1989. [9] E. D. Palik: Handbook of Optical Constants of Solids, San Diego & London: Academic Press, 1998.
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