Fundamental Test Methods for Passive Optical Components

Since the early years of fiber optics, there has been a need to passively switch, tap, split, and multiplex optical signals. Today’s technology allows designers to expand on these original tasks by also providing the ability to direct, control, and filter optical signals for more creative and cost-effective system designs using innovative passive devices. This article presents basic test methods of passive optical components and gives some test device options for passive component testing.

Basis of Passive Optical Components

In fiber optics, the word “passive” refers to those devices do not require electrical-to-optical or optical-to-electrical conversion during its operation. Passive optical components include simple optical connectors, splitters, circulators, isolators, and fixed optical attenuators, but also more complicated, electronically controlled, optical devices like switches and variable optical attenuators. Passive optical components are the foundation stone of fiber optic network systems. They allow capacity upgrades at a relatively low cost without additional construction works in existing network.

Common Test Parameters of Passive Optical Components

The parameters to be tested depend on the purpose of the particular Device Under Test (DUT). The most common test parameters you will consider in passive component testing are illustrated in the following:

1. Insertion Loss

Insertion loss is the basic passive component test. Figure below illustrates one way to do this test. The first step of this test is to measure power through the same lead-in fiber that you will use in step two. This gives you the input power to the DUT. The output power from the DUT is measured in the second step. Insertion loss is then equal to 10log(Input Power/Output Power).

Since this is a relative measurement, the meter’s accuracy does not directly affect the results. Meter stability and linearity, plus connection repeatability, will primarily determine accuracy. Long-term meter stability will determine how often you need to take the lead-in fiber reference measurement. Here are some examples of components that may require an insertion loss test:

Connectors Splitters Isolators, forward direction Circulators, appropriate path Lithium niobate modulators, on-state Switches, selected to this port Attenuators, set to minimum Filters, in the passband wavelength Waveguides, appropriate path and wavelength

2. Isolation

If your DUT is required to block light transmission, then you may need to do isolation test. Isolation is simply an insertion loss test, except that a good result is a high-value loss, so you are measuring a very low signal level. Here are some examples of components that may require an isolation test:

Isolators, reverse direction Circulators, reverse path Lithium niobate modulator, off state Switches, selected to another port Attenuators, set to maximum Filters, outside the passband wavelength Waveguides, adjacent path (crosstalk test)

3. Split Ratio

A splitter, or tap, divides light into two or more outputs without wavelength selection (all channels go to each output). Split ratio can vary from 50/50 to 99/1. A splitter with a high ratio (e.g., 99/1) is usually called a tap. Split ratio is a straightforward measurement. Figure below illustrates one way to do this test.

In this test, you first measure light output through each DUT port, then compute the ratio. You can eliminate the switch by using two power meters or by simply reconnecting a patch cord. Some two-channel power meters have a ratio function to display this result directly. If the splitter has more channels, switching will probably make more sense. Like the insertion loss test, split ratio is a relative measurement. Linearity, stability, and connection repeatability determine measurement uncertainty.

Test Tools Options for Passive Optical Components

We usually use an optical power meter (OPM) to measure the power in an optical signal. Sometimes optical power meters are used combined with a different test function such as an optical light source (OLS) or visual fault locator (VFL) to test the optical loss. Besides, we also use an optical time domain reflectometer (OTDR) to measure optical link loss if its markers are set at the terminus points for which the fiber loss is desired. All these devices are the basic test tools in fiber optics testing. When we are planning to do a passive optical component testing, all these test tools play a very important role.



Passive optical components become more and more essential in toady’s fiber optic network. An accurate passive optical component testing is directly related to the stability of the whole network. Hope the fundamental test methods for passive optical components provided in this article is helpful for your network. FS.COM offers a comprehensive line of passive optical components and relevant test tools with high quality and long lifetime for your options. For more details, please contact us over