Low penalty cascaded operation of a monolithically integrated
quantum dot 1×8 port optical switch
Citation for published version (APA):
Wang, H., Williams, K. A., Wonfor, A., Vries, de, T., Smalbrugge, E., Oei, Y. S., Smit, M. K., Nötzel, R., Liu, S.,
Penty, R. V., & White, I. H. (2009). Low penalty cascaded operation of a monolithically integrated quantum dot
1×8 port optical switch. In Proceedings of the 35th European Conference on Optical Communication (ECOC
2009) 20 - 24 September 2009, Vienna (pp. 6.2.3-1/2). Institute of Electrical and Electronics Engineers.
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Published: 01/01/2009
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Low Penalty Cascaded Operation of a Monolithically Integrated Quantum
Dot 1x8 Port Optical Switch
H. Wang(1), K.A Williams(1,2), A. Wonfor(1), T. de Vries(2), E. Smalbrugge(2), Y.S. Oei(2), M.K. Smit(2), R. Noetzel(2), S. Liu(1), R.V. Penty(1), I.H. White(1)
(1) Centre for Photonic Systems, Electrical Engineering Division, Department of Engineering, University of
Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK, hw288@cam.ac.uk
(2) Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands
Abstract A novel Quantum Dot monolithically integrated 1x8 switch is shown to provide robust routing of data at 10Gb/s modulation rates. Two cascaded switches providing, 1x64 functionality, operate with a power penalty of only 0.9dB.
Introduction
There has recently been much interest in the development of high performance semiconductor optical amplifier (SOA) based switches for computing and other optical networking applications as they offer high on-off switching ratios and provide inherent optical gain. Demonstrations have been made both for switches based on discrete components1 and also partially populated integrated solutions2. To date however monolithic crosspoint devices have been formed with port counts of 4x4, and recently 1x8 port splitters made by active passive integration in quantum well material3 have been demonstrated as enabling components for 8x8 switches4.
The use of quantum well active regions has proved to be successful, but recently quantum dot (QD) materials have been found to be of particular interest in SOA switches partly because of the enhanced performance that QD amplifiers have shown in terms of higher saturation powers, better distortion and lower noise figures. For example, QD amplifiers5 have been shown to exhibit significantly enhanced dynamic range, gain and reduced distortion allowing an increase in loss budget in PONs for example, of 17dB. In addition, other recent work has lead to the demonstration of 4x4 QD-based switches6 and shown the potential for uncooled QD SOA operation7.
In this work therefore we report the first QD 1x8 port integrated switch, suitable for a variety of reconfigurable star, PON and switch applications. We demonstrate the potential for cascading this device to enable the construction of larger bespoke switch architectures.
The 1x8 switch, which is made of a five stack QD active layer embedded in a Q1.15m InGaAsP separate confinement heterostructure6, comprises an input waveguide with a tree of 3dB couplers providing 8 output branches. Each of the output waveguides has separate electrical contacts, allowing independent control of each of the outputs. The input and output waveguides are angled to minimise back reflections
Experiment
A chip containing two 1x8 integrated switches, shown in figure 1A, is used as this allows cascaded performance of multiple devices to be investigated. 4 controllable lensed fibres are used, to allow simultaneous access of an input and output waveguide of both devices and to test different paths.
Figure 1B shows the experimental setup in which a 10Gb/s 231-1 pseudo random bit sequence from a bit error rate test set (BERT) is used to modulate a Mach-Zehnder modulator whose optical input is from a tunable laser. The output from the modulator is then amplified with an EDFA, optically filtered and attenuated to provide a variable input power to the switch. The output is then filtered, and fed via a variable attenuator to an optically pre-amplified receiver allowing the performance of a 1x8 switch to be investigated using the error analyser on the BERT. The output from the first optical switch can be looped back to the second 1x8 switch, allowing cascaded performance to be measured.
Fig. 1: A, Chip schematic. B, Experimental schematic.
The electrical contacts of the optical switches are driven by a switchable power supply so that the temporal performance of the switch can be studied. In the case of a cascaded switch an EDFA and optical filter are used to compensate for the fibre coupling loss at the angled facets.
Results
The on chip performance of the switch has been characterised after removing fibre coupling losses, which are estimated from photocurrent measurements to be 12dB in this device.
The 1x8 switch has 3 cascaded 3dB splitters in each path, introducing 9dB of loss, and, together with some excess loss within the switch, this yields a net
MZI Mod Tunable Laser EDFA 1x8 Switch 1x8 Switch EDFA EDFA Photo diode BERT
ECOC 2009, 20-24 September, 2009, Vienna, Austria Paper 6.2.3
978-3-8007-3173-2 © VDE VERLAG GMBH
facet to facet gain of -3dB at 10°C. Multiple paths within the switch have been measured and show a gain uniformity of ±1dB.
The 3dB spectral bandwidth of the switch is in excess of 65nm as shown in fig 2. This measurement is limited by the range of the tunable laser. At 20°C the on-chip gain is still greater than -6dB, with a similar spectral bandwidth of more than 60nm.
Fig. 2: Gain spectrum of integrated 1x8 switch at 10°C
Power penalty measurements are carried out, though being limited by the available power which can be coupled into the devices and hence observe distortion. Figure 3 shows the penalty to vary very little with input power, being less than 1dB for a 10dB input variation, for an input signal at 1550nm. Measurements at 1535nm and 1560nm also show similar ranges, with a minimum penalty of 0.05dB.
Fig. 3: Penalty measurements through the device as a
function of input power at a 1550nm signal wavelength.
The potential of the 1x8 switch can be seen by cascading two devices, as might be used in the input stage of a large port count switch (say a 1x64 router), or in a large fan-out selector switch for a PON. In this case the power penalty of individual switches and the cascaded pair of switches has been studied.
In this case the waveguides connecting the integrated 8x8 switch must be substituted by fibre, an EDFA and filter between the two switch stages to compensate for the coupling losses. Despite this additional loss and additional EDFA ASE noise the cascaded demonstration shows great promise. With an on-chip input power of 0dBm, each single switch element shows a penalty of less than 0.4dB at a bit error rate (BER) of 10-9, as can be seen in figure 4.
When both switches are cascaded, the penalty is 0.9dB at a BER of 10-9.
Fig. 4: Bit error rates for single switches and cascades
operation, showing less than 0.9dB penalty.
The dynamic operation of the switch is demonstrated by switching packets to a chosen output port of the switch. In this case, the output port drive current is switched from 0mA in the off state to 180mA in the on state. This current swing is currently limited to 10ns by the driver and electrical drive parasitics, which limits the switching times. However improved performance can be expected as QD based switches have been shown to have rise and fall times of 1ns. Figure 5 shows the output of the switch under dynamic operation. The switch produces an optical extinction of 14dB.
Fig. 5: Data switching output from a single 1x8 switch. 1μs per division.
Conclusions
A Quantum Dot monolithically integrated 1x8 switch is presented as a component for large integrated optical switches and optical selectors. The switch has a 3dB spectral bandwidth of > 65nm and a penalty of less than 0.4dB, with a 1dB input saturation power of 5dBm. Two such cascaded switches exhibit a power penalty of only 0.9dB making them suitable candidates for a 64 way low-loss splitter in, for example, PON applications.
References
1 R Grzybowski et al. Proc PiS ’07 TuA1.4 (2007) 2 H Wang et al. Proc OFC ’09. OWQ2 (2009) 3 S Tanaka et al. Proc OFC ’08. OWE2 (2008) 4 Y. Kai et al. Proc ECOC ’08 We.2.D.4 (2008) 5 R Bonk et al. Proc OFC ’09 OWQ1 (2009) 6 K.A. Williams et al. Proc PiS ’08 D-06-4 (2008) 7 E.T. Aw et al. Proc CLEO ’08 CME6 (2008) ! "# $%& -36 -35 -34 -33 -32 -31 B it E rro r R at e Received Power / dBm Back to Back Switch 1 Switch 2 Both switches ' ' () $%& *& &$ &( )+$ )+ ,%) ,%)
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978-3-8007-3173-2 © VDE VERLAG GMBH
