Small Phase Pattern 2D Beam Steering and A Single LCOS Design of 40 1×12 Stacked Wavelength Selective Switches

: Two-dimensional beam steering by small, square, phase patterns as small as 50×50 pixels on a phase-only liquid crystal on silicon (LCOS) device is experimentally verified as suitable for the application of wavelength selective switches (WSSs), in terms of the diffraction efficiency and steering accuracy. This enables a proposed highly functional and versatile stacked switch architecture, where 40 independent 1×12 WSSs can be realised on a single 4k LCOS device. They can be configured to support a 1×N WSSs with N ≤ 144, or an N×N wavelength crossconnect with N ≤ 12.


Introduction
A wavelength selective switch (WSS) [1,2] is one of the key enabling technologies for reconfigurable optical networks [3,4].A typical WSS is able to selectively route individual wavelength division multiplexing (WDM) channels entering its input fibre port to any of the output fibre ports according to the software configuration that is remotely controlled by the service providers.In recent years, phase-only liquid crystal on silicon (LCOS) spatial light modulators (SLMs) [5] have become the technology of choice for WSSs, due to its software upgradable nature and support for flexible spectrum switching [6], dispersion compensation [7], multicasting [8][9][10], and adaptive alignment [11][12][13].Various efforts have been made to improve both the optical performance and the switching functionalities of the LCOS WSSs, especially in terms of static [14][15][16][17][18] and transient [19][20] crosstalk reduction, passband shape optimisation [21] and port count increase [22][23][24][25].
WSSs are usually based on the 'disperse-and-select' optical design, where the WDM channels from the input port are diffracted by a static grating along the dispersion axis at the plane of the optical engine, i.e.LCOS device, before being subsequently switched to the target output ports according to the sub-holograms, i.e. diffractive phase patterns, displayed on the corresponding areas of the LCOS device.Due to the limited number of pixels available on the current generation LCOS devices, however, anamorphic optics are invariably used in these designs to convert the beams of individual WDM channels into elongated shape at the LCOS plane.Correspondingly, output ports are arranged along the switching axis, which is orthogonal to the dispersion axis.Although such an approach is able to increase the port count in one axis, it fails to fully exploit the two dimensional (2D) nature of the pixel array on the LCOS device.Moreover, for such a configuration, all the undesirable diffraction orders due to the LCOS defects will also appear along this switching axis, which makes it fundamentally difficult to suppress crosstalk, especially in WSSs with high port counts.It should also be pointed out 2D output port arrangement and beam steering has been previously demonstrated in WSSs based on 2-axis analogue micro mirror array technologies [26][27][28][29].However, WSSs based on micro mirror array technologies became less attractive in recent years due to its incompatibility with the flexible spectrum switching that is required by next generation networks.
In this paper, we demonstrate the 2D beam steering capability of the small sub-hologram with a square shape in a proof-of-concept LCOS WSS setup without using anamorphic optics.We further propose a stacked WSS design with 2D output port arrangement, which enables a single 4k LCOS device to realise up to 40 independent 1×12 WSSs within one module.Applications of this WSS module are also discussed.

Principle of 2D beam steering
The conventional Fourier transform optical system shown in Fig. 1 is used to illustrate the beam steering principle of the LCOS device.In this system, the IN/OUT plane and the LCOS device are pla The Gaussian where ω IN/OUT and λ the wav Fig. 1 The LCO the IN/OUT p 3ω SLM so that various algor commonly us between the o be expressed a The holog that the input the blazed gra therefore the a pixels within on the beam s Although the design, in gen efficiency (>9 dimension of which leads to In the WS pixels along th band with 80 beam size of result, it is im optics, the sig dispersion ax number of sw With the allocated to ea Therefore, at Cartesian grid non-integral p aced at the fron beam waist at T is the input G velength. . Beam steering by OS device displ plane.The dim at least 99% o rithms [30,31] ed in case of s offset position as gram should al and output bea ating displayed actual diffracti one period.An steering perfor performance neral, a minim 90%) [33]   The relat the LCOS dev there can be quantisation displayed in p First of all, 20 with periods b to these phas Fourier transf profiles, the n It can be seen the phase leve      n the C-band, w ixels [38]) ind multiple LCOS be stacked bas dwidth increm andwidth varia n-modulated in designed to co The small nu o 10 pixels, wh nt switching eff a given directi ports arrange utput ports can s are arranged hat the input po ult, the input b diffraction, the ded to various e WSS will ha y reduce maxim ever, the offse ompared with on the LCOS is maintained ng the y-axis i cover at least 9 with this effect e WSSs that can OS blazed grat where the elec els.The edge of a blazed gr ics design; 2D be hexagonal grid.
ystem.The obje or of a wavele xis.The angle utput fibre, the steering accu is paper is cr avelength chann 4k LCOS devi which means th dependent WSS S devices are sed on this desi ment is 1 GHz able wavelengt nput signal to e over 31×31 pix umber of pixe hich is larger t fficiency and re ion.The circul d on a Cartes n be further inc in a hexagonal orts of these st eams will ente spectra of the degrees on th ave different po mum number et with respecti optical path, i plane.As a r less than 3 is allocated to 99% power of t.Therefore, c n be stacked in ing occurs due ctric field due effect in the L ating, which m eam steering over ective lens in t ength channel, e is controlled ereby maximis uracy by smal ritical for this nnels into the ou ice, we can all hat at least 40 ( Ss can be stac tiled along the ign principle c per pixel, wh th selective sw each sub-hologr xels and achie els covered by than the minim easonably low lar beam on P ian grid, as sh creased, given l pattern as sho tacked WSSs i er the static gra input WDM s he LCOS plane ositions along t of WSSs that ive to the opti i.e. 300 mm, r result, such a pixels in th each stacked f each beam, ou conical diffrac n this design.e to quantizati to the voltage LCOS device may coincide w r the fibre ports a the output opti , k(ρ, φ, λ), to d such that the sing coupling e ll square subpart of opera utput fibre arra locate 50×50 p (48 in the case cked on a sing e y-axis, the n can be further i hich complies w witching.In the ram has a circu eve the 4 th ord y the beam l mum period of w crosstalk leve P SLM allows 2D hown in Fig. 9 the same beam own in Fig. 9(c is off the optic ating (P g ) with signals from th e, i.e. the beam the y-axis on t t can be stacke ical axis, i.e. 1 required to dis difference in e worst-case WSS and the ur design has ction will not l ion of the phas applied to a p can lead to a with a number nsional linear ated in Fig. 8, h an un-targete the -1 st diffract sed by using [18].In addi onstrated to be o the system [

Fig. 6 .
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Fig. 9
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