Active management of 100-GHz-spaced WDM channels - IEEE Xplore

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Active management of 100-GHz-spaced WDM channels A. D. Cohen Formerly at Cambridge Univ. Engineering Dept. (where this work was performed), now with JDS FITEL Inc., 570 West Hunt Club Road, Nepean, Ontario, K2G 5W8, Canada. Tel. (613) 727 1304. Fax (613) 727 1571. E-mail: [email protected]

M. C. Parker Fujitsu Telecommunications Europe Ltd. , Northgate House, St. Peter's St. , Colchester, Essex, CO1 lHH, UK. Tel. +44 (0)1206 363007. Fax +44 (0)1206 363009. E-mail: M. [email protected]

R. J. Mears Cambridge University Engineering Department, Trumpington St. , Cambridge, CB2 lPZ, UK. Tel. +44 (0)1223 332784. Fax +44 (0)1223 332662. E-mail: [email protected]

ABSTRACT We demonstrate the active management of eight 0.8-nm-spaced WDM channels using a polarisation-insensitive spatial-light-modulator-based holographic filter. Arbitrary permutations of dropped and passed channels are possible, as are weighted-amplitude passbands that effect power equalisation. Passband spectral engineering, yielding 'top hat' passbands, has also been demonstrated. Suppression of single dropped channels, consistently > 15dB in these experiments, can be straightforwardly improved by deploying an SLM of greater resolution and higher pixel number. The subsystem we have demonstrated could form the key element in both an optical add-drop multiplexer (OADM) and a dynamic multi-channel equaliser. The passband 3-dB width can be straightforwardly reduced to manage multiple channels at the 50-GHz ( N 0.4-nm) spacing of future WDM systems. N

INTRODUCTION Dynamically reconfigurable multi-channel equalising add-drop WDM filter elements are likely to be key components in wavelength-switched networks.' Acousto-optic tunable filters can achieve 0.8nm resolution by adopting a more complex architecture.2 By contrast, the holographic technique attains such resolution by straightforward selection of appropriate filter design parameters, without substantial redesign. The in-line holographic filter is based on a pixellated programmable binary phase diffractive element - a ferroelectric liquid crystal spatial light modulator (FLC-SLM) of relatively coarse spatial resolution - which provides wavelength dispersion in conjunction with a higher resolution fixed diffraction grating. This technique is polarisation-in~ensitive,~ robust and scalable to many more channels. With an optimised electronic drive scheme the fast FLC response can be exploited to yield device reconfiguration in 20 ps.* EXPERIMENT The experimental configuration (see Figure 1) has a folded architecture to accommodate the reflective blazed grating of 300 line-pairs/mm. The SLM consists of a 2-D array of 128 x 128 pixels on a 165-pm pitch with 15-pm dead space. In this application the device is simply used to display 1-D binary phase holograms. Hologram Design Design calculations were based on the following equation (derived by consideration of summed diffraction angles):

where X is the filter wavelength, IC the displacement of the output fibre from the optical axis, f = 96.1 mm the lens focal length, N = 128 the 1-D hologram pixel count, D = 165 pm the SLM

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Channel equaliser and manager e blazed grating,

l-order diffracted ligh 128 transmissive pixel pitch = 166 j11

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Output singlemode fibre

Figure 1. Schematic diagram of experimental architecture. pixel pitch and d 2~ 3.3 pm the period of the fixed grating. \zr is the inclination of the grating normal to the input optical axis. The hologram spatial frequency parameters, n, lie in the range O-N/2 and typically take integer values. The individual filter passband 3-dB width, determined by the overlap integral of the dispersed input spectrum over the output fibre core, is 0.34 nm. The filter wavelength stepping resolution is given by differentiating Equation 1 with respect to n:

Substitution of filter parameters implies AA 2~ 0.215 nm (- 28 GHz) for An = 1, i.e. a stepping resolution well below the current 100-GHz ITU channel spacing standard. The experimental mean AA was 0.22 nm. For AA = 0.2 nm, the blazed grating pitch, d, must simply be reduced to 3.14p.m. However, while the stepping resolution of the holographic filter is within 5% of linearity in wavelength, it is in fact within 0.1% of linearity in frequency, so a slight reduction in d will provide excellent conformance to the ITU grid. Holograms were designed using a simulated annealing algorithm adapted to give multiple, arbitrary-amplitude pass band^.^>^ Input of multiple n values yielded holograms of mixed spatial frequency; a target function An = 4 yielded a mean centre-tocentre passband spacing of 0.88 nm. Control of a total of C channels incurs an excess filter loss of 1Ologl0 C, or 10 loglo 3C for top hat passbands, but at the high channel counts anticipated in future systems the incremental penalty will be small.

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RESULTS Figures 2(a) and 2(b) show, respectively, examples of the dropping of both a single channel and two channels, with the remaining channels passed and equalised. The dynamic range of equalisation over the 7-nm wavelength range is 4dB. For hologram 1, inter-channel ASE suppression is consistently 2 15dB and the dropped channel #3 is suppressed by approximately 19dB. Figure 2(b) demonstrates two channels dropped - #2 and #3 - with suppression greater than 12-15dB in both cases and passed channels uniform to within 1.8 dB following equalisation. Figure 3(a) illustrates an all-pass filter function with transmission equalised from 4 dB to within 1.7 dB. Inter-channel ASE suppression ranges from 12-23 dB. The top hat holographic filter passband (Figure 3(b)) has AA-3dB > 0.6 nm. The -20 dB : -3 dB-width merit ratio derived in the new configuration is 1.45, a dramatic improvement over the 2.2 ratio typically demonstrated in previous experiments.

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CONCLUSIONS

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We have demonstrated active holographic management of eight WDM channels spaced by 0.8nm. The computer-based hologram design algorithm allows design for arbitrary add/drop permutations in which the filter passbands have arbitrary relative transmission. Suppression of single dropped channels, consistently > 15 dB, can be improved by deploying an SLM of greater resolution and higher pixel number. The holographic technique can also produce a near-rectangular ‘top hat’

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Figure 2. Equalising filter output with holograms displayed to drop (a)channel #3 (hologram 1) and (b) channels #2 and #3 (hologram 2).

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Figure 3. (a) Equalising filter function for all eight 0.8-nm-spaced channels passed (hologram 3), and (b) Spectral ‘top-hat’ shaping of the filter passband (hologram 4). passband. The FLC-SLM-based holographic filter is robust and can incorporate an SLM capable of reconfiguration as fast as w 20 ,us. The technology is based upon elements which are low cost in volume production - it could form the key component in optical add-drop and equalisation nodes of future WDM networks. REFERENCES 1. A. E. Willner. “Systems Issues for WDM Components.” In IEEE-LEOS Summer Topical Meeting on WDM Components Technology, pages 5-6 (WBl), Montrkal, Quebec, Canada, August 13-15, 1997. 2. T. Nakazawa, M. Doi, S. Taniguchi, Y. Takasu and M. Seino. “Ti:LiNbOs AOTF for 0.8 nm Channel-Spaced WDM.” Post-deadline paper in OSA Conference on Optical Fiber Communication, San Jose, California, USA, February 22-27, 1998. 3. S. T. Warr and R. J. Mears. “Polarisation-insensitive operation of ferroelectric liquid crystal devices.” Electronics Letters, 31(9):714-716, 1995. 4. H. 3. White, G. M. Proudley, C. Stace, N. A. Brownjohn, R. Dawkins, A. C. Walker, M. R. Taghizadeh, C. P. Barrett, D. T. Neilson, W. A. Crossland, J. R. Brocklehurst and M. J. Birch. “The OCPM demonstrator system.” In OSA Topical Meeting on Photonics in Switching, page P P d l , Salt Lake City, Utah, USA, April 17-20, 1995. 5. M. C. Parker, A. D. Cohen and R. J. Mears. “Dynamic Holographic Spectral Equalization for WDM.” IEEE Photonics Technology Letters, 9(4):529-531, 1997. 6. A. D. Cohen. Spatial Light Modulator Technologies for WDM. PhD thesis, University of Cambridge, 1998.