How can you measure backreflection in optical devices?

1 Optical power meter

One of the simplest methods for measuring backreflection is to use an optical power meter (OPM) that can detect the reflected light from the device under test (DUT). The OPM can be connected to the source port of the DUT, or to a tap port of a circulator or a directional coupler that isolates the reflected light from the incident light. The OPM can display the backreflection in units of power (dBm) or loss (dB). However, this method has some limitations, such as the accuracy and resolution of the OPM, the sensitivity to polarization and wavelength, and the inability to distinguish multiple reflections from different parts of the DUT.

2 Optical time-domain reflectometer

Another method for measuring backreflection is to use an optical time-domain reflectometer (OTDR) that can send a pulse of light into the DUT and measure the time and intensity of the reflections. The OTDR can display the backreflection as a trace that shows the location and magnitude of each reflection along the DUT. The OTDR can also calculate the total backreflection or the backreflection at a specific point of interest. This method has some advantages, such as the ability to identify and localize multiple reflections, the immunity to polarization and wavelength, and the high resolution and dynamic range. However, this method also has some drawbacks, such as the complexity and cost of the OTDR, the dead zone and attenuation effects, and the need for calibration and interpretation of the trace.

3 Optical spectrum analyzer

A third method for measuring backreflection is to use an optical spectrum analyzer (OSA) that can measure the spectrum of the reflected light from the DUT. The OSA can be connected to the same port as the OPM, or to a different port of a circulator or a directional coupler. The OSA can display the backreflection as a spectrum that shows the wavelength and power of each reflection. The OSA can also calculate the backreflection in units of power or loss. This method has some benefits, such as the ability to measure the wavelength dependence and coherence of the reflections, the high accuracy and sensitivity, and the compatibility with tunable sources and filters. However, this method also has some challenges, such as the high cost and complexity of the OSA, the resolution and bandwidth limitations, and the need for proper alignment and stabilization of the source and the DUT.