OTuF2.pdf
Highly programmable Wavelength Selective Switch
based on Liquid Crystal on Silicon switching elements
Glenn Baxter, Steven Frisken, Dmitri Abakoumov, Hao Zhou,
Ian Clarke, Andrew Bartos and Simon Poole
Engana Pty Ltd, Locomotive Workshop, Australian Technology Park, Eveleigh, NSW 1430, Australia
[email protected]
Abstract: We present a novel Wavelength Selective Switch (WSS) based on a Liquid
Crystal on Silicon (LCOS) switching element. The unit operates simultaneously at both
50 and 100 GHz channel spacing and is compatible with 40 G transmission requirements.
(© 2005 Optical Society of America
OCIS codes: (060.0060) Fiber optics and optical communications, (060.1810) Couplers, switches and
multiplexers
1 Introduction
Reconfigurable networks allow service providers to create a flexible, quickly provisionable DWDM
transport layer in the core of their networks, scalable in both distance and number of nodes. Key to the
creation of a reconfigurable network is a Wavelength Selective Switch (WSS) which allows individual
wavelengths to be switched between multiple fibres. A number of technical approaches to creating a
WSS have been demonstrated including MEMS arrays [1] integrated silica waveguides [2] and circulators
with tunable fiber gratings [3]. We report here a novel design of WSS based on a Liquid Crystal on
Silicon (LCOS) switching element which enables a highly flexible Wavelength Selective Switch with 10
ports and more than 5 THz of bandwidth which can be software configured on a 50 GHz or 100 GHz ITU
grid or other proprietary channel spacing.
2 LCOS background and principle of operation
Conventional LC components used in telecom applications employ control of polarisation state to pass or
transmit light to create for example wavelength blockers or attenuators. Switching applications can be
achieved through polarization dependent deflection or displacement. These components have generally
been limited by having inflexible configurations with one pixel per channel and the requirement to preconfigure the channel plan in advance.
Liquid Crystal on Silicon (LCOS) is
a display technology which combines
Liquid Crystal and semiconductor
,
~~~~~~~~~ ~~~~~~~~Al M rror
Eloctrodo
technologies, to create a solid-state
F
11 Schematic of LCOS structure
display engine with up to WUXGA
Figure
resolution. Figure 1 which shows the
structure of an LCOS display with the Liquid Crystal (LC) layer sandwiched between the Active Matrix
silicon backplane and the ITO-coated top glass.
OTuF2.pdf
MoNeoutput
t 1lll ipurtsE 8.
expres.s input
fib1l-le
E-
arraLCO
LCOS can be employed to control the
phase of light at each pixel to produce
iL
lE
0I-_7 beam-steering
\KiE
[4]. In our
I
gimating
imnaing
p(larisi(Jn
optics
Wiversity
design,
a
large
number of phase steps are used to create a
highly efficient, low-insertion loss switch
shown schematically in Figure 2. This
simple optical design incorporates
polarisation diversity, control of mode size
and a 4-f wavelength optical imaging in the
dispersive axis of the LCOS providing
integrated switching and optical power
control. In operation, the light passes from a
imairro
rmff or
Figure 2 Schematic of Optical Design of LCOS-based WSS
fibre array through the polarisation imaging
optics which separates physically and aligns
orthogonal polarisation states to be in the high efficiency s-polarisation state of the diffraction grating.
The input light from a chosen fibre of the array is reflected from the imaging mirror and then angularly
dispersed by the grating which is at near Littrow incidence, reflecting the light back to the imaging optics
which directs each channel to a different portion of the LCOS. The path for each wavelength is then
retraced upon reflection from the LCOS, with the beam-steering image applied on the LCOS directing the
light to a particular port of the fibre array. As the wavelength channels are separated on the LCOS the
switching of each wavelength is independent of all others and can be switched without interfering with
the light on other channels. There are many different algorithms that can be implemented to achieve a
given coupling between ports including less efficient "images" for attenuation or power splitting.
Results
3
0
0
Group of Channels
on Express
100 GHz Channels
50 GHz Channels
-5~~~~~~~~~~~~~~~~~~~~~~~~~~ n__n_n
-10~~~~~~~~~~~~~~-15
-10
-20_
-1'0
Channels
Channels
_
_
_
_
U)~~~~~~~~~~
-J
-30__
_
-36
-40
-4
-50
LJ_
1525
1530
_
_
Channels
Switched to _ ____
~~~~~~Port
1540
_
_
_
_
_
_
_
-20
~~~~Channels
~~~~Switched to
1535
_
-
Channels-20_____
_
Channels
Blocked
Port 2
1545
1550
Wavelength (nm)
-2 5
1555
LI7LiIiLLII11LL
1560
1565
1570
L
-3
0__
1525
1530
1535
1540
1545
1550
1555
1560
1565
1570
Wavelength (nm)
Figure 3 Measured Spectra showing Express Output Port (left) and Drop Port with per-channel power control (right)
Figure 3 shows the C-band optical response of a typical unit configured as 1 x 9 drop WSS withl express
output port and 8 output drop ports. This illustrates a number of aspects of the unit operation:
(1) A mix of channels on both 50 and 100 GHz spacing, with alternate channels dropped. The choice
of which channel is set to which bandwidth is arbitrary and can be controlled by the customer
with the unit in service.
(2) Blocks of channels which are switched to different drop ports (in this case ports 2 and 6) showing
the high extinction (low coherent crosstalk) obtained.
OTuF2.pdf
(3) The flat response of the unit for un-switched express channels. As can be clearly seen, the
response is very uniform and there is no channel narrowing. This is also the case for all the drop
ports.
(4) High extinction for blocked ports. An extinction of > 40 dB is obtained across blocked channels.
(5) Low insertion loss. The measured insertion loss is < 4.65 dB including all connector losses
A further requirement of any wavelength management system is the ability to control optical power
on a per-channel and per-port basis. Figure 3 also shows the ability of a typical drop port (in this case port
8) to provide per-channel, per-port amplitude control. The unit has an amplitude resolution of 0.1dB
with an accuracy of +/- 0.3dB. . The power control is achieved through the writing of "less efficient"
beam-steering images on the LCOS which are chosen to achieve the desired attenuation. The attenuation
doesn't rely on displacement of the image (as is required for MEMs applications) and so does not require
feedback mechanisms to stabilize the attenuation.
Not only must any future
system be able to mix long-haul
0X
(50GHz) and Metro (100GHz) -lH-10
traffic but it must also support
150
4OGBit/sec data rates. Figure 4
shows the overlay of 40 x 100
, -25
J-V_:___V
GHz channels, showing the
, -30
excellent channel registrationonaetd
Response
40
and uniformity. The graph also
shows
the
concatenated
-50
-100 -75
-50 -25
0
25
50
75
100
performance of 24 channels.
Frequency
Offset
from
ITU
Grid
Center
(GHz)
The concatenation is made
using measured channel shapes,
Figure 4 Overlay of 40 x 100 GHz channels
each of which is measured with
the channel on either side
switched, giving a worst-case band-narrowing. The result shows a 0.5dB clear channel bandwidth of
greater than 80 GHz for a single node and greater than 52GHz for a simulated concatenation of 24 nodes.
4
Conclusions
We have presented for the first time a high- channel- count and high- port- count Wavelength Selective
Switch based on LCOS phase based beam steering. The performance is suitable for both high
performance 50 GHz and 100 GHz ITU grid for Long Haul and Metro, but the simple optical design will
lead to cost effective ROADMS for all applications. The solution is highly programmable in both the
wavelength and switching dimensions
5
References
1. J.E. Ford, V.A. Aksyuk, D.J. Bishop and J.A. Walker, "Wavelength add-drop switching using tilting micromirrors" Journal of
Lightwave Technology, 17, 904 - 911 (1999)
2. C.R. Doerr, "Proposed WDM Cross Connect Using A Planar Arrangement of Waveguide Grating Routers and Phase Shifters",
Photonics Technology Letters, 10, 528-530 (1998)
3. C.R. Giles and V. Mizrahi, "Low-Loss ADD/DROP Multiplexers for WDM Lightwave Networks," in Proc Tenth International
Conference on Integrated Optics and Optical Fibre Communication, 66-67 (1995)
4. K. M. Johnson, D. J. McKnight, and I. Underwood, "Smart spatial light modulators using liquid crystals on silicon", IEEE J. Quant.
Electron., 29, 699 (1993).