In-Fibre Bragg Gratings (FBGs)
In-Fibre Bragg Gratings (FBGs)
What is FBG? An in-fibre Bragg grating is constructed by varying the refractive index of the core lengthwise along the fibre Light of the specified wavelength traveling along the fibre is reflected from the grating back in the direction from which it came Wavelengths which are not selected are passed through with little or no attenuation
• An in-fibre Bragg grating is constructed by varying the refractive index of the core lengthwise along the fibre. • Light of the specified wavelength traveling along the fibre is reflected from the grating back in the direction from which it came. • Wavelengths which are not selected are passed through with little or no attenuation. What is FBG?
Fiber Bragg Grating 200 um Long-period grating 500 nm Bragg grating 125um Cladding 8 Core 1 mm to 1500 mm Singlemode fibre Regions with higher refractive index than that of core’s
Fiber Bragg Grating ~ 500 nm (Bragg grating) ~ 200 µm (Long-period grating) 125 µm 8 µm 1 mm to 1500 mm Singlemode fibre Core Cladding Regions with higher refractive index than that of core’s
It works like this Input signal 4IIIIIIID Selected NON. Sooted Wavelengh wawdengshs resonant wavelengths are reflected back toward the source non-resonant wavelengths are transmitted through the device without loss The centre wavelength is given by =2n eff
It works like this • resonant wavelengths are reflected back toward the source • non-resonant wavelengths are transmitted through the device without loss. • The centre wavelength is given by: = 2neff
Principle of Operation 辑辑菲 The grating forms an electromagnetic resonant circuit. Power from the forward direction is coupled into the resonant circuit and then reflected back. Non-resonant wavelengths are not afected very much
Principle of Operation The grating forms an electromagnetic resonant circuit. Power from the forward direction is coupled into the resonant circuit and then reflected back. Non-resonant wavelengths are not affected very much
Parameters Length unTIl Period The grating period is the distance between modulations of the refractive index in the grating ° The grating length. The " modulation depth, determined by the Ri contrast within the grating ° The RI contrast profile
Parameters • The grating period is the distance between modulations of the refractive index in the grating. • The grating length. • The “modulation depth” , determined by the RI contrast within the grating. • The RI contrast profile
Characteristics of In-Fibre Bragg Gratings Centre Wavelength This is the wavelength at the centre of the gratings reflection band Bandwidth This is the width of the reflection band and specifies the range of wavelengths reflected Reflectance Peak This is a measure of the proportion of incident light reflected at the centre wavelength
Characteristics of In-Fibre Bragg Gratings • Centre Wavelength – This is the wavelength at the centre of the grating's reflection band. • Bandwidth – This is the width of the reflection band and specifies the range of wavelengths reflected. • Reflectance Peak – This is a measure of the proportion of incident light reflected at the centre wavelength
Typical Reflection Spectra (b) unwanted reflections (a typical reflection spectrum of a 1 cm long FBG with relatively low RI contrast. The height of the peak is 100% reflection and the width of the reflection band is. 2 nm;(b) same grating with a stronger contrast. The reflection band has been broadened; (c) same grating as part (a)but after apodisation. Note the reflection peak is now not quite 100%
Typical Reflection Spectra (a) typical reflection spectrum of a 1 cm long FBG with relatively low RI contrast. The height of the peak is 100% reflection and the width of the reflection band is .2 nm; (b) same grating with a stronger contrast. The reflection band has been broadened; (c) same grating as part (a) but after apodisation. Note the reflection peak is now not quite 100%
Apodisation the grating Apodisation is a process of tapering the strength of the grating at either end so that the apparent Ri change is gradual rather than abrupt. The reflection band of an apodised grating is shown in part (c)of the figure
Apodisation the grating • Apodisation is a process of tapering the strength of the grating at either end so that the apparent RI change is gradual rather than abrupt. The reflection band of an apodised grating is shown in part (c) of the figure
Chirped FBGs 匚 I Longer Wavelengths Shorter Wavelengths A"is where you get a variation in the period of the le grating (and hence a variation in its response to different wavelengths)along the length of the grating vary the period of the grating or vary the average Ri of the grating
Chirped FBGs • A “chirp” is where you get a variation in the period of the grating (and hence a variation in its response to different wavelengths) along the length of the grating: – vary the period of the grating or – vary the average RI of the grating