polarization controllers

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Phoenix Photonics 2004 TB_pol_controllers_0204.doc

Technical Brief #6

Polarization Controllers

What is a variable waveplate?........................................................................... 2

Poincar sphere representation of polarization .............................................. 2

Principle of operation ........................................................................................ 3Variable waveplates ........................................................................................... 4

Polarization state scanner and controller ........................................................ 6

Polarization switch............................................................................................. 7

Temperature dependence.................................................................................. 8

Wavelength dependence ................................................................................... 8

Which device to use?......................................................................................... 9

This note is one of a series of technical briefs developed from customer FAQs and intended toanswer the majority of questions concerning the operation of Phoenix products. They are targetedat engineers to assist in incorporating Phoenix products into designs. Any detailed technicalquestions should be forwarded to Phoenix support.


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Phoenix Photonics Polarization controllers Technical Briefing

Phoenix Photonics 2004- TB_pol_controllers_0204.doc

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What is a variable waveplate?

The basis of the range of polarization control components is the fiber waveplate.Fabricated from a length of polarization maintaining fiber the input state of polarization ismodified depending on the birefringence and length of fiber.

A waveplate creates a differential phase shift between the two orthogonal linearpolarization modes. Changing the birefringence of the waveplate enables a variabledifferential phase shift to be created from 0 to 3600.

Poincar sphere representation of polarization

The state of polarization (SOP) of an electromagnetic wave can be described by theamplitude and phase relationship between two orthogonal linear vectors at right angles

to the propagation direction (a). The general state of polarization is elliptical (c) whichevolves along the propagation axis in a helical (b) fashion, the handedness determinedby whether there is a phase lead or lag between the two resolved axes.

The SOP is represented graphically by the Poincar sphere (diagram below) on thesurface of which any possible SOP can be plotted. The equator of the sphererepresents linear polarization states, the vertical (V) and the horizontal (H) correspondingto the two defined orthogonal linear polarization axes. The poles of the sphere representright and left circular polarization. In the co-ordinate system shown in the diagram, thesurface of the northern hemisphere represents any left-handed elliptical state and thesurface of the southern hemisphere represents any right handed elliptical state.


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Principle of operation

The principle of operation of the range of Phoenix polarization control devices is to varythe birefringence of a length of polarization maintaining optical fiber. The two axes of thefiber resolve the incident SOP into two orthogonal components, varying the birefringencealters the phase difference between the two components as they propagate along thefiber which when recombined produces another SOP. In terms of Poincar sphererepresentation the devices move the SOP from one point on the surface of the sphere toanother point along a route defined by the particular device type. This technique formsthe basis of a range of devices which enable the definition of any output SOP from anyinput SOP.


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Variable waveplates

Option 1 single mode fiber in and single mode fiber out

The SOP input to the waveplate from the single mode fiber is arbitrary, thewaveplate will provide a complete cycle of the Poincar sphere.

Condition 1 Variation of input fiber SOP

With no birefringence induced in the outputsingle mode fiber lead the variable

waveplate will describe a circle around thePoincar sphere which will pass through twolinear polarization states. The position of thecircle described is defined by the input SOPand can vary from a fixed linear statecorresponding to one of the PM fiber axisthrough the great circle corresponding to alinear launch at 45 deg to the PM fiber axesto a fixed linear state corresponding to thesecond axis. Plots on the sphere showsome examples of the circles described as

the input SOP is varied and the generalizedelliptical states from four points around the sphere are shown below.


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Polarization state scanner and controller

The SOP scanner/controller has three cascaded variable waveplates and canconvert any arbitrary SOP to any other SOP.

Polarization control

The unit has three currentcontrols and adjusting thecurrent to each of these allowsaccess to any position on thesphere surface from any otherposition, i.e. conversion of anyinput SOP to any output SOP.

The device can be used in SOPcontrol loop to maintain a fixedpolarization state.

Polarization scanning

Applying a time varying current tothe three sections providescontinuous SOP scanning over thefull surface of the Poincar sphere.

In scanning mode the device canbe used for component polarizationparameter measurement, such asPDL or PDR.


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Polarization switch

Varying the control current to thepolarization switch provides a cyclearound the great circle of the Poincarsphere, similar to the PM waveplate.However, in this case the linear statesare aligned to the axes of the outputfiber tail. The cycle around the spherecuts the equator through the verticaland horizontal linear states (relative tothe input PM fiber axes. Therefore theinput linear state on slow axis can beswitched between the fast and slowaxis at the device output, by switchingbetween two drive currents. An option isavailable with a fiber polarizer at the

input to ensure a high extinction ratio for the input polarization state.

Change of linear output state, i.e. switching between the two axes, can beachieved by switching the drive current.

Current Ivapplied to switch

PM fiber PM fiber


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Current IHapplied to switch

Temperature dependence

The waveplates are thermally controlled devices and the change in birefringenceinduced is ambient temperature dependent. The packaging has been designed toreduce response time to external thermal fluctuations, giving a good stabilitywithin the package dimension constraints. For situations in which the stability ofthe waveplate polarization transfer function is critical, for example in the switch,the device cases should be thermally controlled.

The units have been designed for OEM use, easy incorporation into PCB and flatpacked for thermal control with for example a Peltier.

Wavelength dependence

All of the waveplates will operate within the specified wavelength range. Thebirefringence and consequently retardation of the waveplate is wavelengthdependent giving a variation in polarization state transfer function with change inwavelength.

PM fiber PM fiber


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Which device to use?

Requirement DeviceSingle mode fiber systems

SOP scanning around asingle plane of thePoincar sphere

Variable waveplate,single mode fiber inputand output

Conversion of any SOPto any other SOP

Polarization scanner

Scanning of all possiblepolarization states


Control of output SOP forslowly varying input SOP



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Requirement DevicePolarization maintaining fiber systemsConversion of on-axislinear state to orthogonallinear state

Polarization switch

Change input linear stateto either circular stateand two linear states @45deg to fiber axes

Variable waveplate,polarization maintainingfiber input and output.

Change input linear stateto either circular stateand two linear statesaligned to fiber axes

Polarization switch

Scanning input linearpolarization state aroundthe great circle of thePoincar sphere

Variable waveplate,polarization maintainingfiber input and output.

Polarization switch

polarization controllers

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