Method of changing the refractive index in a region of a core of a photonic crystal fiber using a laser

Abstract

Fiber Bragg gratings were written in pure silica photonic crystal fibers and photonic crystal fiber tapers with 125 fs, 800 nm IR radiation. High reflectivites were achieved with short exposure times in the tapers. Both multimode and single mode grating reflections were achieved in the fiber tapers. By tapering the photonic crystal fibers scattering that would otherwise have occurred was lessened and light external to the fiber could reach the core effectively to write a grating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described in conjunction with the drawings in which:

FIG. 1 a photograph in the form of a cross-sectional view of a first PCF.

FIG. 1 b is a cross-sectional view of an alternate type of PCF having a different diameter, number of holes and hole spacing than the PCF of FIG. 1a.

FIG. 2 is a view of a photonic crystal fiber shown being tapered and having a collapsed region.

FIG. 3 is a graph of a transmission reflection spectrum vs wavelength for an ESM-12-01 PCF.

FIG. 4 a is photograph of a microscope image of a tapered region of a PCF with collapsed holes, that is, no visible holes collapsed along entire 12 mm long 30 μm diameter taper waist;

FIG. 4 b is a photograph of a microscope image of grating inscribed in hole collapsed region as viewed normal to irradiating IR beam;

FIG. 4 c is a photograph of a microscope image of grating inscribed in hole collapsed region as viewed along irradiating IR beam axis;

FIG. 5 is a graph of a transmission spectrum for grating written in 30 μm diameter LMA-15 fiber taper shown in FIGS. 4b and 4c.

FIG. 6 a and FIG. 6b show transmission and reflection spectra respectively of a grating in a 55-μm LMA-15 fiber taper.

FIG. 6 c is a photograph of a microscope image of the 1.6 μm period grating in the taper.

FIG. 7 is a graph of a transmission spectra transmission (gray) and reflection (black) spectra of a grating in 48 μm LMA-15 fiber taper without hole closure.

FIG. 8 is a graph of a mode-field profile of the mode exiting the SMF-28 fiver (black trace) and the 55-μm diameter LMA-15 PCF taper (gray trace).

Claims

1. A method of changing the refractive index in a region of the core of a photonic crystal waveguide comprising the steps of: a) providing a photonic crystal waveguide having a core and having a cladding with holes therein;b) changing an aspect of the response of the cladding to light, so as to lessen unwanted scattering in the cladding that would otherwise occur when directing laser light to the core through the cladding; and,c) irradiating the photonic crystal waveguide from a side with laser light having an intensity and duration so as to effect a permanent refractive index change within the region of the core of the photonic crystal waveguide of at least 1×10−5.

2. A method as defined in claim 1, wherein the photonic crystal waveguide is a PCF, and wherein step (b) comprises the step of tapering the PCF to form a tapered region, and wherein step (c) comprises the step of irradiating the tapered region.

3. A method as defined in claim 2 wherein the step of tapering the PCF includes tapering so as to at least partially deform the holes within the PCF.

4. A method as defined in claim 2 wherein the step of tapering includes tapering so as to collapse the holes within the PCF.

5. A method as defined in claim 4 wherein the laser light is pulsed.

6. A method as defined in claim 5 wherein the pulses of laser light are of femtosecond pulse durations.

7. A method as defined in claim 5 wherein the pulses of light are pulses in the infrared band of wavelengths.

8. A method as defined in claim 1 wherein the photonic crystal waveguide is a PCF and wherein the PCF is tapered to an extent that allows some light to propagate through the cladding that would otherwise have scattered in the absence of tapering.

9. A method as defined in claim 8, wherein the PCF is tapered so that a waist region results that is sufficiently narrow so as to allow at least 10% of light propagating from a side directed to the core, to reach the core.

10. A method as defined in claim 8, wherein the PCF is tapered sufficiently so as to mismatch the hole size and spacing with an irradiating wavelength so as to allow at least 10% of light propagating from a side directed to the core, to reach the core.

11. A method of inducing a spatially modulated refractive index pattern in a photonic crystal optical fiber, comprising the steps of: providing the photonic crystal optical fiber;tapering the photonic crystal optical fiber adiabatically such that the photonic crystal optical fiber propagates in single mode electromagnetic radiation having a predetermined wavelength range andcollapsing the holes in the tapered region of said photonic crystal optical fiber such that light propagating along the collapsed region fills the collapsed region cross-section;wherein the collapsed region is transmissive to electromagnetic radiation having a predetermined wavelength range;disposing a mask to be used as an interferometer, adjacent the photonic crystal optical fiber such that light incident upon the mask is transmitted directly into said optical fiber; and,providing electromagnetic radiation on a surface of the mask, the electromagnetic radiation having a predetermined wavelength range and having a pulse duration of less than or equal to 500 picoseconds, wherein the mask is disposed to permit a portion of the electromagnetic radiation to interact with the mask and be incident on the photonic crystal optical fiber, the interaction of the electromagnetic radiation with the mask for producing a spatial intensity modulation pattern within the photonic crystal optical fiber, the electromagnetic radiation incident on the photonic crystal optical fiber or waveguide being sufficiently intense to cause a change in an index of refraction of the photonic crystal optical fiber, wherein electromagnetic radiation interacting with the surface of the mask having a sufficiently low intensity to not significantly alter produced spatial intensity modulation properties of the mask.

12. A method as defined in claim 1 wherein step (b) comprises the step of introducing a refractive index matching fluid into the cladding of the photonic crystal waveguide in at least the region of the cladding to be irradiated, and wherein step (b) is preformed before step (c).

13. A method as defined in claim 12, wherein the index matching fluid is introduced via the holes and wherein the refractive index matching fluid is more closely matched in refractive index to the photonic crystal waveguide material, than to air.