Low-Loss and High-Bandwidth Multimode Polymer
Waveguide Components Using Refractive Index Engineering
Jian Chen, Nikos Bamiedakis, Peter Vasil’ev, Richard V. Penty, and Ian H. White
Electrical Engineering Division, Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK
Author e-mail address: [email protected]
Abstract: Low-loss and high-bandwidth (>47 GHz×m) multimode polymer waveguide crossings
(<0.02 dB/crossing) and bends (<1dB) are demonstrated. The performance of passive optical
backplanes comprising such components is also optimised using refractive-index engineering and
launch conditioning.
OCIS codes: (130.5460) Polymer waveguides, (200.4650) Optical interconnects, (260.2030) Dispersion
1. Introduction
To meet the ever increasing demand for higher interconnection data rates within data centres and high performance
computing environments, short-reach optical interconnects have attracted significant research interest over the past
few years. Optical interconnects provide significant advantages over their conventional electrical counterparts:
higher information capacity, lower power consumption, lower crosstalk and higher data density [1]. In particular,
passive optical backplanes based on multimode polymer waveguides are considered to be a cost-effective solution in
achieving high-speed interconnection between electrical cards in blade servers and data storage systems. Such
backplanes benefit from the use of multimode polymer waveguides that allow direct integration onto low-cost
printed circuit boards (PCBs). The waveguides typically have relatively large dimensions (30-70 µm) in order to
offer relaxed alignment tolerances and therefore enable system assembly using common pick-and-place tools [2, 3].
Such optical backplanes may implement non-blocking interconnection architectures and feature complex waveguide
layouts which include large numbers of on-board passive waveguide components such as bends and crossings [4, 5].
For example, the 1-Tb/s aggregate-capacity 10-card optical backplane presented in [4] features ~1800 waveguide
crossings and 100 90° waveguide bends. These two elementary waveguide components however exhibit differing
behaviour with respect to the waveguide refractive index (RI) difference ∆n: waveguide bend loss benefits from
strong optical confinement, whereas waveguide crossings exhibit lower loss and crosstalk for smaller ∆n values. The
RI profile of the particular siloxane waveguides employed in this work can be readily adjusted by slightly changing
their fabrication parameters [6]. In this paper therefore, we present studies on these waveguide components and
investigate their loss and bandwidth performance for different RI profiles. Excellent optical transmission properties
are obtained while it is shown that appropriate RI engineering and launch conditioning can provide optimised loss
and bandwidth performance for a waveguide layout that comprises a number of such components. The results
demonstrate the strong potential of this multimode polymer waveguide technology and highlight their highly
flexible structural design in forming high-capacity passive optical backplanes.
2. Waveguide samples and experimental setup
Three waveguide samples (denoted as WG01, WG02 and WG03) are fabricated with a slightly different RI profile
and size using standard photolithographic processes on 8-inch silicon substrates from siloxane materials (Dow
Corning WG-1020 Optical Waveguide Core and XX-1023 Optical Waveguide Clad). Each waveguide sample
comprises waveguide components with (i) two 90° bends with a varying radius of curvature, (ii) a varying number
of 90° crossings and (iii) reference waveguides [Fig. 1(a)]. The reference waveguides are used as control samples to
compare the loss performance of the waveguide components and measure their bandwidth (BW). Fig. 1(b) shows
the RI profile of the 3 samples at 850 nm while Fig. 1(c) summarises their key characteristics.
(b)
output
reference waveguides
length: 16.25 cm
90o bends with
varying radius R:
5, 6, 8, 11, 15, 20 mm
1.522
20
1.518
0
1.514
-20
-40
-40
13.7 cm
1.526
1.51
-20
0
20
40 1.506
Horizontal offset (m)
(c)
40
WG02
1.516
20
1.514
0
1.512
1.51
-20
1.508
-40
-40
-20
0
20
40
Horizontal offset (m)
40
Vertical offset (m)
input
number of crossings:
1, 5, 10, 20, 40, 80
WG01
Vertical offset (m)
(a)
Vertical offset (m)
40
1.506
WG03
1.526
1.522
20
1.518
0
1.514
-20
-40
-40
Parameter
WG01
WG02
max Δn
0.020
0.010
0.019
Size (µm2)
35 × 40
55 × 56
32 × 53
1.51
-20
0
20
40
Horizontal offset (m)
1.506
WG03
Fig. 1 (a) Schematic of the components studied and (b) the RI profile at 850 nm and (c) table with key parameters of the 3 waveguide samples.
8
(b)
WG01: 9 μm SMF
7WG_A: 9 μm SMF
WG01: 50 μm MMF
6WG_Α: 50 μm MMF
WG02: 9 μm SMF
5
WG02: 50 μm MMF
4WG_Β: 9 μm SMF
WG03: 9 μm SMF
(c)
Performance metric
WG01
WG02
SMF input
(a)
IL ref. WGs
1.1
1.4
1.0
XL (dB/crossing)
0.093
0.007
0.033
Radius for BL<1 dB (mm)
>6
> 10
>6
BLP ref. WGs (GHz m)
107
154
125
10x
lens
9
8
7
6
5
4
3
2
1
0
Bending loss (dB)
Crossing loss (dB)
The loss and BW performance of the waveguide components and reference waveguides is investigated under two
different launches (a) a restricted excitation: a 9/125 μm single-mode fibre (SMF) input (loss measurements) or a
10× lens input (BW measurements) and (b) a typical multimode launch encountered in real systems: a 50/125 μm
multimode fibre (MMF) input [numerical aperture (NA): 0.2]. All loss measurements are carried out at 850 nm
using a VCSEL, while for the BW measurements, a 1574 nm mode-locked fibre laser (TOPTICA FFS) and a SHG
crystal are used to generate short pulses (~400 fs) at 787 nm. A pair of microscope objectives is used to couple the
emitted light into the input fibre, while the other end of the fibre is butt-coupled with the waveguide input. At the
waveguide output, a 16× microscope objective (NA: 0.32) is used to collect the transmitted light and deliver it either
to an optical power meter for loss measurements or to a matching autocorrelator for pulse broadening measurements.
The waveguide BW is estimated based on the observed pulse broadening after transmission over the waveguide.
3. Experimental results
The bending (BL) or crossing loss (XL) of the waveguide components is obtained by normalizing their insertion loss
(IL) with respect to the IL value obtained for the respective reference waveguides under the same launch condition.
Fig. 2(a) and 2(b) show the excess loss in the components due to the waveguide crossings and bends for the two
launch conditions studied. The MMF input couples a larger percentage of optical power at the waveguide input to
the higher order modes, which are more susceptible to radiation loss in the bends and at the crossings, and therefore
result in higher component losses than the SMF input. WG02 exhibits the highest BL as it has the largest width (~55
μm) and lower index difference ∆n (~0.01). The other 2 samples exhibit a BL < 1 dB for a bend radius of > 6 mm
for both launches. On the other hand, WG02 exhibits the lowest XL (~0.007 and 0.02 dB/crossing for the SMF and
MMF launches respectively) due to its low ∆n, whereas WG01 shows the highest XL value: ~0.093 and 0.1
dB/crossing for the SMF and MMF launches respectively. Fig. 2(c) summarises the obtained results as well as the IL
and bandwidth-length product (BLP) of the reference waveguides for the 3 samples and inputs studied. For the 50
µm MMF input, WG02 exhibits the largest IL (~1.5 dB higher than that of WG01/WG03), but also the largest BW
(~×2.5 over WG01/WG03) due to its lower ∆n. A restricted launch however, results in similar IL for all samples and
high BLP >100 GHz×m due to the low input coupling loss and the excitation of lower order modes.
WG03
50 µm MMF input
3
WG_Β: 50 μm MMFWG03: 50 μm MMF
IL ref. WGs
1.6
3.2
1.7
2
1WG_C: 9 μm SMF
XL (dB/crossing)
0.099 0.019 0.046
0
Radius for BL<1 dB (mm)
>6
> 11
>6
WG_C:
50
0
20
40
60
80
5
8 μm MMF
11
14
17
20
BLP ref. WGs (GHz m)
47
122
48
Number of crossings
Radius (mm)
Fig. 2. Excess loss of (a) the 90° crossings, and (b) the 90° bends for both launch conditions and (c) summary of results including the insertion
loss and BLP (GHz×m) of the reference waveguides for the 3 samples.
4. Conclusions
Multimode polymer waveguide crossings and bends with low loss performance and high bandwidth are
demonstrated (XL<0.02 dB/crossing, BL<1 dB for R > 6 mm, BLP > 47 GHz×m). The component studies highlight
the design trade-offs with respect to the RI profile and indicate the potential to combine RI engineering and launch
conditioning in optimising the loss and bandwidth performance of complex waveguide paths that include a number
of such components. Given the stringent power budget requirements in high-speed (≥ 25 Gb/s) optical links, the
optimisation of the layout and launch becomes particularly important in the design of passive optical backplanes.
5. Acknowledgements
The authors would like to acknowledge Dow Corning for providing the waveguide samples and EPSRC for
supporting the work. Additional data related to this publication is available at the University of Cambridge data
repository (https://www.repository.cam.ac.uk/handle/1810/253476).
6. References
[1]. D. Miller, "Device Requirements for Optical Interconnects to Silicon Chips," in Proceedings of the IEEE, Vol. 97, pp. 1166-1185 (2009).
[2]. N. Bamiedakis, et al., "Low-Cost PCB-Integrated 10-Gb/s Optical Transceiver Built With a Novel Integration Method," in IEEE
Transactions on Components, Packaging and Manufacturing Technology, Vol. 3, pp. 592-600 (2013).
[3]. R. Dangel, et al., "Polymer-Waveguide-Based Board-Level Optical Interconnect Technology for Datacom Applications," in IEEE
Transactions on Advanced Packaging, Vol. 31, pp. 759-767 (2008).
[4]. J. Beals, et al., "A terabit capacity passive polymer optical backplane based on a novel meshed waveguide architecture," in Applied Physics
A: Materials Science & Processing, Vol. 95, pp. 983-988 (2009).
[5]. N. Bamiedakis et al., "A 40 Gb/s Optical Bus for Optical Backplane Interconnections," in J. of Lightw. Techn., Vol. 32, pp.1526-1537 (2014).
[6]. B. W. Swatowski, et al., "Graded Index Silicone Waveguides for High Performance Computing," in Opt. Intercon. Conf., pp. 1-3 (2014).