Insertion Loss, Return Loss, and Reflectance

Insertion loss is composed of the connection coupling loss and additional fiber losses in the fiber following the connection. In multimode fiber, fiber joints can increase fiber attenuation following the joint by disturbing the fiber's mode power distribution (MPD).

Fiber joints may increase fiber attenuation because disturbing the MPD may excite radiative modes. Radiative modes are unbound modes that radiate out of the fiber contributing to joint loss. In single mode fibers, fiber joints can cause the second-order mode to propagate in the fiber following the joint. As long as the coupling loss of the connection is small, neither radiative modes (multimode fiber) or the second-order mode (single modefiber) are excited. Insertion loss of both multimode and single mode interconnection devices is measured using EIA/TIA-455-34. For military applications, an overfill launch condition is used at the input fiber. For other applications a mandrel wrap may be used to strip out high-order mode power. The length of fiber before the connection and after the connection may be specified for some applications.

Power measurements are made on an optical fiber or fiber optic cable before the joint is inserted and after the joint is made. The picture below illustrates the mandrel wrap method of measuring the insertion loss of an interconnecting device inEIA/TIA-455-34.

Insertion loss measurement of an interconnecting device.

Initial power measurements at the detector (P0) and at the source monitoring equipment (PM0) are taken before inserting the interconnecting device into the test setup. The test fiber is then cut at the location specified by the end user. The cut results in a fiber of lengths L1 and L2 before and after theinterconnection device that simulates the actual system configuration. After interconnection, the power at the detector (P 1) and at the source monitoring equipment (PM1) is measured. The insertion loss is calculated as shown below:

If the source power is constant, then the calculation of the insertion loss is similar to that of fiber attenuation.

Return Loss and Reflectance

Reflections occur at optical fiber connections. Optical power may be reflected back into the source fiber when connecting two optical fibers. In laser-based systems, reflected power reaching the optical source can reduce system performance by affecting the stability (operation) of the source. In addition, multiple reflections occur in fiber optic data links containing more than one connection. Multiple reflections can reduce data link performance by increasing the signal noise present at the optical detector.

Reflectance is a measure of the portion of incident light that is reflected back into the source fiber at the point of con-nection. Reflectance is given as a ratio (R) of the reflected light intensity to the incident light intensity.

Return loss and reflectance are measured using EIA/TIA-455-107. They are measured using an optical source connected to one input of a 2 ´ 2 fiber optic coupler. Light is launched into the component under test through the fiber optic coupler. The light reflected from the component under test is transmitted back through the fiber optic coupler to a detector connected to the other input port.

The optical power is measured at the output of the device under test (Po) and at the input port of the coupler where the de-tector is located (Pr). Po is corrected to account for the loss in power through the device under test. Pr is corrected to account for the loss in power through the coupler and any other connection losses in the path. The reflectance is then given by the ratio Pr/Po.

Return loss is the amount of loss of the reflected light com-pared with the power of the incident beam at the interface. The optical return loss at the fiber interface is defined as;

Return loss = -10 Log R

Return loss is only the amount of optical power reflected and does not include power that is transmitted, absorbed, or scattered.

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