More and more aerospace applications are incorporating fiber optics technology into their designs due to its many advantages over copper. The thinner fiber solutions provide higher speed over a longer distance, are more reliable, offer higher noise immunity and, in many cases, lower the cost of ownership. Additionally, for the same diameter, fiber can pack more data than copper. Fiber is faster than the category 5 and 6 copper cables, approaching the speed of light (31% lower). For copper, pushing the speed beyond 1G is a challenge, but for fiber 10G is quite common. Copper is limited by distance. Usually, signal degradation with copper will occur after about 90 meters (2.7 km maximum for custom systems), while fiber can achieve more than 1.5 km without a problem and can deliver over 80 km depending on transmission signal quality.

Perhaps the most significant advantage of fiber is that it is not affected by electrical noise because the transmission uses light instead of electrical signals. The typical electromagnetic interference (EMI) that affects copper cables will not be encountered with fiber optics. Over time, the copper will also degrade and have worse signal-to-noise ratio.

Compared with copper, a fiber system can be very efficient. In the case of a fiber-based Ethernet connection, more than 99.5% of the signal can be delivered to the Ethernet hub. Different types of convertors can be used to convert signals from the popular unshielded twisted pair (UTP) Ethernet connections over fiber cable, so many lower speed UTPs can be combined to achieve, for example, 100/120 Gigabits.

Challenges of Fiber Interconnect Design in Space

According to NASA, space radiation is made up of three kinds of radiation: particles trapped in the Earth’s magnetic field; particles shot into space during solar flares (solar particle events); and galactic cosmic rays, which are high-energy protons and heavy ions from outside our solar system (Figure 1). This adds up to ionizing radiation, proton and gamma ray attacks. These attacks have a major impact on electronic circuits, described as the total Ionizing Dose (TID) effects, which is measured in rad (radiation absorbed dose). Note that 1 rad = an absorbed energy of 0.01 J/kg of material, and 1 gray = 100 rads. The impact of exposure to space radiation ranges from degradation of performance to total malfunction. In space, one would imagine that the results can be quite serious.

The environment in space is harsh and demanding. Commercial-off-the-shelf (COTS) devices have to be able to endure the extreme temperature swings and the constant vibration. Failure is not an option in a space mission. Adding to this is the challenge to deliver maximum performance with minimum size, weight and power (SWaP), high mean-time-between-failure (MTBF), and reliability.

Designing for aeronautics is very different than designing for the earth environment. Aeronautical applications, such as spacecraft, satellites, and military aircraft are much more challenging. Designers of fiber interconnect solutions have to consider specific requirements to deal with those challenges. The three major challenges are:

• Space radiation attacks
• Operation in harsh environment
• Achieving space, weight and power requirements (SWaP) and reliability

Figure 1. Spacecrafts experience constant attacks of space radiation from magnetic fields, solar flares and galactic cosmic rays.

Best Practices for Optical Interconnect Design

Figure 2. SpaceABLE SM is a radiation-resistant optical transceiver created by Reflex Photonics. The modules measure less than 3 cm² and weigh less than 15 g.

Defend Against Radiation with Radiation-Resistant Design

What are the design considerations to meet the requirements as described above? It is important to defend against the radiation from ionizing, gamma, and other attacks. There are several methods to protect the device from radiation, including shielding, error correction, and using radiation-resistant components, which some refer to as radiation hardening. Shielding works for low-level radiation. Error correction works if the amount of radiation only temporarily impacts the device. However, heavy error correction will slow down the performance of the device.

Increasingly, more designs are incorporating radiation-resistant components to protect the device. Radiation-resistant silicon uses a different approach from the typical semiconductor wafers. The common approach is silicon on insulator (SOI) and silicon on sapphire (SOS), which enable radiation-resistant components to withstand an attack of ionizing radiation. While commercial-grade silicon can withstand between 50 and 100 gray (5 and 10 krad), radiation-resistant solutions can withstand 5 to 1000 times more depending on the types of components involved (Figure 2).

Design to Work in Harsh Environments and Follow Standardization

For the interconnect devices to survive in harsh environments, in addition to radiation resistance, they must include other parameters that may not be required for commercial-grade components. This includes meeting requirements for shock and vibration as specified in MIL-STD 883. It is strongly recommended that the devices be sealed from moisture and thermal shock within a wide range of operating temperature (typical -40°C to +100°C). Keep in mind that some devices may slow down when the temperature goes to the extreme, so it is important to measure sustained performance at those temperatures.

Figure 3. A different view of the SpaceABLE SM fiber-optic transceiver shows the connector for fiber-optic cable connection. At the bottom is the view of the ball grid array (BGA) for surface mount soldering.

Designing or selecting open standard-based (VITA 66) interconnect devices ensures that the solutions will follow the lifespan of the standards and will not be easily obsoleted, as is often the case in proprietary or custom designs. To ensure that the devices meet minimum standards, they should meet – but are not limited to – the following industry standards:

• MIL-STD-883, Method 2007.3 (vibration tests)
• MIL-STD-883, Method 2002.4 (mechanical shock tests
• MIL-STD-883, Method 1011.9 (thermal shock tests)
• MIL-STD-202, Method 103B (damp heat tests)
• MIL-STD-810, Method 502.5 (cold storage tests)
• MIL-STD-883, Method 1010.8 (thermal cycling tests)
• MIL-STD 883 (shock and vibration)
• MIL-STD-883G, Method 1019.7 (total Ionizing Dose and Cobalt 60 gamma rays tests)
• Total Non-Ionizing Dose (TNID) tests
• Open VITA 66 standards
• ECSS-Q-ST-60-15 Space Assurance

Achieving Small SWaP and Reliability

Weight becomes increasingly significant in space transportation and applications. The cost of sending 1 kg is estimated to be $50,000. Designing products to achieve optimal SWaP and high reliability with high MTBF is always the ultimate goal.

In space and military missions, failure cannot be tolerated. Satellites will be in orbit for many years, and repairing failed parts is not only difficult but also very costly. Therefore, designing for compact-size, ruggedness and high reliability will help developers stay competitive in the race to space. For example, the SpaceABLE interconnect solution with multiple lanes can yield as much as 150 Gbps. For reliability, a combination of sealing, ruggedness and radiation-resistant design plays into the longevity of the device. Its lifespan can range from a few years to over 20 years. The total cost of ownership including maintenance can be kept to a minimum with high-reliability devices.

Conclusions

Aeronautical applications face many design challenges that are unique to their intended environment. The best practices for optical interconnect design for space applications include the use of radiation-resistant technology to defend against space radiation, the use of components and devices that are designed to operate in harsh environments, and meeting small SWaP and long-term reliability requirements. Finally, it is recommended to follow open standards like VPX and to look for solutions that comply with MIL and quality standards.