Parallel Optics: The Next Leap for Embedded System

Gérald Persaud, VP Business Development and Michel Têtu, Senior Business Development Advisor

C4ISR applications

Command, control, compute, communicate, intelligence, surveillance, and reconnaissance systems (C4ISR) rely on accurate views of local situations for decisions that are critical to national defense. Fiber optics interconnect has emerged as the only viable technology to carry the massive amount of information generated by high resolution radars, infra-red cameras and other sensors.  Fiber optic-interconnect are small, immune to EMI and has superior bandwidth to traditional copper interconnect.
Reflex Photonics optical embedded transceivers are small, rugged, lower power components enabling the transmission and processing of high bandwidth sensor information.

Eyes and ears everywhere

Command, Control, Compute, Communicate, Intelligence, Surveillance, Reconnaissance
Computerized Command, Control, Communications, Intelligence, Surveillance

Illustration of the relation between the different elements of C4ISR systems (Command, Control, Compute, Communicate, Intelligence, Surveillance, Reconnaissance).

ISR trend

Intelligence, surveillance, and reconnaissance (ISR) trend is for more information and accurate views of the situations with longer mission times.  For example, UAVs with high resolution sensors and high performance embedded computing have become an invaluable tool to defense.  They can survey large areas quickly and at much lower cost than manned aircraft.

ISR systems need:

  • Higher resolution sensor arrays with high BW fiber optic interfaces
  • Enormous signal processing with scalable computers
  • Lower SWaP-C
  • Rugged and reliable components to survive extreme temperature, vibration, and moisture …
UAVs with high resolution sensors and high performance embedded computing have become an invaluable tool to defense. These drones can survey large areas quickly and at much lower cost than manned aircraft.
Phased array radar require massive amount of interconnect bandwidth

Parallel optics – The embedded leap for C4ISR

Parallel optics provides multiple high bandwidth interconnects in a space ten time smaller than co-axial copper interconnects. With almost unlimited bandwidth it is clear that all future interconnects for embedded systems will use parallel optics.
Reflex Photonics supplies chip size rugged parallel optics transceivers to operate in harsh military environments. These embedded parallel optical modules are qualified to MIL-STD-883E for severe environmental conditions. 

Surface mount

  • SMT construction provides high resistance to shock and vibration via low CG and solder attach
  • SMT support heat sinking to host board to reduce height and weight

Optical Connector

  • MT connector enables pick-and-place part
  • MT connector simplifies manufacturability (no pigtail)

Cable

  • Low-mass cable and retainer tolerates high shock and vibration
  • High temp materials/simple structure = reliable performance in harsh environments
High temperature materials and simple structure equals reliable performance in harsh environments.

High temperature materials and simple structure equals reliable performance in harsh environments.

System performance - BER

System performance of an optical link is determined by the quality of the signal generated by the TX, channel impairments (fiber optics cable and interconnects) and the sensitivity of the receiver over a bandwidth range. Rugged transceivers must operate over wide temperatures (at least -40 ºC to 85 ºC) which makes it challenging to maintain low bit error rates (BER) at the high operating speed.  For example, the laser response slows with temperature making it difficult to maintain an open eye at speed beyond 6 Gbps. At high temperatures the laser output power declines and causes a decrease in signal to noise ratio. As well, the response time decreases which can cause a high level of ringing. The TX eye diagram is a useful method to assess the quality of the signal generated over temperature. Open eyes correlates to low BER as the receiver is given more bit time to accurately discriminate a high signal from a low signal.
The eye diagram in figure below shows the LightABLE transmitting 10 Gbps at -40 ºC.  It uses the 802.3ab Ethernet mask to show there is a huge bit time margin for the receiver to accurately detect a high from a low. This is why Reflex Photonics transceivers can deliver BER better than 10-15.

System performance -BER
10G Challenge. The closing eye has a significant impact on BER

10G Challenge

  • Laser response slows significantly below -30 ºC causing eye to close at 10G
  • The closing eye has a significant impact on BER
  • IEEE802.3ab specifies a BER of 10-12 - high performance systems expect 10-15

Rugged optics requirements

Reflex Photonics LightABLE embedded transceivers offer small SWaP-C, operation over industrial temperature range (-40 °C to 85 °C), a bit error rate (BER) as low as 10-15, survivability to storage temperature from - 57 °C to 125 °C. The LightABLE can be surface mounted using leaded or RoHS reflow processes or it can be plugged into a board with a Meg-Array socket. Mounting the LightABLE close to the electrical driver delivers the best signal integrity and lowest power operation. The optical fiber interface is a standard MT ferrule directly attached to the module for compatibility with standard connectors and cables.

Operating temperature

  • -40 ºC to 85 ºC or wider
  • Considerations: BER at 10G – due to laser response over temperature

Storage temperature

  • -57 ºC to 125 ºC
  • Considerations: Reliability – mechanical stress, laser alignment

Shock and vibration

  • MIL-STD-810xx – aircraft, land vehicles, gun shock
  • Considerations:
    • Socket with low wipe contact is a concern
    • Mechanical attach strength – SMT vs socket

SWaP-C

  • SMT offers low height without bulky heat sinks for tightly stacked blades
  • Embedded optics typically consumes 100 mW/10G channel
  • Weight is typically 5 g

Moisture

  • Seal to avoid moisture from obstructing optics
  • For example, rapid decompression condenses air moisture

Bit error rate (BER)

  • IEEE802.3ab for 10G Ethernet is specified as 10-12
  • High performance systems expect 10-15 to avoid power hungry FEC, CDR, or equalizers.
  • Higher the BW, lower the expected BER!

Link budget

  • Link budget is the loss that can be tolerated between the transmitter and the receiver for a certain BER
  • Main sources of loss are connector return loss and mode dispersion for multimode fiber
  • TX output should be derated based on mask margin – jitter power penalty

Performance

  • Scalable BW – up to 28G
  • Signal integrity – BER of 10-15
  • Low loss – 0.003 dB/m (OM3 @10G)
  • Reach – 300 m (OM3 @10G)
480G full duplex I/O card

SWaP-C

  • Small – 125 µm diameter fiber
  • Light weight –  <1.5 g/m (OM3)
  • High I/O density – 48 fibers in MT connector
  • Lower power – 100 mW/10Gbps
Optic fiber offer much better I/O density that copper interconnect.

Rugged

  • -40 ºC to 85 ºC operation @ 10 Gbps
  • MIL-STD-810xx Shock and vibration
  • Moisture resistant
  • EMI and nuclear radiation immune
Optic fiber are immune to electro magnetic interferences.

Rugged Optic: New Possibilities for HPEC in Harsh Environment

High input/output interconnects are essential to high performance embedded computing systems (HPEC) and optical technology offering small size and weight and requiring low power consumption is becoming the preferred technology. However for harsh environmental conditions as encountered in defense and aerospace applications rugged optical systems must be devised.
by Michel Têtu, Reflex Photonics Inc.

High Performance Embedded Computing Systems (HPEC)

High performance embedded computing (HPEC) systems are essential to decisional systems where a huge amount of data must be collected and processed in a very short time to guide proper decisions and urgent actions. These systems are generally made of multiple electronic boards interconnected in a box through a backplane circuitry. Most of this circuitry is made of copper wiring but optical interconnects start to be used when high bandwidth high density I/O are requested.

C4ISR Applications

In the defense world, HPEC plays a major role in C4ISR systems (Command, Control, Compute, Communicate, Intelligence, Surveillance, Reconnaissance). For some applications, like active electronically scanned array radar, the information is generated by thousands of sensors. This information is usually in the form of analog signal and has to be digitized before being transmitted to the processing unit. The analog-to-digital conversion has to be high resolution and the communication link to the processing element has to be at high bit rate. (figure 1)
These C4ISR systems are often mounted on mobile platforms and used in harsh environment where extreme storage temperature, wide operating temperature range, high mechanical shocks and vibration are encountered. These operational constraints mandate the use of rugged systems and components. Other important characteristics of these systems are that they must be of small size and weight and consume as little operating power as possible.

Small SWaP Optical Interconnects

The optical interconnects can be used to carry the information from the sensors site to the computing site, between the computing boards, and between the computing system and the communication system. Optical interconnects are perfectly suited to meet the requirements of small SWaP in harsh environment.
It is well known that the size of lasers and photodetectors is of the order of a millimeter. The wavelength involved is of the order of a micron, so the fiber diameter required to guide the light is less than a millimeter. Made out of silica the weight per meter of a fiber is negligible. The weight of an optical transceiver results is mainly made of the electronic board needed to drive the laser and amplify the current generated by the photodetector, the optical connector, and the mechanical housing.
Because the light is guided through a highly homogeneous material, the signal attenuation resulting from scattering is extremely low (2.3 dB/km). This low fiber attenuation and the high efficiency of signal conversion (from electrical-to-optical of the laser, and from optical-to-electrical of the photodetector) generate very low electrical power requirements in order to drive a transceiver and carry the signal over hundreds of meters.
In addition, the fiber is dielectric so there is no susceptibility to electromagnetic interference (EMI). All of these benefits offer great advantages over copper interconnections.

 

Computerized Command, Control, Communications, Intelligence, Surveillance

Figure 1
Illustration of the relation between the different elements of C4ISR systems (Command, Control, Compute, Communicate, Intelligence, Surveillance, Reconnaissance).

LightABLE embedded transceiver in surface mount and pluggable variants

Figure 2
LightABLE products (transmitter, receiver, or transceivers) can be surface mounted or plugged. They are fully qualified for harsh environment.

Rugged Parallel Optic Transceiver

Reflex Photonics has developed the LightABLE products family to meet the demanding requirements of optical interconnects for HPEC used in harsh environment as encountered in defense and aerospace applications. The LightABLE 40G SR4 is a 4-lane full duplex transceiver operating at 10 Gbps per lane and the LightABLE 120G SR12 is a 12-lane transmitter or receiver operating at 10 Gbps per lane. (figure 2) These embedded parallel optic modules have been fully qualified following the Telcordia GR-468-CORE and MIL-STD- 883E standards and includes severe environmental, mechanical and long-term reliability tests. They offer: small SWaP, operation under industrial temperature range (-40°C to 85°C), a bit error rate (BER) as low as 10-15, survivability to storage temperature from - 57°C to 125°C. The optical fiber interface is a standard MT ferrule directly attached to the module for compatibility with standard die mounting processes. The LightABLE products can be surface mounted with regular lead or RoHS reflow process or plugged in close proximity to high-speed electronics and support high temperature reflow process; a unique feature for such products. (figure 3)

LightABLE MicroClip

Figure 3
The MicroClip is a low-profile, low-mass spring loaded MT ferrule.

A proprietary MicroClip MT ferrule has been also devised by Reflex Photonics to connect the LightABLE module to a 12-fiber ribbon cable pigtail. The MicroClip is a low-profile, lowmass spring loaded mechanical assembly that offers a rugged optical connection that is resistant for shock and vibration and is suitable for harsh environment. The MicroClip has proven it can withstand a 1 kg live traffic fiber pull test when mated to its products (40G SR4 and 120G SR12), without any signal performance degradation. This result exceeds by a factor of 2 the requirements of Telecordia GR-468-CORE Fiber Integrity Side Pull Test and confirms the reliability of the Reflex Photonics fiber ribbon interface with the LightABLE and its MicroClip ferrule.
To achieve such performances the LightABLE products are designed with unique features and assembly processes in order to:

  • Maintain laser response over the temperature range;
  • Avoid mechanical stress between parts;
  • Use surface mount technology and low height parts for high resistance to shock and vibration;
  • Use no heat sink or pigtail fiber for pick and place manufacturability;
  • Use sealed enclosure to avoid moisture from obstructing optics.

The future of optical interconnects in HPEC applications

Although there is a large interest for optical interconnects, it is fair to say that we are only at the beginning of their use in the development of high performance embedded computing systems. We see, in open standards organization like VITA, many working groups considering modifications to standardized board-to-backplane connectors in order to include optical interconnects.