Motorized Fiber Polarization Controllers

Features

• Convert Between Linear, Circular, and Elliptical Polarization
• Motorized Paddle Rotation with 0.12° Minimum Step Size
• Two- or Three-Paddle Versions Available with 18 mm Outer Loop Diameter
• Driven with Kinesis^® Software Package
• USB Powered (USB A to Micro USB Type B Cable Provided)
• Compatible with Single Mode Ø900 µm Jacketed Fibers
• Compact Footprint

Thorlabs' Motorized Fiber Polarization Controllers are paddle-based polarization controllers that use stress-induced birefringence within a fiber to dynamically control the output polarization state of light. Their compact size and integrated DC servo motors allow these controllers to be easily incorporated into larger, more complex systems. Each individual paddle can be rotated 170° with a minimum step size of 0.12°, providing full coverage of the Poincaré sphere. These controllers, made from Black Acrylonitrile Butadiene Styrene (ABS), are empty and designed to be used with a single mode fiber or fiber patch cable with a Ø900 µm jacket.

The motorized polarization controllers are available in either two- or three-paddle configurations, with each paddle accommodating up to four 18 mm diameter fiber loops. Stress-induced birefringence is created by bending the fiber; this creates two principle axes in the fiber, one perpendicular to the plane of the loop (slow axis) and the other in the plane of the loop (fast axis). As a result, wrapping the fiber around the fiber spools creates independent wave plates that alter the state of polarization, while rotating the paddles produces a change in polarization by adjusting the fast axis of the fiber with respect to the transmitted polarization. Please see the Operations tab for more information on the operating principle, as well as the recommended fiber types and number of loops needed to achieve specific retardation behaviors.

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Paddle numbers are etched into the base plate at two locations and in two orientations.

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A mounted MPC220 controller is loaded with a Ø900 µm jacketed FC/PC patch cable. The connected USB cable is used for power and communication with the Kinesis software.

Fiber Organization and Mounting
To securely hold the fiber in place, clamps are included before and after the paddles. These can be removed for easy fiber loading and unloading by loosening the M2.5 screws with a 2 mm hex key or balldriver. The controller body, as well as the fiber spools, also features guides to aid in fiber organization. These fiber guides along the base plate are visible in the application image shown to the right.

Both the two- and three-paddle motorized polarization controllers include a base plate featuring clearance slots that accept 1/4"-20 or M6 cap screws. This allows the controller to be mounted to either 1" or 25 mm pitch optical tables and breadboards at any orientation. The paddles are numbered at their base in two locations to help identify the paddles, as shown in the image to the right.

Paddle Rotation with Kinesis Software
The motorized polarization controllers are powered via USB, which connects to a control PC or a powered hub. A USB A to Micro USB Type B cable is included with each controller.

Paddle rotation is driven by Thorlabs' user-friendly Kinesis software package. For 'out of the box' operation, paddle movements such as homing, jogging, and absolute positioning can be manually controlled through a graphical user interface (GUI) panel. To control the polarization paddles without user intervention, the Kinesis package also includes a set of programming interfaces, which allow move sequences to be programmed in several development languages. For more information on the Kinesis software or creating custom applications, please see the Kinesis Software and Kinesis Tutorial tabs, respectively.

Fiber Polarization Control Selection Guide
Polarization Controllers				Linear Polarizers
Motorized Paddle Fiber Polarization Controllers	Manual Paddle Fiber Polarization Controllers	In-Line Manual Fiber Polarization Controller	Free-Space FiberBench	In-Line Fiber Polarizers	Polarizing Fiber

Operating Principle
These motorized polarization controllers utilize stress-induced birefringence to alter the polarization state of light traveling through a single mode fiber. By looping the fiber around each spool, two or three independent fractional wave plates (fiber retarders) are created. The amount of birefringence induced in the fiber is a function of the fiber cladding diameter, the spool diameter (fixed), the number of fiber loops per spool, and the wavelength of the light. (NOTE: the desired birefringence is induced by the loop in the fiber, not by the twisting of the fiber paddles). The fast axis of the fiber, a principle axis of birefringence generated by bending the fiber, is in the plane of the spool; rotating the paddles adjusts the orientation of this axis with respect to the transmitted polarization vector. To transform an arbitrary input polarization state into a fixed and defined output polarization state, the fiber should ideally be looped to create a quarter-wave plate, a half-wave plate, and a quarter-wave plate for the three paddle controller or two quarter-wave plates for the two paddle controller. Because a three-paddle configuration decouples the two quarter-wave plates, more polarization states can be achieved compared to a two-paddle configuration. The retardance of each paddle may be estimated from the following equation:

Here, φ is the retardance, a is a constant (0.133 for silica fiber), N is the number of loops, d is the fiber cladding diameter, λ is the wavelength, and D is the loop diameter. While this equation is for bare fiber, the solution for Ø900 µm jacketed fiber will be similar enough that the results for this equation can still be used (i.e., the solution will not vary by a complete loop N for Ø900 µm jacketed fiber).

Three-Paddle Polarization Controllers
A three-paddle polarization controller combines a quarter-wave plate, half-wave plate, and quarter-wave plate in series to transform an arbitrary polarization state into another polarization state. The first quarter-wave plate would transform the input polarization state into a linear polarization state. The half-wave plate would rotate the linear polarization state, and the last quarter-wave plate would transform the linear state into an a fixed and defined output polarization state. Therefore, adjusting each of the three paddles (fiber retarders) in the MPC320 polarization controller allows complete control of the output polarization state over a broad range of wavelengths from 300 to 2100 nm.

Two-Paddle Polarization Controller
The miniature two-paddle polarization controllers use two quarter-wave plates to transform an arbitrary polarization state into another polarization state. In the two-paddle configuration, however, the control of the polarization will be coupled between the two paddles and therefore it may be difficult to achieve a specific polarization state. The design of the MPC220 polarization controller allows complete control of the output polarization state over a broad range of wavelengths from 300 to 2100 nm.

Recommended Number of Loops
The retardation per paddle is a function of loop number and the cladding diameter of the fiber if the loop diameter is fixed. Figures 1 and 2 show the calculated retardation per paddle for Ø80 µm and Ø125 µm clad fiber, respectively. Due to their small size, the MPC220 and MPC320 motorized polarization controllers cannot accomodate more than four loops, each with an 18 mm diameter, per paddle.

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Figure 2: Plot of the retardance per paddle for bare silica fiber with Ø125 µm cladding on an 18 mm loop diameter.

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Figure 1: Plot of the retardance per paddle for bare silica fiber with Ø80 µm cladding
on an 18 mm loop diameter.

Recommendations for the number of loops and fibers for several wavelengths are given in the following table. The number of loops, calculated from the equation above or determined from Figure 2, are the values that best approximate quarter-wave and half-wave retardation:

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Kinesis GUI Screen for the Motorized Fiber Polarization Controllers

Thorlabs Kinesis^® software package can be used to control devices in the Kinesis or APT family, which covers a wide range of motion controllers ranging from small, low-powered, single-channel drivers (such as the K-Cubes™ and T-Cubes™) to high-power, multi-channel, modular 19" rack nanopositioning systems (the APT Rack System).

The Kinesis Software features .NET controls which can be used by 3rd party developers working in the latest C#, Visual Basic, LabVIEW^®, or any .NET compatible languages to create custom applications. Low-level DLL libraries are included for applications not expected to use the .NET framework. A Central Sequence Manager supports integration and synchronization of all Thorlabs motion control hardware.

The software packages allow two methods of usage: graphical user interface (GUI) utilities for direct interaction with and control of the controllers 'out of the box', and a set of programming interfaces that allow custom-integrated positioning and alignment solutions to be easily programmed in the development language of choice.

Software

Kinesis Version 1.14.18

The Kinesis Software Package, which includes a GUI for control of Thorlabs' Kinesis and APT™ system controllers.

Also Available:

• Communications Protocol

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Figure 2: Poincaré sphere showing the polarization rotation from a three paddle polarization controller.

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Figure 1: Forces produced by the fiber controller paddle

Thorlabs Lab Facts: Using Fiber Paddles to Manipulate Polarization

We present laboratory measurements of the influence on the output polarization state from a fiber due to rotation and twist forces from the fiber polarization controller (FPC). This controller utilizes the effects of stress-induced birefringence to create changes in the polarization of light traveling through a fiber under stress. The stress can be caused either through twisting or rotating [1], as shown in Figure 1. It was found that by using the appropriate number of loops on each paddle that the stress-induced birefringence can be adjusted continuously. This then allows for any arbitrary input polarization state to be rotated into any desired output polarization state. We detail the procedures necessary to achieve a desired output polarization, and plot the change in the polarization on a Poincaré sphere to illustrate the steps necessary in reaching a desired polarization state.

For our experiment, we used the S1FC1310 Fabry-Perot Benchtop Laser (1310 nm) as the light source and couple it into a Ø900 µm tight-buffer fiber. The fiber was mounted through a FPC030 Fiber Polarization Controller, and the output was collimated into a free-space beam with a fiber collimator. From here the beam was measured, either directly by a polarimeter or through an analyzer assembly consisting of a λ/4 wave plate, a linear polarizer, and a power meter.

Figure 2 summarizes the measured results for manipulating the polarization of light in a fiber as a function of rotation and twist forces and is shown on the Poincaré sphere. The colored lines represent one of the three paddles of the FPC030 and correspond to the colored numbers of Figure 1. To produce quarter-wave plate behavior, the fiber needed to be looped around a paddle two times, and for half-wave plate behavior it was 3 times. For the results presented in Figure 2, we used the FPC030 FPC in a 2-3-2 loop configuration. As shown in Figure 2, starting at any arbitrary polarization state, it is possible to achieve any desired polarization state through rotating each paddle through a number of iterations. This manipulation of the polarization by the FPC does not produce intrinsic loss nor back reflections; instead stress-induced birefringence is utilized as a mechanism for rotating the polarization of light in fiber. Data is presented for each of the paddles of the FPC and the polarization changes due to these are mapped out on Poincaré spheres. For details on the experimental setup employed and the results obtained, please click here.

[1] R. Ulrich, A. Simon, “Polarization optics of twisted single-mode fibers” Appl. Opt. 18, 2241-2251 (1979).