OCWR vs OTDR-Difference between OCWR method and OTDR method

This page on OCWR method vs OTDR method describes difference between OCWR method and OTDR method used to measure optical return loss(ORL).

OCWR Method-Optical Continuous Wave Reflectometer

OCWR method

The figure-1 depicts setup used for OCWR method to measure Optical Return Loss(ORL). As shown light source is used to launch single wavelength of light into the optical fiber system. The known power level Po is used and wavelength should be kept as required for communication system as per application.

The directional coupler is used to couple back reflections to the optical power meter. Following two steps are performed during ORL (Optical Return Loss) measurement using OCWR method.

Step-1:Reference optical power measurement is conducted with non-reflective termination plug. The plug is replaced by mandrel wrap or index matching gel.

Step-2: One step-1 is completed, the jumper is closed with DUT (Device Under Test). Care should be undertaken to avoid any unwanted glass to air back reflection. If this is not taken care then it will affect ORL measurement. The ORL is calculated after measuring level of reflected optical power in DUT.

optical return loss (ORL) measurement using OTDR Method-Optical Time Domain Reflectometer

OTDR method

The figure-2 depicts setup used for OTDR method to measure Optical Return Loss(ORL). As shown light pulses are launched into fiber under test. The backscatter information and fresnel reflections are collected. The light received by OTDR method equals to reflected power according to pulsewidth injected.

Optical Return Loss ORL Equation

ORL (Optical Return Loss) equation is mentioned above. ORL is calculated after measuring level of reflected optical power according to pulsewidth.

Comparison between OCWR method and OTDR method

ORL measurement using OTDR method is easier compare to OCWR method as no power output referencing is needed.

OCWR method is more accurate (approx. +/-0.5dB) compare to OTDR method (approx. +/-2dB). OCWR also allows measurement of very short fiber lengths about 1-2 meters. Following table summarizes key comparison between OCWR and OTDR methods. OCWR supports wide dynamic range of upto about 70 dB.

Specifications OCWR OTDR
Full form Optical Continuous Wave Reflectometer Optical Time Domain Reflectometer
Spatial resolution Not supported Supported
typical Accuracy +/-0.5 dB +/- 2dB
Advantages (i.e. merits) Accurate, fast, provides real time results, simple and easy locates reflective events, single ended measurement
Disadvantages (i.e. demerits) reference measurements are needed long acquisition times
Applications Total link ORL and isolated event reflectance measurements during fiber installation as well as commissioning spatial characterization of reflective events and estimation of total ORL during installation, perfect tool for troubleshooting when discrete elements contributing to the ORL must be identified.


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