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