All-Optical Switches - Background & Technologies

Optical interconnection has become a hot topic in the industry as it grapples with the challenges of scaling up to meet the needs of AI and disaggregation for HPC. In this Blog Post Rohit Kunjappa, Head of Product Management and Application Engineering at HUBER+SUHNER Polatis, explains the technology options available for all-optical switching and weighs up the merits of each.

“Switch when you can, route when you must.”
This is well understood in the telecom industry since circuit switching is cheaper and simpler than packet routing. With data rates increasing, optical circuit switching is more relevant now than ever before. All- optical switches (OOO) function by selectively switching the entire optical signal on one optical fiber to another optical fiber.

All-optical switches have a unique value proposition over traditional OEO (optical-electrical-optical) switches since they transmit the original input light signal through a transparent all-optical switch core, without converting it into electrical format. The transparent nature of all-optical switches makes them protocol, format and data rate agnostic.

Any place that is a fiber rich environment is a potential application for all-optical switching - in data centers for aggregation, disaggregation, protection and interconnection; in the government and defense space for cyber security, Lawful Intercept, RFoF (Radio Frequency over Fiber) distribution in satellite ground stations; automation in DevOps and test laboratories; and in the service provider space for fiber layer virtualization and protection.

The applications mentioned above require high-radix non-blocking matrix switching rather than simple 1x2 or 2x2 optical switches. There are many technologies that can enable this. The three that have enjoyed commercial success are:
  • Robotic switching
  • 3D-MEMS
  • Beam-steering using piezoelectric actuators

Robotic switches
Robotic switches take the humans out of manually patching new crossconnections but it is still a physical, albeit automated, process of moving fibers from one port to another. They do have the advantage that the crossconnections inherently latch and the insertion loss is lower than other types of all-optical switches. Robotic switches also have a slight price advantage over the other two technologies.  However they do come with some features that make them less suitable for applications where switching time and frequency is paramount:
  • They can only make one connection at a time and multiple cross connections cannot be made in parallel or simultaneously.
  • It can take 30 seconds to 2 minutes to make a single crossconnection and potentially hours to reconfigure the entire switch.
  • Multiple plugging and re-plugging of fiber connectors leads to poor connections over time.
  • In some types of robotic switches, after a certain number of crossconnections, the fibers tend to get tangled. This requires the switch to be recalibrated – this is a very disruptive process that impacts service uptime.
  • Robotic switches are not able to provide value-add features required in many applications like variable optical attenuation, optical power meters and automatic protection switching.

Though 3D-MEMS and beam-steering are very different optical switching technologies, both result in crossconnections being made in free-space with no manual intervention. 3D-MEMS uses micro-mirrors to deflect the optical beam from an incoming fiber to an outgoing fiber. Beam-steering uses piezoelectric actuators to align or point the optical fibers so signals can be switched between the fibers.

For a 3D-MEMS switch to be operational, it has some unique requirements which may be seen as drawbacks in certain applications:
  • 3D-MEMS requires light to be present in the fiber to make and maintain a connection.  This issue creates lag in circuit availability in some applications.
  • 3D-MEMS requires continuous feedback to keep the mirrors in their optimal alignment for maximum signal strength. This feedback is provided by power meters – so power meters are a prerequisite for 3D-MEMS all-optical switches. To overcome this limitation, some manufactures are using an in-band signal path with cameras to assist with the alignment/tuning of mirrors.
  • The mirrors require dithering based on the power meter feedback to keep them in their optimal position. In certain applications like RFoF (Radio Frequency over Fiber), dithering degrades the optical signal.
  • Current 3D-MEMs systems are typically marketed in one matrix style (symmetrical i.e. an equal number of input and output fibers) with limited port count options.

Beam steering
Beam steering creates high-radix, non-blocking all-optical circuit switches, providing the highest overall optical performance over a wide portfolio of matrix sizes. The largest size now available is a symmetrical 576x576 matrix.

At the core of POLATIS all-optical switches is the patented DirectLight TM beam-steering technology that makes connections using compact piezoelectric actuators to align collimated beams of light from opposing arrays of input and output fibers with minimal loss of the optical signals. Alignment is maintained using feedback from integrated position sensors to ensure connection stability over time, temperature and external disturbances. Switching occurs completely independently of the power level, color or direction of light on the path, enabling pre-provisioning of dark fiber and avoiding concatenation of switching delays across mesh or multi-stage switched optical networks.

The POLATIS Advantage – all optical switches are not created equal
  • Industry's lowest optical loss and superior performance, critical to the success of so many applications, such as ensuring the most accurate test data in a test lab.
  • The broadest range of symmetric, asymmetric and custom-configurable matrix configurations required to meet the evolving needs of customers’ applications.
  • Core component of a system architecture that can scale to support any size network - to tens of thousands of fibers.
  • True dark fiber switching, requiring no light to make and hold connections, is critical when wanting to pre-provision for new services or testing with low power, bi-directional (on a single fiber) or intermittent signals.
  • Accurate Optical Power Meters (OPMs) precisely monitor signal strength with programmable degrade and loss of service alarms.
  • Integrated Variable Optical Attenuation (VOA) enables rapid simulation of multiple link and span loss permutations.
  • Programmable shutter feature allows introduction of intermittent and repetitive fiber breaks to test system response to unusual fault conditions.
  • Optical Switch Partitioning enables different user groups to share the same switch without interference.
  • Integrated with leading test orchestration and DevOps software to replicate and schedule tests across multiple organizations and coordinate with higher layer equipment and devices.
  • Easy to control, with support for the most popular network management interfaces including NETCONF and RESTCONF Software Defined Network (SDN) protocols.
  • Latching can be achieved with battery backups, either via external UPS or integrated solutions.

If you would like to know more about POLATIS optical circuit switches and how the technology would benefit your application, you can contact us by email or phone:

Americas: +1 781 275 5080
EMEA/Rest of World: +44 (0)1223 424200
Further information can also be found at www.polatis.com