OCWR vs OTDR: Understanding Optical Return Loss Measurement

This article clarifies the differences between the Optical Continuous Wave Reflectometer (OCWR) and Optical Time Domain Reflectometer (OTDR) methods, both commonly used to measure Optical Return Loss (ORL).

OCWR Method: Optical Continuous Wave Reflectometer

Figure 1: Setup for OCWR method to measure Optical Return Loss (ORL)

As shown in Figure 1, the OCWR method involves launching a single wavelength of light into the optical fiber system using a light source. The power level ($P_o$) is known and should match the operational wavelength of the communication system. A directional coupler directs back reflections to an optical power meter.

The ORL measurement using OCWR involves two key steps:

Step 1: A reference optical power measurement is taken using a non-reflective termination plug. This plug is then replaced with a mandrel wrap or index matching gel to minimize reflections.

Step 2: After completing Step 1, the jumper is connected to the Device Under Test (DUT). Care must be taken to prevent unwanted glass-to-air back reflections, as these can significantly impact the accuracy of the ORL measurement.

The ORL is calculated based on the measured level of reflected optical power from the DUT.

OTDR Method: Optical Time Domain Reflectometer

Figure 2: Setup for OTDR method to measure Optical Return Loss (ORL)

Figure 2 illustrates the OTDR method, where light pulses are launched into the fiber under test. The instrument then collects backscattered information and Fresnel reflections. The light received by the OTDR corresponds to the reflected power based on the pulse width injected.

Optical Return Loss (ORL) Equation

The ORL is calculated by measuring the level of reflected optical power in relation to the pulse width.

Comparison between OCWR and OTDR Methods

ORL measurement using the OTDR method is generally considered easier than the OCWR method because it eliminates the need for power output referencing. However, OCWR is typically more accurate (approximately +/-0.5dB) compared to OTDR (approximately +/-2dB). OCWR also allows for the measurement of very short fiber lengths, on the order of 1-2 meters.

The following table summarizes the key comparisons between OCWR and OTDR methods:

OCWR supports a wide dynamic range of up to approximately 70 dB.