Monolithic multiband wavelength router
for fast reconfigurable data networking
A. Rohit, K.A. Williams, X.J.M. Leijtens, T. de Vries, Y.S. Oei,
M.J.R. Heck, L.M. Augustin, R. Notzel, D.J. Robbins and M.K. Smit
COBRA, Technische Universiteit Eindhoven, PO Box 513, 5600MB, Eindhoven, The Netherlands
Abstract: A compact and highly scaleable reconfigurable
wavelength router is proposed and demonstrated using an
electronically-gated cyclic wavelength router. Power
penalties for wavelength
wavelength channels of order 0.2dB are achieved.
Keywords: Integrated optoelectronics, Photonic switching
systems; Wavelength division multiplexing
Nanosecond time-scale, wavelength-tunable, optical filters
offer an important enabler for a wavelength agile
networking in metropolitan area telecommunications ,
interconnection systems  and access provisioning .
Increased data rate and connectivity requirements in
combination with stringent cost and energy constraints
have lead to a renewed effort to develop such integrated
photonic sub-systems. Arrayed waveguide gratings offer
loss-independent scaling and large scale integration.
However electronically programmable routing is required
for nanosecond timescale reconfigurability in low-latency
data networking . Electronically controlled phase delays
have been proposed and demonstrated [3,4], but the
analogue control in combination with precalibrated look-up
tables can be cumbersome, and the available end-to-end
bandwidth can be artificially constrained.
In this work we propose binary control with a loss-
compensating amplifier gate
combination with a multiband cyclic arrayed waveguide
router. The cyclic nature of the router enables a mode of
parallel operation suited to space and energy saving
network architectures but such circuits have not so far
been integrated. We study the end-to-end routing of
wavelength multiplexed data through a proof of principle
1x4 gated reconfigurable router using both wave-band and
coarse-grid wavelength multiplexing. Performance is
considered in terms of power penalty to assess the
scalability to more complex on-chip optical networks.
array integrated in
2. Multiband wavelength router
The reconfigurable wavelength router circuit has been
fabricated in a multi-project wafer process using an active-
passive regrown base epitaxy . A composite image
taken using a scanning electron microscope is shown in
figure 1. The highly compact arrayed waveguide grating
on the left is integrated with the densely integrated gate
array of four amplifier gates. The input is split to four
channels using cascaded
couplers to the right of the displayed image. The four
outputs are shown in the bottom left of the image. Inputs
and outputs are angled at 7 degrees to the as-cleaved
Output waveguides from cyclic
arrayed waveguide grating filter
Figure 1 Reconfigurable wavelength router: Microscope
image shows the integrated router with input selector
using a semiconductor optical amplifier gate array.
Four semiconductor optical
amplifier gate electrodes
across single active stripe
The circuit is reduced in size through the selective use of
low-bend-radius, deep-etched waveguides . The four
shallow etched amplifiers are angled across the same
active island. The circuit elements are accommodated
within a highly compact area of under 5mm2, including the
waveguide fan-out for the fiber interface.
3. Spectral response
The spectral response of the semiconductor optical
amplifier gates and cyclic waveguide router are shown in
figure 2. Here one gate is biased to a current of 50mA
and the amplified spontaneous emission is collected at
the input and each of the four outputs.
Transfer function [dB]
Spontaneous emission [dB]
Figure 2: Spectral response of cyclic router:
Upper graph: Theoretically predicted transfer function.
Lower graph: Measured spontaneous emission at input
9781-4244-3856-3/09/$25.00 ©2009 IEEE
Lensed fibers are used for fiber-chip coupling. The Download full-text
measured spontaneous emission spectra are overlaid for
each of the four outputs and show close agreement and
wavelength alignment with the simulated predictions. A
uniform spectral performance with 2dB variation is
observed across the measured 100nm bandwidth. The
channel spacing and the free spectral range were
measured to be 3.2nm and 16nm respectively.
An amplifier transparency current of 1.8kA/cm2 (5mA) was
estimated by measuring the transition from loss to gain in
the photocurrent for a modulated optical input. The total
fibre-fibre loss at amplifier transparency was measured to
be 29dB. This is expected to include 2x6dB facet-fiber
coupling loss, 2x3.5dB splitter loss, 2x0.5dB mode filter
losses, 4dB filter loss and 3dB total waveguide loss. The
remaining 2dB may be attributable to uncertainty in these
values. The amplifier gates do compensate some of this
loss. Importantly, filter losses do not increase with
increased circuit scaling, and facet-fiber losses are not
intrinsic to the design and may be significantly reduced.
4. Multiplexed wavelength routing
The assessment of wavelength multiplexed routing is
performed with data at 10Gb/s on up to three wavelength
channels. Three tunable lasers are multiplexed,
modulated and the optically imprinted data is then
decorrelated with 10km standard dispersion fiber prior to
the circuit input. Two of the wavelength channels are
separated by 0.8nm (100GHz) and are located within the
same pass-band to study waveband routing. The third
channel is offset by one free spectral range for coarse grid
routing. Bit error rate measurements were performed
using a pseudo random bit sequence of 231-1 bit length.
Appropriate biasing of gate
wavelength comb allocation to each of the output ports.
The routed signals are subsequently de-multiplexed and
assessed with a bit error rate test set.
Bit error rate test
Circuit under test
AWG: Arrayed waveguide grating; MMI: Multimode interference coupler;
SOA: Semiconductor optical amplifier; VOA: Variable optical attenuator
Figure 3 Experimental arrangement. Polarisation
controllers before the multiplexer and circuit under test are
not shown for clarity.
Bit error rate measurements are performed for all
channels and combinations at a gate bias point of 26mA.
Data for the wavelength channel at 1554.2nm are shown
in figure 4 to study wavelength multiplexed operation. It
can be seen from figure 4 that low power penalty between
0.2-0.8dB can be achieved for multiple channel operation.
Bit error rate measurements for the shorter wavelength
channels were also performed and also show a low WDM
penalty between 0.2–0.8dB. It is noted that the
performance for three wavelengths leads to a penalty of
only 0.2dB. Such improvement has been previously
observed for multiwavelength switching and is attributable
to reduced aggregate patterning .
Mean receiver power [dBm]
Bit error rate
Back to back
1 x 10Gb/s
2 x 10Gb/s
3 x 10Gb/s
Figure 4 Power penalty assessment for wavelength
multiplexed payloads. Data shown for 1554.2nm channel.
A gated cyclic wavelength router suited to nanosecond
architectures has been demonstrated for wavelength
multiplexed end to end operation. A broad 100nm gain
bandwidth with under 2dB gain variation is observed. Low
power penalty routing of only 0.2dB is achieved for
3x10Gb/s multi-wavelength data.
The chip has been fabricated on the Photonic Integration
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9781-4244-3856-3/09/$25.00 ©2009 IEEE