# US 8,274,647 B2 · Method of classifying a graded-index multimode optical fiber

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- Number: US 8,274,647 B2
- Document ID: US8274647B2
- Status: Granted
- Kind: B2
- Application number: 12/683,775
- Filed: 2010-01-07
- Published: 2010-07-08
- Granted: 2012-09-25
- Earliest claimed: 2009-01-08
- Assignees: Draka Comteq B.V.
- Inventors: Asghar Gholami; Denis Molin; Pierre Sillard; Yves Lumineau
- Web page: https://patentgrep.com/patent/US8274647B2

## CPC

- G02B 6/0288: {Multimode fibre, e.g. graded index core for compensating modal dispersion}
- G01M 11/338: {by measuring dispersion other than PMD, e.g. chromatic dispersion}
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G01M: TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR

## Abstract

A method of classifying a graded-index multimode optical fiber includes taking a series of individual measurements at a single wavelength, and using the measurements to characterize the departure of the multimode optical fiber's actual index profile from the corresponding nominal index profile. The measurements, coupled with intermodal dispersion or EMB measurement, may be used to predict the approximate transmission properties of the optical fiber at wavelengths other than the measurement wavelength.

## Claims

### Claim 1

A method of classifying a graded-index multimode optical fiber, comprising:
(i) injecting into the core of a multimode optical fiber a light pulse having a predefined wavelength, wherein the light pulse is injected at a radial offset from the center of the optical fiber's inlet face;
(ii) detecting the light pulse at the optical fiber's outlet face to determine the time characteristic of the light pulse after propagation through the optical fiber from the optical fiber's inlet face to the optical fiber's outlet face; and
(iii) repeating steps (i) and (ii) for a plurality of radial-offset values; and
(iv) determining the radial offset bandwidth (ROB) as a function of the radial offset.

### Claim 2 (depends on 1)

A method according to claim 1, wherein each light pulse is injected into the optical fiber with a propagation direction that is substantially parallel to the optical fiber's central axis and perpendicular to the optical fiber's inlet face.

### Claim 3 (depends on 1)

A method according to claim 1, wherein each light pulse is spatially localized on the optical fiber's inlet face.

### Claim 4 (depends on 3)

A method according to claim 3, wherein:
a spatial extent of each light pulse is substantially equivalent to the extent of a single spatial mode of a single-mode optical fiber at the predefined wavelength; and/or
the radial-offset values vary from the center of the multimode optical fiber's inlet face over a range of at least 0 to a, where a is the core radius of the multimode optical fiber.

### Claim 5 (depends on 1)

A method according to claim 1, wherein steps (i) and (ii) are implemented in compliance with the FOTP-220 standard.

### Claim 6 (depends on 1)

A method according to claim 1, wherein the predefined wavelength of the light pulse is 850 nanometers ±10 nanometers.

### Claim 7 (depends on 1)

A method according to claim 1, wherein the step of determining the radial offset bandwidth comprises determining the optical fiber's transfer function.

### Claim 8 (depends on 1)

A method according to claim 1, wherein the step of determining the radial offset bandwidth does not include determining the optical fiber's entire transfer function.

### Claim 9 (depends on 1)

A method according to claim 1, wherein the optical fiber possesses an index profile given by the relationship:
n
⁡
(
r
)
=
{
n
0
·
1
-
2
⁢
Δ
⁢
⁢
(
r
a
)
α
r
≤
a
n
0
·
1
-
2
⁢
Δ
r
≥
a
wherein:
n0 is the refractive index on the optical axis of the optical fiber;
r is the distance from the optical axis of the optical fiber;
a is the radius of the core of the optical fiber;
Δ is a non-dimensional parameter indicative of an index difference between the core and the cladding of the optical fiber; and
α is a non-dimensional parameter, indicative of the shape of the index profile.

### Claim 10 (depends on 9)

A method according to claim 9, wherein a value of the parameter α is between about 2.0 and 2.1.

### Claim 11 (depends on 9)

A method according to claim 9, wherein a value of the parameter α is between about 2.04 and 2.06.

### Claim 12 (depends on 1)

A method according to claim 1, comprising determining an effective modal bandwidth at the predefined wavelength.

### Claim 13 (depends on 12)

A method according to claim 12, comprising:
verifying whether the effective modal bandwidth is within a predetermined range of values; and
verifying whether the radial offset bandwidth as a function of the radial offset determined in step (iv) is greater than a predetermined threshold for all radial-offset values between about 0 and R1, where R1 is less than or equal to the core radius of the optical fiber.

### Claim 14 (depends on 13)

A method according to claim 13, wherein:
steps (i) and (ii) are implemented in compliance with the FOTP-220 standard;
the predefined wavelength is 850 nanometers ±10 nanometers;
the optical fiber possesses an index profile given by the relationship:
n
⁡
(
r
)
=
{
n
0
·
1
-
2
⁢
Δ
⁡
(
r
a
)
α
r
≤
a
n
0
·
1
-
2
⁢
Δ
r
≥
a
wherein:
n0 is the refractive index on the optical axis of the optical fiber;
r is the distance from the optical axis of the optical fiber;
a is the radius of the core of the optical fiber;
Δ is a non-dimensional parameter indicative of an index difference between the core and the cladding of the optical fiber; and
α is a non-dimensional parameter, indicative of the shape of the index profile, wherein α is between about 2.04 and 2.06; and
the effective modal bandwidth is a calculated effective modal bandwidth determined in compliance with the FOTP-220 standard.

### Claim 15 (depends on 13)

A method according to claim 13, comprising the step of classifying the optical fiber as satisfactory if the optical fiber meets the effective-bandwidth verification and the radial-offset bandwidth verification.

### Claim 16 (depends on 15)

A method according to claim 15, comprising the step of selecting the optical fiber classified as satisfactory.

### Claim 17 (depends on 15)

A method according to claim 15, comprising the step of deploying the optical fiber classified as satisfactory.

### Claim 18 (depends on 12)

A method according to claim 12, comprising:
determining whether the effective modal bandwidth is between about 3000 MHz·km and 6000 MHz·km; and
determining whether the radial offset bandwidth as a function of the radial offset values determined in step (iv) is greater than or equal to 6000 MHz·km for all radial-offset values between about 0 and R1, where R1 is between about 70 percent and 90 percent of the radius of the optical fiber's core.

### Claim 19

A method of evaluating and selecting a multimode optical fiber, comprising:
providing a multimode optical fiber having a core, an inlet face, and an outlet face;
injecting into the multimode optical fiber a light pulse of a testing wavelength, the light pulse being injected at a radial offset from the center of the multimode optical fiber's inlet face;
detecting the light pulse at the multimode optical fiber's outlet face to measure the time characteristic of the light pulse after propagation through the multimode optical fiber;
repeating for a plurality of radial-offset values the steps of injecting a light pulse and detecting the light pulse;
determining, at the testing wavelength, the effective modal bandwidth (EMB) of the multimode optical fiber via the measured time characteristics of the light pulse;
assessing if the multimode optical fiber's determined effective modal bandwidth is within a predetermined range;
determining the multimode optical fiber's radial offset bandwidth (ROB) as a function of the radial offset via the measured time characteristics of the light pulse; and
assessing if the multimode optical fiber's determined radial offset bandwidth meets or exceeds a threshold value for all radial-offset values between 0 and R1, wherein R1 is less than or equal to the core radius of the multimode optical fiber.

### Claim 20 (depends on 19)

A method according to claim 19, wherein the step of assessing the multimode optical fiber's determined effective modal bandwidth comprises determining whether the multimode optical fiber's determined effective modal bandwidth is between about 3000 MHz·km and 6000 MHz·km.

### Claim 21 (depends on 19)

A method according to claim 19, wherein the step of assessing if the multimode optical fiber's determined radial offset bandwidth meets or exceeds a threshold value comprises determining whether the multimode optical fiber's radial offset bandwidth is at least 6000 MHz·km for all radial-offset values between 0 and R1.

### Claim 22 (depends on 19)

A method according to claim 19, wherein R1 is between about 70 percent and 90 percent of the core radius of the multimode optical fiber.

### Claim 23

A method of classifying a graded-index multimode optical fiber, comprising:
(i) injecting into the core of a multimode optical fiber a light pulse having a predefined wavelength, wherein the light pulse is injected at a radial offset from the center of the optical fiber's inlet face;
(ii) detecting the light pulse at the optical fiber's outlet face to determine the time characteristic of the light pulse after propagation through the optical fiber from the optical fiber's inlet face to the optical fiber's outlet face; and
(iii) repeating steps (i) and (ii) for a plurality of radial-offset values; and
(iv) determining the radial offset bandwidth (ROB) as a function of the radial offset; and
(v) determining whether the radial offset bandwidth as a function of the radial offset values determined in step (iv) is greater than or equal to 6000 MHz·km for all radial-offset values between about 0 and R1, where R1 is about 70 percent of the radius of the optical fiber's core.

### Claim 24 (depends on 23)

A method according to claim 23, wherein R1 is about 90 percent of the radius of the optical fiber's core.

### Claim 25 (depends on 23)

A method according to claim 23, comprising:
determining an effective modal bandwidth at the predefined wavelength; and
determining whether the effective modal bandwidth is between about 3000 MHz·km and 6000 MHz·km.