Determining a polarization-related characteristic of an optical link

Patent metadata

Field	Value
Publication number	US-9829429-B2
Application number	US-201313767851-A
Country	US
Kind code	B2
Filing date	Feb 14, 2013
Priority date	Sep 29, 2005
Publication date	Nov 28, 2017
Grant date	Nov 28, 2017

Abstract

A polarization-related characteristic of an optical path is determined from a predetermined function of the mean-square of a plurality of differences between polarization-analyzed optical power parameters corresponding to pairs of wavelengths mutually spaced about a midpoint wavelength by a small optical frequency difference. At least some of the said differences correspond to wavelength pairs measured under conditions where at least one of midpoint wavelength, input state of polarization (I-SOP) or analyzed state of polarization (A-SOP) of a pair is different.

First claim

The invention claimed is:

1. A method of measuring a polarization-related characteristic of an optical path ( 18 ) wherein light comprising polarized light is propagated, the method comprising the steps of using:
polarization-controller-and-analyzer means ( 14 , 20 A; 14 A, 14 B, 20 ; 14 A, 14 B, 20 A; 14 , 20 ; 14 , 20 C; 14 A, 45 ) connected to the optical path at or adjacent either a proximal end thereof or a distal end thereof or both the proximal end and the distal end to control at least one of state of polarization (I-SOP) of light launched in the optical path and state of polarization (A-SOP) used to analyze light leaving the optical path,
detecting means ( 22 ; 22 A, 22 B; 22 D) to detect the analyzed light and provide corresponding detection signals, and
processing means ( 40 ) to process the detection signals to derive said polarization-related characteristic,
wherein said light leaving the optical path is analyzed to provide transmitted coherent optical power at each wavelength of light in each of at least two groups of wavelengths,
and wherein the lowermost (λ L ) and uppermost (λ U ) said wavelengths in each said group of wavelengths are separated by a first small optical frequency difference;
and wherein each of the said at least two groups comprises a wavelength pair, said pair in each group defining a midpoint wavelength therebetween, and being mutually spaced by a second small optical-frequency difference, the second small optical-frequency difference being equal to or less than the first optical-frequency difference, said second small optical-frequency difference (δν) being the same for corresponding wavelength pairs in different groups, and wherein the I-SOP and A-SOP are substantially constant for each coherent optical power at each said wavelength in each said group, and wherein at least one of the midpoint wavelength, I-SOP and A-SOP is different between the respective said groups, the step of using processing means ( 40 ) to process the detection signals including the steps of:
(i) computing at least one difference between a pair of measured power parameters each corresponding to a respective one of the wavelengths in said wavelength pair for each of the said at least two groups, each said measured power parameter being proportional to transmitted coherent optical power of the said analyzed and subsequently detected light, thereby defining for said at least two groups a set of at least two measured power parameter differences;
(ii) computing a mean-square value of said set of at least two measured power parameter differences; and
(iii) calculating the polarization-related optical path characteristic as a predetermined function of said mean-square value, said predetermined function being dependent upon the said second small optical-frequency difference between the wavelengths corresponding to the said each of at least said two pairs of wavelengths.

2. A method according to claim 1 , further comprising using light source means connected to the optical path at or adjacent the proximal end thereof to launch light comprising polarized light into the optical path, and wherein the said polarization-controller-and-analyzer means comprises input controller means connected to the optical path at or adjacent the proximal end thereof and analyzer means connected to the optical path at or adjacent the distal end thereof.

3. A method according to claim 2 , wherein:
(a) corresponding wavelength pairs in different groups have substantially the same midpoint wavelength,
(b) the second small optical frequency difference (δν) multiplied by differential group delay (DGD) is less than 0.5, and
(c) the said polarization-related optical path characteristic comprises a differential group delay (DGD) at the said midpoint wavelength.

4. A method according to claim 3 , wherein the said measured power parameter is normalized power T(ν), and the limit of said predetermined function as said second small optical-frequency difference (δν) tends to zero is expressed according to the following differential formula:

DGD
⁡

(
v
)

=

α
ds

πδ
⁢

⁢
v

·

〈

⁢

T
2

⁡

(
v
)

〉

SOP

where the constant

α

d
⁢

⁢
s

=

9
2

,

ν is the optical frequency corresponding to the said midpoint wavelength, and ΔT(ν) is the difference in the normalized power obtained for a particular I-SOP and A-SOP couple.

5. A method according to claim 3 , wherein the said measured power parameter is normalized power T(ν), and the mean-square value computing step (ii) further comprises computation of relative variance (σ r 2 (ν)) of normalized powers T(ν) according to the expression:

σ
r
2

⁡

(
v
)

=

(

1

σ
20

)

2

⁡

[

〈

T
2

⁡

(
v
)

〉

SOP

-

〈

T
⁡

Assignees

• Exfo Inc
• Exfo Inc

Inventors

• Cyr Normand
• Chen Hongxin
• Schinn Gregory Walter

Classifications

• G01N21/21 Primary

  Polarisation-affecting properties (G01N21/19 takes precedence) · CPC title

• G01M11/3181

  Reflectometers dealing with polarisation · CPC title

• G01M11/3163 Primary

  by measuring dispersion · CPC title

• G06F17/18

  for evaluating statistical data {, e.g. average values, frequency distributions, probability functions, regression analysis (forecasting specially adapted for a specific administrative, business or logistic context G06Q10/04)} · CPC title

• G01M11/335

  using two or more input wavelengths · CPC title