Method and apparatus for measuring polarization mode dispersion
View Patent ↗Apparatus for measuring polarization mode dispersion (PMD) of a device, e.g. a waveguide, comprises a broadband light source ( 10,12 ) for passing polarized broadband light through the device ( 14 ), an interferometer ( 20 ) for dividing and recombining light that has passed through the device to form interferograms, a polarization separator ( 30 ) for receiving the light from the interferometer and separating such received light along first and second orthogonal Feb. 25, 2003 Feb. 25, 2003 polarization states, detectors ( 32 x ,32 y ) for converting the first and second orthogonal polarization states, respectively, into corresponding first and second electrical signals (P x (τ),P y (τ)), and a processor ( 36 ) for computing the modulus of the difference and such, respectively, of the first and second electrical signals to produce a cross-correlation envelope (E C (τ)) and an auto-correlation envelope (E c (τ)), and determining the polarization mode dispersion according to the expression PMD=where and τ is the delay difference between the paths of the interferometer.
1. Apparatus for measuring polarization mode dispersion (PMD) of an optical waveguide, comprising:
(i) broadband light source means ( 10 , 12 ) for applying polarized broadband light to one end of the waveguide ( 14 ),
(ii) an interferometer ( 20 ) having an input port ( 18 ) for receiving said light from the waveguide ( 14 ), an output port ( 28 ), means ( 22 ) for dividing the light into first and second components ( 24 A, 24 B), first and second paths for conveying the first and second components, respectively, to said output port ( 28 ) for recombination, and means ( 26 B) for varying the length of one of the first and second paths relative to the other to cause interference between the components upon recombination,
(iii) a polarization separator ( 30 ) for receiving the recombined light from the output port and separating said recombined light into first and second interferograms having orthogonal polarization states,
(iv) detection means ( 32 X , 32 Y ) for converting the first and second interferograms, respectively, into corresponding first and second electrical interferogram signals (P X (τ),P Y (τ)), and
(v) means ( 36 ) for processing the first and second electrical interferogram signals to produce a cross-correlation envelope (E C (τ)) and an auto-correlation envelope (E A (τ)), and determining from the cross-correlation envelope and the auto-correlation envelope the polarization mode dispersion (PMD) of the waveguide.
2. Apparatus according to claim 1 , wherein the processing means ( 36 ) is operable to derive the cross-correlation envelope (E C (τ)) as the modulus of the difference between the first and second electrical interferogram signals and the auto-correlation envelope E A (τ)) as the modulus of the sum of the first and second electrical interferogram signals.
3. Apparatus according to claim 2 , wherein the processing means ( 36 ) is operable to compute polarization mode dispersion (PMD) from the cross-correlation envelope (P C (τ)) and the auto-correlation envelope E A (τ)) according to the expression
PMD
=
3
2
(
σ
2
-
σ
0
2
)
where
σ
2
=
∫
τ
2
E
C
2
(
τ
)
ⅆ
τ
∫
E
C
2
(
τ
)
ⅆ
τ
and
σ
0
2
=
∫
τ
2
E
A
2
(
τ
)
ⅆ
τ
∫
E
A
2
(
τ
)
ⅆ
τ
and τ is the delay between the first and second paths of the interferometer.
4. A method of measuring polarization mode dispersion (PMD) of an optical waveguide comprising the steps of:
(i) passing polarized broadband light through the waveguide;
(ii) using an interferometer ( 20 ), dividing and recombining the light leaving the waveguide to produce interferograms;
(iii) separating the recombined light into first and second interferograms having orthogonal polarization states,
(iv) converting the first and second interferograms, respectively, into corresponding first and second electrical interferogram signals (P X (τ),P Y (τ)), and
(v) processing the first and second electrical interferogram signals to produce a cross-correlation envelope (E C (τ)) and an auto-correlation envelope (E A (τ) and determining from the cross-correlation envelope and the auto-correlation envelope the polarization mode dispersion (PMD) of the waveguide, and
(vi) providing a signal representing the measured PMD of the waveguide.
5. A method according to claim 4 , wherein the cross-correlation (E C (τ)) is derived as the modulus of the difference between the first and second electrical interferogram signals and the auto-correlation envelope E A (τ)) is derived as the modulus of the sum of the first and second electrical interferogram signals.
6. A method according to claim 5 , wherein the polarization mode dispersion (PMD) is computed from the cross-correlation envelope (E C (−)) and the auto-correlation envelope (E A (τ)) according to the expression
PMD
=
3
2
(
σ
2
-
σ
0
2
)
where
σ
2
=
∫
τ
2
E
C
2
(
τ
)
ⅆ
τ
∫
E
C
2
(
τ
)
ⅆ
τ
and
σ
0
2
=
∫
τ
2
E
A
2
(
τ
)
ⅆ
τ
∫
E
A
2
(
τ
)
ⅆ
τ
and τ is the delay between the first and second paths of the interferometer.