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In-Line Fiber Optic Polarization Controllers
Polarization Controller for Ø250 µm Bare Fiber
CPC900 with Ø900 µm Tight-Buffer Fiber Mounted on an Optical Table
Installation and Operation of the Fiber Polarization Controllers
We offer compact, in-line polarization controllers for Ø250 µm bare fiber or Ø900 µm tight-buffer fiber. Each device consists of a rotatable fiber squeezer and two fiber holding clamps. They create stress-induced birefringence within SM fiber by mechanically compressing the fiber. This acts like a variable, rotatable wave plate. Both the angle and retardance of the wave plate can be continuously, independently adjusted, which allows any arbitrary input polarization state to be converted to any desired output polarization state. See the Operation tab for details.
The all-fiber design produces low intrinsic loss and back reflections, making these controllers a good alternative to traditional free-space polarization controllers, which consist of two quarter-wave plates and one half-wave plate. Fiber can be dropped into the devices without disconnecting either end from a setup, and the units can be operated without the use of any tools. The polarization controllers have a compact, 0.98" x 2.95" (24.9 mm x 74.9 mm) footprint with four 1/4" (M6) clearance slots for securing to an optical table with imperial or metric hole spacing. Please note that these controllers are not intended for use with loose-tube furcation tubing.
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Figure 1: Features of the In-Line Fiber Polarization Controller
Figure 2: f and s correspond to the fast and slow axes, respectively, of the fiber wave plate. They define a coordinate system used in the calculations below. (a) The squeezing force on the fiber creates stress-induced birefringence, creating a wave plate. (b) The fiber can be twisted to rotate the fast and slow axes of the wave plate.
Recommended Adjustment Procedure
Each Fiber Polarization Controller consists of two fiber holding clamps with a rotatable fiber squeezer mounted between them. The procedure below details how to convert an unknown, arbitrary elliptical input polarization state (common in single mode fiber) to a linear output polarization state, to be input into a polarization-sensitive optical device.
To insert the fiber into a polarization controller, open the two fiber holding clamps by unscrewing the locking screws and raising the clamp arms. Rotate the center section to align the slots along the top of the device. Close the fiber clamps and tighten the screws to secure the fiber in place. Rotate the center section until the thumbscrew (pressure adjustment knob) is upright before tightening the thumbscrew.
Tighten the pressure adjustment knob to apply pressure to the center portion of the fiber. If this causes a significant increase in the monitored optical power, then continue to increase pressure until the optical power starts to decrease. If applying pressure initially causes the monitored optical power to decrease or remain the same, loosen the pressure adjustment knob and rotate the center section in order to compress the fiber in a different direction. For increased control over the amount of pressure applied, the pressure adjustment knob includes a slot for use with the included 5/64" (2.0 mm) hex key.
Rotate the fiber squeezer while maintaining the pressure to fine tune the output polarization. Adjust the pressure and orientation of the rotatable fiber squeezer iteratively until a maximum optical power is obtained. This indicates that the desired polarization is achieved.
Fiber Polarization Control
The pressure applied by the fiber squeezer produces a linear birefringence in a short section of the fiber, which creates a fiber wave plate. The amount of birefringence per unit length, δ, is proportional to the applied pressure and is given by
where F is the applied force in newtons, λ is the wavelength of light in meters, and d is the diameter of the fiber in meters.1, 2
The fiber's core acts as a birefringent wave plate with a slow axis that is defined by the direction of the applied pressure, as shown in Figure 2a. By changing the applied pressure, the retardation of the fiber wave plate can be continuously varied between 0 and 2π.
Rotating the fiber squeezer while applying pressure causes the birefringent portion of the fiber to rotate. This also causes the fiber at the left and right sides of the birefringent section to twist. This twisted fiber rotates the incident polarization in the direction of the twist by an angle given by
where θ is the physical rotation angle shown in Figure 2b and η is a coefficient of twist-induced optical activity. For single mode fibers, η is on the order of 0.083, 4, 5. For a physical rotation of θ degrees, the net change of the incident angle between the slow axis of the fiber wave plate and the input polarization, in degrees, is
For coarse angular adjustments, the fiber squeezer should be rotated without twisting the fiber. This can be accomplished by releasing the pressure from the fiber squeezer before rotating it. Once the desired rotation is achieved, pressure can be reapplied by the fiber squeezer. Thus, for a physical rotation of θ degrees, the net change of the incident angle between the slow axis of the fiber wave plate and the input polarization is also θ degrees.
We recommend this procedure for the coarse adjustment of the output polarization state. When the output polarization is close to that desired, the fiber squeezer can be rotated slightly while applying pressure in order to fine tune the output polarization angle.
Achieving the Desired Output Polarization State
The rotatable fiber squeezer allows the optical fiber to act as a wave plate of variable retardation and rotatable birefringent axes. This is equivalent to a Soleil-Babinet compensator6. Using the slow and fast axes of fiber wave plate as a coordinate system, as shown in Figure 2, the Jones matrix describing the birefringence of the fiber wave plate can be written as
where is the phase retardation of the fiber. In this expression, l is the length of the birefringent fiber and Δn is the index of refraction difference between the slow and fast axes. In the same coordinate system, the Jones vector of an arbitrary input polarization is
where Es and Ef are the amplitudes of the light field projected on the slow and fast axes, respectively, φ is the phase retardation between Es and Ef,
After propagation through the squeezed fiber, the Jones vector of the output polarization state is
where . The birefringent axis of the fiber can be rotated, and thus α can be continuously varied from 0 to π/2. Similarly, the phase retardation, Γ, can be continuously varied from 0 to 2π by changing the pressure on the fiber. Thus, χ can take any value on the complex plane Re(χ) vs. Im(χ). Because each point of the complex plane is associated with a polarization state7, the rotatable fiber squeezer is capable of generating any output polarization from any arbitrary input polarization.
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Figure 2: Poincaré sphere showing the polarization rotation from the polarization controller.
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Figure 1: Forces produced by the polarization controller.
Thorlabs Lab Fact: Using the In-Line Fiber Optic Polarization Controller
We present laboratory measurements of the influence on the output polarization state from a fiber due to rotational and compression forces from the former generation PLC-900 polarization controller. The CPC900 performs in the same way as the PLC-900. The CPC250 performs similarly as well, but it accepts Ø250 µm bare fiber. The 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 compression  or rotation , as shown in Figure 1. It was found that the stress-induced birefringence can be adjusted continuously, thus allowing 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 is mounted through the PLC-900, 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 rotational and compression forces and is shown on the Poincaré sphere. The blue lines represent compression and the red lines represent rotation of the PLC-900 (see Fig. 1); the numbers indicate the step. As shown in Fig. 2, starting at any arbitrary polarization state, it is possible to achieve any desired polarization state through a series of rotations and compressions. This manipulation of the polarization by the PLC-900 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 both compression and rotation forces, 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.
 A.M. Smith, “Single-mode fibre pressure sensitivity,” Electron Lett. 16, 773-774 (1980).
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