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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.

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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

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.

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 space, weight and power (SWaP), high mean-time-between-failure (MTBF), and reliability.

Best Practices for Optical Interconnect Design

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.

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

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).

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