! Polarization-mode dispersion is defined and characterized, using Poincaré sphere and Jones matrix concepts. Interferometric, wavelength scanning, and Jones matrix eigenanalysis measurement methods are described. Instrumentation, especially the HP 8509B lightwave polarization analyzer, is discussed. (: *(1(5$7,216 2) +,*+?63((' 81'(56($ 7(/(&20081,&$7,21 6<67(06 $1' &$%/( ! 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'(9,&( 7+( 32/$5,=$7,21 $7 7+( '(9,&( 287387 :,// ,1 *(1(5$/ 326,7( 6(&21'?25'(5 ',67257,21 25 +,*+?63((' /21*?+$8/ 75$&( 287 $1 ,55(*8/$5 3$7+ 21 7+( 2,1&$5> 63+(5( 6(( 3$*( 7(/(&20081,&$7,216 $1' +,*+?&+$11(/?&$3$&,7< ! 6<6? $6 7+( 237,&$/ 5$',$1 )5(48(1&< ,6 781(' $6 6+2:1 ,1 7(06 72 5($/,=( 7+(,5 327(17,$/ 0867 %( 81'(56722' $1' ,* 9(5 $ 60$// 5$1*( 2) )5(48(1&< $1< 6(&7,21 2) 7+( &21752//(' ,55(*8/$5 3$7+ &$1 %( $3352;,0$7(' $6 $1 $5& 2) $ &,5&/( 21 7+( 685)$&( 2) 7+( 63+(5( +( &(17(5 2) 68&+ $ &,5&/( 352? 2/$5,=$7,21?02'( ',63(56,21 ,6 $ )81'$0(17$/ 3523(57< 2) -(&7(' 72 7+( 685)$&( 2) 7+( 63+(5( /2&$7(6 $ 35,1&,3$/ 67$7( 2) 6,1*/(?02'( 237,&$/ ),%(5 $1' &20321(176 ,1 :+,&+ 6,*1$/ 32/$5,=$7,21 6(&21' 257+2*21$/ 35,1&,3$/ 67$7( 2) 32/$5,=$? (1(5*< $7 $ *,9(1 :$9(/(1*7+ ,6 5(62/9(' ,172 7:2 257+2*21$/ 7,21 ,6 /2&$7(' ',$0(75,&$//< 23326,7( 21 7+( 63+(5( +( 32/$5,=$7,21 02'(6 2) 6/,*+7/< ',))(5(17 3523$*$7,21 9(/2&,7< 35,1&,3$/ 67$7(6 2) 32/$5,=$7,21 $5( 6,*1,),&$17 %(&$86( 7+(< +( 5(68/7,1* ',))(5(1&( ,1 3523$*$7,21 7,0( %(7:((1 32/$5? ,=$7,21 02'(6 ,6 &$//(' 7+( ',))(5(17,$/ *5283 '(/$< &20021/< 6800$5,=( +2: $1< 287387 67$7( 2) 32/$5,=$7,21 (92/9(6 :,7+ )5(48(1&< 6 $ )81&7,21 2) )5(48(1&< $// 287387 67$7(6 6<0%2/,=(' $6 * 25 6,03/< 1 0267 237,&$/ &20321(176 Hewlett-Packard Company 1995 (%58$5< (:/(77?$&.$5' 2851$/ Between 1941 and 1948, R. Clark Jones published a series of papers describing a new polarization calculus based upon optical fields rather than intensities. This approach, although more removed from direct observation than previous methods, allowed calculation of interference effects and in some cases provided a simpler description of optical physics. A completely polarized optical field can be represented by a two-element complex vector, each element specifying the magnitude and phase of the x and y components of the field at a particular point in space. The effect of transmission through an optical device is modeled by multiplying the input field vector by a complex two-by-two device matrix to obtain an output field vector. The matrix representation of an unknown device can be found by measuring three output Jones vectors in response to three known stimulus polarizations. Calculation of the matrix is simplest when the stimuli are linear polarizations oriented at 0, 45, and 90 degrees (Fig. 1), but any three distinct stimuli may be used. The matrix calculated in this manner is related to the true Jones matrix by a multiplicative complex constant c. The magnitude of this constant can be calculated from intensities measured with the device removed from the optical path, but the phase is relatively difficult to calculate, requiring a stable interferometric measurement. Fortunately, measurements of many characteristics such as PMD do not require determination of this constant. ? ? ? X1 Y2 X2 Y2 X3 Y3 k1 = k3 = X1 Y1 k2 = X3 Y3 M=c k4 = k1k4 k2 k4 1 X2 Y2 k3 – k2 k1 – k3 Fig. 1. Measurement of the Jones matrix requires application of three known states of polarization. Output electric field descriptions and ratios kx are complex quantities. The Jones matrix M is found to within a complex constant c, whose phase represents the absolute propagation delay and is not required for PMD measurements. )&++ &,+ !$+) &%%+!% + +.& ')!%!'# *++* & '&#)!1+!&% )+ & )&++!&% !* +)$!% 0 + !)%+!# )&,' #0 '&#)!1+!&% !*')*!&% -+&) !* ')&#0 + $&*+ !%+,!+!-#0 $%!%,# )')*%++!&% & ,* !+ !* !% !% + *$ )# + )3!$%*!&%# *' * + &!%)2 *' ) !* -+&) &)!!%+* + + %+) & + &!%)2 *' ) % '&!%+* +&.) + ')!%!'# *++ & '&#)!1+!&% &,+ . ! + &,+',+ *++* & '&#)!1+!&% )&++ !% &,%+)#&".!* *%* .!+ !%)*!% + ),)0 .#++3") &,)%# s() () () Principal State of Polarization #+!&%* !' +.% + '&#)!1+!&% !*')*!&% -+&) % + &,+',+ *++ & '&#)!1+!&% &% + &!%)2 *' ) -0 #!% * &.* + '+ & + &,+',+ '&#)!1+!&% * + &'+!# )(,%0 %* &,+',+ '+ !* '')&/!$+ &-) *$## )% & )(,%0 * !),#) ) %)+ 0 )&++!&% &,+ + '&#)!1+!&% !*')*!&% -+&) -+&) '&!%+* !% + !)+!&% & &% ')!%!'# *++ & '&#)!1+!&% % *&% ')!%!'# *++ & '&#)!1+!&% !* #&+ !$+)!##0 &''&*!+ &% + *' ) #%+ & !* + !)%+!# )&,' #0 % + &,+',+ *++ & '&#)!1+!&% !* /')** * + )3!$%*!&%# -3 +&) &$'&* & %&)$#!1 +&"* ')$+)* #&+!% + *++ & '&#)!1+!&% &% + *' ) + )&++!&% &,+ + ')!%!'# *++* & '&#)!1+!&% % .)!++% * )&** ')&,+ )#+!&% * × % + $&*+ %)# * !* ,%+!&% & + &'+!# )(,%0 % *&$ &'+!# &$'&%%+* *, * !*&#+&)* % .-3 ,! $&,#+&)* &)!!%+* !% )0*+#* &) .-,!* + )&, . ! #! + ')&'+* .!+ !)%+ )&,' #0* &) !)%+ '&#)!1+!&%* * + &'+!# )(,%0 +)%*3 $!++ + )&, + * &$'&%%+* !* -)! )$!%* !/ !% &)!%++!&% . !# $! + -)0 *#! +#0 * )*,#+ & )&$+! !*')*!&% !% '&#)!1+!&% $& -!* *, * + * !% . ! !* **%+!##0 !%'%%+ & ) *'!##0 *!$'# +& *)! % +& $*,) % ')+!,3 #) + !)%+!# )&,' #0 !* &%*+%+ &-) +!$ % % ,)+#0 )+)!1 * ." ,%+!&% & )(,%0 , -!* % ,*,# * *+%)* ,* + !) )3 +)!*+!* % /'+ +& )$!% *+# )&$ &% +!$ % #&+!&% +& + %/+ &)% *!%#3$& !)* !- -)0 #&. #-# & #&# !))!%% !% !+!&% +& #&. ')&'+!&% #&** ,+ + +* & *$## #&# !))!%%* #&% + !) % 3 ,$,#+ +& ,* *!%!!%+ + )&, !) $%0 "!#&$+)* #&% !))!%% !* ')&')+0 & !#+)! *)!!% + !)% !% + !%/* & ))+!&% &) !3 )%+ '&#)!1+!&%* &# !))!%% !% !) % ,* 0 -!+!&%* )&$ ')+ !),#) *0$$+)0 & + !) &) &) 0 *0$$+)!# *+)** !% + &) )!&% &.!% +& + $%,+,)!% ')&** %* !% + !) &) +$')3 +,) )!%+* +)*** % % .!+ +$')+,) % .!+ +!$ * + #** % + *,))&,%!% # )#/ #3 !% +& !))!%% #&% + !) #%+ + + -&#-* Hewlett-Packard Company 1995 The Poincaré sphere is a graphical tool in real, three-dimensional space that allows convenient description of polarized signals and of polarization transformations caused by propagation through devices. Any state of polarization can be uniquely represented by a point on or within a unit sphere centered on a rectangular (x,y,z) coordinate system. The coordinates of the point are the three normalized Stokes parameters describing the state of polarization. Partially polarized light can be considered a combination of purely polarized light of intensity IP and unpolarized light of intensity IU. Right Circular –45 Linear Vertical The degree of polarization IP/(IP + IU) corresponding to a point is the distance of that point from the coordinate origin, and can vary from zero at the origin (unpolarized light) to unity at the sphere surface (completely polarized light). Points close together on the sphere represent polarizations that are similar, in the sense that the interferometric contrast between two polarizations is related to the distance between the corresponding two points on the sphere. Orthogonal polarizations with zero interferometric contrast are located diametrically opposite on the sphere. As shown in Fig. 1, linear polarizations are located on the equator. Circular states are located at the poles, with intermediate elliptical states continuously distributed between the equator and the poles. Right-hand and lefthand elliptical states occupy the northern and southern hemispheres, respectively. Because a state of polarization is represented by a point, a continuous evolution of polarization can be represented as a continuous path on the Poincaré sphere. For example, the evolution of polarization for light traveling through a waveplate or birefringent crystal is represented by a circular arc about an axis drawn through the two points representing the eigenmodes of the medium. (Eigenmodes are polarizations that propagate unchanged through the medium.) A path can also record the polarization history of a signal, for example in response to changing strain applied to a birefringent fiber. Horizontal +45 Linear Left Circular The real, three-dimensional space of the Poincaré sphere surface is closely linked to the complex, two-dimensional space of Jones vectors (see page 28). Most physical ideas can be expressed in either context, the mathematical links between the two spaces having previously been established for dealing with angular momentum. The graphical Poincaré description allows for a more intuitive approach to polarization mathematics. Fig 1. Points displayed on the surface of the Poincaré sphere represent the polarized portion of a lightwave. Linear polarizations are located on the equator. Circular states are located at the poles, with intermediate elliptical states continuously distributed between the equator and the poles. Right-hand and left-hand elliptical states occupy the northern and southern hemispheres, respectively. Example states are shown ascending the front of the sphere. (/+ -#& "#, "/#(+ %, -( &,.+% -"- #, .'-#(' ( (-" +*.'2 ' -#& ,( -"- ,--#,-#% )#6 -.+ (&, -" &(,- ))+()+#- /#0 ( (+ -" ,2,-& ,#!'+ ,--#,-#% &(% 0"' ,/+% %('! #+ ,-#(', + ('6 -'- -" 1)- /%. 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With the advent of the lightwave polarimeter, engineers in the fields of high-speed telecommunications, cable TV distribution, optical sensing, optical recording, and materials science can characterize polarization phenomena with the ease and graphical simplicity of the common oscilloscope. Supplemented by an optical source, a polarization state generator, and comprehensive measurement software, the polarimeter becomes a polarization analyzer, producing comprehensive measurements of both optical signals and two-port optical devices. The HP 8509B lightwave polarization analyzer consists of an optical unit and a 66-MHz HP Vectra PC. The main display window, shown in Fig. 1, conveniently displays the polarization parameters of an optical signal and provides access to commonly used controls. The Measurements menu provides access to a variety of integrated measurement solutions addressing polarization-mode dispersion (PMD), polarization dependent loss, the Jones matrix, and optimization of optical launch into polarization maintaining fiber. The Display menu allows customization of the display window and the System menu enables the user to reconfigure system operating parameters, optimize performance at a particular wavelength, and automatically check the functional integrity of the instrument. The heart of the HP 8509A/B is a high-speed polarimeter (see Fig. 4a in the accompanying article). A passive optical assembly (see cover) divides the optical signal into four beams and passes each beam through polarization filters to photodiode detectors. Autoranging amplifiers and 16-bit ADCs complete the circuitry. A series of calibration coefficients are determined at manufacture and stored in UV-PROM. The instrument interpolates among these coefficients to provide operation from 1200 to 1600 nm. Parallel filtering and detection combined with high-speed conversion and computation result in a measurement rate of 3000 polarization states per second. A second optical assembly inserts three polarizing filters in the optical source path to allow measurement of the Jones matrix. The Jones matrix eigenanalysis PMD measurement method is based upon Jones matrixes (see page 28) measured at a series of wavelengths. Polarization dependent loss is also derived from the Jones matrix. In addition, the user can use external polarizers to define a physical reference frame, analytically removing the birefringence and polarization dependent loss of components between the polarizer and the polarimeter receiver. Once defined, the reference frame allows the measurement of absolute polarization state at a point far from the instrument itself. Fig. 1. Main measurement window of the HP 8509B lightwave polarization analyzer. Displays of average power and degree of polarization (DOP), along with elliptical and Poincaré sphere displays, fully characterize the polarization state of a lightwave. Shown on the sphere are the loci of output polarization states of a polarization maintaining fiber as the fiber is gently stretched. Red traces are on the front of the sphere, blue on the back. Different circles correspond to different states at the input of the fiber. 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