Passive Component Testing Overview

A whole family of subsystems and components is needed to enable and facilitate the development, manufacturing and deployment of systems using dense WDM technology. Specifications and measurement techniques have been developed to characterize each of these elements. This section explores key measurement and testing techniques used to characterize the major passive network elements of a dense WDM system.

Optical Light Sources for Testing Purposes

In real life, ideal sources (i.e., infinitely variable in wavelength, able to produce spectrally pure multiple wavelengths, perfectly stable) and ideal measuring instruments (high resolution and accuracy, stable calibration, infinitely tunable) do not exist. The test engineer must therefore carefully select equipment, to ensure that the desired parameter is actually being measured and that the measurement technique itself does not introduce undesirable side effects.

Figure 1: Representation of the basic differences between an LED (red curve), a Fabry-Perot laser (yellow curve) and a DFB laser (green curve).

The table below describes the parameters that should be considered in the choice of an optical source that will be used to test passive components.

Parameters	Description
Type of source	Broadband Tunable laser Fabry-Perot Distributed-feedback (DFB) laser
Source type vs. wavelength or wavelength range	For multimode applications: LEDs provide a low-cost test solution, ensuring an averaged measurement over the spectral bandwidth of the source. Although more expensive, the VCSEL offers the linewidth of a fine laser and its central wavelength is closer to the target wavelength. For singlemode applications: LEDs are also an excellent low-cost solution for singlemode applications, and for testing at a single wavelength, lasers are the best tools. DFB lasers have the narrowest linewidth of all sources. They are generally used for DWDM devices testing. Fabry-Perot lasers are general-purpose lasers; they have larger spectral linewidth than DFB lasers higher tolerance on central wavelength, but lower coherence.
Power level and stability	Adequate power levels must be available so that reliable measurements can be taken on high-loss components, or at the extreme transmission limits of wavelength-sensitive devices where two OR more signals may have to be compared, each attenuated by at least 40 dB. Power stability is important to reduce the uncertainty of the setup but also to reduce the downtime due to frequent referencing process.
Spectral width	For components with high spectral variation over wavelength, characterization requires a means to verify the spectral response of the device over a broad wavelength range. This is usually achieved with a broadband source or a tunable laser source.
Launch conditions	Launch conditions are of great interest for multimode sources. In a multimode fiber, there are many possible optical paths for the light to travel through (the modes). When light from an optical source is coupled into a multimode fiber, the light launch conditions determine which modes will be excited or filled, and to what extent. Depending on the device to be tested, different launch conditions will be preferred.

Below are the main applications and characteristics associated with each type of optical light source used in test and measurement applications.

Source Type	Applications	Important Characteristics
Broadband sources	Broadband sources are used with optical spectrum analyzers when only IL or transmission as a function of wavelength needs to be measured (as it is very tedious to measure PDL or ORL with an OSA). These sources can also be used with a power meter, when an average loss measurement over the range of the source is required.	Cover all the spectral ranges of interest; they are reasonably flat over their spectral range (ASE is the flattest, LEDs and SLEDs show Gaussian spectra). Main types: LEDs, SLEDs, ASE source LEDs and ASE sources are non-polarized and thus can reduce polarization uncertainties in test setups (SLEDs are generally polarized). Power output is low for LED, higher for SLED and highest for ASE sources. For multimode sources, launching conditions are important, as repeatable results come with controlled and well-adapted launch conditions.
Tunable narrowband sources	These sources are used with a bank of power meters for automated IL, ORL and PDL measurements on WDM components They are also used to simulate any ITU grid transmitter in the C and L bands.	Continuously tunable or not, polarized sources, generally with high power and low ASE noise (needed for passive component characterization) Main types: fiber lasers and external-cavity (ECL) lasers.
Fabry-Perot sources	Fabry-Perot sources are used for general-purpose loss measurements. They are usually employed with a power meter for IL, ORL and PDL testing of passive devices within a few nm from the nominal wavelength of interest	Polarized sources with spectral envelope of 3 to 5 nm (FWHM) ~ 0 dBm power; not exact wavelength Can be temperature-stabilized for improved stability
DFB lasers	DFB lasers are also used with a power meter for IL, ORL and PDL testing of passive devices at ITU-grid wavelengths. They are also used to emulate transmission lasers. In addition, DFB lasers can be used as a reference source for calibration of power meters and OSAs.	High-power sources, nearly spectrally pure Can be temperature-tuned. Its high coherence may induce interference problems in setups with internal reflections, causing power fluctuations in the setup.

Receivers for Testing Purposes

The most common receiver for test systems is the power meter, which measures the total power reaching its surface, independent of the emission wavelength, provided it is within its calibrated wavelength range.

Testing WDM components almost invariably involves addressing the wavelength and loss sensitivity of the device, so measurement techniques usually include either a wavelength-selecting detection system with a broadband source or a broadband detection system with a tunable light source. The desirable characteristics of receivers in test applications are analogous to those for sources. Broadband receivers (or power meters) should be spectrally flat. They should respond linearly over as wide a dynamic range as possible, and they should contribute as little noise as possible to the measurement. Their polarization sensitivity should be as low as possible.

Receiver Type	Applications	Important Characteristics
Power Meters	Power meters are used with different types of sources, provided that the total power, independent of the wavelength, remains within the calibrated wavelength range. Their main applications are IL, ORL and PDL testing of passive devices. When PDL is to be measured, selecting a power meter with low polarization-dependant response is recommended to reduce test setup uncertainty.	Power and wavelength range have to correspond to those of the sources in the setup. Measurement accuracy greatly depends on the power meter calibration process and on the standards used. You should also look for the possibility of including a high-precision power meter as an option. Linearity is a key precision specification for loss measurements; it refers to the relative difference between the response at a given power P and the response at a reference power P0. Polarization-dependent response refers to the response variation of a power meter with respect to all possible polarization states of the input light. Detector size has to correspond to the spot size (usually 1, 2, 3 or 5 mm)
Optical Spectrum Analyzers	OSAs are used with a broadband source to determine the spectral characteristics of passive components. In general, they are used when testing to determine the power-related characteristics of a DUT (e.g., SNR).	Dynamic range refers to the ability of the instrument to measure a wide range of signal strengths needed; for example, when characterizing the band shape of an optical channel in which sideband anomalies that are 50 dB down may be important. Sensitivity refers to the ability of the instrument to measure very-low-intensity optical signals. Resolution bandwidth (RBW) refers to the ability of the instrument to resolve closely spaced wavelengths, needed to investigate the detailed properties of DWDM channels—a parameter of increasing importance as higher channel densities come into service. Accuracy refers to the ability of the instrument to indicate precisely and correctly the measurement wavelength and power.
Wavelength Meters	Same as those defined for the OSA. In general, wavelength meters are used when testing to determine the spectral characteristics of a DUT.	Same as those defined for the OSA.

Finding the Best Way to Test Passive Components

Establishing the best way to test passive components can be quite challenging. Nonetheless, the choice becomes a lot easier once the exact nature and uses of the devices that will be tested are known. The first step in finding the ideal product combination is to answer the questions below

Question	If you answered yes	If you answered no
In your application, is it relevant to verify the spectral response of the device under test (DUT)?	Spectral characterization is needed (IL as a function of wavelength) Go to the WDM sub-page	List the appropriate wavelengths - singlemode or multimode - and measure IL at those wavelengths. Go to the Broadband sub-page
Is your DUT very sensitive to polarization changes and is it important to quantify this effect?	Polarization-dependent loss (PDL) should be quantified at every part. It is possible that a thorough qualification may eliminate the need for 100% testing.	PDL does not have to be measured, but should at least be measured on a representative sample (qualification).
Is your DUT used in a system where reflections are problematic?	ORL OR reflectance should be measured. This test can usually be combined with IL.	ORL does not have to be measured, but should at least be measured on a representative sample (qualification).