Large Scale Optical Cross Connect
The development consists of a MEMS-based Optical Cross Connect (OXC) with about 50 input and 50 output fibres. One important application of this OXC is an enabling component for a reconfigurable switching network within a communication satellite. The 2 year objective was to deliver a breadboard to prove the soundness of the concept and to demonstrate a working MEMS-based OXC.
An important longer-term objective is to use the knowledge developed in this program to create a Swiss industrial source for MEMS-based OXCs.
The following schematic shows the general concept of the complete switching system. Input and output of the system is in the RF domain, while the core switching is in the optical domain. The RF signal are converted to optical signals prior to the actual optical switching. After the optical switching the are re-converted into the RF domain. The optical switching makes the overall system potentially very compact at very low cross talk and low insertion loss levels.
The key issue in the project is to develop a 2D electrostatic actuated MEMS micromirror array of 128 mirrors which is extreme stable in position over time to avoid heavy and complex feed back loop. Another task is to develop the necessary high port count collimator. And last but not least a training strategy and software for the switch matrix.
One expected benefit is to have spin offs with commercial value. Improvement of the knowledge for space application needs in the MEMS based optical domain. High port count optical switches for ground base applications with potential of satellite integration.
The main development of the OXC system was the optical-fibre-cross-connector core, which is schematically shown below.
The light exits the fibre and is collimated by a microlens, hits on one of the 128 micromirrors and is redirected by tip/tilt of this micromirror to a fixed mirror. The light reflects to the output port micromirror and enters the fibre through the output microlens.
3D representation of the OXC depicting ray tracing results of the optical simulations:
- “folded” optical design with only one micromirror chip - compactness
- 50x50 ports fully reconfigurable
- Extreme low drift design of the MEMS chip and the electronics - No Feedback loop
- High optical isolation because of free space architecture - No crosstalk
Detailed view of the three arrays of fibres, micro lenses, and micromirrors, respectively.
For the bread board, a compact alignment system was constructed which supports the electrical and mechanical interface:
On the MEMS side the main development was the microfabrication of the oval micromirror arrays comprising 128 micromirrors, which are arranged in 4 rows of 32 mirrors each. The micromirrors are oval, since they are illuminated at 45° AoI. A detailed view shows the 3 gold coated MEMS micromirrors. The mirrors measure about 600x900 µm². The mirrors are actuated parallel-plate electrostatic actuators. The mirror itself is the ground electrode, while the three actuation electrodes are arranged compactly underneath the mirror.
Bread Boards Details
The fully assembled and tested breadboard shows the OXC in the upper centre and a close up on the right side of the image. The input and output fibres connect on one end to the optical switch and the other end to the E/O and O/E converters.
Breadboard with close-up of the OXC core.
Top View of the breadboard depicting the fibre interfaces and the OXC.
|Work Package||Expected Duration||Main Actor||Other Actors|
|WP1000: Assessment of large MOEMS switch architecture||2 Months||Alcatel Space||Sercalo, IMT, EPFL|
|WP1100: trade-off architectures||2 Months||Alcatel Space||IMT, EPFL|
|WP1200: Conceptual design||2 Months||Sercalo||IMT, EPFL|
|WP2000: Preliminary Design of breadboard||2 Months||Sercalo||IMT, EPFL|
|WP3000: Detailed design of breadboard and test set||14 Months||Sercalo|
|WP3100: Detailed design of breadboard||14 Months||Sercalo||IMT, EPFL|
|WP3200: Detailed design of test set||3 Months||Alcatel Space||EPFL, Sercalo|
|WP4000: manufacturing||15 Months||Sercalo||IMT, Alcatel Space, EPFL|
|WP4100: MOEMS manufacturing||15 Months||Sercalo||IMT, EPFL|
|WP4200: test bed manufacturing||5 Months||Alcatel Space||Sercalo|
|WP5000: Test campaign||3 Months||Alcatel Space||Sercalo|
|WP6000: Appraisal||1 Month||EPFL||Alcatel Space, Sercalo, IMT|
Completed. We could successfully demonstrate the function of our design and deliver a demo switch system. The testing on the MOEMS switch breadboard and tests-bed was performed on a 10x10 sub-matrix selected among full connected optical inputs and outputs.
|Requirement Object||Target Spec.||Comments|
|Frequency band||Ka-band||OK alsofor S band|
|RF Input Power||>0 dBm||OK fr Ka and S band|
|Opto-microwave gain||0 dB||OK for Ka and S band|
|Opto-microwave noise figure||<28 dB||OK for Ka band|
|C/I ratio (2 tones)||>50dBc||OK for S band, not for Ka band|
|Return Loss||>20 dB||OK for Ka band|
|Static Isolation||>50 dB||>70 dB|
|Requirement Object||Target Spec.||Comments|
|System Level Requirements|
|Number of I/O ports||>50 x 50||Partial 24x24
Full connectivity 10x10.
|Switching time||< 5 ms||65 ms|
|Time for reconfiguration||< 50 ms||100 ms|
|Mass||< 1kg||0.86 kg|
|Size||< 1000 cm3||1400 cm3|
|Power consumption||< 5 W||6.01 W|
|Wavelength range||1.25–1.63µm||1.25-1.63 µm (measurements performed @ 1.55µm)|
|Insertion loss absolute value||< 7dB||< 6.2 dB|
|Insertion Loss variation||TBD||< 0.5 dB|
|Repeatability of loss||< 1 dB||< 0.06dB|
|Stability of loss||< 1 dB||< 0.9 dB|
|Max Power||21 dBm|
|Return Loss (back-reflection)||> 30 dB||15 - 30 dB|
|Isolation (crosstalk)||> 50 dB||> 70 dB|
|Polarization dependent loss (PDL)||< 0.1 dB||< 0.17 dB|
The results obtained during the project encouraged sercalo to start a new project on single 2D analogue driven tilt mirrors for ground based applications, like tunable filters, micro scanners or low cost spectrometers and to commercialize a 25 port fiber collimator for rotary joint applications. Future work will tackle environmental testing at chip and equipment level, yield of MEMS chips, etc.