5.2 Optical Loss Testing

5.2 Optical Loss Testing

Optical loss testing measures the total insertion loss of a fiber optic link. Insertion loss is the cumulative drop in optical power that occurs when components—such as fibers, connectors, adapters, splices, and splitters—are introduced into a light path. Measuring insertion loss is the primary method for certifying that a newly installed cable plant complies with design specifications and can support the transmission requirements of networking equipment.

The Optical Loss Test Set (OLTS)

An Optical Loss Test Set (OLTS) is the primary tool used to measure insertion loss. It is a dual-instrument system consisting of:

1. Stabilized Light Source: Emits a continuous, constant level of optical power at specific wavelengths. The light source must match the fiber type and network application:
   • Multimode Fiber: Typically tested using light-emitting diodes (LEDs) operating at 850 nm and 1300 nm. LEDs have a wide spectral width and fill the multiple propagation modes of the fiber, replicating the launch conditions of standard multimode transceivers.
   • Singlemode Fiber: Typically tested using laser diodes operating at 1310 nm and 1550 nm. Lasers provide a narrow spectral width and inject light into a single mode, matching the behavior of singlemode transmitters.
2. Optical Power Meter: Contains a calibrated photodetector (often made of Indium Gallium Arsenide [InGaAs] or Silicon) that measures the absolute level of optical power received. The power meter must be set to the same wavelength as the light source.

An OLTS can consist of two separate handheld units (often called a Light Source and Power Meter, or LSPM) or a pair of integrated, smart test units that communicate bidirectionally to test multiple wavelengths and fibers simultaneously.

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Understanding Wavelengths and Standards

Fiber optic networks operate at different wavelengths, and loss characteristics vary significantly across the spectrum. According to industry standards:

• Multimode Attenuation: Typically ~3.0 dB/km at 850 nm and ~1.0 dB/km at 1300 nm. Loss is higher at 850 nm due to Rayleigh scattering.
• Singlemode Attenuation: Typically ~0.35 dB/km at 1310 nm and ~0.20 dB/km at 1550 nm. The 1550 nm wavelength has the lowest attenuation but is highly sensitive to bending losses (macrobends).

Because loss is wavelength-dependent, standards require that all fiber links be tested at both of their operating wavelengths (e.g., both 850 nm and 1300 nm for multimode) to ensure no anomalies exist at either wavelength.

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Decibels (dB) vs. Decibel-milliwatts (dBm)

To analyze test results, a technician must understand the distinction between decibel-milliwatts (dBm) and decibels (dB).

dBm (Absolute Power)

dBm is an absolute unit of optical power referenced to 1 milliwatt (mW). It is used to measure the exact amount of optical energy present at a specific point in a network.

• Formula: P(dBm) = 10 * log10(P(mW) / 1 mW)
• Key Reference Points:
  • 1 mW = 0 dBm
  • 2 mW = +3 dBm (doubling the power adds 3 dB)
  • 10 mW = +10 dBm
  • 0.1 mW = -10 dBm (reducing power by a factor of 10 subtracts 10 dB)
  • 0.01 mW = -20 dBm
• Because receivers measure very small light levels, received power readings are almost always negative numbers (e.g., -22.5 dBm).

dB (Relative Loss or Gain)

dB is a relative unit of measurement. It does not represent an absolute amount of power but rather the ratio between two power levels (such as input power versus output power). It is used to express loss or gain.

• Formula: Loss (dB) = P_input(dBm) - P_output(dBm)
• Because it is logarithmic:
  • A loss of 3 dB corresponds to a 50% loss of optical power (half the power is lost).
  • A loss of 10 dB corresponds to a 90% loss of optical power (only 10% of the power remains).
  • A loss of 20 dB corresponds to a 99% loss of optical power (only 1% of the power remains).

Worked Example

Suppose a stabilized light source launches -10.0 dBm of power into a fiber link. The optical power meter at the far end of the link measures -14.2 dBm. $\text{Insertion Loss} = P_{\text{source}} - P_{\text{received}} = -10.0\text{ dBm} - (-14.2\text{ dBm}) = 4.2\text{ dB}$ The received power is an absolute value of -14.2 dBm, while the insertion loss of the link is a relative value of 4.2 dB.

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The 1-Cable Reference Method (ANSI/TIA-526-14-B)

To measure the insertion loss of a link accurately, you must first 'zero' the test instruments to isolate the loss of the link under test from the output of the light source and the loss of the launch cables. The ANSI/TIA standards outline three reference methods. Among these, the 1-Cable Reference Method (Method B) is the most common, accurate, and preferred method for certifying cabling systems.

Step-by-Step Procedure

1. Referencing (Zeroing): Connect a single, high-quality test reference cable (TRC 1, the launch cable) directly between the light source and the optical power meter. Set the power meter to read 0.0 dB (or record the absolute power in dBm as the reference baseline).
2. Maintaining the Source Connection: Disconnect the launch cable only from the power meter. Critical Rule: Never disconnect the launch cable from the light source. The coupling between the light source and the connector is highly variable. If you disconnect and reconnect it, the launched power will change, invalidating your reference and causing inaccurate (optimistic or negative) loss readings.
3. Adding the Receive Cable: Connect a second test reference cable (TRC 2, the receive cable) to the power meter.
4. Measuring the Link: Connect the fiber link under test between TRC 1 and TRC 2 using mating adapters. The power meter will now display the total insertion loss of the link in dB.

Why the 1-Cable Method is Preferred

The 1-Cable Reference Method is the only method that includes the loss of the connections at both ends of the link under test (the connection between TRC 1 and the link, and the connection between the link and TRC 2). Because these connections represent the real-world mating of the cable plant to active transceivers, this method provides the most realistic measurement of the link's insertion loss. TIA standards mandate the 1-cable method for certification testing.

Other Referencing Methods

• 2-Cable Reference (Method A): Uses two reference cables joined by an adapter during the referencing step. This method only measures the loss of one end connection of the link under test, making the final loss measurement look artificially low (optimistic) by about 0.3 to 0.5 dB.
• 3-Cable Reference (Method C): Uses three reference cables and two adapters during the referencing step. It excludes the loss of both end connections of the link under test. This method is used when the connector on the cable plant does not match the test equipment ports, requiring patch cords to adapt them.

Test Reference Cables (TRCs)

Standard patch cords should not be used as reference cables. TRCs are built to extremely tight tolerances, with high-precision fiber geometry and ultra-low-loss connectors (maximum connector loss of 0.1 dB for singlemode and 0.2 dB for multimode). Using standard patch cords as reference cables introduces variable losses, leading to inaccurate test results.