# US 5,708,740 A · Optical notch filter manufacture in optical fibre waveguide by plastic deformation

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- Number: US 5,708,740 A
- Document ID: US5708740A
- Status: Granted
- Kind: A
- Application number: 08/628,579
- Filed: 1996-04-04
- Granted: 1998-01-13
- Earliest claimed: 1995-04-04
- Assignees: Northern Telecom Limited
- Inventors: Thomas John Cullen
- Web page: https://patentgrep.com/patent/US5708740A

## CPC

- G02B 6/29352: {in a light guide}
- G02B 6/02071: {Mechanically induced gratings, e.g. having microbends (G02B6/02066 takes precedence; mode converters G02B6/14)}
- G02B 6/0208: {characterised by their structure, wavelength response (G02B6/02114, G02B6/02171, G02B6/02195 take precedence; multiple layer cores or claddings G02B6/036, protective coverings G02B6/4429)}
- G02B 6/14: Mode converters
- G02B 6/2552: {reshaping or reforming of light guides for coupling using thermal heating, e.g. tapering, forming of a lens on light guide ends}
- G02B 6/02052: {comprising optical elements other than gratings, e.g. filters (comprising gratings G02B6/02057)}
- G02B 6/02066: {Gratings having a surface relief structure, e.g. repetitive variation in diameter of core or cladding}
- G02B 6/02147: {Point by point fabrication, i.e. grating elements induced one step at a time along the fibre, e.g. by scanning a laser beam, arc discharge scanning (G02B6/02133 and G02B6/02142 take precedence)}
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

An optical notch filter is created in a bared length of single mode fibre by using a microburner and longitudinal stretching of the fibre to form a set of equispaced sharply localized non-adiabatic biconical tapers at a pitch corresponding to the beat length between the core-guided HE.sub.11 mode and the cladding-guided HE.sub.12 mode, the latter being stripped downstream of the tapers by a plastics protective coating surrounding the fibre.

## Claims

### Claim 1

A method of generating an optical notch filter in an optical fibre by creating therein a resonantly coupled iterative set of axially spaced perturbations each of which is created by axially stretching the fibre to produce plastic deformation in a heat softened zone thereof which is sufficiently sharply localised to result in the formation thereat of a non-adiabatic taper which weakly couples the zero order mode of the fibre with a higher order mode to provide a coupling coefficient small compared with 3 dB and substantially uniform over a wavelength range large compared with the FWHM bandwidth of the notch filter.

### Claim 2 (depends on 1)

A method as claimed in claim 1, wherein the perturbations are substantially identical and created at substantially equispaced intervals along the axial direction of this fibre.

### Claim 3 (depends on 1)

A method as claimed in claim 1, wherein the number of perturbations is at least ten.

### Claim 4 (depends on 1)

An optical fibre notch filter made by the method claimed in claim 1.

### Claim 5 (depends on 4)

An optical fibre amplifier that includes a notch filter as claimed in claim 4.

### Claim 6

A method of generating an optical notch filter in an optical fibre by creating therein a resonantly coupled iterative set of axially spaced perturbations each of which is created by axially stretching the fibre to produce plastic deformation in a heat softened zone thereof which is sufficiently sharply localised to result in the formation thereat of a non-adiabatic taper which is weakly couples the zero order mode of the fibre with a higher order mode to provide a coupling coefficient small compared with 3 dB and substantially uniform over a wavelength range large compared with the FWHM bandwidth of the notch filter wherein the optical fibre is a single mode optical fibre which, prior to the creation of the spaced perturbations, is pretapered to provide, between two full-sized portions of the single mode fibre, two adiabatic tapers joined by a reduced diameter portion of the single mode fibre which zero order mode power is guided substantially exclusively by the interface between the material of the fibre and the ambient surrounding the reduced diameter portion of the fibre, and wherein the spaced perturbations are created in the reduced diameter portion.

### Claim 7 (depends on 6)

A method as claimed in claim 6, wherein the perturbations are substantially identical and created at substantially equispaced intervals along the axial direction of this fibre.

### Claim 8 (depends on 6)

A method as claimed in claim 6, wherein the number of perturbations is at least ten.

### Claim 9 (depends on 6)

An optical fibre notch filter made by the method claimed in claim 6.

### Claim 10 (depends on 9)

An optical fibre amplifier that includes a notch filter as claimed in claim 9.