Beneath the sleek exterior of a modern jet fighter lie the sensors and control systems required for high-performance flight. And everything is connected by miles of electrical cabling. In new versions of planes such as the F/A-18 Hornet, a significant portion of nonflight-critical wiring has been replaced by fiber-optic cable.
Each optical fiber can carry many signals, weighs less than an equivalent copper wire, and has zero RF emissions. So, it’s easy to see why aerospace manufacturers have found them attractive to the point of mixing copper and fiber in a single cable. But as more fibers have appeared in each new design, it has become clear that manual test methods are impractical.
Enter DIT-MCO’s Model 2500.FTE Fiber Test Equipment. The company has been developing optical-fiber test technology for several years, culminating in the demonstration of a prototype in 2001. The recently released Model 2500.FTE provides 32 to 500 bidirectional ports—enough capacity to handle the simultaneous test of 250 optical fibers in both directions.
Initially intended for use with 100/140-µm multimode fiber operating at 850 nm, each test channel comprises a source LED with a monitoring photodiode and a separate receive photodiode. The elements are linked to the optical port by a 2 × 2 coupler.
An optical coupler branches optical power on a single fiber into two parts in a designated ratio or combines the optical powers on two fibers into one. The coupler ratio chosen is 50:50. This ratio splits the output power into two fibers, each with 50% of the source transmitter diode output.
With this architecture, each port can either receive or transmit without reversing connections to the fiber under test. Attenuation loss measurements are performed on optical fibers connected between two tester ports. The basic instrumentation accuracy is ±0.5 dB, and the uncertainty associated with the required test connectors adds about ±0.25 dB per connector.
A test consists of connecting an 850-nm, multimode, LED light source to one end of the optical fiber and transmitting a known value of light (power Pout) into the fiber. The actual value of transmitted power is determined by a photodiode and an associated logarithmic operational amplifier that converts the received light into a reference monitor voltage (Vmon).
The other end of the fiber is connected to a receiver module that measures the amount of light received (power Pin). The receiver converts the light into an electrical signal, Vref, using a logarithmic operational amplifier. A differential voltage measurement between Vmon and Vref is performed.
Proprietary optical filters ensure overall accuracy by creating precise optical launch conditions. The 2500.FTE meets the AS-100 (M80) specification that requires optical power to be concentrated in the center of the fiber. This approach supports consistent loss measurement over a large range of 100-µm connector variations. Alternatively, you can select a fully filled launch condition, which also is supported.
An optical tare feature allows the loss of the interconnecting patch cords to be subtracted from the test measurement. This capability, implemented through a simple TAR,xx.xdB command, makes it easier to interpret results directly, an important benefit when you are looking for losses less than 1 dB per fiber or 1.5 dB across an entire system.
Field retrofit of the optical test capability to existing Model 2500 analyzers takes less than a day. Retrofitted units and new units with combined electrical and optical capabilities are referred to as Model 2500.FTE. Contact company for price. DIT-MCO International, www.rsleads.com/211ee-184
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