Single fiber bragg grating as delay line interferometer

Patent metadata

Field	Value
Publication number	US-9709745-B2
Application number	US-201414786821-A
Country	US
Kind code	B2
Filing date	Apr 17, 2014
Priority date	Apr 24, 2013
Publication date	Jul 18, 2017
Grant date	Jul 18, 2017

Abstract

A delay line interferometer comprising an optical waveguide having a distributed Bragg reflector, e.g. Bragg grating, fabricated therein. The distributed Bragg reflector has a refractive index modulation with a period variation Λ(z) along its length z that is arranged to output in transmission an output optical signal f out (t) in response to a input optical signal f in (t), wherein the output optical signal f out (t) is the result of temporal interference between one or more time-delayed replicas of the input optical signal f in (t). In other words, the distributed Bragg reflector is operable to generate and permit temporal interference between two or more time-delayed replicas of the input optical signal f in (t). The invention may thus mimic the behaviour of one or more MZIs.

First claim

The invention claimed is:

1. A delay line interferometer, comprising:
an optical waveguide having a Bragg grating fabricated therein, the Bragg grating having a refractive index modulation with a period variation Λ(z) along its length z that is arranged to output in transmission an output optical signal f out (t) in response to a input optical signal f in (t), wherein the output optical signal f out (t) is the result of temporal interference between one or more time-delayed replicas of the input optical signal f in (t),
wherein the optical waveguide is an optical fibre and the Bragg grating is a fibre Bragg grating (FBG), the FBG being a phase-modulated FBG and having a substantially uniform coupling strength amplitude |κ(z)| along its length.

2. The delay line interferometer according to claim 1 , wherein the optical waveguide is arranged to support the propagation of optical radiation between an input portion for receiving the input optical signal f in (t) and an output portion for transmitting the output optical signal f out (t), wherein the Bragg grating is fabricated in the optical waveguide between the input portion and the output portion.

3. The delay line interferometer according to claim 1 , wherein the optical fibre comprises:
a core having a first refractive index, and
a cladding layer surrounding the core, the cladding layer having a second refractive index, the second refractive index being lower than the first refractive index, and
wherein the FBG comprises a refractive index modulation inscribed within the core.

4. The delay line interferometer according to claim 3 , wherein the refractive index modulation is confined within the core.

5. A delay line interferometer according to claim 1 , wherein the spectral response |H T (ω)| of the phase-modulated FBG substantially corresponds to a Fourier transform of an objective impulse response h T,obj (t) of the phase-modulated FBG, where the objective impulse response corresponds to a minimum phase system.

6. The delay line interferometer according to claim 1 , wherein the period variation Λ(z) of the phase-modulated FBG is arranged to output in transmission a time-spaced series of two or more optical pulses in response to a single input pulse.

7. A method of fabricating a phase-modulated fibre Bragg grating (FBG) for a delay line interferometer, the method comprising:
obtaining an objective spectral response |H T,obj (ω)| that is a Fourier transform of an objective impulse response h T,obj (t) of the phase-modulated FBG, where the objective impulse response corresponds to a minimum phase system;
selecting a coupling strength |κ(z)| to be exhibited by the phase-modulated FBG along its length z;
calculating a grating period variation Λ(z) for the phase-modulated FBG using the objective spectral response |H T,obj (ω)| and the coupling strength |κ(z)|; and
inscribing a refractive index modulation having the grating period variation Λ(z) in an optical fibre,
whereby the phase-modulated FBG is operable in transmission to output an output optical signal f out (t) in response to an input optical pulse f in (t).

8. A method according to claim 7 , wherein calculating the grating period variation Λ(z) for the phase-modulated FBG includes performing an iterative numerical optimisation process to bring a calculated spectral response |H T (ω)| towards the objective spectral response |H T,obj (ω)|.

9. A method according to claim 8 , wherein the iterative numerical optimisation process includes:
obtaining the calculated spectral response |H T (ω)| from the coupling strength |κ(z)| and a candidate grating period variation function Λ i (z),
calculating an error between the calculated spectral response |H T (ω)| and the objective spectral response |H T,obj (ω)|, and
selecting the next candidate grating period variation function Λ i+1 (z) based on the error between the calculated spectral response |H T (ω)| and the objective spectral response |H T,obj (ω)|.

10. A method according to claim 7 , wherein inscribing the refractive index modulation includes irradiating the optical fibre with ultraviolet radiation through a phase mask that has the grating period variation Λ(z) encoded thereon.

11. A method according to claim 7 , wherein the objective impulse response h T,obj (t) is Σ n=0 N−1 α n δ(t−nT), where α n and N are selected to ensure correspondence to a minimum phase function.

12. The method according to claim 7 , wherein objective impulse response is selected to correspond to a time-spaced series of two or more optical pulses in response to a single input pulse.

13. A method of fabricating a phase mask for inscribing a phase-modulated fibre Bragg grating (FBG) in an optical fibre, the method comprising:
obtaining an objective spectral response |H T,obj (ω)| that is a Fourier transform of an objective impulse response h T,obj (t) of the phase-modulated FBG, where the objective impulse response corresponds to a minimum phase system;
selecting a coupling strength |κ(z)| to be exhibited by the phase-modulated FBG along its length z;
calculating a grating period variation Λ(z) for the phase-modulated FBG using the objective spectral response |H T,obj (ω)| and the coupling strength |κ(z)|; and
fabricating a phase mask, whereby the grating period variation Λ(z) is encoded across the phase mask.

Assignees

• Univ Huazhong Science Tech

Inventors

• Shu Xuewen
• Preciado Miguel Angel

Classifications

• C03B37/14

  Re-forming fibres or filaments, {i.e. changing their shape}(C03B37/025 takes precedence) · CPC title

• G02B2006/02166

  Methods of designing the gratings, i.e. calculating the structure, e.g. algorithms, numerical methods · CPC title

• G03F1/26

  Phase shift masks [PSM]; PSM blanks; Preparation thereof · CPC title

• G02B6/2861

  using fibre optic delay lines and optical elements associated with them, e.g. for use in signal processing, e.g. filtering (delay lines in general H03H, H01P9/00; optical computing devices G06E) · CPC title

• G02B6/02085

  characterised by the grating profile, e.g. chirped, apodised, tilted, helical · CPC title