WDM CHANNEL MANAGEMENT USING PROGRAMMABLE HOLOGRAPHIC ELEMENTS
`
`R J Mean', A D Cohen," and M C Parker.
`
`We describe results of a high-resolution (0.8nm) holographic, digital multi-wavelength filter,
`based on a ferroelectric liquid crystal (FLC) spatial light modulator (SLM). The filter has
`applications as a wavelength-division-multiplexing (WDM) technology for use in optical
`telecommunications. The polarisation-insensitive FLC SLM acting as a programmable
`holographic element in conjunction with a highly wavelength-dispersive fixed diffractive element
`has been used to perform a number of important WDM functions: Demultiplexing of single and
`multiple (up to 4), segmented passbands spaced by 0.8nm, and dynamic erbium-doped fibre
`ampifier (EDFA) gain equalisation. Apodisation of the filter passband has been demonstrated, and
`optical adddrop multiplexing is also possible using the holographic technique. The filter offers
`potential low loss, excellent crosstalk characterisitcs, and a high resolution over a large tuning
`range.
`
`Introduction
`
`The development of the erbium-doped fibre amplifier (EDFA) [ 11 has opened up the possibility of very high
`bandwidth data pipes, for example by using WDM [2], as well as allowing new optically transparent
`architectures, such as wavelength-routed networks [3]. These new systems require specialised functional
`components, such as tunable sources, receivers, switches and routers, reconfigurable optical amplifiers and
`wavelength converters. Optical telecommunications networks require components which are optically
`transparent and polarisation-insensitive, have a low crosstalk and low loss, achieve high resolution tuning, are
`compact and operate at low powers. In this paper we describe how holographic filtering already satisfies most
`of these demands, and is becoming increasingly attractive as a polarisation-insensitive [4] WDM technology for
`active channel management [5,6]. To date, holographic filtering has been used to demonstrate 4, 6 and 8 WDM
`channel equalisation spaced by 4nm [6,7,8]. However, by using a high-spatial frequency (300 lines/")
`blazed
`grating, a resolution of 0.22nm has been achieved, which allows the filter to manage WDM channels spaced by
`the ITU 0.8nm standard.
`
`Holographic Filter Operation
`
`The operation of the high-resolution tunable holographic wavelength filter, shown schematically in figure 1, is
`based on the wavelength-dispersive nature of diffraction gratings. On its own, the SLM pixel pitch ( 1 6 5 ~ ) is
`
`too large for useful tuning to be obtained. However, a fixed blazed diffraction grating of high spatial frequency
`(300 lines/mm, i.e. line-pair width of 6 . 6 6 ~ ) used in conjunction with the SLM yields a compact high
`resolution filter. The use of an electrically addressed SLM (EASLM), to display a desired phase pattern,
`provides a programmable grating (Le. a hologram) whose spatial period can be altered at will. In addition,
`holograms can be designed to have multiple spatial periods, to allow multiple wavelength tuning. A lens placed
`after the SLM and fixed diffraction grating converts the angular separation of wavelen,&s
`to a spatial
`separation, and a single-moded (SM) optical fibre acts as a fixed spatial filter to select the desired wavelengths.
`
`+
`
`Department of Engineering, University of Cambridge, Trumpington St., Cambridge,
`CB2 IPZ, U.K. Email: rjm@eng.cam.ac.uk,
`JDS Fitel, 570 West Hunt Club Rd, Nepean, Ont. K2G 5W8, Canada.
`Email: Adam-Cohen@jdsfitel.com
`++
`* Fujitsu Telecommunications Europe Ltd., Research, Northgate House, St Peter's St, Colchester, Essex, CO1
`IHH, U.K., Email: M.Parker@ftel.co.uk
`
`0 1998 The Institution of Electrical Engineers.
`Printed and published by the IEE, Savoy Place, London WC2R OBL, UK.
`
`1
`
`THOMAS SWAN 2001
`Finisar v. Thomas Swan
`IPR2014-00461
`
`

`

`Reflective blazed grating
`of line-pair width 6.66pn
`
`Len!
`
`ansmissive, binary
`128x1 28 R C SLM,
`
`Figure 1: Schematic diagram of polarisation-insensitive holographic wavelength filter
`
`FLC SLM PC Controller
`
`Tunable Holographic Wavelength Filter
`
`The current filter has a tuning range of 1 2 . 4 ~ 1 in steps averaging 0.22nm, with a 3dB passband of 0.34~1, and
`is polarisation-insensitive. Figure 2 shows the spectral profile of the filter transmission using a hologram (a).
`Without apodisation, it is close to Gaussian in shape. However, beyond 0.66nm either side of the centre
`Hologram (a)
`Transmission Loss = 21.7dB
`
`
`
`O
`
`S
`0
`.-
`.- e
`(I)
`(I)
`
`5?. " t
`
`
`
`
`
`-3dB width
`= 0.34nm
`
`J
`
`= 0.95nm 1,
`
`1542.0
`1543.0
`Wavelength (nm) 0.2nm/div
`
`Figure 2: Logarithmic plot of filter passband with FWHM=0.34nm
`
`. wavelength, the filter extremities depart from Gaussian behaviour and has the larger 'tails', as illustrated in the
`figure, of a Bessel function, which converges to zero more slowly than a Gaussian. The diagram shows that the
`filter has an optical signal-to-noise ratio, SNR >30cU3. However, this is only achieved for wavelengths greater
`than - 0 . 7 ~ 1 away from the central wavelength, owing to the convolution arising from the GaussiadBessel
`coupling efficiency into the fibre end The 21.7dB loss of the filter is accounted for in the following table:
`
`2
`
`

`

`SLM Losses
`FLC switching angle (28=28O)
`Diffraction efficiency (q=36.5%)
`Aperturing of SLM
`Blazed Grating Losses
`Diffraction efficiency (q=-65%)
`Phase depth optimised for b 1 pm
`Sundry Losses
`10 reflecting surfaces, each contributing 4% loss
`FC/PC patchcord uniter losses (x2)
`fibrebexu coupling efficiency (-42%)
`TOTAL
`
`dB
`6.57
`4.38
`0.79
`
`1.90
`1.43
`
`1.77
`1.14
`3.72
`
`21.7
`
`Optimisation of the FLC and optical components, use of a 1.55pn blazed grating and careful design should
`allow a total optical loss of only -7dB.
`
`Since the FLC is not fully bistable it is necessary to periodically update all the pixels, with the frame being
`downloaded row by row. A practical device, however, would make use of either a bistable FLC or an alternative
`addressing scheme, removing the need for the update process other than when changing between different
`holograms. The effect of the periodic updating is to cause a small modulation during normal transmission of
`approximately 0.035dB. This is illustrated in Figure 3. The -la loss of the signal that occurs during the
`periodic frame update is undesirable, but new pixel addressing schemes currently under development for silicon
`backplane FLC SLMs should eliminate the need for this process, even with an FLC material that is not fully
`bistable. A fully bistable FLC used within the SLM would avoid all temporal modulation of the light and also
`allow fail-safe operation of the device. In the event of a power failure, the SLM would still continue to diffract
`the light and the device still operate, albeit without reconfigurability.
`
`SLM Update
`
`Static Hologram Display
`
`40
`
`80
`
`time
`
`120
`
`160
`
`1
`200
`20mddN
`
`Figure 3: Temporal modulation of filtered light
`
`Passband Apodisation
`
`The Gaussian spectral-profile may be tailored to achieve a more apodised, passband-flattened response, by
`modlfylng the hologram. This is shown in figure 4, which shows the normalised transmission characteristic for a
`different hologram (b). The -3dB width is now 0.59~1, increasing to 1.35nm at -2OdB, Le. a more
`rectangular response. This is at the expense of an additional 2.3dB loss, and a reduction in the noise
`suppression to 18 dB.
`
`1113
`
`3
`
`

`

`- Unbroadened Passband (a)
`- Broadened Passband (b)
`(2.3dB excess loss)
`0 -
`-3dB width
`= 0.59nm
`
`E!
`c
`__ 0 .-
`v) .- E
`-10 -
`v) t: m
`I-
`-0 a
`cn .- -
`-20
`E
`0 z L
`-3 0
`1540
`
`1543
`1542
`1541
`Wavelength (nm)
`
`1545
`1544
`0.5nm/div
`
`Figure 4: Holographic Filter Passband Apodisation
`
`EDFA Gain Equalisation
`
`Multiple wavelength filtering is one of the distinguishing features of holographic wavelength filtering and is
`important for WDM demultiplexing and EDFA gain equalisation. Figure 5 shows the transmission of four
`passbands separated by about 0.8m. The 3dB width of each passband is still about 0.34m, and noise
`supression is generally substantially greater than 8dB, with inter-channel M E suppression reaching 20dB.
`Passband uniformity is within 2dB. There is an associated higher loss due to the available light being divided
`into 4 passbands, and the reduced diffraction efficiency of binary-phase holograms when they function to fan-
`out light. The average optical SNR or channel isolation for a binary hologram is proportional to the number of
`hologram pixels N, and inversely proportional to the number of filtered channels C, such that:
`N
`SNR 2 -
`2c
`
`(1)
`
`Thus the S N R performance of a hologram reduces as it is required to control more channels, but improves with
`more pixels. Likewise, binary-phase holograms show an additional transmission loss of -lOloglo(C), when
`filtering C channels [9], hence the 7.3dB excess loss when filtering 4 channels.
`Hologram ( c )
`
`(excess loss = 7.3dB)
`
`.-
`0
`v1
`
`1538
`
`1544
`1542
`1540
`Wavelength ( n m )
`
`1546
`1 nm/div
`
`Figure 5: Filtering of 4 WDM channels spaced by 0.8nm
`
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`

`

`Tunable Fibre Laser
`
`Tunable fibre lasers may potentially serve an important function in WDM telecommunications networks, acting
`as stable and pure laser sources. They have a very narrow linewidth, high output powers and large tuning
`ranges. We have already published the results of a tunable erbium-doped fibre laser [lo], tuned using a
`holographic wavelength filter. The holographic filter and a high-gain EDFA were placed within a unidirectional
`fibre ring-resonator. A 3dB coupler was used to access the output power. Tuning over 38.5nm, in the range
`1528.6-1567.1nm with steps of 1.3nm was achieved, with output powers of up to -13dBm. The inherent EDFX
`3dl3 lasing linewidth was found to be of the order of 3lcHz, and the long term wavelength stability was about
`0.lnm. A hologram with a mixed spatial frequency has also been designed to allow the EDFL to simultaneously
`lase at 1562.5nm and 1556.Onm, as shown in Figure 6. Due to the gain medium being relatively homogeneous
`and dependent at the two wavelengths, mode competition means that the lasing mode powers fluctuated
`considerably. The power in each mode is also considerably lower than usual, since only half the EDFA power
`is available to each mode, the hologram has less than half the usual diffraction efficiency for each of the two
`wavelengths, and a 10DO coupler is used for the laser output.
`
`- 0.1m
`
`t . 3 7 t p
`mol“
`
`1.5571p
`Wavelength
`
`1.5671p
`
`M d V
`
`Figure 6: Multiple lasing wavelengths
`
`Future Work
`
`The currently unused extra dimension of the SLM can also be used to add functionality, such as in a space-
`wavelength switch. This could serve as an add-drop multiplexer (ADM) in dynamic wavelength-routed optical
`networks. Figure 7 shows an ‘exploded’ concept for a polarisation-insensitive, optically transparent, compact,
`low-loss space-wavelength switch, using a reflective FLC SLM. The switch acts as a 3x3 fibre cross-connect,
`which can also perfectly shuffle wavelengths between the various fibres. The integrated design incorporates a
`graded-index (GRIN) lens instead of a bulk refractive lens, but the limited numerical aperture of a GRIN lens
`will tend to limit the number of fibres possible to interconnect. Figure 8 shows how the packaged, integrated
`device might look.
`
`-
`-- -
`
`Input fibre array
`_1
`
`Space
`
`Input fibres-
`and wavelengths
`
`Switched output
`
`Output fibre array
`
`Transmissive P ixe llated Mirror
`blazed grating FLC SLM
`
`Integrated SLM unit
`
`Figure 7: ‘Exploded’ 3x3 space-wavelength switch
`
`Figure 8: Packaged, integrated space-h switch
`
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`

`

`Conclusions
`
`In this paper, we have presented holographic filtering as a potential technology for dynamic WDM channel
`management. Holographic wavelength tuning may also find application in WDM telecommunications systems,
`where tunable sources, filters and receivers are required. The advantages of holographic tuning are:
`
`0 optical transparency
`0 polarisation insensitivity
`0 digital, fast (-lOp), low power operation
`0 fail-safe operation and robustness
`fine resolution over a large wavelength range
`0 multiple wavelength operation
`low crosstalk
`
`Dynamic gain equalisation, an important issue in WDM networks, has been addressed using holographic tuning.
`The holographically tunable, multiwavelength erbium-doped fibre laser may also find use as a source in a
`WDM network. The technique can be used within a holographic space-wavelength switch, allowing arbitrary
`switching and shuffling of the wavelengths between the fibres.
`
`References:
`
`[ 11 R.J.Mears, L.Reekie, I.M.Jauncey, and D.N.Payne, ‘Low-noise erbium-doped fibre amplifier at 1.54p”,
`Electronics Letters, 23 (19), p.1026-1028, 1987
`[2] C. ABrackett, “Dense Wavelength Division Multiplexing Networks: Principles and Applications”, IEEE
`Journal on Selected Areas In Communications, 8(6), p.948-964, 1990
`[3] G.R.HiU, ”A wavelength routeing approach to optical communication networks”, Br. Telecom. Technol. J.,
`6(3) , p.24-31, 1988
`[4] S.T. Warr and R.J.Mears, ‘Polarisation-insensitive operation of ferroelectric liquid-crystal devices’,
`Electronic Letters, 3 1, p.714-716, 1995
`[SI R.J.Mears, W.ACrossland, M.P.Dames, J.Collington, M.C.Parker, S.T.Warr, T.D.Willcinson, and
`AB.Davey, ‘Telecommunications applications of FLC smart pixels (invited)’, IEEE Journal Selected Topics in
`Quantum Electronics, April 1996, p.35-46
`[6] AD.Cohen, M.C.Parker, R.J.Mears, “Active holographic spectral equalisation and channel management for
`WDM”, OSA TOPS Vol. 12 System Technologies, p.344-349, 1997.
`[7] M.C.Parker, A.D.Cohen, R.J.Mears, ‘‘Dynamic digital holographic wavelength filtering”, to be published in
`IEEE Journal of Lightwave Technology, July 1998.
`[SI AD.Cohen and R.J.Mears, “Dynamic Holographic Eight-Channel Spectral Equaliser for WDM’ ZEEE-
`LEOS Topical Meeting on WDM Components Technology, Paper ThC2, MontrCal, Canada, August 1997
`[9] M.C.Parker, AD.Cohen, and R.J.Mears, ‘Dynamic Holographic Spectral Equalization for WDM’, IEEE
`Photonics Technology Letters, Vol. 9, No. 4, p.529-531, April 1997
`[ 101 M.C.Parker and R.J.Mears, ‘Digitally tunable wavelength Nter and laser’, IEEE Photonics Technology
`Letters, Vol.8, No.8, p.1007-1008, August 1996
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