Fiber Bragg Gratings

Periodic refraction index change Fiber Grating Fiber grating is made by periodically changing the refraction index in the glass core of the fiber. The refraction changes are made by exposing the fiber to the UV-light with a fixed pattern. Glass core Glass cladding Periodic refraction index change (Gratings) Plastic jacket

Fiber Grating Basics When the grating period is half of the input light wavelength, this wavelength signal will be reflected coherently to make a large reflection. The Bragg Condition Reflection spectrum reflect Transmission spectrum trans. in   n (refraction index difference) r = 2neff 

Fiber Bragg Grating: Theory 1978 – Hill et. all Phenomenon of photosensitivity in optical fibers Exposed Ge-doped core fibers to intense light at 488 or 514 nm Induced permanent refractive index changes to the core. ‘The sinusoidal modulation of the index of refraction in the core due to spatial variation in the writing beam gives rise to a refractive index grating that can be used to couple energy of the fundamental guide mode to various guided and lossy modes.

Fiber Bragg Grating: Theory FBG is a longitudinal periodic variation of the index of refraction in the core of an optical fiber. The spacing of the variation is determined by the wavelength of the light to be reflected. Bragg Wavelength

Fiber Bragg Grating: Theory The Bragg Condition is the result of two requirements: Energy Conservation: Frequency of incident radiation and reflected radiation is the same. Momentum Conservation: Sum of incident wave vector and grating wave vector equal the wave vector of the scattered radiation. K + ki = kf The resulting Bragg Condition is: lB = 2L neff The grating reflects the light at the Bragg wavelength (lB) lB is a function of the grating periodicity (L) and effective index (neff). Typically; lB= 1.5 mm, L = 0.5mm The Bragg wavelength is approx. 2-3 x the periodicity. 1300 – 1500nm (infrared)

Fiber Bragg Grating: Theory The spectral component reflected (not transmitted) typically has a bandwidth of 0.05 – 0.3 nm. A general expression for the approximate Full Width Half Maximum bandwidth of a standard grating is given by (S = grating parameter (.5 to 1), N = numbers of grating pains): Δλ =λ B S( (Δn/2n0)2 + (1/N)2 )1/2

Fiber Bragg Grating: Theory The shift in Bragg Wavelength with strain and temperature can be expressed using: DlB = 2nL({1-(n2/2)[P12 – n(P11 + P12)]}e + [a + (dn/dT)/n]DT Where: e = applied strain Pi,j = Pockel’s coef. of the stress-optic tensor n = Pisson’s ratio a = coef. of thermal expansion DT = temperature change [P12 – n(P11 + P12)] ~ 0.22 The shift in Bragg Wavelength is approximately linear with respect to strain and temperature. Delta-Bragg-Wvlgth/Bragg-Wvlgth = Delta-Grating/Grating + Delta-index/index 1) When fiber is strained, Grating period inc. and index of refraction decreases. Thus they have contrasting effects on Bragg Wavelength. 2)With a perturbation, typically the change in grating index of refraction has the largest effect.

Fiber Bragg Grating: Theory The measured strain response at a constant temperature is found to be: (1/lB)dlB/ de = 0.78 x 10-6me-1 Sensitivity Rule of thumb at lB = 1300nm: 0.001nm/me

Fiber Bragg Grating: Theory The measured temperature response at a constant strain is found to be: (1/lB)dlB/ dT = 6.67 x 10-6 oC-1 Sensitivity Rule of thumb at lB = 1300nm: 0.009nm/ oC

Fiber Bragg Grating: Theory – Blazed Grating Bragg grating planes are tilted at an angle to the fiber axis. Light which otherwise would be guided in the fiber core, is coupled into the loosely bound, guided cladding or radiation modes. The bandwidth of the trapped out light is dependent on the tilt angle of the grating planes and the strength of the index modulation. As shown above, the vector diagram is a result of the conservation of momentum and conservation of energy requirement. The results of applying the law of cosines yealds: Cos(θb) = ׀K׀/2v

Fiber Bragg Grating: Theory – Chirped Grating Bragg grating has a monotonically varying period as illustrated above. These gratings can be realized by axially varying either the period of the grating or the index of refraction of the core or both. The Bragg Condition becomes: λB = 2neff(z)Λ(z) The simplest type of chirped grating is one which the grating period varies linearly with axial length: Λ(z) = Λ0 + Λ(z)

Chirped FBG 0 0 f2 f1 f3 f1 f2 f3 Chirped FBG Dispersion comp. at Incident f1 f2 f3 Chirped FBG Reflected 0 Dispersion comp. at Relative Time Delay (ps) 0 Wavelength (nm) Linearly Chirped Dispersion = dT/d  (ps/nm)

Creating Gratings on Fiber One common way to make gratings on fiber is using Phase Mask for UV-light to expose on the fiber core.

Characteristics of FBG It is a reflective type filter Not like to other types of filters, the demanded wavelength is reflected instead of transmitted It is very stable after annealing The gratings are permanent on the fiber after proper annealing process The reflective spectrum is very stable over the time It is transparent to through wavelength signals The gratings are in fiber and do not degrade the through traffic wavelengths, very low loss It is an in-fiber component and easily integrates to other optical devices

Temperature Impact on FBG The fiber gratings is generally sensitive to temperature change (10pm/°C) mainly due to thermo-optic effect of glass. Athermal packaging technique has to be used to compensate the temperature drift

Types of Fiber Gratings CHARACTERS APPLICATIONS Simple reflective gratings Creates gratings on the fiber that meets the Bragg condition Filter for DWDM, stabilizer, locker Long period gratings Significant wider grating periods that couples the light to cladding Gain flattening filter, dispersion compensation Chirped fiber Bragg gratings A sequence of variant period gratings on the fiber that reflects multiple wavelengths Slanted fiber gratings The gratings are created with an angle to the transmission axis Gain flattening filter

Typical FBG Production Procedures Select Proper fiber H2 loading Laser writing Annealing Athermal packaging Testing Different FBG requires different specialty fiber Increase photo sensitivity for easier laser writing Optical alignment & appropriate laser writing condition Enhance grating stability For temperature variation compensation Spec test

Current Applications of FBG FBG for DWDM FBG for OADM FBG as EDFA Pump laser stabilizer FBG as Optical amplifier gain flattening filter FBG as Laser diode wavelength lock filter FBG as Tunable filter FBG for Remote monitoring FBG as Sensor ….

Possible Use of FBG in System ITU FBG filter Pump stabilizer & Gain flattening filter Wave locker Dispersion compensation filter E/O Dispersion control EDFA Multiplexer OADM Demux EDFA Switch ITU FBG filter Tunable filter Pump stabilizer & Gain flattening filter Monitor sensor ITU FBG filter Monitor

ITU FBG Filter for DWDM ... ... Multiplexer De-multiplexer l1, l2 … ln FBG at l1 l1 l2 Circulator FBG at l2 l3 FBG at l3 ... Multiplexer l1, l2 … ln FBG at l1 l1 l2 Circulator FBG at l2 l3 FBG at l3 ... De-multiplexer

ITU FBG Filter for OADM Circulator Circulator FBG Incoming signal Outgoing signal Through signal Dropped signal Circulator Circulator FBG Added signal

Dispersion Compensation Filter circulator Dispersed pulse Chirped FBG

FBG and Dispersion Compensation l5 l4 l3 l2 l1 t t Fiber Dispersion t t l5 l4 l3 l2 l1 FBG Disp. Comp.

Pump Laser Stabilizer Focal lens Fiber 980 Stabilizer + - Pump Laser spectrum

Gain Flattening Filter Gain profile GFF profile Output