Some basic terms
Let’s try to sort out some of the critical terms first. The telcos have relied on Telcordia Technologies to write (with participation from the industry) Generic Requirements documents (GRs) that define performance requirements on the component and system level. These GRs are not standards and Telcordia is not an American National Standards Institute (ANSI)-accredited standards developer (or Standards Development Organization-SDO). These GRs cover design and performance criteria for common fiber-optic components and other products of interest to the telecom industry.
That enforcement frequently includes verification, performed either by the carrier or supplier. When it comes to verification methods, Telcordia references the appropriate test and measurement standards. These standards are predominantly the work of the Telecommunications Industry Association (TIA), which is an ANSI-accredited SDO. These widely used test and measurement standards are also called Fiber Optic Test Procedures (FOTPs). FOTPs standardize a range of tests from simple optical insertion loss measurements to more complex polarization-mode dispersion measurements. They also address environmental tests ranging from vibration and shock to fluid and altitude immersion tests for fiber-optic components (more information about these standards is available at www.tiaonline.org).
If the commercial industry turns to Telcordia GRs to identify performance requirements for optical components and systems, is there an equivalent body for the military industry? The answer is not immediately clear.
The Defense Supply Center Columbus (DSCC), among other duties, oversees the government’s purchasing of fiber-optic components. DSCC has the responsibility for military standard (MIL-STD) documents that define test methods for testing various components and products. In addition, DSCC controls the performance and design parameters of some fiber-optic components via MIL-PRF performance standards. These MIL-PRF documents are available at the DSCC web site (www.dscc.dla.mil/Programs/MilSpec/DocSearch.asp) and cover optical fiber, fiber-optic cable, splices, and various kinds of fiber-optic interconnects. Testing conditions specified for such components might address several environments (e.g., military aerospace and shipboard) within one document or are specific to one application (e.g., ground tactical connector).
Where possible, DSCC references TIA FOTPs as test methods for the verification of specified performance requirements, even if the specified test conditions are often harsher than those referenced in Telcordia GRs. However, there are many performance requirements for which no standardized verification methods exist within TIA FOTPs. In these cases, DSCC references MIL-STDs or combines MIL-STD conditions with FOTP test methods. MIL-STD test methods historically exist where no adequate commercial standards are available or where DSCC saw a need to tightly control the content, including revisions to such as well as the test methods. Because adequate test methods for fiber-optic components (the FOTPs) have already been developed within the TIA, DSCC references such test standards as much as possible wherever applicable and appropriate. In short, MIL-STD documents define test methods and MIL-PRF documents specify component design and performance criteria.
Fiber-optic component standards for military aerospace applications are also published by the Society of Automotive Engineers (SAE), which is another ANSI-accredited SDO. Fiber-optic connector standards are addressed in the subcommittee Aerospace Electrical/Electronic Distribution Systems AE8, for example. These published performance requirements documents reference a mixture of TIA FOTPs and MIL-STDs to be used as valid test methods. Performance specifications for components used in the commercial aerospace industry are developed by ARINC Inc.
As one can see, there is no single agency, standards body, or industry organization writing the performance and design criteria for fiber-optic components for all military environments. The defense industry considers a much broader and more complex range of environments for its fiber-optic component specifications. It does not have the luxury of solely defining an “outside plant” and “central office” environment. The environmental conditions for each application (ground, flight, space, shipboard, etc.) can be vastly different-and they demand many different test conditions and test methods. Defining performance requirements for components and systems is therefore less centralized and often left to those who develop the final products for these varying environments (the major defense contractors). These defense contractors individually write internal component specifications including performance and testing requirements. Such requirements obviously vary from contractor to contractor and from application to application.
After looking at examples of where these fiber-optic component standards for the commercial and military industry are developed, let’s point out some of the differences between specified performance parameters and the associated environmental conditions and test methods for fiber-optic components for communication systems.
While this probably comes as no surprise, the test conditions and performance requirements for military applications are generally harsher than those for the commercial telecom industry. Optical components are often installed in vehicles rather than in stationary settings. Hence, they are subject to various vibration and shock levels. Environments also change over time, requiring verification at various atmospheric pressure levels and exposure to various fluids.
As an example, let’s take a look at one of the most common fiber-optic components in communication systems, the singlemode connector. The table compares an excerpt of Telcordia GR-326 test conditions against requirements for MIL-PRF-29504 fiber-optic termini.
The required measurement of optical circuit discontinuities highlighted in the table for the military application is described in FOTP-32A (EIA/TIA-455-32A) and is typically not required for commercial telecom fiber-optic components. This measurement requires a continuous-wave light source to excite the connector under test and a high-speed optical receiver to monitor the optical signal for short-duration optical discontinuities of specific magnitude during a vibration or shock test. Specifications are as harsh as requiring no discontinuities of ≥1 µsec and ≥2 dB to occur during a test (as specified in a draft of MIL-PRF-83526/16). The figure shows the results of an optical signal discontinuity test.
In the above example, signal discontinuities occur on channels 1 and 3 during the test. The discontinuities seen on channels 2 and 4 do not exceed the set pass criteria for time and amplitude and are therefore not indicated as a fail by the software algorithm.
Mechanical shock testing represents another area of difference between Telcordia performance requirements for a singlemode fiber-optic connector and DSCC specifications. While Telcordia requires no mechanical shock testing within GR-326, the test is a requirement for military connectors. Fiber-optic connectors for shipboard applications are even required to undergo a harsher shock test called “hammer shock.” This test is specified in MIL-S-901 and involves a 180-kg (400-lb) steel hammer swinging against a steel anvil from 1-, 3-, and 5-ft heights (see photo). The connector assembly under test is mounted to the opposite side of the anvil. The optical circuit discontinuities described previously are also measured during the hammer shock test.
Resisting the various fluids that are used in vehicles is another common military requirement. The fluids include (but are not limited to) hydraulic oil, coolant, jet fuel, cleaning fluids and solvents, and seawater. Fiber-optic connectors generally shall not exhibit any swelling or softening of materials, no loss of sealing capability, and must retain their markings and coloration during immersion in such fluids to meet military requirements.
Knowing what performance requirements are applicable for the target markets is obviously important. Manufacturers should also be aware of the market-specific product certification requirements (if any) applicable to fiber-optic components.
While Telcordia does not maintain a laboratory certification program (there is no such thing as a “Telcordia-certified” testing facility), DSCC does. To support the government’s procurement activity for components, DSCC maintains a test-facility certification program and only admits those products for procurement that have been tested by a DSCC-approved facility. A list of approved testing facilities can be found at www.dscc.dla.mil/offices/Sourcing_and_Qualification/labsuit.asp.
Overall, while meeting Telcordia generic requirements for fiber-optic components is a good first step for companies looking to expand into the defense market, military requirements typically demand the supplier to go further. While Telcordia requirements only consider the outside plant and central office (uncontrolled and controlled environments), military specifications address many more applications and environments. Therefore, one can expect to see a vastly more diverse set of requirements with harsher conditions when investigating military requirements for fiber-optic components. However, knowing where to find applicable standards and requirements is always a good first step.Lorenz Cartellieri is president of Experior Photonics Inc. (Newbury Park, CA; www.experiorphotonics.com), a DSCC-approved testing facility specializing in test services for fiber-optics and testing of fiber-optic components. He is also chairman of the TIA subcommittee for Fiber Optic Metrology FO-4.5 and a member of the SAE AS-3 committee on Fibre Optics and Applied Photonics.