Understanding Fiber Optic Transceiver Design and Test Rules
The design of avionics systems must balance several factors. First, engineers want to increase the performance of the systems installed in aircraft or spacecraft. Second, they want to reduce the size and weight of the equipment that must be carried. Third, they want to maximize the safety of the in-flight systems and maintain communication between the aircraft or spacecraft and other flight vessels and the command center, regardless of the circumstances.
Benefits of Fiber Optics
Radio is the preferred method of communication in space. Traditionally, the military and aerospace sectors use copper wire in radio communication because copper is cost-effective and well-established in this application. How ever, copper is heavy and can initiate fire due to overheating.
More importantly, copper is susceptible to electromagnetic interference, which comes from natural sources such as lightning and solar flares. Also, operating a high-voltage system like a spacecraft through the plasma environment of low-Earth orbit will accumulate charged plasmas on the surface of the spacecraft, resulting in electrostatic discharge akin to a lightning strike. Electromagnetic interference may cause equipment degradation and malfunction, affect cockpit radios and radar signals, and interrupt the communication between pilot and control tower, thus jeopardizing the survival of the spacecraft.
Also, as engineers add more features into military aircraft and spacecraft, physical space becomes a precious commodity. More functionality also means a greater need for data bandwidth and processing power, increasing the amount of data the copper wire must transfer at any given time. Hence there's a need for a transfer material that is safer and lighter, and offers enhanced transmission speed.
Because of these considerations, the design of avionics has been moving towards using fiber optics. Although more expensive than copper wiring, fiber optics offer faster transmission, more bandwidth, and lower data loss (attenuation). Made from drawn glass, fiber optics is also more flexible and lightweight. More importantly, being a non-metal, fiber optics are immune to electromagnetic interference.
What is a Fiber Optic Transceiver (FOT)?
Switching from copper wire to fiber optics, however, also means that engineers need to design specific fiber optic transceivers (FOT).
Each FOT contains a transmission and a receiver module. On one end of the optical cable, an FOT transforms electrical signals to light for transmission; then on the other end of the optical cable, another FOT converts light back to electrical signals. The receiver module uses semiconductor detectors (photodiodes or photodetectors) to convert optical signals to electrical signals (laser or LED). The transceivers can be plugged or embedded into another device within a data network.
Optical transceivers come in various shapes and sizes (form factors) to account for different needs in data type or transmission speed and distance. Different protocols, or rules, determine how specific kinds of data are transmitted. Lastly, a hermetic, or airtight, seal is often used to protect the transceiver from humidity created by condensation.
Fiber Optic Transceiver Design Considerations
Primarily, there are three options for electrical interfaces:
Ball grid array (BGA), which offers the lowest possible profile (vertical space used), and is most compatible with board assembly as it can be directly soldered onto the host board; however, it presents the most difficulty in parts replacement.
MegArray™ offers the easiest parts installation and removal but has the highest profile and requires a push-in mechanism if used in a high amplitude vibration environment.
Land grid array (LGA) interposer, which is a compromise between BGA and MegArray™, has a small profile, offers multiple possibilities of installation/removal, but requires more labor during these processes because attachment is done with screws (Figure 1).
The three electrical interface options can be combined with four different types of optical interfaces:
MicroClip™ interface, which reduces fiber management during board assembly and offers a very light and small size interface solution. It is compatible with embedded high-density printed-circuit boards, but not with blind-mate, board-edge backplane mount.
The screw-in connector option is more rugged than the MicroClip but weighs more and has a bigger footprint. Screw terminals are low-cost but require more time to wrap it around a screw head properly. Also, vibration or corrosion can cause a screw-in connector to degrade over time.
The blind-mate, board-edge backplane mount interface is compliant with the OpenVPX™ standards and can be installed without tools.
Fiber pigtailed interface has the best size and weight performance since it has a fiber connector installed at only one end. However, it requires fiber management during assembly and maintenance (Figure 2).
Reliability Test Considerations
Because the spacecraft or aircraft has to maintain communication with other flight vessels and the ground control during operation, malfunctioning of the FOTs is unacceptable. Since fiber optic transceivers age more rapidly during extreme conditions, it is crucial to test and confirm the reliability of FOTs in extreme stress scenarios. The two basic stress conditions to be evaluated include thermal and vibrational stresses.
Thermal Stress: Inside the cockpit and underneath the skin of the aircraft, the primary source of heat is the avionics themselves. The stacking of the instruments not only makes cooling a challenge but also exacerbates the heat. As the aircraft climbs in altitude, the air temperature drops but a great deal of heat is generated from flying at supersonic speed; as a result, the plane experiences both extreme heat and cold. During the descent, the opposite occurs. Space shuttles also undergo significant thermal stress, since re-entry into the atmosphere will create a large amount of heat due to atmospheric friction. Lastly, different rates of thermal expansion by adjacent structures will produce more wear and tear.
Vibrational Stress: For rockets and spacecraft, the burning of solid rocket fuel during launch leads to vibration. Since most of the fighter jets have fixed wings, they will encounter vibration and shock waves during ascent, cruise, and descent. Also, atmospheric turbulence and mechanical malfunctions cause vibration, and the cabin pressurization and depressurization that occurs once per flight also causes stress variation.
Reliability Verification Tests
Due to the significant stresses the aircraft undergoes, and the consequences of failure, it is crucial to test the reliability of fiber optic transceivers (FOTs) under extreme and stressful conditions with five critical tests:
Mechanical and Environmental Tests,
Space Application Test,
Mechanical and Environmental Tests
Mechanical tests include vibration tests across all three axes, mechanical shock tests (for all six orientations), and thermal shock tests.
Environmental tests include a temperature cycling test that evaluates mechanical fatigue over the transceiver's lifetime to identify a thermal expansion-induced mismatch between materials, and a damp heat test to test the level of moisture resistance of the sealed FOT. A sequence of tests that are specifically designed for transceivers with a BGA electrical interface compatible with solder reflow is used to test for possible impact to reflow profile and cold temperature storage.
Life tests are used to forecast the lifetime degradation of performance of an FOT. Several units of transceivers (from different lots) are tested. Under the simulation of 20-plus years of operation, an FOT is tested in an accelerated manner by significantly exceeding its maximum operating conditions for at least 1000 hours.
Live tests evaluate the performance of the electrical and optical interface configurations on a FOT by running high-speed digital signals through every channel, measuring the transmission errors and making sure the bit error rate (BER) is below one error in one trillion bits even under the harshest conditions. A significant signal degradation of an FOT suggests a cumulative effect from both the transmitter and the receiver (Figure 4).
Space Applications Tests
If the FOT is to be used inside a satellite, it will also be submitted to additional tests to ensure that it maintains its performance under levels of radiation and vacuum conditions similar to those in space.
First, a non-operational FOT at rest will be bombarded with a Total Non-Ionizing Dose (TNID) of 100 MeV energy protons. Second, a non-operational FOT, under biased and unbiased conditions, is exposed with Total Ionizing Dose (TID) of gamma rays (100 krad cumulative dose). The difference in error rates between before and after tests must be negligible. Third, the FOT undergoes a Live Test, Single Event Effects (SEE), at both room temperature and 85°C, to detect the presence of Single Event Upset and/or Single Event Latch-ups. Fourth, an encapsulated ruggedized FOT must undergo an outgassing test to ensure cross-contamination of proximity circuit elements will not occur in a vacuum environment. Finally, the same FOT has to be submitted to a Live Thermal Vacuum Test, where thermal fatigue in the same vacuum environment is to be assessed.
The previous tests are associated to a qualification plan to be implemented on a significant sampling of units produced from at least two independent fabrication lots. The qualification plan will confirm the reliability of the design. To confirm the quality of a production lot, the FOT will go through screening tests on 100% of the units that are manufactured. These screening tests usually include thermal cycling covering the complete operating temperature range and a burn-in test at the maximum operating temperature for at least 168 hours, to eliminate units that may suffer from infant mortality.
The design challenges of FOTs used in rugged environments such as space and avionics require small form factors to fit in limited space. Three electrical interface options can be combined with four different types of optical interfaces, each having its own pros and cons. Therefore, it is important to select the design with the right parameters to fit the specific applications.
The ability of FOTs to deliver reliable performance despite extreme thermal, radiation, and mechanical stress is mission-critical. The passage of the FOTs to the previously identified critical tests (mechanical and environmental tests, life tests, live tests, space applications tests, and screening tests) must be applied to ensure the ultimate success of the missions.
This article was written by Jocelyn “Justin” Lauzon, Senior Technical Advisor, Reflex Photonics, Inc. (Kirkland, QC, Canada). For more information, visit here .