SOP Module in
Capture Cage
(US patent 6,061,190),
2mm aperture

The accuracy of the FB-PAM is ensured by the tight manufacturing tolerances of the SOP Module. It is a precision machined 45¢ª-45¢ª-90¢ª triangle, the simplest of geometric shapes. Measurements are made with the Module in each of three positions, corresponding to the three faces of the triangle. The design eliminates human error.
An added safety feature of the Module Capture Cage is that it does not allow the SOP Module to tilt during installation/removal, thus eliminating any chance of stray reflections from opticalsurfaces.
Finally, all optical surfaces are deeply recessed in the Module, thus preventing inadvertent fingerprints on optical apertures.


 

Operation

In order to analyze the SOP of light, the FB-PAM requires a collimated beam. Normally, the light-under-test is from an optical fiber. This fiber is connected to the FiberPort Collimator. From this, a collimated beam transmits through the optical components in the SOP Module and directly to a detector or power meter.
The FB-PAM can be bolted directly to the optical table, or it can be post-mounted to simplify alignment. Any detector or power meter can be used to measure the transmitted intensity.  

FB-PAM, FiberBench
Polarization Analyzer

 
SOP Module is manually set in 4 positions.

A high strength magnet is imbedded in the Cage Base, and a magnetic disc is imbedded in each of the three facets of the SOP Module. When the Module is set in any of its three positions, the Cage Base magnet grabs against the corresponding magnetic disc in the SOP Module. This force of attraction, in combination with tight-tolerance machining, assures absolutely repeatable positioning, time-after-time, as the Module is set in sequence in the Cage.

The Sequence of Measurements is as follows.

Measurement #1 : the SOP Module is reset in the Cage so that the engraved '1' is at the top position, facing the beam. In this position, the horizontal, linearly polarized portion of the light-under-test is measured. The value on the power meter is entered into the appropriate box in the software.

Measurement #2 : the SOP Module

is reset in the Cage with the engraved '2' at the top position. The vertical, linearly polarized portion of the light-under-test is measured. The value is entered.

SOP Module sets into Module
Capture Cage, in
Positions 1 thru 3. 

Position 4 is on the reverse
side of Module,
behind position 3. 

Measurement #3 : the SOP Module

is reset with the engraved '3' at the top position. The 45¢ª,linearly polarized portion of the light-under-test is measured. The value on the power meter is entered.

Measurement #4 : during the previous

three measurements, the engraved #4 has faced away from the input beam. Now, it is necessary to turn the SOP Module "backwards" so that the #4 faces forward, and is at the top. In this position, the right-circularly polarized portion of the light-under-test is measured. The value is entered.

Custom Polarization Analyzer
with 100 meter-phase delay line. 

The following quantities are calculated and displayed in the software :

. Stokes Parameters
. Jones Vector
. Degree of Polarization
. Degree of Linear Polarization
. Linear Polarization Extinction
. Azimuth Angle
. Degree of Circular Polarization
. Ellipticity, Helicity (handedness)

Plots of the Degree of Polarization, Stokes Parameters, and Polarization Ellipse are generated, and can be printed or exported to otherapplications.

In addition, the ExcelTM platform allows the user to customize the program and to add calculations, plots, and graphs.
Sample formula(calculates degree of polarization). Formulas can be added or modified to suit specific needs or applications.

Applications

In addition to measuring the SOP of light-under-test, the FB-PAM can also characterize fiber-pigtailed devices and bulk optics. For example, the FB-PAM can answer the following questions:

. What is the SOP of a signal (free-beam or fiber-optic)?
. What is the Polarization-Dependent Loss/Gain of a fiber device?
. What is the extinction ratio after propagating through PM or PZ fiber?
. What is the retardance of a retarder?
. Has a signal been depolarized after propagating through a long single mode fiber? Through a multi-mode fiber?
. Is the SOP changed after propagating through short lengths of single-mode fiber?
. What are the effects of multilayer coatings on the SOP?
. What are the polarizing effects of a beamsplitter?
. Does a beamsplitter change the phase of the polarization?
. How effective is a depolarizer at reducing degree of polarization?

Theory

The State of Polarization (SOP) of light describes the orientation and oscillation of its electric field. It is uniquely specified by four quantities known as the Stokes Parameters. These Parameters, designated as S0,S1,S2 and S3, are defined as:

Where:

. Ix¢ª refers to the intensity of the light that transmits through a Linear Polarizer oriented at x¢ª.
. IRCP and ILCP are the intensities transmitted through a right-hand and left-hand circular polarizer, respectively.
. S0 is the total intensity of the light.
. S1 represents the dominance of horizontally polarized light vs. vertically-polarized light.
. S2 represents the dominance of 45¢ªpolarized light vs. 135¢ª polarized light.
. S3 represents the dominance of right-circular polarization vs. left-circular polarization.

Although six intensity values are required to define the Stokes Parameters, a fundamental relationship allows the four measurements:
I0¢ª+ I90¢ª= I45¢ª+ I135¢ª= IRCP + ILCP.

When the Stokes Parameters are known, the SOP can be completely described. It is a remarkable result that the electric field can be determined solely by these intensity values.