N7786B Polarization Synthesizer

Introduction

The Keysight Technologies N7786B contains a high-speed lithium-niobate based polarization controller and a polarization analyzer plus a microcontroller-based driving circuitry.

This unit can operate in various modes:

As a polarization stabilizer, it provides a stable output state of polarization (SOP) even with fluctuations and drifts of the input SOP. The stabilized output signal is guided in a standard single-mode fiber (SMF). The output SOP can be defined in the following ways:

• Set-and-forget: When the front button is pushed, the current SOP is stored and maintained, even if polarization changes occur on the instrument input.
• Defined stokes: The target output SOP can be defined by the user using the Stokes parameters.
• As a synchronous scrambler, the device switches the SOP of the output signal in a random (pseudo) way with a cycling speed of up to 100 KSOPs/s.

SOP switching occurs within a few microseconds. An electrical trigger input can be used to synchronize the scrambler with external events.

 As an SOP switch, the N7786B cycles through a sequence of SOPs with a speed of more than 40 KHz, which corresponds to a cycle time of less than 25 microseconds. The sequence of SOPs can easily be defined by the user using Stokes coefficients.

As a traditional scrambler, the N7786B varies the output SOP in a random way.

Full coverage of the Poincaré Sphere is achieved within a few ms.

As a polarization analyzer, the instrument provides truly high-speed capabilities: More than 500K samples can be taken with a sample rate of up to 1 Megasample per second.

As fast-switching polarization controller for single-sweep wavelength dependent PDL measurements in combination with the N7700A PDL software and an Keysight tunable laser and power meter. See the N7700A literature for details.

All above-mentioned applications of the N7786B are supported by Keysight PC software that comes with this instrument.

Various instrument drivers and connectivity to external applications are provided through a DLL interface. Examples are included.

Applications

• Swept-wavelength PDL measurements with tunable laser and N7700A software
• Transmission system test: Polarization sensitivity analysis on link/transmission quality
• Recirculating loop experiments: Loop-synchronous polarization scrambling
• Interferometry: Polarization stabilization to maximize contrast ratio
• Polarization analysis

The instrument setup is shown in Figure 2. A DSP-based electronics controls the polarization analyzer as well as the polarization controller. The SOP can be stabilized by means of a closed-loop operation of the DSP.

The transmission quality of links is known to depend on the state of polarization (SOP) of the launching signal. The N7786B is well suited to launch the modulated signal with predefined SOPs into the link. This allows probing for a range of SOPs on the Poincaré sphere to get information about particular polarization issues of a link.