Mainframe test chassis adapts to varied laboratory measurements

Mainframe test chassis adapts to varied laboratory measurements

Manufacturing, production and research laboratories opt for a mainframe test chassis that provides flexibility, programmability, interface capabilities and automated control of complex measurement procedures for fiber-optic components

A. girard, p. leonard,

f. hébert and M. carlson

exfo electro-optical engineering inc.

By using an integrated modular test system, users can perform multiple, complex and diverse fiber-optic component and system measurements. Moreover, by incorporating a personal computer and a Windows-type software interface, the test system can automate measurement procedures, thereby saving labor, time and cost.

Manufacturing and production testing laboratories prefer test systems that are easy to use, prove cost-effective and can handle repetitive tasks. For example, research and development laboratories require flexibility, programmability, interface capabilities and complete control of complex measurement procedures.

A mainframe test chassis that accepts a variety of modular plug-in units performing as individual test instruments meets these requirements. It also provides capabilities for performing new and innovative measurement applications in a changing and demanding laboratory test environment.

Efficient and cost-effective laboratory measurements of fiber-optic components generally deal with structuring a suitable test system. Some test systems are dedicated to specific applications and, therefore, lack flexibility. Others incorporate a proprietary architecture and consequently suffer expansion problems. To overcome these shortcomings, the mainframe test chassis provides modularity, hardware and software integration and extensive expansion, storage and adaptable measurement capabilities.

Under computer control, a laboratory mainframe test chassis accepts and directs a range of plug-in modular measurement units. Demonstrating diverse testing capabilities, these units individually furnish power meter, light-emitting diode, laser transmitter, variable attenuator, return-loss meter, variable reflector, spectrum analyzer, multimeter, fiber amplifier or optical switch functions. Along with the hardware, a graphical user interface and compatible application software add test control, flexibility and ease of use.

The mainframe test chassis usually comes in either a rack-mount or a benchtop configuration. To support test data presentations, it includes a high-resolution liquid crystal display monitor and multi-purpose function keys. In addition, the software contains several tools, such as a software keyboard, a numeric keypad and function buttons, so that, when rack-mounted, peripheral components are not required. A rack-mount configuration suits industrial, manufacturing and production test environments.

When used in a laboratory benchtop environment, a mainframe test chassis generally accommodates an external keyboard, mouse, color monitor and other computer accessories. If the system is Windows-software-based, compatible software can be installed for handling spreadsheets, databases and word processing. With these capabilities, data recorded during various test measurements can be imported into a spreadsheet or database program for study, analysis and interpretation

A macro programming language is often included. Consequently, customized applications can be programmed to perform complex automatic or interactive fiber-optic measurement procedures. Moreover, a mainframe test chassis might include a port for configuring an attached general-purpose interface bus device or an automated controller and drivers for customizing laboratory application software.

Innovative applications

A mainframe`s integrated test functionality, combined with a variety of plug-in units, offers an automated solution to complex laboratory fiber-optic measurements. With this concentrated measurement support, users can set up and perform complicated test procedures with minimum manual intervention.

An array of plug-in units enables rapid parallel-testing applications, such as insertion-loss measurement, optical return-loss or reflectance measurement, instrument calibration and certification, and component quality control check.

To demonstrate the variety of available measurement capabilities, four test applications demonstrate how a laboratory mainframe test chassis and its associated plug-in units offer efficient and cost-effective measurement alternatives to traditional test instrument checkout methods.

Optical return-loss testing

In fiber-optic systems, reflected light is produced by Rayleigh scattering and Fresnel reflections. Fresnel reflections come from the operation of discrete components and develop because of air gaps, misalignment and non-matching refractive indexes. Rayleigh scattering occurs along the fiber and is related to its composition.

Reflections in fiber-optic systems are undesirable for three main reasons:

They contribute to overall transmission power loss.

High-performance laser transmitters are sensitive to reflected light, which can degrade laser stability and system signal-to-noise ratio. Under extreme conditions, the laser transmitter can be permanently damaged.

Reflected light can be re-reflected in the forward direction. These forward propagating reflections lag behind the original signal and create interference during communication and video signal processing.

A return-loss meter plug-in unit comprises a low-drift indium gallium arsenide photodetector, coupled to a source port, a measurement port and an internal zero-reflection termination. Because it is a component of a modular test system, the meter is designed to operate using an external source. Other expansion modules can be installed to serve as a 1310-nanometer, a 1550-nm or both 1310- and 1550-nm thermo-electrically cooled Fabry-Perot laser sources with high stability.

Mainframe application software provides a user-friendly interface and flexible operation by implementing:

Detailed step-by-step optical return-loss measurement procedure

Manual (expert) measurement mode

Dual-wavelength testing

Flexible reference calibration

User offset

Data storage

Detailed test reporting.

When integrated with a dual-wavelength source expansion module and either a 1䂄 or 1䂔 programmable optical-switch chassis, the optical return-loss meter module can perform automated test monitoring of 16 or 32 different devices, respectively, at two wavelengths.

Because the programmable optical switches possess low backreflection, this test setup can monitor or verify multiple component optical return-loss characteristics, as commonly required to perform quality control and batch testing. The application software also includes the procedure for performing the optical return-loss meter calibration, documenting the measurement environmental conditions and producing detailed test reports and labels.

Test jumper acceptance

When performing inspection or final acceptance of fiber-optic test jumpers (or other similar connectors), insertion loss and reflectance become important measurements. The test instruments used to make these two basic measurements should include the following capabilities:

Perform both insertion loss and reflectance measurements without disconnecting and reconnecting the device under test

Test at two wavelengths

Generate test reports and labels.

These measurement can be performed in the following sequence:

Calibrate the optical return-loss meter at two wavelengths.

Calibrate the optical power meter.

Perform a power meter reference measurement with a bulkhead connector (inline coupler) installed in place of the device under test.

Configure the power meter to display relative power.

Request the user to install the device under test.

Store the insertion-loss value for the first wavelength.

Request that the user terminate the connecting fiber using a mandrel.

Store the reflectance value for the first wavelength.

Repeat the previous three steps for the second wavelength.

Again, the application software documents the operating conditions and generates detailed reports and labels.

Measuring source stability

In designing fiber-optic systems, network planners need to know at what value of reflected light interference signal transmitter stability begins to deteriorate. This measurement calls for an optical power meter and a variable backreflection meter.

The optical power meter is an established fiber-optic test instrument. The variable backreflection meter, however, is less understood. It consists of a variable attenuator and a mirror coupled to an input port, an output port and a monitoring port. The input power is coupled to the output port (incurs an approximate 5-decibel loss including connectors) and to the attenuator/mirror assembly. In adjusting the variable attenuator, the user varies the intensity of the reflected light that is coupled back to the transmitter. The monitor port provides direct access to the reflected power for measurement and calibration.

The measurement procedure for testing laser source stability proceeds as follows:

Set the variable backreflection meter to a minimum reflection setting (typically -55 dB).

Connect a test jumper between the source under test and the input port of the meter.

Connect a second test jumper between the optical power meter detector and the output port of the variable backreflection meter.

Increase the reflection until the optical power meter measurement starts to show signs of instability.

A mainframe chassis test system can automate the previous manual source stability measurement. Its programmable variable reflector can provide automatic reflection adjustments at fine or coarse increments. In addition, its power meter data acquisition software can be configured to trigger at a defined condition--by setting an x and y value in trigger acquisition mode--and to record source operation when it starts to show signs of instability. Alternately, the software can be configured to record the entire event (timer acquisition mode). Because all the test data is stored on a hard or floppy disk, storage space is not a problem.

A wavemeter module that can measure optical wavelength could also be connected into the measurement setup to simultaneously monitor frequency stability.

Note that the variable backreflection meter introduces a less than 5-dB loss between the input and output ports. Many fiber-optic systems can tolerate this additional loss and would not be affected when the meter is connected into the system. With this meter inline, the overall system can be tested to determine sensitivity to reflections. Once the sensitivity is known, appropriate parts, such as connectors or directional couplers, can be installed to safeguard the system. Conversely, the variable backreflection meter test can also prove that a system is relatively insensitive to reflections and that costly protective components are unnecessary.

Three test approaches are available to perform multichannel power measurements-using multichannel power meter(s), single-channel power meter(s) with optical switch(es) or a combination of multichannel power meter(s) with optical switch(es). The choice depends on how many test points or devices are being monitored and whether each device requires continuous monitoring.

If continuous monitoring is required, the best approach is to use a multichannel power meter. Approximately 160 channels can be continuously monitored. Using software-directed trigger conditions, abnormal power trends can be detected, recorded and analyzed.

Where continuous monitoring is not critical, a single-channel power meter can be used with a programmable optical switch. In this configuration, the switch can be controlled to continuously cycle the connected devices under test and provide periodic power monitoring.

To continuously monitor critical components while periodically monitoring other less-critical test points or devices, a hybrid test monitoring system can be configured. In this setup, a multichannel optical power meter is integrated with a programmable optical switch. For example, a hybrid test might involve continuously monitoring three channels, while periodically monitoring 16 devices under test. Once again, application software can provide the required degree of automation. u

A. Girard, P. Leonard, F. Hébert and M. Carlson are members of the Laboratory Product Support Group at EXFO Electro-Optical Engineering Inc. in Vanier, Quebec, Canada..

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