Redundant fiber-optic transceivers have been incorporated into the High Speed Data Acquisition System (HSDAS) at Stennis Space Center, where they are used to communicate data acquired by remotely located instrumentation during tests of spacecraft engines. These and similar transceivers could also prove useful in other situations in which there are requirements for acquiring data at safe distances from instrumentation located in hostile or potentially dangerous environments.

A primary consideration in the design of these transceivers is to separate the analog-to-digital conversion hardware from the data recording device.The analog signals from a test stand could be transmitted to a safe location over long copper cables, but the signals on the cables can become corrupted through pickup of electromagnetic noise from the environment. Another approach would be to avoid long cables by tape-recording the analog signals on the test stand and subsequently transporting the tapes to a safe location for processing. This method would require extra time to digitize the data after tests. Analog-to-digital conversion on the test stand, digital transmission, and recording at a remote location make it possible to avoid both the degradation of analog electrical signals over long cables and the delays incurred through post-test digitization.

Fiber-Optic Transceivers like this one are used in pairs to transmit digital signals from remote test locations at rates up to 125 Mb/s. Whereas the maximum usable length of a coaxial cable for transmission of analog signals in the original application is 50 m, the maximum length of a fiber-optic digital link in the same application is 25 km.

With these transceivers, the analog instrumentation outputs are digitized and preprocessed on the test stand and then recorded remotely on a commercially available high-speed data recorder. Previously, the digitizer was connected to the recorder through a 75-Ω coaxial cable that could be no longer than 50 m. The electrical output of the recorder - a modified TAXI (Transparent Asynchronous Transmit/Receive Interface) signal containing a stream of digital data at a rate of 125 Mb/s -is fed through a standard duplex coaxial interface to the first of two fiber-optic transceivers. In the first transceiver, the electrical signal is converted to an optical one, then transmitted over one of two duplex fiber-optic links to the second transceiver, which is in a safe location far from the test stand. The fiber-optic link can be as long as 25 km - much longer than the coaxial cable. In the second transceiver, the optical signal is converted back to the original electrical signal and this data is then stored on the high-speed data recorder.

Each transceiver (see figure) contains two fiber-optic transmitters and two fiber-optic receivers. Under either manual or automatic control, electronic switching circuitry in the transceivers selects the transmitter/receiver pair that generates the best received signal. The redundancy of transmitters and receivers also helps to prevent communication errors that could arise from defects in the fiber-optic links.

This work was done by Joey V. Kirkpatrick of Stennis Space Centerand Francis Grosz, Jr., Kenny Lannes, and David Maniscalco of Omni Technologies, Inc.

This invention is owned by NASA, and a patent application has been filed. Omni Technologies, Inc., has an exclusive license for this technology. All inquiries should to addressed to

Omni Technologies
Inc.
Attn: Sean Griffin
7412 Lake Shore Drive
New Orleans
LA 70124
Tel No: (504)288-8211

Refer to SSC-00052