State of Polarization (SOP) Testing: Controllers vs. Analyzers

Polarization controllers are used to deliberately change the SOP of an input light source. This is typically achieved using one of two mechanical designs: motorized waveplates or motorized fiber loops.

For highly precise polarization state synthesis, instruments like the Keysight 8169A employ a fixed linear polarizer followed by independently motorized quarter-wave and half-wave plates. Both waveplates offer a full rotation range of 0.0° to 360.0°, enabling exact, repeatable positioning on the Poincaré sphere with an insertion loss variation of less than ±0.03 dB.

Alternatively, controllers can use motorized fiber loops to alter polarization without introducing bulk optics. The Keysight 11896A utilizes a motorized four-loop single-mode fiber design. By adjusting these loops, it covers the entire Poincaré sphere. This loop-based method is highly efficient for scanning, providing eight selectable scan rates for polarization scrambling while maintaining a low insertion loss variation of ±0.002 dB.

Polarization analyzers do not alter the polarization state; instead, they measure and mathematically characterize the light entering the instrument. They are critical for evaluating components like optical isolators, modulators, and single-mode fibers.

A typical analyzer, such as the Keysight 8509B, uses a receiver with a four-diode detection scheme to calculate the Stokes parameters of the signal. This design allows the system to determine the exact SOP and the Degree of Polarization (DOP) across a broad wavelength operating range of 1200 to 1600 nm.

Because analyzers must resolve dynamic optical changes, their measurement speed is a key parameter. For instance, the Keysight 8509B achieves measurement rates greater than 2000 per second with a display update rate of over 15 per second. This allows engineers to view real-time adjustments on a Poincaré sphere with a representation uncertainty of ±1.5° depending on the wavelength band.

Choosing between a controller and an analyzer depends on whether your application requires optical signal generation or optical signal analysis.

Use a polarization controller when you need to sweep through polarization states to measure polarization-dependent loss (PDL), or when scrambling polarization to eliminate polarization sensitivity in a test system. These instruments accept high input powers, such as up to +20 dBm for both the Keysight 8169A and Keysight 11896A.

Use a polarization analyzer when you need to quantify the polarization performance of a device under test (DUT). An analyzer requires high receiver sensitivity, operating down to low power levels—such as the +10 to -55 dBm operating range of the Keysight 8509B. Some analyzers also feature internal optical sources, such as the 1300 nm and 1550 nm Fabry-Perot lasers in the Keysight 8509B, to provide an integrated test signal for components.